Some polypeptides in the nervous system of the marine worm, Nereis diversicolor, are related to the sodium influx stimulating peptide of the pulmonate freshwater snail, Lymnaea stagnalis.

Total mRNA, extracted from brain of the marine worm, Nereis diversicolor (Annelida, Polychaeta), was translated either in vitro using a rabbit reticulocyte lysate or in ovo (Xenopus laevis oocyte). The synthesized polypeptides were analyzed by electrophoresis and Western blotting techniques using polyclonal antisera raised against three peptides: sodium influx stimulating peptide (SISP) sequences 10-19 and 67-76 and a monoclonal antibody raised against purified native SISP (1-77) of Lymnaea stagnalis. Among the products translated in vitro, three polypeptides of 80, 72, and 64 kDa were recognized by the anti-SISP (10-19) polyclonal antiserum and by the monoclonal antiserum, but not by anti-SISP (67-76). Some of the in ovo translated products showed almost identical immunoreactivity to both the anti-SISP (10-19) and the monoclonal antibody. These polypeptides have molecular masses of 80, 72, and 43 kDa. No polypeptides were recognized by anti-SISP (67-76). Western blotting analysis of brain extracts revealed a number of proteins that reacted with the antiserum raised against SISP (10-19) and the monoclonal antiserum. Several perikarya of brain ganglionic nuclei and ventral nerve cord were immunoreactive to anti-SISP (10-19). The monoclonal antiserum gave similar results, although with a less intense immunoreaction. The infracerebral region was also stained, suggesting that the immunoreactive material is released as a true neurohormone into the hemolymph. The largest polypeptides, in particular those translated from brain mRNA, could be neuropeptide precursors containing a SISP-related sequence.