Effect of Narcotic Analgesics and Naloxone on Proestrous Surges of LH, FSH and Prolactin in Rats

Abstract
Morphine [Mor] (50 mg/kg), given at 1200 h and 1400 h to proestrous rats, completely inhibited the proestrous surges of serum luteinzing hormone (LH), follicle-stimulating hormone (FSH) and prolactin (Prl). When it was given at 1600 h, the inhibitory effect of Mor was decreased. Pethidine and nalorphine inhibited the proestrous surges; pentazocine showed a weaker inhibitory effect. Naloxone (Nx), given simultaneously or 2 h after Mor, at 1/20 the dose of Mor, inhibited the blockade of the proestrous gonadotropin release exerted by Mor. The onset of the surges was delayed when it was given 3 h after Mor.