Abstracts

Rationale: The GABA-A receptor is modulated by a wide variety of compounds, including benzodiazepines, neurosteroids and zinc. Zinc is a negative allosteric modulator of GABA-A receptors with preferential sensitivity towards extrasynaptic receptors. Neurosteroids are powerful antiseizure agents because they are allosteric agonists of both synaptic (gamma2-containing) and extrasynaptic (delta-containing) GABA-A receptors. Therefore, neurosteroids have promising potential for the treatment of epilepsy. Combination of zinc and neurosteroids may result in drug-drug interactions that may affect the therapeutic outcomes. Here we report that zinc pretreatment blocks the seizure protective effects of the synthetic neurosteroid 3߭Me-AP. in a mouse model of temporal lobe epilepsy. Methods: Adult male mice were used for electrophysiology and kindling pharmacology studies. GABA currents were recorded from dentate gyrus granule cells (DGGCs) using whole-cell patch clamp electrophysiology. Antiseizure effect of the synthetic neurosteroid and its modulation by zinc was evaluated in fully-kindled mice expressing stage 5 seizures. Zinc was infused directly into the hippocampus 10-min prior to drug administration. Results: Our results show that zinc blocks the GABA-gated chloride currents in dentate granule cells, similar to the GABA-A receptor competitive antagonist bicuculline. In our pharmacological studies, the synthetic neurosteroid produces a dose-dependent suppression of behavioral and electrographic seizures in fully hippocampus-kindled female mice, with nearly complete seizure protection at the 10 mg/kg dose. Zinc significantly prevented the kindling seizure-suppressing activity elicited by the synthetic neurosteroid (10 mg/kg) in a dose-dependent manner with an ED50 of 152 M. This proconvulsant-like blocking response was reversible as animals showed stage 5 seizures 24 h post-zinc perfusion. Conclusions: These results demonstrate that zinc blocks the antiseizure effects of neurosteroids, possibly by modulating extrasynaptic delta-subunit GABA-A receptors in the hippocampus; this drug-drug interaction may have clinical implications in neurosteroid therapy. Funding: Supported by NIH grant NS051398