The effects of methionine- and leucine-enkephalin on spinal neurones of the cat.

Extra- and intracellular recordings were obtained from physiologically identified, spinal neurones in the 6th and 7th lumbar segments of the pentobarbitone-anaesthetized cat. Microiontophoretically applied methionine- and leucine-enkephalin reversibly inhibited the spontaneous, synaptically induced, and L-glutamate-induced activity in the majority of dorsal horn neurones studied in laminae 4, 5 and 6 of Rexed. Most of these depressant effects were antagonized by the prior microiontophoretic application of the opiate antagonist naloxone. Intracellular studies performed on dorsal horn neurones and motoneurones revealed that microiontophoretically applied methionine- and leucine-enkephalin caused no change in the resting membrane potential or the membrane resistance. Neither spike initiation nor spike overshoot were detectably altered by either enkephalin. The membrane depolarization and associated decrease in membrane resistance following microiontophoretic L-glutamate application were effectively blocked by the prior application of enkephalin. Naloxone, which by itself had no detectable effect on the membrane resistance, antagonized this effect. We propose that [enkephalinergic] cells in lamina II and III may modulate cells subserving somatosensory perception, including pain.