A mosaic of functional kainate receptors in hippocampal interneurons

Channelopathies linked to plasma membrane phosphoinositides

The plasma membrane phosphoinositide phosphatidylinositol 4,5-bisphosphate (PIP2) controls the activity of most ion channels tested thus far through direct electrostatic interactions. Mutations in channel proteins that change their apparent affinity to PIP2 can lead to channelopathies. Given the fundamental role that membrane phosphoinositides play in regulating channel activity, it is surprising that only a small number of channelopathies have been linked to phosphoinositides. This review proposes that for channels whose activity is PIP2-dependent and for which mutations can lead to channelopathies, the possibility that the mutations alter channel-PIP2 interactions ought to be tested. Similarly, diseases that are linked to disorders of the phosphoinositide pathway result in altered PIP2 levels. In such cases, it is proposed that the possibility for a concomitant dysregulation of channel activity also ought to be tested. The ever-growing list of ion channels whose activity depends on interactions with PIP2 promises to provide a mechanism by which defects on either the channel protein or the phosphoinositide levels can lead to disease.

Control of cortical GABA circuitry development by Nrg1 and ErbB4 signalling

Schizophrenia is a complex disorder that interferes with the function of several brain systems required for cognition and normal social behaviour. Although the most notable clinical aspects of the disease only become apparent during late adolescence or early adulthood, many lines of evidence suggest that schizophrenia is a neurodevelopmental disorder with a strong genetic component. Several independent studies have identified neuregulin 1 (NRG1) and its receptor ERBB4 as important risk genes for schizophrenia, although their precise role in the disease process remains unknown. Here we show that Nrg1 and ErbB4 signalling controls the development of inhibitory circuitries in the mammalian cerebral cortex by cell-autonomously regulating the connectivity of specific GABA (gamma-aminobutyric acid)-containing interneurons. In contrast to the prevalent view, which supports a role for these genes in the formation and function of excitatory synapses between pyramidal cells, we found that ErbB4 expression in the mouse neocortex and hippocampus is largely confined to certain classes of interneurons. In particular, ErbB4 is expressed by many parvalbumin-expressing chandelier and basket cells, where it localizes to axon terminals and postsynaptic densities receiving glutamatergic input. Gain- and loss-of-function experiments, both in vitro and in vivo, demonstrate that ErbB4 cell-autonomously promotes the formation of axo-axonic inhibitory synapses over pyramidal cells, and that this function is probably mediated by Nrg1. In addition, ErbB4 expression in GABA-containing interneurons regulates the formation of excitatory synapses onto the dendrites of these cells. By contrast, ErbB4 is dispensable for excitatory transmission between pyramidal neurons. Altogether, our results indicate that Nrg1 and ErbB4 signalling is required for the wiring of GABA-mediated circuits in the postnatal cortex, providing a new perspective to the involvement of these genes in the aetiology of schizophrenia.

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

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

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

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

Synaptic activation of kainate receptors gates presynaptic CB(1) signaling at GABAergic synapses

Glutamate can control inhibitory synaptic transmission through activation of presynaptic kainate receptors. We found that glutamate released by train stimulation of Schaffer collaterals could lead to either short-term depression or short-term facilitation of inhibitory synaptic transmission in mouse CA1 pyramidal neurons, depending on the presence of cannabinoid type 1 (CB(1)) receptors on GABAergic afferents. The train-induced depression of inhibition (t-Di) required the mobilization of 2-arachidonoylglycerol through postsynaptic activation of metabotropic glutamate receptors and [Ca(2+)] rise. GluK1 (GluR5)-dependent depolarization of GABAergic terminals enabled t-Di by facilitating presynaptic CB(1) signaling. Thus, concerted activation of presynaptic CB(1) receptors and kainate receptors mediates short-term depression of inhibitory synaptic transmission. In contrast, in inhibitory connections expressing GluK1, but not CB(1), receptors, train stimulation of Schaffer collaterals led to short-term facilitation. Thus, activation of kainate receptors by synaptically released glutamate gates presynaptic CB(1) signaling, which in turn controls the direction of short-term heterosynaptic plasticity.

Deletion of the GluR5 subunit of kainate receptors affects cocaine sensitivity and preference

We have demonstrated previously that mice expressing a constitutive deletion of the kainate receptor subunit GluR5 (GluR5 KO) do not differ from wildtype (WT) littermates of a congenic C57BL/6 background with regard to both the development of morphine physical dependence as measured by naloxone-precipitated withdrawal signs and to morphine reward as revealed by the expression of conditioned place preference (CPP). However, unlike WT, GluR5 KO mice fail to develop antinociceptive tolerance following repeated systemic morphine administration. In this report, we examined the impact of GluR5 deletion on cocaine-mediated CPP and locomotor sensitization. Expression of CPP was evident in WT mice following repeated daily administration of 20mg/kg (but not 10mg/kg) i.p. cocaine. Interestingly, GluR5 KO mice exhibited enhanced cocaine preference as compared with WT mice at both 10 and 20mg/kg doses. In addition, while GluR5 KO mice did not differ from WT with respect to baseline locomotor activity, mutant mice demonstrated increased locomotor hyperactivity versus WT mice after repeated injection of 15mg/kg i.p. cocaine. Collectively, these data indicate that GluR5 appears to negatively modulate some psychostimulant and rewarding properties of cocaine, as demonstrated by heightened sensitization and salience in mutant mice.

Age-dependent enhancement of inhibitory synaptic transmission in CA1 pyramidal neurons via GluR5 kainate receptors

Changes in hippocampal synaptic networks during aging may contribute to age-dependent compromise of cognitive functions such as learning and memory. Previous studies have demonstrated that GABAergic synaptic transmission exhibits age-dependent changes. To better understand such age-dependent changes of GABAergic synaptic inhibition, we performed whole-cell recordings from pyramidal cells in the CA1 area of acute hippocampal slices on aged (24-26 months old) and young (2-4 months old) Brown-Norway rats. We found that the frequency and amplitude of spontaneous inhibitory postsynaptic current (IPSCs) were significantly increased in aged rats, but the frequency and amplitude of mIPSCs were decreased. Furthermore, the regulation of GABAergic synaptic transmission by GluR5 containing kainate receptors was enhanced in aged rats, which was revealed by using LY382884 (a GluR5 kainate receptor antagonist) and ATPA (a GluR5 kainate receptor agonist). Moreover, we demonstrated that vesicular glutamate transporters are involved in the kainate receptor dependent regulation of sIPSCs. Taken together, these results suggest that GABAergic synaptic transmission is potentiated in aged rats, and GluR5 containing kainate receptors regulate the inhibitory synaptic transmission through endogenous glutamate. These alterations of GABAergic input with aging could contribute to age-dependent cognitive decline.

GluR5-mediated glutamate signaling regulates hypothalamo-pituitary-adrenocortical stress responses at the paraventricular nucleus and median eminence

Glutamate is the main excitatory neurotransmitter in the central nervous system, and plays an excitatory role in generation of hypothalamic-pituitary-adrenocortical (HPA) axis responses to stress. The current study assesses the role of kainate-preferring receptors in glutamatergic excitation of the HPA axis. In situ hybridization and immunohistochemical analyses confirmed the existence of the GluR5 kainate subunit in the paraventricular nucleus of the hypothalamus (PVN). Importantly, GluR5 immunoreactivity was enriched in the external lamina of the median eminence, where it is co-localized with corticotropin releasing hormone (CRH). Intra-PVN infusion of LY382884 increased plasma adrenocorticotropin (ACTH), corticosterone and PVN c-Fos immunoreactivity. Infusions of LY382884 into the median eminence region, on the other hand, reduced restraint induced ACTH release without altering c-Fos expression. Together, these findings provide evidence for glutamate-mediated signaling in control of CRH release at the PVN and median eminence, mediated by way of kainate-preferring receptor complexes.

Kainate receptors act as conditional amplifiers of spike transmission at hippocampal mossy fiber synapses

Hippocampal mossy fiber (Mf) synapses are viewed as conditional detonators, assisting CA3 cells in complex network functions. By analyzing mice deficient for GluK2 (GluR6), GluK3 (GluR7) and GluK5 (KA2) genes we show that kainate receptors (KARs) play a crucial role in the control of synaptic integration and spike transmission efficacy at Mf synapses. We dissected out the role of the different KAR functions at Mf synapses and we show that presynaptic and postsynaptic KARs concur to amplify unitary Mf synaptic inputs to trigger spike discharge within a wide range of frequencies (from 1 to 50 Hz). Moreover, KARs strongly favor spike transmission in response to patterns of presynaptic activity mimicking in vivo dentate granule cell activity. By amplifying spike transmission, KARs also facilitate the induction of associative long-term potentiation in CA3. Hence the actions of KARs as amplifiers of spike transmission contribute largely to the "conditional detonator" function of Mf synapses and are likely important for spatial information processing.

Neuroprotection of ethanol against cerebral ischemia/reperfusion induced brain injury through GABA receptor activation

In this study we investigated whether ethanol could play neuroprotective effects against ischemic brain injury and the related mechanism. Cresyl violet staining results demonstrated that moderate dose of ethanol administered intracerebroventricularly (i.c.v.) had neuroprotective effect against ischemia-reperfusion induced neuronal cell death. Ethanol also inhibited the phosphorylation of JNK3 induced by cerebral ischemia-reperfusion. Three separate drugs, NS3763 (the selective antagonist of GluR5), GluR5 antisense oligodeoxynucleotides (AS-ODNs) and Bicuculline (an antagonist of GABA receptors), were found to inhibit the neuroprotective effect of ethanol. Moreover, the GABA receptor agonist muscimol could attenuate the JNK3 phosphorylation. Taken together, the results suggest that during ischemia-reperfusion ethanol may activate presynaptic GluR5-KA and postsynaptic GABA receptors continuously, and the activation of GABA receptors inhibits the JNK3 signal pathway. The results show a novel potential mechanism underlying ethanol protective effects.

Subunit-dependent postsynaptic expression of kainate receptors on hippocampal interneurons in area CA1

Kainate receptors (KARs) contribute to postsynaptic excitation in only a select subset of neurons. To define the parameters that specify the postsynaptic expression of KARs, we examined the contribution of KARs to EPSCs on hippocampal interneurons in area CA1. Interneurons in stratum radiatum/lacunosum-moleculare express KARs both with and without the GluR5 subunit, but KAR-mediated EPSCs are generated mainly, if not entirely, by GluR5-containing KARs. Extrasynaptic glutamate spillover profoundly recruits AMPA receptors (AMPARs) with little effect on KARs, indicating that KARs are targeted at the synapse more precisely than AMPARs. However, spontaneous EPSCs with a conventional AMPAR component did not have a resolvable contribution of KARs, suggesting that the KARs that contribute to the evoked EPSCs are at a distinct set of synapses. GluR5-containing KARs on interneurons in stratum oriens do not contribute substantially to the EPSC. We conclude that KARs are localized to synapses by cell type-, synapse-, and subunit-selective mechanisms.

A transmembrane accessory subunit that modulates kainate-type glutamate receptors

Glutamate receptors play major roles in excitatory transmission in the vertebrate brain. Among ionotropic glutamate receptors (AMPA, kainate, NMDA), AMPA receptors mediate fast synaptic transmission and require TARP auxiliary subunits. NMDA receptors and kainate receptors play roles in synaptic transmission, but it remains uncertain whether these ionotropic glutamate receptors also have essential subunits. Using a proteomic screen, we have identified NETO2, a brain-specific protein of unknown function, as an interactor with kainate-type glutamate receptors. NETO2 modulates the channel properties of recombinant and native kainate receptors without affecting trafficking of the receptors and also modulates kainate-receptor-mediated mEPSCs. Furthermore, we found that kainate receptors regulate the surface expression of NETO2 and that NETO2 protein levels and surface expression are decreased in mice lacking the kainate receptor GluR6. The results show that NETO2 is a kainate receptor subunit with significant effects on glutamate signaling mechanisms in brain.

Calcium-permeable presynaptic kainate receptors involved in excitatory short-term facilitation onto somatostatin interneurons during natural stimulus patterns

Schaffer collateral synapses in hippocampus show target-cell specific short-term plasticity. Using GFP-expressing Inhibitory Neuron (GIN) transgenic mice that express enhanced green fluorescent protein (EGFP) in a subset of somatostatin-containing interneurons (SOM interneurons), we previously showed that Schaffer collateral synapses onto SOM interneurons in stratum (S.) radiatum have unusually large (up to 6-fold) paired-pulse facilitation. This results from a low initial release probability and the enhancement of facilitation by synaptic activation of presynaptic kainate receptors. Here we further investigate the properties of these kainate receptors and examine their effects on short-term facilitation during physiologically derived stimulation patterns, using excitatory postsynaptic currents recorded in S. radiatum interneurons during Schaffer collateral stimulation in acute slices from juvenile GIN mice. We find that GluR5 and GluR6 antagonists decrease short-term facilitation at Schaffer collateral synapses onto SOM interneurons with no additive effects, suggesting that the presynaptic kainate receptors are heteromers containing both GluR5 and GluR6 subunits. The calcium-permeable receptor antagonist 1-napthyl acetyl spermine (NASPM) both mimics and occludes the effect of the kainate receptor antagonists, indicating that the presynaptic kainate receptors are calcium permeable. Furthermore, Schaffer collateral synapses onto SOM interneurons show up to 11-fold short-term facilitation during physiologically derived stimulus patterns, in contrast to other interneurons that have less than 1.5-fold facilitation. Blocking the kainate receptors reduces facilitation in SOM interneurons by approximately 50% during the physiologically derived patterns and reduces the dynamic range. Activation of calcium-permeable kainate receptors containing GluR5/GluR6 causes a dramatic increase in short-term facilitation during physiologically derived stimulus patterns, a mechanism likely to be important in regulating the strength of Schaffer collateral synapses onto SOM interneurons in vivo.

The role of GLU K5-containing kainate receptors in entorhinal cortex gamma frequency oscillations

Using in vitro brain slices of hippocampus and cortex, neuronal oscillations in the frequency range of 30-80 Hz (gamma frequency oscillations) can be induced by a number of pharmacological manipulations. The most routinely used is the bath application of the broad-spectrum glutamate receptor agonist, kainic acid. In the hippocampus, work using transgenic kainate receptor knockout mice have revealed information about the specific subunit composition of the kainate receptor implicated in the generation and maintenance of the gamma frequency oscillation. However, there is a paucity of such detail regarding gamma frequency oscillation in the cortex. Using specific pharmacological agonists and antagonists for the kainate receptor, we have set out to examine the contribution of kainate receptor subtypes to gamma frequency oscillation in the entorhinal cortex. The findings presented demonstrate that in contrast to the hippocampus, kainate receptors containing the GLU(K5) subunit are critically important for the generation and maintenance of gamma frequency oscillation in the entorhinal cortex. Future work will concentrate on determining the exact nature of the cellular expression of kainate receptors in the entorhinal cortex.

Neuroprotection of GluR5-containing kainate receptor activation against ischemic brain injury through decreasing tyrosine phosphorylation of N-methyl-D-aspartate receptors mediated by Src kinase

Previous studies indicate that cerebral ischemia breaks the dynamic balance between excitatory and inhibitory inputs. The neural excitotoxicity induced by ionotropic glutamate receptors gain the upper hand during ischemia-reperfusion. In this paper, we investigate whether GluR5 (glutamate receptor 5)-containing kainate receptor activation could lead to a neuroprotective effect against ischemic brain injury and the related mechanism. The results showed that (RS)-2-amino-3-(3-hydroxy-5-tert-butylisoxazol-4-yl) propanoic acid (ATPA), a selective GluR5 agonist, could suppress Src tyrosine phosphorylation and interactions among N-methyl-D-aspartate (NMDA) receptor subunit 2A (NR2A), postsynaptic density protein 95 (PSD-95), and Src and then decrease NMDA receptor activation through attenuating tyrosine phosphorylation of NR2A and NR2B. More importantly, ATPA had a neuroprotective effect against ischemia-reperfusion-induced neuronal cell death in vivo. However, four separate drugs were found to abolish the effects of ATPA. These were selective GluR5 antagonist NS3763; GluR5 antisense oligodeoxynucleotides; CdCl(2), a broad spectrum blocker of voltage-gated calcium channels; and bicuculline, an antagonist of gamma-aminobutyric acid A (GABA(A)) receptor. GABA(A) receptor agonist muscimol could attenuate Src activation and interactions among NR2A, PSD-95 and Src, resulting the suppression of NMDA receptor tyrosine phosphorylation. Moreover, patch clamp recording proved that the activated GABA(A) receptor could inhibit NMDA receptor-mediated whole-cell currents. Taken together, the results suggest that during ischemia-reperfusion, activated GluR5 may facilitate Ca(2+)-dependent GABA release from interneurons. The released GABA can activate postsynaptic GABA(A) receptors, which then attenuates NMDA receptor tyrosine phosphorylation through inhibiting Src activation and disassembling the signaling module NR2A-PSD-95-Src. The final result of this process is that the pyramidal neurons are rescued from hyperexcitability.

Tonic activation of GLUK5 kainate receptors decreases neuroblast migration in whole-mounts of the subventricular zone

In the postnatal subventricular zone (SVZ), neuroblasts migrate in chains along the lateral ventricle towards the olfactory bulb. AMPA/kainate receptors as well as metabotropic glutamate receptors subtype 5 (mGluR5) are expressed in SVZ cells. However, the cells expressing these receptors and the function of these receptors remain unexplored. We thus examined whether SVZ neuroblasts express mGluR5 and Ca(2+)-permeable kainate receptors in mouse slices. Doublecortin (DCX)-immunopositive cells (i.e. neuroblasts) immunostained positive for mGluR5 and GLU(K5-7)-containing kainate receptors. RT-PCR from approximately 10 GFP-fluorescent cell aspirates obtained in acute slices from transgenic mice expressing green fluorescent protein (GFP) under the DCX promoter showed mGluR5 and GLU(K5) receptor mRNA in SVZ neuroblasts. Patch-clamp data suggest that approximately 60% of neuroblasts express functional GLU(K5)-containing receptors. Activation of mGluR5 and GLU(K5)-containing receptors induced Ca(2+) increases in 50% and 60% of SVZ neuroblasts, respectively, while most neuroblasts displayed GABA(A)-mediated Ca(2+) responses. To examine the effects of these receptors on the speed of neuroblast migration, we developed a whole-mount preparation of the entire lateral ventricle from postnatal day (P) 20-25 DCX-GFP mice. The GABA(A) receptor (GABA(A)R) antagonist bicuculline increased the speed of neuroblast migration by 27%, as previously reported in acute slices. While the mGluR5 antagonist MPEP did not affect the speed of neuroblast migration, the homomeric and heteromeric GLU(K5) receptor antagonists, NS3763 and UB302, respectively, increased the migration speed by 38%. These data show that although both GLU(K5) receptor and mGluR5 activations increase Ca(2+) in neuroblasts, only GLU(K5) receptors tonically reduce the speed of neuroblast migration along the lateral ventricle.

Molecular determinants of kainate receptor trafficking

Glutamate receptors of the kainate subtype are ionotropic receptors that play a key role in the modulation of neuronal network activity. The role of kainate receptors depends on their precise membrane and subcellular localization in presynaptic, extrasynaptic and postsynaptic domains. These receptors are composed of the combination of five subunits, three of them having several splice variants. The subunits and splice variants show great divergence in their C-terminal cytoplasmic tail domains, which have been implicated in intracellular trafficking of homomeric and heteromeric receptors. Differential trafficking of kainate receptors to specific neuronal compartments likely relies on interactions between the different kainate receptor subunits with distinct subsets of protein partners that interact with C-terminal domains. These C-terminal domains have also been implicated in the degradation of kainate receptors. Finally, the phosphorylation of the C-terminal domain regulates receptor trafficking and function. This review summarizes our knowledge on the regulation of membrane delivery and trafficking of kainate receptors implicating C-terminal domains of the different isoforms and focuses on the identification and characterization of the function of interacting partners.

Presynaptic lonotropic receptors

The release of transmitters through vesicle exocytosis from nerve terminals is not constant but is subject to modulation by various mechanisms, including prior activity at the synapse and the presence of neurotransmitters or neuromodulators in the synapse. Instantaneous responses of postsynaptic cells to released transmitters are mediated by ionotropic receptors. In contrast to metabotropic receptors, ionotropic receptors mediate the actions of agonists in a transient manner within milliseconds to seconds. Nevertheless, transmitters can control vesicle exocytosis not only via slowly acting metabotropic, but also via fast acting ionotropic receptors located at the presynaptic nerve terminals. In fact, members of the following subfamilies of ionotropic receptors have been found to control transmitter release: ATP P2X, nicotinic acetylcholine, GABA(A), ionotropic glutamate, glycine, 5-HT(3), andvanilloid receptors. As these receptors display greatly diverging structural and functional features, a variety of different mechanisms are involved in the regulation of transmitter release via presynaptic ionotropic receptors. This text gives an overview of presynaptic ionotropic receptors and briefly summarizes the events involved in transmitter release to finally delineate the most important signaling mechanisms that mediate the effects of presynaptic ionotropic receptor activation. Finally, a few examples are presented to exemplify the physiological and pharmacological relevance of presynaptic ionotropic receptors.

Altered GABAergic neurotransmission is associated with increased kainate-induced seizure in prostaglandin-endoperoxide synthase-2 deficient mice

Excitotoxicity involves over activation of brain excitatory glutamate receptors and has been implicated in neurological, neurodegenerative and neuropsychiatric diseases. Metabolism of arachidonic acid (AA) through the phospholipase A(2) (PLA(2))/prostaglandin-endoperoxide synthase (PTGS) pathway is increased after excitotoxic stimulation. However, the individual roles of the PTGS isoforms in this process are not well established. We assessed the role of the PTGS isoforms in the process of excitotoxicity by exposing mice deficient in either PTGS-1 (PTGS-1(-/-)) or PTGS-2 (PTGS-2(-/-)) to the prototypic excitotoxin, kainic acid (KA). Seizure intensity and neuronal damage were significantly elevated in KA-exposed PTGS-2(-/-), but not in PTGS-1(-/-), mice. The increased susceptibility was not associated with an alteration in KA receptor binding activity or mediated through the CB1 endocannabinoid receptor. The frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) was decreased in the CA1 pyramidal neurons of PTGS-2(-/-) mice, suggesting an alteration of GABAergic function. In wild-type mice, six weeks treatment with the PTGS-2 selective inhibitor celecoxib recapitulated the increased susceptibility to KA-induced excitotoxicity observed in PTGS-2(-/-) mice, further supporting the role of PTGS-2 in the excitotoxic process. The increased susceptibility to KA was also associated with decreased brain levels of PGE(2), a biomarker of PTGS-2 activity. Our results suggest that PTGS-2 activity and its specific products may modulate neuronal excitability by affecting GABAergic neurotransmission. Further, inhibition of PTGS-2, but not PTGS-1, may increase the susceptibility to seizures.

Kainate receptors with a metabotropic modus operandi

Kainate receptors (KARs), together with AMPA and NMDA, are typically described as ionotropic glutamate receptors. The functions of KARs have begun to be elucidated only in the last decade. Although some the actions of KARs are classically ionotropic, surprisingly others seem to involve the activation of second-messenger cascades and invoke metabotropic roles for this type of glutamate receptor. In this review, we describe these metabotropic actions of KARs in relation to the putative signalling cascades involved. Although it is still a mystery how KARs activate G proteins to stimulate second-messenger cascades, intriguingly, in very recent studies, specific subunits of KARs have been demonstrated to associate with G proteins. Altogether, the body of evidence supports the hypothesis that, together with the canonical ionotropic operation, KARs expedite long-lasting signalling by novel metabotropic modes of action.

Differential alterations of kainate receptor subunits in inhibitory interneurons in the anterior cingulate cortex in schizophrenia and bipolar disorder

The aim of this study was to examine whether glutamatergic inputs onto GABA interneurons via the kainate receptor in the anterior cingulate cortex may be altered in schizophrenia and bipolar disorder. Hence, in a cohort of 60 post-mortem human brains from schizophrenia, bipolar disorder, and normal control subjects, we simultaneously labeled the mRNA for the GluR5 or GluR6 subunit of the kainate receptor with [(35)S] and the mRNA for the 67 kD isoform of the GABA synthesizing enzyme glutamic acid decarboxylase (GAD)(67) with digoxigenin using an immunoperoxidase method. The density of the GAD(67) mRNA-containing neurons that co-expressed GluR5 mRNA was decreased by 43% and 40% in layer 2 of the anterior cingulate cortex in schizophrenia and bipolar disorder, respectively. In contrast, the density of the GAD(67) mRNA-containing cells that expressed GluR6 mRNA was unaltered in either condition. Furthermore, the amount of GluR5 or GluR6 mRNA in the GAD(67) mRNA-expressing cells that contained a detectable level of these transcripts was also unchanged. Finally, the density of cells that did not contain GAD(67) mRNA, which presumably included all pyramidal neurons, but expressed the mRNA for the GluR5 or GluR6 subunit was not altered. Thus, glutamatergic modulation of inhibitory interneurons, but not pyramidal neurons, via kainate receptors containing the GluR5 subunit appears to be selectively altered in the anterior cingulate cortex in schizophrenia and bipolar disorder.

Functional cooperation between KA2 and GluR6 subunits is involved in the ischemic brain injury

We investigated the possible relationships between KA2 subunit and GluR6 subunit, as well as the role of KA2 subunit in neuronal death induced by cerebral ischemia/reperfusion. Our results indicated that intracerebroventricular infusion of KA2 antisense oligodeoxynucleotides (AS) not only knocked down the expressions of KA2 and GluR6, but also suppressed the assembly of the GluR6/KA2-PSD95-MLK3 signaling module, and inhibited JNK activation and phosphorylation of c-jun. In addition, infusion of KA2 AS increased neuronal survival in CA1 region after 5 days of reperfusion. More interestingly, we found that the combination of KA2 and GluR6 AS exerted more significant effects than when pretreated with KA2 AS or GluR6 AS alone. Our results suggest that the KA2 subunit is involved in delayed neuronal death induced by cerebral ischemia, at the same time, it is noteworthy that the functional cooperation between KA2 and GluR6 subunits plays a critical role in the ischemic brain injury by PSD95-MLK3-MKK4/7-JNK3 signal pathway.

Metabotropic actions of kainate receptors in the CNS

Kainate receptors (KARs), together with NMDA and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate receptors (AMPA), are typically described as ionotropic glutamate receptors. Although ionotropic functions for KARs are beginning to be characterized in multiple brain regions, both, in the pre- and post-synaptic compartments of the synapse, there is accumulating evidence that KARs mediate some of their effects without invoking ion-fluxes. Thus, since 1998, when the first metabotropic action of KARs was described in the modulation of GABA release in hippocampal interneurons, there have been increasing reports that some of the functions of KARs involve the participation of intracellular signalling cascades and depend on G protein activation. These surprising observations, attesting metabotropic actions of KARs, akin to those usually attributed to seven transmembrane region G protein-coupled receptors, make the physiological classification and description of glutamate receptors more complex. In the present review, we describe the metabotropic roles of KARs in the CNS and discuss the intriguing properties of this receptor which, structurally shows all the facets of a typical ionotropic receptor, but appears to express a metabotropic remit at some key synapses.

Structural consequences of Kcna1 gene deletion and transfer in the mouse hippocampus

PURPOSE

Mice lacking the Kv1.1 potassium channel alpha subunit encoded by the Kcna1 gene develop recurrent behavioral seizures early in life. We examined the neuropathological consequences of seizure activity in the Kv1.1(-/-) (knock-out) mouse, and explored the effects of injecting a viral vector carrying the deleted Kcna1 gene into hippocampal neurons.

METHODS

Morphological techniques were used to assess neuropathological patterns in hippocampus of Kv1.1(-/-) animals. Immunohistochemical and biochemical techniques were used to monitor ion channel expression in Kv1.1(-/-) brain. Both wild-type and knockout mice were injected (bilaterally into hippocampus) with an HSV1 amplicon vector that contained the rat Kcna1 subunit gene and/or the E. coli lacZ reporter gene. Vector-injected mice were examined to determine the extent of neuronal infection.

RESULTS

Video/EEG monitoring confirmed interictal abnormalities and seizure occurrence in Kv1.1(-/-) mice. Neuropathological assessment suggested that hippocampal damage (silver stain) and reorganization (Timm stain) occurred only after animals had exhibited severe prolonged seizures (status epilepticus). Ablation of Kcna1 did not result in compensatory changes in expression levels of other related ion channel subunits. Vector injection resulted in infection primarily of granule cells in hippocampus, but the number of infected neurons was quite variable across subjects. Kcna1 immunocytochemistry showed "ectopic" Kv1.1 alpha channel subunit expression.

CONCLUSIONS

Kcna1 deletion in mice results in a seizure disorder that resembles--electrographically and neuropathologically--the patterns seen in rodent models of temporal lobe epilepsy. HSV1 vector-mediated gene transfer into hippocampus yielded variable neuronal infection.

Activation of kainate receptors controls the number of functional glutamatergic synapses in the area CA1 of rat hippocampus

The expression and functions of kainate-type glutamate receptors (KARs) in the hippocampus are developmentally regulated. In particular, presynaptic KARs depressing glutamate release are tonically activated during early postnatal development, and this activity is down-regulated in parallel with maturation of the synaptic circuitry. In order to understand the physiological relevance of the tonic KAR-mediated signalling, we have here studied the effect of long-term pharmacological activation of KARs on glutamatergic synaptic connectivity in hippocampal slice cultures where presynaptic KARs are expressed but not endogenously activated. Prolonged (16-20 h) activation of the GluR5 subunit-containing KARs using the agonist ATPA (1 microM) caused a specific and enduring increase in the number of glutamatergic synapses in area CA1, evidenced as an increase in the frequency of action potential-independent spontaneous EPSCs (mEPSCs) and in immunostaining against synaptic marker proteins. The long-term ATPA treatment had no detectable effect on GABAergic transmission or on glutamate release probability. Further, the effect of ATPA on synaptic density was independent of action potential firing and dependent on protein kinase C. A critical role of endogenous KAR activity in synaptic development was revealed by chronic treatment of the cultures with the selective GluR5 antagonist LY382884, which caused a significant impairment of glutamatergic transmission to CA1 pyramidal neurons. Together, these data suggest a role for the GluR5 subunit-containing KARs in the formation and/or stabilization of functional glutamatergic synapses in area CA1.

Genetic and pharmacological studies of GluR5 modulation of inhibitory synaptic transmission in the anterior cingulate cortex of adult mice

In the anterior cingulate cortex (ACC), GluR5-containing kainate receptor mediated the small portion of excitatory postsynaptic current. However, little is known about its role in modulation of neurotransmitter release in this brain region. In the present study, we address this question by using selective GluR5 agonist and antagonist, as well as GluR5(-/-) mice. Our results showed that activation of GluR5 induced action potential-dependent GABA release, which is also required for the activation of voltage-dependent calcium channel and Ca(2+) influx. The effect of GluR5 activation is selective to the GABAergic, but not glutamatergic synaptic transmission. Endogenous activation of GluR5 also enhanced GABA release to ACC pyramidal neurons and the corresponding postsynaptic tonic GABA current. Our results suggest the somatodendritic, but not presynaptic GluR5, in modulation of GABA release. The endogenous GluR5 activation and the subsequent tonic GABA current may play an inhibitory role in ACC-related brain functions.

Anxiolytic-like effects through a GLUK5 kainate receptor mechanism

The hypothesis that kainate receptor blockade would be associated with anxiolytic-like effects was tested with a selective ligand, 3S,4aR,6S,8aR-6-((4-carboxyphenyl)methyl)-1,2,3,4,4a,5,6,7,8,8a-decahydroisoquinoline-3-carboxylic acid (LY382884). LY382884 selectively binds the GLU(K5) kainate receptor subunit (K(b)=0.6 microM) and has 30 microM or greater affinity for cloned human AMPA receptor subtypes. The anxiolytic potential of LY382884 was tested in rats responding under a Vogel conflict procedure, a pharmacologically validated model for the prediction of antianxiety efficacy in humans. Both the benzodiazepine anxiolytic chlordiazepoxide and LY382884 increased suppressed licking without affecting rates of non-suppressed licking. In contrast, an AMPA receptor selective antagonist, 7H-1,3-dioxolo[4,5-h][2,3]benzodiazepine-7-carboxamide, 5-(4-aminophenyl)-8,9-dihydro-N,8-dimethyl-, monohydrochloride (9CI) (GYKI53655), did not increase suppressed responding. The finding that a selective GLU(K5) receptor antagonist produced anxiolytic-like effects in an animal model predictive of efficacy in humans combined with data in the literature on glutamatergic modulation of anxiety suggests that kainate receptor sensitivity to glutamate might be an important mediating event in the pathophysiological expression of anxiety states. The selective targeting of kainate receptors with an antagonist could therefore be a novel pharmacological mechanism to treat anxiety disorders.

Actinfilin is a Cul3 substrate adaptor, linking GluR6 kainate receptor subunits to the ubiquitin-proteasome pathway

Kainate receptors have been implicated in excitotoxic neuronal death induced by diseases such as epilepsy and stroke. Actinfilin, a synaptic member of the BTB-Kelch protein family, is known to bind to the actin cytoskeleton. However, little is understood about its function at the synapse. Here, we report that actinfilin is able to bind to GluR6, a kainate-type glutamate receptor subunit, and target GluR6 for degradation. Like many members of its protein family, actinfilin acts as a substrate adaptor, binding Cullin 3 (Cul3) and linking GluR6 to the E3 ubiquitin-ligase complex. We map this interaction to the Kelch repeat domain of actinfilin and the GluR6 C terminus. Co-immunoprecipitation and immunofluorescence studies show that GluR6 is ubiquitinated, and that GluR6 levels are decreased by actinfilin overexpression but increased when actinfilin levels are reduced by specific RNA interference. Furthermore, actinfilin-Cul3 interactions appear to be important for regulating surface GluR6 expression. Synaptic GluR6 levels are elevated in mice with lowered neuronal Cul3 expression and when dominant-negative forms of Cul3 are transfected into hippocampal neurons. Together our data demonstrate that actinfilin acts as a scaffold, linking GluR6 to the Cul3 ubiquitin ligase to provide a novel mechanism for kainate receptor degradation.

Protein kinase signalling requirements for metabotropic action of kainate receptors in rat CA1 pyramidal neurones

Hippocampal pyramidal neurones display a Ca(2+)-dependent K(+) current responsible for the slow afterhyperpolarization (I(sAHP)), a prominent regulator of excitability. There is considerable transmitter convergence onto I(sAHP) but little information about the interplay between the kinase-based transduction mechanisms underlying transmitter action. We have added to existing information about the role of protein kinase C (PKC) in kainate receptor actions by demonstrating that direct postsynaptic activation of PKC with either 1-oleoyl-2-acethylsn-glycerol (OAG) or indolactam is sufficient to inhibit I(sAHP). The physiological correlate of this action - activation of PKC by kainate receptors - requires G alpha(i/o) proteins. The cAMP/PKA system is well documented to subserve the actions of monoamine transmitters. We have found an additional role for the cAMP/PKA system as a requirement for kainate receptor-mediated inhibition of I(sAHP). Inhibition of adenylyl cyclase with dideoxyadenosine or PKA with either H89 or RpcAMPs blocked kainate receptor-mediated actions but did not prevent the actions of direct PKC activation with either OAG or indolactam. We therefore propose that the PKA requirement is upstream from the actions of PKC. We additionally report a downstream link in the form of increased mitogen-activated protein (MAP) kinase activity, which may explain the long duration of metabotropic actions of kainate receptors on I(sAHP).

The expression and binding of kainate receptors is modified in different brain regions by glutamate neurotoxicity during postnatal rat development

Kainic acid receptor (KA-R) subunits are differentially expressed during brain development, and they modulate both neural growth and survival. High concentrations of glutamate in the brain can induce neuronal injury through these receptors, altering normal development. However, it is unclear whether KAR subunit expression itself is also modified by neonatal exposure to high glutamate. To analyze this, monosodium glutamate (4mg/g of body weight) was subcutaneously administered on postnatal days 1, 3, 5 and 7, and the expression of GluR5, GluR6, KA1 and KA2, as well as [(3)H]-kainic acid (KA-R) binding, was evaluated on postnatal days 14, 21, 30 and 60 in different regions of rat brain. As a result, high levels of GluR5 expression associated with strong [(3)H]-kainic acid binding were observed on postnatal days 30 and 60 in the cerebral cortex of rats exposed to glutamate. Similarly, the changes induced by glutamate administration in the expression of the KA1 and KA2 subunits were paralleled by those of [(3)H]-kainic acid binding in the striatum at postnatal days 21 and 30. In contrast, while KAR subunits were over expressed in the hippocampus, no changes were observed in [(3)H]-kainic acid binding in adult rats that had been exposed to glutamate. Therefore, glutamate modifies both the expression of kainic acid receptor subunits and kainic acid binding in a determined spatial and temporal manner, which may be indicative of a regional susceptibility to glutamate neurotoxicity.

Presynaptic kainate receptor activation is a novel mechanism for target cell-specific short-term facilitation at Schaffer collateral synapses

Target cell-specific differences in short-term plasticity have been attributed to differences in the initial release probability of synapses. Using GIN (GFP-expressing inhibitory neurons) transgenic mice that express enhanced green fluorescent protein (EGFP) in a subset of interneurons containing somatostatin, we show that Schaffer collateral synapses onto the EGFP-expressing somatostatin interneurons in CA1 have very large short-term facilitation, even larger facilitation than onto pyramidal cells, in contrast to the majority of interneurons that have little or no facilitation. Using a combination of electrophysiological recordings and mathematical modeling, we show that the large short-term facilitation is caused both by a very low initial release probability and by synaptic activation of presynaptic kainate receptors that increase release probability on subsequent stimuli. Thus, we have discovered a novel mechanism for target cell-specific short-term plasticity at Schaffer collateral synapses in which the activation of presynaptic kainate receptors by synaptically released glutamate contributes to large short-term facilitation, enabling selective enhancement of the inputs to a subset of interneurons.

Presynaptic inhibition by kainate receptors converges mechanistically with presynaptic inhibition by adenosine and GABAB receptors

Kainate receptors are widely reported to regulate the release of neurotransmitter in the CNS, but the mechanisms involved remain controversial. Previous studies have found that the kainate receptor agonist ATPA, which selectively activates Glu(K5)-containing kainate receptors, depresses glutamate release at Schaffer-collateral synapses in the hippocampus. In the present study, we provide pharmacological evidence that this depressant effect is mediated by Glu(K5)-containing heteromers, but is distinct from a similar depressant effect engaged by the kainate receptor agonist domoate. The depressant effect of ATPA is insensitive to antagonists for GABA(A), GABA(B), and adenosine receptors, and is also unaffected by lowering the release probability by reducing extracellular calcium. However, the effect of ATPA is partly occluded by prior activation of GABA(B) receptors and completely occluded by prior activation of adenosine receptors, suggesting a mechanistic convergence of heteromeric Glu(K5) kainate receptor signaling with GABA(B) receptors and adenosine receptors. The effects of domoate are partially occluded by both adenosine and GABA(B) receptor agonists, indicating at least a partial convergence of Glu(K5)-lacking kainate receptor signaling with these other pathways. The depressant effect of ATPA is not blocked by inhibition of serine/threonine protein kinases. These results suggest that ATPA and domoate inhibit glutamate release through mechanisms that converge with those of classical metabotropic receptor agonists, although they do so through different receptors.

Glutamate receptors and pain

Pain is an important survival and protection mechanism for animals. However, chronic/persistent pain may be differentiated from normal physiological pain in that it confers no obvious advantage. An accumulating body of pharmacological, electrophysiological, and behavioral evidence is emerging in support of the notion that glutamate receptors play a crucial role in pain pathways and that modulation of glutamate receptors may have potential for therapeutic utility in several categories of persistent pain, including neuropathic pain resulting from injury and/or disease of central (e.g., spinal cord injury) or peripheral nerves (e.g., diabetic neuropathy, radiculopathy) and inflammatory or joint-related pain (e.g., rheumatoid arthritis, osteoarthritis). This review focuses on the role of glutamate receptors, including both ionotropic (AMPA, NMDA and kainate) and metabotropic (mGlu1-8) receptors in persistent pain states with particular emphasis on their expression patterns in nociceptive pathways and their potential as targets for pharmacological intervention strategies.

Kainate receptors

Kainate receptors form a family of ionotropic glutamate receptors that appear to play a special role in the regulation of the activity of synaptic networks. This review first describes briefly the molecular and pharmacological properties of native and recombinant kainate receptors. It then attempts to outline the general principles that appear to govern the function of kainate receptors in the activity of synaptic networks under physiological conditions. It subsequently describes the way that kainate receptors are involved in synaptic integration, synaptic plasticity, the regulation of neurotransmitter release and the control of neuronal excitability, and the manner in which they might play an important role in synaptogenesis and synaptic maturation. These functions require the proper subcellular localization of kainate receptors in specific functional domains of the neuron, necessitating complex cellular and molecular trafficking events. We show that our comprehension of these mechanisms is just starting to emerge. Finally, this review presents evidence that implicates kainate receptors in pathophysiological conditions such as epilepsy, excitotoxicity and pain, and that shows that these receptors represent promising therapeutic targets.

Identification of an endoplasmic reticulum-retention motif in an intracellular loop of the kainate receptor subunit KA2

Neuronal kainate receptors are typically heteromeric complexes composed of GluR5-7 and KA1-2 subunits. Although GluR5-7 can exist as functional homomeric channels, the KA subunits cannot. KA2 is widely expressed in the CNS, and KA2/GluR6 heteromers are the most prevalent subunit composition in brain. Previous work has identified endoplasmic reticulum (ER)-retention motifs in the C terminus of KA2, which prevent surface expression of KA2 homomers. However, we find that, when these motifs are mutated, only a small fraction of KA2 is surface expressed. We now identify an additional ER retention motif in the intracellular loop region of KA2, which, when mutated together with the C-terminal motifs, significantly increases the level of KA2 surface expression. However, electrophysiological analysis of surface-expressed KA2 homomers indicates that they do not form functional ion channels. In heterologous cells, a large fraction of KA2 remains intracellular even when the trafficking motifs are mutated or when GluR6 is coexpressed. Therefore, we analyzed the trafficking of endogenous KA2 in vivo. We find that native KA2 surface expression is dramatically reduced in GluR6 knock-out mice compared with wild-type mice. In contrast, KA2 trafficking was unaffected in the GluR5 knock-out. Thus, our study demonstrates that trafficking motifs in both the intracellular loop and C terminus regulate KA2 surface expression; however, in neurons, GluR6 oligomerization is required for egress of KA2 from the ER and transport to the cell surface. The combination of these mechanisms likely prevents surface expression of nonfunctional KA2 homomers and ensures a high level of GluR6/KA2 heteromeric kainate receptors.

Distinct subunits in heteromeric kainate receptors mediate ionotropic and metabotropic function at hippocampal mossy fiber synapses

Heteromeric kainate receptors (KARs) containing both glutamate receptor 6 (GluR6) and KA2 subunits are involved in KAR-mediated EPSCs at mossy fiber synapses in CA3 pyramidal cells. We report that endogenous glutamate, by activating KARs, reversibly inhibits the slow Ca2+-activated K+ current I(sAHP) and increases neuronal excitability through a G-protein-coupled mechanism. Using KAR knockout mice, we show that KA2 is essential for the inhibition of I(sAHP) in CA3 pyramidal cells by low nanomolar concentrations of kainate, in addition to GluR6. In GluR6(-/-) mice, both ionotropic synaptic transmission and inhibition of I(sAHP) by endogenous glutamate released from mossy fibers was lost. In contrast, inhibition of I(sAHP) was absent in KA2(-/-) mice despite the preservation of KAR-mediated EPSCs. These data indicate that the metabotropic action of KARs did not rely on the activation of a KAR-mediated inward current. Biochemical analysis of knock-out mice revealed that KA2 was required for the interaction of KARs with Galpha(q/11)-proteins known to be involved in I(sAHP) modulation. Finally, the ionotropic and metabotropic actions of KARs at mossy fiber synapses were differentially sensitive to the competitive glutamate receptor ligands kainate (5 nM) and kynurenate (1 mM). We propose a model in which KARs could operate in two modes at mossy fiber synapses: through a direct ionotropic action of GluR6, and through an indirect G-protein-coupled mechanism requiring the binding of glutamate to KA2.

Pharmacological Characterization of the Competitive GLUK5 Receptor Antagonist Decahydroisoquinoline LY466195 in Vitro and in Vivo

The excitatory neurotransmitter glutamate has been implicated in both migraine and persistent pain. The identification of the kainate receptor GLU(K5) in dorsal root ganglia, the dorsal horn, and trigeminal ganglia makes it a target of interest for these indications. We examined the in vitro and in vivo pharmacology of the competitive GLU(K5)-selective kainate receptor antagonist LY466195 [(3S,4aR,6S,8aR)-6-[[(2S)-2-carboxy-4,4-difluoro-1-pyrrolidinyl]-methyl]decahydro-3-isoquinolinecarboxylic acid)], the most potent GLU(K5) antagonist described to date. Comparisons were made to the competitive GLU(K5)/alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor antagonist LY293558 [(3S,4aR,6R,8aR)-6-[2-(1(2)H-tetrazole-5-yl)ethyl]-decahydroisoquinoline-3-carboxylic acid], other decahydroisoquinoline GLU(K5) receptor antagonists, and the noncompetitive AMPA receptor antagonist LY300168 [1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodi-azepine]. When characterized electrophysiologically in rat dorsal root ganglion neurons, LY466195 antagonized kainate (30 microM)-induced currents with an IC50 value of 0.045 +/- 0.011 microM. In HEK293 cells transfected with GLU(K5), GLU(K2)/GLU(K5), or GLU(K5)/GLU(K6) receptors, LY466195 produced IC50 values of 0.08 +/- 0.02, 0.34 +/- 0.17, and 0.07 +/- 0.02 microM, respectively. LY466195 was efficacious in a dural plasma protein extravasation (PPE) model of migraine with an ID100 value of 100 microg/kg i.v. LY466195 was also efficacious in the c-fos migraine model, with a dose of 1 microg/kg i.v. significantly reducing the number of Fos-positive cells in the rat nucleus caudalis after electrical stimulation of the trigeminal ganglion. Furthermore, LY466195 showed no contractile activity in the rabbit saphenous vein in vitro. The diethyl ester prodrug of LY466195 was also efficacious in the same PPE and c-fos models after oral administration at doses of 10 and 100 microg/kg, respectively while having no N-methyl-D-aspartate antagonist-like behavioral effects at oral doses up to 100 mg/kg.

Kainate receptor activation potentiates GABAergic synaptic transmission in the nucleus accumbens core

Inhibitory synaptic transmission plays an important role in regulating the activity of medium spiny neurons (MSNs) in the nucleus accumbens (NAcc). The kainate (KA) subtype of ionotropic glutamate receptor has been shown to potently modulate GABAergic synaptic transmission in several brain regions. Although KA receptor subunits are expressed in the NAcc, KA receptor modulation of GABAergic synaptic transmission in this brain region has not been previously examined. In the current study, we sought to determine if KA receptor activation could alter inhibitory synaptic transmission in the NAcc as it has been shown to do in other brain regions. Using the whole cell patch-clamp technique, we demonstrate that KA receptor activation potentiates evoked GABAergic synaptic transmission and increases the frequency of spontaneous, but not miniature, GABA(A)-receptor-mediated IPSCs in the NAcc. In contrast, KA has no effect on currents evoked by exogenous application of GABA onto MSNs. Taken together, these data suggest that activation of KA receptors in the NAcc core potently facilitates action-potential-dependent GABAergic synaptic transmission, likely via an excitation of presynaptic GABAergic interneurons.

Altered long-term synaptic plasticity and kainate-induced Ca2+ transients in the substantia gelatinosa neurons in GLUK6-deficient mice

Functional kainate receptors are expressed in the spinal cord substantia gelatinosa region, and their activation contributes to bi-directional regulation of excitatory synaptic transmission at primary afferent synapses with spinal cord substantia gelatinosa neurons. However, no study has reported a role(s) for kainate receptor subtypes in long-term synaptic plasticity phenomena in this region. Using gene-targeted mice lacking glutamate receptor 5 (GLU(K5)) or GLU(K6) subunit, we here show that GLU(K6) subunit, but not GLU(K5) subunit, is involved in the induction of long-term potentiation of excitatory postsynaptic potentials, evoked by two different protocols: (1) high-frequency primary afferent stimulation (100 Hz, 3 s) and (2) low-frequency spike-timing stimulation (1 Hz, 200 pulses). In addition, GLU(K6) subunit plays an important role in the expression of kainate-induced Ca2+ transients in the substantia gelatinosa. On the other hand, genetic deletion of GLU(K5) or GLU(K6) subunit does not prevent the induction of long-term depression. These results indicate that unique expression of kainate receptors subunits is important in regulating spinal synaptic plasticity and thereby processing of sensory information, including pain.

A pharmacological investigation of the role of GLUK5-containing receptors in kainate-driven hippocampal gamma band oscillations

Low concentrations of kainate can induce gamma frequency (25-80 Hz) oscillations in hippocampal slices as well as other brain structures in vitro. Little is known, however, about the kainate receptor (KAR) subtypes that underlie this type of rhythmic neuronal network activity. In this study, the role of GLU(K5) subunit-containing KARs in kainate-induced hippocampal gamma frequency oscillations was assessed using GLU(K5)-selective pharmacological ligands. Activation of GLU(K5)-containing subunits using the selective agonists (RS)-2-amino-3-(3-hydroxy-5-tert-butylisoxazol-4-yl)propanoic acid (ATPA; 0.1-1 microM) or iodowillardiine (0.1-1 microM) failed to induce gamma frequency oscillations in area CA3 of the rat hippocampal slice. Likewise, preincubation with a selective GLU(K5) antagonist, (RS)-3-(2-carboxybenzyl)willardiine (UBP296), did not prevent the appearance of gamma oscillations induced by 150 nM kainate. However, addition of UBP296 (10 microM) to hippocampal slices in which kainate-driven gamma oscillations were pre-established resulted in an approximately 50% reduction in gamma frequency power. These effects occurred in the absence of any effect on AMPA receptor-mediated synaptic transmission. Furthermore, carbachol-induced gamma oscillations were also unaffected by application of UBP296. These results suggest that GLU(K5)-containing KARs are not alone sufficient to generate gamma frequency oscillations, but are involved in maintaining neuronal network activity induced by the actions of kainate at other KARs such as GLU(K6).

Kainate receptor-mediated synaptic transmission in the adult anterior cingulate cortex

Kainate (KA) receptors are expressed widely in the CNS. However, little is known about their functional characterization, molecular identity, and role in synaptic transmission in the forebrain of adult mice. Patch-clamp recordings in genetically modified mice show that postsynaptic KA receptors contribute to fast synaptic transmission in pyramidal neurons in the anterior cingulate cortex (ACC), a forebrain region critical for higher-order cognitive brain functions such as memory and mental disorders. Single-shock stimulation could induce small KA receptor-mediated excitatory postsynaptic currents (KA EPSCs) in the presence of picrotoxin, D-2-amino-5-phosphono-pentanoic acid, and a selective AMPA receptor antagonist, GYKI 53655. KA EPSCs had a significantly slower rise time course and decay time constant compared with AMPA receptor-mediated EPSCs. High-frequency repetitive stimulation significantly facilitated the KA EPSCs. Genetic deletion of the GluR6 or GluR5 subunit significantly reduced, and GluR5 and 6 double knockout completely abolished, KA EPSCs and KA-activated currents in ACC pyramidal neurons. Our results show that KA receptors contribute to synaptic transmission in adult ACC pyramidal neurons and provide a synaptic basis for the physiology and pathology of KA receptors in ACC-related functions.

Differential trafficking of GluR7 kainate receptor subunit splice variants

Kainate receptors (KARs) are heteromeric ionotropic glutamate receptors that play a variety of roles in the regulation of synaptic network activity. The function of glutamate receptors (GluRs) is highly dependent on their surface density in specific neuronal domains. Alternative splicing is known to regulate surface expression of GluR5 and GluR6 subunits. The KAR subunit GluR7 exists under different splice variant isoforms in the C-terminal domain (GluR7a and GluR7b). Here we have studied the trafficking of GluR7 splice variants in cultured hippocampal neurons from wild-type and KAR mutant mice. We have found that alternative splicing regulates surface expression of GluR7-containing KARs. GluR7a and GluR7b differentially traffic from the ER to the plasma membrane. GluR7a is highly expressed at the plasma membrane, and its trafficking is dependent on a stretch of positively charged amino acids also found in GluR6a. In contrast, GluR7b is detected at the plasma membrane at a low level and retained mostly in the endoplasmic reticulum (ER). The RXR motif of GluR7b does not act as an ER retention motif, at variance with other receptors and ion channels, but might be involved during the assembly process. Like GluR6a, GluR7a promotes surface expression of ER-retained subunit splice variants when assembled in heteromeric KARs. However, our results also suggest that this positive regulation of KAR trafficking is limited by the ability of different combinations of subunits to form heteromeric receptor assemblies. These data further define the complex rules that govern membrane delivery and subcellular distribution of KARs.

Profound regulation of neonatal CA1 rat hippocampal GABAergic transmission by functionally distinct kainate receptor populations

Neonatal hippocampus exhibits distinct patterns of network activity that are dependent on the interaction between inhibitory and excitatory transmission. Kainate receptors are ideally positioned to regulate this activity by virtue of their ability to regulate presynaptic function in GABAergic interneurones. Indeed, kainate receptors are highly expressed in neonatal hippocampal interneurones, yet the role and mechanisms by which they might regulate neonatal circuitry are unexplored. To address this we investigated the kainate receptor-dependent regulation of GABAergic transmission onto neonatal CA1 pyramidal neurones. Kainate receptor activation produced two distinct opposing effects, a very large increase in the frequency of spontaneous IPSCs, and a robust depression of evoked GABAergic transmission. The up-regulation of spontaneous transmission was due to activation of somatodendritic and axonal receptors while the depression of evoked transmission could be fully accounted for by a direct regulation of GABA release by kainate receptors located at the terminals. None of the effects of kainate receptor agonists were sensitive to GABAB receptor antagonists, nor was there any postsynaptic kainate receptor-dependent effects observed in CA1 pyramidal cells that could account for our findings. Our data demonstrate that kainate receptors profoundly regulate neonatal CA1 GABAergic circuitry by two distinct opposing mechanisms, and indicate that these two effects are mediated by functionally distinct populations of receptors. Thus kainate receptors are strategically located to play a critical role in shaping early hippocampal network activity and by virtue of this have a key role in hippocampal development.