Regional increases in ornithine decarboxylase mRNA levels in the rat brain after partial mesodiencephalic hemitransection as revealed by in situ hybridization histochemistry

Changes in the level of ornithine decarboxylase mRNA after partial mesodiencephalic hemitransection were evaluated in various regions of the rat brain by means of in situ hybridization histochemistry coupled with computer-assisted image analysis. On days 1 and 2 after the lesion, increased accumulation of ornithine decarboxylase mRNA was observed on the lesioned side in various telencephalic regions (e.g. neostriatum and frontoparietal cortex), and in the pars compacta of the substantia nigra. Both in the frontoparietal cortex and substantia nigra a decreasing gradient of ornithine decarboxylase mRNA activation was observed going far from the site of the lesion. Seven days after the operation, ornithine decarboxylase mRNA levels returned to control values on the lesioned side but increased in some regions, such as the frontoparietal cortex, on the intact side. The present results demonstrate that the parent cell body biosynthetic machinery is activated by the mechanical lesion of the axons at the level of ornithine decarboxylase gene expression. The increase of ornithine decarboxylase mRNA is not as large as the enhancement in ornithine decarboxylase activity previously shown, suggesting that the response to the lesion may also involve changes in the rate of translation of ornithine decarboxylase mRNA and/or in the rate of degradation of ornithine decarboxylase protein.