NOVELTY - Conjugate (I) of an agonist ligand directed to ligand-gated ion channels (LGICs) of the Cys loop-family with a chromophore, is new. USE - (I) is useful in the receptor proteins and/or their interaction with the agonistic ligand, where the interaction with the agonistic ligand is measured in a competition experiment with an unlabelled compound (claimed). (I) is useful in unconjugated ligands in competition experiments to study the effect of such unlabelled ligands on a receptor, to study receptor activation, to characterize the receptors, the compounds and other unlabelled ligands for competition or modulation. ADVANTAGE - (I) allows sensitive on-line and in vivo data acquisition. (I) comprise a ligand giving rise to a selective receptor recognition allowing specific receptor binding assays and the study of receptor properties. (I) are agonists thus allowing one to study receptor activation and/or effect. (I) allows combined measurement of receptor binding and activation. (I) has high efficacy in binding to the cognate receptors. DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for method of characterisation and quantification of receptors, where (I) is contacted with the corresponding receptor and the chromophore bound to the receptor or the activation of the receptor is measured.