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 bala nce that are controlled by the photoperiod (daylength). In Siberian ha msters, 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 incr ease food intake in laboratory rats also increase feeding in Siberian hamsters. Specifically, we asked the questions: (a) Is food intake inc reased following a fast?; (b) Is food intake increased following treat ments that block metabolic fuel utilization, such as the glucose utili zation blocker 2-deoxy-D-glucose (2 DG), or that enhance storage of me tabolic fuels, such as short acting insulin (regular insulin)?; (c) Do es the combination of food deprivation and decreased metabolic fuel ut ilization increase food intake?; and (c) Does the photoperiod affect t he feeding and physiological responses to metabolic challenges? Food i ntake was measured in response to fasting, insulin or 2 DG treatment i n adult female Siberian hamsters housed in long photoperiods. Followin g exposure to a short photoperiod, these hamsters were tested for thei r response to insulin and 2 DG, but not to fasting. Food intake did no t increase following fasts of 12, 24, or 48 h. Food intake was increas ed in long day-housed hamsters given the lowest dose of 2 DG tested (1 25 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 in creased 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 effec ts of these treatments on food intake were negated. Finally, although insulin decreased plasma glucose and gastric contents in both photoper iods, it did so to greater relative degrees in long than in short days . Collectively, these data compliment our previous report that the eff ectiveness of some satiety peptides (i.e., cholecystokinin and bombesi n) also are photoperiod dependent. These data suggest that the physiol ogical and behavioral systems underlying these treatments that stimula te or inhibit food intake exhibit complementary responses to the natur ally occurring, photoperiod-induced seasonal cycle of energy intake in Siberian hamsters.