Effect of narcotic analgesics and naloxone on proestrous surges of LH, FSH and prolactin in rats.

Morphine (50 mg/kg), given at 12:00 h and 14:00 h to proestrous rats, completely inhibited the proestrous surges of serum luteinizing hormone (LH), follicle-stimulating hormone (FSH) and prolactin (Prl). However, when it was given at 16:00 h, the inhibitory effect of morphine (Mor) was decreased. Pethidine and nalorphine inhibited the proestrous surges, while pentazocine showed a weaker inhibitory effect. Naloxone (Nx), given simultaneously or 2 h after morphine (Mor), at 1/20 the dose of Mor, inhibited the blockade of the proestrous gonadotropin release exerted by Mor, while the onset of the surges was delayed when it was given 3 h after Mor.