Neuroendocrinology

"LITTLE BRAIN" IN THE GUT: ENTERIC NERVOUS SYSTEM

"LITTLE BRAIN" IN THE GUT: ENTERIC NERVOUS SYSTEM

Quick note on 5-HT released by Enterochromaffin cells: link with depression and anxiety

5-HT signalling acts locally to reduce GI motility - which is thought to be disrupted in Irritable Bowel Syndrome. However, disrupted 5-HT signalling on the afferent vagus nerve is thought to be a potential underlying cause of depression.

Leptin

A hormone predominantly produced and secreted from adipose cells that helps to regulate energy balance by inhibiting hunger. It acts on receptors in the arcuate nucleus (AgRP/NPY; POMC/CART) of the hypothalamus to regulate energy homeostatsis

Vasopressin

A hormone synthesized as a peptide prohormone or produced in the magnocellular neurosecretory cells of the Supraoptic and Paraventricular nucleus, and is then released into the blood from the posterior lobe (neurohypophysis) of the pituitary glan with a neurophysin (a carrier protein) into the posterior pituitary where is hydrolyzed into AVP.

Oxyntomodulin

A naturally occuring 37 amino acid peptide secreted from colonic L-cells in proportion to meal size. Like GLP-1, it also binds to the GLP-1 receptor in the arcuate nucleus, however it does not bind to GLP-1 receptors in pancreatic Beta cells. As a result, it only functions to suppress appetite.

Oxytocin

A peptide and neurohormone normally produced in the Paraventricular Nucleus of the Hypothalamus and secreted by the posterior pituitary.

Ghrelin

A peptide hormone produced by ghrelinergic cells in the stomach which functions as an orexigenic neuropeptide in the CNS.

Arcuate nucleus

A region of the hypothalamus which is the main regulatory organ for human appetite.

Roles of AVP

AVP increases the amount of solute-free water reabsorbed back into the circulation from the filtrate in the kidney tubules of the nephrons. AVP also constricts arterioles, which increases peripheral vascular resistance and raises arterial blood pressure. AVP is also directly released into the brain from the hypothalamus. and may play an important role in social beaviour, sexual motivation, pair bonding, and maternal responses to stress.

Sensory Neurons - IPANs (Intrinsic Primary Afferent Nerves):

All have Type II morphology with AH properties. --> Muscosal chemosensors: Stimuli include acid or alkaline solutions and 5-HT. All initiate intrinsic reflexes (5HT via 5HT3 receptors on IPANS). -- Mucosal mechanoreceptors: Stroking mucosa can distort myenteric as well as submucous IPANS and this initiates the peristaltic reflex

α-MSH

An agonist of Melanocortin Receptors, MC1, MC3, MC4, MC5 but not MC2 Activation of the MC1 receptor is responsible for its effect on Pigmentation MC3 and MC4 receptors are responsible for its effect on appetite and metabolism. Agouti acts only on MC4R to increase food intake. MC2 is only for Adrenocorticotropic hormone (ACTH)

Types of Enteric Neurons

Approximately 20 types of enteric neurons can be defined by their functions (Brookes and Costa 2002; Furness 2006). Combinations of features (morphology, neurochemical properties, cell physiology and projections to targets) help to define each type. Amongst the 20 types, three classes can be identified, intrinsic primary afferent neurons (IPANs, also referred to as intrinsic sensory neurons), interneurons and motor neurons. IPANs detect the physical state of the organs (for example, tension in the gut wall) and chemical features of the luminal contents (Furness et al. 2004). They react to these signals to initiate appropriate reflex control of motility, secretion and blood flow. IPANs connect with each other, with interneurons and directly with motor neurons. Interneurons connect with other interneurons and with motor neurons. Amongst the motor neurons are muscle motor neurons, secretomotor neurons, secretomotor/ vasodilator neurons and vasodilator neurons.

Ghrelin action on mice

Asakawa et al BACKGROUND AND AIMS: Ghrelin, an endogenous ligand for growth hormone secretagogue receptor (GHS-R), is an appetite stimulatory signal from the stomach with structural resemblance to motilin. We examined the effects of the gastric peptide ghrelin and GHS-R antagonists on energy balance and glycaemic control in mice. MATERIALS AND METHODS: Body weight, fat mass, glucose, insulin, and gene expression of leptin, adiponectin, and resistin in white adipose tissue (WAT) were measured after repeated administrations of ghrelin under a high fat diet. Gastric ghrelin gene expression was assessed by northern blot analysis. Energy intake and gastric emptying were measured after administration of GHS-R antagonists. Repeated administration of GHS-R antagonist was continued for six days in ob/ob obese mice. RESULTS: Ghrelin induced remarkable adiposity and worsened glycaemic control under a high fat diet. Pair feeding inhibited this effect. Ghrelin elevated leptin mRNA expression and reduced resistin mRNA expression. Gastric ghrelin mRNA expression during fasting was increased by a high fat diet. GHS-R antagonists decreased energy intake in lean mice, in mice with diet induced obesity, and in ob/ob obese mice; it also reduced the rate of gastric emptying. Repeated administration of GHS-R antagonist decreased body weight gain and improved glycaemic control in ob/ob obese mice. CONCLUSIONS: Ghrelin appears to be closely related to excess weight gain, adiposity, and insulin resistance, particularly under a high fat diet and in the dynamic stage. Gastric peptide ghrelin and GHS-R may be promising therapeutic targets not only for anorexia-cachexia but also for obesity and type 2 diabetes, which are becoming increasingly prevalent worldwide.

OT antagonist atosiban used as tocolytic agent

Atosiban is a nonapeptide, desamino-oxytocin analogue, and a competitive vasopressin/oxytocin receptor antagonist (VOTra). Atosiban inhibits the oxytocin-mediated release of inositol trisphosphate from the myometrial cell membrane. As a result, there is reduced release of intracellular, stored calcium from the sarcoplasmic reticulum of myometrial cells, and reduced influx of Ca2+ from the extracellular space through voltage gated channels. In addition, atosiban suppresses oxytocin-mediated release of PGE and PGF from the decidua.

Ghrelin levels in Anorexia and Obesity

Barrios et al. OBJECTIVE: To study the influence of dietary intervention on plasma ghrelin levels in obese children and adolescents with anorexia nervosa (AN). Study design Prepubertal obese children (n=16) and 16 anorexic adolescents were studied at three different points: clinical diagnosis and after a reduction or recuperation of 25% and 50% of the initial BMI SD score. We analyzed plasma ghrelin levels and their correlation with plasma levels of leptin, insulin, insulin-like growth factor I (IGF-I), insulin-like growth factor binding protein 1 (IGFBP-1), insulin-like growth factor binding protein 2 (IGFBP-2), insulin-like growth factor binding protein 3 (IGFBP-3), and integrated concentration of growth hormone (ICGH) before and after dietary intervention. RESULTS: At diagnosis, ghrelin levels were significantly decreased in obese children (52% of control levels) and significantly increased in adolescents with AN (164% of control levels). After dietary intervention, ghrelin increased in obese patients but without reaching control levels even after a 50% reduction in body mass index (BMI). In adolescents with AN, ghrelin levels normalized after a 25% increase in BMI. Ghrelin correlated negatively with the BMI and positively with IGFBP-1 levels in controls but not in obese patients or in patients with AN at diagnosis. The positive correlation between ghrelin and IGFBP-1 was recovered after dietary intervention. CONCLUSIONS: Plasma ghrelin levels present opposite changes in obesity and AN after dietary intervention, suggesting that ghrelin is a good marker of nutritional status.

PYY3-36 inhibition of food intake in obese patients

Batterham et al. BACKGROUND: The gut hormone fragment peptide YY3-36 (PYY) reduces appetite and food intake when infused into subjects of normal weight. In common with the adipocyte hormone leptin, PYY reduces food intake by modulating appetite circuits in the hypothalamus. However, in obesity there is a marked resistance to the action of leptin, which greatly limits its therapeutic effectiveness. We investigated whether obese subjects were also resistant to the anorectic effects of PYY. METHODS: We compared the effects of PYY infusion on appetite and food intake in 12 obese and 12 lean subjects in a double-blind, placebo-controlled, crossover study. The plasma levels of PYY, ghrelin, leptin, and insulin were also determined. RESULTS: Caloric intake during a buffet lunch offered two hours after the infusion of PYY was decreased by 30 percent in the obese subjects (P<0.001) and 31 percent in the lean subjects (P<0.001). PYY infusion also caused a significant decrease in the cumulative 24-hour caloric intake in both obese and lean subjects. *PYY infusion reduced plasma levels of the appetite-stimulatory hormone ghrelin*. Endogenous fasting and postprandial levels of PYY were significantly lower in obese subjects (the mean [+/-SE] fasting PYY levels were 10.2+/-0.7 pmol per liter in the obese group and 16.9+/-0.8 pmol per liter in the lean group, P<0.001). Furthermore, the fasting PYY levels correlated negatively with the body-mass index (r = -0.84, P<0.001). CONCLUSIONS: We found that obese subjects were not resistant to the anorectic effects of PYY. Endogenous PYY levels were low in the obese subjects, suggesting that PYY deficiency may contribute to the pathogenesis of obesity.

Phasic release of AVP vs burst OT release

Because AVP is a controlled mechanism used for restoring osmolarity when it is too low. As a homestatic mechanism, it is almost always active, and therefore a highly-efficient mechanism needs to be in place. The release of AVP is seen to be phasic, and there is a maintained release to restore osmolarity

Post-embryonic ablation of AgRP neurons in mice leads to a lean, hypophagic phenotype

Bewick Agouti-related protein (AgRP) and neuropeptide Y (NPY) are colocalized in arcuate nucleus (arcuate) neurons implicated in the regulation of energy balance. Both AgRP and NPY stimulate food intake when administered into the third ventricle and are up-regulated in states of negative energy balance. However, mice with targeted deletion of either NPY or AgRP or both do not have major alterations in energy homeostasis. Using bacterial artificial chromosome (BAC) transgenesis we have targeted expression of a neurotoxic CAG expanded form of ataxin-3 to AgRP-expressing neurons in the arcuate. This resulted in a 47% loss of AgRP neurons by 16 weeks of age, a significantly reduced body weight, (wild-type mice (WT) 34.7+/-0.7 g vs. transgenic mice (Tg) 28.6+/-0.6 g, P<0.001), and reduced food intake (WT 5.0+/-0.2 vs. Tg 3.6+/-0.1 g per day, P<0.001). Transgenic mice had significantly reduced total body fat, plasma insulin, and increased brown adipose tissue UCP1 expression. Transgenic mice failed to respond to peripherally administered ghrelin but retained sensitivity to PYY 3-36. These data suggest that postembryonic partial loss of AgRP/NPY neurons leads to a lean, hypophagic phenotype. PROS: same seen for mice engineered to be receptive for diphteria toxin in AgRP neurons - same weight loss

Osmsoreceptors regulate AVP release

Boone Osmoreceptors in the collecting duct sense water levels If osmolarity is high, osmoreceptors in the collecting duct signals to the hypothalamus to release more AVP from the magnocellular neurons to the posterior pituitary, and then peripherally Likewise, if osmolarity is too low the collecting duct sends afferent signals to the hypothalamus to inhibit AVP release, and internalise some of the AVP present

CONTROL OF THE ANTERIOR PITUITARY

CONTROL OF THE ANTERIOR PITUITARY

Identification of diabetes (db) gene which encodes Ob-R

Chen et al Up to now we have started to understand that Leptin is secreted by adipose tissue, is expressed by the Ob gene, and plays roles in energy expenditure. However, we have not been able to find the gene which encodes the receptor for Leptin (OB-R) In this experiment, researchers identified the gene responsible for coding the high-affinity receptor for Leptin, OB-R. This gene found to encode this receptor was the diabetes (db) gene Mice with homozygous recessive mutations in the db gene also produce OB-R, but these receptors have a pre-maturely terminated intracellular domain As a result, these OB-R are unable to initiate intracellular signal transduction As a result, db/db mice, whilst they produce Leptin, are incapable of signalling bindings to receptors in the arcuate nuceus, which results in an obesity phenotype like observed in Ob/Ob mice

GPR119 sensing fat stimulates GLP-1 release

Chu et al. We recently showed that activation of G protein-coupled receptor 119 (GPR119) (also termed glucose dependent insulinotropic receptor) improves glucose homeostasis via direct cAMP-mediated enhancement of glucose-dependent insulin release in pancreatic beta-cells. Here we show that GPR119 also stimulates incretin hormone release (GLP-1) and thus may regulate glucose homeostasis by this additional mechanism. GPR119 mRNA was found to be expressed at significant levels in intestinal subregions that produce glucose-dependent insulinotropic peptide and glucagon-like peptide (GLP)-1. Furthermore, in situ hybridization studies indicated that most GLP-1-producing cells coexpress GPR119 mRNA. In GLUTag cells, a well-established model of intestinal L-cell function, the potent GPR119 agonist AR231453 stimulated cAMP accumulation and GLP-1 release. When administered in mice, AR231453 increased active GLP-1 levels within 2 min after oral glucose delivery and substantially enhanced total glucose-dependent insulinotropic peptide levels. Blockade of GLP-1 receptor signaling with exending reduced the ability of AR231453 to improve glucose tolerance in mice. Conversely, combined administration of AR231453 and the DPP-4 inhibitor sitagliptin to wild-type mice significantly amplified both plasma GLP-1 levels and oral glucose tolerance, relative to either agent alone. In mice lacking GPR119, no such enhancement was seen. Thus, GPR119 regulates glucose tolerance by acting on intestinal endocrine cells as well as pancreatic beta-cells. These data also suggest that combined stimulation of incretin hormone release and protection against incretin hormone degradation may be an effective antidiabetic strategy.

CART

Cocaine and amphetamine-regulated transcript (CART) Is an anorexigenic peptide co-expressed with POMC in arcuate nucleus neurons Food-deprived animals (low leptin) have decreased CART expression CART absent in animal models of obesity with disrupted leptin signalling (ob/ob or db/db) Peripheral administration of leptin to ob/ob mice increases CART mRNA expression When injected into rat brain CART decreases normal and starvation-induced feeding and blocks the feeding response induced by Neuropeptide Y (NPY)

Serume Leptin concentrations in normal-weight and Obese Humans --> Obese patients become insensitive to Leptin

Considine et al BACKGROUND: Leptin, the product of the ob gene, is a hormone secreted by adipocytes. Animals with mutations in the ob gene are obese and lose weight when given leptin, but little is known about the physiologic actions of leptin in humans. METHODS: Using a newly developed radioimmunoassay, wer measured serum concentrations of leptin in 136 normal-weight subjects and 139 obese subjects (body-mass index, > or = 27.3 for men and > or = 27.8 for women; the body-mass index was defined as the weight in kilograms divided by the square of the height in meters). The measurements were repeated in seven obese subjects after weight loss and during maintenance of the lower weight. The ob messenger RNA (mRNA) content of adipocytes was determined in 27 normal-weight and 27 obese subjects. RESULTS: The mean (+/- SD) serum leptin concentrations were 31.3 +/- 24.1 ng per milliliter in the obese subjects and 7.5 +/- 9.3 ng per milliliter in the normal-weight subjects (P < 0.001). There was a strong positive correlation between serum leptin concentrations and the percentage of body fat (r = 0.85, P < 0.001). The ob mRNA content of adipocytes was about twice as high in the obese subjects as in the normal-weight subjects (P < 0.001) and was correlated with the percentage of body fat (r = 0.68, P < 0.001) in the 54 subjects in whom it was measured. In the seven obese subjects studied after weight loss, both serum leptin concentrations and ob mRNA content of adipocytes declined, but these measures increased again during the maintenance of the lower weight. CONCLUSIONS: Serum leptin concentrations are correlated with the percentage of body fat, suggesting that most obese persons are insensitive to endogenous leptin production. CONS: did not differentiate participants by whether or not they had Ob/Ob gene mutations

ANS vs ENS

Conventional textbook descriptions of the autonomic nervous system depict efferent pathways from the CNS as two neurons in series, a preganglionic and a postganglionic neuron, and depict sensory information flowing from the periphery to the CNS through spinal and cranial primary afferent neurons. The organization of the ENS and of neuronal pathways to and from the intestine does not, however, follow these conventional concepts of the organization of the nervous system. For example, axons of neurons with cell bodies in the ENS (called intestinofugal neurons) project to sympathetic ganglia, the pancreas, gallbladder and trachea, and to the spinal cord and brain stem.

PYY3-36 inhibits food intake (antagonizes NPY gene expression -> increasing POMC gene expression)

Cowley et al. Food intake is regulated by the hypothalamus, including the melanocortin and neuropeptide Y (NPY) systems in the arcuate nucleus. The NPY Y2 receptor (Y2R), a putative inhibitory presynaptic receptor, is highly expressed on NPY neurons in the arcuate nucleus, which is accessible to peripheral hormones. Peptide YY(3-36) (PYY(3-36)), a Y2R agonist, is released from the gastrointestinal tract postprandially in proportion to the calorie content of a meal. Here we show that peripheral injection of PYY(3-36) in rats inhibits food intake and reduces weight gain. PYY(3-36) also inhibits food intake in mice but not in Y2r-null mice, which suggests that the anorectic effect requires the Y2R. Peripheral administration of PYY(3-36) increases c-Fos immunoreactivity in the arcuate nucleus and *decreases hypothalamic Npy messenger RNA*. Intra-arcuate injection of PYY(3-36) inhibits food intake. PYY(3-36) also *inhibits electrical activity of NPY nerve terminals, thus activating adjacent pro-opiomelanocortin (POMC) neurons. In humans, infusion of normal postprandial concentrations of PYY(3-36) significantly decreases appetite and reduces food intake by 33% over 24 h. Thus, postprandial elevation of PYY(3-36) may act through the arcuate nucleus Y2R to inhibit feeding in a gut-hypothalamic pathway.

Ghrelin and actions on Arcuate Nucleus

Cowley et al. The gastrointestinal peptide hormone ghrelin stimulates appetite in rodents and humans via hypothalamic actions. In this experiment, they discovered expression of ghrelin in a group of neurons adjacent to arcuate hypothalamic nuclei. These neurons send efferents onto key hypothalamic circuits. These circuits obviously include NPY/AgRP neurons, as well as POMC neurons. Within the hypothalamus, ghrelin binds mostly on pre-synaptic terminals of NPY neurons. Using electrophysiological measurements, they showed that ghrelin stimulated the activity of arcuate NPY neurons and mimicked the effect of NPY in the paraventricular hypothalamus. At these sites, release of ghrelin may stimulate the release of orexigenic peptides (MCH, orexin) and neurotransmitters, thus representing anovel regulatory circuit controlling energy homeostasis.

Peptide YY K.O. mice showed no GPR119 response

Cox et al Peptide YY (PYY) is released following food intake and regulates intestinal function and glucose homeostasis, but the mechanisms underpinning these processes are unclear. Enteroendocrine L cells contain PYY and express the acylethanolamine receptor, Gpr119. Here, we show that Gpr119 activation inhibited epithelial electrolyte secretion in human and mouse colon in a glucose-sensitive manner. Endogenous PYY selectively mediated these effects, since *PYY(-/-) mice showed no Gpr119 response*, but responses were observed in NPY(-/-) mice. Importantly, *Gpr119 responses in wild-type (WT) mouse tissue and human colon were abolished by Y(1) receptor antagonism*, but were not enhanced by dipeptidylpeptidase IV blockade, indicating that PYY processing to PYY(3-36) was not important. In addition, Gpr119 agonism reduced glycemic excursions after oral glucose delivery to WT mice but not PYY(-/-) mice. Taken together, these data demonstrate a previously unrecognized role of PYY in mediating intestinal Gpr119 activity and an associated function in controlling glucose tolerance.

Inflammation: the link between obesity and Leptin resistance

Dandona et al Recent data have revealed that the plasma concentration of inflammatory mediators, such as tumour necrosis factor-alpha (TNF-alpha) and interleukin-6 (IL-6), is increased in the insulin resistant states of obesity and type 2 diabetes, raising questions about the mechanisms underlying inflammation in these two conditions. It is also intriguing that an increase in inflammatory mediators or indices predicts the future development of obesity and diabetes. Two mechanisms might be involved in the pathogenesis of inflammation. Firstly, glucose and macronutrient intake causes oxidative stress and inflammatory changes. Chronic overnutrition (obesity) might thus be a proinflammatory state with oxidative stress. Secondly, the increased concentrations of TNF-alpha and IL-6, associated with obesity and type 2 diabetes, might interfere with Leptin or Insulin action by suppressing Leptin or Insulin signal transduction. --> interesting note: signal transduction also impeded in db/db mice who have shortened OB-R mRNA which is able to transduce throught the plasma membrane This might interfere with the anti-inflammatory effect of insulin, which in turn might promote inflammation.

Dogiel Type I vs Dogiel Type II

Dogiel Type I are synaptic neurons --> Similar to other neurons in ANS (low resting potential, fast ACh EPSPs) Dogiel Type II are after-polarising neurons: Only found in ENS, multi-axonal and recently shown to express Calcium-sensitive K+ channels --> These neurons are Intrinsic Primary Afferent Neurons (IPANs) ((Type III: no ap or AH, but have fast synaptic inputs Type IV: low excitability, no AH, but do exhibit TTX-sensitive single spikes))

Dogiel Classification of Nerves

Dogiel Type I: Flattened with 4-20 dendrite, 1 long axon - MOTOR Dogiel Type II: Angular, star-shaped, 3-10 longer dendrites, multiaxonal - SENSORY 14% of submucosal plexus neurons are sensory neurons - Dogiel type II, also known as enteric primary afferent neurons or intrinsic primary afferent neurons. Dogiel Type III: Oval cell bodies, 2-10 dendrites that end within ganglion - NO SINGLE FUNCTION

Agouti antagonism of MC4-R

Fan et al Dominant alleles at the agouti locus (A) cause an obesity syndrome in the mouse, as a consequence of ectopic expression of the agouti peptide. This peptide, normally only found in the skin, is a high-affinity antagonist of the melanocyte-stimulating hormone receptor (MC1-R, thus explaining the inhibitory effect of agouti on eumelanin pigment synthesis. The agouti peptide is also an antagonist of the hypothalamic melanocortin-4 receptor (MC4-R), ((but not MC3-R)). To test the hypothesis that agouti causes obesity by antagonism of hypothalamic melanocortin receptors, we identified cyclic melanocortin analogues that are potent agonists or antagonists of the neural MC3 and MC4 receptors. ICV administration of the MC4-R agonist, MTII, inhibited feeding in four models of hyperphagia: fasted C57BL/6J, ob/ob, and AY mice, and mice injected with neuropeptide Y. Co-administration of the specific melanocortin antagonist completely blocked this inhibition. Furthermore, administration of melanocortin antagonist significantly enhanced nocturnal feeding, or feeding stimulated by a prior fast. Our data show that melanocortinergic neurons exert a tonic inhibition of feeding behaviour. Chronic disruption of this inhibitory signal is a likely explanation of the agouti obesity syndrome. CONS: why nocturnal feeding?

Role of MC4 Receptor

Fan et al The melanocortin-4 receptor (MC4-R) is a G protein-coupled, seven-transmembrane receptor expressed in the brain. Inactivation of this receptor by gene targeting results in mice that develop a maturity onset obesity syndrome associated with hyperphagia, hyperinsulinemia, and hyperglycemia. This syndrome recapitulates several of the characteristic features of the agouti obesity syndrome, which results from ectopic expression of agouti protein, a pigmentation factor normally expressed in the skin. Our data identify a novel signaling pathway in the mouse for body weight regulation and support a model in which the primary mechanism by which agouti induces obesity is chronic antagonism of the MC4-R

Leptin replacement therapy in Leptin-deficient adults

Farooqi et al Administration of Leptin in Ob/Ob mice corrects their obesity by reducing food intake and increasing energy expenditure. These mice also have hyperinsulinemia, corticosterone excess, and infertility, which also are reversed by treatment with Leptin In humans, serum leptin concentrations , in general, correlate positively with indexes of obesity In this case they investigated the efficacy of Leptin replacement therapy on a Leptin-deficient child. The patient had normal weight at birth but began gaining weight excessively at about four months of age. She had marked hyperphagia, was constantly hungry, demanded food continually, and was disruptive when denied food. She was a homozygous recessive carrier of a point mutation in the Ob gene. In an attempt to improve her mobility, liposuction was performed to remove fat from her legs when she was six years old. She came from a highly consanguineous family of Pakistani origin; her parents were first cousins who were not severely obese. Leptin injections given everyday resulted in drastic weight loss as early as two weeks. This was seen by increased energy expenditure (basal metabolic rate) and reduced calorie intake. Body fat index of the child also decreased in the child. CONS: child have different symptoms to Ob/Ob mice, such as no reduced oxygen intake, no change in temperature, no pubertal deficiency / insterility. Most people do not have a defective Leptin gene.

MC4-R deficient humans have obesity phenotype

Farooqi et al BACKGROUND Melanocortin 4 receptor (MC4R) deficiency is the commonest monogenic form of obesity. However, the clinical spectrum and mode of inheritance have not been defined, pathophysiological mechanisms leading to obesity are poorly understood, and there is little information regarding genotype-phenotype correlations. METHODS We determined the nucleotide sequence of the MC4R gene in 500 probands with severe childhood obesity. Family studies were undertaken to examine cosegregation of identified mutations with obesity. Subjects with MC4R deficiency underwent metabolic and endocrine evaluation; the results were correlated with the signaling properties of mutant receptors. RESULTS Twenty-nine probands (5.8 percent) had mutations in MC4R; 23 were heterozygous, and 6 were homozygous. Mutation carriers had severe obesity, increased lean mass, increased linear growth, hyperphagia, and severe hyperinsulinemia; homozygotes were more severely affected than heterozygotes. Subjects with mutations retaining residual signaling capacity had a less severe phenotype. CONCLUSIONS Mutations in MC4R result in a distinct obesity syndrome that is inherited in a codominant manner. Mutations leading to complete loss of function are associated with a more severe phenotype. The correlation between the signaling properties of these mutant receptors and energy intake emphasizes the key role of this receptor in the control of eating behavior in humans. PROS: MC4-R KO mice show heavily increased body weight by as early as 8 weeks

Future: Combination of Neuropeptides?

Field et al OBJECTIVE: Peptide YY(3-36) (PYY(3-36)), a Y2 receptor agonist, and oxyntomodulin, a glucagon-like peptide 1 (GLP-1) receptor agonist, are cosecreted by intestinal L-cells after each meal. Separately each hormone acts as an endogenous satiety signal and reduces appetite in humans when infused intravenously. The aim of the current study was to investigate whether the anorectic effects of PYY(3-36) and oxyntomodulin can be additive. METHODS: Twelve overweight or obese human volunteers underwent a randomized, double-blinded, placebo-controlled study. An ad libitum test meal was used to measure energy intake during intravenous infusions of either PYY(3-36) or oxyntomodulin or combined PYY(3-36)/oxyntomodulin. RESULTS: Energy intake during coadministration of PYY(3-36) and oxyntomodulin was reduced by 42.7% in comparison with the saline control and was significantly lower than that during infusions of either hormone alone. CONCLUSIONS: The anorectic effects of PYY(3-36) and oxyntomodulin can be additive in overweight and obese humans. Coadministration of Y2 receptor agonists and GLP-1 receptor agonists may be a useful treatment strategy for obesity.

GLP-1 Receptors

For the pro-insulin (incretin) effects: GLP-1 binds to GLP-1 receptors expressed on the pancreatic β cells, which stimulates insulin secretion into blood. Besides lowering blood glucose, GLP-1 also reduces body weight (anorexigenic effects). It is not fully understood how GLP-1 induces weight loss or to what degree GLP-1 acts directly in the brain. In the ARC, GLP-1 was internalized in neurons expressing proopiomelanocortin (POMC) and cocaine- and amphetamine-regulated transcript (CART). Electrophysiological measurements of murine brain slices revealed that GLP-1 directly stimulates POMC/CART neurons and indirectly inhibits neurotransmission in neurons expressing neuropeptide Y (NPY) and agouti-related peptide (AgRP) via GABA-dependent signaling. Collectively, our findings indicate that the GLP-1R on POMC/CART-expressing ARC neurons likely mediates liraglutide-induced weight loss.

Oxytocin and Partuition

Fuchs et al The concentration of oxytocin receptors increased in the myometrium of pregnant women and reached maximum levels in early labor. Concentrations of oxytocin receptors were also high in the decidua and reached a maximum at parturition. In vitro, prostaglandin production by the decidua, but not by the myometrium, was increased by the addition of oxytocin. Oxytocin may therefore stimulate uterine contractions by acting both directly on the myometrium and indirectly on decidual prostaglandin production. Oxytocin receptors are probably crucial for the onset of human labor, and the stimulus for the increase in uterine prostaglandins may be oxytocin originating from the fetus. PROS: this is why oxytocin agonists are usually given IV during labour to facilitate childbirth - but it can occur without it

Ghrelin receptor

GHS-R1 (Growth Hormone Secretagogue Receptor) is involved in a variety of biological effects of ghrelin. These receptors are primarily located in neurons found in the arcuate nucleus.

GUT HORMONES CONTROLLING APPETITE

GUT HORMONES CONTROLLING APPETITE

ENS: Regulation of Gastric and Pancreatic Secretion

Gastric acid secretion is regulated both by neurons and by hormones. Neural regulation is through cholinergic neurons with cell bodies in the wall of the stomach. These receive excitatory inputs both from enteric sources and from the vagus nerves. Gastric secretion of HCl and pepsinogen in the stomach, and secretion of pancreatic enzymes, is largely dependent on vago-vagal reflexes. Enteric motor neurons are the final common pathway, but the roles of intrinsic reflexes are minor. Pancreatic secretion of bicarbonate, to neutralise the duodenal contents, is controlled secretin, a hormone released from the duodenum, in synergy with activity of cholinergic and non-cholinergic enteric neurons. Secretion into the gall-bladder and bicarbonate secretion in the distal stomach are also nerve controlled.

Ghrelin in medical management of obesity and/or anorexia nervosa?

Ghrelin is not FDA approved for any indication. This is largely due to the adverse effects associated with ghrelin injections. Adverse effects occurred in 20% of participants, with a predominance of flushing and gastric rumbles and a mild degree of severity. The few serious adverse events occurred in patients with advanced illness and were not clearly attributable to ghrelin. Route of administration may affect the pattern of adverse effects.

Activation of Ghrelin

Ghrelin only becomes active when octanoic acid is linked posttransationally by the enzyme ghrelin O-acyltransferase (GOAT). Ghrelin cells are found mainly in the stomach and duodenum, put also in the jejunum.

GLP-1

Glucagon-like peptide-1 (GLP-1) is a 30 amino acid peptidehormone deriving from the tissue-specific posttranslational processing of the proglucagon gene. It is released post-prandially from the small intestine L-cells (ileum, jejunum, duodenum). GLP-1 has the ability to decrease blood sugar levels by enhancing the secretion of insulin, which also cuts appetite.

Excessive AgRP expression leads to Agouti obesity syndrome

Graham et al Agouti, a gene that normally controls pigmentation inthe mouse by antaonizing the MC1R, causes obesity and diabetes when ectopically expressed. In this study, they designed transgenic mice which over-expressed AgRP. The expression of AgRP positively correlated to the weight gain in the mice No change in skin pigmentation was found, which agrees with the results that AgRP does not act on MC1-R, but acts uniquely on MC4-R. Results show that mutations leading to overexpression of AgRP produce an agouti-phenotype for obesity

Future research: discovering other gut hormone involved in appetite regulation -e.g. Insulin-like Peptide 1 in Appetite control

Grosse et al. Background: This paper was primarily designed to provide an initial glimpse into the relevance of Insulin-like peptide 5 (Insl5) in appetite control in mice. This paper hypothesized that Insl5, a hormone previously identified in colonic tissue in humans, would like other gut hormones also to be secreted by enteroendocrine L-cells. If then Insl5 was found to be secreted by L-cells, this paper hypothesized that the secretion of Insl5 would either be up-regulated or down-regulated in response to calorie restriction. Lastly, Rxfp4-/- knockout mice lacking the cognate receptor for Insl5 (Rxfp4) were hypothesized to show modulated appetite behaviours compared to their wild-type littermates. Methods: Quantitative Real-Time Polymerase Chain Reaction (qRT-PCR) captured mouse Insl5 transcript expression in L-cells throughout the gut. Enzyme-linked Immunosorbent Assays (ELISAs) measured mouse Insl5 plasma concentrations during fasting and following feeding. Intraperitoneal (i.p.) Insl5 injections versus sham controls measured the dose-dependent response of Insl5 injections on food intake. Food intake and food preference between a high-fat diet (HFD) or high-carbohydrate diet (HCD) were compared between Rxfp4-/- mice and their wild-type littermates. Results: Insl5 was primarily expressed in colonic enteroendocrine cells. Mouse Insl5 plasma concentrations were elevated during fasting and fell significantly after feeding (p<0.001; n=10). i.p. Insl5 injections produced a dose-dependent stimulation in food intake which was significant compared to sham injections (p<0.001; n=8). Rxfp4-/- mice (n=4) showed significantly shorter meal durations (p<0.001) and reduced body weight at 11 weeks of age (p<0.001) compared to wild-type littermates (n=4). Rxfp4-/- mice (n=4) displayed a preference for HCD, and consumed significantly less HFD after fasting compared to wild-type littermates (n=4). Conclusion: Overall, these findings provide strong evidence to support this paper's hypotheses. Specifically, this paper has demonstrated that Insl5 is an orexigenic hormone secreted from colonic L-cells during conditions of energy deprivation to promote appetite in mice. Insl5 therefore should join ghrelin as the only second identified gut hormone known to enhance appetite. Ideally, further research into modulating the Insl5 axis in humans might provide a new strategy for the treatment of metabolic disease and obesity.

Oxytocin increases eye gazing performance

Guastella et al BACKGROUND: In nonhuman mammals, oxytocin has a critical role in peer recognition and social approach behavior. In humans, oxytocin has been found to enhance trust and the ability to interpret the emotions of others. It has been suggested that oxytocin may enhance facial processing by increasing focus on the eye region of human faces. METHODS: In a double-blind, randomized, placebo-controlled, between-subject design, we tracked the eye movements of 52 healthy male volunteers who were presented with 24 neutral human faces after intranasal administration of 24 IU oxytocin or placebo. RESULTS: Participants given oxytocin showed an increased number of fixations and total gaze time toward the eye region compared with placebo participants. CONCLUSIONS: Oxytocin increases gaze specifically toward the eye region of human faces. This may be one mechanism by which oxytocin enhances emotion recognition, interpersonal communication, and social approach behavior in humans. Findings suggest a possible role for oxytocin in the treatment of disorders characterized by eye-gaze avoidance and facial processing deficits. PROS: might help conditions such as Autism Spectrum Disorder

The peristaltic reflex

Hansen Following mucosal stimulation 5-HT is released from enterochromaffin cells to intrinsic primary afferent neurons (IPANs with 5-HT1P, 5-HT3 and 5-HT4 receptors) and extrinsic vagal and spinal afferents (with 5-HT3 receptors). IPANs release substance (SP) acetylcholine (ACh), glutamate and calcitonin gene-regulated peptide (CGRP) to interneurons. Excitatory interneurons release SP and ACh orally to excitatory motorneurons, while 5-HT and ACh is released aborally to inhibitory motorneurons. Excitatory motorneurons release SP and ACh to muscles, while inhibitory motorneurons release nitric oxide (NO), vasoactive intestinal peptide (VIP) and adenosine triphosphate (ATP) to muscles. Efferent sympathetics release norepinephrine (NE), somatostatin (SOM) and neuropeptide I, while efferent parasympathetics release ACh.

Disruption of Myenteric Plexis: Hirshsprung's disease

Hirschsprung's disease is a congenital disorder of the colon in which nerve cells of the myenteric plexus in its walls, also known as ganglion cells, are absent. Hirschsprung's disease is a form of functional low bowel obstruction due to failure of caudal migration of neuroblasts within developing bowel - this results in an absence of parasympathetic intrinsic ganglion cells in both Auerbach's and Meissner's plexi. The distal large bowel from the point of neuronal arrest to the anus is continuously aganglionic. It is a rare disorder (1:5000), with prevalence among males being four times that of females.

Like Ghrelin, PYY3-36 has many adverse effects

If you give too much participants want to throw up. It has a very short half life IV administration of PYY3-36 impractical Makking PYY3-36 analogues with longer half-lifes? Possible but difficult.

OT neural sources

In the hypothalamus, OT is made in magnocellular neurosecretory cells of the Supraoptic and Paraventricular nucleus, and is then released into the blood from the posterior lobe (neurohypophysis) of the pituitary gland.

Mutation in Ob gene

Ingalls et al Experiment stemmed from some mice who were found to eat voraciously and became largely obese within 6 weeks of birth compared to littermates These mice also developed a wide range of health problems including hyperinsulinemia, decreased energy expenditure, hyperglycaemia Parents of these mice were not obese, but some of their offspring were. These mice were found to be homozygous for mutations in a gene, which they called the Obese gene, whereas their parents did not have this mutation. These findings suggested that the parents were heretozygous carriers for Ob gene mutations. Researchers suggested that a recessive homozygous mutation in a hypothesized "Obese" (Ob) gene was responsible for the rapid onset of obesity in these mice CONS: does not tell us what the Ob gene does or what it encodes

Oxytocin receptor distribution reflects social organization in monogamous and polygamous voles

Insel et al ABSTRACT The neuropeptide oxytocin has been implicated in the mediation of several forms of affiliative behavior including parental care, grooming, and sex behavior. Here we demonstrate that species from the genus Microtus (voles) selected for differences in social affiliation show contrasting patterns of oxytocin receptor expression in brain. By in vitro receptor autoradiography with an iodinated oxytocin analogue, specific binding to brain oxytocin receptors was observed in both the monogamous prairie vole (Microtus ochrogaster) and the polygamous montane vole (Microtus montanus). In the prairie vole, oxytocin receptor density was highest in the prelimbic cortex, bed nucleus of the stria terminalis, nucleus accumbens, midline nuclei of the thalamus, and the lateral aspects of the amygdala. These brain areas showed little binding in the montane vole, in which oxytocin receptors were localized to the lateral septum, ventromedial nucleus of the hypothalamus, and cortical nucleus of the amygdala. Similar differences in brain oxytocin receptor distribution were observed in two additional species, the monogamous pine vole (Microtus pinetorum) and the polygamous meadow vole (Microtus pennsylvanicus). Receptor distributions for two other neurotransmitter systems implicated in the mediation of social behavior, benzodiazepines, and ,u opioids did not show comparable species differences. Furthermore, in the montane vole, which shows little affiliative behavior except during the postpartum period, brain oxytocin receptor distribution changed within 24 hr of parturition, concurrent with the onset of maternal behavior. We suggest that variable expression of the oxytocin receptor in brain may be an important mechanism in evolution of species-typical differences in social bonding and affiliative behavior.

Gut Enteroendocrine Cells

Intestinal enteroendocrine cells (IECs) are specialised cells of the GI tract which together constitute the enteric endocrine system. The enteric endocrine system is a subset of the endocrine system, just as the enteric nervous system is a subset of the nervous system. In a sense, Enteroendocrine cells are known to act as chemoreceptors, initiating digestive actions and detecting harmful substances, and intiating protective responses. In another sense, Enteroendocrine cells are neuromodulators, especially L cells which release neuropeptides such as Ghrelin, PYY and GLP-1 into the bloodstream to act on receptors on the hypothalamic appetite control circuits, as well as Enterochromaffin cells which release serotonin.

Different Types of Cells

Intestinal enteroendocrine cells are not clustered together but spread as single cells throughout the intestinal tract. K cell K cells secrete gastric inhibitory peptide, an incretin, which also promotes triglyceride storage. L cell L cells secrete glucagon-like peptide-1, an incretin, pancreatic peptide YY3-36, oxyntomodulin and glucagon-like peptide-2. L cells are primarily found in the ileum and large intestine (colon), but some are also found in the duodenum and jejunum. I cell I cells secrete cholecystokinin (CCK), and are located in the duodenum and jejunum. They modulate bile secretion, exocrine pancreas secretion, and satiety. G cell Stomach enteroendocrine cells, which release gastrin, and stimulate gastric acid secretion. Enterochromaffin cell Enterochromaffin cells are enteroendocrine and neuroendocrine cells with a close similarity to adrenomedullary chromaffin cells secreting serotonin. Enterochromaffin-like cell Enterochromaffin-like cells or ECL cells are a type of neuroendocrine cell secreting histamine. N cell Located in the jejunum, N cells release neurotensin, and control smooth muscle contraction. S cell S cells secrete secretin from the duodenum and jejunum, and stimulate exocrine pancreatic secretion. D cell also called Delta cells, secrete somatostatin M cell Secrete motilin

GRP119

Is a "fat sensor", sensing dietary triglyceride metabolites

OT in maternal care

It is released into the bloodstream as a hormone in response to stretching of the cervix and uterus during labour and with stimulation of the nipples from breastfeeding.

Roles of OT

It plays a role in social bonding, sexual reproduction, and during (stretching of uterus and cervix during labour) and after childbirth (maternal care; lactation).

alpha-MSH increases plasma Thyrotropin Releasing Hormone (TRH)

Kim et al ICV administration of a stable α-MSH analogue increased plasma TRH in 24-hour-fasted rats. In vitro, α-MSH increased thyrotropin releasing hormone (TRH) release from hypothalamic explants. Agrp (83-132) alone caused no change in TRH release but antagonized the effect of α-MSH on TRH release. Leptin increased TRH release from hypothalami harvested from 48-hour-fasted rats. Agrp (83-132) blocked this effect. These data suggest a role for the hypothalamic melanocortin system in the fasting-induced suppression of the H-P-T axis. CONS: did not examine whether prolonged fasting induces a suppression of the H-P-T axis in the POMC- and MC4-R-deficient mice.

OT increases trust in humans

Kosfeld et al Trust pervades human societies. Trust is indispensable in friendship, love, families and organizations, and plays a key role in economic exchange and politics. In the absence of trust among trading partners, market transactions break down. In the absence of trust in a country's institutions and leaders, political legitimacy breaks down. Much recent evidence indicates that trust contributes to economic, political and social success. Little is known, however, about the biological basis of trust among humans. Here we show that intranasal administration of oxytocin, a neuropeptide that plays a key role in social attachment and affiliation in non-human mammals, causes a substantial increase in trust among humans, thereby greatly increasing the benefits from social interactions- seen in trust risk game (giving more or less money to investor). We also show that the effect of oxytocin on trust is not due to a general increase in the readiness to bear risks. On the contrary, oxytocin specifically affects an individual's willingness to accept social risks arising through interpersonal interactions. These results concur with animal research suggesting an essential role for oxytocin as a biological basis of prosocial approach behaviour.

POMC mutations in humans lead to severe early-onset obesity, adrenal insufficiency and red hair pigmentation

Krude et al Sequential cleavage of the precursor protein pre-pro-opiomelanocortin (POMC) generates the melanocortin peptides adrenocorticotrophin (ACTH), melanocyte-stimulating hormones (MSH) alpha, beta and gamma as well as the opioid-receptor ligand beta-endorphin. While a few cases of isolated ACTH deficiency have been reported, an inherited POMC defect has not been described so far. Recent studies in animal models elucidated a central role of alpha-MSH in the regulation of food intake by activation of the brain melanocortin-4-receptor and the linkage of human obesity to chromosome 2 in close proximity to the POMC locus, led to the proposal of an association of POMC with human obesity. The dual role of alpha-MSH in regulating food intake and influencing hair pigmentation predicts that the phenotype associated with a defect in POMC function would include obesity, alteration in pigmentation and ACTH deficiency. The observation of these symptoms in two probands prompted us to search for mutations within their POMC genes. Patient 1 was found to be a compound heterozygote for two mutations in exon 3 (G7013T, C7133delta) which interfere with appropriate synthesis of ACTH and alpha-MSH. Patient 2 was homozygous for a mutation in exon 2 (C3804A) which abolishes POMC translation. These findings represent the first examples of a genetic defect within the POMC gene and define a new monogenic endocrine disorder resulting in early-onset obesity, adrenal insufficiency and red hair pigmentation. CONS: very rare mutations, again

LEPTIN

LEPTIN

LEPTIN AND NEUROHORMONAL CONTROL OF APPETITE

LEPTIN AND NEUROHORMONAL CONTROL OF APPETITE

Lack of sleep on ghrelin

Lack of sleep increases ghrelin and decreases leptin, both of which result in increased hunger and obesity

Liraglutide: GLP-1 on the market

Liraglutide is a derivative of GLP-1 that is used as a long-lasting GLP-1 receptor agonist, binding to the same receptors as does the endogenous GLP-1, that is GLP-1 R in pancreatic Beta cells, that stimulates insulin secretion It is an injectible drug used for the treatment of Type 2 diabetes. In 2015, is has been used as a treatment for adults who are obese or overweight Collectively, findings indicate that the GLP-1R on POMC/CART-expressing ARC neurons likely mediates liraglutide-induced weight loss. CONS: adverse effects of Liraglutide, including a) risk of developing pancreatitis b) risk of developing thyroid cancer

AVP and Depression

Londen et al Mentally healthy subjects show increased plasma concentrations of the neuropeptides, arginine vasopressin (AVP) and oxytocin (OT), under conditions of stress, but data are lacking about plasma concentrations of AVP and OT in patients with major depression. We thus assessed plasma concentrations of AVP and OT in patients with major depression (n = 52) and healthy controls (n = 37). Mean plasma AVP concentrations were higher in the group of depressed patients than in controls. A subgroup of 16 patients showed very high levels of plasma AVP, but no other feature differentiating this subgroup from the other patients was found. In-patients showed higher plasma AVP levels than out-patients, and melancholic patients had higher plasma AVP levels than did nonmelancholic patients. Plasma AVP levels were slightly related to psychomotor retardation and significantly inversely to neuroticism. Patients' plasma OT concentrations had a wider range than in controls. AVP and AVP-mediated functions may be a factor in the clinical picture of depression, possibly by influencing the activity of the hypothalamic-pituitary-adrenal axis.

Differences between MC3-R and MC4-R

MC3-R is expressed in the brain in regions of the hypothalamus and limbic system, as well as in the placenta and gut -> Not inhibited by agouti HOWEVER MC3 is known to bind to the MC4-R expression is restricted primarily to the brain, where it is widely expressed -> inhibited by agouti --> Although MC3R also present in brain, MC4-R is thought to be behind the agouti obesity syndrome.

Difference in Action of

MC3R are primarily found on NPY/AgRP neurons - Alpha-MSH stimulation of these receptors inhibits gene expression of NPY/AgRP neurons - and serves to inhibit food intake MC4R are found throughout the hypothalamus and limbic system, and alpha-MSH binding to them serves to inhibit food intake. Similarly, AgRP released from AgRP neurons serve to inhibit MC4-R to stop alpha-MSH from binding, andserves to decrease food intake.

Chemosensors outside Enteroendocrine cells (GLUT-2; CasR) regulate L-cell activity in response to nutrient and non-nutrient stimuli

Mace et al In this experiment they isolated loops of rat small intestine to study the secretion of GLP-1 and PYY. GLUT-2: Sweetners given to rats increased expresssion of GLUT2 receptors and lead to increased levels of PYY and GLP-1. Inhibition of GLUT2 completely inhibited the secretion of PYY and GLP-1 from GLUT2 in response to sweetners. GLUT2 disruption found to affect membrane depolarisation through the closure of K+ channels. Casr (calcium-sensing receptor): The presence of L-amino acids including L-glutamine and L-tryptophan lead to higher activation of Casr. Higher expression of CasR was paired by higher release of GLP-1 and PYY secretion. CasR was also found to be sensitive to glucose, and the presence of glucose lead to increased expression CasR inhibitor completely abolished GLP-1 and PYY secretion by CasR CasR agonists also increased GLP-1 and PYY secretion ERGO GLUT2 and CasR regulate L-cell activity in response to nutrient and non-nutrient stimuli In addition to both stimulating PYY and GLP-1, CasR has been found to modulate the L-cells to decrease ghrelin secretion

Pulsatile elease of OT and prolactin in response to, and prior to, suckling

McNeilly The oxytocin and prolactin responses to suckling were measured in 10 women in early (n = 5) and established lactation (n = 5). Oxytocin was released in a pulsatile manner during suckling in all women, but the response was not related to milk volume, prolactin response, or parity of the mother. In all 10 women plasma oxytocin concentrations increased three to 10 minutes before suckling began. In five women this occurred in response to the baby crying, in three it coincided with the baby becoming restless in expectation of the feed, while in two it corresponded with the mother preparing for the feed. There was no prolactin response to stimuli other than stimulation of the nipple associated with suckling. These results clearly indicate that the milk ejection reflex, with release of oxytocin, occurs in most women before the tactile stimulus of suckling. A second release of oxytocin follows in response to the suckling stimulus itself. Thus it is important that care is taken to protect breast feeding mothers from stress not only during suckling but also immediately before nursing, when conditioned releases of oxytocin will occur.

NUTRIENT SENSING IN THE GUT

NUTRIENT SENSING IN THE GUT

NUTRIENT-SENSING BY THE NEUROENDOCRINE GUT

NUTRIENT-SENSING BY THE NEUROENDOCRINE GUT

ENS: regulation of Enteroendocrine cells

Nerve fibres run close to endocrine cells of the mucosa of the gastro-intestinal tract, some of which are under neural control. For example, gastrin cells in the antrum of the stomach are innervated by excitatory neurons that utilize gastrin releasing peptide as the primary neurotransmitter. Conversely, hormones released by gastrointestinal endocrine cells influence the endings of enteric neurons. In a sense, the endocrine cells act like taste cells, that sample the luminal environment, and release messenger molecules into the tissue of the mucosa, where the nerve endings are found. This is a necessary communication, because the nerve endings are separated from the lumen by the mucosal epithelium. An important communication is with serotonin (5-hydroxytryptamine, 5-HT) containing endocrine cells which activate motility reflexes. Excessive release of serotonin can cause nausea and vomiting, and antagonists of the 5-HT3 receptor are anti-nauseants.

Neuropeptide Y

Neuropeptide Y, like AgRP, is a powerful appetite stimulant NPY injection in the brain increases food intake and decreases energy expenditure Continuous administration of NPY leads to obestiy Starvation leads to increase in NPY expression

Vasopressin increases water permeability of kidney collecting duct

Nielsen et al Water excretion by the kidney is regulated by the peptide hormone vasopressin. Vasopressin increases the water permeability of the renal collecting duct cells, allowing more water to be reabsorbed from collecting duct urine to blood. Despite long-standing interest in this process, the mechanism of the water permeability increase has remained undetermined. Recently, a molecular water channel (AQP-CD) has been cloned whose expression appears to be limited to the collecting duct. Previously, we immunolocalized this water channel to the apical plasma membrane (APM) and to intracellular vesicles (IVs) of collecting duct cells. Here, we test the hypothesis that vasopressin increases cellular water permeability by inducing exocytosis of AQP-CD-laden vesicles, transferring water channels from IVs to APM. Rat collecting ducts were perfused in vitro to determine water permeability and subcellular distribution of AQP-CD in the same tubules. The collecting ducts were fixed for immunoelectron microscopy before, during, and after exposure to vasopressin. Vasopressin exposure induced increases in water permeability and the absolute labeling density of AQP-CD in the APM. In parallel, the APM:IV labeling ratio increased. Furthermore, in response to vasopressin withdrawal, AQP-CD labeling density in the APM and the APM:IV labeling ratio decreased in parallel to a measured decrease in osmotic water permeability. We conclude that vasopressin increases the water permeability of collecting duct cells by inducing a reversible translocation of AQP-CD water channels from Intracellular vesicles (IVs) to the Apical Plasma Membrane (APM).

OXYTOCIN

OXYTOCIN

Obesity in Humans

Obesity is a medical condition in which excess body fat has accumulated to the extent that it may have an adverse effect on health. It is one of the leading preventable causes of death worldwide. Obesity is usually accompanied by other co-morbidities such as type II diabetes, cardiovascular diseases, obstructive sleep apnea, and depression.

Causes of Obesity

Obesity is most commonly caused by a combination of excessive food intake, lack of physical exercise, and *genetic susceptibility*. At a societal level, increases rates of obesity are felt to be due to an easily accessible and cheap unhealthy diet, increased sedentary lifestyle caused to increasing use of easy transporting, as well as increases in mechanized manufacturing.

Only real drug on market for treating Obesity: Orilstat

Orilstate is a drug deisgned to treat obesity. Its primary function is preventing the absorption of fats from the human diet by acting as a potent natural inhibitor of pancreated lipases. Pancreatic lipases are enzymes that break down triglycerides in the intestine. The effect on promoting weight loss is very modest. However, it is notorious for having bad gastrointestinal side effects which can include stomach upsets and diarrhea, as well as oily feces. Also associated with higher incidence of breast cancer.

Two major intrinsic networks of ganglia

Outer myenteric plexi (MP): - a.k.a. Auerbach's plexus - Mainly controls musculature - Many different cell bodies/fibres - The myenteric plexus is positioned between the outer longitudinal and circular muscle layers throughout the digestive tract, from the oesophagus to the rectum. Inner submucous plexi (SMP): - a.k.a Meissner's plexus - Mainly controls muscosal function andmucosal blood flow - Fewer neuronal types than MP - The submucous plexus is positioned in the submucosa, being only prominent in the intestines - where nutrient absorption occurs

First Order Neurons involved in Appetite Regulation

POMC/CART neurons are responsible for secreting anorexigenic peptides NPY/AgRP are responsible for secreting orexigenic peptides

Crosstalk between First Order Neurons

POMC/CART neurons inhibit NPY/AgRP neurons and vice versa: - High circulating levels of leptin in fed state inhibit release of appetite stimulant NPY and AgRP through alpha-MSH bindng to MC3R on NPY/AgRP neurons - Only when leptin levels fall during starvation, is the inhibition of NPY and AgRP released. Then NPY inhibits POMC expression through binding to Y1R on POMC/CART neurons - Also, NPY/AgRP neurons are found to have gabaergic connections with POMC gene, such that high NPY/AgRP expression inhibits POMC gene expression

Peptide YY action

PYY exerts its action through Y2-R on NPY receptors. Activation of these receptors are thought to have an inhibitory function on NPY receptors.

Paracrine roles of PYY and GLPs when released from L-cells

PYY is pro-absorptive (=anti-secretory) "the defender against diarrhoea" - mediates the ileal and colonic brakes Endocrine functions: - PYY3-36 anorexigenic neuropeptide GLPs slow gastric emptying and are: - pro-absorptive - proliferative and anti-apoptotic in GI tract - GLP-1 and GLP-2 mediate glucose sensing Endocrine functions: - key controllers of blood glucose (incretins) - GLP-1 anorexigenic peptide

Oxytocin as a stimulator for maternal behaviour in virgin rats

Pederson Intracerebroventricular administration of oxytocin to virgin female rats that had been ovariectomized and primed with estrogen 48 hours previously induced a rapid onset of full maternal behavior. The maternal behavior persisted and its incidence was dose-related. Tocinoic acid, the ring structure of oxytocin, also rapidly induced the onset of persistent, full maternal behavior. Arginine vasopressin induced persistent maternal behavior, but this behavior had a later onset. Prostaglandin F2 alpha induced strong partial maternal behavior, which showed early onset but did not persist. These findings suggest that the release of oxytocin and prostaglandin F2 alpha during labor may promote maternal behavior in rats.

AVP induces maternal behavior

Pederson et al Arginine vasopressin induced persistent maternal behavior, but this behavior had a later onset. Injections of AVP induced a delayed onset of maternal behavior induced by arginine vasopressin (AVP) compared to OT. Five categories of maternal behavior were increased in female rats: grouping and regrouping of pups; licking of pups; crouching over grouped pups; nest building; and retrieval of pups. At the end of each hour each animal was assigned a behavioral score based on one point for each category in which criteria were met during that hour. The five categories of maternal behavior and the criteria for each have been described elsewhere

Peptide YY

Peptide YY is a short peptide released from L-cells in the ileum and colon in response to feeding. PYY acts to reduce appetite. The main circulating form is PYY3-36, which is a selective Y2R agonist..

Posterior vs Anterior Pituitary

Posterior pituitary is controlled directly by neuronal projections from the brain Anterior pituitary has no neuronal projections. It is controlled by hormones released from the hypothalamus which which travel through a dense network of blood vessels from the hypothalamus to the anterior pituitary.

OAP-89: oxyntomodulin analogue as diet pill

Promising pill which showed decreased food intake in mice. Bought for $100 million by Pfizer but it ended up having quite a few issues so never released on the market.

GLP-1 stimulation of parabrachial nucleus reduces food intake

Richard et al. The parabrachial nucleus (PBN) is a key nucleus for the regulation of feeding behavior. Inhibitory inputs from the hypothalamus to the PBN play a crucial role in the normal maintenance of feeding behavior, because their loss leads to starvation. Viscerosensory stimuli result in neuronal activation of the PBN. However, the origin and neurochemical identity of the excitatory neuronal input to the PBN remain largely unexplored. Here, we hypothesize that hindbrain glucagon-like peptide 1 (GLP-1) neurons provide excitatory inputs to the PBN, activation of which may lead to a reduction in feeding behavior. Our data, obtained from mice expressing the yellow fluorescent protein in GLP-1-producing neurons, revealed that hindbrain GLP-1-producing neurons project to the lateral PBN (lPBN). Stimulation of lPBN GLP-1 receptors (GLP-1Rs) reduced the intake of chow and palatable food and decreased body weight in rats. It also activated lPBN neurons, reflected by an increase in the number of c-Fos-positive cells in this region. Further support for an excitatory role of GLP-1 in the PBN is provided by electrophysiological studies showing a remarkable increase in firing of lPBN neurons after Exendin-4 application. We show that within the PBN, GLP-1R activation increased gene expression of 2 energy balance regulating peptides, calcitonin gene-related peptide (CGRP) and IL-6. Moreover, nearly 70% of the lPBN GLP-1 fibers innervated lPBN CGRP neurons. Direct intra-lPBN CGRP application resulted in anorexia. Collectively, our molecular, anatomical, electrophysiological, pharmacological, and behavioral data provide evidence for a functional role of the GLP-1R for feeding control in the PBN.

CART deficiency and obesity phenotype

Santoro et al Cocaine- and amphetamine-regulated transcript (CART) inhibits feeding and induces the expression of c-Fos in hypothalamic areas implicated in appetite regulation. Furthermore, the CART peptide is found in neurons regulating sympathetic outflow, which in turn play an integral role in regulating body temperature and energy expenditure. The Leu34Phe mutation on the CART gene might alter the susceptibility to proteolysis of this potential processing site, likely altering the CART effect on thermogenesis and energy expenditure. CONS: very rare mutation

alpha-MSH reduces food intake

Schwartz et al alpha-Melanocyte-stimulating hormone (alpha-MSH) is a hypothalamic neuropeptide proposed to play a key role in energy homeostasis. To investigate the behavioral, metabolic, and hypothalamic responses to chronic central alpha-MSH administration, alpha-MSH was infused continuously into the third cerebral ventricle of rats for 6 days. Chronic alpha-MSH infusion reduced cumulative food intake and body weight co,pared to saline, which in turn lowered plasma insulin levels- However, alpha-MSH did not cause adipose-specific wasting nor did it alter hypothalamic neuropeptide mRNA levels. Central alpha-MSH infusion acutely activated neurons in forebrain areas such as the hypothalamic paraventricular nucleus, as measured by a 254% increase in c-Fos-like immunoreactivity (P < 0.01 vs. saline), as well as satiety pathways in the hindbrain. Our findings suggest that, although an increase of central melanocortin receptor signaling acutely reduces food intake and body weight, its anorectic potency wanes during chronic infusion and causes only a modest decrease of body weight.

Opposing effects of Ghrelin and GLP-1/PYY by metabolite and neuroendocrine GPCR

Schwartz et al The gut hormones ghrelin and GLP-1 have opposite functions in basically all endocrine and metabolic target organs, as well as opposite secretion patterns in response to food intake. GPCRs that sense nutrients and endogenous metabolites, in general, inhibit ghrelin and stimulate GLP-1 release - as does glucose independent of GPCRs. Triglyceride metabolites inhibit ghrelin secretion through FFA4 (GPR120), but stimulate GLP-1 secretion through (GPR119) The same metabolite GPCRs, for example, the short chain fatty acid receptor FFA2 (GPR43) and the CaSR signal in a context- and cell-dependent manner to inhibit ghrelin release and stimulate GLP-1 release. Gut hormones send information about incoming nutrients to the rest of the body and thereby control many aspects of metabolism. The secretion of ghrelin and glucagon-like protein (GLP)-1, two hormones with opposite secretory patterns and opposite actions on multiple targets, is controlled by a limited number of G-protein coupled receptors (GPCRs); half of which recognize and bind dietary nutrient metabolites, metabolites generated by gut microbiota, and metabolites of the host's intermediary metabolism. Most metabolite GPCRs controlling ghrelin secretion are inhibitory, whereas all metabolite receptors controlling GLP-1 secretion are stimulatory. This dichotomy in metabolite sensor function, which is obtained through a combination of differential expression and cell-dependent signaling bias, offers pharmacological targets to stimulate GLP-1 and inhibit ghrelin through the same mechanism.

Leptin receptors in arcuate nucleus

Schwartz et al The hypothesis that leptin (OB protein) acts in the hypothalamus to reduce food intake and body weight is based primarily on evidence from leptin-deficient, ob/ob mice. To investigate the anatomic basis for these effects, we measured leptin receptor gene (OB-R) expression in rat brain by ISH using a probe complementary to mRNA for all leptin receptor splice variants. OB-R mRNA was densely concentrated in the arcuate nucleus, with lower levels present in the ventromedial and dorsomedial hypothalamic nuclei and other brain areas involved in energy balance. These findings suggest that leptin action in rat hypothalamus involves altered expression of key neuropeptide genes, and implicate leptin in the hypothalamic response to fasting. To investigate whether leptin exerts similar effects in normal animals, we administered leptin intracerebroventricularly (icv) to Long-Evans rats. Leptin administration (3.5 microg icv) at the onset of nocturnal feeding reduced food intake by 50% at 1 h and by 42% at 4 h, as compared with vehicle-treated controls (both P < 0.05). To investigate the basis for this effect, we used in situ hybridization (ISH) to determine whether leptin alters expression of hypothalamic neuropeptides involved in energy homeostasis. Two injections of leptin (3.5 microg icv) during a 40 h fast significantly *decreased levels of mRNA for neuropeptide Y* (NPY, which stimulates food intake) in the arcuate nucleus (-24%) and *increased levels of mRNA for corticotrophin releasing hormone (CRH, an inhibitor of food intake)* in the paraventricular nucleus (by 38%) (both P < 0.05 vs. vehicle-treated controls). CONS: did not talk about activation of POMC/CART network

AVP release in response to haemorrhage - regulation of blood pressure

Silva et al The release of vasopressin in response to haemorrhage and the effects of vasopressin infusions on blood pressure and heart rate have been investigated in anaesthetized dogs. Haemorrhage was produced by the method of Lamson & de Turk (1945), which allows for a precise control of the changes in arterial blood pressure. Blood samples were collected from an external jugular vein, from a femoral vein or from a femoral artery and extracted with alcohol; blood extracts were assayed for antidiuretic activity. Haemorrhage experiments showed that vasopressin secretion is increased when the fall in diastolic blood pressure (diastolic AP) is 25 mm Hg or more. Mild hypotensions (diastolic AP ranging from 21 to 30 mm Hg) produce an average fourfold increase in the concentration of vasopressin in blood. Such increase is maintained throughout the oligaemic period. Severe hypotensions produce, in most cases, a biphasic secretary response, with an initial high peak followed by a lower, constant, secretary plateau. In all experiments, the retransfusion of blood restored vasopressin to control levels. Vasopressin infusion experiments showed that the amounts of hormone secreted in response to haemorrhage are sufficient to cause vasopressor response, provided that the buffering action of blood pressure regulation mechanisms is suppressed. It was also found that the amounts of vasopressin secreted in response to haemorrhage are apparently adequate, if the function of such secretion is to combat the hypotension which follows haemorrhage. The effect of hypophysectomy on the blood pressure of animals previously submitted to bilateral division of the vagi and sinus nerves (deafferented animals) was also investigated. It was found that *hypophysectomy is followed by a fall in arterial blood pressure* which is positively correlated to the previous existing amounts of vasopressin. The time course of this hypotension is similar to that following the stopping of an infusion in a deafferented hypophysectomized animal. In some experiments it was shown that, following hypophysectomy, blood pressure can be restored to its pre-hypophysectomy level by an adequate infusion of vasopressin. It is proposed that the release of vasopressin in response to stimuli arising from cardiovascular sensory receptors plays a part in the mechanism of blood pressure regulation.

K.O. NPY has no phenotype

Similarly to knockout out AgRP, postnatal but not germline manipulation of NPY or its receptors affect energy balance. Due to some sort of compensation very early on, since growth is paramount Also tells us that there's an unidentified appetite regulator

Anterior Pituitary Hormones

Somatotropes: Human growth hormone (HGH), also referred to as 'growth hormone' (GH), and also as somatotropin, is released under the influence of hypothalamic growth hormone-releasing hormone (GHRH), and is inhibited by hypothalamic somatostatin . Corticotropes: Cleaved from the precursor proopiomelanocortin protein, and include adrenocorticotropic hormone (ACTH), and beta-endorphin, and melanocyte-stimulating hormone are released under the influence of hypothalamic corticotropin-releasing hormone (CRH). Thyrotropes: Thyroid-stimulating hormone (TSH), is released under the influence of hypothalamic thyrotropin-releasing hormone (TRH) and is inhibited by somatostatin. Gonadotropes: Luteinizing hormone (LH): Follicle-stimulating hormone (FSH), both released under influence of Gonadotropin-Releasing Hormone (GnRH) Lactotropes: Prolactin (PRL): Whose release is inconsistently stimulated by hypothalamic TRH, oxytocin, vasopressin, vasoactive intestinal peptide, angiotensin II, neuropeptide Y, galanin, substance P, bombesin-like peptides (gastrin-releasing peptide, neuromedin B and C), and neurotensin, and inhibited by hypothalamic dopamine.[8] These hormones are released from the anterior pituitary under the influence of the hypothalamus. Hypothalamic hormones are secreted to the anterior lobe by way of a special capillary system, called the hypothalamic-hypophysial portal system.

First Discovery of Independence of ENS

Starling The idea of an independent nervous system in the gut began about a century ago with the functional discovery of the enteric nervous system by W.M. Bayliss and E.H. Starling. They isolated a loop of dog intestine, increased the pressure within the loop, and observed a muscular movement, which caused forward movement of the contents. They called it "the law of the intestine", which is now known as the peristaltic reflex. They cut all nerve‐mediated communication between the gut and the CNS, since the reflex activity was unaffected, they introduced the term "the local nervous mechanism".

OT biosynthesis

Sythesized as an inactive precursor protein from the OXT gene. This precursor protein includes the OT carrier protein, neurophysin. It is progressively hydrolyzed into OT.

Roles of the ENS

The ENS has multiple roles: determining the patterns of movement of the gastrointestinal tract; controlling gastric acid secretion; regulating movement of fluid across the lining epithelium; changing local blood flow; modifying nutrient handling; and interacting with the immune and endocrine systems of the gut.

Enteric Nervous System

The ENS is a division of the ANS, the other divisions being the sympathetic and parasympathetic, with which it has extensive connections.

Anterior pituitary

The anterior pituitary is a major organ of the endocrine system. This organ regulates several physiological processes including stress, growth, reproduction, and lactation.

ENS: Regulation of Fluid exchange and Local Blood Flow

The enteric nervous system regulates the movement of water and electrolytes between the gut lumen and tissue fluid compartments. It does this by directing the activity of secretomotor neurons that innervate the mucosa in the small and large intestines and control its permeability to ions. Neurotransmitters of secretomotor neurons are vasoactive intestinal peptide (VIP) and acetylcholine. Secretion is integrated with vasodilatation, which provides some of the fluid that is secreted. Most secretomotor neurons have cell bodies in submucosal ganglia. Fluxes of fluid, greater than the total blood volume of the body, cross the epithelial surfaces of the gastrointestinal tract each day. Control of this fluid movement via the enteric nervous system is of prime importance for the maintenance of whole-body fluid and electrolyte balance. The largest fluxes are across the epithelium of the small intestine, with significant fluid movement also occurring in the large intestine, stomach, pancreas and gall-bladder. Water moves between the lumens of digestive organs and body fluid compartments in response to transfer of osmotically active molecules. The greatest absorption of water, 8-9 litres per day, accompanies inward flux of nutrient molecules and Na+ through the activation of nutrient co-transporters, and the greatest secretion accompanies outward fluxes of Cl¯ and HCO3¯ in the small and large intestine, gall-bladder and pancreas. In each of these organs, fluid secretion is controlled by enteric reflexes. In the small intestine and most of the colon the reflexes circuits are intrinsic, in the enteric nervous system. They balance secretion with absorptive fluxes, and draw water from the absorbed fluid and from the circulation. The activity of the secretomotor reflexes is under a physiologically important control from inhibitory sympathetic nerve pathways that respond to changes in blood pressure and blood volume through central reflex centres. Local blood flow to the mucosa is regulated through enteric vasodilator neurons so that the mucosal blood flow is appropriate to balance the nutritive needs of the mucosa and to accommodate the fluid exchange between the vasculature, interstitial fluid and gut lumen. There are no intrinsic vasoconstrictor neurons. Overall blood flow to the gut is regulated from the CNS, via sympathetic vasoconstrictor neurons. The sympathetic vasoconstrictor neurons act in concert with the autonomic control of other vascular beds, to distribute cardiac output in relation to the relative needs of all organs. Thus in times of need, even during digestion, the sympathetic can divert blood flow away from the gastrointestinal tract.

ENS: Control of Motility

The gastrointestinal tract has an external muscle coat whose purposes are to mix the food so that it is exposed to digestive enzymes and to the absorptive lining of the intestine, and to propel the contents of the digestive tube. The muscle also relaxes to accommodate increased bulk of contents, notably in the stomach. In human, in particular, the colon also has an important reservoir function to retain the feces until defecation. The enteric reflex circuits regulate movement by controlling the activity of both excitatory and inhibitory neurons that innervate the muscle. These neurons have co-transmitters, for the excitatory neurons, acetylcholine and tachykinins, and for the inhibitory neurons nitric oxide, vasoactive intestinal peptide (VIP) and ATP. There is also evidence that pituitary adenylate cyclase activating peptide (PACAP) and carbon monoxide (CO) contribute to inhibitory transmission. The times for passage of the contents through the gastrointestinal tract vary depending on the nature of the food, including its amount and nutrient content. The peristaltic activity of the esophagus takes food from the mouth to stomach in about 10 seconds, where the food is mixed with digestive juices. Gastric emptying proceeds over periods of approximately 1-2 hours after a meal, the liquefied contents being propelled by gastric peristaltic waves as small aspirates into the jejunum during this time. The fluid from the stomach is mixed with pancreatic and biliary secretions to form the liquid content of the small intestine, known as chyme. Chyme is mixed and moves slowly along the intestine, under the control of mixing and propulsive movements orchestrated by the ENS, while digestion and absorption of nutrients occurs. The average transit time through the human small intestine is 3-4 hours. Colonic transit in healthy humans takes 1-2 days. Intrinsic reflexes of the enteric nervous system are essential to the generation of the patterns of motility of the small and large intestines. The major muscle movements in the small intestine are: mixing activity; propulsive reflexes that travel for only small distances; the migrating myoelectric complex; peristaltic rushes; and retropulsion associated with vomiting. The enteric nervous system is programmed to produce these different outcomes. In contrast to the intestine, peristalsis in the stomach is a consequence of conducted electrical events (slow waves) that are generated in the muscle. The intensity of gastric contraction is determined by the actions of the vagus nerves, which form connections with enteric neurons in the myenteric ganglia. The proximal stomach relaxes to accommodate the arrival of food. This relaxation is also mediated through vagus nerve connections with enteric neurons. Thus, the primary integrative centres for control of gastric motility are in the brainstem, whereas those for control of the small and large intestines are in the enteric nervous system. In most mammals, the contractile tissue of the external wall of the esophagus is striated muscle, and in others, including humans, the proximal half or more is striated muscle. The striated muscle part of the esophagus is controlled, via the vagus, by an integrative circuitry in the brainstem. Thus, although the myenteric ganglia are prominent in the striated muscle part of the esophagus, they are modifiers, not essential control centres, for esophageal peristalsis. The smooth muscle sphincters restrict and regulate the passage of the luminal contents between regions. In general, reflexes that are initiated proximal to the sphincters relax the sphincter muscle and facilitate the passage of the contents, whereas reflexes that are initiated distally restrict retrograde passage of contents into more proximal parts of the digestive tract. The progress of the contents in an oral to anal direction is inhibited when sympathetic nerve activity increases. To achieve this, transmission from enteric excitatory reflexes to the muscle is inhibited and the sphincters are contracted. The post-ganglionic sympathetic neurons utilise noradrenaline as the primary transmitter. Under resting conditions, the sympathetic pathways exert little influence on motility. They come into action when protective reflexes are activated. Regulation of fluid exchange and local blood flow The enteric nervous system regulates the movement of water and electrolytes between the gut lumen and tissue fluid compartments. It does this by directing the activity of secretomotor neurons that innervate the mucosa in the small and large intestines and control its permeability to ions. Neurotransmitters of secretomotor neurons are vasoactive intestinal peptide (VIP) and acetylcholine. Secretion is integrated with vasodilatation, which provides some of the fluid that is secreted. Most secretomotor neurons have cell bodies in submucosal ganglia. Fluxes of fluid, greater than the total blood volume of the body, cross the epithelial surfaces of the gastrointestinal tract each day. Control of this fluid movement via the enteric nervous system is of prime importance for the maintenance of whole-body fluid and electrolyte balance. The largest fluxes are across the epithelium of the small intestine, with significant fluid movement also occurring in the large intestine, stomach, pancreas and gall-bladder. Water moves between the lumens of digestive organs and body fluid compartments in response to transfer of osmotically active molecules. The greatest absorption of water, 8-9 litres per day, accompanies inward flux of nutrient molecules and Na+ through the activation of nutrient co-transporters, and the greatest secretion accompanies outward fluxes of Cl¯ and HCO3¯ in the small and large intestine, gall-bladder and pancreas. In each of these organs, fluid secretion is controlled by enteric reflexes. In the small intestine and most of the colon the reflexes circuits are intrinsic, in the enteric nervous system. They balance secretion with absorptive fluxes, and draw water from the absorbed fluid and from the circulation. The activity of the secretomotor reflexes is under a physiologically important control from inhibitory sympathetic nerve pathways that respond to changes in blood pressure and blood volume through central reflex centres. Local blood flow to the mucosa is regulated through enteric vasodilator neurons so that the mucosal blood flow is appropriate to balance the nutritive needs of the mucosa and to accommodate the fluid exchange between the vasculature, interstitial fluid and gut lumen. There are no intrinsic vasoconstrictor neurons. Overall blood flow to the gut is regulated from the CNS, via sympathetic vasoconstrictor neurons. The sympathetic vasoconstrictor neurons act in concert with the autonomic control of other vascular beds, to distribute cardiac output in relation to the relative needs of all organs. Thus in times of need, even during digestion, the sympathetic can divert blood flow away from the gastrointestinal tract.

ENS-CNS interactions

The gastrointestinal tract is in two way communication with the CNS. Afferent neurons convey information about the state of the gastrointestinal tract. Some of this reaches consciousness, including pain and discomfort from the gut and the conscious feelings of hunger and satiety, which are integrated perceptions derived from the gastrointestinal tract and other signals (blood glucose, for example). Other afferent signals, concerning, for example, the nutrient load in the small intestine, or the acidity of the stomach, do not normally reach consciousness. In turn, the CNS provides signals to control the intestine, which are, in most cases, relayed through the ENS. For example, the sight and smell of food elicits preparatory events in the gastro-intestinal tract, including salivation and gastric acid secretion. This is termed the cephalic phase of digestion. Swallowed food stimulates the pharynx and upper esophagus, eliciting afferent signals that are integrated in the brainstem, and subsequently provide efferent signals to enteric neurons in the stomach that cause acid secretion and increased gastric volume, in preparation for the arrival of the food. At the other end of the gut, signals from the colon and rectum are relayed to defecation centres in the spinal cord, from which a programmed set of signals is conveyed to the colon, rectum and anal sphincter to cause defecation. The defecation centres are under inhibitory control from higher CNS regions, and inhibition that can be released when it is chosen to defecate. The other central influences are through sympathetic pathways, which have been discussed under the sections on control of motility and regulation of fluid exchange and local blood flow, above.

POMC gene

The large molecule of POMC is the source of several important biologically active substances. POMC gene is activated by Leptin binding to OB-R receptors. POMC gene expression is inhibited by expression of NPY/AgRP neurons - and vice-versa POMC can be cleaved enzymatically into the following peptides: - α-Melanocyte-Stimulating-Hormone (α-MSH) - Adenocorticotropic hormone (ACTH)

Similarities of OT and AVP

The structure of oxytocin is very similar to that of the vasopressins: It is also a nonapeptide with a disulfide bridge and its amino acid sequence differs at only two positions. The two genes are located on the same chromosome separated by a relatively small distance of less than 15,000 bases in most species. The magnocellular neurons that secrete vasopressin are adjacent to magnocellular neurons that secrete oxytocin, and are similar in many respects. The similarity of the two peptides can cause some cross-reactions: oxytocin has a slight antidiuretic function, and high levels of AVP can cause uterine contractions

ENS pathology

There are a large number of pathologies associated with the neural regulation of digestion, many of these arising from abnormalities of the enteric nervous system (De Giorgio and Camilleri 2004; Spiller and Grundy 2004). One neuropathology of the gut is Hirschprung's disease, in which an agenesis of the enteric nervous system, that extends proximally from the rectum for various distances, occurs. It is fatal if untreated. Other enteric neuropathologies include hypertrophic pyloric stenosis, esophageal atresia, gastroparesis, slow transit constipation, some cases of esophageal reflux, and Chagas' disease. The irritable bowel syndrome (IBS) is sometimes considered to be an enteric neuropathy, although IBS covers a spectrum of conditions.

VASOPRESSIN

VASOPRESSIN

AgRP gene disruption and anorexia nervosa

Vink et al Anorexia nervosa (AN) is a life threatening disorder affecting mostly adolescent women. It is a dramatic psychiatric syndrome accompanied by severe weight loss, hyperactivity and neuroendocrine changes. Several studies have shown a strong genetic component in AN. Recent advances in unraveling the mechanisms of weight control point to a crucial role of the melanocortin-4 receptor (MC4-r) system in regulating body weight. The orexigenic neuropeptide agouti-related protein (AGRP), a MC4-r antagonist, plays a crucial role in maintaining body weight, by inducing food intake. The sequence of the coding region of the human AGRP gene (AGRP) was determined and the AGRP of 100 patients with AN was screened for variations. Three single nucleotide polymorphisms (SNPs) were identified and screened in a further 45 patients and 244 controls. Two alleles were in complete linkage disequilibrium and were significantly enriched in anorectic patients compared to controls . These data indicate that variations of AGRP are associated with susceptibility for AN. This is possibly caused by defective suppression of the MC4-r by the variant AGRP, leading to a decreased feeding signal, increasing the risk of developing AN. These results implicate that antagonism of the MC4-r might be considered as pharmacotherapy for patients with AN. AgRP (-/-) or NPY (-/-) or AgRP/NPY (-/-) knockouts in mice do not show any apparent anorexic phenotypes - meaning that there is some sort of developmental compensation throughout embryonic development. HOWEVER, bacteria-induced ablation of AgRP neurons in mice leads to a lean, hypophagic phenotype

AVP enhances paternal behaviour

Wang et al ABSTRACT After being paired with females, male prairie voles show major changes in their social behaviors among which is an increase in paternal responsiveness. These changes are accompanied by fluctuations in the density of the [Argelvasopressin-immunoreactive (AVP-ir) fibers in the lateral septum, suggesting that septal AVP might be involved in these changes. To explore a possible involvement of septal AVP in paternal responsiveness, we tested whether injections of saline, AVP, or the Vla receptor antagonist into the lateral septum influenced the four most prominent paternal activities displayed by male prairie voles; grooming, crouching over, contacting, and retrieving pups. In a first experiment, sexually inexperienced males received a single injection of AVP, saline, or AVP antagonist in the lateral septum, after which their paternal responsiveness was recorded during a 10-min period. AVP-injected animals spent more time contacting and crouching over pups, while AVP antagonist injected animals spent less time grooming pups than saline-injected animals. In a follow-up study, one group of animals received an injection of AVP preceded by an injection of saline or AVP antagonist into the lateral septum. A second group of animals received an inijection of saline preceded by an injection of saline or AVP antagonist into the lateral septum. In both groups, animals spent less time grooming, crouching over, and contacting pups if they had first been injected with AVP antagonist. Control experiments suggested that the effects of AVP on paternal responsiveness were dose- and site-specific. These data suggest that septal AVP enhances paternal responsiveness by a Via receptor-mediated mechanism.

Ghrelin enhances appetite and increases food intake in humans (prepranial rise in Ghrelin levels)

Wren et al Ghrelin is a recently identified endogenous ligand for the growth hormone secretagogue receptor. It is synthesized predominantly in the stomach and found in the circulation of healthy humans. Ghrelin has been shown to promote increased food intake, weight gain and adiposity in rodents. The effect of ghrelin on appetite and food intake in man has not been determined. We investigated the effects of intravenous ghrelin (5.0 pmol/kg/min) or saline infusion on appetite and food intake in a randomised double-blind cross-over study in nine healthy volunteers. There was a clear-cut increase in energy consumed by every individual from a free-choice buffet (mean increase ) during ghrelin compared with saline infusion. Visual analogue scores for appetite were greater during ghrelin compared to saline infusion. Ghrelin had no effect on gastric emptying as assessed by the paracetamol absorption test. Ghrelin is the first circulating hormone demonstrated to stimulate food intake in man. Endogenous ghrelin is a potentially important new regulator of the complex systems controlling food intake and body weight.

Oxyntomodulin reduces body weight in obese subjects

Wren et al This study investigated the effect of subcutaneously administered oxyntomodulin on body weight in healthy overweight and obese volunteers. Participants self-administered saline or oxyntomodulin subcutaneously in a randomized, double-blind, parallel-group protocol. Injections were self-administered for 4 weeks, three times daily, 30 min before each meal. The volunteers were asked to maintain their regular diet and level of physical exercise during the study period. Subjects' body weight, energy intake, and levels of adipose hormones were assessed at the start and end of the study. Body weight was reduced by 2.3 +/- 0.4 kg in the treatment group over the study period compared with 0.5 +/- 0.5 kg in the control group (P = 0.0106). On average, the treatment group had an additional 0.45-kg weight loss per week. The treatment group demonstrated a reduction in leptin and an increase in adiponectin. Energy intake by the treatment group was significantly reduced at the initial study meal) and at the final study meal, with no change in subjective food palatability. Oxyntomodulin treatment resulted in weight loss and a change in the levels of adipose hormones consistent with a loss of adipose tissue. The anorectic effect was maintained over the 4-week period. Oxyntomodulin represents a potential therapy for obesity. CONS: why is Leptin reduced?? Sub-cutaneous injections not ideal.

OT disruption and deficits in lactation and maternal nurturing, increase in male aggressive behavior

Young et al The oxytocin receptor (OXTR) and its ligand, oxytocin (OXT), regulate reproductive physiology (i.e., parturition and lactation) and sociosexual behaviors. To define the essential functions of OXTR, we generated mice with a null mutation in the Oxtr gene (Oxtr-/-) and compared them with OXT-deficient (Oxt-/-) mice. Oxtr-/- mice were viable and had no obvious deficits in fertility or reproductive behavior. Oxtr-/- dams exhibited normal parturition but demonstrated defects in lactation and maternal nurturing. Infant Oxtr-/- males emitted fewer ultrasonic vocalizations than wild-type littermates in response to social isolation. Adult Oxtr-/- males also showed deficits in social discrimination and elevated aggressive behavior. Ligand Oxt-/- males from Oxt-/- dams, but not from Oxt+/- dams, showed similar high levels of aggression. These data suggest a developmental role for the OXT/OXTR system in shaping adult aggressive behavior. Our studies demonstrate that OXTR plays a critical role in regulating several aspects of social behavior and may have important implications for developmental psychiatric disorders characterized by deficits in social behavior.

Cloning of mouse Ob gene and its human homologue

Zhang et al The cloning and sequencing of the mouse Ob gene and its human homologue. Ob encodes an *adipose tissue messenger RNA* with a highly conserved open reading frame. The protein was a 167aa protein identified as a member of the cytokine family - which we now know as leptin. They then looked at mRNA expression of this protein in tissue samples throughout the mose body. In situ mRNA expression for Leptin was found to be highest in adipose tissue. Comparisons between littermates found that mice with the lowers circulating levels of Leptin had lower adipose fat mass, whilst larger mice with higher adipose fat mass had higher circulating Leptin. They also found that Leptin injected ICV decreased feeding and increased energy expenditure in both wild-type and Ob/Ob mice. Over time, this lead to significant weight loss in the mice. This was also found to reverse the co-morbid problems such as hyperinsulineamia, elevated corticosterone levels, and infertility. Leptin circulating concentrations were found to be increased within hours after a meal in the mice, but after several days following overfeeding in humans. Conclusion: shows that the Ob gene product, Leptin, plays important physiological roles in energy expenditure and adipose fat mass homeostasis. Ob/Ob mice are not able to produce leptin, which accounts for the phenotype. CONS: functional sites of leptin? where does it act and how?