D1 DOPAMINE-RECEPTORS CAN INTERACT WITH BOTH STIMULATORY AND INHIBITORY GUANINE-NUCLEOTIDE BINDING-PROTEINS

Pretreatment of striatal membranes with N-ethyl-maleimide in the presence of a D1-specific agonist inactivated endogenous guanine nucleotide binding proteins (G proteins), but not D1 dopamine receptors, resulting in a loss of high-affinity agonist binding sites. Such D1 receptors were solubilized, mixed with exogenous G proteins from cells not containing D1 receptors, and reconstituted into phospholipid vesicles. These reconstituted receptors were able to couple to the exogenous G proteins, and the proportion of agonist high-affinity sites of the receptor (40-57%) was similar to levels obtained with naive receptors coupling to endogenous G proteins (40%) upon solubilization and reconstitution. These hybrid high-affinity sites were fully modulated by guanine nucleotides. Pretreatment of cells with pertussis toxin prior to extraction of G proteins resulted in a 50% decrease in the proportion of high-affinity sites; these sites remained sensitive to guanine nucleotides. When D1 receptors were reconstituted with extracts of cyc- cells, which lack stimulatory G proteins, the proportion of high-affinity sites was reduced to 31% of the total. Pertussis toxin treatment of the cyc- cells completely abolished the formation of high-affinity sites. These results demonstrate that D1-dopaminergic receptors are able to couple to not only stimulatory G proteins (G(s)) but also to inhibitory G proteins (G(i)).