Meeting Abstract

P3.122  Wednesday, Jan. 6  Temporally distinct effects of stress and corticosterone on diel melatonin rhythms of red-sided garter snakes (Thamnophis sirtalis). LUTTERSCHMIDT, D.I.*; MASON, R.T.; Georgia State University, Atlanta; Oregon State University, Corvallis lutterschmidt@gsu.edu

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 in 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, but not exogenous corticosterone (15 and 60 µg), significantly increases photophasic melatonin and decreases scotophasic melatonin concentrations of male snakes. To determine if the effect of stress on scotophasic melatonin is due to a depletion of 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 abolishes the 5-hydroxytryptophan-induced increase in scotophasic melatonin. These experiments indicate that the different phases of an acute physiological stress response have temporally distinct effects on pineal melatonin synthesis: activation of the HPA axis increases melatonin, while increased glucocorticoid concentrations inhibit melatonin synthesis. Collectively, we demonstrate that a physiological coupling between melatonin, glucocorticoids, and the HPA axis is conserved in this ectothermic model and propose that such a coupling plays a functional role in integrating seasonal rhythms.