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 Ca(2+) is a key regulator of neuronal excitability and is largely controlled by the Ca(2+) stores of the endoplasmic reticulum. In other cell types store-operated channels (SOC) have been shown to contribute to the homeostatic control of intracellular Ca(2+). Here, using Ca(2+) imaging, we have shown that ER depletion in vagal sensory neurons (using thapsigargin or caffeine) in the absence of extracellular Ca(2+) evoked Ca(2+) influx upon re-introduction of Ca(2+) into the extracellular buffer. This store-operated Ca(2+) entry (SOCE) was observed in approximately 25-40% of vagal neurons, equally distributed among nociceptive and non-nociceptive sensory subtypes. SOCE was blocked by Gd(3+) 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 Ca(2+) 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.