Effects of melatonin in vivo upon luteinizing hormone and prolactin releases induced by opiate receptor antagonists in adult male rats

Abstract

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.

Influence of Sinusoidal 50-Hz Magnetic Field on Cultured Human Ovarian Granulosa Cells

“…It is known that the hypothalamic-pituitary axis and the pineal gland play important regulatory roles in the function of gonads (4). ELF magnetic fields are known to influence gonadal function via these indirect mechanisms.…”

Ultrastructure and melatonin 1a receptor distribution in the ovaries of African ostrich chicks

“…Studies in chickens have shown that melatonin functions as an antioxidant to improve the glutathione peroxidase activity and enhance immunity (Pablos et al 1995). The melatonin can reduce the weight of the gonad and its accessory organs, so it can delay the sexual maturity of the immaturity animals (Shacoori et al 1996). Ayre et al (1992) pointed out that the melatonin binding site in the chicken ovary is located in the follicular cell layer.…”