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

The Effects of Chronic Marijuana Administration on 6-OHDA-Induced Learning & Memory Impairment and Hippocampal Dopamine and Cannabinoid Receptors Interaction in Male Rats

There are general inhibitory effects of exo-cannabinoids on dopamine-mediated behaviors. Many studies suggested the interaction between cannabinoid receptors and dopamine receptors in the brain that affect cognition behaviors. In this paper, we investigate the effects of marijuana on 6-OHDA-induced cognitive impairments and the expression of dopamine and cannabinoid receptors in the hippocampus of male rats. 42 rats were divided into six groups. 6-hydroxy dopamine (6-OHDA) was administrated into the substantia nigra. Marijuana (60 mg/kg; i.p.) was administered 28 days, one week after the 6-OHDA injection. Morris water maze (MWM) and novel object recognition tests were performed. The hippocampal expression levels of cannabinoid receptors and D1 and D2 dopamine receptors evaluate by real-time PCR. The results showed marijuana improved the spatial learning and memory disorders caused by 6-OHDA in the MVM task and novel object recognition test. Additionally, the level of both D1 and D2 mRNA was decreased in 6-OHDA-treated animals and marijuana consumption only increased the hippocampal level of D1 mRNA. Moreover, the level of hippocampal CB1 mRNA in 6-OHDA- treated rats was higher than in control rats. However, the hippocampal level of CB2 mRNA was decreased in 6-OHDA- treated rats. Marijuana consumption caused a significant decrease in CB1 mRNA level and an increase in CB2 mRNA level in 6-OHDA + marijuana group. Therefore, marijuana may be helpful for learning & memory disorders, D1, and D2 dopamine receptors, and cannabinoid receptor alteration in patients with Parkinson's disease.

Marijuana improved motor impairments and changes in synaptic plasticity-related molecules in the striatum in 6-OHDA-treated rats

Using marijuana has become popular and is allowed for medical purposes in some countries. The effect of marijuana on Parkinson's disease is controversial and Medical marijuana may benefit for motor and non-motor symptoms of patients with Parkinson's disease. No research has been conducted to fully prove the benefits, risks, and uses of marijuana as a treatment for patients with Parkinson's disease. In the present study, several different approaches, including behavioral measures and the western blot method for protein level assay, were used to investigate whether exposure to marijuana affects the motor and synaptic plasticity impairment induced by 6-OHDA. Marijuana consumption significantly decreased apomorphine-induced contralateral rotation, beam travel time, beam freeze time, and catalepsy time, but significantly increased latency to fall in the rotarod test, balance time, and protein level of PSD-95 and dopamine receptor D1 in the 6-OHDA + marijuana group. These results suggest that marijuana may be helpful for motor disorders and synaptic changes in patients with Parkinson's disease.

Molecular Imaging of the Dopamine Transporter

Dopamine transporter (DAT) single-photon emission tomography (SPECT) with (123)Ioflupane is a widely used diagnostic tool for patients with suspected parkinsonian syndromes, as it assists with differentiating between Parkinson’s disease (PD) or atypical parkinsonisms and conditions without a presynaptic dopaminergic deficit such as essential tremor, vascular and drug-induced parkinsonisms. Recent evidence supports its utility as in vivo proof of degenerative parkinsonisms, and DAT imaging has been proposed as a potential surrogate marker for dopaminergic nigrostriatal neurons. However, the interpretation of DAT-SPECT imaging may be challenged by several factors including the loss of DAT receptor density with age and the effect of certain drugs on dopamine uptake. Furthermore, a clear, direct relationship between nigral loss and DAT decrease has been controversial so far. Striatal DAT uptake could reflect nigral neuronal loss once the loss exceeds 50%. Indeed, reduction of DAT binding seems to be already present in the prodromal stage of PD, suggesting both an early synaptic dysfunction and the activation of compensatory changes to delay the onset of symptoms. Despite a weak correlation with PD severity and progression, quantitative measurements of DAT binding at baseline could be used to predict the emergence of late-disease motor fluctuations and dyskinesias. This review addresses the possibilities and limitations of DAT-SPECT in PD and, focusing specifically on regulatory changes of DAT in surviving DA neurons, we investigate its role in diagnosis and its prognostic value for motor complications as disease progresses.

Management of Psychosis in Parkinson's Disease: Emphasizing Clinical Subtypes and Pathophysiological Mechanisms of the Condition

Investigation into neuropsychiatric symptoms in Parkinson's disease (PD) is sparse and current drug development is mainly focused on the motor aspect of PD. The tight association of psychosis with an impaired quality of life in PD, together with an important underreporting of this comorbid condition, contributes to its actual insufficient assessment and management. Furthermore, the withdrawal from access to readily available treatment interventions is unacceptable and has an impact on PD prognosis. Despite its impact, to date no standardized guidelines to the adequate management of PD psychosis are available and they are therefore highly needed. Readily available knowledge on distinct clinical features as well as early biomarkers of psychosis in PD justifies the potential for its timely diagnosis and for early intervention strategies. Also, its specific characterisation opens up the possibility of further understanding the underlying pathophysiological mechanisms giving rise to more targeted therapeutic developments in the nearer future. A literature review on the most recent knowledge with special focus on specific clinical subtypes and pathophysiological mechanisms will not only contribute to an up to date practical approach of this condition for the health care providers, but furthermore open up new ideas for research in the near future.

Pharmacological treatment with L-DOPA may reduce striatal dopamine transporter binding in in vivo imaging studies

Numerous neurologic and psychiatric conditions are treated with pharmacological compounds, which lead to an increase of synaptic dopamine (DA) levels. One example is the DA precursor L-3,4-dihydroxyphenylalanine (L-DOPA), which is converted to DA in the presynaptic terminal. If the increase of DA concentrations in the synaptic cleft leads to competition with exogenous radioligands for presynaptic binding sites, this may have implications for DA transporter (DAT) imaging studies in patients under DAergic medication. This paper gives an overview on those findings, which, so far, have been obtained on DAT binding in human Parkinson's disease after treatment with L-DOPA. Findings, moreover, are related to results obtained on rats, mice or non-human primates. Results indicate that DAT imaging may be reduced in the striata of healthy animals, in the unlesioned striata of animal models of unilateral Parkinson's disease and in less severly impaired striata of Parkinsonian patients, if animal or human subjects are under acute or subchronic treatment with L-DOPA. If also striatal DAT binding is susceptible to alterations of synaptic DA levels, this may allow to quantify DA reuptake in analogy to DA release by assessing the competition between endogenous DA and the administered exogenous DAT radioligand.

Dyskinesias and impulse control disorders in Parkinson's disease: From pathogenesis to potential therapeutic approaches

Dopaminergic treatment in Parkinson's disease (PD) reduces the severity of motor symptoms of the disease. However, its chronic use is associated with disabling motor and behavioral side effects, among which levodopa-induced dyskinesias (LID) and impulse control disorders (ICD) are the most common. The underlying mechanisms and pathological substrate of these dopaminergic complications are not fully understood. Recently, the refinement of imaging techniques and the study of the genetics and molecular bases of LID and ICD indicate that, although different, they could share some features. In addition, animal models of parkinsonism with LID have provided important knowledge about mechanisms underlying such complications. In contrast, animal models of parkinsonism and abnormal impulsivity, although useful regarding some aspects of human ICD, do not fully resemble the clinical phenotype of ICD in patients with PD, and until now have provided limited information. Studies on animal models of addiction could complement the previous models and provide some insights into the background of these behavioral complications given that ICD are regarded as behavioral addictions. Here we review the most relevant advances in relation to imaging, genetics, biochemistry and pharmacological interventions to treat LID and ICD in patients with PD and in animal models with a view to better understand the overlapping and unique maladaptations to dopaminergic therapy that are associated with LID and ICD.

Functional neuroimaging in Parkinson's disease

The use of functional imaging in neurodegenerative diseases has increased in recent years, with applications in research into the underlying pathophysiology, aiding in diagnosis, or evaluating new treatments. In Parkinson's disease (PD), these imaging methods have expanded our understanding of the disease beyond dopaminergic deficits. Moreover, functional imaging methods have described alterations in functional networks relating not only to the motor symptoms, but also to many nonmotor features of PD, such as cognitive dysfunction. From a clinical viewpoint, functional imaging methods can assist in monitoring disease progression, such as in the context of clinical trials, and holds the potential to aid in early diagnosis of PD and differentiation from other parkinsonian disorders.

Estrogen receptors and lesion-induced response of striatal dopamine receptors

Neuroprotection by 17β-estradiol and an estrogen receptor (ER) agonist against 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) lesion were shown to implicate protein kinase B (Akt) signaling in mice. In order to evaluate the associated mechanisms, this study compared estrogen receptor alpha (ERα) and estrogen receptor beta (ERβ) intact or knockout (KO) and wild-type (WT) C57Bl/6 male mice following MPTP treatment of 7, 9, 11mg/kg and/or 17β-estradiol. Striatal D1 and D2 dopamine (DA) receptors were measured by autoradiography with the specific ligands [(3)H]-SCH 23390 and [(3)H]-raclopride, respectively and signaling by Western blot for Akt, glycogen synthase kinase 3β (GSK3β) and extracellular-regulated signal kinases (ERK1 and ERK2). Control ERKOβ mice had lower striatal [(3)H]-SCH 23390 specific binding than WT and ERKOα mice; both KO mice had lower [(3)H]-raclopride specific binding. Striatal D1 receptors decreased with increasing doses of MPTP in correlation with striatal DA concentrations in ERKOα mice and remained unchanged in WT and ERKOβ mice. Striatal D2 receptors decreased with increasing doses of MPTP in correlation with striatal DA concentrations in WT and ERKOα mice and increased in ERKOβ mice. In MPTP-lesioned mice, 17β-estradiol treatment increased D1 receptors in ERKOα and ERKOβ mice and D2 receptors in WT and ERKOβ mice. MPTP did not affect striatal pAkt/Akt and pGSK3β/GSK3β levels in WT and ERKOα mice, while in vehicle-treated ERKOβ mice these levels were higher and increased with MPTP lesioning. Striatal pERK1/ERK1 and pERK2/ERK2 levels showed to a lesser extent a similar pattern. In conclusion, ERs affected the response of striatal DA receptors to a MPTP lesion and post receptor signaling.

Dopamine receptors, motor responses, and dopaminergic agonists

Dopamine receptors are widely distributed within the central nervous system with its highest expression in the striatum. Two different families of dopamine receptors have been identified. The D₁ family comprises D₁ and D5 receptors, whereas D₂, D₃, and D₄ receptors form the D₂ family. These 2 families mediate different behavior patterns that are linked to activation of specific transduction pathways. The functional relevance of dopamine receptors derives from the reduced dopamine content found in the striatum of Parkinson disease (PD) patients and the ability of dopamine and dopamine receptors to reverse the motor deficits exhibited by PD patients. During the last 2 decades dopamine receptor agonists have been used either in de novo PD patients to prevent the appearance of dyskinesias or in PD patients with motor fluctuations to reduce the number of daily "off" hours. It seems that all dopamine receptors agonists produce similar motor responses and adverse effects, but data comparing their effectiveness in the treatment of PD are not available. In this article we summarize the main characteristics of dopamine receptors, their structure, their signaling pathways, and the responses mediated by their independent activation. Here is also described the therapeutic value of the different dopamine receptor agonists in the treatment of PD.

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.

Dopamine transporter binding is unaffected by L-DOPA administration in normal and MPTP-treated monkeys

Background

Radiotracer imaging of the presynaptic nigrostriatal dopaminergic system is used to assess disease progression in Parkinson's disease (PD) and may provide a useful adjunct to clinical assessment during therapeutic trials of potential neuroprotective agents. Several clinical trials comparing dopamine agonists to L-DOPA or early vs. late L-DOPA have revealed differences between clinical assessment and imaging of the presynaptic dopaminergic system, hence questioning the comparability of these measures as neuroprotection outcome variables. Thus, results of these studies may have been affected by factors other than the primary biological process investigated.

Methodology/Principal Findings

We tested the possibility that L-DOPA might interfere with DAT binding. Post-mortem DAT binding was conducted in normal and MPTP-treated macaque monkeys that were administered L-DOPA, acutely or chronically. In parallel, DAT SPECT was conducted in MPTP-treated animals that were administered chronic L-DOPA. [99mTc]TRODAT-1 SPECT binding was similarly reduced in all MPTP monkeys regardless of L-DOPA treatment. L-DOPA had no significant effect on post-mortem DAT binding either in saline or in MPTP-lesioned animals.

Conclusions/Significance

These data indicate that L-DOPA does not induce modifications of DAT expression detectable by SPECT of by DAT binding autoradiography, suggesting that differences between clinical assessment and radiotracer imaging in clinical trials may not be specifically related to L-DOPA treatment.

Compensation in pre-synaptic dopaminergic function following nigrostriatal damage in primates

Clinical symptoms of Parkinson's disease only become evident after 70-80% reductions in striatal dopamine. To investigate the importance of pre-synaptic dopaminergic mechanisms in this compensation, we determined the effect of nigrostriatal damage on dopaminergic markers and function in primates. MPTP treatment resulted in a graded dopamine loss with moderate to severe declines in ventromedial striatum (approximately 60-95%) and the greatest reductions (approximately 95-99%) in dorsolateral striatum. A somewhat less severe pattern of loss was observed for striatal nicotinic receptor, tyrosine hydroxylase and vesicular monoamine transporter expression. Declines in striatal dopamine uptake and transporter sites were also less severe than the reduction in dopamine levels, with enhanced dopamine turnover in the dorsolateral striatum after lesioning. The greatest degree of adaptation occurred for nicotine-evoked [(3)H]dopamine release from striatal synaptosomes, which was relatively intact in ventromedial striatum after lesioning, despite > 50% declines in dopamine. This maintenance of evoked release was not due to compensatory alterations in nicotinic receptor characteristics. Rather, there appeared to be a generalized preservation of release processes in ventromedial striatum, with K(+)-evoked release also near control levels after lesioning. These combined compensatory mechanisms help explain the finding that Parkinson's disease symptomatology develops only with major losses of striatal dopamine.

Disease-related and drug-induced changes in dopamine transporter expression might undermine the reliability of imaging studies of disease progression in Parkinson's disease

Parkinson's disease (PD) is a progressive neurodegenerative disorder. Standard therapeutic interventions are aimed at replenishment of empty dopamine stores with levodopa or substitution with dopamine receptor agonists. However, in the long term this symptomatic therapy fails. Currently, various neuroprotective agents are being developed, with the intention to slow down the degeneration of dopaminergic neurons. In this context, the early identification of persons at risk to develop the disease as well as the assessment of the effectiveness of putative neuroprotective agents, are critical issues. Dopamine transporter (DAT) scintigraphy with single photon emission computed tomography (SPECT) has been used to assess the dopaminergic function in PD. Initial studies with several radioligands show significant loss of DAT binding in PD patients as compared to controls. In this paper we review the evidence on the utility of DAT imaging with SPECT in early PD detection as well as in monitoring neurprotection.

Positron emission tomography and single-photon emission computed tomography in central nervous system drug development

In this review, the value of functional imaging [positron emission tomography (PET)/single-photon emission computed tomography (SPECT)] in drug development is considered. Radionuclide imaging can help establish the diagnosis of neurodegenerative disorders where this is in doubt and provides a potential biomarker for following drug effects on disease progression. PET and SPECT can help understand mechanisms of disease and determine the functional effects of therapeutic approaches on neurotransmission and metabolism. Synthesizing radiotracer analogs of novel drugs can provide proof of principle that these agents reach their enzyme or receptor targets and delineate their regional brain distribution. If such radiotracers do not prove to have ideal properties for imaging, the concept of microdosing potentially allows multiple other drug analogs to be tested with less stringent regulatory requirements than for novel medicinals. Finally, PET tracers can provide receptor and enzyme active site dose occupancy profiles, thereby guiding dosage selection for phase 1 and phase 2 trials. The eventual hope is that radiotracer imaging will provide a surrogate marker for drug efficacy, although this has yet to be realized, and progress the concept of personalized medicine where receptor/enzyme binding profiles help predict therapeutic outcome.

Psychotic symptoms in Parkinson’s disease

Psychotic symptoms are common in Parkinson's disease (PD) and occur in at least 20% of medication-treated patients. Benign visual hallucinations usually appear earlier, while malignant hallucinations, confusional states, delusions, paranoid beliefs, agitation, and delirium become more frequent with disease progression. Virtually all antiparkinsonian drugs may produce psychotic symptoms. Cognitive impairment, increased age, disease duration and severity, depression, and sleep disorders have been consistently identified as independent risk factors for their development. Although the precise pathoetiologic mechanisms remain unknown, we review evidence that links ventral dopaminergic pathway dysfunction (overactivity) together with the involvement of other neurotransmitter system imbalances as likely contributors. The clinical importance of the proposed mechanism is that successful management of psychotic symptoms in PD may rely on a multitarget approach to restore neurotransmitter imbalances rather than focusing exclusively on the dopaminergic dysfunction.

Different responsiveness of striatonigral and striatopallidal neurons to L-DOPA after a subchronic intermittent L-DOPA treatment

Early gene induction by L-DOPA in the striatum of dopamine denervated rats represents a useful way to study long-term modifications produced by this drug. The effects of acute and subchronic L-DOPA administration on zif-268 mRNA expression were compared in 6-hydroxydopamine-lesioned rats. Rats received a subchronic intermittent L-DOPA (6 mg/kg) treatment, which produces behavioural sensitization, a correlate of dyskinetic movements. Three days after interruption of subchronic treatment, zif-268 mRNA was evaluated after an L-DOPA challenge. Zif-268 mRNA levels increased in the lesioned dorsolateral striatum after either acute or subchronic L-DOPA administration. Double labelling of striatal cells with zif-268 and enkephalin or dynorphin mRNA probes was performed to assess neuronal activation in the indirect and direct output pathway. Single acute L-DOPA significantly increased zif-268 in all striatal neurons reflecting a hyperresponsiveness of dopamine-depleted striatum. After subchronic L-DOPA, zif-268 mRNA labelling was still increased in the striatonigral pathway, limited to dynorphin(+) neurons, whereas in all other neurons it was similar to the control value. Results suggest that striatal neurons responding to acute L-DOPA differ from those responding to subchronic L-DOPA. L-DOPA-induced behavioural sensitization was associated to a down-regulation in the responsiveness of striatopallidal and striatonigral dynorphin(-) neurons, whereas in striatonigral neurons containing dynorphin a hyperresponsiveness to L-DOPA was observed. High levels of zif-268, together with a persistent hyperresponsiveness of striatonigral dymorphinergic neurons and hyporesponsiveness of striatopallidal neurons, by creating an unbalanced state of striatal efferent neurons, may be implicated in dyskinetic movements observed in Parkinson's disease (PD).

Levodopa effect on [18F]fluorodopa influx to brain: normal volunteers and patients with Parkinson's disease

OBJECTIVES

Levodopa is the immediate precursor of dopamine and the substrate for DOPA decarboxylase, an enzyme subject to regulation in living brain. To test whether this regulation changes in disease, we used Positron Emission Tomography (PET) with parametric mapping to measure the effect of levodopa on the net clearance of [(18)F]fluorodopa to brain (K, ml/g/min).

METHODS

Five patients with early Parkinson's disease with pause of medication for 3 days and six age-matched healthy volunteers were studied in a baseline condition and after levodopa challenge.

RESULTS

Levodopa (200 mg as Sinemet) increased the magnitude of the net clearance K in the left and right putamen of the healthy volunteers by 11% relative to the baseline condition. In contrast, resumption of medication with levodopa did not significantly alter the magnitude of K in putamen of the Parkinson's disease patients. Compartmental analysis was used to probe the physiological basis of the activation of K: levodopa treatment increased by 15% the apparent distribution volume of [(18)F]fluorodopa in cerebellum (, ml/g) of both patients and control subjects, without significantly altering the unidirectional blood-brain clearance (, ml/g/min) or the relative activity of DOPA decarboxylase (, min(-1)) in putamen.

CONCLUSION

We conclude that levodopa treatment increases the distribution volume of [(18)F]fluorodopa in brain, increasing its availability for utilization in dopamine terminals. We speculate that levodopa act as a direct beta-adrenergic agonist at receptors regulating the permeability of the blood-brain barrier to levodopa. However, the PET analytical method was without sufficient power to detect the consequent increase in magnitude of K in brain of only five Parkinson's disease subjects.

Development of levodopa-induced dyskinesias in parkinsonian monkeys may depend upon rate of symptom onset and/or duration of symptoms

Levodopa-induced dyskinesias (LIDs) present a major problem for the long-term management of Parkinson's disease (PD) patients. Due to the interdependence of risk factors in clinical populations, it is difficult to independently examine factors that may influence the development of LIDs. Using macaque monkeys with different types of MPTP-induced parkinsonism, the current study evaluated the degree to which rate of symptom progression, symptom severity, and response to and duration of levodopa therapy may be involved in the development of LIDs. Monkeys with acute (short-term) MPTP exposure, rapid symptom onset and short symptom duration prior to initiation of levodopa therapy developed dyskinesia between 11 and 24 days of daily levodopa administration. In contrast, monkeys with long-term MPTP exposure, slow symptom progression and/or long symptom duration prior to initiation of levodopa therapy were more resistant to developing LIDs (e.g., dyskinesia developed no sooner than 146 days of chronic levodopa administration). All animals were similarly symptomatic at the start of levodopa treatment and had similar therapeutic responses to the drug. These data suggest distinct differences in the propensity to develop LIDs in monkeys with different rates of symptom progression or symptom durations prior to levodopa and demonstrate the value of these models for further studying the pathophysiology of LIDs.

Worsened motor test performance following acute apomorphine injection in previously untreated patients with Parkinson’s disease

Instrumental tests and clinical rating assess motor disability in Parkinson's disease (PD) patients. Previous long-term dopaminergic substitution influences the behaviour following acute dopaminergic stimulation. Objective of this study was to investigate the motor response following an apomorphine application in previously untreated-, treated- and before treated PD patients, who received placebo. Outcomes of instrumental tests worsened in previously untreated-, but not in before treated PD patients after apomorphine injection and in the PD subjects under the placebo condition. Generally, rating scores of motor symptoms significantly improved after apomorphine administration, whereas placebo application showed no effects. Tolerance to sedative effects of apomorphine in treated PD patients or sensitivity of employed motor tests to presynaptic dopaminergic autoreceptor mediated inhibition of endogenous dopamine release or postsynaptic dopaminergic overstimulation with resulting decreased cognitive function in previously untreated PD patients hypothetically caused this discrepancy between outcomes of subjective clinical rating and objective motor test performance.

L-DOPA treatment modulates nicotinic receptors in monkey striatum

Nicotinic acetylcholine receptor (nAChR) activation is well known to stimulate dopamine release in the striatum. This phenomenon may be physiologically significant in the control of motor function, as well as in pathological conditions such as Parkinson's disease. An understanding of the mechanisms that influence nAChR expression and function is therefore important. Because the dopamine precursor l-DOPA is the most commonly used therapeutic agent for Parkinson's disease, we investigated the effects of l-DOPA treatment on striatal nAChR expression in unlesioned and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-lesioned monkeys. In unlesioned animals, l-DOPA (15 mg/kg) administered twice daily for 2 weeks decreased both 125I-epibatidine and [125I]iodo-3-[2(S)-azetidinylmethoxy]pyridine (A-85380) binding sites in the caudate and putamen, but did not affect 125I-alpha-CtxMII sites. alpha-CtxMII inhibition of striatal 125I-epibatidine and [125I]A-85380 binding with alpha-CtxMII suggest that there are both high- (Ki < 0.2 nM) and low-affinity (Ki > 100 nM) alpha-CtxMII-sensitive sites, as well as alpha-CtxMII-resistant sites, and that l-DOPA treatment influences only the low-affinity alpha-CtxMII-sensitive subtype. The l-DOPA effect was selective for striatal nAChRs with no change in cortical sites. Monkeys with severe nigrostriatal damage did not exhibit l-DOPA-induced declines in striatal nAChRs, suggesting that l-DOPA primarily affects nAChRs associated with dopaminergic terminals. In summary, these data show that l-DOPA treatment decreases nAChR expression, in contrast with the well established up-regulation of these sites by chronic nicotine exposure. Furthermore, they demonstrate preferential l-DOPA regulation of a novel low-affinity alpha-CtxMII-sensitive site. These declines in nAChRs with l-DOPA may be relevant to both the therapeutic and side effect profiles of l-DOPA therapy in Parkinson's disease.

Slowing Parkinson's disease progression: recent dopamine agonist trials

In recent clinical trials, chronic treatment of patients with PD with pramipexole or ropinirole was associated with a slower decline of imaged striatal dopaminergic signal, compared to levodopa monotherapy. Although this could reflect slowed progression of PD, equally plausible is a pharmacologic effect on proteins that interact with the imaging radioligands. To date, there is no compelling evidence favoring dopamine agonists over levodopa; either is an appropriate choice for initial treatment of PD.

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.

Differential nicotinic receptor expression in monkey basal ganglia: effects of nigrostriatal damage

Our previous work showed that there were marked declines in (125)I-alpha-conotoxin MII labeled nicotinic receptors in monkey basal ganglia after nigrostriatal damage, findings that suggest alpha3/alpha6 containing nicotinic receptors sites may be of relevance to Parkinson's disease. We now investigate whether there are differential changes in the distribution pattern of nicotinic receptor subtypes in the basal ganglia in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned animals compared to controls to better understand the changes occurring with nigrostriatal damage. To approach this we used (125)I-alpha-conotoxin MII, a marker for alpha3/alpha6 nicotinic receptors, and (125)I-epibatidine, a ligand that labels multiple nicotinic subtypes. The results demonstrate that there were medial to lateral gradients in nicotinic receptor distribution in control striatum, as well as ventromedial to dorsolateral gradients in the substantia nigra, which resembled those of the dopamine transporter in these same brain regions. Treatment with MPTP, a neurotoxin that selectively destroys dopaminergic nigrostriatal neurons, led to a relatively uniform decrease in nicotinic receptor sites in the striatum, but a differential effect in the substantia nigra with significantly greater declines in the ventrolateral portion. Competition analysis in the striatum showed that alpha-conotoxin MII sensitive sites were primarily affected after lesioning, whereas multiple nicotinic receptor populations were decreased in the substantia nigra. From these data we suggest that in the striatum alpha3/alpha6 nicotinic receptors are primarily localized on dopaminergic nerve terminals, while multiple nicotinic receptor subtypes are present on dopaminergic cell bodies in the substantia nigra. Thus, if activation of striatal nicotinic receptors is key in the regulation of basal ganglia function, alpha3/alpha6-directed nicotinic receptor ligands may be more relevant for Parkinson's disease therapy. However, nicotinic receptor ligands with a broader specificity may be more important if receptors in the substantia nigra play a dominant role in controlling nigrostriatal activity.

The monoamine reuptake blocker brasofensine reverses akinesia without dyskinesia in MPTP-treated and levodopa-primed common marmosets

The common marmoset develops motor deficits after MPTP treatment and exhibits dyskinesia after chronic levodopa (L-dopa) dosing and subsequent re-challenge with L-dopa and other dopaminergic agents. We report on the actions of the potent monoamine reuptake blocker brasofensine on motor disability, locomotor activity, and dyskinesia in the 1-methyl-4-1, 2,3,6-tetrahydropyridine (MPTP) -treated marmoset model of Parkinson's disease. Oral administration of brasofensine (0.25, 0.5, 1.0, or 2.5 mg/kg) to MPTP-treated marmosets produced a long-lasting, dose-dependent increase in locomotor activity and reduction in disability scores. In addition, coadministration of the lowest dose of brasofensine (0.25 mg/kg orally) with a threshold oral dose of L-dopa (2.5 mg/kg) caused a marked increase in locomotor activity, greater than that produced by either drug alone. In other MPTP-treated marmosets previously primed to exhibit dyskinesia by repeated L-dopa dosing, brasofensine effectively reversed akinesia with a naturalistic and prolonged motor response without the appearance of dyskinesia or stereotypy. This finding contrasts with the severe dyskinesia, stereotypy, and hyperkinesis produced by equivalent doses of L-dopa. The ability of brasofensine to produce a prolonged and naturalistic antiparkinsonian response without eliciting dyskinesia after previous L-dopa priming may relate to actions on D(1) receptor-linked pathways. These findings suggest that monoamine reuptake blockade may be of value in the treatment of Parkinson's disease, both early in the disease course and when L-dopa-induced dyskinesias complicate treatment.

Tapping and peg insertion after levodopa intake in treated and de novo parkinsonian patients

BACKGROUND

Investigators use instrumental tasks for objective assessment of parkinsonian motor disability and its drug response. To date, such studies on treated parkinsonian patients have not addressed acute and long-term effects of dopaminergic drugs.

OBJECTIVES

To determine the impact of long-term dopaminergic therapy within a standardized levodopa challenge test design in combination with two repeatedly performed instrumental tasks, peg insertion and tapping, in previously treated and untreated parkinsonian patients.

RESULTS

Tapping significantly deteriorated in previously untreated, but not in treated parkinsonian patients after levodopa intake. In contrast, motor symptoms and peg insertion significantly improved in both groups of parkinsonian patients. Results of both tests differed between parkinsonian patients and matched controls.

CONCLUSION

Worsening of cognitively less demanding tapping may result from upregulated presynaptic inhibitory feedback regulation, sedative effects of levodopa or dopamine overflow in untreated parkinsonian patients, who are sensitive to these effects in contrast to treated parkinsonian patients. Tapping is a task with autonomic repetitive performance and programming of standardised movements with a low need for cognitive effort. This autonomic functioning of attentional control and selective processing is intact in Parkinson's disease. Peg insertion depends on more complex movements and thus hypothetically on dopamine-associated cognitive processes. Therefore, impairment of peg insertion responded to dopaminergic stimulation in both groups of parkinsonian patients. Future studies on the efficacy of antiparkinsonian drugs, using instrumental tasks for objective assessment, should consider long-term impact of antiparkinsonian drug therapy and associated cognitive efforts.

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.

Dopamine D3 receptor as a therapeutic target for antipsychotic and antiparkinsonian drugs

The cloning of the gene for the D3 receptor and subsequent identification of its distribution in brain and pharmacology allowed for serious consideration of the possibility that it might be a target for drugs used to treat schizophrenia and Parkinson's disease (PD). That is because it is highly expressed in limbic regions of the brain, exhibits low expression in motor divisions, and has pharmacologic similarity to the D2 receptor. Thus, antipsychotics that were presumed to block D2 receptors also had high affinity for the D3 receptor. Dopamine agonists used to treat the clinical symptoms of PD also have high affinity for the D3 receptor, and two D3 receptor-preferring agonists were found to be effective for treatment of PD. Many compounds achieving high potency and selectivity are now available, but few have reached clinical testing. Recent findings with respect to the anatomy of this receptor in human brain, altered expression in schizophrenia and PD, and biological models to study its function support the proposal that it is a target for development of drugs to alleviate symptoms in neuropsychiatric and neurologic disorders. Because of distinct aspects of regulation of the D3 receptor, it represents a unique target for therapeutic intervention in schizophrenia without high potential for unintended side effects such as tardive dyskinesia. It may also be that D3 receptor agonists can provide neuroprotective effects in PD and can modify clinical symptoms that D2 receptor-preferring agonists cannot provide.

Time-course and dose-response study on the effects of chronic L-DOPA administration on striatal dopamine levels and dopamine transporter following MPTP toxicity

Despite a long-lasting therapeutic use of L-DOPA in Parkinson's disease, doubts still remain concerning the possibility that chronic L-DOPA might accelerate the progression of this movement disorder. To address this point, in the present study we examined the effects of chronic L-DOPA administration either in intact or MPTP-treated parkinsonian mice. We produced an intermediate striatal dopamine loss by administering a low dose of MPTP (30 mg/kg); then, we treated mice chronically, for different time intervals, with a daily dose of L-DOPA (50 mg/kg). In particular, to study the time-course of the effects of L-DOPA on the recovery of nigrostriatal dopamine axons, mice were sacrificed at 5, 30, 60, and 90 days after a daily L-DOPA administration. To evaluate presynaptic integrity of the nigrostriatal pathway we measured dopamine, metabolite levels, and dopamine uptake sites. In the same animals, we measured striatal serotonin levels and we analysed monoamine content in the olfactory bulb. Administration of MPTP produced a neurotoxic effect, which fully recovered in 2-3 months. Daily L-DOPA administration did not modify this recovery process. Additionally, there was no significant effect of L-DOPA in intact mice, despite a slight decrease in striatal dopamine levels at 5 and 30 days. However, this effect was neither worsened nor reproduced by administering higher doses of L-DOPA (up to 400 mg/kg) for the same amount of time. These data rule out neurotoxic effects induced by prolonged L-DOPA administration, both in intact and MPTP-treated mice. Moreover, administration of L-DOPA does not change the recovery process which takes place after a nigrostriatal lesion.

Expression of D(3) receptor messenger RNA and binding sites in monkey striatum and substantia nigra after nigrostriatal degeneration: effect of levodopa treatment

D(3) receptors are prominently localized in the primate caudate-putamen, and D(3) receptor agonist properties may offer an advantage in Parkinson's disease therapy. In the present experiments, we investigated the relationship between D(3) receptor mRNA, D(3) receptor sites and the dopamine transporter in monkey basal ganglia by comparing their distribution in the brain of control and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated monkeys (Samirai sciureus). In control monkeys, D(3) receptor mRNA appears to be widely expressed throughout the brain, with a distribution similar to that observed in both man and rodent. D(3) receptors are present in areas which express mRNA but also in some which do not, an observation which suggests they may be both pre- and postsynaptic in the monkey brain. Chronic MPTP administration, which selectively destroys the nigrostriatal system, resulted in a 70 to 99% depletion of the dopamine transporter in the basal ganglia. Autoradiographic analysis showed that after MPTP treatment there was a significant decline in D(3) receptors in the caudate, but not putamen, globus pallidus, substantia nigra or other dopaminergic regions. D(3) receptor mRNA expression was not changed in any region after nigrostriatal lesioning. Two weeks of L-3,4-dihydroxyphenylalanine (levodopa, L-DOPA) treatment, which alleviated Parkinsonism but also induced dyskinesias, reversed the MPTP-induced decline in caudate D(3) receptors. These results show that there is a selective decline in D(3) receptors in the caudate after nigrostriatal degeneration, which is reversed by L-DOPA treatment. Since the majority of dopaminergic nerve terminals were destroyed after MPTP lesioning, the reversal in D(3) receptors after L-DOPA treatment may represent an increase in caudate postsynaptic receptors, which could conceivably contribute to an imbalance in striatal circuitry and the development of dyskinesias.

Effects of GM1 ganglioside treatment on pre- and postsynaptic dopaminergic markers in the striatum of parkinsonian monkeys

GM1 ganglioside administration has previously been shown to increase striatal dopamine levels and to enhance the density of tyrosine hydroxylase-positive fibers in the striatum of monkeys made parkinsonian by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). The present study examined the extent to which GM1 administration promotes recovery of dopamine terminals and reverses lesion-induced changes in postsynaptic receptors in the striatum of MPTP-treated monkeys. All MPTP-treated animals developed severe parkinsonism. GM1-treated monkeys exhibited significant functional recovery after 6 weeks of treatment, whereas saline-treated controls remained parkinsonian over the same time period. MPTP exposure resulted in profound decreases in [(3)H]-mazindol binding to dopamine transporters in the caudate and putamen and increased D1 and D2 receptor binding in several striatal regions. GM1 treatment resulted in significant increases in striatal [(3)H]-mazindol binding and decreases in D1 binding compared to control animals in many striatal regions. GM1 treatment did not significantly affect D2 binding. These results show that GM1 treatment can partially restore striatal dopaminergic terminals and partially reverse postsynaptic changes in dopamine receptors in a nonhuman primate model of parkinsonism.

Differential regulation of striatal dopamine D(1) and D(2) receptors in acute and chronic parkinsonian monkeys

The contribution of the duration of the striatal dopamine (DA) depletion and the expression of parkinsonian signs to changes in D(1) and D(2) receptor number was investigated in the present study. Some animals (N=4) received large doses of 1-methyl-4-phenyl-1,2,3, 6-tetrahydropyridine (MPTP) over short periods of time and were symptomatic for a short period of time (1-3 months; acute parkinsonian group). Other animals (N8 months; chronic parkinsonian group). Despite similar symptomatology and similar degrees of striatal DA denervation, only acute parkinsonian animals had significantly increased numbers of D(1) receptors in most striatal regions. Striatal D(2) receptor binding was elevated in acute parkinsonian monkeys but only in some lateral striatal subregions at mid and caudal levels. These findings further suggest that the duration of parkinsonism is a critical factor in modulating changes in striatal neurochemistry.

Levodopa in Parkinson's disease: neurotoxicity issue laid to rest?

Orally administered levodopa remains the most effective symptomatic treatment for Parkinson's disease. The introduction of levodopa therapy is often delayed, however, because of the fear that it might be toxic for the remaining dopaminergic neurons, and thus accelerate the deterioration of the patient's condition. Evidence for levodopa toxicity comes mainly from in vitro studies which have demonstrated that levodopa can damage dopaminergic neurons by a mechanism that probably involves oxidative stress. It is widely accepted, however, that levodopa is not toxic for healthy animals and humans who do not have Parkinson's disease. It has been argued that the lesioned mesostriatal dopaminergic system could be more vulnerable to levodopa-induced toxicity, because the brain extracellular concentrations attained by levodopa are higher when the dopaminergic system is damaged, and remaining dopaminergic neurons experience a process of compensatory hyperactivity. Evidence for in vivo levodopa toxicity in animal models of Parkinson's disease is scarce and contradictory. A comprehensive recent study failed to find any evidence of levodopa toxicity in rats with either moderate or severe lesions of the mesostriatal dopaminergic system. Concerning the hypothesis of toxicity, some recent reports have shown that levodopa can have trophic effects on dopaminergic neurons in vitro, and our own work has shown that long term levodopa therapy promotes recovery of striatal dopaminergic markers in rats with moderate nigrostriatal lesions. Given that neither epidemiological nor clinical studies have ever provided evidence to support that long term levodopa administration can accelerate the progression of Parkinson's disease, we believe that levodopa therapy should not be delayed on the basis of an unconfirmed hypothesis.

Striatal preproenkephalin gene expression is upregulated in acute but not chronic parkinsonian monkeys: implications for the contribution of the indirect striatopallidal circuit to parkinsonian symptomatology

This study examined the extent of striatal dopamine (DA) denervation and coincident expression of preproenkephalin (PPE) mRNA in monkeys made parkinsonian by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) administration. Some animals (n = 4) became moderately parkinsonian after receiving large doses of MPTP over short periods of time and were symptomatic for only a short period of time (1-3 months; acute parkinsonian group). Other animals became moderately parkinsonian after receiving either escalating doses of MPTP over long periods (4-6 months; n = 5) or a high dose of MPTP over a short period (<1 month; n = 1) and remained symptomatic for an extended period (>8 months; chronic parkinsonian group). Despite similar symptomatology and similar degrees of striatal DA denervation at the time of their deaths, only acute parkinsonian animals had significantly increased PPE expression in sensorimotor striatal regions. PPE expression in chronic parkinsonian animals was either not changed or significantly decreased in most striatal regions. These findings suggest that the duration and not the extent of striatal DA denervation is a critical factor in modulating changes in striatal PPE expression. Furthermore, these results question the role of increased activity in the enkephalin-containing indirect striatopallidal pathway in the expression of parkinsonian symptoms.

Striatal dopaminergic markers in dementia with Lewy bodies, Alzheimer's and Parkinson's diseases: rostrocaudal distribution

Dementia with Lewy bodies (DLB) is a neuropsychiatric disease associated with extrapyramidal features which differ from those of Parkinson's disease, including reduced effectiveness of L-dopa and severe sensitivity reactions to neuroleptic drugs. Distinguishing Alzheimer's disease from DLB is clinically relevant in terms of prognosis and appropriate treatment. Dopaminergic activities have been investigated at coronal levels along the rostrocaudal striatal axis from a post-mortem series of 25 DLB, 14 Parkinson's disease and 17 Alzheimer's disease patients and 20 elderly controls. [(3)H]Mazindol binding to the dopamine uptake site was significantly reduced in the caudal putamen in DLB compared with controls (57%), but not as extensively as in Parkinson's disease (75%), and was unchanged in Alzheimer's disease. Among three dopamine receptors measured (D1, D2 and D3), the most striking changes were apparent in relation to D2. In DLB, [(3)H]raclopride binding to D2 receptors was significantly reduced in the caudal putamen (17%) compared with controls, and was significantly lower than in Parkinson's disease at all levels. D2 binding was significantly elevated at all coronal levels in Parkinson's disease compared with controls, most extensively in the rostral putamen (71%). There was no change from the normal pattern of D2 binding in Alzheimer's disease. The only significant alteration in D1 binding ([(3)H]SCH23390) in the groups examined was an elevation (30%) in the caudal striatum in Parkinson's disease. There were no differences in D3 binding, measured using [(3)H]7-OH-DPAT, in DLB compared with controls. A slight, significant decrease in D3 binding in the caudal striatum of Parkinson's disease (13%) patients and an increase in Alzheimer's disease (20%) in the dorsal striatum at the level of the nucleus accumbens were found. The concentration and distribution of dopamine were disrupted in both DLB and Parkinson's disease, although in the caudate nucleus the loss of dopamine in DLB was uniform whereas in Parkinson's disease the loss was greater caudally. In the caudal putamen, dopamine was reduced by 72% in DLB and by 90% in Parkinson's disease. The homovanillic acid : dopamine ratio, a metabolic index, indicated compensatory increased turnover in Parkinson's disease, which was absent in DLB despite the loss of substantia nigra neurons (49%), dopamine and uptake sites. These differences between DLB, Parkinson's disease and Alzheimer's disease may explain some characteristics of the extrapyramidal features of DLB and its limited response to L-dopa and severe neuroleptic sensitivity. The distinct changes in the rostrocaudal pattern of expression of dopaminergic parameters are relevant to the interpretation of the in vivo imaging and diagnosis of DLB.

Dopaminergic activities in the human striatum: rostrocaudal gradients of uptake sites and of D1 and D2 but not of D3 receptor binding or dopamine

The human striatum, which receives dopaminergic innervation from the substantia nigra and ventral tegmental area (cell groups A8, A9 and A10), has structural and functional subdivisions both rostrocaudally and dorsoventrally. These relate to motor and non-motor origins of cortical projections and the specific areas of the substantia nigra and ventral tegmental area providing dopaminergic innervation. In the present study, we have evaluated the distribution of a number of dopaminergic parameters in the caudate, putamen and nucleus accumbens at separate coronal levels in a post mortem study in a series of elderly normal individuals aged 55-94 years, with analysis of the effect of post mortem variables. Dopamine D1 receptor density displayed a rostrocaudally declining gradient in the putamen but not in the caudate, such that at levels posterior to the anterior commissure, there was significantly lower D1 binding in the putamen compared to the caudate. The density of dopamine D2 receptors was similar in the putamen and caudate, increasing rostrocaudally. The density of dopamine uptake sites exhibited an increasing rostrocaudal gradient in the caudate, especially ventrally, but not in the putamen, where binding was more constant. The dopamine D3 receptor was concentrated in the ventral striatum, particularly the nucleus accumbens, although there was no evidence of a rostrocaudal gradient. With respect to striosome-matrix compartmentalization, there was no complete segregation, although D1 and D3 receptors were concentrated in striosomes, whereas D2 receptors and uptake sites showed higher density in the matrix. Levels of dopamine were similar in the caudate and putamen, and were significantly elevated at levels including the nucleus accumbens and the anterior commissure. Homovanillic acid and the metabolic index (homovanillic acid/dopamine ratio) were significantly higher in the putamen compared to the caudate, especially at levels from and caudal to the anterior commissure. These distributions of dopamine receptors and metabolic indicators, reflecting the different functional domains of the striatum, are relevant to the interpretation of current in vivo imaging of the dopamine transporter and receptors in neurological and psychiatric disorders. They provide information to assist in the detection of perturbations in expression, in specific diseases, at particular points on rostrocaudal, lateromedial and dorsoventral axes, a level of resolution beyond current neuroimaging capability.