To visualize the 5-hydroxytryptamine(2A) (5-HT2A) receptor, we developed a 5-HT2A receptor fused with yellow fluorescent protein (5-HT2A-YFP) and expressed this receptor in HEK293 cells. In 5-HT2A-YFP-expressing cells, but not in YFP-expressing or non-expressing cells, 5-HT induced a transient increase in the intracellular Ca2+ concentration (Ca2+ transient) in the Fluo 3 assay, suggesting that 5-HT2A-YFPpossesses a function similar to the wild-type 5-HT2A receptor. Interestingly, not only 5-HT but also insulin induced the internalization of 5-HT2A-YFP. Insulin also inhibited the 5-HT-induced Ca2+ transient. Genistein, an inhibitor of tyrosine kinase, blocked these insulin effects. Our results provide the first evidence that insulin receptor signaling via tyrosine kinase activation induces internalization of the plasma membrane 5-HT2A receptor, and demonstrate crosstalk between the 5-HT2A receptor and the insulin receptor. (c) 2005 Elsevier B.V. All rights reserved.