Spatially organized response zones in rat olfactory epithelium

Electroolfactogram recordings were made with a four-electrode assembly from the olfactory epithelium overlying the endoturbinate bones facing the nasal septum. In this study we tested whether odors of different chemical structures produce maximal responses along longitudinally oriented regions following the olfactory receptor gene expression zones described in the literature. The distribution of responses along the dorsal-to-ventral direction of this epithelium (i.e., across the expression zones) was tested in two types of experiments. In one, four electrodes were fixed along the dorsal-to-ventral axis of one turbinate bone. In the other, four electrodes were placed in corresponding positions on four turbinate bones and moved together up toward the top of the bone. These experiments compared the odorants limonene and alpha-terpinene, which are simple hydrocarbons, with carvone and menthone, which differ from the hydrocarbons by the presence of ketone groups. All responses were standardized to an amyl acetate or ethyl butyrate standard. The responses to limonene and alpha-terpinene were often larger for the ventral electrodes. The responses to carvone and menthone were largest for the dorsal electrodes. Intermediate electrodes gave responses that were intermediate in amplitude for these odors. The possibility that direction of air flow caused the observed response distributions was directly tested in experiments with odor nozzles placed in two positions. The relatively larger dorsal responses to carvone and relatively larger ventral responses to limonene were present despite odor nozzle position. We conclude that the responses to this set of odors vary systematically in a fashion parallel to the four gene expression zones. The odorant property that governs this response distribution may be related to the presence of oxygen-containing functional groups. Certain odors evoked larger responses at the intermediate electrode sites than at other sites. Cineole was the best example of this effect. This observation shows that not all oxygen-containing functional groups produce the same effect. Although we cannot exclude other possible mechanisms, these three response gradients may be produced by the four receptor expression zones described for many of the putative olfactory receptor genes. Therefore many of the receptors in each zone may share common properties. It remains to be determined whether this zonal input is significant in central odor processing. However, the correlation of odor chemical properties with the structure of receptor molecules in each zone may provide significant leads to structure-function relationships in vertebrate olfaction.