Evidence for high-affinity binding sites for the adenosine A_2A receptor agonist [³H] CGS 21680 in the rat hippocampus and cerebral cortex that are different from striatal A_2A receptors

Abstract

The binding of the adenosine A_2A receptor selective agonist 2-[4-(2-p-carboxyethyl) phenylamino]-5′-N-ethylcarboxamidoadenosine (CGS 21680) to the rat hippocampal and cerebral cortical membranes was studied and compared with that to striatal membranes. [³H] CGS 21680, in the concentration range tested (0.2–200 nM), bound to a single site with a K _d of 58 nM and a B _max of 353 fmol/mg protein in the hippocampus, and with a K _d of 58 nM and a B _max of 264 fmol/mg protein in the cortex; in the striatum, the single high-affinity [³H] CGS 21680 binding site had a K _d of 17 nM and a B _max of 419 fmol/mg protein. Both guanylylimidodiphosphate (100 μM) and Na⁺ (100 mM) reduced the affinity of [³H] CGS 21680 binding in the striatum by half and virtually abolished [³H] CGS 21680 binding in the hippocampus and cortex. The displacement curves of [³H] CGS 21680 binding with 1,3-dipropyl-8-cyclopentylxanthine (DPCPX), N ⁶-cyclohexyladenosine (CHA), 5′-N-ethyl-carboxamidoadenosine (NECA) and 2-chloroadenosine (CADO) were biphasic in the hippocampus and cortex as well as in the striatum. The predominant [³H]CGS 21680 binding site in the striatum (80%) had a pharmacological profile compatible with A_2A receptors and was also present in the hippocampus and cortex, representing 10–25% of [³H]CGS 21680 binding. The predominant [³H]CGS 21680 binding site in the hippocampus and cortex had a pharmacological profile distinct from A_2A receptors: the relative potency order of adenosine antagonists DPCPX, 1,3-dipropyl8-{4-[(2-aminoethyl)amino]carbonylmethyloxyphenyl} xanthine (XAC), 8-(3-chlorostyryl) caffeine (CSC), and (E)-1,3-dipropyl-8-(3,4-dimethoxystyryl)-methylxanthine (KF 17,837) as displacers of [³H] CGS 21680 (5 nM) binding in the hippocampus and cerebral cortex was DPCPX > XAC ≫ CSC ≈ KF 17,837, and the relative potency order of adenosine agonists CHA, NECA, CADO, 2-[(2-aminoethylamino)carbonylethylphenylethylamino]-5′-N-ethylcar-boxamidoadenosine (APEC), and 2-phenylaminoadenosine (CV 1808) was CHA ≈ NECA ⩾ CADO > APEC ≈ CV1808 > CGS 21680. In the presence of DPCPX (20 nM), [³H] CGS 21680 (0.2-200 nM) bound to a site (A_2A-like) with a K _d of 20 nM and a B _max of 56 fmol/mg protein in the hippocampus and with a K _d of 22 nM and a B _max of 63 fmol/mg protein in the cortex. In the presence of CSC (200 nM), [³H]CGS 21680 (0.2–200 nM) bound to a second high-affinity site with a K _d of 97 nM and a B _max of 255 fmol/mg protein in the hippocampus and with a K _d of 112 nM and a B _max of 221 fmol/mg protein in the cortex. Two pharmacologically distinct [³H]CGS 21680 binding sites were found in synaptosomal membranes of the hippocampus and cortex and in the striatum, one corresponding to A_2A receptors and the other to the second high-affinity [³H]CGS 21680 binding site. In contrast, the pharmacology of [³H]CHA binding was similar in synaptosomal membranes of the three brain areas. The present results establish the existence of at least two high-affinity [³H]CGS 21680 binding sites in the CNS and demonstrate that the [³H]CGS 21680 binding site predominant in the hippocampus and cerebral cortex has different binding characteristics from the classic A_2A adenosine receptor, which predominates in the striatum.