TRP channels expression on thermosensitive primary afferents: inference from brain evoked potentials modulation by topical agonists

Objective: By recording contact heat (CHEPs) and cool evoked brain potentials (CCEPs) after topical application of chemical agonists of TRPM8, TRPA1 or TRPV1 or vehicle, this study aimed to assess the expression of these thermosensitive TRP channels by noxious heat sensitive type II AMH and cool sensitive Aδ fibres in healthy young volunteers. Method: 12 healthy young volunteers 5x5 cm surface on the volar forearm was pre-treated with either capsaicin (0.005%), CA (0.5%) or menthol (5%) solution. Vehicle on the other arm. Patch removed after 10’. Each subject attended 3 sessions (1 week between each session) and was treated with each product. 30 hot (60°C) and 30 cool (5°C below baseline) stimulation on each forearm in the pre-treated area, slope: 300°C/s, duration: 150 ms, stimulation surface: 115 mm² Verbal rating of intensity (on a 0-10 scale) was asked to the subject after each stimulus. Skin temperature and spontaneous sensation were monitored. 32 channel EEG recording. Automated single trial analysis. Results: CCEP P2 latency was delayed following menthol application. CHEP N2P2 amplitude was affected by the factor vehicle vs product. There is a tendency in absolute values towards a decrease of amplitude following menthol and capsaicin application, however, post-hoc yielded no significant results. Conclusion: Very subtle changes in evoked brain responses were observed following capsaicin and menthol application. However, further studies with larger sample size and more selective/potent agonists are needed