Mechanical strain injury increases intracellular sodium and reverses Na+/Ca2+ exchange in cortical astrocytes

Voltage-gated calcium channel antagonists and traumatic brain injury

Traumatic brain injury (TBI) is a leading cause of death and disability in the United States. Despite more than 30 years of research, no pharmacological agents have been identified that improve neurological function following TBI. However, several lines of research described in this review provide support for further development of voltage gated calcium channel (VGCC) antagonists as potential therapeutic agents. Following TBI, neurons and astrocytes experience a rapid and sometimes enduring increase in intracellular calcium ([Ca²⁺]_i). These fluxes in [Ca²⁺]_i drive not only apoptotic and necrotic cell death, but also can lead to long-term cell dysfunction in surviving cells. In a limited number of in vitro experiments, both L-type and N-type VGCC antagonists successfully reduced calcium loads as well as neuronal and astrocytic cell death following mechanical injury. In rodent models of TBI, administration of VGCC antagonists reduced cell death and improved cognitive function. It is clear that there is a critical need to find effective therapeutics and rational drug delivery strategies for the management and treatment of TBI, and we believe that further investigation of VGCC antagonists should be pursued before ruling out the possibility of successful translation to the clinic.

Sodium fluxes and astroglial function

Astrocytes exhibit their excitability based on variations in cytosolic Ca(2+) levels, which leads to variety of signalling events. Only recently, however, intracellular fluctuations of more abundant cation Na(+) are brought in the limelight of glial signalling. Indeed, astrocytes possess several plasmalemmal molecular entities that allow rapid transport of Na(+) across the plasma membrane: (1) ionotropic receptors, (2) canonical transient receptor potential cation channels, (3) neurotransmitter transporters and (4) sodium-calcium exchanger. Concerted action of these molecules in controlling cytosolic Na(+) may complement Ca(2+) signalling to provide basis for complex bidirectional astrocyte-neurone communication at the tripartite synapse.

Primary cultures of astrocytes: their value in understanding astrocytes in health and disease

During the past few decades of astrocyte research it has become increasingly clear that astrocytes have taken a central position in all central nervous system activities. Much of our new understanding of astrocytes has been derived from studies conducted with primary cultures of astrocytes. Such cultures have been an invaluable tool for studying roles of astrocytes in physiological and pathological states. Many central astrocytic functions in metabolism, amino acid neurotransmission and calcium signaling were discovered using this tissue culture preparation and most of these observations were subsequently found in vivo. Nevertheless, primary cultures of astrocytes are an in vitro model that does not fully mimic the complex events occurring in vivo. Here we present an overview of the numerous contributions generated by the use of primary astrocyte cultures to uncover the diverse functions of astrocytes. Many of these discoveries would not have been possible to achieve without the use of astrocyte cultures. Additionally, we address and discuss the concerns that have been raised regarding the use of primary cultures of astrocytes as an experimental model system.

Guilty molecules, guilty minds? The conflicting roles of the innate immune response to traumatic brain injury

Traumatic brain injury (TBI) is a complex disease in the most complex organ of the body, whose victims endure lifelong debilitating physical, emotional, and psychosocial consequences. Despite advances in clinical care, there is no effective neuroprotective therapy for TBI, with almost every compound showing promise experimentally having disappointing results in the clinic. The complex and highly interrelated innate immune responses govern both the beneficial and deleterious molecular consequences of TBI and are present as an attractive therapeutic target. This paper discusses the positive, negative, and often conflicting roles of the innate immune response to TBI in both an experimental and clinical settings and highlights recent advances in the search for therapeutic candidates for the treatment of TBI.

Multi-scale mechanics of traumatic brain injury: predicting axonal strains from head loads

The length scales involved in the development of diffuse axonal injury typically range from the head level (i.e., mechanical loading) to the cellular level. The parts of the brain that are vulnerable to this type of injury are mainly the brainstem and the corpus callosum, which are regions with highly anisotropically oriented axons. Within these parts, discrete axonal injuries occur mainly where the axons have to deviate from their main course due to the presence of an inclusion. The aim of this study is to predict axonal strains as a result of a mechanical load at the macroscopic head level. For this, a multi-scale finite element approach is adopted, in which a macro-level head model and a micro-level critical volume element are coupled. The results show that the axonal strains cannot be trivially correlated to the tissue strain without taking into account the axonal orientations, which indicates that the heterogeneities at the cellular level play an important role in brain injury and reliable predictions thereof. In addition to the multi-scale approach, it is shown that a novel anisotropic equivalent strain measure can be used to assess these micro-scale effects from head-level simulations only.

In vitro models of traumatic brain injury

In vitro models of traumatic brain injury (TBI) are helping elucidate the pathobiological mechanisms responsible for dysfunction and delayed cell death after mechanical stimulation of the brain. Researchers have identified compounds that have the potential to break the chain of molecular events set in motion by traumatic injury. Ultimately, the utility of in vitro models in identifying novel therapeutics will be determined by how closely the in vitro cascades recapitulate the sequence of cellular events that play out in vivo after TBI. Herein, the major in vitro models are reviewed, and a discussion of the physical injury mechanisms and culture preparations is employed. A comparison between the efficacy of compounds tested in vitro and in vivo is presented as a critical evaluation of the fidelity of in vitro models to the complex pathobiology that is TBI. We conclude that in vitro models were greater than 88% predictive of in vivo results.

Dual inhibition of sodium-mediated proton and calcium efflux triggers non-apoptotic cell death in malignant gliomas

Malignant glioma cells maintain an elevated intracellular pH (pH(i)) within hypoxic-ischemic tumor microenvironments through persistent activation of sodium-proton transport (McLean et al., 2000). Amiloride has been reported to selectively kill human malignant glioma cell lines but not primary astrocytes (Hegde et al., 2004). While amiloride reduces pH(i) of malignant gliomas by inhibiting isoform 1 of sodium-proton exchange (NHE1), direct acidification was shown to be cytostatic rather than cytotoxic. At cytotoxic concentrations, amiloride has multiple drug targets including inhibition of NHE1 and sodium-calcium exchange. Amiloride's glioma cytotoxicity can be explained, at least in part, by dual inhibition of NHE1 and of Na(+)-dependent calcium efflux by isoform 1.1 of the sodium-calcium exchanger (NCX1.1), which increases [Ca(2+)](i) and initiates glioma cell demise. As a result of persistent NHE1 activity, cytosolic free levels of sodium ([Na(+)](i)) in U87 and C6 glioma cells are elevated 3-fold, as compared with normal astrocytes. Basal cytosolic free calcium levels ([Ca(2+)](i)) also are increased 5-fold. 2', 4'-dichlorobenzamil (DCB) inhibits the sodium-dependent calcium transporter (NCX1.1) much more potently than NHE1. DCB was employed in a concentration-dependent fashion in glioma cells to selectively inhibit the forward mode of NCX1.1 at ≤1μM, while dually inhibiting both NHE1 and NCX1.1 at ≥20μM. DCB (1μM) was not cytotoxic to glioma cells, while DCB (20μM) further increased basal elevated levels of [Ca(2+)](i) in glioma cells that was followed by cell demise. Cariporide and SEA0400 are more selective inhibitors of NHE1 and NCX1.1 than amiloride or DCB, respectively. Individually, Cariporide and SEA0400 are not cytotoxic, but in combination induced glioma cell death. Like amiloride, the combination of Cariporide and SEA0400 produced glioma cell death in the absence of demonstrable caspase activation.

In-vitro approaches for studying blast-induced traumatic brain injury

Traumatic brain injury caused by explosive or blast events is currently divided into four phases: primary, secondary, tertiary, and quaternary blast injury. These phases of blast-induced traumatic brain injury (bTBI) are biomechanically distinct, and can be modeled in both in-vivo and in-vitro systems. The purpose of this review is to consider the mechanical phases of bTBI, how these phases are reproduced with in-vitro models, and to review findings from these models to assess how each phase of bTBI can be examined in more detail. Highlighted are some important gaps in the literature that may be addressed in the future to better identify the exact contributing mechanisms for bTBI. These in-vitro models, viewed in combination with in-vivo models and clinical studies, can be used to assess both the mechanisms and possible treatments for this type of trauma.

Membrane trauma and Na+ leak from Nav1.6 channels

During brain trauma, white matter experiences shear and stretch forces that, without severing axons, nevertheless trigger their secondary degeneration. In central nervous system (CNS) trauma models, voltage-gated sodium channel (Nav) blockers are neuroprotective. This, plus the rapid tetrodotoxin-sensitive Ca2+ overload of stretch-traumatized axons, points to "leaky" Nav channels as a pivotal early lesion in brain trauma. Direct effects of mechanical trauma on neuronal Nav channels have not, however, been tested. Here, we monitor immediate responses of recombinant neuronal Nav channels to stretch, using patch-clamp and Na+-dye approaches. Trauma constituted either bleb-inducing aspiration of cell-attached oocyte patches or abrupt uniaxial stretch of cells on an extensible substrate. Nav1.6 channel transient current displayed irreversible hyperpolarizing shifts of steady-state inactivation [availability(V)] and of activation [g(V)] and, thus, of window current. Left shift increased progressively with trauma intensity. For moderately intense patch trauma, a approximately 20-mV hyperpolarizing shift was registered. Nav1.6 voltage sensors evidently see lower energy barriers posttrauma, probably because of the different bilayer mechanics of blebbed versus intact membrane. Na+ dye-loaded human embryonic kidney (HEK) cells stably transfected with alphaNav1.6 were subjected to traumatic brain injury-like stretch. Cytoplasmic Na+ levels abruptly increased and the trauma-induced influx had a significant tetrodotoxin-sensitive component. Nav1.6 channel responses to cell and membrane trauma are therefore consistent with the hypothesis that mechanically induced Nav channel leak is a primary lesion in traumatic brain injury. Nav1.6 is the CNS node of Ranvier Nav isoform. When, during head trauma, nodes experienced bleb-inducing membrane damage of varying intensities, nodal Nav1.6 channels should immediately "leak" over a broadly left-smeared window current range.

Ion transporters and ischemic mitochondrial dysfunction

Ischemia-induced ionic imbalance leads to the activation of numerous events including mitochondrial dysfunction and eventual cell death. Dysregulation of mitochondrial Ca(2+) (Ca(2+)(m)) plays a critical role in cell damage under pathological conditions including traumatic brain injury and stroke. High Ca(2+)(m) levels can induce the persistent opening of the mitochondrial permeability transition pore and trigger mitochondrial membrane depolarization, Ca(2+) release, cessation of oxidative phosphorylation, matrix swelling and eventually outer membrane rupture with release of cytochrome c and other apoptogenic proteins. Thus, the dysregulation of mitochondrial Ca(2+) homeostasis is now recognized to play a crucial role in triggering mitochondrial dysfunction and subsequent apoptosis. Recent studies show that some secondary active transport proteins, such as Na(+)-dependent chloride transporter and Na(+)/Ca(2+) exchanger, contribute to ischemia-induced dissipation of ion homeostasis including Ca(2+)(m).

Differential hippocampal protection when blocking intracellular sodium and calcium entry during traumatic brain injury in rats

This study investigated the contributions of the reverse mode of the sodium-calcium exchanger (NCX) and the type 1 sodium-proton antiporter (NHE-1) to acute astrocyte and neuronal pathology in the hippocampus following fluid percussion traumatic brain injury (TBI) in the rat. KB-R7943, EIPA, or amiloride, which respectively inhibit NCX, NHE-1, or NCX, NHE-1, and ASIC1a (acid-sensing ion channel type 1a), was infused intraventricularly over a 60-min period immediately prior to TBI. Astrocytes were immunostained for glial fibrillary acidic protein (GFAP), and degenerating neurons were identified by Fluoro-Jade staining at 24 h after injury. Stereological analysis of the CA2/3 sub-regions of the hippocampus demonstrated that higher doses of KB-R7943 (2 and 20 nmoles) significantly reduced astrocyte GFAP immunoreactivity compared to vehicle-treated animals. EIPA (2-200 nmoles) did not alter astrocyte GFAP immunoreactivity. Amiloride (100 nmoles) significantly attenuated the TBI-induced acute reduction in astrocyte GFAP immunoreactivity. Of the three compounds examined, only amiloride (100 nmoles) reduced hippocampal neuronal degeneration assessed with Fluoro-Jade. The results provide additional evidence of acute astrocyte pathology in the hippocampus following TBI, while suggesting that activation of NHE-1 and the reverse mode of NCX contribute to both astrocyte and neuronal pathology following experimental TBI.

Biomechanics of traumatic brain injury: influences of the morphologic heterogeneities of the cerebral cortex

Traumatic brain injury (TBI) can be caused by accidents and often leads to permanent health issues or even death. Brain injury criteria are used for assessing the probability of TBI, if a certain mechanical load is applied. The currently used injury criteria in the automotive industry are based on global head kinematics. New methods, based on finite element modeling, use brain injury criteria at lower scale levels, e.g., tissue-based injury criteria. However, most current computational head models lack the anatomical details of the cerebrum. To investigate the influence of the morphologic heterogeneities of the cerebral cortex, a numerical model of a representative part of the cerebral cortex with a detailed geometry has been developed. Several different geometries containing gyri and sulci have been developed for this model. Also, a homogeneous geometry has been made to analyze the relative importance of the heterogeneities. The loading conditions are based on a computational head model simulation. The results of this model indicate that the heterogeneities have an influence on the equivalent stress. The maximum equivalent stress in the heterogeneous models is increased by a factor of about 1.3-1.9 with respect to the homogeneous model, whereas the mean equivalent stress is increased by at most 10%. This implies that tissue-based injury criteria may not be accurately applied to most computational head models used nowadays, which do not account for sulci and gyri.

A concerted role of Na+ -K+ -Cl- cotransporter and Na+/Ca2+ exchanger in ischemic damage

Na+-K+-Cl(-) cotransporter isoform 1 (NKCC1) and Na+/Ca2+ exchanger isoform 1 (NCX1) were expressed in cortical neurons. Three hours of oxygen and glucose deprivation (OGD) significantly increased expression of full-length NCX1 protein ( approximately 116 kDa), which remained elevated during 1 to 21 h reoxygenation (REOX) and was accompanied with concurrent cleavage of NCX1. Na+/Ca2+ exchanger isoform 1 heterozygous (NCX1+/-) neurons with approximately 50% less of NCX1 protein exhibited approximately 64% reduction in NCX-mediated Ca2+ influx. Expression of NCX1 and NKCC1 proteins was reduced in double heterozygous (NCX1+/-/NKCC1+/-) neurons. NCX-mediated Ca2+ influx was nearly abolished in these neurons. Three-hour OGD and 21-h REOX caused approximately 80% mortality rate in NCX1+/+ neurons and in NCX1+/- neurons. In contrast, NKCC1+/- neurons exhibited approximately 45% less cell death. The lowest mortality rate was found in NCX1+/-/NKCC1+/- neurons ( approximately 65% less neuronal death). The increased tolerance to ischemic damage was also observed in NCX1+/-/NKCC1+/- brains after transient cerebral ischemia. NCX1+/-/NKCC1+/- mice had a significantly reduced infarct volume at 24 and 72 h reperfusion. In conclusion, these data suggest that NKCC1 in conjunction with NCX1 plays a role in reperfusion-induced brain injury after ischemia.

Na+/Ca2+ exchange and regulation of cytoplasmic concentration of calcium in rat cerebellar neurons treated with glutamate

In the present work, the forward and/or reversed Na+/Ca2+ exchange in cerebellar granular cells was suppressed by substitution of Na+o by Li+ before, during, and after exposure to glutamate for varied time and also using the inhibitor KB-R7943 of the reversed exchange. After glutamate challenge for 1 min, Na+o/Li+ substitution did not influence the recovery of low [Ca2+]i in a calcium-free medium. A 1-h incubation with 100 microM glutamate induced in the neurons a biphasic and irreversible [Ca2+]i rise (delayed calcium deregulation (DCD)), enhancement of [Na+]i, and decrease in the mitochondrial potential. If Na+o had been substituted by Li+ before the application of glutamate, i.e. the exchange reversal was suppressed during the exposure to glutamate, the number of cells with DCD was nearly fourfold lowered. However, addition of the Na+/K+-ATPase inhibitor ouabain (0.5 mM) not preventing the exchange reversal also decreased DCD in the presence of glutamate. Both exposures decreased the glutamate-caused loss of intracellular ATP. Glucose deprivation partially abolished protective effects of the Na+o/Li+ substitution and ouabain. KB-R7943 (10 microM) increased 7.4-fold the number of cells with the [Ca2+]i decreased to the basal level after the exposure to glutamate. Thus, reversal of the Na+/Ca2+ exchange reinforced the glutamate-caused perturbations of calcium homeostasis in the neurons and slowed the recovery of the decreased [Ca2+]i in the post-glutamate period. However, for development of DCD, in addition to the exchange reversal, other factors are required, in particular a decrease in the intracellular concentration of ATP.

Linking impact to cellular and molecular sequelae of CNS injury: modeling in vivo complexity with in vitro simplicity

Traumatic brain injury (TBI) represents one of most common disorders to the central nervous system (CNS). Despite significant efforts, though, an effective clinical treatment for TBI is not yet available. The complexity of human TBI is modeled with a broad group of experimental models, with each model matching some aspect of the human condition. In the past 15 years, these in vivo models were complemented with a group of in vitro models, with these in vitro models allowing investigators to more precisely identify the mechanism(s) of TBI, the different intracellular events that occur in acute period following injury, and the possible treatment of this injury in vitro. In this paper, we review the available in vitro models to study TBI, discuss their biomechanical basis for human TBI, and review the findings from these in vitro models. Finally, we synthesize the current knowledge and point out possible future directions for this group of models, especially in the effort toward developing new therapies for the traumatically brain injured patient.

Pathophysiology of traumatic brain injury

The knowledge of the pathophysiology after traumatic head injury is necessary for adequate and patient-oriented treatment. As the primary insult, which represents the direct mechanical damage, cannot be therapeutically influenced, target of the treatment is the limitation of the secondary damage (delayed non-mechanical damage). It is influenced by changes in cerebral blood flow (hypo- and hyperperfusion), impairment of cerebrovascular autoregulation, cerebral metabolic dysfunction and inadequate cerebral oxygenation. Furthermore, excitotoxic cell damage and inflammation may lead to apoptotic and necrotic cell death. Understanding the multidimensional cascade of secondary brain injury offers differentiated therapeutic options.

The role of the Na(+)/Ca(2+) exchanger (NCX) in neurons following ischaemia

The Na(+)/Ca(2+) exchanger (NCX) is a bi-directional membrane ion transporter. Under normal conditions, the exchanger transports one calcium ion out of the cell and three sodium ions into the cell. This is known as the calcium exit, or "forward" mode. Under certain conditions, however, the exchanger can reverse and transport calcium ions into the cell (calcium entry mode). Because dysregulation of sodium and calcium homeostasis is an integral feature of ischaemic brain injury, the role of the NCX in neurons following ischaemia has been investigated using a number of in vitro and in vivo models. Studies using in vitro ischaemia-related models (hypoxia, glutamate) have produced conflicting results, with some showing that NCX activity is neuroprotective while others indicate that it is neurodamaging. The majority of in vivo studies using the focal cerebral ischaemia model indicate that blocking NCX activity is neurodamaging while increasing NCX activity is neuroprotective. We have reviewed the major in vitro and in vivo neuronal ischaemia-related NCX studies in an attempt to clarify the reason for the conflicting findings. The use of different ischaemia models and doubts as to the specificity of pharmacological NCX inhibitors and stimulators has contributed to the confusion over the role of the NCX in ischaemic brain injury. The development of NCX transgenic animals may help our understanding of the role of this ion exchanger in neurons following ischaemia and aid the development of an effective stroke treatment.

Regulation of the subthreshold chloride conductance in the rat sympathetic neuron

The mechanisms that control chloride conductance (gCl) in the rat sympathetic neuron have been studied by the two-electrode voltage-clamp technique in mature, intact superior cervical ganglia in vitro. In addition to voltage dependence in the membrane potential range -120/-50 mV, gCl displays time- and activity-dependent regulation (sensitization). The resting membrane potential is governed by voltage-dependent gK and gCl, which determine values of cell input conductance ranging from 7 to 18 nS (full deactivation) to an upper value of about 130 nS (full activation and maximal gCl sensitization). The quiescent neuron, held at constant membrane potential, spontaneously and gradually moved from a low- to a high-conductance status. An increase (about 40 nS) in gCl accounted for this phenomenon, which could be prevented by imposing intermittent hyperpolarizing episodes. Following spike firing, gCl increased by 20-33 nS, independent of the cell conductance value preceding tetanization, and thereafter decayed to the pre-stimulus level within 5 min. Intracellular sodium depletion and its successive ionophoretic restoration moved the neuron from a stable low-conductance state to maximum gCl sensitization, pointing to a link between gCl sensitization and [Na+]i. The dependence of gCl build-up on [Na+]i and the time-course of such Na+-related modulation have been examined: gCl sensitization was absent at 0 [Na+]i, was well developed (20 nS) at 15 mM and tended towards a saturating value of 60 nS for higher [Na+]i. Sensitization was transient in response to neuron activity. In the silent neuron, sensitization of gCl shifted membrane potential over a range of about 15 mV.

Strain rate-dependent induction of reactive astrogliosis and cell death in three-dimensional neuronal-astrocytic co-cultures

A mechanical insult to the brain drastically alters the microenvironment as specific cell types become reactive in an effort to sequester severely damaged tissue. Although injury-induced astrogliosis has been investigated, the relationship between well-defined biomechanical inputs and acute astrogliotic alterations is not well understood. We evaluated the effects of strain rate on cell death and astrogliosis using a three-dimensional (3-D) in vitro model of neurons and astrocytes within a bioactive matrix. At 21 days post-plating, co-cultures were deformed to 0.50 shear strain at strain rates of 1, 10, or 30 s(-1). We found that cell death and astrogliotic profiles varied differentially based on strain rate at 2 days post-insult. Significant cell death was observed after moderate (10 s(-1)) and high (30 s(-1)) rate deformation, but not after quasi-static (1 s(-1)) loading. The vast majority of cell death occurred in neurons, suggesting that these cells are more susceptible to high rate shear strains than astrocytes for the insult parameters used here. Injury-induced astrogliosis was compared to co-cultures treated with transforming growth factor beta, which induced robust astrocyte hypertrophy and increased glial fibrillary acidic protein (GFAP) and chondroitin sulfate proteoglycans (CSPGs). Quasi-static loading resulted in increased cell density and CSPG secretion. Moderate rate deformation increased cell density, GFAP reactivity, and hypertrophic process density. High rate deformation resulted in increased GFAP reactivity; however, other astrogliotic alterations were not observed at this time-point. These results demonstrate that the mode and degree of astrogliosis depend on rate of deformation, demonstrating astrogliotic augmentation at sub-lethal injury levels as well as levels inducing cell death.

Role of Na+-K+-Cl−cotransport and Na+/Ca2+exchange in mitochondrial dysfunction in astrocytes following in vitro ischemia

Na+-K+-Cl−cotransporter isoform 1 (NKCC1) and reverse mode operation of the Na+/Ca2+exchanger (NCX) contribute to intracellular Na+and Ca2+overload in astrocytes following oxygen-glucose deprivation (OGD) and reoxygenation (REOX). Here, we further investigated whether NKCC1 and NCX play a role in mitochondrial Ca2+(Cam2+) overload and dysfunction. OGD/REOX caused a doubling of mitochondrial-releasable Ca2+( P < 0.05). When NKCC1 was inhibited with bumetanide, the mitochondrial-releasable Ca2+was reduced by ∼42% ( P < 0.05). Genetic ablation of NKCC1 also reduced Cam2+accumulation. Moreover, OGD/REOX in NKCC1+/+astrocytes caused dissipation of the mitochondrial membrane potential (Ψm) to 42 ± 3% of controls. In contrast, when NKCC1 was inhibited with bumetanide, depolarization of Ψmwas attenuated significantly (66 ± 10% of controls, P < 0.05). Cells were also subjected to severe in vitro hypoxia by superfusion with a hypoxic, acidic, ion-shifted Ringer buffer (HAIR). HAIR/REOX triggered a secondary, sustained rise in intracellular Ca2+that was attenuated by reversal NCX inhibitor KB-R7943. The hypoxia-mediated increase in Cam2+was accompanied by loss of Ψmand cytochrome c release in NKCC1+/+astrocytes. Bumetanide or genetic ablation of NKCC1 attenuated mitochondrial dysfunction and astrocyte death following ischemia. Our study suggests that NKCC1 acting in concert with NCX causes a perturbation of Cam2+homeostasis and mitochondrial dysfunction and cell death following in vitro ischemia.

Astroglia: important mediators of traumatic brain injury

Traumatic brain injury (TBI) research to date has focused almost exclusively on the pathophysiology of injured neurons with very little attention paid to non-neuronal cells. However in the past decade, exciting discoveries have challenged this century-old view of passive glial cells and have led to a reinterpretation of the role of glial cells in central nervous system (CNS) biology and pathology. In this chapter we review several lines of evidence, indicating that glial cells, particularly astrocytes, are active partners to neurons in the brain, and summarize recent findings that detail the significance of astrocyte pathology in traumatic brain injury.

Effects of 17beta-estradiol on intracellular calcium changes and neuronal survival after mechanical strain injury in neuronal-glial cultures

The neuroprotective effects of 17beta-estradiol (E2) were investigated using an in vitro model of traumatic brain injury in which cortical neuronal cultures were subjected to mechanical strain-injury. The rise in intracellular calcium ([Ca(2+)](i)) following neuronal injury was reduced by addition of 10 or 100 nM E2 to the cultures immediately following injury. Neuronal damage was measured 24 h after injury by propidium iodide uptake and cell viability by carboxyfluorescein diacetate uptake. Addition of 1, 10, or 100 nM E2 to cell cultures immediately following injury decreased neuronal damage and increased neuronal viability compared to vehicle-treated neurons. These results demonstrate the neuroprotective activity of E2 in an in vitro model of neuronal injury, and suggest that such effects may be related to the ability of E2 to modulate [Ca(2+)](i).

Dopamine-induced graded intracellular Ca2+ elevation via the Na+Ca2+ exchanger operating in the Ca2+-entry mode in cockroach salivary ducts

Stimulation with the neurotransmitter dopamine causes an amplitude-modulated increase in the intracellular Ca(2+) concentration ([Ca(2+)](i)) in epithelial cells of the ducts of cockroach salivary glands. This is completely attributable to a Ca(2+) influx from the extracellular space. Additionally, dopamine induces a massive [Na(+)](i) elevation via the Na(+)K(+)2Cl(-) cotransporter (NKCC). We have reasoned that Ca(2+)-entry is mediated by the Na(+)Ca(2+) exchanger (NCE) operating in the Ca(2+)-entry mode. To test this hypothesis, [Ca(2+)](i) and [Na(+)](i) were measured by using the fluorescent dyes Fura-2, Fluo-3, and SBFI. Inhibition of Na(+)-entry from the extracellular space by removal of extracellular Na(+) or inhibition of the NKCC by 10 microM bumetanide did not influence resting [Ca(2+)](i) but completely abolished the dopamine-induced [Ca(2+)](i) elevation. Simultaneous recordings of [Ca(2+)](i) and [Na(+)](i) revealed that the dopamine-induced [Na(+)](i) elevation preceded the [Ca(2+)](i) elevation. During dopamine stimulation, the generation of an outward Na(+) concentration gradient by removal of extracellular Na(+) boosted the [Ca(2+)](i) elevation. Furthermore, prolonging the dopamine-induced [Na(+)](i) rise by blocking the Na(+)/K(+)-ATPase reduced the recovery from [Ca(2+)](i) elevation. These results indicate that dopamine induces a massive NKCC-mediated elevation in [Na(+)](i), which reverses the NCE activity into the reverse mode causing a graded [Ca(2+)](i) elevation in the duct cells.