Neurobiology of Eating Unit 2
External Influence on Food Intake
"Mostly, it seems that people are influenced in their food choice by learned aspects and attitudes". -Said in Study, "Effect of Ambience on Food Intake and Food Choice"
Examples for Modulation of Satiety by factors not (directly) related to calories
-(Hot) Soup -Sensory-Specific Satiety -Time of Day
Glucoprivation
= Depriving cells of glucose -Hypoglycemia can be experimentally induced with Insulin injections2-DG which interferes with glucose metabolism
Lipoprivation
= Depriving cells of lipids -Experimentally can be induced with Mercaptoacetate (MA), which interferes with the ability to metabolize fatty acids.
Neuropeptide Y (NPY)
= Extremely potent stimulant of food intake. *Can induce a dose-dependent 10-fold increase in Food Intake (Study Levine & Morley, 1984)
Post-Prandial Insulin
= Insulin secretion that occurs after ingestion of food. -Post-prandial insulin is released in response to an increase in blood glucose (physiological stimulus). However, -if insulin only released in response to an increase in glucose in the blood from a meal (post-prandial insulin) we would be experiencing post-prandial *hyperglycemia* after each meal.
First look at Short-term Signals: Two things that determine a meal size (From Gill's Review - Decerebrate Rat Model)
= Taste & GI satiety signals (post-ingestive feedback) codetermine a meal size! -In intact rats (that have brainstem-forebrain communication)
Brainstem (and the Control of Feeding & Body Weight Regulation)
= The brainstem receives sensory inputs & has motor outputs (motor neurons and motor pattern generators located in the brainstem)
Dale's Principle
=Each neurotransmitter produced by a neuron is released from all terminals (endings of the axon) of that neuron. -by Sir Henry Dale -won The Nobel Prize in Physiology or Medicine 1936
Social Modeling
=Effect of Family and Peers -*Learning*: Children's intake influenced "not only by what is served at home, but also the communication style". -Social-Affective Context =*The effect of emotion on learning and memory.
Hypothalamus
A neural structure lying below the thalamus; it directs several maintenance activities (eating, drinking, body temperature), helps govern the endocrine system via the pituitary gland, and is linked to emotion and reward.
Learning/Conditioning:
A novel tasting food is consumed; the taste is a conditioned stimulus (CS). After eating, animal is injected with LiCl to induce malaise, which mimics stomach illness normally induced by spoiled food. -LiCl is an unconditioned stimulus (US). *innate responding (without learning): --palatable taste induces eating: CS (taste) --> eating ---LiCl induces illness: US (LiCl) --> illness *learning (conditioning): --novel taste consumption is paired (followed) with LiCl injections --CS --> eating + US --> illness *after learning (conditioning): --CS alone --> food avoidance/aversion (Conditioned Response, CR)
The 'Taste Reactivity 'Test
A test of Acceptance/Rejection of fluids. -When sweet liquid is infused... (sweet generally preferred, set up to like it for survival - sweetness related to calories) -When bitter liquid is infused ... (Hedonic Reaction - Sweet) (Aversive Reaction, set up to spit it out/react to it if it's poison - Bitter) **Looking at Oral-Motor Responses! - to swallow or spit out!
Testing Ability to respond to GI signals
Ability to Regulate Meal Size in response to Postingestive GI signals can be tested by -Sham Feeding -Nutrient Preload -CCK Infusions
Experiment - Effect of Food Deprivation on Energy Expenditure
After 48hr deprivation both Intact and Decerebrate rats reduced metabolic energy expenditure & mobilized fat, decreased serum leptin, and insulin. -even if brainstem doesn't have connections with forebrain, it can reduce metabolic expenditure -shows energy expenditure is a reflex type response (automatically happening, not based on behavior)
"Flatmap" of the Central Nervous System
Brain Forebrain Brainstem Midbrain Hindbrain Spinal Cord
Brainstem
Brainstem: -Midbrain (Mesencephalon): Tectum Tegmentum -Hindbrain (Rhombencephalon): Pons Medulla
Evidence for CART action in the brain? (from same study)
CART peptide was administered centrally (intracerebroventricularly, ICV) to non-fasted rats
Anorexigenic Peptides
CART: Cocaine and Amphetamine Regulated Transcript -CART expressing neurons are located in the Arcuate nucleus of the Hypothalamus
Does the effect of CCK on individual meals translate to an effect on body weight?
CCK & Body Weight? Evidence from 2 animal models without CCK-A receptors (missing CCK-A receptors, and targeted deletion) -CCK- A receptor is a peripheral receptor;the receptor subtype that mediates the satiety effects of CCK.
CART & α-MSH
Classic Homeostatic (catabolic) signaling molecules in the brain As defined by Schwartz et al., 2000. *Food deprivation --> (-) CART/α-MSH (+) <-- Leptin* From all the studies: -Food-deprivation decreases CART mRNA inthe Arcuate nucleus of the Hypothalamus of normal animals. -CART mRNA is absent from the Arcuate nucleus of the Hypothalamus in Ob/Ob mice compared to controls that have a normal Ob gene (Ob/+). Thus, Leptin stimulates CART production.
Can the brainstem function without the forebrain?
Decerebrate Rat = forebrain disconnected from the hindbrain
When Decerebrate Rat given sucrose (sweet) solution?
Decerebrate Rat: Trial 1: Decebrate rats show acceptance when given sucrose solution in Trial 1. Trial 2: Decerebrate rats continue to show acceptance; the same pattern as in Trial 1 (start of training). *shows the Decerebrated rats do not learn from LiCl injections/illness to reject food in the second trail!
CCK - Study: Cholecystokinin decreases sucrose intake in chronic decerebrate rats
Decerebrate rats are responsive to CCK. -the intake-reducing effect of CCK-8 in the chronic decerebrate rat provides compelling evidence that the caudal brain stem is of sufficient neural complexity to mediate this response
External Effects: Review article - Effect of Ambience on Food Intake and Food Choice
Environment (Ambience/Context) Physical (environment/surroundings) Social (environment/variables)
External Signals influence on Food Intake & Body Weight
Environment (physical) Social (environment) factors The effects can be innate and/or dependent on: -Learning -Perception/Expectations/Cultural -Stress, Emotion, Mood -Reward
Another Summary of Evidence 1 and 2
Evidence 1: -from inbred line of rats (OLETF) that lack CCK-A receptors, because of a spontaneous deletion of the corresponding gene. *They may have other spontaneous mutations or impairments caused by the mutation on CCK-A Receptor gene.* Evidence 2: -from mice with targeted deletion of CCK-A receptor. *They have a selective mutation or CCK-A Receptor only, and properly functioning compensatory mechanism/s to counteract for the missing CCK-A receptors.*
Experiment for Effect of "Meal omission" on Energy Intake - "The Neuroanatomical Axis for Control of Energy Balance"
Experiment: -First week: rats given 3 meals ("intraoral intake")/day. -Second week: the second meal was omitted. -They can eat as much as they like (ad libitum) for each meal. Results: -When rats went from 3 meals a day to 2 meals a day intact rats compensated by increasing the average meal size, while decerebrated rats did not (no change in average meal size). -Decerebrates cannot integrate the information of behavioral patterns -Decerebrates in fact lost weight in second week
LHA compared with ARC/ARH
Lateral Hypothalamic Area (LHA): -Distribution of MCH- and ORX-neurons within the LHA *MCH & ORX are NOT co-localized (co-expressed) in the same neurons. Arcuate Nucleus (ARC/ARH): -Distribution of NPY- and AGRP-neurons within the ARC/ARH *NPY & AGRP are co-localized (co-expressed) in the same neurons.
Complex, Long-Term Regulation of Food Intake?
Long-Term Control -integration of Energy Intake & Expenditure- -Maintenance of Energy Balance is accomplished by integration & coordination of two systems: energy Intake (behavioral) & energy Expenditure (physical & metabolic) -In an intact brain: Food deprivation triggers compensatory responses in both. --involves behavior (Forebrain) and metabolism
In same study - MCH injections into the brain (ICV)
MCH injections into the brain (ICV) --> *Increase Food intake
Social Facilitation con't
Many factors could contribute to social facilitation in humans -Duration of meal -Rewarding component -meals are rated higher in *palatability* when eaten with others- -Affective component -more eaten with friends than with strangers- -Learned component
What are the signals (factors) that contribute to the motivation to eat?
Metabolic/Homeostatic signals = Intrinsic physiological signals - Signals from the body and the brain related to metabolic control and maintenance of energy balance (homeostasis). Non-Metabolic/Homeostatic signals = Extrinsic (external) signals;from the environment or other brain areas (other than hypothalamus-brainstem feeding circuitry)
A comparison of effects of NTS lesions (bilateral) vs Decerebration on lipoprivation and glucoprivation induced feeding.
NTS lesions: brain stem has no NTS Decerebration: no communications with forebrain but NTS is intact *NTS is part of both circuitries (generating response to glucoprivaion and lipoprivation), but the circuitry for glucoprivation is within the brainstem,while the circuitry for lipoprivation is across the NTS and forebrain. -Lesion evidence: NTS critical forboth lipo- and gluco-privation. -Decerebration experiment: Brainstem on its own (NTS+other areas) can only respond to glucoprivation
Study - Coexpression of Agrp and NPY in fasting-activated hypothalamic neurons
Neurons in the Arcuate Nucleus that express Neuropeptide Y (NPY) also express Agouti-Related Peptide (AGRP) -Dale's principle! - going to be released together! (NPY and AgrP act together to drive feeding and to shut down unnecessary energy expenditure.)
Short term vs long term vs learning
Short-Term Control -control of Meal Size- Long-Term Control -integration of Energy Intake & Expenditure- Learning -Associative Learning-
Dual-Centers Hypothesis E. Stellar, 1954
The hypothalamus contains centers for initiation and cessation of responses/behaviors (model system: feeding).
Other examples of external factors:
-Time of Day -Distraction/Boredom
Study - Role of the Brain Stem in Hunger
This study demonstrates that the NTS is critical for Lipoprivation- and Glucoprivation-induced food intake, -BUT it does not provide evidence whether the NTS is functioning only within the brainstem or within a network with the forebrain. graphs: (NST=NTSnucleus of the solitary tract for graph) -food intake much more in control group for MA (lipoprivation) and 2-DG (glucoprivation) drug treatment -for NST lesion group, food intake was low for lipoprivation and glucoprivation
What is the Primary function of MC4-Receptor?
To inhibit feeding, since eliminating these receptors results in obesity. -Study: Targeted Disruption of the Melanocortin-4 Receptor Results in Obesity in Mice
Responses to sweet vs. bitter liquids
When sweet liquid is infused (e.g., sugar solution): the response is *acceptance* (rhythmic tongue protrusions, lateral tongue licks, and ingestion). When bitter liquid is infused (e.g., quinine solution): the response is *rejection* ("aversive profile": chin rubs, head shakes, face washes, paw shakes, paw wipes, and rejection of liquid).
Cascade diagram
decrease in fat cell mass --> decrease in leptin/insulin expression --> decrease in leptin/insulin action in hypothalamus --> activates NPY/AgRP neuron and INHIBITS POMC neuron/CART/MSH --> decrease in alpha MSH expression and release --> increase in AgRP release blocks binding of alpha-MSH to melanocortin receptors --> decreases activity of melanocrortin anorexia pathways --> Increase Food intake -Activation of NPY/AgRP cascade would eventually result in obesity, if not regulated -Inhibition of NPY/AgRP cascade would eventually result in anorexia, if not regulated
Targeted mutation of MCH gene Study - Mice lacking melanin-concentrating hormone
mutants: MCH-/- red arrows point to filled circles (a-c), filled bars (d) controls: MCH+/+empty circles (a-c), bars (d) MCH+/-empty triangles (a,b) results: -decrease in body weight! (don't die because other components in system that regulate eating) -doesn't block eating! --> shows that several mediators/driver of eating -essential homeostatic mechanism -eat less (food intake goes down) -fat gain is less -we know there's orexin too, so we know doesn't stop them from eating -also don't eat when they're supposed to, since results show they eat later in the day
Study - The Satiating Efficiency of Foods
-Used different foods as preloads to test hypothesis that some foods are more satiating then others. -Highly satiating foods when eaten prior to/at start of a meal should decrease overall calories consumed in the meal.
Manipulations to test if Vagus nerve is the pathway for CCK to communicate to the brain
-Vagotomy or application of neurotoxin (Capsaicin) Block action of peripherally administered CCK -Lesions of NTS interfere with CCK-induced satiety -That means CCK signals to the brain via neuronal pathways traveling through the Vagus Nerve. --Lesion: Vagotomy or Neurotoxin --take out NTS
Could chronic NPY stimulation cause weight gain? - Studies: Neuropeptide Y Chronically Injected Into the Hypothalamus: A Powerful Neurochemical Indicuer of Hyperphagia and Obesity
-Where is NPY injected?: Hypothalamus -Where does the NPY act (where is it released from axons of NPY neurons) ?: Hypothalamus -NPY is made by neurons located within the Arcuate Nucleus of the Hypothalamus -Cell Bodies of neurons in the Arcuate Nucleus of the Hypothalamus make Neuropeptide Y (NPY) Modified Fig. 1.Baskin et al., 1999, Diabetes vol 48 *NPY is the most potent in the Lateral Hypothalamus (LHA)
dorsal vs ventral
top vs bottom
medial
toward the midline
Learning and Homeostatic Mechanisms
"When individuals are able to predict accurately that a regulated bodily parameter is going to be altered by an external event, they can learn to initiate a conditioned response in anticipation of the perturbation that minimizes its impact." *Anticipatory Motivation is a Learned Response in anticipation of a predictable event. The purpose: to minimizes homeostatic disturbance.
Study - 2-Deoxy-d-glucose, but not mercaptoacetate, increases food intake in decerebrate rats
(-Lipoprivation is more complicated) -Decerebration experiment: Brainstem on its own (NTS+other areas) can only respond to glucoprivation
CART
(Cocaine and Amphetamine Regulated Transcript) is co-expressed with another anorexigenic peptide: α-MSH: α-Melanocyte-Stimulating Hormone
Brainstem & Integration & Coordination of Energy Intake & Energy Expenditure
(From Grill's review:) 1. Effect of Food Deprivation on Energy Intake 2. Effect of "Meal omission" on Energy Intake 3. Effect of Food Deprivation on Energy Expenditure
Short-term Hunger Signals Meal initiation (hunger signals; orexigenic)
(Long-term --> adiposity signals: Fat mass translated to Insulin and Leptin as signals to the brain) Short-term --> satiety signals *Ghrelin: Ghrelin -Levels in human blood plasma. (stimulates GH secretion) -Peptide hormone produced by stomach stimulates feeding -Study: Ghrelin induces adiposity in rodents (Peripheral injections and Central injections)
What happens to Brain substrates that stimulate eating under food deprivation? NPY mRNA levels?
(from Fed vs. Fasted study from before with ob/ob mice) -NPY mRNA is increased by food deprivation (NPY mRNA levels in ob/ob?) -NPY is over-expressed in ob/ob 1. Food deprivation increases mRNA levels for NPY 2. NPY is over-expressed in ob/ob *NPY mRNA is increased by food deprivation & by lack of adiposity signal (leptin), similar to MCH.
Examples for Modulation of Satiety by factors not (directly) related to calories (now in more detail)
(from all the studies just discussed) -Hot Soup (temperature, volume, fat molecules) -Sensory-Specific Satiety (eating or just chewing or smelling food reduces subsequent pleasantness rating of that food) -Time of Day (early in the day easier to reach satiety)
α-MSH
(α-Melanocyte-Stimulating Hormone) -Melanocortins are a family of peptides such as α-MSH that are [cleaved from pro-opiomelanocortin (POMC) precursor molecule]
Food and Food-Cues can induce appetite.
***Priming with cues (or food itself) is a brief exposure prior to a meal. -Different from a longer or more intense exposure That induces sensory-specific satiation.
Study Results
*CCK is critical for regulation of short-term food intake, but may not be essential for maintaining body weight. (affects short-term meal size, but not long-term) -confirmed only in both studies/models that CCK and receptors are need to mediate meal size, but not confirmed in both studies/models that CCK is essential for maintaining body weight Discussion points: -OLETF vs targeted deletion -Other mechanisms provide functional redundancy (ex. metabolism - mice compensated metabolically for body weight to be maintained)
Overall results of Decerebrate
*Decerebrate have intact taste and oral-motor responses related to licking and swallowing. 1a. Able to recognize & respond to taste 1b. Able to ingest variable amounts of liquid* Based on The 'Taste Reactivity' Test The "Intraoral Intake" Test
Study - Hypothalamic CART is a new anorectic peptide regulated by leptin
*Expression of CART mRNA -ob/ob vs. control ob/+ -CART in DMH not significantly different between control and ob/ob (DMH is brain region that is not different in CART between ob/ob and control) -*CART mRNA is absent (suppressed) from the Arcuate nucleus of the Hypothalamus (Arc) in Ob/Ob mice compared to controls (Ob/+).(shows the Arcuate nucleus is primary place for CART) -ob/ob has more drive to eat, and suppressed satiety To test whether CART mRNA is regulated by circulating Leptin Kristensen et al., 1998: -Injected ob/ob mice with leptin daily for 10days (IP; intraperitoneal injections) -Control 1: received Saline (giving nothing) -Control 2: was Pair-fed (eating less but not by leptin, control the amount consumed, to compare with the leptin food intake decrease)
Study - Administration of exogenous CCK reduces food intake.
*Food intake decreases with increasing dose of CCK but does not block a meal
Leptin and Neuropeptide Y
*Leptin regulates Neuropeptide Y -Leptin suppresses Neuropeptide
Lateral Hypothalamic Area (LHA)
*Orexigenic Peptides* = 'appetite-inducing' The two orexigenic peptides: -Melanin-Concentrating Hormone (MCH) -Orexin/Hypocretin (ORX) MCH and ORX are peptides produced by the hypothalamus but they are released within the brain, so they are not hormones.
In the LHA,Is NPY-neuron input directly connected with neurons that make Orexigenic peptides (MCH & ORX) ?
*Structural evidence that MCH & ORX neurons (in the LHA) receive input from NPY neurons (NPY neurons cell bodies are in ARC) -Images show rat & human LHA where NPY axons (black) innervate MCH and ORX cell bodies and dendrites (brown).
Test Intake during 1-hour post Saline or NPY injections
*The NPY/Saline group did not receive NPY prior to test, but ate similar amounts to those that received NPY prior to test (NPY/NPY and Saline/NPY).* -There were no significant changes in peripheral parameters, Insulin, Glucose, or Ghrelin levels. -*The effect in NPY/Saline group was accomplished by conditioning NPY targets in the brain!* Takeaway: -Early work of Pavlov showed that salivation can be conditioned. -Later work showed that Peripheral (insulin) and central (brain; NPY) feeding substrates can be conditioned.
MCH...
*is a critical homeostatic molecule -this reading is relevant to this figure below. Classic Homeostatic signaling molecule in the brainas defined by Schwartz et al., 2000. 1. Leptin and insulin circulate in the blood in concentrations proportional to body fat content and energy balance 2. Leptin and insulin act on central effector pathways in the hypothalamus, repressing brain anabolic neural circuits that stimulate eating and inhabit energy expenditure, while simultaneously activating catabolic circuits that inhibit food intake and increase energy expenditure 3. Low leptin and insulin levels in the brain during weight loss increase activity of anabolic neural pathways that stimulate eating an suppress energy expenditure... 4. Ingestion of food generates neural and hormonal satiety signals to the hindbrain. Leptin/insulin-sensitive central effector pathways interact with the hindbrain satiety circuits to regulate the meal size, thereby modulating food intake and energy balance.
Sham Feeding
-*Sham feeding permits the investigation of the orosensory controls of meal size in the absence of postingestive controls.* -Sham feeding occurs when ingested food is prevented from accumulating in the stomach and small intestine by surgical intervention. -The simplest and most widely used method involves implantation of a gastric cannula in a rat to provide a chronic, reversible gastric fistula. Results -*Like intact rats, Decerebrate consumed more food (sucrose solution) during sham feeding (e.g., when food is drained). --able to detect that stomach didn't get nutrients (as a result, no CCK signal) -This shows that Decerebrate rats respond to GI satiety signals.
Brainstem in the control of feeding & BW regulation
-Brainstem is sufficient to mediate *Reflex* responses to taste (oral-motor responses) & to GI satiety signals --It can regulate meal size without Forebrain -Brainstem is sufficient to mediate *Metabolic* responses to food deprivation (also can be considered a reflex response) -Brainstem & Forebrain communication is necessary for *Behavioral* responses to complex (non-reflex) stimuli, such as food deprivation, meal omission, as well as for *Learning* (conditioned taste aversion)
Same Study - Effect of Anti-CART on feeding
-CART antiserum administration *increased* night-time food intake. -Administration of the Antibody against CART increased feeding indicating that CART is an endogenous inhibitor of food intake. *This work provided evidence that CART is an endogenous inhibitor of food intake that is closely associated with the actions of Leptin and Neuropeptide Y.
CART Distribution in HY
-CART distribution in the Arcuate nucleus of the hypothalamus -Image shows dots that are cells that express CART -Arcuate is sensitive to Leptin (primary place in the hypothalamus)
Both missing CCK-A receptors & have resistance to CCK but only OLETF become obese
-OLETF is an inbred line of rats, missing CCK-A receptors but possibly additional genetic deficits become obese & develop Type II diabetes -Mice with targeted deletion of only CCK-A receptors do not get obese and do not develop Type II diabetes
Same Study - Effect of CART on food intake
-CART peptide ICV injections inhibited feeding in animals food-deprived for 24h (data not shown) --CART dose increase makes average food intake go down -CART peptide ICV injections inhibited feeding in fasted and non-fasted animals & completely blocked NPY-induced feeding. --now animals given CART and NPY, when just given NPY, food intake goes up. Then when CART is given as well, average food intake goes down significantly. (if CART produced, shuts down NPY)
Administration of exogenous CCK reduces food intake.
-CCK & Therapeutic Potential for Obesity Treatment -Short half-life (1-2min). -When injected continuously it rapidly becomes ineffective.
Decerebrate with sweet & bitter taste
-Capable of reflex responding to sweet & bitter taste (from Taste Reactivity Test) which showed thatoral-motor responses related to licking and swallowing are intact How about if larger amounts of liquid are infused?
Learning and Homeostatic Mechanisms - can be conditioned!
-Cephalic Insulin -NPY
Effect of Food Deprivation on Energy Intake Study - Neurological Dissociation of Gastrointestinal and Metabolic Contributions to Meal Size Control
-Decerebrated rats cannot integrate information that they haven't eaten all day, so do not eat more/compensation after food deprivation -In test, respond to preload at eat less, but do not respond to deprivation and eat the same as no deprivation and no preload -The decerebrate rats did not show hyperphagic responses to food deprivation.
Decerebration
-Decerebration is accomplished by transecting the brain stem -Neural circuits in the forebrain cannot affect behaviors controlled by motor neuron caudal to the transection -Neural circuits in the hindbrain can affect behaviors controlled by motor neurons caudal -Control of muscles involved in ingestive behavior *With Decerebration: the brainstem (still) receives sensory input & has motor output, BUT it does not have connections with the Forebrain.*
Social Facilitation
-Eating with others increases a meal size. (study shows this) -The presence of other people during food consumption enhances the actual intake. -Social Facilitation occurs regardless of Age, Time/meal of day, Place (home, restaurant) or Species
Summary of NPY
-Extremely potent stimulant of food intake. -Decreases energy expenditure. -Chronic NPY stimulation causes weight gain. -*NPY is regulated by Food Deprivation & Leptin.*
Ability to recognize & respond to taste and ingest/control variable amounts of liquid is tested in Decerebrate rats by:
-The 'Taste Reactivity' Test (how they react to the taste) -The "Intraoral Intake" Test (can the rats swallow, how well can they do that)
Hypocretin/Orexin (H/O; ORX) Study - Orexins and Orexin Receptors: A Family of Hypothalamic Neuropeptides and G Protein-Coupled Receptors that Regulate Feeding Behavior
-Fasting (Food deprivation) Up-Regulates mRNA for Orexin (more Orexin made when we are food deprived) -Intracerebroventricular Injection of Orexin (ORX) Stimulates Food Consumption (the more you have it, the hungrier you are) -Degeneration of Orexin neurons leads to Narcolepsy. why need both MCH and ORX? (MCH critical for homeostasis): --In addition to Stimulatory role on Eating, Orexin is also critical for Arousal/Wakefulness & Degeneration of Orexin neurons leads to Narcolepsy.
What are the pathway for Ghrelin from the stomach to reach the Hypothalamic neurons?
-Ghrelin is released in the stomach where it stimulates the Vagus nerve, which communicates with the NTS. -Neurons in the NTS also contain Ghrelin, and send axons to the hypothalamus, where they release Ghrelin, which induces feeding. *Ghrelin (stomach cells) --> Vagus nerve --> NTS Ghrelin Neurons --> *HY
Metabolic Hunger Signals
-Glucoprivation -Lipoprivation
Limitations for Decerebrate Rat?
-Impairments from experiments with decerebrated animals provide important evidence about what the brainstem can do on its own -BUT they do not provide evidence about how the brainstem functions within the network with forebrainin an intact animal. -An example regarding limitations of the decerebrate model: -A comparison of effects of lesions of the NTS vs Decerebration on lipoprivation and glucoprivation induced feeding.
Information Flow
-Information flow along individual neurons is by way of electrical signals conducted via the plasma membrane of the dendrites, cell body, and axon. -This information is transferred to another cell (neuron or muscle) via the release of neurotransmitters from axons specialized endings: synapses. -A neuron only has one axon.But each axon has many branches/terminals (endings) with synapses.
Hormones released by the Pancreas
-Insulin -Glucagon
Cephalic Insulin release
-Insulin secretion occurs prior to ingestion of food. Released in response to cues that predict eating, including taste; -Anticipatory/preparatory response, *driven by the brain*
Study - From Lesions to Leptin: Hypothalamic Control of Food Intake and Body Weight
-LHA lesion produces aphasia and weight loss -VMH-ARC lesion produces hyperphagia and obesity
Dual Center Hypothesis for Hypothalamic Control of Feeding
-Lateral Hypothalamic Area (LHA) "Feeding/Hunger Center" -Ventromedial (Arcuate nucleus!!) Nucleus of the Hypothalamus (VMH) "Satiety Center"
Nutrient Preload
-Like intact rats, Decerebrate consumed less food after nutrient preload (appetizer right before the meal, size of meal typically goes down with appetizer) -This shows that Decerebrate rats respond to GI satiety signals.
Study - Hunger in Humans induced by MK-329, a Specific Peripheral-Type Cholecystokinin Receptor Antagonist
-MK-329 is a CCK receptor *antagonist* that is specific to peripheral receptors (CCK-A receptors). -MK-329 *induced hunger*, which is evidence that CCK-A receptors normally mediate satiety. -significant increase in subjective hunger ratios produced by MK-329 -evidence that endogenous CCK activity at the peripheral type receptor physiologically curtails hunger
Study - Increased Food Intake by Neuropeptide Y is Due to an Increased Motivation to Eat
-NPY administration ICV (injection) -lever pressing: animal presses lever and get reward (food) -mimics how much more hungry we are (press more) results: -NPY administration significantly increased lever pressing for milk reinforcement. -NPY-injected mice tolerated shocks to the tongue during milk drinking (better than control mice). -- occurs naturally too! --willing to endure unpleasant shock/grid to get to food/milk -NPY administration increased drinking of milk mixed with quinine (bitter taste). -- hungrier we are, the more likely we will eat anything!
NPY in ob/ob
-No leptin! So no regulating from leptin to NPY (-), just from Ghrelin (+)
Based on 'Taste Reactivity' Test
-Oral Motor Responses are intact/fully functioning in Decerebrate rats -In response to fluids (intra-oral infusion) normally ingested or rejected (sugar solution vs. quinine) -Decerebrate rats show ingestive behaviors similar to those of intact rats. *shows that accepting and rejecting food is a reflex!
Learning -Associative Learning-
-People and animals avoid foods that made them ill. -We learn to avoid the food (taste) that made us sick. ***This learning is most efficient if the taste is novel.
Glucose & Hunger
-Pre-meal Glucose decline: could be due lack of nutrients/energy (metabolic hunger signal) OR -could be because cues predicting meals stimulate (small bursts) of insulin, which in turns produces a decline of glucose in blood. --glucose goes down before the meal! (because of cephalic insulin release) -Study: Coupling Between Glucose and Meal Initiation
Same Study - Soup
-Preload Soup or Equicaloric Appetizer (crackers, cheese, apple juice) -No-Preload Followed by a meal Meal ("Intake") -If hot soup (chicken noodle) consumed first (preload), overall fewer calories are consumed during the meal.
Decerebrate Rat
-Receives sensory input & has access to motor/output neurons. It does NOT have input from the Forebrain. -Maintains posture & grooms spontaneously. Does NOT spontaneously walk, run and jump, but can if stimulated. -Does NOT approach food & must be fed (by gavage) to survive.
Decerebrate with this test
-Respond appropriately to changes in infusion rate. -With an increase in oral infusion both intact and decerebrate rats responded by an increase in both swallow frequency and swallow volume.
Time of Day - Study: The Time of Day of Food Intake Influences Overall Intake in Humans
-Satiety Ratio defined as the duration of the inter-meal interval divided by the meal size *Satiety ratio decreased over the day -largest amount needed to satiate at dinner- (biggest meal at dinner!) *the proportion of intake in the morning was negatively correlated with overall intake, whereas the proportion ingested late in the evening was positively correlated with overall intake. -The more consumed in the morning, the less consumed overall. --when you eat breakfast, you feel less hungry throughout the day and eat less throughout the day! -The more consumed in the evening, the more consumed overall.
Conditioned Taste Aversion
... is a form of Associative Learning -Experimentally an animal can learn to avoid the taste of food that was eaten before illness. -To do that, illness is artificially induced with a toxin/malaise-inducing agent (e.g., LiCl).
How about sham-fed? How about stomach-tube feeding?
-Sham feeding: wouldn't have the second peak (because no glucose/nutrients influx for second peak) -Stomach-tube feeding: wouldn't have the "first peak" (small anticipatory peak), just getting the nutrients so only the "second peak" again: -First peak: early elevation before the nutrients, so due to anticipation of nutrients; driven by sensory cues (taste, smell, sight), so due to food cues. -Second peak: late elevation due to nutrients (glucose rise).
Social Environment/Variables
-Social Facilitation -Social Modeling
Distribution of MCH- and ORX-neurons within the LHA
-Study: "Chemically Define Projections Linking the Mediobasal Hypothalamus and the Lateral Hypothalamic Area -MCH & ORX are expressed within the same region of the LHA but in different neurons. -These two peptides are NOT colocalized! -they can function independently!! -If they do come from the same neuron (colocalized), then will both be released everywhere together (Dale's principle)
Results of testing Ability to Regulate Meal Size in response to Postingestive GI signals (Based on sham feeding, nutrient preload, and CCK Infusions)
-all these studies show that decerebrate rats are able to respond to GI signals -*Decerebrate rats are sensitive to postingestive inhibition. They respond properly to GI signals.* -Decerebrate show fundamentally normal oral motor organization of meal taking (licking; swallowing). -They are sensitive to the same Taste & GI (postingestive feedback/satiety signals) that codetermine meal size in the intact rats. -*(Caudal) Brainstem in neural isolation from the Forebrain is sufficient for and capable of Simple, Reflex, Short-Term Regulation of Food Intake -Decerebrate can: Mediate Reflex, Oral-motor Ingestive Responses -Regulate Meal Size
Study - Administration of CCK to rats every time they begun to eat a meal
-decreased size of each meal (relative to placebo). However!: -The number of meals increased (compare to placebo). -Rats compensated for CCK-induced reduction in meal size by increasing number of meals. -CCK would NOT be a viable weight loss strategy!
Stephen Walter Ranson 1880-1942
-image of where you can cut in the brain (a, b, c) -a further up, b is lower, c is where you cut for decerebration -Levels of central nervous system transection where animals can (a,b) and cannot (c) survive independently and display spontaneous behavior, including eating, drinking, and locomotion. -If cut a (a, b), then animals survives - structure that connects with the brainstem that allows animals to survive (Hypothalamus!)
The "Intraoral Intake" Test
-is Intraoral infusion of liquid that is sustained until the rat ceases to ingest. -The amount consumed is analogous to the meal size in standard short-term tests.
Study - Peripheral Insulin in Response to the Sight and Smell of Food
-measured Blood Insulin -Viewed food for 5min; told they would it in 15 min. *Occurs if/when the sight & smell predictable of a meal. -Insulin goes up after looking at food for 5 minutes --mediating glucose, so goes back down again (starts with cue, then goes down because glucose goes down) -In this study all subjects were women. -Obese had higher insulin responses to sight/smell of food (food cues) (but error was high when replicated). -Overall there were individual differences ....
Study - Melanin-concentrating hormone over expression in transgenic mice leads to obesity and insulin resistance
-overexpression of MCH (artificially making a lot) causes them to eat more, gain more weight -natural drive to eat, in overdrive -body weight increased -serum insulin increased
Anticipatory Mechanisms -Anticipatory Motivation-
-role in ending eating- Satiety mechanisms stop eating before BW is gained in anticipation of imminent replenishment
Food deprivation (Ghrelin) & Leptin regulate AgRP
-same arrows and +/- for AGRP -NPY & AGRP: Classic Homeostatic (anabolic) signaling molecules in the brain as defined by Schwartz et al., 2000.
Melanin-Concentrating Hormone (MCH) Study - A role for melanin-concentrating hormone in the central regulation of feeding behaviour
-tested ob/ob mice vs. +/+ mice -Northern Blot showing mRNA of mice -measured in FED and FASTED situations -when they are not hungry (fed), ob/ob in an overdrive of/has a lot more of MCH mRNA -for when fasted, ob/ob goes up even more (in an overdrive) --still responding to hunger, so their MCH goes up even more than when they are not hungry (so drive for eating is much stronger for ob/ob mice!) --and this is because leptin is not there in ob/ob mice! takeaways: *MCH mRNA increased by food deprivation & by lack of adiposity signal (leptin). *MCH is regulated by Food Deprivation & Leptin.*
Results of leptin and CCK study
-with CCK injection, meal size went down compared to control -with Leptin injection alone, no change in meal size because it is not a short-term signal -Leptin and CCK together, even lower meal size! --*showing that there is integration of the short-term and long-term signals *Leptin (from fat cells) enhanced CCK satiety effects -Leptin enhanced CCK satiety effect is dose-dependent Brain site of action? --> topic 3! Brainstem and Control of Feeding and Body Weight Regulation
MC-R
-α-MSH (and other Melanocortins) binds to Melanocortin Receptors (MC-R) *α-MSH is an agonist for MC4-R* -Agonists for Melanocortin receptor can induce weight loss by increasing energy expenditure (metabolic rate) & decreasing food intake *α-MSH binds to MC4-R and Inhibits eating*
Summary
1. Effect of Food Deprivation on Energy Intake - The decerebrate rats did not eat more after food deprivation. 2. Effect of "Meal omission" on Energy Intake -When rats went from 3 meals a day to 2 meals a dayintact rats compensated by increasing the average meal size, while decerebrated rats did not. 3. Effect of Food Deprivation on Energy Expenditure -After 48hr deprivation both Intact and Decerebrate rats reduced metabolic energy expenditure & mobilized fat, (& decreased serum leptin, and insulin).
Criteria for endogenous satiety signal (factor/hormone)
1. The signal is generated after food is ingested 2. It acts within a single meal (short duration) 3. Exogenous administration should decrease meal size (without causing illness) should be effective in physiological dose 4. Removing or antagonizing the endogenous factor should increase meal size
Smell - Study: Olfactory Sensory-Specific
1.) -First: Banana Eaten to satiety or Chicken Eaten to satiety -Then other foods smelled and pleasantness rated -Results: Pleasantness rating was low for food they had eaten, and higher for others! (very specific) -Then other foods tasted and pleasantness rated -Results: the same, low pleasantness for eating same food (banana or chicken) 2.) *Just chewing food for a few minutes (5min) without eating produced similar effects as eating the food (taste was there) --produced the same effects!! 3.) -First: Banana Smelled or Chicken Smelled -Then other foods tasted and pleasantness rated -Results: Banana and Chicken tasted had the lowest pleasantness rate (because anticipate the food we're about to eat) *Smelling food without caloric intake is sufficient to modulate subsequent perception of taste (and smell) similar to the effects when the food had been eaten. -Anticipatory Mechanisms-
Different tests to examine meal size control
1a. Ability to recognize & respond to taste (*taste alone can drive eating!! - point shown from these experiments) 1b. Ability to ingest/control variable amounts of liquid 2. Ability to respond to GI signals
Anorexigens in HY?
Anorexigens: "appetite-suppressing"
Physical Environment/Surroundings
Food Presentation and Location -availability -variety -physical proximity -portion size Colors Light Sound Temperature Smell
Regulation of MCH diagram
Food deprivation & Leptin regulate MCH but in opposite direction
Would you expect the levels of CART (mRNA)to be higher or lower under food deprivation compared to baseline/sated state?
Food-deprivation decreases CART mRNA in the Arcuate nucleus.
Forebrain
Forebrain -Telencephalon: Cerebral cortex Cerebral nuclei (Basal Ganglia): Striatum; Pallidum -Diencephalon: Thalamus Hypothalamus
Ghrelin and Leptin with NPY
Ghrelin & Leptin: -A decrease in energy (food deprivation) & an increase in energy (fat) regulate NPY but in opposite direction -(+) from Food deprivation/Ghrlin to NPY, and (-) from Leptin to NPY
Ghrelin and Neuropeptide Y
Ghrelin stimulates Neuropeptide Y
During/After a meal
Glucose is abundant in the blood and this triggers ... -glucose will trigger insulin release -insulin will signal to the liver to store glucose -insulin will allow other cells to use glucose
Ivan Petrovich Pavlov
He is widely known for his detailed examination, based on the conditioned reflex, of a type of learning known as Pavlovian or classical conditioning.
Hypothalamus (HY) (and Ghrelin)
How is Ghrelin integrated within the Hypothalamus? What neurons/substrates in the HY does Ghrelin act on? -To identify Ghrelin targets, examined co-expression of Ghrelin Receptor & mRNA for specific peptides (which identifies specific neurons). -They found: Co-expression of Ghrelin Receptor & Neuropeptide Y (NPY) mRNA in Hypothalamic neurons. *Ghrelin acts via Neuropeptide Y neurons
Food Deprivation
In intact brain: compensation in BOTH intake & expenditure. -Decerebrate: do NOT compensate in energy intake (behavior). do compensate in energy expenditure (metabolic).
Diagram of regulation process for NPY/AgRP neuron
Increase in Food intake: decrease in cell mass --> decrease in leptin/insulin expression --> decrease in leptin/insulin action in hypothalamus --> (+) Activates NPY/AgRP neuron --> increase in NPY/AgRP expression --> increase in NPY and AgRP release --> increase in food intake Opposite way for when decreasing Food Intake and NPY/AgRP neuron is (-) Inhibited by hypothalamus
What should happen under food deprivation to a brain substrate that stimulates eating?
Increased!
Is eating solely controlled by physiological signals related to energy balance?
Influence of *External* Signals on Food Intake and Body Weight
Cephalic Insulin
Insulin is also released in anticipation of a meal! -If meal is already predictable, body prepares for it
Postprandial Insulin release
Insulin secretion occurs after ingestion of food. Released in response to a rise in blood glucose; *physiologically* driven.
Conditioned Taste Aversion Study - Chronically Decerebrate Rats Demonstrate Satiation But Not Bait Shyness Intact Rats:
Intact Rats: Procedure Trial 1: Given sucrose solution. -They show acceptance pattern. -Immediately after consumption of sucrose solution, rats given LiCl injections, which made them ill. Trial 2: Given the same sucrose solution, as in Trial 1. --Response pattern: Intact rats show rejection pattern;they respond as if they were given a bitter (e.g., quinine) solution.
Other Study Results
Other graphs shown: Insulin release in response to sight and smell of food differed in different people: -High increase (top panel), -Low increase (middle panel), -& A decrease (bottom panel) *Individual differences .... Majority of subjects had a biphasic response, some had one smaller peak, and some no response or a decrease Cephalic Insulin Results (first graph shown): *A biphasic insulin (2 phases) response also observed during meals. -First peak: early elevation before the nutrients, so due to anticipation of nutrients; driven by sensory cues (taste, smell, sight), so due to food cues. -Second peak: late elevation due to nutrients (glucose rise). --*If they started eating, second peak would be higher, as insulin levels would rise, due to glucose influx.
Study - CCK & Body Weight
Part I: Evidence from inbred line of rats (Otsuka Long-Evans Tokushima Fatty; OLETF) that do not have CCK-A receptors, because of a spontaneous deletion of the corresponding gene. Part II: Evidence from mice with targeted deletion of CCK-A receptor.
Integration between short- and long-term signals
Peripheral Regulatory Signals: -Adiposity (long-term) Signals (Leptin, Insulin) -Short-term Signals (Gherlin, CCK) integration --> through the central nervous system! *Behavioral Evidence! (if gaining weight and not always maintaining the weight)
Overexpression of MCH gene
Photomicrographs demonstrate MCH-immunoreactive neurons inthe lateral hypothalamic area (LHA) in the brain of (a) an MCH-OE mouse (Tg-12) image shows overexpression mouse of MCH (b) of a wild-type mouse (Tg-7). -showing neurons on image! (have axons and dendrites) -more of the processes of the MCH in the MCH-OE mouse
Cephalic Insulin defined
Refers to Insulin secretion in anticipation of a meal by cues associated with meals/feeding
Social-Affective Context Study - The Influence of Social-affective Context on the Formation of Children's Food Preferences
Snack food was presented to pre-school children: (1) As a reward (2) Non-contingently, paired with adult attention (3) In a nonsocial context (4) At snack time (familiarity control) -Snack reference was assessed with pictures of cartoon faces showing liking or disliking. -Rank ordered foods (1-8); foods used were in middle range (4,5,6). Results: -Children developed preference for snacks that were given as a reward or paired with adult attention. -Compared to snacks that were given in in a nonsocial context, or at snack time.
How is Cephalic Insulin accomplished/via which pathways/nerves?
Stimulated (regulated) by the brain via the Vagus nerve. *Visceromotor Information -from brain to intestine (DMX - brain to intestine) In turn, this could reduce the post-prandial increase in glucose (and prevent hyperglycemia), but GLUCOSE!
CCK & Body Weight: Part II
Study - A cholecystokinin-A receptor mediates inhibition of food intake yet is not essential for the maintenance of body weight -(inhibition of food intake = mediating satiety) Evidence from mice with targeted deletion of CCK-A receptor. -The CCK receptor mediates inhibition of food intake yet is not essential for the maintenance of body weight -In these mice, similar to rats, the meal size did not change, showing that they also are not responding to CCK because they don't have the receptors (confirmed in both studies) -*the difference is that the body weight of the mice for males or females did not change
CCK & Body Weight: Part I (Study 1)
Study - Disordered food intake and obesity in rats lacking cholecystokinin A receptors -Evidence from OLETF rats that *do not* express *CCK-A receptors* -*OLETF* is an inbred line of rats, lack CCK-A receptors because of a spontaneous deletion of the corresponding gene. *These rats develop obesity, and type II diabetes.* -Disordered food intake and obesity in rats lacking CCK A receptors -OLETF rats were completely resistant to exogenous CCK. -Not having CCK-A receptors in OLETF rats resulted in a satiety deficit leading to increases in meal size and obesity. BUT Is the absence of CCK-A receptors & hyperphagia the sole cause of obesity in OLETF rats? --- no! because with homeostasis, body weight should be maintained! (shown in study with mice)
Could NPY release be conditioned?
Study - Neuropeptide Y prepares rats or scheduled feeding Training: -7 consecutive days of brain (ICV) injections of Saline or NPYat the same time of day ***No food given at time of injections!!! -During the training rats were fed at a particular time (at 10-2), and then at a later time received injections of Saline or NPY (at 4). -There was no difference in cumulative food intake or body weight over the 7 day training period. -getting NPY at a particular time of day!! (trained) -Test: Food intake (1-hour) measured after ICV injections of Saline or NPY Training / Testing: Saline / Saline - control Saline / NPY ** NPY / Saline ** -- shows if eating was learned/conditioned NPY / NPY - control, we know they will eat!
Study - Human Obesity, Dieting, and Anticipatory Salvation in Food
Subjects were primed with food cues -smell and sight of food- before a meal Study 1: Subjects were presented with freshly baked pizza and informed that they would be allowed to eat it shortly. -Data showed mean/average salivary response to food as the difference between exposure and baseline response: Dieters vs. Non-dieters --**Dieters over-responding! -- most salivary response (no difference between average weight and obese participants) Study 2: -Compared responses to palatable food (freshly baked chocolate chip cookies) with responses to non-palatable food (cookies made visually unappealing by adding green food coloring). -this effect is only related to *palatable* food!
Orexin & Relationship between Eating and Sleep/Wakefulness - Study: Hypothalamic Orexin Neurons Regulate Arousal According to Energy Balance in Mice
Tg/+ are ORX transgenic mice - mutant +/+ are WT weight matched Summary of Results: -Food deprivation makes WT mice more active/energetic -When not food deprived, not more active, and need orexin to do this! -Activity of isolated orexin neurons is inhabited by glucose and leptin and stimulated by ghrelin. -Transgenic mice, in which orexin neurons are ablated, failed to respond to fasting with increased wakefulness and activity
Summary of Study
The decerebrate rats did not learnan association of a taste with illness produced by LiCl. -Intact rats Trial 1 *before LiCl -Intact rats Trial 2 *after LiCl (movements of mouth and tongue) -Decerebrate rats response the same in Trial 1 & 2 (mouth open) *The communication between the Brainstem & Forebrain is necessary for Learning (conditioned taste aversion)*
Distraction/Boredom
The time spent watching television correlates significantly to body fat content, and to snacking and caloric intake. -Study on this on mostly female undergrad students
External Effects
These external effects could be driven by *innate* mechanisms and/or could depend on *previous experience and expectations*.
How does CCK reach the CNS?
Vagus Nerve (X) -Innervates the thoracic and abdominal viscera. -Visceral Sensory Information *Nucleus of Solitary Tract (NTS)* -from intestine to brain to get to NTS in the brainstem
Could food consumption be conditioned?
We already discussed that -Learning can influence what children prefer to eat. (Study - The influence of Social-affective context on the formation of children's food preferences) -Learning can influence how much they eat (Study - Conditioned Meal Initiation in Young Children) -and based on earlier works in rats!
lateral
away from the midline
Does NPY have an effect on Energy Metabolism?
Yes! It decreases energy expenditure. -old study showed that when injected lots of NPY can make you gain weight
Could Cephalic Insulin release be conditioned to initially arbitrary cues?
Yes. If cues are paired with food, after repeated pairings they can stimulate insulin release. -making meals predictable!
Same Study - Effect of Leptin on CART mRNA
in ob/ob mice: -Leptin treatment Increased CART in Arc brain region (only group that showed CART bc leptin is stimulating CART) -Leptin treatment Effective in Decreasing BW *Leptin administration to obese mice (ob/ob) stimulates CART mRNA expression. *Leptin treatment was injected by IP (systemically). Evidence for CART action in the brain?
Study - Central leptin modulates behavioral and neural responsively to CCK (by Michael Edmond)
injections: VEH: vehicle/buffer (control) LEPTIN: brain injections (i.c.v.) 1hr prior to test CCK: body injections (i.p.) immediately before test
Convergence of Inhibitory and Stimulatory peptides on Melanocortin Receptor
α-MSH binds to MC4-R and *Inhibits eating AGRP binds to MC4-R and *Stimulates eating (AGRP also binds to MC4-R) -*AgRP binds to MC4-R, which blocks α-MSH
