Binding of 4(5)-[2-(4-Azido-2-nitroanilino)ethyl]imidazole to Histamine Receptors in Guinea Pig Cerebral Cortical Membranes 1

Abstract The interaction of 4(5)-[2-(4-azido-2-nitroanilino)ethyl]imidazole (AAH), a photolabile histamine receptor antagonist, with the binding of histamine, mepyramine, and tiotidine to guinea pig cerebral cortical membranes was examined to evaluate the specificity of AAH for histamine H1 and H2 receptors. Saturable, specific binding of [3H]histamine, [3H]mepyramine, and [3H]tiotidine to the membranes was observed. Competition assays were used to assess the relative affinity of AAH for H1- and H2-receptors. The rank order of IC50 values obtained was (most to least potent) (i) for competing with [3H]histamine binding: histamine > AAH ≫ mepyramine ≃ tiotidine; (ii) for competing with [3H]mepyramine binding: mepyramine ≫ AAH ≃ histamine > tiotidine; and (iii) for competing with [3H]tiotidine binding: tiotidine ≫ mepyramine > histamine ≃ AAH. The affinity of AAH for H1 receptors was ca. 14-fold greater than for H2 receptors. These findings support previous evidence obtained in isolated smooth muscle preparations that AAH shows H1-receptor selectivity as an antagonist.