Decreases in mineralocorticoid but not glucocorticoid receptor mRNA expression during the short Arctic breeding season in free-living Gambel's white-crowned sparrow (Zonotrichia leucophrys gambelii).

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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. Facultative changes of physiology and behaviour are mediated through changes in circulating levels of glucocorticoids (corticosterone, cortisol) and their subsequent binding to the high-affinity mineralocorticoid receptor (MR) or the low-affinity glucocorticoid receptor (GR). Free-living male wild Gambel's white-crowned sparrows (Zonotrichia leucophrys gambelii) display annual fluctuations in the stress response with marked attenuation during the transition from the pre-parental to the parental stage. 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 found to be significantly lower in the hippocampus as the male birds became parental. No changes were observed in GR mRNA expression in the paraventricular nucleus (PVN) or preoptic area (POA) at this time. No significant correlations were found between initial capture levels of corticosterone and GR or MR mRNA expression. No differences were found in basal levels of corticosterone between pre-parental and parental in birds collected for in situ hybridisation. Stress response data revealed no difference at baseline but reductions in peak levels of corticosterone as birds became parental. These data suggest that changes in MR expression may be important for the regulation of the stress response or behavioural stress sensitivity with respect to promoting parental care and investment.