Multiple (four or eight) electrode arrays were placed for simultaneous electro-olfactogram (EOG) recordings of responses to a series of odors applied directly to the olfactory epithelium. Three different surfaces of the epithelium were exposed in rats immediately after death by anesthetic overdose. We tested three terpene compounds (carvone, limonene and 1,8-cineole) across the epithelium along the medial surface of the endoturbinate bones. Carvone, a ketone, evoked larger responses dorsally on the epithelium. The largest responses to 1,8-cineole (an ether) were seen in an intermediate-ventral region. The responses to limonene (a hydrocarbon) did not vary greatly across the regions, although they were often larger ventrally. The response distributions deviated from this simple pattern on the caudal part of endoturbinate IV, where the carvone responses were small and the limonene responses were larger. These differences were evident across a substantial concentration range. Similar distributions were seen for these three odors in tests along the dorsal-to-ventral direction across the nasal septum and in the medial-to-lateral direction across the dorsal aspect of one of the endoturbinate bones reaching out into the lateral recess. We argue that the spatial distributions of responses are correlated with the olfactory receptor gene expression zones.

The chemical properties that determine the distribution of the electro-olfactogram were studied after exposure of a large area of the rat olfactory epithelium. Multiple electrodes were placed along the rostral border of endoturbinate IV on the midline of the nasal cavity. This array of electrodes spanned a region containing the four receptor gene expression zones described for the rat. The response to a series of odorants containing only carbon, hydrogen, and oxygen was strongly related to electrode position. For most hydrocarbons, the responses were progressively larger toward the ventral epithelium. The only exceptions were aromatic hydrocarbons, which evoked nearly equal response sizes across the epithelium. Ketones and aldehydes evoked relatively larger dorsal responses than did hydrocarbons with similar structures. Aromatic ketones and aldehydes evoked systematically larger responses from the dorsal part of the epithelium. The response profiles for most odorants were well described by a linear fit to the electrode position along the dorsal-ventral position on the epithelium. However, a few bicyclic odorants and carboxylic acids evoked significantly nonlinear profiles. It is concluded that there is a systematic distribution of odorant sensitivity across this part of the epithelium and that this sensitivity is related to general chemical properties. Other evidence suggests that these properties extend to other parts of the epithelium. This spatial sensitivity of the epithelium to odorants probably contributes to olfactory coding in parallel with the convergence of axons from olfactory sensory neurons expressing the same receptor type.