Melatonin mediates photoperiod control of endocrine adaptations and humoral immunity in male Siberian hamsters.

Effects of photoperiod are mediated by the pineal gland in male Siberian hamsters. The hypothesis that the pineal hormone melatonin mediates the effects of short days (SD) to blunt select humoral and endocrine functions was tested. In the first study, regressed testes were found in pineal-intact controls transferred from long days (LD) to SDs (16 hr to 8 hr light/day); the rise in antigen-induced serum immunoglobulin (Ig) M was blunted and serum cortisol concentrations elevated compared with long-day controls. These effects of short-day were blocked in pinealectomized males moved from long to SDs, but restored by melatonin treatments. In a second study, males in LD were exposed to constant light (LL) to abolish the nighttime melatonin rhythm. In hamsters in LL, melatonin induced testicular regression as in males in SDs. Large testes were present in vehicle-treated controls in LL and in males that remained in LDs. Antigen-induced increases in serum IgM in vehicle and melatonin treatment males in LL were intermediate between concentrations in long- or short-day controls and not significantly different from each other. However, serum cortisol was again elevated in hamsters in SDs or in LL when treated with melatonin compared with males in LL or LDs. These findings indicate that melatonin treatments mimicked the effects of SDs to regulate adaptive physiologic functions in hamsters lacking the nocturnal melatonin rhythm. Thus, the photoneuroendocrine mechanism regulating reproductive responses to photoperiod also mediates short-day effects on T cell-dependent B-cell antibody production and processes that regulate cortisol in circulation.