A novel Ca(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(2+) influx pathway in adult rabbit vagal sensory neurons. Acutely dissociated nodose ganglion neurons (NGNs) exhibit robust Ca(2+)-induced Ca(2+) release (CICR) that can be triggered by 10 mM caffeine, the classic agonist of CICR. A caffeine-induced increase in cytosolic-free Ca(2+) concentration ([Ca(2+)](i)) is considered diagnostic evidence of the existence of CICR. However, when CICR was disabled through depletion of intracellular Ca(2+) stores or pharmacological blockade of intracellular Ca(2+) release channels (ryanodine receptors), caffeine still elicited a significant rise in [Ca(2+)](i) in approximately 50% of NGNs. The same response was not elicited by pharmacological agents that elevate cyclic nucleotide concentrations. Moreover, extracellular Ca(2+) was obligatory for such caffeine-induced [Ca(2+)](i) rises in this population of NGNs, suggesting that Ca(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(2+)](i). The inward current had a reversal potential of +8.1 +/- 6.1 (SE) mV (n = 4), a mean peak amplitude of -126 +/- 24 pA (n = 4) at E(m) = -50 mV, and a slope conductance of 1.43 +/- 0.79 nS (n = 4). Estimated EC(50) values for caffeine-induced CICR and for caffeine-activated current were 1.5 and approximately 0.6 mM, respectively. These results indicate that caffeine-induced rises in [Ca(2+)](i), in the presence of extracellular Ca(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(2+) influx pathway.
Hoesch
R E
RE
Department of Physiology, Medical Biotechnology Center, University of Maryland Biotechnology Institute, Baltimore, Maryland 21201, USA.
Weinreich
D
D
Kao
J P
JP
D000818:Animals; D002110:Caffeine / Q000494:pharmacology*; D002118:Calcium / Q000493:pharmacokinetics*; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors / Q000378:metabolism; D000697:Central Nervous System Stimulants / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D004594:Electrophysiology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005260:Female; D006873:Hydroquinones / Q000494:pharmacology; D008297:Male; D008322:Mammals; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D009475:Neurons, Afferent / Q000187:drug effects / Q000378:metabolism*; D011817:Rabbits; D012433:Ryanodine / Q000494:pharmacology
D000697:Central Nervous System Stimulants; D004791:Enzyme Inhibitors; D006873:Hydroquinones; D012433:Ryanodine; C019359:2,5-di-tert-butylhydroquinone; D002110:Caffeine; D000252:Calcium-Transporting ATPases; D002118:Calcium