[3H]SCH 58261, a selective adenosine A2A receptor antagonist, is a useful ligand in autoradiographic studies

We have characterized the new potent and selective nonxanthine adenosine A(2A) receptor antagonist SCH 58261 as a new radioligand for receptor autoradiography. In autoradiographic studies using agonist radioligands for A(2A) receptors ([H-3]CGS 21680) or A(1) receptors (N-6-[H-3]cyclohexyladenosine), it was found that SCH 58261 is close to 800-fold selective for rat brain A(2A) versus A(1) receptors (K-i values of 1.2 nM versus 0.8 mu M). Moreover, receptor autoradiography showed that [H-3]SCH 58261, in concentrations below 2 nM, binds only to the dopamine-rich regions of the rat brain, with a K-D value of 1.4 (0.8-1.8) nM. The maximal number of binding sites was 310 fmol/mg of protein in the striatum. Below concentrations of 3 nM, the nonspecific binding was <15%. Three adenosine analogues displaced all specific binding of [H-3]SCH 58261 With the following estimated K-i values (nM): 2-hex-1-ynyl-5'-N-ethylcarboxamidoadenosine, 3.9 (1.8-8.4); CGS 21680, 130 (42-405); N-6-cyclohexyladenosine, 9,985 (3,169-31,462). The binding of low concentrations of SCH 58261 was not influenced by either GTP (100 mu M) or Mg2+ (10 mM). The present results show that in its tritium-labeled form, SCH 58261 appears to be a good radioligand for autoradiographic studies, because it does not suffer from some of the problems encountered with the currently used agonist radioligand [H-3]CGS 21680.