Conduction-block induced by capsaicin in crayfish giant axon.

The effects of capsaicin on the crayfish giant axon were examined by using microelectrode and double sucrose gap-voltage clamp methods. Capsaicin (1-3 X 10(-4) M), when applied externally, had no effect on the resting membrane potential but gradually suppressed the action potential. The rate of rise of the action potential was simultaneously decreased. Voltage clamp experiments revealed the following: capsaicin (10(-4) M) reduced more effectively the transient sodium current than the steady-state potassium current. Inhibition of the sodium current was derived from a decrease in maximum sodium conductance, as the equilibrium potential remained much the same. The inhibitory effects of capsaicin on action potentials and membrane ionic currents were slowly reversible after removal of capsaicin. These results indicate that capsaicin seems to produce a conduction block in the crayfish giant axon due to an inhibition of sodium channels. The significance of these findings is discussed in relation to the sensory neurone-blocking action of capsaicin.