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  • 1
    The δ-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 δ-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 δ-opioid site. However, anomalies were seen with displacement assays using μ-ligands, which may suggest some common high affinity site for δ- and μ-opioid receptor agonists in the spinal cord.
  • 4
    The results add further evidence for a role of the δ-opioid receptor in spinally-mediated antinociception.