Neuronal activity in the subcortically denervated hippocampus: a chronic model for epilepsy

Spontaneous and evoked field potentials and cellular discharges were studied in the subcortically denervated hippocampus of the freely moving rat. The fimbria fornix, the ventral hippocampal commissure, and the supracallosal afferent fibers were removed by aspiration, and recordings were made 3-5 months after the lesion. Two types of spontaneous interictal spikes were observed. Type 1 interictal spike had identical depth distribution to physiological sharp waves but they were shorter in duration (less than 40 ms), larger in amplitude (greater than 2.5 mV) and population spikes were riding on the main deflection. Type 2 interictal spikes were negative in the stratum oriens and positive in the pyramidal layer and stratum radiatum of both CA1 and CA3. The amplitude of both types of interictal spikes could exceed 6 mV. We suggest that interictal spikes were initiated randomly in different subpopulations of the CA2-3 region and the location of the initiating population burst determined the polarity and amplitude of the extracellular interictal spike. Repetitive stimulation of the perforant path (5 Hz, 6 s) evoked markedly uniform afterdischarges in both intact and fimbria fornix-deprived rats. The threshold of afterdischarges was significantly lower, the seizure spread to the contralateral hippocampus was slower, and secondary afterdischarges lasted significantly longer in the lesioned rats. We suggest that under physiological conditions the electrical stability of the hippocampus is ensured by the feed-forward inhibitory action of subcortical afferents. Removal of tonic inhibitory influences and/or sprouting of local axon collaterals allows extreme synchronization and reverberation of information in the entorhinal-hippocampal-entorhinal cortex circuitry. The presence of interictal spikes and increased susceptibility to seizures for several months after the lesion offers the fimbria-fornix-deprived hippocampus a useful chronic preparation to study the mechanisms of limbic epilepsy.