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

Exposure of olfactory receptor cells to odour stimulates the influx of Ca(2+) through cyclic nucleotide-gated channels into the small volume within the cilia, the site of olfactory transduction. The consequent rise in intraciliary Ca(2+) 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 Ca(2+) performs this dual function, and how the spatial and temporal dynamics of Ca(2+) modulate the odour response. The feedback actions of Ca(2+) 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.