THE EVALUATION OF INSULIN AS A METABOLIC SIGNAL INFLUENCING BEHAVIOR VIA THE BRAIN

The intent of this paper is to evaluate decreases of food intake and b ody weight that occur when a peptide is administered to an animal. Usi ng the pancreatic hormone insulin as an example, the case is made that endogenous insulin is normally secreted in response to circulating nu trients as well as in proportion to the degree of adiposity. Hence, it s levels in the blood are a reliable indicator of adiposity. A further case is then made demonstrating that insulin is transported through t he blood-brain barrier into the brain, where it gains access to neuron s containing specific insulin receptors that are important in the cont rol of feeding and metabolism. Finally, experimentally-induced changes of insulin in the brain cause predictable changes of food intake and body weight. Given these observations, the question is then asked: sin ce endogenous insulin, acting within the brain, appears to decrease fo od intake, can a decrease of food intake caused by exogenous insulin a dministered into the same area of the brain be ascribed to the same, n aturally-occurring response system, or should it be attributed to mala ise or a non-specific depression of behavior? Arguments are presented supporting the former position that exogenous insulin, when administer ed in small quantities directly into the brain, taps into the natural caloric/metabolic system and hence influences food intake and body wei ght.