Photoperiodism and Reproduction in Golden Hamsters: Circadian Organization and the Termination of Photorefractoriness1

Abstract

The photorefractory period of the adult golden hamster is characterized by failure of the reproductive system to respond to short-day treatment with gonadal regression. The reproductive system of photorefractory hamsters remains functional irrespective of ambient photoperiod. Prolonged treatment with long days terminates photorefractoriness in hamsters, restoring the ability of the animals’ hypothalamo-hypophysio-gonadal axis to respond to short daylengths. The basis of the photorefractory hamster’s ability to discriminate long from short days in terminating the refractory period is a circadian oscillation of photosensitivity. Photorefractory hamsters were treated for 22 weeks with LD 14:10 (14 h of light daily), LD 6:18, LD 6:30, LD 6:42, or LD 6:54. They were then treated for 10 weeks with LD 6:18. Exposure for 22 weeks to LD 14:10, LD 6:30 or LD 6:54 terminated photorefractoriness; when challenged with 10 weeks of short day treatment these animals underwent testicular regression. Exposure for 22 weeks to LD 6:18 or LD 6:42 did not terminate photorefractoriness; further short day treatment of these animals had no effect on their fully mature reproductive systems. Thus, whether a photoperiod is interpreted as a long day or a short day in these hamsters depends on the