Opiate Agonists Inhibit Ca2+ Influx in Rat Spinal Cord-Dorsal Root Ganglion Cocultures

The aim of the present study has been to characterize the regulation by opiates of “Ca2+ influx in rat spinal cord-dorsal root ganglion cocultures. We have demonstrated that K*-induced depolarization, in the presence of the Ca” channel agonist Bay K8644, stimulated Ca” influx (3-4-fold) via the dihydropyridine class of voltage-dependent Ca” channels. While p and 6 opiates had no effect, K opiate agonists (e.g. US0488, dynorphin) profoundly depressed the stimulated Ca2+ influx (86% inhibition at 100 pM US0488). The K agonist action was stereospecific and could be reversed by the opiate antagonist naloxone. The inhibition produced by K agonists was greatly diminished following pertussis toxin treatment, and this effect was accompanied by toxin-induced ADP-ribosylation of a 40-41-kDa protein. This suggests that K opiate receptors are negatively coupled to voltage-dependent Ca” channels, via a pertussis toxin-sensitive GTP-binding protein. Basal “Caz+ uptake, stimulated by adenylate cyclase activators (forskolin and cholera toxin), was potently inhibited by K opiates suggesting that, under conditions of neurohormonal stimulation of adenylate cyclase, K receptors are coupled to Ca2+ channels indirectly via the adenylate cyclase complex. In addition, CAMPindependent coupling pathways may also be involved.