Inhibitory effect of capsaicin evoked trigeminal pain on warmth sensation and warmth evoked potentials

The aim of the study was to evaluate the effect of tonic pain evoked by topical application of capsaicin on the somatosensory sensation of warmth. The warmth pathways were studied by recording scalp potentials evoked by non-painful warm laser stimuli delivered on both the right and left perioral region (warmth C-fiber laser evoked potentials-C-LEPs). Amplitudes of the vertex C-LEPs evoked from the secondary hyperalgesic area on the right side and from a symmetrical non-hyperalgesic area on the left perioral region were significantly decreased during capsaicin application and after capsaicin removal, as compared with the baseline recordings. This inhibitory effect might be related to a central mechanism occurring at brainstem level. Probably due to a peripheral inhibitory mechanism, neither reliable C-LEP components nor warmth sensation were evoked by laser pulses delivered to the primary hyperalgesic area. This is the first neurophysiological evidence in humans of an inhibitory effect of pain on warmth sensation.