Morphine and met-enkephalin effects on sural Adelta afferent terminal excitability

Morphine (30--80 nA) and met-enkephalin (30--80 nA) decreased the excitability of single sural Adelta afferent terminals and potentiated the enhancement of the terminal excitability produced by superficial peroneal nerve stimulation, in mid-collicular decerebrate and spinalized cats. Naloxone (20--40 nA) antagonized the actions of both substances on the unconditioned and the conditioned terminal excitabilities. These results indicate that morphine and met-enkephalin hyperpolarize Adelta sural afferent terminals and facilitate the terminal depolarization during presynaptic inhibition. This enhancement of presynaptic inhibition may be, at least partly, responsible for the analgesic action of these agents.