Histological properties of the glomerular layer in the mouse accessory olfactory bulb

Histological evaluation of the alpaca (Vicugna pacos) vomeronasal organ

Little is known about the neuronal structure of the vomeronasal organ (VNO), a receptor organ responsible for pheromone perception, in the alpaca (Vicugna pacos). This study was performed to determine the localization of neuronal elements, including protein gene product 9.5 (PGP 9.5), a pan-neuronal marker, olfactory marker protein (OMP), a marker of mature olfactory receptor cells, and phospholipase C beta 2 (PLC-β2), a marker of solitary chemoreceptor cells (SCCs), in the VNO. OMP was identified in receptor cells of the vomeronasal sensory epithelium (VSE), while PGP 9.5 and PLC-β2 were localized in both the VSE and vomeronasal non-sensory epithelium. Collectively, these results suggested that the alpaca VNO possesses SCCs and olfactory receptor cells, which recognize both harmful substances and pheromones.

Comparative ultrastructure of the olfactory system in the East African root rat (Tachyoryctes splendens) and the naked mole rat (Heterocephalus glaber)

The ultrastructure of the olfactory system of most fossorial rodents remains largely unexplored. This study sought to investigate the functional structure of the olfactory mucosa and olfactory bulb of two species of fossorial rodents that have distinct behaviour and ecology, the East African root rat (RR) and the naked mole rat (NMR). Transmission electron microscopy and scanning electron microscopy were employed. The basic ultrastructural design of the olfactory system of the two species was largely comparable. In both species, the olfactory mucosa comprised an olfactory epithelium and an underlying lamina propria. The olfactory epithelium revealed olfactory knobs, cilia and microvilli apically and sustentancular cells, olfactory receptor neurons and basal cells in the upper, middle and basal zones, respectively. The lamina propria was constituted by Bowman's glands, olfactory nerve bundles and vasculature supported by loose connective tissue. Within the olfactory bulb, intracellular and extracellular structures including cell organelles, axons and dendrites were elucidated. Notable species differences were observed in the basal zone of the olfactory epithelium and on the luminal surface of the olfactory mucosa. The basal zone of the olfactory epithelium of the RR consisted of a single layer of flattened electron-dense horizontal basal cells while the NMR had juxtaposed electron-dense and electron-lucent heterogenous cells, an occurrence seen as being indicative of quiescent and highly proliferative states of the olfactory epithelia in the two species, respectively. The olfactory epithelial surface of the NMR comprised an elaborate cilia network that intertwined extensively forming loop-like structures whereas in the RR, the surface was rugged and consisted of finger-like processes and irregular masses. With gross and histological studies showing significant differences in the olfactory structures of the two species, these findings are a further manifestation that the olfactory system of the RR and the NMR have evolved differently to reflect their varied olfactory functional needs.

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.

Olfactory Stimulation as Environmental Enrichment for Domestic Horses-A Review

Horses constantly face several challenges inherent to the domestic environment, and it is common for the expression of their natural behavior to be drastically limited. Environmental enrichment has been suggested as an alternative to improve the captive situation of domestic horses. Among the recently proposed enrichment strategies, olfactory stimulation has emerged as a method for improving several aspects related to animal behavior. Olfaction is a sensory modality that plays a significant role in the expression of equine behavior, and in recent years, studies have shown that olfactory stimulation can influence the physiological and behavioral parameters of horses. This review provides current information on the anatomical particularities of the equine olfactory system, presents the physiological mechanisms involved in the odor detection process, and demonstrates how stress can interfere with this process. Finally, the use of olfactory stimulation as an environmental enrichment for domestic horses (Equus ferus caballus) is explored. The need for new studies that answer pertinent questions related to this topic is discussed throughout the manuscript.

Collagen XIX is required for pheromone recognition and glutamatergic synapse formation in mouse accessory olfactory bulb

In mammals, the accessory olfactory bulb (AOB) receives input from vomeronasal sensory neurons (VSN) which detect pheromones, chemical cues released by animals to regulate the physiology or behaviors of other animals of the same species. Cytoarchitecturally, cells within the AOB are segregated into a glomerular layer (GL), mitral cell layer (MCL), and granule cell layer (GCL). While the cells and circuitry of these layers has been well studied, the molecular mechanism underlying the assembly of such circuitry in the mouse AOB remains unclear. With the goal of identifying synaptogenic mechanisms in AOB, our attention was drawn to Collagen XIX, a non-fibrillar collagen generated by neurons in the mammalian telencephalon that has previously been shown to regulate the assembly of synapses. Here, we used both a targeted mouse mutant that lacks Collagen XIX globally and a conditional allele allowing for cell-specific deletion of this collagen to test if the loss of Collagen XIX causes impaired synaptogenesis in the mouse AOB. These analyses not only revealed defects in excitatory synapse distribution in these Collagen XIX-deficient mutants, but also showed that these mutant mice exhibit altered behavioral responses to pheromones. Although this collagen has been demonstrated to play synaptogenic roles in the telencephalon, those roles are at perisomatic inhibitory synapses, results here are the first to demonstrate the function of this unconventional collagen in glutamatergic synapse formation.

Neuropeptide S Attenuates the Alarm Pheromone-Evoked Defensive and Risk Assessment Behaviors Through Activation of Cognate Receptor-Expressing Neurons in the Posterior Medial Amygdala

Neuropeptide S (NPS) acts by activating its cognate receptor (NPSR). High level expression of NPSR in the posterior medial amygdala suggests that NPS-NPSR system should be involved in regulation of social behaviors induced by social pheromones. The present study was undertaken to investigate the effects of central administration of NPS or with NPSR antagonist on the alarm pheromone (AP)-evoked defensive and risk assessment behaviors in mice. Furthermore, H129-H8, a novel high-brightness anterograde multiple trans-synaptic virus, c-Fos and NPSR immunostaining were employed to reveal the involved neurocircuits and targets of NPS action. The mice exposed to AP displayed an enhancement in defensive and risk assessment behaviors. NPS (0.1–1 nmol) intracerebroventricular (i.c.v.) injection significantly attenuated the AP-evoked defensive and risk assessment behaviors. NPSR antagonist [D-Val⁵]NPS at the dose of 40 nmol completely blocked the effect of 0.5 nmol of NPS which showed the best effective among dose range. The H129-H8-labeled neurons were observed in the bilateral posterodorsal medial amygdala (MePD) and posteroventral medial amygdala (MePV) 72 h after the virus injection into the unilateral olfactory bulb (OB), suggesting that the MePD and MePV receive olfactory information inputs from the OB. The percentage of H129-H8-labeled neurons that also express NPSR were 90.27 ± 3.56% and 91.67 ± 2.46% in the MePD and MePV, respectively. NPS (0.5 nmol, i.c.v.) remarkably increased the number of Fos immunoreactive (-ir) neurons in the MePD and MePV, and the majority of NPS-induced Fos-ir neurons also expressed NPSR. The behavior characteristic of NPS or with [D-Val⁵]NPS can be better replicated in MePD/MePV local injection within lower dose. The present findings demonstrated that NPS, via selective activation of the neurons bearing NPSR in the posterior medial amygdala, attenuates the AP-evoked defensive and risk assessment behaviors in mice.

Central endothelin ETB receptor activation reduces blood pressure and catecholaminergic activity in the olfactory bulb of deoxycorticosterone acetate-salt hypertensive rats

Endothelins regulate catecholaminergic activity in the olfactory bulb (OB) in normotensive and hypertensive animals. Administration of an endothelin ET_A receptor antagonist decreases blood pressure in deoxycorticosterone acetate-salt (DOCA-salt) rats along with a reduction in tyrosine hydroxylase (TH) activity and expression. In the present work, we sought to establish the role of brain endothelin ET_B receptor on blood pressure regulation and its relationship with the catecholaminergic system within the OB of DOCA-Salt rats. Sprague-Dawley male rats were divided into control and DOCA-Salt groups. Blood pressure, heart rate and TH activity as well as neuronal nitric oxide synthase (nNOS) expression were assessed following IRL-1620 (selective endothelin ET_B receptor agonist) applied to be brain. IRL-1620 significantly reduced systolic, diastolic, and mean arterial pressure in DOCA-Salt hypertensive rats. It also decreased TH activity, TH total and phosphorylated forms expression as well as its mRNA in the OB of hypertensive animals. The expression of phospho-Ser1417-nNOS, which reflects nNOS activation, was significantly decreased in the of OB of DOCA-salt rats, but it was enhanced by IRL-1620. These findings suggest that DOCA-Salt hypertension depends on endogenous central endothelin ET_A receptor activity, rather than on ET_B, and that low endothelin ET_B stimulation is essential for blood pressure elevation in this animal model. The effect of endothelin ET_A receptor antagonism may also result from endothelin ET_B receptor overstimulation. The present study shows that endothelin receptors are involved in the regulation of TH in the OB and that such changes are likely implicated in the hemodynamic control and sympathetic outflow.

Subpopulations of Projection Neurons in the Olfactory Bulb

Generation of neuronal diversity is a biological strategy widely used in the brain to process complex information. The olfactory bulb is the first relay station of olfactory information in the vertebrate central nervous system. In the olfactory bulb, axons of the olfactory sensory neurons form synapses with dendrites of projection neurons that transmit the olfactory information to the olfactory cortex. Historically, the olfactory bulb projection neurons have been classified into two populations, mitral cells and tufted cells. The somata of these cells are distinctly segregated within the layers of the olfactory bulb; the mitral cells are located in the mitral cell layer while the tufted cells are found in the external plexiform layer. Although mitral and tufted cells share many morphological, biophysical, and molecular characteristics, they differ in soma size, projection patterns of their dendrites and axons, and odor responses. In addition, tufted cells are further subclassified based on the relative depth of their somata location in the external plexiform layer. Evidence suggests that different types of tufted cells have distinct cellular properties and play different roles in olfactory information processing. Therefore, mitral and different types of tufted cells are considered as starting points for parallel pathways of olfactory information processing in the brain. Moreover, recent studies suggest that mitral cells also consist of heterogeneous subpopulations with different cellular properties despite the fact that the mitral cell layer is a single-cell layer. In this review, we first compare the morphology of projection neurons in the olfactory bulb of different vertebrate species. Next, we explore the similarities and differences among subpopulations of projection neurons in the rodent olfactory bulb. We also discuss the timing of neurogenesis as a factor for the generation of projection neuron heterogeneity in the olfactory bulb. Knowledge about the subpopulations of olfactory bulb projection neurons will contribute to a better understanding of the complex olfactory information processing in higher brain regions.

Co-localization of two-color rAAV2-retro confirms the dispersion characteristics of efferent projections of mitral cells in mouse accessory olfactory bulb

The accessory olfactory bulb (AOB), located at the posterior dorsal aspect of the main olfactory bulb (MOB), is the first brain relay of the accessory olfactory system (AOS), which can parallelly detect and process volatile and nonvolatile social chemosignals and mediate different sexual and social behaviors with the main olfactory system (MOS). However, due to its anatomical location and absence of specific markers, there is a lack of research on the internal and external neural circuits of the AOB. This issue was addressed by single-color labeling and fluorescent double labeling using retrograde rAAVs injected into the bed nucleus of the stria terminalis (BST), anterior cortical amygdalar area (ACo), medial amygdaloid nucleus (MeA), and posteromedial cortical amygdaloid area (PMCo) in mice. We demonstrated the effectiveness of this AOB projection neuron labeling method and showed that the mitral cells of the AOB exhibited efferent projection dispersion characteristics similar to those of the MOB. Moreover, there were significant differences in the number of neurons projected to different brain regions, which indicated that each mitral cell in the AOB could project to a different number of neurons in different cortices. These results provide a circuitry basis to help understand the mechanism by which pheromone information is encoded and decoded in the AOS.

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.

Interneuron Functional Diversity in the Mouse Accessory Olfactory Bulb

In the mouse accessory olfactory bulb (AOB), inhibitory interneurons play an essential role in gating behaviors elicited by sensory exposure to social odors. Several morphological classes have been described, but the full complement of interneurons remains incomplete. In order to develop a more comprehensive view of interneuron function in the AOB, we performed targeted patch clamp recordings from partially overlapping subsets of genetically labeled and morphologically defined interneuron types. Gad2 (GAD65), Calb2 (calretinin), and Cort (cortistatin)-cre mouse lines were used to drive selective expression of tdTomato in AOB interneurons. Gad2 and Calb2-labeled interneurons were found in the internal, external, and glomerular (GL) layers, whereas Cort-labeled interneurons were enriched within the lateral olfactory tract (LOT) and external cellular layer (ECL). We found that external granule cells (EGCs) from all genetically labeled subpopulations possessed intrinsic functional differences that allowed them to be readily distinguished from internal granule cells (IGCs). EGCs showed stronger voltage-gated Na⁺ and non-inactivating voltage-gated K⁺ currents, decreased I_H currents, and robust excitatory synaptic input. These specific intrinsic properties did not correspond to any genetically labeled type, suggesting that transcriptional heterogeneity among EGCs and IGCs is not correlated with expression of these particular marker genes. Intrinsic heterogeneity was also seen among AOB juxtaglomerular cells (JGCs), with a major subset of Calb2-labeled JGCs exhibiting spontaneous and depolarization-evoked plateau potentials. These data identify specific physiological features of AOB interneurons types that will assist in future studies of AOB function.

Cocaine- and amphetamine-regulated transcript (CART): A multifaceted neuropeptide

Over the last 35 years, the continuous discovery of novel neuropeptides has been the key to the better understanding of how the central nervous system has integrated with neuronal signals and behavioral responses. Cocaine and amphetamine-regulated transcript (CART) was discovered in 1995 in the rat striatum but later was found to be highly expressed in the hypothalamus. The widespread distribution of CART peptide in the brain complicated the understanding of the role played by this neurotransmitter. The main objective of the current compact review is to piece together the fragments of available information about origin, expression, distribution, projection, and function of CART peptides. Accumulative evidence suggests CART as a neurotransmitter and neuroprotective agent that is mainly involved in regulation of feeding, addiction, stress, anxiety, innate fear, neurological disease, neuropathic pain, depression, osteoporosis, insulin secretion, learning, memory, reproduction, vision, sleep, thirst and body temperature. In spite of the vast number of studies about the CART, the overall pictures about the CART functions are sketchy. First, there is a lack of information about cloned receptor, specific agonist and antagonist. Second, CART peptides are detected in discrete sets of neurons that can modulate countless activities and third; CART peptides exist in several fragments due to post-translational processing. For these reasons the overall picture about the CART peptides are sketchy and confounding.

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.

The Accessory Olfactory System: Innately Specialized or Microcosm of Mammalian Circuitry?

In mammals, the accessory olfactory system is a distinct circuit that has received attention for its role in detecting and responding to pheromones. While the neuroscientific investigation of this system is comparatively new, recent advances and its compact size have made it an attractive model for developing an end-to-end understanding of such questions as regulation of essential behaviors, plasticity, and individual recognition. Recent discoveries have indicated a need to reevaluate our conception of this system, suggesting that ( a) physical principles—rather than biological necessity—play an underappreciated role in its raison d'être and that ( b) the anatomy of downstream projections is not dominated by unique specializations but instead consists of an abbreviated cortical/basal ganglia motif reminiscent of other sensorimotor systems. These observations suggest that the accessory olfactory system distinguishes itself primarily by the physicochemical properties of its ligands, but its architecture is otherwise a microcosm of mammalian neurocircuitry.

Immunolocalization of Receptor and Chemoreceptor Modules in the Sheep Vomeronasal Organ

The vomeronasal organ (VNO) is the peripheral receptor organ of the accessory olfactory system, which is responsible for both sexual and innate behaviors. The degree of neuronal differentiation and maturation of the vomeronasal receptor cells together with the verification of the presence of the solitary chemoreceptor cells (SCCs) in the VNO of Corriedale sheep were assessed using immunofluorescence. A protein gene product 9.5 (PGP 9.5), which is a neuronal marker recognized to be expressed in most neurons of vertebrate species, an olfactory marker protein (OMP) that is precise for mature olfactory receptor cells, and lastly phospholipase C-β2 (PLC-β2), a marker in the signal transduction pathway of SCCs, were all tested. The cell bodies and dendrites of almost all receptor cells in the sensory epithelium were strongly positive for PGP 9.5 and to a lesser extent for OMP. In the nonsensory wall, all cells were negative for both PGP 9.5 and OMP; however, some positive PGP 9.5 immunoreactive fibers were identified. For PLC-β2, only 1 basally situated SCC could be identified in the sensory epithelium. A higher number was demonstrated in the nonsensory wall. Corriedale sheep possess matured, fully differentiated vomeronasal receptor cells in their sensory wall, suggesting an appropriate pheromone perception. Additionally, the VNO in sheep may participate in the usual transduction mechanisms, though it is seemingly not a chemoreceptor organ.

Glycan diversity in the vomeronasal organ of the Korean roe deer, Capreolus pygargus: A lectin histochemical study

Glycans in the epithelium play an important role in cell-to-cell communication and adhesion. No detailed evaluation of glycoconjugates in the vomeronasal organs (VNO) of the roe deer has been published previously. The aim of this study was to characterize glycan epitopes in the vomeronasal sensory epithelium (VSE) and non-sensory epithelium (VNSE) using lectin histochemistry. Glycan epitopes identified by lectin histochemistry were grouped as follows: N-acetylglucosamine (s-WGA, WGA, BSL-II, DSL, LEL, STL), mannose (Con A, LCA, PSA), galactose (RCA₁₂₀, BSL-I, Jacalin, PNA, ECL), N-acetylgalactosamine (VVA, DBA, SBA, and SJA), fucose (UEA-I) and complex type N-glycan (PHA-E and PHA-L) groups. The free border of the VSE was positive for all 21 lectins, and 18 of the lectins (excluding DBA, SJA, and PHA-L) showed weak and/or moderate staining in the receptor cells. The supporting cells were weakly positive for 19 lectins (excluding PNA and SJA). Moreover, 17 lectins (excluding BSL-II, Jacalin, PNA, and SJA) were expressed in the basal cells. In the VNSE of roe deer, the free border showed staining for all 21 lectins examined. The ciliated cells were positive for 16 lectins (excluding BSL-II, DSL, PNA, VVA, and SJA). Furthermore, 15 lectins (excluding DSL, LEL, ECL, UEA-I, PHA-E, and PHA-L) were expressed in goblet cells. Twenty lectins (excluding SJA) were expressed in the acini of the vomeronasal glands. Collectively, both VSE and VNSE were rich in N-acetylglucosamine, mannose, galactose, N-acetylgalactosamine, fucose, and complex-type N-glycans, although the different cell types of the VSE and VNSE expressed different glycoconjugates of varying intensities, suggesting that these carbohydrate residues may be involved in odor perception as well as cell-to-cell communication in the VNO.

Nuclear organisation of cholinergic, catecholaminergic, serotonergic and orexinergic neurons in two relatively large-brained rodent species-The springhare (Pedetes capensis) and Beecroft's scaly-tailed squirrel (Anomalurus beecrofti)

The present study describes the nuclear organization of the cholinergic, catecholaminergic, serotonergic and orexinergic systems in the brains of the springhare and Beecroft's scaly-tailed squirrel following immunohistochemical labelling. We aimed to investigate any differences in the nuclear organization of these neural systems when compared to previous data on other species of rodents, as these two rodent species have relatively large brains - 1.2 to 1.4 times larger than would be expected for mammals of their body mass and 1.7-1.9 times larger than would be expected for rodents of their body mass. A series of coronal sections were taken through two brains of each species and immunohistochemically labelled with antibodies against choline acetyltransferase, tyrosine hydroxylase, serotonin and orexin-A. Generally, the nuclear complement of these systems revealed extensive similarities between both species and to previously studied rodents. While no differences were observed in the nuclear complement of the serotonergic and orexinergic systems, some differences were observed in the nuclear complement of the cholinergic and catecholaminergic systems. These include the presence of cholinergic neurons in the cerebral cortex and nucleus of the trapezoid body in the springhare; while the Beecroft's scaly-tailed squirrel exhibited cholinergic neurons in the pretectal area of the midbrain. For the catecholaminergic system it was observed that Beecroft's scaly-tailed squirrel possessed immunoreactive neurons in the accessory olfactory bulb. Despite these four differences, most not previously observed in rodents, the remaining complement of cholinergic and catecholaminergic nuclei were identical to that observed in other rodents, including the presence of the rodent specific catecholaminergic rostral dorsal midline medullary (C3) nucleus in the medulla oblongata. Thus, even with a significant increase in relative brain size, the overall complement of nuclei forming these systems shows minimal changes in complexity within a specific mammalian order.

Region- and Cell-Specific Expression of Transmembrane Collagens in Mouse Brain

Unconventional collagens are nonfribrillar proteins that not only contribute to the structure of extracellular matrices but exhibit unique bio-activities. Although roles for unconventional collagens have been well-established in the development and function of non-neural tissues, only recently have studies identified roles for these proteins in brain development, and more specifically, in the formation and refinement of synaptic connections between neurons. Still, our understanding of the full cohort of unconventional collagens that are generated in the mammalian brain remains unclear. Here, we sought to address this gap by assessing the expression of transmembrane collagens (i.e., collagens XIII, XVII, XXIII and XXV) in mouse brain. Using quantitative PCR and in situ hybridization (ISH), we demonstrate both region- and cell-specific expression of these unique collagens in the developing brain. For the two most highly expressed transmembrane collagens (i.e., collagen XXIII and XXV), we demonstrate that they are expressed by select subsets of neurons in different parts of the brain. For example, collagen XXIII is selectively expressed by excitatory neurons in the mitral/tufted cell layer of the accessory olfactory bulb (AOB) and by cells in the inner nuclear layer (INL) of the retina. On the other hand, collagen XXV, which is more broadly expressed, is generated by subsets of excitatory neurons in the dorsal thalamus and midbrain and by inhibitory neurons in the retina, ventral thalamus and telencephalon. Not only is col25a1 expression present in retina, it appears specifically enriched in retino-recipient nuclei within the brain (including the suprachiasmatic nucleus (SCN), lateral geniculate complex, olivary pretectal nucleus (OPN) and superior colliculus). Taken together, the distinct region- and cell-specific expression patterns of transmembrane collagens suggest that this family of unconventional collagens may play unique, yet-to-be identified roles in brain development and function.

Dichotomous Distribution of Putative Cholinergic Interneurons in Mouse Accessory Olfactory Bulb

Sensory information processing in the olfactory bulb (OB) relies on diverse populations of bulbar interneurons. In rodents, the accessory OB (AOB) is divided into two bulbar regions, the anterior (aAOB) and posterior (pAOB), which differ substantially in their circuitry connections and associated behaviors. We previously identified and characterized a large number of morphologically diverse cholinergic interneurons in the main OB (MOB) using transgenic mice to visualize the cell bodies of choline acetyltransferase (ChAT-expressing neurons and immunolabeling (Krosnowski et al., 2012)). However, whether there are cholinergic neurons in the AOB is controversial and there is no detailed characterization of such neurons. Using the same line of ChAT^{(bacterial artificial chromosome, BAC)}-enhanced green fluorescent protein (eGFP) transgenic mice, we investigated cholinergic neurons in the AOB. We found significant differences in the number and location of GFP-expressing (GFP+), putative cholinergic interneurons between the aAOB and pAOB. The highest numbers of GFP+ interneurons were found in the aAOB glomerular layer (aGL) and pAOB mitral/tufted cell layer (pMCL). We also noted a high density of GFP+ interneurons encircling the border region of the pMCL. Interestingly, a small subset of glomeruli in the middle of the GL receives strong MCL GFP+ nerve processes. These local putative cholinergic-innervated glomeruli are situated just outside the aGL, setting the boundary between the pGL and aGL. Many but not all GFP+ neurons in the AOB were weakly labeled with antibodies against ChAT and vesicular acetylcholine transporter (VAChT). We further determined if these GFP+ interneurons differ from other previously characterized interneuron populations in the AOB and found that AOB GFP+ interneurons express neither GABAergic nor dopaminergic markers and most also do not express the glutamatergic marker. Similar to the cholinergic interneurons of the MOB, some AOB GFP+ interneurons express the calcium binding protein, calbindin-D28K. Moreover, exposure to either a male intruder or soiled bedding from a mating cage leads to an increase in the number of c-Fos-expressing MCL GFP+ neurons. Taken together, our data reveal a population of largely unidentified putative cholinergic neurons in the AOB. Their dichotomous distribution in the aAOB and pAOB suggests region-specific cholinergic involvement in olfactory information processing.

Histological and lectin histochemical studies of the vomeronasal organ of horses

The morphological characteristics and glycoconjugate composition of the vomeronasal organ (VNO) of the horse was investigated using histological, immunohistochemical, and lectin histochemical methods. The VNO is bilaterally located at the base of the nasal septum, has a tubular structure surrounded by cartilage, and consists of sensory and non-sensory epithelia. Immunohistochemical examination showed that the vomeronasal sensory epithelium (VSE) consisted of receptor cells positive for both olfactory marker protein (OMP) and protein gene product 9.5 (PGP 9.5), supporting cells, and basal cells. VNO receptor cells were positive for G protein Gαi2 (vomeronasal receptor type 1 marker), but not Gαo (vomeronasal receptor type 2 marker). Lectin histochemical studies using 21 biotinylated lectins showed that the free border of the VSE was positive for 20 lectins. The receptor and supporting cells reacted with 16 lectins while the basal cells reacted with 15 lectins, with varying intensities. In the vomeronasal non-sensory epithelium, the free border was positive for 19 lectins. The cilated cells were positive for 17 lectins and the basal cells were positive for 15 lectins. The vomeronasal glands, positioned in the lamina propria, were stained with both periodic acid Schiff (PAS) and alcian blue (pH 2.5). Eighteen lectins stained the acinar cells of the vomeronasal glands with various binding patterns. These findings suggest that horse VNO receptor cells express vomeronasal receptor type 1, and the VNO glands have mucous to seromucous characteristics. Moreover, each lectin differentially binds each cell type in both the VNO sensory and non-sensory epithelia.

Histochemical study of the olfactory mucosae of the horse

The olfactory mucosae of the horse were examined by using histology and lectin histochemistry to characterize the carbohydrate sugar residues therein. Histological findings revealed that olfactory epithelium (OE) consisted of both olfactory marker protein (OMP)- and protein gene product (PGP) 9.5-positive receptor cells, supporting cells and basal cells with intervening secretory ducts from Bowman's glands. Mucus histochemistry showed that Bowman's gland acini contain periodic acid-Schiff (PAS) reagent-positive neutral mucins and alcian blue pH 2.5-positive mucosubstances. Lectin histochemistry revealed that a variety of carbohydrate sugar residues, including N-acetylglucosamine, mannose, galactose, N-acetylgalactosamine, fucose and complex type N-glycan groups, are present in the various cell types in the olfactory mucosa at varying levels. Collectively, this is the first descriptive study of horse olfactory mucosa to characterize carbohydrate sugar residues in the OE and Bowman's glands.

Immunohistochemical localization of GABAergic key molecules in the main olfactory bulb of the Korean roe deer, Capreolus pygargus

Gamma-amino butyric acid (GABA) negatively regulates the excitatory activity of neurons and is a predominant neurotransmitter in the nervous system. The olfactory bulb, the main center in the olfactory system, is modulated by inhibitory interneurons that use GABA as their main neurotransmitter. The present study aimed to evaluate GABAergic transmission in the main olfactory bulb (MOB) of the Korean roe deer (Capreolus pygargus) by examining the immunohistochemical localization of GABAergic key molecules, including glutamic acid decarboxylase (GAD), vesicular GABA transporter (VGAT), GABA transporters (GATs; GAT-1 and GAT-3), and potassium sodium chloride co-transporter 2 (KCC2). GAD, VGAT, and KCC2 were expressed in the glomerular layer (GL), external plexiform layer (ePL), mitral cell layer (ML), and granule cell layer (GrL). Intense GAT-1 expression was observed in the GL; GAT-1 expression was discernible in the ePL, ML, and GrL. However, intense GAT-3 expression was extensively observed in all layers of the MOB. These results suggest that substantial GABAergic synapses are present in the GL, ePL, ML, and GrL. Furthermore, the released GABA may be removed by GAT-1 and GAT-3 in the GL, and the majority of GABA, which is present in the ePL to GrL, may undergo reuptake by GAT-3. This is the first morphological and descriptive study of GABAergic transmission in the MOB of Korean roe deer.

Expression profile and down-regulation of argininosuccinate synthetase in hepatocellular carcinoma in a transgenic mouse model

BACKGROUND

Argininosuccinate synthetase (ASS) participates in urea and nitric oxide production and is a rate-limiting enzyme in arginine biosynthesis. Regulation of ASS expression appears complex and dynamic. In addition to transcriptional regulation, a novel post-transcriptional regulation affecting nuclear precursor RNA stability has been reported. Moreover, many cancers, including hepatocellular carcinoma (HCC), have been found not to express ASS mRNA; therefore, they are auxotrophic for arginine. To study when and where ASS is expressed and whether post-transcriptional regulation is undermined in particular temporal and spatial expression and in pathological events such as HCC, we set up a transgenic mouse system with modified BAC (bacterial artificial chromosome) carrying the human ASS gene tagged with an EGFP reporter.

RESULTS

We established and characterized the transgenic mouse models based on the use of two BAC-based EGFP reporter cassettes: a transcription reporter and a transcription/post-transcription coupled reporter. Using such a transgenic mouse system, EGFP fluorescence pattern in E14.5 embryo was examined. Profiles of fluorescence and that of Ass RNA in in situ hybridization were found to be in good agreement in general, yet our system has the advantages of sensitivity and direct fluorescence visualization. By comparing expression patterns between mice carrying the transcription reporter and those carrying the transcription/post-transcription couple reporter, a post-transcriptional up-regulation of ASS was found around the ventricular zone/subventricular zone of E14.5 embryonic brain. In the EGFP fluorescence pattern and mRNA level in adult tissues, tissue-specific regulation was found to be mainly controlled at transcriptional initiation. Furthermore, strong EGFP expression was found in brain regions of olfactory bulb, septum, habenular nucleus and choroid plexus of the young transgenic mice. On the other hand, in crossing to hepatitis B virus X protein (HBx)-transgenic mice, the Tg (ASS-EGFP, HBx) double transgenic mice developed HCC in which ASS expression was down-regulated, as in clinical samples.

CONCLUSIONS

The BAC transgenic mouse model described is a valuable tool for studying ASS gene expression. Moreover, this mouse model is a close reproduction of clinical behavior of ASS in HCC and is useful in testing arginine-depleting agents and for studies of the role of ASS in tumorigenesis.

Immunohistochemical studies for the neuronal elements in the vomeronasal organ of the one-humped camel

The neuronal elements of the vomeronasal organ (VNO) of camel were investigated immunohistochemically. PGP 9.5 labeled the receptor cells in the vomeronasal sensory epithelium, but not the supporting or basal cells. OMP stained some receptor cells, but no immunoreactive signals for OMP were detected in the non-sensory epithelium. PLCβ2 labeled scattered cells in the sensory epithelium and a larger number of cells in the non-sensory epithelium. Double labeling immunohistochemistry revealed that the PLCβ2-positive cells were surrounded by substance P-positive nerve fibers. Collectively, these data suggest that the camel VNO bears, in addition to the mature vomeronasal receptor cells, trigeminally-innervated solitary chemosensory cells which are expected to play a substantial role in the control of stimulus access to the VNO.

CART in the brain of vertebrates: circuits, functions and evolution

Cocaine- and amphetamine-regulated transcript peptide (CART) with its wide distribution in the brain of mammals has been the focus of considerable research in recent years. Last two decades have witnessed a steady rise in the information on the genes that encode this neuropeptide and regulation of its transcription and translation. CART is highly enriched in the hypothalamic nuclei and its relevance to energy homeostasis and neuroendocrine control has been understood in great details. However, the occurrence of this peptide in a range of diverse circuitries for sensory, motor, vegetative, limbic and higher cortical areas has been confounding. Evidence that CART peptide may have role in addiction, pain, reward, learning and memory, cognition, sleep, reproduction and development, modulation of behavior and regulation of autonomic nervous system are accumulating, but an integration has been missing. A steady stream of papers has been pointing at the therapeutic potentials of CART. The current review is an attempt at piecing together the fragments of available information, and seeks meaning out of the CART elements in their anatomical niche. We try to put together the CART containing neuronal circuitries that have been conclusively demonstrated as well as those which have been proposed, but need confirmation. With a view to finding out the evolutionary antecedents, we visit the CART systems in sub-mammalian vertebrates and seek the answer why the system is shaped the way it is. We enquire into the conservation of the CART system and appreciate its functional diversity across the phyla.

A morphological study of the vomeronasal organ and the accessory olfactory bulb in the Korean roe deer, Capreolus pygargus

The vomeronasal organ (VNO) and accessory olfactory bulb (AOB) of the Korean roe deer (Capreolus pygargus) were studied histologically to evaluate their morphological characteristics. Grossly, the VNO, encased by cartilage, has a paired tubular structure with a caudal blind end and a rostral connection through incisive ducts on the hard palate. In the VNO, the vomeronasal sensory epithelium (VSE) consists of galectin-3-positive supporting cells, protein gene product (PGP) 9.5-positive receptor cells, and basal cells. The vomeronasal respiratory epithelium (VRE) consists of a pseudostratified epithelium. The AOB strata included a vomeronasal nerve layer (VNL), a glomerular layer (GL), a mitral/tufted cell layer, and a granular cell layer. All lectins used in this study, including Bandeiraea simplicifolia agglutinin isolectin B4 (BSI-B4), soybean agglutinin (SBA), Ulex europaeus agglutinin I (UEA-I), and Triticum vulgaris wheat germ agglutinin (WGA), labeled the VSE with varying intensity. In the AOB, both the VNL and the GL reacted with BSI-B4, SBA, and WGA with varying intensity, but not with UEA-I. This is the first morphological study of the VNO and AOB of the Korean roe deer, which are similar to those of goats.

Mating increases neuronal tyrosine hydroxylase expression and selectively gates transmission of male chemosensory information in female mice

Exposure to chemosensory signals from unfamiliar males can terminate pregnancy in recently mated female mice. The number of tyrosine hydroxylase-positive neurons in the main olfactory bulb has been found to increase following mating and has been implicated in preventing male-induced pregnancy block during the post-implantation period. In contrast, pre-implantation pregnancy block is mediated by the vomeronasal system, and is thought to be prevented by selective inhibition of the mate’s pregnancy blocking chemosignals, at the level of the accessory olfactory bulb. The objectives of this study were firstly to identify the level of the vomeronasal pathway at which selective inhibition of the mate’s pregnancy blocking chemosignals occurs. Secondly, to determine whether a post-mating increase in tyrosine hydroxylase-positive neurons is observed in the vomeronasal system, which could play a role in preventing pre-implantation pregnancy block. Immunohistochemical staining revealed that mating induced an increase in tyrosine-hydroxylase positive neurons in the arcuate hypothalamus of BALB/c females, and suppressed c-Fos expression in these neurons in response to mating male chemosignals. This selective suppression of c-Fos response to mating male chemosignals was not apparent at earlier levels of the pregnancy-blocking neural pathway in the accessory olfactory bulb or corticomedial amygdala. Immunohistochemical staining revealed an increase in the number of tyrosine hydroxylase-positive neurons in the accessory olfactory bulb of BALB/c female mice following mating. However, increased dopamine-mediated inhibition in the accessory olfactory bulb is unlikely to account for the prevention of pregnancy block to the mating male, as tyrosine hydroxylase expression did not increase in females of the C57BL/6 strain, which show normal mate recognition. These findings reveal an association of mating with increased dopaminergic modulation in the pregnancy block pathway and support the hypothesis that mate recognition prevents pregnancy block by suppressing the activation of arcuate dopamine release.

Three-dimensional reconstruction of electron micrographs reveals intrabulbar circuit differences between accessory and main olfactory bulbs

Three-dimensional (3D) reconstruction of synaptic arrangement on a particular dendrite provides essential information regarding neuronal properties and neural microcircuits. Unconventional synapses are particularly good candidates for such steric attribution. In main and accessory olfactory bulbs (MOBs and AOBs), there are dendrodendritic reciprocal synapses (RSs) between excitatory projection neurons and inhibitory interneurons. Although the fine structure and configuration of these synapses have been investigated in MOB, their characteristics in AOB were unknown. In this study, we performed 3D AOB reconstruction using serial section transmission electron microscopy. We found numerous RSs on primary dendrites from glomeruli to mitral/tufted (MT) cell somas. These synapses formed between dendritic shafts of MT cells and large dendritic spines, or so-called gemmules, of granule (Gr) cells. This indicates that chemical signals received by a glomerulus are regulated in the primary dendrite of an MT cell before reaching its soma. In MOB, RSs are located on secondary dendrites and act as lateral and self-inhibiting following mitral cell depolarization. Our results indicate that AOB intrabulbar microcircuitry is quite different from that in the MOB.