Anticonvulsant and behavioral effects of neuroactive steroids alone and in conjunction with diazepam

Epilepsy continues to be a significant clinical problem as current medications neither adequately control seizures nor are free of untoward side-effects. Modulation of the neuroactive steroid site on the gamma-aminobutyric acid (GABA)A receptor complex may be an important new direction for pharmaceutical interventions in epilepsy. In this study we evaluated the protective actions of four neuroactive steroids, 3alpha-hydroxy-5alpha-pregnan-20-one, the 3beta-methylated analog, ganaxolone (3alpha-hydroxy-3beta-methyl-5alpha-pregnan-20-one), 3alpha-hydroxy-5beta-pregnan-20-one and Co 2-1068 (3beta-(4acetylphenyl)ethynyl-3alpha,21-dihydroxy-5beta++ +-20-one-21-hemisuccinate), against several standard convulsive tests in male, Swiss-Webster mice. Consistent with their GABAergic actions, the neuroactive steroids as well as diazepam and phenobarbital dose-dependently protected against clonic convulsions induced by pentylenetetrazol; the N-methyl-D-aspartate receptor antagonist, dizocilpine, was ineffective. In contrast to diazepam and phenobarbital, however, all of the neuroactive steroids and dizocilpine produced full protection against cocaine-induced convulsions. Some of the neuroactive steroids, as well as dizocilpine, were efficacious against the seizures and lethality induced by N-methyl-D-aspartate. Pregnenolone, a steroid devoid of GABAergic activity, was not effective in any of the convulsant models. Although all of the compounds produced motor toxicity in high doses as measured by the inverted-screen test, the neuroactive steroids demonstrated an equivalent or improved separation between anticonvulsant potency and motoric impairment. Inactive doses of the neuroactive steroids markedly enhanced the anticonvulsant effects of diazepam against pentylenetetrazol without significantly increasing motor toxicity. This adjunct treatment resulted in protective indices ranging from 60 to 360 compared to 12 for diazepam alone. The distinct profile of anticonvulsant activity of the neuroactive steroids may be related to their combined actions on gamma-aminobutyric acid, N-methyl-D-aspartate receptors, or voltage-operated Ca++ channels. These results help to define the neuroactive steroids as a novel class of antiepileptic agents and suggest their potential in clinical practice.