Inhibition of spinal nitric oxide synthase by Nω-nitro- l-arginine blocks the release of met-enkephalin and antinociception induced by supraspinally administered β-endorphin in the rat

Abstract

The antinociception induced by β-endorphin given supraspinally has been demonstrated previously to be mediated by the release of Met-enkephalin acting on δ 2-opioid receptors in the spinal cord. The present study was designed to determine the role of nitric oxide in the spinal cord on β-endorphin-induced release of Met-enkephalin and antinociception. The experiments were performed in pentobarbital-anesthetized rats. The release of Met-enkephalin was performed using a spinal cord perfusion technique and the Met-enkephalin released in the spinal perfusates was measured by radioimmunoassay. Antinociception was assessed by the tail-flick test. β-Endorphin (2 μg) given intraventricularly induced the release of Met-enkephalin from the spinal cord. The release of Met-enkephalin was dose-dependently attenuated by N ω -nitro- l-arginine (0.1 nM–1 μM) added into spinal perfusates and the attenuation was reversed by intrathecally applied l-arginine. The stereoisomer N ω -nitro- d-arginine given intrathecally, however, did not inhibit the release of Met-enkephalin induced by intraventricularly administered β-endorphin. β-Endorphin (4 μg) given intraventricularly produced antinociception in rats pretreated intrathecally with saline. The antinociception induced by β-endorphin was blocked by intrathecally administered N ω -nitro- l-arginine (5 μg) and the blockade of antinociception was reversed by intrathecal injection of l-arginine (50 μg). N ω -Nitro- d-arginine (5 μg) given intrathecally did not block the intraventricularly administered β-endorphin-induced antinociception. N ω -Nitro- l-arginine (10 μg) given intraventricularly did not affect intraventricularly administered β-endorphin-induced Met-enkephalin release nor did it affect intraventricular β-endorphin-induced antinociception, indicating that the effect of N ω -nitro- l-arginine is not at supraspinal sites. Intrathecal pretreatment with N ω -nitro- l-arginine did not affect intrathecally administered [ d-Ala 2]deltorphin II-induced antinociception. Our results indicate that N ω -nitro- l-arginine given intrathecally attenuates intraventricular β-endorphin-administered inhibition of the tail-flick response by presynaptically inhibiting the release of Met-enkephalin.