Odor degrading enzymes and signal termination

Abstract Several molecular steps are involved in odorant detection within insect olfactory sensilla, including various proteins that could interfere with ligand-receptor interaction, either before or after ligand binding. Among these “perireceptor events”, the step of signal termination is crucial to sustain the kinetics of the olfactory response. Antennal enzymes called Odorant-Degrading Enzymes (ODEs) present in the vicinity of receptors may participate in signal inactivation by rapid degradation of odorant molecules into inactive compounds, i.e. compounds that would no longer be able to activate receptors. These enzymes are also proposed to degrade excess of odorant chemicals in the sensilla in order to avoid overstimulation of the receptors, thus contributing to olfactory sensitivity and background noise reduction. ODEs could also take part in the general catabolism of odorant molecules within the sensilla, allowing chemicals to be removed from the olfactory hairs and protecting olfactory neurons from possible harmful molecules. Despite this wide range of possible roles, few antennal enzymes have been characterized as ODEs in insects, a full description implying the determination of both enzymatic activity toward odorants in vitro and participation in odorant responses in vivo, at the electrophysiological and/or behavioral level. In this chapter, we will present the pioneer works on the identification of ODEs, describe their diversity in antenna on the basis of recent transcriptomic studies, and discuss their involvement in odorant processing and termination.