FUNCTIONAL EXPRESSION OF CA2-MOBILIZING OPIOID RECEPTORS IN XENOPUS-OOCYTES INJECTED WITH RAT-BRAIN MESSENGER-RNA()

Functional expression of opioid receptors was detected in the Xenopus oocyte translation system by a voltage-clamp recording. After injectio n of poly(A)(+) RNA isolated from 3-week-old rat striatum or whole bra in, the oocytes often demonstrated intracellular Ca2+-mediated oscilla tory responsiveness to [D-Ala(2), N-methyl-Phe(4), Gly(5)-ol]enkephali n (DAMGO), [D-Pen(2), D-Pen(5)]enkephalin (DPDPE) and U50488H at a con centration of 1 mu M. These responses were very transiently expressed after injection of the mRNA, however, water-injected oocytes never res ponded to any of these opioid agonists. After fractionation by a sucro se-density gradient, an RNA size of about 3-4 kb encoded these opioid receptors. In the oocytes injected with size-selected striatal mRNA, D PDPE evoked the fluctuating current with higher probability and larger amplitude than other agonists, whereas oocytes injected with size-sel ected whole brain mRNA produced DAMGO and U50488H responses predominan tly. The DPDPE response of striatal mRNA-injected oocytes was antagoni zed by naloxone as well as the delta-specific antagonist ICI 174864. T he DAMGO and U50488H responses have not been characterized yet because of a strong desensitizing property making repeated recordings impossi ble. These observations suggest that putative mu, delta and kappa subt ypes of opioid receptors mobilizing intracellular Ca2+ are expressed i n Xenopus oocytes by rat brain mRNA.