Effects of photoperiod duration and melatonin signal characteristics on the reproductive system of male Syrian hamsters.

Abstract

Three experiments tested effects of photoperiod and the pineal hormone melatonin (MEL) on reproductive function among male Syrian-hamsters. In Experiment 1, hamsters were exposed for 32 weeks to 1 of 4 short photoperiods which varied in duration (11.5 L; 10 L; 8 L; 6 L). A fifth group was shifted from 11.5 L to 6 L after 6 weeks. Shorter photoperiods were associated with more rapid regression of the testes, but all groups eventually regressed to the same extent. In contrast, the temporal profile of testicular recrudescence, expressed as males became photorefractory, was not significantly different between groups. A decrease in photoperiod from 11.5 L to 6 L after 6 weeks did not delay the onset of recrudescence. The 11.5 L group was subdivided at week 32 and transferred to either 13 L or 16 L for the next 8 weeks to break photorefractoriness. Upon subsequent exposure to 8 L, both subgroups regressed their testes in similar fashion over weeks 40-52, indicating that the two long photoperiods were equally effective in breaking photorefractoriness. Nevertheless, FSH and prolactin were more consistently suppressed in the 16 L group following the switch to 8 L. Experiment 2 tested whether differing durations of MEL, administered s.c. each night for 9 weeks, elicit graded rates of reproductive regression in pinealectomized males. Testicular regression was more rapid in the group receiving MEL for 12 h than it was in the group receiving MEL for 8.5 h, thus supporting the hypothesis that the faster rates of testicular regression in the shorter photoperiods of Experiment 1 were due to their concomitant longer durations of nightly MEL secretion. Experiment 3 tested the hypothesis that rates of testicular regression in males receiving exogenous MEL would be affected by their prior photoperiodic history. Males were exposed to 18 L or 14 L for 7 weeks, then pinealectomized and administered 9.5 h MEL infusions s.c. each night for 9 weeks. In contrast to predictions, photoperiodic history had only transitory effects on MEL-induced testicular regression. Although the differences in MEL duration that accompany different short photoperiods have reproductive consequences (Experiment 1), the extent to which MEL duration expands during the transition from stimulatory to inhibitory photoperiods appears to be a less significant variable (Experiment 3).