Transfer of radioactive material between electrically coupled neurons of the leech central nervous system

Intracellular application of tritiated precursors by means of microiontophoresis was performed on nerve cells in isolated segmental ganglia of the leech ventral nerve cord. Incorporation as well as intra- and interneuronal transport were studied by autoradiography after injection of fucose, glucosamine, glycine, leucine, orotic acid and uridine. With several minutes of intraneuronal injection the precursors were incorporated into macromolecules. Depending upon the tracer used, the radioactive material was distributed in a specific pattern over the cell somata and then released into the nerve processes. After application of orotic acid and uridine a transport of radioactive material, presumably RNA, could be observed in the processes of the injected neurons at a distance of about 200-500 mum. Fucose and glucosamine injection resulted in the most extended labeling of the nerve cell processes, indicating a transport rate of about 11 mm/day. When the radiochemicals were injected into one of the two electrically coupled giant nerve cells -- the so-called Retzius cells (Rc) -- a specific labeling not only of the injected Rc but also of the coupled but not injected Rc was found. Injection of protein or glycoprotein precursors into one Rc produced heavy labeling of both Rcs including their processes; a slight labeling of other ganglion compartments was only found after increasing the dosage of the amino acids glycine and leucine. With orotic acid and uridine this interneuronal transfer was confined to the electrically coupled Rc twin. Intracellular injection of one Rc with puromycin followed by injection of amino acids or fucose into the same Rc or into the coupled Rc resulted in an inhibition of precursor incorporation within the puromycin-injected Rc and an exclusive labeling of the coupled Rc, thus indicating that the precursors themselves were transferred. It is suggested that after microiontophoretic application an interneuronal transfer of relatively low molecular weight material takes place, probably across the low-resistance junction through which the Rcs are electrically coupled.