title: Evidence for turnover of DNA in the nucleus and in the cytoplasm of adult post-mitotic neurons in vivo.
abstract: Experiments were designed to determine if turnover of DNA occurs in post-mitotic neurons of adult mammals. Three-month-old laboratory mice, Mus musculus, and white footed mice, Peromyscus leucopus, were given three injections of 3H-thymidine (3H-TdR, 16 muCi/g body weight) at noon, 4 pm and 8 pm, and were killed serially beginning 1 h after the last injection. The brain stem was removed from the animals and autoradiographed. Some of the sections were treated with DNase prior to autoradiography. A sensitive and accurate method of autoradiographic grain count analysis was used to measure the grain counts over areas of neuron nuclei, neuron cytoplasm and background areas of the slide (Cameron, Pool and Hoage, 1979). There was a significant elevation of grains per unit area above background over both the nucleus and the cytoplasm in animals killed 24 h after the last 3H-TdR injection.  Counts were reduced to background level by prior DNase treatment. This shows that the grains over the nucleus and the cytoplasm were due to label in the DNA. Changes in the grain counts with time were subjected to least squares regression analysis. In the mouse both the nucleus and the cytoplasmic grain counts showed a decreasing slope which was significantly different from a slope of zero when the data were fitted to either a linear model or a log-linear model. The data showed a somewhat better fit to the log-linear model. The labelled nuclear DNA in the mouse had a calculated half-life of about 502 h while the labelled cytoplasmic DNA had a calculated half-life of about 97 h. Our ability to measure and to characterize DNA turnover in specific in vivo cell types allows us to test theories about its functional role.
mesh_terms: D000818:Animals; D001933:Brain Stem / Q000166:cytology; D002467:Cell Nucleus / Q000378:metabolism; D003593:Cytoplasm / Q000378:metabolism; D004247:DNA / Q000378:metabolism*; D003851:Deoxyribonucleases / Q000494:pharmacology; D007700:Kinetics; D008297:Male; D051379:Mice; D008805:Mice, Inbred A; D008938:Mitosis; D009474:Neurons / Q000166:cytology / Q000378:metabolism*; D010542:Peromyscus
chemical_list: D004247:DNA; D003851:Deoxyribonucleases