Abstracts

Rationale: Synaptic reorganization and the formation of new excitatory circuits are proposed to be important mechanisms for acquired epilepsy. Dual and triple whole-cell recordings in hippocampal slices from chronically epileptic rats (i.e., several months after status epilepticus) are technically difficult for several reasons, but they have the potential to provide direct evidence for epilepsy-associated alterations in synaptic circuitry. Triple whole-cell recordings from rats that had experienced chronic epilepsy for up to nearly 1 yr were used to analyze network behavior at the single cell level in slices containing the isolate dentate gyrus. Methods: Three micromanipulators for whole-cell recording and another manipulator for extracellular stimulation were mounted on a movable XY-platform, and positioning of the three micromanipulators was under remote control. A multiclamp-700A and an axopatch-1D amplifier were used simultaneously to acquire electrical signals. Hippocampal slices were cut from rats with kainate-induced epilepsy ( up to ~11 months after kainate treatment) at a thickness of 350 μm. Neurons were visualized using infrared illumination and differential interference contrast optics and recorded simultaneously. GABAA and GABAB receptors were blocked to allow investigation of excitatory interactions independent of changes in GABA-mediated inhibition. Results: Triple whole-cell recordings (Fig.1A) were consistently obtained from visualized dentate granule cells in most rats. Spontaneous EPSCs and network bursts of action potentials occurred synchronously in the triple recordings (Fig. 1B). The frequency of spontaneous EPSCs increased before the spontaneous bursts, was maintained during the bursts, and slowly decreased after the synchronous bursts. Possible synaptic and electrical connections between the neurons were examined in the triple recordings (i.e., each triplet allows testing of 6 pairs of neurons). Attempts were also made to influence cellular and network activity by simultaneously and alternatively exciting two or more neurons with depolarizing current pulses. Conclusions: Triple recording from chronically epileptic animals provides unique possibilities for exploring the network behavior and cellular mechanisms of synchronous activity in the granule cell population. These experiments have so far shown that spontaneous network bursts in the dentate gyrus, which occur after synaptic reorganization when GABA receptors are blocked, involve local excitatory synaptic interactions. (Supported by NS16683 from the NIH).