Delta-opioid receptor binding sites in rodent spinal cord

1. The delta-opioid receptor agonist [D-Pen2,D-Pen5]enkephalin showed an antinociceptive effect in the mouse tail-flick test, following intrathecal administration. This action was reversed by naloxone and by the selective delta-opioid receptor antagonist ICI 174864. 2. High affinity, saturable binding of [3H]-[D-Pen2,D-Pen5]enkephalin has been demonstrated in spinal cord homogenates from guinea-pig, hamster, rat and both adult and young (18-20 g) mice. The binding was shown by autoradiography to be concentrated in the superficial laminae of the dorsal horn. 3. Competition studies confirmed that the binding of [3H]-[D-Pen2,D-Pen5]enkephalin was to the delta-opioid site. However, anomalies were seen with displacement assays using mu-ligands, which may suggest some common high affinity site for delta- and mu-opioid receptor agonists in the spinal cord. 4. The results add further evidence for a role of the delta-opioid receptor in spinally-mediated antinociception.