Age-dependent alterations of the rate of RNA synthesis in rat brain cell nuclei.

Endogenous Mn2+ and Mg2+ activated RNA polymerase activity was measured in isolated cell nuclei of brain from rats of different age-groups. It was established that the activity of the nucleoplasmic RNA polymerase is maintained at the level of young animals up to an age of 16 months but is decreased after 24 months. The nucleolar RNA polymerase activity decreases already at 16 months and a higher ratio of the Mn2+:Mg2+ activated RNA polymerase activities has been found to be characteristic for the older animals. By means of stepwise sucrose density gradient centrifugation fractions were obtained from the nuclear preparations highly enriched in cell nuclei of neuronal and glial origin respectively. By measuring the activity of the nucleoplasmic and nucleolar RNA polymerases in these fractions it was found that the elevated ratio of the nucleoplasmic to nucleolar RNA polymerase activity at 16 months of age is a characteristic of the neuronal nuclei while the glial nuclei behave by the opposite manner. A parallelism existing between the age-dependent change of the endogenous RNA polymerase activity and that of perichromatic granules of rat brain cortical cells is discussed.