The effect of capsaicin on C-fiber reflex and heat evoked discharge in the acute spinal cat.

The effects of capsaicin were studied electrophysiologically on C-fiber reflex and heat-evoked discharge in adult spinal cats. The results can be summarized as follows: 1) 125 micrograms/Kg of capsaicin, administered intravenously, induced an increase in spontaneous discharge in the L7 ventral root; 2) 250 micrograms/Kg of capsaicin, administered intravenously, induced an increase in the C-fiber reflex response; 3) 125 micrograms/Kg of capsaicin, administered intravenously, induced an increase in the heat-evoked discharge; but 4) 25 micrograms/Kg of capsaicin, administered through the femoral artery, did not show significant alteration of the heat-evoked discharge. These findings suggest that capsaicin which produced a release of substance-P from primary afferent nerves may principally act on C-fiber and A delta-fiber. Capsaicin was much more effective on the heat-evoked discharge than on the C-fiber reflex when administered intravenously.