Identification and characterization of RNA aptamers: A long aptamer blocks the AMPA receptor and a short aptamer blocks both AMPA and kainate receptors

AMPA and kainate receptors, along with NMDA receptors, represent different subtypes of glutamate ion channels. AMPA and kainate receptors share a high degree of sequence and structural similarities, and excessive activity of these receptors has been implicated in neurological diseases such as epilepsy. Therefore, blocking detrimental activity of both receptor types could be therapeutically beneficial. Here, we report the use of an in vitro evolution approach involving systematic evolution of ligands by exponential enrichment with a single AMPA receptor target (i.e. GluA1/2R) to isolate RNA aptamers that can potentially inhibit both AMPA and kainate receptors. A full-length or 101-nucleotide (nt) aptamer selectively inhibited GluA1/2R with a KI of ∼5 μm, along with GluA1 and GluA2 AMPA receptor subunits. Of note, its shorter version (55 nt) inhibited both AMPA and kainate receptors. In particular, this shorter aptamer blocked equally potently the activity of both the GluK1 and GluK2 kainate receptors. Using homologous binding and whole-cell recording assays, we found that an RNA aptamer most likely binds to the receptor's regulatory site and inhibits it noncompetitively. Our results suggest the potential of using a single receptor target to develop RNA aptamers with dual activity for effectively blocking both AMPA and kainate receptors.

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title: The Molecular Pharmacology and Cell Biology of α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid Receptors
abstract: α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors (AMPARs) are of fundamental importance in the brain. They are responsible for the majority of fast excitatory synaptic transmission, and their overactivation is potently excitotoxic. Recent findings have implicated AMPARs in synapse formation and stabilization, and regulation of functional AMPARs is the principal mechanism underlying synaptic plasticity. Changes in AMPAR activity have been described in the pathology of numerous diseases, such as Alzheimer's disease, stroke, and epilepsy. Unsurprisingly, the developmental and activity-dependent changes in the functional synaptic expression of these receptors are under tight cellular regulation. The molecular and cellular mechanisms that control the postsynaptic insertion, arrangement, and lifetime of surface-expressed AMPARs are the subject of intense and widespread investigation. For example, there has been an explosion of information about proteins that interact with AMPAR subunits, and these interactors are beginning to provide real insight into the molecular and cellular mechanisms underlying the cell biology of AMPARs. As a result, there has been considerable progress in this field, and the aim of this review is to provide an account of the current state of knowledge.

title: Glutamate Receptor Ion Channels: Structure, Regulation, and Function
abstract: The mammalian ionotropic glutamate receptor family encodes 18 gene products that coassemble to form ligand-gated ion channels containing an agonist recognition site, a transmembrane ion permeation pathway, and gating elements that couple agonist-induced conformational changes to the opening or closing of the permeation pore. Glutamate receptors mediate fast excitatory synaptic transmission in the central nervous system and are localized on neuronal and non-neuronal cells. These receptors regulate a broad spectrum of processes in the brain, spinal cord, retina, and peripheral nervous system. Glutamate receptors are postulated to play important roles in numerous neurological diseases and have attracted intense scrutiny. The description of glutamate receptor structure, including its transmembrane elements, reveals a complex assembly of multiple semiautonomous extracellular domains linked to a pore-forming element with striking resemblance to an inverted potassium channel. In this review we discuss International Union of Basic and Clinical Pharmacology glutamate receptor nomenclature, structure, assembly, accessory subunits, interacting proteins, gene expression and translation, post-translational modifications, agonist and antagonist pharmacology, allosteric modulation, mechanisms of gating and permeation, roles in normal physiological function, as well as the potential therapeutic use of pharmacological agents acting at glutamate receptors.

title: Structure and symmetry inform gating principles of ionotropic glutamate receptors
abstract: Ionotropic glutamate receptors (iGluRs) transduce signals derived from release of the excitatory neurotransmitter glutamate from pre‐synaptic neurons into excitation of post‐synaptic neurons on a millisecond time‐scale. In recent years, the elucidation of full‐length iGluR structures of NMDA, AMPA and kainate receptors by X‐ray crystallography and single particle cryo‐electron microscopy has greatly enhanced our understanding of the interrelationships between receptor architecture and gating mechanism. Here we briefly review full‐length iGluR structures and discuss the similarities and differences between NMDA receptors and non‐NMDA iGluRs. We focus on distinct conformations, including ligand‐free, agonist‐bound active, agonist‐bound desensitized and antagonist‐bound conformations as well as modulator and auxiliary protein‐bound states. These findings provide insights into structure‐based mechanisms of iGluR gating and modulation which together shape the amplitude and time course of the excitatory postsynaptic potential. This article is part of the Special Issue entitled ‘Ionotropic glutamate receptors’.

title: Kainate receptors coming of age: milestones of two decades of research
abstract: Two decades have passed since the first report of the cloning of a kainate-type glutamate receptor (KAR) subunit. The intervening years have seen a rapid growth in our understanding of the biophysical properties and function of KARs in the brain. This research has led to an appreciation that KARs play very distinct roles at synapses relative to other members of the glutamate-gated ion channel receptor family, despite structural and functional commonalities. The surprisingly diverse and complex nature of KAR signaling underlies their unique impact upon neuronal networks through their direct and indirect effects on synaptic transmission, and their prominent role in regulating cell excitability. This review pieces together highlights from the two decades of research subsequent to the cloning of the first subunit, and provides an overview of our current understanding of the role of KARs in the CNS and their potential importance to neurological and neuropsychiatric disorders.

title: Kainate receptors and the induction of mossy fibre long-term potentiation.
abstract: 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.

title: A family of AMPA-selective glutamate receptors.
abstract: Four cloned cDNAs encoding 900-amino acid putative glutamate receptors with approximately 70 percent sequence identity were isolated from a rat brain cDNA library. In situ hybridization revealed differential expression patterns of the cognate mRNAs throughout the brain. Functional expression of the cDNAs in cultured mammalian cells generated receptors displaying alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-selective binding pharmacology (AMPA = quisqualate greater than glutamate greater than kainate) as well as cation channels gated by glutamate, AMPA, and kainate and blocked by 6,7-dinitroquinoxaline-2,3-dione (CNQX).

title: Expression of transcripts encoding AMPA receptor subunits and associated postsynaptic proteins in the macaque brain
abstract: Glutamate is the primary excitatory neurotransmitter in the central nervous system, regulating numerous cellular signaling pathways and controlling the excitability of central synapses both pre‐ and postsynaptically. Localization, cell surface expression, and activity‐dependent regulation of glutamate receptors in both neurons and glia are performed and maintained by a complex network of protein–protein interactions associated with targeting, anchoring, and spatially organizing synaptic proteins at the cell membrane. Using in situ hybridization, we examined the expression of transcripts encoding the AMPA receptor subunits (GluR1–GluR4) and a family of AMPA‐related intracellular proteins. We focused on PDZ‐proteins that are involved in the regulated pool and anchoring AMPA subunits to the cell membrane (PICK1, syntenin), and those maintaining the constitutive pool of AMPA receptors at the glutamatergic synapse (NSF, stargazin). In addition, we studied a fifth protein, KIAA1719, with high homology to the rat PDZ protein ABP, associated with the clustering of AMPA receptors at the glutamate synapse. The AMPA subunits showed significant differences in regional expression, especially in the neocortex, thalamus, striatum, and cerebellum. The expression of other proteins, even those related to a specific AMPA subunit (such as ABP and PICK1 to GluR2 and GluR3), often had different distributions, whereas others (like NSF) are ubiquitously distributed in the brain. These results suggest that AMPA subunits and related intracellular proteins are differentially distributed in the macaque brain, and in numerous structures there are significant mismatches, suggesting additional functional properties of the associated intracellular proteins.  2004.

title: A complex mosaic of high-affinity kainate receptors in rat brain
abstract: The significance for CNS function of glutamate-gated cation channels that exhibit high-affinity kainate sites is not understood. Such receptors, which on dorsal root ganglia and in recombinant systems exhibit currents that rapidly desensitize to kainate application, have not been detected electrophysiologically in the brain. However, a comparison of the distribution of mRNAs encoding five glutamate receptor subunits exhibiting high-affinity kainate sites (GluR-5-GluR- 7, KA-1, and KA-2) indicates that high-affinity kainate receptors are most likely involved in all central neuronal circuits of the rat brain. The KA-1 mRNA occurs mainly in the CA3 field of the hippocampus and dentate gyrus, with much lower amounts being found in inner cortical layers, cerebellar Purkinje cells, and white matter (e.g., corpus callosum and anterior commissure). The KA-2 gene is widely expressed in many neuronal nuclei including layers II-VI of neocortex, hippocampal pyramidal (CA1-CA3) and dentate granule cells, septal nuclei such as the bed nucleus of the stria terminalis, medial preoptic, suprachiasmatic, and ventral medial hypothalamic nuclei, dorsal raphe, locus coeruleus, and cerebellar granule cells. KA-2 mRNA is also found in the pineal gland. GluR-5 transcripts are in the cingulate and piriform cortex, the subiculum, lateral septal nuclei, anteroventral thalamus, suprachiasmatic nucleus, the tegmental nuclei, pontine nuclei, and Purkinje cells. GluR-6 mRNA is most abundant in cerebellar granule cells, with lower levels in caudate-putamen and the pyramidal cell layers and dentate granule cells of hippocampus. The GluR-7 gene is prominently expressed in the inner neocortical layers and some cells in layer II, subiculum, caudate-putamen, reticular thalamus, ventral medial hypothalamic nucleus, pontine nuclei, and in putative stellate/basket cells in the cerebellum. These findings suggest that a complex mosaic of receptor variants underlies the high-affinity kainate receptor in the vertebrate brain.

title: Distribution of kainate receptor subunit mRNAs in human hippocampus, neocortex and cerebellum, and bilateral reduction of hippocampal GluR6 and KA2 transcripts in schizophrenia
abstract: The mRNAs encoding kainic acid (KA) preferring glutamate receptor subunits (GluR5-7, KA1 and KA2) are differentially expressed in rat brain. We have used regional and cellular in situ hybridization histochemistry with subunit-specific 35S-labelled oligodeoxyribonucleotides to examine these mRNAs in adult human hippocampus, neocortex and cerebellum. GluR5 mRNA was detected only in Purkinje cells and a few scattered hippocampal neurons. GluR6 mRNA was relatively abundant in all areas, notably in dentate gyrus, pyramidal neurons of CA3, and cerebellar granule cells, as well as being present in superficial and deep laminae of the neocortex. Moderate signal for GluR7 mRNA was seen in deep laminae of the neocortex with a weak signal in the dentate gyrus; in dipped sections GluR7 mRNA was also apparent over some pyramidal and non-pyramidal cells in hippocampus and over putative cerebellar stellate/basket cells. KA1 mRNA was detected in the dentate gyrus but not reliably elsewhere. The expression profile and abundance of KA2 mRNA was similar to that of GluR6 mRNA. For all five transcripts, concurrent hybridization of rat brain sections produced the anticipated distribution of signal. The data indicate that the regional and cellular distribution of KA receptor subunit mRNAs in human hippocampus, neocortex and cerebellum largely parallels that in the corresponding areas of rat brain, albeit at lower levels, especially with regard to GluR5 and KA1 transcripts. In schizophrenia there is a partial loss of hippocampal non-NMDA receptors, but there are no data concerning KA receptor subunit expression. KA2 and GluR6 mRNAs were sufficiently abundant for a comparison in the left and right hippocampus between 11 schizophrenics and 13 controls. Using film autoradiography, both mRNAs were significantly reduced in the schizophrenics, having controlled for the effects of brain pH, post mortem interval and age. GluR6 mRNA was also quantitated in cerebellum, wherein no differences were found between cases and controls. In conjunction with earlier findings of reduced hippocampal GluR1 and GluR2 expression and a loss of [3H]KA binding sites, these data show that schizophrenia is associated with impaired expression of both AMPA- and KA-preferring ionotropic glutamate receptors. These deficits are likely to contribute to the glutamatergic component of the disease pathophysiology.

title: Determination of kainate receptor subunit ratios in mouse brain using novel chimeric protein standards
abstract: Kainate‐type glutamate receptors (KARs) are tetrameric channels assembled from GluK1‐5. GluK1‐3 are low‐affinity subunits that form homomeric and heteromeric KARs, while GluK4 and GluK5 are high‐affinity subunits that require co‐assembly with GluK1‐3 for functional expression. Although the subunit composition is thought to be highly heterogeneous in the brain, the distribution of KAR subunits at the protein level and their relative abundance in given regions of the brain remain largely unknown. In the present study, we titrated C‐terminal antibodies to each KAR subunit using chimeric GluA2‐GluK fusion proteins, and measured their relative abundance in the P2 and post‐synaptic density (PSD) fractions of the adult mouse hippocampus and cerebellum. Analytical western blots showed that GluK2 and GluK3 were the major KAR subunits, with additional expression of GluK5 in the hippocampus and cerebellum. In both regions, GluK4 was very low and GluK1 was below the detection threshold. The relative amount of low‐affinity subunits (GluK2 plus GluK3) was several times higher than that of high‐affinity subunits (GluK4 plus GluK5) in both regions. Of note, the highest ratio of high‐affinity subunits to low‐affinity subunits was found in the hippocampal PSD fraction (0.32), suggesting that heteromeric receptors consisting of high‐ and low‐affinity subunits highly accumulate at hippocampal synapses. In comparison, this ratio was decreased to 0.15 in the cerebellar PSD fraction, suggesting that KARs consisting of low‐affinity subunits are more prevalent in the cerebellum. Therefore, low‐affinity KAR subunits are predominant in the brain, with distinct subunit combinations between the hippocampus and cerebellum.

title: Kainate receptor-mediated synaptic transmission in the adult anterior cingulate cortex.
abstract: Kainate (KA) receptors are expressed widely in the CNS. However, little is known about their functional characterization, molecular identity, and role in synaptic transmission in the forebrain of adult mice. Patch-clamp recordings in genetically modified mice show that postsynaptic KA receptors contribute to fast synaptic transmission in pyramidal neurons in the anterior cingulate cortex (ACC), a forebrain region critical for higher-order cognitive brain functions such as memory and mental disorders. Single-shock stimulation could induce small KA receptor-mediated excitatory postsynaptic currents (KA EPSCs) in the presence of picrotoxin, D-2-amino-5-phosphono-pentanoic acid, and a selective AMPA receptor antagonist, GYKI 53655. KA EPSCs had a significantly slower rise time course and decay time constant compared with AMPA receptor-mediated EPSCs. High-frequency repetitive stimulation significantly facilitated the KA EPSCs. Genetic deletion of the GluR6 or GluR5 subunit significantly reduced, and GluR5 and 6 double knockout completely abolished, KA EPSCs and KA-activated currents in ACC pyramidal neurons. Our results show that KA receptors contribute to synaptic transmission in adult ACC pyramidal neurons and provide a synaptic basis for the physiology and pathology of KA receptors in ACC-related functions.

title: Physiopathology of kainate receptors in epilepsy.
abstract: Kainate receptors (KARs) are tetrameric ionotropic glutamate receptors composed of the combinations of five subunits GluK1-GluK5. KARs are structurally related to AMPA receptors but they serve quite distinct functions by regulating the activity of synaptic circuits at presynaptic and postsynaptic sites, through either ionotropic or metabotropic actions. Although kainate is a potent neurotoxin known to induce acute seizures through activation of KARs, the actual role of KARs in the clinically-relevant chronic phase of temporal lobe epilepsy (TLE) has long been elusive. Recent evidences have described pathophysiological mechanisms of heteromeric GluK2/GluK5 KARs in generating recurrent seizures in chronic epilepsy. The role of the other major subunit GluK1 in epileptogenic activity is still a matter of debate. This review will present the current knowledge on the subtype-specific pharmacology of KARs and highlight recent results linking KARs to epileptic conditions.

title: Altered synaptic physiology and reduced susceptibility to kainate-induced seizures in GluR6-deficient mice
abstract: l-glutamate, the neurotransmitter of the majority of excitatory synapses in the brain, acts on three classes of ionotropic receptors: NMDA (N-methyl-d-aspartate), AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) and kainate receptors. Little is known about the physiological role of kainate receptors because in many experimental situations it is not possible to distinguish them from AMPA receptors,. Mice with disrupted kainate receptor genes enable the study of the specific role of kainate receptors in synaptic transmission as well as in the neurotoxic effects of kainate. We have now generated mutant mice lacking the kainate-receptor subunit GluR6. The hippocampal neurons in the CA3 region of these mutant mice are much less sensitive to kainate. In addition, a postsynaptic kainate current evoked in CA3 neurons by a train of stimulation of the mossy fibre system is absent in the mutant,. We find that GluR6-deficient mice are less susceptible to systemic administration of kainate, as judged by onset of seizures and by the activation of immediate early genes in the hippocampus. Our results indicate that kainate receptors containing the GluR6 subunit are important in synaptic transmission as well as in the epileptogenic effects of kainate.

title: Repeated Application of 4-Aminopyridine Provoke an Increase in Entorhinal Cortex Excitability and Rearrange AMPA and Kainate Receptors
abstract: AbstractEntorhinal cortex is a highly epilepsy-prone brain region. Effects of repetitive seizures on ionotropic glutamate receptors (iGluRs) were investigated in rat entorhinal cortex slices. Seizures were induced by daily administration of 4-aminopyridine (4-AP). Electrophysiological, pharmacological and histological investigations were carried out to determine changes in synaptic efficacy and in sensitivity of iGluRs due to recurring seizures. Repeated 4-AP-induced seizures increased the amplitude of evoked synaptic field responses in rat entorhinal cortical slices. While vulnerability to inhibition of AMPA receptors by the specific antagonist GYKI 52466 was slightly reduced, responsiveness to NMDA receptor antagonist APV remained unaffected. Testing of bivalent cation permeability of iGluRs revealed reduced Ca2+-influx through non-NMDA receptors. According to the semi-quantitative histoblot analysis GluA1–4, GluA1, GluA2, GluK5, GluN1 and GluN2A subunit protein expression differently altered. While there was a marked decrease in the level of GluA1–4, GluA2 and GluK5 receptor subunits, GluA1 and GluN2A protein levels moderately increased. The results indicate that brief convulsions, repeated daily for 10 days can increase overall entorhinal cortex excitability despite a reduction in AMPA/kainate receptor activity, probably through the alteration of local network susceptibility.

title: Expression of the AMPA Receptor Subunits GluR1 and GluR2 is Associated with Granule Cell Maturation in the Dentate Gyrus
abstract: The dentate gyrus produces new granule neurons throughout adulthood in mammals from rodents to humans. During granule cell maturation, defined markers are expressed in a highly regulated sequential process, which is necessary for directed neuronal differentiation. In the present study, we show that α-amino-3-hydroxy-5-methy-4-isoxazole propionate (AMPA) receptor subunits GluR1 and GluR2 are expressed in differentiated granule cells, but not in stem cells, in neonatal, and adult dentate gyrus. Using markers for neural progenitors, immature and mature granule cells, we found that GluR1 and GluR2 were expressed mainly in mature cells and in some immature cells. A time-course analysis of 5-bromo-2′-deoxyuridine staining revealed that granule cells express GluR1 around 3 weeks after being generated. In mice heterozygous for the alpha-isoform of calcium/calmodulin-dependent protein kinase II, a putative animal model of schizophrenia and bipolar disorder in which dentate gyrus granule cells fail to mature normally, GluR1 and GluR2 immunoreactivities were substantially downregulated in the dentate gyrus granule cells. In the granule cells of mutant mice, the expression of both presynaptic and postsynaptic markers was decreased, suggesting that GluR1 and GluR2 are also associated with synaptic maturation. Moreover, GluR1 and GluR2 were also expressed in mature granule cells of the neonatal dentate gyrus. Taken together, these findings indicate that GluR1 and GluR2 expression closely correlates with the neuronal maturation state, and that GluR1 and GluR2 are useful markers for mature granule cells in the dentate gyrus.

title: Synaptic plasticity in the anterior cingulate cortex in acute and chronic pain
abstract: The anterior cingulate cortex (ACC) is activated in both acute and chronic pain. In this Review, we discuss increasing evidence from rodent studies that ACC activation contributes to chronic pain states and describe several forms of synaptic plasticity that may underlie this effect. In particular, one form of long-term potentiation (LTP) in the ACC, which is triggered by the activation of NMDA receptors and expressed by an increase in AMPA-receptor function, sustains the affective component of the pain state. Another form of LTP in the ACC, which is triggered by the activation of kainate receptors and expressed by an increase in glutamate release, may contribute to pain-related anxiety.

title: Targeting AMPA and kainate receptors in neurological disease: therapies on the horizon?
abstract: Aberrant excitatory neurotransmission is a prominent pathological component in many neurological and psychiatric diseases. Not surprisingly, the proteins that mediate the majority of excitatory signaling, ionotropic glutamate receptors (iGluRs), represent tempting targets for drug development efforts. This potential remains largely unrealized, however, despite a wealth of promising preclinical data. Here I discuss briefly the new applications and candidate drugs acting on the AMPA and kainate subtypes of iGluRs that suggest that a renaissance might be underway. 
 
AMPA and kainate receptors sub-serve different roles in the brain. AMPA receptors mediate the majority of fast excitatory neurotransmission and are critical cellular constituents of learning and memory processes. Over-activation of AMPA receptors, however, can be damaging to the nervous system, producing convulsions or neuronal death. Kainate receptors play more modulatory roles, fine-tuning the balance between neuronal excitation and inhibition. 
 
Positive AMPA receptor modulators strengthen excitatory transmission, enhance synaptic plasticity, and preclinical and preliminary clinical research suggested efficacy as cognition enhancers (Lynch, 2006; O’Neill and Dix, 2007). The first potentiator tested in large clinical trials was CX516 (Cortex Pharmaceuticals), which did not show efficacy in a variety of pathologies (eg Berry-Kravis et al, 2006). In contrast, a second-generation ampakine, CX717, normalized behaviors associated with attention deficit hyperactivity disorder (ADHD). Further testing of CX717 for ADHD was not approved by the US Food and Drug Administration due to toxicological concerns, although approval was granted to continue trials of CX717 in Alzheimer’s disease. The outcome of this project is uncertain, however, given that a chemically distinct potentiator, LY415395 (Eli Lilly), failed to improve cognitive performance in an Alzheimer’s disease trial (Chappell et al, 2007). Recently, compelling preclinical data prompted initiation of two Phase II trials in Germany to determine if CX717 reverses or prevents respiratory depression during opiate analgesia. These appear to be the only ongoing studies of efficacy for positive AMPA receptor modulators in humans, as clinical studies for similar molecules have been suspended (Schering-Plough) or the results remain undisclosed (Servier, GlaxoSmithKline). 
 
Noncompetitive inhibitors of AMPA receptors, such as talampanel (Teva Pharmaceuticals) and perampanel (Eisai Medical Research), reduce over-excitation and potentially slow neuro-degeneration. These drugs were efficacious as adjunct therapies for refractory partial complex seizures (Howes and Bell, 2007); perampanel also alleviated diabetic and post-herpetic neuropathic pain and will be further tested for these indications. Results released from an in-progress study suggested that talampanel decreased mortality from glioblastoma, and an examination of its efficacy in amyotrophic lateral sclerosis is planned. Perampanel was not effective as an add-on therapy to levodopa in Parkinson’s disease, however, and this program was terminated by Eisai. 
 
Preclinical data suggest that kainate receptors represent an untapped and attractive target for drug development. A nonselective AMPA/kainate receptor inhibitor, tezampanel (NGX424; Torrey Pines Pharmaceutics), reduced both migraine pain and other symptoms in a recent Phase II trial. This clinical efficacy is likely attributable to inhibition of kainate receptors, based on preclinical evidence with more selective antagonists developed by Eli Lilly. A chemically distinct AMPA/kainate receptor antagonist, NS1209 (NeuroSearch A/S), also alleviated refractory status epilepticus and neuro-pathic pain in small Phase II studies, but further research into this molecule was suspended. The apparent success of the first representatives of this new class of drugs provides a strong impetus for further development and clinical testing. 
 
It is evident from this overview that there is reason for both optimism and healthy skepticism regarding the clinical prospects of drugs targeting AMPA and kainate receptors. Cusp of a renaissance or a false dawn? Perhaps a Magic 8-Ball offers the best advice for would-be prognosticators: ‘Ask again later.’

title: The different ways through which specificity works in orthosteric and allosteric drugs.
abstract: Currently, there are two types of drugs on the market: orthosteric, which bind at the active site; and allosteric, which bind elsewhere on the protein surface, and allosterically change the conformation of the protein binding site. In this perspective we argue that the different mechanisms through which the two drug types affect protein activity and their potential pitfalls call for different considerations in drug design. The key problem facing orthosteric drugs is side effects which can occur by drug binding to homologous proteins sharing a similar binding site. Hence, orthosteric drugs should have very high affinity to the target; this would allow a low dosage to selectively achieve the goal of target-only binding. By contrast, allosteric drugs work by shifting the free energy landscape. Their binding to the protein surface perturbs the protein surface atoms, and the perturbation propagates like waves, finally reaching the binding site. Effective drugs should have atoms in good contact with the 'right' protein atoms; that is, the contacts should elicit propagation waves optimally reaching the protein binding site target. While affinity is important, the design should consider the protein conformational ensemble and the preferred propagation states. We provide examples from functional in vivo scenarios for both types of cases, and suggest how high potency can be achieved in allosteric drug development.

title: Kainate receptors: Pharmacology, function and therapeutic potential
abstract: Compared to the other glutamate receptors, progress in the understanding of the functions of kainate receptors (KARs) has lagged behind, due mainly to the relative lack of specific pharmacological tools. Over the last decade subunit selective agonists (e.g. ATPA and 5-iodowillardiine) and orthosteric (e.g. LY382884 and ACET) and allosteric antagonists for KARs that contain GluK1 (GluR5) subunits have been developed. However, no selective ligands for the other KAR subunits have been identified. The use of GluK1 antagonists has enabled several functions of KARs, that contain this subunit, to be identified. Thus, KARs have been shown to regulate excitatory and inhibitory synaptic transmission. In the case of the regulation of L-glutamate release, they can function as facilitatory autoreceptors or inhibitory autoreceptors during repetitive synaptic activation and can respond to ambient levels of L-glutamate to provide a tonic regulation of L-glutamate release. KARs also contribute a component of excitatory synaptic transmission at certain synapses. They can also act as triggers for both long-term potentiation (LTP) and long-term depression (LTD) and rapid alterations in their trafficking can result in altered synaptic transmission during both synaptic plasticity and neuronal development. KARs also contribute to synchronised rhythmic activity in the brain and are involved in forms of learning and memory. With respect to therapeutic indications, antagonists for GluK1 have shown positive activity in animal models of pain, migraine, epilepsy, stroke and anxiety. This potential has now been confirmed in dental pain and migraine in initial studies in man.

title: Non-competitive AMPA antagonists of 2,3-benzodiazepine type.
abstract: The discovery of the selective AMPA antagonist character of 2,3-benzodiazepine derivative GYKI 52466 (5) in the late eighties and the recognition of the non-competitive nature of its mode of action some years later set off the world-wide search for novel class of drugs. Notably the quest to develop new antiepileptic and neuroprotective medicines, which allosterically inhibit the AMPA sensitive glutamate operated channels. This review summarises our present knowledge about the allosteric site, dubbed "GYKI site" where the 2,3-benzodiazepines are supposed to bind to. The structure-activity relationships among AMPA antagonist 2,3-benzodiazepines and their structural analogues with similar biological profile are reviewed in a possibly comprehensive fashion. The chemical synthesis of 2,3-benzodiazepines is shortly described. The in vitro and in vivo experimental methods used for pharmacological characterisation of the biologically active compounds are briefly explained. Finally the therapeutic potential of 2,3-benzodiazepines i.e. the main fields of their clinical utility are outlined with special regard to talampanel (20) in the light of the ongoing clinical trials with this new drug candidate.

title: Systematic evolution of ligands by exponential enrichment: RNA ligands to bacteriophage T4 DNA polymerase.
abstract: High-affinity nucleic acid ligands for a protein were isolated by a procedure that depends on alternate cycles of ligand selection from pools of variant sequences and amplification of the bound species. Multiple rounds exponentially enrich the population for the highest affinity species that can be clonally isolated and characterized. In particular one eight-base region of an RNA that interacts with the T4 DNA polymerase was chosen and randomized. Two different sequences were selected by this procedure from the calculated pool of 65,536 species. One is the wild-type sequence found in the bacteriophage mRNA; one is varied from wild type at four positions. The binding constants of these two RNA's to T4 DNA polymerase are equivalent. These protocols with minimal modification can yield high-affinity ligands for any protein that binds nucleic acids as part of its function; high-affinity ligands could conceivably be developed for any target molecule.

title: From the RNA world to the clinic
abstract: The study of RNA has continually emphasized the structural and functional versatility of RNA molecules. This versatility has inspired translational and clinical researchers to explore the utility of RNA-based therapeutic agents for a wide variety of medical applications. Several RNA therapeutics, with diverse modes of action, are being evaluated in large late-stage clinical trials, and many more are in early clinical development. Hundreds of patients are enrolled in large trials testing messenger RNAs to combat cancer, small interfering RNAs to treat renal and hepatic disorders, and aptamers to combat ocular and cardiovascular disease. Results from these studies are generating considerable interest among the biomedical community and the public and will be important for the future development of this emerging class of therapeutic agents.

title: Aptamers as promising molecular recognition elements for diagnostics and therapeutics in the central nervous system.
abstract: Oligonucleotide aptamers are short, synthetic, single-stranded DNA or RNA able to recognize and bind to a multitude of targets ranging from small molecules to cells. Aptamers have emerged as valuable tools for fundamental research, clinical diagnosis, and therapy. Due to their small size, strong target affinity, lack of immunogenicity, and ease of chemical modification, aptamers are an attractive alternative to other molecular recognition elements, such as antibodies. Although it is a challenging environment, the central nervous system and related molecular targets present an exciting potential area for aptamer research. Aptamers hold promise for targeted drug delivery, diagnostics, and therapeutics. Here we review recent advances in aptamer research for neurotransmitter and neurotoxin targets, demyelinating disease and spinal cord injury, cerebrovascular disorders, pathologies related to protein aggregation (Alzheimer's, Parkinson's, and prions), brain cancer (glioblastomas and gliomas), and regulation of receptor function. Challenges and limitations posed by the blood brain barrier are described. Future perspectives for the application of aptamers to the central nervous system are also discussed.

title: Evidence for multiple AMPA receptor complexes in hippocampal CA1/CA2 neurons
abstract: The AMPA receptor, which is involved in most fast glutamatergic transmission in the mammalian brain and is expressed in most neurons, is made up of four subunits, GluR1–4. In situ hybridzation, immunocytochemistry studies, and single-cell PCR analyses show that the number and type of AMPA receptor subunits expressed vary among neuronal populations and that two to four subunits usually are expressed in each neuron. Neurons that express two or more subunits theoretically could produce multiple pentameric receptor complexes that differ in their subunit compositions, and these complexes could be targeted to different synaptic populations. To determine whether a single neuronal population produces multiple AMPA receptor complexes, we used a preparation of CA1/CA2 hippocampal pyramidal neurons and immunoprecipitation with subunit-specific antibodies to characterize the receptor complexes. The CA1/CA2 pyramidal neurons express high levels of GluR1–3 and receive multiple excitatory inputs, offering the possibility that distinct receptor complexes may be assembled and expressed selectively at different synaptic populations. Our results suggest the presence of two major populations of AMPA receptor complexes: those made up of GluR1 and GluR2 and those made up of GluR2 and GluR3. Very few complexes contained both GluR1 and GluR3, whereas approximately 8% of the total AMPA receptor complexes was homomeric GluR1. The integrity of the receptor complex was verified by measuring [3H]AMPA binding activity in the immunoprecipitated fractions. These results show that AMPA receptor complexes with different subunit compositions are present in CA1/CA2 pyramidal neurons and suggest an additional mechanism to regulate receptor expression in neurons.

title: Subunit-Specific Rules Governing AMPA Receptor Trafficking to Synapses in Hippocampal Pyramidal Neurons
abstract: AMPA-type glutamate receptors (AMPA-Rs) mediate a majority of excitatory synaptic transmission in the brain. In hippocampus, most AMPA-Rs are hetero-oligomers composed of GluR1/GluR2 or GluR2/GluR3 subunits. Here we show that these AMPA-R forms display different synaptic delivery mechanisms. GluR1/GluR2 receptors are added to synapses during plasticity; this requires interactions between GluR1 and group I PDZ domain proteins. In contrast, GluR2/GluR3 receptors replace existing synaptic receptors continuously; this occurs only at synapses that already have AMPA-Rs and requires interactions by GluR2 with NSF and group II PDZ domain proteins. The combination of regulated addition and continuous replacement of synaptic receptors can stabilize long-term changes in synaptic efficacy and may serve as a general model for how surface receptor number is established and maintained.

title: Subunit Composition of Synaptic AMPA Receptors Revealed by a Single-Cell Genetic Approach
abstract: The precise subunit composition of synaptic ionotropic receptors in the brain is poorly understood. This information is of particular importance with regard to AMPA-type glutamate receptors, the multimeric complexes assembled from GluA1-A4 subunits, as the trafficking of these receptors into and out of synapses is proposed to depend upon the subunit composition of the receptor. We report a molecular quantification of synaptic AMPA receptors (AMPARs) by employing a single-cell genetic approach coupled with electrophysiology in hippocampal CA1 pyramidal neurons. In contrast to prevailing views, we find that GluA1A2 heteromers are the dominant AMPARs at CA1 cell synapses (approximately 80%). In cells lacking GluA1, -A2, and -A3, synapses are devoid of AMPARs, yet synaptic NMDA receptors (NMDARs) and dendritic morphology remain unchanged. These data demonstrate a functional dissociation of AMPARs from trafficking of NMDARs and neuronal morphogenesis. This study provides a functional quantification of the subunit composition of AMPARs in the CNS and suggests novel roles for AMPAR subunits in receptor trafficking.

title: RNA aptamers selected against the GluR2 glutamate receptor channel.
abstract: The excessive activation of AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) receptors, a subtype of glutamate ion channels, has been implicated in various neurological diseases such as cerebral ischemeia and amyotrophic lateral sclerosis. Inhibitors of AMPA receptors are drug candidates for potential treatment of these diseases. Using the systematic evolution of ligands by exponential enrichment (SELEX), we have selected a group of RNA aptamers against the recombinant GluR2Qflip AMPA receptor transiently expressed in HEK-293 (human embryonic kidney) cells. One of the aptamers, AN58, is shown to competitively inhibit the receptor. The nanomolar affinity of AN58 rivals that of NBQX (6-nitro-7-sulfamoyl-benzo[f]quinoxaline-2,3-dione), one of the best competitive inhibitors. Like NBQX, AN58 has the highest affinity for GluR2, the selection target, among all AMPA receptor subunits. However, AN58 has a higher selectivity for the GluR4 AMPA receptor subunit and remains potent even at pH = 6.8 (i.e., a clinically relevant acidic pH), as compared with NBQX. Furthermore, this RNA molecule possesses stable physical properties. Therefore, AN58 serves as a unique lead compound for developing water-soluble inhibitors with a nanomolar affinity for GluR2 AMPA receptors.

title: Interpretation of binding curves obtained with high receptor concentrations: practical aid for computer analysis.
abstract: Typical equilibrium binding experiments cannot be quantitatively analyzed on the basis of classical mathematical equations when the receptor concentration is so high that a significant fraction of the added radioligand concentration is in the bound form. In this paper, the appropriate equations are derived and used in a commercial graphics package to estimate the binding parameters, by applying nonlinear regression to pseudo-experimental data. The analysis of saturation and homologous displacement curves obtained with high receptor concentrations reveals that the empirical determination of nonspecific binding by addition of an excess of unlabeled ligand is incorrect.

title: Potent and Selective Inhibition of the Open-Channel Conformation of AMPA Receptors by an RNA Aptamer
abstract: Inhibitors targeting the α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors are useful as biochemical probes for structure-function studies and as drug candidates for a number of neurological disorders and diseases. Here we describe the identification of an RNA inhibitor or aptamer by an in vitro evolution approach, i.e., systematic evolution of ligands by exponential enrichment (SELEX), with the GluA2Qflip AMPA receptor subunit as the selection target. This aptamer inhibits only the open-channel, but not the closed-channel, conformation of the AMPA receptors with a nanomolar affinity (for example, the KI value was found to be 0.95 ± 0.2 µM with GluA2Qflip. Furthermore, the aptamer has no effect on either kainate or NMDA receptors. By using a laser-pulse photolysis technique and a caged glutamate, we also determined that the aptamer inhibited the channel-closing rate constant or kcl, which reflects the lifetime of the open channel, but it did not affect the channel opening rate constant or kop, which reflects the closed-channel state. In a homologous competitive binding experiment, the aptamer was found to bind to both the open-channel and the closed-channel state with Kd of 80 ± 23 nM and 68 ± 40 nM respectively. Taken together, these results are consistent with the aptamer being a noncompetitive inhibitor. Our results further suggest the possibility of developing RNA aptamers that are conformation-selective, potent and water-soluble as a new type of inhibitors against AMPA receptors.

title: Potent and Selective Inhibition of a Single α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid (AMPA) Receptor Subunit by an RNA Aptamer*
abstract: Inhibitors of AMPA-type glutamate ion channels are useful as biochemical probes for structure-function studies and as drug candidates for a number of neurological disorders and diseases. Here, we describe the identification of an RNA inhibitor or aptamer by an in vitro evolution approach and a characterization of its mechanism of inhibition on the sites of interaction by equilibrium binding and on the receptor channel opening rate by a laser-pulse photolysis technique. Our results show that the aptamer is a noncompetitive inhibitor that selectively inhibits the GluA2Qflip AMPA receptor subunit without any effect on other AMPA receptor subunits or kainate or NMDA receptors. On the GluA2 subunit, this aptamer preferentially inhibits the flip variant. Furthermore, the aptamer preferentially inhibits the closed-channel state of GluA2Qflip with a KI = 1.5 μm or by ∼15-fold over the open-channel state. The potency and selectivity of this aptamer rival those of small molecule inhibitors. Together, these properties make this aptamer a promising candidate for the development of water-soluble, highly potent, and GluA2 subunit-selective drugs.

title: Expression of NR1/NR2B N‐Methyl‐d‐Aspartate Receptors Enhances Heroin Toxicity in HEK293 Cells
abstract: Repeated use of drugs of abuse, namely opiates, has been shown to affect glutamate‐releasing neurons. Moreover, blockade of N‐methyl‐D‐aspartate (NMDA) receptors (NMDAR) prevents cell death by apoptosis induced by morphine, a heroin metabolite. Thus, in this article we investigated the involvement of different NMDAR subunits in heroin cytotoxicity. Human embryonic kidney (HEK293) cells, which do not express native NMDAR, were transfected with NR1/NR2A or NR1/NR2B subunits. As a control, cells were transfected with NR1 alone, which does not form functional channels. Incubation with heroin for 24 h induced a dose‐dependent decrease in cell viability both in NR1‐transfected and nontransfected cells. The loss of membrane integrity induced by heroin was more evident in cells transfected with NR1/NR2B than in cells transfected with NR1 alone or NR1/NR2A. This decrease in cell viability was blocked by MK‐801, a selective and noncompetitive antagonist of NMDAR. Nevertheless, no significant changes in intracellular adenosine 5′‐triphosphate (ATP) were observed in cells treated with heroin. These data implicate NR2B‐composed NMDAR as important mediators of heroin neurotoxicity.

title: γ-d-glutamylaminomethyl sulfonic acid (GAMS) distinguishes kainic acid- from AMPA-induced responses in Xenopus oocytes expressing chick brain glutamate receptors
abstract: The effects of the glutamate receptor antagonist gamma-D-glutamylaminomethyl sulfonic acid (GAMS) on inward currents induced by bath application of kainic acid (KA) or alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) were studied with single-electrode voltage clamp methods in Xenopus oocytes injected 3-5 days previously with mRNA from the brain of E16-17 chick embryos. Both AMPA and KA induced smooth inward currents, with Hill coefficients of 1.5 (AMPA) and 2.1 (KA). GAMS, at concentrations up to 1 mM, produced no reliable antagonism of AMPA-induced currents but showed a consistent, dose-dependent and reversible antagonism of KA-induced responses; the slope of the Schild plot was 0.76 and the pA2 value 4.32. In the presence of GAMS, however, the Hill coefficient for AMPA is reduced significantly and approaches unity, suggesting that AMPA interacts with both KA and AMPA binding sites on chick brain glutamate receptors. The selectivities of three quinoxalinedione antagonists (6,7-dinitroquinoxaline-2,3-dione [DNQX], 6-cyano-7- nitroquinoxaline-2,3-dione [CNQX] and 6-nitro-7-sulfamoyl-benzo(F)quinoxaline-2,3-dione [NBQX]) were then compared with that shown by GAMS. DNQX, CNQX and NBQX all blocked the effects of both KA and AMPA completely, competitively, reversibly and dose-dependently, with Schild-plot slopes very close to 1.0. Against AMPA, observed pA2 values were 6.58 for DNQX, 6.43 for CNQX and 6.77 for NBQX. Against KA, pA2 values were 6.42 for DNQX, 6.56 for CNQX and 7.21 for NBQX.(ABSTRACT TRUNCATED AT 250 WORDS)

title: Role of excitatory amino acid receptors in mono- and polysynaptic excitation in the cat spinal cord
abstract: SummaryThree excitatory amino acid antagonists, 2-amino-5-phosphonovalerate (APV), γ-D-glutamyl-glycine (γDGG) and cis-2,3-piperidine dicarboxylate (PDA) have been compared with respect to their ability to block the action of amino acid excitants and both mono- and polysynaptic excitation in the cat spinal cord evoked by stimulation of primary afferent fibres.Each of the three antagonists depressed polysynaptic excitation of dorsal horn neurones. This action correlated with the ability of all of the substances to antagonize responses evoked by N-methyl-D-aspartate (NMDA) and L-aspartate. The order of potency of the antagonists in producing these effects was APV > γDGG = PDA.PDA (particularly) and γDGG also proved to be effective depressants of monosynaptic excitation. This action correlated with the ability of these substances to antagonize quisqualate- and L-glutamate-induced excitation. Antagonism of kainate-induced excitation was usually (but not always) also associated with depression of monosynaptic excitation.These results suggest that, following impulses in low threshold afferent fibres, a transmitter is released from primary afferent terminals which acts at quisqualate- (and possibly kainate-) type receptors and that excitatory interneurones, activated by similar impulses, release a transmitter which acts at NMDA receptors. L-Glutamate and L-aspartate may be the transmitters involved in these monosynaptic and polysynaptic responses, respectively.

title: Depressant actions of γ-d-glutamylaminomethyl sulfonate (GAMS) on amino acid-induced and synaptic excitation in the cat spinal cord
abstract: We have investigated the effects of iontophoretically administered γ- d -glutamylaminomethyl sulfonate (GAMS) on excitation of dorsal horn neurons and Renshaw cells of the cat spinal cord induced by exogenous excitants and by synaptic activation following stimulation of low threshold primary afferent fibers. Comparisons were made between the synaptic depressant effects of GAMS and those of γ- d -glutamylglycine (γDGG) and (±)4-2-amino-5-phosphonovalerate (APV). At low iontophoretic ejection currents, GAMS showed clear selectivity in antagonizing responses to excitatory amino acids in the order kainate > quisqualate > l -aspartate > NMDA > l -glutamate. This selectivity was decreased at high ejection currents, when acetylcholine-induced excitation of Renshaw cells was also reduced. GAMS was equieffective with γDGG in depressing both APV-sensitive polysynaptic excitation and APV-resistant monosynaptic excitation of spinal neurons. Ventral root evoked excitation of Renshaw cells was not reduced by GAMS. In some cells a depression of synaptic excitation by GAMS was observed in the absence of an effect on either l -glutamate- or l -aspartate-induced excitation. This raises the possibility that some other endogenous substance may be a transmitter acting at kainate/quisqualate type receptors in the cat spinal cord. However, other factors are discussed which may explain this observation.

title: Antagonist pharmacology of kainate- and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-preferring receptors.
abstract: Whole-cell recordings were used to study the antagonist pharmacology of two subtypes of non-N-methyl-D-aspartate glutamate receptors: the kainate-preferring subtype expressed by rat dorsal root ganglion (DRG) neurons and the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-preferring subtype expressed by neurons from rat cerebral cortex. A series of quinoxaline derivatives were tested for the ability to distinguish between AMPA and kainate receptors, as determined by differential potency. Of the nine compounds studied, 2,3-dihydroxy-6-nitro-7-sulfamoylbenzo(f)quinoxaline (NBQX) showed the highest selectivity for AMPA-preferring receptors, whereas 5-chloro-7-trifluoromethyl-2,3-quinoxalinedione (ACEA-1011) showed the highest selectivity for the kainate-preferring subtype. NBQX blocked non-N-methyl-D-aspartate currents in cortical cells with a Kb of 0.3 mircroM, but in DRG neurons the Kb for NBQX was 3-fold higher (0.9 microM). ACEA-1011 also blocked the currents in DRG cells with a Kb of approximately 1 microM, but in cortical neurons the kb for this drug was 10-12 microM. Several the Kb for this drug was 10-12 micron. Several additional compounds were tested for selective potency, including 5-nitro-6,7,8,9-tetrahydrobenzo[G]indole-2,3-dione-3-oxime, gamma-D-glutamylaminomethylsulphonic acid, and derivatives of kynurenic acid and 1-benzazepine. 5-Nitro-6,7,8,9-tetrahydrobenzo- [G]indole-2,3-dione-3-oxime displayed the highest selectivity in this group, blocking kainate receptors with a Kb of 6 microM while inhibiting AMPA receptors with a Kb of >100 microM. The remaining antagonists showed <3-fold selectivity between AMPA and kainate receptor subtypes. Our results suggest that most competitive antagonists block native AMPA and kainate receptors with approximately similar potencies, which is in marked contrast to the substantial differences in potency that have been observed with receptor agonists.

title: Spinal antinociceptive effects of excitatory amino acid antagonists: quisqualate modulates the action of N-methyl-D-aspartate.
abstract: Blockade of N-methyl-D-aspartate (NMDA) receptors in the spinal cord of rodents has been shown to produce antinociceptive effects and motor impairment. To find out whether other receptors for excitatory amino acids (EAA) can influence spinal pathways utilizing the NMDA receptors we compared, in mice, the behavioral consequences of intrathecal injection of four EAA antagonists, 2-amino-5-phosphono valerate (APV), kynurenate, gamma-D-glutamyl glycine (DGG) and glutamylaminomethyl sulphonate (GAMS). The selectivity of these antagonists at different concentrations was evaluated behaviorally by assessing their ability to block the biting behavior elicited by intrathecal EAA agonists. Blockade of the NMDA receptor was necessary to elicit antinociceptive effects and motor impairment. Thus, APV produced antinociception at concentrations selective for the action of NMDA. The wide spectrum EAA antagonists, DGG and kynurenate, and the quisqualate/kainate antagonist, GAMS, all produced antinociception and motor impairment at concentrations which also blocked NMDA-induced bites. However, an inhibitory modulation of the action of NMDA by quisqualate-sensitive systems was also observed. Thus, high concentrations of APV (greater than 1 mM), which also blocked quisqualate-elicited bites, produced a surprising, sharp decrease in APV antinociception and motor impairment, effects which were reversed by quisqualate. Furthermore, quisqualate significantly inhibited NMDA-induced bites. Additional evidence for such an inhibitory-modulatory effect of quisqualate can be gathered from the antinociceptive potency of DGG. This antagonist, which blocks the action of both NMDA and quisqualate, was less potent as an antinociceptive agent than APV. No such discrepancy between the ability to inhibit the action of NMDA and to elicit antinociceptive effects and motor impairment was noted for either kynurenate or GAMS. Evidence is provided that these different profiles of action are due to the fact that DGG and high concentrations of APV act at different subpopulations of quisqualate receptors than do kynurenate and GAMS, and that the former subpopulation is involved in the modulation of the action of NMDA.

title: Mechanism of inhibition of the GluR2 AMPA receptor channel opening by 2,3-benzodiazepine derivatives.
abstract: 2,3-Benzodiazepine derivatives are drug candidates synthesized for potential treatment of various neurodegenerative diseases involving the excessive activity of AMPA receptors. Here we describe a rapid kinetic investigation of the mechanism of inhibition of the GluR2Qflip AMPA receptor channel opening by two 2,3-benzodiazepine derivatives that are structurally similar (BDZ-2 and BDZ-3). Using a laser-pulse photolysis technique with a time resolution of approximately 60 mus, we measured the effects of these inhibitors on both the channel opening rate and the whole-cell current amplitude. We found that both compounds preferably inhibit the open-channel state, although BDZ-2 is a more potent inhibitor in that it inhibits the open-channel state approximately 5-fold stronger than BDZ-3 does. Both compounds bind to the same noncompetitive site. Binding of an inhibitor to the receptor involves the formation of a loose, partially conducting channel intermediate, which rapidly isomerizes to a tighter complex. The isomerization reaction is identified as the main step at which the receptor distinguishes the structural difference between the two compounds. These results suggest that addition of a bulky group at the N-3 position on the diazepine ring, as in BDZ-3, does not alter the mechanism of action, or the site of binding, but does lower the inhibitory potency, possibly due to an unfavorable interaction of a bulky group at the N-3 position with the receptor site. The new mechanistic revelation about the structure-reactivity relationship is useful in designing conformation-specific, more potent noncompetitive inhibitors for the GluR2 AMPA receptor.

title: Intrathecal delivery of protein therapeutics to the brain: a critical reassessment.
abstract: Disorders of the central nervous system (CNS), including stroke, neurodegenerative diseases, and brain tumors, are the world's leading causes of disability. Delivery of drugs to the CNS is complicated by the blood-brain barriers that protect the brain from the unregulated leakage and entry of substances, including proteins, from the blood. Yet proteins represent one of the most promising classes of therapeutics for the treatment of CNS diseases. Many strategies for overcoming these obstacles are in development, but the relatively straightforward approach of bypassing these barriers through direct intrathecal administration has been largely overlooked. Originally discounted because of its lack of usefulness for delivering small, lipid-soluble drugs to the brain, the intrathecal route has emerged as a useful, in some cases perhaps the ideal, route of administration for certain therapeutic protein and targeted disease combinations. Here, we review blood-brain barrier functions and cerebrospinal fluid dynamics and their relevance to drug delivery via the intrathecal route, discuss animal and human studies that have investigated intrathecal delivery of protein therapeutics, and outline several characteristics of protein therapeutics that can allow them to be successfully delivered intrathecally.

title: Ziconotide: a new nonopioid intrathecal analgesic for the treatment of chronic pain
abstract: Ziconotide is a new nonopioid intrathecal agent recently approved for the treatment of chronic pain. Ziconotide is indicated for the management of severe chronic pain in patients for whom intrathecal therapy is warranted and who are intolerant of or refractory to other treatment, such as systemic analgesics, adjunctive therapies or intrathecal morphine. Ziconotide blocks the N-type calcium channels located in the superficial dorsal horn of the spinal cord, resulting in potent analgesia. The efficacy of ziconotide has been demonstrated in three randomized, placebo-controlled trials in over 500 patients. In addition, its safety has been demonstrated in over 1200 subjects. Ziconotide is a potent analgesic with a narrow therapeutic window. The drug requires a slow titration in order to achieve analgesia while avoiding dose-limiting side effects. This review examines the currently available information on this new analgesic.

title: Aptamers as therapeutics
abstract: Drug Discovery 9, 537–550 (2010) The address provided for Anthony D. Keefe was incorrect. The correct address is: X-Chem Incorporated, 100 Beaver Street, Waltham, Massachusetts 02453, USA.

title: RNA editing in brain controls a determinant of ion flow in glutamate-gated channels
abstract: L-glutamate, the principal excitatory transmitter in the brain, gates ion channels mediating fast neurotransmission. Subunit components of two related classes of glutamate receptor channels have been characterized by cDNA cloning and shown to carry either an arginine or a glutamine residue in a defined position of their putative channel-forming segment. The arginine residue in this segment profoundly alters, and dominates, the properties of ion flow, as demonstrated for one channel class. We now show that the genomic DNA sequences encoding the particular channel segment of all subunits harbor a glutamine codon (CAG), even though an arginine codon (CGG) is found in mRNAs of three subunits. Multiple genes and alternative exons were excluded as sources for the arginine codon; hence, we propose that transcripts for three subunits are altered by RNA editing. This process apparently edits subunit transcripts of the two glutamate receptor classes with different efficiency and selectivity.

title: The Role of RNA Editing in Controlling Glutamate Receptor Channel Properties
abstract: An unexpected finding in the area of excitatory neurotransmission was that glutamate-activated channels shaping the fast component of the excitatory postsynaptic currents (EPSCs) in the mammalian brain can be molecularly altered in functionally critical amino acid positions as a consequence of RNA editing. This review summarizes what is known about this process and how it affects glutamate-mediated signaling.

title: Divalent ion permeability of AMPA receptor channels is dominated by the edited form of a single subunit
abstract: Functionally diverse GluR channels of the AMPA subtype are generated by the assembly of GluR-A, -B, -C, and -D subunits into homo- and heteromeric channels. The GluR-B subunit is dominant in determining functional properties of heteromeric AMPA receptors. This subunit exists in developmentally distinct edited and unedited forms, GluR-B(R) and GluR-B(Q), which differ in a single amino acid in transmembrane segment TM2 (Q/R site). Homomeric GluR-B(R) channels expressed in 293 cells display a low divalent permeability, whereas homomeric GluR-B(Q) and GluR-D channels exhibit a high divalent permeability. Mutational analysis revealed that both the positive charge and the size of the amino acid side chain located at the Q/R site control the divalent permeability of homomeric channels. Coexpression of Q/R site arginine- and glutamine-containing subunits generates cells with varying divalent permeabilities depending on the amounts of expression vectors used for cell transfection. Intermediate divalent permeabilities were traced to the presence of both divalent permeant homomeric and impermeant heteromeric channels. It is suggested that the positive charge contributed by the arginine of the edited GluR-B(R) subunit determines low divalent permeability in heteromeric GluR channels and that changes in GluR-B(R) expression regulate the AMPA receptor-dependent divalent permeability of a cell.

title: AMPA Receptor Tetramerization Is Mediated by Q/R Editing
abstract: AMPA-type glutamate receptors (AMPARs) play a major role in excitatory synaptic transmission and plasticity. Channel properties are largely dictated by their composition of the four subunits, GluR1-4 (or A-D). Here we show that AMPAR assembly and subunit stoichiometry are determined by RNA editing in the pore loop. We demonstrate that editing at the GluR2 Q/R site regulates AMPAR assembly at the step of tetramerization. Specifically, edited R subunits are largely unassembled and ER retained, whereas unedited Q subunits readily tetramerize and traffic to synapses. This assembly mechanism restricts the number of the functionally critical R subunits in AMPAR tetramers. Therefore, a single amino acid residue affects channel composition and, in turn, controls ion conduction through the majority of AMPARs in the brain.

title: Single-Channel Properties of Recombinant AMPA Receptors Depend on RNA Editing, Splice Variation, and Subunit Composition
abstract: Non-NMDA glutamate receptor subunits of the AMPA-preferring subfamily combine to form ion channels with heterogeneous functional properties. We have investigated the effects of RNA editing at the Q/R site, splice variation of the “flip/flop” cassette, and multimeric subunit assembly on the single-channel conductance and kinetic properties of the recombinant AMPA receptors formed from GluR2 and GluR4 expressed in HEK 293 cells. We found that AMPA receptor single-channel conductance was dependent on the Q/R site editing state of the subunits comprising the channel. Calcium-permeable (unedited) channels had resolvable single-channel events with main conductance states of 7–8 pS, whereas fully edited GluR2 channels had very low conductances of ∼300 fS (estimated from noise analysis). Additionally, the flip splice variant of GluR4 conferred agonist-dependent conductance properties reminiscent of those found for a subset of AMPA receptors in cultured cerebellar granule cells. These results provide a description of the single-channel properties of certain recombinant AMPA receptors and suggest that the single-channel conductance may be determined by the expression of edited GluR2 subunits in neurons.

title: Regulation of Kinetic Properties of GluR2 AMPA Receptor Channels by Alternative Splicing
abstract: The four subunits of the AMPA-type glutamate receptor (GluR1–GluR4 or GluR-A–GluR-D) exist in two distinct forms, flip and flop, generated by alternative splicing of a 115 bp region. The GluR2 subunit plays a key role in determining the functional properties of the AMPA receptor channel. In this study, we examined the differences in kinetic properties between the flip and flop splice variants of the GluR2 subunit expressed in Xenopusoocytes using fast agonist application techniques. Glutamate was applied to outside-out patches from oocytes with piezo-driven double-barreled application pipettes. Because homomeric receptor channels composed of the edited form of GluR2 (GluR2R) produce no appreciable current responses, we expressed the unedited form of GluR2 (GluR2Q) in oocytes, which produced large current responses sufficient for analysis of the kinetic properties. The time constant for desensitization during application of 1 mm glutamate was 5.89 ± 0.17 msec (n = 50) in flip and 1.18 ± 0.05 msec (n = 37) in flop. The deactivation time constant was 0.62 ± 0.06 msec (n = 10) in flip and 0.54 ± 0.05 msec (n = 10) in flop. The steady-state nondesensitizing current was 6.8 ± 0.4% (n = 53) of the peak current in flip, whereas it was almost negligible in flop, being only 1.1 ± 0.1% (n = 36). The slower desensitization kinetics and larger steady-state current responses in the flip variant were also observed in heteromeric receptors assembled from GluR2Q/GluR2R. Thus, desensitization occurred much more prominently in the flop variant in the recombinant GluR2 receptor channels.

title: Heteromeric AMPA Receptors Assemble with a Preferred Subunit Stoichiometry and Spatial Arrangement
abstract: AMPA receptors are thought to be a tetrameric assembly of the subunits GluR1-4. We have examined whether two coexpressed subunits (GluR1/2) combine at random to form channels, or preferentially assemble with a specific stoichiometry and spatial configuration. The subunits carried markers controlling ion permeation and desensitization, and these properties were monitored as a function of relative expression level and subunit composition. Homomeric receptors assembled stochastically while heteromeric receptors preferentially formed with a stoichiometry of two GluR1 and two GluR2 subunits, and with identical subunits positioned on opposite sides of the channel pore. This structure will predominate if GluR1 binds to GluR2 more rapidly during receptor assembly than other subunit combinations. The practical outcome of selective heteromeric assembly is a more homogenous receptor population in vivo.

title: Activation Kinetics of AMPA Receptor Channels Reveal the Number of Functional Agonist Binding Sites
abstract: AMPA and NMDA receptor channels are closely related molecules, yet they respond to glutamate with distinct kinetics, attributable to differences in ligand binding and channel gating steps (for review, seeEdmonds et al., 1995). We used two complementary approaches to investigate the number of functional binding sites on AMPA channels on outside-out patches from cultured hippocampal neurons. The activation kinetics of agonist binding were measured during rapid steps into low concentrations of selective AMPA receptor agonists and during steps from a competitive AMPA receptor antagonist, 6-cyano-7-nitro-quinoxaline-2,3-dione, into a saturating concentration of agonist. Both approaches revealed sigmoidal kinetics, which suggests that multiple agonist binding steps or antagonist unbinding steps are needed for channel activation. A kinetic model with two independent binding sites gave a better fit to the activation phase than models with one or three independent sites. A more refined analysis incorporating cooperative interaction between the two binding sites significantly improved the fits to the responses. The affinity of the first binding step was two to three times higher than the second step. These results demonstrate that binding of two agonist molecules are needed to activate AMPA receptors, but the two binding sites are not identical and independent. Because NMDA receptors require four ligand molecules for activation (two glycine and two glutamate; Benveniste and Mayer, 1991; Clements and Westbrook, 1991), it may be that some binding sites on AMPA receptors are functionally silent.

title: Ligand binding assays at equilibrium: validation and interpretation
abstract: The focus of this review paper is factors affecting data interpretation in ligand binding assays under equilibrium conditions. Protocols for determining Kd (the equilibrium dissociation constant) and KdA (the equilibrium inhibitor constant) for receptor ligands are discussed. The basic theory describing the interaction of a radiotracer and an unlabelled competitor ligand with a receptor is developed. Inappropriate experimental design may result in ligand depletion and non‐attainment of equilibrium, distorting the calculation of Kd and KdA. Strategies, both theoretical and practical, will be given to avoid and correct such errors, thus leading to the determination of reliable values for these constants. In determining KdA from competition binding studies, two additional concepts are discussed. First, the necessity to measure an adequate specific binding signal from the bound radiotracer ligand limits the range of affinity constants that can be measured: a particular set of assay conditions may lead to an upper limit on the apparent affinity of unlabelled ligands. Second, an extension of the basic assay methodology can indicate whether the interaction between the tracer and a test ligand is mediated by a competitive or an allosteric mechanism. Finally, the review ends with a discussion of two factors that are often overlooked: buffer composition and the temperature at which the assay is conducted, and the impact these can have on affinity measurements and the understanding of drug interactions. British Journal of Pharmacology (2010) 161, 1219–1237; doi:10.1111/j.1476‐5381.2009.00604.x; published online 2 February 2010

title: Single-nucleotide resolution of RNAs up to 59 nucleotides by high-performance liquid chromatography.
abstract: Ion-pair, reverse-phase high-performance liquid chromatography (HPLC) is a standard analytical platform for separating, purifying, and analyzing RNAs. However, a single-nucleotide resolution by using HPLC is currently limited to RNAs shorter than 25 nucleotides (nt). Here we describe a method of separating three RNA aptamers with 57, 58, and 59nt on an XBridge ion-pair, reverse-phase HPLC column by a single-nucleotide resolution. Under a similar condition, we also show the capability of our method to resolve two structurally different, yet sequence or mass identical, 59-nt aptamers. We establish that the optimal condition to achieve a single-nucleotide resolution correlates to 50°C and zero magnesium concentration in mobile phases. The ion-pairing agent, the buffer, and the solvent we use are also compatible for post-HPLC analysis such as mass spectrometry. Therefore, our method provides a new way of detecting, analyzing, and separating RNAs by conformation or structure and extends the ability to separate RNAs that are longer than 25nt by single-nucleotide resolution.

title: Mechanism of Inhibition of the GluA2 AMPA Receptor Channel Opening: the Role of 4-Methyl versus 4-Carbonyl Group on the Diazepine Ring of 2,3-Benzodiazepine Derivatives.
abstract: 2,3-Benzodiazepine derivatives are synthesized as drug candidates for a potential treatment of various neurodegenerative diseases involving the excessive activity of AMPA receptors. Here, we describe a rapid kinetic investigation of the mechanism of inhibition of the GluA2Q(flip) AMPA receptor channel opening by two 2,3-benzodiazepine derivatives, i.e. the prototypic 2,3-benzodiazepine compound GYKI 52466 [(1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine)] and 1-(4-aminophenyl)-3,5-dihydro-7,8-methylenedioxy-4H-2,3-benzodiazepin-4-one (BDZ-2). GYKI 52466 and BDZ-2 are structurally similar in that the 4-methyl group in the diazepine ring of GYKI 52466 is replaced by a carbonyl group, yielding BDZ-2. Using a laser-pulse photolysis technique with ∼60 μs time resolution, we characterize the effect of the two compounds individually on the channel-opening process of the GluA2Q(flip) receptor expressed in HEK-293 cells. We find that BDZ-2 preferentially inhibits the open-channel state, whereas GYKI 52466 is more selective for the closed-channel state of the GluA2Q(flip) receptors. Each inhibitor binds independently to its own noncompetitive site, yet the two sites do not interact allosterically. The significance of these results in the context of both the structure-activity relationship and the properties of the GluA2Q(flip) receptor channels is presented.