κ Opiate agonists inhibit Ca²⁺ influx in rat spinal cord-dorsal root ganglion cocultures. Involvement of a GTP-binding protein

The aim of the present study has been to characterize the regulation by opiates of ⁴⁵Ca²⁺ influx in rat spinal cord-dorsal root ganglion cocultures. We have demonstrated that K⁺-induced depolarization, in the presence of the Ca²⁺ channel agonist Bay K8644, stimulated Ca²⁺ influx (3-4-fold) via the dihydropyridine class of voltage-dependent Ca²⁺ channels. While μ and δ opiates had no effect, κ opiate agonists (e.g. U50488, dynorphin) profoundly depressed the stimulated Ca²⁺ influx (86% inhibition at 100 μM U50488). The κ agonist action was stereospecific and could be reversed by the opiate antagonist naloxone. The inhibition produced by κ 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 κ opiate receptors are negatively coupled to voltage-dependent Ca²⁺ channels, via a pertussis toxin-sensitive GTP-binding protein. Basal ⁴⁵Ca²⁺ uptake, stimulated by adenylate cyclase activators (forskolin and cholera toxin), was potently inhibited by κ opiates suggesting that, under conditions of neurohormonal stimulation of adenylate cyclase, κ receptors are coupled to Ca²⁺ channels indirectly via the adenylate cyclase complex. In addition, cAMP-independent coupling pathways may also be involved.