Molecular mechanisms underlying calcium current modulation by nociceptin

Nociceptin is a non-opioid peptide that modulates pain response. One of mechanism underlying its analgesic action is the inhibition of voltage-dependent calcium current (I-Ca), similar to that of opioids. We investigated the molecular mechanism by which nociceptin inhibits I-Ca using sensory neurons and a heterologous expression system. I-Ca inhibition by nociceptin was voltage-dependent, exhibited the slowing of activation kinetics and prepulse facilitation, and was blocked by N-ethylmaleimide, indicating the involvement of Gi/Go protein. I-Ca inhibition by nociceptin was primarily mediated through binding to its own receptor, ORL-I, but not through affecting other mu-opioid receptors. Thus, our results strongly demonstrate that heterologous cross-talk between ORLI and muOR is not involved in the I-Ca inhibition by nociceptin.