The Journal of Neurophysiology Vol. 86 No. 1 July 2001, pp. 190-196
Copyright ©2001 by the American Physiological Society

A Novel Ca²⁺ Influx Pathway in Mammalian Primary Sensory Neurons Is Activated by Caffeine

 ¹Department of Physiology, Medical Biotechnology Center, University of Maryland Biotechnology Institute; and  ²Department of Pharmacology and Experimental Therapeutics, University of Maryland School of Medicine, Baltimore, Maryland 21201

Hoesch, Robert E., Daniel Weinreich, and Joseph P. Y. Kao. A Novel Ca²⁺ Influx Pathway in Mammalian Primary Sensory Neurons Is Activated by Caffeine. J. Neurophysiol. 86: 190-196, 2001. Single-cell microfluorimetry and electrophysiology techniques were used to identify and characterize a novel Ca²⁺ influx pathway in adult rabbit vagal sensory neurons. Acutely dissociated nodose ganglion neurons (NGNs) exhibit robust Ca²⁺-induced Ca²⁺ release (CICR) that can be triggered by 10 mM caffeine, the classic agonist of CICR. A caffeine-induced increase in cytosolic-free Ca²⁺ concentration ([Ca²⁺]_i) is considered diagnostic evidence of the existence of CICR. However, when CICR was disabled through depletion of intracellular Ca²⁺ stores or pharmacological blockade of intracellular Ca²⁺ release channels (ryanodine receptors), caffeine still elicited a significant rise in [Ca²⁺]_i in ~50% of NGNs. The same response was not elicited by pharmacological agents that elevate cyclic nucleotide concentrations. Moreover, extracellular Ca²⁺ was obligatory for such caffeine-induced [Ca²⁺]_i rises in this population of NGNs, suggesting that Ca²⁺ 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 [Ca²⁺]_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 EC₅₀ values for caffeine-induced CICR and for caffeine-activated current were 1.5 and ~0.6 mM, respectively. These results indicate that caffeine-induced rises in [Ca²⁺]_i, in the presence of extracellular Ca²⁺, can no longer be interpreted as unequivocal diagnostic evidence for CICR in neurons. These results also indicate that sensory neurons possess a novel Ca²⁺ influx pathway.