Graduate Thesis Or Dissertation
 

The role of luteinizing hormone-releasing hormone in the neuroendocrine control of seasonal reproduction in male rough-skinned newts, Taricha granulosa

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/t435gh41v

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  • One hypothesis proposed to explain how the endocrine system controls reproductive cycles in seasonally breeding animals is that the hypothalamus undergoes seasonal changes in sensitivity to negative feedback by gonadal steroids. The result is seasonal changes in the secretion of luteinizing hormone-releasing hormone (LHRH) and gonadotropin secretion. The present experiments were designed to test this hypothesis in a male amphibian, the rough-skinned newt. To determine whether seasonal changes in testicular function are correlated with changes in LHRH secretion, LHRH was measured in specific brain areas and androgen and corticosterone were measured in plasma of male newts collected over a 13-month period. To determine whether the hypothalamus undergoes seasonal changes in sensitivity to negative feedback, the effect of castration and testosterone (T) replacement on brain LHRH, plasma androgen and corticosterone concentrations were measured in male newts at the beginning and end of the breeding season. The concentration of LHRH in the infundibulum (I), rostral hypothalamus (RH), and preoptic area (POA) fluctuated throughout the course of the reproductive cycle. LHRH was not detected in any brain area prior to the end of the breeding season when plasma androgen levels began to fall. Because the sensitivity of the pituitary-gonad axis did not change at the end of the breeding season, these results indicate that LHRH synthesis and release declines precipitously at the end of the breeding season. Castration decreased and T replacement maintained LHRH concentrations in hypothalamic areas of male newts at the beginning of the breeding season, but not at the end of the breeding season. Thus, plasma androgens can influence brain LHRH concentrations. The sensitivity of the hypothalamus to negative feedback appears to be greater at the beginning of the breeding season, indicating that the abrupt decline in brain LHRH concentration before the end of the breeding season probably does not depend upon an increase in sensitivity of the hypothalamus to negative feedback.
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