Determinants of the G protein-dependent opioid modulation of neuronal calcium channels

The modulation of a family of cloned neuronal calcium channels by stimulation of a coexpressed mu opioid receptor was studied by transient expression 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) and alpha(1E) currents were not significantly affected. The opioid-induced effects on alpha(1A) and alpha(1B) currents were blocked by pertussis toxin and the GTP analogue guanosine 5'-[beta-thio] diphosphate. Similar to modulation of native calcium currents, DAMGO induced a slowing of the activation kinetics and exhibited a voltage-dependent inhibition that was partially relieved by application of strong depolarizing pulses. alpha(1A) currents were still inhibited in the absence of coexpressed Ca channel alpha(2) and beta subunits, suggesting that the response is mediated by the alpha(1) subunit. Furthermore, the sensitivity of alpha(1A) currents to DAMGO-induced inhibition was increased approximate to 3-fold in the absence of a beta subunit. Overall, the results show that the alpha(1A) (P/Q type) and the alpha(1B) (N type) calcium channels are selectively modulated by a GTP-binding protein (G protein). The results raise the possibility of competitive interactions between beta subunit and G protein binding to the alpha(1) subunit, shifting gating in opposite directions. At presynaptic terminals, the G protein-dependent inhibition may result in decreased synaptic transmission and play a key role in the analgesic effect of opioids and morphine.