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 trans cription factor belonging to a large gene family that includes recepto rs for steroids, retinoids, and thyroid hormone, we generated Nurr1-nu ll mice by homologous recombination. Mice, heterozygous for a single m utated Nurr1 allele, appear normal, whereas mice homozygous for the nu ll 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 ami no acid decarboxylase, and other DA neuron markers. TH immunoreactivit y and mRNA expression in hypothalamic, olfactory, and lower brain stem regions were unaffected. I-Dihydroxyphenylalanine treatments, whether given to the pregnant dams or to the newborns, failed to rescue the N urr1-null mice. We were unable to discern differences between null and wild-type mice in the cellularity, presence of neurons, or axonal pro jections to the SN and VTA. These findings provide evidence for a new mechanism of DA depletion in vivo and suggest a unique role for Nurr1 in fetal development and/or postnatal survival.