Effects of camphor and related compounds on slowly adapting mechanoreceptors in the rat sinus hair follicle

INTRODUCTION: Camphor is a popular compound for therapeutic and cosmetic use with a distinctive odour, and somatosensory warming and cooling properties. The mechanisms for its action remain unclear. OBJECTIVE: The current study examined the effects of two enantiomers of camphor and related monoterpenoid compounds on mechanoreceptors. METHODS: Extracellular recordings were made in an in vitro bath preparation. Camphor, borneol, eugenol, carveol, and thymol were tested on the neural activity of St I and St II slowly adapting mechanoreceptors in the rat vibrissal hair follicle preparation. RESULTS: All compounds tested (0.5 – 2 mM bath concentrations) resulted in dose-dependent depression of spontaneous and mechanically evoked firing (dynamic and static phases). The mean latency of responses also increased. Both St I and St II were similarly affected, although (-)-camphor had a greater depressant effect on St II than on St I units. Differences were found across the different compounds for their effect on the dynamic and static phases. Thymol was found to have the greatest depressant effect on these phases. The broad spectrum TRP blocker ruthenium red did not reverse the depressant effects of camphor. The depressant effects of the compounds appeared similar to those obtained using the local anaesthetic lignocaine. The depressant effects of camphor and of lignocaine were partially reversed by the K(+) channel blocker tetraethylammonium. CONCLUSIONS: The results question whether the depressant effects of camphor and related compounds act through TRP channels. Perhaps the use of more selective blockers may reveal the molecular mechanisms through which these compounds act.