Abstracts

Rationale: The development of epilepsy is linked to complex alterations in neuroplastic mechanisms. Dysregulation of neurosteroid synthesis may also play a role. Neurosteroids, such as allopregnanolone, are synthesized within the brain and regulates neuronal excitability and tonic inhibition by modulating GABA-A receptor function. Neurosteroids are allosteric modulators of synaptic (αβγ₂-containing) and extrasynaptic (αβδ-containing) GABA-A receptors and exhibit greater sensitivity towards δ-containing extrasynaptic receptors present in the hippocampus. However, the pathophysiological roles of neurosteroids remain unclear. Here, we report that the prototype endogenous neurosteroid allopregnanolone is involved in controlling limbic epileptogenesis. Methods: Adult female mice of wildtype and δ-subunit knockout (DKO) strains were used in the study. We utilized the hippocampus kindling, which is a model of functional limbic epileptogenesis since seizures develop after repeated subthreshold stimulation of hippocampus without induction of evident brain lesions. Adult mice were subjected to hippocampus kindling via implanted electrode in the hippocampus and the rate of kindling, behavioral seizure intensity, and afterdischarge (AD) duration were evaluated as indices of epileptogenesis. We investigated the novel role of neurosteroids in epileptogenesis using different approaches for manipulating neurosteroid levels in the brain: (i) gonadotropin-induced elevations in neurosteroids; and (ii) exogenous administration of allopregnanolone. Plasma and brain levels of neurosteroids were measured by LC-MS method. Results: The neurosteroid allopregnanolone, at doses that produce plasma levels similar to physiological conditions, decreased epileptogenesis in WT mice. Dose-response study of allopregnanolone effect on seizure expression in fully-kindled animals shows that a lower dose of allopregnanolone (0.5 mg/kg) did not affect behavioral seizure stage or AD duration. However, such disease-modifying or antiepileptogenic effect of allopregnanolone was diminished in DKO mice. Gonadotropin protocol resulted in persistent elevations in allopregnanolone for at least 10 days as measured by LC-MS method. The gonadotropin-induced augmentation of allopregnanolone was associated with significant decrease in epileptogenesis in WT mice. The gonadotropin-induced allopregnanolone-mediated retardation of epileptogenesis was diminished in DKO mice, indicating the role of δ-containing extrasynaptic GABA-A receptors in the antiepileptogenic effects of neurosteroids. Conclusions: These results demonstrate the antiepileptogenic or disease-modifying properties of neurosteroids, possibly by augmenting the δ-containing extrasynaptic GABA-A receptor-mediated tonic inhibition in the hippocampus. ** Supported by NIH grant NS051398 **