SPERRY, TS; WINGFIELD, JC; Univ of Washington; Univ of Washington: Serotonergic Regulation of Glucocorticoid Release in Male White-Crowned Sparrows

To maximize fitness, free-living animals modulate their stress response in anticipation of predictable conditions and to cope with unpredictable, environmental perturbations. Modulation of the hypothalamic-pituitary-adrenal axis (HPA) can occur at multiple levels with two principle endpoints being baseline and maximum levels of plasma glucocorticoids (CORT). Little is known about the neuroendocrine control of modulation of the HPA axis. We investigated the role of the serotonergic system in regulating CORT release in both short-day (SD) and long-day (LD) male white-crowned sparrows, Zonotrichia leucophrys. Serotonin (5-HT) regulates the release of corticotropin releasing hormone in mammals via 5-HT1A receptors. To test whether the 5-HT system is involved in regulating CORT release in birds we measured the effect of 8-OH-DPAT, a 5-HT1A agonist, on baseline and stress-induced release of CORT. To deliver 8-OH-DPAT in the absence of external stressors laboratory held birds were trained to eat mealworms from remotely opening dishes. The mealworms were injected with saline, 10 mg/kg or 40 mg/kg 8-OH-DPAT. Thirty minutes after ingesting mealworms, the birds were subjected to a standard capture stress protocol providing samples for both baseline and stress induced CORT levels. 8-OH-DPAT significantly lowered baseline CORT levels in LD but not SD birds. There was a trend for the total CORT released during the stressor to be reduced in both doses of 8-OH-DPAT in LD birds only. Thus, the 5-HT1A receptors mediate both the baseline and stress induced release of CORT in a photoperiod dependent manner. Further studies will investigate whether this photoperiod dependent modulation of 5-HT control of CORT release is related to the seasonal changes in the stress-response seen in free-living white-crowned sparrows.