Ifenprodil and ethanol enhance NMDA receptor-dependent long-term depression

Fingolimod administration following hypoxia induced neonatal seizure can restore impaired long-term potentiation and memory performance in adult rats

Neonatal seizures commonly caused by hypoxia can lead to long-term neurological outcomes. Early inflammation plays an important role in the pathology of these outcomes. Therefore, in the current study, we explored the long-term effects of Fingolimod (FTY720), an analog of sphingosine and potentsphingosine 1-phosphate(S1P) receptors modulator, as an anti-inflammatory and neuroprotective agent in attenuating anxiety, memory impairment, and possible alterations in gene expression of hippocampal inhibitory and excitatory receptors following hypoxia-induced neonatal seizure (HINS). Seizure was induced in 24 male and female pups (6 in each experimental group) at postnatal day 10 (P10) by premixed gas (5% oxygen/ 95% nitrogen) in a hypoxic chamber for 15 minutes. Sixty minutes after the onset of hypoxia, FTY720 (0.3 mg/kg) or saline (100 µl) was administered for 12 days (from P10 up to P21). Anxiety-like behavior and hippocampal memory function were assessed at P90 by elevated plus maze (EPM) and novel object recognition (NOR), respectively. Long-term potentiation (LTP) was recorded from hippocampal dentate gyrus region (DG) following stimulation of perforant pathway (PP). In addition, the hippocampal concentration of superoxide dismutase activity (SOD), malondialdehyde (MDA), and thiol as indices of oxidative stress were evaluated. Finally, the gene expression of NR2A subunit of N-Methyl-D-aspartic acid (NMDA) receptor, GluR2 subunit of (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) AMPA receptor and γ2 subunit of γ-Aminobutyric acid (GABA_A) receptor were assessed at P90 by the quantitative real-time PCR. FTY720 significantly reduced later-life anxiety-like behavior, ameliorated object recognition memory and increased the amplitude and slope of the field excitatory postsynaptic potential (fEPSP) in the rats following HINS. These effects were associated with restoration of the hippocampal thiol content to the normal values and the regulatory role of FTY720 in the expression of hippocampal GABA and glutamate receptors subunits. In conclusion, FTY720 could restore the dysregulated gene expression of excitatory and inhibitory receptors. It also increased the reduced hippocampal thiol content, which was accompanied with attenuation of HINS-induced anxiety, reduced the impaired hippocampal related memory, and prevented hippocampal LTP deficits in later life following HINS.

Forebrain GluN2A overexpression impairs fear extinction and NMDAR-dependent long-term depression in the lateral amygdala

N-methyl-d-aspartic acid receptor (NMDAR)-dependent synaptic plasticity at the thalamus-lateral amygdala (T-LA) synapses is related to acquisition and extinction of auditory fear memory. However, the roles of the NMDAR GluN2A subunit in acquisition and extinction of auditory fear memory as well as synaptic plasticity at T-LA synapses remain unclear. Here, using electrophysiologic, molecular biological techniques and behavioral methods, we found that the forebrain specific GluN2A overexpression transgenic (TG) mice exhibited normal acquisition but impaired extinction of auditory fear memory. In addition, in vitro electrophysiological data showed normal basal synaptic transmission and NMDAR-dependent long-term potentiation (LTP) at T-LA synapses, but deficit in NMDAR-dependent long-term depression (LTD) at T-LA synapses in GluN2A TG mice. Consistent with the reduced NMDAR-dependent LTD, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) internalization was also weakened during NMDAR-dependent LTD in GluN2A TG mice. Taken together, our findings for the first time indicate that GluN2A overexpression impairs extinction of auditory fear memory and NMDAR-dependent LTD at T-LA synapses, which further confirms the close relationship between NMDAR-dependent LTD and fear extinction.

Chronic administration of Tat-GluR23Y ameliorates cognitive dysfunction targeting CREB signaling in rats with amyloid beta neurotoxicity

Alzheimer's disease (AD) is behaviorally characterized by memory impairments, and pathologically by amyloid β1-42 (Aβ1-42) plaques and tangles. Aβ binds to excitatory synapses and disrupts their transmission due to dysregulation of the glutamate receptors. Here we hypothesized that chronic inhibition of the endocytosis of AMPA receptors together with GluN2B subunit of NMDA receptors might improve cognition deficit induced by Aβ(1-42) neurotoxicity. Forty male Wistar rats were used in this study and divided into 5 groups: Saline + Saline, Aβ+Saline, Aβ+Ifen (Ifenprodil, 3 nmol /2 weeks), Aβ+GluR23Y (Tat-GluR23Y 3 μmol/kg/2 weeks) and Aβ+Ifen+GluR23Y (same doses and durations). Aβ(1-42) neurotoxicity was induced by intracerebroventricular (ICV) injection of Aβ1-42 (2 μg/μl/side), and then animals received the related treatments for 14 days. Cognitive performance of rats and hippocampal level of cAMP-response element-binding (CREB) were evaluated using Morris Water Maze (MWM), and western blotting respectively. Obtained data from the acquisition trials were analyzed by two way Anova and Student T test. Also one way Analysis of variance (ANOVA) with post hoc Tuckey were used to clarify between groups differences in probe test. The Group receiving Aβ, showed significant cognition deficit (long latency to platform and short total time spent in target quadrant (TTS), parallel with lower level of hippocampal CREB, versus vehicle group. While, Aβ+ GluR23Y exhibited the shortest latency to platform and the longest TTS during the probe test, parallel with the higher hippocampal level of CREB compared with other groups. The present study provides evidence that chronic administration of Tat-GluR23Y; an inhibitor of GluA2-AMPARs endocytosis, successfully restores spatial memory impaired by amyloid beta neurotoxicity targeting CREB signaling pathway.

Phosphorylation of GluN2B subunits of N-methyl-d-aspartate receptors in the frontal association cortex involved in morphine-induced conditioned place preference in mice

Morphine is one of the most abused drugs in the world, which has resulted in serious social problems. The frontal association cortex (FrA) has been shown to play a key role in memory formation and drug addiction. N-Methyl-d-aspartate receptors (NMDARs) are abundant in the prefrontal cortex (PFc) and much evidence indicates that GluN2B-containing NMDARs are involved in morphine-induced conditioned place preference (CPP). However, the function of GluN2B in the FrA during morphine-induced CPP has yet to be fully investigated. In the present work, a CPP animal model was employed to measure the expression of phosphorylated (p-) GluN2B (Serine; Ser 1303) in the FrA and NAc in different phases of morphine-induced CPP. We found that p-GluN2B (Ser 1303) was increased in the FrA during the development and reinstatement phases but unchanged in the extinction phase. The use of ifenprodil, a GluN2B-specific antagonist, to block the activity of GluN2B in the two phases attenuated morphine-induced CPP and reinstatement. Furthermore, ifenprodil also blocked morphine-induced upregulation of p-GluN2B (Ser 1303) in the FrA in both phases. These results indicate that GluN2B-containing NMDARs in the FrA may be involved in the regulation of morphine-induced CPP and reinstatement.

Synaptic Plasticity and its Modulation by Alcohol

Alcohol is one of the oldest pharmacological agents used for its sedative/hypnotic effects, and alcohol abuse and alcohol use disorder (AUD) continues to be major public health issue. AUD is strongly indicated to be a brain disorder, and the molecular and cellular mechanism/s by which alcohol produces its effects in the brain are only now beginning to be understood. In the brain, synaptic plasticity or strengthening or weakening of synapses, can be enhanced or reduced by a variety of stimulation paradigms. Synaptic plasticity is thought to be responsible for important processes involved in the cellular mechanisms of learning and memory. Long-term potentiation (LTP) is a form of synaptic plasticity, and occurs via N-methyl-D-aspartate type glutamate receptor (NMDAR or GluN) dependent and independent mechanisms. In particular, NMDARs are a major target of alcohol, and are implicated in different types of learning and memory. Therefore, understanding the effect of alcohol on synaptic plasticity and transmission mediated by glutamatergic signaling is becoming important, and this will help us understand the significant contribution of the glutamatergic system in AUD. In the first part of this review, we will briefly discuss the mechanisms underlying long term synaptic plasticity in the dorsal striatum, neocortex and the hippocampus. In the second part we will discuss how alcohol (ethanol, EtOH) can modulate long term synaptic plasticity in these three brain regions, mainly from neurophysiological and electrophysiological studies. Taken together, understanding the mechanism(s) underlying alcohol induced changes in brain function may lead to the development of more effective therapeutic agents to reduce AUDs.

Effects of Ethanol on Synaptic Plasticity and NMDA Currents in the Juvenile Rat Dentate Gyrus

Background and Objectives:

We examined how acute ethanol (EtOH) exposure affects long term depression (LTD) in the dentate gyrus (DG) of the hippocampus in juvenile rats. EtOH is thought to directly modulate n-methyl-D-aspartate receptor (NMDAr) currents, which are believed important for LTD induction. LTD in turn is believed to play an important developmental role in the hippocampus by facilitating synaptic pruning.

Methods:

Hippocampal slices (350μm) were obtained at post-natal day (PND) 14, 21, or 28. Field EPSPs (excitatory post-synaptic potential) or whole-cell EPSCs (excitatory post-synaptic conductance) were recorded from the DG (dentate gyrus) in response to medial perforant path activation. Low-frequency stimulation (LFS; 900 pulses; 120 s pulse) was used to induce LTD.

Results:

Whole-cell recordings indicated that EtOH exposure at 50mM did not significantly impact ensemble NMDAr EPSCs in slices obtained from animals in the PND14 or 21 groups, but it reliably produced a modest inhibition in the PND28 group. Increasing the concentration to 100 mM resulted in a modest inhibition of NMDAr EPSCs in all three groups. LTD induction and maintenance was equivalent in magnitude in all three age groups in control conditions, however, and surprisingly, NMDA antagonist AP5 only reliably blocked LTD in the PND21 and 28 age groups. The application of 50 mM EtOH attenuated LTD in all three age groups, however increasing the concentration to 100 mM did not reliably inhibit LTD.

Conclusions:

These results indicate that the effect of EtOH on NMDAr-EPSCs recorded from DGCs is both age and concentration dependent in juveniles. Low concentrations of EtOH can attenuate, but did not block LTD in the DG. The effects of EtOH on LTD do not align well with it’s effects on NNMDA receptors.

Co-treatment of AMPA endocytosis inhibitor and GluN2B antagonist facilitate consolidation and retrieval of memory impaired by β amyloid peptide

BACKGROUND

Glutamate neurotransmission stands as an important issue to minimize memory impairment. We investigated the effects of an inhibitor of α-amino-3-hydroxy-5-methyl-4-isozazole propionic acid receptors (AMPA) endocytosis and GluN2B subunit of N-methyl-d-aspartate receptors (NMDA), either isolated or combined, on memory impairments induced by Amyloid beta1-42 (Aβ).

METHODS

Eighty male Wistar rats were used for two experiments of consolidation and retrieval of memory. Memory impairment was induced by intracerebroventricular (ICV) injection of Aβ1-42 (2 μg/μl), and evaluated using Morris Water Maze (MWM). Each experiment consisted of 5 groups: Saline + Saline, Aβ + Saline, Aβ + Ifenprodil (Ifen, 3 nmol/ICV), Aβ +Tat-GluR23Y (3 µmol/kg/IP), and Aβ1 +Ifen + Tat-GluR23Y. Then, hippocampal cAMP-response element-binding protein (CREB) was measured by western blotting. Data were analyzed by Analysis of variance (ANOVA) repeated measure, and one-way Anova followed by Tukey's post hoc test.

RESULTS

During retrieval, Aβ+ Tat-GluR23Y showed significant improvement in total time spent (TTS) in the target quadrant (p = 0.009), escape latency to a platform (p = 0.008) and hippocampal level of CREB (p = 0.006) compared with Aβ + saline. Also, coadministration of Tat-GluR23Yand Ifen similar to Tat-GluR23Y alone caused significant improvement in TTS (p = 0.014) and latency to platform (p = 0.013). During consolidation, shorter escape latency (p = 0.001), longer TTS (p = 0.002) and higher level of hippocampal CREB were observed in the Aβ + Tat-GluR23Y (p = 0.001) and Aβ+ Tat-GluR23Y + Ifen (p = 0.017), respectively.

CONCLUSION

The present study provides pieces of evidence that inhibition of AMPARs endocytosis using Tat-GluR23Y facilitates memory consolidation and retrieval in Aβ induced memory impairment via the CREB signaling pathway.[Formula: see text].

Lack of Pannexin 1 Alters Synaptic GluN2 Subunit Composition and Spatial Reversal Learning in Mice

Long-term potentiation (LTP) and long-term depression (LTD) are two forms of synaptic plasticity that have been considered as the cellular substrate of memory formation. Although LTP has received considerable more attention, recent evidences indicate that LTD plays also important roles in the acquisition and storage of novel information in the brain. Pannexin 1 (Panx1) is a membrane protein that forms non-selective channels which have been shown to modulate the induction of hippocampal synaptic plasticity. Animals lacking Panx1 or blockade of Pannexin 1 channels precludes the induction of LTD and facilitates LTP. To evaluate if the absence of Panx1 also affects the acquisition of rapidly changing information we trained Panx1 knockout (KO) mice and wild type (WT) littermates in a visual and hidden version of the Morris water maze (MWM). We found that KO mice find the hidden platform similarly although slightly quicker than WT animals, nonetheless, when the hidden platform was located in the opposite quadrant (OQ) to the previous learned location, KO mice spent significantly more time in the previous quadrant than in the new location indicating that the absence of Panx1 affects the reversion of a previously acquired spatial memory. Consistently, we observed changes in the content of synaptic proteins critical to LTD, such as GluN2 subunits of N-methyl-D-aspartate receptors (NMDARs), which changed their contribution to synaptic plasticity in conditions of Panx1 ablation. Our findings give further support to the role of Panx1 channels on the modulation of synaptic plasticity induction, learning and memory processes.

Long-Term Depression Is Independent of GluN2 Subunit Composition

NMDA receptors (NMDARs) mediate both long-term potentiation and long-term depression (LTD) and understanding how a single receptor can initiate both phenomena remains a major question in neuroscience. A prominent hypothesis implicates the NMDAR subunit composition, specifically GluN2A and GluN2B, in dictating the rules of synaptic plasticity. However, studies testing this hypothesis have yielded inconsistent and often contradictory results, especially for LTD. These inconsistent results may be due to challenges in the interpretation of subunit-selective pharmacology and in dissecting out the contributions of differential channel properties versus the interacting proteins unique to GluN2A or GluN2B. In this study, we address the pharmacological and biochemical challenges by using a single-neuron genetic approach to delete NMDAR subunits in conditional knock-out mice. In addition, the recently discovered non-ionotropic nature of NMDAR-dependent LTD allowed the rigorous assessment of unique subunit contributions to NMDAR-dependent LTD while eliminating the variable of differential charge transfer. Here we find that neither the GluN2A nor the GluN2B subunit is strictly necessary for either non-ionotropic or ionotropic LTD.SIGNIFICANCE STATEMENT NMDA receptors are key regulators of bidirectional synaptic plasticity. Understanding the mechanisms regulating bidirectional plasticity will guide development of therapeutic strategies to treat the dysfunctional synaptic plasticity in multiple neuropsychiatric disorders. Because of the unique properties of the NMDA receptor GluN2 subunits, they have been postulated to differentially affect synaptic plasticity. However, there has been significant controversy regarding the roles of the GluN2 subunits in synaptic long term depression (LTD). Using single-neuron knock-out of the GluN2 subunits, we show that LTD requires neither GluN2A nor GluN2B.

Alcohol and the Brain: Neuronal Molecular Targets, Synapses, and Circuits

Ethanol is one of the most commonly abused drugs. Although environmental and genetic factors contribute to the etiology of alcohol use disorders, it is ethanol's actions in the brain that explain (1) acute ethanol-related behavioral changes, such as stimulant followed by depressant effects, and (2) chronic changes in behavior, including escalated use, tolerance, compulsive seeking, and dependence. Our knowledge of ethanol use and abuse thus relies on understanding its effects on the brain. Scientists have employed both bottom-up and top-down approaches, building from molecular targets to behavioral analyses and vice versa, respectively. This review highlights current progress in the field, focusing on recent and emerging molecular, cellular, and circuit effects of the drug that impact ethanol-related behaviors. The focus of the field is now on pinpointing which molecular effects in specific neurons within a brain region contribute to behavioral changes across the course of acute and chronic ethanol exposure.

Chronic intermittent alcohol disrupts the GluN2B-associated proteome and specifically regulates group I mGlu receptor-dependent long-term depression

N-Methyl-d-aspartate receptors (NMDARs) are major targets of both acute and chronic alcohol, as well as regulators of plasticity in a number of brain regions. Aberrant plasticity may contribute to the treatment resistance and high relapse rates observed in alcoholics. Recent work suggests that chronic alcohol treatment preferentially modulates both the expression and subcellular localization of NMDARs containing the GluN2B subunit. Signaling through synaptic and extrasynaptic GluN2B-NMDARs has already been implicated in the pathophysiology of various other neurological disorders. NMDARs interact with a large number of proteins at the glutamate synapse, and a better understanding of how alcohol modulates this proteome is needed. We employed a discovery-based proteomic approach in subcellular fractions of hippocampal tissue from chronic intermittent alcohol (CIE)-exposed C57Bl/6J mice to gain insight into alcohol-induced changes in GluN2B signaling complexes. Protein enrichment analyses revealed changes in the association of post-synaptic proteins, including scaffolding, glutamate receptor and PDZ-domain binding proteins with GluN2B. In particular, GluN2B interaction with metabotropic glutamate (mGlu)1/5 receptor-dependent long-term depression (LTD)-associated proteins such as Arc and Homer 1 was increased, while GluA2 was decreased. Accordingly, we found a lack of mGlu1/5 -induced LTD while α1 -adrenergic receptor-induced LTD remained intact in hippocampal CA1 following CIE. These data suggest that CIE specifically disrupts mGlu1/5 -LTD, representing a possible connection between NMDAR and mGlu receptor signaling. These studies not only demonstrate a new way in which alcohol can modulate plasticity in the hippocampus but also emphasize the utility of this discovery-based proteomic approach to generate new hypotheses regarding alcohol-related mechanisms.

Afferent Input Selects NMDA Receptor Subtype to Determine the Persistency of Hippocampal LTP in Freely Behaving Mice

The glutamatergic N-methyl-D-aspartate receptor (NMDAR) is critically involved in many forms of hippocampus-dependent memory that may be enabled by synaptic plasticity. Behavioral studies with NMDAR antagonists and NMDAR subunit (GluN2) mutants revealed distinct contributions from GluN2A- and GluN2B-containing NMDARs to rapidly and slowly acquired memory performance. Furthermore, studies of synaptic plasticity, in genetically modified mice in vitro, suggest that GluN2A and GluN2B may contribute in different ways to the induction and longevity of synaptic plasticity. In contrast to the hippocampal slice preparation, in behaving mice, the afferent frequencies that induce synaptic plasticity are very restricted and specific. In fact, it is the stimulus pattern and not variations in afferent frequency that determine the longevity of long-term potentiation (LTP) in vivo. Here, we explored the contribution of GluN2A and GluN2B to LTP of differing magnitudes and persistence in freely behaving mice. We applied differing high-frequency stimulation (HFS) patterns at 100 Hz to the hippocampal CA1 region, to induce NMDAR-dependent LTP in wild-type (WT) mice, that endured for <1 h (early (E)-LTP), (LTP, 2–4 h) or >24 h (late (L)-LTP). In GluN2A-knockout (KO) mice, E-LTP (HFS, 50 pulses) was significantly reduced in magnitude and duration, whereas LTP (HFS, 2 × 50 pulses) and L-LTP (HFS, 4 × 50 pulses) were unaffected compared to responses in WT animals. By contrast, pharmacological antagonism of GluN2B in WT had no effect on E-LTP but significantly prevented LTP. E-LTP and LTP were significantly impaired by GluN2B antagonism in GluN2A-KO mice. These data indicate that the pattern of afferent stimulation is decisive for the recruitment of distinct GluN2A and GluN2B signaling pathways that in turn determine the persistency of hippocampal LTP. Whereas brief bursts of patterned stimulation preferentially recruit GluN2A and lead to weak and short-lived forms of LTP, prolonged, more intense, afferent activation recruits GluN2B and leads to robust and persistent LTP. These unique signal-response properties of GluN2A and GluN2B enable qualitative differentiation of information encoding in hippocampal synapses.

Multiple roles of GluN2B-containing NMDA receptors in synaptic plasticity in juvenile hippocampus

In the CA1 area of the hippocampus N-methyl-d-aspartate receptors (NMDARs) mediate the induction of long-term depression (LTD), short-term potentiation (STP) and long-term potentiation (LTP). All of these forms of synaptic plasticity can be readily studied in juvenile hippocampal slices but the involvement of particular NMDAR subunits in the induction of these different forms of synaptic plasticity is currently unclear. Here, using NVP-AAM077, Ro 25-6981 and UBP145 to target GluN2A-, 2B- and 2D-containing NMDARs respectively, we show that GluN2B-containing NMDARs (GluN2B) are involved in the induction of LTD, STP and LTP in slices prepared from P14 rat hippocampus. A concentration of Ro (1 μM) that selectively blocks GluN2B-containing diheteromers is able to block LTD. It also inhibits a component of STP without affecting LTP. A higher concentration of Ro (10 μM), that also inhibits GluN2A/B triheteromers, blocks LTP. UBP145 selectively inhibits the Ro-sensitive component of STP whereas NVP inhibits LTP. These data are consistent with a role of GluN2B diheretomers in LTD, a role of both GluN2B- and GluN2D- containing NMDARs in STP and a role of GluN2A/B triheteromers in LTP.

This article is part of the Special Issue entitled ‘Ionotropic glutamate receptors’.

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Inhibition of GluN2Bs in P14 is sufficient for blockade of NMDAR-LTD.

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GluN2A and GluN2D subunits are not required for the induction of LTD.

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Induction of STP involves GluN2B and GluN2D subunits.

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Induction of LTP depends on GluN2A/2B triheteromers.

Using In Vitro Electrophysiology to Screen Medications: Accumbal Plasticity as an Engram of Alcohol Dependence

The nucleus accumbens (NAc) is a central component of the mesocorticolimbic reward system. Increasing evidence strongly implicates long-term synaptic neuroadaptations in glutamatergic excitatory activity of the NAc shell and/or core medium spiny neurons in response to chronic drug and alcohol exposure. Such neuroadaptations likely play a critical role in the development and expression of drug-seeking behaviors. We have observed unique cell-type-specific bidirectional changes in NAc synaptic plasticity (metaplasticity) following acute and chronic intermittent ethanol exposure. Other investigators have also previously observed similar metaplasticity in the NAc following exposure to psychostimulants, opiates, and amazingly, even following an anhedonia-inducing experience. Considering that the proteome of the postsynaptic density likely contains hundreds of biochemicals, proteins and other components and regulators, we believe that there is a large number of potential molecular sites through which accumbal metaplasticity may be involved in chronic alcohol abuse. Many of our companion laboratories are now engaged in identifying and screening medications targeting candidate genes and its products previously linked to maladaptive alcohol phenotypes. We hypothesize that if manipulation of such target genes and their products change NAc plasticity, then that observation constitutes an important validation step for the development of novel therapeutics to treat alcohol dependence.

Low Doses of Ethanol Enhance LTD-like Plasticity in Human Motor Cortex

Humans liberally use ethanol for its facilitating effects on social interactions but its effects on central nervous system function remain underexplored. We have recently described that very low doses of ethanol abolish long-term potentiation (LTP)-like plasticity in human cortex, most likely through enhancement of tonic inhibition [Lücke et al, 2014, Neuropsychopharmacology 39:1508-18]. Here, we studied the effects of low-dose ethanol on long-term depression (LTD)-like plasticity. LTD-like plasticity was induced in human motor cortex by paired associative transcranial magnetic stimulation (PASLTD), and measured as decreases of motor evoked potential input-output curve (IO-curve). In addition, sedation was measured by decreases in saccade peak velocity (SPV). Ethanol in two low doses (EtOH<10mM, EtOH<20mM) was compared to single oral doses of alprazolam (APZ, 1mg) a classical benzodiazepine, and zolpidem (ZLP, 10 mg), a non-benzodiazepine hypnotic, in a double-blinded randomized placebo-controlled crossover design in ten healthy human subjects. EtOH<10mM and EtOH<20mM but not APZ or ZLP enhanced the PASLTD-induced LTD-like plasticity, while APZ and ZLP but not EtOH<10mM or EtOH<20mM decreased SPV. Non-sedating low doses of ethanol, easily reached during social drinking, enhance LTD-like plasticity in human cortex. This effect is most likely explained by the activation of extrasynaptic α4-subunit containing gamma-aminobutyric type A receptors by low-dose EtOH, resulting in increased tonic inhibition. Findings may stimulate cellular research on the role of tonic inhibition in regulating excitability and plasticity of cortical neuronal networks.

Nampt is required for long-term depression and the function of GluN2B subunit-containing NMDA receptors

Nicotinamide adenine dinucleotide (NAD(+)) is an essential coenzyme/cosubstrate for many biological processes in cellular metabolism. The rate-limiting step in the major pathway of mammalian NAD(+) biosynthesis is mediated by nicotinamide phosphoribosyltransferase (Nampt). Previously, we showed that mice lacking Nampt in forebrain excitatory neurons (CamKIIαNampt(-/-) mice) exhibited hyperactivity, impaired learning and memory, and reduced anxiety-like behaviors. However, it remained unclear if these functional effects were accompanied by synaptic changes. Here, we show that CamKIIαNampt(-/-) mice have impaired induction of long-term depression (LTD) in the Schaffer collateral pathway, but normal induction of long-term potentiation (LTP), at postnatal day 30. Pharmacological assessments demonstrated that CamKIIαNampt(-/-) mice also display dysfunction of synaptic GluN2B (NR2B)-containing N-methyl-d-aspartate receptors (NMDARs) prior to changes in NMDAR subunit expression. These results support a novel, important role for Nampt-mediated NAD(+) biosynthesis in LTD and in the function of GluN2B-containing NMDARs.

Recent progress in understanding subtype specific regulation of NMDA receptors by G Protein Coupled Receptors (GPCRs)

G Protein Coupled Receptors (GPCRs) are the largest family of receptors whose ligands constitute nearly a third of prescription drugs in the market. They are widely involved in diverse physiological functions including learning and memory. NMDA receptors (NMDARs), which belong to the ionotropic glutamate receptor family, are likewise ubiquitously expressed in the central nervous system (CNS) and play a pivotal role in learning and memory. Despite its critical contribution to physiological and pathophysiological processes, few pharmacological interventions aimed directly at regulating NMDAR function have been developed to date. However, it is well established that NMDAR function is precisely regulated by cellular signalling cascades recruited downstream of G protein coupled receptor (GPCR) stimulation. Accordingly, the downstream regulation of NMDARs likely represents an important determinant of outcome following treatment with neuropsychiatric agents that target selected GPCRs. Importantly, the functional consequence of such regulation on NMDAR function varies, based not only on the identity of the GPCR, but also on the cell type in which relevant receptors are expressed. Indeed, the mechanisms responsible for regulating NMDARs by GPCRs involve numerous intracellular signalling molecules and regulatory proteins that vary from one cell type to another. In the present article, we highlight recent findings from studies that have uncovered novel mechanisms by which selected GPCRs regulate NMDAR function and consequently NMDAR-dependent plasticity.

Acute and chronic effects of ethanol on learning-related synaptic plasticity

Alcoholism is associated with acute and long-term cognitive dysfunction including memory impairment, resulting in substantial disability and cost to society. Thus, understanding how ethanol impairs cognition is essential for developing treatment strategies to dampen its adverse impact. Memory processing is thought to involve persistent, use-dependent changes in synaptic transmission, and ethanol alters the activity of multiple signaling molecules involved in synaptic processing, including modulation of the glutamate and gamma-aminobutyric acid (GABA) transmitter systems that mediate most fast excitatory and inhibitory transmission in the brain. Effects on glutamate and GABA receptors contribute to ethanol-induced changes in long-term potentiation (LTP) and long-term depression (LTD), forms of synaptic plasticity thought to underlie memory acquisition. In this paper, we review the effects of ethanol on learning-related forms of synaptic plasticity with emphasis on changes observed in the hippocampus, a brain region that is critical for encoding contextual and episodic memories. We also include studies in other brain regions as they pertain to altered cognitive and mental function. Comparison of effects in the hippocampus to other brain regions is instructive for understanding the complexities of ethanol's acute and long-term pharmacological consequences.

GluN2A and GluN2B subunit-containing NMDA receptors in hippocampal plasticity

N-Methyl-d-aspartate receptor (NMDAR)-dependent synaptic plasticity is a strong candidate to mediate learning and memory processes that require the hippocampus. This plasticity is bidirectional, and how the same receptor can mediate opposite changes in synaptic weights remains a conundrum. It has been suggested that the NMDAR subunit composition could be involved. Specifically, one subunit composition of NMDARs would be responsible for the induction of long-term potentiation (LTP), whereas NMDARs with a different subunit composition would be engaged in the induction of long-term depression (LTD). Unfortunately, the results from studies that have investigated this hypothesis are contradictory, particularly in relation to LTD. Nevertheless, current evidence does suggest that the GluN2B subunit might be particularly important for plasticity and may make a synapse bidirectionally malleable. In particular, we conclude that the presence of GluN2B subunit-containing NMDARs at the postsynaptic density might be a necessary, though not a sufficient, condition for the strengthening of individual synapses. This is owing to the interaction of GluN2B with calcium/calmodulin-dependent protein kinase II (CaMKII) and is distinct from its contribution as an ion channel.

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).

Impaired glutamate recycling and GluN2B-mediated neuronal calcium overload in mice lacking TGF-β1 in the CNS

Transforming growth factor β1 (TGF-β1) is a pleiotropic cytokine expressed throughout the CNS. Previous studies demonstrated that TGF-β1 contributes to maintain neuronal survival, but mechanistically this effect is not well understood. We generated a CNS-specific TGF-β1-deficient mouse model to investigate the functional consequences of TGF-β1-deficiency in the adult mouse brain. We found that depletion of TGF-β1 in the CNS resulted in a loss of the astrocyte glutamate transporter (GluT) proteins GLT-1 (EAAT2) and GLAST (EAAT1) and decreased glutamate uptake in the mouse hippocampus. Treatment with TGF-β1 induced the expression of GLAST and GLT-1 in cultured astrocytes and enhanced astroglial glutamate uptake. Similar to GLT-1-deficient mice, CNS-TGF-β1-deficient mice had reduced brain weight and neuronal loss in the CA1 hippocampal region. CNS-TGF-β1-deficient mice showed GluN2B-dependent aberrant synaptic plasticity in the CA1 area of the hippocampus similar to the glutamate transport inhibitor DL-TBOA and these mice were highly sensitive to excitotoxic injury. In addition, hippocampal neurons from TGF-β1-deficient mice had elevated GluN2B-mediated calcium signals in response to extrasynaptic glutamate receptor stimulation, whereas cells treated with TGF-β1 exhibited reduced GluN2B-mediated calcium signals. In summary, our study demonstrates a previously unrecognized function of TGF-β1 in the CNS to control extracellular glutamate homeostasis and GluN2B-mediated calcium responses in the mouse hippocampus.

Synaptic effects induced by alcohol

Ethanol (EtOH) has effects on numerous cellular molecular targets, and alterations in synaptic function are prominent among these effects. Acute exposure to EtOH activates or inhibits the function of proteins involved in synaptic transmission, while chronic exposure often produces opposing and/or compensatory/homeostatic effects on the expression, localization, and function of these proteins. Interactions between different neurotransmitters (e.g., neuropeptide effects on release of small molecule transmitters) can also influence both acute and chronic EtOH actions. Studies in intact animals indicate that the proteins affected by EtOH also play roles in the neural actions of the drug, including acute intoxication, tolerance, dependence, and the seeking and drinking of EtOH. This chapter reviews the literature describing these acute and chronic synaptic effects of EtOH and their relevance for synaptic transmission, plasticity, and behavior.

Hippocampal long-term depression mediates spatial reversal learning in the Morris water maze

Synaptic plasticity at hippocampal excitatory synapses has been proposed as the cellular mechanism underlying spatial learning and memory. However, most previous studies have focused on the role of long-term potentiation (LTP) in learning and memory, and much less is known about the role of long-term depression (LTD). Here, we report that hippocampal-dependent spatial learning in the Morris water maze facilitated hippocampal CA1 LTD induction in vivo. The LTD can be blocked by systemic application of the selective GluN2B antagonist Ro25-6981 (6 mg/kg, i.p.) or a synthetic peptide Tat-GluA2(3Y) (3 μmol/kg, i.p.) that interferes with the endocytosis of AMPA receptors. In addition, systemic or intrahippocampal administration of these two mechanistically and structurally distinct inhibitors impaired spatial reversal learning of a novel target location, when the hidden platform was moved to the quadrant opposite the initial target location. Notably, acute elevated-platform stress, which facilitates hippocampal LTD induction, enhanced both acquisition and retrieval of spatial reversal memory. The present study demonstrates that reversal learning is impaired by blocking hippocampal LTD, and enhanced by facilitating hippocampal LTD, suggesting that hippocampal LTD may be necessary and sufficient to mediate new information processing. This article is part of a Special Issue entitled 'Cognitive Enhancers'.

Frequency dependency of NMDA receptor-dependent synaptic plasticity in the hippocampal CA1 region of freely behaving mice

Hippocampal synaptic plasticity in the form of long-term potentiation (LTP) and long-term depression (LTD) is likely to enable synaptic information storage in support of memory formation. The mouse brain has been subjected to intensive scrutiny in this regard; however, a multitude of studies has examined synaptic plasticity in the hippocampal slice preparation, whereas very few have addressed synaptic plasticity in the freely behaving mouse. Almost nothing is known about the frequency or N-methyl-D-aspartate receptor (NMDAR) dependency of hippocampal synaptic plasticity in the intact mouse brain. Therefore, in this study, we investigated the forms of synaptic plasticity that are elicited at different afferent stimulation frequencies. We also addressed the NMDAR dependency of this phenomenon. Adult male C57BL/6 mice were chronically implanted with a stimulating electrode into the Schaffer collaterals and a recording electrode into the Stratum radiatum of the CA1 region. To examine synaptic plasticity, we chose protocols that were previously shown to produce either LTP or LTD in the hippocampal slice preparation. Low-frequency stimulation (LFS) at 1 Hz (900 pulses) had no effect on evoked responses. LFS at 3 Hz (ranging from 200 up to 2 × 900 pulses) elicited short-term depression (STD, <45 min). LFS at 3 Hz (1,200 pulses) elicited slow-onset potentiation, high-frequency stimulation (HFS) at 100 Hz (100 or 200 pulses) or at 50 Hz was ineffective, whereas 100 Hz (50 pulses) elicited short-term potentiation (STP). HFS at 100 Hz given as 2 × 30, 2 × 50, or 4 × 50 pulses elicited LTP (>24 h). Theta-burst stimulation was ineffective. Antagonism of the NMDAR prevented STD, STP, and LTP. This study shows for the first time that protocols that effectively elicit persistent synaptic plasticity in the slice preparation elicit distinctly different effects in the intact mouse brain. Persistent LTD could not be elicited with any of the protocols tested. Plasticity responses are NMDAR dependent, suggesting that these phenomena are relevant for hippocampus-dependent learning.

Effects of exposure to low-level lead on spatial learning and memory and the expression of mGluR1, NMDA receptor in different developmental stages of rats

The effect of different periods of lead exposure on deficits of learning and memory is still unclear. In this study, we conduct in vivo experiment to investigate the critical stages when lead induced neurotoxicity in rats and its underlying mechanisms in some critical stages. Rats were exposed to 0.2% mg/ml lead acetate solution via drinking water during gestation, lactation and ablactation periods. Behavior deficits were found in gestation and lactation. N-Methyl-d-aspartate (NMDA) receptor subunit 2A (NR2A) increased during gestation both in hippocampus and cerebral cortex compared to the control group; in all treatment groups NR2B decreased in hippocampus and in cerebral cortex during the lactation period. Meanwhile, in hippocampus metabotropic glutamate receptor 1 (mGluR1) decreased during gestation and lactation periods but increased during the ablactation period. These observations suggest that exposure to lead in gestation and lactation periods could cause neurobehavioral deficits which extend to adulthood, and lactation was a more sensitive period for lead exposure. Furthermore, the abnormal expression of NMDA receptor 2 (NMDAR 2) subunits and mGluR1 are likely to be associated with the impairment.

Slice orientation and muscarinic acetylcholine receptor activation determine the involvement of N-methyl D-aspartate receptor subunit GluN2B in hippocampal area CA1 long-term depression

Background

The contribution of different GluN2 subunits of the N-methyl D-aspartate (NMDA) receptor to the induction of bidirectional hippocampal synaptic plasticity is a controversial topic. As both supporting and refuting evidence for the hypothesis of subunit specialization in opposing directions of plasticity has accumulated since it was first proposed a few years ago, we hypothesize that differences in experimental conditions may have in part contributed to some of the inconsistent results from these studies. Here we investigate the controversial hypothesis that long-term depression (LTD) is preferentially induced by GluN2B-containing NMDA receptors in area CA1 of hippocampal slices.

Results

We find that brain slices from 2-3 week old rats prepared in the sagittal orientation have GluN2B-independent LTD whereas slices prepared in the coronal orientation have GluN2B-dependent LTD. There was no difference between the orientations in the fraction of the NMDAR EPSC sensitive to a GluN2B-selective antagonist, leading us to believe that the intracellular signaling properties of the NMDARs were different in the two preparations. Coronal slices had greater association of LTD-related intracellular signaling protein RasGRF1 with GluN2B relative to sagittal slices. Antagonism of muscarinic acetylcholine receptors (mAChRs) in the sagittal slices returned LTD to a GluN2B-dependent form and increased the association of GluN2B with RasGRF1.

Conclusions

These results suggest a novel form of NMDAR modulation by mAChRs and clarify some disagreement in the literature.

Regulation of long-term plasticity induction by the channel and C-terminal domains of GluN2 subunits

Conventional long-term potentiation (LTP) and long-term depression (LTD) are induced by different patterns of synaptic stimulation, but both forms of synaptic modification require calcium influx through NMDA receptors (NMDARs). A prevailing model (the "calcium hypothesis") suggests that high postsynaptic calcium elevation results in LTP, whereas moderate elevations give rise to LTD. Recently, additional evidence has come to suggest that differential activation of NMDAR subunits also factors in determining which type of plasticity is induced. While the growing amount of data suggest that activation of NMDARs containing specific GluN2 subunits plays an important role in the induction of plasticity, it remains less clear which subunit is tied to which form of plasticity. Additionally, it remains to be determined which properties of the subunits confer upon them the ability to differentially induce long-term plasticity. This review highlights recent studies suggesting differential roles for the subunits, as well as findings that begin to shed light on how two similar subunits may be linked to the induction of opposing forms of plasticity.

Ethanol modulation of synaptic plasticity

Synaptic plasticity in the most general terms represents the flexibility of neurotransmission in response to neuronal activity. Synaptic plasticity is essential both for the moment-by-moment modulation of neural activity in response to dynamic environmental cues and for long-term learning and memory formation. These temporal characteristics are served by an array of pre- and post-synaptic mechanisms that are frequently modulated by ethanol exposure. This modulation likely makes significant contributions to both alcohol abuse and dependence. In this review, I discuss the modulation of both short-term and long-term synaptic plasticity in the context of specific ethanol-sensitive cellular substrates. A general discussion of the available preclinical, animal-model based neurophysiology literature provides a comparison between results from in vitro and in vivo studies. Finally, in the context of alcohol abuse and dependence, the review proposes potential behavioral contributions by ethanol modulation of plasticity.

In vivo chronic intermittent ethanol exposure reverses the polarity of synaptic plasticity in the nucleus accumbens shell

Glutamatergic synaptic plasticity in the nucleus accumbens (NAc) is implicated in response to sensitization to psychomotor-stimulating agents, yet ethanol effects here are undefined. We studied the acute in vitro and in vivo effects of ethanol in medium spiny neurons from the shell NAc subregion of slices of C57BL/6 mice by using whole-cell voltage-clamp recordings of α-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA) excitatory postsynaptic current (EPSCs). Synaptic conditioning (low-frequency stimulation with concurrent postsynaptic depolarization) reliably depressed AMPA EPSCs by nearly 30%; this accumbal long-term depression (LTD) was blocked by a nonselective N-methyl-D-aspartate (NMDA) receptor antagonist (DL-2-amino-5-phosphonovaleric acid) and a selective NMDA receptor 2B antagonist [R-(R*,S*)-α-(4-hydroxyphenyl)-β-methyl-4-(phenylmethyl)-1-piperidine propanol]. Acute ethanol exposure inhibited the depression of AMPA EPSCs differentially with increasing concentrations, but this inhibitory action of ethanol was occluded by a D1-selective dopamine receptor agonist. Ethanol dependence was elicited in C57BL/6 mice by two separate 4-day bouts of chronic intermittent ethanol (CIE) vapor exposure. When assessed 24 h after a single bout of in vivo CIE vapor exposure, NAc LTD was absent, and instead NMDA receptor-dependent synaptic potentiation [long-term potentiation (LTP)] was reliably observed. It is noteworthy that both LTP and LTD were completely absent after an extended withdrawal (72 h) after a single 3-day CIE vapor bout. These observations demonstrate that 1) accumbal synaptic depression is mediated by NR2B receptors, 2) accumbal synaptic depression is highly sensitive to both acute and chronic ethanol exposure, and 3) alterations in this synaptic process may constitute a neural adaptation that contributes to the induction and/or expression of ethanol dependence.

Acute ethanol exposure elevates muscarinic tone in the septohippocampal system

The septohippocampal system has been implicated in the cognitive deficits associated with ethanol consumption, but the cellular basis of ethanol action awaits full elucidation. In the medial septum/diagonal band of Broca (MS/DB), a muscarinic tone, reflective of firing activity of resident cholinergic neurons, regulates that of their noncholinergic, putatively GABAergic, counterparts. Here we tested the hypothesis that ethanol alters this muscarinic tone. The spontaneous firing activity of cholinergic and noncholinergic MS/DB neurons were monitored in acute MS/DB slices from C57Bl/6 mice. Exposing the entire slice to ethanol increased firing in both cholinergic and noncholinergic neurons. However, applying ethanol focally to individual MS/DB neurons increased firing only in cholinergic neurons. The differential outcome suggested different mechanisms of ethanol action on cholinergic and noncholinergic neurons. Indeed, with bath-perfused ethanol, the muscarinic antagonist methyl scopolamine prevented the increase in firing in noncholinergic, but not cholinergic, MS/DB neurons. Thus, the effect on noncholinergic neuronal firing was secondary to ethanol's direct action of acutely increasing muscarinic tone. We propose that the acute ethanol-induced elevation of muscarinic tone in the MS/DB contributes to the altered net flow of neuronal activity in the septohippocampal system that underlies compromised cognitive function.

Effects of postnatal exposure to methylmercury on spatial learning and memory and brain NMDA receptor mRNA expression in rats

The extreme vulnerability of developing nervous system to methylmercury (MeHg) is well documented. Still unclear is the consequence of different postnatal period exposure to MeHg. We investigated the critical postnatal phase when MeHg induced neurotoxicity in rats and the underlying mechanism. Rats were given 5mg/(kg day) methylmercury chloride (MMC) orally on postnatal day (PND) 7, PND14, PND28, and PND60 for consecutive 7 days. A control group was treated with 0.9% sodium chloride solution 5 ml/(kg day) instead. On PND69, spatial learning and memory was evaluated by Morris water maze test. Behavior deficits were found in MMC-treated rats of PND7 and PND14 groups (p<0.01). N-methyl-D-aspartate (NMDA) receptor 2 subunits mRNA expressions were evaluated 3 days after the last administration. In hippocampus, the mRNA expression of NR2A and NR2B decreased, but the NR2C expression increased in PND14 group following MMC-treatment (p<0.01). In cerebral cortex, mRNA expression of NR2A decreased, with NR2C expression elevating in PND14 group following MMC-treatment (p<0.05). These observations suggest that the postnatal exposure to MeHg during PND7-20 could cause neurobehavioral deficits which extend to adulthood. Furthermore, the abnormal expression of NMDAR 2 subunits might associate with the impairment.

Platelet-derived growth factor selectively inhibits NR2B-containing N-methyl-D-aspartate receptors in CA1 hippocampal neurons

Platelet-derived growth factor (PDGF) beta receptor activation inhibits N-methyl-d-aspartate (NMDA)-evoked currents in hippocampal and cortical neurons via the activation of phospholipase Cgamma, PKC, the release of intracellular calcium, and a rearrangement of the actin cytoskeleton. In the hippocampus, the majority of NMDA receptors are heteromeric; most are composed of 2 NR1 subunits and 2 NR2A or 2 NR2B subunits. Using NR2B- and NR2A-specific antagonists, we demonstrate that PDGF-BB treatment preferentially inhibits NR2B-containing NMDA receptor currents in CA1 hippocampal neurons and enhances long-term depression in an NR2B subunit-dependent manner. Furthermore, treatment of hippocampal slices or cultures with PDGF-BB decreases the surface localization of NR2B but not of NR2A subunits. PDGFbeta receptors colocalize to a higher degree with NR2B subunits than with NR2A subunits. After neuronal injury, PDGFbeta receptors and PDGF-BB are up-regulated and PDGFbeta receptor activation is neuroprotective against glutamate-induced neuronal damage in cultured neurons. We demonstrate that the neuroprotective effects of PDGF-BB are occluded by the NR2B antagonist, Ro25-6981, and that PDGF-BB promotes NMDA signaling to CREB and ERK1/2. We conclude that PDGFbetaR signaling, by preferentially targeting NR2B receptors, provides an important mechanism for neuroprotection by growth factors in the central nervous system.

Modulation of ethanol state-dependent learning by dorsal hippocampal NMDA receptors in mice

The possible role of N-methyl-D-aspartate (NMDA) receptors of dorsal hippocampus on ethanol state-dependent learning was studied in adult male mice (Pasteur Institute, Iran). As a model of memory, a single-trial step-down passive avoidance task was used. All animals were bilaterally implanted with cannulae into the CA1 regions of dorsal hippocampi. Results show that intraperitoneal (i.p.) administration of ethanol (0.5 and 1 g/kg) 30 min before training impaired memory performance in animals when tested 24h later. Pretest administration of the same doses of ethanol-induced state-dependent retrieval of the memory acquired under pretraining ethanol (1 g/kg, i.p.) influence. Pretest intra-CA1 microinjection of NMDA (0.001, 0.01, and 0.1 microg/mouse) by itself had no effect on memory retrieval and ethanol-induced amnesia. However, pretest intra-CA1 administration of the same doses of NMDA with an ineffective dose of ethanol (0.25 g/kg, i.p.) significantly restored the retrieval and potentiated ethanol state-dependent learning. On the other hand, pretest administration of a competitive NMDA receptor antagonist D-AP5 (D-(-)-2-Amino-5-phosphonopentanoic acid) (0.01, 0.1, and 1 microg/mouse, intra-CA1) or a noncompetitive NMDA receptor antagonist MK-801 maleate [(5S, 10R)-(+)-5-Methyl-10, 11-dihydro-5H-dibenzo [a, d] cyclohepten-5, 10-imine maleate] (0.25, 0.5, and 1 g/mouse, intra-CA1) 5 min before the administration of ethanol (1 g/kg, i.p.) significantly inhibited ethanol state-dependent learning. Intra-CA1 pretest administration of D-AP5 (0.01, 0.1, and 1 microg/mouse) or MK-801 maleate [5S, 10R)-(+)-5-Methyl-10, 11-dihydro-5H-dibenzo [a, d] cyclohepten-5, 10-imine maleate] (0.25, 0.5, and 1 microg/mouse) alone did not affect memory retention. It may be concluded that dorsal hippocampal NMDA receptors are involved in mediating ethanol state-dependent learning.

Effects of the glucocorticoid antagonist, mifepristone, on the consequences of withdrawal from long term alcohol consumption

BACKGROUND

Studies were carried out to test the hypothesis that administration of a glucocorticoid Type II receptor antagonist, mifepristone (RU38486), just prior to withdrawal from chronic alcohol treatment, would prevent the consequences of the alcohol consumption and withdrawal in mice.

MATERIALS AND METHODS

The effects of administration of a single intraperitoneal dose of mifepristone were examined on alcohol withdrawal hyperexcitability. Memory deficits during the abstinence phase were measured using repeat exposure to the elevated plus maze, the object recognition test, and the odor habituation/discrimination test. Neurotoxicity in the hippocampus and prefrontal cortex was examined using NeuN staining.

RESULTS

Mifepristone reduced, though did not prevent, the behavioral hyperexcitability seen in TO strain mice during the acute phase of alcohol withdrawal (4 hours to 8 hours after cessation of alcohol consumption) following chronic alcohol treatment via liquid diet. There were no alterations in anxiety-related behavior in these mice at 1 week into withdrawal, as measured using the elevated plus maze. However, changes in behavior during a second exposure to the elevated plus maze 1 week later were significantly reduced by the administration of mifepristone prior to withdrawal, indicating a reduction in the memory deficits caused by the chronic alcohol treatment and withdrawal. The object recognition test and the odor habituation and discrimination test were then used to measure memory deficits in more detail, at between 1 and 2 weeks after alcohol withdrawal in C57/BL10 strain mice given alcohol chronically via the drinking fluid. A single dose of mifepristone given at the time of alcohol withdrawal significantly reduced the memory deficits in both tests. NeuN staining showed no evidence of neuronal loss in either prefrontal cortex or hippocampus after withdrawal from chronic alcohol treatment.

CONCLUSIONS

The results suggest mifepristone may be of value in the treatment of alcoholics to reduce their cognitive deficits.

Disruption of AMPA receptor endocytosis impairs the extinction, but not acquisition of learned fear

Synaptic plasticity in the form of long-term potentiation (LTP) plays a critical role in the formation of a Pavlovian fear association. However, the role that synaptic plasticity plays in the suppression of a learned fear response remains to be clarified. Here, we assessed the role that long-term depression (LTD) plays in the acquisition, expression, and extinction of a conditioned fear response. We report that blockade of LTD with a GluR2-derived peptide (Tat-GluR2(3Y); 1.5 micromol/kg, i.v.) that blocks regulated alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA) receptor endocytosis during an initial extinction training session disrupted both the expression and recall of extinction learning. A similar impairment of extinction during training, but not recall, was observed when NMDA receptor-dependent LTD was inhibited through the selective blockade of NMDA NR2B receptors with Ro 25-6981. In contrast, blockade of LTD with Tat-GluR2(3Y) during fear conditioning or during a fear recall test did not effect the expression or recall of either contextual or cue-induced conditioned fear. Similarly, administration of Tat-GluR2(3Y) prior to an extinction recall test did not affect spontaneous recovery or rate of re-extinction in previously extinguished rats. These data demonstrate that AMPA receptor endocytosis does not mediate acquisition or expression of conditioned fear, but may play a role in the extinction of fear memories. Furthermore, these findings suggest that LTD may be a molecular mechanism that facilitates the selective modification of a learned association while leaving intact the ability to form a new memory.

Synaptic plasticity in learning and memory: stress effects in the hippocampus

Synaptic plasticity has often been argued to play an important role in learning and memory. The discovery of long-term potentiation (LTP) and long-term depression (LTD), the two most widely cited cellular models of synaptic plasticity, significantly spurred research in this field. Although correlative evidence suggesting a role for synaptic changes such as those seen in LTP and LTD in learning and memory has been gained in a number of studies, definitive demonstrations of a specific role for either LTP or LTD in learning and memory are lacking. In this review, we discuss a number of recent advancements in the understanding of the mechanisms that mediate LTP and LTD in the rodent hippocampus and focus on the use of subunit-specific N-methyl-d-aspartate receptor antagonists and interference peptides as potential tools to study the role of synaptic plasticity in learning and memory. By using the modulation of synaptic plasticity and hippocampal-dependent learning and memory by acute stress as an example, we review a large body of convincing evidence indicating that alterations in synaptic plasticity underlie the changes in learning and memory produced by acute stress.

Regulation of NMDA receptor subunit expression and its implications for LTD, LTP, and metaplasticity

NMDA-type glutamate receptors (NMDARs) mediate many forms of synaptic plasticity. These tetrameric receptors consist of two obligatory NR1 subunits and two regulatory subunits, usually a combination of NR2A and NR2B. In the neonatal neocortex NR2B-containing NMDARs predominate, and sensory experience facilitates a developmental switch in which NR2A levels increase relative to NR2B. In this review, we clarify the roles of NR2 subunits in synaptic plasticity, and argue that a primary role of this shift is to control the threshold, rather than determining the direction, for modifying synaptic strength. We also discuss recent studies that illuminate the mechanisms regulating NR2 subunits, and suggest that the NR2A/NR2B ratio is regulated by multiple means, which may control the ratio both locally at individual synapses and globally in a cell-wide manner. Finally, we use the visual cortex as a model system to illustrate how activity-dependent modifications in the NR2A/NR2B ratio may contribute to the development of cortical functions.

Long-term depression in vivo: effects of sex, stress, diet, and prenatal ethanol exposure

Long-term depression (LTD) of synaptic efficacy has proven a difficult phenomenon to examine in vivo, despite the ease with which it is induced in a variety of in vitro preparations. Prior exposure to an acute stressful episode does however seem to enhance the capacity of the hippocampus to exhibit LTD in vivo in male animals. In the present experiments, we examined the capacity for low-frequency stimuli (low-frequency stimulation (LFS)) to induce LTD in juvenile male and female animals following an acute stress episode. Interestingly, prior exposure to stress was only required for the induction of LTD in male animals, while both control and stressed female animals exhibited equivalent LTD. In animals that were exposed to ethanol in utero, a similar requirement for prior exposure to stress to elicit LTD was found for male, but not female animals. This prenatal ethanol exposure did not in itself alter the capacity for LTD induction in either sex; however, in utero food restriction did enhance LTD induction in both male and female animals, irrespective of whether they were exposed to stress just prior to being administered LFS. These results indicate that in utero dietary restriction more drastically affects CA1 LTD than in utero ethanol exposure. In addition, female animals seem to exhibit LTD in vivo in the absence of stress much more easily than their male counterparts.

The magnitude of hippocampal long term depression depends on the synaptic location of activated NR2-containing N-methyl-D-aspartate receptors

Activation of N-methyl-D-aspartate receptors (NMDARs) is the first step in the induction of certain forms of synaptic plasticity in the hippocampus. In the adult rat hippocampus, NMDARs are composed almost exclusively of NR1 and NR2 subunits with NR1 subunits being mainly associated with either NR2A and/or NR2B subunits. The role played by the different subunits in synaptic plasticity is still controversial. In the present study, we used two different long term depression (LTD) -inducing protocols (electrical and chemical stimulation) to show that activation of NR2A-containing NMDAR subunits leads to the induction of LTD. We also demonstrated that extrasynaptic NR2B-containing NMDARs regulate the magnitude of LTD by exerting a control over the function of synaptic NR2A-containing NMDARs while having no effect on plasticity in the absence of synaptic receptor activation. Taken as a whole, these experiments demonstrate that NMDAR subunits play different roles according to their nature (NR2A or NR2B) and location (synaptic versus extrasynaptic). This sheds new light on the functional role of extrasynaptic NR2B containing-NMDARs. These results are particularly important for a better understanding of certain pathological disorders associated with glutamatergic overactivity.

Effects of exercise on NMDA receptor subunit contributions to bidirectional synaptic plasticity in the mouse dentate gyrus

We examined synaptic plasticity in the dentate gyrus (DG) of the hippocampus in vitro in juvenile C57Bl6 mice (28-40 days of age), housed in control conditions with minimal enrichment (Controls) or with access to an exercise wheel (Runners). LTP expression was significantly greater in slices from Runners than in those from Controls, but could be blocked by APV in both groups. LTP was significantly reduced by NR2B subunit antagonists in both groups. NVP-AAM077, an antagonist with a higher preference for NR2A subunits over NR2B subunits, blocked LTP in slices from Runners and produced a slight depression in Control animals. LTD in the DG was also blocked by APV, but not by either of the NR2B specific antagonists. Strikingly, NVP-AAM077 prevented LTD in Runners, but not in Control animals, suggesting an increased involvement of NR2A subunits in LTD in animals that exercise. NVP-AAM077 did not block LTD in NR2A Knock Out (KO) animals that exercised, as expected. In an attempt to discern whether NMDA receptors located at extrasynaptic sites could play a role in the induction of LTD, DL-TBOA was used to block excitatory amino acid transport and increase extracellular glutamate levels. Under these conditions, LTD was not blocked by the co-application of a specific NR2B subunit antagonist in either group, but NVP-AAM077 again blocked LTD selectively in Runners. These results indicate that NR2A and NR2B subunits play a significant role in LTP in the DG, and that exercise can significantly alter the contribution of NMDA NR2A subunits to LTD.

Glutamatergic substrates of drug addiction and alcoholism

The past two decades have witnessed a dramatic accumulation of evidence indicating that the excitatory amino acid glutamate plays an important role in drug addiction and alcoholism. The purpose of this review is to summarize findings on glutamatergic substrates of addiction, surveying data from both human and animal studies. The effects of various drugs of abuse on glutamatergic neurotransmission are discussed, as are the effects of pharmacological or genetic manipulation of various components of glutamate transmission on drug reinforcement, conditioned reward, extinction, and relapse-like behavior. In addition, glutamatergic agents that are currently in use or are undergoing testing in clinical trials for the treatment of addiction are discussed, including acamprosate, N-acetylcysteine, modafinil, topiramate, lamotrigine, gabapentin and memantine. All drugs of abuse appear to modulate glutamatergic transmission, albeit by different mechanisms, and this modulation of glutamate transmission is believed to result in long-lasting neuroplastic changes in the brain that may contribute to the perseveration of drug-seeking behavior and drug-associated memories. In general, attenuation of glutamatergic transmission reduces drug reward, reinforcement, and relapse-like behavior. On the other hand, potentiation of glutamatergic transmission appears to facilitate the extinction of drug-seeking behavior. However, attempts at identifying genetic polymorphisms in components of glutamate transmission in humans have yielded only a limited number of candidate genes that may serve as risk factors for the development of addiction. Nonetheless, manipulation of glutamatergic neurotransmission appears to be a promising avenue of research in developing improved therapeutic agents for the treatment of drug addiction and alcoholism.

Hippocampal long-term depression mediates acute stress-induced spatial memory retrieval impairment

Acute stress impairs memory retrieval and facilitates the induction of long-term depression (LTD) in the hippocampal CA1 region of the adult rodent brain. However, whether such alterations in synaptic plasticity cause the behavioral effects of stress is not known. Here, we report that two selective inhibitors of the induction or expression of stress-enabled, N-methyl-D-aspartate receptor-dependent hippocampal LTD also block spatial memory retrieval impairments caused by acute stress. Additionally, we demonstrate that facilitating the induction of hippocampal LTD in vivo by blockade of glutamate transport mimics the behavioral effects of acute stress by impairing spatial memory retrieval. Thus, the present study demonstrates that hippocampal LTD is both necessary and sufficient to cause acute stress-induced impairment of spatial memory retrieval and provides a new perspective from which to consider the nature of cognitive deficits in disorders whose symptoms are aggravated by stress.

Ethanol inhibits the sensory responses of cerebellar granule cells in anesthetized cats

BACKGROUND

Granule cells occupy a strategic position in the transmission of afferent information to the cerebellar cortex. They are also the most abundant type of neurons in the cerebellum. The functions of the cerebellum are thought to be sensitive to acute alcohol intoxication. The effects of acute alcohol intoxication on the in vivo physiology of cerebellar granule cells are, however, not completely known.

METHODS

We studied chloralose-anesthetized cats at ethanol doses relevant to human drinking (0.3-1.2 g/kg). We recorded the electrophysiological responses of granule cell clusters to auditory and visual stimulation, and simultaneously monitored the concentration of ethanol in the cerebrospinal fluid (CSF).

RESULTS

At an intravenous ethanol dose of 0.3 g/kg, CSF ethanol concentration peaked in 10 minutes at 17 mM, equivalent to a blood alcohol concentration (BAC) of about 0.08 g/dL. Ethanol quickly and almost completely abolished both auditory and visual responses from granule cells. Complete or near-complete inhibition lasted 15 to 20 minutes; approximately 50% recovery required an additional 15 minutes, and a full recovery yet another 15 minutes. A higher ethanol dose at 1.2 g/kg resulted in a more severe inhibition and required longer time for recovery. The relationship between ethanol dose, CSF ethanol concentration, and granule cell responses was dynamic and nonlinear, critically depending upon the elapsed time.

CONCLUSIONS

Cerebellar granule cell sensory responses are highly sensitive to ethanol inhibition. A rapid development of acute tolerance appears to be a major factor contributing to the dynamic and nonlinear relationship among ethanol dosage, CSF ethanol concentration, and granule cell responses. It is likely that a generalized de-afferentation of the cerebellum from its mossy fiber afferents, followed by the subsequent development of acute tolerance may play major roles by which alcohol intoxication affects cerebellar functions.

Corticotropin-releasing factor (CRF) receptor type 1-dependent modulation of synaptic plasticity

CRF receptor type (CRHR) 1 exerts neuroregulatory control on associative learning processes such as fear and anxiety like behaviour. Using hippocampal slices, we investigated the neuronal excitability in mice lacking CRHR1 (Crhr1(-/-)). Compared to wild-type mice, long-term potentiation (LTP) elicited by 100 pulses at 100Hz was not different. Unexpectedly, at lower frequencies (1, 5 or 10Hz), the resulting synaptic changes in CA1 neurons of Crhr1(-/-) were systematically shifted towards long-term depression (LTD). Furthermore, testing paired-pulse paradigm revealed a GABA receptor-dependent decrease of paired-pulse ratio in Crhr1(-/-). It might be assumed that a lack of CRHR1 induce developmental changes which resulted in altered GABAergic activity, producing attenuated synaptic potentiation after repetitive stimulation and thus favouring LTD in principal neurons. Since CRHR1 are located in GABAergic somata, axons and boutons the activity of these receptor types rather might contribute to the development of the neuronal ability for plasticity like processes on the level of NMDAR subunit composition and GABAergic activity.

Differential roles of NR2A and NR2B-containing NMDA receptors in LTP and LTD in the CA1 region of two-week old rat hippocampus

The role of NMDA receptors in the induction of long-term potentiation (LTP) and long-term depression (LTD) is well established but which particular NR2 subunits are involved in these plasticity processes is still a matter of controversy. We have studied the effects of subtype selective NMDA receptor antagonists on LTP induced by high frequency stimulation (100 Hz for 1s) and LTD induced by low frequency stimulation (1 Hz for 15 min) in the CA1 region of hippocampal slices from 14 day old Wistar rats. Against recombinant receptors in HEK293 cells NVP-AAM077 (NVP) was approximately 14-fold selective for NR2A vs NR2B receptors, whilst Ro 25-6981 (Ro) was highly selective for NR2B receptors. On NMDA receptor-mediated EPSCs from Schaffer collaterals in CA1 neurones, NVP and Ro both reduced the amplitude but differentially affected the time constant of decay. The data are compatible with the selective effect of NVP (0.1 microM) and Ro (4 microM) on native NR2A and NBR2B receptors, respectively. NVP reduced both LTP and LTD whereas Ro reduced only LTP. Thus, LTP was reduced by 63% at 0.1 microM NVP and almost completely at 0.4 microM whereas 5 microM Ro reduced LTP by 45%. These data are consistent with a role for both NR2A and NR2B in the induction of LTP, under our experimental conditions. In comparison, LTD was unaffected by Ro (5 microM) even in the presence of a glutamate uptake inhibitor threo-beta-benzylaspartic acid (TBOA) to increase the concentration of glutamate at NR2B containing receptors. NVP (0.2-0.4 microM), however, produced a concentration dependent inhibition of LTD which was complete at 0.4 microM. The lack of effect of 0.1 microM NVP on LTD contrasts with its marked effect on LTP and raises the possibility that different NVP-sensitive NR2 subunit-containing NMDA receptors are required for LTP and LTD in this preparation.