Hypothalamic-pituitary-adrenal axis regulation and organization in urban and rural song sparrows

Abstract

Urban habitats present animals with persistent disturbances and acute stressors not present in rural habitats or present at significantly lower levels. Differences in the glucocorticoid stress response could underlie colonization of these novel habitats. Despite urban habitats characterization as more stressful, previous comparisons of urban and rural birds have failed to find consistent differences in baseline and stress induced glucocorticoid levels. Another aspect of glucocorticoid regulation that could underlie an animal’s ability to inhabit novel habitats, but has yet to be well examined, is more efficient termination of the glucocorticoid stress response which would allow birds in urban habitats to recover more quickly after a disturbance. The glucocorticoid stress response is terminated by negative feedback achieved primarily through their binding of receptors in the hippocampus and hypothalamus and subsequent decreased synthesis and release from the adrenals. We investigated if male song sparrows (Melospiza melodia) in urban habitats show more efficient termination of the glucocorticoid stress response than their rural counterparts using two approaches. First, we measured glucocorticoid receptor, mineralocorticoid receptor and 11β-HSD2 (an enzyme that inactivates corticosterone) mRNA expression in negative feedback targets of the brain (the hippocampus and hypothalamus) as a proxy measure of sensitivity to negative feedback. Second, we measured plasma corticosterone levels after standardized restraint and again following a challenge with the synthetic glucocorticoid, dexamethasone, as a means of assessing how quickly birds decreased glucocorticoid synthesis and release. Though there were no differences in the hypothalamus of urban and rural song sparrows, urban birds had lower glucocorticoid receptor and 11β-HSD2 mRNA expression in the hippocampus. Further, urban and rural birds had similar reductions in corticosterone following the dexamethasone challenge, suggesting that they do not differ in how quickly they decrease glucocorticoid synthesis and release. Thus, urban and rural song sparrows display similar termination of the glucocorticoid stress response even though urban birds have decreased hippocampal glucocorticoid receptor and 11β-HSD2 abundance.