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 have continued studies examining the role of GluR5 kainate receptors as mediators of epileptic activity in the amygdala, a common primary focus for seizures in human epilepsy. We have also examined the possibility that GluR5 kainate receptors could represent novel targets for antiepileptic drug (AED) development. It is well recognized that low concentrations of kainate can induce spontaneous epileptiform activity in in vitro brain slice preparations of hippocampus and neocortex. However, because kainate is a nonselective agonist, whether its proconvulsant activity occurs through activation of kainate or AMPA receptors is uncertain. Using extracellular field recordings in the amygdala slice, we carried out pharmacological studies demonstrating that selective activation of GluR5 kainate receptors can induce spontaneous epileptiform discharges. The results also indicated that the in vitro proepileptic activity of kainate may relate to effects specifically on kainate receptors and not AMPA receptors. Since activation of kainate receptors elicits epileptiform activity (at least in the BLA), it is conceivable that blockade of kainate receptors could suppress seizure activity, in which case kainate receptors would represent potential AED targets. In fact, we found that topiramate, a broad spectrum AED whose mechanism of action is poorly understood, selectively inhibits GluR5 kainate receptor-mediated synaptic currents in the BLA, but is much less effective against AMPA receptor responses. Studies of spontaneous and miniature synaptic currents confirmed that the effects of topiramate on GluR5 kainate receptor-mediated synaptic responses occur at least in part via a postsynaptic action. Topiramate block of GluR5 kainate receptor responses was slow, suggesting that it could act indirectly, perhaps by affecting second messenger mechanisms that modulate the activity of kainate receptors. The demonstration that topiramate, a highly effective AED, is able to block kainate receptors at low concentrations supports the concept that kainate receptors may represent a worthwhile target for AED development. To investigate the possibility that selective activation of GluR5 kainate receptors mediates epileptogenesis, we conducted studies with the GluR5 agonist ATPA in the rat in vivo. Microinjection of ATPA into the amygdala resulted in acute seizures. In addition, some animals followed for several months developed spontaneous seizures. These studies demonstrate that activation of GluR5 kainate receptors in the amygdala not only induces seizures, but may also lead to enduring alterations in synaptic physiology that predisposes to spontaneous seizures. Therefore, kainate receptor antagonists may have long-term antiepileptogenic actions in addition to their ability to protect acutely against seizures.
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