CHARACTERIZATION OF THE CONVULSANT ACTION OF PREGNENOLONE SULFATE. Pregnenolone sulfate (PS) is an endogenous neurosteroid synthesized by glial cells, which acts as a potent convulsant when injected intracerebroventricularly and intraperitoneally. PS is found in relatively high concentrations in the hippocampus. But its convulsant action in the hippocampus has not been characterized. A range of PS doses were infused directly into the right hippocampus of 42 rats, which were subsequently monitored for behavioral and electrographic seizures. At the highest dose (4 micromol), PS produced status epilepticus (SE) and severe behavioral convulsions. As the dose of PS was reduced, the fraction of rats having SE diminished (ED50 for SE = 2.7 micromol). At doses lower than 300 nmol, PS infusion produced discrete electrographic seizures (ED50 = 68 nmol) associated with mild behavioral seizures. Both the behavioral seizure score (BSS) and the total number of seizures during the observation period changed in a dose-dependent manner. In separate experiments in cultured hippocampal neurons, PS enhanced NMDA-evoked whole-cell currents (EC50 = 16 microM). The results demonstrate that the hippocampus is highly sensitive to the convulsant effects of PS and that the enhancement of NMDA currents could contribute to the convulsant action of PS.