[3H]zacopride: ligand for the identification of 5-HT3 recognition sites.

[3H]Zacopride displayed saturable binding to homogenates of the rat entorhinal cortex as measured by the inclusion of the 5-HT3 receptor antagonist BRL43694 in the incubation media. Scatchard analysis indicated a single high affinity binding site (KD 0.76 +/- 0.08 nM, Bmax 77.5 +/- 6.5 fmol (mg protein)-1) with a Hill slope close to unity. Other 5-HT3 receptor antagonists (zacopride, ICS 205-930, GR38032F, GR65630, metoclopramide and cocaine) also competed for the binding site displacing 60% of the total [3H]zacopride binding. 5-HT and 2-methyl-5-HT also were competitive antagonists for [3H]zacopride binding whereas 5-HT1/5-HT2 agonists and antagonists, and agents acting on other neurotransmitter receptors had Ki values greater than 10(-5) M. It is concluded that [3H]zacopride may prove a useful ligand for the study of 5-HT3 recognition sites.