Importance of olfactory and vomeronasal systems for male sexual function

The vomeronasal organ of the tammar wallaby

The vomeronasal organ is the primary olfactory organ that detects sexual pheromones in mammals. We investigated the anatomy of the vomeronasal organ of the tammar wallaby (Macropus eugenii), a small macropodid marsupial. Pheromones may be important for activation of the hypothalamo-pituitary axis of tammar males at the start of the breeding season because plasma testosterone and luteinizing hormone concentration in males rise concurrently with pregnancy and the post-partum ovulation in females. The gross anatomy and the connection to the brain of the vomeronasal organ were examined by light and electron microscopy in adult male and female tammars. The vomeronasal organ was well developed in both sexes. The vomeronasal organ is a tubular organ connected at the rostral end via the nasopalatine duct (incisive duct) to the mouth and nasal cavity. At the rostral end the lumen of the vomeronasal organ was crescent shaped, changing to a narrow oval shape caudally. Glandular tissue associated with the vomeronasal organ increased towards the blind end of the organ. The tammar has the typical pattern of mammalian vomeronasal organs with electron-dense supporting cells and electron-lucent receptor cells. Microvilli were present on the surface of both epithelia while cilia were only found on the surface of the non-receptor epithelium. Some non-receptor epithelial cells appeared to secrete mucus into the vomeronasal organ lumen. The vomeronasal organ shows a high degree of structural conservation compared with eutherian mammals. The degree of vomeronasal organ development makes it likely that, as in other mammals, pheromones are important in the reproduction of the tammar.

Lesions that functionally disconnect the anterior and posterodorsal sub-regions of the medial amygdala eliminate opposite-sex odor preference in male Syrian hamsters (Mesocricetus auratus)

In many rodent species, such as Syrian hamsters, reproductive behavior requires neural integration of chemosensory information and steroid hormone cues. The medial amygdala (MA) processes both of these signals through anatomically distinct sub-regions; the anterior region (MeA) receives substantial chemosensory input, but contains few steroid receptor-labeled neurons, whereas the posterodorsal region (MePD) receives less chemosensory input, but contains a dense population of steroid receptors. Importantly, these sub-regions have considerable reciprocal connections, and the goal of this experiment was therefore to determine whether interactions between MeA and MePD are required for male hamsters' preference to investigate female over male odors. To functionally disconnect MeA and MePD, males received unilateral lesions of MeA and MePD within opposite brain hemispheres. Control males received either unilateral lesions of MeA and MePD within the same hemisphere or sham surgery. Odor preferences were measured using a 3-choice apparatus, which simultaneously presented female, male and clean odor stimuli; all tests were done under conditions that either prevented or allowed contact with the odor sources. Under non-contact conditions, males with asymmetrical lesions investigated female and male odors equally, whereas males in both control groups preferred to investigate female odors. Under contact conditions, all groups investigated female odors longer than male odors, although males with asymmetrical lesions displayed decreased investigation of female odors compared to sham males. These data suggest that MeA-MePD interactions are critical for processing primarily the volatile components of social odors and highlight the importance of input from the main olfactory system (MOS) to these nuclei in the regulation of reproductive behavior. More broadly, these results support the role of the MA in integrating chemosensory and hormone information, a process that may underlie social odor processing in a variety of behavioral contexts.

Sensing odorants and pheromones with chemosensory receptors

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

Simple behavioral assessment of mouse olfaction

This unit presents two basic protocols that offer rapid assessments of anosmia (the absence of a sense of smell) in mice. The buried food test is used to check for the ability to smell volatile odors. The olfactory habituation/dishabituation test is used to test whether the animal can detect and differentiate different odors, including both nonsocial and social odors. A non-contact method of odor presentation, along with a general method for collecting urine samples, is given as an alternate protocol. The tests described in this unit only require simple equipment and can be adopted readily by most laboratories.

The underestimated role of olfaction in avian reproduction?

Until the second half of the 20th century, it was broadly accepted that most birds are microsmatic if not anosmic and unable to detect and use olfactory information. Exceptions were eventually conceded for species like procellariiforms, vultures or kiwis that detect their food at least in part based on olfactory signals. During the past 20-30 years, many publications have appeared indicating that this view is definitely erroneous. We briefly review here anatomical, electrophysiological and behavioral data demonstrating that birds in general possess a functional olfactory system and are able to use olfactory information in a variety of ethological contexts, including reproduction. Recent work also indicates that brain activation induced by sexual interactions with a female is significantly affected by olfactory deprivation in Japanese quail. Brain activation was measured via immunocytochemical detection of the protein product of the immediate early gene c-fos. Changes observed concerned two brain areas that play a key role in the control of male sexual behavior, the medial preoptic nucleus and the bed nucleus of the stria terminalis therefore suggesting a potential role of olfaction in the control of reproduction. The widespread idea that birds are anosmic or microsmatic is thus not supported by the available experimental data and presumably originates in our anthropomorphic view that leads us to think that birds do not smell because they have a rigid beak and nostrils and do not obviously sniff. Experimental analysis of this phenomenon is thus warranted and should lead to a significant change in our understanding of avian biology.

The main and the accessory olfactory systems interact in the control of mate recognition and sexual behavior

In the field of sensory perception, one noticeable fact regarding olfactory perception is the existence of several olfactory subsystems involved in the detection and processing of olfactory information. Indeed, the vomeronasal or accessory olfactory system is usually conceived as being involved in the processing of pheromones as it is closely connected to the hypothalamus, thereby controlling reproductive function. By contrast, the main olfactory system is considered as a general analyzer of volatile chemosignals, used in the context of social communication, for the identification of the status of conspecifics. The respective roles played by the main and the accessory olfactory systems in the control of mate recognition and sexual behavior are at present still controversial. We summarize in this review recent results showing that both the main and accessory olfactory systems are able to process partially overlapping sets of sexual chemosignals and that both systems support complimentary aspects in mate recognition and in the control of sexual behavior.

Increased apoptosis during neonatal brain development underlies the adult behavioral deficits seen in mice lacking a functional paternally expressed gene 3 (Peg3)

Inactivation of the maternally imprinted, paternally expressed gene 3 (Peg3) induces deficits in olfactory function, sexual and maternal behaviors, oxytocin neuron number, metabolic homeostasis and growth. Peg3 is expressed in a number of developing hypothalamic and basal forebrain structures and is a component of the P53 apoptosis pathway. Peg3 inactivation in neuronal cell culture lines inhibits P53 mediated apoptosis, which is important in the early postnatal development and sexual differentiation of the brain. In this study, we investigated the effect of inactivating the Peg3 gene on the incidence of caspase 3 positive cells (a marker of apoptosis) in 4- and 6-day postpartum mouse brain. Inactivating the Peg3 gene resulted in an increase in the incidence of total forebrain caspase 3 positive cells at 4 and 6 days postpartum. Increases in specific neuroanatomical regions including the bed nucleus of the stria terminalis, nucleus accumbens, caudate putamen, medial pre-optic area, arcuate nucleus, medial amygdala, anterior cortical and posteriodorsal amygdaloid nuclei, were also observed. In wild-type mice, sex differences in the incidence of caspase 3 positive cells in the medial amygdala, bed nucleus of the stria terminalis, nucleus accumbens, arcuate nucleus and the M2 motor cortex, were also observed. This neural sex difference was ameliorated in the Peg-3 mutant. These findings suggest that the neuronal and behavioral deficits seen in mice lacking a functional Peg3 gene are mediated by increases in the incidence of early neonatal apoptosis in neuroanatomical regions important for reproductive behavior, olfactory and pheromonal processing, thermoregulation and reward.

Age-dependent spatial distribution of bromodeoxyuridine-immunoreactive cells in the main olfactory bulb

The subventricular zone (SVZ) generates an immense number of neurons, which migrate to the main olfactory bulb (MOB) and differentiate into granule cells and periglomerular cells in the MOB, even during adulthood. Pheromonal signals, which are mainly received by the vomeronasal organ, provide specific information concerning the reproductive state in a variety of mammal. Vomeronasal sensory neurons project to the accessory olfactory bulb (AOB) located on the dorso-caudal surface of the MOB. In the present study, bromodeoxyuridine-immunoreactive (BrdU-ir) structures in the sagittal section of the MOB including the AOB of young and old male rats were studied to explore the roles of newly generated cells at the region near the rostral end of AOB in the MOB. The density of BrdU-ir cells in the granule cell layer of the MOB of young rats was higher than that of old rats. In young rats, the density of BrdU-ir cells at the region near the rostral end of the AOB was higher than that at the region distant from the AOB. In old rats, the density of BrdU-ir cells at the near region was lower than that at the distant region. The density of BrdU-ir cells at the region near the AOB in the MOB in old rats may be concerned with age-dependent changes in ability of discrimination and memory of general odors and odors related to the reproduction.

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

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

Olfactory cues are sufficient to elicit social approach behaviors but not social transmission of food preference in C57BL/6J mice

Mouse models for the study of autistic-like behaviors are increasingly needed to test hypotheses about the causes of autism, and to evaluate potential treatments. Both the automated three-chambered social approach test and social transmission of food preference have been proposed as mouse behavioral assays with face validity to diagnostic symptoms of autism, including aberrant reciprocal social interactions and impaired communication. Both assays measure aspects of normal social behavior in the mouse. However, little is known regarding the salient cues present in each assay that elicit normal social approach and communication. To deconstruct the critical components, we focused on delivering discrete social and non-social olfactory and visual cues within the context of each assay. Results indicate that social olfactory cues were sufficient to elicit normal sociability in the three-chambered social approach test. On social transmission of food preference, isolated social olfactory cues were sufficient to induce social investigation, but not sufficient to induce food preference. These findings indicate that olfactory cues are important in mouse social interaction, but that additional sensory cues are necessary in certain situations. The present evidence that both the three-chambered social approach assay and the social transmission of food preference assay require socially relevant cues to elicit normal behavior supports the use of these two assays to investigate autism-related behavioral phenotypes in mice.

Sexual communication via peptide and protein pheromones

Pheromones are specific substances utilized by various organisms for intraspecific communication about sex, strain, or species. Although pheromones in terrestrial animals tend to be volatile airborne chemicals, large non-volatile molecules such as peptides and proteins are also utilized for sociosexual communication. Peptide pheromones are recognized by specific receptors expressed in the vertebrate vomeronasal organ that comprises a unique chemosensory system. The information is sent to the hypothalamic area wherein the signal is further integrated, leading to various pheromonal outputs. In this review, current knowledge on the structure and function of peptide and protein pheromones in vertebrates as well as the mechanisms underlying receptor-mediated signal processing will be summarized. The present review will also discuss why, from chemical and ecological points of view, peptide pheromones evolved.

Impaired sexual behavior in male mice deficient for the beta1-3 N-acetylglucosaminyltransferase-I gene

The beta1-3 N-acetylglucosaminyltransferase-1 (B3gnt1) gene encodes a poly-N-acetyllactosamine synthase which can initiate and extend poly-N-acetyllactosamine chains [Gal(beta1-4)GlcNAc (beta1-3)(n)]. Previous investigations with heterozygous and homozygous null mice for this gene have revealed the importance of poly-N-acetyllactosamine chains for the formation of olfactory axon connections with the olfactory bulb and the migration of gonadotropin releasing hormone neurons to the hypothalamus. The possible long-term effects of these developmental defects, however, has not yet been studied. Here we have examined a reproductive phenotype observed in B3gnt1-null mice. Whereas the B3gnt1 null females were fertile, the B3gnt1 null males were not able to sire litters at the expected rate when mated to either wildtype or B3gnt1-null females. We assessed male sexual behavior as well as male reproduction parameters such as testes size, spermatogenesis, sperm number, morphology, and the development of early embryos in order to identify the source of a reduced rate of reproduction. Our findings show that the B3gnt1 null male reproductive organs were functional and could not account for the lower rate at which they produced offspring with wildtype conspecifics. Hence, we propose that the phenotype observed resulted from an impaired sexual response to female mating partners.

Site-specific effects of anosmia and cloacal gland anesthesia on Fos expression induced in male quail brain by sexual behavior

In rats, expression of the immediate early gene, c-fos observed in the brain following male copulatory behavior relates mostly to the detection of olfactory information originating from the female and to somatosensory feedback from the penis. However, quail, like most birds, are generally considered to have a relatively poorly developed sense of smell. Furthermore, quail have no intromittent organ (e.g., penis). It is therefore intriguing that expression of male copulatory behavior induces in quail and rats a similar pattern of c-fos expression in the medial preoptic area (mPOA), bed nucleus of the stria terminalis (BSTM) and parts of the amygdala. We analyzed here by immunocytochemistry Fos expression in the mPOA/BSTM/amygdala of male quail that had been allowed to copulate with a female during standardized tests. Before these tests, some of the males had either their nostrils plugged, or their cloacal area anesthetized, or both. A control group was not exposed to females. These manipulations did not affect frequencies of male sexual behavior and all birds exposed to a female copulated normally. In the mPOA, the increased Fos expression induced by copulation was not affected by the cloacal gland anesthesia but was markedly reduced in subjects deprived of olfactory input. Both manipulations affected copulation-induced Fos expression in the BSTM. No change in Fos expression was observed in the amygdala. Thus immediate early gene expression in the mPOA and BSTM of quail is modulated at least in part by olfactory cues and/or somatosensory stimuli originating from the cloacal gland. Future work should specify the nature of these stimuli and their function in the expression of avian male sexual behavior.

[The existence vomeronasal organ in adult humans]

The influence of chemical substances (feromones) on human emotional and physical condition has fascinated psychologists, sexuologists and laryngologists since centurie. Literature conveys inconsistent information on vomeronasal organ (VNO) occurrence in humans. This organ is often called Jacobson's, and 2 symmetrical openings leading into it, located on both sides of septum, are called Ruyasch's ducts. The aim of the study was to analyze vomeronasal organ occurrence in humans in relation to age and sex. The study was conducted in a group of 634 patients, aged 18-80 years. All patients underwent routine ENT examination including rhinoscopy, nasal cavity examination with usage of 2.5x magnification lens (surgical glasses) and surgical microscope with 10x magnification. All persons had nasal cavities examined endoscopically. Every time presence of vomeronasal organ openings, along with localization, size and symmetry of these was noted. Subjects, who presented Jacobson's organ, were asked to fill a questionnaire concerning influence of smells on erotic sensations. Vomeronasal organ was fund in 312 persons, that is 49.21%. In 83.65% of cases vomeronasal organ opening size was smaller than 0.2 mm, what restricted its visibility to usage of magnifying lens, microscope, or endoscope. In 16.34% of cases only vomeronasal organ ducts openings were well visible in routine rhinoscopy without magnification. Vomeronasal organ was found more often in men than women. VNO was significantly more rare in patients with nasal septal deviation. In these cases, vomeronasal organ was usually found unilaterally, in all the cases on the concave side of deviated nasal septum.

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

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

The function of Dmrt genes in vertebrate development: It is not just about sex

The Dmrt genes encode a large family of transcription factors whose function in sexual development has been well studied in invertebrates and vertebrates. Their expression pattern is not restricted to the developing gonads, indicating that Dmrt genes might regulate other developmental processes. Here we review the expression pattern of several members of this family across species and summarize recent findings on the function of a subset of these genes in non-gonadal tissues.

Neurotransmitter candidates in the vomeronasal organ of the rat

CONCLUSION

The rich supply of nerve fibres containing neurotransmitters, particularly those containing SP and CGRP, is suggested to be a prerequisite for the recognition of chemical irritants as part of a chemical sense.

OBJECTIVE

The present study was designed to examine the distribution of different neurotransmitter candidates in the vomeronasal organ (VNO) of rats.

MATERIALS AND METHODS

The distribution of neurotransmitter candidates was studied in the vomeronasal organ of the rat using immunocytochemistry.

RESULTS

The neuronal marker protein gene product 9.5 revealed a very rich supply of nerve fibres within and beneath the sensory epithelium, around blood vessels and glands. A moderate supply of nerve fibres containing tyrosine hydroxylase and neuropeptide Y was mostly seen close to blood vessels. Numerous nerve fibres containing nitric oxide synthase and vasoactive intestinal peptide were seen around blood vessels and in the subepithelial layer, with occasional fibres within the epithelium. Only few fibres located in the subepithelial layer contained pituitary adenylate cyclase activating peptide. Nerve fibres containing substance P and in particular calcitonin gene-related peptide were abundant in and beneath the epithelium and scattered in the submucosal layers around blood vessels.

Sex differences in catechol contents in the olfactory bulb of control and unilaterally deprived rats

The dopaminergic system plays important roles in the modulation of olfactory transmission. The present study examines the distribution of dopaminergic cells and the content of dopamine (DA) and its metabolites in control and deprived olfactory bulbs (OB), focusing on the differences between sexes. The content of DA and of its metabolites, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), were measured by HPLC. The morphology and distribution of dopaminergic neurons were studied using tyrosine hydroxylase (TH) immunohistochemistry. Cells were typified with TH-parvalbumin, TH-cholecystokinin or TH-neurocalcin double-immunofluorescence assays. Biochemical analyses revealed sex differences in the content of DA and of its metabolites. In normal conditions, the OBs of male rats had higher concentrations of DA, DOPAC and HVA than the OBs of females. The immunohistochemical data pointed to sex differences in the number of TH-immunopositive cells (higher in male than in female rats). Colocalization analyses revealed that dopaminergic cells constitute a different cell subpopulation from those labelled after parvalbumin, cholecystokinin or neurocalcin immunostaining. Unilateral olfactory deprivation caused dramatic alterations in the dopaminergic system. The DA content and the density of dopaminergic cells decreased, the contents of DA and DOPAC as well as TH immunoreactivity were similar in deprived males and females and, finally, the metabolite/neurotransmitter ratio increased. Our results show that the dopaminergic modulation of olfactory transmission seems to differ between males and females and that it is regulated by peripheral olfactory activity. A possible role of the dopaminergic system in the sexually different olfactory sensitivity, discrimination and memory is discussed.

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.

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.

Identification and characterization of new protein chemoattractants in the frog skin secretome

The vomeronasal organ is a chemosensory organ present in most vertebrates and involved in chemical communication. In the last decade, the deciphering of the signal transduction process of this organ has progressed. However, less is known about the vomeronasal organ ligands and their structure-function relationships. Snakes possess a highly developed vomeronasal system that is used in various behaviors such as mating, predator detection, or prey selection, making this group a suitable model for study of the vomeronasal chemoreception. In this work, we used a proteomics approach to identify and characterize proteins from frog cutaneous mucus proteome involved in prey recognition by snakes of the genus Thamnophis. Herein we report the purification and characterization of two proteins isolated from the frog skin secretome that elicit the vomeronasal organ-mediated predatory behavior of Thamnophis marcianus. These proteins are members of the parvalbumin family, which are calcium-binding proteins generally associated to muscular and nervous tissues. This is the first report that demonstrates parvalbumins are not strictly restricted to intracellular compartments and can also be isolated from exocrine secretions. Purified parvalbumins from frog muscle and mucus revealed identical chemoattractive properties for T. marcianus. Snake bioassay revealed the Ca(2+)/Mg(2+) dependence of the bioactivity of parvalbumins. So parvalbumins appear to be new candidate ligands of the vomeronasal organ.

Accessory chemosignaling mechanisms in primates

Accessory olfaction is defined as the chemoreceptive system that employs the vomeronasal complex (VNC) and its distinct central projections to the accessory olfactory bulb (AOB) and limbic/cortical systems. Comparisons of the structural and functional features of primate accessory olfaction can now be made at many levels. Advances in the understanding of molecular mechanisms of odorant transfer and detection, physiological analyses of signal processing, and appreciation of ontogenetic timetables have clarified the contribution of accessory chemoreception to the sensory map. Two principal functions dominate: the decoding of social information through the uptake of signals (often fluid-borne), and the provision of an essential pathway for the "migration" of presumptive neurocrine (GnRH) cells from the olfactory placode to the hypothalamus. VN "smelling" (vomerolfaction) is now seen to overlap with primary olfaction. Both systems detect signal compounds along the spectrum of volatility/molecular weight, and neither is an exclusive sensor. Both main and accessory chemoreception seem to require collaborative molecular devices to assist in odorant transfer (binding proteins) and (for the VNO) signal recognition (MHC1 proteins). Most adaptive-selective features of primate chemocommunication variously resemble those of other terrestrial mammals. VN function, along with its genome, has been maintained within the Strepsirrhines and tarsiers, reduced in Platyrrhines, and nearly extinguished at the Catarrhine up to hominin levels. It persists as an intriguing ancient sense that retains key features of past evolutionary events.

Amygdaloid lesion-induced obesity: relation to sexual behavior, olfaction, and the ventromedial hypothalamus

Lesions of the amygdala have long been known to produce hyperphagia and obesity in cats, dogs, and monkeys, but only recently have studies with rats determined that the effective site is the posterodorsal amygdala (PDA)-the posterodorsal medial amygdaloid nucleus and the intra-amygdaloid bed nucleus of the stria terminalis. There is a sex difference; female rats with PDA lesions display greater weight gain than male rats. In the brains of female rats with obesity-inducing PDA lesions, there is a dense pattern of axonal degeneration in the capsule of the ventromedial hypothalamus (VMH) and other targets of the stria terminalis. Transections of the dorsal component of the stria terminalis also result in hyperphagia and obesity in female rats. Similar to rats with VMH lesions, rats with PDA lesions are hyperinsulinemic during food restriction and greatly prefer high-carbohydrate diets. The PDA is also a critical site for some aspects of rodent sexual behavior, particularly those that depend on olfaction, and the pattern of degeneration observed after obesity-inducing PDA lesions is remarkably parallel to the circuit that has been proposed to mediate sexual behavior. Medial amygdaloid lesions disrupt the normal feeding pattern and result in impaired responses to caloric challenges, and there is evidence that these behavioral changes are also due to a disruption of olfactory input. With its input from the olfactory bulbs and connections to the VMH, the PDA may be a nodal point at which olfactory and neuroendocrine stimuli are integrated to affect feeding behavior.

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.

Decreases in pheromonal responses at the accessory olfactory bulb of mice with a deficiency of the alpha1B or beta3 subunits of voltage-dependent Ca2+-channels

Pheromones affect gonadal functions and sexual behaviors. Information in regard to pheromones is received by the vomeronasal organ (VNO) and transmitted to the accessory olfactory bulb (AOB). We investigated the physiological role of the alpha1B and beta3 subunits of the N (neuronal)-type voltage-dependent Ca2+ channel in the neurotransduction in the accessory olfactory (vomeronasal) system using alpha1B-deficient mice and beta3-deficient mice. RT-PCR studies showed the existence of beta1, beta2, beta3, beta4, alpha1A, alpha1B, and alpha1C subunits of voltage-dependent Ca2+ channels in the mouse VNO. Immunohistochemical studies showed that the alpha1A, alpha1B, and alpha1C subunits of voltage-dependent Ca2+ channels exist in the sensory neurons and supporting cells of the mouse VNO. Exposure of the VNO to urine samples excreted from male mice induced lower Fos-immunoreactivity in the periglomerular (PG) cells of the AOBs in alpha1B-deficient female mice than in those of wild mice. The density of Fos-immunoreactive (Fos-ir) cells after exposure to female urine samples at the periglomerular cell layer of alpha1B-deficient male mice was lower than that of wild mice. Exposure of the VNO of beta3-deficient female mice to male urine samples also induced low Fos-ir cells in the periglomerular cell layer of the AOB. These data suggest the importance of the alpha1B and beta3 subunits of the N-type voltage-dependent Ca2+ channel for the pheromone signal transduction system.

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.

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.

Female choice and the MHC

In animals, it is the female that typically selects a mating partner. This decision can occur before, during and after copulation. Here, recent evidence for the involvement of genes within the MHC in female choice is reviewed and the roles of MHC I and II antigens, various types of chemoreceptors, as well as MHC-encoded transcription factors, in securing an optimal genetic constitution of the offspring are discussed. Some particularly interesting and as yet unanswered questions are raised and some experiments that could provide deeper insight into the molecular mechanisms underlying female choice are suggested.

Gonadotropin-releasing hormone (GnRH) immunoreactive neurons in male mouse lemurs following removal of the vomeronasal organ

Removal of the vomeronasal organ (VNO) in male mouse lemurs led to an increase in the number of immunoreactive gonadotropin-releasing hormone (GnRH) neurons in the medial preoptic area, compared to control males. No difference was found in the mediobasal hypothalamus. In this primate, which presents a fully functional VNO, the anterior part of the hypothalamus could be the major target for VNO-mediated regulation of GnRH function and the subsequent modulation of chemosensory dependent reproductive behavior.

Age effects on pheromone induced Fos expression in olfactory bulbs of a primate

In the gray mouse lemur, a prosimian primate, aging is associated with a reduction of olfactory behaviors and sexual stimulation. To assess the effect of aging on the central response to pheromone stimulation in this primate, we measured the c-fos expression in the main and accessory olfactory bulbs of adult and aged male mouse lemurs, following exposure to the volatile phase of urine from proestrous females. In adults, pheromone exposure increased the number of Fos-positive neurons in the main olfactory bulb without changes in the accessory olfactory bulb. Fos expression was not increased by the odorant stimulation in aged mouse lemurs. Our results may explain the age-related decrease in behaviors associated with olfactory stimulation in this primate.