The Biological Nature of Appetite

The brain is responsible for a wide range of appetites or biological drives. The limbic system, and especially the hypothalamus, has been identified as a complex neuroendocrine system that is responsible for physiological regulations and homeostasis, biological drives, appetites, rewards, and hedonic experiences. Two examples are discussed. 1) Hunger and the regulation of food intake are controlled by lateral and medial hypothalamic mechanisms that start and stop eating, but that also exert a neuroendocrine control of metabolism and fat deposition. When these mechanisms fail, there is overeating and obesity, seen in the study of brain-lesioned animals and humans and in genetic predisposition to obesity. The mechanisms involved, however, are extraordinarily complex. 2) Salt appetite represents a simpler neuroendocrine model that has promise of revealing the basic biological principles at work in all appetites. The drive of animals to seek salt licks when they are depleted of NaCl illustrates the significance of this appetite. We now know that depletion of the body of NaCl leads to a synergistic action in the brain of angiotensin and aldosterone, the hormones that operate in salt retention at the kidney. Repeated depletions enhance salt appetite. Blocking angiotensin and aldosterone in the brain eliminates it, even in the face of great need. Selective brain lesions in different parts of the limbic system block the induction of salt appetite by angiotensin and aldosterone separately, and we believe that their synergy may depend on interconnecting pathways between them. Having found critical loci for these neuroendocrine effects, it is possible that identification of the neuronal receptors involved will open the door to a cell and molecular analysis of the mechanism of salt appetite.