Postsynaptic protein phosphorylation and LTP

Neuronal plasticity in the cingulate cortex of rats following esophageal acid exposure in early life

BACKGROUND & AIMS

The cingulate cortex has been reported to be involved in processing pain of esophageal origin. However, little is known about molecular changes and cortical activation that arise from early-life esophageal acid reflux. Excitatory neurotransmission via activation of the N-methyl-d-aspartate (NMDA) receptor and its interaction with postsynaptic density protein 95 (PSD-95) at the synapse appear to mediate neuronal development and plasticity. We investigated the effect of early-life esophageal acid exposure on NMDA receptor subunits and PSD-95 expression in the developing cingulate cortex.

METHODS

We assessed NMDA receptor subunits and PSD-95 protein expression in rostral cingulate cortex (rCC) tissues of rats exposed to esophageal acid or saline (control), either during postnatal day (P) 7 to 14 and/or acutely at adult stage (P60) using immunoblot and immunoprecipitation analyses.

RESULTS

Compared with controls, acid exposure from P7 to P14 significantly increased expression of NR1, NR2A, and PSD-95, measured 6 weeks after exposure. However, acute exposure at P60 caused a transient increase in expression of NMDA receptor subunits. These molecular changes were more robust in animals exposed to acid neonatally and rechallenged, acutely, at P60. Esophageal acid exposure induced calcium calmodulin kinase II-mediated phosphorylation of the subunit NR2B at Ser1303.

CONCLUSIONS

Esophageal acid exposure during early stages of life has long-term effects as a result of phosphorylation of the NMDA receptor and overexpression in the rCC. This molecular alteration in the rCC might mediate sensitization of patients with acid-induced esophageal disorders.

Neurotoxicity of brominated flame retardants: (in)direct effects of parent and hydroxylated polybrominated diphenyl ethers on the (developing) nervous system

BACKGROUND/OBJECTIVE

Polybrominated diphenyl ethers (PBDEs) and their hydroxylated (OH-) or methoxylated forms have been detected in humans. Because this raises concern about adverse effects on the developing brain, we reviewed the scientific literature on these mechanisms.

DATA SYNTHESIS

Many rodent studies reported behavioral changes after developmental, neonatal, or adult exposure to PBDEs, and other studies documented subtle structural and functional alterations in brains of PBDE-exposed animals. Functional effects have been observed on synaptic plasticity and the glutamate-nitric oxide-cyclic guanosine monophosphate pathway. In the brain, changes have been observed in the expression of genes and proteins involved in synapse and axon formation, neuronal morphology, cell migration, synaptic plasticity, ion channels, and vesicular neurotransmitter release. Cellular and molecular mechanisms include effects on neuronal viability 
(via apoptosis and oxidative stress), neuronal differentiation and migration, neurotransmitter release/uptake, neurotransmitter receptors and ion channels, calcium (Ca²⁺) homeostasis, and intracellular signaling pathways.

DISCUSSION

Bioactivation of PBDEs by hydroxylation has been observed for several endocrine end points. This has also been observed for mechanisms related to neurodevelopment, including binding to thyroid hormone receptors and transport proteins, disruption of Ca²⁺ homeostasis, and modulation of GABA and nicotinic acetylcholine receptor function.

CONCLUSIONS

The increased hazard for developmental neurotoxicity by hydroxylated (OH-)PBDEs compared with their parent congeners via direct neurotoxicity and thyroid disruption clearly warrants further investigation into a) the role of oxidative metabolism in producing active metabolites of PBDEs and their impact on brain development; b) concentrations of parent and OH-PBDEs in the brain; and c) interactions between different environmental contaminants during exposure to mixtures, which may increase neurotoxicity.

Sigma-1 receptor stimulation by dehydroepiandrosterone ameliorates cognitive impairment through activation of CaM kinase II, protein kinase C and extracellular signal-regulated kinase in olfactory bulbectomized mice

Dehydroepiandrosterone (DHEA) is one of the most abundant neurosteroids synthesized de novo in the CNS. We here found that sigma-1 receptor stimulation by DHEA improves cognitive function through phosphorylation of synaptic proteins in olfactory bulbectomized (OBX) mouse hippocampus. We have previously reported that calcium/calmodulin-dependent protein kinase II (CaMKII), protein kinase C (PKC) and extracellular signal-regulated kinase (ERK) were impaired in OBX mouse hippocampus. OBX mice were administered once a day for 7-8 days with DHEA (30 or 60 mg/kg p.o.) 10 days after operation. The spatial, cognitive and conditioned fear memories in OBX mice were significantly improved as assessed by Y-maze, novel object recognition and passive avoidance task, respectively. DHEA also improved impaired hippocampal long-term potentiation in OBX mice. Notably, DHEA treatment restored PKCα (Ser-657) autophosphorylation and NR1 (Ser-896) and myristoylated alanine-rich protein kinase C substrate (Ser-152/156) phosphorylation to the control levels in the hippocampal CA1 region. Likewise, DHEA treatment improved CaMKIIα (Thr-286) autophosphorylation and GluR1 (Ser-831) phosphorylation to the control levels in the CA1 region. Furthermore, DHEA treatment improved ERK and cAMP-responsive element-binding protein (Ser-133) phosphorylation to the control levels. Finally, NE-100, sigma-1 receptor antagonist, significantly inhibited the DHEA-induced improvement of memory-related behaviors and CaMKII, PKC and ERK phosphorylation in CA1 region. Taken together, sigma-1 receptor stimulation by DHEA ameliorates OBX-induced impairment in memory-related behaviors and long-term potentiation in the hippocampal CA1 region through activation of CaMKII, PKC and ERK.

Reversible palmitoylation regulates surface stability of AMPA receptors in the nucleus accumbens in response to cocaine in vivo

BACKGROUND

Palmitoylation is emerging as one of the most important posttranslational modifications of excitatory synaptic proteins in mammalian brain cells. As a reversible and regulatable modification sensitive to changing synaptic inputs, palmitoylation of ionotropic glutamate receptors contributes not only to the modulation of normal receptor and synaptic activities but also to the pathogenesis of various neuropsychiatric disorders. Here we report that palmitoylation of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor is regulated by the psychostimulant, cocaine, and such regulation is involved in cocaine action.

METHODS

We tested palmitoylation and surface expression of AMPA receptors in striatal neurons and psychomotor behavior in response to cocaine in rats.

RESULTS

All four AMPA receptor subunits (GluA1-4 or GluR1-4) are palmitoylated in the nucleus accumbens (NAc) of adult rats. Among them, GluA1 and GluA3 are preferentially upregulated in their palmitoylation levels by a systemic injection of cocaine. The upregulated GluA1 and 3 palmitoylation is a transient and reversible event. Consequently, it increases the susceptibility of surface-expressed GluA1 and 3 to internalization trafficking, leading to a temporal loss of surface receptor expression. Blockade of the regulated GluA1/3 palmitoylation with a palmitoylation inhibitor in the local NAc reverses the loss of surface GluA1/3. The inhibition of palmitoylation concurrently sustains behavioral responsivity to cocaine as well.

CONCLUSIONS

Our data identify a novel drug-palmitoylation coupling in the center of limbic reward circuits. Through palmitoylating selective AMPA receptor subunits, cocaine activity dependently regulates trafficking and subcellular localization of the receptor in NAc neurons and dynamically controls psychomotor sensitivity to the psychoactive drug in vivo.

Ultra-low exposure to α-7 nicotinic acetylcholine receptor partial agonists elicits an improvement in cognition that corresponds with an increase in α-7 receptor expression in rodents: implications for low dose clinical efficacy

Αlpha-7 neuronal nicotinic receptors (NNRs) are considered targets for cognitive enhancement in schizophrenia and Alzheimer's disease. AZD0328 is an alpha-7 NNR partial agonist that enhances cognition in rodents and nonhuman primates at sub-microgram to microgram doses. We hypothesized that increased expression of the alpha-7 receptor contributes to this beneficial activity at low doses and tested this by examining the effect of AZD0328 using in vivo and ex vivo binding, RT-PCR and cognitive function in rodents. AZD0328 (0.00178 mg/kg) was subcutaneously administered to mice 4, 24, 48 and 72 hours prior to testing in novel object recognition and produced a significant increase in cognition at 4, 24 and 48 h post-dosing. In vivo binding was examined in rat brain using [(3)H]AZ11637326 and there was a dose-dependent reduction in receptor binding at higher doses of AZD0328 (0.001-3 mg/kg), and a second alpha-7 partial agonist, SSR180711 (0.01-30 mg/kg). Lower doses of both compounds (0.0001 mg/kg) produced a significant increase in binding of [(3)H]AZ11637326. Ex vivo binding using [(125)I]-α-bungarotoxin, showed a significant increase in receptor number (B(max.)) in the frontal cortex or hippocampus with no significant effect on receptor affinity (K(d)) 2 h post administration of AZD0328. [(3)H]AZ11637326 administered 1.5 h following AZD0328 produced a significant increase in specific binding in rat brain regions. We found that the effect on receptor number was long-lasting, with [(125)I]-α-bungarotoxin binding increased in rats given AZD0328 for 2-48 h, but this was not accompanied by increased mRNA synthesis. SSR180711 produced a similar increase in B(max.) and specific binding with no effect on K(d). Therefore, trace dose of alpha-7 partial agonists has rapid onset and produces a profound, sustained effect on novel object recognition in mice that corresponds by dose to an increase in receptor number in rat brain. These findings provide an explanation for the acute and sustained benefit of alpha-7 receptor activation in working memory in nonhuman primates and guidance for drug development initiatives and treatment regimens for nicotinic partial agonists.

Cross-talk between P2X4 and gamma-aminobutyric acid, type A receptors determines synaptic efficacy at a central synapse

The essence of neuronal function is to generate outputs in response to synaptic potentials. Synaptic integration at postsynaptic sites determines neuronal outputs in the CNS. Using immunohistochemical and electrophysiological approaches, we first reveal that steroidogenic factor 1 (SF-1) green fluorescent protein (GFP)-positive neurons in the ventromedial nucleus of the hypothalamus express P2X4 subunits that are activated by exogenous ATP. Increased membrane expression of P2X4 channels by using a peptide competing with P2X4 intracellular endocytosis motif enhances neuronal excitability of SF-1 GFP-positive neurons. This increased excitability is inhibited by a P2X receptor antagonist. Furthermore, increased surface P2X4 receptor expression significantly decreases the frequency and the amplitude of GABAergic postsynaptic currents of SF-1 GFP-positive neurons. Co-immunopurification and pulldown assays reveal that P2X4 receptors complex with aminobutyric acid, type A (GABA(A)) receptors and demonstrate that two amino acids in the carboxyl tail of the P2X4 subunit are crucial for its physical association with GABA(A) receptors. Mutation of these two residues prevents the physical association, thereby blocking cross-inhibition between P2X4 and GABA(A) receptors. Moreover, disruption of the physical coupling using competitive peptides containing the identified motif abolishes current inhibition between P2X4 and GABA(A) receptors in recombinant system and P2X4 receptor-mediated GABAergic depression in SF-1 GFP-positive neurons. Our present work thus provides evidence for cross-talk between excitatory and inhibitory receptors that appears to be crucial in determining GABAergic synaptic strength at a central synapse.

Glycosides, depression and suicidal behaviour: the role of glycoside-linked proteins

Nowadays depression and suicide are two of the most important worldwide public health problems. Although their specific molecular mechanisms are still largely unknown, glycosides can play a fundamental role in their pathogenesis. These molecules act presumably through the up-regulation of plasticity-related proteins: probably they can have a presynaptic facilitatory effect, through the activation of several intracellular signaling pathways that include molecules like protein kinase A, Rap-1, cAMP, cADPR and G proteins. These proteins take part in a myriad of brain functions such as cell survival and synaptic plasticity. In depressed suicide victims, it has been found that their activity is strongly decreased, primarily in hippocampus and prefrontal cortex. These studies suggest that glycosides can regulate neuroprotection through Rap-1 and other molecules, and may play a crucial role in the pathophysiology of depression and suicide.

Calcium/calmodulin-dependent protein kinases as potential targets of nitric oxide

Nitric oxide (NO) synthesis is controlled by Ca(2+)/calmodulin (CaM) binding with and kinase-dependent phosphorylation of constitutive NO synthases, which catalyze the formation of NO and L-citrulline from L-arginine. NO operates as a mediator of important cell signaling pathways, such as cGMP signaling cascade. Another mechanism by which NO exerts biological effects is mediated via post-translational modification of redox-sensitive cysteine thiols of proteins. The Ca(2+)/CaM-dependent protein kinases (CaM kinases) such as CaM kinase I, CaM kinase II, and CaM kinase IV, are a family of protein kinases which requires binding of Ca(2+)/CaM to and subsequent phosphorylation of the enzymes to initiate its activation process. We report other regulation mechanisms of CaM kinases, such as S-glutathionylation of CaM kinase I at Cys(179) and S-nitrosylation of CaM kinase II at Cys(6/30). Such unique post-translational modification of CaMKs by NO shed light on a new area of mutual regulation of NO- and CaM kinases-signals. Based on the novel direct regulation of these kinases, we propose that CaM kinases/NO signaling would be good targets for understanding how they can participate in neuronal physiology and disease.

Postsynaptic signal transduction models for long-term potentiation and depression

More than a hundred biochemical species, activated by neurotransmitters binding to transmembrane receptors, are important in long-term potentiation (LTP) and long-term depression (LTD). To investigate which species and interactions are critical for synaptic plasticity, many computational postsynaptic signal transduction models have been developed. The models range from simple models with a single reversible reaction to detailed models with several hundred kinetic reactions. In this study, more than a hundred models are reviewed, and their features are compared and contrasted so that similarities and differences are more readily apparent. The models are classified according to the type of synaptic plasticity that is modeled (LTP or LTD) and whether they include diffusion or electrophysiological phenomena. Other characteristics that discriminate the models include the phase of synaptic plasticity modeled (induction, expression, or maintenance) and the simulation method used (deterministic or stochastic). We find that models are becoming increasingly sophisticated, by including stochastic properties, integrating with electrophysiological properties of entire neurons, or incorporating diffusion of signaling molecules. Simpler models continue to be developed because they are computationally efficient and allow theoretical analysis. The more complex models permit investigation of mechanisms underlying specific properties and experimental verification of model predictions. Nonetheless, it is difficult to fully comprehend the evolution of these models because (1) several models are not described in detail in the publications, (2) only a few models are provided in existing model databases, and (3) comparison to previous models is lacking. We conclude that the value of these models for understanding molecular mechanisms of synaptic plasticity is increasing and will be enhanced further with more complete descriptions and sharing of the published models.

The biochemistry of memory: The 26year journey of a 'new and specific hypothesis'

This Special Issue of Neurobiology of Learning and Memory dedicated to Dr. Richard Thompson to celebrate his 80th birthday and his numerous contributions to the field of learning and memory gave us the opportunity to revisit the hypothesis we proposed more than 25years ago regarding the biochemistry of learning and memory. This review summarizes our early 1980s hypothesis and then describes how it was tested and modified over the years following its introduction. We then discuss the current status of the hypothesis and provide some examples of how it has led to unexpected insights into the memory problems that accompany a broad range of neuropsychiatric disorders.

Rules of engagement: factors that regulate activity-dependent synaptic plasticity during neural network development

Overproduction and pruning during development is a phenomenon that can be observed in the number of organisms in a population, the number of cells in many tissue types, and even the number of synapses on individual neurons. The sculpting of synaptic connections in the brain of a developing organism is guided by its personal experience, which on a neural level translates to specific patterns of activity. Activity-dependent plasticity at glutamatergic synapses is an integral part of neuronal network formation and maturation in developing vertebrate and invertebrate brains. As development of the rodent forebrain transitions away from an over-proliferative state, synaptic plasticity undergoes modification. Late developmental changes in synaptic plasticity signal the establishment of a more stable network and relate to pronounced perceptual and cognitive abilities. In large part, activation of glutamate-sensitive N-methyl-d-aspartate (NMDA) receptors regulates synaptic stabilization during development and is a necessary step in memory formation processes that occur in the forebrain. A developmental change in the subunits that compose NMDA receptors coincides with developmental modifications in synaptic plasticity and cognition, and thus much research in this area focuses on NMDA receptor composition. We propose that there are additional, equally important developmental processes that influence synaptic plasticity, including mechanisms that are upstream (factors that influence NMDA receptors) and downstream (intracellular processes regulated by NMDA receptors) from NMDA receptor activation. The goal of this review is to summarize what is known and what is not well understood about developmental changes in functional plasticity at glutamatergic synapses, and in the end, attempt to relate these changes to maturation of neural networks.

Mechanisms of induction and maintenance of spike-timing dependent plasticity in biophysical synapse models

We review biophysical models of synaptic plasticity, with a focus on spike-timing dependent plasticity (STDP). The common property of the discussed models is that synaptic changes depend on the dynamics of the intracellular calcium concentration, which itself depends on pre- and postsynaptic activity. We start by discussing simple models in which plasticity changes are based directly on calcium amplitude and dynamics. We then consider models in which dynamic intracellular signaling cascades form the link between the calcium dynamics and the plasticity changes. Both mechanisms of induction of STDP (through the ability of pre/postsynaptic spikes to evoke changes in the state of the synapse) and of maintenance of the evoked changes (through bistability) are discussed.

Phosphorylation-state-dependent regulation of NMDA receptor short-term plasticity modifies hippocampal dendritic Ca2+ transients

N-methyl-D-aspartate (NMDA) receptor-mediated currents are enhanced by phosphorylation. We have investigated effects of phosphorylation-dependent short-term plasticity of NMDA receptor-mediated excitatory postsynaptic currents (EPSCs) on the induction of long-term depression (LTD). We confirmed in whole cell clamped CA1 pyramidal neurons that LTD is induced by pairing stimulus protocols. However, after serine-threonine phosphorylation was modified by postsynaptic introduction of a protein phosphatase-1 (PP1) inhibitor, the same pairing protocol evoked long-term potentiation (LTP). We determined effects of modification of phosphatase activity on evoked NMDA EPSCs during LTD induction protocols. During LTD induction, using a protocol pairing depolarization to -40 mV and 0.5 Hz stimulation, NMDA receptor-mediated EPSCs undergo a short-term enhancement at the start of the protocol. In neurons in which PP1 activity was inhibited, this short-term enhancement was markedly amplified. We then investigated the effect of this enhancement on Ca(2+) entry during the start of the LTD induction protocol. Enhancement of NMDA receptor-mediated responses was accompanied by an amplification of induction protocol-evoked Ca(2+) transients. Furthermore, this amplification required synaptic activation during the protocol, consistent with an enhancement of Ca(2+) entry mediated by NMDA receptor activation. The sign of NMDA receptor-mediated long-term plasticity, whether potentiation or depression depends on the amplitude of the synaptic Ca(2+) transient during induction. We conclude that short-term phosphorylation-dependent plasticity of the NMDA receptor-mediated EPSCs contributes significantly to the effect of phosphatase inhibition on the subsequent induction of LTD or LTP.

The direction of synaptic plasticity mediated by C-fibers in spinal dorsal horn is decided by Src-family kinases in microglia: the role of tumor necrosis factor-alpha

Previous studies have shown that Src-family kinases (SFKs) are selectively activated in spinal microglia following peripheral nerve injury and the activated SFKs play a key role for the development of neuropathic pain. To investigate the underlying mechanism, in the present study the effect of SFKs on long-term potentiation (LTP) at C-fiber synapses in spinal dorsal horn, which is believed as central mechanism of neuropathic pain, was investigated in adult rats. Electrophysiological data revealed that pretreatment with either microglia inhibitor (minocycline, 200 microM) or SFKs inhibitors (PP2, 100 microM and SU6656, 200 microM) reversed the effect of high frequency stimulation (HFS), that is, HFS, which induces long-term potentiation (LTP) normally, induced long-term depression (LTD) after inhibition of either microglia or SFKs. Western blotting analysis showed that the level of phosphorylated SFKs (p-SFKs) in ipsilateral spinal dorsal horn was transiently increased after LTP induced by HFS, starting at 15 min and returning to control level at 60 min after HFS. Double-labeled immunofluorescence staining demonstrated that p-SFKs were highly restricted to microglia. Furthermore, we found that the inhibitory effects of minocycline or SU6656 on spinal LTP were reversed by spinal application of rat recombinant tumor necrosis factor-alpha (TNF-alpha 0.5 ng/ml, 200 microl). HFS failed to induce LTP of C-fiber evoked field potentials in TNF receptor-1 knockout mice and in rats pretreated with TNF-alpha neutralization antibody (0.6 microg/ml, 200 microl). The results suggested that in spinal dorsal horn activation of SFKs in microglia might control the direction of plastic changes at C-fiber synapses and TNF-alpha might be involved in the process.

Mu-opioid receptors transiently activate the Akt-nNOS pathway to produce sustained potentiation of PKC-mediated NMDAR-CaMKII signaling

BACKGROUND

In periaqueductal grey (PAG) matter, cross-talk between the Mu-opioid receptor (MOR) and the glutamate N-methyl-D-Aspartate receptor (NMDAR)-CaMKII pathway supports the development of analgesic tolerance to morphine. In neurons, histidine triad nucleotide binding protein 1 (HINT1) connects the regulators of G protein signaling RGSZ1 and RGSZ2 to the C terminus of the MOR. In response to morphine, this HINT1-RGSZ complex binds PKCgamma, and afterwards, the interplay between PKCgamma, Src and Gz/Gi proteins leads to sustained potentiation of NMDAR-mediated glutamate responses.

METHODOLOGY/PRINCIPAL FINDINGS

Following an intracerebroventricular (icv) injection of 10 nmol morphine, Akt was recruited to the synaptosomal membrane and activated by Thr308 and Ser473 phosphorylation. The Akt activation was immediately transferred to neural Nitric Oxide Synthase (nNOS) Ser1417. Afterwards, nitric oxide (NO)-released zinc ions recruited PKCgamma to the MOR to promote the Src-mediated phosphorylation of the Tyr1325 NMDAR2A subunit. This action increased NMDAR calcium flux and CaMKII was activated in a calcium-calmodulin dependent manner. CaMKII then acted on nNOS Ser847 to produce a sustained reduction in NO levels. The activation of the Akt-nNOS pathway was also reduced by the binding of these proteins to the MOR-HINT1 complex where they remained inactive. Tolerance to acute morphine developed as a result of phosphorylation of MOR cytosolic residues, uncoupling from the regulated G proteins which are transferred to RGSZ2 proteins. The diminished effect of morphine was prevented by LNNA, an inhibitor of nNOS function, and naltrindole, a delta-opioid receptor antagonist that also inhibits Akt.

CONCLUSIONS/SIGNIFICANCE

Analysis of the regulatory phosphorylation of the proteins included in the study indicated that morphine produces a transient activation of the Akt/PKB-nNOS pathway. This activation occurs upstream of PKCgamma and Src mediated potentiation of NMDAR activity, ultimately leading to morphine tolerance. In summary, the Akt-nNOS pathway acts as a primer for morphine-triggered events which leads to the sustained potentiation of the NMDAR-CaMKII pathway and MOR inhibition.

[Cellular and molecular mechanisms of memory]

A defining characteristic of the brain is its remarkable capacity to undergo activity-dependent functional and morphological remodelling via mechanisms of plasticity that form the basis of our capacity to encode and retain memories. Today, it is generally accepted that one key neurobiological mechanism underlying the formation of memories reside in activity-driven modifications of synaptic strength and structural remodelling of neural networks activated during learning. The discovery and detailed report of the phenomenon generally known as long-term potentiation, a long-lasting activity-dependent form of synaptic strengthening, opened a new chapter in the study of the neurobiological substrate of memory in the vertebrate brain, and this form of synaptic plasticity has now become the dominant model in the search for the cellular bases of learning and memory. To date, the key events in the cellular and molecular mechanisms underlying synaptic plasticity and memory formation are starting to be identified. They require the activation of specific receptors and of several molecular cascades to convert extracellular signals into persistent functional changes in neuronal connectivity. Accumulating evidence suggests that the rapid activation of neuronal gene programs is a key mechanism underlying the enduring modification of neural networks required for the laying down of memory. The recent developments in the search for the cellular and molecular mechanisms of memory storage are reviewed.

Critical involvement of postsynaptic protein kinase activation in long-term potentiation at hippocampal mossy fiber synapses on CA3 interneurons

Hippocampal mossy fiber (MF) synapses on area CA3 lacunosum-moleculare (L-M) interneurons are capable of undergoing a Hebbian form of NMDA receptor (NMDAR)-independent long-term potentiation (LTP) induced by the same type of high-frequency stimulation (HFS) that induces LTP at MF synapses on pyramidal cells. LTP of MF input to L-M interneurons occurs only at synapses containing mostly calcium-impermeable (CI)-AMPA receptors (AMPARs). Here, we demonstrate that HFS-induced LTP at these MF-interneuron synapses requires postsynaptic activation of protein kinase A (PKA) and protein kinase C (PKC). Brief extracellular stimulation of PKA with forskolin (FSK) alone or in combination with 1-Methyl-3-isobutylxanthine (IBMX) induced a long-lasting synaptic enhancement at MF synapses predominantly containing CI-AMPARs. However, the FSK/IBMX-induced potentiation in cells loaded with the specific PKA inhibitor peptide PKI(6-22) failed to be maintained. Consistent with these data, delivery of HFS to MFs synapsing onto L-M interneurons loaded with PKI(6-22) induced posttetanic potentiation (PTP) but not LTP. Hippocampal sections stained for the catalytic subunit of PKA revealed abundant immunoreactivity in interneurons located in strata radiatum and L-M of area CA3. We also found that extracellular activation of PKC with phorbol 12,13-diacetate induced a pharmacological potentiation of the isolated CI-AMPAR component of the MF EPSP. However, HFS delivered to MF synapses on cells loaded with the PKC inhibitor chelerythrine exhibited PTP followed by a significant depression. Together, our data indicate that MF LTP in L-M interneurons at synapses containing primarily CI-AMPARs requires some of the same signaling cascades as does LTP of glutamatergic input to CA3 or CA1 pyramidal cells.

Alpha7 nAChR-mediated activation of MAP kinase pathways in PC12 cells

The alpha7 nicotinic acetylcholine receptor (alpha7 nAChR) plays a fundamental role in Ca(2+)-dependent activation of signaling pathways that can modulate intracellular events involved in learning and memory. Activation of extracellular signal-regulated kinase-1 and -2 (ERK1/2) are well documented Ca(2+) signaling events, but these have not been well characterized in response to alpha7 nAChR-selective ligands. The present study examined activation of ERK1/2 and explored pathways leading to CREB phosphorylation utilizing alpha7 nAChR-selective ligands in PC12 cells endogenously expressing alpha7 nAChRs. Robust concentration-dependent increase in ERK1/2 phosphorylation was triggered by structurally diverse alpha7 nAChR agonists such as nicotine, choline, GTS-21, SSR-180711A and PNU-282987 in the presence of the positive allosteric modulator (PAM) PNU-120596. This effect was attenuated by selective alpha7 nAChR antagonists or by chelation of extracellular Ca(2+). ERK1/2 phosphorylation was also attenuated by inhibitors of calmodulin-dependent protein kinase II (CaMKII), p38 MAP kinase and mitogen-activated protein kinase kinase1/2 (MEK1/2), indicating the involvement of these kinases upstream of ERK1/2. This was confirmed by direct measurement of p38 MAPK and MEK1/2 phosphorylation. These data suggest that alpha7 nAChR agonist-triggered Ca(2+) transient in PC12 cells induces activation of CaMKII, leading to sequential phosphorylation of p38 MAPK, MEK1/2, ERK1/2 and CREB. Such mechanisms may endow the alpha7 nAChRs with roles in modulating Ca(2+)-dependent intracellular second messenger events implicated in diverse aspects of cognition.

Regulation of AMPA receptor trafficking by O-glycosylation

The present study investigated the role of O-linked beta-N-acetylglucosamine (O-GlcNAc) glycosylation (O-GlcNAcylation) in AMPA receptor trafficking. Alloxan, an inhibitor of O-GlcNAc transferase, potentiated responses of AMPA receptors composed of the GluR1 subunit expressed in Xenopus oocytes. No potentiating effect of alloxan was obtained with mutant GluR1 (S831A) receptor lacking CaMKII phosphorylation site. Alloxan facilitated basal synaptic transmission to approximately 120% of basal levels and enhanced Schaffer collateral-CA1 long-term potentiation (LTP) in rat hippocampal slices, especially in the late phase of the LTP. Alloxan stimulated translocation of the GluR1 and GluR2 subunit from the cytosol towards the plasma membrane in rat hippocampal slices with the LTP, although it had no effect on subcellular distribution of the NR1 subunit. Taken together, the results of the present study show that alloxan regulates AMPA receptor trafficking by inhibiting O-GlcNAcylation, to modulate hippocampal synaptic transmission and synaptic plasticity.

X-ray structure, symmetry and mechanism of an AMPA-subtype glutamate receptor

Ionotropic glutamate receptors mediate most excitatory neurotransmission in the central nervous system and function by opening a transmembrane ion channel upon binding of glutamate. Despite their crucial role in neurobiology, the architecture and atomic structure of an intact ionotropic glutamate receptor are unknown. Here we report the crystal structure of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-sensitive, homotetrameric, rat GluA2 receptor at 3.6 A resolution in complex with a competitive antagonist. The receptor harbours an overall axis of two-fold symmetry with the extracellular domains organized as pairs of local dimers and with the ion channel domain exhibiting four-fold symmetry. A symmetry mismatch between the extracellular and ion channel domains is mediated by two pairs of conformationally distinct subunits, A/C and B/D. Therefore, the stereochemical manner in which the A/C subunits are coupled to the ion channel gate is different from the B/D subunits. Guided by the GluA2 structure and site-directed cysteine mutagenesis, we suggest that GluN1 and GluN2A NMDA (N-methyl-d-aspartate) receptors have a similar architecture, with subunits arranged in a 1-2-1-2 pattern. We exploit the GluA2 structure to develop mechanisms of ion channel activation, desensitization and inhibition by non-competitive antagonists and pore blockers.

Regulation of AMPA receptors in spinal nociception

The functional properties of α-amino-3-hydroxy-5-methy-4-isoxazole propionate (AMPA) receptors in different brain regions, such as hippocampus and cerebellum, have been well studied in vitro and in vivo. The AMPA receptors present a unique characteristic in the mechanisms of subunit regulation during LTP (long-term potentiation) and LTD (long-term depression), which are involved in the trafficking, altered composition and phosphorylation of AMPA receptor subunits. Accumulated data have demonstrated that spinal AMPA receptors play a critical role in the mechanism of both acute and persistent pain. However, less is known about the biochemical regulation of AMPA receptor subunits in the spinal cord in response to painful stimuli. Recent studies have shown that some important regulatory processes, such as the trafficking of AMPA receptor subunit, subunit compositional changes, phosphorylation of AMPA receptor subunits, and their interaction with partner proteins may contribute to spinal nociceptive transmission. Of all these regulation processes, the phosphorylation of AMPA receptor subunits is the most important since it may trigger or affect other cellular processes. Therefore, these study results may suggest an effective strategy in developing novel analgesics targeting AMPA receptor subunit regulation that may be useful in treating persistent and chronic pain without unacceptable side effects in the clinics.

Regulation of CaMKII by phospho-Thr253 or phospho-Thr286 sensitive targeting alters cellular function

Calcium/calmodulin-stimulated protein kinase II (CaMKII) is an important mediator of synaptic function that is regulated by multi-site phosphorylation and targeting through interactions with proteins. A new phosphorylation site at Thr253 has been identified in vivo, that does not alter CaMKII activity, but does alter CaMKII function through interactions with binding proteins. To identify these proteins, as well as to examine the specific effects following Thr253 or Thr286 phosphorylation on these interactions, we developed an in vitro overlay binding assay. We demonstrated that the interaction between CaMKII and its binding proteins was altered by the phosphorylation state of both the CaMKII and the partner, and identified a CaMKII-specific sequence that was responsible for the interaction between CaMKII and two interacting proteins. By comparing CaMKII binding profiles in tissue and cell extracts, we demonstrated that the CaMKII binding profiles varied with cell type, and also showed that overexpression of a CaMKII Thr253 phospho-mimic mutant in human neuroblastoma and breast cancer cells dramatically altered the morphology and growth rates when compared to overexpression of non-phosphorylated CaMKII. This data highlights the importance of the microenvironment in regulating CaMKII function, and describes a potentially new mechanism by which the functions of CaMKII can be regulated.

Positive allosteric activation of GABAA receptors bi-directionally modulates hippocampal glutamate plasticity and behaviour

Long-term BZ (benzodiazepine) anxiolytic therapy increases the risk of physical dependence manifested as withdrawal anxiety. BZ-induced potentiation of GABA(A)R (gamma-aminobutyric acid type-A receptor) function by 1-week oral administration of FZP (flurazepam) bi-directionally modulates excitatory glutamatergic synaptic transmission in hippocampal CA1 neurons during drug withdrawal. Previous electrophysiological studies on acutely isolated and intact CA1 neurons, as well as immunofluorescence and post-embedding immunogold electron microscopy studies, suggest increased synaptic insertion of GluR (glutamate receptor) 2-lacking AMPARs (alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptors) in 2-day FZP-withdrawn rats. Preliminary studies indicated a similar increase in GluR1, then phospho-Ser(831)-GluR1, as well as CaMKIIalpha (Ca(2+)/calmodulin-dependent protein kinase IIalpha), but not phospho-Thr(286)-CaMKII levels at the same time point. In our studies, whole-cell recordings in hippocampal slices revealed that AMPAR mEPSC [miniature EPSC (excitatory postsynaptic current)] amplitude was increased in 1-day FZP-withdrawn rats followed by an increase in estimated single-channel conductance in 2-day-FZP-withdrawn rats. Enhanced conductance was no longer observed in slices pre-incubated for 2 h in the CaMKII inhibitor KN-93, but not the inactive analogue KN-92. To evaluate whether CaMKII-mediated AMPA potentiation could occlude LTP (long-term potentiation), LTP was induced by TBS (theta burst stimulation) and recorded using whole-cell and extracellular techniques. LTP was induced in both groups, but only maintained for <15 min in 2-day FZP-withdrawn rats. LTP was fully restored after 7-day withdrawal. Despite the lack of LTP maintenance, impairment of object recognition, place and context was not observed in 2-day-FZP-withdrawn rats. Since L-VGCC (L-type voltage-gated calcium channel) current density was doubled on drug withdrawal and up to 2 days, Ca(2+) entry through L-VGCCs and perhaps subsequently through Ca(2+)-permeable AMPARs are proposed to be responsible for enhanced CaMKIIalpha levels and AMPAR potentiation. Mechanisms associated with several different models of activity-dependent plasticity may underlie BZ physical dependence.

Dopamine-stimulated dephosphorylation of connexin 36 mediates AII amacrine cell uncoupling

Gap junction proteins form the substrate for electrical coupling between neurons. These electrical synapses are widespread in the CNS and serve a variety of important functions. In the retina, connexin 36 (Cx36) gap junctions couple AII amacrine cells and are a requisite component of the high-sensitivity rod photoreceptor pathway. AII amacrine cell coupling strength is dynamically regulated by background light intensity, and uncoupling is thought to be mediated by dopamine signaling via D(1)-like receptors. One proposed mechanism for this uncoupling involves dopamine-stimulated phosphorylation of Cx36 at regulatory sites, mediated by protein kinase A. Here we provide evidence against this hypothesis and demonstrate a direct relationship between Cx36 phosphorylation and AII amacrine cell coupling strength. Dopamine receptor-driven uncoupling of the AII network results from protein kinase A activation of protein phosphatase 2A and subsequent dephosphorylation of Cx36. Protein phosphatase 1 activity negatively regulates this pathway. We also find that Cx36 gap junctions can exist in widely different phosphorylation states within a single neuron, implying that coupling is controlled at the level of individual gap junctions by locally assembled signaling complexes. This kind of synapse-by-synapse plasticity allows for precise control of neuronal coupling, as well as cell-type-specific responses dependent on the identity of the signaling complexes assembled.

Protein phosphatase 1 regulates the histone code for long-term memory

Chromatin remodeling through histone posttranslational modifications (PTMs) and DNA methylation has recently been implicated in cognitive functions, but the mechanisms involved in such epigenetic regulation remain poorly understood. Here, we show that protein phosphatase 1 (PP1) is a critical regulator of chromatin remodeling in the mammalian brain that controls histone PTMs and gene transcription associated with long-term memory. Our data show that PP1 is present at the chromatin in brain cells and interacts with enzymes of the epigenetic machinery including HDAC1 (histone deacetylase 1) and histone demethylase JMJD2A (jumonji domain-containing protein 2A). The selective inhibition of the nuclear pool of PP1 in forebrain neurons in transgenic mice is shown to induce several histone PTMs that include not only phosphorylation but also acetylation and methylation. These PTMs are residue-specific and occur at the promoter of genes important for memory formation like CREB (cAMP response element-binding protein) and NF-kappaB (nuclear factor-kappaB). These histone PTMs further co-occur with selective binding of RNA polymerase II and altered gene transcription, and are associated with improved long-term memory for objects and space. Together, these findings reveal a novel mechanism for the epigenetic control of gene transcription and long-term memory in the adult brain that depends on PP1.

Increased Fos expression among midbrain dopaminergic cell groups during birdsong tutoring

During avian vocal learning, birds memorize conspecific song patterns and then use auditory feedback to match their vocal output to this acquired template. Some models of song learning posit that during tutoring, conspecific visual, social and/or auditory cues activate neuromodulatory systems that encourage acquisition of the tutor's song and attach incentive value to that specific acoustic pattern. This hypothesis predicts that stimuli experienced during social tutoring activate cell populations capable of signaling reward. Using immunocytochemistry for the protein product of the immediate early gene c-Fos, we found that brief exposure of juvenile male zebra finches to a live familiar male tutor increased the density of Fos+ cells within two brain regions implicated in reward processing: the ventral tegmental area (VTA) and substantia nigra pars compacta (SNc). This activation of Fos appears to involve both dopaminergic and non-dopaminergic VTA/SNc neurons. Intriguingly, a familiar tutor was more effective than a novel tutor in stimulating Fos expression within these regions. In the periaqueductal gray, a dopamine-enriched cell population that has been implicated in emotional processing, Fos labeling also was increased after tutoring, with a familiar tutor again being more effective than a novel conspecific. As several neural regions implicated in song acquisition receive strong dopaminergic projections from these midbrain nuclei, their activation in conjunction with hearing the tutor's song could help to establish sensory representations that later guide motor sequence learning.

Domoic acid induces a long-lasting enhancement of CA1 field responses and impairs tetanus-induced long-term potentiation in rat hippocampal slices

Domoic acid (DOM) is known to cause hippocampal neuronal damage and produces amnesic effects. We examined synaptic plasticity changes induced by DOM exposure in rat hippocampal CA1 region. Brief bath application of DOM to hippocampal slices produces a chemical form of long-term potentiation (LTP) of CA1 field synaptic potentials. The potentiation cannot be blocked by NMDA receptor antagonist MK-801 but can be blocked by the calcium-calmodulin-dependent protein kinase II (CaMKII) inhibitor KN-62 or cAMP-dependent protein kinase (PKA) inhibitor H-89. DOM-potentiated slices show decreased autophosphorylated CaMKII (p-Thr286), an effect that is also dependent on the activity of CaMKII and PKA. Increased phosphorylation of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor subunit GluR1 (p-Ser831) was seen in DOM-potentiated slices. Therefore, aberrant regulation of CaMKII and GluR1 phosphorylation occurs after DOM application. In addition, tetanus-induced LTP as well as the increase of phosphorylation of CaMKII (p-Thr286) were reduced in DOM-potentiated slices. Compared with brief exposure, slices recovering from prolonged exposure did not show potentiation or altered levels of CaMKII (p-Thr286) or GluR (p-Ser831). However, decreased phosphorylation of GluR1 at Ser845 was seen. These results describe a new chemical form of LTP and uncover novel molecular changes induced by DOM. The observed impairment of tetanus LTP and misregulation of CaMKII and GluR1 phosphorylation may partially account for DOM neurotoxicity and underlie the molecular basis for DOM-induced memory deficit.

Substance P enhances excitatory synaptic transmission on spinally projecting neurons in the rostral ventromedial medulla after inflammatory injury

It has been proposed, but not directly tested, that persistent inflammatory nociception enhances excitatory glutamatergic inputs to neurons in the rostral ventromedial medulla (RVM), altering the activity and function of these neurons. This study used whole cell patch-clamp methods to record evoked excitatory postsynaptic currents (eEPSCs) in spinally projecting RVM neurons from rats injected with saline or complete Freund's adjuvant (CFA) 3-4 days earlier and to examine the role of substance P (SP) in modulating excitatory synaptic transmission. Input-output relationships demonstrated that CFA treatment facilitated fast excitatory glutamatergic inputs to type 1 and type 2 nonserotonergic spinally projecting RVM neurons, but not to type 3 neurons. The facilitation in type 1 and 2 neurons was dependent on neurokinin-1 (NK1) and N-methyl-d-aspartate (NMDA) receptors and prevented by the PKC inhibitor GF109203X. In a subset of neurons from naïve rats, SP mimicked the effects of CFA and increased the potency and efficacy of glutamatergic synaptic transmission. The facilitation was prevented by 10 microM GF109203X, but not by 10 microM KN93, a CaMKII inhibitor. SP (0.3-3 microM) by itself produced concentration-dependent inward currents in most nonserotonergic, but not serotonergic neurons. The present study is the first demonstration, at the cellular level, that persistent inflammatory nociception leads to a sustained facilitation of fast excitatory glutamatergic inputs to RVM neurons by an NK1 and NMDA receptor-dependent mechanism that involves PKC. Further, it demonstrates that the facilitation is restricted to specific populations of RVM neurons that by inference may be pain facilitatory neurons.

Pain and learning in a spinal system: contradictory outcomes from common origins

The long-standing belief that the spinal cord serves merely as a conduit for information traveling to and from the brain is changing. Over the past decade, research has shown that the spinal cord is sensitive to response-outcome contingencies, demonstrating that spinal circuits have the capacity to modify behavior in response to differential environmental cues. If spinally transected rats are administered shock contingent on leg extension (controllable shock), they will maintain a flexion response that minimizes shock exposure. If, however, this contingency is broken, and shock is administered irrespective of limb position (uncontrollable shock), subjects cannot acquire the same flexion response. Interestingly, each of these treatments has a lasting effect on behavior; controllable shock enables future learning, while uncontrollable shock produces a long-lasting learning deficit. Here we suggest that the mechanisms underlying learning and the deficit may have evolved from machinery responsible for the spinal processing of noxious information. Experiments have shown that learning and the deficit require receptors and signaling cascades shown to be involved in central sensitization, including activation of NMDA and neurokinin receptors, as well as CaMKII. Further supporting this link between pain and learning, research has also shown that uncontrollable stimulation results in allodynia. Moreover, systemic inflammation and neonatal hindpaw injury each facilitate pain responding and undermine the ability of the spinal cord to support learning. These results suggest that the plasticity associated with learning and pain must be placed in a balance in order for adaptive outcomes to be observed.

Regulation of CaMKII in vivo: the importance of targeting and the intracellular microenvironment

CaMKII (calcium/calmodulin-stimulated protein kinase II) is a multifunctional protein kinase that regulates normal neuronal function. CaMKII is regulated by multi-site phosphorylation, which can alter enzyme activity, and targeting to cellular microdomains through interactions with binding proteins. These proteins integrate CaMKII into multiple signalling pathways, which lead to varied functional outcomes following CaMKII phosphorylation, depending on the identity and location of the binding partner. A new phosphorylation site on CaMKII (Thr253) has been identified in vivo. Thr253 phosphorylation controls CaMKII purely by targeting, does not effect enzyme activity, and occurs in response to physiological and pathological stimuli in vivo, but only in CaMKII molecules present in specific cellular locations. This new phosphorylation site offers a potentially novel regulatory mechanism for controlling functional responses elicited by CaMKII that are restricted to specific subcellular locations and/or certain cell types, by controlling interactions with proteins that are expressed in the cell at that location.

Separating kindling and LTP: lessons from studies of PKM zeta in developing and adult rats

The kindling model of temporal lobe epilepsy (TLE) and the memory model of long-term potentiation (LTP) may have common underlying mechanisms. This is evident by the demonstration that certain signaling molecules play a key role in both. Recently, a brain specific isoform of protein kinase C (PKMzeta) has been shown to play a significant role in both maintaining LTP and memory storage. We were interested in determining if this kinase had a crossover role in kindling-induced epileptogenesis. Using developing and adult rats we examined the role of PKMzeta in kindling. In developing (P15) rats we determined the effect of PKMzeta on retention of amygdala kindling and kindling rate by intra-amygdala administration of a selective PKMzeta antagonist, ZIP (10 nmol). In adult rats we examined the effect of PKMzeta inhibition, ZIP (10 nmol), on after discharge (AD) thresholds and kindling retention using rapid hippocampal kindling. Inhibition of PKMzeta by the antagonist ZIP did not affect kindling rate or retention in developing rats. In addition there was also no observed effect on AD thresholds and kindling retention in adult rats. Our results show that, despite the similarities between kindling and LTP in their induction, there is dissociation in the role that PKMzeta plays within the two in maintenance. This may be of importance in establishing a separation between the pathophysiological processes involved in sustaining kindling and the physiological mechanisms involved in maintaining LTP and memory storage.

The molecular and cellular biology of enhanced cognition

Most molecular and cellular studies of cognitive function have focused on either normal or pathological states, but recent research with transgenic mice has started to address the mechanisms of enhanced cognition. These results point to key synaptic and nuclear signalling events that can be manipulated to facilitate the induction or increase the stability of synaptic plasticity, and therefore enhance the acquisition or retention of information. Here, we review these surprising findings and explore their implications to both mechanisms of learning and memory and to ongoing efforts to develop treatments for cognitive disorders. These findings represent the beginning of a fundamental new approach in the study of enhanced cognition.

Oxaloacetate restores the long-term potentiation impaired in rat hippocampus CA1 region by 2-vessel occlusion

Various acute brain pathological conditions are characterized by the presence of elevated glutamate concentrations in the brain interstitial fluids. It has been established that a decrease in the blood glutamate level enhances the brain-to-blood efflux of glutamate, removal of which from the brain may prevent glutamate excitotoxicity and its contribution to the long-lasting neurological deficits seen in stroke. A decrease in blood glutamate level can be achieved by exploiting the glutamate-scavenging properties of the blood-resident enzyme glutamate-oxaloacetate transaminase, which transforms glutamate into 2-ketoglutarate in the presence of the glutamate co-substrate oxaloacetate. The present study had the aim of an evaluation of the effects of the blood glutamate scavenger oxaloacetate on the impaired long-term potentiation (LTP) induced in the 2-vessel occlusion ischaemic model in rat. Transient (30-min) incomplete forebrain ischaemia was produced 72 h before LTP induction. Although the short transient brain hypoperfusion did not induce histologically identifiable injuries in the CA1 region (Fluoro-Jade B, S-100 and cresyl violet), it resulted in an impaired LTP function in the hippocampal CA1 region without damaging the basal synaptic transmission between the Schaffer collaterals and the pyramidal neurons. This impairment could be fended off in a dose-dependent manner by the intravenous administration of oxaloacetate in saline (at doses between 1.5 mmol and 0.1 mumol) immediately after the transient hypoperfusion. Our results suggest that oxaloacetate-mediated blood and brain glutamate scavenging contributes to the restoration of the LTP after its impairment by brain ischaemia.

Optical induction of plasticity at single synapses reveals input-specific accumulation of alphaCaMKII

Long-term potentiation (LTP), a form of synaptic plasticity, is a primary experimental model for understanding learning and memory formation. Here, we use light-activated channelrhodopsin-2 (ChR2) as a tool to study the molecular events that occur in dendritic spines of CA1 pyramidal cells during LTP induction. Two-photon uncaging of MNI-glutamate allowed us to selectively activate excitatory synapses on optically identified spines while ChR2 provided independent control of postsynaptic depolarization by blue light. Pairing of these optical stimuli induced lasting increase of spine volume and triggered translocation of alphaCaMKII to the stimulated spines. No changes in alphaCaMKII concentration or cytoplasmic volume were observed in neighboring spines on the same dendrite, providing evidence that alphaCaMKII accumulation at postsynaptic sites is a synapse-specific memory trace of coincident activity.

[Acquiring new information in a neuronal network: from Hebb's concept to homeostatic plasticity]

Synaptic plasticity is the cellular mechanism underlying the phenomena of learning and memory. Much of the research on synaptic plasticity is based on the postulate of Hebb (1949) who proposed that, when a neuron repeatedly takes part in the activation of another neuron, the efficacy of the connections between these neurons is increased. Plasticity has been extensively studied, and often demonstrated through the processes of LTP (Long Term Potentiation) and LTD (Long Term Depression), which represent an increase and a decrease of the efficacy of long-term synaptic transmission. This review summarizes current knowledge concerning the cellular mechanisms of LTP and LTD, whether at the level of excitatory synapses, which have been the most studied, or at the level of inhibitory synapses. However, if we consider neuronal networks rather than the individual synapses, the consequences of synaptic plasticity need to be considered on a large scale to determine if the activity of networks are changed or not. Homeostatic plasticity takes into account the mechanisms which control the efficacy of synaptic transmission for all the synaptic inputs of a neuron. Consequently, this new concept deals with the coordinated activity of excitatory and inhibitory networks afferent to a neuron which maintain a controlled level of excitability during the acquisition of new information related to the potentiation or to the depression of synaptic efficacy. We propose that the protocols of stimulation used to induce plasticity at the synaptic level set up a "homeostatic potentiation" or a "homeostatic depression" of excitation and inhibition at the level of the neuronal networks. The coordination between excitatory and inhibitory circuits allows the neuronal networks to preserve a level of stable activity, thus avoiding episodes of hyper- or hypo-activity during the learning and memory phases.

Control of excitatory synaptic transmission by C-terminal Src kinase

The induction of long-term potentiation at CA3-CA1 synapses is caused by an N-methyl-d-aspartate (NMDA) receptordependent accumulation of intracellular Ca(2+), followed by Src family kinase activation and a positive feedback enhancement of NMDA receptors (NMDARs). Nevertheless, the amplitude of baseline transmission remains remarkably constant even though low frequency stimulation is also associated with an NMDAR-dependent influx of Ca(2+) into dendritic spines. We show here that an interaction between C-terminal Src kinase (Csk) and NMDARs controls the Src-dependent regulation of NMDAR activity. Csk associates with the NMDAR signaling complex in the adult brain, inhibiting the Src-dependent potentiation of NMDARs in CA1 neurons and attenuating the Src-dependent induction of long-term potentiation. Csk associates directly with Src-phosphorylated NR2 subunits in vitro. An inhibitory antibody for Csk disrupts this physical association, potentiates NMDAR mediated excitatory postsynaptic currents, and induces long-term potentiation at CA3-CA1 synapses. Thus, Csk serves to maintain the constancy of baseline excitatory synaptic transmission by inhibiting Src kinase-dependent synaptic plasticity in the hippocampus.

CaM kinase II and protein kinase C activations mediate enhancement of long-term potentiation by nefiracetam in the rat hippocampal CA1 region

Nefiracetam is a pyrrolidine-related nootropic drug exhibiting various pharmacological actions such as cognitive-enhancing effect. We previously showed that nefiracetam potentiates NMDA-induced currents in cultured rat cortical neurons. To address questions whether nefiracetam affects NMDA receptor-dependent synaptic plasticity in the hippocampus, we assessed effects of nefiracetam on NMDA receptor-dependent long-term potentiation (LTP) by electrophysiology and LTP-induced phosphorylation of synaptic proteins by immunoblotting analysis. Nefiracetam treatment at 1-1000 nM increased the slope of fEPSPs in a dose-dependent manner. The enhancement was associated with increased phosphorylation of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor through activation of calcium/calmodulin-dependent protein kinase II (CaMKII) without affecting synapsin I phosphorylation. In addition, nefiracetam treatment increased PKCalpha activity in a bell-shaped dose-response curve which peaked at 10 nM, thereby increasing phosphorylation of myristoylated alanine-rich protein kinase C substrate and NMDA receptor. Nefiracetam treatment did not affect protein kinase A activity. Consistent with the bell-shaped PKCalpha activation, nefiracetam treatment enhanced LTP in the rat hippocampal CA1 region with the same bell-shaped dose-response curve. Furthermore, nefiracetam-induced LTP enhancement was closely associated with CaMKII and PKCalpha activation with concomitant increases in phosphorylation of their endogenous substrates except for synapsin I. These results suggest that nefiracetam potentiates AMPA receptor-mediated fEPSPs through CaMKII activation and enhances NMDA receptor-dependent LTP through potentiation of the post-synaptic CaMKII and protein kinase C activities. Together with potentiation of nicotinic acetylcholine receptor function, nefiracetam-enhanced AMPA and NMDA receptor functions likely contribute to improvement of cognitive function.

Nutritional factors in a mouse model of Rett syndrome

Environmental factors such as nutrition and housing can influence behavioral and anatomical characteristics of several neurological disorders, including Rett syndrome (RTT). RTT is associated with mutations in the X-linked gene encoding MeCP2, a transcriptional repressor that binds methylated DNA. While direct genetic intervention in humans is impossible at this time, motor and cognitive deficits in RTT may be ameliorated through manipulations of epigenetic/environmental factors. For example, studies in rodents suggest that choline nutrient supplementation during critical periods of brain development enhances cholinergic neurotransmission, alters neuronal size and distribution, and facilitates performance of memory and motoric tasks. Recent work in a mouse model of RTT shows that enhancing maternal nutrition through choline supplementation improves both anatomical and behavioral symptoms in the mutant offspring. We describe here cellular and molecular mechanisms that may underlie this specific enhancement and may provide more general insights into mechanisms underlying gene-environment interactions in neurological disorders.

Integrin regulation of cytoplasmic calcium in excitatory neurons depends upon glutamate receptors and release from intracellular stores

Integrins regulate cytoplasmic calcium levels ([Ca(2+)]i) in various cell types but information on activities in neurons is limited. The issue is of current interest because of the evidence that both integrins and changes in [Ca(2+)]i are required for Long-Term Potentiation. Accordingly, the present studies evaluated integrin ligand effects in cortical neurons. Integrin ligands or alpha5beta1 integrin activating antisera rapidly increased [Ca(2+)]i with effects greater in glutamatergic than GABAergic neurons, absent in astroglia, and blocked by beta1 integrin neutralizing antisera and the tyrosine kinase antagonist genistein. Increases depended upon extracellular calcium and intracellular store release. Ligand-induced effects were reduced by voltage-sensitive calcium channel and NMDA receptor antagonists, but blocked by tetrodotoxin or AMPA receptor antagonists. These results indicate that integrin ligation triggers AMPA receptor/depolarization-dependent calcium influx followed by intracellular store release and suggest the possibility that integrin modulation of activity-induced changes in [Ca(2+)]i contributes importantly to lasting synaptic plasticity in forebrain neurons.

Removal of S6K1 and S6K2 leads to divergent alterations in learning, memory, and synaptic plasticity

Protein synthesis is required for the expression of enduring memories and long-lasting synaptic plasticity. During cellular proliferation and growth, S6 kinases (S6Ks) are activated and coordinate the synthesis of de novo proteins. We hypothesized that protein synthesis mediated by S6Ks is critical for the manifestation of learning, memory, and synaptic plasticity. We have tested this hypothesis with genetically engineered mice deficient for either S6K1 or S6K2. We have found that S6K1-deficient mice express an early-onset contextual fear memory deficit within one hour of training, a deficit in conditioned taste aversion (CTA), impaired Morris water maze acquisition, and hypoactive exploratory behavior. In contrast, S6K2-deficient mice exhibit decreased contextual fear memory seven days after training, a reduction in latent inhibition of CTA, and normal spatial learning in the Morris water maze. Surprisingly, neither S6K1- nor S6K2-deficient mice exhibited alterations in protein synthesis-dependent late-phase long-term potentiation (L-LTP). However, removal of S6K1, but not S6K2, compromised early-phase LTP expression. Furthermore, we observed that S6K1-deficient mice have elevated basal levels of Akt phosphorylation, which is further elevated following induction of L-LTP. Taken together, our findings demonstrate that removal of S6K1 leads to a distinct array of behavioral and synaptic plasticity phenotypes that are not mirrored by the removal of S6K2. Our observations suggest that neither gene by itself is required for L-LTP but instead may be required for other types of synaptic plasticity required for cognitive processing.

Mechanisms Underlying Dedepression of Synaptic NMDA Receptors in the Hippocampus

N-Methyl-d-aspartate receptor (NMDAR)–mediated synaptic responses in hippocampal CA1 pyramidal cells are depressed during NMDAR-dependent long-term depression (LTD) due to mechanisms, in part, distinct from those underlying LTD of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR)–mediated synaptic responses. The mechanisms underlying dedepression of synaptic NMDARs, however, are not known. We find that dedepression of NMDAR-mediated synaptic responses in the CA1 region of the rat hippocampus is input specific and does not require synaptic stimulation to be maintained. The induction of dedepression does not require activation of metabotropic glutamate receptors, L-type Ca2+ channels, or release of Ca2+ from intracellular stores. It does, however, rely on activation of NMDARs. In contrast to the dedepression of AMPAR-mediated synaptic responses, dedepression of NMDAR-mediated synaptic responses does not depend on activation of calcium/calmodulin-dependent protein kinase II, protein kinase C, cAMP-dependent protein kinase, or Src kinases. However, dedepression of synaptic NMDARs is significantly impaired by inhibitors of mitogen-activated protein kinase signaling. Specifically, inhibitors of extracellular signal-regulated kinase 1/2 prevented normal dedepression of synaptic NMDARs by a mechanism that did not require protein synthesis. These results provide further evidence that synaptic NMDARs can be bidirectionally modified by activity but by mechanisms distinct from those responsible for the activity-dependent, bidirectional modulation of synaptic AMPARs.