Chondroitin sulfate in the extracellular matrix of the medial and lateral superior olivary nuclei in the dog

Neuroanatomical characterization of perineuronal net components in the human cochlear nucleus and superior olivary complex

The human auditory brainstem, especially the cochlear nucleus (CN) and the superior olivary complex (SOC) are characterized by a high density of neurons associated with perineuronal nets (PNs). PNs build a specific form of extracellular matrix surrounding the neuronal somata, proximal dendrites and axon initial segments. They restrict synaptic plasticity and control high-frequency synaptic activity, a prominent characteristic of neurons of the auditory brainstem. The distribution of PNs within the auditory brainstem has been investigated in a number of mammalian species. However, much less is known regarding PNs in the human auditory brainstem. The present study aimed at the immunohistochemical identification of PNs in the cochlear nucleus (CN) and superior olivary complex (SOC) in the human brainstem. We focused on the complex nature and molecular variability of PNs in the CN and SOC by using specific antibodies against the main PN components (aggrecan, brevican, neurocan and hyaluronan and proteoglycan link protein 1). Virtually all subnuclei within the ventral CN and SOC were found to be associated with PNs. Direct comparison between gerbil and human yielded similar fine structure of PNs and confirmed the typical tight interdigitation of PNs with synaptic terminals in both species. Noticeably, an elaborate combination of immunohistochemical labelings clearly supports the still debated existence of the medial nucleus of trapezoid body (MNTB) in the human brain. In conclusion, the present study demonstrates that PNs form a prominent extracellular structure on CN and SOC neurons in the human brain, potentially stabilizing synaptic contacts, which is in agreement with many other mammalian species.

Perineuronal nets in subcortical auditory nuclei of four rodent species with differing hearing ranges

Perineuronal nets (PNs) are aggregates of extracellular matrix molecules that surround some neurons in the brain. While PNs occur widely across many cortical areas, subcortical PNs are especially associated with motor and auditory systems. The auditory system has recently been suggested as an ideal model system for studying PNs and their functions. However, descriptions of PNs in subcortical auditory areas vary, and it is unclear whether the variation reflects species differences or differences in staining techniques. Here, we used two staining techniques (one lectin stain and one antibody stain) to examine PN distribution in the subcortical auditory system of four different species: guinea pigs (Cavia porcellus), mice (Mus musculus, CBA/CaJ strain), Long-Evans rats (Rattus norvegicus), and naked mole-rats (Heterocephalus glaber). We found that some auditory nuclei exhibit dramatic differences in PN distribution among species while other nuclei have consistent PN distributions. We also found that PNs exhibit molecular heterogeneity, and can stain with either marker individually or with both. PNs within a given nucleus can be heterogeneous or homogenous in their staining patterns. We compared PN staining across the frequency axes of tonotopically organized nuclei and among species with different hearing ranges. PNs were distributed non-uniformly across some nuclei, but only rarely did this appear related to the tonotopic axis. PNs were prominent in all four species; we found no systematic relationship between the hearing range and the number, staining patterns or distribution of PNs in the auditory nuclei.

Characterization of the superior olivary complex of Canis lupus domesticus

The superior olivary complex (SOC) is a collection of brainstem auditory nuclei which play essential roles in the localization of sound sources, temporal coding of vocalizations and descending modulation of the cochlea. Notwithstanding, the SOC nuclei vary considerably between species in accordance with the auditory needs of the animal. The canine SOC was subjected to anatomical and physiological examination nearly 50 years ago and was then virtually forgotten. Herein, we aimed to characterize the nuclei of the canine SOC using quantitative morphometrics, estimation of neuronal number, histochemistry for perineuronal nets and immunofluorescence for the calcium binding proteins calbindin and calretinin. We found the principal nuclei to be extremely well developed: the lateral superior olive (LSO) contained over 20,000 neurons and the medial superior olive (MSO) contained over 15,000 neurons. In nearly all non-chiropterian terrestrial mammals, the MSO exists as a thin, vertical column of neurons. The canine MSO was folded into a U-shaped contour and had associated with the ventromedial tip a small, round collection of neurons we termed the tail nucleus of the MSO. Further, we found evidence within the LSO, MSO and medial nucleus of the trapezoid body (MNTB) for significant morphological variations along the mediolateral or rostrocaudal axes. Finally, the majority of MNTB neurons were calbindin-immunopositive and associated with calretinin-immunopositive calyceal terminals. Together, these observations suggest the canine SOC complies with the basic plan of the mammalian SOC but possesses a number of unique anatomical features.

Distribution of perineuronal nets in the human superior olivary complex

Perineuronal nets (PNNs) are specialized assemblies of chondroitin sulfate proteoglycans (CSPGs) in the central nervous system that form a lattice-like covering over the cell body, primary dendrites and initial axon segment of select neuronal populations. PNNs appear to play significant roles in development of the central nervous system, neuronal protection, synaptic plasticity and local ion homeostasis. In seven human brainstems (average age=81 years), we have utilized Wisteria floribunda (WFA) histochemistry and immunocytochemistry for CSPG to map the distribution of PNNs within the nuclei of the human superior olivary complex (SOC). Within the SOC, the majority of net-bearing neurons are situated in the most medially situated nuclei, especially the superior paraolivary nucleus and medial nucleus of the trapezoid body. Net-bearing neurons are consistently found in the ventral nucleus of the trapezoid body and posterior periolivary nucleus, but to a lesser extent in the lateral nucleus of the trapezoid body. Finally, perineuronal nets are typically absent from the lateral and medial superior olives.

Histochemical study of perineuronal nets in the retrosplenial cortex of adult rats

The retrosplenic cortex of rats, similar to many cortical or subcortical regions, is provided with special subsets of neurons that exhibited a fenestrated or reticular coat of condensed extracellular matrix on their soma, initial dendrites and proximal axon segment. This pericellular coating, currently termed "Perineuronal Nets", was detected on the surfaces of some neurons distributing throughout the cortical layers II-V. They presented direct interconnections with each other, and appeared in close association to the astroglial processes. In addition to their collagenous ligands, the perineuronal nets (PNs) were enriched with proteoglycans (PGs, sulfated glycoconjugates) and/or glycoproteins (GPs, unsulfated glycoconjugates with terminal N-acetylgalactosamine). Accordingly, the PNs were differentially identified as belonging to three categories, depending upon their organic nature or chemical composition. First, coats exclusively formed of PGs (stained with iron colloid); second, coats formed of GPs (labeled with plant lectins binding to terminal N-acetylgalactosamine); and third, complex coats formed of PG networks intermingled with glycoprotein molecules (double stained with iron colloid and lectin). Since differential distribution of protein containing substances (GPs and/or PGs) in the extracellular matrix contributes to functional terms, we suggest that these biochemical or morphological differences in the microenvironment of some retrosplenial neurons might reflect certain functional aspects concerned with processing of navigation or episodic memory.

Postnatal development of perineuronal nets in wild-type mice and in a mutant deficient in tenascin-R

The extracellular matrix glycoprotein tenascin-R (TN-R), colocalizing with hyaluronan, phosphacan, and aggregating chondroitin sulphate proteoglycans in the white and grey matter, is accumulated in perineuronal nets that surround different types of neurons in many brain regions. To characterize the role of TN-R in the formation of perineuronal nets, we studied their postnatal development in wild-type mice and in a TN-R knock-out mutant by using the lectin Wisteria floribunda agglutinin and an antibody to nonspecified chondroitin sulphate proteoglycans as established cytochemical markers. We detected the matrix components TN-R, hyaluronan, phosphacan, neurocan, and brevican in the perineuronal nets of cortical and subcortical regions. In wild-type mice, lectin-stained, immature perineuronal nets were first seen on postnatal day 4 in the brainstem and on day 14 in the cerebral cortex. The staining intensity of these nets for TN-R, hyaluronan, phosphacan, neurocan, and brevican was extremely weak or not distinguishable from that of the surrounding neuropil. However, all markers showed an increase in staining intensity of perineuronal nets reaching maximal levels between postnatal days 21 and 40. In TN-R-deficient animals, the perineuronal nets tended to show a granular component within their lattice-like structure at early stages of development. Additionally, the staining intensity in perineuronal nets was reduced for brevican, extremely low for hyaluronan and neurocan, and virtually no immunoreactivity was detectable for phosphacan. The granular configuration of perineuronal nets became more predominant with advancing age of the mutant animals, indicating the continued abnormal aggregation of chondroitin sulphate proteoglycans complexed with hyaluronan. As shown by electron microscopy in the cerebral cortex, the disruption of perineuronal nets was not accompanied by apparent changes in the synaptic structure on net-bearing neurons. The regional distribution patterns and the temporal course of development of perineuronal nets were not obviously changed in the mutant. We conclude that the lack of TN-R initially and continuously disturbs the molecular scaffolding of extracellular matrix components in perineuronal nets. This may interfere with the development of the specific micromilieu of the ensheathed neurons and adjacent glial cells and may also permanently change their functional properties.

Development of chondroitin sulfate proteoglycans in the central auditory system of rats correlates with acquisition of mature properties

Chondroitin sulfate proteoglycans (CSPGs) are macromolecules which regulate the structural organization of the extracellular matrix and can mediate cell migration and axonal growth. Here the spatiotemporal distribution of CSPGs in the central auditory system of rats was investigated using a polyclonal antiserum. In adult brains, CSPGs surrounded many neuronal cell bodies and proximal dendrites at all stations of the auditory pathway except the medial geniculate body. During development, CSPG expression became visible at postnatal day (P) 4 in the superior olivary complex, at P8 in the midbrain, and at P18 in the cortex. Immunoreactivity increased strongly until P12 in the brainstem and until P24 in the cortex. The adult-like pattern in the pontine nuclei, the midbrain, and the cortex was seen at P12, P29, and P35, respectively. The relatively late expression of strong immunoreactivity indicates that the CSPGs are involved in the maturation of axonal connections, but not in early processes such as cell migration or neurite outgrowth.

Immunohistochemical localization of neurocan in the lower auditory nuclei of the dog

Chondroitin sulfate proteoglycans are present at high levels in the lower auditory system of mammals. Axon terminals on the principal neurons in the superior olivary nuclei contain chondroitin 4- and 6-sulfate, while the broad extracellular matrix around axon terminals contains chondroitin sulfate D, a highly sulfated chondroitin sulfate rich in the disaccharide unit of GlcA(2S)beta1 --> 3GalNAc(6S), in the dog. In the present study, we investigated the immunohistochemical staining of neurocan, a brain-specific proteoglycan, in the lower auditory tract of the dog, including an analysis by immunoelectron microscopy. Immunolocalization of neurocan was conspicuous in the medial and lateral superior olivary nuclei and much less intense immunostaining was seen in the cochlear nucleus and posterior colliculus. No immunoreactivity were found in other nuclei. The immunostaining in the medial and lateral superior olivary nuclei was observed as perineuronal nets around large principal neurons at the light-microscopic level, while no immunostaining was observed in the upper segment of the medial superior olivary nucleus and the medial segment of the lateral superior olivary nucleus, in which medium-sized and small neurons were located. Immunoelectron microscopy revealed the reaction products of immunostaining on cell membranes of the perikarya of principal neurons and on cell membranes of presynaptic terminals which made axo-somatic synapses on the principal cells. No immunoreactivity was detected at synaptic junctions, in the extracellular matrix or within axon terminals. In the cochlear nucleus, immunoreactive perineuronal nets were found around a small number of neurons and immunoreactive nerve fibers were scattered in the anterior ventral cochlear nucleus. In the posterior colliculus, perineuronal nets, which were weakly immunostained, were sparsely distributed in the central nucleus. These results suggest that different locations of chondroitin sulfate proteoglycans, including neurocan, may be associated with focal sites composed of neuronal surface, terminal boutons and extracellular matrix in the lower auditory tract of the adult dog.

Extracellular matrix organization in various regions of rat brain grey matter

Previous studies revealed the concentration of extracellular matrix proteoglycans in the so-called perineuronal nets on the one hand and in certain zones of the neuropil on the other. This nonhomogeneous distribution suggested a non-random chemical and spatial heterogeneity of the extracellular space. In the present investigation, regions dominated by one of both distribution patterns, i.e. piriform and parietal cortex, reticular thalamic nucleus, medial septum/diagonal band complex and cerebellar nuclei, were selected for correlative light and electron microscopic analysis. The labelling was performed by the use of the N-acetylgalactosamine-binding plant lectin Wisteria floribunda agglutinin visualized by peroxidase staining and additionally by photoconversion of red carbocyanine fluorescence labelling for electron microscopy. The intense labelling of the neuropil of a superficial piriform region, presumably identical with sublayer Ia, was confined to a fine meshwork spreading over the extracellular space between non-myelinated axons, dendrites and glial profiles. In the reticular thalamic nucleus the neuronal cell bodies were embedded in zones of labelled neuropil. In contrast to these patterns, the labelled extracellular matrix in different cortical layers and in the other subcortical regions was concentrated in perineuronal nets as large accumulations at surface areas of the neuronal perikarya and dendrites and the attached presynaptic boutons. Astrocytic processes usually were separated from the neuronal surface by the interposed extracellular material. Despite a great variability, the width of the extracellular space containing the labelled matrix components in all perineuronal nets appeared to be considerably larger than that in the labelled zones of neuropil and the non-labelled microenvironment of other neurons. Our results support the view that differences expressed in topographical and spatial peculiarities of the extracellular matrix constituents are related to neuron-type and system-specific functional properties.

The presence of chondroitin sulfate A and C within axon terminals in the superior olivary nuclei of the adult dog

Localization of chondroitin sulfate A and C [GlcA beta 1--> 3GalNAc(4S) and GlcA beta 1--> GalNAc(6S)] has been determined in the medial and lateral superior olivary nuclei of the adult dog by light and electron microscopic immunocytochemistry. Neuropil around large neurons were heavily immunostained, whereas neuropil around small neurons showed only weak or no immunoreactivity. Electron microscopy revealed that presence of chondroitin sulfate A and C proteoglycan in axon terminals around neuronal cell bodies.

Cortical areas are revealed by distribution patterns of proteoglycan components and parvalbumin in the Mongolian gerbil and rat

Cortical areas in rodents have been basically characterized by its cytoarchitecture, connectivity or by physiological parameters. In this study we show that they are revealed by distribution patterns of proteoglycans and parvalbumin-immunoreactivity. Brains of young adult Mongolian gerbils (Meriones unguiculatus) and Wistar rats were cut into series of transversal sections. Proteoglycan components were detected using the N-acetylgalactosamine binding Wisteria floribunda agglutinin (WFA) and antibodies against chondroitin sulphate proteoglycan (CSPG). Differences between cortical areas were found to exist with regard to the occurrence and the density of perineuronal nets, but were also expressed in varying staining intensities for WFA and CSPG of the neuropil. Primary neocortical areas (somatosensory, auditory, visual cortex) were characterized by an intense neuropil staining in layer IV and the upper part of layer VI. Using the same methods strong labelling was also typical of the neuropil in the retrosplenial cortex, of layer Ia in the prepiriform cortex and the hippocampal CA3 field. In tangential sections cut from gerbil cortical hemispheres, some of the heavily lectin-stained cortical areas were sharply delineated from adjacent faintly labelled regions, others showed more diffuse borders. In the rat, the area-specific staining for WFA was less clearly expressed than in the gerbil. Immunocytochemistry of the calcium-binding protein parvalbumin in alternate sections showed labelling patterns of neuropil which resembled those of WFA-binding and CSPG-immunoreactivity in the entire neocortex and hippocampus. From these results it can be concluded that functional peculiarities of cortical fields may not only be determined by neuronal network parameters but also by the spatial arrangement of extracellular matrix proteoglycans.

Chondroitin sulfate proteoglycan-immunoreactivity of lectin-labeled perineuronal nets around parvalbumin-containing neurons

Perineuronal nets represent highly specialized glial and glia-associated structures. In this study, a triple fluorescence labeling of chondroitin sulfate proteoglycan-immunoreactive (CSPG-ir) and N-acetylgalactosamine (GalNac)-specific plant lectin Wisteria floribunda agglutinin (WFA) binding net components as well as parvalbumin-immunoreactivity (-ir) was performed. It was shown in the rat cortex, that the same nets frequently surrounding parvalbumin-ir neurons are stained by CSPG-ir as well as by the lectin binding method.

Perineuronal nets--a specialized form of extracellular matrix in the adult nervous system

One century ago, Camillo Golgi described 'perineuronal nets' enwrapping the cell bodies and proximal dendrites of certain neurons in the adult mammalian central nervous system and suggested that they represent a supportive and protective scaffolding. Although other neuroanatomists validated the existence of these nets on selected neurons in the adult brain, there was a lack of agreement on their origins, composition and function. The application of modern molecular and ultrastructural methods has brought new insights and a renewed interest in these classic observations. Recent data suggest that perineuronal nets result from the visualization of extracellular matrix molecules that are confined to the space interposed between glial processes and the nerve cells that they outline. The material confined to these spaces can be visualized selectively by antibodies directed to glycoproteins (e.g., tenascin and restrictin/janusin), proteoglycans (e.g., chondroitin sulfates), markers for hyaluronan as well as by lectins recognizing N-acetylgalactosamine and by monoclonal antibodies directed to epitopes on unknown molecules (e.g., HNK-1, VC1.1 and Cat 301). This review examines the emerging clarification of classical observations of perineuronal nets and the functional implications suggested by their molecular composition. Also discussed are studies that further extend observations on the time of development and of the specificity in the occurrence of perineuronal nets. In the adult brain the molecules constituting the 'perineuronal nets of matrix' could serve as recognition molecules between certain neurons and their surrounding cells and participate in the selection and consolidation of their relationship.