Effects of Morphine and Naloxone on Serum LH, FSH and Prolactin Levels and on Hypothalamic Content of LH-RF in Proestrous Rats

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Abstract
Proestrus surges of serum LH [luteinizing hormone], FSH [follicle stimulating hormone] and prolactin (PRL) were significantly reduced when morphine HCl (50 and 10 mg/kg) was administered to 4-day cycling rats just prior to the proestrous critical period. The inhibitory effect of morphine was reversed by naloxone, a morphine antagonist, at the dose which had no effect on the proestrus surges of serum LH, FSH or PRL. The hypothalamic LH-RF [luteinizing hormone-releasing hormone] content of proestrous rats at 1800 h (during the proestrus surge) was not significantly different from that at 1400 h (before the surge) and was not affected by pretreatment with morphine or naloxone. Naloxone appears to reverse the antiovulatory effect of morphine by antagonizing the inhibitory effect of morphine on preovulatory surges of gonadotropins or PRL.