Loss of inhibition precedes delayed spontaneous seizures in the hippocampus after tetanic electrical stimulation.

1. The electrophysiological consequences of a "continuous" hippocampal stimulation protocol consisting of 10-s-long stimulus trains administered every 11 s for 90 min to the hippocampus of urethane-anesthetized rats were studied. During the period of stimulation, there was an initial phase with tonic discharges occurring every 1-2 min followed by a secondary phase in which there were well-demarcated recurrent clonic seizures. 2. At the end of the 90 min stimulation period, 7 out of a total of 14 animals studied show epileptiform activity that lasted one-half hour. The other half show disappearance of epileptiform activity within 2 min after terminating the stimulus. In all the animals a quiescent period then followed. In 11 of the animals studied, spontaneous seizures reappeared after the quiescent period had been present for several hours. 3. A paired-pulse method that in previous studies had been shown to reflect the functional state of GABAergic inhibition was employed. Paired-pulse depression in the CA1 region of the hippocampus was measured before administration of stimulus trains, in the quiescent period following the trains, and during and after spontaneous seizures. By comparing indices of maximal inhibition under the various experimental conditions, estimates of the relative amount of inhibition lost could be made. 4. All animals showed reduction of paired-pulse depression in the quiescent period following stimulation when compared with prestimulation values.(ABSTRACT TRUNCATED AT 250 WORDS)