Physiology of the normal and dopamine-depleted basal ganglia: insights into levodopa pharmacotherapy

Pain in Parkinson disease: mechanistic substrates, main classification systems, and how to make sense out of them

ABSTRACT

Parkinson disease (PD) affects up to 2% of the general population older than 65 years and is a major cause of functional loss. Chronic pain is a common nonmotor symptom that affects up to 80% of patients with (Pw) PD both in prodromal phases and during the subsequent stages of the disease, negatively affecting patient's quality of life and function. Pain in PwPD is rather heterogeneous and may occur because of different mechanisms. Targeting motor symptoms by dopamine replacement or with neuromodulatory approaches may only partially control PD-related pain. Pain in general has been classified in PwPD according to the motor signs, pain dimensions, or pain subtypes. Recently, a new classification framework focusing on chronic pain was introduced to group different types of PD pains according to mechanistic descriptors: nociceptive, neuropathic, or neither nociceptive nor neuropathic. This is also in line with the International Classification of Disease-11 , which acknowledges the possibility of chronic secondary musculoskeletal or nociceptive pain due to disease of the CNS. In this narrative review and opinion article, a group of basic and clinical scientists revise the mechanism of pain in PD and the challenges faced when classifying it as a stepping stone to discuss an integrative view of the current classification approaches and how clinical practice can be influenced by them. Knowledge gaps to be tackled by coming classification and therapeutic efforts are presented, as well as a potential framework to address them in a patient-oriented manner.

To restrict or not to restrict? Practical considerations for optimizing dietary protein interactions on levodopa absorption in Parkinson's disease

Administration of levodopa for Parkinson's disease (PD) has remained the most effective therapy for symptom management despite being in use for over 50 years. Advancing disease and age, changing tolerability and gastrointestinal (GI) dysfunction may result in change in dietary habits and body weight, as well as unpredictable motor fluctuations and dyskinesias. Dietary proteins which convert into amino acids after digestion are implicated as major factors that inhibit levodopa absorption. For people living with PD (PwP) who experience motor fluctuations, low protein diets (LPD) and protein redistribution diets (PRD) may be effective and are often recommended as a non-pharmacologic approach for improving levodopa bioavailability. However, there is a lack of consensus on a standard definition of these diets and appropriate treatment algorithms for usage. This may be due to the paucity of high-level evidence of LPD and PRD in PwP and whether all or specific subgroups of patients would benefit from these strategies. Managing diet and protein intake with proper education and monitoring may reduce complications associated with these diets such as dyskinesias and unintentional weight loss. Additionally, alterations to medications and GI function may alter levodopa pharmacokinetics. In this narrative review we focus on 1) mechanisms of dietary protein and levodopa absorption in the intestine and blood brain barrier, 2) dietetic approaches to manage protein and levodopa interactions and 3) practical issues for treating PwP as well as future directions to be considered.

L-DOPA modulates activity in the vmPFC, nucleus accumbens, and VTA during threat extinction learning in humans

Learning to be safe is central for adaptive behaviour when threats are no longer present. Detecting the absence of an expected threat is key for threat extinction learning and an essential process for the behavioural treatment of anxiety-related disorders. One possible mechanism underlying extinction learning is a dopaminergic mismatch signal that encodes the absence of an expected threat. Here we show that such a dopamine-related pathway underlies extinction learning in humans. Dopaminergic enhancement via administration of L-DOPA (vs. Placebo) was associated with reduced retention of differential psychophysiological threat responses at later test, which was mediated by activity in the ventromedial prefrontal cortex that was specific to extinction learning. L-DOPA administration enhanced signals at the time-point of an expected, but omitted threat in extinction learning within the nucleus accumbens, which were functionally coupled with the ventral tegmental area and the amygdala. Computational modelling of threat expectancies further revealed prediction error encoding in nucleus accumbens that was reduced when L-DOPA was administered. Our results thereby provide evidence that extinction learning is influenced by L-DOPA and provide a mechanistic perspective to augment extinction learning by dopaminergic enhancement in humans.

The Renin-Angiotensin System Modulates Dopaminergic Neurotransmission: A New Player on the Scene

Parkinson's disease (PD) is an extrapyramidal disorder characterized by neuronal degeneration in several regions of the peripheral and central nervous systems. It is the second most frequent neurodegenerative disease after Alzheimer's. It has become a major health problem, affecting 1% of the world population over 60 years old and 3% of people beyond 80 years. The main histological findings are intracellular Lewy bodies composed of misfolded α-synuclein protein aggregates and loss of dopaminergic neurons in the central nervous system. Neuroinflammation, apoptosis, mitochondrial dysfunction, altered calcium homeostasis, abnormal protein degradation, and synaptic pathobiology have been put forward as mechanisms leading to cell death, α-synuclein deposition, or both. A progressive loss of dopaminergic neurons in the substantia nigra late in the neurodegeneration leads to developing motor symptoms like bradykinesia, tremor, and rigidity. The renin-angiotensin system (RAS), which is involved in regulating blood pressure and body fluid balance, also plays other important functions in the brain. The RAS is involved in the autocrine and paracrine regulation of the nigrostriatal dopaminergic synapses. Dopamine depletion, as in PD, increases angiotensin II expression, which stimulates or inhibits dopamine synthesis and is released via AT1 or AT2 receptors. Furthermore, angiotensin II AT1 receptors inhibit D1 receptor activation allosterically. Therefore, the RAS may have an important modulating role in the flow of information from the brain cortex to the basal ganglia. High angiotensin II levels might even aggravate neurodegeneration, activating the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase complex, which leads to increased reactive oxygen species production.

The structural connectivity of discrete networks underlies impulsivity and gambling in Parkinson’s disease

See O’Callaghan (doi:10.1093/brain/awz349) for a scientific commentary on this article.

Mosley et al. examine impulsivity and naturalistic gambling behaviours in patients with Parkinson’s disease. They link within-patient differences to the structural connectivity of networks subserving reward evaluation and response inhibition, and reveal pivotal roles for the ventral striatum and subthalamic nucleus within these networks.

Neurochemical changes in basal ganglia affect time perception in parkinsonians

BACKGROUND

Parkinson's disease is described as resulting from dopaminergic cells progressive degeneration, specifically in the substantia nigra pars compacta that influence the voluntary movements control, decision making and time perception.

AIM

This review had a goal to update the relation between time perception and Parkinson's Disease.

METHODOLOGY

We used the PRISMA methodology for this investigation built guided for subjects dopaminergic dysfunction in the time judgment, pharmacological models with levodopa and new studies on the time perception in Parkinson's Disease. We researched on databases Scielo, Pubmed / Medline and ISI Web of Knowledge on August 2017 and repeated in September 2017 and February 2018 using terms and associations relevant for obtaining articles in English about the aspects neurobiology incorporated in time perception. No publication status or restriction of publication date was imposed, but we used as exclusion criteria: dissertations, book reviews, conferences or editorial work.

RESULTS/DISCUSSION

We have demonstrated that the time cognitive processes are underlying to performance in cognitive tasks and that many are the brain areas and functions involved and the modulators in the time perception performance.

CONCLUSIONS

The influence of dopaminergic on Parkinson's Disease is an important research tool in Neuroscience while allowing for the search for clarifications regarding behavioral phenotypes of Parkinson's disease patients and to study the areas of the brain that are involved in the dopaminergic circuit and their integration with the time perception mechanisms.

Efficacy and safety of amantadine for the treatment of L-DOPA-induced dyskinesia

L-DOPA induced dyskinesias (LIDs) may affect up to 40% of Parkinson's disease (PD) and impact negatively health-related quality of life. Amantadine has demonstrated significant antidyskinetic effects in animal PD models and in randomized double-blind placebo-controlled trials (RCTs) in patients with PD. These effects are thought to be related to the blockade of NMDA receptors modulating cortico-striatal glutamatergic-dopaminergic interactions involved in the genesis of LIDs. There are three pharmaceutical forms of amantadine currently available in the market: an oral immediate-release (IR) formulation, which is widely available; an extended-release (ER) formulation (ADS-5102) which has been recently developed and approved by the FDA; and an intravenous infusion (IV) solution, which is not commonly used in clinical practice. RCTs with amantadine IR or ER, involving more than 650 patients have shown consistent and long-lasting reductions in LIDs. Interestingly, ADS-5102 not only reduced LIDs, but also reduced significantly at the same time the duration of daily OFF-time, a unique finding compared with other antiparkinsonian medications that usually reduce time spent OFF at the cost of worsening of LIDs. Amantadine IR might also have possible effects on other PD symptoms such as apathy or fatigue. The most common adverse reactions with amantadine are constipation, cardiovascular dysfunction including QT prolongation, orthostatic hypotension and edema, neuropsychiatric symptoms such as hallucinations, confusion and delirium, nausea and livedo reticularis. Corneal degeneration is rare but critical. In summary, amantadine immediate and extended-release are effective and safe for the treatment of LIDs.

Neuroprotection and neurorestoration as experimental therapeutics for Parkinson's disease

Disease-modifying treatments remain an unmet medical need in Parkinson's disease (PD). Such treatments can be operationally defined as interventions that slow down the clinical evolution to advanced disease milestones. A treatment may achieve this outcome by either inhibiting primary neurodegenerative events ("neuroprotection") or boosting compensatory and regenerative mechanisms in the brain ("neurorestoration"). Here we review experimental paradigms that are currently used to assess the neuroprotective and neurorestorative potential of candidate treatments in animal models of PD. We review some key molecular mediators of neuroprotection and neurorestoration in the nigrostriatal dopamine pathway that are likely to exert beneficial effects on multiple neural systems affected in PD. We further review past and current strategies to therapeutically stimulate these mediators, and discuss the preclinical evidence that exercise training can have neuroprotective and neurorestorative effects. A future translational task will be to combine behavioral and pharmacological interventions to exploit endogenous mechanisms of neuroprotection and neurorestoration for therapeutic purposes. This type of approach is likely to provide benefit to many PD patients, despite the clinical, etiological, and genetic heterogeneity of the disease.

Renin-angiotensin system as a potential target for new therapeutic approaches in Parkinson's disease

INTRODUCTION

Currently, available therapies for Parkinson's disease (PD) are symptomatic. Therefore, the search for neuroprotective drugs remains a top priority. Areas covered: In this review, the potential symptomatic or disease-modifying effect of drugs targeting the Renin-Angiotensin System (RAS) in PD will be explored. Expert opinion: The importance of nigrostriatal local RAS has only begun to be unraveled in the last decades. On one hand, there is a complex feedback cycle between RAS and dopamine (DA). On the other hand, RAS affects dopaminergic neurons vulnerability. Neuroprotective effects in animal PD models have been shown for the angiotensin-converting enzyme (ACE) inhibitors captopril and perindopril, and the AT1 receptor antagonists losartan, candesartan and telmisartan. These effects appear to be mediated by a reduction in the overproduction of reactive oxygen species. In a proof-of-concept, randomized, double-blind, crossover study in PD patients, perindopril enhanced the effect of levodopa without inducing dyskinesias. There has not been any clinical trial exploring the neuroprotective effect of RAS drugs, but one cohort study in hypertensive patients suggested a protective effect of ACE inhibitors on PD risk. RAS is a promising target for symptomatic and neuroprotective therapies in PD. Further studies in PD animal models and patients are warranted.

Effect of Levodopa on Reward and Impulsivity in a Rat Model of Parkinson's Disease

The use of dopamine replacement therapies (DRT) in the treatment of Parkinson’s disease (PD) can lead to the development of dopamine dysregulation syndrome (DDS) and impulse control disorders (ICD), behavioral disturbances characterized by compulsive DRT self-medication and development of impulsive behaviors. However, the mechanisms behind these disturbances are poorly understood. In animal models of PD, the assessment of the rewarding properties of levodopa (LD), one of the most common drugs used in PD, has produced conflicting results, and its ability to promote increased impulsivity is still understudied. Moreover, it is unclear whether acute and chronic LD therapy differently affects reward and impulsivity. In this study we aimed at assessing, in an animal model of PD with bilateral mesostriatal and mesocorticolimbic degeneration, the behavioral effects of LD therapy regarding reward and impulsivity. Animals with either sham or 6-hydroxydopamine (6-OHDA)-induced bilateral lesions in the substantia nigra pars compacta (SNc) and ventral tegmental area (VTA) were exposed to acute and chronic LD treatment. We used the conditioned place preference (CPP) paradigm to evaluate the rewarding effects of LD, whereas impulsive behavior was measured with the variable delay-to-signal (VDS) task. Correlation analyses between behavioral measurements of reward or impulsivity and lesion extent in SNc/VTA were performed to pinpoint possible anatomical links of LD-induced behavioral changes. We show that LD, particularly when administered chronically, caused the development of impulsive-like behaviors in 6-OHDA-lesioned animals in the VDS. However, neither acute or chronic LD administration had rewarding effects in 6-OHDA-lesioned animals in the CPP. Our results show that in a bilateral rat model of PD, LD leads to the development of impulsive behaviors, strengthening the association between DRT and DDS/ICD in PD.

Neurotransmission systems in Parkinson’s disease

Parkinson’s disease (PD) is histologically characterized by the accumulation of α-synuclein particles, known as Lewy bodies. The second most common neurodegenerative disorder, PD is widely known because of the typical motor manifestations of active tremor, rigidity, and postural instability, while several prodromal non-motor symptoms including REM sleep behavior disorders, depression, autonomic disturbances, and cognitive decline are being more extensively recognized. Motor symptoms most commonly arise from synucleinopathy of nigrostriatal pathway. Glutamatergic, γ-aminobutyric acid (GABA)ergic, cholinergic, serotoninergic, and endocannabinoid neurotransmission systems are not spared from the global cerebral neurodegenerative assault. Wide intrabasal and extrabasal of the basal ganglia provide enough justification to evaluate network circuits disturbance of these neurotransmission systems in PD. In this comprehensive review, English literature in PubMed, Science direct, EMBASE, and Web of Science databases were perused. Characteristics of dopaminergic and non-dopaminergic systems, disturbance of these neurotransmitter systems in the pathophysiology of PD, and their treatment applications are discussed.

Piribedil for the Treatment of Motor and Non-motor Symptoms of Parkinson Disease

Dopamine agonists are well-established symptomatic medications for treating early and advanced Parkinson disease (PD). Piribedil was one of the first agonists to be marketed (1969) and is widely used as an extended-release oral formulation in European, Latin-American, and Asian countries. Piribedil acts as a non-ergot partial dopamine D2/D3-selective agonist, blocks alpha2-adrenoreceptors and has minimal effects on serotoninergic, cholinergic, and histaminergic receptors. Animal models support the efficacy of piribedil to improve parkinsonian motor symptoms with a lower propensity than levodopa to induce dyskinesia. In PD patients, randomized double-blind studies show that piribedil (150-300 mg/day, three times daily) is superior to placebo in improving motor disability in early PD patients. Based on such evidence, piribedil was considered in the last Movement Disorder Society Evidence-Based Medicine review as "efficacious" and "clinically useful" for the symptomatic treatment of PD, either as monotherapy or in conjunction with levodopa, in non-fluctuating early PD patients. This effect appears comparable to what is known from other D2 agonists. However, randomized controlled trials are not available to assess the effect of piribedil in managing levodopa-induced motor complications. Pilot clinical studies suggest that piribedil may improve non-motor symptoms, such as apathy, but confirmatory trials are needed. The tolerability and safety profile of piribedil fits with that of the class of dopaminergic agonists. As for other non-ergot agonists, pneumo-pulmonary, retroperitoneal, and valvular fibrotic side effects are not a concern with piribedil. The original combination of piribedil D2 dopaminergic and alpha-2 adrenergic properties deserve further investigations to better understand its antiparkinsonian profile.

A review of current and novel levodopa formulations for the treatment of Parkinson's disease

Parkinson's disease treatment is characterized by the nearly inevitable development of motor complications, including fluctuations and dyskinesias, in which the duration of benefit of a dose of medication is offset by involuntary movements that can be more disabling than the Parkinsonian features themselves. While levodopa remains the gold standard of therapy, it is the most likely to be associated with these complications. The concept of continuous dopaminergic stimulation has gained increasing acceptance as a potential mechanism by which to avoid or delay the development of motor complications, or to minimize their impact once they have already occurred. This article will explore existing and novel formulations of levodopa to identify their role in the spectrum of Parkinson's disease therapeutics.

Effects of combined MAO-B inhibitors and levodopa vs. monotherapy in Parkinson's disease

Background: Prior studies report that monoamine oxidases inhibitors (MAO-I) when used as an adjunct to levodopa ameliorate motor symptoms in Parkinson’s disease (PD), but this was not tested in relation to cognitive or psychiatric measures.

Objective: Here, we tested the effects of MAO-I as an adjunct to levodopa, in comparison to levodopa or dopamine (DA) agonists alone, on various cognitive, affective and quality of life measures.

Methods: We studied three groups of subjects: healthy controls, PD patients on combined levodopa and MAO-I, and PD patients on levodopa or DA agonists only.

Results: We found that compared to monotherapy, combined MAO-I and levodopa seemed to improve cognition, including probabilistic learning, working memory and executive functions. There were no differences between the different medication regimes on deterministic learning, attention or memory recall. It was also found that MAO-I as an adjunct to levodopa improves affective measures such as depression, apathy, anxiety and quality of life. Interestingly, this enhancing effect of combined levodopa and MAO-I was more pronounced in PD patients with severe akinesia, compared to patients with severe tremor.

Conclusion: Our data are in agreement with (a) the Continuous Dopaminergic Stimulation (CDS) theory which states that continuous stimulation of the basal ganglia enhances motor, psychiatric and cognitive functions in PD patients; and/or (b) findings that MAO-I increase the bioavailability of monoamines that have beneficial effects on motor and behavioral dysfunction in PD.

Effect of peripheral D₂ dopamine receptor antagonist domperidone on metabolism, feeding behavior, and locomotor activity of rats

We studied the possibility of activation of the central dopaminergic system compartment by modulating activity of D2 dopamine receptors in the gastrointestinal tract with domperidone, an antagonist not crossing the blood-brain barrier. Intragastric administration of 0.1 mg/kg domperidone to rats was followed by a significant decrease in feeding behavior and stimulation of basal metabolism, but had no effect on locomotor activity of animals in a Phenomaster system. These effects are typical of psychostimulant agents that stimulate dopamine release from nerve endings in the nucleus accumbens and some regions of the brain cortex. Our results indicate that physiological functions associated with activity of the central dopaminergic system can be modulated through peripheral dopamine receptors.

Treating Parkinson's disease in the 21st century: can stem cell transplantation compete?

The characteristic and selective degeneration of a unique population of cells—the nigrostriatal dopamine (DA) neurons—that occurs in Parkinson’s disease (PD) has made the condition an iconic target for cell replacement therapies. Indeed, transplantation of fetal ventral mesencephalic cells into the DA-deficient striatum was first trialled nearly 30 years ago, at a time when other treatments for the disease were less well developed. Over recent decades standard treatments for PD have advanced, and newer biological therapies are now emerging. In the 21st century, stem cell technology will have to compete alongside other sophisticated treatments, including deep brain stimulation and gene therapies. In this review we examine how stem cell–based transplantation therapies compare with these novel and emerging treatments in the management of this common condition. J. Comp. Neurol. 522:2802–2816, 2014.

Neural correlates of motor vigour and motor urgency during exercise

This article reviews the brain structures and neural circuitry underlying the motor system as it pertains to endurance exercise. Some obvious phenomena that occur during endurance racing events that need to be explained neurophysiologically are variable pacing strategies, the end spurt, motivation and the rating of perceived exertion. Understanding the above phenomena physiologically is problematic due to the sheer complexity of obtaining real-time brain measurements during exercise. In those rare instances where brain measurements have been made during exercise, the measurements have usually been limited to the sensory and motor cortices; or the exercise itself was limited to small muscle groups. Without discounting the crucial importance of the primary motor cortex in the execution of voluntary movement, it is surprising that very few exercise studies pay any attention to the complex and dynamic organization of motor action in relation to the subcortical nuclei, given that they are essential for the execution of normal movement patterns. In addition, the findings from laboratory-based exercise performance trials are hampered by the absence of objective measures of the motivational state of subjects. In this review we propose that some of the above phenomena may be explained by distinguishing between voluntary, vigorous and urgent motor behaviours during exercise, given that different CNS structures and neurotransmitters are involved in the execution of these different motor behaviours.

Regulation of dopamine D1 receptor dynamics within the postsynaptic density of hippocampal glutamate synapses

Dopamine receptor potently modulates glutamate signalling, synaptic plasticity and neuronal network adaptations in various pathophysiological processes. Although key intracellular signalling cascades have been identified, the cellular mechanism by which dopamine and glutamate receptor-mediated signalling interplay at glutamate synapse remain poorly understood. Among the cellular mechanisms proposed to aggregate D1R in glutamate synapses, the direct interaction between D1R and the scaffold protein PSD95 or the direct interaction with the glutamate NMDA receptor (NMDAR) have been proposed. To tackle this question we here used high-resolution single nanoparticle imaging since it provides a powerful way to investigate at the sub-micron resolution the dynamic interaction between these partners in live synapses. We demonstrate in hippocampal neuronal networks that dopamine D1 receptors (D1R) laterally diffuse within glutamate synapses, in which their diffusion is reduced. Disrupting the interaction between D1R and PSD95, through genetical manipulation and competing peptide, did not affect D1R dynamics in glutamatergic synapses. However, preventing the physical interaction between D1R and the GluN1 subunit of NMDAR abolished the synaptic stabilization of diffusing D1R. Together, these data provide direct evidence that the interaction between D1R and NMDAR in synapses participate in the building of the dopamine-receptor-mediated signalling, and most likely to the glutamate-dopamine cross-talk.

Motor and nonmotor complications in Parkinson's disease: an argument for continuous drug delivery?

The complications of long-term levodopa therapy for Parkinson’s disease (PD) include motor fluctuations, dyskinesias, and also nonmotor fluctuations—at least equally common, but less well appreciated—in autonomic, cognitive/psychiatric, and sensory symptoms. In seeking the pathophysiologic mechanisms, the leading hypothesis is that in the parkinsonian brain, intermittent, nonphysiological stimulation of striatal dopamine receptors destabilizes an already unstable system. Accordingly, a major goal of PD treatment in recent years has been the attainment of continuous dopaminergic stimulation (CDS)—or, less theoretically (and more clinically verifiable), continuous drug delivery (CDD). Improvements in the steadiness of the plasma profiles of various dopaminergic therapies may be a signal of progress. However, improvements in plasma profile do not necessarily translate into CDS, or even into CDD to the brain. Still, it is reassuring that clinical studies of approaches to CDD have generally been positive. Head-to-head comparative trials have often failed to uncover evidence favoring such approaches over an intermittent therapy. Nevertheless, the findings among recipients of subcutaneous apomorphine infusion or intrajejunal levodopa/carbidopa intestinal gel suggest that nonmotor PD symptoms or complications may improve in tandem with motor improvement. In vivo receptor binding studies may help to determine the degree of CDS that a dopaminergic therapy can confer. This may be a necessary first step toward establishing whether CDS is, in fact, an important determinant of clinical efficacy. Certainly, the complexities of optimal PD management, and the rationale for an underlying strategy such as CDS or CDD, have not yet been thoroughly elucidated.

Ventral striatal dopamine synthesis capacity predicts financial extravagance in Parkinson's disease

Impulse control disorders (ICDs), including disordered gambling, can occur in a significant number of patients with Parkinson’s disease (PD) receiving dopaminergic therapy. The neurobiology underlying susceptibility to such problems is unclear, but risk likely results from an interaction between dopaminergic medication and a pre-existing trait vulnerability. Impulse control and addictive disorders form part of a broader psychopathological spectrum of disorders, which share a common underlying genetic vulnerability, referred to as externalizing. The broad externalizing risk factor is a continuously varying trait reflecting vulnerability to various impulse control problems, manifested at the overt level by disinhibitory symptoms and at the personality level by antecedent traits such as impulsivity and novelty/sensation seeking. Trait “disinhibition” is thus a core endophenotype of ICDs, and a key target for neurobiological investigation. The ventral striatal dopamine system has been hypothesized to underlie individual variation in behavioral disinhibition. Here, we examined whether individual differences in ventral striatal dopamine synthesis capacity predicted individual variation in disinhibitory temperament traits in individuals with PD. Eighteen early-stage male PD patients underwent 6-[¹⁸F]Fluoro-l-DOPA (FDOPA) positron emission tomography scanning to measure striatal dopamine synthesis capacity, and completed a measure of disinhibited personality. Consistent with our predictions, we found that levels of ventral, but not dorsal, striatal dopamine synthesis capacity predicted disinhibited personality, particularly a propensity for financial extravagance. Our results are consistent with recent preclinical models of vulnerability to behavioral disinhibition and addiction proneness, and provide novel insights into the neurobiology of potential vulnerability to impulse control problems in PD and other disorders.

Dissociating the cognitive effects of levodopa versus dopamine agonists in a neurocomputational model of learning in Parkinson's disease

BACKGROUND/AIMS

Levodopa and dopamine agonists have different effects on the motor, cognitive, and psychiatric aspects of Parkinson's disease (PD).

METHODS

Using a computational model of basal ganglia (BG) and prefrontal cortex (PFC) dopamine, we provide a theoretical synthesis of the dissociable effects of these dopaminergic medications on brain and cognition. Our model incorporates the findings that levodopa is converted by dopamine cells into dopamine, and thus activates prefrontal and striatal D(1) and D(2) dopamine receptors, whereas antiparkinsonian dopamine agonists directly stimulate D(2) receptors in the BG and PFC (although some have weak affinity to D(1) receptors).

RESULTS

In agreement with prior neuropsychological studies, our model explains how levodopa enhances, but dopamine agonists impair or have no effect on, stimulus-response learning and working memory.

CONCLUSION

Our model explains how levodopa and dopamine agonists have differential effects on motor and cognitive processes in PD.

Dual control of dopamine synthesis and release by presynaptic and postsynaptic dopamine D2 receptors

Dysfunctions of dopaminergic homeostasis leading to either low or high dopamine (DA) levels are causally linked to Parkinson's disease, schizophrenia, and addiction. Major sites of DA synthesis are the mesencephalic neurons originating in the substantia nigra and ventral tegmental area; these structures send major projections to the dorsal striatum (DSt) and nucleus accumbens (NAcc), respectively. DA finely tunes its own synthesis and release by activating DA D2 receptors (D2R). To date, this critical D2R-dependent function was thought to be solely due to activation of D2Rs on dopaminergic neurons (D2 autoreceptors); instead, using site-specific D2R knock-out mice, we uncover that D2 heteroreceptors located on non-DAergic medium spiny neurons participate in the control of DA levels. This D2 heteroreceptor-mediated mechanism is more efficient in the DSt than in NAcc, indicating that D2R signaling differentially regulates mesolimbic- versus nigrostriatal-mediated functions. This study reveals previously unappreciated control of DA signaling, shedding new light on region-specific regulation of DA-mediated effects.

Striatal plasticity in Parkinson's disease and L-dopa induced dyskinesia

Striatal function adapts to the loss of nigrostriatal dopaminergic input in Parkinson's disease (PD) to initially maintain voluntary movement, but subsequently changes in response to drug treatment leading to the onset of motor complications, notably dyskinesia. Alterations in presynaptic dopaminergic function coupled to changes in the response of post-synaptic dopaminergic receptors causing alterations in striatal output underlie attempts at compensation and the control of movement in early PD. However, eventually compensation fails and persistent changes in striatal function ensue that involve morphological, biochemical and electrophysiological change. Key alterations occur in cholinergic and glutamatergic transmission in the striatum and there are changes in motor programming controlled by events involving LTP/LTD. Dopamine replacement therapy with L-dopa modifies altered striatal function and restores motor function but non-physiological dopamine receptor stimulation leads to altered signalling through D1 and D2 receptor systems and changes in striatal function causing abnormalities of LTP/LTD mediated through glutamatergic/nitric oxide (NO) mechanisms. These lead to the onset of dyskinesia and underlie the priming process that characterise dyskinesia and its persistence.

Treatment of Parkinson disease, time and dosage: "does simple dosage facilitate compliance and therapeutic goals?"

Current available treatment for Parkinson disease has many drawbacks: single action on the nigrostriatal pathway, no halt of disease progression, pulsatile dopaminergic stimulation, complex treatment regimens, and motor complications. Compliance with treatment may be irregular in a variable number of patients. Factors such as age, education, complexity of posology, stage of disease, disease comprehension, cognitive function, or family support significantly influence compliance either in a positive or negative way. The consequences of noncompliance include withdrawal symptoms, increase in number of admissions, or, in severe cases, hyperthermia syndrome secondary to levodopa deficit in the case of infradose. In situations of overdose, dyskinesia or psychiatric complications may arise. The ideal treatment should have the potency of levodopa but not its side effects, act on striatal D2 receptors in a continuous way with a single dose and have low potential of addiction. Thus far, simple regimens are only applicable in early stages.

Retigabine, a K(V)7 (KCNQ) potassium channel opener, attenuates L-DOPA-induced dyskinesias in 6-OHDA-lesioned rats

L-DOPA-induced dyskinesias (LID) represent a severe complication of long-time pharmacotherapy in Parkinson's disease that necessitates novel therapeutics. The acute and chronic effects of K(V)7.2-7.5 channel openers (retigabine, flupirtine) on the severity of LID and parkinsonian signs were examined in comparison to the glutamate receptor antagonist amantadine (positive control) in a rat model of LID. Acute treatment with retigabine (2.5, 5 mg/kg i.p.) and flupirtine (5, 10 mg/kg i.p.) significantly reduced the severity of abnormal involuntary movements (AIM) to a comparable extent as amantadine (20, 40 mg/kg s.c.), but flupirtine delayed the disappearance of AIM. Chronic treatment with retigabine (daily 5 mg/kg i.p. over 19 days combined with l-DOPA 10 mg i.p.) did not prevent or delay the development of LID, but reduced the severity of AIM, while antidyskinetic effects of amantadine (40 mg/kg i.p.) were restricted to the first day of treatment. Retigabine caused sedation and ataxia which declined during the chronic treatment, but did not reduce the antiparkinsonian effects of l-DOPA in these experiments. Acute co-injections of retigabine (5 mg) together with l-DOPA (10 mg/kg) neither reduced the motor performance in the rotarod test nor exerted negative effects on the antiparkinsonian efficacy of l-DOPA in the block and stepping test. Nevertheless, the sedative effects of retigabine may limit its therapeutic potential for the treatment of LID. The present data indicate that K(V)7 channels deserve attention in the research of the pathophysiology of dyskinesias. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.

Lyn kinase regulates mesolimbic dopamine release: implication for alcohol reward

We report here that the Src family tyrosine kinase Lyn negatively regulates the release of dopamine (DA) in the mesolimbic system, as well as the rewarding properties of alcohol. Specifically, we show that RNA interference-mediated knockdown of Lyn expression results in an increase in KCl-induced DA release in DAergic-like SH-SY5Y cells, whereas overexpression of a constitutively active form of Lyn (CA-Lyn) leads to a decrease of DA release. Activation of ventral tegmental area (VTA) DAergic neurons results in DA overflow in the nucleus accumbens (NAc), and we found that the evoked release of DA was higher in the NAc of Lyn knock-out (Lyn KO) mice compared with wild-type littermate (Lyn WT) controls. Acute exposure of rodents to alcohol causes a rapid increase in DA release in the NAc, and we show that overexpression of CA-Lyn in the VTA of mice blocked alcohol-induced (2 g/kg) DA release in the NAc. Increase in DA levels in the NAc is closely associated with reward-related behaviors, and overexpression of CA-Lyn in the VTA of mice led to an attenuation of alcohol reward, measured in a conditioned place preference paradigm. Conversely, alcohol place preference was increased in Lyn KO mice compared with Lyn WT controls. Together, our results suggest a novel role for Lyn kinase in the regulation of DA release in the mesolimbic system, which leads to the control of alcohol reward.

Computational cognitive models of prefrontal-striatal-hippocampal interactions in Parkinson's disease and schizophrenia

Disruption to different components of the prefrontal cortex, basal ganglia, and hippocampal circuits leads to various psychiatric and neurological disorders including Parkinson's disease (PD) and schizophrenia. Medications used to treat these disorders (such as levodopa, dopamine agonists, antipsychotics, among others) affect the prefrontal-striatal-hippocampal circuits in a complex fashion. We have built models of prefrontal-striatal and striatal-hippocampal interactions which simulate cognitive dysfunction in PD and schizophrenia. In these models, we argue that the basal ganglia is key for stimulus-response learning, the hippocampus for stimulus-stimulus representational learning, and the prefrontal cortex for stimulus selection during learning about multidimensional stimuli. In our models, PD is associated with reduced dopamine levels in the basal ganglia and prefrontal cortex. In contrast, the cognitive deficits in schizophrenia are associated primarily with hippocampal dysfunction, while the occurrence of negative symptoms is associated with frontostriatal deficits in a subset of patients. In this paper, we review our past models and provide new simulation results for both PD and schizophrenia. We also describe an extended model that includes simulation of the different functional role of D1 and D2 dopamine receptors in the basal ganglia and prefrontal cortex, a dissociation we argue is essential for understanding the non-uniform effects of levodopa, dopamine agonists, and antipsychotics on cognition. Motivated by clinical and physiological data, we discuss model limitations and challenges to be addressed in future models of these brain disorders.

Levodopa-induced dyskinesia is associated with increased thyrotropin releasing hormone in the dorsal striatum of hemi-parkinsonian rats

Background

Dyskinesias associated with involuntary movements and painful muscle contractions are a common and severe complication of standard levodopa (L-DOPA, L-3,4-dihydroxyphenylalanine) therapy for Parkinson's disease. Pathologic neuroplasticity leading to hyper-responsive dopamine receptor signaling in the sensorimotor striatum is thought to underlie this currently untreatable condition.

Methodology/Principal Findings

Quantitative real-time polymerase chain reaction (PCR) was employed to evaluate the molecular changes associated with L-DOPA-induced dyskinesias in Parkinson's disease. With this technique, we determined that thyrotropin releasing hormone (TRH) was greatly increased in the dopamine-depleted striatum of hemi-parkinsonian rats that developed abnormal movements in response to L-DOPA therapy, relative to the levels measured in the contralateral non-dopamine-depleted striatum, and in the striatum of non-dyskinetic control rats. ProTRH immunostaining suggested that TRH peptide levels were almost absent in the dopamine-depleted striatum of control rats that did not develop dyskinesias, but in the dyskinetic rats, proTRH immunostaining was dramatically up-regulated in the striatum, particularly in the sensorimotor striatum. This up-regulation of TRH peptide affected striatal medium spiny neurons of both the direct and indirect pathways, as well as neurons in striosomes.

Conclusions/Significance

TRH is not known to be a key striatal neuromodulator, but intrastriatal injection of TRH in experimental animals can induce abnormal movements, apparently through increasing dopamine release. Our finding of a dramatic and selective up-regulation of TRH expression in the sensorimotor striatum of dyskinetic rat models suggests a TRH-mediated regulatory mechanism that may underlie the pathologic neuroplasticity driving dopamine hyper-responsivity in Parkinson's disease.

Neurotransmission in Parkinson's disease: beyond dopamine

Parkinson's disease (PD) is most frequently associated with characteristic motor symptoms that are known to arise with degeneration of dopaminergic neurons. However, patients with this disease also experience a multitude of non-motor symptoms, such as sleep disturbances, fatigue, apathy, anxiety, depression, cognitive impairment, dementia, olfactory dysfunction, pain, sweating and constipation, some of which can be at least as debilitating as the movement disorders and have a major impact on patients' quality of life. Many of these non-motor symptoms may be evident prior to the onset of motor dysfunction. The neuropathology of PD has shown that complex, interconnected neuronal systems, regulated by a number of different neurotransmitters in addition to dopamine, are involved in the aetiology of motor and non-motor symptoms. This review focuses on the non-dopaminergic neurotransmission systems associated with PD with particular reference to the effect that their modulation and interaction with dopamine has on the non-motor symptoms of the disease. PD treatments that focus on the dopaminergic system alone are unable to alleviate both motor and non-motor symptoms, particularly those that develop at early stages of the disease. The development of agents that interact with several of the affected neurotransmission systems could prove invaluable for the treatment of this disease.

The dopaminergic basis of human behaviors: A review of molecular imaging studies

This systematic review describes human molecular imaging studies which have investigated alterations in extracellular DA levels during performance of behavioral tasks. Whilst heterogeneity in experimental methods limits meta-analysis, we describe the advantages and limitations of different methodological approaches. Interpretation of experimental results may be limited by regional cerebral blood flow (rCBF) changes, head movement and choice of control conditions. We revisit our original study of striatal DA release during video-game playing [Koepp, M.J., Gunn, R.N., Lawrence, A.D., Cunningham, V.J., Dagher, A., Jones, T., Brooks, D.J., Bench, C.J., Grasby, P.M., 1998. Evidence for striatal dopamine release during a video game. Nature 393, 266-268] to illustrate the potentially confounding influences of head movement and alterations in rCBF. Changes in [(11)C]raclopride binding may be detected in extrastriatal as well as striatal brain regions-however we review evidence which suggests that extrastriatal changes may not be clearly interpreted in terms of DA release. Whilst several investigations have detected increases in striatal extracellular DA concentrations during task components such as motor learning and execution, reward-related processes, stress and cognitive performance, the presence of potentially biasing factors should be carefully considered (and, where possible, accounted for) when designing and interpreting future studies.

Personality, addiction, dopamine: insights from Parkinson's disease

In rare instances, patients with Parkinson's disease (PD) may become addicted to their own medication or develop behavioral addictions such as pathological gambling. This is surprising because PD patients typically have a very low incidence of drug abuse and display a personality type that is the polar opposite of the addictive personality. These rare addictive syndromes, which appear to result from excessive dopaminergic medication use, illustrate the link between dopamine, personality, and addiction. We describe the clinical phenomena and attempt to relate them to current models of learning and addiction. We conclude that persistently elevated dopaminergic stimulation promotes the development and maintenance of addictive behaviors.