Elsa Y. Pioli

RGS9–2 Negatively Modulates l-3,4-Dihydroxyphenylalanine-Induced Dyskinesia in Experimental Parkinson's Disease

Chronic l-dopa treatment of Parkinsons disease (PD) often leads to debilitating involuntary movements, termed l-dopa-induced dyskinesia (LID), mediated by dopamine (DA) receptors. RGS9–2 is a GTPase accelerating protein that inhibits DA D2 receptor-activated G proteins. Herein, we assess the functional role of RGS9–2 on LID. In monkeys, Western blot analysis of striatal extracts shows that RGS9–2 levels are not altered by MPTP-induced DA denervation and/or chronic l-dopa administration. In MPTP monkeys with LID, striatal RGS9–2 overexpression – achieved by viral vector injection into the striatum – diminishes the involuntary movement intensity without lessening the anti-parkinsonian effects of the D1/D2 receptor agonist l-dopa. In contrasts, in these animals, striatal RGS9–2 overexpression diminishes both the involuntary movement intensity and the anti-parkinsonian effects of the D2/D3 receptor agonist ropinirole. In unilaterally 6-OHDA-lesioned rats with LID, we show that the time course of viral vector-mediated striatal RGS9–2 overexpression parallels the time course of improvement of l-dopa-induced involuntary movements. We also find that unilateral 6-OHDA-lesioned RGS9−/− mice are more susceptible to l-dopa-induced involuntary movements than unilateral 6-OHDA-lesioned RGS9+/+ mice, albeit the rotational behavior – taken as an index of the anti-parkinsonian response – is similar between the two groups of mice. Together, these findings suggest that RGS9–2 plays a pivotal role in LID pathophysiology. However, the findings also suggest that increasing RGS9–2 expression and/or function in PD patients may only be a suitable therapeutic strategy to control involuntary movements induced by nonselective DA agonist such as l-dopa.

Study of the antidyskinetic effect of eltoprazine in animal models of levodopa-induced dyskinesia

The serotonin (5‐hydroxytryptamine [5HT]) system has recently emerged as an important player in the appearance of l‐3,4‐dihydroxyphenylalanine (levodopa [l‐dopa])–induced dyskinesia in animal models of Parkinsons disease. In fact, dopamine released as a false transmitter from serotonin neurons appears to contribute to the pulsatile stimulation of dopamine receptors, leading to the appearance of the abnormal involuntary movements. Thus, drugs able to dampen the activity of serotonin neurons hold promise for the treatment of dyskinesia. The authors investigated the ability of the mixed 5‐HT 1A/1B receptor agonist eltoprazine to counteract l‐dopa–induced dyskinesia in 6‐hydroxydopamine‐lesioned rats and in 1‐methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine (MPTP)‐treated macaques. The data demonstrated that eltoprazine is extremely effective in suppressing dyskinesia in experimental models, although this effect was accompanied by a partial worsening of the therapeutic effect of l‐dopa. Interestingly, eltoprazine was found to (synergistically) potentiate the antidyskinetic effect of amantadine. The current data indicated that eltoprazine is highly effective in counteracting dyskinesia in preclinical models. However, the partial worsening of the l‐dopa effect observed after eltoprazine administration represents a concern; whether this side effect is due to a limitation of the animal models or to an intrinsic property of eltoprazine needs to be addressed in ongoing clinical trials. The data also suggest that the combination of low doses of eltoprazine with amantadine may represent a valid strategy to increase the antidyskinetic effect and reduce the eltoprazine‐induced worsening of l‐dopa therapeutic effects.

Anti-dyskinetic effect of anpirtoline in animal models of L-DOPA-induced dyskinesia

The serotonin system has emerged as a potential target for anti-dyskinetic therapy in Parkinsons disease. In fact, serotonin neurons can convert L-DOPA into dopamine, and mediate its synaptic release. However, they lack a feedback control mechanism able to regulate synaptic dopamine levels, which leads to un-physiological stimulation of post-synaptic striatal dopamine receptors. Accordingly, drugs able to dampen the activity of serotonin neurons can suppress L-DOPA-induced dyskinesia in animal models of Parkinsons disease. Here, we investigated the ability of the 5-HT1A/1B receptor agonist anpirtoline to counteract L-DOPA-induced dyskinesia in L-DOPA-primed 6-OHDA-lesioned rats and MPTP-treated macaques. Results suggest that anpirtoline dose-dependently reduced dyskinesia both in rats and monkeys; however, the effect in MPTP-treated macaques was accompanied by a worsening of the Parkinsons disease score at significantly effective doses (1.5 and 2.0mg/kg). At a lower dose (0.75mg/kg), anpirtoline markedly reduced dyskinesia in 4 out of 5 subjects, but statistical significance was prevented by the presence of a non-responsive subject. These results provide further evidence that the serotonin neurons contribute both to the pro-dyskinetic effect of L-DOPA and to its therapeutic efficacy in the rat and monkey models of Parkinsons disease.

The mGluR5 negative allosteric modulator dipraglurant reduces dyskinesia in the MPTP macaque model

Blocking metabotropic glutamate receptor type 5 (mGluR5) has been proposed as a target for levodopa‐induced dyskinesias (LID) in Parkinsons disease (PD). We assessed the effect on LID of dipraglurant, a potent selective mGluR5 receptor negative allosteric modulator in the gold‐standard LID macaque model.

Levodopa improves motor deficits but can further disrupt cognition in a macaque parkinson model

Levodopa effectively relieves motor symptoms in Parkinsons disease (PD), but has had inconsistent effects on cognition, even worsening some aspects of cognitive functioning. Therefore, remediation of PD cognitive deficits is a major unmet need. However, drug development efforts have been hampered by lack of an animal model in which motor and cognitive deficits can be examined simultaneously.

Combined fenobam and amantadine treatment promotes robust antidyskinetic effects in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned primate model of Parkinson's disease

Amantadine, an N‐methyl‐D‐aspartate glutamate receptor antagonist, is currently the only pharmacological treatment for levodopa‐induced dyskinesia (LID) in Parkinsons disease (PD), but causes adverse effects on the central nervous system at therapeutic doses. Fenobam, a negative modulator of metabotropic glutamate receptor subtype 5, has recently been reported to attenuate LID in MPTP‐treated macaques. The aim of the current study was to investigate the treatment interactions of fenobam and amantadine on LID in the MPTP‐treated macaque model of PD. The antidyskinetic and ‐parkinsonian effects were measured after administration of fenobam (10‐30 mg/kg) and amantadine (10‐30 mg/kg) alone and in combination. Fenobam (30 mg/kg) and amantadine (30 mg/kg) alone reduced LID, whereas lower doses of either drug did not cause any significant effects. A combined treatment of fenobam and amantadine at subthreshold doses (10 and 20 mg/kg) significantly reduced LID without worsening PD disability. These data suggest that a low‐dose combination of fenobam and amantadine can be used for alleviating dyskinesia without causing adverse motor effects. Such combined therapies may offer a new therapeutic strategy for treatment of LID in PD patients.

Effects of memantine and galantamine on cognitive performance in aged rhesus macaques

Current pharmacotherapies for Alzheimers disease (AD) are focused on improving performance of daily activities, personal care, and management of problematic behaviors. Both memantine, a noncompetitive N-methyl-D-aspartate channel blocker and galantamine, a selective acetylcholinesterase inhibitor, are currently prescribed as symptomatic therapies for AD. However, drugs that progressed directly from testing in rodent models to testing in AD patients in clinical trials failed to demonstrate consistent effects on cognitive symptoms. Considering the lack of nonhuman primate data on the effects of memantine and galantamine alone or in combination on cognitive dysfunction in aged nonhuman primates, the present study examined how closely data derived from aged nonhuman primates reflects data obtained in humans. Mild beneficial effects on aspects of cognitive performance in aged primates were found, in general agreement with the human clinical experience with these drugs but in contrast to the more positive effects reported in the rodent literature. These data suggest that the nonhuman primate might have more predictive validity for drug development in this area than comparable rodent assays.

Asymmetrically lesioned mesencephalon in healthy rodents: call for caution

Stereological counting of tyrosine-hydroxylase immunoreactive (TH-IR) neurons in the mesencephalon is a pivotal parameter in assessing the extent of lesioning in animal models of Parkinsons disease. We here show that the number of TH-IR neurons often appears abnormally decreased in healthy--commercially available--mice and rats, although both the number of Nissl-stained cells and the striatal dopaminergic innervation are unaffected. This potential bias in assessing extent of neurotoxin-induced lesion and subsequent protection by pharmacological manipulation prompts us to call for caution in setting up experimental designs.

Partial bilateral mesencephalic lesions affect D1 but not D2 binding in both the striatum and cortex

The substantia nigra pars compacta (SNc) and the ventral tegmental area (VTA) are the two major mesencephalic dopaminergic systems. Mesencephalic dopamine denervation is followed by long-term modifications in striatum and cortex that preserve dopamine functions. Here, we have studied the impact of isolated bilateral 6-hydroxydopamine lesioning of the SNc or the VTA on D(1) and D(2) dopamine receptor binding in striatal and cortical areas of rat. Neither SNc nor VTA bilateral partial lesioning changed D(2) binding at the striatal or cortical level. Intriguingly, only VTA lesioning increased D(1) binding in the cortex, whereas both bilateral partial lesioning of the SNc or the VTA increased striatal D(1) binding. This suggests that increased cortical D(1) binding could be an indicator of VTA lesioning. Further behavioural experiments may explain the pathophysiological meaning of increased cortical D(1) binding, and determine whether this observation is involved in compensatory mechanisms.

The deafferented nonhuman primate is not a reliable model of intractable pain.

Abstract Before extending the application of motor cortex stimulation it is important to investigate the intimate mechanisms by which it alleviates intractable pain and to consider possible side effects. Self-mutilation in animals following extensive neurectomy or posterior rhizotomy of a limb is thought to reveal severe dysesthesias in the deafferented zone suggesting its usefulness as an animal model of chronic pain in humans. We here show in deafferented nonhuman primates that the autotomy behavior immediately follows the surgery and disappears after 28 days. In keeping with the experience of Y. Lamarre, the simple but careful care of all wounds is sufficient to abolish this behavior. Our results do not exclude the possibility that the deafferentation is still painful for the monkeys, but they definitely rule out that autotomy is a consistent response to deafferentation.