Molecular anatomy of the development of the human substantia nigra

A series of 15 fetal and perinatal human brains (from week 12 of fetal life to day 2 after birth) was studied in order to describe the anatomical and molecular correlates of the substantia nigra ontogeny. In situ hybridization, immunohistochemistry and binding studies were used to detect D2 dopamine receptor (D2R) mRNA, D2R binding sites, dopamine membrane transporter (DAT) mRNA, tyrosine hydroxylase (TH) protein D1 dopamine receptor (D1R) protein and D1R binding sites. Dopaminergic (DA) neurons of the substantia nigra were detected through TH immunoreactivity from week 12. At week 16, the substantia nigra was clearly delineated as a compact group of intermingled neurons and fibers. From week 19, groups of DA neurons were segregated from the pars reticulata. These groups have been divided into the substantia nigra pars compacta, the ventral tegmental area and the retrorubral area. The DA neurons exhibited a gradual increase in size and branching development until birth. From week 12 onward they expressed several other markers of dopamine transmission, i.e., D2R mRNA, D2R binding sites and DAT mRNA. The ventral tegmental area expressed lower levels of mRNA for DAT and D2R than the pars compacta. From week 12, D1R immunoreactivity and D1R binding sites were also present in the substantia nigra pars reticulata. This suggests that projecting striatonigral neurons, known to express the D1R gene, have developed pathways connecting with the substantia nigra by week 12. Our results demonstrate that the developing substantia nigra in human displays early transcriptional and translational activity for the main constituents of dopaminergic transmission from week 12 and receives at this time dopaminoceptive inputs bearing D1 receptors from the striatum.