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Relationship of the length of exposure to short photoperiod to the effect of prolactin on pituitary and testicular function in the golden hamster.

Adult male golden hamsters were given ectopic pituitary transplants at the time of transfer from a long (stimulatory) photoperiod (14 h L:10 h D) to a short photoperiod (5L:19D) or at various intervals thereafter. Grafts given at the time of transfer to 5L:19D failed to maintain plasma FSH levels significantly above those observed in sham-operated controls or to prevent regression of the testes. Grafts given after 4 weeks in 5L:19D increased plasma FSH levels and significantly diminished the decrease in testicular volume. Grafts given 14 weeks after transfer to 5L:19D caused a modest increase in plasma FSH levels and did not significantly alter the time course of testicular redevelopment. Similarly, grafts given 18 weeks after transfer to 5L:19D had no effect on testicular weight measured 2.5 weeks later. These results indicate that the ability of prolactin (secreted by the grafted pituitary) to increase testicular size and the responsiveness of the hypothalamic-pituitary system to prolactin gradually increase during short photoperiod-induced testicular regression, are maximal when the testes are fully regressed and decrease again during spontaneous redevelopment of the testes. We suggest that the effects of prolactin on the testes in golden hamsters are partly mediated by prolactin-induced stimulation of FSH release from the in-situ pituitary.