Induction of mitosis in mature neurons in central nervous system by sustained depolarization.

DNA synthesis and mitosis have been induced in vitro in fully differentiated neurons from the central nervous system by depolarization with a variety of agents that produce a sustained rise in the intracellular sodium ion concentration and a decrease in the potassium ion concentration. Depolarization was followed in less than 1 hour by an increase in RNA synthesis and in 3 hours by initiation of DNA synthesis. Apparently normal nuclear mitosis ensued, but cytokinesis was not completed in most cells; this resulted in the formation of binucleate neurons. The daughter nuclei each contained the same amount of DNA as the diploid preinduction parental neurons; this implies that true mitogenic replication was induced.

Blockage of depolarization-induced mitogenesis in CNS neurons by 5-fluoro-2'-deoxyuridine.

Experiments designed to provide further evidence, at the basic metabolic level, that true mitogenesis and mitotic activity are being induced in CNS neurons in response to sustained ionic depolarization were conducted. The ability of 5-fluoro-2'-deoxyuridine (FUdR), a well-studied inhibitor of normal mitogenesis in naturally proliferating cells, to block induction of DNA synthesis (and subsequent nuclear division) in culture-matured neurons depolarized with ouabain was ascertained, as well as the ability of exogenously supplied thymidine to permit effective bypass of such blockage. Observations of the sequence of intracellular morphological changes induced by ouabain were also made, along with a determination of the alterations in this sequence introduced by FUdR. The results indicate that ouabain mediated depolarization rapidly induces and/or activates the key mitogenic enzyme thymidylate synthetase in the mitotically quiescent neurons, along with all other mitogenesis-specific enzymes required for DNA synthesis and nuclear division. A probable mechanism by which such mitogenic induction may proceed is elaborated. The early morphological changes observed correlate well with the early time-sequence of mitogenic metabolic events, while development of the later changes appears to be dependent upon the progress of mitogenesis activity. The results support the possibility that CNS neurons of adult origin may also be induced to initiate normal mitogenesis by appropriately imposed depolarization treatments.