The Roles of BK Potassium Channels in Neuronal Excitability Differ in Excitatory and Inhibitory Hippocampal Neurons During Early Development

Abstract

BK potassium channels are are activated by intracellular Ca+ and depolarization, opening during action potential firing and speeding repolarization. Because of this, they are key regulators of fast spiking activity and important determinants of neuronal excitability. Due to their role in regulating excitability as well as their broad expression in both excitable and non‐excitable tissues, BK channels are implicated in many pathological states including epilepsy. The BK channel antagonist, paxilline, has been shown to delay seizure induction in adult rodents that have had prior seizures, but not in seizure naïve rodents. History of seizures has been shown induce expression of more Ca2+ sensitive BK channel splice variants, which may underlie the difference in paxilline efficacy between the two states. Because expression of more Ca2+ sensitive isoforms predominates in neonatal rodents ‐similar to in the epileptic adults, we hypothesize that paxilline may be an effective inhibitor of seizure activity even in seizure‐naïve neonatal rodents. Our first goal is to understand how developmental shifts in BK channel expression and function map to changes in neuronal and circuit excitability. Our second goal is to test whether BK channel blockade can modulate seizure activity in neonates, as there is a need to identify pharmacological targets for seizures in neonates and children, which are often intractable to conventional treatments.To test how BK channel function changes with age and by cell type we measured the effect of paxilline on action potential (AP) firing ‐both on single spike kinetics and prolonged firing‐ in two classes of hippocampal neurons, CA1 pyramidal neurons and parvalbumin interneurons (PVIns), a key inhibitory component of local hippocampal circuits. We used P4‐5, P9‐10, and P14‐15 C57BL/6J mice with labelled PVIns. We also tested the efficacy of paxilline in reducing hyperexcitability evoked by application of 0 Mg2+ aCSF in brain slices, an established seizure model.We patched pyramidal neurons and PVIns in whole‐cell current‐clamp configuration and evoked APs using square depolarizing pulses. BK channel blockade with 1mM paxilline lengthens duration of APs in P4‐5 and P9‐10 pyramidal neurons and P4‐5 PVIns but does not affect P9‐10 PVIn AP duration (paired t‐tests, p<0.05). While BK blockade has the least effect on single APs in the P9‐10 PVIns, it has the most effect on the input‐output relationship and spike‐frequency accommodation in these neurons (3‐way ANOVA, p<0.001 for interaction of age and cell type in input‐output; 3‐way ANOVA p<0.05 for main effect of age in frequency adaptation).We measured the effect of BK channel blockade in controlling hyperexcitability in P4‐15 mice. We found that 0 Mg2+ aCSF application induced spontaneous AP firing in patched hippocampal pyramidal neurons within 3.86±0.87 minutes but when 0 Mg2+ aCSF was coapplied with 1 mM paxilline the average onset of spontaneous activity occurred at 8.43±1.40 minutes (t‐test p<0.05).For the first goal, we show that BK channels’ relative importance in regulating AP firing changes with age and by neuronal population. Toward the second goal we report that BK channel blockade can reduce hyperexcitability in a brain slice model of epilepsy.