Lower nucleus accumbens α-synuclein load and D3 receptor levels in Parkinson's disease with impulsive compulsive behaviours

Behavioral disorders in Parkinson disease: current view

Patients with Parkinson disease (PD) frequently experience several behavioral symptoms, such as anxiety, apathy, irritability, agitation, impulsive control and obsessive-compulsive or REM sleep behavior disorders, which can cause severe psychosocial problems and impair quality of life. Occurring in 30-70% of PD patients, these symptoms can manifest at early stages of the disease, sometimes even before the appearance of classic motor symptoms, while others can develop later. Behavioral changes in PD show distinct patterns of brain atrophy, dopaminergic and serotonergic deterioration, altered neuronal connectivity in frontostriatal, corticolimbic, default mode and other networks due to a cascade linking molecular pathologies and deficits in multiple behavior domains. The changes suggest a multi-system neurodegenerative process in the context of a specific α-synucleinopathy inducing a variety of biochemical and functional changes, the neurobiological basis and clinical relevance of which await further elucidation. This paper is intended to review the recent literature with focus on the main behavioral disturbances in PD patients, their epidemiology, clinical features, risk factors, animal models, neuroimaging findings, pathophysiological backgrounds, and treatment options of these deleterious lesions.

Imaging of impulse control disorders in Parkinson's disease

Impulse control disorders (ICD) are frequent and cumbersome behavioral disorders in patients with Parkinson's disease (PD). Understanding their pathophysiological underpinnings is crucial. Molecular imaging using positron emission tomography (PET) and single-photon emission computed tomography (SPECT) clearly indicates preexisting vulnerability and abnormal sensitization of the pre- and postsynaptic dopaminergic system. Functional magnetic resonance imaging (fMRI) studies reveal abnormal connectivity within the reward system involving the ventral striatum and orbitofrontal cortex. These alterations pinpoint the dysfunction of reinforcement learning in ICD, which is biased toward the overvaluation of reward and underestimation of risk, and the deficit in inhibitory control mechanisms related to abnormal connectivity within and between the limbic and the associative and motor networks.

Involvement of dopamine D3 receptor in impulsive choice decision-making in male rats

Impulsive decision-making has been linked to impulse control disorders and substance use disorders. However, the neural mechanisms underlying impulsive choice are not fully understood. While previous PET imaging and autoradiography studies have shown involvement of dopamine and D2/3 receptors in impulsive behavior, the roles of distinct D1, D2, and D3 receptors in impulsive decision-making remain unclear. In this study, we used a food reward delay-discounting task (DDT) to identify low- and high-impulsive rats, in which low-impulsive rats exhibited preference for large delayed reward over small immediate rewards, while high-impulsive rats showed the opposite preference. We then examined D1, D2, and D3 receptor gene expression using RNAscope in situ hybridization assays. We found that high-impulsive male rats exhibited lower levels of D2 and D3, and particularly D3, receptor expression in the nucleus accumbens (NAc), with no significant changes in the insular, prelimbic, and infralimbic cortices. Based on these findings, we further explored the role of the D3 receptor in impulsive decision-making. Systemic administration of a selective D3 receptor agonist (FOB02-04) significantly reduced impulsive choices in high-impulsive rats but had no effects in low-impulsive rats. Conversely, a selective D3 receptor antagonist (VK4-116) produced increased both impulsive and omission choices in both groups of rats. These findings suggest that impulsive decision-making is associated with a reduction in D3 receptor expression in the NAc. Selective D3 receptor agonists, but not antagonists, may hold therapeutic potentials for mitigating impulsivity in high-impulsive subjects.

The Overtreatment Trap: Navigating Dopamine Dysregulation Syndrome in Parkinson's Disease

Dopamine dysregulation syndrome (DDS) is a rare but significant complication of Parkinson's disease (PD), affecting approximately 3-4% of patients on long-term dopamine replacement therapy (DRT). It is characterized by an addictive pattern of DRT use that exceeds the necessary dosages for managing motor symptoms. Patients may engage in self-medication, escalating their DRT doses beyond prescribed limits, and strongly resist attempts to reduce medication. This syndrome often leads to impulsive behaviors, severe dyskinesia, and notable disruptions in social and occupational functioning. DDS is associated with a range of neuropsychiatric symptoms, including punding behaviors, hallucinations, and delusions. The management of DDS presents significant challenges, requiring a delicate balance between adequate symptom control and preventing medication overuse. We present a case of a 68-year-old woman with DDS, highlighting her symptoms and the tailored management strategies we employed to address this challenging condition.

Impulse Control Disorders in Parkinson's Disease: An Overview of Risk Factors, Pathogenesis and Pharmacological Management

Impulse control disorders in Parkinson's disease are relatively common drug-induced addictive behaviours that are usually triggered by the dopamine agonists pramipexole, ropinirole and rotigotine. This narrative review aimed to provide a comprehensive overview of the current knowledge of impulse control disorders in Parkinson's disease. We summarised the prevalence, clinical features, risk factors and potential underlying mechanisms of impulse control disorders in Parkinson's disease. Moreover, recent advances in behavioural and imaging characteristics and management strategies are discussed. Early detection as well as a tailored multidisciplinary approach, which typically includes careful adjustment of the dopaminergic therapy and the treatment of associated neuropsychiatric symptoms, are necessary. In some cases, a continuous delivery of levodopa via a pump or the dopamine D1 receptor agonist, apomorphine, can be considered. In selected patients without cognitive or speech impairment, deep brain stimulation of the subthalamic nucleus can also improve addictions. Finding the right balance of tapering dopaminergic dose (usually dopamine agonists) without worsening motor symptoms is essential for a beneficial long-term outcome.

Future Directions for Developing Non-dopaminergic Strategies for the Treatment of Parkinson's Disease

The symptomatic treatment of Parkinson's disease (PD) has been dominated by the use of dopaminergic medication, but significant unmet need remains, much of which is related to non-motor symptoms and the involvement of non-dopaminergic transmitter systems. As such, little has changed in the past decades that has led to milestone advances in therapy and significantly improved treatment paradigms and patient outcomes, particularly in relation to symptoms unresponsive to levodopa. This review has looked at how pharmacological approaches to treatment are likely to develop in the near and distant future and will focus on two areas: 1) novel non-dopaminergic pharmacological strategies to control motor symptoms; and 2) novel non-dopaminergic approaches for the treatment of non-motor symptoms. The overall objective of this review is to use a 'crystal ball' approach to the future of drug discovery in PD and move away from the more traditional dopamine-based treatments. Here, we discuss promising non-dopaminergic and 'dirty drugs' that have the potential to become new key players in the field of Parkinson's disease treatment.

Impulsive-compulsive behaviour in early Parkinson's disease is determined by apathy and dopamine receptor D3 polymorphism

Impulsive-compulsive behaviour (ICB) is a frequently observed non-motor symptom in early Parkinson's disease after initiating dopamine replacement therapy. At the opposite end of the motivated behaviour spectrum, apathy occurs in early Parkinson's disease even before dopamine replacement is started. The co-occurrence of these behavioural conditions in Parkinson's disease raises questions about their relationship and underlying pathophysiological determinants. In previous imaging or genetic studies, both conditions have been associated with the limbic dopaminergic system. The risk variant of the Ser9Gly polymorphism of the dopamine receptor D3 (DRD3) is linked to increased dopamine affinity in the limbic striatum. With this in mind, we investigated how ICB expression is explained by apathy and DRD3 polymorphisms and their effects on grey matter volume and dopamine synthesis capacity. Fifty-four patients with early Parkinson's disease took part in anatomical T1-weighted MRI. Forty of them also underwent dynamic PET imaging using [18F]DOPA to measure striatal dopamine synthesis capacity. Further, Ser9Gly (rs6280) gene polymorphism influencing the DRD3 dopamine-binding affinity was determined in all patients. The severity of impulsive-compulsive behaviour and apathy was assessed using the Questionnaire for Impulsive-Compulsive Disorders Rating Scale and the Apathy Evaluation Scale. ICB and the severity of apathy were indeed positively correlated. Apathy and the DRD3 polymorphism were interactive risk factors for ICB severity. Apathy was significantly linked to atrophy of the bilateral putamen. Patients with the DRD3 risk type had reduced dopamine synthesis capacity in the putamen and limbic striatum, apathy was associated with reduced dopamine synthesis capacity in the limbic striatum. The results of [18F]DOPA reached only trend significance. Apathy in drug-naïve PD patients might be a consequence of impaired striatal dopaminergic tone. This may represent a predisposing factor for the development of ICB after the initiation of dopamine replacement therapy. The risk type of DRD3 could further amplify this predisposition due to its higher affinity to dopamine.

Neural correlates of impulsive compulsive behaviors in Parkinson's disease: A Japanese retrospective study

BACKGROUND

Impulsive compulsive behaviors (ICBs) often disturb patients with Parkinson's Disease (PD), of which impulse control disorder (ICD) and dopamine dysregulation syndrome (DDS) are two major subsets. The nucleus accumbens (NAcc) is involved in ICB; however, it remains unclear how the NAcc affects cortical function and defines the different behavioral characteristics of ICD and DDS.

OBJECTIVES

To identify the cortico-striatal network primarily involved in ICB and the differences in these networks between patients with ICD and DDS using structural and resting-state functional magnetic resonance imaging.

METHODS

Patients with PD were recruited using data from a previous cohort study and divided into those with ICB (ICB group) and without ICB (non-ICB group) using the Japanese version of the Questionnaire for Impulsive Compulsive Disorders in Parkinson's Disease (J-QUIP). From these two groups, we extracted 37 pairs matched for age, sex, disease duration, and levodopa equivalent daily dose of dopamine agonists. Patients with ICB were further classified as having ICD or DDS based on the J-QUIP subscore. General linear models were used to compare gray matter volume and functional connectivity (FC) of the NAcc, caudate, and putamen between the ICB and non-ICB groups and between patients with ICD and those with DDS.

RESULTS

We found no significant differences in gray matter volumebetween the ICB and non-ICB groups or between patients with ICD and those with DDS. Compared with the non-ICB group, the FC of the right NAcc in the ICB group was lower in the bilateral ventromedial prefrontal cortex and higher in the left middle occipital gyrus. Furthermore, patients with DDS showed higher FC between the right putamen and left superior temporal gyrus and higher FC between the left caudate and bilateral middle occipital gyrus than patients with ICD. In contrast, patients with ICD exhibited higher FC between the left NAcc and the right posterior cingulate cortex than patients with DDS.

CONCLUSIONS

The functionally altered network between the right NAcc and ventromedial prefrontal cortex was associated with ICB in PD. In addition, the surrounding cortico-striatal networks may differentiate the behavioral characteristics of patients with ICD and those with DDS.

Tissue specific LRRK2 interactomes reveal a distinct striatal functional unit

Mutations in LRRK2 are the most common genetic cause of Parkinson's disease. Despite substantial research efforts, the physiological and pathological role of this multidomain protein remains poorly defined. In this study, we used a systematic approach to construct the general protein-protein interactome around LRRK2, which was then evaluated taking into consideration the differential expression patterns and the co-expression behaviours of the LRRK2 interactors in 15 different healthy tissue types. The LRRK2 interactors exhibited distinct expression features in the brain as compared to the peripheral tissues analysed. Moreover, a high degree of similarity was found for the LRRK2 interactors in putamen, caudate and nucleus accumbens, thus defining a potential LRRK2 functional cluster within the striatum. The general LRRK2 interactome paired with the expression profiles of its members constitutes a powerful tool to generate tissue-specific LRRK2 interactomes. We exemplified the generation of the tissue-specific LRRK2 interactomes and explored the functions highlighted by the "core LRRK2 interactors" in the striatum in comparison with the cerebellum. Finally, we illustrated how the LRRK2 general interactome reported in this manuscript paired with the expression profiles can be used to trace the relationship between LRRK2 and specific interactors of interest, here focusing on the LRRK2 interactors belonging to the Rab protein family.

Inhibitory framing in hypersexual patients with Parkinson's disease. An fMRI pilot study

Hypersexuality in medicated patients with PD is caused by an increased influence of motivational drive areas and a decreased influence of inhibitory control areas due to dopaminergic medication. In this pilot study, we test a newly developed paradigm investigating the influence of dopaminergic medication on brain activation elicited by sexual pictures with and without inhibitory contextual framing. Twenty PD patients with and without hypersexuality were examined with fMRI either OFF or ON standardized dopaminergic medication. The paradigm consisted of a priming phase where either a neutral context or an inhibitory context was presented. This priming phase was either followed by a sexual or a neutral target. Sexual, compared to neutral pictures resulted in a BOLD activation of various brain regions implicated in sexual processing. Hypersexual PD patients showed increased activity compared to PD controls in these regions. There was no relevant effect of medication between the two groups. The inhibitory context elicited less activation in inhibition-related areas in hypersexual PD, but had no influence on the perception of sexual cues. The paradigm partially worked: reactivity of motivational brain areas to sexual cues was increased in hypersexual PD and inhibitory contextual framing lead to decreased activation of inhibitory control areas in PD. We could not find a medication effect and the length of the inhibitory stimulus was not optimal to suppress reactivity to sexual cues. Our data provide new insights into the mechanisms of hypersexuality and warrant a replication with a greater cohort and an optimized stimulus length in the future.

More than Addiction—The Nucleus Accumbens Contribution to Development of Mental Disorders and Neurodegenerative Diseases

Stress and negative emotions evoked by social relationships and working conditions, frequently accompanied by the consumption of addictive substances, and metabolic and/or genetic predispositions, negatively affect brain function. One of the affected structures is nucleus accumbens (NAc). Although its function is commonly known to be associated with brain reword responses and addiction, a growing body of evidence also suggests its role in some mental disorders, such as depression and schizophrenia, as well as neurodegenerative diseases, such as Alzheimer’s, Huntington’s, and Parkinson’s. This may result from disintegration of the extensive connections based on numerous neurotransmitter systems, as well as impairment of some neuroplasticity mechanisms in the NAc. The consequences of NAc lesions are both morphological and functional. They include changes in the NAc’s volume, cell number, modifications of the neuronal dendritic tree and dendritic spines, and changes in the number of synapses. Alterations in the synaptic plasticity affect the efficiency of synaptic transmission. Modification of the number and structure of the receptors affects signaling pathways, the content of neuromodulators (e.g., BDNF) and transcription factors (e.g., pCREB, DeltaFosB, NFκB), and gene expression. Interestingly, changes in the NAc often have a different character and intensity compared to the changes observed in the other parts of the basal ganglia, in particular the dorsal striatum. In this review, we highlight the role of the NAc in various pathological processes in the context of its structural and functional damage, impaired connections with the other brain areas cooperating within functional systems, and progression of the pathological processes.

Neuropsychiatric Features of Punding and Hobbyism in Parkinson's Disease

BACKGROUND

Little is known about the cognitive and neuropsychiatric profile associated with punding and hobbyism in Parkinson's disease (PD).

OBJECTIVE

To compare the clinical and neuropsychological features of PD patients with punding and hobbyism to PD controls.

METHODS

The Questionnaire for Impulsive-Compulsive Disorders in Parkinson's Disease-Rating Scale (QUIP-RS) was used as a screening tool, and a structured interview was used to diagnose punding/hobbyism. Clinical and neuropsychological assessment was conducted with validated questionnaires/scales.

RESULTS

Twenty-one patients with PD and punding (PD + pu) were compared to 26 with hobbyism (PD + h) and 25 PD controls. PD + pu patients showed higher levels of anxiety, non-motor symptoms and motor symptoms, and lower Frontal Assessment Battery scores. The PD + h group exhibited similar levels of anxiety and motor fluctuations to the PD + pu group.

CONCLUSION

PD + pu showed increased anxiety and frontal lobe dysfunction, similar to PD + h. Hobbyism could be a prodromal phase with increased risk of leading to punding.

Unlucky punches: the vulnerability-stress model for the development of impulse control disorders in Parkinson's disease

Impulse-control disorders are commonly observed during dopamine-replacement therapy in Parkinson’s disease, but the majority of patients seems “immune” to this side effect. Epidemiological evidence suggests that a major risk factor may be a specific difference in the layout of the dopaminergic-reinforcement system, of which the ventral striatum is a central player. A series of imaging studies of the dopaminergic system point toward a presynaptic reduction of dopamine-reuptake transporter density and dopamine synthesis capacity. Here, we review current evidence for a vulnerability-stress model in which a relative reduction of dopaminergic projections to the ventral striatum and concomitant sensitization of postsynaptic neurons represent a predisposing (hypodopaminergic) vulnerability. Stress (hyperdopaminergic) is delivered when dopamine replacement therapy leads to a relative overdosing of the already-sensitized ventral striatum. These alterations are consistent with consecutive changes in reinforcement mechanisms, which stimulate learning from reward and impede learning from punishment, thereby fostering the development of impulse-control disorders. This vulnerability-stress model might also provide important insights into the development of addictions in the non-Parkinsonian population.

Reply to 'Impulse control disorders are associated with lower ventral striatum dopamine D3 receptor availability in Parkinson's disease: A [11C]-PHNO PET study.'

Pagano and collaborators have recently reported lower ventral striatum D3 receptor availability in Parkinson's disease using PET scan. Our group conducted the first postmortem study of individuals with PD who had ICD and related behaviours in life and reported lower alpha-synuclein pathology and D3R levels in the nucleus accumbens of such individuals. The findings by Pagano and co-authors of low D3R binding in PD patients at baseline, when taken together with our findings of lower Lewy pathology and D3R in the nucleus accumbens, favour the hypothesis that D3R levels are downregulated because of excessive synaptic dopamine.

Advanced Therapies for the Management of Dopamine Dysregulation Syndrome in Parkinson's Disease

Background

Dopamine Dysregulation Syndrome (DDS) is an adverse non-motor complication of dopamine replacement therapy in Parkinson's disease. The current literature on this syndrome is limited, and it remains underdiagnosed and challenging to manage.

Objective

To assess the role of advanced therapies in the management of DDS.

Methods

We performed a retrospective chart review and identified patients who fit the inclusion criteria for DDS. They were classified according to risk factors that have been identified in the literature, motor and complication scores, intervention (medical or surgical) and outcome. Multivariate analyses were performed to analyze these characteristics.

Results

Twenty-seven patients were identified (23 males, mean age of onset: 49 ± 8.8 years). Average levodopa equivalent daily dose was 1916.7 ± 804 mg and a history of impulse control disorders, psychiatric illness, and substance abuse was present in 89%, 70% and 3.7% of the patients, respectively. Overall 81.5% of patients had symptom resolution at follow up, on average 4.8 ± 3.5 years after management, with medication only (7/9), levodopa-carbidopa intestinal gel (1/3), deep brain stimulation of subthalamic nucleus (10/13), or globus pallidus pars interna (2/2). Reduction of medications occurred with deep brain stimulation of subthalamic nucleus (P = 0.01) but was associated with a relapse in two patients.

Conclusion

Although the small sample size of some subgroups limits our ability to draw meaningful conclusions, our results did not suggest superiority of a single treatment option. Advanced therapies including deep brain stimulation can be considered in patients with DDS refractory to conservative measures, but outcome is variable and relapse is possible.

Reduced Interaction of Aggregated α-Synuclein and VAMP2 by Environmental Enrichment Alleviates Hyperactivity and Anxiety in a Model of Parkinson's Disease

Parkinson’s disease (PD) is a prevalent motor disease caused by the accumulation of mutated α-synuclein (α-Syn); however, its early stages are also characterized by non-motor symptoms, such as olfactory loss, cognitive decline, depression, and anxiety. The therapeutic effects of environmental enrichment (EE) on motor recovery have been reported, but its effects on non-motor symptoms remain unclear. Herein, we reveal the beneficial effects of EE on PD-related non-motor symptoms and changes in synaptic plasticity in the nucleus accumbens. To investigate its therapeutic effects in the early phase of PD, we randomly assigned eight-month-old mice overexpressing human A53T (hA53T) α-Syn to either the EE or standard condition groups for two months. Next, we performed behavioral tests and biochemical and histological analyses at 10 months of age. EE significantly alleviated locomotor hyperactivity and anxiety during the early stages of PD. It normalized the levels of tyrosine hydroxylase, phosphorylated and oligomeric α-Syn, and soluble N-ethylmaleimide-sensitive factor attachment protein receptor complex-forming proteins, including synaptosomal-associated protein, 25 kDa, syntaxin1, and vesicle-associated membrane protein 2 (VAMP2). Moreover, the interactions between VAMP2 and pSer129 α-Syn were markedly reduced following EE. The restoration of synaptic vesicle transportation status may underlie the neuroprotective effects of EE in hA53T α-Syn mice.

The role of dopamine pharmacotherapy and addiction-like behaviors in Parkinson's disease

Addictions involve a spectrum of behaviors that encompass features of impulsivity and compulsivity, herein referred to as impulsive-compulsive spectrum disorders (ICSDs). The etiology of ICSDs likely involves a complex interplay among neurobiological, psychological and social risk factors. Neurobiological risk factors include the status of the neuroanatomical circuits that govern ICSDs. These circuits can be altered by disease, as well as exogenous influences such as centrally-acting pharmacologics. The 'poster child' for this scenario is Parkinson's disease (PD) medically managed by pharmacological treatments. PD is a progressive neurodegenerative disease that involves a gradual loss of dopaminergic neurons largely within nigrostriatal projections. Replacement therapy includes dopamine receptor agonists that directly activate postsynaptic dopamine receptors (bypassing the requirement for functioning presynaptic terminals). Some clinically useful dopamine agonists, e.g., pramipexole and ropinirole, exhibit high affinity for the D2/D3 receptor subtypes. These agonists provide excellent relief from PD motor symptoms, but some patients exhibit debilitating ICSD. Teasing out the neuropsychiatric contribution of PD-associated pathology from the drugs used to treat PD motor symptoms is challenging. In this review, we posit that modern clinical and preclinical research converge on the conclusion that dopamine replacement therapy can mediate addictions in PD and other neurological disorders. We provide five categories of evidences that align with this position: (i) ICSD prevalence is greater with D2/D3 receptor agonist therapy vs PD alone. (ii) Capacity of dopamine replacement therapy to produce addiction-like behaviors is independent of disease for which the therapy is being provided. (iii) ICSD-like behaviors are recapitulated in laboratory rats with and without PD-like pathology. (iv) Behavioral pathology co-varies with drug exposure. (v) ICSD Features of ICSDs are consistent with agonist pharmacology and neuroanatomical substrates of addictions. Considering the underpinnings of ICSDs in PD should not only help therapeutic decision-making in neurological disorders, but also apprise ICSDs in general.