Calcium, the two-faced messenger of olfactory transduction and adaptation

Exposure of olfactory receptor cells to odour stimulates the influx of Ca2+ through cyclic nucleotide-gated channels into the small volume within the cilia, the site of olfactory transduction. The consequent rise in intraciliary Ca2+ concentration has two opposing effects: activation of an unusual excitatory Cl- conductance, and negative feedback actions on various stages of the odour transduction mechanism. Recent studies are beginning to unravel how Ca2+ performs this dual function, and how the spatial and temporal dynamics of Ca2+ modulate the odour response. The feedback actions of Ca2+ on different elements of the transduction cascade seem to occur on different timescales, and are therefore responsible for shaping different parts of the receptor current response to odour stimulation.