Expression of mRNAs encoding for two different olfactory receptors in a subset of olfactory receptor neurons

Evidence has accumulated to support a model for odorant detection in which individual olfactory receptor neurons (ORNs) express one of a large family of G protein-coupled receptor proteins that are activated by a small number of closely related volatile chemicals. However, the issue of whether an in dividual ORN expresses one or multiple types of receptor proteins has yet t o be definitively addressed. Physiological data indicate that some individu al ORNs can be activated by odorants differing substantially in structure a nd/or perceived quality, suggesting multiple receptors or one nonspecific r eceptor per cell, In contrast, molecular biological studies favor a scheme with a single, fairly selective receptor per cell. The present studies dire ctly assessed whether individual rat ORNs can express multiple receptors us ing single-cell PCR techniques with degenerate primers designed to amplify a wide variety of receptor sequences. We found that whereas only a single O R sequence was obtained from most ORNs examined, one ORN produced two disti nct receptor sequences that represented different receptor gene families. D ouble-label in situ hybridization studies indicated that a subset of ORNs c o-express two distinct receptor mRNAs. A laminar segregation analysis of th e cell nuclei of ORNs labeled with the two OR mRNA probes showed that for o ne probe, the histogram of the distribution of the cell nuclei along the de pth of the epithelium was bimodal, with one peak overlapping the (unimodal) histogram for the other probe. These results are consistent with co-expres sion of two OR mRNAs in a population of single ORNs.