PHOSPHORYLATION-DEPENDENT AND VOLTAGE-DEPENDENT INHIBITION OF NEURONAL CALCIUM CURRENTS BY ACTIVATION OF HUMAN D-2(SHORT) DOPAMINE-RECEPTORS

1 Activation of human D-2(s) dopamine receptors with quinpirole (10 nM) inhibits omega-conotoxin GVIa-sensitive, high-threshold calcium currents when expressed in differentiated NG108-15 cells (55% inhibition at + 10 mV). This inhibition was made irreversible following intracellular dialysis with the non-hydrolysable guanosine triphosphate analogue GTP-gamma-S (100 mu M), and was prevented by pretreatment with pertussis toxin (1 mu g ml(-1) for 24 h). 2 Stimulation of protein kinase C with the diacylglycerol analogue, 1-oleoyl-2-acetyl-sn-glycerol (100 mu M), also attenuated the inhibition of the sustained calcium current but did not affect the receptor-mediated decrease in rate of current activation. Similarly, okadaic acid (100 nM), a protein phosphatase 1/2A inhibitor, selectively occluded the inhibition of the sustained current. 3 The depression of calcium currents by quinpirole (10 nM) was enhanced following intracellular dialysis with 100 mu M cyclic adenosine monophosphate (cyclic AMP, 72.8 +/- 9.8% depression), but was not mimicked by the membrane permeant cyclic GMP analogue, Sp-8 -bromoguanosine-3,5':cyclic monophosphorothioate (100 mu M). 4 Inhibition of calcium currents was only partly attenuated by 100 ms depolarizing prepulses to + 100 mV immediately preceding the test pulse. However, following occlusion of the sustained depression with okadaic acid (100 nM) the residual kinetic slowing was reversed in a voltage-dependent manner (P < 0.05). 5 Thus pertussis toxin-sensitive G-proteins liberated upon activation of human D-2(short) dopamine receptor inhibited high-threshold calcium currents in two distinct ways. The decrease in rate of calcium current activation involved a voltage-dependent pathway, whereas the sustained inhibition of calcium current involved, in part, the voltage-resistant phosphorylation by cyclic AMP-dependent protein kinases and subsequent dephosphorylation by protein phosphatases 1/2A.