Clavulanic acid reduces rewarding, hyperthermic and locomotor-sensitizing effects of morphine in rats: a new indication for an old drug?

Effects of Beta Lactams on Behavioral Outcomes of Substance Use Disorders: A Meta-Analysis of Preclinical Studies

INTRODUCTION

Preclinical studies demonstrated that beta-lactams have neuroprotective effects in conditions involving glutamate neuroexcitotoxicity, including substance use disorders (SUDs). This meta-analysis aims to analyze the existing evidences on the effects of beta-lactams as glutamate transporter 1 (GLT-1) upregulators in animal models of SUDs, identification of gaps in the literature, and setting the stage for potential translation into clinical phases.

METHODS

Meta-analysis was conducted on preclinical studies retrieved systematically from MEDLINE and ScienceDirect databases. Abused substances were identified by refereeing to the National Institute on Drug Abuse (NIDA). The results were quantitatively described with a focus on the behavioral outcomes. Treatment effect sizes were described using standardized mean difference, and they were pooled using random effect model. I2-statistic was used to assess heterogeneity, and Funnel plot and Egger's test were used for assessment of publication bias.

RESULTS

Literature search yielded a total of 71 studies that were eligible to be included in the analysis. Through these studies, the effects of beta-lactams were evaluated in animal models of nicotine, cannabis, amphetamines, synthetic cathinone, opioids, ethanol, and cocaine use disorders as well as steroids-related aggressive behaviors. Meta-analysis showed that treatments with beta-lactams consistently reduced the pooled undesired effects of the abused substances in several paradigms, including drug-self administration, conditioned place preference, drug seeking behaviors, hyperlocomotion, withdrawal syndromes, tolerance to analgesic effects, hyperalgesia, and hyperthermia.

CONCLUSION

This meta-analysis revealed that enhancing GLT-1 expression in the brain through beta-lactams seemed to be a promising treatment approach in the context of substance use disorders, as indicated by results in animal models.

Clavulanic Acid and its Potential Therapeutic Effects on the Central Nervous System

Clavulanic acid (CLAV) is a non-antibiotic β-lactam that has been used since the late 1970s as a β-lactamase inhibitor in combination with amoxicillin, another ß-lactam with antibiotic activity. Its long-observed adverse reaction profile allows it to say that CLAV is a well-tolerated drug with mainly mild adverse reactions. Interestingly, in 2005, it was discovered that β-lactams enhance the astrocytic expression of GLT-1, a glutamate transporter essential for maintaining synaptic glutamate homeostasis involved in several pathologies of the central nervous system (CNS). This finding, along with a favorable pharmacokinetic profile, prompted the appearance of several studies that intended to evaluate the effect of CLAV in preclinical disease models. Studies have revealed that CLAV can increase GLT-1 expression in the nucleus accumbens (NAcc), medial prefrontal cortex (PFC), and spinal cord of rodents, to affect glutamate and dopaminergic neurotransmission, and exert an anti-inflammatory effect by modulating the levels of the cytokines TNF-α and interleukin 10 (IL-10). CLAV has been tested with positive results in preclinical models of epilepsy, addiction, stroke, neuropathic and inflammatory pain, dementia, Parkinson's disease, and sexual and anxiety behavior. These properties make CLAV a potential therapeutic drug if repurposed. Therefore, this review aims to gather information on CLAV's effect on preclinical neurological disease models and to give some perspectives on its potential therapeutic use in some diseases of the CNS.

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.

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.

Clavulanic Acid Attenuating Effect on the Diabetic Neuropathic Pain in Rats

Diabetic neuropathy is one of the most common complications of diabetes mellitus. Excess glutamate release and oxidative stress are hypothesized to be involved in the pathophysiology of diabetes-induced neuropathy. This study was designed to investigate the effect of clavulanic acid (CLAV), a competitive beta-lactamase inhibitor, on the streptozocin (STZ)-induced neuropathic pain and possible mechanisms in the spinal cord of rats. Male Wistar rats were divided into naive group; control group which got a single dose of STZ (50 mg/kg, i.p.), as a model of diabetic neuropathic pain; prophylactic groups: animals received CLAV (10, 20 and 40 mg/kg, i.p.) 1 week after STZ for 10 days; and therapeutic group: animals received 20 mg/kg CLAV, 21 days after STZ for 10 days. Study of pain behaviors was started on days 0, 7, 14, 21, 28, 35 and 42 after STZ. The expression of the glutamate transport 1 (GLT1), genes of oxidative stress including inducible nitric oxide synthase (iNOS), proinflammatory cytokine, tumor necrosis factor alpha (TNF-α), as well as genes involved in the apoptosis including bcl2, bcl2-associated x (bax) were measured in the spinal cord tissue by Real Time PCR, on day 42. On day 21 post injection of STZ, diabetic animals showed significant mechanical allodynia, cold allodynia and thermal hyperalgesia. CLAV in all doses of 10, 20 and 40 mg/kg reduced symptoms of allodynia and hyperalgesia, in both prophylactic and therapeutic regimens. While iNOS, TNF-α, bax/bcl2 were found significantly overexpressed in spinal cord of diabetic animals, their expression in animals received CLAV had been reduced. In contrast, GLT1 that had decreased in the spinal cord of diabetic animals, significantly increased in those received CLAV. CLAV was found a promising candidate for reliving neuropathic pain in diabetes mellitus. Such beneficial effect of CLAV could be, in part, attributed to the increased expression of GLT 1, inhibition of nitrosative stress, anti-inflammation, and inhibition of some apoptotic mediators followed by administration into diabetic animals.

Evidence for Modulation of Substance Use Disorders by the Gut Microbiome: Hidden in Plain Sight

The gut microbiome modulates neurochemical function and behavior and has been implicated in numerous central nervous system (CNS) diseases, including developmental, neurodegenerative, and psychiatric disorders. Substance use disorders (SUDs) remain a serious threat to the public well-being, yet gut microbiome involvement in drug abuse has received very little attention. Studies of the mechanisms underlying SUDs have naturally focused on CNS reward circuits. However, a significant body of research has accumulated over the past decade that has unwittingly provided strong support for gut microbiome participation in drug reward. β-Lactam antibiotics have been employed to increase glutamate transporter expression to reverse relapse-induced release of glutamate. Sodium butyrate has been used as a histone deacetylase inhibitor to prevent drug-induced epigenetic alterations. High-fat diets have been used to alter drug reward because of the extensive overlap of the circuitry mediating them. This review article casts these approaches in a different light and makes a compelling case for gut microbiome modulation of SUDs. Few factors alter the structure and composition of the gut microbiome more than antibiotics and a high-fat diet, and butyrate is an endogenous product of bacterial fermentation. Drugs such as cocaine, alcohol, opiates, and psychostimulants also modify the gut microbiome. Therefore, their effects must be viewed on a complex background of cotreatment-induced dysbiosis. Consideration of the gut microbiome in SUDs should have the beneficial effects of expanding the understanding of SUDs and aiding in the design of new therapies based on opposing the effects of abused drugs on the host's commensal bacterial community. SIGNIFICANCE STATEMENT: Proposed mechanisms underlying substance use disorders fail to acknowledge the impact of drugs of abuse on the gut microbiome. β-Lactam antibiotics, sodium butyrate, and high-fat diets are used to modify drug seeking and reward, overlooking the notable capacity of these treatments to alter the gut microbiome. This review aims to stimulate research on substance abuse-gut microbiome interactions by illustrating how drugs of abuse share with antibiotics, sodium butyrate, and fat-laden diets the ability to modify the host microbial community.

Glutamatergic Systems and Memory Mechanisms Underlying Opioid Addiction

Glutamate is the main excitatory neurotransmitter in the brain and is of critical importance for the synaptic and circuit mechanisms that underlie opioid addiction. Opioid memories formed over the course of repeated drug use and withdrawal can become powerful stimuli that trigger craving and relapse, and glutamatergic neurotransmission is essential for the formation and maintenance of these memories. In this review, we discuss the mechanisms by which glutamate, dopamine, and opioid signaling interact to mediate the primary rewarding effects of opioids, and cover the glutamatergic systems and circuits that mediate the expression, extinction, and reinstatement of opioid seeking over the course of opioid addiction.

Clavulanic acid improves ethanol withdrawal symptoms in rats

Objective(s):

Ethanol withdrawal following chronic use, is an important challenge clinically. In this study, the effect of clavulanic acid was evaluated on the symptoms of ethanol withdrawal in rats.

Materials and Methods:

Alcohol dependence was induced by the gavage of ethanol (10% v/v, 2 g/kg), twice daily for 10 days. Clavulanic acid (10, 20, 40, and 80 mg/kg) was administered concurrently with ethanol (sub-acute study), or a single dose after ethanol withdrawal (acute study). Six hours after the last dose of ethanol, anxiety was assessed by the elevated plus-maze (EPM). Seizure-like behavior was evaluated by a sub-convulsive dose of pentylenetetrazol (PTZ, 25 mg/kg/IP). Locomotor activity and motor coordination were measured by the open field and rotarod tests, respectively. Lipid peroxidation marker and antioxidant content were assessed through measuring malondialdehyde (MDA) and glutathione (GSH), respectively.

Results:

The number of entries and time spent on the open arms of EPM decreased during the withdrawal state. Motor coordination and locomotor activity were significantly decreased. In the sub-acute study, clavulanic acid 80 mg/kg increased time spent and the number of entries to the open arms of EPM, in withdrawn animals. Both motor incoordination and locomotor activity reduction were normalized by clavulanic acid (10, 20, 40 and 80 mg/kg). Withdrawal-induced PTZ kindling seizure was also suppressed by all of the doses. MDA increased, while GSH decreased after withdrawal. Clavulanic acid attenuated such changes.

Conclusion:

Clavulanic acid could prevent the development of alcohol withdrawal-induced anxiety and seizure. Alcohol withdrawal causes oxidative stress which can be prevented by clavulanic acid.

The Opioid-Addicted Tetrapartite Synapse

Opioid administration in preclinical models induces long-lasting adaptations in reward and habit circuitry. The latest research demonstrates that in the nucleus accumbens, opioid-induced excitatory synaptic plasticity involves presynaptic and postsynaptic elements as well as adjacent astroglial processes and the perisynaptic extracellular matrix. We outline opioid-induced modifications within each component of the tetrapartite synapse and provide a neurobiological perspective on how these adaptations converge to produce addiction-related behaviors in rodent models. By incorporating changes observed at each of the excitatory synaptic compartments into a unified framework of opioid-induced glutamate dysregulation, we highlight new avenues for restoring synaptic homeostasis that might limit opioid craving and relapse vulnerability.

Neuroprotective effects of clavulanic acid following permanent bilateral common carotid artery occlusion in rats

We investigated whether clavulanic acid could improve learning and memory, in rats underwent bilateral occlusion of common carotid artery (2VO). Seventy male Wistar rats were subjected to 2VO, with a 1-week interval between right and left artery occlusions. After 2VO, animals received clavulanic acid (10, 20, 40 mg/kg, intraperitoneally), from day 8 to 20. Spatial memory was assessed in the Morris water maze, 1 week after the induction of 2VO (day 15). The mRNA expression levels of bcl-2, bcl2-associated x protein (bax), caspase-3, inducible nitric oxide synthase (iNOS), and amyloid beta precursor protein (APP) were measured in the neocortex and hippocampus. Clavulanic acid significantly decreased the escape latency and swimming time in the training trial days. As well, it increased time and distance percentage in the target quadrant, while it decreased such factors in the opposite quadrant in the final trial day, compared to 2VO + normal saline animals. Real time-PCR data showed a significant higher mRNA expression of bax, caspase 3, and iNOS in the hippocampus and neocortex of 2VO animal compared to nonoccluded rats. APP increased in the neocortex but not hippocampus. Compared with 2VO animals, clavulanic acid significantly down-regulated the expression of iNOS, caspase 3, and APP, accompanied by diminishing the bax/bcl2 ratio. Our results reveal a potential therapeutic use of clavulanic acid for cognitive dysfunction associated with cerebral hypoperfusion in vascular dementia and Alzheimer disease.

Upregulation of Glutamate Transporter 1 by Clavulanic Acid Administration and Attenuation of Allodynia and Hyperalgesia in Neuropathic Rats

Introduction:

Clavulanic acid (CLAV) is structurally similar to ceftriaxone, a potent stimulator of glial GlutamateTransporter-1 (GLT-1) expression. The present study aims at exploring the anti-nociceptive effects of CLAV, a beta-lactamase inhibitor in rats underwent sciatic nerve Chronic Constriction Injury (CCI).

Methods:

CLAV (12.5, 25, 50 mg/kg) was administered intraperitoneally after the surgery for 14 consecutive days. Behavioral pain parameters were evaluated before and 3, 5, 7, 10 and 14 days after injury. Spinal GLT-1 level was measured via western blotting at days 7 and 14.

Results:

CCI led to mechanical allodynia, cold allodynia and thermal hyperalgesia which started on postoperative days 3 and continued until the end of study. We found that CLAV (12.5 and 25 mg/kg) significantly attenuated all pain related behaviors as compared to the CCI animals treated with normal saline. Protein level of GLT-1 was down-regulated on day 14 following CCI and this phenomenon was reversed by fourteen days treatment of CLAV at the low doses of 12.5 and 25 mg/kg.

Conclusion:

These results suggest that CLAV might provide a new therapeutic strategy for neuropathic pain and its effect might be partially associated with the up-regulation of GLT-1.

Neuroimmune signaling in alcohol use disorder

Alcohol use disorder (AUD) is a widespread disease with limited treatment options. Targeting the neuroimmune system is a new avenue for developing or repurposing effective pharmacotherapies. Alcohol modulates innate immune signaling in different cell types in the brain by altering gene expression and the molecular pathways that regulate neuroinflammation. Chronic alcohol abuse may cause an imbalance in neuroimmune function, resulting in prolonged perturbations in brain function. Likewise, manipulating the neuroimmune system may change alcohol-related behaviors. Psychiatric disorders that are comorbid with AUD, such as post-traumatic stress disorder, major depressive disorder, and other substance use disorders, may also have underlying neuroimmune mechanisms; current evidence suggests that convergent immune pathways may be involved in AUD and in these comorbid disorders. In this review, we provide an overview of major neuroimmune cell-types and pathways involved in mediating alcohol behaviors, discuss potential mechanisms of alcohol-induced neuroimmune activation, and present recent clinical evidence for candidate immune-related drugs to treat AUD.

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.

Effects of Clavulanic Acid on the Acquisition and Reinstatement Following Morphine-induced Conditioned Place Preference in Mice

Introduction:

β-Lactam antibiotics like Clavulanic Acid (CA) enhances cellular glutamate uptake through activation of Glutamate Transporter subtype 1 (GLT-1) and decreases the level of glutamate in the nervous system. Based on studies, blocking the glutamate activity inhibits morphine-induced Conditioned Place Preference (CPP) in animals. Therefore, the effects of CA on the acquisition of morphine craving were evaluated using the CPP model in the current study.

Methods:

CA (1, 50 and 150 mg/kg, ip) was co-administered with morphine (40 mg/kg) for 4 days in the conditioning phase. On day 8, the effects of CA on morphine preference was assessed. In another experiment, the effect of CA on reinstatement of morphine preference by a single morphine injection (10 mg/kg) was evaluated after an extinction period.

Results:

In the first method, the morphine-induced place preference was markedly reduced following administration of CA (50 and 150 mg/kg). In the second experiment, a single administration of CA (50 and 150 mg/kg) markedly inhibited the reinstatement of morphine preference on day 16. The results indicated that CA (50, 150 mg/kg) can block both morphine-induced CPP and the reinstatement of place preference following priming dose of morphine. Also memantine (as a positive control) (10 mg/kg) significantly inhibited both acquisition and reinstatement of morphine CPP.

Conclusion:

Considering the important role of glutamate neurotransmission in morphine dependence, the effects of CA may be partly due to decrease in glutamate level in synaptic space and blockade of N-Methyl-D-aspartate Acid (NMDA) receptors. Although, we need further studies to determine exact cellular mechanism.

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.

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.

Endogenous Opiates and Behavior: 2015

This paper is the thirty-eighth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2015 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia, stress and social status, tolerance and dependence, learning and memory, eating and drinking, drug abuse and alcohol, sexual activity and hormones, pregnancy, development and endocrinology, mental illness and mood, seizures and neurologic disorders, electrical-related activity and neurophysiology, general activity and locomotion, gastrointestinal, renal and hepatic functions, cardiovascular responses, respiration and thermoregulation, and immunological responses.

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

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

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.

Clavulanic acid enhances glutamate transporter subtype I (GLT-1) expression and decreases reinforcing efficacy of cocaine in mice

The β-lactam antibiotic ceftriaxone (CTX) reduces cocaine reinforcement and relapse in preclinical assays through a mechanism involving activation of glutamate transporter subtype 1 (GLT-1). However, its poor brain penetrability and intravenous administration route may limit its therapeutic utility for indications related to CNS diseases. An alternative is clavulanic acid (CA), a structural analog of CTX that retains the β-lactam core required for GLT-1 activity but displays enhanced brain penetrability and oral activity relative to CTX. Here, we tested the hypothesis that CA (1, 10 mg/kg ip) would enhance GLT-1 expression and decrease cocaine self-administration (SA) in mice, but at lower doses than CTX. Experiments revealed that GLT-1 transporter expression in the nucleus accumbens of mice treated with repeated CA (1, 10 mg/kg) was enhanced relative to saline-treated mice. Repeated CA treatment (1 mg/kg) reduced the reinforcing efficacy of cocaine (0.56 mg/kg/inf) in mice maintained on a progressive-ratio (PR) schedule of reinforcement but did not affect acquisition of cocaine SA under fixed-ratio responding or acquisition or retention of learning. These findings suggest that the β-lactamase inhibitor CA can activate the cellular glutamate reuptake system in the brain reward circuit and reduce cocaine's reinforcing efficacy at 100-fold lower doses than CTX.

Effect of ceftriaxone and topiramate treatments on naltrexone-precipitated morphine withdrawal and glutamate receptor desensitization in the rat locus coeruleus

RATIONALE

Morphine withdrawal is associated with a hyperactivity of locus coeruleus (LC) neurons by an elevated glutamate neurotransmission in this nucleus. We postulate that reductions in the amount of glutamate in the LC by enhancing its reuptake or inhibiting its release could attenuate the behavioral and cellular consequences of morphine withdrawal.

OBJECTIVES

We investigated the effect of chronic treatment with ceftriaxone (CFT), an excitatory amino acid transporter (EAAT2) enhancer, and acute administration of topiramate (TPM), a glutamate release inhibitor, on morphine withdrawal syndrome and withdrawal-induced glutamate receptor (GluR) desensitization in LC neurons from morphine-dependent rats.

METHODS

Morphine withdrawal behavior was measured after naltrexone administration in rats implanted with a morphine (200 mg kg(-1)) emulsion for 3 days. GluR desensitization in the LC was assessed by performing concentration-effect curves for glutamate by extracellular electrophysiological recordings in vitro.

RESULTS

Treatments with CFT or TPM reduced, in a dose-related manner, the total behavioral score of naltrexone-precipitated morphine withdrawal. CFT and TPM, at doses that were effective in behavioral tests, also reduced the induction of GluR desensitization normally occurring in LC neurons from morphine-dependent rats. Acute treatment with the specific EAAT2 inhibitor dihydrokainic acid (DHK) prevented the effect of CFT on withdrawal syndrome and GluR desensitization. Perfusion with TPM inhibited KCl-evoked but not glutamate-induced activation of LC neurons in vitro.

CONCLUSIONS

Our results suggest that a reduction of synaptic concentrations of glutamate by enhancing EAAT2-mediated uptake or inhibiting glutamate release alleviates the behavioral response and the cellular changes in the LC during opiate withdrawal.

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.

Levamisole enhances the rewarding and locomotor-activating effects of cocaine in rats

BACKGROUND

The Drug Enforcement Agency estimates that 80% of cocaine seized in the United States contains the veterinary pharmaceutical levamisole (LVM). One problem with LVM is that it is producing life-threatening neutropenia in an alarming number of cocaine abusers. The neuropharmacological profile of LVM is also suggestive of an agent with modest reinforcing and stimulant effects that could enhance cocaine's addictive effects.

METHODS

We tested the hypothesis that LVM (ip) enhances the rewarding and locomotor stimulant effects of cocaine (ip) using rat conditioned place preference (CPP) and locomotor assays. Effects of LVM by itself were also tested.

RESULTS

LVM (0-10 mg/kg) produced CPP at 1mg/kg (P<0.05) and locomotor activation at 5mg/kg (P < 0.05). For CPP combination experiments, a statistically inactive dose of LVM (0.1 mg/kg) was administered with a low dose of cocaine (2.5 mg/kg). Neither agent produced CPP compared to saline (P > 0.05); however, the combination of LVM and cocaine produced enhanced CPP compared to saline or either drug by itself (P < 0.01). For locomotor experiments, the same inactive dose of LVM (0.1mg/kg, ip) was administered with low (10 mg/kg) and high doses (30 mg/kg) of cocaine. LVM (0.1 mg/kg) enhanced locomotor activation produced by 10mg/kg of cocaine (P < 0.05) but not by 30 mg/kg (P>0.05).

CONCLUSIONS

LVM can enhance rewarding and locomotor-activating effects of low doses of cocaine in rats while possessing modest activity of its own.

Designing Novel Nanoformulations Targeting Glutamate Transporter Excitatory Amino Acid Transporter 2: Implications in Treating Drug Addiction

Chronic drug abuse is associated with elevated extracellular glutamate concentration in the brain reward regions. Deficit of glutamate clearance has been identified as a contributing factor that leads to enhanced glutamate concentration following extended drug abuse. Importantly, normalization of glutamate level through induction of glutamate transporter 1 (GLT1)/ excitatory amino acid transporter 2 (EAAT2) expression has been described in several in vivo studies. GLT1 upregulators including ceftriaxone, a beta-lactam antibiotic, have been effective in attenuating drug-seeking and drug-consumption behavior in rodent models. However, potential obstacles toward clinical translation of GLT1 (EAAT2) upregulators as treatment for drug addiction might include poor gastrointestinal absorption, serious peripheral adverse effects, and/or suboptimal CNS concentrations. Given the growing success of nanotechnology in targeting CNS ailments, nanoformulating known GLT1 (EAAT2) upregulators for selective uptake across the blood brain barrier presents an ideal therapeutic approach for treating drug addiction. In this review, we summarize the results obtained with promising GLT1 (EAAT2) inducing compounds in animal models recapitulating drug addiction. Additionally, the various nanoformulations that can be employed for selectively increasing the CNS bioavailability of GLT1 (EAAT2) upregulators are discussed. Finally, the applicability of GLT1 (EAAT2) induction via central delivery of drug-loaded nanoformulations is described.