Search for new strategies for the therapeutic management of temporal lobe epilepsy, using an experimental animal model.

Project Details

Description

This fundamental research project concerns the search for unravelled sites of action involved in epilepsy and new strategies for the therapeutic management of temporal lobe epilepsy. We use an experimental animal model for this neurologic disorder, which was developed and characterised in our laboratory. Intracerebral microdialysis in freely moving rats is the sampling technique and microbore liquid chromatography is used for sample analysis. The microdialysis probe is implanted in the hippocampus, a brain area involved in epileptogenesis. This way, we are able to monitor alterations in the extracellular neurotransmitter concentrations, such as glutamate, GABA and dopamine. In a former research project, we characterised in a similar way some of the neurobiochemical changes during an epileptic seizure. With this knowledge, we can now interact pharmacologically, using sub-type selective receptor ligands, and monitor simultaneously the neurobiochemical and electrocortographic alterations.
Aims
In this project, we will focus on sub-type selective antagonists for ionotropic glutamate receptors and sub-type selective ligands for metabotropic glutamate receptors and adenosine receptors. We will also study the mechanisms of action and the effectiveness of a few recently developed valproic acid derivatives. Finally, we are interested in the neurotoxicological side effects which might clinically occur after administration of quinolone antibiotics.
1. Study of sub-type selective glutamate receptor ligands.
Glutamate is the main central excitatory neurotransmitter. Abnormaly high extracellular glutamate and/or excessive activation of glutamate receptors can induce excitotoxic brain damage, morphological alterations and cell death. An important hypothesis for the pharmacotherapy of epilepsy is the search for alternative strategies to attenuate the excessive glutamate mediated processes. There are to date no anti-epileptics interfering with t glutamate receptors. We demonstated that the GLUK5 kainate receptor sub-type plays a key role in limbic epilepsy. Our experiments with GLUK5 kainate receptor antagonists, LY377770 en LY382884, showed that GLUK5 receptors are implicated in both the initiation and maintenance of the convulsions in the rat and epileptiform activity in hippocampal slices. The in vitro work was performed in cooperation with Prof. Collingridge, Bristol University (U.K.) and the compounds were obtained by cooperation with Dr. Lodge, Lilly, Surrey (U.K.). GLUK5 receptors seem not involved in basal spontaneous hippocampal activity, which makes this receptor sub-type an even more interesting target. It is indeed important (to minimalize side effects) that basal normal synaptic neurotransmission is not disturbed but that the drugs effectively act during high freqency 'epileptic' transmission. The underlying molecular mechanisms of action of these antagonists are presumably a combination of pre- and postsynaptic effects, such as a diminuition of the GLUK5 mediated excitatory postsynaptic potentials and a blockade of GLUK5 mediated inhibition of GABA release. The used anticonvulsant doses did not induce behavioural nor motoric disturbances in the rats and possibly similar GLUK5 kainate receptor antagonists will be further developed as potential antiepileptic drugs (manuscript submitted for publication).
2. Efficacy of the derivatives of valproic acid, valnoctamide en valpromide.
Despite the progress in antiepileptic therapy in the last years, still about 25 % of the epileptic population is not seizure free. This means that the search for better therapies keeps going on and one of the approaches is to develop improved derivatives of the existing antiepileptic drugs. Valproic acid is one of the major anticonvulsants clinically used. Several hypotheses have been proposed to explain the antiepileptic activity of valproate. The first proposes that valproate acts by enhancing GABA levels in the brain and thus by GABA-mediated inhibition; the second suggests a phenytoin like effect on voltage-dependent Na+ channels; furthermore, valproate seems to attenuate neuronal excitation induced by the NMDA type of glutamate receptors. It is likely to accept that the drug's clinical activity relates to a combination of mechanisms, considering its broad spectrum of anticonvulsant activity. However, in animal models, valproate showed lower anticonvulsant potency than the other three established antiepileptic drugs: phenobarbital, phenytoin and carbamazepine. In addition, two major side effects, teratogenicity and hepatotoxicity, have been associated with valproate therapy. This has led to a substantial need to develop improved derivatives of valproate. Two of these derivatives are valpromide, used as antiepileptic and antipsychotic agent in several European countries, and valnoctamide, used as anxiolytic and occasionally as anticonvulsant drug. We evaluated the effectiveness of sodium valproate (400 mg/kg) and its amide derivatives, valpromide and valnoctamide (both 100 mg/kg), in the in vivo pilocarpine rat model of focal epilepsy. Our main interest was to get insight into possible changes in extracellular amino acid neurotransmitter levels following administration of the drugs, both in control and in epileptic conditions. The focally evoked pilocarpine-induced seizures were completely prevented by acute intraperitoneal pretreatment with each of the three drugs in the respective doses. Effective protection was reflected in the electrocorticographic recordings and in the lack of sustained elevations of the extracellular glutamate levels after pilocarpine perfusion. Little effects were seen on the basal extracellular amino acid levels after systemic administration of each of the compounds, nor after the intrahippocampal administration of sodium valproate. Valnoctamide and valpromide (100 mg/kg) proved to be at least as effective as their parent compound sodium valproate (400 mg/kg) against pilocarpine-induced seizures. All three compounds however failed to induce significant initial alterations in extracellular hippocampal GABA release. This questions the enhancement of GABA-mediated inhibition as being one of their mechanisms of action (manuscript published Pharm. Res. 17 (2000) 1408-1413).
3. Characterisation of chinolone-induced convulsant potential.
Fluoroquinolones (FQs) are used in the treatment of various infectious diseases because of their broad and strong antibacterial activity, especially against Gram-negative bacteria. Although generally well tolerated, they may induce central nervous system (CNS) adverse symptoms including headache, confusion, hallucinations, anxiety, nervousness and nightmares. Epileptic seizures have occasionally been reported, most frequently in patients with a history of epilepsy or when FQs had been co-administered with theophylline or non-steroidal anti-inflammatory drugs such as fenbufen. These adverse effects presumably result from inhibition of GABA binding to GABAA receptors. This GABA antagonistic effect is greatly potentiated by the active metabolite of fenbufen, biphenylacetic acid (BPAA). Nevertheless, it remains questionable whether GABA receptor antagonism alone can explain the convulsant potential of these antimicrobial agents. The present study was undertaken to investigate possible effects of norfloxacin, both in the absence and presence of BPAA, on the extracellular hippocampal levels of GABA and glutamate, respectively the main central inhibitory and excitatory amino acid neurotransmitters. This in vivo microdialysis approach in conscious rats allows monitoring behavioural alterations and concomitant transmitter modulation in hippocampus. Peroral administration of 100 mg/kg BPAA was without behavioural effects and did not significantly alter extracellular GABA or glutamate concentrations. Intravenous perfusion of 300 mg/kg norfloxacin did not change the rat's behaviour nor the concomitant neurotransmitter levels in about half of the experiments, while the remaining animals exhibited severe seizures. These norfloxacin-induced convulsions did not affect extracellular hippocampal GABA levels but were accompanied by enhanced glutamate concentrations. Half of the rats receiving both 100 mg/kg BPAA and 50 mg/kg norfloxacin displayed lethal seizures, while the remaining animals showed no seizure-related behaviour. In the latter subgroup, again no significant alterations in extracellular GABA levels were observed but glutamate overflow remained significantly elevated for at least 3 hours. In conclusion, norfloxacin exerts convulsant activity in rats, accompanied by elevations of extracellular hippocampal glutamate but not GABA levels, even in the presence of BPAA. We hypothesise that norfloxacin alone or as an intermolecular complex that it could form with BPAA presumably inhibits postsynaptic GABAA receptors which then leads to a disinhibition of the hippocampal glutamatergic neurones (manuscript submitted for publication).
AcronymOZR487
StatusFinished
Effective start/end date1/01/0031/12/02

Flemish discipline codes

  • Basic sciences
  • Pharmaceutical sciences

Keywords

  • biochemical analysis
  • neurotoxicity
  • therapeutic drug research
  • neuropharmacology