Homeostasis, mood, circadian rhythms

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Psychology 3226A/B
Scott Mac Dougall- Shackleton

Hormones & Behaviour Final Exam Notes Post Midterm 2 Homeostasis (Eating and Drinking Behaviour) Homeostasis, which is a term first coined by Walter B. Cannon in 1929, is the process by which organisms maintain a relatively constant internal environment/optimal level. Across time, the physiological parameter (i.e. water, temperature, Na chloride, etc.) will vary, but will always be brought back to a set point by the bodys internal regulators and/or behaviours that are often hormonally regulated. In order for these homeostatic regulators to work, they must be able to detect upper and lower variation from a set point, not just one or the other (real-life example is a HVAC temperature system). It is often the case that behavioural homeostatic mechanisms are only engaged if the physiological mechanisms fail; for example, if a rats adrenal is removed, it cannot naturally retain enough Na through the action of aldosterone, but if given the choice of drinking salt water, it can maintain its internal Na levels behaviourally. As was the case with reproductive hormones being co-opted to facilitate/regulate mating behaviour, so hormones involved in physiological homeostasis are co-opted to facilitate homeostatic behaviours. For any system to maintain homeostasis, it must have the following: (1) a reference value/set point for the controlled variable; (2) some sort of detection mechanism; (3) the ability to mobilize the organism to make changes that will return the variable to the normal range; (4) the ability to recognize when the change has occurred and then shut off the mobilization through negative feedback. Homeostasis in terms of thermoregulation is especially important in endothermic/homeothermic/warm-blooded animals, which cannot rely on the environment to maintain a constant internal temperature. On the contrary, cold- blooded/ectothermic animals must manipulate their environment to control body temperature. An example with iguanas showcases this example. If iguanas are placed in a terrarium with a heating source at one end and a cooling source at another such that there is a relatively smooth temperature gradient along the terrarium, they will tend to settle down in an area where they can maintain a constant internal temperature of 37 C, which seems to be the set point for most normal animals. However, the set point can be changed according to environmental conditions. For example, if an animal is infected with bacteria, an adaptive response would be to increase its body temperature in order to kill as many of the pathogens as possible and increase chances of survival; indeed, in the terrarium example outlined above,0iguanas injected with bacteria that maintain an internal temperature of about 42 C are much 0 more likely to survive than iguanas who maintain temperature of only 34-36 C. Fluid balance: Water is extremely important to all animals because it is important for many metabolic processes, and acts as a solvent for many physiologically relevant ions, vitamins, macromolecules, etc. However, water is not often stored, and is readily lost through the above processes, as well as perspiration and respiration, which means that water must be retained by the kidney and/or periodically replenished. Water and salt balance are tightly linked because within the kidney, if salt moves one way, water usually follows by osmosis (salt sucks) in order to maintain a relatively constant body composition. Thirst is unitary perceptual experience that is NOT caused by a dry mouth, but by internal cues; proof of this statement comes from the fact that if fluids are infused directly into the body (i.e. bypass the mouth, so the mouth stays dry), drinking behaviour decreases. There are two distinct types of thirst caused by two different mechanisms: (1) osmotic thirst is caused by a decrease in the intracellular fluid volume brought about by consumption of salty foods, which creates a hypertonic extracellular fluid that draws water out of the cell; (2) hypovolemic thirst is caused by a decrease in blood volume or blood pressure, and because it doesnt simply involve a shift in interstitial osmolarity, water, Na, and other ions must also be replaced to alleviate hypovolemic thirst. However, kidney function is compromised by hypovolemia, and causes a decrease in plasma osmolarity (a potent stop-drinking stimulus), so hypovolemic rats dont drink enough to attain normal plasma osmolarity levels. Hypovolemia can be induced experimentally either by inducing haemorrhage, or via subcutaneous injection a large molecule called polyethylene glycol (PEG) that reduces blood pressure and induces drinking behaviour. Endocrine regulation of fluid balance: Vasopressin (AVP or ADH) acts on the kidney tubules (specifically the distal convoluted tubule) to increase water retention, so a lack of ADH leads to diuresis, a major symptom of diabetes insipidus (DI). An animal model of DI are Brattleboro rats; these rats urinate copiously and must drink lots of water to maintain fluid balance, but exogenous ADH treatment in these rats restores level of drinking back to normal levels. There are two types of stimuli associated with a need to balance body fluids that stimulate ADH release from the posterior pituitary: (1) dehydration of cerebral osmoreceptors (often located near 3 ventricle) causes them to send signals to the paraventricular nucleus (PVN) and supraoptic nucleus (SON) of the hypothalamus, which is where ADH is made. In response to mild dehydration, the signal to release ADH is sent; however, if dehydration persists, then the signal to dis-inhibit drinking behaviour is sent; (2) reduction in blood volume/pressure is detected by baroreceptors in cardiac blood vessels, which send signals to the PVN and SON to secrete ADH, which causes vasoconstriction, thus increasing blood pressure closer to normal values. These receptors also send a direct signal to the brain via the vagus nerve to stimulate drinking. Angiotensin II is another potent vasoconstrictor that comes from the blood (i.e. not an endocrine gland) and is released in response to hypovolemia. In response to low blood volume, the brain signals the kidney to release an enzyme called renin, which acts on a circulating precursor called angiotensinogen to form, among other things, angiotensin II. The role of angiotensin in drinking behaviour is controversial, but it certainly does stimulate the release of aldosterone, a hormone with an unequivocal connection to drinking (see below). Aldosterone (produced in zona glomerulosa of adrenal cortex) acts on the ascending Loop of Henle to increase Na retention, which causes more water to be reabsorbed as well. It is chronically stimulated in response to angiontensin II and is tonically maintained by ACTH from the anterior pituitary. Na balance: There are large species variations in Na levels (i.e. freshwater vs. saltwater fish, humans from industrialized vs. developing countries), and terrestrial animals, who, unlike marine animals, arent constantly surrounded by Na, have developed adaptations to deal with fleeting Na supplies, such as excretion of uric acid instead of urea. There may also be radical environmental variation in Na levels (recall the European rabbits brought over to Australia). Rabbits living in the snowy mountains (extremely low Na) crave salt (i.e. salt licks), have low Na content in their urine, high aldosterone and a corresponding increase in the size of the zona glomerulosa; on the other hand, rabbits living in the desert (extremely high Na) do not show Na hunger, have high Na content in their urine, low aldosterone, and no increase in the size of the zona glomerulosa. Overall, these rabbits show physiological (urine Na content), morphological (size of zona glomerulosa), and behavioural (salt licking) adaptations to varying salt conditions. Some animals have odd adaptations to deal with low Na. (1) Syrian hamsters will refuse salt water after adrenalectomy (and therefore die) unless it is infused with saccharine; apparently saccharine masks the salty taste of the salt water. These results are reflective of that fact that Syrian hamsters evolved in the desert, where salt is high and water is scarce, so they have evolved behavioural mechanisms to avoid sodium. (2) Kangaroo rats live in the desert and rarely drink water. Rather, they liberate water from the seeds that they eat, and in order to conserve this water, they rarely urinate (and when they do, it is highly concentrated) because of their extremely long kidney tubules. (3) Elephant seals get all their water from the fish that they consume. Termination of drinking behaviour typically occurs before the water has been absorbed, which suggests that feedback from
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