IT has been suggested that enkephalin is the endogenous ligand for the opiate receptor1–3. This substance is unevenly distributed throughout the brain3,4 and occurs as two pentapeptides—met- and leu-enkephalin5. These substances have been shown to possess analgesic activity when injected intra-cerebroventricularly into rodents6,7 and to depress the firing rate of single neurones in the brainstem of cats8 and rats9. In the present experiments the effects of synthetic metenkephalin have been compared with those of morphine on Renshaw cells in the feline spinal cord using the micro-electrophoretic technique. This cholinoceptive interneurone is particularly suitable for such a comparison since it possesses stereospecific opiate receptor sites10,11. The results obtained show that both met-enkephalin and morphine directly excite Renshaw cells and that these actions are antagonised by naloxone.
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