[3H]tryptamine autoradiography in rat brain and choroid plexus reveals two distinct sites

In vitro autoradiographic techniques were used to examine the distribution of [3H]tryptamine binding sites in rat brain. The gross distribution and pharmacological characteristics of binding to brain sections resembled those seen in homogenate studies. Binding sites were found throughout the brain, with a preponderance of sites in the forebrain and limbic structures; highest levels were seen in the choroid plexus and the interpeduncular nucleus. Other regions exhibiting high levels of [3H]tryptamine binding include the cortex (especially lamina I), caudate putamen, hippocampus, anterior olfactory nucleus, olfactory tubercle, nucleus accumbens, amygdala, superior colliculus (superficial gray layer), locus ceruleus, the nucleus of the solitary tract and the pineal body. Although there were similarities in this distribution to that for binding sites of [3H]5-hydroxytryptamine ([3H]serotonin), the overall patterns were distinct. The binding site for [3H] tryptamine in the choroid plexus (termed T-2) was pharmacologically distinct from that in the rest of the brain (termed T-1); several compounds, including kynuramine, were potent inhibitors of [3H]tryptamine binding at the brain site, but not at the choroid plexus site. [3H]Serotonin also labels a site in the rat choroid plexus; this site was different from both [3H]tryptamine sites. Knowledge of the distribution of tryptamine binding sites in the brain will aid in efforts to ascertain the function of these sites.