Odorant ligands for the CO₂ receptor in two Anopheles vectors of malaria

Exhaled CO₂ is an important host-seeking cue for Anopheles mosquitoes, which is detected by a highly conserved heteromeric receptor consisting of three 7-transmembrane proteins Gr22, Gr23, and Gr24. The CO₂ receptor neuron has been shown to also respond sensitively to a variety of odorants in Aedes aegypti. The detection of CO₂ is important for upwind navigation and for enhancing the attraction to body heat as well as to skin odorants. The orthologs of the CO₂ receptor proteins are present in malaria-transmitting mosquitoes like Anopheles coluzzii and Anopheles sinensis. Activators and inhibitors of the CO₂-neuron were tested on the maxillary palps in these two species by single-sensillum electrophysiology. The electrophysiological testing of three prolonged-activator odorants identified originally in Aedes aegypti also showed varying ability to reduce the CO₂-ellicited increase in spikes. These findings provide a foundation for comparing the functional conservation with the evolutionary conservation of an important class of odorant receptor. The identification of a suite of natural odorants that can be used to modify the CO₂-detection pathway may also contribute to odor-blends that can alter the behavior of these disease transmitting mosquitoes.