Differential effects of Na⁺-K⁺ ATPase blockade on cortical layer V neurons

Sodium-potassium ATPase ('Na⁺-K⁺ ATPase') contributes to the maintenance of the resting membrane potential and the transmembrane gradients for Na⁺ and K⁺ in neurons. Activation of Na⁺-K⁺ ATPase may be important in controlling increases in intracellular sodium during periods of increased neuronal activity. Down-regulation of Na⁺-K⁺ ATPase activity is implicated in numerous CNS disorders, including epilepsy. Although Na⁺-K⁺ ATPase is present in all neurons, little is known about its activity in different subclasses of neocortical cells. We assessed the physiological properties of Na⁺-K⁺ ATPase in fast-spiking (FS) interneurons and pyramidal (PYR) cells to test the hypothesis that Na⁺-K⁺ ATPase activity would be relatively greater in neurons that generated high frequency action potentials (the FS cells). Whole-cell patch clamp recordings were made from FS and PYR neurons in layer V of rat sensorimotor cortical slices maintained in vitro using standard techniques. Bath perfusion of Na⁺-K⁺ ATPase antagonists (ouabain or dihydro-ouabain) induced either a membrane depolarization in current clamp, or inward current under voltage clamp in both cell types. PYR neurons were divided into two subpopulations based on the amplitude of the voltage or current shift in response to Na⁺-K⁺ ATPase blockade. The two PYR cell groups did not differ significantly in electrophysiological properties including resting membrane potential, firing pattern, input resistance and capacitance. Membrane voltage responses of FS cells to Na⁺-K⁺ ATPase blockade were intermediate between the two PYR cell groups (P < 0.05). The resting Na⁺-K⁺ ATPase current density in FS interneurons, assessed by application of blockers, was 3- to 7-fold larger than in either group of PYR neurons. Na⁺-K⁺ ATPase activity was increased either through direct Na⁺ loading via the patch pipette or by focal application of glutamate (20 mm puffs). Under these conditions FS interneurons exhibited the largest increase in Na⁺-K⁺ ATPase activity. We conclude that resting Na⁺-K⁺ ATPase activity and sensitivity to changes in internal Na⁺ concentration vary between and within classes of cortical neurons. These differences may have important consequences in pathophysiological disorders associated with down-regulation of Na⁺-K⁺ ATPase and hyperexcitability within cortical networks. The capacity of neurons to respond appropriately to normal and pathophysiological levels of excitation is dependent on their ability to establish, regulate and maintain their ionic and electrical homeostasis. The sodium-potassium ATPase (the 'sodium pump') is the enzyme primarily responsible for this task through its active transport of Na⁺ and K⁺ ions across the cell membrane. Sodium pump dysfunction has been implicated in numerous CNS disorders and often leads to enhanced neuronal excitation, as may occur in epilepsy. The sodium pump is activated by increases in intracellular Na⁺ that result from synaptic depolarization and cell firing, making its actions particularly important in cells that discharge at high frequencies, such as fast-spiking inhibitory interneurons. The primary neuronal output of the cerebral cortex is through excitatory pyramidal neurons in layer V, whose activity is in turn tightly regulated by these inhibitory interneurons. We demonstrate that fast-spiking inhibitory interneurons possess uniquely high levels of Na⁺-K⁺ ATPase activity that may be vital in maintaining and regulating normal neuronal activity during periods of enhanced excitability.