melatonin influences gonadotropin ii secretion in the atlantic croaker micropogonias undulatus 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 daynight cycle and seasonal reproductive cycle intraperitoneal injection of melatonin during the latelight phase of the daynight 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 2hr sampling time the stimulatory effect of melatonin was dosedependent over the range 5 to 500 ngg 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 daynight cycle with the maximum levels during the earlydark phase interestingly melatonin stimulated gth ii release during the latelight and early and middark phases the increased gonadotropic response to melatonin preceded latelight 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 lhrhainduced gth ii release during the middark phase of the daynight cycle in a dosedependent manner the physiological significance of this inhibitory influence is not understood at present no significant effect of melatonin on basal or lhrhainduced gth ii release was observed in regressed croaker melatonin injection 1 or 10 ngg 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 02 ngml 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