Ionic currents underlying spontaneous action potentials in isolated cerebellar Purkinje neurons

Acutely dissociated cell bodies of mouse Purkinje neurons spontaneously fir ed action potentials at similar to 50 Hz (25 degrees C). To directly measur e the ionic currents underlying spontaneous activity, we voltage-clamped th e cells using prerecorded spontaneous action potentials (spike trains) as v oltage commands and used ionic substitution and selective blockers to isola te individual currents. The largest current flowing during the interspike i nterval was tetrodotoxin-sensitive sodium current (approximately -50 pA bet ween -65 and -60 mV). Although the neurons had large voltage-dependent calc ium currents, the net current blocked by cobalt substitution for calcium wa s outward at all times during spike trains. Thus, the electrical effect of calcium current is apparently dominated by rapidly activated calcium-depend ent potassium currents. Under current clamp, all cells continued firing spo ntaneously (though similar to 30% more slowly) after block of T-type calciu m current by mibefradil, and most cells continued to fire after block of al l calcium current by cobalt substitution. Although the neurons possessed hy perpolarization-activated cation current (I-h), little current flowed durin g spike trains, and block by 1 mM cesium had no effect on firing frequency. The outward potassium currents underlying the repolarization of the spikes were completely blocked by 1 mM TEA. These currents deactivated quickly (< 1 msec) after each spike. We conclude that the spontaneous firing of Purkin je neuron cell bodies depends mainly on tetrodotoxin-sensitive sodium curre nt flowing between spikes. The high firing rate is promoted by large potass ium currents that repolarize the cell rapidly and deactivate quickly, thus preventing strong hyperpolarization and restoring a high input resistance f or subsequent depolarization.