Role of altered structure and function of NMDA receptors in development of alcohol dependence

Region-specific alterations in the expression and phosphorylation of NMDA receptor subunits in the rat prefrontal cortex and dorsal striatum accompanying behavioral sensitization induced by cocaine and ethanol

Behavioral sensitization is defined as the enhanced behavioral response to drugs of abuse after repeated exposure, which can serve as a behavioral model of addiction. Our previous study demonstrated that behavioral cross-sensitization occurs between cocaine and ethanol, suggesting commonalities between these drugs. N-methyl-d-aspartate (NMDA) receptors play important roles in synaptic plasticity, learning, memory, and addiction-associated behaviors. However, little is known about whether NMDA receptor-mediated signaling regulation is a common feature following behavioral sensitizations induced by cocaine and ethanol. Thus, the present study examined the expression of phospho-S896-NR1, NR2A, and NR2B subunits in the prefrontal cortex and dorsal striatum following reciprocal cross-sensitization between cocaine and ethanol. We also examined the mRNA expression of the NR2A and NR2B subunits. In the ethanol-sensitized state, phosphorylation of NR1 and expression of NR2A and NR2B subunits were increased in both the prefrontal cortex and dorsal striatum. In the cocaine-sensitized state, phosphorylation of NR1 and expression of the NR2A and NR2B subunits were increased in the prefrontal cortex but not in the dorsal striatum. Corresponding changes in mRNA expression were observed in the ethanol-sensitized state but not in the cocaine-sensitized state. Acute treatment with either cocaine or ethanol had no effect on the phosphorylation and expression of NMDA receptor subunits in either the prefrontal cortex or dorsal striatum, regardless of the sensitization state. These results indicate a partially overlapping neural mechanism for cocaine and ethanol that may induce the development of behavioral sensitization.

Brain proteomic atlas of alcohol use disorder in adult males

Alcohol use disorder (AUD) affects transcriptomic, epigenetic and proteomic expression in several organs, including the brain. There has not been a comprehensive analysis of altered protein abundance focusing on the multiple brain regions that undergo neuroadaptations occurring in AUD. We performed a quantitative proteomic analysis using a liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis of human postmortem tissue from brain regions that play key roles in the development and maintenance of AUD, the amygdala (AMG), hippocampus (HIPP), hypothalamus (HYP), nucleus accumbens (NAc), prefrontal cortex (PFC) and ventral tegmental area (VTA). Brain tissues were from adult males with AUD (n = 11) and matched controls (n = 16). Across the two groups, there were >6000 proteins quantified with differential protein abundance in AUD compared to controls in each of the six brain regions. The region with the greatest number of differentially expressed proteins was the AMG, followed by the HYP. Pathways associated with differentially expressed proteins between groups (fold change > 1.5 and LIMMA p < 0.01) were analyzed by Ingenuity Pathway Analysis (IPA). In the AMG, adrenergic, opioid, oxytocin, GABA receptor and cytokine pathways were among the most enriched. In the HYP, dopaminergic signaling pathways were the most enriched. Proteins with differential abundance in AUD highlight potential therapeutic targets such as oxytocin, CSNK1D (PF-670462), GABAB receptor and opioid receptors and may lead to the identification of other potential targets. These results improve our understanding of the molecular alterations of AUD across brain regions that are associated with the development and maintenance of AUD. Proteomic data from this study is publicly available at www.lmdomics.org/AUDBrainProteomeAtlas/ .

Memantine Prevents the WIN 55,212-2 Evoked Cross-Priming of Ethanol-Induced Conditioned Place Preference (CPP)

The activation of the endocannabinoid system controls the release of many neurotransmitters involved in the brain reward pathways, including glutamate. Both endocannabinoid and glutamate systems are crucial for alcohol relapse. In the present study, we hypothesize that N-methyl-D-aspartate (NMDA) glutamate receptors regulate the ability of a priming dose of WIN 55,212-2 to cross-reinstate ethanol-induced conditioned place preference (CPP). To test this hypothesis, ethanol-induced (1.0 g/kg, 10% w/v, i.p.) CPP (unbiased method) was established using male adult Wistar rats. After CPP extinction, one group of animals received WIN 55,212-2 (1.0 and 2.0 mg/kg, i.p.), the cannabinoid receptor 1 (CB1) agonist, or ethanol, and the other group received memantine (3.0 or 10 mg/kg, i.p.), the NMDA antagonist and WIN 55,212-2 on the reinstatement day. Our results showed that a priming injection of WIN 55,212-2 (2.0 mg/kg, i.p.) reinstated (cross-reinstated) ethanol-induced CPP with similar efficacy to ethanol. Memantine (3.0 or 10 mg/kg, i.p.) pretreatment blocked this WIN 55,212-2 effect. Furthermore, our experiments indicated that ethanol withdrawal (7 days withdrawal after 10 days ethanol administration) down-regulated the CNR1 (encoding CB1), GRIN1/2A (encoding GluN1 and GluN2A subunit of the NMDA receptor) genes expression in the prefrontal cortex and dorsal striatum, but up-regulated these in the hippocampus, confirming the involvement of these receptors in ethanol rewarding effects. Thus, our results show that the endocannabinoid system is involved in the motivational properties of ethanol, and glutamate may control cannabinoid induced relapse into ethanol seeking behavior.

Pharmacogenetics of drug dependence: Polymorphisms of genes involved in glutamate neurotransmission

Multiple studies provide evidence to support dysfunction of glutamate neurotransmission in the pathogenesis of drug dependence. Pharmacogenetic investigation of glutamate-related genes has provided further support for the involvement of this neurotransmitter in the risk of, and consequences of, drug abuse and dependence. This paper aims to provide a brief review of these association studies. Findings involving single nucleotide polymorphisms (SNPs) in glutamate receptor genes (GRIN, GRIA) and glutamate transporter genes (SLC1A, SLC17A) are reviewed as potential risk factors. As yet a clear perspective of the functional consequences and interactions of the various reported findings is lacking.

Glutamate Receptor-Mediated Neurotoxicity in a Model of Ethanol Dependence and Withdrawal in Rat Organotypic Hippocampal Slice Cultures

Long-term alcohol use can lead to alterations in brain structure and functions and, in some cases, to neurodegeneration. Several mechanisms have been proposed to explain ethanol (EtOH)-related brain injury. One of the most relevant mechanisms of alcohol-induced neurodegeneration involves glutamatergic transmission, but their exact role is not yet fully understood. We investigated the neurochemical mechanisms underlying the toxicity induced by EtOH dependence and/or withdrawal by exposing rat organotypic hippocampal slices to EtOH (100–300 mM) for 7 days and then incubating the slices in EtOH-free medium for the subsequent 24 h. EtOH withdrawal led to a dose-dependent CA1 pyramidal cell injury, as detected with propidium iodide fluorescence. Electron microscopy of hippocampal slices revealed that not only EtOH withdrawal but also 7 days chronic EtOH exposure elicited signs of apoptotic cell death in CA1 pyramidal cells. These data were supported by electrophysiological recordings of spontaneus Excitatory Post Synaptic Currents (sEPSCs) from CA1 pyramidal cells. The average amplitude of sEPSCs in slices treated with EtOH for 7 days was significantly increased, and even more so during the first 30 min of EtOH withdrawal, suggesting that the initial phase of the neurodegenerative process could be due to an excitotoxic mechanism. We then analyzed the expression levels of presynaptic (vGlut1, vGlut2, CB1 receptor, synaptophysin) and postsynaptic (PSD95, GluN1, GluN2A, GluN2B, GluA1, GluA2, mGluR1 and mGluR5) proteins after 7 days EtOH incubation or after EtOH withdrawal. We found that only GluA1 and mGluR5 expression levels were significantly increased after EtOH withdrawal and, in neuroprotection experiments, we observed that AMPA and mGluR5 antagonists attenuated EtOH withdrawal-induced toxicity. These data suggest that chronic EtOH treatment promotes abnormal synaptic transmission that may lead to CA1 pyramidal cell death after EtOH withdrawal through glutamate receptors and increased excitotoxicity.

Altered relation between lipopolysaccharide-induced inflammatory response and excitotoxicity in rat organotypic hippocampal slice cultures during ethanol withdrawal

BACKGROUND

Ethanol (EtOH) causes neurotoxicity by several mechanisms including excitotoxicity and neuroinflammation, but little is known about the interaction between these mechanisms. Because neuroinflammation is known to enhance excitotoxicity, we hypothesized that neuroinflammation contributes to the enhanced excitotoxicity, which is associated with EtOH withdrawal (EWD). The aim of this study was to evaluate the lipopolysaccharide (LPS)-induced inflammatory response of cultured hippocampal tissue during EWD and its effects on the enhanced N-methyl-d-aspartate (NMDA) receptor-mediated excitotoxicity, which occurs at this time.

METHODS

Using a neonatal organotypic hippocampal slice culture (OHSC) model, we assessed the effects of NMDA and LPS (separately or combined) during EWD after 10 days of EtOH exposure. Neurotoxicity was assessed using propidium iodide uptake, and the inflammatory response was evaluated by measuring the release of tumor necrosis factor (TNF)-alpha (quantified by enzyme-linked immunosorbent assay) and nitric oxide (NO; quantified by the Griess reaction) into culture media. Furthermore, we explored the potential role of the microglial cell type using immortalized BV2 microglia treated with EtOH for 10 days and challenged with LPS during EWD.

RESULTS

As predicted, NMDA-induced toxicity was potentiated by LPS under control conditions. However, during EWD, the reverse was observed and LPS inhibited peak NMDA-induced toxicity. Additionally, LPS-induced release of TNF-alpha and NO during EWD was reduced compared to control conditions. In BV2 microglia, following EtOH exposure, LPS-induced release of NO was reduced, whereas TNF-alpha release was potentiated.

CONCLUSIONS

During EWD following chronic EtOH exposure, OHSC exhibited a desensitized inflammatory response to LPS and the effects of LPS on NMDA toxicity were reversed. This might be explained by a change in microglia to an anti-inflammatory and neuroprotective phenotype. In support, studies on BV2 microglia indicate that EtOH exposure and EWD do alter the response of these cells to LPS, but this cannot fully explain the changes observed in the OHSC. The data suggest that neuroinflammation and excitotoxicity do interact during EWD. However, the interaction is not as simple as we originally proposed. This in turn illustrates the need to assess the extent, importance, and relation of these mechanisms in models of EtOH exposure producing neurotoxicity.

Ethanol inhibits long-term potentiation in hippocampal CA1 neurons, irrespective of lamina and stimulus strength, through neurosteroidogenesis

Ethanol inhibits memory encoding and the induction of long-term potentiation (LTP) in CA1 neurons of the hippocampus. Hippocampal LTP at Schaffer collateral synapses onto CA1 pyramidal neurons has been widely studied as a cellular model of learning and memory, but there is striking heterogeneity in the underlying molecular mechanisms in distinct regions and in response to distinct stimuli. Basal and apical dendrites differ in terms of innervation, input specificity, and molecular mechanisms of LTP induction and maintenance, and different stimuli determine distinct molecular pathways of potentiation. However, lamina or stimulus-dependent effects of ethanol on LTP have not been investigated. Here, we tested the effect of acute application of 60 mM ethanol on LTP induction in distinct dendritic compartments (apical versus basal) of CA1 neurons, and in response to distinct stimulation paradigms (single versus repeated, spaced high frequency stimulation). We found that ethanol completely blocks LTP in apical dendrites, whereas it reduces the magnitude of LTP in basal dendrites. Acute ethanol treatment for just 15 min altered pre- and post-synaptic protein expression. Interestingly, ethanol increases the neurosteroid allopregnanolone, which causes ethanol-dependent inhibition of LTP, more prominently in apical dendrites, where ethanol has greater effects on LTP. This suggests that ethanol has general effects on fundamental properties of synaptic plasticity, but the magnitude of its effect on LTP differs depending on hippocampal sub-region and stimulus strength.

Analysis of variations in the glutamate receptor, N-methyl D-aspartate 2A (GRIN2A) gene reveals their relative importance as genetic susceptibility factors for heroin addiction

The glutamate receptor, N-methyl D-aspartate 2A (GRIN2A) gene that encodes the 2A subunit of the N-methyl D-aspartate (NMDA) receptor was recently shown to be involved in the development of opiate addiction. Genetic polymorphisms in GRIN2A have a plausible role in modulating the risk of heroin addiction. An association of GRIN2A single-nucleotide polymorphisms (SNPs) with heroin addiction was found earlier in African Americans. To identify markers that contribute to the genetic susceptibility to heroin addiction, we examined the potential association between heroin addiction and forty polymorphisms of the GRIN2A gene using the MassARRAY system and GeneScan in this study. The frequency of the (GT)26 repeats (rs3219790) in the heroin addiction group was significantly higher than that in the control group (χ² = 5.360, P = 0.021). The allele frequencies of three polymorphisms (rs1102972, rs1650420, and rs3104703 in intron 3) were strongly associated with heroin addiction (P<0.001, 0.0002, and <0.001, after Bonferroni correction). Three additional SNPs from the same intron (rs1071502, rs6497730, and rs1070487) had nominally significant P values for association (P<0.05), but did not pass the threshold value. Haplotype analysis revealed that the G-C-T-C-C-T-A (block 6) and T-T (block 10) haplotypes of the GRIN2A gene displayed a protective effect (P = <0.001 and 0.003). These findings point to a role for GRIN2A polymorphisms in heroin addiction among the Han Chinese from Shaanxi province, and may be informative for future genetic or neurobiological studies on heroin addiction.

Effects of nitric oxide synthase inhibition in the dorsolateral periaqueductal gray matter on ethanol withdrawal-induced anxiety-like behavior in rats

RATIONALE

Nitric oxide (NO)-mediated transmission in the dorsolateral periaqueductal gray matter (dlPAG) has been involved in the expression of anxiety-like behaviors. Ethanol withdrawal sensitizes the dlPAG and results in increased anxiety-like responses.

OBJECTIVES

The objective of the study was to test the hypothesis that NO in the dlPAG is involved in the expression of ethanol withdrawal-induced anxiety.

METHODS

Male Wistar rats were implanted with guide cannulae aimed at the dlPAG. The animals were forced to consume a liquid diet containing ethanol 6-8 % (v/v) for 15 days as their only source of diet. Six days after surgery and 24 h after ethanol discontinuation, the animals received microinjections of the NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (carboxy-PTIO), nonselective nitric oxide synthase inhibitor N (G)-nitro-L-arginine methyl ester (L-NAME), selective neuronal nitric oxide synthase inhibitor 1-(2-[trifluoromethyl]phenyl) imidazole (TRIM), or selective inducible nitric oxide synthase (iNOS) inhibitor N-([3-(aminomethyl)phenyl]methyl) ethanimidamide dihydrochloride (1400W) into the dlPAG. Ten minutes later, the animals were tested in the light/dark box.

RESULTS

Carboxy-PTIO (1 nmol), L-NAME (200 nmol), TRIM (20 nmol), and 1400W (0.3 and 1 nmol) decreased the anxiogenic-like effects of ethanol withdrawal in rats in the light/dark box test. The NO precursor L-arginine reversed the effects of L-NAME.

CONCLUSIONS

NO production in the dlPAG may play a role in the modulation of ethanol withdrawal-induced anxiety-like behavior in rats. Furthermore, iNOS-mediated NO synthesis in the dlPAG is predominantly involved in the behavioral expression of anxiety-like behavior during ethanol withdrawal.

mGluR1/5 receptor densities in the brains of alcoholic subjects: a whole-hemisphere autoradiography study

Increased glutamatergic neurotransmission and hyper-excitability during alcoholic withdrawal and abstinence are associated with increased risk for relapse, in addition to compensatory changes in the glutamatergic system during chronic alcohol intake. Type 5 metabotropic glutamate receptor (mGlur5) is abundant in brain regions known to be involved in drug reinforcement, yet very little has been published on mGluR1/5 expression in alcoholics. We evaluated the densities of mGluR1/5 binding in the hippocampus and striatum of post-mortem human brains by using [(3)H]Quisqualic acid as a radioligand in whole hemispheric autoradiography of Cloninger type 1 (n=9) and 2 (n=8) alcoholics and healthy controls (n=10). We observed a 30-40% higher mGluR1/5 binding density in the CA2 area of hippocampus in type 1 alcoholics when compared with either type 2 alcoholics or healthy subjects. Although preliminary, and from a relatively small number of subjects from these diagnostic groups, these results suggest that the mGluR1/5 receptors may be increased in type 1 alcoholics in certain brain areas.

Differential phosphorylation of GluN1-MAPKs in rat brain reward circuits following long-term alcohol exposure

The effects of long-term alcohol consumption on the mitogen-activated protein kinases (MAPKs) pathway and N-methyl-D-aspartate-type glutamate receptor 1 (GluN1) subunits in the mesocorticolimbic system remain unclear. In the present study, rats were allowed to consume 6% (v/v) alcohol solution for 28 consecutive days. Locomotor activity and behavioral signs of withdrawal were observed. Phosphorylation and expression of extracellular signal-regulated protein kinase (ERK), c-Jun N-terminal kinase (JNK), p38 protein kinase and GluN1 in the nucleus accumbens, caudate putamen, amygdala, hippocampus and prefrontal cortex of these rats were also measured. Phosphorylation of ERK, but not JNK or p38, was decreased in all five brain regions studied in alcohol-drinking rats. The ratio of phospho/total-GluN1 subunit was reduced in all five brain regions studied. Those results suggest that the long-term alcohol consumption can inhibits GluN1 and ERK phosphorylation, but not JNK or p38 in the mesocorticolimbic system, and these changes may be relevant to alcohol dependence. To differentiate alcohol-induced changes in ERK and GluN1 between acute and chronic alcohol exposure, we have determined levels of phospho-ERK, phospho-GluN1 and total levels of GluN1 after acute alcohol exposure. Our data show that 30 min following a 2.5 g/kg dose of alcohol (administered intragastrically), levels of phospho-ERK are decreased while those of phospho-GluN1 are elevated with no change in total GluN1 levels. At 24 h following the single alcohol dose, levels of phospho-ERK are elevated in several brain regions while there are no differences between controls and alcohol treated animals in phospho-GluN1 or total GluN1. Those results suggest that alcohol may differentially regulate GluN1 function and ERK activation depending on alcohol dose and exposure time in the central nervous system.

Regulation of NMDA receptor subunits after acute ethanol treatment in rat brain

AIMS

Tolerance to ethanol-induced inhibition of N-methyl-D-aspartate receptors (NMDARs) is thought to underlie the acute adaptive mechanisms against ethanol. To explore these compensatory upregulating mechanisms of NMDARs, we investigated the expression and phosphorylation of NMDAR subunits in vivo following an acute ethanol treatment.

METHODS

Male Sprague-Dawley rats were given 4 g/kg ethanol, and the phospho-S896-NR1, NR2A and NR2B subunits of NMDAR were immunoblotted from the cerebral cortex and hippocampus. We also examined the mRNAs and ubiquitinated forms of the NR2A and NR2B subunits.

RESULTS

Acute ethanol treatment increased phospho-S896-NR1 at 30 min in the cerebral cortex and hippocampus, and the increase was maintained until 2 h in the hippocampus. Ethanol increased total NR2A and NR2B expression at 30 min in the cortex and hippocampus, and the NR2A increase was maintained until 2 h in the hippocampus. The increased expression of the NR2A and NR2B subunits was not associated with statistically significant alterations in mRNA expression or protein ubiquitination.

CONCLUSION

Acute ethanol treatment increased NR1 subunit phosphorylation and NR2A and NR2B subunit expression in the cerebral cortex and hippocampus of rats. These effects of ethanol on the NMDAR subunits may underlie the mechanisms that compensate for ethanol-induced inhibition of NMDARs. However, the regulation of NR2A and NR2B in this paradigm is not dependent on transcriptional changes.

Downregulation in the human and mice cerebella after ketamine versus ketamine plus ethanol treatment

To study the deleterious effects of ketamine and the potential interaction effects between ethanol and ketamine on the cerebellum, functional magnetic resonance imaging (fMRI) tests were performed on the habitual ketamine users (n = 3) when they flexed and extended their upper limbs. Another fMRI test was performed on the same participants in which they consumed alcohol (12%, 200 mL) 1 h before the test. Downregulation on the activity of cerebellum was observed and the level of activation in the cerebellum decreased dramatically in habitual ketamine users with alcohol consumption before the test. Further studies were performed by using male ICR mice receiving treatment of ketamine only [30 mg kg(-1) intraperitoneally (i.p.)] or ethanol only everyday (0.5 mL 12% orally) and those with coadministration of the above dosages of ketamine and ethanol for 3 months. Fewer Purkinje cells were observed in the cerebellar sections of ketamine treated mice under silver staining. For TUNEL test, a significant increase in the apoptotic cells were observed in the cerebella of the ketamine treated mice (P = 0.016) and of those with co-administration of ketamine and ethanol (P < 0.001), when compared with the control. A statistical significance (P < 0.001) in two-way ANOVA test indicated that there might be an interactive mechanism between ethanol and ketamine acting on the cerebellum.

N-methyl-d-aspartic acid receptors are altered by stress and alcohol in Wistar-Kyoto rat brain

Previous studies have shown that the Wistar-Kyoto (WKY) rat strain is more sensitive to stressors and consumes significant quantities of alcohol under basal as well as stressful conditions when compared to other strains. Given that the glutamate neurotransmitter system has been implicated in depression and addiction, the goals of the present study were to investigate the effects of stress and stress-alcohol interactions on N-methyl-d-aspartate (NMDA) receptors in the rat brain. Thus this study measured the binding of [(3)H] MK-801 to NMDA receptors in the prefrontal cortex (PFC), caudate putamen (CPu), nucleus accumbens (NAc), hippocampus (HIP) and basolateral amygdala (BLA) in WKY rats in comparison to the Wistar (WIS) rat strain. Our results suggested that while voluntary alcohol consumption did not alter NMDA receptors in the PFC, CPu or NAc in either rat strain, it increased NMDA receptors in the HIP and BLA in both strains. In contrast, chronic stress increased NMDA receptors in the PFC, CPu, NAc in WKY rats but not in WIS rats. Chronic stress also decreased NMDA receptors in the HIP and increased NMDA receptors in the BLA in both strains. Alcohol co-treatment with stress increased NMDA receptors in the PFC, CPu and NAc in WKY rats but not in WIS rats. Interestingly, while alcohol co-treatment did not reverse stress induced decreases in NMDA receptors in the HIP, it reduced the binding of NMDA receptors in the BLA to control levels in both strains. Thus it appears that NMDA receptors in the PFC, CPu and NAc may be more sensitive to the effects of stress and could be implicated in the stress-induced alcohol consumption behavior seen in WKY rats. In contrast, NMDA receptors in the HIP and BLA may reflect an adaptive response and may not be responsible for the stress susceptible phenotype of the WKY rat strain.

Determination of amino acid levels in the rat striatum, after administration of ethanol alone and associated with ketamine, a glutamatergic antagonist

The main goal of this study was to determine the amino acids (glutamate, aspartate, glutamine and tyrosine) levels in the rat striatum, after ethanol administration alone and/or associated with ketamine. In protocol 1 (Et+ketamine-1), ethanol was administered to male Wistar rats until the 7th day, and at the next day the group received only ketamine (25mg/kg, i.p.) up to the 14th day. In protocol 2 (Et+ketamine-2), ethanol was also administered up to the 7th day, and was associated with ketamine from the 8th up to the 14th day. In other groups, animals were treated daily with ethanol (4 g/kg, p.o.), for 7 or 14 days or ketamine daily for 7 days. Controls were administered with distilled water for 7 days. Results showed that, in protocol 1, aspartate (ASP) levels increased after ketamine administration, as compared to the controls. This effect was inhibited in the group Et+ketamine-1. Ethanol (7 days) increased glutamate (GLU) levels, as compared to control, and this effect did not differ significantly from that observed in the ketamine group. When ketamine was administered after the ethanol withdrawal (protocol 1), no alterations in those amino acid concentrations were seen, as compared to the control and ketamine groups. A tendency for increasing GLU levels was observed, after administration of ethanol (14 days) or ketamine alone or associated (protocol 2), when compared to control values. In protocol 2, TYR levels decreased as related to controls and to the 14-day ethanol-treated group. We can assume that ketamine presents only an antagonist effect, in animals pretreated with ethanol, followed by ketamine administered from the 8th day on. This is due to the fact that NMDA receptors are already sensitized, leading to a decrease in these receptors functions and consequently to ASP and GLU releases.