Effect of iontophoretically applied 5-hydroxytryptamine on the excitability of single primary afferent C- and A-fibers in the cat spinal cord.

Excitability testing of single sural afferent C- and A-fibers was employed to study possible presynaptic effects of 5-hydroxytryptamine (5-HT) applied iontophoretically at the intraspinal point of lowest threshold for their antidromic activation in anesthetized or decerebrate spinalized cats. Threshold for single fibers recorded in the sural nerve was measured prior to and during iontophoretic application of 5-HT through a micropipette positioned in close proximity to the intraspinal stimulating electrode. 5-HT produced dose-related increases in threshold for antidromic activation in 21 or 30 C-fibers. Six of 9 A delta, and 4 of 7 A beta-fibers were similarly affected.