The alpha2-adrenergic receptor antagonist idazoxan reduces dyskinesia and enhances anti-parkinsonian actions of L-dopa in the MPTP-lesioned primate model of Parkinson's disease

From the cell to the clinic: a comparative review of the partial D₂/D₃receptor agonist and α2-adrenoceptor antagonist, piribedil, in the treatment of Parkinson's disease

Though L-3,4-dihydroxyphenylalanine (L-DOPA) is universally employed for alleviation of motor dysfunction in Parkinson's disease (PD), it is poorly-effective against co-morbid symptoms like cognitive impairment and depression. Further, it elicits dyskinesia, its pharmacokinetics are highly variable, and efficacy wanes upon long-term administration. Accordingly, "dopaminergic agonists" are increasingly employed both as adjuncts to L-DOPA and as monotherapy. While all recognize dopamine D(2) receptors, they display contrasting patterns of interaction with other classes of monoaminergic receptor. For example, pramipexole and ropinirole are high efficacy agonists at D(2) and D(3) receptors, while pergolide recognizes D(1), D(2) and D(3) receptors and a broad suite of serotonergic receptors. Interestingly, several antiparkinson drugs display modest efficacy at D(2) receptors. Of these, piribedil displays the unique cellular signature of: 1), signal-specific partial agonist actions at dopamine D(2)and D(3) receptors; 2), antagonist properties at α(2)-adrenoceptors and 3), minimal interaction with serotonergic receptors. Dopamine-deprived striatal D(2) receptors are supersensitive in PD, so partial agonism is sufficient for relief of motor dysfunction while limiting undesirable effects due to "over-dosage" of "normosensitive" D(2) receptors elsewhere. Further, α(2)-adrenoceptor antagonism reinforces adrenergic, dopaminergic and cholinergic transmission to favourably influence motor function, cognition, mood and the integrity of dopaminergic neurones. In reviewing the above issues, the present paper focuses on the distinctive cellular, preclinical and therapeutic profile of piribedil, comparisons to pramipexole, ropinirole and pergolide, and the core triad of symptoms that characterises PD-motor dysfunction, depressed mood and cognitive impairment. The article concludes by highlighting perspectives for clarifying the mechanisms of action of piribedil and other antiparkinson agents, and for optimizing their clinical exploitation.

Calcium-permeable AMPA receptors are involved in the induction and expression of l-DOPA-induced dyskinesia in Parkinson's disease

Overactivity of striatal alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) glutamate receptors is implicated in the pathophysiology of L-DOPA-induced dyskinesia (LID) in Parkinson's disease (PD). In this study, we evaluated the behavioural and molecular effects of acute and chronic blockade of Ca(2+)-permeable AMPA receptors in animal models of PD and LID. The acute effects of the Ca(2+)-permeable AMPA receptor antagonist 1-trimethylammonio-5-(1-adamantane-methylammoniopentane) dibromide hydrobromide (IEM 1460) on abnormal involuntary movements (AIMs) in the 6-hydroxydopamine (6-OHDA)-lesioned rat and LID in the MPTP-lesioned non-human primate were assessed. Subsequently, the effects of chronic treatment of 6-OHDA-lesioned rats with vehicle, L-DOPA/benserazide (6/15 mg/kg, i.p.) + vehicle or L-DOPA + IEM 1460 (3 mg/kg, i.p.) on behavioural and molecular correlates of priming for LID were evaluated. In the 6-OHDA-lesioned rat and MPTP-lesioned non-human primate, acute treatment with IEM 1460 (1-3 mg/kg) dose-dependently reduced LID without adverse effects on motor performance. Chronic co-treatment for 21 days with IEM 1460 reduced the induction of AIMs by L-DOPA in the 6-OHDA-lesioned rat without affecting peak rotarod performance, and attenuated AIMs score by 75% following l-DOPA challenge (p < 0.05). Chronic IEM 1460 treatment reversed L-DOPA-induced up-regulation of pre-proenkephalin-A, and normalised pre-proenkephalin-B mRNA expression in the lateral striatum, indicating an inhibition of both behavioural and molecular correlates of priming. These data suggest that Ca(2+)-permeable AMPA receptors are critically involved in both the induction and subsequent expression of LID, and represent a potential target for anti-dyskinetic therapies.

Binding of dopamine and 3-methoxytyramine as l-DOPA metabolites to human alpha(2)-adrenergic and dopaminergic receptors

The ability of l-3,4-dihydroxyphenylalanine (l-DOPA), l-DOPA-methyl ester and their major metabolites, dopamine, dihydroxyphenylacetic acid (DOPAC), homovanillic (HVA), 3-O-methyldopa and 3-methoxytyramine (3-MT) to bind to alpha(2) adrenergic and D1 and D2 dopamine receptors was assessed by radioligand binding to cloned human receptors expressed in cell lines. As anticipated, dopamine bound with high affinity to D1 (IC(50) 1.1 + or - 0.16 microM) and D2 (IC(50) 0.7 + or - 0.3 microM) dopamine receptors. However, dopamine also bound with high affinity to alpha(2A) (IC(50) was 2.6 + or - 0.5 microM), alpha(2C) (IC(50) 3.2 + or - 0.7 microM). 3-MT bound to alpha(2A) with high affinity (IC(50), 3.6 + or - 0.2 microM) though moderate affinity to alpha(2)c, D1 and D2 receptors (values of IC(50) were 55 + or - 14, 121 + or - 43, 36 + or - 14 microM, respectively). l-DOPA-methyl ester bound with high affinity to alpha(2) (IC(50) 17-36 microM) but not dopamine receptors (IC(50) 0.9-2.5 mM). l-DOPA, 3-O-methyldopa and DOPAC had no observable effect on binding to any of the receptors tested. These data suggest that the effects of l-DOPA in Parkinson's disease may result from actions of its metabolites dopamine and 3-MT on both dopaminergic and non-dopaminergic receptors. These findings may provide explanations for the differences between l-DOPA and dopamine receptor agonists in mediating anti-parkinsonian effects and propensity to be associated with dyskinesia and motor complications such as wearing-off and on-off.

Functional models of Parkinson's disease: a valuable tool in the development of novel therapies

Functional models of Parkinson's disease (PD) have led to effective treatment for the motor symptoms. Toxin-based models, such as the 6-hydroxydopamine-lesioned rat and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated primate, have resulted in novel dopaminergic therapies and new therapeutic strategies. They have also been used to study processes underlying motor complications, particularly dyskinesia, and for developing pharmacological approaches to dyskinesia avoidance and suppression. Symptomatic models of PD based on nigrostriatal degeneration have a high degree of predictability of clinical effect of dopaminergic drugs on motor symptoms in humans. However, the effects of nondopaminergic drugs in these models do not translate effectively into clinical efficacy. Newer experimental models of PD have attempted to reproduce the pathogenic process and to involve all areas of the brain pathologically affected in humans. In addition, models showing progressive neuronal death have been sought but so far unsuccessfully. Pathogenic modeling has been attempted using a range of toxins, as well as through the use of transgenic models of gene defects in familial PD and mutant rodent strains. However, there are still no accepted progressive models of PD that mimic the processes known to occur during cell death and that result in the motor deficits, pathology, biochemistry, and drug responsiveness as seen in humans. Nevertheless, functional models of PD have led to many advances in treating the motor symptoms of the disorder, and we have been fortunate to have them available. They are an important reason the treatment of PD is so much better compared with treatments for related illnesses.

Comparison of intrastriatal administration of noradrenaline and l-DOPA on dyskinetic movements: a bilateral reverse in vivo microdialysis study in 6-hydroxydopamine-lesioned rats

l-DOPA-induced dyskinesia is known as involuntary debilitating movement, which limits quality of life in patients suffering from Parkinson's disease. The present study focuses on the role of the neurotransmitter noradrenaline (NA) on dyskinetic movements in comparison to the effect of l-DOPA. Rats were unilaterally lesioned with 6-hydroxydopamine and treated with l-DOPA/benserazide (6/15 mg/kg, p.o.) to induce stable dyskinetic movements. On the day of the experiment, NA (0.04 nmol/min, 0.4 nmol/min) and l-DOPA (0.04 nmol/min, 0.4 nmol/min) were perfused into the lesioned and non-lesioned striatum of dyskinetic rats using the reverse in vivo microdialysis technique. Neither NA nor l-DOPA treatment of the non-lesioned striatum produced any dyskinetic behavior. In contrast, administration of l-DOPA 0.4 nmol/min into the lesioned striatum led to a significant increase in dyskinesia indicated by abnormal axial, limb and orolingual movements. Notably, perfusion with NA 0.4 nmol/min into the lesioned striatum revealed a highly significant induction of dyskinetic movements, which are similar to the dyskinesia subtype profile of l-DOPA. In conclusion, NA is as potent as l-DOPA to express dyskinetic movements in l-DOPA-primed rats.

Alpha1-adrenoceptors mediate dihydroxyphenylalanine-induced activity in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-lesioned macaques

The mechanisms underlying actions of dihydroxyphenylalanine (L-DOPA) in Parkinson's disease remain to be fully elucidated. Noradrenaline formed from L-DOPA may stimulate alpha(1)-adrenoceptors. We assessed the involvement of alpha(1)-adrenoceptors in actions of L-DOPA in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-lesioned macaques. In each animal, the minimal dose of L-DOPA required to alleviate parkinsonian symptoms was defined (12.5-25 mg/kg p.o.). The effects of coadministration of the alpha(1)-adrenoceptor antagonist prazosin ([4-(4-amino-6,7-dimethoxy-quinazolin-2-yl) piperazin-1-yl]-(2-furyl)methanone) on motor activity, parkinsonism, and dyskinesia were assessed. Antiparkinsonian benefit was accompanied by mild dyskinesia. L-DOPA also elicited hyperactivity, i.e., activity greater than that seen in normal animals. Coadministration of prazosin (0.16-0.63 mg/kg p.o.) with L-DOPA did not significantly affect either its antiparkinsonian actions or dyskinesia. However, prazosin significantly and dose-dependently attenuated L-DOPA-induced activity, reducing it to a level equivalent to that of normal animals. More specifically, during periods of pronounced L-DOPA-induced activity, prazosin attenuated the total and duration of activity by 80 and 76%, respectively. These actions of prazosin were expressed in the absence of sedation. Although activation of alpha(1)-adrenoceptors plays no major role in the antiparkinsonian and dyskinetic effects of L-DOPA per se, it does contribute to the induction of hyperactivity. alpha(1)-Adrenoceptors may be involved in pathological responses to L-DOPA treatment, including the dopamine dysregulation syndrome.

Pharmacological treatment of Parkinson's disease: life beyond dopamine D2/D3 receptors?

Parkinson's disease (PD) is a multisystemic disorder in which several neurotransmitters other than dopamine are affected. Drugs acting on non-dopaminergic systems are envisaged as promising agents to treat PD and levodopa-induced dyskinesias (LID). However, compounds targeting glutamate, adenosine, noradrenaline, 5-hydroxytryptamine, cannabinoid, and opioid transmitter systems have been assessed in human studies showing negative, inconsistent or unsatisfactory results. Most of these drugs had been tested previously in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-lesioned monkeys, as well as in the classic 6-hydroxydopamine-lesioned rat model. These failures raise several questions and concerns about the true reliability of animal studies, the adequacy of the working hypotheses and design of clinical trials, the validity of tools in current use to evaluate a particular effect, and the selectivity of the drugs used. More importantly, observed discrepancies between the results in models and patients, could challenge the validity of current ideas about the pathophysiology of parkinsonism and LID.

Noradrenergic modulation of subthalamic nucleus activity: behavioral and electrophysiological evidence in intact and 6-hydroxydopamine-lesioned rats

The subthalamic nucleus (STN) plays a key role in the pathophysiology of Parkinson's disease. The modulation of the STN by norepinephrine, however, is unknown. The present study aims at characterizing the effects of systemic administration of noradrenergic agents on locomotor activity and on in vivo extracellularly recorded STN neuronal activity in intact and 6-hydroxydopamine (6-OHDA)-lesioned rats. Using selective agonists and antagonists of alpha1 and alpha2 adrenergic receptors (ARs), we show that STN neurons have functional alpha1- and alpha2-AR controlling STN firing with an impact on locomotor activity. We further demonstrate that those systemic effects are supported, at least in part, by a direct modulation of STN neuronal activity, using patch-clamp recordings of STN neurons in brain slices. These findings support the premise that hypokinesia is associated with an increased STN neuronal activity, and that improvements of parkinsonian motor abnormalities are associated with a decrease in STN activity. Our data challenge assumptions about the role of alpha1-AR and alpha2-AR in the regulation of STN neurons in both intact and 6-OHDA-lesioned rats and further ground the rationale for using alpha2-AR noradrenergic antagonists in Parkinson's disease, albeit via an unexpected mechanism.

Novel 4-(6,7-dimethoxy-1,2,3,4-tetrahydroisoquinolin-2-yl)methylbenzofuran derivatives as selective α2C-adrenergic receptor antagonists

The synthesis of a series of 4-(6,7-dimethoxy-1,2,3,4-tetrahydroisoquinolin-2-yl)methyl-2-arylbenzofuran and 4-(6,7-dimethoxy-1,2,3,4-tetrahydroisoquinolin-2-yl)methylbenzofuran-2-carboxamide derivatives as novel alpha(2C)-adrenergic receptor antagonists are described. Their affinity at three different human alpha(2)-adrenergic receptors is reported, and some of these compounds exhibited high affinity for the alpha(2C)-adrenergic receptor with high subtype selectivity. Among them, compound 10e has been found to show the anti-L-dopa-induced dyskinetic activity in marmosets. The structure-activity relationship of these compounds is also discussed.

Abnormal involuntary movements (AIMs) following pulsatile dopaminergic stimulation: Severe deterioration and morphological correlates following the loss of locus coeruleus neurons

Parkinsonian patients are treated with dopamine replacement therapy (typically, intermittent administration of the dopamine precursor L-DOPA); however, this is associated with the onset of abnormal involuntary movements, which seriously impair the quality of life. The molecular mechanisms underlying abnormal involuntary movements represent an intense field of investigation in the area of neurobiology of disease, although their aetiology remains unclear. Apart from the fine cellular mechanisms, the pathways responsible for the generation of abnormal involuntary movements may involve changes in neurotransmitter systems. A potential candidate is noradrenaline, since a severe loss of this neurotransmitter characterizes Parkinson's disease, and noradrenergic drugs produce a symptomatic relief of L-DOPA-induced dyskinesia. In previous studies we found that pulsatile dopamine release, in the absence of the physiological noradrenaline innervation, produces motor alterations and ultrastructural changes within striatal neurons. In the present study we demonstrate that a unilateral damage to the noradrenaline system anticipates the onset and worsens the severity of L-DOPA-induced contralateral abnormal involuntary movements in hemi-parkinsonian rats. Similarly, ubiquitin-positive striatal ultrastructural changes occur in unilaterally dopamine-depleted, noradrenaline-deficient rats following chronic L-DOPA administration. This study confirms a significant impact of the noradrenergic system in the natural history of Parkinson's disease and extends its role to the behavioural and morphological effects taking place during pulsatile dopamine replacement therapy.

Translation of nondopaminergic treatments for levodopa-induced dyskinesia from MPTP-lesioned nonhuman primates to phase IIa clinical studies: keys to success and roads to failure

Studies in MPTP-lesioned nonhuman primates have demonstrated the potential of nondopaminergic drugs in reducing the problems of levodopa-induced dyskinesia (LID). Here we review the process of translating findings from the monkey to man. Agents targeting glutamate, adensosine, noradrenaline, 5-hydroxytryptamine, cannabinoid, and opioid transmitter systems have been assessed for antidyskinetic potential in human studies. Eleven nondopaminergic drugs with antidyskinetic efficacy in the MPTP primate have been advanced to proof-of-concept phase IIa trials in PD patients (amantadine, istradefylline, idazoxan, fipamezole, sarizotan, quetiapine, clozapine, nabilone, rimonabant, naloxone, and naltrexone). For all six nondopaminergic transmitter systems reviewed, the MPTP-lesioned primate correctly predicted phase II efficacy of at least one drug. Of the 11 specific molecules tested in both monkeys and humans, 8 showed clear antidyskinetic properties in both human and monkey. In the instances where the primate studies did not, or did not consistently, predict the outcome of the human studies, the discrepancy may reflect limitations in the validity of the model or limitations in the design of either the clinical or the preclinical studies. We find that the major determinant of success in predicting efficacy is to ensure that primate studies are conducted in a statistically rigorous way and incorporate designs and outcome measures with clinical applicability. On the other hand, phase IIa trials should strive to replicate the preclinical study, especially in terms of protocol, drug dose equivalence, and outcome measure, so as to test the same hypothesis. Failure to meet these criteria carries the risk of false negative conclusions in phase IIa trials.

Modulation of L-DOPA-induced abnormal involuntary movements by clinically tested compounds: further validation of the rat dyskinesia model

L-DOPA-induced dyskinesia (LID) is a major complication of the pharmacotherapy of Parkinson's Disease. A model of LID has recently been described in rats with unilateral 6-hydroxydopamine (6-OHDA) lesions. In the present study, the model was used in order to compare the efficacies of some clinically available compounds that have shown antidyskinetic effects in nonhuman primate models of LID and/or in patients, namely, amantadine (20 and 40 mg/kg), buspirone (1, 2 and 4 mg/kg), clonidine (0.01, 0.1 and 1 mg/kg), clozapine (4 and 8 mg/kg), fluoxetine (2.5 and 5 mg/kg), propranolol (5, 10 and 20mg/kg), riluzole (2 and 4 mg/kg), and yohimbine (2 and 10 mg/kg). Rats were treated for 3 weeks with L-DOPA for an induction and monitoring of abnormal involuntary movements (AIMs) prior to the drug screening experiments. The antidyskinetic drugs or their vehicles were administered together with L-DOPA, and their effects were evaluated according to a randomized cross-over design both on the AIM rating scale and on the rotarod test. Most of the compounds under investigation attenuated the L-DOPA-induced axial, limb and orolingual AIM scores. However, the highest doses of many of these substances (but for amantadine and riluzole) had also detrimental motor effects, producing a reduction in rotarod performance and locomotor scores. Since the present results correspond well to existing clinical and experimental data, this study indicates that axial, limb and orolingual AIMs possess predictive validity for the preclinical screening of novel antidyskinetic treatments. Combining tests of general motor performance with AIMs ratings in the same experiment allows for selecting drugs that specifically reduce dyskinesia without diminishing the anti-akinetic effect of L-DOPA.

Striatal delta opioid receptor binding in experimental models of Parkinson's disease and dyskinesia

Enhanced delta opioid receptor transmission may represent an endogenous compensatory mechanism in parkinsonism to reduce the activity of the indirect striatopallidal pathway following dopamine depletion. Furthermore, increased delta opioid receptor transmission may be causative in the production of dyskinesia following repeated dopaminergic treatment in Parkinson's disease. The present study employed radioligand receptor autoradiography, using [3H]naltrindole, a ligand selective for the delta opioid receptor, to assess delta opioid receptor binding sites in forebrain regions of reserpine-treated rats, and in parkinsonian nondyskinetic, and dyskinetic MPTP-lesioned macaques. In reserpine-treated animals, specific delta opioid binding was increased in premotor cortex (+30%), sensorimotor striatum (+20%), and associative striatum (+17%) rostrally, but was not changed in caudal forebrain. In contrast, delta opioid receptor binding was not significantly altered at any region analyzed, in either nondyskinetic or dyskinetic, MPTP-lesioned macaques, compared to normal. These results suggest that transient changes in delta opioid receptor binding may occur in motor circuits following acute dopamine depletion. However, in the more chronic MPTP-lesioned macaque model, simple changes in delta opioid receptor number or affinity are unlikely to contribute to mechanisms for abnormal opioid transmission in Parkinson's disease and dyskinesia.

Drug Insight: new drugs in development for Parkinson's disease

For many years, levodopa has given most patients with Parkinson's disease excellent symptomatic benefit. This agent does not slow down the progression of the disease, however, and it can induce motor fluctuations and dyskinesias in the long term. The other available antiparkinsonian agents also have drawbacks, and as a consequence research into antiparkinsonian drugs is expected to take new and different directions in the coming years. The most promising approaches include the development of 'neuroprotective' drugs that are capable of blocking or at least slowing down the degenerative process that is responsible for cellular death; 'restorative' strategies intended to restore normal brain function; more-effective agents for replacing dopamine loss; and symptomatic and antidyskinetic drugs that act on neurotransmitters other than dopamine or target brain areas other than the striatum. In this Review, we discuss the numerous drugs in development that target the primary motor disorder in Parkinson's disease.

The dopamine agonist piribedil with L-DOPA improves attentional dysfunction: relevance for Parkinson's disease

Cognitive deficits are often associated with motor symptoms in Parkinson's disease. This study investigates the ability of piribedil ([(methylenedioxy-3,4 benzyl)-4 pyperazinyl-1]-2 pyrimidine), a D(2)/D(3) dopamine (DA) receptor agonist with antagonist activity at alpha(2A)-adrenoceptors, to restore motor and attentional deficits in nigrostriatal 6-hydroxydopamine-lesioned rats. Subjects were trained to depress a lever, detect a stimulus occurring after variable foreperiods, and release the lever quickly afterward. Striatal DA depletions produce deficits in the timing of foreperiods and prolong reaction times. Although a subchronic treatment with piribedil (0.1-2 mg/kg) is not effective, a dose of 0.3 mg/kg administered for 3 weeks significantly reverses the akinetic deficits produced by the striatal dopamine depletion and progressively improves attentional deficits. When coadministered with the dopamine prodrug l-3,4-dihydroxyphenylalanine (l-DOPA) (3 mg/kg), piribedil (0.3 mg/kg) promotes a rapid and full recovery of preoperative performance. These results suggest that administration of l-DOPA in combination with piribedil in a chronic treatment as either initial or supplemental therapy for Parkinson's disease might improve cognitive functions while reducing the risk for motor complications.

Switching from Levodopa to the Long-acting Dopamine D2/D3 Agonist Piribedil Reduces the Expression of Dyskinesia While Maintaining Effective Motor Activity in MPTP-treated Primates

BACKGROUND

The control of motor complications following dopaminergic medication in late-stage Parkinson disease remains problematic.

OBJECTIVE

We now investigate the potential of oral administration of the long-acting dopamine D2/D3 agonist piribedil to decrease the expression of dyskinesia induced by prior exposure to levodopa in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine hydrochloride (MPTP)-treated primates.

METHODS

MPTP-treated common marmosets were treated with equieffective doses of levodopa (10.0-12.5 mg/kg PO, twice daily) or piribedil (3.0-4.0 mg/kg PO, once daily) for 30 days and then switched to the alternative treatment for a further 35 days.

RESULTS

Levodopa administration markedly improved motor function, but dyskinesia rapidly appeared and intensified as treatment progressed. Administration of piribedil produced a similar reversal of MPTP-induced motor deficits but with comparatively mild dyskinesia. On switching from levodopa to piribedil, the intensity of dyskinesia decreased without altering the improvement in motor deficits. However, on switching from piribedil to levodopa, the rapid increase in dyskinesia despite the improvement in motor function being maintained suggests that piribedil also primes for but does not markedly express dyskinesia.

CONCLUSION

The study confirms the low dyskinesia expression resulting from piribedil treatment compared with an equieffective dose of levodopa. Importantly, the results show that switching from levodopa to piribedil rapidly results in a sustained decrease in dyskinesia intensity.

Histamine H3 receptor agonists reduce L-dopa-induced chorea, but not dystonia, in the MPTP-lesioned nonhuman primate model of Parkinson's disease

L-dopa-induced dyskinesia (LID) remains a major complication of the treatment of Parkinson's disease. The neural mechanisms underlying LID are thought to involve overactivity of striatal glutamatergic neurotransmission, with resultant underactivation of the output regions of the basal ganglia. Histamine H3 heteroreceptors can reduce glutamate and gamma-aminobutyric acid (GABA) transmission in the striatum and substantia nigra reticulata, respectively. Thus, we tested whether the histamine H3 receptor agonists immepip and imetit can alleviate LID in the MPTP-lesioned marmoset model of Parkinson's disease. Coadministration of immepip (1 mg/kg) with L-dopa (15 mg/kg) was associated with significantly less total dyskinesia than L-dopa alone. When dyskinesia was separately rated as chorea and dystonia, coadministration of L-dopa with either immepip or imetit (both 10 mg/kg) significantly reduced chorea but had no effect on dystonia. The antidyskinetic actions of the H3 agonists were not accompanied by alteration of the antiparkinsonian actions of L-dopa. However, immepip (10 mg/kg), when administered as monotherapy, significantly increased parkinsonian disability compared to vehicle. Overall, the results obtained in this study suggest that histamine H3 receptors may be involved in the neural mechanisms underlying L-dopa-induced dyskinesia in Parkinson's disease.

Present and future drug treatment for Parkinson's disease

Considerable advances made in defining the aetiology, pathogenesis, and pathology of Parkinson's disease (PD) have resulted in the development and rapid expansion of the pharmacopoeia available for treatment. Anticholinergics were used before the introduction of levodopa which is now the drug most commonly used. Dopamine agonists are effective when used alone or as an adjunct to levodopa, while monoamine oxidase B inhibitors improve motor function in early and advanced PD. However, treatment mainly addresses the dopaminergic features of the disease and leaves its progressive course unaffected; the drug treatment available for the management of non-motor symptoms is limited. This article seeks to set current treatment options in context, review emerging and novel drug treatments for PD, and assess the prospects for disease modification. Surgical therapies are not considered.

Nondopaminergic mechanisms in levodopa-induced dyskinesia

It has become increasingly apparent that Parkinson's disease involves many transmitter systems other than dopamine. This nondopaminergic involvement impacts on the generation of symptoms, on the neurodegenerative process, but, most tellingly, in the generation of side effects of current treatments, in particular, levodopa-induced dyskinesia (LID). Such mechanisms contribute not only to the expression of LID once it has been established but also to the mechanisms responsible for the development, or priming, of the dyskinetic state and the subsequent maintenance of the brain in that primed state. Within the basal ganglia, abnormalities in different nondopaminergic components of the circuitry have been defined in LID. In particular, a role for enhanced inhibition of basal ganglia outputs by the GABAergic direct pathway has been suggested as a basic mechanism generating LID. We speculate that the external globus pallidus and subthalamic nucleus may play distinct roles in different forms of dyskinesia, e.g., chorea/dystonia; peak/diphasic/off. At the cellular level, an appreciation of abnormal signaling by, among others, glutamatergic (NMDA and AMPA receptors in particular), alpha2 adrenergic, serotonergic (5HT), cannabinoid and opioid mechanisms in both priming and expression of LID has begun to emerge over the last decade. This is being consolidated, though in many cases questions remain regarding the specific sites of such abnormality within the circuitry. Very recently, at the molecular level, mechanisms controlling neurotransmitter release and impacting on the ability of neurons to maintain particular forms of firing patterning and synchronization, e.g., SV2A, have been identified. This increased understanding has already delivered and will continue to define novel approaches to treatment that target both pre- and postsynaptic signaling molecules throughout the basal ganglia circuitry.

Topiramate reduces levodopa-induced dyskinesia in the MPTP-lesioned marmoset model of Parkinson's disease

Overactive AMPA receptor-mediated transmission may be involved in the pathogenesis of levodopa-induced dyskinesia. The mechanism of action of the anticonvulsant drug topiramate involves attenuation of AMPA receptor-mediated transmission. In this study, the potential antidyskinetic action of topiramate was examined in the MPTP-lesioned marmoset model of Parkinson's disease and levodopa-induced dyskinesia. Topiramate significantly reduced levodopa-induced dyskinesia, without affecting the antiparkinsonian action of levodopa. Topiramate represents an exciting potential novel therapeutic approach to levodopa-induced dyskinesia in patients with Parkinson's disease.

The effects of a selective dopamine D2 receptor agonist on behavioral and pathological outcome in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated squirrel monkeys

In this study, we investigated antiparkinsonian activity of the novel, highly selective dopamine D(2) receptor agonist sumanirole compared with two clinically effective dopaminergic therapies in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) primate model of Parkinson's disease. Squirrel monkeys were rendered parkinsonian by chronic administration of MPTP and subsequently dosed with vehicle, L-DOPA plus carbidopa (L-DOPA), ropinirole, or sumanirole over a duration of 8 weeks. Antiparkinsonian effects measured with a parkinsonian primate rating scale (PPRS) showed that sumanirole elicited improved functional outcome compared with vehicle. The dopamine D2/D3 agonist ropinirole improved behavioral outcome similar to sumanirole, whereas L-DOPA treatment yielded the most significant symptomatic improvement. The relative rank of therapies that elicited normalization of PPRS was L-DOPA > sumanirole; ropinirole did not normalize PPRS in any of the treated monkeys. Dyskinesias were present with L-DOPA treatment but were not observed in sumanirole-, ropinirole-, or placebo-treated primates. Pathologically, all MPTP-treated animals displayed neurodegeneration of dopaminergic neurons in the substantia nigra pars compacta and reactive astrocytosis. Neurons immunoreactive with antibodies to the nuclear transcription factor DeltaFosB were most significantly increased in the striatum of L-DOPA-treated monkeys. These results suggest that sumanirole can exert antiparkinsonian effects similar to L-DOPA without the behavioral and morphological consequences of the latter.

A role for endocannabinoids in the generation of parkinsonism and levodopa-induced dyskinesia in MPTP-lesioned non-human primate models of Parkinson's disease

Endocannabinoids and cannabinoid CB1 receptors play a role in the control of movement by modulating GABA, glutamate, and other neurotransmitters throughout the basal ganglia. Roles for abnormalities in endocannabinoid signaling in Parkinson's disease (PD) and the major side effect of current treatments, levodopa-induced dyskinesia (LID), have been suggested by rodent studies. Here we show that signaling by endocannabinoids contributes to the pathophysiology of parkinsonism and LID in MPTP-lesioned, non-human primate models of Parkinson's disease. In MPTP-lesioned marmosets previously treated with levodopa to establish LID, attenuation of CB1 signaling by systemic administration of rimonabant (1 and 3 mg/kg) had anti-parkinsonian actions, equivalent to a 71% increase in motor activity at 3 mg/kg. Rimonabant did not elicit dyskinesia. Co-administration of levodopa (8 mg/kg) and rimonabant (1 and 3 mg/kg) resulted in significantly less dyskinesia than levodopa alone, without significantly affecting the anti-parkinsonian action of levodopa. These data suggest that enhanced endocannabinoid signaling may be involved in the pathophysiology of both parkinsonism and LID. To define potential mechanisms by which such a role might be mediated, we determined the levels of the endocannabinoids anandamide and 2-arachidonyl glycerol (2-AG) throughout the basal ganglia in normal and three groups of MPTP-lesioned cynomolgus monkeys (untreated; acutely treated with L-DOPA, non-dyskinetic; long-term treated, with levodopa-induced dyskinesia). In the untreated, MPTP-lesioned primate, parkinsonism was associated with increases in both 2-AG (+88%) and anandamide (+49%) in the striatum, and of 2-AG (+97%) in the substantia nigra, changes that are consistent with the previously suggested role for endocannabinoids in mechanisms attempting to compensate for loss of dopamine in untreated parkinsonism. Increased levels of anandamide (+34%) in the external globus pallidus of MPTP-lesioned animals were normalized by levodopa treatment and may contribute to the generation of parkinsonian symptoms. However, no clear alteration in endocannabinoid levels could be correlated with the expression of LID. These data highlight the potential roles played by endocannabinoids and CB1 in PD and LID and suggest the need for further research to pursue the multiple therapeutic opportunities for manipulating this system in movement disorders.

Novel pharmacological strategies for motor complications in Parkinson’s disease

In advanced Parkinson's disease, the combination of disease progression and levodopa therapy leads to the development of motor problems complicating the therapeutic response, known as motor response complications. The nonphysiological, pulsatile stimulation produced by most currently available dopaminergic therapies triggers a complicated series of responses resulting in the dysregulation of glutamate receptors and many other neurotransmitter systems on striatal neurons. Although a number of novel compounds that provide a more continuous dopaminergic stimulation are becoming available, no practical way to accomplish this in a truly physiological manner currently exists. Novel strategies for pharmacological intervention with the use of nondopaminergic treatments, with drugs targeting selected transmitter receptors expressed on striatal neurons appear more promising. These include NMDA or AMPA antagonists, or drugs acting on 5-hydroxytryptamine subtype 2A, alpha2-adrenergic, adenosine A2A and cannabinoid CB1 receptors. Future strategies may also target pre- and postsynaptic components that regulate firing pattern, like synaptic vesicle proteins, or nonsynaptic gap junction communication mechanisms, or drugs with actions at the signal transduction systems that modulate the phosphorylation state of NMDA receptors. These new therapeutic strategies, alone or in combination, hold the promise of providing effective control or reversal of motor response complications.

Dopamine Uptake Inhibitor-Induced Rotation in 6-Hydroxydopamine-Lesioned Rats Involves Both D1 and D2 Receptors but Is Modulated through 5-Hydroxytryptamine and Noradrenaline Receptors

Dopamine uptake inhibitors may provide a means of sustaining endogenous and exogenous striatal dopamine levels in Parkinson's disease, but most are not selective and also inhibit the noradrenaline and 5-hydroxytryptamine (5-HT) transporters. To determine the involvement of the individual monoamine transporters in the production of motor activity, the effect of the nonselective monoamine uptake inhibitor BTS 74 398 1-([1-(3,4-dichlorophenyl)cyclobutyl]-2-(3-diaminethylaminopropylthio) ethanone monocitrate) and the selective dopamine, GBR 12909 [1-(2-(bis-(4-fluorphenyl)-methyl)ethyl)-4-(3-phenylpropyl)piperazine) dihydrochloride], noradrenaline (nisoxetine), and 5-HT (fluvoxamine) reuptake inhibitors on circling in the unilateral 6-hydroxydopamine-lesioned rat was investigated. GBR 12909 induced ipsilateral circling, but fluvoxamine and nisoxetine were without effect. However, when administered with GBR 12909, fluvoxamine enhanced rotation, whereas nisoxetine had no effect. The results suggest that 5-HT, but not noradrenaline, reuptake inhibition facilitates dopamine-mediated motor activity. To test this hypothesis, BTS 74 398 was administered in combination with selective dopamine, 5-HT, and noradrenaline receptor antagonists. Both D(1) and D(2) receptor antagonists, SCH 23390 [R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine] and raclopride, inhibited BTS 74 398-induced circling. In contrast, the 5-HT(1A) 5-HT(1A/B) antagonists, WAY 100635 (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinyl-cyclohexane-carboxamide maleate) and pindolol, and the 5-HT(2A) antagonist, ketanserin, had no effect. The nonspecific 5-HT((1/2)) antagonists, methysergide and metergoline, and the specific 5-HT(2C) antagonist, N-desmethylclozapine, enhanced BTS 74 398-induced circling, as did the alpha(2)-adrenoceptor antagonist idazoxan. Overall, the data suggest that inhibition of the 5-HT and noradrenaline transporters modulate dopamine uptake inhibitor-mediated motor activity. However, the mechanism of this interaction is complex, involving opposing effects of noradrenaline and 5-HT agonism and antagonism.

Recent failures of new potential symptomatic treatments for Parkinson's disease: causes and solutions

One major goal of current research in Parkinson's disease (PD) is the discovery of novel agents to improve symptomatic management. The object of these new treatments should be to provide effective symptom control throughout the course of the disease without the development of side effects such as motor and psychiatric complications. Results of several clinical trials of new treatment options reported in the past 2 years have shown negative or unsatisfactory results. Most of the drugs and surgical procedures used in these studies had been tested previously in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) monkeys as well as in the classic 6-hydroxydopamine-lesioned rat model. They raise several questions about the true reliability of animal studies, the adequacy of the working hypotheses and design of clinical trials, the validity of tools in current use to evaluate a specific effect, and the selectivity of the drugs used. All these factors may explain failure. This review focuses on pharmacological and surgical treatments tested to improve the management of patients with motor fluctuations and dyskinesias. Some of the recent trials and possible reasons for their lack of success are critically analysed. Finally, some suggestions to avoid further failures and improve results are proposed.

Treatment with alpha2-adrenoceptor antagonist, 2-methoxy idazoxan, protects 6-hydroxydopamine-induced Parkinsonian symptoms in rats: neurochemical and behavioral evidence

Noradrenaline, not only functions as a synaptic transmitter, but also promotes neural differentiation and regenerative processes. In Parkinson's disease, besides the dopaminergic degeneration, noradrenergic neurons of locus coeruleus origin degenerate as well. Drugs enhancing noradrenergic transmission in the locus coeruleus (e.g. alpha2-adrenoceptor antagonists) have been shown to be neuroprotective against Huntington's and ischemic animal models. However, in Parkinsonian animal models, most of the studies evaluated the worsening of experimental nigral neurodegeneration after locus coeruleus lesions. Here, it has been tested, whether treatment with the selective alpha2-adrenoceptor antagonist, 2-methoxy idazoxan (2.5 mg/kg i.p., twice daily for 5 days), before an experimental lesion to nigra, protects dopaminergic neurodegeneration. Dopaminergic degeneration was produced by 6-hydroxydopamine lesion in the median forebrain bundle. The concentrations of dopamine, 5-hydroxytryptamine and its metabolites were analysed in the various regions of the basal ganglia. The concentrations of noradrenaline and dopamine were measured in the regions innervated by locus coeruleus neurons and in the basal ganglia respectively, after 2-methoxy idazoxan treatment. The Parkinsonian behavior was assessed by catalepsy and activity test. 2-Methoxy idazoxan specifically increased the concentration of noradrenaline in the brain regions, innervated by locus coeruleus neurons. 6-OHDA lesion strongly depleted the concentration of dopamine and its metabolites in the striatum and SN, producing catalepsy and hypoactivity. Multiple treatments with 2-methoxy idazoxan reduced some of the observed neurochemical and behavioral indices of 6-hydroxydopamine-induced Parkinsonism, indicating neuroprotection. Although the mechanism underlying the neuroprotective property remains elusive, the therapeutic usage of alpha2-antagonists might be helpful in slowing the neuronal death and progression of Parkinson's disease.

The NR2B-selective NMDA receptor antagonist CP-101,606 exacerbates L-DOPA-induced dyskinesia and provides mild potentiation of anti-parkinsonian effects of L-DOPA in the MPTP-lesioned marmoset model of Parkinson's disease

In Parkinson's disease (PD), degeneration of the dopaminergic nigrostriatal pathway leads to enhanced transmission at NMDA receptors containing NR2B subunits. Previous studies have shown that some, but not all, NR2B-containing NMDA receptor antagonists alleviate parkinsonian symptoms in animal models of PD. Furthermore, enhanced NMDA receptor-mediated transmission underlies the generation of L-DOPA-induced dyskinesia (LID). The subunit content of NMDA receptors responsible for LID is not clear. Here, we assess the actions of the NMDA antagonist CP-101,606 in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned marmoset model of Parkinson's disease. CP-101,606 is selective for NMDA receptors containing NR2B subunits, with higher affinity for NR1/NR2B complexes compared to ternary NR1/NR2A/NR2B complexes. CP-101,606 had no significant effect on parkinsonian symptoms when administered as monotherapy over a range of doses (0.1-10 mg/kg). CP-101,606 provided a modest potentiation of the anti-parkinsonian actions of L-DOPA (8 mg/kg), although, at doses of 1 and 3 mg/kg, CP-101,606 exacerbated LID. Results of this study provide further evidence of differences in the anti-parkinsonian activity and effects on LID of the NR2B subunit selective NMDA receptor antagonists. These distinctions may reflect disparities in action on NR1/NR2B as opposed to NR1/NR2A/NR2B receptors.

Levetiracetam Interferes With the l-Dopa Priming Process in MPTP-Lesioned Drug-Naive Marmosets

OBJECTIVE

Levetiracetam (LEV; Keppra, UCB Pharma) has been shown to reduce established l-3,4 dihydroxyphenylalanine (l-dopa)-induced dyskinesia. This study investigated whether LEV can modify induction of dyskinesia by l-dopa or the process of priming.

METHODS

Drug-naive MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) -lesioned marmosets were treated for 21 days with l-dopa/LEV or l-dopa alone. Subsequently, the animals were left untreated for 1 week and then both groups were challenged with a single dose of l-dopa alone on day 29. Behavior was assessed by automated activity counts and by post hoc analysis of videotapes using validated rating scales.

RESULTS

LEV had no significant effect on the appearance of dyskinesia when administered de novo in combination with l-dopa. However, after a week of drug holiday, the 2 groups exhibited a different response to an acute l-dopa challenge. Thus, animals previously treated with l-dopa alone exhibited a similar level of dyskinesia to that seen on day 21 of the repeated treatment phase of the study. However, animals previously treated with l-dopa/LEV demonstrated significantly reduced dyskinesia compared with day 21 of the repeated treatment phase of the study.

CONCLUSIONS

LEV does not modify the onset of dyskinesia following de novo treatment with l-dopa. However, concomitant treatment with l-dopa/LEV reduces the level of dyskinesia induced by l-dopa following a drug holiday. Thus, prior treatment with LEV appears to modify the mechanisms responsible for the maintenance of l-dopa-induced dyskinesia.

Selective blockade of D3 dopamine receptors enhances the anti-parkinsonian properties of ropinirole and levodopa in the MPTP-lesioned primate

To date, the lack of highly selective antagonists at the dopamine D(3) receptor has hampered clarification of their involvement in the actions of currently used therapies in Parkinson's disease. However, the novel benzopyranopyrrole, S33084, displays greater than 100-fold selectivity as an antagonist for D(3) versus D(2) receptors and all other sites tested. S33084 was administered to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned marmosets previously primed with levodopa to elicit dyskinesia. Administered alone, S33084 exerted a modest, but significant, anti-parkinsonian effect without provoking dyskinesia. At low D(3)-selective doses (0.16 and 0.64 mg/kg), S33084 potentiated, though to different extents and in qualitatively different ways, the anti-parkinsonian actions of both ropinirole and levodopa. At these doses, S33084 did not significantly modify levodopa-induced or ropinirole-induced dyskinesia. These data suggest that ropinirole and levodopa do not exert their anti-parkinsonian or pro-dyskinetic actions via D(3) receptor stimulation. Indeed, stimulation of D(3) receptors may be detrimental to the anti-parkinsonian properties of D(2)/D(3) agonists. Selectivity for stimulation of D(2), over D(3), receptors may therefore be a beneficial property of dopamine receptor agonists in management of motor symptoms of Parkinson's disease patients with established dyskinesia.

Noradrenergic mechanisms in neurodegenerative diseases: a theory

A deficiency in the noradrenergic system of the brain, originating largely from cells in the locus coeruleus (LC), is theorized to play a critical role in the progression of a family of neurodegenerative disorders that includes Parkinson's disease (PD) and Alzheimer's disease (AD). Consideration is given here to evidence that several neurodegenerative diseases and syndromes share common elements, including profound LC cell loss, and may in fact be different manifestations of a common pathophysiological process. Findings in animal models of PD indicate that the modification of LC-noradrenergic activity alters electrophysiological, neurochemical and behavioral indices of neurotransmission in the nigrostriatal dopaminergic system, and influences the response of this system to experimental lesions. In models related to AD, noradrenergic mechanisms appear to play important roles in modulating the activity of the basalocortical cholinergic system and its response to injury, and to modify cognitive functions including memory and attention. Mechanisms by which noradrenaline may protect or promote recovery from neural damage are reviewed, including effects on neuroplasticity, neurotrophic factors, neurogenesis, inflammation, cellular energy metabolism and excitotoxicity, and oxidative stress. Based on evidence for facilitatory effects on transmitter release, motor function, memory, neuroprotection and recovery of function after brain injury, a rationale for the potential of noradrenergic-based approaches, specifically alpha2-adrenoceptor antagonists, in the treatment of central neurodegenerative diseases is presented.

Noradrenergic Drugs for Levodopa-Induced Dyskinesia

Dyskinesia frequently mars the long-term therapeutic response to levodopa (LD) in Parkinson's disease (PD). New treatment strategies for levodopa-induced dyskinesia (LID) currently being investigated include some that target the nondopaminergic pathways. Indeed, LID in parkinsonism can be modulated by drugs acting on different neurotransmitters including glutamate, gamma-aminobutyric acid, noradrenaline, acetylcholine, serotonin, adenosine, and cholecystokinin. In many cases, the possibility of using specific compounds to counteract LID was raised by the previously shown efficacy of such compounds in the treatment of other types of dyskinesia. More data are now available on drugs that act on the noradrenergic system. Two studies have recently shown how the alpha-2 adrenoreceptor antagonist idazoxan can significantly reduce LID in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned primate model of parkinsonism and in patients with advanced PD. The experimental paper, which studied the antagonistic action of idazoxan on dyskinesia induced by both LD and apomorphine in marmosets with MPTP-induced parkinsonism, showed that the pharmacologic mechanisms underlying LID and apomorphine-induced dyskinesia in PD are probably distinct. LD, although not apomorphine-induced, dyskinesia was found to be influenced by adrenoreceptor antagonists. Indeed, the action of alpha-2 adrenoreceptor antagonists may involve the blockade of the action of noradrenaline synthesized from LD. The hypothesis is that because dopamine agonists are not metabolized to noradrenaline, alpha-2 adrenoreceptor antagonists do not reduce dyskinesia produced by such agents. This finding is particularly relevant in planning clinical studies in which LD or dopamine agonist challenges are used to assess the potential antidyskinetic properties of new drugs. The clinical study assessed the effects of idazoxan on LID in 18 patients with advanced PD: An improvement in LID, without the reappearance of parkinsonian symptoms, was observed. The practical outcome of this research is that, although the mechanisms underlying the manifestations and the priming process for dyskinesia have yet to be fully elucidated, a nondopaminergic approach to therapy may provide an effective way of preventing, or at least limiting, the expression of involuntary movements in PD.

Novel antiepileptic drug levetiracetam decreases dyskinesia elicited byL-dopa and ropinirole in the MPTP-lesioned marmoset

Long-term dopamine replacement therapy of Parkinson's disease leads to the occurrence of dyskinesias. Altered firing patterns of neurons of the internal globus pallidus, involving a pathological synchronization/desynchronization process, may contribute significantly to the genesis of dyskinesia. Levetiracetam, an antiepileptic drug that counteracts neuronal (hyper)synchronization in animal models of epilepsy, was assessed in the MPTP-lesioned marmoset model of Parkinson's disease, after coadministration with (1) levodopa (L-dopa) or (2) ropinirole/L-dopa combination. Oral administration of levetiracetam (13-60 mg/kg) in combination with either L-dopa (12 mg/kg) alone or L-dopa (8 mg/kg)/ropinirole (1.25 mg/kg) treatments was associated with significantly less dyskinesia, in comparison to L-dopa monotherapy during the first hour after administration. Thus, new nondopaminergic treatment strategies targeting normalization of abnormal firing patterns in basal ganglia structures may prove useful as an adjunct to reduce dyskinesia induced by dopamine replacement therapy without affecting its antiparkinsonian action.

Fipamezole (JP-1730) is a potent alpha2 adrenergic receptor antagonist that reduces levodopa-induced dyskinesia in the MPTP-lesioned primate model of Parkinson's disease

Previous studies in the MPTP-lesioned primate model of Parkinson's disease have demonstrated that alpha(2) adrenergic receptor antagonists such as idazoxan, rauwolscine, and yohimbine can alleviate L-dopa-induced dyskinesia and, in the case of idazoxan, enhance the duration of anti-parkinsonian action of L-dopa. Here we describe a novel alpha(2) antagonist, fipamezole (JP-1730), which has high affinity at human alpha(2A) (K(i), 9.2 nM), alpha(2B) (17 nM), and alpha(2C) (55 nM) receptors. In functional assays, the potent antagonist properties of JP-1730 were demonstrated by its ability to reduce adrenaline-induced (35)S-GTPgammaS binding with K(B) values of 8.4 nM, 16 nM, 4.7 nM at human alpha(2A), alpha(2B), and alpha(2C) receptors, respectively. Assessment of the ability of JP-1730 to bind to a range of 30 other binding sites showed that JP-1730 also had moderate affinity at histamine H1 and H3 receptors and the serotonin (5-HT) transporter (IC(50) 100 nM to 1 microM). In the MPTP-lesioned marmoset, JP-1730 (10 mg/kg) significantly reduced L-dopa-induced dyskinesia without compromising the anti-parkinsonian action of L-dopa. The duration of action of the combination of L-dopa and JP-1730 (10 mg/kg) was 66% greater than that of L-dopa alone. These data suggest that JP-1730 is a potent alpha(2) adrenergic receptor antagonist with potential as an anti-dyskinetic agent in the treatment of Parkinson's disease.

The alpha 2-adrenoceptor antagonist idazoxan reverses catalepsy induced by haloperidol in rats independent of striatal dopamine release: role of serotonergic mechanisms

The alpha(2)-adrenoceptor antagonist idazoxan may improve motor symptoms in Parkinson's disease and experimental Parkinsonism. We studied the effect of idazoxan on haloperidol-induced catalepsy in rats, an animal model of the drug-induced extrapyramidal side effects in man. Catalepsy was induced by a subcutaneous (s.c.) injection of haloperidol (1 mg/kg) and measured by the bar test for a maximum of 5 min. At 3 h after haloperidol, rats were given 0.16-5.0 mg/kg s.c. idazoxan, and descent latency was measured 1 h later. Idazoxan potently reversed haloperidol-induced catalepsy with an ED(50) of 0.25 mg/kg. This effect was mimicked by the selective alpha(2)-adrenoceptor antagonist RS-15385-197 (0.3 and 1 mg/kg orally). We assessed how dopaminergic mechanisms were involved in the anticataleptic effect of idazoxan by studying its effect on dopamine (DA) release in the striatum, with the microdialysis technique in conscious rats. Idazoxan (0.3 and 2.5 mg/kg) had no effect on extracellular DA and did not modify the rise of extracellular DA induced by haloperidol, indicating that changes of striatal DA release were not involved in the reversal of catalepsy. The anticataleptic effect of 2.5 mg/kg idazoxan (haloperidol+vehicle 288+/-8 s, haloperidol+idazoxan 47+/-22 s) was attenuated in rats given an intraventricular injection of 150 microg of the serotonin (5-HT) neurotoxin 5,7-dihydroxytryptamine (haloperidol+vehicle 275+/-25 s, haloperidol+idazoxan 137+/-28 s). The 5-HT(1A) receptor antagonist WAY100 635 (0.1 mg/kg s.c.) did not affect the anticataleptic effect of idazoxan. The results suggest that idazoxan reversed haloperidol-induced catalepsy by a mechanism involving blockade of alpha(2)-adrenoceptors and, at least in part, 5-HT neurons.

Stimulation of cannabinoid receptors reduces levodopa-induced dyskinesia in the MPTP-lesioned nonhuman primate model of Parkinson's disease

Long-term treatment with levodopa in Parkinson's disease results in the development of motor fluctuations, including reduced duration of antiparkinsonian action and involuntary movements, i.e., levodopa-induced dyskinesia. Cannabinoid receptors are concentrated in the basal ganglia, and stimulation of cannabinoid receptors can increase gamma-aminobutyric acid transmission in the lateral segment of globus pallidus and reduce glutamate release in the striatum. We thus tested the hypothesis that the cannabinoid receptor agonist nabilone (0.01, 0.03, and 0.10 mg/kg) would alleviate levodopa-induced dyskinesia in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine hydrochloride (MPTP) -lesioned marmoset model of Parkinson's disease. Coadministration of nabilone (0.1 mg/kg) with levodopa was associated with significantly less total dyskinesia (dyskinesia score, 12; range, 6-17; primate dyskinesia rating scale) than levodopa alone (22; range, 14-23; P < 0.05). This effect was more marked during the onset period (0-20 minutes post levodopa). There was no reduction in the antiparkinsonian action of levodopa. Furthermore, the intermediate dose of nabilone used (0.03 mg/kg) increased the duration of antiparkinsonian action of levodopa by 76%. Thus, cannabinoid receptor agonists may be useful in the treatment of motor complications in Parkinson's disease.

Quetiapine attenuates levodopa-induced motor complications in rodent and primate parkinsonian models

The contribution of serotoninergic mechanisms to motor dysfunction in Parkinson's disease (PD) has yet to be fully elucidated. Recent clinical observations increasingly suggest that drugs able to block serotonin 5HT2A/C receptors can benefit patients with certain extrapyramidal movement disorders. To further explore the roles of these and other neurotransmitter receptors in the pathogenesis of parkinsonian signs and levodopa-induced dyskinesias; we evaluated the effects of quetiapine, an atypical antipsychotic with 5HT2A/C and D2/3 antagonistic activity, on motor behavior in 6-hydroxydopamine-lesioned rats and MPTP-lesioned nonhuman primates. In hemiparkinsonian rats, quetiapine (5 mg/kg, po) reversed the shortened motor response to levodopa challenge produced by 3 weeks of twice-daily levodopa treatment (P < 0.01). Quetiapine (5 mg/kg po) also normalized the shortened response to the acute injection of either a dopamine D1 receptor agonist (SKF 38392) or a D2 agonist (quinpirole) in rats that had received chronic levodopa treatment. Quetiapine had no effect on parkinsonian dysfunction when given alone or with levodopa to parkinsonian rats and monkeys. Quetiapine (4 mg/kg, po) did, however, substantially reduce levodopa-induced dyskinesias when coadministered with levodopa (P < 0.05). These results suggest that quetiapine could confer therapeutic benefits to patients with levodopa-induced motor complications. Moreover, our findings may indicate that 5HT2A/C receptor-mediated mechanisms, alone or in combination with other mechanisms, contribute to the pathogenesis of the altered motor responses associated with the treatment of PD.

Emerging therapies in the pharmacological treatment of Parkinson's disease

The pharmacological management of Parkinson's disease is a complex and dynamic task; there is no one 'right' strategy indicating which drugs should be used at a particular stage of the disease. There are now many different drugs belonging to several classes that may be effective, and there are still differences of opinion among leading clinicians about the best course of treatment. This review focuses on drug therapy for the motor impairment in Parkinson's disease. Current and future research directions are summarised by taking inventory of recent and innovative areas of development in the field, representing each category with at least one of its featured treatments. The main research efforts are being directed towards delaying the use of levodopa or finding therapies to be used as adjunct to it, in order to postpone motor complications and, in particular, dyskinesias. One of the recent trends is early employment of dopamine agonists. Additional efforts are being directed towards protecting and restoring dopamine neurons. Novel therapies acting on non-dopaminergic systems are also being researched.

Striatal AMPA receptor binding is unaltered in the MPTP-lesioned macaque model of Parkinson's disease and dyskinesia

Long-term levodopa or dopamine agonist treatment in the MPTP-lesioned primate model of Parkinson's disease elicits dyskinesia, which is phenotypically similar to levodopa-induced dyskinesia in patients with Parkinson's disease. AMPA receptor antagonists have previously been shown to have both anti-parkinsonian and anti-dyskinetic actions in MPTP-lesioned primates, suggesting that AMPA receptor transmission is functionally overactive under these conditions. In this study, we investigated the level of striatal AMPA receptor binding in the MPTP lesioned primate using the selective AMPA ligand (3)H-(S)-5-fluorowillardiine. AMPA receptor binding was studied in non-parkinsonian, non-dyskinetic parkinsonian, and dyskinetic macaques. Striatal AMPA receptor binding was not different in any of the treatment groups (P > 0.05). Although AMPA receptor-mediated transmission is functionally overactive in Parkinson's disease and dyskinesia, changes in striatal AMPA receptor levels are not likely to be the cause of such movement disorders.

Dystonia in multiple system atrophy

OBJECTIVE

To delineate the frequency and nature of dystonia in multiple system atrophy (MSA).

METHODS

A cohort of 24 patients with clinically probable MSA over the past 10 years were prospectively followed up. Motor features were either dominated by parkinsonism (MSA-P subtype, n=18) or cerebellar ataxia (MSA-C, n=6). Classification of dystonic features and their changes with time was based on clinical observation during 6-12 monthly follow up visits. Parkinsonian features and complications of drug therapy were assessed. Most patients (22/24) died during the observation period. Neuropathological examination was confirmatory in all of the five necropsied patients.

RESULTS

At first neurological visit dystonia was present in 11 (46%) patients all of whom had been levodopa naive at this time point. Six patients (25%) exhibited cervical dystonia (antecollis) (MSA-P n=4, MSA-C n=2), five patients (21%) showed unilateral limb dystonia (MSA-P n=4; MSA-C n=1). A definite initial response to levodopa treatment was seen in 15/18 patients with MSA-P, but in none of the six patients with MSA-C. A subgroup of 12 patients with MSA-P developed levodopa induced dyskinesias 2.3 years (range 0.5-4) after initiation of levodopa therapy. Most patients had peak dose craniocervical dystonia; however, some patients experienced limb or generalised dystonia. Isolated peak dose limb chorea occurred in only one patient.

CONCLUSION

The prospective clinical study suggests that dystonia is common in untreated MSA-P. This finding may reflect younger age at disease onset and putaminal pathology in MSA-P. Levodopa induced dyskinesias were almost exclusively dystonic affecting predominantly craniocervical musculature. Future studies are required to elucidate the underlying pathophysiology of dystonia in MSA.

Pharmacological validation of behavioural measures of akinesia and dyskinesia in a rat model of Parkinson's disease

In an attempt to define clinically relevant models of akinesia and dyskinesia in 6-hydroxydopamine (6-OHDA)-lesioned rats, we have examined the effects of drugs with high (L-DOPA) vs. low (bromocriptine) dyskinesiogenic potential in Parkinson's disease on three types of motor performance, namely: (i) abnormal involuntary movements (AIMs) (ii) rotational behaviour, and (iii) spontaneous forelimb use (cylinder test). Rats with unilateral 6-OHDA lesions received single daily i.p. injections of L-DOPA or bromocriptine at therapeutic doses. During 3 weeks of treatment, L-DOPA but not bromocriptine induced increasingly severe AIMs affecting the limb, trunk and orofacial region. Rotational behaviour was induced to a much higher extent by bromocriptine than L-DOPA. In the cylinder test, the two drugs initially improved the performance of the parkinsonian limb to a similar extent. However, L-DOPA-treated animals showed declining levels of performance in this test because the drug-induced AIMs interfered with physiological limb use, and gradually replaced all normal motor activities. L-DOPA-induced axial, limb and orolingual AIM scores were significantly reduced by the acute administration of compounds that have antidyskinetic efficacy in parkinsonian patients and/or nonhuman primates (-91%, yohimbine 10 mg/kg; -19%, naloxone 4-8 mg/kg; -37%, 5-methoxy 5-N,N-dimethyl-tryptamine 2 mg/kg; -30%, clozapine 8 mg/kg; -50%, amantadine 40 mg/kg). L-DOPA-induced rotation was, however, not affected. The present results demonstrate that 6-OHDA-lesioned rats do exhibit motor deficits that share essential functional similarities with parkinsonian akinesia or dyskinesia. Such deficits can be quantified using novel and relatively simple testing procedures, whereas rotometry cannot discriminate between dyskinetic and antiakinetic effects of antiparkinsonian treatments.

Yohimbine: a clinical review

Although yohimbine (YOH) has been available for the treatment of male erectile dysfunction (ED) for longer than Viagra, there is a perception that little is known about the clinical performance of the drug. This review attempts, by comprehensive analysis of the literature, to cover the clinical, pharmacological, and therapeutic profiles of YOH, relevant to its potential utility in the management of patients with ED. Relatively few well-designed studies have been completed. From these, however, it can be concluded that YOH as monotherapy possesses only modest efficacy in ED patients. In acute and chronic (long-term) studies, YOH has been found to be relatively free of side effects over the dose range predicted to be effective in ED. At much higher doses, the most frequently observed effects, consistent with the primary pharmacological action of the drug, are elevation of blood pressure, a slight anxiogenic action, and increased frequency of urination. These side effects are all easily reversible on termination of YOH therapy. There is increasing evidence that the erectogenic action of YOH can be augmented by concomitant administration of agents that augment the release and/or action of nitric oxide in the corpus cavernosum. YOH has yet to be studied in female sexual dysfunction. Overall, the benefit risk profile of YOH would indicate that it has potential, more probably as part of a combination strategy, e.g., with a drug that enhances the nitric oxide pathway, in the treatment of ED.

Mu- and delta-opioid receptor antagonists reduce levodopa-induced dyskinesia in the MPTP-lesioned primate model of Parkinson's disease

Long-term treatment of Parkinson's disease with levodopa is complicated by the emergence of involuntary movements, known as levodopa-induced dyskinesia. It has been hypothesized that increased opioid transmission in striatal output pathways may be responsible for the generation of dyskinesia. In this study, we have investigated the effect of blockade of opioid peptide transmission on levodopa-induced dyskinesia in a primate model of Parkinson's disease-the MPTP-lesioned marmoset. Coadministration of nonselective and mu- or delta-subtype-selective opioid receptor antagonists with levodopa resulted in a significant decrease in dyskinesia. There was no attenuation of the anti-parkinsonian actions of levodopa. These data suggest that specific mu- or delta-opioid receptor antagonists might be applicable clinically in the treatment of levodopa-induced dyskinesia in Parkinson's disease.