Exam 3—PSYCH 337 Lecture 12

Peripheral signals—Leptin

Adipokine (hormone produced by adipose cells) When stored fat is being used for energy, the blood levels of leptin fall faster than thee levels of fat being metabolized This drop in leptin signals for increase in food intake Used to be considered a "satiety" signal, it is now thought to be a "starvation" signal to increase food intake when leptin levels fall Leptin receptors are in the periphery as well as brain regions, especially in the arcuate nucleus of hypothalamus Leptin also affects sexual motivation Elevated leptin levels signal to hypothalamus that fat stores are increased and to inhibit eating, and signals to reproductive system that enough energy is around to support reproduction

Metabolism during the well-fed stage

After a meal, there are two phases of energy utilization and storage: the postprandial phase and the postabsorptive phase Hormonal involvement in the postabsorptive phase: insulin •Levels rise and promote the uptake of glucose into tissues •Stimulates the conversion of glucose to glycogen, a stored form of sugar •Facilitates the transport of glucose into muscle and fat cells, and transport of amino acids into muscle cells

The postprandial phase

Occurs immediately after ingestion of food when the supply of metabolic fuels in the form of glucose, fatty acids, and amino acids enters the blood stream almost immediately

The importance of sodium

The kidneys use sodium to conserve water Sodium is also important for the movement of water between the two major fluid compartments in the body: the intracellular and extracellular compartments

Ectotherms

get most of their heat from the environment

Nutrient regulation helps prepare for future needs

Aside from obtaining energy from ingested food, nutrients are needed for growth, maintenance, and repair of the body Because of its important, there are homeostatic mechanisms in place to ensure a continuous supply of fuels without organisms having to eat constantly Basal metabolism is energy used for heat production, maintenance of membrane potentials, and life-sustaining processes

Hypovolemic thirst

Baroreceptors in major blood vessels detect pressure drop from fluid loss This also triggers ADH which also acts as a vasoconstrictor to increase blood pressure and reduce water flow to bladder The baroreceptors in the heart also signal to the brain directly via the vagus nerve to stimulate thirst Hypovolemia (drop in blood pressure) also stimulates angiotensin II production via action of renin (from the kidneys) •Angiotensin II causes vasoconstriction; •It also causes release of aldosterone (from the adrenal gland, helps with sodium retention in the kidneys) and vasopressin (to construct blood vessels), and stimulates drinking behavior at the OVLT and SFO

Behavioral control of body temperature in endotherms and ectotherms

Behavioral control of the body temperature uses three strategies: •Change exposure of the body surface •Change external insulation •Change surroundings

Endotherms

Generate their own heat through internal processes

Control of food intake

Hunger is a strong motivation to seek out and ingest food Peripheral signals are endocrine signals that come from outside the nervous system, while central signals are those that originate from within the NS

Endotherms can use internal and behavioral processes to control body temperature

In the basic mammalian thermoregulatory system: •Receptors in the skin, body core, and hypothalamus detect temperature and transmit that information to the spinal cord, brainstem, and hypothalamus •If body temperature is outside of the set zone, these neural regions can initiate behavioral and physiological responses to return temperature to the set zone

Osmotic Thirst

Osmosensory neurons in the hypothalamus detect increased osmolality of extracellular fluid, which draws water out of intracellular compartment •These neurons signal to the paraventricular nucleus (PVN) and supraoptic nucleus (SON) of the hypothalamus to release vasopressin (or antidiuretic hormone, ADH); this acts to help conserve water as blood moves through the kidneys •Aldosterone is released from the adrenal glands and causes kidneys to conserve Na+ (to help retain water) If that doesn't fix the dehydration, a signal is sent from the OVLT and SFO to stimulate drinking behavior This dual control allows physiological water-saving systems to be engaged prior to behavioral responses, freeing us from having to drink frequently

Peripheral signals—PYY3-36

PYY3-36 is secreted by cells in the intestines •Its levels are low before eating but rise rapidly after a meal •It also acts in opposition to gherkin, acting as an appetite suppressor

Central signals and the role of the hypothalamus

Shorter-term energy balance—presence or absence of food in the gut—is reported by hormones from the digestive organs Two hormones important for appetite control are: •Ghrelin—synthesized and released by endocrine cells of the stomach; reaches high levels before eating and drops off after eating; works as an appetite stimulant •PYY3-36—released by intestinal cells; reaches high levels after eating and works as an appetite suppressant No single brain region has control of appetite, but the hypothalamus is important to regulation of: •Metabolic rate •Food intake •Body weight "Appetite centers" in the hypothalamus: •Ventromedial hypothalamus (VMH) is a satiety center •Lateral hypothalamus (LH) is a hunger center •The arcuate nucleus of the hypothalamus contains an appetite controller governed by hormones, like insulin The arcuate appetite system relies on two sets of neurons with opposing effects: •POMC neurons produce the peptides pro-opiomelanocortin (POMC) and cocaine- and amphetamine-related transcript (CART) ->These neurons inhibit appetite and raise metabolism when stimulated, promoting weight loss •NPY neurons produce neuropeptide Y (NPY) and agouti-related peptide (AgRP) ->These neurons stimulate appetite and lower metabolism, promoting weight gain POMC and NPY neuron projections leave the arcuate and make connections with hypothalamic structures, ultimately modulating food intake Leptin activates POMC neurons but inhibits NPY neurons, so leptin works to suppress hunger Ghrelin and PYY3-36, act mainly on NPY neurons, work in opposition to provide short term appetite control •Ghrelin stimulates the cells, leading to an increase in appetite •PYY3-36 inhibits the cells, reducing appetite

Thirst

The extracellular fluid indicates the state of the intracellular space and is monitored by the nervous system Two states signal a need for water: 1. High extracellular solute concentrations, or very salty fluids, stimulates osmotic thirst 2. Low extracellular volume from a loss of bodily fluid stimulates hypovolemic thirst

Metabolism during the fasting state

The body must shift from putting energy into storage to getting it out of storage Glycogenolysis breaks down stored glycogen from the liver to provide a rapid supply of glucose via glucagon Lipolysis acts to breakdown triglycerides stored in adipose tissue into free fatty acid and glycerol, which can be used in the periphery (also done by glucagon) Another emergency system is gluconeogenesis—production of glucose from amino acids •Takes place in liver, resulting in production of ketone bodies as a side reaction, called ketosis •Ketone bodies are used as fuel instead of glucose (including in the brain) •Too many ketone bodies in the blood can change blood pH and may be dangerous for some individuals

Satiety

The feeling of fullness •inhibit/suppress food intake

Homeostasis

The maintenance of a steady state within an organism by means of physiological or behavioral feedback control mechanisms •For humans, we maintain an optimal body temperature between 36-38° C •There are also optimal concentrations of sugars, proteins, sodium, potassium, blood pH Some homeostatic systems are completely physiological processes, some utilize only behavioral processes, and some combine both Negative feedback systems are the primary homeostatic mechanisms If a desired value, the set point, is deviated from too much, compensatory action begins The set zone refers to the range of tolerance in a system •This makes it so that the system isn't going on and off too frequently

The postabsorptive phase

The storage of the excess energy from the meal

Short- and long-term energy balance

There are homeostatic mechanisms in place to ensure long-term energy balance •These functions to keep body mass within a relatively fixed range over weeks, months, or years, Other mechanisms exist to regulate short-term energy balance, switching on or off feeding behavior

Complications with the study of energy balance

There are many different systems sending many different signals Lots of different hormones have lots of different effects Additionally, many other substances may indirectly affect food intake by doing something else •Example: orexin increases general arousal and reduces sleep, resulting in animals eating more In humans, many different things influence eating

Fluctuations in energy requirements and intake: storing fat

There is always a persistent need for energy, but fluctuations in requirements •Rate of energy use varies throughout the day and with different seasons •There are also fluctuations in energy acquisition The balance between the amount of energy stored in the body, energy expenditure, and energy intake is controlled but not as highly "regulated" as with water and salt or temp Animals need a minimum of stored energy to survive and reproduce Over the course of evolution, stored fat likely enhanced survival in environments where energy availability fluctuated, helping animals to survive especially harsh winters, drought, or famine It also allowed individuals to do other things like engage in courtship, mating, gestation, lactation, and other reproductive behaviors that conflict with foraging, hoarding, and eating Therefore, there are more mechanisms to promote eating and weight gain (hunger signals) than there are to stop eating (satiety signals) Excess energy is stored in the form of adipose tissue, or fat These fat stores can then be tapped later when fuel after a meal wanes

Maintaining temperature in endotherms

Thermoregulatory systems show redundancy—they are monitored by more than one mechanism There are two separate thermoregulatory systems in the rat •Preoptic area (POA): responsible for the physiological responses to cold, such as shivering and constriction of the blood vessels •Lateral hypothalamus: controls behavioral regulation of temperature, such as turning on heat lamps or cooling fans

Issues with energy metabolism

Type 1 diabetes (also called insulin-dependent diabetes) •B-cells of the pancreas produce very little (if any) insulin •Requires individuals to exogenously deliver insulin Type 2 diabetes •When tissues are no longer sensitive, or do not respond, to insulin •Used to be called adult-onset diabetes, but since it is being seen among younger kids it no longer applies Both result in higher-than-usual glucose in the blood, which can be damaging, while starving cells of energy

Balancing intracellular and extracellular fluid levels: Dynamics between sodium and water

Water can pass freely through semipermeable biological membranes, but many solutes (chemical substances dissolved in the water) cannot When one compartment has a greater concentration of solutes than the other, the water will redistribute itself so the solute concentration is more even This movement of water to an area of higher concentration is called osmosis The concentration of solutes in a solution is its osmolality, and the control of this osmotic concentration is called osmoregulation

Fluid balance

Water is important for virtually all metabolic processes By weight, mammals are approximately 67% water Water also serves as the solvent for sodium, chloride, potassium, sugars, amino acids, proteins, vitamins, and others Water is constantly lost through perspiration, respiration, and urination and must be continuously replaced Very little water is stored in the body When water use exceeds water intake, the body conserves water by reducing the amount excreted from kidneys When physiological water conservation can no longer compensate for water use and loss, behavioral mechanisms kick in so the animal seeks out water sources