Effects of Potassium Concentration on Firing Patterns of Low‐Calcium Epileptiform Activity in Anesthetized Rat Hippocampus: Inducing of Persistent Spike Activity

Purpose: It has been shown that a low‐calcium high‐potassium solution can generate ictal‐like epileptiform activity in vitro and in vivo. Moreover, during status epileptiform activity, the concentration of [K+]o increases, and the concentration of [Ca2+]o decreases in brain tissue. Therefore we tested the hypothesis that long‐lasting persistent spike activity, similar to one of the patterns of status epilepticus, could be generated by a high‐potassium, low‐calcium solution in the hippocampus in vivo. Methods: Artificial cerebrospinal fluid was perfused over the surface of the exposed left dorsal hippocampus of anesthetized rats. A stimulating electrode and a recording probe were placed in the CA1 region. Results: By elevating K+ concentration from 6 to 12 mM in the perfusate solution, the typical firing pattern of low‐calcium ictal bursts was transformed into persistent spike activity in the CA1 region with synaptic transmission being suppressed by calcium chelator EGTA. The activity was characterized by double spikes repeated at a frequency ∼4 Hz that could last for >1 h. The analysis of multiple unit activity showed that both elevating [K+]o and lowering [Ca2+]o decreased the inhibition period after the response of paired‐pulse stimulation, indicating a suppression of the after‐hyperpolarization (AHP) activity. Conclusions: These results suggest that persistent status epilepticus–like spike activity can be induced by nonsynaptic mechanisms when synaptic transmission is blocked. The unique double‐spike pattern of this activity is presumably caused by higher K+ concentration augmenting the frequency of typical low‐calcium nonsynaptic burst activity.