Effects of ethanol on phosphorylation site mutants of recombinant N-methyl-D-aspartate receptors

Neurochemical Evidence of Preclinical and Clinical Reports on Target-Based Therapy in Alcohol Used Disorder

Alcohol use disorder (AUD) is a chronic relapsing disorder, which enforces a person to compulsively seek alcohol, restricting control over alcohol intake leads to emergence of an undesired emotional state during abstinence. There are recent advances for better understanding of neurocircuitry involved in the pathophysiology of AUD. Alcohol interaction with neuronal membrane proteins results in changes in neuronal circuits. It is also linked with the potential medication and their clinical validation concerning their pharmacological targets for alcoholic abstinence. This review covers research work from the past few decades on the therapeutic advances on treatment of alcohol dependence; further detailing the fundamental neurochemical mechanisms after alcohol administration. It also covers interaction of alcohol with GABAergic, glutaminergic, dopaminergic, serotonergic and opioid systems. This review further elaborated the neurobiology of noradrenergic, cholinergic and cannabinoid systems and their interaction with AUD. Elaborative information of potential drug targets under current exploration for AUD treatment with their mechanisms are reported here along with clinical outcomes and the associated side effects.

Effects of Repeated Ethanol Exposures on NMDA Receptor Expression and Locomotor Sensitization in Mice Expressing Ethanol Resistant NMDA Receptors

Evidence from a large number of preclinical studies suggests that chronic exposure to drugs of abuse, such as psychostimulants or ethanol induces changes in glutamatergic transmission in key brain areas associated with reward and control of behavior. These changes include alterations in the expression of ionotropic glutamate receptors including N-methyl-D-aspartate receptors (NMDAR) that are important for regulating neuronal activity and synaptic plasticity. NMDA receptors are inhibited by ethanol and reductions in NMDA-mediated signaling are thought to trigger homestatic responses that limit ethanol's effects on glutamatergic transmission. Following repeated exposures to ethanol, these homeostatic responses may become unstable leading to an altered glutamatergic state that contributes to the escalations in drinking and cognitive deficits observed in alcohol-dependent subjects. An important unanswered question is whether ethanol-induced changes in NMDAR expression are modulated by the intrinsic sensitivity of the receptor to ethanol. In this study, we examined the effects of ethanol on NMDAR subunit expression in cortical (orbitofrontal, medial prefrontal), striatal (dorsal and ventral striatum) and limbic (dorsal hippocampus, basolateral amygdala) areas in mice genetically modified to express ethanol-resistant receptors (F639A mice). These mice have been previously shown to drink more ethanol than their wild-type counterparts and have altered behavioral responses to certain actions of ethanol. Following long-term voluntary drinking, F639A mice showed elevations in GluN2A but not GluN1 or GluN2B expression as compared to wild-type mice. Mice treated with repeated injections with ethanol (2-3.5 g/kg; i.p.) showed changes in NMDAR expression that varied in a complex manner with genotype, brain region, subunit type and exposure protocol all contributing to the observed response. F639A mice, but not wild-type mice, showed enhanced motor activity following repeated ethanol injections and this was associated with differences in NMDAR subunit expression across brain regions thought to be involved in drug sensitization. Overall, while the results of the study suggest that NMDARs with reduced sensitivity to ethanol favor the development of locomotor sensitization, they also show that intrinsic ethanol sensitivity is not the sole determinant underlying changes in NMDAR expression following repeated exposures to ethanol.

Separate intramolecular targets for protein kinase A control N-methyl-D-aspartate receptor gating and Ca2+ permeability

Protein kinase A (PKA) enhances synaptic plasticity in the central nervous system by increasing NMDA receptor current amplitude and Ca(2+) flux in an isoform-dependent yet poorly understood manner. PKA phosphorylates multiple residues on GluN1, GluN2A, and GluN2B subunits in vivo, but the functional significance of this multiplicity is unknown. We examined gating and permeation properties of recombinant NMDA receptor isoforms and of receptors with altered C-terminal domain (CTDs) prior to and after pharmacological inhibition of PKA. We found that PKA inhibition decreased GluN1/GluN2B but not GluN1/GluN2A gating; this effect was due to slower rates for receptor activation and resensitization and was mediated exclusively by the GluN2B CTD. In contrast, PKA inhibition reduced NMDA receptor-relative Ca(2+) permeability (PCa/PNa) regardless of the GluN2 isoform and required the GluN1 CTD; this effect was due primarily to decreased unitary Ca(2+) conductance, because neither Na(+) conductance nor Ca(2+)-dependent block was altered substantially. Finally, we show that both the gating and permeation effects can be reproduced by changing the phosphorylation state of a single residue: GluN2B Ser-1166 and GluN1 Ser-897, respectively. We conclude that PKA effects on NMDA receptor gating and Ca(2+) permeability rely on distinct phosphorylation sites located on the CTD of GluN2B and GluN1 subunits. This separate control of NMDA receptor properties by PKA may account for the specific effects of PKA on plasticity during synaptic development and may lead to drugs targeted to alter NMDA receptor gating or Ca(2+) permeability.

The neurobiology of alcohol consumption and alcoholism: an integrative history

Studies of the neurobiological predisposition to consume alcohol (ethanol) and to transition to uncontrolled drinking behavior (alcoholism), as well as studies of the effects of alcohol on brain function, started a logarithmic growth phase after the repeal of the 18th Amendment to the United States Constitution. Although the early studies were primitive by current technological standards, they clearly demonstrated the effects of alcohol on brain structure and function, and by the end of the 20th century left little doubt that alcoholism is a "disease" of the brain. This review traces the history of developments in the understanding of ethanol's effects on the most prominent inhibitory and excitatory systems of brain (GABA and glutamate neurotransmission). This neurobiological information is integrated with knowledge of ethanol's actions on other neurotransmitter systems to produce an anatomical and functional map of ethanol's properties. Our intent is limited in scope, but is meant to provide context and integration of the actions of ethanol on the major neurobiologic systems which produce reinforcement for alcohol consumption and changes in brain chemistry that lead to addiction. The developmental history of neurobehavioral theories of the transition from alcohol drinking to alcohol addiction is presented and juxtaposed to the neurobiological findings. Depending on one's point of view, we may, at this point in history, know more, or less, than we think we know about the neurobiology of alcoholism.

A novel alcohol-sensitive position in the N-methyl-D-aspartate receptor GluN2A subunit M3 domain regulates agonist affinity and ion channel gating

Abundant evidence supports a role for N-methyl-d-aspartate (NMDA) receptor inhibition in the behavioral actions of ethanol, but the underlying molecular mechanisms have not been fully elucidated. We recently found that clusters of five positions in the third and fourth membrane-associated domains (M3 and M4) at the intersubunit interfaces form putative sites of alcohol action. In the present study, we found that one of these positions, NMDA receptor subunit, GluN2A(F636), can strongly regulate ethanol sensitivity, glutamate potency, and apparent desensitization: ethanol IC50 values, peak (Ip) and steady-state (Iss) glutamate EC50 values, and steady-state to peak current ratio (Iss:Ip) values differed significantly among the mutants tested. Changes in glutamate affinity among the various mutants were not attributable to agonist trapping due to desensitization, as glutamate peak EC50 values were correlated with values of both steady-state EC50 and Iss:Ip. The mean open times determined in selected mutants could be altered up to 4-fold but did not account for the changes in ethanol sensitivity. Ethanol sensitivity was significantly correlated with glutamate EC50 and Iss:Ip values, but the changes in ethanol IC50 among mutants at this position do not appear to be secondary to changes in ion channel kinetics. Substitution of the isomeric amino acids leucine and isoleucine had markedly different effects on ethanol sensitivity, agonist potency, and desensitization, which is consistent with a stringent structural requirement for ion channel modulation by the side chain at this position. Our results indicate that GluN2A(F636) plays an important role in both channel function and ethanol inhibition in NMDA receptors.

Dephosphorylation of GluN2B C-terminal tyrosine residues does not contribute to acute ethanol inhibition of recombinant NMDA receptors

N-methyl-d-aspartate (NMDA) receptors are ion channels activated by the neurotransmitter glutamate and are highly expressed by neurons. These receptors are critical for excitatory synaptic signaling and inhibition of NMDA receptors leads to impaired cognition and learning. Ethanol inhibits NMDA currents at concentrations associated with intoxication and this action may underlie some of the behavioral effects of ethanol. Although numerous sites and mechanisms of action have been tested, the manner in which ethanol inhibits NMDA receptors remains unclear. Recent findings in the literature suggest that ethanol, via facilitation of tyrosine phosphatase activity, may dephosphorylate key tyrosine residues in the C-terminus of GluN2B subunits resulting in diminished channel function. To directly test this hypothesis, we engineered GluN2B mutants that contained phenylalanine in place of tyrosine at three different sites and transiently expressed them with the GluN1 subunit in human embryonic kidney (HEK) cells. Whole-cell patch clamp electrophysiology was used to record glutamate-activated currents in the absence and presence of ethanol (10-600 mM). All mutants were functional and did not differ from one another with respect to current amplitude, steady-state to peak ratio, or magnesium block. Analysis of ethanol dose-response curves showed no significant difference in IC50 values between wild-type receptors and Y1252F, Y1336F, Y1472F or triple Y-F mutants. These findings suggest that dephosphorylation of C-terminal tyrosine residues does not account for ethanol inhibition of GluN2B receptors.

Ethanol effects on N-methyl-D-aspartate receptors in the bed nucleus of the stria terminalis

The extended amygdala is a series of interconnected, embryologically similar series of nuclei in the brain that are thought to play key roles in aspects of alcohol dependence, specifically in stress-induced increases in alcohol-seeking behaviors. Plasticity of excitatory transmission in these and other brain regions is currently an intense area of scrutiny as a mechanism underlying aspects of addiction. N-methyl-D-aspartate (NMDA) receptors (NMDARs) play a critical role in plasticity at excitatory synapses and have been identified as major molecular targets of ethanol. Thus, this article will explore alcohol and NMDAR interactions first at a general level and then focusing within the extended amygdala, in particular on the bed nucleus of the stria terminalis (BNST).

Ethanol inhibition of constitutively open N-methyl-D-aspartate receptors

N-Methyl-D-aspartate (NMDA) receptors gate a slow and calcium-rich component of the postsynaptic glutamate response. Like all ionotropic glutamate receptors, NMDA subunits contain a highly conserved motif (SYTANLAAF) in the transmembrane (TM) 3 domain that is critically involved in channel gating. Mutation of an alanine in this domain (A7; underlined above) results in constitutively open receptors that show reduced sensitivity to several allosteric modulators. In this study, we examined the effects of ethanol, a substance that inhibits NMDA currents via an unknown mechanism, on tonically active NMDA receptors expressed in human embryonic kidney 293 cells. Ethanol (100 mM) inhibited currents from GluN1(A7R)/GluN2A and GluN1(A7R)/GluN2B receptors by approximately 50%, whereas those from GluN1/GluN2B(A7R) receptors were reduced by less than 10%. In cysteine-substituted GluN1 and GluN2 A7 mutants, estimated ethanol IC₅₀ values for agonist-gated currents were 101, 117, 103, and 69 mM for GluN1(A7C)/GluN2A, GluN1(A7C)/GluN2B, GluN1/GluN2A(A7C), and GluN1/GluN2B(A7C) receptors, respectively. After exposure to the thiol-modifying reagent 2-(trimethylammonium)ethyl methanethiosulfonate (MTSET), A7C mutants showed robust agonist-independent currents and reduced sensitivity to ethanol (IC₅₀ values of 371, 256, 715, and 958 mM, respectively, as above). In contrast, cysteine modification of the ligand-binding domain resulted in constitutively open receptors that showed robust ethanol inhibition. Ethanol inhibition of MTSET-treated GluN1(A7C) receptors was further reduced by TM3/TM4 mutations previously shown to reduce ethanol sensitivity of agonist-gated receptors. Overall, these results show that ethanol affects NMDA receptor function at a site distal from agonist binding and appears to exert greater effects via perturbation of GluN2 subunits.

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.