The pineal gland-opioid system relation: melatonin-naloxone interactions in regulating GH and LH releases in man

Several observations have demonstrated that the opioid system can modulate melatonin secretion from the pineal gland and that the effects of opioids may require a pineal participation. In contrast, the role played by the pineal gland in regulating the synthesis and secretion of endorphins and enkephalins in still obscure. To establish whether the neuroendocrine activity of melatonin are mediated by endogenous opioids and to demonstrate a possible action exerted by the pineal gland on opioid peptides, GH and LH serum mean levels were evaluated by RIA in 12 healthy subjects of both sexes after melatonin injection alone (0.4 mg/kg bw im at 09:00 h) and on a separate occasion after a simultaneous administration of melatonin and naloxone (1.2 mg iv as a bolus, followed by an iv infusion of 1.6 mg/h for 3 h). On an other occasion, the study was performed during saline or naloxone infusion alone. In each test, venous blood samples were collected at −20, 0, 30, 60, 90, 120 and 180 min. A significant rise of GH was observed after melatonin injection alone. The simultaneous infusion of naloxone blocked melatonin-induced GH rise. Melatonin did not affected LH serum levels, while it was able to reduce LH increase induced by naloxone. These preliminary results suggest that some neuroendocrine effects of melatonin might be mediated by a modulation on the opioid tone.