Synaptic plasticity and phosphorylation

CaMKII antisense oligodeoxynucleotides protect against ischemia-induced neuronal death in the rat hippocampus

The present study was performed to investigate the effects of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) antisense oligodeoxynucleotides (ODNs) on the assembly of the CaMKII·GluR6·PSD-95 signaling module, GluR6 serine phosphorylation and c-Jun N-terminal kinase 3 (JNK3) activation. A further aim was to determine the neuroprotective mechanism of CaMKII antisense ODNs against ischemia-reperfusion (I/R)-induced neuronal death in the rat hippocampus. CaMKII antisense ODNs were intracerebroventricularly infused to inhibit CaMKII expression once daily for 3 days prior to the induction of ischemia. Transient cerebral ischemia (15 min) and reperfusion were induced by four-vessel occlusion in Sprague-Dawley rats as an animal model for transient cerebral I/R. The expression of related proteins was examined by immunoprecipitation and immunoblotting. Neuronal death in the rat hippocampus was detected by histology and histochemistry. The results indicate that CaMKII antisense ODNs inhibit several of the processes that are normally induced by cerebral I/R, including CaMKII expression, increased CaMKII·GluR6·PSD-95 signaling module assembly, GluR6 serine phosphorylation and JNK3 activation. Alternatively, CaMKII antisense ODNs also exhibit a significant neuroprotective role against cerebral I/R-induced cell death. These results provide the first evidence that CaMKII antisense ODNs can exert neuroprotective effects on cerebral I/R-induced cell death. The possible molecular mechanisms underlying this effect include 1) an inhibition of CaMKII expression and subsequent suppression of the assembly of the CaMKII·GluR6·PSD-95 signaling module, 2) GluR6 serine phosphorylation, and 3) reduced JNK3 activation.

NMDA receptor mediated phosphorylation of GluR1 subunits contributes to the appearance of calcium-permeable AMPA receptors after mechanical stretch injury

Alterations in neuronal cytosolic calcium is a key mediator of the traumatic brain injury (TBI) pathobiology, but less is known of the role and source of calcium in shaping early changes in synaptic receptors and neural circuits after TBI. In this study, we examined the calcium source and potential phosphorylation events leading to insertion of calcium-permeable AMPARs (CP-AMPARs) after in vitro traumatic brain injury, a receptor subtype that influences neural circuit dynamics for hours to days following injury. We found that both synaptic and NR2B-containing NMDARs contribute significantly to the calcium influx following stretch injury. Moreover, an early and sustained phosphorylation of the S-831 site of the GluR1 subunit appeared after mechanical injury, and this phosphorylation was blocked with the inhibition of either synaptic NMDARs or NR2B-containing NMDARs. In comparison, mechanical injury led to no significant change in the S-845 phosphorylation of the GluR1 subunit. Although no change in S-845 phosphorylation appeared in injured cultures, we observed that inhibition of NR2B-containing NMDARs significantly increased S-845 phosphorylation 1h after injury while blockade of synaptic NMDARs did not change S-845 phosphorylation at any time point following injury. These findings show that a broad class of NMDARs are activated in parallel and that targeting either subpopulation will reverse some of the consequences of mechanical injury, providing distinct paths to treat the effects of mechanical injury on neural circuits after TBI.

Ca-permeable AMPA receptors in homeostatic synaptic plasticity

Neurons possess diverse mechanisms of homeostatic adaptation to overall changes in neural and synaptic activity, which are critical for proper brain functions. Homeostatic regulation of excitatory synapses has been studied in the context of synaptic scaling, which allows neurons to adjust their excitatory synaptic gain to maintain their activity within a dynamic range. Recent evidence suggests that one of the main mechanisms underlying synaptic scaling is by altering the function of postsynaptic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs), including synaptic expression of Ca²⁺-permeable (CP-) AMPARs. CP-AMPARs endow synapses with unique properties, which may benefit adaptation of neurons to periods of inactivity as would occur when a major input is lost. This review will summarize how synaptic expression of CP-AMPARs is regulated during homeostatic synaptic plasticity in the context of synaptic scaling, and will address the potential functional consequences of altering synaptic CP-AMPAR content.

Exposure to novel environment is characterized by an interaction of D1/NMDA receptors underlined by phosphorylation of the NMDA and AMPA receptor subunits and activation of ERK1/2 signaling, leading to epigenetic changes and gene expression in rat hippocampus

Interactions between dopamine and glutamate receptors are essential for prefrontal cortical (PFC) and hippocampal cognitive functions. The hippocampus has been identified as a detector of a novel stimulus, where an association between incoming information and stored memories takes place. Further to our previous results which showed a strong synergistic interaction of dopamine D1 and glutamate NMDA receptors, the present study is going to investigate the functional status of that interaction in rats, following their exposure to a novel environment. Our results showed that the "spatial" novelty induced in rat hippocampus and PFC (a) a significant increase in phosphorylation of NMDA and AMPA receptor subunits, as well as a robust phosphorylation/activation of ERK1/2 signaling, which are both dependent on the concomitant stimulation of D1/NMDA receptors and are both abolished by habituation procedure, (b) chromatin remodeling events (phosphorylation-acetylation of histone H3) and (c) an increase in the immediate early genes (IEGs) c-Fos and zif-268 expression in the CA1 region of hippocampus, which is dependent on the co-activation of D1/NMDA and acetylcholine muscarinic receptors. In conclusion, our results clearly show that a strong synergistic interaction of D1/NMDA receptor is required for the novelty-induced phosphorylation of NMDA and AMPA receptor subunits and for the robust activation of ERK1/2 signaling, leading to chromatin remodeling events and the expression of the IEGs c-Fos and zif-268, which are involved in the regulation of synaptic plasticity and memory consolidation.

Posttranslational regulation of AMPA receptor trafficking and function

In the mammalian central nervous system, the majority of fast excitatory synaptic transmission is mediated by glutamate acting on AMPA-type ionotropic glutamate receptors. The abundance of AMPA receptors at the synapse can be modulated through receptor trafficking, which dynamically regulates many fundamental brain functions, including learning and memory. Reversible posttranslational modifications, including phosphorylation, palmitoylation and ubiquitination of AMPA receptor subunits are important regulatory mechanisms for controlling synaptic AMPA receptor expression and function. In this review, we highlight recent advances in the study of AMPA receptor posttranslational modifications and discuss how these modifications regulate AMPA receptor trafficking and function at synapses.

Influence of matrix metalloproteinase MMP-9 on dendritic spine morphology

An increasing body of data has shown that matrix metalloproteinase-9 (MMP-9), an extracellularly acting, Zn(2+)-dependent endopeptidase, is important not only for pathologies of the central nervous system but also for neuronal plasticity. Here, we use three independent experimental models to show that enzymatic activity of MMP-9 causes elongation and thinning of dendritic spines in the hippocampal neurons. These models are: a recently developed transgenic rat overexpressing autoactivating MMP-9, dissociated neuronal cultures, and organotypic neuronal cultures treated with recombinant autoactivating MMP-9. This dendritic effect is mediated by integrin β1 signalling. MMP-9 treatment also produces a change in the decay time of miniature synaptic currents; however, it does not change the abundance and localization of synaptic markers in dendritic protrusions. Our results, considered together with several recent studies, strongly imply that MMP-9 is functionally involved in synaptic remodelling.

Acute ethanol exposure prevents PMA-mediated augmentation of N-methyl-D-aspartate receptor function in primary cultured cerebellar granule cells

Many intracellular proteins and signaling cascades contribute to the ethanol sensitivity of native N-methyl-D-aspartate receptors (NMDARs). One putative protein is the serine/threonine kinase, protein kinase C (PKC). The purpose of this study was to assess if PKC modulates the ethanol sensitivity of native NMDARs expressed in primary cultured cerebellar granule cells (CGCs). With the whole-cell patch-clamp technique, we assessed if ethanol inhibition of NMDA-induced currents (I(NMDA)) (100 μM NMDA plus 10 μM glycine) were altered in CGCs in which the novel and classical PKC isoforms were activated by phorbol-12-myristate-13-acetate (PMA). Percent inhibition by 10, 50, or 100 mM ethanol of NMDA-induced steady-state current amplitudes (I(SS)) or peak current amplitudes (I(Pk)) of NMDARs expressed in CGCs in which PKC was activated by a 12.5 min, 100 nM PMA exposure at 37°C did not differ from currents obtained from receptors contained in control cells. However, PMA-mediated augmentation of I(Pk) in the absence of ethanol was abolished after brief applications of 10 or 1 mM ethanol coapplied with agonists, and this suppression of enhanced receptor function was observed for up to 8 min post-ethanol exposure. Because we had previously shown that PMA-mediated augmentation of I(NMDA) of NMDARs expressed in these cells is by activation of PKCα, we assessed the effect of ethanol (1, 10, 50, and 100 mM) on PKCα activity. Ethanol decreased PKCα activity by 18% for 1 mM ethanol and activity decreased with increasing ethanol concentrations with a 50% inhibition observed with 100 mM ethanol. The data suggest that ethanol disruption of PMA-mediated augmentation of I(NMDA) may be due to a decrease in PKCα activity by ethanol. However, given the incomplete blockade of PKCα activity and the low concentration of ethanol at which this phenomenon is observed, other ethanol-sensitive signaling cascades must also be involved.

Brain area specific regulation of synaptic AMPA receptors by phosphorylation

Regulation of synaptic AMPA receptors (AMPARs) is one of the key elements that allow the nervous system to adapt to changes in the sensory environment as well as for memory formation. One way to regulate AMPAR function is by reversible changes in the phosphorylation of its subunits. We recently reported that phosphorylation of the AMPAR subunit GluA1 (or GluR1) on serine-845 (S845) is a pre-requisite step for sensory experience-dependent homeostatic synaptic plasticity in the visual cortex. In particular, increasing GluA1-S845 phosphorylation upregulated cell surface and synaptic AMPAR levels. Here we report that this is rather specific to the visual cortex, in that increasing GluA1-S845 phosphorylation in hippocampal slices only increase cell surface expression, but not synaptic AMPAR function. Our results suggest that depending on the brain region divergent mechanisms may exist to regulate synaptic AMPAR function with phosphorylation.

Striatal-enriched protein tyrosine phosphatase expression and activity in Huntington's disease: a STEP in the resistance to excitotoxicity

Striatal-enriched protein tyrosine phosphatase (STEP) is highly expressed in striatal projection neurons, the neuronal population most affected in Huntington's disease. Here, we examined STEP expression and phosphorylation, which regulates its activity, in N-terminal exon-1 and full-length mutant huntingtin mouse models. R6/1 mice displayed reduced STEP protein levels in the striatum and cortex, whereas its phosphorylation was increased in the striatum, cortex, and hippocampus. The early increase in striatal STEP phosphorylation levels correlated with a deregulation of the protein kinase A pathway, and decreased calcineurin activity at later stages further contributes to an enhancement of STEP phosphorylation and inactivation. Accordingly, we detected an accumulation of phosphorylated ERK2 and p38, two targets of STEP, in R6/1 mice striatum at advanced stages of the disease. Activation of STEP participates in excitotoxic-induced cell death. Because Huntington's disease mouse models develop resistance to excitotoxicity, we analyzed whether decreased STEP activity was involved in this process. After intrastriatal quinolinic acid (QUIN) injection, we detected higher phosphorylated STEP levels in R6/1 than in wild-type mice, suggesting that STEP inactivation could mediate neuroprotection in R6/1 striatum. In agreement, intrastriatal injection of TAT-STEP increased QUIN-induced cell death. R6/2, Tet/HD94, and Hdh(Q7/Q111) mice striatum also displayed decreased STEP protein and increased phosphorylation levels. In Tet/HD94 mice striatum, mutant huntingtin transgene shutdown reestablished STEP expression. In conclusion, the STEP pathway is severely downregulated in the presence of mutant huntingtin and may participate in compensatory mechanisms activated by striatal neurons that lead to resistance to excitotoxicity.

AMPA receptor regulation during synaptic plasticity in hippocampus and neocortex

Discovery of long-term potentiation (LTP) in the dentate gyrus of the rabbit hippocampus by Bliss and Lømo opened up a whole new field to study activity-dependent long-term synaptic modifications in the brain. Since then hippocampal synapses have been a key model system to study the mechanisms of different forms of synaptic plasticity. At least for the postsynaptic forms of LTP and long-term depression (LTD), regulation of AMPA receptors (AMPARs) has emerged as a key mechanism. While many of the synaptic plasticity mechanisms uncovered in at the hippocampal synapses apply to synapses across diverse brain regions, there are differences in the mechanisms that often reveal the specific functional requirements of the brain area under study. Here we will review AMPAR regulation underlying synaptic plasticity in hippocampus and neocortex. The main focus of this review will be placed on postsynaptic forms of synaptic plasticity that impinge on the regulation of AMPARs using hippocampal CA1 and primary sensory cortices as examples. And through the comparison, we will highlight the key similarities and functional differences between the two synapses.

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.

The expanding social network of ionotropic glutamate receptors: TARPs and other transmembrane auxiliary subunits

Ionotropic glutamate receptors (iGluRs) underlie rapid, excitatory synaptic signaling throughout the CNS. After years of intense research, our picture of iGluRs has evolved from them being companionless in the postsynaptic membrane to them being the hub of dynamic supramolecular signaling complexes, interacting with an ever-expanding litany of other proteins that regulate their trafficking, scaffolding, stability, signaling, and turnover. In particular, the discovery that transmembrane AMPA receptor regulatory proteins (TARPs) are AMPA receptor auxiliary subunits that are critical determinants of their trafficking, gating, and pharmacology has changed the way we think about iGluR function. Recently, a number of novel transmembrane proteins have been uncovered that may also serve as iGluR auxiliary proteins. Here we review pivotal developments in our understanding of the role of TARPs in AMPA receptor trafficking and gating, and provide an overview of how newly discovered transmembrane proteins expand our view of iGluR function in the CNS.

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.

Changes in calcineurin message, enzyme activity and protein content in the spinal dorsal horn are associated with chronic constriction injury of the rat sciatic nerve

Plasticity in the spinal dorsal horn is thought to underlie the development of neuropathic pain. Calcineurin (protein phosphatase 3) plays an important role in plasticity in the brain. Here we examined whether chronic constriction injury (CCI) of the sciatic nerve modifies calcineurin expression in the spinal dorsal horn. Male rats were assigned to control (uninjured), sham-operated or CCI groups. CCI animals exhibited both a shift in weight bearing and a reduction in paw withdrawal latencies as signs of pain behavior. At 3 days (3D) the pain behavior was associated with a significant increase in calcineurin gene expression, enzyme activity and content of its Aα isoform in the ipsilateral spinal dorsal horn. In contrast, while the pain behavior persisted at 7 days (7D) calcineurin gene expression returned to control levels and activity and protein content decreased. A single intrathecal injection of MK-801 15 min before the ligation attenuated both signs of pain behavior in 3D but not 7D CCI animals. The same pre-treatment also prevented the CCI-associated increases in calcineurin in these animals. These data suggested an involvement of calcineurin in CCI-elicited neuropathic pain. The time-dependent divergent changes in calcineurin expression may underlie the different phases of neuropathic pain development.

Routes, destinations and delays: recent advances in AMPA receptor trafficking

Postsynaptic AMPA-type glutamate receptors (AMPARs) mediate most fast excitatory synaptic transmission and are crucial for many aspects of brain function, including learning, memory and cognition. The number, synaptic localization and subunit composition of synaptic AMPARs are tightly regulated by network activity and by the history of activity at individual synapses. Furthermore, aberrant AMPAR trafficking is implicated in neurodegenerative diseases. AMPARs therefore represent a prime target for drug development and the mechanisms that control their synaptic delivery, retention and removal are the subject of extensive research. Here, we review recent findings that have provided new insights into AMPAR trafficking and that might lead to the development of novel therapeutic strategies.

Striatal-enriched protein tyrosine phosphatase (STEP) knockout mice have enhanced hippocampal memory

Striatal-enriched protein tyrosine phosphatase (STEP) is a brain-specific phosphatase that opposes synaptic strengthening by the regulation of key synaptic signaling proteins. Previous studies suggest a possible role for STEP in learning and memory. To demonstrate the functional importance of STEP in learning and memory, we generated STEP knockout (KO) mice and examined the effect of deletion of STEP on behavioral performance, as well as the phosphorylation and expression of its substrates. Here we report that loss of STEP leads to significantly enhanced performance in hippocampal-dependent learning and memory tasks. In addition, STEP KO mice displayed greater dominance behavior, although they were normal in their motivation, motor coordination, visual acuity and social interactions. STEP KO mice displayed enhanced tyrosine phosphorylation of extracellular-signal regulated kinase 1/2 (ERK1/2), the NR2B subunit of the N-methyl-D-aspartate receptor (NMDAR) and proline-rich tyrosine kinase (Pyk2), as well as an increased phosphorylation of ERK1/2 substrates. Concomitant with the increased phosphorylation of NR2B, synaptosomal expression of NR1/NR2B NMDARs was increased in STEP KO mice, as was the GluR1/GluR2 containing α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptors (AMPARs), providing a potential molecular mechanism for the improved cognitive performance. The data support a role for STEP in the regulation of synaptic strengthening. The absence of STEP improves cognitive performance, and may do so by the regulation of downstream effectors necessary for synaptic transmission.

Phosphorylation of AMPA receptors is required for sensory deprivation-induced homeostatic synaptic plasticity

Sensory experience, and the lack thereof, can alter the function of excitatory synapses in the primary sensory cortices. Recent evidence suggests that changes in sensory experience can regulate the synaptic level of Ca²⁺-permeable AMPA receptors (CP-AMPARs). However, the molecular mechanisms underlying such a process have not been determined. We found that binocular visual deprivation, which is a well-established in vivo model to produce multiplicative synaptic scaling in visual cortex of juvenile rodents, is accompanied by an increase in the phosphorylation of AMPAR GluR1 (or GluA1) subunit at the serine 845 (S845) site and the appearance of CP-AMPARs at synapses. To address the role of GluR1-S845 in visual deprivation-induced homeostatic synaptic plasticity, we used mice lacking key phosphorylation sites on the GluR1 subunit. We found that mice specifically lacking the GluR1-S845 site (GluR1-S845A mutants), which is a substrate of cAMP-dependent kinase (PKA), show abnormal basal excitatory synaptic transmission and lack visual deprivation-induced homeostatic synaptic plasticity. We also found evidence that increasing GluR1-S845 phosphorylation alone is not sufficient to produce normal multiplicative synaptic scaling. Our study provides concrete evidence that a GluR1 dependent mechanism, especially S845 phosphorylation, is a necessary pre-requisite step for in vivo homeostatic synaptic plasticity.

Haloperidol disrupts opioid-antinociceptive tolerance and physical dependence

Previous studies from our laboratory and others have implicated a critical role of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) in opioid tolerance and dependence. Translational research targeting the CaMKII pathway is challenging, if not impossible, because of a lack of selective inhibitors. We discovered in a preliminary study that haloperidol, a butyrophenone antipsychotic drug, inhibited CaMKII, which led us to hypothesize that haloperidol can attenuate opioid tolerance and dependence by inhibiting CaMKII. The hypothesis was tested in two rodent models of opioid tolerance and dependence. Pretreatment with haloperidol (0.2-1.0 mg/kg i.p.) prevented the development of morphine tolerance and dependence in a dose-dependent manner. Short-term treatment with haloperidol (0.06-0.60 mg/kg i.p.) dose-dependently reversed the established morphine-antinociceptive tolerance and physical dependence. Correlating with behavioral effects, pretreatment or short-term treatment with haloperidol dose-dependently inhibited morphine-induced up-regulation of supraspinal and spinal CaMKIIα activity. Moreover, haloperidol given orally was also effective in attenuating morphine-induced CaMKIIα activity, antinociceptive tolerance, and physical dependence. Taken together, these data suggest that haloperidol attenuates opioid tolerance and dependence by suppressing CaMKII activity. Because haloperidol is a clinically used drug that can be taken orally, we propose that the drug may be of use in attenuating opioid tolerance and dependence.

[Curing mental retardation: searching for balance]

Mental retardation (MR) occurs in 2 to 3 % of the general population and is still not therapeutically addressed. Milder forms of MR result from deficient synaptogenesis and/or impaired synaptic plasticity during childhood. These alterations would result from disequilibrium in signalling pathways regulating the balance between long term potentiation (LTP) and long term depression (LTD) in certain neurons such as hippocampus neurons. To provide mentally retarded children with increased cognitive abilities, novel experimental approaches are currently being developed to characterize signalling status associated with MR and to identify therapeutic targets that would restore lost equilibrium. Several studies also highlighted the major role played by molecular switches like kinases, phosphatases, small G proteins and their regulators in the coordination and integration of signalling pathways associated with synaptic plasticity. These proteins may therefore constitute promising therapeutic targets for a number of cognitive deficiencies.

AMPA receptor subunit GluR1 (GluA1) serine-845 site is involved in synaptic depression but not in spine shrinkage associated with chemical long-term depression

The structure of dendritic spines is highly plastic and can be modified by neuronal activity. In addition, there is evidence that spine head size correlates with the synaptic α-amino-3-hydroxy-5-methylisoxazole propionic acid (AMPA) receptor (AMPAR) content, which suggests that they may be coregulated. Although there is evidence that there are overlapping mechanisms for structural and functional plasticity, the extent of the overlap needs further investigation. Specifically, it is unknown whether AMPAR levels determine spine size or whether both are regulated via parallel pathways. We studied the correlation between spine structural plasticity and long-term synaptic plasticity following chemical-induced long-term depression (chemLTD). In particular, we examined whether the regulation of AMPARs, which is implicated in LTD, is critical for spine morphological plasticity. We used mutant mice specifically lacking the serine-845 site on the type 1 glutamate receptor (GluR1, or GluA1) subunit of AMPARs (mutants). These mice specifically lack N-methyl-D-aspartate (NMDA) receptor (NMDAR)-dependent LTD and NMDAR activation-induced AMPAR endocytosis. We found that chemLTD causes a rapid and persistent shrinkage in spine head volume of hippocampal CA1 pyramidal neurons in wild types similar to that reported in other studies using low-frequency stimulation (LFS)-induced LTD. Surprisingly, we found that although S845A mutant mice display impaired chemLTD, the shrinkage of spine head volume occurred to a similar magnitude to that observed in wild types. Our results suggest that there is dissociation in the molecular mechanisms underlying functional LTD and spine shrinkage and that GluR1-S845 regulation is not necessary for spine morphological plasticity.

Beta-adrenergic receptor activation rescues theta frequency stimulation-induced LTP deficits in mice expressing C-terminally truncated NMDA receptor GluN2A subunits

Through protein interactions mediated by their cytoplasmic C termini the GluN2A and GluN2B subunits of NMDA receptors (NMDARs) have a key role in the formation of NMDAR signaling complexes at excitatory synapses. Although these signaling complexes are thought to have a crucial role in NMDAR-dependent forms of synaptic plasticity such as long-term potentiation (LTP), the role of the C terminus of GluN2A in coupling NMDARs to LTP enhancing and/or suppressing signaling pathways is unclear. To address this issue we examined the induction of LTP in the hippocampal CA1 region in mice lacking the C terminus of endogenous GluN2A subunits (GluN2AΔC/ΔC). Our results show that truncation of GluN2A subunits produces robust, but highly frequency-dependent, deficits in LTP and a reduction in basal levels of extracellular signal regulated kinase 2 (ERK2) activation and phosphorylation of AMPA receptor GluA1 subunits at a protein kinase A site (serine 845). Consistent with the notion that these signaling deficits contribute to the deficits in LTP in GluN2AΔC/ΔC mice, activating ERK2 and increasing GluA1 S845 phosphorylation through activation of β-adrenergic receptors rescued the induction of LTP in these mutants. Together, our results indicate that the capacity of excitatory synapses to undergo plasticity in response to different patterns of activity is dependent on the coupling of specific signaling pathways to the intracellular domains of the NMDARs and that abnormal plasticity resulting from mutations in NMDARs can be reduced by activation of key neuromodulatory transmitter receptors that engage converging signaling pathways.

Synaptic plasticity in the pathophysiology and treatment of bipolar disorder

Emerging evidence suggests that synaptic plasticity is intimately involved in the pathophysiology and treatment of bipolar disorder (BPD). Under certain conditions, over-strengthened and/or weakened synapses at different circuits in the brain could disturb brain functions in parallel, causing manic-like or depressive-like behaviors in animal models. In this chapter, we summarize the regulation of synaptic plasticity by medications, psychological conditions, hormones, and neurotrophic factors, and their correlation with mood-associated animal behaviors. We conclude that increased serotonin, norepinephrine, dopamine, brain-derived neurotrophic factor (BDNF), acute corticosterone, and antidepressant treatments lead to enhanced synaptic strength in the hippocampus and also correlate with antidepressant-like behaviors. In contrast, inhibiting monoaminergic signaling, long-term stress, and pathophysiological concentrations of cytokines weakens glutamatergic synaptic strength in the hippocampus and is associated with depressive-like symptoms.

Ca2+/calmodulin-dependent protein kinase II inhibitors disrupt AKAP79-dependent PKC signaling to GluA1 AMPA receptors

GluA1 (formerly GluR1) AMPA receptor subunit phosphorylation at Ser-831 is an early biochemical marker for long-term potentiation and learning. This site is a substrate for Ca(2+)/calmodulin (CaM)-dependent protein kinase II (CaMKII) and protein kinase C (PKC). By directing PKC to GluA1, A-kinase anchoring protein 79 (AKAP79) facilitates Ser-831 phosphorylation and makes PKC a more potent regulator of GluA1 than CaMKII. PKC and CaM bind to residues 31-52 of AKAP79 in a competitive manner. Here, we demonstrate that common CaMKII inhibitors alter PKC and CaM interactions with AKAP79(31-52). Most notably, the classical CaMKII inhibitors KN-93 and KN-62 potently enhanced the association of CaM to AKAP79(31-52) in the absence (apoCaM) but not the presence of Ca(2+). In contrast, apoCaM association to AKAP79(31-52) was unaffected by the control compound KN-92 or a mechanistically distinct CaMKII inhibitor (CaMKIINtide). In vitro studies demonstrated that KN-62 and KN-93, but not the other compounds, led to apoCaM-dependent displacement of PKC from AKAP79(31-52). In the absence of CaMKII activation, complementary cellular studies revealed that KN-62 and KN-93, but not KN-92 or CaMKIINtide, inhibited PKC-mediated phosphorylation of GluA1 in hippocampal neurons as well as AKAP79-dependent PKC-mediated augmentation of recombinant GluA1 currents. Buffering cellular CaM attenuated the ability of KN-62 and KN-93 to inhibit AKAP79-anchored PKC regulation of GluA1. Therefore, by favoring apoCaM binding to AKAP79, KN-62 and KN-93 derail the ability of AKAP79 to efficiently recruit PKC for regulation of GluA1. Thus, AKAP79 endows PKC with a pharmacological profile that overlaps with CaMKII.

Activation of D1/D5 dopamine receptors protects neurons from synapse dysfunction induced by amyloid-beta oligomers

Soluble oligomers of the amyloid-β peptide (AβOs) accumulate in the brains of Alzheimer disease (AD) patients and are implicated in synapse failure and early memory loss in AD. AβOs have been shown to impact synapse function by inhibiting long term potentiation, facilitating the induction of long term depression and inducing internalization of both AMPA and NMDA glutamate receptors, critical players in plasticity mechanisms. Because activation of dopamine D1/D5 receptors plays important roles in memory circuits by increasing the insertion of AMPA and NMDA receptors at synapses, we hypothesized that selective activation of D1/D5 receptors could protect synapses from the deleterious action of AβOs. We show that SKF81297, a selective D1/D5 receptor agonist, prevented the reduction in surface levels of AMPA and NMDA receptors induced by AβOs in hippocampal neurons in culture. Protection by SKF81297 was abrogated by the specific D1/D5 antagonist, SCH23390. Levels of AMPA receptor subunit GluR1 phosphorylated at Ser(845), which regulates AMPA receptor association with the plasma membrane, were reduced in a calcineurin-dependent manner in the presence of AβOs, and treatment with SKF81297 prevented this reduction. Establishing the functional relevance of these findings, SKF81297 blocked the impairment of long term potentiation induced by AβOs in hippocampal slices. Results suggest that D1/D5 receptors may be relevant targets for development of novel pharmacological approaches to prevent synapse failure in AD.

Protein phosphatases 1 and 2A are both required for long-term depression and associated dephosphorylation of cAMP response element binding protein in hippocampal area CA1 in vivo

Evidence shows that the serine/threonine protein phosphatase 1 (PP1) plays a critical role in synaptic plasticity and memory. Little is known about the contribution of the serine/threonine phosphatase 1 (PP2A) to synaptic plasticity. Both protein phosphatases can target the transcription factor cAMP response element binding protein (CREB), whose phosphorylation at Ser133, we previously found, was downregulated during long-term depression (LTD) of glutamatergic transmission in area CA1 of the adult hippocampus in vivo. Other work from our group showed that the activity of PP2A, as well as that of PP1, is increased after LTD induction in area CA1 in vivo. We therefore investigated here whether both protein phosphatases are necessary for LTD in area CA1, and whether they both are involved in the LTD-associated modification of CREB. We found that inhibition of either PP1 or PP2A interferes with the establishment of LTD. Furthermore, inhibition of either enzyme alone abrogated the LTD-associated dephosphorylation of CREB. Interestingly, inhibition of PP1 disrupted CREB dephosphosphorylation rapidly after LTD-inducing stimulation, whereas inhibition of PP2A did not blunt the CREB modification until a later time point. Thus, both PP1 and PP2A regulate CREB during LTD in area CA1, although possibly through different signaling pathways. Our results demonstrate that PP2A, similar to PP1, plays an essential role in the molecular events that underlie LTD at glutamatergic synapses in hippocampal area CA1 in vivo. We propose that one of the mechanisms through which these protein phosphatases may contribute to the prolonged maintenance of LTD is through the regulation of CREB.

Similar intracellular peptide profile of TAP1/β2 microglobulin double-knockout mice and C57BL/6 wild-type mice as revealed by peptidomic analysis

Cells produce and use peptides in distinctive ways. In the present report, using isotope labeling plus semi-quantitative mass spectrometry, we evaluated the intracellular peptide profile of TAP1/β2m⁻(/)⁻ (transporter associated with antigen-processing 1/ß2 microglobulin) double-knockout mice and compared it with that of C57BL/6 wild-type animals. Overall, 92 distinctive peptides were identified, and most were shown to have a similar concentration in both mouse strains. However, some peptides showed a modest increase or decrease (~2-fold), whereas a glycine-rich peptide derived from the C-terminal of neurogranin (KGPGPGGPGGAGGARGGAGGGPSGD) showed a substantial increase (6-fold) in TAP1/β2m⁻(/)⁻ mice. Thus, TAP1 and β2microglobulin have a small influence on the peptide profile of neuronal tissue, suggesting that the presence of peptides derived from intracellular proteins in neuronal tissue is not associated with antigens of the class I major histocompatibility complex. Therefore, it is possible that these intracellular peptides play a physiological role.

Glutamate receptor phosphorylation and trafficking in pain plasticity in spinal cord dorsal horn

Glutamate is the major excitatory neurotransmitter in the central nervous system. Considerable evidence suggests that both ionotropic and metabotropic glutamate receptors are involved in pain hypersensitivity. However, glutamate receptor-based therapies are limited by side-effects because the activities of glutamate receptors are essential for many important physiological functions. Here, we review recent key findings in molecular and cellular mechanisms of glutamate receptor regulation and their roles in triggering and sustaining pain hypersensitivity. Targeting these molecular mechanisms could form the basis for new therapeutic strategies for the treatment of chronic pain.

Dopamine D1 receptor-mediated NMDA receptor insertion depends on Fyn but not Src kinase pathway in prefrontal cortical neurons

BACKGROUND

Interactions between dopamine and glutamate in the prefrontal cortex are essential for cognitive functions such as working memory. Modulation of N-methyl-D-aspartic acid (NMDA) receptor functions by dopamine D1 receptor is believed to play a critical role in these functions. The aim of the work reported here is to explore the signaling pathway underlying D1 receptor-mediated trafficking of NMDA receptors in cultured rat prefrontal cortical neurons.

RESULTS

Activation of D1 receptor by selective agonist SKF-81297 significantly increased the expression of NR2B subunits. This effect was completely blocked by small interfering RNA knockdown of Fyn, but not Src. Under control conditions, neither Fyn nor Src knockdown exhibited significant effect on basal NR2B expression. D1 stimulation significantly enhanced NR2B insertion into plasma membrane in cultured PFC neurons, a process obstructed by Fyn, but not Src, knockdown.

CONCLUSIONS

Dopamine D1 receptor-mediated increase of NMDA receptors is thus Fyn kinase dependent. Targeting this signaling pathway may be useful in treating drug addiction and schizophrenia.

Viagra for your synapses: Enhancement of hippocampal long-term potentiation by activation of beta-adrenergic receptors

Beta-adrenergic receptors (beta-ARs) critically modulate long-lasting synaptic plasticity and long-term memory storage in the mammalian brain. Synaptic plasticity is widely believed to mediate memory storage at the cellular level. Long-term potentiation (LTP) is one type of synaptic plasticity that has been linked to memory storage. Activation of beta-ARs can enhance LTP and facilitate long-term memory storage. Interestingly, many of the molecular signaling pathways that are critical for beta-adrenergic modulation of LTP mirror those required for the persistence of memory. In this article, we review the roles of signaling cascades and translation regulation in enabling beta-ARs to control expression of long-lasting LTP in the rodent hippocampus. These include the cyclic-AMP/protein kinase-A (cAMP-PKA) and extracellular signal-regulated protein kinase cascades, two key pathways known to link transmitter receptors with translation regulation. Future research directions are discussed, with emphasis on defining the roles of signaling complexes (e.g. PSD-95) and glutamatergic receptors in controlling the efficacy of beta-AR modulation of LTP.

Involvement of cyclin-dependent kinase-like 2 in cognitive function required for contextual and spatial learning in mice

Cyclin-dependent kinase-like 2 (Cdkl2) is a cdc2-related serine/threonine protein kinase that is postnatally expressed in various brain regions, including the cerebral cortex, entorhinal cortex, hippocampus, amygdala, and dorsal thalamus. The extremely high Cdkl2 expression in these regions suggests that it has a role in cognition and emotion. Recent genetic studies indicate that mutations of Cdkl family kinases are associated with neurodevelopmental and neuropsychiatric disorders in humans. To elucidate the physiologic role of Cdkl2, we behaviorally analyzed Cdkl2(LacZ/LacZ) mice lacking Cdkl2. Cdkl2(LacZ/LacZ) mice had reduced latencies to enter the dark compartment after electric footshock in an inhibitory avoidance task and attenuated contextual fear responses when exposed to mild training conditions. Hippocampal spatial learning in the Morris water maze was slightly anomalous with mice exhibiting an abnormal swimming pattern. The aversive response in a two-way avoidance task was slightly, but not significantly, enhanced. On the other hand, Cdkl2(LacZ/LacZ) mice did not exhibit altered sensitivity to aversive stimuli, such as electric footshock and heat, or deficits in the elevated plus maze or rotating rod test. These findings suggest that Cdkl2 is involved in cognitive function and provide in vivo evidence for the function of Cdkl family kinases expressed in terminally differentiated neurons in mice.

Temporal sensitivity of protein kinase a activation in late-phase long term potentiation

Protein kinases play critical roles in learning and memory and in long term potentiation (LTP), a form of synaptic plasticity. The induction of late-phase LTP (L-LTP) in the CA1 region of the hippocampus requires several kinases, including CaMKII and PKA, which are activated by calcium-dependent signaling processes and other intracellular signaling pathways. The requirement for PKA is limited to L-LTP induced using spaced stimuli, but not massed stimuli. To investigate this temporal sensitivity of PKA, a computational biochemical model of L-LTP induction in CA1 pyramidal neurons was developed. The model describes the interactions of calcium and cAMP signaling pathways and is based on published biochemical measurements of two key synaptic signaling molecules, PKA and CaMKII. The model is stimulated using four 100 Hz tetani separated by 3 sec (massed) or 300 sec (spaced), identical to experimental L-LTP induction protocols. Simulations show that spaced stimulation activates more PKA than massed stimulation, and makes a key experimental prediction, that L-LTP is PKA-dependent for intervals larger than 60 sec. Experimental measurements of L-LTP demonstrate that intervals of 80 sec, but not 40 sec, produce PKA-dependent L-LTP, thereby confirming the model prediction. Examination of CaMKII reveals that its temporal sensitivity is opposite that of PKA, suggesting that PKA is required after spaced stimulation to compensate for a decrease in CaMKII. In addition to explaining the temporal sensitivity of PKA, these simulations suggest that the use of several kinases for memory storage allows each to respond optimally to different temporal patterns.

The hippocampus is a part of the cerebral cortex intimately involved in learning and memory behavior. A common cellular model of learning is a long lasting form of long term potentiation (L-LTP) in the hippocampus, because it shares several characteristics with learning. For example, both learning and long term potentiation exhibit sensitivity to temporal patterns of synaptic inputs and share common intracellular events such as activation of specific intracellular signaling pathways. Therefore, understanding the pivotal molecules in the intracellular signaling pathways underlying temporal sensitivity of L-LTP in the hippocampus may illuminate mechanisms underlying learning. We developed a computational model to evaluate whether the signaling pathways leading to activation of the two critical enzymes: protein kinase A and calcium-calmodulin-dependent kinase II are sufficient to explain the experimentally observed temporal sensitivity. Indeed, the simulations demonstrate that these enzymes exhibit different temporal sensitivities, and make a key experimental prediction, that L-LTP is dependent on protein kinase A for intervals larger than 60 sec. Measurements of hippocampal L-LTP confirm this prediction, demonstrating the value of a systems biology approach to computational neuroscience.

Electroconvulsive seizure increases phosphorylation of PKC substrates, including GAP-43, MARCKS, and neurogranin, in rat brain

Protein kinase C (PKC) has been suggested as a molecular target related to the pathogenetic and therapeutic mechanisms of mood disorders in which electroconvulsive seizure (ECS) is effective. However, the reports concerning the effects of ECS on PKC are anecdotal and need further clarification. In this study, we examined the effects of ECS treatment on the phosphorylation of PKC substrates, including GAP-43, MARCKS, and neurogranin. Immunoblot using anti-p-PKC substrate antibodies revealed that a single ECS treatment induced temporal changes in the phosphorylation level of PKC substrates in rat brain, reflecting the effects on PKC activity. Phosphorylation of GAP-43 and MARCKS, representative PKC substrates related to synaptic remodeling, increased from 5 to 30 min, after a transient decrease at 0 min immediately after ECS, and returned to basal levels at 60 min in rat frontal cortex, hippocampus, and cerebellum. Phosphorylation of neurogranin, another PKC substrate, showed a similar pattern of temporal changes in the frontal cortex and hippocampus. Immunohistochemical analysis revealed that p-GAP-43 and p-MARCKS were densely stained throughout the neuronal cells of the prefrontal cortex and hippocampus, and the Purkinje cells of cerebellum, after ECS treatment. Brief and transient activation of PKC may be translated into long-term biochemical changes, resulting in synaptic plasticity. Taken together, the acute effects of ECS on PKC activity, which could be an underpinning of long-term biochemical changes induced by ECS, may contribute to understand the molecular mechanism of ECS.

Differential regulation of kainate receptor trafficking by phosphorylation of distinct sites on GluR6

Kainate receptors are widely expressed in the brain, and are present at pre- and postsynaptic sites where they play a prominent role in synaptic plasticity and the regulation of network activity. Within individual neurons, kainate receptors of different subunit compositions are targeted to various locations where they serve distinct functional roles. Despite this complex targeting, relatively little is known about the molecular mechanisms regulating kainate receptor subunit trafficking. Here we investigate the role of phosphorylation in the trafficking of the GluR6 kainate receptor subunit. We identify two specific residues on the GluR6 C terminus, Ser(846) and Ser(868), which are phosphorylated by protein kinase C (PKC) and dramatically regulate GluR6 surface expression. By using GluR6 containing phosphomimetic and nonphosphorylatable mutations for these sites expressed in heterologous cells or in neurons lacking endogenous GluR6, we show that phosphorylation of Ser(846) or Ser(868) regulates receptor trafficking through the biosynthetic pathway. Additionally, Ser(846) phosphorylation dynamically regulates endocytosis of GluR6 at the plasma membrane. Our findings thus demonstrate that phosphorylation of PKC sites on GluR6 regulates surface expression of GluR6 at distinct intracellular trafficking pathways, providing potential molecular mechanisms for the PKC-dependent regulation of synaptic kainate receptor function observed during various forms of synaptic plasticity.

Short trains of theta frequency stimulation enhance CA1 pyramidal neuron excitability in the absence of synaptic potentiation

Although plasticity at excitatory synapses is widely studied as a mechanism for memory formation, less is known about the properties and mechanisms underlying activity-dependent changes in excitability. Using extracellular and intracellular recordings in hippocampal slices, we find that short trains (2-3 s) of Schaffer collateral fiber stimulation delivered at 5 Hz induce a robust and persistent increase in the excitability of CA1 pyramidal cells in the absence of synaptic potentiation. This change in excitability is input specific, NMDA receptor dependent, and is not accompanied by lasting changes in either inhibitory synaptic transmission or somatic excitability. Although many of these properties are similar to those seen in synaptic long-term potentiation (LTP), the increase in CA1 pyramidal cell excitability was not blocked by inhibitors of several protein kinases required for the induction of LTP by theta frequency stimulation. Instead, 5 Hz stimulation-induced changes in neuronal excitability were blocked by inhibitors of the protein phosphatase calcineurin. Together, our results suggest that very brief bouts of theta frequency synaptic activity induce a selective, persistent, and dendritically localized increase in CA1 pyramidal cell excitability that might have an important role in both information storage and metaplasticity.

Signal transduction mechanisms underlying group I mGluR-mediated increase in frequency and amplitude of spontaneous EPSCs in the spinal trigeminal subnucleus oralis of the rat

Group I mGluRs (mGluR1 and 5) pre- and/or postsynaptically regulate synaptic transmission at glutamatergic synapses. By recording spontaneous EPSCs (sEPSCs) in the spinal trigeminal subnucleus oralis (Vo), we here investigated the regulation of glutamatergic transmission through the activation of group I mGluRs. Bath-applied DHPG (10 μM/5 min), activating the group I mGluRs, increased sEPSCs both in frequency and amplitude; particularly, the increased amplitude was long-lasting. The DHPG-induced increases of sEPSC frequency and amplitude were not NMDA receptor-dependent. The DHPG-induced increase in the frequency of sEPSCs, the presynaptic effect being further confirmed by the DHPG effect on paired-pulse ratio of trigeminal tract-evoked EPSCs, an index of presynaptic modulation, was significantly but partially reduced by blockades of voltage-dependent sodium channel, mGluR1 or mGluR5. Interestingly, PKC inhibition markedly enhanced the DHPG-induced increase of sEPSC frequency, which was mainly accomplished through mGluR1, indicating an inhibitory role of PKC. In contrast, the DHPG-induced increase of sEPSC amplitude was not affected by mGluR1 or mGluR5 antagonists although the long-lasting property of the increase was disappeared; however, the increase was completely inhibited by blocking both mGluR1 and mGluR5. Further study of signal transduction mechanisms revealed that PLC and CaMKII mediated the increases of sEPSC in both frequency and amplitude by DHPG, while IP₃ receptor, NO and ERK only that of amplitude during DHPG application. Altogether, these results indicate that the activation of group I mGluRs and their signal transduction pathways differentially regulate glutamate release and synaptic responses in Vo, thereby contributing to the processing of somatosensory signals from orofacial region.

Growth factor-dependent trafficking of cerebellar NMDA receptors via protein kinase B/Akt phosphorylation of NR2C

NMDA receptor subunit composition varies throughout the brain, providing molecular diversity in NMDA receptor function. The NR2 subunits (NR2A-D) in large part dictate the distinct functional properties of NMDA receptors and differentially regulate receptor trafficking. Although the NR2C subunit is highly enriched in cerebellar granule cells and plays a unique role in cerebellar function, little is known about NR2C-specific regulation of NMDA receptors. Here, we demonstrate that PKB/Akt directly phosphorylates NR2C on serine 1096 (S1096). In addition, we identify 14-3-3epsilon as an NR2C interactor, whose binding is dependent on S1096 phosphorylation. Both growth factor stimulation and NMDA receptor activity lead to a robust increase in both phosphorylation of NR2C on S1096 and surface expression of cerebellar NMDA receptors. Finally, we find that NR2C expression, unlike NR2A and NR2B, supports neuronal survival. Thus, our data provide a direct mechanistic link between growth factor stimulation and regulation of cerebellar NMDA receptors.

Spinal NMDA receptor activation is necessary for de novo, but not the maintenance of, A2a receptor-mediated phrenic motor facilitation

Adenosine 2a (A2a) receptor agonists elicit persistent increases in phrenic nerve activity by transactivating the neurotrophin receptor, TrkB, near phrenic motoneurons. Our working model proposes that A2a receptor-mediated TrkB receptor activation strengthens glutamatergic synapses onto phrenic motoneurons. Activation of glutamate N-methyl d-aspartate (NMDA) receptors has been implicated in other models of phrenic motor plasticity. Thus we hypothesized that NMDA receptor activation also would contribute to A2a receptor-mediated phrenic motor facilitation. Adult male Sprague-Dawley rats were anesthetized with urethane, mechanically ventilated, neuromuscularly paralyzed, and bilaterally vagotomized. The A2a receptor agonist CGS-21680 and the NMDA receptor-channel blocker MK-801 were administered intrathecally over the C4 spinal segment. Phrenic nerve activity was recorded before, during, and after drug administration. MK-801 (concentration range 0.1, 1.0, 10.0, and 100 microM) was administered 30 min before CGS-21680 (50 microM). MK-801 dose-dependently blocked A2a receptor-mediated phrenic motor facilitation. When administered at 60 min post-CGS-21680, MK-801 prevented further increases in phrenic nerve activity compared with the CGS-21680 alone (CGS-21680 alone at 120 min: 114 +/- 19%; CGS-21680 and MK-801 at 60 min post-CGS-21680: 61 +/- 11%, above baseline, P < 0.05) but did not return phrenic motor output to baseline values. Our data suggest that NMDA receptor activation is necessary for de novo A2a receptor-mediated phrenic motor facilitation and that the maintenance of preexisting phrenic motor facilitation does not involve NMDA receptor-dependent mechanisms.

Phosphorylation of the homer-binding domain of group I metabotropic glutamate receptors by cyclin-dependent kinase 5

Phosphorylation of neurotransmitter receptors can modify their activity and regulate neuronal excitability. Cyclin-dependent kinase 5 (cdk5) is a proline-directed serine/threonine kinase involved not only in neuronal development, but also in synaptic function and plasticity. Here we demonstrate that group I metabotropic glutamate receptors (mGluRs), which modulate post-synaptic signaling by coupling to intracellular signal transduction pathways, are phosphorylated by cdk5. In vitro kinase assays reveal that cdk5 phosphorylates mGluR5 within the domain of the receptor that interacts with the scaffolding protein homer. Using a novel phosphospecific mGluR antibody, we show that the homer-binding domain of both mGluR1 and mGluR5 are phosphorylated in vivo, and that inhibition of cdk5 with siRNA decreases the amount of phosphorylated receptor. Furthermore, kinetic binding analysis, by surface plasmon resonance, indicates that phosphorylation of mGluR5 enhances its association with homer. Homer protein complexes in the post-synaptic density, and their disruption by an activity-dependent short homer 1a isoform, have been shown to regulate the trafficking and signaling of the mGluRs and impact many neuroadaptive processes. Phosphorylation of the mGluR homer-binding domain, in contrast to homer 1a induction, provides a novel mechanism for potentially regulating a subset of homer interactions.

PKCgamma-induced trafficking of AMPA receptors in embryonic zebrafish depends on NSF and PICK1

The trafficking of AMPA receptors (Rs) to and from synaptic membranes is a key component underlying synaptic plasticity mechanisms such as long-term potentiation (LTP) and long-term depression (LTD), and is likely important for synaptic development in embryonic organisms. However, some of the key biochemical components required for receptor trafficking in embryos are still unknown. Here, we report that in embryonic zebrafish, the activation of PKCgamma by phorbol 12-myristate 13-acetate, strongly potentiates the amplitude of AMPAR-mediated miniature excitatory postsynaptic currents (AMPA-mEPSCs) via a N-ethylmaleimide-sensitive fusion (NSF) and protein interacting with C-kinase-1 (PICK1)-dependent process. We found that the mEPSC potentiation is DAG- and Ca(2+)-dependent, and occurs on application of active PKCgamma. Peptides that prevent the association of NSF and PICK1 with the GluR2 subunit, and the actin-polymerization blocker, latrunculin B, prevented the increase in mEPSC amplitude. Also, application of tetanus toxin (TeTx), which cleaves SNARE proteins, also blocked the increase in mEPSC amplitude. Last, application of a 5 mM K(+) medium led to an enhancement in mEPSC amplitude that was prevented by addition of the PKCgamma and NSF-blocking peptides, and the NMDA receptor blocker, 2-amino-5-phosphonovaleric acid (APV). Thus, activation of PKCgamma is necessary for the activity-dependent trafficking of AMPARs in embryonic zebrafish. This process is NMDA and SNARE-dependent and requires AMPARs to associate with both NSF and PICK1. The present data further our understanding of AMPAR trafficking, and have important implications for synaptic development and synaptic plasticity.

AMPA receptors in the medial amygdala are critical for establishing a neuroendocrine memory in the female rat

We sought to examine AMPA receptor (AMPAR) function in the medial posterodorsal amygdala (MePD), as glutamate neurotransmission is critical for the neural response to vaginal-cervical stimulation that initiates pregnancy or pseudopregnancy. Female rats were infused with the AMPAR antagonist CNQX or vehicle directly into the MePD via bilaterally implanted cannulae, then either returned to their homecage (HC), or received 15 mounts-without-intromissions (MO) or 15 intromissions (15I) from a male. Expression of the activity marker EGR-1 was used to determine the CNQX concentration which would prevent mating-induced activation of MePD neurons. Separate cannulated females received CNQX infusions into the MePD prior to receiving 15I, and the oestrous cycle length was monitored by daily vaginal lavages. Infusion of CNQX (500 nm) blocked mating-induced neural activation and lengthened the oestrous cycle, demonstrating AMPAR involvement in the formation of pseudopregnancy. To further explore this involvement, separate groups of 15I, MO and HC females were killed 90 min or 3 h after testing treatment. Brain sections were immunolabeled for AMPAR-subunit GluR1 phosphorylated at one of two sites (Serine-831 or Serine-845), or total GluR1 and GluR2, and immunofluorescence intensity was measured in the MePD, hippocampus and hypothalamus. A mating-induced increase in Serine-831 phosphorylation after 3 h was observed only in the MePD, whereas there was no effect on Serine-845 phosphorylation. Additionally, we observed a time-dependent increase in total GluR1 staining intensity. These results suggest an increased AMPAR function in the MePD after receipt of VCS, and a role for AMPAR in the neural response to VCS resulting in pseudopregnancy.

Neuronal activity regulates phosphorylation-dependent surface delivery of G protein-activated inwardly rectifying potassium channels

G protein-activated inwardly rectifying K(+) (GIRK) channels regulate neuronal excitability by mediating inhibitory effects of G protein-coupled receptors for neurotransmitters and neuromodulators. Notwithstanding many studies reporting modulation of GIRK channel function, whether neuronal activity regulates GIRK channel trafficking remains an open question. Here we report that NMDA receptor activation in cultured dissociated hippocampal neurons elevates surface expression of the GIRK channel subunits GIRK1 and GIRK2 in the soma, dendrites, and dendritic spines within 15 min. This activity-induced increase in GIRK surface expression requires protein phosphatase-1-mediated dephosphorylation of a serine residue (Ser-9) preceding the GIRK2 Val-13/Leu-14 (VL) internalization motif, thereby promoting channel recycling. Because activation of GIRK channels hyperpolarizes neuronal membranes, the NMDA receptor-induced regulation of GIRK channel trafficking may represent a dynamic adjustment of neuronal excitability in response to inhibitory neurotransmitters and/or neuromodulators.

G protein-activated inwardly rectifying potassium channels mediate depotentiation of long-term potentiation

Excitatory synapses in the brain undergo activity-dependent changes in the strength of synaptic transmission. Such synaptic plasticity as exemplified by long-term potentiation (LTP) is considered a cellular correlate of learning and memory. The presence of G protein-activated inwardly rectifying K(+) (GIRK) channels near excitatory synapses on dendritic spines suggests their possible involvement in synaptic plasticity. However, whether activity-dependent regulation of GIRK channels affects excitatory synaptic plasticity is unknown. In a companion article we have reported activity-dependent regulation of GIRK channel density in cultured hippocampal neurons that requires activity of NMDA receptors (NMDAR) and protein phosphatase-1 (PP1) and takes place within 15 min. In this study, we performed whole-cell recordings of cultured hippocampal neurons and found that NMDAR activation increases basal GIRK current and GIRK channel activation mediated by adenosine A(1) receptors, but not GABA(B) receptors. Given the similar involvement of NMDARs, adenosine A(1) receptors, and PP1 in depotentiation of LTP caused by low-frequency stimulation that immediately follows LTP-inducing high-frequency stimulation, we wondered whether NMDAR-induced increase in GIRK channel surface density and current may contribute to the molecular mechanisms underlying this specific depotentiation. Remarkably, GIRK2 null mutation or GIRK channel blockade abolishes depotentiation of LTP, demonstrating that GIRK channels are critical for depotentiation, one form of excitatory synaptic plasticity.

Expression and activation of N-methyl-D-aspartate receptor subunit-1 receptor subunits in gonadotrophin-releasing hormone neurones of young and middle-aged mice during the luteinising hormone surge

Glutamate is an important excitatory neurotransmitter that stimulates the release of gonadotrophin-releasing hormone (GnRH) and participates in the generation of the luteinising hormone (LH) surge. To determine the mechanisms of action of glutamate and possible changes in the glutamatergic input to GnRH neurones during reproductive ageing, we measured the expression and activation of the mandatory N-methyl-D-aspartate receptor subunit-1 (NMDAR1) in GnRH neurones of young and middle-aged mice prior to and during a steroid-induced LH surge. The results show that, in young animals, approximately 55% of all GnRH neurones contain immunoreactive NMDAR1 protein and this percentage does not change during the day of the LH surge. In approximately 10% of the GnRH neurones, NMDA receptor protein is phosphorylated at Ser 890 prior to the surge, whereas, in approximately 55% of the GnRH neurones, NMDAR1 subunits are phosphorylated during the LH surge. Activation of NMDAR1 receptor subunits stimulates the calcium-calmodulin-kinase IV (CaMK IV). pathway, which leads to the translocation of CaMK IV into the nucleus where this enzyme can phosphorylate the cAMP response element-binding protein (CREB) and CREB-binding protein. We show that, in young animals, approximately 20% of the GnRH neurones contain CaMK IV in their nuclei 7 h prior to the LH surge; this percentage increases to 60% at the beginning of the surge and decreases to approximately 40% some 2 h into the LH surge. In middle-aged animals, approximately 25% of the GnRH neurones contain NMDAR1 protein and only 10% of the GnRH neurones contain phosphorylated NMDAR1 protein prior to and during the surge; however 2 h after the peak of the surge, 20% of the GnRH neurones contain phosphorylated NMDAR1 subunits. Similarly, 20% of GnRH neurones contain nuclear CaMK IV and this percentage does not change during the day of the LH surge. The results suggest that, in the young animal, glutamatergic innervation of GnRH neurones during the LH surge causes the activation and phosphorylation of NMDAR1 receptor subunits which results in the translocation of CaMK IV into the nucleus. However, both, the expression and activation of NMDAR1 receptor subunits are greatly reduced in the middle-aged animals, which could result in the absence of LH surges.

The role of post-translational modifications for learning and memory formation

Learning and memory depend on molecular mechanisms involving the protein machinery. Recent evidence proposes that post-translational modifications (PTMs) play a major role in these cognitive processes. PTMs including phosphorylation of serine, threonine, and tyrosine are already well-documented to play a role for synaptic plasticity of the brain, neurotransmitter release, vesicle trafficking and synaptosomal or synaptosomal-associated proteins are substrates of a series of specific protein kinases and their counterparts, protein phosphatases. But protein phosphorylation is only one out of many possible PTMs and first work shows a role of palmitoylation as well as glycosylation for proteins involved in memory formation. Recent technology may now allow reliable detection and even quantification of PTMs of proteins involved in the cognitive system. This will contribute to the understanding of mechanisms for learning and memory formation at the chemical level and has to complement determination of protein levels and indeed determination of protein expression per se generates limited information. The many other PTMs expected including protein nitrosylation and alkylation will even represent targets for pharmacological interventions but in turn increase the complexity of the system. Nevertheless, determination of the presence and the function of PTMs is mandatory and promising cognitive research at the protein chemical level.