Kynurenine diminishes the ischemia-induced histological and electrophysiological deficits in the rat hippocampus.

The neuroprotective effect of L-kynurenine sulfate (KYN), a precursor of kynurenic acid (KYNA, a selective N-methyl-D-aspartate receptor antagonist), was studied. KYN (300 mg/kg i.p., applied daily for 5 days) appreciably decreased the number of injured pyramidal cells from 1850+/-100/mm(2) to 1000+/-300/mm(2) (p<0.001) in the CA1 region of the hippocampus in the four-vessel occlusion (4VO)-induced ischemic adult rat brain. A parallel increase in the number of intact, surviving neurons was demonstrated. Post-treatment with KYN (applied immediately right after reperfusion) proved to be much less effective. In parallel with the histology, a protective effect of KYN on the functioning of the CA1 region was observed: long-term potentiation was abolished in the 4VO animals, but its level and duration were restored by pretreatment with KYN. It is concluded that the administration of KYN elevates the KYNA concentration in the brain to neuroprotective levels, suggesting its potential clinical usefulness for the prevention of neuronal loss in neurodegenerative diseases.

Neuroprotection achieved in the ischaemic rat cortex with L-kynurenine sulphate.

L-kynurenine is a metabolic precursor of kynurenic acid, which is one of the few known endogenous N-methyl-D-aspartate receptor inhibitors. In contrast with kynurenic acid, L-kynurenine is transported across the blood-brain barrier, and it may therefore come into consideration as a therapeutic agent in certain neurobiological disorders, e.g. ischaemia-induced events. The present study evaluated the effect of L-kynurenine administration (300 mg/kg i.p.) on the global ischaemic brain cortex both pre- and post-ischemic intervention. The statistical evaluation revealed that L-kynurenine administration beneficially decreased the number of neurones injured per mm(2) in the cortex, not only in the pre-treated animals, but also in those which received L-kynurenine after the ischaemic insult. It is concluded that even the post-traumatic administration of L-kynurenine may be of substantial therapeutic benefit in the treatment of global brain ischaemia. This is the first histological proof of the neuroprotective effect achieved by the post-traumatic administration of L-kynurenine in the global ischaemic cortex.

Persistent therapeutic effect of a novel α5-GABAA receptor antagonist in rodent preclinical models of vascular cognitive impairment.

This study examined the potential of the selective extra-synaptic α5-GABAA receptor inhibitor S44819 (Egis-13529) to improve cognitive performance in preclinical models of vascular cognitive impairment (VCI). Chronic hypoperfusion of the brain in mice was induced by permanent occlusion of the right common carotid artery (rUCO). rUCO induced impairments of cognitive function in the object recognition test (OR) and the rewarded T-maze (RTM). In both tests, a single oral treatment with S44819 (OR - 0.1-3 mg/kg, RTM - 1-3 mg/kg p.o.) significantly reduced the effect of rUCO. Long-term treatment with S44819 (1-10 mg/kg twice daily p.o. for 14 days), that was initiated 24 h after surgery and was followed by a 10- or 13-day wash-out period, fully prevented the decline of cognitive performance of rUCO mice. In rats, occlusion of the middle cerebral artery (MCA) for 30 min caused a significantly diminished performance in the OR. This was prevented by S44819 given p.o. 15 mg/kg twice daily for 8 days, starting 7 days after surgery and tested following a 7-day wash-out period. Taken together, S44819 markedly and stably improved reference and working memory impaired by rUCO in mice. In rats, the compound effectively suppressed the development of cognitive impairment after mild stroke. In conclusion, as longer-term administration led to a persistent reversal of the cognitive deficits, it appears that S44819 may have symptomatic, as well as disease-modifying effects in models of VCI. Proof of concept is therefore provided for testing S44819 in the therapy of VCI and post-stroke dementia in humans.

Loop F of the GABAA receptor alpha subunit governs GABA potency.

Gamma-amino butyric acid (GABA) is an abundant neurotransmitter in the CNS. GABAergic interneurons orchestrate pyramidal neurons in the cerebral cortex, and thus control learning and memory. Ionotropic receptors for GABA (GABAAR) are heteropentameric complexes of α, ß and γ integral membrane-protein subunits forming Cl- -channels operated by GABA, which are vital for brain function and are important drug targets. However, knowledge on how GABAAR bind GABA is controversial. Structural biology versus functional modelling combined with site-directed mutagenesis suggest markedly different roles for loop F of the extracellular domain of the α-subunit when complexed with GABA. Here, we report that contrary to the results of structural studies, loop F of the α-subunit controls the potency of GABA on GABAAR. We examined the effect of replacing a short, variable segment of loop F of the GABAA α5-subunit with the corresponding segment of the α2-subunit (GABAA5_LF2) and vice versa (GABAA2-LF5). When compared with their respective wild-type counterparts, GABAA5_LF2 receptors displayed enhanced sensitivity towards GABA, whilst in GABAA2-LF5 sensitivity was diminished. Mice homozygous for the genetic knock-in of the GABAA5_LF2 subunit showed a marked deficit in long- but not short-term object recognition memory. Working memory in place learning, spontaneous alternation and the rewarded T-maze were all normal. The deficit in long-term recognition memory was reversed by an α5-GABAA negative allosteric modulator compound. The data show that loop F governs GABA potency in a receptor isoform-specific manner in vitro. Moreover, this mechanism of ligand recognition appears to be operative in vivo and impacts cognitive performance.

Neuroprotective effect of L-kynurenine sulfate administered before focal cerebral ischemia in mice and global cerebral ischemia in gerbils.

Excessive stimulation of N-methyl-D-aspartate (NMDA) receptors during ischemia contributes to apoptotic and excitotoxic nerve cell death. Kynurenic acid is a selective antagonist at the glycine co-agonist site of the NMDA receptor complex at low concentration, and it is a broad-spectrum excitatory amino acid receptor blocker at high concentration. Kynurenic acid provides neuroprotection in animal models of cerebral ischemia only at very high doses as it hardly crosses the blood-brain barrier. The neuroprotective effect of L-kynurenine sulfate, a precursor of kynurenic acid, was therefore studied because L-kynurenine readily crosses the blood-brain barrier. L-kynurenine sulfate was administered 15 min before permanent focal cerebral ischemia produced by electrocoagulation of the distal middle cerebral artery in mice. L-kynurenine sulfate induced a small decrease in the surface area of the brain infarction (10%, P<0.05) at 30 mg/kg i.p., and it caused strong reductions in infarct size (24-25%, P<0.01) at 100 and 300 mg/kg i.p. Treatment of gerbils with L-kynurenine sulfate at 300 mg/kg i.p. 2 h before a 3-min bilateral carotid occlusion decreased (40%, P<0.01) the pyramidal cell loss in the CA1 area of the hippocampus. Furthermore, L-kynurenine sulfate inhibited the ischemia-induced hypermotility (77%, P<0.001), and decreased (50%, P<0.01) the ischemia-induced deterioration of spontaneous alternation, a measure of spatial memory, without altering the rectal temperature. In conclusion, the administration of L-kynurenine can elevate the brain concentration of kynurenic acid to neuroprotective levels, suggesting the potential clinical usefulness of L-kynurenine for the prevention of neuronal loss.

Behavioural pharmacology of the α5-GABAA receptor antagonist S44819: Enhancement and remediation of cognitive performance in preclinical models.

Previous work has shown that S44819 is a novel GABAA receptor (GABAAR) antagonist, which is selective for extrasynaptic GABAARs incorporating the α5 subunit (α5-GABAARs). The present study reports on the preclinical neuropsychopharmacological profile of S44819. Significantly, no sedative or pro-convulsive side effects of S44819 were found at doses up to 30 mg/kg i.p. Object recognition (OR) memory in intact mice was enhanced by S44819 (0.3 mg/kg p.o.) given before the acquisition trial. Mice treated with phencyclidine for two weeks and tested six days after the cessation of treatment failed to show OR memory. This deficit was corrected by a single administration of S44819 (0.1, 0.3 or 1 mg/kg p.o.) prior to the acquisition trial. The amnestic effect of ketamine in rats tested in the eight-arm radial maze (reference and working memory versions) was blocked by S44819 (3 mg/kg p.o.). Extinction of cued fear was preserved during treatment with S44819 (3 mg/kg/diem i.p.). Administration of S44819 had no significant effect in the Vogel-conflict test, the elevated plus maze, the forced swim, the marble-burying and the tail-suspension tests. In contrast, anxiolytic/antidepressant-like effects of the compound were found in paradigms that have mnemonic components, such as social interaction, fear-potentiated startle and social avoidance induced by negative life experience. In summary, S44819 enhanced intact recognition memory and ameliorated memory deficits induced by inhibition of NMDA receptors. Anxiolytic/antidepressant efficacy was limited to paradigms involving cognitive function. In conclusion, S44819 is a novel psychoactive pro-cognitive compound with potential as a therapeutic agent in dementia.

Therapeutic time window of neuroprotection by non-competitive AMPA antagonists in transient and permanent focal cerebral ischemia in rats.

EGIS-8332 and GYKI 53405 are selective, non-competitive AMPA (2-amino-3[3-hydroxy-5-methyl-4-isoxazolyl] propionic acid) antagonists that effectively protected against tissue injury caused by global and focal cerebral ischemia in laboratory animals. This study evaluated the therapeutic time window of neuroprotection by EGIS-8332 and GYKI 53405 in permanent and transient middle cerebral artery occlusion (MCAO) in Sprague-Dawley rats. Infarct size was measured by TTC staining 48 h after permanent MCAO (electrocoagulation), and 24 h after reperfusion following a 1-h transient MCAO carried out using the intraluminal filament technique. Treatment with EGIS-8332 (10 mg/kg, i.p.) 60 or 120 min after permanent MCAO, decreased infarct size by 30% and 36%, respectively, and the effect of GYKI 53405 (10 mg/kg, i.p.) was similar (30% and 33%, respectively; p<0.01 all). Neither compound was effective if administered 180 or 240 min after permanent MCAO. Both EGIS-8332 and GYKI 53405 (20 mg/kg, i.p.) reduced the core and total (core plus penumbra) volumes of tissue injury in the whole brain and the cerebral cortex when administered 120 or 180 min after transient MCAO. The compounds did not alter tissue damage in the striatum. No neuroprotective effect was obtained at 240 min after transient MCAO. In conclusion, the therapeutic time window of neuroprotection by EGIS-8332 and GYKI 53405 was 2 h in permanent and 3 h in transient focal cerebral ischemia in rats. The results suggest that treatment with non-competitive AMPA antagonists can only be expected to produce a neuroprotective action in humans if administered shortly after the appearance of stroke symptoms.

Use of TTC staining for the evaluation of tissue injury in the early phases of reperfusion after focal cerebral ischemia in rats.

BACKGROUND AND PURPOSE: 2,3,5-Triphenyltetrazolium chloride (TTC) staining measures tissue viability used to evaluate infarct size. The goal of this study was to compare viability of neuronal tissue during the early phases of ischemia-reperfusion assessed either by perfusion of the brain with TTC solution transcardially, in vivo, or by staining brain slices, in vitro. METHODS: The middle cerebral artery was occluded for 1 h in male SPRD rats and then reperfused for 0, 1, 4, 8, 16 and 24 h. Ischemic damage was evaluated by TTC staining, in vivo and in vitro, and by histology (Luxol Fast Blue and Fluoro-Jade staining, electron microscopy). RESULTS: Core volume of tissue injury measured in vivo was large at 0 h and steadily decreased by 50% (p<0.001) up to 16 h, but substantially increased from 16 to 24 h of reperfusion. In contrast, a significant core volume appeared at 4 h only, in vitro, and gradually increased up to 24 h. Core volume was larger in vivo than in vitro at all times except at 16 h when the opposite was observed. Evans blue administered intracardially stained TTC-negative areas at 1 and 24 h. Histology covered the evolution of serious tissue injury but also demonstrated some morphologically preserved neurons in the infracted area at 24 h. CONCLUSIONS: Formation of formazan from TTC can depend on both the staining method and the metabolic burden of the brain tissue causing uncertainties in the volume of ischemia-induced brain injury measured by TTC staining.

The effects of AMPA receptor antagonists in models of stroke and neurodegeneration.

Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonists have been shown to have neuroprotective effects in stroke models and although clinical trials with some agents are still ongoing, published results have not been favourable. We therefore wished to compare the effects of GYKI 52466, GYKI 53405, EGIS-8332 and EGIS-10608, non-competitive AMPA receptor antagonists with homophthalazine chemical structures, in standard animal stroke models with effects in a neurodegenerative model--excitoxicity in newborn mice. All compounds inhibited the S-AMPA-induced spreading depression in the chicken retina, in vitro, and were potent anticonvulsants against maximal electroshock in mice, in vivo. The AMPA receptor antagonists prevented domoate-induced cell death of motoneurons, in vitro, and reduced infarct size in a dose-dependent manner in the permanent middle cerebral artery occlusion model in mice, in vivo. In newborn mice (P5, histopathology at P10), local injection of the AMPA receptor agonist S-bromo-willardiine at day 5 after birth induced cortical damage and white matter damage, which was reduced in a dose-dependent manner by the AMPA receptor antagonists. EGIS 10608 was a very powerful receptor antagonist of white matter damage. In contrast, GYKI 52466 did not antagonize cortical and white matter damage induced by ibotenic acid. These models allow quantification of the effects of AMPA receptor antagonists in vitro and in vivo.

A novel GABA(A) alpha 5 receptor inhibitor with therapeutic potential.

Novel 2,3-benzodiazepine and related isoquinoline derivatives, substituted at position 1 with a 2-benzothiophenyl moiety, were synthesized to produce compounds that potently inhibited the action of GABA on heterologously expressed GABAA receptors containing the alpha 5 subunit (GABAA α5), with no apparent affinity for the benzodiazepine site. Substitutions of the benzothiophene moiety at position 4 led to compounds with drug-like properties that were putative inhibitors of extra-synaptic GABAA α5 receptors and had substantial blood-brain barrier permeability. Initial characterization in vivo showed that 8-methyl-5-[4-(trifluoromethyl)-1-benzothiophen-2-yl]-1,9-dihydro-2H-[1,3]oxazolo[4,5-h][2,3]benzodiazepin-2-one was devoid of sedative, pro-convulsive or motor side-effects, and enhanced the performance of rats in the object recognition test. In summary, we have discovered a first-in-class GABA-site inhibitor of extra-synaptic GABAA α5 receptors that has promising drug-like properties and warrants further development.

Reduction of cerebral infarct size by non-competitive AMPA antagonists in rats subjected to permanent and transient focal ischemia.

Antagonists of 2-amino-3(3-hydroxy-5-methyl-4-isoxazolyl) propionic acid (AMPA) receptors can considerably reduce brain damage after cerebral ischemia, but effectiveness of selective AMPA antagonists has been questioned recently. Therefore, we evaluated the antiischemic efficacy of [+/-]-7-acetyl-5-[4-aminophenyl]-7,8-dihydro-8-cyano-8-methyl-9H-1,3-dioxolo-[4,5-h]-2,3-benzodiazepine (EGIS-8332) and GYKI 53405, two selective, non-competitive AMPA antagonists in two rat models of focal cerebral ischemia. Permanent focal ischemia was produced by electrocoagulation of the middle cerebral artery (MCA). EGIS-8332 and GYKI 53405 were administered 30 min after MCA occlusion at doses of 1, 3 or 10 mg/kg i.p. In transient focal ischemia, MCA was occluded for 1 h and reperfused for 24 h using the intraluminal filament technique and the compounds were given at 3x10 mg/kg i.p. 60, 120 and 180 min following occlusion. In permanent focal ischemia, EGIS-8332 decreased the volume of cerebral infarction both at 10 mg/kg i.p. (36.4%, p<0.01) and at 3 mg/kg i.p. (26.4%, p<0.05) in a dose-dependent manner. GYKI 53405 produced a similar antiischemic effect at 10 mg/kg i.p. (36.4%, p<0.01), but it was ineffective at 3 mg/kg i.p. (6.5%, p=0.57). In transient focal ischemia, EGIS-8332 reduced the volume of necrotic brain tissue (38.7%, p<0.01) and GYKI 53405 was similarly effective (32.6%, p<0.05). Both compounds afforded neuroprotection in the cortical and subcortical regions of the MCA territory. Selective, non-competitive AMPA antagonists administered after the ischemic insult can produce effective neuroprotective action in experimental models of focal cerebral ischemia; therefore, these compounds may be useful as therapeutic agents for the treatment of stroke and neurodegenerative disorders.

Egis-11150: a candidate antipsychotic compound with procognitive efficacy in rodents.

Classical antipsychotics, e.g. haloperidol, chlorpromazine, are potent at controlling the positive symptoms of schizophrenia but frequently elicit extrapyramidal motor side-effects. The introduction of atypical antipsychotics such as risperidone, olanzapine and clozapine has obviated this problem, but none of the current drugs seem to improve the cognitive deficits accompanying schizophrenia. Thus there is an unmet need for agents that not only suppress the psychotic symptoms but also ameliorate the impairment of cognition. Here, we report the preclinical properties of a candidate antipsychotic, Egis-11150, that shows marked pro-cognitive efficacy. Egis-11150 displayed high affinity for adrenergic α(1), α(2c), 5-HT(2A) 5-HT7, moderate affinity for adrenergic α(2a) and D2 receptors. It was a functional antagonist on all of the above receptors, with the exception of 5-HT7 receptors, where it was an inverse agonist. Phencyclidine-induced hypermotility in mice and inhibition of conditioned avoidance response in rats were assessed to estimate efficacy against the positive and social withdrawal test in rats was used to predict efficacy against the negative symptoms of schizophrenia. Passive-avoidance learning, novel object recognition and radial maze tests in rats were used to assess pro-cognitive activity, while phencyclidine-induced disruption of prepulse inhibition in mice was examined to test for effects on attention. Egis-11150 (0.01-0.3 mg/kg, ip.) was effective in all of the preclinical models of schizophrenia examined. Moreover, a robust pro-cognitive profile was apparent. In summary, work in preclinical models indicates that Egis-11150 is a potential treatment for controlling the psychosis as well as the cognitive dysfunction in schizophrenia. This article is part of a Special Issue entitled 'Cognitive Enhancers'.

Comparison of the AMPA antagonist action of new 2,3-benzodiazepines in vitro and their neuroprotective effects in vivo.

PURPOSE: AMPA receptor-mediated excitotoxicity is thought to be a critical process in diseases accompanied by neuronal cell loss following a hypoxic/anoxic state of the central nervous system. It has been suggested that blockade of AMPA receptors might result in significant protection of neurons against cellular damage. For testing the hypothesis, in vitro efficacy and in vivo neuroprotective action of new 2,3-benzodiazepine (2,3BDZ) AMPA antagonists have been compared. METHODS: 2.3BDZs were tested on kainate-evoked whole-cell currents in cultured neurons as well as on population spikes (PS) in rat hippocampal slices. Data were correlated with those obtained from the spreading depression (SD) experiments in chicken retina. Compounds were also examined in the gerbil bilateral carotid occlusion model (BCO), where percentage decrease of ischemia-related hypermotility (HM), impaired spatial memory (SA), and hypoxia-induced hippocampal CA1 neuronal cell death (CA1) were evaluated. RESULTS: Certain structural modifications of classical 2,3BDZs resulted in increased in vitro activity and improved in vivo efficacy. In particular, the halogen-substituted compounds EGIS-9879 and EGIS-9883 showed the highest neuroprotective efficacy (84% and 47% protection in CA1, 71% and 82% decrease in HM, respectively; 4 x 5 mg/kg i.p.) in BCO. PS and SD were correlated to the decrease of neuronal loss in the CA1 area. Lack of significant correlation was found between PS and CA1 (r = 0.437, p = 0.079) or SD and CA1 (r = 0.380, p = 0.146). CONCLUSIONS: Several new 2.3BDZ AMPA receptor antagonists have been synthesized at EGIS Pharmaceuticals characterized by remarkable in vitro and corresponding in vivo neuroprotective properties.