Functional coupling of dopamine D2 and muscarinic cholinergic receptors to their respective G proteins assessed by agonist-induced activation of high-affinity GTPase activity in rat striatal membranes.

Agonist-induced high-affinity GTPase activity was investigated using the crude membrane preparation from rat striatum. High-affinity GTPase activity was stimulated by dopamine and carbachol in a Mg(2+)-dependent manner and with possible optimum NaCl concentrations of 50-100 mM to detect the percent increase induced by each agonist. Dopamine and selective (as well as non-selective) D2 receptor agonists, but not selective D1 receptor agonists, stimulated activity in a concentration-dependent manner, with affinities which were significantly correlated with those for adenylate cyclase inhibition as previously reported in the literature. Maximal percent stimulation above basal high-affinity GTPase activity was 9.8 +/- 0.6% and 4.4-7.6% for dopamine and other synthetic dopamine D2 receptor agonists, respectively. Dopamine-stimulated activity was inhibited by several dopamine receptor antagonists with the following rank order of potency: (+)-butaclamol > spiperone > raclopride > S(-)-sulpiride; but not by (-)-butaclamol or SCH 23390. High-affinity GTPase activity was also stimulated by carbachol and acetylcholine through the pirenzepine-insensitive muscarinic receptors. Preincubation of the membranes with AS/7, a specific antiserum to Gi1 and Gi2, appeared to attenuate dopamine-sensitive activity, suggesting that Gi1 and/or Gi2 may be at least partially involved. These results indicate that high-affinity GTPase activity in rat striatal membranes is activated through dopamine D2-like receptors and pirenzepine-insensitive muscarinic receptors, both of which are negatively coupled to adenylate cyclase via Gi proteins.