Naltrexone Enhances Ovulation and Prostaglandin Synthesis in the Rat Ovary

We explored the action of β-endorphin (βE) and naltrexone (Na1) on the number of oocytes and on prostaglandins (PGE and PGF_2α) production by the ovaries from PMSG/hCG-primed immature and cycling rats. Superovulated rats were injected with β-endorphin (0.5 μg) intraperitoneally 4 hours after hCG. The number of ova ovulated was inhibited and this effect was blocked with naltrexone injected into the ovarian bursa (0.1 μg) 30 minutes before β-endorphin. Furthermore, β-endorphin (10⁻⁸ M) decreased prostaglandins production by ovaries isolated 4 hours after hCG. Intraperitoneal injection of β-endorphin (0.5 μg) at 17:00 hr on proestrus decreased (−23%) the number of ova within oviducts on the day after (estrus). Naltrexone injected intraperitoneally (5 μg) at 16:30 hr on proestrus increased the number of ova (+23%). On the other hand, β-endorphin increased the number of oocytes obtained by puncture of antral follicles (+37%) and naltrexone decreased the number of oocytes (−33%). Prostaglandins content in the ovary of adult rats at 23:00 hr, approximately 4 hr before the onset of ovulation, was diminished when the rats received β-endorphin at proestrus. Moreover, when the rats were injected with naltrexone, ovarian production of prostaglandins was increased. Our results further support the hypothesis that β-endorphin affects ovulation at the level of the ovary in the rat and that endogenous opioids may be modulating this physiological process.