The paradoxical stimulatory effect of morphine on LH secretion is dose-dependent and naloxone-reversible

We previously observed that morphine stimulated luteinizing hormone (LH) secretion from ovariectomized rats when administered intravenously at a dose of 10 mg/kg body weight. The objectives of the present study were to determine: (1) if this paradoxical effect of morphine on LH secretion could be antagonized by naloxone; (2) whether beta-endorphin also stimulated LH secretion under similar conditions; (3) what influence, if any, the ovaries have on the expression of this opiate-induced LH secretion, and (4) whether this paradoxical effect of morphine extended to prolactin (PRL) secretion. An intravenous injection of morphine, 10 mg/kg body weight, to ovariectomized rats acutely increased both plasma LH and PRL concentrations. The LH and PRL responses were completely antagonized by the concurrent administration of the opiate antagonist naloxone (1 mg/kg body weight). In contrast, morphine suppressed LH concentrations and had no effect on PRL levels when injected at a dose of 1.0 mg/kg body weight. Intravenous injections of beta-endorphin, 1 mg/kg body weight, increased PRL concentrations to a level comparable to that observed following morphine, 10 mg/kg body weight, and produced a transient but insignificant inhibition of LH release. Intraventricular injections of much lower doses of beta-endorphin resulted in a dose-dependent suppression of LH release and a dose-dependent stimulation of PRL release in ovariectomized rats. Intravenous administrations of morphine (10 mg/kg), but not beta-endorphin (1 mg/kg), to normal female rats resulted in a 2-fold increase in LH concentrations similar to that observed in ovariectomized rats, whereas both treatments similarly increased PRL concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)