Department of Molecular Physiology
One of the unusual features of the olfactory signal transduction cascade is that the receptor current is mainly carried by chloride. In brain and spinal neurons, Cl currents cause inhibition because Cl channels conduct a hyperpolarizing Cl influx. How is it possible that Cl goes the other wav in OSNs and causes excitation? The reason is a very special Cl distribution across the ciliary membrane. The mucus, which covers the cilia on the surface of the olfactory mucosa, has a much lower Cl concentration than the fluid that surrounds cells inside the tissue. We measured a mean value of 55 mM in mucus using EDXA (energy dispersive x-ray microanalysis, left images), while neurons inside the brain see about 150 mM Cl on their outside. The Cl concentration inside the knobs (and inside the cilia) is very similar: We obtained a mean value of 46 mM by surface scan 2P-FLIM (2P-FLIM: two-photon fluorescence lifetime imaging, center image). This is much higher than the intracellular Cl concentration of CNS neurons (< 10 mM). To uphold such a high Cl concentration in the cytosol, OSNs must actively accumulate Cl against its electrochemical potential. We are currently trying to learn more about the molecular mechanism of this Cl accumulation.