Structural and molecular heterogeneity of astrocytes and oligodendrocytes in the gerbil lateral superior olive

Functional anisotropic panglial networks in the lateral superior olive

Astrocytes form large gap junctional networks that contribute to ion and neurotransmitter homeostasis. Astrocytes concentrate in the lateral superior olive (LSO), a prominent auditory brainstem center. Compared to the LSO, astrocyte density is lower in the region dorsal to the LSO (dLSO) and in the internuclear space between the LSO, the superior paraolivary nucleus (SPN). We questioned whether astrocyte networks exhibit certain properties that reflect the precise neuronal arrangement. Employing whole-cell patch-clamp and concomitant injection of a gap junction-permeable tracer, we analyzed size and orientation of astrocyte networks in LSO, dLSO, and SPN-LSO in acute brainstem slices of mice at postnatal days 10-20. The majority of LSO networks exhibited an oval topography oriented orthogonally to the tonotopic axis, whereas dLSO networks showed no preferred orientation. This correlated with the overall astrocyte morphology in both regions, i.e. LSO astrocyte processes were oriented mainly orthogonally to the tonotopic axis. To assess the spread of small ions within LSO networks, we analyzed the diffusion of Na(+) signals between cells using Na(+) imaging. We found that Na(+) not only diffused between SR101(+) astrocytes, but also from astrocytes into SR101(-) cells. Using PLP-GFP mice for tracing, we could show that LSO networks contained astrocytes and oligodendrocytes. Together, our results demonstrate that LSO astrocytes and LSO oligodendrocytes form functional anisotropic panglial networks that are oriented predominantly orthogonally to the tonotopic axis. Thus, our results point toward an anisotropic ion and metabolite diffusion and a limited glial crosstalk between neighboring isofrequency bands in the LSO. GLIA 2016;64:1892-1911.

Different tonotopic regions of the lateral superior olive receive a similar combination of afferent inputs

The mammalian lateral superior olive (LSO) codes disparities in the intensity of the sound that reaches the two ears by integrating ipsilateral excitation and contralateral inhibition, but it remains unclear what particular neuron types convey acoustic information to the nucleus. It is also uncertain whether the known conspicuous morphofunctional differences and gradients along the tonotopic axis of the LSO relate to qualitative and/or quantitative regional differences in its afferents. To clarify these issues, we made small, single injections of the neuroanatomical tracer biotinylated dextran amine (BDA) into different tonotopic regions of the LSO of albino rats and analyzed the neurons labeled retrogradely in brainstem auditory nuclei. We demonstrate that the LSO is innervated tonotopically by four brainstem neuron types: spherical bushy cells and planar multipolar neurons of the ipsilateral ventral cochlear nucleus, principal neurons of the ipsilateral medial nucleus of the trapezoid body, and small multipolar neurons of the contralateral ventral nucleus of the trapezoid body. Unexpectedly, the proportion of labeled neurons of each type was virtually identical in all cases, thus indicating that all tonotopic regions of the LSO receive a similar combination of inputs. Even more surprisingly, our data also suggest that the representation of frequencies in the LSO differs from that of the nuclei that innervate it: compared to the latter nuclei, the LSO seems to possess a relatively larger portion of its volume devoted to processing frequencies in the lower-middle part of the spectrum, and a relative smaller portion devoted to higher frequencies. J. Comp. Neurol. 524:2230-2250, 2016. © 2015 Wiley Periodicals, Inc.

Astrocytes are involved in trigeminal dynamic mechanical allodynia: potential role of D-serine

Trigeminal neuropathic pain affects millions of people worldwide. Despite decades of study on the neuronal processing of pain, mechanisms underlying enhanced pain states after injury remain unclear. N-methyl-D-aspartate (NMDA) receptor-dependent changes play a critical role in triggering central sensitization in neuropathic pain. These receptors are regulated at the glycine site through a mandatory endogenous co-agonist D-serine, which is synthesized by astrocytes. Therefore, the present study was carried out to determine whether astrocytes are involved, through D-serine secretion, in dynamic mechanical allodynia (DMA) obtained after chronic constriction of the infraorbital nerve (CCI-IoN) in rats. Two weeks after CCI-IoN, an important reaction of astrocytes was present in the medullary dorsal horn (MDH), as revealed by an up-regulation of glial fibrillary acidic protein (GFAP) in allodynic rats. In parallel, an increase in D-serine synthesis, which co-localized with its synthesis enzyme serine racemase, was strictly observed in astrocytes. Blocking astrocyte metabolism by intracisternal delivery of fluorocitrate alleviated DMA. Furthermore, the administration of D-amino-acid oxidase (DAAO), a D-serine-degrading enzyme, or that of L-serine O-sulfate (LSOS), a serine racemase inhibitor, significantly decreased pain behavior in allodynic rats. These results demonstrate that astrocytes are involved in the modulation of orofacial post-traumatic neuropathic pain via the release of the gliotransmitter D-serine.

Astroglial heterogeneity closely reflects the neuronal-defined anatomy of the adult murine CNS

Astroglia comprise an extremely morphologically diverse cell type that have crucial roles in neural development and function. Nonetheless, distinct regions of the CNS have traditionally been defined by the phenotypic characteristics and connectivity of neuros. In a complementary fashion, we present evidence that discrete regions of the adult CNS can be delineated based solely on the morphology, density and proliferation rates of astroglia. We used transgenic hGFAP-GFP mice in which robust expression of GFP in adult astroglia enables detailed morphological characterization of this diversely heterogeneous cell population with 3D confocal microscopy. By using three complementary methods for labeling adult astroglia (hGFAP-GFP expression, and GFAP and S100beta immunostaining), we find that there is a remarkably diverse, regionally stereotypical array of astroglial morphology throughout the CNS, and that discrete anatomical regions can be defined solely on the morphology of astroglia within that region. Second, we find that the density of astroglia varies dramatically across the CNS, and that astroglial density effectively delineates even the sub-regions of complex structures, such as the thalamus. We also find that regional astroglial density varies depending on how astroglia are labeled. To quantify and illustrate these broad differences in astroglial density, we generated an anatomical density atlas of the CNS. Third, the proliferation rate, or mitotic index, of astroglia in the adult CNS also effectively defines anatomical regions. These differences are present regardless of the astroglial-labeling method used. To supplement our atlas of astroglial density we generated an atlas of proliferation density for the adult CNS. Together, these studies demonstrate that the morphology, density and proliferation rate of astroglia can independently define the discrete cytoarchitecture of the adult mammalian CNS, and support the concept that regional astroglial heterogeneity reflects important molecular and functional differences between distinct classes of astroglia, much like the long-accepted heterogeneity of neuronal populations.

Determinants of regional and local diversity within the astroglial lineage of the normal central nervous system

Astrocytes are a major component of the resident non-neuronal glial cell population of the CNS. They are ubiquitously distributed throughout the brain and spinal cord, where they were initially thought to function in both structural and homeostatic capacities, providing the framework and environment in which neurons performed their parenchymal duties. However, this stroma-like view of astrocytes is no longer satisfactory. Mounting evidence particularly within the last decade indicates that astrocytes do not simply support neuronal activity but directly contribute to it. Congruent with this evolving view of astrocyte function in information processing is the emergent notion that these glial cells are not a homogeneous population of cells. Thus, astrocytes in various anatomically distinct regions of the normal CNS possess unique phenotypic characteristics that may directly influence the particular neuronal activities that define these regions. Remarkably, regional populations of astrocytes appear to exhibit local heterogeneity as well. Many phenotypic traits of the astrocyte lineage are responsive to local environmental cues (i.e., are adaptable), suggesting that plasticity contributes to this diversity. However, compelling evidence suggests that astrocytes arise from multiple distinct progenitor pools in the developing CNS, raising the intriguing possibility that some astrocyte heterogeneity may result from intrinsic differences between these progenitors. The purpose of this review is to explore the evidence for and mechanistic determinants of regional and local astrocyte diversity.

Is otospiralin inner ear specific? Evidence for its expression in mouse brain

The small protein otospiralin has initially been identified as an inner ear specific molecule. However, compelling evidence from high throughput sequencing projects suggested that otospiralin is likely expressed in the central nervous system. Here, we tested this hypothesis using a combination of molecular biology, immunological, and histological techniques, and found that otospiralin is expressed in numerous regions of the central nervous system in mouse. In situ hybridization and immunohistochemistry revealed that otospiralin is widely expressed in neuronal cell bodies and glia. Ultrastructural observations in the cerebral cortex located the small protein in close proximity to membranous organelles in perikarya, the inner face of post-synaptic neuronal membranes, and in astrocytic processes. These results are in agreement with the predicted structure of the protein which revealed a single N-terminal transmembrane helix domain followed by a C-terminus cytosolic tail. Interestingly, 2 weeks after a mechanical trauma in the cerebral cortex, otospiralin expression increased in reactive astrocytes located within the vicinity of the site of injury, but not in neurons. Collectively, our observations suggest that otospiralin is possibly involved in signaling pathways, and could play a role in repair mechanisms subsequent to an injury in the central nervous system.

CNTFRalpha and CNTF expressions in the auditory brainstem: light and electron microscopy study

CNTF receptor alpha (CNTFRalpha) is involved in the development, the maintenance and the regeneration of a variety of brain structures. However, its in vivo distribution has not been determined in the auditory system. CNTFRalpha expression was studied in developing and adult rat brainstem auditory nuclei using immunohistochemistry. At birth, the CNTFRalpha immunolabeling was clearly present in somata of the external nucleus of the inferior colliculus but was diffuse throughout brainstem auditory nuclei. The labeling was present in most brainstem auditory nuclei by post-natal day (PND) 6. The intensity of the staining subsequently increased to its highest level at PND21 and decreased to an adult-like appearance by the fourth post-natal week. In adult, CNTFRalpha labeling occurred in most neurons of the cochlear nucleus (CN), the lateral superior olive (LSO), the medial superior olive (MSO), and the medial nucleus of the trapezoid body (MNTB). CNTFRalpha labeling first appeared in the central nucleus of the inferior colliculus (IC) by the end of the fourth week. There was a general increase in the expression of CNTFRalpha that begins prior to the onset of hearing and reaches its highest level after this important developmental stage. Ultrastructural analysis in the adult ventral CN revealed the presence of CNTFR in post-synaptic sites. The presence of CNTF has been investigated in the adult using both Western blot and immunohistochemistry. Western blot showed the presence of CNTF in both peripheral and central auditory structures. The CNTF label was generally localized to the somatic compartment, in axons and as puncta surrounding neuronal cell bodies and dendrites. Differential CNTF labeling was observed between the different auditory nuclei. CNTF staining is present in neurons of the CN, the MNTB and the LSO, while it is restricted to axons and puncta surrounding neuronal somata in the IC. The clear presence of CNTFRalpha at post-synaptic terminals and that of its ligand the CNTF in axons and puncta surrounding neuronal cell bodies suggest an anterograde mode of action for CNTF in the central auditory system.

Macroglia distribution in the developing and adult inferior colliculus

Macroglia distribution in the developing and adult gerbil inferior colliculus (IC) was investigated using two oligodendrocytic [myelin-associated-glycoprotein (MAG) and oligodendrocyte-specific molecule (Rip)] and two astrocytic [glial fibrillary acidic protein (GFAP) and S100] markers and immunohistochemistry. There was a spatio-temporal pattern of myelin marker expression starting in the ventral area of the IC and continuing to the dorsal part of the nucleus. Myelination, as revealed by MAG and Rip markers, starts in the IC during the second postnatal week. The intensity of myelination increased between stages P15 and P21 and extended to the whole IC. The appearance of myelin proteins in the IC may suggest a possible axonal outgrowth inhibition by oligodendrocytes in this structure. A differential pattern of staining was obtained with S100 and GFAP antibodies. Astrocytes identified as S100 immunoreactive cells were observed in the IC by birth and the staining was localized to their cell body and processes. S100 positive cells were homogeneously distributed within the IC nucleus. S100 pattern of staining remained the same in stages P7, P15 and P21. In adult IC, S100 positive cell processes were in contact with neuronal cell bodies, other S100 positive cells and blood vessels. Quantitative analysis showed an increase in the density of positive cells during the first postnatal week and a decrease then after through to adulthood. Unlike S100, GFAP immunoreactivity showed a different pattern of staining. At birth GFAP positive astrocytes were observed along the collicular brain midline and around the IC nucleus delimiting its boundaries. The GFAP pattern of labelling remained the same during development and in the adult. This data suggests the presence of two astrocytes subtypes with different locations in the IC nucleus. The GFAP positive astrocytes were located along the edge of the nucleus, while the S100 positive ones displayed a homogeneous distribution across the nucleus.

Development of GABA, glycine, and their receptors in the auditory brainstem of gerbil: a light and electron microscopic study

Inhibitory synaptic transmission is known to play an important role during the maturation of central auditory pathways. While there is a lot of information on the modulatory role of glycine (Gly) on the postsynaptic target nuclei in the developing auditory brain stem, such a role for gamma-aminobutyric acid (GABA) in the lateral superior olive (LSO) of neonatal gerbil has been only recently reported (Kotak and Sanes [1997] Soc Neurosci Abst 23:1549; Kotak et al. [1998] J Neurosci 18:4646-4655). Here we present further immunohistochemical findings and the first ultrastructural evidence documenting a significant decrease in the postsynaptic localization of the beta2,3 subunit of the GABA(A) receptor from postnatal day (P)4 to P14 in the LSO of gerbil and the shift in the location of most of the staining from dendritic to astroglial over the same time course. There was a concomitant increase in staining for the Gly receptor (GlyR) anchoring protein, gephyrin. At the same time, GABA and Gly did not show a significant change in their staining pattern, suggesting that the transmitter levels are not particularly indicative of the inhibitory function in the neonatal gerbil LSO, but their receptors on the postsynaptic cells are. The observations of the present study suggest that the early GABAergic inhibition may be important in establishing appropriate synaptic contacts in the LSO of gerbil.

Laminar redistribution of a glial subtype in the chick optic tectum

Lamination is a central feature of structural organization and segregation within the central nervous system. Afferent fibers typically restrict their synapses to only one or a few specific laminae in the target region. Astroglial cells act as boundary markers for functional segregation of inputs in somatosensory cortex and the olfactory bulb and might also help to segregate particular connections in the neostriatum. This work presents evidence that a subset of astroglial cells expressing the carbohydrate recognized by tomato lectin are enriched in retino-non-recipient laminae of the chick optic tectum. This segregation is dependent upon retinal innervation; enucleated chick tecta contain cells that bind tomato lectin but do not segregate into their normal laminae. These results suggest that tomato lectin positive astrocytes of the superficial chick tectum play a role in defining or restricting lamina specific connections of retinal axons.

A developmental shift from GABAergic to glycinergic transmission in the central auditory system

GABAergic and glycinergic circuits are found throughout the auditory brainstem, and it is generally assumed that transmitter phenotype is established early in development. The present study documents a profound transition from GABAergic to glycinergic transmission in the gerbil lateral superior olive (LSO) during the first 2 postnatal weeks. Whole-cell voltage-clamp recordings were obtained from LSO neurons in a brain slice preparation, and IPSCs were evoked by electrical stimulation of the medial nucleus of the trapezoid body (MNTB), a known glycinergic projection in adult animals. GABAergic and glycinergic components were identified by blocking transmission with bicuculline and strychnine (SN), respectively. In the medial limb of LSO, there was a dramatic change in the GABAergic IPSC component, decreasing from 78% at postnatal day 3 (P3)-P5 to 12% at P12-P16. There was an equal and opposite increase in the glycinergic component during this same period. Direct application of GABA also elicited significantly larger amplitude and longer duration responses in P3-P5 neurons compared with glycine-evoked responses. In contrast, MNTB-evoked IPSCs in lateral limb neurons were more sensitive to SN throughout development. Consistent with the electrophysiological observations, there was a reduction in staining for the beta2,3-GABAA receptor subunit from P4 to P14, whereas staining for the glycine receptor-associated protein gephyrin increased. Brief exposure to baclofen depressed transmission at excitatory and inhibitory synapses for approximately 15 min, suggesting a GABAB-mediated metabotropic signal. Collectively, these data demonstrate a striking switch from GABAergic to glycinergic transmission during postnatal development. Although GABA and glycine elicit similar postsynaptic ionotropic responses, our results raise the possibility that GABAergic transmission in neonates may play a developmental role distinct from that of glycine.

Afferent regulation of glycine receptor distribution in the gerbil LSO

Synaptic activity plays an important role in many aspects ofneuronal development, particularly the expression of proteins. In this study, the influence of inhibitory and excitatory afferents on the development of glycine receptor density in the lateral superior olive (LSO) of Mongolian gerbils was investigated. Afferent activity was manipulated by removing one or both cochleas at postnatal day 7, prior to the onset of sound-evoked responses. Due to the anatomy of the LSO, these manipulations result in either excitatory denervation, inhibitory denervation, or both. The density of glycine receptors in the LSO was determined at 21 days postnatal. Glycine receptors were either labeled with tritiated strychnine (3H-SN) or with an antibody directed against gephyrin, a protein closely associated with the receptor complex. Antibody binding was used to quantify the differential glycine receptor density between the medial limb (high frequency area) and the lateral limb (low frequency area) of the LSO. 3H-SN was used to quantify the amount of glycine receptors in each part of the LSO in control and experimental animals. In addition, changes in neuron density and neuron cross-sectional area were quantified following cochlear ablations. In control animals, the amount of glycine receptors is about 2- to 3-fold higher in the high-frequency than in the low-frequency region. In bilaterally ablated animals, the same density of glycine receptors was measured in the high- and low-frequency region. Unilateral ablations had no significant effect on glycine receptor distribution, either ipsi- or contralateral to the ablation. The neuron cross-sectional area decreased about 30% in the ipsilateral LSO of unilaterally ablated animals and in bilaterally ablated animals. However, alterations of soma density and cross-sectional area were similar in the high- and low-frequency projection region. These results suggest that the distribution of glycine receptors is only changed when excitatory and inhibitory afferents have been denervated.

Differential expression of MAG, MBP and L1 in the developing lateral superior olive

The aim of this study was to investigate whether glial-associated molecules exhibit a pattern of expression that could influence oriented dendrite outgrowth in the gerbil lateral superior olive (LSO). In particular, we have previously noted that axon fascicles are oriented parallel to isofrequency laminae in the medial limb of the LSO, as are LSO dendrites, a phenotype that emerges postnatally. Therefore, we examined the immunocytochemical staining pattern of antibodies directed against three proteins that are found along axons: myelin basic protein (MBP), myelin-associated glycoprotein (MAG), and neuron-glia cell adhesion molecule (L1). MAG staining was first observed at postnatal day (P) 4 on the axon fibers surrounding the LSO. By P7 there was a differential pattern of MAG staining within the LSO, and immunopositive fibers were observed solely in the medial limb (e.g., high frequency projection region). Between P7 and P12, MAG staining was restricted largely to fascicles in the medial limb, and these were oriented parallel to the isofrequency axes. Few positive fibers of irregular orientation were observed in the lateral limb (e.g., low frequency projection region). Significant MAG-staining was not observed in the lateral limb until P15. The MAG immunoreactivity extended throughout the LSO by P21, although it was no longer restricted to axon fascicles. In contrast, MBP-positive fibers were uniformly distributed within the LSO by P12. Finally, L1 was found on oriented axon fascicles at P0, but became sparsely distributed throughout the LSO neuropil after P7, and was restricted to neuron cell bodies in the adult. Taken together, the results suggest that oriented axon fascicles bearing MAG and L1 may contribute to the developmental refinement of dendrite and axon arbors within the LSO.

Immunocytochemical analyses of astrocyte development in the olfactory bulb

Several lines of evidence suggest an important role for glia in establishing boundaries during development of mammalian cortex and insect olfactory lobe. In the adult rat olfactory bulb distinct morphological categories of astroglial cells with clear laminar specificity are easily recognized following immunocytochemical staining of glial fibrillary acidic protein (GFAP). To explore the developmental distribution of olfactory bulb astrocytes and their possible role in establishing the segregation of neurons in specific olfactory bulb laminae, we used immunocytochemical localization of GFAP in rats at 0, 6, 9, 12, 15 and 21 days postnatal plus the adult. In the adult we confirmed prior observations and identified five morphological categories of astrocytes: linear, wedge, elongate, semicircular, and circular. Each category had a unique sublaminar distribution across the olfactory bulb, although categories could occur in more than one lamina. Between 0 and 21 days postnatal a 6th category was apparent, radial glial cells. The mature astrocyte morphologies did not emerge uniformly. Astrocytes found in the outermost glomerular layer developed first with the appearance of the linear, wedge and elongate morphologies. Deeper laminate of the olfactory bulb followed in a successive fashion until the adult pattern was evident around 15 days postnatal. As radial glia disappeared, the mature morphologies assumed their final position. The data suggest that the maturation of olfactory bulb astrocytes may be linked to the final migration and maturation of olfactory bulb neurons.

Localization of NADPH diaphorase/nitric oxide synthase in the optic nerve of the normal guinea pig: a light and electron microscopic study

The aim of this study is to determine the presence and subcellular distribution of NADPH diaphorase (NADPH-d)/nitric oxide synthase (NOS) in the optic nerve of the normal guinea pig. Optic nerve specimens were stained by NADPH-d histochemistry, and double labeled by combining NADPH-d histochemistry with immunostaining for (a) anti-glial fibrillary acidic protein (GFAP) antibody for recognition of astrocytes, (b) griffonia simplicifilia B4-isolectin (GSA-IB4) horse radish peroxidase (HRP)-conjugate for identification of microglia, or (c) oligodendrocyte-associated antibodies to carbonic anhydrase isoenzyme II (CA-II) or to galactocerebroside (GalC) for visualization of oligodendrocytes. In addition, constitutive NOS (cNOS) and inducible NOS (iNOS) immunostaining were used for colocalization with NADPH-d histochemistry. Light microscopy revealed NADPH-d reaction product in the blood vessels and neuroglia of the unmyelinated optic nerve head and myelinated retrobulbar optic nerve. Double labeling with GFAP immunoperoxidase combined with NADPH-d histochemistry revealed both activities in astrocytes. Microglia were labeled with GSA-IB4 isolectin HRP-conjugate, but they did not have NADPH-d activity. Oligodendroglia were immunolabeled with anti CA-II or anti GalC antibodies, but they did not have NADPH-d activity. Both iNOS and cNOS immunoperoxidase labeled astrocytes, but not microglia or oligodendroglia. Under transmission electron microscopy, NADPH-d reaction product appeared as electron-dense particles. These particles were seen in the cytoplasm of endothelial cells, perivascular smooth muscle cells and fibrous astrocytes. Axons and myelin were devoid of NADPH-d activity. This study demonstrates the existence and cellular distribution of NADPH-d/NOS activity in endothelial cells, perivascular smooth muscle cells and fibrous astrocytes of the optic nerve of the normal guinea pig. The presence of these non-neuronal sources of NOS in the optic nerve provides the foundation for future comparative studies of the functional role of reactive oxygen induced toxicity in disorders affecting the optic nerve.