[(3)H]ZM241385--an Antagonist Radioligand for Adenosine A(2A) Receptors in Rat Brain

Binding of the novel adenosine A(2A) receptor-selective antagonist radioligand [2-(3)H]-4-(2-[7-amino-2-(2-furyl)(1,2,4)triazolo(2,3-a)(1,3,5,)triazin-5-yl amino]ethyl)phenol ([(3)H]ZM241385) was examined using particulate preparations and frozen sections of rat brain. In membranes from the rat striatum, binding was saturable, reversible and temperature-dependent. Analysis of saturation isotherms indicated that [(3)H]ZM241385 bound with high affinity (K(d) of 0.84 nM), high density (1680 fmol mg protein(-1)) and with a high proportion of specific binding (93% at 1 nM radioligand). Examination of competition profiles indicated that [(3)H]ZM241385 bound to sites with an A(2A) adenosine receptor-like rank order. The presence of guanosine 5'-(3-thio)-triphosphate failed to alter either [(3)H]ZM241385 binding or agonist competition for [(3)H]ZM241385 binding. Autoradiographic analysis of [(3)H]ZM241385 binding to frozen sections of rat brain indicated specific binding to the rat striatum of similar affinity (K(d) of 0.43 nM) and susceptibility to adenosine receptor ligands. At 2 nM [(3)H]ZM241385, specific binding comprised 95+/-1% total binding. In the hippocampus and frontal cortex, binding of [(3)H]ZM241385 failed to saturate and was of lower density. Taken together, these results indicate that [(3)H]ZM241385 should prove to be a useful radioligand in the characterisation of adenosine A(2A) receptors.