Loss of Ethanol Inhibition of N-Methyl-D-Aspartate Receptor-Mediated Currents and Plasticity of Cerebellar Synapses in Mice Expressing the GluN1(F639A) Subunit

Chemogenetic Perturbation of the Posterior But Not Anterior Cerebellum Reduces Voluntary Ethanol Consumption

The cerebellum communicates with brain areas critically involved in control of goal-directed behaviors including the prefrontal and orbitofrontal cortices and midbrain and basal ganglia structures. In particular, the posterior cerebellum is important for cognitive flexibility and has been implicated in alcohol and drug-related memory. We hypothesized that the cerebellum, through its multiple connections to reward-related brain circuitry, regulates alcohol consumption. To test this, we expressed inhibitory designer receptors exclusively activated by designer drugs (DREADDs) in molecular layer interneurons (MLIs) in anterior (IV-V) or posterior (VI-VIII) cerebellar lobules of male and female mice and activated them during alcohol drinking sessions. In a home-cage drinking paradigm, alcohol consumption was significantly decreased by clozapine-N-oxide (CNO) or deschloroclozapine (DCZ) administration in male mice expressing DREADDs in posterior but not anterior lobules. CNO/DCZ injections did not affect drinking in DREADD expressing female mice or in male mice expressing the control vector. Activation of DREADDs expressed in anterior or posterior lobules had no effect on sucrose or quinine consumption in male or female mice. During operant self-administration sessions, DCZ decreased the number of licks and bouts in male but not female mice expressing DREADDs in posterior lobules with no effect in control vector mice. Performance on an accelerated rotarod was unaffected by chemogenetic manipulation while distance traveled in the open field was decreased by DREADD activation in anterior but not posterior lobules. These results indicate that neuronal activity within the posterior cerebellar cortex plays an important role in the control of alcohol consumption in male mice.

The positive allosteric modulator of NMDA receptors, GNE-9278, blocks the ethanol-induced decrease of excitability in developing retrosplenial cortex neurons from mice

Binge-like exposure to ethanol during the brain growth spurt triggers apoptotic neurodegeneration in multiple brain regions, including the retrosplenial cortex, a brain region that is part of the hippocampal-diencephalic-cingulate memory network. This is mediated, in part, by reduced Ca²⁺ influx through N-methyl-d-aspartate (NMDA) receptors followed by a decrease in the activation of pro-survival genes. Here, we tested whether a positive allosteric modulator of NMDA receptors could counteract the inhibitory effect of ethanol on developing retrosplenial cortex pyramidal neurons. We used patch-clamp electrophysiological techniques in acute slices from postnatal day 6-8 mice to test the effect of the positive allosteric modulator GNE-9278 on ethanol-induced inhibition of NMDA receptor function. GNE-9278 dose-dependently increased the amplitude, decay time, and total charge of NMDA excitatory postsynaptic currents. At a concentration of 5 μmol L^-1 , GNE-9278 significantly reduced the 90 mmol L^-1 ethanol-induced inhibition of NMDA excitatory postsynaptic current amplitude, decay time, and total charge. Current-clamp experiments showed that 5 μmol L^-1 GNE-9278 ameliorated the 90 mmol L^-1 ethanol-induced inhibition of synaptically-evoked action potential firing and compound excitatory postsynaptic potential amplitude. These findings indicate that positive allosteric modulators mitigate ethanol-induced hypofunction of NMDA receptors in developing cerebral cortex neurons, an effect that could ameliorate its pro-apoptotic effects during the late stages of fetal development.

The escalation in ethanol consumption following chronic intermittent ethanol exposure is blunted in mice expressing ethanol-resistant GluN1 or GluN2A NMDA receptor subunits

N-Methyl-D-aspartate receptors (NMDARs) are glutamate-gated ion channels essential for glutamatergic transmission and plasticity. NMDARs are inhibited by acute ethanol and undergo brain region-specific adaptations after chronic alcohol exposure. In previous studies, we reported that knock-in mice expressing ethanol-insensitive GluN1 or GluN2A NMDAR subunits display altered behavioral responses to acute ethanol and genotype-dependent changes in drinking using protocols that do not produce dependence. A key unanswered question is whether the intrinsic ethanol sensitivity of NMDARs also plays a role in determining behavioral adaptations that accompany the development of dependence. To test this, we exposed mice to repeated cycles of chronic intermittent ethanol (CIE) vapor known to produce a robust escalation in ethanol consumption and preference. As expected, wild-type mice showed a significant increase from baseline in ethanol consumption and preference after each of the four weekly CIE cycles. In contrast, ethanol consumption in male GluN2A(A825W) mice was unchanged following cycles 1, 2, and 4 of CIE with a modest increase appearing after cycle 3. Wild-type and GluN2A(A825W) female mice did not show a clear or consistent escalation in ethanol consumption or preference following CIE treatment. In male GluN1(F639A) mice, the increase in ethanol consumption observed with their wild-type littermates was delayed until later cycles of exposure. These results suggest that the acute ethanol sensitivity of NMDARs especially those containing the GluN2A subunit may be a critical factor in the escalation of ethanol intake in alcohol dependence.

Alcohol and IL-6 Alter Expression of Synaptic Proteins in Cerebellum of Transgenic Mice with Increased Astrocyte Expression of IL-6

Recent studies indicate that neuroimmune factors, including the cytokine interleukin-6 (IL-6), play a role in the CNS actions of alcohol. The cerebellum is a sensitive target of alcohol, but few studies have examined a potential role for neuroimmune factors in the actions of alcohol on this brain region. A number of studies have shown that synaptic transmission, and in particular inhibitory synaptic transmission, is an important cerebellar target of alcohol. IL-6 also alters synaptic transmission, although it is unknown if IL-6 targets are also targets of alcohol. This is an important issue because alcohol induces glial production of IL-6, which could then covertly influence the actions of alcohol. The persistent cerebellar effects of both IL-6 and alcohol typically involve chronic exposure and, presumably, altered gene and protein expression. Thus, in the current studies we tested the possibility that proteins involved in inhibitory and excitatory synaptic transmission in the cerebellum are common targets of alcohol and IL-6. We used transgenic mice that express elevated levels of astrocyte produced IL-6 to model persistently elevated expression of IL-6, as would occur in alcohol use disorders, and a chronic intermittent alcohol exposure/withdrawal paradigm (CIE/withdrawal) that is known to produce alcohol dependence. Multiple cerebellar synaptic proteins were assessed by Western blot. Results show that IL-6 and CIE/withdrawal have both unique and common actions that affect synaptic protein expression. These common targets could provide sites for IL-6/alcohol exposure/withdrawal interactions and play an important role in cerebellar symptoms of alcohol use such as ataxia.

Alcohol Intoxication and Cognition: Implications on Mechanisms and Therapeutic Strategies

Binge alcohol drinking is highly prevalent in young adults and results in 30% deaths per year in young males. Binge alcohol drinking or acute alcohol intoxication is a risk factor for developing alcohol use disorder (AUD). Three FDA approved drugs are currently in use as therapy for AUD; however, all of them have contra-indications and limitations. Structural brain imaging studies in alcoholics have shown defects in the brain regions involved in memory, cognition and emotional processing. Positron emission tomography (PET) using radiotracers (e.g., 18FDG) and measuring brain glucose metabolism have demonstrated diagnostic and prognostic utility in evaluating patients with cognitive impairment. Using PET imaging, only a few exclusive human studies have addressed the relationship between alcohol intoxication and cognition. Those studies indicate that alcohol intoxication causes reduction in brain activity. Consistent with prior findings, a recent study by us showed that acute alcohol intoxication reduced brain activity in the cortical and subcortical regions including the temporal lobe consisting the hippocampus. Additionally, we have observed a strong correlation between reduction in metabolic activity and spatial cognition impairment in the hippocampus after binge alcohol exposure. We have also demonstrated the involvement of a stress response protein, cold inducible RNA binding protein (CIRP), as a potential mechanistic mediator in acute alcohol intoxication. In this review, we will first discuss in detail prior human PET imaging studies on alcohol intoxication as well as our recent study on acute alcohol intoxication, and review the existing literature on potential mechanisms of acute alcohol intoxication-induced cognitive impairment and therapeutic strategies to mitigate these impairments. Finally, we will highlight the importance of studying brain regions as part of a brain network in delineating the mechanism of acute alcohol intoxication-induced cognitive impairment to aid in the development of therapeutics for such indication.

Knock-in Mice Expressing an Ethanol-Resistant GluN2A NMDA Receptor Subunit Show Altered Responses to Ethanol

BACKGROUND

N-methyl-D-aspartate receptors (NMDARs) are glutamate-activated, heterotetrameric ligand-gated ion channels critically important in virtually all aspects of glutamatergic signaling. Ethanol (EtOH) inhibition of NMDARs is thought to mediate specific actions of EtOH during acute and chronic exposure. Studies from our laboratory, and others, identified EtOH-sensitive sites within specific transmembrane (TM) domains involved in channel gating as well as those in subdomains of extracellular and intracellular regions of GluN1 and GluN2 subunits that affect channel function. In this study, we characterize for the first time the physiological and behavioral effects of EtOH on knock-in mice expressing a GluN2A subunit that shows reduced sensitivity to EtOH.

METHODS

A battery of tests evaluating locomotion, anxiety, sedation, motor coordination, and voluntary alcohol intake were performed in wild-type mice and those expressing the GluN2A A825W knock-in mutation. Whole-cell patch-clamp electrophysiological recordings were used to confirm reduced EtOH sensitivity of NMDAR-mediated currents in 2 separate brain regions (mPFC and the cerebellum) where the GluN2A subunit is known to contribute to NMDAR-mediated responses.

RESULTS

Male and female mice homozygous for the GluN2A(A825W) knock-in mutation showed reduced EtOH inhibition of NMDAR-mediated synaptic currents in mPFC and cerebellar neurons as compared to their wild-type counterparts. GluN2A(A825W) male but not female mice were less sensitive to the sedative and motor-incoordinating effects of EtOH and showed a rightward shift in locomotor-stimulating effects of EtOH. There was no effect of the mutation on EtOH-induced anxiolysis or voluntary EtOH consumption in either male or female mice.

CONCLUSIONS

These findings show that expression of EtOH-resistant GluN2A NMDARs results in selective and sex-specific changes in the behavioral sensitivity to EtOH.

Involvement of metabotropic glutamate receptor 5 in ethanol regulation of NMDA receptor activity in rat substantia gelatinosa neurons

AIMS

Glutamatergic receptors are important targets of ethanol. Intake of ethanol may produce analgesic effects. The present study examined the effects of ethanol on the activity of ionotropic glutamate receptors in spinal cord substantia gelatinosa (SG) neurons, critical neurons involved in nociceptive transmission.

MAIN METHODS

Whole-cell recordings were made from SG neurons of the lumbar spinal cord slices from 15 to 20-day-old rats. Ethanol and glutamate receptor agonists or antagonists were applied by superfusion.

KEY FINDING

Ethanol (50 and 100 mM) applied by superfusion for 5 min dose-dependently decreased the amplitude of evoked excitatory postsynaptic potential in SG neurons. Superfusion of ethanol (100 mM) for 15 min consistently inhibited NMDA- or AMPA-induced depolarizations in SG neurons. Ethanol (100 mM) also inhibited the depolarizations induced by glutamate. However, ethanol inhibition of glutamate-induced responses significantly decreased at 10-15 min following continuous superfusion, suggesting the development of acute tolerance to the inhibition during prolonged exposure. Application of MPEP hydrochloride (an antagonist of metabotropic glutamate receptor [mGluR] 5) or GF109203X (a protein kinase C [PKC] inhibitor), together with ethanol significantly blocked the tolerance. The inhibition by ethanol of the NMDA-induced, but not AMPA-induced, depolarizations significantly decreased at 15 min during continuous superfusion while ACPD (a mGluR agonist) was co-applied with ethanol.

SIGNIFICANCE

The results suggest that (1) ethanol exposure may inhibit ionotropic glutamate receptor-mediated neurotransmission; (2) regulation of NMDA receptor function by mGluR5/PKC pathways may be involved in the development of the tolerance to ethanol inhibition of glutamate-induced responses during prolonged exposure in SG neurons.

Acute alcohol and cognition: Remembering what it causes us to forget

Addiction has been conceptualized as a specific form of memory that appropriates typically adaptive neural mechanisms of learning to produce the progressive spiral of drug-seeking and drug-taking behavior, perpetuating the path to addiction through aberrant processes of drug-related learning and memory. From that perspective, to understand the development of alcohol use disorders, it is critical to identify how a single exposure to alcohol enters into or alters the processes of learning and memory, so that involvement of and changes in neuroplasticity processes responsible for learning and memory can be identified early. This review characterizes the effects produced by acute alcohol intoxication as a function of brain region and memory neurocircuitry. In general, exposure to ethanol doses that produce intoxicating effects causes consistent impairments in learning and memory processes mediated by specific brain circuitry, whereas lower doses either have no effect or produce a facilitation of memory under certain task conditions. Therefore, acute ethanol does not produce a global impairment of learning and memory, and can actually facilitate particular types of memory, perhaps particular types of memory that facilitate the development of excessive alcohol use. In addition, the effects on cognition are dependent on brain region, task demands, dose received, pharmacokinetics, and tolerance. Additionally, we explore the underlying alterations in neurophysiology produced by acute alcohol exposure that help to explain these changes in cognition and highlight future directions for research. Through understanding the impact that acute alcohol intoxication has on cognition, the preliminary changes potentially causing a problematic addiction memory can better be identified.

The Cerebellar GABAAR System as a Potential Target for Treating Alcohol Use Disorder

In the brain, fast inhibitory neurotransmission is mediated primarily by the ionotropic subtype of the gamma-aminobutyric acid (GABA) receptor subtype A (GABA_AR). It is well established that the brain's GABA_AR system mediates many aspects of neurobehavioral responses to alcohol (ethanol; EtOH). Accordingly, in both preclinical studies and some clinical scenarios, pharmacologically targeting the GABA_AR system can alter neurobehavioral responses to acute and chronic EtOH consumption. However, many of the well-established interactions of EtOH and the GABA_AR system have been identified at concentrations of EtOH ([EtOH]) that would only occur during abusive consumption of EtOH (≥40 mM), and there are still inadequate treatment options for prevention of or recovery from alcohol use disorder (AUD, including abuse and dependence). Accordingly, there is a general acknowledgement that more research is needed to identify and characterize: (1) neurobehavioral targets of lower [EtOH] and (2) associated brain structures that would involve such targets in a manner that may influence the development and maintenance of AUDs.Nearly 15 years ago it was discovered that the GABA_AR system of the cerebellum is highly sensitive to EtOH, responding to concentrations as low as 10 mM (as would occur in the blood of a typical adult human after consuming 1-2 standard units of EtOH). This high sensitivity to EtOH, which likely mediates the well-known motor impairing effects of EtOH, combined with recent advances in our understanding of the role of the cerebellum in non-motor, cognitive/emotive/reward processes has renewed interest in this system in the specific context of AUD. In this chapter we will describe recent advances in our understanding of cerebellar processing, actions of EtOH on the cerebellar GABA_AR system, and the potential relationship of such actions to the development of AUD. We will finish with speculation about how cerebellar specific GABA_AR ligands might be effective pharmacological agents for treating aspects of AUD.