Tyrosine Dephosphorylation and Ethanol Inhibition of N-Methyl-d-aspartate Receptor Function

Abstract

NMDA1 receptors are ionotropic glutamate receptors that mediate calcium and sodium entry into neurons. In the CA1 region of the hippocampus, activation of these receptors is known to be required for induction of long term potentiation (LTP), a cellular process proposed by many as a possible mechanism of memory formation. Aberrant NMDA receptor function can result in cell death and may have implications in pathophysiologic disorders such as Parkinson's disease and epilepsy as well as age-related dementias like Alzheimer's disease (7-11). NMDA receptors are heteromeric assemblies of NR1 and NR2 subunits. The NR1 family is composed of a single gene, which can be alternatively spliced to generate eight theoretical splice variants (12). The NR2 family includes four genes, each encoding a unique subunit: NR2A, NR2B, NR2C, and NR2D (13). A third gene family, NR3, has been discovered, but the function of these subunits is not well understood. Whereas the NR1 subunit is required for a functional channel, differential incorporation of NR2 subunits regulates receptor function. The NR2 subunits, particularly NR2A and NR2B, have been shown to influence the sensitivity of NMDA receptors to ethanol block in cultured neurons as wells as transfected HEK293 cells and oocytes (14 -17). More recent studies have indicated that NR1 subunits may also play a key role in mediating the effects of ethanol on channel activity (18 -20).Ethanol is known to have widespread effects on the central nervous system. Ethanol inhibits transmission at the NMDAR subtype of glutamatergic excitatory synapses and enhances inhibitory GABAergic synaptic transmission. In addition, we and others have demonstrated that ethanol blocks LTP in the hippocampus (6,(21)(22)(23). Given that NMDA receptors are both required for induction of LTP and inhibited by ethanol, it is likely that inhibition of LTP by ethanol in the hippocampus is due, at least in part, to inhibition of NMDA receptors. The molecular mechanisms of this inhibition however, remain unknown.One hypothesis that we favor is that ethanol may inhibit the NMDAR by reducing its phosphorylation. Indeed, ethanol has been shown to enhance protein-tyrosine phosphatase activity and also inhibit receptor tyrosine kinase activity (24, 25). Moreover, previous studies suggest that Fyn, a member of the Src family of tyrosine kinases, may be a key player in regulating the sensitivity of NMDARs to ethanol in the hippocampus (26 -28

Alcohol inhibition of the NMDA receptor function, long-term potentiation, and fear learning requires striatal-enriched protein tyrosine phosphatase

“…We investigated whether dephosphorylation of NMDARs still occurs in STEP KO mice following ethanol treatment. In agreement with previous studies in hippocampal slices from rats (13,20), ethanol produced a significant decrease in overall NR2B tyrosine phosphorylation in hippocampal slices from WT mice (Fig. 4A) (−25.98 ± 12.14%; P < 0.01).…”

“…TAT-Myc was used as a control because it was necessary to identify any potential effects of intracellular injection either TAT or the protein portion of a fusion protein on NMDAR currents. The ethanol inhibition in both control and TAT-Myc neurons was similar to that seen in previous reports (13,20). In contrast, CA1 neurons pretreated intracellularly with 30 nM STEP C/S did not show ethanol inhibition of NMDAR EPSCs (Fig.…”

“…It has previously been shown that activation of members of the Src-family of kinases (SFKs) enhances NMDAR currents (12). Previous studies have demonstrated a correlation between ethanol inhibition of NMDAR function and dephosphorylation of tyrosine residues on the NR2A and NR2B subunits (13). In particular, ethanol was found to reduce the phosphorylation of a site on the NR2B subunit Tyr 1472 that has been shown to regulate endocytosis and function of NMDARs (14,15).…”

“…In particular, ethanol was found to reduce the phosphorylation of a site on the NR2B subunit Tyr 1472 that has been shown to regulate endocytosis and function of NMDARs (14,15). Inhibition of protein tyrosine phosphatase activity using a nonspecific inhibitor, bpV(phen), significantly reduced ethanol inhibition of NMDAR field excitatory postsynaptic potentials (fEPSPs), suggesting that ethanol may inhibit NMDARs via activation of a tyrosine phosphatase (13). FerraniKile et al independently provided additional support for this hypothesis by demonstrating that ethanol decreased NMDAR tyrosine phosphorylation in the cortex (16).…”

Ethanol Reduces Neuronal Excitability of Lateral Orbitofrontal Cortex Neurons Via a Glycine Receptor Dependent Mechanism

“…However, unlike NMDA currents in those reports that were recorded from central amygdala and BNST, NMDA EPSCs in lOFC neurons and hippocampus (Suvarna et al, 2005) are significantly inhibited by EtOH even in the presence of Ro-25-6981 that by itself reduces the amplitude of NMDA evoked responses. These findings point out that while subunit composition may influence the overall EtOH sensitivity of NMDA receptors in some brain regions, other factors such as those related to receptor phosphorylation and trafficking (Alvestad et al, 2003;Wu et al, 2011;Yaka et al, 2003) may contribute to EtOH's action on neuronal NMDA receptors.…”