LUTTERSCHMIDT, D.I.*; LEMASTER, M.P.; MASON, R.T.; Oregon State University, Corvallis; Western Oregon University, Monmouth; Oregon State University, Corvallis: Melatonin modulates the response of red-sided garter snakes to exogenous corticosterone.

The pineal gland and its major secretory product, melatonin (MEL), function in neuroendocrine transduction by converting environmental stimuli into appropriate endocrine signals. MEL has been implicated in the modulation of many behavioral and physiological functions, including reproduction. Interactions between MEL and other hormones are also important in modulating and synchronizing physiological and behavioral parameters. For example, MEL is a potent modulator of the hypothalamic-pituitary-adrenal axis, which mediates an animal�s responses to stressors. We investigated possible interactions between MEL and corticosterone (CORT, the major glucocorticoid in snakes) in modulating the reproductive behavior of male red-sided garter snakes (Thamnophis sirtalis parietalis) following spring emergence. Exogenous CORT significantly reduced courtship behavior of male snakes in a dose-dependent manner. Pretreatment with MEL prior to administering CORT treatments further suppressed courtship behavior of red-sided garter snakes. These results indicate synergistic actions between MEL and CORT in modulating reproductive behavior. Snakes receiving a serotonin antagonist followed by CORT also showed reduced courtship behavior. Our data support the hypothesis that MEL may function as a serotonin receptor antagonist. We speculate that a serotonin system is involved in the modulation of male courtship behavior by MEL and CORT. Further research is necessary to discern whether the actions of MEL and CORT are converging on the same pathway or if their effects on different pathways are having additive inhibitory effects on courtship behavior.