New therapeutic approaches for epilepsies, focusing on reorganization of the GABAA receptor subunits by neurosteroids

Abstract

Neurosteroids such as allopregnanolone (THP) act as positive allosteric modulators of gamma-aminobutyric acid (GABA)A receptors and have exerted anticonvulsant properties. However, their role in the regulation of epileptogenesis is unclear. It has been shown that circulating levels of THP fluctuate during development and seizure episodes. Furthermore, both chronic administration of THP and its withdrawal transiently increase expression of the alpha4 subunit of the GABAA receptor in the brain. The steroidogenic enzymes, 5-alpha-reductase (5aR) and 3-alpha-hydroxysteroid dehydrogenase (3aHSD) have been identified as well, indicating that various cell types are involved in the biosynthesis of neuroactive steroids in the brain. The purpose of the present study is to examine how GABAA receptor-modulating neurosteroids contribute to the epileptogenesis by using the epileptic mutant EL mouse. Male EL mice and control animals, DDY mice, were used. EL mice show secondary generalized seizures, which initiate primarily at the parietal cortex and generalize through the hippocampus. In the interictal period during development, changes of THP, 5aR, 3aSDH, and GABAA receptor alpha4, gamma2, and delta subunits were investigated by western blotting in the hippocampus. In EL mice, levels of the neurosteroid THP and the steroidogenic enzymes 5aR and 3aSDH significantly increased at 3 weeks of age, and rapidly decreased thereafter (5-10 weeks). The sharp withdrawal was observed before mice experienced frequent seizures. In contrast, GABAA alpha4, gamma2, and delta expressions were upregulated (3-8 weeks). In the brain of EL mice, positive neurosteroids such as THP were withdrawn before mice experienced repetitive seizures, which may likely be a trigger for ictogenesis and epileptogenesis. Furthermore, reorganization of the GABAA receptor subunits may lead to a hypersensitivity of the receptor to neurosteroids. Therefore, GABAA receptor-regulating neurosteroids may be a promising target for the development of novel antiepileptic agents.