Migratory state is not associated with differences in neural glucocorticoid or mineralocorticoid receptor expression in pine siskins

Abstract Although the endocrine system likely plays an important role in orchestrating the transition to a migratory state, the specific mechanisms by which this occurs remain poorly understood. Changes in glucocorticoid signaling are one proposed mechanism that may be important in migratory transitions. Although previous work has focused on the role of changes in circulating glucocorticoids, another potential mechanism is changes in the expression of its cognate receptors. Here, we test this hypothesis by comparing mRNA expression of the genes for the mineralocorticoid receptor (MR) and glucocorticoid receptor (GR) in two brain regions implicated in the regulation of migratory behavior (the hippocampus and hypothalamus) in pine siskins (Spinus pinus) sampled before or after the transition to a spring nomadic migratory state. Compared to pre-migratory birds, migratory birds had body conditions more indicative of physiological preparations for migration (e.g., larger body mass), and greater levels of nocturnal migratory restlessness. However, we found no differences between pre-migratory and migratory birds in the expression of GR or MR mRNA in either the hippocampus or hypothalamus. Thus, differences in expression of receptors for glucocorticoids do not appear to underly the observed differences in physiology and behavior across a migratory transition. Taken together with previous results showing no change in circulating corticosterone levels during this transition, our findings provide no evidence for a role of glucocorticoid signaling in the spring migratory transition of this species.