Diurnal changes in sensitivity to melatonin in intact and pinealectomized Djungarian hamsters: effects on thermogenesis, cold tolerance, and gonads.

Djungarian hamsters kept in long photoperiod (16:8 L:D) were injected daily at 0800, 1200, or 1600 with 25 micrograms of melatonin. During 90 days of treatment, body weight and fur coloration were checked at weekly intervals, and at the end of the treatment the reproductive status of the hamsters and their thermoregulatory properties (could limit, maximum thermoregulatory heat production, nonshivering thermogenesis, cytochrome oxidase activity in brown adipose tissue) were measured. Hamsters injected at 1600 changed from summer to winter status with regard to all functions investigated responding simultaneously; i.e., their body weights decreased, their fur became white, their gonads regressed, and their thermoregulatory properties improved. All these changes were identical to the effects of short photoperiod (8:16 L:D) exposure. Injections of melatonin at 0800 and 1200 were ineffective for reproductive functions, but the injection of melatonin at 0800 caused slight improvements of thermogenesis. The response to melatonin injected at 1600 could be suppressed by an additional injection of melatonin at 0800 (75 micrograms). Pinealectomized or ganglionectomized hamsters kept in long photoperiod did not respond to daily injections of melatonin at 1600 for the first 60 days of treatment, but during a prolonged treatment their sensitivity to melatonin was restored. Similarly, pinealectomized or ganglionectomized hamsters failed to respond to short photoperiod for about 40 days, but during prolonged exposure their sensitivity to short photoperiod was restored.