Calcium(2+) physiology in primary vagal sensory neurons

Abstract

Microfluorimetric, electrophysiological, pharmacological, and intracellular photorelease techniques were used to study Ca2+ physiology in rabbit vagal sensory neurons (nodose ganglion neurons, NGNs). NGNs exhibit robust Ca2+-induced Ca2+ release (CICR) that can be triggered by caffeine, the classic CICR agonist. A caffeine-induced increase in cytosolic free Ca2+ concentration ([Ca2+ ]i) was traditionally taken as diagnostic of the existence of CICR. However, when CICR was disabled through depletion of intracellular Ca2+ stores or pharmacological blockade of intracellular Ca 2+ release channels (ryanodine receptors, RyRs), caffeine still elicited a significant rise in [Ca2+]i in ∼50% of NGNs. We demonstrated that this rise in [Ca2+]i results from Ca2+ influx, and that Ca2+ influx is one component of a non-selective cation current that is activated by caffeine. Therefore, in approximately half of all NGNs, caffeine elicits both CICR and Ca2+ influx. We determined that d-myo-inositol 1,4,5-trisphosphate receptors (IP3Rs), another type of intracellular Ca2+ release channel, coexist with RyRs in NGNs. ATP, an extracellular physiological signaling molecule, consistently evoked robust transient increases in [Ca 2+]i (Ca2+ transients) that have a dual origin: (1) Ca2+ influx via P2X receptors and voltage-gated Ca2+ channels; and (2) intracellular Ca2+ release, via IP3Rs and RyRs, that requires activation of P2Y receptors and phospholipase C. We determined that Ca2+ transients evoked by IP3 photorelease can gate a Ca2+-activated K+ current (I IP3) in NGNs. IIP3 is unaffected by three common antagonists of Ca2+-activated K+ currents: iberiotoxin, apamin, and 8-Br-CAMP. Using caffeine to selectively deplete intracellular Ca2+ stores, we show that while CICR does not contribute significantly to global IP3-evoked Ca2+ transients, surprisingly, ∼20% of IIP3 is activated by CICR. We propose a model of Ca2+ signaling microdomains that rationalize these observations. We showed that ATP can evoke a Ca2+-activated K + current in 57% of NGNs. ATP-evoked Ca2+ influx activates ∼75% of this current, while ATP-evoked intracellular Ca2+ release activates the other ∼25%. We also determined that NGNs express P2X receptors that mediate robust influx, therefore our data suggest that ATP can exert both excitatory and inhibitory effects in NGNs.