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Melatonin Inhibits Naloxone‐Induced Luteinizing Hormone Release in Ovariectomized Estrogen‐Primed Rats
Author(s) -
Akema Tatsuo,
Chiba Atsuhiko,
Ikeda Tomoyuki,
Nagami Yuta,
Kimura Fukuko,
Toyoda Junichi
Publication year - 1997
Publication title -
journal of neuroendocrinology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.062
H-Index - 116
eISSN - 1365-2826
pISSN - 0953-8194
DOI - 10.1046/j.1365-2826.1997.00652.x
Subject(s) - endocrinology , medicine , melatonin , (+) naloxone , luteinizing hormone , ovariectomized rat , estrogen , chemistry , opioid , hormone , receptor
The present study aimed to examine the effect of melatonin on naloxone‐induced luteinizing hormone (LH) secretion in ovariectomized estrogen‐primed rats. A single intracerebroventricular (ICV) injection of naloxone (&mgr; opioid receptor blocker, 15 &mgr;g) or an intravenous (IV) injection of LH‐releasing hormone (LHRH, 50 ng/kg) elicited a transient and significant increase in the serum LH concentration within 10 min. While an ICV injection of 100 ng melatonin by itself did not change the basal LH release, it almost completely inhibited the naloxone‐induced LH release. Melatonin (10 ng) also significantly reduced the effect of naloxone. However, an ICV injection of 100 ng melatonin did not affect the LHRH‐induced LH release. In separate experiments, the effect of melatonin on naloxone‐induced pulsatile LH secretion was studied in estrogen‐treated rats. A continuous IV infusion of naloxone (20 mg/kg/h) induced LH pulses in rats treated ICV with saline. An ICV administration of 100 ng melatonin, which by itself did not affect basal LH secretion, significantly reduced the frequency, but not the amplitude, of LH pulses induced by the naloxone infusion. These results show that melatonin has a suprapituitary site of action to inhibit naloxone‐induced LH release, and suggest that melatonin has an effect in inhibiting the activity of the hypothalamic LHRH pulse generator, either directly or indirectly, in female rats.