Changes in Characteristics of the Specific Binding of [³H]LY-278584, a 5-HT₃–Receptor Antagonist, on Differentiated NG108-15 Cells

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We have reported previously that the concentration of intracellular Ca²⁺ evoked by serotonin (5-HT) was significantly augmented in differentiated NG108-15 (NG) cells treated with dibutyryl cAMP and the enhanced response occurred via 5-HT₃ receptors. We investigated changes in the characteristics for specific binding of [³H]LY-278584 (a specific antagonist of the 5-HT₃ receptor) on membranes from differentiated NG cells. The results indicated that the K_d and B_max values for the specific binding to differentiated NG cells were significantly smaller and larger, respectively, than those for undifferentiated NG cells. The binding was significantly inhibited by 10 nM tropisetron, a specific 5-HT₃–receptor antagonist, but not by any other types of 5-HT–receptor antagonists. These results suggested that the enhanced response by 5-HT in differentiated NG cells was due to both qualitative and quantitative changes in the 5-HT₃ receptor.