Autoradiographic localization of H1-histamine receptors in brain using 3H-mepyramine: preliminary studies

3H-Mepyramine (3H-MEP) binds to slide-mounted tissue sections with characteristics indicating the labelling of a H1-histamine receptor. It is saturable (Bmax 12 pg/g tissue), of a high affinity (KD 0.5 mM) and possesses a drug specificity similar to that observed for H1 receptors in a variety of systems. In agreement with other biochemical studies, the association constant and the number of binding sites appear lower in the rat than in the guinea pig. Once the optimal conditions for labelling the receptors were found in the biochemical studies, tissue sections were prepared for autoradiographic studies. The receptor density varied markedly in the different brain regions. For example, there was a high density of H1 receptors in the molecular layer of the guinea pig cerebellum. In the guinea pig hippocampal formation, the highest densities were found in the molecular layer of the dentate gyrus while lower levels were found in the dendritic fields of the pyramidal cells. Significant densities of receptors were found in other areas of the rat and guinea pig brain as well.