Chronic intrahypothalamic insulin infusion in the rat: behavioral specificity

This study examines whether chronic intrahypothalamic (IH) insulin infusions suppress body weight and food intake directly or via effects on water intake or activity. Insulin (15 microU/h) was infused into the ventromedial hypothalamic nucleus of rats for 1 week. If IH insulin infusions primarily suppress water intake, animals should consume less water during insulin infusion in the absence of food. In the first experiment in this study, rats food deprived during IH insulin infusion did not drink significantly less than during vehicle infusion. This implies that IH insulin affects water intake secondarily to its impact on food intake. Insulin might suppress food intake and body weight by decreasing overall activity levels, including activity involved in ingestive behavior. In the second experiment, rats' activity on a running wheel was measured during IH insulin and vehicle infusion; activity increased during insulin infusion compared to vehicle infusion. These findings suggest that insulin's effects on food intake and body weight are via a mechanism that does not appear to directly influence water intake, and does not reduce overall activity levels.