• R V Olsen, H H Andersen, H G Møller, P W Eskelund, and L Arendt-Nielsen.
• Center for Sensory-Motor Interaction (SMI), Department of Health Science and Technology, Faculty of Medicine, Aalborg University, Denmark.
• Eur J Pain. 2014 Oct 1; 18 (9): 1333-42.

BackgroundActivation of TRPM8 and TRPA1 receptors generates cold and cold pain sensations, respectively, and is presumably important in clinical pain manifestations, such as cold hyperalgesia. This study investigated the interaction between TRPM8 and TRPA1 receptors through stimulation of glabrous human skin (volar forearm) by topical administration of 40% L-menthol and 10% trans-cinnamaldehyde (CA), individually and in combination.MethodsSensory manifestations were assessed in 10 healthy volunteers via a platform of 11 quantitative sensory (thermal and mechanical stimuli) and vasomotor tests (skin temperature, perfusion and axon-reflex-flare) in a double-blinded randomized crossover design.ResultsCold pain threshold was increased (p < 0.01, cold allodynia) by L-menthol alone and L-menthol + CA in combination but unaffected by CA. Mechanical pain threshold was significantly decreased (mechanical hyperalgesia) by all three substances (p < 0.01), with a significant intergroup difference found between CA alone and the less decreased L-menthol + CA (p < 0.05). Application of CA alone and L-menthol + CA in combination showed an increase in skin temperature and perfusion significantly larger than that induced by L-menthol alone (p < 0.05). An axon-reflex-flare was present after CA administration, but was significantly reduced upon addition of L-menthol (p < 0.01).ConclusionThis study elucidates the potential of L-menthol as a counter-irritant to secondary neurogenic inflammation and provides evidence of an intricate interplay between cold receptors TRPA1 and TRPM8, warranting further investigation of the neural coding of cold pain perception.© 2014 European Pain Federation - EFIC®

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