EFFECTS OF FOOD-DEPRIVATION AND METABOLIC FUEL UTILIZATION ON THE PHOTOPERIODIC CONTROL OF FOOD-INTAKE IN SIBERIAN HAMSTERS

Unlike rats, Siberian hamsters exhibit seasonal changes in energy balance that are controlled by the photoperiod (daylength). In Siberian hamsters, body weight and fat, and food intake peak in long, summer-like days and reach nadirs in short, winter-like days. The purpose of the present experiments was to test whether metabolic challenges that increase food intake in laboratory rats also increase feeding in Siberian hamsters. Specifically, we asked the questions: (a) Is food intake increased following a fast?; (b) Is food intake increased following treatments that block metabolic fuel utilization, such as the glucose utilization blocker 2-deoxy-D-glucose (2 DG), or that enhance storage of metabolic fuels, such as short acting insulin (regular insulin)?; (c) Does the combination of food deprivation and decreased metabolic fuel utilization increase food intake?; and (c) Does the photoperiod affect the feeding and physiological responses to metabolic challenges? Food intake was measured in response to fasting, insulin or 2 DG treatment in adult female Siberian hamsters housed in long photoperiods. Following exposure to a short photoperiod, these hamsters were tested for their response to insulin and 2 DG, but not to fasting. Food intake did not increase following fasts of 12, 24, or 48 h. Food intake was increased in long day-housed hamsters given the lowest dose of 2 DG tested (125 mg/kg) 2, 4, and 6 h after treatment, but not in short days nor in long days with larger doses of the 2 DG. Similarly, food intake was increased by treatment with regular insulin in long days, but not short days. The most effective dose of insulin to stimulate food intake was 50 U/kg body weight. Combining fasts in long days with insulin or 2 DG treatment did not increase food intake. Instead the stimulatory effects of these treatments on food intake were negated. Finally, although insulin decreased plasma glucose and gastric contents in both photoperiods, it did so to greater relative degrees in long than in short days. Collectively, these data compliment our previous report that the effectiveness of some satiety peptides (i.e., cholecystokinin and bombesin) also are photoperiod dependent. These data suggest that the physiological and behavioral systems underlying these treatments that stimulate or inhibit food intake exhibit complementary responses to the naturally occurring, photoperiod-induced seasonal cycle of energy intake in Siberian hamsters.