Adenosine A2A Receptor Binding Profile of Two Antagonists, ST1535 and KW6002: Consideration on the Presence of Atypical Adenosine A2A Binding Sites

Adenosine A(2A) receptors seem to exist in typical (more in striatum) and atypical (more in hippocampus and cortex) subtypes. In the present study, we investigated the affinity of two adenosine A(2A) receptor antagonists, ST1535 [2 butyl -9-methyl-8-(2H-1,2,3-triazol 2-yl)-9H-purin-6-xylamine] and KW6002 [(E)-1,3-diethyl-8-(3,4-dimethoxystyryl)-7-methyl-3,7-dihydro-1H-purine-2,6,dione] to the "typical" and "atypical" A(2A) binding sites. Affinity was determined by radioligand competition experiments in membranes from rat striatum and hippocampus. Displacement of the adenosine analog [(3)H]CGS21680 [2-p-(2-carboxyethyl)phenethyl-amino-5'-N-ethylcarbox-amidoadenosine] was evaluated in the absence or in the presence of either CSC [8-(3-chlorostyryl)-caffeine], an adenosine A(2A) antagonist that pharmacologically isolates atypical binding sites, or DPCPX (8-cyclopentyl-1,3-dipropylxanthine), an adenosine A(1) receptor antagonist that pharmacologically isolates typical binding site. ZM241385 [84-(2-[7-amino-2-(2-furyl) [1,2,4]-triazol[2,3-a][1,3,5]triazin-5-yl amino]ethyl) phenol)] and SCH58261 [(5-amino-7-(β-phenylethyl)-2-(8-furyl)pyrazolo(4,3-e)-1,2,4-triazolo(1,5-c) pyrimidine], two other adenosine A(2A) receptor antagonists, which were reported to differently bind to atypical and typical A(2A) receptors, were used as reference compounds. ST1535, KW6002, ZM241385 and SCH58261 displaced [(3)H]CGS21680 with higher affinity in striatum than in hippocampus. In hippocampus, no typical adenosine A(2A) binding was detected, and ST1535 was the only compound that occupied atypical A(2A) adenosine receptors. Present data are explained in terms of heteromeric association among adenosine A(2A), A(2B) and A(1) receptors, rather than with the presence of atypical A(2A) receptor subtype.