Selective blockade of the mGluR1 receptor reduces traumatic neuronal injury in vitro and improvesoOutcome after brain trauma

Preso enhances mGluR1-mediated excitotoxicity by modulating the phosphorylation of mGluR1-Homer1 complex and facilitating an ER stress after traumatic brain injury

Glutamate receptor (GluR)-mediated excitotoxicity is an important mechanism causing delayed neuronal injury after traumatic brain injury (TBI). Preso, as a core scaffolding protein of postsynaptic density (PSD), is considered an important regulator during excitotoxicity and TBI and combines with glutamate receptors to form functional units for excitatory glutamatergic neurotransmission, and elucidating the mechanisms of these functional units will provide new targets for the treatment of TBI. As a multidomain scaffolding protein, Preso directly interacts with metabotropic GluR (mGluR) and another scaffold protein, Homer. Because the mGluR-Homer complex plays a crucial role in TBI, modulation of this complex by Preso may be an important mechanism affecting the excitotoxic damage to neurons after TBI. Here, we demonstrate that Preso facilitates the interaction between metabotropic mGluR1 and Homer1 to activate mGluR1 signaling and cause excitotoxic neuronal injury and endoplasmic reticulum (ER) stress after TBI. The regulatory effect of Preso on the mGluR1-Homer1 complex is dependent on the direct association between Preso and this complex and also involves the phosphorylation of the interactive binding sites of mGluR1 and Homer1 by Preso. Further studies confirmed that Preso, as an adaptor of cyclin-dependent kinase 5 (CDK5), promotes the phosphorylation of the Homer1-binding site on mGluR1 by CDK5 and thereby enhances the interaction between mGluR1 and Homer1. Preso can also promote the formation of the mGluR1-Homer1 complex by inhibiting the phosphorylation of the Homer1 hinge region by Ca2+/calmodulin-dependent protein kinase IIα (CaMKIIα). Based on these molecular mechanisms, we designed several blocking peptides targeting the interaction between Preso and the mGluR1-Homer1 complex and found that directly disrupting the association between mGluR1 and scaffolding proteins significantly promotes the recovery of motor function after TBI.

Olfactory Bulb Excitotoxicity as a Gap-Filling Mechanism Underlying the Link Between Traumatic Brain Injury-Induced Secondary Neuronal Degeneration and Parkinson's Disease-Like Pathology

There is increasing preclinical and clinical data supporting a potential association between Traumatic Brain Injury (TBI) and Parkinson's disease (PD). It has been suggested that the glutamate-induced excitotoxicity underlying TBI secondary neuronal degeneration (SND) might be associated with further development of PD. Interestingly, an accumulation of extracellular glutamate and olfactory dysfunction are both sharing pathological conditions in TBI and PD. The possible involvement of glutamate excitotoxicity in olfactory dysfunction has been recently described, however, the role of olfactory bulbs (OB) glutamate excitotoxicity as a possible mechanism involved in the association between TBI and PD-related neurodegeneration has not been investigated yet. We examined the number of nigral dopaminergic neurons (TH +), nigral α-synuclein expression, the striatal dopamine transporter (DAT) expression, and motor performance after bilateral OB N-Methyl-D-Aspartate (NMDA)-induced excitotoxic lesions in rodents. Bulbar NMDA administration induced a decrease in the number of correct choices in the discrimination tests one week after lesions (p < 0.01) and a significant decrease in the number of nigral DAergic neurons (p < 0.01) associated with an increase in α-synuclein expression (p < 0.01). No significant striatal changes in DAT expression or motor alterations were observed. Our results show an association between TBI-induced SND and PD-related neurodegeneration suggesting that the OB excitotoxicity occurring in TBI SND may be a filling gap mechanism underlying the link between TBI and PD-like pathology.

G-Protein-Coupled Receptors and Ischemic Stroke: a Focus on Molecular Function and Therapeutic Potential

In ischemic stroke, there is only one approved drug, tissue plasminogen activator, to be used in clinical conditions for thrombolysis. New neuroprotective therapies for ischemic stroke are desperately needed. Several targets and pathways have been shown to confer neuroprotective effects in ischemic stroke. G-protein-coupled receptors (GPCRs) are one of the most frequently targeted receptors for developing novel therapeutics for central nervous system disorders. GPCRs are a large family of cell surface receptors that response to a wide variety of extracellular stimuli. GPCRs are involved in a wide range of physiological and pathological processes. More than 90% of the identified non-sensory GPCRs are expressed in the brain, where they play important roles in regulating mood, pain, vision, immune responses, cognition, and synaptic transmission. There is also good evidence that GPCRs are implicated in the pathogenesis of stroke. This review narrates the pathophysiological role and possible targeted therapy of GPCRs in ischemic stroke.

International Union of Basic and Clinical Pharmacology. CXI. Pharmacology, Signaling, and Physiology of Metabotropic Glutamate Receptors

Metabotropic glutamate (mGlu) receptors respond to glutamate, the major excitatory neurotransmitter in the mammalian brain, mediating a modulatory role that is critical for higher-order brain functions such as learning and memory. Since the first mGlu receptor was cloned in 1992, eight subtypes have been identified along with many isoforms and splice variants. The mGlu receptors are transmembrane-spanning proteins belonging to the class C G protein-coupled receptor family and represent attractive targets for a multitude of central nervous system disorders. Concerted drug discovery efforts over the past three decades have yielded a wealth of pharmacological tools including subtype-selective agents that competitively block or mimic the actions of glutamate or act allosterically via distinct sites to enhance or inhibit receptor activity. Herein, we review the physiologic and pathophysiological roles for individual mGlu receptor subtypes including the pleiotropic nature of intracellular signal transduction arising from each. We provide a comprehensive analysis of the in vitro and in vivo pharmacological properties of prototypical and commercially available orthosteric agonists and antagonists as well as allosteric modulators, including ligands that have entered clinical trials. Finally, we highlight emerging areas of research that hold promise to facilitate rational design of highly selective mGlu receptor-targeting therapeutics in the future. SIGNIFICANCE STATEMENT: The metabotropic glutamate receptors are attractive therapeutic targets for a range of psychiatric and neurological disorders. Over the past three decades, intense discovery efforts have yielded diverse pharmacological tools acting either competitively or allosterically, which have enabled dissection of fundamental biological process modulated by metabotropic glutamate receptors and established proof of concept for many therapeutic indications. We review metabotropic glutamate receptor molecular pharmacology and highlight emerging areas that are offering new avenues to selectively modulate neurotransmission.

Revisiting Traumatic Brain Injury: From Molecular Mechanisms to Therapeutic Interventions

Studying the complex molecular mechanisms involved in traumatic brain injury (TBI) is crucial for developing new therapies for TBI. Current treatments for TBI are primarily focused on patient stabilization and symptom mitigation. However, the field lacks defined therapies to prevent cell death, oxidative stress, and inflammatory cascades which lead to chronic pathology. Little can be done to treat the mechanical damage that occurs during the primary insult of a TBI; however, secondary injury mechanisms, such as inflammation, blood-brain barrier (BBB) breakdown, edema formation, excitotoxicity, oxidative stress, and cell death, can be targeted by therapeutic interventions. Elucidating the many mechanisms underlying secondary injury and studying targets of neuroprotective therapeutic agents is critical for developing new treatments. Therefore, we present a review on the molecular events following TBI from inflammation to programmed cell death and discuss current research and the latest therapeutic strategies to help understand TBI-mediated secondary injury.

Pre-treatment with microRNA-181a Antagomir Prevents Loss of Parvalbumin Expression and Preserves Novel Object Recognition Following Mild Traumatic Brain Injury

Mild traumatic brain injury (mTBI) can result in permanent impairment in memory and learning and may be a precursor to other neurological sequelae. Clinical treatments to ameliorate the effects of mTBI are lacking. Inhibition of microRNA-181a (miR-181a) is protective in several models of cerebral injury, but its role in mTBI has not been investigated. In the present study, miR-181a-5p antagomir was injected intracerebroventricularly 24 h prior to closed-skull cortical impact in young adult male mice. Paw withdrawal, open field, zero maze, Y maze, object location and novel object recognition tests were performed to assess neurocognitive dysfunction. Brains were assessed immunohistologically for the neuronal marker NeuN, the perineuronal net marker wisteria floribunda lectin (WFA), cFos, and the interneuron marker parvalbumin. Protein quantification was performed with immunoblots for synaptophysin and postsynaptic density 95 (PSD95). Fluorescent in situ hybridization was utilized to localize hippocampal miR-181a expression. MiR-181a antagomir treatment reduced neuronal miR-181a expression after mTBI, restored deficits in novel object recognition and increased hippocampal parvalbumin expression in the dentate gyrus. These changes were associated with decreased dentate gyrus hyperactivity indicated by a relative reduction in PSD95 and cFos expression. These results suggest that miR-181a inhibition may be a therapeutic approach to reduce hippocampal excitotoxicity and prevent cognitive dysfunction following mTBI.

Knockdown of RTN1-C attenuates traumatic neuronal injury through regulating intracellular Ca2+ homeostasis

Reticulons (RTNs) are a family of membrane-bound proteins that are dominantly localized to the endoplasmic reticulum (ER) membrane. RTN1-C is one member of RTNs abundantly expressed in the brain and has been shown to mediate neuronal injury in cerebral ischemia models. In the present study, we investigated the role of RTN1-C in an in vitro brain trauma model mimicked by traumatic neuronal injury (TNI) in primary cultured cortical neurons. TNI increased the expression of RTN1-C in cortical neurons but had no effect on RTN1-A and RTN1-B. Knockdown of RTN1-C with specific siRNA (Si-RTN1-C) significantly decreased cytotoxicity and apoptosis after TNI. The results of Ca²⁺ imaging showed that intracellular Ca²⁺ overload induced by TNI was attenuated by RTN1-C knockdown. Furthermore, the activation of metabotropic glutamate receptor 1 (mGluR1)-induced Ca²⁺ response was partially prevented by Si-RTN1-C transfection. We also evaluated the role of RTN1-C in store-operated Ca²⁺ entry (SOCE) in cortical neurons using the ER Ca²⁺ inducer thapsigargin (Tg). The results showed that knockdown of RTN1-C alleviated the SOCE-mediated Ca²⁺ influx and decreased the expression of stromal interactive molecule 1 (STIM1). In summary, the present study found that knockdown of RTN1-C protected neurons against TNI via preservation of intracellular Ca²⁺ homeostasis, which was associated with the inhibition of mGluR1-mediated ER Ca²⁺ release and suppression of STIM1-related SOCE. Thus, RTN1-C might represent a therapeutic target for traumatic brain injury (TBI) research.

Peroxiredoxin-3 attenuates traumatic neuronal injury through preservation of mitochondrial function

Peroxiredoxins (PRDXs) are a highly conserved family of thiol peroxidases that scavenge peroxides in cells. PRDX3 is one member of PRDXs localized in the mitochondria, and has been shown to be involved in antioxidant defense and redox signaling. In this study, we investigated the role of PRDX3 in neuronal trauma using a traumatic neuronal injury (TNI) model in primary cultured cortical neurons. We found that TNI significantly decreased the expression of PRDX3 at both mRNA and protein levels. Overexpression of PRDX3 by lentivirus (LV-PRDX3) transfection attenuated lactate dehydrogenase (LDH) release and neuronal apoptosis after TNI. The results of immunostaining showed that LV-PRDX3 transfection markedly reduced TNI-induced intracellular ROS production, protein radical formation and lipid peroxidation. In addition, overexpression of PRDX3 preserved mitochondrial membrane potential (MMP) levels and ATP generation, and inhibited mitochondrial cytochrome c release in TNI-injured neurons. The results of polymerase chain reaction (PCR) showed that PRDX3 overexpression also increased mitochondrial DNA (mtDNA) content and upregulated the expression of mitochondrial biogenesis-related factors. Taken together, our data demonstrate that PRDX3 protects against TNI insult by preserving mitochondrial function and mitochondrial biogenesis, and may have potential therapeutic value for traumatic brain injury (TBI).

Down-Regulation of IRF6 Protects Cortical Neurons Against Traumatic Neuronal Injury Through Activating Akt-eNOS Pathway

Interferon regulatory factor 6 (IRF6) is a novel and unique member of the IRF family of transcription factors, and the regulation and function of IRF6 remain unknown. Recently, IRF6 was shown to be upregulated after TBI and could promote neuronal apoptosis under oxidative stress conditions. This study aimed to investigate the role of IRF6 in traumatic neuronal injury (TNI) in primary cultured mouse cortical neurons. We found that the expression of IRF6 was significantly increased within 48 after TNI, and peaked at 24 h. Knockdown of IRF6 using specific targeted small interfering RNA (siRNA) attenuated TNI-induced loss of neuronal viability and release of lactate dehydrogenase. The results of TUNEL staining showed that IRF6 knockdown markedly reduced neuronal apoptosis, which was accompanied by decreased activity of caspase-3. Furthermore, downregulation of IRF6 inhibited lipid peroxidation, promoted the activity of endogenous antioxidative enzymes, and differently regulated the expression of inflammatory cytokines after TNI. In addition, IRF6 knockdown significantly increased phosphorylation of Akt and endothelial nitric oxide synthase (eNOS), whereas blocking Akt-eNOS pathway via selective antagonists partly prevented the protective effects of IRF6 knockdown. These data show that downregulation of IRF6 affords protection against TNI through Akt-eNOS pathway-mediated antioxidative and anti-inflammatory activity.

Increased expression of ApoA1 after neuronal injury may be beneficial for healing

ApoA1 is a player in reverse cholesterol transport that initiates multiple cellular pathways on binding to its receptor ABCA1. Its relation to neuronal injury is however unclear. We found ApoA1 to be increasingly abundant at a later time point in the secondary phase of traumatic spinal cord injury. In a cellular injury model of neuroblastoma, ApoA1 showed an initial diminished expression after infliction of injury, which sharply increased thereafter. Subsequently, ApoA1 was shown to alter wound healing dynamics in neuroblastoma injury model. It was observed that an initial lag in scratch wound closure was followed by rapid healing in the ApoA1 treatment group. Activation of ERK pathway and Actin polymerisation by ApoA1 corroborated its role in healing after neuronal injury. We propose that ApoA1 is increasingly expressed and secreted as a delayed response to neuronal injury, and this is a self-protecting mechanism of the injured system.

Altered expression of metabotropic glutamate receptor 1 alpha after acute diffuse brain injury: Effect of the competitive antagonist 1-aminoindan-1, 5-dicarboxylic acid

The diffuse brain injury model was conducted in Sprague-Dawley rats, according to Marmarou's free-fall attack. The water content in brain tissue, expression of metabotropic glutamate receptor 1α mRNA and protein were significantly increased after injury, reached a peak at 24 hours, and then gradually decreased. After treatment with the competitive antagonist of metabotropic glutamate receptor 1α, (RS)-1-aminoindan-1, 5-dicarboxylic acid, the water content of brain tissues decreased between 12–72 hours after injury, and neurological behaviors improved at 2 weeks. These experimental findings suggest that the 1-aminoindan-1, 5-dicarboxylic acid may result in marked neuroprotection against diffuse brain injury.

Knockdown of STIM1 improves neuronal survival after traumatic neuronal injury through regulating mGluR1-dependent Ca(2+) signaling in mouse cortical neurons

Activation of glutamate receptors and followed increase of intracellular calcium concentration is a key pathological mechanism involved in secondary neuronal injury after traumatic brain injury (TBI). Stromal interaction molecule (STIM) proteins are considered to be important players in regulating neuronal Ca(2+) homeostasis under normal aging and pathological conditions. Here, we investigated the role of STIM1 in regulating metabotropic glutamate receptor 1 (mGluR1)-related Ca(2+) signaling and neuronal survival by using an in vitro traumatic neuronal injury (TNI) model. The expression of STIM1 was significantly increased at both mRNA and protein levels after TNI. Down-regulation of STIM1 by specific small interfere RNA significantly preserved neuronal viability, decreased lactate dehydrogenase release, and inhibited apoptotic cell death after traumatic injury. Moreover, knockdown of STIM1 significantly alleviated the mGluR1-related increase of cytoplasmic Ca(2+) levels after TNI. By analyzing Ca(2+) imaging in Ca(2+)-free conditions, we demonstrated that the mGluR1-dependent inositol trisphosphate receptor and/or ryanodine receptor-mediated Ca(2+) release from the endoplasmic reticulum after TNI is strongly attenuated in the absence of STIM1. Together, our results demonstrate that in the mammalian nervous system, STIM1 is a key regulator of mGluR1-dependent Ca(2+) signaling and knockdown of STIM1 might be an effective intervention target in TBI.

Long-Term Effects of Enriched Environment on Neurofunctional Outcome and CNS Lesion Volume After Traumatic Brain Injury in Rats

To determine whether the exposure to long term enriched environment (EE) would result in a continuous improvement of neurological recovery and ameliorate the loss of brain tissue after traumatic brain injury (TBI) vs. standard housing (SH). Male Sprague-Dawley rats (300-350 g, n=28) underwent lateral fluid percussion brain injury or SHAM operation. One TBI group was held under complex EE for 90 days, the other under SH. Neuromotor and sensorimotor dysfunction and recovery were assessed after injury and at days 7, 15, and 90 via Composite Neuroscore (NS), RotaRod test, and Barnes Circular Maze (BCM). Cortical tissue loss was assessed using serial brain sections. After day 7 EE animals showed similar latencies and errors as SHAM in the BCM. SH animals performed notably worse with differences still significant on day 90 (p<0.001). RotaRod test and NS revealed superior results for EE animals after day 7. The mean cortical volume was significantly higher in EE vs. SH animals (p=0.003). In summary, EE animals after lateral fluid percussion (LFP) brain injury performed significantly better than SH animals after 90 days of recovery. The window of opportunity may be wide and also lends further credibility to the importance of long term interventions in patients suffering from TBI.

Down-regulation of Homer1b/c expression protects cultured neurons after traumatic injury

Activation of metabotropic glutamate receptor 1a aggravates traumatic brain injury. The constitutively expressed protein Homer1b/c participates in delivering and anchoring metabotropic glutamate receptors in neurons. Here, we aimed to verify whether down-regulation of Homer1b/c by RNA interference could protect cultured rat cortical neurons from traumatic injury. We showed that 36 hours after transfection of Homer1b/c small interfering RNA, metabotropic glutamate receptor 1a was present only in the neuronal cytoplasm, but not in the dendrites. Calcium fluorescence intensity was also decreased significantly. Moreover, lactate dehydrogenase concentration was significantly decreased in Homer1b/c small interfering RNA-transfected cells compared with that in untransfected and control small interfering RNA-transfected cells 24 hours after traumatic neuronal injury. Our findings indicate that down-regulation of Homer1b/c could reduce metabotropic glutamate receptor 1a transfer from the cell body to the dendrite, relieve calcium overload, and protect neurons from traumatic injury.

Postsynaptic scaffold protein Homer 1a protects against traumatic brain injury via regulating group I metabotropic glutamate receptors

Traumatic brain injury (TBI) produces excessive glutamate, leading to excitotoxicity via the activation of glutamate receptors. Postsynaptic density scaffold proteins have crucial roles in mediating signal transduction from glutamate receptors to their downstream mediators. Therefore, studies on the mechanisms underlying regulation of excitotoxicity by scaffold proteins can uncover new treatments for TBI. Here, we demonstrated that the postsynaptic scaffold protein Homer 1a was neuroprotective against TBI in vitro and in vivo, and this neuroprotection was associated with its effects on group I metabotropic glutamate receptors (mGluRs). Upon further study, we found that Homer 1a mainly affected neuronal injury induced by mGluR1 activation after TBI and also influenced mGluR5 function when its activity was restored. The ability of Homer 1a to disrupt mGluR-ERK signaling contributed to its ability to regulate the functions of mGluR1 and mGluR5 after traumatic injury. Intracellular Ca(2+) and PKC were two important factors involved in the mediation of mGluR-ERK signaling by Homer 1a. These results define Homer 1a as a novel endogenous neuroprotective agent against TBI.

Downregulation of Homer1b/c improves neuronal survival after traumatic neuronal injury

Homer protein, a member of the post-synaptic density protein family, plays an important role in the neuronal synaptic activity and is extensively involved in neurological disorders. The present study investigates the role of Homer1b/c in modulating neuronal survival by using an in vitro traumatic neuronal injury model, which was achieved by using a punch device that consisted of 28 stainless steel blades joined together and produced 28 parallel cuts. Downregulation of Homer1b/c by specific siRNA significantly (p<0.05) alleviated the cytoplasmic calcium levels and neuron lactate dehydrogenase release, and ultimately decreased the apoptotic rate after traumatic neuronal injury compared with non-targeting siRNA control treatment in cultured rat cortical neurons. Moreover, the expression of metabotropic glutamate receptor 1a (mGluR1a) was significantly (p<0.05) reduced in the Homer1b/c siRNA-transfected neurons after injury. Therefore, Homer1b/c not only modulated the mGluR1a-inositol 1,4,5-triphosphate receptors-Ca(2+) signal transduction pathway, but also regulated the expression of mGluR1a in mechanical neuronal injury. These findings indicate that the suppression of Homer1b/c expression potentially protects neurons from glutamate excitotoxicity after injury and might be an effective intervention target in traumatic brain injury.

PPAR agonists as therapeutics for CNS trauma and neurological diseases

Traumatic injury or disease of the spinal cord and brain elicits multiple cellular and biochemical reactions that together cause or are associated with neuropathology. Specifically, injury or disease elicits acute infiltration and activation of immune cells, death of neurons and glia, mitochondrial dysfunction, and the secretion of substrates that inhibit axon regeneration. In some diseases, inflammation is chronic or non-resolving. Ligands that target PPARs (peroxisome proliferator-activated receptors), a group of ligand-activated transcription factors, are promising therapeutics for neurologic disease and CNS injury because their activation affects many, if not all, of these interrelated pathologic mechanisms. PPAR activation can simultaneously weaken or reprogram the immune response, stimulate metabolic and mitochondrial function, promote axon growth and induce progenitor cells to differentiate into myelinating oligodendrocytes. PPAR activation has beneficial effects in many pre-clinical models of neurodegenerative diseases and CNS injury; however, the mechanisms through which PPARs exert these effects have yet to be fully elucidated. In this review we discuss current literature supporting the role of PPAR activation as a therapeutic target for treating traumatic injury and degenerative diseases of the CNS.

Inhibition of the group I mGluRs reduces acute brain damage and improves long-term histological outcomes after photothrombosis-induced ischaemia

Group I mGluRs (metabotropic glutamate receptors), including mGluR1 and mGluR5, are GPCRs (G-protein coupled receptors) and play important roles in physiology and pathology. Studies on their role in cerebral ischaemia have provided controversial results. In this study, we used a PT (photothrombosis)-induced ischaemia model to investigate whether antagonists to the group I mGluRs may offer acute and long-term protective effects in adult mice. Our results demonstrated that administration with mGluR5 antagonist MPEP [2-methyl-6-(phenylethynyl)-pyridine] or mGluR1 antagonist LY367385 by intraperitoneal injection at 3 h after PT decreased brain infarct volume evaluated one day after ischaemia. Additive effects on infarct volume were observed upon co-injection with MPEP and LY367385. These antagonists also significantly alleviated neurodegeneration and apoptosis in the penumbra. In addition, when evaluated 2 weeks after PT, they reduced infarct volume and tissue loss, attenuated glial scar formation, and inhibited cell proliferation in the penumbra. Importantly, co-injection with MPEP and LY367385 reduced the expression levels of calpain, a Ca2+-activated protease known to mediate ischaemia-induced neuronal death. Injection of calpeptin, a calpain inhibitor, could inhibit neuronal death and brain damage after PT but injection of calpeptin together with MPEP and LY367385 did not further improve the protective effects mediated by MPEP and LY367385. These results suggest that inhibition of group I mGluRs is sufficient to protect ischaemic damage through the calpain pathway. Taken together, our results demonstrate that inhibition of group I mGluRs can mitigate PT-induced brain damage through attenuating the effects of calpain, and improve long-term histological outcomes.

Effects of antagonists of glutamate receptors on pro-inflammatory cytokines in the brain cortex of rats subjected to experimental autoimmune encephalomyelitis

Multiple sclerosis (MS) is an inflammatory demyelinating disease of the central nervous system (CNS). Inflammatory cytokines and glutamate neurotoxicity have been proposed as major determinants accompanying the demyelination and axonal degeneration observed during the course of MS. The present study using the animal model of MS known as experimental autoimmune encephalomyelitis (EAE) demonstrates that pharmacological inhibition of ionotropic NMDA glutamate receptors by their antagonists (amantadine and memantine) suppresses neurological symptoms of disease in EAE rats and reduces expression of pro-inflammatory cytokines in the brain. Conversely, antagonists of group I metabotropic glutamate receptors, mGluRs (LY 367385 and MPEP), do not affect the inflammatory process and the neurological condition of EAE rats.

A review of neuroprotection pharmacology and therapies in patients with acute traumatic brain injury

Traumatic brain injury (TBI) affects 1.6 million Americans annually. The injury severity impacts the overall outcome and likelihood for survival. Current treatment of acute TBI includes surgical intervention and supportive care therapies. Treatment of elevated intracranial pressure and optimizing cerebral perfusion are cornerstones of current therapy. These approaches do not directly address the secondary neurological sequelae that lead to continued brain injury after TBI. Depending on injury severity, a complex cascade of processes are activated and generate continued endogenous changes affecting cellular systems and overall outcome from the initial insult to the brain. Homeostatic cellular processes governing calcium influx, mitochondrial function, membrane stability, redox balance, blood flow and cytoskeletal structure often become dysfunctional after TBI. Interruption of this cascade has been the target of numerous pharmacotherapeutic agents investigated over the last two decades. Many agents such as selfotel, pegorgotein (PEG-SOD), magnesium, deltibant and dexanabinol were ineffective in clinical trials. While progesterone and ciclosporin have shown promise in phase II studies, success in larger phase III, randomized, multicentre, clinical trials is pending. Consequently, no neuroprotective treatment options currently exist that improve neurological outcome after TBI. Investigations to date have extended understanding of the injury mechanisms and sites for intervention. Examination of novel strategies addressing both pathological and pharmacological factors affecting outcome, employing novel trial design methods and utilizing biomarkers validated to be reflective of the prognosis for TBI will facilitate progress in overcoming the obstacles identified from previous clinical trials.

In vitro stretch injury induces time- and severity-dependent alterations of STEP phosphorylation and proteolysis in neurons

Striatal-enriched tyrosine phosphatase (STEP) has been identified as a component of physiological and pathophysiological signaling pathways mediated by N-methyl-d-aspartate (NMDA) receptor/calcineurin/calpain activation. Activation of these pathways produces a subsequent change in STEP isoform expression or activation via dephosphorylation. In this study, we evaluated changes in STEP phosphorylation and proteolysis in dissociated cortical neurons after sublethal and lethal mechanical injury using an in vitro stretch injury device. Sublethal stretch injury produces minimal changes in STEP phosphorylation at early time points, and increased STEP phosphorylation at 24 h that is blocked by the NMDA-receptor antagonist APV, the calcineurin-inhibitor FK506, and the sodium channel blocker tetrodotoxin. Lethal stretch injury produces rapid STEP dephosphorylation via NR2B-containing NMDA receptors, but not calcineurin, and a subsequent biphasic phosphorylation pattern. STEP(61) expression progressively increases after sublethal stretch with no change in calpain-mediated STEP(33) formation, while lethal stretch injury results in STEP(33) formation via a NR2B-containing NMDA receptor pathway within 1 h of injury. Blocking calpain activation in the initial 30 min after stretch injury increases the ratio of active STEP in cells and blocks STEP(33) formation, suggesting that STEP is an early substrate of calpain after mechanical injury. There is a strong correlation between the amount of STEP(33) formed and the degree of cell death observed after lethal stretch injury. In summary, these data demonstrate that previously characterized pathways of STEP regulation via the NMDA receptor are generally conserved in mechanical injury, and suggest that calpain-mediated cleavage of STEP(33) should be further examined as an early marker of neuronal fate after stretch injury.

Selective mGluR1 antagonist EMQMCM inhibits the kainate-induced excitotoxicity in primary neuronal cultures and in the rat hippocampus

Abundant evidence suggests that indirect inhibitory modulation of glutamatergic transmission, via metabotropic glutamatergic receptors (mGluR), may induce neuroprotection. The present study was designed to determine whether the selective antagonist of mGluR1 (3-ethyl-2-methyl-quinolin-6-yl)-(4-methoxy-cyclohexyl)-methanone methanesulfonate (EMQMCM), showed neuroprotection against the kainate (KA)-induced excitotoxicity in vitro and in vivo. In in vitro studies on mouse primary cortical and hippocampal neuronal cultures, incubation with KA (150 μM) induced strong degeneration [measured as lactate dehydrogenase (LDH) efflux] and apoptosis (measured as caspase-3 activity). EMQMCM (0.1-100 μM) added 30 min to 6 h after KA, significantly attenuated the KA-induced LDH release and prevented the increase in caspase-3 activity in the cultures. Those effects were dose- and time-dependent. In in vivo studies KA (2.5 nmol/1 μl) was unilaterally injected into the rat dorsal CA1 hippocampal region. Degeneration was calculated by counting surviving neurons in the CA pyramidal layer using stereological methods. It was found that EMQMCM (5-10 nmol/1 μl) injected into the dorsal hippocampus 30 min, 1 h, or 3 h (the higher dose only) after KA significantly prevented the KA-induced neuronal degeneration. In vivo microdialysis studies in rat hippocampus showed that EMQMCM (100 μM) significantly increased γ-aminobutyric acid (GABA) and decreased glutamate release. When perfused simultaneously with KA, EMQMCM substantially increased GABA release and prevented the KA-induced glutamate release. The obtained results indicate that the mGluR1 antagonist, EMQMCM, may exert neuroprotection against excitotoxicity after delayed treatment (30 min to 6 h). The role of enhanced GABAergic transmission in the neuroprotection is postulated.

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.

Metabotropic glutamate receptor-mediated signaling in neuroglia

Metabotropic glutamate (mGlu) receptors are G-protein-coupled receptors, which include eight subtypes that have been classified into three groups (I-III) based upon sequence homology, signal transduction mechanism and pharmacological profile. Although most studied with regard to neuronal function and modulation, mGlu receptors are also expressed by neuroglia-including astrocytes, microglia and oligodendrocytes. Activation of mGlu receptors on neuroglia under both physiologic and pathophysiologic conditions mediates numerous actions that are essential for intrinsic glial cell function, as well as for glial-neuronal interactions. Astrocyte mGlu receptors play important physiological roles in regulating neurotransmission and maintaining neuronal homeostasis. However, mGlu receptors on astrocytes and microglia also serve to modulate cell death and neurological function in a variety of pathophysiological conditions such as acute and chronic neurodegenerative disorders. The latter effects are complex and bi-directional, depending on which mGlu receptor sub-types are activated.

Protective effects of mGluR5 positive modulators against traumatic neuronal injury through PKC-dependent activation of MEK/ERK pathway

Several previous studies utilizing selective pharmacological antagonists have demonstrated that type 5 metabotropic glutamate receptors (mGluR5) are potential therapeutic targets for the treatment of numerous disorders of the central nervous system, but the role of mGluR5 activation in traumatic brain injury (TBI) is not fully understood. Here in an in vitro TBI model, the mGluR5 agonist (RS)-2-chloro-5- hydroxyphenylglycine (CHPG) and the positive allosteric modulators 3-cyano-N-(1,3- diphenyl-1H-pyrazol-5-yl) benzamide (CDPPB) were used to investigate the neuroprotective potency of mGluR5 activation. Data showed that CHPG and CDPPB suppressed the increase of LDH release and caspase-3 activation induced by traumatic neuronal injury in a dose-dependent manner, and the salutary effects were also present when these compounds were added 1 h after injury. Western blot was used to examine the activation of three members of mitogen-activated protein kinases: extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and p38 kinase (p38). CHPG and CDPPB enhanced the activation of ERK after traumatic neuronal injury, and PD98059 and U0126, two selective MEK/ERK inhibitors, partly revised the protective effects. Furthermore, we also investigated the role of protein kinase C (PKC) in CHPG and CDPPB-induced neuroprotection. With the pretreatment of chelerythrine chloride, a PKC inhibitor, the surpressing effects of CHPG and CDPPB on traumatic injury-evoked LDH release and caspase-3 activation were blocked. All of these findings extended the protective role of mGluR5 activation in an in vitro model of TBI and suggested that these protective effects might be mediated by the PKC-dependent activation of MEK/ERK pathway. These results may have important implications for the development of mGluR5 modulators to treat TBI.

The selective mGluR5 agonist CHPG protects against traumatic brain injury in vitro and in vivo via ERK and Akt pathway

Group I metabotropic glutamate receptors (mGluRs) have been implicated in the pathophysiology of central nervous system injury, but the role of mGluR5 in traumatic brain injury (TBI) remains unclear. In the present study, we investigated the neuroprotective potency of (R,S)-2-chloro-5-hydroxyphenylglycine (CHPG), a selective mGluR5 agonist, for protecting against TBI in both in vitro and in vivo models. Primary cortical neurons were treated with 1 mM CHPG in an in vitro preparation 30 min before TBI, and 250 nM CHPG was injected into the right lateral ventricle of rats 30 min before TBI was induced in in vivo studies. The results showed that CHPG significantly attenuated lactate dehydrogenase (LDH) release and neuronal apoptosis and reduced lesion volume. Compared to the control or vehicle group, the phosphorylation levels of extracellular signal-regulated kinase (ERK) and Akt were increased in the presence of CHPG, even following the induction of TBI. Furthermore, treatment with either the ERK inhibitor PD98059 or Akt inhibitor LY294002 partially reversed the CHPG's neuroprotective effects. These data suggest that CHPG minimizes brain damage after induction of TBI both in vitro and in vivo, and that these protective effects were possibly mediated by activation of the ERK and Akt signaling pathways. Thus, potentiating mGluR5 activity with selective agonists such as CHPG may be useful for the treatment of traumatic brain injury.

Neuroadaptive changes in cerebellar neurons induced by chronic exposure to IL-6

IL-6 is an important signaling molecule in the CNS. CNS neurons express IL-6 receptors and their signal transduction molecules, consistent with a role for IL-6 in neuronal physiology. Research indicates that IL-6 levels are low in the normal brain but can be significantly elevated in CNS injury and disease. Relatively little is known about how the elevated levels of IL-6 affect neurons. In the current study we show that under conditions of chronic exposure, IL-6 induces alterations in the level of protein expression in developing CNS cells. Such changes may play a role in the altered CNS function observed in CNS conditions associated with elevated levels of IL-6 in the CNS.

The role of glutamate receptors in traumatic brain injury: implications for postsynaptic density in pathophysiology

Traumatic brain injury (TBI) is the major cause of death and disability, and the incidence of TBI continues to increase rapidly. In recent years, increasing attention has been paid to an important structure at the postsynaptic membrane: the postsynaptic density (PSD). Glutamate receptors, as major components of the PSD, are highly responsive to alterations in the glutamate concentration at excitatory synapses and activate intracellular signal transduction via calcium and other second messengers following TBI. PSD scaffold proteins (PSD-95, Homer, and Shank), which anchor glutamate receptors and form a network structure, also have potential effects on these downstream signaling pathways. The changes in the function and structure of these major PSD proteins are also induced by TBI, indicating that there is a more complicated mechanism associated with PSD proteins in the pathophysiological process of TBI.

Novel neuroinflammatory targets in the chronically injured spinal cord

Injury to the spinal cord is known to result in inflammation. To date, the preponderance of research has focused on the acute neuroinflammatory response, which begins immediately and is believed to terminate within hours to (at most) days after the injury. However, recent studies have demonstrated that postinjury inflammation is not restricted to the first few hours or days after injury, but can last for months to years after a spinal cord injury (SCI). These chronic studies have revealed that increased numbers of inflammatory cells, such as microglia and macrophages, and inflammatory factors, including cytokines, chemokines, and enzyme products are found at markedly delayed times after injury. Here we review experimental work on a selection of the novel inflammatory factors observed chronically after SCI, including the nicotinamide adenine dinucleotide phosphate-oxidase (NADPH) oxidase enzyme and galectin-3. We will discuss the role of these proteins in inflammation with regard to both detrimental and beneficial effects of neuroinflammation after injury. Finally, the potential of these proteins to serve as therapeutic targets will be considered, and a novel therapeutic approach (i.e., the agonist for metabotropic glutamate receptor 5 [mGluR5], [RS]-2-Chloro-5-hydroxyphenylglycine [CHPG]) will be discussed. This review will demonstrate the expression and activity profiles, roles in potentiation of injury, and therapy studies of these inflammatory factors suggest that not only are these chronically expressed factors viable targets for SCI treatment, but that the therapeutic window is broader than has previously been thought.

Effect of the class I metabotropic glutamate receptor antagonist AIDA on certain behaviours in rats with experimental chronic hyperammonemia

PURPOSE

This study examines possible interactions between behavioral effects and mGluR1 (class I metabotropic glutamate receptor) by injecting AIDA [(RS)-1-aminoindan-1,5-dicarboxylic acid] in rats with experimental chronic hyperammonemia (chHA).

MATERIAL/METHODS

The effects of mGluR1 antagonist on some behaviors were tested in control groups of rats and in rats with chHA. Experimental chHA was induced by intraperitoneal injection of ammonium acetate (12 mmol/kg) for five consecutive days. We used the following behavioural tests: the open field test, the passive avoidance test and the elevated "plus" maze.

RESULTS

In control rats AIDA administered intracerebroventricularly (i.c.v.) at the dose 100 nmol decreased the number of crossings and bar approaches in the open field test and impaired acquisition and recall in the passive avoidance situation. ChHA significantly inhibited locomotor and exploratory activity and profoundly impaired acquisition and recall processes in the passive avoidance test and significantly increased acute stress responses. AIDA increased locomotor activity in chHA rats (especially number of crossed fields and rearings) and produced anxiety enhancement in rats with chHA. AIDA used in rats with chHA significantly improved acquisition and retrieval processes.

CONCLUSIONS

The obtained results suggest that AIDA, the antagonist of mGluR1, had beneficial effects on learning and memory in rats with experimental chronic hyperammonemia.

Effects of TRH and its analogues on primary cortical neuronal cell damage induced by various excitotoxic, necrotic and apoptotic agents

The tripeptide thyrotropin-releasing hormone (TRH, pGlu-His-Pro-NH2) has been shown to possess neuroprotective activity in in vitro and in vivo models. Since its potential utility is limited by relatively rapid metabolism, metabolically stabilized analogues have been constructed. In the present study we investigated the influence of TRH and its three stable analogues: Montirelin (MON, CG-3703), RGH-2202 (L-6-keto-piperidine-2carbonyl-l-leucyl-l-prolinamide) and Z-TRH (N-carbobenzyloxy-pGlutamyl-Histydyl-Proline) in various models of mouse cortical neuronal cell injury. Twenty four hour pre-treatment with TRH and its analogues in low micromolar concentrations attenuated the neuronal cell death evoked by excitatory amino acids (EAAs: glutamate, NMDA, kainate, quisqualate) and hydrogen peroxide. All the peptides showed neuroprotective action on staurosporine (St)-evoked apoptotic neuronal cell death, but this effect was caspase-3 independent. Interestingly, in mixed neuronal-glial cell preparations only MON decreased St- and glutamate-evoked neurotoxicity. None of the peptides inhibited the doxorubicin- and lactacystin-induced neuronal cortical cell death, agents acting via activation of death receptor (FAS) or inhibition of proteasome function, respectively. Furthermore, we found that neither inhibitors of PI3-K (wortmannin, LY 294002) nor MAPK/ERK1/2 (PD 098059, U 0126) were able to inhibit neuroprotective properties of TRH and MON in St model of apoptosis. The protection mediated by TRH and MON it that model was also not connected with influence of peptides on the pro-apoptotic GSK-3beta and JNK protein kinase expression and activity. Further studies showed that calpains, calcium-activated proteases were induced by Glu, but not by St in cortical neurons. Moreover, the Glu-evoked increase in spectrin alpha II cleavage product induced by calpains was blocked by TRH. The obtained data showed that the potency of TRH and its analogues in inhibiting EAAs- and H(2)O(2)-induced neuronal cell death from the highest to lowest activity was: MON>TRH>Z-TRH>RHG. Interestingly, all peptides were active against St-induced apoptosis, however, on concentration basis MON was far more potent than the other peptides. None of the peptides inhibited Dox- and LC-evoked apoptotic cell death. Additionally, the data exclude potential role of pro-survival (PI3-K/Akt and MAPK/ERK1/2) and pro-apoptotic (GSK-3beta and JNK) pathways in neuroprotective effects of TRH and its analogues on St-induced neuronal apoptosis. Moreover, the results point to involvement of the inhibition of calpains in the TRH neuroprotective effect in Glu model of neuronal cell death.

Metabotropic glutamate receptor 5 activation inhibits microglial associated inflammation and neurotoxicity

The Group I metabotropic glutamate receptor 5 (mGluR5) can modulate addiction, pain, and neuronal cell death. Expression of some mGluRs, such as Group II and III mGluRs, has been reported in microglia and may affect their activation. However, the expression and role of mGluR5 in microglia is unclear. Using immunocytochemistry and Western blot, we demonstrate that mGluR5 protein is expressed in primary microglial cultures. Activation of mGluR5 using the selective agonist (RS)-2-chloro-5-hydroxyphenylglycine (CHPG) significantly reduces microglial activation in response to lipopolysaccharide, as indicated by a reduction in nitric oxide, reactive oxygen species, and TNFalpha production. Microglial induced neurotoxicity is also markedly reduced by CHPG treatment. The anti-inflammatory effects of CHPG are not observed in microglial cultures from mGluR5 knockout mice and are blocked by selective mGluR5 antagonists, suggesting that these actions are mediated by the mGluR5 receptor. Anti-inflammatory actions of mGluR5 activation are attenuated by phospholipase C and protein kinase C inhibitors, as well as by calcium chelators, suggesting that the mGluR5 activation in microglia involves the G(alphaq)-protein signal transduction pathway. These data indicate that microglial mGluR5 may represent a novel target for modulating neuroinflammation, an important component of both acute and chronic neurodegenerative disorders.

Activation of metabotropic glutamate receptor 5 modulates microglial reactivity and neurotoxicity by inhibiting NADPH oxidase

Microglial-related factors have been implicated in the signaling cascades that contribute to neuronal cell death in various neurodegenerative disorders. Thus, strategies that reduce microglial activation and associated neurotoxicity may have therapeutic benefit. Group II and III metabotropic glutamate receptors (mGluRs) are expressed in microglia and can modulate microglial activity in primary cell cultures. We demonstrate that the group I receptor member mGluR5 is highly expressed in primary microglial cultures and the BV2 microglial cell line. Activation of mGluR5 using the selective agonist (RS)-2-chloro-5-hydroxyphenylglycine (CHPG) significantly attenuates microglial activation in response to lipopolysaccharide and interferon-gamma, as indicated by a reduction in the expression of inducible nitric-oxide synthase, production of nitric oxide and tumor necrosis factor-alpha, and intracellular generation of reactive oxygen species. In addition, microglial-induced neurotoxicity is also markedly reduced by CHPG treatment. The anti-inflammatory effects of CHPG are mediated by the mGluR5 receptor, because either a selective mGluR5 antagonist or small interference RNA knockdown attenuated the actions of this drug. CHPG blocked the lipopolysaccharide-induced increase in expression and enzymatic activity of NADPH oxidase. Moreover, the protective effects of CHPG were significantly reduced when the NADPH oxidase subunits p22(phox) or gp91(phox) were knocked down by small interference RNA. These data suggest that mGluR5 represents a novel target for modulating microglial-dependent neuroinflammation, and may have therapeutic relevance for neurological disorders that exhibit microglial-mediated neurodegeneration.

Metabotropic glutamate receptors as targets for multipotential treatment of neurological disorders

Glutamate is a major excitatory neurotransmitter in the CNS that is involved in numerous cellular functions, including cell death and survival. Metabotropic glutamate receptors (mGluR) are G-protein coupled receptors that have been classified into three groups on the basis of signal transduction pathways and pharmacological profiles. Group I, II, and III mGluRs are found on cell types within and peripheral to the CNS, including neurons, microglia, astrocytes, oligodendrocytes, T- and B-cell lymphocytes, osteoblasts, hepatocytes, and endothelial cells, among others. These receptors have a number of effects on cells that can influence outcome after trauma, including reducing neuronal and oligodendroglial cell death, inflammation, and endothelial permeability. Thus, mGluRs are a promising multipotential therapeutic approach. Because the pathology of CNS trauma and neurodegeneration is multifactorial (including, for example, oxidative stress, mitochondrial breakdown, and inflammation), therapies that serve to modulate multiple pathophysiological pathways may prove more effective than those directed at a single target. This review examines the multipotential therapeutic utility of mGluR modulation in acute and chronic injury and neurodegeneration.

Interleukin-1beta: a bridge between inflammation and excitotoxicity?

Interleukin-1 (IL-1) is a proinflammatory cytokine released by many cell types that acts in both an autocrine and/or paracrine fashion. While IL-1 is best described as an important mediator of the peripheral immune response during infection and inflammation, increasing evidence implicates IL-1 signaling in the pathogenesis of several neurological disorders. The biochemical pathway(s) by which this cytokine contributes to brain injury remain(s) largely unidentified. Herein, we review the evidence that demonstrates the contribution of IL-1beta to the pathogenesis of both acute and chronic neurological disorders. Further, we highlight data that leads us to propose IL-1beta as the missing mechanistic link between a potential beneficial inflammatory response and detrimental glutamate excitotoxicity.

The injured nervous system: a Darwinian perspective

Much of the permanent damage that occurs in response to nervous system damage (trauma, infection, ischemia, etc.) is mediated by endogenous secondary processes that can contribute to cell death and tissue damage (excitotoxicity, oxidative damage and inflammation). For humans to evolve mechanisms to minimize secondary pathophysiological events following CNS injuries, selection must occur for individuals who survive such insults. Two major factors limit the selection for beneficial responses to CNS insults: for many CNS disease states the principal risk factor is advanced, post-reproductive age and virtually all severe CNS traumas are fatal in the absence of modern medical intervention. An alternative hypothesis for the persistence of apparently maladaptive responses to CNS damage is that the secondary exacerbation of damage is the result of unavoidable evolutionary constraints. That is, the nervous system could not function under normal conditions if the mechanisms that caused secondary damage (e.g., excitotoxicity) in response to injury were decreased or eliminated. However, some vertebrate species normally inhabit environments (e.g., hypoxia in underground burrows) that could potentially damage their nervous systems. Yet, neuroprotective mechanisms have evolved in these animals indicating that natural selection can occur for traits that protect animals from nervous system damage. Many of the secondary processes and regeneration-inhibitory factors that exacerbate injuries likely persist because they have been adaptive over evolutionary time in the healthy nervous system. Therefore, it remains important that researchers consider the role of the processes in the healthy or developing nervous system to understand how they become dysregulated following injury.

Examining lactate in severe TBI using proton magnetic resonance spectroscopy

PRIMARY OBJECTIVE

Clinical management of acute traumatic brain injury (TBI) has emphasized identification of secondary mechanisms of pathophysiology. An important objective in this study is to use proton magnetic resonance spectroscopy (pMRS) to examine early metabolic disturbance due to TBI.

RESEARCH DESIGN

The current design is a case study with repeated measures.

METHOD AND PROCEDURE

Proton magnetic resonance imaging was used to examine neurometabolism in this case of very severe brain trauma at 9 and 23 days post-injury. MRI was performed on a clinical 1.5 Tesla scanner.

MAIN OUTCOMES AND RESULTS

These data also reveal that pMRS methods can detect lactate elevations in an adult surviving severe head trauma and are sensitive to changes in basic neurometabolism during the first month of recovery.

CONCLUSIONS

The current case study demonstrates the sensitivity of pMRS in detecting metabolic alterations during the acute recovery period. The case study reveals that lactate elevations may be apparent for weeks after severe neurotrauma. Further work in this area should endeavour to determine the ideal time periods for pMRS examination in severe TBI as well as the ideal locations of data acquisition (e.g. adjacent or distal to lesion sites).

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.

Glutamate receptor pathology is present in the hippocampus following repeated sub-lethal soman exposure in the absence of spatial memory deficits

Much is still unknown about the long-term effects of repeated, sub-lethal exposure to organophosphorus (OP) nerve agents, such as soman (GD), on learning and memory tasks and related protein expression in the hippocampus. In the present study, guinea pigs were exposed to sub-lethal doses of GD for 10 days and cognitive performance assessed using the Morris water maze (MWM) up to 88 days post-exposure to investigate spatial learning. Additionally, hippocampal lysates were probed for cytoskeletal, synaptic and glutamate receptor proteins using Western blot analyses. No significant difference in MWM performance was observed between repeated sub-lethal GD exposed and saline control groups. However, Western blot analyses revealed significant changes in glutamate receptor protein immunoreactivity for subunits GluR2, NMDAR1, NMDAR2a and NMDAR2b in the hippocampi of GD-exposed guinea pigs. Levels of GluR2, NMDAR2a and NMDAR2b increased by 3 months post-initial exposure and returned to control levels by 6 months while NMDAR1 decreased by 6 months. No significant differences in neurofilament medium (NFM), neurofilament light (NFL) or synaptophysin densitometry were detected and alpha-II-spectrin proteolytic breakdown was also absent. These results reveal that repeated, sub-lethal exposure to GD affects glutamate receptor subunit expression but does not affect cytoskeletal protein immunoreactivity or the proteolytic state in the hippocampus. Though these changes do not affect spatial memory, they may contribute to other cognitive deficits previously observed following sub-lethal OP exposure.

Neuroprotective activity of selective mGlu1 and mGlu5 antagonists in vitro and in vivo

The neuroprotective potential of allosteric mGlu5 and mGlu1 antagonists such as 6-methyl-2-(phenylethynyl)-pyridin (MPEP)/[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP) and (3-ethyl-2-methyl-quinolin-6-yl)-(4-methoxy-cyclohexyl)-methanone methanesulfonate (EMQMCM), was tested in vitro in organotypic hippocampal cultures and in the middle cerebral artery occlusion model of stroke in vivo. Both classes of agent have high selectivity toward mGlu sub-types and are active in animal models of various diseases indicating satisfactory CNS penetration. In organotypic hippocampal cultures MPEP showed high neuroprotective potency against sub-chronic (12 days) insult produced by 3-NP with an IC50 of c.a. 70 nM. In contrast, although the mGlu1 antagonist EMQMCM was also protective, it seems to be weaker yielding an IC50 of c.a. 1 microM. Similarly, in the transient (90 min) middle cerebral artery occlusion model of ischaemia in rats, MTEP seems to be more effective than EMQMCM. MTEP, at 2.5 mg/kg and at 5 mg/kg provided 50 and 70% neuroprotection if injected 2 h after the onset of ischaemia. At a dose of 5 mg/kg, significant (50%) neuroprotection was also seen if the treatment was delayed by 4 h. EMQMCM was not protective at 5 mg/kg (given 2 h after occlusion) but at 10 mg/kg 50% of neuroprotection was observed. The present data support stronger neuroprotective potential of mGlu5 than mGlu1 antagonists.

Metabotropic glutamate receptor subtype 5 antagonists MPEP and MTEP

Glutamate regulates the function of central nervous system (CNS), in part, through the cAMP and/or IP3/DAG second messenger-associated metabotropic glutamate receptors (mGluRs). The mGluR5 antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) has been extensively used to elucidate potential physiological and pathophysiological functions of mGluR5. Unfortunately, recent evidence indicates significant non-specific actions of MPEP, including inhibition of NMDA receptors. In contrast, in vivo and in vitro characterization of the newer mGluR5 antagonist 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP) indicates that it is more highly selective for mGluR5 over mGluR1, has no effect on other mGluR subtypes, and has fewer off-target effects than MPEP. This article reviews literature on both of these mGluR5 antagonists, which suggests their possible utility in neurodegeneration, addiction, anxiety and pain management.

Late effects of enriched environment (EE) plus multimodal early onset stimulation (MEOS) after traumatic brain injury in rats: Ongoing improvement of neuromotor function despite sustained volume of the CNS lesion

Recently we showed that the combination between MEOS and EE applied to rats for 7-15 days after traumatic brain injury (TBI) was associated with reduced CNS lesion volume and enhanced reversal of neuromotor dysfunction. In a continuation of this work, we tested whether these effects persisted for longer post-operative periods, e.g. 30 days post-injury (dpi). Rats were subjected to lateral fluid percussion (LFP) or to sham injury. After LFP, one third of the animals (injured and sham) was placed under conditions of standard housing (SH), one third was kept in EE-only, and one third received EE+MEOS. Standardized composite neuroscore (NS) for neurological functions and computerized analysis of the vibrissal motor performance were used to assess post-traumatic neuromotor deficits. These were followed by evaluation of the cortical lesion volume (CLV) after immunostaining for neuron-specific enolase, caspase 3 active, and GFAP. Finally, the volume of cortical lesion containing regeneration-associated proteins (CLV-RAP) was determined in sections stained for GAP-43, MAP2, and neuronal class III beta-tubulin. We found (i) no differences in the vibrissal motor performance; (ii) EE+MEOS rats performed significantly better than SH rats in NS; (iii) EE-only and EE+MEOS animals, but not SH rats, showed better recovery at 30 dpi than at 15 dpi; (iv) no differences among all groups in CLV (larger than that at 15 dpi) and CLV-RAP, despite a clear tendency to reduction in the EE-only and EE+MEOS rats. We conclude that EE+MEOS retards, but cannot prevent the increase of lesion volume. This retardation is sufficient for a continuous restoration of neurological functions.

Neuroprotective effects of selective group II mGluR activation in brain trauma and traumatic neuronal injury

The effects of group II mGluR activation by selective agonist (-)-2-oxa-4-aminobicyclo[3.1. 0]hexane-4,6-dicarboxylate (LY379268) were examined in a mouse model of controlled cortical impact (CCI)-induced brain injury and in primary neuronal/glial and neuronal cultures subjected to mechanical trauma. Systemic administration of LY379268 to mice at 30 min after CCI significantly improved both motor and cognitive recovery as compared with vehicle-treated control animals. LY379268 also significantly reduced cell death induced by mechanical injury in rat neuronal/glial and neuronal cultures, as measured by lactate dehydrogenase (LDH) release assay. The neuroprotective effect of LY379268 in vitro was abolished by co-administration of the mGluR2/3 antagonist (s)-alpha-ethylglutamic acid (EGLU); however, co-application of selective mGluR3 antagonist beta-N-acetyl-aspartyl-glutamate (NAAG) had no significant influence in the same system. Together, these findings demonstrate the neuroprotective activity of group II mGluR activation and underscore the role of the mGluR2 subtype for this effect.