Seasonal changes in hypothalamic and pituitary feedback sensitivity in sparrows

Environmental conditions can activate avian stress physiology through higher corticosterone secretion in birds coping with harsh environments. Circulating corticosterone levels are ultimately regulated by negative feedback through mineralocorticoid (MR) and glucocorticoid (GR) receptors, although the amount of hormone that reaches the receptors is also regulated by 11βHydroxysteroid dehydrogenase (11βHSD1) which reactivates and 11βHSD2 which can deactivate the hormone.. Here we compared plasma corticosterone concentrations, hypothalamic and pituitary mRNA expression of receptors and enzymes using qPCR in two sub-species of free-living White-crowned sparrow (Zonotrichia leucophrys) during breeding, molt, and non-breeding wintering stages. Z. l. gambelii, migrates from California to breed in Alaska and Z. l. nuttalli is a non-migratory resident of coastal California. Corticosterone was highest in migrants during breeding compared to molt and wintering while residents showed higher corticosterone levels during breeding and winter compared to molt . During breeding, migrant males had higher corticosterone compared to residents. No differences were found for females. Pituitary MR and GR mRNA for both sexes of migrants and male residents were lowest during breeding when plasma corticosterone was elevated. Concordantly, to reduce local availability of hormone for negative feedback, 11βHSD1 expression was lower and both hypothalamic and pituitary11βHSD2 higher during breeding compared to wintering in male migrants. Together these data suggest exquisite plasticity of pituitary and hypothalamic cellular feedback controls as necessitated by unique stress profiles between resident and migrant white-crowned sparrows at key stages of the annual cycle.