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Neurosteroids and stress physiology in adult songbirds Newman, Amy Elida Margaret

Abstract

Stress increases adrenal glucocorticoid secretion, and chronic elevation of glucocorticoids can have detrimental effects on the brain. Dehydroepiandrosterone (DHEA) is an androgen precursor synthesized in the adrenal glands, gonads or the brain and has anti-glucocorticoid properties. However, little is known about the role of DHEA in the stress response, particularly in the brain. In Chapter 2, I validated a solid phase extraction technique for extracting steroids from lipid-rich brain tissue and plasma of songbirds. In Chapter 3, I demonstrated that acute stress had statistically significant effects on plasma corticosterone and DHEA in wild adult male song sparrows that were season and vein specific. For corticosterone, acute stress increased jugular levels more than brachial levels during the molt. For DHEA, acute stress did not affect brachial DHEA but decreased jugular DHEA during the breeding season and increased jugular DHEA during the molt. These results suggest that corticosterone and DHEA are locally synthesized in the brain during molt. In Chapter 4, I measured the effects of acute stress and season on corticosterone and DHEA in brain tissue and jugular plasma. Compared to jugular plasma, corticosterone levels were up to 10Ã lower in brain, whereas DHEA levels were up to 5Ã higher in brain and were highest in the hippocampus. Acute stress increased corticosterone levels in jugular plasma and brain, except during molt, when stress decreased corticosterone levels in the hippocampus. In Chapter 5, I tested the effects of corticosterone and DHEA treatments on the brain. Corticosterone and DHEA had additive effects on the volume, neuron number and recruitment of new cells into HVC. Elsewhere in the brain, DHEA increased BrdU+ cells only in the absence of corticosterone suggesting that corticosterone can interfere with the action of DHEA. Together, these studies demonstrate that acute stress and season have distinct effects on corticosterone and DHEA in plasma and brain. Furthermore, I demonstrate that corticosterone and DHEA can have additive effects on cell survival and recruitment in the adult brain and that, in some cases, corticosterone can inhibit the actions of DHEA in the brain.

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