Ontogeny of tyrosine hydroxylase and cholecystokinin gene expression in the rat mesencephalon.

The ontogeny of tyrosine hydroxylase and cholecystokinin gene expression was studied in the rat mesencephalon using hybridization histochemistry. Both transcripts appeared on E13 in the ventrocaudal mesencephalon. The levels of both transcripts increased synchronously during the second half of gestation. The locations of neurons containing either transcript changed similarly during development with a rostral transposition and a lateral extension of the respective areas covered with grains. On the day after birth, the patterns of expression for both genes, although at lower transcript levels, were similar to the patterns seen in the adult.