ACTIONS OF CAPSAICIN ON MOUSE DORSAL-ROOT GANGLION-CELLS IN-VITRO

The effects of capsaicin were investigated on different populations of dorsal root ganglion cells in the in vitro mouse spinal cord-dorsal r oot ganglion preparation using intracellular electrodes. Dorsal root g anglion cells were characterised by the conduction velocity of their p ropagated action potential evoked by electrical stimulation of the dor sal root, and by the shape of their action potential. All cells with C -fiber characteristics (conduction velocity < 0.6 m/s; broad action po tential with shoulder on the descending slope) were depolarised and ge nerated action potentials when capsaicin (100-700 nM) was added to the bathing solution for 30 s. At these concentrations the membrane poten tial of DRG cells with myelinated fibers (conduction velocity > 2.0 m/ s) was unaffected. Concentrations of capsaicin of 1.0-5.0 muM depolari sed 50% of cells with conduction velocity > 10 m/s. During the depolar ization of the membrane no action potentials were generated. In 50% of the capsaicin-sensitive neurons with conduction velocity faster than 10 m/s there was an initial hyperpolarization. Electrical stimulation of the dorsal root failed to evoke action potentials during the depola rization in 38% of the DRG cells with myelinated fibers and in all C-f ibers tested within 10 min of the onset of the capsaicin effect. Passi ve depolarization of the membrane by intrasomal current injection mimi cked the conduction block in neurons with large myelinated fibers. The se observations confirm that capsaicin applied directly to the dorsal root ganglion affects, in a dose-dependent manner, both myelinated and unmyelinated primary afferents with a higher potency for C-neurons. C apsaicin evoked action potentials in C-neurons but not in neurons with myelinated fibers. Action potential block by capsaicin was a common f eature in all types of fibers, indicating that capsaicin may impair th e propagation of sensory signals generated from the periphery.