Regulation of extracellular potassium in the developing hippocampus

It has long been felt that one of the reasons that the immature brain has an increased propensity to seize is due to an inability to regulate the extracellular potassium ([K+]o). However, the data supporting this hypothesis is controversial. Here, we tested the regulation of [K+]o in urethane-anesthetized juvenile and adult rats during and after synchronized epileptiform activity induced by 20 Hz stimulus trains to the CA3 region. The regulation of [K+]o in CA3 was compared to the same measurements in CA1. Across all age groups, there was no difference between CA1 and CA3. There was a slight decrease in the peak level of [K+]o reached during stimulation only in the youngest age groups tested (PN9-11). There was an age-dependent change in the rate of recovery of [K+]o from the elevated levels. This recovery was slowest in the PN9-11 age group. In all the animals in the PN9-11 group and 3/7 animals in the PN14-15 group, there was a secondary increase in the [K+]o during the afterdischarge. This secondary rise was never observed in animals over 15 days of age. These data confirm that there is altered regulation of [K+]o in the developing brain that takes two forms: (1) the ability to recover from elevated [K+]o levels, suggesting maturation of regulatory, or uptake, mechanisms; and (2) that which is related to mechanisms of synchronization and initiation of afterdischarges and the level of [K+]o that is produced during the discharge.