THE EFFECTS OF MELATONIN ON VASOPRESSIN SECRETION IN-VIVO - INTERACTIONS WITH ACETYLCHOLINE AND PROSTAGLANDINS

The pineal hormone melatonin influences the neurohypophysial hormone r elease from the isolated hypothalamus in vitro through the effect on t he cholinergic pathways as well as the biosynthesis of prostaglandins. The aim of the present study was, therefore, to investigate the effec ts of melatonin (0.5, 1, or 5 ng) administered in vivo on the vasopres sin and oxytocin release as well as to examine whether similar interac tions between melatonin and acetylcholine or prostaglandins occur in v ivo. In the initial study on the effect of melatonin male Sprague-Dawl ey rats were implanted under anaesthesia with an arterial and venous c annula. Melatonin in a dose of 0.5 ng injected intravenously had no ef fect on plasma vasopressin concentration. The higher dose of 1 ng caus ed a significant decrease in vasopressin release 10 min after injectio n, whereas 5 ng melatonin caused an increase in plasma hormone concent rations, the difference being significant 20 min after injection. No s ignificant effects of melatonin on the oxytocin release was found. In the second study in which an ICV cannula was additionally implanted, t he cholinergic muscarinic receptor antagonist atropine (10 mu g) injec ted ICV abolished the melatonin-induced effects on plasma vasopressin level. On the other hand, a cyclo-oxygenase inhibitor ibuprofen (75 mu g) injected ICV blocked the vasopressin release induced by 5 ng melat onin and reversed the inhibitory effect of 1 ng melatonin. These resul ts demonstrate that melatonin affects the neurosecretory function of t he hypothalamoneurohypophysial complex in vivo possibly via mechanisms involving cholinergic transmission and/or prostaglandin biosynthesis. (C) 1997 Elsevier Science Inc.