Dopamine D2-receptor activation differentially inhibits N- and R-type Ca2+ channels in Xenopus melanotrope cells

Neuroendocrinology. 2004;80(6):368-78. doi: 10.1159/000084144. Epub 2005 Feb 24.

Abstract

Dopamine inhibits pituitary melanotrope cells of the amphibian Xenopus laevis through activation of a dopamine (D2) receptor that couples to a Gi protein. Activated Gi protein subunits are known to affect voltage-operated Ca2+ currents (ICa). In the present study we investigated which Ca2+ currents are regulated by D2-receptor activation and which Gi 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 ICa 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 omega-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 omega-conotoxin GVIA was still observed. Blocking L-type current by nifedipine or P/Q-type current by omega-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 Gbeta/gamma proteins, and this part of the inhibited ICa had fast activating and inactivating kinetics. However, the other part of the current inhibited by D2-receptor activation may proceed through Galpha-PKA phosphorylation.

MeSH terms

  • Animals
  • Apomorphine / pharmacology
  • Calcium Channel Blockers / pharmacology
  • Calcium Channels / drug effects
  • Calcium Channels / metabolism*
  • Cells, Cultured
  • Dopamine Agonists / pharmacology
  • Electrophysiology
  • Membrane Potentials / drug effects
  • Membrane Potentials / physiology
  • Patch-Clamp Techniques
  • Pituitary Gland / cytology*
  • Pituitary Gland / drug effects
  • Pituitary Gland / metabolism*
  • Receptors, Dopamine D2 / drug effects
  • Receptors, Dopamine D2 / metabolism*
  • Xenopus laevis

Substances

  • Calcium Channel Blockers
  • Calcium Channels
  • Dopamine Agonists
  • Receptors, Dopamine D2
  • Apomorphine