FURTHER CHARACTERIZATION OF [H-3] CGS-21680 BINDING-SITES IN THE RAT STRIATUM AND CORTEX

1 The putative high affinity binding site for the adenosine A(2A) receptor agonist 2-p-(2-carboxyethyl)phenethyl-amino-5'-N- ethylcarboxamidoadenosine (CGS 21680) in the rat cerebral cortex was characterized by use of a number of selective A(1) and A(2) adenosine receptor ligands, and compared to the characteristics of the more abundant striatal A(2A) receptor. 2 The binding of [H-3]-CGS 21680 to cortical membranes was performed at pH 5.5, in order to increase the amount of specific binding. 3 Reduction of the pH from 7.4 to 5.5 increased the apparent affinity of the striatal binding side for both agonists and antagonists. The relative order of potencies of both groups of ligands were the same at both pH values, and were consistent with binding to the A(2A) receptor. There was no observable change in the B-max the values being 415 and 446 fmol mg(-1) protein at pH 5.5 and 7.4 respectively. 4 The cortical binding site yielded a B-max value of 117 fmol mg(-1) protein. The relative order of potencies of the adenosine receptor ligands observed at this binding site were not the same as those observed in the striatum, exhibiting a profile with both A(1) and A(2) characteristics. 5 Further characterization of this cortical binding site in the presence of the A(1) selective antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) revealed a more typical A(2A) profile. This indicated that under the conditions used there were two components of [H-3]-CGS 21680 binding, approximately 20% of the A(1) receptor and 80% to the A(2A) receptor. 6 It is concluded that in the cerebral cortex there is a CGS 21680 binding site showing the characteristic properties of the striatal A(2A) receptor, and no evidence was obtained for the existence of a novel A(2A)-like binding site.