dOr83b-Receptor or Ion Channel?

Odorant signals are detected by binding of odor molecules to odorant receptors. These belong to the G protein-coupled receptor family. They in turn couple to G proteins, most of which induce cAMP production. This second messenger activates ion channels to depolarize the olfactory sensory neuron, thus providing a signal for further neuronal processing. Recent findings challenge this concept of olfactory signal transduction in insects, since their odorant receptors, which lack any sequence similarity to other G protein-coupled receptors, are composed of conventional odorant receptors (e.g., Or22a), dimerized with a ubiquitously expressed chaperone protein, such as Or83b in Drosophila. Or83b has a structure similar to G protein-coupled receptors, but has an inverted orientation in the plasma membrane. Still, G proteins are expressed in insect olfactory receptor neurons, and olfactory perception is modified by mutations affecting the cAMP transduction pathway. In our experiments we demonstrated that application of odorants to mammalian cells co-expressing Or22a and Or83b results in nonselective cation currents activated via both an ionotropic and a metabotropic pathway, and a subsequent increase in the intracellular Ca2+ concentration. Expression of Or83b alone leads to functional ion channels not directly responding to odorants, but directly activated by intracellular cAMP or cGMP. Insect odorant receptors thus form ligand-gated channels as well as complexes of odorant-sensing units and cyclic nucleotide-activated nonselective cation channels.