CELLULAR-LOCALIZATION OF PUTATIVE ODORANT RECEPTOR MESSENGER-RNAS IN OLFACTORY AND CHEMOSENSORY NEURONS - A NONRADIOACTIVE IN-SITU HYBRIDIZATION STUDY

The precise cellular localization of mRNAs for putative odorant recept ors was investigated in the mouse chemosensory system (olfactory epith elium, septal organ and vomeronasal organ). Four additional members of the odorant receptor family were cloned from mouse olfactory mucosa a nd in situ hybridization was performed with paraffin-embedded tissue u sing digoxigenin labelled, non-radioactive antisense RNA probes for th ese individual receptor genes. The results clearly demonstrated expres sion of odorant receptors within single individual receptor neurons an d there was no receptor expression either in the basal cells (stem cel ls) or supporting cells (sustentacular cells). In contrast to the unif orm expression of olfactory marker protein mRNA within the layer of ma ture neurons, odorant receptor expression was localized in scattered i ndividual 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 str ingent hybridization and washing conditions used here, even mixed RNA probes prepared from 4 different odorant receptor genes were only expr essed in a maximum of 20-60 neurons per section (i.e. less than 0.1% o f the population of total receptor neurons) suggesting the size of odo rant receptor superfamilies to be larger than previously estimated. So me chemoreceptor neurons in the septal organ and vomeronasal organ als o expressed odorant receptor mRNAs suggesting that these two additiona l non-olfactory chemosensory systems share the same chemoreceptive pat hway as the olfactory system.