Chloride influx aggravates Ca2+-dependent AMPA receptor-mediated motoneuron death

Roles of GABAA receptor α5 subunit on locomotion and working memory in transient forebrain ischemia in mice

The γ-aminobutyric acid A (GABA_A) receptor, which contains a chloride channel, is a typical inhibitory neurotransmitter receptor in the central nervous system. Although the GABAergic neurotransmitter system has been discovered to be involved in various psychological behaviors, such as anxiety, convulsions, and cognitive function, its functional changes under conditions of ischemic pathological situation are still uncovered. In the present study, we attempted to elucidate the functional changes in the GABAergic system after transient forebrain ischemia in mice. A bilateral common carotid artery occlusion for 20 min was used to establish a model of transient forebrain ischemia/reperfusion (tI/R). Delayed treatment with diazepam, a positive allosteric modulator of the GABA_A receptor, increased locomotor activity in the open field test and spontaneous alternations in the Y-maze test in tI/R mice, but not in shams. Delayed treatment with diazepam did not alter neuronal death or the number of GABAergic neurons in tI/R mice. However, tI/R induced changes in the protein levels of GABA_A receptor subunits in the hippocampus. In particular, the most marked increase in the tI/R group was found in the level of α5 subunit of the GABA_A receptor. Similar to delayed treatment with diazepam, delayed treatment with imidazenil, an α5-sensitive benzodiazepine, increased spontaneous alternations in the Y-maze in tI/R mice, whereas zolpidem, an α5-insensitive benzodiazepine, failed to show such effects. These results suggest that tI/R-induced changes in the level of the α5 subunit of the GABA_A receptor can alter the function of GABAergic drugs in a mouse model of forebrain ischemia.

LRRC8A-dependent volume-regulated anion channels contribute to ischemia-induced brain injury and glutamatergic input to hippocampal neurons

Volume-regulated anion channels (VRACs) are critically involved in regulating cell volume, and leucine-rich repeat-containing protein 8A (LRRC8A, SWELL1) is an obligatory subunit of VRACs. Cell swelling occurs early after brain ischemia, but it is unclear whether neuronal LRRC8a contributes to ischemia-induced glutamate release and brain injury. We found that Lrrc8a conditional knockout (Lrrc8a-cKO) mice produced by crossing Nestin^Cre+/- with Lrrc8a^flox+/+ mice died 7-8 weeks of age, indicating an essential role of neuronal LRRC8A for survival. Middle cerebral artery occlusion (MCAO) caused an early increase in LRRC8A protein levels in the hippocampus in wild-type (WT) mice. Whole-cell patch-clamp recording in brain slices revealed that oxygen-glucose deprivation significantly increased the amplitude of VRAC currents in hippocampal CA1 neurons in WT but not in Lrrc8a-cKO mice. Hypotonicity increased the frequency of spontaneous excitatory postsynaptic currents (sEPSCs) in hippocampal CA1 neurons in WT mice, and this was abolished by DCPIB, a VRAC blocker. But in Lrrc8a-cKO mice, hypotonic solution had no effect on the frequency of sEPSCs in these neurons. Furthermore, the brain infarct volume and neurological severity score induced by MCAO were significantly lower in Lrrc8a-cKO mice than in WT mice. In addition, MCAO-induced increases in cleaved caspase-3 and calpain activity, two biochemical markers of neuronal apoptosis and death, in brain tissues were significantly attenuated in Lrrc8a-cKO mice compared with WT mice. These new findings indicate that cerebral ischemia increases neuronal LRRC8A-dependent VRAC activity and that VRACs contribute to increased glutamatergic input to hippocampal neurons and brain injury caused by ischemic stroke.

Interferon-γ Receptor 1 and GluR1 upregulated in motor neurons of symptomatic hSOD1G93A mice

Motor neurons are markedly vulnerable to excitotoxicity mostly by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic receptor (AMPAR) stimulation and are principal targets in the neurodegenerative disease Amyotrophic Lateral Sclerosis. Interferon-gamma (IFN-γ), a pro-inflammatory cytokine, can independently cause neuronal dysfunction by triggering calcium influx through a calcium-permeable complex of IFN-γ receptor 1(IFNGR1) subunit and AMPAR subunit GluR1. This receptor complex is formed via a non-canonical neuron-specific IFN-γ pathway that involves Jak1/Stat1 and Protein Kinase A. In this study, we explore the expression of the pathway's participants for the first time in the hSOD1G93A Amyotrophic Lateral Sclerosis mouse model. Elevated IFNGR1 and GluR1 are detected in motor neurons of hSOD1G93A symptomatic mice ex vivo, unlike the downstream targets - Jak1, Stat1, and Protein Kinase A. We, also, determine effects of IFN-γ alone or in the presence of an excitotoxic agent, kainate, on motor neuron survival in vitro. IFN-γ induces neuronal damage, but does not influence kainate-mediated excitotoxicity. Increased IFNGR1 can most likely sensitize motor neurons to excitotoxic insults involving GluR1 and/or pathways mediated by IFN-γ, thus, serving as a potential direct link between neurodegeneration and inflammation in Amyotrophic Lateral Sclerosis.

Sigma 1 receptor activation modifies intracellular calcium exchange in the G93AhSOD1 ALS model

Aberrations in intracellular calcium (Ca2+) have been well established within amyotrophic lateral sclerosis (ALS), a severe motor neuron disease. Intracellular Ca2+ concentration is controlled in part through the endoplasmic reticulum (ER) mitochondria Ca2+ cycle (ERMCC). The ER supplies Ca2+ to the mitochondria at close contacts between the two organelles, i.e. the mitochondria-associated ER membranes (MAMs). The Sigma 1 receptor (Sig1R) is enriched at MAMs, where it acts as an inter-organelle signaling modulator. However, its impact on intracellular Ca2+ at the cellular level remains to be thoroughly investigated. Here, we used cultured embryonic mice spinal neurons to investigate the influence of Sig1R activation on intracellular Ca2+ homeostasis in the presence of G93AhSOD1 (G93A), an established ALS-causing mutation. Sig1R expression was increased in G93A motor neurons relative to non-transgenic (nontg) controls. Furthermore, we demonstrated significantly reduced bradykinin-sensitive intracellular Ca2+ stores in G93A spinal neurons, which were normalized by the Sig1R agonist SA4503. Moreover, SA4503 accelerated cytosolic Ca2+ clearance following a) AMPAR activation by kainate and b) IP3R-mediated ER Ca2+ release following bradykinin stimulation in both genotypes. PRE-084 (another Sig1R agonist) did not exert any significant effects on cytosolic Ca2+. Both Sig1R expression and functionality were altered by the G93A mutation, indicating the centrality of Sig1R in ALS pathology. Here, we showed that intracellular Ca2+ shuttling can be manipulated by Sig1R activation, thus demonstrating the value of using the pharmacological manipulation of Sig1R to understand Ca2+ homeostasis.

DNA strand breaks and TDP-43 mislocation are absent in the murine hSOD1G93A model of amyotrophic lateral sclerosis in vivo and in vitro

Mutations in the human Cu/Zn superoxide dismutase type-1 (hSOD1) gene are common in familial amyotrophic lateral sclerosis (fALS). The pathophysiology has been linked to, e.g., organelle dysfunction, RNA metabolism and oxidative DNA damage conferred by SOD1 malfunction. However, apart from metabolically evoked DNA oxidation, it is unclear whether severe genotoxicity including DNA single-strand breaks (SSBs) and double-strand breaks (DSBs), originates from loss of function of nuclear SOD1 enzyme. Factors that endogenously interfere with DNA integrity and repair complexes in hSOD1-mediated fALS remain similarly unexplored. In this regard, uncontrolled activation of transposable elements (TEs) might contribute to DNA disintegration and neurodegeneration. The aim of this study was to elucidate the role of the fALS-causing hSOD1^G93A mutation in the generation of severe DNA damage beyond well-characterized DNA base oxidation. Therefore, DNA damage was assessed in spinal tissue of hSOD1^G93A-overexpressing mice and in corresponding motor neuron-enriched cell cultures in vitro. Overexpression of the hSOD1^G93A locus did not change the threshold for severe DNA damage per se. We found that levels of SSBs and DSBs were unaltered between hSOD1^G93A and control conditions, as demonstrated in post-mitotic motor neurons and in astrocytes susceptible to replication-dependent DNA breakage. Analogously, parameters indicative of DNA damage response processes were not activated in vivo or in vitro. Evidence for a mutation-related elevation in TE activation was not detected, in accordance with the absence of TAR DNA binding protein 43 (TDP-43) proteinopathy in terms of cytoplasmic mislocation or nuclear loss, as nuclear TDP-43 is supposed to silence TEs physiologically. Conclusively, the superoxide dismutase function of SOD1 might not be required to preserve DNA integrity in motor neurons, at least when the function of TDP-43 is unaltered. Our data establish a foundation for further investigations addressing functional TDP-43 interaction with ALS-relevant genetic mutations.

Imaging of glial cell morphology, SOD1 distribution and elemental composition in the brainstem and hippocampus of the ALS hSOD1G93A rat

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder affecting motor and cognitive domains of the CNS. Mutations in the Cu,Zn-superoxide dismutase (SOD1) cause 20% of familial ALS and provoke formation of intracellular aggregates and copper and zinc unbinding, leading to glial activation and neurodegeneration. Therefore, we investigated glial cell morphology, intracellular SOD1 distribution, and elemental composition in the brainstem and hippocampus of the hSOD1^G93A transgenic rat model of ALS. Immunostaining for astrocytes, microglia and SOD1 revealed glial proliferation and progressive tissue accumulation of SOD1 in both brain regions of ALS rats starting already at the presymptomatic stage. Glial cell morphology analysis in the brainstem of ALS rats revealed astrocyte activation occurring before disease symptoms onset, followed by activation of microglia. Hippocampal ALS astrocytes exhibited an identical reactive profile, while microglial morphology was unchanged. Additionally, ALS brainstem astrocytes demonstrated progressive SOD1 accumulation in the cell body and processes, while microglial SOD1 levels were reduced and its distribution limited to distal cell processes. In the hippocampus both glial cell types exhibited SOD1 accumulation in the cell body. X-ray fluorescence imaging revealed decreased P and increased Ca, Cl, K, Ni, Cu and Zn in the brainstem, and higher levels of Cl, Ni and Cu, but lower levels of Zn in the hippocampus of symptomatic ALS rats. These results bring new insights into the glial response during disease development and progression in motor as well as in non-motor CNS structures, and indicate disturbed tissue elemental homeostasis as a prominent hallmark of disease pathology.

What Is Being Trained? How Divergent Forms of Plasticity Compete To Shape Locomotor Recovery after Spinal Cord Injury

Spinal cord injury (SCI) is a devastating syndrome that produces dysfunction in motor and sensory systems, manifesting as chronic paralysis, sensory changes, and pain disorders. The multi-faceted and heterogeneous nature of SCI has made effective rehabilitative strategies challenging. Work over the last 40 years has aimed to overcome these obstacles by harnessing the intrinsic plasticity of the spinal cord to improve functional locomotor recovery. Intensive training after SCI facilitates lower extremity function and has shown promise as a tool for retraining the spinal cord by engaging innate locomotor circuitry in the lumbar cord. As new training paradigms evolve, the importance of appropriate afferent input has emerged as a requirement for adaptive plasticity. The integration of kinematic, sensory, and loading force information must be closely monitored and carefully manipulated to optimize training outcomes. Inappropriate peripheral input may produce lasting maladaptive sensory and motor effects, such as central pain and spasticity. Thus, it is important to closely consider the type of afferent input the injured spinal cord receives. Here we review preclinical and clinical input parameters fostering adaptive plasticity, as well as those producing maladaptive plasticity that may undermine neurorehabilitative efforts. We differentiate between passive (hindlimb unloading [HU], limb immobilization) and active (peripheral nociception) forms of aberrant input. Furthermore, we discuss the timing of initiating exposure to afferent input after SCI for promoting functional locomotor recovery. We conclude by presenting a candidate rapid synaptic mechanism for maladaptive plasticity after SCI, offering a pharmacological target for restoring the capacity for adaptive spinal plasticity in real time.

NSAIDs modulate GABA-activated currents via Ca2+-activated Cl- channels in rat dorsal root ganglion neurons

The ability of non-steroidal anti-inflammatory drugs (NSAIDs) to modulate γ-aminobutyrate (GABA)-activated currents via Ca²⁺-activated Cl⁻ channels in rat dorsal root ganglion neurons (DRG), was examined in the present study. During the preparation of DRG neurons harvested from Sprague-Dawley rats, the whole-cell recording technique was used to record the effect of NSAIDs on GABA-activated inward currents, and the expression levels of the TMEM16A and TMEM16B subunits were revealed. In the event that DRG neurons were pre-incubated for 20 sec with niflumic acid (NFA) and 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB) prior to the administration of GABA, the GABA-induced inward currents were diminished markedly in the majority of neurons examined (96.3%). The inward currents induced by 100 µmol/l GABA were attenuated by (0±0.09%; neurons = 4), (5.32±3.51%; neurons = 6), (21.3±4.00%; neurons = 5), (33.8±5.20%; neurons = 17), (52.2±5.10%; neurons = 4) and (61.1±4.12%; neurons = 12) by 0.1, 1, 3, 10, 30 and 100 µmol/l NFA, respectively. The inward currents induced by 100 µmol/l GABA were attenuated by (13.8±6%; neurons = 6), (23.2±14.7%; neurons = 6) and (29.7±9.1%; neurons = 9) by 3, 10 and 30 µmol/l NPPB, respectively. NFA and NPPB dose-dependently inhibited GABA-activated currents with half maximal inhibitory concentration (IC₅₀) values of 6.7 and 11 µmol/l, respectively. The inhibitory effect of 100 µmol/l NFA on the GABA-evoked inward current were also strongly inhibited by nitrendipine (NTDP; an L-type calcium channel blocker), 1,2-bis(2-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid tetrakis (a highly selective calcium chelating reagent), caffeine (a widely available Ca²⁺ consuming drug) and calcium-free extracellular fluid, in a concentration-dependent manner. Immunofluorescent staining indicated that TMEM16A and TMEM16B expression was widely distributed in DRG neurons. The results suggest that NSAIDs may be able to regulate Ca²⁺-activated chloride channels to reduce GABA_A receptor-mediated inward currents in DRGs.

Novel computer vision algorithm for the reliable analysis of organelle morphology in whole cell 3D images--A pilot study for the quantitative evaluation of mitochondrial fragmentation in amyotrophic lateral sclerosis

The function of intact organelles, whether mitochondria, Golgi apparatus or endoplasmic reticulum (ER), relies on their proper morphological organization. It is recognized that disturbances of organelle morphology are early events in disease manifestation, but reliable and quantitative detection of organelle morphology is difficult and time-consuming. Here we present a novel computer vision algorithm for the assessment of organelle morphology in whole cell 3D images. The algorithm allows the numerical and quantitative description of organelle structures, including total number and length of segments, cell and nucleus area/volume as well as novel texture parameters like lacunarity and fractal dimension. Applying the algorithm we performed a pilot study in cultured motor neurons from transgenic G93A hSOD1 mice, a model of human familial amyotrophic lateral sclerosis. In the presence of the mutated SOD1 and upon excitotoxic treatment with kainate we demonstrate a clear fragmentation of the mitochondrial network, with an increase in the number of mitochondrial segments and a reduction in the length of mitochondria. Histogram analyses show a reduced number of tubular mitochondria and an increased number of small mitochondrial segments. The computer vision algorithm for the evaluation of organelle morphology allows an objective assessment of disease-related organelle phenotypes with greatly reduced examiner bias and will aid the evaluation of novel therapeutic strategies on a cellular level.

Endoplasmic reticulum stress is accompanied by activation of NF-κB in amyotrophic lateral sclerosis

BACKGROUND

Recent studies have indicated that endoplasmic reticulum (ER) stress is involved in the pathogenesis of amyotrophic lateral sclerosis (ALS). ER stress occurs when the ER-mitochondria calcium cycle is disturbed and misfolded proteins accumulate in the ER. To cope with ER stress, cells activate the unfolded protein response (UPR). Accumulating evidence from non-neuronal cell models suggests that there is extensive cross-talk between the UPR and the NF-κB pathway.

METHODS

Here we investigated the expression of NF-κB and the main UPR markers X-box binding protein 1 (XBP1), basic leucine-zipper transcription factor 6 (ATF6) and phosphorylated eukaryotic initiation factor-2α (p-eIF2) in mutated SOD1(G93A) cell models of ALS, as well as their modulation by lipopolysaccharide and ER-stressing (tunicamycin) stimuli.

RESULTS

Expression of NF-κB was enhanced in the presence of SOD1(G93A). Lipopolysaccharide did not induce the UPR in NSC34 cells and motor neurons in a mixed motor neuron-glia coculture system. The induction of the UPR by tunicamycin was accompanied by activation of NF-κB in NSC34 cells and motor neurons.

CONCLUSION

Our data linked two important pathogenic mechanisms of ALS, ER stress and NF-κB signalling, in motor neurons.

Overexpression of human mutated G93A SOD1 changes dynamics of the ER mitochondria calcium cycle specifically in mouse embryonic motor neurons

Motor neurons vulnerable to the rapidly progressive deadly neurodegenerative disease amyotrophic lateral sclerosis (ALS) inherently express low amounts of calcium binding proteins (CaBP), likely to allow physiological motor neuron firing frequency modulation. At the same time motor neurons are susceptible to AMPA receptor mediated excitotoxicity and internal calcium deregulation which is not fully understood. We analysed ER mitochondria calcium cycle (ERMCC) dynamics with subsecond resolution in G93A hSOD1 overexpressing motor neurons as a model of ALS using fluorescent calcium imaging. When comparing vulnerable motor neurons and non-motor neurons from G93A hSOD1 mice and their non-transgenic littermates, we found a decelerated cytosolic calcium clearance in the presence of G93A hSOD1. While both non-transgenic as well as G93A hSOD1 motor neurons displayed large mitochondrial calcium uptake by the mitochondrial uniporter (mUP), the mitochondrial calcium extrusion system was altered in the presence of G93A hSOD1. In addition, ER calcium uptake by the sarco-/endoplasmic reticulum ATPase (SERCA) was increased in G93A hSOD1 motor neurons. In survival assays, blocking the mitochondrial sodium calcium exchanger (mNCE) by CGP37157 as well as inhibiting SERCA by cyclopiazonic acid showed protective effects against kainate induced excitotoxicity. Thus, our study shows for the first time that the functional consequence of G93A hSOD1 overexpression in intact motor neurons is indeed a disturbance of the ER mitochondria calcium cycle, and identified two promising targets for therapeutic intervention in the pathology of ALS.

Dantrolene is neuroprotective in vitro, but does not affect survival in SOD1(G⁹³A) mice

Amyotrophic Lateral Sclerosis (ALS) is a devastating progressive neurodegenerative disease. One of the proposed disease mechanisms is excitotoxicity, in which excessive cytosolic calcium causes neuronal death. Although most calcium may originate from the extracellular space through activation of calcium-permeable AMPA receptors, we investigated in this study the contribution of endoplasmic reticulum calcium release by blocking the ryanodine receptor (RyR) using dantrolene. In vitro, dantrolene provides a significant protection to motor neurons exposed to a brief excitotoxic insult. However, daily administration of dantrolene to mice overexpressing superoxide dismutase 1 glycine to alanine at position 93 (SOD1(G93A)) does affect neither survival nor the number of motor neurons and ubiquitin aggregates indicating that calcium release through RyRs does not contribute to the selective motor neuron death in this animal model for ALS.

SOD1 and TDP-43 animal models of amyotrophic lateral sclerosis: recent advances in understanding disease toward the development of clinical treatments

Amyotrophic lateral sclerosis (ALS) is a fatal motor neuron disease with no cure. Breakthroughs in understanding ALS pathogenesis came with the discovery of dominant mutations in the superoxide dismutase 1 gene (SOD1) and other genes, including the gene encoding transactivating response element DNA binding protein-43 (TDP-43). This has led to the creation of animal models to further our understanding of the disease and identify a number of ALS-causing mechanisms, including mitochondrial dysfunction, protein misfolding and aggregation, oxidative damage, neuronal excitotoxicity, non-cell autonomous effects and neuroinflammation, axonal transport defects, neurotrophin depletion, effects from extracellular mutant SOD1, and aberrant RNA processing. Here we summarise the SOD1 and TDP-43 animal models created to date, report on recent findings supporting the potential mechanisms of ALS pathogenesis, and correlate this understanding with current developments in the clinic.

Downregulation of the potassium chloride cotransporter KCC2 in vulnerable motoneurons in the SOD1-G93A mouse model of amyotrophic lateral sclerosis

The balance between excitatory and inhibitory synaptic inputs is critical for the physiological control of motoneurons. The maintenance of a low-intracellular chloride concentration by the potassium chloride cotransporter 2 (KCC2) is essential for the efficacy of fast synaptic inhibition of mature motoneurons in response to the activation of ionotropic γ-aminobutyric acid A and glycine receptors. Altered synaptic balance and excitotoxicity have been proposed as candidate pathophysiological processes in amyotrophic lateral sclerosis (ALS). Therefore, we investigated the expression patterns of KCC2 and its functional opponent, the chloride influx-mediating sodium-potassium chloride cotransporter 1 (NKCC1), in the superoxide dismutase 1 (SOD1-G93A) mouse model of ALS. We detected reduced KCC2 messenger RNA levels and less membrane-bound KCC2 immunoreactivity in ALS-vulnerable motoneurons in lumbar spinal cord and hypoglossal nuclei of SOD1-G93A mice but not in degeneration-resistant oculomotor nuclei. Downregulation of KCC2 started during late presymptomatic stages and accelerated in parallel to hind limb and tongue motor function deficits. In contrast, NKCC1 messenger RNA levels were unaltered in postnatal lumbar spinal cord motoneurons. Our data indicate that reductions in KCC2 gene expression may contribute to selective motor deficits and disease progression in vulnerable motoneurons in a mouse model of ALS.

GAB(A) receptors present higher affinity and modified subunit composition in spinal motor neurons from a genetic model of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis is a neurodegenerative disease characterized by the selective degeneration of motor neurons in the spinal cord, brainstem and cerebral cortex. In this study we have analysed the electrophysiological properties of GABA(A) receptors and GABA(A) alpha1 and alpha2 subunits expression in spinal motor neurons in culture obtained from a genetic model of ALS (G93A) and compared with transgenic wild type SOD1 (SOD1) and their corresponding non transgenic litter mates (Control). Although excitotoxic motor neuron death has been extensively studied in relation to Ca(2+)-dependent processes, strong evidence indicates that excitotoxic cell death is also remarkably dependent on Cl(-) ions and on GABA(A) receptor activation. In this study we have analysed the electrophysiological properties of GABA(A) receptors and the expression of GABA(A)alpha(1) and alpha(2) subunits in cultured motor neurons obtained from a genetic model of amyotrophic lateral sclerosis (G93A) and compared them with transgenic wild-type Cu,Zn superoxide dismutase and their corresponding non-transgenic littermates (Control). In all tested motor neurons, the application of gamma-aminobutyric acid (GABA) (0.5-100 mum) evoked an inward current that was reversibly blocked by bicuculline (100 mum), thus indicating that it was mediated by the activation of GABA(A) receptors. Our results indicate that the current density at high GABA concentrations is similar in control, Cu,Zn superoxide dismutase and G93A motor neurons. However, the dose-response curve significantly shifted toward lower concentration values in G93A motor neurons and the extent of desensitization also increased in these neurons. Finally, multiplex single-cell real-time polymerase chain reaction and immunofluorescence revealed that the amount of GABA(A)alpha(1) subunit was significantly increased in G93A motor neurons, whereas the levels of alpha(2) subunit were unchanged. These data show that the functionality and expression of GABA(A) receptors are altered in G93A motor neurons inducing a higher Cl(-) influx into the cell with a possible consequent neuronal excitotoxicity acceleration.

Differential contribution of the Na(+)-K(+)-2Cl(-) cotransporter NKCC1 to chloride handling in rat embryonic dorsal root ganglion neurons and motor neurons

Plasma membrane chloride (Cl(-)) pathways play an important role in neuronal physiology. Here, we investigated the role of NKCC1 cotransporters (a secondary active Cl(-) uptake mechanism) in Cl(-) handling in cultured rat dorsal root ganglion neurons (DRGNs) and motor neurons (MNs) derived from fetal stage embryonic day 14. Gramicidin-perforated patch-clamp recordings revealed that DRGNs accumulate intracellular Cl(-) through a bumetanide- and Na(+)-sensitive mechanism, indicative of the functional expression of NKCC1. Western blotting confirmed the expression of NKCC1 in both DRGNs and MNs, but immunocytochemistry experiments showed a restricted expression in dendrites of MNs, which contrasts with a homogeneous expression in DRGNs. Both MNs and DRGNs could be readily loaded with or depleted of Cl(-) during GABA(A) receptor activation at depolarizing or hyperpolarizing membrane potentials. After loading, the rate of recovery to the resting Cl(-) concentration (i.e., [Cl(-)](i) decrease) was similar in both cell types and was unaffected by lowering the extracellular Na(+) concentration. In contrast, the recovery on depletion (i.e., [Cl(-)](i) increase) was significantly faster in DRGNs in control conditions but not in low extracellular Na(+). The experimental observations could be reproduced by a mathematical model for intracellular Cl(-) kinetics, in which DRGNs show higher NKCC1 activity and smaller Cl(-)-handling volume than MNs. On the basis of these results, we conclude that embryonic DRGNs show a higher somatic functional expression of NKCC1 than embryonic MNs. The high NKCC1 activity in DRGNs is important for maintaining high [Cl(-)](i), whereas lower NKCC1 activity in MNs allows large [Cl(-)](i) variations during neuronal activity.

Progranulin functions as a neurotrophic factor to regulate neurite outgrowth and enhance neuronal survival

Recently, mutations in the progranulin (PGRN) gene were found to cause familial and apparently sporadic frontotemporal lobe dementia (FTLD). Moreover, missense changes in PGRN were identified in patients with motor neuron degeneration, a condition that is related to FTLD. Most mutations identified in patients with FTLD until now have been null mutations. However, it remains unknown whether PGRN protein levels are reduced in the central nervous system from such patients. The effects of PGRN on neurons also remain to be established. We report that PGRN levels are reduced in the cerebrospinal fluid from FTLD patients carrying a PGRN mutation. We observe that PGRN and GRN E (one of the proteolytic fragments of PGRN) promote neuronal survival and enhance neurite outgrowth in cultured neurons. These results demonstrate that PGRN/GRN is a neurotrophic factor with activities that may be involved in the development of the nervous system and in neurodegeneration.

Survey of ALS-associated factors potentially promoting Ca2+ overload of motor neurons

The deleterious consequences of Ca(2+) overload are thought to be a probable cause of motoneuronal death in ALS, although the overloading mechanism is currently unclear. In this paper some ALS-linked factors are analysed with regard to their influence on Ca(2+ )influx into neurons. Intensive cortex activity can render motor neurons susceptible to stimulation of calcium-permeable glutamate NMDA-receptors; increase in CSF concentrations of glutamate, glycine, and norepinephrine supposedly can intensify these receptors' activity. Elevated CSF levels of GABA and reduced levels of serotonin can promote Ca(2+ )influx through glutamate AMPA-receptors and voltage-gated channels of L-, N-, and P-type. Additionally, brain ischaemia can contribute to Ca(2+ )overload of motor neurons. Thus, ALS is characterized by the unique combination of factors potentially able to promote the overload of motor neurons with calcium.

Disruption of ionic and cell volume homeostasis in cerebral ischemia: The perfect storm

The mechanisms of brain tissue damage in stroke are strongly linked to the phenomenon of excitotoxicity, which is defined as damage or death of neural cells due to excessive activation of receptors for the excitatory neurotransmitters glutamate and aspartate. Under physiological conditions, ionotropic glutamate receptors mediate the processes of excitatory neurotransmission and synaptic plasticity. In ischemia, sustained pathological release of glutamate from neurons and glial cells causes prolonged activation of these receptors, resulting in massive depolarization and cytoplasmic Ca(2+) overload. High cytoplasmic levels of Ca(2+) activate many degradative processes that, depending on the metabolic status, cause immediate or delayed death of neural cells. This traditional view has been expanded by a number of observations that implicate Cl(-) channels and several types of non-channel transporter proteins, such as the Na(+),K(+),2Cl(-) cotransporter, Na(+)/H(+) exchanger, and Na(+)/Ca(2+) exchanger, in the development of glutamate toxicity. Some of these ion transporters increase tissue damage by promoting pathological cell swelling and necrotic cell death, while others contribute to a long-term accumulation of cytoplasmic Ca(2+). This brief review is aimed at illustrating how the dysregulation of various ion transport processes combine in a 'perfect storm' that disrupts neural ionic homeostasis and culminates in the irreversible damage and death of neural cells. The clinical relevance of individual transporters as targets for therapeutic intervention in stroke is also briefly discussed.

Astrocytes regulate GluR2 expression in motor neurons and their vulnerability to excitotoxicity

Influx of Ca(2+) ions through alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors contributes to neuronal damage in stroke, epilepsy, and neurodegenerative disorders such as ALS. The Ca(2+) permeability of AMPA receptors is largely determined by the glutamate receptor 2 (GluR2) subunit, receptors lacking GluR2 being permeable to Ca(2+) ions. We identified a difference in GluR2 expression in motor neurons from two rat strains, resulting in a difference in vulnerability to AMPA receptor-mediated excitotoxicity both in vitro and in vivo. Astrocytes from the ventral spinal cord were found to mediate this difference in GluR2 expression in motor neurons. The presence of ALS-causing mutant superoxide dismutase 1 in astrocytes abolished their GluR2-regulating capacity and thus affected motor neuron vulnerability to AMPA receptor-mediated excitotoxicity. These results reveal a mechanism through which astrocytes influence neuronal functioning in health and disease.

Role of mitochondria in kainate-induced fast Ca2+ transients in cultured spinal motor neurons

Motor neuron death in amyotrophic lateral sclerosis (ALS) has been linked to selective vulnerability towards AMPA receptor-mediated excitotoxicity. We investigated intracellular mechanisms leading to impairment of motor neuron Ca2+ homeostasis with near physiological AMPA receptor activation. Using fast solution exchange on patch-clamped cultured neurons, kainate (KA) was applied for 2s. This induced a transient increase in the cytosolic Ca2+ concentration ([Ca2+]c) for seconds. Inhibition of the mitochondrial uniporter by RU-360 abolished the decay of the Ca2+ transient and caused immediate [Ca2+]c overload. Repetitive short KA stimulation caused a slowing of the decay of the Ca2+ transient and a gradual increase in peak and baseline [Ca2+]c in motor neurons, but not in other neurons, indicating saturation of the mitochondrial buffer. Furthermore, mitochondrial density was lower in motor neurons and, in a network of neurons with physiological synaptic AMPA receptor input, RU-360 acutely induced an increase in Ca2+ transients. We conclude that motor neurons have an insufficient mitochondrial capacity to buffer large Ca2+ elevations which is partly due to a reduced mitochondrial density per volume compared to non-motor neurons. This may exert deleterious effects in motor neuron disease where mitochondrial function is thought to be compromised.

Ivermectin inhibits AMPA receptor-mediated excitotoxicity in cultured motor neurons and extends the life span of a transgenic mouse model of amyotrophic lateral sclerosis

alpha-Amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor-mediated excitotoxicity contributes to the selective motor neuron death in amyotrophic lateral sclerosis (ALS). In this study, we investigated the effect of P2 receptor-influencing substances on kainate-induced motor neuron death in an in vitro model for AMPA receptor-mediated excitotoxicity. Complete protection was found after preincubation of the motor neurons with ivermectin or Cibacron Blue 3G-A. Preincubation with both P2X4 modulators did not influence the number or Ca2+ permeability of the AMPA receptors and addition during kainate stimulation alone had no effect. Preincubation with a low concentration of ATP, the natural agonist of the P2X4 receptor, also protected the motor neurons against a subsequent excitotoxic stimulation, while high concentrations of ATP were toxic. Moreover, ivermectin increased the toxicity of low ATP concentrations, indicating that ivermectin can potentiate the effect of ATP on its receptor. Ivermectin and ATP also protected against hypoxia/hypoglycemia. To further investigate the relevance of these findings for ALS, we treated SOD1(G93A)-mice, a transgenic animal model for familial ALS, with ivermectin. This resulted in an extension of the life span of these mice with almost 10%. We conclude that ivermectin induces a mechanism in motor neurons, in vivo and in vitro, that protects against subsequent excitotoxic insults. Our in vitro data indicate that this protective mechanism is due to the potentiation by ivermectin of an effect of ATP mediated by the P2X4 receptor.

Excitotoxicity and amyotrophic lateral sclerosis

Since its description by Charcot more than 130 years ago, the pathogenesis of selective motor neuron degeneration in amyotrophic lateral sclerosis (ALS) remains unsolved. Over the years, many pathogenic mechanisms have been proposed. Amongst others these include: oxidative stress, excitotoxicity, aggregate formation, inflammation, growth factor deficiency and neurofilament disorganization. This multitude of contributing factors indicates that ALS is a complex disease and also suggests that ALS is a multifactorial disorder. Excitotoxicity is not the newest and most spectacular hypothesis in the ALS field, but it is undoubtedly one of the most robust pathogenic mechanisms supported by an impressive amount of evidence. Moreover, the therapeutic efficacy of riluzole, the only drug proven to slow disease progression in ALS, is most likely related to its anti-excitotoxic properties. In this review, we will give an overview of the arguments in favor of the involvement of excitotoxicity in ALS and of the possible mechanisms leading to motor neuron death. We will also summarize the intrinsic properties of motor neurons that render these cells particularly vulnerable to excitotoxicity and could explain the selective vulnerability of motor neurons in ALS. All this information could help to develop new and better therapeutic strategies that could protect motor neurons from excitotoxicity.

The role of excitotoxicity in the pathogenesis of amyotrophic lateral sclerosis

Unfortunately and despite all efforts, amyotrophic lateral sclerosis (ALS) remains an incurable neurodegenerative disorder characterized by the progressive and selective death of motor neurons. The cause of this process is mostly unknown, but evidence is available that excitotoxicity plays an important role. In this review, we will give an overview of the arguments in favor of the involvement of excitotoxicity in ALS. The most important one is that the only drug proven to slow the disease process in humans, riluzole, has anti-excitotoxic properties. Moreover, consumption of excitotoxins can give rise to selective motor neuron death, indicating that motor neurons are extremely sensitive to excessive stimulation of glutamate receptors. We will summarize the intrinsic properties of motor neurons that could render these cells particularly sensitive to excitotoxicity. Most of these characteristics relate to the way motor neurons handle Ca(2+), as they combine two exceptional characteristics: a low Ca(2+)-buffering capacity and a high number of Ca(2+)-permeable AMPA receptors. These properties most likely are essential to perform their normal function, but under pathological conditions they could become responsible for the selective death of motor neurons. In order to achieve this worst-case scenario, additional factors/mechanisms could be required. In 1 to 2% of the ALS patients, mutations in the SOD1 gene could shift the balance from normal motor neuron excitation to excitotoxicity by decreasing glutamate uptake in the surrounding astrocytes and/or by interfering with mitochondrial function. We will discuss point by point these different pathogenic mechanisms that could give rise to classical and/or slow excitotoxicity leading to selective motor neuron death.

Disrupted function and axonal distribution of mutant tyrosyl-tRNA synthetase in dominant intermediate Charcot-Marie-Tooth neuropathy

Charcot-Marie-Tooth (CMT) neuropathies are common disorders of the peripheral nervous system caused by demyelination or axonal degeneration, or a combination of both features. We previously assigned the locus for autosomal dominant intermediate CMT neuropathy type C (DI-CMTC) to chromosome 1p34-p35. Here we identify two heterozygous missense mutations (G41R and E196K) and one de novo deletion (153-156delVKQV) in tyrosyl-tRNA synthetase (YARS) in three unrelated families affected with DI-CMTC. Biochemical experiments and genetic complementation in yeast show partial loss of aminoacylation activity of the mutant proteins, and mutations in YARS, or in its yeast ortholog TYS1, reduce yeast growth. YARS localizes to axonal termini in differentiating primary motor neuron and neuroblastoma cultures. This specific distribution is significantly reduced in cells expressing mutant YARS proteins. YARS is the second aminoacyl-tRNA synthetase found to be involved in CMT, thereby linking protein-synthesizing complexes with neurodegeneration.

Excitotoxicity and ALS: what is unique about the AMPA receptors expressed on spinal motor neurons?

It has been repeatedly reported that spinal motor neurons are selectively vulnerable to AMPA receptor-mediated excitotoxicity. Therefore, identifying the uniqueness of AMPA receptors that are expressed on motor neurons, especially in individuals affected with sporadic amyotrophic lateral sclerosis (ALS) is essential for elucidating the etiology of this disorder. The mechanism that initiates motor neuronal death appears to be an exaggerated influx of Ca(2+) through AMPA receptors. The determinants that affect this Ca(2+) influx are Ca(2+) permeability, which is regulated by the presence of the GluR2 subunit and by RNA editing at the Q/R site of GluR2; channel desensitization, which is regulated by alternative splicing at the flip/flop site and by RNA editing at the R/G site of GluR subunits; and receptor density on the cell surface, which is controlled by many factors including regulatory proteins, direct phosphorylation and RNA editing at the Q/R site. This review focuses on recent progress on the molecular dynamics of AMPA receptors and discusses the pathophysiology of selective motor neuron death mediated by AMPA receptors in individuals affected with sporadic ALS.

GluR2 Deficiency Accelerates Motor Neuron Degeneration in a Mouse Model of Amyotrophic Lateral Sclerosis

AMPA receptor-mediated excitotoxicity has been implicated in the selective degeneration of motor neurons in amyotrophic lateral sclerosis (ALS). Motor neurons in vitro are particularly vulnerable to excessive AMPA receptor stimulation and one of the factors underlying this selective vulnerability is the presence of a large proportion of Ca2+ -permeable (i.e. GluR2-lacking) AMPA receptors. However, the precise role of GluR2-lacking AMPA receptors in motor neuron degeneration remains to be defined. We therefore studied the impact of GluR2 deficiency on motor neuron death in vitro and in vivo. Cultured motor neurons from GluR2-deficient embryos displayed an increased Ca2+ influx through AMPA receptors and an increased vulnerability to AMPA receptor-mediated excitotoxicity. We deleted the GluR2 gene in mutant SOD1G93A mice by crossbreeding them with GluR2 knockout mice. GluR2 deficiency clearly accelerated the motor neuron degeneration and shortened the life span of mutant SOD1G93A mice. These findings indicate that GluR2 plays a pivotal role in the vulnerability of motor neurons in vitro and in vivo, and that therapies that limit Ca2+ entry through AMPA receptors might be beneficial in ALS patients.

Rat embryonic motoneurons in long-term co-culture with Schwann cells—a system to investigate motoneuron diseases on a cellular level in vitro

Investigations of motoneuron diseases on a cellular and molecular level require long-term cultivation of primary cells. Here we present a new culture system in which matured motoneurons interact with their physiological partners like interneurons, astroglia and peripheral glia cells. This enables motoneuron-maturation for up to 3 weeks, while motoneurons consistently reached large diameters of their somata of 30-45 microm, occasionally more than 80 microm. Dissociated rat embryonic ventral spinal cord cells were enriched for motoneurons by density gradient centrifugation and seeded on a non-confluent mono-layer of highly enriched neonatal rat Schwann cells. Immunocytochemical visualization of neuron specific betaIII-tubulin in all neurons and of motoneuron specific non-phosphorylated neurofilament H/M, respectively, revealed that after 3 days in vitro >70% of all neurons were motoneurons. After 20 days in vitro, a motoneuron fraction of 12% was maintained. Motoneurons were susceptible to transient transfection with green fluorescent protein cDNA when liposomal transfection and an enhancer substance were combined. Synaptic connections enabled formation of spontaneously active neuronal networks which provide a culture model to study glutamate excitotoxicity and calcium deregulation on a molecular level. Both mechanisms are implied in the pathophysiology of amyotrophic lateral sclerosis, a neurodegenerative motoneuron disorder.

Spinal myoclonus responsive to topiramate

Spinal segmental myoclonus rarely occurs in peripheral neurological disorders. There are no data about the efficacy of topiramate (TPM) on spinal myoclonus. We describe a patient whose segmental myoclonus in amputation stump was ameliorated markedly by TPM.

Excitotoxic death induced by released glutamate in depolarized primary cultures of mouse cerebellar granule cells is dependent on GABAAreceptors and niflumic acid-sensitive chloride channels

Excitotoxic neuronal death has been linked to neurological and neurodegenerative diseases. Several studies have sought to clarify the involvement of Cl(-) channels in neuronal excitotoxicity using either N-methyl-D-aspartic acid (NMDA) or alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate/kainic acid agonists. In this work we induced excitotoxic death in primary cultures of cerebellar granule cells by means of endogenously released glutamate. Excitotoxicity was provoked by exposure to high extracellular K(+) concentrations ([K(+)](o)) for 5 min. Under these conditions, a Ca(2+)-dependent release of glutamate was evoked. When extracellular glutamate concentration rose to between 2 and 4 microM, cell viability was significantly reduced by 30-40%. The NMDA receptor antagonists (MK-801 and D-2-amino-5-phosphonopentanoic acid) prevented cell death. Exposure to high [K(+)](o) produced a (36)Cl(-) influx which was significantly reduced by picrotoxinin. In addition, the GABA(A) receptor antagonists (bicuculline, picrotoxinin and SR 95531) protected cells from high [K(+)](o)-triggered excitotoxicity and reduced extracellular glutamate concentration. The Cl(-) channel blockers niflumic acid and 5-nitro-2-(3-phenylpropylamino)benzoic acid also exerted a neuroprotective effect and reduced extracellular glutamate concentration, even though they did not reduce high [K(+)](o)-induced (36)Cl(-) influx. Primary cultures of cerebellar granule cells also contain a population of GABAergic neurons that released GABA in response to high [K(+)](o). Chronic treatment of primary cultures with kainic acid abolished GABA release and rendered granule cells insensitive to high [K(+)](o) exposure, even though NMDA receptors were functional. Altogether, these results demonstrate that, under conditions of membrane depolarization, low micromolar concentrations of extracellular glutamate might induce an excitotoxic process through both NMDA and GABA(A) receptors and niflumic acid-sensitive Cl(-) channels.

Increased GABAergic function in mouse models of Huntington's disease: Reversal by BDNF

Huntington's disease (HD) is characterized by loss of striatal gamma-aminobutyric acid (GABA)ergic medium-sized spiny projection neurons (MSSNs), whereas some classes of striatal interneurons are relatively spared. Striatal interneurons provide most of the inhibitory synaptic input to MSSNs and use GABA as their neurotransmitter. We reported previously alterations in glutamatergic synaptic activity in the R6/2 and R6/1 mouse models of HD. In the present study, we used whole-cell voltage clamp recordings to examine GABAergic synaptic currents in MSSNs from striatal slices in these two mouse models compared to those in age-matched control littermates. The frequency of spontaneous GABAergic synaptic currents was increased significantly in MSSNs from R6/2 transgenics starting around 5-7 weeks (when the overt behavioral phenotype begins) and continuing in 9-14-week-old mice. A similar increase was observed in 12-15-month-old R6/1 transgenics. Bath application of brain-derived neurotrophic factor, which is downregulated in HD, significantly reduced the frequency of spontaneous GABAergic synaptic currents in MSSNs from R6/2 but not control mice at 9-14 weeks. Increased GABA current densities also occurred in acutely isolated MSSNs from R6/2 animals. Immunofluorescence demonstrated increased expression of the ubiquitous alpha1 subunit of GABA(A) receptors in MSSNs from R6/2 animals. These results indicate that increases in spontaneous GABAergic synaptic currents and postsynaptic receptor function occur in parallel to progressive decreases in glutamatergic inputs to MSSNs. In conjunction, both changes will severely alter striatal outputs to target areas involved in the control of movement.

Long-lasting changes in GABA responsiveness in cultured neurons

In neuronal cells, GABA evokes an increase in chloride conductance by activating GABA(A) and GABA(C) receptors. In mature neurons, this increase in conductance generally has a hyperpolarizing and inhibitory action. Using gramicidin-based perforated patch recordings, we show that in cultured motor neurons GABA-induced currents are significantly altered following activation of GABA receptors coinciding with changes in membrane potential. Changes in intracellular chloride concentration constituted the mechanism for this modulation. Because of low resting chloride conductance and low activity of chloride transporters, changes in intracellular chloride concentration and hence GABA response were long-lasting (time constant of recovery was about 2.5 min). Cultured dorsal horn and hippocampal neurons exhibited a similar response pattern, suggesting a general property of cultured neuronal cells. These long-lasting changes in GABA responsiveness may have major implications on neuronal excitability.

Glutamate-induced elevations in intracellular chloride concentration in hippocampal cell cultures derived from EYFP-expressing mice

The homeostasis of intracellular Cl(-) concentration ([Cl(-)](i)) is critical for neuronal function, including gamma-aminobutyric acid (GABA)ergic synaptic transmission. Here, we investigated activity-dependent changes in [Cl(-)](i) using a transgenetically expressed Cl(-)-sensitive enhanced yellow-fluorescent protein (EYFP) in cultures of mouse hippocampal neurons. Application of glutamate (100 microm for 3 min) in a bath perfusion to cell cultures of various days in vitro (DIV) revealed a decrease in EYFP fluorescence. The EYFP signal increased in amplitude with increasing DIV, reaching a maximal response after 7 DIV. Glutamate application resulted in a slight neuronal acidification. Although EYFP fluorescence is sensitive to pH, EYFP signals were virtually abolished in Cl(-)-free solution, demonstrating that the EYFP signal represented an increase in [Cl(-)](i). Similar to glutamate, a rise in [Cl(-)](i) was also induced by specific ionotropic glutamate receptor agonists and by increasing extracellular [K(+)], indicating that an increase in driving force for Cl(-) suffices to increase [Cl(-)](i). To elucidate the membrane mechanisms mediating the Cl(-) influx, a series of blockers of ion channels and transporters were tested. The glutamate-induced increase in [Cl(-)](i) was resistant to furosemide, bumetanide and 4,4'-diisothiocyanato-stilbene-2,2'-disulphonic acid (DIDS), was reduced by bicuculline to about 80% of control responses, and was antagonized by niflumic acid (NFA) and 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB). We conclude that membrane depolarization increases [Cl(-)](i) via several pathways involving NFA- and NPPB-sensitive anion channels and GABA(A) receptors, but not through furosemide-, bumetanide- or DIDS-sensitive Cl(-) transporters. The present study highlights the vulnerability of [Cl(-)](i) homeostasis after membrane depolarization in neurons.

Synaptic signaling between neurons and glia

Rapid signaling between vertebrate neurons occurs primarily at synapses, intercellular junctions where quantal release of neurotransmitter triggers rapid changes in membrane conductance through activation of ionotropic receptors. Glial cells express many of these same ionotropic receptors, yet little is known about how receptors in glial cells become activated in situ. Because synapses were thought to be the sole provenance of neurons, it has been assumed that these receptors must be activated following diffusion of transmitter out of the synaptic cleft, or through nonsynaptic mechanisms such as transporter reversal. Two recent reports show that a ubiquitous class of progenitors that express the proteoglycan NG2 (NG2 cells) engage in rapid signaling with glutamatergic and gamma-aminobutyric acid (GABA)ergic neurons through direct neuron-glia synapses. Quantal release of transmitter from neurons at these sites triggers rapid activation of aminomethylisoxazole propionic acid (AMPA) or GABA(A) receptors in NG2 cells. These currents exhibit properties consistent with direct rather than spillover-mediated transmission, and electron micrographic analyses indicate that nerve terminals containing clusters of synaptic vesicles form discrete junctions with NG2 cell processes. Although activation of AMPA or GABA(A) receptors depolarize NG2 cells, these receptors are more likely to serve as routes for ion flux rather than as current sources for depolarization, because the amplitudes of the synaptic transients are small and the resting membrane potential of NG2 cells is highly negative. The ability of both glutamate and GABA to influence the morphology, physiology, and development of NG2 cells in vitro suggests that this rapid form of signaling may play important roles in adapting the behavior of these cells to the needs of surrounding neurons in vivo.

Synaptic signaling between GABAergic interneurons and oligodendrocyte precursor cells in the hippocampus

Oligodendrocyte precursor cells (OPCs) express receptors for many neurotransmitters, but the mechanisms responsible for their activation are poorly understood. We have found that quantal release of GABA from interneurons elicits GABA(A) receptor currents with rapid rise times in hippocampal OPCs. These currents did not exhibit properties of spillover transmission or release by transporters, and immunofluorescence and electron microscopy suggest that interneuronal terminals are in direct contact with OPCs, indicating that these GABA currents are generated at direct interneuron-OPC synapses. The reversal potential of OPC GABA(A) currents was -43 mV, and interneuronal firing was correlated with transient depolarizations induced by GABA(A) receptors; however, GABA application induced a transient inhibition of currents mediated by AMPA receptors in OPCs. These results indicate that OPCs are a direct target of interneuronal collaterals and that the GABA-induced Cl(-) flux generated by these events may influence oligodendrocyte development by regulating the efficacy of glutamatergic signaling in OPCs.