FURTHER CHARACTERIZATION OF THE BINDING OF THE ADENOSINE RECEPTOR AGONIST [H-3] CGS-21680 TO RAT-BRAIN USING AUTORADIOGRAPHY

2-[p-(2-carboxyethyl)-phenylethylamino]-5'-N-ethylcarboxamidoadenosine (CGS 21680) is considered a selective ligand for adenosine A(2A) receptors, which are known to be enriched in striatum and olfactory tubercle. We have investigated the characteristics of [H-3]CGS 21680 binding in several brain regions using quantitative autoradiography. In agreement with previous data the radioligand was found to label the caudate-putamen, accumbens nucleus, olfactory tubercle and globus pallidus, but also many other structures, e.g. cerebral and cerebellar cortex, hippocampus, thalamus and some brainstem nuclei, were labelled. Cortical and striatal binding of [H-3]CGS 21680 was unaltered by high concentrations of the adenosine transport inhibitor dipyridamole or the phosphodiesterase inhibitor rolipram but was displaced by 1,3-diethyl-8-phenylxanthine, the A(2) selective adenosine antagonist CP 66,713, and the A(2A) selective agonist SHA 118. These three agents were approximately equipotent in striatum, cortex and hippocampus. The A(2) selective agonist CV 1808 was a 4-5 times more potent displacer in cortex and hippocampus than in the striatum. [H-3]CGS 21680 binding was strongly magnesium-dependent in all the studied brain regions, in contrast to the binding of adenosine A(1) agonists. The binding of [H-3]CGS 21680 to cerebral cortex and hippocampus, but not the binding to striatum, was displaced by the adenosine receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine in nanomolar concentrations. The present study provides evidence that in cerebral cortex and hippocampus, most of the [H-3]CGS 21680 binds to a receptor site that is distinct from the striatal A(2A) receptor and the classical adenosine A(1) receptor and may represent a hitherto unrecognized binding site.