Molecular messengers of olfaction.

Our knowledge of olfactory signal transduction has been greatly clarified by several recent advances. Molecular cloning has revealed a large family of putative odorant receptors localized to olfactory epithelium that display a seven-transmembrane-domain motif suggesting an association with G proteins. Very potent and rapid enhancement of both adenylyl cyclase and phosphoinositide turnover has been demonstrated in response to odorants both in isolated olfactory cilia and primary olfactory receptor neuronal cultures. A Ca(2+)-calmodulin-dependent phosphodiesterase has been localized to olfactory cilia. A key role for Ca2+ is evident from many investigations. More recently, odorants have also been shown to affect the levels of cGMP in olfactory receptor neurons. The involvement of multiple second messengers may provide mechanisms for both fine-tuning and desensitization of olfaction.