Binding of adenosine receptor ligands to brain of adenosine receptor knock-out mice: evidence that CGS 21680 binds to A1 receptors in hippocampus

摘要

The adenosine receptor agonist 2-[p-(2-carboxyethyl)phenylethylamino]-5′-N-ethylcarboxamidoadenosine (CGS 21680) is generally considered to be a selective adenosine A2A receptor ligand. However, the compound has previously been shown to exhibit binding characteristics that are not compatible with adenosine A2A receptor binding, at least in brain regions other than the striatum. We have examined binding of [3H]CGS 21680 and of antagonist radioligands with high selectivity for adenosine A1 or A2A receptors to hippocampus and striatum of mice lacking either adenosine A1 (A1R(−/−)) or A2A (A2AR(−/−)) receptors. Both receptor autoradiography and membrane binding techniques were used for this purpose and gave similar results. There were no significant changes in the binding of the A1 receptor antagonist [3H]DPCPX in mice lacking A2A receptors, or in the binding of the A2A receptor antagonists [3H]SCH 58261 and [3H]ZM 241385 in mice lacking A1 receptors. Furthermore, [3H]CGS 21680 binding in striatum was abolished in the A2AR(−/−), and essentially unaffected in striatum from mice lacking A1 receptors. In hippocampus, however, binding of [3H]CGS 21680 remained in the A2AR(−/−), whereas binding was virtually abolished in the A1R(−/−). There were no adaptive alterations in A2A receptor expression in this region in A1R(−/−) mice. Thus, most of the [3H]CGS 21680 binding in hippocampus is dependent on the presence of adenosine A1 receptors, but not on A2A receptors, indicating a novel binding site or novel binding mode.