Opiate-induced enhancement of the effects of naloxone on serum luteinizing hormone levels in the male rat: Specificity for mu agonists

We have shown previously that acute morphine administration markedly enhances naloxone-induced increases in serum luteinizing hormone (LH) levels in the male rat. The purposes of the present studies were to determine whether this effect was opiate-specific and, if so, whether it was mediated by mu, kappa or sigma opiate receptors. In agreement with our previous reports, we found that naloxone-induced increases in serum LH levels were markedly enhanced (> 400%) in morphine-pretreated rats, relative to controls, 6 to 8 hr after a single injection; furthermore, similar effects were observed with all mu agonists assessed with the order of potency being etorphine > levorphanol > morphine > methadone > codeine. In contrast, we were unable to demonstrate any enhancement of the effects of naloxone on serum LH levels by ketocyclazocine, cyclazocine or SKF 10,047, prototypic ligands for kappa and sigma binding sites in brain. Finally, we observed that neither ethanol nor Nembutal induced a period of supersensitivity to the effects of naloxone on LH, even though both compounds transiently depressed serum LH levels over a time course similar to that observed for the opiates. On the basis of these results, it appears that the phenomenon of opiate-induced enhancement of the effects of naloxone on serum LH levels is opiate specific and, most importantly, is a unique feature of mu opiate agonists. The mechanisms underlying this phenomenon are unclear, but our results suggest that as yet unidentified events occurring within the hypothalamus must be responsible.