Neuropharmacological insight into preventive intervention in posttraumatic epilepsy based on regulating glutamate homeostasis

Abstract Background Posttraumatic epilepsy (PTE) is one of the most critical complications of traumatic brain injury (TBI), significantly increasing TBI patients' neuropsychiatric symptoms and mortality. The abnormal accumulation of glutamate caused by TBI and its secondary excitotoxicity are essential reasons for neural network reorganization and functional neural plasticity changes, contributing to the occurrence and development of PTE. Restoring glutamate balance in the early stage of TBI is expected to play a neuroprotective role and reduce the risk of PTE. Aims To provide a neuropharmacological insight for drug development to prevent PTE based on regulating glutamate homeostasis. Methods We discussed how TBI affects glutamate homeostasis and its relationship with PTE. Furthermore, we also summarized the research progress of molecular pathways for regulating glutamate homeostasis after TBI and pharmacological studies aim to prevent PTE by restoring glutamate balance. Results TBI can lead to the accumulation of glutamate in the brain, which increases the risk of PTE. Targeting the molecular pathways affecting glutamate homeostasis helps restore normal glutamate levels and is neuroprotective. Discussion Taking glutamate homeostasis regulation as a means for new drug development can avoid the side effects caused by direct inhibition of glutamate receptors, expecting to alleviate the diseases related to abnormal glutamate levels in the brain, such as PTE, Parkinson's disease, depression, and cognitive impairment. Conclusion It is a promising strategy to regulate glutamate homeostasis through pharmacological methods after TBI, thereby decreasing nerve injury and preventing PTE.


| INTRODUC TI ON
Traumatic brain injury (TBI) is an alteration in brain function or other pathological injuries of brain tissue caused by external force. 1 Each year, an estimated 69 million individuals worldwide suffer from TBI, most of which are mild (81%) and moderate (11%). 2 Posttraumatic epilepsy (PTE) is recurrent epilepsy secondary to TBI and one of the most critical complications of TBI. Compared with the general population, patients with severe TBI have a 30-fold increase in the incidence rate of epilepsy, and this risk is higher among soldiers exposed to missile and blast injuries. 3  The current theory holds that the development of acquired epilepsy (AE), including PTE, mainly goes through three phases: acute brain injury, the latent period of epilepsy, and the spontaneous recurrent period of chronic epilepsy. 5 After the acute brain injury phase, the brain will gradually change at the histological, cytological and molecular levels, contributing to recurrent epilepsy. With the discovery of risk factors and biomarkers of PTE, people can now better predict PTE. If we can take corresponding measures in the window between TBI and late recurrent epilepsy, we may be able to prevent the onset of PTE. 6 Therefore, from the theoretical point of view, PTE is potentially preventable epilepsy. 7 In current clinical practice, the prevention and treatment of PTE mainly include the following strategies: (1) after the occurrence of severe TBI, take preventive measures to prevent the occurrence of early PTS in time; (2) when late PTE occurs, take first-line antiepileptic drugs (AEDs) that are effective for focal epilepsy; (3) for patients with drug-resistant PTE or unable to tolerate drug side effects, surgical treatment can be selected. 8 Unfortunately, the existing AEDs do not seem ideal for reducing the risk of late PTE. Phenytoin sodium and levetiracetam are the most commonly used AEDs to prevent PTE in clinical applications. 9,10 Still, previous studies have shown that early administration of AEDs can reduce the risk of early PTS compared with placebo or standard care. Still, there was no evidence that AEDs can reduce the risk of late seizures or mortality. 11 Another retrospective study also showed that the occurrence of late PTS is not relieved by preventive treatment. 12 Craniotomy and decompressive surgery showed the same situation. 13 The complex and unclear pathogenesis may have contributed to the failure of traditional AEDs in preventing PTE.
The mechanism of PTE is complex, involving blood-brain barrier damage, inflammatory reaction, and excitotoxicity. The imbalance between excitatory and inhibitory neurotransmitter transmission and the consequent excitotoxicity is one of the fundamental causes of symptomatic epilepsy and the development and maintenance of PTE. Glutamate is the most abundant neurotransmitter in the nervous system, closely related to excitatory signal transmission between neurons and neuroplasticity of the normal brain. 14 TBI causes increased release and transport abnormalities of glutamate in the brain, leading to the accumulation of glutamate in the neuronal extracellular space. Abnormally accumulated glutamate mediates excitotoxic effects via glutamate receptors on cells, leading to neuronal damage and even death. In the acute phase of TBI, glutamateinduced neuronal injury and alterations in neural networks may increase the susceptibility to epilepsy and contribute to recurrent seizures. Elucidating the role of the glutamate system in the development of PTE and its intervention pathways will facilitate the screening and discovery of preventive drugs for PTE.
In this review, we will systematically expound on the effects of TBI on glutamate homeostasis and the related role of glutamate involvement in the onset and development of PTE. In addition, we will review the pharmacological research on the molecular pathways regulating glutamate homeostasis after TBI to provide a reference for drug research and development for the intervention of PTE.

| Glutamate-glutamine cycle
The complete glutamate system involves glutamate synthesis, presynaptic glutamate release, the interaction between glutamate and glutamate receptors, and glutamate uptake by glutamate transporters. Glutamate is released from the presynapse to the synaptic cleft in a calcium-dependent manner in response to neuronal depolarization. Glutamate released into the extracellular space can exert excitatory synaptic transmission by binding to several ionotropic glutamate receptors (iGluRs) and metabotropic glutamate receptors (mGluRs) subtypes at the postsynapse or spillover from the synaptic cleft. In contrast, ambient glutamate participates in physiological and pathological changes. 15 Postsynaptic ionotropic receptors include N-methyl-D-aspartate receptor (NMDAR), α-Amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor (AMPAR), and kainic acid receptor (KAR). 16 Under normal physiological conditions, glutamate released from glutamatergic nerve terminals acts on postsynaptic receptors, causing depolarization and releasing Mg 2+ cations from NMDA receptor channels, allowing Ca 2+ to pass through pores. AMPAR is usually co-expressed with NMDAR at the synapse. They participate in synaptic plasticity, involving learning, memory, excitotoxicity, and neuroprotection. Metabotropic glutamate receptors (mGluRs) are Class C 7-transmembrane domain (7-TMD) G-protein coupled receptors (GPCRs). mGluR dimers bind glutamate, which triggers a conformational change of the receptor and the initiation of intracellular signaling, participating in the modulation of neurotransmission throughout the nervous system. 17 According to G-protein coupling, sequence homology, and ligand selectivity, mGluRs can be subdivided into three groups. Group I (mGlu1 and mGlu5) are coupled with Gq/11 and related signal pathways, group II (mGlu2 and mGlu3) and group III (mGlu4, 6,7,8) are coupled with Gi/o signaling. 18 Compared with ligand-gated ion channels (NMDA, AMPA, and KAR) responsible for rapid excitatory transmission, mGluRs have a greater regulatory effect, helping finetune synaptic efficacy and control the accuracy and sharpness of transmission. Its allosteric regulator has various pharmacological effects on central system diseases. 19 The extrasynaptic glutamate mainly originates from the spillover from synapses, 15,20 as well as astrocytic 21  ists in neurons and astrocytes and is most abundant in Bergman glial cells in the cerebellar molecular layer but also in the cortex, hippocampus, and deep cerebellar nucleus. 25 The heterozygous mutation of EAAT1 can lead to a decrease in glutamate uptake, which causes neuronal hyperexcitability and induces seizures, hemiplegia, and episodic ataxia. 26 EAAT2 is specifically localized in astrocytes and distributed throughout the brain and spinal cord. 25 EAAT2 is the main glutamate transporter in the central nervous system of adult rodents. It absorbs the most extracellular glutamate (~90%) in almost all brain regions. 27,28 Studies have shown that EAAT2 keeps extracellular glutamate concentrations low and prevents neurotoxicity of glutamate, which would otherwise be elevated enough to cause epilepsy and cell death. 29 EAAT3 protein is widely distributed in the somata and dendrites of all hippocampal neurons, and its concentration in the hippocampus of adult rats is 100 times lower than that of EAAT2. 30 EAAT3 mainly mediates the uptake of cysteine and glutamate in the brain and plays an important role in regulating glutamatergic and GABAergic neurotransmission and neuronal redox homeostasis. 31 Neuronal death and the number of degenerating neurons in the hippocampus more than doubled after TBI in EAAT3 (−/−) mice compared to wild-type mice, as did superoxide production, zinc translocation, and microglial activation. 32 EAAT4 and EAAT5 expressions are almost exclusively restricted to specific neuronal populations in the cerebellum and retina, respectively. 33 The efficiency with which EAATs take up and transport glutamate into the intracellular astrocyte space is related to the intracellular concentration of sodium and glutamate, which may slow the cycling rate of EAATs. Glutamate absorbed into astrocytes is catalyzed by glutamine synthetase (GS) to synthesize glutamine via ammonia and ATP metabolism. Specific enzymes such as GS maintain glutamate concentrations at low levels. Its conversion to glutamate is essential to ensure the efficient clearance of glutamate by EAATs and prevent glutamate spillover. 34,35 In addition, glutamate can also enter mitochondria for oxidative metabolism.
Glutamate dehydrogenase (GDH) catalyzes the initial oxidation step by deaminating glutamate to the tricarboxylic acid (TCA) cycle intermediate a-ketoglutarate using NAD (P) 1 as a cofactor. 36 Studies in mutant mice and allosteric drugs have shown that loss or overexpression of GDH activity in the brain modulates wholebody energy metabolism and affects the early onset of Parkinson's disease, Alzheimer's disease, temporal lobe epilepsy, and spinocerebellar atrophy. 37 Glutamine is released from astrocytes to extracellular space by various Na + dependent and Na + independent transporter pathways. 38 The SNAT (sodium-coupled neutral amino acid transporter) family encoded by solute carrier family 38 (SLC38) is involved in this process, with SNAT3 and SNAT5 being the primary mediators of glutamine efflux in astrocytes. 39 Studies have shown that astrocytes adjacent to glutamatergic synapses can release glutamine in a temporally precise, controlled manner in response to the activation of glial glutamate transporters.
Astrocytes can respond to synaptic activation by this mechanism and in ways that maintain or enhance further communication. 40 The release of glutamine caused by EAATs activation is mainly driven by increased intracellular Na + concentration. 41 In addition, glutamate uptake by neurons is mediated by SNAT1 and SNAT2 on the plasma membrane of neurons, 42 and SNAT6 has recently been shown to have a potential role in glutamate uptake at the presynaptic end of excitatory neurons. 43 Glutamine is taken up by excitatory neurons and catalyzed by phosphate-activated glutaminase (PAG) to produce glutamate, which continues to play its role as an excitatory neurotransmitter. It has been shown that PAG concentration and activity per neuron increase in the hippocampal formation in patients with mesial temporal lobe epilepsy (MTLE).
This increase contributes to the disruption of glutamate homeostasis in the hippocampus of MTLE patients. 44 The process by which glutamate is efficiently cleared from the extracellular space and replenished at axon terminals is collectively referred to as the glutamate-glutamine cycle and includes glutamate release, uptake, conversion, and elimination ( Figure 1). 45 Under pathological conditions, the excessive release of glutamate and the inhibition of its reuptake will cause abnormal accumulation of extracellular glutamate. High glutamate concentrations will cause excessive stimulation of ionic glutamate receptors, cause excitotoxicity, and lead to neuronal injury and death. 46

| Effect of TBI on glutamate release
The excessive release of glutamate and the abnormality of glutamate transporter after TBI will lead to a series of central nervous system disorders, such as Parkinson's disease 50 and epilepsy, and some mental disorders, such as depression, 51 cognitive impairment, 52 anxiety, and alcohol consumption. 53 The changes in glutamate after TBI show specific characteristics. There is a significant gender difference in glutamate production after severe TBI. Within 48 h after injury, compared with females, the production of glutamate in the cerebrospinal fluid (CSF) of males is significantly increased. 54,55 Changes in glutamate concentrations in the brain may depend on the time elapsed after injury and region-specific. 56 In mice, susceptibility to early and late seizures increased after repeated mild TBI. At 1 week after TBI, the increase of glutamate dominated excitation/inhibition (E/I) ratio, but 6 weeks after TBI, the glutamate levels were normalized and the imbalance of E/I ratio was characterized by the decrease of GABA. 57 Tonic glutamate release was measured in the prefrontal cortex, dentate gyrus and striatum 2 days after mild or moderate brain injury in adult male rats using enzyme-based microelectrode arrays. The results showed that tonic glutamate levels significantly correlated with injury severity in the dentate gyrus and striatum. 58 TBI also affects kir4.1 and EAAT2 gene expression age-and time-dependent, leading to more glutamate accumulation at early synapses in aged mice compared to adult mice, which may be related to the early and severe neuronal depolarization and excitotoxicity caused by TBI in older adults. 59 A prospective study of 165 severe TBI patients showed that initially high glutamate values after TBI predicted poor outcomes.
Compared to patients whose glutamate levels tended to normalize over the monitoring period (120 h), patients whose glutamate levels tended to increase with time or remain abnormally elevated had a higher mortality rate (39.6 vs 17.1%) and a worse 6-month functional outcome among survivors (20.7 vs 41.2%). 60 Glutamate chemical exchange saturation transfer (GluCEST) MRI can noninvasively image glutamate in vivo with high sensitivity and spatial resolution. Sequential GluCEST imaging scans were performed in adult male Sprague Dawley rats before TBI and 1, 3, 7, and 14 days after TBI. Compared with the baseline and control groups, GluCEST% increased and peaked on the first day after TBI in the injured cortical core lesion and peaked on the third day in the ipsilateral hippocampus. GluCEST% gradually decreased to baseline on the 14th day after TBI. 61 GluCEST was used to evaluate glutamate changes in patients with acute mild to moderate TBI. The results showed that the level of Glutamate increased significantly during acute TBI and was closely related to the cognitive effects 1 month after injury. 62 A prospective study was conducted on 42 severely injured adults using proton magnetic resonance spectroscopy (MRS) to measure the quantitative metabolite changes in the early (7 days) after typical brain injury and to predict the long-term prognosis of the nervous system. The results showed that glutamate/glutamine and choline in occipital gray matter and parietal F I G U R E 1 Glutamate-glutamine cycle.
white matter were significantly increased in patients with poor long-term (6-12 months) prognosis, which may be a reflection of early excitotoxic injury and of injury associated with membrane disruption secondary to diffuse axonal injury. 63 The glutamate value of extracellular fluid (ECF) increased immediately after TBI, and the sustained activation of microglia was prolonged 10 days after TBI. This activation occurs after significantly increasing glutamate levels in ECF after trauma. 64 Glutamate content increased significantly 20 min after controlled cortical impact (CCI), which coincided with the decrease of extracellular glucose (20 min) and the increase of lactic acid (40-60 min) in both brain regions after CCI. 65 Moderate FPI (fluid percussion injury, 3.8-4.8 atm) did not change the excitability of the postsynaptic membrane of CA1 pyramidal neurons; However, glutamatergic excitatory synaptic transmission was enhanced in CA1 pyramidal neurons after FPI.
Paired pulse facilitation (PPF) was significantly inhibited in the ipsilateral FPI group and contralateral FPI group. The frequency of MEPSP in neurons of the bilateral FPI group was higher than that of the control group. These results suggest that glutamatergic synaptic transmission in the hippocampal CA1 region is promoted by the presynaptic mechanism after brain injury. 66

| Effects of TBI on glutamate transports
Morphometric immunohistochemical analysis showed that EAAT1 and EAAT2 were mainly expressed in astrocytes of the nor- showed that the absolute number of GFAP-positive astrocytes co-expressing EAAT1 or EAAT2 decreased when the survival time was up to 7 days. In addition, the relative proportion of astrocytes co-expressing glutamate transporters decreased after TBI, which indicates that the decrease of EAAT1 and EAAT2 expression in posttraumatic cells is mainly due to the degeneration of astrocytes and the down-regulation of living astrocytes. The reduction of glutamate uptake by astrocytes will aggravate the increase of extracellular glutamate after human trauma. 67 Adult male Wistar rats received unilateral moderate FPI. The hippocampus and cortex were analyzed 3 h after the injury. It was found that the mitochondrial function was abnormal, characterized by potential membrane interruption, imbalance of the redox system, decreased mitochondrial activity, and complex I inhibition. In addition, it was also noted that the destruction of calcium homeostasis and the increase of mitochondrial swelling and mitochondrial dysfunction led to the decrease of glutamate uptake and glial glutamate transporters (EAAT1 and EAAT2), resulting in excitotoxicity. 68 The TBI model of adult male rats was constructed by LFPI. The results showed no difference in the expression of EAAT2 in the ipsilateral cortex and hippocampus 24 h after TBI. In contrast, glutamate uptake in the cerebral cortex decreased 24 h after injury, but glutamate uptake in the hippocampus did not decrease. 69 In addition, TBI also affects the activity of glutamate transporters. As early as 5 min after brain injury in rats with brain injury, the activity (Vmax) of EAAT2 in the cortex and hippocampus was significantly lower than that in sham-injured animals. Still, its affinity for glutamate (Km) and the expression of glutamate transporter 1 (EAAT2) and glutamate aspartate transporter 1 (EAAT1) was not changed due to injury. 70 Excitatory amino acid carrier type 1 (EAAC1, EAAT3) is a highaffinity glutamate transporter that can consume energy to transfer glutamate to neurons. However, under normal physiological conditions, EAAT3 has no significant effect on glutamate clearance but participates in the cysteine uptake by neurons. This process is crucial for maintaining neuronal antioxidant function by providing cysteine to synthesize glutathione. 32 The expression of glutamate transporter EAAT4 in the hippocampus increased 7 days after FPI.
Immunohistochemical analysis showed that this increase was limited to the cells showing the morphological characteristics of astrocytes.
In addition, on the third day after the injury, increased EAAT4 immunoreactivity was observed in astrocytes of the ipsilateral cortex and cerebellum, which lasted until the seventh day after the injury, indicating that this protein may play an essential role in the pathophysiology of TBI. 71

| Glutamate excitotoxicity and PTE
There is a close relationship between neuronal injury triggered by glutamate and excessive calcium influx. Calcium overload can trigger a series of downstream neurotoxic cascades, including the decoupling of mitochondrial electron transfer in ATP synthesis, enzyme activation, and over stimulation, such as calpain and other proteases, protein kinase, nitric oxide synthase (NOS), calcineurin and endonuclease. Changes in the activity of these enzymes will lead to (1) increased production of toxic reactive oxygen species The concentration of glutamate in microdialysate of 6 patients with complex partial epilepsy before and during seizures was measured by microdialysis in the bilateral hippocampus. Before seizures, the concentration of glutamate in the epileptic hippocampus was higher. During seizures, the concentration of extracellular glutamate in the epileptic hippocampus increases continuously. In addition, this increase precedes seizures, which indicates that the increase in extracellular glutamate concentration may lead to seizures. 75 Brain microdialysis probes were implanted into 17 adults with severe brain injury, and systematic sampling was performed 1-9 days after injury. In each patient, a transient increase in glutamate was seen daily. However, these elevations were most common on day 3. These increases are often associated with seizure activity. 76 In animal experiments, it was found that microperfusion of 200 μM glutamate into the hippocampus of free-moving rats was sufficient to lower the picrotoxin seizure threshold down to 50% in 60% of the animals studied and prolonged seizure duration (180 ± 23%). 77 High-speed glutamate biosensor imaging showed that the glutamate signal transduction in the damaged cortex of TBI animals was significantly increased, and the increased glutamate response was related to epilepsy-like activity, which was the highest when it was directly adjacent to the injury and spread The development of acquired epilepsy, represented by PTE, usually includes three stages: the first is the injury of the central nervous system, the second is the incubation period of epilepsy, and third is the development into spontaneous recurrent epilepsy, that is chronic epilepsy. The damage leading to acquired epilepsy often has a common molecular mechanism; that is, the increase of extracellular glutamate concentration leads to the increase of intracellular neuronal calcium, which leads to neuronal damage and/or death.
Neurons that survive glutamate-induced injury and are exposed to which has become a prominent feature of epilepsy phenotype. 5 In vitro experiments show that TBI leads to an abnormal response of neurons to glutamate stimulation. This change in sensitivity to stimulation may be related to the changes in excitability in the first few hours to days after TBI. It may play a role in the early posttraumatic seizures in TBI patients. 81

| P OTENTIAL INTERVENTI ON APPROACHE S TO RE S TORE G LUTAMATE HOMEOS TA S IS AF TER TB I
Numerous studies have reported the molecular pathways that affect glutamate homeostasis after TBI (Table 1). These studies mainly focused on reducing glutamate excitotoxicity by reducing brain glutamate concentration after TBI, thereby reducing further damage and related complications caused by TBI. These molecular pathways may serve as potential drug targets to reduce the risk of PTE after TBI.

| NAAG peptidase
N-acetyl aspartate glutamate (NAAG) is a ubiquitous neuropeptide in the central nervous system. It is released with glutamate and inhibits presynaptic glutamate release by acting on mGluR3. Extracellular NAAG is hydrolyzed to N-acetyl aspartic acid and glutamate by peptidase [glutamate carboxypeptidase II (GCP II) and GCP III] activity. 82 Studies have shown that NAAG peptidase inhibitor ZJ-43 can increase the extracellular NAAG level and reduce the extracellular levels of amino acid neurotransmitters, including glutamate, aspartic acid, and GABA in the dialysate of the dorsal hippocampus of TBI through the mechanism mediated by group II metabotropic glutamate receptor (mGluR). 83 Systemic injection of GCP II inhibitor ZJ-43 and its prodrug PGI-02776 within the first 30 min after the injury can reduce the acute neuropathological and TBI-related long-term cognitive impairment caused by excessive extracellular glutamate, 84 significantly reduce the harmful effect of TBI combined with secondary hypoxic injury, 85 and reduce the exercise and cognitive impairment combined with TBI and the second hypoxic injury within a few weeks after injury. 86 GCP II is a transmembrane zinc metallopeptidase that mainly exists in the nervous system, prostate, and small intestine. In the nervous system, glial cell-bound GCP II mediates the neurotransmitter NAAG hydrolysis to glutamate and N-acetyl aspartate. Studies have shown that inhibition of GCP II can attenuate excitotoxicity associated with enhanced glutamate transmission under pathological conditions. The sensitivity of GCP II-KO mice to moderate TBI was significantly reduced. 87 Two non-invasive methods, 1H-MRS and T2 MR imaging, as well as in vitro brain water content measurement, showed that the reduction of glutamate and N-acetyl aspartate levels were less obvious in GCP II-KO mice than in WT mice, proving that the potential mechanism of the neuroprotective effect of GCP II-KO on TBI brain swelling involves the changes of glutamate and N-acetyl aspartate levels. 88 In the animal model of TBI, inhibition of GCP II can increase the extracellular level of NAAG, inhibit the release of glutamate, and have neuroprotective effects. This effect was confirmed in GCP II-KO mice by reducing oxidative stress. 89

| Adenosine receptor
After the severe craniocerebral injury, the concentration of adenosine in cerebrospinal fluid increased in a time-dependent and severity-dependent manner. There is a strong correlation between increased CSF adenosine and glutamate concentrations, which may reflect endogenous neuroprotective measures against excitatory toxicity after severe TBI. 90 Evidence suggests that early release of adenosine after TBI can inhibit seizures and brain inflammation, while glutamate can increase intracellular and extracellular adenosine release. 91 Adenosine plays a dual role in glutamate cytotoxicity, in which A 2A adenosine receptor (A 2A R) and A 2B adenosine receptor (A 2B R) activation intensify glutamineinduced cell injury, while A 1 adenosine receptor activation TA B L E 1 Potential intervention pathways for restoring glutamate homeostasis after TBI and brain-protective effects.

Intervention pathway Mode of action Effects on brain and glutamate homeostasis References
NAAG peptidase

NAAG peptidase inhibition
Inhibited glutamate release and reduced acute neuropathological injury and longterm cognitive deficits associated with TBI induced by glutamate neurotoxicity after TBI

A 2A R and A 2B R inhibition
Inhibited glutamate release and reduced cell injury induced by glutamate; antiinflammatory effect; increased the activity of Na + -K + -ATPase, reduced its interaction with EAAT in isolated cerebral capillaries, and restored the normal transport function of EAAT

Intravenous injection of GOT and GPT
Mediate the redistribution of excess glutamate from brain extracellular fluid to blood, and reduce the secondary brain injury caused by glutamate neurotoxicity 103

Administration of estrogen
Reduce the level of blood glutamate in TBI rats to improve neurological prognosis 104 Administration of blood glutamate scavengers (oxaloacetic acid, pyruvic, GOT) Reduced glutamate levels, improved neurologic recovery, and improved depressivelike behavior; reduced the loss of hippocampal neurons and improved neurological function; reversed LTP inhibition by enhancing glutamate homeostasis in BBB

Administration of glucagon
Activated gluconeogenesis by increasing the liver uptake of amino acids such as glutamate and promoting their conversion to glucose 109 Exogenous lactic acid supplementation Reduced cerebral glucose consumption; Increased brain extracellular pyruvate and glucose availability and decreased brain glutamate and intracranial pressure; exerts beneficial effects on brain metabolism and hemodynamics after TBI Caffeine is the most consumed psychoactive drug and nonspecific adenosine receptor antagonist. In the CCI-induced TBI mouse model, long-term (3 weeks) caffeine preconditioning in drinking water significantly reduced neurological deficits, brain edema, and inflammatory cell infiltration compared with the control group.
Its mechanism of action may be achieved by upregulating brain A 1 receptor mRNA and inhibiting excessive glutamate and inflammatory cytokine release. 99

| Blood glutamate clearance
Studies have shown that excessive glutamate in the brain can be eliminated by reducing the level of glutamate in the blood and accelerating the outflow of brain-blood glutamate. 100 After intravenous injection of non-metabolic analogs of glutamate to juvenile rats, it can be observed that glutamate is rapidly cleared from the blood to peripheral tissues in a non-metabolic form. Among them, the liver plays a central role in glutamate metabolism. It is the source of glutamate metabolites, which will be redistributed to skeletal muscle and intestine, which suggests that drug manipulation reduces the release rate of glutamate in the liver or increases the pumping rate of glutamate in skeletal muscle, may achieve neuroprotective effects in the brain. 101 Some studies have shown that the transient decrease of blood glutamate after TBI is a stress response, and β2 is a result of the activation of the sympathetic nervous system by adrenergic receptors. The observed increase in glutamate brain blood outflow leads to a high level of glutamate in the brain after brain injury. 102 Intravenous injection of glutamic oxaloacetic transaminase (GOT) and glutamic pyruvate transaminase (GPT) can reduce the level of blood glutamate. The lowering effect of GOT and GPT on glutamate can mediate the redistribution of excess glutamate from brain extracellular fluid to blood and reduce the secondary brain injury caused by glutamate neurotoxicity. 103 Injection of Premarin into male rats can significantly reduce the blood glutamate level in TBI rats. This decrease is related to the improvement of neurological prognosis, suggesting the role of estrogen in neuroprotection. 104 Administration of pyruvate to clear blood glutamate after TBI has been widely proven to be an effective method to provide neuroprotection by reducing blood glutamate and subsequent brain glutamate levels. Studies have shown that prophylactic or therapeutic administration of pyruvate after TBI can reduce glutamate levels and improve the recovery of the nervous system and TBI-induced depression-like behavior. 105 Using blood glutamate scavengers, oxaloacetic acid, and pyruvic acid can also provide a neuroprotective effect after TBI, which reduces the loss of hippocampal neurons and improves neurological function. 106 Delayed cognitive impairment after brain injury is more related to transient glutamate enhancement in the hippocampus. The peripheral glutamate scavenger rGOT can reverse LTP inhibition by enhancing glutamate homeostasis in BBB. 107

| Regulation of metabolism
The endogenous mechanism of decreasing glutamate concentration in the brain parenchyma after brain injury is unclear. Mass spectrometric evidence shows that glutamate oxidation can reduce glutamate concentration through a "truncated" tricarboxylic acid cycle coupled with the urea cycle. This process reduces glutamate levels, generates carbon for energy metabolism, leads to citrulline accumulation, and produces nitric oxide. 108 Glucagon can activate gluconeogenesis by increasing the liver uptake of amino acids such as glutamate and promoting their conversion to glucose. Studies have shown that glucagon plays a prominent neuroprotective role after TBI by reducing glutamate in the central nervous system. Despite elevated blood glucose, glucagon is also beneficial. 109 Exogenous lactic acid supplementation can be used as the priority energy substrate of the injured human brain to save brain glucose consumption. It can also increase the availability of brain extracellular pyruvate and glucose and reduce brain glutamate and intracranial pressure, indicating that hypertonic lactate treatment has beneficial brain metabolism and hemodynamics after TBI. 110 In addition, there is experimental evidence that brain glutamate was lowest measured at brain glucose 3-5 mM and increased significantly at brain glucose below 3 mM or above 6 mM. Insulin administration could increase brain glutamate at low brain glucose. 111 However, reducing excitatory amino acids after ischemia by adding glucose to dialysate had no significant effect on dialysate glutamate in the cortex or hippocampus after TBI. 112

| Calcium influx pathway
Calcium can mediate the release of glutamate and the interruption of glial regulation of extracellular glutamate, resulting in the increase of extracellular glutamate in the striatum 2 days after diffuse brain injury. 113 TBI-induced glutamate release increases mitochondrial Ca 2+ cycle/ overload, resulting in mitochondrial dysfunction. 114 Mitochondrial dysfunction will further reduce glutamate uptake and glial glutamate transporters (EAAT1 and EAAT2), leading to excitotoxicity. Guanosine (GUO) treatment can improve mitochondrial damage and glutamatergic dysfunction as an effective strategy secondary to acute physiology after TBI. 68,115 In addition, the acute increase of intracellular [Ca 2+ ] i after TBI can also trigger the cellular mechanism leading to neuronal dysfunction and death. Administration of the N-type voltage-gated calcium channel (VGCCs) blocker SNX-185 before or immediately after cell injury reduced glutamate release and increased neuronal and as- the risk of epilepsy after sTBI, which may serve as potential drug targets for PTS. 120,121 Glutamate is converted into GABA through two types of glutamate decarboxylase, GAD67 and GAD65, encoded by the GAD1 and GAD2 genes, respectively. In the GAD1 gene, the tagging SNP (tSNP) rs3828275 was associated with an increased risk of PTS within 1 week of TBI. In comparison, tSNP rs769391 and functional SNP rs3791878 in the GAD1 gene were associated with an increased risk of PTS from 1 week to 6 months post-TBI. Both risk variants conferred increased susceptibility to PTS compared to subjects with 0-1 risk variants. 122 Therefore, the discovery and early identification of relevant genetic biomarkers are of great significance for preventing and treating PTE.

| PHARMACOLOG IC AL S TUDY ON PRE VENTING P TE BY RE S TORING G LUTAMATE HOMEOS TA S IS
A series of preclinical studies have explored the preventive and protective effects of drugs on the occurrence and development of epilepsy after TBI based on the mechanism of regulating glutamate homeostasis ( Table 2).
It has long been known that β-lactam antibiotics are an effective stimulator of EAAT2 expression, which may be mediated by increasing the transcription of the EAAT2 gene. Ceftriaxone can increase the expression and biochemical and functional activity of EAAT2 in animal brains and play a neuroprotective role in animal disease models based on glutamate toxicity, such as ischemic brain injury, motor neuron degeneration, amyotrophic lateral sclerosis (ALS). 123 Based on this mechanism, the role of ceftriaxone in post-TBI has also