Preference for Exercise vs. More Sedentary Reinforcers: Validation of an Animal Model of Tetrabenazine-Induced Anergia

Refining the study of decision-making in animals: differential effects of d-amphetamine and haloperidol in a novel touchscreen-automated Rearing-Effort Discounting (RED) task and the Fixed-Ratio Effort Discounting (FRED) task

Effort-based decision-making is impaired in multiple psychopathologies leading to significant impacts on the daily life of patients. Preclinical studies of this important transdiagnostic symptom in rodents are hampered, however, by limitations present in currently available decision-making tests, including the presence of delayed reinforcement and off-target cognitive demands. Such possible confounding factors can complicate the interpretation of results in terms of decision-making per se. In this study we addressed this problem using a novel touchscreen Rearing-Effort Discounting (RED) task in which mice choose between two single-touch responses: rearing up to touch an increasingly higher positioned stimulus to obtain a High Reward (HR) or touching a lower stimulus to obtain a Low Reward (LR). To explore the putative advantages of this new approach, RED was compared with a touchscreen version of the well-studied Fixed Ratio-based Effort Discounting (FRED) task, in which multiple touches are required to obtain an HR, and a single response is required to obtain an LR. Results from dopaminergic (haloperidol and d-amphetamine), behavioral (changes in the order of effort demand; fixed-ratio schedule in FRED or response height in RED), and dietary manipulations (reward devaluation by pre-feeding) were consistent with the presence of variables that may complicate interpretation of conventional decision-making tasks, and demonstrate how RED appears to minimize such variables.

Sex and age differences in mice models of effort-based decision-making and anergia in depression: the role of dopamine, and cerebral-dopamine-neurotrophic-factor

Mesolimbic dopamine (DA) regulates vigor in motivated behavior. While previous results have mainly been performed in male rodents, the present studies compared CD1 male and female mice in effort-based decision-making tests of motivation. These tests offered choices between several reinforcers that require different levels of effort (progressive ratio/choice task and 3-choice-T-maze task). Sweet reinforcers were used in both tasks. In the operant tasks, females worked harder as the task required more effort to access a 10% sucrose solution. Although males and females did not differ in preference for 10% vs 3% solutions under free concurrent presentation, females consumed more of the 10% solution when tested alone. The operant task requires a long period of training and changes in the DA system due to age can be mediating long-term changes in effort. Thus, age and sex factors were evaluated in the T-maze task, which requires only a short training period. Both sexes and ages were equally active when habituated to the running wheel (RW), but females consumed more sweet pellets than males, especially at an older age. Both sexes had a strong preference for the RW compared to more sedentary reinforcers in the 3-choice-T-maze test, but older animals spent less time running and ate more than the young ones. The DA-depleting agent tetrabenazine reduced time running in older mice but not in adolescents. Cerebral-dopamine-neurotrophic-factor was reduced in older mice of both sexes compared to adolescent mice. These results emphasize the importance of taking into account differences in sex and age when evaluating willingness to exert effort for specific reinforcers.

Effects of the dopamine depleting agent tetrabenazine in tests evaluating different components of depressive-like behavior in mice: sex-dependent response to antidepressant drugs with SERT and DAT blocker profiles

BACKGROUND

Depression is a disorder twice as common in women than in men. There are sex differences in the symptomatology and treatment response to this disorder. Impairments in behavioral activation (i.e. anergia, fatigue) are often seen in people with depression and are highly resistant to treatment. The role of mesolimbic dopamine (DA) in regulating behavioral activation has been extensively studied in male rodents, but little is known in female rodents.

OBJECTIVE

The present studies assessed potential sex differences in rodent paradigms used to study different components of depressive-like behavior, and in the treatment response to antidepressants with different mechanisms of action.

METHODS

Male and female CD1 mice received Tetrabenazine (TBZ), a VMAT-2 blocker that depletes DA and induces depressive symptoms in humans. Mice were tested on the Forced Swim Test, (FST), the Dark-Light box (DL), the elevated plus maze (EPM), Social Interaction (SI) test, and sucrose preference and consumption using the two bottles test. In addition, bupropion (a DA reuptake inhibitor) or fluoxetine (a serotonin reuptake inhibitor) were used to reverse TBZ-induced anergia.

RESULTS

In the FST, bupropion reversed TBZ effects in both sexes but fluoxetine was only effective in female mice. DA depletion did not affect other aspects of depression such as anxiety, sociability or sucrose consumption, and there was no interaction with bupropion on these parameters. In TBZ treated-females SERT-blockers may be effective at reversing anergia in aversive contexts (FST), and potentiating avoidance of anxiogenic stimuli.

CONCLUSIONS

Pro-dopaminergic antidepressants seem more efficacious at improving anergia in both sexes than SERT-blockers.

Critical review of RDoC approaches to the study of motivation with animal models: effort valuation/willingness to work

The NIMH research domain criteria (RDoC) approach was instigated to refocus mental health research on the neural circuits that mediate psychological functions, with the idea that this would foster an understanding of the neural basis of specific psychiatric dysfunctions (i.e. 'symptoms and circuits') and ultimately facilitate treatment. As a general idea, this attempt to go beyond traditional diagnostic categories and focus on neural circuit dysfunctions related to specific symptoms spanning multiple disorders has many advantages. For example, motivational dysfunctions are present in multiple disorders, including depression, schizophrenia, Parkinson's disease, and other conditions. A critical aspect of motivation is effort valuation/willingness to work, and several clinical studies have identified alterations in effort-based decision making in various patient groups. In parallel, formal animal models focusing on the exertion of effort and effort-based decision making have been developed. This paper reviews the literature on models of effort-based motivational function in the context of a discussion of the RDoC approach, with an emphasis on the dissociable nature of distinct aspects of motivation. For example, conditions associated with depression and schizophrenia blunt the selection of high-effort activities as measured by several tasks in animal models (e.g. lever pressing, barrier climbing, wheel running). Nevertheless, these manipulations also leave fundamental aspects of hedonic reactivity, food motivation, and reinforcement intact. This pattern of effects demonstrates that the general emphasis of the RDoC on the specificity of the neural circuits mediating behavioral pathologies, and the dissociative nature of these dysfunctions, is a valid concept. Nevertheless, the specific placement of effort-related processes as simply a 'sub-construct' of 'reward processing' is empirically and conceptually problematic. Thus, while the RDoC is an excellent general framework for new ways to approach research and therapeutics, it still needs further refinement.

Vigor, Effort-Related Aspects of Motivation and Anhedonia

In this chapter we provide an overview of the pharmacological and circuit mechanisms that determine the willingness to expend effort in pursuit of rewards. A particular focus will be on the role of the mesolimbic dopamine system, as well the contributing roles of limbic and cortical brains areas involved in the evaluation, selection, and invigoration of goal-directed actions. We begin with a review of preclinical studies, which have provided key insights into the brain systems that are necessary and sufficient for effort-based decision-making and have characterized novel compounds that enhance selection of high-effort activities. Next, we summarize translational studies identifying and expanding this circuitry in humans. Finally, we discuss the relevance of this work for understanding common motivational impairments as part of the broader anhedonia symptom domain associated with mental illness, and the identification of new treatment targets within this circuitry to improve motivation and effort-expenditure.

The selective 5-HT 1A receptor agonist, NLX-112, overcomes tetrabenazine-induced catalepsy and depression-like behavior in the rat

Tetrabenazine, a preferential inhibitor of the vesicular monoamine transporter type 2, depletes the brain monoamines dopamine, serotonin and norepinephrine. Tetrabenazine and deutetrabenazine (Austedo ®) are used to treat chorea associated with Huntington's disease. However, both compounds are known to aggravate Parkinsonism and depression observed in Huntington's disease patients. NLX-112 (a.k.a. befiradol/F13640) is a highly selective, potent and efficacious serotonin 5-HT 1A agonist. In animal models, it has robust efficacy in combating other iatrogenic motor disorders such as L-DOPA-induced dyskinesia and has marked antidepressant-like activity in rodent tests. In the present study, we investigated, in rats, the efficacy of NLX-112 to counteract tetrabenazine-induced catalepsy (a model of Parkinsonism) and tetrabenazine-induced potentiation of immobility in the forced swim test (FST, a model to detect antidepressant-like activity). The prototypical 5-HT 1A agonist, (±)8-OH-DPAT, and the 5-HT 1A partial agonist/dopamine D2 receptor blocker, buspirone, were used as comparators. Both NLX-112 and (±)8-OH-DPAT (0.16-2.5 mg/kg p.o. or s.c., respectively) abolished catalepsy induced by tetrabenazine (2 mg/kg i.p.). In comparison, buspirone (0.63-5.0 mg/kg p.o.) was ineffective and even tended to potentiate tetrabenazine-induced catalepsy at 0.63 mg/kg. In the FST, NLX-112 and (±)8-OH-DPAT (0.63 mg/kg) strongly reduced immobility when administered alone but also significantly opposed potentiation of immobility induced by tetrabenazine (1.5 mg/kg i.p.). Buspirone (0.63 and 2.5 mg/kg p.o.) had no effect by itself or against tetrabenazine. These results strongly suggest that selective and highly efficacious 5-HT 1A agonists, such as NLX-112, may be useful in combating tetrabenazine-induced Parkinsonism and/or depression in Huntington's disease patients.

Impact of Fluoxetine on Behavioral Invigoration of Appetitive and Aversively Motivated Responses: Interaction With Dopamine Depletion

Impaired behavioral activation and effort-related motivational dysfunctions like fatigue and anergia are debilitating treatment-resistant symptoms of depression. Depressed people show a bias towards the selection of low effort activities. To determine if the broadly used antidepressant fluoxetine can improve behavioral activation and reverse dopamine (DA) depletion-induced anergia, male CD1 mice were evaluated for vigorous escape behaviors in an aversive context (forced swim test, FST), and also with an exercise preference choice task [running wheel (RW)-T-maze choice task]. In the FST, fluoxetine increased active behaviors (swimming, climbing) while reducing passive ones (immobility). However, fluoxetine was not effective at reducing anergia induced by the DA-depleting agent tetrabenazine, further decreasing vigorous climbing and increasing immobility. In the T-maze, fluoxetine alone produced the same pattern of effects as tetrabenazine. Moreover, fluoxetine did not reverse tetrabenazine-induced suppression of RW time but it reduced sucrose intake duration. This pattern of effects produced by fluoxetine in DA-depleted mice was dissimilar from devaluing food reinforcement by pre-feeding or making the food bitter since in both cases sucrose intake time was reduced but animals compensated by increasing time in the RW. Thus, fluoxetine improved escape in an aversive context but decreased relative preference for active reinforcement. Moreover, fluoxetine did not reverse the anergic effects of DA depletion. These results have implications for the use of fluoxetine for treating motivational symptoms such as anergia in depressed patients.

Sedentary time is related to deficits in response inhibition among adults with overweight and obesity: An accelerometry and event-related brain potentials study

Excessive sedentariness has been related to poorer cognitive control in adults. Sedentariness may compound obesity-related impairments in response inhibition, but its relationship to response inhibition remains poorly understood. This study investigated the relationship between accelerometer-measured sedentary time (ST, min/day), performance on the Oddball and NoGo tasks, N2 and P3-ERP indices of response inhibition and attentional control in 80 adults with overweight and obesity (55 females, M_age  = 35.2 ± 5.8 years, BMI = 32.8 ± 5.3 kg/m² ). ST was not related to performance on the Oddball task. However, more sedentary adults had larger P3b amplitude to targets. Higher ST was also related to increased attentional resource allocation during NoGo target and nontarget trials as indicated by higher P3b amplitudes across centroparietal sites (C1, Cz, C2, CP1, CPz, CP2; ps ≤ .03). ST was negatively indirectly related to target accuracy on NoGo trials through its association with faster response times to nontargets (95% percentile bootstrap CI for a standardized effect: -0.182, -0.014). ST was not related to N2 amplitude on either Oddball or NoGo target trials. Adjustment for moderate-to-vigorous physical activity (MVPA; all models), age (models with P3b NoGo target amplitude, N2 NoGo target amplitude and latency), and % fat mass (models with target NoGo accuracy and N2 NoGo target amplitude) did not modulate behavioral findings. MVPA did not significantly predict P3b amplitude. Our results suggest suboptimal response inhibition due to trading accuracy for speed and despite the upregulation of attentional resources among more sedentary adults with overweight and obesity.

Sex differences in lever pressing and running wheel tasks of effort-based choice behavior in rats: Suppression of high effort activity by the serotonin transport inhibitor fluoxetine

Selective serotonin transport (SERT) inhibitors such as fluoxetine are the most commonly prescribed treatments for depression. Although efficacious for many symptoms of depression, motivational impairments such as psychomotor retardation, anergia, fatigue and amotivation are relatively resistant to treatment with SERT inhibitors, and these drugs have been reported to exacerbate motivational deficits in some people. In order to study motivational dysfunctions in animal models, procedures have been developed to measure effort-related decision making, which offer animals a choice between high effort actions leading to highly valued reinforcers, or low effort/low reward options. In the present studies, male and female rats were tested on two different tests of effort-based choice: a fixed ratio 5 (FR5)/chow feeding choice procedure and a running wheel (RW)/chow feeding choice task. The baseline pattern of choice differed across tasks for males and females, with males pressing the lever more than females on the operant task, and females running more than males on the RW task. Administration of the SERT inhibitor and antidepressant fluoxetine suppressed the higher effort activity on each task (lever pressing and wheel running) in both males and females. The serotonin receptor mediating the suppressive effects of fluoxetine is uncertain, because serotonin antagonists with different patterns of receptor selectivity failed to reverse the effects of fluoxetine. Nevertheless, these studies uncovered important sex differences, and demonstrated that the suppressive effects of fluoxetine on high effort activities are not limited to tasks involving food reinforced behavior or appetite suppressive effects. It is possible that this line of research will contribute to an understanding of the neurochemical factors regulating selection of voluntary physical activity vs. sedentary behaviors, which could be relevant for understanding the role of physical activity in psychiatric disorders.

Energizing effects of bupropion on effortful behaviors in mice under positive and negative test conditions: modulation of DARPP-32 phosphorylation patterns

Motivational symptoms such as anergia, fatigue, and reduced exertion of effort are seen in depressed people. To model this, nucleus accumbens (Nacb) dopamine (DA) depletions are used to induce a low-effort bias in rodents tested on effort-based decision-making. We evaluated the effect of the catecholamine uptake blocker bupropion on its own, and after administration of tetrabenazine (TBZ), which blocks vesicular storage, depletes DA, and induces depressive symptoms in humans. Male CD1 mice were tested on a 3-choice-T-maze task that assessed preference between a reinforcer involving voluntary physical activity (running wheel, RW) vs. sedentary activities (sweet food pellet intake or a neutral non-social odor). Mice also were tested on the forced swim test (FST), two anxiety-related measures (dark-light box (DL), and elevated plus maze (EPM)). Expression of phosphorylated DARPP-32 (Thr34 and Thr75) was evaluated by immunohistochemistry as a marker of DA-related signal transduction. Bupropion increased selection of RW activity on the T-maze. TBZ reduced time running, but increased time-consuming sucrose, indicating an induction of a low-effort bias, but not an effect on primary sucrose motivation. In the FST, bupropion reduced immobility, increasing swimming and climbing, and TBZ produced the opposite effects. Bupropion reversed the effects of TBZ on the T-maze and the FST, and also on pDARPP32-Thr34 expression in Nacb core. None of these manipulations affected anxiety-related parameters. Thus, bupropion improved active behaviors, which were negatively motivated in the FST, and active behaviors that were positively motivated in the T-maze task, which has implications for using catecholamine uptake inhibitors for treating anergia and fatigue-like symptoms.

Maternal Motivation: Exploring the Roles of Prolactin and Pup Stimuli

Motherhood entails increased motivation for pups, which become strong reinforcers and guide maternal behaviours. This depends on steroids and lactogens acting on the brain of females during pregnancy and postpartum. Since virgin female mice exposed to pups are nearly spontaneously maternal, the specific roles of endocrine and pup-derived signals in the induction of maternal motivation remain unclear. This work investigates maternal motivation in dams and virgin female mice, using a novel variant of the pup retrieval paradigm, the motivated pup retrieval test. We also analyse the role of prolactin (PRL) and of stimuli derived from a litter of pups and its mother, in the acquisition of maternal motivation. Experimental design included female mice in 3 conditions: lactating dams, comothers (virgins housed and sharing pup care with dams) and pup-naïve virgins. Females underwent 3 motivated-pup-retrieval trials, with pups displaced behind a 10-cm-high wire-mesh barrier. Dams retrieved with significantly lower latencies than comothers or virgins, indicating that full maternal motivation appears only after pregnancy. Although initially comothers and virgins showed no retrieval, comothers significantly improved throughout the experiment, suggesting an induced sensitization process. Lengthening exposure of comothers to the dyad pups-dam (from 2 to 5 days at the beginning of testing) had no strong effects on maternal sensitization. PRL responsiveness was analysed in these animals using immunohistochemical detection of phosphorylated signal transducer and activator of transcription 5 (pSTAT5, PRL-derived signalling marker). As expected, dams showed significantly higher pSTAT5 expression in most of the analysed nuclei. Moreover, comothers displayed significantly higher PRL responsiveness than pup-naïve virgins in the medial preoptic nucleus, even if they display similar circulating PRL levels, which are significantly lower than those of dams. Given the instrumental role of this nucleus in the relay and integration of pup-derived stimuli to facilitate proactive maternal responses, this increase in PRL responsiveness likely reflects the mechanism underlying the maternal sensitization process reported in this work. Since the analyses of maternal motivation and PRL signalling in the brain were performed in the same animals, we were able to explore correlation between both set of data. The results shed light on the neuroendocrine mechanisms underlying maternal motivation and other aspects of maternal behaviour.

The novel atypical dopamine transport inhibitor CT-005404 has pro-motivational effects in neurochemical and inflammatory models of effort-based dysfunctions related to psychopathology

Depressed individuals suffer from effort-related motivational symptoms such as anergia and fatigue, which are resistant to treatment with many common antidepressants. While drugs that block dopamine transport (DAT) reportedly have positive motivational effects, DAT inhibitors such as cocaine and amphetamines produce undesirable side effects. Thus, there is a need to develop and characterize novel atypical DAT inhibitors with unique and selective binding profiles. Rodent effort-based choice tasks provide useful models of motivational dysfunctions. With these tasks, animals choose between a high-effort instrumental action leading to highly valued reinforcement vs. a low effort/low reward option. The present studies focused on the initial characterization of a novel atypical DAT inhibitor, CT-005404, which binds to DAT with high selectivity relative to serotonin and norepinephrine transport, and produces long-term elevations of extracellular DA. CT-005404 was assessed for its ability to attenuate the effort-related motivational effects of the DA depleting agent tetrabenazine and the pro-inflammatory cytokine interleukin-1β (IL-1β) using a fixed ratio 5/chow feeding choice test. Tetrabenazine (1.0 mg/kg i.p.) shifted choice behavior, decreasing lever pressing and increasing chow intake. IL-1β (4.0 μg/kg i.p.) also decreased lever pressing. CT-005404 was co-administered (7.5-30.0 mg/kg p.o.) with either tetrabenazine or IL-1β, and the 15.0 and 30.0 mg/kg doses significantly reversed the effects of tetrabenazine and IL-1β. CT-005404 administered alone produced a dose-related increase in lever pressing in rats tested on a progressive ratio/chow feeding choice task. Atypical DAT inhibitors such as CT-005404 offer potential as a new avenue for drug treatment of motivational dysfunctions in humans.

The dopamine depleting agent tetrabenazine alters effort-related decision making as assessed by mouse touchscreen procedures

RATIONALE

Effort-based decision-making tasks allow animals to choose between preferred reinforcers that require high effort to obtain vs. low-effort/low reward options. Mesolimbic dopamine (DA) and related neural systems regulate effort-based choice. Tetrabenazine (TBZ) is a vesicular monoamine transport type-2 inhibitor that blocks DA storage and depletes DA. In humans, TBZ induces motivational dysfunction and depression. TBZ has been shown reliably to induce a low-effort bias in rats, but there are fewer mouse studies.

OBJECTIVES

The present studies used touchscreen operant procedures (Bussey-Saksida chambers) to assess the effects of TBZ on effort-based choice in mice.

METHODS

C57BL6 mice were trained to press an elevated lit panel on the touchscreen on a fixed ratio 1 schedule reinforced by strawberry milkshake, vs. approaching and consuming a concurrently available but less preferred food pellets (Bio-serv).

RESULTS

TBZ (2.0-8.0 mg/kg IP) shifted choice, producing a dose-related decrease in panel pressing but an increase in pellet intake. In contrast, reinforcer devaluation by pre-feeding substantially decreased both panel pressing and pellet intake. In free-feeding choice tests, mice strongly preferred the milkshake vs. the pellets, and TBZ had no effect on milkshake intake or preference, indicating that the TBZ-induced low-effort bias was not due to changes in primary food motivation or preference. TBZ significantly decreased tissue levels of nucleus accumbens DA.

CONCLUSION

The DA depleting agent TBZ induced an effort-related motivational dysfunction in mice, which may have clinical relevance for assessing novel drug targets for their potential use as therapeutic agents in patients with motivation impairments.