The regional metabolism of high-molecular-weight RNA in the developing female rat brain was investigated after the intracranial injection of [32P]P1. The synthesis of polyadenylated RNA relative to high-molecular-weight RNA was determined after oligo(dT)-cellulose chromatography of total cellular high-molecular-weight RNA labelled after 4h. In both hypothalamus and cortex this synthesis was significantly higher during the first 10 days post partum than at subsequent ages. In both regions apparently more mRNA is synthesized in the young. The ratio of the specific radioactivity of cytoplasmic high-molecular-weight RNA relative to that of the nucleus, measured after a 48 h period of labelling, was considered to be an index of the nucleocytoplasmic transport of newly synthesized RNA [Berthold & Lim (1976) Biochem. J. 154, 529–539]. In the cortex, nucleo-cytoplasmic RNA transport in rats aged up to 20 days was significantly higher than in older rats, with the maximal value being attained between 16 and 19 days post partum. In contrast, in the hypothalamus, nucleo-cytoplasmic transport of RNA was low during the neonatal period and comparable with that of the mature animal. However, there were two periods of increased transport at later stages of development, the first between 15 and 19 days post partum and the second between 25 and 29 days post partum. These prepubertal changes in the nucleo-cytoplasmic transport of RNA in the female hypothalamus during weeks 3 and 4 post partum are coincident with other reported changes occurring during sexual differentiation. Differences in the timing of the maturational changes of the two brain regions thus appear to be reflected in developmental changes in RNA transport.

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