Extracellular potassium influences DNA and protein syntheses and glial fibrillary acidic protein expression in cultured glial cells

Gender Differences in Oxidative Stress in Relation to Cancer Susceptibility and Survival

Genetic, developmental, biochemical, and environmental variables interact intricately to produce sex differences. The significance of sex differences in cancer susceptibility is being clarified by numerous studies. Epidemiological research and cancer registries have revealed over the past few years that there are definite sex variations in cancer incidence, progression, and survival. However, oxidative stress and mitochondrial dysfunction also have a significant impact on the response to treatment of neoplastic diseases. Young women may be more protected from cancer than men because most of the proteins implicated in the regulation of redox state and mitochondrial function are under the control of sexual hormones. In this review, we describe how sexual hormones control the activity of antioxidant enzymes and mitochondria, as well as how they affect several neoplastic diseases. The molecular pathways that underlie the gender-related discrepancies in cancer that have been identified may be better understood, which may lead to more effective precision medicine and vital information on treatment options for both males and females with neoplastic illnesses.

The Interaction between hERG1 and β1 Integrins Modulates hERG1 Current in Different Pathological Cell Models

Ion channels are implicated in various diseases, including cancer, in which they modulate different aspects of cancer progression. In particular, potassium channels are often aberrantly expressed in cancers, a major example being provided by hERG1. The latter is generally complexed with β1 integrin in tumour cells, and such a molecular complex represents a new druggable hub. The present study focuses on the characterization of the functional consequences of the interaction between hERG1 and β1 integrins on different substrates over time. To this purpose, we studied the interplay alteration on the plasma membrane through patch clamp techniques in a cellular model consisting of human embryonic kidney (HEK) cells stably transfected with hERG1 and in a cancer cell model consisting of SH-SY5Y neuroblastoma cells, endogenously expressing the channel. Cells were seeded on different substrates known to stimulate β1 integrins, such as fibronectin (FN) for HEK-hERG1 and laminin (LMN) for SH-SY5Y. In HEK cells stably overexpressing hERG1, we observed a hERG1 current density increase accompanied by V_rest hyperpolarization after cell seeding onto FN. Notably, a similar behaviour was shown by SH-SY5Y neuroblastoma cells plated onto LMN. Interestingly, we did not observe this phenomenon when plating the cells on substrates such as Bovine Serum Albumin (BSA) or Polylysine (PL), thus suggesting a crucial involvement of ECM proteins as well as of β1 integrin activation.

Role of ion channels during cell division

Ion channels are transmembrane proteins whose canonical function is the transport of ions across the plasma membrane to regulate cell membrane potential and play an essential role in neural communication, nerve conduction, and muscle contraction. However, over the last few years, non-canonical functions have been identified for many channels, having active roles in phagocytosis, invasiveness, proliferation, among others. The participation of some channels in cell proliferation has raised the question of whether they may play an active role in mitosis. There are several reports showing the participation of channels during interphase, however, the direct participation of ion channels in mitosis has received less attention. In this article, we summarize the current evidence on the participation of ion channels in mitosis. We also summarize some tools that would allow the study of ion channels and cell cycle regulatory molecules in individual cells during mitosis.

Bioelectric Properties of Myogenic Progenitor Cells

Modern stem cell research has mainly focused on protein expression and transcriptional networks. However, transmembrane voltage gradients generated by ion channels and transporters have demonstrated to be powerful regulators of cellular processes. These physiological cues exert influence on cell behaviors ranging from differentiation and proliferation to migration and polarity. Bioelectric signaling is a fundamental element of living systems and an untapped reservoir for new discoveries. Dissecting these mechanisms will allow for novel methods of controlling cell fate and open up new opportunities in biomedicine. This review focuses on the role of ion channels and the resting membrane potential in the proliferation and differentiation of skeletal muscle progenitor cells. In addition, findings relevant to this topic are presented and potential implications for tissue engineering and regenerative medicine are discussed.

A novel role for estrogen-induced signaling in the colorectal cancer gender bias

Colorectal cancer (CRC) is a malignancy whose incidence is increasing globally, and there is a gender difference in the increasing risk. Evidence from hormone replacement therapy studies points to a role for circulating estrogens in suppressing the development of CRC. Estrogen receptor-β has been identified as a tumor suppressor, but other actions of estrogen may also contribute to the difference in CRC incidence between men and women. The KCNQ1/KCNE3 potassium channel is regulated by estrogen in order to modulate chloride secretion during the menstrual cycle; the effect of estrogen on the colon is to promote fluid conservation during the implantation window. KCNQ1 is also a tumor suppressor in CRC, and its sustained expression has been linked to suppression of the Wnt/β-catenin signaling pathway that contributes to CRC tumor progression. KCNQ1 regulation may represent a link between the normal physiological actions of estrogen in the colon and the hormone's apparent tumor-suppressive effects in CRC development.

Type-specific photoreceptor loss in pigeons after disruption of parasympathetic control of choroidal blood flow by the medial subdivision of the nucleus of Edinger-Westphal

The medial part of the nucleus of Edinger–Westphal (EWM) in birds mediates light-regulated adaptive increases in choroidal blood flow (ChBF). We sought to characterize the effect of loss of EWM-mediated ChBF regulation on photoreceptor health in pigeons housed in either moderate intensity diurnal or constant light (CL). Photoreceptor abundance following complete EWM destruction was compared to that following a lesion in the pupil control circuit (as a control for spread of EWM lesions to the nearby pupil-controlling lateral EW) or following no EW damage. Birds were housed post-lesion in a 12 h 400 lux light/12 h dark light cycle for up to 16.5 months, or in constant 400 lux light for up to 3 weeks. Paraformaldehyde–glutaraldehyde fixed eyes were embedded in plastic, sectioned, slide-mounted, and stained with toluidine blue/azure II. Blinded analysis of photoreceptor outer segment abundance was performed, with outer segment types distinguished by oil droplet tint and laminar position. Brains were examined histologically to assess lesion accuracy. Disruption of pupil control had no adverse effect on photoreceptor outer segment abundance in either diurnal light or CL, but EWM destruction led to 50–60% loss of blue/violet cone outer segments in both light conditions, and a 42% loss of principal cone outer segments in CL. The findings indicate that adaptive regulation of ChBF by the EWM circuit plays a role in maintaining photoreceptor health and mitigates the harmful effect of light on photoreceptors, especially short wavelength-sensitive cone photoreceptors.

Wide field fluorescent imaging of extracellular spatiotemporal potassium dynamics in vivo

Potassium homeostasis is fundamental for the physiological functioning of the brain. Increased [K(+)] in the extracellular fluid has a major impact on neuronal physiology and can lead to ictal events. Compromised regulation of extracellular [K(+)] is involved in generation of seizures in animal models and potentially also in humans. For this reason, the investigation of K(+) spatio-temporal dynamics is of fundamental importance for neuroscientists in the field of epilepsy and other related pathologies. To date, the majority of studies investigating changes in extracellular K(+) have been conducted using a micropipette filled with a K(+) sensitive solution. However, this approach presents a major limitation: the area of the measurement is circumscribed to the tip of the pipette and it is not possible to know the spatiotemporal distribution or origin of the focally measured K(+) signal. Here we propose a novel approach, based on wide field fluorescence, to measure extracellular K(+) dynamics in neural tissue. Recording the local field potential from the somatosensory cortex of the mouse, we compared responses obtained from a K(+)-sensitive microelectrode to the spatiotemporal increases in fluorescence of the fluorophore, Asante Potassium Green-2, in physiological conditions and during 4-AP induced ictal activity. We conclude that wide field imaging is a valuable and versatile tool to measure K(+) dynamics over a large area of the cerebral cortex and is capable of capturing fast dynamics such as during ictal events. Moreover, the present technique is potentially adaptable to address questions regarding spatiotemporal dynamics of other ionic species.

Membrane potential and cancer progression

Membrane potential (V_m), the voltage across the plasma membrane, arises because of the presence of different ion channels/transporters with specific ion selectivity and permeability. V_m is a key biophysical signal in non-excitable cells, modulating important cellular activities, such as proliferation and differentiation. Therefore, the multiplicities of various ion channels/transporters expressed on different cells are finely tuned in order to regulate the V_m. It is well-established that cancer cells possess distinct bioelectrical properties. Notably, electrophysiological analyses in many cancer cell types have revealed a depolarized V_m that favors cell proliferation. Ion channels/transporters control cell volume and migration, and emerging data also suggest that the level of V_m has functional roles in cancer cell migration. In addition, hyperpolarization is necessary for stem cell differentiation. For example, both osteogenesis and adipogenesis are hindered in human mesenchymal stem cells (hMSCs) under depolarizing conditions. Therefore, in the context of cancer, membrane depolarization might be important for the emergence and maintenance of cancer stem cells (CSCs), giving rise to sustained tumor growth. This review aims to provide a broad understanding of the V_m as a bioelectrical signal in cancer cells by examining several key types of ion channels that contribute to its regulation. The mechanisms by which V_m regulates cancer cell proliferation, migration, and differentiation will be discussed. In the long term, V_m might be a valuable clinical marker for tumor detection with prognostic value, and could even be artificially modified in order to inhibit tumor growth and metastasis.

Physical exercise increases GFAP expression and induces morphological changes in hippocampal astrocytes

Physical exercise has an important influence on brain plasticity, which affects the neuron-glia interaction. Astrocytes are susceptible to plasticity, and induce and stabilize synapses, regulate the concentration of various molecules, and support neuronal energy metabolism. The aim of our study was to investigate whether physical exercise is capable of altering the morphology, density and expression of glial fibrillary acidic protein (GFAP) in astrocytes from the CA1 region of rat hippocampus. Thirteen male rats were divided in two groups: sedentary (n = 6) and exercise (n = 7). The animals in the exercise group were submitted to a protocol of daily physical exercise on a treadmill for four consecutive weeks. GFAP immunoreactivity was evaluated using optical densitometry and the morphological analyses were an adaptation of Sholl's concentric circles method. Our results show that physical exercise is capable of increasing the density of GFAP-positive astrocytes as well as the regional and cellular GFAP expression. In addition, physical exercise altered astrocytic morphology as shown by the increase observed in the degree of ramification in the lateral quadrants and in the length of the longest astrocytic processes in the central quadrants. Our data demonstrate important changes in astrocytes promoted by physical exercise, supporting the idea that these cells are involved in regulating neural activity and plasticity.

Bioelectric controls of cell proliferation: ion channels, membrane voltage and the cell cycle

All cells possess long-term, steady-state voltage gradients across the plasma membrane. These transmembrane potentials arise from the combined activity of numerous ion channels, pumps and gap junction complexes. Increasing data from molecular physiology now reveal that the role of changes in membrane voltage controls, and is in turn controlled by, progression through the cell cycle. We review recent functional data on the regulation of mitosis by bioelectric signals, and the function of membrane voltage and specific potassium, sodium and chloride ion channels in the proliferation of embryonic, somatic and neoplastic cells. Its unique properties place this powerful, well-conserved, but still poorly-understood signaling system at the center of the coordinated cellular interactions required for complex pattern formation. Moreover, disregulation of ion channel expression and function is increasingly observed to be not only a useful marker but likely a functional element in oncogenesis. New advances in genomics and the development of in vivo biophysical techniques suggest exciting opportunities for molecular medicine, bioengineering and regenerative approaches to human health.

Activation of satellite glial cells in rat trigeminal ganglion after upper molar extraction

The neurons in the trigeminal ganglion (TG) are surrounded by satellite glial cells (SGCs), which passively support the function of the neurons, but little is known about the interactions between SGCs and TG neurons after peripheral nerve injury. To examine the effect of nerve injury on SGCs, we investigated the activation of SGCs after neuronal damage due to the extraction of the upper molars in rats. Three, 7, and 10 days after extraction, animals were fixed and the TG was removed. Cryosections of the ganglia were immunostained with antibodies against glial fibrillary acidic protein (GFAP), a marker of activated SGCs, and ATF3, a marker of damaged neurons. After tooth extraction, the number of ATF3-immunoreactive (IR) neurons enclosed by GFAP-IR SGCs had increased in a time-dependent manner in the maxillary nerve region of the TG. Although ATF3-IR neurons were not detected in the mandibular nerve region, the number of GFAP-IR SGCs increased in both the maxillary and mandibular nerve regions. Our results suggest that peripheral nerve injury affects the activation of TG neurons and the SGCs around the injured neurons. Moreover, our data suggest the existence of a neuronal interaction between maxillary and mandibular neurons via SGC activation.

Early blockade of injured primary sensory afferents reduces glial cell activation in two rat neuropathic pain models

Satellite glial cells in the dorsal root ganglion (DRG), like the better-studied glia cells in the spinal cord, react to peripheral nerve injury or inflammation by activation, proliferation, and release of messengers that contribute importantly to pathological pain. It is not known how information about nerve injury or peripheral inflammation is conveyed to the satellite glial cells. Abnormal spontaneous activity of sensory neurons, observed in the very early phase of many pain models, is one plausible mechanism by which injured sensory neurons could activate neighboring satellite glial cells. We tested effects of locally inhibiting sensory neuron activity with sodium channel blockers on satellite glial cell activation in a rat spinal nerve ligation (SNL) model. SNL caused extensive satellite glial cell activation (as defined by glial fibrillary acidic protein [GFAP] immunoreactivity) which peaked on day 1 and was still observed on day 10. Perfusion of the axotomized DRG with the Na channel blocker tetrodotoxin (TTX) significantly reduced this activation at all time points. Similar findings were made with a more distal injury (spared nerve injury model), using a different sodium channel blocker (bupivacaine depot) at the injury site. Local DRG perfusion with TTX also reduced levels of nerve growth factor (NGF) in the SNL model on day 3 (when activated glia are an important source of NGF), without affecting the initial drop of NGF on day 1 (which has been attributed to loss of transport from target tissues). Local perfusion in the SNL model also significantly reduced microglia activation (OX-42 immunoreactivity) on day 3 and astrocyte activation (GFAP immunoreactivity) on day 10 in the corresponding dorsal spinal cord. The results indicate that early spontaneous activity in injured sensory neurons may play important roles in glia activation and pathological pain.

Sustained upregulation of glial fibrillary acidic protein in Müller cells in pigeon retina following disruption of the parasympathetic control of choroidal blood flow

Choroidal blood flow in pigeon eyes is light driven and controlled by a parasympathetic input from ciliary ganglion (CG) neurons that receive input from the medial subdivision of the ipsilateral nucleus of Edinger-Westphal (EWM). EWM lesions diminish basal ChBF and irreversibly prevent ipsilateral light-evoked increases in ChBF, presumably rendering the retina mildly ischemic. To characterize the location, severity, and time course of the retinal abnormality caused by an EWM lesion, we quantitatively analyzed the cellular and regional extent of Müller cell glial fibrillary acidic protein (GFAP) immunolabeling up to nearly a year after an EWM lesion. We found that unilateral EWM lesions greatly increased Müller cell GFAP throughout the entire retinal depth and topographic extent of the affected eye, up to nearly a year post lesion. By contrast, destruction of the pupilloconstrictive pretectum or of the pupilloconstrictive part of lateral EW (EWL) did not appreciably increase Müller cell GFAP. Thus, the large increase in Müller cell GFAP following an EW lesion is attributable to an ongoing defect in choroidal vasodilatory function rather than to chronic pupil dilation. The Müller cell GFAP increase was greater ipsilateral than contralateral to the EWM destruction for the retinal territory deep to the heavily CG-innervated superior and temporal choroid, but not for the retinal territory deep to the poorly CG-innervated inferior and nasal choroid. The GFAP increase was light-dependent, since it did not occur in EW-lesioned birds housed in dim illumination. Our results show that the chronic vascular insufficiency caused by the loss of the EWM-mediated parasympathetic control of choroidal blood flow leads to a significant and sustained increase in retinal Müller cell GFAP. This increase could be a sign of a disturbance in retinal homeostasis that eventually leads to retinal injury and impaired visual function.

Lower packing density of glial fibrillary acidic protein-immunoreactive astrocytes in the prelimbic cortex of alcohol-naive and alcohol-drinking alcohol-preferring rats as compared with alcohol-nonpreferring and Wistar rats

BACKGROUND

Low packing density of glial cells, possibly astrocytes, has been described in the prefrontal cortex and hippocampus of "uncomplicated" alcoholics. Astrocytes perform crucial support functions in the processing of neurotransmitters and transfer of energy substrates from blood to cortical neurons. It is still unknown whether attrition in the numbers of astrocytes is only a consequence of prolonged alcohol abuse or also predates the exposure to alcohol in subjects at risk for alcohol dependence.

METHODS

We used alcohol-preferring (P) rats exposed ad libitum for 2 or 6 months to either water only or 10% ethanol and alcohol-nonpreferring (NP) rats and nonselected Wistar rats exposed only to water for 2 months. Sections through the rat frontal cortex were immunostained for glial fibrillary acidic protein (GFAP), a specific marker of astrocytes. The packing density of GFAP-immunoreactive (IR) astrocytes and the area fraction of GFAP immunoreactivity were measured in the prelimbic cortex (PLC) using the dissector probe and analysis of binary images of GFAP immunostaining, respectively.

RESULTS

The packing density of GFAP-IR astrocytes was significantly lower in both alcohol-naive and alcohol-exposed P rats than in NP rats or Wistar rats. The area fraction of GFAP immunoreactivity was significantly lower in the alcohol-exposed P rats than in NP rats, Wistar rats, and alcohol-naive P rats.

CONCLUSION

These results suggest that low density of GFAP-IR astrocytes in the PLC of P rats predates the exposure to alcohol and might be a factor contributing to the increased risk for alcohol dependence. In addition, prolonged free-choice alcohol drinking may reduce the extent of GFAP-IR processes in the PLC of P rats.

The differential response of astrocytes within the vestibular and cochlear nuclei following unilateral labyrinthectomy or vestibular afferent activity blockade by transtympanic tetrodotoxin injection in the rat

In this study, we investigated whether changes in the vestibular neuronal activity per se influence the pattern of astrocytes morphology, glial fibrillary acidic protein (GFAP) expression and ultimately their activation within the vestibular nuclei after unilateral transtympanic tetrodotoxin (TTX) injections and after unilateral inner ear lesion. The rationale was that, theoretically the noninvasive pharmacological functional blockade of peripheral vestibular inputs with TTX, allowed us to dissociate the signals exclusively related to the shutdown of the resting activity of the first-order vestibular neurons and from neuronal signals associated with trans-ganglionic changes in first order vestibular neurons induced by unilateral labyrinthectomy (UL). Since the cochlea was removed during the surgical procedure, we also studied the astrocytic reaction within the deafferented cochlear nuclei. No significant changes in the distribution or relative levels of GFAP mRNA expression, relative levels of GFAP protein or immunoreactivity for GFAP were found in the ipsilateral vestibular nuclei at any post-TTX injection times studied. In addition, no sign of microglia activation was observed. In contrast, a robust increase of the distribution and relative levels of GFAP mRNA expression, protein levels and immunoreactivity was observed in the deafferented vestibular and cochlear nuclei beginning at 1 day after inner ear lesion. GFAP mRNA expression and immunoreactivity in the cochlear nucleus was qualitatively stronger than in the ipsilateral vestibular nuclei. The results suggest that astrocyte activation in the vestibular nuclei is not related to drastic changes of vestibular nuclei neuronal activity per se. Early trans-ganglionic changes due to vestibular nerve dendrites lesion provoked by the mechanical destruction of vestibular receptors, most probably induced the glial reaction. Its functional role in the vestibular compensation process remains to be elucidated.

Proliferation of glial cells in vivo induced in the neural lobe of the rat pituitary by lithium

Lithium salts are widely used for treatment of psychiatric illness. Lithium also affects cell proliferation. During investigation of the effect of lithium chloride on the central nervous system (CNS) of nephrectomized rats, we noted numerous mitotic figures in the neural lobe of the pituitary. Morphologic criteria established that the mitotic cells were astrocytes, the supporting glial cells of the CNS, also known as pituicytes. Equimolar doses of chlorides of chemically related cations (sodium, potassium, rubidium) had no such effect.

The exact expression of glial fibrillary acidic protein (GFAP) in trigeminal ganglion and dental pulp

The expression in various cell types of peripheral tissues of glial fibrillary acidic protein (GFAP), first discovered as an intermediate filament specific for astrocytes, remains controversial owing to numerous reports of a wide distribution for GFAP-immunoreactivity in various cells. The present study employed immunohistochemistry to investigate the precise expression of GFAP in the dental pulp and trigeminal ganglion of adult rats and wild-type mice as well as GFAP-knockout mice. The exhibition of GFAP-immunoreactivity in the trigeminal ganglion was further examined by a reverse transcription polymerase chain reaction (RT-PCR) technique, and in situ hybridization histochemistry using a specific cRNA probe prepared by us. The immunoreaction for GFAP was recognizable in the axons, Schwann cells, and the fibroblasts in the dental pulp of rats and wild-type littermate mice. However, mice with null mutations in the GFAP gene remained immunoreactive for GFAP in all these locations. Intense GFAP-immunoreactivity was found in a small number of satellite cells in the trigeminal ganglion in all animals examined in this study. RT-PCR analysis demonstrated bands for the GFAP gene corresponding to the length expected from the primer design in the samples of trigeminal ganglion and dental pulp. In situ hybridization histochemistry also showed intense signals for GFAP mRNA in some satellite cells of the trigeminal ganglion, but never in the neurons. These data suggest that the GFAP-immunoreactive molecules in the pulpal axons and fibroblasts react non-specifically with the polyclonal antibody and are probably a closely related type of intermediate filament.

Rapid morphological changes in astrocytes are accompanied by redistribution but not by quantitative changes of cytoskeletal proteins

Astrocytes have the potential to acquire very different morphologies, depending on their regional location in the CNS and on their functional interactions with other cell types. Morphological changes between a flat or a fibroblast-like and a stellate or process-bearing appearance, and vice versa, can occur rapidly, but very little is known as to whether morphological transformations are based on quantitative changes of cytoskeletal proteins in microfilaments, intermediate filaments, and/or microtubules. Using a cell culture of selective type 1 astrocytes, we compared the distribution and protein amounts of a number of cytoskeletal proteins both during primary process growth induced by specific media conditions and after secondary transformations induced by dBcAMP. Our data presented in this report support the idea that astrocytes can undergo dramatic changes in their morphology requiring subcellular redistribution of most cytoskeletal proteins but no quantitative modifications of the amount of the respective proteins. After pharmacological treatment with lysophosphatic acid and genistein we show that astrocytes can acquire intermediate morphologies reminiscent of both fibroblast and stellate-like cells. These experiments demonstrate that the recently described RhoA-mediated signaling cascade between the cell surface and cytoskeletal proteins is only one of several signaling pathways acting on the astrocytic cytoskeleton.

Effect of elevated K(+), hypotonic stress, and cortical spreading depression on astrocyte swelling in GFAP-deficient mice

Glial fibrillary acidic protein (GFAP) is the main component of intermediate filaments in astrocytes. To assess its function in astrocyte swelling, we compared astrocyte membrane properties and swelling in spinal cord slices of 8- to 10-day-old wild-type control (GFAP(+/+)) and GFAP-knockout (GFAP(-/-)) mice. Membrane currents and K(+) accumulation around astrocytes after a depolarizing pulse were studied using the whole-cell patch-clamp technique. In vivo cell swelling was studied in the cortex during spreading depression (SD) in 3 to 6-month-old animals. Swelling-induced changes of the extracellular space (ECS) diffusion parameters, i.e., volume fraction alpha and tortuosity lambda, were studied by the real-time iontophoretic tetramethylammonium (TMA(+)) method using TMA(+)-selective microelectrodes. Morphological analysis using confocal microscopy and quantification of xy intensity profiles in a confocal plane revealed a lower density of processes in GFAP(-/-) astrocytes than in GFAP(+/+) astrocytes. K(+) accumulation evoked by membrane depolarization was lower in the vicinity of GFAP(-/-) astrocytes than GFAP(+/+) astrocytes, suggesting the presence of a larger ECS around GFAP(-/-) astrocytes. Astrocyte swelling evoked by application of 50 mM K(+) or by hypotonic solution (HS) produced a larger increase in [K(+)](e) around GFAP(+/+) astrocytes than around GFAP(-/-) astrocytes. No differences in alpha and lambda in the spinal cord or cortex of GFAP(+/+) and GFAP(-/-) mice were found; however, the application of either 50 mM K(+) or HS in spinal cord, or SD in cortex, evoked a large decrease in alpha and an increase in lambda in GFAP(+/+) mice only. Slower swelling in GFAP(-/-) astrocytes indicates that GFAP and intermediate filaments play an important role in cell swelling during pathological states.

Glutamate, NMDA, and AMPA induced changes in extracellular space volume and tortuosity in the rat spinal cord

Glutamate release, particularly in pathologic conditions, may result in cellular swelling. The authors studied the effects of glutamate, N-methyl-D-aspartate (NMDA), and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) on extracellular pH (pH(e)), extracellular potassium concentration ([K(+)](e)), and changes in extracellular space (ECS) diffusion parameters (volume fraction alpha, tortuosity lambda) resulting from cellular swelling. In the isolated spinal cord of 4-to 12-day-old rats, the application of glutamate receptor agonists induced an increase in [K(+)](e), alkaline-acid shifts, a substantial decrease in alpha, and an increase in lambda. After washout of the glutamate receptor agonists, alpha either returned to or overshot normal values, whereas lambda remained elevated. Pretreatment with 20 mmol/L Mg(++), MK801, or CNQX blocked the changes in diffusion parameters, [K(+)](e) and pH(e) evoked by NMDA or AMPA. However, the changes in diffusion parameters also were blocked in Ca(2+)-free solution, which had no effect on the [K(+)](e) increase or acid shift. The authors conclude that increased glutamate release may produce a large, sustained and [Ca(2+)](e)-dependent decrease in alpha and increase in lambda. Repetitive stimulation and pathologic states resulting in glutamate release therefore may lead to changes in ECS volume and tortuosity, affecting volume transmission and enhancing glutamate neurotoxicity and neuronal damage.

Increased potassium concentration inhibits stimulation of vascular smooth muscle proliferation by PDGF-BB and bFGF

The effects of changes in extracellular potassium concentration on the rate of vascular smooth muscle cell proliferation stimulated by cytokines and serum were analyzed in vitro. To analyze the DNA synthesis response, cells from swine coronary artery were grown in DMEM medium containing 3, 4, 5, or 6 mmol/L potassium together with 20 ng/mL platelet-derived growth factor BB (PDGF-BB), 25 ng/mL basic fibroblast growth factor (bFGF), or 5% fetal bovine serum (FBS), with [methyl 3H] thymidine added, for 24 h. Proliferation responses were analyzed in cells grown in medium with potassium concentrations of 3, 4, 5, or 6 mmol/L, together with either 20 ng/mL PDGF-BB, 25 ng/mL bFGF, or 5% FBS, for 7 days, then harvested and counted. Highly significant inverse relationships were observed between potassium concentration and 3H-thymidine incorporation stimulated by each of the three agonists (P < .01 for each, ANOVA), and between potassium concentration and proliferation (all P < .01, ANOVA). Elevation of potassium concentration within the physiologic range inhibits vascular smooth muscle cell DNA synthesis and proliferation.

Quantitative immunochemistry on neuronal loss, reactive gliosis and BBB damage in cortex/striatum and hippocampus/amygdala after systemic kainic acid administration

Cell specific markers were quantified in the hippocampus, the amygdala/pyriform cortex, the frontal cerebral cortex and the striatum of the rat brain after systemic administration of kainic acid. Neuron specific enolase (NSE) reflects loss of neurons, glial fibrillary acidic protein (GFAP) reflects reactive gliosis, and brain levels of serum proteins measures blood-brain-barrier permeability. While the concentration of NSE remained unaffected in the frontal cerebral cortex and the striatum, their GFAP content increased during the first three days. In the hippocampus and amygdala, NSE levels decreased significantly. GFAP levels in the hippocampus were unaffected after one day and decreased in the amygdala/pyriform cortex. After that, GFAP increased strikingly until day 9 or, in the case of amygdala/pyriform cortex, even longer. This biphasic time course for GFAP was accompanied by a decrease of S-100 during days 1-9 followed by a significant increase at day 27 above the initial level. The regional differences in GFAP and S-100 could result from the degree of neuronal degeneration, the astrocytic receptor set-up and/or effects on the blood-brain barrier.

Glial swelling and astrogliosis produce diffusion barriers in the rat spinal cord

Cell swelling and astrogliosis (manifested as an increase in GFAP) were evoked in isolated rat spinal cords of 4-21-day-old rats by incubation in either 50 mM K+ or hypotonic solution (235 mosmol kg(-1)). Application of K+ and hypotonic solution resulted at first in a decrease of extracellular space (ECS) volume fraction alpha (ECS volume/total tissue volume) and an increase in tortuosity lambda (lambda2 = free/apparent diffusion coefficient) in spinal gray (GM) and white matter (WM). These changes resulted from cell swelling, since the total water content (TW) in spinal cord was unchanged and the changes were blocked in Cl- -free solution and slowed down by furosemide and bumetanide. Diffusion in WM was anisotropic, i.e., more facilitated along fibers (x-axis) than across them (y- or z-axis). The increase of lambda(y,z) was greater than that of lambda(x), reaching unusually high values above 2.4. In GM only, during continuous 45 min application, alpha and lambda started to return towards control values, apparently due to cell shrinkage of previously swollen cells since TW remained unchanged. This return was blocked by fluoroacetate, suggesting that most of the changes were due to the swelling of glia. A 45 min application of 50 mM K+ and, to a lesser degree, of hypotonic solution evoked astrogliosis, which persisted after washing out these solutions with physiological saline. During astrogliosis lambda increased again to values as high as 2.0, while alpha either returned to or increased above control values. This persistent increase in lambda after washout was also found in WM, and, in addition, the typical diffusion anisotropy was diminished. Our data show that glial swelling and astrogliosis are associated with a persistent increase in ECS diffusion barriers. This could lead to the impairment of the diffusion of neuroactive substances, extrasynaptic transmission, "crosstalk" between synapses and neuron-glia communication.

Molecular characterization of glioblastoma cell differentiation

OBJECTIVE

Induction of cellular differentiation continues to be an attractive therapeutic strategy for malignant glioma. The purpose of this study was to develop a convenient in vitro model system for glioblastoma differentiation and to then characterize it using conventional techniques and flow cytometry.

METHODS

A subline of U138 MG cells ("U138B") was treated with 0 to 4 mmol/L sodium butyrate (or serum deprivation) for up to 96 hours. Cells were initially studied for effects on proliferation, morphology, and glial fibrillary acidic protein (GFAP) staining. Northern blot and immunoblot analyses of c-myc expression were performed. Multiparameter flow cytometry was then used to analyze GFAP, c-myc protein, and total cellular protein fluorescence and to relate them to changes in cell cycle distribution.

RESULTS

Butyrate treatment produced a dose-dependent inhibition of cellular proliferation and changes in morphology, GFAP staining, and c-myc expression consistent with a differentiation response. Detailed flow cytometric studies, including subpopulation analysis, showed that during 72 hours of treatment with 2 mmol/L butyrate, mean GFAP fluorescence increased to 420%, whereas c-myc protein decreased to 45 +/- 13% and total cellular protein increased to 181 +/- 17%. The effects of butyrate were distinct from those of serum deprivation and were not simply the result of cells shifting into Gzero/G1.

CONCLUSION

The butyrate-induced responses of the U138B cell line provide a convenient model system for studying the molecular events accompanying the differentiation of glioblastoma cells. Multiparameter flow cytometry is a useful technique for characterizing such differentiation.

The reaction of astrocytes and neurons in the hippocampus of adult rats during chronic ethanol treatment and correlations to behavioral impairments

Chronic ethanol treatment of Wistar rats to 10% (v/v) ethanol over a period of 4, 12, and 36 weeks produced distinct alterations of the glial fibrillary acidic protein immunoreactivity (GFAP-IR) of dorsal hippocampal astrocytes. Ethanol consumption over a period of 4 weeks caused an increase in the total GFAP-IR of the astrocytes. Down-regulation of the total GFAP-IR was measured in all examined brain regions after 36 weeks of ethanol treatment. Prolonged ethanol treatment induced a significant loss of the total number of hippocampal pyramidal and dentate gyrus granule cells. Regional differences in the vulnerability to the neurotoxic effects of chronic ethanol intake over 36 weeks were found: CA3 > CA1 + CA2 > > CA4 > GD. In agreement with the degree of neuronal cell loss, ethanol-induced behavioral impairments were found. The acquisition of maze performance using a complex elevated labyrinth was deteriorated after 36 weeks of ethanol treatment, suggesting a deficit in learning and memory. These findings illustrate the importance of time-response analysis when determining the structural and functional changes produced by chronic ethanol treatment.

Age-related changes in GFAP-immunoreactive astrocytes in the rat ventral cochlear nucleus

The age-related changes in the ventral cochlear nucleus (VCN) as revealed by glial fibrillary acid protein (GFAP) immunoreactivity were analyzed in the following age groups: 3-, 6-, 12-, 18-, and 24-month-old Sprague-Dawley rats. A cartographic and a quantitative analysis showed a significant increase in the number of GFAP positive astrocytes during the first year of life and a significant decrease in older rats. We also observed an age-induced modification in the spatial distribution of GFAP positive astrocyte. In the anterior part of the VCN of the 3- and 6-month-old rats, we observed a significant decrease in the rostro-caudal as well in the dorso-ventral axes. In the posterior part of the VCN, a significant decrease in the dorso-ventral axis could be also observed, but no significant difference in the spatial distribution was obtained in the rostro-caudal axis. In older rats, the distribution appeared homogeneous throughout the nucleus. Additionally, aging was associated with a significant increase in GFAP positive astrocyte sizes, except for immunolabelled astrocytes in the granule cell layer. The different levels of GFAP expression occurring in the VCN during normal aging could reflect a progressive decline of cellular activity in the VCN, without severe cell degeneration or synaptic loss.

K+ channel expression and cell proliferation are regulated by intracellular sodium and membrane depolarization in oligodendrocyte progenitor cells

The effects of a variety of antiproliferative agents on voltage-dependent K+ channel function in cortical oligodendrocyte progenitor (O-2A) cells were studied. Previously, we had shown that glutamate receptor activation reversibly inhibited O-2A cell proliferation stimulated by mitogenic factors and prevented lineage progression by attenuating outward K+ currents in O-2A cells. We now show that the antiproliferative actions of glutamate receptor activation are Ca2+-independent and arise from an increase in intracellular Na+ and subsequent block of outward K+ currents. In support of this mechanism, agents that acted to depolarize O-2A cells or increase intracellular sodium similarly had an antiproliferative effect, attributable at least in part to a reduction in voltage-gated K+ currents. Also, these effects were reversible and Ca2+-independent. Chronic treatment with glutamate agonists was without any long-term effect on K+ current function. Cells cultured in elevated K+, however, demonstrated an upregulation of inward rectifier K+ currents, concomitant with an hyperpolarization of the resting membrane potential. This culture condition therefore promoted a current phenotype typical of pro-oligodendroblasts. Finally, cells chronically treated with the mitotic inhibitor retinoic acid displayed a selective downregulation of outward K+ currents. In conclusion, signals that affect O-2A cell proliferation do so by regulating K+ channel function. These data indicate that the regulation of K+ currents in cells of the oligodendrocyte lineage plays an important role in determining their proliferative potential and demonstrate that O-2A cell K+ current phenotype can be modified by long-term depolarization of the cell membrane.

Soman-induced seizures rapidly activate astrocytes and microglia in discrete brain regions

Neurons in the piriform cortex and the pontine nucleus locus coeruleus express elevated levels of the immediate early gene protein product, Fos, within 30-45 minutes of a seizurogenic dose of the anticholinesterase, soman (Zimmer et al., [1997] J. Comp. Neurol. 378:468-481). By 24 hours following soman injection, there is marked neuropathology in the piriform cortex. These findings suggest selective, regional vulnerability in response to the seizurogenic actions of soman. In the present study, we determined that soman-induced seizures also cause selective, rapid activation of astrocytes and microglia in the piriform cortex and other brain regions. Animals were killed at different intervals between 1 hour and 24 hours after a convulsive dose of soman. Brain sections were processed for immunocytochemical detection of astrocytes with antibodies against glial fibrillary acidic protein, and microglia and macrophages with antibodies against the complement receptor 3 protein, OX-42. The results demonstrate that following soman administration: (1) there is a rapid increase in glial fibrillary acidic protein staining in astrocytes of the piriform cortex (1 hour); (ii) reactive astrocytes are specifically restricted to layer II and the superficial boundaries of layer III of the piriform cortex. These are the same layers in which neurons express Fos within 30-45 minutes following soman administration; (3) between 1 and 4 hours, resting (ramified) microglia in the piriform cortex and the hippocampus alter their morphology to resemble active microglia. From 4-8 hours, active microglia undergo morphological changes characteristic of reactive microglia that resemble macrophages. Taken together, these observations indicate that astrocytes and microglia in brain regions susceptible to soman become rapidly "reactive" in response to seizures. The highly specific anatomical codistribution of reactive glia and Fos-expressing neurons suggests that intensely active neurons provide local signals that trigger reactive changes in neighboring glia.

Evidence for reactive astrocytes in rat vestibular and cochlear nuclei following unilateral inner ear lesion

We investigated whether unilateral removal of the labyrinthine and cochlear receptors induces a macroglial reaction in rat vestibular and cochlear nuclei using vimentin and glial fibrillary acidic protein (GFAP) immunochemical markers. Antibody binding was visualized using the avidin-biotin method and 3,3'-diaminobenzidine as the peroxidase substrate. In addition, double-labelling experiments were performed using specific secondary fluorescent antibodies. Potentially degenerating axon terminals were also studied using a silver impregnation method. In normal adult rats, vimentin was found only in ependymal cells, tanicytes around the fourth ventricle, endothelial cells in the blood vessels and Bergmann glia in the molecular layer of the cerebellum. In lesioned rats, all deafferented vestibular and ventral cochlear nuclei showed strong vimentin immunoreactivity. Furthermore, double-labelling experiments demonstrated that these vimentin-positive cells were also GFAP-positive. The reaction became evident on the second day after the lesion, was intense for 3-8 days and then declined until day 21. No vimentin immunoreactivity could be detected at the level of the ipsilateral dorsal cochlear nucleus. Therefore, unilateral inner ear lesion induced an astroglial reaction within the deafferented vestibular and cochlear nuclei. The decrease in the resting discharge of the primary vestibular afferents and/or in the deafferented central vestibular neurons may induce the glial reaction in the vestibular complex, whereas both degeneration and silence of the cochlear nerve and central cochlear neurons are most probably responsible for the cochlear vimentin-immunoreactive staining. The role of the reactive astrocytes in the vestibular compensation process remains to be determined.

Selective ablation of astrocytes by intracerebral injections of alpha-aminoadipate

The efficacy and the specificity of the putative astrotoxin, alpha-aminoadipate, were examined in this study. The integrity of astrocytes was evaluated at several time points following a single injection of alpha-aminoadipate into amygdala of adult rats using immunohistochemistry. The density and the morphological appearance of neurons and the response of microglia were also examined. The injection of alpha-aminoadipate disrupted the astrocytic network in that region. There was a profound loss of glial fibrillary acidic protein-positive and S100 beta-positive astrocytes, normally present in the region, while vimentin immunohistochemistry revealed the presence of deformed cell processes, presumably astrocytic. The presence of reactive microglia at the injection site was suggestive of an active degenerative process, while the normal neuronal density and appearance, as compared to controls, suggested that the damage was confined to astrocytes. The confirmed effectiveness and cellular specificity of alpha-aminoadipate in vivo makes it a potentially important experimental tool for attempting to decipher the functional significance of astrocytes.

Effect of extracellular K+ on the intracellular free Ca2+ concentration in leech glial cells and Retzius neurones

The effects of extracellular K+ on the intracellular free Ca2+ concentration ([Ca2+]i) of neuropile glial cells and Retzius neurones in intact segmental ganglia of the leech Hirudo medicinalis were investigated by using iontophoretically injected fura-2. In both cell types, an elevation of the extracellular K+ concentration ([K+]o) caused an increase in [Ca2+]i, which was blocked by Co2+, Ni2+ and menthol, whereas nicardipine, flunarizine, omega-conotoxin GVIA and omega-agatoxin IVA were ineffective. In Ca(2+)-free solution, the K(+)-induced [Ca2+]i increase was largely suppressed in neuropile glial cells and completely abolished in Retzius neurones. The results indicate that the K(+)-induced [Ca2+]i increase was mainly due to Ca2+ influx through voltage-dependent Ca2+ channels. The Ca2+ channels of the two cell types were activated at different membrane potentials but at the same [K+]o. In both cell types, the recovery from a K(+)-induced [Ca2+]i increase was unaltered in Na(+)-free solution, indicating that active Ca2+ transport across the plasma membrane is mediated by Na(+)-independent mechanisms.

Dehydration-induced proliferation of identified pituicytes in fully adult rats

The mitotic activity astrocytes in the adult central nervous system (CNS) is of interest due to their roles in gliosis and tumorigenesis, and their potential in aiding recovery of function following injury or disease. The posterior pituitary offers a potentially powerful model to study proliferation in vivo, since its resident astrocytes, called pituicytes, have been reported to divide concurrently with hormone release from the neurosecretory terminals there. our aim in this study was to confirm and characterize this proliferative response during dehydration and rehydration in fully adult animals using contemporary techniques. Adult male rats were given 2% saline in substitution for water for 0-9 days. Proliferation of pituicytes was quantified in tissue sections triple-labeled with the proliferation marker, 5-bromodeoxyuridine (BrdU), the astrocyte marker glial fibrillary acidic protein (GFAP), and the DNA marker 4,6,diamidino-2-phenylindole, HCL(DAPI). A robust proliferative response began within three days of dehydration and continued at a constant rate thereafter. In animals allowed to rehydrate, this response continued. After 9 days of dehydration, approximately 35% of pituicytes had participated in mitosis. While cell density remained constant across conditions, a reversible increase in posterior pituitary area was seen, suggesting that some cell death also occurs simultaneously. A significant proportion of non-pituicytes also underwent similar changes. These results indicate that pituicytes in the adult posterior pituitary retain characteristics necessary for reentering the cell cycle in response to local factors present during neurosecretory activity. We hypothesize that this proliferative response is directly related to the morphological changes previously reported for these cells under activating conditions.

Voltage-dependent clamp of intracellular pH of identified leech glial cells

1. The intracellular pH (pHi) was measured in voltage-clamped, giant neuropile glial cells in isolated segmental ganglia of the leech Hirudo medicinalis, using double-barrelled, pH-sensitive microelectrodes and a slow, two-electrode voltage-clamp system. The potential sensitivity of the pHi regulation in these glial cells was found to be due to an electrogenic Na(+)-HCO3- cotransporter (Deitmer & Szatkowski, 1990). 2. In the presence of 5% CO2 and 24 mM HCO3- (pH 7.4), pHi shifted by 1 pH unit per 110 mV, corresponding to a stoichiometry of 2HCO3-: 1 Na+ of the cotransporter, while in Hepes-buffered CO2-HCO3(-)-free saline (pH 7.4), pHi changed by 1 pH unit per 274 mV. The potential sensitivity of pHi decreased at lower pHo, being 1 pH unit per 216 mV at external pH (pHo) 7.0. 3. Changing pHo between 7.8 and 6.6 induced pHi shifts with a slope of 0.72 pHi units per pHo unit in non-clamped, and of 0.80 pHi units per pHo unit in voltage-clamped cells, indicating that pHi largely followed pHo. The electrochemical gradient of H(+)-HCO3- across the glial membrane was around 56 mV, and remained almost constant over this pHo range. 4. The membrane potential-dependent and pHo-sensitive shifts of pHi were unaffected by amiloride, an inhibitor of Na(+)-H+ exchange. 5. The intracellular acidification upon lowering pHo could be reversed by depolarizing the membrane as predicted from a cotransporter, whose equilibrium follows the membrane potential by resetting pHi. 6. The results indicate that the pHi of leech glial cells is dominated by the electrogenic Na(+)-HCO3- cotransporter, and is hence a function of the membrane potential, and the Na+ and H(+)-HCO3- gradients, across the cell membrane.

Postnatal development of glial fibrillary acidic protein, vimentin and S100 protein in monkey visual cortex: evidence for a transient reduction of GFAP immunoreactivity

In the cerebral cortex of some species, the gradual appearance of glial fibrillary acidic protein (GFAP) is often interpreted as reflecting the parallel maturation of neuronal connectivity. We studied the postnatal maturation of astrocytes in the primary visual cortex of Callithrix jacchus using antibodies against GFAP, vimentin and S100 protein as immunohistochemical markers. In the cortical grey matter of this species, the overall GFAP-immunoreactivity (IR) as measured by image analysis is high at birth (130% of the adult value), decreases until about 3 months (80%) and increases again towards adult values (100%). Vimentin-IR was high at birth, and declined towards 3 months and later. In contrast, S100-IR augmented postnatally in neuropil, and showed a laminar shift of maximum IR from layer IV to supragranular layers during ontogenesis. The decrease of GFAP-IR is predominantly due to changes in density of GFAP-positive (+) astrocytes within cortical tissue (newborn: 18,600 GFAP+astrocytes/mm3; 1 month: 11,600/mm3; 3 months: 5,700/mm3; adult: 10,200/mm3), while the overall number of astrocytes remained relatively constant as shown by the number of S100-positive astrocytic cell bodies. At times of low GFAP-IR a reduced area density of intermediate filaments was found in astrocytes by electron microscopy. The period of reduced GFAP-expression coincides with the time of prominent synapse remodeling in the visual cortex of marmosets. These data suggest that GFAP-expression may depend on functional conditions rather than time-dependent maturation.

Potassium-dependent calcium influx in acutely isolated hippocampal astrocytes

Potassium depolarization can increase the intracellular ionized calcium concentration ([Ca2+]i) of cultured astrocytes, but it is not known if astrocytes that have matured in the intact CNS also exhibit voltage-dependent [Ca2+]i signalling. To address this issue, fluorometric measurements of [Ca2+]i were obtained from astrocytes acutely isolated from young adult rat hippocampus. In control artificial cerebrospinal fluid containing 5 mM [K+]o, average resting [Ca2+]i was 195 nM. Elevation of [K+]o to 50 mM caused [Ca2+]i to increase 150 nM to 1 microM above resting levels. The threshold [K+]o necessary to evoke an elevation in [Ca2+]i was 20-25 mM, and the magnitude of the [Ca2+]i signal grew progressively with increasing [K+]o (up to 50 mM). These [Ca2+]i increases were blocked completely by removal of external Ca2+, and markedly suppressed by the calcium channel blockers verapamil (30 microM and greater) and Co2+ (1 mM). Neither reversal of Na(+)-Ca2+ exchange, nor Ca(2+)-activated Ca2+ release, nor Ca2+ influx through stretch-activated channels contributed to the [Ca2+]i increase. These results suggest that [K+]o-evoked [Ca2+]i signals are mediated by influx through voltage-gated calcium channels. In contrast to results from cultured astrocytes and acutely isolated neurons, these [Ca2+]i increases were insensitive to dihydropyridine compounds. We conclude that increases in interstitial [K+], observed in situ during several pathological conditions, trigger voltage-dependent [Ca2+]i signals in astroglial cells. This may constitute an important form of neuron-to-glial communication.

GFAP mRNA increases with age in rat and human brain

Glial fibrillary acidic protein (GFAP) mRNA was examined by RNA blot hybridization in three age groups of two cohorts of male F-344 rats and in 47 human postmortem brain samples. GFAP mRNA increased in the hippocampus and striatum of 24 versus 6- to 7-month-old rats. Another astrocytic molecular marker, glutamine synthetase mRNA, did not change with age in rat brain. Rat GFAP mRNA prevalence was inversely correlated with serum testosterone but not correlated with serum corticosterone. In human hippocampus, frontal and temporal cortex, GFAP mRNA also increased in older (60-79 years) compared with middle-aged (25-59 years) individuals. In contrast, mitochondrial cytochrome oxidase subunit 1 mRNA did not change between age groups in any region. By combining the three regions for further analysis, GFAP mRNA increased with age irregardless of gender, alcoholism in the middle-aged group, or whether brains were classified as normal or neuropathologic (excluding Alzheimer's disease pathology). These data indicate that increased GFAP protein or GFAP-immunoreactive astrocytes in rats and humans may result from transcriptional or post-transcriptional regulation and extend the number to three species (including mouse) showing an increase in GFAP mRNA with age. Factors that are known to regulate GFAP mRNA expression in young brains are considered as possible causes of age-related increases.

MK-801 affects the potassium-induced increase of glial fibrillary acidic protein immunoreactivity in rat brain

Exposure of a limited brain surface to a high potassium (K+) concentration produces an injury limited to the underlying cortex, without apparently affecting other brain areas. Such a treatment produces an increased expression of glial fibrillary acidic protein (GFAP) in astrocytes, as assessed by immunohistochemical techniques, throughout the cortex ipsilateral to K+ exposure. This effect is evident 2 days after treatment and persists up to, at least, day 7. Thirty days after K+ exposure GFAP immunostaining is similar in both hemispheres. Administration of the non-competitive NMDA antagonist MK-801 (4 mg/kg i.p.) prior to the injury prevented the rise in GFAP immunoreactivity (IR) at 2 but not 7 days after the treatment. Administration of MK-801 after the injury appeared to have no effect on GFAP expression. This work confirms that brain injury, associated with spreading depression, can induce a glial response far from the lesion site. Furthermore, the fact that this phenomenon can be modified by an NMDA receptor antagonist suggests that glutamate may play a role, in vivo, in the regulation of astrocytic response to injury and introduces the possibility that brain injury-induced gliosis may be pharmacologically manipulated.

Rapid growth of astrocytic processes in N. magnocellularis following cochlea removal

Removal of the cochlea or pharmacological blockade of eighth nerve activity in young postnatal chickens results in rapid transneuronal cell death and atrophy in neurons of n. magnocellularis. The present experiments were designed to examine the influence of afferent input on astrocyte structure in n. magnocellularis. Young chickens were subjected to unilateral cochlea removal. At times ranging from 5 minutes to 72 hours later, the brainstems were histologically processed with a polyclonal antibody against glial fibrillary acidic protein (GFAP). A second group of chick brainstems was impregnated by a Golgi method 6 hours after unilateral cochlea removal and impregnated three-dimensional reconstructions were made of glial cells in n. magnocellularis (NM). Analyses of GFAP positive processes in NM revealed an observable increase in the number of astrocytic processes at the borders of the nucleus within 30 minutes of cochlea removal and a twofold increase in GFAP + glial processes by 6 hours. A secondary increase in the number and density of GFAP + processes occurred between 24 and 72 hours following cochlea removal, during the period of axonal degeneration, and transneuronal cell atrophy and death. Analyses of astrocytes impregnated by the Golgi method revealed that individual glial cells had increased their total process length and the number of processes by approximately twofold by 6 hours after cochlea removal. These results suggest that the structure of astrocytes is rapidly and dramatically influenced by the level of excitatory activity in a neuronal system. Furthermore, the similarity of results obtained with GFAP immunohistochemistry and three-dimensional reconstruction of astrocytes provides evidence that the short-term changes observed following cochlea removal represent the actual growth of glial processes. We speculate that modulations in glial processes as a function of afferent activity may act to influence synaptic efficacy.

Neuronal activity up-regulates astroglial gene expression

Neuronal gene expression is known to be modulated by functional activity. This modulation is thought to play a key role in determining the differentiation of developing neurons and regulating the operation of mature neurons. Here we describe a regulation of astroglial gene expression by neuronal activity. We report that intense neuronal activity (electrically induced seizures) in rat hippocampus leads to rapid and dramatic increases in mRNA for glial fibrillary acidic protein (GFAP), an astroglia-specific intermediate filament protein. GFAP mRNA levels increased at sites of stimulation as well as in areas that were synaptically activated by the resultant seizures. When seizures were induced repetitively for many days, levels of GFAP mRNA remained chronically elevated. However, GFAP mRNA returned to control levels within a few days after the cessation of stimulation. The coupling between astroglial gene expression and neuronal activity may be a mechanism through which neuronal activity modulates the function of supporting cells that are responsible for regulating the extracellular microenvironment of the brain.