Cellular localization of putative odorant receptor mRNAs in olfactory and chemosensory neurons: a non radioactive in situ hybridization study.

The precise cellular localization of mRNAs for putative odorant receptors was investigated in the mouse chemosensory system (olfactory epithelium, septal organ and vomeronasal organ). Four additional members of the odorant receptor family were cloned from mouse olfactory mucosa and in situ hybridization was performed with paraffin-embedded tissue using digoxigenin labelled, non-radioactive antisense RNA probes for these individual receptor genes. The results clearly demonstrated expression of odorant receptors within single individual receptor neurons and there was no receptor expression either in the basal cells (stem cells) or supporting cells (sustentacular cells). In contrast to the uniform expression of olfactory marker protein mRNA within the layer of mature neurons, odorant receptor expression was localized in scattered individual cells but with a bilateral symmetry. The number of positive cells was far less than the number detected with the olfactory marker protein probe. Interestingly, rostro-caudal and dorso-ventral sites of expression were specific to each receptor probe. Under the highly stringent hybridization and washing conditions used here, even mixed RNA probes prepared from 4 different odorant receptor genes were only expressed in a maximum of 20-60 neurons per section (i.e. less than 0.1% of the population of total receptor neurons) suggesting the size of odorant receptor superfamilies to be larger than previously estimated. Some chemoreceptor neurons in the septal organ and vomeronasal organ also expressed odorant receptor mRNAs suggesting that these two additional non-olfactory chemosensory systems share the same chemoreceptive pathway as the olfactory system.