Inhibition of TRPM8 by icilin distinct from desensitization induced by menthol and menthol derivatives.

TRPM8 is a cation channel activated by cold temperatures and the chemical stimuli menthol and icilin. Both compounds use different mechanisms of current activation; amino acid residues within the S2-S3 linker have been identified critical for current activation by icilin but not by menthol. Current decline in the course of menthol stimulation reflects Ca(2+)-dependent desensitization attributed to phosphatidylinositol 4,5-bisphosphate depletion. Carboxyamide derivatives chemically resembling menthol have been described as activators of TRPM8 analogous to icilin. Our aim was a detailed analysis of whether differences exist between all these substances with respect to their activation and inactivation of currents. We studied wild-type TRPM8 as well as an s3-TRPM8 mutant with mutations in the S2-S3 linker region that could not be activated by icilin. Menthol and menthol derivatives behaved indistinguishable in evoking currents through both channels in a Ca(2+)-independent manner as well as inducing Ca(2+)-dependent desensitization. Icilin, in contrast, activated currents only in wild type TRPM8 and in the presence of Ca(2+). Moreover, it completely reversed currents induced by menthol, menthol derivatives, and cold temperatures in wild type TRPM8 and s3-TRPM8; this current inhibition was independent of Ca(2+). Finally, icilin suppressed current activation by the other agonists. None of the inhibiting effects of icilin occurred in the cation channel TRPA1 that is also stimulated by both menthol and icilin. Thus, icilin specifically inhibits TRPM8 independently of its interaction site within the S2-S3 linker through a process distinct from desensitization.