Regulation of kainate receptors by protein kinase C and metabotropic glutamate receptors

Kainate receptors in the developing neuronal networks

Kainate receptors (KARs) are highly expressed in the immature brain and have unique developmentally regulated functions that may be important in linking neuronal activity to morphogenesis during activity-dependent fine-tuning of the synaptic connectivity. Altered expression of KARs in the developing neural network leads to changes in glutamatergic connectivity and network excitability, which may lead to long-lasting changes in behaviorally relevant circuitries in the brain. Here, we summarize the current knowledge on physiological and morphogenic functions described for different types of KARs at immature neural circuitries, focusing on their roles in modulating synaptic transmission and plasticity as well as circuit maturation in the rodent hippocampus and amygdala. Finally, we discuss the emerging evidence suggesting that malfunction of KARs in the immature brain may contribute to the pathophysiology underlying developmentally originating neurological disorders.

The emerging role of kainate receptor functional dysregulation in pain

Pain is a serious clinical challenge, and is associated with a significant reduction in quality of life and high financial costs for affected patients. Research efforts have been made to explore the etiological basis of pain to guide the future treatment of patients suffering from pain conditions. Findings from studies using KA (kainate) receptor agonist, antagonists and receptor knockout mice suggested that KA receptor dysregulation and dysfunction may govern both peripheral and central sensitization in the context of pain. Additional evidence showed that KA receptor dysfunction may disrupt the finely-tuned process of glutamic acid transmission, thereby contributing to the onset of a range of pathological contexts. In the present review, we summarized major findings in recent studies which examined the roles of KA receptor dysregulation in nociceptive transmission and in pain. This timely overview of current knowledge will help to provide a framework for future developing novel therapeutic strategies to manage pain.

Reduced synaptic function of Kainate receptors in the insular cortex of Fmr1 Knock-out mice

Fragile X syndrome is caused by the loss of fragile X mental retardation protein (FMRP). Kainate receptor (KAR) is a subfamily of ionotropic glutamate receptors (iGluR) that acts mainly as a neuromodulator of synaptic transmission and neuronal excitability. However, little is known about the changes of synaptic KAR in the cortical area of Fmr1 KO mice. In this study, we performed whole-cell patch-clamp recordings from layer II/III pyramidal neurons in the insular cortex of Fmr1 KO mice. We found that KARs mediated currents were reduced in Fmr1 KO mice. KARs were mainly located in the synaptosomal fraction of the insular cortex. The abundance of KAR subunit GluK1 and GluK2/3 in the synaptosome was reduced in Fmr1 KO mice, whereas the total expressions of these KARs subunits were not changed. Finally, lack of FMRP impairs subsequent internalization of surface GluK2 after KAR activation, while having no effect on the surface GluK2 expression. Our studies provide evidence indicating that loss of FMRP leads to the abnormal function and localization of KARs. This finding implies a new molecular mechanism for Fragile X syndrome.

Glutamate heteroreceptor complexes in the brain

The existence of mGluR, NMDAR, AMPAR and putative KAR heteroreceptor complexes in synaptic and extrasynaptic regions of brain glutamate synapses represents a major integrative mechanism. Our aim in the current article is to analyze if the formation of the different types glutamate hetereceptor complexes involves the contribution of triplet amino acid homologies (protriplets) in a postulated receptor interface based on the triplet puzzle theory. Seven main sets (lists) of receptor pairs in databases were used containing various sets (lists) of human receptor heteromers and nonheteromers obtained from the available scientific publications including the publically available GPCR-hetnet database. Brain mGluR1-mGluR5 and mGluR2-mGluR4 isoreceptor complexes were demonstrated with a predominant extrasynaptic localization at a post- and prejunctional localization. The existence of putative mGluR4-mGluR7 heteroreceptor complexes in the basal ganglia is proposed. Metabotropic glutamate receptor subtypes also participated in the formation of a large number of heteroreceptor complexes like mGluR1-A1R, mGluR5-A2AR, mGluR5-D2R and D2R-A2AR-mGluR5, located in relation to glutamate synapses, especially in the basal ganglia. A putative mGluR1-GABAB1/2 heterocomplex may also exist. NMDAR heteroreceptor complexes were also demonstrated as a fundamental integrative mechanism in the glutamate synapse and its extrasynaptic membranes. It represented fundamental work on inter alia NMDAR-mGluR5, NMDAR-D1R and NMDAR-D2R heteroreceptor complexes involving both antagonistic and facilitatory allosteric receptor-receptor interactions. As to AMPA receptors, a heterocomplex was found for the interaction between IFNgR1 and the AMPAR mediated via the subunit GluA1 which may be of relevance for neuroinflammation. AMPAR-D2R heteroreceptor complexes were also demonstrated. Besides glutamate heteroreceptor complexes and their allosteric receptor-receptor interactions, a significant mechanism for the functional crosstalk can also be phosphorylation and/or reorganization of adapter proteins with dynamic binding to the two receptors modulating the allosteric receptor mechanism.

PKC SUMOylation inhibits the binding of 14-3-3τ to GluK2

Phosphorylation and SUMOylation of the kainate receptor (KAR) subunit GluK2 have been shown to regulate KAR surface expression, trafficking and synaptic plasticity. In addition, our previous study has shown that a phosphorylation-dependent interaction of 14-3-3τ and GluK2a-containing receptors contributes to the slow decay kinetics of native KAR-EPSCs. However, it is unknown whether SUMOylation participates in the regulation of the interaction between 14-3-3τ and GluK2a-containing receptors. Here we report that SUMOylation of PKC, but not GluK2, represses the binding of 14-3-3τ to GluK2a via decreasing the phosphorylation level of GluK2a. These results suggest that PKC SUMOylation is an important regulator of the 14-3-3 and GluK2a protein complex and may contribute to regulate the decay kinetics of KAR-EPSCs.

Mechanisms of tau and Aβ-induced excitotoxicity

Excitotoxicity was originally postulated to be a late stage side effect of Alzheimer׳s disease (AD)-related neurodegeneration, however more recent studies indicate that it may occur early in AD and contribute to the neurodegenerative process. Tau and amyloid beta (Aβ), the main components of neurofibrillary tangles (NFTs) and amyloid plaques, have been implicated in cooperatively and independently facilitating excitotoxicity. Our study investigated the roles of tau and Aβ in AD-related excitotoxicity. In vivo studies showed that tau knockout (tau(-/-)) mice were significantly protected from seizures and hippocampal superoxide production induced with the glutamate analog, kainic acid (KA). We hypothesized that tau accomplished this by facilitating KA-induced Ca(2+) influx into neurons, however lentiviral tau knockdown failed to ameliorate KA-induced Ca(2+) influx into primary rat cortical neurons. We further investigated if tau cooperated with Aβ to facilitate KA-induced Ca(2+) influx. While Aβ biphasically modulated the KA-induced Cacyt(2+) responses, tau knockdown continued to have no effect. Therefore, tau facilitates KA-induced seizures and superoxide production in a manner that does not involve facilitation of Ca(2+) influx through KA receptors (KAR). On the other hand, acute pretreatment with Aβ (10 min) enhanced KA-induced Ca(2+) influx, while chronic Aβ (24 h) significantly reduced it, regardless of tau knockdown. Given previously published connections between Aβ, group 1 metabotropic glutamate receptors (mGluRs), and KAR regulation, we hypothesized that Aβ modulates KAR via a G-protein coupled receptor pathway mediated by group 1 mGluRs. We found that Aβ did not activate group 1 mGluRs and inhibition of these receptors did not reverse Aβ modulation of KA-induced Ca(2+) influx. Therefore, Aβ biphasically regulates KAR via a mechanism that does not involve group 1mGluR activation.

Trafficking of kainate receptors

Ionotropic glutamate receptors (iGluRs) mediate the vast majority of excitatory neurotransmission in the central nervous system of vertebrates. In the protein family of iGluRs, kainate receptors (KARs) comprise the probably least well understood receptor class. Although KARs act as key players in the regulation of synaptic network activity, many properties and functions of these proteins remain elusive until now. Especially the precise pre-, extra-, and postsynaptic localization of KARs plays a critical role for neuronal function, as an unbalanced localization of KARs would ultimately lead to dysregulated neuronal excitability. Recently, important advances in the understanding of the regulation of surface expression, function, and agonist-dependent endocytosis of KARs have been achieved. Post-translational modifications like PKC-mediated phosphorylation and SUMOylation have been reported to critically influence surface expression and endocytosis, while newly discovered auxiliary proteins were shown to shape the functional properties of KARs.

Modulation of GluK2a subunit-containing kainate receptors by 14-3-3 proteins

Kainate receptors (KARs) are one of the ionotropic glutamate receptors that mediate excitatory postsynaptic currents (EPSCs) with characteristically slow kinetics. Although mechanisms for the slow kinetics of KAR-EPSCs are not totally understood, recent evidence has implicated a regulatory role of KAR-associated proteins. Here, we report that decay kinetics of GluK2a-containing receptors is modulated by closely associated 14-3-3 proteins. 14-3-3 binding requires PKC-dependent phosphorylation of serine residues localized in the carboxyl tail of the GluK2a subunit. In transfected cells, 14-3-3 binding to GluK2a slows desensitization kinetics of both homomeric GluK2a and heteromeric GluK2a/GluK5 receptors. Moreover, KAR-EPSCs at mossy fiber-CA3 synapses decay significantly faster in the 14-3-3 functional knock-out mice. Collectively, these results demonstrate that 14-3-3 proteins are an important regulator of GluK2a-containing KARs and may contribute to the slow decay kinetics of native KAR-EPSCs.

Postsynaptic kainate receptor recycling and surface expression are regulated by metabotropic autoreceptor signalling

Kainate receptors (KARs) play fundamentally important roles in controlling synaptic function and regulating neuronal excitability. Postsynaptic KARs contribute to excitatory neurotransmission but the molecular mechanisms underlying their activity-dependent surface expression are not well understood. Strong activation of KARs in cultured hippocampal neurons leads to the downregulation of postsynaptic KARs via endocytosis and degradation. In contrast, low-level activation augments postsynaptic KAR surface expression. Here, we show that this increase in KARs is due to enhanced recycling via the recruitment of Rab11-dependent, transferrin-positive endosomes into spines. Dominant-negative Rab11 or the recycling inhibitor primaquine prevents the kainate-evoked increase in surface KARs. Moreover, we show that the increase in surface expression is mediated via a metabotropic KAR signalling pathway, which is blocked by the protein kinase C inhibitor chelerythrine, the calcium chelator BAPTA and the G-protein inhibitor pertussis toxin. Thus, we report a previously uncharacterized positive feedback system that increases postsynaptic KARs in response to low- or moderate-level agonist activation and can provide additional flexibility to synaptic regulation.

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.

Activation of group I metabotropic glutamate receptors potentiates heteromeric kainate receptors

Kainate receptors (KARs), a family of ionotropic glutamate receptors, are widely expressed in the central nervous system and are critically involved in synaptic transmission. KAR activation is influenced by metabotropic glutamate receptor (mGlu) signaling, but the underlying mechanisms are not understood. We undertook studies to examine how mGlu modulation affects activation of KARs. Confocal immunohistochemistry of rat hippocampus and cultured rat cortex revealed colocalization of the high-affinity KAR subunits with group I mGlu receptors. In hippocampal and cortical cultures, the calcium signal caused by activation of native KARs was potentiated by activation of group I mGlu receptors. In Xenopus laevis oocytes, activation of group I mGlu receptors potentiated heteromeric but not homomeric KAR-mediated currents, with no change in agonist potency. The potentiation of heteromeric KARs by mGlu1 activation was attenuated by GDPβS, blocked by an inhibitor of phospholipase C or the calcium chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), prolonged by the phosphatase inhibitor okadaic acid, but unaffected by the tyrosine kinase inhibitor lavendustin A. Protein kinase C (PKC) inhibition reduced the potentiation by mGlu1 of GluK2/GluK5, and conversely, direct activation of PKC by phorbol 12-myristate,13-acetate potentiated GluK2/GluK5. Using site-directed mutagenesis, we identified three serines (Ser833, Ser836, and Ser840) within the membrane proximal region of the GluK5 C-terminal domain that, in combination, are required for mGlu1-mediated potentiation of KARs. Together, these data suggest that phosphorylation of key residues in the C-terminal domain changes the overall charge of this domain, resulting in potentiated agonist responses.

Modification and movement: Phosphorylation and SUMOylation regulate endocytosis of GluK2-containing kainate receptors

Kainate receptors (KARs) are tetrameric glutamate-gated ion channels composed of combinations of the subunits GluK1-5. Depending on their precise localization and subunit composition, KARs can regulate neurotransmitter release, synaptic function and neuronal excitability. Because of these diverse roles, the regulated and precisely targeted trafficking of KARs is of crucial importance to neuronal function. We previously reported that the KAR subunit GluK2 is post-translationally modified by attachment of Small Ubiquitin-like Modifier 1 (SUMO-1) and that SUMOylation is required for agonist-dependent endocytosis of GluK2. We recently extended these findings to demonstrate that agonist activation leads to PKC-mediated phosphorylation of GluK2 at serine 868, which directly enhances GluK2 SUMOylation and, in turn, leads to endocytosis of the receptor. These new data demonstrate the importance of interplay between two post-translational modifications in orchestrating the temporal and spatial regulation of kainate receptor trafficking.

SUMOylation and phosphorylation of GluK2 regulate kainate receptor trafficking and synaptic plasticity

Phosphorylation or SUMOylation of the kainate receptor (KAR) subunit GluK2 have both individually been shown to regulate KAR surface expression. However, it is unknown whether phosphorylation and SUMOylation of GluK2 are important for activity-dependent KAR synaptic plasticity. We found that protein kinase C–mediated phosphorylation of GluK2 at serine 868 promotes GluK2 SUMOylation at lysine 886 and that both of these events are necessary for the internalization of GluK2-containing KARs that occurs during long-term depression of KAR-mediated synaptic transmission at rat hippocampal mossy fiber synapses. Conversely, phosphorylation of GluK2 at serine 868 in the absence of SUMOylation led to an increase in KAR surface expression by facilitating receptor recycling between endosomal compartments and the plasma membrane. Our results suggest a role for the dynamic control of synaptic SUMOylation in the regulation of KAR synaptic transmission and plasticity.

Role of the vesicular transporter VGLUT3 in retrograde release of glutamate by cerebellar Purkinje cells

In the cerebellum, retrograde release of glutamate (Glu) by Purkinje cells (PCs) participates in the control of presynaptic neurotransmitter release responsible for the late component of depolarization-induced suppression of excitation (DSE), as well as for depolarization-induced potentiation of inhibition (DPI). It might also participate in the depolarization-induced slow current (DISC) in PCs, although this contribution was later challenged. We also know that both DPI and DISC are soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE)-dependent processes, although the molecular nature of the vesicular transporter was not determined. In PCs, VGLUT3 is the only known vesicular glutamate transporter identified and is expressed during the same developmental frame as when DPI, DISC, and the Glu-dependent component of DSE are observed. We therefore tested the hypothesis that all these processes depend on the presence of VGLUT3 by comparing the Glu-dependent component of DSE, DPI, and DISC in nearly mature (2- to 3-wk-old) wild-type and VGLUT3 knockout mice. Our data demonstrate that, in nearly mature mice, the slow component of DSE occurs through vesicular release of Glu that involves VGLUT3. This Glu-dependent component of DSE is no longer present in fully mature mice. This study also establishes that, in nearly mature mice, DPI also depends on the presence of VGLUT3, whereas this is not the case for DISC. Finally, the unusually large basal paired-pulse facilitation observed in nearly mature VGLUT3(-/-) mice but not in adult ones suggests that some basal retrograde release of Glu occurs during development and contributes to basal concentrations of extracellular Glu.

Subunit-dependent modulation of kainate receptors by muscarinic acetylcholine receptors

Interactions between cholinergic and glutamatergic neurotransmitter systems influence synaptic transmission and plasticity. While previous studies have examined cross-talk between acetylcholine (ACh) and NMDA or AMPA receptors, little is known about the effect of ACh on kainate receptors (KARs). We show that stimulation of m1 or m3 muscarinic ACh receptors (mAChRs) for 2min potentiates recombinant KAR currents in a long lasting fashion. Muscarinic AChR activation potentiates heteromeric GluK2/GluK4 and GluK2/GluK5 receptors, but not homomeric GluK2 receptors. In hippocampal slices kainate potentiates mossy fiber axon excitability. Transient mAChR activation enhances this action of kainate, suggesting a novel mechanism through which acetylcholine could modulate synaptic transmission in the hippocampus. KAR over-activation has been implicated in excitotoxic cell death. To establish the functional significance of the interaction between mAChRs and KARs we examined the effect of mAChR activation on KAR-mediated excitotoxicity. We find that during pharmacological blockade of NMDA and AMPA receptors, KAR activation with AMPA produces significant cell death in primary cortical culture. Concanavalin A (Con A), which selectively blocks KAR desensitization, markedly increases this KAR-mediated neurotoxicity. Brief activation of mAChRs with pilocarpine significantly enhances KAR-mediated excitotoxicity both in the presence and absence of Con A. We conclude that KARs are modulated in a subunit dependent manner by mAChRs. We suggest that ACh may induce long lasting alterations in neuronal excitability and enhance excitotoxicity in part by potentiating KAR function.

Brain-derived neurotrophic factor controls activity-dependent maturation of CA1 synapses by downregulating tonic activation of presynaptic kainate receptors

Immature hippocampal synapses express presynaptic kainate receptors (KARs), which tonically inhibit glutamate release. Presynaptic maturation involves activity-dependent downregulation of the tonic KAR activity and consequent increase in release probability; however, the molecular mechanisms underlying this developmental process are unknown. Here, we have investigated whether brain derived neurotrophic factor (BDNF), a secreted protein implicated in developmental plasticity in several areas of the brain, controls presynaptic maturation by regulating KARs. Application of BDNF in neonate hippocampal slices resulted in increase in synaptic transmission that fully occluded the immature-type KAR activity in area CA1. Conversely, genetic ablation of BDNF was associated with delayed synaptic maturation and persistent presynaptic KAR activity, suggesting a role for endogenous BDNF in the developmental regulation of KAR function. In addition, our data suggests a critical role for BDNF TrkB signaling in fast activity-dependent regulation of KARs. Selective acute inhibition of TrkB receptors using a chemical-genetic approach prevented rapid change in synapse dynamics and loss of tonic KAR activity that is typically seen in response to induction of LTP at immature synapses. Together, these data show that BDNF-TrkB-dependent maturation of glutamatergic synapses is tightly associated with a loss of endogenous KAR activity. The coordinated action of these two receptor mechanisms has immediate physiological relevance in controlling presynaptic efficacy and transmission dynamics at CA3-CA1 synapses at a stage of development when functional contact already exists but transmission is weak.

Role of presynaptic kainate receptors at parallel fiber-purkinje cell synapses in induction of cerebellar LTD: interplay with climbing fiber input

Until recently, except for A1 adenosine, N-methyl-d-aspartate, and cannabinoid receptors, little effort has been made to unravel possible roles of parallel fiber (PF) presynaptic receptors in long-term depression (LTD) of synaptic transmission at PF-Purkinje cell (PC) synapses. Presynaptic kainate (KA) receptors are also present on PFs and might also influence LTD induction by modulating glutamate (Glu) release at PF-PC synapses. This hypothesis was tested by comparing the efficacy of two pairing protocols in inducing LTD in adult wild-type and knock-out mice for the Glu receptor 6 (GluR6) subunit of KA receptors. Activation of presynaptic KA receptors was unnecessary for LTD induction when PF inputs were paired with climbing fiber (CF) stimulation but became crucial when CF input was replaced by direct depolarization of PCs. By comparing basal paired-pulse facilitation of PF-excitatory postsynaptic currents (EPSCs) and depolarization-induced suppression of excitation in adult wild-type and GluR6 knock-out mice, it was shown that the participation of PF presynaptic KA receptors in LTD induction was likely to mainly result from their tonic activation by basal extracellular Glu, rather than from their activation by retrograde release of Glu by PCs during pairing protocols. Finally, this study suggests that, in adult mice, CFs not only participate in LTD induction by depolarizing postsynaptic cells but also by activating postsynaptic mGluR1alpha metabotropic glutamate receptors at CF-PC synapses.

Impact of protein kinase C activation on epileptiform activity in the hippocampal slice

There is evidence suggesting that protein kinase C (PKC) activation can prevent the enhanced network excitability associated with status epilepticus and group I metabotropic glutamate receptor (mGluR)-induced epileptogenesis. However, we observed no suppression of mGluR-induced burst prolongation in the guinea pig hippocampal slice when applied in the presence of the PKC activator phorbol-12,13-dibutyrate (PDBu). Furthermore, PDBu alone converted picrotoxin-induced interictal bursts into ictal-length discharges ranging from 2 to 6s in length. This effect could not be elicited by the inactive analog 4-alpha-PDBu and was suppressed with the PKC inhibitor chelerythrine, indicating PKC dependence. PKC activation can enhance neurotransmitter release, and both glutamate and acetylcholine are capable of eliciting similar prolonged synchronized discharges. However, neither mGluR1 nor NMDA receptor antagonist suppressed PDBu-driven burst prolongation, suggesting that increased glutamate release alone is unlikely to account for the PKC-induced expression of ictaform discharges. Similarly, atropine, a broad-spectrum muscarinic receptor antagonist, had no effect on PKC-induced burst prolongation. By contrast, AMPA/kainate receptor antagonist abolished PKC-induced burst prolongation, and mGluR5 antagonist significantly blunted the maximum burst length induced by PKC. These data suggest that PKC-induced prolongation of epileptiform bursts is dependent on changes specific to mGluR5 and AMPA/kainate receptors and not mediated simply by a generalized increase in transmitter release.

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.

Expression of functional Kir6.1 channels regulates glutamate release at CA3 synapses in generation of epileptic form of seizures

The Kir6.1 channels are a subtype of ATP-sensitive inwardly rectifying potassium (K(ATP)) channels that play an essential role in coupling the cell's metabolic events to electrical activity. In this study, we show that functional Kir6.1 channels are located at excitatory pre-synaptic terminals as a complex with type-1 Sulfonylurea receptors (SUR1) in the hippocampus. The mutant mice with deficiencies in expressing the Kir6.1 or the SUR1 gene are more vulnerable to generation of epileptic form of seizures, compared to wild-type controls. Whole-cell patch clamp recordings demonstrate that genetic deletion of the Kir6.1/SUR1 channels enhances glutamate release at CA3 synapses. Hence, expression of functional Kir6.1/SUR1 channels inhibits seizure responses and possibly acts via limiting excitatory glutamate release.

Developmental changes in agonist-induced retrograde signaling at parallel fiber-Purkinje cell synapses: role of calcium-induced calcium release

In cerebellar Purkinje cells (PCs), activation of postsynaptic mGluR1 receptors inhibits parallel fiber (PF) to PC synaptic transmission by retrograde signaling. However, results were conflicting with respect to whether endocannabinoids or glutamate (Glu) is the retrograde messenger involved. Experiments in cerebellar slices from 10- to 12-day-old rats and mice confirmed that suppression of PF-excitatory postsynaptic currents (EPSCs) by mGluR1 agonists was entirely blocked by cannabinoid receptor antagonists at this early developmental stage. In contrast, suppression of PF-EPSCs by mGluR1 agonists was only partly blocked by cannabinoid receptor antagonists in 18- to 22-day-old rats, and the remaining suppression was accompanied by an increase in paired-pulse facilitation. This endocannnabinoidindependent suppression of PF-EPSCs was potentiated by the Glu uptake inhibitor D-threo-beta-benzyloxyaspartate (D-TBOA) and blocked by the desensitizing kainate (KA) receptors agonist SYM 2081, by nonsaturating concentrations of 6-cyano-7-nitroquinoxaline-2-3-dione (CNQX) [but not by GYKI 52466 hydrochloride (GYKI)] and by dialyzing PCs with guanosine 5'-[beta-thio]diphosphate (GDP-betaS). An endocannnabinoid-independent suppression of PF-EPSCs was also present in nearly mature wild-type mice but was absent in GluR6(-/-) mice. The endocannnabinoid-independent suppression of PF-EPSCs induced by mGluR1 agonists and the KA-dependent component of depolarization-induced suppression of excitation (DSE) were blocked by ryanodine acting at a presynaptic level. We conclude that retrograde release of Glu by PCs participates in mGluR1 agonist-induced suppression of PF-EPSCs at nearly mature PF-PC synapses and that Glu operates through activation of presynaptic KA receptors located on PFs and prolonged release of calcium from presynaptic internal calcium stores.

Short-term plasticity of kainate receptor-mediated EPSCs induced by NMDA receptors at hippocampal mossy fiber synapses

Kainate receptors (KARs) are heteromeric ionotropic glutamate receptors that play a variety of functions in the regulation of the activity of synaptic networks. Little is known about the regulation of the function of synaptic KARs in the brain. In the present study, we found that a conditioning activation of synaptic NMDA receptors (NMDARs) induces short-term depression of KAR-EPSCs but not of AMPA receptor-EPSCs at synapses between mossy fibers and CA3 pyramidal cells. Short-term depression of KAR-EPSCs by synaptic NMDARs peaked at 1 s and reversed within 20 s, was likely induced and expressed postsynaptically, and was homosynaptic. It depended on a rise of Ca2+ in the postsynaptic cell and on the activation of the phosphatase calcineurin that likely binds to the GluR6b (glutamate receptor subunit 6b) subunit splice variant allowing the dephosphorylation of KARs and inhibition of activity. Finally, we show in the current-clamp mode that short-term depression of KAR-EPSPs is induced by the coincident discharge of action potentials in the postsynaptic cell together with synaptic stimulation. Hence, this study describes a form of short-term synaptic plasticity that is postsynaptic, depends on the temporal order of presynaptic and postsynaptic spiking, and likely affects the summation properties of mossy fiber EPSPs.

Effects of the kainate receptor agonist ATPA on glutamatergic synaptic transmission and plasticity during early postnatal development

Kainate type of glutamate receptors (KARs) modulate synaptic transmission in a developmentally regulated manner at several synapses in the brain. Previous studies have shown that KARs depress glutamatergic transmission at CA3-CA1 synapses in the hippocampus and these receptors are tonically active during early postnatal development. Here we use the GluR5 subunit specific agonist ATPA to further characterize the role of KARs in the modulation of synaptic transmission and plasticity in area CA1 during the first two weeks of life. We find that the depressant effect of ATPA on evoked fEPSPs/EPSCs is smaller in the neonate (P3-P6) than in the juvenile (P14-P18) rat CA1, due to endogenous activity of KAR in the neonate. Further, in the neonate but not juvenile CA1, ATPA downregulates action-potential independent transmission (mEPSCs) and its effects are dependent on protein kinase C activity. ATPA-induced depression of fEPSPs in the neonate occludes the presynaptic component of long-term depression (LTD). In contrast, at P14-P18, ATPA prevents LTD indirectly via GABAergic mechanisms. These data show that GluR5 signaling mechanisms are developmentally regulated and suggest distinct functional role for KARs in the modulation of synaptic transmission and plasticity at different stages of development.

Homer 1a uncouples metabotropic glutamate receptor 5 from postsynaptic effectors

Metabotropic glutamate receptors (mGluRs) and Homer proteins play critical roles in neuronal functions including plasticity, nociception, epilepsy, and drug addiction. Furthermore, Homer proteins regulate mGluR1/5 function by acting as adapters and facilitating coupling to effectors such as the inositol triphosphate receptor. However, although Homer proteins and their interaction with mGluRs have been the subject of intense study, direct measurements of Homer-induced changes in postsynaptic mGluR-effector coupling have not been reported. This question was addressed here by examining glutamatergic excitatory postsynaptic currents (EPSCs) in rat autaptic hippocampal cultures. In most neurons, the group I mGluR agonist (S)-3,5-dihydroxyphenylglycine strongly inhibited the EPSC acutely. This modulation occurred postsynaptically, was mediated primarily by mGluR5, and was inositol triphosphate receptor-dependent. Expression of the dominant negative, immediate early form of Homer, Homer 1a, strongly reduced EPSC modulation, but the W24A mutant of Homer 1a, which cannot bind mGluRs, had no effect. (S)-3,5-dihydroxyphenylglycine-mediated intracellular calcium responses in the processes of Homer 1a-expressing neurons were reduced compared with those in Homer 1a W24A-expressing cells. However, neither the distribution of mGluR5 nor the modulation of somatic calcium channels was altered by Homer 1a expression. These data demonstrate that Homer 1a can reduce mGluR5 coupling to postsynaptic effectors without relying on large changes in the subcellular distribution of the receptor. Thus, alteration of mGluR signaling by changes in Homer protein expression may represent a viable mechanism for fine-tuning synaptic strength in neurons.

mGluR5 is involved in dendrite differentiation and excitatory synaptic transmission in NTERA2 human embryonic carcinoma cell-derived neurons

The pluripotent human embryonic carcinoma cell line NTERA2 readily differentiates into neurons when exposed to retinoic acid in vitro. These neurons show characteristic morphology with long processes and they express neuronal markers TUJ-1 and NeuN. NTERA2-derived neurons can regulate Ca2+ signalling through ionotropic glutamate (iGluR) and muscarinic receptors (mAChRs). Little is known, however, about the role of metabotropic glutamate receptors (mGluRs) in these neurons. Here we show that NTERA2-derived neurons express functional mGluR5, which is involved in Ca2+ signalling. Blocking mGluR5 activity at early stages of differentiation leads to fewer dendrites and a reduction in miniature excitatory postsynaptic currents (mEPSCs). Furthermore, cells cultured in the presence of the mGluR5 antagonist 2-methyl-6-(phenylethynyl)pyridine (MPEP) show reduced N-methyl-D-aspartate (NMDA) receptor-mediated Ca2+ mobilisation but increased alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor Ca2+ permeability. During normal neuronal development, the edited GluR2 renders AMPARs Ca2+ impermeable. The increased Ca2+ permeability of AMPARs in MPEP-treated neurons is due to the reduced expression of GluR2 subunit protein. Thus, mGluR5 activity at early stages of differentiation is likely to play a role in the development of multipotent cell-derived neurons.

Synaptic plasticity of kainate receptors

Synaptic plasticity of ionotropic glutamate receptors has been extensively studied with a particular focus on the role played by NMDA (N-methyl-D-aspartate) receptors in the induction of synaptic plasticity and the subsequent movement of AMPA (alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid) receptors. The third subtype of ionotropic glutamate receptor, kainate receptors, has not been studied to the same extent, but recent evidence shows that these receptors also exhibit synaptic plasticity in response to activity. There is also a growing body of data on the mechanisms underlying kainate receptor trafficking and the proteins they interact with. This review summarizes the current state of knowledge on this topic, focusing on the evidence for the removal or insertion of functional kainate receptors in response to synaptic activity and the cellular mechanisms that underlie this regulation of neuronal kainate receptor function.

Developmental changes in retrograde messengers involved in depolarization-induced suppression of excitation at parallel fiber-Purkinje cell synapses in rodents

At parallel fiber (PF) to Purkinje cell (PC) synapses, depolarization-induced suppression of excitation (DSE) and suppression of PF-excitatory postsynaptic currents (EPSCs) by activation of postsynaptic mGluR1 glutamate (Glu) receptors involve retrograde release of endocannabinoids. However, Levenes et al. suggested instead that Glu was the retrograde messenger in this latter case. Because the study by Levenes et al. was performed in nearly mature rats, whereas most others were performed in juvenile animals, DSE was re-investigated in juvenile versus nearly mature rats and mice. Indeed, DSE was preferred here to agonist-induced suppression of PF-EPSCs, to avoid possible indirect effects in this latter case. In 10- to 12-day-old rats, DSE of PF-EPSCs was entirely mediated through retrograde release of endocannabinoids. In 18- to 22-day-old-rats, DSE was partly resistant to CB1 cannabinoid receptor antagonists. The remaining component was potentiated by the Glu uptake inhibitor d-threo-beta-benzyloxyaspartate (d-TBOA) and blocked by the desensitizing kainate (KA) receptor agonist (2S,4R)-4-methylglutamic acid (SYM 2081). This SYM-2081-sensitive component of DSE was accompanied by a paired-pulse facilitation increase that was also potentiated by d-TBOA and blocked by SYM 2081. In nearly mature wild-type and GluR6 -/- mice, results fully confirmed the presence of an endocannabinoid-independent component of DSE that involves retrograde release of Glu and activation of presynaptic KA receptors including GluR6 receptor subunits. Therefore retrograde release of Glu by PCs participates to DSE at PF-PC synapses in nearly mature rodents but not in juvenile ones, and Glu probably operates through activation of presynaptic KA receptors that include GluR6 receptor subunits.

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).

Differential activation of protein kinase C isoforms following chemical ischemia in rat cerebral cortex slices

The aim of the current study was to characterize the effects of chemical ischemia and reperfusion at the transductional level in the brain. Protein kinase C isoforms (alpha, beta(1), beta(2), gamma, delta and epsilon) total levels and their distribution in the particulate and cytosolic compartments were investigated in superfused rat cerebral cortex slices: (i) under control conditions; (ii) immediately after a 5-min treatment with 10mM NaN(3), combined with 2mM 2-deoxyglucose (chemical ischemia); (iii) 1h after chemical ischemia (reperfusion). In control samples, all the PKC isoforms were detected; immediately after chemical ischemia, PKC beta(1), delta and epsilon isoforms total levels (cytosol+particulate) were increased by 2.9, 2.7 and 9.9 times, respectively, while alpha isoform was slightly reduced and gamma isoform was no longer detectable. After reperfusion, the changes displayed by alpha, beta(1), gamma, delta and epsilon were maintained and even potentiated, moreover, an increase in beta(2) (by 41+/-12%) total levels became significant. Chemical ischemia-induced a significant translocation to the particulate compartment of PKC alpha isoform, which following reperfusion was found only in the cytosol. PKC beta(1) and delta isoforms particulate levels were significantly higher both in ischemic and in reperfused samples than in the controls. Conversely, following reperfusion, PKC beta(2) and epsilon isoforms displayed a reduction in their particulate to total level ratios. The intracellular calcium chelator, 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, 1mM, but not the N-methyl-d-asparate receptor antagonist, MK-801, 1muM, prevented the translocation of beta(1) isoform observed during ischemia. Both drugs were effective in counteracting reperfusion-induced changes in beta(2) and epsilon isoforms, suggesting the involvement of glutamate-induced calcium overload. These findings demonstrate that: (i) PKC isoforms participate differently in neurotoxicity/neuroprotection events; (ii) the changes observed following chemical ischemia are pharmacologically modulable; (iii) the protocol of in vitro chemical ischemia is suitable for drug screening.

Kainate receptor-interacting proteins and membrane trafficking

Kainate receptors are composed of several subunits and splice variants, but the relevance of this diversity is still not well understood. The subunits and splice variants show great divergence in their C-terminal cytoplasmic tail region, which has been identified as a region of interaction with a number of protein partners. Differential trafficking of kainate receptors to neuronal compartments is likely to rely on interactions with distinct subsets of protein partners. This review summarizes our knowledge of the regulation of trafficking of kainate receptors and focuses on the identification and characterization of functions of interacting partners.

Synaptic strength at the thalamocortical input to layer IV neonatal barrel cortex is regulated by protein kinase C

Long-term synaptic plasticity is an important mechanism underlying the development of cortical circuits in a number of brain regions. In barrel cortex NMDA receptor (NMDAR)-dependent long-term potentiation (LTP) and long-term depression (LTD) play a critical role in the development and experience-dependent plasticity of the topographical map of the rodent whiskers. However, the mechanisms underlying the induction and expression of these forms of plasticity are poorly characterised. Here we investigate the role of PKC in the regulation of synaptic strength in neonatal barrel cortex using patch-clamp recordings in brain slices. We demonstrate that PKC activity tonically maintains AMPA receptor-mediated transmission at thalamocortical synapses, and that basal transmission can be potentiated by PKC activation using postsynaptic infusion of phorbol ester. Furthermore, we show that induction of NMDAR-dependent LTP requires PKC activity. These findings demonstrate that PKC is required for the regulation of transmission at thalamocortical synapses, the major ascending sensory input to barrel cortex. Thalamocortical inputs in barrel cortex only express LTP during the first postnatal week during a critical period for experience-dependent plasticity in layer IV. Therefore, the requirement for PKC in LTP suggests an important role for this kinase in the development of the barrel cortex sensory map.

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.

Different composition of glutamate receptors in corticothalamic and lemniscal synaptic responses and their roles in the firing responses of ventrobasal thalamic neurons in juvenile mice

Thalamic ventrobasal (VB) relay neurons receive information via two major types of glutamatergic synapses, that is, from the medial lemniscus (lemniscal synapses) and primary somatosensory cortex (corticothalamic synapses). These two synapses influence and coordinate firing responses of VB neurons, but their precise operational mechanisms are not yet well understood. In this study, we compared the composition of glutamate receptors and synaptic properties of corticothalamic and lemniscal synapses. We found that the relative contribution of NMDA receptor-mediated excitatory postsynaptic currents (EPSCs) to non-NMDA receptor-mediated EPSCs was significantly greater in corticothalamic synapses than in lemniscal synapses. Furthermore, NMDA receptor 2B-containing NMDA receptor- and kainate receptor-mediated currents were observed only in corticothalamic synapses, but not in lemniscal synapses. EPSCs in corticothalamic synapses displayed the postsynaptic summation in a frequency-dependent manner, in which the summation of the NMDA receptor-mediated component was largely involved. The summation of kainate receptor-mediated currents also partially contributed to the postsynaptic summation in corticothalamic synapses. In contrast, the contribution of NMDA receptor-mediated currents to the postsynaptic summation of lemniscal EPSCs was relatively minor. Furthermore, our results indicated that the prominent NMDA receptor-mediated component in corticothalamic synapses was the key determinant for the late-persistent firing of VB neurons in response to corticothalamic stimuli. In lemniscal synapses, in contrast, the onset-transient firing in response to lemniscal stimuli was regulated mainly by AMPA receptors.

Protein-protein coupling/uncoupling enables dopamine D2 receptor regulation of AMPA receptor-mediated excitotoxicity

here is considerable evidence that dopamine D2 receptors can modulate AMPA receptor-mediated neurotoxicity. However, the molecular mechanism underlying this process remains essentially unclear. Here we report that D2 receptors inhibit AMPA-mediated neurotoxicity through two pathways: the activation of phosphoinositide-3 kinase (PI-3K) and downregulation of AMPA receptor plasma membrane expression, both involving a series of protein-protein coupling/uncoupling events. Agonist stimulation of D2 receptors promotes the formation of the direct protein-protein interaction between the third intracellular loop of the D2 receptor and the ATPase N-ethylmaleimide-sensitive factor (NSF) while uncoupling the NSF interaction with the carboxyl tail (CT) of the glutamate receptor GluR2 subunit of AMPA receptors. Previous studies have shown that full-length NSF directly couples to the GluR2CT and facilitates AMPA receptor plasma membrane expression. Furthermore, the CT region of GluR2 subunit is also responsible for several other intracellular protein couplings, including p85 subunit of PI-3K. Therefore, the direct coupling of D2-NSF and concomitant decrease in the NSF-GluR2 interaction results in a decrease of AMPA receptor membrane expression and an increase in the interaction between GluR2 and the p85 and subsequent activation of PI-3K. Disruption of the D2-NSF interaction abolished the ability of D2 receptor to attenuate AMPA-mediated neurotoxicity by blocking the D2 activation-induced changes in PI-3K activity and AMPA receptor plasma membrane expression. Furthermore, the D2-NSF-GluR2-p85 interactions are also responsible for the D2 inhibition of ischemia-induced cell death. These data may provide a new avenue to identify specific targets for therapeutics to modulate glutamate receptor-governed diseases, such as stroke.

Surface clustering of metabotropic glutamate receptor 1 induced by long Homer proteins

Background

Metabotropic glutamate receptors (mGluRs) regulate neuronal excitability and synaptic strength. The group I mGluRs, mGluR1 and 5, are widespread in the brain and localize to post-synaptic sites. The Homer protein family regulates group I mGluR function and distribution. Constitutively expressed 'long' Homer proteins (Homer 1b, 1c, 2 and 3) induce dendritic localization of group I mGluRs and receptor clustering, either internally or on the plasma membrane. Short Homer proteins (Homer 1a, Ania-3) exhibit regulated expression and act as dominant negatives, producing effects on mGluR distribution and function that oppose those of the long Homer proteins.

There remains some controversy over whether long Homer proteins induce receptor internalization by inducing retention in the endoplasmic reticulum, or induce mGluR clustering on the plasma membrane. Further, an exhaustive study of the effects of each long Homer isoform on mGluR distribution has not been published.

Results

The distribution of a GFP-tagged group I mGluR, mGluR1-GFP, was examined in the absence of Homer proteins and in the presence of several Homer isoforms expressed in sympathetic neurons from the rat superior cervical ganglion (SCG) using total internal reflection fluorescence (TIRF-M) and confocal microscopy. Quantitative analysis of mGluR1-GFP fluorescence using TIRF-M revealed that expression of each long Homer isoform tested (Homer 1b, 1c, 2b and 3) induced a significant degree of surface clustering. Using confocal imaging, Homer-induced mGluR clusters were observed intra-cellularly as well as on the plasma membrane. Further, in approximately 40% of neurons co-expressing mGluR1-GFP and Homer 1b, intracellular inclusions were observed, but plasma membrane clusters were also documented in some Homer 1b coexpressing cells.

Conclusion

All long Homer proteins examined (Homer 1b, 1c, 2b and 3) induced a significant degree of mGluR1-GFP clustering on the plasma membrane compared to cells expressing mGluR1-GFP alone. Clusters induced by long Homers appeared on the plasma membrane and intracellularly, suggesting that clusters form prior to plasma membrane insertion and/or persist after internalization. Finally, while Homer 1b induced surface clustering of mGluR1 in some cells, under some conditions intracellular retention may occur.

Long-Term Depression of Kainate Receptor-Mediated Synaptic Transmission

Kainate receptors (KARs) have been shown to be involved in hippocampal mossy fiber long-term potentiation (LTP); however, it is not known if KARs are involved in the induction or expression of long-term depression (LTD), the other major form of long-term synaptic plasticity. Here we describe LTD of KAR-mediated synaptic transmission (EPSC(KA) LTD) in perirhinal cortex layer II/III neurons that is distinct from LTD of AMPAR-mediated transmission, which also coexists at the same synapses. Induction of EPSC(KA) LTD requires a rise in postsynaptic Ca(2+) but is independent of NMDARs or T-type voltage-gated Ca(2+) channels; however, it requires synaptic activation of inwardly rectifying KARs and release of Ca(2+) from stores. The synaptic KARs are regulated by tonically activated mGluR5, and expression of EPSC(KA) LTD occurs via a mechanism involving mGluR5, PKC, and PICK1 PDZ domain interactions. Thus, we describe the induction and expression mechanism of a form of synaptic plasticity, EPSC(KA) LTD.

Kainate-induced excitation and sensitization of nociceptors in normal and inflamed rat glabrous skin

This study investigates contributions of peripheral kainate receptors to acute nociception and persistent inflammatory pain in rat. Immunohistochemical analysis of kainate receptor expression using antibodies recognizing glutamate receptor subunits 5, 6, and 7 demonstrates that 28% of unmyelinated axons in normal digital nerve are positively labeled. Following intraplantar injection of complete Freund's adjuvant, a significant increase in glutamate receptor subunits 5, 6, and 7-labeled axons occurs at 2 days (40%), but not 7 (31%) or 14 days (28%) post-complete Freund's adjuvant. In behavioral studies, we confirm an increased mechanical sensitivity in complete Freund's adjuvant-injected hind paws. Furthermore, activation of kainate receptors following intraplantar injection of 1.0 mM kainate in normal animals results in a mechanical sensitivity similar to that observed in inflamed animals. A 1.0 mM kainate injection into inflamed hind paws further enhances the mechanical sensitivity. Injection of the non-N-methyl-D-aspartate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (0.1 mM) reverses complete Freund's adjuvant-induced mechanical sensitivity through a local effect. In single unit recordings from nociceptors in a glabrous skin-nerve preparation, mechanical sensitization is present in inflamed skin evidenced by a decrease in mechanical threshold and an increase in discharge rate during a suprathreshold, constant force stimulus. Thermal sensitization is also present evidenced by a decrease in heat threshold. There is a dose-dependent increase in kainate-induced nociceptor activity in both normal and inflamed skin but the kainate required to induce activation is reduced in inflamed skin. Although proportions of kainate-activated nociceptors are the same in normal and inflamed skin, the kainate-induced mean discharge rate is significantly enhanced in inflamed skin. Exposure of normal and inflamed nociceptors to 0.3 mM kainate sensitizes fibers to re-application of kainate and heat. This sensitization is blocked in the presence of 6-cyano-7-nitroquinoxaline-2,3-dione or the glutamate receptor subunit 5 selective antagonist 3S,4aR,6S,8aR-6-[4-carboxy-phenyl] methyl-1,2,3,4,4a,5,6,7,8,8a-deca-hydroisoquinoline-3-carboxylic acid. The data indicate that peripheral kainate receptors not only play an important role in normal nociception but also contribute to mechanical sensitivity and heat sensitization accompanying inflammatory pain.

Long-term potentiation of neuronal glutamate transporters

Persistent, use-dependent modulation of synaptic strength has been demonstrated for fast synaptic transmission mediated by glutamate and has been hypothesized to underlie persistent behavioral changes ranging from memory to addiction. Glutamate released at synapses is sequestered by the action of excitatory amino acid transporters (EAATs) in glia and postsynaptic neurons. So, the efficacy of glutamate transporter function is crucial for regulating glutamate spillover to adjacent presynaptic and postsynaptic receptors and the consequent induction of plastic or excitotoxic processes. Here, we report that tetanic stimulation of cerebellar climbing fiber-Purkinje cell synapses results in long-term potentiation (LTP) of a climbing fiber-evoked glutamate transporter current recorded in Purkinje cells. This LTP is postsynaptically expressed and requires activation of an mGluR1/PKC cascade. Together with a simultaneously induced long-term depression (LTD) of postsynaptic AMPA receptors, this might reflect an integrated antiexcitotoxic cellular response to strong climbing fiber synaptic activation, as occurs following an ischemic episode.

Activity-dependent endocytic sorting of kainate receptors to recycling or degradation pathways

Kainate receptors (KARs) play important roles in the modulation of neurotransmission and plasticity, but the mechanisms that regulate their surface expression and endocytic sorting remain largely unknown. Here, we show that in cultured hippocampal neurons the surface expression of GluR6-containing KARs is dynamically regulated. Furthermore, internalized KARs are sorted into recycling or degradative pathways depending on the endocytotic stimulus. Kainate activation causes a Ca2+- and PKA-independent but PKC-dependent internalization of KARs that are targeted to lysosomes for degradation. In contrast, NMDAR activation evokes a Ca2+-, PKA- and PKC-dependent endocytosis of KARs to early endosomes with subsequent reinsertion back into the plasma membrane. These results demonstrate that GluR6-containing KARs are subject to activity-dependent endocytic sorting, a process that provides a mechanism for both rapid and chronic changes in the number of functional receptors.

Cell surface expression of GluR5 kainate receptors is regulated by an endoplasmic reticulum retention signal

Kainate receptors (KARs) are mediators of excitatory neurotransmission in the mammalian central nervous system, and their efficient targeting and trafficking is critical for normal synaptic function. A key step in the delivery of KARs to the neuronal plasma membrane is the exit of newly assembled receptors from the endoplasmic reticulum (ER). Here we report the identification of a novel ER retention signal in the alternatively spliced C-terminal domain of the GluR5-2b subunit, which controls receptor trafficking in both heterologous cells and neurons. The ER retention motif consists of a critical arginine (Arg-896) and surrounding amino acids, disruption of which promotes ER exit and surface expression of the receptors, as well as altering their physiological properties. The Arg-896-mediated ER retention of GluR5 is regulated by a mutation that mimics phosphorylation of Thr-898, but not by PDZ interactions. Furthermore, two positively charged residues (Arg-900 and Lys-901) in the C terminus were also found to regulate ER export of the receptors. Taken together, our results identify novel trafficking signals in the C-terminal domain of GluR5-2b and demonstrate that alternative splicing is an important mechanism regulating KAR function.