Inhibition of hormonally induced inositol trisphosphate production in transfected GH(4)C(1) cells: A novel role for the D-5 subtype of the dopamine receptor

We have previously found that the D-5 dopamine receptor couples to a G-prot ein other than G(s)alpha, and could be involved in signaling pathways other than regulation of adenylyl cyclase. To describe interactions of the D-5 r eceptor with cellular effecters, we used GH(4)C(1) cells transfected with c DNA for the human Dg receptor. Thyrotropin-releasing hormone (TRH, 100 nM) stimulated accumulation of inositol phosphates (IPs) fivefold in D(5)GH(4)C (1) cells. Dopamine (DA, 10 mu M) inhibited TRH-stimulated IP values by 29% ; at higher concentrations (100 mu M), maximal inhibition of 61% was observ ed. The D-5 agonist SKF R-38393 (10 mu M) mimicked this effect (28% inhibit ion). SCH 23390, a D-5 antagonist, blocked the inhibition caused by both DA and SKF R-38393, Spiperone, a D-2 receptor antagonist, did not block the i nhibition. The D-2 agonist (+/-)-2-(N-phenylethyl-N-propyl)amino-5-hydroxyt etralin (PPHT) did not inhibit TRH-stimulated IP production, nor did it aug ment the effect of D-5 agonists. The DA-mediated suppression of IP levels w as not sensitive to pertussis toxin; cholera toxin blocked both TRH stimula tion and DA suppression of IP accumulation in response to 100 nM TRH. Neith er dibutyryl cAMP nor forskolin lowered IP formation in response to TRH. Ph orbol ester decreased TRH-stimulated IP accumulation in D(5)GH(4)C(1) cells ; however, an inhibitor of protein kinase C (PKC) did not block the effect of DA.