The potent 5-HT3 receptor antagonist (R)-zacopride labels an additional high affinity site in the central nervous system

The binding characteristics of [3H](S)-zacopride were investigated in membranes from the rat entorhinal cortex and NG 108-15 clonal cells. In contrast to [3H](S)-zacopride which bound solely to 5-HT3 receptors, [3H](R)-zacopride recognized another class of binding sites, called the (R)-sites, in both membrane preparations. In addition to (R)-zacopride (Ki=3–11 nM), only (R)-iodo-zacopride, (R)-dechloro-zacopride, prazosin and mianserin exhibited high to moderate affinity for the (R)-sites, whose possible functions remain to be established