Enhancement of persistent sodium current by internal fluorescence in isolated hippocampal neurons

Following up on an earlier chance observation, voltage-dependent whole-cell currents were recorded from isolated hippocampal neurons filled with the f luorescent dyes Fluo-3 and Fura-red, that were intermittently excited by 48 8 nm laser light. In the absence of any ion channel blocking drugs, in most cells depolarizing voltage steps initially evoked only the 'Hodgkin-Huxley ' type early, fast inward surge followed by sustained outward current. Over 5-20 min of intermittent electrical stimulation and laser-excited fluoresc ence pulses, a voltage-dependent, slowly inactivating inward current also a ppeared and grew, while sustained outward current diminished. When K+ curre nts were blocked, a small persistent inward current was usually detectable immediately, and then it increased in amplitude. This current was blocked b y tetrodotoxin (TTX) and it had current-voltage (I-V) characteristics of a persistent sodium current, I-Na,I-P. In cells not filled with dye but illum inated by laser, and in cells with dye but not illuminated, I-Na,I-P remain ed small. There was a more than 12-fold difference in the maximal amplitude of I-Na,I-P of fluorescent compared to non-fluorescent cells. Once induced I-Na,I-P decreased very slowly. Fluorescence increased the duration but no t the amplitude of the transient Na+ current, I-Na,I-T. With membrane poten tial clamped to a constant voltage, the laser-induced fluorescence did not evoke a membrane current. It is not certain whether fluorescence-induced I- Na,I-P potentiation is related to photodynamic action. (C) 2000 Elsevier Sc ience B.V. All rights reserved.