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H Nonaka, A Mori, M Ichimura, T Shindou, K Yanagawa, J Shimada and H Kase
Pharmaceutical Research Laboratories, Kyowa Hakko Kogyo Co., Ltd., Shizouka, Japan.
The potential of 8-(3,4-dimethoxystyryl)-1,3-dipropyl-7-[3H] methylxanthine ([3H]Kf17837S) as a highly selective antagonist radioligand for the adenosine A2A receptor was examined and compared with the properties of the adenosine A2A receptor agonist radioligand 2- [p-(2-[3H]carboxyethyl)phenethylamino]-5'-N-ethyl- carboxamidoadenosine ([3H]CGS21680). [3H]KF17837S specific binding to rat striatal membranes was saturable and reversible. Saturation studies showed that the binding of [3H]KF17837S occurred at a single site, with high affinity (Kd, 7.1 +/- 0.91 nM) and limited capacity (Bmax, 1.3 +/- 0.23 pmol/mg of protein). Adenosine receptor antagonist ligands competed with the binding of 1 nM [3H]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)benzenesul fonamide > or = xanthine amine congener > 8-cyclopentyl-1,3-dipropylxanthine > 8- (noradamantan-3-yl)-1,3-dipropylxanthine > caffeine. Adenosine receptor agonists inhibited [3H] KF17837S binding in the following order: 5'-N- ethylcarboxamidoadenosine > or = CGS21680 > 2-phenylaminoadenosine > or = (R)- N6-phenylisopropyladenosine > N6-cyclopentyladenosine > (S)- N6- phenylisopropyladenosine. The Ki values of the antagonists for [3H]KF17837S binding and the rank order of potency were similar to those for [3H]CGS21680 binding. The affinities of the agonists were lower with [3H]KF17837S binding than with [3H] 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 [3H]KF17837S labeled two agonist coupling states. Other pharmacological agents had negligible affinities for the [3H]KF17837S binding site. Autoradiographic study of [3H]KF17837S binding using rat brain sections revealed that the binding site was highly enriched in the striatal region. These data indicate that [3H] KF17837S labels the adenosine A2A receptor in rat brain.
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