BINDING OF [H-3] KF17837S, A SELECTIVE ADENOSINE A(2) RECEPTOR ANTAGONIST, TO RAT-BRAIN MEMBRANES

The potential of 8-(3,4-dimethoxystyryl)-1,3-dipropyl-7-[H-3]methylxanthine ([H-3]KF17837S) as a highly selective antagonist radioligand for the adenosine A(2A) receptor was examined and compared with the properties of the adenosine A(2A) receptor agonist radioligand 2-[p-(2-[H-3]carboxyethyl)phenethylamino]-5'-N-ethylcarboxamidoadenosine ([H-3]CGS21680). [H-3]KF17837S specific binding to rat striatal membranes was saturable and reversible. Saturation studies showed that the binding of [H-3]KF17837S occurred at a single site, with high affinity (K-d, 7.1 +/- 0.91 nM) and limited capacity (B-max, 1.3 +/- 0.23 pmol/mg of protein). Adenosine receptor antagonist ligands competed with the binding of 1 nM [H-3]KF17837S with the following order of activity: CGS15943 > KF17837S > N-[2-(dimethylamino)ethyl]-N-methyl-4-(2,3,6,7-tetrahydro-2,6-dioxo-1,3-dipropyl-1H-purin-8-yl)benzenesulfonamide greater than or equal to xanthine amine congener > 8-cyclopentyl-1,3-dipropylxanthine > 8-(noradamantan-3-yl)-1,3-dipropylxanthine > caffeine. Adenosine receptor agonists inhibited [H-3]KF17837S binding in the following order: 5'-N-ethylcarboxamidoadenosine greater than or equal to CGS21680 > 2-phenylaminoadenosine greater than or equal to (R)-N-6-phenylisopropyladenosine > N-6-cyclopentyladenosine > (S)-N-6-phenylisopropyladenosine. The K-i values of the antagonists for [H-3]KF17837S binding and the rank order of potency were similar to those for [H-3]CGS21680 binding. The affinities of the agonists were lower with [H-3]KF17837S binding than with [H-3]CGS21680 binding. However, a strong positive correlation (r = 0.98) was observed between the pharmacological profiles for these two radioligand assays. The inhibition curve for CGS21680 was best fitted to a two-component binding model and addition of GTP shifted the inhibition curve to the right, suggesting that [H-3]KF17837S labeled two agonist coupling states. Other pharmacological agents had negligible affinities for the [H-3]KF17837S binding site. Autoradiographic study of [H-3]KF17837S binding using rat brain sections revealed that the binding site was highly enriched in the striatal region. These data indicate that [H-3]KF17837S labels the adenosine A(2A) receptor in rat brain.