Dopamine-like immunoreactivity in sponge larvae

: Sponges differ from the majority of multicellular animals by lack of specialized neural cells. Although sponge genomes show the toolkits for metabolism of some neuroactive substances, only few genes of monoamines metabolism were found. We studied larvae of the freshwater sponge Eunapius fragilis to analyze the immunoreaction to one of the monoamines – dopamine (DA). We found dopamine-like immunoreactivity in structures located under every flagellum in the larval epithelial cells. Double labeling with anti-DA and anti-58K Golgi protein antibodies, confocal microscopy with improved signal-to-noise ratio and super-resolution (Airyscan), and ultrastructural electron microscopy analysis revealed that the DA-like-immunopositive structures are closely associated with the Golgi apparatus. A similar pattern of immunolabeling was reported in the blastulae of sea urchins, whose ciliary activity is known to be affected by monoamines. Our finding of DA-like immunoreactive structures in sponge ciliated cells provide morphological basis for considering monoamines as potential intracellular regulators of flagellar/ciliary activity.