Imaging odor-induced calcium transients in single olfactory cilia: specificity of activation and role in transduction

The possibility that odor stimuli trigger distinct Ca2+ elevations within the cilia of vertebrate olfactory receptor neurons (ORNs) is a widely proposed concept. However, because of the small size of the olfactory cilia, the existence and properties of such Ca2+ elevations and their role in odor transduction are still unknown. We investigate odor-induced Ca2+ changes in individual olfactory cilia from salamander using the Ca2+ indicator dye fluo-3 in combination with laser scanning confocal microscopy. Single brief applications of odor ligand produce highly localized Ca2+ elevations in individual cilia lasting for several seconds. These Ca2+ signals originate in the cilia and depend entirely on Ca2+ entry through ciliary cyclic nucleotide-gated ion channels. The odor specificity of the Ca2+ rises implies a receptor-operated mechanism underlying odor detection. Each of the cilia on a receptor neuron functions as an independent biochemical compartment that can detect odorants and produce a Ca2+ transient with remarkably uniform properties in terms of kinetics and odor specificity. The rate of recovery of the odor-induced Ca2+ transients matches recovery from a short-term form of odor adaptation. Application of the membrane-permeant intracellular Ca2+ chelator BAPTA AM eliminates this odor adaptation. The results indicate that an olfactory cilium serves as a basic functional unit at the input level of the olfactory system, controlling both the specificity and sensitivity of odor detection.