The C-fibre conduction block caused by capsaicin on rat vagus nerve in vitro.

Capsaicin (0.01-50 microM) was applied to adult rat vagus nerve in vitro to examine the C-fibre conduction block and to compare its time course with that of the depolarisation caused by this drug. The conduction block was assessed by the reduction in the C-wave of the compound action potential. Capsaicin caused a dose-dependent decrease in the height and area under the C-wave; the threshold dose was between 0.01 and 0.3 microM and maximum C-wave reduction of about 85% occurred with doses of 5 microM or above. The C-wave reduction was divided into reversible and irreversible components which differed in dose dependency. The threshold for the reversible block was below 0.3 microM and for the irreversible block it was about 1 microM. The onset of the block took about 5 min. regardless of dose. Where there was more than 50% recovery after the block the reversible component was measured. This was dose dependent and its duration was 10-90 min. Removal of external calcium did not affect the magnitude of the C-wave block, although it did seem to prevent recovery of the C-wave.