The Hypothalamus and Adrenal Regulate Modulation of Corticosterone Release in Redpolls (Carduelis flammea—An Arctic-Breeding Song Bird)

Abstract

Free-living redpolls (Carduelis flammea—a species that breeds in the Alaskan Arctic), modulate corticosterone release in response to capture and restraint depending upon the breeding site. We extended these findings to adults undergoing a prebasic molt (the energetically costly replacement of feathers) and to juveniles. Results indicate not only that the stress response is dramatically reduced at one breeding site, but that the stress response during molt and in juveniles is lower still. In fact, juveniles failed to secrete any corticosterone in response to capture and handling, suggesting a stress hyporesponsive period. We also examined possible mechanisms underlying stress modulation. Corticosterone binding protein capacity does not change, and the stress response is only correlated with the overall condition of the bird (assessed by fat storage) at one site, suggesting that neither can explain the different corticosterone responses. Adrenal insensitivity also does not appear to fully explain reduced maximal output since exogenous ACTH enhanced corticosterone release. Exogenous ACTH, however, cannot stimulate corticosterone to stress-induced levels at the high-response site, implying reduced adrenal capacity. Redpoll pituitaries responded to exogenous corticotrophin-releasing factor and arginine vasotocin, suggesting a mechanism upstream from the pituitary blunts corticosterone release. Taken together, these results indicate that corticosterone release in this species is modulated depending upon the ecological and physiological state of the animal, and that the maximal corticosterone response is controlled at multiple sites in the hypothalamic–pituitary–adrenal axis.