Cross-adaptation between Olfactory Responses Induced by Two Subgroups of Odorant Molecules

It has long been believed that vertebrate olfactory signal transduction is mediated by independent multiple pathways (using cAMP and InsP₃ as second messengers). However, the dual presence of parallel pathways in the olfactory receptor cell is still controversial, mainly because of the lack of information regarding the single-cell response induced by odorants that have been shown to produce InsP₃ exclusively (but not cAMP) in the olfactory cilia. In this study, we recorded activities of transduction channels of single olfactory receptor cells to InsP₃-producing odorants. When the membrane potential was held at −54 mV, application of InsP₃-producing odorants to the ciliary region caused an inward current. The reversal potential was 0 ± 7 mV (mean ± SD, n = 10). Actually, InsP₃-producing odorants generated responses in a smaller fraction of cells (lilial, 3.4%; lyral, 1.7%) than the cAMP-producing odorant (cineole, 26%). But, fundamental properties of responses were surprisingly homologous; namely, spatial distribution of the sensitivity, waveforms, I-V relation, and reversal potential, dose dependence, time integration of stimulus period, adaptation, and recovery. By applying both types of odorants alternatively to the same cell, furthermore, we observed cells to exhibit symmetrical cross-adaptation. It seems likely that even with odorants with different modalities adaptation occurs completely depending on the amount of current flow. The data will also provide evidence showing that olfactory response generation and adaptation are regulated by a uniform mechanism for a wide variety of odorants.