OLFACTORY RECEPTORS AND NEURAL CODING OF ODORS

Current views on the neural coding of olfactory information may have t o be reconsidered in the light of the discovery of a huge family of ge nes coding for olfactory receptors. The extreme diversity of these rec eptors confers a striking singularity to the olfactory system in that it apparently provides the sensory epithelium itself with a high capac ity for stimulus recognition. The large number of receptors makes it p ossible that some odorants be predominantly recognized by receptors of a single type, which offers a basis for understanding specific anosmi as. However, individual molecular sensitivities are mixed at the recep tor cell lever. It follows that the current concept of odor coding in terms of across-neuron pattern of activity remains a valid assumption. Odor coding patterns define a chemotopy at both epithelium and bulb l evels. Several data from 2-deoxyglucose metabolic studies, immunohisto logy and electrophysiology suggest that olfactory glomeruli act as con vergence centers for related inputs. Receiving various efferent inputs from noradrenergic, serotoninergic and cholinergic projection systems along with feedback innervation from its paleocortical projection are as, the olfactory bulb plays a key role in olfactory processing. Its d irect involvement in olfactory learning and memory is a promising line of research. All these domains will benefit from a better understandi ng of the principles of molecular recognition in olfactory receptor ce lls.