Discovery and pleiotropic effects of a "Feeding Circuit-Activating Peptide"-type neuropeptide in the mollusk Lymnaea stagnalis.

Neuropeptides play a central role in the regulation of animal physiology and behavior. Mollusks have been pivotal in the discovery of neuropeptide signaling, yet the diversity and functional roles of neuropeptides remain incompletely understood across the phylum. In this study, we identified and characterized a novel Feeding Circuit-Activating Peptide (FCAP)-type neuropeptide in the widely used molluscan model, the great pond snail (Lymnaea stagnalis). Using mass spectrometry imaging (MSI), we detected a new peptide, ALDLSGGFNVHGW, which differs by a single amino acid from FCAPb (ALDLSGGFQVHGW) of Aplysia californica. Sequence analysis revealed that the FCAP precursor of L. stagnalis contains 20 copies of ALDLSGGFNVHGW and one copy of an active peptide identical to Ap-FCAPb. Subsequent MSI analysis indicated that ALDSLGGFNVHGW is the main active FCAP-type peptide, predominantly localized in the anterior lobe of the right cerebral ganglion and sparsely in the buccal ganglia. Furthermore, ALDLSGGFNVHGW co-localized with APGWamide, but not with FMRFamide or cardioexcitatory peptide. Behavioral assays demonstrated dose-dependent, pleiotropic effects of ALDLSGGFNVHGW: transient suppression of feeding, delayed enhancement of locomotor activity, and robust stimulation of egg-laying. Together, these anatomical, molecular, and behavioral data support the view that peptides of the FCAP family are not restricted to feeding modulation but instead act as multifunctional regulators. Future studies, including identification of the receptor(s) that mediate the effects of FCAP-type peptides, are required to deepen our understanding of their functions and evolutionary relationships with neuropeptides in other phyla.