Direct demonstration of persistent Na+ channel activity in dendritic processes of mammalian cortical neurones.

1. Single Na+ channel activity was recorded in patch-clamp, cell-attached experiments performed on dendritic processes of acutely isolated principal neurones from rat entorhinal-cortex layer II. The distances of the recording sites from the soma ranged from approximately 20 to approximately 100 microm. 2. Step depolarisations from holding potentials of -120 to -100 mV to test potentials of -60 to +10 mV elicited Na+ channel openings in all of the recorded patches (n = 16). 3. In 10 patches, besides transient Na+ channel openings clustered within the first few milliseconds of the depolarising pulses, prolonged and/or late Na+ channel openings were also regularly observed. This 'persistent' Na+ channel activity produced net inward, persistent currents in ensemble-average traces, and remained stable over the entire duration of the experiments ( approximately 9 to 30 min). 4. Two of these patches contained < or = 3 channels. In these cases, persistent Na+ channel openings could be attributed to the activity of one single channel. 5. The voltage dependence of persistent-current amplitude in ensemble-average traces closely resembled that of whole-cell, persistent Na+ current expressed by the same neurones, and displayed the same characteristic low threshold of activation. 6. Dendritic, persistent Na+ channel openings had relatively high single-channel conductance ( approximately 20 pS), similar to what is observed for somatic, persistent Na+ channels. 7. We conclude that a stable, persistent Na+ channel activity is expressed by proximal dendrites of entorhinal-cortex layer II principal neurones, and can contribute a significant low-threshold, persistent Na+ current to the dendritic processing of excitatory synaptic inputs.