Evidence for persistent Na+ current in apical dendrites of rat neocortical neurons from imaging of Na+-sensitive dye.

Evidence for a persistent Na+ current (I(NaP)) in the apical dendrite of neocortical neurons was sought with the use of fluorescence imaging to measure changes in intradendritic Na+ concentration. Neurons in neocortical brain slices were filled iontophoretically through an intracellular recording microelectrode with the Na+-sensitive dye benzofuran isophthalate (SBFI), and fluorescence images were recorded with a cooled charge-coupled device camera system using 380-nm illumination. In the presence of Ca2+ and K+ channel blockers, a short depolarizing current pulse evoked a single action potential followed by a plateau depolarization (PD) lasting >1 s. This tetrodotoxin (TTX)-sensitive PD is known to be maintained by I(NaP). A single action potential caused no detectable SBFI fluorescence change, whereas the PD was associated with an SBFI fluorescence change in the soma and apical dendrite indicating increased intracellular Na+ concentration. Determination of the full spatial extent of the dendritic fluorescence change was prevented by our inability to detect the dim fluorescence signal in the distal regions of the apical dendrite. In each experiment the fluorescence change extended into the apical dendrite as far as dye could be visualized (50-300 microm). A slow, depolarizing voltage-clamp ramp that activated I(NaP) caused similar fluorescence changes that were eliminated by TTX, indicating that the SBFI fluorescence changes are caused by Na+ influx due to I(NaP) activation. We conclude that I(NaP) can be generated by the apical dendritic membrane to at least 300 microm from the soma.