A functionally active presynaptic high-affinity kainate receptor in the rat hippocampal CA3 subregion

Presynaptic kainate receptor-mediated facilitation of glutamate release involves Ca2+ -calmodulin at mossy fiber-CA3 synapses

Presynaptic kainate receptors (KARs) modulate the release of glutamate at synapses established between mossy fibers (MF) and CA3 pyramidal cells in the hippocampus. The activation of KAR by low, nanomolar, kainate concentrations facilitates glutamate release. KAR-mediated facilitation of glutamate release involves the activation of an adenylate cyclase/cyclic adenosine monophosphate/protein kinase A cascade at MF-CA3 synapses. Here, we studied the mechanisms by which KAR activation produces this facilitation of glutamate release in slices and synaptosomes. We find that the facilitation of glutamate release mediated by KAR activation requires an increase in Ca(2+) levels in the cytosol and the formation of a Ca(2+) -calmodulin complex to activate adenylate cyclase. The increase in cytosolic Ca(2+) underpinning this modulation is achieved, both, by Ca(2+) entering via Ca(2+) -permeable KARs and, by the mobilization of intraterminal Ca(2+) stores. Finally, we find that, congruent with the Ca(2+) -calmodulin support of KAR-mediated facilitation of glutamate release, induction of long-term potentiation at MF-CA3 synapses has an obligate requirement for Ca(2+) -calmodulin activity.

Neuroprotective efficiency of NMDA receptor blockade in the striatum and CA3 hippocampus after various durations of cerebral ischemia in gerbils

The purpose of this study was to investigate neuroprotective efficiency of N-methyl D-aspartate (NMDA) receptor (NMDAR) blockade on the neuronal damage in the less studied and allegedly less affected CA3 hippocampus and striatum in the Mongolian gerbil model of global cerebral ischemia. The common carotid arteries of gerbils were occluded for 5, 10 or 15 minutes. Gerbils were given a low dose of non-competitive NMDA antagonist (MK-801, 3 mg/kg i.p.) or saline immediately after the occlusion in normothermic conditions. Neuronal damage was examined on 4th, 14th and 28th day after reperfusion. The effect of NMDAR blockade was followed in vivo by monitoring the neurological status of whole animals or at the cellular level by standard light- and confocal microscopy on brain slices. Increased duration of cerebral ischemia resulted in a progressive loss of striatal and CA3 hippocampal neurons. The most beneficial NMDAR blockade effect was observed when the neuronal damage was most severe - on the 28th day after 15-min ischemia. As judged by morphological and neurological data, the effect of ischemia is also apparent in the presumed less vulnerable regions (CA3 and striatum) which are functionally important in stroke plasticity. So, NMDAR blockade in normothermic conditions showed neuroprotective efficiency.

Role of ionotropic glutamate receptors in the regulation of hippocampal norepinephrine output in vivo

In vitro evidence indicates that norepinephrine release in the mammalian hippocampus is modulated by glutamate receptors. With the use of microdialysis, we have now evaluated the role of ionotropic glutamate receptors in the regulation of hippocampal norepinephrine output in vivo. Stimulation of N-methyl-D-aspartate (NMDA)-sensitive glutamate receptors by local administration of NMDA (1-100μM) resulted in a concentration-dependent decrease in the extracellular concentration of norepinephrine in the hippocampus of freely moving rats, whereas the blockade of these receptors with MK801 (1-100μM) induced a concentration-dependent increase in norepinephrine output. Activation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-sensitive glutamate receptors with AMPA (1-100μM) resulted in a biphasic effect on the extracellular norepinephrine concentration, with a decrease in this parameter apparent at 10μM and an increase at 100μM. The AMPA receptor antagonist 6,7-dinitroquinoxaline-2,3-dione had no effect on norepinephrine output. The GABA(A) receptor antagonist bicuculline (10μM) prevented the decrease in hippocampal norepinephrine output induced by either NMDA or 10μM AMPA. Our results thus implicate ionotropic glutamate receptors as key regulators of norepinephrine release in the hippocampus and may therefore provide a basis for the development of new drugs for stress-related disorders.

Mobility of NMDA autoreceptors but not postsynaptic receptors at glutamate synapses in the rat entorhinal cortex

NMDA receptors (NMDAr) are known to undergo recycling and lateral diffusion in postsynaptic spines and dendrites. However, NMDAr are also present as autoreceptors on glutamate terminals, where they act to facilitate glutamate release, but it is not known whether these receptors are also mobile. We have used functional pharmacological approaches to examine whether NMDA receptors at excitatory synapses in the rat entorhinal cortex are mobile at either postsynaptic sites or in presynaptic terminals. When NMDAr-mediated evoked EPSCs (eEPSCs) were blocked by MK-801, they showed no evidence of recovery when the irreversible blocker was removed, suggesting that postsynaptic NMDAr were relatively stably anchored at these synapses. However, using frequency-dependent facilitation of AMPA receptor (AMPAr)-mediated eEPSCs as a reporter of presynaptic NMDAr activity, we found that when facilitation was blocked with MK-801 there was a rapid (approximately 30-40 min) anomalous recovery upon removal of the antagonist. This was not observed when global NMDAr blockade was induced by combined perfusion with MK-801 and NMDA. Anomalous recovery was accompanied by an increase in frequency of spontaneous EPSCs, and a variable increase in frequency-facilitation. Following recovery from blockade of presynaptic NMDAr with a competitive antagonist, frequency-dependent facilitation of AMPAr-mediated eEPSCs was also transiently enhanced. Finally, an increase in frequency of miniature EPSCs induced by NMDA was succeeded by a persistent decrease. Our data provide the first evidence for mobility of NMDAr in the presynaptic terminals, and may point to a role of this process in activity-dependent control of glutamate release.

Kainate receptors with a metabotropic modus operandi

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

Dual presynaptic control by ATP of glutamate release via facilitatory P2X1, P2X2/3, and P2X3 and inhibitory P2Y1, P2Y2, and/or P2Y4 receptors in the rat hippocampus

ATP is released in a vesicular manner from nerve terminals mainly at higher stimulation frequencies. There is a robust expression of ATP (P2) receptors in the brain, but their role is primarily unknown. We report that ATP analogs biphasically modulate the evoked release of glutamate from purified nerve terminals of the rat hippocampus, the facilitation being mediated by P2X1, P2X2/3, and P2X3 [antagonized by 8-(benzamido)naphthalene-1,3,5-trisulfonate and 2',3'-O-(2,4,6-trinitrophenyl)-ATP] and the inhibition by P2Y1, P2Y2, and/or P2Y4 [antagonized by reactive blue 2 and 2'deoxy-N6-methyladenosine-3',5'-bisphosphate and mimicked by P1-(urinine 5'-),P4-(inosine 5'-) tetraphosphate and 2-methylthio-ADP] receptors. The combination of single-cell PCR analysis of rat hippocampal pyramidal neurons, Western blot analysis of purified presynaptic active zone fraction, and immunocytochemical analysis of hippocampal glutamatergic terminals revealed that the P2 receptors expressed in glutamatergic neurons, located in the active zone and in glutamatergic terminals, were precisely P2X1, P2X2, and P2X3 subunits and P2Y1, P2Y2, and P2Y4 receptors. This provides coincident functional and molecular evidence that P2 receptors are present and act presynaptically as a modulatory system controlling hippocampal glutamate release.

Presynaptic kainate receptors are localized close to release sites in rat hippocampal synapses

The subsynaptic distribution of kainate receptors is still a matter of much debate given its importance to understand the way they influence neuronal communication. Here, we show that, in synapses of the rat hippocampus, presynaptic kainate receptors are localized within the presynaptic active zone close to neurotransmitter release sites. The activation of these receptors with low concentrations of agonists induces the release of [(3)H]glutamate in the absence of a depolarizing stimulus. Furthermore, this modulation of [(3)H]glutamate release by kainate is more efficient when compared with a KCl-evoked depolarization that causes a more than two-fold increase in the intra-terminal calcium concentration but no apparent release of [(3)H]glutamate, suggesting a direct receptor-mediated process. Using a selective synaptic fractionation technique that allows for a highly efficient separation of presynaptic, postsynaptic and non-synaptic proteins we confirmed that, presynaptically, kainate receptors are mainly localized within the active zone of hippocampal synapses where they are expected to be in a privileged position to modulate synaptic phenomena.

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.

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.

Regulation of vesicle traffic and neurotransmitter release in isolated nerve terminals

In this overview current insights in the regulation of presynaptic transmitter release, mainly acquired in studies using isolated CNS nerve terminals are highlighted. The following aspects are described. (i) The usefulness of pinched-off nerve terminals, so-called synaptosomes, for biochemical and ultrastructural studies of presynaptic stimulus-secretion coupling. (ii) The regulation of neurotransmitter release by multiple Ca2+ channels, with special emphasis on the specificity of different classes of these channels with respect to the release of distinct types of neurotransmitters, that are often co-localized, such as amino acids and neuropeptides. (iii) Possible molecular mechanisms involved in targeting synaptic vesicle (SV) traffic toward the active zone. (iv) The role of presynaptic receptors in regulating transmitter release, with special emphasis on different glutamate subtype receptors. Isolated nerve terminals are of great value as model system in order to obtain a better understanding of the regulation of the release of distinct classes of neurotransmitters in tiny CNS nerve endings.

Implication of ionotropic glutamate receptors in the release of noradrenaline in hippocampal CA1 and CA3 subregions under oxygen and glucose deprivation

A strong linkage between adrenergic and glutamatergic systems exists in the CNS but it is still unclear whether the excessive release of noradrenaline under ischemic conditions is modulated by excitatory amino acids. We studied the effect of selective glutamate receptor antagonists on the release of [3H]noradrenaline evoked by glucose and oxygen deprivation in hippocampal CA1, CA3 and dentate gyrus subregions. The release of glutamate, aspartate and GABA was measured by HPLC. Omission of oxygen and glucose increased the release of [3H]noradrenaline as well as the release of amino acids. Maximum effect on noradrenaline release was observed in CA1 region. The relative increase of the release after 30 min energy deprivation (R(2)) versus the basal release under normal conditions (R(1)), i.e. the R(2)/R(1) ratio was 7.1+/-1.0, 3.87+/-0.4 and 3.26+/-0.27 for CA1, CA3 and dentate gyrus, respectively. The [3H]noradrenaline outflow in response to glucose and oxygen deprivation was abolished at low temperature, but not by Ca(2+) removal, suggesting a cytoplasmic release process. In CA1 and CA3 [3H]noradrenaline release was significantly attenuated by MK-801, an NMDA receptor antagonist. The AMPA receptor antagonist GYKI-53784 had no effect in CA3, but partly reduced noradrenaline release in CA1. Our results suggest that ionotropic glutamate receptors seem to be implicated in the massive cytoplasmic release of noradrenaline in CA1 what may contribute to its selective vulnerability.

Zinc chelation during non-lesioning overexcitation results in neuronal death in the mouse hippocampus

In the hippocampus, chelatable zinc is accumulated in vesicles of glutamatergic presynaptic terminals, abounding specially in the mossy fibers, from where it is released with activity and can exert a powerful inhibitory action upon N-methyl-D-aspartate receptors. Zinc is therefore in a strategic situation to control overexcitation at the zinc-rich excitatory synapses, and consequently zinc removal during high activity might result in excitotoxic neuronal damage. We analyzed the effect of zinc chelation with sodium dietyldithiocarbamate under overexcitation conditions induced by non-lesioning doses of kainic acid in the mouse hippocampus, to get insight into the role of zinc under overexcitation. Swiss male mice were injected with kainic acid (15 mg/kg, i.p.) 15 min prior to sodium dietyldithiocarbamate (150 mg/kg, i.p.), and left to survive for 6 h, 1 day, 4 days, or 7 days after the treatment. Cell damage was analyzed with the hematoxylin-eosin and acid fuchsin stainings. Neither control animals treated only with kainic acid nor those treated only with sodium dietyldithiocarbamate suffered seizures or neuronal damage. By contrast, the kainic acid+sodium dietyldithiocarbamate-treated animals showed convulsive behavior and cell death involving the hilus, CA3, and CA1 regions. Pretreatment with the N-methyl-D-aspartate receptor antagonist MK801 (1 mg/kg, i.p.) completely prevented neuronal damage. Experiments combining different doses of sodium dietyldithiocarbamate and kainic acid with different administration schedules demonstrated that the overlap of zinc chelation and overexcitation is necessary to trigger the observed effects. Moreover, the treatment with a high dose of sodium dietyldithiocarbamate (1000 mg/kg), which produced a complete bleaching of the Timm staining for approximately 12 h, highly increased the sensitivity of animals to kainic acid. Altogether, our results indicate that the actions of sodium dietyldithiocarbamate are based on a reduction of zinc levels, which under overexcitation conditions induce seizures and neuronal damage. These findings fully support a protective role for synaptically released zinc during high neuronal activity, most probably mediated by its inhibitory actions on N-methyl-D-aspartate receptors, and argue against a direct action of synaptic zinc on the observed neuronal damage.

Cobalt staining of hippocampal neurons mediated by non-desensitizing activation of AMPA but not kainate receptors

Activation of calcium permeable glutamate receptors is likely to be important for neuronal death associated with brain trauma, stroke and neurodegenerative diseases. Cobalt uptake can be used to identify cells containing Ca2+-permeable non-NMDA ionotropic glutamate receptors. However, the relative contribution of AMPA and kainate receptors, and also the role of receptor desensitization on the influx of Co2+, remain to be established. We found that the selective non-desensitizing activation of AMPA receptors was efficient in promoting Co2+ staining. However, the selective activation of kainate receptors, even under non-desensitizing conditions, did not result in Co2+ staining. Taken together, our results show that non-desensitizing stimulation of AMPA, but not of kainate receptors, mediates the influx of Co2+ in cultured rat hippocampal neurons.

Increased density of hippocampal kainate receptors but normal density of NMDA and AMPA receptors in a rat model of prenatal protein malnutrition

The postnatal development of excitatory amino acid receptor types including kainate, N-methyl-D-aspartate (NMDA), and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) was assessed in the hippocampus, entorhinal cortex, and adjacent neocortex in normal and prenatally protein malnourished rats ages 15, 30, 90, and 220 postnatal days by quantitative autoradiography. Tritiated ligands used to measure binding site density were (3)[H]kainate, (3)[H]MK-801, and (3)[H]AMPA, respectively. Kainate receptors showed statistically significant increases in binding density in stratum lucidum of CA3 (hippocampal mossy fiber zone) in 90- and 220-day-old malnourished rats compared with age- and sex-matched controls but not in 15- or 30-day-old malnourished rats. Compared with previous anatomic studies, these results are mostly in agreement with a significantly decreased hippocampal mossy fiber plexus in 15-, 90-, and 220-day-old rats but not in 30-day-old rats. These results suggested that the increased density of postsynaptic kainate receptors located mainly on proximal apical dendrites of CA3 pyramidal cells may be compensatory to decreased glutamate release due to the reduction in mossy fiber plexus. In contrast, the density of putative NMDA and AMPA receptors quantified in prenatally malnourished rats was comparable to the density quantified in age- and sex-matched control rats, as were all three receptor types in entorhinal cortex and adjacent neocortex. Thus, the selectivity of the compensation of (3)[H]kainate-labeled mossy fiber plexus in adult but not in early postnatal developing malnourished rats may help ensure continued breeding and survival of the species under otherwise adverse environmental conditions.

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.

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.

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.

Comparative behavioural toxicity of domoic acid and kainic acid in neonatal rats

Cumulative behavioural toxicity was measured in groups of male and female rat pups (n=6/sex) at different stages of postnatal development. Dose-response curves (DRCs) for toxicity produced by domoic acid (DOM) were generated using animals on postnatal days (PND) 0, 5, 14, and 22, using a behavioural rating scale. In a subsequent experiment, DRCs for toxicity generated by either DOM or kainic acid were produced in rats at PND 8 and 14 for comparison between the two toxins. DOM was found to be a very potent neurotoxin in newborn rats and the potency of DOM progressively decreased with increasing age (interpolated ED(50)=0.12, 0.15, 0.30, and 1.06 mg/kg at PND 0, 5, 14, and 22, respectively). In addition, the patterns of behavioural expression were found to differ with age. Comparisons between DOM and kainic acid revealed that DOM was approximately six-fold more potent than kainate at both PND 8 and PND 14 and that both toxins were approximately two-fold less potent in PND 14 rats, compared to PND 8. This implies that the mechanism(s) responsible for reduced potency is/are similar between the two compounds. Consistent with previous reports, however, there were both similarities and differences in the observed patterns of behavioural toxicity produced by the two toxins at both ages.

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.

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.

Presynaptic kainate receptors in the monkey striatum

Although kainate has long been known as a powerful axon-sparing neurotoxin, the localization and functions of kainate receptors in the CNS are largely unknown. In the present study we examined the distribution of kainate receptor subunits in the monkey striatum using kainate receptor subunits GluR6/7 and kainate receptor subunit KA2 subunit antibodies at the electron microscope level. We found that kainate receptor subunits GluR6/7 immunoreactivity is expressed not only in neuronal perikarya and dendritic processes, but also in a large population of terminals which form axospinous and axodendritic asymmetric synapses. The ultrastructural features of these terminals resembled those of glutamatergic corticostriatal boutons. In contrast, very few kainate receptor subunit KA2-containing terminals were encountered. Although the functions of these presynaptic kainate receptors remain to be established, the present data suggest the possibility that they are located to modulate the release of glutamate from cortical afferents in the monkey striatum, and that an abnormal regulation of these presynaptic receptors might be involved in the death of striatal neurons in Huntington's disease. Accordingly, recent findings demonstrated that the variance in the age of onset of Huntington's disease could be attributed to the genotype variation of kainate receptor subunit GluR6 in humans.

A high-affinity presynaptic kainate-type glutamate receptor facilitates glutamate exocytosis from cerebral cortex nerve terminals (synaptosomes)

Ionotropic glutamate receptor agonists, kainate, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and domoate, all facilitated 4-aminopyridine-evoked glutamate release from rat cerebrocortical nerve terminals (synaptosomes). The non-selective, non-N-methyl-D-aspartate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione blocked kainate facilitation of glutamate release. AMPA responses were non-desensitizing and insensitive to the AMPA receptor desensitization inhibitor, cyclothiazide. The AMPA receptor antagonist GYKI 52466 failed to block ionotropic glutamate receptor-mediated facilitation, but the ionotropic glutamate receptor 6 kainate receptor subunit antagonist NS-102 was a potent blocker. Furthermore, kainate and AMPA responses were not additive. Taken together, our results indicate that, in the cerebral cortex, both kainate and AMPA may be facilitating glutamate release through the activation of a high-affinity kainate receptor containing glutamate receptor 6 kainate subunits. Kainate enhanced 4-aminopyridine-evoked depolarization of the synaptosomal plasma membrane potential, indicating that a ligand-gated ion channel that conducts cations may underlie the mechanism by which kainate mediates facilitation of glutamate release. While the facilitatory effect of kainate on glutamate release is consistent with a classical ionotropic action of ionotropic glutamate receptors, our observation that kainate inhibits GABA release suggests that alternative presynaptic mechanisms may operate in cerebrocortical nerve terminals to mediate the ionotropic glutamate receptor modulation of glutamate and GABA release. We conclude that high-affinity kainate-type glutamate autoreceptors represent a positive feed-forward system for potentiating the release of glutamate from cerebrocortical nerve terminals.

Presynaptic ionotropic receptors and the control of transmitter release

The quantity of neurotransmitter released into the synaptic cleft, the reliability with which it is released, and the response of the postsynaptic cell to that transmitter all contribute to the strength of a synaptic connection. The presynaptic nerve terminal is a major regulatory site for activity-dependent changes in synaptic function. Ionotropic receptors for the inhibitory amino acid GABA, expressed on the presynaptic terminals of crustacean motor axons and vertebrate sensory neurons, were the first well-defined mechanism for the heterosynaptic transmitter-mediated regulation of transmitter release. Recently, presynaptic ionotropic receptors for a large range of transmitters have been found to be widespread throughout the central and peripheral nervous systems. In this review, we first consider some general theoretical issues regarding whether and how presynaptic ionotropic receptors are important regulators of presynaptic function. We consider the criteria that should be met to identify a presynaptic ionotropic receptor and its regulatory function and review several examples of presynaptic receptors that meet at least some of those criteria. We summarize the classic studies of presynaptic inhibition mediated by GABA-gated Cl channels and then focus on presynaptic nicotinic ACh receptors and presynaptic glutamate receptors. Finally, we briefly discuss evidence for other types of presynaptic ionotropic receptors.

Neurochemistry and pharmacology of the major hippocampal transmitter systems: synaptic and nonsynaptic interactions

Hippocampus plays a crucial role in important brain functions (e.g. memory, learning) thus in the past two decades this brain region became a major objective of neuroscience research. During this period large number of anatomical, neurochemical and electrophysiological data have been accumulated. While excellent reviews have been published on the anatomy and electrophysiology of hippocampal formation, the neurochemistry of this area has not been thoroughly surveyed. Therefore the aim of this review is to summarize the neurochemical and pharmacological data on the release of the major neurotransmitters found in the hippocampal region: glutamate (GLU), gamma-amino butyric acid (GABA), acetylcholine (ACh), noradrenaline (NA) and serotonin (5-HT). In addition, this review analyzes the synaptic and nonsynaptic interactions between hippocampal neuronal elements and overviews how auto- and heteroreceptors are involved in the presynaptic modulation of transmitter release. The presented data clearly show that transmitters released from axon terminals without synaptic contact play an important role in the fine tuning of communication between neurons within a neuronal circuit.

Glutamate receptors in the mammalian central nervous system

Glutamate receptors (GluRs) mediate most of the excitatory neurotransmission in the mammalian central nervous system (CNS). In addition, they are involved in plastic changes in synaptic transmission as well as excitotoxic neuronal cell death that occurs in a variety of acute and chronic neurological disorders. The GluRs are divided into two distinct groups, ionotropic and metabotropic receptors. The ionotropic receptors (iGluRs) are further subdivided into three groups: alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), kainate and N-methyl-D-aspartate (NMDA) receptor channels. The metabotropic receptors (mGluRs) are coupled to GTP-binding proteins (G-proteins), and regulate the production of intracellular messengers. The application of molecular cloning technology has greatly advanced our understanding of the GluR system. To date, at least 14 cDNAs of subunit proteins constituting iGluRs and 8 cDNAs of proteins constituting mGluRs have been cloned in the mammalian CNS, and the molecular structure, distribution and developmental change in the CNS, functional and pharmacological properties of each receptor subunit have been elucidated. Furthermore, the obtained clones have provided valuable tools for conducting studies to clarify the physiological and pathophysiological significances of each subunit. For example, the generation of gene knockout mice has disclosed critical roles of some GluR subunits in brain functions. In this article, we review recent progress in the research for GluRs with special emphasis on the molecular diversity of the GluR system and its implications for physiology and pathology of the CNS.

Hyperexcitability in combined entorhinal/hippocampal slices of adult rat after exposure to brain-derived neurotrophic factor

Effects of brain-derived neurotrophic factor (BDNF) in area CA3, the dentate gyrus, and medial entorhinal cortex were examined electrophysiologically by bath application of BDNF in slices containing the hippocampus and entorhinal cortex. Bath application of 25-100 ng/ml BDNF for 30-90 min increased responses to single afferent stimuli in selective pathways in the majority of slices. In area CA3, responses to mossy fiber stimulation increased in 73% of slices and entorhinal cortex responses to white matter stimulation increased in 64% of slices. After exposure to BDNF, these areas also demonstrated evidence of hyperexcitability, because responses to repetitive stimulation (1-Hz paired pulses for several s) produced multiple population spikes in response to mossy fiber stimulation in CA3 or multiple field potentials in response to white matter stimulation in the entorhinal cortex. Repetitive field potentials persisted after repetitive stimulation ended and usually were followed by spreading depression. Enhancement of responses to single stimuli and hyperexcitability were never evoked in untreated slices or after bath application of boiled BDNF or cytochrome C. The tyrosine kinase antagonist K252a (2 microM) blocked the effects of BDNF. In area CA3, both the potentiation of responses to single stimuli and hyperexcitability showed afferent specificity, because responses to mossy fiber stimulation were affected but responses to fimbria or Schaffer collateral stimulation were not. In addition, regional specificity was demonstrated in that the dentate gyrus was much less affected. The effects of BDNF in area CA3 were similar to those produced by bath application of low doses of kainic acid, which is thought to modulate glutamate release from mossy fiber terminals by a presynaptic action. These results suggest that BDNF has acute effects on excitability in different areas of the hippocampal-entorhinal circuit. These effects appear to be greatest in areas that are highly immunoreactive for BDNF, such as the mossy fibers and the entorhinal cortex. Although the present experiments do not elucidate mechanism(s) definitively, the afferent specificity, similarity to the effects of kainic acid, and block by K252a are consistent with previous hypotheses that BDNF affects acute excitability by a presynaptic action on trkB receptors that modulate excitatory amino acid transmission. However, we cannot rule out actions on inhibitory synapses or postsynaptic processes.

Kainate receptors mediate a slow postsynaptic current in hippocampal CA3 neurons

Glutamate, the neurotransmitter at most excitatory synapses in the brain, activates a variety of receptor subtypes that can broadly be divided into ionotropic (ligand-gated ion channels) and metabotropic (G-protein-coupled) receptors. Ionotropic receptors mediate fast excitatory synaptic transmission, and based on pharmacological and molecular biological studies are divided into NMDA and non-NMDA subtypes. The non-NMDA receptor group is further divided into AMPA and kainate subtypes. Virtually all fast excitatory postsynaptic currents studied so far in the central nervous system are mediated by the AMPA and NMDA subtypes of receptors. Surprisingly, despite extensive analysis of their structure, biophysical properties and anatomical distribution, a synaptic role for kainate receptors in the brain has not been found. Here we report that repetitive activation of the hippocampal mossy fibre pathway, which is associated with high-affinity kainate binding and many of the kainate receptor subtypes, generates a slow excitatory synaptic current with all of the properties expected of a kainate receptor. This activity-dependent synaptic current greatly augments the excitatory drive of CA3 pyramidal cells.

Inhibition of [3H] gamma-aminobutyric acid release by kainate receptor activation in rat hippocampal synaptosomes

We studied the modulation of gamma-aminobutyric acid (GABA) release by activation of kainate receptor in rat whole hippocampal synaptosomes. Kainate (10-300 microM) inhibited [3H]GABA release in a concentration-dependent manner with an EC50 of 25 microM. This effect of kainate (30 microM) was prevented by the ionotropic non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM) and by the selective kainate receptor antagonist 5-nitro-6,7,8,9-tetrahydrobenzo(g)indole-2,3-dione-3-oxime (NS-102, 10 microM), but not by the selective non-competitive AMPA receptor antagonist 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5 H-2,3-benzodiazepine (GYKI 52466, 100 microM). Other kainate receptor agonists, such as domoic acid (0.3-10 microM) and (2S,4R)-4-methylglutamic acid (MGA, 0.3-3 microM), also inhibited [3H]GABA release in a concentration-dependent manner with EC50 values of 4.0 microM and 0.90 microM, respectively, whereas alpha-amino-3-hydroxy-5-methyl-4-oxazolepropionate (AMPA, 10-100 microM) was devoid of effect. These inhibitory effects of both domoic acid (3 microM) and MGA (1 microM) were antagonized by CNQX (10 microM). These results indicate that GABA release can be modulated directly by presynaptic high-affinity kainate heteroreceptors.

Regulation of glutamate release by presynaptic kainate receptors in the hippocampus

Most reported actions of kainate are mediated by AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate) receptors. Here we report that, unlike AMPA which stimulates, kainate elicits a dose-dependent decrease in L-glutamate release from rat hippocampal synaptosomes and also depresses glutamatergic synaptic transmission. Brief exposure to kainate inhibited Ca(2+)-dependent [3H]L-glutamate release by up to 80%. Inhibition was reversed by kainate antagonists but not by the AMPA-selective non-competitive antagonist 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine (GYKI 52466). A corresponding reversible kainate-evoked depression of NMDA (N-methyl-D-aspartate) receptor-mediated excitatory postsynaptic currents (e.p.s.cs) was observed when AMPA receptors were blocked by GYKI 52466. The synaptic depression was preceded by a brief period of enhanced release and a small inward current was also observed. The effects of kainate were unaffected by metabotropic glutamate (mGlu), GABAA, GABAB, glycine and adenosine receptor antagonists. These results indicate that glutamate release can be modulated directly by kainate autoreceptors.