Chlormethiazole antagonises seizures induced by N-methyl-DL-aspartate without interacting with the NMDA receptor complex

Rapid and long-lasting tolerance to clomethiazole-induced hypothermia in the rat

Mechanism, onset and duration of tolerance development to clomethiazole-induced hypothermia were investigated in rats using telemetry. The hypothermic effect of clomethiazole was completely abolished for 10 days after an s.c. injection of 300 micromol/kg and the effect returned to approximately 50% in 32 days. The gamma-aminobutyric acidA (GABA(A)) receptor agonist muscimol induced hypothermia at 88 micromol/kg without any (cross-) tolerance. GABA(A) receptor antagonists, bicuculline (5.4 micromol/kg) and picrotoxin (3.3 micromol/kg), did not inhibit clomethiazole-induced hypothermia nor the tolerance. The noncompetitive N-methyl-d-aspartate (NMDA) receptor antagonist, dizocilpine, counteracted clomethiazole-induced hypothermia at 3 micromol/kg but not the tolerance. Tolerance to the 5-hydroxytryptamine(1A) (5-HT(1A)) receptor agonist R-(+)-8-hydroxy-2-(di-n-propylamino)tetralin (R-8-OH-DPAT)-induced hypothermia was blocked by dizocilpine and clomethiazole but not vice versa. No pharmacokinetic interaction was observed. In conclusion, long-lasting tolerance to clomethiazole-induced hypothermia does not involve GABA(A) or 5-HT(1A) receptor functions. Glutamate via NMDA receptors may be involved in the hypothermic response but not in the tolerance.

Differential neuroprotective effects for three GABA-potentiating compounds in a model of hypoxia–ischemia

Clomethiazole (CMZ) is a GABA(A)-potentiating compound; however, it is unclear whether this mode of action is responsible for its neuroprotective effects in animal models of ischemia. This study compared the neuroprotective efficacies of muscimol and midazolam, two potent GABA(A)-potentiating compounds, to that of CMZ in a model of hypoxia-ischemia (H-I). To establish a neuroprotective profile for CMZ, CMZ (60, 95, or 125 mg kg-1, i.p.) was administered to post-natal day 25 male rats at numerous post-hypoxic time points and the rats were sacrificed 1 or 4 weeks later. Varying degrees of histological protection were evident when CMZ was administered 1, 2, or 3 h post-hypoxia with the 125 mg kg-1 dose producing complete histological protection if administered 3 h post-hypoxia. To determine whether midazolam or muscimol could match the protection provided by CMZ administered 3 h post-hypoxia, H-I rats received varying doses of these compounds 3 h post-hypoxia and were sacrificed 1 week later. Under identical conditions, no dose of muscimol or midazolam provided equivalent neuroprotection to that provided by CMZ. In fact, muscimol showed no neuroprotective ability whatsoever. Thus, CMZ, administered as late as 3 h post-hypoxia, was able to completely prevent H-I-induced cell death while a full dose range of other GABA-potentiating agents did not. Such direct comparison of these compounds in this model suggests the mechanism underlying the protective effects of CMZ may not rely solely on GABA(A)-potentiating properties. Elucidation of a novel mechanism of action for CMZ may expose new therapeutic targets in stroke treatment.

Chlormethiazole inhibits epileptiform activity by potentiating GABA(A) receptor function

Chlormethiazole has sedative, hypnotic, anticonvulsant and neuroprotective properties. Using in vitro grease-gap recordings, we show that it inhibits epileptiform activity in neocortical slices superfused with Mg(2+)-free medium (IC(50) approximately 200 microM). At an antiepileptic concentration (300 microM), chlormethiazole potentiated the action of exogenously applied GABA (1 mM) but did not affect responses to the glutamate receptor agonists N-methyl-D-aspartate (10 microM) or L-quisqualic acid (3 microM). The GABA(A) receptor antagonist N-methyl-bicuculline (50 microM) reduced chlormethiazole's potency to inhibit the epileptiform activity. These results indicate that chlormethiazole's anticonvulsant action is likely mediated by potentiating GABA(A)ergic inhibition rather than by antagonising glutamatergic excitation.

GABA potentiation: a logical pharmacological approach for the treatment of acute ischaemic stroke

It has been shown that enhancing the function of the major inhibitory neurotransmitter GABA decreases glutamatergic activity in the brain. Since increased glutamatergic activity is the major primary event that results in cell death following an acute hypoxic-ischaemic stroke, GABAmimetic drugs might therefore be expected to be neuroprotective. This review examines the evidence that GABAergic function is acutely depressed following an ischaemic insult, and also reviews the data that suggest that increasing cerebral GABA concentration has a neuroprotective effect, as does the administration of some (but not all) GABAmimetic agents. The GABA uptake inhibitor CI-966, the GABA(A) agonist muscimol and the GABA(A)mimetic clomethiazole have all been shown to be neuroprotective in animal models of stroke when given after the ischaemic insult. In contrast, benzodiazepines and particularly barbiturates, although potent GABA(A) potentiators, have shown little promise as neuroprotectants. The diversity of GABA(A) receptor subtypes and the in vivo efficacy of certain GABA(A) receptor ligands in animal models of stroke suggests that GABAmimetic drugs are an undervalued approach to stroke therapy.

An investigation of the possible interaction of clomethiazole with glutamate and ion channel sites as an explanation of its neuroprotective activity

The activity of the neuroprotective agent clomethiazole at glutamate and ion channel sites has been investigated. Dizocilpine (3.25 mg/kg intraperitoneally) provided almost total protection against the damage produced by infusion of N-methyl-DL-aspartate (NMDLA; 75 micrograms) into the right hippocampus. In contrast, clomethiazole (96 mg/kg intraperitoneally) was without effect. Using ligand binding techniques, no evidence was found for clomethiazole interacting with NMDA, AMPA or sigma binding sites. Clomethiazole did inhibit the stimulatory effect of the metabotropic glutamate receptor agonist 1S3R-aminocyclopentone-1,3-dicarboxylic acid (ACPD) on phosphoinositol hydrolysis, but only at a concentration of 10(-3) M, which is unlikely to have functional relevance. Clomethiazole was also without effect on ligand binding to Ca2+ channels (N- or L- type), Na+ channels or ATP-sensitive K+ channels. Potentiation of GABA function therefore remains the most plausible explanation for the neuroprotective activity of clomethiazole.

Role of hyperthermia in the protective action of clomethiazole against MDMA ('ecstasy')-induced neurodegeneration, comparison with the novel NMDA channel blocker AR-R15896AR

1. The immediate effect of administration of 3,4-methylenedioxymethamphetamine (MDMA or 'ecstasy') on rectal temperature and the effect of putative neuroprotective agents on this change has been examined in rats. The influence of the temperature changes on the long term MDMA-induced neurodegeneration of cerebral 5-hydroxytryptamine (5-HT) nerve terminals was also examined. 2. The novel low affinity N-methyl-D-aspartate (NMDA) receptor channel blocker AR-R15896AR (20 mg kg(-1), i.p.) given 5 min before and 55 min after MDMA (15 mg kg(-1), i.p.) did not prevent the MDMA-induced hyperthermia and did not alter either the MDMA-induced neurodegenerative loss of 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) in cortex, striatum and hippocampus or loss of [3H]-paroxetine binding in cortex 7 days later. 3. The neuroprotective agent clomethiazole (50 mg kg(-1), i.p.) given 5 min before and 55 min after MDMA (15 mg kg(-1)) abolished the MDMA-induced hyperthermic response and markedly attenuated the loss of 5-HT, 5-HIAA and [3H]-paroxetine binding in the brain regions examined 7 days later. 4. When rats treated with MDMA plus clomethiazole were kept at high ambient temperature for 5 h post-MDMA, thereby keeping their body temperature elevated to near that seen in rats given MDMA alone, the MDMA-induced loss of 5-HT, 5-HIAA and [3H]-paroxetine was still attenuated. However, the protection (39%) afforded by the clomethiazole administration was less than seen in rats kept at normal ambient temperature (75%). 5. These data support the proposals of others that NMDA receptor antagonists are neuroprotective against MDMA-induced degeneration only if they induce hypothermia and further suggest that increased glutamate activity may not be involved in the neurotoxic action of MDMA. 6. These data further demonstrate that a proportion of the neuroprotective action of clomethiazole is due to an effect on body temperature but that, in addition, the compound protects against MDMA-induced damage by an unrelated mechanism.

Chlormethiazole anticonvulsive efficacy diminished by N-methyl-D-aspartate but not kainate in mice

The aim of this study was to evaluate the effect of N-methyl-D-aspartate (NMDA) and kainate used at nonconvulsive doses upon protective efficacy of chlormethiazole against maximal electroshock-induced seizures. NMDA (50 mg/kg, i.p.) reduced the anticonvulsant potency of chlormethiazole increasing its ED50 value from 126.9 to 155.0 mg/kg. The effect of NMDA was completely reversed by the competitive NMDA receptor antagonist D-(E)-2-amino-4-methyl-5-phosphono-3-pentenoic acid (CGP 40116) (0.06 mg/kg i.p.). Kainic acid (9 mg/kg i.p.) did not affect the anticonvulsive properties of chlormethiazole. Our results suggest that NMDA but not kainate receptor-mediated events participate in the anticonvulsant action of chlormethiazole.

The attenuation of kainate-induced neurotoxicity by chlormethiazole and its enhancement by dizocilpine, muscimol, and adenosine receptor agonists

Systemically administered kainate (10 mg.kg-1) caused neuronal loss in both the hippocampus and the entorhinal regions of the rat brain. This resulted in a loss of 68.3 +/- 13.8 and 53.3 +/- 12.8% of pyramidal neurones in the hippocampal CA1 and CA3a regions, respectively. Chlormethiazole attenuated the loss of neurones in the hippocampal cell layers CA1 (cell loss 10 +/- 3.2%) and CA3a (cell loss 10 +/- 7.7%). The neuroprotective activity of chlormethiazole was apparent in the presence or absence of a low dose of clonazepam (200 micrograms.kg-1 i.p.). The kainate-induced damage could also be measured by the increase in binding of the peripheral benzodiazepine ligand ([3H]PK11195) in the hippocampus. In kainate-treated rats there was a 350-500% increase in binding indicative of reactive gliosis. Chlormethiazole prevented this elevation in a dose- and time-dependent manner, with an ED50 of 10.64 mg.kg-1 and an effective therapeutic window from 1 to 4 h posttreatment. Dizocilpine also attenuated damage significantly. The GABAA agonist muscimol was also able to attenuate the increase in [3H]PK11195 binding in a dose-dependent manner, with an ED50 of approximately 0.1 mg.kg-1. If muscimol, dizocilpine, or the adenosine A1 receptor agonist R-N6-phenylisopropyl-adenosine were administered together with chlormethiazole at their respective ED25 doses, a potentiation was apparent in the degree of neuroprotection. It is concluded that the combination of neuroprotective agents with different mechanisms of action can lead to a synergistic protection against excitotoxicity.

Is chlormethiazole neuroprotective in experimental global cerebral ischemia? A microdialysis and behavioral study

Chlormethiazole, an anticonvulsive agent, has been shown to have a possible neuroprotective effect against cerebral ischemia. In addition, chlormethiazole inhibits methamphetamine-induced release of dopamine, protecting against this neurotransmitter's neurotoxicity. The aim of this work was to ascertain whether, in experimental cerebral ischemia, chlormethiazole administration attenuated the ischemia-induced rise of the extracellular concentration of aminergic neurotransmitters and whether it reduces ischemia-induced deficits in memory and learning. Histology for assessment of ischemic damage was a so included. The four-vessel occlusion rat model was used to induce global cerebral ischemia. Aminergic neurotransmitters and their metabolites in the striatal extracellular fluid obtained by microdialysis were assayed by high-performance liquid chromatography-electrochemical detection. The drug was administered either IP (50 mg/kg-1) or directly through the dialysis probe (30 microM) 80 min before ischemia. For the behavioral test and histology, the drug was given IP (100 mg/kg-1) 1 h postischemia. The results obtained did not demonstrate any statistically significant evidence that chlormethiazole has an effect on the ischemia-induced rise in extracellular dopamine and serotonin levels. There was also no variation in metabolite levels. Behavioral measures (learning, recall) were not changed appreciably by the treatment. We observed no significant cell protection in the hippocampus (CA1, CA1), striatum, and entorhinal cortex in animals treated with chlormethiazole. We conclude that, under our experimental conditions, chlormethiazole has little or no effect on the neurochemical, neurobehavioral, and histological consequences of global cerebral ischemia.

A study of the mechanism of MDMA ('ecstasy')-induced neurotoxicity of 5-HT neurones using chlormethiazole, dizocilpine and other protective compounds

1. An investigation has been made in rats into the neurotoxic effect of the relatively selective 5-hydroxytryptamine (5-HT) neurotoxin, 3,4-methylenedioxymethamphetamine (MDMA or 'Ecstasy') using chlormethiazole and dizocilpine, both known neuroprotective compounds and also gamma-butyrolactone, ondansetron and pentobarbitone. 2. Administration of MDMA (20 mg kg-1, i.p.) resulted in a 50% loss of cortical and hippocampal 5-HT and 5-hydroxyindole acetic acid (5-HIAA) 4 days later. This reflects the long term neurotoxic loss of 5-HT that occurs. Injection of gamma-butyrolactone (GBL; 400 mg kg-1, i.p.) 5 min before and 55 min after the MDMA provided substantial protection. Pentobarbitone (25 mg kg-1, i.p.) using the same dose regime was also protective, but ondansetron (0.5 mg kg-1 or 0.1 mg kg-1, i.p.) was without effect. 3. MDMA (20 mg kg-1) had no significant effect on striatal dopamine concentration 4 days later but did produce a small decrease in 3,4-dihydroxyphenylacetic acid (DOPAC) content. There were few significant changes in rats given MDMA plus GBL, ondansetron or pentobarbitone. 4. A single injection of MDMA (20 mg kg-1, i.p.) resulted in a greater than 80% depletion of 5-HT in hippocampus and cortex 4 h later, reflecting the initial rapid release that had occurred. None of the neuroprotective compounds (chlormethiazole, 50 mg kg-1; dizocilpine, 1 mg kg-1; GBL, 400 mg kg-1; pentobarbitone, 25 mg kg-1) given 5 min before and 55 min after the MDMA injection, altered the degree of 5-HT loss. 5. Acute MDMA injection increased striatal dopamine content (28%) and decreased the DOPAC content. In general, administration of the drugs under investigation did not significantly alter these MDMA-induced changes. Both chlormethiazole and GBL produced a greater increase in dopamine than MDMA alone, but this was apparently an additive effect to the action of either drug alone. 6. The 5-HT loss 4 h following administration of the neurotoxin p-chloroamphetamine (2.5 mg kg-1,i.p.) was not affected by chlormethiazole or dizocilpine. p-Chloroamphetamine did not appear to alter striatal dopamine metabolism.7. None of the protective drugs inhibited the initial 5-HT loss following MDMA, rendering unlikely any proposal that they are protective because they inhibit 5-HT release and the subsequent formation ofa toxic indole derivative. All the protective compounds (unlike ondansetron) probably inhibit dopamine release in the striatum. Since the neurotoxic action of some substituted amphetamines is dependent on the integrity of nigro-striatal neurones, this fact may go some way to explain the protective action of this diverse group of compounds.