Capsaicin desensitization of peripheral nociceptive fibres does not impair sensitivity to other noxious stimuli

Capsaicin was tested on peripheral fibres in vitro to determine whether evoked depolarization or desensitization were likely to explain the antinociception observed after acute systemic capsaicin in vivo. The activation of peripheral fibres by noxious (capsaicin, bradykinin, heat) and innocuous (light brush) stimuli was recorded as a depolarization of a spinal ventral root (L3-L5) in the neonatal rat spinal cord with attached tail. Prolonged superfusion of the tail with low doses (0.2-2 microM) of capsaicin produced a short lasting depolarization followed by a complete loss of sensitivity to capsaicin without changes in sensitivity to other noxious or innocuous stimuli. Partial recovery from this selective desensitization could be observed 3-5 h later. In most preparations superfusion of the tail with 20 microM capsaicin produced a prolonged and non-selective impairment of sensitivity to all noxious stimuli. These data suggest that neither depolarization by capsaicin nor the selective desensitization of peripheral fibres to capsaicin are likely to account for the acute antinociceptive effect of systemic capsaicin. On the other hand the non-selective reduction in sensitivity to noxious stimuli induced by capsaicin may contribute to its antinociceptive action.