Effects of morphine on luteinizing hormone secretion and catecholamine turnover in the hypothalamus of estrogen-treated rats

Abstract

This study examined the effects of morphine sulfate and naloxone alone or in combination on phasic luteinizing hormone (LH) secretion in estrogen-treated ovariectomized rats. Thereafter, the effects of morphine on initial concentrations, rate constants and rates of turnovers of norepinephrine and dopamine were evaluated in untreated or morphine-injected, estrogen-primed rats. Morphine, when given at 12.30 h, completely suppressed the spontaneous LH surges which occur in steroid-treated rats. The opiate antagonist, naloxone, (12.15 h) markedly amplified and advanced the time of LH release and a combination of morphine and naloxone produced peak afternoon LH values which were intermediate between those obtained in controls and in rats receiving only naloxone. Norepinephrine (NE) and dopamine (DA) turnover were calculated from data obtained in groups of rats sacrificed 0,45 or 90 min after administering 300 mg/kg b. wt. i.p. ofα-methyl-p-tyrosine (α-MPT) at 10.00 or 15.00 h. In these experiments, the medial preoptic nucleus (MPN) and the median eminence (ME) were microdissected and analyzed for changes in NE and DA concentrations by a radioenzymatic procedure. In estrogen-treated rats, NE rate constants and turnover significantly increased at 15.00 vs 10.00 h in MPN and ME concomitant with increases in serum LH. Morphine blocked both increases in rate constants and turnover in the MPN and ME and also significantly reduced initial concentrations of NE in the MPN. None of the DA parameters measured in MPN or ME changed in estrogen-treated controls between morning and afternoon. Further, while morphine did not affect DA turnover in the MPN, DA turnover declined in the ME. These data add to accumulating evidence which demonstrates an important modulatory role for hypothalamic opiate neurons in regulating catecholamine activity and gonadotropin secretion.