EVIDENCE FOR HIGH-AFFINITY BINDING-SITES FOR THE ADENOSINE A(2A) RECEPTOR AGONIST [H-3] CGS-21680 IN THE RAT HIPPOCAMPUS AND CEREBRAL-CORTEX THAT ARE DIFFERENT FROM STRIATAL A(2A) RECEPTORS

The binding of the adenosine A(2A) receptor selective agonist 2-[4-(2- p-carboxyethyl) phenylamino] -5'-N-ethylcarboxamidoadenosine (CGS 2168 0) to the rat hippocampal and cerebral cortical membranes was studied and compared with that to striatal membranes. [H-3] 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, an d 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 [H-3] CGS 21680 binding si te had a K-d of 17 nM and a B-max of 419 fmol/mg protein. Both guanyly limidodiphosphate (100 mu M) and Na+ (100 mM) reduced the affinity of [H-3] CGS 21680 binding in the striatum by half and virtually abolishe d [H-3] CCS 21680 binding in the hippocampus and cortex. The displacem ent curves of [H-3] CGS 21680 binding with 1,3-dipropyl-8-cyclopentylx anthine (DPCPX), N-6-cyclohexyladenosine (CHA), 5'-N-ethyl-carboxamido adenosine (NECA) and 2-chloro-adenosine (CADO) were biphasic in the hi ppocampus and cortex as well as in the striatum. The predominant [H-3] CGS 21680 binding site in the striatum (80%) had a pharmacological pro file compatible with A(2A) receptors and was also present in the hippo campus and cortex, representing 10-25% of [H-3]CGS 21680 binding. The predominant [H-3]CGS 21680 binding site in the hippocampus and cortex had a pharmacological profile distinct from A(2A) receptors: the relat ive potency order of adenosine antagonists DPCPX, dipropyl-8-{4-[(2-am inoethyl)amino]carbonylmethyl- oxyphenyl} xanthine (XAC), 8-(3-chloros tyryl) caffeine (CSC), and (E)-1,3-dipropyl-8-(3,4-dimethoxystyryl)- m ethylxanthine (KF 17,837) as displacers of [H-3] CGS 21680 (5 nM) bind ing in the hippocampus and cerebral cortex was DPCPX > XAC much greate r than CSC approximate to KF 17,837, and the relative potency order of adenosine agonists CHA, NECA, CADO, ylphenylethylamino]-5'-N-ethylcar boxamidoadenosine (APEC), and 2-phenylaminoadenosine (CV 1808) was CHA approximate to NECA greater than or equal to CADO > APEC approximate to CV1808 > CGS 21680. In the presence of DPCPX (20 nM), [H-3] CGS 216 80 (0.2-200 nM)a B-max of 56 fmol/mg protein in the hippocampus and wi th 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), [H-3]CGS 21680 (0.2-200 nM) bound to a s econd high-affinity site with a K-d of 97 nM and a B-max of 255 fmol/m g protein in the hippocampus and with a K-d of 112 nM and a B-max of 2 21 fmol/mg protein in the cortex. Two pharmacologically distinct [H-3] CGS 21680 binding sites were found ill synaptosomal membranes of the h ippocampus and cortex and in the striatum, one corresponding to A(2A) receptors and the other to the second high-affinity [H-3]CGS 21680 bin ding site. In contrast, the pharmacology of [H-3]CHA binding was simil ar in synaptosomal membranes of the three brain areas. The present res ults establish the existence of at least two high-affinity [H-3]CGS 21 680 binding sites in the CNS and demonstrate that the [H-3]CGS 21680 b inding site predominant in the hippocampus and cerebral cortex has dif ferent binding characteristics from the classic AZA adenosine receptor , which predominates in the striatum.