NEUROSTEROID WITHDRAWAL REGULATES GABA-A RECEPTOR α4-SUBUNIT EXPRESSION AND SEIZURE SUSCEPTIBILITY BY ACTIVATION OF PROGESTERONE RECEPTOR-INDEPENDENT EARLY GROWTH RESPONSE FACTOR-3 PATHWAY

Neurosteroids regulate GABA-A receptor plasticity. Neurosteroid withdrawal occurs during menstruation and is associated with a marked increase in expression of GABA-A receptor alpha 4-subunit, a key subunit linked to enhanced neuronal excitability, seizure susceptibility and benzodiazepine resistance. However, the molecular mechanisms underlying the upregulation of alpha 4-subunit expression remain unclear. Here we utilized the progesterone receptor (PR) knockout mouse to investigate molecular pathways of PR and the transcription factor early growth response factor-3 (Egr3) in regulation of the GABA-A receptor alpha 4-subunit expression in the hippocampus in a mouse neurosteroid withdrawal paradigm. Neurosteroid withdrawal induced a threefold increase in alpha 4-subunit expression in wild-type mice, but this upregulation was unchanged in PR knockout mice. The expression of Egr3, which controls alpha 4-subunit transcription, was increased significantly following neurosteroid withdrawal in wild-type and PR knockout mice. Neurosteroid withdrawal-induced alpha 4-subunit upregulation was completely suppressed by antisense Egr3 inhibition. In the hippocampus kindling model of epilepsy, there was heightened seizure activity, significant reduction in the antiseizure sensitivity of diazepam (a benzodiazepine insensitive at alpha 4 beta gamma-receptors) and conferral of increased seizure protection of flumazenil (a low-affinity agonist at alpha 4 beta gamma-receptors) in neurosteroid-withdrawn wild-type and PR knockout mice. These observations are consistent with enhanced alpha 4-containing receptor abundance in vivo. Neurosteroid withdrawal-induced seizure exacerbation, diazepam insensitivity, and flumazenil efficacy in the kindling model were reversed by inhibition of Egr3. These results indicate that neurosteroid withdrawal-induced upregulation of GABA-A receptor alpha 4-subunit expression is mediated by the Egr3 via a PR-independent signaling pathway. These findings help advance our understanding of the molecular basis of catamenial epilepsy, a neuroendocrine condition that occurs around the perimenstrual period and is characterized by neurosteroid withdrawal-linked seizure exacerbations in women with epilepsy. (C) 2010 IBRO. Published by Elsevier Ltd. All rights reserved.