Low ambient temperature accelerates short-day responses in Siberian hamsters by altering responsiveness to melatonin

Exposure to low ambient temperatures (T-a) accelerates appearance of the wi nter phenotype in Siberian hamsters transferred from long to short day leng ths. Because melatonin transduces the effects of day length on the neuroend ocrine axis, the authors assessed whether low T-a promotes the transition t o winterlike traits by accelerating the onset of increased nocturnal melato nin secretion or by enhancing responsiveness to melatonin in short day leng ths. Male hamsters were transferred from 16L (16 h light/day) to 8L (8 h li ght/day) photoperiods and held at 5 degreesC or 22 degreesC. Locomotor acti vity was recorded continuously, and body mass, testis size, and pelage colo r were determined biweekly for 8 weeks. The duration of nocturnal locomotio n (alpha), a reliable indicator of the duration of nocturnal melatonin secr etion, lengthened significantly earlier in hamsters exposed to a T-a of 5 d egreesC than 22 degreesC. Cold exposure increased the proportion of hamster s that were photoresponsive: gonadal regression in short days increased fro m 44% at 22 degreesC to 81% at 5 degreesC (p < 0.05); low T-a did not, howe ver, accelerate testicular regression in animals that were photoresponsive. Nonphotoresponsive animals at 5 <degrees>C temporarily had longer alphas d uring the first 4 weeks in short days and significant decreases in body mas s and testicular size that were reversed during the ensuing weeks when a de creased. In a 2nd experiment, pinealectomized male hamsters infused for 10 h/day with melatonin for 2 weeks had significantly lower body and testes ma sses when maintained at 5 degreesC but not 22 degreesC. Low-ambient tempera ture appears to accelerate the appearance of the winter phenotype primarily by increasing target tissue responsiveness to melatonin and to a lesser ex tent by augmenting the rate at which the duration of nocturnal melatonin se cretion increases in short day lengths.