INTRAVENTRICULAR INSULIN AND THE LEVEL OF MAINTAINED BODY-WEIGHT IN RATS

To determine whether central insulin administration lowers the level a round which body weight is regulated, insulin (6 mU/day) or saline was infused into the third ventricles of four groups of rats. One insulin -infused and one saline-infused group were food-deprived for 3 days an d were then returned to an ad lib feeding schedule. The other two grou ps were maintained on ad lib feeding throughout. Insulin-infused food- deprived rats lost weight at a significantly greater rate than saline- infused food-deprived rats. In ad lib fed rats, insulin infusion cause d a significant reduction of food intake and weight relative to saline -infused controls. When formerly food-deprived rats were returned to a d lib feeding, they gained weight, and this was significantly more pro nounced in the saline-infused than the insulin-infused group. The body weights of the two insulin-infused groups converged on a value approx imately 9% below the average of the two saline infused groups, with on e group increasing its weight and the other decreasing its weight to a chieve that weight. These findings suggest that the third-ventricular infusion of insulin does not incapacitate the rats and that they can a lter their food intake either upward or downward to attain a new weigh t. The results are also consistent with the hypothesis that direct adm inistration of insulin into the brain determines the level of weight m aintained by the animal.