The synaptic activation of kainate receptors

Involvement of P2X7 receptors in the regulation of neurotransmitter release in the rat hippocampus

Although originally cloned from rat brain, the P2X7 receptor has only recently been localized in neurones, and functional responses mediated by these neuronal P2X7 receptors (P2X7 R) are largely unknown. Here we studied the effect of P2X7 R activation on the release of neurotransmitters from superfused rat hippocampal slices. ATP (1-30 mm) and other ATP analogues elicited concentration-dependent [3 H]GABA outflow, with the following rank order of potency: benzoylbenzoylATP (BzATP) > ATP > ADP. PPADS, the non-selective P2-receptor antagonist (3-30 microm), Brilliant blue G (1-100 nm) the P2X7 -selective antagonist and Zn2+ (0.1-30 microm) inhibited, whereas lack of Mg2+ potentiated the response by ATP. In situ hybridization revealed that P2X7 R mRNA is expressed in the neurones of the cell body layers in the hippocampus. P2X7 R immunoreactivity was found in excitatory synaptic terminals in CA1 and CA3 region targeting the dendrites of pyramidal cells and parvalbumin labelled structures. ATP (3-30 microm) and BzATP (0.6-6 microm) elicited concentration-dependent [14 C]glutamate efflux, and blockade of the kainate receptor-mediated transmission by CNQX (10-100 microm) and gadolinium (100 microm), decreased ATP evoked [3 H]GABA efflux. The Na+ channel blocker TTX (1 microm), low temperature (12 degrees C), and the GABA uptake blocker nipecotic acid (1 mm) prevented ATP-induced [3 H]GABA efflux. Brilliant blue G and PPADS also reduced electrical field stimulation-induced [3 H]GABA efflux. In conclusion, P2X7 Rs are localized to the excitatory terminals in the hippocampus, and their activation regulates the release of glutamate and GABA from themselves and from their target cells.

RNA editing of neurotransmitter receptors in the mammalian brain

RNA editing refers to various posttranscriptional mechanisms that alter the nucleotide sequence of RNA. In the mammalian brain, RNA editing results in significant changes in the functional properties of receptors for the important neurotransmitters glutamate and serotonin. These changes result from site-specific deamination of single adenosines in the pre-messenger RNA encoding these receptors. Here, we review what is known about the mechanisms underlying this editing, the consequences of RNA editing for glutamate and serotonin receptor function, and recent studies on transgenic mice and human post-mortem tissue that have begun to elucidate the role of RNA editing in the intact mammalian brain.

Functional characterization of kainate receptors in the mouse nucleus accumbens

Kainate receptors are abundantly expressed in the nucleus accumbens but their functional characterization and their role in synaptic transmission has not yet been investigated. Using patch-clamp recordings in mouse nucleus accumbens slices, we show the presence of functional kainate receptors activated by low concentrations of kainate (100-300 nM) in medium size neurons. These somatodendritic receptors are comprised of the GluR6 subunit, since they are absent in GluR6-deficient mice. Kainate receptors do not directly participate in glutamatergic synaptic transmission evoked by electrical stimulation of cortical afferent fibers in nucleus accumbens neurons. However, application of low concentrations of kainate inhibits cortico-accumbens synaptic transmission, by increasing synaptic failure rate and increasing variation coefficient, thus indicating a presynaptic site of action. Presynaptic kainate receptors are observed both in GluR6 and in GluR5-deficient mice, but are absent in mice devoid of both subunits. Hence, at variance with somatodendritic kainate receptors, presynaptic kainate receptors on cortical afferents are composed of both GluR5 and GluR6 kainate receptor subunits. These results indicate that different subtypes of kainate receptors, representing distinct pharmacological targets, should play important roles in the synaptic integration properties of nucleus accumbens neurons.

Metabotropic-mediated kainate receptor regulation of IsAHP and excitability in pyramidal cells

Kainate receptors (KARs) on CA1 pyramidal cells make no detectable contribution to EPSCs. We report that these receptors have a metabotropic function, as shown previously for CA1 interneurons. Brief kainate exposure caused long-lasting inhibition of a postspike potassium current (I(sAHP)) in CA1 pyramidal cells. The pharmacological profile was independent of AMPA receptors or the GluR5 subunit, indicating a possible role for the GluR6 subunit. KAR inhibition of I(sAHP) did not require ionotropic action or network activity, but was blocked by the inhibitor of pertussis toxin-sensitive G proteins, N-ethylmaleimide (NEM), or the PKC inhibitor calphostin C. These data suggest how KARs, putatively containing GluR6, directly increase excitability of CA1 pyramidal cells and help explain the propensity for seizure activity following KAR activation.

Intrinsic membrane properties and synaptic inputs regulating the firing activity of the dopamine neurons

Dopamine (DA) neurones of the ventral mesencephalon are involved in the control of reward related behaviour, cognitive functions and motor performances, and provide a critical site of action for major categories of neuropsychiatric drugs, such as antipsychotic agents, dependence producing drugs and anti-Parkinson medication. The midbrain DA neurones are mainly located in the substantia nigra pars compacta (SNPC) and the ventral tegmental area (VTA). Intrinsic membrane properties regulate the activity of these neurones. In fact, they possess several conductances that allow them to fire in a slow pacemaker-like mode. The internal set of membrane currents interact with afferent synaptic inputs which, especially in in vivo conditions, contribute to accelerate or decelerate the firing activity of the cells in accordance with the necessity to optimise the release of dopamine in the terminal fields. In particular, discrete excitatory and inhibitory inputs transform the firing from a low regular into a bursting pattern. The bursting activity promotes dopamine release being very important in cognition and motor performances. In the present paper we review electrophysiological data regarding the role of glutamatergic and cholinergic and GABAergic afferent inputs in regulating the midbrain DAergic neuronal activity.

Correlation of the expression of kainate receptor subtypes to responses evoked in cultured cortical and spinal cord neurones

Responses of cultured rat cortical and spinal cord neurones to kainate (KA) have been related to the expression of KA receptor subunits revealed by single-cell reverse transcription polymerase chain reaction (RT-PCR). Semi-rapid application of KA evoked non-desensitizing responses with EC(50) values of 82 microM for cortical and 67 microM for spinal cord neurones. In the presence of concanavalin A, GYKI 53655 (100 microM) reduced responses of both types of neurone to KA by about 80% to leave a KA receptor-mediated response with an EC(50) of 4 microM on spinal cord neurones and 27 microM (P<0.001) on cortical neurones. Ultra-fast application of KA to outside-out patches of cortical neurones evoked a non-decaying response which was reduced by about 30% by GYKI 53655 to reveal a transient response that desensitized by 92.5% with a time constant (tau(des)) of 26.2 ms. Responses of spinal cord patches decayed by 47.8%. GYKI 53655 reduced the peak response by 8.3% and the residual response desensitized by 75.8%, with a tau(des) of 17.3 ms, all values being significantly smaller than for cortical neurones. Single-cell RT-PCR showed relative abundances of mRNAs for the KA receptors, GluR5, GluR6 and GluR7, of 12, 33 and 54% for cortical neurones and 38, 10 and 54% for spinal cord neurones, respectively. The relative abundances of KA1 and KA2 were 12 and 88% for cortical neurones, and 19 and 79% for spinal cord neurones, respectively. The most likely expression patterns of functional KA receptors is GluR6/KA2 for cortical neurones and GluR5/KA2 for spinal cord neurones.

Kainate receptor-dependent presynaptic modulation and plasticity

Kainate-type ionotropic glutamate receptors (KARs) distribute widely and heterogenously throughout the central nervous system (CNS). There is now increasing evidence showing that, in addition to conventional action to mediate postsynaptic excitation, KAR also exerts presynaptic action modulating the amount of transmitter release at certain synapses in the CNS. The mechanism and physiological function of presynaptic KARs have been studied most extensively at the hippocampal mossy fiber (MF)-CA3 synapse, one of the CNS regions where the highest density of KAR subunits is expressed. One unique feature of presynaptic KARs is that their activation modulates transmitter release bi-directionally; weak activation enhances glutamate release, while strong activation leads to inhibition. These findings may be explained by their possible ionotropic action leading to axonal depolarization, which in turn regulates several voltage-dependent channels involved in action potential-dependent Ca2+ entry processes. Furthermore, physiological activation of presynaptic KAR involves an activity-dependent process. Large frequency facilitation, a form of short-term plasticity characteristic of the MF-CA3 synapse, is mediated, at least partly, by presynaptic KAR. Bi-directional and activity-dependent regulation of transmitter release by kainate autoreceptors might have physiological significance in information processing in the hippocampus and other CNS regions, as well as its well-known pathological action contributing to epileptogenesis.

Subcellular and subsynaptic localization of presynaptic and postsynaptic kainate receptor subunits in the monkey striatum

The localization and functions of kainate receptors (KARs) in the CNS are still poorly known. In the striatum, GluR6/7 and KA2 immunoreactivity is expressed presynaptically in a subpopulation of glutamatergic terminals and postsynaptically in dendrites and spines. The goal of this study was to further characterize the subcellular and subsynaptic localization of kainate receptor subunits in the monkey striatum. Immunoperoxidase data reveal that the relative abundance of GluR6/7- and KA2-immunoreactive terminals is homogeneous throughout the striatum irrespective of the differential degree of striatal degeneration in Huntington's disease. Pre-embedding and post-embedding immunogold data indicate that >70% of the presynaptic or postsynaptic GluR6/7 and KA2 labeling is expressed intracellularly. In material stained with the post-embedding immunogold method, approximately one-third of plasma membrane-bound gold particles labeling in axon terminals and spines is associated with asymmetric synapses, thereby representing synaptic kainate receptor subunits. On the other hand, >60% of the plasma-membrane bound labeling is extrasynaptic. Both GluR6/7 and KA2 labeling in glutamatergic terminals often occurs in clusters of gold particles along the membrane of large vesicular organelles located at various distances from the presynaptic grid. Anterograde labeling from the primary motor cortex or the centromedian thalamic nucleus indicate that both corticostriatal and thalamostriatal terminals express presynaptic GluR6/7 and KA2 immunoreactivity in the postcommissural putamen. In conclusion, these data demonstrate that kainate receptors in the striatum display a pattern of subcellular distribution different from other ionotropic glutamate receptor subtypes, but consistent with their metabotropic-like functions recently shown in the hippocampus.

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

The development of GluR5-selective kainate receptor ligands is helping to elucidate the functions of kainate receptors in the CNS. Here we have further characterised the actions of a GluR5 selective agonist, ATPA, and a GluR5 selective antagonist, LY382884, in the CA3 region of rat hippocampal slices. In addition, we have used LY382884 to study a novel synaptic mechanism. This antagonist substantially reduces frequency facilitation of mossy fibre synaptic transmission, monitored as either AMPA or NMDA receptor-mediated EPSCs. This suggests that GluR5-containing kainate receptors on mossy fibres function as autoreceptors to facilitate the synaptic release of L-glutamate, in a frequency-dependent manner.

A critical role of a facilitatory presynaptic kainate receptor in mossy fiber LTP

The mechanisms involved in mossy fiber LTP in the hippocampus are not well established. In the present study, we show that the kainate receptor antagonist LY382884 (10 microM) is selective for presynaptic kainate receptors in the CA3 region of the hippocampus. At a concentration at which it blocks mossy fiber LTP, LY382884 selectively blocks the synaptic activation of a presynaptic kainate receptor that facilitates AMPA receptor-mediated synaptic transmission. Following the induction of mossy fiber LTP, there is a complete loss of the presynaptic kainate receptor-mediated facilitation of synaptic transmission. These results identify a central role for the presynaptic kainate receptor in the induction of mossy fiber LTP. In addition, these results suggest that the pathway by which kainate receptors facilitate glutamate release is utilized for the expression of mossy fiber LTP.

Characterization of the rat GRIK5 kainate receptor subunit gene promoter and its intragenic regions involved in neural cell specificity

The GRIK5 (glutamate receptor ionotropic kainate-5) gene encodes the kainate-preferring glutamate receptor subunit KA2. The GRIK5 promoter is TATA-less and GC-rich, with multiple consensus initiator sequences. Transgenic mouse lines carrying 4 kilobases of the GRIK5 5'-flanking sequence showed lacZ reporter expression predominantly in the nervous system. Reporter assays in central glial (CG-4) and non-neural cells indicated that a 1200-base pair (bp) 5'-flanking region could sustain neural cell-specific promoter activity. Transcriptional activity was associated with the formation of a transcription factor IID-containing complex on an initiator sequence located 1100 bp upstream of the first intron. In transfection studies, deletion of exonic sequences downstream of the promoter resulted in reporter gene activity that was no longer neural cell-specific. When placed downstream of the GRIK5 promoter, a 77-bp sequence from the deleted fragment completely silenced reporter expression in NIH3T3 fibroblasts while attenuating activity in CG-4 cells. Analysis of the 77-bp sequence revealed a functional SP1-binding site and a sequence resembling a neuron-restrictive silencer element. The latter sequence, however, did not display cell-specific binding of REST-like proteins. Our studies thus provide evidence for intragenic control of GRIK5 promoter activity and suggest that elements contributing to tissue-specific expression are contained within the first exon.

Long-term depression: a cascade of induction and expression mechanisms

The aims of this paper are to provide a comprehensive and up to date review of the mechanisms of induction and expression of long-term depression (LTD) of synaptic transmission. The review will focus largely on homosynaptic LTD and other forms of LTD will be considered only where appropriate for a fuller understanding of LTD mechanisms. We shall concentrate on what are felt to be some of the most interesting recent findings concerning LTD in the central nervous system. Wherever possible we shall try to consider some of the disparities in results and possible reasons for these. Finally, we shall briefly consider some of the possible functional consequences of LTD for normal physiological function.

Glutamate receptor involvement in dentate granule cell epileptiform activity evoked by mossy fiber stimulation

In many persons with temporal lobe epilepsy, dentate granule cells form an interconnected synaptic network. This recurrent mossy fiber circuit mediates reverberating excitation that may facilitate seizure propagation by synchronizing granule cell discharge. The involvement of specific glutamate receptors in granule cell epileptiform activity evoked by stimulating the mossy fibers was investigated with use of rat hippocampal slices superfused with bicuculline, with or without increasing [K+](o) to 6 mM. The occurrence of short-latency mossy fiber-evoked granule cell epileptiform activity in slices from pilocarpine-treated rats correlated with the presence and extent of recurrent mossy fiber growth. Blockade of AMPA receptors nearly abolished the orthodromic component of the response; subsequent antagonism of kainate receptors as well appeared to have no further action. Antagonism of NMDA receptors reduced the duration of epileptiform discharge, but increased the amplitude of population spikes within the evoked burst. Thus AMPA and NMDA, but perhaps not kainate, receptors play an important role in this type of epileptiform activity. Activation of type II metabotropic glutamate receptors, which inhibits the release of glutamate from mossy fiber boutons, reduced the magnitude of epileptiform discharge. This action was reversed by a partial agonist of these receptors. However, neither an agonist nor an antagonist of type III metabotropic glutamate receptors significantly altered the response. Considering the importance of synchronous granule cell discharge for seizure propagation from the entorhinal cortex to the hippocampus, agonists of type II metabotropic glutamate receptors may be useful in suppressing such discharge both experimentally and clinically.

Presynaptic kainate receptors at hippocampal mossy fiber synapses

Hippocampal mossy fibers, which are the axons of dentate granule cells, form powerful excitatory synapses onto the proximal dendrites of CA3 pyramidal cells. It has long been known that high-affinity binding sites for kainate, a glutamate receptor agonist, are present on mossy fibers. Here we summarize recent experiments on the role of these presynaptic kainate receptors (KARs). Application of kainate has a direct effect on the amplitude of the extracellularly recorded fiber volley, with an enhancement by low concentrations and a depression by high concentrations. These effects are mediated by KARs, because they persist in the presence of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor-selective antagonist GYKI 53655, but are blocked by the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/KAR antagonist 6-cyano-7-nitroquinoxaline-2,3-dione and the KAR antagonist SYM2081. The effects on the fiber volley are most likely caused by a depolarization of the fibers via the known ionotropic actions of KARs, because application of potassium mimics the effects. In addition to these effects on fiber excitability, low concentrations of kainate enhance transmitter release, whereas high concentrations depress transmitter release. Importantly, the synaptic release of glutamate from mossy fibers also activates these presynaptic KARs, causing an enhancement of the fiber volley and a facilitation of release that lasts for many seconds. This positive feedback contributes to the dramatic frequency facilitation that is characteristic of mossy fiber synapses. It will be interesting to determine how widespread facilitatory presynaptic KARs are at other synapses in the central nervous system.

Excitatory stimulation of neurons in the arcuate nucleus inhibits gastric acid secretion via vagal pathways in anesthetized rats

It is well established that autonomic control of gastrointestinal function is modulated by central autonomic neurotransmission. In this context it has been shown that gastrointestinal motility and secretion can be modulated by exogenous neuropeptides microinjected into the paraventricular nucleus of the hypothalamus (PVN). Furthermore, there is considerable evidence suggesting that neurons projecting from the arcuate nucleus (Arc) to the PVN may be the source of endogenous neuropeptide release in the PVN. This poses the question whether stimulation of neurons in the arcuate nucleus, e.g. by an excitatory amino acid, alters gastrointestinal function. In the present study, we investigated the effect of an excitatory amino acid, kainate, microinjected into the arcuate nucleus on gastric acid secretion in urethane-anesthetized rats. Kainate (140 pmol/rat) bilaterally microinjected into the Arc induced an significant inhibition of pentagastrin (PG) stimulated (16 mg/kg per h) gastric acid secretion throughout an observation period of 120 min after microinjection. Microinjection of kainate into hypothalamic areas outside the arcuate nucleus did not modify gastric secretion. Bilateral cervical vagotomy blocked the effect of kainate injected into the Arc on PG-stimulated gastric acid secretion. These data show that gastric secretory function can be modulated by stimulation of neuronal activity in the Arc via efferent vagal pathways. The results suggest that the arcuate nucleus is a forebrain area involved in the CNS regulation of gastrointestinal function.

Role of kainate receptor activation and desensitization on the [Ca(2+)](i) changes in cultured rat hippocampal neurons

We investigated the role of kainate (KA) receptor activation and desensitization in inducing the increase in the intracellular free Ca(2+) concentration ([Ca(2+)](i)) in individual cultured rat hippocampal neurons. The rat hippocampal neurons in the cultures were shown to express kainate receptor subunits, KA2 and GluR6/7, either by immunocytochemistry or by immunoblot analysis. The effect of LY303070, an alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA) receptor antagonist, on the alterations in the [Ca(2+)](i) caused by kainate showed cell-to-cell variability. The [Ca(2+)](i) increase caused by kainate was mostly mediated by the activation of AMPA receptors because LY303070 inhibited the response to kainate in a high percentage of neurons. The response to kainate was potentiated by concanavalin A (Con A), which inhibits kainate receptor desensitization, in 82.1% of the neurons, and this potentiation was not reversed by LY303070 in about 38% of the neurons. Also, upon stimulation of the cells with 4-methylglutamate (MGA), a selective kainate receptor agonist, in the presence of Con A, it was possible to observe [Ca(2+)](i) changes induced by kainate receptor activation, because LY303070 did not inhibit the response in all neurons analyzed. In toxicity studies, cultured rat hippocampal neurons were exposed to the drugs for 30 min, and the cell viability was evaluated at 24 hr using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. The selective activation of kainate receptors with MGA, in the presence of Con A, induced a toxic effect, which was not prevented by LY303070, revealing a contribution of a small subpopulation of neurons expressing kainate receptors that independently mediate cytotoxicity. Taken together, these results indicate that cultured hippocampal neurons express not only AMPA receptors, but also kainate receptors, which can modulate the [Ca(2+)](i) and toxicity.

Kinetics and activation of postsynaptic kainate receptors at thalamocortical synapses: role of glutamate clearance

Kainate (KA) receptor-mediated excitatory postsynaptic currents (EPSCs) exhibit slow kinetics at the great majority of synapses. However, native or heterologously expressed KA receptors exhibit rapid kinetics in response to agonist application. One possibility to explain this discrepancy is that KA receptors are extrasynaptic and sense glutamate diffusing from the synaptic cleft. We investigated this by studying the effect of three manipulations that change glutamate clearance on evoked KA EPSCs at thalamocortical synapses. First, we used high-frequency stimulation to increase extrasynaptic glutamate levels. This caused an apparent increase in the relative contribution of the KA EPSC to transmission and slowed the decay kinetics. However, scaling and summing the EPSC evoked at low frequency reproduced this, demonstrating that the effect was due to postsynaptic summation of KA EPSCs. Second, we applied inhibitors of high-affinity glutamate transport. This caused a depression in both AMPA and KA EPSC amplitude due to the activation of a presynaptic glutamatergic autoreceptor. However, transport inhibitors had no selective effect on the amplitude or kinetics of the KA EPSC. Third, to increase glutamate clearance, we raised temperature during recordings. This shortened the decay of both the AMPA and KA components and increased their amplitudes, but this effect was the same for both. Therefore these data provide evidence against glutamate diffusion out of the synaptic cleft as the mechanism for the slow kinetics of KA EPSCs. Other possibilities such as interactions of KA receptors with other proteins or novel properties of native synaptic heteromeric receptors are required to explain the slow kinetics.

Excitotoxic profiles of novel, low-affinity kainate receptor agonists in primary cultures of murine cerebellar granule cells

The involvement of low-affinity kainate (KA) receptors in neuronal injury was investigated by employing a variety of agonists active at GluR5-7. Their excitotoxic profiles were determined in primary cultures of cerebellar granule cells, which abundantly expressed low-affinity KA receptors, and in the absence of any AMPA receptor-mediated neurotoxicity. Neurotoxicity induced by these compounds was analysed by phase contrast microscopy, a cell viability assay, the TUNEL technique (apoptosis), and by employing propidium iodide (PI; necrosis). All agonists induced concentration-dependent neurotoxicity, with rank order (EC(50) values; microM): (S)-iodowillardiine (IW) 0.2>(2S,4R)-4-methylglutamate (4-MG) 36>(2S,4R,6E)-2-amino-4-carboxy-7-(2-naphthyl)hept-6-enoic acid (LY339434) 46>KA 74>(RS)-2-amino-3-(hydroxy-5-tert-butylisoxazol-4yl)propanoic acid (ATPA) 88. IW exposure resulted in apoptosis at lower concentrations (<30 microM) and necrosis at higher concentrations, both of which were attenuated by CNQX (50 microM), but not MK-801 (10 microM). ATPA-mediated neurotoxicity was purely apoptotic and was attenuated by the non-NMDA receptor antagonists. Both IW and ATPA induced injury with the morphological characteristics of apoptosis shown by the presence of TUNEL-positive neurones. LY339434-mediated neuronal injury was only attenuated by MK-801 and was necrotic in nature. Similarly, 4-MG (>30 microM) exposure caused necrosis that was partially attenuated by MK-801 (10 microM) and CNQX (50 microM). The patterns of neurotoxicity possessed a complex pharmacological profile, demonstrated an apoptotic-necrotic continuum and were inconsistent with past findings, further outlining the importance of characterizing novel compounds at native receptors. ATPA and to a lesser extent IW appear to be suitable drugs for low-affinity KA receptors. Since toxicity-mediated by low-affinity KA receptors seem likely to contribute to neurodegenerative conditions, our study importantly examines the excitotoxic profile of these novel agonists.

Molecular mechanisms of glutamate receptor-mediated excitotoxic neuronal cell death

Excitotoxicity is one of the most extensively studied processes of neuronal cell death, and plays an important role in many central nervous system (CNS) diseases, including CNS ischemia, trauma, and neurodegenerative disorders. First described by Olney, excitotoxicity was later characterized as an excessive synaptic release of glutamate, which in turn activates postsynaptic glutamate receptors. While almost every glutamate receptor subtype has been implicated in mediating excitotoxic cell death, it is generally accepted that the N-methyl-D-aspartate (NMDA) subtypes play a major role, mainly owing to their high calcium (Ca2+) permeability. However, other glutamate receptor subtypes such as 2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl) propionate (AMPA) or kainate receptors have also been attributed a critical role in mediating excitotoxic neuronal cell death. Although the molecular basis of glutamate toxicity is uncertain, there is general agreement that it is in large part Ca(2+)-dependent. The present review is aimed at summarizing the molecular mechanisms of NMDA receptor and AMPA/kainate receptor-mediated excitotoxic neuronal cell death.

Kainate receptors: knocking out plasticity

There is increasing evidence that kainate receptors contribute to both postsynaptic and presynaptic signaling. Studies of knockout mice have played a pivotal role in defining the functions of kainate receptors, including a recent study that implicates kainate receptors in frequency-dependent facilitation and long-term potentiation of hippocampal mossy fiber synapses.

Kainate receptor-dependent axonal depolarization and action potential initiation in interneurons

Kainate receptor agonists are powerful chemoconvulsants and excitotoxins. These properties are in part explained by depolarization of hippocampal principal neurons. However, kainate also depresses evoked inhibitory signals in pyramidal neurons, and promotes spontaneous GABA release from interneurons. The mechanisms underlying these phenomena are not fully understood, nor are the consequences for the inhibitory traffic among interneurons. We report that both the amplitude and the frequency of spontaneous IPSCs recorded in interneurons were enhanced by low concentrations of kainate, but action potential-independent IPSCs were unaffected. In the presence of GABA(A) receptor antagonists, kainate lowered the threshold for antidromic action potential generation, suggesting that interneuron axons are directly depolarized; this effect was mimicked by synaptically released glutamate. Kainate application also induced spontaneous antidromic action potentials. Axonal receptors are thus important in initiating the intense interneuronal activity triggered by kainate, which in turn influences inhibitory signaling to principal cells.

Molecular physiology of kainate receptors

A decade ago, our understanding of the molecular properties of kainate receptors and their involvement in synaptic physiology was essentially null. A plethora of recent studies has altered this situation profoundly such that kainate receptors are now regarded as key players in the modulation of transmitter release, as important mediators of the postsynaptic actions of glutamate, and as possible targets for the development of antiepileptic and analgesic drugs. In this review, we summarize our current knowledge of the properties of kainate receptors focusing on four key issues: 1) their structural and biophysical features, 2) the important progress in their pharmacological characterization, 3) their pre- and postsynaptic mechanisms of action, and 4) their involvement in a series of physiological and pathological processes. Finally, although significant progress has been made toward the elucidation of their importance for brain function, kainate receptors remain largely an enigma and, therefore, we propose some new roads that should be explored to obtain a deeper understanding of this young, but intriguing, class of proteins.

Postsynaptic calcium transients evoked by activation of individual hippocampal mossy fiber synapses

Control of Ca(2+) within dendritic spines is critical for excitatory synaptic function and plasticity, but little is known about Ca(2+) dynamics at thorny excrescences, the complex spines on hippocampal CA3 pyramidal cells contacted by mossy fiber terminals of dentate granule cell axons. We have monitored subthreshold stimulus-dependent postsynaptic Ca(2+) transients in optically and ultrastructurally characterized complex spines and find that such spines can act as discrete units of Ca(2+) response. In contrast to the more common "simple" spines, synaptically evoked Ca(2+) transients at complex spines have only a small NMDA receptor-dependent component and do not involve release of calcium from internal stores. Instead, they result mainly from AMPA receptor-gated Ca(2+) influx through voltage-activated calcium channels on the spine; these channels provide graded amplification of the response of thorny excrescences to individual mossy fiber synaptic events.

Kainate receptors regulate unitary IPSCs elicited in pyramidal cells by fast-spiking interneurons in the neocortex

Unitary IPSCs elicited by fast-spiking (FS) interneurons in layer V pyramidal cells of the neocortex were studied by means of dual whole-cell recordings in acute slices. FS to pyramidal cell unitary IPSCs were depressed by (RS)-S-amino-3-(3-hydroxy-5-tert-butylisoxazol-4-yl) (ATPA), a kainate (KA) receptor agonist, and by the endogenous agonist l-glutamate in the presence of AMPA, NMDA, mGluR, and GABA(B) receptor antagonists. This effect was accompanied by an increase in failure rate of synaptic transmission, in the coefficient of variation, and in the paired pulse ratio, indicating a presynaptic origin of the IPSC depression. Pairing the activation of the presynaptic neuron with a depolarization of the postsynaptic cell mimicked the decrease of unitary IPSCs, and this effect persisted when postsynaptic sodium action potentials were blocked with the local anesthetic QX314. The effects of ATPA, glutamate, and of the pairing protocol were almost totally blocked by CNQX. These data suggest that KA receptors located on presynaptic FS cell terminals decrease the release of GABA and can be activated by glutamate released from the somatodendritic compartment of the postsynaptic pyramidal cells.

Regulation of spontaneous inhibitory synaptic transmission by endogenous glutamate via non-NMDA receptors in cultured rat hippocampal neurons

The regulation of gamma-aminobutyric acid (GABA)-mediated spontaneous inhibitory synaptic transmission by endogenously released glutamate was studied in cultured rat hippocampal neurons. After 7 days in vitro (DIV), both spontaneous excitatory postsynaptic currents (sEPSCs) and spontaneous inhibitory postsynaptic currents (sIPSCs) could be detected. After 15 DIV, most postsynaptic spontaneous currents occurred as sEPSC/sIPSC sequences when recorded at a holding voltage of -30 mV. In the presence of the glutamate alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor subtype antagonist LY303070, both the frequency and amplitude of sIPSC were strongly and reversibly reduced. The N-methyl-D-aspartate (NMDA) receptor antagonist, 2-amino-5-phosphonopentanoic acid (AP5), had no effect on sIPSC while cyclothiazide strongly increased sIPSC frequency. Under blockade of AMPA receptors, the kainate- and GluR5-selective kainate receptor agonists, (RS)-2-amino-3-(3-hydroxy-5-tert-butylisoxazol-4-yl) propanoic acid) (ATPA) and (S)-5-iodowillardiine (5IWill), induced a large enhancement of the frequency of small-amplitude sIPSC which was blocked by the non-NMDA receptor antagonist, 2,3-dihydro-6-nitro-7-sulfamoyl-benzo(f)quinoxaline (NBQX). All of these effects were sensitive to tetrodotoxin (TTX). In the presence of LY303070 and TTX, kainate could induce a small inward current while GluR5 agonists had no effect. In the presence of NMDA and AMPA receptor antagonists, the glutamate uptake inhibitor L-trans-pyrrolidine-2,4-dicarboxylic acid (t-PDC) could restore sIPSC. When NBQX was used as an AMPA antagonist, the stimulatory effect of t-PDC was blocked while the group I metabotropic glutamate agonist, 3,5-dihydroxyphenylglycine (DHPG), induced a strong enhancement of sIPSC. Therefore, both AMPA and kainate receptors can regulate inhibitory synaptic transmission in cultured hippocampal neurons, the former by tonic activation, the latter when the glutamate concentration is increased by impairing glutonate uptake.

Electrophysiological analysis of G protein-coupled receptors in mammalian neurons

This unit describes general techniques that are useful for recording electrophysiological responses that are mediated via the activation of G-protein coupled receptors (GPCRs). It includes a brief description of preparations, but focuses primarily on experiments using hippocampal brain slice preparations. Techniques for the preparation of brain slices, electrodes, filling solutions, and the recording protocols that are suitable for assessing the activity of GPCRs using electrophysiological techniques are summarized, and various protocols for the activation of these receptors are discussed.

Kindling Induces Transient NMDA Receptor–Mediated Facilitation of High-Frequency Input in the Rat Dentate Gyrus

To elucidate the gating mechanism of the epileptic dentate gyrus on seizure-like input, we investigated dentate gyrus field potentials and granule cell excitatory postsynaptic potentials (EPSPs) following high-frequency stimulation (10–100 Hz) of the lateral perforant path in an experimental model of temporal lobe epilepsy (i.e., kindled rats). Although control slices showed steady EPSP depression at frequencies greater than 20 Hz, slices taken from animals 48 h after the last seizure presented pronounced EPSP facilitation at 50 and 100 Hz, followed by steady depression. However, 28 days after kindling, the EPSP facilitation was no longer detectable. Using the specific N-methyl-d-aspartate (NMDA) and RS-α-amino-3-hydroxy-5-methyl-4-isoxazoleproponic acid (AMPA) receptor antagonists 2-amino-5-phosphonovaleric acid and SYM 2206, we examined the time course of alterations in glutamate receptor–dependent synaptic currents that parallel transient EPSP facilitation. Forty-eight hours after kindling, the fractional AMPA and NMDA receptor–mediated excitatory postsynaptic current (EPSC) components shifted dramatically in favor of the NMDA receptor–mediated response. Four weeks after kindling, however, AMPA and NMDA receptor–mediated EPSCs reverted to control-like values. Although the granule cells of the dentate gyrus contain mRNA-encoding kainate receptors, neither single nor repetitive perforant path stimuli evoked kainate receptor–mediated EPSCs in control or in kindled rats. The enhanced excitability of the kindled dentate gyrus 48 h after the last seizure, as well as the breakdown of its gating function, appear to result from transiently enhanced NMDA receptor activation that provides significantly slower EPSC kinetics than those observed in control slices and in slices from kindled animals with a 28-day seizure-free interval. Therefore, NMDA receptors seem to play a critical role in the acute throughput of seizure activity and in the induction of the kindled state but not in the persistence of enhanced seizure susceptibility.

A kainate receptor increases the efficacy of GABAergic synapses

Brain functions are based on the dynamic interaction of excitatory and inhibitory inputs. Spillover of glutamate from excitatory synapses may diffuse to and modulate nearby inhibitory synapses. By recording unitary inhibitory postsynaptic currents (uIPSCs) from cell pairs in CA1 of the hippocampus, we demonstrated that low concentrations of Kainate receptor (KAR) agonists increased the success rate (P(s)) of uIPSCs, whereas high concentrations of KAR agonists depressed GABAergic synapses. Ambient glutamate released by basal activities or stimulation of the stratum radiatum increases the efficacy of GABAergic synapses by activating presynaptic KARs, which facilitate Ca(2+)-dependent GABA release. The results suggest that glutamate released from excitatory synapses may also function as an intermediary between excitatory and inhibitory synapses to protect overexcitation of local circuits.

Functional diversity and developmental changes in rat neuronal kainate receptors

1. Whole-cell currents evoked by kainate and the GluR5-selective agonist (RS)-2-amino-3-(3-hydroxy-5-tertbutylisoxazol-4-yl)propanoic acid (ATPA) were used to compare the physiological properties of kainate receptors expressed by neurons from rat hippocampus, spinal cord and dorsal root ganglia. 2. In contrast to kainate, which evoked desensitizing currents with similar decay rates and steady-state components in all three cell types, responses to ATPA were distinctly different in the three cell populations. Currents evoked by ATPA displayed a significant steady-state component in hippocampal neurons, but decayed rapidly to baseline in dorsal root ganglion (DRG) cells. ATPA failed to evoke current in many of the spinal neurons. 3. ATPA caused steady-state desensitization in DRG cells with an IC50 of 41 nM. Recovery from desensitization of DRG cell receptors by ATPA was significantly slower than for any previously described agonist. In contrast, hippocampal kainate receptors recovered from desensitization by ATPA within a few seconds. 4. Half-maximal activation of kainate receptors in hippocampal neurons required 938 nM ATPA. In DRG cells treated with concanavalin A the EC50 for ATPA was 341 nM. ATPA evoked current in embryonic hippocampal neurons but with lower amplitude relative to kainate than in cultured postnatal neurons. 5. Collectively, these results highlight functional differences between neuronal kainate receptors that may reflect their distinct subunit composition and their diverse roles in synaptic transmission.

On the cause of mental retardation in Down syndrome: extrapolation from full and segmental trisomy 16 mouse models

Down syndrome (DS, trisomy 21, Ts21) is the most common known cause of mental retardation. In vivo structural brain imaging in young DS adults, and post-mortem studies, indicate a normal brain size after correction for height, and the absence of neuropathology. Functional imaging with positron emission tomography (PET) shows normal brain glucose metabolism, but fewer significant correlations between metabolic rates in different brain regions than in controls, suggesting reduced functional connections between brain circuit elements. Cultured neurons from Ts21 fetuses and from fetuses of an animal model for DS, the trisomy 16 (Ts16) mouse, do not differ from controls with regard to passive electrical membrane properties, including resting potential and membrane resistance. On the other hand, the trisomic neurons demonstrate abnormal active electrical and biochemical properties (duration of action potential and its rates of depolarization and repolarization, altered kinetics of active Na(+), Ca(2+) and K(+) currents, altered membrane densities of Na(+) and Ca(2+) channels). Another animal model, the adult segmental trisomy 16 mouse (Ts65Dn), demonstrates reduced long-term potentiation and increased long-term depression (models for learning and memory related to synaptic plasticity) in the CA1 region of the hippocampus. Evidence suggests that the abnormalities in the trisomy mouse models are related to defective signal transduction pathways involving the phosphoinositide cycle, protein kinase A and protein kinase C. The phenotypes of DS and its mouse models do not involve abnormal gene products due to mutations or deletions, but result from altered expression of genes on human chromosome 21 or mouse chromosome 16, respectively. To the extent that the defects in signal transduction and in active electrical properties, including synaptic plasticity, that are found in the Ts16 and Ts65Dn mouse models, are found in the brain of DS subjects, we postulate that mental retardation in DS results from such abnormalities. Changes in timing and synaptic interaction between neurons during development can lead to less than optimal functioning of neural circuitry and signaling then and in later life.

Presynaptic kainate receptor mediation of frequency facilitation at hippocampal mossy fiber synapses

Inhibition of transmitter release by presynaptic receptors is widespread in the central nervous system and is typically mediated via metabotropic receptors. In contrast, very little is known about facilitatory receptors, and synaptic activation of a facilitatory autoreceptor has not been established. Here we show that activation of presynaptic kainate receptors can facilitate transmitter release from hippocampal mossy fiber synapses. Synaptic activation of these presumed ionotropic kainate receptors is very fast (<10 ms) and lasts for seconds. Thus, these presynaptic kainate receptors contribute to the short-term plasticity characteristics of mossy fiber synapses, which were previously thought to be an intrinsic property of the synapse.

Identification of the kainate receptor subunits underlying modulation of excitatory synaptic transmission in the CA3 region of the hippocampus

To understand the physiological role of kainate receptors and their participation in seizure induction in animal models of epilepsy, it will be necessary to develop a comprehensive description of their action in the CA3 region of the hippocampus. Activation of presynaptic kainate receptors depresses excitatory synaptic transmission at mossy fiber and associational-commissural inputs to CA3 pyramidal neurons (Vignes et al., 1998; Bortolotto et al., 1999; Kamiya and Ozawa, 2000). In this study, we use gene-targeted mice lacking glutamate receptor 5 (GluR5) or GluR6 kainate receptor subunits to identify the receptor subunits that comprise the kainate receptors responsible for presynaptic modulation of CA3 transmission. We found that bath application of kainate (3 microm) profoundly reduced EPSCs at mossy fiber and collateral synapses in neurons from wild-type and GluR5(-/-) mice but had no effect on EPSCs in neurons from GluR6(-/-) mice. These results therefore contrast with previous studies that supported a role for GluR5-containing receptors at mossy fiber and associational-commissural synapses (Vignes et al., 1998; Bortolotto et al., 1999). Surprisingly, at perforant path synapses kainate receptor activation enhanced transmission; this potentiation was abolished in both GluR5 and GluR6 knock-out mice. Kainate receptors thus play multiple and complex roles to modulate excitatory synaptic transmission in the CA3 region of the hippocampus.

In vivo, the direct and seizure-induced neuronal cytotoxicity of kainate and AMPA is modified by the non-competitive antagonist, GYKI 52466

The 2,3-benzodiazepine GYKI 52466, administered intracerebrally or systemically, was assessed for its ability to protect against the neuronal death in the brain caused by intra-hippocampal injections of the non-N-methyl-D-aspartate (NMDA) receptor agonists, kainate and L-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA). In contrast to a previous report, a low intra-hippocampal dose of GYKI 52466 (25 nmol) did not protect against kainate toxicity. In order to achieve higher doses of GYKI 52466, solubilization in 2-hydroxypropyl-beta-cyclodextrin was used, and limited protection against AMPA, but not kainate toxicity was found. There was a commensurate reduction in seizure-related neuronal loss in the limbic regions of the brain. When diazepam was used to prevent seizures, GYKI 52466 had no effect on hippocampal neuronal loss caused by the direct toxicity of AMPA and kainate on hippocampal neurons. Systemic administration of GYKI 52466 had only a minimal effect on preventing neuronal death caused by AMPA. In vivo, GYKI 52466 is only weakly effective as a neuroprotective agent.

Kainate receptors are involved in short- and long-term plasticity at mossy fiber synapses in the hippocampus

Kainate receptors alter the excitability of mossy fiber axons and have been reported to play a role in the induction of long-term potentiation (LTP) at mossy fiber synapses in the hippocampus. These previous studies have relied primarily on the use of compounds whose selectivity is unclear. In this report, we investigate short- and long-term facilitation of mossy fiber synaptic transmission in kainate receptor knockout mice. We find that LTP is reduced in mice lacking the GluR6, but not the GluR5, kainate receptor subunit. Additionally, short-term synaptic facilitation is impaired in GluR6 knockout mice, suggesting that kainate receptors act as presynaptic autoreceptors on mossy fiber terminals to facilitate synaptic transmission. These data demonstrate that kainate receptors containing the GluR6 subunit are important modulators of mossy fiber synaptic strength.

GluR5,6,7 subunit immunoreactivity on apical pyramidal cell dendrites in hippocampus of schizophrenics and manic depressives

Recent postmortem studies have suggested that changes in the regulation of kainate-sensitive glutamate receptors (kainate receptors) in the hippocampus may play a role in schizophrenia. To explore this possibility further, the distribution of immunoreactivity (IR) for the GluR5,6,7 subunits of the KR was assessed in a cohort consisting of 15 normal controls, 15 schizophrenics, and 9 manic depressives matched for age and postmortem interval (PMI). Cross sections of hippocampus showed abundant GluR5,6,7-IR on apical dendrites of pyramidal neurons in the stratum radiatum and stratum moleculare. In normal controls, both the numerical and length density of IR dendrites were much higher in sector CA2 than in sectors CA3 or CA1. When data for the individual groups were separately examined, the schizophrenics showed a 30-35% reduction in the density of GluR5,6,7-IR dendrites found in both stratum radiatum and stratum moleculare of sectors CA3 and CA2, as well as proximal and middle portions of CA1. In CA2, the magnitude of this decrease in schizophrenia was 2.5 times larger than that seen in any of the other sectors. For the manic depressive group, no significant differences were observed in any sectors or laminae examined. The potential confounding effects of either age, PMI, or neuroleptic exposure do not explain the reduced density of IR dendrites detected in the schizophrenic group. Taken together, the preferential reduction of GluR5,6,7-IR observed on apical dendrites of pyramidal neurons is consistent with a functional downregulation of the kainate receptor in the hippocampus of schizophrenic brain.

NMDA and AMPA/kainate glutamate receptors modulate dentate neurogenesis and CA3 synapsin-I in normal and ischemic hippocampus

The effect of N-methyl-D-aspartate (NMDA) and 2-(aminomethyl)phenylacetic acid/kainate (AMPA/kainate) glutamate receptors on dentate cell proliferation and hippocampal synapsin-I induction was examined after global ischemia. Cell proliferation was assessed using BrdU labeling, and synaptic responses were assessed using synapsin-I expression. Systemic glutamate receptor antagonists (MK-801 and NBQX) increased BrdU-labeled cells in the dentate subgranular zone (SGZ) of control adult gerbils (30% to 90%, P < 0.05). After global ischemia (at 15 days after 10 minutes of ischemia), most CA1 pyramidal neurons died, whereas the numbers of BrdU-labeled cells in the SGZ increased dramatically (>1000%, P < 0.0001). Systemic injections of MK801 or NBQX, as well as intrahippocampal injections of either drug, when given at the time of ischemia completely blocked the birth of cells in the SGZ and the death of CA1 pyramidal neurons at 15 days after ischemia. Glutamate receptor antagonists had little effect on cell birth and death when administered 7 days after ischemia. The induction of synapsin-I protein in stratum moleculare of CA3 at 7 and 15 days after global ischemia was blocked by pretreatment with systemic or intrahippocampal MK-801 or NBQX. It is proposed that decreased dentate glutamate receptor activation--produced by glutamate receptor antagonists in normal animals and by chronic ischemic hippocampal injury--may trigger dentate neurogenesis and synaptogenesis. The synapsin-I induction in mossy fiber terminals most likely represents re-modeling of dentate granule cell neuron presynaptic elements in CA3 in response to the ischemia. The dentate neurogenesis and synaptogenesis that occur after ischemia may contribute to memory recovery after hippocampal injury caused by global ischemia.

The excitatory amino acid glutamate mediates reflexly increased tracheal blood flow and airway submucosal gland secretion

In six decerebrated and in eight alpha-chloralose anesthetized, paralyzed and mechanically ventilated beagle dogs, we have studied involvement of glutamate and glutamate receptors in transmission of excitatory inputs from the airway sensory receptors to the nucleus tractus solitarius and from this site to airway-related vagal preganglionic cells that regulate the tracheal circulation and the submucosal gland secretion. Stimulation of airway sensory fibers by lung deflation-induced reflex increase in tracheal blood flow and submucosal gland secretion. These responses were diminished by prior administration of AMPA/kainate receptor antagonist CNQX into the fourth ventricle (n=6). Furthermore, topical application or microinjection of AMPA/kainate receptor blockers, into the region of the ventrolateral medulla, where airway-related vagal preganglionic neurons are located, abolished the reflex changes in tracheal submucosal gland secretion (n=8); in these dogs mucosal blood flow was not measured). These findings indicate that reflex increase in tracheal blood flow and submucosal gland secretions are mediated mainly via release of glutamate and activation of the AMPA/kainate subtype of glutamate receptors.

Larger intercellular variation in (Q/R) editing of GluR6 than GluR5 revealed by single cell RT-PCR

RNA editing of the pre-mRNA encoding the kainate receptor subtypes determines the Ca2+ permeability and the rectifying properties of the receptors in which these are assembled. GluR6 pre-mRNA contains three characterized editing sites: Q/R, IN and the Y/C, whereas GluR5 pre-mRNA contains only the (Q/R) site. Single cell RT-PCR was used on cultured cortical neurons to determine the relative expression and editing levels of the kainate receptor subunits encoding mRNA. The analysis showed a large intercellular variation in editing efficiency. The overall lower level of GluR5 editing, in the culture, compared to GluR6 editing is a result of an approximately 60% lower editing efficiency of GluR5 pre-mRNA, within single cells, compared with GluR6.

Kainate, a double agent that generates seizures: two decades of progress

Studies using kainate, an excitatory amino acid extracted from a seaweed, have provided major contributions to the understanding of epileptogenesis. Here we review pioneering and more recent studies aimed at determining how kainate generates seizures and, in particular, how inhibition is altered during seizures. We focus on target and subunit-specific effects of kainate on hippocampal pyramidal neurons and interneurons that lead to an excitation of both types of neurons and thus to the parallel increase of glutamatergic and GABAergic spontaneous currents. We propose that kainate excites all its targets, the net consequence depending on the level of activity of the network.

Subunit composition of kainate receptors in hippocampal interneurons

Kainate receptor activation affects GABAergic inhibition in the hippocampus by mechanisms that are thought to involve the GluR5 subunit. We report that disruption of the GluR5 subunit gene does not cause the loss of functional KARs in CA1 interneurons, nor does it prevent kainate-induced inhibition of evoked GABAergic synaptic transmission onto CA1 pyramidal cells. However, KAR function is abolished in mice lacking both GluR5 and GluR6 subunits, indicating that KARs in CA1 stratum radiatum interneurons are heteromeric receptors composed of both subunits. In addition, we show the presence of presynaptic KARs comprising the GluR6 but not the GluR5 subunit that modulate synaptic transmission between inhibitory interneurons. The existence of two separate populations of KARs in hippocampal interneurons adds to the complexity of KAR localization and function.

KA1-like kainate receptor subunit immunoreactivity in neurons and glia using a novel anti-peptide antibody

Functional kainate receptors can be formed by various combinations of subunits with low (GluR5, GluR6 and GluR7) or high affinity (KA1 and KA2) for kainate. The precise contribution of each subunit to native receptors, as well as their distribution within the central nervous system (CNS) is still unclear. Here, we describe the presence of KA1-like immunoreactivity in both neurons and glial cells of the CNS, using a newly developed antiserum to a specific carboxy terminus epitope of the KA1 subunit. Intense immunoreactivity was observed in the CA3 area of the rat hippocampus. Electron microscopy revealed that immunostaining was present in dendritic structures postsynaptic to commissural-associational fibers, rather than in those contacted by mossy fiber terminals. We also observed immunostaining of CA1 pyramidal cell apical dendrites. In the cerebral cortex, KA1-like immunostaining was observed in many pyramidal neuron somata, mainly in layer V, and along their apical dendrites. A subset of gamma-amino-butyric acidic cells were also intensely stained. In the cerebellum, the antiserum selectively stained Purkinje cell somata and their dendrites as well as Bergmann glial processes. Other types of macroglia were also labeled by the KA1 antiserum. Thus, optic nerve oligodendrocytes both in vitro and in situ and cultured astrocytes were densely stained. Our results indicate that KA1-type subunits are more widely distributed throughout the CNS than previously thought. This newly developed antiserum may help to clarify the properties of kainate receptors containing KA1 or KA1-type subunits within the normal and pathological brain.

The multifarious hippocampal mossy fiber pathway: a review

The hippocampal mossy fiber pathway between the granule cells of the dentate gyrus and the pyramidal cells of area CA3 has been the target of numerous scientific studies. Initially, attention was focused on the mossy fiber to CA3 pyramidal cell synapse because it was suggested to be a model synapse for studying the basic properties of synaptic transmission in the CNS. However, the accumulated body of research suggests that the mossy fiber synapse is rather unique in that it has many distinct features not usually observed in cortical synapses. In this review, we have attempted to summarize the many unique features of this hippocampal pathway. We also have attempted to reconcile some discrepancies that exist in the literature concerning the pharmacology, physiology and plasticity of this pathway. In addition we also point out some of the experimental challenges that make electrophysiological study of this pathway so difficult.Finally, we suggest that understanding the functional role of the hippocampal mossy fiber pathway may lie in an appreciation of its variety of unique properties that make it a strong yet broadly modulated synaptic input to postsynaptic targets in the hilus of the dentate gyrus and area CA3 of the hippocampal formation.

Modulation of kainate--activated currents by diazoxide and cyclothiazide analogues (IDRA) in cerebellar granule neurons

1. Patch-clamp technique was used in primary cultures of cerebellar granule neurons to study the modulation of the cyclothiazide analogue (IDRA21) and of the diazoxide derivative (IDRA 5) on KA-evoked currents. 2. The dose-response of kainic acid (KA) reveals an EC50=90 microM and an Hill coefficient of 1.3. IDRA 21 and cyclothiazide potentiate KA-evoked current in a dose dependent way, being cyclothiazide more potent but less efficacious than IDRA 21. Conversely IDRA 5 acts as a negative modulator of KA evoked -current. 3. Application of IDRA 21 and cyclothiazide results in a current potentiation of 125+/-18% and 80+/-12% respectively, while IDRA 5 decreases KA-current (-21+/-5%). Coapplication of cyclothiazide and IDRA 21 produces a potentiation of 110+/-17%, suggesting a competition of the two drugs for the same site. 4. In the same experimental model we studied the ability of IDRA compounds of promoting toxicity through AMPA-receptor activation. Under basal conditions AMPA treatment (50 microM for 1 hour) results in a negligible excitotoxicity. 5. In contrast similar treatment with AMPA + IDRA 21 (1 mM) or + IDRA 5 (1 mM) or + cyclothiazide (100 microM) induces citotoxicity. The neurotoxic damage induced by IDRA 21 and cyclothiazide is blocked by GYKI 53655 (50 microM) and by NBQX (10 microM). Interestingly GYKI and NBQX are ineffective in reducing IDRA 5 toxicity.

Modulation of fast synaptic transmission by presynaptic ligand-gated cation channels

There is now considerable evidence demonstrating that ligand-gated cation channels (i.e., P2X, nicotinic, kainate, NMDA, AMPA and 5-HT(3) receptors), in addition to mediating fast excitatory neurotransmission, may be located presynaptically on nerve terminals in the peripheral and central nervous systems where they function to modulate neurotransmitter release. This modulation can be facilitation, inhibition or both. In this article, we first outline the multiple mechanisms by which activation of presynaptic ligand-gated cation channels can modulate spontaneous and evoked neurotransmitter release, before reviewing in detail published electrophysiological studies of presynaptic P2X, nicotinic, kainate, NMDA, AMPA and 5-HT(3) receptors.

Prolonged synaptic currents and glutamate spillover at the parallel fiber to stellate cell synapse

Although neurons often fire in bursts, most of what is known about glutamate signaling and postsynaptic receptor activation is based on experiments using single stimuli. Here we examine the activation of ionotropic glutamate receptors by bursts at the parallel fiber to stellate cell synapse. We show that brief stimulus trains generate prolonged AMPA receptor (AMPAR)- and NMDA receptor (NMDAR)-mediated EPSCs recorded in whole-cell voltage clamp. These EPSCs contrast with the rapid AMPAR-mediated EPSC evoked by a single stimulus. The prolonged AMPAR-mediated EPSC is promoted by high-frequency and high-intensity trains and can persist for hundreds of milliseconds. This EPSC is also increased by l-trans-2,4-PDC, an inhibitor of glutamate transporters, suggesting that these transporters usually limit the synaptic response to trains. These prolonged EPSCs reflect both receptor properties and a long-lasting glutamate signal. In addition, several experiments demonstrate that glutamate spillover can contribute to receptor activation. First, imaging stimulus-evoked changes in presynaptic calcium establishes that distinct parallel fiber bands can be activated. Second, activation of parallel fibers that do not directly synapse onto a given stellate cell can evoke indirect AMPAR- and NMDAR-mediated EPSCs in that cell. Third, experiments using the use-dependent NMDAR blocker MK-801 show that these indirect EPSCs reflect glutamate spillover in response to trains. Together, these findings indicate that stimulus trains can generate a sustained and widespread glutamate signal that can in turn evoke large and prolonged EPSCs mediated by ionotropic glutamate receptors. These synaptic properties may have important functional consequences for stellate cell firing.

Kainate receptor-mediated synaptic currents in cerebellar Golgi cells are not shaped by diffusion of glutamate

We report the presence of kainate receptors (KARs) in cerebellar Golgi cells of wild-type but not GluR6-deficient mice. Parallel fiber stimulation activates KAR-mediated synaptic currents [KAR-excitatory postsynaptic currents (EPSCs)] of small amplitude. KAR-EPSCs greatly differ from synaptic currents mediated by alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors (AMPAR-EPSCs) at the same synapse. KAR-EPSCs display slow rise and decay time and summate in response to a train of stimulations. By using PDA, a low-affinity competitive antagonist and agents that modify the clearance of glutamate, we show that these properties cannot be explained by diffusion of glutamate outside of the synaptic cleft and activation of extrasynaptic KARs. These data suggest that the slow kinetic of KAR-EPSCs is due to intrinsic properties of KARs being localized at postsynaptic sites. The contrasting properties of KAR- and AMPAR-EPSCs in terms of kinetics and summation offer the possibility for a glutamatergic synapse to integrate excitatory inputs over two different time scales.