Control of GluR1 AMPA receptor function by cAMP-dependent protein kinase

Ionotropic glutamate receptors: regulation by G-protein-coupled receptors

The function of many ion channels is under dynamic control by coincident activation of G-protein-coupled receptors (GPCRs), particularly those coupled to the Gαs and Gαq family members. Such regulation is typically dependent on the subunit composition of the ionotropic receptor or channel as well as the GPCR subtype and the cell-specific panoply of signaling pathways available. Because GPCRs and ion channels are so highly represented among targets of U.S. Food and Drug Administration-approved drugs, functional cross-talk between these drug target classes is likely to underlie many therapeutic and adverse effects of marketed drugs. GPCRs engage a myriad of signaling pathways that involve protein kinases A and C (PKC) and, through PKC and interaction with β-arrestin, Src kinase, and hence the mitogen-activated-protein-kinase cascades. We focus here on the control of ionotropic glutamate receptor function by GPCR signaling because this form of regulation can influence the strength of synaptic plasticity. The amino acid residues phosphorylated by specific kinases have been securely identified in many ionotropic glutamate (iGlu) receptor subunits, but which of these sites are GPCR targets is less well known even when the kinase has been identified. N-methyl-d-aspartate, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, and heteromeric kainate receptors are all downstream targets of GPCR signaling pathways. The details of GPCR-iGlu receptor cross-talk should inform a better understanding of how synaptic transmission is regulated and lead to new therapeutic strategies for neuropsychiatric disorders.

AMPA receptor trafficking in inflammation-induced dorsal horn central sensitization

Activity-dependent postsynaptic receptor trafficking is critical for long-term synaptic plasticity in the brain, but it is unclear whether this mechanism actually mediates the spinal cord dorsal horn central sensitization (a specific form of synaptic plasticity) that is associated with persistent pain. Recent studies have shown that peripheral inflammation drives changes in α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) subunit trafficking in the dorsal horn and that such changes contribute to the hypersensitivity that underlies persistent pain. Here, we review current evidence to illustrate how spinal cord AMPARs participate in the dorsal horn central sensitization associated with persistent pain. Understanding these mechanisms may allow the development of novel therapeutic strategies for treating persistent pain.

Biophysical mechanisms regulating AMPA receptor accumulation at synapses

Controlling the number of AMPA receptors at synapses is fundamental for fast synaptic transmission as well as for long term adaptations in synaptic strength. In this review, we examine the biophysical mechanisms implicated in regulating AMPAR levels at the cell surface and at synapses. We first describe the structure and function of AMPARs, as well as their interactions with various proteins regulating their traffic and function. Second we review the vesicular trafficking mechanism involving exocytosis and endocytosis, by which AMPARs reach the cell surface and are internalized, respectively. Third, we examine the properties of lateral diffusion of AMPARs and their trapping at post-synaptic densities. Finally, we discuss how these two parallel mechanisms are integrated in time and space to control changes in synaptic AMPAR levels in response to plasticity protocols. This review highlights the important role of the extra-synaptic AMPAR pool, which makes an obligatory link between vesicular trafficking and trapping or release at synapses.

Alterations in expression and phosphorylation of GluA1 receptors following cocaine-cue extinction learning

Brain regional analyses of total GluA1 and GluA1-pSer(845) were used to delineate plasticity of the AMPA receptor in conjunction with cocaine-cue extinction learning. Rats were trained to self-administer cocaine paired with a 2-s light cue and later underwent a single 2 h extinction session for which cocaine was withheld but response-contingent cues were presented. Control groups received yoked-saline sessions or received cocaine self-administration training without undergoing extinction training. Extinction-related increases and decreases, respectively, in total GluA1 were observed in the ventromedial prefrontal cortex (vmPFC) and basolateral amygdala (BLA). Phosphorylation of GluA1 at Ser(845) was increased in the vmPFC and nucleus accumbens (NAc). Though total GluA1 did not change in NAc, there was a positive association between the number of responses during extinction training and the magnitude of total GluA1 in NAc. No significant changes were evident in the dorsal hippocampus. We conclude that the BLA and vmPFC, in particular, appear to be loci for the inhibition of learned behavior induced via extinction training, but each site may have different signaling functions for cocaine-cue extinction learning.

Equipotent inhibition of fatty acid amide hydrolase and monoacylglycerol lipase - dual targets of the endocannabinoid system to protect against seizure pathology

Advances in the understanding of the endogenous cannabinoid system have led to several therapeutic indications for new classes of compounds that enhance cannabinergic responses. Endocannabinoid levels are elevated during pathogenic conditions, and inhibitors of endocannabinoid inactivation promote such on-demand responses. The endocannabinoids anandamide and 2-arachidonoyl glycerol have been implicated in protective signaling against excitotoxic episodes, including seizures. To better understand modulatory pathways that can exploit such responses, we used the new generation compound AM6701 that blocks both the anandamide-deactivating enzyme fatty acid amide hydrolase (FAAH) and the 2-arachidonoyl glycerol-deactivating enzyme monoacylglycerol lipase (MAGL) with equal potency. Also studied was the structural isomer AM6702 which is 44-fold more potent for inhibiting FAAH versus MAGL. When applied before and during kainic acid (KA) exposure to cultured hippocampal slices, AM6701 protected against the resulting excitotoxic events of calpain-mediated cytoskeletal damage, loss of presynaptic and postsynaptic proteins, and pyknotic changes in neurons. The equipotent inhibitor was more effective than its close relative AM6702 at protecting against the neurodegenerative cascade assessed in the slice model. In vivo, AM6701 was also the more effective compound for reducing the severity of KA-induced seizures and protecting against behavioral deficits linked to seizure damage. Corresponding with the behavioral improvements, cytoskeletal and synaptic protection was elicited by AM6701, as found in the KA-treated hippocampal slice model. It is proposed that the influence of AM6701 on FAAH and MAGL exerts a synergistic action on the endocannabinoid system, thereby promoting the protective nature of cannabinergic signaling to offset excitotoxic brain injury.

TARP-associated AMPA receptors display an increased maximum channel conductance and multiple kinetically distinct open states

Fast excitatory synaptic transmission in the CNS is mediated mainly by AMPA-type glutamate receptors (AMPARs), whose biophysical properties are dramatically modulated by the presence of transmembrane AMPAR regulatory proteins (TARPs). To help construct a kinetic model that will realistically describe native AMPAR/TARP function, we have examined the single-channel properties of homomeric GluA1 AMPARs in combination with the TARPs, γ-2, γ-4 and γ-5. In a saturating concentration of agonist, each of these AMPAR/TARP combinations gave rise to single-channel currents with multiple conductance levels that appeared intrinsic to the receptor-channel complex, and showed long-lived subconductance states. The open time and burst length distributions of the receptor complexes displayed multiple dwell-time components. In the case of γ-2- and γ-4-associated receptors, these distributions included a long-lived component lasting tens of milliseconds that was absent from both GluA1 alone and γ-5-associated receptors. The open time distributions for each conductance level required two dwell-time components, indicating that at each conductance level the channel occupies a minimum of two kinetically distinct open states. We have explored how these data place novel constraints on possible kinetic models of TARP-associated AMPARs that may be used to define AMPAR-mediated synaptic transmission.

Neural functions of calcineurin in synaptic plasticity and memory

Major brain functions depend on neuronal processes that favor the plasticity of neuronal circuits while at the same time maintaining their stability. The mechanisms that regulate brain plasticity are complex and engage multiple cascades of molecular components that modulate synaptic efficacy. Protein kinases (PKs) and phosphatases (PPs) are among the most important of these components that act as positive and negative regulators of neuronal signaling and plasticity, respectively. In these cascades, the PP protein phosphatase 2B or calcineurin (CaN) is of particular interest because it is the only Ca(2+)-activated PP in the brain and a major regulator of key proteins essential for synaptic transmission and neuronal excitability. This review describes the primary properties of CaN and illustrates its functions and modes of action by focusing on several representative targets, in particular glutamate receptors, striatal enriched protein phosphatase (STEP), and neuromodulin (GAP43), and their functional significance for synaptic plasticity and memory.

GluA1 phosphorylation alters evoked firing pattern in vivo

AMPA and NMDA receptors convey fast synaptic transmission in the CNS. Their relative contribution to synaptic output and phosphorylation state regulate synaptic plasticity. The AMPA receptor subunit GluA1 is central in synaptic plasticity. Phosphorylation of GluA1 regulates channel properties and trafficking. The firing rate averaged over several hundred ms is used to monitor cellular input. However, plasticity requires the timing of spiking within a few ms; therefore, it is important to understand how phosphorylation governs these events. Here, we investigate whether the GluA1 phosphorylation (p-GluA1) alters the spiking patterns of CA1 cells in vivo. The antidepressant Tianeptine was used for inducing p-GluA1, which resulted in enhanced AMPA-evoked spiking. By comparing the spiking patterns of AMPA-evoked activity with matched firing rates, we show that the spike-trains after Tianeptine application show characteristic features, distinguishing from spike-trains triggered by strong AMPA stimulation. The interspike-interval distributions are different between the two groups, suggesting that neuronal output may differ when new inputs are activated compared to increasing the gain of previously activated receptors. Furthermore, we also show that NMDA evokes spiking with different patterns to AMPA spike-trains. These results support the role of the modulation of NMDAR/AMPAR ratio and p-GluA1 in plasticity and temporal coding.

mGlu5R promotes glutamate AMPA receptor phosphorylation via activation of PKA/DARPP-32 signaling in striatopallidal medium spiny neurons

Group I metabotropic glutamate receptors (mGluRs), which comprise mGlu1Rs and mGlu5Rs, are enriched in striatal medium spiny neurons (MSNs), where they modulate glutamatergic transmission. Here, we have examined the effect of group I mGluRs on the regulation of the state of phosphorylation of the GluA1 subunit of the AMPA glutamate receptor. We found that incubation of mouse striatal slices with the group I mGluR agonist (R,S)-3,5-dihydroxyphenylglycine (DHPG) promotes GluA1 phosphorylation at the cAMP-dependent protein kinase (PKA) site, Ser845. This effect is prevented by 2-methyl-6-(phenylethynyl)pyridine hydrochloride (MPEP), a selective mGlu5R antagonist. The increase in GluA1 phosphorylation produced by DHPG is also prevented by blockade of adenosine A2A receptors (A2ARs), which are known to promote cAMP signaling specifically in striatopallidal MSNs, as well as by enzymatic degradation of endogenous adenosine, achieved with adenosine deaminase. The ability of DHPG to increase PKA-dependent phosphorylation of GluA1 depends on concomitant activation of the dopamine- and cAMP-regulated phosphoprotein of 32kDa (DARPP-32). Thus, inactivation of the PKA phosphorylation site of DARPP-32 abolishes the effect of DHPG. Moreover, cell-specific knock out of DARPP-32 in striatopallidal, but not in striatonigral, MSNs prevents the increase in Ser845 phosphorylation induced by DHPG. These results indicate that activation of mGlu5Rs promotes PKA/DARPP-32-dependent phosphorylation of downstream target proteins in striatopallidal MSNs and that this effect is exerted via potentiation of tonic A2AR transmission. This article is part of a Special Issue entitled 'Metabotropic Glutamate Receptors'.

Developmental nicotine exposure induced alterations in behavior and glutamate receptor function in hippocampus

In the developing brain, nicotinic acetylcholine receptors (nAChRs) are involved in cell survival, targeting, formation of neural and sensory circuits, and development and maturation of other neurotransmitter systems. This regulatory role is disrupted when the developing brain is exposed to nicotine, which occurs with tobacco use during pregnancy. Prenatal nicotine exposure has been shown to be a strong risk factor for memory deficits and other behavioral aberrations in the offspring. The molecular mechanisms underlying these neurobehavioral outcomes are not clearly elucidated. We used a rodent model to assess behavioral, neurophysiological, and neurochemical consequences of prenatal nicotine exposure in rat offspring with specific emphasis on the hippocampal glutamatergic system. Pregnant dams were infused with nicotine (6 mg/kg/day) subcutaneously from the third day of pregnancy until birth. Results indicate that prenatal nicotine exposure leads to increased anxiety and depressive-like effects and impaired spatial memory. Synaptic plasticity in the form of long-term potentiation (LTP), basal synaptic transmission, and AMPA receptor-mediated synaptic currents were reduced. The deficit in synaptic plasticity was paralleled by declines in protein levels of vesicular glutamate transporter 1 (VGLUT1), synaptophysin, AMPA receptor subunit GluR1, phospho(Ser845) GluR1, and postsynaptic density 95 (PSD-95). These results suggest that prenatal nicotine exposure by maternal smoking could result in alterations in the glutamatergic system in the hippocampus contributing to the abnormal neurobehavioral outcomes.

PKA increases in the olfactory bulb act as unconditioned stimuli and provide evidence for parallel memory systems: pairing odor with increased PKA creates intermediate- and long-term, but not short-term, memories

Neonatal odor-preference memory in rat pups is a well-defined associative mammalian memory model dependent on cAMP. Previous work from this laboratory demonstrates three phases of neonatal odor-preference memory: short-term (translation-independent), intermediate-term (translation-dependent), and long-term (transcription- and translation-dependent). Here, we use neonatal odor-preference learning to explore the role of olfactory bulb PKA in these three phases of mammalian memory. PKA activity increased normally in learning animals 10 min after a single training trial. Inhibition of PKA by Rp-cAMPs blocked intermediate-term and long-term memory, with no effect on short-term memory. PKA inhibition also prevented learning-associated CREB phosphorylation, a transcription factor implicated in long-term memory. When long-term memory was rescued through increased β-adrenoceptor activation, CREB phosphorylation was restored. Intermediate-term and long-term, but not short-term odor-preference memories were generated by pairing odor with direct PKA activation using intrabulbar Sp-cAMPs, which bypasses β-adrenoceptor activation. Higher levels of Sp-cAMPs enhanced memory by extending normal 24-h retention to 48-72 h. These results suggest that increased bulbar PKA is necessary and sufficient for the induction of intermediate-term and long-term odor-preference memory, and suggest that PKA activation levels also modulate memory duration. However, short-term memory appears to use molecular mechanisms other than the PKA/CREB pathway. These mechanisms, which are also recruited by β-adrenoceptor activation, must operate in parallel with PKA activation.

Selective cholinergic depletion in medial septum leads to impaired long term potentiation and glutamatergic synaptic currents in the hippocampus

Cholinergic depletion in the medial septum (MS) is associated with impaired hippocampal-dependent learning and memory. Here we investigated whether long term potentiation (LTP) and synaptic currents, mediated by alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA) and N-methyl-D-aspartate (NMDA) receptors in the CA1 hippocampal region, are affected following cholinergic lesions of the MS. Stereotaxic intra-medioseptal infusions of a selective immunotoxin, 192-saporin, against cholinergic neurons or sterile saline were made in adult rats. Four days after infusions, hippocampal slices were made and LTP, whole cell, and single channel (AMPA or NMDA receptor) currents were recorded. Results demonstrated impairment in the induction and expression of LTP in lesioned rats. Lesioned rats also showed decreases in synaptic currents from CA1 pyramidal cells and synaptosomal single channels of AMPA and NMDA receptors. Our results suggest that MS cholinergic afferents modulate LTP and glutamatergic currents in the CA1 region of the hippocampus, providing a potential synaptic mechanism for the learning and memory deficits observed in the rodent model of selective MS cholinergic lesioning.

Mechanisms of CaMKII action in long-term potentiation

Long-term potentiation (LTP) of synaptic strength occurs during learning and can last for long periods, making it a probable mechanism for memory storage. LTP induction results in calcium entry, which activates calcium/calmodulin-dependent protein kinase II (CaMKII). CaMKII subsequently translocates to the synapse, where it binds to NMDA-type glutamate receptors and produces potentiation by phosphorylating principal and auxiliary subunits of AMPA-type glutamate receptors. These processes are all localized to stimulated spines and account for the synapse-specificity of LTP. In the later stages of LTP, CaMKII has a structural role in enlarging and strengthening the synapse.

PKC phosphorylates GluA1-Ser831 to enhance AMPA receptor conductance

AMPA receptors mediate fast excitatory synaptic transmission in the brain, and are dynamically regulated by phosphorylation of multiple residues within the C-terminal domain. CaMKII phosphorylates Ser831 within the AMPA receptor GluA1 subunit to increase single channel conductance, and biochemical studies show that PKC can also phosphorylate this residue. In light of the discovery of additional PKC phosphorylation sites within the GluA1 C-terminus, it remains unclear whether PKC phosphorylation of Ser831 increases GluA1 conductance in intact receptors. Here, we report that the purified, catalytic subunit of PKC significantly increases the conductance of wild-type GluA1 AMPA receptors expressed in the presence of stargazin in HEK293T cells. Furthermore, the mutation GluA1-S831A blocks the functional effect of PKC. These findings suggest that GluA1 AMPA receptor conductance can be increased by activated CaMKII or PKC, and that phosphorylation at this site provides a mechanism for channel modulation via a variety of protein signaling cascades.

Redefining the classification of AMPA-selective ionotropic glutamate receptors

AMPA-type ionotropic glutamate receptors (iGluRs) represent the major excitatory neurotransmitter receptor in the developing and adult vertebrate CNS. They are crucial for the normal hardwiring of glutamatergic circuits but also fine tune synaptic strength by cycling into and out of synapses during periods of sustained patterned activity or altered homeostasis. AMPARs are grouped into two functionally distinct tetrameric assemblies based on the inclusion or exclusion of the GluA2 receptor subunit. GluA2-containing receptors are thought to be the most abundant AMPAR in the CNS, typified by their small unitary events, Ca(2+) impermeability and insensitivity to polyamine block. In contrast, GluA2-lacking AMPARs exhibit large unitary conductance, marked divalent permeability and nano- to micromolar polyamine affinity. Here, I review evidence for the existence of a third class of AMPAR which, though similarly Ca(2+) permeable, is characterized by its near-insensitivity to internal and external channel block by polyamines. This novel class of AMPAR is most notably found at multivesicular release synapses found in the avian auditory brainstem and mammalian retina. Curiously, these synapses lack NMDA-type iGluRs, which are conventionally associated with controlling AMPAR insertion. The lack of NMDARs suggests that a different set of rules may govern AMPAR cycling at these synapses. AMPARs with similar functional profiles are also found on some glial cells suggesting they may have a more widespread distribution in the mammalian CNS. I conclude by noting that modest changes to the ion-permeation pathway might be sufficient to retain divalent permeability whilst eliminating polyamine sensitivity. Consequently, this emerging AMPAR subclass need not be assembled from novel subunits, yet to be cloned, but could simply occur by varying the stoichiometry of existing proteins.

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.

Amyloid beta peptide-(1-42) induces internalization and degradation of beta2 adrenergic receptors in prefrontal cortical neurons

Emerging evidence indicates that amyloid β peptide (Aβ) initially induces subtle alterations in synaptic function in Alzheimer disease. We have recently shown that Aβ binds to β(2) adrenergic receptor (β(2)AR) and activates protein kinase A (PKA) signaling for glutamatergic regulation of synaptic activities. Here we show that in the cerebrums of mice expressing human familial mutant presenilin 1 and amyloid precursor protein genes, the levels of β(2)AR are drastically reduced. Moreover, Aβ induces internalization of transfected human β(2)AR in fibroblasts and endogenous β(2)AR in primary prefrontal cortical neurons. In fibroblasts, Aβ treatment also induces transportation of β(2)AR into lysosome, and prolonged Aβ treatment causes β(2)AR degradation. The Aβ-induced β(2)AR internalization requires the N terminus of the receptor containing the peptide binding sites and phosphorylation of β(2)AR by G protein-coupled receptor kinase, not by PKA. However, the G protein-coupled receptor kinase phosphorylation of β(2)AR and the receptor internalization are much slower than that induced by βAR agonist isoproterenol. The Aβ-induced β(2)AR internalization is also dependent on adaptor protein arrestin 3 and GTPase dynamin, but not arrestin 2. Functionally, pretreatment of primary prefrontal cortical neurons with Aβ induces desensitization of β(2)AR, which leads to attenuated response to subsequent stimulation with isoproterenol, including decreased cAMP levels, PKA activities, PKA phosphorylation of serine 845 on α-amino-2,3-dihydro-5-methyl-3-oxo-4-isoxazolepropanoic acid (AMPA) receptor subunit 1 (GluR1), and AMPA receptor-mediated miniature excitatory postsynaptic currents. This study indicates that Aβ induces β(2)AR internalization and degradation leading to impairment of adrenergic and glutamatergic activities.

Casein kinase 1 enables nucleus accumbens amphetamine-induced locomotion by regulating AMPA receptor phosphorylation

The closely related δ and ε isoforms of the serine/threonine protein kinase casein kinase 1 (Csnk1) have been implicated in the generation of psychostimulant-induced behaviors. In this study, we show that Csnk1δ/ε produces its effects on behavior by acting on the Darpp-32-PP1 signaling pathway to regulate AMPA receptor phosphorylation in the nucleus accumbens (NAcc). Inhibiting Csnk1δ/ε in the NAcc with the selective inhibitor PF-670462 blocks amphetamine induced locomotion and its ability to increase phosphorylation of Darpp-32 at S137 and T34, decrease PP1 activity and increase phosphorylation of the AMPA receptor subunit at S845. Consistent with these findings, preventing GluR1 phosphorylation with the alanine mutant GluR1(S845A) reduces glutamate-evoked currents in cultured medium spiny neurons and blocks the locomotor activity produced by NAcc amphetamine. Thus, Csnk1 enables the locomotor and likely the incentive motivational effects of amphetamine by regulating Darrp-32-PP1-GlurR1(S845) signaling in the NAcc. As such, Csnk1 may be a critical target for intervention in the treatment of drug use disorders.

Colocalization of protein kinase A with adenylyl cyclase enhances protein kinase A activity during induction of long-lasting long-term-potentiation

The ability of neurons to differentially respond to specific temporal and spatial input patterns underlies information storage in neural circuits. One means of achieving spatial specificity is to restrict signaling molecules to particular subcellular compartments using anchoring molecules such as A-Kinase Anchoring Proteins (AKAPs). Disruption of protein kinase A (PKA) anchoring to AKAPs impairs a PKA-dependent form of long term potentiation (LTP) in the hippocampus. To investigate the role of localized PKA signaling in LTP, we developed a stochastic reaction-diffusion model of the signaling pathways leading to PKA activation in CA1 pyramidal neurons. Simulations investigated whether the role of anchoring is to locate kinases near molecules that activate them, or near their target molecules. The results show that anchoring PKA with adenylyl cyclase (which produces cAMP that activates PKA) produces significantly greater PKA activity, and phosphorylation of both inhibitor-1 and AMPA receptor GluR1 subunit on S845, than when PKA is anchored apart from adenylyl cyclase. The spatial microdomain of cAMP was smaller than that of PKA suggesting that anchoring PKA near its source of cAMP is critical because inactivation by phosphodiesterase limits diffusion of cAMP. The prediction that the role of anchoring is to colocalize PKA near adenylyl cyclase was confirmed by experimentally rescuing the deficit in LTP produced by disruption of PKA anchoring using phosphodiesterase inhibitors. Additional experiments confirm the model prediction that disruption of anchoring impairs S845 phosphorylation produced by forskolin-induced synaptic potentiation. Collectively, these results show that locating PKA near adenylyl cyclase is a critical function of anchoring.

The hippocampus is a part of the cerebral cortex involved in formation of certain types of long term memories. Activity-dependent change in the strength of neuronal connections in the hippocampus, known as synaptic plasticity, is one mechanism used to store memories. The ability to form crisp and distinguishable memories of different events implies that learning produces plasticity of specific and distinct subsets of synapses within each neuron. Synaptic activity leads to production of intracellular signaling molecules, which ultimately cause changes in the properties of the synapses. The requirement for synaptic specificity seems incompatible with the diffusibility of intracellular signaling molecules. Anchoring proteins restrict signaling molecules to particular subcellular compartments thereby combating the indiscriminate spread of intracellular signaling molecules. To investigate whether the critical function of anchoring proteins is to localize proteins near their activators or their targets, we developed a stochastic reaction-diffusion model of signaling pathways leading to synaptic plasticity in hippocampal neurons. Simulations demonstrate that colocalizing proteins with their activator molecules is more important due to inactivation mechanisms that limit the spatial extent of the activator molecules.

Pituitary adenylate cyclase-activating peptide induces long-lasting neuroprotection through the induction of activity-dependent signaling via the cyclic AMP response element-binding protein-regulated transcription co-activator 1

Pituitary adenylate cyclase-activating peptide (PACAP) is a neuroprotective peptide which exerts its effects mainly through the cAMP-protein kinase A (PKA) pathway. Here, we show that in cortical neurons, PACAP-induced PKA signaling exerts a major part of its neuroprotective effects indirectly, by triggering action potential (AP) firing. Treatment of cortical neurons with PACAP induces a rapid and sustained PKA-dependent increase in AP firing and associated intracellular Ca²⁺ transients, which are essential for the anti-apoptotic actions of PACAP. Transient exposure to PACAP induces long-lasting neuroprotection in the face of apoptotic insults which is reliant on AP firing and the activation of cAMP response element (CRE) binding protein (CREB)-mediated gene expression. Although direct, activity-independent PKA signaling is sufficient to trigger phosphorylation on CREB’s activating serine-133 site, this is insufficient for activation of CREB-mediated gene expression. Full activation is dependent on CREB-regulated transcription co-activator 1 (CRTC1), whose PACAP-induced nuclear import is dependent on firing activity-dependent calcineurin signaling. Over-expression of CRTC1 is sufficient to rescue PACAP-induced CRE-mediated gene expression in the face of activity-blockade, while dominant negative CRTC1 interferes with PACAP-induced, CREB-mediated neuroprotection. Thus, the enhancement of AP firing may play a significant role in the neuroprotective actions of PACAP and other adenylate cyclase-coupled ligands.

A kinase anchor protein 150 (AKAP150)-associated protein kinase A limits dendritic spine density

The A kinase anchor protein AKAP150 recruits the cAMP-dependent protein kinase (PKA) to dendritic spines. Here we show that in AKAP150 (AKAP5) knock-out (KO) mice frequency of miniature excitatory post-synaptic currents (mEPSC) and inhibitory post-synaptic currents (mIPSC) are elevated at 2 weeks and, more modestly, 4 weeks of age in the hippocampal CA1 area versus litter mate WT mice. Linear spine density and ratio of AMPAR to NMDAR EPSC amplitudes were also increased. Amplitude and decay time of mEPSCs, decay time of mIPSCs, and spine size were unaltered. Mice in which the PKA anchoring C-terminal 36 residues of AKAP150 are deleted (D36) showed similar changes. Furthermore, whereas acute stimulation of PKA (2-4 h) increases spine density, prolonged PKA stimulation (48 h) reduces spine density in apical dendrites of CA1 pyramidal neurons in organotypic slice cultures. The data from the AKAP150 mutant mice show that AKAP150-anchored PKA chronically limits the number of spines with functional AMPARs at 2-4 weeks of age. However, synaptic transmission and spine density was normal at 8 weeks in KO and D36 mice. Thus AKAP150-independent mechanisms correct the aberrantly high number of active spines in juvenile AKAP150 KO and D36 mice during development.

Enhanced cocaine-conditioned place preference and associated brain regional levels of BDNF, p-ERK1/2 and p-Ser845-GluA1 in food-restricted rats

Previously, a learning-free measure was used to demonstrate that chronic food restriction (FR) increases the reward magnitude of a wide range of abused drugs. Moreover, a variety of striatal neuroadaptations were detected in FR subjects, some of which are known to be involved in synaptic plasticity but have been ruled out as modulators of acute drug reward magnitude. Little is known about effects of FR on drug-conditioned place preference (CPP) and brain regional mechanisms that may enhance CPP in FR subjects. The purpose of the present study was to compare the expression and persistence of a conditioned place preference (CPP) induced by a relatively low dose of cocaine (7.0mg/kg, i.p.) in ad libitum fed (AL) and FR rats and take several brain regional biochemical measures following the first CPP conditioning session to probe candidate mechanisms that may underlie the more robust CPP observed in FR subjects. Behaviorally, AL subjects displayed a CPP upon initial testing which extinguished rapidly over the course of subsequent test sessions while CPP in FR subjects persisted. Despite previous reports of elevated BDNF protein in forebrain regions of FR rats, the FR protocol used in the present study did not alter BDNF levels in dorsal hippocampus, nucleus accumbens or medial prefrontal cortex. On the other hand, FR rats, whether injected with cocaine or vehicle, displayed elevated p-ERK1/2 and p-Ser845-GluA1 in dorsal hippocampus. FR rats also displayed elevated p-ERK1/2 in medial prefrontal cortex and elevated p-ERK1 in nucleus accumbens, with further increases produced by cocaine. The one effect observed exclusively in cocaine-treated FR rats was increased p-Ser845-GluA1 in nucleus accumbens. These findings suggest a number of avenues for continuing investigation with potential translational significance.

Enhanced synaptic plasticity in mice with phosphomimetic mutation of the GluA1 AMPA receptor

Phosphorylation of the GluA1 subunit of AMPA receptors has been proposed to regulate receptor trafficking and synaptic transmission and plasticity. However, it remains unclear whether GluA1 phosphorylation is permissive or sufficient for enacting these functional changes. Here we investigate the role of GluA1 phosphorylation at S831 and S845 residues in the hippocampus through the analyses of GluA1 S831D/S845D phosphomimetic knock-in mice. S831D/S845D mice showed normal total and surface expression and subcellular localization of GluA1 as well as intact basal synaptic transmission. In addition, theta-burst stimulation, a protocol that was sufficient to induce robust long-term potentiation (LTP) in WT mice, resulted in LTP of similar magnitude in S831D/S845D mice. However, S831D/S845D mice showed LTP induced with 10-Hz stimulation, a protocol that is weaker than theta-burst stimulation and was not sufficient to induce LTP in WT mice. Moreover, S831D/S845D mice exhibited LTP induced with spike-timing-dependent plasticity (STDP) protocol at a long pre-post interval that was subthreshold for WT mice, although a suprathreshold STDP protocol at a short pre-post interval resulted in similarly robust LTP for WT and S831D/S845D mice. These results indicate that phosphorylation of GluA1 at S831 and S845 is sufficient to lower the threshold for LTP induction, increasing the probability of synaptic plasticity.

Differential regulation of AMPA receptor GluA1 phosphorylation at serine 831 and 845 associated with activation of NMDA receptor subpopulations

AMPA receptors and NMDA receptors are the main subtypes of ionotropic glutamate receptors in the vertebrate central nervous system. Accumulating evidence demonstrates that two serine sites, S831 and S845, on the AMPA receptor GluA1 subunit, are phosphorylation-regulated and profoundly involved in NMDA receptor-dependent synaptic plasticity. On the other hand, recent studies have revealed distinct functional consequences of activating synaptic or extrasynaptic NMDA receptors, or of activating GluN2A- or GluN2B-containing NMDA receptors. Therefore, it is essential to determine how phosphorylation of the GluA1 at S831 and S845 is regulated by NMDA receptor subpopulations. In this study, we demonstrated transiently increased phosphorylation of GluA1 at S831 and persistently decreased phosphorylation of GluA1 at S845 by bath application of NMDA to hippocampal slices from rats. Interestingly, we also found a differential regulation of phosphorylation of GluA1 at S831 and S845 by activation of NMDA receptor subpopulations: the synaptic and/or the GluN2A-containing NMDA receptors were more likely to mediate up-regulation of GluA1 phosphorylation at S831 and down-regulation of GluA1 phosphorylation at S845, while the extrasynaptic NMDA receptors down-regulated GluA1 phosphorylation at S831. Taken together, our results suggest the NMDA receptor subpopulations differentially regulate GluA1 phosphorylation, which may contribute to NMDA receptor-dependent synaptic plasticity.

Food scarcity, neuroadaptations, and the pathogenic potential of dieting in an unnatural ecology: binge eating and drug abuse

In the laboratory, food restriction has been shown to induce neuroadaptations in brain reward circuitry which are likely to be among those that facilitate survival during periods of food scarcity in the wild. However, the upregulation of mechanisms that promote foraging and reward-related learning may pose a hazard when food restriction is self-imposed in an ecology of abundant appetitive rewards. For example, episodes of loss of control during weight-loss dieting, use of drugs with addictive potential as diet aids, and alternating fasting with alcohol consumption in order to avoid weight gain, may induce synaptic plasticity that increases the risk of enduring maladaptive reward-directed behavior. In the present mini-review, representative basic research findings are outlined which indicate that food restriction alters the function of mesoaccumbens dopamine neurons, potentiates cellular and behavioral responses to D-1 and D-2 dopamine receptor stimulation, and increases stimulus-induced synaptic insertion of AMPA receptors in nucleus accumbens. Possible mechanistic underpinnings of increased drug reward magnitude, drug-seeking, and binge intake of sucrose in food-restricted animal subjects are discussed and possible implications for human weight-loss dieting are considered.

Calcium-dependent phosphorylation regulates neuronal stability and plasticity in a highly precise pacemaker nucleus

Specific types of neurons show stable, predictable excitability properties, while other neurons show transient adaptive plasticity of their excitability. However, little attention has been paid to how the cellular pathways underlying adaptive plasticity interact with those that maintain neuronal stability. We addressed this question in the pacemaker neurons from a weakly electric fish because these neurons show a highly stable spontaneous firing rate as well as an N-methyl-D-aspartate (NMDA) receptor-dependent form of plasticity. We found that basal firing rates were regulated by a serial interaction of conventional and atypical PKC isoforms and that this interaction establishes individual differences within the species. We observed that NMDA receptor-dependent plasticity is achieved by further activation of these kinases. Importantly, the PKC pathway is maintained in an unsaturated baseline state to allow further Ca(2+)-dependent activation during plasticity. On the other hand, the Ca(2+)/calmodulin-dependent phosphatase calcineurin does not regulate baseline firing but is recruited to control the duration of the NMDA receptor-dependent plasticity and return the pacemaker firing rate back to baseline. This work illustrates how neuronal plasticity can be realized by biasing ongoing mechanisms of stability (e.g., PKC) and terminated by recruiting alternative mechanisms (e.g., calcineurin) that constrain excitability. We propose this as a general model for regulating activity-dependent change in neuronal excitability.

Mechanism of Ca2+/calmodulin-dependent kinase II regulation of AMPA receptor gating

The function, trafficking and synaptic signalling of AMPA receptors are tightly regulated by phosphorylation. CaMKII phosphorylates the GluA1 AMPA subunit at Ser831 to increase single channel conductance. We show for the first time that CaMKII increases the conductance of native heteromeric AMPA receptors in mouse hippocampal neurons via phosphorylation of Ser831. In addition, co-expression of TARPs with recombinant receptors is required for phosphoSer831 to increase conductance of heteromeric GluA1/GluA2 receptors. Finally, phosphorylation of Ser831 increases the efficiency with which each subunit can activate, independent of agonist efficacy, thereby increasing the likelihood that more receptor subunits will be simultaneously activated during gating. This underlies the observation that phosphoSer831 increases the frequency of openings to larger conductance levels rather than altering unitary conductance. Together, these findings suggest that CaMKII phosphorylation of GluA1-Ser831 decreases the activation energy for an intrasubunit conformational change that regulates the conductance of the receptor when the channel pore opens.

AKAP signaling complexes in regulation of excitatory synaptic plasticity

Plasticity at excitatory glutamatergic synapses in the central nervous system is believed to be critical for neuronal circuits to process and encode information, allowing animals to perform complex behaviors such as learning and memory. In addition, alterations in synaptic plasticity are associated with human diseases, including Alzheimer disease, epilepsy, chronic pain, drug addiction, and schizophrenia. Long-term potentiation (LTP) and depression (LTD) in the hippocampal region of the brain are two forms of synaptic plasticity that increase or decrease, respectively, the strength of synaptic transmission by postsynaptic AMPA-type glutamate receptors. Both LTP and LTD are induced by activation of NMDA-type glutamate receptors but differ in the level and duration of Ca(2+) influx through the NMDA receptor and the subsequent engagement of downstream signaling by protein kinases, including PKA, PKC, and CaMKII, and phosphatases, including PP1 and calcineurin-PP2B (CaN). This review addresses the important emerging roles of the A-kinase anchoring protein family of scaffold proteins in regulating localization of PKA and other kinases and phosphatases to postsynaptic multiprotein complexes that control NMDA and AMPA receptor function during LTP and LTD.

Odor preference learning and memory modify GluA1 phosphorylation and GluA1 distribution in the neonate rat olfactory bulb: testing the AMPA receptor hypothesis in an appetitive learning model

An increase in synaptic AMPA receptors is hypothesized to mediate learning and memory. AMPA receptor increases have been reported in aversive learning models, although it is not clear if they are seen with memory maintenance. Here we examine AMPA receptor changes in a cAMP/PKA/CREB-dependent appetitive learning model: odor preference learning in the neonate rat. Rat pups were given a single pairing of peppermint and 2 mg/kg isoproterenol, which produces a 24-h, but not a 48-h, peppermint preference in the 7-d-old rat pup. GluA1 PKA-dependent phosphorylation peaked 10 min after the 10-min training trial and returned to baseline within 90 min. At 24 h, GluA1 subunits did not change overall but were significantly increased in synaptoneurosomes, consistent with increased membrane insertion. Immunohistochemistry revealed a significant increase in GluA1 subunits in olfactory bulb glomeruli, the targets of olfactory nerve axons. Glomerular increases were seen at 3 and 24 h after odor exposure in trained pups, but not in control pups. GluA1 increases were not seen as early as 10 min after training and were no longer observed 48 h after training when odor preference is no longer expressed behaviorally. Thus, the pattern of increased GluA1 membrane expression closely follows the memory timeline. Further, blocking GluA1 insertion using an interference peptide derived from the carboxyl tail of the GluA1 subunit inhibited 24 h odor preference memory providing causative support for our hypothesis. PKA-mediated GluA1 phosphorylation and later GluA1 insertion could, conjointly, provide increased AMPA function to support both short-term and long-term appetitive memory.

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.

Propofol pretreatment increases antidepressant-like effects induced by acute administration of ketamine in rats receiving forced swimming test

Animal and clinical studies have demonstrated that ketamine has quick, obvious and persistent antidepressant-like effect, but such side effects as cardiovascular system excitement and hallucinogenic activity have limited its clinical application. Intravenous anesthetic propofol can remarkably inhibit or eliminate ketamine's side effects including cardiovascular system excitement and hallucinogenic activity even in a sub-anesthesia dose. However, effect of propofol on rapid antidepressant-like action induced by ketamine still remains unclear. The present study aimed to investigate effect of pretreatment with propofol on the ketamine-induced rapid antidepressant-like action in rats receiving forced swimming test. Open field test and forced swimming test were used to investigate behavior changes of rats receiving different medication. Expression of brain derived neurotrophic factors (BDNF) and a-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) pGluR1-Ser845 in hippocampus was measured with sandwich-ELISA and Western Blot, respectively. Results demonstrated that rats receiving propofol alone showed neither antidepressant-like effects nor increased BDNF content; pretreatment with propofol could increase the ketamine-induced antidepressant-like effects and the expression of AMPA pGluR1-Ser845 in hippocampus, but could not further reinforce the increased BDNF content induced by ketamine in hippocampus; after AMPA receptor was antagonized, the strengthening effect of propofol on ketamine-induced antidepressant-like action significantly decreased. The results indicated that propofol in a sub-anesthetic dose could increase the ketamine-induced antidepressant-like effect.

Alterations in AMPA receptor subunits and TARPs in the rat nucleus accumbens related to the formation of Ca²⁺-permeable AMPA receptors during the incubation of cocaine craving

Cue-induced cocaine seeking intensifies or incubates after withdrawal from extended access cocaine self-administration, a phenomenon termed incubation of cocaine craving. The expression of incubated craving is mediated by Ca²⁺-permeable AMPA receptors (CP-AMPARs) in the nucleus accumbens (NAc). Thus, CP-AMPARs are a potential target for therapeutic intervention, making it important to understand mechanisms that govern their accumulation. Here we used subcellular fractionation and biotinylation of NAc tissue to examine the abundance and distribution of AMPAR subunits, and GluA1 phosphorylation, in the incubation model. We also studied two transmembrane AMPA receptor regulatory proteins (TARPs), γ-2 and γ-4. Our results, together with earlier findings, suggest that some of the new CP-AMPARs are synaptic. These are probably associated with γ-2, but they are loosely tethered to the PSD. Levels of GluA1 phosphorylated at serine 845 (pS845 GluA1) were significantly increased in biotinylated tissue and in an extrasynaptic membrane-enriched fraction. These results suggest that increased synaptic levels of CP-AMPARs may result in part from an increase in pS845 GluA1 in extrasynaptic membranes, given that S845 phosphorylation primes GluA1-containing AMPARs for synaptic insertion and extrasynaptic AMPARs supply the synapse. Some of the new extrasynaptic CP-AMPARs are likely associated with γ-4, rather than γ-2. The maintenance of CP-AMPARs in NAc synapses during withdrawal is accompanied by activation of CaMKII and ERK2 but not CaMKI. Overall, AMPAR plasticity in the incubation model shares some features with better described forms of synaptic plasticity, although the timing of the phenomenon and the persistence of related neuroadaptations are significantly different.

Attention-deficit/hyperactivity phenotype in mice lacking the cyclin-dependent kinase 5 cofactor p35

BACKGROUND

Attention-deficit/hyperactivity disorder (ADHD) may result from delayed establishment of corticolimbic circuitry or perturbed dopamine (DA) neurotransmission. Despite the widespread use of stimulants to treat ADHD, little is known regarding their long-term effects on neurotransmitter levels and metabolism. Cyclin-dependent kinase 5 (Cdk5) regulates DA signaling through control of synthesis, postsynaptic responses, and vesicle release. Mice lacking the Cdk5-activating cofactor p35 are deficient in cortical lamination, suggesting altered motor/reward circuitry.

METHODS

We employed mice lacking p35 to study the effect of altered circuitry in vivo. Positron emission tomography measured glucose metabolism in the cerebral cortex using 2-deoxy-2-[¹⁸F] fluoro-d-glucose as the radiotracer. Retrograde dye tracing and tyrosine hydroxylase immunostains assessed the effect of p35 knockout on the medial prefrontal cortex (PFC), especially in relation to mesolimbic circuit formation. We defined the influence of Cdk5/p35 activity on catecholaminergic neurotransmission and motor activity via examination of locomotor responses to psychostimulants, monoamine neurotransmitter levels, and DA signal transduction.

RESULTS

Here, we report that mice deficient in p35 display increased glucose uptake in the cerebral cortex, basal hyperactivity, and paradoxical decreased locomotion in response to chronic injection of cocaine or methylphenidate. Knockout mice also exhibited an increased susceptibility to changes in PFC neurotransmitter content after chronic methylphenidate exposure and altered basal DAergic activity in acute striatal and PFC slices.

CONCLUSIONS

Our findings suggest that dysregulation of Cdk5/p35 activity during development may contribute to ADHD pathology, as indicated by the behavioral phenotype, improperly established mesolimbic circuitry, and aberrations in striatal and PFC catecholaminergic signaling in p35 knockout mice.

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.

A single brief burst induces GluR1-dependent associative short-term potentiation: a potential mechanism for short-term memory

Recent work showed that short-term memory (STM) is selectively reduced in GluR1 knockout mice. This raises the possibility that a form of synaptic modification dependent on GluR1 might underlie STM. Studies of synaptic plasticity have shown that stimuli too weak to induce long-term potentiation induce short-term potentiation (STP), a phenomenon that has received little attention. Here we examined several properties of STP and tested the dependence of STP on GluR1. The minimal requirement for inducing STP was examined using a test pathway and a conditioning pathway. Several closely spaced stimuli in the test pathway, forming a single brief burst, were sufficient to induce STP. Thus, STP is likely to be induced by the similar bursts that occur in vivo. STP induction is associative in nature and dependent on the NMDAR. STP decays with two components, a fast component (1.6 +/- 0.26 min) and a slower one (19 +/- 6.6 min). To test the role of GluR1 in STP, experiments were conducted on GluR1 knockout mice. We found that STP was greatly reduced. These results, taken together with the behavioral work of D. Sanderson et al. [Sanderson, D., Good, M. A., Skelton, K., Sprengel, R., Seeburg, P. H., Nicholas, J., et al. Enhanced long-term and impaired short-term spatial memory in GluA1 AMPA receptor subunit knockout mice: Evidence for a dual-process memory model. Learning and Memory, 2009], provide genetic evidence that STP is a likely mechanism of STM.

Glutamate receptor ion channels: structure, regulation, and function

The mammalian ionotropic glutamate receptor family encodes 18 gene products that coassemble to form ligand-gated ion channels containing an agonist recognition site, a transmembrane ion permeation pathway, and gating elements that couple agonist-induced conformational changes to the opening or closing of the permeation pore. Glutamate receptors mediate fast excitatory synaptic transmission in the central nervous system and are localized on neuronal and non-neuronal cells. These receptors regulate a broad spectrum of processes in the brain, spinal cord, retina, and peripheral nervous system. Glutamate receptors are postulated to play important roles in numerous neurological diseases and have attracted intense scrutiny. The description of glutamate receptor structure, including its transmembrane elements, reveals a complex assembly of multiple semiautonomous extracellular domains linked to a pore-forming element with striking resemblance to an inverted potassium channel. In this review we discuss International Union of Basic and Clinical Pharmacology glutamate receptor nomenclature, structure, assembly, accessory subunits, interacting proteins, gene expression and translation, post-translational modifications, agonist and antagonist pharmacology, allosteric modulation, mechanisms of gating and permeation, roles in normal physiological function, as well as the potential therapeutic use of pharmacological agents acting at glutamate receptors.

Binding of amyloid beta peptide to beta2 adrenergic receptor induces PKA-dependent AMPA receptor hyperactivity

Progressive decrease in neuronal function is an established feature of Alzheimer's disease (AD). Previous studies have shown that amyloid beta (Abeta) peptide induces acute increase in spontaneous synaptic activity accompanied by neurotoxicity, and Abeta induces excitotoxic neuronal death by increasing calcium influx mediated by hyperactive alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptors. An in vivo study has revealed subpopulations of hyperactive neurons near Abeta plaques in mutant amyloid precursor protein (APP)-transgenic animal model of Alzheimer's disease (AD) that can be normalized by an AMPA receptor antagonist. In the present study, we aim to determine whether soluble Abeta acutely induces hyperactivity of AMPA receptors by a mechanism involving beta(2) adrenergic receptor (beta(2)AR). We found that the soluble Abeta binds to beta(2)AR, and the extracellular N terminus of beta(2)AR is critical for the binding. The binding is required to induce G-protein/cAMP/protein kinase A (PKA) signaling, which controls PKA-dependent phosphorylation of GluR1 and beta(2)AR, and AMPA receptor-mediated excitatory postsynaptic currents (EPSCs). beta(2)AR and GluR1 also form a complex comprising postsynaptic density protein 95 (PSD95), PKA and its anchor AKAP150, and protein phosphotase 2A (PP2A). Both the third intracellular (i3) loop and C terminus of beta(2)AR are required for the beta(2)AR/AMPA receptor complex. Abeta acutely induces PKA phosphorylation of GluR1 in the complex without affecting the association between two receptors. The present study reveals that non-neurotransmitter Abeta has a binding capacity to beta(2)AR and induces PKA-dependent hyperactivity in AMPA receptors.

Lithium acts as a potentiator of AMPAR currents in hippocampal CA1 cells by selectively increasing channel open probability

Recent evidence suggests that lithium, which is used in the treatment of bipolar disorders, may act by influencing AMPAR properties at central glutamatergic synapses. While it is clear that lithium potentiates recombinant AMPAR responses in a subunit specific way, the origin of this potentiation is not known. We examined the effects of lithium on native AMPAR channels in CA1 pyramidal cells in hippocampal slices where AMPARs are expected to be associated with auxiliary subunits. We found that lithium produced a selective increase in single-channel open probability (P(open)), with little effect on single-channel conductance or burst length. From the present and previous finding it is likely that lithium causes a reduction in the time to recovery from desensitization, resulting in the observed increase in P(open). This would be consistent with the view that lithium acts like certain other allosteric AMPAR modulators to reduce the time spent in the desensitized state, but differs from those that act by slowing dissociation of glutamate.

Signal integration through blending, bolstering and bifurcating of intracellular information

A cell's response to its environment is often determined by signaling through the actions of enzyme cascades. The ability to organize these enzymes into multiprotein complexes allows for a high degree of fidelity, efficiency and spatial precision in signaling responses.

The function of GluR1 and GluR2 in cerebellar and hippocampal LTP and LTD is regulated by interplay of phosphorylation and O-GlcNAc modification

Long-term potentiation (LTP) and long-term depression (LTD) are the current models of synaptic plasticity and widely believed to explain how different kinds of memory are stored in different brain regions. Induction of LTP and LTD in different regions of brain undoubtedly involve trafficking of AMPA receptor to and from synapses. Hippocampal LTP involves phosphorylation of GluR1 subunit of AMPA receptor and its delivery to synapse whereas; LTD is the result of dephosphorylation and endocytosis of GluR1 containing AMPA receptor. Conversely the cerebellar LTD is maintained by the phosphorylation of GluR2 which promotes receptor endocytosis while dephosphorylation of GluR2 triggers receptor expression at the cell surface and results in LTP. The interplay of phosphorylation and O-GlcNAc modification is known as functional switch in many neuronal proteins. In this study it is hypothesized that a same phenomenon underlies as LTD and LTP switching, by predicting the potential of different Ser/Thr residues for phosphorylation, O-GlcNAc modification and their possible interplay. We suggest the involvement of O-GlcNAc modification of dephosphorylated GluR1 in maintaining the hippocampal LTD and that of dephosphorylated GluR2 in cerebral LTP.

Expression of protein phosphatase 2B (calcineurin) subunit A isoforms in rat hippocampus after traumatic brain injury

Calcineurin (CaN) is a calcium/calmodulin-dependent phosphatase directly activated by calcium as a result of neuronal activation that is important for neuronal function. CaN subunit isoforms are implicated in long-term potentiation (LTP), long-term depression (LTD), and structural plasticity. CaN inhibitors are also beneficial to cognitive outcomes in animal models of traumatic brain injury (TBI). There are known changes in the CaN A (CnA) subunit following fluid percussion injury (FPI). The CnA subunit has two isoforms: CnAalpha and CnAbeta. The effect of moderate controlled cortical impact (CCI) on distribution of CnA isoforms was examined at 2 h and 2 weeks post-injury. CnA distribution was assayed by immunohistochemistry and graded for non-parametric analysis. Acutely CnA isoforms showed reduced immunoreactivity in stratum radiatum processes of the ipsilateral CA1 and CA1-2. There was also a significant alteration in the immunoreactivity of both CnA isoforms in the ipsilateral dentate gyrus, predominantly within the hidden blade. Alterations in CnA isoform regional distribution within the CA1, CA1-2, and dentate gyrus may have significant implications for persistent hippocampal dysfunction following TBI, including dysfunction in hippocampal plasticity. Understanding alterations in CnA isoform distribution may help improve the targeting of current therapeutic interventions and/or the development of new treatments for TBI.