CHANGES IN EXTRACELLULAR ION CONCENTRATIO NS DURING EPILEPTIFORM ACTIVITY IN THE RAT HIPPOCAMPAL-ENTORHINAL CORTEX SLICE

Changes in extracellular ion concentrations during epileptiform activi ty were investigated in different models using a combined hippocampal- entorhinal cortex slice preparation and ion-sensitive microelectrodes. During convulsive activity an increase in extracellular K+ was found but concentrations of Na+, Mg2+ and Ca2+ decreased. In addition, a sli ght increase in extracellular Cl- was noted. During epileptiform activ ity a shrinkage of the extracellular space of approximately 20% occure d and a prolonged acidotic pH-shift was found after an initial brief a lkaline shift. The consequences of extracellular K+ accumulation inclu de depolarization of membrane potentials, reduced efficacy of K+ curre nts, increased synaptic transmission and decreased inhibition. In cont rast, decreased extracellular Ca2+ and Mg2+ cause activation of NMDA-r eceptors. Such changes caused by convulsive activity may themselves in duce epileptiform activity.