Electrophysiological studies on the spinal effects of dermorphin, an endogenous mu-opioid agonist

The intrathecal administration of dermorphin, an endogenous heptopeptide first discovered in amphibia, produces dose-dependent selective inhibitions of C fibre-evoked responses in rat dorsal horn nociceptive neurones (ED50 0.11 micrograms). Naloxone (10 micrograms) but not ICI 174,864 (125 micrograms) antagonised the effects of the peptide. A beta-fibre-evoked activity was relatively unaffected. Thus dermorphin can profoundly inhibit nociceptive afferent input in the spinal cord, and in this preparation is more potent (approximately 40X) than morphine.