High Conductance Sustained Single-Channel Activity Responsible for the Low-Threshold Persistent Na+ Current in Entorhinal Cortex Neurons

Stellate cells from entorhinal cortex (EC) layer II express both a transient Na+ current (INa) and a low-threshold persistent Na+ current (INaP) that helps to generate intrinsic theta-like oscillatory activity. We have used single-channel patch-clamp recording to investigate the Na+ channels responsible forINaP in EC stellate cells. Macropatch (more than six channels) recordings showed high levels of transient Na+ channel activity, consisting of brief openings near the beginning of depolarizing pulses, and lower levels of persistent Na+ channel activity, characterized by prolonged openings throughout 500 msec long depolarizations. The persistent activity contributed a noninactivating component to averaged macropatch recordings that was comparable with whole-cellINaP in both voltage dependence of activation (10 mV negative to the transient current) and amplitude (1% of the transient current at −20 mV). In 14 oligochannel (less than six channels) patches, the ratio of transient to persistent channel activity varied from patch to patch, with 10 patches exhibiting exclusively transient openings and one patch showing exclusively persistent openings. In two patches containing only a single persistent channel, prolonged openings were observed in >50% of test depolarizations. Moreover, persistent openings had a significantly higher single-channel conductance (19.7 pS) than transient openings (15.6 pS). We conclude that this stable high-conductance persistent channel activity is responsible for INaP in EC stellate cells. This persistent channel behavior is more enduring and has a higher conductance than the infrequent and short-lived transitions to persistent gating modes that have been described previously in brain neurons.