Nuclear Ca2+/calmodulin translocation activated by mu-opioid (OP3) receptor

Previous evidence has suggested a role for calmodulin (CaM) in opioid recep tor signaling. We demonstrate here that morphine stimulation of the mu-opio id (OP3) receptor causes rapid CaM translocation to the nucleus in OP3-tran sfected human embryonic kidney (HEK)-293 cells and in SH-SY5Y human neurobl astoma cells. Ca2+ influx into the cells resulting from OP3 receptor activa tion was required for nuclear CaM translocation. Moreover, in HEK-OP3 and S H-SY5Y cells, increased nuclear CaM content was associated with enhanced ph osphorylation of the nuclear transcription factor cyclic AMP-responsive ele ment-binding protein. This appeared to be mediated by Ca2+/CaM kinases and also by a pathway involving protein kinase C. CaM was previously shown to b ind directly to the OP3 receptor and to be released from the plasma membran e on agonist stimulation. To test whether OP3-mediated CaM release contribu tes to nuclear CaM signaling, we used a mutant OP3 receptor (K273A) with re duced affinity for CaM that fails to release CaM from the plasma membrane. K273A-OP3 activated Ca2+ influx to a similar extent as wild-type OP3; howev er, CaM translocation to the nucleus was attenuated. These results indicate that OP3-stimulated Ca2+ influx results in nuclear CaM translocation, whic h appears to be enhanced by simultaneous CaM release by OP3 wild-type recep tor from plasma membranes. These results suggest a novel Ca2+/CaM signaling pathway of opioid receptors in the regulation of transcriptional activity.