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

Graduation date: 1984One hypothesis proposed to explain how the endocrine\ud system controls reproductive cycles in seasonally breeding\ud animals is that the hypothalamus undergoes seasonal\ud changes in sensitivity to negative feedback by gonadal\ud steroids. The result is seasonal changes in the secretion\ud of luteinizing hormone-releasing hormone (LHRH) and\ud gonadotropin secretion. The present experiments were\ud designed to test this hypothesis in a male amphibian, the\ud rough-skinned newt.\ud To determine whether seasonal changes in testicular\ud function are correlated with changes in LHRH secretion,\ud LHRH was measured in specific brain areas and androgen and\ud corticosterone were measured in plasma of male newts\ud collected over a 13-month period. To determine whether\ud the hypothalamus undergoes seasonal changes in sensitivity\ud to negative feedback, the effect of castration and\ud testosterone (T) replacement on brain LHRH, plasma\ud androgen and corticosterone concentrations were measured\ud in male newts at the beginning and end of the breeding\ud season.\ud The concentration of LHRH in the infundibulum (I),\ud rostral hypothalamus (RH), and preoptic area (POA)\ud fluctuated throughout the course of the reproductive\ud cycle. LHRH was not detected in any brain area prior to\ud the end of the breeding season when plasma androgen levels\ud began to fall. Because the sensitivity of the\ud pituitary-gonad axis did not change at the end of the\ud breeding season, these results indicate that LHRH\ud synthesis and release declines precipitously at the end of\ud the breeding season.\ud Castration decreased and T replacement maintained\ud LHRH concentrations in hypothalamic areas of male newts at\ud the beginning of the breeding season, but not at the end\ud of the breeding season. Thus, plasma androgens can\ud influence brain LHRH concentrations. The sensitivity of\ud the hypothalamus to negative feedback appears to be\ud greater at the beginning of the breeding season,\ud indicating that the abrupt decline in brain LHRH\ud concentration before the end of the breeding season\ud probably does not depend upon an increase in sensitivity\ud of the hypothalamus to negative feedback