Olfactory receptors: molecular basis for recognition and discrimination of odors

The daunting task of our nose to detect and discriminate among thousands of low-molecular-weight organic compounds with diverse chemical structures and properties requires an enormous molecular recognition capacity. This is based on distinct proteins, capable of recognizing and binding odorous compounds, including odorant-binding proteins, which are supposed to shuttle odorous compounds through the nasal mucus, and most notably the odorant receptors, which are heptahelical membrane proteins coupling via G-proteins onto intracellular transduction cascades. From more than a thousand genes each olfactory neuron is supposed to express only one receptor subtype. Receptors appear to be selective but rather non-specific—i.e. a distinct odorant activates multiple receptors and individual receptors respond to multiple odorants. It is the molecular receptive range of its receptor type which determines the reaction spectrum of a sensory neuron. Populations of cells equipped with the same receptor type project their axons to common glomeruli, thereby transmitting the molecular receptive range of a receptor type into the receptive field of glomerulus. Recent insight into the molecular basis of odor recognition and the combinatorial coding principles of the olfactory system may provide some clues for the design and development of technical sensors, electronic noses. In this review more emphasis has been placed on physiological rather than analytical aspects.