Protein kinase C modulates NMDA receptor trafficking and gating

Protein kinase C modulates NMDA receptors in the myenteric plexus of the guinea pig ileum during in vitro ischemia and reperfusion

BACKGROUND

Ischemic episodes lead to profound functional and structural alterations of the gastrointestinal tract which may contribute to disorders of intestinal motility. Enhancement of glutamate overflow and the consequent activation of NMDA (N-methyl-D-aspartate) receptors may participate to such changes by modulating different enteric neurotransmitter systems, including cholinergic motor pathways.

METHODS

The molecular mechanism/s underlying activation of NMDA receptors in the guinea pig ileum were investigated after glucose/oxygen deprivation (in vitro ischemia) and during reperfusion.

KEY RESULTS

The number of ileal myenteric neurons positive for NR1, the functional subunit of NMDA receptors, and its mRNA levels were unchanged after in vitro ischemia/reperfusion. In these conditions, the protein levels of NR1, and of its phosphorylated form by protein kinase C (PKC), significantly increased in myenteric neurons, whereas, the levels of NR1 phosphorylated by protein kinase A (PKA) did not change, with respect to control values. Spontaneous glutamate overflow increased during in vitro ischemia/reperfusion. In these conditions, the NMDA receptor antagonists, D(-)-2-amino-5-phosphonopentanoic acid [(D)-AP5] (10 μmol L(-1)) and 5,7-dichlorokynurenic acid (5,7-diClKyn acid) (10 μmol L(-1)) and the PKC antagonist, chelerythrine (1 μmol L(-1)), but not the PKA antagonist, H-89 (1 μmol L(-1)), were able to significantly depress the increased glutamate efflux.

CONCLUSIONS & INFERENCES

The present data suggest that in the guinea pig ileum during in vitro ischemia/reperfusion, NR1 protein levels increase. Such event may rely upon posttranscriptional events involving NR1 phosphorylation by PKC. Increased NR1 levels may, at least in part, explain the ability of NMDA receptors to modulate a positive feedback on ischemia/reperfusion-induced glutamate overflow.

Glutamate pharmacology and metabolism in peripheral primary afferents: physiological and pathophysiological mechanisms

In addition to using glutamate as a neurotransmitter at central synapses, many primary sensory neurons release glutamate from peripheral terminals. Primary sensory neurons with cell bodies in dorsal root or trigeminal ganglia produce glutaminase, the synthetic enzyme for glutamate, and transport the enzyme in mitochondria to peripheral terminals. Vesicular glutamate transporters fill neurotransmitter vesicles with glutamate and they are shipped to peripheral terminals. Intense noxious stimuli or tissue damage causes glutamate to be released from peripheral afferent nerve terminals and augmented release occurs during acute and chronic inflammation. The site of action for glutamate can be at the autologous or nearby nerve terminals. Peripheral nerve terminals contain both ionotropic and metabotropic excitatory amino acid receptors (EAARs) and activation of these receptors can lower the activation threshold and increase the excitability of primary afferents. Antagonism of EAARs can reduce excitability of activated afferents and produce antinociception in many animal models of acute and chronic pain. Glutamate injected into human skin and muscle causes acute pain. Trauma in humans, such as arthritis, myalgia, and tendonitis, elevates glutamate levels in affected tissues. There is evidence that EAAR antagonism at peripheral sites can provide relief in some chronic pain sufferers.

Insulin, synaptic function, and opportunities for neuroprotection

A steadily growing number of studies have begun to establish that the brain and insulin, while traditionally viewed as separate, do indeed have a relationship. The uptake of pancreatic insulin, along with neuronal biosynthesis, provides neural tissue with the hormone. As well, insulin acts upon a neuronal receptor that, although a close reflection of its peripheral counterpart, is characterized by unique structural and functional properties. One distinction is that the neural variant plays only a limited part in neuronal glucose transport. However, a number of other roles for neural insulin are gradually emerging; most significant among these is the modulation of ligand-gated ion channel (LGIC) trafficking. Notably, insulin has been shown to affect the tone of synaptic transmission by regulating cell-surface expression of inhibitory and excitatory receptors. The manner in which insulin regulates receptor movement may provide a cellular mechanism for insulin-mediated neuroprotection in the absence of hypoglycemia and stimulate the exploration of new therapeutic opportunities.

PKC activator therapeutic for mild traumatic brain injury in mice

Traumatic brain injury (TBI) is a frequent consequence of vehicle, sport and war related injuries. More than 90% of TBI patients suffer mild injury (mTBI). However, the pathologies underlying the disease are poorly understood and treatment modalities are limited. We report here that in mice, the potent PKC activator bryostatin1 protects against mTBI induced learning and memory deficits and reduction in pre-synaptic synaptophysin and post-synaptic spinophylin immunostaining. An effective treatment has to start within the first 8h after injury, and includes 5 × i.p. injections over a period of 14 days. The treatment is dose dependent. Exploring the effects of the repeated bryostatin1 treatment on the processing of the amyloid precursor protein, we found that the treatment induced an increase in the putative α-secretase ADAM10 and a reduction in β-secretase activities. Both these effects could contribute towards a reduction in β-amyloid production. These results suggest that bryostatin1 protects against mTBI cognitive and synaptic sequela by rescuing synapses, which is possibly mediated by an increase in ADAM10 and a decrease in BACE1 activity. Since bryostatin1 has already been extensively used in clinical trials as an anti-cancer drug, its potential as a remedy for the short- and long-term TBI sequelae is quite promising.

The muscarinic long-term enhancement of NMDA and AMPA receptor-mediated transmission at Schaffer collateral synapses develop through different intracellular mechanisms

We had described a muscarinic-mediated long-term synaptic enhancement at Schaffer collateral synapses caused by the insertion of AMPARs in spines of rat hippocampal CA1 pyramidal neurons that requires Ca(2+) release from IP3-sensitive stores (Fernández de Sevilla et al., 2008). We now show that this AMPA-mediated LTP(IP3) is precisely matched by an amplification of NMDAR-mediated transmission. The enhanced AMPAR transmission involves SNARE protein activity and CaMKII activation. The amplification of NMDA transmission requires combined CaMKII, PKC, and SRC kinase activity without detectable surface incorporation of NMDARs, suggesting that changes in receptor properties mediate this process. The enhanced AMPAR- and NMDAR-mediated transmission markedly reduce the induction threshold of "Hebbian" LTP. We conclude that both modes of glutamatergic synaptic potentiation may play a critical functional role in the regulation of the learning machinery of the brain by adding flexibility to the demands of the hippocampal network.

Vasoconstriction resulting from dynamic membrane trafficking of TRPM4 in vascular smooth muscle cells

The melastatin (M) transient receptor potential (TRP) channel TRPM4 mediates pressure and protein kinase C (PKC)-induced smooth muscle cell depolarization and vasoconstriction of cerebral arteries. We hypothesized that PKC causes vasoconstriction by stimulating translocation of TRPM4 to the plasma membrane. Live-cell confocal imaging and fluorescence recovery after photobleaching (FRAP) analysis was performed using a green fluorescent protein (GFP)-tagged TRPM4 (TRPM4-GFP) construct expressed in A7r5 cells. The surface channel was mobile, demonstrating a FRAP time constant of 168 +/- 19 s. In addition, mobile intracellular trafficking vesicles were readily detected. Using a cell surface biotinylation assay, we showed that PKC activation with phorbol 12-myristate 13-acetate (PMA) increased (approximately 3-fold) cell surface levels of TRPM4-GFP protein in <10 min. Similarly, total internal reflection fluorescence microscopy demonstrated that stimulation of PKC activity increased (approximately 3-fold) the surface fluorescence of TRPM4-GFP in A7r5 cells and primary cerebral artery smooth muscle cells. PMA also caused an elevation of cell surface TRPM4 protein levels in intact arteries. PMA-induced translocation of TRPM4 to the plasma membrane was independent of PKCalpha and PKCbeta activity but was inhibited by blockade of PKCdelta with rottlerin. Pressure-myograph studies of intact, small interfering RNA (siRNA)-treated cerebral arteries demonstrate that PKC-induced constriction of cerebral arteries requires expression of both TRPM4 and PKCdelta. In addition, pressure-induced arterial myocyte depolarization and vasoconstriction was attenuated in arteries treated with siRNA against PKCdelta. We conclude that PKCdelta activity causes smooth muscle depolarization and vasoconstriction by increasing the number of TRPM4 channels in the sarcolemma.

SNARE force synchronizes synaptic vesicle fusion and controls the kinetics of quantal synaptic transmission

Neuronal communication relies on rapid and discrete intercellular signaling but neither the molecular mechanisms of the exocytotic machinery that define the timing of the action potential-evoked response nor those controlling the kinetics of transmitter release from single synaptic vesicles are known. Here, we investigate how interference with the putative force transduction between the complex-forming SNARE (soluble N-ethylamide-sensitive factor attachment protein receptor) domain and the transmembrane anchor of synaptobrevin II (SybII) affects action potential-evoked currents and spontaneous, quantal transmitter release at mouse hippocampal synapses. The results indicate that SybII-generated membrane stress effectively determines the kinetics of the action potential-evoked response and show that SNARE force modulates the concentration profile of cleft glutamate by controlling the rate of transmitter release from the single synaptic vesicle. Thus, multiple SybII actions determine the exquisite temporal regulation of neuronal signaling.

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.

Glutamate and GABA in the balance: convergent pathways sustain seizures during status epilepticus

Seizures rapidly become self-sustaining and pharmacoresistant to benzodiazepines during status epilepticus (SE). A decrease in the number of postsynaptic gamma-aminobutyric acid (GABA)(A) receptors with SE causes a loss of synaptic inhibition, whereas increases in postsynaptic glutamatergic receptors further upset the balance between excitation and inhibition. Although extracellular GABA levels may increase during SE and contribute to postsynaptic GABA(A) receptor desensitization, other pathways involving glutamatergic activation ultimately may be responsible for the persistent down-regulation of postsynaptic GABA(A) receptors and erosion of synaptic inhibition.

Rapid activation of dormant presynaptic terminals by phorbol esters

Presynaptic stimulation stochastically recruits transmission according to the release probability (P(r)) of synapses. The majority of central synapses have relatively low P(r), which includes synapses that are completely quiescent presynaptically. The presence of presynaptically dormant versus active terminals presumably increases synaptic malleability when conditions demand synaptic strengthening or weakening, perhaps by triggering second messenger signals. However, whether modulator-mediated potentiation involves recruitment of transmission from dormant terminals remains unclear. Here, by combining electrophysiological and fluorescence imaging approaches, we uncovered rapid presynaptic awakening by select synaptic modulators. A phorbol ester phorbol 12,13-dibutyrate (PDBu) (a diacylglycerol analog), but not forskolin (an adenylyl cyclase activator) or elevated extracellular calcium, recruited neurotransmission from presynaptically dormant synapses. This effect was not dependent on protein kinase C activation. After PDBu-induced awakening, these previously dormant terminals had a synaptic P(r) spectrum similar to basally active synapses naive to PDBu treatment. Dormant terminals did not seem to have properties of nascent or immature synapses, judged by NR2B NMDAR (NMDA receptor) receptor subunit contribution after PDBu-stimulated awakening. Strikingly, synapses rendered inactive by prolonged depolarization, unlike basally dormant synapses, were not awakened by PDBu. These results suggest that the initial release competence of synapses can dictate the acute response to second messenger modulation, and the results suggest multiple pathways to presynaptic dormancy and awakening.

Metabotropic glutamate mGlu5 receptor-mediated serine phosphorylation of NMDA receptor subunit NR1 in hippocampal CA1 region after transient global ischemia in rats

Phosphorylation of the NR1 subunit of the N-methyl-d-aspartate (NMDA) receptor has been implicated in the regulation of the receptor's ion channel. The contribution of metabotropic glutamate receptors to the NMDA receptor function after brain ischemia remains to be determined. Presently we investigated the effects of an antagonist of the metabotropic glutamate mGlu5 receptor on cell death and serine phosphorylation of the NR1 subunit of the NMDA receptor in the hippocampal CA1 region after transient global ischemia and sought to explore the mechanisms involved. Phosphorylation of serine residues at 890 and 896 of NR1 was increased predominantly in the deoxycholate (DOC)-insoluble fraction after transient global ischemia in rats; and the increase in the phosphorylation of S890, but not that of S896, of NR1 in this fraction was attenuated by the mGlu5 receptor antagonist. The administration of this antagonist also reduced the increase in the amount of protein kinase C (PKC)gamma, but not that of PKCalpha, in the DOC-insoluble fraction. The results suggest that the mGlu5 receptor in the hippocampal CA1 region is involved in the phosphorylation of S890 of NR1 subunit via PKCgamma following transient ischemia. As treatment with the mGlu5 receptor antagonist reduced cell death in the hippocampal CA1 region on day 3 after the start of the reperfusion, these changes in intracellular signaling through mGlu5 receptor may be linked to the pathogenesis of cerebral ischemia.

Opposing functions of two sub-domains of the SNARE-complex in neurotransmission

The SNARE-complex consisting of synaptobrevin-2/VAMP-2, SNAP-25 and syntaxin-1 is essential for evoked neurotransmission and also involved in spontaneous release. Here, we used cultured autaptic hippocampal neurons from Snap-25 null mice rescued with mutants challenging the C-terminal, N-terminal and middle domains of the SNARE-bundle to dissect out the involvement of these domains in neurotransmission. We report that the stabilities of two different sub-domains of the SNARE-bundle have opposing functions in setting the probability for both spontaneous and evoked neurotransmission. Destabilizing the C-terminal end of the SNARE-bundle abolishes spontaneous neurotransmitter release and reduces evoked release probability, indicating that the C-terminal end promotes both modes of release. In contrast, destabilizing the middle or deleting the N-terminal end of the SNARE-bundle increases both spontaneous and evoked release probabilities. In both cases, spontaneous release was affected more than evoked neurotransmission. In addition, the N-terminal deletion delays vesicle priming after a high-frequency train. We propose that the stability of N-terminal two-thirds of the SNARE-bundle has a function for vesicle priming and limiting spontaneous release.

Metabotropic glutamate receptors in the trafficking of ionotropic glutamate and GABA(A) receptors at central synapses

The trafficking of ionotropic glutamate (AMPA, NMDA and kainate) and GABA(A) receptors in and out of, or laterally along, the postsynaptic membrane has recently emerged as an important mechanism in the regulation of synaptic function, both under physiological and pathological conditions, such as information processing, learning and memory formation, neuronal development, and neurodegenerative diseases. Non-ionotropic glutamate receptors, primarily group I metabotropic glutamate receptors (mGluRs), co-exist with the postsynaptic ionotropic glutamate and GABA(A) receptors. The ability of mGluRs to regulate postsynaptic phosphorylation and Ca(2+) concentration, as well as their interactions with postsynaptic scaffolding/signaling proteins, makes them well suited to influence the trafficking of ionotropic glutamate and GABA(A) receptors. Recent studies have provided insights into how mGluRs may impose such an influence at central synapses, and thus how they may affect synaptic signaling and the maintenance of long-term synaptic plasticity. In this review we will discuss some of the recent progress in this area: i) long-term synaptic plasticity and the involvement of mGluRs; ii) ionotropic glutamate receptor trafficking and long-term synaptic plasticity; iii) the involvement of postsynaptic group I mGluRs in regulating ionotropic glutamate receptor trafficking; iv) involvement of postsynaptic group I mGluRs in regulating GABA(A) receptor trafficking; v) and the trafficking of postsynaptic group I mGluRs themselves.

Nerve growth factor-regulated emergence of functional delta-opioid receptors

Sorting of intracellular G-protein-coupled receptors (GPCRs) either to lysosomes for degradation or to plasma membrane for surface insertion and functional expression is a key process regulating signaling strength of GPCRs across the plasma membrane in adult mammalian cells. However, little is known about the molecular mechanisms governing the dynamic process of receptor sorting to the plasma membrane for functional expression under normal and pathological conditions. In this study, we demonstrate that delta-opioid receptor (DOPr), a GPCR constitutively targeted to intracellular compartments, is driven to the surface membrane of central synaptic terminals and becomes functional by the neurotrophin nerve growth factor (NGF) in native brainstem neurons. The NGF-triggered DOPr translocation is predominantly mediated by the signaling pathway involving the tyrosine receptor kinase A, Ca(2+)-mobilizing phospholipase C, and Ca(2+)/calmodulin-dependent protein kinase II. Importantly, it requires interactions with the cytoplasmic sorting protein NHERF-1 (Na(+)/H(+) exchange regulatory factor-1) and N-ethyl-maleimide-sensitive factor-regulated exocytosis. In addition, this NGF-mediated mechanism is likely responsible for the emergence of functional DOPr induced by chronic opioids. Thus, NGF may function as a key molecular switch that redirects the sorting of intracellularly targeted DOPr to plasma membrane, resulting in new functional DOPr on central synapses under chronic opioid conditions.

Dopamine D(1) receptor-mediated enhancement of NMDA receptor trafficking requires rapid PKC-dependent synaptic insertion in the prefrontal neurons

Understanding the interaction between dopamine and glutamate, particularly the interaction of dopamine and NMDA receptors, may enable a more rational approach to the treatment of schizophrenia, drug addiction, and other psychiatric disorders. We show that, in prefrontal cortical neurons, dopamine D(1)-induced enhancement of NMDA receptor function depends on rapid insertion of new NMDA receptor 2B subunits on the synaptic surface. Protein kinase A (PKA) inhibitor, but not protein kinase C (PKC) inhibitor, completely blocked dopamine D(1) agonist SKF-81297-induced increase of the total expression of NMDA receptors. Furthermore, SKF-81297 failed to alter the surface expression and synaptic insertion of NMDA receptors in the presence of PKA inhibitor, phospholipase C inhibitor, PKC inhibitor, or Src family kinase inhibitor. Our data suggest that D(1)-mediated enhancement of NMDA current depends on the NMDA receptor trafficking through rapid synaptic insertion and both PKA and PKC signaling pathways play important roles in the regulatory process. Although both PKA and PKC mediate the D(1)-induced enhancement of NMDA receptors, the phospholipase C-PKC-Src pathway is only required for surface expression and new synaptic insertion of NMDA receptors.

Alcoholism and alternative splicing of candidate genes

Gene expression studies have shown that expression patterns of several genes have changed during the development of alcoholism. Gene expression is regulated not only at the level of transcription but also through alternative splicing of pre-mRNA. In this review, we discuss some of the evidence suggesting that alternative splicing of candidate genes such as DRD2 (encoding dopamine D2 receptor) may form the basis of the mechanisms underlying the pathophysiology of alcoholism. These reports suggest that aberrant expression of splice variants affects alcohol sensitivities, and alcohol consumption also regulates alternative splicing. Thus, investigations of alternative splicing are essential for understanding the molecular events underlying the development of alcoholism.

SNAP-25 is a target of protein kinase C phosphorylation critical to NMDA receptor trafficking

Protein kinase C (PKC) enhances NMDA receptor (NMDAR)-mediated currents and promotes NMDAR delivery to the cell surface via SNARE-dependent exocytosis. Although the mechanisms of PKC potentiation are established, the molecular target of PKC is unclear. Here we show that synaptosomal-associated protein of 25 kDa (SNAP-25), a SNARE protein, is functionally relevant to PKC-dependent NMDAR insertion, and identify serine residue-187 as the molecular target of PKC phosphorylation. Constitutively active PKC delivered via the patch pipette potentiated NMDA (but not AMPA) whole-cell currents in hippocampal neurons. Expression of RNAi targeting SNAP-25 or mutant SNAP-25(S187A) and/or acute disruption of the SNARE complex by treatment with BoNT A, BoNT B or SNAP-25 C-terminal blocking peptide abolished NMDAR potentiation. A SNAP-25 peptide and function-blocking antibody suppressed PKC potentiation of NMDA EPSCs at mossy fiber-CA3 synapses. These findings identify SNAP-25 as the target of PKC phosphorylation critical to PKC-dependent incorporation of synaptic NMDARs and document a postsynaptic action of this major SNARE protein relevant to synaptic plasticity.

A neuronal role for SNAP-23 in postsynaptic glutamate receptor trafficking

Regulated exocytosis is essential for many biological processes and many components of the protein trafficking machinery are ubiquitous. However, there are also exceptions, such as SNAP-25, a neuron-specific SNARE protein that is essential for synaptic vesicle release from presynaptic nerve terminals. In contrast, SNAP-23 is a ubiquitously expressed SNAP-25 homolog that is critical for regulated exocytosis in non-neuronal cells. However, the role of SNAP-23 in neurons has not been elucidated. We found that SNAP-23 was enriched in dendritic spines and colocalized with constituents of the postsynaptic density, whereas SNAP-25 was restricted to axons. In addition, loss of SNAP-23 using genetically altered mice or shRNA targeted to SNAP-23 led to a marked decrease in NMDA receptor surface expression and NMDA receptor currents, whereas loss of SNAP-25 did not. SNAP-23 is therefore important for the functional regulation of postsynaptic glutamate receptors.

Regulation of HERG (KCNH2) potassium channel surface expression by diacylglycerol

The HERG (KCNH2) channel is a voltage-sensitive potassium channel mainly expressed in cardiac tissue, but has also been identified in other tissues like neuronal and smooth muscle tissue, and in various tumours and tumour cell lines. The function of HERG has been extensively studied, but it is still not clear what mechanisms regulate the surface expression of the channel. In the present report, using human embryonic kidney cells stably expressing HERG, we show that diacylglycerol potently inhibits the HERG current. This is mediated by a protein kinase C-evoked endocytosis of the channel protein, and is dependent on the dynein-dynamin complex. The HERG protein was found to be located only in early endosomes and not lysosomes. Thus, diacylglycerol is an important lipid participating in the regulation of HERG surface expression and function.

Dual palmitoylation of NR2 subunits regulates NMDA receptor trafficking

Modification of NMDA receptor function and trafficking contributes to the regulation of synaptic transmission and is important for several forms of synaptic plasticity. Here, we report that NMDA receptor subunits NR2A and NR2B have two distinct clusters of palmitoylation sites in their C-terminal region. Palmitoylation within the first cluster on a membrane-proximal region increases tyrosine phosphorylation of tyrosine-based internalization motifs by Src family protein tyrosine kinases, leading to enhanced stable surface expression of NMDA receptors. In addition, palmitoylation of these sites regulates constitutive internalization of the NMDA receptor in developing neurons. In marked contrast, palmitoylation of the second cluster in the middle of C terminus by distinct palmitoyl transferases causes receptors to accumulate in the Golgi apparatus and reduces receptor surface expression. These data suggest that regulated palmitoylation of NR2 subunits differentially modulates receptor trafficking and might be important for NMDA-receptor-dependent synaptic plasticity.

Synaptic NR2A- but not NR2B-Containing NMDA Receptors Increase with Blockade of Ionotropic Glutamate Receptors

NMDA receptors (NMDAR) are key molecules involved in physiological and pathophysiological brain processes such as plasticity and excitotoxicity. Neuronal activity regulates NMDA receptor levels in the cell membrane. However, little is known on which time scale this regulation occurs and whether the two main diheteromeric NMDA receptor subtypes in forebrain, NR1/NR2A and NR1/NR2B, are regulated in a similar fashion. As these differ considerably in their electrophysiological properties, the NR2A/NR2B ratio affects the neurons’ reaction to NMDA receptor activation. Here we provide evidence that the basal turnover rate in the cell membrane of NR2A- and NR2B-containing receptors is comparable. However, the level of the NR2A subtype in the cell membrane is highly regulated by NMDA receptor activity, resulting in a several-fold increased insertion of new receptors after blocking NMDAR for 8 h. Blocking AMPA receptors also increases the delivery of NR2A-containing receptors to the cell membrane. In contrast, the amount of NR2B-containing receptors in the cell membrane is not affected by ionotropic glutamate receptor block. Moreover, electrophysiological analysis of synaptic currents in hippocampal cultures and CA1 neurons of hippocampal slices revealed that after 8 h of NMDA receptor blockade the NMDA EPSCs increase as a result of augmented NMDA receptor-mediated currents. In conclusion, synaptic NR2A- but not NR2B-containing receptors are dynamically regulated, enabling neurons to change their NR2A/NR2B ratio within a time scale of hours.

Activity-dependent synaptic plasticity of NMDA receptors

Activity-dependent, bidirectional control of synaptic efficacy is thought to contribute to many forms of experience-dependent plasticity, including learning and memory. Although most excitatory synapses contain both AMPA and N-methyl-d-aspartate receptors (AMPARs and NMDARs), most studies have focused on the plasticity of synaptic AMPARs, and on the pivotal role of NMDA receptors for its induction. Here we review evidence that synaptic NMDARs themselves are subject to long-term activity-dependent changes by mechanisms that may differ from that of synaptic AMPARs. The bidirectional modulation of NMDAR-mediated synaptic responses is likely to have important functional implications for NMDAR-dependent forms of synaptic plasticity.

Excitatory roles of protein kinase C in striatal cholinergic interneurons

Protein kinase C (PKC) plays critical roles in neuronal activity and is widely expressed in striatal neurons. However, it is not clear how PKC activation regulates the excitability of striatal cholinergic interneurons. In the present study, we found that PKC activation significantly inhibited A-type potassium current (I(A)), but had no effect on delayed rectifier potassium currents. Consistently, application of PKC activator caused an increase of firing in response to depolarizing currents in cholinergic interneurons, which was persistent in the presence of both excitatory and inhibitory neurotransmission blockers. These excitatory effects of PKC could be partially mimicked and occluded by blockade of I(A) with potassium channel blocker 4-aminopyridine. In addition, immunostaining demonstrated that PKCalpha, but not PKCgamma and PKCepsilon, was expressed in cholinergic interneurons. Furthermore, activation of group I metabotropic glutamate receptors (mGluRs) led to an inhibition of I(A) through a PKC-dependent pathway. These data indicate that activation of PKC, most likely PKCalpha, increases the neuronal excitability of striatal cholinergic interneurons by down-regulating I(A). Group I mGluR-mediated I(A) inhibition might be important for the glutamatergic regulation of cholinergic tone in the neostriatum.

Involvement of protein kinase C and Src tyrosine kinase in acute tolerance to ethanol inhibition of spinal NMDA-induced pressor responses in rats

BACKGROUND AND PURPOSE

The present study was carried out to examine the role of protein kinases in the development of acute tolerance to the effects of ethanol on spinal N-methyl-D-aspartate (NMDA) receptor-mediated pressor responses during prolonged ethanol exposure.

EXPERIMENTAL APPROACH

Blood pressure responses induced by intrathecal injection of NMDA were recorded. The levels of several phosphorylated residues on NMDA receptor NR1 (GluN1) (NR1) and NMDA receptor NR2B (GluN2B) (NR2B) subunits were determined by immunohistochemistry and Western blot analysis.

KEY RESULTS

Ethanol inhibited spinal NMDA-induced pressor responses at 10 min, but the inhibition was significantly reduced at 40 min following continuous infusion. This effect was dose-dependently blocked by chelerythrine [a protein kinase C (PKC) inhibitor, 1-1000 pmol] or PP2 (a Src family tyrosine kinase inhibitor, 1-100 pmol) administered intrathecally 10 min following ethanol infusion. A significant increase in the immunoreactivity of phosphoserine 896 of NR1 subunits (pNR1-Ser896) and phosphotyrosine 1336 of NR2B subunits (pNR2B-Tyr1336) was found in neurons of intermediolateral cell column during the development of tolerance. Levels of pNR1-Ser896 and pNR2B-Tyr1336 were also significantly increased in lateral horn regions of the spinal cord slices incubated with ethanol for 40 min in vitro. The increases in pNR1-Ser896 and pNR2B-Tyr1336 levels were inhibited by post-treatment with chelerythrine and PP2, respectively, both in the in vivo and in vitro studies.

CONCLUSIONS AND IMPLICATIONS

The results suggest that activation of PKC and Src tyrosine kinase during prolonged ethanol exposure leading to increases in the levels of pNR1-Ser896 and pNR2B-Tyr1336 may contribute to acute tolerance to inhibition by ethanol of NMDA receptor function.

Phorbol 12-myristate 13-acetate potentiation of N-methyl-D-aspartate-induced currents in primary cultured cerebellar granule cells is mediated by protein kinase C alpha

We have previously reported that activation of protein kinase C (PKC) by phorbol 12-myristate 13-acetate (PMA) results in potentiation of N-methyl-D-aspartate-induced currents (I(NMDA))of receptors contained in primary cultured cerebellar granule cells (CGCs). The purpose of this study was to identify which PKC isoform(s) was responsible for this effect by using the whole-cell patch-clamp technique. Experiments were conducted on CGCs that expressed both the NR2A and NR2B NMDA receptor subunits as well as the PMA-sensitive PKC isoforms alpha, betaI, betaII, delta, epsilon, gamma, and . As observed previously, N-methyl-D-aspartate-induced peak currents (I(Pk)) were enhanced by a 12.5-min, 100 nM PMA exposure at 37 degrees C under normal recording conditions. Potentiation of receptor function was not observed when extracellular Ca(2+) was removed and 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid was present inside the cell. PMA-induced potentiation of I(Pk) did not occur when PKCalpha-specific antibody was introduced into the cell via the recording electrode. However, in similar experiments with antibodies specific for PKCbetaII, delta, epsilon, gamma, and , PMA potentiation of I(Pk) was observed. Down-regulation of PMA-sensitive PKC isoforms by an overnight exposure of 100 nM PMA resulted in lack of potentiation by PMA that was rescued when catalytically active PKCalpha was introduced into the cell via the patch electrode. PMA potentiation of I(Pk) was not recovered when catalytically active PKCbetaI, PKCbetaII, or PKCgamma was introduced into the cell via the patch electrode. Collectively, our data provide strong evidence that PMA-enhanced function of native NMDA receptors expressed in primary cultured CGCs is mediated by activation of PKCalpha.

Potentiation of spinal NMDA-mediated nociception by cocaine- and amphetamine-regulated transcript peptide via PKA and PKC signaling pathways in rats

Our previous study showed that cocaine- and amphetamine-regulated transcript peptide fragment 55-102 (CARTp) specifically potentiated spinal N-methyl-D-aspartate (NMDA)-mediated nociceptive transmission in vivo and in vitro. The cellular mechanisms underlying CARTp potentiation of NMDA receptor function remains unclear. The present study was carried out to test the hypothesis that CARTp changes the phosphorylated state of NMDA receptors by activating intracellular signals and subsequently increasing the function of NMDA receptors. We found that the potentiating effect of CARTp on spinal NMDA-induced hyperalgesia in rats was reduced by intrathecal pretreatment with KT5720 (a selective PKA inhibitor) or GF109206X (a selective PKC inhibitor), but was increased by pretreatment with calyculin A (a protein phosphatase inhibitor). In the in vitro electrophysiological study, CARTp potentiation of NMDA-induced depolarizations was blocked by superfusion of PKA or PKC inhibitor applied 10 min before the application of CARTp. The levels of phosphoserine 897 on the NR1 subunit (pNR1-ser897) and phosphoserine 896 on the NR1 subunit (pNR1-ser896) in the dorsal horn of spinal lumbar segments significantly increased following CARTp superfusion in vitro or intrathecal injection in vivo. The increases in pNR1-ser897 and pNR1-ser896 in the in vivo and in vitro studies were inhibited by pretreatment with KT5720 and GF109206X, respectively. The results provide the first evidence that CARTp increases the phosphorylation of NMDA receptor NR1 subunit via activation of PKA and PKC signals, which may play a crucial role in CARTp regulation of spinal NMDA receptor-mediated nociceptive responses.

Tumor necrosis factor-alpha-induced neutral sphingomyelinase-2 modulates synaptic plasticity by controlling the membrane insertion of NMDA receptors

The insertion and removal of NMDA receptors from the synapse are critical events that modulate synaptic plasticity. While a great deal of progress has been made on understanding the mechanisms that modulate trafficking of NMDA receptors, we do not currently understand the molecular events required for the fusion of receptor containing vesicles with the plasma membrane. Here, we show that sphingomyelin phosphodiesterase 3 (also known as neutral sphingomyelinase-2) is critical for tumor necrosis factor (TNF) alpha-induced trafficking of NMDA receptors and synaptic plasticity. TNFalpha initiated a rapid increase in ceramide that was associated with increased surface localization of NMDA receptor NR1 subunits and a specific clustering of NR1 phosphorylated on serines 896 and 897 into lipid rafts. Brief applications of TNFalpha increased the rate and amplitude of NMDA-evoked calcium bursts and enhanced excitatory post-synaptic currents. Pharmacological inhibition or genetic mutation of neutral sphingomyelinase-2 prevented TNFalpha-induced generation of ceramide, phosphorylation of NR1 subunits, clustering of NR1, enhancement of NMDA-evoked calcium flux and excitatory post-synaptic currents.

siRNA-mediated knockdown of the NR1 subunit gene of the NMDA receptor attenuates formalin-induced pain behaviors in adult rats

NMDA receptors in the spinal cord dorsal horn (SCDH) mediate some inflammatory pain behaviors. Here, we used rAAV vectors expressing an active small interfering RNA (siRNA) (vector 6) targeting the essential NR1 subunit of the NMDA receptor or a mismatch siRNA (vector MM-6) sequence to determine the consequences of RNAi-mediated knockdown of NR1 expression on NMDA receptor levels and formalin-induced pain behaviors in adult rats. Three weeks after intraparenchymal administration of the vector 6 into the right lumbar SCDH, NR1 mRNA and protein levels were significantly reduced (P < .01) in the ipsilateral SCDH compared with the contralateral SCDH but not in vector MM-6 or non-vector control animals. Formalin-induced phase 2 nociceptive response was significantly reduced (P < .05) in vector 6 animals compared with controls. Although neither vector affected normal mechanical threshold, vector 6 provided protection from the mechanical allodynia seen in controls at 24 hours after intraplantar formalin. Vector 6 also prevented the increase in phosphorylated NR1 levels seen in the ipsilateral SCDH of control rats 45 minutes after formalin. These results indicate that vector-derived siRNAs can effectively produce spatial knockdown of NR1 gene expression, and this knockdown selectively attenuates in vivo NMDA receptor-mediated formalin behaviors and NR1 phosphorylation in the rat.

PERSPECTIVE

This study reveals that a single administration of an siRNA-expressing viral vector produces significant knockdown of the NR1 gene in the SCDH of adult rats. This preclinical study demonstrates the use of RNAi to target the expression of genes mediating pain and the therapeutic potential of this approach.

Mechanisms of Group I mGluR-Dependent Long-Term Depression of NMDA Receptor–Mediated Transmission at Schaffer Collateral–CA1 Synapses

The mechanisms underlying group I metabotropic glutamate receptor (mGluR)-dependent long-term depression (LTD) of N-methyl-d-aspartate receptor (NMDAR)-mediated synaptic currents (EPSCsNMDAR) are poorly understood. Here we investigated the effects of ( R,S)-3,5-dihydroxyphenylglycine (DHPG), a selective agonist of group I mGluRs, on the EPSCsNMDAR in area CA1 of acute hippocampal slices from 6- to 8-wk Sprague-Dawley rats. DHPG acutely and persistently depressed the isolated EPSCNMDAR and transiently slowed its decay rate. Combined antagonism of mGluR1 and mGluR5 blocked the effects of DHPG. Strong calcium buffering with intracellular BAPTA did not reduce the acute depression or LTD, making the involvement of elevated postsynaptic calcium unlikely. The acute depression and LTD were not mediated by activation of tyrosine kinases or phosphatases, nor were they dependent on protein synthesis. However, the LTD was prevented by the intracellular actin-stabilizer jasplakinolide, raising the possibility that it was associated with a lateral movement of NMDARs. Supporting this hypothesis, when the effective spatial spread of synaptically released glutamate was increased using the glutamate transporter inhibitor TBOA, the resultant EPSCNMDAR did not undergo LTD in response to DHPG. Importantly, isolation of the extrasynaptic EPSCNMDAR by blockade of synaptic NMDARs with MK-801 showed that this was not due to a potentiation of the preexisting extrasynaptic component. These findings indicate that LTD of NMDAR-mediated synaptic transmission occurs via lateral movement of receptors away from the synapse.

Inhibitory effects of Group I metabotropic glutamate receptors antagonists on the expression of NMDA receptor NR1 subunit in morphine tolerant rats

N-methyl-D-aspartate receptor (NMDAR) and Group I metabotropic glutamate receptors (mGluRs) are involved in the process of morphine tolerance. Previous studies have shown that Group I mGluRs can modulate NMDAR functions in the central nervous system. The aim of the present study was to examine the influence of Group I mGluRs antagonists on the expression of NMDA receptor NR1 subunit (NR1) in the rat spinal cord. Morphine tolerance was induced in rats by repeated administration of 10 microg morphine (intrathecal, i.t.) twice a day for 7 consecutive days. Tail flick test was used to assess the effect of Group I mGluRs antagonist, AIDA ((RS)-1-Aminoindan-1,5 dicarboxylic acid) or mGluR5 antagonist, MPEP (2-methyl-6-(phenylethynyl)pyridine) on morphine antinociceptive tolerance. The expression of NR1 was measured by immunofluorescence and Western blot. Behavioral tests revealed that both AIDA and MPEP attenuated the development of morphine tolerance. The expression of NR1 was upregulated in the dorsal horn of spinal cord after chronic morphine treatment. AIDA or MPEP co-administered with morphine attenuated morphine induced upregulation of NR1. These findings suggest that the development of morphine tolerance partly prevented by Group I mGluRs antagonists may due to its inhibitory effect on the expression of NR1 subunit.

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.

Ethanol inhibition of recombinant NMDA receptors is not altered by coexpression of CaMKII-alpha or CaMKII-beta

Previous studies have shown that the N-methyl-d-aspartate (NMDA) receptor is an important target for the actions of ethanol in the brain. N-methyl-d-aspartate receptors are glutamate-activated ion channels that are highly expressed in neurons. They are activated during periods of significant glutamatergic synaptic activity and are an important source of the signaling molecule calcium in the postsynaptic spine. Alterations in the function of NMDA receptors by drugs or disease are associated with deficits in motor, sensory and cognitive processes of the brain. Acutely, ethanol inhibits ion flow through NMDA receptors whereas sustained exposure to ethanol can induce compensatory changes in the density and localization of the receptor. Defining factors that govern the acute ethanol sensitivity of NMDA receptors is an important step in understanding how an individual responds to ethanol. In the present study, we investigated the effect of calcium-calmodulin dependent protein kinase II (CaMKII) on the ethanol sensitivity of recombinant NMDA receptors. Calcium-calmodulin dependent protein kinase II is a major constituent of the postsynaptic density and is critically involved in various forms of learning and memory. NMDA receptor subunits were transiently expressed in human embryonic kidney 293 cells along with CaMKII-alpha or CaMKII-beta tagged with the green fluorescent protein. Whole cell currents were elicited by brief exposures to glutamate and were measured using patch-clamp electrophysiology. Neither CaMKII-alpha or CaMKII-beta had any significant effect on the ethanol inhibition of NR1/2A or NR1/2B receptors. Ethanol inhibition was also unaltered by deletion of CaMKII binding domains in NR1 or NR2 subunits or by phospho-site mutants that mimic or occlude CaMKII phosphorylation. Chronic treatment of cortical neurons with ethanol had no significant effect on the expression of CaMKII-alpha or CaMKII-beta. The results of this study suggest that CaMKII is not involved in regulating the acute ethanol sensitivity of NMDA receptors.

Estrogen alters spinal NMDA receptor activity via a PKA signaling pathway in a visceral pain model in the rat

Pain symptoms in several chronic pain disorders in women, including irritable bowel syndrome, fluctuate with the menstrual cycle suggesting a gonadal hormone component. In female rats, estrogens modulate visceral sensitivity although the underlying mechanism(s) are unknown. In the present study the effects of 17-beta estradiol on N-methyl-D-aspartate (NMDA) receptor signaling of colorectal nociceptive processing in the spinal cord were examined. Estrogen receptor alpha and the NR1 subunit of the NMDA receptor are co-expressed in dorsal horn neurons, supporting a direct action of estradiol on NMDA receptors. Intrathecal administration of the NMDA receptor antagonist D(-)-2-amino-5-phosphonopentanoic acid (APV) dose-dependently attenuated the visceromotor response with greater potency in ovariectomized (OVx) rats compared to OVx with estradiol replacement (E2) rats. Estradiol significantly increased protein expression of NR1 in the lumbosacral spinal cord compared to OVx rats. Colorectal distention significantly increased phosphorylation of NR1ser-897, a PKA phosphorylation site on the NR1 subunit in E2, but not OVx rats. Intrathecal administration of a PKA inhibitor significantly attenuated the visceromotor response, decreased NR1 phosphorylation and increased the potency of APV to attenuate the visceromotor response compared to vehicle-treated E2 rats. These data suggest that estradiol increases spinal processing of visceral nociception by increasing NMDA receptor NR1 subunit expression and increasing site-specific receptor phosphorylation on the NR1 subunit contributing to an increase in NMDA receptor activity.

Trafficking properties and activity regulation of the neuronal glycine transporter GLYT2 by protein kinase C

The neuronal glycine transporter GLYT2 controls the availability of the neurotransmitter in glycinergic synapses, and the modulation of its function may influence synaptic transmission. The active transporter is located in membrane rafts and reaches the cell surface through intracellular trafficking. In the present study we prove that GLYT2 constitutively recycles between the cell interior and the plasma membrane by means of a monensin-sensitive trafficking pathway. Also, a regulated trafficking can be triggered by PMA. We demonstrate that PMA inhibits GLYT2 transport by causing net accumulation of the protein in internal compartments through an increase of the internalization rate. In addition, a small increase of plasma membrane delivery and a redistribution of the transporter to non-raft domains is triggered by PMA. A previously identified phorbol-ester-resistant mutant (K422E) displaying an acidic substitution in a regulatory site, exhibits constitutive traffic but, in contrast with the wild-type, fails to show glycine uptake inhibition, membrane raft redistribution and trafficking modulation by PMA. We prove that the action of PMA on GLYT2 involves PKC (protein kinase C)-dependent and -independent pathways, although an important fraction of the effects are PKC-mediated. We show the additional participation of signalling pathways triggered by the small GTPase Rac1 on PMA action. GLYT2 inhibition by PMA and monensin also take place in brainstem primary neurons and synaptosomes, pointing to a GLYT2 trafficking regulation in the central nervous system.

Membrane trafficking of aquaporin 3 induced by epinephrine

We investigated the membrane trafficking of AQP3 induced by epinephrine in Caco-2 cells to clarify the digestive absorption of glycerol permeated by AQP3. Epinephrine was found to promote within 60 min the translocation of AQP3 from the cytoplasmic fraction to the plasma membrane. This increased trafficking of AQP3 was suppressed by phospholipase C and protein kinase C (PKC) inhibitors and a phorbol ester accelerated the trafficking of AQP3 to the membrane fraction. In contrast, adenylyl cyclase (AC) and protein kinase A (PKA) inhibitors did not have any effect on the increased in trafficking of AQP3 by epinephrine and an AC activator did not affect the trafficking of AQP3. Phosphorylation of a threonine (514) residue in PKC was detected upon the treatment with epinephrine and the temporal transitional pattern of this phosphorylation paralleled that of the increased trafficking of AQP3. These results suggest that PKC modulates the trafficking of AQP3.

Orexin-A modulates glutamatergic NMDA-dependent spinal reflex potentiation via inhibition of NR2B subunit

Glucose-sensitive neurons in the lateral hypothalamic area produce orexin-A (OxA) as well as orexin-B (OxB) and send their axons to the spinal dorsal horn, which predominantly expresses orexin receptor-1 (OX-1), showing a higher sensitivity to OxA. The purpose of the present study was to assess the effects of OxA on the induction of a novel form of activity-dependent reflex potentiation, spinal reflex potentiation (SRP), in the pelvic-urethral reflex activity. External urethra sphincter electromyogram in response to pelvic afferent nerve test stimulation (TS; 1/30 Hz) or repetitive stimulation (RS; 1 Hz) was recorded in anesthetized rats. TS evoked a baseline reflex activity, whereas RS produced SRP, which was abolished by intrathecal OxA (30 nM, 10 mul). Intrathecal SB-408124 (10 muM, 10 mul), an OX-1 antagonist, reversed the abolition on SRP caused by OxA. Although there is, so far, no NR2A- and NR2B-specific agonist available, N-methyl-d-aspartate (NMDA) reversed the abolition on the RS-induced SRP caused by the co-administration of OxA and Co-101244 (30 nM, 10 mul; an NMDA NR2B subunit antagonist), but it did not reverse the abolition by the co-administration of OxA and PPPA (300 nM, 10 mul; an NMDA NR2A subunit antagonist). In conclusion, the activation of descending orexinergic fibers may inhibit the repetitive afferent input-induced central sensitization of pelvic-urethral reflex activity and urethra hyperactivity, indicating that spinal orexinergic neural transmission may be a novel target for the treatment of patients with neuropathetic or postinflammatory pain of pelvic origin.

Preferential relocation of the N-methyl-D-aspartate receptor NR1 subunit in nucleus accumbens neurons that contain dopamine D1 receptors in rats showing an apomorphine-induced sensorimotor gating deficit

Sensorimotor gating as measured by prepulse inhibition (PPI) to startle-evoking auditory stimulation (AS) is disrupted in schizophrenia and in rodents receiving systemic administration of apomorphine, a dopamine D1/D2 receptor agonist, or MK-801, an N-methyl-d-aspartate (NMDA) receptor antagonist. The functional analogies and our prior results showing apomorphine- and AS-induced relocation of the dopamine D1 receptor (D1R) in the nucleus accumbens (Acb) shell suggest that apomorphine and AS may affect the subcellular distribution of the NMDA receptor NR1 subunit, a protein that forms protein-protein interactions with the D1R. We quantitatively compared the electron microscopic immunogold labeling for NR1 in dendritic profiles distinguished with respect to presence of D1R immunoreactivity and location in the Acb shell or core of rats receiving a single s.c. injection of vehicle (VEH) or apomorphine (APO) alone, or combined with AS (VEH+AS, APO+AS). The rats in the APO+AS group were previously shown to have PPI deficits, whereas the rats in the VEH+AS group had normal PPI. A significantly higher percentage of plasmalemmal and a lower percentage of cytoplasmic NR1 immunogold particles were seen in D1R-labeled dendritic spines in the Acb shell of the APO+AS group compared with all other groups. D1R-containing small dendrites in the Acb shell of the APO+AS group also showed a significantly higher density of plasmalemmal and a lower density of cytoplasmic NR1 immunogold particles compared with VEH or APO groups. In the Acb core, the APO+AS group had significantly fewer dendritic spines co-expressing NR1 and D1R compared with VEH or VEH+AS groups. These results, together with our earlier findings, suggest that NMDA receptors are preferentially mobilized in D1R-containing Acb neurons of rats showing apomorphine-induced disruption of PPI in a paradigm using acoustic stimulation.

Surface trafficking of N-methyl-D-aspartate receptors: physiological and pathological perspectives

The N-methyl-D-aspartate receptor (NMDAR) plays a crucial role in shaping the strength of synaptic connections. Over the last decades, extensive studies have defined the cellular and molecular mechanisms by which synaptic NMDARs control the maturation and plasticity of synaptic transmission, and how altered synaptic NMDAR signaling is implicated in neurodegenerative and psychiatric disorders. It is now clear that activation of synaptic or extrasynaptic NMDARs produces different signaling cascades and thus neuronal functions. Our current understanding of NMDAR surface distribution and trafficking is only emerging. Exchange of NMDARs between synaptic and extrasynaptic areas through surface diffusion is a highly dynamic and regulated process. The aim of this review is to describe the identified mechanisms that regulate surface NMDAR behaviors and discuss the impact of this new trafficking pathway on the well-established NMDAR-dependent physiological and pathophysiological processes.

Complexins facilitate neurotransmitter release at excitatory and inhibitory synapses in mammalian central nervous system

Complexins (Cplxs) are key regulators of synaptic exocytosis, but whether they act as facilitators or inhibitors is currently being disputed controversially. We show that genetic deletion of all Cplxs expressed in the mouse brain causes a reduction in Ca(2+)-triggered and spontaneous neurotransmitter release at both excitatory and inhibitory synapses. Our results demonstrate that at mammalian central nervous system synapses, Cplxs facilitate neurotransmitter release and do not simply act as inhibitory clamps of the synaptic vesicle fusion machinery.

NMDA receptor desensitization regulated by direct binding to PDZ1-2 domains of PSD-95

Regulation of N-methyl-d-aspartate receptor (NMDAR) activity by desensitization is important in physiological and pathological states; NMDAR desensitization contributes in shaping synaptic responses and may be protective by limiting calcium influx during sustained glutamate insults. We previously reported that glycine-independent desensitization decreases during hippocampal neuronal development, correlating with NMDAR synaptic localization and association with postsynaptic density 95 (PSD-95). PSD-95/Discs large/zona occludens (PDZ)-1,2 domains of PSD-95 bind to the C-terminus of NMDAR NR2 subunits. The role of PSD-95 in anchoring signaling proteins near NMDARs is well documented. To determine if PSD-95-induced changes in NMDAR desensitization occur because of direct binding to NR2 or due to recruitment of regulatory proteins, we tested the effects of various PSD-95 constructs on NMDAR currents in human embryonic kidney 293 (HEK293) cells and neurons. In HEK cells, wild-type PSD-95 significantly reduced wild-type NMDAR desensitization without altering currents of NMDARs containing NR2A-S1462A, a mutation that abolishes PSD-95 binding. The PSD-95 N-terminus truncated after the PDZ1-2 domains was sufficient for this effect in neurons with low endogenous PSD-95 levels; in NMDAR-expressing HEK cells, the effect persisted when PSD-95 multimerization was eliminated. Moreover other PSD-95 family members with highly homologous PDZ1-2 domains significantly reduced NMDAR desensitization. In mature neurons, disruption of PSD-95/NMDAR interaction through protein kinase C (PKC) activation increased desensitization to levels found in immature neurons, and this effect was not due to PKC direct regulation of NMDAR activity. We conclude that direct binding of PSD-95 increases stability of NMDAR responses to agonist exposure in neuronal and nonneuronal cells.

Water and ion channels: crucial in the initiation and progression of apoptosis in central nervous system?

Programmed cell death (PCD), is a highly regulated and sophisticated cellular mechanism that commits cell to isolated death fate. PCD has been implicated in the pathogenesis of numerous neurodegenerative disorders. Countless molecular events underlie this phenomenon, with each playing a crucial role in death commitment. A precedent event, apoptotic volume decrease (AVD), is ubiquitously observed in various forms of PCD induced by different cellular insults. Under physiological conditions, cells when subjected to osmotic fluctuations will undergo regulatory volume increase/decrease (RVI/RVD) to achieve homeostatic balance with neurons in the brain being additionally protected by the blood-brain-barrier. However, during AVD following apoptotic trigger, cell undergoes anistonic shrinkage that involves the loss of water and ions, particularly monovalent ions e.g. K(+), Na(+) and Cl(-). It is worthwhile to concentrate on the molecular implications underlying the loss of these cellular components which posed to be significant and crucial in the successful propagation of the apoptotic signals. Microarray and real-time PCR analyses demonstrated several ion and water channel genes are regulated upon the onset of lactacystin (a proteosomal inhibitor)-mediated apoptosis. A time course study revealed that gene expressions of water and ion channels are being modulated just prior to apoptosis, some of which are aquaporin 4 and 9, potassium channels and chloride channels. In this review, we shall looked into the molecular protein machineries involved in the execution of AVD in the central nervous system (CNS), and focus on the significance of movements of each cellular component in affecting PCD commitment, thus provide some pharmacological advantages in the global apoptotic cell death.

Long-term potentiation at C-fibre synapses by low-level presynaptic activity in vivo

Inflammation, trauma or nerve injury trigger low-level activity in C-fibres and may cause long-lasting hyperalgesia. Long-term potentiation (LTP) at synapses of primary afferent C-fibres is considered to underlie some forms of hyperalgesia. In previous studies, high- but not low-frequency conditioning stimulation of C-fibres has, however, been used to induce LTP in pain pathways. Recently we could show that also conditioning low-frequency stimulation (LFS) at C-fibre intensity induces LTP in vitro as well as in the intact animal, i.e. with tonic descending inhibition fully active. In the slice preparation, this form of LTP requires a rise in postsynaptic Ca2+-concentration and activation of Ca2+-dependent signalling pathways. Here, we investigated the signalling mechanisms underlying this novel form of LTP in vivo. We found that the signal transduction pathways causing LFS-induced LTP in vivo include activation of neurokinin 1 and N-methyl-D-aspartate receptors, rise of [Ca2+]i from intracellular stores and via T-type voltage-dependent Ca2+ channels, activation of phospholipase C, protein kinase C and Ca2+-calmodulin dependent kinase II. These pathways match those leading to hyperalgesia in behaving animals and humans. We thus propose that LTP induced by low-level activity in C-fibres may underlie some forms of hyperalgesia.

Hypocretin-1 potentiates NMDA receptor-mediated somatodendritic secretion from locus ceruleus neurons

Our previous observations showed that several stimuli, including high-K(+) solution, glutamate, and voltage pulses, induce somatic noradrenaline (NA) secretion from locus ceruleus (LC) neurons. Hypocretin (orexin), a hypothalamic peptide critical for normal wakefulness, has been shown to evoke NA release from the axon terminals of LC neurons. Here, we used amperometry to test the effect of hypocretin-1 (HCRT) on NMDA receptor-mediated somatodendritic release in LC neurons. Either HCRT or NMDA applied alone dose-dependently induced somatodendritic secretion. Bath application of HCRT notably potentiated NMDA receptor-mediated somatodendritic NA release. This potentiation was blocked by SB 334867, a selective HCRT receptor (Hcrtr 1) antagonist, or bisindolylmaleimide, a specific protein kinase C (PKC) inhibitor, indicating the involvement of Hcrtr 1 and PKC. Consistent with this, phorbol 12-myristate 13-acetate, a PKC activator, mimicked the HCRT-induced potentiation. Furthermore, HCRT enhanced NMDA-induced intracellular Ca(2+) elevation via activation of Hcrtr 1 and PKC, which may contribute to HCRT-potentiated somatodendritic secretion. These results suggest that HCRT modulates LC activity not only by regulating noradrenergic input to its targets, but also by affecting noradrenergic communication in the soma and dendrites.

Emergence of brain-derived neurotrophic factor-induced postsynaptic potentiation of NMDA currents during the postnatal maturation of the Kolliker-Fuse nucleus of rat

The Kölliker-Fuse nucleus (KF) contributes essentially to respiratory pattern formation and adaptation of breathing to afferent information. Systems physiology suggests that these KF functions depend on NMDA receptors (NMDA-R). Recent investigations revealed postnatal changes in the modulation of glutamatergic neurotransmission by brain-derived neurotrophic factor (BDNF) in the KF. Therefore, we investigated postnatal changes in NMDA-R subunit composition and postsynaptic modulation of NMDA-R-mediated currents by BDNF in KF slice preparations derived from three age groups (neonatal: postnatal day (P) 1-5; intermediate: P6-13; juvenile: P14-21). Immunohistochemistry showed a developmental up-regulation of the NR2D subunit. This correlated with a developmental increase in decay time of NMDA currents and a decline of desensitization in response to repetitive exogenous NMDA applications. Thus, developmental up-regulation of the NR2D subunit, which reduces the Mg(2+) block of NMDA-R, causes these specific changes in NMDA current characteristics. This may determine the NMDA-R-dependent function of the mature KF in the control of respiratory phase transition. Subsequent experiments revealed that bath-application of BDNF progressively potentiated these repetitively evoked NMDA currents only in intermediate and juvenile age groups. Pharmacological inhibition of protein kinase C (PKC), as a downstream component of the BDNF-tyrosine kinase B receptor (trkB) signalling, prevented BDNF-induced potentiation of NMDA currents. BDNF-induced potentiation of NMDA currents in later developmental stages might be essential for synaptic plasticity during the adaptation of the breathing pattern in response to peripheral/central commands. The lack of plasticity in neonatal neurones strengthens the hypothesis that the respiratory network becomes permissive for activity-dependent plasticity with ongoing postnatal development.