In vitro melatonin treatment enhances splenocyte proliferation in prairie voles

The seasonal effects of photoperiod on reproduction are mediated by melatonin, and it is hypothesized that increased immune function in short days is due to the increase in the duration of nightly melatonin secretion. Melatonin can act both directly and indirectly on target tissue within the immune system. The present study sought to tease apart the direct and indirect effects of melatonin on one aspect of immune function by examining the influence of in vitro melatonin on splenocyte proliferation in female prairie voles held in long (LD 16:8) or short (LD 8:16) days. Splenocyte proliferation in response to the T-cell mitogen concanavalin A was enhanced by the addition of melatonin in vitro, as compared to cultures receiving no melatonin. Body mass increased in short-day housed prairie voles, indicating that the animals were responsive to photoperiod. However, photoperiod did not affect splenocyte proliferation in the present study. These results support the hypothesis that melatonin exerts a direct effect on splenocyte proliferation, potentially via high-affinity melatonin receptors localized on splenocytes. The findings also indicate that, irrespective of photoperiod, melatonin exerts direct effects on splenocytes to enhance immune function.