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Steroids and sexual differentiation of the songbird brain Chin, Eunice Hannah


Sexual differentiation of the songbird brain has traditionally thought to be driven by gonadal testosterone (T) and estradiol (E2). Recent studies have indicated that other sources of sex steroids may be responsible. Moreover, these alternative sources may release precursors, such as dehydroepiandrosterone (DHEA), which are subsequently converted downstream to sex steroids, such as E2, only at target tissues. This thesis examines plasma and tissue hormone profiles during development and possible alternative sources of sex steroids, including the adrenals, brain and liver, during the nestling phase in the European starling (Sturnus vulgaris). In the first experiment, wild starling chicks were blood sampled from either the brachial wing vein or the jugular vein from hatch (PO) to fledging (P20). In the second experiment, brain and peripheral tissues were collected from wild starling chicks at PO, P6 and P8. Hormones were extracted from samples using solid phase extraction, and then measured using radioimmunassays. Plasma DHEA was higher in males than females at P4 in the brachial vein, whereas plasma DHEA was higher in females than males at PO and PIO in the jugular vein. No sex difference was found in plasma E2. Brain DHEA was low, but detectable, in all regions at all ages, and a sex difference was only detected at PO. Brain E2 was nondetectable in all regions at all ages. DHEA was detectable in all peripheral tissues, with the gonadal tissue containing the highest levels of DHEA. Taken together, these data suggest that the gonads may be the primary source of circulating DHEA, and that the adrenals may be a secondary source during the nestling phase.

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