Chloride accumulation in mammalian olfactory sensory neurons

The generation of an excitatory receptor current in mammalian olfactory sensory neurons (OSNs) involves the sequential activation of two distinct types of ion channels: cAMP-gated Ca2+-permeable cation channels and Ca2+-gated Cl- channels, which conduct a depolarizing Cl- efflux. This unusual transduction mechanism requires an outward-directed driving force for Cl-, established by active accumulation of Cl- within the lumen of the sensory cilia. We used two-photon fluorescence lifetime imaging microscopy of the Cl--sensitive dye 6-methoxy-quinolyl acetoethyl ester to measure the intracellular Cl- concentration in dendritic knobs of OSNs from mice and rats. We found a uniform intracellular Cl- concentration in the range of 40 - 50 mM, which is indicative of active Cl- accumulation. Functional assays and PCR experiments revealed that NKCC1-mediated Cl- uptake through the apical membrane counteracts Cl- depletion in the sensory cilia, and thus maintains the responsiveness of OSNs to odor stimulation. To permit Cl- accumulation, OSNs avoid the chloride switch: they do not express KCC2, the main Cl- extrusion cotransporter operating in neurons of the adult CNS. Cl- accumulation provides OSNs with the driving force for the depolarizing Cl- current that is the basis of the low-noise receptor current in these neurons.