Photorefractoriness of Immune Function in Male Siberian Hamsters (Phodopus sungorus)

Short days induce multiple changes in reproductive and immune function in Siberian hamsters. Short‐day reproductive inhibition in this species is regulated by an endogenous timing mechanism; after approximately 20 weeks in short days, neuroendocrine refractoriness to short‐day patterns of melatonin develops, triggering spontaneous recrudescence of the reproductive system. It is unknown whether analogous mechanisms control immune function, or if photoperiodic changes in immune function are masked by prevailing photoperiod. In Experiment 1, 3 weeks of exposure to long days was not sufficient to induce long‐day‐like enhancement of in vitro lymphocyte proliferation in short‐day adapted male Siberian hamsters. Experiment 2 tested the hypothesis that immunological photorefractoriness is induced by prolonged exposure to short days. Adult male hamsters were gonadectomized or sham‐gonadectomized and housed in long (14 h light/day) or short (10 h light/day) photoperiods for 12, 32 or 40 weeks. Somatic and reproductive regression occurred after 12 weeks in short days, and spontaneous recrudescence was complete after 32–40 weeks in short days, indicative of somatic and reproductive photorefractoriness. In gonad‐intact hamsters, 12 weeks of exposure to short days decreased the number of circulating granulocytes and increased the number of B‐like lymphocytes. After 32 weeks in short days, these measures were restored to long‐day values, indicative of photorefractoriness; castration eliminated these effects of photoperiod. In both intact and castrated hamsters, in vitro proliferation of splenic lymphocytes was inhibited by 12 weeks of exposure to short days. After 40 weeks in short days lymphocyte proliferation was restored to long‐day values in intact hamsters, but remained suppressed in castrated hamsters. These results suggest that short‐day‐induced inhibition of lymphocyte function does not depend on gonadal regression, but that spontaneous recrudescence of this measure is dependent on gonadal recrudescence. In Experiment 3, in vitro treatment with melatonin enhanced basal proliferation of lymphocytes from male hamsters exposed to short days for 12 weeks, but had no effect on lymphocytes of photorefractory hamsters or long‐day control hamsters. Lymphocytes of castrated hamsters were unresponsive to in vitro melatonin, suggesting that photoperiodic changes in gonadal hormone secretion may be required to activate mechanisms which permit differential responsiveness to m