Khan I A, Hawkins M B, Thomas P
The University of Texas at Austin, Marine Science Institute, Port Aransas, Texas 78373, USA.
Biol Reprod. 1999 Sep;61(3):834-41. doi: 10.1095/biolreprod61.3.834.
Involvement of gonadal steroids in the control of gonadotropin II (GTH II) (homologous to LH) secretion was investigated in the Atlantic croaker (Micropogonias undulatus) using gonadectomy (Gx) and steroid replacement paradigms. Gonadectomy in males and females during the late gonadal recrudescence phase elicited significant increases in the gonadotropin response to stimulation by an LHRH analog (LHRHa), without altering basal GTH II secretion. Slow-release silicone elastomer implants of testosterone or estradiol significantly inhibited LHRHa-induced GTH II secretion in gonad-intact and Gx males, and in Gx females, whereas 5alpha-dihydrotestosterone, a nonaromatizable androgen, was ineffective. Pretreatment of fish with an aromatase inhibitor, 1,4, 6-androstatrien-3,17-dione, 2 days before the administration of testosterone implants, completely blocked the negative effect of testosterone on LHRHa-induced GTH II secretion in males, but only partially restored it in females. This suggests that the negative feedback of testosterone in males is primarily mediated by its conversion to estradiol at the level of the hypothalamus and/or pituitary gland, while in females the androgen may also exert a direct inhibitory effect on GTH II secretion, probably mediated via an androgen receptor. In addition, estradiol and testosterone exerted positive effects on basal and LHRHa-induced GTH II secretion during the early-recrudescence phase of the gonadal cycle. The steroids switched to a negative effect on LHRHa-induced GTH II secretion once the fish had fully developed gonads, possibly as a mechanism that prevents a precocious surge in GTH II secretion and final gamete maturation until gametogenesis is complete and the environmental conditions are appropriate for spawning.
利用性腺切除术（Gx）和类固醇替代模式，研究了性腺类固醇在大西洋鲷（Micropogonias undulatus）中对促性腺激素II（GTH II）（与促黄体生成素同源）分泌的控制作用。在性腺晚期再发育阶段对雄性和雌性进行性腺切除，可显著增加促性腺激素对促性腺激素释放激素类似物（LHRHa）刺激的反应，而不改变基础GTH II分泌。睾酮或雌二醇的缓释硅橡胶植入物可显著抑制性腺完整和性腺切除的雄性以及性腺切除雌性中LHRHa诱导的GTH II分泌，而5α-二氢睾酮（一种不可芳香化的雄激素）则无效。在植入睾酮前2天用芳香化酶抑制剂1,4,6-雄甾三烯-3,17-二酮预处理鱼类，可完全阻断睾酮对雄性LHRHa诱导的GTH II分泌的负面影响，但在雌性中仅部分恢复。这表明，雄性中睾酮的负反馈主要通过其在下丘脑和/或垂体水平转化为雌二醇来介导，而在雌性中，雄激素可能也对GTH II分泌发挥直接抑制作用，可能通过雄激素受体介导。此外，在性腺周期的早期再发育阶段，雌二醇和睾酮对基础和LHRHa诱导的GTH II分泌具有正向作用。一旦鱼类性腺完全发育，这些类固醇就会对LHRHa诱导的GTH II分泌产生负向作用，这可能是一种防止GTH II分泌过早激增和最终配子成熟的机制，直到配子发生完成且环境条件适合产卵。
