Tyrosine hydroxylase‐positive neurons intrinsic to the human striatum express the transcription factorNurr1

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Abstract
The putative dopaminergic (DA) neurons intrinsic to human striatum were studied to determine their similarity with DA neurons of the substantia nigra pars compacta (SNpc). The comparison was based on morphological features and on the presence or absence ofNurr1, an orphan receptor of the nuclear receptor family that is essential for the expression of DA phenotype by developing SNpc neurons. Immunohistochemistry for the neuronal nuclear protein (NeuN; a neuronal marker) andin situhybridization for tyrosine hydroxylase (TH) and/orNurr1were applied topost‐mortemtissue obtained from seven normal individuals. On one hand, the TH‐positive multipolar neurons in the human striatum, which were subdivided into three groups according to their size and pattern of dendritic arborization, were found to be morphologically similar to TH‐positive neurons of the SNpc. The distribution frequency of striatal TH‐positive neurons, according to their diameter, closely matches the frequency observed for multipolar TH‐positive cells in the SNpc. On the other hand, the proportion of neurons expressingNurr1and TH mRNA transcripts on single striatal section was similar to the proportion of TH‐immunoreactive neurons observed on adjacent sections. More importantly, in each striatum analysed, virtually all cells that stained for TH also expressed NeuN andNurr1. This study provides novel data that confirm the existence of DA neurons intrinsic to the human striatum. It also provides the first evidence for the existence of striking morphological and chemical similarities between the DA neurons present at striatal level and those that populate the SNpc.