Dopamine-D2S Receptor Inhibition of Calcium Influx, Adenylyl Cyclase, and Mitogen-Activated Protein Kinase in Pituitary Cells: Distinct Gα and Gβγ Requirements

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Abstract
The G protein specificity of multiple signaling pathways of the dopamine-D2S (short form) receptor was investigated in GH4ZR7 lactotroph cells. Activation of the dopamine-D2S receptor inhibited forskolin-induced cAMP production, reduced BayK8644- activated calcium influx, and blocked TRH-mediated p42/p44 MAPK phosphorylation. These actions were blocked by pretreatment with pertussis toxin (PTX), indicating mediation by Gi/o proteins. D2S stimulation also decreased TRH-induced MAPK/ERK kinase phosphorylation. TRH induced c-Raf but not B-Raf activation, and the D2S receptor inhibited both TRH-induced c-Raf and basal B-Raf kinase activity. After PTX treatment, D2S receptor signaling was rescued in cells stably transfected with individual PTX-insensitive Gα mutants. Inhibition of adenylyl cyclase was partly rescued by Gαi2 or Gαi3, but Gαo alone completely reconstituted D2S-mediated inhibition of BayK8644-induced L-type calcium channel activation. Gαo and Gαi3 were the main components involved in D2S-mediated p42/44 MAPK inhibition. In cells transfected with the carboxyl-terminal domain of G protein receptor kinase to inhibit Gβγ signaling, only D2S-mediated inhibition of calcium influx was blocked, but not inhibition of adenylyl cyclase or MAPK. These results indicate that the dopamine-D2S receptor couples to distinct Gi/o proteins, depending on the pathway addressed, and suggest a novel Gαi3/Gαo-dependent inhibition of MAPK mediated by c-Raf and B-Raf-dependent inhibition of MAPK/ERK kinase.