NMDAR-dependent Synaptic Plasticity Research Articles

Separate Ionotropic and Metabotropic Glutamate Receptor Functions in Depotentiation vs. LTP: A Distinct Role for Group1 mGluR Subtypes and NMDARs

Depotentiation (DP) is a mechanism by which synapses that have recently undergone long-term potentiation (LTP) can reverse their synaptic strengthening within a short time-window after LTP induction. Group 1 metabotropic glutamate receptors (mGluRs) were shown to be involved in different forms of LTP and long-term depression (LTD), but little is known about their roles in DP. Here, we generated DP by applying low-frequency stimulation (LFS) at 5 Hz after LTP had been induced by a single train of theta-burst-stimulation (TBS). While application of LFS for 2 min (DP2′) generated only a short-lasting DP that was independent of the activation of N-methyl-D-aspartate receptors (NMDARs) and group 1 mGluRs, LFS given for 8 min (DP8′) induced a robust DP that was maintained for at least 2 h. This strong form of DP was contingent on NMDAR activation. Interestingly, DP8′ appears to include a metabotropic NMDAR function because it was blocked by the competitive NMDAR antagonist D-AP5 but not by the use-dependent inhibitor MK-801 or high Mg2+. Furthermore, DP8′ was enhanced by application of the mGluR1 antagonist (YM 298198, 1 μM). The mGluR5 antagonist 2-Methyl-6(phenylethynyl) pyridine (MPEP, 40 μM), in contrast, failed to affect it. The induction of LTP, in turn, was NMDAR dependent (as tested with D-AP5), and blocked by MPEP but not by YM 298198. These results indicate a functional dissociation of mGluR1 and mGluR5 in two related and consecutively induced types of NMDAR-dependent synaptic plasticity (LTP → DP) with far-reaching consequences for their role in plasticity and learning under normal and pathological conditions.

P.1.h.007 - Peripheral immunization of mice to produce antibodies against NMDA receptor as a potential approach to create a rodent model of schizophrenia

We have investigated the relevance of d-aspartate oxidase, the only enzyme known to selectively degrade d-aspartate (d-Asp), in modulating glutamatergic system homeostasis. Interestingly, the lack of the Ddo gene, by raising d-Asp content, induces a substantial increase in extracellular glutamate (Glu) levels in Ddo-mutant brains. Consistent with an exaggerated and persistent N-methyl-d-aspartate receptor (NMDAR) stimulation, we documented in Ddo knockouts severe age-dependent structural and functional alterations mirrored by expression of active caspases 3 and 7 along with appearance of dystrophic microglia and reactive astrocytes. In addition, prolonged elevation of d-Asp triggered in mutants alterations of NMDAR-dependent synaptic plasticity associated to reduction of hippocampal GluN1 and GluN2B subunits selectively located at synaptic sites and to increase in the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-to-N-methyl-d-aspartate ratio. These effects, all of which converged on a progressive hyporesponsiveness at NMDAR sites, functionally resulted in a greater vulnerability to phencyclidine-induced prepulse inhibition deficits in mutants. In conclusion, our results indicate that d-aspartate oxidase, by strictly regulating d-Asp levels, impacts on the homeostasis of glutamatergic system, thus preventing accelerated neurodegenerative processes.

Cocaine Experience Enhances Thalamo-Accumbens N-Methyl-D-Aspartate Receptor Function

BackgroundExcitatory synaptic transmission in the nucleus accumbens (NAc) is a key biological substrate underlying behavioral responses to psychostimulants and susceptibility to relapse. Studies have demonstrated that cocaine induces changes in glutamatergic signaling at distinct inputs to the NAc. However, consequences of cocaine experience on synaptic transmission from the midline nuclei of the thalamus (mThal) to the NAc have yet to be reported. MethodsTo examine synapses from specific NAc core inputs, we recorded light-evoked excitatory postsynaptic currents following viral-mediated expression of channelrhodopsin-2 in the mThal, prefrontal cortex (PFC), or basolateral amygdala from acute brain slices. To identify NAc medium spiny neuron subtypes, we used mice expressing tdTomato driven by the promoter for dopamine receptor subtype 1 (D1). We recorded N-methyl-D-aspartate receptor (NMDAR) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor (AMPAR) properties to evaluate synaptic adaptations induced by cocaine experience, a 5-day cocaine exposure followed by 2 weeks of abstinence. ResultsExcitatory inputs to the NAc core displayed differential NMDAR properties, and cocaine experience uniquely altered AMPAR and NMDAR properties at mThal-D1(+), mThal-D1(−), and PFC-D1(+) synapses, but not at PFC-D1(−) synapses. Finally, at mThal-D1(+) synapses, cocaine enhanced GluN2C/D function and NMDAR-dependent synaptic plasticity. ConclusionsOur results identify contrasting cocaine-induced AMPAR and NMDAR modifications at mThal-NAc and PFC-NAc core synapses. These changes include an enhancement of NMDAR function and plasticity at mThal-D1(+) synapses. Incorporation of GluN2C/D-containing NMDARs most likely underlies these phenomena and represents a potential therapeutic target for psychostimulant use disorders.

Temporal Memory and Its Enhancement by Estradiol Requires Surface Dynamics of Hippocampal CA1 N-Methyl-D-Aspartate Receptors

BackgroundIdentifying the underlying cellular mechanisms of episodic memory is an important challenge, since this memory, based on temporal and contextual associations among events, undergoes preferential degradation in aging and various neuropsychiatric disorders. Memory storage of temporal and contextual associations is known to rely on hippocampal N-methyl-D-aspartate receptor (NMDAR)-dependent synaptic plasticity, which depends ex vivo on dynamic organization of surface NMDARs. Whether NMDAR surface trafficking sustains the formation of associative memory, however, remains unknown. MethodsWe tested this hypothesis, using single nanoparticle imaging, electrophysiology, and behavioral approaches, in hippocampal networks challenged with a potent modulator of NMDAR-dependent synaptic plasticity and memory, 17β-estradiol (E2). ResultsWe demonstrate that E2 modulates NMDAR surface trafficking, a necessary condition for E2-induced potentiation at hippocampal cornu ammonis 1 synapses. Strikingly, cornu ammonis 1 NMDAR surface trafficking controls basal and E2-enhanced mnemonic retention of temporal, but not contextual, associations. ConclusionsNMDAR surface trafficking and its modulation by the sex hormone E2 is a cellular mechanism critical for a major component of episodic memory, opening a new and noncanonical research avenue in the physiopathology of cognition.

From synaptic plasticity to spatial maps and sequence learning.

The entorhinal-hippocampal circuit is crucial for several forms of learning and memory, especially sequence learning, including spatial navigation. The challenge is to understand the underlying mechanisms. Pioneering discoveries of spatial selectivity in this circuit, i.e. place cells and grid cells, provided a major step forward in tackling this challenge. Considerable research has also shown that sequence learning relies on synaptic plasticity, especially the Hebbian or the NMDAR-dependent synaptic plasticity. This raises several questions: Are spatial maps plastic? If so, what is the contribution of Hebbian plasticity to spatial map plasticity? How does the spatial map plasticity contribute to sequence learning? A combination of computational and experimental studies has shown that NMDAR-mediated plasticity and theta rhythm can have specific effects on the formation and experiential modification of spatial maps to facilitate predictive coding. Advances in transgenic techniques have provided further support for these mechanisms. Although many exciting challenges remain, these findings have brought us closer to solving the puzzle of how the hippocampal system contributes to spatial memory, and point to a way forward.

Biased mGlu5-Positive Allosteric Modulators Provide In Vivo Efficacy without Potentiating mGlu5 Modulation of NMDAR Currents

Schizophrenia is associated with disruptions in N-methyl-D-aspartate glutamate receptor subtype (NMDAR)-mediated excitatory synaptic signaling. The metabotropic glutamate receptor subtype 5 (mGlu5) is a closely associated signaling partner with NMDARs and regulates NMDAR function in forebrain regions implicated in the pathology of schizophrenia. Efficacy of mGlu5 positive allosteric modulators (PAMs) in animal models of psychosis and cognition was previously attributed to potentiation of NMDAR function. To directly test this hypothesis, we identified VU0409551 as a novel mGlu5 PAMthat exhibits distinct stimulus bias and selectively potentiates mGlu5 coupling to Gαq-mediated signaling but not mGlu5 modulation of NMDAR currents or NMDAR-dependent synaptic plasticity in the rat hippocampus. Interestingly, VU0409551 produced robust antipsychotic-like and cognition-enhancing activity in animal models. These data provide surprising new mechanistic insights into the actions of mGlu5 PAMs and suggest that modulation of NMDAR currents is not critical for invivo efficacy. VIDEO ABSTRACT.

D-Aspartate: An endogenous NMDA receptor agonist enriched in the developing brain with potential involvement in schizophrenia

Free d-aspartate and d-serine occur at substantial levels in the mammalian brain. d-Serine is a physiological endogenous co-agonist for synaptic N-Methyl d-Aspartate (NMDA) receptors (NMDARs), and is involved in the pathophysiology of schizophrenia. Much less is known about the biological meaning of d-aspartate. d-Aspartate is present at high levels in the embryo brain and strongly decreases at post-natal phases. Temporal reduction of d-aspartate levels depends on the post-natal onset of d-aspartate oxidase (DDO), an enzyme able to selectively catabolize this d-amino acid. Pharmacological evidence indicates that d-aspartate binds to and activates NMDARs. Characterization of genetic and pharmacological mouse models with abnormally higher levels of d-aspartate has evidenced that increased d-aspartate enhances hippocampal NMDAR-dependent synaptic plasticity, dendritic morphology and spatial memory. In line with the hypothesis of a hypofunction of NMDARs in the pathogenesis of schizophrenia, it has been shown that increased d-aspartate levels also improve brain connectivity, produce corticostriatal adaptations resembling those observed after chronic haloperidol treatment, and protects against prepulse inhibition deficits and abnormal circuits activation induced by psychotomimetic drugs. In healthy humans, genetic variation predicting reduced expression of DDO in post-mortem prefrontal cortex is associated with greater prefrontal gray matter and activity during working memory. On the other side, evaluation of d-aspartate content in post-mortem patients with schizophrenia has shown a significant reduction of this d-amino acid in the prefrontal cortex and striatum. Generation of mouse models with reduced embryonic levels of d-aspartate may disclose unprecedented role for d-aspartate in developmental brain processes associated with vulnerability to psychotic-like symptoms.

D-Aspartate oxidase influences glutamatergic system homeostasis in mammalian brain

We have investigated the relevance of d-aspartate oxidase, the only enzyme known to selectively degrade d-aspartate (d-Asp), in modulating glutamatergic system homeostasis. Interestingly, the lack of the Ddo gene, by raising d-Asp content, induces a substantial increase in extracellular glutamate (Glu) levels in Ddo-mutant brains. Consistent with an exaggerated and persistent N-methyl-d-aspartate receptor (NMDAR) stimulation, we documented in Ddo knockouts severe age-dependent structural and functional alterations mirrored by expression of active caspases 3 and 7 along with appearance of dystrophic microglia and reactive astrocytes. In addition, prolonged elevation of d-Asp triggered in mutants alterations of NMDAR-dependent synaptic plasticity associated to reduction of hippocampal GluN1 and GluN2B subunits selectively located at synaptic sites and to increase in the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-to-N-methyl-d-aspartate ratio. These effects, all of which converged on a progressive hyporesponsiveness at NMDAR sites, functionally resulted in a greater vulnerability to phencyclidine-induced prepulse inhibition deficits in mutants. In conclusion, our results indicate that d-aspartate oxidase, by strictly regulating d-Asp levels, impacts on the homeostasis of glutamatergic system, thus preventing accelerated neurodegenerative processes.

Channel-mediated astrocytic glutamate modulates hippocampal synaptic plasticity by activating postsynaptic NMDA receptors.

BackgroundActivation of G protein coupled receptor (GPCR) in astrocytes leads to Ca2+-dependent glutamate release via Bestrophin 1 (Best1) channel. Whether receptor-mediated glutamate release from astrocytes can regulate synaptic plasticity remains to be fully understood.ResultsWe show here that Best1-mediated astrocytic glutamate activates the synaptic N-methyl-D-aspartate receptor (NMDAR) and modulates NMDAR-dependent synaptic plasticity. Our data show that activation of the protease-activated receptor 1 (PAR1) in hippocampal CA1 astrocytes elevates the glutamate concentration at Schaffer collateral-CA1 (SC-CA1) synapses, resulting in activation of GluN2A-containing NMDARs and NMDAR-dependent potentiation of synaptic responses. Furthermore, the threshold for inducing NMDAR-dependent long-term potentiation (LTP) is lowered when astrocytic glutamate release accompanied LTP induction, suggesting that astrocytic glutamate is significant in modulating synaptic plasticity.ConclusionsOur results provide direct evidence for the physiological importance of channel-mediated astrocytic glutamate in modulating neural circuit functions.

Dysbindin-1 loss compromises NMDAR-dependent synaptic plasticity and contextual fear conditioning

Genetic variants in DTNBP1 encoding the protein dysbindin-1 have often been associated with schizophrenia and with the cognitive deficits prominent in that disorder. Because impaired function of the hippocampus is thought to play a role in these memory deficits and because NMDAR-dependent synaptic plasticity in this region is a proposed biological substrate for some hippocampal-dependent memory functions in schizophrenia, we hypothesized that reduced dysbindin-1 expression would lead to impairments in NMDAR-dependent synaptic plasticity and in contextual fear conditioning. Acute slices from male mice carrying 0, 1, or 2 null mutant alleles of the Dtnbp1 gene were prepared, and field recordings from the CA1 striatum radiatum were obtained before and after tetanization of Schaffer collaterals of CA3 pyramidal cells. Mice homozygous for the null mutation in Dtnbp1 exhibited significantly reduced NMDAR-dependent synaptic potentiation compared to wild type mice, an effect that could be rescued by bath application of the NMDA receptor coagonist glycine (10 μM). Behavioral testing in adult mice revealed deficits in hippocampal memory processes. Homozygous null mice exhibited lower conditional freezing, without a change in the response to shock itself, indicative of a learning and memory deficit. Taken together, these results indicate that a loss of dysbindin-1 impairs hippocampal plasticity which may, in part, explain the role dysbindin-1 plays in the cognitive impairments of schizophrenia.

Rescue of NMDAR-Dependent Synaptic Plasticity in Fmr1 Knock-Out Mice

Fragile X Syndrome (FXS) is the most common form of inherited intellectual disability and results from a loss of Fragile X mental retardation protein (FMRP). FMRP is important for mRNA shuttling and translational control and binds to proteins important for synaptic plasticity. Like many developmental disorders, FXS is associated with alterations in synaptic plasticity that may impair learning and memory processes in the brain. However, it remains unclear whether FMRP plays a ubiquitous role in synaptic plasticity in all brain regions. We report that a loss of FMRP leads to impairments in N-methyl-D-aspartate receptor (NMDAR)-dependent synaptic plasticity in the dentate gyrus (DG), but not in the cornu ammonis area 1 (CA1) subregion of the hippocampus of adult mice. DG-specific deficits are accompanied by a significant reduction in NMDAR GluN1, GluN2A, and GluN2B subunit levels and reduced serine 831 GluA1 phosphorylation specifically in this region. Importantly, we demonstrate that treatment with NMDAR co-agonists (glycine or D-serine) independently rescue impairments in NMDAR-dependent synaptic plasticity in the DG of the Fragile X mental retardation 1 (Fmr1) knockout mouse. These findings implicate the NMDAR in the pathophysiology of FXS and suggest that indirect agonists of the NMDAR may be a successful therapeutic intervention in FXS.

MHC class I immune proteins are critical for hippocampus-dependent memory and gate NMDAR-dependent hippocampal long-term depression

Memory impairment is a common feature of conditions that involve changes in inflammatory signaling in the brain, including traumatic brain injury, infection, neurodegenerative disorders, and normal aging. However, the causal importance of inflammatory mediators in cognitive impairments in these conditions remains unclear. Here we show that specific immune proteins, members of the major histocompatibility complex class I (MHC class I), are essential for normal hippocampus-dependent memory, and are specifically required for NMDAR-dependent forms of long-term depression (LTD) in the healthy adult hippocampus. In β2m−/−TAP−/−mice, which lack stable cell-surface expression of most MHC class I proteins, NMDAR-dependent LTD in area CA1 of adult hippocampus is abolished, while NMDAR-independent forms of potentiation, facilitation, and depression are unaffected. Altered NMDAR-dependent synaptic plasticity in the hippocampus of β2m−/−TAP−/−mice is accompanied by pervasive deficits in hippocampus-dependent memory, including contextual fear memory, object recognition memory, and social recognition memory. Thus normal MHC class I expression is essential for NMDAR-dependent hippocampal synaptic depression and hippocampus-dependent memory. These results suggest that changes in MHC class I expression could be an unexpected cause of disrupted synaptic plasticity and cognitive deficits in the aging, damaged, and diseased brain.

Light-Induced Plasticity of Synaptic AMPA Receptor Composition in Retinal Ganglion Cells

Light-evoked responses of all three major classes of retinal ganglion cells (RGCs) are mediated by NMDA receptors (NMDARs) and AMPA receptors (AMPARs). Although synaptic activity at RGC synapses is highly dynamic, synaptic plasticity has not been observed in adult RGCs. Here, using patch-clamp recordings in dark-adapted mouse retina, we report a retina-specific form of AMPAR plasticity. Both chemical and light activation of NMDARs caused the selective endocytosis of GluA2-containing, Ca(2+)-impermeable AMPARs on RGCs and replacement with GluA2-lacking, Ca(2+)-permeable AMPARs. The plasticity was expressed in ON but not OFF RGCs and was restricted solely to the ON responses in ON-OFF RGCs. Finally, the plasticity resulted in a shift in the light responsiveness of ON RGCs. Thus, physiologically relevant light stimuli can induce a change in synaptic receptor composition of ON RGCs, providing a mechanism by which the sensitivity of RGC responses may be modified under scotopic conditions.

Neuronal Stimulation Induces Autophagy in Hippocampal Neurons That Is Involved in AMPA Receptor Degradation after Chemical Long-Term Depression

Many studies have reported the roles played by regulated proteolysis in synaptic plasticity and memory, but the role of autophagy in neurons remains unclear. In mammalian cells, autophagy functions in the clearance of long-lived proteins and organelles and in adaptation to starvation. In neurons, although autophagy-related proteins (ATGs) are highly expressed, autophagic activity markers, autophagosome (AP) number, and light chain protein 3-II (LC3-II) are low compared with other cell types. In contrast, conditional knock-out of ATG5 or ATG7 in mouse brain causes neurodegeneration and behavioral deficits. Therefore, this study aimed to test whether autophagy is especially regulated in neurons to adapt to brain functions. In cultured rat hippocampal neurons, we found that KCl depolarization transiently increased LC3-II and AP number, which was partially inhibited with APV, an NMDA receptor (NMDAR) inhibitor. Brief low-dose NMDA, a model of chemical long-term depression (chem-LTD), increased LC3-II with a time course coincident with Akt and mammalian target of rapamycin (mTOR) dephosphorylation and degradation of GluR1, an AMPA receptor (AMPAR) subunit. Downstream of NMDAR, the protein phosphatase 1 inhibitor okadaic acid, PTEN inhibitor bpV(HOpic), autophagy inhibitor wortmannin, and short hairpin RNA-mediated knockdown of ATG7 blocked chem-LTD-induced autophagy and partially recovered GluR1 levels. After chem-LTD, GFP-LC3 puncta increased in spines and in dendrites when AP-lysosome fusion was blocked. These results indicate that neuronal stimulation induces NMDAR-dependent autophagy through PI3K-Akt-mTOR pathway inhibition, which may function in AMPAR degradation, thus suggesting autophagy as a contributor to NMDAR-dependent synaptic plasticity and brain functions.

Pull-Push Neuromodulation of LTP and LTD Enables Bidirectional Experience-Induced Synaptic Scaling in Visual Cortex

Neuromodulatory input, acting on G protein-coupled receptors, is essential for the induction of experience-dependent cortical plasticity. Here we report that G-coupled receptors in layer II/III of visual cortex control the polarity of synaptic plasticity through a pull-push regulation of LTP and LTD. In slices, receptors coupled to Gs promote LTP while suppressing LTD; conversely, receptors coupled to Gq11 promote LTD and suppress LTP. In vivo, the selective stimulation of Gs- or Gq11-coupled receptors brings the cortex into LTP-only or LTD-only states, which allows the potentiation or depression of targeted synapses with visual stimulation. The pull-push regulation of LTP/LTD occurs via direct control of the synaptic plasticity machinery and it is independent of changes in NMDAR activation or neuronal excitability. We propose these simple rules governing the pull-push control of LTP/LTD form a general metaplasticity mechanism that may contribute to neuromodulation of plasticity in other cortical circuits.

Frequency-Dependent Changes in NMDAR-Dependent Synaptic Plasticity

The NMDAR-dependent synaptic plasticity is thought to mediate several forms of learning, and can be induced by spike trains containing a small number of spikes occurring with varying rates and timing, as well as with oscillations. We computed the influence of these variables on the plasticity induced at a single NMDAR containing synapse using a reduced model that was analytically tractable, and these findings were confirmed using detailed, multi-compartment model. In addition to explaining diverse experimental results about the rate and timing dependence of synaptic plasticity, the model made several novel and testable predictions. We found that there was a preferred frequency for inducing long-term potentiation (LTP) such that higher frequency stimuli induced lesser LTP, decreasing as 1/f when the number of spikes in the stimulus was kept fixed. Among other things, the preferred frequency for inducing LTP varied as a function of the distance of the synapse from the soma. In fact, same stimulation frequencies could induce LTP or long-term depression depending on the dendritic location of the synapse. Next, we found that rhythmic stimuli induced greater plasticity then irregular stimuli. Furthermore, brief bursts of spikes significantly expanded the timing dependence of plasticity. Finally, we found that in the ∼5–15-Hz frequency range both rate- and timing-dependent plasticity mechanisms work synergistically to render the synaptic plasticity most sensitive to spike timing. These findings provide computational evidence that oscillations can have a profound influence on the plasticity of an NMDAR-dependent synapse, and show a novel role for the dendritic morphology in this process.

GABA A receptor inhibition does not affect mGluR-dependent LTD at hippocampal Schaffer collateral-CA1 synapses

Hippocampal synaptic plasticity between Schaffer collaterals and CA1 pyramidal neurons can be induced by activation of N-methyl- d-aspartate receptors (NMDARs) or of metabotropic glutamate receptors (mGluRs). Inhibitory GABAergic interneurons in this region abundantly terminate on pyramidal neurons and may thus influence synaptic plasticity. Although NMDAR-dependent synaptic plasticity is known to be influenced by inhibitory interneurons, little is known about the role of GABA on mGluR-dependent plasticity. Here, we used field potential recordings of the Schaffer collateral-CA1 synapses in rat hippocampal slices in order to study the effect of GABA A receptor (GABA AR) inhibition on mGluR-dependent long-term depression (LTD). Without GABA AR blockade, mGluR-dependent LTD was induced pharmacologically by the group I mGluR agonist (RS)-3,5-dihydroxyphenylglycine (DHPG, 100 μM, 10 min) as well as electrically by paired-pulse low-frequency stimulation (PP-LFS, 900 paired pulses at 1 Hz) resulting in a stable depression of the field response lasting at least 80 min after LTD induction. The GABA AR antagonist gabazine (5 μM) itself caused an increase of field responses suggesting an endogenous GABA release inhibiting CA1 field potentials. However, when either DHPG or PP-LFS was applied during GABA AR inhibition, the field responses were significantly reduced. Moreover, normalizing these responses to experiments without GABA AR blockade, there was no significant effect of gabazine on both DHPG- and PP-LFS-induced LTD. Thus, our results show that mGluR-dependent LTD at Schaffer collateral-CA1 synapses is unaffected by GABA AR mediated synaptic transmission.

NR2A-containing NMDA receptors are required for LTP induction in rat dorsolateral striatum in vitro

N-methyl- d-aspartate receptors (NMDARs) have been implicated in various forms of synaptic plasticity. In recent years, studies have been shown that NMDA receptor subunits play different roles in several forms of NMDAR-dependent synaptic plasticity. However, the contribution of NR2A and NR2B subunits in the induction of long-term potentiation (LTP) in the corticostriatal pathway remains unclear. The present study used patch-clamp recordings to study the role of NR2A-containing and NR2B-containing NMDARs in LTP induction in corticostriatal slices from 13–14-day old rats. High-frequency stimulation (HFS) of the corticostriatal pathway readily induced LTP of excitatory postsynaptic currents (EPSCs), and D-APV, a selective NMDAR antagonist, blocked LTP. Moreover, NR2B-containing NMDAR antagonists (Ro 25-6981 and ifenprodil) displayed no influence on LTP induction. However, LTP was not inducible in the presence of Zn 2+, an NR2A-containing NMDAR antagonist. These results suggest that the induction of LTP by HFS in the dorsolateral striatum is NMDAR-dependent and requires NR2A-containing NMDARs, not NR2B-containing NMDARs.

Src‐protein tyrosine kinases are required for cocaine‐induced increase in the expression and function of the NMDA receptor in the ventral tegmental area

Cocaine-induced long-term potentiation of glutamatergic synapses in the ventral tegmental area (VTA) has been proposed as a key process that contributes to the development of addictive behaviors. In particular, the activation of ionotrophic glutamate NMDA receptor (NMDAR) in the VTA is critical for the initiation of cocaine sensitization. Here we show that application of cocaine both in slices and in vivo induced an increase in tyrosine phosphorylation of the NR2A, but not the NR2B subunit of the NMDAR in juvenile rats. Cocaine induced an increase in the activity of both Fyn and Src kinases, and the Src-protein tyrosine kinase (Src-PTKs) inhibitor, 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP2), abolished both cocaine-induced increase in tyrosine phosphorylation of the NR2A subunit and the increase in the expression of NR1, NR2A, and NR2B in the VTA. Moreover, cocaine-induced enhancement in NMDAR-mediated excitatory post-synaptic currents was completely abolished by PP2. Taken together, these results suggest that acute cocaine induced an increase in the expression of NMDAR subunits and enhanced tyrosine phosphorylation of NR2A-containing NMDAR through members of the Src-PTKs. This in turn, increased NMDAR-mediated currents in VTA dopamine neurons. These results provide a potential cellular mechanism by which cocaine triggers NMDAR-dependent synaptic plasticity of VTA neurons that may underlie the development of behavioral sensitization.

Developmental switch in requirement for PKA RIIβ in NMDA-receptor-dependent synaptic plasticity at Schaffer collateral to CA1 pyramidal cell synapses

The cAMP/protein kinase A (PKA) signaling cascade is crucial for synaptic plasticity in a wide variety of species. PKA regulates Ca2+ permeation through NMDA receptors (NMDARs) and induction of NMDAR-dependent synaptic plasticity at the Schaffer collateral to CA1 pyramidal cell synapse. Whereas the role of PKA in induction of NMDAR-dependent LTP at CA1 synapses is established, the identity of PKA isoforms involved in this phenomenon is less clear. Here we report that protein synthesis-independent NMDAR-dependent LTP at the Schaffer collateral–CA1 synapse in the hippocampus is deficient, but NMDAR-dependent LTD is normal, in young (postnatal day 10 (P10)–P14) mice lacking PKA RIIβ, the PKA regulatory protein that links PKA to NMDARs at synaptic sites. In contrast, in young adult (P21–P28) mice lacking PKA RIIβ, LTP is normal and LTD is abolished. These findings indicate that distinct PKA isoforms may subserve distinct forms of synaptic plasticity and are consistent with a developmental switch in the signaling cascades required for LTP induction.