Abstracts

RATIONALE: Brain malformations are found in nearly 25% of all epilepsy patients. Recognition of the association between intractable epilepsy and an identified brain malformation, has generated considerable interest in how the epileptic brain develops. One current area of active research is to identify the precise phenotype of cells located within a brain malformation. To address this complicated issue, we have been studying an animal model characterized by distinct nodular heterotopia and enhanced seizure susceptibility e.g., rats exposed to MAM in utero.METHODS: To induce neuronal heterotopia, pregnant S-D rats were injected with methylazoxymethanol acetate (MAM; 25 mg/kg, i.p.) on E15. Control animals were injected with saline. Non-radioactive in situ hybridization of hippocampal tissue sections was performed using probes for cortical- or hippocampal-specific molecules. Whole-cell current- or voltage-clamp recordings were obtained from visualized cells.RESULTS: First, we examined the intrinsic neuronal properties of heterotopic or normotopic pyramidal cells (MAM animals) and CA1 pyramidal cells or Layer II-III supragranular pyramidal cells (Control animals). In current-clamp mode, Rin and firing frequency were similar for heterotopic and supragranular neurons. These cells exhibited prominent depolarizing afterpotentials and very little [dsquote]sag[dsquote] potential. Sag potentials were prominent for normotopic (MAM) and CA1 pyramidal cells. Second, we examined hyperpolarization-activated (Ih) currents. Ih on heterotopic and cortical neurons were nearly identical. Ih was blocked by 5 mM cesium chloride or 100 [mu]M ZD7288. Third, we examined the molecular identity of heterotopic neurons. Consistent with previous reports [JCN 394:520-536, 1998; PNAS 95:10263-10268, 1998], hippocampal-specific gene markers e.g., SCIP, Math-2, Prox-1, and Neuropilin-2 were not expressed in regions of hippocampus containing nodular heterotopia. Tissue sections from control animals showed prominent expression of these markers in appropriate cell regions. However, Id-2, a molecule prominently expressed in Layer II-III supragranular neurons, was observed in nodular heterotopia. Markers for Layer III-VI cortical neurons, e.g., Cad-6, Cad-8, Rzr-b and Tbr-1, were not prominently expressed in heterotopia.CONCLUSIONS: Our results suggest that nodular heterotopia contain a population of cortical pyramidal neurons. This conclusion is based on our observation that heterotopic neurons have physiological and molecular properties that are nearly identical to those of Layer II-III supragranular pyramidal neurons. Although the functional consequence of having a [dsquote]cortical[dsquote] neuron in a hippocampal location is not entirely clear, it is conceivable that these cells alter the network properties of this brain region.
Support: Lucille Packard CHI, Parents Against Childhood Epilepsy and NIH