Presence of Ca -Dependent K Channels in Chemosensory Cilia Support a Role in Odor Transduction

Delgado, Ricardo, M. Verónica Saavedra, Oliver Schmachtenberg, Jimena Sierralta, and Juan Bacigalupo. Presence of Ca dependent K channels in chemosensory cilia support a role in odor transduction. J Neurophysiol 90: 2022–2028, 2003. First published June 11, 2003; 10.1152/jn.01167.2002. Olfactory receptor neurons (ORNs) respond to odorants with changes in the action potential firing rate. Excitatory responses, consisting of firing increases, are mediated by a cyclic AMP cascade that leads to the activation of cationic nonselective cyclic nucleotide-gated (CNG) channels and Ca -dependent Cl (ClCa) channels. This process takes place in the olfactory cilia, where all protein components of this cascade are confined. ORNs from various vertebrate species have also been shown to generate inhibitory odor responses, expressed as decreases in action potential discharges. Odor inhibition appears to rely on Ca -dependent K (KCa) channels, but the underlying transduction mechanism remains unknown. If these channels are involved in odor transduction, they are expected to be present in the olfactory cilia. We found that a specific antibody against a large conductance KCa recognized a protein of approximately 116 kDa in Western blots of purified rat olfactory ciliary membranes. Moreover, the antibody labeled ORN cilia in isolated ORNs from rat and toad (Caudiverbera caudiverbera). In addition, single-channel recordings from inside-out membrane patches excised from toad chemosensory cilia showed the presence of 4 different types of KCa channels, with unitary conductances of 210, 60, 12, and 29 and 60 pS, high K -selectivity, and Ca sensitivities in the low micromolar range. Our work demonstrates the presence of K channels in the ORN cilia and supports their participation in odor transduction.