Physiology of glia: glial-neuronal interactions

In Vitro Testing of Neurotoxicity

Many of the toxic compounds that are at large in the environment represent a risk to our neuronal functions. Chemicals may have a direct or indirect effect on the nervous system and they may interfere with general biochemical properties or specific neuronal structures and processes. In this review, a brief presentation of the major neurotoxicological targets is given, together with a discussion of some aspects of the use of different in vitro models for screening purposes and mechanistic studies. It is believed that in vitro methods offer special opportunities for the development of new neurotoxicological assays, and that this development will mainly involve cultured model systems. Therefore, a presentation of nerve and glia tissue culture methods is given, followed by an overview of how information on the action of mercury and mercurials, excitotoxins and acrylamide has been obtained through the use of cultured cell models. It is concluded that the developmental potential in cell neurotoxicology lies within the areas of separation and identification of cells representative for different structures in the nervous system, co-cultivation of different cell types, in vivo/in vitro (ex vivo) procedures, chemically defined media, metabolic competent cultures of human cells and improved physiological conditions for cultivation and exposure.

Cortical spreading depression in traumatic brain injuries: is there a role for astrocytes?

Cortical spreading depression (CSD) is a presumably pathophysiological phenomenon that interrupts local cortical function for periods of minutes to hours. This phenomenon is important due to its association with different neurological disorders such as migraine, malignant stroke and traumatic brain injury (TBI). Glial cells, especially astrocytes, play an important role in the regulation of CSD and in the protection of neurons under brain trauma. The correlation of TBI with CSD and the astrocytic function under these conditions remain unclear. This review discusses the possible link of TBI and CSD and its implication for neuronal survival. Additionally, we highlight the importance of astrocytic function for brain protection, and suggest possible therapeutic strategies targeting astrocytes to improve the outcome following TBI-associated CSD.

Local Gene Expression in Axons and Nerve Endings: The Glia-Neuron Unit

Neurons have complex and often extensively elongated processes. This unique cell morphology raises the problem of how remote neuronal territories are replenished with proteins. For a long time, axonal and presynaptic proteins were thought to be exclusively synthesized in the cell body, which delivered them to peripheral sites by axoplasmic transport. Despite this early belief, protein has been shown to be synthesized in axons and nerve terminals, substantially alleviating the trophic burden of the perikaryon. This observation raised the question of the cellular origin of the peripheral RNAs involved in protein synthesis. The synthesis of these RNAs was initially attributed to the neuron soma almost by default. However, experimental data and theoretical considerations support the alternative view that axonal and presynaptic RNAs are also transcribed in the flanking glial cells and transferred to the axon domain of mature neurons. Altogether, these data suggest that axons and nerve terminals are served by a distinct gene expression system largely independent of the neuron cell body. Such a local system would allow the neuron periphery to respond promptly to environmental stimuli. This view has the theoretical merit of extending to axons and nerve terminals the marginalized concept of a glial supply of RNA (and protein) to the neuron cell body. Most long-term plastic changes requiring de novo gene expression occur in these domains, notably in presynaptic endings, despite their intrinsic lack of transcriptional capacity. This review enlightens novel perspectives on the biology and pathobiology of the neuron by critically reviewing these issues.

Isoflurane enhances glutamate uptake via glutamate transporters in rat glial cells

In this study I examined whether isoflurane, an inhalational anesthetic used commonly in clinical practice, affected glutamate uptake via glutamate transporters, proteins expressed in the plasma membrane of cells in the central nervous system. Isoflurane at clinically relevant concentrations (1-3%) caused a time-, sodium- and concentration-dependent increase of glutamate uptake in primary cultures of rat cerebral mixed glial cells. This enhancement was inhibited by a specific glutamate transporter inhibitor. The study also demonstrated that 2.0% isoflurane significantly increased both Vmax and Km of transporter-mediated glutamate uptake. Thus, isoflurane enhances glutamate uptake by a pathway that requires function of glutamate transporters. This represents a novel pharmacological effect of inhalational anesthetics and may contribute to isoflurane-induced anesthesia and neuroprotective effects.

Infarct tolerance accompanied enhanced BDNF-like immunoreactivity in neuronal nuclei

A prolonged period (48 h) of cortical spreading depression (CSD) induced resistance against severe focal cerebral ischemia (infarct tolerance), however, the mechanism behind this is unknown. The infarct tolerance was a transient phenomenon; the resistance increased linearly for the initial 12 days, peaking from 12 to 15 days after a preconditioning of CSD, and was decreased thereafter. This study examined the time course of brain-derived neurotrophic factor (BDNF), heat shock protein (hsp)27 and 70, and glial fibrillary acidic protein (GFAP) expressions after CSD in the brain. Immunohistochemical expression of BDNF, hsp27, hsp70, or GFAP following a prolonged period of CSD induced by KCl-infusion, or following NaCl-infusion was analyzed by regional densitometry for 24 days in the rat neocortex. In addition, BDNF protein was measured quantitatively by two-site ELISA assay in the neocortex (n=6 at each time point). The GFAP expression was elevated in astrocytes (compared to the normal level of immunodensity) during the period peaking on day 3-6 following the CSD. The hsp27 immunoreactivity was also elevated in astrocytes from day 1 to 12 peaking on day 1 and 6, but there was no expression of hsp70 during the period following CSD. The immunoreactivity for BDNF was elevated in neurons from day 0 to 18 peaking on day 1 and 6. The protein levels of BDNF in the neocortex were significantly elevated from day 0 to 12 peaking on days 0 and 6 (compared to the normal level) (P<0.05). Using a laser-scanning confocal imaging system, the BDNF-like immunoreactivity in neuronal nuclei was found to increase linearly peaking on day 12, which correlated well with the development of infarct tolerance. The intranuclear increase in BDNF-like protein might contribute to the induction of infarct tolerance in the brain.

In vitro techniques for the assessment of neurotoxicity

Risk assessment is a process often divided into the following steps: a) hazard identification, b) dose-response assessment, c) exposure assessment, and d) risk characterization. Regulatory toxicity studies usually are aimed at providing data for the first two steps. Human case reports, environmental research, and in vitro studies may also be used to identify or to further characterize a toxic hazard. In this report the strengths and limitations of in vitro techniques are discussed in light of their usefulness to identify neurotoxic hazards, as well as for the subsequent dose-response assessment. Because of the complexity of the nervous system, multiple functions of individual cells, and our limited knowledge of biochemical processes involved in neurotoxicity, it is not known how well any in vitro system would recapitulate the in vivo system. Thus, it would be difficult to design an in vitro test battery to replace in vivo test systems. In vitro systems are well suited to the study of biological processes in a more isolated context and have been most successfully used to elucidate mechanisms of toxicity, identify target cells of neurotoxicity, and delineate the development and intricate cellular changes induced by neurotoxicants. Both biochemical and morphological end points can be used, but many of the end points used can be altered by pharmacological actions as well as toxicity. Therefore, for many of these end points it is difficult or impossible to set a criterion that allows one to differentiate between a pharmacological and a neurotoxic effect. For the process of risk assessment such a discrimination is central. Therefore, end points used to determine potential neurotoxicity of a compound have to be carefully selected and evaluated with respect to their potential to discriminate between an adverse neurotoxic effect and a pharmacologic effect. It is obvious that for in vitro neurotoxicity studies the primary end points that can be used are those affected through specific mechanisms of neurotoxicity. For example, in vitro systems may be useful for certain structurally defined compounds and mechanisms of toxicity, such as organophosphorus compounds and delayed neuropathy, for which target cells and the biochemical processes involved in the neurotoxicity are well known. For other compounds and the different types of neurotoxicity, a mechanism of toxicity needs to be identified first. Once identified, by either in vivo or in vitro methods, a system can be developed to detect and to evaluate predictive ability for the type of in vivo neurotoxicity produced. Therefore, in vitro tests have their greatest potential in providing information on basic mechanistic processes in order to refine specific experimental questions to be addressed in the whole animal.

Secretion of GDNF by glial cells does not account for the neurotrophic effect of bFGF on dopamine neurons in vitro

The neurotrophic effect of bFGF on dopamine (DA) neurons in vitro depends upon its mitogenic activity on glial cells. We examined whether glial secretion of the dopaminergic growth factor GDNF is responsible for the trophic activity of bFGF on mesencephalic DA neurons. When added with lower concentrations of bFGF, GDNF had an additive trophic effect, which disappeared at optimal bFGF concentrations. However, addition of GDNF-neutralizing antibodies could not modify the bFGF-induced survival and growth of DA neurons. These results indicate that although GDNF could be secreted by bFGF stimulated glia, its secretion alone does not account for the trophic effect of bFGF on mesencephalic DA neurons.

Intercellular signaling in neuronal-glial networks

Glial cells have recently been found to exhibit electrophysiological and metabolic responses to many neurotransmitters and neuromodulators. These findings have focused attention on the possibility that active signaling between neurons and glia could represent an important form of intercellular communication within the brain. Since glial and neuronal networks are both physically and metabolically interlinked, such intercellular signaling may represent a mechanism for inducing collective changes in the cellular physiology of neuronal and glial cell populations. Within the nervous tissue of both vertebrate and invertebrate organisms, glial cells are known to secrete extracellular signal molecules, modulate carbohydrate metabolism, and control the volume and ionic composition of extracellular space. In this paper, the roles that cytoplasmic [Ca2+] transients may play in regulating these glial cell functions are reviewed. Mechanisms by which intracellular Ca oscillations and intercellular Ca waves may be generated in neurotransmitter-stimulated glial cells are also discussed. In addition, it is proposed that rhythmic glial cell contractions and shape changes, which have been observed for many decades, are linked to Ca-induced secretion of ions, water, and neuroactive compounds. These activities represent mechanisms by which Ca-induced changes in glial cell physiology could potentially alter the excitability of neuronal networks.

Immunolocalization of granulocyte-colony-stimulating factor in human glial and primitive neuroectodermal tumors

Granulocyte-colony-stimulating factor (G-CSF) is a hematopoietic cytokine that regulates the differentiation of myeloid progenitors and the function of mature neutrophils. It is produced in vitro by monocytes/macrophages, mesothelial cells, fibroblasts and endothelial cells after appropriate induction by inflammatory mediators like IL-1 and TNF. Normal as well as tumorous glial cells can also be induced to produce CSFs in vitro. However, little is yet known about the in vivo expression of G-CSF as a mediator in inflammation and malignancy within the human central nervous system. The aim of the present study was to investigate by immunostaining the expression of the G-CSF protein within non-tumorous and tumorous glial tissues, and primitive neuroectodermal tumors. Using the murine monoclonal anti-G-CSF TM 82/60 antibody, we found high G-CSF expression in astrocytoma WHO grades I and II and reactive brain tissue, low expression in astrocytoma WHO grade III, and none in glioblastoma, oligodendroglioma WHO grades II and III, and medulloblastoma. In consecutive sections of the tissue samples, G-CSF protein was localized in GFAP-positive glial cells, but not in macrophages/microglial cells, which expressed HLA-DR, detected by the antibody CR3/43. Computer-assisted microdensitometric evaluation of the intensity of immunostaining for G-CSF and statistic analysis of the data revealed significant differences between the diagnostic entities studied (p < 0.0001). We conclude that in vivo expression of G-CSF is a characteristic of reactive as well as tumorous astrocytes, with the latter losing this feature at higher degrees of dedifferentiation.

Immunohistochemical localization of glutathione peroxidase in infarcted human brain

This is the first report which demonstrates the presence of glutathione peroxidase in the autopsied brain of 5 patients without cerebral infarction and 21 patients with cerebral infarction by the indirect enzyme-labeled antibody technique with monoclonal antibody to human glutathione peroxidase. In 2 out of 5 patients without cerebral infarction, a weak reaction for glutathione peroxidase was demonstrated both in neurons and glia. In 6 patients who had died within 5 days after stroke, no staining was observed in infarcted brain tissue except in macrophages. In all 15 patients who had died more than 6 days after stroke, however, a reaction for glutathione peroxidase was demonstrated in the cytoplasm of glial cells in the marginal area around the infarction, and there was a patchy reaction in the cytoplasm of macrophages in the core lesion. These results suggest that glutathione peroxidase in glial cells of the marginal area around the infarction may play a protective role against lipid peroxidation after cerebral infarction, or alternatively, may be involved in the healing process after ischemia.

Neuronal activity triggers calcium waves in hippocampal astrocyte networks

The recent discovery that the neurotransmitter glutamate can trigger actively propagating Ca2+ waves in the cytoplasm of cultured astrocytes suggests the possibility that synaptically released glutamate may trigger similar Ca2+ waves in brain astrocytes in situ. To explore this possibility, we used confocal microscopy and the Ca2+ indicator fluo-3 to study organotypically cultured slices of rat hippocampus, where astrocytic and neuronal networks are intermingled in their normal tissue relationships. We find that astrocytic Ca2+ waves are present under these circumstances and that these waves can be triggered by the firing of glutamatergic neuronal afferents with latencies as short as 2 s. The Ca2+ waves closely resemble those previously observed in cultured astrocytes: they propagate both within and between astrocytes at velocities of 7-27 microns/s at 21 degrees C. The ability of tissue astrocyte networks to respond to neuronal network activity suggests that astrocytes may have a much more dynamic and active role in brain function than has been generally recognized.

Biochemical changes in the developing rat central nervous system due to hyperthermia

Rat embryos at 10 days of gestation were exposed to 43 degrees C for 8 minutes by submerging the exteriorized right uterine horn in heated saline solution and then reinserting the uterine horn into the abdominal cavity. At 15 days, the fetuses were removed, and cells from the cerebral hemispheres were dissociated and grown as primary cultures. Embryos from the left uterine horn served as controls. No morphological changes were observed between the cultures of cells from control and heat-exposed embryos at different days in culture. However, exposure of embryos to hyperthermia at 10 days significantly affected the developmental pattern of activities of acetylcholine esterase associated with cholinergic neurons and of 2',3'-cyclic nucleotide phosphohydrolase associated with oligodendrocytes and myelin membrane formation. These results suggest that hyperthermia at 10 days of gestation in the rat may lead to an impairment in the development of neurons and oligodendrocytes in the central nervous system.

VIP modulation of cultured glial cells of the rat retina

Cultured retinal glial cells from the rat are responsive to modulation by vasoactive intestinal peptide (VIP). VIP (1 X 10(-6) M) elevated the intracellular cyclic AMP concentration from the basal level of (4.4-11.1) p mole/mg protein to (354-440) p mole/mg protein in three minutes at 25 degrees C. The half-maximal concentration is 4.8 X 10(-8) M, which is similar to that observed in the cultured retinal glial cells from the chick embryo.

Attenuation of neurotoxicity following anoxia or glutamate receptor activation in EGF- and hippocampal extract-treated neuronal cultures

Neurotoxicity following anoxia or glutamate receptor activation was studied in primary neuronal cultures grown in serum-free, chemically defined CDM R12 medium. Exposure to 1 mM KCN, 0.5 mM kainic acid and 0.5 mM N-methyl-D-aspartate led to progressive neuronal degeneration. This damage was quantified by measuring lactate dehydrogenase released in the culture medium. The toxic effects were observed early during the development of the neuronal culture (from 4 days in vitro on) and seemed to be neuron-specific since astrocyte cultures were not affected. Chronic treatment of the neuronal cultures with epidermal growth factor at 10 ng/ml and hippocampal extract at dil. 1/833 (w/v) induced morphological alterations, increased beta-adrenergic receptor coupled adenylate cyclase activity, increased level of total lactate dehydrogenase activity in the case of epidermal growth factor-treated cultures, and attenuation of lactate dehydrogenase release following exposure to KCN or glutamate receptor agonists. The alterations observed are probably due to the proliferation and differentiation of glial cells in these treated cultures. This suggests that glial cells protect neurons in vitro from degeneration induced by anoxia or glutamate receptor activation.

An immunocytochemical comparison of Müller cells and astrocytes in the cat retina

Immunocytochemical localization, at the light and electron microscopic levels, of five different known glial proteins was used to compare Müller cells with astrocytes in the adult cat retina. Retina from two different areas of the eye was examined. A region of retina on the border of the optic nerve was used because of its large population of astrocytes, and a region away from the optic nerve was used to examine Müller cells (astrocytes are sparse in this region). Antibodies to cellular retinaldehyde binding protein and glutamine synthetase labeled the Müller cells but not the astrocytes, while labeling with anti-carbonic anhydrase C, anti-alpha crystallin and anti-glial fibrillary acidic protein was found in both Müller cells and astrocytes.

Neurochemical correlates of cyanide-induced hypoxic neuronal damage in vitro

Neuronal cortical cell cultures obtained from fetal mice were subjected to an hypoxic insult produced by sodium cyanide (1 mM) for 24 h. Neurochemical assays were performed 13-14 days after plating on intact cells in situ to determine if there was a specific pattern of cellular dysfunction in addition to morphologic change. Ro5-4864-displaceable benzodiazepine (BDZ) binding and high-affinity [3H] beta-alanine uptake were not reduced when compared to control values. However, specific and clonazepam-displaceable BDZ binding (81 +/- 4% and 50 +/- 9% of control values, respectively), high-affinity [3H]GABA uptake (75 +/- 2%), and choline acetyltransferase activity (82 +/- 2%) were significantly lower. When the data were expressed in terms of protein content, high-affinity [3H] beta-alanine uptake was significantly increased in cyanide-exposed and magnesium-treated cultures (123 +/- 5% and 117 +/- 3%, respectively) as was R05-4864-displaceable BDZ binding (152 +/- 14%), consistent with stimulation of nonneuronal BDZ binding and increased glial neurotransmitter uptake. Moreover, pretreatment of the cultures with magnesium effectively prevented both the morphologic and neurochemical evidence of hypoxic injury. These data lend further support to the notion that the release of excitatory neurotransmitters may mediate neurotoxicity in developing brain.

Astrocytes protect cultured neurons from degeneration induced by anoxia

Neurons grown in cultures of dissociated brain cells degenerate when exposed to anoxia and deprived of glucose. We have developed culture systems in which neurons can be grown in the presence or absence of astrocytes and have used them to study the influence of astrocytes on the neuronal degeneration induced by anoxia and glucopenia. Cultures were prepared from fetal rat forebrains. Mixed cultures contained neurons (identified by immunocytochemical staining of neuron-specific enolase, NSE) and about an equal number of non-neuronal cells (identified by glial fibrillary acid protein). Pure neuronal cultures were prepared by adding a cytostatic compound (cytosine arabinoside) to the medium. Treated cultures were exposed for 4 h to glucose-free medium and an atmosphere of 95% N2 and 5% CO2, whereas control cultures were left in the usual medium containing glucose and in an atmosphere composed of 95% air and 5% CO2. After an interval of 24 h, cultures were fixed, taken for NSE staining, and the number of surviving neurons was counted. Exposure to anoxia and glucopenia reduced the number of surviving neurons in pure neuronal cultures to 5-10% of control levels. In contrast, in mixed cultures 40-60% of the neurons survived these conditions. Anoxia without glucose deprivation reduced the number of surviving neurons in both types of cultures to the same extent as anoxia combined with glucopenia. Glucose deprivation alone was ineffective. The findings suggest a protective influence of astrocytes on neurons under anoxic conditions. gamma-D-Glutamylglycine protected neurons in both types of cultures from anoxia-induced degeneration.(ABSTRACT TRUNCATED AT 250 WORDS)

Susceptibility of fetal guinea pig brain cell cultures to replicating guinea pig cytomegalovirus infection is increased with increasing fetal age: infection of astrocytes

Guinea pig brain cell cultures were established from fetuses at 25, 31, and 37 days of gestation (DG). After 7 days in vitro, the cultures were infected with guinea pig cytomegalovirus (GPCMV). Based on cytopathic effect, immunofluorescence staining for GPCMV by using virus-specific antiserum, and the amount of virus recovered, cultures established from fetuses at 25 DG were least susceptible to replicating infection, and cultures established from fetuses at 37 DG were most susceptible. Using cell-type-specific markers, it was determined that the increase in susceptibility to replicating infection paralleled an increase in the number of differentiated cells. Astrocytes were the most abundant cell type identified and were susceptible to replicating GPCMV infection, whereas neurons were not.

Fetal brain tissue transplants reduce visual deficits in adult rats with bilateral lesions of the occipital cortex

Fetal brain tissue from the occipital or the frontal cortex was implanted into the damaged occipital cortex of adult rats. The animals receiving grafts of embryonic frontal cortex showed partial restoration of brightness discrimination while recipients given homologous implants of occipital cortex were as impaired as those animals with lesions alone. Neither frontal nor occipital grafts aided in the performance of a pattern discrimination problem; both groups of brain-damaged animals were unable to learn the task. Nonetheless, both groups of animals had viable and enlarged grafts with similar neuronal and glial profiles.

A comparative analysis of glial and neuronal markers in the retina of fish: variable character of horizontal cells

The immunohistochemical localizations of the enzymes glutamine synthetase, carbonic anhydrase-C, and the intermediate filament protein GFA were examined for potential neuroglial specificity in the retinas of several types of fish. Both glutamine synthetase and carbonic anhydrase-C appear to be characteristic markers for retinal Müller cells. However, the horizontal neurons of most fish examined also possess high levels of carbonic anhydrase. Furthermore, GFA, the characteristic marker for fibrous astroglia in higher vertebrates, was found specifically localized in the horizontal neurons of several teleost fish. The identity of the GFA antigens was qualified by immunochemical as well as cytological examinations. Furthermore, specific antisera to other intermediate filament proteins, including neurofilaments, validated and contrasted with the observations made with antisera to GFA. The presence of GFA in horizontal neurons of fish is widespread but not typical of all fish. These observations indicate an evolutionary constancy of retinal Müller glial cells. However, these results again focus attention on whether horizontal cells are truly neurons or rather represent an intermediate cell type that may prove useful in studying the evolution, ontogeny, and functional significance of the neuronal-glial phenotypic dichotomy.

Immunoblot identification of glial fibrillary acidic protein in rat sciatic nerve, brain, and spinal cord during development

The appearance of the glial fibrillary acidic protein (GFAP) during embryonic and postnatal development of the rat brain and spinal cord and in rat sciatic nerve during postnatal development was examined by the immunoblot technique. Cytoskeletal proteins were isolated from the central and peripheral nervous system and separated by SDS slab gel electrophoresis or two-dimensional gel electrophoresis. Proteins from the acrylamide gels were transferred to nitrocellulose sheets which were treated with anti-bovine GFAP serum and GFAP was identified by the immunoblot technique. GFAP was present in the embryonic rat brain and spinal cord at 14 and 16 days of gestation respectively. The appearance of GFAP at this stage of neural development suggests that the synthesis of GFAP may be related to the proliferation of radial glial cells from which astrocytes are derived. It is also feasible that GFAP provides structural support for the radial glial cell processes analogous to its role in differentiated astrocytes. GFAP was found to be present in rat sciatic nerves at birth and at all subsequent stages of development. These results indicate that some cellular elements in the rat sciatic nerve, such as Schwann cells, are capable of synthesizing GFAP which is immunochemically indistinguishable from its counterpart in the central nervous system. Thus it appears that GFAP is present both in the central and peripheral nervous system of the rat when the glial cells synthesizing GFAP are still undergoing differentiation.

Ionic and electrophysiological properties of retinal Müller (glial) cells of the turtle

The ionic and electrophysiological properties of Müller cells, the principal glial element of the vertebrate retina, were investigated. The membrane potential of enzymatically dissociated and in situ Müller cells was about -80 mV and depended on external K+ concentration in a manner that was described by the Goldman-Hodgkin-Katz equation with a Na+-K+ permeability ratio of 0.037. The current-voltage relation showed marked inward rectification, with the input resistance at the resting potential being about 30 M omega for dissociated cells and about 3 M omega for in situ cells. In situ Müller cells were found to be electrically coupled to each other which could explain their lower resistance. We conclude that Müller cells are similar to other types of glia. In spite of a finite Na+ permeability their membrane potential is determined mainly by K+, they are electrically inexcitable and form an electrically coupled network in the retina.

Determinants of deoxyglucose uptake in cultured astrocytes: the role of the sodium pump

Glucose utilization in primary cell cultures of mouse cerebral astrocytes was studied by measuring uptake of tracer concentrations of [3H]2-deoxyglucose ([3H]2-DG). The resting rate of glucose utilization, estimated at an extracellular K+ concentration ([K+]o) of 5.4 mM, was high (7.5 nmol glucose/mg protein/min) and was similar in morphologically undifferentiated and "differentiated" (dibutyryl cyclic AMP-pretreated) cultures. Resting uptake of [3H]2-DG was depressed by ouabain, by reducing [K+]o, and by cooling. These observations suggest that resting glucose utilization in astrocytes was dependent on sodium pump activity. Sodium pump-dependent uptake in 2-3-week-old cultures was about 50% of total [3H]2-DG uptake but this fraction declined with culture age from 1 to 5 weeks. Uptake was not affected by changes in extracellular bicarbonate concentration ([HCO3-]o) in the range of 5-50 mM but was significantly reduced in bicarbonate-free solution. At high [HCO3-]o (50 mM) uptake was insensitive to pH (pH 6-8), whereas at low [HCO3-]o (less than 5 mM) uptake was markedly pH-dependent. Elevation of [K+]o from 2.3 mM to 14.2-20 mM (corresponding to extremes of the physiological range of [K+]o) resulted in a 35-43% increase in [3H]2-DG uptake that was not affected by culture age or by morphological differentiation. Our results indicate a high apparent rate of glucose utilization in astrocytes. This rate is dynamically responsive to changes in extracellular K+ concentration in the physiological range and is partially dependent on sodium pump activity.

The development of glio-interstitial tissue in Mytilus retractor muscle depends on Na+-Ca2+ antagonism

The development of glio-interstitial cell processes has been studied by quantitative electron microscopy in the anterior byssal retractor muscle of mussels kept in various artificial sea-waters. After 20 days, the number of glio-interstitial processes per unit area of muscle section from animals adapted to diluted sea-water (700 mosM) is not significantly different from the control (1100 mosM) but it is almost doubled in mussels adapted to concentrated sea-water (1400 mosM). The diluted sea-water has a high [Ca2+]/[Na+]2 molar ratio (6.81 X 10(-5)) and the concentrated sea-water a relatively low one (3.34 X 10(-5)); all the ions are present in the same proportions as in the control. In a second experiment, diluted sea-water (700 mosM) with a low [Ca2+]/[Na+]2 (3.34 X 10(-5)) and concentrated sea-water (1400 mosM) with a high ratio (6.81 X 10(-5)) are tested: the results agree with the prediction that the development of glio-interstitial processes depends on the relative concentrations of Na+ and Ca2+ rather than on osmotic pressure or ionic strength. In the third experiment, five artificial sea-waters are employed with decreasing [Ca2+]/[Na+]2 ratios, all at the same osmotic pressure of 1100 mosM: the results suggest that the salinity-induced proliferation of glio-interstitial processes is directly dependent on the [Ca2+]/[Na+]2 ratio. Glial proliferation thus occurs in reaction to the relative lack of Ca2+, or excess of Na+, in the environment; it is proposed that the glio-interstitial tissue plays a role in regulating the concentration of Ca2+ in the vicinity of the muscle and/or the nerve cells.

Intracellular Ca2+ elevation induced by a neurotransmitter in a glial cell clone

By fluorometry using a Ca2+-indicator Quin 2, we found an elevation of intracellular Ca2+ concentration ([Ca2+]i) in response to an application of serotonin in a rat clonal glial cell (C6BU-1). The [Ca2+]i rise depended on the dose of applied serotonin and the level of environmental Ca2+. The possibility was suggested that neuron-glia interactions might be controlled by a receptor-coupled [Ca2+]i-regulation system.

Interaction of neuropeptides and cultured glial (Müller) cells of the chick retina: elevation of intracellular cyclic AMP by vasoactive intestinal peptide and glucagon

Vasoactive intestinal peptide (VIP) and, to a lesser extent, glucagon were found to increase intracellular cyclic AMP rapidly in cultured glial (Müller) cells of the chick embryo retina. Although VIP elicited higher cyclic AMP accumulation than glucagon at each concentration tested, the half-maximal concentrations were similar, i.e., 6 X 10(-8) M for VIP and 8 X 10(-8) M for glucagon. Secretin had a minimal effect on cyclic AMP accumulation even at a very high (5 X 10(-6) M) concentration. Several other peptide and nonpeptide putative agonists also had little effect on cyclic AMP accumulation. The cultured Müller cell may thus be a useful model for examining VIP and glucagon effects on glial elements of the CNS.

CNS stimulation and PGE2 release. I. Depolarizing agents

Radioimmunoassayable PGE2 was reliably detected in the ventriculocisternal perfusates of chloralose-urethanized cats (557-1481 pg/min; +/- 1 S.E.). Examination of the levels of PGE2 in sequential 30 min samples for periods of up to 6 h revealed that the basal secretion rates of this prostaglandin did not fluctuate ina statistically significant fashion during this extended interval. The addition of either K+ (50 mM) or veratridine (7.5 X 10(-5) M) were shown to reliably evoke a long lasting increase (190-210%) in radioimmunoassayable PGE2 in the ventriculocisternal superfusates. In the majority of experiments, the elevated PGE2 showed a return to prestimulation levels within 60 min after the removal of the depolarizing agent. The ability of two depolarizing agents which act by different mechanisms to produce PGE2 release, and the likelihood that the stimulus-dependent increases were not due to altered clearance, metabolism or dilution makes it likely that the PGE2 levels in the extravascular-extracellular fluid were elevated secondary to increases in neuronal activity.

Variable CA II compartmentalization in vertebrate retina

We have generated a series of polyclonal and monoclonal antibodies to mammalian, avian, and osteichthian CA II for the purpose of studying its distribution in vertebrate nervous systems. In mature chicken retina, CA II is immunohistochemically detectable only in Müller glial cells. However, during embryonic development, CA II expression is suddenly "switched-on" early as a general constituent of all retinoblasts, later becoming restricted to Müller cells and transiently to a distinct type of amacrine neuron. A similar developmental pattern occurs in mouse. However, at maturity high CA II levels remain in certain amacrine neurons in addition to Müller cells. Comparative analyses of mature retinas of lower vertebrates show that reptiles parallel chicken with high CA II only in Müller cells, certain amphibians show CA II staining in Müller cells, amacrine neurons as in mouse, and in horizontal neurons, teleost and elasmobranch fish possess high CA II in Müller cells and the horizontal neurons, and lamprey eel shows CA II staining primarily in horizontal cells. An evolutionary sequence that will be discussed is thus suggested.

Cellular composition of primary cultures from cerebral cortex, striatum, hippocampus, brainstem and cerebellum

Primary cultures from newborn rat cerebral cortex, striatum, hippocampus, brainstem and cerebellum were grown for 14 days. There was a linear relationship between the amount of material seeded and the protein content of the respective culture. The amount of tissue material seeded was selected so that the different cultures reached confluence at 6-7 days and contained similar amounts of protein when 7 and 14 days old. The cellular content was evaluated by astroglial markers, such as the glial fibrillary acidic protein (GFAp; alpha-albumin) and the S-100 protein, and by markers for other cells expected to be in the cultures (14-3-2 protein, macrophage acidic protein (MAP), alkaline phosphatase, myelin basic protein (MBP), 2',3'-cyclic nucleotide 3'-phosphohydrolase (CNP]. Astroglial-like cells represented 60-70% of the cells present in the different cultures. Quantitation of GFAp (alpha-albumin) showed similar amounts to be present in cultures from cerebral cortex, hippocampus and striatum; however, on lower levels expressed in soluble proteins than in the corresponding brain regions of adult rats. Brainstem of adult rat contained large amounts of GFAp (alpha-albumin), while low levels were found in brainstem culture. Also, phagocytic cells (macrophages), endothelial-like cells, mesenchymal-like cells, ependymal-like cells and oligoblasts were found. Neither mature neurons, nor oligodendroglial cells were observed. It is concluded that although there might be some differences in the degree of maturation or in the cellular composition of the various cultures, they could serve as a good model system for studying the characteristics of astroglial cells from various brain regions.

Cerebral circulation and metabolism

Recent developments in the field of cerebral circulation and metabolism are reviewed, with emphasis on circulatory and metabolic events that have a bearing on brain damage incurred in ischemia. The first part of the treatise reviews aspects of cerebral metabolism that provide a link to the coupling of metabolism and blood flow, notably those that lead to a perturbation of cellular energy state, ionic homeostasis, and phospholipid metabolism. In the second part, attention is focused on the derangement of energy metabolism and its effects on ion fluxes, acid-base homeostasis, and lipid metabolism. It is emphasized that gross brain damage, involving edema formation and infarction, is enhanced by tissue acidosis, and that neuronal damage, often showing a pronounced selectivity in localization, appears related to a disturbed Ca2+ homeostasis, and to Ca2+-triggered events such as lipolysis and proteolysis.

Cellular compartmentalization of carbonic anhydrase-C and glutamine synthetase in developing and mature mouse neural retina

Using immunohistochemical methods, we have determined the cellular localization of the enzymes, glutamine synthetase (GS) and carbonic anhydrase-C (CA-C), in mouse neural retina during development and in the mature tissue. GS is always confined exclusively to the Müller glial cells; it is first detectable in these cells post-natally on about day 12, i.e. shortly before the eyes open. Also CA-C in the mature retina is localized in the Müller cells but, in addition, it is found in certain amacrine neurons as well. CA-C is first detectable in the retina already several days before birth; at that time it is found in most of the cells, with the exception of the emerging ganglion cells. However, with advancing differentiation, CA-C becomes progressively restricted to Müller cells and to a sub-category of amacrine neurons, and persists only in these cells in the mature retina. The present results extend our previous studies on these enzymes in the avian retina; they demonstrate that also in mammalian retina, different temporal and cellular patterns of GS and CA-C expression and localization earmark distinct phases of structural and functional differentiation of the retina. The striking developmental changes in the cellular localization of CA-C, and the finding of this enzyme in certain amacrine neurons as well as in Müller cells, raise questions about the role of CA-C in the retina, and about mechanisms regulating its expression in specific cell types.

Cellular composition of a cerebral hemisphere primary culture

In this overview attention is given to available markers and methods for characterizing cell elements in a culture system. Primary cultures from newborn rat cerebral hemispheres were grown for 14 days. The population of cells was dominated by astrocytic glial cells (60-70%), but cells with properties of macrophages, endothelial-like cells, mesenchymal-like cells, ependymal-like cells, and oligoblasts were also found. Neither mature neurons nor oligodendroglial cells were observed. The enrichment in astroglial-like cells makes the cultures a satisfactory astroglial-cell model, at least for some purposes.

Potassium activity in leech neuropile glial cells changes with external potassium concentration

The effect of the external K+ concentration on the intracellular K+ activity of neuropile glial cells and of sensory neurons in the central nervous system of the leech (Hirudo medicinalis L.) was determined directly with double-barreled ion-sensitive microelectrodes. As the external K+ concentration was raised, the intracellular K+ activity of the neuropile glial cells increased considerably, and was accompanied by the uptake and/or intracellular synthesis of an, as yet, unidentified substance. In contrast, the intracellular K+ activity of sensory neurons was unchanged when the external K+ concentration was increased. These neurons appeared not to accumulate a second substance.

Specific messenger RNA changes in Joseph disease cerebella

Joseph disease is an autosomal-dominant, spinocerebellar degeneration characterized at the biochemical level by elevations in the steady-state levels of several abundant proteins (H, J, and L) in affected brain areas such as the cerebellar cortex. The increased levels of these proteins could either be a consequence of a relative increase in their de novo synthesis or result from altered rates of proteolysis in degenerating brain cells. These alternatives can be distinguished by comparing the in vitro protein-synthetic capacities of the messenger ribonucleic acid populations isolated from cerebellar cortex of control subjects and patients with Joseph disease. Protein H (glial fibrillary acidic protein) is synthesized at detectable levels by all messenger ribonucleic acid isolates, and the levels of its translatable messenger ribonucleic acid are reproducibly increased in ribonucleic acids isolated from cerebellar cortex of patients with Joseph disease as compared with those isolated from cerebellar cortex of control subjects. Thus, the increased level of protein H in Joseph disease is a consequence of an increase in its de novo synthesis and is correlated with the increased number of cerebellar glial cells. In contrast to these results, there is no detectable synthesis of proteins J and L by messenger ribonucleic acid populations isolated from cerebellar cortex of either Joseph disease patients or control subjects, suggesting that the increased levels of these proteins in affected cerebellar cortex are a consequence of posttranslational protein modifications.

Melanin: the organizing molecule

The hypothesis is advanced that (neuro)melanin (in conjunction with other pigment molecules such as the isopentenoids) functions as the major organizational molecule in living systems. Melanin is depicted as an organizational "trigger" capable of using established properties such as photon-(electron)-phonon conversions, free radical-redox mechanisms, ion exchange mechanisms, and semiconductive switching capabilities to direct energy to strategic molecular systems and sensitive hierarchies of protein enzyme cascades. Melanin is held capable of regulating a wide range of molecular interactions and metabolic processes primarily through its effective control of diverse covalent modifications. To support the hypothesis, established and proposed properties of melanin are reviewed (including the possibility that (neuro)melanin is capable of self-synthesis). Two "melanocentric systems"--key molecular systems in which melanin plays a central if not controlling role--are examined: 1) the melanin-purine-pteridine (covalent modification) system and 2) the APUD (or diffuse neuroendocrine) system. Melanin's role in embryological organization and tissue repair/regeneration via sustained or direct current is considered in addition to its possible control of the major homeostatic regulatory systems--autonomic, neuroendocrine, and immunological.

A qualitative and quantitative ultrastructural study of glial cells in the developing visual cortex of the rat

(i) This paper provides new information on the time course and fine structural features of glial cell differentiation, on the relative frequencies of glioblasts, astroblasts, astrocytes, oligodendrocytes and microglial cells, and on neuron: glia ratios in visual cortex of the rat between birth and maturity. The analyses were done on montages of electron micrographs of 75 pm wide strips extending the full depth of the cortex from animals 12 h and 4, 6, 8, 10, 12, 14, 20, 24, 90 and 180 days old (six montages from two or three animals at each age). (ii) At birth, and up to 4 days, most non-neuronal cells are poorly differentiated, irregularly shaped cells with dark nuclei (glioblasts). A few at this stage and progressively larger numbers over the next few days, can be recognized asastroblastsby the presence of a distinctive form of granular reticulum (distended cisterns with a moderately electron dense content), and some also by their position in contact with the subpial or perivascular basal laminae. Astroblasts enlarge, develop processes and transform into immature astrocytes: their nuclei become paler, the granular reticulum is no longer distended, and glial filaments begin to accumulate.Mature astrocyteswith pale nuclei, filaments and a low concentration of perikaryal organelles in a pale cytoplasmic matrix predominate at 24 days, and at 3-6 months 51 % of all glial cells are astrocytes. (iii) Concentrations of glioblasts (at 0 and 4 days) and subsequently of cells of the astrocytic lineage are apparent in the most superficial and in the deepest cortical layers, and an additional small peak is seen at the level of layer IV in the adult animals. The superficial concentration is probably associated with the subpial glia limitans and the layer IV concentration with the high density of synapses in this region; several probable explanations are considered for the concentration in layer VI. (iv) Processes ofradial glial cellsare apparent from birth to day 8 but not thereafter. No evidence was found for transformation of radial glia into astrocytes. A peak in phagocytic activity by immature microglial cells at days 6-8 suggests the possibility of loss of radial glial processes by degeneration rather than transformation. (v)Oligodendroblasts, intermediate in morphology between glioblasts and light oligodendrocytes, appear suddenly in the deep cortex and subcortical white matter at day 6 and are rapidly replaced bylight oligodendrocytes. These are large, organelle-rich cells with characteristically distended Golgi saccules, and are the only oligodendrocytes present during early myelination, which begins at day 10. Early in the 3rd postnatal week some light oligodendrocytes are replaced bymedium oligodendrocytes, which are smaller and darker, with abundant orderly stacks of granular reticulum.Dark oligodendrocytesare first apparent at the end of the 3rd week, account for about one-third of all oligodendrocytes at day 24, predominate at day 40 and constitute 90 % of all oligodendrocytes at 3 and 6 months, at which time oligodendrocytes comprise 39% of all cortical glial cells. We suggest that the progression from light to medium oligodendrocytes does not simply represent a diminution in the overall level of synthetic activity but that different components of the myelin sheath are being synthesized at the two stages. (vi)Microgliaare present from birth but are seen in significant numbers at days 6—10 and thereafter. Some are relatively mature in appearance, even in the youngest animals, and almost all are similar to the resting microglia of adult brain by day 16. At 3-6 months, 8 % of all cortical glial cells are identified as microglia and these cells are fairly evenly distributed throughout the cortical depth but are surprisingly and consistently poorly represented in layer VI. From day 6 to the end of the 2nd postnatal week, cells with poorly differentiated cytoplasm (many free polyribosomes), but containing phagocytosed products of cell degeneration, are identified asimmature microglia. However, it is possible that such cells do not mature into classical resting microglia but that they represent a different cell type. (vii) Theneuron: glia ratiois 4.54 at birth, rises to 5.09 at 4 days, and falls to approximately 2.5 at days 12-24. At 3-6 months the ratio is 2.13.

Hormonal induction of glutamine synthetase in cultures of embryonic retina cells: requirement for neuron-glia contact interactions

Cortisol induces glutamine synthetase (GS) in gliocytes of chick embryo neural retina. Using adherent cultures of retina cells we have demonstrated that responsiveness of the gliocytes to GS induction by the hormone requires contact with neurons. GS is not inducible in high-density cultures depleted of neurons and consisting only of gliocytes. In neuron-containing cultures, induced GS was detected immunohistochemically only in those gliocytes that were closely juxtaposed with clusters of neurons. Unlike the induction of GS, the expression of carbonic anhydrase-C (which does not require cortisol) persisted in these glia cells also in the absence of neurons. The nature and role of glia-neuron interactions in the hormonal induction of GS are briefly discussed.

Enkephalin immunoreactivity in synaptoid elements on glial cells in the rat neural lobe

Opioid peptides were localized in fibres of the rat neural lobe using various immunocytochemical methods at the light- and electron-microscopical level. Leu-enkephalin immunoreactivity was present in beaded fibres distributed throughout the neural lobe. These fibres surround the neurohypophyseal glial cells (pituicytes) and make synaptoid contacts upon their soma and processes. The reaction product was localized both in dense-core vesicles of about 100 nm in diameter and diffusely spread over the cytoplasm. No arguments in support of the co-existence of enkephalins and the neurohypophyseal hormones vasopressin and oxytocin in the same terminal were found. It is suggested that pituicytes might mediate the inhibitory effect of opiod peptides on vasopressin and oxytocin release from the neural lobe.

Specific chromosome changes associated with viral transformation of rat glial cells

Karyotypes of three malignant cell lines derived from Wistar and WKA/Mk fetal rat glioblasts, transformed by murine sarcoma virus (MSV-M-os) as well as those of four cell lines derived from C6 glioma cells of Wistar origin, retransformed by MSV-M-os, were analyzed in early culture passages. The C6 line had a modal number of 42 chromosomes with a normal male karyotype, and only a minor population of cells with 43 chromosomes. The modal chromosome number in every transformed glial cell line shifted from 42 to 43. The G-banding pattern revealed consistent chromosome abnormalities. Structural chromosome changes occurred in one chromosome No. 2 (2q-) and in one No. 4 (4q+). The cells with a 43 chromosome karyotype showed trisomy of chromosome No. 12 and its heteromorphism, a finding also confirmed by silver staining. Identical chromosome changes were found in transformed C6 cell lines. A further interesting feature was that all malignant cells had different distribution patterns of silver-stained nucleolar organizer regions (Ag-NORs) among particular chromosomes (Nos. 3, 11 and 12) from normal cells, showing an increased frequency of chromosome No. 12 with Ag-NORs. These results suggested that the gain and/or loss of specific segments involved in chromosomes Nos. 2, 4 and 12 contain(s) genes favorable to malignant transformation in rat glial cells.

Glutamine synthetase activity in Huntington's disease

Glutamine synthetase activity was measured in seven brain areas post-mortem from control patients, and those with Huntington's disease. The activity of the enzyme was reduced in the frontal and temporal cortex, putamen and cerebellum, but not in the hippocampus, thalamus or olivary nucleus. The results do not suggest a generalised deficiency of glutamine synthetase in Huntington's disease. However, as this enzyme is localised to astrocytic cells, the reduction in activity in areas of neuronal devastation, where the ration of astrocytes to neurones is increased, may reflect a greater functional deficit. The enzyme plays a crucial role in cerebral ammonia assimilation and its inhibition in laboratory animals is known to produce neuronal toxicity. A reduction in its activity in Huntington's disease may well contribute to the neuronal pathology in certain areas.

Morphology and distribution of postnatally generated glial cells in the somatosensory cortex of the rat: an autoradiographic and electron microscopic study

Postnatal cytogenesis in rat somatosensory cortex was examined by the technique combining light microscopic autoradiography with electron microscopy. Tritiated thymidine was injected intraperitoneally to rats on different days (0-21 days). All animals were sacrificed on the twenty-fifth day after birth. Coronal slices including somatosensory cortex were embedded in epoxy resin. Semithin sections for autoradiography and ultrathin ones for electron microscopy were made alternately. The labeled cortical cells were found mainly in the cases injected with [3H]thymidine during the first and the second weeks. Examination of laminar distribution of the labeled cells revealed that the cells in deeper laminae were labeled on earlier postnatal days than those in more superficial laminae. The labeled cells were examined with electron microscope to identify their nature. By this, it was revealed that ultrastructural morphology of the labeled cells were that of glial cells (either astrocyte or oligodendrocyte). Time and space pattern of this neocortical postnatal gliogenesis shows the tendency of the inside-out sequence, though the pattern is not as distinctive as that of prenatal neocortical neurogenesis. The relationships between the pattern of gliogenesis and the maturation of cortical neurons is suggested.

Receptor-associated changes of the catecholamine-sensitive adenylate cyclase in glioma cells doubly transformed with Moloney sarcoma virus

A doubly transformed rat glioma cell line, designated C6V-1, was obtained from rat glioma C6 cells by infection with a rat-adapted variant of Moloney sarcoma virus (MSV-M-os). The C6V-1 cells show karyotypic changes in chromosome number (43) and structure, while C6 cells possess a normal male karyotype. C6V-1 and C6 cells were employed for characterization of a receptor-adenylate cyclase system of the surface membrane. C6V-1 cells showed lower adenylate cyclase activity than that of C6 cells, though the apparent Km for ATP in both types of cells was the same. The maximal stimulation of adenylate cyclase by isoproterenol was significantly reduced, and Kact for isoproterenol was approximately 18-fold lower in C6V-1 cells. When the concentration of beta-adrenergic receptors was measured by various concentrations of [3H] dihydroalprenolol (DHA), the maximal binding sites of C6 and C6V-1 cells were 760 and 230 fmol/mg protein, respectively, without any changes in the association constant for DHA. The concentration of isoproterenol required for 50% displacement of the [3H] DHA binding (Kd) was the same (around 1.5 X 10(-6)M) in both cells, measured in the presence of GTP. Thus the 19-fold drop in the Kd/Kact ratio in C6V-1 cells suggests an incomplete coupling of beta-receptors to adenylate cyclase. Cyclic AMP phosphodiesterase activity and cAMP content in C6V-1 were lower than in C6 cells. Mitochondrial monoamine oxidase and cytosomal enolase activities, however, were somewhat higher in C6V-1 cells. The results indicate that a set of changes in the receptors and in the cyclic AMP system of C6V-1 is one of the specific alterations by transformation, even though those may not be the cause of cell transformation.

Uptake and release of choline in cultures of human glioma cells

Human glioma cells (138MG) have a low-affinity uptake system for choline (Km = 20 microM; Vmax = 56 pmol/min/10(6) cells). The uptake is reduced by acetylcholine, hemicholinium-3, HgCl2, and phosphodiesterase inhibitors. Release of [3H]choline from preloaded cultures showed two pools with half-lives of 1.3 and 160 min. Choline release was stimulated by 8-bromo-cAMP or isobutylmethylxanthine. The results suggest that release of choline occurs by a facilitated diffusion transport system and is increased by elevations of intracellular cAMP.