Evidence that the atypical 5‐HT₃ receptor ligand, [³H]‐BRL46470, labels additional 5‐HT₃ binding sites compared to [³H]‐granisetron

The radioligand binding characteristics of the ³H‐derivative of the novel 5‐HT₃ receptor antagonist BRL46470 were investigated and directly compared to the well characterized 5‐HT₃ receptor radioligand [³H]‐granisetron, in tissue homogenates prepared from rat cerebral cortex/hippocampus, rat ileum, NG108‐15 cells, HEK‐5‐HT₃As cells and human putamen In rat cerebral cortex/hippocampus, rat ileum, NG108‐15 cell and HEK‐5‐HT₃As cell homogenates, [³H]‐BRL46470 bound with high affinity (K_d (nM): 1.57 ± 0.18, 2.49 ± 0.30, 1.84 ± 0.27, 3.46 ± 0.36, respectively; mean ± s.e. mean, n = 3–4) to an apparently homogeneous saturable population of sites (B_max (fmol mg⁻¹ protein): 102 ± 16, 44 ± 4, 968 ± 32 and 2055 ± 105, respectively; mean ± s.e.mean, n = 3–4) but failed to display specific binding in human putamen homogenates In the same homogenates of rat cerebral cortex/hippocampus, rat ileum, NG108‐15 cells, HEK‐5‐HT₃As cells and human putamen as used for the [³H]‐BRL46470 studies, [³H]‐granisetron also bound with high affinity (K_d (nM): 1.55 ± 0.61, 2.31 ± 0.44, 1.89 ± 0.36, 2.03 ± 0.42 and 6.46 ± 2.58 respectively; mean ± s.e.mean, n = 3–4) to an apparently homogeneous saturable population of sites (B_max (fmol mg⁻¹ protein): 39 ± 4, 20±2, 521 ± 47, 870 ± 69 and 18 ± 2, respectively; mean±s.e.mean, n = 3–4) Competition studies with a range of structurally different 5‐HT₃ receptor ligands indicated that in both rat cerebral cortex/hippocampus and rat ileum homogenates, [³H]‐BRL46470 binding exhibited a pharmacological profile consistent with the labelling the 5‐HT₃ receptor with compounds competing with Hill coefficients close to unity In HEK‐5‐HT₃As cell homogenates, [³H]‐BRL46470 and [³H]‐granisetron associated rapidly ((3.84 ± 0.4)10⁶ M⁻¹s⁻¹ and (5.85 ± 0.2)10⁶ M^−ls^−l respectively, mean ± s.e.mean, n = 3–4) in an apparently monophasic manner. Following the establishment of equilibrium, both [³H]‐BRL46470 and [³H]‐granisetron at a saturating concentration ([³H]‐BRL46470 approximately 16 nM; [³H]‐granisetron approximately 18 nM) and at a sub‐K_d concentration (approximately 1 nM for both radioligands) dissociated biphasically in HEK‐5‐HT₃As cell homogenates (saturating concentration; [³H]‐BRL46470 4.05 × 10⁻³±2.53 × 10⁻³ s⁻¹ and 5.83 × 10⁻⁵ ± 0.91 × 10⁻⁵ s⁻¹; [³H]‐granisetron 3.20 × 10⁻³± 1.70 × 10⁻³ s⁻¹ and 18.58 × 10⁻⁵±4.19 × 10⁻⁵ s⁻¹: sub‐K_d concentration; [³H]‐BRL46470 2.47 × 10⁻³± 1.18 × 10⁻³ s⁻¹ and 9.30 × 10⁻⁵ ± 2.59 × 10⁻⁵ s⁻¹; [³H]‐granisetron 65.91 × 10⁻³±22.14 × 10⁻³ s⁻¹ and 49.96 × 10⁻⁵± 12.26 × 10⁻⁵ s⁻¹, mean ± s.e.mean, n= 4–8) when induced by a 300 fold dilution in ice‐cold Tris/Krebs In conclusion, the present study provides evidence that [³H]‐BRL46470 specifically labels the 5‐HT₃ receptor in rat cerebral cortex/hippocampus, rat ileum, NG108‐15 cell and HEK‐5‐HT₃As cell homogenates, but fails to label the 5‐HT₃ receptor expressed in human putamen. Whilst the pharmacological profile of the site labelled by [³H]‐BRL46470 is directly comparable to that labelled by [³H]‐granisetron, [³H]‐BRL46470 consistently labelled approximately twice the density of sites compared to [³H]‐granisetron in the same tissue homogenates prepared from rat cortex/hippocampus, rat ileum, NG108‐15 cells and HEK‐5‐HT₃As cells. 1995 British Pharmacological Society