EVIDENCE FOR PERSISTENT NA-SENSITIVE DYE( CURRENT IN APICAL DENDRITESOF RAT NEOCORTICAL NEURONS FROM IMAGING OF NA+)

Evidence for a persistent Na+ current (I-NaP) in the apical dendrite o f neocortical neurons was sought with the use of fluorescence imaging to measure changes in intradendritic Na+ concentration. Neurons in neo cortical brain slices were tilled iontophoretically through an intrace llular recording microelectrode with the Na+-sensitive dye benzofuran isophthalate (SBFI), and fluorescence images were recorded with a cool ed charge-coupled device camera system using 380-nm illumination. In t he presence of Ca2+ and K+ channel blockers, a short depolarizing curr ent pulse evoked a single action potential followed by a plateau depol arization (PD) lasting >1 s. This tetrodotoxin (TTX)-sensitive PD is k nown to be maintained by I-NaP. A single action potential caused no de tectable 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 o ur inability to detect the dim fluorescence signal in the distal regio ns of the apical dendrite. In each experiment the fluorescence change extended into the apical dendrite as far as dye could be visualized (5 0-300 mu m). A slow, depolarizing voltage-clamp ramp that activated I- NaP caused similar fluorescence changes that were eliminated by mt, in dicating that the SBFI fluorescence changes are caused by Na+ influx d ue to I-Nap activation. We conclude that I-NaP can be generated by the apical dendritic membrane to at least 300 mu m from the soma.