Effects of repeated, short-term, corticosterone administration on the hypothalamo–pituitary–adrenal axis of the white-crowned sparrow ( Zonotrichia leucophrys gambelii)

Abstract

Our knowledge of glucocorticoid actions in vertebrates comes primarily from laboratory studies, which are often conducted with little consideration of how animals experience changes in glucocorticoid secretion in natural contexts. Typically, free-living animals are exposed to acute perturbations of the environment, ranging from a few minutes to a few hours duration, with varying frequency. The cumulative effects of these perturbations and their resultant glucocorticoid surges are not well known. To investigate the possible cumulative effects of repeated, acute surges in glucocorticoid secretion, we developed an ecologically relevant methodology for treating captive white-crowned sparrows ( Zonotrichia leucophrys gambelii) with corticosterone (CORT). We dissolved CORT in dimethylsulfoxide (DMSO) and administered this cocktail directly on the skin. Treatments resulted in small elevations of CORT within the physiological range. In our first experiment at the end of the breeding life stage, birds were treated three times a day (3×). Two control groups were used: one treated with DMSO 3× and one not handled nor treated. In a second study at the beginning of the breeding life stage, one group was treated once a day and a second group 3×. A DMSO-control group was used for each dosage regime. Repeated, acute administration of CORT resulted in higher baseline CORT levels and a down-regulation of the endogenous adrenocortical response to a standardized stress. Maximum CORT and plasma corticosterone binding globulin levels increased in response to the CORT treatments only at the end of the breeding season. CORT treatment did not alter adrenal size, adrenal response to ACTH, or hepatic CORT metabolism.