Protein synthesis in the neuropil of the rat dentate gyrus during synapse development

Previous studies have shown that there are dramatic accumulations of polyribosomes under developing synapses on dendrites of CNS neurons. The present study was designed to evaluate what types of proteins might be synthesized by the synapse-associated polyribosomes. Hippocampal slices from rat pups sacrificed at 4, 7, 10, 12, 14, and 21 days after birth as well as slices from adult animals were incubated in a modified Eagle's medium containing 3H-leucine. After a 30 min exposure to radiolabeled amino acids, the slices were microdissected, separating the dendritic enriched molecular layer from the cell bodies of the dentate gyrus and the hippocampus proper. The level of protein synthetic activity was assessed by comparing the incorporation in cell body and dendritic laminae. Polypeptides present in each dissected zone were separated electrophoretically on 1D SDS-polyacrylamide gels according to their molecular weight and the newly synthesized proteins were analyzed through gel fluorography. The overall level of 3H-amino acid incorporation into protein (measured as cpm/microgram protein) was higher than that of the adult at all postnatal ages. When the entire slice was analyzed, the maximum incorporation was at 12 days after birth. In the dissected subregions of the slice the peak protein synthetic activity in cell-body-enriched regions of dentate gyrus and hippocampus proper was at 4-7 days postnatal, declining between 7 and 21 days to values comparable to the adult. By contrast, protein synthesis in the molecular layer of the dentate gyrus did not peak until 12 days after birth, decreasing toward adult rates after 14 days. The overall pattern of Coomassie stained polypeptides present in the dentate molecular layer was comparable at all ages examined. Moreover, one-dimensional gel analysis showed no qualitative differences in the proteins that were synthesized in the three dissected zones across ages.(ABSTRACT TRUNCATED AT 250 WORDS)