Expression of amino acid decarboxylase in proliferating cells of the neural tube and notochord of developing rat embryo

In vertebrate embryos, the catecholamine (CA) neurons of brain and periphery arise from undifferentiated precursors in the ventral neural tube and neural crest to migrate to their final location where they first express tyrosine hydroxylase (TH). We sought to determine by immunocytochemistry and autoradiography whether the next enzyme of the CA pathway, aromatic L-amino acid decarboxylase (AADC) (1) is expressed by CA precursors before or, as TH, after reaching their final site and (2) is present in proliferating cells. AADC was first observed on day 12 of development (E 12) in all cells of the notochord along its entire length and in neuroepithelial cells of the ventral neural tube from the mesencephalic to the lumbar region. At E 14 the cells of the sacral basal plate of the neural tube also contained AADC. At this stage, some cells of the notochord and neural tube containing AADC were also labeled with tritiated thymidine. At E 16 the cells of the notochord lacked AADC immunoreactivity, and at E 21 the enzyme was no longer detected in the ventricular layer of the developing brain. At this stage and in the adult stage, however, a few cells containing AADC were found scattered in the spinal cord. AADC was not detected in migrating neural crest cells. The studies indicate that in rat embryo: (1) AADC is expressed by cells of the ventral neural tube and notochord from E 12 until late gestation; (2) the onset of expression of AADC occurs prior to the last cell division; and (3) at least in brain, the precursors of CA neurons express AADC while still in their site of origin and TH after arriving at their final location. During early vertebrate development, cells which in adults populate the autonomic or central nervous system and synthesize and store catecholamines (CAs), arise from precursor cells localized, respectively, in neural crest or the ventricular layer of neural tube. These cellular precursors contain neither CAs nor tyrosine hydroxylase (TH), the first enzyme of the CA-biosynthetic pathway and hence have yet to express their CA phenotype. It is only after the undifferentiated precursor cells leave their tissue of origin to emigrate either from neural crest into the sympathetic ganglia or from the ventricular to intermediate (mantle) layer of brain that TH and the capacity to synthesize and store CAs are expressed (Enemar et al., 1965; Olson and Sieger, 1972; Golden, 1973; Lauder and ’ We express appreciation for the excellent technical assistance of Leila Grayson. This research was supported by Research Grant HL18974 and Training Grant HL07379 from the National Heart, Lung and Blood Insititue. * To whom correspondence should be addressed. Bloom, 1974; Cochard et al., 1978; Teitelman et al., 1978, 1979; Specht et al., 1981). It is therefore, paradoxical that administration into chick embryos of the CA precursor L-DOPA results, in advance of the appearance of CA neurons, in specific CA histofluorescence in neuroepithelial cells of the ventral neural tube, cells of the underlying notochord, and cells presumed to be of neural crest origin (Pearse and Polak, 1971; Kirby and Gilmore, 1972; Laurence and Burden, 1973; Allan and Newgreen, 1977). Because L-DOPA itself does not fluoresce, these findings imply that it must be first converted to a CA product, dopamine (DA) or conceivably norepinephrine or even epinephrine. The major route of conversion of L-DOPA in CA cells is by decarboxylation to dopamine by the second enzyme of the CA biosynthetic pathway, aromatic L-amino acid decarboxylase (AADC). Thus the production of CA fluorescence after administration of L-DOPA in precursor tissues suggests either: (1) that AADC is first expressed in CA precursor cells of brain and periphery prior to their