Presynaptic localization of histamine H 3-receptors in rat brain

Abstract

The localization of histamine H 3-receptors in subcellular fractions from the rat brain was examined in a [ 3H](R)α-methylhistamine binding assay and compared with those of histamine H 1- and adrenaline α 1- and α 2-receptors. Major [ 3H](R)α-methylhistamine binding sites with increased specific activities ([ 3H]ligand binding vs. protein amount) were recovered from the P 2 fraction by differential centrifugation. Minor [ 3H](R)α-methylhistamine binding sites with increased specific activities were also detected in the P 3 fraction. Further subfractionation of the P 2 fraction by discontinuous sucrose density gradient centrifugation showed major recoveries of [ 3H](R)α-methylhistamine binding in myelin (MYE) and synaptic plasma membrane (SPM) fractions. A further increase in specific activity was observed in the MYE fraction, but the SPM fraction showed no significant increase in specific activity. Adrenaline α 2-receptors, the pre-synaptic autoreceptors, in a [ 3H] yohimbine binding assay showed distribution patterns similar to histamine H 3-receptors. On the other hand, post-synaptic histamine H 1- and adrenaline α 1-receptors were closely localized and distributed mainly in the SPM fraction with increased specific activity. Only a negligible amount was recovered in the MYE fraction, unlike the histamine H 3- and adrenaline α 2-receptors.