Relapse-like ethanol-drinking behavior depends on increased glutamate transmission in the mesocorticolimbic motive circuit. Extracellular glutamate is regulated by a number of glutamate transporters. Of these transporters, glutamate transporter 1 (GLT1) is responsible for the majority of extracellular glutamate uptake. We have recently reported that ceftriaxone (CEF) treatment (i.p.), a β-lactam antibiotic known to elevate GTL1 expression, reduced ethanol intake in male alcohol-preferring (P) rats. We investigated here whether CEF treatment attenuates relapse-like ethanol-drinking behavior. P rats were exposed to free choice of 15% and 30% ethanol for 5 weeks and treated with CEF (50 and 100 mg/kg, i.p.) during the last 5 days of the 2-week deprivation period. Rats treated with CEF during the deprivation period showed a reduction in ethanol intake compared with saline-treated rats upon re-exposure to ethanol; this effect persisted for 9 days. Moreover, CEF-mediated attenuation in relapse to ethanol-drinking behavior was associated with upregulation of GLT1 level in prefrontal cortex and nucleus accumbens core. GLT1 upregulation was revealed only at the higher dose of CEF. In addition, CEF has no effect on relapse-like sucrose-drinking behavior. These findings suggest that ceftriaxone might be used as a potential therapeutic treatment for the attenuation of relapse-like ethanol-drinking behavior.
Relapse; glutamate; ethanol intake; EAAT2; alcohol dependence
We have previously shown that ceftriaxone, β-lactam antibiotic known to upregulate glutamate transporter 1 (GLT1), reduced ethanol intake in alcohol-preferring (P) rats. GLT1 is a glial glutamate transporter that regulates the majority of extracellular glutamate uptake. We tested in this study the effects of neuroimmunophilin GPI-1046 (3-(3-pyridyl)-1-propyl (2S)-1-(3,3-dimethyl-1,2-dioxopentyl)-2-pyrrolidinecarboxylate), known also to upregulate GLT1 expression, in ethanol intake in P rats. Male P rats had concurrent access to free choice of 15% and 30% ethanol, water, and food for five weeks. On Week 6, P rats continued in this drinking and food regimen and they were administered either 10 or 20 mg/kg GPI-1046 (i.p.), or a vehicle for five consecutive days. Body weight, ethanol intake, and water consumption were measured daily for 8 days starting on Day 1 of GPI-1046 or vehicle i.p. injections. We have also tested the effect of GPI-1046 (20 mg/kg) on daily sucrose (10%) intake. The data revealed significant dose-dependent effects in the reduction of ethanol intake starting 48 h after the first treatment with GPI-1046 throughout treatment and post-treatment periods. There were also dose-dependent increases in water intake. However, GPI-1046 treatment did not affect the body weight of all animals nor sucrose intake. Importantly, GPI-1046 (20 mg/kg) increased GLT1 level compared to all groups in nucleus accumbens core (NAc-core). Alternatively, GPI-1046 (10 mg/kg) upregulated GLT1 level in NAc-core compared to vehicle (ethanol naïve) group. Moreover, both doses of GPI-1046 increased significantly GLT1 level in the prefrontal cortex (PFC) compared to ethanol naïve vehicle group. GPI-1046 (20 mg/kg) increased GLT1 level in PFC compared to naïve control group that was exposed to water and food only. These findings demonstrated that neuroimmunophilin GPI-1046 attenuates ethanol intake in part through the upregulation of GLT1 in PFC and NAc-core.
GLT1; EAAT2; glutamate; alcohol dependence; GPI-1046; neuroimmunophilin
Relapse to cocaine-seeking behavior depends on increased glutamate transmission in key regions of the mesocorticolimbic motive circuit, including prefrontal cortex (PFC) and nucleus accumbens (NAcc). Because GLT1 is responsible for the uptake of ≥90% of extracellular glutamate, we tested the hypothesis that increased GLT1 expression attenuates cocaine relapse. Rats were trained to self-administer cocaine (0.125 mg per iv infusion) in a lever-pressing task in a daily two-hour session for 10–14 days followed by five days of extinction training. Immediately after each extinction session, rats received ceftriaxone (ip), a β-lactam antibiotic believed to increase GLT1 expression, or vehicle. On the following day, presentation of the cue (light and tone) previously associated with cocaine self-administration reinstated lever-pressing in rats treated with vehicle, whereas 100 or 200, but not 50 mg/kg ceftriaxone blocked this response. Immunoblotting confirmed that the ceftriaxone-induced blockade of cocaine relapse was associated with an increase in GLT1 expression in both PFC and NAcc. In separate groups of rats, 200 mg/kg ceftriaxone failed to block cue-induced food seeking, arguing against a ceftriaxone-induced effect unique to extinction training or lever pressing. Our results suggest that glutamate plays a key role in cue-induced relapse to cocaine-seeking behavior, implicating GLT1 as a potential therapeutic target for cocaine addiction.
ceftriaxone; cocaine; GLT1; glutamate; prefrontal cortex; nucleus accumbens
Glutamate transporter subtype 1 (GLT-1) activation is a promising – and understudied - approach for managing aspects of morphine tolerance caused by increased glutamatergic transmission. Identification of beta-lactam antibiotics as pharmaceuticals which activate GLT-1 transporters prompted us to hypothesize that repeated beta-lactam antibiotic (ceftriaxone) administration blocks development of tolerance to morphine antinociception through GLT-1 activation. Here, we injected rats with morphine (10 mg/kg, s.c.) twice daily for 7 days to induce tolerance and used the hot-plate assay to determine antinociception on days 1, 4 and 7 of repeated morphine administration. Ceftriaxone and a selective GLT-1 transporter inhibitor dihydrokainate (DHK) were co-administered with morphine to determine if GLT-1 activation mediated the ceftriaxone effect. Tolerance was present on days 4 and 7 of repeated morphine administration. Ceftriaxone (50, 100 or 200 mg/kg, i.p.) administration dose-dependently blocked development of morphine tolerance. DHK (10 mg/kg, s.c.), administered 15 min before each morphine injection, prevented inhibition of morphine tolerance by ceftriaxone (200 mg/kg, i.p.). These results identify an interaction between ceftriaxone and morphine in opioid-tolerant rats and suggest beta-lactam antibiotics preserve analgesic efficacy during chronic morphine exposure.
beta-lactam antibiotic; GLT-1; ceftriaxone; glutamate; morphine; opioid; tolerance
Emerging evidence indicates that many aspects of alcohol and drug dependence involve changes in glutamate transmission. A number of studies have reported that drugs of abuse, including alcohol and cocaine, alter glutamate transport. Extracellular glutamate is regulated by a number of glutamate transporters in various brain regions. Of these transporters, glutamate transporter (GLT1) is a key player in the removal of most of the extracellular glutamate. Similar to neurodegenerative disease models, in which there is dysfunction of the glutamatergic excitatory system, the role of GLT1 has been tested in drug dependence models that show dysfunction of glutamate transmission. We and others have recently found that ceftriaxone, an FDA-approved drug known to elevate GLT1 expression, attenuates cue-induced cocaine relapse. Moreover, we recently found that alcohol-preferring rats treated with ceftriaxone showed a significant dose-dependent reduction in alcohol consumption. We also demonstrated that ceftriaxone-induced upregulation of GLT1 expression was associated with increases in glutamate uptake in Huntington’s disease mouse model. Importantly, ceftriaxone is currently in clinical trials for the treatment of amyotrophic lateral sclerosis. This review provides information about the potential therapeutic role of GLT1 for the treatment of alcohol abuse and dependence.
GLT1; EAAT2; glutamate; alcohol dependence; alcohol addiction; cocaine; GLAST; EAAT1; glutamate transporters; alcohol-preferring rats; glutamate uptake; cystine-glutamate exchanger; basal extracellular glutamate; nucleus accumbens; prefrontal cortex
We investigated the short- and long-term effects of ceftriaxone on GLT-1 transporter activity and extracellular glutamate in the rat nucleus accumbens. Repeated ceftriaxone administration (50, 100 or 200 mg/kg, i.p.) produced a dose-dependent reduction in glutamate levels that persisted for 20 days following discontinuation of drug exposure. The ceftriaxone effect was prevented bythe GLT-1 transporter inhibitor dihydrokainate (DHK) (1 μM, intra-accumbal). These results suggest β-lactam antibiotics produce an enduring reduction in glutamatergic transmission in the brain reward center.
ceftriaxone; glutamate; nucleus accumbens; β-lactam antibiotic; microdialysis; GLT-1
Analgesic tolerance is partially mediated by enhanced glutamatergic transmission in the CNS. Beta lactam antibiotics, through glutamate transporter subtype 1 (GLT-1) activation, reduce extracellular glutamate levels and attenuate tolerance to morphine analgesia in rats. Like opioids, nicotine has potent analgesic properties that are subject to tolerance. The purpose of this study was to evaluate the effects of ceftriaxone (CTX), a beta lactam antibiotic and GLT-1 activator on nicotine antinociception and its tolerance. Rats were pre-treated for five days with CTX (200 mg/kg, ip) prior to evaluating their analgesic response to nicotine (1.0 or 2.5 mg/kg sc) for seven consecutive days using the tail flick assay. CTX-treated rats displayed an enhanced antinociceptive response to nicotine and unlike saline injected controls, did not develop tolerance to nicotine’s analgesic effects. These results suggest that GLT-1 transporter activation enhances and preserves nicotine antinociception and identify beta lactam antibiotics as potential complementary therapeutic agents for the treatment of chronic pain.
nicotine; analgesia; glutamate; GLT-1; tolerance; beta lactam
A corticostriatal-dependent deficit in the release of ascorbate (AA), an antioxidant vitamin and neuromodulator, occurs concurrently in striatum with dysfunctional GLT1-dependent uptake of glutamate in the R6/2 mouse model of Huntington's disease (HD), an autosomal dominant condition characterized by overt corticostriatal dysfunction. To determine if deficient striatal AA release into extracellular fluid is related to altered GLT1 activity in HD, symptomatic R6/2 mice between 6 and 9 weeks of age and age-matched wild-type (WT) mice received single daily injections of 200 mg/kg ceftriaxone, a β-lactam antibiotic that elevates the functional expression of GLT1, or saline vehicle for 5 consecutive days. On the following day, in vivo voltammetry was coupled with corticostriatal afferent stimulation to monitor evoked release of AA into striatum. In saline-treated mice, we found a marked decrease in evoked extracellular AA in striatum of R6/2 relative to WT. Ceftriaxone, in contrast, restored striatal AA in R6/2 mice to WT levels. In addition, intra-striatal infusion of either the GLT1 inhibitor dihydrokainic acid (DHK) or DL-threo-beta-benzyloxyaspartate (TBOA) blocked evoked striatal AA release. Collectively, our results provide compelling evidence for a link between GLT1 activation and release of AA into the striatal extracellular fluid, and suggest that dysfunction of this system is a key component of HD pathophysiology.
Huntington's disease; ceftriaxone; ascorbate; glutamate uptake; striatum
Ceftriaxone is a beta-lactam antibiotic which has been found to increase the expression and function of the major glutamate transporter, GLT-1. It has previously been shown that GLT-1 expression is decreased in the nucleus accumbens following cocaine self-administration and extinction training; ceftriaxone given in the days immediately prior to reinstatement testing attenuates both cue- and cocaine-primed reinstatement. Here we tested the ability of ceftriaxone pre-treatment (for 5 days prior to the first cocaine exposure) to prevent the induction of cocaine sensitization and the acquisition of cocaine self-administration. We also tested whether ceftriaxone administered only during self-administration attenuates the reinstatement of extinguished cocaine-seeking. We found that ceftriaxone did not affect the acquisition of cocaine self-administration but was able to attenuate reinstatement weeks after ceftriaxone administration ceased. This attenuation in reinstatement was accompanied by a restoration of GLT-1 expression in the nucleus accumbens. Ceftriaxone also attenuated locomotor behavior following the first cocaine injection and prevented the induction of cocaine but not caffeine sensitization. While ceftriaxone-treated animals did not sensitize to caffeine, they displayed reduced caffeine-induced locomotion following repeated caffeine treatment, indicating a possible dopaminergic effect of ceftriaxone. Taken together, these results indicate that ceftriaxone produces enduring changes in glutamate homeostasis in the nucleus accumbens which counteract addiction-related behaviors.
The β-lactam antibiotic and glutamate transporter subtype 1 (GLT-1) activator ceftriaxone prevents relapse to cocaine-seeking and inhibits morphine-induced physical dependence and tolerance in rats, but its efficacy against amphetamine-induced behaviors is unknown.
Here, we tested the hypothesis that ceftriaxone (200 mg/kg, i.p.) inhibits hyperactivity produced by acute amphetamine administration (2 mg/kg, i.p.) and sensitization of hyperactivity induced by repeated amphetamine exposure (2 mg/kg, i.p.). For acute experiments, rats treated with ceftriaxone for 5 days were injected with amphetamine or saline on day 6.
Amphetamine elicited less ambulatory and stereotypical activity in ceftriaxone-treated rats than in ceftriaxone-naïve rats. For chronic experiments, rats injected with ceftriaxone or saline for 8 days were also injected with amphetamine or saline on days 6–8 and then challenged with amphetamine 5 days later. Amphetamine produced greater ambulatory and stereotypical activity in amphetamine-pretreated rats than in rats previously naïve to amphetamine. Amphetamine challenge produced less ambulatory and stereotypical activity in rats pretreated with a combination of ceftriaxone (200 mg/kg) and amphetamine than in rats pretreated with only amphetamine.
The present demonstration that ceftriaxone attenuates amphetamine-induced hyperactivity and behavioral sensitization suggests its documented efficacy against adverse cocaine and morphine effects extends to amphetamine.
amphetamine; β-lactam antibiotic; GLT-1; ceftriaxone; glutamate; sensitization; addiction
No medication is approved to treat cocaine addiction, but mounting evidence suggests glutamate-directed approaches may reduce cocaine dependence and relapse. We tested the hypotheses that the glutamate transporter subtype 1 (GLT-1) activator ceftriaxone disrupts acquisition of cocaine self-administration, motivation to self-administer cocaine, and conditioned place preference in mice. Repeated ceftriaxone (200 mg/kg) reduced the ability of mice to acquire cocaine and the motivation to self-administer cocaine following successful acquisition without affecting acquisition of or motivation for sweet food. Repeated ceftriaxone had no effect on cocaine-conditioned place preference. These results suggest that a β-lactam antibiotic reduces the direct reinforcing strength of cocaine without producing non-specific deficits in conditioned learning processes.
cocaine; glutamate; GLT-1; ceftriaxone; self-administration; place preference; β-lactam; addiction; mouse
The striatum, which processes cortical information for behavioral output, is a key target of Huntington's disease (HD), an autosomal dominant condition characterized by cognitive decline and progressive loss of motor control. Increasing evidence implicates deficient glutamate uptake caused by a down-regulation of GLT1, the primary astroglial glutamate transporter. To test this hypothesis, we administered ceftriaxone, a β-lactam antibiotic known to elevate GLT1 expression (200 mg/kg, ip, for 5 days), to symptomatic R6/2 mice, a widely studied transgenic model of HD. Relative to vehicle, ceftriaxone attenuated several HD behavioral signs: paw clasping and twitching were reduced, while motor flexibility, as measured in a plus maze, and open-field climbing were increased. Assessment of GLT1 expression in striatum confirmed a ceftriaxone-induced increase relative to vehicle. To determine if the change in behavior and GLT1 expression represented a change in striatal glutamate handling, separate groups of behaving mice were evaluated with no-net-flux microdialysis. Vehicle treatment revealed a glutamate uptake deficit in R6/2 mice relative to wild-type controls that was reversed by ceftriaxone. Vehicle-treated animals, however, did not differ in GLT1 expression, suggesting that the glutamate uptake deficit in R6/2 mice reflects dysfunctional rather than missing GLT1. Our results indicate that impaired glutamate uptake is a major factor underlying HD pathophysiology and symptomology. The glutamate uptake deficit, moreover, is present in symptomatic HD mice and reversal of this deficit by up-regulating the functional expression of GLT1 with ceftriaxone attenuates the HD phenotype.
Huntington's disease; ceftriaxone; R6/2; striatum; microdialysis; glutamate
Glutamate neurotransmission is highly regulated, largely by glutamate transporters. In the spinal cord, the glutamate transporter GLT-1 is primarily responsible for glutamate clearance. Downregulation of GLT-1 can occur in activated astrocytes, and is associated with increased extracellular glutamate and neuroexcitation. Among other conditions, astrocyte activation occurs following repeated opioids and in models of chronic pain. If GLT-1 downregulation occurs in these states, GLT-1 could be a pharmacological target for improving opioid efficacy and controlling chronic pain. The present studies explored whether daily intrathecal treatment of rats with ceftriaxone, a β-lactam antibiotic that upregulates GLT-1 expression, could prevent development of hyperalgesia and allodynia following repeated morphine, reverse pain arising from central or peripheral neuropathy, and reduce glial activation in these models. Ceftriaxone pre-treatment attenuated the development of hyperalgesia and allodynia in response to repeated morphine, and prevented associated astrocyte activation. In a model of multiple sclerosis (experimental autoimmune encephalomyelitis; EAE), ceftriaxone reversed tactile allodynia and halted the progression of motor weakness and paralysis. Similarly, ceftriaxone reversed tactile allodynia induced by chronic constriction nerve injury (CCI). EAE and CCI each significantly reduced the expression of membrane-bound, dimerized GLT-1 protein in lumbar spinal cord, an effect normalized by ceftriaxone. Lastly, ceftriaxone normalized CCI- and EAE-induced astrocyte activation in lumbar spinal cord. Together, these data indicate that increasing spinal GLT-1 expression attenuates opioid-induced paradoxical pain, alleviates neuropathic pain, and suppresses associated glial activation. GLT-1 therefore may be a therapeutic target that could improve available treatment options for patients with chronic pain.
opioid; spinal cord; multiple sclerosis; astrocyte; allodynia; hyperalgesia
Homer proteins are constituents of scaffolding complexes that regulate the trafficking and function of central Group1 metabotropic glutamate receptors (mGluRs) and N-methyl-D-aspartate (NMDA) receptors. Research supports the involvement of these proteins in ethanol-induced neuroplasticity in mouse. In this study, we examined the effects of short versus long-term withdrawal from chronic ethanol consumption on Homer and glutamate receptor protein expression within striatal and amygdala subregions of selectively bred, alcohol-preferring P rats.
For 6 months, male P rats had concurrent access to 15% and 30% ethanol solutions under intermittent (IA: 4 d/wk) or continuous (CA: 7 d/wk) access conditions in their home cage. Rats were killed 24 hours (short withdrawal: SW) or 4 weeks (long withdrawal: LW) after termination of ethanol access, subregions of interest were micropunched and tissue processed for detection of Group1 mGluRs, NR2 subunits of the NMDA receptor and Homer protein expression.
Within the nucleus accumbens (NAC), limited changes in NR2a and NR2b expression were detected in the shell (NACsh), whereas substantial changes were observed for Homer2a/b, mGluRs as well as NR2a and NR2b subunits in the core (NACc). Within the amygdala, no changes were detected in the basolateral subregion, whereas substantial changes, many paralleling those observed in the NACc, were detected in the central nucleus (CeA) subregion. In addition, most of the changes observed in the CeA, but not NACc, were present in both SW and LW rats.
Overall, these subregion specific, ethanol-induced increases in mGluR/Homer2/NR2 expression within the NAC and amygdala suggest changes in glutamatergic plasticity had taken place. This may be a result of learning and subsequent memory formation of ethanol’s rewarding effects in these brain structures, which may, in part, mediate the chronic relapsing nature of alcohol abuse.
Homer Proteins; Group1 Metabotropic Glutamate Receptors; NMDA Receptors; Nucleus Accumbens; Amygdala; Ethanol Consumption
In outbred rats, increases in brain neuropeptide Y (NPY) activity suppress ethanol consumption in a variety of access conditions, but only following a history of ethanol dependence. NPY reliably suppresses ethanol drinking in alcohol-preferring (P) rats and this effect is augmented following a period of ethanol abstinence. The purpose of this experiment was to examine the effects of NPY on 2-bottle choice ethanol drinking and feeding in Wistar rats that had undergone chronic ethanol vapor exposure, cycles of ethanol abstinence, or both. Ethanol-drinking Wistars were given six weeks of access to 15% (v/v) ethanol and water followed by either: two cycles of one week ethanol vapor exposure and two weeks with no ethanol; two cycles of one week ethanol bottle availability and two weeks with no ethanol; or two weeks of ethanol vapor exposure. Rats were infused ICV with one of four NPY doses (0.0, 2.5, 5.0, or 10.0 µg) following the ethanol exposure patterns described above, and tested for ethanol drinking and feeding in a 2-bottle choice situation. NPY dose-dependently increased food intake regardless of ethanol exposure history, but suppressed ethanol drinking only in rats that underwent cycles of ethanol access and ethanol abstinence. These results support the notion that dysregulation of brain NPY systems during chronic intermittent ethanol exposure is important in the motivational drive for subsequent relapse to ethanol drinking.
Neuropeptide Y; Dependence; Ethanol Vapor; Ethanol Abstinence
The main glutamate transporter GLT-1 is responsible for clearing synaptically released glutamate from the extracellular space and contributes to the shaping of glutamatergic transmission. Recently, it has been shown that ceftriaxone (CEF)-induced GLT-1 upregulation is associated with an impairment of the prepulse inhibition (PPI) of the startle reflex, a simple form of information processing that is reduced in schizophrenia, and determines a strong reduction in hippocampal metabotropic glutamate receptor (mGluR)2/3-dependent long-term depression. In this study, we tested the hypothesis that administration of the mGluR2/3 agonist LY379268 blocks the effect of GLT-1 upregulation on PPI of the startle. We showed that administration of LY379268 (1 mg/kg) prevented PPI alterations associated with GLT-1 upregulation, suggesting that CEF-induced PPI impairment was mGluR2/3 dependent. In addition, we showed that CEF-induced GLT-1 upregulaton did not alter the expression of mGluR2/3, and also that it occurred at sites of mGluR2/3 expression. These results indicate a novel mechanism by which GLT-1 upregulation modulates PPI of the startle.
glutamate transporters; GLT-1; metabotropic glutamate receptors; prepulse inhibition; Glutamate; Neurochemistry; Schizophrenia/Antipsychotics; Transporters; glutamate transporters; GLT-1; MEtabotropic glutamate receptors; Prepulse inhibition of the startle reflex
The glutamate transporter GLT-1 is responsible for the largest proportion of total glutamate transport. Recently, it has been demonstrated that ceftriaxone (CEF) robustly increases GLT-1 expression. In addition, physiological studies have shown that GLT-1 up-regulation strongly affects synaptic plasticity, and leads to an impairment of the prepulse inhibition, a simple form of information processing, thus suggesting that GLT-1 over-expression may lead to dysfunctions of large populations of neurons. To test this possibility, we assessed whether CEF affects cortical electrical activity by using chronic electroencephalographic (EEG) recordings in male WKY rats. Spectral analysis showed that 8 days of CEF treatment resulted in a delayed reduction in EEG theta power (7–9 Hz) in both frontal and parietal derivations. This decrease peaked at day 10, i.e., 2 days after the end of treatment, and disappeared by day 16. In addition, we found that the same CEF treatment increased motor activity, especially when EEG changes are more prominent. Taken together, these data indicate that GLT-1 up-regulation, by modulating glutamatergic transmission, impairs the activity of widespread neural circuits. In addition, the increased motor activity and prepulse inhibition alterations previously described suggest that neural circuits involved in sensorimotor control are particularly sensitive to GLT-1 up-regulation.
Growing evidence supports a role of metabotropic glutamate receptors (mGluRs) in ethanol reinforcement, ethanol-seeking, and ethanol withdrawal. To extend the understanding of the role of mGluRs in the addiction-relevant effects of ethanol as well as of the treatment target potential of these receptors for alcohol abuse, the effects of a selective mGlu2/3 agonist (LY379268) and a selective mGlu5 antagonist (MTEP) were tested on two processes central to alcohol addiction: ethanol reinforcement and stress-induced reinstatement of ethanol-seeking in rats with a history of ethanol dependence.
Following operant ethanol self-administration training, male Wistar rats were made dependent by intragastric ethanol intubations. Ethanol dependence was confirmed by the presence of somatic withdrawal signs. Following 2 weeks of withdrawal, stable ethanol self-administration was re-established, and the effects of LY379268 (0-3 mg/kg, SC) and MTEP (0-3 mg/kg, IP) on ethanol self-administration were determined in both non-dependent and post-dependent rats. A second set of rats underwent extinction training and then was tested for the effects of LY379268 or MTEP on reinstatement of ethanol-seeking induced by footshock stress.
LY379268 and MTEP dose-dependently reduced both ethanol self-administration and reinstatement of ethanol-seeking induced by footshock stress. Additionally, LY379268 was more effective than MTEP in inhibiting both behaviors in post-dependent than in non-dependent animals.
These findings suggest that neuroadaptation associated with chronic ethanol exposure or withdrawal alters the sensitivity of mGlu2/3 receptors, with implications for the understanding of the neural basis of alcohol dependence and the treatment target potential of these receptors.
ethanol dependence; LY379268; MTEP; metabotropic glutamate receptor; self-administration; relapse
Huntington's disease (HD) is an inherited neurodegenerative disorder characterized by cortico-striatal dysfunction and loss of glutamate uptake. At 7 weeks of age, R6/2 mice, which model an aggressive form of juvenile HD, show a glutamate-uptake deficit in striatum that can be reversed by treatment with ceftriaxone, a β-lactam antibiotic that increases GLT1 expression. Only at advanced ages (> 11 weeks), however, do R6/2 mice show an actual loss of striatal GLT1. Here, we tested whether ceftriaxone can reverse the decline in GLT1 expression that occurs in older R6/2s.
Western blots were used to assess GLT1 expression in both striatum and cerebral cortex in R6/2 and corresponding wild-type (WT) mice at 9 and 13 weeks of age. Mice were euthanized for immunoblotting 24 hr after five consecutive days of once daily injections (ip) of ceftriaxone (200 mg/kg) or saline vehicle. Despite a significant GLT1 reduction in saline-treated R6/2 mice relative to WT at 13, but not 9, weeks of age, ceftriaxone treatment increased cortical and striatal GLT1 expression relative to saline in all tested mice.
The ability of ceftriaxone to up-regulate GLT1 in R6/2 mice at an age when GLT1 expression is significantly reduced suggests that the mechanism for increasing GLT1 expression is still functional. Thus, ceftriaxone could be effective in modulating glutamate transmission even in late-stage HD.
Prenatal ethanol significantly heightens later alcohol consumption, but the mechanisms that underlie this phenomenon are poorly understood. Little is known about the basis of this effect of prenatal ethanol on the sensitivity to ethanol’s reinforcing effects. One possibility is that prenatal ethanol exposure makes subjects more sensitive to the appetitive effects of ethanol or less sensitive to ethanol’s aversive consequences. The present study assessed ethanol-induced second-order conditioned place preference (CPP) and aversion and ethanol-induced conditioned taste aversion (CTA) in infant rats prenatally exposed to ethanol (2.0 g/kg) or vehicle (water) or left untreated. The involvement of the κ opioid receptor system in ethanol-induced CTA was also explored. When place conditioning occurred during the ascending limb of the blood-ethanol curve (Experiment 1), the pups exposed to ethanol in utero exhibited greater CPP than untreated controls, with a shift to the right of the dose-response curve. Conditioning during a later phase of intoxication (30–45 min post-administration; Experiment 2) resulted in place aversion in control pups exposed to vehicle during late gestation but not in pups that were exposed to ethanol in utero. Ethanol induced a reliable and similar CTA (Experiment 3) in the pups treated with vehicle or ethanol during gestation, and CTA was insensitive to κ antagonism. These results suggest that brief exposure to a moderate ethanol dose during late gestation promotes ethanol-mediated reinforcement and alters the expression of conditioned aversion by ethanol. This shift in the motivational reactivity to ethanol may be an underlying basis of the effect of prenatal ethanol on later ethanol acceptance.
prenatal ethanol exposure; ethanol; second-order conditioning; aversion; preference
The cellular mechanisms underlying alcohol addiction are poorly understood. In several brain areas, ethanol depresses glutamatergic excitatory transmission, but how it affects excitatory synapses on dopamine neurons of the ventral tegmental area (VTA), a crucial site for the development of drug addiction, is not known. We report here that in midbrain slices from rats, clinically relevant concentrations of ethanol (10–80 mM) increase the amplitude of evoked EPSCs and reduce their paired-pulse ratio in dopamine neurons in the VTA. The EPSCs were mediated by glutamate α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors. In addition, ethanol increases the frequency but not the amplitude of spontaneous EPSCs. Furthermore, ethanol increases extracellular glutamate levels in the VTA of midbrain slices. The effects of ethanol are mimicked by SKF 38393, a dopamine D1 receptor agonist, and by GBR 12935, a dopamine reuptake inhibitor, and they are blocked by SKF 83566, a D1 antagonist, or by reserpine, which depletes dopamine stores. The enhancement of sEPSC frequency reaches a peak with 40mM ethanol and declines with concentrations ≥80mM ethanol, which is quite likely a result of D2 receptor activation as raclopride, a D2 receptor blocker, significantly enhanced 80mM ethanol-induced enhancement of sEPSCs. Finally, 6, 7-dinitroquinoxaline-2, 3-dione (DNQX), an AMPA receptor antagonist, attenuates ethanol-induced excitation of VTA DA neurons. We therefore conclude that, acting via presynaptic D1 receptors, ethanol at low concentrations increases glutamate release in the VTA, thus raising somatodendritic dopamine release, which further activates the presynaptic D1 receptors. Enhancement of this positive feedback loop may significantly contribute to the development of alcohol addiction.
mesolimbic system; addiction; alcohol; glutamate; D1 and D2 receptors; raclopride
Cocaine addiction is characterized by compulsive drug seeking, including relapse after a period of withdrawal. The relapse response requires increased glutamate transmission in the nucleus accumbens (NAc). Consistent with this view, GLT1, the transporter responsible for >90% of glutamate uptake, is down-regulated in NAc after several days of withdrawal in rats previously trained to self-administer cocaine under limited access conditions (1–2 hr/day). Human addiction, however, appears to be better modeled by extending daily drug access (6–8 hr/day) and introducing long periods of withdrawal. Here, we determined the combined effects of manipulating cocaine access and withdrawal on GLT1 expression in NAc core and shell. Rats were trained to self-administer cocaine (0.25 mg per intravenous infusion) in daily limited or extended access sessions for 11 days followed by a period of short (1 day) or long (40–45 days) withdrawal. We found that although cocaine withdrawal decreases GLT1 expression in both core and shell, only in core is GLT1 down-regulation sensitive to both access and withdrawal. In fact, after long withdrawal, GLT1 in core is down-regulated more than in shell in either the limited or extended access condition. Thus, glutamate regulation in core appears to be a critical factor in the drug-seeking behavior that follows relatively long periods of cocaine withdrawal.
cocaine; GLT1; nucleus accumbens; self-administration; withdrawal; glutamate
The cystine-glutamate exchanger is down-regulated after chronic cocaine, resulting in reduced extracellular levels of nucleus accumbens glutamate. The importance of cocaine-induced loss of glutamate homeostasis is revealed by N-acetylcysteine restoring cystine-glutamate exchange and attenuating reinstatement to cocaine-seeking. Another regulator of extracellular glutamate is the glial glutamate transporter GLT-1. We hypothesized that cocaine self-administration reduces GLT-1, and that GLT-1 up-regulation inhibits cocaine-seeking.
We measured [3H] glutamate uptake and protein expression of GLT-1 and xCT, the catalytic subunit of the cystine-glutamate exchanger, following cocaine self-administration and 3 weeks of extinction training. We also examined the affect of ceftriaxone (previously shown to increase GLT-1) and N-acetylcysteine treatment on the expression of GLT-1 and xCT. Ceftriaxone was also tested for the capacity to inhibit cue- and cocaine-induced relapse.
Cocaine self-administration reduced glutamate uptake and the expression of both GLT-1 and xCT. Ceftriaxone restored GLT-1 and xCT levels and prevented cue- and cocaine-induced reinstatement of drug-seeking. NAC also restored GLT-1 and xCT levels.
These results indicate that glutamate transport and cystine-glutamate exchange may be co-regulated, and provide further evidence that targeting glutamate homeostasis is a potential method for treating cocaine relapse.
cocaine; accumbens; cystine-glutamate exchange; glutamate uptake; GLT-1; self-administration
Although alcoholism is a worldwide problem resulting in millions of deaths, only a small percentage of alcohol users become addicted. Notably, the specific neural substrates responsible for individual differences in vulnerability to alcohol addiction are not known. In these studies, we used rodent models to study behavioral and synaptic correlates related to individual differences in the development of ethanol locomotor sensitization, a form of drug-dependent behavioral plasticity associated with addiction vulnerability. Male Swiss mice were treated daily with saline or 1.8 g/kg ethanol for 21 days. Locomotor activity tests were performed once a week for 15 min immediately after saline or ethanol injections. After at least eleven days of withdrawal, cohorts of saline and ethanol-treated mice were used to characterize the relationships between locomotor sensitization, ethanol drinking, and glutamatergic synaptic transmission in the nucleus accumbens. Ethanol-treated mice that expressed locomotor behavioral sensitization to ethanol drank significantly more ethanol than saline-treated subjects and ethanol-treated animals resilient to this form of behavioral plasticity. Moreover, ethanolsensitized mice also had reduced accumbal NMDA receptor function and expression, as well as deficits in NMDA receptor-dependent long term depression in the nucleus accumbens core after a protracted withdrawal. These findings suggest that disruption of accumbal core NMDA receptor-dependent plasticity may represent a synaptic correlate associated with ethanol-induced locomotor sensitization and increased propensity to consume ethanol.
Long-Term Depression; Nucleus Accumbens; Behavioral sensitization; Ethanol; Voluntary drinking; Glutamate receptors
Perinatal brain injury is the leading cause of subsequent neurological disability in both term and preterm baby. Glutamate excitotoxicity is one of the major factors involved in perinatal hypoxic-ischemic encephalopathy (HIE). Glutamate transporter GLT1, expressed mainly in mature astrocytes, is the major glutamate transporter in the brain. HIE induced excessive glutamate release which is not reuptaked by immature astrocytes may induce neuronal damage. Compounds, such as ceftriaxone, that enhance the expression of GLT1 may exert neuroprotective effect in HIE.
We used a neonatal rat model of HIE by unilateral ligation of carotid artery and subsequent exposure to 8% oxygen for 2 hrs on postnatal day 7 (P7) rats. Neonatal rats were administered three dosages of an antibiotic, ceftriaxone, 48 hrs prior to experimental HIE. Neurobehavioral tests of treated rats were assessed. Brain sections from P14 rats were examined with Nissl and immunohistochemical stain, and TUNEL assay. GLT1 protein expression was evaluated by Western blot and immunohistochemistry.
Pre-treatment with 200 mg/kg ceftriaxone significantly reduced the brain injury scores and apoptotic cells in the hippocampus, restored myelination in the external capsule of P14 rats, and improved the hypoxia-ischemia induced learning and memory deficit of P23-24 rats. GLT1 expression was observed in the cortical neurons of ceftriaxone treated rats.
These results suggest that pre-treatment of infants at risk for HIE with ceftriaxone may reduce subsequent brain injury.
β-lactam antibiotics; ceftriaxone; hypoxic-ischemic injury; neonatal rat; GLT1; EAAT2