Neuroimmunophilin GPI-1046 reduces ethanol consumption in part through activation of GLT1 in alcohol-preferring rats

Effects of MC-100093 on Ethanol Drinking and the Expression of Astrocytic Glutamate Transporters in the Mesocorticolimbic Brain Regions of Male and Female Alcohol-Preferring Rats

Chronic ethanol consumption increased extracellular glutamate concentrations in several reward brain regions. Glutamate homeostasis is regulated in majority by astrocytic glutamate transporter 1 (GLT-1) as well as the interactive role of cystine/glutamate antiporter (xCT). In this study, we aimed to determine the attenuating effects of a novel beta-lactam MC-100093, lacking the antibacterial properties, on ethanol consumption and GLT-1 and xCT expression in the subregions of nucleus accumbens (NAc core and NAc shell) and medial prefrontal cortex (Infralimbic, mPFC-IL and Prelimbic, mPFC-PL) in male and female alcohol-preferring (P) rats. Female and male rats were exposed to free access to ethanol (15% v/v) and (30% v/v) and water for five weeks, and on Week 6, rats were administered 100 mg/kg (i.p) of MC-100093 or saline for five days. MC-100093 reduced ethanol consumption in both male and female P rats from Day 1-5. Additionally, MC-100093 upregulated GLT-1 and xCT expression in the mPFC and NAc subregions as compared to ethanol-saline groups in female and male rats. Chronic ethanol intake reduced GLT-1 and xCT expression in the IL and PL in female and male rats, except there was no reduction in GLT-1 expression in the mPFC-PL in female rats. Although, MC-100093 upregulated GLT-1 and xCT expression in the subregions of NAc, we did not observe any reduction in GLT-1 and xCT expression with chronic ethanol intake in female rats. These findings strongly suggest that MC-100093 treatment effectively reduced ethanol intake and upregulated GLT-1 and xCT expression in the mPFC and NAc subregions in male and female P rats.

Glutamate-Mediated Excitotoxicity in the Pathogenesis and Treatment of Neurodevelopmental and Adult Mental Disorders

Glutamate is the main excitatory neurotransmitter in the brain wherein it controls cognitive functional domains and mood. Indeed, brain areas involved in memory formation and consolidation as well as in fear and emotional processing, such as the hippocampus, prefrontal cortex, and amygdala, are predominantly glutamatergic. To ensure the physiological activity of the brain, glutamatergic transmission is finely tuned at synaptic sites. Disruption of the mechanisms responsible for glutamate homeostasis may result in the accumulation of excessive glutamate levels, which in turn leads to increased calcium levels, mitochondrial abnormalities, oxidative stress, and eventually cell atrophy and death. This condition is known as glutamate-induced excitotoxicity and is considered as a pathogenic mechanism in several diseases of the central nervous system, including neurodevelopmental, substance abuse, and psychiatric disorders. On the other hand, these disorders share neuroplasticity impairments in glutamatergic brain areas, which are accompanied by structural remodeling of glutamatergic neurons. In the current narrative review, we will summarize the role of glutamate-induced excitotoxicity in both the pathophysiology and therapeutic interventions of neurodevelopmental and adult mental diseases with a focus on autism spectrum disorders, substance abuse, and psychiatric disorders. Indeed, glutamatergic drugs are under preclinical and clinical development for the treatment of different mental diseases that share glutamatergic neuroplasticity dysfunctions. Although clinical evidence is still limited and more studies are required, the regulation of glutamate homeostasis is attracting attention as a potential crucial target for the control of brain diseases.

Effects of novel GLT-1 modulator, MC-100093, on neuroinflammatory and neurotrophic biomarkers in mesocorticolimbic brain regions of male alcohol preferring rats exposed chronically to ethanol

Chronic ethanol consumption can lead to increased extracellular glutamate concentrations in key reward brain regions, such as medial prefrontal cortex (mPFC) and nucleus accumbens (NAc), and consequently leading to oxidative stress and neuroinflammation. Previous studies from our lab tested β-lactam antibiotics and novel beta-lactam non-antibiotic, MC-100093, and showed these β-lactam upregulated the major astrocytic glutamate transporter, GLT-1, and consequently reduced ethanol intake and normalized glutamate homeostasis. This present study tested the effects of novel synthetic β-lactam non-antibiotic drug, MC-100093, in chronic ethanol intake and neuroinflammatory and trophic factors in subregions of the NAc (NAc core and shell) and mPFC (Prelimbic, PL; and Infralimbic, IL) of male P rats. MC-100093 treatment reduced ethanol intake after 5-week drinking regimen. Importantly, MC-100093 attenuated ethanol-induced downregulation of brain derived neurotrophic factor (BDNF) expression in these brain regions. In addition, MC-100093 attenuated ethanol-induced upregulation of pro-inflammatory cytokines such as TNF-a and HMGB1 in all these brain regions. Furthermore, MC-100093 treatment attenuated ethanol-induced increase in RAGE in these brain regions. MC-100093 prevented neuroinflammation caused by ethanol intake as well as increased neurotrophic factor in mesocorticolimbic brain regions. MC-100093 treatment reduced ethanol intake and this behavioral effect was associated with attenuation of reduced trophic factors and increased pro-inflammatory factors. MC-100093 is considered a small molecule that may have potential therapeutic effects for the treatment of the effects of chronic exposure to ethanol.

β_lactam antibiotics against drug addiction: A novel therapeutic option

Drug addiction as a problem for the health of the individual and the society is the result of a complex process in which there is an interaction between brain nuclei and neurotransmitters (such as glutamate). β-lactam antibiotics, due to their enhancing properties on the glutamate transporter glutamate transporter-1, can affect and counteract the addictive mechanisms of drugs through the regulation of extracellular glutamate. Since glutamate is a key neurotransmitter in the development of drug addiction, it seems that β-lactams can be considered as a promising treatment for addiction. However, more research in this field is necessary to identify other mechanisms involved in their effectiveness. This article is a review of the studies conducted on the effect of β-lactam administration in preventing the development of drug addiction, as well as their possible cellular and molecular mechanisms. This review suggests the clinical use of β-lactam antibiotics that have weak antimicrobial properties (such as clavulanic acid) in the treatment of drug dependence.

Effects of a Novel Beta Lactam Compound, MC-100093, on the Expression of Glutamate Transporters/Receptors and Ethanol Drinking Behavior of Alcohol-Preferring Rats

Chronic ethanol exposure affects the glutamatergic system in several brain reward regions including the nucleus accumbens (NAc). Our laboratory has shown that chronic exposure to ethanol reduced the expression of glutamate transporter 1 (GLT-1) and cystine/glutamate exchanger (xCT) and, as a result, increased extracellular glutamate concentrations in the NAc of alcohol-preferring (P) rats. Moreover, previous studies from our laboratory reported that chronic ethanol intake altered the expression of certain metabotropic glutamate receptors in the brain. In addition to central effects, chronic ethanol consumption induced liver injury, which is associated with steatohepatitis. In the present study, we investigated the effects of chronic ethanol consumption in the brain and liver. Male P rats had access to a free choice of ethanol and water bottles for five weeks. Chronic ethanol consumption reduced GLT-1 and xCT expression in the NAc shell but not in the NAc core. Furthermore, chronic ethanol consumption increased fat droplet content as well as peroxisome proliferator-activated receptor alpha (PPAR-α) and GLT-1 expression in the liver. Importantly, treatment with the novel beta-lactam compound, MC-100093, reduced ethanol drinking behavior and normalized the levels of GLT-1 and xCT expression in the NAc shell as well as normalized GLT-1 and PPAR-α expression in the liver. In addition, MC-100093 attenuated ethanol-induced increases in fat droplet content in the liver. These findings suggest that MC-100093 may be a potential lead compound to attenuate ethanol-induced dysfunction in the glutamatergic system and liver injury. SIGNIFICANCE STATEMENT: This study identified a novel beta-lactam, MC-100093, that has demonstrated upregulatory effects on GLT-1. MC-100093 reduced ethanol drinking behavior and normalized levels of GLT-1 and xCT expression in the NAc shell as well as normalized GLT-1 and PPAR-α expression in the liver. In addition, MC-100093 attenuated ethanol-induced increases in fat droplet content in the liver.

The GLT-1 enhancer clavulanic acid suppresses cocaine place preference behavior and reduces GCPII activity and protein levels in the rat nucleus accumbens

The β-lactam antibiotic ceftriaxone (CTX) is a glutamate transporter subtype 1 (GLT-1) enhancer that reduces cocaine reinforcing efficacy and relapse in rats, but pharmacokinetic liabilities limit translational utility. An attractive alternative is clavulanic acid (CLAV), a structurally related β-lactamase inhibitor and component of FDA-approved Augmentin. CLAV retains the GLT-1 enhancing effects of CTX but displays greater oral bioavailability, brain penetrability and negligible antibacterial activity. CLAV reduces morphine conditioned place preference (CPP) and ethanol consumption in rats, but knowledge about the efficacy of CLAV in preclinical models of drug addiction remains sparse. Here, we investigated effects of CLAV (10 mg/kg, IP) on the acquisition, expression, and maintenance of cocaine CPP in rats, and on two glutamate biomarkers associated with cocaine dependence, GLT-1 and glutamate carboxypeptidase II (GCPII). CLAV administered during cocaine conditioning (10 mg/kg, IP x 4 d) did not affect the development of cocaine CPP. However, a single CLAV injection, administered after the conditioning phase, reduced the expression of cocaine CPP. In rats with established cocaine preference, repeated CLAV administration facilitated extinction of cocaine CPP. In the nucleus accumbens, acute CLAV exposure reduced GCPII protein levels and activity, and a 10-d CLAV treatment regimen enhanced GLT-1 levels. These results suggest that CLAV reduces expression and maintenance of cocaine CPP but lacks effect against development of CPP. Moreover, the ability of a single injection of CLAV to reduce both GCPII activity and protein levels, as well as expression of cocaine CPP, points toward studying GCPII as a therapeutic target of CLAV.

Amino Acid Solute Carrier Transporters in Inflammation and Autoimmunity

The past decade exposed the importance of many homeostasis and metabolism related proteins in autoimmunity disease and inflammation. Solute carriers (SLCs) are a group of membrane channels that can transport amino acids, the building blocks of proteins, nutrients, and neurotransmitters. This review summarizes the role of SLCs amino acid transporters in inflammation and autoimmunity disease. In detail, the importance of Glutamate transporters SLC1A1, SLC1A2, and SLC1A3, mainly expressed in the brain where they help prevent glutamate excitotoxicity, is discussed in the context of central nervous system disorders such as multiple sclerosis. Similarly, the cationic amino acid transporter SLC7A1 (CAT1), which is an important arginine transporter for T cells, and SLC7A2 (CAT2), essential for innate immunity. SLC3 family proteins, which bind with light chains from the SLC7 family (SLC7A5, SLC7A7 and SLC7A11) to form heteromeric amino acid transporters, are also explored to describe their roles in T cells, NK cells, macrophages and tumor immunotherapies. Altogether, the link between SLC amino acid transporters with inflammation and autoimmunity may contribute to a better understanding of underlying mechanism of disease and provide novel potential therapeutic avenues. Significance Statement SIGNIFICANCE STATEMENT In this review, we summarize the link between SLC amino acid transporters and inflammation and immune responses, specially SLC1 family members and SLC7 members. Studying the link may contribute to a better understanding of related diseases and provide potential therapeutic targets and useful to the researchers who have interest in the involvement of amino acids in immunity.

Substance abuse and neurotransmission

The number of people who suffer from a substance abuse disorder has continued to rise over the last decade; particularly, the number of drug-related overdose deaths has sharply increased during the COVID-19 pandemic. Converging lines of clinical observations, supported by imaging and neuropsychological performance testing, have demonstrated that substance abuse-induced dysregulation of neurotransmissions in the brain is critical for development and expression of the addictive properties of abused substances. Recent scientific advances have allowed for better understanding of the neurobiological processes that mediates drugs of abuse and addiction. This chapter presents the past classic concepts and the recent advances in our knowledge about how cocaine, amphetamines, opioids, alcohol, and nicotine alter multiple neurotransmitter systems, which contribute to the behaviors associated with each drug. Additionally, we discuss the interactive effects of HIV-1 or COVID-19 and substance abuse on neurotransmission and neurobiological pathways. Finally, we introduce therapeutic strategies for development of pharmacotherapies for substance abuse disorders.

Knowledge atlas of the involvement of glutamate and GABA in alcohol use disorder: A bibliometric and scientometric analysis

INTRODUCTION

Abnormal neurotransmission of glutamate and γ-aminobutyric acid (GABA) is a key characteristic of alcohol-related disorders. To track research output, we conducted a bibliometric analysis to explore the current status and trends in this field over the past decades.

METHODS

Studies related to neurotransmitters and alcohol use disorder published in English from 2005-2021 were retrieved from the Web of Science Core Collection and Scopus databases. The R-bibliometrix package was used for a descriptive analysis of the publications. Citespace, WOSviewer, and R-bibliometrix were used to construct networks of countries/institutions/authors based on co-authorship, co-citation analysis of cited references and co-occurrence as well as burst detection of keywords.

RESULTS

A total of 4,250 unique articles and reviews were included in the final analysis. The annual growth rate of publications was 5.4%. The USA was the most productive country in this field, contributing nearly half of the total documents. The top ten most productive institutions were all located in the USA. The most frequent worldwide collaboration was between the USA and Italy. The most productive and influential institution was the University of California. The author contributing the most productions to this field was Marisa Roberto from the Scripps Research Institute. The top co-cited reference was a review titled "Neurocircuitry of addiction." The top journal in terms of the number of records and citations was Alcoholism: Clinical and Experimental Research. Comprehensive analyses have been conducted over past decades based on co-cited reference analysis, including modulators, transporters, receptor subtypes, and animal models. In recent years, the research frontiers have been shifting to the identification of risk factors/biomarkers, drug development for alcohol use disorder, and mechanisms related to alcoholic and non-alcoholic fatty liver.

CONCLUSION

Our bibliometric analysis shows that glutamate and GABA continue to be of interest in alcohol use disorder. The focus has evolved from mechanisms and medications related to glutamate and GABA in alcohol use disorder, to novel drug development, risk factor/biomarker identification targeting neurotransmitters, and the mechanisms of related diseases.

Extrasynaptic therapeutic targets in substance use and stress disorders

Treatments for substance use and stress disorders are based on ameliorating behavioral symptoms, not on reversing the synaptic pathology that has the potential to cure disorders. This failing arises in part from a research focus on how pre- and postsynaptic physiology is changed even though key neuropathology exists in the perisynaptic neuropil that homeostatically regulates synaptic transmission. We explore recent findings from the substance use and stress disorder literature pointing to a key role for perisynaptic astroglia and signaling in the extracellular matrix (ECM) in regulating synaptic pathology. We conclude that drugs and stress initiate long-lasting changes in brain synapses via enduring neuroadaptations in astroglia and the ECM, and that modulating extrasynaptic regulators may be therapeutically useful.

Behavioral consequences of the downstream products of ethanol metabolism involved in alcohol use disorder

Research concerning Alcohol Use Disorder (AUD) has previously focused primarily on either the behavioral or chemical consequences experienced following ethanol intake, but these areas of research have rarely been considered in tandem. Compared with other drugs of abuse, ethanol has been shown to have a unique metabolic pathway once it enters the body, which leads to the formation of downstream metabolites which can go on to form biologically active products. These metabolites can mediate a variety of behavioral responses that are commonly observed with AUD, such as ethanol intake, reinforcement, and vulnerability to relapse. The following review considers the preclinical and chemical research implicating these downstream products in AUD and proposes a chemobehavioral model of AUD.

Effect of Modulation of the Astrocytic Glutamate Transporters' Expression on Cocaine-Induced Reinstatement in Male P Rats Exposed to Ethanol

AIM

Reinforcing properties of ethanol and cocaine are mediated in part through the glutamatergic system. Extracellular glutamate concentration is strictly maintained through several glutamate transporters, such as glutamate transporter 1 (GLT-1), cystine/glutamate transporter (xCT) and glutamate aspartate transporter (GLAST). Previous findings revealed that cocaine and ethanol exposure downregulated GLT-1 and xCT, and that β-lactam antibiotics restored their expression.

METHODS

In this study, we investigated the effect of ampicillin/sulbactam (AMP/SUL) (200 mg/kg, i.p.), a β-lactam antibiotic, on cocaine-induced reinstatement and locomotor activity in male alcohol preferring (P) rats using free choice ethanol (15 and 30%, v/v) and water. We also investigated the effect of co-exposure to ethanol and cocaine (20 mg/kg, i.p.) on GLT-1, xCT and GLAST expression in the nucleus accumbens (NAc) core, NAc shell and dorsomedial prefrontal cortex (dmPFC).

RESULTS

Cocaine exposure decreased ethanol intake and preference. Cocaine and ethanol co-exposure acquired place preference and increased locomotor activity compared to ethanol-exposed rats. GLT-1 and xCT expression were downregulated after cocaine and ethanol co-exposure in the NAc core and shell, but not in dmPFC. AMP/SUL attenuated reinstatement to cocaine as well attenuated the decrease in locomotor activity and ethanol intake and preference. These effects were associated with upregulation of GLT-1 and xCT expression in the NAc core/shell and dmPFC. GLAST expression was not affected after ethanol and cocaine co-exposure or AMP/SUL treatment.

CONCLUSION

Our findings demonstrate that astrocytic glutamate transporters within the mesocorticolimbic area are critical targets in modulating cocaine-seeking behavior while being consuming ethanol.

Effects of chronic ethanol consumption on the expression of GLT-1 and neuroplasticity-related proteins in the nucleus accumbens of alcohol-preferring rats

Chronic ethanol exposure induces impairments in CNS excitatory and inhibitory activity. These impairments are associated with glutamatergic dysfunction, including altered neuroplasticity. This study examined the effects of 6-week ethanol (15% and 30% v/v) consumption, by male alcohol-preferring P rats, on protein expression associated with neuroplasticity and glutamate transporter-1 (GLT-1) function. The latter regulates intra- and extra-synaptic glutamate levels. We focused on the shell and core subregions of the nucleus accumbens (Acb); i.e., shell (AcbSh) and core (AcbCo), for these measures. Chronic ethanol exposure increased the expression of BDNF, Arc and phosphorylated (p)-post-synaptic density protein-95 (p-PSD-95) in the AcbSh of P rats. Moreover, the ratio of phospho-neuronal nitric oxide synthase (p-nNOS) to total nNOS was also increased in the AcbSh. These changes in BDNF, Arc and p-nNOS/nNOS ratio were not observed in the AcbCo. Furthermore, chronic ethanol consumption reduced GLT-1 expression in the AcbSh. Alternatively, treatment with ceftriaxone (CEF), a known GLT-1 upregulator, abolished the effect of chronic ethanol consumption on BDNF expression in the AcbSh. Overall, the present findings confirm that chronic ethanol consumption modulates activity-associated synaptic proteins, including BDNF, Arc and nNOS in a subregion-specific (i.e., in the AcbSh but not AcbCo) manner. Thus, alterations in mesocorticolimbic glutamatergic homeostasis and neuroplasticity are possible functional targets for the treatment of alcohol use disorders.

Ethanol-Induced Changes in Brain of Transgenic Mice Overexpressing DYRK1A

Alcoholism is a chronic relapsing disorder defined by loss of control over excessive consumption of ethanol despite damaging effects on the liver and brain. We previously showed that the overexpression in mice of Dyrk1A (TgDyrk1A, for dual-specificity tyrosine (Y) phosphorylation-regulated kinase 1A) reduces the severity of alcohol mediated liver injury. Ethanol consumption has also been associated with increased brain glutamate concentration that led to therapies targeting glutamatergic receptors and normalization of glutamatergic neurotransmission. Interestingly, mice overexpressing Dyrk1A (TgDyrk1A mice) present a reduction of glutamatergic brain transmission, which we propose could be protective against alcohol intake. To answer this question, we investigated the ethanol preference in TgDyrk1A mice using a two-bottle choice paradigm. TgDyrk1A mice showed a non-significant decrease of voluntary ethanol intake and ethanol preference compared with wild-type mice. At the peripheral level, mice overexpressing Dyrk1A show lower ethanol plasma levels, indicating a faster ethanol metabolism. At the end of the protocol, lasting 21 days, brains were extracted for protein analysis. Ethanol reduced levels of the synaptic protein PSD-95 and increased the glutamate decarboxylase GAD65, specifically in the cortex of TgDyrk1A mice. Our results suggest that overexpression of DYRK1A may cause different ethanol-induced changes in the brain.

Effects of Cocaine Exposure on Astrocytic Glutamate Transporters and Relapse-Like Ethanol-Drinking Behavior in Male Alcohol-Preferring Rats

AIM

Glutamate has been considered as neurotransmitter that is critical in triggering relapse to drugs of abuse, including ethanol and cocaine. Extracellular glutamate concentrations are tightly regulated by several mechanisms, including reuptake through glutamate transporters. Glutamate transporter type 1 (GLT-1) is responsible for clearing the majority of extracellular glutamate. The astrocytic cystine/glutamate antiporter (xCT) regulates also glutamate homeostasis. In this study, we investigated the effects of cocaine exposure and ampicillin/sulbactam (AMP/SUL), a β-lactam antibiotic known to upregulate GLT-1 and xCT, on relapse-like ethanol intake and the expression of astrocytic glutamate transporters in mesocorticolimbic brain regions.

METHODS

Male alcohol-preferring (P) rats had free access to ethanol for 5 weeks. On Week 6, rats were exposed to either cocaine (20 mg/kg, i.p.) or saline for 12 consecutive days. Ethanol bottles were then removed for 7 days; during the last 5 days, either AMP/SUL (100 or 200 mg/kg, i.p.) or saline was administered to the P rats. Ethanol bottles were reintroduced, and ethanol intake was measured for 4 days.

RESULTS

Cocaine exposure induced an alcohol deprivation effect (ADE), which was associated in part by a decrease in the expression of GLT-1 and xCT in the nucleus accumbens (NAc) core. AMP/SUL (100 mg/kg, i.p.) attenuated the ADE, while AMP/SUL (200 mg/kg, i.p.) reduced ethanol intake during 4 days of ethanol re-exposure and upregulated GLT-1 and xCT expression in the NAc core, NAc shell and dorsomedial prefrontal cortex (dmPFC).

CONCLUSION

This study suggests that these astrocytic glutamate transporters might be considered as potential targets for the treatment of polysubstance abuse.

The neurobiology of addiction

Substance and alcohol use disorders impose large health and economic burdens on individuals, families, communities, and society. Neither prevention nor treatment efforts are effective in all individuals. Results are often modest. Advances in neuroscience and addiction research have helped to describe the neurobiological changes that occur when a person transitions from recreational substance use to a substance use disorder or addiction. Understanding both the drivers and consequences of substance use in vulnerable populations, including those whose brains are still maturing, has revealed behavioral and biological characteristics that can increase risks of addiction. These findings are particularly timely, as law- and policymakers are tasked to reverse the ongoing opioid epidemic, as more states legalize marijuana, as new products including electronic cigarettes and newly designed abused substances enter the legal and illegal markets, and as "deaths of despair" from alcohol and drug misuse continue.

Alcohol and Cocaine Exposure Modulates ABCB1 and ABCG2 Transporters in Male Alcohol-Preferring Rats

Two efflux transporters, ATP-binding cassettes B1 (ABCB1) and G2 (ABCG2), are highly expressed in the endothelial cells of the brain, where they regulate the bioavailability and distribution of several endogenous and xenobiotic compounds. However, whether ABCB1 or ABCG2 has any link with drug dependence, drug withdrawal effects, or the incidence of adverse effects in drug abuser is not known. In this study, we determined the effects of voluntary ethanol consumption following repeated exposure to cocaine or vehicle on the relative mRNA and protein expression of Abcg2/ABCG2 and Abcb1/ABCB1 in the nucleus accumbens (NAc) and medial prefrontal cortex (mPFC) of male alcohol-preferring (P) rats. Male P rats were allowed free choice access to ethanol (15 and 30% v/v) and water for 5 weeks to establish baseline drinking behavior. The following week, rats were either injected with 20 mg/kg i.p. of cocaine or saline, once a day, for 7 days. The relative mRNA and protein expression of Abcb1/ABCB1 and Abcg2/ABCG2 in the NAc and mPFC were significantly decreased in ethanol-saline- and ethanol-cocaine-exposed rats compared to control rats that received neither ethanol nor cocaine. Thus, prolonged exposure to commonly abused drugs, ethanol and cocaine, alters the expression of Abcb1/ABCB1 and Abcg2/ABCG2 mRNA and protein levels in brain areas that play a role in drug dependence.

Effects of Clavulanic Acid Treatment on Reinstatement to Methamphetamine, Glial Glutamate Transporters, and mGluR 2/3 Expression in P Rats Exposed to Ethanol

Evidence demonstrated that the glutamatergic system is implicated in mediating relapse to several drugs of abuse, including methamphetamine (METH). Glutamate homeostasis is maintained by a number of glutamate transporters, such as glutamate transporter type 1 (GLT-1), cystine/glutamate transporter (xCT), and glutamate aspartate transporter (GLAST). In addition, group II metabotropic glutamate receptors (mGluR2/3) were found to be implicated in relapse-seeking behavior. Ample evidence showed that β-lactam antibiotics are effective in upregulating GLT-1 and xCT expression, thus improving glutamate homeostasis and attenuating relapse to drugs of abuse. In this study, we investigated the reinstatement of METH using conditioned place preference (CPP) in male alcohol-preferring (P) rats exposed to home-cage free choice ethanol drinking. Here, we tested the effect of clavulanic acid (CA), a β-lactam, on the reinstatement of METH-seeking and ethanol drinking. In addition, we examined the expression of GLT-1, xCT, and GLAST as well as metabotropic glutamate receptor (mGluR2/3) in the nucleus accumbens (NAc) shell, NAc core, and dorsomedial prefrontal cortex (dmPFC). A priming i.p. injection of METH reinstated preference in METH-paired chamber following extinction. Chronic exposure to ethanol decreased the expression of GLT-1 and xCT in the NAc shell, but not in the NAc core or dmPFC. CA treatment blocked the reinstatement of METH-seeking, decreased ethanol intake, and restored the expression of GLT-1 and xCT in the NAc shell. In addition, the expression of mGluR2/3 was increased by CA treatment in the NAc shell and dmPFC. These findings suggest that these glutamate transporters and mGluR2/3 might be potential therapeutic targets for the attenuation of reinstatement to METH-seeking.

Role of glutamatergic system and mesocorticolimbic circuits in alcohol dependence

Emerging evidence demonstrates that alcohol dependence is associated with dysregulation of several neurotransmitters. Alterations in dopamine, glutamate and gamma-aminobutyric acid release are linked to chronic alcohol exposure. The effects of alcohol on the glutamatergic system in the mesocorticolimbic areas have been investigated extensively. Several studies have demonstrated dysregulation in the glutamatergic systems in animal models exposed to alcohol. Alcohol exposure can lead to an increase in extracellular glutamate concentrations in mesocorticolimbic brain regions. In addition, alcohol exposure affects the expression and functions of several glutamate receptors and glutamate transporters in these brain regions. In this review, we discussed the effects of alcohol exposure on glutamate receptors, glutamate transporters and glutamate homeostasis in each area of the mesocorticolimbic system. In addition, we discussed the genetic aspect of alcohol associated with glutamate and reward circuitry. We also discussed the potential therapeutic role of glutamate receptors and glutamate transporters in each brain region for the treatment of alcohol dependence. Finally, we provided some limitations on targeting the glutamatergic system for potential therapeutic options for the treatment alcohol use disorders.

Involvement of extrasynaptic glutamate in physiological and pathophysiological changes of neuronal excitability

Glutamate is the most abundant neurotransmitter of the central nervous system, as the majority of neurons use glutamate as neurotransmitter. It is also well known that this neurotransmitter is not restricted to synaptic clefts, but found in the extrasynaptic regions as ambient glutamate. Extrasynaptic glutamate originates from spillover of synaptic release, as well as from astrocytes and microglia. Its concentration is magnitudes lower than in the synaptic cleft, but receptors responding to it have higher affinity for it. Extrasynaptic glutamate receptors can be found in neuronal somatodendritic location, on astroglia, oligodendrocytes or microglia. Activation of them leads to changes of neuronal excitability with different amplitude and kinetics. Extrasynaptic glutamate is taken up by neurons and astrocytes mostly via EAAT transporters, and astrocytes, in turn metabolize it to glutamine. Extrasynaptic glutamate is involved in several physiological phenomena of the central nervous system. It regulates neuronal excitability and synaptic strength by involving astroglia; contributing to learning and memory formation, neurosecretory and neuromodulatory mechanisms, as well as sleep homeostasis.The extrasynaptic glutamatergic system is affected in several brain pathologies related to excitotoxicity, neurodegeneration or neuroinflammation. Being present in dementias, neurodegenerative and neuropsychiatric diseases or tumor invasion in a seemingly uniform way, the system possibly provides a common component of their pathogenesis. Although parts of the system are extensively discussed by several recent reviews, in this review I attempt to summarize physiological actions of the extrasynaptic glutamate on neuronal excitability and provide a brief insight to its pathology for basic understanding of the topic.

Effects of sequential ethanol exposure and repeated high-dose methamphetamine on striatal and hippocampal dopamine, serotonin and glutamate tissue content in Wistar rats

Alcohol (ethanol) and methamphetamine (METH) co-abuse is a major public health issue. Ethanol or METH exposure has been associated with changes in neurotransmitter levels in several central brain regions. However, little is known about the effect of sequential exposure to ethanol and METH on glutamate, dopamine and serotonin tissue content in striatum and hippocampus. In this study, we investigated the effects of sequential exposure to ethanol and METH on tissue content of these neurotransmitters. Male Wistar rats were orally gavaged with either ethanol (6g/kg) or water for seven days. Rats were administered with high dose of METH (10mg/kg, i.p. every 2h×4) or saline on Day 8 and euthanized 48h of last METH or saline i.p. injection. In the striatum, sequential exposure to ethanol and METH increased glutamate tissue content while reducing dopamine and serotonin tissue content as compared to the group exposed to ethanol alone. In the hippocampus, sequential exposure to ethanol and METH decreased serotonin tissue content as compared to the group that was exposed to ethanol alone. However, this study showed that ethanol has no additive effect to METH on tissue content of dopamine and serotonin as compared to METH in the striatum and hippocampus. This study demonstrated that sequential exposure of ethanol and METH has an additive effect on tissue content of certain neurotransmitters in the brain.

β-Lactamase inhibitor, clavulanic acid, attenuates ethanol intake and increases glial glutamate transporters expression in alcohol preferring rats

Studies from our laboratory showed that upregulation of glutamate transporter 1 (GLT-1) and cystine-glutamate exchanger (xCT) expression with ceftriaxone, β-lactam antibiotic, in the brain was associated with attenuation of ethanol consumption. In this study, we tested clavulanic acid, which is another β-lactam compound with negligible antimicrobial activity, on ethanol consumption and expression of GLT-1, xCT and glutamate aspartate transporter (GLAST) in male alcohol-preferring (P) rats. Clavulanic acid has the central β-lactam pharmacophore that is critical for the upregulation of GLT-1 and xCT expression. We found that clavulanic acid, at 5mg/kg (i.p.) dose, significantly attenuated ethanol consumption and ethanol preference in P rats as compared to vehicle-treated group. This effect was associated with a significant increase in water intake in clavulanic acid treated group. Importantly, we found that clavulanic acid increased the expression of GLT-1 and xCT in nucleus accumbens. However, there was no effect of clavulanic acid on GLAST expression in the nucleus accumbens. Clavulanic acid treatment did not upregulate the expression of GLT-1, xCT and GLAST in prefrontal cortex. These findings revealed that clavulanic acid at 20-40 fold lower dose than ceftriaxone can attenuate ethanol consumption, in part through upregulation of GLT-1 and xCT expression in the nucleus accumbens. Thus, we suggest that clavulanic acid might be used as an alternative option to ceftriaxone to attenuate ethanol drinking behavior.

Rat animal models for screening medications to treat alcohol use disorders

The purpose of this review is to present animal research models that can be used to screen and/or repurpose medications for the treatment of alcohol abuse and dependence. The focus will be on rats and in particular selectively bred rats. Brief introductions discuss various aspects of the clinical picture, which provide characteristics of individuals with alcohol use disorders (AUDs) to model in animals. Following this, multiple selectively bred rat lines will be described and evaluated in the context of animal models used to screen medications to treat AUDs. Next, common behavioral tests for drug efficacy will be discussed particularly as they relate to stages in the addiction cycle. Tables highlighting studies that have tested the effects of compounds using the respective techniques are included. Wherever possible the Tables are organized chronologically in ascending order to describe changes in the focus of research on AUDs over time. In general, high ethanol-consuming selectively bred rats have been used to test a wide range of compounds. Older studies usually followed neurobiological findings in the selected lines that supported an association with a propensity for high ethanol intake. Most of these tests evaluated the compound's effects on the maintenance of ethanol drinking. Very few compounds have been tested during ethanol-seeking and/or relapse and fewer still have assessed their effects during the acquisition of AUDs. Overall, while a substantial number of neurotransmitter and neuromodulatory system targets have been assessed; the roles of sex- and age-of-animal, as well as the acquisition of AUDs, ethanol-seeking and relapse continue to be factors and behaviors needing further study. This article is part of the Special Issue entitled "Alcoholism".

Ceftriaxone reduces alcohol intake in outbred rats while upregulating xCT in the nucleus accumbens core

Alcohol addiction is a chronic disease characterized by an inability to regulate drinking. A critical brain region involved in alcohol consumption is the nucleus accumbens (NA). Glutamate transmission in this region regulates alcohol consumption and relapse to alcohol-seeking. Across multiple alcohol-administration rodent models, basal extracellular glutamate levels are increased in the NA during early withdrawal. Glutamate transporter 1 (GLT-1) and system x_C-, containing the subunit xCT, regulate NA glutamate levels. Ceftriaxone (Cef) increases expression and function of both transporters following extinction from cocaine self-administration and here we sought to determine if Cef would similarly decrease alcohol consumption while increasing xCT and GLT-1 in the NA core. We used the intermittent access to alcohol (IAA) paradigm to induce drinking in outbred Sprague-Dawley rats; this paradigm permits rats access to alcohol (20%v/v) for 24-h without water deprivation, followed by 24-h of abstinence. Following 17 24-h drinking sessions, Cef treatment (200mg/kg IP) was initiated and continued for 5days while a control group received vehicle (0.9% saline IP). Alcohol consumption was assessed for two 24-h periods during Cef and two 24-h periods after cessation of Cef treatment. In a separate cohort of rats, Cef's ability to alter blood alcohol levels (BALs) after a non-contingent alcohol injection (1g/kg) was assessed. We found that Cef decreased alcohol consumption during the period of Cef treatment and on the two days following injections, and this was accompanied by an increase in NA core xCT expression. Furthermore, a history of alcohol consumption did not alter xCT and GLT-1 expression relative to alcohol-naïve controls. Cef did not alter BALs, indicating that the reduction in alcohol consumption was not caused by altered alcohol clearance. These results indicate that while Cef reduces alcohol consumption in outbred rats, its ability to do so is not associated with an increase in GLT-1 expression.

Modulatory effects of Ampicillin/Sulbactam on glial glutamate transporters and metabotropic glutamate receptor 1 as well as reinstatement to cocaine-seeking behavior

Glutamatergic system has an important role in cocaine-seeking behavior. Studies have reported that chronic exposure to cocaine induces downregulation of glutamate transporter-1 (GLT-1) and cystine/glutamate exchanger (xCT) in the central reward brain regions. Ceftriaxone, a β-lactam antibiotic, restored GLT-1 expression and consequently reduced cue-induced reinstatement of cocaine-seeking behavior. In this study, we investigated the reinstatement to cocaine (20mg/kg, i.p.) seeking behavior using a conditioned place preference (CPP) paradigm in male alcohol-preferring (P) rats. In addition, we investigated the effects of Ampicillin/Sulbactam (AMP/SUL) (200mg/kg, i.p.), a β-lactam antibiotic, on cocaine-induced reinstatement. We also investigated the effects of AMP/SUL on the expression of glial glutamate transporters and metabotropic glutamate receptor 1 (mGluR1) in the nucleus accumbens (NAc) core and shell and the dorsomedial prefrontal cortex (dmPFC). We found that AMP/SUL treatment reduced cocaine-triggered reinstatement. This effect was associated with a decrease in locomotor activity. Moreover, GLT-1 and xCT were downregulated in the NAc core and shell, but not in the dmPFC, following cocaine-primed reinstatement. However, cocaine exposure increased the expression of mGluR1 in the NAc core, but not in the NAc shell or dmPFC. Importantly, AMP/SUL treatment normalized GLT-1 and xCT expression in the NAc core and shell; however, the drug normalized mGluR1 expression in the NAc core only. Additionally, AMP/SUL increased the expression of GLT-1 and xCT in the dmPFC as compared to the water naïve group. These findings demonstrated that glial glutamate transporters and mGluR1 in the mesocorticolimbic area could be potential therapeutic targets for the attenuation of reinstatement to cocaine-seeking behavior.

Glutamate Transport: A New Bench to Bedside Mechanism for Treating Drug Abuse

Drug addiction has often been described as a "hijacking" of the brain circuits involved in learning and memory. Glutamate is the principal excitatory neurotransmitter in the brain, and its contribution to synaptic plasticity and learning processes is well established in animal models. Likewise, over the past 20 years the addiction field has ascribed a critical role for glutamatergic transmission in the development of addiction. Chronic drug use produces enduring neuroadaptations in corticostriatal projections that are believed to contribute to a maladaptive deficit in inhibitory control over behavior. Much of this research focuses on the role played by ionotropic glutamate receptors directly involved in long-term potentiation and depression or metabotropic receptors indirectly modulating synaptic plasticity. Importantly, the balance between glutamate release and clearance tightly regulates the patterned activation of these glutamate receptors, emphasizing an important role for glutamate transporters in maintaining extracellular glutamate levels. Five excitatory amino acid transporters participate in active glutamate reuptake. Recent evidence suggests that these glutamate transporters can be modulated by chronic drug use at a variety of levels. In this review, we synopsize the evidence and mechanisms associated with drug-induced dysregulation of glutamate transport. We then summarize the preclinical and clinical data suggesting that glutamate transporters offer an effective target for the treatment of drug addiction. In particular, we focus on the role that altered glutamate transporters have in causing drug cues and contexts to develop an intrusive quality that guides maladaptive drug seeking behaviors.

Effects of repeated cocaine exposure and withdrawal on voluntary ethanol drinking, and the expression of glial glutamate transporters in mesocorticolimbic system of P rats

Glutamatergic neurotransmission within the brain's reward circuits plays a major role in the reinforcing properties of both ethanol and cocaine. Glutamate homeostasis is regulated by several glutamate transporters, including glutamate transporter type 1 (GLT-1), cystine/glutamate transporter (xCT), and glutamate aspartate transporter (GLAST). Cocaine exposure has been shown to induce a dysregulation in glutamate homeostasis and a decrease in the expression of GLT-1 and xCT in the nucleus accumbens (NAc). In this study, alcohol preferring (P) rats were exposed to free-choice of ethanol (15% and 30%) and/or water for five weeks. On Week 6, rats were administered (i.p.) cocaine (10 and 20mg/kg) or saline for 12 consecutive days. This study tested two groups of rats: the first group was euthanized after seven days of repeated cocaine i.p. injection, and the second group was deprived from cocaine for five days and euthanized at Day 5 after cocaine withdrawal. Only repeated cocaine (20mg/kg, i.p.) exposure decreased ethanol intake from Day 3 through Day 8. Co-exposure of cocaine and ethanol decreased the relative mRNA expression and the expression of GLT-1 in the NAc but not in the medial prefrontal cortex (mPFC). Importantly, co-exposure of cocaine and ethanol decreased relative expression of xCT in the NAc but not in the mPFC. Our findings demonstrated that chronic cocaine exposure affects ethanol intake; and ethanol and cocaine co-abuse alters the expression of glial glutamate transporters.

The Nucleus Accumbens: Mechanisms of Addiction across Drug Classes Reflect the Importance of Glutamate Homeostasis

The nucleus accumbens is a major input structure of the basal ganglia and integrates information from cortical and limbic structures to mediate goal-directed behaviors. Chronic exposure to several classes of drugs of abuse disrupts plasticity in this region, allowing drug-associated cues to engender a pathologic motivation for drug seeking. A number of alterations in glutamatergic transmission occur within the nucleus accumbens after withdrawal from chronic drug exposure. These drug-induced neuroadaptations serve as the molecular basis for relapse vulnerability. In this review, we focus on the role that glutamate signal transduction in the nucleus accumbens plays in addiction-related behaviors. First, we explore the nucleus accumbens, including the cell types and neuronal populations present as well as afferent and efferent connections. Next we discuss rodent models of addiction and assess the viability of these models for testing candidate pharmacotherapies for the prevention of relapse. Then we provide a review of the literature describing how synaptic plasticity in the accumbens is altered after exposure to drugs of abuse and withdrawal and also how pharmacological manipulation of glutamate systems in the accumbens can inhibit drug seeking in the laboratory setting. Finally, we examine results from clinical trials in which pharmacotherapies designed to manipulate glutamate systems have been effective in treating relapse in human patients. Further elucidation of how drugs of abuse alter glutamatergic plasticity within the accumbens will be necessary for the development of new therapeutics for the treatment of addiction across all classes of addictive substances.

Metabotropic and ionotropic glutamate receptors as potential targets for the treatment of alcohol use disorder

Emerging evidence indicates that dysfunctional glutamate neurotransmission is critical in the initiation and development of alcohol and drug dependence. Alcohol consumption induced downregulation of glutamate transporter 1 (GLT-1) as reported in previous studies from our laboratory. Glutamate is the major excitatory neurotransmitter in the brain, which acts via interactions with several glutamate receptors. Alcohol consumption interferes with the glutamatergic signal transmission by altering the functions of these receptors. Among the glutamate receptors involved in alcohol-drinking behavior are the metabotropic receptors such as mGluR1/5, mGluR2/3, and mGluR7, as well as the ionotropic receptors, NMDA and AMPA. Preclinical studies using agonists and antagonists implicate these glutamatergic receptors in the development of alcohol use disorder (AUD). Therefore, the purpose of this review is to discuss the neurocircuitry involving glutamate transmission in animals exposed to alcohol and further outline the role of metabotropic and ionotropic receptors in the regulation of alcohol-drinking behavior. This review provides ample information about the potential therapeutic role of glutamatergic receptors for the treatment of AUD.

Effects of orally administered Augmentin on glutamate transporter 1, cystine-glutamate exchanger expression and ethanol intake in alcohol-preferring rats

Alcohol dependence is associated with deficits in glutamate uptake and impairment of glutamate homeostasis in different brain reward regions. Glutamate transporter subtype 1 (GLT-1), cystine-glutamate exchanger (xCT) and glutamate/aspartate transporter (GLAST) are one of the key players in regulating extracellular glutamate concentration in the brain. Parenteral treatment with ceftriaxone, β-lactam antibiotic, has been reported to attenuate ethanol consumption and reinstatement to cocaine-seeking behavior, in part, by restoring the expression of GLT-1 and xCT in mesocorticolimbic brain regions in rats. In this study, we focused to test Augmentin (amoxicillin/clavulanate), which can be administered orally to subjects. Therefore, we examined the effects of orally administered Augmentin on ethanol intake as well as GLT-1, xCT and GLAST expression in male alcohol-preferring (P) rats. We found that orally administered Augmentin significantly attenuated ethanol consumption in P rats as compared to the vehicle-treated group. Importantly, the attenuation in ethanol consumption was associated with a significant upregulation of GLT-1 and xCT expression in nucleus accumbens (NAc) and prefrontal cortex (PFC). There was no effect of orally administered Augmentin on GLAST expression in either NAc or PFC. These findings present strong evidence that oral administration of Augmentin can be used as an alternative to parenteral treatment.

Effects of Administered Ethanol and Methamphetamine on Glial Glutamate Transporters in Rat Striatum and Hippocampus

Exposure to ethanol (EtOH) or methamphetamine (MA) can lead to increase in extracellular glutamate concentration in the brain. Although studies from ours showed the effects of EtOH exposure on key glial glutamate transporters, little is known about the effects of sequential exposure to EtOH and MA or MA alone on certain glial glutamate transporters. In this study, we investigated the effects of sequential exposure to EtOH and MA on the expression of the major glutamate transporters, glutamate transporter 1 (GLT-1), as well as cystine/glutamate antiporter (xCT) and glutamate aspartate transporter (GLAST) in striatum and hippocampus. We also tested the effects of ceftriaxone (CEF), known to upregulate GLT-1, in animals administered EtOH and MA. Wistar rats were orally gavaged with EtOH (6 g/kg) or water for 7 days. On the following day (day 8), the rats received four intraperitoneal (i.p.) injections of MA (10 mg/kg) or saline (vehicle) occurring every 2 h. The rats were then treated with CEF (200 mg/kg/day, i.p.) or saline on days 8, 9, and 10. EtOH or MA exposure caused a significant downregulation of GLT-1 expression as compared to control groups in striatum and hippocampus. Furthermore, sequential exposure of EtOH and MA caused a significant downregulation of GLT-1 expression as compared to either drug administered alone in both brain regions. Importantly, GLT-1 expression was restored following CEF treatment. There were no significant differences on xCT and GLAST expression in striatum and hippocampus between all groups. These findings demonstrated that sequential exposure to EtOH and MA has additive effect in downregulation of GLT-1 and this effect can be attenuated by CEF treatment.

Effects of repeated high-dose methamphetamine and ceftriaxone post-treatments on tissue content of dopamine and serotonin as well as glutamate and glutamine

Repeated exposure to high doses of methamphetamine (METH) is known to alter several neurotransmitters in certain brain regions. Little is known about the effects of ceftriaxone (CEF), a β-lactam antibiotic, known to upregulate glutamate transporter subtype 1, post-treatment on METH-induced depletion of dopamine and serotonin (5-HT) tissue content in brain reward regions. Moreover, the effects of METH and CEF post-treatment on glutamate and glutamine tissue content are not well understood. In this study, Wistar rats were used to investigate the effects of METH and CEF post-treatment on tissue content of dopamine/5-HT and glutamate/glutamine in the nucleus accumbens (NAc) and prefrontal cortex (PFC). Rats received either saline or METH (10mg/kg, i.p. every 2h×4) followed by either saline or CEF (200mg/kg, i.p, every day×3) post-treatment. METH induced a significant depletion of dopamine and 5-HT in the NAc and PFC. Importantly, dopamine tissue content was completely restored in the NAc following CEF post-treatment. Additionally, METH caused a significant decrease in glutamate and glutamine tissue content in PFC, and this effect was attenuated by CEF post-treatment. These findings demonstrate for the first time the attenuating effects of CEF post-treatment on METH induced alterations in the tissue contents of dopamine, glutamate, and glutamine.

Effects of Ceftriaxone on Glial Glutamate Transporters in Wistar Rats Administered Sequential Ethanol and Methamphetamine

Methamphetamine (METH) is one of the psychostimulants that is co-abused with ethanol. Repeated exposure to high dose of METH has been shown to cause increases in extracellular glutamate concentration. We have recently reported that ethanol exposure can also increase the extracellular glutamate concentration and downregulate the expression of glutamate transporter subtype 1 (GLT-1). GLT-1 is a glial transporter that regulates the majority of extracellular glutamate. A Wistar rat model of METH and ethanol co-abuse was used to examine the expression of GLT-1 as well as other glutamate transporters such as cystine/glutamate exchanger (xCT) and glutamate aspartate transporter (GLAST). We also examined the body temperature in rats administered METH, ethanol or both drugs. We further investigated the effects of ceftriaxone (CEF), a β-lactam antibiotic known to upregulate GLT-1, in this METH/ethanol co-abuse rat model. After 7 days of either ethanol (6 g/kg) or water oral gavage, Wistar rats received either saline or METH (10 mg/kg i.p. every 2 h × 4), followed by either saline or CEF (200 mg/kg) posttreatment. METH administered alone decreased GLT-1 expression in the nucleus accumbens (NAc) and prefrontal cortex (PFC) and increased body temperature, but did not reduce either xCT or GLAST expression in ethanol and water-pretreated rats. Interestingly, ethanol and METH were found to have an additive effect on the downregulation of GLT-1 expression in the NAc but not in the PFC. Moreover, ethanol alone caused GLT-1 downregulation in the NAc and elevated body temperature compared to control. Finally, CEF posttreatment significantly reversed METH-induced hyperthermia, restored GLT-1 expression, and increased xCT expression. These findings suggest the potential therapeutic role of CEF against METH- or ethanol/METH-induced hyperglutamatergic state and hyperthermia.

Alcohol consumption increases basal extracellular glutamate in the nucleus accumbens core of Sprague-Dawley rats without increasing spontaneous glutamate release

Glutamate neurotransmission in the nucleus accumbens core (NAc) mediates ethanol consumption. Previous studies using non-contingent and voluntary alcohol administration in inbred rodents have reported increased basal extracellular glutamate levels in the NAc. Here, we assessed basal glutamate levels in the NAc following intermittent alcohol consumption in male Sprague-Dawley rats that had access to ethanol for 7 weeks on alternating days. We found increased basal NAc glutamate at 24 h withdrawal from ethanol and thus sought to identify the source of this glutamate. To do so, we employed a combination of microdialysis, slice electrophysiology and western blotting. Reverse dialysis of the voltage-gated sodium channel blocker tetrodotoxin did not affect glutamate levels in either group. Electrophysiological recordings in slices made after 24 h withdrawal revealed a decrease in spontaneous excitatory postsynaptic current (sEPSC) frequency relative to controls, with no change in sEPSC amplitude. No change in metabotropic glutamate receptor 2/3 (mGlu2/3) function was detected as bath application of the mGlu2/3 agonist LY379268 decreased spontaneous and miniature EPSC frequency in slices from both control and ethanol-consuming rats. The increase in basal glutamate was not associated with changes in the surface expression of GLT-1, however, a decrease in slope of the no-net-flux dialysis function was observed following ethanol consumption, indicating a potential decrease in glutamate reuptake. Taken together, these findings indicate that the increase in basal extracellular glutamate occurring after chronic ethanol consumption is not mediated by an increase in action potential-dependent glutamate release or a failure of mGlu2/3 autoreceptors to regulate such release.

Effects of Amoxicillin and Augmentin on Cystine-Glutamate Exchanger and Glutamate Transporter 1 Isoforms as well as Ethanol Intake in Alcohol-Preferring Rats

Alcohol dependence is associated with alteration of glutamate transport and glutamate neurotransmission. Glutamate transporter 1 (GLT-1) is a major transporter that regulates the majority of extracellular glutamate concentration, which is also regulated by cystine-glutamate exchanger (xCT). Importantly, we recently reported that amoxicillin and Augmentin (amoxicillin/clavulanate) upreglulated GLT-1 expression in nucleus accumbens (NAc) and prefrontal cortex (PFC) as well as reduced ethanol consumption in male P rats. In this study, we examined the effects of amoxicillin and Augmentin on GLT-1 isoforms (GLT-1a and GLT-1b), xCT, and glutamate/aspartate transporter (GLAST) expression in NAc and PFC as well as ethanol intake in male P rats. We found that both compounds significantly reduced ethanol intake, and increased GLT-1a, GLT-1b, and xCT expression in NAc. However, only Augmentin increased GLT-1a, GLT-1b, and xCT expression in PFC. There were no effects of these compounds on GLAST expression in NAc and PFC. These findings demonstrated that Augmentin and amoxicillin have the potential to upregulate GLT-1 isoforms and xCT expression, and consequently attenuate ethanol dependence.

Effects of ceftriaxone on ethanol, nicotine or sucrose intake by alcohol-preferring (P) rats and its association with GLT-1 expression

Increased glutamatergic neurotransmission appears to mediate the reinforcing properties of drugs of abuse, including ethanol (EtOH). We have shown that administration of ceftriaxone (CEF), a β-lactam antibiotic, reduced EtOH intake and increased glutamate transporter 1 (GLT-1) expression in mesocorticolimbic regions of male and female alcohol-preferring (P) rats. In the present study, we tested whether CEF administration would reduce nicotine (NIC) and/or EtOH intake by adult female P rats. P rats were randomly assigned to 4 groups: (a) 5% sucrose (SUC) or 10% SUC [SUC], (b) 5% SUC+0.07mg/ml NIC and 10% SUC+0.14mg/ml NIC [NIC-SUC], 15% EtOH and 30% EtOH [EtOH] and (d) 15% EtOH+0.07mg/ml NIC and 30% EtOH+0.14mg/ml NIC [NIC-EtOH]. After achieving stable intakes (4weeks), the rats were administered 7 consecutive, daily i.p. injections of either saline or 200mg/kg CEF. The effects of CEF on intake were significant but differed across the reinforcers; such that ml/kg/day SUC was reduced by ∼30%, mg/kg/day NIC was reduced by ∼70% in the NIC-SUC group and ∼40% in the EtOH-NIC group, whereas g/kg/day EtOH was reduced by ∼40% in both the EtOH and EtOH-NIC group. The effects of CEF on GLT-1 expression were also studied. We found that CEF significantly increased GLT-1 expression in the prefrontal cortex and the nucleus accumbens of the NIC and NIC-EtOH rats as compared to NIC and NIC-EtOH saline-treated rats. These findings provide further support for GLT-1-associated mechanisms in EtOH and/or NIC abuse. The present results along with previous reports of CEF's efficacy in reducing cocaine self-administration in rats suggest that modulation of GLT-1 expression and/or activity is an important pharmacological target for treating polysubstance abuse and dependence.

Glutamate Transporter GLT-1 as a Therapeutic Target for Substance Use Disorders

The development of new treatments for substance use disorders requires identification of targetable molecular mechanisms. Pathology in glutamatergic neurotransmission system in brain reward circuitry has been implicated in relapse to multiple classes of drugs. Glutamate transporter 1 (GLT-1) crucially regulates glutamatergic signaling by removing excess glutamate from the extrasynaptic space. The purpose of this review is to highlight the effects of addictive drug use on GLT-1 and glutamate uptake, and using GLT-1 as a target in addiction pharmacotherapy. Cocaine, opioids, ethanol, nicotine, amphetamines, and cannabinoids each affect GLT-1 expression and glutamate uptake, and restoring GLT-1 expression with N-acetylcysteine or ceftriaxone shows promise in correcting pre-clinical and clinical manifestations of drug addiction.

A Genetic Animal Model of Alcoholism for Screening Medications to Treat Addiction

The purpose of this review is to present up-to-date pharmacological, genetic, and behavioral findings from the alcohol-preferring P rat and summarize similar past work. Behaviorally, the focus will be on how the P rat meets criteria put forth for a valid animal model of alcoholism with a highlight on its use as an animal model of polysubstance abuse, including alcohol, nicotine, and psychostimulants. Pharmacologically and genetically, the focus will be on the neurotransmitter and neuropeptide systems that have received the most attention: cholinergic, dopaminergic, GABAergic, glutamatergic, serotonergic, noradrenergic, corticotrophin releasing hormone, opioid, and neuropeptide Y. Herein, we sought to place the P rat's behavioral and neurochemical phenotypes, and to some extent its genotype, in the context of the clinical literature. After reviewing the findings thus far, this chapter discusses future directions for expanding the use of this genetic animal model of alcoholism to identify molecular targets for treating drug addiction in general.

Binge ethanol withdrawal: Effects on post-withdrawal ethanol intake, glutamate-glutamine cycle and monoamine tissue content in P rat model

Alcohol withdrawal syndrome (AWS) is a medical emergency situation which appears after abrupt cessation of ethanol intake. Decreased GABA-A function and increased glutamate function are known to exist in the AWS. However, the involvement of glutamate transporters in the context of AWS requires further investigation. In this study, we used a model of ethanol withdrawal involving abrupt cessation of binge ethanol administration (4 g/kg/gavage three times a day for three days) using male alcohol-preferring (P) rats. After 48 h of withdrawal, P rats were re-exposed to voluntary ethanol intake. The amount of ethanol consumed was measured during post-withdrawal phase. In addition, the expression of GLT-1, GLAST and xCT were determined in both medial prefrontal cortex (mPFC) and nucleus accumbens (NAc). We also measured glutamine synthetase (GS) activity, and the tissue content of glutamate, glutamine, dopamine and serotonin in both mPFC and NAc. We found that binge ethanol withdrawal escalated post-withdrawal ethanol intake, which was associated with downregulation of GLT-1 expression in both mPFC and NAc. The expression of GLAST and xCT were unchanged in the ethanol-withdrawal (EW) group compared to control group. Tissue content of glutamate was significantly lower in both mPFC and NAc, whereas tissue content of glutamine was higher in mPFC but unchanged in NAc in the EW group compared to control group. The GS activity was unchanged in both mPFC and NAc. The tissue content of DA was significantly lower in both mPFC and NAc, whereas tissue content of serotonin was unchanged in both mPFC and NAc. These findings provide important information of the critical role of GLT-1 in context of AWS.

A New Outlook on Mental Illnesses: Glial Involvement Beyond the Glue

Mental illnesses have long been perceived as the exclusive consequence of abnormalities in neuronal functioning. Until recently, the role of glial cells in the pathophysiology of mental diseases has largely been overlooked. However recently, multiple lines of evidence suggest more diverse and significant functions of glia with behavior-altering effects. The newly ascribed roles of astrocytes, oligodendrocytes and microglia have led to their examination in brain pathology and mental illnesses. Indeed, abnormalities in glial function, structure and density have been observed in postmortem brain studies of subjects diagnosed with mental illnesses. In this review, we discuss the newly identified functions of glia and highlight the findings of glial abnormalities in psychiatric disorders. We discuss these preclinical and clinical findings implicating the involvement of glial cells in mental illnesses with the perspective that these cells may represent a new target for treatment.

Ethanol-Associated Changes in Glutamate Reward Neurocircuitry: A Minireview of Clinical and Preclinical Genetic Findings

Herein, we have reviewed the role of glutamate, the major excitatory neurotransmitter in the brain, in a number of neurochemical, -physiological, and -behavioral processes mediating the development of alcohol dependence. The findings discussed include results from both preclinical as well as neuroimaging and postmortem clinical studies. Expression levels for a number of glutamate-associated genes and/or proteins are modulated by alcohol abuse and dependence. These changes in expression include metabotropic receptors and ionotropic receptor subunits as well as different glutamate transporters. Moreover, these changes in gene expression parallel the pharmacologic manipulation of these same receptors and transporters. Some of these gene expression changes may have predated alcohol abuse and dependence because a number of glutamate-associated polymorphisms are related to a genetic predisposition to develop alcohol dependence. Other glutamate-associated polymorphisms are linked to age at the onset of alcohol-dependence and initial level of response/sensitivity to alcohol. Finally, findings of innate and/or ethanol-induced glutamate-associated gene expression differences/changes observed in a genetic animal model of alcoholism, the P rat, are summarized. Overall, the existing literature indicates that changes in glutamate receptors, transporters, enzymes, and scaffolding proteins are crucial for the development of alcohol dependence and there is a substantial genetic component to these effects. This indicates that continued research into the genetic underpinnings of these glutamate-associated effects will provide important novel molecular targets for treating alcohol abuse and dependence.

Glutamatergic transmission in drug reward: implications for drug addiction

Individuals addicted to drugs of abuse such as alcohol, nicotine, cocaine, and heroin are a significant burden on healthcare systems all over the world. The positive reinforcing (rewarding) effects of the above mentioned drugs play a major role in the initiation and maintenance of the drug-taking habit. Thus, understanding the neurochemical mechanisms underlying the reinforcing effects of drugs of abuse is critical to reducing the burden of drug addiction in society. Over the last two decades, there has been an increasing focus on the role of the excitatory neurotransmitter glutamate in drug addiction. In this review, pharmacological and genetic evidence supporting the role of glutamate in mediating the rewarding effects of the above described drugs of abuse will be discussed. Further, the review will discuss the role of glutamate transmission in two complex heterogeneous brain regions, namely the nucleus accumbens (NAcc) and the ventral tegmental area (VTA), which mediate the rewarding effects of drugs of abuse. In addition, several medications approved by the Food and Drug Administration that act by blocking glutamate transmission will be discussed in the context of drug reward. Finally, this review will discuss future studies needed to address currently unanswered gaps in knowledge, which will further elucidate the role of glutamate in the rewarding effects of drugs of abuse.

Amoxicillin and amoxicillin/clavulanate reduce ethanol intake and increase GLT-1 expression as well as AKT phosphorylation in mesocorticolimbic regions

Studies have shown that administration of the β-lactam antibiotic ceftriaxone (CEF) attenuates ethanol consumption and cocaine seeking behavior as well as prevents ethanol-induced downregulation of glutamate transporter 1 (GLT-1) expression in central reward brain regions. However, it is not known if these effects are compound-specific. Therefore, the present study examined the effects of two other β-lactam antibiotics, amoxicillin (AMOX) and amoxicillin/clavulanate (Augmentin, AUG), on ethanol drinking, as well as GLT-1 and phosphorylated-AKT (pAKT) levels in the nucleus accumbens (Acb) and medial prefrontal cortex (mPFC) of alcohol-preferring (P) rats. P rats were exposed to free-choice of ethanol (15% and 30%) for five weeks and were given five consecutive daily i.p. injections of saline vehicle, 100 mg/kg AMOX or 100mg/kg AUG. Both compounds significantly decreased ethanol intake and significantly increased GLT-1 expression in the Acb. AUG also increased GLT-1 expression in the mPFC. Results for changes in pAKT levels matched those for GLT-1, indicating that β-lactam antibiotic-induced reductions in ethanol intake are negatively associated with increases in GLT-1 and pAKT levels within two critical brains regions mediating drug reward and reinforcement. These findings add to a growing literature that pharmacological increases in GLT-1 expression are associated with decreases in ethanol intake and suggest that one mechanism mediating this effect may be increased phosphorylation of AKT. Thus, GLT-1 and pAKT may serve as molecular targets for the treatment of alcohol and drug abuse/dependence.

Effects of ceftriaxone on GLT1 isoforms, xCT and associated signaling pathways in P rats exposed to ethanol

RATIONALE

Several studies have demonstrated a correlation between extracellular glutamate concentration in the mesolimbic reward pathway and alcohol craving. Extracellular glutamate concentration is regulated by several glutamate transporters. Glial glutamate transporter 1 (GLT1) is one of them that regulates the majority of extracellular glutamate concentration. In addition, cystine/glutamate antiporter (xCT) is another transporter that regulates extracellular glutamate.

OBJECTIVES

We focus in this study to determine the effects of ceftriaxone, β-lactam antibiotic, on glial proteins such as GLT1 isoforms, xCT, glutamate aspartate transporter (GLAST), and several associated signaling pathways as well as ethanol intake in P rats. Additionally, to examine the onset of signaling pathways associated with GLT1 upregulation following ceftriaxone treatment, we tested 2- versus 5-day daily dosing of ceftriaxone.

RESULTS

Ceftriaxone treatment (100 mg/kg), 2 and 5 days, resulted in about five fold reduction in ethanol intake by P rats. The reduction in ethanol intake was associated with significantly enhanced expression of GLT1, GLT1a, GLT1b, and xCT in the nucleus accumbens (NAc) and prefrontal cortex (PFC) of 5-day ceftriaxone-treated P rats. Two-day-treated P rats showed marked changes in expression of these glutamate transporters in the PFC but not in the NAc. Importantly, ceftriaxone-treated P rats (2 and 5 days) demonstrated enhanced phosphorylation of Akt and nuclear translocation of nuclear factor kappaB (NFκB) in the NAc and PFC compared to control animals.

CONCLUSIONS

These findings demonstrate that ceftriaxone treatment induced upregulation of GLT1, GLT1 isoforms, and xCT in association with activation of the Akt-NFκB signaling pathway.

Ceftriaxone attenuates ethanol drinking and restores extracellular glutamate concentration through normalization of GLT-1 in nucleus accumbens of male alcohol-preferring rats

Alteration of glutamatergic-neurotransmission is a hallmark of alcohol dependence. We have previously reported that chronic ethanol-drinking downregulated glutamate transporter 1 (GLT-1) in nucleus accumbens (NAc) in male P rats in a manner that was reversed by ceftriaxone treatment. However, the effect of ceftriaxone on extracellular glutamate concentrations in NAc after chronic ethanol-drinking has not yet been studied. In the present study, male P rats were treated with ceftriaxone (100 mg/kg/day, i.p.) for five consecutive days following five-weeks of free choice ethanol (15% and 30%) drinking. In vivo microdialysis was performed to measure the extracellular glutamate concentrations in NAc and the effect of blockade of GLT-1 with dihydrokainic acid (DHK) on extracellular glutamate in NAc of ceftriaxone-treated rats was determined. Ceftriaxone treatment attenuated ethanol intake as well as ethanol preference. Extracellular glutamate was significantly higher in NAc after five-weeks of ethanol drinking in saline-treated compared to water control rats. Ceftriaxone treatment blocked the increase extracellular glutamate produced by ethanol intake. Blockade of GLT-1 by DHK reversed the effects of ceftriaxone on glutamate and implicated the role of GLT-1 in the normalization of extracellular glutamate by ceftriaxone. In addition, GLT-1 protein was decreased in ethanol exposed animals and ceftriaxone treatment reversed this deficit. Ceftriaxone treatment also increased glutamine synthetase activity in NAc but not in PFC as compared to ethanol drinking saline-treated rats. Our present study demonstrates that ceftriaxone treatment prevents ethanol drinking in part through normalization of extracellular glutamate concentrations in NAc of male P rats via GLT-1.

Targeting glutamate uptake to treat alcohol use disorders

Alcoholism is a serious public health concern that is characterized by the development of tolerance to alcohol's effects, increased consumption, loss of control over drinking and the development of physical dependence. This cycle is often times punctuated by periods of abstinence, craving and relapse. The development of tolerance and the expression of withdrawal effects, which manifest as dependence, have been to a great extent attributed to neuroadaptations within the mesocorticolimbic and extended amygdala systems. Alcohol affects various neurotransmitter systems in the brain including the adrenergic, cholinergic, dopaminergic, GABAergic, glutamatergic, peptidergic, and serotonergic systems. Due to the myriad of neurotransmitter and neuromodulator systems affected by alcohol, the efficacies of current pharmacotherapies targeting alcohol dependence are limited. Importantly, research findings of changes in glutamatergic neurotransmission induced by alcohol self- or experimenter-administration have resulted in a focus on therapies targeting glutamatergic receptors and normalization of glutamatergic neurotransmission. Glutamatergic receptors implicated in the effects of ethanol include the ionotropic glutamate receptors (AMPA, Kainate, and NMDA) and some metabotropic glutamate receptors. Regarding glutamatergic homeostasis, ceftriaxone, MS-153, and GPI-1046, which upregulate glutamate transporter 1 (GLT1) expression in mesocorticolimbic brain regions, reduce alcohol intake in genetic animal models of alcoholism. Given the hyperglutamatergic/hyperexcitable state of the central nervous system induced by chronic alcohol abuse and withdrawal, the evidence thus far indicates that a restoration of glutamatergic concentrations and activity within the mesocorticolimbic system and extended amygdala as well as multiple memory systems holds great promise for the treatment of alcohol dependence.