Dopamine Biosynthesis Is Selectively Abolished in Substantia Nigra/Ventral Tegmental Area but Not in Hypothalamic Neurons in Mice with Targeted Disruption of the Nurr1 Gene

To ascertain the function of an orphan nuclear receptor Nurr1, a transcription factor belonging to a large gene family that includes receptors for steroids, retinoids, and thyroid hormone, we generated Nurr1-null mice by homologous recombination. Mice, heterozygous for a single mutated Nurr1 allele, appear normal, whereas mice homozygous for the null allele die within 24 h after birth. Dopamine (DA) was absent in the substantia nigra (SN) and ventral tegmental area (VTA) of Nurr1-null mice, consistent with absent tyrosine hydroxylase (TH),l-aromatic amino acid decarboxylase, and other DA neuron markers. TH immunoreactivity and mRNA expression in hypothalamic, olfactory, and lower brain stem regions were unaffected.l-Dihydroxyphenylalanine treatments, whether given to the pregnant dams or to the newborns, failed to rescue the Nurr1-null mice. We were unable to discern differences between null and wild-type mice in the cellularity, presence of neurons, or axonal projections to the SN and VTA. These findings provide evidence for a new mechanism of DA depletionin vivoand suggest a unique role for Nurr1 in fetal development and/or postnatal survival.