The antinociception induced by β-endorphin given supraspinally has been previously demonstrated to be mediated by the release of [Met5]enkephalin acting on δ-opioid receptors in the spinal cord. The present study was designed to determine what type of opioid receptors in the spinal cord is involved in β-endorphin-induced antinociception in the rat. Antinociception was induced by β-endorphin (0.6 nmol) given into nucleus raphe obscurus and was assessed by the tail-flick test in pentobarbital-anesthesized rats. Naltriben (0.6-6.0 nmol), a selective δ2-opioid receptor antagonist, given intrathecally dose-dependently attenuated β-endorphin-induced inhibition of the tail-flick response. On the other hand, 7-benzylidene naltrexone (2.1-64.3 nmol), CTOP (d-Phe-Cys-Tyr-d-Trp-Orn-Thr-Pen-Thr-NH2, 0.09-2.8 nmol), or nor-binaltorphimine (1.4-40.8 nmol), selective δ1-, μ-, and κ-opioid receptor antagonists, respectively, did not block β-endorphin-induced antinociception. The results of present study in rats are consistent with previous experiments in mice indicating that spinal δ2-, but not δ1-, μ- or κ-opioid receptors are involved in β-endorphin-induced inhibition of the tail-flick response.
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- Tail-flick inhibition
- δ-Opioid receptor