Synaptic activation of a presynaptic kainate receptor facilitates AMPA receptor-mediated synaptic transmission at hippocampal mossy fibre synapses

GluK2 Q/R editing regulates kainate receptor signaling and long-term potentiation of AMPA receptors

Q/R editing of the kainate receptor (KAR) subunit GluK2 radically alters recombinant KAR properties, but the effects on endogenous KARs in vivo remain largely unexplored. Here, we compared GluK2 editing-deficient mice that express ∼95% unedited GluK2(Q) to wild-type counterparts that express ∼85% edited GluK2(R). At mossy fiber-CA3 (MF-CA3) synapses GluK2(Q) mice displayed increased postsynaptic KAR function and KAR-mediated presynaptic facilitation, demonstrating enhanced ionotropic function. Conversely, GluK2(Q) mice exhibited reduced metabotropic KAR function, assessed by KAR-mediated inhibition of slow after-hyperpolarization currents (ISAHP). GluK2(Q) mice also had fewer GluA1-and GluA3-containing AMPA receptors (AMPARs) and reduced postsynaptic AMPAR currents at both MF-CA3 and CA1-Schaffer collateral synapses. Moreover, long-term potentiation of AMPAR-mediated transmission at CA1-Schaffer collateral synapses was reduced in GluK2(Q) mice. These findings suggest that GluK2 Q/R editing influences ionotropic/metabotropic balance of KAR signaling to regulate synaptic expression of AMPARs and plasticity.

Whole-Transcriptome Analysis of Dermal Fibroblasts, Derived from Three Pairs of Monozygotic Twins, Discordant for Parkinson's Disease

Parkinson's disease (PD) is one of the most common neurodegenerative diseases. In most cases, the development of the disease is sporadic and is not associated with any currently known mutations associated with PD. It is believed that changes associated with the epigenetic regulation of gene expression may play an important role in the pathogenesis of this disease. The study of individuals with an almost identical genetic background, such as monozygotic twins, is one of the best approaches to the analysis of such changes. A whole-transcriptome analysis of dermal fibroblasts obtained from three pairs of monozygotic twins discordant for PD was carried out in this work. Twenty-nine differentially expressed genes were identified in the three pairs of twins. These genes were included in seven processes within two clusters, according to the results of an enrichment analysis. The cluster with the greatest statistical significance included processes associated with the regulation of the differentiation of fat cells, the action potential, and the regulation of glutamatergic synaptic transmission. The most significant genes, which occupied a central position in this cluster, were PTGS2, SCN9A, and GRIK2. These genes can be considered as potential candidate genes for PD.

Kainate Receptors, Homeostatic Gatekeepers of Synaptic Plasticity

Extensive research over the past decades has characterized multiple forms of synaptic plasticity, identifying them as key processes that allow the brain to operate in a dynamic manner. Within the wide variety of synaptic plasticity modulators, kainate receptors are receiving increasing attention, given their diversity of signaling mechanisms and cellular expression profile. Here, we summarize the experimental evidence about the involvement of kainate receptor signaling in the regulation of short- and long-term plasticity, from the perspective of the regulation of neurotransmitter release. In light of this evidence, we propose that kainate receptors may be considered homeostatic modulators of neurotransmitter release, able to bidirectionally regulate plasticity depending on the functional history of the synapse.

Differential Effect on Hippocampal Synaptic Facilitation by the Presynaptic Protein Mover

Neurotransmitter release relies on an evolutionarily conserved presynaptic machinery. Nonetheless, some proteins occur in certain species and synapses, and are absent in others, indicating that they may have modulatory roles. How such proteins expand the power or versatility of the core release machinery is unclear. The presynaptic protein Mover/TPRGL/SVAP30 is heterogeneously expressed among synapses of the rodent brain, suggesting that it may add special functions to subtypes of presynaptic terminals. Mover is a synaptic vesicle-attached phosphoprotein that binds to Calmodulin and the active zone scaffolding protein Bassoon. Here we use a Mover knockout mouse line to investigate the role of Mover in the hippocampal mossy fiber (MF) to CA3 pyramidal cell synapse and Schaffer collateral to CA1. While Schaffer collateral synapses were unchanged by the knockout, the MFs showed strongly increased facilitation. The effect of Mover knockout in facilitation was both calcium- and age-dependent, having a stronger effect at higher calcium concentrations and in younger animals. Increasing cyclic adenosine monophosphate (cAMP) levels by forskolin equally potentiated both wildtype and knockout MF synapses, but occluded the increased facilitation observed in the knockout. These discoveries suggest that Mover has distinct roles at different synapses. At MF terminals, it acts to constrain the extent of presynaptic facilitation.

Kainate Receptor-Mediated Depression of Glutamate Release Involves Protein Kinase A in the Cerebellum

Kainate (KA) receptors (KAR) have important modulatory roles of synaptic transmission. In the cerebellum, the action mechanisms of KAR-mediated glutamatergic depression are unknown. We studied these mechanisms by recording evoked excitatory postsynaptic currents (eEPSCs) from cerebellar slices using the whole-cell configuration of the patch-clamp technique. We observed that 3 μM KA decreased the amplitude of eEPSCs and increased the number of failures at the synapses established between parallel fibers (PF) and Purkinje neurons, and the effect was antagonized by NBQX under the condition where AMPA receptors were previously blocked. The inhibition of protein kinase A (PKA) suppressed the effect of KAR activation on eEPSC, and effect was not prevented by protein kinase C inhibitors. Furthermore, in the presence of Pertussis toxin, the depression of glutamate release mediated by KAR activation was prevented, invoking the participation of a G_i/o protein in this modulation. Finally, the KAR-mediated depression of glutamate release was not prevented by blocking calcium-permeable KARs or by treatments that affect calcium release from intracellular stores. We conclude that KARs present at these synapses mediate an inhibition of glutamate release through a mechanism that involves the activation of G-protein and protein kinase A.

Kainate Receptors Play a Role in Modulating Synaptic Transmission in the Olfactory Bulb

Glutamate is the neurotransmitter used at most excitatory synapses in the mammalian brain, including those in the olfactory bulb (OB). There, ionotropic glutamate receptors including N-methyl-d-aspartate receptors (NMDARs) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) play a role in processes such as reciprocal inhibition and glomerular synchronization. Kainate receptors (KARs) represent another type of ionotropic glutamate receptor, which are composed of five (GluK1-GluK5) subunits. Whereas KARs appear to be heterogeneously expressed in the OB, evidence as to whether these KARs are functional, found at synapses, or modify synaptic transmission is limited. In the present study, coapplication of KAR agonists (kainate, SYM 2081) and AMPAR antagonists (GYKI 52466, SYM 2206) demonstrated that functional KARs are expressed by OB neurons, with a subset of receptors located at synapses. Application of kainate and the GluK1-selective agonist ATPA had modulatory effects on excitatory postsynaptic currents (EPSCs) evoked by stimulation of the olfactory nerve layer. Application of kainate and ATPA also had modulatory effects on reciprocal inhibitory postsynaptic currents (IPSCs) evoked using a protocol that evokes dendrodendritic inhibition. The latter finding suggests that KARs, with relatively slow kinetics, may play a role in circuits in which the relatively brief duration of AMPAR-mediated currents limits the role of AMPARs in synaptic transmission (e.g., reciprocal inhibition at dendrodendritic synapses). Collectively, our findings suggest that KARs, including those containing the GluK1 subunit, modulate excitatory and inhibitory transmission in the OB. These data further suggest that KARs participate in the regulation of synaptic circuits that encode odor information.

Cerebellar Kainate Receptor-Mediated Facilitation of Glutamate Release Requires Ca2+-Calmodulin and PKA

We elucidated the mechanisms underlying the kainate receptor (KAR)-mediated facilitatory modulation of synaptic transmission in the cerebellum. In cerebellar slices, KA (3 μM) increased the amplitude of evoked excitatory postsynaptic currents (eEPSCs) at synapses between axon terminals of parallel fibers (PF) and Purkinje neurons. KA-mediated facilitation was antagonized by NBQX under condition where AMPA receptors were previously antagonized. Inhibition of protein kinase A (PKA) suppressed the effect of KA on glutamate release, which was also obviated by the prior stimulation of adenylyl cyclase (AC). KAR-mediated facilitation of synaptic transmission was prevented by blocking Ca²⁺ permeant KARs using philanthotoxin. Furthermore, depletion of intracellular Ca²⁺ stores by thapsigargin, or inhibition of Ca²⁺-induced Ca²⁺-release by ryanodine, abrogated the synaptic facilitation by KA. Thus, the KA-mediated modulation was conditional on extracellular Ca²⁺ entry through Ca²⁺-permeable KARs, as well as and mobilization of Ca²⁺ from intracellular stores. Finally, KAR-mediated facilitation was sensitive to calmodulin inhibitors, W-7 and calmidazolium, indicating that the increased cytosolic [Ca²⁺] sustaining KAR-mediated facilitation of synaptic transmission operates through a downstream Ca²⁺/calmodulin coupling. We conclude that, at cerebellar parallel fiber-Purkinje cell synapses, presynaptic KARs mediate glutamate release facilitation, and thereby enhance synaptic transmission through Ca²⁺-calmodulin dependent activation of adenylyl cyclase/cAMP/protein kinase A signaling.

Non-canonical Mechanisms of Presynaptic Kainate Receptors Controlling Glutamate Release

A metabotropic modus operandi for kainate receptors (KARs) was first discovered in 1998 modulating GABA release. These receptors have been also found to modulate glutamate release at different synapses in several brain regions. Mechanistically, a general biphasic mechanism for modulating glutamate release by presynaptic KARs with metabotropic actions has emerged, with low KA concentrations invoking an increase in glutamate release, whereas higher concentrations of KA mediate a decrease in the release of this neurotransmitter. The molecular mechanisms underpinning the opposite modulation of glutamate release are distinct, with a G-protein-independent, adenylate cyclase (AC)- and protein kinase A (PKA)-dependent mechanism mediating the facilitation of glutamate release, while a G-protein dependent mechanism (with or without protein kinase recruitment) is involved in the decrease of neurotransmitter release. In the present review, we revisit the mechanisms underlying the non-canonical modus operandi of KARs effecting the bimodal control of glutamatergic transmission in different brain regions, and address the possible functions that this modulation may support.

Aging-related impairments of hippocampal mossy fibers synapses on CA3 pyramidal cells

The network interaction between the dentate gyrus and area CA3 of the hippocampus is responsible for pattern separation, a process that underlies the formation of new memories, and which is naturally diminished in the aged brain. At the cellular level, aging is accompanied by a progression of biochemical modifications that ultimately affects its ability to generate and consolidate long-term potentiation. Although the synapse between dentate gyrus via the mossy fibers (MFs) onto CA3 neurons has been subject of extensive studies, the question of how aging affects the MF-CA3 synapse is still unsolved. Extracellular and whole-cell recordings from acute hippocampal slices of aged Wistar rats (34 ± 2 months old) show that aging is accompanied by a reduction in the interneuron-mediated inhibitory mechanisms of area CA3. Several MF-mediated forms of short-term plasticity, MF long-term potentiation and at least one of the critical signaling cascades necessary for potentiation are also compromised in the aged brain. An analysis of the spontaneous glutamatergic and gamma-aminobutyric acid-mediated currents on CA3 cells reveal a dramatic alteration in amplitude and frequency of the nonevoked events. CA3 cells also exhibited increased intrinsic excitability. Together, these results demonstrate that aging is accompanied by a decrease in the GABAergic inhibition, reduced expression of short- and long-term forms of synaptic plasticity, and increased intrinsic excitability.

SIRT3 overexpression antagonizes high glucose accelerated cellular senescence in human diploid fibroblasts via the SIRT3-FOXO1 signaling pathway

Sirtuin 3 (SIRT3) is one of the seven mammalian sirtuins, which are homologs of the yeast Sir2 gene. SIRT3 is the only sirtuin reported to be associated with human life span. Many recent studies have indicated that SIRT3 levels are elevated by exercise and caloric restriction, but whether SIRT3 influences cell senescence under stressed conditions in human diploid fibroblasts has not been established. Our data showed that expression of SIRT3 is elevated in human diploid fibroblasts under low glucose (3.3 mM glucose) growth conditions and decreased under high glucose (25 mM glucose) growth conditions. We have demonstrated that SIRT3 interacts with forkhead box protein O1 (FOXO1). High glucose levels also increased aging phenotypes and FOXO1 acetylation level. We have demonstrated that overexpression of SIRT3 under high glucose conditions reduces FOXO1 acetylation, suggesting that deacetylation of FOXO1 by SIRT3 elevates the expression of the FOXO1 target genes, catalase, and manganese superoxide dismutase (MnSOD) while decreasing senescence phenotypes. We studied the effects of SIRT3 protein knockdown by shRNA under low glucose conditions. The data showed that shRNA-SIRT3 accelerated senescence phenotypes and acetylation of FOXO1; the expression level of catalase and MnSOD decreased compared with the control group. As a consequence, SIRT3 antagonized cellular senescence with the characteristic features of delayed SA-β-gal staining, senescence-associated heterochromatin foci (SAHF) formation, and p16(INK4A) expression. These results demonstrate for the first time that SIRT3 overexpression antagonizes high glucose-induced cellular senescence in human diploid fibroblasts via the SIRT3-FOXO1 signaling pathway.

Presynaptic kainate receptor-mediated bidirectional modulatory actions: mechanisms

Kainate receptors (KARs) are members of the glutamate receptor family, which also includes two other ionotropic subtypes, i.e. NMDA- and AMPA-type receptors, and types I, II and III metabotropic glutamate receptors. KARs mediate synaptic transmission postynaptically through their ionotropic capacity, while presynaptically, they modulate the release of both GABA and glutamate through operationally diverse modus operandi. At hippocampal mossy fiber (MF)-CA3 synapses, KARs have a biphasic effect on glutamate release, such that, depending on the extent of their activation, a facilitation or depression of glutamate release can be observed. This modulation is posited to contribute to important roles of KARs in short- and long-term plasticity. Elucidation of the modes of action of KARs in their depression and facilitation of glutamate release is beginning to gather impetus. Here we will focus on the cellular mechanisms involved in the modulation of glutamate release by presynaptic KAR activation at MF-CA3 synapses, a field that has seen significant progress in recent years.

Differences in kainate receptor involvement in hippocampal mossy fibre long-term potentiation depending on slice orientation

Long-term potentiation (LTP) is a well-established experimental model used to investigate the synaptic basis of learning and memory. LTP at mossy fibre - CA3 synapses in the hippocampus is unusual because it is normally N-methyl-d-aspartate (NMDA) receptor-independent. Instead it seems that the trigger for mossy fibre LTP involves kainate receptors (KARs). Although it is generally accepted that pre-synaptic KARs play an essential role in frequency facilitation and LTP, their subunit composition remains a matter of significant controversy. We have reported previously that both frequency facilitation and LTP can be blocked by selective antagonism of GluK1 (formerly GluR5/Glu(K5))-containing KARs, but other groups have failed to reproduce this effect. Moreover, data from receptor knockout and mRNA expression studies argue against a major role of GluK1, supporting a more central role for GluK2 (formerly GluR6/Glu(K6)). A potential reason underlying the controversy in the pharmacological experiments may reside in differences in the preparations used. Here we show differences in pharmacological sensitivity of synaptic plasticity at mossy fibre - CA3 synapses depend critically on slice orientation. In transverse slices, LTP of fEPSPs was invariably resistant to GluK1-selective antagonists whereas in parasagittal slices LTP was consistently blocked by GluK1-selective antagonists. In addition, there were pronounced differences in the magnitude of frequency facilitation and the sensitivity to the mGlu2/3 receptor agonist DCG-IV. Using anterograde labelling of granule cells we show that slices of both orientations possess intact mossy fibres and both large and small presynaptic boutons. Transverse slices have denser fibre tracts but a smaller proportion of giant mossy fibre boutons. These results further demonstrate a considerable heterogeneity in the functional properties of the mossy fibre projection.

Kainate receptors with a metabotropic signature enhance hippocampal excitability by regulating the slow after-hyperpolarization in CA3 pyramidal neurons

Most of our knowledge of the synaptic function of kainate receptors stems from a detailed analysis of synaptic transmission between dentate granule cells and CA3 pyramidal neurons, where kainate receptors mediate a slow excitatory current with integrative properties ideally suited for repetitive neuronal firing. Besides this well characterized ionotropic effect of kainate receptors, they can also enhance neuronal excitability by inhibiting the slow Ca(2+) activated K(+) current I(sAHP) via a G-protein coupled mechanism. This phenomenon is associated with Ca(2+) mobilization and protein-kinase activation and ultimately leads to modulation of ion channels responsible for intrinsic electrical properties such as firing adaptation. The significance for CNS function of these newly emerging metabotropic kainate receptors is poorly understood and as yet proteomic analysis of kainate receptors has yielded little information on signaling molecules associated with the kainate receptor ionophore. This chapter covers the key findings that have led to the proposal that high-affinity postsynaptic kainate receptors trigger a form of metabotropic signaling regulating I(sAH P) and neuronal firing in CA3 hippocampal neurons.

Metabotropic actions of kainate receptors in the control of glutamate release in the hippocampus

Kainate-type glutamate receptors (KARs) structurally present the credentials of the other ionotropic glutamate receptor (iGluR) family members (NMDA and AMPA receptors), but functionally often purport examples of a metabotropic mode of operation. In the present chapter, we describe these metabotropic roles of KARs in the modulation of glutamate release in the hippocampus at CA3 Schaffer Collateral (SC)-CA1 Pyramidal Cell (PC) synapses and dentate gyrus granule cell Mossy Fiber (MF)-CA3 PC synapses. As autoreceptors on SC terminals, KARs inhibit the release of glutamate at SC-CA1 PC synapses through a mechanism dependent on a pertussis toxin-sensitive G(i/o) protein thought to couple via its Gβγ subunit to a decrease in Ca(2+) channel function. At MF-CA3 PC synapses, autoreceptors on MF terminals respond diametrically depending on the agonist concentration. At low KA concentrations (< 100 nM), a G-protein-independent process invokes the activation of proteins kinase A (PKA) to effect a facilitation of glutamate release. This facilitation possibly involves the Ca(2+)-dependent (rather than GPCR-dependent) activation of adenylate cyclase (AC). At high KA concentrations (<100 nM), a mechanism involving a pertussis toxin-sensitive G(i/o) protein is invoked to inhibit AC activity and thereby suppress PKA activity. Taken together with the heterosynaptic regulation of GABA release by KARs working with a metabotropic modus operandi, there is therefore compelling evidence that these ionotropic glutamate receptors are involved in a noncanonical modulation of glutamate release that does not rely on their typical ionotropic activity.

In the developing rat hippocampus, endogenous activation of presynaptic kainate receptors reduces GABA release from mossy fiber terminals

Presynaptic kainate receptors regulate synaptic transmission in several brain areas but are not known to have this action at immature mossy fiber (MF) terminals, which during the first week of postnatal life release GABA, which exerts into targeted cells a depolarizing and excitatory action. Here, we report that, during the first week of postnatal life, endogenous activation of GluK1 receptors by glutamate present in the extracellular space severely depresses MF-mediated GABAergic currents [GABA(A)-mediated postsynaptic currents (GPSCs)]. Activation of GluK1 receptors was prevented by treating the slices with enzymatic glutamate scavengers that enhanced the clearance of glutamate from the extracellular space. The depressant effect of GluK1 on MF-GPSCs was mediated by a metabotropic process sensitive to pertussis toxin. In the presence of U73122 (1-[6-[[(17b)-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-1H-pyrrole-2,5-dione), a selective inhibitor of phospholipase C, along the transduction pathway downstream to G-protein, GluK1 activation increased the probability of GABA release, thus unveiling the ionotropic action of this receptor. In line with this type of action, we found that GluK1 enhanced MF excitability by directly depolarizing MF terminals via calcium-permeable cation channels. Furthermore, GluK1 dynamically regulated the direction of spike time-dependent plasticity occurring by pairing MF stimulation with postsynaptic spiking and switched spike time-dependent potentiation into depression. The GluK1-induced depression of MF-GPSCs would prevent excessive activation of the CA3 associative network by the excitatory action of GABA and the emergence of seizures in the immature brain.

Amygdalocortical circuitry in schizophrenia: from circuits to molecules

Schizophrenia is a disorder in which disturbances in the integration of emotion with cognition plays a central role and probably involves several different regions, including the dorsolateral prefrontal cortex, the rostral anterior cingulate cortex, the hippocampal formation, and basolateral amygdala (BLA). Recent brain imaging studies have reported changes in volume, whereas postmortem studies point to dysfunction of the GABA and glutamate systems in these regions. Microarray-based profiles indicate that complex changes in the expression of genes associated with synaptic transmission and ion channels are involved in GABA cell dysfunction in schizophrenics. Molecular abnormalities vary considerably on the basis of sector and layer, suggesting that the unique connectivity of intrinsic and extrinsic afferents may critical in regulating the activity of genes in specific subpopulations of GABA cells. Projections of the BLA may be of particular importance to the induction of abnormal circuitry in schizophrenia, as their ingrowth during late adolescence and early adulthood may help to 'trigger' the onset of illness in susceptible individuals. A preponderance of cellular and molecular abnormalities has been found in the stratum oriens (SO) of sectors CA3/2 in which BLA afferents provide a robust innervation. These observations have lead to the development of a rodent model for the study of abnormal circuitry in this disorder. For example, single-cell recordings in hippocampal slices exposed to increased activation from the BLA have shown decreases in GABA currents in pyramidal neurons in SO of CA3/2, but not CA1, and support the validity of this model. Overall, the postmortem studies of neural circuitry abnormalities in schizophrenia are beginning to implicate specific cellular, molecular, and electrophysiological mechanism in specific subtypes of cortical neurons defined by their afferent and efferent connectivity within key corticolimbic regions.

When and why amino acids?

This article reviews especially the early history of glutamate and GABA as neurotransmitters in vertebrates. The proposal that some amino acids could mediate synaptic transmission in the CNS initially met with much resistance. Both GABA and its parent glutamate are abundant in the brain; but, unlike glutamate, GABA had no obvious metabolic function. By the late 1950s, the switch of interest from electrical to chemical transmission invigorated the search for central transmitters. Its identification with Factor I, a brain extract that inhibited crustacean muscle, focused interest on GABA as a possible inhibitory transmitter. In the first microiontophoretic tests, though GABA strongly inhibited spinal neurons, these effects were considered 'non-specific'. Strong excitation by glutamate (and other acidic amino acids) led to the same conclusion. However, their great potency and rapid actions on cortical neurons convinced other authors that these endogenous amino acids are probably synaptic transmitters. This was partly confirmed by showing that both IPSPs and GABA greatly increased Cl() conductance, their effects having similar reversal potentials. Many anticonvulsants proving to be GABA antagonists, by the 1970s GABA became widely accepted as a mediator of IPSPs. Progress was much slower for glutamate. Being generated on distant dendrites, EPSPs could not be easily compared with glutamate-induced excitation, and the search for specific antagonists was long hampered by the lack of blockers and the variety of glutamate receptors. These difficulties were gradually overcome by the application of powerful techniques, such as single channel recording, cloning receptors, as well as new pharmacological tools.

Up-regulation of GLT-1 severely impairs LTD at mossy fibre--CA3 synapses

Glutamate transporters are responsible for clearing synaptically released glutamate from the extracellular space. By this action, they maintain low levels of ambient glutamate, thus preventing excitotoxic damage, and contribute to shaping synaptic currents. We show that up-regulation of the glutamate transporter GLT-1 by ceftriaxone severely impaired mGluR-dependent long-term depression (LTD), induced at rat mossy fibre (MF)-CA3 synapses by repetitive stimulation of afferent fibres. This effect involved GLT-1, since LTD was rescued by the selective GLT-1 antagonist dihydrokainate (DHK). DHK per se produced a modest decrease in fEPSP amplitude that rapidly regained control levels after DHK wash out. Moreover, the degree of fEPSP inhibition induced by the low-affinity glutamate receptor antagonist gamma-DGG was similar during basal synaptic transmission but not during LTD, indicating that in ceftriaxone-treated rats LTD induction did not alter synaptic glutamate transient concentration. Furthermore, ceftriaxone-induced GLT-1 up-regulation significantly reduced the magnitude of LTP at MF-CA3 synapses but not at Schaffer collateral-CA1 synapses. Postembedding immunogold studies in rats showed an increased density of gold particles coding for GLT-1a in astrocytic processes and in mossy fibre terminals; in the latter, gold particles were located near and within the active zones. In both CEF-treated and untreated GLT-1 KO mice used for verifying the specificity of immunostaining, the density of gold particles in MF terminals was comparable to background levels. The enhanced expression of GLT-1 at release sites may prevent activation of presynaptic receptors, thus revealing a novel mechanism by which GLT-1 regulates synaptic plasticity in the hippocampus.

Regulation of kainate receptor subunit mRNA by stress and corticosteroids in the rat hippocampus

Kainate receptors are a class of ionotropic glutamate receptors that have a role in the modulation of glutamate release and synaptic plasticity in the hippocampal formation. Previous studies have implicated corticosteroids in the regulation of these receptors and recent clinical work has shown that polymorphisms in kainate receptor subunit genes are associated with susceptibility to major depression and response to anti-depressant treatment. In the present study we sought to examine the effects of chronic stress and corticosteroid treatments upon the expression of the mRNA of kainate receptor subunits GluR5-7 and KA1-2. Our results show that, after 7 days, adrenalectomy results in increased expression of hippocampal KA1, GluR6 and GluR7 mRNAs, an effect which is reversed by treatment with corticosterone in the case of KA1 and GluR7 and by aldosterone treatment in the case of GluR6. 21 days of chronic restraint stress (CRS) elevated the expression of the KA1 subunit, but had no effect on the expression of the other subunits. Similarly, 21 days of treatment with a moderate dose of corticosterone also increased KA1 mRNA in the dentate gyrus, whereas a high corticosterone dose has no effect. Our results suggest an interaction between hippocampal kainate receptor composition and the hypothalamic-pituitary-adrenal (HPA) axis and show a selective chronic stress induced modulation of the KA1 subunit in the dentate gyrus and CA3 that has implications for stress-induced adaptive structural plasticity.

Contrary roles of kainate receptors in transmitter release at corticothalamic synapses onto thalamic relay and reticular neurons

Corticothalamic fibres, which originate from layer VI pyramidal neurons in the cerebral cortex, provide excitatory synaptic inputs to both thalamic relay neurons and reticular neurons; reticular neurons in turn supply inhibitory inputs to thalamic relay neurons. Pyramidal cells in layer VI in the mouse somatosensory cortex highly express mRNA encoding kainate receptors, which facilitate or depress transmitter release at several synapses in the central nervous system. We report here that contrary modulation of transmitter release from corticothalamic fibres onto thalamic relay and reticular neurons is mediated by activation of kainate receptors in mouse thalamic ventrobasal complex and thalamic reticular nucleus. Exogenous kainate presynaptically depresses the synaptic transmission at corticothalamic synapses onto thalamic relay neurons, but facilitates it at corticothalamic synapses onto reticular neurons. Meanwhile, the lemniscal synaptic transmission, which sends primary somatosensory inputs to relay neurons, is not affected by kainate. In addition, GluR5-containing kainate receptors are involved in the depression of corticothalamic synaptic transmission onto relay neurons, but not onto reticular neurons. Furthermore, synaptically activated kainate receptors mimic these effects; high-frequency stimulation of corticothalamic fibres depresses synaptic transmission onto relay neurons, but facilitates it onto reticular neurons. Our results suggest that the opposite sensitivity of kainate receptors at the two corticothalamic synapses is governed by cortical activity and regulates the balance of excitatory and inhibitory inputs to thalamic relay neurons and therefore their excitability.

ACET is a highly potent and specific kainate receptor antagonist: characterisation and effects on hippocampal mossy fibre function

Kainate receptors (KARs) are involved in both NMDA receptor-independent long-term potentiation (LTP) and synaptic facilitation at mossy fibre synapses in the CA3 region of the hippocampus. However, the identity of the KAR subtypes involved remains controversial. Here we used a highly potent and selective GluK1 (formerly GluR5) antagonist (ACET) to elucidate roles of GluK1-containing KARs in these synaptic processes. We confirmed that ACET is an extremely potent GluK1 antagonist, with a Kb value of 1.4+/-0.2 nM. In contrast, ACET was ineffective at GluK2 (formerly GluR6) receptors at all concentrations tested (up to 100 microM) and had no effect at GluK3 (formerly GluR7) when tested at 1 microM. The X-ray crystal structure of ACET bound to the ligand binding core of GluK1 was similar to the UBP310-GluK1 complex. In the CA1 region of hippocampal slices, ACET was effective at blocking the depression of both fEPSPs and monosynaptically evoked GABAergic transmission induced by ATPA, a GluK1 selective agonist. In the CA3 region of the hippocampus, ACET blocked the induction of NMDA receptor-independent mossy fibre LTP. To directly investigate the role of pre-synaptic GluK1-containing KARs we combined patch-clamp electrophysiology and 2-photon microscopy to image Ca2+ dynamics in individual giant mossy fibre boutons. ACET consistently reduced short-term facilitation of pre-synaptic calcium transients induced by 5 action potentials evoked at 20-25Hz. Taken together our data provide further evidence for a physiological role of GluK1-containing KARs in synaptic facilitation and LTP induction at mossy fibre-CA3 synapses.

Circuitry-based gene expression profiles in GABA cells of the trisynaptic pathway in schizophrenics versus bipolars

Significant reductions in GABAergic cell numbers and/or activity have been demonstrated in the hippocampus of subjects with schizophrenia and bipolar disorder. To understand how different subpopulations of interneurons are regulated, laser microdissection and gene expression profiling have been used to "deconstruct" the trisynaptic pathway, so that subtypes of GABA cells could be defined by their location in various layers of CA3/2 and CA1. The results suggest that the cellular endophenotypes for SZ and BD may be determined by multiple factors that include unique susceptibility genes for the respective disorders and altered integration among hippocampal GABA cells with extrinsic and intrinsic afferent fiber systems. The extensive and intricate data that has come from this study has provided insights into how a complex circuit, like the trisynaptic pathway, may be regulated in human hippocampus in both health and disease.

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.

PKC-dependent autoregulation of membrane kainate receptors

Agonists of kainate receptors (KARs) cause both the opening of the associated ion channels and the activation of signalling pathways driven by G-proteins and PKC. Here we report the existence of an unknown mechanism of KAR autoregulation, involving the interplay of this two signalling mechanisms. Repetitive activation of native KARs evoked the rundown of the ionotropic responses in a manner that was dependent on the activation of PKC. Experiments on recombinant GluR5 expressed in neuroblastoma cells indicated that KARs trigger the activation of PKC and induce the internalization of membrane receptors. This phenomenon depends on the PKC-mediated phosphorylation of serines 879 and 885 of the GluR5-2b subunits, since mutation of these two residues abolished internalization. These results reveal that the non-canonical signalling of KARs is associated with a sensitive mechanism that detects afferent activity. Such a mechanism represents an active way to limit overactivation of the KAR system, by regulating the number of KARs in the cell membrane.

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.

GluR7 is an essential subunit of presynaptic kainate autoreceptors at hippocampal mossy fiber synapses

Presynaptic ionotropic glutamate receptors are emerging as key players in the regulation of synaptic transmission. Here we identify GluR7, a kainate receptor (KAR) subunit with no known function in the brain, as an essential subunit of presynaptic autoreceptors that facilitate hippocampal mossy fiber synaptic transmission. GluR7(-/-) mice display markedly reduced short- and long-term synaptic potentiation. Our data suggest that presynaptic KARs are GluR6/GluR7 heteromers that coassemble and are localized within synapses. We show that recombinant GluR6/GluR7 KARs exhibit low sensitivity to glutamate, and we provide evidence that presynaptic KARs at mossy fiber synapses are likely activated by high concentrations of glutamate. Overall, from our data, we propose a model whereby presynaptic KARs are localized in the presynaptic active zone close to release sites, display low affinity for glutamate, are likely Ca(2+)-permeable, are activated by single release events, and operate within a short time window to facilitate the subsequent release of glutamate.

Kainate receptor-mediated presynaptic inhibition converges with presynaptic inhibition mediated by Group II mGluRs and long-term depression at the hippocampal mossy fiber-CA3 synapse

Kainate receptors (KARs) effect depression of glutamate release at hippocampal mossy fiber-CA3 (MF-CA3) synapses by a metabotropic action involving adenylyl cyclase (AC) inhibition, cAMP reduction, and diminished protein kinase A (PKA) activation. Using hippocampal slices, we show here that KAR activation interferes with the depression of glutamate release produced by Group II metabotropic glutamate receptor stimulation and low frequency stimulation (LFS)-induced long-term depression (LTD), also expressed through presynaptic AC/cAMP/PKA at MF-CA3 synapses. The mutual occlusion of depression mediated by presynaptic KARs, Group II mGluR and LFS-induced LTD suggests their mechanistic convergence at the MF-CA3 synapse and thus invokes KARs in synaptic plasticity manifest in LTD.

Expression and function of kainate receptors in the rat olfactory bulb

Although recent results suggest roles for NMDA and AMPA receptors in odor encoding, little is known about kainate receptors (KARs) in the olfactory bulb (OB). Molecular, immunological, and electrophysiological techniques were used to provide a functional analysis of KARs in the OB. Reverse transcriptase-polymerase chain reaction revealed that the relative level of expression of KAR subunits was GluR5 approximately GluR6 approximately KA2 > KA1 >> GluR7. In situ hybridization data imply that mitral/tufted cells express mostly GluR5 and KA2, whereas interneurons express mostly GluR6 and KA2. Immunohistochemical double-labeling experiments (GluR5/6/7 or GluR5 + synapsin) suggest that KARs are expressed at both synaptic and extrasynaptic loci. This heterogeneous expression of KAR subunits suggests that KARs may play a multitude of roles in odor processing, each tailored to the function of specific OB circuits. A functional analysis, using whole-cell electrophysiology, suggests that one such role is to increase the frequency of glutamate transmission while attenuating the amplitude of individual events, likely via a presynaptic depolarizing mechanism. Such effects would be important to odor processing particularly by OB glomeruli. In these highly compartmentalized structures, an increase in the frequency of glutamate release and the high density of extrasynaptic KARs, activated by spillover, could enhance glomerular synchronization and thus the transfer of more specific sensory information to cortical structures.

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.

Kainate Receptor–Mediated Inhibition of Glutamate Release Involves Protein Kinase A in the Mouse Hippocampus

The mechanisms involved in the inhibition of glutamate release mediated by the activation of presynaptic kainate receptors (KARs) at the hippocampal mossy fiber–CA3 synapse are not well understood. We have observed a long-lasting inhibition of CA3 evoked excitatory postsynaptic currents (eEPSCs) after a brief application of kainate (KA) at concentrations ranging from 0.3 to 10 μM. The inhibition outlasted the change in holding current caused by the activation of ionotropic KARs in CA3 pyramidal cells, indicating that this action is not contingent on the opening of the receptor channels. The inhibition of the eEPSCs by KA was prevented by G protein and protein kinase A (PKA) inhibitors and was enhanced after stimulation of the adenylyl cyclase (AC) with forskolin. We conclude that KARs present at mossy fiber terminals mediate the inhibition of glutamate release through a metabotropic mechanism that involves the activation of an AC-second messenger cAMP-PKA signaling cascade.

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.

Functional Maturation of CA1 Synapses Involves Activity-Dependent Loss of Tonic Kainate Receptor-Mediated Inhibition of Glutamate Release

Early in development, excitatory synapses transmit with low efficacy, one mechanism for which is a low probability of transmitter release (Pr). However, little is known about the developmental mechanisms that control activity-dependent maturation of the presynaptic release. Here, we show that during early development, transmission at CA3-CA1 synapses is regulated by a high-affinity, G protein-dependent kainate receptor (KAR), which is endogenously activated by ambient glutamate. By tonically depressing glutamate release, this mechanism sets the dynamic properties of neonatal inputs to favor transmission during high frequency bursts of activity, typical for developing neuronal networks. In response to induction of LTP, the tonic activation of KAR is rapidly down regulated, causing an increase in Pr and profoundly changing the dynamic properties of transmission. Early development of the glutamatergic connectivity thus involves an activity-dependent loss of presynaptic KAR function producing maturation in the mode of excitatory transmission from CA3 to CA1.

Endogenous activation of kainate receptors regulates glutamate release and network activity in the developing hippocampus

Kainate receptors (KARs) are highly expressed throughout the neonatal brain, but their function during development is unclear. Here, we show that the maturation of the hippocampus is associated with a switch in the functional role of presynaptic KARs. In a developmental period restricted to the first postnatal week, endogenous L-glutamate tonically activates KARs at CA3 glutamatergic synapses to regulate release in an action potential-independent manner. At synapses onto pyramidal cells, KARs inhibit glutamate release via a G-protein and PKC-dependent mechanism. In contrast, at glutamatergic terminals onto CA3 interneurons, presynaptic KARs can facilitate release in a G-protein-independent mechanism. In both cell types, however, KAR activation strongly upregulates inhibitory transmission. We show that, through the interplay of these novel diverse mechanisms, KARs strongly regulate the characteristic synchronous network activity observed in the neonatal hippocampus. By virtue of this, KARs are likely to play a central role in the development of hippocampal synaptic circuits.

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).

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.

Attenuated plasticity of postsynaptic kainate receptors in hippocampal CA3 pyramidal neurons

Kainate receptor-mediated components of postsynaptic currents at hippocampal mossy fiber synapses have markedly slower kinetics than currents arising from AMPA receptors. Here, we demonstrate that other aspects of kainate and AMPA receptor function at this synapse are distinct; in particular, kainate receptor currents are less sensitive to short- and long-term increases in presynaptic strength. EPSCs arising predominantly from AMPA receptors exhibited well characterized paired-pulse facilitation, frequency facilitation, and NMDA receptor-independent long-term potentiation, whereas isolated kainate receptor synaptic currents (KA-EPSCs) exhibited attenuated facilitation and long-term potentiation. In addition, KA-EPSCs varied in their sensitivity to a low-affinity competitive antagonist, suggestive of a synaptic heterogeneity greater than that of EPSCs comprised predominantly of AMPA receptors. These data suggest that the proportional contribution of AMPA and kainate receptors to ensemble synaptic currents will vary depending on the firing frequency of mossy fiber afferents. These synaptic features may be a mechanism for limiting activation of kainate receptors at mossy fiber synapses, which has been shown to be involved in seizurogenic firing of the CA3 network.

Kainate receptor trafficking: physiological roles and molecular mechanisms

Recently, there has been intense interest in the mechanisms regulating the trafficking and synaptic targeting of kainate receptors in neurons. This topic is still in its infancy when compared with studies of trafficking of other ionotropic glutamate receptors; however, it is already clear that mechanisms exist for subunit- and splice variant-specific trafficking of kainate receptors. There is also enormous diversity of kainate receptor targeting, with the best-studied neurons in this regard being hippocampal CA3 pyramidal neurons and CA1 GABAergic interneurons. This review summarizes the current state of knowledge on this topic, focusing on the molecular mechanisms of kainate receptor trafficking and the potential for these mechanisms to regulate neuronal kainate receptor function.

Assessing the role of GLUK5 and GLUK6 at hippocampal mossy fiber synapses

It has been suggested recently that presynaptic kainate receptors (KARs) are involved in short-term and long-term synaptic plasticity at hippocampal mossy fiber synapses. Using genetic deletion and pharmacology, we here assess the role of GLU(K5) and GLU(K6) in synaptic plasticity at hippocampal mossy fiber synapses. We found that the kainate-induced facilitation was completely abolished in the GLU(K6)-/- mice, whereas it was unaffected in the GLU(K5)-/-. Consistent with this finding, synaptic facilitation was reduced in the GLU(K6)(-/-) and was normal in the GLU(K5)-/-. In agreement with these results and ruling out any compensatory effects in the genetic deletion models, application of the GLU(K5)-specific antagonist LY382884 [(3S,4aR,6S,8aR)-6-(4-carboxyphenyl)methyl-1,2,3,4,4a,5,6,7,8,8a-decahydroisoquinoline-3-carboxylic acid] did not affect short-term and long-term synaptic plasticity at the hippocampal mossy fiber synapses. We therefore conclude that the facilitatory effects of kainate on mossy fiber synaptic transmission are mediated by GLU(K6)-containing KARs.

The effect of a kainate GluR5 receptor antagonist on responses of spinothalamic tract neurons in a model of peripheral neuropathy in primates

The responses of antidromically identified spinothalamic tract (STT) neurons to mechanical and thermal stimuli were compared in anesthetized normal and neuropathic monkeys before and after administration of a GluR5 kainate receptor antagonist (LY382884) into the spinal cord dorsal horn through a microdialysis fiber. Peripheral neuropathy was induced by tight ligation of the L7 spinal nerve 13-15 days prior to the experiment. STT neurons recorded in the animals with neuropathy showed increased responsiveness to weak mechanical stimuli and to heating and cooling of the skin compared to STT cells in normal animals. In both normal and the neuropathic monkeys the responses of the STT neurons to mechanical and thermal stimuli were attenuated by LY382884 application in a concentration-dependent manner. Intraspinal application of LY382884 in the neuropathic animals led to a potent reduction of those responses of the STT neurons that were aggravated by the peripheral neuropathy (weak mechanical, heat and innocuous cooling stimuli). These results suggest that kainate receptors are involved in synaptic activation of STT cells in the normal state and may also play an important role in pathological pain states such as peripheral neuropathy in primates. Kainate receptor antagonists could thus be useful for the treatment of certain forms of allodynia and hyperalgesia.

Presynaptic kainate receptor facilitation of glutamate release involves protein kinase A in the rat hippocampus

We have explored the mechanisms involved in the facilitation of glutamate release mediated by the activation of kainate receptors in the rat hippocampus using isolated nerve terminal (synaptosome) and slice preparations. In hippocampal nerve terminals, kainate (KA) produced an increase of glutamate release at concentrations of agonist ranging from 10 to 1000 microm. In hippocampal slices, KA at low nanomolar concentrations (20-50 nm) also produced an increase of evoked excitatory postsynaptic currents (eEPSCs) at mossy fibre-CA3 synapses. In both, synaptosomes and slices, the effect of KA was antagonized by CNQX, and persisted after pretreatment with a cocktail of antagonists for other receptors whose activation could potentially have produced facilitation of release. These data indicate that the facilitation of glutamate release observed is mediated by the activation of presynaptic glutamate receptors of the kainate type. Mechanistically, the observed effects of KA appear to be the same in synaptosomal and slice preparations. Thus, the effect of KA on glutamate release and mossy fibre-CA3 synaptic transmission was occluded by the stimulation of adenylyl cyclase by forskolin and suppressed by the inhibition of protein kinase A by H-89 or Rp-Br-cAMP. We conclude that kainate receptors present at presynaptic terminals in the rat hippocampus mediate the facilitation of glutamate release through a mechanism involving the activation of an adenylyl cyclase-second messenger cAMP-protein kinase A signalling cascade.

Kainate receptors and synaptic transmission

Excitatory glutamatergic transmission involves a variety of different receptor types, each with distinct properties and functions. Physiological studies have identified both post- and presynaptic roles for kainate receptors, which are a subtype of the ionotropic glutamate receptors. Kainate receptors contribute to excitatory postsynaptic currents in many regions of the central nervous system including hippocampus, cortex, spinal cord and retina. In some cases, postsynaptic kainate receptors are co-distributed with alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptors, but there are also synapses where transmission is mediated exclusively by postsynaptic kainate receptors: for example, in the retina at connections made by cones onto off bipolar cells. Modulation of transmitter release by presynaptic kainate receptors can occur at both excitatory and inhibitory synapses. The depolarization of nerve terminals by current flow through ionotropic kainate receptors appears sufficient to account for most examples of presynaptic regulation; however, a number of studies have provided evidence for metabotropic effects on transmitter release that can be initiated by activation of kainate receptors. Recent analysis of knockout mice lacking one or more of the subunits that contribute to kainate receptors, as well as studies with subunit-selective agonists and antagonists, have revealed the important roles that kainate receptors play in short- and long-term synaptic plasticity. This review briefly addresses the properties of kainate receptors and considers in greater detail the physiological analysis of their contributions to synaptic transmission.

Presynaptic modulation controlling neuronal excitability and epileptogenesis: role of kainate, adenosine and neuropeptide Y receptors

Based on the idea that seizures may arise from an overshoot of excitation over inhibition, all substances that may decrease glutamatergic function while having no effect or even increasing GABAergic neurotransmission are likely to be effective anticonvulsants. We now review the possible role of three such neuromodulators, kainate, adenosine, and neuropeptide Y receptors in controlling hyperexcitability and epileptogenesis. Particular emphasis is given on the robust neuromodulatory role of these three groups of receptors on the release of glutamate in the hippocampus, a main focus of epilepsy. Moreover, we also give special attention to the mechanisms of receptor activation and coupled signaling events that can be explored as attractive targets for the treatment of epilepsy and excitotoxicity. The present paper is a tribute to Arsélio Pato de Carvalho who has been the main driving force for the development of Neuroscience in Portugal, notably with a particular emphasis on the presynaptic mechanisms of modulation of neurotransmitter release.

Distribution of kainate receptor subunits at hippocampal mossy fiber synapses

Kainate receptors function as mediators of postsynaptic currents and as presynaptic modulators of synaptic transmission at mossy fiber synapses. Despite intense research into the physiological properties of mossy fiber kainate receptors, their subunit composition in the presynaptic and postsynaptic compartments is unclear. Here we describe the distribution of kainate receptor subunits in mossy fiber synapses using subunit-selective antibodies and knock-out mice. We provide morphological evidence for the presynaptic localization of KA1 and KA2 receptor subunits at mossy fiber synapses. Immunogold staining for KA1 and KA2 was commonly seen at synaptic contacts and in vesicular structures. Postsynaptic labeling in dendritic spines was also observed. Although KA1 predominantly showed presynaptic localization, KA2 was concentrated to a greater degree on postsynaptic membranes. Both subunits coimmunoprecipitated from hippocampal membrane extracts with GluR6 but not GluR7 subunits. These results demonstrate that KA1 and KA2 subunits are localized presynaptically and postsynaptically at mossy fiber synapses where they most likely coassemble with GluR6 subunits to form functional heteromeric kainate receptor complexes.

Kainate receptor (GluR5)-mediated disinhibition of responses in rat ventrobasal thalamus allows a novel sensory processing mechanism

Kainate receptors have been studied extensively in vitro, but how they might function physiologically remains unclear. We studied kainate receptor modulation of synaptic responses in the rat ventrobasal thalamus using the novel antagonist LY382884 and the agonist ATPA (selective for GluR5-containing kainate receptors) as tools. No evidence could be found for a direct contribution of kainate receptors to responses of thalamic relay cells to lemniscal (sensory) input in thalamic slices studied with the aid of intracellular and field potential recordings, using selective AMPA and NMDA receptor antagonists and LY382884. However, the GluR5 agonist ATPA reduced the IPSPs originating from the thalamic reticular nucleus. Extracellular single-neurone recordings in anaesthetised rats showed that excitatory responses evoked by physiological vibrissa afferent stimulation were reduced by LY382884 applied iontophoretically at the recording site. This action of the antagonist was occluded when GABA receptors were blocked, indicating that the reduction in excitatory sensory responses by LY382884 is due to an action on GABAergic inhibition arising from the thalamic reticular nucleus. Further experiments showed that these actions depended on whether inhibition was evoked during activation of the excitatory receptive field rather than when inhibition was evoked from a surround vibrissa. We suggest that GluR5 is located presynaptically on inhibitory GABAergic terminals of thalamic reticular nucleus neurones, and that it is normally activated by glutamate spillover from synapses between excitatory afferents and relay neurones during physiological stimulation. We propose that this GluR5-activated disinhibition has an important novel role in extracting sensory information from background noise.

A role for Ca2+ stores in kainate receptor-dependent synaptic facilitation and LTP at mossy fiber synapses in the hippocampus

Compared with NMDA receptor-dependent LTP, much less is known about the mechanism of induction of NMDA receptor-independent LTP; the most extensively studied form of which is mossy fiber LTP in the hippocampus. In the present study we show that Ca2+-induced Ca2+ release from intracellular stores is involved in the induction of mossy fiber LTP. This release also contributes to the kainate receptor-dependent component of the pronounced synaptic facilitation that occurs during high-frequency stimulation. We also present evidence that the trigger for this Ca2+ release is Ca2+ permeation through kainate receptors. However, these novel synaptic mechanisms can be bypassed when the Ca2+ concentration is raised (from 2 to 4 mM), via a compensatory involvement of L-type Ca2+ channels. These findings suggest that presynaptic kainate receptors at mossy fiber synapses can initiate a cascade involving Ca2+ release from intracellular stores that is important in both short-term and long-term plasticity.

Activation of presynaptic glycine receptors facilitates glycine release from presynaptic terminals synapsing onto rat spinal sacral dorsal commissural nucleus neurons

Glycine is a major inhibitory neurotransmitter in the spinal cord and brainstem. Here we report the novel finding that presynaptic glycine autoreceptors modulate release from terminals synapsing onto rat spinal sacral dorsal commissural nucleus (SDCN) neurons. In mechanically dissociated SDCN neurons, in which functional presynaptic nerve terminals remain adherent to the isolated neurons, exogenously applied glycine (3 microM) increased the frequency of glycinergic spontaneous inhibitory postsynaptic currents (sIPSCs) without affecting their amplitudes or decay times. This suggests that glycine acts presynaptically to increase glycine release probability. Picrotoxin, at a concentration that had little direct effect on sIPSC frequency and amplitude (30 microM), significantly attenuated glycine-induced presynaptic sIPSC facilitation. The glycine-induced sIPSC frequency facilitation was completely abolished either in a Ca(2+)-free external solution or in the presence of 100 microM Cd2+, suggesting the involvement of extracellular Ca2+ influx into the nerve terminals. The glycine action was also completely occluded in the presence of 300 nM tetrodotoxin. In recordings from SDCN neurons in spinal cord slices, glycine (10 microM) increased evoked IPSC (eIPSC) amplitude and decreased the extent of paired-pulse facilitation. In response to brief high frequency stimulus trains the eIPSCs displayed a profound frequency-dependent facilitation that was greatly reduced by picrotoxin (30 microM). These results indicate that glycine acts at presynaptic autoreceptors, causing depolarization of the glycinergic nerve terminals, the subsequent activation of voltage-dependent Na+ and Ca2+ channels, and facilitation of glycine release. Furthermore, this presynaptic facilitation was observed under more physiological conditions, suggesting that these glycinergic autoreceptors may contribute to the integration of local inhibitory inputs to SDCN neurons.