The objective of this project is to explore new strategies for the rational development of antiepileptic drugs based upon their interaction with neuronal ion channel systems. Cellular electrophysiological recording techniques are used to study drug modulation of neurotransmitter-gated and voltage-activated ion channels in brain slices, cultured neurons and heterologous cells transfected with cloned ion channel subunit genes. Correlative studies are carried out in animal models. Recent studies have focused on kainate-type glutamate receptors. We have demonstrated that a component of the excitatory synaptic response evoked in basolateral amygdala neurons by external capsule stimulation is mediated by kainate receptors containing the GluR5 subunit and we have shown that these receptors elicit a novel form of synaptic plasticity that could mediate some types of epileptogenesis in the amygdala. In the present reporting period, we investigated the interaction of topiramate with GluR5 kainate receptor mediated synaptic responses in basolateral amygdala neurons. Topiramate is a widely used antiepileptic agent whose mechanism of action is poorly understood. The drug has been reported to interact with various ion channel types, including AMPA/kainate receptors. The potential action of topiramate on AMPA/kainate receptors is intriguing inasmuch as drugs that block these receptors are highly effective in animal models used in the screening of antiepileptic drugs, but no other clinically used antiseizure medication targets these receptors at therapeutic concentrations. In whole-cell voltage-clamp recordings from principal neurons of the rat basolateral amygdala in the in vitro brain slice, topiramate at low concentrations selectively inhibited pharmacologically isolated excitatory synaptic currents mediated by kainate receptors containing the GluR5 subunit. Topiramate also partially depressed predominantly AMPA-receptor-mediated EPSCs, but with lower efficacy. Topiramate did not alter the degree of facilitation in paired-pulse experiments, and it reduced the amplitude of miniature EPSCs without affecting their frequency, demonstrating that the block of synaptic responses occurs postsynaptically. Inhibition of GluR5 kainate receptors could represent a key mechanism underlying the anticonvulsant activity of topiramate. Moreover, these results support the concept that GluR5 kainate receptors represent a novel target for antiepileptic drug development. To determine if the inhibitory action of topiramate on GluR5 kainate receptors as shown in brain slice recordings is relevant to the anticonvulsant effects of the drug in vivo, we determined the protective activity of topiramate against seizures induced by intravenous infusion of various ionotropic glutamate receptor agonists in mice. Topiramate produced a dose-dependent elevation in the threshold for clonic seizures induced by infusion of ATPA, a selective agonist of GluR5 kainate receptors. Topiramate was less effective in protecting against clonic seizures induced by kainate, a mixed agonist of AMPA and kainate receptors. Topiramate did not affect clonic seizures induced by AMPA or NMDA. In contrast, the thresholds for tonic seizures induced by higher doses of these various glutamate receptor agonists were all elevated by topiramate. Our results are consistent with the possibility that the effects of topiramate on clonic seizure activity are due to its specific blockade of GluR5 kainate receptors. Protection from tonic seizures may be mediated by other actions of the drug. Together with our in vitro cellular electrophysiological results, the present observations strongly support a unique mechanism of action of topiramate, which involves selective blockade of GluR5 kainate receptors.
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