PHOSPHOINOSITIDE 2ND-MESSENGERS IN OLFACTION

Olfactory stimuli (odorants) are detected and recognized by binding to receptors belonging to the G-protein-coupled receptor superfamily. Th e binding of odorants to some receptors stimulates the activity of an odorant-sensitive phospholipase C (PLC) thereby generating the second messengers inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG) . IP3 plays a key role in membrane depolarization by binding to a rece ptor that is itself a cation channel. The formation of DAG is expected to stimulate the activity of protein kinase C (PKC). PKC, together wi th G-protein-coupled receptor kinases, mediates signal termination by phosphorylation of odorant receptors and possibly other substrates. Th is review summarizes recent evidence regarding the role of phosphoinos itide-derived second messengers in the molecular events underlying olf actory signaling. In addition, the role of calcium as a ''third messen ger'' that provides a mechanism for interaction between phosphoinositi de second messengers and components of the cyclic AMP signaling pathwa y is also discussed.