Binding of the prototypical adenosine A 2A receptor agonist CGS 21680 to the cerebral cortex of adenosine A 1 and A 2A receptor knockout mice

2‐ p ‐(2‐carboxyethylphenethylamino‐5′‐ethylcarboxamidoadenosine) (CGS 21680) is considered the reference compound to study adenosine A 2A receptors. However, CGS 21680 binding in the cerebral cortex, where adenosine A 1 receptors are predominant, displays a mixed A 2A /A 1 receptor pharmacology. We now use adenosine A 1 and A 2A receptor knockout mice to investigate the characteristics of cortical [ 3 H]CGS 21680 binding. [ 3 H]CGS 21680 binding to the cerebral cortex was strongly reduced in adenosine A 1 receptor knockout mice, but only slightly reduced in A 2A receptor knockout mice compared with the corresponding wild‐type littermates. Another selective A 2A receptor ligand, [ 3 H]‐5‐amino‐7‐(2‐phenylethyl)‐2‐(2‐furyl)‐pyrazolo[4,3‐ e ]‐1,2,4‐triazolo[1,5‐ c ]pyrimidine ([ 3 H]SCH 58261), displayed a saturable binding to mouse cortical membranes, albeit with a binding density 20 times lower than that of striatal membranes, and this [ 3 H]SCH58261 binding was abolished in both striatal and cortical membranes of A 2A receptor knockout mice and unchanged in A 1 receptor knockout mice. The presence of A 2A receptors in cortical neurons was further confirmed by Western blot in mouse cortical nerve terminal membranes. It is concluded that, although A 2A receptors are present in the cerebral cortex, the purportedly selective A 2A receptor agonist [ 3 H]CGS 21680 binds in the cerebral cortex to an entity that requires the presence of adenosine A 1 receptors. Thus, CGS 21680 should be used with care in all preparations where adenosine A 1 receptors out‐number A 2A receptors.British Journal of Pharmacology (2004) 141 , 1006–1014. doi: 10.1038/sj.bjp.0705692