In vitro melatonin treatment enhances splenocyte proliferation in prairie voles

The seasonal effects of photoperiod on reproduction are mediated by melaton in, and it is hypothesized that increased immune function in short days is due to the increase in the duration of nightly melatonin secretion. Melaton in can act both directly and indirectly on target tissue within the immune system. The present study sought to tease apart the direct and indirect eff ects of melatonin on one aspect of immune function by examining the influen ce of in vitro melatonin on splenocyte proliferation in female prairie vole s 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 additi on of melatonin in vitro, as compared to cultures receiving no melatonin. B ody mass increased in short-day housed prairie voles, indicating that the a nimals were responsive to photoperiod. However, photoperiod did not affect splenocyte proliferation in the present study. These results support the hy pothesis 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 e xerts direct effects on splenocytes to enhance immune function.