THE BINDING OF THE ADENOSINE-A2 RECEPTOR-SELECTIVE AGONIST [H-3] CGS-21680 TO RAT CORTEX DIFFERS FROM ITS BINDING TO RAT STRIATUM

The binding of the reportedly A2A selective agonist CGS 21680 (2-[p-(2-carboxyethyl)phenylethylamino]-5'-N-ethylcarboxamidoadenosine) to cortex and striatum was examined in parallel using quantitative receptor autoradiography. [H-3]CGS 21680 bound to a single site in rat striatum with K(D) 2.3 nM and B(max) 320 fmol/mg grey matter. In addition [H-3]CGS 21680 bound to a single site in the cerebral cortex with K(D) 47 nM and B(max) 100 fmol/mg grey matter. In cat cortex [H-3]CGS 21680 (2 nM) binding was strong and particularly evident in the most superficial layers. The potency order for inhibition of 2 nM [H-3]CGS 21680 binding to rat striatum was NECA (5'-N-ethylcarboxamidoadenosine; IC50 9.0 nM) > 2-CADO (2-chloroadenosine; 87 nM) > R-PIA (N6-(R)-phenylisopropyladenosine; 110 nM). The potency order for inhibition of 2 nM [H-3]CGS 21680 binding to rat cortex was NECA (3.0 nM) > 2-CADO (14 nM) greater-than-or-equal-to R-PIA (16 nM). Gpp(NH)p (5'-guanylyl imidodiphosphate) inhibited [H-3]CGS 21680 binding to both cortex and striatum, but more potently in cortex (IC50 100 nM vs. 470 nM). The present results show that there is a cortical binding site for [H-3]CGS 21680 which appears to be different from the the striatal A2A receptor, the A2B receptor and the A1 receptor.