Dopaminergic neurons intrinsic to the striatum.

The striatum -- the largest integrative component of the basal ganglia -- harbors a population of neurons that express the enzyme tyrosine hydroxylase (TH), a faithful marker of dopaminergic neurons. The dopaminergic nature of these neurons is further supported by the fact that they express the dopamine (DA) transporter (DAT) and the nuclear orphan receptor Nurr1, a transcription factor essential for the expression of the DA phenotype by midbrain neurons. The vast majority of these neurons are morphologically similar to the medium-sized aspiny striatal interneurons and they all express the enzyme GAD(65). The striatal TH-positive neurons increase markedly in number in animal models of Parkinson's disease (PD), where striatal DA concentrations are low, but this increase is abolished by L-dopa treatment. Hence, local DA concentrations appear to regulate the numerical density of this ectopic neuronal population, a phenomenon that is more likely the result of a shift in the phenotype of preexistent striatal interneurons rather than the recruitment of newborn neurons that will develop a DA phenotype. Altogether, these findings suggest that striatal TH-positive neurons act as a local source of DA and, as such, are part of a compensatory mechanism that could be artificially enhanced to alleviate or delay PD symptoms.