PHYSIOLOGICAL REVIEWS   Vol. 78 No. 2 April 1998, pp. 429-466
Copyright ©1998 The American Physiological Society

Transduction Mechanisms in Vertebrate Olfactory Receptor Cells

DETLEV SCHILD AND DIEGO RESTREPO

Physiologisches Institut, Universität Göttingen, Göttingen, Germany; and Department of Cellular and Structural Biology, School of Medicine, University of Colorado Health Sciences Center, Denver, Colorado

Schild, Detlev, and Diego Restrepo. Transduction Mechanisms in Vertebrate Olfactory Receptor Cells. Physiol. Rev. 78: 429-466, 1998.  Considerable progress has been made in the understanding of transduction mechanisms in olfactory receptor neurons (ORNs) over the last decade. Odorants pass through a mucus interface before binding to odorant receptors (ORs). The molecular structure of many ORs is now known. They belong to the large class of G protein-coupled receptors with seven transmembrane domains. Binding of an odorant to an OR triggers the activation of second messenger cascades. One second messenger pathway in particular has been extensively studied; the receptor activates, via the G protein G_olf , an adenylyl cyclase, resulting in an increase in adenosine 3',5'-cyclic monophosphate (cAMP), which elicits opening of cation channels directly gated by cAMP. Under physiological conditions, Ca²⁺ has the highest permeability through this channel, and the increase in intracellular Ca²⁺ concentration activates a Cl current which, owing to an elevated reversal potential for Cl, depolarizes the olfactory neuron. The receptor potential finally leads to the generation of action potentials conveying the chemosensory information to the olfactory bulb. Although much less studied, other transduction pathways appear to exist, some of which seem to involve the odorant-induced formation of inositol polyphosphates as well as Ca²⁺ and/or inositol polyphosphate-activated cation channels. In addition, there is evidence for odorant-modulated K⁺ and Cl conductances. Finally, in some species, ORNs can be inhibited by certain odorants. This paper presents a comprehensive review of the biophysical and electrophysiological evidence regarding the transduction processes as well as subsequent signal processing and spike generation in ORNs.