The δ-Opioid Receptor Regulates Activity of Ryanodine Receptors in the Human Neuroblastoma Cell Line SK-N-BE

Abstract: Recent studies have demonstrated that opioid agonists affect the cytosolic Ca²⁺ concentration (Ca²⁺]_i) either by regulating plasma membrane Ca²⁺-channel activity or by mobilizing intracellular Ca²⁺ stores. The present report documents the Ca²⁺]_i increase induced by opioid agonists in a human neuroblastoma cell line, SK-N-BE, expressing δ-opioid receptors. In the presence, as well as in the absence, of extracellular Ca²⁺, opioid agonists enhanced significantly Ca²⁺]_i, whereas carbachol, known to mobilize specifically inositol 1,4,5-trisphosphate-sensitive intracellular Ca²⁺ stores, acted only in the presence of extracellular Ca²⁺. The opioid-induced increase in Ca²⁺]_i was not affected by treatments modifying the trimeric G_i, G_o, and G_s protein transduction mechanisms or the activity of adenylyl cyclase. The Ca²⁺-ATPase pump-inhibiting sesquiterpene lactone, thapsigargin, did not modify the opioid-induced Ca²⁺]_i response, whereas it abolished the effects of carbachol. The Ryana speciosa alkaloid, ryanodine, at concentrations known to block endoplasmic reticulum ryanodine receptors, decreased significantly the response to opioids without affecting the effects of carbachol. Thus, our results suggest that, in SK-N-BE cells, δ-opioid receptors mobilize Ca²⁺ from intracellular ryanodine-sensitive stores and the mechanism involved is independent of G_i/G_o and G_s proteins and protein kinase A activation.