Our long term goal is to learn how an inherited gene error produces a specific pattern of epilepsy in the developing brain, and to provide an exact description of subsequent seizure-induced plasticity within affected neural networks. Generalized absence seizures of the spike-wave pattern comprise a major category of inherited epilepsy in children. Although genes for this phenotype are known in mice, the underlying basic neuronal excitability mechanisms, their degree of overlap, and the effects of spike-wave hypersynchrony on developing neural circuitry have not been clearly defined. The primary goal of this project is to examine specific network abnormalities identified in isolated thalamocortical brain slices of stargazer (gamma2) mutant mice, and compare them to 3 other mutants of voltage-paled calcium channel subunits, tottering (alpha1A), lethargic (beta4), and ducky (alpha2delta2), all with gene-linked spike-wave seizures. The stargazer gene product gamma2 (stargazin), has homology to the gamma1 subunit of the muscle voltage-gated calcium channel, and interacts with neuronal alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) type glutamate receptors. In the previous project period, we identified specific excitability defects in cortical, thalamic, and hippocampal neurons in these mutants. We hypothesize: (1) that spike-wave discharges in these mutant mice arise from multiple distinct thalamocortical network excitability defects that alter membrane currents, synaptic transmission, and gene expression, and (2) that inhibitory and excitatory synapses are differentially affected by these channelopathies. Using slice patch clamp, optical recordings, molecular anatomy, and transgenic methods to study the mutant neurons, we will test specific hypotheses regarding the membrane and synaptic mechanisms underlying this network defect. We will explore the functional role of the gene-linked defects in epileptogenesis by determining which components arise developmentally as a primary cellular expression of the channelopathy, and which arise secondarily as a product of seizure-induced neuroplasticity.
In specific aim 1, we will analyze intrinsic membrane properties and calcium (Ca)2+ currents of control and mutant thalamic neurons to quantify changes in burst firing properties linked to each genotype. We will explore the basis for T-type current alterations in non-T-type gene mutants.
In specific aim 2, we will examine the role of specific calcium channel subunits in neurotransmitter release from presynaptic terminals of mutant cortical neurons.
In specific aim 3, we will explore the molecular basis for an abnormal response to glutamate receptor antagonists in stargazer cortical circuits.
In specific aim 4 we will use transgenic rescue strategies to dissect the role of inhibitory brain pathways in the mutant phenotype. In each specific aim, we will examine the development of these properties relative to the postnatal onset of spike-wave epilepsy. These studies will directly test key hypotheses concerning basic mechanisms of thalamocortical oscillations produced by calcium ion channelopathies, and the degree of long-term cellular and molecular neuroplasticity that may accompany early seizures of the spike-wave pattern.
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