Pramipexole-induced impulsivity in mildparkinsonian rats: a model of impulse control disorders in Parkinson's disease

Pramipexole restores behavioral inhibition in highly impulsive rats through a paradoxical modulation of frontostriatal networks

Impulse control disorders (ICDs), a wide spectrum of maladaptive behaviors which includes pathological gambling, hypersexuality and compulsive buying, have been recently suggested to be triggered or aggravated by treatments with dopamine D2/3 receptor agonists, such as pramipexole (PPX). Despite evidence showing that impulsivity is associated with functional alterations in corticostriatal networks, the neural basis of the exacerbation of impulsivity by PPX has not been elucidated. Here we used a hotspot analysis to assess the functional recruitment of several corticostriatal structures by PPX in male rats identified as highly (HI), moderately impulsive (MI) or with low levels of impulsivity (LI) in the 5-choice serial reaction time task (5-CSRTT). PPX dramatically reduced impulsivity in HI rats. Assessment of the expression pattern of the two immediate early genes C-fos and Zif268 by in situ hybridization subsequently revealed that PPX resulted in a decrease in Zif268 mRNA levels in different striatal regions of both LI and HI rats accompanied by a high impulsivity specific reduction of Zif268 mRNA levels in prelimbic and cingulate cortices. PPX also decreased C-fos mRNA levels in all striatal regions of LI rats, but only in the dorsolateral striatum and nucleus accumbens core (NAc Core) of HI rats. Structural equation modeling further suggested that the anti-impulsive effect of PPX was mainly attributable to the specific downregulation of Zif268 mRNA in the NAc Core. Altogether, our results show that PPX restores impulse control in highly impulsive rats by modulation of limbic frontostriatal circuits.

Methadone maintenance treatment and impulsivity: premature responding

INTRODUCTION

Previous research showed that methadone maintenance treatment (MMT) is linked to impulsivity, with higher impulsivity levels being associated with for example, increased drug use. One aspect of impulsivity, most commonly studied in rodent research, is premature responding, the failure to wait for a starting signal. Premature responding is of high translational significance since it predicts the development of addiction-like behaviors in rodents.

METHODS

We assessed 45 MMT patients and 46 demographically matched (age, sex, education, and handedness) healthy volunteers (HVs) on premature responding alongside action and inhibition of instructed and intentional trials using the Intentional Hand Task (IHT).

RESULTS

The results showed markedly enhanced premature responses in the MMT vs. the HV group, which correlated positively with methadone dosage in the MMT patients. Throughout the task, MMT patients were faster across all trial parts and less accurate in response to instructed trials compared to HVs.

CONCLUSIONS

The increase in premature motor reactions during variable waiting periods alongside increased motion speed and lower accuracy might reflect a specific motor inhibition deficit in MMT, a subcomponent of impulsivity not previously assessed in MMT. Incorporating an experimentally defined measure of impulsivity, such as premature responding, into existing test batteries used by clinicians might enable more tailored treatments addressing the increased impulsivity levels and associated dysfunctional behaviors in MMT.

D2/3 Agonist during Learning Potentiates Cued Risky Choice

Impulse control and/or gambling disorders can be triggered by dopamine agonist therapies used to treat Parkinson's disease, but the cognitive and neurobiological mechanisms underlying these adverse effects are unknown. Recent data show that adding win-paired sound and light cues to the rat gambling task (rGT) potentiates risky decision-making and impulsivity via the dopamine system, and that changing dopaminergic tone has a greater influence on behavior while subjects are learning task contingencies. Dopamine agonist therapy may therefore be potentiating risk-taking by amplifying the behavioral impact of gambling-related cues on novel behavior. Here, we show that ropinirole treatment in male rats transiently increased motor impulsivity but robustly and progressively increased choice of the high-risk/high-reward options when administered during acquisition of the cued but not uncued rGT. Early in training, ropinirole increased win-stay behavior after large unlikely wins on the cued rGT, indicative of enhanced model-free learning, which mediated the drug's effect on later risk preference. Ex vivo cFos imaging showed that both chronic ropinirole and the addition of win-paired cues suppressed the activity of dopaminergic midbrain neurons. The ratio of midbrain:prefrontal cFos+ neurons was lower in animals with suboptimal choice patterns and tended to predict risk preference across all rats. Network analyses further suggested that ropinirole induced decoupling of the dopaminergic cells of the VTA and nucleus accumbens but only when win-paired cues were present. Frontostriatal activity uninformed by the endogenous dopaminergic teaching signal therefore appeared to perpetuate risky choice, and ropinirole exaggerated this disconnect in synergy with reward-paired cues.SIGNIFICANCE STATEMENT D2/3 receptor agonists, used to treat Parkinson's disease, can cause gambling disorder through an unknown mechanism. Ropinirole increased risky decision-making in rats, but only when wins were paired with casino-inspired sounds and lights. This was mediated by increased win-stay behavior after large unlikely wins early in learning, indicating enhanced model-free learning. cFos imaging showed that ropinirole suppressed activity of midbrain dopamine neurons, an effect that was mimicked by the addition of win-paired cues. The degree of risky choice rats exhibited was uniquely predicted by the ratio of midbrain dopamine:PFC activity. Depriving the PFC of the endogenous dopaminergic teaching signal may therefore drive risky decision-making on-task, and ropinirole acts synergistically with win-paired cues to amplify this.

Motivational disturbances in rodent models of neuropsychiatric disorders

Motivated behavior is integral to the survival of individuals, continuously directing actions toward rewards or away from punishments. The orchestration of motivated behavior depends on interactions among different brain circuits, primarily within the dopaminergic system, that subserve the analysis of factors such as the effort necessary for obtaining the reward and the desirability of the reward. Impairments in motivated behavior accompany a wide range of neuropsychiatric disorders, decreasing the patients’ quality of life. Despite its importance, motivation is often overlooked as a parameter in neuropsychiatric disorders. Here, we review motivational impairments in rodent models of schizophrenia, depression, and Parkinson’s disease, focusing on studies investigating effort-related behavior in operant conditioning tasks and on pharmacological interventions targeting the dopaminergic system. Similar motivational disturbances accompany these conditions, suggesting that treatments aimed at ameliorating motivation levels may be beneficial for various neuropsychiatric disorders.

Striatal synaptic bioenergetic and autophagic decline in premotor experimental parkinsonism

Synaptic impairment might precede neuronal degeneration in Parkinson’s disease. However, the intimate mechanisms altering synaptic function by the accumulation of presynaptic α-synuclein in striatal dopaminergic terminals before dopaminergic death occurs, have not been elucidated. Our aim is to unravel the sequence of synaptic functional and structural changes preceding symptomatic dopaminergic cell death. As such, we evaluated the temporal sequence of functional and structural changes at striatal synapses before parkinsonian motor features appear in a rat model of progressive dopaminergic death induced by overexpression of the human mutated A53T α-synuclein in the substantia nigra pars compacta, a protein transported to these synapses. Sequential window acquisition of all theoretical mass spectra proteomics identified deregulated proteins involved first in energy metabolism and later, in vesicle cycling and autophagy. After protein deregulation and when α-synuclein accumulated at striatal synapses, alterations to mitochondrial bioenergetics were observed using a Seahorse XF96 analyser. Sustained dysfunctional mitochondrial bioenergetics was followed by a decrease in the number of dopaminergic terminals, morphological and ultrastructural alterations, and an abnormal accumulation of autophagic/endocytic vesicles inside the remaining dopaminergic fibres was evident by electron microscopy. The total mitochondrial population remained unchanged whereas the number of ultrastructurally damaged mitochondria increases as the pathological process evolved. We also observed ultrastructural signs of plasticity within glutamatergic synapses before the expression of motor abnormalities, such as a reduction in axospinous synapses and an increase in perforated postsynaptic densities. Overall, we found that a synaptic energetic failure and accumulation of dysfunctional organelles occur sequentially at the dopaminergic terminals as the earliest events preceding structural changes and cell death. We also identify key proteins involved in these earliest functional abnormalities that may be modulated and serve as therapeutic targets to counterbalance the degeneration of dopaminergic cells to delay or prevent the development of Parkinson’s disease.

Assessment of Repetitive and Compulsive Behaviors Induced by Pramipexole in Rats: Effect of Alpha-Synuclein-Induced Nigrostriatal Degeneration

Treatment with dopamine agonists in Parkinson’s disease (PD) is associated with debilitating neuropsychiatric side-effects characterized by impulsive and compulsive behaviors. The vulnerability to develop such impairments is thought to involve interactions between individual vulnerability traits, types of antiparkinsonian medications, and the neurodegenerative process. We investigated the effect of the dopamine D3/D2 agonist pramipexole (PPX) and selective nigrostriatal degeneration achieved by viral-mediated expression of alpha-synuclein on the expression of repetitive and compulsive-like behaviors in rats. In a task assessing spontaneous food hoarding behavior, PPX increased the time spent interacting with food pellets at the expense of hoarding. This disruption of hoarding behavior was identical in sham and lesioned rats. In an operant post-training signal attenuation task, the combination of nigrostriatal lesion and PPX decreased the number of completed trials and increased the number of uncompleted trials. The lesion led to an increased compulsive behavior after signal attenuation, and PPX shifted the overall behavioral output towards an increased proportion of compulsive lever-presses. Given the magnitude of the behavioral effects and the lack of strong interaction between PPX and nigral degeneration, these results suggest that extra-nigral pathology may be critical to increase the vulnerability to develop compulsive behaviors following treatment with D3/D2 agonists.

Neuropsychiatric and Cognitive Deficits in Parkinson's Disease and Their Modeling in Rodents

Parkinson’s disease (PD) is associated with a large burden of non-motor symptoms including olfactory and autonomic dysfunction, as well as neuropsychiatric (depression, anxiety, apathy) and cognitive disorders (executive dysfunctions, memory and learning impairments). Some of these non-motor symptoms may precede the onset of motor symptoms by several years, and they significantly worsen during the course of the disease. The lack of systematic improvement of these non-motor features by dopamine replacement therapy underlines their multifactorial origin, with an involvement of monoaminergic and cholinergic systems, as well as alpha-synuclein pathology in frontal and limbic cortical circuits. Here we describe mood and neuropsychiatric disorders in PD and review their occurrence in rodent models of PD. Altogether, toxin-based rodent models of PD indicate a significant but non-exclusive contribution of mesencephalic dopaminergic loss in anxiety, apathy, and depressive-like behaviors, as well as in learning and memory deficits. Gene-based models display significant deficits in learning and memory, as well as executive functions, highlighting the contribution of alpha-synuclein pathology to these non-motor deficits. Collectively, neuropsychiatric and cognitive deficits are recapitulated to some extent in rodent models, providing partial but nevertheless useful options to understand the pathophysiology of non-motor symptoms and develop therapeutic options for these debilitating symptoms of PD.

Increased emissions of 50-kHz ultrasonic vocalizations in hemiparkinsonian rats repeatedly treated with dopaminomimetic drugs: A potential preclinical model for studying the affective properties of dopamine replacement therapy in Parkinson's disease

Dopamine replacement therapy used in Parkinson's disease (PD) may induce alterations in the emotional state that can underlie the manifestation of iatrogenic psychiatric-like disturbances. The preclinical investigation of these disturbances is limited, also because few reliable paradigms are available to study the affective properties of dopaminomimetic drugs in parkinsonian animals. To provide a relevant experimental tool in this respect, we evaluated whether dopaminomimetic drugs modified the emission of 50-kHz ultrasonic vocalizations (USVs), a behavioral marker of positive affect, in rats bearing a unilateral lesion with 6-hydroxydopamine in the medial forebrain bundle. Apomorphine (2 or 4 mg/kg, i.p.), L-3,4-dihydroxyphenilalanine (L-DOPA, 6 or 12 mg/kg, i.p.), or pramipexole (2 or 4 mg/kg, i.p.) were administered in a test cage (× 5 administrations) on alternate days. Seven days after treatment discontinuation, rats were re-exposed to the test cage to measure conditioned calling behavior and thereafter received a drug challenge. Hemiparkinsonian rats treated with either apomorphine or L-DOPA, but not pramipexole, markedly vocalized during repeated treatment and after challenge, and showed conditioned calling behavior. Moreover, apomorphine, L-DOPA and pramipexole elicited different patterns of 50-kHz USV emissions and rotational behavior, indicating that calling behavior in hemiparkinsonian rats treated with dopaminomimetic drugs is not a byproduct of motor activation. Taken together, these results suggest that measuring 50-kHz USV emissions may be a relevant experimental tool for studying how dopaminomimetic drugs modify the affective state in parkinsonian rats, with possible implications for the preclinical investigation of iatrogenic psychiatric-like disturbances in PD.

Comparison of pramipexole and levodopa/benserazide combination therapy versus levodopa/benserazide monotherapy in the treatment of Parkinson's disease: a systematic review and meta-analysis

The purpose of this research was to evaluate the clinical efficacy and safety of pramipexole plus levodopa/benserazide (P+LB) combination therapy in the treatment of Parkinson's disease (PD) compared to that of LB monotherapy, in order to confer a reference for clinical practice. Randomized controlled trials (RCTs) of P+LB for PD published up to April 2020 were retrieved. Heterogeneity and sensitivity analysis were executed. Twenty-nine RCTs with 3017 participants were included. Clinical efficacy of P+LB combination therapy was significantly better than LB monotherapy (RR 1.27, 95% CI 1.22 to 1.32, P<0.00001). Compared with LB monotherapy, the pooled effects of P+LB combination therapy on UPDRS score were (SMD -1.41, 95% CI -1.71 to -1.11, P<0.00001) for motor UPDRS score, (SMD -1.65, 95% CI -2.25 to -1.04, P<0.00001) for activities of daily living UPDRS score, (SMD -2.20, 95% CI -3.32 to -1.09, P=0.0001) for mental UPDRS score, and (SMD -1.60, 95% CI -2.06 to -1.15, P<0.00001) for complication UPDRS score. The HAMD score showed significant decrease in the P+LB combination therapy compared to LB monotherapy (SMD -1.32, 95% CI -1.80 to -0.84, P<0.00001). In contrast to LB monotherapy, P+LB combination therapy decreased the number of any adverse events obviously in PD patients (RR 0.53, 95% CI 0.45 to 0.63, P<0.00001). In conclusion, P+LB combination therapy is superior to LB monotherapy for improvement of clinical symptoms in PD patients. Moreover, the safety profile of P+LB combination therapy is better than that of LB monotherapy. Further well-designed, multi-center RCTs needed to identify these findings.

Impulse Control Disorders in Parkinson's Disease: From Bench to Bedside

Parkinson’s disease (PD) is a neurodegenerative disorder that is characterized by symptoms that impact both motor and non-motor domains. Outside of motor impairments, PD patients are at risk for impulse control disorders (ICDs), which include excessively disabling impulsive and compulsive behaviors. ICD symptoms in PD (PD + ICD) can be broadly conceptualized as a synergistic interaction between dopamine agonist therapy and the many molecular and circuit-level changes intrinsic to PD. Aside from discontinuing dopamine agonist treatment, there remains a lack of consensus on how to best address ICD symptoms in PD. In this review, we explore recent advances in the molecular and neuroanatomical mechanisms underlying ICD symptoms in PD by summarizing a rapidly accumulating body of clinical and preclinical studies, with a special focus on the utility of rodent models in gaining new insights into the neurochemical basis of PD + ICD. We also discuss the relevance of these findings to the broader problem of impulsive and compulsive behaviors that impact a range of neuropsychiatric syndromes.

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.

Motor impulsivity and delay intolerance are elicited in a dose-dependent manner with a dopaminergic agonist in parkinsonian rats

RATIONALE

Impulse control disorders (ICD) and other impulsive-compulsive behaviours are frequently found in Parkinson's disease (PD) patients treated with dopaminergic agonists. To date, there are no available animal models to investigate their pathophysiology and determine whether they can be elicited by varying doses of dopaminergic drugs. In addition, there is some controversy regarding the predispositional pattern of striatal dopaminergic depletion.

OBJECTIVES

To study the effect of two doses of pramipexole (PPX) on motor impulsivity, delay intolerance and compulsive-like behaviour.

METHODS

Male rats with mild dopaminergic denervation in the dorsolateral striatum (bilateral injections of 6-hydroxidopamine (6-OHDA)) treated with two doses of PPX (0.25 mg/kg and 3 mg/kg) and tested in the variable delay-to-signal paradigm.

RESULTS

Partial (50%) dopaminergic depletion did not induce significant changes in motor impulsivity or delay intolerance. However, 0.25 mg/kg of PPX increased motor impulsivity, while 3 mg/kg of PPX increased both motor impulsivity and delay intolerance. These effects were independent of the drug's antiparkinsonian effects. Importantly, impulsivity scores before and after dopaminergic lesion were positively associated with the impulsivity observed after administering 3 mg/kg of PPX. No compulsive-like behaviour was induced by PPX administration.

CONCLUSIONS

We described a rat model, with a moderate dorsolateral dopaminergic lesion resembling that suffered by patients with early PD, that develops different types of impulsivity in a dose-dependent manner dissociated from motor benefits when treated with PPX. This model recapitulates key features of abnormal impulsivity in PD and may be useful for deepening our understanding of the pathophysiology of ICD.

The adverse effects of pramipexole on probability discounting are not reversed by acute D2 or D3 receptor antagonism

Pramipexole (PPX) is a D₂ and D₃ dopamine receptor agonist approved for clinical use, which is associated with a higher risk of impulse-control disorders. Using a rat model, we recently found that low doses of the monoamine-depleting agent reserpine (RES; 1 mg/kg/day, SC) dramatically increased the untoward effects of PPX (0.3 mg/kg/day, SC) on probability discounting, a key impulsivity function. To further understand the neurobehavioral mechanisms underlying these effects, we first tested whether the combination of PPX and RES may lead to a generalized enhancement in risk taking, as tested in the suspended wire-beam paradigm. The association of RES and PPX did not augment the proclivity of rats to cross the bridge in order to obtain a reward, suggesting that the effects of RES and PPX on probability discounting do not reflect a generalized increase in impulsivity. We then studied what receptors mediate the effects of PPX in RES-treated rats. The combination of RES and PPX increased membrane expression and binding of D₃, but not D₂ dopamine receptors, in the nucleus accumbens. However, the behavioral effects of PPX and RES were not reduced by acute treatments with the D₂/D₃ receptor antagonist raclopride (0.01-0.05 mg/kg, SC), the highly selective D₂ receptor antagonist L-741,626 (0.1-1 mg/kg, SC) or the D₃ receptor antagonists GR 103691 (0.1-0.3 mg/kg, SC) and SB 277011A (1-10 mg/kg, SC). These findings collectively suggest that the effects of PPX in probability discounting do not reflect generalized enhancements in impulsivity or acute dopamine D₂ or D₃ receptor activation.

Unilateral accumbal dopamine depletion affects decision-making in a side-specific manner

Mechanisms underlying affective and cognitive deficits in Parkinson's disease (PD) remain less studied than motor symptoms. Nucleus accumbens (NAc) is affected in PD and due to its well-known involvement in motivation is an interesting target in this context. Furthermore, PD is frequently asymmetrical, with side-specific deficits aligning with evidences of accumbal laterality. We therefore used a 6-hydroxydopamine (6-OHDA) model to study the role of left and right NAc dopamine depletion in a battery of behavioral tasks. 2 months old male rats were used in all experiments. Habitual-based and goal-directed decision-making, impulsivity, anxiety- and depressive-like behavior and motor performance were tested 3 weeks after left (6-OHDA L) or right (6-OHDA R) NAc lesion was induced. Upon contingency degradation, 6-OHDA R decrease their lever press rate less than Sham and 6-OHDA L, indicating an impairment in the shift from habit-based to goal-directed strategies. On the other hand, 6-OHDA L lesions lead to increased rates of premature responding when delays where increased in the variable delay-to-signal test. Importantly, in both paradigms task acquisition was similar between groups. In the same line we found no differences in the amount of sugared pellets eaten when freely available as well as in both general and fine motor behaviors. In conclusion, left and right NAc play distinct roles in the contingency degradation and impulsivity. More studies are needed to understand the mechanisms behind this functional lateralization and its implications for PD patients.

Neuropsychiatric Disorders in Parkinson's Disease: What Do We Know About the Role of Dopaminergic and Non-dopaminergic Systems?

Besides the hallmark motor symptoms (rest tremor, hypokinesia, rigidity, and postural instability), patients with Parkinson’s disease (PD) have non-motor symptoms, namely neuropsychiatric disorders. They are frequent and may influence the other symptoms of the disease. They have also a negative impact on the quality of life of patients and their caregivers. In this article, we will describe the clinical manifestations of the main PD-related behavioral disorders (depression, anxiety disorders, apathy, psychosis, and impulse control disorders). We will also provide an overview of the clinical and preclinical literature regarding the underlying mechanisms with a focus on the role of the dopaminergic and non-dopaminergic systems.