The acute effects of capsaicin on rat primary afferents and spinal neurons.

Capsaicin applied to the soma of fast and slow conducting primary afferent fibers had a depolarizing action associated with an increase in membrane conductance. The depolarizing effect desensitized rapidly and was only partially reversible in slow conducting fibers. Capsaicin application to the entry zone of the appropriate dorsal rootlets led to strong excitation of dorsal horn neurons receiving multimodal input. After acute capsaicin treatment the response to noxious heat was attenuated or totally absent, while other noxious stimuli were still effective. The present data suggest that capsaicin exerts its specific degenerative action by a long-lasting depolarizing action on small-calibre fibers.