Capsaicin and nociception

The antinociceptive effect of capsaicin to noxious chemical stimuli has been invariably verified. As to thermal or mechanical nociception, however, routine pharmacological methods resulted in conflicting findings. Therefore, using new techniques the nociceptive thresholds of different stimuli were determined on the hindpaw of the rat. After systemic (400 mg/kg s.c.), perineural (1% on the sciatic nerve) and local (5 micrograms into the hindpaw) application of capsaicin the threshold for noxious heat (47.4 +/- 0.08) was shifted upwards by 3.3 degrees C, 4.1 degrees C and 2.9 degrees C, respectively. The changes in mechanonociceptive threshold evoked by pin prick (186 +/- 9 mN force) were more variable. The response to percutaneous xylene application was abolished or markedly inhibited. After systemic application the responsiveness to noxious heat recovered faster than the effect of xylene. C-polymodal nociceptors and some A-delta mechanoheat-sensitive nociceptors isolated from the saphenous nerve of the rat were activated by capsaicin in nanogram doses given close arterially. Five micrograms capsaicin excited few slowly adapting A mechanoreceptors after a long latency, but not A-delta mechanonociceptors or other cutaneous receptors. Proportion of C-polymodal nociceptors was decreased, that of the C-mechanoreceptors was increased after systemic treatment. The role of polymodal-type nociceptors, interaction of other nociceptors, as well as secondary dynamic changes are stressed to explain the antinociceptive effect of capsaicin.