[C fiber is not necessary in electroacupuncture analgesia, but necessary in diffuse noxious inhibitory controls...

Experiments were carried on rats. We applied capsaicin topically on sciatic nerve and used the techniques of extracellular recording and nerve trunk recording, Our results showed that the size of C compound action potentials in nerve trunk and C fiber response of spinal cord WDR neurons were decreased by at least 70% (mean) after topical application of capsaicin (250 micrograms) on the nerve, but A compound action potentials and A fiber response did not change significantly. It indicated that capsaicin blocked C fiber conduction selectively. Electroacupuncture (EA: 100 Hz, 0.1 ms, 3V) applied on left Zusanli (S36) and Sanyinjiao (Sp6) points inhibited C fiber response of spinal WDR neurons in the right side. The effect was similar to animal behavior analgesia elicited by EA. After applying capsaicin (250 micrograms) topically on left sciatic nerve, the inhibitory effect of EA on WDR neurons remained essentially intact (from 61.3 +/- 12.0% to 59.0 +/- 11.6%, n = 6, P greater than 0.05). It indicated that C fiber was not important in EA analgesia. Noxious heat (NH) applied on left hind paw by immersing the hind paw into 52 degrees C water inhibited C fiber response of spinal WDR neurons in the right side. It was called diffuse noxious inhibitory controls (DNIC). After applying capsaicin (250 micrograms) topically on left sciatic nerve, the inhibitory effect of NH on WDR neurons was dramatically decreased (from 77.7 +/- 8.5% to 8.1 +/- 8.9%, n = 6, P less than 0.001). It indicated that C fiber was important in DNIC. Both inhibitory effects of NH and EA were not changed by vehicle treatment.(ABSTRACT TRUNCATED AT 250 WORDS)