ANTAGONISTIC EFFECT OF ORPHANIN FQ ON OPIOID ANALGESIA IN RAT

AIM: To study the effect of orphanin FQ (OFQ), a newly discovered hept adecapeptide, on nociception and opioid analgesia. METHODS: The intrac erebroventricular (icv) and intrathecal (ith) injections were used to give the drugs. The tail-flick model of rats were used to test the pai n threshold. RESULTS: OFQ (icv or ith) 0.1 mu g had no effect on nocic eption but 0.5 - 10 mu g induces hyper-reaction of mt to noxious elect ric stimulus; the decapeptide (OFQ(1-10) icv), a fragment of the OFQ, did not affect the pain reaction of rats. Fentanyl (1 mu g, icv or ith ), a selective mu-receptor agonist, DSLET (5 mu g, icv or ith), a sele ctive delta-receptor agonist, or U50488H (1 mu g, ith), a kappa-recept or agonist, induced an increase in pain threshold, when OFQ (0.1 or 1 mu g) was added together with one of them (except for the ith injectio n of DSLET), the increase of pain threshold was reduced obviously. CON CLUSION: OFQ induces hyperalgesia and antagonizes opioid analgesia med iated by mu- and delta-receptors in the brain and by mu- and kappa- bu t not delta-receptors in the spinal cord of rats.