Prolonged odor exposure causes a specific, reversible adaptation of olfactory responses. A genetic screen for negative regulators of olfaction uncovered mutations in the cGMP-dependent protein kinase EGL-4 that disrupt olfactory adaptation in C. elegans. G protein-coupled olfactory receptors within the AWC olfactory neuron signal through cGMP and a cGMP-gated channel. The cGMP-dependent kinase functions in AWC neurons during odor exposure to direct adaptation to AWC-sensed odors, suggesting that adaptation is a cell intrinsic process initiated by cGMP. A predicted phosphorylation site on the beta subunit of the cGMP-gated channel is required for adaptation after short odor exposure, suggesting that phosphorylation of signaling molecules generates adaptation at early time points. A predicted nuclear localization signal within EGL-4 is required for adaptation after longer odor exposure, suggesting that nuclear translocation of EGL-4 triggers late forms of adaptation.

Animals in complex environments must discriminate between salient and uninformative sensory cues. Caenorhabditis elegans uses one pair of olfactory neurons called AWC to sense many different odorants, yet the animal can distinguish each odorant from the others in discrimination assays. We demonstrate that the transmembrane guanylyl cyclase ODR-1 is essential for responses to all AWC-sensed odorants. ODR-1 appears to be a shared signaling component downstream of odorant receptors. Overexpression of ODR-1 protein indicates that ODR-1 can influence odor discrimination and adaptation as well as olfaction. Adaptation to one odorant, butanone, is disrupted by ODR-1 overexpression. Olfactory discrimination is also disrupted by ODR-1 overexpression, probably by overproduction of the shared second messenger cGMP. We propose that AWC odorant signaling pathways are insulated to permit odor discrimination.