Assessment of reward responsiveness in the response bias probabilistic reward task in rats: implications for cross-species translational research

Reward-Related Brain Activity Mediates the relationship between Decision-Making Deficits and Pediatric Depression Symptom Severity

BACKGROUND

The mechanisms linking neural and behavioral indices of reduced reward sensitivity in depression, particularly in children, remain unclear. Reward positivity (RewP), a neural index of reward processing, has been consistently associated with depression. Separately, recent studies using the drift-diffusion model (DDM) on behavioral data have delineated computational indices of reward sensitivity. Therefore, the present study examined whether RewP is a neural mediator of DDM-based indices of reward processing in predicting pediatric depression across varying levels of symptom severity.

METHODS

A community sample of 166 girls, aged 8 to 14 years, completed two tasks. The first was a reward guessing task from which RewP was computed using electroencephalography; the second was a probabilistic reward-based decision-making task. On this second task, DDM analysis was applied to behavioral data to quantify the efficiency of accumulating reward-related evidence (drift rate) and potential baseline bias (starting point) towards the differently rewarded choices. Depression severity was measured using the self-report Children's Depression Inventory (CDI).

RESULTS

RewP was correlated with drift rate, but not starting point bias, towards the more rewarded choice. Furthermore, RewP completely mediated the association between a slower drift rate towards the more rewarded option and higher depression symptom severity.

CONCLUSION

Our findings suggest that reduced neural sensitivity to reward feedback might be a neural mechanism underscoring behavioral insensitivity to reward in children and adolescents with higher depression symptom severity, offering novel insights into the relationship between neural and computational indices of reward processing in this context.

Chronic ecologically relevant stress effects on reverse-translated touchscreen assays of reward responsivity and attentional processes in male rats: Implications for depression

Stagnation in the development of novel therapeutic strategies for treatment-resistant depression has encouraged continued interest in improving preclinical methods. One tactic prioritizes the reverse translation of behavioral tasks developed to objectively quantify depressive phenotypes in patient populations for their use in laboratory animals via touchscreen technology. After cross-species concordance in task outcomes under healthy conditions is confirmed, construct validity can be further enhanced by identifying environmental stressors that reliably produce deficits in task performance that resemble those in depressive participants. The present studies characterized in male rats the ability of two chronic ecologically relevant stressors, inescapable ice water or isolated restraint, to produce depressive-like behavioral phenotypes in the Probabilistic Reward Task (PRT) and Psychomotor Vigilance Task (PVT). These tasks previously have been reverse-translated using touchscreen technology for rodents and nonhuman primates to objectively quantify, respectively, reward responsivity (anhedonia) and attentional processes (impaired cognitive function), each of which are core features of major depressive disorder. In the PRT, both inescapable ice water and isolated restraint produced persistent anhedonic phenotypes compared to non-stressed control performance (i.e., significantly blunted response bias for the richly rewarded stimulus). In the PVT, both chronic stressors impaired attentional processing, revealed by increases in titrated reaction times; however, these deficits largely subsided by the end of the chronic condition. Taken together, these findings confirm the ability of reverse-translated touchscreen tasks to effectively generate behavioral phenotypes that exhibit expected deficits in performance outcomes following exposure to chronic ecologically relevant stress. In turn, this approach is well positioned to appraise the ability of candidate therapeutics to attenuate or reverse such behavioral deficits and, thereby, contribute to preclinical medications development for treatment-resistant depression.

Characterisation of behaviours relevant to apathy syndrome in the aged male rat

Apathy is a complex psychiatric syndrome characterised by motivational deficit, emotional blunting and cognitive changes. It occurs alongside a broad range of neurological disorders, but also occurs in otherwise healthy ageing. Despite its clinical prevalence, apathy does not yet have a designated treatment strategy. Generation of a translational animal model of apathy syndrome would facilitate the development of novel treatments. Given the multidimensional nature of apathy, a model cannot be achieved with a single behavioural test. Using a battery of behavioural tests we investigated whether aged rats exhibit behavioural deficits across different domains relevant to apathy. Using the effort for reward and progressive ratio tasks we found that aged male rats (21-27 months) show intact reward motivation. Using the novelty supressed feeding test and position-based object exploration we found aged rats showed increased anxiety-like behaviour inconsistent with emotional blunting. The sucrose preference test and reward learning assay showed intact reward sensitivity and reward-related cognition in aged rats. However, using a bowl-digging version of the probabilistic reversal learning task, we found a deficit in cognitive flexibility in aged rats that did not translate across to a touchscreen version of the task. While these data reveal important changes in cognitive flexibility and anxiety associated with ageing, aged rats do not show deficits across other behavioural domains relevant to apathy. This suggests that aged rats are not a suitable model for age-related apathy syndrome. These findings contrast with previous work in mice, revealing important species differences in behaviours relevant to apathy syndrome in ageing.

Rodent tests of depression and anxiety: Construct validity and translational relevance

Behavioral testing constitutes the primary method to measure the emotional states of nonhuman animals in preclinical research. Emerging as the characteristic tool of the behaviorist school of psychology, behavioral testing of animals, particularly rodents, is employed to understand the complex cognitive and affective symptoms of neuropsychiatric disorders. Following the symptom-based diagnosis model of the DSM, rodent models and tests of depression and anxiety focus on behavioral patterns that resemble the superficial symptoms of these disorders. While these practices provided researchers with a platform to screen novel antidepressant and anxiolytic drug candidates, their construct validity-involving relevant underlying mechanisms-has been questioned. In this review, we present the laboratory procedures used to assess depressive- and anxiety-like behaviors in rats and mice. These include constructs that rely on stress-triggered responses, such as behavioral despair, and those that emerge with nonaversive training, such as cognitive bias. We describe the specific behavioral tests that are used to assess these constructs and discuss the criticisms on their theoretical background. We review specific concerns about the construct validity and translational relevance of individual behavioral tests, outline the limitations of the traditional, symptom-based interpretation, and introduce novel, ethologically relevant frameworks that emphasize simple behavioral patterns. Finally, we explore behavioral monitoring and morphological analysis methods that can be integrated into behavioral testing and discuss how they can enhance the construct validity of these tests.

Validation of a touchscreen probabilistic reward task for mice: A reverse-translated assay with cross-species continuity

The Probabilistic Reward Task (PRT) is a laboratory-based technique used to objectively quantify responsivity to reward. The PRT was initially designed to identify reinforcement learning deficits in clinical populations and subsequently was reverse-translated for use in preclinical studies with rats and monkeys. In this task, subjects make visual discriminations and asymmetric probabilistic contingencies are arranged such that correct responses to one stimulus (rich) are reinforced more often than correct responses to the other (lean). Numerous studies have demonstrated that healthy subjects reliably develop a response bias toward the richly rewarded stimulus, whereas humans with anhedonia and laboratory animals with a history of chronic stress exhibit a blunted response bias. This is important because anhedonia, the loss of responsivity to previously rewarding stimuli, is a behavioral phenotype that is a cardinal feature of multiple neuropsychiatric conditions and is without approved pharmacotherapeutic options. To aid in addressing this critical treatment gap, this report describes validation of the first PRT designed for mice, which are a commonly utilized species in preclinical research toward neuropsychiatric medications development. Results reveal orderly psychophysical functions in response to asymmetric probabilistic contingencies in mice, with signal detection outcomes comparable to previous PRT findings in humans, rats, and monkeys. Taken together, such robust cross-species continuity in task performance confirms that the mouse is well-positioned to serve in bidirectional research efforts between human and animal laboratories. These efforts may accelerate the development of treatment options for anhedonia in the different neuropsychiatric conditions in which it is prominent.

Neuropsychological Effects of Antidepressants: Translational Studies

Pharmacological treatments that improve mood were first identified serendipitously, but more than half a century later, how these drugs induce their antidepressant effects remains largely unknown. With the help of animal models, a detailed understanding of their pharmacological targets and acute and chronic effects on brain chemistry and neuronal function has been achieved, but it remains to be elucidated how these effects translate to clinical efficacy. Whilst the field has been dominated by the monoamine and neurotrophic hypotheses, the idea that the maladaptive cognitive process plays a critical role in the development and perpetuation of mood disorders has been discussed since the 1950s. Recently, studies using objective methods to quantify changes in emotional processing found acute effects with conventional antidepressants in both healthy volunteers and patients. These positive effects on emotional processing and cognition occur without a change in the subjective ratings of mood. Building from these studies, behavioural methods for animals that quantify similar cognitive affective processes have been developed. Integrating these behavioural approaches with pharmacology and targeted brain manipulations, a picture is beginning to emerge of the underlying mechanisms that may link the pharmacology of antidepressants, these neuropsychological constructs and clinical efficacy. In this chapter, we discuss findings from animal studies, experimental medicine and patients investigating the neuropsychological effects of antidepressant drugs. We discuss the possible neural circuits that contribute to these effects and discuss whether a neuropsychological model of antidepressant effects could explain the temporal differences in clinical benefits observed with conventional delayed-onset antidepressants versus rapid-acting antidepressants.

Repetitive Negative Thinking-Specific and -Nonspecific White Matter Tracts Engaged by Historical Psychosurgical Targets for Depression

BACKGROUND

Repetitive negative thinking (RNT) is a frequent symptom of major depressive disorder (MDD) that is associated with poor outcomes and treatment resistance. While most studies on RNT have focused on structural and functional characteristics of gray matter, this study aimed to examine the association between white matter (WM) tracts and interindividual variability in RNT.

METHODS

A probabilistic tractography approach was used to characterize differences in the size and anatomical trajectory of WM fibers traversing psychosurgery targets historically useful in the treatment of MDD (anterior capsulotomy, anterior cingulotomy, and subcaudate tractotomy) in patients with MDD and low (n = 53) or high (n = 52) RNT, and healthy control subjects (n = 54). MDD samples were propensity matched on depression and anxiety severity and demographics.

RESULTS

WM tracts traversing left hemisphere targets and reaching the ventral anterior body of the corpus callosum (thus extending to contralateral regions) were larger in the high-RNT MDD group compared with low-RNT (effect size D = 0.27, p = .042) and healthy control (D = 0.23, p = .02) groups. MDD was associated with greater size of tracts that converge onto the right medial orbitofrontal cortex regardless of RNT intensity. Other RNT-nonspecific findings in MDD involved tracts reaching the left primary motor and right primary somatosensory cortices.

CONCLUSIONS

This study provides the first evidence to our knowledge that WM connectivity patterns, which could become targets of intervention, differ between high- and low-RNT participants with MDD. These WM differences extend to circuits that are not specific to RNT, possibly subserving reward mechanisms and psychomotor activity.

Objective measures of reward sensitivity and motivation in people with high v. low anhedonia

BACKGROUND

Anhedonia - a diminished interest or pleasure in activities - is a core self-reported symptom of depression which is poorly understood and often resistant to conventional antidepressants. This symptom may occur due to dysfunction in one or more sub-components of reward processing: motivation, consummatory experience and/or learning. However, the precise impairments remain elusive. Dissociating these components (ideally, using cross-species measures) and relating them to the subjective experience of anhedonia is critical as it may benefit fundamental biology research and novel drug development.

METHODS

Using a battery of behavioural tasks based on rodent assays, we examined reward motivation (Joystick-Operated Runway Task, JORT; and Effort-Expenditure for Rewards Task, EEfRT) and reward sensitivity (Sweet Taste Test) in a non-clinical population who scored high (N = 32) or low (N = 34) on an anhedonia questionnaire (Snaith-Hamilton Pleasure Scale).

RESULTS

Compared to the low anhedonia group, the high anhedonia group displayed marginal impairments in effort-based decision-making (EEfRT) and reduced reward sensitivity (Sweet Taste Test). However, we found no evidence of a difference between groups in physical effort exerted for reward (JORT). Interestingly, whilst the EEfRT and Sweet Taste Test correlated with anhedonia measures, they did not correlate with each other. This poses the question of whether there are subgroups within anhedonia; however, further work is required to directly test this hypothesis.

CONCLUSIONS

Our findings suggest that anhedonia is a heterogeneous symptom associated with impairments in reward sensitivity and effort-based decision-making.

Development of cross-species translational paradigms for psychiatric research in the Research Domain Criteria era

The past 30 years of IBNS has included research attempting to treat the cognitive and behavioral deficits observed in people with psychiatric conditions. Early work utilized drugs identified from tests thought to be cognition-relevant, however the high failure rate crossing the translational-species barrier led to focus on developing valid cross-species translational tests. The face, predictive, and neurobiological validities used to assess animal models of psychiatry can be used to validate these tests. Clinical sensitivity is another important aspect however, for if the clinical population targeted for treatment does not exhibit task deficits, then why develop treatments? This review covers some work validating cross-species translational tests and suggests future directions. Also covered is the contribution IBNS made to fostering such research and my role in IBNS, making it more available to all including fostering mentor/mentee programs plus spearheading diversity and inclusivity initiatives. All science needs support and IBNS has supported research recreating the behavioral abnormalities that define psychiatric conditions with the aim to improve the lives of people with such conditions.

Electrophysiological signatures of reward learning in the rodent touchscreen-based Probabilistic Reward Task

Blunted reward learning and reward-related activation within the corticostriatal-midbrain circuitry have been implicated in the pathophysiology of anhedonia and depression. Unfortunately, the search for more efficacious interventions for anhedonic behaviors has been hampered by the use of vastly different preclinical and clinical assays. In a first step in addressing this gap, in the current study, we used event-related potentials and spectral analyses in conjunction with a touchscreen version of the rodent Probabilistic Reward Task (PRT) to identify the electrophysiological signatures of reward learning in rats. We trained 11 rats (5 females and 6 males) on the rodent touchscreen-based PRT and subsequently implanted them with deep electrodes in the anterior cingulate cortex (ACC) and nucleus accumbens (NAc) for local field potentials recordings during the PRT. Behaviorally, the expected responsivity-to-reward profile was observed. At the electrophysiological level, we identified a negative amplitude deflection 250-500 ms after feedback in the ACC and NAc electrodes, as well as power increase in feedback-locked delta (1-5 Hz) and alpha/beta (9-17 Hz) bands in both electrodes for rewarded trials. Using a reverse-translational approach, we identified electrophysiological signatures of reward learning in rats similar to those described in humans. These findings and approaches might provide a useful translational platform to efficiently evaluate novel therapeutics targeting anhedonia.

Clinical and Preclinical Assessments of Anhedonia in Psychiatric Disorders

Anhedonia is a prevalent symptom across many psychiatric disorders. The contemporary scope of anhedonia across various models includes interest, reward anticipation, motivation, effort expenditure, reward valuation, expectation, pleasure, satiation, and learning. In order to further elucidate the impact of anhedonia on treatment outcomes, quality of life, as well as brain function, validated tools to probe the various facets of anhedonia are necessary. This chapter evaluates assessment tools for anhedonia in clinical populations and in animals. Subjective clinical scales have been in use for decades, and as the construct of anhedonia evolved, contemporary scales were developed to integrate these new concepts. Clinical scales are useful for understanding the subjective experience of anhedonia but do not account for objective aspects of anhedonia, including implicit learning. Behavioral tasks that probe responses to rewarding stimuli have been useful to fill this gap and to delineate the specific brain processes underlying facets of anhedonia. Although there have been translational challenges in the assessments of anhedonia and reward deficits from preclinical to clinical (and vice versa), the multifaceted clinical scales and reward tasks provide valuable insights into the conceptualization of anhedonia and its neural basis across psychiatric disorders.

Effects of dopamine modulation on chronic stress-induced deficits in reward learning

Anhedonia is characteristically preceded by chronic stress, likely involving downstream effects of glucocorticoid alterations on dopamine (DA) function. To elucidate the neural underpinnings of this interaction, we examined whether acute pharmacological modulation of DA alters reward learning after chronic mild stress (CMS). Forty-eight male Wistar rats were exposed to a 21-day CMS regime (n = 48 no stress controls) before completing the probabilistic reward task (PRT), a well-validated cross-species test of reward learning. We first examined whether stress-induced reward dysfunction could be restored by systemic injections of low-dose amisulpride (AMI), which increases DA transmission via D2-like autoreceptor blockade. Then, we investigated region-specific effects through bilateral infusions of quinpirole (QUIN), a D2-like receptor agonist, into either the nucleus accumbens core (NAcc) or medial prefrontal cortex (mPFC). Blunted reward learning in CMS animals was reversed by acute AMI administration, but this treatment did not alter reward learning in the no stress group. Elevated adrenal gland weight, a proxy for stress reactivity, predicted lower reward learning in the untreated CMS group. This effect was extinguished following AMI treatment. These findings might be attributed to significantly higher D2 receptor density in the NAcc of high stress reactive animals. To this end, NAcc QUIN infusions potentiated reward learning relative to mPFC QUIN infusions in CMS rats, but there was no effect in no stress control rats. Collectively, these findings suggest that DA modulation reverses stress-induced reward dysfunction, even among the most stress-reactive animals. The effect might depend on D2-like receptor activation in the mesolimbic system.

Toward a Better Understanding of the Mechanisms and Pathophysiology of Anhedonia: Are We Ready for Translation?

Anhedonia-the loss of pleasure or lack of reactivity to pleasurable stimuli-remains a formidable treatment challenge across neuropsychiatric disorders. In major depressive disorder, anhedonia has been linked to poor disease course, worse response to psychological, pharmacological, and neurostimulation treatments, and increased suicide risk. Moreover, although some neural abnormalities linked to anhedonia normalize after successful treatment, several persist-for example, blunted activation of the ventral striatum to reward-related cues and reduced functional connectivity involving the ventral striatum. Critically, some of these abnormalities have also been identified in unaffected, never-depressed children of parents with major depressive disorder and have been found to prospectively predict the first onset of major depression. Thus, neural abnormalities linked to anhedonia may be promising targets for prevention. Despite increased appreciation of the clinical importance of anhedonia and its underlying neural mechanisms, important gaps remain. In this overview, the author first summarizes the extant knowledge about the pathophysiology of anhedonia, which may provide a road map toward novel treatment and prevention strategies, and then highlights several priorities to facilitate clinically meaningful breakthroughs. These include a need for 1) appropriately controlled clinical trials, especially those embracing an experimental therapeutics approach to probe target engagement; 2) novel preclinical models relevant to anhedonia, with stronger translational value; and 3) clinical scales that incorporate neuroscientific advances in our understanding of anhedonia. The author concludes by highlighting important future directions, emphasizing the need for an integrated, collaborative, cross-species, and multilevel approach to tackling anhedonic phenotypes.

A cross-species assay demonstrates that reward responsiveness is enduringly impacted by adverse, unpredictable early-life experiences

Exposure to early-life adversity (ELA) is associated with several neuropsychiatric conditions, including major depressive disorder, yet causality is difficult to establish in humans. Recent work in rodents has implicated impaired reward circuit signaling in anhedonic-like behavior after ELA exposure. Anhedonia, the lack of reactivity to previously rewarding stimuli, is a transdiagnostic construct common to mental illnesses associated with ELA. Here, we employed an assay of reward responsiveness validated across species, the Probabilistic Reward Task (PRT). In the PRT, healthy participants reliably develop a response bias toward the more richly rewarded stimulus, whereas participants with anhedonia exhibit a blunted response bias that correlates with current and future anhedonia. In a well-established model of ELA that generates a stressful, chaotic, and unpredictable early-life environment, ELA led to blunted response biases in the PRT in two separate cohorts, recapitulating findings in humans with anhedonia. The same ELA rats had blunted sucrose preference, further supporting their anhedonic-like phenotypes. Probing the aspects of ELA that might provoke these deficits, we quantified the unpredictability of dam/pup interactions using entropy measures and found that the unpredictability of maternal care was significantly higher in the ELA groups in which PRT and sucrose preference reward deficits were present later in life. Taken together, these data position the PRT, established in clinical patient populations, as a potent instrument to assess the impact of ELA on the reward circuit across species. These findings also implicate the unpredictability of maternal signals during early life as an important driver of reward sensitivity deficits.

New Horizons for Phenotyping Behavior in Rodents: The Example of Depressive-Like Behavior

The evolution of the field of behavioral neuroscience is significantly dependent on innovative disruption triggered by our ability to model and phenotype animal models of neuropsychiatric disorders. The ability to adequately elicit and measure behavioral parameters are the fundaments on which the behavioral neuroscience community establishes the pathophysiological mechanisms of neuropsychiatric disorders as well as contributes to the development of treatment strategies for those conditions. Herein, we review how mood disorders, in particular depression, are currently modeled in rodents, focusing on the limitations of these models and particularly on the analyses of the data obtained with different behavioral tests. Finally, we propose the use of new paradigms to study behavior using multidimensional strategies that better encompasses the complexity of psychiatric conditions, namely depression; these paradigms provide holistic phenotyping that is applicable to other conditions, thus promoting the emergence of novel findings that will leverage this field.

Environmental Contributions to Anhedonia

Anhedonia is a core feature of psychopathological conditions that have recent exposure to stress and trauma as central to their etiology. Indeed, evolutionary accounts of depression suggest that decreased motivation to pursue reward may be an adaptive strategy in the face of social stress, in particular, as it may serve to defuse interpersonal conflict. Through a review of rodent models and research with humans, we show that exposure to stress, particularly when it is chronic, repeated, and/or involves themes of social rejection or defeat, is consistently associated with reduced hedonic capacity ("liking"), motivation to pursue reward ("wanting"), and ability to learn from reward ("reward learning"). Further, across rodent and human research, there is evidence that females show greater stress-induced blunting of reward processing than males. In humans, this sex difference emerges most strongly when examining individual differences in the stress response rather than group differences in stress exposure. We discuss the implications of these findings for understanding the etiology of, and sex differences in, stress-related psychopathology, including depression and addiction.

Developmental Trajectories of Anhedonia in Preclinical Models

This chapter discusses how the complex concept of anhedonia can be operationalized and studied in preclinical models. It provides information about the development of anhedonia in the context of early-life adversity, and the power of preclinical models to tease out the diverse molecular, epigenetic, and network mechanisms that are responsible for anhedonia-like behaviors.Specifically, we first discuss the term anhedonia, reviewing the conceptual components underlying reward-related behaviors and distinguish anhedonia pertaining to deficits in motivational versus consummatory behaviors. We then describe the repertoire of experimental approaches employed to study anhedonia-like behaviors in preclinical models, and the progressive refinement over the past decade of both experimental instruments (e.g., chemogenetics, optogenetics) and conceptual constructs (salience, valence, conflict). We follow with an overview of the state of current knowledge of brain circuits, nodes, and projections that execute distinct aspects of hedonic-like behaviors, as well as neurotransmitters, modulators, and receptors involved in the generation of anhedonia-like behaviors. Finally, we discuss the special case of anhedonia that arises following early-life adversity as an eloquent example enabling the study of causality, mechanisms, and sex dependence of anhedonia.Together, this chapter highlights the power, potential, and limitations of using preclinical models to advance our understanding of the origin and mechanisms of anhedonia and to discover potential targets for its prevention and mitigation.

Probabilistic Reinforcement Learning and Anhedonia

Despite the prominence of anhedonic symptoms associated with diverse neuropsychiatric conditions, there are currently no approved therapeutics designed to attenuate the loss of responsivity to previously rewarding stimuli. However, the search for improved treatment options for anhedonia has been reinvigorated by a recent reconceptualization of the very construct of anhedonia, including within the Research Domain Criteria (RDoC) initiative. This chapter will focus on the RDoC Positive Valence Systems construct of reward learning generally and sub-construct of probabilistic reinforcement learning specifically. The general framework emphasizes objective measurement of a subject's responsivity to reward via reinforcement learning under asymmetrical probabilistic contingencies as a means to quantify reward learning. Indeed, blunted reward responsiveness and reward learning are central features of anhedonia and have been repeatedly described in major depression. Moreover, these probabilistic reinforcement techniques can also reveal neurobiological mechanisms to aid development of innovative treatment approaches. In this chapter, we describe how investigating reward learning can improve our understanding of anhedonia via the four RDoC-recommended tasks that have been used to probe sensitivity to probabilistic reinforcement contingencies and how such task performance is disrupted in various neuropsychiatric conditions. We also illustrate how reverse translational approaches of probabilistic reinforcement assays in laboratory animals can inform understanding of pharmacological and physiological mechanisms. Next, we briefly summarize the neurobiology of probabilistic reinforcement learning, with a focus on the prefrontal cortex, anterior cingulate cortex, striatum, and amygdala. Finally, we discuss treatment implications and future directions in this burgeoning area.

Prefrontal cortex and depression

The prefrontal cortex (PFC) has emerged as one of the regions most consistently impaired in major depressive disorder (MDD). Although functional and structural PFC abnormalities have been reported in both individuals with current MDD as well as those at increased vulnerability to MDD, this information has not translated into better treatment and prevention strategies. Here, we argue that dissecting depressive phenotypes into biologically more tractable dimensions - negative processing biases, anhedonia, despair-like behavior (learned helplessness) - affords unique opportunities for integrating clinical findings with mechanistic evidence emerging from preclinical models relevant to depression, and thereby promises to improve our understanding of MDD. To this end, we review and integrate clinical and preclinical literature pertinent to these core phenotypes, while emphasizing a systems-level approach, treatment effects, and whether specific PFC abnormalities are causes or consequences of MDD. In addition, we discuss several key issues linked to cross-species translation, including functional brain homology across species, the importance of dissecting neural pathways underlying specific functional domains that can be fruitfully probed across species, and the experimental approaches that best ensure translatability. Future directions and clinical implications of this burgeoning literature are discussed.

Deconstructing and reconstructing behaviour relevant to mental health disorders: The benefits of a psychological approach, with a focus on addiction

RUTHERFORD, L.G. and Milton, A.L. Deconstructing and reconstructing behaviour relevant to mental health disorders: what can psychology offer? NEUROSCI BIOBEHAV REV XX(X)XXX-XXX, 2021. - Current treatments for mental health disorders are successful only for some patients, and there is an unmet clinical need for new treatment development. One challenge for treatment development has been how best to model complex human conditions in animals, where mechanism can be more readily studied with a range of neuroscientific techniques. We suggest that an approach to modelling based on associative animal learning theory provides a good framework for deconstructing complex mental health disorders such that they can be studied in animals. These individual simple models can subsequently be used in combination to 'reconstruct' a more complex model of the mental health disorder of interest. Using examples primarily from the field of drug addiction, we explore the 'psychological approach' and suggest that in addition to facilitating translation and backtranslation of tasks between animal models and patients, it is also highly concordant with the concept of triangulation.

Examining the effects of psychoactive drugs on complex behavioral processes in laboratory animals

Behavioral pharmacology has been aided significantly by the development of innovative cognitive tasks designed to examine complex behavioral processes in laboratory animals. Performance outcomes under these conditions have provided key metrics of drug action which serve to supplement traditional in vivo assays of physiologic and behavioral effects of psychoactive drugs. This chapter provides a primer of cognitive tasks designed to assay different aspects of complex behavior, including learning, cognitive flexibility, memory, attention, motivation, and impulsivity. Both capstone studies and recent publications are highlighted throughout to illustrate task value for two distinct but often interconnected translational strategies. First, task performance in laboratory animals can be utilized to elucidate how drugs of abuse affect complex behavioral processes. Here, the expectation is that adverse effects on such processes will have predictive relevance to consequences that will be experienced by humans. Second, these same task outcomes can be used to evaluate candidate therapeutics. In this case, the extent to which drug doses with medicinal value perturb task performance can contribute critical information for a more complete safety profile appraisal and advance the process of medications development. Methodological and theoretical considerations are discussed and include an emphasis on determining selectivity in drug action on complex behavioral processes.

Acute drug effects differentially predict desire to take dextroamphetamine again for work and recreation

RATIONALE

Misuse of dextroamphetamine occurs in work and recreational contexts. While acute drug effects broadly predict abuse liability, few studies have considered the relationship between acute effects and context.

OBJECTIVES

This study examined how individual differences in acute effects of dextroamphetamine relate to desire to take dextroamphetamine again in different contexts.

METHODS

This secondary analysis used data from healthy adults with no history of moderate-to-severe substance use disorder, who received oral doses of placebo and dextroamphetamine (10 and 20 mg) over 3 sessions under double-blind, randomized conditions. Subjects rated subjective effects and completed reward-related behavioral tasks. Subjects rated their desire to take dextroamphetamine again in hypothetical work and recreational contexts. Multilevel models examined within-subjects change scores (10 mg-placebo; 20 mg-placebo) to determine how subjective effects and behavioral outcomes predicted desire to take dextroamphetamine again for work versus recreation.

RESULTS

Subjects reported more desire to take 20 mg dextroamphetamine again for work than for recreation. At 20 mg, there was an interaction between context and liking/wanting, such that liking/wanting predicted desire to use dextroamphetamine for work only. There was also an interaction at 20 mg between context and psychomotor speed, such that psychomotor speed predicted interest in using dextroamphetamine for recreation only.

CONCLUSIONS

We found that positive subjective effects predicted desire to use dextroamphetamine again for work, while increased motor effects predicted desire to use dextroamphetamine recreationally. Hedonic effects may be perceived as advantageous when working, while increased physical energy may be preferred during recreation, suggesting that context of intended use is important when examining abuse liability.

Anhedonia as a central factor in depression: Neural mechanisms revealed from preclinical to clinical evidence

Anhedonia is one of the core symptoms of major depressive disorder (MDD), which is often inadequately treated by traditional antidepressants. The modern framework of anhedonia extends the definition from impaired consummatory pleasure or interest in rewards to a broad spectrum of deficits that impact functions such as reward anticipation, approach motivation, effort expenditure, reward valuation, expectation, and reward-cue association learning. Substantial preclinical and clinical research has explored the neural basis of reward deficits in the context of depression, and has implicated mesocorticolimbic reward circuitry comprising the nucleus accumbens, ventral pallidum, ventral tegmental area, amygdala, hippocampus, anterior cingulate, insula, orbitofrontal cortex, and other prefrontal cortex regions. Dopamine modulates several reward facets including anticipation, motivation, effort, and learning. As well, serotonin, norepinephrine, opioids, glutamate, Gamma aminobutyric acid (GABA), and acetylcholine are also involved in anhedonia, and medications targeting these systems may also potentially normalize reward processing in depression. Unfortunately, whereas reward anticipation and reward outcome are extensively explored by both preclinical and clinical studies, translational gaps remain in reward motivation, effort, valuation, and learning, where clinical neuroimaging studies are in the early stages. This review aims to synthesize the neurobiological mechanisms underlying anhedonia in MDD uncovered by preclinical and clinical research. The translational difficulties in studying the neural basis of reward are also discussed.

Dose-response effects of d-amphetamine on effort-based decision-making and reinforcement learning

Effort-related decision-making and reward learning are both dopamine-dependent, but preclinical research suggests they depend on different dopamine signaling dynamics. Therefore, the same dose of a dopaminergic medication could have differential effects on effort for reward vs. reward learning. However, no study has tested how effort and reward learning respond to the same dopaminergic medication within subjects. The current study aimed to test the effect of therapeutic doses of d-amphetamine on effort for reward and reward learning in the same healthy volunteers. Participants (n = 30) completed the Effort Expenditure for Reward Task (EEfRT) measure of effort-related decision-making, and the Probabilistic Reward Task (PRT) measure of reward learning, under placebo and two doses of d-amphetamine (10 mg, and 20 mg). Secondarily, we examined whether the individual characteristics of baseline working memory and willingness to exert effort for reward moderated the effects of d-amphetamine. d-Amphetamine increased willingness to exert effort, particularly at low to intermediate expected values of reward. Computational modeling analyses suggested this was due to decreased effort discounting rather than probability discounting or decision consistency. Both baseline effort and working memory emerged as moderators of this effect, such that d-amphetamine increased effort more in individuals with lower working memory and lower baseline effort, also primarily at low to intermediate expected values of reward. In contrast, d-amphetamine had no significant effect on reward learning. These results have implications for treatment of neuropsychiatric disorders, which may be characterized by multiple underlying reward dysfunctions.

Affective Bias Through the Lens of Signal Detection Theory

Affective bias – a propensity to focus on negative information at the expense of positive information – is a core feature of many mental health problems. However, it can be caused by wide range of possible underlying cognitive mechanisms. Here we illustrate this by focusing on one particular behavioural signature of affective bias – increased tendency of anxious/depressed individuals to predict lower rewards – in the context of the Signal Detection Theory (SDT) modelling framework. Specifically, we show how to apply this framework to measure affective bias and compare it to the behaviour of an optimal observer. We also show how to extend the framework to make predictions about bias when the individual holds incorrect assumptions about the decision context. Building on this theoretical foundation, we propose five experiments to test five hypothetical sources of this affective bias: beliefs about prior probabilities, beliefs about performance, subjective value of reward, learning differences, and need for accuracy differences. We argue that greater precision about the mechanisms driving affective bias may eventually enable us to better understand the mechanisms underlying mood and anxiety disorders.

The Importance of Common Currency Tasks in Translational Psychiatry

Purpose of Review

Common currency tasks are tasks that investigate the same phenomenon in different species. In this review, we discuss how to ensure the translational validity of common currency tasks, summarise their benefits, present recent research in this area and offer future directions and recommendations.

Recent Findings

We discuss the strengths and limitations of three specific examples where common currency tasks have added to our understanding of psychiatric constructs—affective bias, reversal learning and goal-based decision making.

Summary

Overall, common currency tasks offer the potential to improve drug discovery in psychiatry. We recommend that researchers prioritise construct validity above face validity when designing common currency tasks and suggest that the evidence for construct validity is summarised in papers presenting research in this area.

Dysregulation of brain dopamine systems in major depressive disorder

Major depressive disorder (MDD or depression) is a debilitating neuropsychiatric syndrome with genetic, epigenetic, and environmental contributions. Depression is one of the largest contributors to chronic disease burden; it affects more than one in six individuals in the United States. A wide array of cellular and molecular modifications distributed across a variety of neuronal processes and circuits underlie the pathophysiology of depression-no established mechanism can explain all aspects of the disease. MDD suffers from a vast treatment gap worldwide, and large numbers of individuals who require treatment do not receive adequate care. This mini-review focuses on dysregulation of brain dopamine (DA) systems in the pathophysiology of MDD and describing new cellular targets for potential medication development focused on DA-modulated micro-circuits. We also explore how neurodevelopmental factors may modify risk for later emergence of MDD, possibly through dopaminergic substrates in the brain.

Reward Learning Through the Lens of RDoC: a Review of Theory, Assessment, and Empirical Findings in the Eating Disorders

PURPOSE OF REVIEW

Reward-related processes may represent important transdiagnostic factors underlying eating pathology. Using the NIMH Research Domain Criteria as a guide, the current article reviews theories, behavioral and self-report assessments, and empirical findings related to reward learning in the eating disorders.

RECENT FINDINGS

Data from behavioral tasks suggest deficits in reinforcement learning, which may become more pronounced with increasing disorder severity and duration. Self-report data strongly implicate positive eating and thinness/restriction expectancies (an element of reward prediction error) in the onset and maintenance of eating pathology. Finally, self-report measures of habit strength demonstrate relationships with eating pathology and illness duration; however, behavioral task data do not support relationships between eating pathology and a propensity towards general habit development. Existing studies are limited, but provide preliminary support for the presence of abnormal reward learning in eating disorders. Continued research is needed to address identified gaps in the literature.

Translational Assessments of Reward Responsiveness in the Marmoset

BACKGROUND

Anhedonia, the loss of pleasure in previously rewarding activities, is a prominent feature of major depressive disorder and often resistant to first-line antidepressant treatment. A paucity of translatable cross-species tasks to assess subdomains of anhedonia, including reward learning, presents a major obstacle to the development of effective therapeutics. One assay of reward learning characterized by orderly behavioral and pharmacological findings in both humans and rats is the probabilistic reward task. In this computerized task, subjects make discriminations across numerous trials in which correct responses to one alternative are rewarded more often (rich) than correct responses to the other (lean). Healthy control subjects reliably develop a response bias to the rich alternative. However, participants with major depressive disorder as well as rats exposed to chronic stress typically exhibit a blunted response bias.

METHODS

The present studies validated a touchscreen-based probabilistic reward task for the marmoset, a small nonhuman primate with considerable translational value. First, probabilistic reinforcement contingencies were parametrically examined. Next, the effects of ketamine (1.0-10.0 mg/kg), a US Food and Drug Administration-approved rapid-acting antidepressant, and phencyclidine (0.01-0.1 mg/kg), a pharmacologically similar N-methyl-D-aspartate receptor antagonist with no known antidepressant efficacy, were evaluated.

RESULTS

Increases in the asymmetry of rich:lean probabilistic contingencies produced orderly increases in response bias. Consistent with their respective clinical profiles, ketamine but not phencyclidine produced dose-related increases in response bias at doses that did not reduce task discriminability.

CONCLUSIONS

Collectively, these findings confirm task and pharmacological sensitivity in the marmoset, which may be useful in developing medications to counter anhedonia across neuropsychiatric disorders.

Using pharmacological manipulations to study the role of dopamine in human reward functioning: A review of studies in healthy adults

Dopamine (DA) plays a key role in reward processing and is implicated in psychological disorders such as depression, substance use, and schizophrenia. The role of DA in reward processing is an area of highly active research. One approach to this question is drug challenge studies with drugs known to alter DA function. These studies provide good experimental control and can be performed in parallel in laboratory animals and humans. This review aimed to summarize results of studies using pharmacological manipulations of DA in healthy adults. 'Reward' is a complex process, so we separated 'phases' of reward, including anticipation, evaluation of cost and benefits of upcoming reward, execution of actions to obtain reward, pleasure in response to receiving a reward, and reward learning. Results indicated that i) DAergic drugs have different effects on different phases of reward; ii) the relationship between DA and reward functioning appears unlikely to be linear; iii) our ability to detect the effects of DAergic drugs varies depending on whether subjective, behavioral, imaging measures are used.

Pretreatment Reward Sensitivity and Frontostriatal Resting-State Functional Connectivity Are Associated With Response to Bupropion After Sertraline Nonresponse

BACKGROUND

Standard guidelines recommend selective serotonin reuptake inhibitors as first-line antidepressants for adults with major depressive disorder, but success is limited and patients who fail to benefit are often switched to non-selective serotonin reuptake inhibitor agents. This study investigated whether brain- and behavior-based markers of reward processing might be associated with response to bupropion after sertraline nonresponse.

METHODS

In a two-stage, double-blinded clinical trial, 296 participants were randomized to receive 8 weeks of sertraline or placebo in stage 1. Individuals who responded continued on another 8-week course of the same intervention in stage 2, while sertraline and placebo nonresponders crossed over to bupropion and sertraline, respectively. Data from 241 participants were analyzed. The stage 2 sample comprised 87 patients with major depressive disorder who switched medication and 38 healthy control subjects. A total of 116 participants with major depressive disorder treated with sertraline in stage 1 served as an independent replication sample. The probabilistic reward task and resting-state functional magnetic resonance imaging were administered at baseline.

RESULTS

Greater pretreatment reward sensitivity and higher resting-state functional connectivity between bilateral nucleus accumbens and rostral anterior cingulate cortex were associated with positive response to bupropion but not sertraline. Null findings for sertraline were replicated in the stage 1 sample.

CONCLUSIONS

Pretreatment reward sensitivity and frontostriatal connectivity may identify patients likely to benefit from bupropion following selective serotonin reuptake inhibitor failures. Results call for a prospective replication based on these biomarkers to advance clinical care.

Selective kappa-opioid antagonism ameliorates anhedonic behavior: evidence from the Fast-fail Trial in Mood and Anxiety Spectrum Disorders (FAST-MAS)

Anhedonia remains a major clinical issue for which there is few effective interventions. Untreated or poorly controlled anhedonia has been linked to worse disease course and increased suicidal behavior across disorders. Taking a proof-of-mechanism approach under the auspices of the National Institute of Mental Health FAST-FAIL initiative, we were the first to show that, in a transdiagnostic sample screened for elevated self-reported anhedonia, 8 weeks of treatment with a kappa-opioid receptor (KOR) antagonist resulted in significantly higher reward-related activation in one of the core hubs of the brain reward system (the ventral striatum), better reward learning in the Probabilistic Reward Task (PRT), and lower anhedonic symptoms, relative to 8 weeks of placebo. Here, we performed secondary analyses of the PRT data to investigate the putative effects of KOR antagonism on anhedonic behavior with more precision by using trial-level model-based Bayesian computational modeling and probability analyses. We found that, relative to placebo, KOR antagonism resulted in significantly higher learning rate (i.e., ability to learn from reward feedback) and a more sustained preference toward the more frequently rewarded stimulus, but unaltered reward sensitivity (i.e., the hedonic response to reward feedback). Collectively, these findings provide novel evidence that in a transdiagnostic sample characterized by elevated anhedonia, KOR antagonism improved the ability to modulate behavior as a function of prior rewards. Together with confirmation of target engagement in the primary report (Krystal et al., Nat Med, 2020), the current findings suggest that further transdiagnostic investigation of KOR antagonism for anhedonia is warranted.

Empirical validation of a touchscreen probabilistic reward task in rats

Anhedonia, the loss of pleasure from previously rewarding activities, is implicated in several neuropsychiatric conditions, including major depressive disorder (MDD). In order to accelerate drug development for mood disorders, quantitative approaches are needed to objectively measure responsiveness to reward as a means to identify deficits. One such approach, the probabilistic reward task (PRT), uses visual discrimination methodology to quantify reward learning. In this computerized task, humans make visual discriminations, and probabilistic contingencies are arranged such that correct responses to one alternative are rewarded more often (rich) than correct responses to the other (lean). Healthy participants consistently develop a response bias in favor of the rich alternative. However, participants with MDD typically exhibit lower response biases, and this blunting correlates with current and future anhedonia. The present studies validated a touchscreen-based PRT in rodents with formal and functional similarity to the human task. First, rats were trained to discriminate between two lines that differed in length. Next, parametric manipulations of probabilistic contingencies, line-length stimuli, and drug treatment (amphetamine, 0.32-3.2 mg/kg; scopolamine, 0.1-1.0 mg/kg; oxycodone, 0.1-1.0 mg/kg) on response bias were evaluated. Results demonstrated orderly shifts in bias and discriminability that varied as a function of, respectively, the asymmetry of rich/lean probabilities and disparity in line lengths. Drugs that enhance reward responsiveness (amphetamine and scopolamine, but not oxycodone) increased bias, verifying pharmacological task sensitivity. Finally, performance outcomes under optimized conditions were replicated in female rats. Collectively, the touchscreen-based rodent PRT appears to have high preclinical value as a quantitative assay of reward learning.

Baseline reward processing and ventrostriatal dopamine function are associated with pramipexole response in depression

The efficacy of dopamine agonists in treating major depressive disorder has been hypothesized to stem from effects on ventrostriatal dopamine and reward function. However, an important question is whether dopamine agonists are most beneficial for patients with reward-based deficits. This study evaluated whether measures of reward processing and ventrostriatal dopamine function predicted response to the dopamine agonist, pramipexole (ClinicalTrials.gov Identifier: NCT02033369). Individuals with major depressive disorder (n = 26) and healthy controls (n = 26) (mean ± SD age = 26.5 ± 5.9; 50% female) first underwent assessments of reward learning behaviour and ventrostriatal prediction error signalling (measured using functional MRI). 11C-(+)-PHNO PET before and after oral amphetamine was used to assess ventrostriatal dopamine release. The depressed group then received open-label pramipexole treatment for 6 weeks (0.5 mg/day titrated to a maximum daily dose of 2.5 mg). Symptoms were assessed weekly, and reward learning was reassessed post-treatment. At baseline, relative to controls, the depressed group showed lower reward learning (P = 0.02), a trend towards blunted reward-related prediction error signals (P = 0.07), and a trend towards increased amphetamine-induced dopamine release (P = 0.07). Despite symptom improvements following pramipexole (Cohen's d ranging from 0.51 to 2.16 across symptom subscales), reward learning did not change after treatment. At a group level, baseline reward learning (P = 0.001) and prediction error signalling (P = 0.004) were both associated with symptom improvement, albeit in a direction opposite to initial predictions: patients with stronger pretreatment reward learning and reward-related prediction error signalling improved most. Baseline D2/3 receptor availability (P = 0.02) and dopamine release (P = 0.05) also predicted improvements in clinical functioning, with lower D2/3 receptor availability and lower dopamine release predicting greater improvements. Although these findings await replication, they suggest that measures of reward-related mesolimbic dopamine function may hold promise for identifying depressed individuals likely to respond favourably to dopaminergic pharmacotherapy.

Dissecting the impact of depression on decision-making

BACKGROUND

Cognitive deficits in depressed adults may reflect impaired decision-making. To investigate this possibility, we analyzed data from unmedicated adults with Major Depressive Disorder (MDD) and healthy controls as they performed a probabilistic reward task. The Hierarchical Drift Diffusion Model (HDDM) was used to quantify decision-making mechanisms recruited by the task, to determine if any such mechanism was disrupted by depression.

METHODS

Data came from two samples (Study 1: 258 MDD, 36 controls; Study 2: 23 MDD, 25 controls). On each trial, participants indicated which of two similar stimuli was presented; correct identifications were rewarded. Quantile-probability plots and the HDDM quantified the impact of MDD on response times (RT), speed of evidence accumulation (drift rate), and the width of decision thresholds, among other parameters.

RESULTS

RTs were more positively skewed in depressed v. healthy adults, and the HDDM revealed that drift rates were reduced-and decision thresholds were wider-in the MDD groups. This pattern suggests that depressed adults accumulated the evidence needed to make decisions more slowly than controls did.

CONCLUSIONS

Depressed adults responded slower than controls in both studies, and poorer performance led the MDD group to receive fewer rewards than controls in Study 1. These results did not reflect a sensorimotor deficit but were instead due to sluggish evidence accumulation. Thus, slowed decision-making-not slowed perception or response execution-caused the performance deficit in MDD. If these results generalize to other tasks, they may help explain the broad cognitive deficits seen in depression.

Rodent models of post-traumatic stress disorder: behavioral assessment

Although the etiology and expression of psychiatric disorders are complex, mammals show biologically preserved behavioral and neurobiological responses to valent stimuli which underlie the use of rodent models of post-traumatic stress disorder (PTSD). PTSD is a complex phenotype that is difficult to model in rodents because it is diagnosed by patient interview and influenced by both environmental and genetic factors. However, given that PTSD results from traumatic experiences, rodent models can simulate stress induction and disorder development. By manipulating stress type, intensity, duration, and frequency, preclinical models reflect core PTSD phenotypes, measured through various behavioral assays. Paradigms precipitate the disorder by applying physical, social, and psychological stressors individually or in combination. This review discusses the methods used to trigger and evaluate PTSD-like phenotypes. It highlights studies employing each stress model and evaluates their translational efficacies against DSM-5, validity criteria, and criteria proposed by Yehuda and Antelman's commentary in 1993. This is intended to aid in paradigm selection by informing readers about rodent models, their benefits to the clinical community, challenges associated with the translational models, and opportunities for future work. To inform PTSD model validity and relevance to human psychopathology, we propose that models incorporate behavioral test batteries, individual differences, sex differences, strain and stock differences, early life stress effects, biomarkers, stringent success criteria for drug development, Research Domain Criteria, technological advances, and cross-species comparisons. We conclude that, despite the challenges, animal studies will be pivotal to advances in understanding PTSD and the neurobiology of stress.

Comparison of conventional and rapid-acting antidepressants in a rodent probabilistic reversal learning task

Deficits in reward processing are a central feature of major depressive disorder with patients exhibiting decreased reward learning and altered feedback sensitivity in probabilistic reversal learning tasks. Methods to quantify probabilistic learning in both rodents and humans have been developed, providing translational paradigms for depression research. We have utilised a probabilistic reversal learning task to investigate potential differences between conventional and rapid-acting antidepressants on reward learning and feedback sensitivity. We trained 12 rats in a touchscreen probabilistic reversal learning task before investigating the effect of acute administration of citalopram, venlafaxine, reboxetine, ketamine or scopolamine. Data were also analysed using a Q-learning reinforcement learning model to understand the effects of antidepressant treatment on underlying reward processing parameters. Citalopram administration decreased trials taken to learn the first rule and increased win-stay probability. Reboxetine decreased win-stay behaviour while also decreasing the number of rule changes animals performed in a session. Venlafaxine had no effect. Ketamine and scopolamine both decreased win-stay probability, number of rule changes performed and motivation in the task. Insights from the reinforcement learning model suggested that reboxetine led animals to choose a less optimal strategy, while ketamine decreased the model-free learning rate. These results suggest that reward learning and feedback sensitivity are not differentially modulated by conventional and rapid-acting antidepressant treatment in the probabilistic reversal learning task.

Affective biases and their interaction with other reward-related deficits in rodent models of psychiatric disorders

Major depressive disorder (MDD) is one of the leading global causes of disability. Symptoms of MDD can vary person to person, and current treatments often fail to alleviate the poor quality of life that patients experience. One of the two core diagnostic criteria for MDD is the loss of interest in previously pleasurable activities, which suggests a link between the disease aetiology and reward processing. Cognitive impairments are also common in patients with MDD, and more recently, emotional processing deficits known as affective biases have been recognised as a key feature of the disorder. Studies in animals have found similar affective biases related to reward. In this review we consider these affective biases in the context of other reward-related deficits and examine how affective biases associated with learning and memory may interact with the wider behavioural symptoms seen in MDD. We discuss recent developments in how analogues of affective biases and other aspects of reward processing can be assessed in rodents, as well as how these behaviours are influenced in models of MDD. We subsequently discuss evidence for the neurobiological mechanisms contributing to one or more reward-related deficits in preclinical models of MDD, identified using these behavioural assays. We consider how the relationships between these selective behavioural assays and the neurobiological mechanisms for affective bias and reward processing could be used to identify potential treatment strategies.

Anhedonia as a key clinical feature in the maintenance and treatment of opioid use disorder

There is a critical need for research on clinical features that may influence response to treatment for opioid use disorder (OUD). Given its neurobiology and relevance to opioid use, anhedonia may be one such promising clinical feature. We identified and reviewed 11 studies that measured anhedonia in humans with OUD to characterize the current state of evidence and highlight potential implications for treatment. The majority of studies were cross-sectional, indicating higher anhedonia scores in opioid-dependent samples compared to healthy controls. Rates of participants with clinically significant anhedonia ranged from 21% to 48%. Anhedonia scores were correlated with opioid craving and use, however there are significant knowledge gaps regarding its time course and impact on treatment adherence and outcomes. Repeated assessment of anhedonia early in treatment for OUD is recommended, as it may be a unique predictor of dropout or non-response, and a potential target for behavioral and/or pharmacological intervention.

Strategies to Address Challenges in Neuroscience Drug Discovery and Development

The paucity of novel drugs for neuropsychiatric indications contrasts with the remarkable recent advances in neuroscience research. We have identified 5 challenges the field needs to address and recommend potential solutions. First, we need to drive discovery efforts based on human data. Second, we need to think more carefully about animal models, embracing them as tools to test pathophysiological alterations. Third, we need to develop strategies to select more homogenous groups of patients in our clinical trials. Fourth, we need to develop and validate translational biomarkers, which can be used for pharmacodynamic assessments as well as for patient selection. Fifth, we need to adopt more reliable and objective measures to capture clinical efficacy. The tools that will allow these solutions to be implemented may already be in place but not routinely adopted or are still being developed. Overall, a change in mindset to adopt science- and data-driven paths is needed.

Inflammation and dimensions of reward processing following exposure to the influenza vaccine

BACKGROUND

Alterations in reward processing are a central feature of depression and may be influenced by inflammation. Indeed, inflammation is associated with deficits in reward-related processes in animal models and with dysregulation in reward-related neural circuitry in humans. However, the downstream behavioral manifestations of such impairments are rarely examined in humans.

METHODS

The influenza vaccination was used to elicit a mild inflammatory response in 41 healthy young adults (age range: 18-22, 30 female). Participants provided blood samples and completed behavioral measures of three key aspects of reward-reward motivation, reward learning, and reward sensitivity-before and 1 day after receiving the influenza vaccine.

RESULTS

The influenza vaccine led to mild but significant increases in circulating levels of the pro-inflammatory cytokine interleukin-6 (IL-6) (p <  .001). Consistent with hypotheses, increases in IL-6 predicted lower reward motivation (p =  .029). However, contrary to hypotheses, increases in IL-6 predicted increased performance on a reward learning task (p =  .043) and were not associated with changes in reward sensitivity (p's > .288).

CONCLUSIONS

These findings contribute to an emerging literature on the nuanced associations between inflammation and reward and demonstrate that even mild alterations in inflammation are associated with multiple facets of reward processing.

Translational new approaches for investigating mood disorders in rodents and what they may reveal about the underlying neurobiology of major depressive disorder

Mood disorders represent one of society's most costly and challenging health burdens. The drug treatments used today were initially discovered serendipitously in the 1950s. Animal models were then developed based on the ability of these drugs to alter specific behaviours. These models have played a major role in the development of the second generation of antidepressants. However, their use has been heavily criticized, particularly in relation to whether they recapitulate similar underlying biology to the psychiatric disorder they are proposed to represent. This article considers our work in the field of affective bias and the development of a translational research programme to try to develop and validate better animal models. We discuss whether the new data that have arisen from these studies support an alternative perspective on the underlying neurobiological processes that lead to major depressive disorder (MDD). Specifically, this article will consider whether a neuropsychological mechanism involving affective biases plays a causal role in the development of MDD and its associated emotional and behavioural symptoms. These animal studies also raise the possibility that neuropsychological mechanisms involving affective biases are a precursor to, rather than a consequence of, the neurotrophic changes linked to MDD.

This article is part of a discussion meeting issue ‘Of mice and mental health: facilitating dialogue between basic and clinical neuroscientists’.

Investigating dopamine and glucocorticoid systems as underlying mechanisms of anhedonia

RATIONALE

Anhedonia, a deficit in reward processing, is an endophenotype of several neuropsychiatric conditions. Despite its prevalence and debilitating effects, treatments for anhedonia are lacking, primarily because its underlying mechanisms are poorly understood. Dopamine (DA) has been implicated in anhedonia through its role in reward-related learning; glucocorticoid systems may also be involved in that anhedonia is often preceded by chronic stress.

OBJECTIVE

This study investigated DA and glucocorticoid systems in anhedonia using a rat version of the probabilistic reward task (PRT).

METHODS

Adult male Wistar rats were trained on the PRT and then tested following: (1) activation or inhibition of DA activity induced by amphetamine (AMPH) or pramipexole (PRAMI) injections, (2) chronic mild stress (CMS), or (3) glucocorticoid system activation (dexamethasone (DEX)) or inhibition (mifepristone (MIFE)).

RESULTS

AMPH increased and PRAMI decreased response bias, pointing to enhanced and diminished reward responsiveness with DA agonism and antagonism, respectively. CMS reduced response bias but only in a subpopulation of rats. DEX also decreased response bias, suggesting that glucocorticoid processes contribute to anhedonia, although glucocorticoid inhibition (MIFE) had no effect. None of the manipulations altered the ability to detect and respond to reward-paired stimuli.

CONCLUSIONS

These results confirm a role of DA in anhedonia and elucidate the contribution of the glucocorticoid system to this effect. In addition, chronic stress may interfere with normal DA functioning, leading to impaired reward-related learning in some animals. These findings may direct future treatment of anhedonia by targeting DA and glucocorticoid systems, as well as a possible interaction between the two.

Animal models in addiction research: A dimensional approach

Drug addiction affects approximately 10% of the population and these numbers are rising. Treatment and prevention of addiction are impeded by current diagnostic systems, such as DSM-5, which are based on outcomes rather than processes. Here, we review the importance of adopting a dimensional framework, specifically the Research Domain Criteria (RDoC), to identify protective and vulnerability mechanisms in addiction. We discuss how preclinical researchers should work within this framework to develop animal models based on domains of function. We highlight RDoC paradigms related to addiction and discuss how these can be used to investigate the biological underpinnings of an addiction cycle (i.e., binge/intoxication, negative affect, and craving). Using this information, we then outline the critical role of animal research in ongoing revisions to the RDoC matrix (specifically the functional significance of domains, constructs and subconstructs) and its contribution to the development and refinement of addiction theories. We conclude with an overview of the contribution that animal research has made to the development of pharmacological and behavioural treatments for addiction.

Making Sense of Rodent Models of Anhedonia

A markedly reduced interest or pleasure in activities previously considered pleasurable is a main symptom in mood disorder and psychosis and is often present in other psychiatric disorders and neurodegenerative diseases. This condition can be labeled as "anhedonia," although in its most rigorous connotation the term refers to the lost capacity to feel pleasure that is one aspect of the complex phenomenon of processing and responding to reward. The responses to rewarding stimuli are relatively easy to study in rodents, and the experimental conditions that consistently and persistently impair these responses are used to model anhedonia. To this end, long-term exposure to environmental aversive conditions is primarily used, and the resulting deficits in reward responses are often accompanied by other deficits that are mainly reminiscent of clinical depressive symptoms. The different components of impaired reward responses induced by environmental aversive events can be assessed by different tests or protocols that require different degrees of time allocation, technical resources, and equipment. Rodent models of anhedonia are valuable tools in the study of the neurobiological mechanisms underpinning impaired behavioral responses and in the screening and characterization of drugs that may reverse these behavioral deficits. In particular, the antianhedonic or promotivational effects are relevant features in the spectrum of activities of drugs used in mood disorders or psychosis. Thus, more than the model, it is the choice of tests that is crucial since it influences which facets of anhedonia will be detected and should be tuned to the purpose of the study.

Rat models of reward deficits in psychiatric disorders

Loss of interest in rewarding activities is a hallmark of many psychiatric disorders and may be relevant for neurodegenerative disorders and patients suffering from brain injury. There is increasing evidence that deficits in reward-related behaviour are more complex than previously described. The traditional view of anhedonia as 'the inability to experience pleasure' may be too limited to fully encompass the types of reward deficit observed in these patients. Developments in methods to measure different aspects of reward processing in humans and animals are starting to provide insights into the complexity of this behaviour. In this article we consider the rodent models which have traditionally been used to study reward deficits in psychiatric disorders and consider their limitations relative to clinical findings. We then discuss work where methods derived from human neuropsychological tests are providing insights into the complexity of reward-related behaviour. Specifically, we consider tasks which investigate different aspects of reward-related behaviour focusing on learning and memory as well as decision-making and consider what these may mean in terms of how we model reward deficits in rodents.

Chronic Social Stress Leads to Reduced Gustatory Reward Salience and Effort Valuation in Mice

Pathology of reward processing is a major clinical feature of stress-related neuropsychiatric disorders including depression. Several dimensions of reward processing can be impacted, including reward valuation/salience, learning, expectancy and effort valuation. To establish the causal relationships between stress, brain changes, and reward processing pathologies, valid animal models are essential. Here, we present mouse experiments investigating behavioral effects of chronic social stress (CSS) in association learning tests of gustatory reward salience and effort valuation. The reward salience test (RST) comprised Pavlovian pairing of a tone with gustatory reward. The effort valuation test (EVT) comprised operant responding for gustatory reinforcement on a progressive ratio schedule (PRS). All testing was conducted with mice at 100% baseline body weight (BBW). In one experiment, mice underwent 15-day CSS or control handling (CON) and testing was conducted using sucrose pellets. In the RST on days 16–17, CSS mice made fewer feeder responses and had a longer tone response latency, than CON mice. In a shallow EVT on days 19–20, CSS mice attained a lower final ratio than CON mice. In a second CSS experiment, mice underwent CSS or CON and testing was conducted with chocolate pellets and in the presence of standard diet (low effort/low reward). In the RST on days 16–18, CSS mice made fewer feeder responses and had a longer tone response latency, than CON mice. In a steep EVT on days 19–20, CSS and CON mice attained less pellets than in the RST, and CSS mice attained a lower final ratio than CON mice. At day 21, blood levels of glucose and the satiety adipokine leptin were similar in CSS and CON mice. Therefore, CSS leads to consistent reductions in reward salience and effort valuation in tests based on association learning. These reward pathology models are being applied to identify the underlying neurobiology and putative molecular targets for therapeutic pharmacology.