THE ANTINOCICEPTION INDUCED BY BETA-ENDORPHIN ADMINISTERED INTRATHECALLY IS MEDIATED BY THE ACTIVATION OF MU-OPIOID AND KAPPA-OPIOID RECEPTORS IN THE RAT

The antinociception induced by beta-endorphin given supraspinally has been previously demonstrated to be mediated by the stimulation of epsilon-, but not mu-, delta- or kappa-opioid receptors in rats and mice. The present study was designed to determine what types of opioid receptors in the spinal cord are involved in the antinociception induced by intrathecally (i.t.) administered beta-endorphin. Antinociception was assessed by the tail-flick test in male Sprague-Dawley rats. CTOP (0.9-6.6 nmol), a selective mu-opioid receptor antagonist, or nor-BNI(13.6-95.3 nmol), a selective rc-opioid receptor antagonist, given i.t. dose-dependently reversed i.t. administered beta-endorphin-induced inhibition of the tail-flick response. On the other hand, naltrindole (6.6-44.4 nmol), a selective delta-opioid receptor antagonist, or beta-endorphin (1-27) (1-6.7 nmol), a selective epsilon-opioid receptor antagonist given i.t., did not antagonize the inhibition of the tail-flick response induced by i.t. administered beta-endorphin. The results are consistent with the previous study in mice [Tseng LF and Collins KA (1992) fur J Pharmacol 214: 59-65] that the antinociception induced by beta-endorphin given i.t. is mediated by the stimulation of mu- and kappa-, but not delta- and epsilon-opioid receptors.