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

Prolonged stimulation of G(i)-coupled receptors often sensitizes adenylate cyclase to subsequent activation by forskolin or G(s)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 β-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-D(2L) cells was 100 nM, ~80-fold higher than the IC₅₀ value for dopamine inhibition of cAMP accumulation. The D₂ receptor agonists quinpirole, 7-hydroxy-dipropylaminotetralin, and pergolide also induced sensitization, whereas the high affinity ergot agonists bromocriptine and lisuride did not. Stimulation of either D(2L) or D(2S) receptors sensitized cAMP accumulation to similar extents, but stimulation of D₃ receptors did not. In C₆-D(2L) 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-D(2L) and HEK- D(2L) cells, indicating an involvement of one or more of the pertussis toxin- sensitive G proteins, G(i)/G(o). D₂ receptor stimulation also sensitized type I and type II adenylate cyclases, each expressed in HEK293 cells together with D(2L) dopamine receptors. Rapid D₂ receptor-mediated heterologous sensitization may be the result of enhanced interaction of G(s) with adenylate cyclase and may represent a novel mechanism for modulation of neural activity by D₂ receptors.