Catecholamine biosynthetic enzymes are expressed in replicating cells of the peripheral but not the central nervous system.
We sought to determine whether the precursors of catecholamine-containing neurons in the developing peripheral and central nervous systems of chickens and rats express the biosynthetic enzymes tyrosine hydroxylase [THase; tyrosine 3-monooxygenase; L-tyrosine, tetrahydropteridine: oxygen oxidoreductase (3-hydroxylating), EC 1.14.16.2] or dopamine beta-hydroxylase [DBHase; 3,4-dihydroxyphenylethylamine, ascorbate:oxygen oxidoreductase (beta-hydroxylating), EC 1.14.17.1], prior to the time they withdraw from the cell cycle. Chicken embryos (stages 26-27) were injected with [3H-thymidine and 4 hr later were prepared for the simultaneous demonstration of radioautographically labeled nuclei in immunoreactive THase cells. The brains and sympathetic chains of rat fetuses (days E12-E14), exposed for 2 hr to [3H]thymidine, were treated similarly except that peripheral tissues were stained with a specific antibody to DBHase as well as anti-THase. In the peripheral nervous system of both chicken and rat, nuclei of THase-containing cells were radioautographically labeled. DBHase-containing cells in the peripheral nervous system of rats were also labeled and thus are noradrenergic. THase was localized in cells of the brain of the same rat fetuses beginning on day E12 (no THase was detected on day E11 or E11.5) in the mantle layer of the ventral mesencephalic and rostrolateral rhombocephalic cellular groups; however. THase-containing cells in the central nervous system did not incorporate [3H]thymidine. We conclude that, during development, the adrenergic neuronal precursors of the peripheral nervous system but not of the central, have the capacity to synthesize catecholamines before they withdraw from the cell cycle. Differences in the maturation of peripheral and central neurons may be related to differences in their embryological origin.