Peripheral odor coding in the rat and frog: Quality and intensity specification

In mammals, two recent studies have shown recently that one odor molecule c an be recognized by several molecular olfactory receptors (ORs), and a sing le OR can recognize multiple odor molecules. In addition, one olfactory rec eptor neuron (ORN) may respond to different stimuli chosen as representativ e of distinct odor qualities. The aim of the present study was to analyze q uality and intensity coding abilities of rat single ORNs, comparing them wi th previous extensive data gathered in the frog to get insight into the gen erality of olfactory coding mechanisms over vertebrates. Response properties of 90 rat ORNs to different odors or to one odor at dif ferent concentrations were analyzed. In the rat and the frog, odor quality appears to be specified through the identity of activated ORNs. However, rat ORNs have higher response thresholds. This lower sensitivity m ay be interpreted as an increase in selectivity of rat ORNs for low or medi um odor intensities. In these conditions, the lower proportion of activated ORNs could be counterbalanced by their number, as well as by their higher glomerular convergence ratio in the olfactory bulb. From amphibians to mamm als, the olfactory system appears to use universal mechanisms based on a co mbinatorial-coding mode that may allow quasi-infinite possibilities of adap tation to various olfactory environments.