Characterization and quantitative autoradiography of [3H]tryptamine binding sites in rat brain

[3H]Tryptamine binds with high affinity (Kd = 9.1 nM, Bmax = 54 fmol/mg wet wt.) to tissue sections of rat brain. The binding occurs rapidly and is reversible. Low concentrations of the beta-carbolines harmaline (IC50 = 25 nM) and tetrahydronorharman (tetrahydro-beta-carboline, IC50 = 50 nM) inhibit [3H]tryptamine binding. Serotonin (5-HT, IC50 = 2600 nM) as well as the 5-HT receptor antagonists methysergide and metergoline displace [3H]tryptamine at much higher concentrations from brain slices. The distribution of [3H]tryptamine binding sites in sections of rat brain has been analyzed by quantitative autoradiography. The highest density of binding sites is found in the nucleus (n.) interpeduncularis, a slightly lower one in the locus coeruleus. Moderately labelled are the n. accumbens septi, n. septi lateralis, n. medialis habenulae, n. tractus olfactorii lateralis, the central region of the amygdala, n. caudatus/putamen, n. reuniens and the hippocampal formation. A low density of binding sites is detected in the cerebral cortex and the subiculum. Even less binding sites are found in the n. dorsalis raphe and the substantia nigra. The pattern of distribution of [3H]tryptamine binding sites differs from that of [3H]5-HT (5-HT1), [3H]ketanserin (5-HT2) as well as [3H]imipramine binding sites. These data suggest unique tryptamine binding sites.