In patients with Parkinson's disease, the therapeutic efficacy of l-DOPA medication is gradually lost over time, and abnormal involuntary movements, dyskinesias, gradually emerge as a prominent side-effect in response to previously beneficial doses of the drug. Here we show that dyskinesia induced by chronic l-DOPA treatment in rats with 6-hydroxydopamine-induced lesions of the nigrostriatal dopamine pathway is critically dependent on the integrity and function of the serotonergic system. Removal of the serotonin afferents, or dampening of serotonin neuron activity by 5-HT_1A and 5-HT_1B agonist drugs, resulted in a near-complete block of the l-DOPA-induced dyskinesias, suggesting that dysregulated dopamine release from serotonin terminals is the prime trigger of dyskinesia in the rat Parkinson's disease model. In animals with complete dopamine lesions, the spared serotonin innervation was unable to sustain the therapeutic effect of l-DOPA, suggesting that dopamine released as a ‘false transmitter’ from serotonin terminals is detrimental rather than beneficial. The potent synergistic effect of low doses of 5-HT_1A and 5-HT_1B agonists to suppress dyskinesia, without affecting the anti-parkinsonian effect of l-DOPA in presence of spared dopamine terminals, suggests an early use of these drugs to counteract the development of dyskinesia in Parkinson's disease patients.

The molecular basis of L-3,4-dihydroxyphenylalanine (L-DOPA)-induced dyskinesia (LID), one of the major hindrances in the current therapy for Parkinson's disease, is still unclear. We show that attenuation of cAMP signaling in the medium spiny neurons of the striatum, achieved by genetic inactivation of the dopamine and cAMP-regulated phosphoprotein of 32 kDa (DARPP-32), reduces LID. We also show that, in dyskinetic mice, sensitized cAMP/cAMP-dependent protein kinase/DARPP-32 signaling leads to phosphorylation/activation of the extracellular signal-regulated protein kinases 1 and 2 (ERK1/2). The increase in ERK1/2 phosphorylation associated with dyskinesia results in activation of mitogen- and stress-activated kinase-1 (MSK-1) and phosphorylation of histone H3, two downstream targets of ERK involved in transcriptional regulation. In line with these observations, we found that c-Fos expression is abnormally elevated in the striata of mice affected by LID. Persistent enhancement of the ERK signaling cascade is implicated in the generation of LID. Thus, pharmacological inactivation of ERK1/2 achieved using SL327 (alpha-[amino[(4-aminophenyl)thio]methylene]-2-(trifluoromethyl)benzeneacetonitrile), an inhibitor of the mitogen-activated kinase/ERK kinase, MEK, during chronic L-DOPA treatment counteracts the induction dyskinesia. Together, these results indicate that a significant proportion of the abnormal involuntary movements developed in response to chronic L-DOPA are attributable to hyperactivation in striatal medium spiny neurons of a signaling pathway including sequential phosphorylation of DARPP-32, ERK1/2, MSK-1, and histone H3.

Human embryonic stem cells (hESCs) have been proposed as a source of dopamine (DA) neurons for transplantation in Parkinson's disease (PD). We have investigated the effect of in vitro predifferentiation on in vivo survival and differentiation of hESCs implanted into the 6-OHDA (6-hydroxydopamine)-lesion rat model of PD. The hESCs were cocultured with PA6 cells for 16, 20, or 23 days, leading to the in vitro differentiation into DA neurons. Grafted hESC-derived cells survived well and expressed neuronal markers. However, very few exhibited a DA neuron phenotype. Reversal of lesion-induced motor deficits was not observed. Rats grafted with hESCs predifferentiated in vitro for 16 days developed severe teratomas, whereas most rats grafted with hESCs predifferentiated for 20 and 23 days remained healthy until the end of the experiment. This indicates that prolonged in vitro differentiation of hESCs is essential for preventing formation of teratomas.

Transcription factors involved in the specification and differentiation of neurons often continue to be expressed in the adult brain, but remarkably little is known about their late functions. Nurr1, one such transcription factor, is essential for early differentiation of midbrain dopamine (mDA) neurons but continues to be expressed into adulthood. In Parkinson's disease, Nurr1 expression is diminished and mutations in theNurr1gene have been identified in rare cases of disease; however, the significance of these observations remains unclear. Here, a mouse strain for conditional targeting of theNurr1gene was generated, andNurr1was ablated either at late stages of mDA neuron development by crossing with mice carrying Cre under control of the dopamine transporter locus or in the adult brain by transduction of adeno-associated virus Cre-encoding vectors.Nurr1deficiency in maturing mDA neurons resulted in rapid loss of striatal DA, loss of mDA neuron markers, and neuron degeneration. In contrast, a more slowly progressing loss of striatal DA and mDA neuron markers was observed after ablation in the adult brain. As in Parkinson's disease, neurons of the substantia nigra compacta were more vulnerable than cells in the ventral tegmental area whenNurr1was ablated at late embryogenesis. The results show that developmental pathways play key roles for the maintenance of terminally differentiated neurons and suggest that disrupted function of Nurr1 and other developmental transcription factors may contribute to neurodegenerative disease.

Appearance of dyskinesia is a common problem of long-term l-DOPA treatment in Parkinson's disease patients and represents a major limitation for the pharmacological management of the motor symptoms in advanced disease stages. We have recently demonstrated that dopamine released from serotonin neurons is responsible for l-DOPA-induced dyskinesia in 6-hydroxydopamine (6-OHDA)-lesioned rats, raising the possibility that blockade of serotonin neuron activity by combination of 5-HT_1A and 5-HT_1B agonists could reduce l-DOPA-induced dyskinesia. In the present study, we have investigated the efficacy of 5-HT_1A and 5-HT_1B agonists to counteract l-DOPA-induced dyskinesia in 1-methyl-4-phenyl 1,2,3,6-tetrahydropyridine (MPTP)-treated macaques, the gold standard model of Parkinson's disease. In addition, we have studied the ability of this treatment to prevent development of l-DOPA-induced dyskinesia in 6-OHDA-lesioned rats. The results demonstrate the existence of a potent synergistic effect between 5-HT_1A and 5-HT_1B agonists in their ability to dampen l-DOPA-induced dyskinesia in the MPTP-treated macaques. Sub-threshold doses of the drugs, which individually produced no effect, were able to reduce the abnormal involuntary movements by up to 80% when administered in combination, without affecting the anti-parkinsonian properties of l-DOPA. Furthermore, chronic administration of low doses of the 5-HT₁ agonists in combination was able to prevent development of dyskinesia, and reduce the up-regulation of FosB after daily treatment with l-DOPA in the rat 6-OHDA model. Our results support the importance of a clinical investigation of the effect of 5-HT_1A and 5-HT_1B agonists, particularly in combination, in dyskinetic l-DOPA-treated Parkinson's disease patients.