JOURNAL OF BIOLOGICAL RHYTHMS / February 2001Larkin et al. / COLD AC E ERATES SHORT-DAY RESPONSES Low Ambient Temperature Accelerates Short-Day Responses in Siberian Hamsters by Altering Responsiveness to Melatonin

Abstract Exposure to low ambient temperatures (Ta) accelerates appearance of the winter phenotype in Siberian hamsters transferred from long to short day lengths. Because melatonin transduces the effects of day length on the neuro-endocrine axis, the authors assessed whether low Ta promotes the transition to winterlike traits by accelerating the onset of increased nocturnal melatonin secre-tion or by enhancing responsiveness to melatonin in short day lengths. Male hamsters were transferred from 16L (16 h light/day) to 8L (8 h light/day) photoperiods and held at 5 °C or 22 °C. Locomotor activity was recorded contin-uously, and body mass, testis size, and pelage color were determined biweekly for 8 weeks. The duration of nocturnal locomotion (α), a reliable indicator of the duration of nocturnal melatonin secretion, lengthened significantly earlier in hamsters exposed to a Ta of 5 °C than 22 °C. Cold exposure increased the propor-tion of hamsters that were photoresponsive: gonadal regression in short days increased from 44 % at 22 °C to 81 % at 5 °C (p < 0.05); low Ta did not, however, accelerate testicular regression in animals that were photoresponsive