Temporally distinct effects of stress and corticosterone on diel melatonin rhythms of red-sided garter snakes (Thamnophis sirtalis)

Circadian and circannual rhythms in physiology and behavior are temporally organized via hormonal signals that reflect changing environmental cues. Interactions between endocrine signals are in turn important for integrating multiple physiological and behavioral rhythms. In the present study, we examined interactions between melatonin, the hypothalamus–pituitary–adrenal (HPA) axis, and corticosterone in a well-studied population of red-sided garter snakes (Thamnophis sirtalis parietalis). We demonstrate that 4 h of capture stress significantly increased photophasic melatonin and decreased scotophasic melatonin concentrations of male snakes. Treatment with exogenous corticosterone (15 and 60 μg) did not mimic the effects of stress on diel melatonin rhythms. To determine if capture stress decreases scotophasic melatonin by depleting the precursors necessary for melatonin synthesis, we used a paradigm in which snakes were treated with the melatonin precursor 5-hydroxytryptophan (0.6 and 1.2 mg) to elevate melatonin concentrations. Pretreatment of snakes with both capture stress and exogenous corticosterone blocked the effect of 5-hydroxytryptophan on scotophasic melatonin. Thus, although corticosterone itself does not influence melatonin rhythms of snakes, corticosterone can inhibit the synthesis of melatonin from 5-hydroxytryptophan. These experiments suggest that the initial versus later phases of an acute physiological stress response have temporally distinct effects on melatonin synthesis: activation of the sympathoadrenal system increases melatonin, while increased glucocorticoids can inhibit melatonin synthesis. Collectively, we demonstrate that a physiological coupling between melatonin, glucocorticoids, and the sympathoadrenal system is conserved in this ectothermic model and propose that such interactions may mediate stress-induced changes in physiology and behavior.