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Department of Anatomy and Neurobiology, University of Vermont College of Medicine, Burlington, VT 05405, USA

Experiments were done using guinea-pig sympathetic neurones dissociated from the stellate ganglia to establish whether calcium-induced calcium release (CICR) modulated action potential (AP) generation in mammalian neurones. Using measurements of intracellular calcium ([Ca²⁺]_i) with the Ca²⁺-sensitive dye fluo-3, we demonstrated that 10 mM caffeine activated ryanodine receptors and caused a rise in [Ca²⁺]_i in both Ca²⁺-containing and Ca²⁺-deficient solutions. We also demonstrated that combined treatment with caffeine and 1 µM thapsigargin or caffeine and 20 µM ryanodine blocked subsequent caffeine-induced elevations of [Ca²⁺]_i. Treatment with thapsigargin, ryanodine or 200 µM Cd²⁺ to disrupt CICR decreased the latency to AP generation during 400 ms depolarizing current ramps using the perforated patch whole cell patch clamp in current clamp mode. Treatment with 500 µM tetraethylammonium also decreased the latency to AP generation during depolarizing current ramps in control cells, but not in cells pretreated with thapsigargin to deplete internal Ca²⁺ stores. In summary, we propose that an outward current, carried at least in part through BK channels, is activated by CICR at membrane voltages approaching the threshold for AP initiation and that this current opposed depolarizing current ramps applied to guinea-pig sympathetic stellate neurones.

(Received 10 December 2003; accepted after revision 14 January 2004; first published online 14 January 2004)
Corresponding author R. L. Parsons: Department of Anatomy and Neurobiology, University of Vermont College of Medicine, Burlington, VT 05405, USA.  Email: Rodney.Parsons{at}uvm.edu

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