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.
Somjen
G G
GG
Department of Cell Biology, Box 3709, Duke University Medical Center, Durham, NC 27710, USA. g.somjen@cellbio.duke.edu
D000818:Animals; D005071:Evoked Potentials / Q000187:drug effects / Q000502:physiology; D006624:Hippocampus / Q000166:cytology*; D066298:In Vitro Techniques; D007834:Lasers; D008856:Microscopy, Fluorescence; D009474:Neurons / Q000166:cytology / Q000378:metabolism*; D011188:Potassium / Q000378:metabolism; D051381:Rats; D012964:Sodium / Q000378:metabolism*; D015222:Sodium Channels / Q000378:metabolism*; D019789:Tetraethylammonium / Q000494:pharmacology; D013779:Tetrodotoxin / Q000494:pharmacology
D015222:Sodium Channels; D013779:Tetrodotoxin; D019789:Tetraethylammonium; D012964:Sodium; D011188:Potassium