LONG-TERM REPRODUCTIVE EFFECTS OF A SINGLE LONG-DAY IN THE SIBERIAN HAMSTER (PHODOPUS-SUNGORUS)

Testicular regression was prevented or attenuated in Siberian hamsters exposed to a single 1- to 4-h extension of the 16-h photophase at 18 days of age and subsequently maintained in a short photoperiod (8L:16D ) through Day 35. Testicular weights on Day 35 were not correlated-wit h the duration of the active phase of wheel running or with the time o f activity onset after transfer to the 8L:16D photoperiod. Wheel-runni ng activity was not stably entrained to the light-dark cycle by 35 day s of age. Progonadal effects of a single 33-h Light pulse were greates t at 18 days of age, still evident at 30 days, but undetectable in old er hamsters. In female hamsters, a single longer day at weaning was as sociated with increased fecundity several weeks later. Long photoperio ds accelerated development of antral ovarian follicles, but exposure t o males was necessary to induce ovulation before 60 days of age. The i nterval beginning shortly after weaning is one of heightened responsiv eness to changes in day length (DL); exposure to increasing DL at this time may prolong the breeding season when DL decreases after the summ er solstice. We suggest that the long-term effects of acute Light trea tments on reproduction are mediated by sustained changes in melatonin secretion induced by reprogramming of circadian oscillators.