Malformations of the developing brain are characteristically associated with mental retardation, schizophrenia, and epilepsy. In fact, with recent advances in neuro-imaging it is now clear that cortical malformations are a common feature of therapy-resistant seizure syndromes in children. Unfortunately, since clinical studies are technically limited in the analysis of cellular mechanisms of epileptogenesis, it is not known how dysplastic brain tissue becomes prone to epileptogenesis. To address this problem, we have been studying a unique rodent model of malformation-associated epilepsy i.e., methylazoxymethanol (MAM)-exposed rats. As in humans, malformations in the rat brain consist of disruptions in lamination, aberrant cell clusters, and microdysgenesis. In vivo and in vitro studies indicate that thresholds for generation of seizure activity are markedly reduced in these MAM-exposed rats. Electrophysiological recordings show that displaced cell clusters are capable of independent seizure generation and displaced neurons exhibit abnormal (hyperexcitable) firing properties. As a logical extension of these studies, the planned experiments in the New Investigator proposal will test potential mechanisms by which dysplastic neurons become hyperexcitable. Techniques will involve use of hippocampal slices maintained in vitro, and application of visualized patch-clamp methods to study whole-cell currents and individual ion channels on dysplastic neurons. Pharmacological experiments will be performed to assess how endogenous neurotransmitters modulate the activity of dysplastic neurons. Molecular studies will be performed on tissue from MAM-exposed rats to examine the expression and distribution of potassium channel sub-units. In some studies, simultaneous fluorescent calcium detection and whole-cell voltage-clamp will be used to examine the kinetics of intracellular Ca2+ mobilization in dysplastic tissue.
Three specific aims are proposed: I) to characterize the function of Ca2+-activated channels on dysplastic neurons, II) to examine the expression of K-Ca channels on dysplastic neurons, and the III) to examine intracellular Ca2+ mobilization mechanisms in dysplastic neurons. The results promise to provide new information about cellular mechanisms of epileptogenesis associated with malformations and may lead to the design of novel anticonvulsant treatments for these otherwise intractable forms of epilepsy.
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