Vomeronasal Organ Neuroepithelium Research Articles

Is the Mole Rat Vomeronasal Organ Functional?

The colonial naked mole rat Heterocephalus glaber is a subterranean, eusocial rodent. The H. glaber vomeronasal organ neuroepithelium (VNE) displays little postnatal growth. However, the VNE remains neuronal in contrast to some mammals that possess nonfunctional vomeronasal organ remnants, for example, catarrhine primates and some bats. Here, we describe the vomeronasal organ (VNO) microanatomy in the naked mole rat and we make preliminary observations to determine if H. glaber shares its minimal postnatal VNE growth with other African mole rats. We also determine the immunoreactivity to the mitotic marker Ki67, growth-associated protein 43 (GAP43), and olfactory marker protein (OMP) in six adult and three subadult H. glaber individuals. VNE volume measurements on a small sample of Cryptomys hottentotus and Fukomys damarensis indicate that the VNE of those African mole rat species are also likely to be growth-deficient. Ki67(+) cells show that the sensory epithelium is mitotically active. GAP43 labelling indicates neurogenesis and OMP(+) cells are present though less numerous compared to GAP43(+) cells. In this respect, the VNO of H. glaber does not appear vestigial. The African mole rat VNE may be unusually variable, perhaps reflecting reduced selection pressure on the vomeronasal system. If so, African mole rats may provide a useful genetic model for understanding the morphological variability observed in the mammalian VNO. Anat Rec, 2019. © 2019 Wiley Periodicals, Inc. Anat Rec, 303:318-329, 2020. © 2019 American Association for Anatomy.

Signaling mechanisms and behavioral function of the mouse basal vomeronasal neuroepithelium.

The vomeronasal organ (VNO) is a sensory organ that is found in most terrestrial vertebrates and that is principally implicated in the detection of pheromones. The VNO contains specialized sensory neurons organized in a pseudostratified neuroepithelium that recognize chemical signals involved in initiating innate behavioral responses. In rodents, the VNO neuroepithelium is segregated into two distinct zones, apical and basal. The molecular mechanisms involved in ligand detection by apical and basal VNO sensory neurons differ extensively. These two VNO subsystems express different subfamilies of vomeronasal receptors and signaling molecules, detect distinct chemosignals, and project to separate regions of the accessory olfactory bulb (AOB). The roles that these olfactory subdivisions play in the control of specific olfactory-mediated behaviors are largely unclear. However, analysis of mutant mouse lines for signal transduction components together with identification of defined chemosensory ligands has revealed a fundamental role of the basal part of the mouse VNO in mediating a wide range of instinctive behaviors, such as aggression, predator avoidance, and sexual attraction. Here we will compare the divergent functions and synergies between the olfactory subsystems and consider new insights in how higher neural circuits are defined for the initiation of instinctive behaviors.

The vomeronasal organ of Lemur catta.

The vomeronasal organ (VNO), also known as the Jacobson's organ, is a bilateral chemosensory organ found at the base of the nasal cavity specialized for the detection of higher-molecular weight (non-volatile) chemostimuli. It has been linked to pheromone detection. The VNO has been well studied in nocturnal lemurs and lorises, but poorly studied in diurnal/cathemeral species despite the large repertoire of olfactory behaviors noted in species such as Lemur catta. Here, the VNO and associated structures were studied microanatomically in one adult female and one adult male L. catta. Traditional and immunohistochemical procedures demonstrate the VNO epithelium consists of multiple rows of sensory neurons. Immunoreactivity to Growth-associated protein 43 (GAP43) indicates the VNO is postnatally neurogenic. In volume, the VNO neuroepithelium scales similarly to palatal length compared to nocturnal strepsirrhines. Numerous taste buds present at the oral opening to the nasopalatine duct, with which the VNO communicates, provide an additional (or alternative) explanation for the flehmen behavior that has been observed in this species. The VNO of L. catta is shown to be microanatomically comparable to that of nocturnal strepsirrhines. Like nocturnal strepsirrhines, the VNO of L. catta may be functional in the reception of high-molecular weight secretions.

The Vomeronasal Organ of New World Monkeys (Platyrrhini)

Although all platyrrhine primates possess a vomeronasal organ (VNO), few species have been studied in detail. Here, we revisit the microanatomy of the VNO and related features in serially sectioned samples from 41 platyrrhine cadavers (14 species) of mixed age. Procedures to identify terminally differentiated vomeronasal sensory neurons (VSNs) via immunolabeling of olfactory marker protein (OMP) were used on selected specimens. The VNO varies from an elongated epithelial tube (e.g., Ateles fusciceps) to a dorsoventrally expanded sac (e.g., Saguinus spp.). The cartilage that surrounds the VNO is J-shaped or U-shaped in most species, and articulates with a groove on the bony palate. Preliminary results indicate a significant correlation between the length of this groove and length of the VNO neuroepithelium, indicating this feature may serve as a skeletal correlate. The VNO neuroepithelium could be identified in all adult primates except Alouatta, in which poor preservation prevented determination. The VNO of Ateles, described in detail for the first time, had several rows of VSNs and nerves in the surrounding lamina propria. Patterns of OMP-reactivity in the VNO of perinatal platyrrhines indicate that few or no terminally differentiated VSNs are present at birth, thus supporting the hypothesis that some platyrrhines may have delayed maturation of the VNO. From a functional perspective, all platyrrhines studied possess structures required for chemoreception (VSNs, vomeronasal nerves). However, some microanatomical findings, such as limited reactivity to OMP in some species, indicate that some lineages of New World monkeys may have a reduced or vestigial vomeronasal system.

Characterization of cAMP degradation by phosphodiesterases in the accessory olfactory system.

To characterize the potential role of cAMP in pheromone transduction, we have examined the occurrence of cyclic nucleotide phosphodiesterases (PDEs) in the mouse vomeronasal organ (VNO). We show that the cAMP-specific isoforms PDE4A and PDE4D are found preferentially in the apical and basal layers, respectively, of the VNO neuroepithelium and in the rostral (PDE4A) and caudal (PDE4D) portions of the accessory olfactory bulb glomerular layer. Assays for cAMP hydrolysis showed that PDE activity in VNO homogenates was about half that measured in the cerebral cortex and olfactory epithelium, and the proportion of total activity inhibited by rolipram, a PDE4-specific inhibitor, was approximately 40%. Activity in the VNO was enhanced 60% by Ca(2+) and calmodulin (CaM), implicating the presence of Ca(2+)/CaM-dependent PDE1. Zaprinast, which is known to inhibit PDE1C isoforms, completely suppressed Ca(2+)/CaM-stimulated activity and, together, zaprinast and rolipram inhibited cAMP hydrolysis by approximately 70%. Our results suggest that PDE1 and PDE4 isoforms are the primary source of cAMP degradation in the VNO.

Sex Difference and Steroid Modulation of Pheromone-Induced Immediate Early Genes in the Two Zones of the Mouse Accessory Olfactory System

Two anatomically and neurochemically distinct zones within the vomeronasal organ (VNO) and accessory olfactory bulb (AOB) have been identified that are responsible for the detection of pheromones. Using markers to distinguish between apical and basal neurons of the VNO neuroepithelium and rostral versus caudal AOB glomeruli, we examined immediate early gene immunoreactivity (IEG-IR) in gonadectomized, steroid-treated mice in response to pheromones of male and female conspecifics. After exposure of estradiol-treated females to soiled male bedding, more VNO neurons in the basal than the apical layer exhibited IEG-IR compared with VNO neurons of estradiol-treated males. Conversely, whereas soiled female bedding failed to induce IEG-IR in VNO neurons of estradiol-treated males or females, both apical and basal neurons were activated in testosterone-treated males. Male and female pheromones also activated mitral and granule cells in the AOBs of all subjects, but responses to different pheromones were distributed across the boundary of the rostral and caudal regions. These data show that differences in the response of males and females to the same pheromonal stimulus are found in the sensory neurons of the VNO. We propose that centrifugal, noradrenergic inputs to VNO neurons, which may differ in the two sexes and respond differently to adult sex steroids, modulate sensitivity to pheromonal stimulation.

Distribution of PDE4A and G(o) alpha immunoreactivity in the accessory olfactory system of the mouse.

Distribution of the cAMP-specific phosphodiesterase PDE4A was examined in the accessory olfactory system by immunohistochemistry. Adjacent sections through the vomeronasal organ (VNO) and accessory olfactory bulb (AOB) were alternately immunostained with antibodies against PDE4A or the G-protein alpha subunit G(o) alpha, which labels basal VNO neurons, in order to determine whether PDE4A occurs preferentially in one of two segregated VNO pathways. We found that PDE4A strongly labeled apical VNO neurons and rostral AOB glomeruli. There was virtually no overlap in G(o) alpha and PDE4A staining, and there were no regions of the VNO neuroepithelium or AOB glomeruli not labeled by either antibody. These results identify a potential member of the pheromone transduction cascade in apical neurons, and provide further evidence that the VNO consists of functionally distinct pathways.

Vomeronasal neuroepithelium and forebrain Fos responses to male pheromones in male and female mice

Male urinary pheromones modulate behavioral and neuroendocrine function in mice after being detected by sensory neurons in the vomeronasal organ (VNO) neuroepithelium. We used nuclear Fos protein immunoreactivity (Fos-IR) as a marker of changes in neuronal activity to examine the processing of male pheromones throughout the VNO projection pathway to the hypothalamus. Sexually naive male and female Balb/c mice were gonadectomized and treated daily with estradiol benzoate (EB) or oil vehicle for 3 weeks. Subjects were then exposed to soiled bedding from gonadally intact Balb/c males or to clean bedding for 90 min prior to sacrifice and processing of their VNOs and forebrains for Fos-IR. Male pheromones induced similar numbers of Fos-IR cells in the VNO neuroepithelium of oil-treated male and female subjects; however, EB-treated females had significantly more Fos-IR neurons in the VNO than any other group. There was an equivalent neuronal Fos response to male odors in the mitral and granule cells of the anterior and posterior accessory olfactory bulb of males and females, regardless of hormone treatment. In central portions of the VNO projection pathway (i.e., bed nucleus of the stria terminalis, medial preoptic area) neuronal Fos responses to male pheromones were present in female but absent in male subjects, regardless of hormone treatment. In a separate experiment, mating induced neuronal Fos-IR in these brain regions at levels in gonadally intact male subjects which were equal to or greater than those seen in ovariectomized females primed with estrogen and progesterone. This suggests that neurons in the central portions of the male's VNO pathway are capable of expressing Fos. Our results suggest that sexually dimorphic central responses to pheromones exist in mice that may begin in the VNO neuroepithelium.

The Effect of Ammonia on Olfactory Epithelium and Vomeronasal Organ Neuroepithelium of Rabbits

In recent studies, the vomeronasal organ (VNO), although vestigial and with unknown function in humans, was reported to be present in almost every person examined. In rabbits, it is a well-developed organ, one lying on each side of the nasal septum. Histologically it was found to contain neuroepithelium which is considered an accessory olfactory system taking charge of an olfactory discrimination different from that of olfactory epithelium. Experimental removal of the VNO in male animals reduces reproductive performance and aggression. In this study, the effects of prolonged exposure to ammonia vapor on the histological pattern and enzymatic activity of the olfactory epithelium and the VNO neuroepithelium of 30 adult male rabbits were investigated and compared with a control group. In the exposed animals, the supporting cells in both types of epithelia showed hyperplasia which was more marked in the olfactory epithelium. Manifestations of cytotoxicity were found more in the bipolar cells of the olfactory epithelium than in the VNO neuroepithelium. The enzymatic activity in the exposed group supported the histological results. The presence of the VNO neuroepithelium of the rabbit in a narrow duct with a minute nasal orifice could have some protective effects on the cells.

Sensory transduction in vomeronasal neurons: evidence for G alpha o, G alpha i2, and adenylyl cyclase II as major components of a pheromone signaling cascade

The mammalian vomeronasal organ (VNO) is an accessory olfactory structure implicated in the sensing of pheromones. Although virtually nothing is known about sensory transduction in the mammalian VNO, recent findings have raised the possibility that it proceeds via a G-protein-coupled mechanism and involves a cyclic nucleotide-gated ion channel as in the nasal olfactory epithelium. To investigate this possibility, we cloned G-protein alpha subunits, adenylyl cyclases, and guanylyl cyclases that are expressed in the VNO and examined their patterns of expression. Of seven G alpha subunits identified as being expressed in the VNO, we found that mRNAs encoding only two, G alpha o and G alpha i2, are highly expressed in VNO neurons. Moreover, G alpha o and G alpha i2 are highly expressed by separate subsets of neurons that are located in different regions of the VNO neuroepithelium. Immunohistochemical studies show that both G alpha o and G alpha i2 are enriched in VNO microvilli, suggesting that G-proteins containing both of these alpha subunits may be involved in VNO sensory transduction. Of the adenylyl and guanylyl cyclases that we cloned, we found that only one, adenylyl cyclase type II, is highly expressed in VNO neurons; furthermore, it is expressed by both G alpha o+ and G alpha i2+ subsets. Our findings suggest that spatially segregated subsets of VNO neurons may use different, but related, sensory transduction pathways in which G-proteins and an adenylyl cyclase play major roles.