A leu-enkephalin depresses transmission from muscle and skin non-nociceptors to first-order feline spinal neurones.

1. The effects of an opioid (D-Ser-Leu-enkephalin-Thr; DSLET) were tested on synaptic actions of non-nociceptive afferents: group I and II muscle afferents and low-threshold skin afferents. They were tested on population EPSPs (field potentials) evoked in the dorsal horn and the intermediate zone of mid-lumbar segments, and on monosynaptically evoked responses of single interneurones at the same location. DSLET was applied locally (ionophoretically) at locations at which the field potentials were maximal and close to the selected neurones. 2. DSLET potently depressed transmission from group II muscle afferents and from low-threshold skin afferents. Transmission to neurones located in the dorsal horn or in the intermediate zone was depressed to a similar extent. The depression was readily antagonized by naloxone. Transmission from group Ia or Ib muscle afferents to neurones located in the intermediate zone was not affected, or was facilitated by DSLET. 3. The results show that DSLET has similar depressive actions on spinal neurones to monoamines, but its actions are more widespread. Like monoamines it affects transmission from nociceptors and group II muscle afferents, but in addition it gates transmission from low-threshold cutaneous afferents. Furthermore its effects do not appear to be restricted to interneurones at particular locations since it depressed responses of dorsal horn interneurones (gated by serotonin) as well as intermediate zone interneurones (gated by noradrenaline).