Synaptic and nonsynaptic ictogenesis occurs at different temperatures in submerged and interface rat brain slices

To investigate the temperature sensitivity of low-Ca2+-induced nonsynaptic and low-Mg2+-induced synaptic ictogenesis under submerged and interface conditions, we compared changes of extracellular field potential and extracellular potassium concentration at room temperature (23 +/- 1 degrees C; mean +/- SD) and at 35 +/- 1 degrees C in hippocampal-entorhinal cortex slices. The induction of spontaneous epileptiform activity under interface conditions occurred at 35 +/- 1 degrees C in both models. In contrast, under submerged conditions, spontaneous epileptiform activity in low-Mg2+ artificial cerebrospinal fluid (ACSF) was observed at 35 +/- 1 degrees C, whereas epileptiform discharges induced by low-Ca2+ ACSF occurred only at room temperature. To investigate the different temperature effects under submerged and interface conditions, measurements of extra- and intracellular pH and extracellular space volume were performed. Lowering the temperature from 35 +/- 1 degrees C to room temperature effected a reduction in extracellular pH under submerged and interface conditions. Under submerged conditions, temperature changes had no significant influence on the intracellular pH in presence of either normal or low-Mg2+ ACSF. In contrast, application of low-Ca2+ ACSF effected a significant increase in intracellular pH at room temperature but not at 35 +/- 1 degrees C under submerged conditions. Therefore increasing intracellular pH by lowering the temperature in low-Ca2+ ACSF may push slices to spontaneous epileptiform activity by opening gap junctions. Finally, extracellular space volume significantly decreased by switching from submerged to interface conditions. The reduced extracellular space volume under interface conditions may lead to an enlarged ephaptic transmission and therefore promotes low-Mg2+- and low-Ca2+-induced spontaneous epileptiform activity. The results of the study indicate that gas-liquid interface and total-liquid submerged slice states impart distinct physiological parameters on brain tissue.