Relative anticonvulsant effects of GABAmimetic and GABA modulatory agents

Anticonvulsant properties of compounds that enhance GABA-mediated inhibition through modulatory sites on the GABAA receptor [phenobarbital (PB), clonazepam (CZP), alpha-ethyl-alpha-methyl-gamma-thiobutyrolactone (alpha-EMTBL)] were compared with anticonvulsant effects of compounds believed to be antagonists at these modulatory sites (Ro15-1788 and alpha-isopropyl-alpha-methyl-gamma-butyrolactone gamma-IMGBL)] and to 4,5,6,7-tetrahydroisoxazolo-[4,5-c]-pyridin-3-ol (THIP, GABAA receptor agonist), (+/-) baclofen (GABAB receptor agonist), and gamma-vinyl GABA, a compound that increases endogenous GABA. The compounds were tested for their ability to block experimental seizures caused by maximal electroshock, pentylenetetrazol, picrotoxin, methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM), bicuculline (BIC), aminophylline, strychnine, and t-butyl-bicyclophosphorothionate (TBPS) in mice. CZP blocked all but strychnine seizures. PB was also highly effective, blocking all but TBPS seizures. alpha-EMTBL, representing a new class of experimental anticonvulsant drugs, prevented all seizures except strychnine (STR)- and aminophylline-induced seizures. The antagonists are effective only against one convulsant stimulus. Ro15-1788 and alpha-IMGBL prevented only DMCM- and pentylenetetrazol (PTZ)-induced seizures, respectively. THIP and gamma-vinyl GABA both blocked only BIC and picrotoxin seizures. Baclofen had no anticonvulsant activity. These data demonstrate that compounds that increase neuronal inhibition by potentiating the action of GABA have a broader spectrum of anticonvulsant action than either antagonists or GABAmimetic agents or compounds that increase endogenous GABA.