Abstracts

Rationale: Seizure activity leads to increases in extracellular potassium concentration [K⁺] and elevated extracellular potassium concentration [K⁺] alters neuronal and network excitability and can cause seizures. Here we investigated the effects of increased [K⁺] on neuronal activity from layers II & III of the somatosensory cortex in juvenile mice during 4-aminopyridine (4-AP) induced seizure like events (SLEs). Methods: In vitro: Whole cell patch clamp recordings were performed from layer II/III pyramidal cells using a K-gluconate based intracellular solution. Local field potentials (LFPs) were simultaneously recorded from the same layer of the slice and within 150 µm from the patched cell. Alternatively, LFP recordings were paired with a [K⁺] sensitive electrode. In vivo, the extracellular [K⁺] was monitored by a [K⁺] sensitive which was inserted within 150 µm from the LFP-recording pipette into the somatosensory cortex of an anesthetized (ketamine/xylazine) mouse. High frequency electrical stimulation (100 Hz and 50-100 pA) was provided via a concentric bipolar tungsten electrode (Microprobes) placed in the proximity of the recording electrodes. Results: In vitro: As the [K⁺] increased from 3.5mM up to 12mM in 4-AP perfusate, neurons demonstrated the following: a moderate rise of extracellular [K⁺] up to 9mM increased the frequency of SLEs and depolarized the membrane potential, whereas 12mM [K⁺] blocked the SLEs in some slices and left only inter-ictal bursts. 15mM [K⁺] caused marked further depolarization and marked depression of cellular excitability and no LFPs could be recorded. Single cell action potentials and inter-ictal bursts followed by SLEs reappeared after changing perfusate back to 4-AP containing aCSF with a [K⁺] of 3.5mM. The blockade of h current by 2-4mM Cs⁺ or ZD7288 enhance the seizure-blocking effects of [K⁺]. In a subset of human temporal cortical slices removed at epilepsy surgery and seizing in 4-AP solution, raised K (12-15 mM) consistently blocked SLEs. In vivo, we used high-frequency electrical stimulation to evoke epileptiform field responses which were accompanied by a local increase in extracellular [K⁺]. When 4-AP (12mM) was applied to the somatosensory cortex, extracellular [K⁺] rose (8-9 mM) alongside with increased neuronal excitability that eventually turned into an SLE. In line with in vitro observations, a further increase in applied extracellular [K⁺] terminated the SLEs, which rapidly returned upon washout of the raised K. Conclusions: These results suggest the extracellular [K⁺] plays a bidirectional role in regulating neuronal excitability and SLEs. In line with what has been observed in other areas of the brain, our in vitro experiments suggest that a moderate increase in [K⁺] promotes neuronal excitability, whereas further increase diminishes neuronal activity by enhanced Ih and other mechanisms as yet undetermined. In vivo experiments confirmed that raised extracellular [K⁺] in seizing cortex can suppress the epileptiform activity.