For many organisms, searching for relevant targets such as food or mates entails active, strategic sampling of the environment. Finding odorous targets may be the most ancient search problem that motile organisms evolved to solve. While chemosensory navigation has been well characterized in micro-organisms and invertebrates, spatial olfaction in vertebrates is poorly understood. We have established an olfactory search assay in which freely-moving mice navigate noisy concentration gradients of airborne odor. Mice solve this task using concentration gradient cues and do not require stereo olfaction for performance. During task performance, respiration and nose movement are synchronized with tens of milliseconds precision. This synchrony is present during trials and largely absent during inter-trial intervals, suggesting that sniff-synchronized nose movement is a strategic behavioral state rather than simply a constant accompaniment to fast breathing. To reveal the spatiotemporal structure of these active sensing movements, we used machine learning methods to parse motion trajectories into elementary movement motifs. Motifs fall into two clusters, which correspond to investigation and approach states. Investigation motifs lock precisely to sniffing, such that the individual motifs preferentially occur at specific phases of the sniff cycle. The allocentric structure of investigation and approach indicate an advantage to sampling both sides of the sharpest part of the odor gradient, consistent with a serial sniff strategy for gradient sensing. This work clarifies sensorimotor strategies for mouse olfactory search and guides ongoing work into the underlying neural mechanisms.
For many organisms, searching for relevant targets such as food or mates entails active, strategic sampling of the environment. Finding odorous targets may be the most ancient search problem that motile organisms evolved to solve. While chemosensory navigation has been well characterized in micro-organisms and invertebrates, spatial olfaction in vertebrates is poorly understood. We have established an olfactory search assay in which freely-moving mice navigate noisy concentration gradients of airborne odor. Mice solve this task using concentration gradient cues and do not require stereo olfaction for performance. During task performance, respiration and nose movement are synchronized with tens of milliseconds precision. This synchrony is present during trials and largely absent during inter-trial intervals, suggesting that sniff-synchronized nose movement is a strategic behavioral state rather than simply a constant accompaniment to fast breathing. To investigate the spatiotemporal structure of these active sensing movements, we used machine learning methods to parse motion trajectories into elementary movement motifs. Motifs fall into two clusters, which correspond to investigation and approach states. Investigation motifs lock precisely to sniffing, such that the individual motifs preferentially occur at specific phases of the sniff cycle. This work clarifies sensorimotor strategies for mouse olfactory search and guides ongoing work into the underlying neural mechanisms. Bhattacharyya U, Singh Bhalla U. Robust and rapid air borne odor tracking without casting. eNeuro. 2015 11; 2(6):Eneuro.0102-15. doi: 10.1523/ENEURO.0102-15.2015. Bi S, Sourjik V. Stimulus sensing and signal processing in bacterial chemotaxis.
Projection neurons (PNs) in the mammalian olfactory bulb (OB) receive input from the nose and project to diverse cortical and subcortical areas. Morphological and physiological studies have highlighted functional heterogeneity, yet no molecular markers have been described that delineate PN subtypes. Here, we used viral injections into olfactory cortex and fluorescent nucleus sorting to enrich PNs for high-throughput single nucleus and bulk RNA deep sequencing. Transcriptome analysis and RNA in situ hybridization identified distinct mitral and tufted cell populations with characteristic transcription factor network topology, cell adhesion and excitability-related gene expression. Finally, we describe a new computational approach for integrating bulk and snRNA-seq data, and provide evidence that different mitral cell populations preferentially project to different target regions. Together, we have identified potential molecular and gene regulatory mechanisms underlying PN diversity and provide new molecular entry points into studying the diverse functional roles of mitral and tufted cell subtypes.
Projection neurons in the mammalian olfactory bulb (OB) not only receive direct input from the nose but also directly project to diverse cortical and subcortical areas. While morphological and physiological studies have highlighted functional heterogeneity, no molecular markers have been described that could reliably delineate subtypes of projection neurons. Here, we used viral injections into olfactory cortex and fluorescent nucleus sorting to enrich OB neuronal populations for projection neurons. We then performed high-throughput single nucleus and bulk RNA deep sequencing of such enriched populations. Transcriptome analysis together with RNA in situ hybridization allowed us to identify three distinct mitral and five distinct tufted cell populations with characteristic cell adhesion and cell excitability-related gene expression patterns. Using a novel computational approach to integrate bulk and snRNA-seq data we propose that different mitral cell populations selectively project to different regions of olfactory cortex. Together, we have identified potential molecular and gene regulatory mechanisms underlying projection neuron diversity and provide critical new molecular entry points into studying the diverse functional roles of mitral and tufted cell subtypes.
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