Access of urinary nonvolatiles to the mammalian vomeronasal organ

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.

A Drosophila Protein Specific to Pheromone-Sensing Gustatory Hairs Delays Males' Copulation Attempts

In insects, increasing evidence suggests that small secreted pheromone binding proteins (PBPs) and odorant binding proteins (OBPs) are important for normal olfactory detection of airborne pheromones and odorants far from their source. In contrast, it is unknown whether extracellular ligand binding proteins participate in perception of less volatile chemicals, including many pheromones, that are detected by direct contact with chemosensory organs. CheB42a, a small Drosophila melanogaster protein unrelated to known PBPs or OBPs, is expressed and likely secreted in only a small subset of gustatory sensilla on males' front legs, the site of gustatory perception of contact pheromones. Here we show that CheB42a is expressed specifically in the sheath cells surrounding the taste neurons expressing Gr68a, a putative gustatory pheromone receptor for female cuticular hydrocarbons that stimulate male courtship. Surprisingly, however, CheB42a mutant males attempt to copulate with females earlier and more frequently than control males. Furthermore, CheB42a mutant males also attempt to copulate more frequently with other males that secrete female-specific cuticular hydrocarbon pheromones, but not with females lacking cuticular hydrocarbons. Together, these data indicate that CheB42a is required for a normal gustatory response to female cuticular hydrocarbon pheromones that modulate male courtship.

Destruction of the main olfactory epithelium reduces female sexual behavior and olfactory investigation in female mice

We studied the contribution of the main olfactory system to mate recognition and sexual behavior in female mice. Female mice received an intranasal irrigation of either a zinc sulfate (ZnSO4) solution to destroy the main olfactory epithelium (MOE) or saline (SAL) to serve as control. ZnSO4-treated female mice were no longer able to reliably distinguish between volatile as well as nonvolatile odors from an intact versus a castrated male. Furthermore, sexual behavior in mating tests with a sexually experienced male was significantly reduced in ZnSO4-treated female mice. Vomeronasal function did not seem to be affected by ZnSO4 treatment: nasal application of male urine induced similar levels of Fos protein in the mitral and granule cells of the accessory olfactory bulb (AOB) of ZnSO4 as well as SAL-treated female mice. Likewise, soybean agglutinin staining, which stains the axons of vomeronasal neurons projecting to the glomerular layer of the AOB was similar in ZnSO4-treated female mice compared to SAL-treated female mice. By contrast, a significant reduction of Fos in the main olfactory bulb was observed in ZnSO4-treated females in comparison to SAL-treated animals, confirming a substantial destruction of the MOE. These results show that the MOE is primarily involved in the detection and processing of odors that are used to localize and identify the sex and endocrine status of conspecifics. By contrast, both the main and accessory olfactory systems contribute to female sexual receptivity in female mice.

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.

Subdivision of the accessory olfactory bulb in the Japanese common toad, Bufo japonicus, revealed by lectin histochemical analysis

Lectin binding patterns in the olfactory bulb of the Japanese common toad, Bufo japonicus, were examined using 21 types of lectin. Ten out of 21 lectins, WGA, s-WGA, LEL, STL, DBA, VVA, SJA, RCA-I, PNA, and PHA-L, stained the olfactory nerve, the glomeruli in the main olfactory bulb (MOB), the vomeronasal nerve, and the glomeruli in the accessory olfactory bulb (AOB). The binding patterns of LEL, STL, DBA, and PHA-L subdivided AOB glomeruli into rostral and caudal regions, where LEL, STL, and DBA stained the rostral region more intensely than the caudal region, and PHA-L had the opposite effect. Another lectin, BSL-I, stained both AOB glomeruli and the vomeronasal nerve, but not MOB glomeruli or the olfactory nerve. This is the first report of histological subdivision in the AOB of an amphibian, which suggests that the AOB development in Bufo may be unique.

Mouse major urinary proteins trigger ovulation via the vomeronasal organ

The major urinary proteins are a species-specific complex of proteins excreted by male mice that influence the reproductive behavior and the neuroendocrine condition of female mice through the olfactory system. The aim of this work is to determine their influence on ovulation. The major urinary proteins isolated from the urine of adult male mice were voided of bound odorants, dissolved at a physiological concentration in urine of prepubertal mice, and put on the nostril of reproductively cycling female mice housed in groups, the first day of estrus at 1100. The eggs shed in the oviducts were counted under dissection the morning of the second day of estrus. The results showed that 1) a single stimulus of the major urinary proteins increased ovulation nearly as much as the whole urine of male mice, 2) the effect was not elicited by male rat urine which contains different proteins, 3) a peptide with four residues of the amino-terminal sequence of the major urinary proteins stimulated ovulation, and 4) mice that had been isolated or had the vomeronasal organ (VNO) removed did not respond to the major urinary proteins and had a high spontaneous ovulation. The results suggest that the major urinary proteins activate the neuroendocrine system through the VNO and trigger ovulation.

The vomeronasal organ is required for the expression of lordosis behaviour, but not sex discrimination in female mice

The role of the vomeronasal organ (VNO) in mediating neuroendocrine responses in female mice is well known; however, whether the VNO is equally important for sex discrimination is more controversial as evidence exists for a role of the main olfactory system in mate recognition. Therefore, we studied the effect of VNO removal (VNOx) on the ability of female mice to discriminate between volatile and non-volatile odours of conspecifics of the two sexes and in different endocrine states using Y-maze tests. VNOx female mice were able to reliably distinguish between male and female or male and gonadectomized (gdx) male volatile odours. However, when subjects had to discriminate between male and female or gdx male non-volatile odours, VNOx females were no longer able to discriminate between sex or different endocrine status. These results thus show that the VNO is primarily involved in the detection and processing of non-volatile odours, and that female mice can use volatile odours detected and processed by the main olfactory system for mate recognition. However, VNO inputs are needed to promote contact with the male, including facilitation of lordosis responses to his mounts. A single subcutaneous injection with gonadotropin-releasing hormone (GnRH) partially reversed the deficit in lordosis behaviour observed in VNOx females suggesting that VNO inputs may reach hypothalamic GnRH neurons to influence the display of sexual behaviour.

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.

Increases in plasma concentration of progesterone by protease-sensitive urinary pheromones in female rats

Plasma progesterone concentrations in female Wistar rats after exposure to urine preparations with and without protease-treatment were measured to explore the effects of protease-sensitive pheromones on the endocrine state. Exposure to crude urine excreted from male rats induced an increase in the plasma progesterone concentration in female rats. The progesterone concentration of oestrous females increased with an increase in the protein concentration in urine samples. Exposure of females in the oestrous state to urine preparations treated with protease did not induce increases in plasma progesterone. These results suggest that the presence of a protease-sensitive component in male urine exerts an influence on the endocrine state of oestrous females.

Attraction to sexual pheromones and associated odorants in female mice involves activation of the reward system and basolateral amygdala

Adult female mice are innately attracted to non-volatile pheromones contained in male-soiled bedding. In contrast, male-derived volatiles become attractive if associated with non-volatile attractive pheromones, which act as unconditioned stimulus in a case of Pavlovian associative learning. In this work, we study the chemoinvestigatory behaviour of female mice towards volatile and non-volatile chemicals contained in male-soiled bedding, in combination with the analysis of c-fos expression induced by such a behaviour to clarify: (i) which chemosensory systems are involved in the detection of the primary attractive non-volatile pheromone and of the secondarily attractive volatiles; (ii) where in the brain male-derived non-volatile and volatile stimuli are associated to induce conditioned attraction for the latter; and (iii) whether investigation of these stimuli activates the cerebral reward system (mesocorticolimbic system including the prefrontal cortex and amygdala), which would support the view that sexual pheromones are reinforcing. The results indicate that non-volatile pheromones stimulate the vomeronasal system, whereas air-borne volatiles activate only the olfactory system. Thus, the acquired preference for male-derived volatiles reveals an olfactory-vomeronasal associative learning. Moreover, the reward system is differentially activated by the primary pheromones and secondarily attractive odorants. Exploring the primary attractive pheromone activates the basolateral amygdala and the shell of nucleus accumbens but neither the ventral tegmental area nor the orbitofrontal cortex. In contrast, exploring the secondarily attractive male-derived odorants involves activation of a circuit that includes the basolateral amygdala, prefrontal cortex and ventral tegmental area. Therefore, the basolateral amygdala stands out as the key centre for vomeronasal-olfactory associative learning.

Importance of olfactory and vomeronasal systems for male sexual function

Chemosensory cues stimulate male sexual arousal and behavior. The main olfactory system has an important role in attracting males to estrous females, and the vomeronasal receptors are important for activating accessory olfactory pathways that engage mating behavior in a sexually dimorphic manner. The gonadotropin releasing hormone (GnRH) neurons like the vomeronasal organ (VNO) neurons take their origin in the olfactory placode and migrate to the basal forebrain along pathfinder axons that take their origin in the developing VNO. The maturation of both systems is synchronized in time such that the early postnatal testosterone surge masculinizes the VNO neural relay en route to the medio preoptic area (MPOA). Although VNO slices and VNO receptor neurons in culture respond to volatile odors, in vivo electrophysiological recordings at the first relay in the accessory olfactory bulb (AOB) are silent until the male makes active nuzzling investigations of the female. The VNO neurons may therefore respond to volatiles that are transported into the organ on carrier peptides that themselves may play a part in receptor activation. In the context of modern molecular phylogenetic studies, it is becoming less likely that pheromones acting via the VNO have any part to play in human sexual behavior, but the possibility exists for conserved VNO genes influencing human reproduction via fertilization.

Information processing in the mammalian olfactory system

Recently, modern neuroscience has made considerable progress in understanding how the brain perceives, discriminates, and recognizes odorant molecules. This growing knowledge took over when the sense of smell was no longer considered only as a matter for poetry or the perfume industry. Over the last decades, chemical senses captured the attention of scientists who started to investigate the different stages of olfactory pathways. Distinct fields such as genetic, biochemistry, cellular biology, neurophysiology, and behavior have contributed to provide a picture of how odor information is processed in the olfactory system as it moves from the periphery to higher areas of the brain. So far, the combination of these approaches has been most effective at the cellular level, but there are already signs, and even greater hope, that the same is gradually happening at the systems level. This review summarizes the current ideas concerning the cellular mechanisms and organizational strategies used by the olfactory system to process olfactory information. We present findings that exemplified the high degree of olfactory plasticity, with special emphasis on the first central relay of the olfactory system. Recent observations supporting the necessity of such plasticity for adult brain functions are also discussed. Due to space constraints, this review focuses mainly on the olfactory systems of vertebrates, and primarily those of mammals.

Modification of Odor Investigation and Discrimination in Female Opossums (Monodelphis domestica) Following the Ablation of the Accessory Olfactory Bulbs

To determine whether the vomeronasal system of the Brazilian short-tailed opossum (Monodelphis domestica) is important to the response to conspecific chemical signals, the authors tested female opossums with conspecific odors, before and after ablation of their accessory olfactory bulbs (AOBs). Anesthesia and sham treatments did not modify females' discrimination of conspecific odors when tested against water, between male and female odors, or between different odors from the same male donors. Odor investigation was partially diminished following partial ablation of the AOB, and complete ablation of the AOBs further impaired the ability of females to discriminate between certain odors. These findings provide the first evidence for the importance of the vomeronasal system in the detection of chemosignals of known origin in opossums.

Pheromone reception in mammals

Pheromonal communication is the most convenient way to transfer information regarding gender and social status in animals of the same species with the holistic goal of sustaining reproduction. This type of information exchange is based on pheromones, molecules often chemically unrelated, that are contained in body fluids like urine, sweat, specialized exocrine glands, and mucous secretions of genitals. So profound is the relevance of pheromones over the evolutionary process that a specific peripheral organ devoted to their recognition, namely the vomeronasal organ of Jacobson, and a related central pathway arose in most vertebrate species. Although the vomeronasal system is well developed in reptiles and amphibians, most mammals strongly rely on pheromonal communication. Humans use pheromones too; evidence on the existence of a specialized organ for their detection, however, is very elusive indeed. In the present review, we will focus our attention on the behavioral, physiological, and molecular aspects of pheromone detection in mammals. We will discuss the responses to pheromonal stimulation in different animal species, emphasizing the complicacy of this type of communication. In the light of the most recent results, we will also discuss the complex organization of the transduction molecules that underlie pheromone detection and signal transmission from vomeronasal neurons to the higher centers of the brain. Communication is a primary feature of living organisms, allowing the coordination of different behavioral paradigms among individuals. Communication has evolved through a variety of different strategies, and each species refined its own preferred communication medium. From a phylogenetic point of view, the most widespread and ancient way of communication is through chemical signals named pheromones: it occurs in all taxa, from prokaryotes to eukaryotes. The release of specific pheromones into the environment is a sensitive and definite way to send messages to other members of the same species. Therefore, the action of an organism can alter the behavior of another organism, thereby increasing the fitness of either or both. Albeit slow in transmission and not easily modulated, pheromones can travel around objects in the dark and over long distances. In addition, they are emitted when necessary and their biosynthesis is usually economic. In essence, they represent the most efficient tool to refine the pattern of social behaviors and reproductive strategies.

Female-induced sexual arousal in male mice and rats: behavioral and testosterone response

Exposure of a male mouse to a female mouse separated from it by a holed partition induced specific behavior and an increase in blood testosterone in the male. The male made more approaches to the partition and spent more time at it. The time spent by the male mouse over the first 10 min at the partition, behind which an estrus female was placed, was increased sixfold compared to the time spent by a male mouse exposed to the vacant neighboring compartment; and 1.5-fold compared to that spent by a male mouse exposed to a nonreceptive female or a male. Increased blood testosterone level was detected at 20 min of exposure to a receptive female in winter and at 40 min in summer. No variation in blood testosterone levels in the male mouse exposed to a nonreceptive female or a male was observed. Similar response to a receptive female placed in the neighboring compartment was shown in a male rat. The time spent by the male rat at the partition was 12 times higher when there was an estrus female behind it than in control. Blood testosterone in the male rat increased in response to a female rat and did not change in response to a male rat indicating female-induced motivation. It was concluded that the partition time might serve as a quantitative measure of sexual motivation in the males and that the model of female-induced sexual arousal used was suitable for studying both motivational and hormonal components of sexual arousal in male mice and rats.

Altered social behavior in pituitary adenylate cyclase-activating polypeptide type I receptor-deficient mice

The olfactory bulb plays a critical role in odor discrimination and in processing olfactory cues controlling social behavior in mammals. Given that the pituitary adenylate cyclase-activating polypeptide (PACAP) type 1 receptor (PAC1) is highly expressed in the olfactory bulb, we examined its role in regulating olfaction and social investigation. We found that olfactory detection of nonsocial stimuli was similar in PAC1-deficient mice and wild-type (WT) littermates. In contrast, PAC1-deficient mice displayed markedly abnormal social behaviors. PAC1-deficient mice exhibited a faster decrease in social investigation after repeated exposure to social cues or ovariectomized female urine compared with WT mice. Moreover, PAC1-deficient females exhibited delayed affiliative behavior when housed with novel males, and PAC1-deficient males displayed excessive sexual mounting toward both females and males as well as reduced aggression and increased licking and grooming toward intruder males. In aggregate, these results uncover PAC1 signaling as an important factor in the development and/or functioning of neural pathways associated with pheromone processing and the regulation of social interactions in mice. In turn, these studies raise the potential clinical relevance of PACAP signaling dysfunctions in neuropsychiatric disorders characterized by social reciprocity impairments such as autism spectrum disorders.

Mice (Mus musculus) lacking a vomeronasal organ can discriminate MHC-determined odortypes

Major histocompatibility complex (MHC) genes in mammals (H-2 in mice) play a major role in regulating immune function. They also bestow individuality in the form of a chemical signature or odortype. At present, the respective contributions of the olfactory epithelium and the vomeronasal organ (VNO) in the recognition of individual odortypes are not well defined. We examined a possible role for the VNO in the recognition of MHC odortypes in mice by first removing the organ (VNX) and then training the mice to distinguish the odors of two congenic strains of mice that differed only in their MHC type. C57BL/6J mice (bb at H-2) and C57BL/6J-H-2(k) (kk at H-2) provided urine for sensory testing. Eight VNX and six sham-operated mice were trained to make the discrimination. Neither the number of training trials-to-criterion nor the rate of learning differed significantly for VNX and sham-operated mice. We conclude that the VNO is not necessary for learning to discriminate between MHC odortypes.

The nose knows who's who: chemosensory individuality and mate recognition in mice

Individual recognition is an important component of behaviors, such as mate choice and maternal bonding that are vital for reproductive success. This article highlights recent developments in our understanding of the chemosensory cues and the neural pathways involved in individuality discrimination in rodents. There appear to be several types of chemosensory signal of individuality that are influenced by the highly polymorphic families of major histocompatibility complex (MHC) proteins or major urinary proteins (MUPs). Both have the capability of binding small molecules and may influence the individual profile of these chemosignals in biological fluids such as urine, skin secretions, or saliva. Moreover, these proteins, or peptides associated with them, can be taken up into the vomeronasal organ (VNO) where they can potentially interact directly with the vomeronasal receptors. This is particularly interesting given the expression of major histocompatibility complex Ib proteins by the V2R class of vomeronasal receptor and the highly selective responses of accessory olfactory bulb (AOB) mitral cells to strain identity. These findings are consistent with the role of the vomeronasal system in mediating individual discrimination that allows mate recognition in the context of the pregnancy block effect. This is hypothesized to involve a selective increase in the inhibitory control of mitral cells in the accessory olfactory bulb at the first level of processing of the vomeronasal stimulus.

Wake up and smell the conspecific!

Mammals regulate their behavior using odor cues called pheromones. These compounds are detected and recognized by neurons in the accessory olfactory system. New electrophysiological recordings in behaving mice by Luo, Fee and Katz reveal aspects of pheromone biology and how these stimuli are represented by single neurons.

Sexual differentiation of the neuroendocrine mechanisms regulating mate recognition in mammals

When in breeding condition, male and female mammals seek out and mate with opposite-sex conspecifics. The neural mechanisms controlling mate recognition and heterosexual partner preference are sexually differentiated by the perinatal actions of sex steroid hormones. Many mammalian species use odours to identify potential mates. Thus, sex differences in partner preference may actually reflect sex differences in how male and female mammals perceive socially relevant odours. Two olfactory systems have evolved in vertebrates that differ considerably in their anatomy and function. It is generally believed that the main olfactory system is used to detect a wide variety of volatile odours derived from food prey among many sources, whereas the accessory olfactory system has evolved to detect and process primarily nonvolatile odours shown to influence reproductive behaviours and neuroendocrine functions. Some recent results obtained in oestradiol-deficient aromatase knockout (ArKO) mice that provide evidence for a developmental role of oestradiol in olfactory investigation of volatile body odours are discussed, suggesting that: (i) oestrogens contribute to the development of the main olfactory system and (ii) mate recognition is mediated by the main as opposed to the accessory olfactory system. Thus, sex differences in mate recognition and sexual partner preference may reflect sex differences in the perception of odours by the main olfactory system.

Encoding pheromonal signals in the accessory olfactory bulb of behaving mice

Many mammalian species rely on pheromones-semiochemicals produced by other members of the same species-to communicate social status and reproductive readiness. To assess how the central nervous system integrates the complex repertoire of pheromones, we recorded from single neurons in the accessory olfactory bulb, a nucleus that processes pheromonal signals, of male mice engaged in natural behaviors. Neuronal firing was robustly modulated by physical contact with male and female conspecifics, with individual neurons activated selectively by specific combinations of the sex and strain of conspecifics. We infer that mammals encode social and reproductive information by integrating vomeronasal sensory activity specific to sex and genetic makeup.

Olfactory signal transduction in the mouse septal organ

The septal organ, a distinct chemosensory organ observed in the mammalian nose, is essentially a small island of olfactory neuroepithelium located bilaterally at the ventral base of the nasal septum. Virtually nothing is known about its physiological properties and function. To understand the nature of the sensory neurons in this area, we studied the mechanisms underlying olfactory signal transduction in these neurons. The majority of the sensory neurons in the septal organ express olfactory-specific G-protein and adenylyl cyclase type III, suggesting that the cAMP signaling pathway plays a critical role in the septal organ as in the main olfactory epithelium (MOE). This is further supported by patch-clamp recordings from individual dendritic knobs of the sensory neurons in the septal organ. Odorant responses can be mimicked by an adenylyl cyclase activator and a phosphodiesterase inhibitor, and these responses can be blocked by an adenylyl cyclase inhibitor. There is a small subset of cells in the septal organ expressing a cGMP-stimulated phosphodiesterase (phosphodiesterase 2), a marker for the guanylyl cyclase-D subtype sensory neurons identified in the MOE. The results indicate that the septal organ resembles the MOE in major olfactory signal transduction pathways, odorant response properties, and projection to the main olfactory bulb. Molecular and functional analysis of the septal organ, which constitutes approximately 1% of the olfactory epithelium, will provide new insights into the organization of the mammalian olfactory system and the unique function this enigmatic organ may serve.

Ion conductances in supporting cells isolated from the mouse vomeronasal organ

The vomeronasal organ (VNO) is a chemosensory structure involved in the detection of pheromones in most mammals. The VNO sensory epithelium contains both neurons and supporting cells. Data suggest that vomeronasal neurons represent the pheromonal transduction sites, whereas scarce information is available on the functional properties of supporting cells. To begin to understand their role in VNO physiology, we have characterized with patch-clamp recording techniques the electrophysiological properties of supporting cells isolated from the neuroepithelium of the mouse VNO. Supporting cells were distinguished from neurons by their typical morphology and by the lack of immunoreactivity for Ggamma8 and OMP, two specific markers for vomeronasal neurons. Unlike glial cells in other tissues, VNO supporting cells exhibited a depolarized resting potential (about -29 mV). A Goldman-Hodgkin-Katz analysis for resting ion permeabilities revealed indeed an unique ratio of P(K):P(Na):P(Cl) = 1:0.23:1.4. Supporting cells also possessed voltage-dependent K(+) and Na(+) conductances that differed significantly in their biophysical and pharmacological properties from those expressed by VNO neurons. Thus glial membranes in the VNO can sustain significant fluxes of K(+) and Na(+), as well as Cl(-). This functional property might allow supporting cells to mop-up and redistribute the excess of KCl and NaCl that often occurs in certain pheromone-delivering fluids, like urine, and that could blunt the sensitivity of VNO neurons to pheromones. Therefore vomeronasal supporting cells could affect chemosensory transduction in the VNO by regulating the ionic strength of the pheromone-containing medium.

Got milk? A pheromonal message for newborn rabbits

A substance in rabbit milk, 2-methylbut-2-enal (2MB2), has been identified as a pheromone that triggers stereotypical searching behavior from rabbit pups. Pups respond to the odor of 2MB2 solutions in concentration-dependent manner, but fail to respond to 20 other volatile components in rabbit milk and 20 additional odorants. The effectiveness of 2MB2 generalizes across strains and breeds of rabbits, but is ineffective in closely related species. Finally, pup responsiveness to 2MB2 is innate and does not require learning. This study, for the first time, identifies a mammary pheromone that provides sufficient sensory cue for nipple attachment by newborns. In addition to contributing to our understanding of pheromonal communication, it provides an advantageous model system for neurobiologists.

Attractive properties of sexual pheromones in mice: innate or learned?

It is generally assumed that chemical signals (sexual pheromones) constitute the primary stimulus for sexual attraction in many mammals. However, it is unclear whether these pheromones are volatile or nonvolatile and which sensory systems are involved in their detection (vomeronasal and/or olfactory). Moreover, it has been demonstrated that experience influences the behavioral response to sexual pheromones and the sensory systems implicated. In order to clarify this issue, the attractive properties of volatile and nonvolatile components of the male-soiled bedding have been analyzed in female mice that had no previous experience with adult male-derived chemical signals (chemically naïve females) using two-choice preference tests. The results indicate that some nonvolatile male-derived substances exert an innate attraction to females, but volatiles derived from male-soiled bedding do not attract chemically nai;ve females. Therefore, the primary attractive sexual pheromone includes a nonvolatile compound (e.g. major urinary proteins, MUPs). On the other hand, male-derived volatiles become attractive to females because of repeated exposure to male-soiled bedding. This represents a Pavlovian-like associative learning in which previously neutral volatiles (very likely odorants) acquire attractive properties by association with the nonvolatile, innately attractive pheromone(s). These findings indicate that not only the sexual but also the 'chemical' experience (previous experience with sexual pheromones) has to be taken into account to interpret the role of chemicals as releaser or primer pheromones. The sensory systems involved in the detection of these stimuli and the neural basis of the odor-pheromone association are discussed.

Lesions of the vomeronasal organ disrupt mating-induced pair bonding in female prairie voles (Microtus ochrogaster)

The prairie vole (Microtus ochrogaster) is a highly social, monogamous species and displays pair bonding that can be assessed by the presence of selective affiliation with the familiar partner versus a conspecific stranger. In female prairie voles, exposure to a male or to male sensory cues is essential for estrus induction, and the subsequent mating facilitates pair bond formation. In the present study, we examined the role of the vomeronasal organ (VNO) in estrus induction and pair bonding in female prairie voles. VNO lesions did not alter olfaction mediated by the main olfactory system, but did prevent male-induced estrus induction. We by-passed the necessity of the VNO for estrus induction by estrogen priming the females. Despite the fact that all subjects displayed similar levels of mating, social contact and locomotor activities, VNO lesioned females failed to show mating-induced pair bonding whereas intact and sham-lesioned females displayed a robust preference for the familiar partner. Our data not only support previous findings that the VNO is important for estrus induction but also indicate that this structure is crucial for mating-induced pair bonding, suggesting an important role for the VNO in reproductive success in prairie voles.

Major urinary proteins, alpha(2U)-globulins and aphrodisin

The major urinary proteins (MUPs) are proteins secreted by the liver and filtered by the kidneys into the urine of adult male mice and rats, the MUPs of rats being also referred to as alpha(2U)-globulins. The MUP family also comprises closely related proteins excreted by exocrine glands of rodents, independently of their sex. The MUP family is an expression of a multi-gene family. There is complex hormonal and tissue-specific regulation of MUP gene expression. The multi-gene family and its outflow are characterized by a polymorphism which extends over species, strains, sexes, and individuals. There is evidence of evolutionary conservation of the genes and their outflow within the species and evidence of change between species. MUPs share the eight-stranded beta-barrel structure lining a hydrophobic pocket, common to lipocalins. There is also a high degree of structural conservation between mouse and rat MUPs. MUPs bind small natural odorant molecules in the hydrophobic pocket with medium affinity in the 10(4)-10(5) M(-1) range, and are excreted in the field, with bound odorants. The odorants are then released slowly in air giving a long lasting olfactory trace to the spot. MUPs seem to play complex roles in chemosensory signalling among rodents, functioning as odorant carriers as well as proteins that prime endocrine reactions in female conspecifics. Aphrodisin is a lipocalin, found in hamster vaginal discharge, which stimulates male copulatory behaviour. Aphrodisin does not seem to bind odorants and no polymorphism has been shown. Both MUPs and aphrodisin stimulate the vomeronasal organ of conspecifics.

Presence of the vomeronasal system in aquatic salamanders

Previous reports have indicated that members of the proteid family of salamanders lack a vomeronasal system, and this absence has been interpreted as representing the ancestral condition for aquatic amphibians. I examined the anatomy of the nasal cavities, nasal epithelia, and forebrains of members of the proteid family, mudpuppies (Necturus maculosus), as well as members of the amphiumid and sirenid families (Amphiuma tridactylum and Siren intermedia). Using a combination of light and transmission electron microscopy, I found no evidence that mudpuppies possess a vomeronasal system, but found that amphiuma and sirens possess both vomeronasal and olfactory systems. Amphiumids and sirenids are considered to be outgroups relative to proteids; therefore, these data indicate that the vomeronasal system is generally present in salamanders and has been lost in mudpuppies. Given that the vomeronasal system is generally present in aquatic amphibians, and that the last common ancestor of amphibians and amniotes is believed to have been fully aquatic, I conclude that the vomeronasal system arose in aquatic tetrapods and did not originate as an adaptation to terrestrial life. This conclusion has important implications for the hypothesis that the vomeronasal organ is specialized for detection of non-volatile compounds.

Increases in Fos-immunoreactivity after exposure to a combination of two male urinary components in the accessory olfactory bulb of the female rat

Exposure to either the dialyzed urine preparation (<500 Da) or the remaining substances (>500 Da) did not induce expression of Fos-immunoreactive cells in the mitral/tufted cell layer of the accessory olfactory bulb (AOB), whereas exposure to a mixture of these preparation did induce expression. These results suggest that a combination of low and high molecular weight substances is necessary for the increases in Fos-immunoreactivity in the AOB.

Glycosylated major urinary protein of the house mouse: characterization of its N-linked oligosaccharides

A minor component of the major urinary protein complex of the house mouse was chromatographically isolated and ascertained to be a previously suspected glycoprotein. Using highly sensitive mass-spectrometric techniques for sequencing and linkage analysis, the N-linked oligosaccharides of this glycoprotein were characterized. They were determined to be of the complex type with a wide heterogeneity. The heterogeneity was due to both the degree of sialylation and the presence of galactose residues in either beta(1-3) or beta(1-4) linkages. The biantennary structures were the most pronounced glycans, while tri- and tetraantennary entities were minor.

Expression patterns of glycoconjugates in the three distinctive olfactory pathways of the clawed frog, Xenopus laevis

Xenopus laevis has three distinctive olfactory neuroepithelia. We examined the axonal projection from each of these epithelia to the olfactory bulb by Di-I labeling, and confirmed that the Xenopus primary olfactory pathways involve the dorsal pathway from the olfactory epithelium to the dorsal region of the main olfactory bulb, the ventral pathway from the middle chamber epithelium to the ventral region of the main olfactory bulb, and the vomeronasal pathway from the vomeronasal epithelium to the accessory olfactory bulb. We next examined expression patterns of glycoconjugates in the three olfactory pathways by lectin-histochemistry using 21 biotinylated lectins. Fourteen out of 21 lectins stained the Xenopus primary olfactory system. RCA-I stained the three olfactory pathways uniformly. PHA-E stained only the dorsal pathway. LEL, STL, PNA, ECL and UEA-I stained the dorsal pathway more intensely than the ventral pathway, and among them, only UEA-I stained the vomeronasal pathway. In contrast, s-WGA, DBA, SBA, BSL-I VVA, SJA and PHA-L showed intense stainings in the ventral pathway and moderate stainings in the vomeronasal pathway, but faint or weak stainings in the dorsal pathway. These observations suggest that the ventral pathway expresses glycoconjugates shared commonly with either the dorsal or the vomeronasal pathway. In addition, from the binding patterns of the lectins with a binding specificity for N-acetylgalactosamine, glycoconjugates containing this saccharide seem to play an important role for the organization of the olfactory pathways.

The anatomy and functional role of the circumvallate papilla/von Ebner gland complex

We suggest that the description of the reciprocal relationships between the circumvallate papilla (CP) and von Ebner glands (VEGs) reported in most textbooks of anatomy must be rewritten. In the past, the VEG was described as a gland ancillary to the taste buds, mainly involved in the washing of the vallum around the CP or in perireceptorial events. Recent data obtained in our laboratory or reported in the literature indicate that both these structures form a single functional unity which could be called circumvallata papilla/von Ebner gland (CP/VEG) complex. The CP/VEG complex seems to represent an important enzyme- and pheromone-producing system composed of a sensitive (taste buds) and an effectory (VEG) branch linked by feedback mechanisms of control. In our hypothesis, the taste buds located in the distal portion of the VEG ductal system can be considered similar to the chemoreceptory cells located in other portions of the digestive apparatus such as pancreatic and bile ducts. Therefore, the CP/VEG complex represents a rare example of chemoreceptor-secretory organ.

Protease-sensitive urinary pheromones induce region-specific Fos-expression in rat accessory olfactory bulb

Vomeronasal organs of female Wistar rats were exposed with sprayed urine preparations of male Wistar rats prior to sacrifice. Exposure to crude urine and ultrafiltrated urine preparation (<5000 Da) induced significant Fos expression, which is correlated with cellular activity, in the mitral/tufted cell layer of the accessory olfactory bulb (AOB), while exposure to the remaining substances after ultrafiltration (>5000 Da) and control salt solution did not. Exposure to urine preparation treated with papain induced expression of Fos-immunoreactive cells in the rostral region of the AOB, but did not induce such expression in the caudal region. Exposure to urine preparation treated with pronase induced urine-specific Fos immunoreactivity neither in the rostral nor in the caudal region. These results suggest that at least two different peptides carrying pheromonal activities are contained in male Wistar rat urine.

Laminar distribution of pheromone-receptive neurons in rat vomeronasal epithelium

1. Responses of vomeronasal sensory neurons to urine excreted from rats, mice and hamsters were studied by the on-cell patch clamp method in slices of sensory epithelium from female Wistar rats. 2. The urine excreted from male and female Wistar rats, male Donryu rats and male C57BL/6 mice induced relatively large responses, while urine from male Sprague-Dawley rats and male Syrian hamsters induced small responses. 3. Of the 62 neurons responding to urine, 57 responded to only one of the urine preparations. 4. The sensory neurons that responded to the male Wistar urine were localized in the apical position of the epithelium where one type of GTP-binding protein, Gi2alpha, is selectively expressed. The neurons in the basal position of the epithelium, which express Goalpha, responded to urine from the other animals. 5. This study demonstrates that sensory neurons responsive to different urinary pheromones are localized in a segregated layer in the rat vomeronasal sensory epithelium.

A unique urinary constituent, 6-hydroxy-6-methyl-3-heptanone, is a pheromone that accelerates puberty in female mice

BACKGROUND

Olfactorily mediated puberty acceleration in female mice (measured by an increase in uterine weight) has been observed since the 1960s without the active chemosignal being structurally identified. There are many controversies in the literature as to whether this male-originated pheromone is a volatile substance. We investigated the chemical nature of the urinary fractions that are responsible for the characteristic uterine weight increases.

RESULTS

The active pheromone was identified as 5,5-dimethyl-2-ethyltetrahydrofuran-2-ol and/or its open-chain tautomer (6-hydroxy-6-methyl-3-heptanone). A series of cyclic vinyl ethers were isolated from chromatographically active fractions of the urine. Because these compounds did not accelerate puberty, we postulated that these ethers were degradation products of a lactol (5,5-dimethyl-2-ethyltetrahydrofuran-2-ol). The lactol was then detected directly in the mouse urine extract using a silylation agent. Synthetic 6-hydroxy-6-methyl-3-heptanone had strong biological activity, whereas its close structural analogs did not.

CONCLUSIONS

The male house mouse excretes into its urine a large quantity of a volatile substance that has a unique lactol/hydroxyketone structure. This substance is capable of binding to the less volatile urinary constituents, such as proteins or peptides, and is active in puberty-acceleration bioassays. The controversies regarding the volatility of the puberty-accelerating pheromones can now be explained by considering a complex of volatile lactol/hydroxyketone and urinary proteins.

Inositol-1,4,5-trisphosphate accumulation induced by urinary pheromones in female rat vomeronasal epithelium

The mechanisms involved in pheromone-induced responses in the vomeronasal neurons, especially in mammals, are still unclear. In the present study, we examined the effects of rat urine samples containing various types of pheromones regulating gonadal functions on the accumulation of cAMP and inositol 1,4,5-trisphosphate (IP3) in a vomeronasal membrane preparation from the female Wistar rat. Stimulation of the preparation with forskolin induced cAMP accumulation, but stimulation with urine samples excreted from the male Wistar rat, the female Wistar rat, and the male Donryu rat did not change cAMP levels. These results were consistent with the electrophysiological results showing that dialysis of a high concentration of cAMP into the vomeronasal neuron does not induce currents. Stimulation with the three urine samples induced the accumulation of IP3 in the membrane preparation. These results are consistent with previous electrophysiological results [K. Inamura, M. Kashiwayanagi, K. Kurihara, Inositol-1,4,5-trisphosphate induces responses in receptor neurons in rat vomeronasal sensory slices, Chem. Senses 22 (1997) 93-103; K. Inamura, M. Kashiwayanagi, K. Kurihara, Blockage of urinary responses by inhibitors for IP3-mediated pathway in rat vomeronasal sensory neurons, Neurosci. Lett. 233 (1997) 129-132]. After the treatment with Pertussis toxin (PTX), the male Wistar urine did not induce IP3 accumulation significantly. Application of the male Wistar urine decreased ADP-ribosylation of Gi with PTX, while that of the male Donryu urine decreased ADP-ribosylation of Go. Thus, the present results support a mechanism by which the responses of the rat vomeronasal neurons to urinary pheromones are mediated by IP3, Gi and/or Go.

Effects of vomeronasal organ removal on individual odor discrimination, sex-odor preference, and scent marking by female hamsters

Removal of the vomeronasal organ (VNX) did not eliminate the ability of female hamsters to discriminate between individual male's flank gland or urine odors in a habituation/discrimination task nor did it impair preference for male odors over female odors from a distance. Vomeronasal organ removal did reduce overall levels of investigation of flank gland odor in the habituation/discrimination task. Although VNX females did not show severe impairments in the frequency of either flank or vaginal marking in response to odors, they did show an abnormal pattern of marking. VNX females, unlike shams, did not flank mark more to female odors than to male odors, nor did they vaginal mark more to male odors than to female odors. Thus, the vomeronasal organ in female hamsters appears to be important for differences in scent marking toward male and female odors, but is not essential for discrimination of individual odors or for preferences for male over female odors.

Pheromones, the vomeronasal system, and communication. From hormonal responses to individual recognition

The functions of the vomeronasal and olfactory systems are evaluated within the broad context of chemical communication in mammals. Five main points are stressed. First, there are a variety of types of chemical signals, ranging from classic pheromones to odor quality signals, all of which are important for communication. Second, the vomeronasal organ is primarily responsible for mediating responses to some, but by no means all, pheromone-like signals. Third, the olfactory system mediates responses to some pheromone-like signals. Fourth, the vomeronasal system mediates some responses to odor quality signals. Thus, not all receptors in the vomeronasal organ are involved in detection of pheromones. Fifth, the vomeronasal and olfactory systems often interact to mediate responses to odor signals. In sum, it is important to maintain a broad, balanced view and to avoid oversimplifications if we are to advance our understanding of the sensory mechanisms underlying responses to chemical signals and other odors.

Amygdala but not hippocampal lesions impair olfactory memory for mate in prairie voles (Microtus ochrogaster)

Exposure to an unfamiliar male conspecific results in pregnancy interruption (i.e., the Bruce effect) in rodents. Unlike most laboratory rodents, female prairie voles (Microtus ochrogaster) are induced into estrus by chemosensory stimuli contained in the urine of male conspecifics while grooming the anogenital (A-G) region of unfamiliar males. Female prairie voles maintain a brief "memory" for the stud male for 8-10 days after mating. Subsequent exposure to the same mate within this 8- to 10-day window does not elicit A-G investigation by the female and pregnancy block does not result. However, exposure to the original male after 10 days evokes A-G investigation and pregnancy block. To determine the neuroanatomic area(s) involved in olfactory memory for mate, female voles received bilateral electrolytic lesions of either the amygdala or hippocampus. Females were subsequently exposed to males for 48 h, separated for 3 days, then reintroduced to their original mate for 24 h. Although pregnancy rate did not differ among the experimental groups, a greater proportion of amygdala-lesioned females displayed pregnancy block when reexposed to their previous mates compared with hippocampal- or sham-lesioned voles. Amygdala-lesioned voles also displayed a greater number of A-G investigations compared with the other groups. Performance on olfactory tests was not impaired. Taken together, these results suggest that the amygdala plays an important role in olfactory memory for mate in prairie voles.

Blockage of urinary responses by inhibitors for IP3-mediated pathway in rat vomeronasal sensory neurons

The mammalian vomeronasal system is involved in the effects of urinary chemicals on gonadal functions and sexual behaviors. For example, exposure to urine affects the timing of oestrous cycles in rats. Rat vomeronasal sensory neurons in slice preparation were studied under on-cell patch clamp conditions. We found that urine excreted from male Wistar rats increased impulse frequency in vomeronasal sensory neurons of female Wistar rats. The urinary responses were blocked by an inositol-1,4,5-trisphosphate (IP3)-channel inhibitor (10 microM ruthenium red) or phospholipase C inhibitors (10 microM U-73122 and 1 mM neomycin), suggesting that pheromone-like substances in the urine induce the response in the rat vomeronasal sensory neurons via the IP3-dependent transduction pathway.

A new multigene family of putative pheromone receptors

The vomeronasal organ (VNO) mediates detection of pheromones related to social and reproductive behavior in most terrestrial vertebrates. We have identified a new multigene family of G protein-linked receptors (V2Rs) that are specifically expressed in the VNO. V2Rs have no significant homology to other putative pheromone receptors (V1Rs) or to olfactory receptors but are related to the Ca2+-sensing receptor and metabotropic glutamate receptors. V2Rs are expressed at high levels in small subpopulations of VNO neurons. V2Rs are primarily expressed in a different layer of VNO neurons from V1Rs, thus both gene families are likely to encode mammalian pheromone receptors.

Pheromone transduction in the vomeronasal organ

Single-cell physiology and cloning efforts have extended studies of the vomeronasal organ to cellular and molecular levels. Recent work has shown that transduction in the vomeronasal organ is probably mediated by signalling pathways distinct from those that mediate transduction in the main olfactory system. An advance in understanding transduction has come with the cloning from rat vomeronasal organ of a family of putative pheromone receptor genes that bear no sequence similarity to previously cloned receptors. Other work has examined the expression of putative signalling components and found a zonal organization of the epithelium. Patch-clamp studies have described the basic electrical properties of vomeronasal neurons and explored second-messenger pathways.

Vomeronasal system involvement in response to conspecific odors in adult male opossums, Monodelphis domestica

The present study was designed to test the hypothesis that preferential nuzzling of unfamiliar conspecific odors by male opossums is vomeronasally mediated. Opossums were presented with vials containing their own odors (Own) and vials with odors from unfamiliar conspecifics (Novel) both before and after (Expt. 1) or only after (Expt. 2) vomeronasal nerve (VNNX) or sham (Sham) surgery. Nuzzling duration was greater for Novel before (Expt. 1) and after (Expts. 1 and 2) surgery both in VNNX and Sham animals, indicating that a functional vomeronasal system is not necessary for the differential nuzzling of novel odors. In contrast, differential scent marking of Novel was lost following VNNX, but not following sham surgery. Since VNNX lesions disrupt differential marking behavior but, not differential nuzzling, it is possible that different components of conspecific odors guide these two chemosensory behaviors and that these components may be detected by different sensory systems. Vomeronasal nerve lesions, confirmed histologically, resulted in the loss of the beta-NADPH-dependent reaction product from the accessory olfactory bulbs (AOB). The pattern of staining with 4 lectins, soybean agglutinin (SBA), Phosphocarpus tetragonolobus (PCT), Vicia villosa agglutinin (VVA), and Griffonia simplicifolia lectin I-isolectin B4 (GS I-B4) remained unchanged in the AOB following vomeronasal nerve lesions, although the size of the AOB appeared to decrease in the VNNX animals.

Differential expression of odorant-binding protein genes in rat nasal glands: implications for odorant-binding proteinII as a possible pheromone transporter

We examined the distribution and ontogeny of two odorant-binding proteins in the rat at various stages of development from newborn to adult using northern blot and in situ hybridization methods. Our results demonstrated spatial segregation between odorant-binding protein and odorant-binding proteinII in nasal glandular tissues. Odorant-binding protein messenger RNA was expressed in the glandular system opening into the nasal vestibule, whereas odorant-binding proteinII messenger RNA was seen in the posterior glands of the nasal septum and in the vomeronasal glands. In addition, odorant-binding protein and odorant-binding proteinII messenger RNA levels increased during early postnatal stages with time courses that paralleled the anatomical development of the main olfactory system and the vomeronasal system, respectively. Our results suggest that odorant-binding proteinII functions as a pheromone transporter in the vomeronasal system.

Analysis of the possible altering function of the septal organ in rats: a lesional and behavioral study

The septal olfactory organ is a small patch of sensory epithelium located on the septal wall at the entry of the nasopharynx. There is a general consensus that by sampling olfactory stimuli during periods of rest, this organ may have an alerting function. To verify this hypothesis, we have lesioned by electrocoagulation the septal organ of male rats and recorded by polygraphy their awakening reaction in response to biologically meaningful (trimethyl-thiazoline, dimethyl sulfite, food) and meaningless (geraniol, eucalyptol) odorants. The awakening reactions of both lesioned and intact rats in response to these odorants were studied according to 3 parameters, frequency, latency and duration of awakening, and were analyzed using three-way analyses of variance. Data show that no significant difference in the awakening reactions was observed between control and lesioned animals. In all cases, the biologically meaningful odors presented the highest awakening influence. In addition, two of these odors (trimethyl-thiazoline and dimethyl sulfite) elicited a later habituation in comparison to biologically meaningless odors. From our results, it could be inferred that the hypothesis regarding an alerting function that would be specific to the septal organ, appears no longer current.