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l-3,4-Dihydroxyphenylalanine (l-DOPA) is the therapeutic gold standard in Parkinson's disease. However, most patients develop debilitating abnormal involuntary movements termed l-DOPA-induced dyskinesia (LID) as therapy-complicating side effects. The underlying mechanisms of LID pathogenesis are still not fully understood. Recent evidence suggests an involvement of striatal tyrosine hydroxylase (TH) protein-expressing neurons, as they are capable of endogenously producing l-DOPA and possibly dopamine. The aim of this study was to elucidate changes of TH transcription in the striatum and nucleus accumbens that occur under experimental conditions of LID. Mice with a unilateral 6-hydroxydopamine-induced lesion of the medial forebrain bundle were treated daily with l-DOPA for 15days to provoke dyskinesia. In situ hybridization analysis revealed a significant numerical decrease of TH mRNA-positive neurons in the striatum and nucleus accumbens of mice not exhibiting LID, whereas dyskinetic animals failed to show this reduction of TH transcription. Interestingly, similar changes were observed in intact non-deafferentiated striata, demonstrating an l-DOPA-responsive transcriptional TH regulation independently from nigrostriatal lesion severity. Consolidation with our previous study on TH protein level (Keber et al., 2015) impressively highlights that LID is associated with both a deficient downregulation of TH transcription and an excessive translation of TH protein in intrastriatal neurons. As TH protein levels in comparison to mRNA levels showed a stronger correlation with development and severity of LID, antidyskinetic treatment strategies should focus on translational and posttranslational modulations of TH as a promising target.

L-3,4-Dihydroxyphenylalanine (L-DOPA) is the therapeutic gold standard in Parkinson's disease. However, long-term treatment is complicated by the induction of debilitating abnormal involuntary movements termed L-DOPA-induced dyskinesias (LIDs). Until today the underlying mechanisms of LID pathogenesis are not fully understood. The aim of this study was to reveal new factors, which may be involved in the induction of LID. We have focused on the expression of striatal tyrosine hydroxylase-positive (TH+) neurons, which are capable of producing either L-DOPA or dopamine (DA) in target areas of ventral midbrain DAergic neurons. To address this issue, a daily L-DOPA dose was administered over the course of 15 days to mice with unilateral 6-hydroxydopamine-induced lesions of the medial forebrain bundle and LIDs were evaluated. Remarkably, the number of striatal TH+ neurons strongly correlated with both induction and severity of LID as well as ΔFosB expression as an established molecular marker for LID. Furthermore, dyskinetic mice showed a marked augmentation of serotonergic fiber innervation in the striatum, enabling the decarboxylation of L-DOPA to DA. Axial, limb and orolingual dyskinesias were predominantly associated with TH+ neurons in the lateral striatum, whereas medially located TH+ neurons triggered locomotive rotations. In contrast, identified accumbal and cortical TH+ cells did not contribute to the generation of LID. Thus, striatal TH+ cells and serotonergic terminals may cooperatively synthesize DA and subsequently contribute to supraphysiological synaptic DA concentrations, an accepted cause in LID pathogenesis.