Drugs and appetite.

Natural feeding behaviour in animals is genetically determined and is characteristic of the species. In the normal adult animal with readily available food supplies, the balance between energy intake from food and over-all energy output from the body is regulated with remarkable accuracy over long periods of time. This homeostatic regulation is controlled by the central nervous system, which monitors the feed-back of information on energy utilization for work, body temperature (in homeotherms) and body energy stores, and adjusts food intake to meet these energy requirements. It has been experimentally demonstrated in many animal species that the co-ordinated activity of the lateral (‘feeding centre’) and ventro-medial (‘satiety centre’) areas of the hypothalamus, together with the limbic system, reticulasformation and other areas of the brain, integrates the incoming information and, via the sensations of hunger, appetite and satiety, initiates the drive to eat and controls feeding behaviour and food intake (Mayer, 1953, 1955a, b ; Anand, 1961; Morgane & Jacobs, 1969; Baile, 1971; Bell, 1971 ; Baile & Forbes, 1974). Various theories have been proposed to explain the nature and mode of action of the signals which affect the central nervous regulation of food intake (Table I). They all have some experimental support, but none alone explains all the facts. There is no doubt, however, that the sensations of hunger, appetite and satiety are involved in the control of food intake, and agents which affect these sensations are therefore of considerable interest to nutritionists.