Utilization of Zacopride and its R- and S-Enantiomers in Studies of 5-HT₃ Receptor “Subtypes”

Summary

5-HT₃ binding sites with high affinity (Kd = 0.3 – 1.0 nM) for [³H]zacopride have been studied in brain, vagus nerve, enteric neurons, and PC12 cells. A novel 5-HT₃ binding site in guinea pig small bowel exhibits decreased affinity for zacopride and other 5-HT₃ antagonists but a similar rank order of potency to that of 5-HT₃ binding sites in rabbit bowel and vagus nerve. The (S)-enantiomer of zacopride exhibits a 10–40 fold greater affinity for 5-HT₃ binding sites than its (R)-enantiomer, a ratio which correlates with their antimetic potency. However (R)-zacopride is > 1000 fold more potent that the (S)-enantiomer as an anxiolytic. Together with our ligand binding and radioautographic studies, this suggests that some of the therapeutic effects of the R isomer may not be due to binding at 5-HT₃ receptors. [³H](S)-zacopride also labelled a 5-HT₃ binding site which is expressed in PC12 cells cultured with nerve growth factor. Finally, a 5-HT₃ binding site was solubilized from rabbit small bowel muscularis membranes, labelled with [³H](S)-zacopride and found to retain the binding properties of the membrane-bound form.