Differential development of binding sites of two lectins in the vomeronasal axons of the rat accessory olfactory bulb

Lectin histochemistry of the olfactory mucosa of Korean native cattle, Bos taurus coreanae

BACKGROUND

The olfactory mucosa (OM) is crucial for odorant perception in the main olfactory system. The terminal carbohydrates of glycoconjugates influence chemoreception in the olfactory epithelium (OE).

OBJECTIVES

The histological characteristics and glycoconjugate composition of the OM of Korean native cattle (Hanwoo, Bos taurus coreae) were examined to characterize their morphology and possible functions during postnatal development.

METHODS

The OM of neonate and adult Korean native cattle was evaluated using histological, immunohistochemical, and lectin histochemical methods.

RESULTS

Histologically, the OM in both neonates and adults consists of the olfactory epithelium and the lamina propria. Additionally, using periodic acid Schiff and Alcian blue (pH 2.5), the mucus specificity of the Bowman's gland duct and acini in the lamina propria was determined. Immunohistochemistry demonstrated that mature and immature olfactory sensory neurons of OEs express the olfactory marker protein and growth associated protein-43, respectively. Lectin histochemistry indicated that numerous glycoconjugates, including as N-acetylglucosamine, mannose, galactose, N-acetylgalactosamine, complex type N-glycan, and fucose groups, were expressed at varied levels in the different cell types in the OMs of neonates and adults at varying levels. According to our observations, the cattle possessed a well-developed olfactory system, and the expression patterns of glycoconjugates in neonatal and adult OMs varied considerably.

CONCLUSIONS

This is the first study to describe the morphological assessment of the OM of Korean native cattle with a focus on lectin histochemistry. The findings suggest that glycoconjugates may play a role in olfactory chemoreception, and that their labeling properties may be closely related to OM development and maturity.

Glycan changes in the olfactory mucosa of rats with experimental autoimmune encephalomyelitis

Glycans are components of glycoconjugates and function in odorant recognition and cell signaling in the olfactory mucosa. However, little is known about glycan expression in the olfactory mucosa in the presence of neuroinflammatory disorders, which can influence olfaction. We evaluated the changes in glycan in the olfactory mucosa of rats with experimental autoimmune encephalomyelitis (EAE) by histochemical analyses of 21 lectins. In the olfactory mucosa of normal control rats, 16 lectins bound to olfactory sensory neurons, supporting cells, basal cells, nerve and Bowman's glands, and their expression did not significantly change during the course of EAE. In rats with paralytic-stage EAE, five lectins showed different reactivities with the olfactory mucosa compared to those of normal control rats. Of them, Bandeiraea simplicifolia lectin (BSL)-II and BSL-I showed transiently downregulated binding to olfactory sensory neurons and supporting cells in rats with EAE. The reactivities of Lens culinaris agglutinin for the basement membrane, Vicia villosa agglutinin for Bowman's glands and Dolichos biflorus agglutinin for all nuclei were upregulated in the olfactory mucosa of EAE rats. These results suggest that BSL-II-binding N-acetyl-glucosamine and BSL-I-binding N-acetyl-galactose are involved in transient olfactory dysfunction in EAE, which may hamper odor perception and/or signal processing in olfactory sensory neurons.

Signal Detection and Coding in the Accessory Olfactory System

In many mammalian species, the accessory olfactory system plays a central role in guiding behavioral and physiological responses to social and reproductive interactions. Because of its relatively compact structure and its direct access to amygdalar and hypothalamic nuclei, the accessory olfactory pathway provides an ideal system to study sensory control of complex mammalian behavior. During the last several years, many studies employing molecular, behavioral, and physiological approaches have significantly expanded and enhanced our understanding of this system. The purpose of the current review is to integrate older and newer studies to present an updated and comprehensive picture of vomeronasal signaling and coding with an emphasis on early accessory olfactory system processing stages. These include vomeronasal sensory neurons in the vomeronasal organ, and the circuitry of the accessory olfactory bulb. Because the overwhelming majority of studies on accessory olfactory system function employ rodents, this review is largely focused on this phylogenetic order, and on mice in particular. Taken together, the emerging view from both older literature and more recent studies is that the molecular, cellular, and circuit properties of chemosensory signaling along the accessory olfactory pathway are in many ways unique. Yet, it has also become evident that, like the main olfactory system, the accessory olfactory system also has the capacity for adaptive learning, experience, and state-dependent plasticity. In addition to describing what is currently known about accessory olfactory system function and physiology, we highlight what we believe are important gaps in our knowledge, which thus define exciting directions for future investigation.

Olfactory ensheathing cells form the microenvironment of migrating GnRH-1 neurons during mouse development

During development, GnRH-1 neurons differentiate extracerebraly from the nasal placode and migrate from the vomeronasal organ to the forebrain along vomeronasal and terminal nerves. Numerous studies have described the influence of different molecules on the migration of GnRH-1 neurons, however, the role of microenvironment cells remains poorly understood. This study used GFAP-GFP transgenic mice to detect glial cells at early developmental stages. Using nasal explant cultures, the comigration of glial cells with GnRH-1 neurons was clearly demonstrated. This in vitro approach showed that glial cells began migrating from the explants before GnRH-1 neurons. They remained ahead of the GnRH-1 migratory front and stopped migrating after the GnRH-1 neurons. The association of these glial cells with the axons combined with gene expression analysis of GFAP-GFP sorted cells enabled them to be identified as olfactory ensheathing cells (OEC). Immunohistochemical analysis revealed the presence of multiple glial cell-type markers showing several OEC subpopulations surrounding GnRH-1 neurons. Moreover, these OEC expressed genes whose products are involved in the migration of GnRH-1 neurons, such as Nelf and Semaphorin 4. In situ data confirmed that the majority of the GnRH-1 neurons were associated with glial cells along the vomeronasal axons in nasal septum and terminal nerves in the nasal forebrain junction as early as E12.5. Overall, these data demonstrate an OEC microenvironment for migrating GnRH-1 neurons during mouse development. The fact that this glial cell type precedes GnRH-1 neurons and encodes for molecules involved in their nasal migration suggests that it participates in the GnRH-1 system ontogenesis.

Lectin histochemical study on the olfactory bulb of the newt, Cynops pyrrhogaster

The function and/or morphological features of the vomeronasal olfactory system remain unclear in aquatic animals, although the system appeared first in urodeles based on phylogenic data. We examined the lectin binding patterns in the olfactory bulb of a semi-aquatic urodele, the Japanese red-bellied newt, Cynops pyrrhogaster, using 22 different lectins. Eleven of the lectins showed specific binding to the nerve fibres and glomeruli in the olfactory bulb. Among these, Wheat germ agglutinin, pokeweed and peanut agglutinin preferentially bound the main olfactory bulb, reflecting variation in the expression of glycoconjugates between the main and accessory olfactory bulbs. By contrast, the types of lectins bound to the Cynops olfactory bulb were considerably different from those reported in other urodele families. These results suggest a histochemical distinction between the main and accessory olfactory bulbs, and that glycoconjugate expression may differ significantly among urodele families.

Diverse systems for pheromone perception: multiple receptor families in two olfactory systems

Traditionally, the olfactory epithelium is considered to recognize conventional odors, while the vomeronasal organ detects pheromones. However, recent advances suggest that vertebrate pheromones can also be detected by the olfactory epithelium. In the vomeronasal organ and the olfactory epithelium, structurally distinct multiple receptor families are expressed. In rodents, two of these receptor families, V1R and V2R, are expressed specifically in the vomeronasal organ and detect pheromones and pheromone candidates. A newly isolated trace amine-associated receptor detects some of the putative pheromones in the mouse olfactory epithelium. In addition, distinct second-messenger pathways and neural circuits are used for pheromone perception mediated by each receptor family. Furthermore, the function of these receptor families in these olfactory organs appears to differ among various vertebrate species. The systems for pheromone perception in vertebrates are far more complex than previously predicted.

Structure and chemical organization of the accessory olfactory bulb in the goat

The structure and chemical composition of the accessory olfactory bulb (AOB) were examined in male and female goats. Sections were subjected to either Nissl staining, Klüver-Barrera staining, lectin histochemistry, or immunohistochemistry for nitric oxide synthase (NOS), neuropeptide Y (NPY), tyrosine hydroxylase (TH), dopamine beta-hydroxylase (DBH), and glutamic acid decarboxylase (GAD). The goat AOB was divided into four layers: the vomeronasal nerve layer (VNL), glomerular layer (GL), mitral/tufted (M/T) cell layer (MTL), and granule cell layer (GRL). Quantitative and morphometric analyses indicated that a single AOB contained 5,000-8,000 putative M/T cells with no sex differences, whereas the AOB was slightly larger in males. Of the 21 lectins examined, 7 specifically bound to the VNL and GL, and 1 bound not only to the VNL, but also to the MTL and GRL. In either of these cases, no heterogeneity of lectin staining was observed in the rostrocaudal direction. NOS-, TH-, DBH-, and GAD-immunoreactivity (ir) were observed in the MTL and GRL, whereas NPY-ir was present only in the GRL. In the GL, periglomerular cells with GAD-ir were found in abundance, and a subset of periglomerular cells containing TH-ir was also found. Double-labeling immunohistochemistry revealed that virtually all periglomerular cells containing TH-ir were colocalized with GAD-ir.

Subdivision of the accessory olfactory bulb in the Japanese common toad, Bufo japonicus, revealed by lectin histochemical analysis

Lectin binding patterns in the olfactory bulb of the Japanese common toad, Bufo japonicus, were examined using 21 types of lectin. Ten out of 21 lectins, WGA, s-WGA, LEL, STL, DBA, VVA, SJA, RCA-I, PNA, and PHA-L, stained the olfactory nerve, the glomeruli in the main olfactory bulb (MOB), the vomeronasal nerve, and the glomeruli in the accessory olfactory bulb (AOB). The binding patterns of LEL, STL, DBA, and PHA-L subdivided AOB glomeruli into rostral and caudal regions, where LEL, STL, and DBA stained the rostral region more intensely than the caudal region, and PHA-L had the opposite effect. Another lectin, BSL-I, stained both AOB glomeruli and the vomeronasal nerve, but not MOB glomeruli or the olfactory nerve. This is the first report of histological subdivision in the AOB of an amphibian, which suggests that the AOB development in Bufo may be unique.

Ultrastructural localization of alpha-galactose-containing glycoconjugates in the rat vomeronasal organ

Binding sites of Griffonia simplicifolia I-B4 isolectin (GS-I-B4), which recognizes terminal alpha-galactose residues of glycoconjugates, were examined in the juxtaluminal region of the rat vomeronasal sensory epithelium and its associated glands of the vomeronasal organ, using a lectin cytochemical technique. Lowicryl K4M-embedded ultra-thin sections, which were treated successively with biotinylated GS-I-B4 and streptavidin-conjugated 10 nm colloidal gold particles, were observed under a transmission electron microscope. Colloidal gold particles, which reflect the presence of terminal alpha-galactose-containing glycoconjugates, were present in vomeronasal receptor neurons in the sensory epithelium and secretory granules of acinar cells of associated glands of the epithelium. Quantitative analysis demonstrated that the density of colloidal gold particles associated with sensory cell microvilli that projected from dendritic endings of vomeronasal neurons was considerably higher than that of microvilli that projected from neighboring sustentacular cells. The same was true for the apical cytoplasms of these cells just below the microvilli. These results suggest that of the sensory microvilli and dendritic endings contained a much larger amount of the alpha-galactose-containing glycoconjugates, compared with those in sustentacular microvilli. Further, biochemical analyses demonstrated several vomeronasal organ-specific glycoproteins with terminal alpha-galactose.

Developmental changes in oscillatory and slow responses of the rat accessory olfactory bulb

Field potential, patch-clamp and optical recordings were performed in accessory olfactory bulb slices of postnatal rats following single electrical stimulation of the vomeronasal nerve layer. On the basis of differences in the components of the field potential, postnatal days were divided into three periods: immature (until postnatal day 11), transitional (postnatal days P12-17) and mature periods (after postnatal day 18). During the immature period, vomeronasal nerve layer stimulation provoked a characteristic damped oscillatory field potential, and the field potential recorded in the glomerular layer consisted of a compound action potential followed by several periodic negative peaks superimposed on slow components. Reduction in [Mg2+] enhanced slow components but did not affect oscillation, whereas an NMDA receptor antagonist, D-2-amino-5-phosphonovalerate, depressed slow components but did not affect the oscillation. During the mature period, slow components and the periodic waves (oscillation) disappeared. The time course of the field potential was similar to that in adults, suggesting that the accessory olfactory bulb reached electrophysiologically maturity at postnatal day 18. A non-NMDA receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione, inhibited vomeronasal nerve layer-induced responses, while D-2-amino-5-phosphonovalerate had no effect, suggesting that NMDA and non-NMDA receptors are active in immature tissues, whereas non-NMDA receptors predominated in mature tissue. Results from whole-cell patch recordings in mitral and granule cells yielded results consistent with those from field potential and optical recordings. Further, a gradual decrease in number and frequency of oscillating waves was observed until postnatal day 17. Analyses of the depth profile of field potentials and current source density in immature tissue suggested that the oscillation and slow components originated in the glomerular layer but not in the external plexiform/mitral cell layer. Further, a new type of oscillation, which was independent of the reciprocal dendrodendritic synapses between mitral and granule cells, was detected. These data indicate that the lack of oscillatory suppression by immature NMDA receptors may play a critical role in the dynamic alteration of bulbar conditions.

Expression of netrin-1 and netrin-1 receptor, DCC, in the rat olfactory nerve pathway during development and axonal regeneration

Netrin-1 is a bifunctional secreted protein that directs axon extension in various groups of developing axonal tracts. The transmembrane DCC (deleted in colorectal cancer) receptor is described as netrin-1 receptor and is involved in the attractive effects of netrin-1. In this study, we examined the spatio-temporal expression patterns of both netrin-1 and DCC in the rat olfactory system at different stages of development and during axonal regeneration following unilateral bulbectomy. High DCC expression was detected on the pioneer olfactory axons as they are extending toward the telencephalon. This expression was transient since from embryonic day 16 onwards, DCC was no longer detected along the olfactory nerve path. From embryonic day 14 until birth, DCC was also expressed within the mesenchyme surrounding the olfactory epithelium. During the same period, netrin-1 protein was detected along the trajectory of olfactory axons up to the olfactory bulb and its expression pattern in the nasal mesenchyme largely overlapped that of DCC. Moreover, netrin-1 continued to be present during the two first post-natal weeks, and a weak protein expression still persisted in the dorso-medial region of the olfactory epithelium in adult rats. While unilateral bulbectomy induced a transient up-regulation of netrin-1 in the lamina propria, particularly in the dorso-medial region of the neuroepithelium, no DCC expression was detected on the regenerating olfactory axons. In the developing olfactory bulb, the extension of mitral cell axons was associated with DCC presence while netrin-1 was absent along this axonal path. DCC was also highly expressed in the newly formed glomeruli after birth, and a weak DCC expression was still detected in the glomerular layer in adult rats. Taken together, these data support the notion that netrin-1, via DCC expressed on axons, may play a role in promoting outgrowth and/or guidance of pioneering olfactory axons toward the olfactory bulb primordium. Moreover, association of netrin-1 with mesenchymal DCC may provide a permissive environment to the growth of both pioneer and later-growing axons. The maintenance of netrin-1 expression in the nasal mesenchyme of adult rats as well as its regional up-regulation following unilateral bulbectomy infer that netrin-1, even in the absence of DCC, may be involved in the process of axonal growth of newly differentiated olfactory receptor neurons probably through the use of other receptors.

A role for the EphA family in the topographic targeting of vomeronasal axons

We have investigated the role of the Eph family of receptor tyrosine kinases and their ligands in the establishment of the vomeronasal projection in the mouse. Our data show intriguing differential expression patterns of ephrin-A5 on vomeronasal axons and of EphA6 in the accessory olfactory bulb (AOB), such that axons with high ligand concentration project onto regions of the AOB with high receptor concentration and vice versa. These data suggest a mechanism for development of this projection that is the opposite of the repellent interaction between Eph receptors and ligands observed in other systems. In support of this idea, when given the choice of whether to grow on lanes containing EphA-F(c)/laminin or F(c)/laminin protein (in the stripe assay), vomeronasal axons prefer to grow on EphA-F(c)/laminin. Analysis of ephrin-A5 mutant mice revealed a disturbance of the topographic targeting of vomeronasal axons to the AOB. In summary, these data, which are derived from in vitro and in vivo experiments, indicate an important role of the EphA family in setting up the vomeronasal projection.

A novel family of candidate pheromone receptors in mammals

Pheromonal activation of the vomeronasal organ (VNO) elicits genetically preprogrammed behaviors and physiological changes in mammals. We have identified a novel gene family encoding over one hundred VNO specific receptors, the V3Rs. V3R sequences are highly similar to each other and appear distantly related to the putative pheromone receptors, V1Rs, and the taste receptors, T2Rs. Within the VNO, V3R-positive neurons are distinct from neurons expressing the pheromone receptor families V1R and V2R. The V3Rs are likely to represent a new large family of pheromone receptors in mammals. Multiple V3R-related human sequences have been identified, including one clone retaining the capacity to create a complete and functional transcript. Our data uncover a striking complexity in the molecular and cellular organization of the VNO and provide an essential framework for the study of pheromone signaling in mammals.

Morphological and histochemical changes in the regenerating vomeronasal epithelium

Receptor cell degeneration and regeneration within the vomeronasal organ (VNO) of the rat was studied using both electron microscopy and histochemical methods. Electron microscopy was employed to examine the morphological changes along the surface of the sensory epithelium, and histochemical markers were used to monitor the changes in the epithelial cell layers. Transection of the vomeronasal nerves induced selective degeneration of the receptor cells, and within six days, a significant decrease in the number of receptor cells was observed. During the subsequent stage of receptor cell regeneration, cilia and bud-like structures characteristic of a developing sensory epithelium were seen. By day 15, thin microvilli covering the surface of the receptor cells reappeared in the sensory epithelium. The neural cell adhesion molecule (NCAM) and two vomeronasal system-specific lectins; 1) Bandeiraea simplicifolia lectin (BSL-I) and 2) Vicia villosa agglutinin (VVA) were used as the histochemical markers. NCAM immunoreactivity on the surface of the epithelium was observed to be decreased significantly six days after nerve transection, and was restored during receptor cell regeneration (day 15). The reactivity of the two lectins, BSL-I and VVA, was decreased slightly during degeneration, but was still detectable at the time of maximum receptor cell degeneration (day 6). Lectin reactivity was restored to control levels by day 15. These findings suggest that (1) NCAM is a useful marker for vomeronasal receptor cells and that the vomeronasal system-specific lectins may bind to both receptor and supporting cells and (2) degeneration of vomeronasal receptor cells occurs during the first week (day 6) following nerve transection and the receptor cell population begins to recover within 15 days. The morphological changes observed during receptor cell regeneration suggest that the stages of VNO receptor cell regeneration are similar to those observed during development.

Sensory coding of pheromone signals in mammals

The vomeronasal organ (VNO) of mammals plays an essential role in the detection of pheromones, chemical cues secreted by animals that elicit genetically programmed sexual and aggressive behaviors among conspecifics. The recent characterization of genes encoding molecular components of the VNO sensory response suggests that VNO neurons express a unique set of molecules to recognize and translate pheromone signals into neuronal electrical activity. Identification of these genes, which include putative pheromone receptor genes, has offered a new opportunity to uncover basic principles of pheromone sensory processing and important aspects of vomeronasal development.

Novel subdivisions of the rat accessory olfactory bulb revealed by the combined method with lectin histochemistry, electrophysiological and optical recordings

Wistaria floribunda agglutinin and peanut agglutinin were found to bind histochemically to the anterior and posterior regions, respectively, of the vomeronasal nerve and glomerular layers in the rat accessory olfactory bulb. Furthermore, Ricinus communis agglutinin showed strong binding to the anterior region of the vomeronasal nerve and glomerular layers, whereas it bound weakly and/or moderately to the rostral two-thirds of the posterior glomerular layer but not at all to the caudal one-third. This suggests that the posterior region is further divided into two subregions. An electrophysiological mapping study in sagittal slice preparations demonstrated that stimulation given within the anterior vomeronasal nerve layer elicited field potentials within the anterior region of the external plexiform layer, whereas shocks to the rostral two-thirds and the caudal one-third of the posterior vomeronasal nerve layer provoked field responses within the rostral two-thirds and within the caudal one-third of the posterior external plexiform layer, respectively, indicating that the posterior external plexiform layer is also divided into two subregions. Real-time optical imaging showed similar results as above, except that neural activity also spread into mitral cell layers. Furthermore, the most anterior and posterior ends of the neural activity evoked in the rostral two-thirds of the posterior region immediately adjoined the posterior border of that evoked in the anterior region and the anterior border of that evoked in the caudal one-third of the posterior region, respectively. Moreover, the granule cell layer was also found to have similar boundaries. Thus, optical imaging studies demonstrated individual precise boundaries of these subdivisions, which were positioned right beneath those defined by Ricinus communis agglutinin histochemistry. The presence of functional segregation in each layer leads us to conclude that there are at least three different input-output pathways in the rat vomeronasal system.

Neurestin: putative transmembrane molecule implicated in neuronal development

We have cloned a novel cDNA encoding a putative transmembrane protein, neurestin, from the rat olfactory bulb. Neurestin was identified based on a sequence similar to that of the second extracellular loops of odorant receptors in the cysteine-rich CC box located immediately after EGF-like motifs. Neurestin shows homology to a neuregulin gene product, human gamma-heregulin, a Drosophila receptor-type pair-rule gene product, Odd Oz (Odz) / Ten(m), and Ten(a), suggesting a possible function in synapse formation and morphogenesis. Recently, a mouse neurestin homolog has independently been cloned as DOC4 from the NIH-3T3 cell line. Northern blot analysis showed that neurestin is highly expressed in the brain and also in other tissues at much lower levels. In situ hybridization studies showed that neurestin is expressed in many types of neurons, including pyramidal cells in the cerebral cortex and tufted cells in the olfactory bulb during development. In adults, neurestin is mainly expressed in olfactory and hippocampal granule cells, which are known to be generated throughout adulthood. Nonetheless, in adults the expression of neurestin was experimentally induced in external tufted cells during regeneration of olfactory sensory neurons. These results suggest a role for neurestin in neuronal development and regeneration in the central nervous system.

Expression of brain-2 in the developing olfactory bulb

The expression of Brain-2, a POU domain transcription factor, was examined in the developing olfactory bulb. Brain-2 was expressed mainly in the output neurons, mitral cell and tufted cells in the main olfactory bulb (MOB), and mitral/tufted cells (MT cells) in the accessory olfactory bulb (AOB). It was not expressed in granular cells in either the MOB or the AOB. Our results suggest that Brain-2 was specifically expressed in output neurons but not in interneurons in the developing olfactory bulb. Brain-2 may play a role in the development of these output neurons.

Lectin characterization of the olfactory system in brachiopterygian fish

Lectin binding was performed on the olfactory system of Polypterus and Erpetoichthys, the living genera of the subclass of Brachiopterygii. The lectin histochemical patterns and the Western-blot analysis indicate that the receptor cells of the olfactory mucosa are characterized by high density of specific glycoconjugate residues. The presence of glycoproteins, whose terminal sugars are detected by lectin binding, might be related to the reception of an odor stimulus and its transduction into a nervous signal or to the histogenesis of the olfactory system.

The accessory olfactory bulb of the mink, Mustela vison: a morphological and lectin histochemical study

The distribution of binding sites for the lectins Ulex europaeus agglutinin I, Soybean agglutinin, Bandeiraea simplicifolia agglutinin I-isolectin B4, and Vicia villosa agglutinin in the mink olfactory bulb was investigated. All lectins except Ulex europaeus agglutinin I bound exclusively and systematically to a single area of the olfactory bulb. This area corresponded to that in which the vomeronasal nerves terminate, indicating that it is the accessory olfactory bulb, as confirmed by microdissection and by the study of transverse and parasagittal series of the olfactory bulb. The results, moreover, indicate that the accessory olfactory bulb of the mink comprises three isolated eminences, the largest in the dorsal part of the olfactory bulb, and the other two in the lateral and medial parts.

A novel family of putative pheromone receptors in mammals with a topographically organized and sexually dimorphic distribution

Mammals have retained two functionally and anatomically independent collections of olfactory neurons located in the main olfactory epithelium and in the vomeronasal organ (VNO). Pheromones activate the VNO in order to elicit fixed action behaviors and neuroendocrine changes involved in animal reproduction and aggression. Differential screening of cDNA libraries constructed from individual rat VNO neurons has led to the isolation of a novel family of approximately 100 genes encoding seven transmembrane receptors with sequence similarity with Ca2+-sensing and metabotropic glutamate receptors. These genes are likely to encode a novel family of pheromone receptors. Patterns of receptor gene expression suggest that the VNO is organized into discrete and sexually dimorphic functional units that may permit segregation of pheromone signals leading to specific arrays of behaviors and neuroendocrine responses.

Subdivisions of the guinea-pig accessory olfactory bulb revealed by the combined method with immunohistochemistry, electrophysiological, and optical recordings

The presence of subgroups in vomeronasal sensory neurons has been known in various animals. To elucidate possible functional subdivisions in the guinea-pig accessory olfactory bulb, the combined studies with GTP-binding protein immunohistochemistry, electrophysiological and optical recordings were carried out. Gi2 alpha and Go alpha proteins were immunohistochemically localized, respectively, in the anterior and posterior regions of the vomeronasal nerve and glomerular layers, indicating that the guinea-pig accessory olfactory bulb receives at least two different inputs. This suggests that an anatomical boundary exists in these two layers. A mapping study of field potentials in sagittal slice preparations demonstrated that stimulation of the anterior vomeronasal nerve layer elicited field potentials with weak oscillatory responses exclusively in the anterior region of the external plexiform layer, whereas shocks to the posterior vomeronasal nerve layer provoked distinct oscillatory responses within the posterior one. The damping factors of oscillations in the anterior and posterior regions were 0.064+/-0.028 and 0.025+/-0.014, respectively. These electrophysiological results suggest that the accessory olfactory bulb consists of two functionally different subdivisions. Real-time optical imaging showed that anterior vomeronasal nerve layer shocks produced neural activity which spread horizontally from anterior to posterior only within the anterior region of the external plexiform and mitral cell layers, whereas shocks to the posterior vomeronasal nerve layer evoked periodic neural activity which spread horizontally from posterior to anterior only within the posterior region. Furthermore, the most posterior extent of the optical response evoked in the anterior region immediately adjoined the most anterior extent of that evoked in the posterior region. The maximal distance of signal propagation in the granule cell layer corresponded to that in the overlying external plexiform and mitral cell layers, indicating that the granule cell layer also has a similar boundary. Thus, these optical imaging studies not only demonstrated a precise boundary in each layer of the accessory olfactory bulb, which was positioned right beneath the boundary defined by GTP-binding protein immunohistochemistry, but also confirmed the observations from electrophysiological mapping that evoked field potentials are independently distributed in each of two subdivisions. The presence of the functional subdivision in each layer leads us to conclude that the accessory olfactory bulb in the guinea-pig is distinctly segregated into the anterior and posterior subdivisions, and to suggest that there are at least two different input output pathways in the vomeronasal system.

Lamina-specific connectivity in the brain: regulation by N-cadherin, neurotrophins, and glycoconjugates

In the vertebrate brain, neurons grouped in parallel laminae receive distinct sets of synaptic inputs. In the avian optic tectum, arbors and synapses of most retinal axons are confined to 3 of 15 laminae. The adhesion molecule N-cadherin and cell surface glycoconjugates recognized by a plant lectin are selectively associated with these "retinorecipient" laminae. The lectin and a monoclonal antibody to N-cadherin perturbed laminar selectivity in distinct fashions. In contrast, neurotrophins increased the complexity of retinal arbors without affecting their laminar distribution. Thus, cell surface molecules and soluble trophic factors may collaborate to shape lamina-specific arbors in the brain, with the former predominantly affecting their position and the latter their size.

Vicia villosa agglutinin labels a subset of neurons coexpressing both the mu opioid receptor and parvalbumin in the developing rat subiculum

Vicia villosa agglutinin (VVA), anti-parvalbumin antiserum and an affinity-purified anti-mu opioid receptor antibody were used to triple-label neurons in the postnatal rat subiculum. VVA labeled a subset of mu opioid receptor-positive neurons that were also immunoreactive for parvalbumin. The morphology of the triple-labeled neurons was heterogeneous, and included multipolar, ovoid and pyramidal-shaped neurons. Neurons single-labeled for the mu opioid receptor, VVA or parvalbumin were also morphologically heterogeneous. The postnatal development of mu opioid receptor immunoreactivity (IR), parvalbumin-IR and VVA binding was investigated using triple-labeling immunocytochemistry. Mu opioid receptor-IR appeared first and was present at postnatal day 1 (P1). Parvalbumin-IR was first observed in somata at P10, followed by proximal and distal dendrites at P15 and P20 respectively. Faint VVA labeling was seen first at P10 and surrounded a limited number of neurons. The intensity of labeling and the number of neurons labeled with VVA increased between P10 and P20; however, both measures remained below adult levels at P20. This study further illustrates the neurochemical heterogeneity of interneurons in the hippocampal formation and shows the developmentally early appearance of mu opioid receptor-IR compared to the late appearance of VVA binding and parvalbumin-IR.

Identification of surface glycoconjugates in the olfactory system of turtle

Lectin binding histochemistry was performed on the olfactory system of Pseudemys scripta to investigate the distribution and density of defined carbohydrate terminals on the cell surface glycoproteins of the olfactory receptors and their terminals in the olfactory bulbs. The lectin staining patterns indicate that the receptor cells of the olfactory mucosa are characterized by glycoconjugates containing alpha-D-galactose and N-acetyl-D-glucosamine terminal residues. The vomeronasal receptor cells contain instead alpha-N-acetyl-D-galactosamine, N-acetyl-D-glucosamine and alpha-D-galactose residues. The results demonstrate that the vomeronasal receptor cells contain high density of alpha-N-acetyl-D-galactosamine sugar residues that are not expressed by receptor cells of the olfactory mucosa. The presence of specific glycoproteins, whose terminal sugars are detected by lectin binding, might be related to the chemoreception and transduction of the odorous message into a nervous signal or in the histogenesis of the olfactory system. In fact, the olfactory receptors are the only known neurons in the vertebrate nervous system that undergo a continual cycle of proliferation not only in developing animals but also in mature ones. Moreover the results show that BSA-I-B4, an alpha-D-galactosyl-specific isolectin, targets the terminal sugar residues in the ramified microglial cells.

Coculture of the vomeronasal organ and olfactory bulb of the fetal rat

The vomeronasal organ and the olfactory bulb of the rat were cocultured from 15-day embryo siblings on collagen-coated membrane in Dulbecco's modified Eagle's medium containing fetal calf serum, horse serum, and antibiotics. At 4 days in vitro (DIV), vomeronasal axons forming two to three large fascicles were seen originating from the explants of the vomeronasal organ. Differential axonal growth was observed. Some fascicles made connections with the explants of the olfactory bulb. Twenty percent of the cocultures studied here showed the formation of connections. At 6-10 DIV many fascicles that did not connect with the olfactory bulb had degenerated, and large fascicles that were connected with the olfactory bulb survived for more than 10 DIV. The formation of connections between the vomeronasal organ and the olfactory bulb in coculture favors the survival of large nerve fascicles, but it could not be determined whether or not the presence of the olfactory bulb affects the initial orientation of the fibers and fascicles from the explants of the vomeronasal organ.

Lectin histochemical identification of carbohydrate moieties in opossum chemosensory systems during development, with special emphasis on VVA-identified subdivisions in the accessory olfactory bulb

Lectins, sugar-binding molecules of nonimmune origin, were used in this study to describe the development of the main olfactory and vomeronasal systems in the Brazilian gray short-tailed opossum, Monodelphis domestica. A battery of seven lectins of the N-acetylgalactosamine/galactose-binding group was used. Of the seven lectins, only two, Vicia villosa agglutinin (VVA) and Griffonia (Bandeiraea) simplicifolia lectin I-isolectin B4 (GS I-B4), were specific to the vomeronasal system. The other five lectins recognized carbohydrates in both chemosensory systems, although the binding was more intense in the accessory olfactory system. Furthermore, whereas six of the lectins stained the adult opossum accessory olfactory bulb (AOB) homogeneously, the VVA lectin distinguished two regions of the AOB. Similar to the expression of olfactory marker protein (OMP) (Shnayder et al. [1993] Neuroreport 5:193-196), the rostral half of the AOB stained much darker with VVA than the caudal half, and the onset of the restricted pattern of staining at age 45 days also coincided. We conclude that 1) GS I-B4 and VVA recognize cell surface carbohydrate moieties specific to the vomeronasal, but not to the main olfactory, system, and 2) the carbohydrate moiety that is recognized by the VVA lectin, presumably terminal N-acetyl-galactosamine, is both temporally and spatially restricted in the opossum AOB. These results are discussed in the framework of other known spatially restricted molecules of the two major nasal chemosensory systems.

Vicia villosa agglutinin inhibits the fasciculation of vomeronasal axons in fetal rat vomeronasal organ culture

The vomeronasal organ of rat was cultured from embryonic 15-day littermates. During 4-8 days in vitro, vomeronasal axons originating from the explants of vomeronasal organ formed 2-3 large fascicles. When Vicia villosa agglutinin (VVA) was added to the culture medium, fasciculation of the vomeronasal axons was inhibited. The timing of addition and the duration of the presence of VVA were related to the inhibition of fasciculation of vomeronasal axons. Glycocojugates that bind with the VVA may therefore play an important role in the fasciculation of developing vomeronasal axons.