Investigating dopamine and glucocorticoid systems as underlying mechanisms of anhedonia

Effects of GABA, Sex, and Stress on Reward Learning in Current and Remitted Major Depression

BACKGROUND

Neurocognitive factors including aberrant reward learning, blunted GABA (gamma-aminobutyric acid), and potentiated stress sensitivity have been linked to anhedonia, a hallmark depressive symptom, possibly in a sex-dependent manner. However, previous research has not investigated the putative associations among these factors or the extent to which they represent trait- or state-based vulnerabilities for depression.

METHODS

Young adults with current major depressive disorder (MDD) (n = 44), remitted MDD (n = 42), and healthy control participants (HCs) (n = 44), stratified by sex assigned at birth, underwent magnetic resonance spectroscopy to assess macromolecular contaminated GABA (GABA+) and then a reward learning task before and after acute stress. We assessed changes in reward learning after stress and associations with GABA+.

RESULTS

Results revealed blunted baseline reward learning in participants with remitted MDD versus participants with current MDD and HCs but, surprisingly, no differences between participants with current MDD and HCs. Reward learning was reduced following acute stress regardless of depressive history. GABA+ in the rostral anterior cingulate cortex, but not the dorsolateral prefrontal cortex, was associated with reduced baseline reward learning only in female participants. GABA+ did not predict stress-related changes in reward learning.

CONCLUSIONS

To our knowledge, this is the first study to investigate associations among GABA, reward learning, and stress reactivity in current versus past depression. Hypothesized depression-related differences in reward learning did not emerge, precluding claims about state versus trait vulnerabilities. However, our finding that blunted GABA was associated with greater reward learning in female participants provides novel insights into sex-selective associations between the frontal GABAergic inhibitory system and reward processing.

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.

Neuroplasticity: Pathophysiology and Role in Major Depressive Disorder

Neuroplasticity is characterized by the brain's ability to change its activity in response to extrinsic and intrinsic factors and is thought to be the mechanism behind all brain functions. Neuroplasticity causes structural and functional changes on a molecular level, specifically the growth of different regions in the brain and changes in synaptic and post-synaptic activities. The four types of neuroplasticity are homologous area adaption, compensatory masquerade, cross-modal reassignment, and map expansion. All of these help the brain work around injuries or new information inputs. In addition to baseline physical functions, neuroplasticity is thought to be the basis of emotional and mental regulations and the impairment of it can cause various mental illnesses. Concurrently, these mental illnesses further the damage of synaptic plasticity in the brain. Major depressive disorder (MDD) is one of the most common mental illnesses. It is affected by and accelerates the impairment of neuroplasticity. It is characterized by a chronically depressed state of mind that can impact the patient's daily life, including work life and interests. This review will focus on highlighting the physiological aspects of the disease and the role of neuroplasticity in the pathogenesis and pathology of the disorder. Moreover, the role of monoamine regulation and ketamine uptake will be discussed in terms of their antidepressant effects on the outcomes of MDD.

From Reward to Anhedonia-Dopamine Function in the Global Mental Health Context

When "hijacked" by compulsive behaviors that affect the reward and stress centers of the brain, functional changes in the dopamine circuitry occur as the consequence of pathological brain adaptation. As a brain correlate of mental health, dopamine has a central functional role in behavioral regulation from healthy reward-seeking to pathological adaptation to stress in response to adversity. This narrative review offers a spotlight view of the transition from healthy reward function, under the control of dopamine, to the progressive deregulation of this function in interactions with other brain centers and circuits, producing what may be called an anti-reward brain state. How such deregulation is linked to specific health-relevant behaviors is then explained and linked to pandemic-related adversities and the stresses they engendered. The long lockdown periods where people in social isolation had to rely on drink, food, and digital rewards via the internet may be seen as the major triggers of changes in motivation and reward-seeking behavior worldwide. The pathological adaptation of dopamine-mediated reward circuitry in the brain is discussed. It is argued that, when pushed by fate and circumstance into a physiological brain state of anti-reward, human behavior changes and mental health is affected, depending on individual vulnerabilities. A unified conceptual account that places dopamine function at the centre of the current global mental health context is proposed.

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.

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.

Effects of stress on endophenotypes of suicide across species: A role for ketamine in risk mitigation

Suicide is a leading cause of death and morbidity worldwide, yet few interventions are available to mitigate its risk. Barriers to effective treatments involve a limited understanding of factors that predict the onset of suicidal thoughts and behaviors. In the context of suicide risk, stress is a precipitating factor that is largely overlooked in the literature. Indeed, the pathophysiology of stress and suicide are heavily interconnected, underscoring the need to target the stress system in suicide prevention. In this review, we integrate findings from the preclinical and clinical literature that links stress and suicide. We focus specifically on the effects of stress on underlying biological functions and processes associated with suicide, allowing for the review of research using animal models. Owing to the rapid anti-suicidal effects of (R,S)-ketamine, we discuss its ability to modulate various stress-related endophenotypes of suicide, as well as its potential role in preventing suicide in those with a history of chronic life stress (e.g., early life adversity). We highlight future research directions that could advance our understanding of stress-related effects on suicide risk, advocating a dimensional, endophenotype approach to suicide research.

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Suicide and chronic stress pathophysiology are interconnected.

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Chronic stress has profound impacts on several endophenotypes of suicide.

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Animal and human research points to stress as a precipitating factor in suicide.

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Ketamine modulates specific biological processes associated with stress and suicide.

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Suicide research into endophenotypes can help inform risk-mitigation strategies.

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.

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.

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.

Anhedonia difference between major depressive disorder and bipolar disorder II

OBJECTIVE

This study aims to explore the difference in anhedonia between Major Depressive Disorder (MDD) and Bipolar Disorder II (BD-II), and attempt to distinguish the two diseases through Snaith-Hamilton Pleasure Scale (SHAPS).

METHODS

A total of 164 drug-free depressive patients (98 MDD patients, 66 BD-II patients) completed the investigation. 17-item Hamilton Depression Scale (HAMD-17) and Hamilton Anxiety Scale (HAMA) and SHAPS were assessed in all participants.

RESULTS

Our results showed that BD-II patients had higher SHAPS scores than MDD patients. The stepwise logistic regression analysis further revealed that SHAPS score, drinking habit, and extroversion as influencing factors for the identification of BD-II. The ROC curve analysis indicated that SHAPS could differentiate BD-II from MDD patients (AUC = 0.655, P = 0.001, 95% CI = 0.568 to 0.742), with the best screening cutoff at 26, and the corresponding sensitivity and specificity was 0.788 and 0.520, respectively.

CONCLUSION

Our results suggest that BD-II patients had more severe anhedonia compared to MDD patients, and the difference in anhedonia may help clinicians preliminary identify BD patients from MDD patients. The preliminary findings are worthly of further exploration.

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.

Cortisol reactivity to stress predicts behavioral responsivity to reward moderation by sex, depression, and anhedonia

Depression is associated with blunted reactivity to acute stress, as well as blunted responsivity to rewards. However, the extent to which responses to stress are associated with responses to reward in individuals meeting criteria for a depressive disorder is unknown. The goal of this study was to examine the relation of responses to stress and reward, and to determine if this relation is moderated by depression diagnosis, anhedonia, and sex. Participants included 114 adults (68 depressed, 46 non-depressed; 75% women) recruited from the community. Stress reactivity was operationalized as the total salivary cortisol output to the Trier Social Stress Test (TSST; Kirschbaum et al., 1993). Response bias to monetary reward was assessed following the TSST recovery period with a probabilistic reward task (PRT; Pizzagalli et al., 2005). In men only, total cortisol output during the TSST was more strongly positively associated with response bias to reward across the three blocks of the PRT. In addition, among depressed participants with high levels of anhedonia, higher cortisol output during the TSST was significantly associated with higher overall response bias to reward. We suggest that in men, the stress and reward systems may both respond quickly, and resolve rapidly, in the face of acute stress. Further, in depression, our findings suggest that anhedonia may represent a specific phenotype in which the stress and reward systems are particularly tuned together.

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.

Allostatic load and mental health: a latent class analysis of physiological dysregulation

This study explored the associations between specific profiles of biological dysregulation and mental health outcomes in a national, community sample of healthy adults in the United States. A latent class analysis of data from the Midlife Development in the United States study (n = 1,757) was conducted to determine classes of biological dysregulation. Multinomial logistic regressions of class membership were employed to determine associations with measures related to depression, including whether or not individuals had sought treatment, Center for Epidemiological Studies Depression Scale, and both the generalized distress and anhedonia subscales of the Mood and Anxiety Symptoms Questionnaire. Four classes of dysregulation emerged: baseline/low dysregulation, metabolic and inflammatory dysregulation, parasympathetic dysregulation, and SAM pathway dysregulation. Individuals who met the criteria for depression measures were more likely to be in the metabolic and immune dysregulation and parasympathetic dysregulation groups as compared to the baseline group. The results suggest that mental health outcomes, such as depression, are differentially associated with specific profiles of biological dysregulation. A more nuanced approach to profiles of dysregulation could better inform treatment decisions.Lay summaryHigher levels of allostatic load, which represents the cumulative wear and tear of exposure to stress, are associated with increased rates of depression and anhedonia. Specifically, parasympathetic dysregulation and immunometabolic dysregulation are associated with negative mental health outcomes.

PTEN in prefrontal cortex is essential in regulating depression-like behaviors in mice

Chronic stress is an environmental risk factor for depression and causes neuronal atrophy in the prefrontal cortex (PFC) and other brain regions. It is still unclear about the molecular mechanism underlying the behavioral alterations and neuronal atrophy induced by chronic stress. We here report that phosphatase and tensin homolog deleted on chromosome ten (PTEN) is a mediator for chronic stress-induced depression-like behaviors and neuronal atrophy in mice. One-month chronic restraint stress (CRS) up-regulated PTEN signaling pathway in the PFC of mice as indicated by increasing levels of PTEN, p-MEK, and p-ERK but decreasing levels of p-AKT. Over-expression of Pten in the PFC led to an increase of depression-like behaviors, whereas genetic inactivation or knockdown of Pten in the PFC prevented the CRS-induced depression-like behaviors. In addition, systemic administration of PTEN inhibitor was also able to prevent these behaviors. Cellular examination showed that Pten over-expression or the CRS treatment resulted in PFC neuron atrophy, and this atrophy was blocked by genetic inactivation of Pten or systemic administration of PTEN inhibitor. Furthermore, possible causal link between Pten and glucocorticoids was examined. In chronic dexamethasone (Dex, a glucocorticoid agonist) treatment-induced depression model, increased PTEN levels were observed, and depression-like behaviors and PFC neuron atrophy were attenuated by the administration of PTEN inhibitor. Our results indicate that PTEN serves as a key mediator in chronic stress-induced neuron atrophy as well as depression-like behaviors, providing molecular evidence supporting the synaptic plasticity theory of depression.

Neural correlates of reward processing: Functional dissociation of two components within the ventral striatum

INTRODUCTION

Rewarding and punishing stimuli elicit BOLD responses in the affective division of the striatum. The responses typically traverse from the affective to the associative division of the striatum, suggesting an involvement of associative processes during the modulation of stimuli valance. In this study, we hypothesized that fMRI responses to rewards versus punishments in a guessing card game can be disassociated into two functional component processes that reflect the convergence of limbic and associative functional networks in the ventral striatum.

METHODS

We used fMRI data of 175 (92 female) subjects from the human connectome project´s gambling task, working memory task, and resting-state scans. A reward > punish contrast identified a ventral striatum cluster from which voxelwise GLM parameter estimates were entered into a k-means clustering algorithm. The k-means analysis supported separating the cluster into two spatially distinct components. These components were used as seeds to investigate their functional connectivity profile. GLM parameter estimates were extracted and compared from the task contrasts reward > punish and 2-back > 0-back from two ROIs in the ventral striatum and one ROI in hippocampus.

RESULTS

The analyses converged to show that a superior striatal component, coupled with the ventral attention and frontal control networks, was responsive to both a modulation of cognitive control in working memory and to rewards, whereas the most inferior part of the ventral striatum, coupled with the limbic and default mode networks including the hippocampus, was selectively responsive to rewards.

CONCLUSION

We show that the fMRI response to rewards in the ventral striatum reflects a mixture of component processes of reward. An inferior ventral striatal component and hippocampus are part of an intrinsically coupled network that responds to reward-based processing during gambling. The more superior ventral striatal component is intrinsically coupled to networks involved with executive functioning and responded to both reward and cognitive control demands.

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.

Facilitation of dopamine-dependent long-term potentiation in the medial prefrontal cortex of male rats follows the behavioral effects of stress

The effect of stress on animal behavior and brain activity has been attracting growing attention in the last decades. Stress dramatically affects several aspects of animal behavior, including motivation and cognitive functioning, and has been used to model human pathologies such as post-traumatic stress disorder. A key question is whether stress alters the plastic potential of synaptic circuits. In this work, we evaluated if stress affects dopamine (DA)-dependent synaptic plasticity in the medial prefrontal cortex (mPFC). On male adolescent rats, we characterized anxiety- and depressive-like behaviors using behavioral testing before and after exposure to a mild stress (elevated platform, EP). After the behavioral protocols, we investigated DA-dependent long-term potentiation (DA-LTP) and depression (DA-LTD) on acute slices of mPFC and evaluated the activation of DA-producing brain regions by western and dot blot analysis. We show that exposure to the EP stress enhances DA-LTP and that desipramine (DMI) treatment abolishes this effect. We also found that DA-LTD is not affected by EP stress unless when this is followed by DMI treatment. In addition, EP stress reduces anxiety, an effect abolished by both DMI and ketamine, while motivation is promoted by previous exposure to EP stress independently of pharmacological treatments. Finally, this form of stress reduces the expression of the early gene cFOS in the ventral tegmental area. These findings support the idea that mild stressors can promote synaptic plasticity in PFC through a dopaminergic mechanism, an effect that might increase the sensitivity of mPFC to subsequent stressful experiences.

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.

Personalized Psychiatry and Depression: The Role of Sociodemographic and Clinical Variables

Despite several pharmacological options, the clinical outcomes of major depressive disorder (MDD) are often unsatisfactory. Personalized psychiatry attempts to tailor therapeutic interventions according to each patient's unique profile and characteristics. This approach can be a crucial strategy in improving pharmacological outcomes in MDD and overcoming trial-and-error treatment choices. In this narrative review, we evaluate whether sociodemographic (i.e., gender, age, race/ethnicity, and socioeconomic status) and clinical [i.e., body mass index (BMI), severity of depressive symptoms, and symptom profiles] variables that are easily assessable in clinical practice may help clinicians to optimize the selection of antidepressant treatment for each patient with MDD at the early stages of the disorder. We found that several variables were associated with poorer outcomes for all antidepressants. However, only preliminary associations were found between some clinical variables (i.e., BMI, anhedonia, and MDD with melancholic/atypical features) and possible benefits with some specific antidepressants. Finally, in clinical practice, the assessment of sociodemographic and clinical variables considered in our review can be valuable for early identification of depressed individuals at high risk for poor responses to antidepressants, but there are not enough data on which to ground any reliable selection of specific antidepressant class or compounds. Recent advances in computational resources, such as machine learning techniques, which are able to integrate multiple potential predictors, such as individual/ clinical variables, biomarkers, and genetic factors, may offer future reliable tools to guide personalized antidepressant choice for each patient with MDD.