Ion Channels in Epilepsy and as Targets for Antiepilepti

Neurological Disorders And Stroke

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Abstract

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 (BLA) 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. Synaptic responses generated by GluR5 kainate receptors in BLA neurons are inwardly rectifying and calcium permeable. In brain slice recordings from BLA neurons, we demonstrated that topiramate, a widely used antiepileptic agent, selectively and potently inhibits GluR5 kainate receptor mediated synaptic responses. The ability of topiramate to antagonize kainate receptors is intriguing inasmuch as no other clinically used antiseizure medication targets these receptors at therapeutic concentrations. In the present reporting period, we sought to characterize the properties of AMPA receptors in BLA principal neurons with respect inward rectification and presumed calcium permeability. AMPA receptors that lack the GluR2 subunit are inwardly rectifying and are calcium permeable. We used immunoelectron microscopy to determine the extent to which synapses in the rat BLA have AMPA receptors with GluR2 subunits; for comparison, a parallel examination was carried out in the hippocampus. We also recorded from amygdala brain slices to examine the voltage-dependent properties of AMPA receptor-mediated evoked synaptic currents in BLA principal neurons. At the light microscopic level, GluR2 immunoreactivity was localized to the perikarya and proximal dendrites of BLA neurons; dense labeling was also present over the pyramidal cell layer of hippocampal subfields CA1 and CA3. In electron micrographs from the BLA, most of the synapses were asymmetrical with pronounced postsynaptic densities (PSD). They contained clear, spherical vesicles apposed to the PSD and were predominantly onto spines (86%), indicating that they are mainly with BLA principal neurons. Only 11% of morphological synapses in the BLA were onto postsynaptic elements that showed GluR2 immunoreactivity, in contrast to hippocampal subfields CA1 and CA3 in which 76% and 71% of postsynaptic elements were labeled. Synaptic staining in the BLA and hippocampus, when it occurred, was exclusively postsynaptic, and particularly heavy over the PSD. In whole-cell voltage clamp recordings, 72% of BLA principal neurons exhibited AMPA receptor-mediated synaptic currents evoked by external capsule stimulation that were inwardly rectifying. Although BLA principal neurons express perikaryal and proximal dendritic GluR2 immunoreactivity, few synapses onto these neurons express GluR2, and a preponderance of principal neurons have inwardly rectifying AMPA-mediated synaptic currents, suggesting that targeting of GluR2 to synapses is restricted. Unlike the hippocampus where AMPA receptors onto principal neurons are calcium impermeable, BLA neuron AMPA receptors are similar to GluR5 kainate receptors in their presumed calcium permeability. Thus, AMPA receptors on BLA principal neurons, like the GluR5 kainate receptors on these neurons, could play roles in synaptic plasticity, epileptogenesis and excitoxicity. The unusual properties of AMPA receptors at principal neuron synapses in the amygdala may contribute to the unique epileptic susceptibility of this brain region.