Abstracts

Rationale: Neurosteroids are endogenous regulators of neuronal excitability and seizure susceptibility. Neurosteroids such as allopregnanolone (AP) exhibit enhanced anticonvulsant activity in perimenstrual catamenial epilepsy, a neuroendocrine condition in which seizures are clustered around menstrual period associated with neurosteroid withdrawal (NSW). However, the molecular mechanisms underlying such enhanced neurosteroid sensitivity remain unclear. Neurosteroids are allosteric modulators of both synaptic (αβγ₂-containing) and extrasynaptic (αβδ-containing) GABA_A receptors, but they display greater sensitivity towards δ-subunit receptors in dentate gyrus granule cells (DGGCs). Here we report a novel plasticity of extrasynaptic δ-containing GABA-A receptors in the dentate gyrus in a mouse perimenstrual model of NSW. Methods: Adult female mice of wildtype and δ-subunit knockout strains were used in the study to investigate the mechanisms of catamenial epilepsy and drug sensitivity. We utilized the mouse model of catamenial epilepsy that our lab developed by using the neurosteroid-withdrawal (NSW) paradigm. A chronic condition of elevated neurosteroid levels was induced by sequential gonadotropin treatment, and withdrawal was induced by the neurosteroid synthesis inhibitor finasteride. Receptor subunits were assayed by TaqMan PCR and western blots. Tonic currents were recorded from brain slices. Mice undergoing NSW were subjected to rapid kindling for evaluation of seizure susceptibility. Results: In molecular and immunofluorescence studies, a significant increase occurred in δ-subunit, but not α₁, α₂, β₂ and γ₂ subunits, in the dentate gyrus of mice subjected to NSW paradigm. Patch-clamp studies confirmed enhanced sensitivity to AP potentiation of GABA-gated currents in DGGCs, but not in CA1 pyramidal cells, in NSW animals. AP produced a greater potentiation of tonic currents in DGGCs of NSW animals as compared to controls. Moreover, such enhanced AP sensitivity was not evident in d-subunit knockout mice subjected to similar withdrawal paradigm. In behavioral studies, mice undergoing NSW exhibited enhanced seizure susceptibility to hippocampus kindling. Conclusions: These results indicate that perimenstrual NSW is associated with striking upregulation of extrasynaptic δ-containing GABA-A receptors that mediate tonic inhibition and neurosteroid sensitivity in the dentate gyrus. These findings may represent a molecular rationale for neurosteroid therapy of catamenial epilepsy. ** Supported by NIH grant NS051398 **