You may consult the following paper for some G proteins in the rat kidney; I assume that the same antibodies may be used (tested) also in the mouse brain:
Stow JL, Sabolić I, Ausiello DA, Brown D: Immunocytochemical localization of G protein alpha subunits in rat kidney. J Am Soc Nephrol 2:465, 1991
Which G-proteins (monomeric or heterotrimeric) you are interested in? There is very good antibodies for monomeric G proteins belong to the Ras super family.
I am seeing mRNA downregulation of , Gnaz, Gnb1, Gng10, Gng11, Gng3, Gng4, Gng8, and Gngt2. I wanted to follow up these changes by measuring protein levels.