Khan I A, Thomas P
University of Texas at Austin, Marine Science Institute, Port Aransas 78373, USA.
Gen Comp Endocrinol. 1996 Nov;104(2):231-42. doi: 10.1006/gcen.1996.0166.
The role of melatonin in the control of gonadotropin II (GtH II) secretion was investigated in Atlantic croaker (Micropogonias undulatus) during different phases of the day-night cycle and seasonal reproductive cycle. Intraperitoneal injection of melatonin during the late-light phase of the day-night cycle elicited a significant elevation in plasma GtH II levels in croaker with fully developed gonads. The increase in GtH II levels was observed 30 min postinjection; the levels remained elevated for 1 hr and started to decline at the 2-hr sampling time. The stimulatory effect of melatonin was dose-dependent over the range 5 to 500 ng/g body weight. Circulating GtH II levels in fish with fully developed gonads showed significant diurnal variation, with elevated levels during the dark phase. The GtH II secretion in response to stimulation by a LHRH analog (LHRHa) also varied during the day-night cycle, with the maximum levels during the early-dark phase. Interestingly, melatonin stimulated GtH II release during the late-light and early- and mid-dark phases. The increased gonadotropic response to melatonin preceded (late-light phase) or corresponded to the period of elevated circulating GtH II levels and increased responsiveness to stimulation by LHRHa during the dark phase, suggesting that melatonin may be involved in the control of daily GtH II cycles in this species. Melatonin inhibited LHRHa-induced GtH II release during the mid-dark phase of the day-night cycle in a dose-dependent manner; the physiological significance of this inhibitory influence is not understood at present. No significant effect of melatonin on basal or LHRHa-induced GtH II release was observed in regressed croaker. Melatonin injection (1 or 10 ng/g body weight) into the third ventricle in the preoptic anterior hypothalamic area (POAH) of croaker with fully developed gonads resulted in an elevation in plasma GtH II concentrations. In addition, a low concentration (0.2 ng/ml) of melatonin stimulated in vitro GtH II release from the pituitary fragments of fish with fully developed gonads. These results suggest that melatonin can influence GtH II secretion by acting at the level of POAH and also directly at the pituitary to stimulate GtH II release.
在昼夜周期和季节性繁殖周期的不同阶段，研究了褪黑素在调控大西洋 croaker（Micropogonias undulatus）促性腺激素 II（GtH II）分泌中的作用。在昼夜周期的暗光后期腹腔注射褪黑素，可使性腺完全发育的 croaker 的血浆 GtH II 水平显著升高。注射后 30 分钟观察到 GtH II 水平升高；水平在 1 小时内保持升高，并在 2 小时采样时开始下降。褪黑素的刺激作用在 5 至 500 ng/g 体重范围内呈剂量依赖性。性腺完全发育的鱼的循环 GtH II 水平呈现显著的昼夜变化，在黑暗阶段水平升高。对促性腺激素释放激素类似物（LHRHa）刺激的 GtH II 分泌在昼夜周期中也有所不同，在黑暗早期阶段水平最高。有趣的是，褪黑素在暗光后期以及黑暗早期和中期刺激 GtH II 释放。对褪黑素促性腺反应的增加在（暗光后期）之前或与循环 GtH II 水平升高的时期相对应，并且在黑暗阶段对 LHRHa 刺激的反应性增加，这表明褪黑素可能参与了该物种每日 GtH II 周期的调控。在昼夜周期的黑暗中期，褪黑素以剂量依赖性方式抑制 LHRHa 诱导的 GtH II 释放；目前尚不清楚这种抑制作用的生理意义。在性腺退化的 croaker 中，未观察到褪黑素对基础或 LHRHa 诱导的 GtH II 释放有显著影响。向性腺完全发育的 croaker 的视前区下丘脑前部（POAH）的第三脑室注射褪黑素（1 或 10 ng/g 体重）导致血浆 GtH II 浓度升高。此外，低浓度（0.2 ng/ml）的褪黑素刺激性腺完全发育的鱼的垂体片段体外释放 GtH II。这些结果表明，褪黑素可通过作用于 POAH 水平并直接作用于垂体来刺激 GtH II 释放，从而影响 GtH II 分泌。
