The binding characteristics of a radiolabelled
5-HT3 receptor agonist, [3H]meta-chlorophenylbiguanide (
mCPBG), were examined in membranes from N1E-115
neuroblastoma cells. Scatchard plots of saturation binding data showed the presence of two populations of binding sites, with Kd = 0.03 +/- 0.01 nM and 4.4 +/- 1.2 nM and Bmax = 11.9 +/- 4.2 and 897.9 +/- 184.7 fmol/mg
protein respectively. Competition studies with a selection of agonists and antagonists revealed the pharmacological profile expected for a
5-HT3 receptor. The rank order of potency for antagonists was
granisetron >
quipazine >
GR65630 >
ondansetron > MDL72222, and for agonists was
mCPBG >
5-HT (5-hydroxytryptamine, serotonin) >
2-methyl-5-HT. IC50 values for
5-HT and
2-methyl-5-HT were lower than those observed using radiolabelled antagonists, and combined with functional experiments, the data suggest that [3H]
mCPBG may label high affinity desensitized states of the receptor. We conclude that [3H]
mCPBG labels 5-HT3 receptors in N1E-115
neuroblastoma cell membranes and may be a useful compound with which to explore 5-HT3 receptors in other systems.