REPRODUCTIVE AND IMMUNE-RESPONSES TO PHOTOPERIOD AND MELATONIN ARE LINKED IN PEROMYSCUS SUBSPECIES

The effects of photoperiod and melatonin treatment on reproductive and immune function were assessed in two subspecies of Peromyscus manicul atus from different latitudes of origin. In experiment 1, P. m. bairid ii (latitude = 42 degrees 51' N) and P. m. luteus (latitude = 30 degre es 37' N) were housed in either long (LD 16:8) or short days (LD 8:16) for 8 weeks. Short-day P. Mt. bairdii displayed reproductive regressi on and elevated splenocyte proliferation in response to the T-cell mit ogen concanavalin A, as compared to long-day mice. In contrast, P. m. luteus did not undergo reproductive regression or exhibit any increase in lymphocyte proliferation in short days. In experiment 2, individua ls of both P. m. bairdii and P. m. luteus were implanted with empty ca psules or capsules that contained melatonin. Individual P. m. bairdii implanted with melatonin underwent reproductive regression. Individual s of this subspecies also displayed elevated lymphocyte proliferation compared to control mice. Conversely, P. m. luteus implanted with mela tonin did not undergo reproductive regression and displayed no signifi cant changes in lymphocyte proliferation. These results suggest that r eproductive responsiveness to melatonin mediates short-day enhancement of immune function in deer mice. These data also imply that melatonin may not possess universal immunoenhancing properties. Rather, the eff ectiveness of melatonin to influence immune responses may be constrain ed by reproductive responsiveness to this indole-amine.