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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SUMMARY In mammals, olfactory sensory neurons (OSNs) expressing a specific odorant receptor (OR) gene project with precise stereotypy onto mitral/tufted (M/T) cells in the main olfactory bulb (MOB). It remains challenging to understand how incoming olfactory signals are transformed into outputs of M/T cells. By recording from OSNs expressing mouse I7 receptor and their postsynaptic neurons in the bulb, we found that I7 OSNs and their corresponding M/T cells exhibit similarly selective tuning profiles at low concentrations. Increasing the concentration significantly reduces response selectivity for both OSNs and M/T cells, although the tuning curve of M/T cells remains comparatively narrow. By contrast, interneurons in the MOB are broadly tuned, and blocking GABAergic neurotransmission reduces selectivity of M/T cells at high odorant concentrations. Our results indicate that olfactory information carried by an OR is channeled to its corresponding M/T cells and support the role of lateral inhibition via interneurons in sharpening the tuning of M/T cells.

The large number of olfactory receptor genes necessitates high throughput methods to analyze their expression patterns. We have therefore designed a high-density oligonucleotide array containing all known mouse olfactory receptor (OR) and V1R vomeronasal receptor genes. This custom array detected a large number of receptor genes, demonstrating specific expression in the olfactory sensory epithelium for Ϸ800 OR genes previously designated as ORs based solely on genomic sequences. The array also enabled us to monitor the spatial and temporal distribution of gene expression for the entire OR family. Interestingly, OR genes showing spatially segregated expression patterns were also segregated on the chromosomes. This correlation between genomic location and spatial expression provides unique insights about the regulation of this large family of genes.

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Respiration and airflow through the nasal cavity are known to be correlated with rhythmic neural activity in the central nervous system. Here we show in rodents that during conditioned fear-induced freezing behavior, mice breathe at a steady rate (~4 Hz), which is correlated with a predominant 4-Hz oscillation in the prelimbic prefrontal cortex (plPFC), a structure critical for expression of conditioned fear behaviors. We demonstrate anatomical and functional connections between the olfactory pathway and plPFC via circuit tracing and optogenetics. Disruption of olfactory inputs significantly reduces the 4-Hz oscillation in the plPFC, but leads to prolonged freezing periods. Our results indicate that olfactory inputs can modulate rhythmic activity in plPFC and freezing behavior.

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.

Odorant Receptor (OR) genes and proteins represent more than 2% of our genome and 4% of our proteome 25 and constitute the largest sub-group of G Protein-Coupled Receptors (GPCRs). The mechanism underlying OR activation remains poorly understood, as they do not share some of the highly conserved motifs critical for activation of non-olfactory GPCRs. By combining site-directed mutagenesis, heterologous expression, and molecular dynamics simulations that capture the conformational change of constitutively active mutants, we tentatively identified crucial residues for the function of these receptors using the mouse MOR256-3 (Olfr124) as a model. The toggle-switch for sensing agonists in-volves a highly conserved tyrosine residue in helix VI. The ionic-lock is located between the `DRY' motif in helix III and a positively charged `R/K' residue in helix VI. This study provides an unprecedented model that captures the main mechanisms of odorant receptor activation.