Naltrexone enhances ovulation and prostaglandin synthesis in the rat ovary

We explored the action of beta-endorphin (beta E) and naltrexone (Nal) on the number of oocytes and on prostaglandins (PGE and PGF2 alpha) production by the ovaries from PMSG/hCG-primed immature and cycling rats. Superovulated rats were injected with beta-endorphin (0.5 microgram) 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 microgram) 30 minutes before beta-endorphin. Furthermore, beta-endorphin (10(-8) M) decreased prostaglandins production by ovaries isolated 4 hours after hCG. Intraperitoneal injection of beta-endorphin (0.5 microgram) at 17:00 hr on proestrus decreased (-23%) the number of ova within oviducts on the day after (estrus). Naltrexone injected intraperitoneally (5 micrograms) at 16:30 hr on proestrus increased the number of ova (+23%). On the other hand, beta-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 beta-endorphin at proestrus. Moreover, when the rats were injected with naltrexone, ovarian production of prostaglandins was increased. Our results further support the hypothesis that beta-endorphin affects ovulation at the level of the ovary in the rat and that endogenous opioids may be modulating this physiological process.