Labelling of 5-Hydroxytryptamine3 Receptors with a Novel 5-HT3 Receptor Ligand, [3H]RS-42358–197

Abstract: RS-42358–197{(S)-N-(1-azabicyclo[2.2.2]oct-3-yl)-2,4,5,6-tetrahydro-1H-benzo[de]isoquinolin-1-one hydrochloride} displaced the prototypic 5-hydroxytryptamine₃ (5-HT₃) receptor ligand [³H]quipazine in rat cerebral cortical membranes with an affinity (pK_i) of 9.8 ± 0.1, while having weak affinity (pK_i < 6.0) in 23 other receptor binding assays. [³H]RS-42358–197 was then utilized to label 5-HT₃ receptors in a variety of tissues. [³H]RS-42358–197 labelled high-affinity and saturable binding sites in membranes from rat cortex, NG108–15 cells, and rabbit ileal myenteric plexus with affinities (K_D) of 0.12 ± 0.01, 0.20 ± 0.01, and 0.10 ± 0.01 nM and densities (B_max) of 16.0 ± 2.0, 660 ± 74, and 88 ± 12 fmol/mg of protein, respectively. The density of sites labelled in each of these tissues with [³H]RS-42358–197 was similar to that labelled with [³H]GR 65630, but was significantly less than that found with [³H]-quipazine. The binding of [³H]RS-42358–197 had a pharmacological profile similar to that of [³H]quipazine, as indicated by the rank order of displacement potencies: RS-42358–197 > (S)-zacopride > tropisetron > (R)-zacopride > ondansetron > MDL72222 > 5-HT. However, differences in 5-HT₃ receptors of different tissues and species were detected on the basis of statistically significant differences in the affinities of phenylbiguanide, and 1-(m-chlorophenyl)biguanide when displacing [³H]RS-42358-197 binding. [³H]RS-42358–197 also labelled a population (B_max= 91 ± 17 fmol/mg of protein) of binding sites in guinea pig myenteric plexus membranes, with lower affinity (K_D= 1.6 ± 0.3 nM) than those in the other preparations. Moreover, the rank order of displacement potencies of 15 5-HT₃ receptor ligands in guinea pig ileum was found not to be identical to that in other tissues. Binding studies carried out with [³H]RS-42358–197 have detected differences in 5-HT₃ receptor binding sites in tissues of different species and further underscore the unique nature of the guinea pig 5-HT₃ receptor.