Autoradiographic localization in polychaete embryos of tritiated mesulergine, a selective antagonist of serotonin receptors that inhibits early polychaete development.

Developing embryos of the polychaete Ophryotrochal labronica were exposed to tritiated mesulergine, a selective antagonist of the serotonin receptors 5-HT1c and 5-HT2, that also has significant affinity to dopamine D-2 sites, and the labeling was analyzed by autoradiography. Already at the earliest developmental stages (1-4 cells), numerous silver grains visualizing 3H-mesulergine binding sites and possibly also serotonin receptors were recorded over the cytoplasm, mostly in association with decomposing yolk granules, but few grains were detected over the nuclear region. In advanced pregastrular embryos (3 days) the number of silver grains was greatly increased over nuclei, cell borders and non-yolk cytoplasmic elements, notably in the animal half of the embryos. For newly gastrulated embryos (4 days), more than 90% of the grains appeared over non-yolk cellular structures. Abundant access to serotonin receptors is probably a fundamental condition not only for gastrulation but also for the high mitotic activity of the cleavage period. An indication hereof is the observation that exposure of cleaving polychaete eggs/embryos to unlabeled mesulergine inhibited cytokinesis and chromosome movements, whereas spindle formation and chromosome duplication were unaffected.