Abstracts

Tuberous sclerosis complex (TSC) is a genetically-based disorder that is often associated with early-onset epilepsy. The mechanisms that link the structural pathology of TSC to seizure activity are poorly understood. We have investigated morphological and physiological features the brain of a TSC animal model - the Eker rat [ndash] in which there is a mutation in the TSC2 gene, and characterized the consequences of a [ldquo]second hit[rdquo] insult, varying the dose of [gamma]-irradiation delivered at different developmental time-points. Eker (TSC2+/-) and wildtype rats were exposed to ionizing irradiation (Cs-based [gamma]- ray) at ages E18, P1, P2, P3, P4 and P5. Irradiation dose ranged from 400 to 750 cGy. Flurothyl seizure threshold testing was performed at 1 and 3 months post-irradiation. Histological procedures were carried out at either one or three months post-irradiation to quantify pathological cell features. At 2-3 months post-irradiation, a sub-group of animals was studied with slice electrophysiological procedures; population responses to paired-pulse stimulation were recorded from the deep layers of neocortex to evaluate cortical excitability. Seizure threshold: In irradiated animals (750 cGy), seizure threshold was reduced (compared to non-irradiated rats) at 3 months post-irradiation but not at 1 month. There was no significant difference between irradiated wildtype and irradiated Eker rats.
Histology: Irradiation (400-750cGy) of Eker (but not wildtype) rat pups, at ages E18 to P6, induced an increase in the number of dysmorphic neurons in the neocortex. This increase was modest at one month post-irradiation, but was substantial at three months (a 4-5-fold increase compared to one month). To induce cell pathology, optimal time of irradiation was 3-4 days postnatal. The higher irradiation doses (700-750cGy) induced a 2-3-fold greater increase in the number of dysmorphic neurons and giant astrocytes compared to the lower doses (400-650cGy) (irrespective of time of irradiation); this difference was found at both one and three months post-irradiation.
Physiology. Non-irradiated wildtype animals showed fEPSP amplitude facilitation at shorter interpulse intervals (20-50ms) and depression at longer intervals (100-140 ms). Irradiated Eker rat cortex showed relatively little change in the fEPSP ratio over the paired-pulse intervals tested. Early postnatal ionizing irradiation of Eker rats exacerbated cortical pathology in a dose-dependent manner. The degree of histopathology increased between one and three months post-irradiation [ndash] a change that was mirrored in the time-dependent decrease of seizure threshold. Preliminary electrophysiological data from irradiated Eker rat cortex suggest that the evolving structural abnormalities are reflected in aberrant cortical (Supported by NIH/NINDS grant # NS 18895 (PAS) and an EF postdoctoral fellowship (NT).)