The Ca-activated Cl Channel and its Control in Rat Olfactory Receptor Neurons

Abstract

Odorants activate sensory transduction in olfactory receptor neurons (ORNs) via a cAMP-signaling cascade, which results in the opening of nonselective, cyclic nucleotide–gated (CNG) channels. The consequent Ca2+ influx through CNG channels activates Cl channels, which serve to amplify the transduction signal. We investigate here some general properties of this Ca-activated Cl channel in rat, as well as its functional interplay with the CNG channel, by using inside-out membrane patches excised from ORN dendritic knobs/cilia. At physiological concentrations of external divalent cations, the maximally activated Cl current was ∼30 times as large as the CNG current. The Cl channels on an excised patch could be activated by Ca2+ flux through the CNG channels opened by cAMP. The magnitude of the Cl current depended on the strength of Ca buffering in the bath solution, suggesting that the CNG and Cl channels were probably not organized as constituents of a local transducisome complex. Likewise, Cl channels and the Na/Ca exchanger, which extrudes Ca2+, appear to be spatially segregated. Based on the theory of buffered Ca2+ diffusion, we determined the Ca2+ diffusion coefficient and calculated that the CNG and Cl channel densities on the membrane were ∼8 and 62 μm−2, respectively. These densities, together with the Ca2+ diffusion coefficient, demonstrate that a given Cl channel is activated by Ca2+ originating from multiple CNG channels, thus allowing low-noise amplification of the olfactory receptor current.

Ca2+-activated Cl− currents are dispensable for olfaction

“…In contrast to Ano -/-mice, Cnga2 -/y mice have severely impaired olfaction (refs. 25,30 and present work).We conclude that CNG-mediated cation currents do not need an additional boost from anion currents for near-physiological levels of olfactory sensitivity.Although fluid-phase EOGs revealed a significant difference between the genotypes, the ~40% reduction of Ano2 -/-EOG amplitudes is much less than expected from the published 80-90% contribution of Cl -to OSN receptor currents 8,11,12 . The contribution of Ca 2+ -activated Cl -channels was often estimated from the inhibition by NFA 11,37 , but this compound also modulates several other ion 8 channels (for example, refs.…”

“…After first publications had shown that Ca 2+ -activated Cl -currents could powerfully amplify olfactory receptor currents [4][5][6][7] , this concept has received support from many groups. Experiments with isolated receptor neurons [4][5][6]8 showed that up to 80-90% of the receptor current is carried by Cl -. An important role for Cl -in olfaction was also inferred from EOGs 11,16,37 .…”

“…These channels are thought to mediate an outward flow of Cl -, which generates a depolarizing current that, in rodents, is five-to tenfold larger than currents through CNG channels 8,11,12 . A prerequisite for Cl -efflux 3 is an inside-out electrochemical Cl --gradient.…”

“…cAMP then opens heteromeric cyclic nucleotide-gated (CNG) cation channels 3 . The resulting influx of Na + and Ca 2+ depolarizes the plasma membrane and raises the cytosolic Ca 2+ concentration ([Ca 2+ ] i ), thereby activating Ca 2+ -activated Cl -channels 1,[4][5][6][7][8][9][10] . All these components of the signal transduction cascade are localized to sensory cilia, which are embedded in the mucus covering the MOE.…”

“…There has been broad consensus that Ca 2+ -activated Cl -channels powerfully amplify olfactory signal transduction 1,[4][5][6][7][8][9][10] . These channels are thought to mediate an outward flow of Cl -, which generates a depolarizing current that, in rodents, is five-to tenfold larger than currents through CNG channels 8,11,12 .…”