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

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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.