Transient incorporation of native GluR2-lacking AMPA receptors during hippocampal long-term potentiation

Estradiol suppresses glutamatergic transmission to gonadotropin-releasing hormone neurons in a model of negative feedback in mice

A surge of gonadotropin-releasing hormone (GnRH) release from the brain triggers the luteinizing hormone (LH) surge that causes ovulation. The GnRH surge is initiated by a switch in estradiol action from negative to positive feedback. Estradiol signals critical for the surge are likely transmitted to GnRH neurons at least in part via estradiol-sensitive afferents. Using an ovariectomized estradiol-treated (OVX+E) mouse model that exhibits daily LH surges, we examined changes in glutamate transmission to GnRH neurons during negative feedback and positive feedback. Spontaneous glutamatergic excitatory postsynaptic currents (EPSCs) mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid/kainate receptors (AMPA/KA Rs) or N-methyl-D-aspartate receptors (NMDARs) were recorded in GnRH neurons from OVX+E and OVX mice. There were no diurnal changes in the percentage of GnRH neurons from OVX mice exhibiting EPSCs. In cells from OVX+E mice, the profile of AMPA/KA R-mediated and NMDAR-mediated EPSCs showed changes dependent on time of day. Comparison of AMPA/KA R-mediated EPSC frequency in OVX+E and OVX cells showed that estradiol suppressed transmission during negative feedback but had no effect during positive feedback. Tetrodotoxin treatment to block action potential firing did not affect AMPA/KA R-mediated EPSC frequency in OVX cells during negative feedback or in OVX+E cells during positive feedback, suggesting that estradiol-induced suppression of glutamate transmission may be primarily due to activity-independent changes. The diurnal removal of estradiol-induced suppression of AMPA/KA R-mediated glutamate transmission to GnRH neurons during positive feedback suggests that the primary role for estradiol-induced changes in glutamate transmission may be in mediating negative feedback.

Increased AMPA receptor GluR1 subunit incorporation in rat hippocampal CA1 synapses during benzodiazepine withdrawal

Prolonged benzodiazepine treatment leads to tolerance and increases the risk of dependence. Flurazepam (FZP) withdrawal is associated with increased anxiety correlated with increased alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-type glutamate receptor (AMPAR)-mediated synaptic function and AMPAR binding in CA1 pyramidal neurons. Enhanced AMPAR synaptic strength is also associated with a shift toward inward rectification of synaptic currents and increased expression of GluR1, but not GluR2, subunits, suggesting augmented membrane incorporation of GluR1-containing, GluR2-lacking AMPARs. To test this hypothesis, the postsynaptic incorporation of GluR1 and GluR2 subunits in CA1 neurons after FZP withdrawal was examined by postembedding immunogold quantitative electron microscopy. The percentage of GluR1 positively labeled stratum radiatum (SR) synapses was significantly increased in FZP-withdrawn rats (88.2% +/- 2.2%) compared with controls (74.4% +/- 1.9%). In addition, GluR1 immunogold density was significantly increased by 30% in SR synapses in CA1 neurons from FZP-withdrawn rats compared with control rats (FZP: 14.1 +/- 0.3 gold particles/mum; CON: 10.8 +/- 0.4 gold particles/mum). In contrast, GluR2 immunogold density was not significantly different between groups. Taken together with recent functional data from our laboratory, the current study suggests that the enhanced glutamatergic strength at CA1 neuron synapses during benzodiazepine withdrawal is mediated by increased incorporation of GluR1-containing AMPARs. Mechanisms underlying synaptic plasticity in this model of drug dependence are therefore fundamentally similar to those that operate during activity-dependent plasticity.

Calcium-permeable AMPA receptors appear in cortical neurons after traumatic mechanical injury and contribute to neuronal fate

Traumatic brain injury (TBI) is one of the most disabling injuries in the population, with 1.5 million Americans new cases each year and 5.3 million Americans overall requiring long-term daily care as a result of their injuries. One critical aspect in developing effective treatments for TBI is determining if new, specific receptor populations emerge in the early phase after injury that can subsequently be targeted to reduce neuronal death after injury. One specific glutamate receptor subtype, the calcium-permeable AMPA receptor (CP-AMPAR), is becoming increasingly recognized for its role in physiological and pathophysiological processes. Although present in relatively low levels in the mature brain, recent studies show that CP-AMPARs can appear following ischemic brain injury or status epilepticus, and the mechanisms that regulate the appearance of these receptors include alterations in transcription, RNA editing, and receptor trafficking. In this report, we use an in vitro model of TBI to show a gradual appearance of CP-AMPARs four hours following injury to cortical neurons. Moreover, the appearance of these receptors is mediated by the phosphorylation of CaMKIIalpha following injury. Selectively blocking CP-AMPARs after mechanical injury leads to a significant reduction in the cell death that occurs 24 h following injury in untreated controls, and is similar in protection offered by broad-spectrum NMDA and AMPA receptor antagonists. These data point to a potentially new and more targeted therapeutic approach for treating TBI.

Sequential changes in AMPA receptor targeting in the developing neocortical excitatory circuit

Many principal neurons undergo an early developmental switch from GluR2-lacking to GluR2-containing synaptic glutamate receptors. We tested the generality and timing of the GluR2 switch in excitatory neurons of rat somatosensory cortex. Previous studies show that the switch occurs between postnatal day 14 (P14) and P16 in layer 5 pyramidal neurons. We show, using sensitivity to intracellular spermine, that a similar switch occurs between P12 and P14 in layer 2/3 pyramidal cells and between P7 and P8 in layer 4 stellate cells. The presence of GluR2-lacking receptors in layer 2/3 pyramidal cells before P12 was confirmed by demonstrating sensitivity to blockade by 1-naphthyl-acetyl-spermine and large single-channel conductances. GluR2 and the postsynaptic protein PSD95 show progressive colocalization in tissue from P10, P14, and P24 rats, mirroring electrophysiological developments. To distinguish whether changes in GluR2 expression or targeting underlie the switch, we characterized dendritic AMPA receptor responses using focal photolysis of caged glutamate. Contrary to synaptic responses, dendritic responses at all ages studied (P6-P40) were characteristic of GluR2-containing receptors. In addition, dendritically and synaptically evoked responses showed a corresponding decrease in NMDA/AMPA ratios in pyramidal cells, suggesting parallel mechanisms that regulate neuronal calcium levels. These data suggest that the GluR2 switch results from changes in AMPA receptor targeting during early postnatal development, and that rather than following the laminar sequence of cortical development, it proceeds sequentially from layer 4 to layer 2/3 and finally to layer 5b.

cGMP signalling in the mammalian brain: role in synaptic plasticity and behaviour

The second messenger cyclic guanosine 3',5'-monophosphate (cGMP) plays a crucial role in the control of cardiovascular and gastrointestinal homeostastis, but its effects on neuronal functions are less established. This review summarizes recent biochemical and functional data on the role of the cGMP signalling pathway in the mammalian brain, with a focus on the regulation of synaptic plasticity, learning, and other complex behaviours. Expression profiling, along with pharmacological and genetic manipulations, indicates important functions of nitric oxide (NO)-sensitive soluble guanylyl cyclases (sGCs), cGMP-dependent protein kinases (cGKs), and cGMP-regulated phosphodiesterases (PDEs) as generators, effectors, and modulators of cGMP signals in the brain, respectively. In addition, neuronal cGMP signalling can be transmitted through cyclic nucleotide-gated (CNG) or hyperpolarization-activated cyclic nucleotide-gated (HCN) ion channels. The canonical NO/sGC/cGMP/cGK pathway modulates long-term changes of synaptic activity in the hippocampus, amygdala, cerebellum, and other brain regions, and contributes to distinct forms of learning and memory, such as fear conditioning, motor adaptation, and object recognition. Behavioural studies indicate that cGMP signalling is also involved in anxiety, addiction, and the pathogenesis of depression and schizophrenia. At the molecular level, different cGK isoforms appear to mediate effects of cGMP on presynaptic transmitter release and postsynaptic functions. The cGKs have been suggested to modulate cytoskeletal organization, vesicle and AMPA receptor trafficking, and gene expression via phosphorylation of various substrates including VASP, RhoA, RGS2, hSERT, GluR1, G-substrate, and DARPP-32. These and other components of the cGMP signalling cascade may be attractive new targets for the treatment of cognitive impairment, drug abuse, and psychiatric disorders.

Leptin reverses long-term potentiation at hippocampal CA1 synapses

The hormone leptin crosses the blood brain barrier and regulates numerous neuronal functions, including hippocampal synaptic plasticity. Here we show that application of leptin resulted in the reversal of long-term potentiation (LTP) at hippocampal CA1 synapses. The ability of leptin to depotentiate CA1 synapses was concentration-dependent and it displayed a distinct temporal profile. Leptin-induced depotentiation was not associated with any change in the paired pulse facilitation ratio or the coefficient of variance, indicating a post-synaptic locus of expression. Moreover, the synaptic activation of NMDA receptors was required for leptin-induced depotentiation as the effects of leptin were blocked by the competitive NMDA receptor antagonist, D-aminophosphovaleric acid (D-AP5). The signaling mechanisms underlying leptin-induced depotentiation involved activation of the calcium/calmodulin-dependent protein phosphatase, calcineurin, but were independent of c-jun NH(2) terminal kinase. Furthermore, leptin-induced depotentiation was accompanied by a reduction in alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor rectification indicating that loss of glutamate receptor 2 (GluR2)-lacking AMPA receptors underlies this process. These data indicate that leptin reverses hippocampal LTP via a process involving calcineurin-dependent internalization of GluR2-lacking AMPA receptors which further highlights the key role for this hormone in regulating hippocampal synaptic plasticity and neuronal development.

The self-tuning neuron: synaptic scaling of excitatory synapses

Homeostatic synaptic scaling is a form of synaptic plasticity that adjusts the strength of all of a neuron's excitatory synapses up or down to stabilize firing. Current evidence suggests that neurons detect changes in their own firing rates through a set of calcium-dependent sensors that then regulate receptor trafficking to increase or decrease the accumulation of glutamate receptors at synaptic sites. Additional mechanisms may allow local or network-wide changes in activity to be sensed through parallel pathways, generating a nested set of homeostatic mechanisms that operate over different temporal and spatial scales.

Calmodulin-kinases: modulators of neuronal development and plasticity

In the nervous system, many intracellular responses to elevated calcium are mediated by CaM kinases (CaMKs), a family of protein kinases whose activities are initially modulated by binding Ca(2+)/calmodulin and subsequently by protein phosphorylation. One member of this family, CaMKII, is well-established for its effects on modulating synaptic plasticity and learning and memory. However, recent studies indicate that some actions on neuronal development and function attributed to CaMKII may instead or in addition be mediated by other members of the CaMK cascade, such as CaMKK, CaMKI, and CaMKIV. This review summarizes key neuronal functions of the CaMK cascade in signal transduction, gene transcription, synaptic development and plasticity, and behavior. The technical challenges of mapping cellular protein kinase signaling pathways are also discussed.

Mechanism and time course of cocaine-induced long-term potentiation in the ventral tegmental area

Synaptic plasticity in the ventral tegmental area (VTA) has been implicated in the acquisition of a drug-dependent state. Even a single exposure to cocaine in naive animals is sufficient to trigger sustained changes on VTA glutamatergic synapses that resemble activity-dependent long-term potentiation (LTP) in other brain regions. However, an insight into its time course and mechanisms of action is limited. Here, we show that cocaine acts locally within the VTA to induce an LTP-like enhancement of AMPA receptor-mediated transmission that is not detectable minutes after drug exposure but is fully expressed within 3 h. This cocaine-induced LTP appears to be mediated via dopamine D(5) receptor activation of NMDA receptors and to require protein synthesis. Increased levels of high-conductance GluR1-containing AMPA receptors at synapses are evident at 3 h after cocaine exposure. Furthermore, our data suggest that cocaine-induced LTP might share the same molecular substrates for expression with activity-dependent LTP induced in the VTA by a spike-timing-dependent (STD) protocol, because we observed that STD LTP is significantly reduced or not inducible in VTA neurons previously exposed to cocaine in vivo or in vitro.

Tamoxifen mediated estrogen receptor activation protects against early impairment of hippocampal neuron excitability in an oxygen/glucose deprivation brain slice ischemia model

Pretreatment of ovarectomized rats with estrogen shows long-term protection via activation of the estrogen receptor (ER). However, it remains unknown whether activation of the ER can provide protection against early neuronal damage when given acutely. We simulated ischemic conditions by applying oxygen and glucose deprived (OGD) solution to acute male rat hippocampal slices and examined the neuronal electrophysiological changes. Pyramidal neurons and interneurons showed a time-dependent membrane potential depolarization and reduction in evoked action potential frequency and amplitude over a 10 to 15 min OGD exposure. These changes were largely suppressed by 10 microM TAM. The TAM effect was neuron-specific as the OGD-induced astrocytic membrane potential depolarization was not altered. The TAM effect was mediated through ER activation because it could be simulated by 17beta-estradiol and was completely inhibited by the ER inhibitor ICI 182, 780, and is therefore an example of TAM's selective estrogen receptor modulator (SERM) action. We further show that TAM's effects on OGD-induced impairment of neuronal excitability was largely due to activation of neuroprotective BK channels, as the TAM effect was markedly attenuated by the BK channel inhibitor paxilline at 10 microM. TAM also significantly reduced the frequency and amplitude of AMPA receptor mediated spontaneous excitatory postsynaptic currents (sEPSCs) in pyramidal neurons which is an early consequence of OGD. Altogether, this study demonstrates that both 17beta-estradiol and TAM attenuate neuronal excitability impairment early on in a simulated ischemia model via ER activation mediated potentiation of BK K(+) channels and reduction in enhanced neuronal AMPA/NMDA receptor-mediated excitotoxicity.

Neuregulin-1 regulates LTP at CA1 hippocampal synapses through activation of dopamine D4 receptors

Neuregulin-1 (NRG-1) is genetically linked with schizophrenia, a neurodevelopmental cognitive disorder characterized by imbalances in glutamatergic and dopaminergic function. NRG-1 regulates numerous neurodevelopmental processes and, in the adult, suppresses or reverses long-term potentiation (LTP) at hippocampal glutamatergic synapses. Here we show that NRG-1 stimulates dopamine release in the hippocampus and reverses early-phase LTP via activation of D4 dopamine receptors (D4R). NRG-1 fails to depotentiate LTP in hippocampal slices treated with the antipsychotic clozapine and other more selective D4R antagonists. Moreover, LTP is not depotentiated in D4R null mice by either NRG-1 or theta-pulse stimuli. Conversely, direct D4R activation mimics NRG-1 and reduces AMPA receptor currents and surface expression. These findings demonstrate that NRG-1 mediates its unique role in counteracting LTP via dopamine signaling and opens future directions to study new aspects of NRG function. The novel functional link between NRG-1, dopamine, and glutamate has important implications for understanding how imbalances in Neuregulin-ErbB signaling can impinge on dopaminergic and glutamatergic function, neurotransmitter pathways associated with schizophrenia.

Activity-dependent coordination of presynaptic release probability and postsynaptic GluR2 abundance at single synapses

The strength of an excitatory synapse depends on both the presynaptic release probability (p(r)) and the abundance of functional postsynaptic AMPA receptors. How these parameters are related or balanced at a single synapse remains unknown. By taking advantage of live fluorescence imaging in cultured hippocampal neurons where individual synapses are readily resolved, we estimate p(r) by labeling presynaptic vesicles with a styryl dye, FM1-43, while concurrently measuring postsynaptic AMPA receptor abundance at the same synapse by immunolabeling surface GluR2. We find no appreciable correlation between p(r) and the level of surface synaptic GluR2 under basal condition, and blocking basal neural activity has no effect on the observed lack of correlation. However, elevating network activity drives their correlation, which accompanies a decrease in mean GluR2 level. These findings provide the direct evidence that the coordination of pre- and postsynaptic parameters of synaptic strength is not intrinsically fixed but that the balance is tuned by synaptic use at individual synapses.

Activity-dependent regulation of synaptic AMPA receptor composition and abundance by beta3 integrins

At synapses, cell adhesion molecules (CAMs) provide the molecular framework for coordinating signaling events across the synaptic cleft. Among synaptic CAMs, the integrins, receptors for extracellular matrix proteins and counterreceptors on adjacent cells, are implicated in synapse maturation and plasticity and memory formation. However, little is known about the molecular mechanisms of integrin action at central synapses. Here, we report that postsynaptic beta3 integrins control synaptic strength by regulating AMPA receptors (AMPARs) in a subunit-specific manner. Pharmacological perturbation targeting beta3 integrins promotes endocytosis of GluR2-containing AMPARs via Rap1 signaling, and expression of beta3 integrins produces robust changes in the abundance and composition of synaptic AMPARs without affecting dendritic spine structure. Importantly, homeostatic synaptic scaling induced by activity deprivation elevates surface expression of beta3 integrins, and in turn, beta3 integrins are required for synaptic scaling. Our findings demonstrate a key role for integrins in the feedback regulation of excitatory synaptic strength.

Recruitment of calcium-permeable AMPA receptors during synaptic potentiation is regulated by CaM-kinase I

Ca(2+)-permeable AMPA receptors (CP-AMPARs) at central glutamatergic synapses are of special interest because of their unique biophysical and signaling properties that contribute to synaptic plasticity and their roles in multiple neuropathologies. However, intracellular signaling pathways that recruit synaptic CP-AMPARs are unknown, and involvement of CP-AMPARs in hippocampal region CA1 synaptic plasticity is controversial. Here, we report that intracellular infusion of active CaM-kinase I (CaMKI) into cultured hippocampal neurons enhances miniature EPSC amplitude because of recruitment of CP-AMPARs, likely from an extrasynaptic pool. The ability of CaMKI, which regulates the actin cytoskeleton, to recruit synaptic CP-AMPARs was blocked by inhibiting actin polymerization with latrunculin A. CaMK regulation of CP-AMPARs was also confirmed in hippocampal slices. CA1 long-term potentiation (LTP) after theta bursts, but not high-frequency tetani, produced a rapid, transient expression of synaptic CP-AMPARs that facilitated LTP. This component of TBS LTP was blocked by inhibition of CaM-kinase kinase (CaMKK), the upstream activator of CaMKI. Our calculations show that adding CP-AMPARs numbering <5% of existing synaptic AMPARs is sufficient to account for the potentiation observed in LTP. Thus, synaptic expression of CP-AMPARs is a very efficient mechanism for rapid enhancement of synaptic strength that depends on CaMKK/CaMKI signaling, actin dynamics, and the pattern of synaptic activity used to induce CA1 LTP.

Silencing-induced metaplasticity in hippocampal cultured neurons

Silencing-induced homeostatic plasticity is usually expressed as a change in the amplitude or the frequency of miniature postsynaptic currents. Here we report that, prolonged (approximately 24 h) silencing of mature (20-22 days in vitro) cultured hippocampal neurons using the voltage-gated sodium channel blocker tetrodotoxin (TTX) produced no effects on the amplitude or frequency of the miniature excitatory postsynaptic currents (mEPSCs). However, the silencing changed the intrinsic membrane properties of the neurons, resulting in an increased excitability and rate of action potentials firing upon TTX washout. Allowing neurons to recover in TTX-free recording solution for a short period of time after the silencing resulted in potentiation of mEPSC amplitudes. This form of activity-dependent potentiation is different from classical long-term potentiation, as similar potentiation was not seen in nonsilenced neurons treated with bicuculline to raise their spiking activity to the same level displayed by the silenced neurons during TTX washout. Also, the potentiation of mEPSC amplitudes after the recovery period was not affected by the N-methyl-d-aspartate receptor blocker d-2-amino-5-phosponopentanoic acid or by the calcium/calmodulin-dependent kinase II (CaMKII) inhibitor KN-62 but was abolished by the L-type calcium channel blocker nifedipine. We thus conclude that the potentiation of mEPSC amplitudes following brief recovery of spiking activity in chronically silenced neurons represents a novel form of metaplasticity that differs from the conventional models of homeostatic synaptic plasticity.

Developmental expression of NMDA and AMPA receptor subunits in vestibular nuclear neurons that encode gravity-related horizontal orientations

We examined the expression profile of subunits of ionotropic glutamate receptors [N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole-proprionate (AMPA)] during postnatal development of connectivity in the rat vestibular nucleus. Vestibular nuclear neurons were functionally activated by constant velocity off-vertical axis rotation, a strategy to stimulate otolith organs in the inner ear. These neurons indicated Fos expression as a result. By immunodetection for Fos, otolith-related neurons that expressed NMDA/AMPA receptor subunits were identified as early as P7, and these neurons were found to increase progressively up to adulthood. Although there was developmental invariance in the percentage of Fos-immunoreactive neurons expressing the NR1, NR2A, GluR1, or GluR2/3 subunits, those expressing the NR2B subunit decreased from P14 onward, and those expressing the GluR4 subunit decreased in adults. These double-immunohistochemical data were corroborated by combined immuno-/hybridization histochemical data obtained from Fos-immunoreactive neurons expressing NR2B mRNA or GluR4 mRNA. The staining of both NR2B and GluR4 in the cytoplasm of these neurons decreased upon maturation. The percentage of Fos-immunoreactive neurons expressing the other ionotropic glutamate receptor subunits (viz. NR1, NR2A, GluR1, and GluR2/3) remained relatively constant throughout postnatal maturation. Triple immunofluorescence further demonstrated coexpression of NR1 and NR2 subunits in Fos-immunoreactive neurons. Coexpression of NR1 subunit with each of the GluR subunits was also observed among the Fos-immunoreactive neurons. Taken together, the different expression profiles of ionotropic glutamate receptor subunits constitute the histological basis for glutamatergic neurotransmission in the maturation of central vestibular connectivity for the coding of gravity-related horizontal head movements.

An essential role for PICK1 in NMDA receptor-dependent bidirectional synaptic plasticity

PICK1 is a calcium-sensing, PDZ domain-containing protein that interacts with GluR2 and GluR3 AMPA receptor (AMPAR) subunits and regulates their trafficking. Although PICK1 has been principally implicated in long-term depression (LTD), PICK1 overexpression in CA1 pyramidal neurons causes a CaMK- and PKC-dependent potentiation of AMPAR-mediated transmission and an increase in synaptic GluR2-lacking AMPARs, mechanisms associated with NMDA receptor (NMDAR)-dependent long-term potentiation (LTP). Here, we directly tested whether PICK1 participates in both hippocampal NMDAR-dependent LTP and LTD. We show that the PICK1 potentiation of AMPAR-mediated transmission is NMDAR dependent and fully occludes LTP. Conversely, blockade of PICK1 PDZ interactions or lack of PICK1 prevents LTP. These observations demonstrate an important role for PICK1 in LTP. In addition, deletion of PICK1 or blockade of PICK1 PDZ binding prevented NMDAR-dependent LTD. Thus, PICK1 plays a critical role in bidirectional NMDAR-dependent long-term synaptic plasticity in the hippocampus.

AKAP79 selectively enhances protein kinase C regulation of GluR1 at a Ca2+-calmodulin-dependent protein kinase II/protein kinase C site

Enhancement of AMPA receptor activity in response to synaptic plasticity inducing stimuli may arise, in part, through phosphorylation of the GluR1 AMPA receptor subunit at Ser-831. This site is a substrate for both Ca(2+)-calmodulin-dependent protein kinase II (CaMKII) and protein kinase C (PKC). However, neuronal protein levels of CaMKII may exceed those of PKC by an order of magnitude. Thus, it is unclear how PKC could effectively regulate this common target site. The multivalent neuronal scaffold A-kinase-anchoring protein 79 (AKAP79) is known to bind PKC and is linked to GluR1 by synapse-associated protein 97 (SAP97). Here, biochemical studies demonstrate that AKAP79 localizes PKC activity near the receptor, thus accelerating Ser-831 phosphorylation. Complementary electrophysiological studies indicate that AKAP79 selectively shifts the dose-dependence for PKC modulation of GluR1 receptor currents approximately 20-fold, such that low concentrations of PKC are as effective as much higher CaMKII concentrations. By boosting PKC activity near a target substrate, AKAP79 provides a mechanism to overcome limitations in kinase abundance thereby ensuring faithful signal propagation and efficient modification of AMPA receptor-mediated responses.

Rapid tumor necrosis factor alpha-induced exocytosis of glutamate receptor 2-lacking AMPA receptors to extrasynaptic plasma membrane potentiates excitotoxicity

The postinjury inflammatory response in the CNS leads to neuronal excitotoxicity. Our previous studies show that a major component of this response, the inflammatory cytokine tumor necrosis factor alpha (TNFalpha), causes a rapid increase in AMPA glutamate receptors (AMPARs) on the plasma membrane of cultured hippocampal neurons. This may potentiate neuron death through an increased vulnerability to AMPAR-dependent excitotoxic stress. Here, we test this hypothesis with an in vitro lactose dehydrogenase death assay and examine in detail the AMPAR surface delivery time course, receptor subtype, and synaptic and extrasynaptic distribution after TNFalpha exposure. These data demonstrate that surface levels of glutamate receptor 2 (GluR2)-lacking Ca2+-permeable AMPARs peak at 15 min after TNFalpha treatment, and the majority are directed to extrasynaptic sites. TNFalpha also induces an increase in GluR2-containing surface AMPARs but with a slower time course. We propose that this activity contributes to excitotoxic neuron death because TNFalpha potentiation of kainate excitotoxicity is blocked by a Ca2+-permeable AMPAR antagonist [NASPM (1-naphthyl acetyl spermine)] and a specific phosphoinositide 3 kinase (PI3 kinase) inhibitor (LY294,002 [2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one]) previously shown to block the TNFalpha-induced increase in AMPAR surface delivery. This information forms the basis for future in vivo studies examining AMPAR-dependent potentiation of excitotoxic neuron death and dysfunction caused by TNFalpha after acute injury and during neurodegenerative or neuropsychiatric disorders.

Endosomal trafficking of AMPA-type glutamate receptors

Many different forms of synaptic plasticity have been shown to ultimately modulate the number of AMPA-type glutamate receptors at the synapse. This trafficking involves lateral movements between synaptic and extrasynaptic sites at the neuron surface, as well as vesicular transport between the plasma membrane and intracellular compartments. Several new studies have shed light on the location and regulation of AMPA-type receptor (AMPAR) endocytosis, their intracellular sorting to divergent pathways at the level of endosomes, and the mechanism and sites of receptor recycling. This review summarizes this recent data on the trafficking along the endocytic pathway, and follows the path of internalized AMPAR from endocytosis up to sites of recycling.

Intracellular machinery for the transport of AMPA receptors

AMPA-type glutamate receptors are one of the most dynamic components of excitatory synapses. Their regulated addition and removal from synapses leads to long-lasting forms of synaptic plasticity, known as long-term potentiation (LTP) and long-term depression (LTD). In addition, AMPA receptors reach their synaptic targets after a complicated journey involving multiple transport steps through different membrane compartments. This review summarizes our current knowledge of the trafficking pathways of AMPARs and their relation to synaptic function and plasticity.

PICK1: a multi-talented modulator of AMPA receptor trafficking

AMPA (alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid) receptor trafficking is a fundamental mechanism for regulating synaptic strength, and hence may underlie cellular processes involved in learning and memory. PICK1 (protein interacting with C-kinase) is a PDZ and BAR domain-containing protein that has recently emerged as a key regulator of AMPA receptor traffic. Via the PDZ domain, PICK1 interacts directly with AMPA receptor subunits and is involved in the regulated removal of AMPA receptors from the synaptic plasma membrane. PICK1 has the ability to functionally interact with a number of cellular processes, including calcium signaling, actin polymerisation and phospholipid membrane architecture. In this review, I summarize recent findings that describe the importance of PICK1 in neurons and its specific molecular characteristics that enable it to regulate AMPA receptor trafficking.

GluR1 links structural and functional plasticity at excitatory synapses

Long-term potentiation (LTP), a cellular model of learning and memory, produces both an enhancement of synaptic function and an increase in the size of the associated dendritic spine. Synaptic insertion of AMPA receptors is known to play an important role in mediating the increase in synaptic strength during LTP, whereas the role of AMPA receptor trafficking in structural changes remains unexplored. Here, we examine how the cell maintains the correlation between spine size and synapse strength during LTP. We found that cells exploit an elegant solution by linking both processes to a single molecule: the AMPA-type glutamate receptor subunit 1 (GluR1). Synaptic insertion of GluR1 is required to permit a stable increase in spine size, both in hippocampal slice cultures and in vivo. Synaptic insertion of GluR1 is not sufficient to drive structural plasticity. Although crucial to the expression of LTP, the ion channel function of GluR1 is not required for the LTP-driven spine size enhancement. Remarkably, a recombinant cytosolic C-terminal fragment (C-tail) of GluR1 is driven to the postsynaptic density after an LTP stimulus, and the synaptic incorporation of this isolated GluR1 C-tail is sufficient to permit spine enlargement even when postsynaptic exocytosis of endogenous GluR1 is blocked. We conclude that during plasticity, synaptic insertion of GluR1 has two functions: the established role of increasing synaptic strength via its ligand-gated ion channel, and a novel role through the structurally stabilizing effect of its C terminus that permits an increase in spine size.

Differential trafficking of AMPA and NMDA receptors during long-term potentiation in awake adult animals

Despite a wealth of evidence in vitro that AMPA receptors are inserted into the postsynaptic membrane during long-term potentiation (LTP), it remains unclear whether this occurs in vivo at physiological concentrations of receptors. To address the issue of whether native AMPA or NMDA receptors undergo such trafficking during LTP in the adult brain, we examined the synaptic and surface expression of glutamate receptor subunits during the early induction phase of LTP in the dentate gyrus of awake adult rats. Induction of LTP was accompanied by a rapid NMDA receptor-dependent increase in surface expression of glutamate receptor 1-3 (GluR1-3) subunits. However, in the postsynaptic density fraction only GluR1 accumulated. GluR2/3-containing AMPA receptors, in contrast, were targeted exclusively to extrasynaptic sites in a protein synthesis-dependent manner. NMDA receptor subunits exhibited a delayed accumulation, both at the membrane surface and in postsynaptic densities, that was dependent on protein synthesis. These data suggest that trafficking of native GluR1-containing AMPA receptors to synapses is important for early-phase LTP in awake adult animals, and that this increase is followed homeostatically by a protein synthesis-dependent trafficking of NMDA receptors.

Communication between the synapse and the nucleus in neuronal development, plasticity, and disease

Sensory experience is critical for the proper development and plasticity of the brain throughout life. Successful adaptation to the environment is necessary for the survival of an organism, and this process requires the translation of specific sensory stimuli into changes in the structure and function of relevant neural circuits. Sensory-evoked activity drives synaptic input onto neurons within these behavioral circuits, initiating membrane depolarization and calcium influx into the cytoplasm. Calcium signaling triggers the molecular mechanisms underlying neuronal adaptation, including the activity-dependent transcriptional programs that drive the synthesis of the effector molecules required for long-term changes in neuronal function. Insight into the signaling pathways between the synapse and the nucleus that translate specific stimuli into altered patterns of connectivity within a circuit provides clues as to how activity-dependent programs of gene expression are coordinated and how disruptions in this process may contribute to disorders of cognitive function.

The cell biology of synaptic plasticity: AMPA receptor trafficking

The cellular processes that govern neuronal function are highly complex, with many basic cell biological pathways uniquely adapted to perform the elaborate information processing achieved by the brain. This is particularly evident in the trafficking and regulation of membrane proteins to and from synapses, which can be a long distance away from the cell body and number in the thousands. The regulation of neurotransmitter receptors, such as the AMPA-type glutamate receptors (AMPARs), the major excitatory neurotransmitter receptors in the brain, is a crucial mechanism for the modulation of synaptic transmission. The levels of AMPARs at synapses are very dynamic, and it is these plastic changes in synaptic function that are thought to underlie information storage in the brain. Thus, understanding the cellular machinery that controls AMPAR trafficking will be critical for understanding the cellular basis of behavior as well as many neurological diseases. Here we describe the life cycle of AMPARs, from their biogenesis, through their journey to the synapse, and ultimately through their demise, and discuss how the modulation of this process is essential for brain function.

Activity-dependent synaptic plasticity in retinal ganglion cells

At many excitatory synapses, AMPA-type receptors (AMPARs) are not statically situated in the membrane, but undergo continuous rounds of endocytosis and exocytosis, referred to as rapid cycling. AMPAR cycling is believed to play a role in certain forms of synaptic plasticity, but the link between cycling and synaptic function is not well understood. We have previously demonstrated that AMPARs cycle in neurons of the inner retina, including amacrine and ganglion cells, and that cycling is inhibited by synaptic activity. Recording from cultured neurons and ON ganglion cells in the flat-mount retina, we now show that rapid cycling is primarily, perhaps exclusively, restricted to AMPARs that contain the GluR2 subunit, and that cycle is confined to extrasynaptic receptors. We also demonstrate a form of plasticity at the ON bipolar cell-ON ganglion cell synapse, whereby synaptic quiescence drives a change in the composition of AMPARs from predominantly GluR2-containing to GluR2-lacking. Finally, we provide evidence linking synaptic receptor composition and cycling, showing that disruption of cycling leads increases the number of GluR2-containing receptors in the ON bipolar-ON ganglion cell synapse. We propose that cycling lowers the number of GluR2-containing receptors at the surface and, consequently, within the synapse. After increased levels of synaptic activity, cycling ceases, and all GluR2-containing receptors are free to go to the surface, where they can be delivered to synapses. Our results suggest that by regulating the cycling of AMPARs, ambient light can modulate the composition of synaptic receptors in ON ganglion cells.

Switch to Ca2+-permeable AMPA and reduced NR2B NMDA receptor-mediated neurotransmission at dorsal horn nociceptive synapses during inflammatory pain in the rat

Glutamate receptor response properties of nociceptive synapses on neurokinin 1 receptor positive (NK1R+) lamina I neurons were determined 3 days after induction of chronic peripheral inflammation with Freund's Complete Adjuvant (CFA). A significant increase in the AMPAR/NMDAR ratio was found during inflammation, which was associated with a significant reduction in the quantal amplitude of NMDAR-mediated synaptic currents. A significant shortening of the quantal AMPA current decay, a greater inward rectification of the AMPAR-mediated eEPSC amplitude and an increased sensitivity to the Ca2+-permeable AMPAR channel blocker 1-naphthylacetyl spermine (NAS) was also observed, indicating an increase in the contribution of Ca2+-permeable AMPARs at this synapse during inflammation. Furthermore the reduced effectiveness of the NR2B-specific antagonist CP-101,606 on NMDAR-mediated eEPSCs together with a decrease in Mg2+ sensitivity suggests a down regulation of the highly Mg2+-sensitive and high conductance NR2B subunit at this synapse. These changes in glutamatergic receptor function during inflammation support the selective effectiveness of Ca2+-permeable AMPAR antagonists in inflammatory pain models and may underlie the reported ineffectiveness of NR2B antagonists in spinal antinociception.

Synaptic plasticity and addiction

Addiction is caused, in part, by powerful and long-lasting memories of the drug experience. Relapse caused by exposure to cues associated with the drug experience is a major clinical problem that contributes to the persistence of addiction. Here we present the accumulated evidence that drugs of abuse can hijack synaptic plasticity mechanisms in key brain circuits, most importantly in the mesolimbic dopamine system, which is central to reward processing in the brain. Reversing or preventing these drug-induced synaptic modifications may prove beneficial in the treatment of one of society's most intractable health problems.

Developmental expression of Ca2+-permeable AMPA receptors underlies depolarization-induced long-term depression at mossy fiber CA3 pyramid synapses

Many central excitatory synapses undergo developmental alterations in the molecular and biophysical characteristics of postsynaptic ionotropic glutamate receptors via changes in subunit composition. Concerning AMPA receptors (AMPARs), glutamate receptor 2 subunit (GluR2)-containing, Ca2+-impermeable AMPARs (CI-AMPARs) prevail at synapses between mature principal neurons; however, accumulating evidence indicates that GluR2-lacking, Ca2+-permeable AMPARs (CP-AMPARs) contribute at these synapses early in development. Here, we used a combination of imaging and electrophysiological recording techniques to investigate potential roles for CP-AMPARs at developing hippocampal mossy fiber-CA3 pyramidal cell (MF-PYR) synapses. We found that transmission at nascent MF-PYR synapses is mediated by a mixed population of CP- and CI-AMPARs as evidenced by polyamine-dependent inwardly rectifying current-voltage (I-V) relationships, and partial philanthotoxin sensitivity of synaptic events. CP-AMPAR expression at MF-PYR synapses is transient, being limited to the first 3 postnatal weeks. Moreover, the expression of CP-AMPARs is regulated by the PDZ (postsynaptic density-95/Discs large/zona occludens-1) domain-containing protein interacting with C kinase 1 (PICK1), because MF-PYR synapses in young PICK1 knock-out mice are philanthotoxin insensitive with linear I-V relationships. Strikingly, MF-PYR transmission via CP-AMPARs is selectively depressed during depolarization-induced long-term depression (DiLTD), a postsynaptic form of MF-PYR plasticity observed only at young MF-PYR synapses. The selective depression of CP-AMPARs during DiLTD was evident as a loss of postsynaptic CP-AMPAR-mediated Ca2+ transients in PYR spines and reduced rectification of MF-PYR synaptic currents. Preferential targeting of CP-AMPARs during DiLTD is further supported by a lack of DiLTD in young PICK1 knock-out mice. Together, these findings indicate that the transient participation of CP-AMPARs at young MF-PYR synapses dictates the developmental window to observe DiLTD.

Age-dependent requirement of AKAP150-anchored PKA and GluR2-lacking AMPA receptors in LTP

Association of PKA with the AMPA receptor GluR1 subunit via the A kinase anchor protein AKAP150 is crucial for GluR1 phosphorylation. Mutating the AKAP150 gene to specifically prevent PKA binding reduced PKA within postsynaptic densities (>70%). It abolished hippocampal LTP in 7-12 but not 4-week-old mice. Inhibitors of PKA and of GluR2-lacking AMPA receptors blocked single tetanus LTP in hippocampal slices of 8 but not 4-week-old WT mice. Inhibitors of GluR2-lacking AMPA receptors also prevented LTP in 2 but not 3-week-old mice. Other studies demonstrate that GluR1 homomeric AMPA receptors are the main GluR2-lacking AMPA receptors in adult hippocampus and require PKA for their functional postsynaptic expression during potentiation. AKAP150-anchored PKA might thus critically contribute to LTP in adult hippocampus in part by phosphorylating GluR1 to foster postsynaptic accumulation of homomeric GluR1 AMPA receptors during initial LTP in 8-week-old mice.

Stargazin attenuates intracellular polyamine block of calcium-permeable AMPA receptors

Endogenous polyamines profoundly affect the activity of various ion channels, including that of calcium-permeable AMPA-type glutamate receptors (CP-AMPARs). Here we show that stargazin, a transmembrane AMPAR regulatory protein (TARP) known to influence transport, gating and desensitization of AMPARs, greatly reduces block of CP-AMPARs by intracellular polyamines. By decreasing CP-AMPAR affinity for cytoplasmic polyamines, stargazin enhances the charge transfer following single glutamate applications and eliminates the frequency-dependent facilitation seen with repeated applications. In cerebellar stellate cells, which express both synaptic CP-AMPARs and stargazin, we found that the rectification and unitary conductance of channels underlying excitatory postsynaptic currents were matched by those of recombinant AMPARs only when the latter were associated with stargazin. Taken together, our observations establish modulatory actions of stargazin that are specific to CP-AMPARs, and suggest that during synaptic transmission the activity of such receptors, and thus calcium influx, is fundamentally changed by TARPs.

Enhanced LTP of primary afferent neurotransmission in AMPA receptor GluR2-deficient mice

Ca(2+)-permeable-AMPA receptors (AMPARs) are expressed in the superficial dorsal horn (SDH, laminae I/II) of the spinal cord, the area involved in transmission and modulation of sensory information, including nociception. A possible role of Ca(2+)-permeable-AMPARs in synaptic strengthening has been suggested in postnatal DH cultures, but their role in the long-lasting activity-dependent synaptic plasticity of primary afferent neurotransmission in the adult mouse SDH has not been investigated. In the present study the role of Ca(2+)-permeable-AMPARs in the regulation of long-lasting synaptic plasticity, specifically long-term potentiation (LTP) and long-term depression (LTD) in the SDH, was investigated using mice deficient in AMPAR GluR2 subunit. We show here that the GluR2 mutants exhibited no changes in passive membrane properties, but a significant increase in rectification of excitatory postsynaptic currents, the finding suggesting increased expression of Ca(2+)-permeable-AMPARs. In the absence of GluR2, high-frequency stimulation (HFS) of small-diameter primary afferent fibers induced LTP that is enhanced and non-saturating in the SDH at both primary afferent Adelta- and/or C-fibers monosynaptic and polysynaptic pathways, whereas neuronal excitability and paired-pulse depression were normal. The LTP could be induced in the presence of the NMDA receptor antagonist d-AP5, and L-type Ca(2+) channel blockers, suggesting that Ca(2+)-permeable-AMPARs are sufficient to induce LTP in the SDH neurons of adult mouse spinal cord. In contrast, the induction of HFS-LTD is reduced in the SDH of GluR2 mutants. These results suggest an important role for AMPAR GluR2 subunit in regulating synaptic plasticity with potential relevance for long-lasting hypersensitivity in pathological states.

Long-term potentiation in the hippocampal CA1 region does not require insertion and activation of GluR2-lacking AMPA receptors

Activity-dependent insertion of AMPA-type glutamate receptors is thought to underlie long-term potentiation (LTP) at Schaffer collateral fiber synapses on pyramidal cells in the hippocampal CA1 region. Although it is widely accepted that the AMPA receptors at these synapses contain glutamate receptor type 2 (GluR2) subunits, recent findings suggest that LTP in hippocampal slices obtained from 2- to 3-wk-old rodents is dependent on the transient postsynaptic insertion and activation of Ca(2+)-permeable, GluR2-lacking AMPA receptors. Here we examined whether LTP in slices prepared from adult animals exhibits similar properties. In contrast to previously reported findings, pausing synaptic stimulation for as long as 30 min post LTP induction had no effect on LTP maintenance in slices from 2- to 3-mo-old mice. LTP was also not disrupted by postinduction application of a selective blocker of GluR2-lacking AMPA receptors or the broad-spectrum glutamate receptor antagonist kynurenate. Although these results suggest that the role of GluR2-lacking AMPA receptors in LTP might be regulated during postnatal development, LTP in slices obtained from 15- to 21-day-old mice also did not require postinduction synaptic stimulation or activation of GluR2-lacking AMPA receptors. Thus the insertion and activation of GluR2-lacking AMPA receptors do not appear to be fundamental processes involved in LTP at excitatory synapses in the hippocampal CA1 region.

Molecular determinants of AMPA receptor subunit assembly

AMPA-type (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate) glutamate receptors (AMPARs) mediate post-synaptic depolarization and fast excitatory transmission in the central nervous system. AMPARs are tetrameric ion channels that assemble in the endoplasmic reticulum (ER) in a poorly understood process. The subunit composition determines channel conductance properties and gating kinetics, and also regulates vesicular traffic to and from synaptic sites, and is thus critical for synaptic function and plasticity. The distribution of functionally different AMPARs varies within and between neuronal circuits, and even within individual neurons. In addition, synapses employ channels with specific subunit stoichiometries, depending on the type of input and the frequency of stimulation. Taken together, it appears that assembly is not simply a stochastic process. Recently, progress has been made in understanding the molecular mechanisms underlying subunit assembly and receptor biogenesis in the ER. These processes ultimately determine the size and shape of the postsynaptic response, and are the subject of this review.

Synaptic plasticity, AMPA-R trafficking, and Ras-MAPK signaling

Synaptic modification of transmission is a general phenomenon expressed at almost every excitatory synapse in the mammalian brain. Over the last three decades, much has been discovered about the cellular, synaptic, molecular, and signaling mechanisms responsible for controlling synaptic transmission and plasticity. Here, we present a brief review of these mechanisms with emphasis on the current understanding of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor (AMPA-R) trafficking and Ras-mitogen-activated protein kinase (MAPK) signaling events involved in controlling synaptic transmission.

Diffusional trapping of GluR1 AMPA receptors by input-specific synaptic activity

Synaptic activity regulates the postsynaptic accumulation of AMPA receptors over timescales ranging from minutes to days. Indeed, the regulated trafficking and mobility of GluR1 AMPA receptors underlies many forms of synaptic potentiation at glutamatergic synapses throughout the brain. However, the basis for synapse-specific accumulation of GluR1 is unknown. Here we report that synaptic activity locally immobilizes GluR1 AMPA receptors at individual synapses. Using single-molecule tracking together with the silencing of individual presynaptic boutons, we demonstrate that local synaptic activity reduces diffusional exchange of GluR1 between synaptic and extraynaptic domains, resulting in postsynaptic accumulation of GluR1. At neighboring inactive synapses, GluR1 is highly mobile with individual receptors frequently escaping the synapse. Within the synapse, spontaneous activity confines the diffusional movement of GluR1 to restricted subregions of the postsynaptic membrane. Thus, local activity restricts GluR1 mobility on a submicron scale, defining an input-specific mechanism for regulating AMPA receptor composition and abundance.

Time-dependent postsynaptic AMPA GluR1 receptor recruitment in the cingulate synaptic potentiation

The anterior cingulate cortex (ACC) is critical for brain functions including learning, memory, fear and pain. Long-term synaptic potentiation (LTP), a cellular model for learning and memory, has been reported in the ACC neurons. Unlike LTP in the hippocampus and amygdala, two key structures for memory and fear, little is known about the synaptic mechanism for the expression of LTP in the ACC. Here we use whole-cell patch clamp recordings to demonstrate that cingulate LTP requires the functional recruitment of GluR1 AMPA receptors; and such events are rapid and completed within 5-10 min after LTP induction. Our results demonstrate that the GluR1 subunit is essential for synaptic plasticity in the ACC and may play critical roles under physiological and pathological conditions.

The Role of the GluR2 Subunit in AMPA Receptor Function and Synaptic Plasticity

The AMPA receptor (AMPAR) GluR2 subunit dictates the critical biophysical properties of the receptor, strongly influences receptor assembly and trafficking, and plays pivotal roles in a number of forms of long-term synaptic plasticity. Most neuronal AMPARs contain this critical subunit; however, in certain restricted neuronal populations and under certain physiological or pathological conditions, AMPARs that lack this subunit are expressed. There is a current surge of interest in such GluR2-lacking Ca2+-permeable AMPARs in how they affect the regulation of synaptic transmission. Here, we bring together recent data highlighting the novel and important roles of GluR2 in synaptic function and plasticity.

Conservation of glutamate receptor 2-containing AMPA receptors during long-term potentiation

Long-term potentiation (LTP) at hippocampal synapses is thought to involve the insertion of AMPA receptors into the postsynaptic membrane. Conflicting evidence exists as to whether calcium-permeable receptors are inserted during LTP and whether synaptic activity mediated by the newly inserted AMPA receptors is required to maintain the increase in synaptic strength. Here, we rigorously test these hypotheses and conclude that calcium-permeable AMPA receptors are not inserted during LTP nor does potentiation require ongoing activity to be maintained.

Novel toxicity of the unedited GluR2 AMPA receptor subunit dependent on surface trafficking and increased Ca2+-permeability

RNA editing modifies the GluR2 AMPA receptor subunit pore loop at the Q/R site and limits receptor Ca(2+) permeability. Editing failure is implicated in neurodegenerative diseases, including amyotrophic lateral sclerosis. We show that channels with unedited GluR2 are highly toxic in cultured hippocampal neurons. Toxicity exceeds that of other Ca(2+)-permeable AMPA receptor types and is influenced by agonist binding site mutations, ability to desensitize, and extracellular Ca(2+) levels. Significantly, toxicity also depends on GluR2's constitutive surface trafficking, a function dependent on GluR2 C-terminal domain interaction with NSF, a specialized chaperone. We have exploited the interaction between unedited GluR2 and NSF to reduce GluR2 surface levels. We show that a peptide that blocks the GluR2-NSF interaction reduces unedited GluR2 toxicity by reducing receptor surface expression. Peptide block of trafficking provides a model for design of drugs to reduce unedited GluR2 excitotoxicity in neurodegenerative diseases that result from editing failure.

AMPA receptor biogenesis and trafficking

AMPA-type glutamate receptors mediate the majority of fast excitatory transmission in the central nervous system. The trafficking of AMPA receptors to and from synapses alters synaptic strength and has been recognized as a central mechanism underlying various forms of synaptic plasticity. Both secretory and endocytic trafficking events seem to be driven by the subunit composition of AMPA receptor tetramers. Moreover, recent work suggests that synapses employ different tetramer combinations in response to altered synaptic input, suggesting the existence of signalling pathways that mediate remodelling of AMPA receptors. These latest developments and recent progress in elucidating the mechanisms that underlie channel assembly and trafficking are the subject of this review.

AMPAR exocytosis through NO modulation of PICK1

The activation of NMDA receptors (NMDARs) triggers long-term changes in AMPA receptor-mediated synaptic transmission in the CNS. These long-lasting changes occur via the addition or removal of AMPA receptors (AMPARs) at the synaptic membrane and are mediated by a number of regulatory proteins including the GluR2 AMPAR-interacting proteins n-ethylmaleimide sensitive factor (NSF) and Protein Interacting with C Kinase (PICK1). We have shown that the potent activation of NMDARs drives unclustering of PICK1 and PICK1-GluR2 dissociation in dendrites resulting in increased surface delivery of AMPARs. Here we show that the dispersal of PICK1 is mediated by the actions of NSF. We find that elevated NMDAR signaling leads to the S-nitrosylation of NSF and increased NSF-GluR2 association. Both NMDAR-dependent unclustering of PICK1 and the delivery of surface AMPARs are dependent on release of nitric oxide (NO). Our data suggest that NMDAR activation can drive the surface delivery of AMPARs from a pool of intracellular AMPARs retained by PICK1 through the NO-dependent modification of NSF.