n-methylnaloxone and nalmefene

The effects of melatonin on LH and PRL releases induced by treatment with naloxone, naloxone methyliodide and nalmefene were studied in adult male rats. Subcutaneous melatonin injection (1.4 mg/Kg) had no effect on LH secretion, but caused an inhibition effect (84%) on LH release induced by naloxone (2.4 mg/Kg). Melatonin too totally inhibited LH secretion induced by naloxone methyliodide (2.8 mg/Kg) and nalmefene (2 mg/Kg) when it was simultaneously administered with each opioid receptor antagonist. Melatonin alone had no significant effect on serum PRL levels, but decreased by 25.5% the inhibitory effect potency of nalmefene on PRL secretion after simultaneous injections. The inhibitory effect potency of naloxone on PRL release increased (16%) when it was administered with melatonin. Simultaneous injection of melatonin with naloxone methyliodide inhibited PRL release (78%) while naloxone methyliodide alone did not modify this secretion. The results obtained with a quaternary opioid antagonist indicate that the opioid receptor type which mediates LH and PRL responses is located respectively outside and inside the blood-brain barrier. Our findings show that opiate antagonists and their quaternary ammonium salts affect secretion of LH and PRL through different mechanisms susceptible to the influence of melatonin.

Topics: Animals; Luteinizing Hormone; Male; Melatonin; Naloxone; Naltrexone; Narcotic Antagonists; Prolactin; Quaternary Ammonium Compounds; Rats; Rats, Wistar

The effect of melatonin on the prolactin (PRL) release induced by treatment with naloxone, naloxone methyliodide, naltrexone and nalmefene were studied in adult male rats. Subcutaneous melatonin injection (1.4 mg/Kg) had no significant effect on serum PRL levels, but decreased by 29% and 26% respectively the inhibitory effect potency of naltrexone (2.5 mg/Kg) and nalmefene (2 mg/Kg) on PRL secretion after simultaneous injections. The inhibitory effect potency of naloxone on PRL release increased (16%) when it was administered with melatonin. Simultaneous injection of melatonin with naloxone methyliodide (2.8 mg/Kg) inhibited PRL release (77.5%) while naloxone methyliodide alone did not modify this secretion. The results obtained with a quaternary opioid antagonist indicate that the opioid receptor type which mediates PRL response is located inside the blood-brain barrier. Our findings show that opiate antagonists and their quaternary ammonium salts affect secretion of PRL through mechanism susceptible to the influence of melatonin.

Topics: Animals; Injections, Subcutaneous; Male; Melatonin; Naloxone; Naltrexone; Narcotic Antagonists; Prolactin; Quaternary Ammonium Compounds; Rats; Rats, Wistar