[ 3 H]SCH 58261, a Selective Adenosine A 2A Receptor Antagonist, Is a Useful Ligand in Autoradiographic Studies

• Published in: Journal of neurochemistry Vol. 70; no. 3; pp. 1210 - 1216
• Main Authors: Fredholm, Bertil B., Lindström, Karin, Dionisotti, Silvio, Ongini, Ennio
• Format: Journal Article
• Language: English
• Published: 01.03.1998
• ISSN: 0022-3042; 1471-4159
• DOI: 10.1046/j.1471-4159.1998.70031210.x

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 ([ 3 H]CGS 21680) or A 1 receptors ( N 6 ‐[ 3 H]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 n M versus 0.8 µ M ). Moreover, receptor autoradiography showed that [ 3 H]SCH 58261, in concentrations below 2 n M , binds only to the dopamine‐rich regions of the rat brain, with a K D value of 1.4 (0.8–1.8) n M . The maximal number of binding sites was 310 fmol/mg of protein in the striatum. Below concentrations of 3 n M , the nonspecific binding was <15%. Three adenosine analogues displaced all specific binding of [ 3 H]SCH 58261 with the following estimated K i values (n M ): 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 µ M ) or Mg 2+ (10 m M ). 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 [ 3 H]CGS 21680.