mGlu receptors in the treatment of Parkinson's disease and L-DOPA-induced dyskinesia

Different Generations of Type-B Monoamine Oxidase Inhibitors in Parkinson's Disease: From Bench to Bedside

Three inhibitors of type-B monoamine oxidase (MAOB), selegiline, rasagiline, and safinamide, are used for the treatment of Parkinson's disease (PD). All three drugs improve motor signs of PD, and are effective in reducing motor fluctuations in patients undergoing long-term L-DOPA treatment. The effect of MAOB inhibitors on non-motor symptoms is not uniform and may not be class-related. Selegiline and rasagiline are irreversible inhibitors forming a covalent bond within the active site of MAOB. In contrast, safinamide is a reversible MAOB inhibitor, and also inhibits voltage- sensitive sodium channels and glutamate release. Safinamide is the prototype of a new generation of multi-active MAOB inhibitors, which includes the antiepileptic drug, zonisamide. Inhibition of MAOB-mediated dopamine metabolism largely accounts for the antiparkinsonian effect of the three drugs. Dopamine metabolism by MAOB generates reactive oxygen species, which contribute to nigro-striatal degeneration. Among all antiparkinsonian agents, MAOB inhibitors are those with the greatest neuroprotective potential because of inhibition of dopamine metabolism, induction of neurotrophic factors, and, in the case of safinamide, inhibition of glutamate release. The recent development of new experimental animal models that more closely mimic the progressive neurodegeneration associated with PD will allow to test the hypothesis that MAOB inhibitors may slow the progression of PD.

Viewpoint: Developing drugs for levodopa-induced dyskinesia in PD: Lessons learnt, what does the future hold?

The drive to develop drugs to treat PD starts and ends with the patient. Herein, we discuss how the experience with drug development for LID has led the field in translational studies in PD with advancing ground-breaking science via rigorous clinical trial design, to deliver clinical proof-of-concepts across multiple therapeutic targets. However, issues remain in advancing drugs efficacious preclinically to the clinic, and future studies need to learn from past successes and failures. Such lessons include implementing better early indicators of tolerability, for instance evaluating non-motor symptoms in preclinical models; improving patient-related outcome measures in clinical trials, as well as considering the unique nature of dyskinesia in an individual patient. The field of translational studies needs to become more patient focused to improve successful outcomes.

Genetic enhancement of Ras-ERK pathway does not aggravate L-DOPA-induced dyskinesia in mice but prevents the decrease induced by lovastatin

Increasing evidence supports a close relationship between Ras-ERK1/2 activation in the striatum and L-DOPA-induced dyskinesia (LID). ERK1/2 activation by L-DOPA takes place through the crosstalk between D1R/AC/PKA/DARPP-32 pathway and NMDA/Ras pathway. Compelling genetic and pharmacological evidence indicates that Ras-ERK1/2 inhibition prevents LID onset and may even revert already established dyskinetic symptoms. However, it is currently unclear whether exacerbation of Ras-ERK1/2 activity in the striatum may further aggravate dyskinesia in experimental animal models. Here we took advantage of two genetic models in which Ras-ERK1/2 signaling is hyperactivated, the Nf1^+/− mice, in which the Ras inhibitor neurofibromin is reduced, and the Ras-GRF1 overexpressing (Ras-GRF1 OE) transgenic mice in which a specific neuronal activator of Ras is enhanced. Nf1^+/− and Ras-GRF1 OE mice were unilaterally lesioned with 6-OHDA and treated with an escalating L-DOPA dosing regimen. In addition, a subset of Nf1^+/− hemi-parkinsonian animals was also co-treated with the Ras inhibitor lovastatin. Our results revealed that Nf1^+/− and Ras-GRF1 OE mice displayed similar dyskinetic symptoms to their wild-type counterparts. This observation was confirmed by the lack of differences between mutant and wild-type mice in striatal molecular changes associated to LID (i.e., FosB, and pERK1/2 expression). Interestingly, attenuation of Ras activity with lovastatin does not weaken dyskinetic symptoms in Nf1^+/− mice. Altogether, these data suggest that ERK1/2-signaling activation in dyskinetic animals is maximal and does not require further genetic enhancement in the upstream Ras pathway. However, our data also demonstrate that such a genetic enhancement may reduce the efficacy of anti-dyskinetic drugs like lovastatin.

Genetic Knockdown of mGluR5 in Striatal D1R-Containing Neurons Attenuates L-DOPA-Induced Dyskinesia in Aphakia Mice

L-DOPA is the main pharmacological therapy for Parkinson's disease. However, long-term exposure to L-DOPA induces involuntary movements termed dyskinesia. Clinical trials show that dyskinesia is attenuated by metabotropic glutamate receptor type 5 (mGluR5) antagonists. Further, the onset of dyskinesia is delayed by nicotine and mGluR5 expression is lower in smokers than in non-smokers. However, the mechanisms by which mGluR5 modulates dyskinesia and how mGluR5 and nicotine interact have not been established. To address these issues, we studied the role of mGluR5 in D1R-containing neurons in dyskinesia and examined whether nicotine reduces dyskinesia via mGluR5. In the aphakia mouse model of Parkinson's disease, we selectively knocked down mGluR5 in D1R-containing neurons (aphakia-mGluR5^KD-D1). We found that genetic downregulation of mGluR5 decreased dyskinesia in aphakia mice. Although chronic nicotine increased the therapeutic effect of L-DOPA in both aphakia and aphakia-mGluR5^KD-D1 mice, it caused a robust reduction in dyskinesia only in aphakia, and not in aphakia-mGluR5^KD-D1 mice. Downregulating mGluR5 or nicotine treatment after L-DOPA decreased ERK and histone 3 activation, and FosB expression. Combining nicotine and mGluR5 knockdown did not have an added antidyskinetic effect, indicating that the effect of nicotine might be mediated by downregulation of mGluR5 expression. Treatment of aphakia-mGluR5^KD-D1 mice with a negative allosteric modulator did not further modify dyskinesia, suggesting that mGluR5 in non-D1R-containing neurons does not play a role in its development. In conclusion, this work suggests that mGluR5 antagonists reduce dyskinesia by mainly affecting D1R-containing neurons and that the effect of nicotine on dyskinetic signs in aphakia mice is likely via mGluR5.