Fig. 1.

Scheme of odorant signal transduction. The insect odorant receptor complex is composed of an odorant-specific receptor protein OrX and the non-selective cation channel Or83 that conducts Na⁺, K⁺ and Ca²⁺. Odor-stimulation of OrX activates Or83 by an ionotropic and a metabotropic pathway. The direct activation of Or83b by OrX (red flash) leads to a fast and transient cation flow. The metabotropic pathway stimulates—via activation of G_s proteins by OrX—the adenylyl cyclase (AC) activity (violet flash) and thus the cAMP production. cAMP in turn activates Or83b (blue-green flash). The ionotropic pathway ensures a very rapid recognition of high odor concentrations while the metabotropic pathway allows highly sensitive odor detection