EFFECT OF ORAL ADMINISTRATION OF MELATONIN ON GH RESPONSES TO GRF 1–44 IN NORMAL SUBJECTS

SUMMARY In order to investigate the role of melatonin on the neuroregulation of GH secretion, eight healthy male volunteers each underwent four separate tests in random order separated by at least 1 week. Following oral administration of melatonin (500 mg at – 60 min and at – 30 min) plasma GH levels were higher than after placebo at 45 min (mean + SEM 2.9 ± 0.8 vs 0.9 ± 0.4 ng/ml, P <0‐01) and 60 min (mean ± SEM 2.9 ± 0.4 vs 0.8 ± 0.1 ng/ml, P <005). Likewise, after prior administration of melatonin, GH responses to GRF 1‐44 (1 μg/kg i.v. at 0 min) were greater than placebo plus GRF at 15 min (mean ± SEM 22‐4 ± 6‐1 ng/ml vs 11.3 ± 2.3 ng/ml, P <005), 45 min (mean ± SEM 26.2 ± 5.3 ng/ml vs 13.3 ± 2.5 ng/ml, P < 0.01) and 60 min (mean ± SEM, 24.7 ± 7.4 ng/ml vs 11.1 ± 2.5 ng/ml, P <0.05). In contrast we did not observe any effect of either 10 −9 M, 10 −7 M or 10 −5 M melatonin on in‐vitro basal GH release and GH responses to 10 −8 M GRF by rat anterior pituitary cells in monolayer culture. These data suggest that melatonin plays a facilitatory role in the neuroregulation of GH secretion, probably by acting at the hypothalamic level.