Figure 5.

The invertebrate olfactory transduction cascade: In invertebrates, the ORs are 7 transmembrane domain receptors, which associate with a co-receptor, Orco. Upon binding of an odorant (step 1), the OR/Orco co-receptor channel pore opens, allowing the influx of cations, Ca²⁺ and Na⁺ (step 2). It is possible that a G protein mediated pathway regulates the sensitivity of the channels, odorant binding activates G_s (step 3). The G_sα subunit then in turn activates adenylyl cyclase to create cAMP from ATP, which may mediate sensitization of the odorant response (step 4). Phosphorylation by PKC of the Orco receptor may mediate the olfactory neuron sensitization (step 3).