Potentiation of presynaptic inhibition of nociceptive pathways as a mechanism for analgesia.

In decerebrated spinal cats, stimulation of cutaneous A delta and C afferents and intraarterial injections of bradykinin into a hind limb produced a reduction in the threshold for antidromic activation in the substantia gelatinosa of single C fibres. Meperidine increased the unconditioned threshold but potentiated the decrease in the threshold produced by A delta fibre stimulation and by bradykinin injection. These results support the suggestion that myelinated cutaneous afferents can depolarize C afferent terminals. The observations also indicate that the depolarization of C afferent terminals produced during pain and by stimulation of other cutaneous afferents is potentiated by meperidine.