Abstracts

Rationale: Rapid cooling can terminate spontaneous epileptiform discharges in animal models of epilepsy. However, the mechanism of this effect has not been clearly established. We hypothesized that hypothermia may interrupt synchrony between the brain networks in the brain. We used a 60-channel perforated multi-electrode array (MEA) system to record local field potentials and multi-units simultaneously from various hippocampal and cortical layers to study the effect of cooling on synchrony. Methods: Simultaneous field recordings through a 60-channel MEA were carried out from P14-15 mouse coronal slices containing hippocampus and cortex. The slices were perfused with 100 μM 4-aminopyridine and a reduced concentration of MgCl2 (0.6 mM), at a baseline temperature of 30-32οC. Bicuculline (BMR) (25 μM) or Gaboxadol (THIP, 10 μM) were added in some experiments. Cooling was induced at a rate of 0.1-0.5οC/s by switching to cold aCSF to lower temperature to <20οC and warming was achieved at a slower rate through an inline heater. Results: We recorded local field potentials from distinct hippocampal regions that showed synchronized runs of interictal discharges initiated in CA3. In contrast, cortical electrodes displayed more frequent ictal-like discharges that were not synchronized with hippocampal regions. Long bursts of multiunit firing were recorded from individual electrodes in cortex and hippocampus that were not synchronous with the epileptiform discharges. Cooling decreased the interictal discharges in hippocampus, abolished the ictal-like discharges in the cortex, and disrupted the long bursting of multiunit activity by changing it to continuous firing. This was particularly true for electrodes located in hippocampus at the border between the stratum radiatum and lacunosum molecularis, a region rich in GABA-ergic interneurons. Perfusion with BMR altered the network activity both in hippocampus and in cortex. Runs of interictal events in hippocampus were replaced by more frequent brief (~6s) ictal-like events. Cortical ictal-like events were much shorter and more frequent but still independent from hippocampal activity. Multiunit activities changed to brief synchronous burst firing limited to individual electrodes or electrodes pairs. Cooling abolished all epileptiform discharges but not the continuous multiunit firing. We also studied the effect of GABAa receptor agonist THIP with preference for extrasynaptic tonic conductance that is shown to be predominant in GABA-ergic interneurons. THIP Increased the interval between runs of interictal events and abolished most multiunit firing. Cooling effects were decreased in the presence of both drugs. Conclusions: MEA recordings reveal independent interictal and ictal-like discharges in cortex and hippocampus. Hypothermia reversibly abolishes 4-AP induced epileptiform discharges in all hippocampal and cortical sites while preserving spontaneous multiunit firing. Blockade or activation of GABAa receptor alters network activity and interferes with the effect of cooling.