Fluorescent antagonists for human 5-HT(4) receptors were synthesized based on ML10302 1, a potent 5-HT(4) receptor agonist and on piperazine analogue 2. These molecules were derived with three fluorescent moieties, dansyl, naphthalimide, and NBD (7-nitrobenz-2-oxa-1,3-diazol-4-yl), through alkyl chains. The synthesized molecules were evaluated in binding assays on the recently cloned human 5-HT(4(e)) receptor isoform stably expressed in C6 glial cells with [(3)H]GR113808 as the radioligand. The affinity values depended upon the basal structure together with the alkyl chain length. The derivatives based on ML10302 were more potent ligands than the derivatives based on piperazine analogue. For ML10302-based ligands, dansyl and NBD derivatives attached through a chain length of one carbon atom 17a and 32, respectively, led to affinities close to the affinity of ML10302. The most potent compounds 17a, 28, and 32 produced an inhibition of the 5-HT stimulated cyclic AMP synthesis in the same cellular system with nanomolar K(b) values. Fluorescent properties of 17a, 28, and 32 were more particularly studied. Interactions of the fluorescent ligand 28 with the h5-HT(4(e)) receptor were indicated using h5-HT(4(e)) receptor transfected C6 glial cell membranes and entire cells. Ligand 28 was also used in fluorescence microscopy experiments in order to label h5-HT(4(e)) receptor transfected C6 glial cells, and subcellular localization of these receptors was more precisely determined using confocal microscopy.