Abstract

Epilepsy is often treated with a combination of antiepileptic drugs. Although neurosteroids are potent anticonvulsants, little is known about their combination potential for the treatment of refractory epilepsy. Here we investigated the combination efficacy of neurosteroids allopregnanolone (AP, brexanolone) and ganaxolone (GX) with the GABA-reuptake inhibitor tiagabine (TG) or the benzodiazepine midazolam (MDZ) on tonic inhibition in dentate gyrus granule cells (DGGCs) and seizure protection in the hippocampus kindling and 6-Hz seizure models. Isobolographic analysis indicated that combinations of GX and TG, or AP and TG at three standard ratios (1:1, 3:1 and 1:3) displayed significant synergism in augmenting tonic inhibition. In pharmacological studies, GX, AP, and TG produced dose-dependent antiseizure effects in mice (ED50 = 1.46, 4.20, and 0.20 mg/kg, respectively). The combination of GX and TG at the fixed-ratio of 1:1 exerted the greatest combination index (CI = 0.53), indicating strong synergistic interaction in seizure protection. In addition, combination regimens of AP and TG showed robust synergism for seizure protection (CI = 0.4). Finally, combination regimens of GX and MDZ elicited synergistic (CI = 0.6) response for seizure protection. These results demonstrate striking synergism of neurosteroids and TG combination for seizure protection, likely due to their effects at extrasynaptic GABA-A receptors from TG-induced elevation in GABA levels. Super-additive antiseizure activity of neurosteroid-midazolam combinations may stem from their actions at both synaptic and extrasynaptic GABA-A receptors. Together, these findings provide potential mechanistic basis for combination potential of neurosteroids with TG or benzodiazepines for the management of refractory epilepsy, status epilepticus, and seizure disorders.