The ventromedial hypothalamic syndrome, satiety, and a cephalic phase hypothesis.

Set points for body weight and fat

As do other avian systems, the digestive system shows adaptations for flight (Farner, 1960). In the mouth area, the teeth and heavy jaw bones and muscles of reptiles and mammals have been replaced by a much lighter beak, jaw bones, and jaw muscles in birds. Since birds do not chew food, the esophagus is large in diameter to accommodate larger food items. The heavy muscular gizzard, or muscular stomach (for mechanical digestion), and the proventriculus, or glandular stomach (Figure 11–1), are located within the main mass of the bird’s body. Less modification is evident in the avian small intestine and rectum; however, a cloaca is present as in reptiles.

Alimentary Canal: Anatomy, Regulation of Feeding, and Motility

Leptin and the Systems Neuroscience of Meal Size Control

Normal adult rats lived on powdered diets adulterated to contain as much as 1.6% quinine sulfate, on a palatable high-fat diet, or in Skinner boxes with 45-mg Noyes pellets available on fixed-ratio (FR) schedules as high as FR 156. They maintained lower body weights over periods of months in proportion to the percentage of quinine adulteration or the fixed ratio. Rats exposed to the high-fat diet overate as much and gained weight as rapidly as rats recovering from food deprivation, and became moderately obese. Rats having become lean or obese contingent on the palatability or accessibility of their diet defended body weight by eating more in the cold, less when force-fed by gavage, and more to restore weight after food deprivation. Yet on chow they restored and defended body weights typical of rats whose diet had been confined to commercially prepared chow. These results are interpreted to be inconsistent with motivational models that rigidly distinguish drive from incentive, that treat body weight changes as evidence for failure to regulate energy balance or body weight, or that rely exclusively on deprivation of food or reduction of body weight for definitions of need for calories. Instead, caloric homeostasis in rats may incorporate ecological constraints.

Rats define different body weights depending on palatability and accessibility of their food.

Although hypothalamic hyperphagia was described and named 25 years ago,: the nature of this phenomenon is still not clear. Presumably it is brought about by some deficiency in the systems controlling food intake. What this deficiency might be was explicitly considered by Kennedy' in the paper where he suggested that the body regulates its fat content. He had confirmed the observation of Brobeck, Tepperman, and Long' that one of the characteristics of this hyperphagia is that it tends to disappear as the animal becomes obese. Kennedy decided that the obesity more or less directly antagonizes the hyperphagia, and wrote as follows: \"The only invariable index of the loss of hypothalamic function is the level of obesity which develops. .. Once the level of fatness is determined, it is actively maintained. .. The only disturbance of appetite which ever does develop, in fact, is the transient increase incidental to getting fat.\"' His view of these phenomena has been accepted into the literature as the \"lipostatic\" hypothesis of control of food intake. According to this hypothesis , the hypothalamus adjusts food intake so as to regulate the size of fat depots. After ventromedial lesions the food intake is increased because the control system is given a new set point that requires a greater amount of fat in the body. When this greater amount has been acquired, the system functions to maintain energy balance as before. In subscribing to this hypothesis , Teitelbaum' has written, \".. . then one might say that the hyper-phagic rat overeats to get fat. Once it is fat, it no longer overeats\" (p. 49). Some animals with hypothalamic lesions become more obese than other animals, presumably because the effectiveness of ventromedial lesions varies from one animal to another; in any given animal, however, the effective

C. L. Hamilton

Papers

Variability of body fat in hyperphagic rats.