Figure. Concentration-dependent activation of HTR4 by serotonin. Reporter cells were transfected with either the expression plasmid for 5-hy­droxy­tryp­ta­mine receptor 4 (HTR4) or the mock plasmid and treated with various concentrations of serotonin. Data points shown are the mean ± SEM of an experiment performed in triplicate, and the curve is a fit to Hill equation with an EC50 of 26 nM.