Seasonal obesity in Syrian hamsters: effects of age, diet, photoperiod, and melatonin.

Two experiments examined the effects of photoperiod, melatonin, and diet on body weight in female Syrian hamsters (Mesocricetus auratus). In experiment 1, daily injections of 25 micrograms melatonin increased body weight when given 3 h before lights-out but not when given at the midpoint of the light phase, in a 16-h light-8-h dark cycle (LD 16:8). Ten micrograms of melatonin, given 3 h before lights-out, were sufficient to increase body weight and fat content, to increase interscapular brown adipose tissue weight, to decrease uterine weight, and to interrupt estrous cyclicity. However, 2.5 micrograms of melatonin increased body weight and fat content without affecting brown adipose tissue weight or reproductive function. In experiment 2, melatonin treatment, exposure to a short photoperiod (LD 8:16), and feeding a high-fat diet increased body weight gain in weanling (25-day-old) female hamsters. When melatonin treatment or high-fat diet were withdrawn, hamsters reduced their food intake, and their body weight and fat content returned to control levels. After 15-17 wk in the short photoperiod, hamsters also began to undereat, and their body weight and fat content returned to control levels. These findings suggest several conclusions. 1) As with the changes in reproductive function, melatonin is effective at increasing body weight only when given at certain times of day. 2) Not all end points are equally responsive to melatonin, suggesting that they are independent of one another. 3) Weanling hamsters respond to photoperiod, melatonin, and diet just as adults do. 4) The striking obesities induced by these manipulations are completely reversible.