DETERMINANTS OF THE G-PROTEIN-DEPENDENT OPIOID MODULATION OF NEURONALCALCIUM CHANNELS

The modulation of a family of cloned neuronal calcium channels by stim ulation of a coexpressed mu opioid receptor was studied by transient e xpression in Xenopus oocytes. Activation of the morphine receptor with the synthetic enkephalin [D-Ala(2),N-Me-Phe(4),Gly-ol(5)] enkephalin (DAMGO) resulted in a rapid inhibition of alpha(1A) (by approximate to 20%) and alpha(1B) (by approximate to 55%) currents while alpha(1C) a nd alpha(1E) currents were not significantly affected. The opioid-indu ced effects on alpha(1A) and alpha(1B) currents were blocked by pertus sis toxin and the GTP analogue guanosine 5'-[beta-thio] diphosphate. S imilar to modulation of native calcium currents, DAMGO induced a slowi ng of the activation kinetics and exhibited a voltage-dependent inhibi tion that was partially relieved by application of strong depolarizing pulses. alpha(1A) currents were still inhibited in the absence of coe xpressed Ca channel alpha(2) and beta subunits, suggesting that the re sponse is mediated by the alpha(1) subunit. Furthermore, the sensitivi ty of alpha(1A) currents to DAMGO-induced inhibition was increased app roximate to 3-fold in the absence of a beta subunit. Overall, the resu lts show that the alpha(1A) (P/Q type) and the alpha(1B) (N type) calc ium channels are selectively modulated by a GTP-binding protein (G pro tein). The results raise the possibility of competitive interactions b etween beta subunit and G protein binding to the alpha(1) subunit, shi fting gating in opposite directions. At presynaptic terminals, the G p rotein-dependent inhibition may result in decreased synaptic transmiss ion and play a key role in the analgesic effect of opioids and morphin e.