Chronic D2/3 agonist ropinirole treatment increases preference for uncertainty in rats regardless of baseline choice patterns

Rationale and Development of Tavapadon, a D1/D5-Selective Partial Dopamine Agonist for the Treatment of Parkinson's Disease

Currently, available therapeutics for the treatment of Parkinson's disease (PD) fail to provide sustained and predictable relief from motor symptoms without significant risk of adverse events (AEs). While dopaminergic agents, particularly levodopa, may initially provide strong motor control, this efficacy can vary with disease progression. Patients may suffer from motor fluctuations, including sudden and unpredictable drop-offs in efficacy. Dopamine agonists (DAs) are often prescribed during early-stage PD with the expectation they will delay the development of levodopa-associated complications, but currently available DAs are less effective than levodopa for the treatment of motor symptoms. Furthermore, both levodopa and DAs are associated with a significant risk of AEs, many of which can be linked to strong, repeated stimulation of D2/D3 dopamine receptors. Targeting D1/D5 dopamine receptors has been hypothesized to produce strong motor benefits with a reduced risk of D2/D3-related AEs, but the development of D1-selective agonists has been previously hindered by intolerable cardiovascular AEs and poor pharmacokinetic properties. There is therefore an unmet need in PD treatment for therapeutics that provide sustained and predictable efficacy, with strong relief from motor symptoms and reduced risk of AEs. Partial agonism at D1/D5 has shown promise for providing relief from motor symptoms, potentially without the AEs associated with D2/D3-selective DAs and full D1/D5-selective DAs. Tavapadon is a novel oral partial agonist that is highly selective at D1/D5 receptors and could meet these criteria. This review summarizes currently available evidence of tavapadon's therapeutic potential for the treatment of early through advanced PD.

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.

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.

Behavioral, Cellular and Molecular Responses to Cold and Mechanical Stimuli in Rats with Bilateral Dopamine Depletion in the Mesencephalic Dopaminergic Neurons

Pain is the major non-motor symptom in Parkinson's disease (PD). Preclinical studies have mostly investigated mechanical pain by considering the decrease in a nociceptive threshold. Only a few studies have focused on thermal pain in animal models of PD. Therefore, the goal of this study was to assess the thermal nociceptive behavior of rats subjected to 6-hydroxydopamine (6-OHDA) administration, which constitutes an animal model of PD. Thermal plate investigation demonstrated significant thermal sensitivity to cold temperatures of 10 °C and 15 °C, and not to higher temperatures, in 6-OHDA-lesioned rats when compared with sham. 6-OHDA-lesioned rats also showed cold allodynia as demonstrated by a significant difference in the number of flinches, latency and reaction time to acetone stimulus. Ropinirole administration, a dopamine receptor 2 (D2R) agonist, blocked the acetone-induced cold allodynia in 6-OHDA-lesioned rats. In addition, mechanical hypersensitivity and static allodynia, as demonstrated by a significant difference in the vocalization threshold and pain score respectively, were noticed in 6-OHDA-lesioned rats. Acetone stimulus induced a significant increase in extracellular signal-regulated protein kinases 1 and 2 (ERK1/2) phosphorylation, a pain process molecular marker, in the spinal dorsal horn (SDH), the insular and cingulate cortices in 6-OHDA-lesioned rats when compared to sham. In 6-OHDA-lesioned rats, there was a significant augmentation in the expression of both protein kinase C gamma (PKCγ) and glutamate decarboxylase 67 (GAD67) in the SDH. This highlighted an increase in excitation and a decrease in inhibition in the SDH. Overall, the present study demonstrated a clear cold thermal hypersensitivity, in addition to a mechanical one, in 6-OHDA-lesioned rats.

Effort-motivated behavior resolves paradoxes in appetitive conditioning

Motivated behavior has long been studied by psychologists, ethologists, and neuroscientists. To date, many scientists agree with the view that cue and reward attraction is the product of a dopamine-dependent unconscious process called incentive salience or "wanting". This process allows the influence of multiple factors such as hunger and odors on motivational attraction. In some cases, however, the resulting motivated behavior differs from what the incentive salience hypothesis would predict. I argue that seeking behavior under reward uncertainty illustrates this situation: Organisms do not just "want" (appetite-based attraction) cues that are inconsistent or associated with reward occasionally, they "hope" that those cues will consistently predict reward procurement in the ongoing trial. Said otherwise, they become motivated to invest time and energy to find consistent cue-reward associations despite no guarantee of success (effort-based attraction). A multi-test comparison of performance between individuals trained under uncertainty and certainty reveals behavioral paradoxes suggesting that the concept of incentive salience cannot fully account for responding to inconsistent cues. A mathematical model explains how appetite-based and effort-based attractions might combine their effects.

Noradrenergic contributions to cue-driven risk-taking and impulsivity

RATIONALE

The flashing lights and sounds of modern casinos are alluring and may contribute to the addictive nature of gambling. Such cues can have a profound impact on the noradrenaline (NA) system, which could therefore be a viable therapeutic target for gambling disorder (GD). While there is substantial evidence to support the involvement of NA in the impulsive symptoms of GD, its function in mediating the "pro-addictive" impact of cues is less understood.

OBJECTIVE

We wished to investigate the role of NA in our rodent assay of decision making and impulsivity, the cued rat gambling task (crGT). Given that sex differences are prominent in addiction disorders, and increasingly reported in the monoaminergic regulation of behaviour, we also prioritised evaluating noradrenergic drugs in both sexes.

METHODS

Female and male rats were trained to stability on the crGT and then given intraperitoneal injections of the noradrenaline reuptake inhibitor atomoxetine, the α_2A receptor agonist guanfacine, the beta receptor antagonist propranolol, and the α₂ receptor antagonist yohimbine.

RESULTS

Atomoxetine dose-dependently improved decision-making score. Guanfacine selectively enhanced decision making in risk-preferring males and optimal performing females. Propranolol and yohimbine did not influence decision making. Atomoxetine and guanfacine reduced premature responses, while yohimbine bi-phasically affected this index of motor impulsivity.

CONCLUSIONS

These results support the hypothesis that NA is an important neuromodulator of the cue-induced deficits in decision making observed in laboratory-based gambling paradigms, and suggest that NAergic drugs like atomoxetine and guanfacine may be useful in treating GD.

Dopamine receptors regulate preference between high-effort and high-risk rewards

RATIONALE

Optimal decision-making necessitates evaluation of multiple rewards that are each offset by distinct costs, such as high effort requirement or high risk of failure. The neurotransmitter dopamine is fundamental toward these cost-benefit analyses, and D1-like and D2-like dopamine receptors differently modulate the reward-discounting effects of both effort and risk. However, measuring the role of dopamine in regulating decision-making between options associated with distinct costs exceeds the scope of traditional rodent economic decision-making paradigms.

OBJECTIVES

We developed the effort vs probability economic conflict task (EvP) to model multimodal economic decision-making in rats. This task measures choice between two rewards of uniform magnitude associated with either a high effort requirement or risk of reward omission. We then tested the modulatory effects of systemic cocaine and D1/D2 blockade or activation on the preference between high-effort and high-risk alternatives.

METHODS

In the EvP, two reinforcers of equal magnitude are associated with either (1) an effort requirement that increases throughout the session (1, 5, 10, and 20 lever presses), or (2) a low probability of reward receipt (25% of probabilistic choices). Critically, the reinforcer for each choice is comparable (one pellet), which eliminates the influence of magnitude discrimination on the decision-making process. After establishing the task, the dopamine transporter blocker cocaine and D1/D2 antagonists and agonists were administered prior to EvP performance.

RESULTS

Preference shifted away from either effortful or probabilistic choice when either option became more costly, and this preference was highly variable between subjects and stable over time. Cocaine, D1 activation, and D2 blockade produced limited, dose-dependent shifts in choice preference contingent on high or low effort conditions. In contrast, D2 activation across multiple doses evoked a robust shift from effortful to risky choice that was evident even when clearly disadvantageous.

CONCLUSIONS

The EvP clearly demonstrates that rats can evaluate distinct effortful or risky costs associated with rewards of comparable magnitude, and shift preference away from either option with increasing cost. This preference is more tightly linked to D2 than D1 receptor manipulation, suggesting D2-like receptors as a possible therapeutic target for maladaptive biases toward risk-taking over effort.

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.

Translating concepts of risk and loss in rodent models of gambling and the limitations for clinical applications

Gambling involves placing something of value at risk in exchange for the opportunity to potentially gain something of greater value in return. A variety of gambling paradigms have been designed to study the maladaptive decision-making that underlies problematic gambling. Central to these gambling models are the definitions of "risk" and "loss", especially when translating the results from rodent studies to clinical applications. Risk and loss are not mutually exclusive but rather share some overlap. With careful interpretation and consideration of the limitations of these behavioral paradigms, results from rodent models may provide insights into the neurobiology of risky decision-making that leads to problematic gambling in humans.

Dopaminergic influences on risk preferences of Parkinson's disease patients

Clinicians are increasingly recognizing impulse control disorders (ICDs) as a complication of dopaminergic treatment in Parkinson's disease (PD). Considering the pivotal role of dopamine in reward information processing, ICDs may originate from dysregulation of reward-oriented behavior, and the behavioral changes may be reflected in shifts of psychological risk preference during decision-making. We used a behavioral economics paradigm to evaluate quantitatively the risk preferences of PD patients in levodopa on and off states. We also examined age-matched healthy controls. We found that levodopa increased the subjective value and prolonged the decision time in PD patients. These effects are apparently not explained by kinematic improvements but are attributed to psychological shifts of risk preferences and increased attention during risky decision-making. The risk preferences of healthy controls were similar to those of PD on levodopa treatment. The risk preferences of PD patients were not correlated with the scores of routine cognitive batteries, suggesting that dopamine-sensitive risk preferences are independent of cognitive capacities as measured by conventional batteries, including general intelligence, memory, and frontal functioning. By contrast, apathy and ICD partially predicted the risk attitude in PD patients, suggesting a common background of limbic origin behind these properties. The present results demonstrated that dopamine deficiency in off-state PD leads to risk-avoiding behavior and levodopa treatment increases the risk preferences. Behavioral economics framework is useful to evaluate short-term psychological changes in response to levodopa in PD patients.

A selective D2 dopamine receptor agonist alleviates depression through up-regulation of tyrosine hydroxylase and increased neurogenesis in hippocampus of the prenatally stressed rats

Prenatal stress (PNS) has its negative impact on both the infant hippocampal neurogenesis and pregnancy outcomes in the neonates that serves as a risk factor for postnatal depression in adult offsprings. Therefore, main objectives of the present study were to evaluate the effect of maternal chronic unpredictable mild stress (CUMS) on behavioural changes, levels of oxidative stress, changes in selective developmental signaling genes and neurogenesis in the adult brain of Wistar rats and its reversal through a selective non-ergoline D2 type dopamine receptor (D2R) agonist Ropinirole (ROPI). Effects of ROPI treatment on CUMS induced adult rats offspring were measured by assessment of behavioural tests (sucrose preference test and forced swim test), biomarkers of oxidative stress, protein expression of tyrosine hydroxylase (TH), mRNA expression of SHH, GSK-3β, β-catenin, Notch, brain-derived neurotrophic factor (BDNF), Dopamine receptor 2 (Drd2) and bromodeoxyuridine (BrdU) cell proliferation assay. The oxidative stress, protein and mRNA expression were determined in the hippocampus and prefrontal cortex while the BrdU cell proliferation was observed in the hippocampus of rat brain. PNS induced changes resulted in depression validated by the depression-like behaviours, increased oxidative stress, decreased TH expression, altered expression of selective developmental genes, along with the reduced hippocampal neurogenesis and BDNF expression in the brain of adult offsprings. Chronic ROPI treatment reversed those effects and was equally effective like Imipramine (IMI) treatment. So, the present study suggested that ROPI can be used as an antidepressant drug for the treatment of depressive disorders.

Can Slot-Machine Reward Schedules Induce Gambling Addiction in Rats?

Similar to drugs of abuse, random-ratio reward schedules are highly motivating and, in humans, are thought to foster gambling addiction. Animal gambling models, however, have not yet demonstrated the compulsivity so characteristic of drug addiction. Three criteria have been used to evaluate addiction-like behavior in drug models: (1) response inhibition when reward is not available, (2) persistence under a progressive ratio schedule, in which the response-to-reward ratio is stretched, and (3) persistence in spite of punishment. We tested whether prolonged exposure (6 weeks) to a gambling-like reward schedule would induce addiction-like symptoms in rats. In two studies, separate groups were trained to respond to either random- or fixed-ratio schedules for food reward. We found that rats trained on random-ratio schedules showed higher response rates and dramatically shorter pauses after rewards. Tests of addiction-like behavior, however, were largely negative. Response rates were not different during cued no-reward periods nor when reward was coupled with punishment. We also found no group differences when food was devalued nor in reinstatement of reward-seeking after a 1-week delay. The sole exception to this pattern was that rats in the second experiment showed greater persistence on a progressive ratio test. After experiment two, subjects were also orally administered pramipexole, which caused increased perseveration during progressive ratio testing, especially in the random ratio group. While, it is possible that longer training or more appetitive rewards might have led to addiction-like behavior, our results, on the surface, suggest that random-ratio schedules are motivating but not addictive.

Using rodent models to understand interactions between gambling and substance use

Substance use is strongly associated with gambling, but the nature of this association can be difficult to determine. Rodents offer the opportunity to test causal models of these relationships through isolation of individual variables of interest. This review describes recent research in rodents showing: a) predisposing factors for both gambling-like behavior and substance use; b) exposure to drugs of abuse increasing gambling-like behavior; c) experience with gambling-like behavior increasing substance use; and d) links between gambling-like behavior and substance use in models of Parkinson's disease therapies. These findings reveal novel relationships between gambling and substance use, and highlight the utility of rodent models for future work in this area.

Increased motor impulsivity in a rat gambling task during chronic ropinirole treatment: potentiation by win-paired audiovisual cues

RATIONALE

Chronic administration of D_2/3 receptor agonists ropinirole or pramipexole can increase the choice of uncertain rewards in rats, theoretically approximating iatrogenic gambling disorder (iGD).

OBJECTIVES

We aimed to assess the effect of chronic ropinirole in animal models that attempt to capture critical aspects of commercial gambling, including the risk of losing rather than failing to gain, and the use of win-paired sensory stimuli heavily featured in electronic gambling machines (EGMs).

METHODS

Male Long-Evans rats learned the rat gambling task (rGT; n = 24), in which animals sample between four options that differ in the magnitude and probability of rewards and time-out punishments. In the cued rGT (n = 40), reward-concurrent audiovisual cues were added that scaled in complexity with win size. Rats were then implanted with an osmotic pump delivering ropinirole (5 mg/kg/day) or saline for 28 days.

RESULTS

Chronic ropinirole did not unequivocally increase preference for more uncertain outcomes in either the cued or uncued rGT. Ropinirole transiently increased premature responses, a measure of motor impulsivity, and this change was larger and more long-lasting in the cued task.

CONCLUSIONS

These data suggest that explicitly signaling loss prevents the increase in preference for uncertain rewards caused by D_2/3 receptor agonists observed previously. The ability of win-paired cues to amplify ropinirole-induced increases in motor impulsivity may explain why compulsive use of EGMs is particularly common in iGD. These data offer valuable insight into the cognitive-behavioral mechanisms through which chronic dopamine agonist treatments may induce iGD and related impulse control disorders.

Dopaminergic Neurotransmission in Patients With Parkinson's Disease and Impulse Control Disorders: A Systematic Review and Meta-Analysis of PET and SPECT Studies

Background: Around 30% Parkinson's disease (PD) patients develop impulse control disorders (ICDs) to D_2/3 dopamine agonists and, to a lesser extent, levodopa. We aim to investigate striatal dopaminergic function in PD patients with and without ICD.

Methods: PubMed, Science Direct, EBSCO, and ISI Web of Science databases were searched (from inception to March 7, 2018) to identify PET or SPECT studies reporting striatal dopaminergic function in PD patients with ICD (ICD+) compared to those without ICD (ICD–). Studies which included drug naïve patients, explored non-pharmacological procedures (e.g., deep brain stimulation), and those using brain blood perfusion or non-dopaminergic markers were excluded. Standardized mean difference (SDM) was used and random-effect models were applied. Separate meta-analyses were performed for dopamine transporter level, dopamine release, and dopamine receptors availability in the putamen, caudate, dorsal, and ventral striatum.

Results: A total of 238 studies were title and abstract screened, of which 19 full-texts were assessed. Nine studies (ICD+: N = 117; ICD–: N = 175 patients) were included in the analysis. ICD+ showed a significant reduction of dopamine transporter binding in the putamen (SDM = −0.46; 95% CI: −0.80, −0.11; Z = 2.61; p = 0.009), caudate (SDM = −0.38; 95% CI: −0.73, −0.04; Z = 2.18; p = 0.03) and dorsal striatum (SDM = −0.45; 95% CI: −0.77, −0.13; Z = 2.76; p = 0.006), and increased dopamine release to reward-related stimuli/gambling tasks in the ventral striatum (SDM = −1.04; 95% CI: −1.73, −0.35; Z = 2.95; p = 0.003). Dopamine receptors availability did not differ between groups. Heterogeneity was low for dopamine transporter in the dorsal striatum (I² = 0%), putamen (I² = 0%) and caudate (I² = 0%), and pre-synaptic dopamine release in the dorsal (I² = 0%) and ventral striatum (I² = 0%); heterogeneity was high for dopamine transporter levels in the ventral striatum (I² = 80%), and for dopamine receptors availability in the ventral (I² = 89%) and dorsal (I² = 86%) striatum, putamen (I² = 93%), and caudate (I² = 71%).

Conclusions: ICD+ patients show lower dopaminergic transporter levels in the dorsal striatum and increased dopamine release in the ventral striatum when engaged in reward-related stimuli/gambling tasks. This dopaminergic imbalance might represent a biological substrate for ICD in PD. Adequately powered longitudinal studies with drug naïve patients are needed to understand whether these changes may represent biomarkers of premorbid vulnerability to ICD.

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

Treatment with dopaminergic agonists such as pramipexole (PPX) contributes to the development of impulse control disorders (ICDs) in patients with Parkinson's disease (PD). As such, animal models of abnormal impulse control in PD are needed to better study the pathophysiology of these behaviors. Thus, we investigated impulsivity and related behaviors using the 5-choice serial reaction time task, as well as FosB/ΔFosB expression, in rats with mild parkinsonism induced by viral-mediated substantia nigra overexpression of human A53T mutated α-synuclein, and following chronic PPX treatment (0.25 mg/kg/d) for 4 weeks. The bilateral loss of striatal dopamine transporters (64%) increased the premature response rate of these rats, indicating enhanced waiting impulsivity. This behavior persisted in the OFF state after the second week of PPX treatment and it was further exacerbated in the ON state throughout the treatment period. The enhanced rate of premature responses following dopaminergic denervation was positively correlated with the premature response rate following PPX treatment (both in the ON and OFF states). Moreover, the striatal dopaminergic deficit was negatively correlated with the premature response rate at all times (pretreatment, ON and OFF states) and it was positively correlated with the striatal FosB/ΔFosB expression. By contrast, PPX treatment was not associated with changes in compulsivity (perseverative responses rate). This model recapitulates some features of PD with ICD, namely the dopaminergic deficit of early PD and the impulsivity traits provoked by dopaminergic loss in association with PPX treatment, making this model a useful tool to study the pathophysiology of ICDs.

Pharmacological insights into impulsive-compulsive spectrum disorders associated with dopaminergic therapy

Impulsive-compulsive spectrum disorders are associated with dopamine agonist therapy in some patients. These untoward outcomes occur with direct-acting, full and partial agonists at D2 dopamine family receptors. The disorders typically emerge during chronic treatment, and exhibit common features that are independent of the neurological or psychiatric pathology for which the initial therapy was indicated. It is well-documented that the brain is 'plastic', changing in response to alterations to internal factors (e.g., disease processes), as well as external factors (e.g., therapies). The complexities of these clinical scenarios have eluded a clear depiction of the neurobiology for impulsive-compulsive spectrum disorders and engendered considerable debate regarding the mechanistic underpinnings of the disorders. In this opinion, we use pharmacological concepts related to homeostatic compensation subsequent to chronic receptor activation to provide a unifying construct. This construct helps explain the occurrence of impulsive-compulsive spectrum disorders across disease states, and during therapy with full and partial agonists.

The β-adrenoceptor blocker propranolol ameliorates compulsive-like gambling behaviour in a rodent slot machine task: implications for iatrogenic gambling disorder

Previous work has shown that chronic administration of the dopamine D_2/3 receptor agonist ropinirole invigorates performance on a rodent slot machine task (rSMT). This behavioural change appears superficially similar to the iatrogenic gambling disorder (GD) observed in a sub-set of patients with Parkinson's disease (PD), and has been associated with increased activation of the intra-cellular signalling proteins GSK3β and CREB in the striatum. Here, we wanted to determine whether this response to ropinirole could be attenuated by targeting these signalling proteins, and if the loss of dopaminergic innervation characteristic of PD would alter ropinirole's effects on the rSMT. Male Long Evans rats were trained on the rSMT. Dopaminergic terminals innervating the dorsolateral striatum were then lesioned bilaterally using the neurotoxin 6-hydroxydopamine hydrochloride (6-OHDA). Subsequently animals were implanted with osmotic mini-pumps delivering ropinirole. Lastly, animals were given dietary lithium (Li⁺ ), to inhibit the activation of GSK3β, or injections of the ß-adrenoceptor antagonist propranolol, which potently inhibits CREB as a secondary mechanism of action, and any changes in ropinirole-induced increases in compulsive-like engagement in the rSMT evaluated. Chronic ropinirole increased the number of trials animals completed, reproducing our original finding. This increase in task engagement was not altered in animals with 6-OHDA lesions, a putative model of early PD. In addition, the effects of ropinirole were not attenuated by administration of Li⁺ , but were ameliorated by propranolol. These data suggest that propranolol may represent a potential pharmacotherapy for the treatment of iatrogenic gambling.

How foraging works: Uncertainty magnifies food-seeking motivation

Food uncertainty has the effect of invigorating food-related responses. Psychologists have noted that mammals and birds respond more to a conditioned stimulus that unreliably predicts food delivery, and ecologists have shown that animals (especially small passerines) consume and/or hoard more food and can get fatter when access to that resource is unpredictable. Are these phenomena related? We think they are. Psychologists have proposed several mechanistic interpretations, while ecologists have suggested a functional interpretation: The effect of unpredictability on fat reserves and hoarding behavior is an evolutionary strategy acting against the risk of starvation when food is in short supply. Both perspectives are complementary, and we argue that the psychology of incentive motivational processes can shed some light on the causal mechanisms leading animals to seek and consume more food under uncertainty in the wild. Our theoretical approach is in agreement with neuroscientific data relating to the role of dopamine, a neurotransmitter strongly involved in incentive motivation, and its plausibility has received some explanatory and predictive value with respect to Pavlovian phenomena. Overall, we argue that the occasional and unavoidable absence of food rewards has motivational effects (called incentive hope) that facilitate foraging effort. We show that this hypothesis is computationally tenable, leading foragers in an unpredictable environment to consume more food items and to have higher long-term energy storage than foragers in a predictable environment.

Impulse control disorders in Parkinson's disease

Impulse control disorders (ICD) are frequent side effects of dopamine replacement therapy (DRT) used in Parkinson's disease (PD) with devastating consequences on the patients and caregivers. ICD are behavioural addictions including compulsive gambling, shopping, sexual behaviour, and binge eating that are mainly associated with dopamine D2/D3 agonists. Their management is a real clinical challenge due to the lack of therapeutic alternative. Clinical studies have identified demographic and clinical risk factors for ICD such as younger age at disease onset, male gender, prior history of depression or substance abuse, REM sleep behaviour disorders and higher rate of dyskinesia. PD patients with ICD may also have a specific pattern of dopaminergic denervation in the ventral striatum. Specific evaluation tools have now been designed to better evaluate the severity and impact of ICD in PD. Patients with ICD display altered processing of reward and loss, and decisional bias associated with altered activity in cortical and subcortical areas such as the orbitofrontal cortex, amygdala, insula, anterior cingular cortex, and ventral striatum. Preclinical studies have demonstrated that D2/D3 agonists induce impairments in behavioural processes likely relevant to ICD such as risk-taking behaviour, preference for uncertainty, perseverative responding and sustained drive to engage in gambling-like behaviour. Whether interactions between dopamine denervation and DRT significantly contribute to the pathogenesis of ICD remains poorly understood so far, although features unique to PD have been identified in patients with ICD. Large-scale longitudinal studies are needed to better identify subjects with increased risk to develop ICD and develop therapeutic options.

The neurobiology of impulse control disorders in Parkinson's disease: from neurotransmitters to neural networks

Impulse control disorders (ICD) are common neuropsychiatric disorders that can arise in Parkinson’s disease (PD) patients after commencing dopamine replacement therapy. Approximately 15% of all patients develop these disorders and many more exhibit subclinical symptoms of impulsivity. ICD is thought to develop due to an interaction between the use of dopaminergic medication and an as yet unknown neurobiological vulnerability that either pre-existed before PD onset (possibly genetic) or is associated with neural alterations due to the PD pathology. This review discusses genes, neurotransmitters and neural networks that have been implicated in the pathophysiology of ICD in PD. Although dopamine and the related reward system have been the main focus of research, recently, studies have started to look beyond those systems to find new clues to the neurobiological underpinnings of ICD and come up with possible new targets for treatment. Studies on the whole-brain connectome to investigate the global alterations due to ICD development are currently lacking. In addition, there is a dire need for longitudinal studies that are able to disentangle the contributions of individual (genetic) traits and secondary effects of the PD pathology and chronic dopamine replacement therapy to the development of ICD in PD.

Deciphering Decision Making: Variation in Animal Models of Effort- and Uncertainty-Based Choice Reveals Distinct Neural Circuitries Underlying Core Cognitive Processes

Maladaptive decision-making is increasingly recognized to play a significant role in numerous psychiatric disorders, such that therapeutics capable of ameliorating core impairments in judgment may be beneficial in a range of patient populations. The field of "decision neuroscience" is therefore in its ascendancy, with researchers from diverse fields bringing their expertise to bear on this complex and fascinating problem. In addition to the advances in neuroimaging and computational neuroscience that contribute enormously to this area, an increase in the complexity and sophistication of behavioral paradigms designed for nonhuman laboratory animals has also had a significant impact on researchers' ability to test the causal nature of hypotheses pertaining to the neural circuitry underlying the choice process. Multiple such decision-making assays have been developed to investigate the neural and neurochemical bases of different types of cost/benefit decisions. However, what may seem like relatively trivial variation in behavioral methodologies can actually result in recruitment of distinct cognitive mechanisms, and alter the neurobiological processes that regulate choice. Here we focus on two areas of particular interest, namely, decisions that involve an assessment of uncertainty or effort, and compare some of the most prominent behavioral paradigms that have been used to investigate these processes in laboratory rodents. We illustrate how an appreciation of the diversity in the nature of these tasks can lead to important insights into the circumstances under which different neural regions make critical contributions to decision making.

Chronic administration of the dopamine D2/3 agonist ropinirole invigorates performance of a rodent slot machine task, potentially indicative of less distractible or compulsive-like gambling behaviour

RATIONALE

Whilst dopamine agonist therapies can successfully manage the symptoms of diseases such as Parkinson's disease (PD), fibromyalgia and restless leg syndrome, they can also cause impulse control and addiction disorders such as gambling disorder (GD). These compulsive behaviours seriously undermine the utility of such treatments.

OBJECTIVES

The objective of the study was to model this phenomenon using a rodent slot machine task (rSMT) in order to investigate the neurobiological basis underlying such behavioural changes.

METHODS

Male Long Evans rats were trained to perform the rSMT. The D₂-like agonist ropinirole, or saline, was then delivered continuously for 28 days via osmotic mini-pump. The effects of ropinirole on baseline rSMT performance, as well as extinction and reinstatement sessions, were determined during this time. Brain samples from key frontostriatal regions implicated in GD and PD were then harvested immediately or after a 4-week washout period during which behaviour returned to pre-drug baseline.

RESULTS

Ropinirole invigorated task performance, in that drug treatment resulted in a robust and sustained increase in the number of trials completed. Ex vivo analyses revealed that chronic ropinirole treatment led to a pattern of changes indicative of upregulation within the β-arrestin-AKT-GSK3β intracellular cascade, recently theorised to dominate D₂-mediated signalling under hyperdopaminergic conditions, in the dorsal striatum, rather than the canonical PKA-dependent signalling pathway associated with D₂ receptor activation.

CONCLUSIONS

Such findings provide novel insight into the role of dopamine signalling in mediating compulsive-like gambling behaviour and may inform more directed pharmacotherapies for the treatment of both idiopathic and iatrogenic GD.