Fyn kinase does not reduce ethanol inhibition of zinc-insensitive NR2A-containing N-methyl-D-aspartate receptors

Synaptic targets: Chronic alcohol actions

Alcohol acts on numerous cellular and molecular targets to regulate neuronal communication within the brain. Chronic alcohol exposure and acute withdrawal generate prominent neuroadaptations at synapses, including compensatory effects on the expression, localization and function of synaptic proteins, channels and receptors. The present article reviews the literature describing the synaptic effects of chronic alcohol exposure and their relevance for synaptic transmission in the central nervous system. This review is not meant to be comprehensive, but rather to highlight the effects that have been observed most consistently and that are thought to contribute to the development of alcohol dependence and the negative aspects of withdrawal. Specifically, we will focus on the major excitatory and inhibitory neurotransmitters in the brain, glutamate and GABA, respectively, and how their neuroadaptations after chronic alcohol exposure contributes to alcohol reinforcement, dependence and withdrawal. This article is part of the Special Issue entitled "Alcoholism".

Correlated changes in NMDA receptor phosphorylation, functional activity, and sedation by chronic ethanol consumption

Alcohol abuse leads to tolerance, dependence, and memory impairments that involve excitatory glutamatergic NMDA synaptic transmission. The NMDA receptor (NMDAR) is known to undergo activity-dependent adaptive functional changes. Since we observed that acute ethanol inhibition of the NMDAR was regulated by protein tyrosine phosphorylation, we investigated the role of protein tyrosine kinases and phosphatases on the NMDAR functions by chronic ethanol treatment. We carried out whole-cell recording, western blotting, and behavioral righting reflex measurements to assess the impact of chronic ethanol treatment on NMDAR function. Our results indicated that these receptors became resistant to the acute ethanol inhibition following chronic ethanol consumption. This resistance occurred without an increase in the NMDAR subunit expression but was associated with decreases in the levels of phospho-Y-1472 NR2B, increases in the levels of STEP33, increases in phospho-p38 mitogen-activated protein kinase (pp38 MAPK), and acquisition of tolerance to the sedative effects of ethanol. These data suggested that altered protein tyrosine phosphorylation of the NMDAR subunits significantly contributes to functional changes of this receptor by chronic ethanol ingestion. Therefore, preservation of the integrity of tyrosine phosphorylation mechanisms of the NMDAR may be important in controlling the progression of alcohol tolerance and dependence.

Ethanol disrupts NMDA receptor and astroglial EAAT2 modulation of Kv2.1 potassium channels in hippocampus

Delayed-rectifier Kv2.1 channels are the principal component of voltage-sensitive K+ currents (I(K)) in hippocampal neurons and are critical regulators of somatodendritic excitability. In a recent study, we demonstrated that surface trafficking and phosphorylation of Kv2.1 channels is modulated by NMDA-type glutamate receptors and that astroglial excitatory amino acid transporters 2 (EAAT2) regulate the coupling of NMDA receptors and Kv2.1 channels. Because ethanol is known to acutely inhibit NMDA receptors, we sought to determine if NMDA receptor and astroglial EAAT2 modulation of Kv2.1 channels is impaired by ethanol in the rodent hippocampus. As expected, bath application of NMDA to hippocampal cultures reduced the size of Kv2.1 clusters and produced a hyperpolarizing shift in the voltage-dependent activation of I(K) that was associated with dephosphorylated Kv2.1 channels. Ethanol, applied acutely, prevented the hyperpolarizing shift in activation of I(K) induced by NMDA and restored Kv2.1 clustering and phosphorylation to near control levels. Ethanol also attenuated the dephosphorylation of Kv2.1 channels produced by the EAAT2 selective inhibitor dihydrokainic acid. These data demonstrate that acute ethanol disrupts changes in Kv2.1 channels that follow NMDA receptor activation and impairs astroglial regulation of the functional coupling between NMDA receptors and Kv2.1 channels.

Ethanol inhibition of recombinant NMDA receptors is not altered by coexpression of CaMKII-alpha or CaMKII-beta

Previous studies have shown that the N-methyl-d-aspartate (NMDA) receptor is an important target for the actions of ethanol in the brain. N-methyl-d-aspartate receptors are glutamate-activated ion channels that are highly expressed in neurons. They are activated during periods of significant glutamatergic synaptic activity and are an important source of the signaling molecule calcium in the postsynaptic spine. Alterations in the function of NMDA receptors by drugs or disease are associated with deficits in motor, sensory and cognitive processes of the brain. Acutely, ethanol inhibits ion flow through NMDA receptors whereas sustained exposure to ethanol can induce compensatory changes in the density and localization of the receptor. Defining factors that govern the acute ethanol sensitivity of NMDA receptors is an important step in understanding how an individual responds to ethanol. In the present study, we investigated the effect of calcium-calmodulin dependent protein kinase II (CaMKII) on the ethanol sensitivity of recombinant NMDA receptors. Calcium-calmodulin dependent protein kinase II is a major constituent of the postsynaptic density and is critically involved in various forms of learning and memory. NMDA receptor subunits were transiently expressed in human embryonic kidney 293 cells along with CaMKII-alpha or CaMKII-beta tagged with the green fluorescent protein. Whole cell currents were elicited by brief exposures to glutamate and were measured using patch-clamp electrophysiology. Neither CaMKII-alpha or CaMKII-beta had any significant effect on the ethanol inhibition of NR1/2A or NR1/2B receptors. Ethanol inhibition was also unaltered by deletion of CaMKII binding domains in NR1 or NR2 subunits or by phospho-site mutants that mimic or occlude CaMKII phosphorylation. Chronic treatment of cortical neurons with ethanol had no significant effect on the expression of CaMKII-alpha or CaMKII-beta. The results of this study suggest that CaMKII is not involved in regulating the acute ethanol sensitivity of NMDA receptors.

Ethanol inhibition of NMDA receptors under conditions of altered protein kinase A activity

N-methyl-D-aspartate receptors (NMDA) are glutamate-activated ligand-gated ion channels that participate in diverse forms of synaptic plasticity as well as glutamate-dependent excitotoxicity. Inhibition of the NMDA receptor function may underlie some of the behavioral actions associated with acute exposure to ethanol. The sensitivity of NMDA receptors to ethanol is influenced by the subunit composition of the receptor and, by association, with certain cytoskeletal proteins. Previous studies have also suggested that phosphorylation may regulate the sensitivity of NMDA receptors to ethanol. In this study, the ethanol inhibition of recombinant NMDA receptor currents was determined under conditions designed to enhance or inhibit the activity of protein kinase A (PKA). Human embryonic kidney 293 (HEK293) cells were transfected with cDNAs encoding NMDA subunits and channel activity was monitored with whole-cell patch-clamp electrophysiology. Under control recording conditions, ethanol (100 mM) inhibited NR1/2A and NR1/2B receptor currents by approximately 25-30%. The degree of ethanol inhibition was not affected or was slightly enhanced under conditions designed to enhance PKA activity. This included treatment of cells with cAMP analogs, inclusion of phosphatase inhibitors or purified PKA in the pipette filling solution, co-expression of catalytically active PKA, expression of the NR1 PKA-site phosphorylation site mimic (S897D) or by co-expression of the PKA scaffolding protein yotiao or the dopamine D(1) receptor. Ethanol inhibition of NR1/2A and NR1/2B receptors was not altered when PKA effects were suppressed, either by co-expression of a PKI inhibitory peptide or the phosphorylation-deficient NR1 mutants (S897A, S896A, S896A/S897A). In addition, ethanol inhibition of NMDA-induced currents in cultured cortical or hippocampal neurons was not affected by modulators of PKA. These results suggest that PKA does not appear to play a major role in determining the acute ethanol sensitivity of NMDA receptors.