Modulation of Adenylyl Cyclase Activity in Rat Striatal Homogenate by Dopaminergic Receptors

Abstract

We have characterized the modulation of adenylyl cyclase (AC) activity by ligands of dopaminergic receptors in rat striatal homogenate and compared the results with receptor-ligand binding affinities. Despite the fact that rat striatum contains high level of both dopamine D1 and D2 receptors, only the D1-specific AC activation by agonists could be determined. All D1-receptor agonists (dopamine, dihydrexidine, and A 77636) used were able to increase cAMP accumulation in a concentration-dependent manner, while D1-receptor antagonists (SCH23390, SKF83566, and butaclamol) blocked the effects induced by the aforementioned agonists. At the same time, the D2-receptor agonist quinpirole and antagonist sulpiride had no effect on cAMP accumulation in striatal homogenate neither on the basal level nor on the activated level of AC, while inhibited [3H]raclopride binding to these membranes. Comparing the ligands of the D1 receptor in modulating the activity of AC and displacing D1-receptor–specific radioligand [3H]SCH23390 binding revealed that the ligands modulate both of these processes with similar affinities. It indicates that under given experimental conditions, only dopamine D1-receptor–mediated stimulation of AC activity can be measured in membrane homogenate of rat striatum, while dopamine D2-receptor effects remain fully hidden.