Tetrahydro-P-carbolines (THBCs), 6-methoxyharmalan and norharman and the corresponding open chain tryptamines studied inhibited 5-hydroxytryptamine (5-HT) and dopamine (DA) uptake in human blood platelets. I-methyl-THBCs were better inhibitors of D A than 5-HT uptake but THBCs generally were far more potent inhibitors of 5-HT uptake than of DA uptake. 6-methoxy-1,2,3,4-tetrahydro-P-carboline was as potent as 5-HT itself in 'H-5-HT uptake inhibition in platelets and the inhibition wascompetitive. All the P-carbolines studied were more potent inhibitors of 'H-DA uptake than DA itself. Contrary to results in rat brain synaptosomes, THBCs were more potent in platelets than the corresponding tryptamines with the freely rotating ethylamine side chain. Unsaturated P-carbolines were weaker inhibitors than THBCs. The clear difference in the rank order of potencies of these compounds in human platelets and rat brain synaptosomes demonstrates that these different model systems for amine uptake studies d o not always give comparable results. The results also suggest that there are differences in the uptake systems for 5-HT and D A in human platelets.

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