ION CHANNELS IN EPILEPSY AND AS TARGETS FOR ANTIEPILEPTIC DRUGS

这个项目的目的是探索新的

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. In the present reporting period, studies were continued examining the interaction of anticonvulsant drugs with NMDA receptors. Felbamate, an anticonvulsant with efficacy in various childhood (Lennox-Gastaut syndrome) and adult seizure disorders, was characterized with regard to its selectivity for NMDA receptors composed of different subunit combinations. Our previous studies indicated that felbamate acts as a channel blocking NMDA receptor antagonist (and also a positive allosteric modulator of GABA-A receptors). However, in contrast to other NMDA receptor antagonists, felbamate has low propensity for behavioral toxicity. To understand the basis of this favorable toxicity profile, whole cell voltage clamp recordings were carried out from recombinant NMDA receptors expressed in HEK cells transfected with NR1a subunits and one of several NR2 subunits. Felbamate produced a concentration-dependent block of NMDA-evoked currents carried by NMDA receptors composed of all subunit combinations. However, felbamate was a more potent antagonist of NMDA receptors containing the NR2B subunit than other NR2 subunits. Felbamate's selectivity for NR2B-containing NMDA receptors may, in part, account for its improved toxicity profile. In addition, since NR2B subunits are preferentially expressed in early brain development, felbamate's selectivity for NMDA receptors containing this subunit may contribute to its utility in treating childhood epilepsy syndromes, such as the Lennox-Gastaut syndrome. Studies were also initiated examining glutamate receptor mediated neurotransmission and synaptic plasticity mechanisms in the amygdala, a key brain site for epileptogenesis in animal models and a common primary focus for seizures in human epilepsy. Experiments focused on the role of kainate-type glutamate receptors whose function until recently has been obscure. The GluR5 kainate receptor is prominently expressed in the amygdala where it could serve as a mediator of epileptic hyperexcitability and epileptogenesis, such as occurs in response to kainate receptor agonists including the neurotoxins kainate and domoate. An excitatory synaptic pathway in the basolateral amygdala (BLA) that utilizes the GluR5 kainate receptor was identified and its role in a novel form of enduring synaptic facilitation was demonstrated. Synaptic potentials were evoked by stimulation of either the external capsule (EC) or basal amygdala (BA) in transverse slices of the rat amygdala. NMDA and GABA-A receptors were blocked by inclusion of APV and bicuculline in the perfusion solution. The AMPA receptor-selective allosteric antagonists GYKI 52466 and GYKI 53655 partially suppressed depolarizing synaptic responses evoked by single shock EC stimulation, but fully blocked synaptic responses evoked by BA stimulation. In recordings carried out in the presence of the AMPA receptor antagonists, EC stimulation with pulse trains evoked a large increase in the amplitude of synaptic responses. Such AMPA receptor-independent, train-evoked synaptic responses were blocked by the GluR5-selective kainate receptor antagonist LY293558. These results indicate that a component of the EC (but not the BA) synaptic response is mediated by kainate receptors containing the GluR5 subunit. Further studies identified several novel forms of activity-dependent synaptic plasticity in the BLA. Low frequency stimulation (LFS) of the EC induced a persistent NMDA receptor-independent (APV insensitive) enhancement in the amplitude of synaptic potentials recorded in BLA neurons. Brief high-frequency EC stimulation induced APV-sensitive short-term potentiation. When LFS was applied after recovery from the short-term potentiating effect of HFS ("HFS/LFS"), there was persistent synaptic depression. This represents the first demonstration of stimulus-dependent long-lasting synaptic depression in the amygdala. Application of the presynaptic (group II) metabotropic glutamate receptor antagonist 2S-alpha-ethylglutamic acid (EGLU) prevented the HFS-dependent switch from synaptic facilitation to depression. Thus, LFS can induce either enduring synaptic potentiation or depression, depending on whether a priming HFS train has been applied. Synaptic depression induced by an HFS/LFS-like mechanism could play a role in diverse amygdala-dependent processes, including the regulation of epileptogenesis. LFS-induced enduring facilitation in BLA neurons is a novel NMDA receptor-independent form of synaptic plasticity. The facilitation persisted in the presence of NMDA or AMPA receptor blockers. However, it was exquisitely sensitive to antagonists of GluR5 kainate receptors including LY293558 and the more selective compound LY377770, indicating that activation of GluR5-containing kainate receptors are required. Interestingly, LFS-induced synaptic facilitation in one input pathway to BLA neurons resulted in potentiation of synaptic responses evoked by the other input pathway. This heterosynaptic facilitation provides a mechanism for spread of hyperexcitability within the amygdala, such as may occur during the development of an epileptic focus. These studies represent the first demonstration of a kainate receptor-dependent synaptic plasticity mechanism. This mechanism could be of importance in the phenomenon of epileptogensis within the amygdala, and raise the possibility that kainate receptor-selective antagonists could be of utility in epilepsy prophylaxis.