Abstracts

RATIONALE: There are few reports of spontaneous interictal-like activity generated by slices prepared from hippocampal tissue resected from patients with intractable temporal lobe epilepsy. We searched for such activity in the subiculum as well as the CA1 and CA3 regions. METHODS: Activity was recorded with both intracellular and extracellular tungsten electrodes. Extracellular signals were filtered to reveal either action potentials or slow potentials. RESULTS: Rhythmic synchronous discharges were generated by the subiculum, but not the CA1 or CA3 regions of intact slices (n=10 slices; 6 patients) or by isolated subicular slices. The duration of rhythmic events was 28 7 ms and their frequency was 1.3 0.9 Hz, similar to that of interictal discharges recorded in previously obtained scalp or deep EEG records. Each recorded slice generated activity continuously for up to 6-8 hrs. Intracellular correlates of population discharges were diverse. Cells showed either large IPSPs (>20mV), EPSP / IPSP sequences, or EPSPs which induced cell firing with each population event. Spontaneous, rhythmic activity was suppressed by either picrotoxin (50 M) or bicuculline (10 M), or by NBQX (5 M) or CNQX (20 M) together with APV (100 M). When fast GABAergic transmission was suppressed, interictal discharges could be rescued by increasing extracellular potassium from its control value of 4mM to 9 mM showing that purely excitatory synaptic mechanisms could suffice for rhythm generation. CONCLUSIONS: These results show that the subiculum, an output region of the hippocampus with less severe cell loss than the CA1 and CA3 zones, generates a synchronous activity in vitro. This rhythmic discharge resembles interictal discharges detected in patient EEG records and involves both excitatory and inhibitory synaptic signalling.