Abstracts

Rationale: The use of the word spike by electrophysiologists can be confusing. At its most basic, spikes are action potentials generated by single neurons. Population spikes ( pop spikes ) are fast (1-5 msec duration) local field potentials generated by synchronously firing neurons. EEG spikes (or interictal discharges (IDs)) are 20-70 msec events recorded mostly by macroelectrodes, that represent the summed currents generated by synaptic events occurring relatively synchronously in the brain region under the recording electrode. IDs can occur during interictal periods or as part of a seizure. Spontaneous pop spikes have been identified in seizures thought to involve dentate gyrus granule cells (DG GCs), but whether they are limited to ictal or peri-ictal periods, is not known. Short bursts of small pop spikes have been hypothesized to underlie high frequency oscillations (HFOs) recorded from epileptic tissue.Methods: We recorded spontaneous and paired pulse stimulation (PPS)-evoked pop spikes from the dentate gyrus of awake behaving rats before and after being induced to develop epilepsy by prolonged perforant path stimulation (8 hour unilateral or 3 hour bilateral) and after pilocarpine-induced status epilepticus. Recordings were made during interictal, ictal, and postictal periods.(2000 kHz, 0.1, 600 Hz filters).Results: Small, spontaneous pop spikes are very difficult to identify in background recordings compared to large evoked pop spikes or large spontaneous pop spikes detected during seizures. Large pop spikes are seen during, and for short periods after, spontaneous seizures. Spontaneous small pop spike-like events are occasionally identified in na ve, newly implanted rats that do not have epilepsy. Spontaneous large pop spikes are seen during interictal periods in some epileptic rats (often riding on interictal discharges) and groups of small pop spike-like events (or HFOs) are also seen in DG electrodes during interictal periods. During prolonged monitoring, with PPS administered once per hour, evoked fEPSPs and pop spikes, and paired pulse depression, are easily detectable and quite invariant.Conclusions: DG GCs fire synchronously (large pop spikes) during seizures and postictal periods and also occasionally during interictal periods. It is likely that smaller groups of GCs fire together (small pop spike-like events), often in brief bursts, during interictal periods and these are recorded as HFOs in the intracranial EEG. Supported by NS 41811.