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

Glutamatergic pathways in the brains of turtles: A comparative perspective among reptiles, birds, and mammals

Glutamate acts as the main excitatory neurotransmitter in the brain and plays a vital role in physiological and pathological neuronal functions. In mammals, glutamate can cause detrimental excitotoxic effects under anoxic conditions. In contrast, Trachemys scripta, a freshwater turtle, is one of the most anoxia-tolerant animals, being able to survive up to months without oxygen. Therefore, turtles have been investigated to assess the molecular mechanisms of neuroprotective strategies used by them in anoxic conditions, such as maintaining low levels of glutamate, increasing adenosine and GABA, upregulating heat shock proteins, and downregulating KATP channels. These mechanisms of anoxia tolerance of the turtle brain may be applied to finding therapeutics for human glutamatergic neurological disorders such as brain injury or cerebral stroke due to ischemia. Despite the importance of glutamate as a neurotransmitter and of the turtle as an ideal research model, the glutamatergic circuits in the turtle brain remain less described whereas they have been well studied in mammalian and avian brains. In reptiles, particularly in the turtle brain, glutamatergic neurons have been identified by examining the expression of vesicular glutamate transporters (VGLUTs). In certain areas of the brain, some ionotropic glutamate receptors (GluRs) have been immunohistochemically studied, implying that there are glutamatergic target areas. Based on the expression patterns of these glutamate-related molecules and fiber connection data of the turtle brain that is available in the literature, many candidate glutamatergic circuits could be clarified, such as the olfactory circuit, hippocampal–septal pathway, corticostriatal pathway, visual pathway, auditory pathway, and granule cell–Purkinje cell pathway. This review summarizes the probable glutamatergic pathways and the distribution of glutamatergic neurons in the pallium of the turtle brain and compares them with those of avian and mammalian brains. The integrated knowledge of glutamatergic pathways serves as the fundamental basis for further functional studies in the turtle brain, which would provide insights on physiological and pathological mechanisms of glutamate regulation as well as neural circuits in different species.

Mechanisms underlying pre- and postnatal development of the vomeronasal organ

The vomeronasal organ (VNO) is sensory organ located in the ventral region of the nasal cavity in rodents. The VNO develops from the olfactory placode during the secondary invagination of olfactory pit. The embryonic vomeronasal structure appears as a neurogenic area where migratory neuronal populations like endocrine gonadotropin-releasing hormone-1 (GnRH-1) neurons form. Even though embryonic vomeronasal structures are conserved across most vertebrate species, many species including humans do not have a functional VNO after birth. The vomeronasal epithelium (VNE) of rodents is composed of two major types of vomeronasal sensory neurons (VSNs): (1) VSNs distributed in the apical VNE regions that express vomeronasal type-1 receptors (V1Rs) and the G protein subunit Gαi2, and (2) VSNs in the basal territories of the VNE that express vomeronasal type-2 receptors (V2Rs) and the G subunit Gαo. Recent studies identified a third subclass of Gαi2 and Gαo VSNs that express the formyl peptide receptor family. VSNs expressing V1Rs or V2Rs send their axons to distinct regions of the accessory olfactory bulb (AOB). Together, VNO and AOB form the accessory olfactory system (AOS), an olfactory subsystem that coordinates the social and sexual behaviors of many vertebrate species. In this review, we summarize our current understanding of cellular and molecular mechanisms that underlie VNO development. We also discuss open questions for study, which we suggest will further enhance our understanding of VNO morphogenesis at embryonic and postnatal stages.

An amygdala circuit that suppresses social engagement

Innate social behaviours, such as mating and fighting, are fundamental to animal reproduction and survival¹. However, social engagements can also put an individual at risk². Little is known about the neural mechanisms that enable appropriate risk assessment and the suppression of hazardous social interactions. Here we identify the posteromedial nucleus of the cortical amygdala (COApm) as a locus required for the suppression of male mating when a female mouse is unhealthy. Using anatomical tracing, functional imaging and circuit-level epistatic analyses, we show that suppression of mating with an unhealthy female is mediated by the COApm projections onto the glutamatergic population of the medial amygdalar nucleus (MEA). We further show that the role of the COApm-to-MEA connection in regulating male mating behaviour relies on the neuromodulator thyrotropin-releasing hormone (TRH). TRH is expressed in the COApm, whereas the TRH receptor (TRHR) is found in the postsynaptic MEA glutamatergic neurons. Manipulating neural activity of TRH-expressing neurons in the COApm modulated male mating behaviour. In the MEA, activation of the TRHR pathway by ligand infusion inhibited mating even towards healthy female mice, whereas genetic ablation of TRHR facilitated mating with unhealthy individuals. In summary, we reveal a neural pathway that relies on the neuromodulator TRH to modulate social interactions according to the health status of the reciprocating individual. Individuals must balance the cost of social interactions relative to the benefit, as deficits in the ability to select healthy mates may lead to the spread of disease.

Cyclophosphamide has Long-Term Effects on Proliferation in Olfactory Epithelia

Chemotherapy patients often experience chemosensory changes during and after drug therapy. The chemotherapy drug, cyclophosphamide (CYP), has known cytotoxic effects on sensory and proliferating cells of the taste system. Like the taste system, cells in the olfactory epithelia undergo continuous renewal. Therefore, we asked if a single injection of 75 mg/kg CYP would affect cell proliferation in the anterior dorsomedial region of the main olfactory epithelium (MOE) and the vomeronasal organ (VNO) from 0 to 125 days after injection. Both epithelia showed a decrease in Ki67-labeled cells compared to controls at day 1 and no Ki67+ cells at day 2 postinjection. In the sensory layer of the MOE, cell proliferation began to recover 4 days after CYP injection and by 6 days, the rate of proliferation was significantly greater than controls. Ki67+ cells peaked 30 days postinjection, then declined to control levels at day 45. Similar temporal sequences of initial CYP-induced suppression of cell proliferation followed by elevated rates peaking 30-45 days postinjection were seen in the sustentacular layer of the MOE and all 3 areas (sensory, sustentacular, marginal) of the VNO. CYP affected proliferation in the sensory layer of the MOE more than the sustentacular layer and all 3 areas of the VNO. These findings suggest that chemotherapy involving CYP is capable of affecting cell renewal of the olfactory system and likely contributes to clinical loss of function during and after chemotherapy.

Structural, morphometric and immunohistochemical study of the rabbit accessory olfactory bulb

The accessory olfactory bulb (AOB) is the first neural integrative centre of the vomeronasal system (VNS), which is associated primarily with the detection of semiochemicals. Although the rabbit is used as a model for the study of chemocommunication, these studies are hampered by the lack of knowledge regarding the topography, lamination, and neurochemical properties of the rabbit AOB. To fill this gap, we have employed histological stainings: lectin labelling with Ulex europaeus (UEA-I), Bandeiraea simplicifolia (BSI-B4), and Lycopersicon esculentum (LEA) agglutinins, and a range of immunohistochemical markers. Anti-G proteins Gαi2/Gαo, not previously studied in the rabbit AOB, are expressed following an antero-posterior zonal pattern. This places Lagomorpha among the small groups of mammals that conserve a double-path vomeronasal reception. Antibodies against olfactory marker protein (OMP), growth-associated protein-43 (GAP-43), glutaminase (GLS), microtubule-associated protein-2 (MAP-2), glial fibrillary-acidic protein (GFAP), calbindin (CB), and calretinin (CR) characterise the strata and the principal components of the BOA, demonstrating several singular features of the rabbit AOB. This diversity is accentuated by the presence of a unique organisation: four neuronal clusters in the accessory bulbar white matter, two of them not previously characterised in any species (the γ and δ groups). Our morphometric study of the AOB has found significant differences between sexes in the numerical density of principal cells, with larger values in females, a pattern completely opposite to that found in rats. In summary, the rabbit possesses a highly developed AOB, with many specific features that highlight the significant role played by chemocommunication among this species.

Mitral cell development in the olfactory bulb of sharks: evidences of a conserved pattern of glutamatergic neurogenesis

In mammals, the development of the olfactory bulb (OB) relies in part on the expression of transcription factors involved in the specifications/differentiation of glutamatergic cells. In a previous study from our group, a high molecular similarity was reported between mammals and cartilaginous fishes regarding the neurogenic mechanisms underlying the development of glutamatergic cells in the telencephalon. However, information about the transcriptional program operating in the development of the glutamatergic system (mainly represented by mitral cells) in the OB is lacking in the catshark Scyliorhinus canicula, a cartilaginous fish. Using immunohistochemistry and in situ hybridization techniques, we have found that, previously to the appearance of the olfactory primordium (OP), proliferating cells expressing Pax6 with molecular hallmarks of progenitor radial glia were located in the ventrolateral pallial ventricular zone. Later in development, when the OP is recognizable, a stream of Pax6-positive cells were observed between the ventricular zone and the OP, where transcription factors involved in mitral cell development in mammals (ScTbr2, ScNeuroD, Tbr1) are expressed. Later in development, these transcription factors became expressed in a layered-like structure where ScVglut1, a marker of mitral cells, is also present. Our data suggest that the transcriptional program related with the specification/differentiation of glutamatergic cells in the telencephalon has been conserved throughout the evolution of vertebrates. These results, in combination with previous studies concerning GABAergic neurogenesis in sharks, have evidenced that the OB of mammals and sharks shares similarities in the timing and molecular programs of development.

Distribution of the neuronal inputs to the ventral premammillary nucleus of male and female rats

The ventral premammillary nucleus (PMV) expresses dense collections of sex steroid receptors and receptors for metabolic cues, including leptin, insulin and ghrelin. The PMV responds to opposite sex odor stimulation and projects to areas involved in reproductive control, including direct innervation of gonadotropin releasing hormone neurons. Thus, the PMV is well positioned to integrate metabolic and reproductive cues, and control downstream targets that mediate reproductive function. In fact, lesions of PMV neurons blunt female reproductive function and maternal aggression. However, although the projections of PMV neurons have been well documented, little is known about the neuronal inputs received by PMV neurons. To fill this gap, we performed a systematic evaluation of the brain sites innervating the PMV neurons of male and female rats using the retrograde tracer subunit B of the cholera toxin (CTb). In general, we observed that males and females show a similar pattern of afferents. We also noticed that the PMV is preferentially innervated by neurons located in the forebrain, with very few projections coming from brainstem nuclei. The majority of inputs originated from the medial nucleus of the amygdala, the bed nucleus of the stria terminalis and the medial preoptic nucleus. A moderate to high density of afferents was also observed in the ventral subiculum, the arcuate nucleus and the ventrolateral subdivision of the ventromedial nucleus of the hypothalamus. Our findings strengthen the concept that the PMV is part of the vomeronasal system and integrates the brain circuitry controlling reproductive functions.

The sea lamprey has a primordial accessory olfactory system

Background

A dual olfactory system, represented by two anatomically distinct but spatially proximate chemosensory epithelia that project to separate areas of the forebrain, is known in several classes of tetrapods. Lungfish are the earliest evolving vertebrates known to have this dual system, comprising a main olfactory and a vomeronasal system (VNO). Lampreys, a group of jawless vertebrates, have a single nasal capsule containing two anatomically distinct epithelia, the main (MOE) and the accessory olfactory epithelia (AOE). We speculated that lamprey AOE projects to specific telencephalic regions as a precursor to the tetrapod vomeronasal system.

Results

To test this hypothesis, we characterized the neural circuits and molecular profiles of the accessory olfactory epithelium in the sea lamprey (Petromyzon marinus). Neural tract-tracing revealed direct and reciprocal connections with the dorsomedial telencephalic neuropil (DTN) which in turn projects directly to the dorsal pallium and the rostral hypothalamus. High-throughput sequencing demonstrated that the main and the accessory olfactory epithelia have virtually identical profiles of expressed genes. Real time quantitative PCR confirmed expression of representatives of all 3 chemoreceptor gene families identified in the sea lamprey genome.

Conclusion

Anatomical and molecular evidence shows that the sea lamprey has a primordial accessory olfactory system that may serve a chemosensory function.

Accessory olfactory bulb function is modulated by input from the main olfactory epithelium

Although it is now established that sensory neurons in both the main olfactory epithelium and the vomeronasal organ may be activated by both general and pheromonal odorants, it remains unclear what initiates sampling by the vomeronasal organ. Anterograde transport of wheat germ agglutinin-horseradish peroxidase was used to determine that adequate intranasal syringing with zinc sulfate interrupted all inputs to the main olfactory bulb but left intact those to the accessory olfactory bulb. Adult male treated mice were frankly anosmic when tested with pheromonal and non-pheromonal odors and failed to engage in aggressive behavior. Treated juvenile females failed to show puberty acceleration subsequent to exposure to bedding from adult males. Activation of the immediate early gene c-Fos and electrovomeronasogram recording confirmed the integrity of the vomeronasal system in zinc sulfate-treated mice. These results support the hypothesis that odor detection by the main olfactory epithelium is required to initiate sampling by the vomeronasal system.

Study of smell and reproductive organs in a mouse model for CHARGE syndrome

CHARGE syndrome is a multiple congenital anomaly syndrome characterised by Coloboma, Heart defects, Atresia of choanae, Retardation of growth and/or development, Genital hypoplasia, and Ear anomalies often associated with deafness. It is caused by heterozygous mutations in the CHD7 gene and shows a highly variable phenotype. Anosmia and hypogonadotropic hypogonadism occur in the majority of the CHARGE patients, but the underlying pathogenesis is unknown. Therefore, we studied the ability to smell and aspects of the reproductive system (reproductive performance, gonadotropin-releasing hormone (GnRH) neurons and anatomy of testes and uteri) in a mouse model for CHARGE syndrome, the whirligig mouse (Chd7^Whi/+). We showed that Chromodomain Helicase DNA-binding protein 7 (Chd7) is expressed in brain areas involved in olfaction and reproduction during embryonic development. We observed poorer performance in the smell test in adult Chd7^Whi/+ mice, secondary either to olfactory dysfunction or to balance disturbances. Olfactory bulb and reproductive organ abnormalities were observed in a proportion of Chd7^Whi/+ mice. Hypothalamic GnRH neurons were slightly reduced in Chd7^Whi/+ females and reproductive performance was slightly less in Chd7^Whi/+ mice. This study shows that the penetrance of anosmia and hypogonadotropic hypogonadism is lower in Chd7^Whi/+ mice than in CHARGE patients. Interestingly, many phenotypic features of the Chd7 mutation showed incomplete penetrance in our model mice, despite the use of inbred, genetically identical mice. This supports the theory that the extreme variability of the CHARGE phenotype in both humans and mice might be attributed to variations in the fetal microenvironment or to purely stochastic events.

Inhalation exposure to acetone induces selective damage on olfactory neuroepithelium in mice

Due to their specific position in the nasal cavity, the cells of olfactory neuroepithelium can be damaged by exposure to environmental airborne chemicals. However, few studies have been focused on selective damage, i.e. olfactory sensory neurons, basal cells, supporting and duct cells. As solvents are known to induce critical effects on olfactory neuroepithelium (OE), this study was designed to characterize histological and immunohistological effects induced by acetone exposure on OE in mice. Behavioral tests were conducted to evaluate olfactory sensitivity. Moreover, olfactory neuroepithelium was examined to evaluate the thickness and the total number of cells. Finally, different markers, olfactory marker protein (OMP) and proliferating cell nuclear antigen (PCNA), were used to characterize respectively olfactory sensory neurons and basal cells, and secondly to evaluate the dynamic of the tissue turnover. Results showed structural modifications, since the thickness and the number of cells in the OE were modified according to the time course of the exposure. Additionally, no changes for OMP-positive cells were observed whereas significant differences appeared for the density of PCNA-positive cells in relation to their location (main-body or basal layer of OE). These findings indicate that acetone exposure induces selective damage in olfactory neuroepithelium.

Pheromone sensing in mice

Beginning with the neuroepithelium of the vomeronasal organ, the accessory olfactory system in rodents runs parallel to the main olfactory system and is specialized in the detection of pheromones. Only a small number of vomeronasal agonists carrying pheromonal information have been identified this far. These structurally diverse classes of chemicals include peptides secreted by exocrine glands and range from small volatile molecules to proteins and fragments thereof present in urine. Most pheromones activate both vomeronasal and main olfactory sensory neurons, making the identification of functionally relevant populations of sensory neurons difficult. Analyses of gene-targeted mice selectively affecting either vomeronasal or main olfactory signaling have attempted to elucidate the functional contribution of the different chemosensory epithelia to pheromone sensing in mice. These mouse models suggest that both the main and the accessory olfactory systems can converge and synergize to express the complex array of stereotyped behaviors and hormonal changes triggered by pheromones.