Emerging views on the distinct but related roles of the main and accessory olfactory systems in responsiveness to chemosensory signals in mice

Role of the vomeronasal system in intersexual attraction in female mice

Although it is generally accepted that rodents' sociosexual behavior relies mainly on chemosignals, the specific roles played by the vomeronasal and olfactory systems in detecting these signals are presently unclear. This work reports the results of three experiments aimed at clarifying the role of the vomeronasal system on gender recognition and intersexual attraction, by analyzing the effects of lesions of the accessory olfactory bulbs (AOB) in chemically naïve female mice. The first experiment demonstrates that lesions of the AOB abolish the preference that females show for male-soiled bedding in tests in which the females can contact the bedding, thus having access to both volatile and involatile male chemosignals. The second experiment shows that airborne male-derived chemosignals are not attractive to intact, chemically naïve females but tend to be preferentially explored by females whose AOB has been lesioned. However, repeated exposure to male-soiled bedding has opposite effects in sham-operated and AOB-lesioned female mice. Whereas after this experience sham-operated females show an (acquired) attraction toward male airborne chemosignals, in AOB-lesioned females the same experience makes male-derived volatiles aversive. Finally, in the third experiment we have confirmed that our AOB-lesioned females are able to detect urine-borne male odorants, as well as to discriminate them from the synthetic terpene geraniol. These findings strongly suggest that in mice, the involatile male sexual pheromone that is intrinsically attractive is detected by the vomeronasal system of the females. In addition, the repeated experience of females with male-soiled bedding would probably allow the association of this pheromone, acting as unconditioned stimulus, with olfactory stimuli (odorants) that therefore would become conditioned attractors to the females.

The olfactory pathway mediates sheltering behavior of Caribbean spiny lobsters, Panulirus argus, to conspecific urine signals

The "noses" of diverse taxa are organized into different subsystems whose functions are often not well understood. The "nose" of decapod crustaceans is organized into two parallel pathways that originate in different populations of antennular sensilla and project to specific neuropils in the brain-the aesthetasc/olfactory lobe pathway and the non-aesthetasc/lateral antennular neuropil pathway. In this study, we investigated the role of these pathways in mediating shelter selection of Caribbean spiny lobsters, Panulirus argus, in response to conspecific urine signals. We compared the behavior of ablated animals and intact controls. Our results show that control and non-aesthetasc ablated lobsters have a significant overall preference for shelters emanating urine over control shelters. Thus the non-aesthetasc pathway does not play a critical role in shelter selection. In contrast, spiny lobsters with aesthetascs ablated did not show a preference for either shelter, suggesting that the aesthetasc/olfactory pathway is important for processing social odors. Our results show a difference in the function of these dual chemosensory pathways in responding to social cues, with the aesthetasc/olfactory lobe pathway playing a major role. We discuss our results in the context of why the noses of many animals contain multiple parallel chemosensory systems.

The combined role of the main olfactory and vomeronasal systems in social communication in mammals

The main olfactory and the vomeronasal systems are the two systems by which most vertebrates detect chemosensory cues that mediate social behavior. Much research has focused on how one system or the other is critical for particular behaviors. This has lead to a vision of two distinct and complexly autonomous olfactory systems. A closer look at research over the past 30 years reveals a different picture however. These two seemingly distinct systems are much more integrated than previously thought. One novel set of chemosensory cues in particular (MHC Class I peptide ligands) can show us how both systems are capable of detecting the same chemosensory cues, through different mechanisms yet provide the same general information (genetic individuality). Future research will need to now focus on how two seemingly distinct chemosensory systems together detect pheromones and mediate social behaviors. Do these systems work independently, synergistically or competitively in communicating between individuals of the same species?

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.

Sex and the nose: human pheromonal responses

The chemosensory functions of the human nose are underappreciated. Traditional teaching is that the sense of smell detects volatile compounds, which may then allow the identification of substances that may be beneficial or harmful--such as good versus putrefied food. However, increasing evidence from research in other animals suggests that olfaction may serve another and more important purpose, that of mate selection in sexual reproduction; indeed, olfaction may be an essential impetus for evolution.

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.

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.

Hypothesis: do homeopathic medicines exert their action in humans and animals via the vomeronasal system?

There is significant debate on the nature of the active therapeutic ingredient in homeopathic medicines and whether the effect of homeopathic medicines is exerted locally. This paper accepts that there is an active therapeutic ingredient in homeopathic medicines that acts pharmacologically in the body and proposes a possible receptor site. The vomeronasal organ (Jacobson's organ) is the receptor site for the detection of non-odorant molecules, eg pheromones, in reptiles, amphibians and mammals. The organ forms the main part of a chemoreceptor system known as the vomeronasal system. This paper proposes that it is this system that constitutes the receptor for homeopathic medicines in both animal and human subjects.

Sex and the nose: human pheromonal responses

The chemosensory functions of the human nose are underappreciated. Traditional teaching is that the sense of smell detects volatile compounds, which may then allow the identification of substances that may be beneficial or harmful--such as good versus putrefied food. However, increasing evidence from research in other animals suggests that olfaction may serve another and more important purpose, that of mate selection in sexual reproduction; indeed, olfaction may be an essential impetus for evolution.

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.

Sex steroid hormones and sexual dimorphism of chemosensory structures in a terrestrial salamander (Plethodon shermani)

The volume of the vomeronasal organ (VNO) in the terrestrial salamander Plethodon shermani was approximately 1.7 times larger in adult males compared to adult females, even though male body size was, on average, slightly smaller than female body size. VNO cell density, however, was the same in adult males and females. The sex difference in VNO volume was found in sexually immature animals as well, indicating that the increase of plasma androgens that occurs at sexual maturity does not produce the sex difference in VNO volume. There was no difference in VNO volume between reproductive and non reproductive adult females, despite differences in plasma estradiol (E2) levels. The volumes of the main olfactory epithelium and muscles regulating diameter of the external nares were similar between males and females, indicating that the VNO per se, and not other aspects of the nasal cavity, was sexually dimorphic. To conclude, the sex difference in VNO volume appears to be a permanent sex difference that develops before sexual maturity. Future studies will examine the functional consequences of this structural sexual dimorphism in a peripheral sensory organ, the VNO.

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.

Chemosensory and steroid-responsive regions of the medial amygdala regulate distinct aspects of opposite-sex odor preference in male Syrian hamsters

In rodent species, such as the Syrian hamster, the expression of sexual preference requires neural integration of social chemosensory signals and steroid hormone cues. Although anatomical data suggest that separate pathways within the nervous system process these two signals, the functional significance of this separation is not well understood. Specifically, within the medial amygdala, the anterior region (MEa) receives input from the olfactory bulbs and other chemosensory areas, whereas the posterodorsal region (MEpd) contains a dense population of steroid receptors and receives less substantial chemosensory input. Consequently, the MEa may subserve a primarily discriminative function, whereas the MEpd may mediate the permissive effects of sex steroids on sexual preference. To test these hypotheses, we measured preference and attraction to female and male odors in males with lesions of either the MEa or MEpd. In Experiment 1, lesions of either region eliminated opposite-sex odor preferences. Importantly, MEpd-lesioned males displayed decreased attraction toward female odors, suggesting decreased sexual motivation. In contrast, MEa-lesioned males displayed high levels of investigation of both male and female odors, suggesting an inability to categorize the relevance of the odor stimuli. In Experiment 2, we verified that both MEa- and MEpd-lesioned males could discriminate between female and male odors, thereby eliminating the possibility that the observed lack of preference reflected a sensory deficit. Taken together, these results suggest that both the MEa and MEpd are critical for the expression of opposite-sex odor preference, although they appear to mediate distinct aspects of this behavior.

Nursing behavior: remembrance of things past

Successful suckling is vital to the survival of mammalian newborns. In many mammals, nursing behavior is triggered by maternally derived odors. Such odors may also promote the learned association of odorant cues present in the environment during nursing.

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.

Mice mutant in the DM domain gene Dmrt4 are viable and fertile but have polyovular follicles

Proteins containing the DM domain, a zinc finger-like DNA binding motif, have been implicated in sexual differentiation in diverse metazoan organisms. Of seven mammalian DM domain genes, only Dmrt1 and Dmrt2 have been functionally analyzed. Here, we report expression analysis and targeted disruption of Dmrt4 (also called DmrtA1) in the mouse. Dmrt4 is widely expressed during embryonic and postnatal development. However, we find that mice homozygous for a putative null mutation in Dmrt4 develop essentially normally, undergo full sexual differentiation in both sexes, and are fertile. We observed two potential mutant phenotypes in Dmrt4 mutant mice. First, ovaries of most mutant females have polyovular follicles, suggesting a role in folliculogenesis. Second, 25% of mutant males consistently exhibited copulatory behavior toward other males. We also tested potential redundancy between Dmrt4 and two other gonadally expressed DM domain genes, Dmrt1 and Dmrt7. We observed no enhancement of gonadal phenotypes in the double mutants, suggesting that these genes function independently in gonadal development.

Sexual experience does not compensate for the disruptive effects of zinc sulfate--lesioning of the main olfactory epithelium on sexual behavior in male mice

Recent studies point to an important role for the main olfactory epithelium (MOE) in regulating sexual behavior in male mice. We asked whether sexual experience could compensate for the disruptive effects of lesioning the MOE on sexual behavior in male mice. Male mice, which were either sexually naive or experienced, received an intranasal irrigation of either a zinc sulfate solution to destroy the MOE or saline. Sexual behavior in mating tests with an estrous female was completely abolished in zinc sulfate-treated male mice regardless of whether subjects were sexually experienced or not before the treatment. Furthermore, zinc sulfate treatment clearly disrupted olfactory investigation of both volatile and nonvolatile odors. Destruction of the MOE by zinc sulfate treatment was confirmed by a significant reduction in the expression of Fos protein in the main olfactory bulb following exposure to estrous female urine. By contrast, vomeronasal function did not seem to be affected by zinc sulfate treatment: nasal application of estrous female urine induced similar levels of Fos protein in the mitral and granule cells of the accessory olfactory bulb (AOB) of zinc sulfate- and saline-treated males. Likewise, the expression of soybean agglutinin, which stains the axons of vomeronasal organ neurons projecting to the glomerular layer of the AOB, was similar in zinc sulfate- and saline-treated male mice. These results show that the main olfactory system is essential for the expression of sexual behavior in male mice and that sexual experience does not overcome the disruptive effects of MOE lesioning on this behavior.

Expression of pheromone receptor gene families during olfactory development in the mouse: expression of a V1 receptor in the main olfactory epithelium

In the mouse, two large gene families, V1R and V2R, encoding putative pheromone receptors have been described. Studies have suggested a homotypic recognition role for V1Rs and V2Rs during development in the targeting of vomeronasal axons to specific sets of glomeruli in the accessory olfactory bulb (AOB). Analysis of the onset of expression of the V1R and V2R gene families in developing vomeronasal neurons using polymerase chain reaction and in situ hybridization now suggests that a role for these receptors in the organization of axon projections is only likely at the final stages of targeting within the AOB. Surprisingly, our studies reveal expression of a V1Rd receptor in scattered cells within the main olfactory epithelium, suggesting that limited pheromone detection may also take place in this structure. The pheromone sensory neurons of the vomeronasal system and the neuroendocrine gonadotrophin-releasing hormone (GnRH) neurons that regulate fertility both arise from progenitor cells of the nasal placode. The development of these two cell types is intimately linked, and the GnRH neuron population migrates into the forebrain during embryogenesis in close association with a subset of vomeronasal sensory axons; how GnRH neurons recognize this axon subset is unknown. We report selective expression of a V1Ra gene in the clonal NLT GnRH cell line, raising the possibility of a similar role for V1Rs or V2Rs in the directed migration of GnRH neurons. However, no expression of this gene or of other V1Rs and V2Rs is detectable at the cellular level in migrating GnRH neurons in the mouse.

A second class of chemosensory receptors in the olfactory epithelium

The mammalian olfactory system detects chemicals sensed as odours as well as social cues that stimulate innate responses. Odorants are detected in the nasal olfactory epithelium by the odorant receptor family, whose approximately 1,000 members allow the discrimination of a myriad of odorants. Here we report the discovery of a second family of receptors in the mouse olfactory epithelium. Genes encoding these receptors, called 'trace amine-associated receptors' (TAARs), are present in human, mouse and fish. Like odorant receptors, individual mouse TAARs are expressed in unique subsets of neurons dispersed in the epithelium. Notably, at least three mouse TAARs recognize volatile amines found in urine: one detects a compound linked to stress, whereas the other two detect compounds enriched in male versus female urine-one of which is reportedly a pheromone. The evolutionary conservation of the TAAR family suggests a chemosensory function distinct from odorant receptors. Ligands identified for TAARs thus far suggest a function associated with the detection of social cues.

Volatile female odors activate the accessory olfactory system of male mice without physical contact

We previously reported that male mice are more attracted to volatile odors from intact female mice than from ovariectomized female mice. In the present study, we investigated male attraction to volatile odors from soiled bedding collected from the cages of estrous or ovariectomized female mice. There was no difference in the total time spent sniffing volatile odors from estrous and ovariectomized female mice, suggesting that female mice emit volatile odors which are not excreted into bedding. To test this possibility, we investigated c-Fos expression in the mitral cell layer and granule cell layer of the accessory olfactory bulb 60 min after exposure of male mice to volatile odors without physical contact. Volatile odors from an estrous female mouse significantly increased the total number of c-Fos positive cells in each of the rostral and caudal granule cell layer, but not in the mitral cell layer. After exposure to volatile odors from estrous bedding, the total number of c-Fos positive cells did not increase. Volatile odors from a male mouse did not increase the total number of c-Fos positive cells. Volatile odors from an ovariectomized female mouse increased c-Fos expression only in the caudal granule cell layer. These results suggest that female mice emit specific volatile odors which are not excreted into bedding, and that the volatile odors activate the accessory olfactory system of male mice without physical contact. To characterize the female-specific volatile odors, we conducted habituation-dishabituation tests. Whereas sham-operated male mice discriminated between volatile odors of estrous and ovariectomized female mice, vomeronasal organ-removed male mice did not. These results suggest that male mice discriminated whether or not female mice were ovariectomized, by volatile odors via the accessory olfactory system, and that the female-specific volatile odors are involved in reproduction.

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.

Vomeronasal sensory neurons from Sternotherus odoratus (stinkpot/musk turtle) respond to chemosignals via the phospholipase C system

The mammalian signal transduction apparatus utilized by vomeronasal sensory neurons (VSNs) in the vomeronasal organ (VNO) has been richly explored, while that of reptiles, and in particular, the stinkpot or musk turtle Sternotherus odoratus, is less understood. Given that the turtle's well-known reproductive and mating behaviors are governed by chemical communication, 247 patch-clamp recordings were made from male and female S. odoratus VSNs to study the chemosignal-activated properties as well as the second-messenger system underlying the receptor potential. Of the total neurons tested, 88 (35%) were responsive to at least one of five complex natural chemicals, some of which demonstrated a degree of sexual dimorphism in response selectivity. Most notably, male VSNs responded to male urine with solely outward currents. Ruthenium Red, an IP3 receptor (IP3R) antagonist, failed to block chemosignal-activated currents, while the phospholipase C (PLC) inhibitor, U73122, abolished the chemosignal-activated current within 2 min, implicating the PLC system in the generation of a receptor potential in the VNO of musk turtles. Dialysis of several second messengers or their analogues failed to elicit currents in the whole-cell patch-clamp configuration, negating a direct gating of the transduction channel by cyclic adenosine monophosphate (cAMP), inositol 1,4,5-trisphosphate (IP3), arachidonic acid (AA), or diacylglycerol (DAG). Reversal potential analysis of chemosignal-evoked currents demonstrated that inward currents reversed at -5.7+/-7.8 mV (mean +/- s.e.m.; N=10), while outward currents reversed at -28.2+/-2.4 mV (N=30). Measurements of conductance changes associated with outward currents indicated that the outward current represents a reduction of a steady state inward current by the closure of an ion channel when the VSN is exposed to a chemical stimulus such as male urine. Chemosignal-activated currents were significantly reduced when a peptide mimicking a domain on canonical transient receptor potential 2 (TRPC2), to which type 3 IP3 receptor (IP3R3) binds, was included in the recording pipette. Collectively these data suggest that there are multiple transduction cascades operational in the VSNs of S. odoratus, one of which may be mediated by a non-selective cation conductance that is not gated by IP3 but may be modulated by the interaction of its receptor with the TRPC2 channel.

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.

Accessory olfactory neural Fos responses to a conditioned environment are blocked in male mice by vomeronasal organ removal

The ability of an anesthetized estrous female to induce a conditioned place preference (CPP) response was assessed in male mice from which the vomeronasal organ (VNO) had either been removed (VNOx) or left intact (VNOi) in an initial effort to assess the possible contribution of VNO-accessory olfactory inputs to the intrinsically rewarding properties of opposite-sex body odorants. Both VNOi and VNOx male mice acquired a CPP after repeated pairing of an initially non-preferred test chamber with an anesthetized estrous female mouse, suggesting that odorants detected by the main olfactory system and/or visual and tactile cues from the anesthetized estrous female can compensate for absent VNO inputs to establish a CPP. Subsequent exposure to this conditioning chamber alone caused significant increases in the number of Fos-immunoreactive cells in the mitral and granule cell layers of the accessory olfactory bulb as well as in the medial amygdala and ventral tegmental area of VNOi but not of VNOx males. These results suggest that activity in distal segments of the VNO-accessory olfactory pathway, in addition to the mesolimbic dopamine reward system, can be conditioned to respond to non-odor cues.

Isolation and characterisation of main olfactory and vomeronasal receptor gene families from the Atlantic salmon (Salmo salar)

The Atlantic salmon (Salmo salar) has been widely used as a model species in studies of olfactory signal transduction and processing. Here we report the isolation and characterisation of salmon olfactory receptor (SOR) and salmon vomeronasal receptor (SVR) partial sequences from Atlantic salmon. Six groups of SOR sequences (SORA-F) and three groups of SVR sequences (SVRA-C) were identified. All SORB, SORF, SVRB and SVRC sequences contained uninterrupted open reading frames. However, all SORA sequences and members of the SVRA sequence family contained multiple stop codons while SORC and SORE sequences were truncated in the 3' region of the sequence. Full length SORF and almost complete SORB sequences displayed amino acid residues and motifs conserved in fish olfactory receptor genes. In sequence phylogenies, SOR sequences fell into the main olfactory receptor (MOR) type I clade and were most closely related to either delta or zeta reference sequences, while all SVR sequences grouped within a clade of fish type 2 vomeronasal receptor (V2R) sequences. A family of sequences (Sasa CaSR1-6), isolated using the same degenerate primers that amplified SVR sequences, clustered within a group of calcium sensing receptor (CaSR) sequences. Analysis of tissue expression patterns of sequences by reverse transcriptase polymerase chain reaction showed that they were transcribed in olfactory epithelium (SORB, SORF, all SVR and Sasa CaSR sequences), testis (SORB, SORD and Sasa CaSR) and/or anterior kidney (SORB and Sasa CaSR). Similar analysis of expression supported the identification of SORA sequences as non-transcribed pseudogene(s). Although the level of occurrence of OR pseudogenes is within the range found for other, well-characterised vertebrate OR genomes, it does not seem to reflect the importance of olfaction in the biology of the Atlantic salmon.

Olfactory evolution and behavioral ecology in primates

Primates are usually thought of as "visual" mammals, and several comparative studies have emphasized the role of vision in primate neural and sociocognitive specialization. Here I explore the role of olfactory systems, using phylogenetic analysis of comparative volumetric data. The relative sizes of the main olfactory bulb (MOB) and accessory olfactory bulb (AOB) tend to show different evolutionary patterns in accordance with their different functions. Although there is some evidence of correlated evolution of the two systems, this is apparent in only one clade (the strepsirhines). As predicted, the MOBs correlate predominantly with ecological factors (activity period and diet), while the AOBs correlate with social and mating systems. Related olfactory structures (i.e., the piriform cortex and amygdala) exhibit correlated evolution with the AOBs but not with the MOBs, and the corticobasolateral part of the amygdala exhibits a correlation with social group size in platyrrhines similar to that observed for the AOB. These social system correlations support the idea that there is an olfactory dimension to the concept of the social brain.

Vomeronasal versus olfactory epithelium: is there a cellular basis for human vomeronasal perception?

The vomeronasal organ (VNO) constitutes an accessory olfactory organ that receives chemical stimuli, pheromones, which elicit behavioral, reproductive, or neuroendocrine responses among individuals of the same species. In many macrosmatic animals, the morphological substrate constitutes a separate organ system consisting of a vomeronasal duct (ductus vomeronasalis, VND), equipped with chemosensory cells, and a vomeronasal nerve (nervus vomeronasalis, VNN) conducting information into the accessory olfactory bulb (AOB) in the central nervous system (CNS). Recent data require that the long-accepted dual functionality of a main olfactory system and the VNO be reexamined, since all species without a VNO are nevertheless sexually active, and species possessing a VNO also can sense other than "vomeronasal" stimuli via the vomeronasal epithelium (VNE). The human case constitutes a borderline situation, as its embryonic VNO anlage exerts a developmental track common to most macrosmatics, but later typical structures such as the VNN, AOB, and probably most of the chemoreceptor cells within the still existent VND are lost. This review also presents recent information on the VND including immunohistochemical expression of neuronal markers, intermediate filaments, lectins, integrins, caveolin, CD44, and aquaporins. Further, we will address the issue of human pheromone candidates.

MHC peptides and the sensory evaluation of genotype

Social interactions, such as finding and identifying a mate, often rely on the ability to sense molecular cues carrying information about genetic relationship and individuality. We summarize recent evidence for an unexpected mechanistic link between the immune and olfactory systems in enabling this identification process. In addition to their established role in the immune response, peptide ligands of major histocompatibility complex (MHC) molecules constitute a previously unknown family of social recognition signals detected by specific subsets of sensory neurons in the mammalian nose. This sensing of MHC peptides can be viewed as a form of functional genome analysis by the nose. Behavioral studies in mice and fish show that MHC peptides are accepted as olfactory cues that influence mate choice decisions and selective pregnancy failure. These findings provide a molecular mechanism by which an individual can sense the composition and compatibility of vital immune system molecules of a conspecific, with direct consequences for social behavior.

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.

Deficits in sexual and aggressive behaviors in Cnga2 mutant mice

Odors detected by the vomeronasal organ or the main olfactory epithelium (MOE) trigger social behaviors in many animals. It is unknown whether MOE neurons detect cues that initiate mating or aggression. We demonstrate that mice lacking functional CNGA2 (cyclic nucleotide-gated channel alpha2), which is required for odor-evoked MOE signaling, fail to mate or fight, suggesting a broad and essential role for the MOE in regulating these behaviors.

Social Signals: The Secret Language of Mice

Animals are known to produce substances that modulate social and sexual behavior of conspecifics, but the mechanistic details underlying these phenomena have been elusive. A recent paper identifies a male-specific compound in mouse urine that activates olfactory bulb neurons and mediates behavioral attraction.

Odors detected by mice deficient in cyclic nucleotide-gated channel subunit A2 stimulate the main olfactory system

It is believed that odor transduction in the mammalian main olfactory system only involves the cAMP-signaling pathway. Here, we report on odor responsiveness in mice with a disrupted cyclic nucleotide-gated (CNG) channel subunit A2. Several odorants, including putative pheromones, can be detected and discriminated by these mice behaviorally. These odors elicit responses in the olfactory epithelium, main olfactory bulb, and olfactory (piriform) cortex of CNGA2 knock-out mice. In addition, responses to odors detected by CNGA2 knock-out mice are relatively insensitive to inhibitors of the cAMP pathway. These results provide strong evidence that cAMP-independent pathways in the main olfactory system of mammals participate in detecting a subset of odors.

Odors detected by mice deficient in cyclic nucleotide-gated channel subunit A2 stimulate the main olfactory system

It is believed that odor transduction in the mammalian main olfactory system only involves the cAMP-signaling pathway. Here, we report on odor responsiveness in mice with a disrupted cyclic nucleotide-gated (CNG) channel subunit A2. Several odorants, including putative pheromones, can be detected and discriminated by these mice behaviorally. These odors elicit responses in the olfactory epithelium, main olfactory bulb, and olfactory (piriform) cortex of CNGA2 knock-out mice. In addition, responses to odors detected by CNGA2 knock-out mice are relatively insensitive to inhibitors of the cAMP pathway. These results provide strong evidence that cAMP-independent pathways in the main olfactory system of mammals participate in detecting a subset of odors.