Antagonists of GLU(K5)-containing kainate receptors prevent pilocarpine-induced limbic seizures

Topiramate selectively protects against seizures induced by ATPA, a GluR5 kainate receptor agonist

Although the mechanism of action of topiramate is not fully understood, its anticonvulsant properties may result, at least in part, from an interaction with AMPA/kainate receptors. We have recently shown that topiramate selectively inhibits postsynaptic responses mediated by GluR5 kainate receptors. To determine if this action of topiramate is relevant to the anticonvulsant effects of the drug in vivo, we determined the protective activity of topiramate against seizures induced by intravenous infusion of various ionotropic glutamate receptor agonists in mice. Topiramate (25-100 mg/kg, i.p.) produced a dose-dependent elevation in the threshold for clonic seizures induced by infusion of ATPA, a selective agonist of GluR5 kainate receptors. Topiramate was less effective in protecting against clonic seizures induced by kainate, a mixed agonist of AMPA and kainate receptors. Topiramate did not affect clonic seizures induced by AMPA or NMDA. In contrast, the thresholds for tonic seizures induced by higher doses of these various glutamate receptor agonists were all elevated by topiramate. Unlike topiramate, carbamazepine elevated the threshold for AMPA- but not ATPA-induced clonic seizures. Our results are consistent with the possibility that the effects of topiramate on clonic seizure activity are due to functional blockade of GluR5 kainate receptors. Protection from tonic seizures may be mediated by other actions of the drug. Together with our in vitro cellular electrophysiological results, the present observations strongly support a unique mechanism of action of topiramate, which involves GluR5 kainate receptors.

In vitro characterization of 5-carboxyl-2,4-di-benzamidobenzoic acid (NS3763), a noncompetitive antagonist of GLUK5 receptors

Accumulating preclinical data suggest that compounds that block the excitatory effect of glutamate on the kainate subtype of glutamate receptors may have utility for the treatment of pain, migraine, and epilepsy. In the present study, the in vitro pharmacological properties of the novel glutamate antagonist 5-carboxyl-2,4-di-benzamido-benzoic acid (NS3763) are described. In functional assays in human embryonic kidney (HEK)293 cells expressing homomeric GLU(K5) or GLU(K6) receptors, NS3763 is shown to display selectivity for inhibition of domoate-induced increase in intracellular calcium mediated through the GLU(K5) subtype (IC(50) = 1.6 microM) of kainate receptors compared with the GLU(K6) subtype (IC(50) > 30 microM). NS3763 inhibits the GLU(K5)-mediated response in a noncompetitive manner and does not inhibit [(3)H]alpha-amino-3-hydroxy-5-tertbutylisoxazole-4-propionic acid binding to GLU(K5) receptors. Furthermore, NS3763 selectively inhibits l-glutamate- and domoate-evoked currents through GLU(K5) receptors in HEK293 cells and does not significantly inhibit alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid- or N-methyl-d-aspartate-induced currents in cultured mouse cortical neurons at 30 microM. This is the first report on a selective and noncompetitive GLU(K5) antagonist.

Anticonvulsant action of hippocampal dopamine and serotonin is independently mediated by D2 and 5-HT1A receptors

The present microdialysis study evaluated the anticonvulsant activity of extracellular hippocampal dopamine (DA) and serotonin (5-HT) with concomitant assessment of the possible mutual interactions between these monoamines. The anticonvulsant effects of intrahippocampally applied DA and 5-HT concentrations were evaluated against pilocarpine-induced seizures in conscious rats. DA or 5-HT perfusions protected the rats from limbic seizures as long as extracellular DA or 5-HT concentrations ranged, respectively, between 70-400% and 80-350% increases compared with the baseline levels. Co-perfusion with the selective D(2) blocker remoxipride or the selective 5-HT(1A) blocker WAY-100635 clearly abolished all anticonvulsant effects. These anticonvulsant effects were mediated independently since no mutual 5-HT and DA interactions were observed as long as extracellular DA and 5-HT levels remained within these protective ranges. Simultaneous D(2) and 5-HT(1A) receptor blockade significantly aggravated pilocarpine-induced seizures. High extracellular DA (> 1000% increases) or 5-HT (> 900% increases) concentrations also worsened seizure outcome. The latter proconvulsive effects were associated with significant increases in extracellular glutamate (Glu) and mutual increases in extracellular monoamines. Our results suggest that, within a certain concentration range, DA and 5-HT contribute independently to the prevention of hippocampal epileptogenesis via, respectively, D(2) and 5-HT(1A) receptor activation.

In vivo modulation of extracellular hippocampal glutamate and GABA levels and limbic seizures by group I and II metabotropic glutamate receptor ligands

The effects of several metabotropic receptor (mGluR) ligands on baseline hippocampal glutamate and GABA overflow in conscious rats and the modulation of limbic seizure activity by these ligands were investigated. Intrahippocampal mGluR group I agonist perfusion via a microdialysis probe [1 mm (R,S)-3,5-dihydroxyphenylglycine] induced seizures and concomitant augmentations in amino acid dialysate levels. The mGlu1a receptor antagonist LY367385 (1 mm) decreased baseline glutamate but not GABA concentrations, suggesting that mGlu1a receptors, which regulate hippocampal glutamate levels, are tonically activated by endogenous glutamate. This decrease in glutamate may contribute to the reported LY367385-mediated anticonvulsant effect. The mGlu5 receptor antagonist 2-methyl-6-(phenylethynyl)-pyridine (50 mg/kg) also clearly abolished pilocarpine-induced seizures. Agonist-mediated actions at mGlu2/3 receptors by LY379268 (100 microm, 10 mg/kg intraperitoneally) decreased basal hippocampal GABA but not glutamate levels. This may partly explain the increased excitation following systemic LY379268 administration and the lack of complete anticonvulsant protection within our epilepsy model with the mGlu2/3 receptor agonist. Group II selective mGluR receptor blockade with LY341495 (1-10 microm) did not alter the rats' behaviour or hippocampal amino acid levels. These data provide a neurochemical basis for the full anticonvulsant effects of mGlu1a and mGlu5 antagonists and the partial effects observed with mGlu2/3 agonists in vivo.

Kainate receptors and synaptic transmission

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

Long-term alterations in glutamate receptor and transporter expression following early-life seizures are associated with increased seizure susceptibility

Prolonged seizures in early childhood are associated with an increased risk of development of epilepsy in later life. The mechanism(s) behind this susceptibility to later development of epilepsy is unclear. Increased synaptic activity during development has been shown to permanently alter excitatory neurotransmission and could be one of the mechanisms involved in this increased susceptibility to the development of epilepsy. In the present study we determine the effect of status-epilepticus induced by lithium/pilocarpine at postnatal day 10 (P10 SE) on the expression of glutamate receptor and transporter mRNAs in hippocampal dentate granule cells and protein levels in dentate gyrus of these animals in adulthood. The results revealed a decrease in glutamate receptor 2 (GluR2) mRNA expression and protein levels as well as an increase in protein levels for the excitatory amino acid carrier 1 (EAAC1) in P10 SE rats compared to controls. Expression of glutamate receptor 1 (GluR1) mRNA was decreased in both P10 SE rats and identically handled, lithium-injected littermate controls compared to naive animals, and GluR1 protein levels were significantly lower in lithium-controls than in naive rats, suggesting an effect of either the handling or the lithium on GluR1 expression. These changes in EAA receptors and transporters were accompanied by an increased susceptibility to kainic acid induced seizures in P10 SE rats compared to controls. The current data suggest that early-life status-epilepticus can result in permanent alterations in glutamate receptor and transporter gene expression, which may contribute to a lower seizure threshold.

Selective antagonism of GluR5 kainate-receptor-mediated synaptic currents by topiramate in rat basolateral amygdala neurons

Topiramate is a widely used antiepileptic agent whose mechanism of action is poorly understood. The drug has been reported to interact with various ion channel types, including AMPA/kainate receptors. In whole-cell voltage-clamp recordings from principal neurons of the rat basolateral amygdala, topiramate at low concentrations (IC50, approximately 0.5 microm) selectively inhibited pharmacologically isolated excitatory synaptic currents mediated by kainate receptors containing the GluR5 subunit. Topiramate also partially depressed predominantly AMPA-receptor-mediated EPSCs, but with lower efficacy. Topiramate did not alter the degree of facilitation in paired-pulse experiments, and it reduced the amplitude of miniature EPSCs without affecting their frequency, demonstrating that the block of synaptic responses occurs postsynaptically. Inhibition of GluR5 kainate receptors could represent a key mechanism underlying the anticonvulsant activity of topiramate. Moreover, these results support the concept that GluR5 kainate receptors represent a novel target for antiepileptic drug development.

Noncanonical signaling by ionotropic kainate receptors

The potent neurotoxin kainate activates ion channel-forming receptors. However, it can also activate a G protein-coupled signaling pathway to inhibit transmitter release in central neurons. It remains unclear whether the same receptor complex is involved in both signaling activities. Here we show that in a population of dorsal root ganglion cells, exposure to kainate elicits a G protein-dependent increase in intracellular Ca2+. Furthermore, in these cells a brief exposure to kainate inhibited the K+-induced Ca2+ increase, a process that was sensitive to the G protein inhibitor Pertussis toxin and inhibitors of protein kinase C. This metabotropic action did not require ion channel activity and was not observed in neurons prepared from mice deficient for the ion channel-forming subunit GluR5. These results indicate that GluR5, an ion channel-forming subunit, signals through a second messenger cascade, inhibiting voltage-dependent Ca2+ channels. Thus, such a system represents a noncanonical signaling route of ion channel-forming receptors.

Kainate receptors and the induction of mossy fibre long-term potentiation

There is intense interest in understanding the molecular mechanisms involved in long-term potentiation (LTP) in the hippocampus. Significant progress in our understanding of LTP has followed from studies of glutamate receptors, of which there are four main subtypes (alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA), N-methyl-D-aspartate (NMDA), mGlu and kainate). This article summarizes the evidence that the kainate subtype of glutamate receptor is an important trigger for the induction of LTP at mossy fibre synapses in the CA3 region of the hippocampus. The pharmacology of the first selective kainate receptor antagonists, in particular the GLU(K5) subunit selective antagonist LY382884, is described. LY382884 selectively blocks the induction of mossy fibre LTP, in response to a variety of different high-frequency stimulation protocols. This antagonist also inhibits the pronounced synaptic facilitation of mossy fibre transmission that occurs during high-frequency stimulation. These effects are attributed to the presence of presynaptic GLU(K5)-subunit-containing kainate receptors at mossy fibre synapses. Differences in kainate receptor-dependent synaptic facilitation of AMPA and NMDA receptor-mediated synaptic transmission are described. These data are discussed in the context of earlier reports that glutamate receptors are not involved in mossy fibre LTP and more recent experiments using kainate receptor knockout mice, that argue for the involvement of GLU(K6) but not GLU(K5) kainate receptor subunits. We conclude that activation of presynaptic GLU(K5)-containing kainate receptors is an important trigger for the induction of mossy fibre LTP in the hippocampus.

(S)-2-Amino-3-(3-hydroxy-7,8-dihydro-6H-cyclohepta[d]isoxazol-4-yl)propionic acid, a potent and selective agonist at the GluR5 subtype of ionotropic glutamate receptors. Synthesis, modeling, and molecular pharmacology

We have previously described (RS)-2-amino-3-(3-hydroxy-7,8-dihydro-6H-cyclohepta[d]isoxazol-4-yl)propionic acid (4-AHCP) as a highly effective agonist at non-N-methyl-d-aspartate (non-NMDA) glutamate (Glu) receptors in vivo, which is more potent than (RS)-2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propionic acid (AMPA) but inactive at NMDA receptors. However, 4-AHCP was found to be much weaker than AMPA as an inhibitor of [(3)H]AMPA binding and to have limited effect in a [(3)H]kainic acid binding assay using rat cortical membranes. To shed light on the mechanism(s) underlying this quite enigmatic pharmacological profile of 4-AHCP, we have now developed a synthesis of (S)-4-AHCP (6) and (R)-4-AHCP (7). At cloned metabotropic Glu receptors mGluR1alpha (group I), mGluR2 (group II), and mGluR4a (group III), neither 6 nor 7 showed significant agonist or antagonist effects. The stereoisomer 6, but not 7, activated cloned AMPA receptor subunits GluR1o, GluR3o, and GluR4o with EC(50) values in the range 4.5-15 microM and the coexpressed kainate-preferring subunits GluR6 + KA2 (EC(50) = 6.4 microM). Compound 6, but not 7, proved to be a very potent agonist (EC(50) = 0.13 microM) at the kainate-preferring GluR5 subunit, equipotent with (S)-2-amino-3-(5-tert-butyl-3-hydroxyisothiazol-4-yl)propionic acid [(S)-Thio-ATPA, 4] and almost 4 times more potent than (S)-2-amino-3-(5-tert-butyl-3-hydroxyisoxazol-4-yl)propionic acid [(S)-ATPA, 3]. Compound 6 thus represents a new structural class of GluR5 agonists. Molecular modeling and docking to a crystal structure of the extracellular binding domain of the AMPA subunit GluR2 has enabled identification of the probable active conformation and binding mode of 6. We are able to rationalize the observed selectivities by comparing the docking of 4 and 6 to subtype constructs, i.e., a crystal structure of the extracellular binding domain of GluR2 and a homology model of GluR5.

Structural requirements for novel willardiine derivatives acting as AMPA and kainate receptor antagonists

1. The natural product willardiine is an AMPA receptor agonist. We have examined the structural changes required to convert willardiine into an antagonist at AMPA and kainate receptors. Structure-activity analysis has been carried out to discover the structural features required to increase the potency and/or selectivity of the antagonists at AMPA or kainate receptors. 2. Reduction of the fast component of the dorsal root-evoked ventral root potential (fDR-VRP) has been used to investigate AMPA receptor antagonist activity. To examine antagonist activity at kainate receptors, the ability of compounds to depress kainate-induced depolarisations of dorsal root fibres was assessed. 3. Blocking ionisation of the uracil ring by adding a methyl group to the N(3) position was not sufficient to convert willardiine into an antagonist. However, willardiine derivatives with a side-chain bearing a carboxylic acid group at the N(3)-position of the uracil ring could antagonise AMPA and kainate receptors. 4. S stereochemistry was optimal for antagonism. When compounds with differing interacidic group chain lengths were compared, a group chain length of two methylene groups was preferable for AMPA receptor antagonism in the series of compounds bearing a carboxyalkyl side chain (UBP275, UBP277 and UBP279 reduced the fDR-VRP with IC(50) values of 287+/-41, 23.8+/-3.9 and 136+/-17 micro M, respectively). For kainate receptor antagonism, two or three methylene groups were almost equally acceptable (UBP277 and UBP279 reduced dorsal root kainate responses with apparent K(D) values of 73.1+/-4.5 and 60.5+/-4.1 micro M, respectively). 5. Adding an iodo group to the 5-position of UBP277 and UBP282 enhanced activity at kainate receptors (UBP291 and UBP301 antagonised kainate responses on the dorsal root with apparent K(D) values of 9.83+/-1.62 and 5.94+/-0.63 micro M, respectively). 6. The most useful antagonist identified in this study was UBP301, which was a potent and approximately 30-fold selective kainate receptor antagonist. UBP282 may also be of use in isolating a non-GluR5-mediated kainate response.