Isolation, Localization, and Bioactivity of the FMRFamide‐Related Neuropeptides GYIRFamide and YIRFamide from the Marine Turbellarian Bdelloura candida

: Two FMRFamide-related neuropeptides, GYIRFamide and YIRFamide, were isolated from the marine turbellarian Bdelloura candida. The peptides elicited a dose-dependent contraction of isolated turbellarian muscle fibers, and both were more potent than FMRFamide. Structure-activity studies, using a range of analogues of the tetrapeptide amide, indicated that the structure of the endogenous peptides was optimal for peak activity. Immunocytochemistry, using an autologous antiserum, revealed a widespread distribution of peptide immunoreactivity within central and peripheral neurons and their processes. This study indicates an important role for GYIRFamide and YIRFamide in the control of neuromuscular function in turbellarians.