Abstract

The planarian flatworm, *Dugesia japonica*, serves as a powerful model for studying the neurobiology of regeneration and muscle function, given its simple yet highly plastic nervous system. Serotonin (5-HT) is a crucial neurotransmitter and neuromodulator involved in diverse physiological processes across the animal kingdom, but its specific role in the muscular system of planaria remains incompletely understood. This study investigated the physiological function of 5-HT in planarian muscle contractility and movement. We employed pharmacological manipulation using 5-HT agonists and antagonists, coupled with behavioral assays and immunofluorescence staining of muscle fibers. Our results demonstrate that exogenous 5-HT significantly increases the frequency and amplitude of spontaneous muscle contractions, a response that is blocked by the 5-HT receptor antagonist, methysergide. Furthermore, depletion of endogenous 5-HT through chemical ablation led to a marked reduction in whole-body motility, which was rescued by the application of a 5-HT precursor. Immunostaining revealed a dense network of 5-HT-positive neurons closely associated with the ventral muscle layers. These findings suggest that 5-HT acts as a potent excitatory neuromodulator of planarian muscle function, likely through direct or indirect signaling pathways to regulate muscle contraction. This work provides critical insight into the conserved role of serotonergic signaling in invertebrate neuromuscular control and contributes to our understanding of the fundamental mechanisms governing movement in this regenerative model organism.