A DEPOLARIZING CHLORIDE CURRENT CONTRIBUTES TO CHEMOELECTRICAL TRANSDUCTION IN OLFACTORY SENSORY NEURONS IN-SITU

Recent biophysical investigations of vertebrate olfactory signal trans duction have revealed that Ca2+-gated Cl- channels are activated durin g odorant detection in the chemosensory membrane of olfactory sensory neurons (OSNs). To understand the role of these channels in chemoelect rical signal transduction, it is necessary to know the Cl--equilibrium potential that determines direction and size of Cl- fluxes across the chemosensory membrane. We have measured Cl-, Na+, and K+ concentratio ns in ultrathin cryosections of rat olfactory epithelium, as well as r elative element contents in isolated microsamples of olfactory mucus, using energy-dispersive x-ray microanalysis. Determination of the Cl- concentrations in dendritic knobs and olfactory mucus yielded an estim ate of the Cl--equilibrium potential E-Cl in situ. With Cl- concentrat ions of 69 mM in dendritic knobs and 55 mM in olfactory mucus, we obta ined an E-Cl value of +6 +/- 12 mV. This indicates that Ca2+-gated Cl- channels in olfactory cilia conduct inward currents in vivo carried b y Cl- efflux into the mucus. Our results show that rat OSNs are among the few known types of neurons that maintain an elevated level of cyto solic Cl-. In these cells, activation of Cl- channels leads to depolar ization of the membrane voltage and can induce electrical excitation. The depolarizing Cl- current in mammalian OSNs appears to contribute a major fraction to the receptor current and may sustain olfactory func tion in sweet-water animals.