L-dopa reverses the elevated density of D2 dopamine receptors in Parkinson's diseased striatum

Boolean analysis shows a high proportion of dopamine D2 receptors interacting with adenosine A2A receptors in striatal medium spiny neurons of mouse and non-human primate models of Parkinson's disease

The antagonistic effect of adenosine on dopaminergic transmission in the basal ganglia indirect motor control pathway is mediated by dopamine D2 (D2R) and adenosine A2A (A2AR) receptors co-expressed on medium spiny striatal neurons. The pathway is unbalanced in Parkinson's disease (PD) and an A2AR blocker has been approved for use with levodopa in the therapy of the disease. However, it is not known whether the therapy is acting on individually expressed receptors or in receptors forming A2A-D2 receptor heteromers, whose functionality is unique. For two proteins prone to interact, a very recently developed technique, MolBoolean, allows to determine the number of proteins that are either non-interacting or interacting. After checking the feasibility of the technique and reliability of data in transfected cells and in striatal primary neurons, the Boolean analysis of receptors in the striatum of rats and monkeys showed a high percentage of D2 receptors interacting with the adenosine receptor, while, on the contrary, a significant proportion of A2A receptors do not interact with dopamine receptors. The number of interacting receptors increased when rats and monkeys were lesioned to become a PD model. The use of a tracer of the indirect pathway in monkeys confirmed that the data was restricted to the population of striatal neurons projecting to the GPe. The results are not only relevant for being the first study quantifying individual versus interacting G protein-coupled receptors, but also for showing that the D2R in these specific neurons, in both control and PD animals, is under the control of the A2AR. The tight adenosine/dopamine receptor coupling suggest benefits of early antiparkinsonian treatment with adenosine receptor blockers.

Recent advances in dopamine D2 receptor ligands in the treatment of neuropsychiatric disorders

Dopamine is a biologically active amine synthesized in the central and peripheral nervous system. This biogenic monoamine acts by activating five types of dopamine receptors (D_1-5 Rs), which belong to the G protein-coupled receptor family. Antagonists and partial agonists of D₂ Rs are used to treat schizophrenia, Parkinson's disease, depression, and anxiety. The typical pharmacophore with high D₂ R affinity comprises four main areas, namely aromatic moiety, cyclic amine, central linker and aromatic/heteroaromatic lipophilic fragment. From the literature reviewed herein, we can conclude that 4-(2,3-dichlorophenyl), 4-(2-methoxyphenyl)-, 4-(benzo[b]thiophen-4-yl)-1-substituted piperazine, and 4-(6-fluorobenzo[d]isoxazol-3-yl)piperidine moieties are critical for high D₂ R affinity. Four to six atoms chains are optimal for D₂ R affinity with 4-butoxyl as the most pronounced one. The bicyclic aromatic/heteroaromatic systems are most frequently occurring as lipophilic appendages to retain high D₂ R affinity. In this review, we provide a thorough overview of the therapeutic potential of D₂ R modulators in the treatment of the aforementioned disorders. In addition, this review summarizes current knowledge about these diseases, with a focus on the dopaminergic pathway underlying these pathologies. Major attention is paid to the structure, function, and pharmacology of novel D₂ R ligands, which have been developed in the last decade (2010-2021), and belong to the 1,4-disubstituted aromatic cyclic amine group. Due to the abundance of data, allosteric D₂ R ligands and D₂ R modulators from patents are not discussed in this review.

Biophysical Modeling of Dopaminergic Denervation Landscapes in the Striatum Reveals New Therapeutic Strategy

Parkinson’s disease (PD) results from a loss of dopaminergic neurons. What triggers the break-down of neuronal signaling, and how this might be compensated, is not understood. The age of onset, progression and symptoms vary between patients, and our understanding of the clinical variability remains incomplete. In this study, we investigate this, by characterizing the dopaminergic landscape in healthy and denervated striatum, using biophysical modeling. Based on currently proposed mechanisms, we model three distinct denervation patterns, and show how this affect the dopaminergic network. Depending on the denervation pattern, we show how local and global differences arise in the activity of striatal neurons. Finally, we use the mathematical formalism to suggest a cellular strategy for maintaining normal dopamine (DA) signaling following neuronal denervation. This strategy is characterized by dual enhancement of both the release and uptake capacity of DA in the remaining neurons. Overall, our results derive a new conceptual framework for the impaired dopaminergic signaling related to PD and offers testable predictions for future research directions.

Neural Basis of Increased Cognitive Control of Impulsivity During the Mid-Luteal Phase Relative to the Late Follicular Phase of the Menstrual Cycle

Hormonal changes across the menstrual cycle have been shown to influence reward-related motivation and impulsive behaviors. Here, with the aim of examining the neural mechanisms underlying cognitive control of impulsivity, we compared event-related monetary delay discounting task behavior and concurrent functional magnetic resonance imaging (fMRI) revealed brain activity as well as resting state (rs)-fMRI activity, between women in the mid-luteal phase (LP) and women in the late follicular phase (FP). The behavioral data were analyzed and related to neural activation data. In the delay discounting task, women in the late FP were more responsive to short-term rewards (i.e., showed a greater discount rate) than women in the mid-LP, while also showing greater activity in the dorsal striatum (DS). Discount rate (transformed k) correlated with functional connectivity between the DS and dorsal lateral prefrontal cortex (dlPFC), consistent with previous findings indicating that DS-dlPFC circuitry may regulate impulsivity. Our rs-fMRI data further showed that the right dlPFC was significantly more active in the mid-LP than in late FP, and this effect was sensitive to absolute and relative estradiol levels during the mid-LP. DS-dlPFC functional connectivity magnitude correlated negatively with psychometric impulsivity scores during the late FP, consistent with our behavioral data and further indicating that relative estradiol levels may play an important role in augmenting cognitive control. These findings provide new insight into the treatment of conditions characterized by hyper-impulsivity, such as obsessive compulsive disorder, Parkinson disease, and attention deficit hyperactivity disorder. In conclusion, our results suggest that cyclical gonadal hormones affect cognitive control of impulsive behavior in a periodic manner, possibility via DS-dlPFC circuitry.

An integrative model of Parkinson's disease treatment including levodopa pharmacokinetics, dopamine kinetics, basal ganglia neurotransmission and motor action throughout disease progression

Levodopa is considered the gold standard treatment of Parkinson's disease. Although very effective in alleviating symptoms at their onset, its chronic use with the progressive neuronal denervation in the basal ganglia leads to a decrease in levodopa's effect duration and to the appearance of motor complications. This evolution challenges the establishment of optimal regimens to manage the symptoms as the disease progresses. Based on up-to-date pathophysiological and pharmacological knowledge, we developed an integrative model for Parkinson's disease to evaluate motor function in response to levodopa treatment as the disease progresses. We combined a pharmacokinetic model of levodopa to a model of dopamine's kinetics and a neurocomputational model of basal ganglia. The parameter values were either measured directly or estimated from human and animal data. The concentrations and behaviors predicted by our model were compared to available information and data. Using this model, we were able to predict levodopa plasma concentration, its related dopamine concentration in the brain and the response performance of a motor task for different stages of disease.

Dopamine-Induced Changes in Gαolf Protein Levels in Striatonigral and Striatopallidal Medium Spiny Neurons Underlie the Genesis of l-DOPA-Induced Dyskinesia in Parkinsonian Mice

The dopamine precursor, l-3,4-dihydroxyphenylalanine (l-DOPA), exerts powerful therapeutic effects but eventually generates l-DOPA-induced dyskinesia (LID) in patients with Parkinson’s disease (PD). LID has a close link with deregulation of striatal dopamine/cAMP signaling, which is integrated by medium spiny neurons (MSNs). Olfactory type G-protein α subunit (Gα_olf), a stimulatory GTP-binding protein encoded by the GNAL gene, is highly concentrated in the striatum, where it positively couples with dopamine D₁ (D₁R) receptor and adenosine A_2A receptor (A_2AR) to increase intracellular cAMP levels in MSNs. In the striatum, D₁Rs are mainly expressed in the MSNs that form the striatonigral pathway, while D₂Rs and A_2ARs are expressed in the MSNs that form the striatopallidal pathway. Here, we examined the association between striatal Gα_olf protein levels and the development of LID. We used a hemi-parkinsonian mouse model with nigrostriatal lesions induced by 6-hydroxydopamine (6-OHDA). Using quantitative immunohistochemistry (IHC) and a dual-antigen recognition in situ proximity ligation assay (PLA), we here found that in the dopamine-depleted striatum, there appeared increased and decreased levels of Gα_olf protein in striatonigral and striatopallidal MSNs, respectively, after a daily pulsatile administration of l-DOPA. This leads to increased responsiveness to dopamine stimulation in both striatonigral and striatopallidal MSNs. Because Gα_olf protein levels serve as a determinant of cAMP signal-dependent activity in striatal MSNs, we suggest that l-DOPA-induced changes in striatal Gα_olf levels in the dopamine-depleted striatum could be a key event in generating LID.

Multivalent approaches and beyond: novel tools for the investigation of dopamine D2 receptor pharmacology

The dopamine D2 receptor (D2R) has been implicated in the symptomology of disorders such as schizophrenia and Parkinson's disease. Multivalent ligands provide useful tools to investigate emerging concepts of G protein-coupled receptor drug action such as allostery, bitopic binding and receptor dimerization. This review focuses on the approaches taken toward the development of multivalent ligands for the D2R recently and highlights the challenges associated with each approach, their utility in probing D2R function and approaches to develop new D2R-targeting drugs. Furthermore, we extend our discussion to the possibility of designing multitarget ligands. The insights gained from such studies may provide the basis for improved therapeutic targeting of the D2R.

Levodopa-Induced Dyskinesia: Facts and Fancy. What Does the MPTP Monkey Model Tell Us?

Levodopa-induced dyskinesia, one of the most frequent long-term side effects of antiparkinsonian therapy, is often attributed to denervation supersensitivity of dopamine receptors and perhaps more specifically the D-1 receptor. The available evidence based not only on clinico-pathological studies in patients but also on results of experiments performed on methyl-phenyl-tetrahydropyridine (MPTP)-treated monkeys suggests that the mechanisms may be more complex than heretofore believed. Thus it appears that no single receptor is the sole culprit, that some form of denervation supersensitivity is probably involved but not in the form of increased density of dopamine receptors. Moreover, other neurotransmitter systems must be considered such as GABA, excitatory aminoacids and peptides. The MPTP monkey model remains very useful for predicting the potential of new drugs for inducing dyskinesia. Such trials however must be performed in drug-naive animals.

New therapeutic strategies targeting D1-type dopamine receptors for neuropsychiatric disease

The neurotransmitter dopamine acts via two major classes of receptors, D1-type and D2-type. D1 receptors are highly expressed in the striatum and can also be found in the cerebral cortex. Here we review the role of D1 dopamine signaling in two major domains: L-DOPA-induced dyskinesias in Parkinson's disease and cognition in neuropsychiatric disorders. While there are many drugs targeting D2-type receptors, there are no drugs that specifically target D1 receptors. It has been difficult to use selective D1-receptor agonists for clinical applications due to issues with bioavailability, binding affinity, pharmacological kinetics, and side effects. We propose potential therapies that selectively modulate D1 dopamine signaling by targeting second messengers downstream of D1 receptors, allosteric modulators, or by making targeted modifications to D1-receptor machinery. The development of therapies specific to D1-receptor signaling could be a new frontier in the treatment of neurological and psychiatric disorders.

Convenient synthesis of 18F-radiolabeled R-(-)-N-n-propyl-2-(3-fluoropropanoxy-11-hydroxynoraporphine

Aporphines are attractive candidates for imaging D2 receptor function because, as agonists rather than antagonists, they are selective for the receptor in the high affinity state. In contrast, D2 antagonists do not distinguish between the high and low affinity states, and in vitro data suggests that this distinction may be important in studying diseases characterized by D2 dysregulation, such as schizophrenia and Parkinson's disease. Accordingly, MCL-536 (R-(-)-N-n-propyl-2-(3-[(18)F]fluoropropanoxy-11-hydroxynoraporphine) was selected for labeling with (18)F based on in vitro data obtained for the non-radioactive ((19)F) compound. Fluorine-18-labeled MCL-536 was synthesized in 70% radiochemical yield, >99% radiochemical purity, and specific activity of 167 GBq/µmol (4.5 Ci/µmol) using p-toluenesulfonyl (tosyl) both as a novel protecting group for the phenol and a leaving group for the radiofluorination.

Dopamine-transporter levels drive striatal responses to apomorphine in Parkinson's disease

Dopaminergic therapy in Parkinson's disease (PD) can improve some cognitive functions while worsening others. These opposite effects might reflect different levels of residual dopamine in distinct parts of the striatum, although the underlying mechanisms remain poorly understood. We used functional magnetic resonance imaging (fMRI) to address how apomorphine, a potent dopamine agonist, influences brain activity associated with working memory in PD patients with variable levels of nigrostriatal degeneration, as assessed via dopamine-transporter (DAT) scan. Twelve PD patients underwent two fMRI sessions (Off-, On-apomorphine) and one DAT-scan session. Twelve sex-, age-, and education-matched healthy controls underwent one fMRI session. The core fMRI analyses explored: (1) the main effect of group; (2) the main effect of treatment; and (3) linear and nonlinear interactions between treatment and DAT levels. Relative to controls, PD-Off patients showed greater activations within posterior attentional regions (e.g., precuneus). PD-On versus PD-Off patients displayed reduced left superior frontal gyrus activation and enhanced striatal activation during working-memory task. The relation between DAT levels and striatal responses to apomorphine followed an inverted-U-shaped model (i.e., the apomorphine effect on striatal activity in PD patients with intermediate DAT levels was opposite to that observed in PD patients with higher and lower DAT levels). Previous research in PD demonstrated that the nigrostriatal degeneration (tracked via DAT scan) is associated with inverted-U-shaped rearrangements of postsynaptic D2-receptors sensitivity. Hence, it can be hypothesized that individual differences in DAT levels drove striatal responses to apomorphine via D2-receptor-mediated mechanisms.

Motivational properties of D2 and D3 dopamine receptors agonists and cocaine, but not with D1 dopamine receptors agonist and L-dopa, in bilateral 6-OHDA-lesioned rat

Dopamine dysregulation syndrome in Parkinson's disease (PD) has been attributed to dopamine replacement therapy (DRT). We hypothesize that DRT can induce a potential rewarding effect in an animal model of PD. Using the conditioned place preference (CPP) paradigm, we investigated the motivational effects of L-dopa, dopamine receptor agonists (DRAs), and cocaine in rat with a bilateral 6-OHDA lesion of the nigrostriatal dopaminergic pathway. In 6-OHDA animals, D1 receptors agonist (SKF81297) revealed significantly a conditioned place aversion (CPA) at 3 mg/kg and 9 mg/kg doses. D2 receptors agonist (bromocriptine) induced both CPP and CPA at 1 mg/kg and 10 mg/kg doses respectively. D3 receptors agonist (PD128907) induced a CPP only at 1 mg/kg, comparable to that of cocaine. Sham animals revealed biphasic CPP curves, with significant dose effect, for the intermediate dose of the 3 DRAs. However, L-dopa induced no significant effect while cocaine induced CPP in both lesioned and sham animals. In conclusion, this study confirms the predominant roles of D2R class, and most specifically D3R subtypes, in rewarding properties of DRT.

Postanoxic akinesia with bilateral pallidal lesions: a PET study

A 70-year-old woman developed marked akinesia after an anoxic event related to bronchiectasia. Magnetic resonance imaging studies revealed lesions in the bilateral globus pallidus and, to a lesser extent, in the putamen. Positron emission tomography studies with (18)F-6-fluoro-L-dopa and (11)C-N-methylspiperone showed a decreased pre- and post-synaptic uptake in the striatum. Consistent with previous reports, the present case demonstrated the basal ganglia, particularly the globus pallidus, to be selectively susceptible to anoxic insults. Furthermore, a PET study indicated a disrupted presynaptic integrity of the dopaminergic terminals and decreased dopamine D(2) receptor binding, which together appear to underlie the pathophysiology of post-anoxic akinesia, at least in the present case.

L-DOPA: a scapegoat for accelerated neurodegeneration in Parkinson's disease?

There is consensus that amelioration of the motor symptoms of Parkinson's disease is most effective with L-DOPA (levodopa). However, this necessary therapeutic step is biased by an enduring belief that L-DOPA is toxic to the remaining substantia nigra dopaminergic neurons by itself, or by specific metabolites such as dopamine. The concept of L-DOPA toxicity originated from pre-clinical studies conducted mainly in cell culture, demonstrating that L-DOPA or its derivatives damage dopaminergic neurons due to oxidative stress and other mechanisms. However, the in vitro data remain controversial as some studies showed neuroprotective, rather than toxic action of the drug. The relevance of this debate needs to be considered in the context of the studies conducted on animals and in clinical trials that do not provide convincing evidence for L-DOPA toxicity in vivo. This review presents the current views on the pathophysiology of Parkinson's disease, focusing on mitochondrial dysfunction and oxidative/proteolytic stress, the factors that can be affected by L-DOPA or its metabolites. We then critically discuss the evidence supporting the two opposing views on the effects of L-DOPA in vitro, as well as the animal and human data. We also address the problem of inadequate experimental models used in these studies. L-DOPA remains the symptomatic 'hero' of Parkinson's disease. Whether it contributes to degeneration of nigral dopaminergic neurons, or is a 'scapegoat' for explaining undesirable or unexpected effects of the treatment, remains a hotly debated topic.

Functional neuroimaging in Parkinson's disease

INTRODUCTION

Functional neuroimaging techniques have greatly contributed to improving our understanding of Parkinson's disease (PD) neurodegeneration and related compensatory mechanisms.

AREAS COVERED

In this paper, the authors analyze the role of functional neuroimaging as a diagnostic tool in PD and review functional neuroimaging studies on PD progression and compensatory adaptations. Through this, the article provides the reader with sensible approaches for the use of functional neuroimaging in the diagnosis of PD. The reader is also provided with knowledge on the time course of nigrostriatal dopamine dysfunction in PD as well as an overview of the potential beneficial and deleterious effects of increased dopamine turnover. Finally, the reader is provided with a critical discussion of the differential effects of levodopa and dopamine agonists on presynaptic dopamine markers and the implications for the interpretation of clinical trials.

EXPERT OPINION

Functional neuroimaging probably plays a limited role in the diagnosis of PD. Parkinson's disease pathology leads to an exponential decline in nigrostriatal dopamine function and a compensatory increase in dopamine turnover, which may help delay symptom onset. On the negative side, increased dopamine turnover contributes to the development of treatment-related motor complications. Presynaptic markers of dopamine function are subject to regulatory changes, compromising the direct interpretation of neuroimaging results in trials of neuroprotective therapies for PD.

Measurement of density and affinity for dopamine D(2) receptors by a single positron emission tomography scan with multiple injections of [(11)C]raclopride

Positron emission tomography (PET) with [(11)C]raclopride has been used to investigate the density (B(max)) and affinity (K(d)) of dopamine D(2) receptors related to several neurological and psychiatric disorders. However, in assessing the B(max) and K(d), multiple PET scans are necessary under variable specific activities of administered [(11)C]raclopride, resulting in a long study period and unexpected physiological variations. In this paper, we have developed a method of multiple-injection graphical analysis (MI-GA) that provides the B(max) and K(d) values from a single PET scan with three sequential injections of [(11)C]raclopride, and we validated the proposed method by performing numerous simulations and PET studies on monkeys. In the simulations, the three-injection protocol was designed according to prior knowledge of the receptor kinetics, and the errors of B(max) and K(d) estimated by MI-GA were analyzed. Simulations showed that our method could support the calculation of B(max) and K(d), despite a slight overestimation compared with the true magnitudes. In monkey studies, we could calculate the B(max) and K(d) of diseased or normal striatum in a 150 mins scan with the three-injection protocol of [(11)C]raclopride. Estimated B(max) and K(d) values of D(2) receptors in normal or partially dopamine-depleted striatum were comparable to the previously reported values.

Effect of menstrual cycle phase on dopamine D2 receptor availability in female cynomolgus monkeys

Sex differences have been reported in a variety of affective and neurodegenerative disorders that involve dysfunctional dopamine (DA) neurotransmission. In addition, there is evidence for differences in sensitivity to the abuse-related effects of psychostimulants across the menstrual cycle which may result from effects of ovarian hormones on DA function. The goal of the present study was to extend previous work examining menstrual cycle-related changes in DA D2 receptor availability in humans to drug-naive female cynomolgus monkeys (n=7) using the selective D2-like receptor ligand [(18)F]fluoroclebopride (FCP) and a high-resolution microPET P4 scanner. Menstrual cycle phase was characterized by daily vaginal swabs and measurements of serum progesterone levels. PET studies were conducted once during the luteal phase and once during the follicular phase. Regions of interest in the caudate nucleus, putamen, and cerebellum were defined on coregistered MRIs. Distribution volumes were calculated for FCP in each structure and the distribution volume ratio (DVR) for both brain regions relative to the cerebellum was used as a measure of D2 receptor availability. FCP DVRs were significantly higher in the luteal phase compared to the follicular phase in both the caudate nucleus (11.7% difference, p=0.02) and putamen (11.6% difference, p=0.03). These findings extend earlier work in humans and suggest that changes in DA receptor availability may be involved in the variation in symptoms of various neuropsychiatric disorders across the menstrual cycle, including differences in sensitivity to the abuse-related effects of stimulants.

Upregulation of dopamine D2 receptors in dopaminergic drug-naive patients with Parkin gene mutations

Medicated patients with Parkinsonism and parkin gene mutations have been reported to show a significant decrease in striatal dopamine D2 receptors (D2R) in comparison to medicated idiopathic Parkinson's disease (IPD) patients with similar age and disease severity. The aim of this study was to verify whether the genetic defect per se is responsible for this decrease. We have studied with [11C]raclopride (RAC) positron emission tomography (PET) in a group of 14 sporadic patients with parkin-linked Parkinsonism, 6 of whom had never received levodopa or dopamine agonists. The remaining 8 patients had been treated with levodopa for at least 5 years. Presynaptic striatal [18F]dopa storage was not significantly different between these two groups of patients. In untreated parkin-positive patients, significant putaminal increases in RAC-binding potential (BP) were found in comparison to an age-matched healthy control group by using a classical region of interest approach and statistical parametric mapping. In contrast, levodopa-treated parkin-positive patients showed significant decreases in RAC-BP in the caudate and putamen when compared to an age-matched healthy control group. The RAC PET findings revealed that striatal D2R upregulation occurs in dopaminergic drug-naive parkin-positive patients, in a similar fashion to the upregulation reported in drug-naive IPD. D2R downregulation observed in medicated parkin-positive patients, therefore, is not caused primarily by the genetic defect itself. Parkin-positive patients appear to have a greater susceptibility to the exposure to dopaminergic medication than IPD patients, which in turn might be an indirect effect of their genetic mutation.

Combination of dopamine transporter and D2 receptor SPECT in the diagnostic evaluation of PD, MSA, and PSP

It is often difficult to differentiate clinically between Parkinson's disease (PD), multiple system atrophy (MSA), and progressive supranuclear palsy (PSP). The objective of this work was to investigate whether combined pre- and postsynaptic dopaminergic single photon emission computed tomography (SPECT) scanning can reliably demonstrate changes in the nigrostriatal dopaminergic system and help differentiate between normal controls, PD, MSA, and PSP patients. We performed SPECT evaluation of the dopamine transporter (DAT) and dopamine D2 receptors (D2). SPECT scans using [123I]beta-CIT (for DAT) and [123I]IBF (for D2) were performed in 18 patients with PD (12 dopa-naïve and 6 on levodopa and/or dopamine agonists), 7 with MSA of the striatonigral degeneration type, 6 with PSP, and 29 normal controls. Antiparkinsonian drugs were withheld for at least 12 hours before the scans. DAT and D2 binding potentials (Rv = V3/V2) were measured for caudate, anterior, and posterior putamen on the sides ipsilateral and contralateral to the worst motor symptoms. DAT binding in the posterior putamen was markedly reduced in all patients. However, D2 binding in posterior putamen was significantly increased in dopa-untreated PD, being greater than the normal range in 4 of 12 (33%), and it was significantly reduced in MSA, being below the normal range in 5 of 7 (71%). None of the patients with PD showed reduced D2 binding below the normal range in posterior putamen. The degree of DAT binding could not discriminate between the patient groups. The ratio of posterior putamen to caudate percentage D2 Rv compared with the controls showed an opposite pattern between PD or PSP and MSA; the caudate was greater in 16 of 18 with PD and 6 of 6 with PSP, whereas caudate was less in 5 of 7 with MSA. These findings suggest that DAT SPECT may be useful in differentiating parkinsonism from controls and D2 SPECT in further differentiating MSA from Parkinson's disease and possibly PSP.

Subthalamic nucleus stimulation for Parkinson disease: benefits observed in levodopa-intolerant patients

OBJECT

A blinded evaluation of the effects of subthalamic nucleus (STN) stimulation was performed in levodopa-intolerant patients with Parkinson disease (PD). These patients (Group I, seven patients) were moderately or severely disabled (Hoehn and Yahr Stages III-V during the off period), but were receiving only a small dose of medication (levodopa-equivalent dose [LED] 0-400 mg/day) because they suffered unbearable side effects. The results were analyzed in comparison with those obtained in patients with advanced PD (Group II, seven patients) who were severely disabled (Hoehn and Yahr Stages IV and V during the off period), but were treated with a large dose of medication (500-990 mg/day).

METHODS

The patients were evaluated twice at 6 to 8 months after surgery. To determine the actual benefits afforded by STN stimulation to their overall daily activities, the patients were maintained on their medication regimen with optimal doses and schedules. Stimulation was turned off overnight for at least 12 hours. It was turned on in the morning (or remained turned off), and each patient's best and worst scores on the Unified Parkinson's Disease Rating Scale during waking daytime activity were recorded as on- and off-period scores, respectively. The order of assessment with respect to whether stimulation was occurring was determined randomly. The STN stimulation markedly improved daily activity and total motor scores in Group I patients. The percentage time of immobility (Hoehn and Yahr Stages IV and V) became 0% in patients who were intermittently immobile while not receiving stimulation. Improvements were demonstrated in tremor, rigidity. akinesia, and gait subscores. The STN stimulation produced less marked but still noticeable improvements in the daily activity and total motor scores in Group II patients. The percentage time of immobility as well as the LED was reduced in patients who displayed intermittent immobility with pronounced motor fluctuations while not receiving stimulation. Improvements were demonstrated in tremor, rigidity, and dyskinesia subscores in these patients. In contrast, STN stimulation did not improve the overall daily activities at all in patients who had become unresponsive to a tolerable dose of levodopa and were continuously immobile, even though these patients' tremor and rigidity subscores were still improved by stimulation.

CONCLUSIONS

Consistent with earlier findings, the great benefit of STN stimulation in levodopa-intolerant patients is that STN stimulation can reduce the level of required levodopa medication. This suggests that STN stimulation could be a therapeutic option for patients with less-advanced PD by allowing levodopa medication to be maintained at as low a dose as possible, and to prevent adverse reactions to the continued use of large-dose levodopa.

The use of pramipexole in Parkinson's disease: are its actions D(3) mediated?

Pramipexole is a non-ergot dopamine agonist recently approved for the treatment of early and advanced Parkinson's disease (PD). It has preferential affinity for the D(3) dopamine receptor, compared to previous dopamine agonists that have higher affinity for D(2) dopamine receptors. The ultimate question is whether its efficacy is linked to its action at the D(3) dopamine site or due to its binding to D(2) dopamine receptors. There is no direct experimental evidence available to answer this question. Based on a review of the pharmacological literature, it is likely that the motor benefits of pramipexole in PD patients are due to D(2) stimulation, whereas its putative effects on mood and apathy may be related to its D(3) agonist properties.

In vivo PET studies of the dopamine D2 receptors in rhesus monkeys with long-term MPTP-induced parkinsonism

Studies of dopamine (DA) receptor binding in early parkinsonian patients, or in models of Parkinson's disease, have revealed a supersensitivity of the D2-like receptor subtype as compared to age-matched controls. The lack of upregulation in advanced patients is often attributed to the effects of prolonged antiparkinsonian therapy, but the impact of therapy vs. intrinsic mechanisms in untreated patients or animals with long-term lesions of the DA nigrostriatal pathway has been difficult to address. We studied, in vivo, by PET using the DA D2 receptor ligand raclopride, the status of the DA receptors in normal rhesus monkeys and those with acute (3 months) or long-term (10 years) MPTP-induced nigrostriatal lesions. Compared to age-matched controls, there was no change in raclopride binding in MPTP-treated animals without parkinsonian symptoms. There was a significant increase in raclopride binding in the putamen (but not caudate nucleus) of all the animals displaying rigidity, hypo- and bradykinesia. This increase was greater in the animals with acute lesions (32%) than with established, long-term lesions (18%). There was no correlation between the postmortem striatal DA concentrations and in vivo raclopride binding but there was a correlation between PET raclopride binding and [(3)H]raclopride binding in vitro. Complex changes in D2 receptor binding occur in various stages of parkinsonism. Antiparkinsonian therapy is unlikely to be solely responsible for the lack of upregulation found in advanced parkinsonian patients but may be a contributing factor.

Effects of repeated methyl levodopa administration on apomorphine sensitivity of rotational behavior and striatal Fos expression of rats with unilateral 6-OHDA lesions

This study was designed to elucidate the mechanism to develop levodopa-induced dyskinesia in patients with Parkinson's disease. For this purpose, we administered methyl levodopa repeatedly to a rat model of Parkinson's disease with unilateral 6-hydroxydopamine (6-OHDA)-induced lesion of the nigrostriatal dopamine pathway. After a washout period, we measured apomorphine sensitivity of contralateral rotation and made parallel determination of Fos expression in the caudate-putamen and globus pallidus of the same animal. Once daily, i.p. injection of methyl levodopa plus benserazide for 10 days increased the number of rotations over time. A challenge dose of apomorphine showed enhanced rotational response in rats pretreated with methyl levodopa. Repeated administration of methyl levodopa resulted in diminished apomorphine sensitivity of Fos expression in the dopamine depleted caudate-putamen and in enhanced sensitivity in the globus pallidus of the same side. Present results may add evidence to the idea that repeated administration of levodopa develops dopaminergic sensitization mediated by augmented activation of pallidal neurons involved in D2-responsive pallidal output pathway.

Effects of oligonucleotide antisense to dopamine D(1A) receptor messenger RNA in a rodent model of levodopa-induced dyskinesia

Dyskinesias are abnormal involuntary movements which develop as a side-effect of long-term treatment with levodopa in patients with Parkinson's disease. The pathophysiology underlying these dyskinesias remains unclear, although, it has been suggested that heightened activity of dopamine D(1) receptor-bearing striatonigral neurons may play a key role. Chronic pulsatile levodopa administration to hemiparkinsonian rats results in sensitization of rotational responses to apomorphine. This sensitization is thought to be analogous to levodopa-induced dyskinesias in humans. In these studies, we further clarify the role of the dopamine D(1A) receptor in this rodent model of levodopa-induced dyskinesias using an in vivo oligonucleotide antisense approach. Hemiparkinsonian rats received twice daily injections of levodopa for three weeks followed by intrastriatal infusion of dopamine D(1A) receptor antisense (7nmol/day, three days), a scrambled missense control sequence, or saline. Those animals treated with antisense displayed significantly fewer apomorphine-induced rotations than saline- or missense-treated controls.By reducing dopamine D(1A) receptor expression, we were able to attenuate sensitization of the response to apomorphine resulting from chronic pulsatile levodopa treatment. Thus, the dopamine D(1A) receptor appears to play a significant role in levodopa-induced dyskinesias and warrants further examination. These findings may have important implications for the development of selective treatment strategies designed to alleviate parkinsonian symptoms, while minimizing motor complications.

123I-beta-CIT and 123I-IBZM-SPECT scanning in levodopa-naive Parkinson's disease

Striatal dopamine transporter function and dopamine D2 receptor status were evaluated in 15 patients with early untreated Parkinson's disease using single photon emission tomography (SPECT) with 123I-Iodo-2beta-carboxymethoxy-3beta-(4-idiophenyl)tropane (beta-CIT) and 123I-Iodobenzamide (IBZM) as pre- and postsynaptic ligands. Symptoms were unilateral in five patients and bilateral but asymmetric in 10 patients. Patients with bilateral symptoms had significantly lower 18-hour striatal/cerebellar beta-CIT binding ratios (3.59 +/- 0.79) than hemiparkinsonian patients (5.76 +/- 1.48, p < 0.05) reflecting more advanced disease in this subgroup. Patients with bilateral parkinsonism were also found to have a significant side-to-side difference in striatal beta-CIT binding with more marked reduction contralateral to the presenting limb (18-hour striatal/cerebellar ratio: 4.13 +/- 0.78 [ipsilateral] versus 3.59 +/- 0.79 [contralateral], p < 0.05). Dopamine D2 receptor binding as measured by IBZM was significantly elevated contralateral to the affected side in hemiparkinsonian patients (striatal/cerebellar ratio: 2.42 +/- 0.90 [contralateral] versus 2.19 +/- 0.80 [ipsilateral], p < 0.05). This asymmetric upregulation was absent in the patients with bilateral parkinsonism (striatal/cerebellar ratio: 1.85 +/- 0.43 [contralateral to more severely affected side] versus 1.83 +/- 0.34 [ipsilateral], p > 0.05). Our data suggest that postsynaptic dopamine receptor upregulation contralateral to the presenting side occurs in untreated unilateral PD and disappears in untreated bilateral (asymmetric) PD despite a greater loss of dopamine transporter function. Combined beta-CIT and IBZM SPECT studies may be helpful to monitor the progression of nigrostriatal dysfunction in early PD.

Continuous or pulsatile chronic D2 dopamine receptor agonist (U91356A) treatment of drug-naive 4-phenyl-1,2,3,6-tetrahydropyridine monkeys differentially regulates brain D1 and D2 receptor expression: in situ hybridization histochemical analysis

The effect of a chronic D2 dopamine receptor agonist (U91356A) treatment on dopamine receptor gene expression in the brain of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned monkeys was investigated using quantitative in situ hybridization histochemistry. U91356A was administered to MPTP-monkeys for 27 days in a pulsatile (n=3) or continuous (n=3) schedule. Animals treated in a pulsatile mode showed progressive sensitization and developed dyskinesia; whereas with the continuous mode behavioural tolerance was observed but no dyskinesia developed. Untreated MPTP as well as naive control animals were also studied. The efficacy and uniformity of the MPTP effect was assessed by measures of dopamine concentrations by high performance liquid chromatography with electrochemical detection in the relevant brain areas. D1 and D2 receptor messenger RNAs levels were examined by in situ hybridization histochemistry using human complementary RNA probes. Intense specific labelling for D1 and D2 receptor messenger RNAs was measured in the caudate and putamen with a rostrocaudal gradient for D2 receptors and a lower density in the cortex for D1 receptors messenger RNA. D1 receptor mRNA levels in rostral striatum and cortex decreased whereas D2 receptor messenger RNA in caudal striatum increased in MPTP-monkeys compared to control animals. Continuous administration of U91356A reversed the MPTP-induced increase of D2 receptor messenger RNA, whereas the pulsatile administration did not significantly correct these messenger RNA changes. U91356A treatment whether continuous or pulsatile partially corrected the D1 receptor messenger RNA lesion-induced decrease in the striatum, whereas no correction was observed in the cortex. All MPTP-monkeys were extensively and similarly denervated suggesting that the D1 and D2 receptor expression changes following U91356A administration were treatment related. Our data show a lesion-induced imbalance of D1 (decrease) and D2 (increase) receptor messenger RNAs in the striatum of MPTP-monkeys. The response of these receptors to D1 agonist treatment showed receptor selectivity and was influenced by the time-course of drug delivery. Hence chronic continuous but not pulsatile administration of U91356A reversed the striatal D1 receptor messenger RNA increase.

The effects of chronic levodopa treatment on pre- and postsynaptic markers of dopaminergic function in striatum of parkinsonian monkeys

Therapeutic treatment of parkinsonian monkeys by chronic administration of levodopa (l-DOPA) leads to the development of dyskinesias and other motor fluctuations. It is unclear whether there are alterations in the dopamine system that are related to the induction of dyskinesias by l-DOPA, but recent attention has focused on the D1 receptor system. The present study assessed the consequences of chronic l-DOPA treatment in monkeys made parkinsonian with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) on indices of the pre- and post-synaptic dopamine (DA) system. Treatment with therapeutic doses of l-DOPA led to the induction of dyskinesias in the MPTP-treated monkeys. High-pressure liquid chromatography was used for measurement of tissue levels of DA and its metabolites, and quantitative autoradiography was used to examine the regional integrity of the presynaptic DA system (by measuring [3H]mazindol binding to DA uptake sites). Quantitative autoradiography was used to measure the number of postsynaptic D1 receptors (using [3H] SCH 23390) in the striatum and pallidum of normal, MPTP alone, and MPTP monkeys treated chronically with l-DOPA. In both MPTP-treated monkeys, levels of DA and metabolites as well as [3H]mazindol binding were greatly reduced in the caudate and putamen, slightly more in dorsal than in ventral areas. However, the lack of increase in striatal DA levels along with higher [3H]mazindol binding in MPTP-plus-l-DOPA-treated monkeys suggested differences in the way DA was used after l-DOPA treatment In MPTP-treated monkeys, a significant increase (141-170% of normals) of D1 receptor numbers was observed in putamen and dorsal caudate. With l-DOPA treatment, the number of D1 receptor numbers was further elevated in caudal putamen (119-123%), dorsal caudate (110-130%), and in the internal segment of the globus pallidus (GPi; 164% of normals) of MPTP-treated monkeys as compared with MPTP treatment alone. This suggested that in MPTP-treated monkeys made dyskinetic by chronic pulsatile delivery of l-DOPA, there was enhanced production of D1 receptors in the direct striatal output to the GPi.

Long-term changes of striatal dopamine D2 receptors in patients with Parkinson's disease: a study with positron emission tomography and [11C]raclopride

We used [11C]raclopride (RACLO) and positron emission tomography (PET) to study longitudinally striatal dopamine D2 receptor binding in nine patients with Parkinson's disease (PD) at an early drug-naive stage and 3-5 years later, when motor fluctuations had appeared in seven of them. Patients were treated with a combination of levodopa and dopamine agonists. Data were compared with 10 healthy controls in the same age range. Initially, patients with PD showed a significant increase of RACLO uptake in the putamen compared with controls (p < 0.04). The caudate nucleus revealed values in the normal range. After 3-5 years, RACLO binding was significantly reduced in the putamen (p < 0.03) and caudate nucleus (p < 0.03) compared with baseline. Values were now in the control range in the putamen and reduced in the caudate nucleus (p < 0.05). The clinical score at "off" had significantly worsened (p < 0.0005) compared with the first PET scan. The nine PD patients reported here had already been investigated 3-4 months after therapy began and that time did not show a reduction of the initially increased RACLO binding capacity (data published previously). These results indicate long-term downregulation of striatal dopamine D2 receptor binding in PD. Receptor changes in the striatum of patients with PD may be induced by chronic dopaminergic therapy or occur independently of treatment, as a result of structural adaptation of the postsynaptic dopaminergic system to the progressive decline of nigrostriatal neurons.

Effects of L-DOPA-therapy on dopamine D2 receptor mRNA expression in the striatum of MPTP-intoxicated parkinsonian monkeys

The cellular expression of dopamine D2 receptor mRNA was examined in striatal (caudate nucleus and putamen) neurones of 9 Macaca fascicularis monkeys rendered parkinsonian by systemic injection of MPTP. Messenger RNA abundance was determined by quantitative in situ hybridization using human-specific 35S-labelled oligonucleotides. Control monkeys were untreated and received neither MPTP nor L-DOPA while the rest were rendered parkinsonian and received chronic levodopa therapy to induce dyskinesia. In the control brains a strong dopamine D2 receptor hybridization signal was detected overlying medium-sized and some large neurons in both the caudate nucleus and putamen. Neurons from the lateral and medial regions of the caudate nucleus, and from the dorsal and ventral regions of the putamen were analysed separately. A significant increase in the cellular abundance of dopamine D2 receptor mRNA was seen in the striatum of MPTP-treated monkeys; this increase being restricted to the population of medium-sized striatal cells. No such increase in dopamine D2 receptor mRNA was observed in (dyskinetic) L-DOPA-treated monkeys suggesting that levodopa-therapy normalises D2 receptor expression in post-synaptic striatal cells. The cellular abundance of dopamine D2 receptor mRNA expressed by large striatal neurons (putative cholinergic cells) was unaffected by either MPTP treatment or levodopa therapy. The implications of these findings for the development of levodopa-induced dyskinesias is discussed.

Interhemispheric modulation of dopamine receptor interactions in unilateral 6-OHDA rodent model

A critical assumption in the unilateral 6-hydroxydopamine (6-OHDA) model is that interactions between the intact and denervated hemispheres do not influence the response to insult. The present study examined this issue by assessing the effects of unilateral substantia nigra 6-OHDA lesions in rats that previously had received corpus callosum transections, a treatment designed to minimize interhemispheric influences. Quantitative autoradiography in the caudate-putamen ipsilateral to the lesion revealed that corpus callosum transection did not alter the increase in D2-like receptors ([125I]-epidepride-labeled sites) that is induced by unilateral 6-OHDA lesion. There were no effects of either 6-OHDA lesion or transection on D1 receptor density ([125I]-SCH23982 autoradiography). As a functional endpoint, dopamine-stimulated cAMP efflux was measured in superfused striatal slices. In this paradigm, the net effect of dopamine (DA) represents a combination of D1 receptor-mediated stimulation and D2 receptor-mediated inhibition. 6-OHDA lesion increased cAMP efflux induced by exposure to 100 microM DA alone; corpus callosum transection did not alter this effect. An interaction between 6-OHDA lesion and transection status was revealed, however, by comparison of results obtained with DA alone vs. DA plus the D2 antagonist sulpiride (to block the D2 inhibitory effects of 100 microM DA). This comparison revealed two important effects of 6-OHDA lesion in rats with an intact corpus callosum: 1) a moderate decrease in dopamine D1 receptor-mediated stimulation; and 2) a dramatic decrease in the ability of D2 receptors to inhibit this stimulation. Corpus callosum transection prevented these effects of 6-OHDA. These results provide a biochemical demonstration of D1:D2 receptor uncoupling in unilateral 6-OHDA lesioned rats, and suggest that interhemispheric influences (e.g., contralateral cortico-striatal glutamatergic projections) may contribute to lesion-induced alterations in D1:D2 receptor interactions.

Levodopa or D2 agonist induced dyskinesia in MPTP monkeys: correlation with changes in dopamine and GABAA receptors in the striatopallidal complex

Dopamine D1 and D2 receptors as well as the GABA/benzodiazepine receptor complex in the striatum and the globus pallidus (internal: GPi and external: GPe) were studied by autoradiography using [3H]SCH 23390, [3H]spiperone, and [3H]flunitrazepam ([3H]FNZ) respectively, in five groups of cynomolgus monkeys. These included (i) untreated 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-monkeys; (ii) MPTP monkeys treated chronically with levodopa injections; (iii) MPTP monkeys treated chronically with injections of the novel D2 agonist U91356A; (iv) MPTP monkeys treated chronically with U91356A delivered through an osmotic mini-pump; and (5) naive controls. Animals treated in a pulsatile mode with U91356A or levodopa injections showed progressive sensitization to their respective drug and developed choreic dyskinesia. In contrast, animals treated in a continuous mode with U91356A showed behavioral tolerance but did not develop dyskinesia. A trend for a down-regulation of putaminal D2 receptors was observed following D2 agonist stimulation with U913356A. Striatal [3H]FNZ binding was significantly decreased only in animals treated in a continuous mode with U91356A. The dopamine receptor decrease in the striatum could be implicated with the development of tolerance but cannot explain the appearance of dyskinesia. Denervation by MPTP was associated with a decrease of the GPe/GPi [3H]FNZ binding ratio which reflects an imbalance of striatal output pathways; this ratio was not reversed by any of the treatments although changes were observed in the GPe and GPi. Indeed, pulsatile U91356A treatment restored the decreased [3H]FNZ binding in the GPe near control values and levodopa showed a similar tendency. A significant increase of [3H]FNZ binding in the GPi only of dyskinetic monkeys, namely those treated with pulsatile U91356A or levodopa was seen compared to untreated MPTP or naive controls. This GABAA receptor up-regulation might lead to a supersensitive state of the GPi to gabaergic input which may be involved in the mechanism underlying the development of dopaminomimetic-induced dyskinesia.

Alterations in dopamine uptake sites and D1 and D2 receptors in cats symptomatic for and recovered from experimental parkinsonism

The administration of the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to adult cats severely disrupts the dopaminergic innervation of the striatum. Animals display a parkinson-like syndrome, consisting of akinesia, bradykinesia, postural instability, and rigidity, which spontaneously recovers by 4-6 weeks after the last administration of MPTP. In this study we used quantitative receptor autoradiography to examine changes in DA uptake sites and DA receptors in the basal ganglia of normal, and symptomatic and recovered MPTP-treated cats. Consistent with the destruction of the nigrostriatal DA pathway, there was a severe loss of DA uptake sites, labeled with [3H]-mazindol, in the caudate nucleus (64-82%), nucleus accumbens (44%), putamen (63%), and substantia nigra pars compacta (SNc, 53%) of symptomatic cats. Following behavioral recovery, there were no significant changes in DA uptake site density. Significant increases of [3H]-SCH 23390 binding to D1 DA receptors were observed in the dorsal caudate (> 24%; P < 0.05) of symptomatic cats and in all regions of the caudate-putamen (> 30%; P < 0.05) of recovered animals. [3H]-SCH 23390 binding in the substantia nigra pars reticulata was half of that in the striatum and showed no changes in symptomatic or recovered animals. No alterations in the binding of [125I]-epidepride to D2 receptors was observed in any region of the striatum in either symptomatic or recovered animals. [125I]-Epidepride binding in the SNc was decreased by > 36% (P < 0.05) following MPTP treatment. These data show that cats made parkinsonian by MPTP exposure have a significant decrease in the number of DA reuptake sites throughout the striatum and that recovery of sensorimotor function in these animals is not correlated with an increase in the number of striatal reuptake sites. Behavioral recovery, however, does seem to be correlated with a general elevation of D1 receptors throughout the striatal complex. The present data also show that direct correlations between changes in DA receptor regulation after a large DA depleting lesion and behavioral deficits or recovery from those deficits are difficult and that the relationships between DA receptors/transporters and behavior require further study.

Transmembrane signalling systems in the brain of patients with Parkinson's disease

The clinical efficacy of dopamine (DA) replacement therapy for patients with Parkinson's disease (PD) depends on the preservation of postsynaptic DA receptors and their intracellular signalling mechanisms in the striatum long after degeneration of the nigrostriatal DA pathway. DA activates adenylyl cyclase (AC) and phospholipase C (PLC) via the D1 receptor, and inhibits through the D2 receptor, thereby regulating the production of intracellular second messengers, cyclic adenosine 3',5'-monophosphate (cAMP), 1,2-diacylglycerol (DAG) and Ca2+. Recent advances in molecular biology have made it possible to monitor the intracellular signal transduction cascade following receptor activation by various transmitters. The authors review the literature addressing this issue, summarized as follows: (1) striatal D1 and D2 receptor densities remain constant, at least in treated and non-demented patients; (2) DA-sensitive AC activity appears to be increased in the putamen of treated patients, although this remains to be confirmed; (3) levels of cAMP-dependent protein kinase (PKA) are normal in non-demented patients, consistent with unchanged levels of DARPP-32 (dopamine- and cAMP-regulated phosphoprotein of M(r) 32,000); (4) levels of Ca2+/phospholipid-dependent protein kinase (PKC) and of inositol 1,4,5-trisphosphate (InsP3) receptor also remain unchanged in non-demented patients; (5) the above three second messenger sites as well as densities of D1 and D2 receptors are decreased in the striatum of demented PD patients (PDD). We tentatively conclude that postreceptor signalling function is intact in the striatum of non-demented PD patients and that there is a clear difference between non-demented patients and PDD, i.e. striatal dopaminoceptive neurons are affected in PDD.

Functional imaging in relation to parkinsonian syndromes

Parkinsonism is a feature not only of Parkinson's disease but also of many other diseases affecting basal ganglia function. Functional imaging (PET and SPECT) can demonstrate the various resting patterns of disruption of regional cerebral blood flow, metabolism, and neuropharmacology associated with different parkinsonian disorders. 18F-dopa PET also has the potential to detect subclinical dysfunction of dopaminergic terminals in at-risk subjects. Finally, functional imaging can help us understand the nature of the networks involved in performing different motor tasks, and can reveal how these networks malfunction in the presence of bradykinesia or parkinsonian tremor.

Differential effect of chronic dopaminergic treatment on dopamine D1 and D2 receptors in the monkey brain in MPTP-induced parkinsonism

Dopamine D1 and D2 receptors located within the striatum (caudate nucleus and putamen) were studied autoradiographically, using [3H]SCH 23390 and [3H]sulpiride respectively, in (i) seven monkeys rendered parkinsonian by the systemic administration of MPTP, four of which were chronically exposed to anti-parkinsonian drugs (levodopa or apomorphine), (ii) two hemi-parkinsonian monkeys (induced by intra-carotid infusion of MPTP), one of which received chronic exposure to apomorphine, and (iii) three control monkeys which received neither MPTP nor dopaminergic drugs. Anti-parkinsonian drug exposure resulted in a reversal of symptoms and was accompanied by the development of limb dyskinesias. In parkinsonian monkeys not chronically exposed to drugs. [3H]SCH 23390 binding was slightly but not significantly elevated above control values, whilst in the same animals [3H]sulpiride binding was significantly increased above that found in the control group. Rostrally [3H]SCH 23390 binding was similar in the control and drug-exposed parkinsonian groups but more caudally there was a small consistent, although not significant, increase in [3H]SCH 23390 binding in the drug-exposed animals as compared to the parkinsonian monkeys not exposed to drugs. In contrast at all rostro-caudal levels [3H]sulpiride binding in the drug-exposed parkinsonian group was lower than the corresponding values from the non-drug exposed animals. [3H]SCH 23390 binding showed no major side-to-side difference in the hemi-parkinsonian animal which was not exposed to levodopa/apomorphine, whilst in the hemi-parkinsonian monkey which received apomorphine there was again an increase in binding on the MPTP-treated side of the brain. In both drug- and non-drug exposed hemi-parkinsonian animals there was a greater density of [3H]sulpiride binding in the parkinsonian side of the brain; the general level of binding in the drug-exposed monkey was less than that seen in the other animal. These results would support the idea that in MPTP-induced parkinsonism, dopaminergic denervation results in a greater change in the D2 receptors, but furthermore would indicate a differential effect of levodopa/apomorphine exposure on the D1 and D2 receptor populations. Drug exposure apparently encourages the reversal of the MPTP-induced increase in the D2 receptor binding, whilst the D1 receptor binding appears to proliferate in response to these drugs. These results may have important implications in relation to the development of dyskinesias, subsequent to the chronic use of some anti-parkinsonian drug treatments.

Sensitization of Dopamine-Stimulated Adenylyl Cyclase in the Striatum of 1-Methyl-4-Phenyl-1,2,3,6-Tetrahydropyridine-Treated Rhesus Monkeys and Patients with Idiopathic Parkinson's Disease

Dopamine-stimulated adenylyl cyclase activity was measured in striatal homogenates of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated rhesus monkeys and humans with idiopathic Parkinson's disease and compared with the activity in control tissue. No differences between parkinsonian and control tissue were found in the presence of 20 mM NaCl. However, when 120 mM NaCl was included in the assay medium, a significantly higher increase in the Vmax of dopamine-stimulated adenylyl cyclase activity was observed in the caudate of MPTP-parkinsonian rhesus monkeys and the putamen of patients with idiopathic Parkinson's disease. No such sensitization was seen in the MPTP-treated rhesus putamen or human Parkinson's disease caudate tissue. A role of D2 receptors in this sensitization could be ruled out by the concomitant use of the D2 antagonist l-sulpiride and by [3H]spiperone saturation analysis of the D2 receptor density, which was found at control level in the caudate tissue of MPTP-treated rhesus monkeys. Similarly, on the basis of saturation binding with the D1 selective ligand 125I-SCH 23982, there was no difference in caudate nucleus D1 receptor densities between control and MPTP-treated monkeys. Our results point to a region-specific functional sensitization of D1 receptors as a consequence of severe dopaminergic denervation of the striatum and suggest the possibility of a therapeutic potential of a D1 agonist with full intrinsic activity in Parkinson's disease.

A post-mortem study on striatal dopamine receptors in Parkinson's disease

Striatal dopamine D1 and D2 receptors were investigated in 49 patients with Parkinson's disease (PD) and 33 age-matched controls with [3H]SCH 23390 and [3H]spiroperidol as ligands respectively. A full Scatchard analysis giving Bmax and Kd values was performed. In the caudate nucleus, a small but significant decrease in the number of D1 and D2 receptors was seen, whereas in the putamen the number of dopamine receptors was unchanged. Treatment with neuroleptics was found to increase the number of D2 receptors both in the caudate nucleus and putamen. The number of neither D1 nor D2 receptors correlated neither with the duration of disease or levodopa treatment of the patients nor with the post-mortem delay or storage time of the samples. Furthermore, no association was found between either D1 or D2 receptor number and clinical variables of the patients. The activity of choline acetyltransferase (ChAT) was found to be unchanged in the striatum, whereas a marked decline was seen in the hippocampus and cortical areas, indicating that intrinsic striatal cholinergic neurons are not affected in PD. The present results suggest that there is a modest decline in the number of striatal dopamine D2 receptors in advanced patients with PD at the end stage of the disease.

Reduced D2 dopamine and muscarinic cholinergic receptor densities in caudate specimens from fluctuating parkinsonian patients

Binding of spiperone and 3-quinuclidinyl benzilate (QNB), both labeled with hydrogen 3 (3H), were measured in caudate tissue obtained from 8 living parkinsonian patients at the time of cerebral transplantation. This was clinically homogeneous group of patients. All remained predominantly responsive to levodopa, although with marked disability secondary to clinical fluctuations (short-duration responses) and medication-induced dyskinesias; all were receiving substantial doses of levodopa and 6 of the 8 patients were additionally receiving bromocriptine or pergolide. Binding densities of dopamine D2 receptors, as measured by [3H]spiperone binding, were reduced in this group of patients, compared to caudate specimens from autopsy control subjects. This findings may reflect medication-induced receptor downregulation. Parallel changes occurred with muscarinic cholinergic receptors; [3H]QNB binding was significantly reduced, compared to autopsy control values. This reduction of muscarinic receptors might be due to loss of nigrostriatal terminals that are known to contain muscarinic receptors. Alternatively, muscarinic receptors may have been downregulated by increased corticostriatal glutamatergic input to cholinergic cells, inferred to be present based on the prominent levodopa-induced dyskinesias. Finally, receptor deficits could have been a reflection of more widespread degenerative cerebral disease, although levodopa-refractory symptoms were generally not pronounced in these patients.

Concentrations of tyrosine, L-dihydroxyphenylalanine, dopamine, and 3-O-methyldopa in the cerebrospinal fluid of Parkinson's disease

We determined the concentrations of tyrosine, L-3,4-dihydroxyphenylalanine (DOPA), dopamine (DA), and 3-O-methyldopa (3-OMD) in the cerebrospinal fluid (CSF) of parkinsonian patients and elevated potential interactions between the substances. We found a significant increase in tyrosine, and a significant decrease in DOPA, DA, and 3-OMD. We also found that for a given concentration of DOPA, DA and 3-OMD were proportional. In addition, the ratio of DA to 3-OMD was significantly shifted in favor of DA in parkinsonian patients.

Effects of L-dopa on extracellular dopamine in striatum of normal and 6-hydroxydopamine-treated rats

In vivo microdialysis was used to examine the effect of L-3,4-dihydroxyphenylalanine (L-DOPA) administration upon dopamine (DA) in extracellular fluid both in intact striatum and in striatum of rats treated with the catecholaminergic neurotoxin 6-hydroxydopamine (6-HDA). Basal extracellular levels of DA were not significantly altered by 6-HDA unless the DA content of striatal tissue was reduced to less than 20% of control. Peripheral aromatic amino acid decarboxylase (AADC) inhibition (RO4-4602, 50 mg/kg i.p.) followed by L-DOPA treatment (100 mg/kg i.p.) elevated extracellular DA in striatum of control rats from 37 +/- 5 to 68 +/- 11 pg/sample (n = 7; values corrected for recovery of the dialysis probe). In animals with severe bilateral depletions of DA in striatal tissue (mean depletion 87%; n = 6), L-DOPA increased extracellular DA in striatum from 8 +/- 3 to 266 +/- 60 pg/sample. In animals with large unilateral depletions of DA in striatal tissue (mean depletion 96%; n = 6), the increase in extracellular DA in striatum after L-DOPA was greater on the lesion side (from 7 +/- 4 to 245 +/- 67 pg/sample) than on the intact side (from 28 +/- 11 to 61 +/- 8 pg/sample). Animals with unilateral DA depletions showed contralateral circling behavior after L-DOPA. Increases in extracellular DA approaching the magnitude of those occurring in DA-depleted striata were observed when intact animals were treated with nomifensine (5 mg/kg i.p.; n = 5), an inhibitor of high-affinity DA uptake, in addition to L-DOPA.

Autoradiographic studies in animal models of hemi-parkinsonism reveal dopamine D2 but not D1 receptor supersensitivity. II. Unilateral intra-carotid infusion of MPTP in the monkey (Macaca fascicularis)

The selective dopaminergic antagonist ligands [3H]SCH 23390 and [3H]sulpiride were used to reveal autoradiographically dopamine D1 and D2 receptors, respectively, in brain sections from monkeys which had received unilateral intracarotid infusions of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), causing loss of dopamine-containing neurones of the substantia nigra pars compacta. The monkeys developed hemi-parkinsonian symptoms (tremor, bradykinesia) in limbs contralateral to the side of the toxin infusion. Administration of apomorphine (0.05-0.25 mg/kg) caused contralateral rotational behaviour, and reversal of the parkinsonian symptoms. Loss of forebrain dopaminergic terminals was assessed autoradiographically using [3H]mazindol to label dopamine uptake sites. A reduction in these sites of 97% (mean brain value) in the caudate nucleus, and 91% in the putamen, as compared with binding values from untreated control monkeys, was accompanied by a significant increase in the binding of [3H]sulpiride (D2) in these structures. In contrast, in the same animals there was no similar increase in [3H]SCH 23390 binding to D1 receptors in the denervated areas. These results suggest that in the parkinsonian brain, where the dopaminergic innervation of the caudate nucleus and putamen has been lost, D2 receptors may be more susceptible than D1 receptors to changes, revealed here as an increase in [3H]sulpiride binding sites.

Autoradiographic studies in animal models of hemi-parkinsonism reveal dopamine D2 but not D1 receptor supersensitivity. I. 6-OHDA lesions of ascending mesencephalic dopaminergic pathways in the rat

The selective dopaminergic antagonist ligands [3H]SCH 23390 and [3H]sulpiride were used to reveal autoradiographically dopamine D1 and D2 receptors, respectively, in brain sections from rats which had received unilateral 6-hydroxydopamine (6-OHDA) injections destroying ascending nigrostriatal neurones. The binding of both ligands to striatal sections was first shown to be saturable, reversible and of high affinity and specificity [( 3H]SCH 23390: Bmax 2.16 pmol/mg protein, Kd 1.4 nM; [3H]sulpiride; Bmax 0.67 pmol/mg protein, Kd 10.7 nM). After unilateral stereotaxic 6-OHDA injections, rats rotated contralaterally when challenged with apomorphine (0.5 mg/kg), or specific D1 or D2 agonists, SKF 38393 (1.0-5.0 mg/kg) and LY 171555 (0.05-0.5 mg/kg), respectively. Loss of forebrain dopaminergic terminals was assessed autoradiographically using [3H]mazindol to label dopamine uptake sites. A loss of approximately 90-95% of uptake sites was reproducibly accompanied by an enhanced density of binding ipsilaterally for the D2 ligand, [3H]sulpiride, in all areas of the striatum, but most markedly in the lateral areas. An increase in the D2 binding site density was also seen in the ipsilateral nucleus accumbens and the olfactory tubercle. In contrast, in the same animals, the striatal D1 receptors were far less affected by dopaminergic denervation, with no consistent changes seen in the binding of [3H]SCH 23390. These results suggest that dopamine D2 receptors are more susceptible than D1 receptors to changes after dopaminergic denervation, which is expressed as an increase in the density of binding sites revealed here with [3H]sulpiride.