Establishing operant conflict tests for the translational study of anxiety in mice

Approach-avoidance reinforcement learning as a translational and computational model of anxiety-related avoidance

Although avoidance is a prevalent feature of anxiety-related psychopathology, differences in the measurement of avoidance between humans and non-human animals hinder our progress in its theoretical understanding and treatment. To address this, we developed a novel translational measure of anxiety-related avoidance in the form of an approach-avoidance reinforcement learning task, by adapting a paradigm from the non-human animal literature to study the same cognitive processes in human participants. We used computational modelling to probe the putative cognitive mechanisms underlying approach-avoidance behaviour in this task and investigated how they relate to subjective task-induced anxiety. In a large online study (n = 372), participants who experienced greater task-induced anxiety avoided choices associated with punishment, even when this resulted in lower overall reward. Computational modelling revealed that this effect was explained by greater individual sensitivities to punishment relative to rewards. We replicated these findings in an independent sample (n = 627) and we also found fair-to-excellent reliability of measures of task performance in a sub-sample retested 1 week later (n = 57). Our findings demonstrate the potential of approach-avoidance reinforcement learning tasks as translational and computational models of anxiety-related avoidance. Future studies should assess the predictive validity of this approach in clinical samples and experimental manipulations of anxiety.

The Evolving Role of Animal Models in the Discovery and Development of Novel Treatments for Psychiatric Disorders

Historically, animal models have been routinely used in the characterization of novel chemical entities (NCEs) for various psychiatric disorders. Animal models have been essential in the in vivo validation of novel drug targets, establishment of lead compound pharmacokinetic to pharmacodynamic relationships, optimization of lead compounds through preclinical candidate selection, and development of translational measures of target occupancy and functional target engagement. Yet, with decades of multiple NCE failures in Phase II and III efficacy trials for different psychiatric disorders, the utility and value of animal models in the drug discovery process have come under intense scrutiny along with the widespread withdrawal of the pharmaceutical industry from psychiatric drug discovery. More recently, the development and utilization of animal models for the discovery of psychiatric NCEs has undergone a dynamic evolution with the application of the Research Domain Criteria (RDoC) framework for better design of preclinical to clinical translational studies combined with innovative genetic, neural circuitry-based, and automated testing technologies. In this chapter, the authors will discuss this evolving role of animal models for improving the different stages of the discovery and development in the identification of next generation treatments for psychiatric disorders.

Evidence for Effects of Extracellular Vesicles on Physical, Inflammatory, Transcriptome and Reward Behaviour Status in Mice

Immune-inflammatory activation impacts extracellular vesicles (EVs), including their miRNA cargo. There is evidence for changes in the EV miRNome in inflammation-associated neuropsychiatric disorders. This mouse study investigated: (1) effects of systemic lipopolysaccharide (LPS) and chronic social stress (CSS) on plasma EV miRNome; and (2) physiological, transcriptional, and behavioural effects of peripheral or central delivered LPS-activated EVs in recipient mice. LPS or CSS effects on the plasma EV miRNome were assessed by using microRNA sequencing. Recipient mice received plasma EVs isolated from LPS-treated or SAL-treated donor mice or vehicle only, either intravenously or into the nucleus accumbens (NAc), on three consecutive days. Bodyweight, spleen or NAc transcriptome and reward (sucrose) motivation were assessed. LPS and CSS increased the expression of 122 and decreased expression of 20 plasma EV miRNAs, respectively. Peripheral LPS-EVs reduced bodyweight, and both LPS-EVs and SAL-EVs increased spleen expression of immune-relevant genes. NAc-infused LPS-EVs increased the expression of 10 immune-inflammatory genes. Whereas motivation increased similarly across test days in all groups, the effect of test days was more pronounced in mice that received peripheral or central LPS-EVs compared with other groups. This study provides causal evidence that increased EV levels impact physiological and behavioural processes and are of potential relevance to neuropsychiatric disorders.

Cross-species anxiety tests in psychiatry: pitfalls and promises

Behavioural anxiety tests in non-human animals are used for anxiolytic drug discovery, and to investigate the neurobiology of threat avoidance. Over the past decade, several of them were translated to humans with three clinically relevant goals: to assess potential efficacy of candidate treatments in healthy humans; to develop diagnostic tests or biomarkers; and to elucidate the pathophysiology of anxiety disorders. In this review, we scrutinise these promises and compare seven anxiety tests that are validated across species: five approach-avoidance conflict tests, unpredictable shock anticipation, and the social intrusion test in children. Regarding the first goal, three tests appear suitable for anxiolytic drug screening in humans. However, they have not become part of the drug development pipeline and achieving this may require independent confirmation of predictive validity and cost-effectiveness. Secondly, two tests have shown potential to measure clinically relevant individual differences, but their psychometric properties, predictive value, and clinical applicability need to be clarified. Finally, cross-species research has not yet revealed new evidence that the physiology of healthy human behaviour in anxiety tests relates to the physiology of anxiety symptoms in patients. To summarise, cross-species anxiety tests could be rendered useful for drug screening and for development of diagnostic instruments. Using these tests for aetiology research in healthy humans or animals needs to be queried and may turn out to be unrealistic.

Neural correlates and determinants of approach-avoidance conflict in the prelimbic prefrontal cortex

The recollection of environmental cues associated with threat or reward allows animals to select the most appropriate behavioral responses. Neurons in the prelimbic (PL) cortex respond to both threat- and reward-associated cues. However, it remains unknown whether PL regulates threat-avoidance vs. reward-approaching responses when an animals’ decision depends on previously associated memories. Using a conflict model in which male Long–Evans rats retrieve memories of shock- and food-paired cues, we observed two distinct phenotypes during conflict: (1) rats that continued to press a lever for food (Pressers) and (2) rats that exhibited a complete suppression in food seeking (Non-pressers). Single-unit recordings revealed that increased risk-taking behavior in Pressers is associated with persistent food-cue responses in PL, and reduced spontaneous activity in PL glutamatergic (PL^GLUT) neurons during conflict. Activating PL^GLUT neurons in Pressers attenuated food-seeking responses in a neutral context, whereas inhibiting PL^GLUT neurons in Non-pressers reduced defensive responses and increased food approaching during conflict. Our results establish a causal role for PL^GLUT neurons in mediating individual variability in memory-based risky decision-making by regulating threat-avoidance vs. reward-approach behaviors.

Effects of chlordiazepoxide on pausing during rich-to-lean transitions

Extended pausing during discriminable transitions from rich-to-lean conditions can be viewed as escape (i.e., rich-to-lean transitions function aversively). Thus, an anxiolytic drug would be predicted to mitigate the aversiveness and decrease pausing. In the current experiment, pigeons' key pecking was maintained by a multiple fixed-ratio fixed-ratio schedule of rich (i.e., larger) or lean (i.e., smaller) reinforcers. Intermediate doses (3.0-10.0 mg/kg) of chlordiazepoxide differentially decreased median pauses during rich-to-lean transitions. Relatively small decreases in pauses occurred during lean-to-lean and rich-to-rich transitions. Effects of chlordiazepoxide on pausing occurred without appreciable effects on run rates. These findings suggest that signaled rich-to-lean transitions function aversively.

Characterizing Different Strategies for Resolving Approach-Avoidance Conflict

The ability of animals to maximize benefits and minimize costs during approach-avoidance conflicts is an important evolutionary tool, but little is known about the emergence of specific strategies for conflict resolution. Accordingly, we developed a simple approach-avoidance conflict task in rats that pits the motivation to press a lever for sucrose against the motivation to step onto a distant platform to avoid a footshock delivered at the end of a 30 s tone (sucrose is available only during the tone). Rats received conflict training for 16 days to give them a chance to optimize their strategy by learning to properly time the expression of both behaviors across the tone. Rats unexpectedly separated into three distinct subgroups: those pressing early in the tone and avoiding later (Timers, 49%); those avoiding throughout the tone (Avoidance-preferring, 32%); and those pressing throughout the tone (Approach-preferring, 19%). The immediate early gene cFos revealed that Timers showed increased activity in the ventral striatum and midline thalamus relative to the other two subgroups, Avoidance-preferring rats showed increased activity in the amygdala, and Approach-preferring rats showed decreased activity in the prefrontal cortex. This pattern is consistent with low fear and high behavioral flexibility in Timers, suggesting the potential of this task to reveal the neural mechanisms of conflict resolution.

The ventral hippocampus is necessary for cue-elicited, but not outcome driven approach-avoidance conflict decisions: a novel operant choice decision-making task

Approach-avoidance conflict is induced when an organism encounters a stimulus that carries both positive and negative attributes. Accumulating evidence implicates the ventral hippocampus (VH) in the detection and resolution of approach-avoidance conflict, largely on the basis of maze-based tasks assaying innate and conditioned responses to situations of conflict. However, its role in discrete trial approach-avoidance decision-making has yet to be elucidated. In this study, we designed a novel cued operant conflict decision-making task in which rats were required to choose and respond for a low reward option or high reward option paired with varying shock intensities on a differential reinforcement of low rates of responding schedule. Post training, the VH was chemogenetically inhibited while animals performed the task with the usual outcomes delivered, and with the presentation of cues associated with the reward vs. conflict options only (extinction condition). We found that VH inhibition led to an avoidance of the conflict option and longer latency to choose this option when decision-making was being made on the basis of cues alone with no outcomes. Consistent with these findings, VH-inhibited animals spent more time in the central component of the elevated plus maze (EPM), indicating a potential deficit in decision-making under innate forms of approach-avoidance conflict. Taken together, these findings implicate the VH in cue-driven approach-avoidance decisions in the face of motivational conflict.

Early life stress causes sex-specific changes in adult fronto-limbic connectivity that differentially drive learning

It is currently unclear whether early life stress (ELS) affects males and females differently. However, a growing body of work has shown that sex moderates responses to stress and injury, with important insights into sex-specific mechanisms provided by work in rodents. Unfortunately, most of the ELS studies in rodents were conducted only in males, a bias that is particularly notable in translational work that has used human imaging. Here we examine the effects of unpredictable postnatal stress (UPS), a mouse model of complex ELS, using high resolution diffusion magnetic resonance imaging. We show that UPS induces several neuroanatomical alterations that were seen in both sexes and resemble those reported in humans. In contrast, exposure to UPS induced fronto-limbic hyper-connectivity in males, but either no change or hypoconnectivity in females. Moderated-mediation analysis found that these sex-specific changes are likely to alter contextual freezing behavior in males but not in females.

Vulnerability to psychological stress-induced anorexia in female mice depends on blockade of ghrelin signal in nucleus tractus solitarius

Background and Purpose

Women have a higher incidence of eating disorders than men. We investigated whether the effects of ghrelin on feeding are affected by sex and stress, and to elucidate the mechanisms that may cause sex differences in stress‐mediated anorexia, focusing on ghrelin.

Experimental Approach

Acylated ghrelin was administered to naïve and psychologically stressed male and female C57BL/6J mice, followed by measurements of food intake and plasma hormone levels. Ovariectomy was performed to determine the effects of ovary‐derived oestrogen on stress‐induced eating disorders in female mice. The numbers of Agrp or c‐Fos mRNA‐positive cells and estrogen receptor α/c‐Fos protein‐double‐positive cells were assessed.

Key Results

Ghrelin administration to naïve female mice caused a higher increase in food intake, growth hormone secretion, Agrp mRNA expression in the arcuate nucleus and c‐Fos expression in the nucleus tractus solitarius (NTS) than in male mice. In contrast, psychological stress caused a more sustained reduction in food intake in females than males. The high sensitivity of naïve females to exogenous ghrelin was attenuated by stress exposure. The stress‐induced decline in food intake was not abolished by ovariectomy. Estrogen receptor‐α but not ‐β antagonism prevented the decrease in food intake under stress. Estrogen receptor‐α/c‐Fos‐double‐positive cells in the NTS were significantly increased by stress only in females.

Conclusion and Implications

Stress‐mediated eating disorders in females may be due to blockade of ghrelin signalling via estrogen receptor‐α activation in the NTS. Targeting the ghrelin signal in the brain could be a new treatment strategy to prevent these disorders.

Hippocampal Representation of Threat Features and Behavior in a Human Approach-Avoidance Conflict Anxiety Task

Decisions under threat are crucial to survival and require integration of distinct situational features, such as threat probability and magnitude. Recent evidence from human lesion and neuroimaging studies implicated anterior hippocampus (aHC) and amygdala in approach-avoidance decisions under threat, and linked their integrity to cautious behavior. Here we sought to elucidate how threat dimensions and behavior are represented in these structures. Twenty human participants (11 female) completed an approach-avoidance conflict task during high-resolution fMRI. Participants could gather tokens under threat of capture by a virtual predator, which would lead to token loss. Threat probability (predator wake-up rate) and magnitude (amount of token loss) varied on each trial. To disentangle effects of threat features, and ensuing behavior, we performed a multifold parametric analysis. We found that high threat probability and magnitude related to BOLD signal in left aHC/entorhinal cortex. However, BOLD signal in this region was better explained by avoidance behavior than by these threat features. A priori ROI analysis confirmed the relation of aHC BOLD response with avoidance. Exploratory subfield analysis revealed that this relation was specific to anterior CA2/3 but not CA1. Left lateral amygdala responded to low and high, but not intermediate, threat probability. Our results suggest that aHC BOLD signal is better explained by avoidance behavior than by threat features in approach-avoidance conflict. Rather than representing threat features in a monotonic manner, it appears that aHC may compute approach-avoidance decisions based on integration of situational threat features represented in other neural structures.SIGNIFICANCE STATEMENT An effective threat anticipation system is crucial to survival across species. Natural threats, however, are diverse and have distinct features. To be able to adapt to different modes of danger, the brain needs to recognize these features, integrate them, and use them to modify behavior. Our results disclose the human anterior hippocampus as a likely arbiter of approach-avoidance decisions harnessing compound environmental information while partially replicating previous findings and blending into recent efforts to illuminate the neural basis of approach-avoidance conflict in humans.

Systematic review and meta-analysis: effects of maternal separation on anxiety-like behavior in rodents

The mechanisms by which childhood maltreatment increases anxiety is unclear, but a propensity for increased defensive behavior in rodent models of early life stress (ELS) suggests that work in rodents may clarify important mechanistic details about this association. A key challenge in studying the effects of ELS on defensive behavior in rodents is the plethora of inconsistent results. This is particularly prominent with the maternal separation (MS) literature, one of the most commonly used ELS models in rodents. To address this issue we conducted a systematic review and meta-analysis, examining the effects of MS on exploratory-defensive behavior in mice and rats using the open field test (OFT) and the elevated plus maze (EPM). This search yielded a total of 49 studies, 24 assessing the effect of MS on behavior in the EPM, 11 tested behavior in the OFT, and 14 studies provided data on both tasks. MS was associated with increased defensive behavior in rats (EPM: Hedge's g = -0.48, p = 0.02; OFT: Hedge's g = -0.33, p = 0.05), effect sizes that are consistent with the anxiogenic effect of early adversity reported in humans. In contrast, MS did not alter exploratory behavior in mice (EPM: Hedge's g = -0.04, p = 0.75; OFT: Hedge's g = -0.03, p = 0.8). There was a considerable amount of heterogeneity between studies likely related to the lack of standardization of the MS protocol. Together, these findings suggest important differences in the ability of MS to alter circuits that regulate defensive behaviors in mice and rats.

Disentangling Hippocampal and Amygdala Contribution to Human Anxiety-Like Behavior

Anxiety comprises a suite of behaviors to deal with potential threat and is often modeled in approach–avoidance conflict tasks. Collectively, these tests constitute a predominant preclinical model of anxiety disorder. A body of evidence suggests that both ventral hippocampus and amygdala lesions impair anxiety-like behavior, but the relative contribution of these two structures is unclear. A possible reason is that approach–avoidance conflict tasks involve a series of decisions and actions, which may be controlled by distinct neural mechanisms that are difficult to disentangle from behavioral readouts. Here, we capitalize on a human approach–avoidance conflict test, implemented as computer game, that separately measures several action components. We investigate three patients of both sexes with unspecific unilateral medial temporal lobe (MTL) damage, one male with selective bilateral hippocampal (HC), and one female with selective bilateral amygdala lesions, and compare them to matched controls. MTL and selective HC lesions, but not selective amygdala lesions, increased approach decision when possible loss was high. In contrast, MTL and selective amygdala lesions, but not selective HC lesions, increased return latency. Additionally, selective HC and selective amygdala lesions reduced approach latency. In a task targeted at revealing subjective assumptions about the structure of the computer game, MTL and selective HC lesions impacted on reaction time generation but not on the subjective task structure. We conclude that deciding to approach reward under threat relies on hippocampus but not amygdala, whereas vigor of returning to safety depends on amygdala but not on hippocampus.

SIGNIFICANCE STATEMENT Approach–avoidance conflict tests are widely investigated in rodents, and increasingly in humans, to understand the neural basis of anxiety-like behavior. However, the contribution of the most relevant brain regions, ventral hippocampus and amygdala, is incompletely understood. We use a human computerized test that separates different action components and find that hippocampus, but not amygdala, lesions impair approach decisions, whereas amygdala, but not hippocampus, lesions impair the vigor of return to safety.