Riluzole increases high-affinity glutamate uptake in rat spinal cord synaptosomes

The role of small extracellular vesicles and microRNA as their cargo in the spinal cord injury pathophysiology and therapy

Spinal cord injury (SCI) is a devastating condition with a complex pathology that affects a significant portion of the population and causes long-term consequences. After primary injury, an inflammatory cascade of secondary injury occurs, followed by neuronal cell death and glial scar formation. Together with the limited regenerative capacity of the central nervous system, these are the main reasons for the poor prognosis after SCI. Despite recent advances, there is still no effective treatment. Promising therapeutic approaches include stem cells transplantation, which has demonstrated neuroprotective and immunomodulatory effects in SCI. This positive effect is thought to be mediated by small extracellular vesicles (sEVs); membrane-bound nanovesicles involved in intercellular communication through transport of functional proteins and RNA molecules. In this review, we summarize the current knowledge about sEVs and microRNA as their cargo as one of the most promising therapeutic approaches for the treatment of SCI. We provide a comprehensive overview of their role in SCI pathophysiology, neuroprotective potential and therapeutic effect.

Neurophysiological, histological, and behavioral characterization of animal models of distraction spinal cord injury: a systematic review

Distraction spinal cord injury is caused by some degree of distraction or longitudinal tension on the spinal cord and commonly occurs in patients who undergo corrective operation for severe spinal deformity. With the increased degree and duration of distraction, spinal cord injuries become more serious in terms of their neurophysiology, histology, and behavior. Very few studies have been published on the specific characteristics of distraction spinal cord injury. In this study, we systematically review 22 related studies involving animal models of distraction spinal cord injury, focusing particularly on the neurophysiological, histological, and behavioral characteristics of this disease. In addition, we summarize the mechanisms underlying primary and secondary injuries caused by distraction spinal cord injury and clarify the effects of different degrees and durations of distraction on the primary injuries associated with spinal cord injury. We provide new concepts for the establishment of a model of distraction spinal cord injury and related basic research, and provide reference guidelines for the clinical diagnosis and treatment of this disease.

Neuronal DSCAM regulates the peri-synaptic localization of GLAST in Bergmann glia for functional synapse formation

In the central nervous system, astrocytes enable appropriate synapse function through glutamate clearance from the synaptic cleft; however, it remains unclear how astrocytic glutamate transporters function at peri-synaptic contact. Here, we report that Down syndrome cell adhesion molecule (DSCAM) in Purkinje cells controls synapse formation and function in the developing cerebellum. Dscam-mutant mice show defects in CF synapse translocation as is observed in loss of function mutations in the astrocytic glutamate transporter GLAST expressed in Bergmann glia. These mice show impaired glutamate clearance and the delocalization of GLAST away from the cleft of parallel fibre (PF) synapse. GLAST complexes with the extracellular domain of DSCAM. Riluzole, as an activator of GLAST-mediated uptake, rescues the proximal impairment in CF synapse formation in Purkinje cell-selective Dscam-deficient mice. DSCAM is required for motor learning, but not gross motor coordination. In conclusion, the intercellular association of synaptic and astrocyte proteins is important for synapse formation and function in neural transmission.

The Disruption of NMDAR/TRPM4 Death Signaling with TwinF Interface Inhibitors: A New Pharmacological Principle for Neuroprotection

With the discovery that the acquisition of toxic features by extrasynaptic NMDA receptors (NMDARs) involves their physical interaction with the non-selective cation channel, TRPM4, it has become possible to develop a new pharmacological principle for neuroprotection, namely the disruption of the NMDAR/TRPM4 death signaling complex. This can be accomplished through the expression of the TwinF domain, a 57-amino-acid-long stretch of TRPM4 that mediates its interaction with NMDARs, but also using small molecule TwinF interface (TI) inhibitors, also known as NMDAR/TRPM4 interaction interface inhibitors. Both TwinF and small molecule TI inhibitors detoxify extrasynaptic NMDARs without interfering with synaptic NMDARs, which serve important physiological functions in the brain. As the toxic signaling of extrasynaptic NMDARs contributes to a wide range of neurodegenerative conditions, TI inhibitors may offer therapeutic options for currently untreatable human neurodegenerative diseases including Amyotrophic Lateral Sclerosis, Alzheimer's disease, and Huntington's disease.

Clinical Trials Targeting Secondary Damage after Traumatic Spinal Cord Injury

Spinal cord injury (SCI) often causes loss of sensory and motor function resulting in a significant reduction in quality of life for patients. Currently, no therapies are available that can repair spinal cord tissue. After the primary SCI, an acute inflammatory response induces further tissue damage in a process known as secondary injury. Targeting secondary injury to prevent additional tissue damage during the acute and subacute phases of SCI represents a promising strategy to improve patient outcomes. Here, we review clinical trials of neuroprotective therapeutics expected to mitigate secondary injury, focusing primarily on those in the last decade. The strategies discussed are broadly categorized as acute-phase procedural/surgical interventions, systemically delivered pharmacological agents, and cell-based therapies. In addition, we summarize the potential for combinatorial therapies and considerations.

Tectorigenin alleviates the apoptosis and inflammation in spinal cord injury cell model through inhibiting insulin-like growth factor-binding protein 6

Since tectorigenin has been reported to possess anti-inflammation, redox balance restoration, and anti-apoptosis properties, we determine to unravel whether tectorigenin has potential in alleviating spinal cord injury (SCI). Herein, PC12 cells were induced by lipopolysaccharide (LPS) to establish in vitro SCI models. The cell viability and apoptosis were detected through cell counting kit-8 and flow cytometry assays. The caspase-3/8/9 content was measured by colorimetric method. Western blot was conducted to quantify the expressions of cleaved caspse-3/8/9, IGFBP6, TLR4, IκBα, p-IκBα, RELA proto-oncogene, p65, and p-p65. Enzyme-linked immunosorbent assay and real-time quantitative polymerase chain reaction were carried out to quantitate expressions of IGFBP6, interleukin-1β (IL-1β), interleukin-6 (IL-6), and tumor necrosis factor-α (TNF-α). SwissTargetPrediction and GSE21497 database were utilized to predict the potential therapeutic targets of tectorigenin. Comparison of IGFBP6 expression in SCI tissues and normal tissues was analyzed by GEO2R. Our study found that LPS induced the declined cell viability, elevated cell apoptosis, upregulation of caspase-3/8/9, cleaved caspase-3/8/9, IL-1β, IL-6, TNF-α, IGFBP6, and TLR4, and the activation of IκBα and p65 in PC12 cells. Tectorigenin reversed the above effects of LPS. IGFBP6 was predicted to be the potential therapeutic target of tectorigenin and was overexpressed in SCI tissues. Notably, IGFBP6 overexpression offset the effects of tectorigenin on PC12 cells. In conclusion, tectorigenin could alleviate the LPS-induced apoptosis, inflammation, and activation of NF-κB signaling in SCI cell models via inhibiting IGFBP6.

Recent Advances in Cell and Functional Biomaterial Treatment for Spinal Cord Injury

Spinal cord injury (SCI) is a devastating central nervous system disease caused by accidental events, resulting in loss of sensory and motor function. Considering the multiple effects of primary and secondary injuries after spinal cord injury, including oxidative stress, tissue apoptosis, inflammatory response, and neuronal autophagy, it is crucial to understand the underlying pathophysiological mechanisms, local microenvironment changes, and neural tissue functional recovery for preparing novel treatment strategies. Treatment based on cell transplantation has become the forefront of spinal cord injury therapy. The transplanted cells provide physical and nutritional support for the damaged tissue. At the same time, the implantation of biomaterials with specific biological functions at the site of the SCI has also been proved to improve the local inhibitory microenvironment and promote axonal regeneration, etc. The combined transplantation of cells and functional biomaterials for SCI treatment can result in greater neuroprotective and regenerative effects by regulating cell differentiation, enhancing cell survival, and providing physical and directional support for axon regeneration and neural circuit remodeling. This article reviews the pathophysiology of the spinal cord, changes in the microenvironment after injury, and the mechanisms and strategies for spinal cord regeneration and repair. The article will focus on summarizing and discussing the latest intervention models based on cell and functional biomaterial transplantation and the latest progress in combinational therapies in SCI repair. Finally, we propose the future prospects and challenges of current treatment regimens for SCI repair, to provide references for scientists and clinicians to seek better SCI repair strategies in the future.

Alternative Targets for Modulators of Mitochondrial Potassium Channels

Mitochondrial potassium channels control potassium influx into the mitochondrial matrix and thus regulate mitochondrial membrane potential, volume, respiration, and synthesis of reactive oxygen species (ROS). It has been found that pharmacological activation of mitochondrial potassium channels during ischemia/reperfusion (I/R) injury activates cytoprotective mechanisms resulting in increased cell survival. In cancer cells, the inhibition of these channels leads to increased cell death. Therefore, mitochondrial potassium channels are intriguing targets for the development of new pharmacological strategies. In most cases, however, the substances that modulate the mitochondrial potassium channels have a few alternative targets in the cell. This may result in unexpected or unwanted effects induced by these compounds. In our review, we briefly present the various classes of mitochondrial potassium (mitoK) channels and describe the chemical compounds that modulate their activity. We also describe examples of the multidirectional activity of the activators and inhibitors of mitochondrial potassium channels.

Hormesis and neural stem cells

This paper provides a detailed identification and assessment of hormetic dose responses in neural stem cells (NSCs) as identified in a number of animal models and human tissues, with particular emphasis on cell proliferation and differentiation. Hormetic dose responses were commonly observed following administration of a number of agents, including dietary supplements [e.g., berberine, curcumin, (-)-epigallocatechin-3-gallate (EGCG), Ginkgo Biloba, resveratrol], pharmaceuticals (e.g., lithium, lovastatin, melatonin), endogenous ligands [e.g., hydrogen sulfide (H₂S), magnesium, progesterone, taurine], environmental contaminants (e.g., arsenic, rotenone) and physical agents [e.g., hypoxia, ionizing radiation, electromagnetic radiation (EMF)]. These data indicate that numerous agents can induce hormetic dose responses to upregulate key functions of such as cell proliferation and differentiation in NSCs, and enhance resilience to inflammatory stresses. The paper assesses both putative mechanisms of hormetic responses in NSCs, and the potential therapeutic implications and application(s) of hormetic frameworks in clinical approaches to neurological injury and disease.

Reduction of glutamate neurotoxicity: A novel therapeutic approach for Niemann-Pick disease, type C1

Niemann-Pick disease, type C1 is a progressive, lethal, neurodegenerative disorder due to endolysosomal storage of unesterified cholesterol. Cerebellar ataxia, as a result of progressive loss of cerebellar Purkinje neurons, is a major symptom of Nieman-Pick disease, type C1. Comparing single cell RNAseq data from control (Npc1^+/+) and mutant (Npc1^-/-) mice, we observed significantly decreased expression of Slc1a3 in Npc1^-/- astrocytes. Slc1a3 encodes a glutamate transporter (GLAST, EAAT1) which functions to decrease glutamate concentrations in the post synaptic space after neuronal firing. Glutamate is an excitatory neurotransmitter and elevated extracellular levels of glutamate can be neurotoxic. Impaired EAAT1 function underlies type-6 episodic ataxia, a rare disorder with progressive cerebellar dysfunction, thus suggesting that impaired glutamate uptake in Niemann-Pick disease, type C1 could contribute to disease progression. We now show that decreased expression of Slc1a3 in Npc1^-/- mice has functional consequences that include decreased surface protein expression and decreased glutamate uptake by Npc1^-/- astrocytes. To test whether glutamate neurotoxicity plays a role in Niemann-Pick disease, type C1 progression, we treated NPC1 deficient mice with ceftriaxone and riluzole. Ceftriaxone is a β-lactam antibiotic that is known to upregulate the expression of Slc1a2, an alternative glial glutamate transporter. Although ceftriaxone increased Slc1a2 expression, we did not observe a treatment effect in NPC1 mutant mice. Riluzole is a glutamate receptor antagonist that inhibits postsynaptic glutamate receptor signaling and reduces the release of glutamate. We found that treatment with riluzole increased median survival in Npc1^-/- by 12%. Given that riluzole is an approved drug for the treatment of amyotrophic lateral sclerosis, repurposing of this drug may provide a novel therapeutic approach to decrease disease progression in Niemann-Pick disease type, C1 patients.

Role of glutamate excitotoxicity and glutamate transporter EAAT2 in epilepsy: Opportunities for novel therapeutics development

Epilepsy is a complex neurological syndrome characterized by seizures resulting from neuronal hyperexcitability and sudden and synchronized bursts of electrical discharges. Impaired astrocyte function that results in glutamate excitotoxicity has been recognized to play a key role in the pathogenesis of epilepsy. While there are 26 drugs marketed as anti-epileptic drugs no current treatments are disease modifying as they only suppress seizures rather than the development and progression of epilepsy. Excitatory amino acid transporters (EAATs) are critical for maintaining low extracellular glutamate concentrations and preventing excitotoxicity. When extracellular glutamate concentrations rise to abnormal levels, glutamate receptor overactivation and the subsequent excessive influx of calcium into the post-synaptic neuron can trigger cell death pathways. In this review we discuss targeting EAAT2, the predominant glutamate transporter in the CNS, as a promising approach for developing therapies for epilepsy. EAAT2 upregulation via transcriptional and translational regulation has proven successful in vivo in reducing spontaneous recurrent seizures and offering neuroprotective effects. Another approach to regulate EAAT2 activity is through positive allosteric modulation (PAM). Novel PAMs of EAAT2 have recently been identified and are under development, representing a promising approach for the advance of novel therapeutics for epilepsy.

Anti-allodynic and promotive effect on inhibitory synaptic transmission of riluzole in rat spinal dorsal horn

Riluzole (2-amino-6-(trifluoromethoxy)benzothiazole) is a drug known for its inhibitory effect on glutamatergic transmission and its anti-nociceptive and anti-allodynic effects in neuropathic pain rat models. Riluzole also has an enhancing effect on GABAergic synaptic transmission. However, the effect on the spinal dorsal horn, which plays an important role in modulating nociceptive transmission, remains unknown. We investigated the ameliorating effect of riluzole on mechanical allodynia using the von Frey test in a rat model of neuropathic pain and analyzed the synaptic action of riluzole on inhibitory synaptic transmission in substantia gelatinosa (SG) neurons using whole-cell patch clamp recordings. We found that single-dose intraperitoneal riluzole (4 mg/kg) administration effectively attenuated mechanical allodynia in the short term in a rat model of neuropathic pain. Moreover, 300 μM riluzole induced an outward current in rat SG neurons. The outward current induced by riluzole was not suppressed in the presence of tetrodotoxin. Furthermore, we found that the outward current was suppressed by simultaneous bicuculline and strychnine application, but not by strychnine alone. Altogether, these results suggest that riluzole enhances inhibitory synaptic transmission monosynaptically by potentiating GABAergic synaptic transmission in the rat spinal dorsal horn.

Corticospinal Motor Circuit Plasticity After Spinal Cord Injury: Harnessing Neuroplasticity to Improve Functional Outcomes

Spinal cord injury (SCI) is a devastating condition that affects approximately 294,000 people in the USA and several millions worldwide. The corticospinal motor circuitry plays a major role in controlling skilled movements and in planning and coordinating movements in mammals and can be damaged by SCI. While axonal regeneration of injured fibers over long distances is scarce in the adult CNS, substantial spontaneous neural reorganization and plasticity in the spared corticospinal motor circuitry has been shown in experimental SCI models, associated with functional recovery. Beneficially harnessing this neuroplasticity of the corticospinal motor circuitry represents a highly promising therapeutic approach for improving locomotor outcomes after SCI. Several different strategies have been used to date for this purpose including neuromodulation (spinal cord/brain stimulation strategies and brain-machine interfaces), rehabilitative training (targeting activity-dependent plasticity), stem cells and biological scaffolds, neuroregenerative/neuroprotective pharmacotherapies, and light-based therapies like photodynamic therapy (PDT) and photobiomodulation (PMBT). This review provides an overview of the spontaneous reorganization and neuroplasticity in the corticospinal motor circuitry after SCI and summarizes the various therapeutic approaches used to beneficially harness this neuroplasticity for functional recovery after SCI in preclinical animal model and clinical human patients' studies.

Demonstrated hormetic mechanisms putatively subserve riluzole-induced effects in neuroprotection against amyotrophic lateral sclerosis (ALS): Implications for research and clinical practice

This paper provides evidence to support that riluzole, an FDA-approved treatment for amyotrophic lateral sclerosis (ALS), like many neuroprotective agents, displays and exerts hormetic biphasic dose responses. These findings have important implications for the experimental study and clinical treatment of ALS.

Current updates on various treatment approaches in the early management of acute spinal cord injury

Spinal cord injury (SCI) is a debilitating condition which often leads to a severe disability and ultimately impact patient's physical, psychological, and social well-being. The management of acute SCI has evolved over the couple of decades due to improved understanding of injury mechanisms and increasing knowledge of disease. Currently, the early management of acute SCI patient includes pharmacological agents, surgical intervention and newly experimental neuroprotective strategies. However, many controversial areas are still surrounding in the current treatment strategies for acute SCI, including the optimal timing of surgical intervention, early versus delayed decompression outcome benefits, the use of methylprednisolone. Due to the lack of consensus, the optimal standard of care has been varied across treatment centres. The authors have shed a light on the current updates on early treatment approaches and neuroprotective strategies in the initial management of acute SCI in order to protect the early neurologic injury and reduce the future disability.

Increased Axon Initial Segment Length Results in Increased Na+ Currents in Spinal Motoneurones at Symptom Onset in the G127X SOD1 Mouse Model of Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease preferentially affecting motoneurones. Transgenic mouse models have been used to investigate the role of abnormal motoneurone excitability in this disease. Whilst an increased excitability has repeatedly been demonstrated in vitro in neonatal and embryonic preparations from SOD1 mouse models, the results from the only studies to record in vivo from spinal motoneurones in adult SOD1 models have produced conflicting findings. Deficits in repetitive firing have been reported in G93A SOD1^{(high copy number)} mice but not in presymptomatic G127X SOD1 mice despite shorter motoneurone axon initial segments (AISs) in these mice. These discrepancies may be due to the earlier disease onset and prolonged disease progression in G93A SOD1 mice with recordings potentially performed at a later sub-clinical stage of the disease in this mouse. To test this, and to explore how the evolution of excitability changes with symptom onset we performed in vivo intracellular recording and AIS labelling in G127X SOD1 mice immediately after symptom onset. No reductions in repetitive firing were observed showing that this is not a common feature across all ALS models. Immunohistochemistry for the Na⁺ channel Nav1.6 showed that motoneurone AISs increase in length in G127X SOD1 mice at symptom onset. Consistent with this, the rate of rise of AIS components of antidromic action potentials were significantly faster confirming that this increase in length represents an increase in AIS Na⁺ channels occurring at symptom onset in this model.

Randomized Controlled Trial of Riluzole Augmentation for Posttraumatic Stress Disorder: Efficacy of a Glutamatergic Modulator for Antidepressant-Resistant Symptoms

OBJECTIVE

Current pharmacologic treatments for posttraumatic stress disorder (PTSD) have shown limited efficacy, prompting a call to investigate new classes of medications. The current study investigated the efficacy of glutamate modulation with riluzole augmentation for combat-related PTSD symptoms resistant to treatment with selective serotonin reuptake inhibitors (SSRIs) or serotonin-norepinephrine reuptake inhibitors (SNRIs).

METHODS

A randomized, double-blind, placebo-controlled, parallel trial was conducted at Walter Reed National Military Medical Center and Syracuse VA Medical Center between December 2013 and November 2017. Veterans and active duty service members with combat-related PTSD (per the Clinician Administered PTSD Scale [CAPS]) who were not responsive to SSRI or SNRI pharmacotherapy were randomized to 8-week augmentation with a starting dose of 100 mg/d of riluzole (n = 36) or placebo (n = 38) and assessed weekly for PTSD symptoms, anxiety, depression, disability, and side effects.

RESULTS

Intent-to-treat analyses (N = 74) of the primary outcome (CAPS for DSM-IV) showed no significant between-group difference in change in overall PTSD symptoms (F = 0.64, P = .422), with a small effect size (d = 0.25). There was clinically significant within-group improvement in overall PTSD symptoms in both groups, with a greater mean (SD) decrease in CAPS score in the riluzole group (-21.1 [18.9]) than in the placebo group (-16.7 [17.2]). Exploratory analyses of PTSD symptom clusters showed significantly greater improvement on hyperarousal symptoms in the riluzole group as measured by the PTSD Checklist-Specific-Subscale D (d = 0.48) and near-significant findings on the CAPS Subscale D. Riluzole augmentation was not superior to placebo on change in depression, anxiety, or disability severity.

CONCLUSIONS

Although preliminary, the exploratory findings of this study offer some evidence that riluzole augmentation of an SSRI or SNRI may selectively improve PTSD hyperarousal symptoms without changes in overall PTSD symptoms, depression, anxiety, or disability. Additional investigation of the mechanism of the efficacy of riluzole for hyperarousal symptoms is warranted.

TRIAL REGISTRATION

ClinicalTrials.gov identifier: NCT02155829.

Efficacy of riluzole in the treatment of spinal cord injury: a systematic review of the literature

OBJECTIVERiluzole is a glutamatergic modulator that has recently shown potential for neuroprotection after spinal cord injury (SCI). While the effects of riluzole are extensively documented in animal models of SCI, there remains heterogeneity in findings. Moreover, there is a paucity of data on the pharmacology of riluzole and its effects in humans. For the present study, the authors systematically reviewed the literature to provide a comprehensive understanding of the effects of riluzole in SCI.METHODSThe PubMed database was queried from 1996 to September 2018 to identify animal studies and clinical trials involving riluzole administration for SCI. Once articles were identified, they were processed for year of publication, study design, subject type, injury model, number of subjects in experimental and control groups, dose, timing/route of administration, and outcomes.RESULTSA total of 37 studies were included in this study. Three placebo-controlled clinical trials were included with a total of 73 patients with a mean age of 39.1 years (range 18-70 years). For the clinical trials included within this study, the American Spinal Injury Association Impairment Scale distributions for SCI were 42.6% grade A, 25% grade B, 26.6% grade C, and 6.2% grade D. Key findings from studies in humans included decreased nociception, improved motor function, and attenuated spastic reflexes. Twenty-six animal studies (24 in vivo, 1 in vitro, and 1 including both in vivo and in vitro) were included. A total of 520 animals/in vitro specimens were exposed to riluzole and 515 animals/in vitro specimens underwent other treatment for comparison. The average dose of riluzole for intraperitoneal, in vivo studies was 6.5 mg/kg (range 1-10 mg/kg). Key findings from animal studies included behavioral improvement, histopathological tissue sparing, and modified electrophysiology after SCI. Eight studies examined the pharmacology of riluzole in SCI. Key findings from pharmacological studies included riluzole dose-dependent effects on glutamate uptake and its modified bioavailability after SCI in both animal and clinical models.CONCLUSIONSSCI has many negative sequelae requiring neuroprotective intervention. While still relatively new in its applications for SCI, both animal and human studies demonstrate riluzole to be a promising pharmacological intervention to attenuate the devastating effects of this condition.

The Effect of Glutamatergic Modulators on Extracellular Glutamate: How Does this Information Contribute to the Discovery of Novel Antidepressants?

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The complexity of glutamatergic signaling challenges glutamate modulator usage.

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Functional biomarkers are needed to understand the MOA of glutamate modulators.

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Evaluating drug effect on EAATs' kinetics may add to antidepressant discovery.

Background

In the search for new antidepressants, clinical researchers have been using drugs that simultaneously modulate multiple targets. During preclinical and clinical trials, the glutamatergic modulators riluzole and ketamine have received particular attention. Glutamatergic agents have a modulatory effect on synaptic transmission, so they can act on both neurons and astrocytes. In addition to influencing the quantity of glutamate released, these modulators can also affect the expression, localization, and functionality of glutamate-binding sites.

Objective

This review discusses the complexity of the glutamatergic system, the ambiguity of data regarding glutamate levels in patients with depression, as well as the mechanisms of action for riluzole and ketamine, which includes their relation to the physiology of glutamatergic transmission. The principal aim is to contribute to the development of novel glutamatergic antidepressant medications whilst emphasizing the need for innovative approaches that evaluate their effects on extracellular glutamate.

Methods

Literature was obtained via PubMed by searching the term depression in combination with each of the following terms: riluzole, ketamine, and glutamate. The search was restricted to full-text articles published in English between 1985 and 2018 relating to both the modulatory mechanisms of glutamatergic-binding proteins and the antidepressant actions of these medicines. Articles about mechanisms associated with synaptic plasticity and antidepressant effects were excluded.

Results

Although experimental data relates glutamatergic signaling to the pathophysiology of major depression and bipolar disorder, the role of glutamate—as well as its extracellular concentration in patients with said disorders—is still unclear. Riluzole's antidepressant action is ascribed to its capacity to reduce glutamate levels in the synaptic cleft, and ketamine's effect has been associated with increased extracellular glutamate levels.

Conclusions

The strategy of using glutamatergic modulators as therapeutic agents requires a better understanding of the role of glutamate in the pathophysiology of depression. Gaining such understanding is a challenge because it entails evaluating different targets as well as the effects of these modulators on the kinetics of glutamate uptake. Essentially, glutamate transport is a dynamic process and, currently, it is still necessary to develop new approaches to assay glutamate in the synaptic cleft. ORCID: 0000-0002-3358-6939.

Riluzole Selective Antioxidant Effects in Cell Models Expressing Amyotrophic Lateral Sclerosis Endophenotypes

Objective

Until recently, riluzole was the only drug licensed for amyotrophic lateral sclerosis (ALS). In spite of its efficacy, the mechanism of action remains elusive, and both blocking of glutamate release and antioxidant properties have been postulated. Here we characterized human SH-SY5Y neuroblastoma cell lines, taking advantage of their insensitivity to excitotoxic insults, in order to selectively assess the presence of a direct antioxidant effect of riluzole.

Methods

SH-SY5Y cells, either parental or overexpressing the G93A SOD1 mutation, were exposed for 24 hours to the selected stimuli.

Results

Riluzole (1–10 μM) was able to counteract the effects of H₂O₂ exposure (200 μM/24 hr), limiting both cell death and whole-cell reactive oxygen species (ROS) increase. The same experiments were repeated using SH-SY5Y cells carrying the familial ALS-related G93A-SOD1 mutation and constitutively expressing two-fold increased whole-cell ROS levels with respect to wild-type cells: riluzole was ineffective in this paradigm. Analogously, riluzole was ineffective in preventing cell death induced by exposing SH-SY5Y cells to 3-morpholino-sydnonimine (SIN-1, 1.5 mM/24 hr), a reactive nitrogen species (RNS) donor.

Conclusion

Our data support a direct antioxidant action of riluzole. Furthermore, the lack of efficacy of riluzole observed in the SOD1 cell model mirrors the lack of efficacy already demonstrated in cognate mouse models of ALS, plausibly reflecting differences in the underlying pathogenic mechanisms. Finally, riluzole inefficacy against nitrosative stress might support the idea that a combined therapeutic intervention may result more effective in ALS patients, as in the case of co-administration of edaravone, a drug known to reduce RNS.

Novel Positive Allosteric Modulators of Glutamate Transport Have Neuroprotective Properties in an in Vitro Excitotoxic Model

Dysfunction of excitatory amino acid transporters (EAATs) has been implicated in the pathogenesis of various neurological disorders, such as stroke, brain trauma, epilepsy, and several neurodegenerative disorders. EAAT2 is the main transporter subtype responsible for glutamate clearance in the brain, and plays a key role in regulating neurotransmission and preventing excitotoxicity. Therefore, compounds that increase the activity of EAAT2 have therapeutic potential for neuroprotection. In previous studies, we used virtual screening approaches to identify novel positive allosteric modulators (PAMs) of EAAT2. These compounds were shown to selectively increase the activity of EAAT2 and increase V_max of transport, without changing substrate affinity. In this work, our major effort was to investigate whether increasing the activity of EAAT2 by allosteric modulation would translate to neuroprotection in in vitro primary culture models of excitotoxicity. To investigate potential neuroprotective effects of one EAAT2 PAM, GT949, we subjected cultures to acute and prolonged excitotoxic insults by exogenous application of glutamate, or oxidative stress by application of hydrogen peroxide. GT949 administration did not result in neuroprotection in the oxidative stress model, likely due to damage of the glutamate transporters. However, GT949 displayed neuroprotective properties after acute and prolonged glutamate-mediated excitotoxicity. We propose that this compound prevents excess glutamate signaling by increasing the rate of glutamate clearance by EAAT2, thereby preventing excitotoxic damage and cell death. This novel class of compounds is therefore an innovative approach for neuroprotection with potential for translation in in vivo animal models of excitotoxicity.

Progressive multiple sclerosis: from pathophysiology to therapeutic strategies

Multiple sclerosis (MS) is a chronic inflammatory disease of the central nervous system that involves demyelination and axonal degeneration. Although substantial progress has been made in drug development for relapsing-remitting MS, treatment of the progressive forms of the disease, which are characterized clinically by the accumulation of disability in the absence of relapses, remains unsatisfactory. This unmet clinical need is related to the complexity of the pathophysiological mechanisms involved in MS progression. Chronic inflammation, which occurs behind a closed blood-brain barrier with activation of microglia and continued involvement of T cells and B cells, is a hallmark pathophysiological feature. Inflammation can enhance mitochondrial damage in neurons, which, consequently, develop an energy deficit, further reducing axonal health. The growth-inhibitory and inflammatory environment of lesions also impairs remyelination, a repair process that might protect axons from degeneration. Moreover, neurodegeneration is accelerated by the altered expression of ion channels on denuded axons. In this Review, we discuss the current understanding of these disease mechanisms and highlight emerging therapeutic strategies based on these insights, including those targeting the neuroinflammatory and degenerative aspects as well as remyelination-promoting approaches.

Treatment with the glutamate modulator riluzole prevents early life stress-induced cognitive deficits and impairments in synaptic plasticity in APPswe/PS1dE9 mice

BACKGROUND

Environmental factors like stress affect age-related cognitive deficits and promote Alzheimer's disease (AD)-related pathology in mice. Excess glutamate has been proposed as a possible mediator underlying these effects in the hippocampus, a vulnerable brain region implicated in learning and memory.

METHODS

Here, we examined a) whether stress applied during a sensitive developmental period early in life affects later synaptic plasticity, learning and memory and plaque load in the APPswe/PS1dE9 mouse model for Alzheimer's disease and b) whether these effects could be rescued using long-term treatment with the glutamate modulator riluzole.

RESULTS

Our results demonstrate that ELS impairs synaptic plasticity in 6-month-old mice and increases plaque load in 12-month-old APPswe/PS1dE9 mice, while impairing flexible spatial learning in the Barnes maze at this age. Notably, spatial learning correlated well with hippocampal expression of the transporter EAAT2, which is important for extracellular glutamate uptake. The changes in LTP, plaque load and cognition after ELS were all prevented by riluzole treatment that started from post-weaning.

CONCLUSION

These results suggest that normalising glutamate signalling may be a viable therapeutic strategy for treating vulnerable individuals at risk of developing stress-aggravated AD, particularly in relation to adverse early life experiences.

The emergence of new antidepressants for clinical use: Agomelatine paradox versus other novel agents

This study was designed with the rational aim of discussing the emerging antidepressant agents that are likely to bring positive landmark, tremendous improvement and significant impact to the management of patients with depression disorders. It also elaborates on the Agomelatine paradox vis-a-vis the other novel antidepressant agents. The emerging antidepressants are: selective monoamine oxidase inhibitors (MAOIs) such as bifemelane, pirlindole, toloxatone, selegiline, rasagiline and safinamide; serotonin-norepinephrine reuptake inhibitors (SNRIs) such as ansofaxine, nefopam and levomilnacipran; norepinephrine reuptake inhibitors (NRIs) such as Reboxetine, viloxazine, teniloxazine (also known as sulfoxazine or sufoxazine), and atomoxetine; Vilazodone (a serotonin 5-HT_1A autoreceptor partial agonist with serotonin reuptake inhibition [SPARI]); Vortioxetine (a serotonin receptors antagonist with serotonin reuptake inhibition [SARI]); atypical antipsychotics such as olanzapine, quetiapine, risperidone, lurasidone, aripiprazole and brexpiprazole; N-methyl-d-aspartate (NMDA)-glutamatergic neurotransmission system blockers such as ketamine, CP-101,606 (traxoprodil), GLYX-13 (rapastinel), NRX-1074 (Apimostinel) and Riluzole. While Agomelatine (a melatonergic MT₁ and MT₂ receptors agonist and a selective serotonergic 5-HT_2B and 5-HT_2C receptors antagonist [MASSA]) remains a paradoxical agent that doesn't fit into any of the currently available classes of antidepressant agents and its pharmacological properties also deemed it unfit and inappropriate to be classified into another separate novel class of antidepressants contrary to the reports published in previous reference literatures. Lastly, this review remarkably advocates for the incorporation of the atypical antipsychotics and NMDA-glutamatergic ionoceptor blockers as new member classes of the antidepressant agents because of their clinically significant roles in the management of depression disorders.

Serum and plasma brain-derived neurotrophic factor and response in a randomized controlled trial of riluzole for treatment resistant depression

BACKGROUND

Serum brain-derived neurotrophic factor (BDNF) is decreased in individuals with major depressive disorder (MDD). Pre-clinical and clinical reports suggest that the glutamate release inhibitor riluzole increases BDNF and may have antidepressant properties. Here we report serum (sBDNF) and plasma (pBDNF) levels from a randomized controlled, adjunctive, sequential parallel comparison design trial of riluzole in MDD.

METHODS

Serum and plasma BDNF samples were drawn at baseline and weeks 6 and 8 from 55 subjects randomized to adjunctive treatment with riluzole or placebo for 8 weeks.

RESULTS

Riluzole responders had lower baseline serum (19.08 ng/ml [SD 9.22] v. 28.80 ng/ml [9.63], p = 0.08) and plasma (2.72 ng/ml [1.07] v. 4.60 ng/ml [1.69], p = 0.06) BDNF compared to non-responders at a trend level. This pattern was nominally seen in placebo responders for baseline pBDNF to some degree (1.21 ng/ml [SD 1.29] v. 3.58 ng/ml [SD 1.67], p = 0.12) but not in baseline sBDNF.

LIMITATIONS

A number of limitations warrant comment, including the small sample size of viable BDNF samples and the small number of riluzole responders.

CONCLUSIONS

Preliminary evidence reported here suggests that lower baseline BDNF may be associated with better clinical response to riluzole.

Prophylactic Riluzole Attenuates Oxidative Stress Damage in Spinal Cord Distraction

Spinal cord injury (SCI) without radiographical abnormalities (SCIWORA) presents a significant challenge because of the loss of function despite an apparent normal anatomy. The cause of dysfunction is not understood, and specific treatment options are lacking. Some scoliosis corrective surgeries result in SCIWORA, where stretching of the spinal cord can lead to vascular compromise and hypoxia. The iatrogenic nature of this injury allows for the implantation of neuroprotective strategies that are designed to prevent damage. We utilized a model of atraumatic SCI to evaluate the efficacy of the sodium-channel blocker, riluzole, as a prophylactic neuroprotectant. As expected, the stretch injury caused a significant reduction in intraparenchymal oxygen in distraction (-53.09 ± 22.23%) and riluzole pre-treated distraction animals (-43.04 ± 22.86%). However, in contrast to the oxidative stress and metabolic impairments observed in vehicle-treated distraction animals, in which protein carbonylation increased significantly (5.88 ± 1.3 nmol/mL), riluzole kept these levels within the normal range (1.8 ± 1.0 nmol/mL). This neurprotection also prevented ventral motor neuron hypoplasia and pyknosis, characteristic features of this atraumatic SCI model, and maintained normal gait function (e.g., stride length and stance time). This study provides evidence for the use of prophylactic neuroprotective strategies in which thoracic or spine surgeries present the risk of causing atraumatic SCI.

Traumatic Spinal Cord Injury-Repair and Regeneration

BACKGROUND

Traumatic spinal cord injuries (SCI) have devastating consequences for the physical, financial, and psychosocial well-being of patients and their caregivers. Expediently delivering interventions during the early postinjury period can have a tremendous impact on long-term functional recovery.

PATHOPHYSIOLOGY

This is largely due to the unique pathophysiology of SCI where the initial traumatic insult (primary injury) is followed by a progressive secondary injury cascade characterized by ischemia, proapoptotic signaling, and peripheral inflammatory cell infiltration. Over the subsequent hours, release of proinflammatory cytokines and cytotoxic debris (DNA, ATP, reactive oxygen species) cyclically adds to the harsh postinjury microenvironment. As the lesions mature into the chronic phase, regeneration is severely impeded by the development of an astroglial-fibrous scar surrounding coalesced cystic cavities. Addressing these challenges forms the basis of current and upcoming treatments for SCI.

MANAGEMENT

This paper discusses the evidence-based management of a patient with SCI while emphasizing the importance of early definitive care. Key neuroprotective therapies are summarized including surgical decompression, methylprednisolone, and blood pressure augmentation. We then review exciting neuroprotective interventions on the cusp of translation such as Riluzole, Minocycline, magnesium, therapeutic hypothermia, and CSF drainage. We also explore the most promising neuroregenerative strategies in trial today including Cethrin™, anti-NOGO antibody, cell-based approaches, and bioengineered biomaterials. Each section provides a working knowledge of the key preclinical and patient trials relevant to clinicians while highlighting the pathophysiologic rationale for the therapies.

CONCLUSION

We conclude with our perspectives on the future of treatment and research in this rapidly evolving field.

Excessive spinal glutamate transmission is involved in oxaliplatin-induced mechanical allodynia: a possibility for riluzole as a prophylactic drug

Oxaliplatin, a chemotherapy medication, causes severe peripheral neuropathy. Although oxaliplatin-induced peripheral neuropathy is a dose-limiting toxicity, a therapeutic strategy against its effects has not been established. We previously reported the involvement of N-methyl-D-aspartate receptors and their intracellular signalling pathway in oxaliplatin-induced mechanical allodynia in rats. The aim of this study was to clarify the involvement of spinal glutamate transmission in oxaliplatin-induced mechanical allodynia. In vivo spinal microdialysis revealed that the baseline glutamate concentration was elevated in oxaliplatin-treated rats, and that mechanical stimulation of the hind paw markedly increased extracellular glutamate concentration in the same rats. In these rats, the expression of glutamate transporter 1 (GLT-1), which plays a major role in glutamate uptake, was decreased in the spinal cord. Moreover, we explored the potential of pharmacological therapy targeting maintenance of extracellular glutamate homeostasis. The administration of riluzole, an approved drug for amyotrophic lateral sclerosis, suppressed the increase of glutamate concentration, the decrease of GLT-1 expression and the development of mechanical allodynia. These results suggest that oxaliplatin disrupts the extracellular glutamate homeostasis in the spinal cord, which may result in neuropathic symptoms, and support the use of riluzole for prophylaxis of oxaliplatin-induced mechanical allodynia.

Neuronal Dysfunction in iPSC-Derived Medium Spiny Neurons from Chorea-Acanthocytosis Patients Is Reversed by Src Kinase Inhibition and F-Actin Stabilization

Chorea-acanthocytosis (ChAc) is a fatal neurological disorder characterized by red blood cell acanthocytes and striatal neurodegeneration. Recently, severe cell membrane disturbances based on depolymerized cortical actin and an elevated Lyn kinase activity in erythrocytes from ChAc patients were identified. How this contributes to the mechanism of neurodegeneration is still unknown. To gain insight into the pathophysiology, we established a ChAc patient-derived induced pluripotent stem cell model and an efficient differentiation protocol providing a large population of human striatal medium spiny neurons (MSNs), the main target of neurodegeneration in ChAc. Patient-derived MSNs displayed enhanced neurite outgrowth and ramification, whereas synaptic density was similar to controls. Electrophysiological analysis revealed a pathologically elevated synaptic activity in ChAc MSNs. Treatment with the F-actin stabilizer phallacidin or the Src kinase inhibitor PP2 resulted in the significant reduction of disinhibited synaptic currents to healthy control levels, suggesting a Src kinase- and actin-dependent mechanism. This was underlined by increased G/F-actin ratios and elevated Lyn kinase activity in patient-derived MSNs. These data indicate that F-actin stabilization and Src kinase inhibition represent potential therapeutic targets in ChAc that may restore neuronal function.

SIGNIFICANCE STATEMENT

Chorea-acanthocytosis (ChAc) is a fatal neurodegenerative disease without a known cure. To gain pathophysiological insight, we newly established a human in vitro model using skin biopsies from ChAc patients to generate disease-specific induced pluripotent stem cells (iPSCs) and developed an efficient iPSC differentiation protocol providing striatal medium spiny neurons. Using patch-clamp electrophysiology, we detected a pathologically enhanced synaptic activity in ChAc neurons. Healthy control levels of synaptic activity could be restored by treatment of ChAc neurons with the F-actin stabilizer phallacidin and the Src kinase inhibitor PP2. Because Src kinases are involved in bridging the membrane to the actin cytoskeleton by membrane protein phosphorylation, our data suggest an actin-dependent mechanism of this dysfunctional phenotype and potential treatment targets in ChAc.

Mechanistic Studies of Capsaicin-Induced Apnea in Rodents

Inhalation of capsaicin-based sprays can cause central respiratory depression and lethal apneas. There are contradictory reports regarding the sites of capsaicin action. Furthermore, an understanding of the neurochemical mechanisms underlying capsaicin-induced apneas and the development of pharmacological interventions is lacking. The main objectives of this study were to perform a systematic study of the mechanisms of action of capsaicin-induced apneas and to provide insights relevant to pharmacological intervention. In vitro and in vivo rat and transient receptor potential vanilloid superfamily member 1 (TRPV1)-null mouse models were used to measure respiratory parameters and seizure-like activity in the presence of capsaicin and compounds that modulate glutamatergic neurotransmission. Administration of capsaicin to in vitro and in vivo rat and wild-type mouse models induced dose-dependent apneas and the production of seizure-like activity. No significant changes were observed in TRPV1-null mice or rat medullary slice preparations. The capsaicin-induced effects were inhibited by the TRPV1 antagonist capsazepine, amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor antagonists CNQX, NBQX, perampanel, and riluzole, a drug that inhibits glutamate release and increases glutamate uptake. The capsaicin-induced effects on breathing and seizure-like activity were accentuated by positive allosteric modulators of the AMPA receptors, CX717 and cyclothiazide. To summarize, capsaicin-induced apneas and seizure-like behaviors are mediated via TRPV1 activation acting at lung afferents, spinal cord-ascending tracts, and medullary structures (including nucleus tractus solitarius). AMPA receptor-mediated conductances play an important role in capsaicin-induced apneas and seizure-like activity. A pharmaceutical strategy targeted at reducing AMPA receptor-mediated glutamatergic signaling may reduce capsaicin-induced deleterious effects.

Using Enzyme-based Biosensors to Measure Tonic and Phasic Glutamate in Alzheimer's Mouse Models

Neurotransmitter disruption is often a key component of diseases of the central nervous system (CNS), playing a role in the pathology underlying Alzheimer's disease, Parkinson's disease, depression, and anxiety. Traditionally, microdialysis has been the most common (lauded) technique to examine neurotransmitter changes that occur in these disorders. But because microdialysis has the ability to measure slow 1-20 minute changes across large areas of tissue, it has the disadvantage of invasiveness, potentially destroying intrinsic connections within the brain and a slow sampling capability. A relatively newer technique, the microelectrode array (MEA), has numerous advantages for measuring specific neurotransmitter changes within discrete brain regions as they occur, making for a spatially and temporally precise approach. In addition, using MEAs is minimally invasive, allowing for measurement of neurotransmitter alterations in vivo. In our laboratory, we have been specifically interested in changes in the neurotransmitter, glutamate, related to Alzheimer's disease pathology. As such, the method described here has been used to assess potential hippocampal disruptions in glutamate in a transgenic mouse model of Alzheimer's disease. Briefly, the method used involves coating a multi-site microelectrode with an enzyme very selective for the neurotransmitter of interest and using self-referencing sites to subtract out background noise and interferents. After plating and calibration, the MEA can be constructed with a micropipette and lowered into the brain region of interest using a stereotaxic device. Here, the method described involves anesthetizing rTg(TauP301L)4510 mice and using a stereotaxic device to precisely target sub-regions (DG, CA1, and CA3) of the hippocampus.

Efficacy of off-label augmentation in unipolar depression: A systematic review of the evidence

Treatment of unipolar depression with currently available antidepressants is still unsatisfactory. Augmentation with lithium or second generation antipsychotics is an established practice in non-responders to antidepressant monotherapy, but is also associated with a substantial non-response rate and with non-tolerance. Based on a systematic review of the literature, including meta-analyses, randomized controlled trials (RCTs), non-randomized comparative studies and case studies, off-label augmentation agents (administered in addition to an antidepressant, without FDA approval for treatment of MDD) were identified and evaluated regarding their efficacy using levels of evidence. The agents had to be added to an existing antidepressant regime with the aim of achieving an improved clinical response to an ongoing antidepressant treatment (augmentation) or an earlier onset of effect when starting antidepressant and augmentation agent simultaneously (acceleration). Five substances, modafinil, ketamine, pindolol, testosterone and estrogen (the latter two in hormone-deficient patients) were shown to be clinically effective in high evidence studies. For the six drugs dexamethasone, mecamylamine, riluzole, amantadine, pramipexole and yohimbine clear proof of efficacy was not possible due to low levels of evidence, small sample sizes or discordant results. For the two agents methylphenidate and memantine only studies with negative outcomes could be found. Overall, the quality of study designs was low and results were often contradictory. However, the use of pindolol, ketamine, modafinil, estrogen and testosterone might be an option for depressed patients who are not responding to antidepressant monotherapy or established augmentation strategies. Further high quality studies are necessary and warranted.

Riluzole But Not Melatonin Ameliorates Acute Motor Neuron Degeneration and Moderately Inhibits SOD1-Mediated Excitotoxicity Induced Disrupted Mitochondrial Ca2+ Signaling in Amyotrophic Lateral Sclerosis

Selective motoneurons (MNs) degeneration in the brain stem, hypoglossal motoneurons (HMNs), and the spinal cord resulting in patients paralysis and eventual death are prominent features of amyotrophic lateral sclerosis (ALS). Previous studies have suggested that mitochondrial respiratory impairment, low Ca²⁺ buffering and homeostasis and excitotoxicity are the pathological phenotypes found in mice, and cell culture models of familial ALS (fALS) linked with Cu/Zn-superoxide dismutase 1 (SOD1) mutation. In our study, we aimed to understand the impact of riluzole and melatonin on excitotoxicity, neuronal protection and Ca²⁺ signaling in individual HMNs ex vivo in symptomatic adult ALS mouse brain stem slice preparations and in WT and SOD1-G93A transfected SH-SY5Y neuroblastoma cell line using fluorescence microscopy, calcium imaging with high speed charged coupled device camera, together with immunohistochemistry, cell survival assay and histology. In our experiments, riluzole but not melatonin ameliorates MNs degeneration and moderately inhibit excitotoxicity and cell death in SH-SY5Y^WT or SH-SY5Y^G93A cell lines induced by complex IV blocker sodium azide. In brain stem slice preparations, riluzole significantly inhibit HMNs cell death induced by inhibiting the mitochondrial electron transport chain by Na-azide. In the HMNs of brainstem slice prepared from adult (14–15 weeks) WT, and corresponding symptomatic SOD1^G93A mice, we measured the effect of riluzole and melatonin on [Ca²⁺]_i using fura-2 AM ratiometric calcium imaging in individual MNs. Riluzole caused a significant decrease in [Ca²⁺]_i transients and reversibly inhibited [Ca²⁺]_i transients in Fura-2 AM loaded HMNs exposed to Na-azide in adult symptomatic SOD1^G93A mice. On the contrary, melatonin failed to show similar effects in the HMNs of WT and SOD1^G93A mice. Intrinsic nicotinamide adenine dinucleotide (NADH) fluorescence, an indicator of mitochondrial metabolism and health in MNs, showed enhanced intrinsic NADH fluorescence in HMNs in presence of riluzole when respiratory chain activity was inhibited by Na-azide. Riluzole’s inhibition of excitability and Ca²⁺ signaling may be due to its multiple effects on cellular function of mitochondria. Therefore formulating a drug therapy to stabilize mitochondria-related signaling pathways using riluzole might be a valuable approach for cell death protection in ALS. Taken together, the pharmacological profiles of the riluzole and melatonin strengthen the case that riluzole indeed can be used as a therapeutic agent in ALS whereas claims of the efficacy of melatonin alone need further investigation as it fail to show significant neuroprotection efficacy.

Glutamate Transport System as a Novel Therapeutic Target in Chronic Pain: Molecular Mechanisms and Pharmacology

The vast majority of peripheral neurons sensing noxious stimuli and conducting pain signals to the dorsal horn of the spinal cord utilize glutamate as a chemical transmitter of excitation. High-affinity glutamate transporter subtypes GLAST/EAAT1, GLT1/EAAT2, EAAC1/EAAT3, and EAAT4, differentially expressed on sensory neurons, postsynaptic spinal interneurons, and neighboring glia, ensure fine modulation of glutamate neurotransmission in the spinal cord. The glutamate transport system seems to play important roles in molecular mechanisms underlying chronic pain and analgesia. Downregulation of glutamate transporters (GluTs) often precedes or occurs simultaneously with development of hypersensitivity to thermal or tactile stimuli in various models of chronic pain. Moreover, antisense knockdown or pharmacological inhibition of these membrane proteins can induce or aggravate pain. In contrast, upregulation of GluTs by positive pharmacological modulators or by viral gene transfer to the spinal cord can reverse the development of such pathological hypersensitivity. Furthermore, some multi-target drugs displaying analgesic properties (e.g., tricyclic antidepressant amitriptyline, riluzole, anticonvulsant valproate, tetracycline antibiotic minocycline, β-lactam antibiotic ceftriaxone and its structural analog devoid of antibacterial activity, clavulanic acid) can significantly increase the spinal glutamate uptake. Thus, mounting evidence points at GluTs as prospective therapeutic target for chronic pain treatment. However, design and development of new analgesics based on the modulation of glutamate uptake will require more precise knowledge of molecular mechanisms underlying physiological or aberrant functioning of this transport system in the spinal cord.

Role of glutamatergic neurotransmission in the enteric nervous system and brain-gut axis in health and disease

Several studies have been carried out in the last 30 years in the attempt to clarify the possible role of glutamate as a neurotransmitter/neuromodulator in the gastrointestinal tract. Such effort has provided immunohistochemical, biomolecular and functional data suggesting that the entire glutamatergic neurotransmitter machinery is present in the complex circuitries of the enteric nervous system (ENS), which participates to the local coordination of gastrointestinal functions. Glutamate is also involved in the regulation of the brain-gut axis, a bi-directional connection pathway between the central nervous system (CNS) and the gut. The neurotransmitter contributes to convey information, via afferent fibers, from the gut to the brain, and to send appropriate signals, via efferent fibers, from the brain to control gut secretion and motility. In analogy with the CNS, an increasing number of studies suggest that dysregulation of the enteric glutamatergic neurotransmitter machinery may lead to gastrointestinal dysfunctions. On the whole, this research field has opened the possibility to find new potential targets for development of drugs for the treatment of gastrointestinal diseases. The present review analyzes the more recent literature on enteric glutamatergic neurotransmission both in physiological and pathological conditions, such as gastroesophageal reflux, gastric acid hypersecretory diseases, inflammatory bowel disease, irritable bowel syndrome and intestinal ischemia/reperfusion injury.

Neuroprotective Effects of the Glutamate Transporter Activator (R)-(-)-5-methyl-1-nicotinoyl-2-pyrazoline (MS-153) following Traumatic Brain Injury in the Adult Rat

Traumatic brain injury (TBI) in humans and in animals leads to an acute and sustained increase in tissue glutamate concentrations within the brain, triggering glutamate-mediated excitotoxicity. Excitatory amino acid transporters (EAATs) are responsible for maintaining extracellular central nervous system glutamate concentrations below neurotoxic levels. Our results demonstrate that as early as 5 min and up to 2 h following brain trauma in brain-injured rats, the activity (Vmax) of EAAT2 in the cortex and the hippocampus was significantly decreased, compared with sham-injured animals. The affinity for glutamate (KM) and the expression of glutamate transporter 1 (GLT-1) and glutamate aspartate transporter (GLAST) were not altered by the injury. Administration of (R)-(-)-5-methyl-1-nicotinoyl-2-pyrazoline (MS-153), a GLT-1 activator, beginning immediately after injury and continuing for 24 h, significantly decreased neurodegeneration, loss of microtubule-associated protein 2 and NeuN (+) immunoreactivities, and attenuated calpain activation in both the cortex and the hippocampus at 24 h after the injury; the reduction in neurodegeneration remained evident up to 14 days post-injury. In synaptosomal uptake assays, MS-153 up-regulated GLT-1 activity in the naïve rat brain but did not reverse the reduced activity of GLT-1 in traumatically-injured brains. This study demonstrates that administration of MS-153 in the acute post-traumatic period provides acute and long-term neuroprotection for TBI and suggests that the neuroprotective effects of MS-153 are related to mechanisms other than GLT-1 activation, such as the inhibition of voltage-gated calcium channels.

Riluzole rescues alterations in rapid glutamate transients in the hippocampus of rTg4510 mice

Those at risk for Alzheimer's disease (AD) often exhibit hippocampal hyperexcitability in the years preceding diagnosis. Our previous work with the rTg(TauP301L)4510 tau mouse model of AD suggests that this increase in hyperexcitability is likely mediated by an increase in depolarization-evoked glutamate release and a decrease in glutamate uptake, alterations of which correlate with learning and memory deficits. Treatment with riluzole restored glutamate regulation and rescued memory deficits in the TauP301L model. Here, we used enzyme-based ceramic microelectrode array technology to measure real-time phasic glutamate release and uptake events in the hippocampal subregions of TauP301L mice. For the first time, we demonstrate that perturbations in glutamate transients (rapid, spontaneous bursts of glutamate) exist in a tau mouse model of AD mouse model and that riluzole mitigates these alterations. These results help to inform our understanding of how glutamate signaling is altered in the disease process and also suggest that riluzole may serve as a clinically applicable therapeutic approach in AD.

Neuronal vs glial glutamate uptake: Resolving the conundrum

Neither normal brain function nor the pathological processes involved in neurological diseases can be adequately understood without knowledge of the release, uptake and metabolism of glutamate. The reason for this is that glutamate (a) is the most abundant amino acid in the brain, (b) is at the cross-roads between several metabolic pathways, and (c) serves as the major excitatory neurotransmitter. In fact most brain cells express glutamate receptors and are thereby influenced by extracellular glutamate. In agreement, brain cells have powerful uptake systems that constantly remove glutamate from the extracellular fluid and thereby limit receptor activation. It has been clear since the 1970s that both astrocytes and neurons express glutamate transporters. However the relative contribution of neuronal and glial transporters to the total glutamate uptake activity, however, as well as their functional importance, has been hotly debated ever since. The present short review provides (a) an overview of what we know about neuronal glutamate uptake as well as an historical description of how we got there, and (b) a hypothesis reconciling apparently contradicting observations thereby possibly resolving the paradox.

The importance of the excitatory amino acid transporter 3 (EAAT3)

The neuronal excitatory amino acid transporter 3 (EAAT3) is fairly ubiquitously expressed in the brain, though it does not necessarily maintain the same function everywhere. It is important in maintaining low local concentrations of glutamate, where its predominant post-synaptic localization can buffer nearby glutamate receptors and modulate excitatory neurotransmission and synaptic plasticity. It is also the main neuronal cysteine uptake system acting as the rate-limiting factor for the synthesis of glutathione, a potent antioxidant, in EAAT3 expressing neurons, while on GABAergic neurons, it is important in supplying glutamate as a precursor for GABA synthesis. Several diseases implicate EAAT3, and modulation of this transporter could prove a useful therapeutic approach. Regulation of EAAT3 could be targeted at several points for functional modulation, including the level of transcription, trafficking and direct pharmacological modulation, and indeed, compounds and experimental treatments have been identified that regulate EAAT3 function at different stages, which together with observations of EAAT3 regulation in patients is giving us insight into the endogenous function of this transporter, as well as the consequences of altered function. This review summarizes work done on elucidating the role and regulation of EAAT3.

Concise Review: Bridging the Gap: Novel Neuroregenerative and Neuroprotective Strategies in Spinal Cord Injury

Spinal cord injuries (SCIs) result in devastating lifelong disability for patients and their families. The initial mechanical trauma is followed by a damaging secondary injury cascade involving proapoptotic signaling, ischemia, and inflammatory cell infiltration. Ongoing cellular necrosis releases ATP, DNA, glutamate, and free radicals to create a cytotoxic postinjury milieu. Long-term regeneration of lost or injured networks is further impeded by cystic cavitation and the formation of an inhibitory glial-chondroitin sulfate proteoglycan scar. In this article, we discuss important neuroprotective interventions currently applied in clinical practice, including surgical decompression, blood pressure augmentation, and i.v. methylprednisolone. We then explore exciting translational therapies on the horizon, such as riluzole, minocycline, fibroblast growth factor, magnesium, and hypothermia. Finally, we summarize the key neuroregenerative strategies of the next decade, including glial scar degradation, Rho-ROCK inhibition, cell-based therapies, and novel bioengineered adjuncts. Throughout, we emphasize the need for combinatorial approaches to this multifactorial problem and discuss relevant studies at the forefront of translation. We conclude by providing our perspectives on the future direction of SCI research.

SIGNIFICANCE

Spinal cord injuries (SCIs) result in devastating, lifelong disability for patients and their families. This article discusses important neuroprotective interventions currently applied in clinical practice, including surgical decompression, blood pressure augmentation, and i.v. methylprednisolone. Translational therapies on the horizon are discussed, such as riluzole, minocycline, fibroblast growth factor, magnesium, and hypothermia. The key neuroregenerative strategies of the next decade are summarized, including glial scar degradation, Rho-ROCK inhibition, cell-based therapies, and novel bioengineered adjuncts. The need for combinatorial approaches to this multifactorial problem is emphasized, relevant studies at the forefront of translation are discussed, and perspectives on the future direction of SCI research are presented.

Transcriptional Regulation of Glutamate Transporters: From Extracellular Signals to Transcription Factors

Glutamate is the predominant excitatory neurotransmitter in the mammalian CNS. It mediates essentially all rapid excitatory signaling. Dysfunction of glutamatergic signaling contributes to developmental, neurologic, and psychiatric diseases. Extracellular glutamate is cleared by a family of five Na(+)-dependent glutamate transporters. Two of these transporters (GLAST and GLT-1) are relatively selectively expressed in astrocytes. Other of these transporters (EAAC1) is expressed by neurons throughout the nervous system. Expression of the last two members of this family (EAAT4 and EAAT5) is almost exclusively restricted to specific populations of neurons in cerebellum and retina, respectively. In this review, we will discuss our current understanding of the mechanisms that control transcriptional regulation of the different members of this family. Over the last two decades, our understanding of the mechanisms that regulate expression of GLT-1 and GLAST has advanced considerably; several specific transcription factors, cis-elements, and epigenetic mechanisms have been identified. For the other members of the family, little or nothing is known about the mechanisms that control their transcription. It is assumed that by defining the mechanisms involved, we will advance our understanding of the events that result in cell-specific expression of these transporters and perhaps begin to define the mechanisms by which neurologic diseases are changing the biology of the cells that express these transporters. This approach might provide a pathway for developing new therapies for a wide range of essentially untreatable and devastating diseases that kill neurons by an excitotoxic mechanism.

Spinal Glutamate Transporters Are Involved in the Development of Electroacupuncture Tolerance

BACKGROUND

Electroacupuncture (EA) tolerance is a gradual decline in EA antinociception because of its repeated or prolonged use. This study aims to explore the role of spinal glutamate transporters (GTs) in EA tolerance (EAT).

METHODS

Rats were treated with EA once per day for eight consecutive days, and their L4-5 spinal cords were collected at days 0, 2, 4, 6 and 8. The levels of three spinal GTs and their mRNAs were detected with Western blot and pPCR, respectively. Then, riluzole, a positive GT regulator, was administered intrathecally in order to observe its effect on EA analgesia after repeated EA.

RESULTS

The expression levels of the spinal GTs increased at days 2 and 4, and gradually decreased as the times of EA increased. At day 8, no difference was observed in the spinal GTs between the sham treatment and the EA treatment. Intrathecal administration of riluzole dose-dependently attenuated the decreased EA analgesia.

CONCLUSION

These results indicated the participation of the spinal GTs in EAT.

Rosiglitazone ameliorates astrocyte over-activation and inflammatory cytokine release induced by global cerebral ischemia/reperfusion

Global cerebral ischemia (GCI) is a leading cause of mortality worldwide and remains the primary cause of long-term neurological disability. Astrocyte over-activation and extensive neuron loss in the ischemic brain are the characteristic pathological features of cerebral ischemia. Rosiglitazone (RSG) is a peroxisome-proliferating activating receptor-γ agonist known for its anti-inflammatory activity. Previous studies have suggested that RSG is able to exert neuroprotection in numerous acute and chronic brain injury models. However, whether RSG treatment is involved in astrocyte over-activation and inflammatory reaction in the cortex remains unclear. The aim of the present study was to investigate whether RSG treatment improved functional impairment induced following GCI and protected against cortex neuron loss, and to elucidate the potential mechanisms underlying these functions. Rats were randomly divided into three groups: Sham-operated, GCI and RSG treatment groups. The RSG treatment group was treated with 2 mg/kg RSG immediately following GCI. The results demonstrated that RSG treatment significantly reduced infarct volume and neuron survival rates in addition to increasing function recovery. Furthermore, these results correlate with a reduction in astrocyte over-activation and inflammatory cytokines in the rat cortex. However, no significant changes in glutamate transporter-1 expression levels were observed following RSG treatment compared with the GCI rats. The results of this investigation provide in vivo evidence that RSG significantly protected rats against ischemia-reperfusion-induced brain injury. In addition, RSG may exert neuroprotective effects by inhibiting astrocyte over-activation, and thereby reducing the levels of inflammatory cytokines in the GCI-injured brain. All data revealed that RSG may be a potential neuroprotective agent for cerebral ischemia.