Structure-activity relationships in the development of excitatory amino acid receptor agonists and competitive antagonists

Synthesis and Biological Activity of Homohypotaurine Obtained by the Enzyme-Based Conversion of Homocysteine Sulfinic Acid Using Recombinant Escherichia Coli Glutamate Decarboxylase

l-Homocysteine, formed from S-adenosyl methionine following demethylation and adenosine release, accumulates when the methionine recycling pathway and other pathways become impaired, thus leading to hyperhomocysteinemia, a biomarker in cardiovascular diseases, neurological/psychiatric disorders, and cancer. The partial oxidation of the l-homocysteine thiol group and its decarboxylation on C-alpha lead to the formation of l-homocysteinesulfinic acid (l-HCSA) and homohypotaurine (HHT), respectively. Both compounds are not readily available from commercial suppliers, which hinders the investigation of their biological activities. Herein, the chemical synthesis of l-HCSA, from l-homocystine, was the starting point for establishing the bio-based synthesis of HHT using recombinant Escherichia coli glutamate decarboxylase (EcGadB), an enzyme already successfully employed for the bio-based synthesis of GABA and its phosphinic analog. Prior to HHT synthesis, kcat (33.92 ± 1.07) and KM (38.24 ± 3.45 mM) kinetic constants were determined for l-HCSA on EcGadB. The results of our study show that the EcGadB-mediated synthesis of HHT can be achieved with good yields (i.e., 40% following enzymatic synthesis and column chromatography). Purified HHT was tested in vitro on primary human umbilical vein endothelial cells and rat cardiomyoblasts and compared to the fully oxidized analog, homotaurine (OT, also known as tramiprosate), in widespread pharmaceutical use. The results show that both cell lines display statistically significant recovery from the cytotoxic effects induced by H2O2 in the presence of HHT.

Revisiting the calpain hypothesis of learning and memory 40 years later

In 1984, Gary Lynch and Michel Baudry published in Science a novel biochemical hypothesis for learning and memory, in which they postulated that the calcium-dependent protease, calpain, played a critical role in regulating synaptic properties and the distribution of glutamate receptors, thereby participating in memory formation in hippocampus. Over the following 40 years, much work has been done to refine this hypothesis and to provide convincing arguments supporting what was viewed at the time as a simplistic view of synaptic biochemistry. We have now demonstrated that the two major calpain isoforms in the brain, calpain-1 and calpain-2, execute opposite functions in both synaptic plasticity/learning and memory and in neuroprotection/neurodegeneration. Thus, calpain-1 activation is required for triggering long-term potentiation (LTP) of synaptic transmission and learning of episodic memory, while calpain-2 activation limits the magnitude of LTP and the extent of learning. On the other hand, calpain-1 is neuroprotective while calpain-2 is neurodegenerative, and its prolonged activation following various types of brain insults leads to neurodegeneration. The signaling pathways responsible for these functions have been identified and involve local protein synthesis, cytoskeletal regulation, and regulation of glutamate receptors. Human families with mutations in calpain-1 have been reported to have impairment in motor and cognitive functions. Selective calpain-2 inhibitors have been synthesized and clinical studies to test their potential use to treat disorders associated with acute neuronal damage, such as traumatic brain injury, are being planned. This review will illustrate the long and difficult journey to validate a bold hypothesis.

Potential of Capric Acid in Neurological Disorders: An Overview

To solve the restrictions of a classical ketogenic diet, a modified medium-chain triglyceride diet was introduced which required only around 60% of dietary energy. Capric acid (CA), a small molecule, is one of the main components because its metabolic profile offers itself as an alternate source of energy to the brain in the form of ketone bodies. This is possible with the combined capability of CA to cross the blood-brain barrier and achieve a concentration of 50% concentration in the brain more than any other fatty acid in plasma. Natural sources of CA include vegetable oils such as palm oil and coconut oil, mammalian milk and some seeds. Several studies have shown that CA has varied action on targets that include AMPA receptors, PPAR-γ, inflammatory/oxidative stress pathways and gut dysbiosis. Based on these lines of evidence, CA has proved to be effective in the amelioration of neurological diseases such as epilepsy, affective disorders and Alzheimer's disease. But these studies still warrant more pre-clinical and clinical studies that would further prove its efficacy. Hence, to understand the potential of CA in brain disease and associated comorbid conditions, an advance and rigorous molecular mechanistic study, apart from the reported in-vitro/in-vivo studies, is urgently required for the development of this compound through clinical setups.

Ketamine-New Possibilities in the Treatment of Depression: A Narrative Review

The SARS-CoV-2 coronavirus epidemic has led to an increase in the number of people with depression. Symptoms related to the mental sphere (mainly depression and anxiety) may be experienced by one third of the worldwide population. This entails the need for the effective and rapid treatment of depressive episodes. An effective drug seems to be s-ketamine, which was accepted in March 2019 by the Food and Drug Administration (FDA) for the treatment of drug-resistant depression. This drug provides a quick antidepressant effect with maximum effectiveness achieved after 24 h. It also appears to reduce the occurrence of suicidal thoughts. However, research into undesirable effects, especially in groups of people susceptible to psychotic episodes or those who use alcohol or psychoactive substances, is necessary.

RNA aptamers for AMPA receptors

RNA aptamers are single-stranded RNA molecules, and they are selected against a target of interest so that they can bind to and modulate the activity of the target, such as inhibiting the target activity, with high potency and selectivity. Antagonists, such as RNA aptamers, acting on AMPA receptors, a major subtype of ionotropic glutamate receptors, are potential drug candidates for treatment of a number of CNS diseases that involve excessive receptor activation and/or elevated receptor expression. Here we review the approach to discover RNA aptamers targeting AMPA receptors from a random sequence library (∼10¹⁴ sequences) through a process called systematic evolution of ligands by exponential enrichment (SELEX). As compared with small-molecule compounds, RNA aptamers are a new class of regulatory agents with interesting and desirable pharmacological properties. Some AMPA receptor aptamers we have developed are presented in this review. The promises and challenges of translating RNA aptamers into potential drugs and treatment options are also discussed. This article is part of the special Issue on 'Glutamate Receptors - AMPA receptors'.

A review on the synthesis of heteroannulated quinolones and their biological activities

The quinoline scaffold has become an important construction motif for the development of new drugs. The quinolones and their heteroannulated derivatives have high importance due to their diverse spectrum of biological activities as antifungal, anti-inflammatory, anti-diabetes, anti-Alzheimer's disease, antioxidant and diuretic activities. This review summarizes the various new, efficient and convenient synthetic approaches to synthesize diverse quinolone-based scaffolds and their biological activities. We also dealt with the important mechanism, the route and type of reactions of the obtained products. The biological activities of some heteroannulated quinolones were also discussed.

Circuits and Synapses: Hypothesis, Observation, Controversy and Serendipity - An Opinion Piece

More than a century of dedicated research has resulted in what we now know, and what we think we know, about synapses and neural circuits. This piece asks to what extent some of the major advances - both theoretical and practical - have resulted from carefully considered theory, or experimental design: endeavors that aim to address a question, or to refute an existing hypothesis. It also, however, addresses the important part that serendipity and chance have played. There are cases where hypothesis driven research has resulted in important progress. There are also examples where a hypothesis, a model, or even an experimental approach - particularly one that seems to provide welcome simplification - has become so popular that it becomes dogma and stifles advance in other directions. The nervous system rejoices in complexity, which should neither be ignored, nor run from. The emergence of testable "rules" that can simplify our understanding of neuronal circuits has required the collection of large amounts of data that were difficult to obtain. And although those collecting these data have been criticized for not advancing hypotheses while they were "collecting butterflies," the beauty of the butterflies always enticed us toward further exploration.

Subunit-selective iGluR antagonists can potentiate heteromeric receptor responses by blocking desensitization

Ionotropic glutamate receptors (iGluRs) are key molecules for synaptic signaling in the central nervous system, which makes them promising drug targets. Intensive efforts are being devoted to the development of subunit-selective ligands, which should enable more precise pharmacologic interventions while limiting the effects on overall neuronal circuit function. However, many AMPA and kainate receptor complexes in vivo are heteromers composed of different subunits. Despite their importance, little is known about how subunit-selective ligands affect the gating of heteromeric iGluRs, namely their activation and desensitization properties. Using fast ligand application experiments, we studied the effects of competitive antagonists that block glutamate from binding at part of the four subunits. We found that UBP-310, a kainate receptor antagonist with high selectivity for GluK1 subunits, reduces the desensitization of GluK1/GluK2 heteromers and fully abolishes the desensitization of GluK1/GluK5 heteromers. This effect is mirrored by subunit-selective agonists and heteromeric receptors that contain binding-impaired subunits, as we show for both kainate and GluA2 AMPA receptors. These findings are consistent with a model in which incomplete agonist occupancy at the four receptor subunits can provide activation without inducing desensitization. However, we did not detect significant steady-state currents during UBP-310 dissociation from GluK1 homotetramers, indicating that antagonist dissociation proceeds in a nonuniform and cooperativity-driven manner, which disfavors nondesensitizing occupancy states. Besides providing mechanistic insights, these results have direct implications for the use of subunit-selective antagonists in neuroscience research and envisioned therapeutic interventions.

d-Aspartate N-methyltransferase catalyzes biosynthesis of N-methyl-d-aspartate (NMDA), a well-known selective agonist of the NMDA receptor, in mice

N-Methyl-d-aspartate (NMDA), which is a selective agonist for the NMDA receptor, has recently been shown to be present in various biological tissues. In mammals, the activity of d-aspartate N-methyltransferase (DDNMT), which produces NMDA from d-aspartate, has been detected only in homogenates prepared from rat tissues. Moreover, the enzymatic properties of DDNMT have been poorly studied and its molecular entity has not yet been identified. In this report, we show for the first time that the activity of DDNMT is present in mouse tissues and succeed in obtaining a partially purified enzyme preparation from a mouse tissue homogenate with a purification fold of 1900 or more, and have characterized the enzymatic activity of this preparation. The results indicate that DDNMT, which is highly specific for d-aspartate and is S-adenosyl-l-methionine-dependent, is a novel enzyme that clearly differs from the known methylamine-glutamate N-methyltransferase (EC 2.1.1.21) and glycine N-methyltransferase (EC 2.1.1.20).

Mechanism and properties of positive allosteric modulation of N-methyl-d-aspartate receptors by 6-alkyl 2-naphthoic acid derivatives

The theory that N-methyl-d-aspartate receptor (NMDAR) hypofunction is responsible for the symptoms of schizophrenia is well supported by many pharmacological and genetic studies. Accordingly, positive allosteric modulators (PAMs) that augment NMDAR signaling may be useful for treating schizophrenia. Previously we have identified several NMDAR PAMs containing a carboxylic acid attached to naphthalene, phenanthrene, or coumarin ring systems. In this study, we describe several functional and mechanistic properties of UBP684, a 2-naphthoic acid derivative, which robustly potentiates agonist responses at each of the four GluN1a/GluN2 receptors and at neuronal NMDARs. UBP684 increases the maximal l-glutamate/glycine response while having minor subunit-specific effects on agonist potency. PAM binding is independent of agonist binding, and PAM activity is independent of membrane voltage, redox state, and the GluN1 exon 5 N-terminal insert. UBP684 activity is, however, markedly pH-dependent, with greater potentiation occurring at lower pHs and inhibitory activity at pH 8.4. UBP684 increases channel open probability (P_o) and slows receptor deactivation time upon removal of l-glutamate, but not glycine. The structurally related PAM, UBP753, reproduced most of these findings, but did not prolong agonist removal deactivation time. Studies using cysteine mutants to lock the GluN1 and GluN2 ligand-binding domains (LBDs) in the agonist-bound states indicate that PAM potentiation requires GluN2 LBD conformational flexibility. Together, these findings suggest that UBP684 and UBP753 stabilize the GluN2 LBD in an active conformation and thereby increase P_o. Thus, UBP684 and UBP753 may serve as lead compounds for developing agents to enhance NMDAR activity in disorders associated with NMDAR hypofunction.

Novel Functional Properties of Drosophila CNS Glutamate Receptors

Phylogenetic analysis reveals AMPA, kainate, and NMDA receptor families in insect genomes, suggesting conserved functional properties corresponding to their vertebrate counterparts. However, heterologous expression of the Drosophila kainate receptor DKaiR1D and the AMPA receptor DGluR1A revealed novel ligand selectivity at odds with the classification used for vertebrate glutamate receptor ion channels (iGluRs). DKaiR1D forms a rapidly activating and desensitizing receptor that is inhibited by both NMDA and the NMDA receptor antagonist AP5; crystallization of the KaiR1D ligand-binding domain reveals that these ligands stabilize open cleft conformations, explaining their action as antagonists. Surprisingly, the AMPA receptor DGluR1A shows weak activation by its namesake agonist AMPA and also by quisqualate. Crystallization of the DGluR1A ligand-binding domain reveals amino acid exchanges that interfere with binding of these ligands. The unexpected ligand-binding profiles of insect iGluRs allows classical tools to be used in novel approaches for the study of synaptic regulation. VIDEO ABSTRACT.

Ketamine-Mediated Inhibition of Rabies Virus Infection in vitro and in Rat Brain

This report describes the effect of the dissociative anaesthetic ketamine, a non-competitive antagonist of the N-methyl-d-aspartate (NMDA) receptor, on the infectivity of the neurotropic rabies virus, in neuronal culture systems and in vivo. Ketamine (1–2 mm) produced an approximate 100–1000-fold decrease in the production of rabies virus in neuronal cell cultures. Rabies virus infection was also inhibited in non-neuronal cell lines (Baby Hamster Kidney fibroblasts) but the inhibitory effects appeared better expressed in cells of neuronal origin (neuroblastoma and primary neuronal cultures). The kainate/quisqualate competitive antagonist CNQX did not modify the course of rabies virus infection and modulators of known NMDA regulatory sites (Mg2+, Zn2+, HA-966) failed to antagonize the ketamine-mediated inhibition of rabies virus production in neuroblastoma cells. Furthermore, Ca2+-mobilization does not appear to be involved because Ca2+-depleted and EGTA-treated medium did not affect the normal production of rabies virus. The action of ketamine on rabies virus infection in vivo was also investigated; peripheral treatment of rats with this drug inhibited rabies virus infection in the thalamus, cortex and hippocampal formation (in particular the pyramidal layer of the CA1 region). Taken together, these data suggest that the ketamine-mediated inhibition of rabies virus production in vitro, although highly selective, is not acting via classic NMDA receptor-mediated mechanisms. However, the ketamine-mediated retardation of rabies virus infection in vivo may offer new prospects for post-exposure rabies antiviral therapy.

Alcohol Withdrawal and Cerebellar Mitochondria

Cerebellar disorders trigger the symptoms of movement problems, imbalance, incoordination, and frequent fall. Cerebellar disorders are shown in various CNS illnesses including a drinking disorder called alcoholism. Alcoholism is manifested as an inability to control drinking in spite of adverse consequences. Human and animal studies have shown that cerebellar symptoms persist even after complete abstinence from drinking. In particular, the abrupt termination (ethanol withdrawal) of long-term excessive ethanol consumption has shown to provoke a variety of neuronal and mitochondrial damage to the cerebellum. Upon ethanol withdrawal, excitatory neurotransmitter molecules such as glutamate are overly released in brain areas including cerebellum. This is particularly relevant to the cerebellar neuronal network as glutamate signals are projected to Purkinje neurons through granular cells that are the most populated neuronal type in CNS. This excitatory neuronal signal may be elevated by ethanol withdrawal stress, which promotes an increase in intracellular Ca(2+) level and a decrease in a Ca(2+)-binding protein, both of which result in the excessive entry of Ca(2+) to the mitochondria. Subsequently, mitochondria undergo a prolonged opening of mitochondrial permeability transition pore and the overproduction of harmful free radicals, impeding adenosine triphosphate (ATP)-generating function. This in turn provokes the leakage of mitochondrial molecule cytochrome c to the cytosol, which triggers a cascade of adverse cytosol reactions. Upstream to this pathway, cerebellum under the condition of ethanol withdrawal has shown aberrant gene modifications through altered DNA methylation, histone acetylation, or microRNA expression. Interplay between these events and molecules may result in functional damage to cerebellar mitochondria and consequent neuronal degeneration, thereby contributing to motoric deficit. Mitochondria-targeting research may help develop a powerful new therapy to manage cerebellar disorders associated with hyperexcitatory CNS disorders like ethanol withdrawal.

Excitotoxicity as a Common Mechanism for Fetal Neuronal Injury with Hypoxia and Intrauterine Inflammation

Excitotoxicity is a mechanism of neuronal injury, implicated in the pathogenesis of many acute and chronic neurologic disorders, including perinatal brain injury associated with hypoxia-ischemia and exposure to intrauterine inflammation. Glutamate, the primary excitatory neurotransmitter, signals through N-methyl-d-aspartic acid (NMDA)/α-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors. Proper functioning of both of these receptors, in conjunction with glutamate signaling, is crucial for normal development. However, even a small imbalance can result in perinatal neuronal injury. Therefore, a mechanistic understanding of the role of excitotoxicity and the NMDA/AMPA receptor functions is critical to establishing the pathogenesis of hypoxic-ischemic encephalopathy (HIE) and perinatal brain injury due to exposure to intrauterine inflammation. Evidence from experimental animal models and clinical studies indicates that both oxygen and glucose deficiencies play a major role in fetal neuronal injury. However, the connection between these deficiencies, excitotoxicity, and HIE is not well established. The excitotoxic mechanisms in animal models and humans have many parallels, suggesting that detailed animal studies can elicit clinically relevant discoveries. While current therapies for HIE include hypothermia and other neuroprotective measures, emphasizing prevention of acute injuries, increase of therapeutic time window, and increased neural repair, there are no effective widely used treatment modalities for fetuses and neonates exposed to intrauterine inflammation. Further studies of HIE and intrauterine inflammation (as in cases of preterm birth and chorioamnionitis) will provide a better insight into development of effective therapeutic interventions for these conditions.

Radial symmetry in a chimeric glutamate receptor pore

Ionotropic glutamate receptors comprise two conformationally different A/C and B/D subunit pairs. Closed channels exhibit fourfold radial symmetry in the transmembrane domain (TMD) but transition to twofold dimer-of-dimers symmetry for extracellular ligand binding and N-terminal domains. Here, to evaluate symmetry in open pores we analysed interaction between the Q/R editing site near the pore loop apex and the transmembrane M3 helix of kainate receptor subunit GluK2. Chimeric subunits that combined the GluK2 TMD with extracellular segments from NMDA receptors, which are obligate heteromers, yielded channels made up of A/C and B/D subunit pairs with distinct substitutions along M3 and/or Q/R site editing status, in an otherwise identical homotetrameric TMD. Our results indicate that Q/R site interaction with M3 occurs within individual subunits and is essentially the same for both A/C and B/D subunit conformations, suggesting that fourfold pore symmetry persists in the open state.

Neuroprotective activity of stiripentol with a possible involvement of voltage-dependent calcium and sodium channels

A growing body of data has shown that recurrent epileptic seizures may be caused by an excessive release of the excitatory neurotransmitter glutamate in the brain. Glutamatergic overstimulation results in massive neuronal influxes of calcium and sodium through N-methyl-D-aspartate (NMDA), α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, and kainic acid glutamate subtype receptors and also through voltage-gated calcium and sodium channels. These persistent and abnormal sodium and calcium entry points have deleterious consequences (neurotoxicity) for neuronal function. The therapeutic value of an antiepileptic drug would include not only control of seizure activity but also protection of neuronal tissue. The present study examines the in vitro neuroprotective effects of stiripentol, an antiepileptic compound with γ-aminobutyric acidergic properties, on neuronal-astroglial cultures from rat cerebral cortex exposed to oxygen-glucose deprivation (OGD) or to glutamate (40 µM for 20 min), two in vitro models of brain injury. In addition, the affinity of stiripentol for the different glutamate receptor subtypes and the interaction with the cell influx of Na(+) and of Ca(2+) enhanced by veratridine and NMDA, respectively, are assessed. Stiripentol (10-100 µM) included in the culture medium during OGD or with glutamate significantly increased the number of surviving neurons relative to controls. Stiripentol displayed no binding affinity for different subtypes of glutamate receptors (IC50  >100 µM) but significantly blocked the entry of Na(+) and Ca(2+) activated by veratridine and NMDA, respectively. These results suggest that Na(+) and Ca(2+) channels could contribute to the neuroprotective properties of sitiripentol.

Response Properties of a Newly Identified Tristratified Narrow Field Amacrine Cell in the Mouse Retina

Amacrine cells were targeted for whole cell recording using two-photon fluorescence microscopy in a transgenic mouse line in which the promoter for dopamine receptor 2 drove expression of green fluorescent protein in a narrow field tristratified amacrine cell (TNAC) that had not been studied previously. Light evoked a multiphasic response that was the sum of hyperpolarizing and depolarization synaptic inputs consistent with distinct dendritic ramifications in the off and on sublamina of the inner plexiform layer. The amplitude and waveform of the response, which consisted of an initial brief hyperpolarization at light onset followed by recovery to a plateau potential close to dark resting potential and a hyperpolarizing response at the light offset varied little over an intensity range from 0.4 to ~10^6 Rh*/rod/s. This suggests that the cell functions as a differentiator that generates an output signal (a transient reduction in inhibitory input to downstream retina neurons) that is proportional to the derivative of light input independent of its intensity. The underlying circuitry appears to consist of rod and cone driven on and off bipolar cells that provide direct excitatory input to the cell as well as to GABAergic amacrine cells that are synaptically coupled to TNAC. Canonical reagents that blocked excitatory (glutamatergic) and inhibitory (GABA and glycine) synaptic transmission had effects on responses to scotopic stimuli consistent with the rod driven component of the proposed circuit. However, responses evoked by photopic stimuli were paradoxical and could not be interpreted on the basis of conventional thinking about the neuropharmacology of synaptic interactions in the retina.

Efficient, complete deletion of synaptic proteins using CRISPR

One of the most powerful ways to test the function of a protein is to characterize the consequences of its deletion. In the past, this has involved inactivation of the gene by homologous recombination either in the germline or later through conditional deletion. RNA interference (RNAi) provides an alternative way to knock down proteins, but both of these approaches have their limitations. Recently, the CRISPR/Cas9 system has suggested another way to selectively inactivate genes. We have now tested this system in postmitotic neurons by targeting two well-characterized synaptic proteins, the obligatory GluN1 subunit of the NMDA receptor and the GluA2 subunit of the AMPA receptor. Expression of CRISPR/Cas9 in hippocampal slice cultures completely eliminated NMDA receptor and GluA2 function. CRISPR/Cas9 thus provides a powerful tool to study the function of synaptic proteins.

Progress toward advanced understanding of metabotropic glutamate receptors: structure, signaling and therapeutic indications

The metabotropic glutamate (mGlu) receptors are a group of Class C seven-transmembrane spanning/G protein-coupled receptors (7TMRs/GPCRs). These receptors are activated by glutamate, one of the standard amino acids and the major excitatory neurotransmitter. By activating G protein-dependent and non-G protein-dependent signaling pathways, mGlus modulate glutamatergic transmission both in the periphery and throughout the central nervous system. Since the discovery of the first mGlu receptor, and especially during the last decade, a great deal of progress has been made in understanding the signaling, structure, pharmacological manipulation and therapeutic indications of the 8 mGlu members.

Paraventricular nucleus of the hypothalamus glutamate neurotransmission modulates autonomic, neuroendocrine and behavioral responses to acute restraint stress in rats

In the present study, the involvement of paraventricular nucleus of the hypothalamus (PVN) glutamate receptors in the modulation of autonomic (arterial blood pressure, heart rate and tail skin temperature) and neuroendocrine (plasma corticosterone) responses and behavioral consequences evoked by the acute restraint stress in rats was investigated. The bilateral microinjection of the selective non-NMDA glutamate receptor antagonist NBQX (2 nmol/ 100 nL) into the PVN reduced the arterial pressure increase as well as the fall in the tail cutaneous temperature induced by the restraint stress, without affecting the stress-induced tachycardiac response. On the other hand, the pretreatment of the PVN with the selective NMDA glutamate receptor antagonist LY235959 (2 nmol/100 nL) was able to increase the stress-evoked pressor and tachycardiac response, without affecting the fall in the cutaneous tail temperature. The treatment of the PVN with LY235959 also reduced the increase in plasma corticosterone levels during stress and inhibited the anxiogenic-like effect observed in the elevated plus-maze 24h after the restraint session. The present results show that NMDA and non-NMDA receptors in the PVN differently modulate responses associated to stress. The PVN glutamate neurotransmission, via non-NMDA receptors, has a facilitatory influence on stress-evoked autonomic responses. On the other hand, the present data point to an inhibitory role of PVN NMDA receptors on the cardiovascular responses to stress. Moreover, our findings also indicate an involvement of PVN NMDA glutamate receptors in the mediation of the plasma corticosterone response as well as in the delayed emotional consequences induced by the restraint stress.

Seizure-induced reduction in PIP3 levels contributes to seizure-activity and is rescued by valproic acid

Phosphatidylinositol (3–5) trisphosphate (PIP₃) is a central regulator of diverse neuronal functions that are critical for seizure progression, however its role in seizures is unclear. We have recently hypothesised that valproic acid (VPA), one of the most commonly used drugs for the treatment of epilepsy, may target PIP₃ signalling as a therapeutic mode of action. Here, we show that seizure induction using kainic acid in a rat in vivo epilepsy model resulted in a decrease in hippocampal PIP₃ levels and reduced protein kinase B (PKB/AKT) phosphorylation, measured using ELISA mass assays and Western blot analysis, and both changes were restored following VPA treatment. These finding were reproduced in cultured rat hippocampal primary neurons and entorhinal cortex–hippocampal slices during exposure to the GABA(A) receptor antagonist pentylenetetrazol (PTZ), which is widely used to generate seizures and seizure-like (paroxysmal) activity. Moreover, VPA's effect on paroxysmal activity in the PTZ slice model is blocked by phosphatidylinositol 3-kinase (PI3K) inhibition or PIP₂ sequestration by neomycin, indicating that VPA's efficacy is dependent upon PIP₃ signalling. PIP₃ depletion following PTZ treatment may also provide a positive feedback loop, since enhancing PIP₃ depletion increases, and conversely, reducing PIP₃ dephosphorylation reduces paroxysmal activity and this effect is dependent upon AMPA receptor activation. Our results therefore indicate that PIP₃ depletion occurs with seizure activity, and that VPA functions to reverse these effects, providing a novel mechanism for VPA in epilepsy treatment.

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In vivo seizure induction (using kainic acid) reduces hippocampal PIP₃ levels.

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In vivo seizure induction (using kainic acid) reduces hippocampal phospho-PKB levels.

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Valproic acid protects against these reductions under seizure conditions only.

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Similar regulation is seen with PTZ-induced in vitro seizure activity.

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Seizure-induced PIP₃ reduction causes a feedback activation of seizure activity.

Modulation of extracellular d-serine content by calcium permeable AMPA receptors in rat medial prefrontal cortex as revealed by in vivo microdialysis

In mammalian brains, d-serine has been shown to be required for the regulation of glutamate neurotransmission as an endogenous co-agonist for the N-methyl-d-aspartate type glutamate receptor that is essential for the expression of higher-order brain functions. The exact control mechanisms for the extracellular d-serine dynamics, however, await further elucidation. To obtain an insight into this issue, we have characterized the effects of agents acting at the α-amino-3-hydroxy-5-methyl-4-isoxazolepropioinic acid (AMPA) type glutamate receptor on the extracellular d-serine contents in the medial prefrontal cortex of freely moving rats by an in vivo microdialysis technique in combination with high-performance liquid chromatography with fluorometric detection. In vivo experiments are needed in terms of a crucial role of d-serine in the neuron-glia communications despite the previous in vitro studies on AMPA receptor-d-serine interactions using the separated preparations of neurons or glial cells. Here, we show that the intra-cortical infusion of (S)-AMPA, an active enantiomer at the AMPA receptor, causes a significant and concentration-dependent reduction in the prefrontal extracellular contents of d-serine, which is reversed by an AMPA/kainate receptor antagonist, 2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[f]quinoxaline-7-sulfonamide disodium salt, and a calcium permeable AMPA receptor antagonist, 1-naphthyl acetyl spermine. The d-serine reducing effects of (S)-AMPA are augmented by co-infusion of cyclothiazide that prevents AMPA receptor desensitization. Our data support the view that a calcium permeable AMPA receptor subtype may exert a phasic inhibitory control on the extracellular d-serine release in the mammalian prefrontal cortex in vivo.

Carboxylic acid (bio)isosteres in drug design

The carboxylic acid functional group can be an important constituent of a pharmacophore, however, the presence of this moiety can also be responsible for significant drawbacks, including metabolic instability, toxicity, as well as limited passive diffusion across biological membranes. To avoid some of these shortcomings while retaining the desired attributes of the carboxylic acid moiety, medicinal chemists often investigate the use of carboxylic acid (bio)isosteres. The same type of strategy can also be effective for a variety other purposes, for example, to increase the selectivity of a biologically active compound or to create new intellectual property. Several carboxylic acid isosteres have been reported, however, the outcome of any isosteric replacement cannot be readily predicted as this strategy is generally found to be dependent upon the particular context (i.e., the characteristic properties of the drug and the drug-target). As a result, screening of a panel of isosteres is typically required. In this context, the discovery and development of novel carboxylic acid surrogates that could complement the existing palette of isosteres remains an important area of research. The goal of this Minireview is to provide an overview of the most commonly employed carboxylic acid (bio)isosteres and to present representative examples demonstrating the use and utility of each isostere in drug design.

Inhibition of NMDA-type glutamate receptors induces arousal from torpor in hibernating arctic ground squirrels (Urocitellus parryii)

Hibernation is an adaptation to overcome periods of resource limitation often associated with extreme climatic conditions. The hibernation season consists of prolonged bouts of torpor that are interrupted by brief interbout arousals. Physiological mechanisms regulating spontaneous arousals are poorly understood, but may be related to a need for gluconeogenesis or elimination of metabolic wastes. Glutamate is derived from glutamine through the glutamate-glutamine cycle and from glucose via the pyruvate carboxylase pathway when nitrogen balance favors formation of glutamine. This study tests the hypothesis that activation of NMDA-type glutamate receptors (NMDAR) maintains torpor in arctic ground squirrel (arctic ground squirrel (AGS); Urocitellus parryii). Administration of NMDAR antagonists MK-801 (5 mg/kg, i.p.) that crosses the blood-brain barrier and AP5 (5 mg/kg, i.p.) that does not cross the blood-brain barrier induced arousal in AGS. Central administration of MK-801 (0.2, 2, 20 or 200 μg; icv) to hibernating AGS failed to induce arousal. Results suggest that activation of NMDAR at a peripheral or circumventricular site is necessary to maintain prolonged torpor and that a decrease in glutamate at these sites may contribute to spontaneous arousal in AGS.

Johansen T, Frydenvang K, Suturkova L. Ionotropic Glutamate Receptors (iGluRs): Overview of iGluR2 ligand binding domain in complex with agonists and antagonists

Ionotropic glutamate receptors (iGluRs) constitute a family of ligand gated ion channels subdivided in three classes, NMDA, AMPA (iGluA1-4) and KA (1-5) according to the agonists that selectively activate them. iGluRs are tetrameric assemblies of highly homologous receptor subunits. They are critically important for normal brain function and are considered to be involved on neurological disorders and degenerative diseases such as schizophrenia, Alzheimer’s disease, brain damage following stroke and epilepsy. Since the first publication of the structure of recombinant soluble protein of ligand binding domain of GluA2 extensive studies on this group of receptors were performed and many crystal structures as complexes of GluA2-LBD with agonists, partial agonists and antagonists were obtained. The structural information in combination with functional data makes good platform for consecutive investigation and design of new selective drugs which will be used in treatment of neurodegerative diseases.

Synthesis of C5-tetrazole derivatives of 2-amino-adipic acid displaying NMDA glutamate receptor antagonism

Five derivatives of 2-amino-adipic acid bearing a tetrazole-substituted in C5 position were synthesized. These compounds displayed selective antagonism towards N-methyl-D: -aspartate (NMDA) receptors compared with AMPA receptors, and they were devoid of any neurotoxicity. Among these five analogues, one exhibited a higher affinity for synaptic NMDA responses than the other four. Therefore, C5 tetrazole-substituted of 2-amino-adipic acid represent an interesting series of new NMDA receptor antagonists. This approach may be considered as a new strategy to develop ligands specifically targeted to synaptic or extra-synaptic NMDA receptors.

Lessons from more than 80 structures of the GluA2 ligand-binding domain in complex with agonists, antagonists and allosteric modulators

Ionotropic glutamate receptors (iGluRs) constitute a family of ligand-gated ion channels that are essential for mediating fast synaptic transmission in the central nervous system. These receptors play an important role for the development and function of the nervous system, and are essential in learning and memory. However, iGluRs are also implicated in or have causal roles for several brain disorders, e.g. epilepsy, Alzheimer's disease, Parkinson's disease and schizophrenia. Their involvement in neurological diseases has stimulated widespread interest in their structure and function. Since the first publication in 1998 of the structure of a recombinant soluble protein comprising the ligand-binding domain of GluA2 extensive studies have afforded numerous crystal structures of wildtype and mutant proteins including different ligands. The structural information obtained combined with functional data have led to models for receptor activation and desensitization by agonists, inhibition by antagonists and block of desensitization by positive allosteric modulators. Furthermore, the structural and functional studies have formed a powerful platform for the design of new selective compounds.

Changes in microtubule turnover accompany synaptic plasticity and memory formation in response to contextual fear conditioning in mice

Synaptic plasticity plays a crucial role in learning, memory, and cognitive disorders. Cytoskeletal reorganization underlies neuronal synaptic plasticity, but little is known about the regulation of cytoskeletal dynamics in living animals. We used stable isotope labeling to measure the turnover of tubulin in defined microtubule (MT) populations in murine brain. Neuronal MTs generally exhibited low turnover rates in vivo. Basal turnover was highest in tau-associated MTs, intermediate in microtubule-associated protein 2 (MAP2)-associated MTs, and lowest in cold-stable MTs. Labeling of MTs in mature neurons in cell culture yielded similar turnover results. Intracerebroventricular glutamate injection stimulated, via N-methyl-D-aspartic acid receptors, label incorporation (turnover) in cold-stable, tau-associated, and MAP2-associated MTs, the last of which was shown to be dependent on cyclic adenosine-3', 5'-monophosphorothioate-protein kinase A. Contextual fear conditioning, a hippocampus-mediated form of memory formation, was accompanied by increased turnover of hippocampal MAP2-associated and cold-stable MTs. Treatment with the MT-depolymerizing drug nocodazole reversed the conditioning-induced increase in label incorporation in MAP2-associated MTs, reduced dendritic spine density, and impaired memory formation. The effects of nocodazole on MT turnover were prevented by the MT-stabilizing agent Taxol (Sigma-Aldrich, St. Louis, MO, USA) and by brain-derived nerve growth factor, both of which also restored dendritic spine density and memory formation in this model. In conclusion, these results suggest that changes in hippocampal MT turnover are required for, and are a biomarker of, the synaptic plasticity that is involved in memory formation.

GABAERGIC NEURON DEATH IN THE STRIATUM FOLLOWING KAINATE-INDUCED DAMAGE OF HIPPOCAMPAL NEURONS: EVIDENCE FOR THE ROLE OF NO IN LOCOMOTION

The authors examined the role of nitric oxide (NO) in the relationship between kainate-induced neuronal death and locomotion changes. Locomotion was significantly increased 1 h after kainate injection, suggesting that kainate induced NO and dopamine release. Cell death occurred in the CA1 (41%) and CA3 (54%) regions at 12 h. At 7 days, GABAergic neurons in striatum were lost, suggesting possible pyramidal neuron synapse with striatal GABAergic neurons, and pyramidal neuron damage leading to deafferentation and degeneration of striatal GABAergic neurons. Pre-administration of Nw-nitro-L-arginine-methyl-ester or 7-nitroindazole reduced these effects. These results indicate that NO may modulate kainate-induced neuronal death and locomotion.

Metabotropic glutamate 1 receptor: current concepts and perspectives

Almost 25 years after the first report that glutamate can activate receptors coupled to heterotrimeric G-proteins, tremendous progress has been made in the field of metabotropic glutamate receptors. Now, eight members of this family of glutamate receptors, encoded by eight different genes that share distinctive structural features have been identified. The first cloned receptor, the metabotropic glutamate (mGlu) receptor mGlu1 has probably been the most extensively studied mGlu receptor, and in many respects it represents a prototypical subtype for this family of receptors. Its biochemical, anatomical, physiological, and pharmacological characteristics have been intensely investigated. Together with subtype 5, mGlu1 receptors constitute a subgroup of receptors that couple to phospholipase C and mobilize Ca(2+) from intracellular stores. Several alternatively spliced variants of mGlu1 receptors, which differ primarily in the length of their C-terminal domain and anatomical localization, have been reported. Use of a number of genetic approaches and the recent development of selective antagonists have provided a means for clarifying the role played by this receptor in a number of neuronal systems. In this article we discuss recent advancements in the pharmacology and concepts about the intracellular transduction and pathophysiological role of mGlu1 receptors and review earlier data in view of these novel findings. The impact that this new and better understanding of the specific role of these receptors may have on novel treatment strategies for a variety of neurological and psychiatric disorders is considered.

Design, synthesis, and biological evaluation of a scaffold for iGluR ligands based on the structure of (-)-kaitocephalin

The design and synthesis of four pyrrolidine scaffolds that are structurally related to the known ionotropic glutamate receptor antagonist, (-)-kaitocephalin, is described. Additionally, preliminary results of the biological evaluation of these compounds are disclosed.

Morphometric analysis of the AMPA-type neurons in the Deiter's vestibular complex of the chick brain

Chicken (Gallus gallus) brains were used to investigate the typology and the immunolabel pattern for the subunits composing the AMPA-type glutamate receptors (GluR) of hindbrain neurons of the dorsal (dND) and ventral nuclei (vND) of the Deiter's vestibular complex (CD), which is the avian correspondent of the lateral vestibular nucleus (LVN) of mammals. Our results revealed that neurons of both divisions were poor in GluR1. The vND, the GluR2/3+ and GluR4+ label presented no area or neuronal size preference, although most neurons were around 75%. The dND neurons expressing GluR2/3 are primarily around 85%, medium to large-sized 85%, and predominantly 60% located in the medial portion of the rostral pole and in the lateral portion of the caudal pole. The majority of dND neurons containing GluR4 are also around 75%, larger (70% are large and giant), exhibiting a distribution that seems to be complementary to that of GluR2/3+ neurons. This distinct arrangement indicates functional differences into and between the DC nuclei, also signaling that such variation could be attributed to the diverse nature of the subunit composition of the GluRs. Discussion addresses the morphological and functional correlation of the avian DC with the LVN of mammals in addition to the high morphological correspondence, To include these data into the modern comparative approach we propose to adopt a similar nomenclature for the avian divisions dND and vND that could be referred as dLVN and vLVN.

NMDA receptor plasticity in the perirhinal and prefrontal cortices is crucial for the acquisition of long-term object-in-place associative memory

A key process for recognition memory is the formation of associations between an object and the place in which it was encountered, a process that has been shown to require the perirhinal (PRH) and medial prefrontal (mPFC) cortices. Here we demonstrate, for the first time, the importance of glutamatergic neurotransmission, within the PRH and mPFC, for object-in-place associative recognition memory. Unilateral blockade of AMPA receptors (by CNQX) in the PRH and mPFC in opposite hemispheres impaired an object-in-place task in rats, confirming that these cortical regions operate within a neural network for object-in-place recognition memory. Intra-mPFC infusions of AP5 (NMDA receptor antagonist) impaired short-term memory and the acquisition of long-term memory, but had no effect on retrieval. AP5 infusions into the PRH disrupted acquisition of long-term memory, but not short-term memory or retrieval. Significantly, crossed AP5 infusions into both the PRH and mPFC disrupted acquisition of long-term memory but were without effect on short-term memory. Finally a unilateral infusion of the selective kainate (GLU(K5)) receptor antagonist UBP302 [(S)-1-(2-amino-2-carboxyethyl)-3-(2-carboxybenzyl)pyrimidine-2,4-dione] into the PRH combined with a unilateral infusion of AP5 into the contralateral mPFC significantly impaired short-term object-in-place associative memory. These data show that the PRH and mPFC make distinct contributions to object-in-place associative memory and that the encoding of long-term but not short-term memory requires concurrent NMDA receptor activation in both cortical regions. In contrast, short-term object-in-place memory appears to be dependent on kainate receptor activation in the PRH and NMDA receptor activation in the mPFC.

Total syntheses of (-)-alpha-kainic acid and (+)-alpha-allokainic acid via stereoselective C-H insertion and efficient 3,4-stereocontrol

Reported herein is a novel approach to the total syntheses of (-)-alpha-kainic acid and (+)-alpha-allokainic acid, where the stereochemistries on C(2), C(3), and C(4) of the pyrrolidine core were introduced efficiently and selectively. A regio- and stereoselective C-H insertion reaction was utilized to prepare the gamma-lactam as an intermediate. A Michael-type cyclization of phenylsulfone with a conjugated acetylenic ketone was developed to prepare the tricyclic ketone as a key intermediate for (-)-alpha-kainic acid. Subsequently, a stereoselective dephenylsulfonylation was carried out successfully to secure the cis relationship at C(3) and C(4) centers. An unprecedented acetylation on the phenylsulfone, followed by a stereoselective dephenylsulfonylation, secured the trans relationship at C(3) and C(4) centers in (+)-alpha-allokainic acid.