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 fluorescent dyes Fluo-3 and Fura-red, that were intermittently excited by 488 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 fluorescence pulses, a voltage-dependent, slowly inactivating inward current also appeared and grew, while sustained outward current diminished. When K + currents were blocked, a small persistent inward current was usually detectable immediately, and then it increased in amplitude. This current was blocked by tetrodotoxin (TTX) and it had current–voltage ( I–V) characteristics of a persistent sodium current, I Na,P. In cells not filled with dye but illuminated by laser, and in cells with dye but not illuminated, I Na,P remained small. There was a more than 12-fold difference in the maximal amplitude of I Na,P of fluorescent compared to non-fluorescent cells. Once induced, I Na,P decreased very slowly. Fluorescence increased the duration but not the amplitude of the transient Na + current, I Na,T. With membrane potential clamped to a constant voltage, the laser-induced fluorescence did not evoke a membrane current. It is not certain whether fluorescence-induced I Na,P potentiation is related to photodynamic action.