The role of melatonin in hypothalamic regulation of pituitary hormone secretion in cattle.

本研究旨在阐明褪黑素在下丘脑调节黄牛垂体前叶生长激素(GH)分泌中的作用。在本研究的第一年(2005年)，利用为牛设计的立体定位仪在第三脑室植入一个慢性插管，并通过插管在青春期前荷斯坦牛的脑室内注射褪黑素的效果进行评估。侧脑室注射褪黑激素显著刺激了生长激素的释放。2006年，为了研究内源性褪黑素与生长激素分泌的关系，进行了一系列的试验。动物饲养于光照控制室(光照6：00~18：00，黑暗18：00~6：00；12L：12D)。分别于白天(光照11：00~15：00)和夜间(23：00~3：00)采血，测定血中褪黑激素、生长激素和催乳素的浓度。并观察各时相点生长激素分泌对生长激素释放激素(GHRH)的反应。褪黑激素的分泌呈典型的昼夜节律(白天低，夜间高)。在本研究中观察到夜间生长激素的增加，这种生长激素分泌模式以前从未在该物种中被报道过。此外，注射GHRH后GH的增加在夜间有高于白天的趋势。综上所述，本研究结果表明褪黑素参与了牛通过下丘脑从垂体前叶分泌GH的调节机制。

The present project was conducted to clarify the role of melatonin in hypothalamic regulation of growth hormone (GH) secretion from the anterior pituitary in cattle.In the first year of the project (2005), a chronic cannula was implanted in the third ventricle using a stereotaxic apparatus designed for cattle, and the effect of intracerebroventricular injection (icv) of melatonin through the cannula was evaluated in prepubertal Holstein steers. The icv injection of melatonin significantly stimulated GH release.In 2006, a series of experiments was performed in order to investigate the relationship between endogenous melatonin and GH secretions in steers. Animals were kept in the light controlled room (LIGHT 6:00-18:00, DARK 18:00-6:00 ; 12L:12D). Blood samples were collected during daytime (LIGHT, 11:00-15:00) and nighttime (DARK, 23:00-3:00), and the concentrations of melatonin, GH and prolactin (PRL) were determined. In addition, the response of GH secretion to GH-releasing hormone (GHRH) injection was investigated in each period. The secretion of melatonin showed a typical circadian rhythm (low during daytime and high during nighttime). A nocturnal GH increase was observed in the present study and this pattern of GH secretion has never been reported in this species before. Furthermore, the increase in GH after GHRH injection tended to be higher during nighttime than that during daytime.In summary, the present results suggest that melatonin is involved in the regulatory mechanism of GH secretion from the anterior pituitary via the hypothalamus in cattle.