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

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.