Simultaneous activation of mouse main and accessory olfactory bulbs by odors or pheromones

MRI in small brains displaying extensive plasticity

Manganese-enhanced magnetic resonance imaging (ME-MRI), blood oxygen-level-dependent functional MRI (BOLD fMRI) and diffusion tensor imaging (DTI) can now be applied to animal species as small as mice or songbirds. These techniques confirmed previous findings but are also beginning to reveal new phenomena that were difficult or impossible to study previously. These imaging techniques will lead to major technical and conceptual advances in systems neurosciences. We illustrate these new developments with studies of the song control and auditory systems in songbirds, a spatially organized neuronal circuitry that mediates the acquisition, production and perception of complex learned vocalizations. This neural system is an outstanding model for studying vocal learning, brain steroid hormone action, brain plasticity and lateralization of brain function.

Integration and sensory experience-dependent survival of newly-generated neurons in the accessory olfactory bulb of female mice

Newborn neurons generated by proliferative progenitors in the adult subventricular zone (SVZ) integrate into the olfactory bulb circuitry of mammals. Survival of these newly-formed cells is regulated by the olfactory input. The presence of new neurons in the accessory olfactory bulb (AOB) has already been demonstrated in some mammalian species, albeit their neurochemical profile and functional integration into AOB circuits are still to be investigated. To unravel whether the mouse AOB represents a site of adult constitutive neurogenesis and whether this process can be modulated by extrinsic factors, we have used multiple in vivo approaches. These included fate mapping of bromodeoxyuridine-labelled cells, lineage tracing of SVZ-derived enhanced green fluorescent protein-positive engrafted cells and neurogenesis quantification in the AOB, in both sexes, as well as in females alone after exposure to male-soiled bedding or its derived volatiles. Here, we show that a subpopulation of SVZ-derived neuroblasts acquires proper neurochemical profiles of mature AOB interneurons. Moreover, 3D reconstruction of long-term survived engrafted neuroblasts in the AOB confirms these cells show features of fully integrated neurons. Finally, exposure to male-soiled bedding, but not to its volatile compounds, significantly increases the number of new neurons in the AOB, but not in the main olfactory bulb of female mice. These data show SVZ-derived neuroblasts differentiate into new functionally integrated neurons in the AOB of young and adult mice. Survival of these cells seems to be regulated by an experience-specific mechanism mediated by pheromones.

A direct main olfactory bulb projection to the 'vomeronasal' amygdala in female mice selectively responds to volatile pheromones from males

The main olfactory system, like the accessory olfactory system, responds to pheromones involved in social communication. Whereas pheromones detected by the accessory system are transmitted to the hypothalamus via the medial ('vomeronasal') amygdala, the pathway by which pheromones are detected and transmitted by the main system is not well understood. We examined in female mice whether a direct projection from mitral/tufted (M/T) cells in the main olfactory bulb (MOB) to the medial amygdala exists, and whether medial amygdala-projecting M/T cells are activated by volatile urinary odors from conspecifics or a predator (cat). Simultaneous anterograde tracing using Phaseolus vulgaris leucoagglutinin and Fluoro-Ruby placed in the MOB and accessory olfactory bulb (AOB), respectively, revealed that axons of MOB M/T cells projected to superficial laminae of layer Ia in anterior and posterodorsal subdivisions of the medial amygdala, whereas projection neurons from the AOB sent axons to non-overlapping, deeper layer Ia laminae of the same subdivisions. Placement of the retrograde tracer cholera toxin B into the medial amygdala labeled M/T cells that were concentrated in the ventral MOB. Urinary volatiles from male mice, but not from female conspecifics or cat, induced Fos in medial amygdala-projecting MOB M/T cells of female subjects, suggesting that information about male odors is transmitted directly from the MOB to the 'vomeronasal' amygdala. The presence of a direct MOB-to-medial amygdala pathway in mice and other mammals could enable volatile, opposite-sex pheromones to gain privileged access to diencephalic structures that control mate recognition and reproduction.

Tactile and non-tactile sensory paradigms for fMRI and neurophysiologic studies in rodents

Functional magnetic resonance imaging (fMRI) has become a popular functional imaging tool for human studies. Future diagnostic use of fMRI depends, however, on a suitable neurophysiologic interpretation of the blood oxygenation level dependent (BOLD) signal change. This particular goal is best achieved in animal models primarily due to the invasive nature of other methods used and/or pharmacological agents applied to probe different nuances of neuronal (and glial) activity coupled to the BOLD signal change. In the last decade, we have directed our efforts towards the development of stimulation protocols for a variety of modalities in rodents with fMRI. Cortical perception of the natural world relies on the formation of multi-dimensional representation of stimuli impinging on the different sensory systems, leading to the hypothesis that a sensory stimulus may have very different neurophysiologic outcome(s) when paired with a near simultaneous event in another modality. Before approaching this level of complexity, reliable measures must be obtained of the relatively small changes in the BOLD signal and other neurophysiologic markers (electrical activity, blood flow) induced by different peripheral stimuli. Here we describe different tactile (i.e., forepaw, whisker) and non-tactile (i.e., olfactory, visual) sensory paradigms applied to the anesthetized rat. The main focus is on development and validation of methods for reproducible stimulation of each sensory modality applied independently or in conjunction with one another, both inside and outside the magnet. We discuss similarities and/or differences across the sensory systems as well as advantages they may have for studying essential neuroscientific questions. We envisage that the different sensory paradigms described here may be applied directly to studies of multi-sensory interactions in anesthetized rats, en route to a rudimentary understanding of the awake functioning brain where various sensory cues presumably interrelate.

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.

Projections of olfactory bulbs to the olfactory and vomeronasal cortices

Projections from the olfactory bulbs have been traditionally described as 'nontopographically organized'. Olfactory and vomeronasal projections have been reported to reach nonoverlapping cortical areas. Four receptor expression zones have been described in the olfactory epithelium, maintained in the main olfactory bulb, but none in the olfactory cortex. Recent data have demonstrated convergence in the basal telencephalon of olfactory and vomeronasal projections. Injections of methanesulfonate hydroxystilbamidine (FluoroGold) in the chemosensory cortex were done to map retrograde labeling in the bulbs. Topography was not observed in the four zones of the main olfactory bulb. Areas of the rostral telencephalon were shown to receive simultaneous inputs from the main and accessory olfactory bulbs.

Profound context-dependent plasticity of mitral cell responses in olfactory bulb

On the basis of its primary circuit it has been postulated that the olfactory bulb (OB) is analogous to the retina in mammals. In retina, repeated exposure to the same visual stimulus results in a neural representation that remains relatively stable over time, even as the meaning of that stimulus to the animal changes. Stability of stimulus representation at early stages of processing allows for unbiased interpretation of incoming stimuli by higher order cortical centers. The alternative is that early stimulus representation is shaped by previously derived meaning, which could allow more efficient sampling of odor space providing a simplified yet biased interpretation of incoming stimuli. This study helps place the olfactory system on this continuum of subjective versus objective early sensory representation. Here we show that odor responses of the output cells of the OB, mitral cells, change transiently during a go-no-go odor discrimination task. The response changes occur in a manner that increases the ability of the circuit to convey information necessary to discriminate among closely related odors. Remarkably, a switch between which of the two odors is rewarded causes mitral cells to switch the polarity of their divergent responses. Taken together these results redefine the function of the OB as a transiently modifiable (active) filter, shaping early odor representations in behaviorally meaningful ways.

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.

The accessory olfactory bulb in the adult rat: a cytological study of its cell types, neuropil, neuronal modules, and interactions with the main olfactory system

The accessory olfactory bulb (AOB) in the adult rat is organized into external (ECL) and internal (ICL) cellular layers separated by the lateral olfactory tract (LOT). The most superficial layer, or vomeronasal nerve layer, is composed of two fiber contingents that distribute in rostral and caudal halves. The second layer, or glomerular layer, is also divided by a conspicuous invagination of the neuropil of the ECL at the junction of the rostral and caudal halves. The ECL contains eight cell types distributed in three areas: a subglomerular area containing juxtaglomerular and superficial short-axon neurons, an intermediate area harboring large principal cells (LPC), or mitral and tufted cells, and a deep area containing dwarf, external granule, polygonal, and round projecting cells. The ICL contains two neuron types: internal granule (IGC) and main accessory cells (MACs). The dendrites and axons of LPCs in the two AOB halves are organized symmetrically with respect to an anatomical plane called linea alba. The LPC axon collaterals may recruit adjacent intrinsic, possibly gamma-aminobutyric acid (GABA)-ergic, neurons that, in turn, interact with the dendrites of the adjacent LPCs. These modules may underlie the process of decoding pheromonal clues. The most rostral ICL contains another neuron group termed interstitial neurons of the bulbi (INBs) that includes both intrinsic and projecting neurons. MACs and INBs share inputs from fiber efferents arising in the main olfactory bulb (MOB) and AOB and send axons to IGCs. Because IGCs are a well-known source of modulatory inputs to LPCs, both MACs and INBs represent a site of convergence of the MOB with the AOB.

Olfactory discrimination of aliphatic odorants at 1 ppm: too easy for CD-1 mice to show odor structure-activity relationships?

Using an operant conditioning paradigm we tested the ability of CD-1 mice to discriminate between 25 odorants comprising members of five homologous series of aliphatic odorants (C4-C8) presented at a gas phase concentration of 1 ppm. We found (a) that all mice significantly discriminated between all 50 stimulus pairs that involved odorants sharing the same functional group, but differing in carbon chain length, as well as between all 50 stimulus pairs that involved odorants sharing the same carbon chain length but differing in functional group, (b) a significant negative correlation between discrimination performance and structural similarity of odorants in terms of differences in carbon chain length with the acetic esters and the 2-ketones, but not with the 1-alcohols, n-aldehydes, and n-carboxylic acids tested, and (c) that odorant pairs differing in functional group were significantly better discriminated than odorant pairs differing in carbon chain length. These findings demonstrate that CD-1 mice have excellent discrimination ability for structurally related aliphatic odorants, that correlations between discrimination performance and structural similarity of odorants are odorant class-specific rather than a general phenomenon, and that both carbon chain length and type of functional group play an important role for odor quality coding in mice.

Increased dopamine after mating impairs olfaction and prevents odor interference with pregnancy

In rodents, social odor sensing influences female reproductive status by affecting neuroendocrine cascades. The odor of male mouse urine can induce ovulation or block pregnancy within 3 d post coitus. Females avoid the action of such olfactory stimuli after embryonic implantation. The mechanisms underlying these changes are unknown. Here we report that shortly after mating, a surge in dopamine in the mouse main olfactory bulb impairs the perception of social odors contained in male urine. Treatment of females at 6.5 d post coitus with a dopamine D2 receptor antagonist restores social odor sensing and favors disruption of pregnancy by inhibition of prolactin release, when administered in the presence of alien male urine odors. These results show that an active sensory barrier blocks social olfactory cues detrimental to pregnancy, consistent with the main olfactory bulb being a major relay through which social odor modulates reproductive status.

Distinct evolutionary patterns between chemoreceptors of 2 vertebrate olfactory systems and the differential tuning hypothesis

Most tetrapod vertebrates have 2 olfactory systems, the main olfactory system (MOS) and the vomeronasal system (VNS). According to the dual olfactory hypothesis, the MOS detects environmental odorants, whereas the VNS recognizes intraspecific pheromonal cues. However, this strict functional distinction has been blurred by recent reports that both systems can perceive both types of signals. Studies of a limited number of receptors suggest that MOS receptors are broadly tuned generalists, whereas VNS receptors are narrowly tuned specialists. However, whether this distinction applies to all MOS and VNS receptors remains unknown. The differential tuning hypothesis predicts that generalist MOS receptors detect an overlapping set of ligands and thus are more likely to be conserved over evolutionary time than specialist VNS receptors, which would evolve in a more lineage-specific manner. Here we test this prediction for all olfactory chemoreceptors by examining the evolutionary patterns of MOS-expressed odorant receptors (ORs) and trace amine-associated receptors (TAARs) and VNS-expressed vomeronasal type 1 receptors (V1Rs) and vomeronasal type 2 receptors (V2Rs) in 7 tetrapods (mouse, rat, dog, opossum, platypus, chicken, and frog). The phylogenies of V1Rs and V2Rs show abundant lineage-specific gene gains/losses and virtually no one-to-one orthologs between species. Opposite patterns are found for ORs and TAARs. Analysis of functional data and ligand-binding sites of ORs confirms that paralogous chemoreceptors are more likely than orthologs to have different ligands and that functional divergence between paralogous chemoreceptors is established relatively quickly following gene duplication. Together, these results strongly suggest that the functional profile of the VNS chemoreceptor repertoire evolves much faster than that of the MOS chemoreceptor repertoire and that the differential tuning hypothesis applies to the majority, if not all, of MOS and VNS receptors.

Olfactory sensory neuron-specific and sexually dimorphic expression of protocadherin 20

Olfactory sensory axons navigate from the nasal cavity to the olfactory bulb and sort from among 1,000 different odorant receptor-expressing types to converge upon the same two or three glomeruli. To achieve this task during development, it is likely that multiple classes of regulatory molecules, including cell adhesion molecules, are involved. Cell adhesion molecules have been shown to be important in controlling axon guidance, fasciculation, and synapse formation. To gain further understanding of the involvement of adhesion molecules in olfactory circuitry development, we examined the dynamic and cell type specific expression of a novel protocadherin, PCDH20, in the olfactory system. PCDH20 is specifically expressed in newly differentiated olfactory sensory neurons and their axons during development. PCDH20 expression is down-regulated in the adult olfactory system, except in a small olfactory sensory neuron population. These small, discrete numbers of PCDH20-positive glomeruli in the adult olfactory bulb are consistently clustered in the ventral-caudal region in both male and female mice. However, adult males have higher numbers of PCDH20-positive glomeruli with a broader distribution, whereas adult females have fewer PCDH20-positive glomeruli with a more restricted distribution. The gender difference in PCDH20 expression may reflect olfactory receptor expression differences for gender-specific social discrimination.

Representation of a mixture of pheromone and host plant odor by antennal lobe projection neurons of the silkmoth Bombyx mori

Pheromone-source orientation behavior can be modified by coexisting plant volatiles. Some host plant volatiles enhance the pheromonal responses of olfactory receptor neurons and increase the sensitivity of orientation behavior in the Lepidoptera species. Although many electrophysiological studies have focused on the pheromonal response of olfactory interneurons, the response to the mixture of pheromone and plant odor is not yet known. Using the silkmoth, Bombyx mori, we investigated the physiology of interneurons in the antennal lobe (AL), the primary olfactory center in the insect brain, in response to a mixture of the primary pheromone component bombykol and cis-3-hexen-1-ol, a mulberry leaf volatile. Application of the mixture enhanced the pheromonal responses of projection neurons innervating the macroglomerular complex in the AL. In contrast, the mixture of pheromone and cis-3-hexen-1-ol had little influence on the responses of projection neurons innervating the ordinary glomeruli whereas other plant odors dynamically modified the response. Together this suggests moths can process plant odor information under conditions of simultaneous exposure to sex pheromone.

Encoding olfactory signals via multiple chemosensory systems

Most animals have evolved multiple olfactory systems to detect general odors as well as social cues. The sophistication and interaction of these systems permit precise detection of food, danger, and mates, all crucial elements for survival. In most mammals, the nose contains two well described chemosensory apparatuses (the main olfactory epithelium and the vomeronasal organ), each of which comprises several subtypes of sensory neurons expressing distinct receptors and signal transduction machineries. In many species (e.g., rodents), the nasal cavity also includes two spatially segregated clusters of neurons forming the septal organ of Masera and the Grueneberg ganglion. Results of recent studies suggest that these chemosensory systems perceive diverse but overlapping olfactory cues and that some neurons may even detect the pressure changes carried by the airflow. This review provides an update on how chemosensory neurons transduce chemical (and possibly mechanical) stimuli into electrical signals, and what information each system brings into the brain. Future investigation will focus on the specific ligands that each system detects with a behavioral context and the processing networks that each system involves in the brain. Such studies will lead to a better understanding of how the multiple olfactory systems, acting in concert, offer a complete representation of the chemical world.

Are pheromones detected through the main olfactory epithelium?

A major sensory organ for the detection of pheromones by animals is the vomeronasal organ (VNO). Although pheromones control the behaviors of various species, the effect of pheromones on human behavior has been controversial because the VNO is not functional in adults. However, recent genetic, biochemical, and electrophysiological data suggest that some pheromone-based behaviors, including male sexual behavior in mice, are mediated through the main olfactory epithelium (MOE) and are coupled to the type 3 adenylyl cyclase (AC3) and a cyclic nucleotide-gated (CNG) ion channel. These recent discoveries suggest the provocative hypothesis that human pheromones may signal through the MOE.

A role for the androgen receptor in the sexual differentiation of the olfactory system in mice

Olfactory signals play a central role in the identification of a mating partner in rodents, and the behavioral response to these cues varies markedly between the sexes. As several other sexually dimorphic traits, this response is thought to differentiate as a result of exposure of the developing individual to gonadal steroids, but both the identity of the specific steroid signal and the neural structures targeted for differentiation on this particular case are largely unknown. The present review summarizes results obtained in our lab using genetic males affected by the testicular feminization syndrome (Tfm) as experimental model, and that led to the identification of a role for non-aromatized gonadal steroids acting through the androgen receptor (AR) in the differentiation of olfactory cues processing in mice. The existing literature about AR-mediated sexual differentiation of the CNS in animal models is discussed, along with potential targets for the action of non-aromatized gonadal steroids in either one of the subsystems that detect and process olfactory information in rodents.

Convergence of olfactory and vomeronasal projections in the rat basal telencephalon

Olfactory and vomeronasal projections have been traditionally viewed as terminating in contiguous non-overlapping areas of the basal telencephalon. Original reports, however, described areas such as the anterior medial amygdala where both chemosensory afferents appeared to overlap. We addressed this issue by injecting dextran amines in the main or accessory olfactory bulbs of rats and the results were analyzed with light and electron microscopes. Simultaneous injections of different fluorescent dextran amines in the main and accessory olfactory bulbs were performed and the results were analyzed using confocal microscopy. Similar experiments with dextran amines in the olfactory bulbs plus FluoroGold in the bed nucleus of the stria terminalis indicate that neurons projecting through the stria terminalis could be integrating olfactory and vomeronasal inputs. Retrograde tracing experiments using FluoroGold or dextran amines confirm that areas of the rostral basal telencephalon receive inputs from both the main and accessory olfactory bulbs. While both inputs clearly converge in areas classically considered olfactory-recipient (nucleus of the lateral olfactory tract, anterior cortical amygdaloid nucleus, and cortex-amygdala transition zone) or vomeronasal-recipient (ventral anterior amygdala, bed nucleus of the accessory olfactory tract, and anteroventral medial amygdaloid nucleus), segregation is virtually complete at posterior levels such as the posteromedial and posterolateral cortical amygdalae. This provides evidence that areas so far considered receiving a single chemosensory modality are likely sites for convergent direct olfactory and vomeronasal inputs. Therefore, areas of the basal telencephalon should be reclassified as olfactory, vomeronasal, or mixed chemosensory structures, which could facilitate understanding of olfactory-vomeronasal interactions in functional studies.

Mammalian pheromone sensing

The traditional distinction that the mammalian main olfactory system recognizes general odor molecules and the accessory (vomeronasal) system detects pheromones is no longer valid. The emerging picture is that both systems have considerable overlap in terms of the chemosignals they detect and the effects that they mediate. Recent investigations have discovered large families of pheromonal signals together with a rich variety of specific receptor systems and nasal detection pathways. Selective genetic targeting of these subsystems should help to unravel their biological role in pheromone-mediated behavioral responses.

Current status of functional MRI on small animals: application to physiology, pathophysiology, and cognition

This review aims to make the reader aware of the potential of functional MRI (fMRI) in brain activation studies in small animal models. As small animals generally require anaesthesia for immobilization during MRI protocols, this is believed to be a serious limitation to the type of question that can be addressed with fMRI. We intend to introduce a fresh view with an in-depth overview of the surprising number of fMRI applications in a wide range of important research domains in neuroscience. These include the pathophysiology of brain functioning, the basic science of activity, and functional connectivity of different sensory circuits, including sensory brain mapping, the challenges when studying the hypothalamus as the major control centre in the central nervous system, and the limbic system as neural substrate for emotions and reward. Finally the contribution of small animal fMRI research to cognitive neuroscience is outlined. This review avoids focusing exclusively on traditional small laboratory animals such as rodents, but rather aims to broaden the scope by introducing alternative lissencephalic animal models such as songbirds and fish, as these are not yet well recognized as neuroimaging study subjects. These models are well established in many other neuroscience disciplines, and this review will show that their investigation with in vivo imaging tools will open new doors to cognitive neuroscience and the study of the autonomous nervous system in experimental animals.

Expression of a vomeronasal receptor gene (V1r) and G protein alpha subunits in goat, Capra hircus, olfactory receptor neurons

Most mammals have two distinct olfactory epithelia, the olfactory epithelium (OE) and vomeronasal epithelium (VNE), containing, respectively, olfactory receptor neurons (ORNs) and vomeronasal receptor neurons (VRNs). Olfactory receptors (ORs), which couple to G alpha olf, are generally expressed by ORNs, whereas two vomeronasal receptor families (V1rs and V2rs) coupled respectively to G alpha i2 and G alpha o, are expressed by VRNs. Previously, we reported that one goat V1rs (gV1ra1) is expressed by ORNs and VRNs. To investigate the characteristics of vomeronasal-receptor-expressing ORNs in mammals we performed double-label in situ hybridization for gV1ra1, G alpha i2, G alpha olf, olfactory marker protein (OMP), and growth association protein 43 (GAP43). Goat V1r-expressing ORNs are categorized into two types situated in different areas of the epithelium. The first type of V1r-expressing ORN coexpressed G alpha i2, but not OMP or GAP43. The second type of V1r-expressing ORN expresses G alpha olf and OMP, but not G alpha i2 or GAP43. These findings suggest that the two types of V1r-expressing ORN in goat OE function using different G protein alpha subunits for chemoreception.

Chemotopic odorant coding in a mammalian olfactory system

Systematic mapping studies involving 365 odorant chemicals have shown that glomerular responses in the rat olfactory bulb are organized spatially in patterns that are related to the chemistry of the odorant stimuli. This organization involves the spatial clustering of principal responses to numerous odorants that share key aspects of chemistry such as functional groups, hydrocarbon structural elements, and/or overall molecular properties related to water solubility. In several of the clusters, responses shift progressively in position according to odorant carbon chain length. These response domains appear to be constructed from orderly projections of sensory neurons in the olfactory epithelium and may also involve chromatography across the nasal mucosa. The spatial clustering of glomerular responses may serve to "tune" the principal responses of bulbar projection neurons by way of inhibitory interneuronal networks, allowing the projection neurons to respond to a narrower range of stimuli than their associated sensory neurons. When glomerular activity patterns are viewed relative to the overall level of glomerular activation, the patterns accurately predict the perception of odor quality, thereby supporting the notion that spatial patterns of activity are the key factors underlying that aspect of the olfactory code. A critical analysis suggests that alternative coding mechanisms for odor quality, such as those based on temporal patterns of responses, enjoy little experimental support.

SenseLab: new developments in disseminating neuroscience information

This article presents the latest developments in neuroscience information dissemination through the SenseLab suite of databases: NeuronDB, CellPropDB, ORDB, OdorDB, OdorMapDB, ModelDB and BrainPharm. These databases include information related to: (i) neuronal membrane properties and neuronal models, and (ii) genetics, genomics, proteomics and imaging studies of the olfactory system. We describe here: the new features for each database, the evolution of SenseLab's unifying database architecture and instances of SenseLab database interoperation with other neuroscience online resources.

Quantitative analysis of mouse urine volatiles: in search of MHC-dependent differences

Genes of the major histocompatibility complex (MHC), which play a critical role in immune recognition, influence mating preference and other social behaviors in mice. Training experiments using urine scent from mice differing only in the MHC complex, from MHC class I mutants or from knock-out mice lacking functional MHC class I molecules (ß2m-deficient), suggest that these behavioral effects are mediated by differences in MHC-dependent volatile components. In search for the physical basis of these behavioral studies, we have conducted a comparison of urinary volatiles in three sub-strains of C57BL/6 mice, a ß2m-deficient mutant lacking functional MHC class I expression and two unrelated inbred strains, using the technique of sorptive extraction with polydimethylsiloxan and subsequent analysis by gas chromatography/mass spectrometry. We show (i) that qualitative differences occur between different inbred strains but not in mice with the C57BL/6 background, (ii) that the individual variability in abundance in the same mouse strain is strongly component-dependent, (iii) that C57BL/6 sub-strains obtained from different provenance show a higher fraction of quantitative differences than a sub-strain and its ß2m-mutant obtained from the same source and (iv) that comparison of the spectra of ß2m mice and the corresponding wild type reveals no qualitative differences in close to 200 major and minor components and only minimal differences in a few substances from an ensemble of 69 selected for quantitative analysis. Our data suggest that odor is shaped by ontogenetic, environmental and genetic factors, and the gestalt of this scent may identify a mouse on the individual and population level; but, within the limits of the ensemble of components analysed, the results do not support the notion that functional MHC class I molecules influence the urinary volatile composition.

Neurodevelopment of C57B/L6 mouse brain assessed by in vivo diffusion tensor imaging

Heterogeneous spatiotemporal patterns of C57B/L6 murine brain maturation during the first 7 weeks after birth (i.e. P15 to P45) were assessed in vivo by diffusion tensor imaging (DTI) at 9.4 T. Maps of apparent diffusion coefficient (ADC) and fractional anisotropy (FA) were used to assess developmental changes. Because directionally encoded color (DEC) maps provide an efficient and straightforward way to visualize anisotropy direction, they were used to highlight the orientation-dominant anisotropic tissues. In the corpus callosum, the increases in FA (approximately 0.4 to approximately 0.6 from P15 to P45) were primarily dominant in the medial-lateral direction, whereas the ADC decreased slightly (approximately 0.8 x 10(-3) to approximately 0.5 x 10(-3) mm(2)/s from P15 to P45). Similar increases in FA (approximately 0.3 to approximately 0.4 from P15 to P45) and decreases in ADC (approximately 0.8 x 10(-3) to approximately 0.5 x 10(-3) mm(2)/s from P15 to P45) were found in the cingulate, but these anisotropic changes were dominant in the anterior-posterior direction. In the caudate putamen, there were significant FA increases (approximately 0.1 to approximately 0.2 from P15 to P45) dominant in the dorsal-ventral and anterior-posterior directions, whereas the ADC increased rapidly early in development (approximately 0.3 x 10(-3) to approximately 0.7 x 10(-3) mm(2)/s from P15 to P17). There were no significant changes in tissue anisotropy in the somatosensory regions (whisker, forelimb), but the ADC decreased slightly (approximately 0.7 x 10(-3) to approximately 0.5 x 10(-3) mm(2)/s from P15 to P45). Although the major differences in DEC values were mainly observed in white matter pathways, other cortical and subcortical regions showed some potential morphological changes that were consistent with classical histological findings. In summary, these results show that high-resolution DTI at high magnetic fields allows detection and quantification of brain structures throughout normal development in C57B/L6 mice in vivo.

Urine from stressed rats increases immobility in receptor rats forced to swim: Role of 2-heptanone

The present study was aimed to determine whether the urine from donor rats, which were physically stressed (UD-PS) by unavoidable electric footshocks, produces despair in receptor partner rats (RP) in the long-term. For each trial, an RP rat was placed during 10 min once per day for 21 days in a small non-movement-restricting cage impregnated with the urine collected from a UD-PS rat. Control rats, free of stimulation, maintained their locomotion and immobility scores at basal values throughout the 21-day test. After 21 days of stressing experience [F(2,90)=15.22, P<0.0001] locomotion significantly increased in RP rats (r=0.938, P<0.01), whereas in the UD-PS group locomotion decreased (r=-0.606, P<0.05). The RP and UD-PS groups displayed the longest time of immobility [F(2,90)=8.83, P<0.001] in the forced-swim test (RP, r=0.886, P<0.05; UD-PS, r=0.962, P<0.001) compared with the control group (r=-0.307, NS). We conclude that the RP became similarly despaired as the UD-PS group through the action of 2-heptanone, a ketonic compound identified in UD-PS urine by HS-GC/MS techniques. This ketone was found to be increased [F(2,15)=3.50, P<0.05] from the 1st day of unavoidable electric footshocks, and to induce despair, an effect reverted [F(2,21)=16.5, P<0.0001] by imipramine (5.0 mg/kg) in another group of rats.

Component-dependent urine responses in the rat accessory olfactory bulb

To investigate how pheromonal information is processed in the rat accessory olfactory bulb, we optically imaged intrinsic signals to obtain high-resolution maps of activation induced by urinary stimulation. Application of volatile components in male urine mainly induced activation in the anterior accessory olfactory bulb, irrespective of the sex, whereas volatile female urine elicited activation not only in the anterior but also to some extent in the caudal part of the posterior accessory olfactory bulb of male, but not female, rats. Nonvolatile components of both male and female urine induced activation mainly in the rostral part of the posterior and to a lesser extent in the anterior accessory olfactory bulb, irrespective of the sex. These results indicate that volatile and nonvolatile urinary components activate the anterior and posterior subdivisions of the accessory olfactory bulb, respectively.

Olfactory neurons expressing transient receptor potential channel M5 (TRPM5) are involved in sensing semiochemicals

Olfactory sensory neurons (OSNs) in the main olfactory epithelium respond to environmental odorants. Recent studies reveal that these OSNs also respond to semiochemicals such as pheromones and that main olfactory input modulates animal reproduction, but the transduction mechanism for these chemosignals is not fully understood. Previously, we determined that responses to putative pheromones in the main olfactory system were reduced but not eliminated in mice defective for the canonical cAMP transduction pathway, and we suggested, on the basis of pharmacology, an involvement of phospholipase C. In the present study, we find that a downstream signaling component of the phospholipase C pathway, the transient receptor potential channel M5 (TRPM5), is coexpressed with the cyclic nucleotide-gated channel subunit A2 in a subset of mature OSNs. These neurons project axons primarily to the ventral olfactory bulb, where information from urine and other socially relevant signals is processed. We find that these chemosignals activate a subset of glomeruli targeted by TRPM5-expressing OSNs. Our data indicate that TRPM5-expressing OSNs that project axons to glomeruli in the ventral area of the main olfactory bulb are involved in processing of information from semiochemicals.

Pheromonal communication in vertebrates

Recent insights have revolutionized our understanding of the importance of chemical signals in influencing vertebrate behaviour. Previously unknown families of pheromonal signals have been identified that are expanding the traditional definition of a pheromone. Although previously regarded as functioning independently, the main olfactory and vomeronasal systems have been found to have considerable overlap in terms of the chemosignals they detect and the effects that they mediate. Studies using gene-targeted mice have revealed an unexpected diversity of chemosensory systems and their underlying cellular and molecular mechanisms. Future developments could show how the functions of the different chemosensory systems are integrated to regulate innate and learned behavioural and physiological responses to pheromones.

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.

Origin and evolution of the vertebrate vomeronasal system viewed through system-specific genes

Tetrapods have two distinct nasal chemosensory systems, the main olfactory system and the vomeronasal system (VNS). Defined by certain morphological components, the main olfactory system is present in all groups of vertebrates, while the VNS is found only in tetrapods. Previous attempts to identify a VNS precursor in teleost fish were limited by functional and morphological characters that could not clearly distinguish between homologous and analogous systems. In the past decade, several genes that specifically function in the VNS have been discovered. Here we first describe recent evolutionary studies of mammalian VNS-specific genes. We then review evidence showing the presence and tissue-specific expression of the VNS-specific genes in teleosts, as well as co-expression patterns of these genes in specific regions of the teleost olfactory epithelium. We propose that a VNS precursor exists in teleosts and that its evolutionary origin predated the separation between teleosts and tetrapods.

Removal of the vomeronasal organ blocks the stress-induced hyperthermia response to alarm pheromone in male rats

Previously, we reported that male Wistar rats release alarm pheromone from their perianal region, which aggravates stress-induced hyperthermia (SIH) in pheromone-recipient rats. The subsequent discovery that this pheromone could be trapped in water enabled us to expose recipients to the pheromone in their home cages. Despite its apparent influence on autonomic and behavioral functions, we still had no clear evidence as to whether the alarm pheromone was perceived by the main olfactory system (MOS) or by the vomeronasal system. In this study, we investigated this question by exposing 3 types of recipients to alarm pheromone in their home cages: intact males (Intact), vomeronasal organ-excised males (VNX), and sham-operated males (Sham). The Intact and Sham recipients showed aggravated SIH in response to alarm pheromone, whereas the VNX recipients did not. In addition, the results of the habituation/dishabituation test and soybean agglutinin binding to the accessory olfactory bulb verified the complete ablation of the vomeronasal organ (VNO) with a functional MOS in the pheromone recipients. These results strongly suggest that male rats perceive alarm pheromone with the VNO.

Cells in the vomeronasal organ express odorant receptors but project to the accessory olfactory bulb

Recent evidence indicates that the vomeronasal organ (VNO) of mice not only responds to pheromones but also to odorants. To analyze whether genes encoding odorant receptors (ORs) are expressed in the VNO, reverse transcriptase-polymerase chain reaction analyses were performed. These led to the identification of 44 different OR genes, comprising class-I and class-II receptors. The genes encoding these receptors were scattered over several gene clusters. The respective OR genes were concomitantly expressed in cells of the main olfactory epithelium (MOE). Although the cells in the MOE were zonally distributed, no such patterns were displayed in the VNO. Cells expressing ORs in the VNO were positive for the TRP2-channel and Galphai, a marker for vomeronasal neurons of the apical layer. In transgenic mice, which coexpress histological markers with the receptor mOR18-2, characteristic morphological differences between cells expressing this receptor in the VNO compared with the MOE became evident. Visualizing the axonal processes of VNO cells expressing distinct ORs revealed that they project to the accessory olfactory bulb (AOB). Axon fibers were visible exclusively in the anterior subdomain; here, they converged into glomerular-like structures positioned at the very rostral tip of the AOB. The findings that a set of ORs is expressed in cells located in the apical layer of the VNO with typical features of VNO sensory neurons that project their axons to the anterior part of the AOB suggest that this population of sensory cells may be considered as a unique facet of the complex chemosensory system.

Olfactory discrimination ability of CD-1 mice for a large array of enantiomers

With use of a conditioning paradigm, the ability of eight CD-1 mice to distinguish between 15 enantiomeric odor pairs was investigated. The results demonstrate a) that CD-1 mice are capable of discriminating between all odor pairs tested, b) that the enantiomeric odor pairs clearly differed in their degree of discriminability and thus in their perceptual similarity, and c) that pre-training with the rewarded stimuli led to improved initial but not terminal or overall performance. A comparison between the proportion of discriminated enantiomeric odor pairs of the CD-1 mice and those of other species tested in earlier studies on the same discrimination tasks (or on subsets thereof) shows a significant positive correlation between discrimination performance and the number of functional olfactory receptor genes. These findings provide the first evidence of a highly developed ability of CD-1 mice to discriminate between an array of non-pheromonal chiral odorants. Further, they suggest that a species' olfactory discrimination capabilities for these odorants may be correlated with its number of functional olfactory receptor genes. The data presented here may provide useful information for the interpretation of findings from electrophysiological or imaging studies in the mouse and the elucidation of odor structure-activity relationships.

Odortypes and MHC peptides: Complementary chemosignals of MHC haplotype?

The olfactory and immune systems must perform optimally in the task of recognizing thousands of molecules to ensure survival. A particularly intriguing link between these systems is that animals can smell differences in the major histocompatibility complex (MHC), a cluster of highly polymorphic genes found on human chromosome 6 and mouse chromosome 17. Two different sets of compounds found in urine have been postulated to convey information on MHC haplotype: volatile compounds (odortypes) and MHC peptides. Here we argue for complementary roles for these chemosignals.

The vomeronasal system in mice: from the nose to the hypothalamus- and back!

The vomeronasal pathway in rodents runs parallel to the main olfactory pathway and mediates responses to different classes of chemosensory stimuli. Both olfactory systems can converge and synergize to control reproductive behaviors and hormonal changes triggered by chemosensory cues. Novel experimental approaches expressing genetic transneuronal tracers in hypothalamic neurons regulating reproduction have set the stage to analyze how chemosensory inputs are integrated in the brain to elicit reproductive behaviors and hormonal changes, and how neuroendocrine status might modulate susceptibility to chemosensory cues.

Essential role of the main olfactory system in social recognition of major histocompatibility complex peptide ligands

Genes of the major histocompatibility complex (MHC), which play a critical role in immune recognition, influence mating preference and other social behaviors in fish, mice, and humans via chemical signals. The cellular and molecular mechanisms by which this occurs and the nature of these chemosignals remain unclear. In contrast to the widely held view that olfactory sensory neurons (OSNs) in the main olfactory epithelium (MOE) are stimulated by volatile chemosignals only, we show here that nonvolatile immune system molecules function as olfactory cues in the mammalian MOE. Using mice with targeted deletions in selected signal transduction genes (CNGA2, CNGA4), we used a combination of dye tracing, electrophysiological, Ca2+ imaging, and behavioral approaches to demonstrate that nonvolatile MHC class I peptides activate subsets of OSNs at subnanomolar concentrations in vitro and affect social preference of male mice in vivo. Both effects depend on the cyclic nucleotide-gated (CNG) channel gene CNGA2, the function of which in the nose is unique to the main population of OSNs. Disruption of the modulatory CNGA4 channel subunit reveals a profound defect in adaptation of peptide-evoked potentials in the MOE. Because sensory neurons in the vomeronasal organ (VNO) also respond to MHC peptides but do not express CNGA2, distinct mechanisms are used by the mammalian main and accessory olfactory systems for the detection of MHC peptide ligands. These results suggest a general role for MHC peptides in chemical communication even in those vertebrates that lack a functional VNO.

Is the vomeronasal system really specialized for detecting pheromones?

Many academics, clinicians and lay readers of science incorrectly assume that vomeronasal processing is equivalent to pheromone processing. We review the abundant data concerning the roles of both the olfactory and the vomeronasal systems in the processing of both pheromones and other odorants, demonstrating that this "equivalency hypothesis" is untenable. This conclusion has important implications for the design and interpretation of experiments examining vomeronasal and olfactory system function. We describe some of the problems that arise from assuming that this equivalency holds. Two alternative hypotheses have been offered, but the available data do not enable us to accept or reject either one. Perhaps no single functional description can adequately characterize the role of the vomeronasal system.

Analysis of training-induced changes in ethyl acetate odor maps using a new computational tool to map the glomerular layer of the olfactory bulb

Odor quality is thought to be encoded by the activation of partially overlapping subsets of glomeruli in the olfactory bulb (odor maps). Mouse genetic studies have demonstrated that olfactory sensory neurons (OSNs) expressing a particular olfactory receptor target their axons to a few individual glomeruli in the bulb. While the specific targeting of OSN axons provides a molecular underpinning for the odor maps, much remains to be understood about the relationship between the functional and molecular maps. In this article, we ask the question whether intensive training of mice in a go/no-go operant conditioning odor discrimination task affects odor maps measured by determining c-fos up-regulation in periglomerular cells. Data analysis is performed using a newly developed suite of computational tools designed to systematically map functional and molecular features of glomeruli in the adult mouse olfactory bulb. This suite provides the necessary tools to process high-resolution digital images, map labeled glomeruli, visualize odor maps, and facilitate statistical analysis of patterns of identified glomeruli in the olfactory bulb. The software generates odor maps (density plots) based on glomerular activity, density, or area. We find that training up-regulates the number of glomeruli that become c-fos positive after stimulation with ethyl acetate.