Store-operated calcium entry in vagal sensory nerves is independent of Orai channels

Abstract

Vagal sensory nerves innervate the majority of visceral organs (e.g., heart, lungs, GI tract, etc) and their activation is critical for defensive and regulatory reflexes. Intracellular Ca2+ is a key regulator of neuronal excitability and is largely controlled by the Ca2+ stores of the endoplasmic reticulum. In other cell types store-operated channels (SOC) have been shown to contribute to the homeostatic control of intracellular Ca2+. Here, using Ca2+ imaging, we have shown that ER depletion in vagal sensory neurons (using thapsigargin or caffeine) in the absence of extracellular Ca2+ evoked Ca2+ influx upon re-introduction of Ca2+ into the extracellular buffer. This store-operated Ca2+ entry (SOCE) was observed in approximately 25–40% of vagal neurons, equally distributed among nociceptive and non-nociceptive sensory subtypes. SOCE was blocked by Gd3+ but not by the Orai channel blocker SKF96365. We found Orai channel mRNA in extracts from whole vagal ganglia, but when using single cell RT-PCR analysis we found only 3 out of 34 neurons expressed Orai channel mRNA, indicating that Orai channel expression in the vagal ganglia was likely derived from non-neuronal cell types. Confocal microscopy of vagal neurons in 3 day cultures demonstrated rich ER tracker fluorescence throughout axonal and neurite structures and ER store depletion (thapsigargin) evoked Ca2+ transients from these structures. However, no SOCE could be detected in the axonal/neurite structures of vagal neurons. We conclude that SOCE occurs in vagal sensory neuronal cell bodies through non-Orai mechanisms but is absent at nerve terminals.