In this work package we address the question of the functionality of the neuropeptides and the way they activate their target cells. Focus is made only on the selected neuropeptides for which putative receptor candidates have been identified by sequence comparisons and phylogenetic studies (see above WP2). Because subtle sequence variability within a given peptide may have functional outcomes, structural characterization and molecular modelling of synthetic replicates of the most promising neuropeptides is studied using spectroscopic analytical methods (circular dichroism and NMR). The matching of the ligand/receptor pair is investigated via the development of a reverse pharmacology assay, a methodological approach originally used for mammalian and insect receptor desorphanisation and recently applied to characterize a new ligand-receptor couple in oyster. Finally, neuropeptide analogues are also designed for deciphering the molecular determinants of the peptide/receptor interactions in order to propose valuable ligands for modulating the reproduction system.