The acute stress response in vertebrates is a highly adaptive suite of physiological and behavioural mechanisms that promote survival in the face of deleterious stimuli from the environment. Physiology, morphology and behaviour are mediated through changes in circulating levels of the glucocorticoid corticosterone and its subsequent binding to the high affinity mineralocorticoid receptor (MR) or the low affinity glucocorticoid receptor (GR). Every spring, migratory songbirds leave their wintering grounds to travel to breeding grounds in the Arctic where they can encounter demanding situations such as intense storms, food shortages, predation and social disputes. Free-living wild migratory Gambel’s White-crowned sparrows (Zonotrichia leucophrys gambelii) display annual fluctuations in the stress response with a marked attenuation occurring during the transition from the pre-parental to the parental stage on their Arctic breeding grounds. In this study we investigated whether this rapid reduction in the stress response is mediated through changes in MR and GR mRNA expression in the brain using in situ hybridisation. MR mRNA expression was significantly lower in the hippocampus as the birds became parental. No changes were observed in GR mRNA expression in the paraventricular nucleus or preoptic area at this time. No significant correlations were found between initial capture levels of corticosterone and GR or MR mRNA expression. These data suggest that changes in MR expression may be important for the regulation of the stress response or reducing stress sensitivity in order to promote parental care and investment in birds that have very short breeding seasons at high latitude.
Research supported by the BBSRC and NSF Office of Polar Programs