Figure 2.

Proposed signal transduction mechanisms. (A) A classical model of insect olfactory transduction that involves a G-protein-mediated PLC-IP3 pathway. (B) Alternative model in which the odorant receptor (OR) forms a heteromeric odorant-gated non-selective cation channel with an Orco family protein. (C) Alternative model postulating two pathways, both of which depend on the cation channel function of Orco. An ionotropic pathway involves the direct activation of Orco by an OR with a bound ligand inducing a rapid but transient cation influx. A metabotropic pathway is coupled to the G protein, and induces slow but prolonged cation currents.