[3H]SCH 23390 labels a novel 5-hydroxytryptamine binding site in human blood platelet membranes

In human blood platelet membranes, 5-HT displaced the binding of the putative selective D-1 dopamine receptor antagonist [3H]SCH 23390 in a competitive manner with a Ki value of 5.7 +/- 0.8 nM, which was about 1000-fold lower than the Ki value for dopamine (Ki = 4400 +/- 150 nM). Thus the 'D-1 dopamine-like' site in human blood platelet membranes described previously corresponds to a 5-HT1-type site. [3H]SCH 23390 competition experiments with a number of serotonergic drugs disclosed a pharmacological profile that was distinct from the four 5-HT1 site subtypes reported previously. We therefore propose that this novel 5-HT site be designated the 5-HT1E site. Binding of [3H]SCH 23390 to 5-HT1-type sites could not be detected in several regions of the human brain. In some regions, however, 5-HT displaced part of the [3H]SCH 23390 binding with a K1 value of 320-380 nM. These sites correspond to 5-HT2 receptors.