Repeated blockade of GABAA receptors in the medial septal region induces epileptiform activity in the hippocampus

Changes in the activity of putative interneurons of the stratum oriens of the hippocampus and hippocampal EEG after the delivery of the GABAA receptor antagonist bicuculline (1.0 nmol/1 μl) to the medial septal region were investigated in awake rabbits. The injection of bicuculline produced a sharp increase in the firing rate in 94.3% of hippocampal cells. The effect of bicuculline on the neuronal theta rhythmicity depended on the number of injections. The first five daily infusions decreased the theta activity in 85.7% of cells. On the fourth to fifth experimental days paroxysmal discharges and 8–15 Hz oscillations were recorded in the hippocampal EEG. Six to seven further daily bicuculline injections following a brief diminution of theta activity produced a sharp augmentation of theta oscillations in 78.9% of cells and provoked seizures. Immediately before seizures, stabilization of theta bursts and an increase in burst frequency was usually observed in putative interneurons. During seizures, neuronal rhythmic activity was either disordered and then turned into seizure discharges or was inhibited, partially or completely. In the hippocampal EEG, the power of theta rhythm before seizures usually strongly increased compared with controls. Injection of the GABAA agonist muscimol (30 nmol/1 μl) 15 min before bicuculline infusion prevented the development of seizures. These findings suggest that the interplay between septal neurons via GABAA receptors is critical in the tuning of septal output signals that insure generation of natural theta rhythm as well as adequate functioning of the hippocampus.