The cellular and molecular basis of odor adaptation

An important recent advance in the understanding of odor adaptation has com e from the discovery that complex mechanisms of odor adaptation already tak e place at the earliest stage of the olfactory system, in the olfactory cil ia. At least two rapid forms and one persistent form of odor adaptation coe xist in vertebrate olfactory receptor neurons. These three different adapta tion phenomena can be dissected on the basis of their different onset and r ecovery time courses and their pharmacological properties, indicating that they are controlled, at least in part, by separate molecular mechanisms. Ev idence is provided for the involvement of distinct molecular steps in these forms of odor adaptation, including Ca2+ entry through cyclic nucleotide-g ated (CNG) channels, Ca2+-dependent CNG channel modulation, Ca2+/calmodulin kinase Ii-dependent attenuation of adenylyl cyclase, and the activity of t he carbon monoxide/cyclic GMP second messenger system. Identification of th ese molecular steps may help to elucidate how the olfactory system extracts temporal and intensity information and to which extent odor perception is influenced by the different mechanisms underlying adaptation.