Effects of positive modulators of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptors in a benzodiazepine-induced deficit of spatial discrimination in mice.

Imbalance between GABAergic and glutamatergic neurotransmission has been recently hypothesized to trigger memory decline related either to ageing or to Alzheimer's disease (AD). Thereby, benzodiazepine-induced anterograde amnesia has been construed as a model of hippocampal-related cognitive dysfunctions. Since spatial memory is altered both by ageing and by benzodiazepines such as alprazolam, we investigated the pharmacological sensitivity of alprazolam-induced deficit in a delayed spatial discrimination (SD) task, notably with positive allosteric modulators of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptors. We showed that alprazolam (0.1 mg/kg intraperitoneally) induced memory impairments as compared with vehicle-treated mice. The oral administration of modulators of AMPA receptors (IDRA-21: 10 mg/kg; S18986: 3 and 10 mg/kg) reversed the alprazolam-induced deficits. This study is first to show evidence that reference treatments of AD, such as memantine (a NMDA receptor antagonist) at 3 mg/kg per os (po) and donepezil (an acetylcholinesterase inhibitor) at 1 mg/kg po, also reversed the alprazolam-induced amnesia. Given such results, the SD task emerges as a valuable novel task to screen pro-cognitive compounds. Thus, we highlight the efficacy of modulators of AMPA-type glutamate receptors to counteract alprazolam-induced spatial deficits. These results could be viewed alongside the imbalance between excitation and inhibition observed during normal and pathological ageing.

Ring-fused thiadiazines as core structures for the development of potent AMPA receptor potentiators.

Amongst ionotropic glutamatergic receptors, the AMPA receptor subtype has been recognized as a major contributor to the fast excitatory neurotransmission in the central nervous system and the expression and maintenance of longterm potentiation. This receptor subtype also represents an interesting target to develop innovative therapeutic drugs such as positive allosteric modulators (AMPA receptor potentiators) since the enhancement of AMPA signals is expected to be beneficial in the management of several neurological disorders such as depression, schizophrenia, Parkinson's disease and learning-memory deficits linked to Alzheimer's disease. This article is dedicated to the use of (hetero) aromatic ring-fused thiadiazines (i.e. benzo- pyrido- and thienothiadiazines) as core structures for the discovery of new positive allosteric modulators of AMPA receptors. Recent advances exploring other chemotypes in the field of AMPA potentiators is the object of a separate review of the present issue.

Pharmacological modulation of contextual "episodic-like" memory in aged mice.

Episodic memory deficits in elderly are due to the progressive weaknesses to use the contextual and temporal cues of the to be remembered information. Since the inability to remember the specific context of events is an important feature of episodic memory deficits in the elderly, we first focused on describing two mice models of contextual "episodic-like" memory. In a second part, we described the effects of aging on memory in the contextual and serial discrimination (CSD) task. We showed more specifically that the CSD task allowed detection of early memory impairments in middle-aged (14-15 months) animals as compared to young (4-5 months) or aged (18-19 months) ones. Interestingly, the very same memory impairments were observed following dorsal hippocampal lesions in young adult mice, which suggest that the CSD task allowed detection of early signs of age-related hippocampal dysfunction. In a third part, we showed that pharmacological reference compounds such as donepezil and memantine (mainly used in the treatment of mild to severe forms of Alzheimer's diseases) reversed the age-induced memory impairments as well as emerging pharmacological compounds acting on different neurotransmitter targets (nicotinic and AMPA receptors). Thus, the CSD task appears to be a reliable behavioural tool for detecting the early emergence of age-related memory dysfunction and for identifying new pharmacological targets and therapeutic strategies in the treatment of age-related amnesia.

Rapid and precise method to locate microdialysis probe implantation in the rodent brain.

An important concern about microdialysis methodology is the histological validation of the dialysis probe implantation site in brain tissue of rodents (rat, mouse). Several methods have been described on standard histological staining (i.e., cresyl violet, formalin fixation, fast green perfusion, etc.). However, this methodology is time consuming. These requirements are not compatible with a histological validation prior to analysis of microdialysis samples. Here, we developed a new method to locate the track of the dialysis probe in the rodent brain. This method is based on a digital photomicrograph of a coronal section of the rodent frozen brain. The fitting of an appropriate coronal diagram of the rats' and mice' brain atlas with this photomicrograph, allowed us to locate precisely and quickly the track of the dialysis probe.

Verbal fluency output in children aged 7-16 as a function of the production criterion: qualitative analysis of clustering, switching processes, and semantic network exploitation.

Developmental changes in children's verbal fluency were explored in this study. One hundred and forty children aged from 7 to 16 completed four verbal fluency tasks, each with a different the production criterion (letter, sound, semantic, and free). The age differences were analyzed both in terms of number of words produced, and clustering, switching, and semantic network exploration. Analysis of the number of words produced showed a larger difference between the 7-8- and the 9-10-year-olds in semantic than in letter fluency, but this difference gradually disappeared with increasing age for semantic fluency while remaining constant for letter fluency. In letter fluency production, age modified both the number of switches and clusters formed whereas in semantic fluency tasks, only cluster size changed with age. Concerning the semantic network exploration indicators derived from the supermarket fluency task, the number of categories sampled increased from 11 to 12 years, but efficient semantic exploitation occurred only after the age of 13-14 years. These results are discussed in terms of the development of strategic retrieval components and categorical knowledge.

[In vivo exploration of cerebral ischemia: use of neuroprotective agents in animal studies]. / Exploration de l'ischémie cérébrale in vivo: application à l'étude des neuroprotecteurs chez l'animal.

A "good" in vivo animal model of stroke must reproduce the etiology, anatomical, functional and metabolic consequences of human pathology and must also permit the study of anti-ischemic drugs in conditions pertinent to the clinical therapeutics. As stroke is a very heterogeneous clinical entity, such a model could only mimic a limited part of stroke. Animal data are usually collected in healthy laboratory rodents of the same age, in which a standardized amount of cerebral ischemia is induced by a reproducible intervention. In contrast, aetiology, location and severity of ischaemic stroke in patients is very heterogeneous. Among the various animal models of stroke, two of them are particularly used: a model of global transient ischemia by occlusion of the 4-vessels in the rat (Pulsinelli's model), which induces a delayed neuronal death in the hippocampus and model(s) of permanent or transient focal cerebral ischemia occluding the middle cerebral artery in rodents. A large number of compounds have been shown to be active using these two animal models, but unfortunately, none of them were found to be active in clinical trials. Various factors could be responsible for this major discrepancy and some of them are not related to pre-clinical studies, but to the complexities of the clinical problem of stroke. Failure in the translation of results from animals models to humans implicates potential limitations of the current drug development process. Retrospective analysis of studies suggests possible improvements at several stages during pre-clinical studies. Standardized guidelines for preclinical evaluation of neuroprotective drugs may improve chances of success. For example, preclinical studies should be performed in at least 2 species and 2 strains for a specific specie in order to take into account known strain and species differences. Moreover, while neuroprotection drug development is dominated by volumetric histology as the outcome measure, the demonstration of functional benefits must be performed both after short and long periods of recovery. Attempts should be made to use multiple models such as stroke-prone spontaneously hypertensive rats, outbred rodents and aged animals that more closely simulate clinical conditions. In addition, treatment in animals should not be given immediately after ischaemia, but after a delay, as most patients are not treated within minutes of stroke onset. Animal models should be used to determine dosage and duration of therapy, which will vary with the pharmacokinetic properties of different agents. Moreover, complete dose-response curves should be established as bell-shaped dose-responses curves may predict dose-limiting adverse effects that hinder subsequent efficacy trials. Finally, physiological monitoring (cerebral blood flow, blood pressure and gazes, body temperature, glycemia, ...) should be performed to eliminate confounding variables and to observe adverse systemic effects. The future of neuroprotection for stroke remains bright in spite of previous disappointments.

Differential inhibition of [3H]-oxotremorine-M and [3H]-quinuclinidyl benzilate binding to muscarinic receptors in rat brain membranes with acetylcholinesterase inhibitors.

The potential interaction of acetylcholinesterase inhibitors with cholinergic receptors may play a significant role in the therapeutic and/or side-effects associated with this class of compound. In the present study, the capacity of acetylcholinesterase inhibitors to interact with muscarinic receptors was assessed by their ability to displace both [3H]-oxotremorine-M and [3H]-quinuclinidyl benzilate binding in rat brain membranes. The [3H]-quinuclinidyl benzilate/[3H]-oxotremorine-M affinity ratios permitted predictions to be made of either the antagonist or agonist properties of the different compounds. A series of compounds, representative of the principal classes of acetylcholinesterase inhibitors, displaced [3H]-oxotremorine-M binding with high-to-moderate potency (ambenonium>neostigmine=pyridostigmine=tacrine>physostigmine> edrophonium=galanthamine>desoxypeganine) whereas only ambenonium and tacrine displaced [3H]-quinuclinidyl benzilate binding. Inhibitors such as desoxypeganine, parathion and gramine demonstrated negligible inhibition of the binding of both radioligands. Scatchard plots constructed from the inhibition of [3H]-oxotremorine-M binding in the absence and presence of different inhibitors showed an unaltered Bmax and a reduced affinity constant, indicative of potential competitive or allosteric mechanisms. The capacity of acetylcholinesterase inhibitors, with the exception of tacrine and ambenonium, to displace bound [3H]-oxotremorine-M in preference to [3H]quinuclinidyl benzilate predicts that the former compounds could act as potential agonists at muscarinic receptors. Moreover, the rank order for potency in inhibiting acetylcholinesterase (ambenonium>neostigmine=physostigmine =tacrine>pyridostigmine=edrophonium=galanthamine >desoxypeganine>parathion>gramine) indicated that the most effective inhibitors of acetylcholinesterase also displaced [3H]-oxotremorine-M to the greatest extent. The capacity of these inhibitors to displace [3H]-oxotremorine-M binding preclude their utilisation for the prevention of acetylcholine catabolism in rat brain membranes, the latter being required to estimate the binding of acetylcholine to [3H]-oxotremorine-M-labelled muscarinic receptors. However, fasciculin-2, a potent peptide inhibitor of acetylcholinesterase (IC50 24 nM), did prevent catabolism of acetylcholine in rat brain membranes with an atypical inhibition isotherm of [3H]-oxotremorine-M binding, thus permitting an estimation of the "global affinity" of acetylcholine (Ki 85 nM) for [3H]-oxotremorine-M-labelled muscarinic receptors in rat brain.

Protective effect of the antioxidant 6-ethoxy-2,2-pentamethylen-1,2-dihydroquinoline (S 33113) in models of cerebral neurodegeneration.

In a previous study Dorey et al. [Bio. Org. Chem. Lett., 10 (2000) 935] a series of novel dihydroquinoline compounds were developed, based on the potent antioxidant 6-ethoxy-1,2-dihydro-2,2,4-trimethylquinoline (ethoxyquin), and permitted the selection of the analogue 6-ethoxy-2,2-pentamethylen-1,2-dihydroquinoline (S 33113) lacking the hypothermic effects associated with ethoxyquin at equivalent doses. Herein, an extensive investigation of the neuroprotective capacity of S 33113 in different in vitro and in vivo paradigms of oxidative stress-mediated cellular degeneration was undertaken. In vitro S 33113 was a potent inhibitor (IC(50) = 0.29 microM) of Fenton-reaction-induced lipid peroxidation in mouse cortical membranes. Administration of S 33113 either intraperitoneally (< or =150 mg/kg i.p.) or orally (< or =600 mg/kg p.o.) did not significantly modify body temperature in NMRI mice. Furthermore, S 33113 (150 mg/kg i.p. or 600 mg/kg p.o.) markedly reduced the lethality induced by an intracerebroventricular injection of t-butylhydroperoxide in NMRI (naval medical research institute) mice for up to 5 h. Oral administration of S 33113, significantly attenuated alloxan-mediated hyperglycaemia in NMRI mice at 400 and 600 mg/kg (60%; P < 0.001). Administration of S 33113 (150 mg/kg i.p.) 30 min before transient global ischaemia significantly prevented delayed neuronal cell death in the CA1 region of the rat hippocampal formation, 7 days post-ischaemia (33% cell loss vs. 88% in ischaemia controls; P < 0.001). Similarly, a single pre-administration of S 33113 (150 mg/kg i.p.) prevented kainic acid-induced cell death in the CA3 hippocampal region at 7 days post-exposure (17% cell loss vs. 52% in kainate-treated controls; P < 0.01). Furthermore, D-methamphetamine-mediated dopamine depletion in the striatum of C57BL/6 mice (39-46%) was significantly prevented with S 33113 administered at either (2 x 150mg/kg i.p.) (11%; P < 0.01) or (2x150 mg/kg p.o.) (17%; P < 0.001). In conclusion, S 33113 represents a novel dihydroquinoline compound with potential for the treatment of cerebral pathologies implicating chronic neurodegeneration.

[Pharmacologic treatment for cognitive disorders: an update]. / Traitements pharmacologiques des troubles cognitifs: pistes actuelles.

Today, the memory is regarded as a network of memories which is sustained by distinct interconnected neuronal pathways. These diverse neuronal pathways are differentially impaired during cerebral ageing. Consequently, according to their molecular and cellular targets, cognitive enhancing therapies will be specifically applied to some neurodegenerative diseases or to age-related cognitive disorders. During recent decades, cognition enhancers have been devised in order to facilitate or to serve as substitutes for the effects of deficient neurotransmitters, particularly acetylcholine. These pharmacological approaches have allowed the proposition of acetylcholinesterase inhibitors and muscarinic agents. At present, these targets remain in progress but research is orientated on the discovery of nicotinic agents and acetylcholine releasers. Simultaneously, intense research activity has been undertaken around the other major systems of neurotransmission and/or neuromodulation such as the glutamatergic, monoaminergic and peptidergic pathways. Although research has always given precedence to symptomatic treatments (cognition enhancers) over organic treatments (neuroprotective agents), those are the subject of intensive research activity as they could be capable of counteracting the neuronal death and decrease of synaptic plasticity that occur during neurodegenerative diseases and ageing, respectively.

Chemical and pharmacological aspects of heteroaryl-nitrones.

Radical induced oxidative damage is extremely harmful to tissues and organs due to molecular modifications brought to polyunsaturated membrane lipids, proteins and nucleic acids. Oxidative stress is believed to be one of the pathophysiological mechanisms that operate in neurodegenerative disorders such as cerebral ischemias, amyotrophic lateral sclerosis, Parkinson s and Alzheimer s diseases. Nitrones oppose oxidative challenges by virtue of their ability to trap very rapidly oxygen or carbon centered radicals thus generating nitroxide radical species which are more stable and biochemically less harmful than the original radical. However the operational mechanism of nitrones might also go beyond direct scavenging of radicals. The chemical and pharmacological properties of nitrones depend strongly on the connectivity as well as on the type and position of the substituents in the compound's architecture. Heteroaryl-nitrones are known, but except for a few cases (for example pyridyl-nitrones) no particular attention has been given to this class of molecules. The following review is a survey of the literature reports on this subject from 1980 to 1999. The structures were classified according to the heterocyclic substituent on the nitrone double bond, and documented pharmaceutical features were emphasized. Whenever possible heteroaromatic and related aromatic nitrones were compared.

(S)-2,3-dihydro-[3,4]cyclopentano-1,2,4-benzothiadiazine-1,1-dioxide: (S18986-1) a positive modulator of AMPA receptors enhances (S)-AMPA-mediated [3H]noradrenaline release from rat hippocampal and frontal cortex slices.

The present study describes the effect of (S)-2,3-dihydro-[3, 4]cyclopentano-1,2,4-benzothiadiazine-1,1-dioxide (S18986-1), a positive allosteric modulator of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors with cognitive-enhancing effects, on (S)-AMPA-induced [3H]noradrenaline release in rat hippocampal and frontal cortex slices. (S)-AMPA significantly increased [3H]noradrenaline release in rat hippocampus and frontal cortex slices, whereas S18986-1 (3-1000 microM) alone, was inactive. However, S18986-1 between 30 and 1000 microM potently enhanced (+200%) (S)-AMPA-mediated [3H]noradrenaline release in both hippocampal and frontal cortex slices. The capacity of S18986-1 to potentiate [3H]noradrenaline release was specific for AMPA receptors as S18986-1 failed to potentiate either kainate and N-methyl-D-aspartate (NMDA)-mediated release of [3H]noradrenaline in rat hippocampal slices. Moreover, 1, 2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide (NBQX) and 1-(4-aminophenyl)-3-methylcarbamoyl-4-methyl-3, 4-dihydro-7,8-methylenedioxy-5H-2,3-benzodiazepine (GYKI-53655) but not (5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5, 10-imine ((+)-MK-801), inhibited (S)-AMPA and S18986-induced stimulation of (S)-AMPA-mediated [3H]noradrenaline release. In addition, S18986-1-induced stimulation of (S)-AMPA-evoked [3H]noradrenaline release was markedly attenuated in the presence of tetrodotoxin (1 microM) and in Ca(2+)-free buffer. S18986-1 enhanced (S)-AMPA-mediated [3H]noradrenaline release to a greater extent than its corresponding (R)-enantiomer S19024-1 and racemic mixture S17951-1. However, positive allosteric modulators of AMPA receptors such as aniracetam failed to potentiate AMPA-mediated noradrenaline release in hippocampal slices, whereas cyclothiazide potently enhanced (S)-AMPA-mediated [3H]noradrenaline release. These results suggest that the capacity of S18986-1 to enhance AMPA receptor-mediated release of noradrenaline in rat hippocampus and frontal cortex, could contribute to the cognition enhancing mechanisms of S18986-1.

Effects of S 18986-1, a novel cognitive enhancer, on memory performances in an object recognition task in rats.

(S)-2,3-dihydro-[3,4]cyclopentano-1,2,4-benzothiadiazine-1,1-dioxi de (S 18986-1) is a new compound that facilitates post-synaptic responses by modulating alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor-mediated synaptic responses and thus promotes long-term potentiation and potentiates (S)-AMPA-induced release of noradrenaline in rat brain slices. In the present study, the effects of S 18986-1 were evaluated on cognitive functions by using a one-trial object-recognition test in the Wistar rat, a test which measures a form of episodic memory in rodents. Recognition was measured by the ability of treated rats to discriminate between a familiar and a new object after a 24-h retention delay. Oral administrations with S 18986-1 (0.3 to 100 mg/kg) 1 h before each session of the test improved object recognition at concentrations as low as 0.3 mg/kg. Under the same conditions, the nootropic drug aniracetam was active at a dose of 10 mg/kg by i.p. route. S 18986-1 was still effective on the object-recognition test when it was administered 4 h before each of the three sessions. Furthermore, subchronic oral pretreatment (7 days) with S 18986-1 (0.3 to 30 mg/kg) also increased the recognition of the familiar object indicating that the animals failed to develop tolerance to repeated administrations with S 18986-1. Finally, the recognition of the familiar object was improved when S 18986-1 was administered before the recognition trial whereas the rats failed to recognise the familiar object when S 18986-1 was administered before the sample presentation trial only. Taken together, the results indicated that S 18986-1 facilitated a form of episodic memory in the rat, by improving the recognition of a familiar information (retention). Furthermore, S 18986-1 was long-acting and demonstrated a good oral bioavailability. These data confer on S 18986-1, a potential role in improving episodic memory impaired in neurodegenerative diseases and during aging.

Synthesis, structure, and neuroprotective properties of novel imidazolyl nitrones.

A new series of imidazolyl nitrones spin traps has been synthesized and evaluated pharmacologically. The salient structural feature of these molecules is the presence of an imidazole moiety substituted by aromatic or heteroaromatic cycles. This connectivity imparts to the nitrone superior neuroprotective properties in vivo and in parallel reduced side effects and toxicity. Thus compound 6a (a 2-phenylimidazolyl nitrone) administered intraperitoneally protects (80%) mice from lethality induced by an intracerebroventricular administration of tert-butyl hydroperoxide (t-BHP) an oxidant capable of inducing neurodegenerative processes. Administration of the archetypal nitrone phenyl-tert-butyl nitrone (PBN) at an equimolar dose also affords some protection (60%) in this test. However, this activity is accompanied by hypothermia, whereas no such effect is apparent for 6a. Moreover, previously prepared nonsubstituted or alkyl-substituted imidazolyl nitrones were shown to be extremely toxic to rats in contrast to the compounds prepared in this study. The observed activities in vivo correlate well with the calculated partition coefficients (ClogP) and HOMO energy level.

New quinolinic derivatives as centrally active antioxidants.

A series of new 1,2-dihydro and 1,2,3,4-tetrahydroquinolines, synthesized from the corresponding propargylaniline intermediates, have been developed as antioxidants for the potential treatment of pathologies implicating central oxidative stress.

Inhibition of L-homocysteic acid and buthionine sulphoximine-mediated neurotoxicity in rat embryonic neuronal cultures with alpha-lipoic acid enantiomers.

In the present report, we have set out to investigate the potential capacity of both the oxidised and reduced forms of RS-alpha-lipoic acid, and its separate R-(+) and S-(-)enantiomers, to prevent cell death induced with L-homocysteic acid (L-HCA) and buthionine sulphoximine (BSO) in rat primary cortical and hippocampal neurons. L-HCA induced a concentration-dependent neurotoxic effect, estimated by cellular 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide (MTT) reduction, in primary neurons, but was significantly more toxic for hippocampal (EC(50)=197 microM) compared with cortical neurons (EC(50)=1016 microM) whereas D-HCA demonstrated only moderate (<20%) toxicity. On the other hand, cortical and hippocampal cultures were equally susceptible (341 and 326 microM, respectively) to the neurotoxic action of BSO. Antioxidants including butylated hydroxyanisole, propyl gallate and vitamin E protected cells against the neurotoxic effect of L-HCA and BSO. However, N-acetyl-cysteine and tert-butylphenyl nitrone, although capable of abrogating L-HCA-mediated cell death showed no protective effect against BSO-mediated toxicity. RS-alpha-lipoic acid, RS-alpha-dihydrolipoic acid and the enantiomers R-alpha-lipoic acid and S-alpha-lipoic acid protected cells against L-HCA-mediated toxicity with EC(50) values between 3.1-8.3 microM in primary hippocampal neurons and 2.6-16.8 microM for cortical neurons. However, RS-alpha-lipoic acid, RS-alpha-dihydrolipoic acid, and S-alpha-lipoic acid failed to protect cells against the degeneration induced by prolonged exposure to BSO, whereas the natural form, R-alpha-lipoic, was partially active under the same conditions. The present results indicate a unique sensitivity of hippocampal neurons to the effect of L-HCA-mediated toxicity, and suggest that RS-alpha-lipoic acid, and in particular the R-alpha-enantiomeric form is capable of preventing oxidative stress-mediated neuronal cell death in primary cell culture.

Involvement of nicotinergic mechanisms in thyrotropin-releasing hormone-induced neurologic recovery after concussive head injury in the mouse.

A behavioral study was performed in an attempt to understand the neuronal mechanisms involved in the thyrotropin-releasing hormone (TRH)-induced improvement of consciousness after concussive head injury in the mouse. Intravenous administration of TRH dose dependently shortened the duration of unconsciousness after concussion in the mouse (ED50 = 3.2 mg/kg). The improvement of recovery evoked by TRH (3 mg/kg i.v.) after concussion was not affected by i.p. pretreatment with N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine, alpha-methyl-para-tyrosine, p-chlorophenylalanine, scopolamine or methylscopolamine. However, mecamylamine or hexamethonium i.p. pretreatment completely inhibited the TRH-induced improvement of outcome in traumatic brain injury. The results imply that TRH-induced improvement of recovery after concussion is not associated with increased activity of monoaminergic neurons in the brain. These results suggest that the inhibitory effect of TRH upon unconsciousness after concussion in mice is mainly produced by activation of central cholinergic systems via nicotinic receptors whereas muscarinic receptors seem to be not implicated.

4H-1,2,4-Pyridothiadiazine 1,1-dioxides and 2,3-dihydro-4H-1,2, 4-pyridothiadiazine 1,1-dioxides chemically related to diazoxide and cyclothiazide as powerful positive allosteric modulators of (R/S)-2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propionic acid receptors: design, synthesis, pharmacology, and structure-activity relationships.

A series of 4H-1,2,4-pyridothiadiazine 1,1-dioxides and 2, 3-dihydro-4H-1,2,4-pyridothiadiazine 1,1-dioxides bearing various alkyl and aryl substituents on the 2-, 3-, and 4-positions was synthesized and tested as possible positive allosteric modulators of the (R/S)-2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propionic acid (AMPA) receptors. Many compounds were found to be more potent than the reference compounds diazoxide and aniracetam as potentiators of the AMPA current in rat cortex mRNA-injected Xenopus oocytes. The most active compound, 4-ethyl-2,3-dihydro-4H-pyrido[3,2-e]-1,2, 4-thiadiazine 1,1-dioxide (31b), revealed an in vitro activity on Xenopus oocytes not far from that of cyclothiazide, the most potent allosteric modulator of AMPA receptors reported to date. Moreover, 31b, but not cyclothiazide, was found to potentiate the duration and the amplitude of the excitatory postsynaptic field potentials induced by electric stimulation in rat hippocampal slices. Such an effect could indicate, for 31b, but not for cyclothiazide, a possible interaction with postsynaptic AMPA receptor binding sites located on hippocampal CA1 neurons. Structure-activity relationships indicated that the structural requirements responsible for a biological activity on AMPA receptors are different from those responsible for an inhibitory activity on the insulin releasing process (putative ATP-sensitive K+-channel openers). For instance, 31b and other related dihydropyridothiadiazines were found to be ineffective as inhibitors of insulin release from rat pancreatic B-cells, in contrast to diazoxide and known pyridothiadiazines reported as ATP-sensitive K+-channel openers. Conversely, the pyridothiadiazines active on B-cells were found to be ineffective as potentiators of the AMPA currents in Xenopus oocytes. Thus, 31b appeared to be more specific than diazoxide as an AMPA receptor modulator. This compound may be considered as a new pharmacological tool, different from diazoxide and cyclothiazide, for studying AMPA receptors. Moreover, 31b can also constitute a new therapeutic agent for the treatment of cognitive disorders.

Structure-activity relationships in a series of 2(1H)-quinolones bearing different acidic function in the 3-position: 6,7-dichloro-2(1H)-oxoquinoline-3-phosphonic acid, a new potent and selective AMPA/kainate antagonist with neuroprotective properties.

Recently, we reported the synthesis of 3-(sulfonylamino)-2(1H)-quinolones, a new series of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)/kainate and N-methyl-D-aspartic acid (NMDA)/glycine antagonists. By exploring the structure-activity relationships (SAR) in this series, we were able to identify the 6,7-dinitro derivative 6 as a potent and balanced antagonist at both receptors. Unfortunately, compound 6 was devoid of in vivo activity in mice anticonvulsant testing. To overcome this critical limitation, new compounds bearing various acidic moieties at the 3-position of the quinolone skeleton were synthesized and evaluated. The SAR of these new analogues indicated that not all acidic groups are acceptable at the 3-position: A rank order of potency going from carboxylic approximately phosphonic > tetrazole > mercaptoacetic > hydroxamic >> other heterocyclic acids was defined. In addition, the selectivity between the AMPA/kainate and NMDA/glycine sites is dependent on the nature of the substitution (nitro > chloro for AMPA selectivity), its position (5,7- > 6,7-pattern for glycine selectivity), and the distance between the quinolone moiety and the heteroatom bearing the acidic hydrogen (the longer the distance the more AMPA selective the compound). Among these new AMPA antagonists, we have identified 6,7-dichloro-2(1H)-oxoquinoline-3-phosphonic acid (24c) as a water soluble and selective compound endowed with an appealing pharmacological profile. Compared with the reference AMPA antagonist NBQX, the phosphonic acid 24c is much less potent in vitro but almost equipotent in vivo in the audiogenic seizures model after intraperitoneal administration. Moreover, unlike NBQX, compound 24c is also active after oral administration. In the gerbil global ischemia model, compound 24c shows a neuroprotective effect at 10 mg/kg/ip, equivalent to the reference NBQX.

The effects of melatonin and pinealectomy upon local cerebral glucose utilization in awake unrestrained rats are restricted to a few specific regions.

The effects of the infusion of melatonin (MT) and/or of pinealectomy upon glucose utilization in anatomically discrete regions of the brain of freely moving rats have been studied by the quantitative autoradiographic 2-deoxy-D-[1-14C]glucose technique. The experiments were made from 14.00 to 16.00 h, when MT is normally not secreted by the pineal gland, in order to compare the local cerebral glucose utilization (LCGU) response to MT from animals with long-term (pinealectomized) or short-term (pineal intact) absence of MT secretion. The majority of the 98 brain areas examined showed no change in LCGU after MT administration and/or pinealectomy. Pinealectomy increased the LCGU in the median mammillary nucleus only, whereas following MT administration, an increase in LCGU was observed in 3 cerebral regions of intact rats (paraventricular nucleus of the hypothalamus, nucleus of the solitary tract, choroid plexus of the third ventricle) and in 5 cerebral regions of pinealectomized rats (paraventricular nucleus of the hypothalamus, nucleus of the solitary tract, choroid plexuses of the lateral ventricles, third and fourth ventricles). Except for the choroid plexuses of the fourth ventricle, there was no difference in LCGU response to MT between pinealectomized and intact animals. This does not favor the hypothesis of receptor supersensitivity under the conditions of this experiment. Our results suggest that the hypothalamus, nucleus of the solitary tract and choroid plexuses represent a neural substrate through which MT could influence the activity of the central nervous system when administered at a low dose under near-physiological conditions.

Apomorphine-induced inhibition of substantia nigra dopamine neurons: effects of unilateral injection through the internal carotid artery.

Possible indirect components in the inhibition of firing of A9 dopamine neurons induced by systemic apomorphine were studied using unilateral drug administration through the internal carotid artery, known to irrigate only the ipsilateral mid- and forebrain. When compared to intravenous injection, unilateral intracarotid administration inhibited ipsilateral neurons with a marked decrease of both the latency (less than 1 s) and the dose required for complete inhibition, whereas contralateral neurons were not affected. This suggests a first-pass central effect of apomorphine, presumably associated with brain extraction. Thus, peripheral and hindbrain targets do not seem to contribute to the inhibitory effect of low doses of systemic apomorphine. An intranigral possible mode of action is discussed in view of the particular arrangement of dopaminergic dendrites within the zona reticulata.