Axonal effects of capsaicin: an electrophysiological study

The effects of axonally applied capsaicin on the discharge activity and compound action potential of the cat vagus, saphenous and phrenic nerves and the cervical sympathetic trunk were studied under in vivo and in vitro conditions. Application of capsaicin (10(-4) M) to the vagus, saphenous and phrenic nerves resulted in the appearance of intense discharge activity which reached its maximum after 3-4 min and lasted for 15-20 min. Parallel with the increase in discharge activity, elicited by orthodromic activation induced by capsaicin, the amplitudes of the A delta and C components of an antidromically evoked compound action potential were significantly reduced. After the excitatory action of capsaicin vanished, an increase in the latency and duration and a decrease in the amplitudes of the components of the compound action potential were observed which might have led to the development of a local block of impulse propagation. These changes proved to be reversible after the removal of capsaicin from the nerve. Compound action potentials recorded from the saphenous or vagus nerves pretreated with capsaicin 3-5 days before the experiments failed to show any significant changes. It is concluded that upon direct axonal application capsaicin results in the excitation of both A delta and C fibres which is followed by a nonspecific but reversible blockade of impulse propagation. The possible significance of these transient effects of axonally applied capsaicin in term of the development of the highly specific functional impairment occurring a few days after perineural capsaicin treatment remains to be elucidated.