Pharmacologically distinct pramipexole-mediated akinesia vs. risk-taking in a rat model of Parkinson's disease

Dopamine activity in the nigrostriatal pathway alters cue-induced risky choice patterns in female rats

Deficits in cost/benefit decision making is a critical risk factor for gambling disorder. Reward-paired cues may play an important role, as these stimuli can enhance risk preference in rats. Despite extensive research implicating the dorsal striatum in the compulsive aspects of addiction, the role of nigrostriatal dopaminergic activity in cue-induced risk preference remains unclear, particularly in females. Accordingly, we examined the effects of manipulating the dopaminergic nigrostriatal pathway on cue-induced risky choice in female rats. TH:Cre rats were trained on the cued version of the rat Gambling Task. This task was designed such that maximal reward is attained by avoiding the high-risk, high-reward options and instead favouring the options associated with lower per-trial gains, as they feature less frequent and shorter time-out penalties. Adding reward-paired audiovisual cues to the task leads to greater risky choice on average. To assess the role of the nigrostriatal pathway, a viral vector carrying either Cre-dependent inhibitory or excitatory DREADD was infused into the substantia nigra. Rats then received clozapine-N-oxide either during task acquisition or after a stable performance baseline was reached. Inhibition of this pathway accelerated the development of risk preference in early sessions and increased risky choice during performance, but long-term inhibition actually improved decision making. Activation of this pathway had minimal effects. These results provide evidence for the involvement of the dopaminergic nigrostriatal pathway in cue-induced risk preference in females, therefore shedding light on its role in cost/benefit decision-making deficits and expanding our knowledge of the female dopaminergic system.

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.

Polyvinylpyrrolidone-Capped Copper Oxide Nanoparticles-Anchored Pramipexole Attenuates the Rotenone-Induced Phenotypes in a Drosophila Parkinson's Disease Model

Parkinson's disease (PD) is a progressive, age-related neurodegenerative disease. The disease is characterized by the loss of dopaminergic neurons in the substantia nigra, pars compacta of the midbrain. Pramipexole (PPX) is a novel drug used for the treatment of PD. It has a high affinity for the dopamine (DA) D2 receptor subfamily and acts as a targeted mitochondrial antioxidant. It is less effective in the treatment of PD due to its short half-life, highly inconvenient dosing schedule, and long-term side effects. In recent years, PPX-loaded nanoformulations have been actively reported to overcome these limitations. In the current study, we focused on increasing the effectiveness of PPX by minimizing the dosing frequency and improving the treatment strategy for PD. Herein, we report the synthesis of biodegradable polyvinylpyrrolidone (PVP)-capped copper oxide nanoparticles (PVP-CuO NPs), followed by PPX anchoring on the surface of the PVP-CuO NPs (PPX-PVP-CuO NC), in a simple and inexpensive method. The newly formulated PPX-PVP-CuO NC complex was analyzed for its chemical and physical properties. The PPX-PVP-CuO NC was tested to protect against rotenone (RT)-induced toxicity in the Drosophila PD model. The in vivo studies using the RT-induced Drosophila PD model showed significant changes in negative geotaxis behavior and the level of DA and acetylcholinesterase. In addition, oxidative stress markers such as glutathione-S-transferase, total glutathione, thiobarbituric acid reactive species, and protein carbonyl content showed significant amelioration. The positive changes of PPX-PVP-CuO NC treatment in behavior, neurotransmitter level, and antioxidant level suggest its potential role in mitigating the PD phenotype. The formulation can be used for treatment or pharmacological intervention against PD.

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.

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.

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.

An update on the use of non-ergot dopamine agonists for the treatment of Parkinson's disease

INTRODUCTION

Long-term treatment of Parkinson's disease (PD) with levodopa is hampered by motor complications related to the inability of residual nigrostriatal neurons to convert levodopa to dopamine (DA) and use it appropriately. This generated a tendency to postpone levodopa, favoring the initial use of DA agonists, which directly stimulate striatal dopaminergic receptors. Use of DA agonists, however, is associated with multiple side effects and their efficacy is limited by suboptimal bioavailability.

AREAS COVERED

This paper reviewed the latest preclinical and clinical findings on the efficacy and adverse effects of non-ergot DA agonists, discussing the present and future of this class of compounds in PD therapy.

EXPERT OPINION

The latest findings confirm the effectiveness of DA agonists as initial treatment or adjunctive therapy to levodopa in advanced PD, but a more conservative approach to their use is emerging, due to the complexity and repercussions of their side effects. As various factors may increase the individual risk to side effects, assessing such risk and calibrating the use of DA agonists accordingly may become extremely important in the clinical management of PD, as well as the availability of new DA agonists with better profiles of safety and efficacy.

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.

Neuropsychiatric aspects of Parkinson disease psychopharmacology: Insights from circuit dynamics

Parkinson disease (PD) is a neurodegenerative disorder with a complex pathophysiology characterized by the progressive loss of dopaminergic neurons within the substantia nigra. Persons with PD experience several motoric and neuropsychiatric symptoms. Neuropsychiatric features of PD include depression, anxiety, psychosis, impulse control disorders, and apathy. In this chapter, we will utilize the National Institutes of Mental Health Research Domain Criteria (RDoC) to frame and integrate observations from two prevailing disease constructions: neurotransmitter anomalies and circuit physiology. When there is available evidence, we posit how unified translational observations may have clinical relevance and postulate importance outside of PD. Finally, we review the limited evidence available for pharmacologic management of these symptoms.

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.

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.

Examining the neurochemical underpinnings of animal models of risky choice: Methodological and analytic considerations

Because risky choice is associated with several psychiatric conditions, recent research has focused on examining the underlying neurochemical processes that control risk-based decision-making. Not surprisingly, several tasks have been developed to study the neural mechanisms involved in risky choice. The current review will briefly discuss the major tasks used to measure risky choice and will summarize the contribution of several major neurotransmitter systems to this behavior. To date, the most common measures of risky choice are the probability discounting task, the risky decision task, and the rat gambling task. Across these three tasks, the contribution of the dopaminergic system has been most studied, although the effects of serotonergic, adrenergic, cholinergic, and glutamatergic ligands will be discussed. Drug effects across these tasks have been inconsistent, which makes determining the precise role of neurotransmitter systems in risky choice somewhat difficult. Furthermore, procedural differences can modulate drug effects in these procedures, and the way data are analyzed can alter the interpretations one makes concerning pharmacological manipulations. By taking these methodological/analytic considerations into account, we may better elucidate the neurochemistry of risky decision-making. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

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.

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.

Mirtazapine and ketanserin alter preference for gambling-like schedules of reinforcement in rats

Drug and behavioral addictions have overlapping features, e.g., both manifest preference for larger, albeit costlier, reinforcement options in cost/benefit decision-making tasks. Our prior work revealed that the mixed-function serotonergic compound, mirtazapine, attenuates behaviors by rats motivated by abused drugs. To extend this work to behavioral addictions, here we determined if mirtazapine and/or ketanserin, another mixed-function serotonin-acting compound, can alter decision-making in rats that is independent of drug (or food)-motivated reward. Accordingly, we developed a novel variable-ratio task in rats wherein intracranial self-stimulation was used as the positive reinforcer. Using lever pressing for various levels of brain stimulation, the operant task provided choices between a small brain stimulation current delivered on a fixed-ratio schedule (i.e., a predictable reward) and a large brain stimulation delivered following an unpredictable number of responses (i.e., a variable-ratio schedule). This task allowed for demonstration of individualized preference and detection of shifts in motivational influences during a pharmacological treatment. Once baseline preference was established, we determined that pretreatment with mirtazapine or ketanserin significantly decreased preference for the large reinforcer presented after gambling-like schedules of reinforcement. When the rats were tested the next day without drug, preference for the unpredictable large reinforcer option was restored. These data demonstrate that mirtazapine and ketanserin can reduce preference for larger, costlier reinforcement options, and illustrate the potential for these drugs to alter behavior.

Brain Stimulation Reward Supports More Consistent and Accurate Rodent Decision-Making than Food Reward

Animal models of decision-making rely on an animal's motivation to decide and its ability to detect differences among various alternatives. Food reinforcement, although commonly used, is associated with problematic confounds, especially satiety. Here, we examined the use of brain stimulation reward (BSR) as an alternative reinforcer in rodent models of decision-making and compared it with the effectiveness of sugar pellets. The discriminability of various BSR frequencies was compared to differing numbers of sugar pellets in separate free-choice tasks. We found that BSR was more discriminable and motivated greater task engagement and more consistent preference for the larger reward. We then investigated whether rats prefer BSR of varying frequencies over sugar pellets. We found that animals showed either a clear preference for sugar reward or no preference between reward modalities, depending on the frequency of the BSR alternative and the size of the sugar reward. Overall, these results suggest that BSR is an effective reinforcer in rodent decision-making tasks, removing food-related confounds and resulting in more accurate, consistent, and reliable metrics of choice.