A Novel Ca super(2+) Influx Pathway in Mammalian Primary Sensory Neurons Is Activated by Caffeine

Single-cell microfluorimetry and electrophysiology techniques were used to identify and characterize a novel Ca super(2+) influx pathway in adult rabbit vagal sensory neurons. Acutely dissociated nodose ganglion neurons (NGNs) exhibit robust Ca super(2+)-induced Ca super(2+) release (CICR) that can be triggered by 10 mM caffeine, the classic agonist of CICR. A caffeine-induced increase in cytosolic-free Ca super(2+) concentration ([Ca super(2+)] sub(i)) is considered diagnostic evidence of the existence of CICR. However, when CICR was disabled through depletion of intracellular Ca super(2+) stores or pharmacological blockade of intracellular Ca super(2+) release channels (ryanodine receptors), caffeine still elicited a significant rise in [Ca super(2+)] sub(i) in similar to 50% of NGNs. The same response was not elicited by pharmacological agents that elevate cyclic nucleotide concentrations. Moreover, extracellular Ca super(2+) was obligatory for such caffeine-induced [Ca super(2+)] sub(i) rises in this population of NGNs, suggesting that Ca super(2+) influx is responsible for this rise. Simultaneous microfluorimetry with whole cell patch-clamp studies showed that caffeine activates an inward current that temporally parallels the rise in [Ca super(2+)] sub(i). The inward current had a reversal potential of +8.1 plus or minus 6.1 (SE) mV (n = 4), a mean peak amplitude of - 126 plus or minus 24 pA (n = 4) at E sub(m) = -50 mV, and a slope conductance of 1.43 plus or minus 0.79 nS (n = 4). Estimated EC sub(50) values for caffeine-induced CICR and for caffeine-activated current were 1.5 and similar to 0.6 mM, respectively. These results indicate that caffeine-induced rises in [Ca super(2+)] sub(i), in the presence of extracellular Ca super(2+), can no longer be interpreted as unequivocal diagnostic evidence for CICR in neurons. These results also indicate that sensory neurons possess a novel Ca super(2+) influx pathway.