Differences in the regulatory adaptation of the 5HT recognition sites labelled by 3H-mianserin or 3H-ketanserin

In crude synaptic membranes prepared from rat brain the sites occupied by 3H-spiroperidol that are displaced by microM concentrations of serotonin (5HT) have been termed 5HT2 receptors (Peroutka and Snyder, 1980). Since the 3H-spiroperidol displaced by 5HT is also displaced very effectively (IC50 in the nM range) by ketanserin and mianserin it was suggested that spiroperidol, mianserin and ketanserin are labelling 5HT2 receptors. Data are presented showing that the 3H-ketanserin and 3H-mianserin bound to crude synaptic membrane in the presence of a H1 receptor blocker are not labelling the same recognition site. Hence from this standpoint the recognition site marked by 3H-mianserin and 3H-ketanserin is not identical. The possibility that allosteric effects are operative in some of these ligand displacements should be entertained.