Improvement of 5-HT3 Receptor Binding Assay: Enhancement of Specific [3H]Quipazine Binding with Triton X-100-Treated Membranes from Rat Cortex

The 5-hydroxytryptamine (5-HT)3 receptor binding assay using [3H]quipazine was examined. It was impossible to obtain specific [3H]quipazine binding with the membrane fractions from rat cortex prepared by the usual procedure. When the membranes were pretreated with detergent Triton X-100, the ratio of specific [3H]quipazine binding markedly increased, depending upon the concentration of Triton X-100 in the range of 0.01-0.1% (w/v). At a concentration of more than 0.05%, the specific binding reached a maximum of 55 to 60% of the total binding. The specific [3H]quipazine binding to the Triton X-100-treated membranes was reversible and was potently inhibited by several 5-HT3 antagonists, while 5-HT1, 5-HT2 receptor antagonists and other receptor-specific ligands had no effect on the binding. Scatchard analysis indicated a single class of binding sites with a Kd of 0.62 nM and Bmax of 97 fmol/mg protein. Thus, the Triton X-100-treated membranes retained the characteristics of 5-HT3 binding sites, making it possible to use [3H]quipazine for a 5-HT3 receptor binding assay with a high ratio of specific binding.