Neuropharmacology of AMPA and kainate receptors

Kainate receptors are involved in synaptic plasticity

The ability of synapses to modify their synaptic strength in response to activity is a fundamental property of the nervous system and may be an essential component of learning and memory. There are three classes of ionotropic glutamate receptor, namely NMDA (N-methyl-D-aspartate), AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole-4-propionic acid) and kainate receptors; critical roles in synaptic plasticity have been identified for two of these. Thus, at many synapses in the brain, transient activation of NMDA receptors leads to a persistent modification in the strength of synaptic transmission mediated by AMPA receptors. Here, to determine whether kainate receptors are involved in synaptic plasticity, we have used a new antagonist, LY382884 ((3S, 4aR, 6S, 8aR)-6-((4-carboxyphenyl)methyl-1,2,3,4,4a,5,6,7,8,8a-decahydro isoquinoline-3-carboxylic acid), which antagonizes kainate receptors at concentrations that do not affect AMPA or NMDA receptors. We find that LY382884 is a selective antagonist at neuronal kainate receptors containing the GluR5 subunit. It has no effect on long-term potentiation (LTP) that is dependent on NMDA receptors but prevents the induction of mossy fibre LTP, which is independent of NMDA receptors. Thus, kainate receptors can act as the induction trigger for long-term changes in synaptic transmission.

High-affinity [3H] kainic acid binding to brain membranes: a re-evaluation of ligand potency and selectivity

[3H]Kainic acid ([3H]KA) is a widely used tool for studying the KA class of excitatory amino acid receptors. [3H]KA of significantly higher specific activity has become available permitting use of radioligand concentrations below the dissociation constant (K(D)) of the high-affinity binding site. We employed low radioligand (0.05-0.2 nM) and receptor concentrations (0.01 nM) to gain new insights into the binding characteristics of the high-affinity KA binding site in a standard preparation of lyzed synaptosomal membranes from the cerebral cortex of male Sprague-Dawley rats. Under these conditions, KA binds to a single class of high-affinity sites with a K(D) of 1.0+/- 0.3 nM. The potencies of competing agents are considerably higher than published reports. Specifically, domoic acid, glutamate, and glutamine exhibit IC(50) values for displacing [3H]KA of 0.37+/-0.02, 94+/-13, and 1500+/-500 nM, respectively. Domoate (1 microM) was tested against a panel of 32 central nervous system binding sites and found to be inactive at each, indicating this toxin displays considerable selectivity. This study illustrates the remarkable potency of domoic acid and underlines the importance of performing radioligand binding studies at concentrations of constituents that permit characterization of high-affinity interactions.

Reversible neural inactivation reveals hippocampal participation in several memory processes

Studies of patients and animals with brain lesions have implicated the hippocampal formation in spatial, declarative/relational and episodic types of memory. These and other types of memory consist of a series of interdependent but potentially dissociable memory processes-encoding, storage, consolidation and retrieval. To identify whether hippocampal activity contributes to these processes independently, we used a novel method of inactivating synaptic transmission using a water-soluble antagonist of AMPA/kainate glutamate receptors. Once calibrated using electrophysiological and two-deoxyglucose techniques in vivo, drug or vehicle was infused chronically or acutely into the dorsal hippocampus of rats at appropriate times during or after training in a water maze. Our findings indicate that hippocampal neural activity is necessary for both encoding and retrieval of spatial memory and for either trace consolidation or long-term storage.

Pharmacological characterization of a GluR6 kainate receptor in cultured hippocampal neurons

We have examined the pharmacology of kainate receptors in cultured hippocampal neurons (6-8 days in vitro (DIV)) from embryonic rats (E17). Cultured neurons were pre-treated with concanavalin A to remove kainate receptor desensitization and whole-cell voltage clamp electrophysiology employed to record inward currents in response to glutamatergic agonists and antagonists. N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid (AMPA) receptor responses were blocked using MK801 (3 microM) and the 2,3-benzodiazepine, LY300168 (GYKI53655, 50 microM), respectively. Inward currents were recorded in hippocampal neurons upon application of kainate and the 2S,4R isomer of 4-methyl glutamic acid (SYM2081) with EC50 values of 3.4 +/- 0.4 microM and 1.6 +/- 0.5 microM, respectively (n = 6 cells). The GluR5 selective agonists, LY339434 (100 microM) and (RS)-2-amino-3-(3-hydroxy-5-tert-butyl-4-isoxazolyl) propionic acid (ATPA) (100 microM), did not evoke detectable inward currents in any cell responding to kainate. LY293558 and the selective GluR5 antagonist, LY382884, had weak antagonist effects on responses evoked by either kainate or (2S,4R)-4-methyl glutamate (IC50 > 300 microM). The quinoxalinedione, 2,3-dihyro-6-nitro-7-sulfamoyl-benzo(f)quinoxaline (NBQX), blocked both kainate and (2S,4R)-4-methyl glutamate-activated currents at much lower concentrations (IC50 approximately 10 microM). These results provide pharmacological evidence that ion channels comprised of GluR6 kainate receptor subunits mediate kainate receptor responses in hippocampal neurons cultured 6-8 DIV.

Excitotoxic injury profiles of low-affinity kainate receptor agonists in cortical neuronal cultures

Neurotoxic profiles of putative agonists for low-affinity kainate subtypes of L-glutamate receptors (GluR5-7) were determined in cultured cortical neurones. Rank order of neurotoxic potency (microM): (S)-5-iodowillardiine (9) approximately = (2S,4R,6E)-2-amino-4-carboxy-7-(2-naphthyl)hept-6-enoic acid (LY339434, 11) > (2S,4R)-4-methylglutamate (33) > kainate (100) > (RS)-2-amino-3-(hydroxy-5-tert-butylisoxazol-4-yl)propanoic acid (ATPA, 360). Using ionotropic glutamate receptor antagonists, neurotoxicity induced by kainate, ATPA and (S)-5-iodowillardiine appeared to involve a GluR5-7 component, unlike LY339434 and (2S,4R)-4-methylglutamate. These putative GluR5-7 agonists exhibited complex excitotoxic profiles highlighting the importance of studying native glutamate receptors.

Developmental and activity-dependent regulation of kainate receptors at thalamocortical synapses

Most of the fast excitatory synaptic transmission in the mammalian brain is mediated by ionotrophic glutamate receptors, of which there are three subtypes: AMPA (alpha-amino-3-hydroxyl-5-methyl-4-isoxazolepropionate), NMDA (N-methyl-D-aspartate) and kainate. Although kainate-receptor subunits (GluR5-7, KA1 and 2) are widely expressed in the mammalian central nervous system, little is known about their function. The development of pharmacological agents that distinguish between AMPA and kainate receptors has now allowed the functions of kainate receptors to be investigated. The modulation of synaptic transmission by kainate receptors and their synaptic activation in a variety of brain regions have been reported. The expression of kainate receptor subunits is developmentally regulated but their role in plasticity and development is unknown. Here we show that developing thalamocortical synapses express postsynaptic kainate receptors as well as AMPA receptors; however, the two receptor subtypes do not colocalize. During the critical period for experience-dependent plasticity, the kainate-receptor contribution to transmission decreases; a similar decrease occurs when long-term potentiation is induced in vitro. This indicates that during development there is activity-dependent regulation of the expression of kainate receptors at thalamocortical synapses.

Ionotropic glutamate receptors

The glutamate-binding sites of ionotropic glutamate receptors are formed from two extracellular domains of a single subunit. Conformational changes induced by agonist binding produce mechanical processes that are translated into ion gating and receptor desensitization. The interactions between macromolecular assemblies of synaptic proteins and ionotropic glutamate receptors, and their subsequent roles in receptor clustering and specificity are being elucidated. Kainate receptor pharmacology is finally revealing its secrets as a result of the availability of selective pharmacological agents.

Synthesis and anticonvulsant activity of 3-aryl-5H-2,3-benzodiazephine AMPA antagonists

A novel series of 3-aryl-5H-2,3-benzodiazepines with N-3 aromatic substituents has been synthesized. Good in vivo anticonvulsant activity of the new compounds has been demonstrated employing the maximal electroshock seizure test in mice. Evaluation of a subset of the compounds in the cortical wedge assay confirmed the new structures to be AMPA antagonists.

Prolonged enhancement of AP-1 DNA binding by blockade of glutamate uptake in cultured neurons

Prolonged blockade of glutamate reuptake by the specific inhibitor of glutamate transporters, L-transpyrrolidine-2,4-dicarboxylate (PDC), produces a dramatic decrease in NMDA-induced neurotoxicity in cerebellar granule cell cultures, and is accompanied by a down-regulation of NMDA receptors. We now report that cultured cerebellar granule cells treated with 100 microM PDC for 1, 2, 4, 8, 16 and 24h, respectively, show increased AP-1 DNA-binding activity as measured by electrophoretic mobility shift assay. This effect was blocked by the NMDA receptor antagonist, CGP 37849, indicative of a pivotal role of NMDA receptors in the PDC-evoked enhancement of AP-1 DNA-binding. Our results suggest that AP-1 may be involved in the transcriptional regulation of neuronal adaptation initiated by prolonged inhibition of glutamate reuptake.

Functional high- and low affinity agonist binding sites at native dorsal horn AMPA receptors

Transmission and processing of nociceptive information in the superficial dorsal horn (DH) of the spinal cord involves activation of AMPA-type glutamate receptors (AMPARs). We have studied the properties of native AMPARs in freshly dissociated laminae I-II neurones from postnatal rats using a modified form of the concentration-clamp technique for fast agonist application. Analysis of kainic acid dose-response curves showed the existence of two types of functional agonist binding sites governing AMPAR activation. These sites differ by their affinity for the agonist. Depending on the neurotransmitter concentration reached in the synaptic cleft, the relative contribution of high affinity and low affinity sites might play an important role in the shaping of AMPAR-mediated postsynaptic currents.