Norepinephrine reduces excitability of single cutaneous primary afferent C-fibers in the cat spinal cord.

An excitability testing procedure of single afferent C-fibers was used to study possible presynaptic actions of norepinephrine (NE) applied to locally at the intraspinal sites of minimal threshold for their antidromic activation in spinalized cats. NE, applied by iontophoresis and/or pressure microinjection, produced dose-related increases in threshold for antidromic activation in 44 of 65 C-fibers. It was concluded that in addition to previously shown postsynaptic depressant action, NE reduces excitability of intraspinal cutaneous C-fibers.