Synthesis and binding assays of H3-receptor ligands.

The preparation of a representative group of derivatives of the known H3-antagonist thioperamide is described. Binding affinity for histamine H3-receptors of thioperamide and its derivatives, which were obtained by substitution on the imidazole ring, was measured on rat brain cortex synaptosomes. Competitive binding assays were performed with two different labelled ligands, the physiological agonist [3H]histamine ([3H]HA) and the potent H3-agonist N alpha-[3H]methyl-histamine ([3]NAMHA). We observed a remarkable difference in Ki values obtained versus the two labelled ligands, both for thioperamide and its derivatives. In particular, 5-methylthioperamide showed a considerable selectivity for the system recognized by [3H]NAMHA, being about 100 times more potent versus this system than versus the system recognized by [3H]HA. On the basis of these observations, we suggest that it is necessary to consider this difference in evaluating the affinity of new compounds for the H3-receptors.