EVIDENCE FOR A DOPAMINE-RECEPTOR SUBTYPE SENSITIVE TO COMBINATION OF D1 WITH D2 ANTAGONIST IN MEASUREMENT OF G-PROTEIN ACTIVITY USING RAT STRIATAL MEMBRANES

In rat striatal membranes, various kinds of dopamine receptor agonists stimulated low-Km GTPase activity in a concentration-dependent manner . This stimulation by bromocriptine, pergolide and apomorphine was par tially inhibited by sulpiride (SUL), a D2-selective antagonist, marked ly inhibited by combination of SUL with SCH 23390 (SCH), a D1-selectiv e antagonist, and not modified by SCH alone. The stimulation by BAM-11 10 was resistant to SUL or SCH alone but abolished by combination of S UL with SCH. These findings suggest the presence of another subtype of a dopamine receptor in a functional in vitro bioassay system in rat s triata.