Functional analysis of broadly tuned odorant receptors expressed in the mouse septal organ through patch-clamp recordings in an intact preparation and luciferase assay in a heterologous expression system
Résumé
The septal organ (SO) is a small island of olfactory epithelium located at the ventral base of the nasal septum. SO neurons express identified odorant receptors, but its function remains unknown. Using perforated patch clamp recordings, we investigated the response properties of SO neurons in the intact epithelium of mice to a panel of 45 odorants or mixtures. Out of 328 neurons tested, approximately 70% responded to odor stimulation. Among the responsive neurons, 72.5 % responded to multiple odorants. Responding neurons were tested with increasing concentrations of 3 odorants (octanoic acid, (+) camphor, amylacetate) and a mixture (fox urine). Dose-response curves show nanomolar threshold and broad dynamic range. The olfactory receptor SR1 (MOR256-3) is expressed in ~50% of SO cells and also in the ventral zone of the main olfactory epithelium (MOE). To analyze the origin of the broad tuning of the SO cells, we recorded from SR1-expressing cells in a genetically engineered mouse strain, SR1-IRES-tauGFP. We observed that all SR1-expressing cells in the SO and in the MOE respond to odorants with diverse structure (n = 8 and 10 respectively). Furthermore, all SR1 cells in both SO and MOE responded to a selected odorant (amyl acetate) with a nanomolar threshold and a broad dynamic range covering three to four log units. We then recorded the responses from the labeled cells in a mouse strain in which the SR1 coding region has been deleted and replaced with RFP, using the same set of odorants. The response properties of RFP-positive cells were radically different from SR1-expressing cells: only 22 % were broadly-tuned (n = 8). We further confirm the broad response profile of SR1 in the heterologous expression system Hana3A: transfected cells also responded to multiple odorants. The consensus model in mammalian olfaction is that the detection of millions of odorants requires a large number of odorant receptors (ORs) and that each OR interacts selectively with a small number of odorants. This study demonstrates that some olfactory receptors are relatively broadly tuned and may serve as general odor detectors. The septal organ, by concentrating some of the broadly-tuned receptors in the air path, may play an alerting role.