L-DOPA reverses altered gene expression of substance P but not enkephalin in the caudate-putamen of common marmosets treated with MPTP

Cervical percutaneous interferential current stimulation improves citric acid cough tests in patients with Parkinson's disease on medication

Aspiration pneumonia is the leading cause of death in patients with Parkinson's disease. The incidence of silent aspiration is high in such patients owing to decreased pharyngeal and laryngeal sensation; thus, interventions for this condition may help prevent pneumonia. In this single-arm, open-label study, we used a cervical percutaneous interferential current stimulation device to activate pharyngeal and laryngeal sensory nerves. We evaluated its effectiveness in patients with Hoehn-Yahr stages 2-4 Parkinson's disease. The primary endpoint was the proportion of patients with a normal cough reflex after consuming 1% citric acid at the end of the intervention compared with baseline measurements. In total, 25 patients received neck percutaneous interferential current stimulation for 20 min twice weekly for 8 weeks. Afterward, the proportion of patients with a normal cough reflex after 1% citric acid consumption increased significantly (p = 0.001), whereas other indicators, such as tongue pressure, peak expiratory flow, and penetration or aspiration during videofluoroscopic examination, remained unchanged. A longer duration of illness, higher Unified Parkinson's Disease Rating Scale total scores, and higher levodopa equivalent daily doses were significantly associated with improved cough test outcomes. Hence, cervical percutaneous interferential current stimulation significantly improved cough reflexes and may improve silent aspiration. Trial Registration: Japan Registry of Clinical Trials, jRCTs062220013, first registered 09/05/2022.

Dyskinesias in Parkinson's disease: views from positron emission tomography studies

Levodopa-induced dyskinesias (LIDs) and graft-induced dyskinesias (GIDs) are serious and common complications of Parkinson's disease (PD) management following chronic treatment with levodopa or intrastriatal transplantation with dopamine-rich foetal ventral mesencephalic tissue, respectively. Positron emission tomography (PET) molecular imaging provides a powerful in vivo tool that has been employed over the past 20 years for the elucidation of mechanisms underlying the development of LIDs and GIDs in PD patients. PET used together with radioligands tagging molecular targets has allowed the functional investigation of several systems in the brain including the dopaminergic, serotonergic, glutamatergic, opioid, endocannabinoid, noradrenergic and cholinergic systems. In this article the role of PET imaging in unveiling pathophysiological mechanisms underlying the development of LIDs and GIDs in PD patients is reviewed.

Comparison of rating scales used to evaluate L-DOPA-induced dyskinesia in the 6-OHDA lesioned rat

Abnormal involuntary movement (AIM) rating scales are frequently used to study the mechanisms underlying L-DOPA-induced dyskinesia (LID) in 6-OHDA lesioned rodents and the propensity of novel treatments for Parkinson's disease to induce or alleviate similar abnormal behaviours. Despite the existence of at least one well validated method, other AIM scales are also in use. Moreover, there have been developments and variations in the original scales and their methods of use, without re-validation. In this study, 6-OHDA medial forebrain bundle lesioned Sprague-Dawley rats were treated with chronic L-DOPA 6 mg/kg/day for 5 weeks followed by 12 mg/kg/day for another 5 weeks. Rats were assessed weekly by simultaneous ratings on four published AIM and stereotypy scales with concurrent recording of rotation, over 3 hours following L-DOPA injection. Three contemporary AIM scales have then been validated pharmacologically using agents that are known to reduce LID clinically and in primates (amantadine) or to interfere with the activity of L-DOPA (the D(1) and D(2) dopamine receptor antagonists, SCH-23390 and raclopride) respectively. We also demonstrate that AIM, stereotypic and rotational behaviour are distinct motor dysfunctions induced by chronic and acute treatment of L-DOPA, and should be assessed separately. The undertaking of assessments at multiple time points is essential especially when testing the efficacy of new potential anti-dyskinetic treatments. Importantly critical to all AIM and rotation testing is the internal validation of both the scale being used and the environment being used.

Role and clinical utility of pramipexole extended release in the treatment of early Parkinson's disease

The aim of this article is to provide a short review of the most relevant pharmacological and clinical data on pramipexole extended release (ER) as well as to address the clinical utility and potential advantages of a once-daily formulation especially in the treatment of early Parkinson's disease (PD). Pramipexole is widely established as a symptomatic treatment in early as well as advanced PD. The development of an ER formulation, with stable pramipexole plasma concentration over 24 hours, now offers a bioequivalent once-daily alternative. Double-blind randomized controlled trials in early and advanced PD, have established noninferiority of pramipexole ER compared with immediate release as well as superiority of both formulations over placebo. The overnight switch from the standard to the once-daily formulation was shown to be successful in >80% of patients without requiring any dose adjustments. Potential benefits of the prolonged-release design, which have not yet been formally demonstrated in the pivotal trial program, include improved compliance and a potential for better symptomatic control, particularly in patients with early disease that can be managed with monotherapy.

The clinical spectrum of levodopa-induced motor complications

After more than 40 years of clinical use, levodopa (LD) still remains the gold standard for symptomatic efficacy in Parkinson's disease (PD). However, long-term treatment with LD is often complicated by the development of various types of motor response oscillations as well as drug-induced dyskinesias. These treatment-related motor complications evolve in approximately one-third of patients after only 2 years of LD exposure and, once established, they are difficult to treat and significantly contribute to overall disability and disease burden. Although first described soon after the introduction of LD, the pathophysiology of motor complications is still not completely understood. In fact, it is most likely that non-physiological pulsatile stimulation of dopamine receptors, which is followed by various downstream alterations, plays a key role in the development of LD-induced motor response oscillations and dyskinesias. This review outlines the various types of motor complications and will also address underlying mechanisms, treatment options, as well as impact on clinical disability and quality of life (QoL).

Co-administration of the partial dopamine D2 agonist terguride with L-dopa attenuates L-dopa-induced locomotor sensitization in hemiparkinsonian mice

While dopamine replacement remains the standard pharmacotherapy for Parkinson's disease, chronic L-dopa treatment is associated with development of debilitating motor fluctuations such as L-dopa-induced dyskinesia (LID). In this study we evaluated the effects of the partial dopamine D(2) agonist terguride on the development of LID in hemiparkinsonian mice (unilaterally lesioned with 6-hydroxydopamine). First, consistent with the partial agonist property, terguride had 1000-fold higher potency than dopamine, yet producing one-third level of maximal activation of dopamine, as assayed by [(35)S]GTPgammaS binding. Furthermore, in the absence and presence of dopamine in vitro, terguride increased and decreased striatal [(35)S]GTPgammaS binding, respectively. Next, we found that co-administration of terguride (at 0.1 and 0.5mg/kg, i.p.) with L-dopa (1.8 mg/kg) daily for 14 days, significantly attenuated the development and expression of L-dopa-induced rotational sensitization. Furthermore, the cross-challenge paradigm revealed that chronic L-dopa treatment (but not terguride) sensitized locomotor response to the dopamine D(1) agonist SKF 81297 while chronic treatment with terguride (but not L-dopa) produced sensitized locomotor responses to the adenosine A(2A) antagonist 8-(3-chlorostyryl)caffeine (CSC). Importantly, the co-administration of terguride with L-dopa did not show locomotor sensitization to either SFK 81297 or CSC upon challenge. Together, these results suggest that co-administration of partial dopamine D(2) agonists with L-dopa may prophylactically attenuate L-dopa-induced abnormal behavioral responses such as LID.

Differential gene expression in the striatum of mice with very low expression of the vesicular monoamine transporter type 2 gene

The vesicular monoamine transporter type 2 (VMAT2) packages pre-synaptic monoamines into vesicles. Previously, we generated mice hypomorphic for the VMAT2 gene (Slc18a2), which results in a approximately 95% reduction in VMAT2 protein, disrupted vesicular storage, severe depletion of striatal dopamine and mice with moderate motor behaviour deficits. Dopamine released from mid-brain dopamine neurons acts on post-synaptic type 1 (D1) and 2 (D2) receptors located on striatal medium spiny neurons to initiate a signalling cascade that leads to altered transcription factor activity, gene expression and neuronal activity. We investigated striatal gene expression changes in VMAT2hypo mice by quantitative real-time PCR and in situ hybridisation. Despite unaltered expression of D1 and D2 dopamine receptors, there were dramatic alterations in striatal mRNAs encoding the neuropeptides substance P, dynorphin, enkephalin and cholecystokinin. The promoters of these genes are regulated by a combination of transcription factors that includes cAMP responsive element binding protein-1 (CREB) and c-Fos. Indeed, the changes in peptide mRNAs were associated with elevated expression of Creb1 and c-Fos. These data indicate that striatal dopamine depletion, as a consequence of deficient vesicular storage in this mouse, triggers a complex program of gene expression, consistent with this mouse being an excellent model of Parkinson's disease.

Imaging Non-Dopaminergic Function in Parkinson’s Disease

In Parkinson's disease (PD), there is degeneration of the cholinergic, noradrenergic, and serotonergic systems in addition to dopaminergic projections. Function of these non-dopaminergic systems can be imaged with positron emission tomography (PET) and single photon emission computed tomography (SPECT) and correlated with motor and nonmotor symptomatology. In addition, neuronal loss in PD is associated with microglial activation. The role of microglia in driving the disease process remains uncertain. This review presents and discusses current findings in these areas.

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.

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.

Phenotype of striatofugal medium spiny neurons in parkinsonian and dyskinetic nonhuman primates: a call for a reappraisal of the functional organization of the basal ganglia

The classic view of anatomofunctional organization of the basal ganglia is that striatopallidal neurons of the "indirect" pathway express D2 dopamine receptors and corelease enkephalin with GABA, whereas striatopallidal neurons of the "direct" pathway bear D1 dopamine receptors and corelease dynorphin and substance P with GABA. Although many studies have investigated the pathophysiology of the basal ganglia after dopamine denervation and subsequent chronic levodopa (L-dopa) treatment, none has ever considered the possibility of plastic changes leading to profound reorganization and/or biochemical phenotype modifications of medium spiny neurons. Therefore, we studied the phenotype of striatal neurons in four groups of nonhuman primates, including the following: normal, parkinsonian, parkinsonian chronically treated with L-dopa without exhibiting dyskinesia, and parkinsonian chronically treated with L-dopa exhibiting overt dyskinesia. To identify striatal cells projecting to external (indirect) or internal (direct) segments of the globus pallidus, the retrograde tracer cholera toxin subunit B (CTb) was injected stereotaxically into the terminal areas. Using immunohistochemistry techniques, brain sections were double labeled for CTb and dopamine receptors, opioid peptides, or the substance P receptor (NK1). We also used HPLC-RIA to assess opioid levels throughout structures of the basal ganglia. Our results suggest that medium spiny neurons retain their phenotype because no variations were observed in any experimental condition. Therefore, it appears unlikely that dyskinesia is related to a phenotype modification of the striatal neurons. However, this study supports the concept of axonal collateralization of striatofugal cells that project to both globus pallidus pars externa and globus pallidus pars interna. Striatofugal pathways are not as segregated in the primate as previously considered.

Enhanced striatal opioid receptor-mediated G-protein activation in L-DOPA-treated dyskinetic monkeys

Long-term l-3,4-dihydroxyphenylalanine (L-DOPA) treatment in Parkinson's disease leads to dyskinesias in the majority of patients. The underlying molecular mechanisms for L-DOPA-induced dyskinesias (LIDs) are currently unclear. However, the findings that there are alterations in opioid peptide mRNA and protein expression and that opioid ligands modulate dyskinesias suggest that the opioid system may be involved. To further understand its role in dyskinesias, we mapped opioid receptor-stimulated G-protein activation using [35S]guanylyl-5'-O-(gamma-thio)-triphosphate ([35S]GTPgammaS) autoradiography in the basal ganglia of normal and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned squirrel monkeys administered water or L-DOPA. Subtype-selective opioid receptor G-protein coupling was investigated using the mu-opioid agonist [D-Ala, N-Me-Phe, Gly-ol]-enkephalin, delta-agonist SNC80 and kappa-agonist U50488H. Our data show that mu-opioid receptor-mediated G-protein activation is significantly enhanced in the basal ganglia and cortex of L-DOPA-treated dyskinetic monkeys, whereas delta- and kappa-receptor-induced increases were limited to only a few regions. A similar pattern of enhancement was observed in both MPTP-lesioned and unlesioned animals with LIDs suggesting the effect was not simply due to a compromised nigrostriatal system. Opioid receptor G-protein coupling was not enhanced in non-dyskinetic L-DOPA-treated animals, or lesioned monkeys not given L-DOPA. The increases in opioid-stimulated [35S]GTPgammaS binding are directly correlated with dyskinesias. The present data demonstrate an enhanced subtype-selective opioid-receptor G-protein coupling in the basal ganglia of monkeys with LIDs. The positive correlation with LIDs suggests this may represent an intracellular signaling mechanism underlying these movement abnormalities.

Neuroprotection induced by the adenosine A2A antagonist CSC in the 6-OHDA rat model of parkinsonism: effect on the activity of striatal output pathways

In Parkinson's disease (PD), the striatal dopamine depletion and the following overactivation of the indirect pathway of the basal ganglia leads to very early disinhibition of the subthalamic nucleus (STN) that may contribute to the progression of PD by glutamatergic overstimulation of the dopaminergic neurons in the substantia nigra. Adenosine A2A antagonism has been demonstrated to attenuate the overactivity of the striatopallidal pathway. To investigate whether neuroprotection exerted by the A2A antagonist 8-(3-chlorostyryl)caffeine (CSC) correlates with a diminution of the striatopallidal pathway activity, we have examined the changes in the mRNA encoding for enkephalin, dynorphin, and adenosine A2A receptors by in situ hybridization induced by subacute systemic pretreatment with CSC in rats with striatal 6-hydroxydopamine(6-OHDA) administration. Animals received CSC for 7 days until 30 min before 6-OHDA intrastriatal administration. Vehicle-treated group received a solution of dimethyl sulfoxide. CSC pretreatment partially attenuated the decrease in nigral tyrosine hydroxylase immunoreactivity induced by 6-OHDA, whereas no modification of the increase in preproenkephalin mRNA expression in the dorsolateral striatum was observed. The neuroprotective effect of the adenosine A2A antagonist CSC in striatal 6-OHDA-lesioned rats does not result from a normalization of the increase in striatal PPE mRNA expression in the DL striatum, suggesting that other different mechanisms may be involved.

Therapeutic potential of adenosine A2A receptor antagonists in Parkinson's disease

In the pursuit of improved treatments for Parkinson's disease (PD), the adenosine A(2A) receptor has emerged as an attractive nondopaminergic target. Based on the compelling behavioral pharmacology and selective basal ganglia expression of this G-protein-coupled receptor, its antagonists are now crossing the threshold of clinical development as adjunctive symptomatic treatment for relatively advanced PD. The antiparkinsonian potential of A(2A) antagonism has been boosted further by recent preclinical evidence that A(2A) antagonists might favorably alter the course as well as the symptoms of the disease. Convergent epidemiological and laboratory data have suggested that A(2A) blockade may confer neuroprotection against the underlying dopaminergic neuron degeneration. In addition, rodent and nonhuman primate studies have raised the possibility that A(2A) receptor activation contributes to the pathophysiology of dyskinesias-problematic motor complications of standard PD therapy--and that A(2A) antagonism might help prevent them. Realistically, despite being targeted to basal ganglia pathophysiology, A(2A) antagonists may be expected to have other beneficial and adverse effects elsewhere in the central nervous system (e.g., on mood and sleep) and in the periphery (e.g., on immune and inflammatory processes). The thoughtful design of new clinical trials of A(2A) antagonists should take into consideration these counterbalancing hopes and concerns and may do well to shift toward a broader set of disease-modifying as well as symptomatic indications in early PD.

Early administration of entacapone prevents levodopa-induced motor fluctuations in hemiparkinsonian rats

The purpose of this study was to investigate the effect of the catechol-O-methyltransferase (COMT) inhibitor, entacapone, in the reversal and prevention of "wearing-off" phenomena in hemiparkinsonian rats. Catechol-O-methyltransferase (COMT) inhibitors increase the half-life and bioavailability of levodopa, providing more continuous dopamine receptor stimulation. This raises the possibility of using levodopa and a COMT inhibitor not only to treat motor complications, but also to prevent their development. Male Sprague-Dawley rats received a unilateral 6-hydroxydopamine (6-OHDA) administration in the nigrostriatal pathway. Two sets of experiments were performed. First, animals were treated with levodopa (50 mg/kg/day with benserazide 12.5 mg/kg/day, twice daily (b.i.d.), intraperitoneally (i.p.) for 22 days. On day 23, animals received either entacapone (30 mg/kg, i.p.) or vehicle with each levodopa dose. In the second set, animals were treated either with levodopa (50 mg/kg/day, i.p.) plus entacapone (30 mg/kg/day, i.p.) or levodopa (50 mg/kg/day, i.p.) plus vehicle, administered two or three times daily [b.i.d. or thrice daily (t.i.d.), respectively] for 22 consecutive days. Entacapone both reversed and prevented the shortening of the motor response duration that defines "wearing-off" motor fluctuations. Entacapone also decreased the frequency of failures to levodopa. The combination of levodopa and entacapone may reduce the likelihood of motor fluctuation development and may thus become a valuable approach to treat Parkinson disease whenever levodopa is needed.

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.

Regulation of striatal preproenkephalin mRNA levels in MPTP-lesioned mice treated with estradiol

We reported previously the protective effect of 17beta-estradiol (17beta-E(2)) on 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced dopamine (DA) depletion. This protection was stereospecific, because 17beta-E(2) showed activity but 17alpha-estradiol (17alpha-E(2)) did not. The mechanisms by which estradiol exerts its beneficial effects, however, remain unknown. We investigated a possible implication of enkephalins (ENK) in neuroprotective activity of 17beta-E(2). Protection against MPTP-induced DA depletion was obtained with 17beta-E(2) but not 17alpha-E(2). MPTP lesion increased striatal preproenkephalin (PPE) mRNA levels and they remained elevated in 17alpha-E(2)-treated MPTP mice whereas 17beta-E(2) treatment decreased these levels to control values. This is the first report of estradiol modulation of striatal PPE mRNA in mice. Negative and significant correlations between DA levels, vesicular monoamine transporter (VMAT(2)) density, and PPE mRNA were observed in the striatum of lesioned animals. This effect of 17beta-E(2) on PPE mRNA after a lesion could be one of many mechanisms by which this steroid exerts its neuroprotective activity.

Alterations in expression of dopamine receptors and neuropeptides in the striatum of GTP cyclohydrolase-deficient mice

The hph-1 mice have defective tetrahydrobiopterin biosynthesis and share many neurochemical similarities with l-dopa-responsive dystonia (DRD) in humans. In both, there are deficiencies in GTP cyclohydrolase I and low brain levels of dopamine (DA). Striatal tyrosine hydroxylase (TH) levels are decreased while the number of DA neurones in substantia nigra (SN) appears normal. The hph-1 mouse is therefore a useful model in which to investigate the biochemical mechanisms underlying dystonia in DRD. In the present study, the density of striatal DA terminals and DA receptors and the expression of D-1, D-2, and D-3 receptors, preproenkephalin (PPE-A), preprotachykinin (PPT), and nitric oxide synthase (NOS) mRNAs in the striatum and nucleus accumbens and nigral TH mRNA expression were examined. Striatal DA terminal density as judged by specific [3H]mazindol binding was not altered while the levels of TH mRNA were elevated in the SN of hph-1 mice compared to control (C57BL) mice. Total and subregional analysis of the striatum and nucleus accumbens showed that D-2 receptor ([3H]spiperone) binding density was increased while D-1 receptor ([3H]SCH 23390) and D-3 receptor ([3H]7-OH-DPAT) binding density was not altered. In the striatum and nucleus accumbens, expression of PPT mRNA was elevated but PPE-A mRNA, D-1, D-2 receptor, and nNOS mRNA were not changed in hph-1 mice compared to controls. These findings suggest that an imbalance between the direct strionigral and indirect striopallidal output pathways may be relevant to the genesis of DRD. However, the pattern of changes observed is not that expected as a result of striatal dopamine deficiency and suggests that other effects of GTP cyclohydrolase I deficiency may be involved.

Striatal preprotachykinin gene expression reflects parkinsonian signs

Striatal preprotachykinin (PPT) gene expression was measured in MPTP-treated cats when symptomatic and during various stages of recovery from parkinsonism using in situ hybridization histochemistry. Animals expressing severe (1 week post-MPTP) or moderate (3 weeks post-MPTP) parkinsonian sensorimotor deficits had significantly reduced striatal PPT mRNA expression. In contrast, fully recovered animals (6 weeks post-MPTP) had striatal PPT mRNA levels that were not significantly different from normal. Thus, PPT gene expression in the striatum appears to reflect presence or absence of sensorimotor deficits in MPTP-treated cats.

Levodopa in the treatment of Parkinson's disease: Current controversies

Levodopa is the most effective symptomatic agent in the treatment of Parkinson's disease (PD) and the "gold standard" against which new agents must be compared. However, there remain two areas of controversy: (1) whether levodopa is toxic, and (2) whether levodopa directly causes motor complications. Levodopa is toxic to cultured dopamine neurons, and this may be a problem in PD where there is evidence of oxidative stress in the nigra. However, there is little firm evidence to suggest that levodopa is toxic in vivo or in PD. Clinical trials have not clarified this situation. Levodopa is also associated with motor complications. Increasing evidence suggests that they are related, at least in part, to the short half-life of the drug (and its potential to induce pulsatile stimulation of dopamine receptors) rather than to specific properties of the molecule. Treatment strategies that provide more continuous stimulation of dopamine receptors provide reduced motor complications in MPTP monkeys and PD patients. These studies raise the possibility that more continuous and physiological delivery of levodopa might reduce the risk of motor complications. Clinical trials to test this hypothesis are underway. We review current evidence relating to these areas of controversy.

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.

Effect of acute and chronic administration of U50,488, a kappa opioid receptor agonist, in 6-OHDA-lesioned rats chronically treated with levodopa

To evaluate the possible involvement of kappa opioid receptor-mediated mechanisms in levodopa-induced motor fluctuations, we have investigated the effects of U50,488, a selective kappa opioid agonist, on levodopa-induced motor alterations in rats with unilateral 6-OHDA lesion. Acute and chronic administration of U50,488 has been studied to evaluate the possible reversion or prevention of these levodopa effects. In a first set of experiments, rats were treated with levodopa (25 mg/kg with benserazide, twice daily, ip) for 22 days and, on Day 23 U50,488 (0.5, 1, or 3 mg/kg, i.p.) was administered immediately before levodopa. In a second set of experiments, rats were treated daily for 22 days with levodopa and U50,488 (1 or 3 mg/kg/day, i.p.). The duration of the rotational behavior induced by chronic levodopa decreased after 22 days (P < 0.05). Acute administration of U50,488 on Day 23 reversed this effect when low doses were administered (P < 0.05). Chronic U50,488 administration did not prevent the shortening in response duration induced by levodopa. Our results demonstrate that the kappa opioid receptor agonist U50,488 reverses but does not prevents levodopa-induced motor alterations in parkinsonian rats. These results suggest a role for kappa opioid receptor-mediated mechanisms in the pathophysiology of levodopa-induced motor response complications. These findings suggest that the stimulation of kappa opioid receptors might confer clinical benefit to parkinsonian patients under levodopa therapy suffering from motor complication syndrome.

Alterations in striatal neuropeptide mRNA produced by repeated administration of L-DOPA, ropinirole or bromocriptine correlate with dyskinesia induction in MPTP-treated common marmosets

Chronic administration of L-DOPA to MPTP-treated common marmosets induces marked dyskinesia while repeated administration of equivalent antiparkisonian doses of ropinirole and bromocriptine produces only mild involuntary movements. The occurrence of dyskinesia has been associated with an altered balance between the direct and indirect striatal output pathways. Using in situ hybridisation histochemistry, we now compare the effects of these drug treatments on striatal preproenkephalin-A (PPE-A) and adenosine A(2a) receptor mRNA expression as markers of the indirect pathway and striatal preprotachykinin (PPT) mRNA and preproenkephalin-B (PPE-B, prodynorphin) mRNA expression as markers of the direct pathway.The equivalent marked losses of specific [3H]mazindol binding in the striatum of all drug treatment groups confirmed the identical nature of the nigral cell loss produced by MPTP treatment. MPTP-induced destruction of the nigro-striatal pathway markedly increased the level of PPE-A mRNA in the caudate nucleus and putamen and decreased the levels of PPT and PPE-B mRNA relative to normal animals. Repeated treatment with L-DOPA for 30 days produced marked dyskinesia but had no effect on the MPTP-induced increase in PPE-A mRNA in the caudate nucleus and putamen. In contrast, L-DOPA treatment normalised the MPTP-induced decrease in the level of PPT and PPE-B mRNA. Repeated treatment with ropinirole produced little or no dyskinesia but markedly reversed the MPTP-induced elevation in PPE-A mRNA in the caudate nucleus and putamen. However, it had no effect on the decrease in PPT or PPE-B mRNA. Similarly, bromocriptine treatment which induced only mild dyskinesia attenuated the MPTP-induced elevation in PPE-A mRNA in the caudate nucleus and putamen with no effect on reduced striatal PPT or PPE-B mRNA. Neither MPTP treatment nor treatment with L-DOPA, bromocriptine or ropinirole had any effect on adenosine A(2a) receptor mRNA in the striatum. These patterns of alteration in striatal PPE-A and PPT and PPE-B mRNA produced by L-DOPA, bromocriptine and ropinirole show differential involvement of markers of the direct and indirect striatal output pathways related to improvement of locomotor activity and mirror the relative abilities of the drugs to induce dyskinesia.

Adenosine A2A antagonism reverses levodopa-induced motor alterations in hemiparkinsonian rats

To evaluate the possible involvement of adenosine A(2A) receptor-mediated mechanisms in levodopa-induced motor fluctuations, we investigated the effects of CSC (8-(3-chlorostryryl) caffeine), a selective adenosine A(2A) receptor antagonist, on levodopa-induced motor alterations in rats with unilateral 6-OHDA lesion. Acute and chronic administration of CSC was studied to evaluate the possible reversion or prevention of these levodopa effects. In a first set of experiments, rats were treated with levodopa (25 mg/kg with benserazide, twice daily, i.p.) for 22 days and on day 23 CSC (5 mg/kg, i.p.) was administered immediately before levodopa. In a second set of experiments, rats were treated daily for 22 days with levodopa and CSC (5 mg/kg/day, i.p.). The duration of the rotational behavior induced by chronic levodopa decreased after 22 days (P < 0.05). Acute administration of CSC on day 23 reversed levodopa-induced shortening in motor response duration (P < 0.01). Chronic CSC administration did not prevent the shortening in response duration induced by levodopa. Our results demonstrate that the adenosine A(2A) receptor antagonist CSC reverses but does not prevent levodopa-induced motor alterations in parkinsonian rats. These results suggest a role for adenosine A(2A) receptor-mediated mechanisms in the pathophysiology of levodopa-induced motor response complications. These findings suggest that the antagonism of adenosine A(2A) receptors might confer clinical benefit to parkinsonian patients under levodopa therapy suffering from motor complication syndrome.

Increases in striatal preproenkephalin gene expression are associated with nigrostriatal damage but not L-DOPA-induced dyskinesias in the squirrel monkey

Changes in preproenkephalin expression in the caudate and putamen have been linked to the development of L-3,4-dihydroxyphenylalanine (L-DOPA)-induced dyskinesias in primate models of Parkinson's disease, although not all investigators have been able to confirm this association. Because nigrostriatal damage per se is associated with increases in striatal preproenkephalin mRNA levels, it is difficult to know if changes in transcript levels are a result of lesioning or concurrent L-DOPA treatment and resulting dyskinesias. To circumvent these difficulties, we measured striatal preproenkephalin mRNA levels in monkeys with L-DOPA-induced dyskinesias both with and without lesions of the nigrostriatal system. The latter model is not confounded by morphological and biochemical changes resulting from nigrostriatal damage. Monkeys were gavaged with L-DOPA (15 mg/kg) twice daily for a 2-week period and killed 3 days after treatment. 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) treatment alone resulted in an increase in preproenkephalin mRNA levels as previously shown. However, striatal transcript levels were similarly elevated in dyskinetic MPTP-lesioned animals treated with L-DOPA. In unlesioned animals, preproenkephalin mRNA levels were also similar in control and L-DOPA-treated dyskinetic monkeys. Because drug-induced changes in mRNA may not be sustained for a prolonged period after treatment, a second series of experiments were done in which animals were killed 3-4 h after the last dose of L-DOPA, but the results were similar to those obtained after 3 days. These data show that, while elevations in striatal preproenkephalin mRNA levels are associated with nigrostriatal damage, they are not linked to the development of L-DOPA-induced dyskinesias. These results thus question the importance of preproenkephalin mRNA in the pathogenesis of this disabling complication of L-DOPA therapy in Parkinson's disease.

Repeated administration of piribedil induces less dyskinesia than L-dopa in MPTP-treated common marmosets: a behavioural and biochemical investigation

Piribedil ([1-(3,4-methylenedioxybenzyl)-4-(2-pyrimidinyl)piperazine]; S 4200) is a dopamine agonist with equal affinity for D(2)/D(3) dopamine receptors effective in treating Parkinson's disease as monotherapy or as an adjunct to levodopa (L-dopa). However, its ability to prime basal ganglia for the appearance of dyskinesia is unknown. We now report on the ability of repeated administration of piribedil to induce dyskinesia in drug naïve 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) -lesioned common marmosets compared with L-dopa and its actions on the direct and indirect striatal outflow pathways. Administration of piribedil (4.0-5.0 mg/kg orally) or L-dopa (12.5 mg/kg orally plus carbidopa 12.5 mg/kg orally twice daily) produced equivalent increases in locomotor activity and reversal of motor deficits over a 28-day study period. Administration of L-dopa resulted in the progressive development of marked dyskinesia over the period of study. In contrast, administration of piribedil produced a significantly lower degree and intensity of dyskinesia. Surprisingly, piribedil caused an increase in vigilance and alertness compared to L-dopa, which may relate to the recently discovered alpha(2)-noradrenergic antagonist properties of piribedil. The behavioural differences between piribedil and L-dopa are reflected in the biochemical changes associated with the direct striatal output pathway. Administration of L-dopa or piribedil did not reverse the MPTP-induced up-regulation of preproenkephalin A mRNA in rostral or caudal areas of the putamen or caudate nucleus. In contrast, administration of either piribedil or L-dopa reversed the downregulation of preprotachykinin mRNA induced by MPTP in rostral and caudal striatum. L-dopa, but not Piribedil, reversed the decrease in preproenkephalin B mRNA produced by MPTP treatment.

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.

Up-regulation of secretoneurin immunoreactivity and secretogranin II mRNA in rat striatum following 6-hydroxydopamine lesioning and chronic L-DOPA treatment

Destruction of the nigro-striatal pathway in Parkinson's disease and treatment with L-DOPA lead to persistent alterations in basal ganglia output pathways that are poorly characterised. Differential display mRNA analysis was used to study the effects of 6-hydroxydopamine-induced lesions of the medial forebrain bundle on gene expression in the rat striatum. One up-regulated cDNA identified in two independent groups of 6-hydroxydopamine-lesioned animals was cloned and sequence analysis showed 97% homology to secretogranin II. Differential up-regulation of secretogranin II following 6-hydroxydopamine lesioning was confirmed in a further group of 6-hydroxydopamine-lesioned rats using TaqMan real time quantitative reverse transcription-polymerase chain reaction. Following chronic L-DOPA treatment of 6-hydroxydopamine-lesioned rats, secretogranin II mRNA was further up-regulated to a similar degree to that observed for preproenkephalin A mRNA expression. Immunohistochemical analysis confirmed the increase in secretogranin II peptide levels in striatal neurones in 6-hydroxydopamine-lesioned rats following chronic L-DOPA treatment. The increase in secretogranin II mRNA occurring following destruction of the nigro-striatal pathway and chronic L-DOPA treatment may result in an increase in secretoneurin levels, which could be important for the regulation of striatal output pathways.

Mice with very low expression of the vesicular monoamine transporter 2 gene survive into adulthood: potential mouse model for parkinsonism

We have created a transgenic mouse with a hypomorphic allele of the vesicular monoamine transporter 2 (Vmat2) gene by gene targeting. These mice (KA1) have profound changes in monoamine metabolism and function and survive into adulthood. Specifically, these animals express very low levels of VMAT2, an endogenous protein which sequesters monoamines intracellularly into vesicles, a process that, in addition to being important in normal transmission, may also act to keep intracellular levels of the monoamine neurotransmitters below potentially toxic thresholds. Homozygous mice show large reductions in brain tissue monoamines, motor impairments, enhanced sensitivity to dopamine agonism, and changes in the chemical neuroanatomy of the striatum that are consistent with alterations in the balance of the striatonigral (direct) and striatopallidal (indirect) pathways. The VMAT2-deficient KA1 mice are also more vulnerable to the neurotoxic effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine in terms of nigral dopamine cell death. We suggest that the mice may be of value in examining, long term, the insidious damaging consequences of abnormal intracellular handling of monoamines. On the basis of our current findings, the mice are likely to prove of immediate interest to aspects of the symptomatology of parkinsonism. They may also, however, be of use in probing other aspects of monoaminergic function and dysfunction in the brain, the latter making important contributions to the pathogenesis of schizophrenia and addiction.

Expression of striatal preprotachykinin mRNA in symptomatic and asymptomatic 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-exposed monkeys is related to parkinsonian motor signs

Striatal preprotachykinin (PPT) gene expression and [(3)H]mazindol binding were examined in monkeys exposed to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Some animals (n = 5) became moderately to severely parkinsonian after receiving large doses of MPTP over 9-30 d and remained symptomatic for a relatively short time (3 weeks to 3 months; acutely symptomatic group). A second group of animals (n = 5) received low doses of MPTP (1.5-12 months), developed cognitive impairments but displayed no gross motor deficits (asymptomatic group), and were killed 3-12 months after their final dose of MPTP. Other animals became moderately to severely parkinsonian after receiving escalating doses of MPTP (>6 months; n = 4) or high doses of MPTP (<1 month; n = 1) and remained symptomatic for 2.5-5.75 years (chronically symptomatic group). All MPTP-treated animals had extensive losses of [(3)H]mazindol binding in dorsal striatal sensorimotor regions with asymptomatic animals generally having a lesser degree of damage. However, PPT mRNA levels differed sharply among treatment groups. Symptomatic animals (acutely and chronically parkinsonian) had significantly decreased PPT mRNA levels in most striatal regions. In asymptomatic animals, PPT mRNA expression was not significantly different from that measured in control animals, despite decreases in [(3)H]mazindol binding in some striatal regions of similar magnitude to those observed in symptomatic animals. These observations suggest that PPT gene expression may be directly related to expression of parkinsonian motor symptomatology regardless of duration of MPTP exposure, duration of the parkinsonism, or extent of dopamine denervation. These results imply that the direct striatal output circuit may have a greater contribution to expression of parkinsonian symptomatology than proposed previously.

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.

L-DOPA produces strong induction of c-fos messenger RNA in dopamine-denervated cortical and striatal areas of the common marmoset

Common marmosets (Callithrix jacchus) with near-complete unilateral 6-hydroxydopamine denervation of the dopaminergic input received a single injection of saline or L-DOPA (15mg/kg plus 6.25mg/kg benserazide). Using in situ hybridization, the effects of these treatments on c-fos messenger RNA expression in the cerebral cortex, the striatal complex and the external layer of the pallidum were studied. Moreover, receptor autoradiography was used to determine the levels of dopamine D(1) and D(2) receptors in these areas. In the cerebral cortex, animals treated with L-DOPA displayed a high expression of c-fos messenger RNA restricted to the dopamine-denervated hemisphere. No changes in the levels of cortical D(1) and D(2) receptors were found in the dopamine-denervated hemisphere. L-DOPA treatment also induced a strong expression of c-fos messenger RNA in the striatal complex in the dopamine-denervated hemisphere. The levels of striatal D(2), but not D(1), receptors were increased in the dopamine-denervated hemisphere. In the external pallidum, the major terminal region for D(2) dopamine receptor-containing striatal projection neurons, L-DOPA treatment induced c-fos messenger RNA expression in both the intact and the dopamine-denervated hemispheres.Thus, using c-fos messenger RNA as a biochemical marker of postsynaptic neuronal activation, these results provide evidence that near-complete dopamine depletion causes a profound supersensitization to L-DOPA treatment in the cerebral cortex and in the striatal complex, but not in the external layer of the pallidum, of the primate brain. The cortical response may be unique to the primate brain, but c-fos messenger RNA activation within the striatum has also been reported in the rodent. The effects of L-DOPA probably depend both on a direct activation of supersensitized dopamine receptors by dopamine produced in the few remaining, but hyperactive, dopaminergic nerve terminals and in serotonergic nerve terminals, as well as on indirect actions of L-DOPA related to activation of circuitries connecting cerebral cortex and basal ganglia structures. These results provide novel information on the mechanisms underlying L-DOPA's action in the cerebral cortex, striatum and external pallidum in a primate model of Parkinson's disease.

The role of the D(2) dopamine receptor (D(2)R) in A(2A) adenosine receptor (A(2A)R)-mediated behavioral and cellular responses as revealed by A(2A) and D(2) receptor knockout mice

The A(2A)R is largely coexpressed with D(2)Rs and enkephalin mRNA in the striatum where it modulates dopaminergic activity. Activation of the A(2A)R antagonizes D(2)R-mediated behavioral and neurochemical effects in the basal ganglia through a mechanism that may involve direct A(2A)R-D(2)R interaction. However, whether the D(2)R is required for the A(2A)R to exert its neural function is an open question. In this study, we examined the role of D(2)Rs in A(2A)R-induced behavioral and cellular responses, by using genetic knockout (KO) models (mice deficient in A(2A)Rs or D(2)Rs or both). Behavioral analysis shows that the A(2A)R agonist 2-4-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine reduced spontaneous as well as amphetamine-induced locomotion in both D(2) KO and wild-type mice. Conversely, the nonselective adenosine antagonist caffeine and the A(2A)R antagonist 8-(3-chlorostyryl)caffeine produced motor stimulation in mice lacking the D(2)R, although the stimulation was significantly attenuated. At the cellular level, A(2A)R inactivation counteracted the increase in enkephalin expression in striatopallidal neurons caused by D(2)R deficiency. Consistent with the D(2) KO phenotype, A(2A)R inactivation partially reversed both acute D(2)R antagonist (haloperidol)-induced catalepsy and chronic haloperidol-induced enkephalin mRNA expression. Together, these results demonstrate that A(2A)Rs elicit behavioral and cellular responses despite either the genetic deficiency or pharmacological blockade of D(2)Rs. Thus, A(2A)R-mediated neural functions are partially independent of D(2)Rs. Moreover, endogenous adenosine acting at striatal A(2A)Rs may be most accurately viewed as a facilitative modulator of striatal neuronal activity rather than simply as an inhibitory modulator of D(2)R neurotransmission.

Early changes in neuropeptide mRNA expression in the striatum following reserpine treatment

Chronic dopamine depletion produces neurochemical changes within the striatum as well as enhanced behavioral and metabolic responses to dopamine agonists. Changes in striatal neuropeptides have been consistently described, including increased expression of preproenkephalin mRNA and decreased expression of preprotachykinin and prodynorphin mRNA. Acute dopamine depletion following treatment with reserpine also produces enhanced behavioral and metabolic responses to agonist treatment which develop rapidly. In the present study, we used in situ hybridization histochemistry to investigate whether acute neurochemical changes occur following reserpine treatment. We evaluated neuropeptide mRNA expression in the striatum and nucleus accumbens at several time points from 6 to 120 h following single doses of reserpine and AMPT. The aim of these studies was to determine if changes in neuropeptide mRNA expression occur following acute dopamine depletion and whether such changes are specific to the striatum. Changes in striatal neuropeptide mRNA expression developed rapidly. Preproenkephalin mRNA expression by striatopallidal neurons was unchanged at 48 h, but increased by 44% at 120 h. Preprotachykinin mRNA expression in striatonigral neurons was increased at 6 h and then fell, with a maximal decrease of 45% at 48 h and partial recovery by 120 h. Prodynorphin mRNA expression was unchanged. Expression of preproenkephalin and preprotachykinin mRNA was also examined in subregions of the striatum and the nucleus accumbens. Expression of preproenkephalin mRNA was uniform in the striatum and higher in the core than the shell of the nucleus accumbens. Preprotachykinin mRNA expression in the striatum was higher in the lateral quadrants and was higher in the shell than in the core of the nucleus accumbens. The changes in neuropeptide mRNA following treatment with reserpine were only found in the striatum. These data provide further evidence for early alterations in neuronal function in the striatum following acute dopamine depletion and suggest that neuropeptide expression by striatonigral neurons may be more rapidly regulated in response to changes in dopamine levels.

GDNF reverses priming for dyskinesia in MPTP-treated, L-DOPA-primed common marmosets

Parkinson's disease (PD) is associated with a progressive loss of dopamine neurons in the substantia nigra and degeneration of dopaminergic terminals in the striatum. Although L-DOPA treatment provides the most effective symptomatic relief for PD it does not prevent the progression of the disease, and its long-term use is associated with the onset of dyskinesia. In rodent and primate studies, glial cell line-derived neurotrophic factor (GDNF) may prevent 6-OHDA- or MPTP-induced nigral degeneration and so may be beneficial in the treatment of PD. In this study, we investigate the effects of GDNF on the expression of dyskinesia in L-DOPA-primed MPTP-treated common marmosets, exhibiting dyskinesia. GDNF or saline was administered by two intraventricular injections, 4 weeks apart, to MPTP-treated, L-DOPA-treated common marmosets primed to exhibit dyskinesia. Prior to GDNF or saline administration, all animals displayed marked dyskinesia when treated with L-DOPA. GDNF administration produced a significant improvement in motor disability and, following the second injection of GDNF, a significant improvement in the locomotor activity was observed. Following the administration of L-DOPA there was a greater reversal of disability and a reduction in the intensity of L-DOPA-induced dyskinesia in GDNF-treated animals compared to saline-treated controls. However, there was no significant difference in L-DOPA's ability to increase locomotor activity between GDNF-treated and saline-treated animals. GDNF treatment caused a significant increase in the number of tyrosine hydroxylase-positive neurons in the substantia nigra, but no change in [(3)H]mazindol binding to dopamine terminals was found in the striatum of GDNF-treated animals compared to saline-treated controls. In GDNF-treated animals a small but significant reduction in enkephalin mRNA was observed in the caudate nucleus but not in the putamen or the nucleus accumbens. Substance P mRNA expression was equally reduced in the caudate nucleus and the putamen of the GDNF-treated animals but not in the nucleus accumbens. Intraventricular administration of GDNF improved MPTP-induced disability and reversed dopamine cell loss in the substantia nigra. GDNF also diminished L-DOPA-induced dyskinesia, which may relate to its ability to partly restore nigral dopaminergic transmission or to modify the activity of striatal output pathways.

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.

Alterations in preproenkephalin and adenosine-2a receptor mRNA, but not preprotachykinin mRNA correlate with occurrence of dyskinesia in normal monkeys chronically treated with L-DOPA

Chronic treatment with L-DOPA induces dyskinesia in patients with Parkinson's disease (PD) and 1-methyl-4-phenyl-1,2,3, 6-tetrahydropyridine (MPTP)-treated monkeys, but is not thought to do so in normal humans or primates. However, we have shown that chronic oral high dose L-DOPA administration, with the peripheral decarboxylase inhibitor, carbidopa and with or without the peripherally acting catechol-O-methyl transferase (COMT) inhibitor, entacapone, to normal macaque monkeys for 13 weeks induced dyskinesia in a proportion of animals. In the present study, in situ hybridization histochemistry was used to investigate the effect of chronic L-DOPA administration on the activity of the direct and indirect striatal output pathways by measuring striatal preprotachykinin (PPT), preproenkephalin-A (PPE-A) and adenosine-2a (A2a) receptor gene expression in these monkeys. Overall there was no significant difference in striatal PPT, PPE-A and A2a receptor mRNA levels between normal animals and all L-DOPA (plus carbidopa and/or entacapone)-treated animals irrespective of whether or not dyskinesia occurred. However, when the level of PPE-A and A2a receptor mRNA was analysed in eight monkeys displaying marked dyskinesias as a result of L-DOPA (plus carbidopa with or without entacapone) treatment, there was a significant increase in PPE-A and A2a receptor mRNA message levels in the striatum compared with animals receiving identical treatment, but displaying few or no involuntary movements, and compared with normal controls. There was no difference in striatal PPT mRNA levels in monkeys exhibiting severe dyskinesia compared with those showing little or no dyskinesia after L-DOPA treatment or to normal controls. These results suggest that prolonged L-DOPA treatment alone has no consistent effect on either the direct or indirect pathways, as judged by striatal PPT, PPE-A or A2a receptor mRNA levels in normal monkeys. However, in monkeys exhibiting marked dyskinesia resulting from chronic L-DOPA treatment, abnormal activity is detected in the indirect striato-pallidal output pathway, as judged by striatal PPE-A and A2a receptor mRNA levels, indicating an imbalance between the direct and indirect striatal pathway which may explain the emergence of dyskinesia in these animals.

Chronic L-DOPA treatment increases striatal cannabinoid CB1 receptor mRNA expression in 6-hydroxydopamine-lesioned rats

The effect of a unilateral 6-hydroxydopamine (6-OHDA) lesion of the left medial forebrain bundle and 3 weeks treatment with L-DOPA of normal and 6-OHDA lesioned rats on CB1r mRNA expression was investigated by in situ hybridization. A 6-OHDA lesion of nigrostriatal pathway alone, confirmed by the loss of nigral tyrosine hydroxylase mRNA, did not alter CB1r mRNA levels in the dopamine depleted striatum. Similarly, chronic L-DOPA treatment of normal rats had no effect on striatal CB1r mRNA expression. In contrast, chronic L-DOPA treatment of 6-OHDA-lesioned rats significantly increased CB1r mRNA expression in the denervated striatum. These results suggest that the CB1r activity may be altered by L-DOPA's action and this may be related to the treatment of Parkinson's disease.

Opiate receptor avidity is reduced bilaterally in rhesus monkeys unilaterally lesioned with MPTP

Opiate receptor avidity (unoccupied receptor density / the receptor dissociation constant), was measured in four animals with unilateral parkinsonian symptoms following MPTP (1-methyl-4-phenyl-1,2,3, 6-tetrahydropyridine) infusions into the internal carotid of one side, and nine normal controls with positron emission tomography (PET) and 6-deoxy-6-beta-[(18)F]fluoronaltrexone (cyclofoxy, CF), a mu- and kappa-opiate receptor antagonist. PET studies of 6-[(18)F]-L-fluoro-L-3,4-dihydroxyphenylalanine ([(18)F]-DOPA) in these parkinsonian animals, although documenting the primarily unilateral nature of the lesion, also demonstrated a milder loss of dopaminergic on the side opposite the infusion. Opiate receptor avidity was found to be reduced by 20-34% in the caudate, anterior putamen, thalamus, and amygdala of these primarily unilaterally MPTP-exposed animals, bilaterally with no statistically significant differences between the two sides. The affected regions are the same as those previously demonstrated to have a 30-35% loss in clinically recovered bilaterally MPTP-lesioned animals. These findings confirm that the opiate pathway can change in response to modest decreases in basal ganglia dopamine innervation. Thus, opiate pathway adaptation is likely to contribute to the dynamic changes in basal ganglia circuits that forestall the initial clinical manifestations of Parkinson's disease. In addition, opiate pathway(s) may contribute to the treatment responsiveness and progression of the disease either directly through effects on basal ganglia function or indirectly through effects on basal ganglia plasticity.

Levodopa-induced dyskinesias treated by pallidotomy

Pallidotomy has been reported to improve parkinsonian symptoms, but its effects on levodopa-induced dyskinesia (LID) have not been thoroughly examined. We describe here the results of stereotactic, unilateral, posteroventral pallidotomy on LID in 42 patients (22 women), who were followed for up to 9 months. Their mean age was 60. 6+/-9.3 (range: 40-74), age at onset was 46.1+/-9.1 (range: 24-46), and duration of symptoms was 14.5+/-5.3 (range: 4-25) years. Three months following pallidotomy, the percent time with dyskinesia decreased from 37.0 to 17.3 (P<0.0001) and the percent time the patients were 'on' with dyskinesias decreased even more, from 71.0 to 22.9 (P<0.0001). Furthermore, the number of patients with troublesome (moderate to violent) dyskinesia had decreased from 36 (86%) prior to surgery to only 5 (12%) after surgery. The mean unified Parkinson disease rating scale (UPDRS) scores for LID-related disability and pain decreased from 1.95 to 0.74 (P<0. 0001) and from 1.02 to 0.17 (P<0.0001), respectively. Since the pre- and post-pallidotomy daily levodopa dosage remained essentially the same, the improvement in LID could not be attributed to a reduction in levodopa. Surgery-related complications occurred in eight (19%) patients, but none of them had persistent disability as a result of these complications. We conclude that pallidotomy is an effective and safe procedure in the treatment of medically intractable LID.

Differential regulation of striatal preproenkephalin and preprotachykinin mRNA levels in MPTP-lesioned monkeys chronically treated with dopamine D1 or D2 receptor agonists

Studies in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned monkeys and in parkinsonian patients show elevated preproenkephalin (PPE) mRNA levels, unaltered by chronic L-DOPA therapy, whereas preprotachykinin (PPT) mRNA levels are decreased by the lesion and corrected by L-DOPA. The relative contributions of the dopamine D1 and D2 receptors for PPE mRNA regulation were investigated in the present study and compared with those for PPT mRNA. In situ hybridization was used to measure peptide mRNA levels in the striatum of MPTP cynomolgus monkeys after chronic 1-month treatment with the D1 agonist SKF-82958, administered subcutaneously in pulsatile or continuous mode, compared with the long-acting D2 agonist cabergoline. Normal as well as untreated MPTP animals were also studied. PPE mRNA levels were elevated in the caudate nucleus and putamen of untreated MPTP monkeys compared with control animals with a more pronounced increase in the lateral as compared with the medial part of both structures. PPT mRNA levels showed a rostrocaudal gradient, with higher values in the middle of the caudate-putamen and more so in the medial versus the lateral parts. PPT mRNA levels were decreased in the caudate and putamen of untreated MPTP monkeys compared with control animals, and this was observed in the middle and posterior parts of these brain areas. Elevated PPE and decreased PPT mRNA levels observed after MPTP exposure were corrected after treatment with cabergoline (0.25 mg/kg, every other day), a dose that had antiparkinsonian effects and did not give sustained dyskinesia. In contrast, elevated PPE mRNA levels observed in untreated MPTP monkeys were markedly increased by pulsatile administration of SKF-82958 (1 mg/kg, three times daily) in two monkeys in which the parkinsonian symptoms were improved and dyskinesias developed, whereas it remained close to control values in a third one that did not display dyskinesias despite a sustained improvement in disability; a shorter duration of motor benefit (wearing off) over time was observed in these three animals. By contrast, pulsatile administration of SKF-82958 corrected the decreased PPT level observed in untreated MPTP monkeys. Continuous treatment with SKF-82958 (equivalent daily dose) produced no clear antiparkinsonian and dyskinetic responses and did not alter the denervation-induced elevation of PPE or decrease of PPT mRNA levels. The present data suggest an opposite contribution of the dopamine D1 receptors (stimulatory) as compared with the dopamine D2 receptors (inhibitory) on PPE mRNA, whereas a similar stimulatory contribution of D1 or D2 receptors is observed for PPT mRNA. An increase in PPE expression could be involved in the induction of dyskinesias and wearing off, whereas our data do not support this link for PPT. The antiparkinsonian response was associated with a correction of the lesion-induced decrease of PPT.

Cellular distribution of adenosine A2A receptor mRNA in the primate striatum

The cellular expression of adenosine A2A receptor mRNA in the adult monkey and human striatum was examined by using single and double in situ hybridization with ribonucleotide probes. Analysis on adjacent sections demonstrated a homogeneous overlapping expression of adenosine A2A receptor and preproenkephalin A mRNAs throughout nucleus caudatus, putamen, and nucleus accumbens. By contrast, high expression of preproenkephalin A mRNA but no expression of adenosine A2A receptor mRNA was found in the nucleus basalis of Meynert. Double in situ hybridization demonstrated an extensive colocalization of adenosine A2A receptor and preproenkephalin A mRNAs in approximately 50% of the medium-sized spiny neurons of the monkey nucleus caudatus, putamen, and nucleus accumbens. A small number of neurons (4-12%) that contained adenosine A2A receptor mRNA but not preproenkephalin A mRNA was found along the ventral borders of the striatum. Virtually all adenosine A2A receptor mRNA-containing neurons co-expressed dopamine D2 receptor mRNA, whereas only very few adenosine A2A receptor mRNA containing neurons co-expressed dopamine D1 receptor or substance P mRNAs. In addition, a sub-population of adenosine A2A receptor mRNA-expressing neurons that also contained preproenkephalin A mRNA was found in the septum in monkeys. These results demonstrate that there is a high expression of adenosine A2A receptor mRNA in the primate striatum that is extensively co-localized with dopamine D2 receptor and preproenkephalin A mRNAs. It is concluded that adenosine A2A receptors are likely to be important for the parallel organization of primate striatal neurotransmission and that these receptors could be a target for drug therapy in Parkinson's disease.

Glutamatergic regulation of striatal peptide gene expression in rats

The mRNA levels encoding enkephalin and substance P were measured in the rat striatum following cortical ablation, blockade of N-methyl-D-aspartate (NMDA) receptors or inhibition of glutamate release by lamotrigine. Unilateral ablation of the cerebral cortex resulted in a decrease of substance P mRNA levels particularly in the rostral dorsolateral and dorsomedial striatum ipsilateral to the lesion. There was a similar trend for a reduction in levels of enkephalin mRNA. Continuous, intrastriatal infusion of the competitive NMDA receptor antagonist, 3-((+/-)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid, (CPP, 0.12 and 1.2microg/day) decreased both enkephalin mRNA and substance P mRNA in dose-dependent manner evenly throughout the striatum adjacent to the infusion site. Following subchronic administration of the presumed glutamate release inhibitor, lamotrigine (5 and 20mg/kg IP) there was no significant alterations in either enkephalin mRNA or substance P mRNA levels in the striatum. Both enkephalin mRNA and substance P mRNA expression in the rat striatum appear tonically stimulated through postsynaptic NMDA receptor mediated mechanisms. This contrasts with differential dopaminergic modulation of peptides in striatal output neurons.

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.

Alterations in opioid receptor binding in Parkinson's disease patients with levodopa-induced dyskinesias

Levodopa-induced dyskinesias remain a major challenge in the therapeutic management of Parkinson's disease (PD). Their etiology is unknown although dysfunction of striatal opioid transmission has been implicated in experimental models of PD. To determine whether the opioid system is involved in human dyskinetic PD, we measured in vivo opioid receptor binding in PD patients with and without levodopa-induced dyskinesias, using positron emission tomography (PET) and the opioid receptor ligand [11C]diprenorphine. Striatal and thalamic/occipital uptake ratios were calculated using a region of interest (ROI) approach. In addition, we used statistical parametric mapping (SPM) and images reflecting the volume of distribution of [11C]diprenorphine to assess changes in cerebral receptor binding on a voxel-by-voxel basis. By using the ROI approach, we found significantly reduced striatal and thalamic opioid binding in dyskinetic, but not in nondyskinetic, PD patients. The SPM approach confirmed reduced availability in these areas and, in addition, showed decreased cingulate and increased prefrontal opioid receptor binding in the dyskinetic patients. Our findings confirm that altered opioid transmission is part of the pathophysiology of levodopa-induced dyskinesias in PD and support further investigation into the role of opioid agents in the management of these involuntary movements.

Preproenkephalin mRNA expression in the caudate-putamen of MPTP monkeys after chronic treatment with the D2 agonist U91356A in continuous or intermittent mode of administration: comparison with L-DOPA therapy

The effect of chronic treatment with the D2 dopamine agonist U91356A or L-DOPA therapy on the regulation of preproenkephalin (PPE) mRNA was investigated in the caudate-putamen of previously drug-naive cynomolgus monkeys Macaca fascicularis rendered parkinsonian by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). In MPTP monkeys, pulsatile treatment with either L-DOPA or U91356A relieved parkinsonian symptoms but caused progressive sensitization to treatment and, as expected, induced choreic dyskinesias. In contrast, U91356A given in a continuous mode led to partial behavioral tolerance without appearance of dyskinesias. Using in situ hybridization histochemistry, lesioning was shown to produce elevation of PPE mRNA levels in the lateral and medial parts of the putamen and in the lateral part of the caudate nucleus compared to control animals at the three rostrocaudal regions analyzed. In general, no change of PPE mRNA levels were observed in the medial caudate after MPTP lesioning with or without L-DOPA or U91356A treatments in the three rostrocaudal regions measured except for an increase in the caudal part of L-DOPA-treated MPTP monkeys. In the putamen and lateral caudate nucleus, elevated PPE mRNA expression by MPTP generally was not corrected (or only partially corrected) by chronic L-DOPA treatment except for the rostral medial putamen where correction to control values was observed. In general, pulsatile administration of U91356A partially corrected the lesion-induced elevation of PPE mRNA levels in the putamen and lateral caudate nucleus whereas the correction was more pronounced and widespread when MPTP monkeys received the continuous administration of this drug. These results indicate that the mode of administration of a D2 dopamine receptor agonist, such as U91356A, although at a roughly equivalent dosage influences the extent of inhibition of the expression of PPE in the denervated striatum of monkeys. In addition, the general lack of correction of the MPTP-induced increase of PPE mRNA in the striatum of L-DOPA-treated monkeys compared to the decreases observed with the D2 agonist treatments suggest that the D1 agonist component of L-DOPA therapy opposes the D2 agonist activity. Hence, D1 receptor agonist activity would stimulate PPE mRNA expression whereas D2 receptor agonists inhibit the expression of this peptide. Increases in PPE expression in the striatum may be implicated in the induction of dyskinesias since both groups of treated MPTP monkeys displaying dyskinesias had elevated striatal PPE mRNA levels whereas the MPTP monkeys with the lowest striatal PPE mRNA levels developed tolerance without dyskinesias.