Functional expression of Ca(2+)-mobilizing opioid receptors in Xenopus oocytes injected with rat brain mRNA

Functional expression of opioid receptors was detected in the Xenopus oocyte translation system by a voltage-clamp recording. After injection of poly(A)+ RNA isolated from 3-week-old rat striatum or whole brain, the oocytes often demonstrated intracellular Ca(2+)-mediated oscillatory responsiveness to [D-Ala2, N-methyl-Phe4, Gly5-ol]enkephalin (DAMGO), [D-Pen2, D-Pen5]enkephalin (DPDPE) and U50488H at a concentration of 1 microM. These responses were very transiently expressed after injection of the mRNA, however, water-injected oocytes never responded to any of these opioid agonists. After fractionation by a sucrose-density gradient, an RNA size of about 3-4 kb encoded these opioid receptors. In the oocytes injected with size-selected striatal mRNA, DPDPE evoked the fluctuating current with higher probability and larger amplitude than other agonists, whereas oocytes injected with size-selected whole brain mRNA produced DAMGO and U50488H responses predominantly. The DPDPE response of striatal mRNA-injected oocytes was antagonized by naloxone as well as the delta-specific antagonist ICI 174864. The DAMGO and U50488H responses have not been characterized yet because of a strong desensitizing property making repeated recordings impossible. These observations suggest that putative mu, delta and kappa subtypes of opioid receptors mobilizing intracellular Ca2+ are expressed in Xenopus oocytes by rat brain mRNA.