No abstract

Treatment of Parkinson disease (PD) with L-3,4-dihydroxyphenylalanine (L-DOPA) dramatically relieves associated motor deficits, but L-DOPA-induced dyskinesias (LID) limit the therapeutic benefit over time. Previous investigations have noted changes in striatal medium spiny neurons, including abnormal activation of extracellular signal-regulated kinase1/2 (ERK). Using two PD models, the traditional 6-hydroxydopamine toxic lesion and a genetic model with nigrostriatal dopaminergic deficits, we found that acute dopamine challenge induces ERK activation in medium spiny neurons in denervated striatum. After repeated L-DOPA treatment, however, ERK activation diminishes in medium spiny neurons and increases in striatal cholinergic interneurons. ERK activation leads to enhanced basal firing rate and stronger excitatory responses to dopamine in striatal cholinergic neurons. Pharmacological blockers of ERK activation inhibit L-DOPA-induced changes in ERK phosphorylation, neuronal excitability, and the behavioral manifestation of LID. In addition, a muscarinic receptor antagonist reduces LID. These data indicate that increased dopamine sensitivity of striatal cholinergic neurons contributes to the expression of LID, which suggests novel therapeutic targets for LID.antagonist D opaminergic drugs are effective treatments for the motor symptoms of Parkinson disease (PD), but long-term therapy is limited by disabling abnormal involuntary movements, referred to as L-DOPA-induced dyskinesias (LID) (1). Thus, understanding the molecular and cellular mechanisms underlying LID will help identify more effective treatments for PD, and may also help elucidate the role of dopamine (DA) signaling in motor control.Several biochemical markers of LID have been studied in striatum using animal models. FosB/δFosB expression show a long-term temporal correlation with LID development in DA denervation PD models (2). The increased FosB expression persists over days or weeks and may contribute to the development of LID, but does not correlate with the L-DOPA-induced episodes of dyskinesia that follow each dose. To mediate expression of LID directly, cell signals should grow stronger with repeated L-DOPA treatment and show temporal correlation with acute dopaminergic stimulation. Acute administration of L-DOPA or dopamine agonists activates ERK1/2 by phosphorylation in striatal neurons of DA-denervated animals (3-7). In animals with unilateral 6-hydroxydopamine (6-OHDA) lesions, coadministration of inhibitors of ERK1/2 phosphorylation during repeated L-DOPA treatments reduce LID development (4,8). Studies on these molecular changes have focused on the predominant cell type in the striatum, medium spiny neurons (MSN). In addition, although the unilateral 6-OHDA lesion has been the standard model for the PD phenotype, and particularly for LID, the abrupt nature of the lesion and extreme depletion of dopaminergic afferents has posed limitations in behavioral and biochemical studies.In this study, we used both a unilateral 6-OHDA lesion model and a g...

Mutations in the DJ-1 gene were recently identified in an autosomal recessive form of early-onset familial Parkinson disease. Structural biology, biochemistry, and cell biology studies have suggested potential functions of DJ-1 in oxidative stress, protein folding, and degradation pathways. However, animal models are needed to determine whether and how loss of DJ-1 function leads to Parkinson disease. We have generated DJ-1 null mice with a mutation that resembles the large deletion mutation reported in patients. Our behavioral analyses indicated that DJ-1 deficiency led to age-dependent and taskdependent motoric behavioral deficits that are detectable by 5 months of age. Unbiased stereological studies did not find obvious dopamine neuron loss in 6-month-and 11-month-old mice. Neurochemical examination revealed significant changes in striatal dopaminergic function consisting of increased dopamine reuptake rates and elevated tissue dopamine content. These data represent the in vivo evidence that loss of DJ-1 function alters nigrostriatal dopaminergic function and produces motor deficits.Mutations in DJ-1 were recently identified in an autosomal recessive form of early-onset familial Parkinson disease (PD) 1 (1). The first reported mutation involves one large deletion of the first 5 exons and part of the promoter and another mutation was a missense mutation (L166P) that might cause instability of the DJ-1 protein by preventing it from folding properly and forming homodimers (2-5). Since this first report, a number of other mutations of DJ-1 including deletion mutations, point mutations, and a frameshift mutation have been found to cause PD (6 -10). These studies suggest that the loss of the normal function of DJ-1 leads to PD.However, the nature of the normal function of DJ-1 and the mechanism by which DJ-1 deficiency leads to PD are not well established. Studies prior to the report of its association with PD suggested that DJ-1 might play a role in oncogenesis (11), male fertility (12, 13), control of protein-RNA interaction (14), and in modulating androgen receptor transcription activity (15,16). In addition, the DJ-1 protein was shown to be responsive to oxidation (17, 18), suggesting a potential role in oxidative stress, a process often implicated in PD. Studies on PD-linked DJ-1 mutations indicate that wild-type, but not mutant, DJ-1 protects cells from oxidative stress (19 -21). Canet-Aviles et al. (22) reported that oxidation of the Cys 106 residue in DJ-1 could lead to its relocalization in mitochondria and protect cells from mitochondrial damage. Structurally, DJ-1 closely resembles the members of the ThiJ/PfpI family that have protease and chaperone activities (23-27). Recent biochemical studies suggested that DJ-1 might have protease (5) and redox-dependent chaperone activities (28). Therefore, putative functions of DJ-1 seem to converge on the common pathogenesis of PD implicated in other genetic and sporadic forms of PD.Despite those new insights into the biochemical and cellular functions of DJ-1, th...

The role of dopamine as a vulnerability factor and a toxic agent in Parkinson's disease (PD) is still controversial, yet the presumed dopamine toxicity is partly responsible for the "DOPA-sparing" clinical practice that avoids using L-3,4-dihydroxyphenylalanine (L-DOPA), a dopamine precursor, in early PD. There is a lack of studies on animal models that directly isolate dopamine as one determining factor in causing neurodegeneration. To address this, we have generated a novel transgenic mouse model in which striatal neurons are engineered to take up extracellular dopamine without acquiring regulatory mechanisms found in dopamine neurons. These mice developed motor dysfunctions and progressive neurodegeneration in the striatum within weeks. The neurodegeneration was accompanied by oxidative stress, evidenced by substantial oxidative protein modifications and decrease in glutathione. Ultrastructural morphologies of degenerative cells suggest necrotic neurodegeneration. Moreover, L-DOPA accelerated neurodegeneration and worsened motor dysfunction. In contrast, reducing dopamine input to striatum by lesioning the medial forebrain bundle attenuated motor dysfunction. These data suggest that pathology in genetically modified striatal neurons depends on their dopamine supply. These neurons were also supersensitive to neurotoxin. A very low dose of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (5 mg/kg) caused profound neurodegeneration of striatal neurons, but not midbrain dopamine neurons. Our results provide the first in vivo evidence that chronic exposure to unregulated cytosolic dopamine alone is sufficient to cause neurodegeneration. The present study has significant clinical implications, because dopamine replacement therapy is the mainstay of PD treatment. In addition, our model provides an efficient in vivo approach to test therapeutic agents for PD.

Objective Dopamine (DA) is critical for motor performance, motor learning, and corticostriatal plasticity. The relationship between motor performance and learning, and the role of DA in the mediation of them, however, remain unclear. Methods To examine this question, we took advantage of PITx3-deficient mice (aphakia mice), in which DA in the dorsal striatum is reduced by 90%. PITx3-deficient mice do not display obvious motor deficits in their home cage, but are impaired in motor tasks that require new motor skills. We used the accelerating rotarod as a motor learning task. Results We show that the deficiency in motor skill learning in PITx3(−/−) is dramatic and can be rescued with levodopa treatment. In addition, cessation of levodopa treatment after acquisition of the motor skill does not result in an immediate drop in performance. Instead, there is a gradual decline of performance that lasts for a few days, which is not related to levodopa pharmacokinetics. We show that this gradual decline is dependent on the retesting experience. Interpretation This observation resembles the long-duration response to levodopa therapy in its slow buildup of improvement after the initiation of therapy and gradual degradation. We hypothesize that motor learning may play a significant, underappreciated role in the symptomatology of Parkinson disease as well as in the therapeutic effects of levodopa. We suggest that the important, yet enigmatic long-duration response to chronic levodopa treatment is a manifestation of rescued motor learning.