Beyond negative valence: 2-week administration of a serotonergic antidepressant enhances both reward and effort learning signals

Direct serotonin release in humans shapes aversive learning and inhibition

The role of serotonin in human behaviour is informed by approaches which allow in vivo modification of synaptic serotonin. However, characterising the effects of increased serotonin signalling in human models of behaviour is challenging given the limitations of available experimental probes, notably selective serotonin reuptake inhibitors. Here we use a now-accessible approach to directly increase synaptic serotonin in humans (a selective serotonin releasing agent) and examine its influence on domains of behaviour historically considered core functions of serotonin. Computational techniques, including reinforcement learning and drift diffusion modelling, explain participant behaviour at baseline and after week-long intervention. Reinforcement learning models reveal that increasing synaptic serotonin reduces sensitivity for outcomes in aversive contexts. Furthermore, increasing synaptic serotonin enhances behavioural inhibition, and shifts bias towards impulse control during exposure to aversive emotional probes. These effects are seen in the context of overall improvements in memory for neutral verbal information. Our findings highlight the direct effects of increasing synaptic serotonin on human behaviour, underlining its role in guiding decision-making within aversive and more neutral contexts, and offering implications for longstanding theories of central serotonin function.

The selective serotonin reuptake inhibitor sertraline alters learning from aversive reinforcements in patients with depression: evidence from a randomized controlled trial

BACKGROUND

Selective serotonin reuptake inhibitors (SSRIs) are first-line pharmacological treatments for depression and anxiety. However, little is known about how pharmacological action is related to cognitive and affective processes. Here, we examine whether specific reinforcement learning processes mediate the treatment effects of SSRIs.

METHODS

The PANDA trial was a multicentre, double-blind, randomized clinical trial in UK primary care comparing the SSRI sertraline with placebo for depression and anxiety. Participants (N = 655) performed an affective Go/NoGo task three times during the trial and computational models were used to infer reinforcement learning processes.

RESULTS

There was poor task performance: only 54% of the task runs were informative, with more informative task runs in the placebo than in the active group. There was no evidence for the preregistered hypothesis that Pavlovian inhibition was affected by sertraline. Exploratory analyses revealed that in the sertraline group, early increases in Pavlovian inhibition were associated with improvements in depression after 12 weeks. Furthermore, sertraline increased how fast participants learned from losses and faster learning from losses was associated with more severe generalized anxiety symptoms.

CONCLUSIONS

The study findings indicate a relationship between aversive reinforcement learning mechanisms and aspects of depression, anxiety, and SSRI treatment, but these relationships did not align with the initial hypotheses. Poor task performance limits the interpretability and likely generalizability of the findings, and highlights the critical importance of developing acceptable and reliable tasks for use in clinical studies.

FUNDING

This article presents research supported by NIHR Program Grants for Applied Research (RP-PG-0610-10048), the NIHR BRC, and UCL, with additional support from IMPRS COMP2PSYCH (JM, QH) and a Wellcome Trust grant (QH).

Dorsal raphe neurons integrate the values of reward amount, delay, and uncertainty in multi-attribute decision-making

The dorsal raphe nucleus (DRN) is implicated in psychiatric disorders that feature impaired sensitivity to reward amount, impulsivity when facing reward delays, and risk-seeking when confronting reward uncertainty. However, it has been unclear whether and how DRN neurons signal reward amount, reward delay, and reward uncertainty during multi-attribute value-based decision-making, where subjects consider these attributes to make a choice. We recorded DRN neurons as monkeys chose between offers whose attributes, namely expected reward amount, reward delay, and reward uncertainty, varied independently. Many DRN neurons signaled offer attributes, and this population tended to integrate the attributes in a manner that reflected monkeys' preferences for amount, delay, and uncertainty. After decision-making, in response to post-decision feedback, these same neurons signaled signed reward prediction errors, suggesting a broader role in tracking value across task epochs and behavioral contexts. Our data illustrate how the DRN participates in value computations, guiding theories about the role of the DRN in decision-making and psychiatric disease.

Optogenetic activation of dorsal raphe serotonin neurons induces brain-wide activation

Serotonin is a neuromodulator that affects multiple behavioral and cognitive functions. Nonetheless, how serotonin causes such a variety of effects via brain-wide projections and various receptors remains unclear. Here we measured brain-wide responses to optogenetic stimulation of serotonin neurons in the dorsal raphe nucleus (DRN) of the male mouse brain using functional MRI with an 11.7 T scanner and a cryoprobe. Transient activation of DRN serotonin neurons caused brain-wide activation, including the medial prefrontal cortex, the striatum, and the ventral tegmental area. The same stimulation under anesthesia with isoflurane decreased brain-wide activation, including the hippocampal complex. These brain-wide response patterns can be explained by DRN serotonergic projection topography and serotonin receptor expression profiles, with enhanced weights on 5-HT1 receptors. Together, these results provide insight into the DR serotonergic system, which is consistent with recent discoveries of its functions in adaptive behaviors.

Dorsal raphe neurons signal integrated value during multi-attribute decision-making

The dorsal raphe nucleus (DRN) is implicated in psychiatric disorders that feature impaired sensitivity to reward amount, impulsivity when facing reward delays, and risk-seeking when grappling with reward uncertainty. However, whether and how DRN neurons signal reward amount, reward delay, and reward uncertainty during multi-attribute value-based decision-making, where subjects consider all these attributes to make a choice, is unclear. We recorded DRN neurons as monkeys chose between offers whose attributes, namely expected reward amount, reward delay, and reward uncertainty, varied independently. Many DRN neurons signaled offer attributes. Remarkably, these neurons commonly integrated offer attributes in a manner that reflected monkeys' overall preferences for amount, delay, and uncertainty. After decision-making, in response to post-decision feedback, these same neurons signaled signed reward prediction errors, suggesting a broader role in tracking value across task epochs and behavioral contexts. Our data illustrate how DRN participates in integrated value computations, guiding theories of DRN in decision-making and psychiatric disease.

Boosting Serotonin Increases Information Gathering by Reducing Subjective Cognitive Costs

Serotonin is implicated in the valuation of aversive costs, such as delay or physical effort. However, its role in governing sensitivity to cognitive effort, for example, deliberation costs during information gathering, is unclear. We show that treatment with a serotonergic antidepressant in healthy human individuals of either sex enhances a willingness to gather information when trying to maximize reward. Using computational modeling, we show this arises from a diminished sensitivity to subjective deliberation costs during the sampling process. This result is consistent with the notion that serotonin alleviates sensitivity to aversive costs in a domain-general fashion, with implications for its potential contribution to a positive impact on motivational deficits in psychiatric disorders.SIGNIFICANCE STATEMENT Gathering information about the world is essential for successfully navigating it. However, sampling information is costly, and we need to balance between gathering too little and too much information. The neurocomputational mechanisms underlying this arbitration between a putative gain, such as reward, and the associated costs, such as allocation of cognitive resources, remain unclear. In this study, we show that week-long daily treatment with a serotonergic antidepressant enhances a willingness to gather information when trying to maximize reward. Computational modeling indicates this arises from a reduced perception of aversive costs, rendering information gathering less cognitively effortful. This finding points to a candidate mechanism by which serotonergic treatment might help alleviate motivational deficits in a range of mental illnesses.

An online experimental medicine trial on the effect of 28-day simvastatin administration on emotional processing, reward learning, working memory and salivary cortisol in healthy volunteers at risk for depression: OxSTEP protocol

BACKGROUND

Evidence suggests inflammation may be a key mechanism by which psychosocial stress, including loneliness, predisposes to depression. Observational and clinical studies have suggested simvastatin, with its anti-inflammatory properties, may have a potential use in the treatment of depression. Previous experimental medicine trials investigating 7-day use of statins showed conflicting results, with simvastatin displaying a more positive effect on emotional processing compared with atorvastatin. It is possible that statins require longer administration in predisposed individuals before showing the expected positive effects on emotional processing.

AIMS

Here, we aim to test the neuropsychological effects of 28-day simvastatin administration versus placebo, in healthy volunteers at risk for depression owing to loneliness.

METHOD

This is a remote experimental medicine study. One hundred participants across the UK will be recruited and randomised to either 28-day 20 mg simvastatin or placebo in a double-blind fashion. Before and after administration, participants will complete an online testing session involving tasks of emotional processing and reward learning, processes related to vulnerability to depression. Working memory will also be assessed and waking salivary cortisol samples will be collected. The primary outcome will be accuracy in identifying emotions in a facial expression recognition task, comparing the two groups across time.

Multispecies probiotic administration reduces emotional salience and improves mood in subjects with moderate depression: a randomised, double-blind, placebo-controlled study

BACKGROUND

The potential antidepressant properties of probiotics have been suggested, but their influence on the emotional processes that may underlie this effect is unclear.

METHODS

Depressed volunteers (n = 71) were recruited into a randomised double-blind, placebo-controlled study to explore the effects of a daily, 4-week intake of a multispecies probiotic or placebo on emotional processing and cognition. Mood, anxiety, positive and negative affect, sleep, salivary cortisol and serum C-reactive peptide (CRP) were assessed before and after supplementation.

RESULTS

Compared with placebo, probiotic intake increased accuracy at identifying faces expressing all emotions (+12%, p < 0.05, total n = 51) and vigilance to neutral faces (mean difference between groups = 12.28 ms ± 6.1, p < 0.05, total n = 51). Probiotic supplementation also reduced reward learning (-9%, p < 0.05, total n = 51), and interference word recall on the auditory verbal learning task (-18%, p < 0.05, total n = 50), but did not affect other aspects of cognitive performance. Although actigraphy revealed a significant group × night-time activity interaction, follow up analysis was not significant (p = 0.094). Supplementation did not alter salivary cortisol or circulating CRP concentrations. Probiotic intake significantly reduced (-50% from baseline, p < 0.05, n = 35) depression scores on the Patient Health Questionnaire-9, but these did not correlate with the changes in emotional processing.

CONCLUSIONS

The impartiality to positive and negative emotional stimuli or reward after probiotic supplementation have not been observed with conventional antidepressant therapies. Further studies are required to elucidate the significance of these changes with regard to the mood-improving action of the current probiotic.

Chronic escitalopram in healthy volunteers has specific effects on reinforcement sensitivity: a double-blind, placebo-controlled semi-randomised study

Several studies of the effects on cognition of selective serotonin reuptake inhibitors (SSRI), administered either acutely or sub-chronically in healthy volunteers, have found changes in learning and reinforcement outcomes. In contrast, to our knowledge, there have been no studies of chronic effects of escitalopram on cognition in healthy volunteers. This is important in view of its clinical use in major depressive disorder (MDD) and obsessive-compulsive disorder (OCD). Consequently, we aimed to investigate the chronic effect of the SSRI, escitalopram, on measures of 'cold' cognition (including inhibition, cognitive flexibility, memory) and 'hot cognition' including decision-making and particularly reinforcement learning. The study, conducted at the University of Copenhagen between May 2020 and October 2021, used a double-blind placebo-controlled design with 66 healthy volunteers, semi-randomised to receive either 20 mg of escitalopram (n = 32) or placebo (n = 34), balanced for age, sex and intelligence quotient (IQ) for at least 21 days. Questionnaires, neuropsychological tests and serum escitalopram measures were taken. We analysed group differences on the cognitive measures using linear regression models as well as innovative hierarchical Bayesian modelling of the Probabilistic Reversal Learning (PRL) task. The novel and important finding was that escitalopram reduced reinforcement sensitivity compared to placebo on both the Sequential Model-Based/Model-Free task and the PRL task. We found no other significant group differences on 'cold' or 'hot' cognition. These findings demonstrate that serotonin reuptake inhibition is involved in reinforcement learning in healthy individuals. Lower reinforcement sensitivity in response to chronic SSRI administration may reflect the 'blunting' effect often reported by patients with MDD treated with SSRIs. Trial Registration: NCT04239339 .

Role of serotonin in modulation of decision-making in Parkinson's disease

BACKGROUND

Dysfunction of dopaminergic pathways has been considered to play a pivotal role in Parkinson's disease (PD), affecting the processing of emotional and rewarding information, and potentially leading to symptoms of depression or apathy. However, some aspects of motivation in PD might be affected by non-dopaminergic mechanisms.

AIM AND METHOD

The objective of this experimental medicine study was to investigate the contribution of serotonergic modulation via administration of citalopram (20 mg) for 7 days on motivated decision-making in twenty PD patients, measured using several different computerised tasks and clinical questionnaires that probe different aspects of decision-making. Twenty healthy controls were additionally tested without medication to assess any baseline differences between the two groups.

RESULTS

Results indicated that PD patients were overall less motivated than controls on an effort- and reward-based decision-making task. Citalopram increased or decreased willingness to exert effort for reward, depending on whether baseline motivation was high or low, respectively. A task assessing decision-making under risk revealed higher levels of risk aversion for potential losses in PD patients, which neither serotonin nor the patient's regular dopaminergic medication seemed to restore. However, citalopram in PD was associated with more risk-seeking choices for gains, although patients and controls did not differ on this at baseline.

CONCLUSION

The results provide evidence for a role of the serotonergic system in influencing some aspects of motivated decision-making in PD processes.

Fluoxetine degrades luminance perceptual thresholds while enhancing motivation and reward sensitivity

Selective serotonin reuptake inhibitors (SSRIs) increase serotonin activity in the brain. While they are mostly known for their antidepressant properties, they have been shown to improve visual functions in amblyopia and impact cognitive functions ranging from attention to motivation and sensitivity to reward. Yet, a clear understanding of the specific action of serotonin to each of bottom-up sensory and top-down cognitive control components and their interaction is still missing. To address this question, we characterize, in two adult male macaques, the behavioral effects of fluoxetine, a specific SSRI, on visual perception under varying bottom-up (luminosity, distractors) and top-down (uncertainty, reward biases) constraints while they are performing three different visual tasks. We first manipulate target luminosity in a visual detection task, and we show that fluoxetine degrades luminance perceptual thresholds. We then use a target detection task in the presence of spatial distractors, and we show that under fluoxetine, monkeys display both more liberal responses as well as a degraded perceptual spatial resolution. In a last target selection task, involving free choice in the presence of reward biases, we show that monkeys display an increased sensitivity to reward outcome under fluoxetine. In addition, we report that monkeys produce, under fluoxetine, more trials and less aborts, increased pupil size, shorter blink durations, as well as task-dependent changes in reaction times. Overall, while low level vision appears to be degraded by fluoxetine, performances in the visual tasks are maintained under fluoxetine due to enhanced top-down control based on task outcome and reward maximization.

Associations Between Statin Use and Negative Affective Bias During COVID-19: An Observational, Longitudinal UK Study Investigating Depression Vulnerability

BACKGROUND

There is growing interest in the antidepressant potential of statins. We tested whether statin use is associated with cognitive markers previously found to indicate psychological vulnerability to depression within the context of the COVID-19 pandemic.

METHODS

Between April 2020 and February 2021, we conducted an observational online study of 2043 adults in the United Kingdom. Participants completed cognitive tasks assessing processes related to depression vulnerability, including affective bias and reward processing. We also measured working memory, medication use, and current psychiatric symptoms. Using mixed analysis of covariance and regression models, we compared participants on statins alone (n = 81), antihypertensive medication alone (n = 126), both medications (n = 111), and on neither medication (n = 1725).

RESULTS

Statin use was associated with reduced recognition of angry and fearful faces (F₁ = 9.19, p = .002; F₁ = 6.9, p = .009) and with increased misclassification of these expressions as positive. Increased recognition of angry faces at baseline predicted increased levels of depression and anxiety 10 months later (β = 3.61, p = .027; β = 2.37, p = .002). Statin use was also associated with reduced learning about stimuli associated with loss (F_1,1418 = 9.90, p = .002). These indicators of reduced negative bias were not seen in participants taking antihypertensive medication alone, suggesting that they were related to statin use in particular rather than nonspecific demographic factors. In addition, we found no evidence of an association between statin use and impairment in working memory.

CONCLUSIONS

Statin use was associated with cognitive markers indicative of reduced psychological vulnerability to depression, supporting their potential use as a prophylactic treatment for depression.

Serotonin modulates asymmetric learning from reward and punishment in healthy human volunteers

Instrumental learning is driven by a history of outcome success and failure. Here, we examined the impact of serotonin on learning from positive and negative outcomes. Healthy human volunteers were assessed twice, once after acute (single-dose), and once after prolonged (week-long) daily administration of the SSRI citalopram or placebo. Using computational modelling, we show that prolonged boosting of serotonin enhances learning from punishment and reduces learning from reward. This valence-dependent learning asymmetry increases subjects' tendency to avoid actions as a function of cumulative failure without leading to detrimental, or advantageous, outcomes. By contrast, no significant modulation of learning was observed following acute SSRI administration. However, differences between the effects of acute and prolonged administration were not significant. Overall, these findings may help explain how serotonergic agents impact on mood disorders.

Role of dopamine and clinical heterogeneity in cognitive dysfunction in Parkinson's disease

Parkinson's disease (PD) is commonly treated with dopaminergic medication, which enhances some, while impairing other cognitive functions. It can even contribute to impulse control disorder and addiction. We describe the history of research supporting the dopamine overdose hypothesis, which accounts for the large within-patient variability in dopaminergic medication effects across different tasks by referring to the spatially non-uniform pattern of dopamine depletion in dorsal versus ventral striatum. However, there is tremendous variability in dopaminergic medication effects not just within patients across distinct tasks, but also across different patients. In the second part of this chapter we review recent studies addressing the large individual variability in the negative side effects of dopaminergic medication on functions that implicate dopamine, such as value-based learning and choice. These studies begin to unravel the mechanisms of dopamine overdosing, thus revising the strict version of the overdose hypothesis. For example, the work shows that the canonical boosting of reward-versus punishment-based choice by medication is greater in patients with depression and a non-tremor phenotype, which both implicate, among other pathology, more rather than less severe dysregulation of the mesolimbic dopamine system. Future longitudinal cohort studies are needed to identify how to optimally combine different clinical, personality, cognitive, neural, genetic and molecular predictors of detrimental medication effects in order to account for as much of the relevant variability as possible. This will provide a useful tool for precision neurology, allowing individual and contextual tailoring of (the dose of) dopaminergic medication in order to maximize its cognitive benefits, yet minimize its side effects.

Different depression: motivational anhedonia governs antidepressant efficacy in Huntington's disease

Depression is more common in neurodegenerative diseases such as Huntington's disease than the general population. Antidepressant efficacy is well-established for depression within the general population: a recent meta-analysis showed serotonin norepinephrine reuptake inhibitors, tricyclic antidepressants and mirtazapine outperformed other antidepressants. Despite the severe morbidity, antidepressant choice in Huntington's disease is based on Class IV evidence. We used complementary approaches to determine treatment choice for depression in Huntington's disease: propensity score analyses of antidepressant treatment outcome using the ENROLL-HD data set, and a dissection of the cognitive mechanisms underlying depression in Huntington's disease using a cognitive battery based on the Research Domain Criteria for Depression. Study 1 included ENROLL-HD 5486 gene-positive adult patients started on an antidepressant medication for depression. Our outcome measures were depression (Hospital Anxiety and Depression Scale or Problem Behaviours Assessment 'Depressed Mood' item) at first follow-up (primary outcome) and all follow-ups (secondary outcome). The intervention was antidepressant class. We used Svyglm&Twang in R to perform propensity scoring, using known variables (disease progression, medical comorbidity, psychiatric morbidity, sedatives, number of antidepressants, demographics and antidepressant contraindications) to determine the probability of receiving different antidepressants (propensity score) and then included the propensity score in a model of treatment efficacy. Study 2 recruited 51 gene-positive adult patients and 26 controls from the South Wales Huntington's Disease Management Service. Participants completed a motor assessment, in addition to measures of depression and apathy, followed by tasks measuring consummatory anhedonia, motivational anhedonia, learning from reward and punishment and reaction to negative outcome. We used generalised linear models to determine the association between task performance and depression scores. Study 1 showed selective serotonin reuptake inhibitors outperformed serotonin norepinephrine reuptake inhibitors on the primary outcome (P = 0.048), whilst both selective serotonin reuptake inhibitors (P = 0.00069) and bupropion (P = 0.0045) were superior to serotonin norepinephrine reuptake inhibitors on the secondary outcome. Study 2 demonstrated an association between depression score and effort for reward that was not explained by apathy. No other mechanisms were associated with depression score. We found that selective serotonin reuptake inhibitors and bupropion outperform serotonin norepinephrine reuptake inhibitors at alleviating depression in Huntington's disease. Moreover, motivational anhedonia appears the most significant mechanism underlying depression in Huntington's disease. Bupropion is improves motivational anhedonia and has a synergistic effect with selective serotonin reuptake inhibitors. This work provides the first large-scale, objective evidence to determine treatment choice for depression in Huntington's disease, and provides a model for determining antidepressant efficacy in other neurodegenerative diseases.

A single oral dose of citalopram increases interoceptive insight in healthy volunteers

RATIONALE

Interoception is the signalling, perception, and interpretation of internal physiological states. Many mental disorders associated with changes of interoception, including depressive and anxiety disorders, are treated with selective serotonin reuptake inhibitors (SSRIs). However, the causative link between SSRIs and interoception is not yet clear.

OBJECTIVES

To ascertain the causal effect of acute changes of serotonin levels on cardiac interoception.

METHODS

Using a within-participant placebo-controlled design, forty-seven healthy human volunteers (31 female, 16 male) were tested on and off a 20 mg oral dose of the commonly prescribed SSRI, citalopram. Participants made judgements on the synchrony between their heartbeat and auditory tones and then expressed confidence in each judgement. We measured three types of interoceptive cognition.

RESULTS

Citalopram increased cardiac interoceptive insight, measured as correspondence of self-reported confidence to the likelihood that interoceptive judgements were actually correct. This effect was driven by enhanced confidence for correct interoceptive judgements and was independent of measured cardiac and reported subjective effects of the drug.

CONCLUSIONS

An acute change of serotonin levels can increase insight into the reliability of inferences made from cardiac interoceptive sensations.

Functional dynamics of dopamine synthesis during monetary reward and punishment processing

The assessment of dopamine release with the PET competition model is thoroughly validated but entails disadvantages for the investigation of cognitive processes. We introduce a novel approach incorporating 6-[¹⁸F]FDOPA uptake as index of the dynamic regulation of dopamine synthesis enzymes by neuronal firing. The feasibility of this approach is demonstrated by assessing widely described sex differences in dopamine neurotransmission. Reward processing was behaviorally investigated in 36 healthy participants, of whom 16 completed fPET and fMRI during the monetary incentive delay task. A single 50 min fPET acquisition with 6-[¹⁸F]FDOPA served to quantify task-specific changes in dopamine synthesis. In men monetary gain induced stronger increases in ventral striatum dopamine synthesis than loss. Interestingly, the opposite effect was discovered in women. These changes were further associated with reward (men) and punishment sensitivity (women). As expected, fMRI showed robust task-specific neuronal activation but no sex difference. Our findings provide a neurobiological basis for known behavioral sex differences in reward and punishment processing, with important implications in psychiatric disorders showing sex-specific prevalence, altered reward processing and dopamine signaling. The high temporal resolution and magnitude of task-specific changes make fPET a promising tool to investigate functional neurotransmitter dynamics during cognitive processing and in brain disorders.

The interpretation of behavior-model correlations in unidentified cognitive models

The rise of computational modeling in the past decade has led to a substantial increase in the number of papers that report parameter estimates of computational cognitive models. A common application of computational cognitive models is to quantify individual differences in behavior by estimating how these are expressed in differences in parameters. For these inferences to hold, models need to be identified, meaning that one set of parameters is most likely, given the behavior under consideration. For many models, model identification can be achieved up to a scaling constraint, which means that under the assumption that one parameter has a specific value, all remaining parameters are identified. In the current note, we argue that this scaling constraint implies a strong assumption about the cognitive process that the model is intended to explain, and warn against an overinterpretation of the associative relations found in this way. We will illustrate these points using signal detection theory, reinforcement learning models, and the linear ballistic accumulator model, and provide suggestions for a clearer interpretation of modeling results.

Lithium modulates striatal reward anticipation and prediction error coding in healthy volunteers

Lithium is one of the most effective mood-stabilizing medications in bipolar disorder. This study was designed to test whether lithium administration may stabilize mood via effects on reward processing. It was hypothesized that lithium administration would modulate reward processing in the striatum and affect both anticipation and outcome computations. Thirty-seven healthy human participants (18 males, 33 with suitable fMRI data) received 11 (±1) days of lithium carbonate or placebo intervention (double-blind), after which they completed the monetary incentive delay task while fMRI data were collected. The monetary incentive delay task is a robust task with excellent test-retest reliability and is well suited to investigate different phases of reward processing within the caudate and nucleus accumbens. To test for correlations with prediction error signals a Rescorla-Wagner reinforcement-learning model was applied. Lithium administration enhanced activity in the caudate during reward anticipation compared to placebo. In contrast, lithium administration reduced caudate and nucleus accumbens activity during reward outcome. This latter effect seems related to learning as reward prediction errors showed a positive correlation with caudate and nucleus accumbens activity during placebo, which was absent after lithium administration. Lithium differentially modulates the anticipation relative to the learning of rewards. This suggests that lithium might reverse dampened reward anticipation while reducing overactive reward updating in patients with bipolar disorder. This specific effect of lithium suggests that a targeted modulation of reward learning may be a viable approach for novel interventions in bipolar disorder.

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.

Prior Activation of 5-HT7 Receptors Modulates the Conditioned Place Preference With Methylphenidate

The serotonin receptor subtype 7 (5-HT7R) is clearly involved in behavioral functions such as learning/memory, mood regulation and circadian rhythm. Recent discoveries proposed modulatory physiological roles for serotonergic systems in reward-guided behavior. However, the interplay between serotonin (5-HT) and dopamine (DA) in reward-related behavioral adaptations needs to be further assessed. TP-22 is a recently developed arylpiperazine-based 5-HT7R agonist, which is also showing high affinity and selectivity towards D1 receptors. Here, we report that TP-22 displays D1 receptor antagonist activity. Moreover, we describe the first in vivo tests with TP-22: first, a pilot experiment (assessing dosage and timing of action) identified the 0.25 mg/kg i.v. dosage for locomotor stimulation of rats. Then, a conditioned place preference (CPP) test with the DA-releasing psychostimulant drug, methylphenidate (MPH), involved three rat groups: prior i.v. administration of TP-22 (0.25 mg/kg), or vehicle (VEH), 90 min before MPH (5 mg/kg), was intended for modulation of conditioning to the white chamber (saline associated to the black chamber); control group (SAL) was conditioned with saline in both chambers. Prior TP-22 further increased the stimulant effect of MPH on locomotor activity. During the place-conditioning test, drug-free activity of TP-22+MPH subjects remained steadily elevated, while VEH+MPH subjects showed a decline. Finally, after a priming injection of TP-22 in MPH-free conditions, rats showed a high preference for the MPH-associated white chamber, which conversely had vanished in VEH-primed MPH-conditioned subjects. Overall, the interaction between MPH and pre-treatment with TP-22 seems to improve both locomotor stimulation and the conditioning of motivational drives to environmental cues. Together with recent studies, a main modulatory role of 5-HT7R for the processing of rewards can be suggested. In the present study, TP-22 proved to be a useful psychoactive tool to better elucidate the role of 5-HT7R and its interplay with DA in reward-related behavior.

Modulation of anterior cingulate cortex reward and penalty signalling in medication-naive young-adult subjects with depressive symptoms following acute dose lurasidone

BACKGROUND

Aberrations in reward and penalty processing are implicated in depression and putatively reflect altered dopamine signalling. This study exploits the advantages of a placebo-controlled design to examine how a novel D2 antagonist with adjunctive antidepressant properties modifies activity in the brain's reward network in depression.

METHODS

We recruited 43 medication-naïve subjects across the range of depression severity (Beck's Depression Inventory-II score range: 0-43), including healthy volunteers, as well as people meeting full-criteria for major depressive disorder. In a double-blind placebo-controlled cross-over design, all subjects received either placebo or lurasidone (20 mg) across two visits separated by 1 week. Functional magnetic resonance imaging with the Monetary Incentive Delay (MID) task assessed reward functions via neural responses during anticipation and receipt of gains and losses. Arterial spin labelling measured cerebral blood flow (CBF) at rest.

RESULTS

Lurasidone altered fronto-striatal activity during anticipation and outcome phases of the MID task. A significant three-way Medication-by-Depression severity-by-Outcome interaction emerged in the anterior cingulate cortex (ACC) after correction for multiple comparisons. Follow-up analyses revealed significantly higher ACC activation to losses in high- v. low depression participants in the placebo condition, with a normalisation by lurasidone. This effect could not be accounted for by shifts in resting CBF.

CONCLUSIONS

Lurasidone acutely normalises reward processing signals in individuals with depressive symptoms. Lurasidone's antidepressant effects may arise from reducing responses to penalty outcomes in individuals with depressive symptoms.

Dissociable temporal effects of bupropion on behavioural measures of emotional and reward processing in depression

Antidepressants remediate negative biases in emotional processing early in treatment, prior to mood improvement. However, the effects on reward processing potentially relevant to the treatment of anhedonia are less clear. Here we investigate the early and sustained effects of the dopamine and noradrenaline reuptake inhibitor bupropion on behavioural measures of emotional and reward processing in currently depressed individuals. Forty-six currently depressed patients and 42 healthy controls participated in a repeated measures study, during which open-label bupropion was administered to only the patient group over a six week period without a placebo group. All participants completed the Emotional Test Battery and a probabilistic instrumental learning task at week 0, week 2 and week 6. Currently depressed patients displayed negative biases in emotional processing and blunted response bias for high-probability wins compared to the healthy controls at baseline. Bupropion was found to reduce the negative biases in emotional processing early in treatment, including a significant decrease in the percentage misclassification of other face emotions as sad and the number of negative self-referent words falsely recalled between baseline and week 2. Conversely, bupropion was found to initially further reduce the response bias for high-probability wins between baseline and week 2. This effect reversed with six weeks' bupropion treatment and reward processing was normalized compared to the healthy controls. Early in treatment, bupropion acts to reduce negative biases in emotional processing but exacerbates impaired reward processing. The beneficial actions of bupropion on reward processing then occur later in treatment. Such dissociation in the temporal effects of bupropion on emotional and reward processing has implications for the treatment of the different symptom domains of negative affect and anhedonia in depression.This article is part of a discussion meeting issue 'Of mice and mental health: facilitating dialogue between basic and clinical neuroscientists'.

Understanding psychiatric disorder by capturing ecologically relevant features of learning and decision-making

Recent research in cognitive neuroscience has begun to uncover the processes underlying increasingly complex voluntary behaviours, including learning and decision-making. Partly this success has been possible by progressing from simple experimental tasks to paradigms that incorporate more ecological features. More specifically, the premise is that to understand cognitions and brain functions relevant for real life, we need to introduce some of the ecological challenges that we have evolved to solve. This often entails an increase in task complexity, which can be managed by using computational models to help parse complex behaviours into specific component mechanisms. Here we propose that using computational models with tasks that capture ecologically relevant learning and decision-making processes may provide a critical advantage for capturing the mechanisms underlying symptoms of disorders in psychiatry. As a result, it may help develop mechanistic approaches towards diagnosis and treatment. We begin this review by mapping out the basic concepts and models of learning and decision-making. We then move on to consider specific challenges that emerge in realistic environments and describe how they can be captured by tasks. These include changes of context, uncertainty, reflexive/emotional biases, cost-benefit decision-making, and balancing exploration and exploitation. Where appropriate we highlight future or current links to psychiatry. We particularly draw examples from research on clinical depression, a disorder that greatly compromises motivated behaviours in real-life, but where simpler paradigms have yielded mixed results. Finally, we highlight several paradigms that could be used to help provide new insights into the mechanisms of psychiatric disorders.

Prospection, Perseverance, and Insight in Sequential Behavior

Real-world decisions have benefits occurring only later and dependent on additional decisions taken in the interim. We investigated this in a novel decision-making task in humans (n = 76) while measuring brain activity with fMRI (n = 24). Modeling revealed that participants computed the prospective value of decisions: they planned their future behavior taking into account how their decisions might affect which states they would encounter and how they themselves might respond in these states. They considered their own likely future behavioral biases (e.g., failure to adapt to changes in prospective value) and avoided situations in which they might be prone to such biases. Three neural networks in adjacent medial frontal regions were linked to distinct components of prospective decision making: activity in dorsal anterior cingulate cortex, area 8 m/9, and perigenual anterior cingulate cortex reflected prospective value, anticipated changes in prospective value, and the degree to which prospective value influenced decisions.

Neural Mechanisms for Adaptive Learned Avoidance of Mental Effort

Humans tend to avoid mental effort. Previous studies have demonstrated this tendency using various demand-selection tasks; participants generally avoid options associated with higher cognitive demand. However, it remains unclear whether humans avoid mental effort adaptively in uncertain and nonstationary environments. If so, it also remains unclear what neural mechanisms underlie such learned avoidance and whether they remain the same regardless of cognitive-demand types. We addressed these issues by developing novel demand-selection tasks where associations between choice options and cognitive-demand levels change over time, with two variations using mental arithmetic and spatial reasoning problems (males/females: 29:4 and 18:2). Most participants showed avoidance, and their choices depended on the demand experienced on multiple preceding trials. We assumed that participants updated the expected cost of mental effort through experience, and fitted their choices by reinforcement learning models, comparing several possibilities. Model-based fMRI analyses revealed that activity in the dorsomedial and lateral frontal cortices was positively correlated with the trial-by-trial expected cost for the chosen option commonly across the different types of cognitive demand. Analyses also revealed a trend of negative correlation in the ventromedial prefrontal cortex. We further identified correlates of cost-prediction error at time of problem presentation or answering the problem, the latter of which partially overlapped with or were proximal to the correlates of expected cost at time of choice cue in the dorsomedial frontal cortex. These results suggest that humans adaptively learn to avoid mental effort, having neural mechanisms to represent expected cost and cost-prediction error, and the same mechanisms operate for various types of cognitive demand.SIGNIFICANCE STATEMENT In daily life, humans encounter various cognitive demands and tend to avoid high-demand options. However, it remains unclear whether humans avoid mental effort adaptively under dynamically changing environments. If so, it also remains unclear what the underlying neural mechanisms are and whether they operate regardless of cognitive-demand types. To address these issues, we developed novel tasks where participants could learn to avoid high-demand options under uncertain and nonstationary environments. Through model-based fMRI analyses, we found regions whose activity was correlated with the expected mental effort cost, or cost-prediction error, regardless of demand type. These regions overlap, or are adjacent with each other, in the dorsomedial frontal cortex. This finding helps clarify the mechanisms for cognitive-demand avoidance, and provides empirical building blocks for the emerging computational theory of mental effort.

A Dissociation of the Acute Effects of Bupropion on Positive Emotional Processing and Reward Processing in Healthy Volunteers

Background: Previous research indicates that antidepressants can restore the balance between negative and positive emotional processing early in treatment, indicating a role of this effect in later mood improvement. However, less is known about the effect of antidepressants on reward processing despite the potential relevance to the treatment of anhedonia. In this study, we investigated the effects of an acute dose of the atypical antidepressant (dual dopamine and noradrenaline reuptake inhibitor) bupropion on behavioral measures of emotional and reward processing in healthy volunteers. Methods: Forty healthy participants were randomly allocated to double-blind intervention with either an acute dose of bupropion or placebo prior to performing the Emotional Test Battery (ETB) and a probabilistic instrumental learning task. Results: Acute bupropion significantly increased the recognition of ambiguous faces as happy, decreased response bias toward sad faces and reduced attentional vigilance for fearful faces compared to placebo. Bupropion also reduced negative bias compared to placebo in the emotional recognition memory task (EMEM). There was no evidence that bupropion enhanced reward processing or learning. Instead, bupropion was associated with reduced likelihood to choose high-probability wins and increased score on a subjective measure of anhedonia. Conclusions: Whilst acute bupropion decreases negative and increases positive emotional processing, it has an adverse effect on reward processing. There seems to be a dissociation of the acute effects of bupropion on positive emotional processing and reward processing, which may have clinical implications for anhedonia early in treatment.

An Update on the Role of Serotonin and its Interplay with Dopamine for Reward

The specific role of serotonin and its interplay with dopamine (DA) in adaptive, reward guided behavior as well as drug dependance, still remains elusive. Recently, novel methods allowed cell type specific anatomical, functional and interventional analyses of serotonergic and dopaminergic circuits, promising significant advancement in understanding their functional roles. Furthermore, it is increasingly recognized that co-release of neurotransmitters is functionally relevant, understanding of which is required in order to interpret results of pharmacological studies and their relationship to neural recordings. Here, we review recent animal studies employing such techniques with the aim to connect their results to effects observed in human pharmacological studies and subjective effects of drugs. It appears that the additive effect of serotonin and DA conveys significant reward related information and is subjectively highly euphorizing. Neither DA nor serotonin alone have such an effect. This coincides with optogenetically targeted recordings in mice, where the dopaminergic system codes reward prediction errors (PE), and the serotonergic system mainly unsigned PE. Overall, this pattern of results indicates that joint activity between both systems carries essential reward information and invites parallel investigation of both neurotransmitter systems.

Downwards Vertical Attention Bias in Conversion Disorder vs Controls: A Pilot Study

BACKGROUND

Conversion disorder (CD) is a largely enigmatic disorder, one that requires a thorough ruling-out process. Prior research suggests that metaphors and conceptualization are rooted in physical experience, and that we interpret our affective world through metaphors. Spatial metaphors (interaction of affect and vertical space) are a prominent example of the grounding of metaphors. This is a relatively unpaved direction of research of CD.

OBJECTIVES

The present pilot study sought to explore this view by investigating the "healthy is up, sick is down" spatial metaphors (e.g., "fell ill" and "top shape") in patients with CD, examining the correlation between the processing of bodily-related words, CD, and vertical space. We hypothesized that patients with CD, who experience their bodies as ill, will demonstrate a downwards bias when processing bodily-related words; corresponding to the "healthy is up, sick is down" spatial metaphor.

METHODS

A total of 8 female patients (ages M-38.13 SD-10.44) and 42 female controls (ages M-36.4 SD-14.57) performed a visual attention task. Participants were asked to identify a spatial probe at the top or the bottom of a screen, following either a bodily related (e.g., arm) or non-bodily related (e.g., clock) prime word.

RESULTS

As predicted, when processing bodily-related words, patients with CD demonstrated a downwards attention bias. Moreover, the higher the patient's level of somatization, the faster the patient detected lower (vs upper) spatial targets.

CONCLUSIONS

This study suggests that the changed health paradigm of patients with CD is grounded in sensorimotor perception. Further research could propose new diagnostic and treatment options for CD.

Frameworking memory and serotonergic markers

The evidence for neural markers and memory is continuously being revised, and as evidence continues to accumulate, herein, we frame earlier and new evidence. Hence, in this work, the aim is to provide an appropriate conceptual framework of serotonergic markers associated with neural activity and memory. Serotonin (5-hydroxytryptamine [5-HT]) has multiple pharmacological tools, well-characterized downstream signaling in mammals’ species, and established 5-HT neural markers showing new insights about memory functions and dysfunctions, including receptors (5-HT1A/1B/1D, 5-HT2A/2B/2C, and 5-HT3-7), transporter (serotonin transporter [SERT]) and volume transmission present in brain areas involved in memory. Bidirectional influence occurs between 5-HT markers and memory/amnesia. A growing number of researchers report that memory, amnesia, or forgetting modifies neural markers. Diverse approaches support the translatability of using neural markers and cerebral functions/dysfunctions, including memory formation and amnesia. At least, 5-HT1A, 5-HT4, 5-HT6, and 5-HT7receptors and SERT seem to be useful neural markers and therapeutic targets. Hence, several mechanisms cooperate to achieve synaptic plasticity or memory, including changes in the expression of neurotransmitter receptors and transporters.

Cognitive performance of juvenile monkeys after chronic fluoxetine treatment

Potential long term effects on brain development are a concern when drugs are used to treat depression and anxiety in childhood. In this study, male juvenile rhesus monkeys (three-four years of age) were dosed with fluoxetine or vehicle (N=16/group) for two years. Histomorphometric examination of cortical dendritic spines conducted after euthanasia at one year postdosing (N=8/group) suggested a trend toward greater dendritic spine synapse density in prefrontal cortex of the fluoxetine-treated monkeys. During dosing, subjects were trained for automated cognitive testing, and evaluated with a test of sustained attention. After dosing was discontinued, sustained attention, recognition memory and cognitive flexibility were evaluated. Sustained attention was affected by fluoxetine, both during and after dosing, as indexed by omission errors. Response accuracy was not affected by fluoxetine in post-dosing recognition memory and cognitive flexibility tests, but formerly fluoxetine-treated monkeys compared to vehicle controls had more missed trial initiations and choices during testing. Drug treatment also interacted with genetic and environmental variables: MAOA genotype (high- and low transcription rate polymorphisms) and testing location (upper or lower tier of cages). Altered development of top-down cortical regulation of effortful attention may be relevant to this pattern of cognitive test performance after juvenile fluoxetine treatment.