Natural bornavirus infections and their resulting diseases are largely restricted to horses and sheep in Central Europe. The disease also occurs naturally in cats, and can be induced experimentally in laboratory rodents and numerous other mammals. Borna disease virus-1 (BoDV-1), the cause of most cases of mammalian Borna disease, is a negative-stranded RNA virus that replicates within the nucleus of target cells. It causes severe, often lethal, encephalitis in susceptible species. Recent events, especially the discovery of numerous new species of bornaviruses in birds and a report of an acute, lethal bornaviral encephalitis in humans, apparently acquired from squirrels, have revived interest in this remarkable family of viruses. The clinical manifestations of the bornaviral diseases are highly variable. Thus, in addition to acute lethal encephalitis, they can cause persistent neurologic disease associated with diverse behavioral changes. They also cause a severe retinitis resulting in blindness. In this review, we discuss both the pathological lesions observed in mammalian bornaviral disease and the complex pathogenesis of the neurologic disease. Thus infected neurons may be destroyed by T-cell-mediated cytotoxicity. They may die as a result of excessive inflammatory cytokine release from microglia. They may also die as a result of a ‘glutaminergic storm’ due to a failure of infected astrocytes to regulate brain glutamate levels.

The few available neuropathological studies of bipolar disorder are reviewed and placed in the context of structural imaging findings. Candidate pathological processes include cy-toarchitectural abnormalities, particularly in the anterior cingulate cortex and affecting glia, and disruption of white matter connections. The possible neuropathological relationship of bipolar disorder with major depression and with schizophrenia is discussed.

Energy filtered transmission electron microscopy techniques are regularly used to build elemental maps of spatially distributed nanoparticles in materials and biological specimens. When working with thick biological sections, electron energy loss spectroscopy techniques involving core-loss electrons often require exposures exceeding several minutes to provide sufficient signal to noise. Image quality with these long exposures is often compromised by specimen drift, which results in blurring and reduced resolution. To mitigate drift artifacts, a series of short exposure images can be acquired, aligned, and merged to form a single image. For samples where the target elements have extremely low signal yields, the use of charge coupled device (CCD)-based detectors for this purpose can be problematic. At short acquisition times, the images produced by CCDs can be noisy and may contain fixed pattern artifacts that impact subsequent correlative alignment. Here we report on the use of direct electron detection devices (DDD’s) to increase the signal to noise as compared with CCD’s. A 3× improvement in signal is reported with a DDD versus a comparably formatted CCD, with equivalent dose on each detector. With the fast rolling-readout design of the DDD, the duty cycle provides a major benefit, as there is no dead time between successive frames.

Advances in microscopy allow one to probe the structure of neurons and their interactions with astrocytes in brain slices and in vivo at ever increasing resolution. Moreover, the dynamic interactions between the cells can be examined in live preparation. In this paper we discuss how a variety of imaging approaches: confocal microscopy, electron microscopy, and multiphoton time-lapse microscopy are employed to probe neuron glia interactions in the developing cerebellum.

Glutamine (Gln) is found abundantly in the central nervous system (CNS) where it participates in a variety of metabolic pathways. Its major role in the brain is that of a precursor of the neurotransmitter amino acids: the excitatory amino acids, glutamate (Glu) and aspartate (Asp), and the inhibitory amino acid, γ-amino butyric acid (GABA). The precursor–product relationship between Gln and Glu/GABA in the brain relates to the intercellular compartmentalization of the Gln/Glu(GABA) cycle (GGC). Gln is synthesized from Glu and ammonia in astrocytes, in a reaction catalyzed by Gln synthetase (GS), which, in the CNS, is almost exclusively located in astrocytes (Martinez-Hernandez et al., 1977). Newly synthesized Gln is transferred to neurons and hydrolyzed by phosphate-activated glutaminase (PAG) to give rise to Glu, a portion of which may be decarboxylated to GABA or transaminated to Asp. There is a rich body of evidence which indicates that a significant proportion of the Glu, Asp and GABA derived from Gln feed the synaptic, neurotransmitter pools of the amino acids. Depolarization-induced-, calcium- and PAG activity-dependent releases of Gln-derived Glu, GABA and Asp have been observed in CNS preparations in vitro and in the brain in situ. Immunocytochemical studies in brain slices have documented Gln transfer from astrocytes to neurons as well as the location of Gln-derived Glu, GABA and Asp in the synaptic terminals. Patch-clamp studies in brain slices and astrocyte/neuron co-cultures have provided functional evidence that uninterrupted Gln synthesis in astrocytes and its transport to neurons, as mediated by specific carriers, promotes glutamatergic and GABA-ergic transmission. Gln entry into the neuronal compartment is facilitated by its abundance in the extracellular spaces relative to other amino acids. Gln also appears to affect neurotransmission directly by interacting with the NMDA class of Glu receptors. Transmission may also be modulated by alterations in cell membrane polarity related to the electrogenic nature of Gln transport or to uncoupled ion conductances in the neuronal or glial cell membranes elicited by Gln transporters. In addition, Gln appears to modulate the synthesis of the gaseous messenger, nitric oxide (NO), by controlling the supply to the cells of its precursor, arginine. Disturbances of Gln metabolism and/or transport contribute to changes in Glu-ergic or GABA-ergic transmission associated with different pathological conditions of the brain, which are best recognized in epilepsy, hepatic encephalopathy and manganese encephalopathy.

Dominant mutations in GJA1, the gene encoding the gap junction protein connexin43 (Cx43), cause oculodentodigital dysplasia (ODDD), a syndrome affecting multiple tissues, including the central nervous system (CNS). We investigated the effects of the G60S mutant, which causes a similar, dominant phenotype in mice (Gja1Jrt/+). Astrocytes in acute brain slices from Gja1Jrt/+ mice transfer sulforhodamine-B comparably to that in their wild-type (WT) littermates. Further, astrocytes and cardiomyocytes cultured from Gja1Jrt/+ mice showed a comparable transfer of lucifer yellow to those from WT mice. In transfected cells, the G60S mutant formed gap junction (GJ) plaques but not functional channels. In co-transfected cells, the G60S mutant co-immunoprecipitated with WT Cx43, but did not diminish GJ coupling as measured by dual patch clamp. Thus, whereas G60S has dominant effects, it did not appreciably reduce GJ coupling.

There is no question about the fact that astrocytes and other glial cells release neurotransmitters that activate receptors on neurons, glia and vascular cells, and that calcium is an important second messenger regulating the release. This occurs in cell culture, tissue slice and in vivo. Negative results from informative experiments designed to test the mechanism of calcium-dependent neurotransmitter release from astrocytes and the ensuing effects on synaptic transmission, have been cited as evidence calling into question whether astrocytes release neurotransmitters under normal circumstances with effects on synaptic transmission. The special feature section in this issue of Neuron Glia Biology addresses these issues and other aspects of neurotransmitter release from astrocytes in communicating with neurons and glial cells. Together these studies suggest that application of vocabulary and concepts developed for synaptic communication between neurons can lead to confusion and apparent paradoxes with respect to communication by extracellular signaling molecules released from glia in response to functional activity.

Recent results indicate that astrocytic β-adrenergic receptors (βAR) participate in noradrenergic modulation of synaptic activity. In this study, we sought to examine whether neural activity can, in turn, regulate astrocytic βAR. To address this question, an antiserum that recognizes β-adrenergic receptors (βAR) specifically in astrocytes was used to assess the distribution of the receptors across ocular dominance columns in VI of two monocular and four visually intact adult monkeys. Cytochrome oxidase histochemistry (CO) was used to identify the position of the cortical laminae and of the ocular dominance columns receiving visual inputs from the intact and enucleated eyes. This stain revealed the expected pattern within V1 of monocular monkeys–i.e. darker and lighter bands of equal widths (ca. 500μm) spanning laminae 4–6, each associated with larger and smaller blobs, respectively, in lamina 2/3;. Alignment of CO sections with adjacent sections stained for astrocytic βAR by the immunoperoxidase method revealed intense βAR-like immunoreactivity (βAR-li) in the superficial laminae, a slightly weaker staining in the infragranular laminae and weakest staining in lamina 4C. Within lamina 4C., a prominent striped pattern was evident. The darker bands of the stripe closely matched widths and positions of the lighter CO columns associated with the enucleated eye. On the other hand, immunocytochemical staining for the astrocytic intermediate filament protein, GFAP, within V1 of monocular monkeys revealed no inter-columnar difference in the density of astrocytic cell bodies or processes. Nissl stain also revealed no overt inter-columnar differences in cell density. V1 of visually intact monkeys exhibited a similar laminar distribution pattern of βAR-li and of CO. Within lamina 4C., βAR-li was uniformly faint and CO staining was uniformly intense. This suggests that the striped pattern of βAR-li seen in lamina 4C of monocular monkeys results from elevation of the βAR-antigen within the inactive columns. The results indicate that astrocytic βAR density is regulated by local neural activity. The mechanisms regulating βAR density are likely to be independent of those regulating glial cell proliferation or GFAP synthesis. In vitro experimental results of others suggest that elevation of astrocytic βAR may be a mechanism compensating for chronic neural inactivity, since the coincident release of noradrenaline with visual stimulation would elevate neuropil excitability via the astrocytic mechanism of (1) decreasing the uptake of neuronally released L-glutamate; (2) increasing GABA uptake; and (3) stimulating glycogenolysis. Alternatively, the changes in βAR-li may reflect an up-regulation of the receptors within inactive columns due to reduced levels of noradrenaline release.

We have studied the glial investment of ganglion cells of the cat's retina, orienting the sections taken for electron microscopy so that the investment could be traced from the soma along the axon. The soma of each ganglion cell is covered by a close-fitting, continuous sheath formed by Müller cells. The axon hillock and the first part of the initial segment are invested by an extension of the somal sheath, and are thus enclosed in the same glial compartment as the soma. The initial segment extends a few microns past the Müller cell sheath; this last length of the initial segment is contacted by numerous processes of astrocytes, which converge on it in a pattern found also on nodes of the same axons, in the optic nerve. Beyond the initial segment, the intraretinal lengths of the axons are invested by both Müller cells and astrocytes, but the investment is strikingly incomplete. Large areas of axonal membrane have no glial cover, and lie close to other axonal membranes. The sequential arrangement of these distinct forms of glial wrapping of the soma, initial segment, and axon is described here for the first time. It is suggested that this pattern of glial investment controls the flow of current between dendrite and initial segment of the ganglion cell, defines the site of initiation of action spikes, and controls the formation of synapses on the soma and initial segment.

The blood–brain barrier (BBB) is a specialized tissue interface that provides an important homeostatic and immunosurveillance role in the CNS. Unlike most microvascular tissues, which readily promote paracellular passage of solutes and macromolecules, the BBB is more analogous to polarized mucosal epithelia that restrict such permeability in order to prevent disease onset. Recent transgenic ablation studies have demonstrated that the BBB and mucosal tissues also share a requirement for astroglial-regulated barrier integrity. This review highlights the emerging concept that astroglia regulate barrier function at markedly different tissue interfaces. It also explores possible lessons that might be learnt by adopting epithelial model paradigms of the BBB. For example, novel glial-derived S-nitrosylation signals that regulate intestinal permeability in the digestive tract might provide new mechanistic insights into the function of the BBB. A better understanding of such universal mechanisms for barrier regulation will facilitate novel therapeutic strategies that target permeability disorders at CNS and mucosal tissue interfaces.

In this review, we briefly summarize what is known about the properties of the three families of gap junction proteins, connexins, innexins and pannexins, emphasizing their importance as intercellular channels that provide ionic and metabolic coupling and as non-junctional channels that can function as a paracrine signaling pathway. We discuss that two distinct groups of proteins form gap junctions in deuterostomes (connexins) and protostomes (innexins), and that channels formed of the deuterostome homologues of innexins (pannexins) differ from connexin channels in terms of important structural features and activation properties. These differences indicate that the two families of gap junction proteins serve distinct, complementary functions in deuterostomes. In several tissues, including the CNS, both connexins and pannexins are involved in intercellular communication, but have different roles. Connexins mainly contribute by forming the intercellular gap junction channels, which provide for junctional coupling and define the communication compartments in the CNS. We also provide new data supporting the concept that pannexins form the non-junctional channels that play paracrine roles by releasing ATP and, thus, modulating the range of the intercellular Ca2+-wave transmission between astrocytes in culture.

Summary

Background and objective: Astrocytes exposed to hypoxia produce pro-inflammatory cytokines and upregulate intercellular adhesion molecule-1 on cerebral endothelium. This study investigated the effects of midazolam on this response. Methods: Mouse astrocytes were exposed to 4 h of hypoxia and 24 h of reoxygenation. Astrocyte-conditioned medium were applied to mouse cerebral endothelial cell cultures for 4 h and 24 h in normoxia. Endothelial cells were pre-incubated for 1 h with midazolam (0, 5, 50 μg L−1). Flow cytometry was used to estimate endothelial ICAM-1 expression. IL-1β concentrations were measured with ELISA. Repeated comparisons were made using ANOVA and post hoc Tukey Test as appropriate. Data are mean (SD). Results: Mouse cerebral endothelial cell ICAM-1 expression was greater after 24 h exposure to hypoxia–reoxygenation astrocyte-conditioned medium compared to normoxic astrocyte-conditioned medium (mean channel flouresence 112.5 (9.5) vs. 81.5 (7.5), P = 0.01). ICAM-1 expression was decreased by midazolam (5 μg L−1) compared to control (mean channel flouresence 81.35 (7.5) vs. 112.5 (9.5), P = 0.01). Supernatant IL-1β concentrations (pg mL−1) in astrocytes exposed to hypoxia–reoxygenation were greater than those exposed to normoxia (16.41 (2.35) vs. 10.5 (2.13), P = 0.01). Conclusion: We conclude that decreased cerebral endothelial ICAM-1 expression in response to activated glial cell compartment by midazolam may decrease post ischaemic brain inflammation and secondary brain injury.

The high level of intercellular communication mediated by gap junctions between astrocytes indicates that, besides individual astrocytic domains, a second level of organization might exist for these glial cells as they form communicating networks. Therefore, the contribution of astrocytes to brain function should also be considered to result from coordinated groups of cells. To evaluate the shape and extent of these networks we have studied the expression of connexin 43, a major gap junction protein in astrocytes, and the intercellular diffusion of gap junction tracers in two structures of the developing brain, the hippocampus and the cerebral cortex. We report that the shape of astrocytic networks depends on their location within neuronal compartments in a defined brain structure. Interestingly, not all astrocytes are coupled, which indicates that connections within these networks are restricted. As gap junctional communication in astrocytes is reported to contribute to several glial functions, differences in the shape of astrocytic networks might have consequences on neuronal activity and survival.

The extension and directionality of neurite outgrowth are key to achieving successful target connections during both CNS development and during the re-establishment of connections lost after neural trauma. The degree of axonal elongation depends, in large part, on the spatial arrangement of astrocytic processes rich in growth-promoting proteins. Because astrocytes in culture align their processes on exposure to an electrical field of physiological strength, we sought to determine the extent to which aligned astrocytes affect neurite outgrowth. To this end, dorsal root ganglia cells were seeded onto cultured rat astrocytes that were pre-aligned by exposure to an electric field of physiological strength (500 mV mm−1). Using confocal microscopy and digital image analysis, we found that neurite outgrowth at 24 hours and at 48 hours is enhanced significantly and directed consistently along the aligned astrocyte processes. Moreover, this directed neurite outgrowth is maintained when grown on fixed, aligned astrocytes. Collectively, these results indicate that endogenous electric fields present within the developing CNS might act to align astrocyte processes, which can promote and direct neurite growth. Furthermore, these results demonstrate a simple method to produce an aligned cellular substrate, which might be used to direct regenerating neurites.

In the CNS, fine processes of astrocytes often wrap around dendrites, axons and synapses, which provides an interface where neurons and astrocytes might interact. We have reported previously that selective Ca2+ elevation in astrocytes, by photolysis of caged Ca2+ by o-nitrophenyl-EGTA (NP-EGTA), causes a kainite receptor-dependent increase in the frequency of spontaneous inhibitory post-synaptic potentials (sIPSCs) in neighboring interneurons in hippocampal slices. However, tetrodotoxin (TTX), which blocks action potentials, reduces the frequency of miniature IPSCs (mIPSCs) in interneurons during Ca2+ uncaging by an unknown presynaptic mechanism. In this study we investigate the mechanism underlying the presynaptic inhibition. We show that Ca2+ uncaging in astrocytes is accompanied by a decrease in the amplitude of evoked IPSCs (eIPSCs) in neighboring interneurons. The decreases in eIPSC amplitude and mIPSC frequency are prevented by CPPG, a group II/III metabotropic glutamate receptor (mGluR) antagonist, but not by the AMPA/kainate and NMDA receptor antagonists CNQX/CPP. Application of either the group II mGluR agonist DCG IV or the group III mGluR agonist L-AP4 decreased the amplitude of eIPSCs by a presynaptic mechanism, and both effects are blocked by CPPG. Thus, activation of mGluRs mediates the effects of Ca2+ uncaging on mIPSCs and eIPSCs. Our results indicate that Ca2+-dependent release of glutamate from astrocytes can activate distinct classes of glutamate receptors and differentially modulate inhibitory synaptic transmission in hippocampal interneurons.

The complement inflammatory cascade is an essential component of the phylogenetically ancient innate immune response and is crucial to our natural ability to ward off infection. Complement is involved in host defence by triggering the generation of a membranolytic complex (the C5b-9 complex) at the surface of the pathogen. Complement fragments (opsonins; C1q, C3b and iC3b) interact with complement cell-surface receptors (C1qRp, CR1, CR3 and CR4) to promote phagocytosis and a local pro-inflammatory response that, ultimately, contributes to the protection and healing of the host. Complement is of special importance in the brain, where entrance of elements of the adaptive immune system is restricted by a blood–brain barrier. There is now compelling evidence that complement is produced locally in response to an infectious challenge. Moreover, complement biosynthesis and activation also occurs in neurodegenerative disorders such as Alzheimer's, Huntington's and Pick's diseases, and the cytolytic/cytotoxic activities of complement are thought to contribute to neuronal loss and brain tissue damage. However, recent data suggest that at least some of the complement components have the ability to contribute to neuroprotective pathways. The emerging paradigm is that complement is involved in the clearance of toxic cell debris (e.g. amyloid fibrils) and apoptotic cells, as well as in promoting tissue repair through the anti-inflammatory activities of C3a. Knowledge of the unique molecular and cellular innate immunological interactions that occur in the development and resolution of pathology in the brain should facilitate the design of effective therapeutic strategies.

Most nerves are attached to the neuraxis by rootlets. The CNS–PNS transitional zone (TZ) is that length of rootlet containing both central and peripheral nervous tissue. The 2 tissues are separated by a very irregular but clearly defined interface, consisting of the surface of the astrocytic tissue comprising the central component of the TZ. Central to this, myelin sheaths are formed by oligodendrocytes and the supporting tissue is astrocytic. Peripheral to it, sheaths are formed by Schwann cells which are enveloped in endoneurium. The features of transitional nodes are a composite of those of central and peripheral type. The interface is penetrated only by axons. It is absent at first. It is formed by growth of processes into the axon bundle from glial cell bodies around its perimeter. These form a barrier across the bundle which fully segregates prospectively myelinated axons. Rat spinal dorsal root TZs have been used extensively to study CNS axon regeneration. The CNS part of the TZ responds to primary afferent axon degeneration and to regenerating axons in ways which constitute a satisfactory model of the gliotic tissue response which occurs in CNS lesions. It undergoes gliosis and the gliotic TZ tissue expands distally along the root. In mature animals axons can regenerate satisfactorily through the endoneurial tubes of the root but cease growth on reaching the gliotic tissue. The general objective of experimental studies is to achieve axon regeneration from the PNS through this outgrowth and into the dorsal spinal cord. Since immature tissue has a greater capacity for regeneration than that of the adult, one approach includes the transplantation of embryonic or fetal dorsal root ganglia into the locus of an extirpated adult ganglion. Axons grow centrally from the transplanted ganglion cells and some enter the cord. Other approaches include alteration of the TZ environment to facilitate axon regeneration, for example, by the application of tropic, trophic, or other molecular factors, and also by transplantation of cultured olfactory ensheathing cells (OECs) into the TZ region. OECs, by association with growing axons, facilitate their extensive regeneration into the cord. Unusually, ventral motoneuron axons may undergo some degree of unaided CNS regeneration. When interrupted in the spinal cord white matter, some grow out to the ventral rootlet TZ and thence distally in the PNS. The DRTZ is especially useful for quantitative studies on regeneration. Since the tissue is anisometric, individual parameters such as axon numbers, axon size and glial ensheathment can be readily measured and compared in the CNS and PNS environments, thereby yielding indices of regeneration across the interface for different sets of experimental conditions.

The immunohistochemical location of glial fibrillary acidic protein (GFAP) was used to study the state of maturation of retinal astrocytes from an 18-mo-old infant and to compare it with the situation in the adult. Infant astrocytes showed intense GFAP immunoreactivity in the perikarya and possessed spindle-like enlargements in their processes, while in the adult immunoreactivity in the perikarya was scarce and the spindle-like enlargements were not evident. Two types of astrocyte were observed in adult and child retinas: elongated and star-shaped. In the adult, the star-shaped type tend to be more stylised and to have longer processes than in the infant. In the infant, numerous astrocyte cell bodies were observed over vessels, while in the adult these were scarce. In the infant, the star-shaped astrocytes made up a honeycomb plexus, but this was not fully developed. These results suggest that at 18 mo of postnatal development the retinal astrocytes are still increasing and growing into the astroglial structure found in adults.