Sensitization of endogenous and recombinant adenylate cyclase by activation of D2 dopamine receptors

Prolonged stimulation of Gi-coupled receptors often sensitizes adenylate cyclase to subsequent activation by forskolin or Gs-coupled receptors. To identify mechanisms of heterologous sensitization, we characterized sensitization of cAMP accumulation that was induced by activation of recombinant dopamine D2 receptors expressed in C6 glioma and human embryonic kidney (HEK)293 cells. Pretreatment with dopamine or other agonists for 2 hr induced heterologous sensitization that was blocked by the D2 antagonist spiperone but not by the beta-adrenergic receptor antagonist propranolol. Sensitization was evident after 15 min of treatment with dopamine and persisted for at least 2 hr after washout. The EC50 value for sensitization by dopamine in HEK-D2L cells was 100 nM(r) approximately 80-fold higher than the IC50 value for dopamine inhibition of cAMP accumulation. The D2 receptor agonists quinpirole, 7-hydroxy-dipropylamin-otetralin, and pergolide also induced sensitization, whereas the high affinity ergot agonists bromocriptine and lisuride did not. Stimulation of either D2L or D2S receptors sensitized cAMP accumulation to similar extents, but stimulation of D3 receptors did not. In C6-D21 cells, sensitization of isoproterenol-stimulated activity was manifested as a > 100% increase in maximal response, with no change in potency. In contrast, the potency for forskolin-stimulated activity was increased 4-fold, with no apparent change in maximal response. Overnight treatment with pertussis toxin (25 ng/ml) had little effect on isoproterenol or forskolin activation of adenylate cyclase per se but prevented D2 receptor-mediated sensitization in both C6-D2L and HEK-D2L cells, indicating an involvement of one or more of the pertussis toxin-sensitive G proteins, Gi/Gzero. D2 receptor stimulation also sensitized type I and type II adenylate cyclases, each expressed in HEK293 cells together with D2L dopamine receptors. Rapid D2 receptor-mediated heterologous sensitization may be the result of enhanced interaction of G6 with adenylate cyclase and may represent a novel mechanism for modulation of neural activity by D2 receptors.