Abstract

In situ hybridization with cRNA probes showed A_2A receptor and G_olf mRNAs to be abundantly expressed in caudate putamen, nucleus accumbens, and olfactory tubercle, whereas G_s mRNA shows a comparatively low expression in regions expressing A_2A receptors. In caudate putamen, 49% of the medium-sized neuron-like cells exhibited a strong signal for adenosine A_2A receptor mRNA, and 98% showed a strong signal for G_olf mRNA. In contrast, G_s mRNA was found in only 12% of the medium-sized neuron-like cells in caudate putamen. The coexpression of adenosine A_2A receptor mRNA with that of G_olf or G_s mRNAs was studied with double in situ hybridization. A large majority (91–95%) of the neurons in caudate-putamen that contained adenosine A_2A receptor mRNA also expressed G_olf mRNA, whereas only 3 to 5% of the neurons with adenosine A_2A receptor mRNA coexpressed G_s mRNA. The A_2A receptor agonist CGS 21680 [2-[p-(2-carbonylethyl)phenylethylamino-5′-N-ethylcarboxamidoadenosine] dose dependently activated G_olf subunits in striatal membranes as shown by photolabeling with [α-³²P]m-acetylanilido-GTP followed by immunoprecipitation with a specific antibody against G_olf. Transfection of G_olf cDNA into Chinese hamster ovary cells, which stably express human adenosine A_2A receptors, led to an increased efficacy of CGS 21680, as evidenced by a stronger cAMP response, indicating that activation of G_olf by A_2A receptors leads to a biological signal. In conclusion, these results provide anatomical and biochemical evidence that adenosine A_2A receptors stimulate G_olf rather than G_s in striatum.