NMDA GluN2A and GluN2B receptors play separate roles in the induction of LTP and LTD in the amygdala and in the acquisition and extinction of conditioned fear

Synaptic plasticity mediated by NMDA glutamate receptors is thought to be a primary mechanism underlying the formation of new memories. Activation of GluN2A NMDA receptor subunits may induce long-term potentiation (LTP), whereas low-frequency stimulation of GluN2B receptors induces long-term depression (LTD). In the present study, we show that blockade of GluN2A, but not GluN2B receptors with NVP-AAM077 and Ro25-6981 respectively, prevented LTP of auditory thalamic inputs to the lateral amygdala. Conversely, LTD induction in this pathway was prevented by blockade of GluN2B, but not GluN2A receptors. As this pathway plays a critical role in the acquisition, retrieval and extinction of a learned auditory-cue fear association, we next examined the effects of blockade of GluN2A and GluN2B receptors on the development and retention of a conditioned fear response. Administration of NVP-AAM077, but not Ro25-6981, prior to conditioning disrupted the expression of conditioned fear 24h later. Conversely, Ro25-6981 but not NVP-AAM077 impaired extinction of the conditioned fear response. These data expand on previous work showing that LTP/D in the thalamic-lateral amygdala pathway is dependent on NMDA receptors, by demonstrating selective roles for GluN2A and GluN2B NMDA receptor subunits in LTP and LTD respectively. Furthermore, GluN2A receptor activation and associated LTP may be involved specifically in the initial formation and/or stabilization of a learned fear response, whereas GluN2B receptor activation and associated LTD may facilitate the suppression of Pavlovian fear responses during extinction. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.     

Direct pharmacological monitoring of the developmental switch in NMDA receptor subunit composition using TCN 213, a GluN2A-selective, glycine-dependent antagonist

BACKGROUND AND PURPOSE

Developmental switches in NMDA receptor subunit expression have been inferred from studies of GluN2 expression levels, changes in kinetics of glutamatergic synaptic currents and sensitivity of NMDA receptor-mediated currents to selective GluN2B antagonists. Here we use TCN 213, a novel GluN2A-selective antagonist to identify the presence of this subunit in functional NMDA receptors in developing cortical neurones.

EXPERIMENTAL APPROACH

Two-electrode voltage-clamp (TEVC) recordings were made from Xenopus laevis oocytes to determine the pharmacological activity of TCN 213 at recombinant NMDA receptors. TCN 213 antagonism was studied in cultures of primary cortical neurones, assessing the NMDA receptor dependency of NMDA-induced excitotoxicity and monitoring developmental switches in NMDA receptor subunit composition.

KEY RESULTS

TCN 213 antagonism of GluN1/GluN2A NMDA receptors was dependent on glycine but independent of glutamate concentrations in external recording solutions. Antagonism by TCN 213 was surmountable and gave a Schild plot with unity slope. TCN 213 block of GluN1/GluN2B NMDA receptor-mediated currents was negligible. In cortical neurones, at a early developmental stage predominantly expressing GluN2B-containing NMDA receptors, TCN 213 failed to antagonize NMDA receptor-mediated currents or to prevent GluN2B-dependent, NMDA-induced excitoxicity. In older cultures (DIV 14) or in neurones transfected with GluN2A subunits, TCN 213 antagonized NMDA-evoked currents. Block by TCN 213 of NMDA currents inversely correlated with block by ifenprodil, a selective GluN2B antagonist.

CONCLUSIONS AND IMPLICATIONS

TCN 213 selectively blocked GluN1/GluN2A over GluN1/GluN2B NMDA receptors allowing direct dissection of functional NMDA receptors and pharmacological profiling of developmental changes in native NMDA receptor subunit composition.     

Dopamine induces a GluN2A-dependent form of long-term depression of NMDA synaptic responses in the nucleus accumbens

Natural rewards and addictive drugs are believed to exert their reinforcing actions by influencing synaptic plasticity in reward-related brain regions such as the nucleus accumbens (NAc). Long-lasting changes in the efficacy of excitatory synaptic transmission in the NAc are critically dependent on efficient interactions between the dopaminergic and the glutamatergic neurotransmitter systems. Potential targets to the actions of dopamine and of addictive drugs include the GluN2 subunits that compose the N-Methyl-d-Aspartate (NMDA) type of glutamate receptors. However, the ability of dopamine to induce synaptic plasticity by modulating specific subunits of the NMDA receptor has not been examined. The present study shows that in the mouse NAc, dopamine produces a long-lasting depression of NMDA responses which occludes long-term depression (LTD) induced by high frequency stimulation (HFS) of glutamatergic fibers. LTD induced by dopamine or by HFS does not involve a change in the subunit composition of NMDA receptors. Although GluN2B contributes to synaptic responses in the NAc and is affected by dopamine, this subunit might not be a direct target to the actions of dopamine. The results, however, identify a critical role for GluN2A in dopamine-induced and HFS-induced synaptic plasticity. This study suggests a possible mechanism of action for dopamine in the regulation of reward-related behaviors.     

Glutamate receptors in preclinical research on Alzheimer's disease: update on recent advances

The cognitive and related symptoms of Alzheimer's disease are mainly attributable to synaptic failure. Here we review recent research on how the Alzheimer's disease amyloid ß-protein (Aß) affects glutamate receptors and fast excitatory synaptic transmission and plasticity of that transmission. l-glutamate, the main excitatory neurotransmitter in the brain, has long been implicated in causing NMDA receptor-mediated excitotoxicity leading to neurodegeneration in the late stages of the disease. However there is now extensive evidence that soluble Aß oligomers disrupt synaptic transmission and especially synaptic plasticity via non-excitotoxic glutamatergic mechanisms. New data highlight the relatively selective involvement of certain glutamate receptor subtypes including GluN2B (NR2B) subunit-containing NMDA receptors and mGlu5 receptors. Aß exerts direct and indirect effects on synaptic plasticity-related glutamate receptor signaling and trafficking between different neuronal compartments. For example, Aß-induced ectopic NMDA and mGlu receptor-mediated signaling coupled with caspase-3 activation may cause inhibition of long-term potentiation and facilitation of long-term depression. Intriguingly, some of the disruptive synaptic actions of Aß have been found to be dependent on endogenous tau located in dendrites or spines. Given the role of glutamatergic transmission in regulating Aß production and release, future therapies targeting glutamate offer the opportunity to remedy both mis-processing of Aß and cellular mechanisms of synaptic failure in early AD.     

Structure-activity relationships for allosteric NMDA receptor inhibitors based on 2-naphthoic acid

Over-activation of N-methyl-d-aspartate (NMDA) receptors is critically involved in many neurological conditions, thus there has been considerable interest in developing NMDA receptor antagonists. We have recently identified a series of naphthoic and phenanthroic acid compounds that allosterically modulate NMDA receptors through a novel mechanism of action. In the present study, we have determined the structure-activity relationships of 18 naphthoic acid derivatives for the ability to inhibit the four GluN1/GluN2(A–D) NMDA receptor subtypes. 2-Naphthoic acid has low activity at GluN2A-containing receptors and yet lower activity at other NMDA receptors. 3-Amino addition, and especially 3-hydroxy addition, to 2-naphthoic acid increased inhibitory activity at GluN1/GluN2C and GluN1/GluN2D receptors. Further halogen and phenyl substitutions to 2-hydroxy-3-naphthoic acid leads to several relatively potent inhibitors, the most potent of which is UBP618 (1-bromo-2-hydroxy-6-phenylnaphthalene-3-carboxylic acid) with an IC₅₀ ～ 2 μM at each of the NMDA receptor subtypes. While UBP618 is non-selective, elimination of the hydroxyl group in UBP618, as in UBP628 and UBP608, leads to an increase in GluN1/GluN2A selectivity. Of the compounds evaluated, specifically those with a 6-phenyl substitution were less able to fully inhibit GluN1/GluN2A, GluN1/GluN2B and GluN1/GluN2C responses (maximal % inhibition of 60–90%). Such antagonists may potentially have reduced adverse effects by not excessively blocking NMDA receptor signaling. Together, these studies reveal discrete structure-activity relationships for the allosteric antagonism of NMDA receptors that may facilitate the development of NMDA receptor modulator agents for a variety of neuropsychiatric and neurological conditions.     

New advances in NMDA receptor pharmacology

N-Methyl-D-aspartate (NMDA) receptors are tetrameric ion channels containing two of four possible GluN2 subunits. These receptors have been implicated for decades in neurological diseases such as stroke, traumatic brain injury, dementia and schizophrenia. The GluN2 subunits substantially contribute to functional diversity of NMDA receptors and are distinctly expressed during development and among brain regions. Thus, subunit-selective antagonists and modulators that differentially target the GluN2 subunit might provide an opportunity to pharmacologically modify the function of select groups of neurons for therapeutic gain. A flurry of clinical, functional and chemical studies have together reinvigorated efforts to identify subunit-selective modulators of NMDA receptor function, resulting in a handful of new compounds that appear to act at novel sites. Here, we review the properties of new emerging classes of subunit-selective NMDA receptor modulators, which we predict will mark the beginning of a productive period of progress for NMDA receptor pharmacology.     

Open-channel blockade is less effective on GluN3B than GluN3A subunit-containing NMDA receptors

The GluN3 subunits of the N-methyl-d-aspartate (NMDA) receptor are known to reduce its Ca(2+) permeability and Mg(2+) sensitivity, however, little is known about their effects on other channel blockers. cRNAs for rat NMDA receptor subunits were injected into Xenopus oocytes and responses to NMDA and glycine were recorded using two electrode voltage clamp. Channel block of receptors containing GluN1-1a/2A, GluN1-1a/2A/3A or GluN1-1a/2A/3B subunits was characterised using Mg(2+), memantine, MK-801, philanthotoxin-343 and methoctramine. IC(50) values for Mg(2+) and memantine increased when receptors contained GluN3A subunits and were further increased when they contained GluN3B, e.g. IC(50)s at -75mV for block of GluN1-1a/2A, GluN1-1a/2A/3A and GluN1-1a/2A/3B receptors respectively were 4.2, 22.4 and 40.1μM for Mg(2+), and 2.5, 7.5 and 17.5μM for memantine. Blocking activity was found to be fully or partially restored when G or R (at the N and N+1 sites respectively) were mutated to N in GluN3A. Thus, the changes cannot be attributed to the loss of the N or N+1 sites alone, but rather involve both sites or residues elsewhere. Block by MK-801 and philanthotoxin-343 was also reduced by GluN3A, most strongly at -100mV but not at -50mV, and by GluN3B at all V(h). Methoctramine was the least sensitive to introduction of GluN3 subunits suggesting a minimal interaction with the N and N+1 sites. We conclude that GluN3B-containing receptors provide increased resistance to channel block compared to GluN3A-containing receptors and this must be due to differences outside the deep pore region (N site and deeper).     

Influence of the intracellular GluN2 C-terminal domain on NMDA receptor function

Excitatory neurotransmission mediated by N-methyl-d-aspartate receptors (NMDARs) is fundamental to learning and memory and, when impaired, causes certain neurological disorders. NMDARs are heterotetrameric complexes composed of two GluN1 [NR1] and two GluN2(A-D) [NR2(A-D)] subunits. The GluN2 subunit is responsible for subunit-specific channel activity and gating kinetics including activation (rise time), peak open probability (peak Po) and deactivation (decay time). The peak Po of recombinant NMDARs was recently described to be controlled by the extracellular GluN2 N-terminal domain (NTD). The cytoplasmic GluN2 C-terminal domain (CTD) could also be involved, because the Po of synaptic NMDARs is reduced in mice expressing C-terminally truncated GluN2 subunits. Here, we examined the role of the GluN2 cytoplasmic tail for NMDAR channel activity and gating in HEK-293 cells. C-terminal truncation of GluN2A, GluN2B or GluN2C did not change the subunit-specific rise time but accelerated the decay time of glutamate-activated currents. Furthermore, the peak Po was reduced by about 50% for GluN2A and GluN2B but not for GluN2C. These results indicated that the CTD of GluN2 has a modulating role in NMDAR gating even in the absence of interacting synaptic proteins. Reduction of peak Po and deactivation kinetics following GluN2 C-terminal truncation were reversed by re-introducing a CTD from a different GluN2 subunit. Thus, the CTDs of GluN2 subunits behave as constitutive structural elements required for normal functioning of NMDARs but are not involved in determining the subunit-specific gating properties of NMDARs.     

Functional evidence for a twisted conformation of the NMDA receptor GluN2A subunit N-terminal domain

Ionotropic glutamate receptors (iGluRs) possess in their extracellular region a large N-terminal domain (NTD) that precedes the agonist-binding domain and displays a clamshell-like architecture similar to the bacterial leucine/isoleucine/valine-binding protein (LIVBP). In addition to their role in receptor assembly, in NMDA receptors (NMDARs), the NTDs of GluN2A and GluN2B subunits form a major site for subunit-specific regulation of ion channel activity, in particular through binding of allosteric modulators such as the synaptically-enriched zinc ion. A recent crystallographic study of the isolated GluN2B NTD has revealed an unexpected twisted closed-cleft conformation caused by a rotation of ∼50° in the interlobe orientation compared with all other known LIVBP-like structures (Karakas et al., 2009). By measuring currents carried by recombinant NMDARs, we now provide functional evidence, through disulfide cross-linking and the identification of a new zinc-binding residue (D283), that the GluN2A NTD of intact GluN1/GluN2A receptors adopts a similar twisted conformation in its closed-cleft state. We propose that the twisted NTD conformation is a distinct structural feature of NMDARs (at least for GluN2A and GluN2B subunits), arguing for interactions between the NTDs in the tetrameric complex that are likely to differ between NMDA and AMPA/kainate receptors.     

Distinct pharmacological and functional properties of NMDA receptors in mouse cortical astrocytes

BACKGROUND AND PURPOSE

Astrocytes of the mouse neocortex express functional NMDA receptors, which are not blocked by Mg²⁺ ions. However, the pharmacological profile of glial NMDA receptors and their subunit composition is far from complete.

EXPERIMENTAL APPROACH

We tested the sensitivity of NMDA receptor-mediated currents to the novel GluN2C/D subunit-selective antagonist UBP141 in mouse cortical astrocytes and neurons. We also examined the effect of memantine, an antagonist that has substantially different affinities for GluN2A/B and GluN2C/d-containing receptors in physiological concentrations of extracellular Mg²⁺.

KEY RESULTS

UBP141 had a strong inhibitory action on NMDA receptor-mediated transmembrane currents in the cortical layer II/III astrocytes with an IC₅₀ of 2.29 µM and a modest inhibitory action on NMDA-responses in the pyramidal neurons with IC₅₀ of 19.8 µM. Astroglial and neuronal NMDA receptors exhibited different sensitivities to memantine with IC₅₀ values of 2.19 and 10.8 µM, respectively. Consistent with pharmacological differences between astroglial and neuronal NMDA receptors, NMDA receptors in astrocytes showed lower Ca²⁺ permeability than neuronal receptors with P_Ca/P_Na ratio of 3.4.

CONCLUSIONS AND IMPLICATIONS

The biophysical and pharmacological properties of the astrocytic NMDA receptors strongly suggest that they have a tri-heteromeric structure composed of GluN1, GluN2C/D and GluN3 subunits. The substantial difference between astroglial and neuronal NMDA receptors in their sensitivity to UBP141 and memantine may enable selective modulation of astrocytic signalling that could be very helpful for elucidating the mechanisms of neuron-glia communications. Our results may also provide the basis for the development of novel therapeutic agents specifically targeting glial signalling.     

NMDA receptor subunit composition determines the polarity of leptin-induced synaptic plasticity

Leptin is a hormone that crosses the blood-brain barrier and regulates numerous CNS functions. The hippocampus in particular is an important site for leptin action. Indeed, leptin markedly influences excitatory synaptic transmission and synaptic plasticity in this brain region. Recent studies indicate that leptin modulation of hippocampal excitatory synaptic transmission is age-dependent however the cellular basis for this is unclear. Here we show that early in development leptin evokes a transient (P11-18) or persistent (P5-8) depression of synaptic transmission, whereas leptin evokes a long lasting increase (LTP) in synaptic strength in adulthood. The synaptic depressions induced by leptin required activation of NMDA receptor GluN2B subunits and the ERK signalling cascade. Conversely, leptin-induced LTP in adult was mediated by GluN2A subunits and involved PI 3-kinase dependent signalling. In addition, low-frequency stimulus (LFS)-evoked LTD occluded the persistent effects of leptin at P5-8 and vice versa. Similarly, synaptically-induced LTP occluded the persistent increase in synaptic transmission induced by leptin, indicating that similar expression mechanisms underlie leptin-induced LTD and LFS-induced LTD at P5-8, and leptin-induced LTP and HFS-induced LTP in adult. These findings have important implications for the role of leptin in hippocampal synaptic function during early neuronal development and in aging.     

Astrocytic NMDA Receptors in the Basolateral Amygdala Contribute to Facilitation of Fear Extinction

BackgroundEnhancement of N-methyl-D-aspartate (NMDA) receptor function using glycine-site agonist D-cycloserine is known to facilitate fear extinction, providing a means to augment cognitive behavioral therapy in anxiety disorders. A novel class of glycine-site agonists has recently been identified, and we have found that the prototype, AICP, is more effective than D-cycloserine in modulating neuronal function.MethodsUsing novel glycine-site agonist AICP, local infusion studies, and genetic models, we elucidated the role of GluN2C-containing receptors in fear extinction.ResultsWe tested the effect of intracerebroventricular injection of AICP on fear extinction and found a robust facilitation of fear extinction. This effect was dependent on GluN2C subunit, consistent with superagonist action of AICP at GluN2C-containing receptors. Local infusion studies in wild-type and GluN2C knockout mice suggested that AICP produces its effect via GluN2C-containing receptors in the basolateral amygdala (BLA). Furthermore, consistent with astrocytic expression of GluN2C subunit in the amygdala, we found that AICP did not facilitate fear extinction in mice with conditional deletion of obligatory GluN1 subunit from astrocytes. Importantly, chemogenetic activation of astrocytes in the basolateral amygdala facilitated fear extinction. Acutely, AICP was found to facilitate excitatory neurotransmission in the BLA via presynaptic GluN2C-dependent mechanism. Immunohistochemical studies suggest that AICP-mediated facilitation of fear extinction involves synaptic insertion of α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor GluA1 subunit.ConclusionThese results identify a unique role of astrocytic NMDA receptors composed of GluN2C subunit in extinction of conditioned fear memory and demonstrate that further development of recently identified superagonists of GluN2C-containing receptors may have utility for anxiety disorders.     

Potentiation of GluN2C/D NMDA Receptor Subtypes in the Amygdala Facilitates the Retention of Fear and Extinction Learning in Mice

NMDA receptors are glutamate receptor ion channels that contribute to synaptic plasticity and are important for many forms of learning and memory. In the amygdala, NMDA receptors are critical for the acquisition, retention, and extinction of classically conditioned fear responses. Although the GluN2B subunit has been implicated in both the acquisition and extinction of conditioned fear, GluN2C-knockout mice show reduced conditioned fear responses. Moreover, D-cycloserine (DCS), which facilitates fear extinction, selectively enhances the activity of GluN2C-containing NMDA receptors. To further define the contribution of GluN2C receptors to fear learning, we infused the GluN2C/GluN2D-selective potentiator CIQ bilaterally into the basolateral amygdala (3, 10, or 30 μg/side) following either fear conditioning or fear extinction training. CIQ both increased the expression of conditioned fear 24 h later and enhanced the extinction of the previously conditioned fear response. These results support a critical role for GluN2C receptors in the amygdala in the consolidation of learned fear responses and suggest that increased activity of GluN2C receptors may underlie the therapeutic actions of DCS.     

Role of GluN2A and GluN2B subunits in the formation of filopodia and secondary dendrites in cultured hippocampal neurons

GluN receptors are heteromers of obligatory GluN1 subunits and GluN2(A-D) subunits. In the present study, we addressed the question whether GluN2A and GluN2B subunits play distinct roles in the formation of filopodia and dendrites during the early development of hippocampal neurons. In hippocampal neurons brought into culture at embryonic day 17, we studied 2–3 days later the effects of N-methyl-d-aspartic acid (NMDA) on the numbers of filopodia, growth cones, and primary as well as secondary dendrites. Antagonists specific for GluN2A and GluN2B subunits were applied together with NMDA. NMDA only induced the formation of filopodia and secondary dendrites. Whereas filopodia were generated within 15 min by NMDA alone, secondary dendrites were only induced by the joint application of NMDA and the Rho kinase inhibitor Y-27632 for 24 h. The GluN2B antagonists ifenprodil and Ro 25-6981 prevented the NMDA-induced formation of filopodia, whereas the GluN2A antagonists NVP-AAM007 and EAA-090 prevented the formation of secondary dendrites. We conclude that both GluN2 subunits have differential roles in dendritic arborization.     

Positive modulation of NMDA receptors by AGN-241751 exerts rapid antidepressant-like effects via excitatory neurons

Dysregulation of the glutamatergic system and its receptors in medial prefrontal cortex (mPFC) has been implicated in major depressive disorder. Recent preclinical studies have shown that enhancing NMDA receptor (NMDAR) activity can exert rapid antidepressant-like effects. AGN-241751, an NMDAR positive allosteric modulator (PAM), is currently being tested as an antidepressant in clinical trials, but the mechanism and NMDAR subunit(s) mediating its antidepressant-like effects are unknown. We therefore used molecular, biochemical, and electrophysiological approaches to examine the cell-type-specific role of GluN2B-containing NMDAR in mediating antidepressant-like behavioral effects of AGN-241751. We demonstrate that AGN-241751 exerts antidepressant-like effects and reverses behavioral deficits induced by chronic unpredictable stress in mice. AGN-241751 treatment enhances NMDAR activity of excitatory and parvalbumin-inhibitory neurons in mPFC, activates Akt/mTOR signaling, and increases levels of synaptic proteins crucial for synaptic plasticity in the prefrontal cortex. Furthermore, cell-type-specific knockdown of GluN2B-containing NMDARs in mPFC demonstrates that GluN2B subunits on excitatory, but not inhibitory, neurons are necessary for antidepressant-like effects of AGN-241751. Together, these results demonstrate antidepressant-like actions of the NMDAR PAM AGN-241751 and identify GluN2B on excitatory neurons of mPFC as initial cellular trigger underlying these behavioral effects.Subject terms: Cellular neuroscience, Prefrontal cortex     

Analysis of neuronal functions in mice lacking the NMDA receptor epsilon 1 subunit

The NMDA subtype of glutamate receptor (GluR) plays an important role in excitatory neurotransmission, synaptic plasticity, brain development, and neurodegeneration. NMDA receptors are inherent high Ca(2+)-permeable channels, which are formed by heteromeric assembly of the GluR zeta 1 subunit (NR1) and any one of four GluR epsilon subunits (GluR epsilon 1-4; NR2A-D). Mice lacking the GluR epsilon 1 subunit exhibited a malfunction of NMDA receptors, as evidenced by reduction of NMDA receptor channel current, hippocampal long-term potentiation, [3H]MK-801 binding, and NMDA-stimulated 45Ca2+ uptake. A biochemical analysis revealed a hyperfunction of dopaminergic and serotonergic neuronal systems in the frontal cortex and striatum of GluR epsilon 1 mutant mice. The enhancement of dopaminergic neuronal activity in the striatum, at least, due to the disinhibition of inhibitory GABAergic neuronal input. GluR epsilon 1 mutant mice showed an increase of locomotor activity in a novel environment attributed to the hyperfunction of the dopaminergic neuronal system, and an impairment of spatial, contextual, and latent learning. These findings provide evidence that NMDA receptors regulate behavior through the modulation of not only glutamatergic but also GABAergic and dopaminergic neuronal systems. Moreover, it is suggested that GluR epsilon 1 mutant mice are useful as an animal model, which is associated with the malfunction of NMDA receptors and hyperfunction of the dopaminergic neuronal system.     

The role of Akt-GSK-3beta signaling and synaptic strength in phencyclidine-induced neurodegeneration.

N-methyl-D-aspartate (NMDA) receptor antagonists such as phencyclidine (PCP) can induce positive and negative symptoms of schizophrenia in humans and related effects in rodents. PCP treatment of developing rats induces apoptotic neurodegeneration and behavioral deficits later in life that mimic some symptoms of schizophrenia. The precise mechanism of PCP-induced neural degeneration is unknown. This study used selective antagonists, siRNA, and Western analysis to investigate the role of the Akt-glycogen synthase kinase-3beta (GSK-3beta) pathway in PCP-induced neuronal apoptosis in both neuronal culture and postnatal day 7 rats. PCP administration in vivo and in vitro reduced the phosphorylation of Akt Ser427 and GSK-3beta Ser9, decreasing Akt activity and increasing GSK-3beta activity. The alteration of Akt-GSK-3beta signaling parallels the temporal profile of caspase-3 activation by PCP. Reducing GSK-3beta activity by application of selective inhibitors or depletion of GSK-3beta by siRNA attenuates caspase-3 activity and blocks PCP-induced neurotoxicity. Moreover, increasing synaptic strength by either activation of L-type calcium channels with BAY K8644 or potentiation of synaptic NMDA receptors with either a low concentration of NMDA or bicuculline plus 4-aminopyridine completely blocks PCP-induced cell death by increasing Akt phosphorylation. These neuroprotective effects are associated with activation of phosphoinositide-3-kinase-Akt signaling, and to a lesser extent, the MAPK signaling pathway. Overall, these data suggest that PCP-induced hypofunction of synaptic NMDA receptors impairs the Akt-GSK-3beta cascade, which is necessary for neuronal survival during development, and that interference with this cascade by PCP or natural factors may contribute to neural pathologies, perhaps including schizophrenia.     

Chronic Intermittent Ethanol Exposure Enhances the Excitability and Synaptic Plasticity of Lateral Orbitofrontal Cortex Neurons and Induces a Tolerance to the Acute Inhibitory Actions of Ethanol

Alcoholism is associated with changes in brain reward and control systems, including the prefrontal cortex. In prefrontal areas, the orbitofrontal cortex (OFC) has been suggested to have an important role in the development of alcohol-abuse disorders and studies from this laboratory demonstrate that OFC-mediated behaviors are impaired in alcohol-dependent animals. However, it is not known whether chronic alcohol (ethanol) exposure alters the fundamental properties of OFC neurons. In this study, mice were exposed to repeated cycles of chronic intermittent ethanol (CIE) exposure to induce dependence and whole-cell patch-clamp electrophysiology was used to examine the effects of CIE treatment on lateral OFC (lOFC) neuron excitability, synaptic transmission, and plasticity. Repeated cycles of CIE exposure and withdrawal enhanced current-evoked action potential (AP) spiking and this was accompanied by a reduction in the after-hyperpolarization and a decrease in the functional activity of SK channels. CIE mice also showed an increase in the AMPA/NMDA ratio, and this was associated with an increase in GluA1/GluA2 AMPA receptor expression and a decrease in GluN2B NMDA receptor subunits. Following CIE treatment, lOFC neurons displayed a persistent long-term potentiation of glutamatergic synaptic transmission following a spike-timing-dependent protocol. Lastly, CIE treatment diminished the inhibitory effect of acute ethanol on AP spiking of lOFC neurons and reduced expression of the GlyT1 transporter. Taken together, these results suggest that chronic exposure to ethanol leads to enhanced intrinsic excitability and glutamatergic synaptic signaling of lOFC neurons. These alterations may contribute to the impairment of OFC-dependent behaviors in alcohol-dependent individuals.     

Differential actions of ethanol and trichloroethanol at sites in the M3 and M4 domains of the NMDA receptor GluN2A (NR2A) subunit

Background and purpose:

Alcohol produces its behavioural effects in part due to inhibition of N-methyl-d-aspartate (NMDA) receptors in the CNS. Previous studies have identified amino acid residues in membrane-associated domains 3 (M3) and 4 (M4) of the NMDA receptor that influence ethanol sensitivity. In addition, in other alcohol-sensitive ion channels, sedative-hypnotic agents have in some cases been shown to act at sites distinct from the sites of ethanol action. In this study, we compared the influence of mutations at these sites on sensitivity to ethanol and trichloroethanol, a sedative-hypnotic agent that is a structural analogue of ethanol.

Experimental approach:

We constructed panels of mutants at ethanol-sensitive positions in the GluN2A (NR2A) NMDA receptor subunit and transiently expressed these mutants in human embryonic kidney 293 cells. We used whole-cell patch-clamp recording to assess the actions of ethanol and trichloroethanol in these mutant NMDA receptors.

Key results:

Ethanol sensitivity of mutants at GluN2A(Ala825) was not correlated with any physicochemical measures tested. Trichloroethanol sensitivity was altered in two of three ethanol-insensitive mutant GluN2A subunits: GluN2A(Phe637Trp) in M3 and GluN2A(Ala825Trp) in M4, but not GluN2A(Met823Trp). Trichloroethanol sensitivity decreased with increasing molecular volume at Phe637 or increasing hydrophobicity at Ala825 and was correlated with ethanol sensitivity at both sites.

Conclusions and implications:

Evidence obtained to date is consistent with a role of GluN2A(Ala825) as a modulatory site for ethanol and trichloroethanol sensitivity, but not as a binding site. Trichloroethanol appears to inhibit the NMDA receptor in a manner similar, but not identical to, that of ethanol.