ИСТИНА

Serotonin (5-HT) is a signaling molecule that acts primarily through its membrane-bound receptors. It also enters cells via membrane transporter and modulates the activity of structural and signaling molecules through post-translational modifications. Previously, we have shown that increased intracellular serotonin during early embryogenesis in Lymnaea stagnalis affects both developmental rate and offspring behavior, in particular feeding pattern. Feeding in L. stagnalis is regulated by neurons of the buccal ganglia and the local intestinal neuronal network, whose genetic specification begins during late gastrulation. To investigate the non-neuronal role of intracellular 5-HT, we focused on pre-neuronal developmental stages. We monitored gastrulation-related morphogenetic events in response to elevated intracellular 5-HT levels and pharmacological modulation of key signaling pathways involved in neuronal differentiation and cell polarity. During the cleavage embryos were treated with 1-azakenpaullone to activate cWnt signaling, U0126 to inhibit the MAPK pathway, and Brefeldin A to disrupt Golgi apparatus function. Intracellular 5-HT level was elevated by incubation in the biochemical precursor – 5-HTP. All treatments resulted in morphologically similar abnormalities — embryos consisting of two connected hemispheres. However, embryos showed distinct internal architectures and differential expression patterns of neuronal transcription factors involved in neurodevelopmental patterning (Otx, SoxB1, Nk-family) and archenteron formation (FoxA, Brachyury). Alterations in signaling pathway activity (cWnt, MAPK) led to radialization, while elevated intracellular 5-HT and Golgi disruption impaired cell-layer interactions. These specific developmental alterations of primary body plan were manifested at morphological and gene expression levels. Similarities between the elevated intracellular 5-HT and Brefeldin A suggest a common mechanism involving the coordination of intracellular trafficking and epithelial polarity, whereas modulation of signaling pathways primarily influences body plan. Comparable mechanisms are also implicated in intercellular communication between neurons. Our experimental model is unique to distinguish between receptor-mediated and non-receptor-mediated actions of serotonin and to evaluate the contribution of intracellular 5-HT to neuronal differentiation and intercellular contact establishment.