Retronasal and Oral-Cavity-Only Responses to TRPM8 Odorants

Retronasal (retro) and/or oral-cavity-only (OeD) smelling of vapor-phase agonists for the trigeminal chemoreceptive ion channel TRPM8 was studied. In a retro and OeD identification experiment. 1 of 6 previously practiced identifications (ID) was selected on a computer display under forced-choice conditions. Analyses of variance (ANOV A) found differences across odorants and retro and OeD. and for retro and OeD separately. Within retro smelling. median correct ID and modal ID (in parentheses) for eucalyptol was 100% (ointment). different from all other odorants. Geraniol 67% (lemon) and linalool 67% (cleaner) differed from dl-menthol. but isopulegol, dl-menthol and l-carvone did not differ. With oeD smelling. ID for geraniol fell to 0% and linalool to 33%, but no significant change for eucalyptol ID at 84%. A second, OeD-only triangle test examined discrimination of geraniol, l-carvone. and isopulegol from sunflower oil. ANOV A found overall differences and Wilcoxon tests found significant pairwise differences between all three odorants. Isopulegol was discriminated by 100% of particpants; l-carvone, by 53%; geraniol, by 13%, implying insignificant OeD geraniol discrimination. but significant discrimination of both isolpulegol and L-carvone in comparison with the non-odorant, sunflower oiL The differential OeD ID and discriminations indicate that TRPM8 channels are not the only oral cavity trigeminal mechanism for these odorants, while the retro versus OeD differences suggest either olfactory involvement or distinct oral and nasal cavity trigeminal arrays.