Calcium-induced calcium release in rat sensory neurons

1. In isolated dorsal root ganglion cells (DRG neurons), changes in the concentration of global cytosolic Ca2+ (delta [Ca2+]c) were measured by the fluorescence of K5-indo-1. Depolarizations from -60 to 0 mV (500 ms) and Ca2+ influx through Ca2+ channels (ICa) increased [Ca2+]c by 480 +/- 113 nM, the peak occurring 542 +/- 76 ms (mean +/- S.E.M.) after repolarization. 2. Ryanodine (10 microM) reduced depolarization-induced delta [Ca2+]c by up to 80% and blocked delta [Ca2+]c induced by 20 mM caffeine. 3. Peak delta [Ca2+]c and peak ICa followed a similar bell-shaped voltage dependence. Removal of extracellular Ca2+ abolished depolarization-induced delta [Ca2+]c; its elevation from 2 to 8 mM increased peak ICa by 30% and delta [Ca2+]c by 108%. 4. Ca2+ influx at 0 mV was graded by pulse durations between 20 and 500 ms. Up to 200 ms, delta [Ca2+]c increased linearly with Ca2+ influx. Depolarizations longer than 200 ms induced a supralinear increase in delta [Ca2+]c that was abolished by caffeine (20 mM). 5. The supralinear increase in delta [Ca2+]c and the caffeine-induced delta [Ca2+]c were measured only in thirteen of nineteen DRG neurons; in the other six of nineteen cells both properties were absent. The results suggest that Ca(2+)-induced Ca2+ release (CICR) is expressed differently in different populations of DRG neurons. 6. A single action potential did not significantly increase [Ca2+]c. Trains of stimuli (20 Hz) induced delta [Ca2+]c that linearly increased with the number of action potentials. Delta [Ca2+]c due to 100 action potentials had a significant ryanodine-sensitive component. 7. It is discussed that CICR can contribute to the depolarization-induced [Ca2+]c, provided the Ca2+ influx lasts for a certain minimum period of time.