[3H]L-657, 743 (MK-912): A new, high affinity, selective radioligand for brain α2-adrenoceptors

Abstract L-657, 743 (MK-912), a highly potent and selective α 2 -adrenoceptor antagonist was tritiated to a high specific activity and its binding characteristics to brain tissue were determined. The specific binding of [ 3 H]L-657, 743 to rat cerebrocortex was saturable, reversible, and dependent on tissue concentration. In saturation studies, [ 3 H]L-657, 743 binding was resolved into two high affinity components exhibiting Kd values of 86 pM and 830 pM with densities of 82 fmol/mg protein and 660 fmol/mg protein, respectively. Based on the binding potencies of a variety of compounds with differing receptor selectivities, the sites labeled by [ 3 H]L-657, 743 were characteristic of α 2 -adrenoceptors. In contrast to α 2 -antagonists, α 2 -agonists displayed shallow competition curves. In the presence of 100 μM GTP, Gpp(NH) p or 150 mM NaCl, the competition curve for epinephrine was shifted to the right, whereas that for yohimbine was unaffected. In studies utilizing human cerebrocortical tissue, [ 3 H]L-657, 743 also bound with high affinity to sites characteristic of α 2 -adrenoceptors.