Dopamine D ₂-Receptor Activation Differentially Inhibits N- and R-Type Ca ²⁺ Channels in Xenopus Melanotrope Cells

Abstract

Dopamine inhibits pituitary melanotrope cells of the amphibian Xenopus laevis through activation of a dopamine (D₂) receptor that couples to a G_i protein. Activated G_i protein subunits are known to affect voltage-operated Ca²⁺ currents (I_Ca). In the present study we investigated which Ca²⁺ currents are regulated by D₂-receptor activation and which G_i protein subunits are involved. Whole-cell voltage-clamp patch-clamp experiments from holding potentials (HPs) of –80 and –30 mV show that 28.6 and 36.9%, respectively, of the total I_Ca was inhibited by apomorphin, a D₂-receptor agonist. The inhibited current had fast activation and inactivation kinetics. From an HP of –80 mV, inhibition of N-type Ca²⁺ currents with ω-conotoxin GVIA and R-type current by SNX-482 reduced the efficacy of the apomorphin-induced inhibition. From an HP of –30 mV this reduction for ω-conotoxin GVIA was still observed. Blocking L-type current by nifedipine or P/Q-type current by ω-agatoxin IVA did not affect apomorphin-induced inhibition at either HP. Our results imply that D₂-receptor activation inhibits both N- and R-type Ca²⁺ currents. Using a strong depolarizing pre-pulse partially reversed the inhibition of the total current by apomorphin. About 50% of this inhibition was achieved through interaction of Gβ/γ proteins, and this part of the inhibited I_Ca had fast activating and inactivating kinetics. However, the other part of the current inhibited by D₂-receptor activation may proceed through Gα-PKA phosphorylation.