An ex vivo preparation of the intact mouse vomeronasal organ and accessory olfactory bulb

Paradoxically Sparse Chemosensory Tuning in Broadly Integrating External Granule Cells in the Mouse Accessory Olfactory Bulb

The accessory olfactory bulb (AOB), the first neural circuit in the mouse accessory olfactory system, is critical for interpreting social chemosignals. Despite its importance, AOB information processing is poorly understood compared with the main olfactory bulb (MOB). Here, we sought to fill gaps in the understanding of AOB interneuron function. We used 2-photon GCaMP6f Ca2+ imaging in an ex vivo preparation to study chemosensory tuning in AOB external granule cells (EGCs), interneurons hypothesized to broadly inhibit activity in excitatory mitral cells (MCs). In ex vivo preparations from mice of both sexes, we measured MC and EGC tuning to natural chemosignal blends and monomolecular ligands, finding that EGC tuning was sparser, not broader, than upstream MCs. Simultaneous electrophysiological recording and Ca2+ imaging showed no differences in GCaMP6f-to-spiking relationships in these cell types during simulated sensory stimulation, suggesting that measured EGC sparseness was not due to cell type-dependent variability in GCaMP6f performance. Ex vivo patch-clamp recordings revealed that EGC subthreshold responsivity was far broader than indicated by GCaMP6f Ca2+ imaging, and that monomolecular ligands rarely elicited EGC spiking. These results indicate that EGCs are selectively engaged by chemosensory blends, suggesting different roles for EGCs than analogous interneurons in the MOB.SIGNIFICANCE STATEMENT The mouse accessory olfactory system (AOS) interprets social chemosignals, but we poorly understand AOS information processing. Here, we investigate the functional properties of external granule cells (EGCs), a major class of interneurons in the accessory olfactory bulb (AOB). We hypothesized that EGCs, which are densely innervated by excitatory mitral cells (MCs), would show broad chemosensory tuning, suggesting a role in divisive normalization. Using ex vivo GCaMP6f imaging, we found that EGCs were instead more sparsely tuned than MCs. This was not due to weaker GCaMP6f signaling in EGCs than in MCs. Instead, we found that many MC-activating chemosignals caused only subthreshold EGC responses. This indicates a different role for AOB EGCs compared with analogous cells in the main olfactory bulb.

Physiology-forward identification of bile acid-sensitive vomeronasal receptors

The mouse accessory olfactory system (AOS) supports social and reproductive behavior through the sensation of environmental chemosignals. A growing number of excreted steroids have been shown to be potent AOS cues, including bile acids (BAs) found in feces. As is still the case with most AOS ligands, the specific receptors used by vomeronasal sensory neurons (VSNs) to detect BAs remain unknown. To identify VSN BA receptors, we first performed a deep analysis of VSN BA tuning using volumetric GCaMP6f/s Ca2+ imaging. These experiments revealed multiple populations of BA-receptive VSNs with submicromolar sensitivities. We then developed a new physiology-forward approach for identifying AOS ligand-receptor interactions, which we call Fluorescence Live Imaging for Cell Capture and RNA sequencing, or FLICCR-seq. FLICCR-seq analysis revealed five specific V1R family receptors enriched in BA-sensitive VSNs. These studies introduce a powerful new approach for ligand-receptor matching and reveal biological mechanisms underlying mammalian BA chemosensation.

Signal Detection and Coding in the Accessory Olfactory System

In many mammalian species, the accessory olfactory system plays a central role in guiding behavioral and physiological responses to social and reproductive interactions. Because of its relatively compact structure and its direct access to amygdalar and hypothalamic nuclei, the accessory olfactory pathway provides an ideal system to study sensory control of complex mammalian behavior. During the last several years, many studies employing molecular, behavioral, and physiological approaches have significantly expanded and enhanced our understanding of this system. The purpose of the current review is to integrate older and newer studies to present an updated and comprehensive picture of vomeronasal signaling and coding with an emphasis on early accessory olfactory system processing stages. These include vomeronasal sensory neurons in the vomeronasal organ, and the circuitry of the accessory olfactory bulb. Because the overwhelming majority of studies on accessory olfactory system function employ rodents, this review is largely focused on this phylogenetic order, and on mice in particular. Taken together, the emerging view from both older literature and more recent studies is that the molecular, cellular, and circuit properties of chemosensory signaling along the accessory olfactory pathway are in many ways unique. Yet, it has also become evident that, like the main olfactory system, the accessory olfactory system also has the capacity for adaptive learning, experience, and state-dependent plasticity. In addition to describing what is currently known about accessory olfactory system function and physiology, we highlight what we believe are important gaps in our knowledge, which thus define exciting directions for future investigation.

In vivo stimulus presentation to the mouse vomeronasal system: Surgery, experiment, setup, and software

BACKGROUND

Achieving controlled stimulus delivery is a major challenge in the physiological analysis of the vomeronasal system (VNS).

NEW METHOD

We provide a comprehensive description of a setup allowing controlled stimulus delivery into the vomeronasal organ (VNO) of anesthetized mice. VNO suction is achieved via electrical stimulation of the sympathetic nerve trunk (SNT) using cuff electrodes, followed by flushing of the nasal cavity. Successful application of this methodology depends on several aspects including the surgical preparation, fabrication of cuff electrodes, experimental setup modifications, and the stimulus delivery and flushing. Here, we describe all these aspects in sufficient detail to allow other researchers to readily adopt it. We also present a custom written MATLAB based software with a graphical user interface that controls all aspects of the actual experiment, including trial sequencing, hardware control, and data logging.

RESULTS

The method allows measurement of stimulus evoked sensory responses in brain regions that receive vomeronasal inputs. An experienced investigator can complete the entire surgical procedure within thirty minutes.

COMPARISON WITH EXISTING METHODS

This is the only approach that allows repeated and controlled stimulus delivery to the intact VNO, employing the natural mode of stimulus uptake. The approach is economical with respect to stimuli, requiring stimulus volumes as low as 1-2μl.

CONCLUSIONS

This comprehensive description will allow other investigators to adapt this setup to their own experimental needs and can thus promote our physiological understanding of this fascinating chemosensory system. With minor changes it can also be adapted for other rodent species.

Odorant organization in the olfactory bulb of the sea lamprey

Olfactory sensory neurons innervate the olfactory bulb, where responses to different odorants generate a chemotopic map of increased neural activity within different bulbar regions. In this study, insight into the basal pattern of neural organization of the vertebrate olfactory bulb was gained by investigating the lamprey. Retrograde labelling established that lateral and dorsal bulbar territories receive the axons of sensory neurons broadly distributed in the main olfactory epithelium and that the medial region receives sensory neuron input only from neurons projecting from the accessory olfactory organ. The response duration for local field potential recordings was similar in the lateral and dorsal regions, and both were longer than medial responses. All three regions responded to amino acid odorants. The dorsal and medial regions, but not the lateral region, responded to steroids. These findings show evidence for olfactory streams in the sea lamprey olfactory bulb: the lateral region responds to amino acids from sensory input in the main olfactory epithelium, the dorsal region responds to steroids (taurocholic acid and pheromones) and to amino acids from sensory input in the main olfactory epithelium, and the medial bulbar region responds to amino acids and steroids stimulating the accessory olfactory organ. These findings indicate that olfactory subsystems are present at the base of vertebrate evolution and that regionality in the lamprey olfactory bulb has some aspects previously seen in other vertebrate species.

Heterogeneous effects of norepinephrine on spontaneous and stimulus-driven activity in the male accessory olfactory bulb

Norepinephrine (NE) release has been linked to experience-dependent plasticity in many model systems and brain regions. Among these is the rodent accessory olfactory system (AOS), which is crucial for detecting and processing socially relevant environmental cues. The accessory olfactory bulb (AOB), the first site of chemosensory information processing in the AOS, receives dense centrifugal innervation by noradrenergic fibers originating in the locus coeruleus. Although NE release has been linked to behavioral plasticity through its actions in the AOB, the impacts of noradrenergic modulation on AOB information processing have not been thoroughly studied. We made extracellular single-unit recordings of AOB principal neurons in ex vivo preparations of the early AOS taken from adult male mice. We analyzed the impacts of bath-applied NE (10 μM) on spontaneous and stimulus-driven activity. In the presence of NE, we observed overall suppression of stimulus-driven neuronal activity with limited impact on spontaneous activity. NE-associated response suppression in the AOB came in two forms: one that was strong and immediate (21%) and one other that involved gradual, stimulus-dependent monotonic response suppression (47%). NE-associated changes in spontaneous activity were more modest, with an overall increase in spontaneous spike frequency observed in 25% of neurons. Neurons with increased spontaneous activity demonstrated a net decrease in chemosensory discriminability. These results reveal that noradrenergic signaling in the AOB causes cell-specific changes in chemosensory tuning, even among similar projection neurons.NEW & NOTEWORTHY Norepinephrine (NE) is released throughout the brain in many behavioral contexts, but its impacts on information processing are not well understood. We studied the impact of NE on chemosensory tuning in the mouse accessory olfactory bulb (AOB). Electrophysiological recordings from AOB neurons in ex vivo preparations revealed that NE, on balance, inhibited mitral cell responses to chemosensory cues. However, NE's effects were heterogeneous, indicating that NE signaling reshapes AOB output in a cell- and stimulus-specific manner.

Faecal bile acids are natural ligands of the mouse accessory olfactory system

The accessory olfactory system (AOS) guides behaviours that are important for survival and reproduction, but understanding of AOS function is limited by a lack of identified natural ligands. Here we report that mouse faeces are a robust source of AOS chemosignals and identify bile acids as a class of natural AOS ligands. Single-unit electrophysiological recordings from accessory olfactory bulb neurons in ex vivo preparations show that AOS neurons are strongly and selectively activated by peripheral stimulation with mouse faecal extracts. Faecal extracts contain several unconjugated bile acids that cause concentration-dependent neuronal activity in the AOS. Many AOS neurons respond selectively to bile acids that are variably excreted in male and female mouse faeces, and others respond to bile acids absent in mouse faeces. These results identify faeces as a natural source of AOS information, and suggest that bile acids may be mammalian pheromones and kairomones.

The accessory olfactory system (AOS) processes social chemosensory information and guides behaviors that are important for survival and reproduction in mammals. Here the authors report that mouse feces are a source of AOS neuronal activity and identify unconjugated bile acids in feces as a class of natural AOS ligands.

Genetic differentiation of hypothalamus parentally biased transcripts in populations of the house mouse implicate the Prader-Willi syndrome imprinted region as a possible source of behavioral divergence

Parentally biased expression of transcripts (genomic imprinting) in adult tissues, including the brain, can influence and possibly drive the evolution of behavioral traits. We have previously found that paternally determined cues are involved in population-specific mate choice decisions between two populations of the Western house mouse (Mus musculus domesticus). Here, we ask whether this could be mediated by genomically imprinted transcripts that are subject to fast differentiation between these populations. We focus on three organs that are of special relevance for mate choice and behavior: The vomeronasal organ (VNO), the hypothalamus, and the liver. To first identify candidate transcripts at a genome-wide scale, we used reciprocal crosses between M. m. domesticus and M. m. musculus inbred strains and RNA sequencing of the respective tissues. Using a false discovery cutoff derived from mock reciprocal cross comparisons, we find a total of 66 imprinted transcripts, 13 of which have previously not been described as imprinted. The largest number of imprinted transcripts were found in the hypothalamus; fewer were found in the VNO, and the least were found in the liver. To assess molecular differentiation and imprinting in the wild-derived M. m. domesticus populations, we sequenced the RNA of the hypothalamus from individuals of these populations. This confirmed the presence of the above identified transcripts also in wild populations and allowed us to search for those that show a high genetic differentiation between these populations. Our results identify the Ube3a–Snrpn imprinted region on chromosome 7 as a region that encompasses the largest number of previously not described transcripts with paternal expression bias, several of which are at the same time highly differentiated. For four of these, we confirmed their imprinting status via single nucleotide polymorphism-specific pyrosequencing assays with RNA from reciprocal crosses. In addition, we find the paternally expressed Peg13 transcript within the Trappc9 gene region on chromosome 15 to be highly differentiated. Interestingly, both regions have been implicated in Prader–Willi nervous system disorder phenotypes in humans. We suggest that these genomically imprinted regions are candidates for influencing the population-specific mate-choice in mice.

Ex vivo preparations of the intact vomeronasal organ and accessory olfactory bulb

The mouse accessory olfactory system (AOS) is a specialized sensory pathway for detecting nonvolatile social odors, pheromones, and kairomones. The first neural circuit in the AOS pathway, called the accessory olfactory bulb (AOB), plays an important role in establishing sex-typical behaviors such as territorial aggression and mating. This small (<1 mm(3)) circuit possesses the capacity to distinguish unique behavioral states, such as sex, strain, and stress from chemosensory cues in the secretions and excretions of conspecifics. While the compact organization of this system presents unique opportunities for recording from large portions of the circuit simultaneously, investigation of sensory processing in the AOB remains challenging, largely due to its experimentally disadvantageous location in the brain. Here, we demonstrate a multi-stage dissection that removes the intact AOB inside a single hemisphere of the anterior mouse skull, leaving connections to both the peripheral vomeronasal sensory neurons (VSNs) and local neuronal circuitry intact. The procedure exposes the AOB surface to direct visual inspection, facilitating electrophysiological and optical recordings from AOB circuit elements in the absence of anesthetics. Upon inserting a thin cannula into the vomeronasal organ (VNO), which houses the VSNs, one can directly expose the periphery to social odors and pheromones while recording downstream activity in the AOB. This procedure enables controlled inquiries into AOS information processing, which can shed light on mechanisms linking pheromone exposure to changes in behavior.

Functional organization of glomerular maps in the mouse accessory olfactory bulb

The mammalian accessory olfactory system (AOS) extracts information about species, sex, and individual identity from social odors, but its functional organization remains unclear. We imaged presynaptic Ca²⁺ signals in vomeronasal inputs to the accessory olfactory bulb (AOB) during peripheral stimulation using light sheet microscopy. Urine- and steroid-responsive glomeruli densely innervated the anterior AOB. Glomerular activity maps for sexually mature female mouse urine overlapped maps for juvenile and/or gonadectomized urine of both sexes, whereas maps for sexually mature male urine were highly distinct. Further spatial analysis revealed a complicated organization involving selective juxtaposition and dispersal of functionally-grouped glomerular classes. Glomeruli that were similarly tuned to urines were often closely associated, whereas more disparately tuned glomeruli were selectively dispersed. Maps to a panel of sulfated steroid odorants identified tightly-juxtaposed groups that were disparately tuned and dispersed groups that were similarly tuned. These results reveal a modular, non-chemotopic spatial organization in the AOB.

Electrical recordings from the accessory olfactory bulb in VNO-AOB ex vivo preparations

Electrical recordings from individual accessory olfactory bulb neurons allow exploration of the functional properties of this important pheromonal processing circuit. Several approaches to performing such recordings have been used. Here, we describe ex vivo methods that we have found useful for recording from accessory olfactory bulb neurons using simple extracellular glass electrodes.

Representation and transformation of sensory information in the mouse accessory olfactory system

In mice, nonvolatile social cues are detected and analyzed by the accessory olfactory system (AOS). Here we provide a first view of information processing in the AOS with respect to individual chemical cues. 12 sulfated steroids, recently-discovered mouse AOS ligands, caused widespread activity among vomeronasal sensory neurons (VSNs), yet VSN responses clustered into a small number of repeated functional patterns or processing streams. Downstream neurons in the accessory olfactory bulb (AOB) responded to these ligands with enhanced signal/noise compared to VSNs. Whereas the dendritic connectivity of AOB mitral cells suggests the capacity for broad integration, most sulfated steroid responses were well-modeled by linear excitatory drive from just one VSN processing stream. However, a significant minority demonstrated multi-stream integration. Most VSN excitation patterns were also observed in the AOB, but excitation by estradiol sulfate processing streams was rare, suggesting AOB circuit organization is specific to the biological relevance of sensed cues.