Responses of vomeronasal neurons to natural stimuli

Impaired pheromone detection and abnormal sexual behavior in female mice deficient for ancV1R

Ancient vomeronasal receptor type-1 (ancV1R), a putative vomeronasal receptor, is highly conserved across a wide range of vertebrates and is expressed in the majority of vomeronasal sensory neurons, co-expressing with canonical vomeronasal receptors, V1Rs and V2Rs. The pseudogenization of ancV1R is closely associated with vomeronasal organ (VNO) degeneration, indicating its critical role in pheromone sensing. However, the specific role of ancV1R remains unknown. In this study, to elucidate the function of ancV1R, we conducted phenotypic analyses of ancV1R-deficient female mice. Behavioral analyses showed that ancV1R-deficient females exhibited rejective responses toward male sexual behavior and displayed no preference for male urine. Physiological analyses demonstrate that the loss-of-function mutation of ancV1R reduced VNO response to various pheromone cues, including male urine, the sexual enhancing pheromone exocrine gland-secreting peptide 1 (ESP1), and β-estradiol 3-sulfate. Pre-exposure to ESP1 did not overcome the rejection behavior caused by ancV1R deficiency. Analysis of neural activity in the vomeronasal system revealed increased responses in the medial amygdala and posteromedial cortical amygdala of mutant females upon contact with males but not in response to male urine alone. Additionally, upon male contacts, ancV1R-deficient females exhibited increased neural activity in the lateral septum, a stress-associated brain region, along with elevated stress hormone levels. Such effects were not observed in females exposed solely to male urine. These findings suggest that, in females, ancV1R facilitates VNO responses to pheromone stimuli and plays a crucial role in perceiving males as mating partners. The absence of ancV1R results in failure of male perception, leading to abnormal sexual behaviors and stress responses upon male contact.

Stem cell expression of CXCR4 regulates tissue composition in the vomeronasal organ

The vomeronasal organ (VNO) detects signaling molecules that often prompt innate behaviors, such as aggression and reproduction. Vomeronasal sensory neurons, classified into apical and basal lineages based on receptor expression, have a limited lifespan and are continuously replaced from a common stem cell niche. Using a combination of single-cell RNA sequencing data, immunofluorescence staining and lineage tracing, we identified CXCR4 expression in proliferative stem cells and the basal neuronal lineage. Mice with a conditional knockout of Cxcr4 showed an increased number of SOX2-positive proliferative stem cells and enhanced basal neuronal lineage maturation. In addition, computational gene perturbation analysis revealed 87 transcription factors that might contribute to neurogenesis, among which was SOX2. Conditional knockout of Cxcr4 did not only disturb neuronal maturation, but also affected non-neuronal cell types, resulting in a decrease of basal lamina lining quiescent stem cells and an increase in sustentacular support cells. Together, these findings enhance our understanding how a common pool of stem cells can give rise to different cell types of the VNO, highlighting the distinct role of CXCR4 in this process.

Molecular, cellular, and developmental organization of the mouse vomeronasal organ at single cell resolution

We have generated single cell transcriptomic atlases of vomeronasal organs (VNO) from juvenile and adult mice. Combined with spatial molecular imaging, we uncover a distinct, previously unidentified class of cells that express the vomeronasal receptors (VRs) and a population of canonical olfactory sensory neurons in the VNO. High-resolution trajectory and cluster analyses reveal the lineage relationship, spatial distribution of cell types, and a putative cascade of molecular events that specify the V1r, V2r, and OR lineages from a common stem cell population. The expression of vomeronasal and olfactory receptors follow power law distributions, but there is high variability in average expression levels between individual receptor and cell types. Substantial co-expression is found between receptors across clades, from different classes, and between olfactory and VRs, with nearly half from pairs located on the same chromosome. Interestingly, the expression of V2r, but not V1r, genes is associated with various transcription factors, suggesting distinct mechanisms of receptor choice associated with the two cell types. We identify association between transcription factors, surface axon guidance molecules, and individual VRs, thereby uncovering a molecular code that guides the specification of the vomeronasal circuitry. Our study provides a wealth of data on the development and organization of the accessory olfactory system at both cellular and molecular levels to enable a deeper understanding of vomeronasal system function.

Noradrenaline modulates sensory information in mouse vomeronasal sensory neurons

During the induction of the body's alert state, the sympathetic system modulates sensory modalities and fine-tunes peripheral organs for improved stimulus detection. We explored noradrenaline (NA)'s role in modulating signaling in vomeronasal sensory neurons (VSNs), responsible for detecting pheromones and other semiochemicals. In current-clamp recordings, NA increased the firing frequency in response to natural stimuli of responsive VSNs and induced spiking activity in previously unresponsive neurons. Current injections into VSNs showed an increase in firing frequency during NA application. Combining transcriptomic analysis, electrophysiology, Ca2+ imaging, and a pharmacological approach, we identified alpha 1 adrenergic receptors as crucial for NA-induced firing frequency increases in VSNs. Immunohistochemistry revealed catecholaminergic fibers in the vomeronasal sensory epithelium, suggesting localized NA release near VSNs. This study unveils NA as a key regulator of VSN signaling, shedding light on the intricate interplay between the sympathetic nervous system and chemosensory processing, advancing our understanding of sensory modulation.

Molecular, Cellular, and Developmental Organization of the Mouse Vomeronasal organ at Single Cell Resolution

We have generated single cell transcriptomic atlases of vomeronasal organs (VNO) from juvenile and adult mice. Combined with spatial molecular imaging, we uncover a distinct, previously unidentified class of cells that express the vomeronasal receptors and a population of canonical olfactory sensory neurons in the VNO. High resolution trajectory and cluster analyses reveal the lineage relationship, spatial distribution of cell types, and a putative cascade of molecular events that specify the V1r, V2r, and OR lineages from a common stem cell population. The expression of vomeronasal and olfactory receptors follow power law distributions, but there is high variability in average expression levels between individual receptor and cell types. Substantial co-expression is found between receptors across clades, from different classes, and between olfactory and vomeronasal receptors, with nearly half from pairs located on the same chromosome. Interestingly, the expression of V2r, but not V1r, genes is associated with various transcription factors, suggesting distinct mechanisms of receptor choice associated with the two cell types. We identify association between transcription factors, surface axon guidance molecules, and individual VRs, thereby uncovering a molecular code that guides the specification of the vomeronasal circuitry. Our study provides a wealth of data on the development and organization of the accessory olfactory system at both cellular and molecular levels to enable a deeper understanding of vomeronasal system function.

Multiple administrations of bovine-appeasing substance during a 42-d preconditioning program followed by feedlot receiving and its effects on physiologic, health, and performance responses of feeder cattle

This experiment evaluated the effects of multiple bovine-appeasing substance (BAS) administration during a 42-d preconditioning program followed by a feedlot receiving period on productivity, health, and physiological variables of feeder cattle. Ninety calves were weaned, weighed, loaded into a livestock trailer, transported for 70 km, and unloaded at the Bozeman Agricultural Research and Teaching Farm for a 42-d preconditioning program. Upon arrival, calf body weight (BW) was recorded again, and both pre- and post-transport BWs were averaged and used as calf weaning initial BW. Calves were ranked by BW, sex, and age in a completely randomized design and assigned to receive 1) multiple administrations of BAS at weaning (day 0), days 14, 28, and before transport and feedlot entry (day 42; BAS; RSEA Group, Quartier Salignan, France; n = 9 pens/treatment), or 2) placebo (diethylene glycol monoethyl ether; CON; n = 9 pens/treatment). Treatments (5 mL) were applied to the nuchal skin area of each animal during the preconditioning period. Calves within treatment groups were ranked again by initial BW, sex, and age, in a manner that pens have similar initial BW, age, and three steers and two heifers and allocated to 1 of the18 drylot pens. On day 42, calves were combined within the treatment group, loaded into two different single double-deck commercial livestock trailers, and transported for 1,000 km (approximately 16 h). Upon arrival (day 43), calves were unloaded at the same feedyard. Blood samples were collected on days 0, 3, 7, 14, 21, 28, 42, 43, 46, 50, 57, 64, and 90. Average daily gain, final BW, and feed efficiency did not differ (P > 0.52) between BAS and CON calves in the preconditioning and receiving phases. A treatment × day interaction was detected (P < 0.001) for plasma haptoglobin concentrations, which was greater (P < 0.01) in CON on days 3 and 7 vs. BAS calves. During the preconditioning phase, serum NEFA concentration was reduced (P < 0.01) in BAS on day 3 compared with CON calves. A treatment × day interaction was detected (P = 0.001) for exit velocity, which was greater (P < 0.001) for CON vs. BAS calves on days 3, 7, 14, and 21 during the preconditioning phase and on day 46 of the receiving phase. Therefore, Applications of BAS reduced immunological responses and exit velocity associated with stress caused by management practices, but did not improve performance during the preconditioning and receiving phases.

From odor to oncology: non-canonical odorant receptors in cancer

Odorant receptors, traditionally associated with olfaction as chemoreceptors, have been increasingly recognized for their presence and diverse functions in various non-nasal tissues throughout the body. Beyond their roles in sensory perception, emerging evidence suggests a compelling interplay between odorant receptors and cancer progression as well. Alongside the canonical GPCR odorant receptors, dysregulation of non-canonical odorant receptors such as trace amine-associated receptors (TAARs), formyl peptide receptors (FPRs), and membrane-spanning 4A family (MS4As) has been observed in various cancer types, suggesting their contributions to cancer progression. The roles of these non-canonical chemoreceptors in cancer are complex, with some receptors promoting tumorigenesis and others acting as tumor-suppressing factors upon activation, depending on the cancer type. These findings shed light on the potential of non-canonical odorant receptors as therapeutic targets and prognostic markers in cancer, inviting further exploration to unravel their precise mechanisms of action and implications in cancer biology. In this review, we provide a comprehensive overview of the intricate relationships between these chemoreceptors and various types of cancer, potentially paving the way for innovative odor-based therapeutics. Ultimately, this review discusses the potential development of novel therapeutic strategies targeting these non-canonical chemoreceptors.

Aptamer-Field-Effect Transistors for Small-Molecule Sensing in Complex Environments

Aptamer-functionalized field-effect transistor (FET) biosensors enable detection of small-molecule targets in complex environments such as tissue and blood. Conventional FET-based platforms suffer from Debye screening in high ionic strength physiological environments where the effective sensing distance is limited to less than a nanometer from the surface of the sensor. Aptamers that undergo significant conformational rearrangement of negatively charged backbones upon target recognition within or in close proximity to the Debye length, facilitate the transduction of electronic signals through the semiconducting channel. Herein, the fabrication of high-performance, ultrathin-film FETs and subsequent aptamer functionalization are described. Moreover, electronic sensing measurement protocols alongside calibration methods to minimize device-to-device variations are covered.

Reprogramming the topology of the nociceptive circuit in C. elegans reshapes sexual behavior

The effect of the detailed connectivity of a neural circuit on its function and the resulting behavior of the organism is a key question in many neural systems. Here, we study the circuit for nociception in C. elegans, which is composed of the same neurons in the two sexes that are wired differently. We show that the nociceptive sensory neurons respond similarly in the two sexes, yet the animals display sexually dimorphic behaviors to the same aversive stimuli. To uncover the role of the downstream network topology in shaping behavior, we learn and simulate network models that replicate the observed dimorphic behaviors and use them to predict simple network rewirings that would switch behavior between the sexes. We then show experimentally that these subtle synaptic rewirings indeed flip behavior. Interestingly, when presented with aversive cues, rewired males were compromised in finding mating partners, suggesting that network topologies that enable efficient avoidance of noxious cues have a reproductive "cost." Our results present a deconstruction of the design of a neural circuit that controls sexual behavior and how to reprogram it.

The Bruce effect: Representational stability and memory formation in the accessory olfactory bulb of the female mouse

In the Bruce effect, a mated female mouse becomes resistant to the pregnancy-blocking effect of the stud. Various lines of evidence suggest that this form of behavioral imprinting results from reduced sensitivity of the female's accessory olfactory bulb (AOB) to the stud's chemosignals. However, the AOB's combinatorial code implies that diminishing responses to one individual will distort representations of other stimuli. Here, we record extracellular responses of AOB neurons in mated and unmated female mice while presenting urine stimuli from the stud and from other sources. We find that, while initial sensory responses in the AOB (within a timescale required to guide social interactions) remain stable, responses to extended stimulation (as required for eliciting the pregnancy block) display selective attenuation of stud-responsive neurons. Such temporal disassociation could allow attenuation of slow-acting endocrine processes in a stimulus-specific manner without compromising ongoing representations that guide behavior.

Slow Inactivation of Sodium Channels Contributes to Short-Term Adaptation in Vomeronasal Sensory Neurons

Adaptation plays an important role in sensory systems as it dynamically modifies sensitivity to allow the detection of stimulus changes. The vomeronasal system controls many social behaviors in most mammals by detecting pheromones released by conspecifics. Stimuli activate a transduction cascade in vomeronasal neurons that leads to spiking activity. Whether and how these neurons adapt to stimuli is still debated and largely unknown. Here, we measured short-term adaptation performing current-clamp whole-cell recordings by using diluted urine as a stimulus, as it contains many pheromones. We measured spike frequency adaptation in response to repeated identical stimuli of 2-10 s duration that was dependent on the time interval between stimuli. Responses to paired current steps, bypassing the signal transduction cascade, also showed spike frequency adaptation. We found that voltage-gated Na+ channels in VSNs undergo slow inactivation processes. Furthermore, recovery from slow inactivation of voltage-gated Na+ channels occurs in several seconds, a time scale similar to that measured during paired-pulse adaptation protocols, suggesting that it partially contributes to short-term spike frequency adaptation. We conclude that vomeronasal neurons do exhibit a time-dependent short-term spike frequency adaptation to repeated natural stimuli and that slow inactivation of Na+ channels contributes to this form of adaptation. These findings not only increase our knowledge about adaptation in the vomeronasal system, but also raise the question of whether slow inactivation of Na+ channels may play a role in other sensory systems.

TMEM16A and TMEM16B Modulate Pheromone-Evoked Action Potential Firing in Mouse Vomeronasal Sensory Neurons

The mouse vomeronasal system controls several social behaviors. Pheromones and other social cues are detected by sensory neurons in the vomeronasal organ (VNO). Stimuli activate a transduction cascade that leads to membrane potential depolarization, increase in cytosolic Ca²⁺ level, and increased firing. The Ca²⁺-activated chloride channels TMEM16A and TMEM16B are co-expressed within microvilli of vomeronasal neurons, but their physiological role remains elusive. Here, we investigate the contribution of each of these channels to vomeronasal neuron firing activity by comparing wild-type (WT) and knock-out (KO) mice. Performing loose-patch recordings from neurons in acute VNO slices, we show that spontaneous activity is modified by Tmem16a KO, indicating that TMEM16A, but not TMEM16B, is active under basal conditions. Upon exposure to diluted urine, a rich source of mouse pheromones, we observe significant changes in activity. Vomeronasal sensory neurons (VSNs) from Tmem16a cKO and Tmem16b KO mice show shorter interspike intervals (ISIs) compared with WT mice, indicating that both TMEM16A and TMEM16B modulate the firing pattern of pheromone-evoked activity in VSNs.

Principles of odor coding in vertebrates and artificial chemosensory systems

The biological olfactory system is the sensory system responsible for the detection of the chemical composition of the environment. Several attempts to mimic biological olfactory systems have led to various artificial olfactory systems using different technical approaches. Here we provide a parallel description of biological olfactory systems and their technical counterparts. We start with a presentation of the input to the systems, the stimuli, and treat the interface between the external world and the environment where receptor neurons or artificial chemosensors reside. We then delineate the functions of receptor neurons and chemosensors as well as their overall I-O relationships. Up to this point, our account of the systems goes along similar lines. The next processing steps differ considerably: while in biology the processing step following the receptor neurons is the "integration" and "processing" of receptor neuron outputs in the olfactory bulb, this step has various realizations in electronic noses. For a long period of time, the signal processing stages beyond the olfactory bulb, i.e., the higher olfactory centers were little studied. Only recently there has been a marked growth of studies tackling the information processing in these centers. In electronic noses, a third stage of processing has virtually never been considered. In this review, we provide an up-to-date overview of the current knowledge of both fields and, for the first time, attempt to tie them together. We hope it will be a breeding ground for better information, communication, and data exchange between very related but so far little connected fields.

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.

Interpreting the Spatial-Temporal Structure of Turbulent Chemical Plumes Utilized in Odor Tracking by Lobsters

Olfactory systems in animals play a major role in finding food and mates, avoiding predators, and communication. Chemical tracking in odorant plumes has typically been considered a spatial information problem where individuals navigate towards higher concentration. Recent research involving chemosensory neurons in the spiny lobster, Panulirus argus, show they possess rhythmically active or ‘bursting’ olfactory receptor neurons that respond to the intermittency in the odor signal. This suggests a possible, previously unexplored olfactory search strategy that enables lobsters to utilize the temporal variability within a turbulent plume to track the source. This study utilized computational fluid dynamics to simulate the turbulent dispersal of odorants and assess a number of search strategies thought to aid lobsters. These strategies include quantification of concentration magnitude using chemosensory antennules and leg chemosensors, simultaneous sampling of water velocities using antennule mechanosensors, and utilization of antennules to quantify intermittency of the odorant plume. Results show that lobsters can utilize intermittency in the odorant signal to track an odorant plume faster and with greater success in finding the source than utilizing concentration alone. However, the additional use of lobster leg chemosensors reduced search time compared to both antennule intermittency and concentration strategies alone by providing spatially separated odorant sensors along the body.

Odor tracking in aquatic organisms: the importance of temporal and spatial intermittency of the turbulent plume

In aquatic and terrestrial environments, odorants are dispersed by currents that create concentration distributions that are spatially and temporally complex. Animals navigating in a plume must therefore rely upon intermittent, and time-varying information to find the source. Navigation has typically been studied as a spatial information problem, with the aim of movement towards higher mean concentrations. However, this spatial information alone, without information of the temporal dynamics of the plume, is insufficient to explain the accuracy and speed of many animals tracking odors. Recent studies have identified a subpopulation of olfactory receptor neurons (ORNs) that consist of intrinsically rhythmically active 'bursting' ORNs (bORNs) in the lobster, Panulirus argus. As a population, bORNs provide a neural mechanism dedicated to encoding the time between odor encounters. Using a numerical simulation of a large-scale plume, the lobster is used as a framework to construct a computer model to examine the utility of intermittency for orienting within a plume. Results show that plume intermittency is reliably detectable when sampling simulated odorants on the order of seconds, and provides the most information when animals search along the plume edge. Both the temporal and spatial variation in intermittency is predictably structured on scales relevant for a searching animal that encodes olfactory information utilizing bORNs, and therefore is suitable and useful as a navigational cue.

Population Numbers and Physiological Response of an Invasive and Native Thrip Species Following Repeated Exposure to Imidacloprid

Frankliniella occidentalis and F. intonsa are devastating pest insects that target Rosa rugosa, Chrysanthemum morifolium, and Phaseolus vulgaris, which are important economical horticultural plants in China. Meanwhile, R. rugosa and C. morifolium are important cash plants in Kunming, South China. We focus on the population performance of these two thrips species on these three host plants with or without repeated exposure to imidacloprid in Kunming. In the field, the population numbers of F. occidentalis developed faster and were larger on these three sampled host plants, especially under imidacloprid exposure, compared with F. intonsa. The activity of the detoxifying enzymes (CarE, AchE, and MFO) and the antioxidant enzymes (CAT and POD) in both thrips species were significantly enhanced under imidacloprid exposure, whereas the activities of SOD in both thrips were significantly decreased on these three host plants, compared with the control. Overall, enzyme activity of F. occidentalis showed a greater increase than that observed in F. intonsa in most cases, which could be exploited in further studies on thrips resistance management.

The Role of Olfactory Genes in the Expression of Rodent Paternal Care Behavior

Olfaction is the dominant sensory modality in rodents, and is crucial for regulating social behaviors, including parental care. Paternal care is rare in rodents, but can have significant consequences for offspring fitness, suggesting a need to understand the factors that regulate its expression. Pup-related odor cues are critical for the onset and maintenance of paternal care. Here, I consider the role of olfaction in the expression of paternal care in rodents. The medial preoptic area shares neural projections with the olfactory and accessory olfactory bulbs, which are responsible for the interpretation of olfactory cues detected by the main olfactory and vomeronasal systems. The olfactory, trace amine, membrane-spanning 4-pass A, vomeronasal 1, vomeronasal 2 and formyl peptide receptors are all involved in olfactory detection. I highlight the roles that 10 olfactory genes play in the expression of direct paternal care behaviors, acknowledging that this list is not exhaustive. Many of these genes modulate parental aggression towards intruders, and facilitate the recognition and discrimination of pups in general. Much of our understanding comes from studies on non-naturally paternal laboratory rodents. Future studies should explore what role these genes play in the regulation and expression of paternal care in naturally biparental species.

Gαi2+ vomeronasal neurons govern the initial outcome of an acute social competition

Pheromone detection by the vomeronasal organ (VNO) mediates important social behaviors across different species, including aggression and sexual behavior. However, the relationship between vomeronasal function and social hierarchy has not been analyzed reliably. We evaluated the role of pheromone detection by receptors expressed in the apical layer of the VNO such as vomeronasal type 1 receptors (V1R) in dominance behavior by using a conditional knockout mouse for G protein subunit Gαi2, which is essential for V1R signaling. We used the tube test as a model to analyze the within-a-cage hierarchy in male mice, but also as a paradigm of novel territorial competition in animals from different cages. In absence of prior social experience, Gαi2 deletion promotes winning a novel social competition with an unfamiliar control mouse but had no effect on an established hierarchy in cages with mixed genotypes, both Gαi2^−/− and controls. To further dissect social behavior of Gαi2^−/− mice, we performed a 3-chamber sociability assay and found that mutants had a slightly altered social investigation. Finally, gene expression analysis in the medial prefrontal cortex (mPFC) for a subset of genes previously linked to social status revealed no differences between group-housed Gαi2^−/− and controls. Our results reveal a direct influence of pheromone detection on territorial dominance, indicating that olfactory communication involving apical VNO receptors like V1R is important for the outcome of an initial social competition between two unfamiliar male mice, whereas final social status acquired within a cage remains unaffected. These results support the idea that previous social context is relevant for the development of social hierarchy of a group. Overall, our data identify two context-dependent forms of dominance, acute and chronic, and that pheromone signaling through V1R receptors is involved in the first stages of a social competition but in the long term is not predictive for high social ranks on a hierarchy.

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.

Limbic Neurons Shape Sex Recognition and Social Behavior in Sexually Naive Males

Sexually naive animals have to distinguish between the sexes because they show species-typical interactions with males and females without meaningful prior experience. However, central neural pathways in naive mammals that recognize sex of other individuals remain poorly characterized. We examined the role of the principal component of the bed nucleus of stria terminalis (BNSTpr), a limbic center, in social interactions in mice. We find that activity of aromatase-expressing BNSTpr (AB) neurons appears to encode sex of other animals and subsequent displays of mating in sexually naive males. Silencing these neurons in males eliminates preference for female pheromones and abrogates mating success, whereas activating them even transiently promotes male-male mating. Surprisingly, female AB neurons do not appear to control sex recognition, mating, or maternal aggression. In summary, AB neurons represent sex of other animals and govern ensuing social behaviors in sexually naive males.

Steroidal pheromones and their potential target sites in the vomeronasal organ

Steroids are important olfactory signals in most mammalian species. The vomeronasal organ has been suspected to be the primary target of pheromones. In rat vomeronasal sensory neurons express steroid binding proteins and nuclear receptors. Some binding globulins were found also in single ciliated cells of the non-sensory vomeronasal epithelium. Immunoelectron microscopy revealed VDR in olfactory microvilli and DPB in apical membrane protrusions of supporting sells within the sensory epithelium. Pilot behavioral studies with dogs showed increased sniffing duration upon exposure to low concentrations of vitamin D while higher concentrations were less effective. It has been shown that vitamin D has pheromone-like properties in lizards. Our histochemical and behavioral observations indicate that the mammalian vomeronasal organ may be a vitamin D target. Olfactory functions of vitamin D involve most likely rapid membrane mediated effects rather than actions through nuclear receptors.

Sex separation induces differences in the olfactory sensory receptor repertoires of male and female mice

Within the mammalian olfactory sensory epithelium, experience-dependent changes in the rate of neuronal turnover can alter the relative abundance of neurons expressing specific chemoreceptors. Here we investigate how the mouse olfactory sensory receptor repertoire changes as a function of exposure to odors emitted from members of the opposite sex, which are highly complex and sexually dimorphic. Upon housing mice either sex-separated or sex-combined until six months of age, we find that sex-separated mice exhibit significantly more numerous differentially expressed genes within their olfactory epithelia. A subset of these chemoreceptors exhibit altered expression frequencies following both sex-separation and olfactory deprivation. We show that several of these receptors detect either male- or female-specific odors. We conclude that the distinct odor experiences of sex-separated male and female mice induce sex-specific differences in the abundance of neurons that detect sexually dimorphic odors.

Gene expression profiling of the olfactory tissues of sex-separated and sex-combined female and male mice

Olfactory experience can alter the molecular and cellular composition of chemosensory neurons within the olfactory sensory epithelia of mice. We sought to investigate the scope of cellular and molecular changes within a mouse's olfactory system as a function of its exposure to complex and salient sets of odors: those emitted from members of the opposite sex. We housed mice either separated from members of the opposite sex (sex-separated) or together with members of the opposite sex (sex-combined) until six months of age, resulting in the generation of four cohorts of mice. From each mouse, the main olfactory epithelium (MOE), vomeronasal organ (VNO), and olfactory bulb (OB) were removed and RNA-extracted. A total of 36 RNA samples, representing three biological replicates per sex/condition/tissue combination, were analyzed for integrity and used to prepare RNA-seq libraries, which were subsequently analyzed via qPCR for the presence of tissue- or sex-specific markers. Libraries were paired-end sequenced to a depth of ~20 million fragments per replicate and the data were analyzed using the Tuxedo suite.

Voluntary urination control by brainstem neurons that relax the urethral sphincter

Voluntary urination ensures that waste is eliminated when safe and socially appropriate, even without a pressing urge. Uncontrolled urination, or incontinence, is a common problem with few treatment options. Normal urine release requires a small region in the brainstem known as Barrington's nucleus (Bar), but specific neurons that relax the urethral sphincter and enable urine flow are unknown. Here we identify a small subset of Bar neurons that control the urethral sphincter in mice. These excitatory neurons express estrogen receptor 1 (Bar^ESR1), project to sphincter-relaxing interneurons in the spinal cord and are active during natural urination. Optogenetic stimulation of Bar^ESR1 neurons rapidly initiates sphincter bursting and efficient voiding in anesthetized and behaving animals. Conversely, optogenetic and chemogenetic inhibition reveals their necessity in motivated urination behavior. The identification of these cells provides an expanded model for the control of urination and its dysfunction.

Sensory Adaptation to Chemical Cues by Vomeronasal Sensory Neurons

Sensory adaptation is a source of experience-dependent feedback that impacts responses to environmental cues. In the mammalian main olfactory system (MOS), adaptation influences sensory coding at its earliest processing stages. Sensory adaptation in the accessory olfactory system (AOS) remains incompletely explored, leaving many aspects of the phenomenon unclear. We investigated sensory adaptation in vomeronasal sensory neurons (VSNs) using a combination of in situ Ca²⁺ imaging and electrophysiology. Parallel studies revealed prominent short-term sensory adaptation in VSNs upon repeated stimulation with mouse urine and monomolecular bile acid ligands at interstimulus intervals (ISIs) less than 30 s. In such conditions, Ca²⁺ signals and spike rates were often reduced by more than 50%, leading to dramatically reduced chemosensory sensitivity. Short-term adaptation was reversible over the course of minutes. Population Ca²⁺ imaging experiments revealed the presence of a slower form of VSN adaptation that accumulated over dozens of stimulus presentations delivered over tens of minutes. Most VSNs showed strong adaptation, but in a substantial VSN subpopulation adaptation was diminished or absent. Investigation of same- and opposite-sex urine responses in male and female VSNs revealed that adaptation to same-sex cues occurred at ISIs up to 180 s, conditions that did not induce adaptation to opposite-sex cues. This result suggests that VSN sensory adaptation can be modulated by sensory experience. These studies comprehensively establish the presence of VSN sensory adaptation and provide a foundation for future inquiries into the molecular and cellular mechanisms of this phenomenon and its impact on mammalian behavior.

Recent Progress in Light Sheet Microscopy for Biological Applications

The introduction of light sheet fluorescence microscopy (LSFM) has overcome the challenges in conventional optical microscopy. Among the recent breakthroughs in fluorescence microscopy, LSFM had been proven to provide a high three-dimensional spatial resolution, high signal-to-noise ratio, fast imaging acquisition rate, and minuscule levels of phototoxic and photodamage effects. The aforementioned auspicious properties are crucial in the biomedical and clinical research fields, covering a broad range of applications: from the super-resolution imaging of intracellular dynamics in a single cell to the high spatiotemporal resolution imaging of developmental dynamics in an entirely large organism. In this review, we provided a systematic outline of the historical development of LSFM, detailed discussion on the variants and improvements of LSFM, and delineation on the most recent technological advancements of LSFM and its potential applications in single molecule/particle detection, single-molecule super-resolution imaging, imaging intracellular dynamics of a single cell, multicellular imaging: cell-cell and cell-matrix interactions, plant developmental biology, and brain imaging and developmental biology.

The Accessory Olfactory System: Innately Specialized or Microcosm of Mammalian Circuitry?

In mammals, the accessory olfactory system is a distinct circuit that has received attention for its role in detecting and responding to pheromones. While the neuroscientific investigation of this system is comparatively new, recent advances and its compact size have made it an attractive model for developing an end-to-end understanding of such questions as regulation of essential behaviors, plasticity, and individual recognition. Recent discoveries have indicated a need to reevaluate our conception of this system, suggesting that ( a) physical principles—rather than biological necessity—play an underappreciated role in its raison d'être and that ( b) the anatomy of downstream projections is not dominated by unique specializations but instead consists of an abbreviated cortical/basal ganglia motif reminiscent of other sensorimotor systems. These observations suggest that the accessory olfactory system distinguishes itself primarily by the physicochemical properties of its ligands, but its architecture is otherwise a microcosm of mammalian neurocircuitry.

In-vivo activation of vomeronasal neurons shows adaptive responses to pheromonal stimuli

In most mammals, the vomeronasal system has a pivotal role in mediating socio-sexual behaviours. The vomeronasal organ senses pheromones through the activation of specific receptors. Pheromone binding to cognate receptors activates Ca-influx via the gating of a cation channel that generates membrane depolarisation. The ex-vivo activation of vomeronasal neurons (VSNs) by pheromonal stimuli has been largely investigated by electrophysiological and imaging techniques; however, few studies have been carried out to determine the physiological responses of VSNs, in-vivo. By tracking the phosphorylation of S6 ribosomal protein as a marker of neuronal activity, we show that S6 becomes phosphorylated (pS6) in mouse VSNs stimulated by intraspecific and heterospecific pheromonal cues. We observed that female scent induces pS6 immunoreactivity in the apical VSNs of male vomeronasal epithelium, whereas male cues stimulate S6 phosphorylation in both the basal and apical VSNs of females. We also show that this dimorphic pattern of pS6 immunoreactivity is reproduced when heterospecific stimuli are used. Moreover, we found that a consistent proportion of VSNs is activated by both heterospecific and intraspecific pheromones. Additionally, we have evidence of adaptive responses to S6 phosphorylation when stimulation with cues of the same and opposite sex and of different species is sustained.

Olfaction and Pheromones: Uncanonical Sensory Influences and Bulbar Interactions

The rodent main and accessory olfactory systems (AOS) are considered functionally and anatomically segregated information-processing pathways. Each system is devoted to the detection of volatile odorants and pheromones, respectively. However, a growing number of evidences supports a cooperative interaction between them. For instance, at least four non-canonical receptor families (i.e., different from olfactory and vomeronasal receptor families) have been recently discovered. These atypical receptor families are expressed in the sensory organs of the nasal cavity and furnish parallel processing-pathways that detect specific stimuli and mediate specific behaviors as well. Aside from the receptor and functional diversity of these sensory modalities, they converge into a poorly understood bulbar area at the intersection of the main- main olfactory bulb (MOB) and accessory olfactory bulb (AOB) that has been termed olfactory limbus (OL). Given the intimate association the OL with specialized glomeruli (i.e., necklace and modified glomeruli) receiving uncanonical sensory afferences and its interactions with the MOB and AOB, the possibility that OL is a site of non-olfactory and atypical vomeronasal sensory decoding is discussed.

Neuronal Representation of Social Information in the Medial Amygdala of Awake Behaving Mice

The medial amygdala (MeA) plays a critical role in processing species- and sex-specific signals that trigger social and defensive behaviors. However, the principles by which this deep brain structure encodes social information is poorly understood. We used a miniature microscope to image the Ca²⁺ dynamics of large neural ensembles in awake behaving mice and tracked the responses of MeA neurons over several months. These recordings revealed spatially intermingled subsets of MeA neurons with distinct temporal dynamics. The encoding of social information in the MeA differed between males and females and relied on information from both individual cells and neuronal populations. By performing long-term Ca²⁺ imaging across different social contexts, we found that sexual experience triggers lasting and sex-specific changes in MeA activity, which, in males, involve signaling by oxytocin. These findings reveal basic principles underlying the brain's representation of social information and its modulation by intrinsic and extrinsic factors.

Type 3 inositol 1,4,5-trisphosphate receptor is dispensable for sensory activation of the mammalian vomeronasal organ

Signal transduction in sensory neurons of the mammalian vomeronasal organ (VNO) involves the opening of the canonical transient receptor potential channel Trpc2, a Ca²⁺-permeable cation channel that is activated by diacylglycerol and inhibited by Ca²⁺-calmodulin. There has been a long-standing debate about the extent to which the second messenger inositol 1,4,5-trisphosphate (InsP₃) and type 3 InsP₃ receptor (InsP₃R3) are involved in the opening of Trpc2 channels and in sensory activation of the VNO. To address this question, we investigated VNO function of mice carrying a knockout mutation in the Itpr3 locus causing a loss of InsP₃R3. We established a new method to monitor Ca²⁺ in the endoplasmic reticulum of vomeronasal sensory neurons (VSNs) by employing the GFP-aequorin protein sensor erGAP2. We also performed simultaneous InsP₃ photorelease and Ca²⁺ monitoring experiments, and analysed Ca²⁺ dynamics, sensory currents, and action potential or field potential responses in InsP₃R3-deficient VSNs. Disruption of Itpr3 abolished or minimized the Ca²⁺ transients evoked by photoactivated InsP₃, but there was virtually no effect on sensory activation of VSNs. Therefore, InsP₃R3 is dispensable for primary chemoelectrical transduction in mouse VNO. We conclude that InsP₃R3 is not required for gating of Trpc2 in VSNs.

The rat vomeronasal organ is a vitamin D target

We studied the expression of vitamin D receptor and of vitamin D binding protein in the rat vomeronasal organ. With immunofluorescence, in situ hybridization and with reverse transcriptase PCR we found both proteins in sensory as well as in non-sensory cells. Sensory neurons contained immunoreactivity for vitamin D3 receptor in nuclei, in portions of the cytoplasm, and in apical dendrites and their microvilli. Vitamin D binding protein was observed in sensory neuron axons and cytoplasm, mostly confined to dendrites. Colocalization appeared in the contact zone of supporting cells and sensory dendrites. Both proteins were also found in single ciliated cells within the non-sensory epithelium. Vitamin D binding protein was also localized in secretory vesicles in a portion of the vomeronasal glands. Our findings suggest that the rat vomeronasal organ is a vitamin D target.

Imorin: a sexual attractiveness pheromone in female red-bellied newts (Cynops pyrrhogaster)

The male red-bellied newt (Cynops pyrrhogaster) approaches the female’s cloaca prior to performing any courtship behaviour, as if he is using some released substance to gauge whether she is sexually receptive. Therefore, we investigated whether such a female sexual attractiveness pheromone exists. We found that a tripeptide with amino acid sequence Ala-Glu-Phe is secreted by the ciliary cells in the epithelium of the proximal portion of the oviduct of sexually developed newts and confirmed that this is the major active substance in water in which sexually developed female newts have been kept. This substance only attracted sexually developed male newts and acted by stimulating the vomeronasal epithelial cells. This is the first female sexual attractiveness peptide pheromone to be identified in a vertebrate.

Olfactory Receptors in Non-Chemosensory Organs: The Nervous System in Health and Disease

Olfactory receptors (ORs) and down-stream functional signaling molecules adenylyl cyclase 3 (AC3), olfactory G protein α subunit (Gαolf), OR transporters receptor transporter proteins 1 and 2 (RTP1 and RTP2), receptor expression enhancing protein 1 (REEP1), and UDP-glucuronosyltransferases (UGTs) are expressed in neurons of the human and murine central nervous system (CNS). In vitro studies have shown that these receptors react to external stimuli and therefore are equipped to be functional. However, ORs are not directly related to the detection of odors. Several molecules delivered from the blood, cerebrospinal fluid, neighboring local neurons and glial cells, distant cells through the extracellular space, and the cells’ own self-regulating internal homeostasis can be postulated as possible ligands. Moreover, a single neuron outside the olfactory epithelium expresses more than one receptor, and the mechanism of transcriptional regulation may be different in olfactory epithelia and brain neurons. OR gene expression is altered in several neurodegenerative diseases including Parkinson’s disease (PD), Alzheimer’s disease (AD), progressive supranuclear palsy (PSP) and sporadic Creutzfeldt-Jakob disease (sCJD) subtypes MM1 and VV2 with disease-, region- and subtype-specific patterns. Altered gene expression is also observed in the prefrontal cortex in schizophrenia with a major but not total influence of chlorpromazine treatment. Preliminary parallel observations have also shown the presence of taste receptors (TASRs), mainly of the bitter taste family, in the mammalian brain, whose function is not related to taste. TASRs in brain are also abnormally regulated in neurodegenerative diseases. These seminal observations point to the need for further studies on ORs and TASRs chemoreceptors in the mammalian brain.

Elevated Cytosolic Cl- Concentrations in Dendritic Knobs of Mouse Vomeronasal Sensory Neurons

In rodents, the vomeronasal system controls social and sexual behavior. However, several mechanistic aspects of sensory signaling in the vomeronasal organ remain unclear. Here, we investigate the biophysical basis of a recently proposed vomeronasal signal transduction component-a Ca(2+)-activated Cl(-) current. As the physiological role of such a current is a direct function of the Cl(-) equilibrium potential, we determined the intracellular Cl(-) concentration in dendritic knobs of vomeronasal neurons. Quantitative fluorescence lifetime imaging of a Cl(-)-sensitive dye at the apical surface of the intact vomeronasal neuroepithelium revealed increased cytosolic Cl(-) levels in dendritic knobs, a substantially lower Cl(-) concentration in vomeronasal sustentacular cells, and an apparent Cl(-) gradient in vomeronasal neurons along their dendritic apicobasal axis. Together, our data provide a biophysical basis for sensory signal amplification in vomeronasal neuron microvilli by opening Ca(2+)-activated Cl(-) channels.

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.

Prolonged Intracellular Na+ Dynamics Govern Electrical Activity in Accessory Olfactory Bulb Mitral Cells

Persistent activity has been reported in many brain areas and is hypothesized to mediate working memory and emotional brain states and to rely upon network or biophysical feedback. Here, we demonstrate a novel mechanism by which persistent neuronal activity can be generated without feedback, relying instead on the slow removal of Na⁺ from neurons following bursts of activity. We show that mitral cells in the accessory olfactory bulb (AOB), which plays a major role in mammalian social behavior, may respond to a brief sensory stimulation with persistent firing. By combining electrical recordings, Ca²⁺ and Na⁺ imaging, and realistic computational modeling, we explored the mechanisms underlying the persistent activity in AOB mitral cells. We found that the exceptionally slow inward current that underlies this activity is governed by prolonged dynamics of intracellular Na⁺ ([Na⁺]_i), which affects neuronal electrical activity via several pathways. Specifically, elevated dendritic [Na⁺]_i reverses the Na⁺-Ca²⁺ exchanger activity, thus modifying the [Ca²⁺]_i set-point. This process, which relies on ubiquitous membrane mechanisms, is likely to play a role in other neuronal types in various brain regions.

An experimental and computational study reveals a novel mechanism for persistent activity of neurons in response to transient stimulation. Instead of involving feedback mechanisms, it relies on slow changes in intracellular sodium ion concentration, leading to prolonged calcium-dependent inward current.

The accessory olfactory system is essential for chemical communication in animals during social interactions. During this process, the principle cells of the accessory olfactory bulb (AOB) may respond to transient stimulation with prolonged activity, sometimes lasting for minutes—a property known as persistent activity. This property, which has been observed in other brain areas, is usually attributed to positive feedback mechanisms either at the cellular or the network level. Here, we show how persistent activity can emerge without feedback, relying on slow changes in internal ionic concentrations, which keep a record of past neuronal activity for long periods of time. We used a combined computational and experimental approach to show that the complex interaction between various ions, their extrusion mechanisms, and the membrane potential leads to stimulus-dependent persistent activity in the AOB. The same mechanism may apply to other neuronal types in various brain regions.

Intracellular chloride concentration of the mouse vomeronasal neuron

Background

The vomeronasal organ (VNO) is specialized in detecting pheromone and heterospecific cues in the environment. Recent studies demonstrate the involvement of multiple ion channels in VNO signal transduction, including the calcium-activated chloride channels (CACCs). Opening of CACCs appears to result in activation of VNO neuron through outflow of Cl⁻ ions. However, the intracellular Cl⁻ concentration remains undetermined.

Results

We used the chloride ion quenching dye, MQAE, to measure the intracellular Cl⁻ concentration of VNO neuron in live VNO slices. The resting Cl⁻ concentration in the VNO neurons is measured at 84.73 mM. Urine activation of the VNO neurons causes a drop in Cl⁻ concentration, consistent with the notion of an efflux of Cl⁻ to depolarize the cells. Similar observation is made for VNO neurons from mice with deletion of the transient receptor potential canonical channel 2 (TRPC2), which have a resting Cl⁻ concentrations at 81 mM.

Conclusions

The VNO neurons rest at high intracellular Cl⁻ concentration, which can lead to depolarization of the cell when chloride channels open. These results also provide additional support of TRPC2-independent pathway of VNO activation.

Neural map formation and sensory coding in the vomeronasal system

Sensory systems enable us to encode a clear representation of our environment in the nervous system by spatially organizing sensory stimuli being received. The organization of neural circuitry to form a map of sensory activation is critical for the interpretation of these sensory stimuli. In rodents, social communication relies strongly on the detection of chemosignals by the vomeronasal system, which regulates a wide array of behaviours, including mate recognition, reproduction, and aggression. The binding of these chemosignals to receptors on vomeronasal sensory neurons leads to activation of second-order neurons within glomeruli of the accessory olfactory bulb. Here, vomeronasal receptor activation by a stimulus is organized into maps of glomerular activation that represent phenotypic qualities of the stimuli detected. Genetic, electrophysiological and imaging studies have shed light on the principles underlying cell connectivity and sensory map formation in the vomeronasal system, and have revealed important differences in sensory coding between the vomeronasal and main olfactory system. In this review, we summarize the key factors and mechanisms that dictate circuit formation and sensory coding logic in the vomeronasal system, emphasizing differences with the main olfactory system. Furthermore, we discuss how detection of chemosignals by the vomeronasal system regulates social behaviour in mice, specifically aggression.

Brca1 Mutations Enhance Mouse Reproductive Functions by Increasing Responsiveness to Male-Derived Scent

We compared the gene expression profiles of ovarian granulosa cells harboring either mutant or wild type Brca1 to follow up on our earlier observation that absence of a functional Brca1 in these important regulators of menstrual/estrous cycle progression leads to prolongation of the pre-ovulatory phase of the estrous cycle and to increased basal levels of circulating estradiol. Here we show that ovarian granulosa cells from mice carrying a conditional Brca1 gene knockout express substantially higher levels of olfactory receptor mRNA than granulosa cells from wild type littermates. This led us to hypothesize that reproductive functions in mutant female mice might be more sensitive to male-derived scent than in wild type female mice. Indeed, it is well established that isolation from males leads to complete cessation of mouse estrous cycle activity while exposure to olfactory receptor ligands present in male urine leads to resumption of such activity. We found that Brca1-/- female mice rendered anovulatory by unisexual isolation resumed ovulatory activity more rapidly than their wild type littermates when exposed to bedding from cages where males had been housed. The prime mediator of this increased responsiveness appears to be the ovary and not olfactory neurons. This conclusion is supported by the fact that wild type mice in which endogenous ovaries had been replaced by Brca1-deficient ovarian transplants responded to male-derived scent more robustly than mutant mice in which ovaries had been replaced by wild type ovarian transplants. Our findings not only have important implications for our understanding of the influence of olfactory signals on reproductive functions, but also provide insights into mechanisms whereby genetic risk factors for breast and extra uterine Müllerian carcinomas may influence menstrual activity in human, which is itself an independent risk factor for these cancers.

A Molecular Code for Identity in the Vomeronasal System

In social interactions among mammals, individuals are recognized by olfactory cues, but identifying the key signals among thousands of compounds remains a major challenge. To address this need, we developed a new technique, component-activity matching (CAM), to select candidate ligands that "explain" patterns of bioactivity across diverse complex mixtures. Using mouse urine from eight different sexes and strains, we identified 23 components to explain firing rates in seven of eight functional classes of vomeronasal sensory neurons. Focusing on a class of neurons selective for females, we identified a novel family of vomeronasal ligands, steroid carboxylic acids. These ligands accounted for much of the neuronal activity of urine from some female strains, were necessary for normal levels of male investigatory behavior of female scents, and were sufficient to trigger mounting behavior. CAM represents the first step toward an exhaustive characterization of the molecular cues for natural behavior in a mammalian olfactory system.

Oxytocin Mediates Entrainment of Sensory Stimuli to Social Cues of Opposing Valence

Meaningful social interactions modify behavioral responses to sensory stimuli. The neural mechanisms underlying the entrainment of neutral sensory stimuli to salient social cues to produce social learning remain unknown. We used odor-driven behavioral paradigms to ask if oxytocin, a neuropeptide implicated in various social behaviors, plays a crucial role in the formation of learned associations between odor and socially significant cues. Through genetic, optogenetic, and pharmacological manipulations, we show that oxytocin receptor signaling is crucial for entrainment of odor to social cues but is dispensable for entrainment to nonsocial cues. Furthermore, we demonstrate that oxytocin directly impacts the piriform, the olfactory sensory cortex, to mediate social learning. Lastly, we provide evidence that oxytocin plays a role in both appetitive and aversive social learning. These results suggest that oxytocin conveys saliency of social stimuli to sensory representations in the piriform cortex during odor-driven social learning.

Tuning properties and dynamic range of type 1 vomeronasal receptors

The mouse vomeronasal organ (VNO) expresses chemosensory receptors that detect intra-species as well as inter-species cues. The vomeronasal neurons are thought to be highly selective in their responses. The tuning properties of individual receptors remain difficult to characterize due to the lack of a robust heterologous expression system. Here, we take a transgenic approach to ectopically express two type 1 vomeronasal receptors in the mouse VNO and characterize their responses to steroid compounds. We find that V1rj2 and V1rj3 are sensitive to two sulfated estrogens (SEs) and can be activated by a broad variety of sulfated and glucuronidated steroids at high concentrations. Individual neurons exhibit narrow range of concentration-dependent activation. Collectively, a neuronal population expressing the same receptor covers a wide dynamic range in their responses to SEs. These properties recapitulate the response profiles of endogenous neurons to SEs.

G-Protein Coupled Receptor Kinase 2 Minimally Regulates Melanopsin Activity in Intrinsically Photosensitive Retinal Ganglion Cells

Phosphorylation is a primary modulator of mammalian G-protein coupled receptor (GPCR) activity. The GPCR melanopsin is the photopigment of intrinsically photosensitive retinal ganglion cells (ipRGCs) in the mammalian retina. Recent evidence from in vitro experiments suggests that the G-protein coupled receptor kinase 2 (GRK2) phosphorylates melanopsin and reduces its activity following light exposure. Using an ipRGC-specific GRK2 loss-of-function mouse, we show that GRK2 loss alters melanopsin response dynamics and termination time in postnatal day 8 (P8) ipRGCs but not in older animals. However, the alterations are small in comparison to the changes reported for other opsins with loss of their cognate GRK. These results suggest GRK2 contributes to melanopsin deactivation, but that other mechanisms account for most of modulation of melanopsin activity in ipRGCs.

Structure and function of a peptide pheromone family that stimulate the vomeronasal sensory system in mice

Mammals use pheromones to communicate with other animals of the same species. In mice, the VNO (vomeronasal organ) has a pivotal role in pheromone detection. We discovered a 7 kDa peptide, ESP1 (exocrine-gland-secreting peptide 1), in tear fluids from male mice that enhances the sexual behaviour of female mice via the VNO. NMR studies demonstrate that ESP1 adopts a compact structure with a helical fold stabilized by an intramolecular disulfide bridge. Functional analysis in combination with docking simulation indicates that ESP1 is recognized by a specific G-protein-coupled vomeronasal receptor, V2Rp5, via charge-charge interactions in the large extracellular region of the receptor. ESP1 is a member of the ESP family, which comprises 38 homologous genes in mice, and some of these genes are expressed in a sex- or age-dependent manner. Most recently, ESP22 was found to be released specifically in juvenile tear fluids and to inhibit the sexual behaviour of adult male mice. These studies demonstrate that peptide pheromones are used for chemical communication in mice, and they indicate a structural basis for the narrowly tuned perception of mammalian peptide pheromones by vomeronasal receptors.

Extracellular pH regulates excitability of vomeronasal sensory neurons

The mouse vomeronasal organ (VNO) plays a critical role in semiochemical detection and social communication. Vomeronasal stimuli are typically secreted in various body fluids. Following direct contact with urine deposits or other secretions, a peristaltic vascular pump mediates fluid entry into the recipient's VNO. Therefore, while vomeronasal sensory neurons (VSNs) sample various stimulatory semiochemicals dissolved in the intraluminal mucus, they might also be affected by the general physicochemical properties of the "solvent." Here, we report cycle stage-correlated variations in urinary pH among female mice. Estrus-specific pH decline is observed exclusively in urine samples from sexually experienced females. Moreover, patch-clamp recordings in acute VNO slices reveal that mouse VSNs reliably detect extracellular acidosis. Acid-evoked responses share the biophysical and pharmacological hallmarks of the hyperpolarization-activated current Ih. Mechanistically, VSN acid sensitivity depends on a pH-induced shift in the voltage-dependence of Ih activation that causes the opening of HCN channels at rest, thereby increasing VSN excitability. Together, our results identify extracellular acidification as a potent activator of vomeronasal Ih and suggest HCN channel-dependent vomeronasal gain control of social chemosignaling. Our data thus reveal a potential mechanistic basis for stimulus pH detection in rodent chemosensory communication.

Conditional knockout of TMEM16A/anoctamin1 abolishes the calcium-activated chloride current in mouse vomeronasal sensory neurons

TMEM16A is an essential component of Ca²⁺-activated Cl⁻ currents in mouse vomeronasal sensory neurons.

Pheromones are substances released from animals that, when detected by the vomeronasal organ of other individuals of the same species, affect their physiology and behavior. Pheromone binding to receptors on microvilli on the dendritic knobs of vomeronasal sensory neurons activates a second messenger cascade to produce an increase in intracellular Ca²⁺ concentration. Here, we used whole-cell and inside-out patch-clamp analysis to provide a functional characterization of currents activated by Ca²⁺ in isolated mouse vomeronasal sensory neurons in the absence of intracellular K⁺. In whole-cell recordings, the average current in 1.5 µM Ca²⁺ and symmetrical Cl⁻ was −382 pA at −100 mV. Ion substitution experiments and partial blockade by commonly used Cl⁻ channel blockers indicated that Ca²⁺ activates mainly anionic currents in these neurons. Recordings from inside-out patches from dendritic knobs of mouse vomeronasal sensory neurons confirmed the presence of Ca²⁺-activated Cl⁻ channels in the knobs and/or microvilli. We compared the electrophysiological properties of the native currents with those mediated by heterologously expressed TMEM16A/anoctamin1 or TMEM16B/anoctamin2 Ca²⁺-activated Cl⁻ channels, which are coexpressed in microvilli of mouse vomeronasal sensory neurons, and found a closer resemblance to those of TMEM16A. We used the Cre–loxP system to selectively knock out TMEM16A in cells expressing the olfactory marker protein, which is found in mature vomeronasal sensory neurons. Immunohistochemistry confirmed the specific ablation of TMEM16A in vomeronasal neurons. Ca²⁺-activated currents were abolished in vomeronasal sensory neurons of TMEM16A conditional knockout mice, demonstrating that TMEM16A is an essential component of Ca²⁺-activated Cl⁻ currents in mouse vomeronasal sensory neurons.