Na-K-ATPase functions in the developing hippocampus: regional differences in CA1 and CA3 neuronal excitability and role in epileptiform network bursting.

The Na-K-ATPase (Na-K pump) is essential for setting resting membrane potential and restoring transmembrane Na and K gradients after neuronal firing, yet its roles in developing neurons are not well understood. This study examined the contribution of the Na-K pump to resting membrane potential and membrane excitability of developing CA1 and CA3 neurons and its role in maintaining synchronous network bursting. Experiments were conducted in postnatal day (P)9 to P13 rat hippocampal slices using whole cell patch-clamp and extracellular field-potential recordings. Blockade of the Na-K pump with strophanthidin caused marked depolarization (23.1mV) in CA3 neurons but only a modest depolarization (3.3mV) in CA1 neurons. Regarding other membrane properties, strophanthidin differentially altered the voltage-current responses, input resistance, action-potential threshold and amplitude, rheobase, and input-output relationship in CA3 vs. CA1 neurons. At the network level, strophanthidin stopped synchronous epileptiform bursting in CA3 induced by 0 Mg and 4-aminopyridine. Furthermore, dual whole cell recordings revealed that strophanthidin disrupted the synchrony of CA3 neuronal firing. Finally, strophanthidin reduced spontaneous excitatory postsynaptic current (sEPSC) bursts (i.e., synchronous transmitter release) and transformed them into individual sEPSC events (i.e., nonsynchronous transmitter release). These data suggest that the Na-K pump plays a more profound role in membrane excitability in developing CA3 neurons than in CA1 neurons and that the pump is essential for the maintenance of synchronous network bursting in CA3. Compromised Na-K pump function leads to cessation of ongoing synchronous network activity, by desynchronizing neuronal firing and neurotransmitter release in the CA3 synaptic network. These findings have implications for the regulation of network excitability and seizure generation in the developing brain. Despite the extensive literature showing the importance of the Na-K pump in various neuronal functions, its roles in the developing brain are not well understood. This study reveals that the Na-K pump differentially regulates the excitability of CA3 and CA1 neurons in the developing hippocampus, and the pump activity is crucial for maintaining network activity. Compromised Na-K pump activity desynchronizes neuronal firing and transmitter release, leading to cessation of ongoing epileptiform network bursting.