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 ph otoperiods 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 photoperio ds were associated with more rapid regression of the testes, but all g roups eventually regressed to the same extent, In contrast, the tempor al profile of testicular recrudescence, expressed as males became phot orefractory, was not significantly different between groups. A decreas e in photoperiod from 11.5 L to 6 L after 6 weeks did not delay the on set of recrudescence, The 11.5 L group was subdivided at week 32 and t ransferred to either 13 L or 16 L for the next 8 weeks to break photor efractoriness. Upon subsequent exposure to 8 L, both subgroups regress ed their testes in similar fashion over weeks 40-52, indicating that t he two long photoperiods were equally effective in breaking photorefra ctoriness, Nevertheless, FSH and prolactin were more consistently supp ressed in the 16 L group following the switch to 8 L. Experiment 2 tes ted whether differing durations of MEL, administered s.c. each night f or 9 weeks, elicit graded rates of reproductive regression in pinealec tomized males. Testicular regression was more rapid in the group recei ving MEL for 12 h than it was in the group receiving MEL for 8.5 h, th us supporting the hypothesis that the faster rates of testicular regre ssion in the shorter photoperiods of Experiment 1 were due to their co ncomitant longer durations of nightly MEL secretion. Experiment 3 test ed the hypothesis that rates of testicular regression in males receivi ng exogenous MEL would be affected by their prior photoperiodic histor y, Males were exposed to 18 L or 14 L for 7 weeks, then pinealectomize d and administered 9.5 h MEL infusions s.c. each night for 9 weeks. In contrast to predictions, photoperiodic history had only transitory ef fects on MEL-induced testicular regression, Although the differences i n MEL duration that accompany different short photoperiods have reprod uctive consequences (Experiment 1), the extent to which MEL duration e xpands during the transition from stimulatory to inhibitory photoperio ds appears to be a less significant variable (Experiment 3).