Versatile regulation of cytosolic Ca2+ by vanilloid receptor I in rat dorsal root ganglion neurons

Analysis of small dorsal root ganglion (DRG) neurons revealed novel functions for vanilloid receptor 1 (VR1) in the regulation of cytosolic Ca2+. The VR1 agonist capsaicin induced Ca2+ mobilization from intracellular stores in the absence of extracellular Ca2+, and this release was inhibited by the VR1 antagonist capsazepine but was unaffected by the phospholipase C inhibitor xestospongins, indicating that Ca2+ mobilization was dependent on capsaicin receptor binding and was not due to intracellular inositol-1,4,5-trisphosphate generation. Confocal microscopy revealed extensive expression of VR1 on endoplasmic reticulum, consistent with VR1 operating as a Ca2+ release receptor. The main part of the capsaicin-releasable Ca2+ store was insensitive to thapsigargin, a selective endoplasmic reticulum Ca2+- ATPase inhibitor, suggesting that VR1 might be predominantly localized to a thapsigargin-insensitive endoplasmic reticulum Ca2+ store. In addition, VR1 was observed to behave as a store-operated Ca2+ influx channel. In DRG neurons, capsazepine attenuated Ca2+ influx following thapsigargin-induced Ca2+ store depletion and inhibited thapsigargin-induced inward currents. Conversely, transfected HEK-293 cells expressing VR1 showed enhanced Ca2+ influx and inward currents following Ca2+ store depletion. Combined data support topographical and functional diversity for VR1 in the regulation of cytosolic Ca2+ with the plasma membrane-associated form behaving as a store-operated Ca2+ influx channel and endoplasmic reticulum-associated VR1 possibly functioning as a Ca2+ release receptor in sensory neurons.