A POSSIBLE ROLE OF THE PINEAL-GLAND IN ACUTE IMMOBILIZATION-RELATED SUPPRESSION OF NALOXONE-INDUCED LH-RELEASE IN OVARIECTOMIZED ESTROGEN-PRIMED RATS

It has been recently reported that acute immobilization stress almost completely suppresses the luteinizing hormone (LH) release induced by naloxone, a mu-opioid antagonist, in ovariectomized estrogen-primed ra ts. The present study examined the possible involvement of the pineal gland in the acute immobilization-related suppression of the naloxone- induced LH release. An intraventricular (ICV) injection of 15 mu g nal oxone produced an abrupt increase in circulating LH concentrations in non-stressed rats. The naloxone-induced LH release was completely elim inated when tested 60 min after the end of a 30 min session of acute i mmobilization. The same stress conditions did not affect LH-releasing hormone (LHRH)-induced LH release, suggesting that the stress-related suppression of the naloxone-induced LH release was a suprapituitary ev ent. In chronically-pinealectomized rats, but not in sham-pinealectomi zed rats, naloxone injected 60 min after the end of the stress session evoked a significant increase in serum LH concentrations. However, na loxone injected ICV during the acute immobilization did not elicit LH release in either pinealectomized or sham-operated rats. Under non-str essed conditions, the LH secretory response to naloxone was similar in pinealectomized and sham-operated animals. The same stress (30 min im mobilization) significantly increased pineal melatonin content as well as plasma melatonin concentrations in rats bearing intact pineal glan ds, indicating that stress actually affected the pineal function. Thes e results provide evidence for a role of the pineal in the suppression of the LH response to naloxone after stress, but not during stress.