Signaling mechanisms of vasopressin/oxytocin-type neuropeptide-induced muscle contraction in the sea cucumber Apostichopus japonicus.

The myoregulatory action of vasopressin/oxytocin (VP/OT)-type neuropeptides is evolutionarily conserved across Bilateria. In vertebrates, the signaling cascades involved have been comprehensively characterized in several muscle types, including uterine and gastrointestinal smooth muscles. VP/OT-type neuropeptide-induced muscle contraction or relaxation has been reported in a variety of invertebrates, but the downstream signaling pathways responsible for these effects have yet to be elucidated. Here, using heterologous cell systems and in vitro pharmacological experiments, we investigated the signaling pathways underlying VP/OT-type neuropeptide (holotocin) induced contraction of the longitudinal muscle of the body wall (LMBW) in the sea cucumber Apostichopus japonicus (phylum Echinodermata), a deuterostome invertebrate. Holotocin-induced contraction of the LMBW comprised two distinct phases: an initial rapid phasic contraction followed by a sustained tonic contraction. Pharmacological experiments revealed that upon binding to its receptor AjHOR, holotocin activates a Gαq-dependent pathway, leading to phospholipase C (PLC) activation and subsequent hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) into inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). IP3 triggers Ca2+ release from intracellular Ca2+ stores via IP₃ receptors (IP3R), but depletion of intracellular Ca2+ does not activate store-operated Ca2+ entry (SOCE). DAG activates protein kinase C (PKC), which may modulate the activity of ion channels in the plasma membrane, resulting in membrane depolarization, opening of voltage-gated Ca2+ channels (VGCCs), and subsequent influx of extracellular Ca2+. Overall, this study reveals similarities and differences in the signaling pathways mediating smooth muscle contraction in invertebrates and vertebrates, providing new insights into the evolution of these mechanisms across the Bilateria.