Exocrine Gland-Secreting Peptide 1 Is a Key Chemosensory Signal Responsible for the Bruce Effect in Mice

Presynaptic GABAB receptors inhibit vomeronasal nerve transmission to accessory olfactory bulb mitral cells

Vomeronasal sensory neurons (VSNs) recognize pheromonal and kairomonal semiochemicals in the lumen of the vomeronasal organ. VSNs send their axons along the vomeronasal nerve (VN) into multiple glomeruli of the accessory olfactory bulb (AOB) and form glutamatergic synapses with apical dendrites of mitral cells, the projection neurons of the AOB. Juxtaglomerular interneurons release the inhibitory neurotransmitter γ-aminobutyric acid (GABA). Besides ionotropic GABA receptors, the metabotropic GABAB receptor has been shown to modulate synaptic transmission in the main olfactory system. Here we show that GABAB receptors are expressed in the AOB and are primarily located at VN terminals. Electrical stimulation of the VN provokes calcium elevations in VSN nerve terminals, and activation of GABAB receptors by the agonist baclofen abolishes calcium influx in AOB slice preparations. Patch clamp recordings reveal that synaptic transmission from the VN to mitral cells can be completely suppressed by activation of GABAB receptors. A potent GABAB receptor antagonist, CGP 52432, reversed the baclofen-induced effects. These results indicate that modulation of VSNs via activation of GABAB receptors affects calcium influx and glutamate release at presynaptic terminals and likely balances synaptic transmission at the first synapse of the accessory olfactory system.

A chemical signal in human female tears lowers aggression in males

Rodent tears contain social chemosignals with diverse effects, including blocking male aggression. Human tears also contain a chemosignal that lowers male testosterone, but its behavioral significance was unclear. Because reduced testosterone is associated with reduced aggression, we tested the hypothesis that human tears act like rodent tears to block male aggression. Using a standard behavioral paradigm, we found that sniffing emotional tears with no odor percept reduced human male aggression by 43.7%. To probe the peripheral brain substrates of this effect, we applied tears to 62 human olfactory receptors in vitro. We identified 4 receptors that responded in a dose-dependent manner to this stimulus. Finally, to probe the central brain substrates of this effect, we repeated the experiment concurrent with functional brain imaging. We found that sniffing tears increased functional connectivity between the neural substrates of olfaction and aggression, reducing overall levels of neural activity in the latter. Taken together, our results imply that like in rodents, a human tear-bound chemosignal lowers male aggression, a mechanism that likely relies on the structural and functional overlap in the brain substrates of olfaction and aggression. We suggest that tears are a mammalian-wide mechanism that provides a chemical blanket protecting against aggression.

In vivo spontaneous activity and coital-evoked inhibition of mouse accessory olfactory bulb output neurons

Little is known about estrous effects on brain microcircuits. We examined the accessory olfactory bulb (AOB) in vivo, in anesthetized naturally cycling females, as model microcircuit receiving coital somatosensory information. Whole-cell recordings demonstrate that output neurons are relatively hyperpolarized in estrus and unexpectedly fire high frequency bursts of action potentials. To mimic coitus, a calibrated artificial vagino-cervical stimulation (aVCS) protocol was devised. aVCS evoked stimulus-locked local field responses in the interneuron layer independent of estrous stage. The response is sensitive to α1-adrenergic receptor blockade, as expected since aVCS increases norepinephrine release in AOB. Intriguingly, only in estrus does aVCS inhibit AOB spike output. Estrus-specific output reduction coincides with prolonged aVCS activation of inhibitory interneurons. Accordingly, in estrus the AOB microcircuit sets the stage for coital stimulation to inhibit the output neurons, possibly via high frequency bursting-dependent enhancement of reciprocal synapse efficacy between inter- and output neurons.

The Bruce effect: Representational stability and memory formation in the accessory olfactory bulb of the female mouse

In the Bruce effect, a mated female mouse becomes resistant to the pregnancy-blocking effect of the stud. Various lines of evidence suggest that this form of behavioral imprinting results from reduced sensitivity of the female's accessory olfactory bulb (AOB) to the stud's chemosignals. However, the AOB's combinatorial code implies that diminishing responses to one individual will distort representations of other stimuli. Here, we record extracellular responses of AOB neurons in mated and unmated female mice while presenting urine stimuli from the stud and from other sources. We find that, while initial sensory responses in the AOB (within a timescale required to guide social interactions) remain stable, responses to extended stimulation (as required for eliciting the pregnancy block) display selective attenuation of stud-responsive neurons. Such temporal disassociation could allow attenuation of slow-acting endocrine processes in a stimulus-specific manner without compromising ongoing representations that guide behavior.

A single vomeronasal receptor promotes intermale aggression through dedicated hypothalamic neurons

The pheromonal information received by the vomeronasal system plays a crucial role in regulating social behaviors such as aggression in mice. Despite accumulating knowledge of the brain regions involved in aggression, the specific vomeronasal receptors and the exact neural circuits responsible for pheromone-mediated aggression remain unknown. Here, we identified one murine vomeronasal receptor, Vmn2r53, that is activated by urine from males of various strains and is responsible for evoking intermale aggression. We prepared a purified pheromonal fraction and Vmn2r53 knockout mice and applied genetic tools for neuronal activity recording, manipulation, and circuit tracing to decipher the neural mechanisms underlying Vmn2r53-mediated aggression. We found that Vmn2r53-mediated aggression is regulated by specific neuronal populations in the ventral premammillary nucleus and the ventromedial hypothalamic nucleus. Together, our results shed light on the hypothalamic regulation of male aggression mediated by a single vomeronasal receptor.

Lymphocytic Choriomeningitis Virus Alters the Expression of Male Mouse Scent Proteins

Mature male mice produce a particularly high concentration of major urinary proteins (MUPs) in their scent marks that provide identity and status information to conspecifics. Darcin (MUP20) is inherently attractive to females and, by inducing rapid associative learning, leads to specific attraction to the individual male’s odour and location. Other polymorphic central MUPs, produced at much higher abundance, bind volatile ligands that are slowly released from a male’s scent marks, forming the male’s individual odour that females learn. Here, we show that infection of C57BL/6 males with LCMV WE variants (v2.2 or v54) alters MUP expression according to a male’s infection status and ability to clear the virus. MUP output is substantially reduced during acute adult infection with LCMV WE v2.2 and when males are persistently infected with LCMV WE v2.2 or v54. Infection differentially alters expression of darcin and, particularly, suppresses expression of a male’s central MUP signature. However, following clearance of acute v2.2 infection through a robust virus-specific CD8 cytotoxic T cell response that leads to immunity to the virus, males regain their normal mature male MUP pattern and exhibit enhanced MUP output by 30 days post-infection relative to uninfected controls. We discuss the likely impact of these changes in male MUP signals on female attraction and mate selection. As LCMV infection during pregnancy can substantially reduce embryo survival and lead to lifelong infection in surviving offspring, we speculate that females use LCMV-induced changes in MUP expression both to avoid direct infection from a male and to select mates able to develop immunity to local variants that will be inherited by their offspring.

Origin and Evolution of the Gene Family of Proteinaceous Pheromones, the Exocrine Gland-Secreting Peptides, in Rodents

The exocrine-gland secreting peptide (ESP)gene family encodes proteinaceous pheromones that are recognized by the vomeronasal organ in mice. For example, ESP1 is a male pheromone secreted in tear fluid that regulates socio-sexual behavior, and ESP22 is a juvenile pheromone that suppresses adult sexual behavior. The family consists of multiple genes and has been identified only in mouse and rat genomes. The coding region of a mouse ESP gene is separated into two exons, each encoding signal and mature sequences. Here, we report the origin and evolution of the ESP gene family. ESP genes were found only in the Muridea and Cricetidae families of rodents, suggesting a recent origin of ESP genes in the common ancestor of murids and cricetids. ESP genes show a great diversity in number, length, and sequence among different species as well as mouse strains. Some ESPs in rats and golden hamsters are expressed in the lacrimal gland and the salivary gland. We also found that a mature sequence of an ESP gene showed overall sequence similarity to the α-globin gene. The ancestral ESP gene seems to be generated by recombination of a retrotransposed α-globin gene with the signal-encoding exon of the CRISP2 gene located adjacent to the ESP gene cluster. This study provides an intriguing example of molecular tinkering in rapidly evolving species-specific proteinaceous pheromone genes.

Coding of pheromones by vomeronasal receptors

Communication between individuals is critical for species survival, reproduction, and expansion. Most terrestrial species, with the exception of humans who predominantly use vision and phonation to create their social network, rely on the detection and decoding of olfactory signals, which are widely known as pheromones. These chemosensory cues originate from bodily fluids, causing attractive or avoidance behaviors in subjects of the same species. Intraspecific pheromone signaling is then crucial to identify sex, social ranking, individuality, and health status, thus establishing hierarchies and finalizing the most efficient reproductive strategies. Indeed, all these features require fine tuning of the olfactory systems to detect molecules containing this information. To cope with this complexity of signals, tetrapods have developed dedicated olfactory subsystems that refer to distinct peripheral sensory detectors, called the main olfactory and the vomeronasal organ, and two minor structures, namely the septal organ of Masera and the Grueneberg ganglion. Among these, the vomeronasal organ plays the most remarkable role in pheromone coding by mediating several behavioral outcomes that are critical for species conservation and amplification. In rodents, this organ is organized into two segregated neuronal subsets that express different receptor families. To some extent, this dichotomic organization is preserved in higher projection areas of the central nervous system, suggesting, at first glance, distinct functions for these two neuronal pathways. Here, I will specifically focus on this issue and discuss the role of vomeronasal receptors in mediating important innate behavioral effects through the recognition of pheromones and other biological chemosignals.

Semiochemical responsive olfactory sensory neurons are sexually dimorphic and plastic

Understanding how genes and experience work in concert to generate phenotypic variability will provide a better understanding of individuality. Here, we considered this in the main olfactory epithelium, a chemosensory structure with over a thousand distinct cell types in mice. We identified a subpopulation of olfactory sensory neurons, defined by receptor expression, whose abundances were sexually dimorphic. This subpopulation of olfactory sensory neurons was over-represented in sex-separated mice and robustly responsive to sex-specific semiochemicals. Sex-combined housing led to an attenuation of the dimorphic representations. Single-cell sequencing analysis revealed an axis of activity-dependent gene expression amongst a subset of the dimorphic OSN populations. Finally, the pro-apoptotic gene Baxwas necessary to generate the dimorphic representations. Altogether, our results suggest a role of experience and activity in influencing homeostatic mechanisms to generate a robust sexually dimorphic phenotype in the main olfactory epithelium.

Maternally inherited peptides as strain-specific chemosignals

Most mammals rely on chemosensory cues for individual recognition, which is essential to many aspects of social behavior, such as maternal bonding, mate recognition, and inbreeding avoidance. Both volatile molecules and nonvolatile peptides secreted by individual conspecifics are detected by olfactory sensory neurons in the olfactory epithelium and the vomeronasal organ. The pertinent cues used for individual recognition remain largely unidentified. Here we show that nonformylated, but not N-formylated, mitochondrially encoded peptides-that is, the nine N-terminal amino acids of NADH dehydrogenases 1 and 2-can be used to convey strain-specific information among individual mice. We demonstrate that these nonformylated peptides are sufficient to induce a strain-selective pregnancy block. We also observed that the pregnancy block by an unfamiliar peptide derived from a male of a different strain was prevented by a memory formed at the time of mating with that male. Our findings also demonstrate that pregnancy-blocking chemosignals in the urine are maternally inherited, as evidenced by the production of reciprocal sons from two inbred strains and our test of their urine's ability to block pregnancy. We propose that this link between polymorphic mitochondrial peptides and individual recognition provides the molecular means to communicate an individual's maternal lineage and strain.

Vulpeculin: a novel and abundant lipocalin in the urine of the common brushtail possum, Trichosurus vulpecula

Lipocalins are a family of secreted proteins. They are capable of binding small lipophilic compounds and have been extensively studied for their role in chemosignalling in rodent urine. Urine of the common brushtail possum (Trichosurus vulpecula) contains a prominent glycoprotein of 20 kDa, expressed in both sexes. We have isolated this protein and determined its primary sequence by mass spectrometry, including the use of metabolic labelling to resolve the leucine/isoleucine isobaric ambiguity. The protein sequence was identified as a lipocalin, and phylogenetic analysis grouped the protein with other marsupial lipocalin sequences in a phylogenetic clade distinct from established cross-species lipocalin sub-families. The pattern of expression in possum urine and the similarity in sequence and structure to other lipocalins suggests this protein may have a role in brushtail possum chemosignalling.

Female mouse tears contain an anti-aggression pheromone

Tears contain pheromones that trigger specific behavioral responses. In the mouse, male tear fluid is involved in long and short-term effects such as the receptive behavior and pregnancy block in females and the aggression in males. In contrast, pup tears exert an inhibitory effect on male mating behavior, also promoting sexual rejection in females. In the rat, a male lacrimal protein acts as an intraspecific and heterospecific signal enhancing sexual behavior in females and evoking avoidance behavior in mouse. However, behavioral effects of female tears on male behavior have yet to be described. Here, we report that female lacrimal fluid of different mouse strains contains a relatively small and involatile factor that abolishes inter-male aggression switching it into a copulatory behavior. The production of this molecule by the lacrimal glands is not affected by the estrous cycle but it is sensitive to ovariectomy, thus suggesting a control mediated by hormones. Moreover, this lacrimal anti-aggression pheromone modulates the activity of the lateral habenula, a brain area responsible for the valence of the aggressive interactions.

Mouse protein coding diversity: What's left to discover?

For over a century, mice have been used to model human disease, leading to many fundamental discoveries about mammalian biology and the development of new therapies. Mouse genetics research has been further catalysed by a plethora of genomic resources developed in the last 20 years, including the genome sequence of C57BL/6J and more recently the first draft reference genomes for 16 additional laboratory strains. Collectively, the comparison of these genomes highlights the extreme diversity that exists at loci associated with the immune system, pathogen response, and key sensory functions, which form the foundation for dissecting phenotypic traits in vivo. We review the current status of the mouse genome across the diversity of the mouse lineage and discuss the value of mice to understanding human disease.

Data quality and the comparative method: the case of pregnancy failure in rodents

In mammalian species where infanticide by males is likely, females exhibit counterstrategies to prevent or mitigate the costs of infanticide. One putative mitigation strategy is the “Bruce effect,” in which pregnant or inseminated females exposed to an unfamiliar male experience pregnancy block or failure. Females then mate with the new male, thus shifting investment from a “doomed” pregnancy to a more fruitful one. However, the Bruce effect may be an adaptive response to other factors besides infanticide. For example, if paternal care is necessary for offspring survival, and an unfamiliar male replacing the original mate is unlikely to provide such care to offspring of a litter it did not sire, then a female may terminate a pregnancy to initiate a new one. The infanticide and paternal care hypotheses have not been rigorously tested because comparative data on the Bruce effect across mammals are scarce. We compiled data on the Bruce effect, infanticide, and paternal care from one particularly rich source of information, rodents, but found the data set to be less rich than expected. The Bruce effect, infanticide, and paternal care were common among rodent species, but we found no clear relationship among the traits. However, this was likely due to 1) a bias toward positive results, 2) missing data, and 3) a reliance on studies of captive animals. These are common problems in comparative research, and we outline standards that should be implemented to successfully answer questions of importance in the field.

Male-mediated prenatal loss: Functions and mechanisms

Sexually selected infanticide has been the subject of intense empirical and theoretical study for decades; a related phenomenon, male-mediated prenatal loss, has received much less attention in evolutionary studies. Male-mediated prenatal loss occurs when inseminated or pregnant females terminate reproductive effort following exposure to a nonsire male, either through implantation failure or pregnancy termination. Male-mediated prenatal loss encompasses two sub-phenomena: sexually selected feticide and the Bruce effect. In this review, we provide a framework that explains the relationship between feticide and the Bruce effect and describes what is known about the proximate and ultimate mechanisms involved in each. Using a simple model, we demonstrate that male-mediated prenatal loss can provide greater reproductive benefits to males than infanticide. We therefore suggest that, compared to infanticide, male-mediated prenatal loss may be more prevalent in mammalian species and may have played a greater role in their social evolution than has previously been documented.

Neural circuits regulating sexual behaviors via the olfactory system in mice

Reproduction is essential for any animal species. Reproductive behaviors, or sexual behaviors, are largely shaped by external sensory cues exchanged during sexual interaction. In many animals, including rodents, olfactory cues play a critical role in regulating sexual behavior. What exactly these olfactory cues are and how they impact animal behavior have been a central question in the field. Over the past few decades, many studies have dedicated to identifying an active compound that elicits sexual behavior from crude olfactory components. The identified substance has served as a tool to dissect the sensory processing mechanisms in the olfactory systems. In addition, recent advances in genetic engineering, and optics and microscopic techniques have greatly expanded our knowledge of the neural mechanisms underlying the control of sexual behavior in mice. This review summarizes our current knowledge about how sexual behaviors are controlled by olfactory cues.

Sexual rejection via a vomeronasal receptor-triggered limbic circuit

Mating drive is balanced by a need to safeguard resources for offspring, yet the neural basis for negative regulation of mating remains poorly understood. In rodents, pheromones critically regulate sexual behavior. Here, we observe suppression of adult female sexual behavior in mice by exocrine gland-secreting peptide 22 (ESP22), a lacrimal protein from juvenile mice. ESP22 activates a dedicated vomeronasal receptor, V2Rp4, and V2Rp4 knockout eliminates ESP22 effects on sexual behavior. Genetic tracing of ESP22-responsive neural circuits reveals a critical limbic system connection that inhibits reproductive behavior. Furthermore, V2Rp4 counteracts a highly related vomeronasal receptor, V2Rp5, that detects the male sex pheromone ESP1. Interestingly, V2Rp4 and V2Rp5 are encoded by adjacent genes, yet couple to distinct circuits and mediate opposing effects on female sexual behavior. Collectively, our study reveals molecular and neural mechanisms underlying pheromone-mediated sexual rejection, and more generally, how inputs are routed through olfactory circuits to evoke specific behaviors.

Sex pheromones that increase mating have been reported across a number of different species, yet there is little known about pheromones that suppress female mating drive. This study reports that juvenile female mice release a pheromone, ESP22, which suppresses sexual receptivity of adult female mice by evoking a robust rejection behavior upon male mounting.

Neural control of sexually dimorphic social behaviors

Sexually reproducing animals display sex differences in behavior. Although many of these sex differences in behavior are acquired with experience, sexually dimorphic behaviors such as mating and aggression are innate in the sense that they can be displayed without prior training or experience. In this review, we present recent advances in our understanding of the neural control of such innate sexually dimorphic social behaviors, with a focus on sexual behavior and aggression in flies and mice. We provide a brief overview of fundamental processes that regulate sexual differentiation in these animals to provide a framework within which more recent advances can be understood. We discuss advances in sensory, neuromodulatory, neural circuit, and experiential regulation of sexually dimorphic social behaviors.

Individual odour signatures that mice learn are shaped by involatile major urinary proteins (MUPs)

Background

Reliable recognition of individuals requires phenotypic identity signatures that are both individually distinctive and appropriately stable over time. Individual-specific vocalisations or visual patterning are well documented among birds and some mammals, whilst odours play a key role in social recognition across many vertebrates and invertebrates. Less well understood, though, is whether individuals are recognised through variation in cues that arise incidentally from a wide variety of genetic and non-genetic differences between individuals, or whether animals evolve distinctive polymorphic signals to advertise identity reliably. As a bioassay to understand the derivation of individual-specific odour signatures, we use female attraction to the individual odours of male house mice (Mus musculus domesticus), learned on contact with a male’s scent marks.

Results

Learned volatile odour signatures are determined predominantly by individual differences in involatile major urinary protein (MUP) signatures, a specialised set of communication proteins that mice secrete in their urine. Recognition of odour signatures in genetically distinct mice depended on differences in individual MUP genotype. Direct manipulation using recombinant MUPs confirmed predictable changes in volatile signature recognition according to the degree of matching between MUP profiles and the learned urine template. Both the relative amount of the male-specific MUP pheromone darcin, which induces odour learning, and other MUP isoforms influenced learned odour signatures. By contrast, odour recognition was not significantly influenced by individual major histocompatibility complex genotype. MUP profiles shape volatile odour signatures through isoform-specific differences in binding and release of urinary volatiles from scent deposits, such that volatile signatures were recognised from the urinary protein fraction alone. Manipulation using recombinant MUPs led to quantitative changes in the release of known MUP ligands from scent deposits, with MUP-specific and volatile-specific effects.

Conclusions

Despite assumptions that many genes contribute to odours that can be used to recognise individuals, mice have evolved a polymorphic combinatorial MUP signature that shapes distinctive volatile signatures in their scent. Such specific signals may be more prevalent within complex body odours than previously realised, contributing to the evolution of phenotypic diversity within species. However, differences in selection may also result in species-specific constraints on the ability to recognise individuals through complex body scents.

Electronic supplementary material

The online version of this article (10.1186/s12915-018-0512-9) contains supplementary material, which is available to authorized users.