Nucleic Acid Metabolism in the Developing Rat Brain

Abstract

In the developing rat brain, the early period after birth is characterized by extensive cellular proliferation and differentiation. This period is followed by one of growth and maturation in which myelination is one of the most significant events taking place. We have begun investigations into the changes in nucleic acid metabolism that arise during the transition from the state of cellular proliferation to the process of myelination in the rat brain. [32P]Pi was injected intracisternally into rats of different ages. Nucleic acids were extracted and purified from cerebral preparations at different time-intervals after the injection. High-molecular-weight RNA was separated from tRNA and DNA by precipitation with 2M-LicI. The nucleic acids were analysed by electrophoresis on polyacrylamide gels of various concentrations, before and after the LiCl step, and uptake of label into the various fractions was assayed. DNA synthesis was marked up to 10 days after birth and barely detectable in the actively myelinating rat (3-4 weeks old). In the newborn, the rate of labelling of DNA was higher than that of ribosomal RNA for several hours after the administration of [32P]Pi. At early time-periods there was extensive uptake into the high-molecular-weight RNA, including ribosomal RNA, with most of the label appearing in the 28s and 185 RNA species 5 h after injection. In the preparations from the myelinating rat, there was also extensive labelling of the highmolecular-weight RNA with a pronounced uptake of label into the ribosomal RNA by 5h. Even at this time, and up to 12h, a highly labelled heavy RNA fraction (>30S) could still be detected, as distinct from the case of the labelled RNA isolated from the newborn. The LiCI-precipitated RNA, labelled for short time-periods, was analysed by oligo(dT) cellulose column chromatography for mRNA content (Aviv & Leder, 1972). Since the RNA analysed was the total cellular high-molecular-weight RNA, the ‘mRNA’ fractions would include both ‘mRNA’ and the precursors to the ‘mRNA’. About 30% of the labelled RNA in the myelinating rat was ‘mRNA’ up to 5h after labelling, whereas the corresponding values for the RNA from the newborn were 12 % in 1 h, falling to 4-5 % by 5h. These results suggest that in the newborn rat cerebral cortex, the synthesis of ribosomal RNA represents a greater proportion of total RNA synthesis than that in the myelinating rat, where a substantial portion of the RNA synthesized is ‘mRNA’, as judged by the presence of poly(A) regions (Lim & Canellakis, 1970).