SPINAL DELTA(2)-OPIOID, BUT NOT DELTA(1)-OPIOID, MU-OPIOID,OR KAPPA-OPIOID RECEPTORS ARE INVOLVED IN THE TAIL-FLICK INHIBITION INDUCED BY BETA-ENDORPHIN FROM NUCLEUS RAPHE OBSCURUS IN THE PENTABARBITAL-ANESTHETIZED RAT

The antinociception induced by beta-endorphin given supraspinally has been previously demonstrated to be mediated by the release of [Met(5)] enkephalin acting on delta-opioid receptors in the spinal cord, The pr esent study was designed to determine what type of opioid receptors in the spinal cord is involved in beta-endorphin-induced antinociception in the rat. Antinociception was induced by beta-endorphin (0.6 nmol) given into nucleus raphe obscurus and was assessed by the tail-flick t est in pentobarbital-anesthesized rats. Naltriben (0.6-6.0 nmol), a se lective delta(2)-opioid receptor antagonist, given intrathecally dose- dependently attenuated beta-endorphin-induced inhibition of the tail-f lick response. On the other hand, 7-benzylidene naltrexone (2.1-64.3 n mol), CTOP (D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2, 0.09-2.8 nmol), o r nor-binaltorphimine (1.4-40.8 nmol), selective delta(1)-, mu-, and k appa-opioid receptor antagonists, respectively, did not block beta-end orphin-induced antinociception. The results of present study in rats a re consistent with previous experiments in mice indicating that spinal delta(2)-, but not delta(1)-, mu- or kappa-opioid receptors are invol ved in beta-endorphin-induced inhibition of the tail-flick response.