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 e xperiments performed on dendritic processes of acutely isolated principal n eurones from rat entorhinal-cortex layer II. The distances of the recording sites from the soma ranged from similar to 20 to similar to 100 mu m. 2. Step depolarisations from holding potentials of -120 to -100 mV to test potentials of -60 to +10 InV elicited Na+ channel openings in all of the re corded patches (n = 16). 3. In 10 patches, besides transient Na+ channel openings clustered within t he first few milliseconds of the depolarising pulses, prolonged and/or late Na+ channel openings were also regularly observed. This 'persistent' Na+ c hannel activity produced net inward, persistent currents in ensemble-averag e traces, and remained stable over the entire duration of the experiments ( similar to 9 to 30 min). 4. Two of these patches contained less than or equal to 3 channels. In thes e cases, persistent Na+ channel openings could be attributed to the activit y of one single channel. 5. The voltage dependence of persistent-current amplitude in ensemble-avera ge traces closely resembled that of whole-cell, persistent Na+ current expr essed by the same neurones, and displayed the same characteristic low thres hold of activation. 6. Dendritic, persistent Na+ channel openings had relatively high single-ch annel conductance (similar to 20 pS), similar to what is observed for somat ic, 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.