GENERATION AND PROPAGATION OF EPILEPTIFORM DISCHARGES IN A COMBINED ENTORHINAL CORTEX HIPPOCAMPAL SLICE

1. The development of epileptiform discharges in response to tetanic s timulation of the Schaeffer collaterals was studied by using extracell ular field potential recordings in CA 1, CA3, dentate gyrus, and entor hinal cortex and intracellular recordings in CA1 neurons in rat hippoc ampal-parahippocampal slices, which were cut so as to maintain recipro cal connections between entorhinal cortex and hippocampus in vitro. 2. The first type of epileptiform discharge to develop was an immediate afterdischarge, which grew in duration and amplitude with repeated sti mulation trains at 10-min intervals, until it plateaued after five to nine trains at 40-s duration, on average. This afterdischarge, when fu lly developed, consisted of an early, high frequency tonic component, followed by a later, lower frequency clonic component. Fully developed primary afterdischarges were all-or-none, in that they had a definite threshold, and varied little in amplitude or duration when activated by threshold or suprathreshold stimulation. The primary discharge coul d be recorded simultaneously throughout the hippocampal-parahippocampa l slice, providing evidence for the intact reciprocal connections betw een hippocampus and entorhinal cortex. Intracellular recordings in CA1 neurons revealed that during the tonic phase of the afterdischarge, n eurons were depolarized by 15-30 mV and gradually repolarized during t he clonic component. 3. After full development of the primary afterdis charge, a delayed secondary epileptiform discharge began to appear aft er five to nine stimulation trains. This late discharge began 2-5 min after the stimulation train and progressed in amplitude and duration w ith repeated stimulation, in some cases to 2-3 h long self-sustained e pileptiform discharges. Like the primary afterdischarge, the secondary discharge could be recorded simultaneously throughout the hippocampal -parahippocampal slice, and individual bursts comprising the secondary discharge occurred at earliest latency in the dentate gyrus, followed by activation in CA3, CA 1, and finally in the entorhinal cortex. Int racellular recordings in CA1 neurons established that the secondary di scharge occurred without an accompanying depolarization. Rather, it ap peared as synaptic bursts developing in an escalating frequency barrag e, initiated 2-5 min after the primary afterdischarge. 4. Lesioning st udies were conducted to begin determining the site of origin of the se condary epileptiform discharge. After appearance of the secondary disc harge, the mossy fibers were cut. This lesion abolished the secondary discharge but did not block the primary afterdischarge. Moving the sti mulating electrodes from the Schaeffer collaterals to the mossy fibers proximal to the cut reestablished a truncated secondary discharge. In a second lesioning experiment, a cut was made through the subicular r egion of the hippocampal-parahippocampal slice before the onset of sti mulation. In these slices, primary afterdischarges progressed normally , but secondary discharges never were seen, even after 12-14 trains. T hese lesion experiments demonstrated the dependence of the secondary d ischarge on intact hippocampal-parahippocampal connections and implica ted the mossy fiber/CA3 synapse, at least in part, as participating in generation of the secondary discharge. 5. The sensitivity of well-dev eloped primary and secondary epileptiform discharges to the N-methyl-D -aspartate antagonist 2-amino-5-phosphonovalerate (APV) was assessed. Both the primary and secondary afterdischarge were reversibly potentia ted in frequency and duration by bath application of 50 muM APV. 6. Th e ability to record these complex, generalized secondary epileptiform discharges in vitro may provide a promising system with which to devel op new insight into mechanisms important in generation of secondary fo ci, in seizure propagation, and in control of long-duration, self-sust ained seizure discharges like those of Status Epilepticus.