Signal Transduction in Vertebrate Olfactory Cilia

The initial steps of olfaction occur in primary sensory neurons located in the olfactory epithelium of the nasal cavity of vertebrates. These neurons are responsible for the detection of odorant molecules present in the surrounding environment and the generation of the neural signal that is transmitted to the brain. The morphology of the primary sensory neurons was described by Max Schultze in the second half of the nineteenth century (for review, see Zippel 1993), but only about 100 years later the first reviews describing some functional properties of these neurons were published (Getchell 1986; Lancet 1986). Primary sensory neurons of the olfactory epithelium, often indicated by various names: olfactory receptor cells (ORCs), olfactory sensory neurons (OSNs), or olfactory receptor neurons (ORNs), are bipolar neurons with a single dendrite that terminates with a knob, from which several tiny cilia protrude, where the transduction of the olfactory signal takes place. Odorant molecules bind to odorant receptors, and this interaction triggers an increase in the intraciliary concentration of cyclic adenosine monophosphate (cAMP) through the activation of the receptor-coupled G-protein and adenylyl cyclase (AC). Cyclic nucleotide-gated (CNG) channels located in the ciliary membrane are directly activated by cytoplasmic cAMP, causing a depolarizing influx of Na and Ca ions. The odorant-induced inward transduction current has been shown to be composed not only of a cation influx through CNG channels, but also of a C efflux through C channels activated by Ca (Cl channels). This chapter will review the molecular mechanisms underlying the functional role of vertebrate olfactory cilia.