Compensatory Processes in Striatal Neurons Expressing the Tyrosine Hydroxylase Gene in Transgenic Mice in a Model of Parkinson’s Disease

The mammalian striatum is known to contain non-dopaminergic neurons that express dopamine (DA)-synthesizing enzymes and produce DA, responsible for the regulation of motor function. This study assessed the expression of DA-synthesizing enzymes in striatal neurons and their role in DA synthesis in transgenic mice expressing the green fluorescent protein (GFP) gene under the tyrosine hydroxylase (TH) gene promoter in a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) model of Parkinson’s disease (PD). We showed that, in Parkinsonian animals, the number of neurons expressing the TH gene increased by 1.9 times compared with the control (0.9% NaCl), which indicates a compensatory response to the DAergic denervation of the striatum. This assumption is supported by a 2.5-fold increase in the expression of genes for TH and transcription factor Nurr1 and a 1.45-fold increase in the expression of the large amino acid transporter 1 gene. It is noteworthy that, in Parkinsonian mice, in contrast to the controls, DA-synthesizing enzymes were found not only in nerve fibers but also in neuronal cell bodies. Indeed, TH or TH and aromatic L-amino acid decarboxylase (AADC) were detected in GFP-positive neurons, and AADC was detected in GFP-negative neurons. These neurons were shown to synthesize DA, and this synthesis is compensatorily increased in Parkinsonian mice. The above data open the prospect of improving the treatment of PD by maintaining DA homeostasis in the striatum.