Single Ca2+-activated Cl- channel currents recorded from toad olfactory cilia

Abstract

Background: Odor transduction, occurring in the chemosensory cilia of vertebrate olfactory sensory neurons, is triggered by guanosine triphosphate-coupled odor receptors and mediated by a cyclic adenosine monophosphate (cAMP) signaling cascade, where cAMP opens cationic non-selective cyclic nucleotide-gated (CNG) channels. Calcium enters through CNG gates Ca2+-activated Cl- channels, allowing a Cl- inward current that enhances the depolarization initiated by the CNG-dependent inward current. The anoctamin channel 2, ANO2, is considered the main Ca2+-activated Cl- channel of olfactory transduction. Although Ca2+-activated Cl- channel-dependent currents in olfactory sensory neurons were reported to be suppressed in ANO2-knockout mice, field potentials from their olfactory epithelium were only modestly diminished and their smell-dependent behavior was unaffected, suggesting the participation of additional Ca2+-activated Cl- channel types. The Bestrophin channel 2, Best2, was also detected in mouse olfactory cilia and ClCa4l, belonging to the ClCa family of Ca2+-activated Cl- channels, were found in rat cilia. Best2 knock-out mice present no electrophysiological or behavioral impairment, while the ClCa channels have not been functionally studied; therefore, the overall participation of all these channels in olfactory transduction remains unresolved.

Results: We explored the presence of detectable Ca2+-activated Cl- channels in toad olfactory cilia by recording from inside-out membrane patches excised from individual cilia and detected unitary Cl-current events with a pronounced Ca2+ dependence, corresponding to 12 and 24 pS conductances, over tenfold higher than the aforementioned channels, and a approx. fivefold higher Ca2+ affinity (K-0.5 = 0.38 mu M). Remarkably, we observed immunoreactivity to anti-ClCa and anti-ANO2 antibodies in the olfactory cilia, suggesting a possible cooperative function of both channel type in chemotransduction.

Conclusions: These results are consistent with a novel olfactory cilia channel, which might play a role in odor transduction.