Abstracts

Rationale: Vigabatrin (VGB) is an antiepileptic drug used for treatment of infantile spasms and refractory complex partial seizures. VGB is especially effective in stopping spasms in patients with tuberous sclerosis complex (TSC), but the mechanism of this unique efficacy of VGB in TSC is unknown. VGB is a structural analog of gamma-aminobutyric acid (GABA) and increases brain GABA concentrations by irreversibly inhibiting GABA transaminase. The pathophysiology of epilepsy in TSC is incompletely understood, but may involve abnormalities in the GABA system, as well as excessive activation of the mammalian target of rapamycin (mTOR) signaling pathway. Given the unique therapeutic relationship of VGB to TSC, we investigated the effects of VGB on seizures and other phenotypic features of a mouse model of TSC, including potential interactions with the mTOR system.Methods: Three-week-old mice with inactivation of the Tsc1 gene in glial fibrillary acid protein (GFAP)-expressing cells (Tsc1^GFAPCKO mice) were treated with vehicle or VGB at different doses (50, 100, 200 mg/kg/day, i.p) for one week. Vehicle-treated non-KO littermates served as additional controls. For assessment of mTOR pathway activity, western blot analysis was used to measure the ratio of phospho-S6 (P-S6) and total S6 in the neocortex and hippocampus of Tsc1^GFAPCKO mice. For assays of glial proliferation, immunostaining for GFAP was performed and GFAP-positive cells in neocortex and hippocampus were counted. In separate experiments, mice were treated with VGB (200mg/kg/day) or vehicle from 3 weeks of age until death. Some mice underwent weekly video-EEG monitoring starting at 3 weeks of age to assess seizure frequency. Other mice were monitored for survival, body weight, and brain weight.Results: Video-EEG studies showed that treatment of VGB at 200 mg/kg/day almost completely inhibited seizures in Tsc1^GFAPCKO mice. VGB also modestly improved the survival of Tsc1^GFAPCKO mice compared to vehicle treated KO mice, but all VGB-treated mice still died by about 3 months of age. Western blot analysis showed that VGB treatment decreased activation of the mTOR pathway by a maximum of 40% in Tsc1^GFAPCKO mice, as reflected by a decrease in the P-S6 to total S6 ratio. VGB treatment also decreased slightly the number of GFAP positive cells in neocortex and hippocampus compared to vehicle-treated Tsc1^GFAPCKO mice, but not back to the levels of control mice. No significant difference was found in brain and body weight between VGB and vehicle-treated Tsc1^GFAPCKO mice, both of which were abnormal compared to control mice.Conclusions: VGB strongly inhibits seizures, but has only modest effects on survival and glial proliferation and no effect on megalencephaly of Tsc1^GFAPCKO mice. In addition to modulating GABA, VGB may act as a weak mTOR inhibitor, which could account for some of its effects on the phenotype of Tsc1^GFAPCKO mice; however, VGB does not have the same effects as the classic mTOR inhibitor, rapamycin. Additional studies are required to determine the unique mechanisms of action of VGB in relation to GABA and mTOR in TSC.