Pharmacological or genetic inactivation of the serotonin transporter improves reversal learning in mice

Serotonin transporter knockdown relieves depression-like behavior and ethanol-induced CPP in mice after chronic social defeat stress

Patients with stress-triggered major depression disorders (MDD) can often seek comfort or temporary relief through alcohol consumption, as they may turn to it as a means of self-medication or coping with overwhelming emotions. The use of alcohol as a coping mechanism for stressful events can escalate, fostering a cycle where the temporary relief it provides from depression can deepen into alcohol dependence, exacerbating both conditions. Although, the specific mechanisms involved in stress-triggered alcohol dependence and MDD comorbidities are not well understood, a large body of literature suggests that the serotonin transporter (SERT) plays a critical role in these abnormalities. To further investigate this hypothesis, we used a lentiviral-mediated knockdown approach to examine the role of hippocampal SERT knockdown in social defeat stress-elicited depression like behavior and ethanol-induced place preference (CPP). The results showed that social defeat stress-pro depressant effects were reversed following SERT knockdown demonstrated by increased sucrose preference, shorter latency to feed in the novelty suppressed feeding test, and decreased immobility time in the tail suspension and forced swim tests. Moreover, and most importantly, social stress-induced ethanol-CPP acquisition and reinstatement were significantly reduced following hippocampal SERT knockdown using short hairpin RNA shRNA-expressing lentiviral vectors. Finally, we confirmed that SERT hippocampal mRNA expression correlated with measures of depression- and ethanol-related behaviors by Pearson's correlation analysis. Taken together, our data suggest that hippocampal serotoninergic system is involved in social stress-triggered mood disorders as well as in the acquisition and retrieval of ethanol contextual memory and that blockade of this transporter can decrease ethanol rewarding properties.

Chronic exposure to imipramine induces a switch from depression-like to mania-like behavior in female serotonin transporter knockout rats: Role of BDNF signaling in the infralimbic cortex

BACKGROUND

Bipolar disorder (BD) is a highly burdensome psychiatric disorder characterized by alternating states of mania and depression. A major challenge in the clinic is the switch from depression to mania, which is often observed in female BD patients during antidepressant treatment such as imipramine. However, the underlying neural basis is unclear.

METHODS

To investigate the potential neuronal pathways, serotonin transporter knockout (SERT KO) rats, an experimental model of female BD patients, were subjected to a battery of behavioral tests under chronic treatment of the antidepressant imipramine. In addition, the expression of brain-derived neurotrophic factor (BDNF) and its downstream signaling was examined in the prefrontal cortex.

RESULTS

Chronic exposure to imipramine reduced anxiety and sociability and problem-solving capacity, and increased thigmotaxis and day/night activity in all animals, but specifically in female SERT KO rats, compared to female wild-type (WT) rats. Further, we found an activation of BDNF-TrkB-Akt pathway signaling in the infralimbic, but not prelimbic, cortex after chronic imipramine treatment in SERT KO, but not WT, rats.

LIMITATIONS

Repeated testing behaviors could potentially affect the results. Additionally, the imipramine induced changes in behavior and in the BDNF system were measured in separate animals.

CONCLUSIONS

Our study indicates that female SERT KO rats, which mirror the female BD patients with the 5-HTTLPR s-allele, are at higher risk of a switch to mania-like behaviors under imipramine treatment. Activation of the BDNF-TrkB-Akt pathway in the infralimbic cortex might contribute to this phenotype, but causal evidence remains to be provided.

Research Report: A Link between Sertraline Treatment and Susceptibility to (Mis)information

Recent research revealed that several psycho-cognitive processes, such as insensitivity to positive and negative feedback, cognitive rigidity, pessimistic judgment bias, and anxiety, are involved in susceptibility to fake news. All of these processes have been previously associated with depressive disorder and are sensitive to serotoninergic manipulations. In the current study, a link between chronic treatment with the selective serotonin reuptake inhibitor (SSRI) sertraline and susceptibility to true and fake news was examined. Herein, a sample of 1162 participants was recruited via Prolific Academic for an online study. Half of the sample reported taking sertraline (Zoloft) for at least 8 weeks (sertraline group), and the other half confirmed not taking any psychiatric medication (control group). The sertraline group was further divided according to their daily dosage (50, 100, 150, and 200 mg/day). All participants completed a susceptibility to misinformation scale, wherein they were asked to determine the veracity of the presented true and fake news and their willingness to behaviorally engage with the news. The results were compared between those of the sertraline groups and the control group. The results showed that sertraline groups did not differ significantly in the assessment of the truthfulness of information or their ability to discern the truth. However, those taking sertraline appeared to have a significantly increased likelihood of behavioral engagement with the information, and this effect was observed for both true and fake news. The research presented here represents the initial endeavor to comprehend the neurochemical foundation of the susceptibility to misinformation. The association between sertraline treatment and increased behavioral engagement with information observed in this study can be explained in light of previous studies showing positive correlations between serotonin (5-HT) system activity and the inclination to engage in social behaviors. It can also be attributed to the anxiolytic effects of sertraline treatment, which mitigate the fear of social judgment. The heightened behavioral engagement with information in people taking sertraline may, as part of a general phenomenon, also shape their interactions with fake news. Future longitudinal studies should reveal the specificity and exact causality of these interactions.

Neonatal hypoxia impairs serotonin release and cognitive functions in adult mice

Children who experienced moderate perinatal asphyxia (MPA) are at risk of developing long lasting subtle cognitive and behavioral deficits, including learning disabilities and emotional problems. The prefrontal cortex (PFC) regulates cognitive flexibility and emotional behavior. Neurons that release serotonin (5-HT) project to the PFC, and compounds modulating 5-HT activity influence emotion and cognition. Whether 5-HT dysregulations contribute to MPA-induced cognitive problems is unknown. We established a MPA mouse model, which displays recognition and spatial memory impairments and dysfunctional cognitive flexibility. We found that 5-HT expression levels, quantified by immunohistochemistry, and 5-HT release, quantified by in vivo microdialysis in awake mice, are reduced in PFC of adult MPA mice. MPA mice also show impaired body temperature regulation following injection of the 5-HT1A receptor agonist 8-OH-DPAT, suggesting the presence of deficits in 5-HT auto-receptor function on raphe neurons. Finally, chronic treatment of adult MPA mice with fluoxetine, an inhibitor of 5-HT reuptake transporter, or the 5-HT1A receptor agonist tandospirone rescues cognitive flexibility and memory impairments. All together, these data demonstrate that the development of 5-HT system function is vulnerable to moderate perinatal asphyxia. 5-HT hypofunction might in turn contribute to long-term cognitive impairment in adulthood, indicating a potential target for pharmacological therapies.

Synthetic surprise as the foundation of the psychedelic experience

Psychedelic agents, such as LSD and psilocybin, induce marked alterations in consciousness via activation of the 5-HT2A receptor (5-HT2ARs). We hypothesize that psychedelics enforce a state of synthetic surprise through the biased activation of the 5-HTRs system. This idea is informed by recent insights into the role of 5-HT in signaling surprise. The effects on consciousness, explained by the cognitive penetrability of perception, can be described within the predictive coding framework where surprise corresponds to prediction error, the mismatch between predictions and actual sensory input. Crucially, the precision afforded to the prediction error determines its effect on priors, enabling a dynamic interaction between top-down expectations and incoming sensory data. By integrating recent findings on predictive coding circuitry and 5-HT2ARs transcriptomic data, we propose a biological implementation with emphasis on the role of inhibitory interneurons. Implications arise for the clinical use of psychedelics, which may rely primarily on their inherent capacity to induce surprise in order to disrupt maladaptive patterns.

A role for the serotonin 2A receptor in the expansion and functioning of human transmodal cortex

Integrating independent but converging lines of research on brain function and neurodevelopment across scales, this article proposes that serotonin 2A receptor (5-HT2AR) signalling is an evolutionary and developmental driver and potent modulator of the macroscale functional organization of the human cerebral cortex. A wealth of evidence indicates that the anatomical and functional organization of the cortex follows a unimodal-to-transmodal gradient. Situated at the apex of this processing hierarchy-where it plays a central role in the integrative processes underpinning complex, human-defining cognition-the transmodal cortex has disproportionately expanded across human development and evolution. Notably, the adult human transmodal cortex is especially rich in 5-HT2AR expression and recent evidence suggests that, during early brain development, 5-HT2AR signalling on neural progenitor cells stimulates their proliferation-a critical process for evolutionarily-relevant cortical expansion. Drawing on multimodal neuroimaging and cross-species investigations, we argue that, by contributing to the expansion of the human cortex and being prevalent at the apex of its hierarchy in the adult brain, 5-HT2AR signalling plays a major role in both human cortical expansion and functioning. Owing to its unique excitatory and downstream cellular effects, neuronal 5-HT2AR agonism promotes neuroplasticity, learning and cognitive and psychological flexibility in a context-(hyper)sensitive manner with therapeutic potential. Overall, we delineate a dual role of 5-HT2ARs in enabling both the expansion and modulation of the human transmodal cortex.

Automating licking bias correction in a two-choice delayed match-to-sample task to accelerate learning

Animals often display choice bias, or a preference for one option over the others, which can significantly impede learning new tasks. Delayed match-to-sample (DMS) tasks with two-alternative choices of lickports on the left and right have been widely used to study sensory processing, working memory, and associative memory in head-fixed animals. However, extensive training time, primarily due to the animals' biased licking responses, limits their practical utility. Here, we present the implementation of an automated side bias correction system in an olfactory DMS task, where the lickport positions and the ratio of left- and right-rewarded trials are dynamically adjusted to counterbalance mouse's biased licking responses during training. The correction algorithm moves the preferred lickport farther away from the mouse's mouth and the non-preferred lickport closer, while also increasing the proportion of non-preferred side trials when biased licking occurs. We found that adjusting lickport distances and the proportions of left- versus right-rewarded trials effectively reduces the mouse's side bias. Further analyses reveal that these adjustments also correlate with subsequent improvements in behavioral performance. Our findings suggest that the automated side bias correction system is a valuable tool for enhancing the applicability of behavioral tasks involving two-alternative lickport choices.

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.

Reinforcer value moderates the effects of prenatal alcohol exposure on learning and reversal

Introduction

Fetal Alcohol Spectrum Disorders (FASD) are the leading cause of preventable developmental disability and are commonly characterized by alterations in executive function. Reversal learning tasks are reliable, cross-species methods for testing a frequently impaired aspect of executive control, behavioral flexibility. Pre-clinical studies commonly require the use of reinforcers to motivate animals to learn and perform the task. While there are several reinforcers available, the most commonly employed are solid (food pellets) and liquid (sweetened milk) rewards. Previous studies have examined the effects of different solid rewards or liquid dietary content on learning in instrumental responding and found that rodents on liquid reward with higher caloric content performed better with increased response and task acquisition rate. The influence of reinforcer type on reversal learning and how this interacts with developmental insults such as prenatal alcohol exposure (PAE) has not been explored.

Methods

We tested whether reinforcer type during learning or reversal would impact an established deficit in PAE mice.

Results

We found that all male and female mice on liquid reward, regardless of prenatal exposure were better motivated to learn task behaviors during pre-training. Consistent with previous findings, both male and female PAE mice and Saccharine control mice were able to learn the initial stimulus reward associations irrespective of the reinforcer type. During the initial reversal phase, male PAE mice that received pellet rewards exhibited maladaptive perseverative responding whereas male mice that received liquid rewards performed comparable to their control counterparts. Female PAE mice that received either reinforcer types did not exhibit any deficits on behavioral flexibility. Female saccharine control mice that received liquid, but not pellet, rewards showed increased perseverative responding during the early reversal phase.

Discussion

These data suggest that reinforcer type can have a major impact on motivation, and therefore performance, during reversal learning. Highly motivating rewards may mask behavioral deficits seen with more moderately sought rewards and gestational exposure to the non-caloric sweetener, saccharine, can impact behavior motivated by those reinforcers in a sex-dependent manner.

Serotonin in the orbitofrontal cortex enhances cognitive flexibility

Cognitive flexibility is a brain's ability to switch between different rules or action plans depending on the context. However, cellular level understanding of cognitive flexibility have been largely unexplored. We probed a specific serotonergic pathway from dorsal raphe nuclei (DRN) to the orbitofrontal cortex (OFC) while animals are performing reversal learning task. We found that serotonin release from DRN to the OFC promotes reversal learning. A long-range connection between these two brain regions was confirmed anatomically and functionally. We further show that spatiotemporally precise serotonergic action directly enhances the excitability of OFC neurons and offers enhanced spike probability of OFC network. Serotonergic action facilitated the induction of synaptic plasticity by enhancing Ca2+ influx at dendritic spines in the OFC. Thus, our findings suggest that a key signature of flexibility is the formation of choice specific ensembles via serotonin-dependent synaptic plasticity.

Effect of psilocybin on decision-making and motivation in the healthy rat

Psilocybin and its active metabolite psilocin are hallucinogenic serotonergic agonists with high affinity for several serotonin receptors. In addition to underlying the hallucinogenic effects of these compounds, serotonin receptor activation also has important effects on decision-making and goal-directed behaviors. The impact of psilocybin and psilocin on these cognitive systems, however, remains unclear. This study investigated the effects of psilocybin treatment on decision-making and motivation in healthy male and female rats. We compared probability and delay discounting performance of psilocybin treated (1 mg/kg) to vehicle rats (n = 10/sex/group), and further assessed motivation in each group using a progressive ratio task. We also confirmed drug action by assessing head twitch responses after psilocybin treatment (1 mg/kg). Results from this study demonstrated that exposure to 1 mg/kg psilocybin did not affect decision-making in the probability and delay discounting tasks and did not reduce response rates in the progressive ratio task. However, psilocybin treatment did cause the expected increase in head twitch responses in both male and female rats, demonstrating that the drug was delivered at a pharmacologically relevant dosage. Combined, these results suggest that psilocybin may not impair or improve decision-making and motivation. Considering recent interest in psilocybin as a potential fast-acting therapeutic for a variety of mental health disorders, our findings also suggest the therapeutic effects of this drug may not be mediated by changes to the brain systems underlying reward and decision-making. Finally, these results may have important implications regarding the relative safety of this compound, suggesting that widespread cognitive impairments may not be seen in subjects, even after chronic treatment.

Acute psilocybin enhances cognitive flexibility in rats

Psilocybin has been shown to improve symptoms of depression and anxiety when combined with psychotherapy or other clinician-guided interventions. To understand the neural basis for this pattern of clinical efficacy, experimental and conceptual approaches that are different than traditional laboratory models of anxiety and depression are needed. A potential novel mechanism is that acute psilocybin improves cognitive flexibility, which then enhances the impact of clinician-assisted interventions. Consistent with this idea, we find that acute psilocybin robustly improves cognitive flexibility in male and female rats using a task where animals switched between previously learned strategies in response to uncued changes in the environment. Psilocybin did not influence Pavlovian reversal learning, suggesting that its cognitive effects are selective to enhanced switching between previously learned behavioral strategies. The serotonin (5HT) 2 A receptor antagonist ketanserin blocked psilocybin's effect on set-shifting, while a 5HT2C-selective antagonist did not. Ketanserin alone also improved set-shifting performance, suggesting a complex relationship between psilocybin's pharmacology and its impact on flexibility. Further, the psychedelic drug 2,5-Dimethoxy-4-iodoamphetamine (DOI) impaired cognitive flexibility in the same task, suggesting that this effect of psilocybin does not generalize to all other serotonergic psychedelics. We conclude that the acute impact of psilocybin on cognitive flexibility provides a useful behavioral model to investigate its neuronal effects relevant to its positive clinical outcome.

Constitutive depletion of brain serotonin differentially affects rats' social and cognitive abilities

Central serotonin appears a promising transdiagnostic marker of psychiatric disorders and a modulator of some of their key behavioral symptoms. In adult male Tph2 ^-/- rats, constitutively lacking central serotonin, we tested individual's cognitive, social and non-social abilities and characterized group's social organization under classical and ethological testing conditions. Using unsupervised machine learning, we identified the functions most dependent on serotonin. Although serotonin depletion did not affect cognitive performances in classical testing, in the home-cage it induced compulsive aggression and sexual behavior, hyperactive and hypervigilant stereotyped behavior, reduced self-care and exacerbated corticosterone levels. This profile recalled symptoms of impulse control and anxiety disorders. Serotonin appeared essential for behavioral adaptation to dynamic social environments. Our animal model challenges the essential role of serotonin in decision-making, flexibility, impulsivity, and risk-taking. These findings highlight the importance of studying everyday life functions within the dynamic social living environment to model complexity in animal models.

Acute psilocybin enhances cognitive flexibility in rats

Psilocybin has been shown to improve symptoms of depression and anxiety when combined with psychotherapy or other clinician-guided interventions. To understand the neural basis for this pattern of clinical efficacy, experimental and conceptual approaches that are different than traditional laboratory models of anxiety and depression are needed. A potential novel mechanism is that acute psilocybin improves cognitive flexibility, which then enhances the impact of clinician-assisted interventions. Consistent with this idea, we find that acute psilocybin robustly improves cognitive flexibility in male and female rats using a task where animals switched between previously learned strategies in response to uncued changes in the environment. Psilocybin did not influence Pavlovian reversal learning, suggesting that its cognitive effects are selective to enhanced switching between previously learned behavioral strategies. The serotonin (5HT) 2A receptor antagonist ketanserin blocked psilocybin's effect on set-shifting, while a 5HT2C-selective antagonist did not. Ketanserin alone also improved set-shifting performance, suggesting a complex relationship between psilocybin's pharmacology and its impact on flexibility. Further, the psychedelic drug 2,5-Dimethoxy-4-iodoamphetamine (DOI) impaired cognitive flexibility in the same task, suggesting that this effect of psilocybin does not generalize to all other serotonergic psychedelics. We conclude that the acute impact of psilocybin on cognitive flexibility provides a useful behavioral model to investigate its neuronal effects relevant to its positive clinical outcome.

Effect of lysergic acid diethylamide (LSD) on reinforcement learning in humans

BACKGROUND

The non-selective serotonin 2A (5-HT2A) receptor agonist lysergic acid diethylamide (LSD) holds promise as a treatment for some psychiatric disorders. Psychedelic drugs such as LSD have been suggested to have therapeutic actions through their effects on learning. The behavioural effects of LSD in humans, however, remain incompletely understood. Here we examined how LSD affects probabilistic reversal learning (PRL) in healthy humans.

METHODS

Healthy volunteers received intravenous LSD (75 μg in 10 mL saline) or placebo (10 mL saline) in a within-subjects design and completed a PRL task. Participants had to learn through trial and error which of three stimuli was rewarded most of the time, and these contingencies switched in a reversal phase. Computational models of reinforcement learning (RL) were fitted to the behavioural data to assess how LSD affected the updating ('learning rates') and deployment of value representations ('reinforcement sensitivity') during choice, as well as 'stimulus stickiness' (choice repetition irrespective of reinforcement history).

RESULTS

Raw data measures assessing sensitivity to immediate feedback ('win-stay' and 'lose-shift' probabilities) were unaffected, whereas LSD increased the impact of the strength of initial learning on perseveration. Computational modelling revealed that the most pronounced effect of LSD was the enhancement of the reward learning rate. The punishment learning rate was also elevated. Stimulus stickiness was decreased by LSD, reflecting heightened exploration. Reinforcement sensitivity differed by phase.

CONCLUSIONS

Increased RL rates suggest LSD induced a state of heightened plasticity. These results indicate a potential mechanism through which revision of maladaptive associations could occur in the clinical application of LSD.

Brain gain—Is the cognitive performance of domestic hens affected by a functional polymorphism in the serotonin transporter gene?

The serotonin transporter (5-HTT) plays an important role in regulating serotonergic transmission via removal of serotonin (5-HT) from synaptic clefts. Alterations in 5-HTT expression and subsequent 5-HT transmission have been found to be associated with changes in behaviour, such as fearfulness or activity, in humans and other vertebrates. In humans, alterations in 5-HTT expression have been suggested to be able to lead to better learning performance, with more fearful persons being better at learning. Similar effects of the variation in the 5-HTT on fearfulness have been found in chickens, and in this study, we investigated effects on learning. Therefore, we tested 52 adult laying hens, differing in their functional 5-HTT genotype (W/W, W/D and D/D) in an operant learning paradigm in three different phases (initial learning, reversal learning and extinction) and in a tonic immobility test for fearfulness. We found that the 5-HTT polymorphism affects the initial learning performance of laying hens, with homogeneous wild-type (W/W) hens being the slowest learners, and the most fearful birds. W/W hens, showed significantly more choices to solve the initial learning task (LME, p = 0.031) and had the highest latencies in a tonic immobility test (p = 0.039), indicating the highest fearfulness. Our results provide interesting first insights into the role of 5-HTT in chickens and its sensitive interaction with the environment. We further suggest that the 5-HTT gene can be an interesting target gene for future breeding strategies as well as for further experimental studies.

Spatiotemporal Pavlovian head-fixed reversal learning task for mice

Our world is full of uncertainty. Animals, including humans, need to behave flexibly to adjust to ever-changing environments. Reversal learning tasks have been used to assess behavioral flexibility in many species. However, there are some limitations in the traditional free-moving methodology, including (1) sessions to train the animals, (2) within-session number of trials associated with reversals, (3) factors of physical movement unrelated to the task in the maze or operant box, and (4) incompatibility with techniques, such as two-photon imaging. Therefore, to address these limitations, we established a novel spatiotemporal Pavlovian head-fixed reversal learning task for mice. Six experimentally naive adult C57BL/6J mice were used in this study. First, we trained head-fixed mice on a fixed-time schedule task. Sucrose solution was delivered every 10 s with a single drinking spout placed within the licking distance of the mice. After the mice showed anticipatory licking toward the timing of sucrose solution delivery, we began training the mice on the fixed-time schedule reversal learning task with two licking spouts. In this task, sucrose solution was delivered through one of the two drinking spouts. The rewarding spout was switched every 10 trials. Mice quickly learned to switch anticipatory licking to the rewarding side of the spouts, suggesting that they learned this head-fixed reversal learning task. Using the head-fixed experimental design, behavioral measures can be simplified by eliminating the complex behavioral sequences observed in free-moving animals. This novel head-fixed reversal learning task is a useful assay for studying the neurobiological mechanism of behavioral flexibility that is impaired in various psychopathological conditions.

Prenatal and postnatal insults differentially contribute to executive function and cognition: Utilizing touchscreen technology for perinatal brain injury research

The use of touchscreen technology to evaluate cognitive deficits in animal models has grown tremendously over the past 20 years. The touchscreen apparatus encompasses many advantages, namely a high level of standardization and translational capability. Improvements in technology in recent years have expanded the versatility of the touchscreen platform, as it is able to test distinct cognitive modalities including working memory, attention, discrimination, and association. Importantly, touchscreen technology has allowed researchers to explore deficits in multiple pillars of cognition in a wide variety of perinatal disorders with neurological sequelae across critical developmental windows. The touchscreen platform has been used to dissect deficits in antenatal CNS injury including fetal alcohol syndrome, prenatal opioid exposure, and chorioamnionitis, to peripartum insults such as term hypoxic-ischemic encephalopathy, to early postnatal insults including infantile traumatic brain injury. Most importantly, touchscreen technology offers the sensitivity necessary to detect subtle injury and treatment-induced changes in cognition and executive function beyond those offered by more rudimentary tests of rodent cognition. Understanding the pathophysiology of these disorders in rodents is paramount to addressing these deficits in human infants and dissecting the neural circuitry essential to perinatal brain injury pathophysiology and responsiveness to novel therapeutics. Touchscreen testing provides an effective, facile, sophisticated technique to accelerate the goal of improving cognitive and behavioral outcomes of children who suffer perinatal brain injury.

Understanding the effects of serotonin in the brain through its role in the gastrointestinal tract

The neuromodulatory arousal system imbues the nervous system with the flexibility and robustness required to facilitate adaptive behaviour. While there are well-understood mechanisms linking dopamine, noradrenaline and acetylcholine to distinct behavioural states, similar conclusions have not been as readily available for serotonin. Fascinatingly, despite clear links between serotonergic function and cognitive capacities as diverse as reward processing, exploration, and the psychedelic experience, over 95% of the serotonin in the body is released in the gastrointestinal tract, where it controls digestive muscle contractions (peristalsis). Here, we argue that framing neural serotonin as a rostral extension of the gastrointestinal serotonergic system dissolves much of the mystery associated with the central serotonergic system. Specifically, we outline that central serotonin activity mimics the effects of a digestion/satiety circuit mediated by hypothalamic control over descending serotonergic nuclei in the brainstem. We review commonalities and differences between these two circuits, with a focus on the heterogeneous expression of different classes of serotonin receptors in the brain. Much in the way that serotonin-induced peristalsis facilitates the work of digestion, serotonergic influences over cognition can be reframed as performing the work of cognition. Extending this analogy, we argue that the central serotonergic system allows the brain to arbitrate between different cognitive modes as a function of serotonergic tone: low activity facilitates cognitive automaticity, whereas higher activity helps to identify flexible solutions to problems, particularly if and when the initial responses fail. This perspective sheds light on otherwise disparate capacities mediated by serotonin, and also helps to understand why there are such pervasive links between serotonergic pathology and the symptoms of psychiatric disorders.

Neuromodulation and Neurophysiology on the Timescale of Learning and Decision-Making

Nervous systems evolved to effectively navigate the dynamics of the environment to achieve their goals. One framework used to study this fundamental problem arose in the study of learning and decision-making. In this framework, the demands of effective behavior require slow dynamics-on the scale of seconds to minutes-of networks of neurons. Here, we review the phenomena and mechanisms involved. Using vignettes from a few species and areas of the nervous system, we view neuromodulators as key substrates for temporal scaling of neuronal dynamics. Expected final online publication date for the Annual Review of Neuroscience, Volume 45 is July 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Parenting and the Serotonin Transporter Gene (5HTTLPR), Is There an Association? A Systematic Review of the Literature

The current systematic review examines whether there is an association between the genetic 5-HTTPLR polymorphism and parenting, and the mechanisms by which this association operates. The literature was searched in various databases such as PubMed, Scopus, and ScienceDirect. In line with our inclusion criteria, nine articles were eligible out of 22. Most of the studies analysed in this review found an association between 5HTTLPR and parenting. Four studies found a direct association between 5-HTTLPR and parenting with conflicting findings: two studies found that mothers carrying the short variant were more sensitive to their infants, while two studies found that parents carrying the S allele were less sensitive. In addition, several studies found strong interaction between genetic and environmental factors, such as childhood stress and disruptive child behaviour, quality of early care experiences, poor parenting environment, and quality of the environment. Only one study found an association between children's 5HTTLPR and parenting. Parenting can be described as a highly complex construct influenced by multiple factors, including the environment, as well as parent and child characteristics. According to the studies, maternal 5-HTTLPR polymorphism is most likely to be associated with sensitive parenting.

A mutant allele of glycoprotein M6-B (Gpm6b) facilitates behavioral flexibility but increases delay discounting

The neuronal membrane glycoprotein M6B (Gpm6b) gene encodes a membrane glycoprotein that belongs to the proteolipid protein family, and is enriched in neurons, oligodendrocytes, and subset of astrocytes in the central nervous system. GPM6B is thought to play a role in neuronal differentiation, myelination, and inactivation of the serotonin transporter via internalization. Recent human genome-wide association studies (GWAS) have implicated membrane glycoproteins (both GPM6B and GPM6A) in the regulation of traits relevant to psychiatric disorders, including neuroticism, depressed affect, and delay discounting. Mouse studies have implicated Gpm6b in sensorimotor gating and regulation of serotonergic signaling. We used CRISPR to create a mutant Glycoprotein M6B (Gpm6b) allele on a C57BL/6J mouse background. Because Gpm6b is located on the X chromosome, we focused on male Gpm6b mutant mice and their wild-type littermates (WT) in two behavioral tests that measured aspects of impulsive or flexible decision-making. We found that Gpm6b deletion caused deficits in a delay discounting task. In contrast, reward sensitivity was enhanced thereby facilitating behavioral flexibility and improving performance in the probabilistic reversal learning task. Taken together these data further delineate the role of Gpm6b in decision making behaviors that are relevant to multiple psychiatric disorders.

Serotonin neurons modulate learning rate through uncertainty

Regulating how fast to learn is critical for flexible behavior. Learning about the consequences of actions should be slow in stable environments, but accelerate when that environment changes. Recognizing stability and detecting change are difficult in environments with noisy relationships between actions and outcomes. Under these conditions, theories propose that uncertainty can be used to modulate learning rates ("meta-learning"). We show that mice behaving in a dynamic foraging task exhibit choice behavior that varied as a function of two forms of uncertainty estimated from a meta-learning model. The activity of dorsal raphe serotonin neurons tracked both types of uncertainty in the foraging task as well as in a dynamic Pavlovian task. Reversible inhibition of serotonin neurons in the foraging task reproduced changes in learning predicted by a simulated lesion of meta-learning in the model. We thus provide a quantitative link between serotonin neuron activity, learning, and decision making.

Impaired cognitive flexibility following NMDAR-GluN2B deletion is associated with altered orbitofrontal-striatal function

A common feature across neuropsychiatric disorders is inability to discontinue an action or thought once it has become detrimental. Reversal learning, a hallmark of executive control, requires plasticity within cortical, striatal and limbic circuits and is highly sensitive to disruption of N-methyl-d-aspartate receptor (NMDAR) function. In particular, selective deletion or antagonism of GluN2B containing NMDARs in cortical regions including the orbitofrontal cortex (OFC), promotes maladaptive perseveration. It remains unknown whether GluN2B functions to maintain local cortical activity necessary for reversal learning, or if it exerts a broader influence on the integration of neural activity across cortical and subcortical systems. To address this question, we utilized in vivo electrophysiology to record neuronal activity and local field potentials (LFP) in the orbitofrontal cortex and dorsal striatum (dS) of mice with deletion of GluN2B in neocortical and hippocampal principal cells while they performed touchscreen reversal learning. Reversal impairment produced by corticohippocampal GluN2B deletion was paralleled by an aberrant increase in functional connectivity between the OFC and dS. These alterations in coordination were associated with alterations in local OFC and dS firing activity. These data demonstrate highly dynamic patterns of cortical and striatal activity concomitant with reversal learning, and reveal GluN2B as a molecular mechanism underpinning the timing of these processes.

The selective 5-HT2A receptor agonist 25CN-NBOH does not affect reversal learning in mice

Psychedelic 5-hydroxytryptamine 2A receptor (5-HT2AR) agonists are showing promise in the treatment of psychiatric disorders, such as treatment-resistant depression and anxiety. Human studies suggest that enhanced cognitive flexibility may contribute to their clinical efficacy. Both improvement and impairment of cognitive flexibility has been reported with 5-HT2AR ligands, making the link between 5-HT2AR pharmacology and cognitive flexibility equivocal. We tested the selective 5-HT2AR agonist 25CN-NBOH in healthy male C57BL/6JOlaHsd mice in a touchscreen-based mouse reversal learning test. No effects were observed on acquisition of the new stimulus-reward contingency, learning errors, or perseverative responses during reversal. Our results suggest that 25CN-NBOH does not affect reversal learning in the schedule used in this study.

Acute vagus nerve stimulation enhances reversal learning in rats

Cognitive flexibility is a prefrontal cortex-dependent neurocognitive process that enables behavioral adaptation in response to changes in environmental contingencies. Electrical vagus nerve stimulation (VNS) enhances several forms of learning and neuroplasticity, but its effects on cognitive flexibility have not been evaluated. In the current study, a within-subjects design was used to assess the effects of VNS on performance in a novel visual discrimination reversal learning task conducted in touchscreen operant chambers. The task design enabled simultaneous assessment of acute VNS both on reversal learning and on recall of a well-learned discrimination problem. Acute VNS delivered in conjunction with stimuli presentation during reversal learning reliably enhanced learning of new reward contingencies. Enhancement was not observed, however, if VNS was delivered during the session but was not coincident with presentation of to-be-learned stimuli. In addition, whereas VNS delivered at 30 HZ enhanced performance, the same enhancement was not observed using 10 or 50 Hz. Together, these data show that acute VNS facilitates reversal learning and indicate that the timing and frequency of the VNS are critical for these enhancing effects. In separate rats, administration of the norepinephrine reuptake inhibitor atomoxetine also enhanced reversal learning in the same task, consistent with a noradrenergic mechanism through which VNS enhances cognitive flexibility.

Impact of specific serotonin receptor modulation on behavioral flexibility

Serotonin (5-HT) is known to play a critical role in regulation of essential neural processes, whereas more recent research highlights serotonin's modulatory effects on cognition and executive functioning. Current examinations have identified specific serotonin receptors for their direct impact on behavioral flexibility. Providing definitive evidence for the impact of specific receptor targets on behavioral flexibility is difficult, due to the range of behavioral tests used. Due to limited studies and the sheer amount of different serotonin receptor targets, beginning to bring these studies together is important for the field. Our current review of the literature aims to differentiate how modulation of specific 5-HT receptors affects behavioral flexibility. Although more studies have examined 5-HT_2A, 5-HT_2C, and 5-HT₆ receptors, it is unclear why this is the case. Above all, there are some paradoxical results pertaining to these receptor targets. There is a clear distinction between 5-HT_2A and 5-HT_2C, which conveys that these two receptor subtypes have inverse effects when compared to each other. In addition, some findings support one another, such as upregulation of 5-HT₆ receptors impairs flexibility, while blockade alleviates this impairment in both drug-induced and disease model rodent studies. Further understanding how modulatory effects of specific 5-HT receptors impact behavioral flexibility is imperative to advance the development of new therapeutics for neuropsychiatric disorders afflicted by behavioral inflexibility.

Sequential reversal learning: a new touchscreen schedule for assessing cognitive flexibility in mice

RATIONALE

The widespread deficits in cognitive flexibility observed across psychiatric disorders call for improved rodent tests to understand the biology of cognitive flexibility and development of better psychotherapeutics. Current reversal learning paradigms have a forced-choice setup that challenges the interpretation of results.

OBJECTIVES

We aimed at developing a free-choice reversal learning test, where images are presented sequentially and animals are free to move, to enable investigation of the cognitive sub-processes that occur during reversal.

METHODS

Behavior in female C57BL/6JOlaHsd mice was characterized using chronic fluoxetine as a reference compound. Additional tests were included to support the interpretation of results and exclude confounding pharmacological effects. Behaviors in vehicle-treated mice were furthermore analyzed for relatedness to deepen the understanding of parameters measured.

RESULTS

We found that exploitation of the previously rewarded image was independent of exploration and acquisition of the new reward contingency and could be differentially modulated by fluoxetine, supporting recent theories that these processes are not mutually exclusive. Specifically, fluoxetine reduced mistake rate, premature and perseverative responses, and promoted conservative strategies during reversal without affecting hit rate. These effects appeared to be most prominent during the late stage of reversal learning, where accuracy was above chance level. Analysis of behaviors in vehicle-treated mice suggested that exploitation was related to an impulsive-like deficit in response inhibition, while exploration was more related to motivation.

CONCLUSIONS

This new schedule was feasible, easy to implement, and can provide a deeper understanding of the cognitive sub-processes during reversal.

General Overview of Organic Cation Transporters in Brain

Inhibitors of Na⁺/Cl^- dependent high affinity transporters for norepinephrine (NE), serotonin (5-HT), and/or dopamine (DA) represent frequently used drugs for treatment of psychological disorders such as depression, anxiety, obsessive-compulsive disorder, attention deficit hyperactivity disorder, and addiction. These transporters remove NE, 5-HT, and/or DA after neuronal excitation from the interstitial space close to the synapses. Thereby they terminate transmission and modulate neuronal behavioral circuits. Therapeutic failure and undesired central nervous system side effects of these drugs have been partially assigned to neurotransmitter removal by low affinity transport. Cloning and functional characterization of the polyspecific organic cation transporters OCT1 (SLC22A1), OCT2 (SLC22A2), OCT3 (SLC22A3) and the plasma membrane monoamine transporter PMAT (SLC29A4) revealed that every single transporter mediates low affinity uptake of NE, 5-HT, and DA. Whereas the organic transporters are all located in the blood brain barrier, OCT2, OCT3, and PMAT are expressed in neurons or in neurons and astrocytes within brain areas that are involved in behavioral regulation. Areas of expression include the dorsal raphe, medullary motoric nuclei, hypothalamic nuclei, and/or the nucleus accumbens. Current knowledge of the transport of monoamine neurotransmitters by the organic cation transporters, their interactions with psychotropic drugs, and their locations in the brain is reported in detail. In addition, animal experiments including behavior tests in wildtype and knockout animals are reported in which the impact of OCT2, OCT3, and/or PMAT on regulation of salt intake, depression, mood control, locomotion, and/or stress effect on addiction is suggested.

Cortical Tuning is Impaired After Perceptual Experience in Primary Visual Cortex of Serotonin Transporter-Deficient Mice

Serotonin (5-hydroxytryptamine) is crucial for the proper development of neuronal circuits early in life and their refinement throughout adulthood. Its signaling is tightly regulated by the serotonin transporter (SERT), alterations of which were implicated in various neurological and psychiatric disorders. Animal models lacking a functional SERT variant display diverse phenotypes, including increased anxiety, social communication deficits, and altered cortical development. However, it remains unclear how SERT disruption affects sensory processing and experience-dependent learning in adulthood. It has been previously shown that perceptual experience leads to the development of visual familiarity-evoked theta oscillations in mouse V1. Here, we discovered that familiarity-evoked theta oscillations were longer and less stimulus specific in SERT knockout (KO) compared with wild-type (WT) mice. Interestingly, while the overall visual response properties were similar in naive mice, orientation and spatial frequency processing were significantly impaired in SERT KO compared with WT or SERT heterozygous mice following perceptual experience. Our findings shed more light on the mechanism of familiarity-evoked oscillations and highlight the importance of serotonin signaling in perceptual learning.

Serotonergic Innervations of the Orbitofrontal and Medial-prefrontal Cortices are Differentially Involved in Visual Discrimination and Reversal Learning in Rats

Cross-species studies have identified an evolutionarily conserved role for serotonin in flexible behavior including reversal learning. The aim of the current study was to investigate the contribution of serotonin within the orbitofrontal cortex (OFC) and medial prefrontal cortex (mPFC) to visual discrimination and reversal learning. Male Lister Hooded rats were trained to discriminate between a rewarded (A+) and a nonrewarded (B−) visual stimulus to receive sucrose rewards in touchscreen operant chambers. Serotonin was depleted using surgical infusions of 5,7-dihydroxytryptamine (5,7-DHT), either globally by intracebroventricular (i.c.v.) infusions or locally by microinfusions into the OFC or mPFC. Rats that received i.c.v. infusions of 5,7-DHT before initial training were significantly impaired during both visual discrimination and subsequent reversal learning during which the stimulus–reward contingencies were changed (A− vs. B+). Local serotonin depletion from the OFC impaired reversal learning without affecting initial discrimination. After mPFC depletion, rats were unimpaired during reversal learning but slower to respond at the stimuli during all the stages; the mPFC group was also slower to learn during discrimination than the OFC group. These findings extend our understanding of serotonin in cognitive flexibility by revealing differential effects within two subregions of the prefrontal cortex in visual discrimination and reversal learning.

Psychological mechanisms and functions of 5-HT and SSRIs in potential therapeutic change: Lessons from the serotonergic modulation of action selection, learning, affect, and social cognition

Uncertainty regarding which psychological mechanisms are fundamental in mediating SSRI treatment outcomes and wide-ranging variability in their efficacy has raised more questions than it has solved. Since subjective mood states are an abstract scientific construct, only available through self-report in humans, and likely involving input from multiple top-down and bottom-up signals, it has been difficult to model at what level SSRIs interact with this process. Converging translational evidence indicates a role for serotonin in modulating context-dependent parameters of action selection, affect, and social cognition; and concurrently supporting learning mechanisms, which promote adaptability and behavioural flexibility. We examine the theoretical basis, ecological validity, and interaction of these constructs and how they may or may not exert a clinical benefit. Specifically, we bridge crucial gaps between disparate lines of research, particularly findings from animal models and human clinical trials, which often seem to present irreconcilable differences. In determining how SSRIs exert their effects, our approach examines the endogenous functions of 5-HT neurons, how 5-HT manipulations affect behaviour in different contexts, and how their therapeutic effects may be exerted in humans - which may illuminate issues of translational models, hierarchical mechanisms, idiographic variables, and social cognition.

Improved visual discrimination learning in mice with partial 5-HT2B gene deletion

The neurotransmitter serotonin (5-hydroxytryptamine, 5-HT) has been linked to multiple aspects of cognition. For example, in rodents, discrimination and reversal learning are altered by experimentally induced changes in brain serotonin levels, and reduced expression of the 5-HT_2B receptor subtype in mice and humans is associated with decreased serotonergic tone and increased behavioral impulsivity. Serotonin modulates cognitive flexibility as well as fear and anxiety, but the specific contributions of 5-HT_2B receptors to these behaviors is unknown. The current study assessed mice with partial Htr2b deletion for performance on a touchscreen-based pairwise visual discrimination and reversal learning task followed by a test of cued fear learning. Male Htr2b heterozygous mice (+/-) and littermate controls (+/+) were trained to discriminate between two visual stimuli presented on a touch-sensitive screen, one which predicted delivery of a 14-mg food pellet and the other which was not rewarded. Once discrimination performance criterion was attained, the stimulus-reward contingencies were reversed. Htr2b +/- mice were faster to reach discrimination criterion than +/+ controls, and made fewer errors. Htr2b +/- mice were also slower to make responses and collect rewards. Conversely, measures of reversal learning were not different between genotypes. Pavlovian cued fear conditioning was also normal in Htr2b +/-mice. These data demonstrate a selective improvement in touchscreen-based discrimination learning in mice with partial deletion of the 5-HT_2B receptor, and provide further insight into the role of the 5-HT_2B receptor in cognition.

Effects of maternal depression and prenatal SSRI exposure on executive functions and susceptibility to household chaos in 6-year-old children: prospective cohort study

BACKGROUND

Maternal depressed mood during pregnancy may shape a child's adaptation to their environment and engagement in goal-directed behaviour such as executive functions. Whether everyday household context also alters executive functions in children with prenatal selective serotonin reuptake inhibitor (SSRI) antidepressant exposure remains to be determined.

AIMS

To examine the impact of prenatal depressed maternal mood and SSRI exposure on child executive functions and to determine whether these exposures shape a susceptibility to household chaos.

METHOD

A prospective cohort study of mothers and their children (118 mother-children dyads (47 SSRI-exposed, 71 non-exposed)) followed from the second trimester to 6 years. Regression models examined relationships between maternal depressed mood and household chaos on maternal report of child executive functions. Competitive-confirmatory regression models examined whether children were susceptible to household chaos or were positively influenced by less chaos.

RESULTS

Prenatal SSRI exposure, third-trimester maternal depressed mood and household chaos in a three-way interaction were associated with executive functions within a model of differential susceptibility. When household chaos was low, children of non-prenatally depressed mothers had better executive function than children of prenatally depressed mothers, regardless of whether the mothers were SSRI-treated. However, when household chaos was high, SSRI-exposed children of mothers who were not depressed during pregnancy had poorer executive functions at 6 years of age compared with SSRI-exposed children whose mothers were symptomatic during pregnancy.

CONCLUSIONS

The impact of household chaos depended on whether mothers were prenatally depressed and whether mothers were SSRI-treated.

Glutamatergic and Serotonergic Modulation of Rat Medial and Lateral Orbitofrontal Cortex in Visual Serial Reversal Learning

Adapting behavior to a dynamic environment requires both steadiness when the environment is stable and behavioral flexibility in response to changes. Much evidence suggests that cognitive flexibility, which can be operationalized in reversal learning tasks, is mediated by cortico-striatal circuitries, with the orbitofrontal cortex (OFC) playing a prominent role. The OFC is a functionally heterogeneous region, and we have previously reported differential roles of lateral (lOFC) and medial (mOFC) regions in a touchscreen serial visual reversal learning task for rats using pharmacological inactivation. Here, we investigated the effects of pharmacological overactivation of these regions using a glutamate transporter 1 (GLT-1) inhibitor, dihydrokainate (DHK), which increases extracellular glutamate by blocking its reuptake. We also tested the impact of antagonism of the serotonin 2A receptor (5-HT_2AR), which modulates glutamate action, in the mOFC and lOFC on the same task. Overactivation induced by DHK produced dissociable effects in the mOFC and lOFC, with more prominent effects in the mOFC, specifically improving performance in the early, perseveration phase. Intra-lOFC DHK increased the number of omitted responses without affecting errors. In contrast, blocking the 5-HT_2AR in the lOFC impaired reversal learning overall, while mOFC 5-HT_2AR blockade had no effect. These results further support dissociable roles of the rodent mOFC and lOFC in deterministic visual reversal learning and indicate that modulating glutamate transmission through blocking the GLT-1 and the 5-HT_2AR have different roles in these two structures.

This study further supports dissociable roles of specific orbitofrontal subregions, as well as glutamatergic and serotonergic transmission in these subregions, in cognitive flexibility. This knowledge will add to the understanding of specific neural mechanisms underlying inflexible behaviour across psychiatric disorders.

Perinatal exposure of rats to the HIV drug efavirenz affects medial prefrontal cortex cytoarchitecture

Efavirenz (EFV) is used for antiretroviral treatment of HIV infection, and successfully inhibits viral replication and mother-to-child transmission of HIV during pregnancy and childbirth. Unfortunately, the drug induces neuropsychiatric symptoms such as anxiety and depressed mood and potentially affects cognitive performance. EFV acts on, among others, the serotonin transporter and serotonin receptors that are expressed in the developing brain. Yet, how perinatal EFV exposure affects brain cytoarchitecture remains unclear. Here, we exposed pregnant and lactating rats to EFV, and examined in the medial prefrontal cortex (mPFC) of their adult offspring the effects of the maternal EFV exposure on cortical architecture. We observed a significant decrease in the number of cells, mainly mature neurons, in the infra/prelimbic and cingulate cortices of adult offspring. Next, we found an altered cortical cytoarchitecture characterized by a significant reduction in deep- and superficial-layer cells. This was accompanied by a sharp increase in programmed cell death, as we identified a significantly higher number of cleaved Caspase-3-positive cells. Finally, the serotonergic and dopaminergic innervation of the mPFC subdomains was increased. Thus, the perinatal exposure to EFV provoked in the mPFC of adult offspring cell death, significant changes in cytoarchitecture, and disturbances in serotonergic and dopaminergic innervation. Our results are important in the light of EFV treatment of HIV-positive pregnant women, and its effect on brain development and cognitive behavior.

The Neurobiology of Impulsive Decision-Making and Reinforcement Learning in Nonhuman Animals

Impulsive decisions are those that favor immediate over delayed rewards, involve the acceptance of undue risk or uncertainty, or fail to adapt to environmental changes. Pathological levels of impulsive decision-making have been observed in individuals with mental illness, but there may be substantial heterogeneity in the processes that drive impulsive choices. Understanding this behavioral heterogeneity may be critical for understanding associated diverseness in the neural mechanisms that give rise to impulsivity. The application of reinforcement learning algorithms in the deconstruction of impulsive decision-making phenotypes can help bridge the gap between biology and behavior and provide insights into the biobehavioral heterogeneity of impulsive choice. This chapter will review the literature on the neurobiological mechanisms of impulsive decision-making in nonhuman animals; specifically, the role of the amine neuromodulatory systems (dopamine, serotonin, norepinephrine, and acetylcholine) in impulsive decision-making and reinforcement learning processes is discussed. Ultimately, the integration of reinforcement learning algorithms with sophisticated behavioral and neuroscience techniques may be critical for advancing the understanding of the neurochemical basis of impulsive decision-making.

Neonatal Hypoxic-Ischemic Encephalopathy Yields Permanent Deficits in Learning Acquisition: A Preclinical Touchscreen Assessment

Neonatal hypoxic-ischemic encephalopathy (HIE) remains a common problem world-wide for infants born at term. The impact of HIE on long-term outcomes, especially into adulthood, is not well-described. To facilitate identification of biobehavioral biomarkers utilizing a translational platform, we sought to investigate the impact of HIE on executive function and cognitive outcomes into adulthood utilizing a murine model of HIE. HIE mice (unilateral common carotid artery occlusion to induce ischemia, followed by hypoxia with a FiO₂ of 0.08 for 45 min) and control mice were tested on discrimination and reversal touchscreen tasks (using their noses) shown to be sensitive to loss of basal ganglia or cortical function, respectively. We hypothesized that the HIE injury would result in deficits in reversal learning, revealing complex cognitive and executive functioning impairments. Following HIE, mice had a mild discrimination impairment as measured by incorrect responses but were able to learn the paradigm to similar levels as controls. During reversal, HIE mice required significantly more total trials, errors and correction trials across the paradigm. Analysis of specific stages showed that reversal impairments in HIE were driven by significant increases in all measured parameters during the late learning, striatal-mediated portion of the task. Together, these results support the concept that HIE occurring during the neonatal period results in abnormal neurodevelopment that persists into adulthood, which can impact efficient associated learning. Further, these data show that utilization of an established model of HIE coupled with touchscreen learning provides valuable information for screening therapeutic interventions that could mitigate these deficits to improve the long-term outcomes of this vulnerable population.

8-OH-DPAT Induces Compulsive-like Deficit in Spontaneous Alternation Behavior: Reversal by MDMA but Not Citalopram

Rodents exhibit natural exploratory behaviors, which can be measured by the spontaneous alternation behavior (SAB) test. Perseverance in this test induced by the 5-hydroxytryptamine 1A receptor (5-HT_1AR) agonist, 8-hydroxy-2-dipropylaminotetralin (8-OH-DPAT), resembles compulsive behaviors observed in humans and manifests as reduced alternation ratio. This study characterized 8-OH-DPAT-induced perseverance in the SAB test in C57BL/6JOlaHsd male mice by coadministration of WAY100635, citalopram and the 5-HT releasing agent, 3,4-methylenedioxymethamphetamine (MDMA), to deepen the understanding of 5-HT-dependent mechanisms. The 5-HT_1AR mechanism of 8-OH-DPAT (1.0 mg/kg, p < 0.01) on perseverance was confirmed by coadministration of the 5-HT_1AR antagonist, WAY100635 (2.0 mg/kg, p < 0.05), which attenuated the effects of 8-OH-DPAT. Such effects could also be reversed by MDMA (1.0 mg/kg, p < 0.05; 10.0 mg/kg, p < 0.001) but not citalopram. These findings confirm the importance of 5-HT in regulating perseverative behavior. Future investigations are required to determine the predictive validity of the 8-OH-DPAT-disrupted SAB test as an inducible mouse model of compulsivity.

Paradoxical effects of exercise on hippocampal plasticity and cognition in mice with a heterozygous null mutation in the serotonin transporter gene

BACKGROUND AND PURPOSE

Exercise is known to improve cognitive function, but the exact synaptic and cellular mechanisms remain unclear. We investigated the potential role of the serotonin (5-HT) transporter (SERT) in mediating these effects.

EXPERIMENTAL APPROACH

Hippocampal long-term potentiation (LTP) and neurogenesis were measured in standard-housed and exercising (wheel running) wild-type (WT) and SERT heterozygous (HET) mice. We also assessed hippocampal-dependent cognition using the Morris water maze (MWM) and a spatial pattern separation touchscreen task.

KEY RESULTS

SERT HET mice had impaired hippocampal LTP regardless of the housing conditions. Exercise increased hippocampal neurogenesis in WT mice. However, this was not observed in SERT HET animals, even though both genotypes used the running wheels to a similar extent. We also found that standard-housed SERT HET mice displayed altered cognitive flexibility than WT littermate controls in the MWM reversal learning task. However, SERT HET mice no longer exhibited this phenotype after exercise. Cognitive changes, specific to SERT HET mice in the exercise condition, were also revealed on the touchscreen spatial pattern separation task, especially when the cognitive pattern separation load was at its highest.

CONCLUSIONS AND IMPLICATIONS

Our study is the first evidence of reduced hippocampal LTP in SERT HET mice. We also show that functional SERT is required for exercise-induced increase in adult neurogenesis. Paradoxically, exercise had a negative impact on hippocampal-dependent cognitive tasks, especially in SERT HET mice. Taken together, our results suggest unique complex interactions between exercise and altered 5-HT homeostasis.

Does Experience Enhance Cognitive Flexibility? An Overview of the Evidence Provided by the Environmental Enrichment Studies

Neuroplasticity accounts for the ability of the brain to change in both structure and function in consequence of life experiences. An enhanced stimulation provided by the environment is able to create a form of brain, neural, and cognitive reserve, which allows an individual to cope better with the environmental demands, also in case of neural damage leading to cognitive decline. With its complex manipulation of several stimuli, the animal experimental paradigm of environmental enrichment (EE) appears particularly effective in modulating the ability to successfully respond to the ever-changing characteristics of the environment. According to this point, it could be very relevant to analyze the specific effects of EE on cognitive flexibility (CF). CF could be defined as the ability to effectively change behavior in response to the environmental condition changing. This review article is specifically aimed to summarize and focus on the available evidence in relation to the effects of EE on CF. To this aim, findings obtained in behavioral tasks specifically structured to investigate animal CF, such as reversal learning and attentional set-shifting tests (tasks based on the request of responding to a rewarding rule that changes, within one or multiple perceptual dimensions), are reviewed. Data provided on the structural and biochemical correlates of these findings are also enumerated. Studies realized in healthy animals and also in pathological models are considered. On the whole, the summarized evidence clearly supports the specific beneficial effects of EE on CF. However, further studies on this key topic are strictly required to gain a comprehensive and detailed framework on the mechanisms by which an enhanced stimulation could improve CF.

Anabolic-androgenic steroids and cognitive effort discounting in male rats

Anabolic-androgenic steroids (AAS) are drugs of abuse that impair behavior and cognition. In a rodent model of AAS abuse, testosterone-treated male rats expend more physical effort, by repeatedly pressing a lever for a large reward in an operant discounting task. However, since modern society prioritizes cognitive over physical effort, it is important to determine if AAS limit cognitive effort. Here we tested the effects of AAS on a novel cognitive-effort discounting task. Each operant chamber had 3 nose-pokes, opposite 2 levers and a pellet dispenser. Rats pressed a lever to illuminate 1 nose-poke; they responded in the illuminated nose-poke to receive sugar pellets. For the 'easy' lever, the light remained on for 1 s, and a correct response earned 1 pellet. For the 'hard' lever, the light duration decreased from 1 s to 0.1 s across 5 blocks of trials, and a correct response earned 4 pellets. As the duration of the nose-poke light decreased, all rats decreased their choice of the hard lever in a modest discounting curve. Task accuracy also decreased significantly across the 5 blocks of trials. However, there was no effect of testosterone on choice of the hard lever or task accuracy. Antagonism of dopamine D1 or D2 receptors had no effect on lever choice or task accuracy. However, serotonin depletion significantly decreased preference for the hard lever, and impaired task accuracy. Thus, physical effort discounting depends on dopamine activity, while cognitive effort discounting task is sensitive to serotonin. AAS impair physical effort discounting, but not cognitive effort discounting.

Assessment of mGluR5 KO mice under conditions of low stress using a rodent touchscreen apparatus reveals impaired behavioural flexibility driven by perseverative responses

Genetic and pharmacological manipulations targeting metabotropic glutamate receptor 5 (mGluR5) affect performance in behavioural paradigms that depend on cognitive flexibility. Many of these studies involved exposing mice to highly stressful conditions including electric foot shocks or water immersion and forced swimming. Because mGluR5 is also implicated in resilience and stress responses, however, apparent impairments in inhibitory learning may have been an artifact of manipulation-induced changes in affective state. To address this, we present here a characterization of cognitive flexibility in mGluR5 knockout (KO) mice conducted with a rodent touchscreen cognitive assessment apparatus in which the animals experience significantly less stress.

Our results indicate a significant reversal learning impairment relative to wild-type (WT) controls in the two-choice Visual Discrimination-Reversal (VDR) paradigm. Upon further analysis, we found that this deficit is primarily driven by a prolonged period of perseveration in the early phase of reversal. We also observed a similar perseveration phenotype in the KO mice in the Extinction (EXT) paradigm. In addition, mGluR5 KO mice show higher breakpoints in the touchscreen Progressive Ratio (PR) and altered decision making in the Effort-related Choice (ERC) tasks. Interestingly, this impairment in PR is an additional manifestation of an increased propensity to perseverate on the emission of relatively simplistic behavioural outputs.

Together, these findings suggest that under conditions of low stress, mGluR5 KO mice exhibit a pronounced perseverative phenotype that blunts cognitive flexibility.

Impaired cognitive flexibility following NMDAR-GluN2B deletion is associated with altered orbitofrontal-striatal function

A common feature across neuropsychiatric disorders is inability to discontinue an action or thought once it has become detrimental. Reversal learning, a hallmark of executive control, requires plasticity within cortical, striatal and limbic circuits and is highly sensitive to disruption of N-methyl-_D-aspartate receptor (NMDAR) function. In particular, selective deletion or antagonism of GluN2B containing NMDARs in cortical regions including the orbitofrontal cortex (OFC), promotes maladaptive perseveration. It remains unknown whether GluN2B functions to maintain local cortical activity necessary for reversal learning, or if it exerts a broader influence on the integration of neural activity across cortical and subcortical systems. To address this question, we utilized in vivo electrophysiology to record neuronal activity and local field potentials (LFP) in the orbitofrontal cortex and dorsal striatum (dS) of mice with deletion of GluN2B in neocortical and hippocampal principal cells while they performed touchscreen reversal learning. Reversal impairment produced by corticohippocampal GluN2B deletion was paralleled by an aberrant increase in functional connectivity between the OFC and dS. These alterations in coordination were associated with alterations in local OFC and dS firing activity. These data demonstrate highly dynamic patterns of cortical and striatal activity concomitant with reversal learning, and reveal GluN2B as a molecular mechanism underpinning the timing of these processes.

Translational tests involving non-reward: methodological considerations

This review is concerned with methods for assessing the processing of unrewarded responses in experimental animals and the mechanisms underlying performance of these tasks. A number of clinical populations, including Parkinson's disease, depression, compulsive disorders, and schizophrenia demonstrate either abnormal processing or learning from non-rewarded responses in laboratory-based reinforcement learning tasks. These effects are hypothesized to result from disturbances in modulatory neurotransmitter systems, including dopamine and serotonin. Parallel work in experimental animals has revealed consistent behavioral patterns associated with non-reward and, consistent with the human literature, modulatory roles for specific neurotransmitters. Classical tests involving an important reward omission component include appetitive extinction, ratio schedules of responding, reversal learning, and delay and probability discounting procedures. In addition, innovative behavioral tests have recently been developed leverage probabilistic feedback to specifically assay accommodation of, and learning from, non-rewarded responses. These procedures will be described and reviewed with discussion of the behavioral and neural determinants of performance. A final section focusses specifically on the benefits of trial-by-trial analysis of responding during such tasks, and the implications of such analyses for the translation of findings to clinical studies.

Prefrontal cortex executive processes affected by stress in health and disease

Prefrontal cortical executive functions comprise a number of cognitive capabilities necessary for goal directed behavior and adaptation to a changing environment. Executive dysfunction that leads to maladaptive behavior and is a symptom of psychiatric pathology can be instigated or exacerbated by stress. In this review we survey research addressing the impact of stress on executive function, with specific focus on working memory, attention, response inhibition, and cognitive flexibility. We then consider the neurochemical pathways underlying these cognitive capabilities and, where known, how stress alters them. Finally, we review work exploring potential pharmacological and non-pharmacological approaches that can ameliorate deficits in executive function. Both preclinical and clinical literature indicates that chronic stress negatively affects executive function. Although some of the circuitry and neurochemical processes underlying executive function have been characterized, a great deal is still unknown regarding how stress affects these processes. Additional work focusing on this question is needed in order to make progress on developing interventions that ameliorate executive dysfunction.

Extended Erythropoietin Treatment Prevents Chronic Executive Functional and Microstructural Deficits Following Early Severe Traumatic Brain Injury in Rats

Survivors of infant traumatic brain injury (TBI) are prone to chronic neurological deficits that impose lifelong individual and societal burdens. Translation of novel interventions to clinical trials is hampered in part by the lack of truly representative preclinical tests of cognition and corresponding biomarkers of functional outcomes. To address this gap, the ability of a high-dose, extended, post-injury regimen of erythropoietin (EPO, 3000U/kg/dose × 6d) to prevent chronic cognitive and imaging deficits was tested in a postnatal day 12 (P12) controlled-cortical impact (CCI) model in rats, using touchscreen operant chambers and regional analysis of diffusion tensor imaging (DTI). Results indicate that EPO prevents functional injury and MRI injury after infant TBI. Specifically, subacute DTI at P30 revealed widespread microstructural damage that is prevented by EPO. Assessment of visual discrimination on a touchscreen operant chamber platform demonstrated that all groups can perform visual discrimination. However, CCI rats treated with vehicle failed to pass reversal learning, and perseverated, in contrast to sham and CCI-EPO rats. Chronic DTI at P90 showed EPO treatment prevented contralateral white matter and ipsilateral lateral prefrontal cortex damage. This DTI improvement correlated with cognitive performance. Taken together, extended EPO treatment restores executive function and prevents microstructural brain abnormalities in adult rats with cognitive deficits in a translational preclinical model of infant TBI. Sophisticated testing with touchscreen operant chambers and regional DTI analyses may expedite translation and effective yield of interventions from preclinical studies to clinical trials. Collectively, these data support the use of EPO in clinical trials for human infants with TBI.

Effects of HIV-1 TAT protein and methamphetamine exposure on visual discrimination and executive function in mice

Mild neurocognitive impairments are common in people with human immunodeficiency virus (HIV) infection. HIV-encoded proteins, such as trans-activator of transcription (TAT), contribute to neuropathology and cognitive function in medicated subjects. The combination of TAT and comorbid methamphetamine use may further impair neurocognitive function in HIV-positive individuals by affecting dopaminergic systems in the brain. The current study examined the effects of TAT protein expression and methamphetamine exposure on cognitive function and dopamine systems in mice. Transgenic mice with inducible brain expression of the TAT protein were exposed to a binge methamphetamine regimen. TAT expression was induced via a doxycycline-containing diet during the final stage of the regimen and maintained throughout cognitive testing. Learning and executive function were assessed using an operant visual discrimination protocol, with a strategy switch and reversal. TAT expression and methamphetamine exposure improved visual discrimination learning. Combined TAT expression and methamphetamine exposure increased perseverative errors during reversal learning. TAT expression altered reversal learning by improving early stage, but impairing late stage, learning. TAT expression was also associated with an increase in dopamine transporter expression in the caudate putamen. These results highlight that TAT expression and methamphetamine exposure likely affect a range of selective cognitive processes, with some potentially improving function under certain conditions.