A brain-gut excitatory peptide/CCHamide homolog regulates satiation and motivational state transitions in the Aplysia feeding circuit.

Excitatory peptide (EP) and CCHamide (CCHa) are protostome neuropeptides originally identified in lophotrochozoans (including annelids and mollusks) and arthropods, respectively, and are homologous to the deuterostome endothelin (ET) and gastrin-releasing peptide (GRP)/neuromedin-B (NMB) systems. These peptides are brain-gut peptides: in vertebrates, GRP/NMB function as satiety peptides, whereas arthropod CCHa displays species-specific actions, either inhibiting or promoting feeding. However, the mechanisms by which these peptides modulate feeding circuits remain unknown. Here, we investigated the EP/CCHa signaling pathway in Aplysia, a mollusk with a well-defined feeding circuit. We identified a single precursor encoding Aplysia EP/CCHa (apEP/CCHa). Mass spectrometry demonstrated that an apEP/CCHa peptide is present in the central ganglia. In situ hybridization and immunohistochemistry revealed apEP/CCHa-positive neurons in the CNS, immunopositive cells in the gut, and immunopositive fibers in the gut-innervating esophageal nerve. To identify potential targets, we cloned two novel apEP/CCHa receptors. Phylogenetically, one receptor clusters with lophotrochozoan EP/CCHa receptors, whereas the other unexpectedly clusters with arthropod receptors, suggesting independent lineages for the two receptors. Single-cell RNA sequencing showed that both receptors are expressed in the key feeding central pattern generator (CPG) interneurons B20 and B34. Functionally, apEP/CCHa inhibited food intake in vivo and converted ingestive motor programs to egestive ones in vitro. At the circuit level, apEP/CCHa modulated excitability of B20 and B34, and two additional interneurons (B40, B64). In summary, we demonstrate that apEP/CCHa is a brain-gut peptide that functions as a satiation signal, and identify specific feeding CPG elements through which apEP/CCHa regulates motivational state transitions.