Levodopa-induced dyskinesias

Evaluation of the D3 dopamine receptor selective agonist/partial agonist PG01042 on L-dopa dependent animal involuntary movements in rats

The substituted 4-phenylpiperazine D3 dopamine receptor selective antagonist PG01037 ((E)-N-(4-(4-(2,3-dichlorophenyl)piperazin-1-yl)but-2-enyl)-4-(pyridin-2-yl)benzamide) was reported to attenuate L-dopa-associated abnormal involuntary movements (AIMs) in unilaterally lesioned rats, a model of L-dopa-dependent dyskinesia in patients with Parkinson's Disease (Kumar et al., 2009a). We now report that PG01042 (N-(4-(4-(2,3-dichlorophenyl)piperazin-1-yl)butyl)-4-(pyridin-3-yl)benzamide), which is a D3 dopamine receptor selective agonist for adenylyl cyclase inhibition and a partial agonist for mitogenesis, is also capable of attenuating AIMs scores. The intrinsic activity of PG01037 and PG01042 were determined using a) a forskolin-dependent adenylyl cyclase inhibition assay and b) an assay for agonist-associated mitogenesis. It was observed that the in vivo efficacy of PG01042 increased when administered by intraperitoneal (i.p.) injection simultaneously with L-dopa/benserazide (8 mg/kg each), as compared to a 60 min or 30 min pretreatment. PG01042 was found to attenuate AIM scores in these animals in a dose dependent manner. While PG01042 did not effectively inhibit SKF 81297-dependent AIMs, it inhibited apomorphine-dependent AIM scores. Rotarod studies indicate that PG01042 at a dose of 10 mg/kg did not adversely affect motor coordination of the unilaterally lesioned rats. Evaluation of lesioned rats using a cylinder test behavioral paradigm indicated that PG01042 did not dramatically attenuate the beneficial effects of L-dopa. These studies and previously published studies suggest that both D3 dopamine receptor selective antagonists, partial agonists and agonists, as defined by an adenylyl cyclase inhibition assay and a mitogenic assay, are pharmacotherapeutic candidates for the treatment of L-dopa-associated dyskinesia in patients with Parkinson's Disease.

Levodopa is not a useful treatment for Lesch-Nyhan disease

Lesch-Nyhan disease (LND) is characterized by dystonia, cognitive abnormalities, and self-injurious behavior. No effective therapies are available. LND is associated with a presynaptic dopaminergic deficit, but the reported effects of dopamine replacement therapy are conflicting. The current prospective open-label study assesses the effects of levodopa on both neurological and behavioral features of LND. All 6 study participants discontinued levodopa early, due to lack of effect and sometimes worsening of motor function. The results provide important clues for pathophysiological mechanisms and suggestions for future treatment options.

Nicotinic receptor agonists decrease L-dopa-induced dyskinesias most effectively in partially lesioned parkinsonian rats

L-dopa therapy for Parkinson's disease leads to dyskinesias or abnormal involuntary movement (AIMs) for which there are few treatment options. Our previous data showed that nicotine administration reduced L-dopa-induced AIMs in parkinsonian monkeys and rats. To further understand how nicotine mediates its antidyskinetic action, we investigated the effect of nicotinic receptor (nAChR) agonists in unilateral 6-OHDA-lesioned rats with varying striatal damage. We first tested the drugs in L-dopa-treated rats with a near-complete striatal dopamine lesion (>99%), the standard rodent dyskinesia model. Varenicline, an agonist that interacts with multiple nAChRs, did not significantly reduce L-dopa-induced AIMs, while 5-iodo-A-85380 (A-85380), which acts selectively at α4β2* and α6β2* subtypes, reduced AIMs by 20%. By contrast, both varenicline and A-85380 reduced L-dopa-induced AIMs by 40-50% in rats with a partial striatal dopamine lesion. Neither drug worsened the antiparkinsonian action of L-dopa. The results show that selective nicotinic agonists reduce dyskinesias, and that they are optimally effective in animals with partial striatal dopamine damage. These findings suggest that presynaptic dopamine terminal α4β2* and α6β2* nAChRs are critical for nicotine's antidyskinetic action. The current data have important implications for the use of nicotinic receptor-directed drugs for L-dopa-induced dyskinesias, a debilitating motor complication of dopamine replacement therapy for Parkinson's disease.

Therapeutic potential of targeting group III metabotropic glutamate receptors in the treatment of Parkinson's disease

Current drugs used in the treatment of Parkinson's disease (PD), for example, L-DOPA and dopamine agonists, are very effective at reversing the motor symptoms of the disease. However, they do little to combat the underlying degeneration of dopaminergic neurones in the substantia nigra pars compacta (SNc) and their long-term use is associated with the appearance of adverse effects such as L-DOPA-induced dyskinesia. Much emphasis has therefore been placed on finding alternative non-dopaminergic drugs that may circumvent some or all of these problems. Group III metabotropic glutamate (mGlu) receptors were first identified in the basal ganglia a decade ago. One or more of these receptors (mGlu4, mGlu7 or mGlu8) is found on pre-synaptic terminals of basal ganglia pathways whose overactivity is implicated not only in the generation of motor symptoms in PD, but also in driving the progressive SNc degeneration. The finding that drugs which activate group III mGlu receptors can inhibit transmission across these overactive synapses has lead to the proposal that group III mGlu receptors are promising targets for drug discovery in PD. This paper provides a comprehensive review of the role and target potential of group III mGlu receptors in the basal ganglia. Overwhelming evidence obtained from in vitro studies and animal models of PD supports group III mGlu receptors as potentially important drug targets for providing both symptom relief and neuroprotection in PD.

l-Dopa-induced dyskinesia-clinical presentation, genetics, and treatment

Levodopa-induced dyskinesia (LID) has been recognized since the introduction of levodopa for the management of Parkinson's disease (PD) and continues to be one of the most clinically challenging factors in long-term management of patients with PD. Most patients develop LID within 10 years of PD onset and the cause has been attributed to various factors including disease demographics, pharmacological, and possibly genetic causes. The clinical pattern of LID varies and shows intra and inter-patient variability and has been classified based upon phenomenology and relation to timing of levodopa. The potential armamentarium to address and manage LID has significantly increased in the last decade. This chapter addresses the current understanding of various clinical aspects and available therapeutics for LID.

Levetiracetam for the management of levodopa-induced dyskinesias in Parkinson's disease

The efficacy and safety of levetiracetam (LEV), administered for management of levodopa-induced dyskinesias (LID) in Parkinson's disease (PD), was examined using a multicenter, double-blind, placebo-controlled, parallel groups, crossover trial. Because of having a period effect, data after crossover point was excluded from analysis. Levodopa-treated PD participants with LID (n = 38) received LEV 500 mg/day, were assessed, titrated to 1,000 mg/day and reassessed, before and after crossover. The placebo group followed the same routine. Primary efficacy was defined from percent change in "On with LID" time from patient diaries. Secondary efficacy assessment used "On without LID," "Off" time, unified PD rating scale (UPDRS), clinical global impression (CGI), and Goetz dyskinesia scale after levodopa challenge. Safety measures were also performed. On with LID time decreased 37 minutes (95% confidence interval [CI] 0.59, 7.15; P = 0.02) at 500 mg/day, 7.85% 75 minutes (95% CI 3.3, 12.4; P = 0.002) at 1,000 mg/day. On without LID time increased by 46 minutes (95% CI -1.55, -0.03; P = 0.04) at 500 mg/day and 55 minutes (95% CI -10.39, -1.14; P = 0.018) at 1,000 mg/day. UPDRS 32 showed decreased dyskinesia duration mean change 0.35 (95% CI 0.09, 0.5; P = 0.009) at 1,000 mg/day. CGI showed LID decreased by 0.7 (95% CI 0.21, 1.18; P = 0.006) at 1,000 mg/day. Patient diaries and UPDRS show no increase in Off time. This exploratory trial provides evidence that LEV in 1,000 mg/day, slowly titrated, could be useful in improving LID as was assessed with patient diaries, UPDRS, and CGI scales, safely, with minimal side effects.

Levodopa-induced dyskinesia is associated with increased thyrotropin releasing hormone in the dorsal striatum of hemi-parkinsonian rats

Background

Dyskinesias associated with involuntary movements and painful muscle contractions are a common and severe complication of standard levodopa (L-DOPA, L-3,4-dihydroxyphenylalanine) therapy for Parkinson's disease. Pathologic neuroplasticity leading to hyper-responsive dopamine receptor signaling in the sensorimotor striatum is thought to underlie this currently untreatable condition.

Methodology/Principal Findings

Quantitative real-time polymerase chain reaction (PCR) was employed to evaluate the molecular changes associated with L-DOPA-induced dyskinesias in Parkinson's disease. With this technique, we determined that thyrotropin releasing hormone (TRH) was greatly increased in the dopamine-depleted striatum of hemi-parkinsonian rats that developed abnormal movements in response to L-DOPA therapy, relative to the levels measured in the contralateral non-dopamine-depleted striatum, and in the striatum of non-dyskinetic control rats. ProTRH immunostaining suggested that TRH peptide levels were almost absent in the dopamine-depleted striatum of control rats that did not develop dyskinesias, but in the dyskinetic rats, proTRH immunostaining was dramatically up-regulated in the striatum, particularly in the sensorimotor striatum. This up-regulation of TRH peptide affected striatal medium spiny neurons of both the direct and indirect pathways, as well as neurons in striosomes.

Conclusions/Significance

TRH is not known to be a key striatal neuromodulator, but intrastriatal injection of TRH in experimental animals can induce abnormal movements, apparently through increasing dopamine release. Our finding of a dramatic and selective up-regulation of TRH expression in the sensorimotor striatum of dyskinetic rat models suggests a TRH-mediated regulatory mechanism that may underlie the pathologic neuroplasticity driving dopamine hyper-responsivity in Parkinson's disease.

Differential genetic susceptibility in diphasic and peak-dose dyskinesias in Parkinson's disease

To examine whether there is a differential genetic susceptibility in the diphasic and peak-dose forms of levodopa-induced dyskinesias (LID) in patients with Parkinson's disease (PD). The study cohort comprised 503 unrelated Korean PD patients who were treated with levodopa and had a disease duration of at least 5 years. The presence of LID was identified during a routine follow-up and special care was taken to separate the two distinct forms of LID into diphasic and peak-dose dyskinesias (PDSK). Genotyping was performed in the 503 patients and in 559 healthy controls to search for polymorphisms of DRD3 p.S9G, DRD2 Taq1A, GRIN2B c.2664C>T, c.366C>G, c.-200T>G, and the promoter region of SLC6A4. A total of 229 patients expressed LID (peak-dose in 205, diphasic in 57, and both in 33). The presence of diphasic dyskinesia (DDSK) was exclusively associated with the DRD3 p.S9G variant after adjusting for gender, age at PD onset, Hoehn & Yahr stage, and duration of levodopa treatment. Carrying the AA genotype was likely to shorten the onset of DDSK according to the duration of levodopa therapy (P = 0.02). The presence of PDSK was not significantly associated with any of the six genetic variants studied. There may be a genetic susceptibility in the development of DDSK in PD patients on chronic levodopa therapy, and its underlying pathophysiological mechanism might be distinct from that of PDSK.

Pros and cons of apomorphine and L-dopa continuous infusion in advanced Parkinson's disease

Motor fluctuations and dyskinesia are common in advanced Parkinson's disease and can be poorly managed by current oral medications. Risk factors include the amount of L-dopa administered, gender and patient age. Continuous duodenal L-dopa or subcutaneous apomorphine infusions are helpful strategies because they can control motor complications by providing continuous dopaminergic drug delivery. Apomorphine subcutaneous infusion provides a motor benefit similar to that of dopamine and is relatively easy to use in advanced PD. However, it commonly requires concomitant administration of oral L-dopa and its long-term use is limited by compliance. Continuous administration of L-dopa/carbidopa by infusion in the duodenum/jejunum is a more complex procedure requiring a gastrostomy for the placement of the infusion tube, but it allows replacement of all oral medications and the achievement of a satisfactory therapeutic response paralleled by a reduction of motor complication severity. It should be noted that although these procedures are effective, most evidence relates to small case series and, particularly in the case of apomorphine, despite its long-term availability, there is a complete lack of randomized blinded studies. In addition, unlike deep brain stimulation, it is unclear which patients are the best candidates for these procedures, making any indirect comparison very complex, given the clinical heterogeneity of reported patients. This has consequences in resource allocation and in estimating cost-benefit ratios for these complex therapies in advanced PD.

Levetiracetam for levodopa-induced dyskinesia in Parkinson's disease: a randomized, double-blind, placebo-controlled trial

The aim of this study was to assess the efficacy, safety and tolerability of the antiepileptic compound levetiracetam (LEV) for the treatment of levodopa-induced dyskinesia (LID) in Parkinson's disease (PD). We thus performed a randomized, double-blind, placebo-controlled, parallel-group pilot study in PD patients with moderate-to-severe LID on stable dopaminergic therapy. Placebo or LEV was administered twice daily (titrated from 250 to 2,000 mg/day) as add-on therapy. Subjects underwent evaluation of the unified-PD-rating scale (UPDRS) and the modified abnormal involuntary movement scale (AIMS). The primary outcome variable was the change of the AIMS score between baseline and end-of-treatment visit. Secondary variables included total UPDRS score and response to levodopa challenge. Of 32 randomized patients (mean age 65.2 years, 62.5% women), 17 received LEV and 15 placebo. After 11 weeks of treatment, mean changes of the modified AIMS from baseline were -1.5 (-26%) for LEV (p = 0.332) and +0.9 (+13%) for placebo (p = 0.588) without significant differences between groups. Mean changes of the UPDRS item 32/33 sum score from baseline showed significant improvement of dyskinesia in the LEV group [-1.0 (-20%); p = 0.012], but not in the placebo group [-0.4 (-8%); p = 0.306]. Treatment had no effects on UPDRS motor score or levodopa response. Frequency and quality of adverse events were similar in both treatment groups. Together, LEV showed only mild antidyskinetic effects without worsening of Parkinsonian symptoms or compromising levodopa efficacy. LEV was well tolerated in doses up to 2,000 mg/day. Further large controlled studies are warranted to evaluate the impact of LEV on LID in PD patients.

Dysregulation of striatal dopamine release in a mouse model of dystonia

Dystonia is a neurological disorder characterized by involuntary movements. We examined striatal dopamine (DA) function in hyperactive transgenic (Tg) mice generated as a model of dystonia. Evoked extracellular DA concentration was monitored with carbon-fiber microelectrodes and fast-scan cyclic voltammetry in striatal slices from non-Tg mice, Tg mice with a positive motor phenotype, and phenotype-negative Tg littermates. Peak single-pulse evoked extracellular DA concentration was significantly lower in phenotype-positive mice than in non-Tg or phenotype-negative mice, but indistinguishable between non-Tg and phenotype-negative mice. Phenotype-positive mice also had higher functional D2 DA autoreceptor sensitivity than non-Tg mice, which would be consistent with lower extracellular DA concentration in vivo. Multiple-pulse (phasic) stimulation (five pulses, 10-100 Hz) revealed an enhanced frequency dependence of evoked DA release in phenotype-positive versus non-Tg or phenotype-negative mice, which was exacerbated when extracellular Ca(2+) concentration was lowered. Enhanced sensitivity to phasic stimulation in phenotype-positive mice was reminiscent of the pattern seen with antagonism of nicotinic acetylcholine receptors. Consistent with a role for altered cholinergic regulation, the difference in phasic responsiveness among groups was lost when nicotinic receptors were blocked by mecamylamine. Together, these data implicate compromised DA release regulation, possibly from cholinergic dysfunction, in the motor symptoms of this dystonia model.

Objective measurement of dyskinesia in Parkinson's disease using a force plate

Clinical investigation of levodopa-induced dyskinesia (LID) in Parkinson's disease (PD) is limited because of lack of objective measurements and no consensus on use of a standard measuring tool. Currently, clinical trials use subject-completed diaries of dyskinesia throughout the day or investigator-administered clinical rating scales. An objective and valid method of measuring LID would reduce bias, variability, and decrease the time and number needed in trials of potential anti-dyskinetic agents. We have investigated using a force plate under standing subjects, which records movement of the center of pressure (CoP) to quantify LID over a levodopa (L-dopa) cycle. Twenty-two PD subjects (15 with LID, 7 without LID) admitted to an inpatient research facility had their PD meds withheld overnight, followed by a 2 hours intravenous L-dopa infusion the next day. The root mean squared of the velocity in the anterior-posterior direction (RMSV) derived from an analysis of the CoP, and, the modified Abnormal Involuntary Movement Scale (mAIMS) were performed repeatedly for 6 hours, initially as subjects were OFF before the infusion, through infusion until OFF again. There was a high correlation between the area under the curve (AUC) of the mAIMS and the RMSV within and between subjects. As a measure of LID, RMSV had excellent validity and reliability between subjects, and using a force plate was feasible. Sensitivity to changes in LID was initially demonstrated but should be repeated. Thus, CoP recordings on a force plate can objectively quantify LID in PD and may be very useful in clinical trials or other investigations of dyskinesia.

The effect of peripheral enzyme inhibitors on levodopa concentrations in blood and CSF

Levodopa combined with a dopa-decarboxylase inhibitor, such as carbidopa, shifts the metabolism to the COMT pathway. Adding the peripheral acting COMT inhibitor entacapone provides improvement for patients with PD suffering from motor fluctuations. We studied the effects of the enzyme inhibitors entacapone and carbidopa on the levodopa concentrations in CSF and in blood. Five PD patients with wearing-off underwent lumbar drainage and intravenous microdialysis. Samples were taken 12 h daily for 3 days. Day 1; intravenous levodopa was given, day 2; additional oral entacapone 200 mg tid, day 3; additional oral entacapone 200 mg tid and carbidopa 25 mg bid. Levodopa in CSF and in dialysates was analysed. The AUC for levodopa increased both in blood and CSF when additional entacapone was given alone and in combination with carbidopa. The C(max) of levodopa in both CSF and blood increased significantly. Additional entacapone to levodopa therapy gives an increase of C(max) in CSF and in blood. The increase is more evident when entacapone is combined with carbidopa.

Insights into pathophysiology of punding reveal possible treatment strategies

Punding is a stereotyped behavior characterized by an intense fascination with a complex, excessive, nongoal oriented, repetitive activity. Men tend to repetitively tinker with technical equipment such as radio sets, clocks, watches and car engines, the parts of which may be analyzed, arranged, sorted and cataloged but rarely put back together. Women, in contrast, incessantly sort through their handbags, tidy continuously, brush their hair or polish their nails. Punders are normally aware of the inapposite and obtuse nature of the behavior; however, despite the consequent self-injury, they do not stop such behavior. The most common causes of punding are dopaminergic replacement therapy in patients affected by Parkinson's disease (PD) and cocaine and amphetamine use in addicts. The vast majority of information about punding comes from PD cases. A critical review of these cases shows that almost all afflicted patients (90%) were on treatment with drugs acting mainly on dopamine receptors D1 and D2, whereas only three cases were reported in association with selective D2 and D3 agonists. Epidemiological considerations and available data from animal models suggest that punding, drug-induced stereotypies, addiction and dyskinesias all share a common pathophysiological process. Punding may be related to plastic changes in the ventral and dorsal striatal structures, including the nucleus accumbens, and linked to psychomotor stimulation and reward mechanisms. Possible management guidelines are proposed.

Alleviation of off-period dystonia in Parkinson disease by a microlesion following subthalamic implantation

Object

A collision/implantation or microlesion effect is commonly described after subthalamic nucleus (STN) implantation for high-frequency stimulation, and this is presumed to reflect disruption of cells and/or fibers. Off-period dystonia, a frequent cause of disability in patients with advanced Parkinson disease, can lead to the need for surgical treatment. The authors assessed the early effect of this microlesion on off-period dystonia.

Methods

The authors assessed 30 consecutive patients with the advanced levodopa-responsive form of Parkinson disease. The patients' symptoms were Hoehn and Yahr Scale score ≥ 3, the mean duration of their disease was 11.4 ± 3.5 years, and they had undergone bilateral implantation of electrodes within the STN for high-frequency stimulation between February 2004 and December 2006. The microlesion effect was defined by the clinical improvement (Unified Parkinson's Disease Rating Scale [UPDRS] Part III score, UPDRS Part IV, item 35) assessed the morning of the 3rd day following STN implantation, after at least a 12-hour withdrawal of dopaminergic treatment and before the programmable pulse generator was switched on (off-drug/off-stimulation mode).

Results

Compared with baseline (off state), the microlesion effect improved the motor score (UPDRS Part III) by 27%. Subscores for tremor, rigidity, and bradykinesia respectively improved by 42, 37, and 25%. Nineteen patients (63%) suffered from off-period dystonia before surgery. Twelve (41%) reported complete relief of their symptoms in the immediate postoperative period and remained free of painful off-period dystonia throughout the 6-month follow-up period.

Conclusions

The author postulated that off-period dystonia alleviation may reflect both a microsubthalamotomy and micropallidotomy effect. They hypothesize, moreover, that the microlesion could play a role in the 6-month postoperative outcome.

Cannabinoid-dopamine interaction in the pathophysiology and treatment of CNS disorders

Endocannabinoids and their receptors, mainly the CB(1) receptor type, function as a retrograde signaling system in many synapses within the CNS, particularly in GABAergic and glutamatergic synapses. They also play a modulatory function on dopamine (DA) transmission, although CB(1) receptors do not appear to be located in dopaminergic terminals, at least in the major brain regions receiving dopaminergic innervation, e.g., the caudate-putamen and the nucleus accumbens/prefrontal cortex. Therefore, the effects of cannabinoids on DA transmission and DA-related behaviors are generally indirect and exerted through the modulation of GABA and glutamate inputs received by dopaminergic neurons. Recent evidence suggest, however, that certain eicosanoid-derived cannabinoids may directly activate TRPV(1) receptors, which have been found in some dopaminergic pathways, thus allowing a direct regulation of DA function. Through this direct mechanism or through indirect mechanisms involving GABA or glutamate neurons, cannabinoids may interact with DA transmission in the CNS and this has an important influence in various DA-related neurobiological processes (e.g., control of movement, motivation/reward) and, particularly, on different pathologies affecting these processes like basal ganglia disorders, schizophrenia, and drug addiction. The present review will address the current literature supporting these cannabinoid-DA interactions, with emphasis in aspects dealing with the neurochemical, physiological, and pharmacological/therapeutic bases of these interactions.

Reduction of L-DOPA-induced dyskinesia by the selective metabotropic glutamate receptor 5 antagonist 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-lesioned macaque model of Parkinson's disease

Long-term motor complications of dopamine replacement, such as L-DOPA-induced dyskinesia (LID) and reduced quality of L-DOPA action, remain obstacles in the treatment of Parkinson's disease. Dysfunctional glutamatergic neurotransmitter systems have been observed in both the untreated parkinsonian and dyskinetic states and represent novel targets for treatment. Here, we assess the pharmacokinetic profile and corresponding pharmacodynamic effects on behavior of the orally active, selective metabotropic glutamate receptor type 5 (mGlu5) antagonist, 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP) (as the hydrochloride salt) in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned macaque. Six parkinsonian, MPTP-lesioned cynomolgus monkeys, with established LID, were administered acute challenges with MTEP (4.5-36 mg/kg p.o.) or vehicle, either alone or in combination with L-DOPA (33 +/- 1 mg/kg p.o.). Motor activity, parkinsonian disability, and dyskinesia were assessed for a 6-h period. Plasma drug levels were assessed by liquid chromatography-tandem mass spectrometry. MTEP had no antiparkinsonian action as monotherapy. However, administration of L-DOPA in combination with MTEP (36 mg/kg) reduced peak dose LID by 96%. Moreover, although total on-time (duration for which L-DOPA exerted an antiparkinsonian effect) was not significantly reduced, MTEP (36 mg/kg) reduced the duration of on-time with disabling LID by 70% compared with that for L-DOPA alone. These effects were associated with a peak plasma concentration of 20.9 microM and an area under the curve from 0 to 24 h of 136.1 h x microM (36 mg/kg). Although total on-time was not reduced, the peak antiparkinsonian benefit of l-DOPA/MTEP (36 mg/kg) was less than that with L-DOPA alone. Selective mGlu5 inhibitors may have significant potential to ameliorate dyskinesia, but care should be taken to ensure that such effects do not come at the expense of the peak antiparkinsonian benefit of L-DOPA.

Therapy for dyskinesias in Parkinson’s disease patients

Dyskinesia hampers the quality of life for most Parkinson’s disease patients following several years of therapy. However, the severity of L-Dopa-induced dyskinesia (LID) varies between patients, being quite tolerable in late-onset patients. Understanding the pathogenesis of LID has contributed to the development of a set of therapeutic strategies, including the choice, in early stages, of the least pulsatile regimen of dopamine-receptor activation. In cases where LIDs are already disabling, there is only a limited number of options: the optimization of ongoing DOPA-centered treatment, the utilization of glutamate antagonists and the exploration of the benefits of antipsychotic agents. More radical solutions are provided by deep brain stimulation in the subthalamic nucleus (or internal pallidus). This approach has proved efficacious in reducing LID, largely because it allows a reduction in dopaminergic daily doses. Stereotactic neurosurgery has fuelled several lines of investigation regarding the crosstalk between the basal ganglia and motor cortex. Here, we will present interesting evidence highlighting the potential for repetitive transcranial stimulation in reducing the occurrence of LID. The future may disclose important new avenues for the treatment of LIDs, given the current development of promising agents that might target different facets of dyskinesia, such as the impairment of striatal plasticity and non-Dopaminergic contributors such as adenosine, nitric oxide and the nucleotide cascade.

Central levodopa influx and the clinical motor response to levodopa in patients with Parkinson disease complicated with motor fluctuations and dyskinesias

OBJECTIVE

To study the possible relationship between central levodopa influx and short-term antiparkinsonian and dyskinetic responses to levodopa in patients with Parkinson disease.

METHODS

The clinical response to a single oral dose of standard and controlled-release levodopa/carbidopa was assessed in 12 patients with Parkinson disease complicated with motor fluctuations and dyskinesias. Plasma concentrations of levodopa and large neutral amino acids were determined, and the theoretical central levodopa influx was calculated using a model based on competitive inhibition of substrates to cross the blood-brain barrier.

RESULTS

The mean (SD) central levodopa influxes at the onset of the antiparkinsonian clinical effect were 19.7 (10.9 x 10(-3) and 19.1 (7.4 x 10(-3) nmol min(-1) g(-)1 (P >0.1) for standard and controlled-release levodopa, respectively. The mean (SD) central levodopa influxes at the onset of choreic dyskinesias were 20.1 (8.2 x 10 (-3) and 19.9 (10.8 x 10(-3) nmol min (-1) g(-)1 (P 9 0.1) for standard and controlled-release levodopa, respectively. During the tests, choreic dyskinesias were associated with a central levodopa influx of 10 x 10(-3) nmol min(-1) g(-1) or greater, and foot dystonia occurred with a central levodopa influx less than 9 x 10(-3) nmol min(-1) g(-1).

CONCLUSIONS

The clinical response to levodopa in patients with advanced Parkinson disease may be related to central levodopa influx. We found no differences in the central levodopa influx threshold for clinical improvement with different levodopa formulations. The central levodopa influxes at the onset of choreic dyskinesias and antiparkinsonian effect were similar. Choreic dyskinesias and foot dystonia were associated with high and low central levodopa influx, respectively.

MK-801 inhibits L-DOPA-induced abnormal involuntary movements only at doses that worsen parkinsonism

Amantadine and dextromethorphan suppress levodopa (L-DOPA)-induced dyskinesia in Parkinson's disease patients and abnormal involuntary movements (AIMs) in the 6-hydroxydopamine (6-OHDA) rat model. These medications have been hypothesized to exert their therapeutic effects by a noncompetitive N-methyl-D-aspartate (NMDA) antagonist mechanism, but they also have known serotonin (5-HT) indirect agonist effects that could suppress AIMs. This raised the possibility that NMDA antagonists lacking 5-HTergic effects would not have the anti-dyskinetic action predicted by previous investigators. To test this hypothesis, we investigated MK-801, the most widely-studied NMDA antagonist. We found that chronic low-dose MK-801 (0.1 mg/kg) had no effect on development of AIMs or contraversive rotation. In addition, in L-DOPA-primed rats, low-dose MK-801 (0.1 mg/kg) had no effect on expression of AIMs, contraversive rotation, or sensorimotor function. Conversely, higher doses of MK-801 (0.2-0.3 mg/kg) suppressed expression of AIMs. However, as we show for the first time, anti-dyskinetic doses of MK-801 also suppressed L-DOPA-induced contralateral rotation and impaired sensorimotor function, likely due to non-specific interference of MK-801 with L-DOPA-induced behavior. We conclude that noncompetitive NMDA antagonists are unlikely to suppress dyskinesia clinically without worsening parkinsonism.

Maladaptive striatal plasticity in L-DOPA-induced dyskinesia

Dopamine (DA) replacement therapy with l-DOPA remains the most effective treatment for Parkinson's disease, but causes dyskinesia (abnormal involuntary movements) in the vast majority of the patients. The basic mechanisms of l-DOPA-induced dyskinesia (LID) have become the object of intense research focusing on neurochemical and molecular adaptations in the striatum. Here we review this vast literature and highlight trends that converge into a unifying pathophysiological interpretation. We propose that the core molecular alteration of striatal neurons in LID consists in an inability to turn down supersensitive signaling responses downstream of DA D1 receptors (where supersensitivity is primarily caused by DA denervation). The sustained activation of intracellular signaling pathways induced by each dose of l-DOPA leads to abnormal cellular plasticity and high bioenergetic expenditure. The over-exploitation of signaling pathways and energy reserves during treatment impairs the ability of striatal neurons to dynamically gate cortically driven motor commands. LID thus exemplifies a disorder where 'too much' molecular plasticity leads to plasticity failure in the striatum.

Imaging the nigrostriatal system to monitor disease progression and treatment-induced complications

Radiotracer imaging (RTI) techniques such as positron emission tomography (PET) allow the in vivo assessment of nigrostriatal DA function in Parkinson's disease and have provided valuable insights into the mechanisms of nigrostriatal degeneration and the consequent compensatory changes. Moreover, functional imaging serves as an excellent tool in the assessment of the progression of PD and the evolution of treatment-related complications. However, various studies have shown discordance between clinical progression of PD and nigrostriatal degeneration estimated by PET or SPECT, and no RTI technique can be reliably used as a biomarker for progression of PD. Presynaptic dopaminergic imaging has consistently demonstrated an anterior-posterior gradient of dopaminergic dysfunction predominantly affecting the putamen, with side-to-side asymmetry in tracer binding. Dopaminergic hypofunction in the striatum follows a negative exponential pattern with the fastest rate of decline in early disease. Evaluation of central pharmacokinetics of levodopa action by PET has demonstrated the role of increased synaptic dopamine turnover and downregulation of the dopamine transporter in the pathophysiology of levodopa-induced dyskinesias. In PD with behavioral complications such as impulse control disorders, increased levels of dopamine release have been observed in the ventral striatum during performance of a positive reward task, as well as loss of deactivation in orbitofrontal cortex in response to negative reward prediction errors. This suggests that there is a pathologically heightened "reward" response in the ventral striatum together with loss of the capacity to respond to negative outcomes. Overall, functional imaging with PET is an excellent tool for understanding the disease and its complications; however, caution must be applied in interpretation of the results.

The sigma-1 antagonist BMY-14802 inhibits L-DOPA-induced abnormal involuntary movements by a WAY-100635-sensitive mechanism

RATIONALE

Levodopa (L-DOPA), the gold standard treatment for Parkinson's disease (PD), eventually causes L-DOPA-induced dyskinesia (LID) in up to 80% of patients. In the 6-hydroxydopamine (6-OHDA) rat model of PD, L-DOPA induces a similar phenomenon, which has been termed abnormal involuntary movement (AIM). We previously demonstrated that BMY-14802 suppresses AIM expression in this model.

OBJECTIVES

Although BMY-14802 is widely used as a sigma-1 antagonist, it is also an agonist at serotonin (5-HT) 1A and adrenergic alpha-1 receptors. The current study was conducted to determine which of these mechanisms underlies BMY-14802's AIM-suppressing effect. This characterization included testing the 5-HT1A agonist buspirone and multiple sigma agents. When these studies implicated a 5-HT1A mechanism, we subsequently undertook a pharmacological reversal study, evaluating whether the 5-HT1A antagonist WAY-100635 counteracted BMY-14802's AIM-suppressing effects.

RESULTS

Buspirone dose-dependently suppressed AIM, supporting past findings. However, no AIM-suppressing effects were produced by drugs with effects at sigma receptors, including BD-1047, finasteride, SM-21, DTG, trans-dehydroandrosterone (DHEA), carbetapentane, and opipramol. Finally, we show for the first time that the AIM-suppressing effect of BMY-14802 was dose-dependently prevented by WAY-100635 but not by the alpha-1 antagonist prazosin.

CONCLUSIONS

BMY-14802 exerts its AIM-suppressing effects via a 5-HT1A agonist mechanism, similar to buspirone. Other 5-HT1A agonists have failed clinical trials, possibly due to submicromolar affinity at other receptors, including D2, which may exacerbate PD symptoms. BMY-14802 is a promising candidate for clinical trials due to its extremely low affinity for the D2 receptor and lack of extrapyramidal effects during prior clinical trials for schizophrenia.

Multiple roles for nicotine in Parkinson's disease

There exists a remarkable diversity of neurotransmitter compounds in the striatum, a pivotal brain region in the pathology of Parkinson's disease, a movement disorder characterized by rigidity, tremor and bradykinesia. The striatal dopaminergic system, which is particularly vulnerable to neurodegeneration in this disorder, appears to be the major contributor to these motor problems. However, numerous other neurotransmitter systems in the striatum most likely also play a significant role, including the nicotinic cholinergic system. Indeed, there is an extensive anatomical overlap between dopaminergic and cholinergic neurons, and acetylcholine is well known to modulate striatal dopamine release both in vitro and in vivo. Nicotine, a drug that stimulates nicotinic acetylcholine receptors (nAChRs), influences several functions relevant to Parkinson's disease. Extensive studies in parkinsonian animals show that nicotine protects against nigrostriatal damage, findings that may explain the well-established decline in Parkinson's disease incidence with tobacco use. In addition, recent work shows that nicotine reduces l-dopa-induced abnormal involuntary movements, a debilitating complication of l-dopa therapy for Parkinson's disease. These combined observations suggest that nAChR stimulation may represent a useful treatment strategy for Parkinson's disease for neuroprotection and symptomatic treatment. Importantly, only selective nAChR subtypes are present in the striatum including the alpha4beta2*, alpha6beta2* and alpha7 nAChR populations. Treatment with nAChR ligands directed to these subtypes may thus yield optimal therapeutic benefit for Parkinson's disease, with a minimum of adverse side effects.

Evaluation of the D3 dopamine receptor selective antagonist PG01037 on L-dopa-dependent abnormal involuntary movements in rats

The D3 dopamine receptor selective antagonist PG01037 has been evaluated for the ability to attenuate L-dopa-associated abnormal involuntary movements (AIMs) in unilaterally lesioned male Sprague-Dawley rats, which is a model of L-dopa-dependent dyskinesia in patients with Parkinson's Disease. The intrinsic activity of PG01037 was determined using a) a forskolin-dependent adenylyl cyclase inhibition assay with transfected HEK 293 cells expressing either the human D2Long or D3 dopamine receptor subtype and b) an assay for agonist-associated mitogenesis. For the initial experiments, the 5-HT1A receptor selective partial agonist buspirone was used as a positive control to verify our ability to quantitate changes in total AIMs and AIMs minus locomotor scores. Subcutaneous (s.c.) administration of PG01037 was found to have minimal effect on AIMs score. However, it was observed that the in vivo efficacy of PG01037 increased when administered by intraperitoneal (i.p.) injection 15 min after L-dopa/benserazide administration, as compared to a 60 min, 30 min or 0 min pretreatment. It was also found that i.p. administration of PG01037 could inhibit involuntary movements after they had achieved maximum intensity. PG01037 was found to attenuate AIM scores in these animals in a dose dependent manner with IC(50) value equal to a) 7.4 mg/kg following L-dopa/benserazide administration (8 mg/kg each, i.p.) and b) 18.4 mg/kg following the administration of apomorphine (0.05 mg/kg, s.c.). However, PG01037 did not effectively inhibit SKF 81297-dependent abnormal involuntary movements. Rotarod studies indicate that PG01037 at a dose of 10 mg/kg did not adversely affect motor coordination of the unilaterally lesioned rats. Evaluation of lesioned rats using a cylinder test behavioral paradigm indicated that PG01037 did not dramatically attenuate the beneficial effects of L-dopa. These studies suggest that D3 dopamine receptor selective antagonists are potential pharmacotherapeutic candidates for the treatment of L-dopa-associated dyskinesia in patients with Parkinson's Disease.

Apomorphine and levodopa infusion therapies for advanced Parkinson's disease

Continuous infusion of levodopa or apomorphine provide constant dopaminergic stimulations are good alternatives to deep brain stimulation to control motor fluctuations in patients with advanced Parkinson’s disease (PD). Apomorphine provides motor benefit similar to dopamine, but its long-term use is limited by compliance, mostly injection site skin reactions. Administration of levodopa/carbidopa by continuous duodenal infusion allows replacement of all oral medications and permits achievement of a satisfactory therapeutic response paralleled by a reduction in motor complication severity. However, this procedure is more invasive than apomorphine as it requires a percutaneous endoscopic gastrostomy Clinical experience with infusions shows that continuous dopaminergic stimulation of dopaminergic medications reduces dyskinesia and widens the therapeutic window in advanced PD.

Molecular mechanisms of L-DOPA-induced dyskinesia

L-DOPA (L-3,4-dihydroxyphenylalanine) remains the most effective drug for the treatment of Parkinson's disease. However, chronic use causes dyskinesia, a complex motor phenomenon that consists of two components: the execution of involuntary movements in response to drug administration, and the 'priming' phenomenon that underlies these movements' establishment and persistence. A reinterpretation of recent data suggests that priming for dyskinesia results from nigral denervation and the loss of striatal dopamine input, which alters glutamatergic synaptic connectivity in the striatum. The subsequent response of the abnormal basal ganglia to dopaminergic drugs determines the manner and timing of dyskinesia expression. The combination of nigral denervation and drug treatment establishes inappropriate signalling between the motor cortex and the striatum, leading to persistent dyskinesia.

Nicotine and Parkinson's disease: implications for therapy

Accumulating evidence suggests that nicotine, a drug that stimulates nicotinic acetylcholine receptors, may be of therapeutic value in Parkinson's disease. Beneficial effects may be several-fold. One of these is a protective action against nigrostriatal damage. This possibility stems from the results of epidemiological studies that consistently demonstrate an inverse correlation between tobacco use and Parkinson's disease. This reduced incidence of Parkinson's disease has been attributed to the nicotine in tobacco products, at least in part, based on experimental work showing a protective effect of nicotine against toxic insults. Second, several studies suggest a symptomatic effect of nicotine in Parkinson's disease, although effects are small and somewhat variable. Third, recent data in nonhuman primates show that nicotine attenuates levodopa-induced dyskinesias, a debilitating side effect that develops in the majority of patients on levodopa therapy. Collectively, these observations suggest that nicotine or CNS selective nicotinic receptor ligands hold promise for Parkinson's disease therapy to reduce disease progression, improve symptoms, and/or decrease levodopa-induced dyskinesias.

L-dopa therapy for Parkinson's disease: past, present, and future

Dopamine (DA) supplementation therapy by l-dopa for Parkinson's disease (PD) was established around 1970. The dose of l-dopa can be reduced by the combined administration of inhibitors of peripheral l-amino acid decarboxylase (AADC), catechol O-methyltransferase (COMT), or monoamine oxidase B (MAO B). DA in the striatum may be produced from exogenously administered l-dopa by various AADC-containing cells, such as serotonin neurons. The long-term administration of l-dopa in PD patients may produce l-dopa-induced dyskinesia (LID), which may be due to chronic overstimulation of supersensitive DA D1 receptors. l-dopa may be used in combination with various new strategies such as gene therapy or transplantation in the future.

L-DOPA neurotoxicity is mediated by up-regulation of DMT1-IRE expression

Background

The mechanisms underlying neurotoxicity caused by L-DOPA are not yet completely known. Based on recent findings, we speculated that the increased expression of divalent metal transporter 1 without iron-response element (DMT1−IRE) induced by L-DOPA might play a critical role in the development of L-DOPA neurotoxicity. To test this hypothesis, we investigated the effects of astrocyte-conditioned medium (ACM) and siRNA DMT-IRE on L-DOPA neurotoxicity in cortical neurons.

Methods and Findings

We demonstrated that neurons treated with L-DOPA have a significant dose-dependent decrease in neuronal viability (MTT Assay) and increase in iron content (using a graphite furnace atomic absorption spectrophotometer), DMT1−IRE expression (Western blot analysis) and ferrous iron (55Fe(II)) uptake. Neurons incubated in ACM with or without L-DOPA had no significant differences in their morphology, Hoechst-33342 staining or viability. Also, ACM significantly inhibited the effects of L-DOPA on neuronal iron content as well as DMT1−IRE expression. In addition, we demonstrated that infection of neurons with siRNA DMT-IRE led to a significant decrease in DMT1−IRE expression as well as L-DOPA neurotoxicity.

Conclusion

The up-regulation of DMT1−IRE and the increase in DMT1−IRE-mediated iron influx play a key role in L-DOPA neurotoxicity in cortical neurons.

Once-daily transdermal rivastigmine in the treatment of Alzheimer's disease

During the past decade, transdermal delivery systems (TDS) have become increasingly important for treating neurologic and psychiatric disorders. The rivastigmine patch was the first patch to be approved to treat Alzheimer’s disease (AD). The 9.5 mg/24 h patch has equal efficacy to the capsules and reduces gastrointestinal adverse events, such as nausea and vomiting, by two-thirds. This treatment is well tolerated by patients because drug delivery is even and continuous, reducing fluctuation in drug plasma level, and attenuating the development of centrally mediated cholinergic side effects. Furthermore, once-a-day application of the patch enables an easy treatment schedule, ease of handling, infrequent skin irritations, and a patient- and caregiver-friendly mode of administration. Improved compliance with a subsequent drug administration may contribute to better clinical efficacy, reduce caregiver burden, result in a slower rate of institutionalization, and lead to a decrease in healthcare and medical costs. Because of these advantages, the rivastigmine patch has enabled great progress in the treatment of AD, and represents an excellent alternative to the orally administered cholinesterase inhibitors.

Evaluation of D2 and D3 dopamine receptor selective compounds on L-dopa-dependent abnormal involuntary movements in rats

A panel of novel D2 and D3 dopamine receptor selective antagonists, partial agonists and full agonists have been evaluated for the ability to attenuate L-dopa-associated abnormal involuntary movements (AIMs) in 6-hydroxydopamine (6-OHDA) unilaterally lesioned male Sprague Dawley rats, which is an animal model of L-dopa-induced dyskinesia (LID). LID is often observed in patients with Parkinson's Disease following chronic treatment with L-dopa. The intrinsic activity of these dopaminergic compounds was determined using a forskolin-dependent adenylyl cyclase inhibition assay with transfected HEK 293 cells expressing either the human D2Long or D3 dopamine receptor subtype. For the initial experiments the 5-HT1A receptor selective partial agonist buspirone was used to verify our ability to quantitate changes in total AIMs and AIMs minus locomotor scores. Two D2 dopamine receptor selective antagonists, SV 156 and SV 293, were evaluated and found to minimally attenuate AIM scores in these animals. Four members of our WC series of D3 dopamine receptor selective compounds of varying intrinsic activity at the D3 dopamine receptor subtype, WC 10, WC 21, WC 26 and WC 44, were also evaluated and found to attenuate AIM scores in a dose dependent manner. The in vivo efficacy of the compounds increased when they were administered simultaneously with L-dopa, as compared to when the compounds were administered 60 min prior to the L-dopa/benserazide. It was also found that the D3 receptor antagonist WC 10 could inhibit the involuntary movements after they had achieved maximum intensity. Unlike the D1-like dopamine receptor selective agonist SKF 81297 and the D2-like dopamine receptor agonist bromocriptine which can precipitate abnormal involuntary movements in these unilaterally lesioned animals, abnormal involuntary movements were not observed after administration of our D3 receptor selective agonist WC 44. In addition, we evaluated the effect of these four D3 dopamine receptor selective compounds for their effect on a) spontaneous locomotion and b) coordination and agility using a rotarod apparatus. We also used a cylinder test to assess the effect of L-dopa on spontaneous and independent use of each of the rat's forelimbs in the presence or absence of test compound. The results of these studies suggest that substituted phenylpiperazine D3 dopamine receptor selective compounds are potential pharmacotherapeutic agents for the treatment of L-dopa-associated dyskinesia in patients with Parkinson's Disease.

Enhancing aromatic L-amino acid decarboxylase activity: implications for L-DOPA treatment in Parkinson's disease

Aromatic L-amino acid decarboxylase (AAAD) is an essential enzyme for the formation of catecholamines, indolamines, and trace amines. Moreover, it is a required enzyme for converting L-DOPA to dopamine when treating patients with Parkinson's disease (PD). There is now substantial evidence that the activity of AAAD in striatum is regulated by activation and induction, and second messengers play a role. Enzyme activity can be modulated by drugs acting on a number of neurotransmitter receptors including dopamine (D1-4), glutamate (NMDA), serotonin (5-HT(1A), 5-HT(2A)) and nicotinic acetylcholine receptors. Generally, antagonists enhance AAAD activity; while, agonists may diminish it. Enhancement of AAAD activity is functional, as the formation of dopamine from exogenous L-DOPA mirrors activity. Following a lesion of nigrostriatal dopaminergic neurons, AAAD in striatum responds more robustly to pharmacological manipulations, and this is true for the decarboxylation of exogenous L-DOPA as well. We review the evidence for parallel modulation of AAAD activity and L-DOPA decarboxylation and propose that this knowledge can be exploited to optimize the formation of dopamine from exogenous L-DOPA. This information can be used as a blue print for the design of novel L-DOPA treatment adjuvants to benefit patients with PD.