Bivalent effects of MK-801 on ethanol-induced sensitization do not parallel its effects on ethanol-induced tolerance

The effect of MK-801 on stress-ethanol cross-sensitization is dissociable from its effects on nNOS activity

Locomotor behavioral sensitization represents an animal model for understanding neuroadaptive processes related to repeated drug exposure. Repeated stress can elicit a cross-sensitization to the stimulant response of ethanol, which involves neuronal nitric oxide synthase (nNOS). Activation of N-methyl d-aspartate (NMDA) glutamate receptors triggers nNOS and the synthesis of nitric oxide (NO). In this study, we investigated the effects of blocking NMDA receptors using the NMDA receptor antagonist MK-801 on the cross-sensitization between restraint stress and ethanol. We also evaluated the nNOS activity in the prefrontal cortex (PFC) and hippocampus. Mice were pretreated with saline or MK-801 30 min before an injection of saline or stress exposure for 14 days. On the following day, they were challenged with either saline or 1.8 g/kg ethanol. Swiss male mice pretreated with 0.25 mg/kg MK-801 exhibited a sensitized response to ethanol. Moreover, MK-801 potentiated the cross-sensitization between stress and ethanol. However, MK-801 prevented the enhanced nNOS activity in stress-exposed groups (challenged with saline or ethanol) in the PFC; the antagonist also prevented the ethanol-induced increase in nNOS activity and reduced this enzyme activity in mice exposed to stress in the hippocampus. These data indicate that systemic treatment with the NMDA antagonist potentiated, rather than blocked, ethanol-induced behavioral sensitization and that this effect is dissociable from the capacity of NMDA antagonists to reduce ethanol/stress-induced NOS stimulation in the PFC and hippocampus.

Ampicillin/Sulbactam Treatment Modulates NMDA Receptor NR2B Subunit and Attenuates Neuroinflammation and Alcohol Intake in Male High Alcohol Drinking Rats

Exposure to ethanol commonly manifests neuroinflammation. Beta (β)-lactam antibiotics attenuate ethanol drinking through upregulation of astroglial glutamate transporters, especially glutamate transporter-1 (GLT-1), in the mesocorticolimbic brain regions, including the nucleus accumbens (Acb). However, the effect of β-lactam antibiotics on neuroinflammation in animals chronically exposed to ethanol has not been fully investigated. In this study, we evaluated the effects of ampicillin/sulbactam (AMP/SUL, 100 and 200 mg/kg, i.p.) on ethanol consumption in high alcohol drinking (HAD1) rats. Additionally, we investigated the effects of AMP/SUL on GLT-1 and N-methyl-d-aspartate (NMDA) receptor subtypes (NR2A and NR2B) in the Acb core (AcbCo) and Acb shell (AcbSh). We found that AMP/SUL at both doses attenuated ethanol consumption and restored ethanol-decreased GLT-1 and NR2B expression in the AcbSh and AcbCo, respectively. Moreover, AMP/SUL (200 mg/kg, i.p.) reduced ethanol-increased high mobility group box 1 (HMGB1) and receptor for advanced glycation end-products (RAGE) expression in the AcbSh. Moreover, both doses of AMP/SUL attenuated ethanol-elevated tumor necrosis factor-alpha (TNF-α) in the AcbSh. Our results suggest that AMP/SUL attenuates ethanol drinking and modulates NMDA receptor NR2B subunits and HMGB1-associated pathways.

Long-term exposure to daily ethanol injections in DBA/2J and Swiss mice: Lessons for the interpretation of ethanol sensitization

Most mice ethanol sensitization studies focused on neurobiology at the expense of its behavioral characterization. Furthermore, relatively short ethanol exposures (10 to 20 injections) were used in these studies. The first aim of the present study is to better characterize the development and expression of ethanol sensitization after an extended exposure of 45 daily injections. In some previous studies, mice were classified as "respondent" and "resistant" to ethanol sensitization. The second aim of the present study is to test the long-term reliability of such categorizations and the consequences of their use on the interpretation of the ethanol sensitization results. Swiss and DBA/2J female mice received 45 consecutive daily ethanol administrations (respectively 2.5 and 2.0 g/kg) and their locomotor activity was daily recorded to test the development of ethanol sensitization. At the end of the procedure, a challenge test assessed the inter-group ethanol sensitization.The results of the present study show that ethanol sensitization continues to develop beyond 20 days to reach maximal levels after about 25 injections in DBA/2J mice and 40 injections in Swiss mice, although the core phase of the development of ethanol sensitization occurred in both strains during the first 20 days. Remarkably, ethanol sensitization after such a long daily ethanol treatment resulted in both an upward shift of the magnitude of ethanol stimulant effects and a prolongation of these effects in time (up to 30 minutes). Mice classified as "resistant to ethanol sensitization" according to previous studies developed very significant levels of ethanol sensitization when tested after 45 ethanol injections and are best described as showing a delayed development of ethanol sensitization. Furthermore, mice classified as respondent or resistant to ethanol sensitization also differ in their acute response to ethanol, such that it is difficult to ascertain whether these classifications are specifically related to the sensitization process.

A Review of Developmental Considerations in Human Laboratory Alcohol Research

PURPOSE OF REVIEW

Human laboratory studies involving alcohol administration have generated critical knowledge about individual differences in risk for alcohol use disorder (AUD), but have primarily involved adult populations and cross-sectional research designs. Ethical constraints have largely precluded human laboratory alcohol research in adolescence, and prospective studies have been rare. This paper provides an overview of developmental considerations in human laboratory alcohol research, with a focus on studies conducted with youth.

RECENT FINDINGS

Recent human laboratory studies from Europe and Canada have examined aspects of alcohol response during late adolescence, while recent survey studies from the United States have highlighted methods for circumventing alcohol administration in studies of adolescents.

SUMMARY

Across several decades of research, exceedingly few laboratory studies have examined developmental differences in alcohol responses or utilized prospective designs. Efforts to prioritize prospective research would further clarify the role of alcohol sensitivity traits as predictors or markers of AUD onset and progression.

A role for nucleus accumbens glutamate in the expression but not the induction of behavioural sensitization to ethanol

Mechanisms underlying differential sensitivity to behavioural sensitization to ethanol (EtOH) remain poorly understood, although accumulating evidence suggests a role for glutamatergic processes in the ventral striatum. Efforts to address this issue can benefit from the well-documented fact that in any given cohort, some of the mice (High sensitized; HS) show robust sensitization, while others (Low sensitized; LS) show little, if any, sensitization. Here, we examined whether this variability might be differentially associated with nucleus accumbens (NAc) glutamate processes. Male DBA mice received 5 EtOH (2.2g/kg) or saline injections twice a week and were challenged with EtOH (1.8g/kg) 2 weeks after injection 5. When an EtOH challenge was administered 2 weeks following the induction of sensitization, HS, but not LS, mice showed a robust increase in glutamate levels (67%, P<0.01) as measured by in vivo microdialysis. In a separate cohort, the mGlu2/3 agonist LY354740 (10mg/kg), given prior to the EtOH challenge, abolished the expression of sensitization. To ascertain whether enhanced release could also be observed during the induction of sensitization, glutamate levels were measured after the 1st and 5th EtOH injection and were found to be unchanged in HS mice, although briefly elevated in LS mice at injection 5. To further assess possible glutamate involvement during the induction of sensitization, sensitizing EtOH injections were co-administered with NMDAR antagonists. At the doses used, MK-801 (0.25mg/kg) and CGS 19755 (10mg/kg) blocked the expression of sensitization, but did not significantly interfere with the development of EtOH sensitization. Within the limitations of the present design, the results suggest an important role for EtOH-induced glutamate release in the NAc when sensitization is well established, but not necessarily during the development of sensitization.

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 nicotine on ethanol-induced locomotor sensitization: A model of neuroadaptation

Co-morbid use of nicotine-containing tobacco products and alcohol (ethanol) is prevalent in young adults initiating use and in alcohol dependent adults, suggesting that these drugs in combination may increase risk to develop dependence on one or both drugs. Neuroadaptations caused by repeated drug exposure are related to the development of drug dependence and vulnerability to relapse. Locomotor sensitization has been used as a behavioral measure used to detect changes in neural drug sensitivity that are thought to contribute to drug dependence and relapse. Locomotor sensitization was measured in the current studies to examine potential differences in the effects of nicotine and ethanol given alone and in combination. Baseline activity levels of DBA/2J mice were assessed on 2 days, then mice were treated for 10 days with saline, nicotine (1 or 2mg/kg of nicotine tartrate), ethanol (1 or 2g/kg), or nicotine plus ethanol and locomotor activity was assessed every third day. On the following day, all mice were challenged with ethanol to measure the expression of sensitization. Mice treated with both nicotine and ethanol exhibited greater stimulation than predicted from the combined independent effects of these drugs, consistent with our previously published results. The combined effects of nicotine and ethanol on locomotor sensitization were dependent on the dose of ethanol and whether testing was performed after the drugs were given together, or after challenge with ethanol alone. These results suggest that nicotine and ethanol in combination can have neuroadaptive effects that differ from the independent effects of these drugs.

Nicotine enhances the locomotor stimulating but not the conditioned rewarding effect of ethanol in DBA/2J mice

BACKGROUND

One hypothesis to explain the high rate of nicotine and alcohol (ethanol [EtOH]) co-abuse is that these drugs have enhanced rewarding effects when taken together. The goal of this work was to use the conditioned place preference (CPP) procedure to determine whether nicotine would enhance the development of EtOH-induced CPP.

METHODS

The conditioned rewarding effects of nicotine (1 or 2 mg/kg of nicotine tartrate), EtOH (1 g/kg), and nicotine plus EtOH in combination were assessed using a well-established CPP procedure chosen specifically for examining alterations in the development of EtOH-induced CPP by nicotine. In addition, the reference dose procedure was used to directly compare the conditioned rewarding effect of EtOH versus nicotine plus EtOH. DBA/2J mice were used because they are an inbred strain that has repeatedly been shown to develop CPP to EtOH.

RESULTS

Neither dose of nicotine alone produced CPP, whereas EtOH did, using the standard EtOH CPP procedure. The magnitude of EtOH-induced CPP was not affected by co-administration of 1 mg/kg nicotine, but 2 mg/kg nicotine interfered with the development of EtOH-induced CPP. Using the reference dose procedure, there was no significant preference or aversion for either nicotine + EtOH dose combination versus EtOH alone. However, combined nicotine and EtOH had a larger effect on locomotor activity, during the conditioning trials, compared to their additive effect when given alone, consistent with previous data.

CONCLUSIONS

These data do not support the hypothesis that nicotine enhances the conditioned rewarding effect of EtOH. This finding differs from the combined locomotor stimulant effects of nicotine and EtOH that were observed in this study and in our previously published work, and suggests that combined stimulant effects of nicotine and EtOH do not predict enhanced reward.

Altered NMDA receptor subunit gene expression in brains of mice showing high vs. low sensitization to ethanol

Repeated administration of ethanol (EtOH) in mice leads to behavioural sensitization, a progressive increase in locomotor activity. Since not all mice sensitize equally to EtOH, the objective of the present study was to determine whether variability in this response is associated with altered subunit gene expression of the N-methyl-d-aspartate receptor (NMDAR), a primary target of EtOH. We examined NR1, NR2A, and NR2B expression throughout the brain during the development phase of EtOH sensitization, as well as after a 14 day withdrawal period. Male DBA/2J mice received 5-6 injections of EtOH (2.2g/kg, i.p.) or saline (SAL) and were categorized as high- (HS) or low-sensitized (LS) on the basis of locomotor activity scores after the final injection. NMDAR subunits were analyzed by in situ hybridization in brains removed either immediately following the final EtOH injection or 14 days thereafter. At the end of development phase, LS mice showed increased NR2A expression in several brain areas compared to saline controls. LS animals also had greater NR1 expression in the nucleus accumbens core (+11%, p=0.05) and shell (+14%, p=0.04) compared to HS mice, and increased NR2B expression in hippocampal CA1 (+20%, p=0.05) relative to saline-treated animals. High-sensitized mice showed increased NR2A expression in the bed nucleus of the stria terminalis when compared to controls (+54%, p=0.02). No differences in gene expression between the treatment groups were seen 14 days after the final injection. These findings suggest that region-specific NMDAR subunits may play an important role in the variability associated with the induction of EtOH sensitization. Low-sensitized mice may be more sensitive to the NMDAR inhibitory effects of EtOH, with the NR1 and NR2A subunits potentially playing a key role in the failure to sensitize upon repeated EtOH exposure.

Glutamatergic targets for new alcohol medications

RATIONALE

An increasingly compelling literature points to a major role for the glutamate system in mediating the effects of alcohol on behavior and the pathophysiology of alcoholism. Preclinical studies indicate that glutamate signaling mediates certain aspects of ethanol's intoxicating and rewarding effects, and undergoes adaptations following chronic alcohol exposure that may contribute to the withdrawal, craving and compulsive drug-seeking that drive alcohol abuse and alcoholism.

OBJECTIVES

We discuss the potential for targeting the glutamate system as a novel pharmacotherapeutic approach to treating alcohol use disorders, focusing on five major components of the glutamate system: the N-methyl-D-aspartate (NMDA) receptor and specific NMDA subunits, the glycineB site on the NMDA receptors (NMDAR), L-alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid ionotropic (AMPA) and kainate (KAR) receptors, metabotropic receptors (mGluR), and glutamate transporters.

RESULTS

Chronic alcohol abuse produces a hyperglutamatergic state, characterized by elevated extracellular glutamate and altered glutamate receptors and transporters. Pharmacologically manipulating glutamatergic neurotransmission alters alcohol-related behaviors including intoxication, withdrawal, and alcohol-seeking, in rodents and human subjects. Blocking NMDA and AMPA receptors reduces alcohol consumption in rodents, but side-effects may limit this as a therapeutic approach. Selectively targeting NMDA and AMPA receptor subunits (e.g., GluN2B, GluA3), or the NMDAR glycineB site offers an alternative approach. Blocking mGluR5 potently affects various alcohol-related behaviors in rodents, and mGluR2/3 agonism also suppresses alcohol consumption. Finally, glutamate transporter upregulation may mitigate behavioral and neurotoxic sequelae of excess glutamate caused by alcohol.

CONCLUSIONS

Despite the many challenges that remain, targeting the glutamate system offers genuine promise for developing new treatments for alcoholism.

Probing the modulation of acute ethanol intoxication by pharmacological manipulation of the NMDAR glycine co-agonist site

BACKGROUND

Stimulating the glycine(B) binding site on the N-methyl-d-aspartate ionotropic glutamate receptor (NMDAR) has been proposed as a novel mechanism for modulating behavioral effects of ethanol (EtOH) that are mediated via the NMDAR, including acute intoxication. Here, we pharmacologically interrogated this hypothesis in mice.

METHODS

Effects of systemic injection of the glycine(B) agonist, d-serine, the GlyT-1 glycine transporter inhibitor, ALX-5407, and the glycine(B) antagonist, L-701,324, were tested for the effects on EtOH-induced ataxia, hypothermia, and loss of righting reflex (LORR) duration in C57BL/6J (B6) and 129S1/SvImJ (S1) inbred mice. Effects of the glycine(B) partial agonist, d-cycloserine (DCS), the GlyT-1 inhibitor, N-[3-(4'-fluorophenyl)-3-(4'-phenylphenoxy)propyl]sarcosine (NFPS), and the glycine(B) antagonist, 5,7-dichlorokynurenic (DCKA), on EtOH-induced LORR duration were also tested. Interaction effects on EtOH-induced LORR duration were examined via combined treatment with d-serine and ALX-5407, d-serine and MK-801, d-serine and L-701,324, as well as L-701,324 and ALX-5407, in B6 mice, and d-serine in GluN2A and PSD-95 knockout mice. The effect of dietary depletion of magnesium (Mg), an element that interacts with the glycine(B) site, was also tested.

RESULTS

Neither d-serine, DCS, ALX-5407, nor NFPS significantly affected EtOH intoxication on any of the measures or strains studied. L-701,324, but not DCKA, dose-dependently potentiated the ataxia-inducing effects of EtOH and increased EtOH-induced (but not pentobarbital-induced) LORR duration. d-serine did not have interactive effects on EtOH-induced LORR duration when combined with ALX-5407. The EtOH-potentiating effects of L-701,324, but not MK-801, on LORR duration were prevented by d-serine, but not ALX-5407. Mg depletion potentiated LORR duration in B6 mice and was lethal in a large proportion of S1 mice.

CONCLUSIONS

Glycine(B) site activation failed to produce the hypothesized reduction in EtOH intoxication across a range of measures and genetic strains, but blockade of the glycine(B) site potentiated EtOH intoxication. These data suggest endogenous activity at the glycine(B) opposes EtOH intoxication, but it may be difficult to pharmacologically augment this action, at least in nondependent subjects, perhaps because of physiological saturation of the glycine(B) site.

Insulin attenuates the acquisition and expression of ethanol-induced locomotor sensitization in DBA/2J mice

AIM

Ethanol-induced locomotor sensitization is a behavioral manifestation of physiological responses to repeated ethanol exposures. While ethanol exerts direct effects on multiple neurotransmitter systems in the brain, ethanol-induced changes in metabolic state, including acute hyperglycemia and inhibition of insulin signaling, also have plausible roles in the expression of ethanol-related behaviors through direct and indirect effects on brain function. The current experiments examined whether insulin administration or the resultant hypoglycemia might attenuate the development of sensitization to the locomotor stimulant effect of ethanol.

MAIN METHODS

Male and female DBA/2J mice received daily injections of 5 or 10 IU/kg insulin before or after a stimulating dose of ethanol and subsequent testing in an automated activity monitor. Blood glucose levels were determined upon the completion of the experiments.

KEY FINDINGS

Insulin injected prior to ethanol blunted the acute stimulant response as well as the acquisition and expression of locomotor sensitization, while insulin given after ethanol did not affect the development of the sensitized response. In a separate experiment, mice given glucose concurrently with insulin developed ethanol-induced locomotor sensitization normally.

SIGNIFICANCE

These experiments suggest that insulin attenuates the development of ethanol-induced locomotor sensitization, and that blood glucose levels can largely account for this effect. Further studies of the role of ethanol-induced metabolic states should provide novel information on the expression of ethanol-related behaviors.

Ethanol drinking-in-the-dark facilitates behavioral sensitization to ethanol in C57BL/6J, BALB/cByJ, but not in mu-opioid receptor deficient CXBK mice

BACKGROUND

Neuroplasticity associated with drug-induced behavioral sensitization has been associated with excessive drug pursuit and consumption characteristic of addiction. Repeated intraperitoneal (ip) injections of ethanol (EtOH) can induce psychomotor sensitization in mice. In terms of its clinical relevance, however, it is important to determine whether this phenomenon can also be produced by voluntary EtOH consumption.

METHODS

The present investigation used a drinking-in-the-dark (DID) methodology to induce high levels of EtOH drinking in mice; EtOH replaces water for 2 or 4h, starting 3h after the beginning of the dark cycle. Animals followed a 3-week DID protocol prior to an evaluation of EtOH-induced locomotor activity (acute and repeated EtOH). For the first week, animals had access to 20% EtOH. On weeks 2 and 3, different concentrations of EtOH (10, 20 or 30%) were used. Three different inbred strains of mice were used: C57BL/6J (B6), BALB/cByJ (BALB), and CXBK. The CXBK mouse line was used because of its reduced expression and functioning of brain mu-opioid receptors, which have been suggested to participate in the development of EtOH-induced sensitization. B6 and BALB mice were used as controls.

RESULTS

B6 and CXBK mice presented comparable levels of EtOH drinking (approx. 3g/kg in 2h), that were higher than those showed by BALB. All animals, regardless of genotype, adjusted volume of EtOH intake to obtain stable g/kg of EtOH across concentrations. Previous EtOH DID produced (B6) or potentiated (BALB) sensitization to EtOH; this effect was not seen in CXBK. Western blot analysis showed a reduced number of mu-opioid receptors in several brain regions of CXBK as compared to that of B6 and BALB mice.

CONCLUSIONS

In summary, here we show that the DID methodology can be used to trigger EtOH-induced neuroplasticity supporting psychomotor sensitization, a process that might require participation of mu-opioid receptors.

NR2B-deficient mice are more sensitive to the locomotor stimulant and depressant effects of ethanol

The NR2B subunit of N-methyl d-aspartate glutamate receptors influences pharmacological properties and confers greater sensitivity to the modulatory effects of ethanol. This study examined behavioral responses to acute ethanol in a conditional knockout mouse model that allowed for a delayed genetic deletion of the NR2B subunit to avoid mouse lethality. Mice lacking the NR2B gene (knockout) were produced by mating NR2B[f/f] mice with CAMKIIa-driven tTA transgenic mice and the tetO-CRE transgenic mice. Adult male and female offspring representing each of the resultant genotypes (knockout, CAM, CRE and wildtype mice) were tested for open-field locomotor activity following acute low- and high-dose ethanol challenge as well as loss of righting reflex. Findings indicate that male and female mice lacking the NR2B subunit exhibited greater overall activity in comparison to other genotypes during the baseline locomotor activity test. NR2B knockout mice exhibited an exaggerated stimulant response to 1.5 g/kg (i.p.) and an exaggerated depressant response to 3.0 g/kg (i.p.) ethanol challenge. In addition, NR2B knockout mice slept longer following a high dose of ethanol (4.0 g/kg, i.p.). To evaluate pharmacokinetics, clearance rates of ethanol (1.5, 4.0 g/kg, i.p.) were measured and showed that female NR2B knockouts had a faster rate of metabolism only at the higher ethanol dose. Western blot analyses confirmed significant reduction in NR2B expression in the forebrain of knockout mice. Collectively, these data indicate that the NR2B subunit of the N-methyl d-aspartate glutamate receptor is involved in regulating low-dose stimulant effects of ethanol and the depressant/hypnotic effects of ethanol.

Inhibition of extracellular signal-regulated kinase (ERK) activity with SL327 does not prevent acquisition, expression, and extinction of ethanol-seeking behavior in mice

Although extracellular signal-regulated kinase (ERK) activity is essential for the acquisition of a variety of associative learning tasks, its involvement in the acquisition and extinction of ethanol (EtOH)-induced conditioned place preference (CPP) remains unknown. Therefore, in these experiments we examined the effects of the ERK-kinase (MEK)-inhibitor SL327 on acquisition and expression of EtOH-CPP as well as the dose- and time-dependent effects of SL327 on CPP extinction. The parametric findings of Experiment 1 showed that three 30-min (but not 15- or 5-min) non-reinforced trials were required to completely extinguish EtOH-CPP in male, DBA/2J mice. In Experiments 2 and 3, SL327 (30 and 50mg/kg), administered 30 or 90min prior to extinction trials, was unable to impair EtOH-CPP extinction. Experiment 4 showed that SL327 (50mg/kg) had no effect on acquisition of EtOH-CPP or the development of EtOH-induced sensitization during conditioning. When administered prior to testing in Experiments 5 and 6, SL327 did not alter expression of EtOH-CPP but did reduce test activity. Importantly, SL327 significantly reduced pERK protein levels when assessed in the dorsal striatum and motor cortex (Experiment 7). Together, these data suggest that EtOH-related learning and EtOH reward in mice, as assessed with CPP, are not impaired by the systemically administered MEK-inhibitor SL327.

Repeated ethanol administration modifies the temporal structure of sucrose intake patterns in mice: effects associated with behavioral sensitization

Neuroadaptations supporting behavioral sensitization to abused drugs are suggested to underlie pathological, excessive motivation toward drugs and drug-associated stimuli. Drug-induced sensitization has also been linked to increased appetitive responses for non-drug, natural reinforcers. The present research investigated whether ethanol (EtOH)-induced neural changes, inferred from psychomotor sensitization, can modify consumption and intake dynamics for the natural reinforcer, sucrose. The effects of EtOH-induced sensitization in mice on the temporal structure of sucrose intake patterns were measured using a lickometer system. After sensitization, sucrose intake dynamics were measured for 1 hour daily for 7 days and indicated more rapid initial approach and consumption of sucrose in EtOH-sensitized groups; animals showed a shorter latency to the first intake bout and an increased number of sucrose bottle licks during the initial 15 minutes of the 1-hour sessions. This effect was associated with increased frequency and size of bouts. For the total 1-hour session, sucrose intake and bout dynamics were not different between groups, indicating a change in patterns of sucrose intake but not total consumption. When sensitization was prevented by the gamma-aminobutyric acid B receptor agonist, baclofen, the increased rate of approach and consumption of sucrose were also prevented. Thus, EtOH-induced sensitization, and not the mere exposure to EtOH, was associated with changes in sucrose intake patterns. These data are consistent with current literature suggesting an enhancing effect of drug-induced sensitization on motivational processes involved in reinforcement.

Effects of topiramate and other anti-glutamatergic drugs on the acute intoxicating actions of ethanol in mice: modulation by genetic strain and stress

Compounds with anti-glutamatergic properties currently in clinical use for various indications (eg Alzheimer's disease, epilepsy, psychosis, mood disorders) have potential utility as novel treatments for alcoholism. Enhanced sensitivity to certain acute intoxicating effects (ataxia, sedative) of alcohol may be one mechanism by which anti-glutamatergic drugs modulate alcohol use. We examined the effects of six compounds (memantine, dextromethorphan, haloperidol, lamotrigine, oxcarbazepine, and topiramate) on sensitivity to acute intoxicating effects of ethanol (ataxia, hypothermia, sedation/hypnosis) in C57BL/6J mice. Analysis of topiramate was extended to determine the influence of genetic background (by comparison of the 129S1, BALB/cJ, C57BL/6J, DBA/2J inbred strains) and prior stress history (by chronic exposure of C57BL/6J to swim stress) on topiramate's effects on ethanol-induced sedation/hypnosis. Results showed that one N-methyl-D-aspartate receptor (NMDAR) antagonist, memantine, but not another, dextromethorphan, potentiated the ataxic but not hypothermic or sedative/hypnotic effects of ethanol. Haloperidol increased ethanol-induced ataxia and sedation/hypnosis to a similar extent as the prototypical NMDAR antagonist MK-801. Of the anticonvulsants tested, lamotrigine accentuated ethanol-induced sedation/hypnosis, whereas oxcarbazepine was without effect. Topiramate was without effect per se under baseline conditions in C57BL/6J, but had a synergistic effect with MK-801 on ethanol-induced sedation/hypnosis. Comparing inbred strains, topiramate was found to significantly potentiate ethanol's sedative/hypnotic effects in BALB/cJ, but not 129S1, C57BL/6J, or DBA/2J strains. Topiramate also increased ethanol-induced sedation/hypnosis in C57BL/6J after exposure to chronic stress exposure. Current data demonstrate that with the exception of MK-801 and haloperidol, the compounds tested had either no significant or assay-selective effects on sensitivity to acute ethanol under baseline conditions in C57BL/6J. However, significant effects of topiramate were revealed as a function of co-treatment with an NMDAR blocker, genetic background, or prior stress history. These findings raise the possibility that topiramate and possibly other anti-glutamatergic drugs could promote the acute intoxicating effects of ethanol in specific subpopulations defined by genetics or life history.

Attenuation of the stimulant response to ethanol is associated with enhanced ataxia for a GABA, but not a GABA, receptor agonist

BACKGROUND

The gamma-aminobutyric acid (GABA) system is implicated in the neurobiological actions of ethanol, and pharmacological agents that increase the activity of this system have been proposed as potential treatments for alcohol use disorders. As ethanol has its own GABA mimetic properties, it is critical to determine the mechanism by which GABAergic drugs may reduce the response to ethanol (i.e., via an inhibition or an accentuation of the neurobiological effects of ethanol).

METHODS

In this study, we examined the ability of 3 different types of GABAergic compounds, the GABA reuptake inhibitor NO-711, the GABA(A) receptor agonist muscimol, and the GABA(B) receptor agonist baclofen, to attenuate the locomotor stimulant response to ethanol in FAST mice, which were selectively bred for extreme sensitivity to ethanol-induced locomotor stimulation. To determine whether these compounds produced a specific reduction in stimulation, their effects on ethanol-induced motor incoordination were also examined.

RESULTS

NO-711, muscimol, and baclofen were all found to potently attenuate the locomotor stimulant response to ethanol in FAST mice. However, both NO-711 and muscimol markedly increased ethanol-induced ataxia, whereas baclofen did not accentuate this response.

CONCLUSIONS

These results suggest that pharmacological agents that increase extracellular concentrations of GABA and GABA(A) receptor activity may attenuate the stimulant effects of ethanol by accentuating its intoxicating and sedative properties. However, selective activation of the GABA(B) receptor appears to produce a specific attenuation of ethanol-induced stimulation, suggesting that GABA(B) receptor agonists may hold greater promise as potential pharmacotherapies for alcohol use disorders.

Role of major NMDA or AMPA receptor subunits in MK-801 potentiation of ethanol intoxication

BACKGROUND

The glutamate system plays a major role in mediating EtOH's effects on brain and behavior, and is implicated in the pathophysiology of alcohol-related disorders. N-methyl-D-aspartate receptor (NMDAR) antagonists such as MK-801 (dizocilpine) interact with EtOH at the behavioral level, but the molecular basis of this interaction is unclear.

METHODS

We first characterized the effects of MK-801 treatment on responses to the ataxic (accelerating rotarod), hypothermic and sedative/hypnotic effects of acute EtOH administration in C57BL/6J and 129/SvImJ inbred mice. Effects of another NMDAR antagonist, phencyclidine, on EtOH-induced sedation/hypnosis were also assessed. Gene knockout of the NMDAR subunit NR2A or l-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate GluR1 or pharmacological antagonism of the NMDAR subunit NR2B (via Ro 25-6981) was employed to examine whether inactivating any one of these glutamate signaling molecules modified MK-801's effect on EtOH-related behaviors.

RESULTS

MK-801 markedly potentiated the ataxic effects of 1.75 g/kg EtOH and the sedative/hypnotic effects of 3.0 g/kg EtOH, but not the hypothermic effects of 3.0 g/kg EtOH, in C57BL/6J and 129/SvImJ mice. Phencyclidine potentiated EtOH-induced sedation/hypnosis in both inbred strains. Neither NR2A nor GluR1 KO significantly altered basal EtOH-induced ataxia, hypothermia, or sedation/hypnosis. Ro 25-6981 modestly increased EtOH-induced sedation/hypnosis. The ability of MK-801 to potentiate EtOH-induced ataxia and sedation/hypnosis was unaffected by GluR1 KO or NR2B antagonism. NR2A KO partially reduced MK-801 + EtOH-induced sedation/hypnosis, but not ataxia or hypothermia.

CONCLUSIONS

Data confirm a robust and response-specific potentiating effect of MK-801 on sensitivity to EtOH's intoxicating effects. Inactivation of three major components of glutamate signaling had no or only partial impact on the ability of MK-801 to potentiate behavioral sensitivity to EtOH. Further work to elucidate the mechanisms underlying NMDAR x EtOH interactions could ultimately provide novel insight into the role of NMDARs in alcoholism and its treatment.

Comparison of ethanol locomotor sensitization in adolescent and adult DBA/2J mice

RATIONALE

The mammalian adolescent period is characterized by enhanced vulnerability to drug-induced neuroadaptations. Epidemiological evidence indicates that individuals who start drinking alcohol during adolescence are four times more likely to develop alcohol dependence in adulthood, but little is known about the adaptive mechanism(s) that may underlie this observation. Behavioral sensitization in rodents is a model of neurobehavioral plasticity that occurs following repeated drug exposure and may underlie components of addiction.

OBJECTIVES

The goal of this study was to determine if adolescent mice are differentially sensitive to ethanol-induced locomotor sensitization as compared to adults.

MATERIALS AND METHODS

Adolescent and adult DBA/2J mice were treated with saline or ethanol (1.0, 1.5, 2.0, 2.5 g/kg) for 7, 11, or 15 days and tested for acute and sensitized locomotor activity. Blood ethanol clearance (BEC) was also assessed 10, 60, and 180 min following treatment with ethanol 2 g/kg.

RESULTS

Adolescent mice were more sensitive than adult mice to the acute locomotor activating effects of ethanol. However, adolescent mice were less sensitive than adult mice to locomotor sensitization, as only the highest dose of ethanol (2.5 g/kg) induced sensitization in the adolescent mice, while lower doses of ethanol elicited sensitization in the adult mice. The differential response to ethanol sensitization was not related to duration of treatment or differential BEC.

CONCLUSIONS

These results indicate that adolescent mice are less sensitive to ethanol sensitization, and this blunted behavioral response in adolescents might reflect differential ethanol-induced neurobehavioral adaptations.

Glutamatergic substrates of drug addiction and alcoholism

The past two decades have witnessed a dramatic accumulation of evidence indicating that the excitatory amino acid glutamate plays an important role in drug addiction and alcoholism. The purpose of this review is to summarize findings on glutamatergic substrates of addiction, surveying data from both human and animal studies. The effects of various drugs of abuse on glutamatergic neurotransmission are discussed, as are the effects of pharmacological or genetic manipulation of various components of glutamate transmission on drug reinforcement, conditioned reward, extinction, and relapse-like behavior. In addition, glutamatergic agents that are currently in use or are undergoing testing in clinical trials for the treatment of addiction are discussed, including acamprosate, N-acetylcysteine, modafinil, topiramate, lamotrigine, gabapentin and memantine. All drugs of abuse appear to modulate glutamatergic transmission, albeit by different mechanisms, and this modulation of glutamate transmission is believed to result in long-lasting neuroplastic changes in the brain that may contribute to the perseveration of drug-seeking behavior and drug-associated memories. In general, attenuation of glutamatergic transmission reduces drug reward, reinforcement, and relapse-like behavior. On the other hand, potentiation of glutamatergic transmission appears to facilitate the extinction of drug-seeking behavior. However, attempts at identifying genetic polymorphisms in components of glutamate transmission in humans have yielded only a limited number of candidate genes that may serve as risk factors for the development of addiction. Nonetheless, manipulation of glutamatergic neurotransmission appears to be a promising avenue of research in developing improved therapeutic agents for the treatment of drug addiction and alcoholism.

Behavioral sensitization to ethanol does not result in cross-sensitization to NMDA receptor antagonists

RATIONALE

Behavioral sensitization to the locomotor stimulant effects of ethanol may be related to neuroadaptations within glutamatergic systems. Previous research has suggested that the N-methyl-D: -aspartate (NMDA) subclass of glutamate receptors is critical for the development of ethanol sensitization. We hypothesized that sensitization to ethanol would be associated with changes in sensitivity to NMDA receptor ligands.

MATERIALS AND METHODS

DBA/2J and heterogeneous stock (HS) mice were injected with ethanol or saline for 12 days and tested for their acute and sensitized responses to the locomotor effects of ethanol in automated activity monitors. After this treatment phase, mice were challenged with MK-801, ethanol, or ketamine, and locomotor activity was measured for 20 to 60 min. Other ethanol-sensitized and nonsensitized mice were assessed for sensitivity to the effects of NMDA after tail-vein infusions.

RESULTS

There was no evidence for cross-sensitization to MK-801 or ketamine, or altered sensitivity to NMDA in ethanol-sensitized animals, in any experiment. In one experiment, previously ethanol-treated HS mice developed tolerance to the locomotor stimulant effects of ketamine.

CONCLUSIONS

These results indicate that ethanol-induced behavioral sensitization is not associated with increased behavioral sensitivity to NMDA receptor antagonists or altered sensitivity to NMDA receptor agonists. To the extent that changes in sensitivity to these ligands reflect changes in NMDA receptors, these results are inconsistent with the hypothesis that ethanol sensitization is associated with alterations in NMDA receptor-mediated processes.

The role of opioid receptor subtypes in the development of behavioral sensitization to ethanol

Nonspecific blockade of opioid receptors has been found to prevent development of behavioral sensitization to ethanol. Whether this effect is achieved through a specific opioid receptor subtype, however, is not clear. The present study investigated, for the first time, the role of specific opioid receptor subtypes in the development of ethanol-(2.5 g/kg/day; six sessions) induced locomotor sensitization in mice. We confirmed previous results showing that the nonspecific antagonism of opioid receptors (naltrexone; 0-2 mg/kg) prevented the development of behavioral sensitization to ethanol, an effect attained at doses presumed to occupy only mu opioid receptors. This was confirmed by using the selective mu opioid receptor antagonist CTOP (0-1.5 mg/kg), which also blocked sensitization to ethanol. The selective delta receptor antagonist, naltrindole (0-10 mg/kg), however, did not alter sensitization. We further assessed the role of mu opioid receptors in sensitization to ethanol by exploring the involvement of mu(1), mu(1+2), and mu(3) opioid receptor subtypes. Results of these experiments revealed that the blockade of mu(1) (naloxonazine; 0-30 mg/kg) or mu(3) opioid receptors (3-methoxynaltrexone; 0-6 mg/kg) did not prevent locomotor sensitization to ethanol. Using naloxonazine under treatment conditions that block mu(1+2) opioid receptor subtypes we observed a retarded sensitization. The present data suggest that the concurrent inactivation of all mu opioid receptor subtypes may be required to prevent the neural adaptations underlying the development of behavioral sensitization to ethanol. In addition, these results support previous data suggesting a putative role for the mu opioid receptor endogenous ligand, beta-endorphin, and the hypothalamic arcuate nucleus in ethanol sensitization.

Involvement of protein kinase A in ethanol-induced locomotor activity and sensitization

RATIONALE

Mutant mice lacking the RIIbeta subunit of protein kinase A (regulatory subunit II beta(-/-)) show increased ethanol preference. Recent evidence suggests a relationship between heightened ethanol preference and susceptibility to ethanol-induced locomotor sensitization. It is currently unknown if protein kinase A signaling modulates the stimulant effects and/or behavioral sensitization caused by ethanol administration. To address this question, we examined the effects of repeated ethanol administration on locomotor activity RIIbeta(-/-) and littermate wild-type (RIIbeta(+/+)) mice on multiple genetic backgrounds.

METHODS

Over three consecutive days, mice were given single i.p. saline injections and immediately placed in a locomotor activity apparatus to establish a composite baseline for locomotor activity. Next, mice maintained on a hybrid 129/SvEvxC57BL/6J or pure C57BL/6J genetic background were given 10 i.p. ethanol injections before being placed in the activity apparatus. Each ethanol injection was separated by 3-4 days. To determine if changes in behavior were specific to ethanol injection, naïve mice were tested following repeated daily saline injections. The effects of ethanol injection on locomotor behavior were also assessed using an alternate paradigm in which mice were given repeated ethanol injections in their home cage environment.

RESULTS

Relative to RIIbeta(+/+) mice, RIIbeta(-/-) mice, regardless of genetic background, consistently showed significantly greater ethanol-induced locomotor activation. RIIbeta(-/-) mice also showed increased sensitivity to ethanol-induced locomotor sensitization resulting from repeated administration, an effect that was dependent on genetic background and testing paradigm. Increased locomotor activity by RIIbeta(-/-) mice was specific to ethanol injections, and was not related to altered blood ethanol levels.

CONCLUSIONS

These data provide novel evidence implicating an influence of protein kinase A signaling on ethanol-induced locomotor activity and behavioral sensitization. The observation that RIIbeta(-/-) mice are more sensitive to the effects of repeated ethanol administration suggests that normal protein kinase A signaling limits, or is protective against, the stimulant effects of ethanol and the plastic alterations that underlie behavioral sensitization.

Delay Discounting Predicts Behavioral Sensitization to Ethanol in Outbred WSC Mice

BACKGROUND

Alcoholic individuals discount the value of future rewards more steeply than social drinkers, which is viewed as symptomatic of higher levels of impulsivity. However, the mechanisms underlying this difference are unknown. This study examined 2 hypotheses about the relationship between discounting and ethanol's effects in mice: (1) steep discounters are less sensitive to the initial stimulant-like effects of ethanol and (2) steep discounters exhibit greater behavioral adaptation to stimulant effects with repeated ethanol exposure.

METHODS

An adjusting amount procedure was used to assess discounting as a function of delay in ethanol-naïve genetically heterogeneous WSC mice. Mice chose between a small amount of sucrose solution delivered immediately and 19.5 microL delivered following a delay (0, 2, 4, 8, or 12 seconds, varied between sessions). Within sessions, the amount (microL) of immediate sucrose was adjusted until animals became indifferent between the immediate and specific delayed reward. Hyperbolic discount functions were fitted to quantify the degree of delay discounting. Then, in a within-subjects design over 13 days, mice received a pattern of daily injections of saline or ethanol, and after certain treatments their locomotor activity was assessed for 15 minutes.

RESULTS

Animals with steeper discount functions (greater impulsivity) tended to exhibit less locomotor stimulation on their initial exposure to ethanol. However, steeper discounting was positively associated with increases in locomotor activity after repeated exposure (sensitization), indicating that steep discounters showed higher levels of sensitization to the stimulating effects of ethanol.

CONCLUSIONS

These results suggest 2 behavioral effects of ethanol, associated with an increased risk for alcohol abuse, that are associated with variations in delay discounting.

Repeated ethanol intoxication induces behavioral sensitization in the absence of a sensitized accumbens dopamine response in C57BL/6J and DBA/2J mice

Repeated exposure to drugs of abuse results in an increased sensitivity to their behavioral effects, a phenomena referred to as behavioral sensitization. It has been suggested that the same neuroadaptations underlying behavioral sensitization contribute to the maintenance and reinstatement of addiction. Dysregulation of dopamine (DA) neurotransmission in the mesoaccumbens system is one neuroadaptation that is thought to lead to the compulsive drug-seeking that characterizes addiction. Evidence that sensitization to psychostimulants and opiates is associated with an enhancement of drug-evoked DA levels in the nucleus accumbens has also been obtained. Like other drugs of abuse, the acute administration of ethanol (ETOH) stimulates DA release in this brain region. Moreover, repeated ETOH experience results in an enhanced behavioral response to a subsequent ethanol challenge. Data regarding the influence of repeated ethanol intoxication and withdrawal upon mesoaccumbal DA neurotransmission is limited. Studies examining ETOH-evoked alterations in mesoaccumbal DA neurotransmission as a function of withdrawal duration are lacking. The present experiments quantified basal and ethanol-evoked DA levels 14 days and 24 h following the cessation of a repeated ETOH intoxication protocol, which results in sensitization to the locomotor activating effects of ethanol. Locomotor activity was assessed in parallel groups of animals. Studies were conducted in two mouse strains, C57BL/6J and DBA/2J, which differ in their behavioral responses to ETOH. The results indicate the development of transient tolerance to both ETOH-induced behavioral activation and evoked accumbens DA release at early withdrawal. Moreover, no enhanced DA response to a subsequent ETOH challenge could be demonstrated in ETOH experienced animals 2 weeks after withdrawal, in spite of the observation of clear behavioral sensitization at this time point. These results suggest that, at least in the case of ethanol, sensitization of the DA mesolimbic system may not be necessary for the development of behavioral sensitization.

Patterns of brain activation associated with contextual conditioning to methamphetamine in mice

Classical conditioning is thought to play a key role in addiction. The authors used c-Fos immunohistochemistry to demonstrate a conditioned physiological response to methamphetamine (meth) in mice. Male outbred mice were placed into an environment where they had previously experienced 2 mg/kg meth or saline. The meth-paired mice displayed increased c-Fos in several brain regions, including the nucleus accumbens, prefrontal cortex, orbitofrontal cortex, basolateral amygdala, and bed nucleus of the stria terminalis. No conditioned locomotor activity was observed, but individual activity levels strongly correlated with c-Fos in many regions. A batch effect among immunohistochemical assays was demonstrated. Results implicate specific brain regions in classical conditioning to meth and demonstrate the importance of considering locomotor activity and batch in a c-Fos study.

Behavioral sensitization to ethanol is modulated by environmental conditions, but is not associated with cross-sensitization to allopregnanolone or pentobarbital in DBA/2J mice

RATIONALE

The ability of ethanol to facilitate GABA(A) receptor-mediated transmission may result in GABA(A) receptor alterations during repeated ethanol administration, and lead to dynamic behavioral changes, including sensitization to the locomotor stimulant effect of ethanol. Since alterations in GABA(A) receptors are likely to alter sensitivity to GABAergic drugs such as 3alpha-hydroxy-5alpha-pregnan-20-one (allopregnanolone) and pentobarbital, we determined whether enhanced sensitivity to ethanol was associated with enhanced sensitivity (cross-sensitization) to these drugs. Two procedures that produced differences in the magnitude of expression of ethanol-induced locomotor sensitization were used.

METHODS

After habituation to testing procedures for 2 days, female DBA/2J mice were injected with ethanol or saline for 12 days. On the following day, locomotion was recorded after a challenge injection of ethanol (2 g/kg), allopregnanolone (10 or 17 mg/kg), or pentobarbital (10 or 20 mg/kg). Due to evidence that exposure to the test chambers influenced sensitization, in some experiments, mice were exposed to the test apparatus on the day prior to challenge.

RESULTS

Exposure to the test apparatus prior to drug challenge attenuated the expression of ethanol sensitization, compared with mice without this pre-exposure. Cross-sensitization was not observed to either allopregnanolone or pentobarbital under any condition; however, some groups of repeated ethanol-treated mice displayed tolerance to the initial stimulant effects of allopregnanolone and pentobarbital.

CONCLUSIONS

These studies indicate that behavioral sensitization to ethanol is not associated with cross-sensitization to pentobarbital or allopregnanolone, and that the expression of ethanol sensitization is influenced by the relative novelty of the test chamber. In addition, these results do not support a mechanism in which alterations in the neurosteroid or barbiturate modulatory sites of the GABA(A) receptor are responsible for the expression of sensitization to the locomotor stimulant effects of ethanol.

Sensitivity to ketamine, alone or in combination with ethanol, is altered in mice selectively bred for sensitivity to ethanol's locomotor effects

BACKGROUND

Sensitivity to erthanol's locomotor activating and reinforcing effects may be influenced by some common neural mechanisms. Mice selectively bred in replicate for increased (FAST-1 and FAST-2) and decreased (SLOW-1 and SLOW-2) sensitivity to ethanol's locomotor stimulant effects are useful for investigating the neural substrates of ethanol's effects. Previous studies have suggested that differences in N-methyl-d-aspartate (NMDA) receptors may underlie differences in ethanol-induced locomotion in these mice. This study examined the responses of FAST and SLOW mice to ketamine, a fast-acting NMDA antagonist. In addition, reverse-selected lines (r-FAST-1, r-FAST-2, r-SLOW-1, and r-SLOW-2) were tested as a means of verifying correlations detected in the forward-selected lines. Two initial studies characterized ketamine-induced locomotion in DBA/2J (D2) mice, an inbred strain chosen for its high sensitivity to ethanol-induced locomotion.

METHODS

After a 2- to 3-day period of habituation to test procedures, mice were given intraperitoneal injections of ketamine alone (0, 5, 10, 20, 30, and 60 mg/kg) or in combination with 1 or 2 g/kg ethanol. Locomotor activity was measured for 20 to 30 min in automated activity monitors.

RESULTS

When administered alone, ketamine dose-dependently stimulated the locomotor activity of D2 mice and also reduced the amount of ethanol-induced stimulation. Ketamine stimulated locomotion more in FAST mice than in SLOW mice. Reverse selection abolished these differences, because r-FAST and r-SLOW mice did not differ in their responses to ketamine. Ketamine potentiated ethanol's locomotor effects within FAST mice and potentiated ethanol's locomotor depressant effect within one replicate of SLOW mice.

CONCLUSIONS

We propose that sensitivities to ethanol- and ketamine-induced locomotion are genetically correlated and that the combined effects of ethanol and ketamine in FAST mice reflect a leftward shift in ethanol's biphasic dose-response curve.

Acute and chronic ethanol alter glutamatergic transmission in rat central amygdala: an in vitro and in vivo analysis

The modulation of glutamatergic transmission by ethanol may contribute to ethanol intoxication, reinforcement, tolerance, and dependence. Therefore, we used in vitro electrophysiological and in vivo microdialysis techniques to investigate the effects of acute and chronic ethanol on glutamatergic transmission in the central nucleus of amygdala (CeA). Superfusion of 5-66 mM ethanol decreased compound glutamatergic EPSPs and EPSCs in CeA neurons, with half-maximal inhibition elicited by 14 mM ethanol. Ethanol (44 mM) decreased both non-NMDAR- and NMDAR-mediated EPSPs and EPSCs by 21%. Both the ethanol- and ifenprodil-induced depression of NMDAR-mediated EPSPs and EPSCs was enhanced in rats that received chronic ethanol treatment (CET). Ifenprodil also occluded the ethanol effect, suggesting that NR2B subunit-containing receptors may be involved. With local applications of NMDA, acute ethanol elicited a greater inhibition of NMDA currents in slices taken from CET (47%) compared with naive (30%) animals, suggesting that CET sensitizes NMDA receptors to ethanol. Acute ethanol also reduced paired pulse facilitation of EPSPs and EPSCs only in CET animals, suggesting acute ethanol-induced increase of glutamate release. This finding was supported by in vivo experiments showing that infusion of ethanol (0.1-1 M) via reverse microdialysis significantly increased glutamate release into the CeA dialysate but only after CET. Moreover, baseline CeA glutamate content was significantly higher in CET compared with naive animals. These combined findings suggest that CET and withdrawal lead to neuroadaptations of glutamatergic transmission at both presynaptic and postsynaptic sites in CeA, and glutamatergic synapses in CeA may play an important role in ethanol dependence.