Most sensory systems are primarily specialized to detect one sensory modality. Here we report that olfactory sensory neurons (OSNs) in the mammalian nose can detect two distinct modalities transmitted by chemical and mechanical stimuli. As revealed by patch-clamp recordings, many OSNs respond not only to odorants, but also to mechanical stimuli delivered by pressure ejections of odorfree Ringer solution. The mechanical responses correlate directly with the pressure intensity and show several properties similar to those induced by odorants, including onset latency, reversal potential and adaptation to repeated stimulation. Blocking adenylyl cyclase or knocking out the cyclic nucleotide-gated channel CNGA2 eliminates the odorant and the mechanical responses, suggesting that both are mediated by a shared cAMP cascade. We further show that this mechanosensitivity enhances the firing frequency of individual neurons when they are weakly stimulated by odorants and most likely drives the rhythmic activity (theta oscillation) in the olfactory bulb to synchronize with respiration.Receptor cells within each sensory system are specialized to detect a certain modality (for example, vision, hearing, touch, taste or smell) and to convert external stimuli into intracellular signals. Odor detection via the olfactory system relies on a large family of odorant receptors expressed in the olfactory sensory neurons (OSNs) within the neuroepithelium of the nose 1 . These odorant receptors initiate a cAMP pathway that enables olfactory signal transduction 2 . Binding of odor molecules to odorant receptors sequentially activates a specific G protein (Golf) and adenylyl cyclase type III (ACIII), which increases the intracellular cAMP level. Later opening of specific cyclic nucleotide-gated (CNG) channels by cAMP depolarizes the sensory neurons and triggers action potentials, which then permits odor information to reach the olfactory bulb in the brain.In addition to processing the convergent odor inputs from numerous OSNs, the olfactory bulb shows rhythmic activity (theta-band oscillation) coupled with respiration even in the absence of odorants, suggesting that the bulb also receives airflow information 3-8 . The mechanism underlying the rhythmic activity is not fully understood, although both peripheral and centrifugal sources are implicated 6,9-12 . Nonetheless, air intake, especially in the form of Correspondence should be addressed to M.M. (minghong@mail.med.upenn.edu.). AUTHOR CONTRIBUTIONS X.G. performed the recordings in the septal organ, L.C.S. performed the recordings in the main olfactory epithelium, J.T. performed the recordings in the olfactory bulb, M.L. supervised the bulb recordings, and M.M. supervised the whole project and drafted the manuscript. COMPETING INTERESTS STATEMENTThe authors declare that they have no competing financial interests. We show that the mechanosensitivity enhances the neuronal activity of individual OSNs when they are weakly stimulated by odorants. Furthermore, the mechanosensitivity of the...

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The current 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 subset of odorants, which are typically related in structure. Here, we report the odorant response properties of an OR that deviates from this model: SR1, a mouse OR that is abundantly expressed in sensory neurons of the septal organ and also of the main olfactory epithelium. Patch-clamp recordings reveal that olfactory sensory neurons (OSNs) that express SR1 respond to many, structurally unrelated odorants, and over a wide concentration range. Most OSNs expressing a gene-targeted SR1 locus that lacks the SR1 coding sequence do not show this broad responsiveness. Gene transfer in the heterologous expression system Hana3A confirms the broad response profile of SR1. There may be other mouse ORs with such broad response profiles.

In mammals, the sense of smell is modulated by the status of satiety, which is mainly signaled by blood-circulating peptide hormones. However, the underlying mechanisms linking olfaction and food intake are poorly understood. Here we investigated the effects of two anorectic peptides, insulin and leptin, on the functional properties of olfactory sensory neurons (OSNs). Using patch-clamp recordings, we analyzed the spontaneous activity of rat OSNs in an in vitro intact epithelium preparation. Bath perfusion of insulin and leptin significantly increased the spontaneous firing frequency in 91.7% (n = 24) and 75.0% (n = 24) of the cells, respectively. When the activity was electrically evoked, both peptides shortened the latency to the first action potential by approximately 25% and decreased the interspike intervals by approximately 13%. While insulin and leptin enhanced the electrical excitability of OSNs in the absence of odorants, they surprisingly reduced the odorant-induced activity in the olfactory epithelium. Insulin and leptin decreased the peak amplitudes of isoamyl acetate-induced electroolfactogram (EOG) signals to 46 and 38%, respectively. When measured in individual cells by patch-clamp recordings, insulin and leptin decreased odorant-induced transduction currents and receptor potentials. Therefore by increasing the spontaneous activity but reducing the odorant-induced activity of OSNs, an elevated insulin and leptin level (such as after a meal) may result in a decreased global signal-to-noise ratio in the olfactory epithelium, which matches the smell ability to the satiety status.

Mechanosensitive cells are essential for organisms to sense the external and internal environments, and a variety of molecules have been implicated as mechanical sensors. Here we report that odorant receptors (ORs), a large family of G protein-coupled receptors, underlie the responses to both chemical and mechanical stimuli in mouse olfactory sensory neurons (OSNs). Genetic ablation of key signaling proteins in odor transduction or disruption of OR-G protein coupling eliminates mechanical responses. Curiously, OSNs expressing different OR types display significantly different responses to mechanical stimuli. Genetic swap of putatively mechanosensitive ORs abolishes or reduces mechanical responses of OSNs. Furthermore, ectopic expression of an OR restores mechanosensitivity in loss-of-function OSNs. Lastly, heterologous expression of an OR confers mechanosensitivity to its host cells. These results indicate that certain ORs are both necessary and sufficient to cause mechanical responses, revealing a previously unidentified mechanism for mechanotransduction. , but our understanding of the mechanical sensors is still limited. We previously discovered that some OSNs in the mammalian nose responded to mechanical stimulation (4), a feature that may allow the nose to carry an afferent signal of breathing to the brain and facilitate binding of orofacial sensation (5). In the current study, we aim to identify the mechanical sensor(s) and mechanotransduction pathway in OSNs.In mammals, smell perception depends on a large family of ORs expressed in OSNs. Out of a repertoire of >1,000 ORs (6, 7), each OSN expresses a single type, which determines its response profile and central target in the brain. Binding of odorant molecules with specific ORs activates the olfactory G protein G olf , which in turn activates type III adenylyl cyclase (ACIII). ACIII activation causes increased production of cAMP, which opens a cyclic nucleotide-gated cation (CNG) channel. The inward current via the CNG channel is further amplified by Cl − outflow through a calcium-activated Cl − channel. This transduction cascade leads to depolarization of OSNs, which fire action potentials carrying the odor information to the brain (8). OSNs expressing the same OR are scattered in one of the few broadly defined zones in the olfactory epithelium, but their axons typically converge onto a pair of glomeruli in the olfactory bulb (9).Here we report that disruption of the olfactory signal transduction cascade completely eliminates mechanical responses in OSNs. OSNs expressing different receptor types display differential responses to mechanical stimuli. For instance, I7, M71, and SR1 neurons have much stronger mechanical responses than MOR23 and mOR-EG neurons. Loss-of-function mutation of the I7 receptor, genetic switch of the M71 receptor, or ablation of the SR1 receptor, abolishes or dramatically reduces mechanical responses in the host OSNs. Furthermore, ectopic expression of the I7 receptor restores mechanosensitivity in loss-of-function mutant I7 cells. ...

Mammalian olfactory sensory neurons (OSNs) form the primary elements of the olfactory system. Inserted in the olfactory mucosa lining of the nasal cavity, they are exposed to the environment and their lifespan is brief. Several reports say that OSNs are regularly regenerated during the entire life and that odorant environment affects the olfactory epithelium. However, little is known about the impact of the odorant environment on OSNs at the cellular level and more precisely in the context of early postnatal olfactory exposure. Here we exposed MOR23-green fluorescent protein (GFP) and M71-GFP mice to lyral or acetophenone, ligands for MOR23 or M71, respectively. Daily postnatal exposure to lyral induces plasticity in the population of OSNs expressing MOR23. Their density decreases after odorant exposure, whereas the amount of MOR23 mRNA and protein remain stable in the whole epithelium. Meanwhile, quantitative PCR indicates that each MOR23 neuron has higher levels of olfactory receptor transcripts and also expresses more CNGA2 and phosphodiesterase 1C, fundamental olfactory transduction pathway proteins. Transcript levels return to baseline after 4 weeks recovery. Patch-clamp recordings reveal that exposed MOR23 neurons respond to lyral with higher sensitivity and broader dynamic range while the responses' kinetics were faster. These effects are specific to the odorant-receptor pair lyral-MOR23: there was no effect of acetophenone on MOR23 neurons and no effect of acetophenone and lyral on the M71 population. Together, our results clearly demonstrate that OSNs undergo specific anatomical, molecular, and functional adaptation when chronically exposed to odorants in the early stage of life.

The septal organ, a distinct chemosensory organ observed in the mammalian nose, is essentially a small island of olfactory neuroepithelium located bilaterally at the ventral base of the nasal septum. Virtually nothing is known about its physiological properties and function. To understand the nature of the sensory neurons in this area, we studied the mechanisms underlying olfactory signal transduction in these neurons. The majority of the sensory neurons in the septal organ express olfactory-specific G-protein and adenylyl cyclase type III, suggesting that the cAMP signaling pathway plays a critical role in the septal organ as in the main olfactory epithelium (MOE). This is further supported by patch-clamp recordings from individual dendritic knobs of the sensory neurons in the septal organ. Odorant responses can be mimicked by an adenylyl cyclase activator and a phosphodiesterase inhibitor, and these responses can be blocked by an adenylyl cyclase inhibitor. There is a small subset of cells in the septal organ expressing a cGMP-stimulated phosphodiesterase (phosphodiesterase 2), a marker for the guanylyl cyclase-D subtype sensory neurons identified in the MOE. The results indicate that the septal organ resembles the MOE in major olfactory signal transduction pathways, odorant response properties, and projection to the main olfactory bulb. Molecular and functional analysis of the septal organ, which constitutes approximately 1% of the olfactory epithelium, will provide new insights into the organization of the mammalian olfactory system and the unique function this enigmatic organ may serve.

The sense of smell deteriorates in normal aging, but the underling mechanisms are still elusive. Here we investigated age-related alterations in expression patterns of odorant receptor (OR) genes and functional properties of olfactory sensory neurons (OSNs)-2 critical factors that define the odor detection threshold in the olfactory epithelium. Using in situ hybridization for 9 representative OR genes, we compared the cell densities of each OR in coronal nose sections at different ages (3-27 months). The cell density for different ORs peaked at different time points and a decline was observed for 6 of 9 ORs at advanced ages. Using patch clamp recordings, we then examined the odorant responses of individual OSNs coexpressing a defined OR (MOR23) and green fluorescent protein. The MOR23 neurons recorded from aged animals maintained a similar sensitivity and dynamic range in response to the cognate odorant (lyral) as those from younger mice. The results indicate that although the cell densities of OSNs expressing certain types of ORs decline at advanced ages, individual OSNs can retain their sensitivity. The implications of these findings in age-related olfactory deterioration are discussed.