Potential of opioid antagonists in the treatment of levodopa-induced dyskinesias in Parkinson's disease

Reduced Levels of Intestinal Neuropeptides and Neurotrophins in Neurotoxin-Induced Parkinson Disease Mouse Models

Intestinal neuropeptides and neurotrophins as endocrine messengers play a key role in the bidirectional gut-brain interaction both in health and disease status. Their alterations in several neurological disorders have been reported, but whether a remarkable change occurs in Parkinson disease (PD) remains unexplored. In this study, we aimed to investigate the levels of 13 neuropeptides and 4 neurotrophins in the intestine of neurotoxin-induced PD mice. The PD mice were obtained by chronic injection of 1-methyl-4-phenyl-1,2,3,6 tetrahydropyridine (MPTP) or MPTP/probenecid (MPTP/p). The levels of mRNA and protein expression in mouse intestines were measured by using real-time reverse transcription polymerase chain reaction and Western blotting, respectively. We found that the mRNA expression of 2 neuropeptides (cholecystokinin [CCK] and dynorphin A [Dyn A]) and 2 neurotrophins (brain-derived neurotrophic factor [BDNF] and neurotrophin-5) was significantly decreased in the colon of MPTP group compared to the vehicle-treated group. The protein levels of CCK, Dyn A, and BDNF were reduced in the colon of MPTP- or MPTP/p-treated mice compared to those of the vehicle-treated group. These data suggest that the intestinal expression of CCK, Dyn A, and BDNF was significantly reduced in PD animal models, and may play a role in the gut-brain axis in PD.

Neuromolecular imaging, a nanobiotechnology for Parkinson's disease: advancing pharmacotherapy for personalized medicine

Evaluating each patient and animal as its own control achieves personalized medicine, which honors the hippocratic philosophy, explaining that "it is far more important to know what person has the disease than what disease the person has." Similarly, individualizing molecular signaling directly from the patient's brain in real time is essential for providing prompt, patient-based treatment as dictated by the point of care. Fortunately, nanotechnology effectively treats many neurodegenerative diseases. In particular, the new medicinal frontier for the discovery of therapy for Parkinson's disease is nanotechnology and nanobiotechnology. Indeed, the unique nanotechnology of neuromolecular imaging combined with the series of nanobiosensors enables continuous videotracking of molecular neurotransmitters in both the normal physiologic and disease states with long-term electrochemical operational stability. This nanobiotechnology is able to track a signal in real time with excellent temporal and spatial resolution directly from each patient's brain to a computer as subjects are behaving during movement, normal and/or dysfunctional including prion-like Parkinson's behavioral biometrics. Moreover, the molecular signaling performed by these nanobiosensors live streams directly online and originates from precise neuroanatomic brain sites such as, in this case, the dorsal striatum in basal ganglia. Thus, the nanobiotechnology studies discussed herein imaged neuromolecules with and without L-3,4-dihydroxyphenylalanine (L-DOPA) in dorsal striatal basal ganglia neurons. Parkinsonian and non-Parkinsonian animals were video-tracked, and images were readily seen on a laptop via a potentiostat using a semiderivative electrical circuit. Administered L-DOPA doses were 50 and 100 mg/kg intraperitoneally (ip); the same experimental paradigm was used to image and then contrast data. Results showed that the baseline release of biogenic amine molecules was significantly above detection limits in non-Parkinsonian animals. After administration of L-DOPA, biogenic amines significantly increased in these non-Parkinson's animals. Nevertheless, it is intriguing to see that L-DOPA could not enable synaptic dopamine release in Parkinson's animals, thereby demonstrating that biogenic amines are biomarkers for Parkinson's disease. Biomarkers are biochemical, genetic, or molecular measures of biological reactions. Importantly, there were other significant biomarkers present in Parkinsonian animals and absent in non-Parkinsonian animals; these were peptide neurotransmitters that include dynorphin and somatostatin in the brain with detection limits of 40 nM for dynorphin and 37 nM for somatostatin (see Table 1). Furthermore, L-DOPA significantly increased these peptide biomarkers, dynorphin and somatostatin, in Parkinson's animals. Targeting biomarkers enables new diagnostic devices and treatments for Parkinson's disease through nanotechnology and nanobiotechnology.

Real Time Imaging of Biomarkers in the Parkinson's Brain Using Mini-Implantable Biosensors. II. Pharmaceutical Therapy with Bromocriptine

We used Neuromolecular Imaging (NMI) and trademarked BRODERICK PROBE^® mini-implantable biosensors, to selectively and separately detect neurotransmitters in vivo, on line, within seconds in the dorsal striatal brain of the Parkinson's Disease (PD) animal model. We directly compared our results derived from PD to the normal striatal brain of the non-Parkinson's Disease (non-PD) animal. This advanced biotechnology enabled the imaging of dopamine (DA), serotonin (5-HT), homovanillic acid (HVA) a metabolite of DA, L-tryptophan (L-TP) a precursor to 5-HT and peptides, dynorphin A 1-17 (Dyn A) and somatostatin (somatostatin releasing inhibitory factor) (SRIF). Each neurotransmitter and neurochemical was imaged at a signature electroactive oxidation/half-wave potential in dorsal striatum of the PD as compared with the non-PD animal. Both endogenous and bromocriptine-treated neurochemical profiles in PD and non-PD were imaged using the same experimental paradigm and detection sensitivities. Results showed that we have found significant neurotransmitter peptide biomarkers in the dorsal striatal brain of endogenous and bromocriptine-treated PD animals. The peptide biomarkers were not imaged in dorsal striatal brain of non-PD animals, either endogenously or bromocriptine-treated. These findings provide new pharmacotherapeutic strategies for PD patients. Thus, our findings are highly applicable to the clinical treatment of PD.

Genotype and smoking history affect risk of levodopa-induced dyskinesias in Parkinson's disease

Parkinson's disease (PD) patients vary widely in their response to levodopa treatment, and this variation may be partially genetic in origin. We determined whether particular dopamine and opioid receptor polymorphisms were associated with risk of earlier onset of dyskinesia side effects during levodopa therapy. Smoking status was also examined. The 92 subjects were recruited from the movement disorders clinic of a neurology practice associated with a medical school. All were adult-onset PD patients who had been taking levodopa at least 5 years and/or had developed levodopa-induced dyskinesia. Carrying the G-allele of the A118G single nucleotide coding region polymorphism of the mu opioid receptor, as well as a history of never smoking, were independently associated with increased risk of earlier onset of dyskinesia (P=0.05 and 0.02, respectively). One genotype of the D2 dopamine receptor intronic dinucleotide repeat polymorphism (14 repeats/15 repeats, with frequency of 6%) was also associated with earlier dyskinesia (P=0.003). History of smoking has previously been associated with reduced risk of developing PD. Our results suggest that smoking history may also influence the response to levodopa, with contribution comparable to those of individual genes including the mu opioid receptor and D2 dopamine receptor.

Do preclinical findings of methamphetamine-induced motor abnormalities translate to an observable clinical phenotype?

This review summarizes the preclinical literature of the effects of methamphetamine (MA) on subcortical dopaminergic and GABAergic mechanisms underlying motor behavior with the goal of elucidating the clinical presentation of human MA-induced movement disorders. Acute and chronic MA exposure in laboratory animal can lead to a variety of motor dysfunctions including increased locomotor activity, stereotypies, diminished or enhanced response times, and parkinsonian-like features. With the exception of psychomotor impairment and hyperkinesia, MA-induced movement disorders are not well documented in humans. This review attempts to draw parallels between the animal and human changes in basal ganglia neurochemistry associated with MA exposure and offers explanations for why a parkinsonian phenotype is not apparent among individuals who use and abuse MA. Significant differences in the expression of neurotoxicity and presence of multiple environmental and pharmacologic confounds may account for the lack of a parkinsonian phenotype in humans despite evidence of altered dopamine function.

Coordinated and Spatial Upregulation of Arc in Striatonigral Neurons Correlates With L-Dopa-Induced Behavioral Sensitization in Dyskinetic Rats

Although oral administration of L-Dopa remains the best therapy for Parkinson disease, its long-term administration causes the appearance of abnormal involuntary movements such as dyskinesia. Although persistent striatal induction of some genes has already been associated with such pathologic profiles in hemiparkinsonian rats, molecular and cellular mechanisms underlying such long-term adaptations remain to be elucidated. In this study, using a rat model of L-Dopa-induced dyskinesia, we report that activity regulated cytoskeletal (Arc)-associated protein is strongly upregulated in the lesioned striatum and that the extent of its induction further varies according to the occurrence or absence of locomotor sensitization. Moreover, Arc is preferentially induced, along with FosB, nur77, and homer-1a, in striatonigral neurons, which express mRNA encoding the precursor of dynorphin. Given the likely importance of Arc in the regulation of cytoskeleton during synaptic plasticity, its upregulation supports the hypothesis that a relationship exists between cytoskeletal modifications and the longlasting action of chronically administrated L-Dopa.

Pallidotomy effect on the cortical excitability in patients with severe Parkinson's disease

Surgical lesions in the medial pallidum have been shown to ameliorate motor deficits in patients with Parkinson's disease (PD). It is believed that interruption of the pallidothalamocortical projections to the motor cortex is required for the satisfactory results. In this report, we adopt cortico-cortical inhibition as the tool to assess the pallidotomy effect on cortical excitability in PD. Interstimulus interval between 1 and 15 msec were investigated. The average peak-to-peak amplitude was measured and calculated at each delay. A total of 8 patients (M:F = 4:4) 54.9 years of age (SD = 9.6) and 10 controls were recruited for the study. In the controls, the inhibitory phenomenon was observed from the 1-msec to the 4-msec delay points and the maximal inhibition was at the 3-msec delay point (33.69% +/- 6.50% of the control response). Mild facilitation was noticed since the 5-msec delay point and thereafter. In patients before operation, a similar trend of inhibition was also observed in the initial 4 msec with the maximal inhibition also at the 3-msec delay point (64.66 +/- 6.77% of the control response). In the postoperative group, the short interstimulus interval inhibition can no longer be observed and the conditioned response was 95.06 +/- 23.68% of the control at the 3-msec delay point. The suppression was gone at and after the 7-msec delay point. Results of repeated-measures analysis of variance show a significant difference among the controls and PD patients before and 3 months after pallidotomy (F = 3.40, P = 0.05). Post hoc examination revealed a significant difference between the controls and PD patients 3 months after pallidotomy at the 3-msec delay point (P = 0.004). However, no correlation was observed between the 3-msec inhibition and the Unified Parkinson's Disease Rating Scale Motor score or the dyskinesia score. The results suggest that pallidotomy can modulate the cortical inhibitory circuitry in patients with PD.

Pathophysiological basis of drug-induced dyskinesias in Parkinson's disease

Drug-induced dyskinesias (DID) represent a troublesome, dose-limiting, and common complication of long-term pharmacotherapy in Parkinson's disease (PD) patients. The pathophysiological basis and clinical nature of DID is of major interest for clinicians and neuroscientists. In this review article, we evaluate the theories of pathophysiology and molecular basis of DID, validity of various animal models used in DID related research, and electrophysiological characteristics of various basal ganglia nuclei during DID. We also discuss the relevance of various treatment strategies to the pathophysiological mechanisms.

The kappa opioid agonist U50,488 potentiates 6-hydroxydopamine-induced neurotoxicity on dopaminergic neurons

Several observations support the hypothesis that kappa opioid (kappa-opioid) receptor agonism may contribute to neurotoxicity, but other reports have suggested that certain kappa-agonists can attenuate neurological dysfunction. Degeneration of dopaminergic neurons in the substantia nigra is the pathological hallmark of Parkinson's disease. Therefore, it is of particular interest to study whether kappa-opioid receptor agonism has an influence on the progressive degeneration of dopaminergic neurons. We have investigated the effect exerted by the selective kappa-agonist U50,488 on the neurotoxicity induced by intrastriatal 6-hydroxydopamine (6-OHDA) administration on dopaminergic neurons. Male Sprague-Dawley rats received an acute (0.5 mg/kg) or subacute (0.5 mg/kg, twice at day, for 7 days) administration of U50,488, receiving the last dose 30 min before intrastriatal 6-OHDA administration. Acute or subacute U50,488 pretreatment potentiated the 6-OHDA-induced decrease in the number of nigral tyrosine hydroxylase immunoreactive neurons (P < 0.05). Acute U50,488 pretreated animals showed a tendency, although not statistically significant to increase striatal mRNA encoding for enkephalin (PPE mRNA). Subacute U50,488 significantly potentiated the increase in PPE mRNA induced by 6-OHDA (P < 0.05). The present results show a neurotoxic effect of the kappa agonist U50,488 on dopaminergic neurons in rats with a striatal lesion induced by 6-OHDA. This neurotoxic effect is associated to an increase in striatal PPE mRNA levels, suggesting that an increase in the indirect pathway activity and consequently an increase in the activity of the subthalamo-nigral pathway might be involved in this phenomenon.

Altered extracellular striatal in vivo biotransformation of the opioid neuropeptide dynorphin A(1-17) in the unilateral 6-OHDA rat model of Parkinson's disease

The in vivo biotransformation of dynorphin A(1-17) (Dyn A) was studied in the striatum of hemiparkinsonian rats by using microdialysis in combination with nanoflow reversed-phase liquid chromatography/electrospray time-of-flight mass spectrometry. The microdialysis probes were implanted into both hemispheres of unilaterally 6-hydroxydopamine (6-OHDA) lesioned rats. Dyn A (10 pmol microl(-1)) was infused through the probes at 0.4 microl min(-1) for 2 h. Samples were collected every 30 min and analyzed by mass spectrometry. The results showed for the first time that there was a difference in the Dyn A biotransformation when comparing the two corresponding sides of the brain. Dyn A metabolites 1-8, 1-16, 5-17, 10-17, 7-10 and 8-10 were detected in the dopamine-depleted striatum but not in the untreated striatum. Dyn A biotransformed fragments found in both hemispheres were N-terminal fragments 1-4, 1-5, 1-6, 1-11, 1-12 and 1-13, C-terminal fragments 2-17, 3-17, 4-17, 7-17 and 8-17 and internal fragments 2-5, 2-10, 2-11, 2-12, and 8-15. The relative levels of these fragments were lower in the dopamine-depleted striatum. The results imply that the extracellular in vivo processing of the dynorphin system is being disturbed in the 6-OHDA-lesion animal model of Parkinson's disease.

Pathobiology of dynorphins in trauma and disease

Dynorphins, endogenous opioid neuropeptides derived from the prodynorphin gene, are involved in a variety of normative physiologic functions including antinociception and neuroendocrine signaling, and may be protective to neurons and oligodendroglia via their opioid receptor-mediated effects. However, under experimental or pathophysiological conditions in which dynorphin levels are substantially elevated, these peptides are excitotoxic largely through actions at glutamate receptors. Because the excitotoxic actions of dynorphins require supraphysiological concentrations or prolonged tissue exposure, there has likely been little evolutionary pressure to ameliorate the maladaptive, non-opioid receptor mediated consequences of dynorphins. Thus, dynorphins can have protective and/or proapoptotic actions in neurons and glia, and the net effect may depend upon the distribution of receptors in a particular region and the amount of dynorphin released. Increased prodynorphin gene expression is observed in several disease states and disruptions in dynorphin processing can accompany pathophysiological situations. Aberrant processing may contribute to the net negative effects of dysregulated dynorphin production by tilting the balance towards dynorphin derivatives that are toxic to neurons and/or oligodendroglia. Evidence outlined in this review suggests that a variety of CNS pathologies alter dynorphin biogenesis. Such alterations are likely maladaptive and contribute to secondary injury and the pathogenesis of disease.

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.

The opioid agonist morphine decreases the dyskinetic response to dopaminergic agents in parkinsonian monkeys

In parkinsonian patients as well as in primate models with levodopa-induced dyskinesias (LID), an increase in the expression of preproenkephalin in the striatal output pathways has been demonstrated. Does this increase contribute to the development of LID, or does it rather act as a protection mechanism? To clarify this question, we have investigated the effect of different doses of morphine on the dyskinetic response to L-DOPA, a D2 agonist, and a D1 agonist. We have used MPTP-treated cynomolgus monkeys with a stable parkinsonian syndrome and reproducible dyskinesias to L-DOPA. Co-administration of morphine with dopaminergic agents produces a significant reduction in the severity of dyskinesias, while it does not affect the anti-parkinsonian efficacy of the treatment. This study suggests that the increased production of opioids in the striatal projection neurons might have a protective role to compensate the changes in synaptic transmissions that are responsible for dyskinesias, rather than be the cause of dyskinesias.

Neuroimaging in Parkinson's disease

In this review, the potential role of positron emission tomography and single photon emission computed tomography as biological markers for diagnosing and following the progression of Parkinson's disease (PD) is discussed. Their value for assessing the efficacy of putative neuroprotective agents in PD and for revealing the pharmacological changes underlying the symptomatology and complications of this disorder is also considered. It is concluded that in the future functional imaging will provide a valuable adjunct to clinical assessment when judging the efficacy of putative neuroprotective approaches to PD.

Non-subtype-selective opioid receptor antagonism in treatment of levodopa-induced motor complications in Parkinson's disease

Opioid peptide transmission is enhanced in the striatum of animal models and Parkinson's disease (PD) patients with levodopa-induced motor complications. Opioid receptor antagonists reduce levodopa-induced dyskinesia in primate models of PD; however, clinical trials to date have been inconclusive. A double-blind, placebo controlled, crossover design study in 14 patients with PD experiencing motor fluctuations was carried out, using the non-subtype-selective opioid receptor antagonist naloxone. Naloxone did not reduce levodopa-induced dyskinesia. The duration of action of levodopa was increased significantly by 17.5%. Non-subtype-selective opioid receptor antagonism may prove useful in the treatment of levodopa-related wearing-off in PD but not in dyskinesia.

Upregulation of striatal adenosine A2A receptor mRNA in 6-hydroxydopamine-lesioned rats intermittently treated with L-DOPA

To determine whether the adenosine A2A receptor might play a role in L-DOPA-induced dyskinesia in Parkinson's disease, we analyzed changes in the expression of A2A receptor mRNA in response to intermittent treatment with L-DOPA in rats with dopaminergic denervation by 6-hydroxydopamine (OHDA) infusion into the medial forebrain bundle. Intermittent treatment with L-DOPA increased A2A receptor mRNA levels in the dopamine-depleted striatum of 6-OHDA-lesioned rats exhibiting behavioral sensitization to L-DOPA. These results suggest that A2A receptor activation is associated with the development of motor complications induced by L-DOPA treatment.

Effect of subthalamic nucleus or entopeduncular nucleus lesion on levodopa-induced neurochemical changes within the basal ganglia and on levodopa-induced motor alterations in 6-hydroxydopamine-lesioned rats

Inactivation of the subthalamic nucleus (STN) or the internal segment of the pallidum (GPi)/entopeduncular nucleus (EP) by deep brain stimulation or lesioning alleviates clinical manifestations of Parkinson's disease (PD) as well as reducing the side-effects of levodopa treatment. However, the effects of STN or entopeduncular nucleus (EP) lesion on levodopa-related motor fluctuations and on neurochemical changes induced by levodopa remain largely unknown. The effects of such lesions on levodopa-induced motor alterations were studied in 6-hydroxydopamine (6-OHDA)-lesioned rats and were assessed neurochemically by analyzing the functional activity of the basal ganglia nuclei, using the expression levels of the mRNAs coding for glutamic acid decarboxylase and cytochrome oxidase as molecular markers of neuronal activity. At the striatal level, preproenkephalin (PPE) mRNA levels were analyzed. We found in 6-OHDA-lesioned rats that a unilateral STN or EP lesion ipsilateral to the 6-OHDA lesion had no effect on either the shortening in the duration of the levodopa-induced rotational response or the levodopa-induced biochemical changes in the basal ganglia nuclei. In contrast, overexpression of PPE mRNA due to levodopa treatment was reversed by the STN or EP lesion. Our study thus shows that lesion of the EP or STN may counteract some of the neurochemical changes induced by levodopa treatment within the striatum.

The fluctuating Parkinsonian patient--clinical and pathophysiological aspects

Although levodopa-related motor response complications remain challenging from a pathophysiological and therapeutic standpoint, major advances have been made in the last decade, supporting the development of several promising drugs. Eventually, these drugs may help us to prevent, alleviate, or even "deprime" these frequent and disabling complications. Knowledge of the basic mechanisms and hypotheses underlying this fascinating conversion in the parkinsonian brain allows neurologists to understand the rationale behind emerging treatment strategies.

AMPA receptor antagonist LY293558 reverses preproenkephalin mRNA overexpression in the striatum of 6-OHDA-lesioned-rats treated with L-dopa

Striatal neurons that contain GABA and enkephalin and project to the external segment of the pallidum are thought to be overactive in Parkinson's disease. Furthermore, it has been shown that the appearance of L-dopa-induced dyskinesias is correlated to an increase of preproenkephalin (PPE) mRNA expression and that some antagonists of glutamate receptors can prevent and reverse L-dopa-induced dyskinesias in parkinsonian rats. The aim of this study was therefore to analyse the effect of a systemic treatment with glutamate receptor antagonists, alone or in combination with L-dopa, on the PPE mRNA level in rats with a 6-hydroxydopamine-induced unilateral lesion of the nigrostriatal pathway. In vehicle-treated animals, PPE mRNA levels were markedly increased in the striatum on the lesioned side. Sub-chronic L-dopa treatment, with bi-daily injections for 22 days, induced a further increase in PPE mRNA expression in the denervated striatum. Administration of the AMPA receptor antagonist, LY293558, partially reversed the lesion-induced and L-dopa-induced increases in PPE mRNA expression. However, although the administration of the NMDA receptor antagonist MK801 showed a tendency to decrease this L-dopa induced overexpression, it did not reach significance. This study provides evidence that glutamatergic antagonists, and particularly AMPA antagonists, tend to reverse PPE neurochemical changes at the striatal level induced by L-dopa in hemiparkinsonian rats.

Levodopa response motor complications--GABA receptors and preproenkephalin expression in human brain

Post-mortem studies in human brain of patients with Parkinson's disease have greatly contributed to our understanding of the disease. However, few human brain studies have focused on levodopa-induced dyskinesias, which considerably limit the beneficial effect of levodopa (LD) in the treatment of Parkinson's disease. We have taken advantage of the fact that some patients develop dyskinesias and other do not to compare biochemical markers between them. In post-mortem samples from LD-treated parkinsonian patients, increased preproenkephalin expression in the putamen and increased GABA(A) receptors content in the internal globus pallidus (GPi) are found in dyskinetic parkinsonian patients compared to non-dyskinetic patients. These data are consistent with previous observations in MPTP monkeys developing dyskinesias following LD or dopamine agonist treatment. This combination of data in an animal model and in humans strongly suggests that increased enkephalinergic activity in the putamen and increased sensitivity of GABA(A) receptors in the GPi are implicated in the pathogenesis of LD-induced dyskinesias in Parkinson's disease.

Persistent behavioral sensitization to chronic L-DOPA requires A2A adenosine receptors

To investigate the role of A(2A) adenosine receptors in adaptive responses to chronic intermittent dopamine receptor stimulation, we compared the behavioral sensitization elicited by repeated l-DOPA treatment in hemiparkinsonian wild-type (WT) and A(2A) adenosine receptor knock-out (A(2A) KO) mice. Although the unilateral nigrostriatal lesion produced by intrastriatal injection of 6-hydroxydopamine was indistinguishable between WT and A(2A) KO mice, they developed strikingly different patterns of behavioral sensitization after daily treatment with low doses of l-DOPA for 3 weeks. WT mice initially displayed modest contralateral rotational responses and then developed progressively greater responses that reached a maximum within 1 week and persisted for the duration of the treatment. In contrast, any rotational behavioral sensitization in A(2A) KO mice was transient and completely reversed within 2 weeks. Similarly, the time to reach the peak rotation was progressively shortened in WT mice but remained unchanged in A(2A) KO mice. Furthermore, daily l-DOPA treatment produced gradually sensitized grooming in WT mice but failed to induce any sensitized grooming in A(2A) KO mice. Finally, repeated l-DOPA treatment reversed the 6-OHDA-induced reduction of striatal dynorphin mRNA in WT but not A(2A) KO mice, raising the possibility that the A(2A) receptor may contribute to l-DOPA-induced behavioral sensitization by facilitating adaptations within the dynorphin-expressing striatonigral pathway. Together these results demonstrate that the A(2A) receptor plays a critical role in the development and particularly the persistence of behavioral sensitization to repeated l-DOPA treatment. Furthermore, they raise the possibility that the maladaptive dyskinetic responses to chronic l-DOPA treatment in Parkinson's disease may be attenuated by A(2A) receptor inactivation.

Increase of preproenkephalin mRNA levels in the putamen of Parkinson disease patients with levodopa-induced dyskinesias

The expression of preproenkephalin messenger RNA was studied in the brain of Parkinson disease (PD) patients using in situ hybridization. All these patients were treated with levodopa (LD) and the development of motor complications was recorded. Eleven normal controls and 14 PD patients were used, of which 4 developed dyskinesias, 3 developed wearing-off, 3 developed both dyskinesias and wearing-off, and 4 developed no adverse effect following dopaminomimetic therapy. Nigrostriatal denervation was similar between the subgroups of PD patients as assessed using 125I-RTI-specific binding to the dopamine transporter and measures of catecholamine concentrations by HPLC. A significant increase of preproenkephalin messenger RNA levels was observed in the lateral putamen of dyskinetic patients in comparison to controls (+210%; p < 0.01) and in comparison to nondyskinetic patients (+112%; p < 0.05). No change was observed in medial parts of the putamen or in the caudate nucleus. No relationship between preproenkephalin messenger RNA levels and other clinical variables such as development of wearing-off, age of death, duration of disease, or duration of LD therapy was found. These findings suggest that increase synthesis of preproenkephalin in the medium spiny output neurons of the striatopallidal pathway play a role in the development of dyskinesias following long-term LD therapy in Parkinson disease.

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.

Pathophysiology of levodopa-induced dyskinesia: potential for new therapies

Involuntary movements--or dyskinesias--are a debilitating complication of levodopa therapy for Parkinson's disease, and is experienced in most patients. Despite the importance of this problem, little was known about the cause of dyskinesia until recently; however, this situation has changed significantly in the past few years. Our increased understanding of levodopa-induced dyskinesia is not only valuable for improving patient care, but also in providing us with new insights into the functional organization of the basal ganglia and motor systems.

Radionuclide Imaging in Parkinson’s Disease: Diagnosis, Treatment, and Disease Progression

This paper reviews the abilities of positron emission tomography (PET) and single photon emission tomography (SPECT) to detect Parkinson’s disease, monitor its progression and the effect of therapy. It also provides insights on the role these two modalities provide in terms of discriminating atypical syndromes from Parkinson’s disease. Both PET and SPECT are sensitive means of detecting alterations in metabolism and blood flow in the brain and impairments in neurotransmitter function, especially dopaminergic, in the striatum and, more recently, in extrastriatal structures. To date, PET presents the added advantage of quantification, better sensitivity and resolution and a greater variety of tracers for both the dopaminergic and nondopaminergic systems.

Endomorphin-1: induction of motor behavior and lack of receptor desensitization

The endomorphins are recently discovered endogenous agonists for the mu-opioid receptor (Zadina et al., 1997). Endomorphins produce analgesia; however, their role in other brain functions has not been elucidated. We have investigated the behavioral effects of endomorphin-1 in the globus pallidus, a brain region that is rich in mu-opioid receptors and involved in motor control. Bilateral administration of endomorphin-1 in the globus pallidus of rats induced orofacial dyskinesia. This effect was dose-dependent and at the highest dose tested (18 pmol per side) was sustained during the 60 min of observation, indicating that endomorphin-1 does not induce rapid desensitization of this motor response. In agreement with a lack of desensitization of mu-opioid receptors, 3 hr of continuous exposure of the cloned mu receptor to endomorphin-1 did not diminish the subsequent ability of the agonist to inhibit adenylate cyclase activity in cells expressing the cloned mu-opioid receptor. Confirming the involvement of mu-opioid receptors, the behavioral effect of endomorphin-1 in the globus pallidus was blocked by the opioid antagonist naloxone and the mu-selective peptide antagonist Cys(2)-Tyr(3)-Orn(5)-Pen(7) amide (CTOP). Furthermore, the selective mu receptor agonist [d-Ala(2)-N-Me-Phe(4)-Glycol(5)]-enkephalin (DAMGO) also stimulated orofacial dyskinesia when infused into the globus pallidus, albeit transiently. Our findings suggest that endogenous mu agonists may play a role in hyperkinetic movement disorders by inducing sustained activation of pallidal opioid receptors.

Antiparkinsonian activity and dyskinesia risk of ropinirole and L-DOPA combination therapy in drug naïve MPTP-lesioned common marmosets (Callithrix jacchus)

De novo administration of long-acting dopamine agonists, such as ropinirole, to patients with Parkinson's disease or to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated subhuman primates produces a lower incidence of dyskinesia than occurs with L-DOPA. This study compares the intensity of dyskinesia produced by combinations of L-DOPA and ropinirole and by these drugs alone, using the MPTP-treated common marmoset model of Parkinson's disease. The objective is to determine the optimum therapeutic strategy for the long-term control of Parkinson's disease with a minimal risk of dyskinesia. MPTP-treated marmosets received either L-DOPA alone, ropinirole alone, or one of two combinations of these drugs (either L-DOPA dominant or ropinirole dominant) daily for 28 days in doses titrated to produce a similar improvement in disability and increase in locomotion. In the group receiving L-DOPA alone, there was a trend for peak dose locomotor activity to increase and the duration of drug effect to decline over the period of the study. L-DOPA alone induced marked dyskinesia over the period of treatment, in contrast to ropinirole which produced a low intensity of involuntary movements. The L-DOPA dominant combination initially produced little dyskinesia, but this became increasingly intense as the study progressed. In contrast, the ropinirole dominant combination produced no greater intensity of dyskinesia than was produced by ropinirole alone. These data suggest that in early Parkinson's disease, the use of ropinirole alone or in combination with a low-dose L-DOPA might delay the induction of dyskinesias while improving motor performance.

Coadministration of (-)-OSU6162 with l-DOPA normalizes preproenkephalin mRNA expression in the sensorimotor striatum of primates with unilateral 6-OHDA lesions

The substituted phenylpiperidine (-)-OSU6162 is a novel modulator of the dopaminergic systems with low affinity for dopamine D(2) receptors and potent normalizing effects on l-DOPA-induced dyskinesias. We studied the effects of coadministration of (-)-OSU6162 with l-DOPA on the regulation of striatal preproenkephalin (PPE) and prodynorphin (PDyn) mRNA expression in the primate brain by in situ hybridization histochemistry. Common marmoset monkeys sustaining unilateral 6-hydroxydopamine lesions of the nigrostriatal pathway received l-DOPA/carbidopa, l-DOPA/carbidopa plus (-)-OSU6162, or vehicle over 14 days. In vehicle-treated animals, PPE mRNA levels were markedly increased in the sensorimotor territory of the lesioned striatum. By contrast, a rather uniform lesion-induced reduction of PDyn mRNA levels was found in the vehicle group. Subchronic l-DOPA treatment induced a further increase in PPE mRNA expression in a number of sensorimotor and associative subregions of the denervated striatum. Coadministration of (-)-OSU6162 with l-DOPA partially reversed the lesion- and l-DOPA-induced elevation of PPE expression and, by affecting PPE mRNA expression differentially on the intact and lesioned striatum, markedly reduced the side-to-side difference in PPE mRNA expression. The effects on PPE mRNA expression were apparent throughout the rostrocaudal extent of the putamen and the dorsal portions of the caudate nucleus. l-DOPA treatment resulted in an enhancement in PDyn mRNA expression in all functional compartments of the striatum. Coadministration of (-)-OSU6162 had no apparent influence on these l-DOPA-induced changes in PDyn mRNA expression. The present results suggest that (-)-OSU6162 acts primarily by modifying striatal output via the indirect pathway.

Alterations in cortical and basal ganglia levels of opioid receptor binding in a rat model of l-DOPA-induced dyskinesia

Opioid receptor-binding autoradiography was used as a way to map sites of altered opioid transmission in a rat model of l-DOPA-induced dyskinesia. Rats with unilateral 6-hydroxydopamine lesions of the nigrostriatal pathways sustained a 3-week treatment with l-DOPA (6 mg/kg/day, combined with 12 mg/kg/day benserazide), causing about half of them to develop dyskinetic-like movements on the side of the body contralateral to the lesion. Autoradiographic analysis of mu-, delta-, and kappa-opioid binding sites was carried out in the caudate-putamen (CPu), the globus pallidus (GP), the substantia nigra (SN), the primary motor area, and the premotor-cingulate cortex. The dopamine-denervating lesion alone caused an ipsilateral reduction in opioid radioligand binding in the CPu, GP, and SN, but not in the cerebral cortex. Chronic l-DOPA treatment affected opioid receptor binding in both the basal ganglia and the cerebral cortex, producing changes that were both structure- and receptor-type specific, and closely related to the motor response elicited by the treatment. In the basal ganglia, the most clear-cut differences between dyskinetic and nondyskinetic rats pertained to kappa opioid sites. On the lesioned side, both striatal and nigral levels of kappa binding densities were significantly lower in the dyskinetic group, showing a negative correlation with the rats' dyskinesia scores on one hand and with the striatal expression of opioid precursor mRNAs on the other hand. In the cerebral cortex, levels of mu and delta binding site densities were bilaterally elevated in the dyskinetic group, whereas kappa radioligand binding was specifically increased in the nondyskinetic cases and showed a negative correlation with the rats' dyskinesia scores. These data demonstrate that bilateral changes in cortical opioid transmission are closely associated with l-DOPA-induced dyskinesia in the rat. Moreover, the fact that dyskinetic and nondyskinetic animals often show opposite changes in opioid radioligand binding suggests that the motor response to l-DOPA is determined, at least in part, by compensatory adjustments of brain opioid receptors.

PET studies and motor complications in Parkinson's disease

Parkinson's disease (PD) patients with motor complications show a greater reduction in putamen [18F]dopa uptake on positron emission tomography (PET) compared with sustained responders to L-dopa, although individual ranges overlap considerably. This implies that, although loss of putamen dopamine storage predisposes motor complications in PD, it cannot be the only factor determining timing of onset. Additional PET studies suggest that loss of striatal dopamine storage capacity along with pulsatile exposure to exogenous L-dopa results in pathologically raised synaptic dopamine levels and deranged basal ganglia opioid transmission.This, rather than altered dopamine receptor binding, then causes inappropriate overactivity of basal ganglia-frontal projections, resulting in breakthrough involuntary movements.

Functional imaging of Parkinson's disease: is it possible to detect brain areas for specific symptoms?

H2(15)O PET activation studies enable the brain systems involved in controlling different aspects of motor function to be defined. In Parkinson's disease (PD) freely chosen limb movements are performed slowly. This bradykinesia is associated with selective underactivity of the supplementary motor area and dorsal prefrontal cortex, frontal association areas that receive subcortical input principally from the basal ganglia. At the same time there is compensatory overactivity of the lateral premotor and parietal cortex, areas that have a primary role in facilitating motor responses to visual and auditory cues. This finding explains why PD patients find it easier to perform cued as opposed to freely chosen actions. Levels of activation of the supplementary motor area and dorsal prefrontal cortex in PD can be restored with dopaminergic medication, implants of fetal mesencephalic tissue, internal pallidotomy or high frequency electrical subthalamic stimulation. Activation studies suggest that Parkinsonian rest tremor arises from a combination of inappropriate overactivity of cerebellar connections and loss of dopaminergic function. When tremor is relieved by ventral thalamotomy or thalamic stimulation this cerebellar overactivity is corrected but at the expense of reducing levels of primary motor cortex activation. It has been hypothesised that dyskinesias in PD arise due to altered dopamine receptor binding following chronic exposure to levodopa stimulation. Functional imaging findings, however, are against this hypothesis and rather suggest that downstream increases in basal ganglia opioid neurotransmission are more likely to be relevant.

Expression of mu, kappa, and delta opioid receptor messenger RNA in the human CNS: a 33P in situ hybridization study

The existence of at least three opioid receptor types, referred to as mu, kappa, and delta, is well established. Complementary DNAs corresponding to the pharmacologically defined mu, kappa, and delta opioid receptors have been isolated in various species including man. The expression patterns of opioid receptor transcripts in human brain has not been established with a cellular resolution, in part because of the low apparent abundance of opioid receptor messenger RNAs in human brain. To visualize opioid receptor messenger RNAs we developed a sensitive in situ hybridization histochemistry method using 33P-labelled RNA probes. In the present study we report the regional and cellular expression of mu, kappa, and delta opioid receptor messenger RNAs in selected areas of the human brain. Hybridization of the different opioid receptor probes resulted in distinct labelling patterns. For the mu and kappa opioid receptor probes, the most intense regional signals were observed in striatum, thalamus, hypothalamus, cerebral cortex, cerebellum and certain brainstem areas as well as the spinal cord. The most intense signals for the delta opioid receptor probe were found in cerebral cortex. Expression of opioid receptor transcripts was restricted to subpopulations of neurons within most regions studied demonstrating differences in the cellular expression patterns of mu, kappa, and delta opioid receptor messenger RNAs in numerous brain regions. The messenger RNA distribution patterns for each opioid receptor corresponded in general to the distribution of opioid receptor binding sites as visualized by receptor autoradiography. However, some mismatches, for instance between mu opioid receptor receptor binding and mu opioid receptor messenger RNA expression in the anterior striatum, were observed. A comparison of the distribution patterns of opioid receptor messenger RNAs in the human brain and that reported for the rat suggests a homologous expression pattern in many regions. However, in the human brain, kappa opioid receptor messenger RNA expression was more widely distributed than in rodents. The differential and region specific expression of opioid receptors may help to identify targets for receptor specific compounds in neuronal circuits involved in a variety of physiological functions including pain perception, neuroendocrine regulation, motor control and reward.

Effect of repeated L-DOPA, bromocriptine, or lisuride administration on preproenkephalin-A and preproenkephalin-B mRNA levels in the striatum of the 6-hydroxydopamine-lesioned rat

Abnormal involuntary movements, or dyskinesias, plague current symptomatic approaches to the treatment of Parkinson's disease. The neural mechanisms underlying the generation of dyskinesia following repeated l-3,4-dihydroxyphenylalanine (L-DOPA) or dopamine agonist administration in Parkinson's disease remain unknown. However, de novo administration of bromocriptine or lisuride to either l-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-lesioned primates or patients can alleviate parkinsonian symptoms without the development of dyskinesia. In this study, we have investigated behavioral responses and alterations in the expression of opioid neuropeptide precursors preproenkephalin-A (PPE-A, encoding methionine- and leucine-enkephalin) and preproenkephalin-B (PPE-B), the precursor encoding dynorphins (dynorphin A1-17 and B1-13, leucine-enkephalin, and alpha-neoendorphin) in striatal output pathways of the 6-hydroxydopamine (6-OHDA)-lesioned rat model of Parkinson's disease. Expression was assessed following repeated L-DOPA, bromocriptine, or lisuride administration. Given the functional organization of basal ganglia circuitry into anatomically discrete parallel circuits, we investigated alterations in peptide expression with reference to the detailed topography of the striatum. Following repeated L-DOPA administration (6.5 mg/kg, b.d., 21 days) in the 6-OHDA-lesioned rat a rotational response was observed. This became markedly enhanced with repeated treatment. We have previously characterized the pharmacology of this enhanced response and have suggested that it is a useful model for the elucidation of the cellular and molecular mechanisms underlying L-DOPA- and dopamine agonist-induced dyskinesia. In contrast to l-DOPA, de novo administration of bromocriptine (1 or 5 mg/kg, b.d., 21 days) or lisuride (0.01 or 0.1 mg/kg, b.d., 21 days) did not lead to an enhanced behavioral response. In vehicle-treated, 6-OHDA-lesioned animals, PPE-A expression was elevated rostrally and dorsally, while PPE-B expression was reduced in the striatum at all rostrocaudal levels. Repeated l-DOPA administration was accompanied by elevations in striatal PPE-B mRNA levels and a further elevation, above lesion-induced levels, in PPE-A expression. This further elevation was restricted to the dorsolateral striatum. However, following repeated bromocriptine or lisuride administration no increase in PPE-B expression was observed and the lesion-induced increase in PPE-A expression was normalized to prelesion levels. Increased PPE-A and PPE-B levels may, through decreasing GABA and glutamate release, respectively, in output nuclei of the basal ganglia, play a role in the development of L-DOPA- and dopamine-agonist induced dyskinesia in Parkinson's disease. These studies suggest that anti-parkinsonian treatments which are not associated with an elevation in PPE-B and/or normalize elevated PPE-A precursor expression, such as NMDA-receptor antagonists or long-acting dopamine D2 receptor agonists, e.g., cabergoline or ropinirole, may reduce dyskinesia in Parkinson's disease.

Adjuncts to dopamine replacement: a pragmatic approach to reducing the problem of dyskinesia in Parkinson's disease

Dyskinesias following long-term dopamine replacement therapy are a major limitation of current treatments for Parkinson's disease. Recently, attention has been focused on the concept of using non-dopaminergic adjuncts to currently available therapies in an attempt to reduce the problem of dyskinesia. Thus, an enhanced understanding of the neural mechanisms underlying dyskinetic symptoms has led to the realization that it might be possible to manipulate non-dopaminergic systems and reduce dyskinesia without compromising the anti-parkinsonian efficacy of drugs such as L-dopa. This article discusses how non-dopaminergic manipulations could reverse the abnormalities in basal ganglia circuitry responsible for generating dyskinesia. It is proposed that potential anti-dyskinetic drugs might include glutamate (NMDA) receptor antagonists, opioid receptor antagonists, cannabinoid receptor agonists or antagonists, alpha2 adrenergic receptor antagonists, and 5-HT-enhancing agents.

Characterization of enhanced behavioral responses to L-DOPA following repeated administration in the 6-hydroxydopamine-lesioned rat model of Parkinson's disease

Long-term treatment of Parkinson's disease with dopamine-replacing agents such as L-3,4-dihydroxyphenylalanine (L-DOPA) is compromised by many side-effects, most notably involuntary movements, L-DOPA-induced dyskinesia. Acute challenge with dopamine-replacing drugs elicits a rotational response in the 6-hydroxydopamine (6-OHDA)-lesioned rat model of Parkinson's disease. This rotation is contraversive to the lesion and is considered to represent an antiparkinsonian effect. More recently, it has become clear that the rotational response shows plasticity and that repeated L-DOPA or apomorphine therapy is accompanied by a marked enhancement in this response. In this study, we demonstrate that the enhanced behavioral response to repeated dopamine-replacement therapy seen in the 6-OHDA-lesioned rat has pharmacological characteristics similar to L-DOPA-induced dyskinesia seen in MPTP-lesioned primates and man. Thus, the magnitude and rate of development of the enhanced response to L-DOPA treatment is related to both the number of doses and the size of the dose of L-DOPA administered. In contrast, de novo administration of dopaminergic drugs that are associated with a lower incidence of dyskinesia, e.g., bromocriptine or lisuride, does not lead to an enhanced behavioral response following repeated treatment. However, following a single "priming" administration of apomorphine, the rotational response elicited by subsequent bromocriptine administrations is enhanced with repeated treatment. Once established, the enhanced behavioral response to repeated L-DOPA-administration (6.5 mg/kg, twice daily) can, like L-DOPA-induced dyskinesia in man and MPTP-treated monkeys, be selectively reduced by coadministration of L-DOPA with the alpha2-adrenergic receptor antagonist yohimbine (10 mg/kg, -95%), the 5-HT uptake inhibitor 5-MDOT (2 mg/kg, -90%), or the beta-adrenergic receptor antagonist propranalol (10 mg/kg, -35%). While these rats do not exhibit symptoms of dyskinesia per se, this rodent model does exhibit behaviors, the underlying mechanism of which is likely to be similar to that underlying L-DOPA-induced dyskinesia and may prove useful in studying the molecular and cellular mechanisms of L-DOPA-induced dyskinesia in Parkinson's disease.

Topographical organization of opioid peptide precursor gene expression following repeated apomorphine treatment in the 6-hydroxydopamine-lesioned rat

Many studies have previously described changes in preproenkephalin-A (PPE-A) and preproenkephalin-B (PPE-B) gene expression in the striatum of the 6-hydroxydopamine (6-OHDA)-lesioned rat model of Parkinson's disease (both with or without dopamine replacement treatment). To date, these studies have either taken the striatum as a whole or have focused on a single subregion of the striatum. However, the striatum is organized into anatomically discrete parallel circuits serving different functions (motor, associative, and limbic). We have therefore employed in situ hybridization to examine the detailed topography of changes in opioid precursor expression following dopamine depletion and subsequent treatment with apomorphine (5 mg/kg twice daily for 10 days). In the untreated 6-OHDA-lesioned striatum PPE-A expression was elevated only in the dorsal (sensorimotor) caudate-putamen. Following apomorphine treatment PPE-A mRNA levels were further raised in the sensorimotor striatum (</=77%) and approximately doubled and tripled in the ventral caudate-putamen (associative) and nucleus accumbens (limbic), respectively. These subsequent elevations were mostly restricted to rostral portions of the striatum. Although unchanged following vehicle treatment, PPE-B gene expression in the lesioned caudate-putamen (sensorimotor and associative) was elevated some 30-fold by apomorphine treatment. A smaller rise (fivefold) was seen in rostral regions of the lesioned nucleus accumbens. Thus, differential regulation of opioid peptide transmission exists in motor, limbic, and associative regions of the striatum and may contribute to the generation of motor and cognitive disturbances following long-term treatment of the dopamine-depleted striatum.

Enhancement of the behavioral response to apomorphine administration following repeated treatment in the 6-hydroxydopamine-lesioned rat is temporally correlated with a rise in striatal preproenkephalin-B, but not preproenkephalin-A, gene expression

Repeated dopamine receptor stimulation in the 6-hydroxydopamine (6-OHDA)-lesioned rat produces a marked enhancement in the behavioral response to a given dose of dopamine agonist. Such treatment also increases the synthesis of preproenkephalin-B (PPE-B) throughout the striatum and preproenkephalin-A (PPE-A) rostrally. To examine the relationship between these changes, 6-OHDA-lesioned rats were treated with apomorphine (5 mg/kg) twice-daily. Between 0.5 and 7 days treatment, behavior was monitored and the levels of PPE-A and PPE-B mRNA were determined by in situ hybridization. A single injection of apomorphine induced a well-known rotational response contraversive to the lesion (314 +/- 22 rotations h-1). However, with repeated injection, the response to apomorphine was markedly enhanced, being significantly higher on Days 3 (704 +/- 60 rotations h-1), 5 (1354 +/- 104 rotations h-1), and 7 (1680 +/- 117 rotations h-1). In the lesioned caudate-putamen, but not in the nucleus accumbens, PPE-B expression rose in a manner that was temporally correlated with the enhanced locomotor response to apomorphine. PPE-A expression in the lesioned striatum, significantly increased only after 7 days treatment, did not correlate with the behavioral response. In conclusion, PPE-B may contribute to the development of behavioral supersensitivity following repeated dopamine-replacement therapy in animal models of Parkinson's disease.