Photoperiod Modulates the Inhibitory Effect of In Vitro Melatonin on Lymphocyte Proliferation in Female Siberian Hamsters

In Siberian hamsters (Phodopus sungorus), short days suppress reproductive function and lymphocyte proliferation. To determine whether melatonin influences cell-mediated immunity through a direct action on lymphocyte proliferation, in vitro responsiveness to mitogens and melatonin was assessed in systemic and splenic lymphocytes from adult female Siberian hamsters housed in either long or short days for 13 weeks. Short days provoked reproductive regression and reduced lymphoctye proliferation. Physiological concentrations of melatonin (50 pg/ml) inhibited in vitro proliferation of circulating lymphocytes, whereas higher concentrations ([.greaterequal] 500 pg/ml) were required to inhibit proliferation of splenic lymphocytes. Immunomodulatory effects of melatonin were restricted to lymphocytes from long-day hamsters—in vitro melatonin had no effect on circulating or splenic lymphocytes from females in short days. Responsiveness to melatonin in short-day lymphocytes may be restrained by the already expanded nightly pattern of melatonin secretion in short days. These data support the hypothesis that melatonin acts directly on lymphocytes from long-day hamsters to suppress blastogenesis.