Comparison of the capsaicin- and amino acid-sensitivity of dorsal root C fibres in the rat and the toad

Abstract 1. The C elevation of the compound action potential (CAP) was recorded with suction electrodes from dorsal roots of rats at 25°C and toads ( Bufo bufo ) at 10°C. The C fibre CAP had a conduction velocity of 0.5 ± 0.07 SE M per sec ( N = 10) and 0.25 ± 0.04 M per sec ( N = 8 in the rat toad nerves respectively. 2. The depressant effect of applied drugs on the amplitude of the C fibres CAP was measured. Nerves from both species had similar sensitivities to GABA EC 50 5.0 μ M ± 0.05 SEM ( N = 3) ( N = 3) for the rat and toad respectively. Maximum depressant effects of GABA produced in rat and toad nerves were 35% ± 5 SEM and 17% ± 2.5 respectively. 3. In five out of ten of the rat nerves tested kainate had a clear depressant effect (maximum 36% ± 4.3 SEM, EC 50 6.8 μ M ± 0.9 SEM, N = 3) on the C fibre CAP. Kainate, at c to 500 μM, had no effect on seven toad nerves. 4. Toad nerves were about 100 times less sensitive, than rat nerves, to capsaicin ( ED 50 values ± 190 SEM and 0.7 μ M ± 0.2 respectively, N = 4). 5. The similar sensitivity of nerves in both species to GABA and differing sensitivies to kainate and capsaicin suggests that amphibian C fibres specifically lack sensitivity to capsaicin and kainate.