Differential Dependence of the D₁ and D₅ Dopamine Receptors on the G Protein γ₇ Subunit for Activation of Adenylylcyclase

The D₁ dopamine receptor, G protein γ₇ subunit, and adenylylcyclase are selectively expressed in the striatum, suggesting their potential interaction in a common sig naling pathway. To evaluate this possibility, a ribozyme strategy was used to suppress the expression of the G protein γ₇ subunit in HEK 293 cells stably expressing the human D₁ dopamine receptor. Prior in vitro analysis revealed that the γ₇ ribozyme possessed cleavage activity directed exclusively toward the γ₇ RNA transcript (Wang, Q., Mullah, B., Hansen, C., Asundi, J., and Robishaw, J. D. (1997) J. Biol. Chem. 272, 26040-26048). In vivo analysis of cells transfected with the γ ₇ ribozyme showed a specific reduction in the expression of the γ₇ protein. Coincident with the loss of the γ ₇ protein, there was a noticeable reduction in the expression of the β₁ protein, confirming their interaction in these cells. Finally, functional analysis of ribozyme-mediated suppression of the β ₁ and γ₇ proteins revealed a significant attenuation of SKF81297-stimulated adenylylcyclase activity in D₁ dopamine receptor-expressing cells. By contrast, ribozyme-mediated suppression of the β₁ and γ₇ proteins showed no reduction of SKF81297-stimulated adenylylcyclase activity in D₅ dopamine receptor-expressing cells. Taken together, these data indicate that the structurally related D₁ and D₅ dopamine receptor subtypes utilize G proteins composed of distinct βγ subunits to stimulate adenylylcyclase in HEK 293 cells. Underscoring the physiological relevance of these findings, single cell reverse transcriptase-polymerase chain reaction analysis revealed that the D₁ dopamine receptor and the G protein γ₇ subunit are coordinately expressed in substance P containing neurons in rat striatum, suggesting that the G protein γ ₇ subunit may be a new target for drugs to selectively alter dopaminergic signaling within the brain.