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Chapter 12

PSYCH 1XX3 Chapter 12: Module 12- Hunger and Chemical Sense

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McMaster University
Joe Kim

Module 12- Hunger and Chemical Sense Hunger and Satiety - There are many signals and complex interactions between the brain and digestive system that drive the feeling of hunger - One of the main reasons you feel hungry is low glucose levels - Glucose is important for keeping your body’s functions operating and is the main source of energy - The brain especially needs glucose as it cannot use fat for energy - Glucose levels relate directly to the feelings of hunger - Some glucose is stored in the muscles but the main supply is in the liver as glycogen - The glucose-glycogen balance is mediated by the liver and a hormone in the pancreas called insulin - The pancreas secretes insulin to promote the uptake of glucose by cells in your body for immediate use, but also to stimulate storage of excess glucose as glycogen - Over time blood glucose levels will being to dip, and once they get low enough the liver begins to breakdown its stored glycogen into glucose and release it into circulation - The liver and pancreas help to buffer swings in glucose levels - As more time passes from the meal, the glycogen reserves will decrease and a signal of hunger is sent to the brain - At some point the glucose and glycogen levels get too low and you will get hungry - Neuropeptide Y or NPY is a hunger cue - NPY activity is in the hypothalamus and are associated with food seeking behaviors - NPY works similarity in fish, reptiles, birds and other mammals - The liver also sends signals to the brain to trigger satiety - The liver monitors your glycogen stores and blood sugar levels - Low blood glucose and low glycogen levels signal hunger - High glycogen levels and high glucose levels signal satiety - The small intestine also plays a key role is satiety - The small intestine produces cholecystokinin or CCK which is a hormone responsible for feeling full - Receptors in the brain detect CCK and signal the person to stop eating - Scientists have found that if you inject individuals with CCK they report feeling satiety sooner - Researchers administered CCK to rats leading to shorter than average meal durations - The rats with CCK had more meals than those without but the overall food intake was the same - CCK regulates short-term satiety Long Term Weight Regulation - Fat has more than twice the carbohydrates like glycogen - For every one gram of fat there are 9 units of kilocalories compared to the 4 in carbohydrates - Fat is found in virtually all parts of the body - For the long term fat is the preferred choice of storage - Adipose tissue (fat) is an active component of your physiology and fairly recent classified as an endocrine organ as well - Adipose tissue secretes a hormone called leptin which is involved in long- term energy balance and correlated with fat mass - When leptin levels rise, they act on receptors in the hypothalamus to reduce appetite and food consumption decreases - Leptin production is controlled by the OB gene - Knock-out mice (those with an altered OB gene) become obese - Studies suggest that a contributing factor to obesity may involve a defective OB gene but there are no clinical findings - Considering evolution leptin was to serve as indicator of low energy stores rather than as a signal to directly reduce intake - Low leptin levels would signal increasing foraging efforts or minimize activity in order to conserve energy - NPY activity in the hypothalamus as the ON switch for appetite, leptin acts to inhibit the actions of NPY - NPY and leptin act to regulate your weight to optimal levels - Evidence in rats suggests that NPYergic neurons can specifically affect reward-driven feeding for high calorie foods - NPY was injected into rats and revealed interesting finding o An increase in sucrose intake o Harder effort put in for cues associated with sucrose o Increased consumption of saccharin (similar taste to sucrose but without the calories) o Rats will also preferentially choose a diet of carbohydrates over fat or protein - The research suggests that NPY action promotes unconditional and conditional behaviors that specifically lead to increased carbohydrate consumption - Welch et al 1994 o Rats with pre-existing preference of carbohydrates had an even greater preference after NPY injection - Endogenous Opioids o Naturally occurring chemical substances that have morphine-like analgesic actions in the body o Contributes to palatability and reward-driven feeding o Naloxone reduces intake of saccharin, sucrose and saline - Knock in mice (mice that lack an opioid receptor) show lower preference for saccharin than control mice Taste Preferences and Food Selection - Foods th
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