EXOGENOUS MELATONIN ENHANCES CELL-MEDIATED, BUT NOT HUMORAL, IMMUNE FUNCTION IN ADULT MALE DEER MICE (PEROMYSCUS-MANICULATUS)

Many nontropical rodent species display seasonal changes in reproducti ve physiology and metabolism, as well as in immune function. Field stu dies of seasonal changes in immune function typically report decreased immune function in the short days of winter compared to summer; presu mably, reduced immunity in winter reflects increased glucocorticoid se cretion in response to environmental stressors. In contrast, laborator y studies of photoperiodic changes in immunity invariably demonstrate increased immune function in short compared to long days. Although the precise mechanisms regulating short-day enhancement of immune functio n are not known, it is hypothesized that increased immunity is due to the increased duration of melatonin secretion in short compared to lon g days. However, melatonin can act both directly (i.e, via melatonin r eceptors located on lymphatic tissue) and indirectly (i.e., via altera tions in gonadal steroids) to affect immune function. The present stud y examined the effects of exogenous melatonin administration on both c ell-mediated and humoral immune function in adult male deer mice (Pero myscus maniculatus), as well as the role of gonadal steroid hormones i n mediating these effects. Mice either were castrated to remove circul ating androgens or received sham operations and were implanted with em pty capsules or capsules containing melatonin. Individual mice implant ed with melatonin underwent reproductive regression and displayed enha nced splenocyte proliferation to the T-cell mitogen concanavalin A; an tigen-specific serum immunoglobulin M production was unaffected by mel atonin treatment. Castration had no effect on either cell-mediated or humoral immune function. Taken together, these results suggest that ex ogenous melatonin enhances cell-mediated, but not humoral, immune func tion in adult male deer mice and that this effect is independent of go nadal steroid hormones. These results are consistent with a direct eff ect of melatonin on immunity.