Fragment-Based Optimization of Dihydropyrazino-Benzimidazolones as Metabotropic Glutamate Receptor-2 Positive Allosteric Modulators against Migraine

Our previous scaffold-hopping attempts resulted in dihydropyrazino-benzimidazoles as metabotropic glutamate receptor-2 (mGluR2) positive allosteric modulators (PAMs) with suboptimal drug-like profiles. Here, we report an alternative fragment-based optimization strategy applied on the new dihydropyrazino-benzimidazolone scaffold. Analyzing published high-affinity mGluR2 PAMs, we used a pharmacophore-guided approach to identify suitable growing vectors and optimize the scaffold in these directions. This strategy resulted in a new fragment like lead (34) with improved druglike properties that were translated to sufficient pharmacokinetics and validated proof-of-concept studies in migraine. Gratifyingly, compound 34 showed reasonable activity in the partial infraorbital nerve ligation, a migraine disease model that might open this indication for mGluR2 PAMs.

Changes of metabolite profile in kainic acid induced hippocampal injury in rats measured by HRMAS NMR

The solid-state high resolution magic angle spinning nuclear magnetic resonance (HRMAS NMR) technique was applied in this work to characterize and quantify the neurochemical changes in the rat hippocampus (CA1 or CA3) after local administration of kainic acid (KA). Intact tissue samples obtained from the KA treated and control brain samples were analyzed using HRMAS NMR. Metabolite profiles from NMR spectra of KA treated and control samples revealed the statistical significant decrease of N-acetylaspartate (NAA) and an increase of choline derivatives in the CA1 and CA3 directly receiving KA injection. Less extensive KA-induced metabolic changes were found in the hippocampi sample from the area contralateral to the site receiving KA administration. These results provided quantitative metabolic information on KA-induced neuronal loss and cell breakdown. In addition, the present study also revealed increased level of gamma-aminobutyric acid (GABA) and glutamate after KA treatment, suggesting that the cellular release of inhibitory and excitatory amino acids can be quantified using this method. KA induced microglia activation was evidenced by increased level of myo-insitol (myo-I). This study demonstrates that ex vivo HRMAS NMR is a useful tool for analyzing and quantifying changes of neurochemistry and cerebral metabolism in the intact brain.

Pharmacological and pharmacokinetic properties of a structurally novel, potent, and selective metabotropic glutamate 2/3 receptor agonist: in vitro characterization of agonist (-)-(1R,4S,5S,6S)-4-amino-2-sulfonylbicyclo[3.1.0]-hexane-4,6-dicarboxylic acid (LY404039)

Group II metabotropic glutamate (mGlu) receptor agonists, including (1S,2S,5R,6S)-2-aminobicyclo[3.1.0]hexane-2,6-dicarboxylate monohydrate (LY354740) and (-)-2-oxa-4-aminobicyclo[3.1.0]hexane-4,6-dicarboxylate (LY379268), have demonstrated efficacy in animal models of anxiety and schizophrenia, and LY354740 decreased anxiety in human subjects. Herein, we report the in vitro pharmacological profile and pharmacokinetic properties of another potent, selective, and structurally novel mGlu2/3 receptor agonist, (-)-(1R,4S,5S,6S)-4-amino-2-sulfonylbicyclo[3.1.0]hexane-4,6-dicarboxylic acid (LY404039) and provide comparisons with LY354740. Similar to LY354740, LY404039 is a nanomolar potent agonist at recombinant human mGlu2 and mGlu3 receptors (K(i) = 149 and 92, respectively) and in rat neurons expressing native mGlu2/3 receptors (Ki = 88). LY404039 is highly selective for mGlu2/3 receptors, showing more than 100-fold selectivity for these receptors, versus ionotropic glutamate receptors, glutamate transporters, and other receptors targeted by known anxiolytic and antipsychotic medications. Functionally, LY404039 potently inhibited forskolin-stimulated cAMP formation in cells expressing human mGlu2 and mGlu3 receptors. Electrophysiological studies indicated that LY404039 suppressed electrically evoked excitatory activity in the striatum, and serotonin-induced l-glutamate release in the prefrontal cortex; effects reversed by LY341495. These characteristics suggest LY404039 modulates glutamatergic activity in limbic and forebrain areas relevant to psychiatric disorders; and that, similar to LY354740, it works through a mechanism that may be devoid of negative side effects associated with current antipsychotics and anxiolytics. Interestingly, despite the slightly lower potency (approximately 2-5-fold) of LY404039 versus LY354740 in binding, functional, and electrophysiological assays, LY404039 demonstrated higher plasma exposure and better oral bioavailability in pharmacokinetic experiments. Collectively, the current data indicate that LY404039 may be valuable in the treatment of neuropsychiatric disorders, including anxiety and psychosis.

Developmental lead (Pb) exposure reduces the ability of the NMDA antagonist MK-801 to suppress long-term potentiation (LTP) in the rat dentate gyrus, in vivo

Chronic developmental lead (Pb) exposure increases the threshold and enhances decay of long-term potentiation (LTP) in the dentate gyrus of the hippocampal formation. MK-801 and other antagonists of the N-methyl-D-aspartate (NMDA) glutamate receptor subtype impair induction of LTP. In addition, Pb exposure reduces presynaptic glutamate release and is associated with alterations in NMDA receptor expression. This study examined LTP in Pb-exposed animals challenged with a low dose of MK-801 to assess the sensitivity of this receptor to inhibition. Pregnant rats received 0.2% Pb acetate in the drinking water beginning on gestational day 16, and this regimen was continued through lactation. Adult male offspring maintained on this solution from weaning were prepared with indwelling electrodes in the perforant path and dentate gyrus. Several weeks later, input/output (I/O) functions were collected in awake animals before and after saline or MK-801 administration (0.05 mg/kg, s.c.). LTP was induced using suprathreshold train stimuli 60 min post-drug. Post-train I/O functions were reassessed 1 and 24 h after train delivery. Upon full decay of any induced LTP, drug conditions were reversed such that each animal was tested under saline and MK-801. I/O functions measured 1 and 24 h after train induction as well as immediate post-train responses revealed significant LTP of comparable magnitude that was induced in both control and Pb-exposed animals tested under saline conditions. In contrast, MK-801 reduced LTP in control but not in Pb-exposed animals. The broadening of the excitatory postsynaptic potential evident in responses evoked by train stimuli is NMDA-dependent. Pb exposure attenuated the MK-801-induced reduction in area of this NMDA component by approximately 50%. These findings are consistent with other neurochemical and behavioural observations and suggest that up-regulation of postsynaptic NMDA receptors produces subsensitivity to the inhibitory effects of MK-801 on hippocampal LTP following chronic developmental Pb exposure.

Metabotropic glutamate receptor/phospholipase C system in female rat heart

Glutamate is the main excitatory neurotransmitter in the central nervous system. This amino acid mediates learning and memory processes acting through ionotropic and metabotropic receptor binding. Metabotropic glutamate receptors (mGluRs) are G protein-coupled receptors that stimulate phospholipase C (PLC) or inhibit adenylyl cyclase (AC). MGluRs have been widely described in CNS. However, little is known about these receptors in peripheral system. The present work describes the mGluR/PLC pathway in membranes from pregnant and non-pregnant rat heart by radioligand binding, Western-blot assays and PLC activity determination. Furthermore, mRNA coding mGluR1, mGluR5, alphaGq/11 and PLCbeta1 was identified by RT-PCR. Binding assays indicated total mGlu receptor numbers of 4.7+/-0.2 pmol/mg protein and 4.2+/-1.0 pmol/mg protein in non-pregnant and pregnant rats respectively, and their corresponding KD values were 545.3+/-85.6 nM and 1062.8+/-393.6 nM. Western blots revealed bands corresponding to mGluR1 and mGluR5 receptors, confirming that these receptors are expressed in heart. The beta1 isoform of PLC, which mediates group I mGluRs (mGluR I) response, was also expressed in rat heart. Moreover, PLC activity was modulated by calcium in a dose-dependent manner. Finally, specific agonists for mGluRs increased the PLC activity and the increase was prevented by specific mGluR antagonists. These results demonstrate the presence of group I mGlu receptors and their functional coupling to the PLC stimulation in female rat heart, suggesting a possible role of mGluR/PLC pathway in this tissue.

Impaired maze performance in aged rats is accompanied by increased density of NMDA, 5-HT1A, and α-adrenoceptor binding in hippocampus

Using quantitative receptor autoradiography, we assessed binding site densities and distribution patterns of glutamate, GABA(A), acetylcholine (ACh), and monoamine receptors in the hippocampus of 32-month-old Fischer 344/Brown Norway rats. Prior to autoradiography, the rats were divided into two groups according to their retention performance in a water maze reference memory task, which was assessed 1 week after 8 days of daily maze training. The animals of the inferior group showed less long-term retention of the hidden-platform task but did not differ from superior rats in their navigation performance during place training and cued trials. The decreased retention performance in the group of inferior learners was primarily accompanied by increased alpha(1)-adrenoceptors in all hippocampal subregions under inspection (CA1-CA4 and dentate gyrus), while elevated alpha(2)-adrenoceptor binding was observed in the CA1 region and DG. Furthermore, inferior learners had higher NMDA binding in the CA2 and CA4 and increased 5-HT(1A) binding sites in the CA2, CA3, and CA4 region. No significant differences between inferior and superior learners were evident with regard to AMPA, kainate, GABA(A), muscarinergic M(1), dopamine D(1), and 5-HT(2) binding densities in any hippocampal region analyzed. These results show that increased NMDA, 5-HT(1A), and alpha-adrenoceptor binding in the hippocampus is associated with a decline in spatial memory. The increased receptor binding observed in the group of old rats with inferior maze performance might be the result of neural adaptation triggered by age-related changes in synaptic connectivity and/or synaptic activity.

9H-Xanthene-9-carboxylic acid [1,2,4]oxadiazol-3-yl- and (2H-tetrazol-5-yl)-amides as potent, orally available mGlu1 receptor enhancers

Small molecule mGluR1 enhancers based on the lead compound (9H-xanthene-9-carbonyl)-carbamic acid butyl ester derived from random-screening hit diphenylacetyl-carbamic acid ethyl ester were designed and synthesized as useful pharmacological tools for the study of the physiological roles mediated by mGlu1 receptors. The synthesis and the structure-activity relationship of this new class of positive allosteric modulators of mGlu1 receptors will be discussed in detail.

Ionotropic glutamate receptor binding in the posterior cingulate cortex in schizophrenia patients

Using quantitative autoradiography, the present study examined ionotropic glutamatergic receptor binding sites using [3H]dizocilpine, [3H]alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate, and [3H]kainate in the posterior cingulate cortex of schizophrenia patients and matched controls. We found a significant increase in [3H]dizocilpine binding in the superficial layers (41%, p<0.001) and deep layers (30%, p=0.004) of the posterior cingulate cortex in the schizophrenia group compared with controls. No significant differences were observed in [3H]alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate and [3H]kainate binding. In summary, the present study has for the first time demonstrated that the glutamatergic system is affected in the posterior cingulate cortex in schizophrenia patients. The fact that only the N-methyl-D-aspartate receptor densities are significantly altered suggests that this is unlikely to be caused by a simple decrease in glutamatergic transmission.

Positive Allosteric Modulators of the Metabotropic Glutamate Receptor Subtype 2 (mGluR2)

This article describes recent medicinal chemistry progress toward selective potentiators of the metabotropic glutamate receptor 2 (mGluR2). Groups at Lilly and Merck have identified new classes of potentiators that exhibit selectivity for mGluR2 over the seven other subtypes of mGluRs. Structure-activity relationships as well as pharmacokinetic properties and in vivo activity are reviewed.

Cortical glutamatergic markers in schizophrenia

Post-mortem studies have yet to produce consistent findings on cortical glutamatergic markers in schizophrenia; therefore, it is not possible to fully understand the role of abnormal glutamatergic function in the pathology of the disorder. To better understand the changes in cortical glutamatergic markers in schizophrenia, we measured the binding of radioligands to the ionotropic glutamate receptors (N-methyl D-aspartate, [3H]CGP39653, [3H]MK-801), amino-3-hydroxy-5-methyl-4-isoxazole ([3H]AMPA), kainate ([3H]kainate), and the high-affinity glutamate uptake site ([3H]aspartate) using in situ radioligand binding with autoradiography and levels of mRNA for kainate receptors using in situ hybridization in the dorsolateral prefrontal cortex from 20 subjects with schizophrenia and 20 controls matched for age and sex. Levels of [3H]kainate binding were significantly decreased in cortical laminae I-II (p = 0.01), III-IV (p < 0.05), and V-VI (p < 0.01) from subjects with schizophrenia. By contrast, levels of [3H]MK-801, [3H]AMPA, [3H]aspartate, or [3H]CGP39653 binding did not differ between the diagnostic cohorts. Levels of mRNA for the GluR5 subunit were decreased overall (p < 0.05), with no changes in levels of mRNA for GluR6, GluR7, KA1, or KA2 in tissue from subjects with schizophrenia. These data indicate that the decreased number of kainate receptors in the dorsolateral prefrontal cortex in schizophrenia may result, in part, from reduced expression of the GluR5 receptor subunits.

METABOLISM AND DISPOSITION OF A POTENT GROUP II METABOTROPIC GLUTAMATE RECEPTOR AGONIST, IN RATS, DOGS, AND MONKEYS

Metabolism and disposition of MGS0028 [(1R,2S,5S,6S)-2-amino-6-fluoro-4-oxobicyclo[3.1.0]hexane-2,6-dicarboxylic acid monohydrate], a potent group II metabotropic glutamate receptor agonist, were examined in three preclinical species (Sprague-Dawley rats, beagle dogs, and rhesus monkeys). In rats, MGS0028 was widely distributed and primarily excreted in urine as parent and as a single reductive metabolite, identified as the 4R-isomer MGS0034 [(1R,2S,4R,5S,6S)-2-amino-6-fluoro-4-hydroxybicyclo[3.1.0]-hexane-2,6-dicarboxylic acid]. MGS0028 had a low brain to plasma ratio at efficacious doses in rats and was eliminated more slowly in rat brain than in plasma. Exposure increased proportionally (1--10 mg/kg p.o.) in rats, with bioavailability>60% at all doses. However, bioavailability was only approximately 20% in monkeys, and MGS0034 was found in relatively high abundance in plasma. In dogs, oral bioavailability was >60%, and the metabolite was not detected. In vitro metabolism was examined in liver subcellular fractions (microsomes and cytosol) from rat, dog, monkey, and human. Reductive metabolism was observed in rat, monkey, and human liver cytosol incubations, but not in dog liver cytosol incubations. No metabolism of MGS0028 was detected in incubations with liver microsomes from any species. Similar to in vivo results, MGS0028 was reduced in cytosol stereospecifically to MGS0034. The rank order of in vitro metabolite formation (monkey >> rat approximately human >> dog) was in agreement with in vivo observations in rats, dogs, and monkeys. Based on the observation of species difference in reductive metabolism, rat and monkey were recommended to be the preclinical species for further characterization prior to testing in humans. Finally, allometric scaling predicts that human pharmacokinetic parameters would be acceptable for further development.

A rat model of neurolathyrism: repeated injection of L-?-ODAP induces the paraparesis of the hind legs

Neurolathyrisim is a motor neuron disease characterized by spastic paraparesis in the hind legs, and is caused by grass pea, Lathyrus sativus, which contains the excitotoxic amino acid, 3-N-oxalyl-L: -2,3-diaminopropanoic acid (L: -beta-ODAP), an alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-type glutamatergic receptor agonist. In an attempt to make a useful model of this disease, the CNS distribution and toxicity of L: -beta-ODAP was studied in rat neonates after parenteral administration. L: -beta-ODAP was detected in the spinal cord as well as in the pons/medulla oblongata, though only small amounts in the latter. Repeated injection of L: -beta-ODAP resulted in rats with paraparesis of the legs, though at a low incidence rate of 0.032. These paralyzed rats displayed the severe atrophy of the ventral root of the lumbar cord as well as degenerations of motor neuron. The rats were useful models for the study of motor neuron degeneration in the spinal cord.

4-Alkylated homoibotenic acid (HIBO) analogues: Versatile pharmacological agents with diverse selectivity profiles towards metabotropic and ionotropic glutamate receptor subtypes

4-Alkylated analogues of homoibotenic acid (HIBO) have previously shown high potency and selectivity at ionotropic and metabotropic glutamic acid receptor (iGluR and mGluR) subtypes. Compounds with different selectivity profiles are valuable pharmacological tools for neuropharmacological studies, and the series of 4-alkyl-HIBO analogues have been extended in this paper in the search for versatile agents. Pharmacological characterization of five new analogues, branched and unbranched 4-alkyl-HIBO analogues, have been carried out. The present compounds are all weak antagonists at Group I mGluRs (mGluR1 and 5) presenting only small differences in potencies (Ki values ranging from 89 to 670 microM). Affinities were studied at native and cloned iGluRs, and the compounds described show preference for the AMPA receptor subtypes GluR1 and 2 over GluR3 and 4. However, compared to previous 4-alkyl-HIBO analogues, these compounds show a remarkably high affinity for the Kain preferring subtype GluR5. The observed GluR5 affinities were either similar or higher compared to their GluR1 and 2 affinity. Isopropyl-HIBO showed the highest affinity for GluR5 (Ki=0.16 microM), and represents a unique compound with high affinity towards the three subtypes GluR1, 2 and 5. In general, these compounds represent new selectivity profiles compared to previously reported Glu receptor analogues.

Effects of birth insult and stress at adulthood on excitatory amino acid receptors in adult rat brain

Birth complications involving fetal hypoxia and stress at adulthood, which are risk factors for schizophrenia, can produce alterations in subcortical dopamine (DA) function in rat models. As adults, rats born either by cesarean section (C-section) or by C-section with added global anoxia show increased stress-induced DA release from nucleus accumbens and increased amphetamine-induced locomotion, compared to vaginally born controls. Moreover, stress at adulthood interacts with these birth insults to modulate DA receptor and transporter levels. Glutamatergic transmission at the level of the nucleus accumbens, prefrontal cortex, and hippocampus are known to modulate subcortical DA activity. Thus, altered excitatory amino acid (EAA) function might contribute to the dopaminergic changes observed in rats after birth insult and/or stress at adulthood. To test this possibility, rats born vaginally, by C-section, or by C-section with 15 min of anoxia, were either repeatedly stressed (15 min of tail pinch daily for 5 days) at adulthood or received no stress, and levels of EAA receptor binding were measured by ligand autoradiography in limbic brain regions. As adults, rats born by C-section showed increases in AMPA receptor binding in nucleus accumbens shell, NMDA receptor binding in cingulate cortex, and kainate receptor binding in the hippocampal CA1 region. Anoxic rats showed increases in CA1 kainate receptor and anterior olfactory NMDA receptor binding. Stress at adulthood increased AMPA receptor binding in several regions of prefrontal cortex and reduced NMDA receptor binding in infralimbic cortex and dentate gyrus, across all birth groups. Two instances of interactions between birth insult and stress at adulthood were observed. Stress reduced cingulate cortex NMDA receptor binding and increased olfactory tubercle kainate receptor binding only in C-sectioned animals, but not in controls. The possibility that the observed EAA receptor changes contribute to dopaminergic dysfunction in these animal models is discussed, in light of known glutamate-DA interactions.

LY-354740 (Eli Lilly)

Lilly is developing LY-354740, the lead compound in a series of derivatives of the metabotropic glutamate receptor group II agonist L-CCG-1, for the potential treatment of anxiety [212536], [276941], [276942].

[3H]muscimol binding to gamma-aminobutyric acid(A) receptors is upregulated in CA1 neurons of the gerbil hippocampus in the ischemia-tolerant state

BACKGROUND AND PURPOSE

Excitotoxic activation of glutamate receptors is currently thought to play a pivotal role in delayed neuronal death (DND) of highly vulnerable CA1 neurons in the gerbil hippocampus after transient global ischemia. Postischemic degeneration of these neurons can be prevented by "preconditioning" with a short sublethal ischemic stimulus. The present study was designed to test whether ischemic preconditioning is associated with specific alterations of ligand binding to excitatory glutamate and/or inhibitory gamma-aminobutyric acid (GABA)A receptors compared with ischemia severe enough to induce DND.

METHODS

With the use of quantitative receptor autoradiography, postischemic ligand binding of [3H]MK-801 and [3H]alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) to excitatory N-methyl-D-aspartate (NMDA) and AMPA receptors as well as [3H]muscimol to inhibitory GABA(A) receptors in hippocampal subfields CA1, CA3, and the dentate gyrus were analyzed in 2 experimental paradigms. Gerbils were subjected to (1) a 5-minute ischemic period resulting in DND of CA1 neurons and (2) a 2.5-minute period of ischemia mediating tolerance induction.

RESULTS

[3H]MK-801 and [3H]AMPA binding values to excitatory NMDA and AMPA receptors showed a delayed decrease in relatively ischemia-resistant CA3 and dentate gyrus despite maintained neuronal cell density. [3H]Muscimol binding to GABA(A) receptors in CA1 neurons was transiently but significantly increased after preconditioning but not after global ischemia with consecutive neuronal death.

CONCLUSIONS

Downregulation of ligand binding to glutamate receptors in relatively ischemia-resistant CA3 and dentate gyrus neurons destined to survive suggests marked synaptic reorganization processes despite maintained structural integrity. More importantly, upregulation of binding to inhibitory GABA(A) receptors in the hippocampus indicates a relative shift between inhibitory and excitatory neurotransmission that we suggest may participate in endogenous postischemic neuroprotection.

Glial transport of the neuromodulator D-serine

D-Serine is an endogenous agonist of NMDA receptors that occurs in astrocytes in gray matter areas of the brain. D-Serine is synthesized from L-serine by the activity of a glial enriched serine racemase, but little is known on the properties of D-serine transport and factors regulating its synaptic concentration. In the present report we characterize the transport of D-serine in astrocytes. In primary astrocyte cultures, D-serine uptake is dependent on sodium ions and exhibits both low affinity and low specificity for D-serine. The kinetics of D-serine transport resembles that of ASCT type transporters as several small neutral amino acids strongly inhibit the uptake of D-serine. D-Serine fluxes are coupled to counter-movement of L-serine and to a less extent to other small neutral amino acids. Thus, addition of D-serine to cell cultures elicits robust efflux of intracellular L-serine. Conversely, physiological concentrations of L-serine induce efflux of preloaded D-serine from astrocytes. L-Serine was more effective than kainate, which have been previously shown to induce D-serine release from astrocytes upon stimulation of non-NMDA type of glutamate receptors. The features of D-serine transport we describe reveal possible new mechanisms controlling the synaptic concentration of D-serine.

The disposition, metabolism, and pharmacokinetics of a selective metabotropic glutamate receptor agonist in rats and dogs

Compound LY354740 [(+)-2-aminobicyclo[3.1.0]hexane-2,6-dicarboxylic acid], an analog of glutamic acid, is a selective group 2 metabotropic glutamate receptor agonist in clinical development for the treatment of anxiety. Studies have been conducted to characterize the absorption, disposition, metabolism, and excretion of LY354740 in rats and dogs after intravenous bolus or oral administration. Plasma concentrations of LY354740 were measured using a validated gas chromatography/mass spectrometry assay. In rats, LY354740 demonstrated linear pharmacokinetics after oral administration from 30 to 1000 mg/kg. The oral bioavailability of LY354740 was approximately 10% in rats and 45% in dogs. In the dog, food decreased the mean area under the plasma concentration-time curve value by approximately 34%, hence, decreasing the oral bioavailability of the compound. Excretion studies in both rats and dogs indicate that the absorbed drug is primarily eliminated via renal excretion. In addition, tissue distribution in rats showed that the highest levels of radioactivity were in the kidney and gastrointestinal tract, which is consistent with the excretion studies. Metabolism of LY354740 was evaluated in vitro using rat and dog liver microsomes and rat liver slices. In addition, urine and fecal samples from rat and dog excretion studies were profiled using HPLC with radio-detection. These evaluations indicated that neither rats nor dogs metabolized LY354740. In summary, LY354740 is poorly absorbed in rats, moderately absorbed in dogs, and rapidly excreted as unchanged drug in the urine.

Presynaptic kynurenate-sensitive receptors inhibit glutamate release

Kynurenic acid is a tryptophan metabolite provided with antagonist activity on ionotropic glutamate and alpha7 nicotinic acetylcholine receptors. We noticed that in rats with a dialysis probe placed in the head of their caudate nuclei, local administration of kynurenic acid (30-100 nM) significantly reduced glutamate output. Qualitatively and quantitatively similar effects were observed after systemic administration of kynurenine hydroxylase inhibitors, a procedure able to increase brain kynurenate concentrations. Interestingly, in microdialysis studies, methyllycaconitine (0.3-10 nM), a selective alpha7 nicotinic receptor antagonist, also reduced glutamate output. In isolated superfused striatal synaptosomes, kynurenic acid (100 nM), but not methyllycaconitine, inhibited the depolarization (KCl 12.5 mM)-induced release of transmitter or previously taken-up [3H]-D-aspartate. This inhibition was not modified by glycine, N-methyl-D-aspartate or subtype-selective kainate receptor agents, while CNQX or DNQX (10 microM), two AMPA and kainate receptor antagonists, reduced kynurenic acid effects. Low concentrations of kynurenic acid, however, did not modify [3H]-kainate (high and low affinity) or [3H]-AMPA binding to rat brain membranes. Finally, because metabotropic glutamate (mGlu) receptors modulate transmitter release in striatal preparations, we evaluated, with negative results, kynurenic acid (1-100 nM) effects in cells transfected with mGlu1, mGlu2, mGlu4 or mGlu5 receptors. In conclusion, our data show that kynurenate-induced inhibition of glutamate release is not mediated by glutamate receptors. Nicotinic acetylcholine receptors, however, may contribute to the inhibitory effects of kynurenate found in microdialysis studies, but not in those found in isolated synaptosomes.

Pharmacological properties of the potent epileptogenic amino acid dysiherbaine, a novel glutamate receptor agonist isolated from the marine sponge Dysidea herbacea

Dysiherbaine (DH) is a marine sponge-derived amino acid that causes seizures upon injection into mice. In this report we investigate the behavioral effects and characterize the pharmacological activity of DH. DH induced convulsive behaviors in mice with ED(50) values of 13 pmol/mouse, i.c.v. and 0.97 mg/kg, i.p. In rat brain synaptic membranes DH displaced binding of [3H]kainic acid (KA) and [3H]alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) with K(i) values of 26 and 153 nM, respectively; in contrast, DH did not displace the N-methyl-D-aspartic acid (NMDA) receptor ligand [3H]CGS-19755. DH displaced [3H]KA from recombinant GluR5 and GluR6 kainate receptor subunits expressed in HEK293 cells with K(i) values of 0.74 and 1.2 nM, respectively. In whole-cell voltage-clamp recordings from cultured rat hippocampal neurons, DH evoked inward currents from both AMPA and KA receptors with EC(50) values of 9.7 microM and 210 nM, respectively. AMPA receptor currents were blocked by GYKI 53655, whereas KA receptor currents were blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Surprisingly, in calcium imaging experiments we found that DH also activated recombinant mGluR5 receptors but did not activate mGluR1 receptors. DH did not activate glutamate transporters or gamma-aminobutyric acid A (GABA(A)) receptors. These results indicate that DH is a potent non-NMDA-type agonist with very high affinity for KA receptors, as well as a subtype-selective mGluR agonist. DH possesses the most potent epileptogenic activity among the amino acids yet identified. This novel excitatory amino acid may prove useful for evaluating the physiological and pathological roles of non-NMDA receptors, especially KA receptors, in the central nervous system.

Interference of S-nitrosoglutathione with the binding of ligands to ionotropic glutamate receptors in pig cerebral cortical synaptic membranes

The interactions of S-nitrosoglutathione (GSNO) with the ionotropic glutamate receptors were studied on synaptic membranes isolated from the pig cerebral cortex. GSNO displaced the binding of [3H]glutamate, 3-[(R)-2-carboxypiperazin-4-yl] [3H]propyl-1-phosphonate ([3H]CPP), a competitive N-methyl-D-aspartate (NMDA) antagonist, and [3H]kainate, with IC50 values in the low micromolar range. It failed to displace (S)-5-fluoro-[3H]willardiine, a selective agonist of 2-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors. Reduced and oxidized glutathione were almost as effective as GSNO in glutamate and CPP binding. Of the three, GSNO was the most potent in kainate binding. They all stimulated [3H]dizocilpine binding in a concentration-dependent manner. This effect was additive to that of glycine and not mimicked by NO donors such as S-nitroso-N-acetylpenicillamine, 5-amino-3-morpholinyl-1,2,3-oxadiazolium chloride (SIN-1) and nitroglycerin. We assume that GSNO may act as an endogenous ligand at the NMDA and non-NMDA classes of glutamate receptors. In this manner it may facilitate NO transfer and target its delivery to specific sites in these receptors.

[Behavior characterization of a model of Huntington's disease in rats, induced by quinolinic acid]

INTRODUCTION

Huntington's disease (HD) is a progressive neurodegenerative disorder, characterized by severe degeneration of basal ganglia neurons. Behavioral symptoms of HD include abnormal, uncontrollable and constant choreiform movements, impaired cognitive function and emotional disturbance.

OBJECTIVE

In order to explore the changes of cognitive and motor functions induced by quinolinate lesion we realized this experiment.

MATERIALS AND METHODS

We studied the behavior of rats with unilateral quinolinate induced lesions of the medial striatum. Intact 3 months old male rats (n = 23) were trained in the Morris Water Maze during three consecutive days, eight trials/day (acquisition), and before surgery they were randomly assigned either to intact or lesion groups. Fifteen days after the lesion the rats were tested using retention test (one day/four trials, with the escape platform in the same position as in acquisition test), on the next three days the rats were tested in the transfer test (three days/eight trials-day, with the platform in the new position). The Paw reaching test and the asymmetrical rotational behavior test in respond to amphetamine were also tested in these rats.

RESULTS

Lesioned animals exhibited deficient retrieval of stored memories of visuospatial skills and impaired transfer of learning. In relation with motor activity the lesioned rats showed a profound impairment in the skill of the left forelimb for reaching food compared with its right forelimb as well as with the forelimb abilities of intact rats. The lesioned animals showed significant rotational behavior induced by amphetamine agonist, ipsilateral to the lesioned striatum.

CONCLUSIONS

These results are consistent with the notion that the striatal degeneration could sufficiently account for the cognitive abnormalities associated with HD, and with the key role played by basal ganglia in enabling voluntary and postural adjustment of the movements.

[3H]ATPA: a high affinity ligand for GluR5 kainate receptors

The pharmacological properties of [3H]ATPA ((RS)-2-amino-3(3-hydroxy-5-tert-butylisoxazol-4-yl)propanoic acid) are described. ATPA is a tert-butyl analogue of AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid) that has been shown to possess high affinity for the GluR5 subunit of kainate receptors. [3H]ATPA exhibits saturable, high affinity binding to membranes expressing human GluR5 (GluR5) kainate receptors (Kd approximately 13 nM). No specific binding was observed in membranes expressing GluR2 and GluR6 receptors. Several compounds known to interact with the GluR5 kainate receptor inhibited [3H]ATPA binding with potencies similar to those obtained for competition of [3H]kainate binding to GluR5. Saturable, high affinity [3H]ATPA binding (Kd approximately 4 nM) was also observed in DRG neuron (DRG) membranes isolated from neonatal rats. The rank order potency of compounds to inhibit [3H]ATPA binding in rat DRG and GluR5 membranes were in agreement. These finding demonstrate that [3H]ATPA can be used as a radioligand to examine the pharmacological properties of GluR5 containing kainate receptors.

Inhibition of uptake and release of a novel mGluR agonist (L-F2CCG-I) by anion transport blockers in the rat spinal cord

A new metabotropic glutamate receptor (mGluR) agonist, (2S,1'S,2'S)-2-(2-carboxy-3,3-difluorocyclopropyl)glycine (L-F2CCG-I), induces a priming effect on (RS)-alpha-aminopimelate in the isolated spinal cord of newborn rats. Similar to (RS)-alpha-aminopimelate, L-glutamate (30-100 microM) neither affected spinal reflexes nor the resting membrane potentials of motoneurones, but preferentially potentiated the depression of monosynaptic excitation caused by L-F2CCG-I (0.4 microM). Following L-F2CCG-I treatment (1-2 microM), L-glutamate decreased the monosynaptic spinal reflexes in a concentration dependent manner, indicating a priming' effect of L-F2CCG-I. Thus L-glutamate is completely compatible with (RS)-alpha-aminopimelate in revealing the priming effect. An anion transport blocker, 4,4'-dinitrostilbene-2,2'-disulphonic acid (DNDS) (100 microM), markedly inhibited both the response to (RS)-alpha-aminopimelate and the induction of the L-F2CCG-I priming effect. The data suggest that L-F2CCG-I is Cl- -dependently incorporated into certain stores, and that (RS)-alpha-aminopimelate or L-glutamate must stimulate the release of L-F2CCG-I from the storage site. There were pharmacological similarities between the quisqualate and L-F2CCG-I priming effect. The physiological significance of the quisqualate or L-F2CCG-I priming is not yet established. L-F2CCG-I would be expected to be a useful pharmacological probe for elucidating the mechanism of the priming.

Differential changes in the subcellular distribution of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate and N-methyl-D-aspartate receptors in neonate and adult rat cortex

We compared the distribution of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptors and their individual subunits in synaptosomal and microsomal fractions prepared from 2-day-old (P2) and adult rat cortex. In P2 cortex more [3H]-(S)-fluorowillardiine ([3H]FW) binding to AMPA receptors was in the intracellular microsomal fraction than in the synaptosomal fraction whereas in adult rats the reverse was observed. Immunoblots with GluR1, GluR2/3, GluR4 and pan-AMPA antibodies showed the same profile. In contrast, the majority of [3H]MK-801 binding to the N-methyl-D-aspartate (NMDA) receptor and NR1 subunit immunoreactivity was present in the synaptosomal fraction at both developmental time points. These results suggest a developmental rearrangement of the distribution of AMPA receptors within neurons, a process which is likely to be important in synaptic stabilization and plasticity.

[3H]CGP 61594, the first photoaffinity ligand for the glycine site of NMDA receptors

Activation of NMDA receptors requires the presence of glycine as a coagonist which binds to a site that is allosterically linked to the glutamate binding site. To identify the protein constituents of the glycine binding site in situ the photoaffinity label [3H]CGP 61594 was synthesized. In reversible binding assays using crude rat brain membranes, [3H]CGP 61594 labeled with high affinity (K(D) = 23 nM) the glycine site of the NMDA receptor. This was evident from the Scatchard analysis, the displacing potencies of various glycine site ligands and the allosteric modulation of [3H]CGP 61594 binding by ligands of the glutamate and polyamine sites. Electrophysiological experiments in a neocortical slice preparation identified CGP 61594 as a glycine antagonist. Upon UV-irradiation, a protein band of 115 kDa was specifically photolabeled by [3H]CGP 61594 in brain membrane preparations. The photolabeled protein was identified as the NR1 subunit of the NMDA receptor by NR1 subunit-specific immunoaffinity chromatography. Thus, [3H]CGP 61594 is the first photoaffinity label for the glycine site of NMDA receptors. It will serve as a tool for the identification of structural elements that are involved in the formation of the glycine binding domain of NMDA receptors in situ and will thereby complement the mutational analysis of recombinant receptors.

Newer anticonvulsant drugs: role of pharmacology, drug interactions and adverse reactions in drug choice

In the last few years a number of new anticonvulsants have been introduced into clinical practice mainly as add-on therapy in patients who do not become seizure-free while receiving established anticonvulsants. Up to now, no single drug has been shown to be more effective at controlling seizures of a particular type than another, so other factors such as mechanism of action, pharmacokinetics, dosage regimens or the spectrum of adverse drug reactions and interactions are used when making a choice between one agent and another. The mechanism of action of tiagabine and vigabatrin is very specific; both agents increase gamma-aminobutyric acid (GABA) levels through inhibition of reuptake and catabolism respectively. However, the mechanism of action of gabapentin is unknown and those of felbamate, lamotrigine and topiramate are not sufficiently clarified as yet, and may be multiple. Great advances have been made in improving the pharmacokinetic characteristics of these newer anticonvulsants. Gabapentin and vigabatrin exhibit relatively ideal pharmacokinetic properties as they are not bound to proteins, are excreted mostly unchanged in the urine and show linear pharmacokinetics. Lamotrigine possesses a highly variable elimination half-life depending on the co-medication. Tiagabine is highly protein bound and zonisamide shows nonlinear pharmacokinetics; both these drugs are extensively metabolised. Problematic drug interactions between newer anticonvulsants and other drugs in general occur rarely when these agents are given concomitantly. However, in common with most new drugs, there are very few data on the use of the newer anticonvulsants in women of childbearing age. Studies done so far on interactions with oral contraceptives used low anticonvulsant dosages for a very short time. The newer anticonvulsants elicit adverse reactions that, while not being unique, are particularly associated with that drug. For example, felbamate may cause aplastic anaemia and fulminant liver failure, lamotrigine is prone to cause skin rash, and oxcarbazepine may cause symptomatic hyponatraemia. Topiramate and zonisamide cause kidney stones, and vigabatrin may induce psychiatric syndromes. Although highly diverse in structure and activity, these newer drugs offer new possibilities for treating refractory epilepsy. However, since no single factor can dictate the choice of drug nor predict the success of treatment, prescribing of these rather expensive drugs has to depend upon careful consideration of the aims of treatment, the characteristics of the drug and the needs of the individual patient.