A multireceptor genetic approach uncovers an ordered integration of VNO sensory inputs in the accessory olfactory bulb

Biological complexity and adaptability of simple mammalian olfactory memory systems

Chemosensory systems play vital roles in the lives of most mammals, including the detection and identification of predators, as well as sex and reproductive status and the identification of individual conspecifics. All of these capabilities require a process of recognition involving a combination of innate (kairomonal/pheromonal) and learned responses. Across very different phylogenies, the mechanisms for pheromonal and odour learning have much in common. They are frequently associated with plasticity of GABA-ergic feedback at the initial level of processing the chemosensory information, which enhances its pattern separation capability. Association of odourant features into an odour object primarily involves anterior piriform cortex for non-social odours. However, the medial amygdala appears to be involved in both the recognition of social odours and their association with chemosensory information sensed by the vomeronasal system. Unusually not only the sensory neurons themselves, but also the GABA-ergic interneurons in the olfactory bulb are continually being replaced, with implications for the induction and maintenance of learned chemosensory responses.

Integrated action of pheromone signals in promoting courtship behavior in male mice

The mammalian vomeronasal organ encodes pheromone information about gender, reproductive status, genetic background and individual differences. It remains unknown how pheromone information interacts to trigger innate behaviors. In this study, we identify vomeronasal receptors responsible for detecting female pheromones. A sub-group of V1re clade members recognizes gender-identifying cues in female urine. Multiple members of the V1rj clade are cognate receptors for urinary estrus signals, as well as for sulfated estrogen (SE) compounds. In both cases, the same cue activates multiple homologous receptors, suggesting redundancy in encoding female pheromone cues. Neither gender-specific cues nor SEs alone are sufficient to promote courtship behavior in male mice, whereas robust courtship behavior can be induced when the two cues are applied together. Thus, integrated action of different female cues is required in pheromone-triggered mating behavior. These results suggest a gating mechanism in the vomeronasal circuit in promoting specific innate behavior.DOI: http://dx.doi.org/10.7554/eLife.03025.001.

Anatomy, histochemistry, and immunohistochemistry of the olfactory subsystems in mice

The four regions of the murine nasal cavity featuring olfactory neurons were studied anatomically and by labeling with lectins and relevant antibodies with a view to establishing criteria for the identification of olfactory subsystems that are readily applicable to other mammals. In the main olfactory epithelium and the septal organ the olfactory sensory neurons (OSNs) are embedded in quasi-stratified columnar epithelium; vomeronasal OSNs are embedded in epithelium lining the medial interior wall of the vomeronasal duct and do not make contact with the mucosa of the main nasal cavity; and in Grüneberg's ganglion a small isolated population of OSNs lies adjacent to, but not within, the epithelium. With the exception of Grüneberg's ganglion, all the tissues expressing olfactory marker protein (OMP) (the above four nasal territories, the vomeronasal and main olfactory nerves, and the main and accessory olfactory bulbs) are also labeled by Lycopersicum esculentum agglutinin, while Ulex europaeus agglutinin I labels all and only tissues expressing G_αi2 (the apical sensory neurons of the vomeronasal organ, their axons, and their glomerular destinations in the anterior accessory olfactory bulb). These staining patterns of UEA-I and LEA may facilitate the characterization of olfactory anatomy in other species. A 710-section atlas of the anatomy of the murine nasal cavity has been made available on line.

Murine pheromone proteins constitute a context-dependent combinatorial code governing multiple social behaviors

During social interactions, an individual's behavior is largely governed by the subset of signals emitted by others. Discrimination of "self" from "other" regulates the territorial urine countermarking behavior of mice. To identify the cues for this social discrimination and understand how they are interpreted, we designed an olfactory-dependent countermarking assay. We find major urinary proteins (MUPs) sufficient to elicit countermarking, and unlike other vomeronasal ligands that are detected by specifically tuned sensory neurons, MUPs are detected by a combinatorial strategy. A chemosensory signature of "self" that modulates behavior is developed via experience through exposure to a repertoire of MUPs. In contrast, aggression can be elicited by MUPs in an experience-independent but context-dependent manner. These findings reveal that individually emitted chemical cues can be interpreted based on their combinatorial permutation and relative ratios, and they can transmit both fixed and learned information to promote multiple behaviors.

Sex-specific processing of social cues in the medial amygdala

Animal–animal recognition within, and across species, is essential for predator avoidance and social interactions. Despite its essential role in orchestrating responses to animal cues, basic principles of information processing by the vomeronasal system are still unknown. The medial amygdala (MeA) occupies a central position in the vomeronasal pathway, upstream of hypothalamic centers dedicated to defensive and social responses. We have characterized sensory responses in the mouse MeA and uncovered emergent properties that shed new light onto the transformation of vomeronasal information into sex- and species-specific responses. In particular, we show that the MeA displays a degree of stimulus selectivity and a striking sexually dimorphic sensory representation that are not observed in the upstream relay of the accessory olfactory bulb (AOB). Furthermore, our results demonstrate that the development of sexually dimorphic circuits in the MeA requires steroid signaling near the time of puberty to organize the functional representation of sensory stimuli.

DOI:http://dx.doi.org/10.7554/eLife.02743.001

Many animals emit and detect chemicals known as pheromones to communicate with other members of their own species. Animals also rely on chemical signals from other species to warn them, for example, that a predator is nearby. Many of these chemical signals—which are present in sweat, tears, urine, and saliva—are detected by a structure called the vomeronasal organ, which is located at the base of the nasal cavity.

When this organ detects a particular chemical signal, it broadcasts this information to a network of brain regions that generates an appropriate behavioral response. Two structures within this network, the accessory olfactory bulb and the medial amygdala, play an important role in modifying this signal before it reaches its final destination—a region of the brain called the hypothalamus. Activation of the hypothalamus by the signal triggers changes in the animal's behavior. Although the anatomical details of this pathway have been widely studied, it is not clear how information is actually transmitted along it.

Now, Bergan et al. have provided insights into this process by recording signals in the brains of anesthetized mice exposed to specific stimuli. Whereas neurons in the accessory olfactory bulb responded similarly in male and female mice, those in the medial amygdala showed a preference for female urine in male mice, and a preference for male urine in the case of females. This is the first direct demonstration of differences in sensory processing in the brains of male and female mammals.

These differences are thought to result from the actions of sex hormones, particularly estrogen, on brain circuits during development. Consistent with this, neurons in the medial amygdala of male mice with reduced levels of estrogen showed a reduced preference for female urine compared to control males. Similarly, female mice that had been previously exposed to high levels of estrogen as pups showed a reduced preference for male urine compared to controls.

In addition to increasing understanding of how chemical signals—including pheromones—influence the responses of rodents to other animals, the work of Bergan et al. has provided clues to the neural mechanisms that underlie sex-specific differences in behaviors.

DOI:http://dx.doi.org/10.7554/eLife.02743.002

Sex-specific processing of social cues in the medial amygdala

Animal-animal recognition within, and across species, is essential for predator avoidance and social interactions. Despite its essential role in orchestrating responses to animal cues, basic principles of information processing by the vomeronasal system are still unknown. The medial amygdala (MeA) occupies a central position in the vomeronasal pathway, upstream of hypothalamic centers dedicated to defensive and social responses. We have characterized sensory responses in the mouse MeA and uncovered emergent properties that shed new light onto the transformation of vomeronasal information into sex- and species-specific responses. In particular, we show that the MeA displays a degree of stimulus selectivity and a striking sexually dimorphic sensory representation that are not observed in the upstream relay of the accessory olfactory bulb (AOB). Furthermore, our results demonstrate that the development of sexually dimorphic circuits in the MeA requires steroid signaling near the time of puberty to organize the functional representation of sensory stimuli.DOI: http://dx.doi.org/10.7554/eLife.02743.001.

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.

Kappe neurons, a novel population of olfactory sensory neurons

Perception of olfactory stimuli is mediated by distinct populations of olfactory sensory neurons, each with a characteristic set of morphological as well as functional parameters. Beyond two large populations of ciliated and microvillous neurons, a third population, crypt neurons, has been identified in teleost and cartilaginous fishes. We report here a novel, fourth olfactory sensory neuron population in zebrafish, which we named kappe neurons for their characteristic shape. Kappe neurons are identified by their G_o-like immunoreactivity, and show a distinct spatial distribution within the olfactory epithelium, similar to, but significantly different from that of crypt neurons. Furthermore, kappe neurons project to a single identified target glomerulus within the olfactory bulb, mdg5 of the mediodorsal cluster, whereas crypt neurons are known to project exclusively to the mdg2 glomerulus. Kappe neurons are negative for established markers of ciliated, microvillous and crypt neurons, but appear to have microvilli. Kappe neurons constitute the fourth type of olfactory sensory neurons reported in teleost fishes and their existence suggests that encoding of olfactory stimuli may require a higher complexity than hitherto assumed already in the peripheral olfactory system.

Olfactory wiring logic in amphibians challenges the basic assumptions of the unbranched axon concept

Olfactory receptor neurons extend axons into the olfactory bulb, where they face the challenge to integrate into existing circuitry. The consensus view is that in vertebrates individual receptor neurons project unbranched axons into one specific glomerulus of the olfactory bulb. We report here that, strikingly different from the generally assumed wiring principle in vertebrate olfactory systems, axons of single receptor neurons of Xenopus laevis regularly bifurcate and project into more than one glomerulus. Specifically, the innervation of multiple glomeruli is present in all ontogenetic stages of this species, from the larva to the postmetamorphic frog. Also, we show that this unexpected wiring pattern is not restricted to axons of immature receptor neurons, but that it is also a feature of mature neurons of both the main and accessory olfactory system. This glomerular innervation pattern is unique among vertebrates investigated so far and represents a new olfactory wiring strategy.

Reliable sex and strain discrimination in the mouse vomeronasal organ and accessory olfactory bulb

Animals modulate their courtship and territorial behaviors in response to olfactory cues produced by other animals. In rodents, detecting these cues is the primary role of the accessory olfactory system (AOS). We sought to systematically investigate the natural stimulus coding logic and robustness in neurons of the first two stages of accessory olfactory processing, the vomeronasal organ (VNO) and accessory olfactory bulb (AOB). We show that firing rate responses of just a few well-chosen mouse VNO or AOB neurons can be used to reliably encode both sex and strain of other mice from cues contained in urine. Additionally, we show that this population code can generalize to new concentrations of stimuli and appears to represent stimulus identity in terms of diverging paths in coding space. Together, the results indicate that firing rate code on the temporal order of seconds is sufficient for accurate classification of pheromonal patterns at different concentrations and may be used by AOS neural circuitry to discriminate among naturally occurring urine stimuli.

Robust encoding of stimulus identity and concentration in the accessory olfactory system

Sensory systems represent stimulus identity and intensity, but in the neural periphery these two variables are typically intertwined. Moreover, stable detection may be complicated by environmental uncertainty; stimulus properties can differ over time and circumstance in ways that are not necessarily biologically relevant. We explored these issues in the context of the mouse accessory olfactory system, which specializes in detection of chemical social cues and infers myriad aspects of the identity and physiological state of conspecifics from complex mixtures, such as urine. Using mixtures of sulfated steroids, key constituents of urine, we found that spiking responses of individual vomeronasal sensory neurons encode both individual compounds and mixtures in a manner consistent with a simple model of receptor-ligand interactions. Although typical neurons did not accurately encode concentration over a large dynamic range, from population activity it was possible to reliably estimate the log-concentration of pure compounds over several orders of magnitude. For binary mixtures, simple models failed to accurately segment the individual components, largely because of the prevalence of neurons responsive to both components. By accounting for such overlaps during model tuning, we show that, from neuronal firing, one can accurately estimate log-concentration of both components, even when tested across widely varying concentrations. With this foundation, the difference of logarithms, log A - log B = log A/B, provides a natural mechanism to accurately estimate concentration ratios. Thus, we show that a biophysically plausible circuit model can reconstruct concentration ratios from observed neuronal firing, representing a powerful mechanism to separate stimulus identity from absolute concentration.

Zebrafish crypt neurons project to a single, identified mediodorsal glomerulus

Crypt neurons are a third type of olfactory receptor neurons with a highly unusual "one cell type--one receptor" mode of expression, the same receptor being expressed by the entire population of crypt neurons. Attempts to identify the target region(s) of crypt neurons have been inconclusive so far. We report that TrkA-like immunoreactivity specifically labeled somata, axons, and terminals of zebrafish crypt neurons and reveal a single glomerulus, mdg2 of the dorsomedial group, as target glomerulus of crypt neurons. Injection of a fluorescent tracing dye into the mdg2 glomerulus retrogradely labeled mostly crypt neurons, as assessed by quantitative morphometry, whereas no crypt neurons were found after injections in neighboring glomeruli. Our data provide strong evidence that crypt neurons converge onto a single glomerulus, and thus form a labeled line consisting of a single sensory cell type, a single olfactory receptor and a single target glomerulus.

Mammalian pheromones

Mammalian pheromones control a myriad of innate social behaviors and acutely regulate hormone levels. Responses to pheromones are highly robust, reproducible, and stereotyped and likely involve developmentally predetermined neural circuits. Here, I review several facets of pheromone transduction in mammals, including (a) chemosensory receptors and signaling components of the main olfactory epithelium and vomeronasal organ involved in pheromone detection; (b) pheromone-activated neural circuits subject to sex-specific and state-dependent modulation; and (c) the striking chemical diversity of mammalian pheromones, which range from small, volatile molecules and sulfated steroids to large families of proteins. Finally, I review (d) molecular mechanisms underlying various behavioral and endocrine responses, including modulation of puberty and estrous; control of reproduction, aggression, suckling, and parental behaviors; individual recognition; and distinguishing of own species from predators, competitors, and prey. Deconstruction of pheromone transduction mechanisms provides a critical foundation for understanding how odor response pathways generate instinctive behaviors.

Convergence of FPR-rs3-expressing neurons in the mouse accessory olfactory bulb

In the mouse, most members of the FPR receptor family are expressed by vomeronasal sensory neurons. The neural circuitry corresponding to this class of chemical sensors is unknown. Taking advantage of the presence of FPR-rs3 on both vomeronasal dendrites and axonal fibers, we visualized the distribution of sensory cells expressing this member of the FPR family, and their corresponding axonal projections in the olfactory bulb. We found a rostrocaudal gradient of receptor choice frequency in the vomeronasal sensory neuroepithelium, and observed a convergence of FPR-rs3 axons into multiple, linked and deeply located glomeruli. These were homogenously innervated, and spatially restricted to the basal portion of the rostral accessory olfactory bulb. This organization, reminiscent of the one that characterizes axonal projections of V1R-expressing neurons, supports a role played by these receptors in the perception of semiochemicals.

Kirrel3 is required for the coalescence of vomeronasal sensory neuron axons into glomeruli and for male-male aggression

The accessory olfactory system controls social and sexual interactions in mice that are crucial for survival. Vomeronasal sensory neurons (VSNs) form synapses with dendrites of second order neurons in glomeruli of the accessory olfactory bulb (AOB). Axons of VSNs expressing the same vomeronasal receptor coalesce into multiple glomeruli within spatially conserved regions of the AOB. Here we examine the role of the Kirrel family of transmembrane proteins in the coalescence of VSN axons within the AOB. We find that Kirrel2 and Kirrel3 are differentially expressed in subpopulations of VSNs and that their expression is regulated by activity. Although Kirrel3 expression is not required for early axonal guidance events, such as fasciculation of the vomeronasal tract and segregation of apical and basal VSN axons in the AOB, it is necessary for proper coalescence of axons into glomeruli. Ablation of Kirrel3 expression results in disorganization of the glomerular layer of the posterior AOB and formation of fewer, larger glomeruli. Furthermore, Kirrel3(-/-) mice display a loss of male-male aggression in a resident-intruder assay. Taken together, our results indicate that differential expression of Kirrels on vomeronasal axons generates a molecular code that dictates their proper coalescence into glomeruli within the AOB.

Three-dimensional reconstruction of electron micrographs reveals intrabulbar circuit differences between accessory and main olfactory bulbs

Three-dimensional (3D) reconstruction of synaptic arrangement on a particular dendrite provides essential information regarding neuronal properties and neural microcircuits. Unconventional synapses are particularly good candidates for such steric attribution. In main and accessory olfactory bulbs (MOBs and AOBs), there are dendrodendritic reciprocal synapses (RSs) between excitatory projection neurons and inhibitory interneurons. Although the fine structure and configuration of these synapses have been investigated in MOB, their characteristics in AOB were unknown. In this study, we performed 3D AOB reconstruction using serial section transmission electron microscopy. We found numerous RSs on primary dendrites from glomeruli to mitral/tufted (MT) cell somas. These synapses formed between dendritic shafts of MT cells and large dendritic spines, or so-called gemmules, of granule (Gr) cells. This indicates that chemical signals received by a glomerulus are regulated in the primary dendrite of an MT cell before reaching its soma. In MOB, RSs are located on secondary dendrites and act as lateral and self-inhibiting following mitral cell depolarization. Our results indicate that AOB intrabulbar microcircuitry is quite different from that in the MOB.

Dual origins of the mammalian accessory olfactory bulb revealed by an evolutionarily conserved migratory stream

The accessory olfactory bulb (AOB) is a critical olfactory structure that has been implicated in mediating social behavior. It receives input from the vomeronasal organ and projects to targets in the amygdaloid complex. Its anterior and posterior components (aAOB and pAOB) display molecular, connectional and functional segregation in processing reproductive and defensive and aggressive behaviors, respectively. We observed a dichotomy in the development of the projection neurons of the aAOB and pAOB in mice. We found that they had distinct sites of origin and that different regulatory molecules were required for their specification and migration. aAOB neurons arose locally in the rostral telencephalon, similar to main olfactory bulb neurons. In contrast, pAOB neurons arose caudally, from the neuroepithelium of the diencephalic-telencephalic boundary, from which they migrated rostrally to reach their destination. This unusual origin and migration is conserved in Xenopus, providing an insight into the origin of a key component of this system in evolution.

Mutual influences between the main olfactory and vomeronasal systems in development and evolution

The sense of smell plays a crucial role in the sensory world of animals. Two chemosensory systems have been traditionally thought to play-independent roles in mammalian olfaction. According to this, the main olfactory system (MOS) specializes in the detection of environmental odorants, while the vomeronasal system (VNS) senses pheromones and semiochemicals produced by individuals of the same or different species. Although both systems differ in their anatomy and function, recent evidence suggests they act synergistically in the perception of scents. These interactions include similar responses to some ligands, overlap of telencephalic connections and mutual influences in the regulation of olfactory-guided behavior. In the present work, we propose the idea that the relationships between systems observed at the organismic level result from a constant interaction during development and reflects a common history of ecological adaptations in evolution. We review the literature to illustrate examples of developmental and evolutionary processes that evidence these interactions and propose that future research integrating both systems may shed new light on the mechanisms of olfaction.

One nose, one brain: contribution of the main and accessory olfactory system to chemosensation

The accessory olfactory system is present in most tetrapods. It is involved in the perception of chemical stimuli, being implicated also in the detection of pheromones. However, it is sensitive also to some common odorant molecules, which have no clear implication in intraspecific chemical communication. The accessory olfactory system may complement the main olfactory system and may contribute different perceptual features to the construction of a unitary representation, which merges the different chemosensory qualities. Crosstalk between the main and accessory olfactory systems occurs at different levels of central processing, in brain areas where the inputs from the two systems converge. Interestingly, centrifugal projections from more caudal brain areas are deeply involved in modulating both main and accessory sensory processing. A high degree of interaction between the two systems may be conceived and partial overlapping appears to occur in many functions. Therefore, the central chemosensory projections merge inputs from different organs to obtain a complex chemosensory picture.

β3GnT2 null mice exhibit defective accessory olfactory bulb innervation

Vomeronasal sensory neurons (VSNs) extend axons to the accessory olfactory bulb (AOB) where they form synaptic connections that relay pheromone signals to the brain. The projections of apical and basal VSNs segregate in the AOB into anterior (aAOB) and posterior (pAOB) compartments. Although some aspects of this organization exhibit fundamental similarities with the main olfactory system, the mechanisms that regulate mammalian vomeronasal targeting are not as well understood. In the olfactory epithelium (OE), the glycosyltransferase β3GnT2 maintains expression of axon guidance cues required for proper glomerular positioning and neuronal survival. We show here that β3GnT2 also regulates guidance and adhesion molecule expression in the vomeronasal system in ways that are partially distinct from the OE. In wildtype mice, ephrinA5(+) axons project to stereotypic subdomains in both the aAOB and pAOB compartments. This pattern is dramatically altered in β3GnT2(-/-) mice, where ephrinA5 is upregulated exclusively on aAOB axons. Despite this, apical and basal VSN projections remain strictly segregated in the null AOB, although some V2r1b axons that normally project to the pAOB inappropriately innervate the anterior compartment. These fibers appear to arise from ectopic expression of V2r1b receptors in a subset of apical VSNs. The homotypic adhesion molecules Kirrel2 and OCAM that facilitate axon segregation and glomerular compartmentalization in the main olfactory bulb are ablated in the β3GnT2(-/-) aAOB. This loss is accompanied by a two-fold increase in the total number of V2r1b glomeruli and a failure to form morphologically distinct glomeruli in the anterior compartment. These results identify a novel function for β3GnT2 glycosylation in maintaining expression of layer-specific vomeronasal receptors, as well as adhesion molecules required for proper AOB glomerular formation.

Calcium-activated sustained firing responses distinguish accessory from main olfactory bulb mitral cells

Many mammals rely on pheromones for mediating social interactions. Recent studies indicate that both the main olfactory system (MOS) and accessory olfactory system (AOS) detect and process pheromonal stimuli, yet the functional difference between these two chemosensory systems remains unclear. We hypothesized that the main functional distinction between the MOS and AOS is the type of sensory information processing performed by each system. Here we compared the electrophysiological responses of mitral cells recorded from the accessory olfactory bulb (AOB) and main olfactory bulb (MOB) in acute mouse brain slices to various stimuli and found them markedly different. The response of MOB mitral cells to brief (0.1 ms, 1-100 V) stimulation of their sensory afferents remained transient regardless of stimulus strength, whereas sufficiently strong stimuli evoked sustained firing in AOB mitral cells lasting up to several minutes. Using EPSC-like current injections (10-100 pA, 10 ms rise time constant, 5 s decay time constant) in the presence of various synaptic blockers (picrotoxin, CGP55845, APV, DNQX, E4CPG, and MSPG), we demonstrated that this difference is attributable to distinct intrinsic properties of the two neuronal populations. The AOB sustained responses were found to be mediated by calcium-activated nonselective cationic current induced by transient intense firing. This current was found to be at least partially mediated by TRPM4 channels activated by calcium influx. We hypothesize that the sustained activity of the AOS induces a new sensory state in the animal, reflecting its social context.

Activity regulates functional connectivity from the vomeronasal organ to the accessory olfactory bulb

The mammalian accessory olfactory system is specialized for the detection of chemicals that identify kin and conspecifics. Vomeronasal sensory neurons (VSNs) residing in the vomeronasal organ project axons to the accessory olfactory bulb (AOB), where they form synapses with principal neurons known as mitral cells. The organization of this projection is quite precise and is believed to be essential for appropriate function of this system. However, how this precise connectivity is established is unknown. We show here that in mice the vomeronasal duct is open at birth, allowing external chemical stimuli access to sensory neurons, and that these sensory neurons are capable of releasing neurotransmitter to downstream neurons as early as the first postnatal day (P). Using major histocompatibility complex class I peptides to activate a selective subset of VSNs during the first few postnatal days of development, we show that increased activity results in exuberant VSN axonal projections and a delay in axonal coalescence into well defined glomeruli in the AOB. Finally, we show that mitral cell dendritic refinement occurs just after the coalescence of presynaptic axons. Such a mechanism may allow the formation of precise connectivity with specific glomeruli that receive input from sensory neurons expressing the same receptor type.

Chemoreception: identifying friends and foes

The vomeronasal organ detects chemical cues that trigger sexual, aggressive and defensive behaviors. An in situ hybridization analysis has identified the specificities of nearly a hundred VNO receptors and elucidated the logic by which they encode these cues.

Odor and pheromone sensing via chemoreceptors

Evolutionally, chemosensation is an ancient but yet enigmatic sense. All organisms ranging from the simplest unicellular form to the most advanced multicellular creature possess the capability to detect chemicals in the surroundings. Conversely, all living things emit some forms of smells, either as communicating signals or as by-products of metabolism. Many species (from worms, insects to mammals) rely on the olfactory systems which express a large number of chemoreceptors to locate food and mates and to avoid danger. Most chemoreceptors expressed in olfactory organs are G-protein coupled receptors (GPCRs) and can be classified into two major categories: odorant receptors (ORs) and pheromone receptors, which principally detect general odors and pheromones, respectively. In vertebrates, these two types of receptors are often expressed in two distinct apparatuses: The main olfactory epithelium (MOE) and the vomeronasal organ (VNO), respectively. Each olfactory sensory neuron (OSN) in the MOE typically expresses one type of OR from a large repertoire. General odors activate ORs and their host OSNs (ranging from narrowly- to broadly-tuned) in a combinatorial manner and the information is sent to the brain via the main olfactory system leading to perception of smells. In contrast, pheromones stimulate relatively narrowly-tuned receptors and their host VNO neurons and the information is sent to the brain via the accessory olfactory system leading to behavioral and endocrinological changes. Recent studies indicate that the functional separation between these two systems is blurred in some cases and there are more subsystems serving chemosensory roles. This chapter focuses on the molecular and cellular mechanisms underlying odor and pheromone sensing in rodents, the best characterized vertebrate models.

Molecular organization of vomeronasal chemoreception

The vomeronasal organ (VNO) has a key role in mediating the social and defensive responses of many terrestrial vertebrates to species- and sex-specific chemosignals. More than 250 putative pheromone receptors have been identified in the mouse VNO, but the nature of the signals detected by individual VNO receptors has not yet been elucidated. To gain insight into the molecular logic of VNO detection leading to mating, aggression or defensive responses, we sought to uncover the response profiles of individual vomeronasal receptors to a wide range of animal cues. Here we describe the repertoire of behaviourally and physiologically relevant stimuli detected by a large number of individual vomeronasal receptors in mice, and define a global map of vomeronasal signal detection. We demonstrate that the two classes (V1R and V2R) of vomeronasal receptors use fundamentally different strategies to encode chemosensory information, and that distinct receptor subfamilies have evolved towards the specific recognition of certain animal groups or chemical structures. The association of large subsets of vomeronasal receptors with cognate, ethologically and physiologically relevant stimuli establishes the molecular foundation of vomeronasal information coding, and opens new avenues for further investigating the neural mechanisms underlying behaviour specificity.

Distinct intrinsic membrane properties determine differential information processing between main and accessory olfactory bulb mitral cells

Most mammals rely on semiochemicals, such as pheromones, to mediate their social interactions. Recent studies found that semiochemicals are perceived by at least two distinct chemosensory systems: the main and accessory olfactory systems, which share many molecular, cellular, and anatomical features. Nevertheless, the division of labor between these systems remained unclear. Previously we suggested that the two olfactory systems differ in the way they process sensory information. In this study we found that mitral cells of the main and accessory olfactory bulbs, the first brain stations of both systems, display markedly different passive and active intrinsic properties which permit distinct types of information processing. Moreover, we found that accessory olfactory bulb mitral cells are divided into three neuronal sub-populations with distinct firing properties. These neuronal sub-populations can be integrated in a simulated neuronal network that neglects episodic stimuli while amplifying reaction to long-lasting signals.

Regulation of adult neurogenesis by behavior and age in the accessory olfactory bulb

The vomeronasal system (VNS) participates in the detection and processing of pheromonal information related to social and sexual behaviors. Within the VNS, two different populations of sensory neurons, with a distinct pattern of distribution, line the epithelium of the vomeronasal organ (VNO) and give rise to segregated sensory projections to the accessory olfactory bulb (AOB). Apical sensory neurons in the VNO project to the anterior AOB (aAOB), while basal neurons project to the posterior AOB (pAOB). In the AOB, the largest population of neurons are inhibitory, the granule and periglomerular cells (GCs and PGs) and remarkably, these neurons are continuously born and functionally integrated in the adult brain, underscoring their role on olfactory function. Here we show that behaviors mediated by the VNS differentially regulate adult neurogenesis across the anterior-posterior axis of the AOB. We used immunohistochemical labeling of newly born cells under different behavioral conditions in mice. Using a resident-intruder aggression paradigm, we found that subordinate mice exhibited increased neurogenesis in the aAOB. In addition, in sexually naive adult females exposed to soiled bedding odorized by adult males, the number of newly born cells was significantly increased in the pAOB; however, neurogenesis was not affected in females exposed to female odors. In addition, we found that at two months of age adult neurogenesis was sexually dimorphic, with male mice exhibiting higher levels of newly born cells than females. Interestingly, adult neurogenesis was greatly reduced with age and this decrease correlated with a decrease in progenitor cells proliferation but not with an increase in cell death in the AOB. These results indicate that the physiological regulation of adult neurogenesis in the AOB by behaviors is both sex and age dependent and suggests an important role of newly born neurons in sex dependent behaviors mediated by the VNS.

Chemosensory burst coding by mouse vomeronasal sensory neurons

The capabilities of any sensory system are ultimately constrained by the properties of the sensory neurons: the ability to detect and represent stimuli is limited by noise due to spontaneous activity, and optimal decoding in downstream circuitry must be matched to the nature of the encoding performed at the input. Here, we investigated the firing properties of sensory neurons in the accessory olfactory system, a distinct sensory system specialized for detection of socially relevant odors. Using multielectrode array recording, we observed that sensory neurons are spontaneously active and highly variable across time and trials and that this spontaneous activity limits the ability to distinguish sensory responses from noise. Sensory neuron activity tended to consist of bursts that maintained remarkably consistent statistics during both spontaneous activity and in response to stimulation with sulfated steroids. This, combined with pharmacological and genetic intervention in the signal transduction cascade, indicates that sensory transduction plays a role in shaping overall spontaneous activity. These findings indicate that as-yet unexplored characteristics of the sensory transduction cascade significantly constrain the representation of sensory information by vomeronasal neurons.

Sexual dimorphism in olfactory signaling

What makes males and females behave differently? Although genetic master-regulators commonly underlie physical differences, sexually dimorphic behavior is additionally influenced by sensory input such as olfactory cues. Olfaction requires both ligands for signaling and sensory neural circuits for detection. Specialized subsets of each interact to generate gender-dimorphic behavior. It has long been accepted that males and females emit sex-specific odor compounds that function as pheromones to promote stereotypic behavior. Significant advances have now been made in purifying and isolating several of these sex-specific olfactory ligands. In contrast, the neural mechanisms that enable a gender-dimorphic response to these odors remain largely unknown. However, first progress has been made in identifying components of sexually dimorphic olfactory circuits in both Drosophila and the mouse.

Distinct signals conveyed by pheromone concentrations to the mouse vomeronasal organ

In mammalian species, detection of pheromone cues by the vomeronasal organ (VNO) at different concentrations can elicit distinct behavioral responses and endocrine changes. It is not well understood how concentration-dependent activation of the VNO impacts innate behaviors. In this study, we find that, when mice investigate the urogenital areas of a conspecific animal, the urinary pheromones can reach the VNO at a concentration of approximately 1% of that in urine. At this level, urinary pheromones elicit responses from a subset of cells that are tuned to sex-specific cues and provide unambiguous identification of the sex and strain of animals. In contrast, low concentrations of urine do not activate these cells. Strikingly, we find a population of neurons that is only activated by low concentrations of urine. The properties of these neurons are not found in neurons responding to putative single-compound pheromones. Additional analyses show that these neurons are masked by high-concentration pheromones. Thus, an antagonistic interaction in natural pheromones results in the activation of distinct populations of cells at different concentrations. The differential activation is likely to trigger different downstream circuitry and underlies the concentration-dependent pheromone perception.

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.

In vivo vomeronasal stimulation reveals sensory encoding of conspecific and allospecific cues by the mouse accessory olfactory bulb

The rodent vomeronasal system plays a critical role in mediating pheromone-evoked social and sexual behaviors. Recent studies of the anatomical and molecular architecture of the vomeronasal organ (VNO) and of its synaptic target, the accessory olfactory bulb (AOB), have suggested that unique features underlie vomeronasal sensory processing. However, the neuronal representation of pheromonal information leading to specific behavioral and endocrine responses has remained largely unexplored due to the experimental difficulty of precise stimulus delivery to the VNO. To determine the basic rules of information processing in the vomeronasal system, we developed a unique preparation that allows controlled and repeated stimulus delivery to the VNO and combined this approach with multisite recordings of neuronal activity in the AOB. We found that urine, a well-characterized pheromone source in mammals, as well as saliva, activates AOB neurons in a manner that reliably encodes the donor animal's sexual and genetic status. We also identified a significant fraction of AOB neurons that respond robustly and selectively to predator cues, suggesting an expanded role for the vomeronasal system in both conspecific and interspecific recognition. Further analysis reveals that mixed stimuli from distinct sources evoke synergistic responses in AOB neurons, thereby supporting the notion of integrative processing of chemosensory information.

Transposition and Intermingling of Galphai2 and Galphao afferences into single vomeronasal glomeruli in the Madagascan lesser Tenrec Echinops telfairi

The vomeronasal system (VNS) mediates pheromonal communication in mammals. From the vomeronasal organ, two populations of sensory neurons, expressing either Galphai2 or Galphao proteins, send projections that end in glomeruli distributed either at the rostral or caudal half of the accessory olfactory bulb (AOB), respectively. Neurons at the AOB contact glomeruli of a single subpopulation. The dichotomic segregation of AOB glomeruli has been described in opossums, rodents and rabbits, while Primates and Laurasiatheres present the Galphai2-pathway only, or none at all (such as apes, some bats and aquatic species). We studied the AOB of the Madagascan lesser tenrec Echinops telfairi (Afrotheria: Afrosoricida) and found that Galphai2 and Galphao proteins are expressed in rostral and caudal glomeruli, respectively. However, the segregation of vomeronasal glomeruli at the AOB is not exclusive, as both pathways contained some glomeruli transposed into the adjoining subdomain. Moreover, some glomeruli seem to contain intermingled afferences from both pathways. Both the transposition and heterogeneity of vomeronasal afferences are features, to our knowledge, never reported before. The organization of AOB glomeruli suggests that synaptic integration might occur at the glomerular layer. Whether intrinsic AOB neurons may make synaptic contact with axon terminals of both subpopulations is an interesting possibility that would expand our understanding about the integration of vomeronasal pathways.

Subsystem organization of the mammalian sense of smell

The mammalian olfactory system senses an almost unlimited number of chemical stimuli and initiates a process of neural recognition that influences nearly every aspect of life. This review examines the organizational principles underlying the recognition of olfactory stimuli. The olfactory system is composed of a number of distinct subsystems that can be distinguished by the location of their sensory neurons in the nasal cavity, the receptors they use to detect chemosensory stimuli, the signaling mechanisms they employ to transduce those stimuli, and their axonal projections to specific regions of the olfactory forebrain. An integrative approach that includes gene targeting methods, optical and electrophysiological recording, and behavioral analysis has helped to elucidate the functional significance of this subsystem organization for the sense of smell.

Sensing odorants and pheromones with chemosensory receptors

Olfaction is a critical sensory modality that allows living things to acquire chemical information from the external world. The olfactory system processes two major classes of stimuli: (a) general odorants, small molecules derived from food or the environment that signal the presence of food, fire, or predators, and (b) pheromones, molecules released from individuals of the same species that convey social or sexual cues. Chemosensory receptors are broadly classified, by the ligands that activate them, into odorant or pheromone receptors. Peripheral sensory neurons expressing either odorant or pheromone receptors send signals to separate odor- and pheromone-processing centers in the brain to elicit distinct behavioral and neuroendocrinological outputs. General odorants activate receptors in a combinatorial fashion, whereas pheromones activate narrowly tuned receptors that activate sexually dimorphic neural circuits in the brain. We review recent progress on chemosensory receptor structure, function, and circuitry in vertebrates and invertebrates from the point of view of the molecular biology and physiology of these sensory systems.

From Pheromones to Behavior

In recent years, considerable progress has been achieved in the comprehension of the profound effects of pheromones on reproductive physiology and behavior. Pheromones have been classified as molecules released by individuals and responsible for the elicitation of specific behavioral expressions in members of the same species. These signaling molecules, often chemically unrelated, are contained in body fluids like urine, sweat, specialized exocrine glands, and mucous secretions of genitals. The standard view of pheromone sensing was based on the assumption that most mammals have two separated olfactory systems with different functional roles: the main olfactory system for recognizing conventional odorant molecules and the vomeronasal system specifically dedicated to the detection of pheromones. However, recent studies have reexamined this traditional interpretation showing that both the main olfactory and the vomeronasal systems are actively involved in pheromonal communication. The current knowledge on the behavioral, physiological, and molecular aspects of pheromone detection in mammals is discussed in this review.

A common gene exclusion mechanism used by two chemosensory systems

Sensory coding strategies within vertebrates involve the expression of a limited number of receptor types per sensory cell. In mice, each vomeronasal sensory neuron transcribes monoallelically a single V1R pheromone receptor gene, chosen from a large V1R repertoire. The nature of the signals leading to this strict receptor expression is unknown, but is apparently based on a negative feedback mechanism initiated by the transcription of the first randomly chosen functional V1R gene. We show, in vivo, that the genetic replacement of the V1rb2 pheromone receptor coding sequence by an unrelated one from the odorant receptor gene M71 maintains gene exclusion. The expression of this exogenous odorant receptor in vomeronasal neurons does not trigger the transcription of odorant receptor-associated signalling molecules. These results strongly suggest that despite the different odorant and vomeronasal receptor expression sites, function and transduction cascades, a common mechanism is used by these chemoreceptors to regulate their transcription.

A centrifugal pathway to the mouse accessory olfactory bulb from the medial amygdala conveys gender-specific volatile pheromonal signals

We previously found that female mice exhibited Fos responses in the accessory olfactory bulb (AOB) after exposure to volatile opposite-sex, but not same-sex, urinary odours. This effect was eliminated by lesioning the main olfactory epithelium, raising the possibility that the AOB receives information about gender via centrifugal inputs originating in the main olfactory system instead of from the vomeronasal organ. We asked which main olfactory forebrain targets send axonal projections to the AOB, and whether these input neurons express Fos in response to opposite-sex urinary volatiles. Female mice received bilateral injections of the retrograde tracer cholera toxin B (CTB), into the AOB, and were exposed to either same- or opposite-sex volatile urinary odours 1 week later. We found CTB-labeled cell bodies in several forebrain sites including the bed nucleus of the accessory olfactory tract, the rostral portion of the medial amygdala (MeA) and the posteromedial cortical nucleus of the amygdala. A significant increase in the percentage of CTB/Fos co-labeled cells was seen only in the MeA of female subjects exposed to male but not to female urinary volatiles. In Experiment 2, CTB-injected females were later exposed to volatile odours from male mouse urine, food, or cat urine. Again, a significant increase in the percentage of CTB/Fos co-labeled cells was seen in the MeA of females exposed to male mouse urinary volatiles but not to food or predator odours. Main olfactory-MeA-AOB signaling may motivate approach behaviour to opposite-sex pheromonal signals that ensure successful reproduction.

On the organization of olfactory and vomeronasal cortices

Classically, the olfactory and vomeronasal pathways are thought to run in parallel non-overlapping axes in the forebrain subserving different functions. The olfactory and vomeronasal epithelia project to the main and accessory olfactory bulbs (primary projections), which in turn project to different areas of the telencephalon in a non-topographic fashion (secondary projections) and so on (tertiary projections). New data indicate that projections arising from the main and accessory olfactory bulbs converge widely in the rostral basal telencephalon. In contrast, in the vomeronasal system, cloning two classes of vomeronasal receptors (V1R and V2R) has led to the distinction of two anatomically and functionally independent pathways that reach some common, but also some different, targets in the amygdala. Tertiary projections from the olfactory and vomeronasal amygdalae are directed to the ventral striatum, which thus becomes a site for processing and potential convergence of chemosensory stimuli. Functional data indicate that the olfactory and vomeronasal systems are able to detect and process volatiles (presumptive olfactory cues) as well as pheromones in both epithelia and bulbs. Collectively, these data indicate that the anatomical and functional distinction between the olfactory and vomeronasal systems should be re-evaluated. Specifically, the recipient cortex should be reorganized to include olfactory, vomeronasal (convergent and V1R and V2R specific areas) and mixed (olfactory and vomeronasal) chemosensory cortices. This new perspective could help to unravel olfactory and vomeronasal interactions in behavioral paradigms.

Cytoarchitecture of the accessory olfactory bulb in the salamander Plethodon shermani

Plethodontid terrestrial salamanders are emerging models in the study of the evolution of chemical communication in vertebrates. Their vomeronasal system is well defined. It comprises sensory neurons in the epithelium of the vomeronasal organ, whose axons form the vomeronasal nerve projecting to the accessory olfactory bulb (AOB), which in turn projects to the vomeronasal amygdala through the accessory olfactory tract. A detailed description of the cellular elements of the urodele AOB is lacking. Neuronal morphology in the AOB was studied by means of biocytin intracellular injections and retrograde tract tracing in the salamander Plethodon shermani. The AOB exhibits the characteristic lamination of olfactory bulbs, except that it displays a mixed periglomerular and mitral somata layer superficially. Mitral cells are the only AOB neurons projecting to the vomeronasal amygdala. Each mitral cell sends multiple axonal branches, generally through both dorsal and ventral portions of the accessory olfactory tract. Some mitral cells additionally send axon collaterals in the white matter immediately ventral to the AOB. AOB interneurons are divided into superficial periglomerular and deep granule cells, each category exhibiting morphological variety. Some neurons in the granule cell layer of the AOB or the region ventral to the AOB have dendritic trees that cover both regions. The present study is the first to highlight the full anatomical extent of single AOB neurons and surprisingly suggests that the ventrolateral telencephalon found below the AOB is part of the salamander vomeronasal system.

Pheromone sensing in mice

Beginning with the neuroepithelium of the vomeronasal organ, the accessory olfactory system in rodents runs parallel to the main olfactory system and is specialized in the detection of pheromones. Only a small number of vomeronasal agonists carrying pheromonal information have been identified this far. These structurally diverse classes of chemicals include peptides secreted by exocrine glands and range from small volatile molecules to proteins and fragments thereof present in urine. Most pheromones activate both vomeronasal and main olfactory sensory neurons, making the identification of functionally relevant populations of sensory neurons difficult. Analyses of gene-targeted mice selectively affecting either vomeronasal or main olfactory signaling have attempted to elucidate the functional contribution of the different chemosensory epithelia to pheromone sensing in mice. These mouse models suggest that both the main and the accessory olfactory systems can converge and synergize to express the complex array of stereotyped behaviors and hormonal changes triggered by pheromones.

Gene cluster lock after pheromone receptor gene choice

In mammals, perception of pheromones is based on the expression in each vomeronasal sensory neuron of a limited set of receptor genes, chosen among a large repertoire. Here, we report an extremely tight control of the monogenic and monoallelic transcription of the V1rb2 receptor gene. Combining genetic and electrophysiological approaches, we show that the transcription of a non-functional V1r allele leads to the coexpression of another, functional V1r gene. The choice of this coexpressed gene surprisingly includes genes located on the cluster homologous to the one from which the mutant allele is transcribed. However, V1r genes located in cis relative to the transcribed mutant allele are excluded from the coexpression choice. Our observations strongly suggest a monogenic regulatory mechanism acting (a) at a general level, via the expression of the V1r receptor itself, and (b) at a more local level, defined by the V1r gene cluster.

Segregated pathways to the vomeronasal amygdala: differential projections from the anterior and posterior divisions of the accessory olfactory bulb

Apically and basally located receptor neurons in the vomeronasal sensory epithelium express G(i2 alpha)- and G(o alpha)-proteins, V1R and V2R vomeronasal receptors, project to the anterior and posterior accessory olfactory bulb and respond to different stimuli, respectively. The extent to which secondary projections from the two portions of the accessory olfactory bulb are convergent in the vomeronasal amygdala is controversial. This issue is addressed by using anterograde and retrograde tract-tracing methods in rats including electron microscopy. Injections of dextran-amines, Fluoro Gold, cholera toxin-B subunit and Fast Blue were delivered to the anterior and posterior accessory olfactory bulb, bed nucleus of the stria terminalis, dorsal anterior amygdala and bed nucleus of the accessory olfactory tract/anteroventral medial amygdaloid nucleus. We have demonstrated that, apart from common vomeronasal-recipient areas, only the anterior accessory olfactory bulb projects to the bed nucleus of the stria terminalis, medial division, posteromedial part, and only the posterior accessory olfactory bulb projects to the dorsal anterior amygdala and deep cell layers of the bed nucleus of the accessory olfactory tract and the anteroventral medial amygdaloid nucleus. These results provide evidence that, excluding areas of convergence, the V1R and V2R vomeronasal pathways project to specific areas of the amygdala. These two vomeronasal subsystems are therefore anatomically and functionally separated in the telencephalon.

Genetic analysis of brain circuits underlying pheromone signaling

Molecular approaches and genetic manipulations have provided novel insights into the processing of pheromone-mediated information by the olfactory and vomeronasal systems of mammals. We will review and discuss the specific contribution of each of the two chemosensory systems that ensure specific behavioral responses to conspecific animals.

Mammalian social odours: attraction and individual recognition

Mammalian social systems rely on signals passed between individuals conveying information including sex, reproductive status, individual identity, ownership, competitive ability and health status. Many of these signals take the form of complex mixtures of molecules sensed by chemosensory systems and have important influences on a variety of behaviours that are vital for reproductive success, such as parent-offspring attachment, mate choice and territorial marking. This article aims to review the nature of these chemosensory cues and the neural pathways mediating their physiological and behavioural effects. Despite the complexities of mammalian societies, there are instances where single molecules can act as classical pheromones attracting interest and approach behaviour. Chemosignals with relatively high volatility can be used to signal at a distance and are sensed by the main olfactory system. Most mammals also possess a vomeronasal system, which is specialized to detect relatively non-volatile chemosensory cues following direct contact. Single attractant molecules are sensed by highly specific receptors using a labelled line pathway. These act alongside more complex mixtures of signals that are required to signal individual identity. There are multiple sources of such individuality chemosignals, based on the highly polymorphic genes of the major histocompatibility complex (MHC) or lipocalins such as the mouse major urinary proteins. The individual profile of volatile components that make up an individual odour signature can be sensed by the main olfactory system, as the pattern of activity across an array of broadly tuned receptor types. In addition, the vomeronasal system can respond highly selectively to non-volatile peptide ligands associated with the MHC, acting at the V2r class of vomeronasal receptor. The ability to recognize individuals or their genetic relatedness plays an important role in mammalian social behaviour. Thus robust systems for olfactory learning and recognition of chemosensory individuality have evolved, often associated with major life events, such as mating, parturition or neonatal development. These forms of learning share common features, such as increased noradrenaline evoked by somatosensory stimulation, which results in neural changes at the level of the olfactory bulb. In the main olfactory bulb, these changes are likely to refine the pattern of activity in response to the learned odour, enhancing its discrimination from those of similar odours. In the accessory olfactory bulb, memory formation is hypothesized to involve a selective inhibition, which disrupts the transmission of the learned chemosignal from the mating male. Information from the main olfactory and vomeronasal systems is integrated at the level of the corticomedial amygdala, which forms the most important pathway by which social odours mediate their behavioural and physiological effects. Recent evidence suggests that this region may also play an important role in the learning and recognition of social chemosignals.

Abstracts of the 15th Annual Meeting of the Israel Society for Neuroscience (Eilat, Israel, December 3–5, 2006)

The Israel Society for Neuroscience (ISFN) was founded in 1993 by a group of Israeli leading scientists conducting research in the area of neurobiology. The primary goal of the society was to promote and disseminate the knowledge and understanding acquired by its members, and to strengthen interactions between them. Since then, the society holds its annual meeting every year in Eilat during the month of December. At these annual meetings the senior Israeli neurobiologists, their teams, and their graduate students, as well as foreign scientists and students, present their recent research findings in platform and poster presentations. The meeting also offers the opportunity for the researchers to exchange information with each other, often leading to the initiation of collaborative studies. Both the number of members of the society and of those participating in the annual meeting is constantly increasing, and it is anticipated that this year about 600 scientists will convene at the Princess Hotel in Eilat, Israel.

Further information concerning the Israel Society for Neuroscience can be found at http://www.isfn.org.il.

Committee:

Zvi Wollberg (President) Tel Aviv University

Edi Barkai University of Haifa

Etti Grauer Israel Institute for Biological Research, Ness Ziona

Yoram Rami Grossman Ben Gurion University of the Negev

Yoel Yaari Hebrew University of Jerusalem

Gal Yadid Bar-Ilan University

Shlomo Rotshenker (President Elect) Hebrew University of Jerusalem

Ettie Grauer (Treasurer) Israel Institute for Biological Research, Ness Ziona

Michal Gilady (Administrator) Rishon Le Zion

Molecular biology of peptide pheromone production and reception in mice

Intraspecies communication via pheromones plays an important role in social and sexual behaviors, which are critical for survival and reproduction in many animal species. In mice, pheromonal signals are processed by the parallel action of two olfactory systems: the main olfactory system and the vomeronasal pathway. Pheromones are recognized by chemosensory receptors expressed in the main olfactory epithelium and by V1R- and V2R-type receptors expressed in the vomeronasal organ (VNO). Mice take advantage of the chemical properties of both types of pheromones (i.e., volatile/nonvolatile) to precisely control the spatial and temporal transmission of their individual signals. The recent discovery of the exocrine gland-secreting peptide (ESP) family, which appears to encode a VNO-specific ligand repertoire, should open a new avenue to understanding peptide pheromone-mediated communication via the vomeronasal pathway in mice. In this chapter, I will review the current knowledge on genetic and molecular aspects of peptide pheromones and their receptors, by focusing primarily on the mouse VNO system. It is also an intriguing aspect to discuss peptide pheromones in the context of the evolutionary importance of species-specific chemical communication.