Hockenbury Discovering Psychology 5th txtbk

326 CHAPTER 8 Motivation and EmotionIs Your Mouth Watering? Does the sight ofthese freshly baked brownies make youfeel hungry? Some foods—like brownies—have a strong positive incentive value.Even if you’ve just eaten a large meal, thereinforcing value of a brownie mighttempt you to keep eating. Although youmight not have been particularly hungrybefore you read this page, looking at thisphotograph might send you to the kitchenin search of a sweet snack—which isexactly what happened to your authors!foods you are eating. So, after you havewolfed down four slices of pizza, thepizza’s appeal begins to diminish. Thisphenomenon is termed sensoryspecificsatiety (Maier & others,2007). Of course, if a different appealingfood becomes available, your willingnessto eat might return. Restaurantsare well aware of this, which iswhy servers will bring a tempting platterof scrumptious desserts to yourtable after you’ve finished a large andotherwise satisfying dinner.Ob/ob Mice, Before and After LeptinLeptin is a hormone produced by body fat.Because of a genetic mutation, these mice,dubbed ob/ob mice, lack the ability to produceleptin. Consequently, ob/ob mice behaveas though their brains were tellingthem that their body fat reserves are completelydepleted and that they are starving.Ob/ob mice have voracious appetitesand five times as much body fat as normalweightmice. Yet they display the characteristicsof starving animals, including decreasedimmune system functioning, lowbody temperatures, and lack of energy.When the ob/ob mouse on the right wasgiven supplemental leptin, it lost the excessfat and began eating normally. Its bodytemperature, immune system, and metabolismalso became normal (Friedman & Halaas,1998). Unfortunately, what worked forobese mice has not worked as easily forobese people, although researchers remainhopeful (Morrison, 2008).Long-Term Signals That Regulate Body WeightIn the past decade, researchers have discovered more than 20 different chemicalmessengers that monitor and help us maintain a stable body weight over time (seeSchwartz & others, 2000; Woods & others, 2000). Three of the best-documentedinternal signals are leptin, insulin, and neuropeptide Y.Leptin is a hormone secreted by the body’s adipose tissue into the bloodstream.The amount of leptin that is secreted is directly correlated with the amount of bodyfat. The brain receptor sites for leptin are located in several areas of the hypothalamus.Neurons in the stomach and the gut also have leptin receptor sites.Leptin is a key element in the feedback loop that regulates energy homeo stasis.Under conditions of positive energy balance, the body’s fat stores increase, and sodo blood levels of leptin. When the leptin level in the brain increases, food intake isreduced and the body’s fat stores shrink over time (Ahima & Osei, 2004). Increasedleptin levels also intensify the satiety-producing effects of CCK, further decreasingthe amount of food consumed (Matson & others, 2000). Should negative energybalance occur, fat stores shrink and there is a corresponding decrease in leptin bloodlevels, which triggers eating behavior.The hormone insulin is also involved in brain mechanisms controlling foodintake and body weight. Like leptin, the amount of insulin secreted by the pancreasis directly proportional to the amount of body fat. In the brain, insulin receptorsare located in the same hypothalamus areas as leptin receptors (Brüning & others,2000). Increased brain levels of insulin are also associated with a reduction in foodintake and body weight. So in much the same way as leptin, insulin levels vary inresponse to positive or negative energy balance, triggering an increase or decreasein eating.