Conformational changes of channel activation: Five enhanced green fluorescent protein (EGFP) mols. (green cylinders) were integrated into the intracellular part of the homopentameric ionotropic 5-HT3 receptor. This allowed the detection of extracellular binding of fluorescent ligands (.bul.) to EGFP by FRET, and also enabled the quantification of agonist-induced conformational changes in the intracellular region of the receptor by homo-FRET between EGFPs. The approach opens novel ways for probing receptor activation and functional screening of therapeutic compds. [on SciFinder (R)]