Differing Distributions of Receptors for Morphine and Met5-enkephalinamide in the Dorsal Horn of the Cat

In the dorsal horn of barbiturate-anaesthetized cats evidence was obtained for a different distribution of enkephalin-preferring and morphine-preferring receptors. When ejected electrophoretically from micropipettes, morphine reduced the nociceptive responses of neurones of laminae IV and V when administered in the substantia gelatinosa but not at more ventral sites including sites near cell bodies. By contrast, Met5-enkephalinamide reduced nociceptive responses at all of these sites of administration often with increased potency near cell bodies. It is proposed that morphine-preferring receptors are located mainly near the terminals of primary afferent fibres whereas those for enkephalin are present on these fibres and also on the dendrites and somata of dorsal horn neurones. These dendritic receptors may be located adjacent to sites of termination of nociceptive afferents, a mechanism which would inhibit nociceptive but not other inputs to these neurones.