Serotonergic projections to the orbitofrontal and medial prefrontal cortices differentially modulate waiting for future rewards

Orbitostriatal encoding of reward delayed gratification and impulsivity in chronic pain

Central robust network functional rearrangement is a characteristic of several neurological conditions, including chronic pain. Preclinical and clinical studies have shown the importance of pain-induced dysfunction in both orbitofrontal cortex (OFC) and nucleus accumbens (NAc) brain regions for the emergence of cognitive deficits. Outcome information processing recruits the orbitostriatal circuitry, a pivotal pathway regarding context-dependent reward value encoding. The current literature reveals the existence of structural and functional changes in the orbitostriatal crosstalk in chronic pain conditions, which have emerged as a possible underlying cause for reward and time discrimination impairments observed in individuals affected by such disturbances. However, more comprehensive investigations are needed to elucidate the underlying disturbances that underpin disease development. In this review article, we aim to provide a comprehensive view of the orbitostriatal mechanisms underlying time-reward dependent behaviors, and integrate previous findings on local and network malplasticity under the framework of the chronic pain sphere.

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

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

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

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

Poor Decision Making and Sociability Impairment Following Central Serotonin Reduction in Inducible TPH2-Knockdown Rats

Serotonin is an essential neuromodulator for mental health and animals' socio-cognitive abilities. However, we previously found that a constitutive depletion of central serotonin did not impair rat cognitive abilities in stand-alone tests. Here, we investigated how a mild and acute decrease in brain serotonin would affect rats' cognitive abilities. Using a novel rat model of inducible serotonin depletion via the genetic knockdown of tryptophan hydroxylase 2 (TPH2), we achieved a 20% decrease in serotonin levels in the hypothalamus after three weeks of non-invasive oral doxycycline administration. Decision making, cognitive flexibility, and social recognition memory were tested in low-serotonin (Tph2-kd) and control rats. Our results showed that the Tph2-kd rats were more prone to choose disadvantageously in the long term (poor decision making) in the Rat Gambling Task and that only the low-serotonin poor decision makers were more sensitive to probabilistic discounting and had poorer social recognition memory than other low-serotonin and control individuals. Flexibility was unaffected by the acute brain serotonin reduction. Poor social recognition memory was the most central characteristic of the behavioral network of low-serotonin poor decision makers, suggesting a key role of social recognition in the expression of their profile. The acute decrease in brain serotonin appeared to specifically amplify the cognitive impairments of the subgroup of individuals also identified as poor decision makers in the population. This study highlights the great opportunity the Tph2-kd rat model offers to study inter-individual susceptibilities to develop cognitive impairment following mild variations of brain serotonin in otherwise healthy individuals. These transgenic and differential approaches together could be critical for the identification of translational markers and vulnerabilities in the development of mental disorders.

Aggression modulator: Understanding the multifaceted role of the dorsal raphe nucleus

Aggressive behavior is instinctively driven behavior that helps animals to survive and reproduce and is closely related to multiple behavioral and physiological processes. The dorsal raphe nucleus (DRN) is an evolutionarily conserved midbrain structure that regulates aggressive behavior by integrating diverse brain inputs. The DRN consists predominantly of serotonergic (5-HT:5-hydroxytryptamine) neurons and decreased 5-HT activity was classically thought to increase aggression. However, recent studies challenge this 5-HT deficiency model, revealing a more complex role for the DRN 5-HT system in aggression. Furthermore, emerging evidence has shown that non-5-HT populations in the DRN and specific neural circuits contribute to the escalation of aggressive behavior. This review argues that the DRN serves as a multifaceted modulator of aggression, acting not only via 5-HT but also via other neurotransmitters and neural pathways, as well as different subsets of 5-HT neurons. In addition, we discuss the contribution of DRN neurons in the behavioral and physiological aspects implicated in aggressive behavior, such as arousal, reward, and impulsivity, to further our understanding of DRN-mediated aggression modulation.

Maturation of cortical input to dorsal raphe nucleus increases behavioral persistence in mice

The ability to persist toward a desired objective is a fundamental aspect of behavioral control whose impairment is implicated in several behavioral disorders. One of the prominent features of behavioral persistence is that its maturation occurs relatively late in development. This is presumed to echo the developmental time course of a corresponding circuit within late-maturing parts of the brain, such as the prefrontal cortex, but the specific identity of the responsible circuits is unknown. Here, we used a genetic approach to describe the maturation of the projection from layer 5 neurons of the neocortex to the dorsal raphe nucleus in mice. Using optogenetic-assisted circuit mapping, we show that this projection undergoes a dramatic increase in synaptic potency between postnatal weeks 3 and 8, corresponding to the transition from juvenile to adult. We then show that this period corresponds to an increase in the behavioral persistence that mice exhibit in a foraging task. Finally, we used a genetic targeting strategy that primarily affected neurons in the medial prefrontal cortex, to selectively ablate this pathway in adulthood and show that mice revert to a behavioral phenotype similar to juveniles. These results suggest that frontal cortical to dorsal raphe input is a critical anatomical and functional substrate of the development and manifestation of behavioral persistence.

The serotonergic neurons derived from rhombomere 2 are localized in the median raphe and project to the dorsal pallium in zebrafish

The serotonergic neurons in the raphe nucleus are implicated in various cognitive functions such as learning and emotion. In vertebrates, the raphe nucleus is divided into the dorsal raphe and the median raphe. In contrast to the abundance of knowledge on the functions of the dorsal raphe, the roles of the serotonergic neurons in the median raphe are relatively unknown. The studies using zebrafish revealed that the median raphe serotonergic neurons receive input from the two distinct pathways from the habenula and the IPN. The use of zebrafish may reveal the function of the Hb-IPN-median raphe pathway. To clarify the functions of the median raphe serotonergic neurons, it is necessary to distinguish them from those in the dorsal raphe. Most median raphe serotonergic neurons originate from rhombomere 2 in mice, and we generated the transgenic zebrafish which can label the serotonergic neurons derived from rhombomere 2. In this study, we found the serotonergic neurons derived from rhombomere 2 are localized in the median raphe and project axons to the rostral dorsal pallium in zebrafish. This study suggests that this transgenic system has the potential to specifically reveal the function and information processing of the Hb-IPN-raphe-telencephalon circuit in learning.

Long-Term Impact of Early Life Stress on Serotonin Connectivity

BACKGROUND

Chronic childhood stress is a prominent risk factor for developing affective disorders, yet mechanisms underlying this association remain unclear. Maintenance of optimal serotonin (5-HT) levels during early postnatal development is critical for the maturation of brain circuits. Understanding the long-lasting effects of early life stress (ELS) on serotonin-modulated brain connectivity is crucial to develop treatments for affective disorders arising from childhood stress.

METHODS

Using a mouse model of chronic developmental stress, we determined the long-lasting consequences of ELS on 5-HT circuits and behavior in females and males. Using FosTRAP mice, we cross-correlated regional c-Fos density to determine brain-wide functional connectivity of the raphe nucleus. We next performed in vivo fiber photometry to establish ELS-induced deficits in 5-HT dynamics and optogenetics to stimulate 5-HT release to improve behavior.

RESULTS

Adult female and male mice exposed to ELS showed heightened anxiety-like behavior. ELS further enhanced susceptibility to acute stress by disrupting the brain-wide functional connectivity of the raphe nucleus and the activity of 5-HT neuron population, in conjunction with increased orbitofrontal cortex (OFC) activity and disrupted 5-HT release in medial OFC. Optogenetic stimulation of 5-HT terminals in the medial OFC elicited an anxiolytic effect in ELS mice in a sex-dependent manner.

CONCLUSIONS

These findings suggest a significant disruption in 5-HT-modulated brain connectivity in response to ELS, with implications for sex-dependent vulnerability. The anxiolytic effect of the raphe-medial OFC circuit stimulation has potential implications for developing targeted stimulation-based treatments for affective disorders that arise from early life adversities.

Transcranial direct current stimulation (tDCS) over the orbitofrontal cortex reduces delay discounting

Delay discounting (DD) is a quantifiable psychological phenomenon that regulates decision-making. Nevertheless, the neural substrates of DD and its relationship with other cognitive domains are not well understood. The orbitofrontal cortex (OFC) is a potential candidate for supporting the expression of DD, but due to its wide involvement in several psychological functions and neural networks, its central role remains elusive. In this study, healthy subjects underwent transcranial direct current stimulation (tDCS) while performing an intertemporal choice task for the quantification of DD and a working memory task. To selectively engage the OFC, two electrode configurations have been tested, namely, anodal Fp1-cathodal Fp2 and cathodal Fp1-anodal Fp2. Our results show that stimulation of the OFC reduces DD, independently from electrode configuration. In addition, no relationship was found between DD measures and either working memory performance or baseline impulsivity assessed through established tests. Our work will direct future investigations aimed at unveiling the specific neural mechanisms underlying the involvement of the OFC in DD, and at testing the efficacy of OFC tDCS in reducing DD in psychological conditions where this phenomenon has been strongly implicated, such as addiction and eating disorders.

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.

Medial Prefrontal Cortex Serotonin Input Regulates Cognitive Flexibility in Mice

The medial prefrontal cortex (mPFC) regulates cognitive flexibility and emotional behavior. Neurons that release serotonin project to the mPFC, and serotonergic drugs influence emotion and cognition. Yet, the specific roles of endogenous serotonin release in the mPFC on neurophysiology and behavior are unknown. We show that axonal serotonin release in the mPFC directly inhibits the major mPFC output neurons. In serotonergic neurons projecting from the dorsal raphe to the mPFC, we find endogenous activity signatures pre-reward retrieval and at reward retrieval during a cognitive flexibility task. In vivo optogenetic activation of this pathway during pre-reward retrieval selectively improved extradimensional rule shift performance while inhibition impaired it, demonstrating sufficiency and necessity for mPFC serotonin release in cognitive flexibility. Locomotor activity and anxiety-like behavior were not affected by either optogenetic manipulation. Collectively, our data reveal a powerful and specific modulatory role of endogenous serotonin release from dorsal raphe-to-mPFC projecting neurons in cognitive flexibility.

Elevated levels of serotonin 5-HT2A receptors in the orbitofrontal cortex of antisocial individuals

Antisocial behavior (ASB) is characterized by frequent violations of the rights and properties of others, as well as aggressive conduct. While ample evidence points to a critical role of serotonin in the emotional modulation of social responses, the implication of this neurotransmitter in ASB is unclear. Here, we performed the first-ever postmortem analysis of serotonergic markers in the orbitofrontal cortex (OFC) of male subjects with ASB (n = 9). We focused on this brain region, given its well-recognized role in social response and ASB pathophysiology. Given that all individuals also had a substance use disorder (SUD) diagnosis, two age-matched control groups were used: SUD only and unaffected controls. Tissues were processed for immunoblotting analyses on eight key serotonergic targets: tryptophan hydroxylase 2 (TPH2), the rate-limiting enzyme of brain serotonin synthesis; serotonin transporter (SERT), the primary carrier for serotonin uptake; monoamine oxidase A (MAOA), the primary enzyme for serotonin catabolism; and five serotonin receptors previously shown to influence social behavior: 5-HT1A, 5-HT1B, 5-HT2A, 5-HT2C, and 5-HT4. Our analyses documented a significant increase in 5-HT2A receptor levels in the ASB + SUD group compared to SUD-only controls. Furthermore, TPH2 levels were significantly reduced in the SUD group (including SUD only and ASB + SUD) compared to unaffected controls. No difference was detected in the expression of any other serotonergic target. These results are in keeping with previous evidence showing high 5-HT2A receptor binding in the OFC of pathologically aggressive individuals and point to this molecule as a potential target for ASB treatment.

Neural Correlates of Delay Discounting in the Light of Brain Imaging and Non-Invasive Brain Stimulation: What We Know and What Is Missed

In decision making, the subjective value of a reward declines with the delay to its receipt, describing a hyperbolic function. Although this phenomenon, referred to as delay discounting (DD), has been extensively characterized and reported in many animal species, still, little is known about the neuronal processes that support it. Here, after drawing a comprehensive portrait, we consider the latest neuroimaging and lesion studies, the outcomes of which often appear contradictory among comparable experimental settings. In the second part of the manuscript, we focus on a more recent and effective route of investigation: non-invasive brain stimulation (NIBS). We provide a comprehensive review of the available studies that applied transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS) to affect subjects’ performance in DD tasks. The aim of our survey is not only to highlight the superiority of NIBS in investigating DD, but also to suggest targets for future experimental studies, since the regions considered in these studies represent only a fraction of the possible ones. In particular, we argue that, based on the available neurophysiological evidence from lesion and brain imaging studies, a very promising and underrepresented region for future neuromodulation studies investigating DD is the orbitofrontal cortex.

Serotonergic and dopaminergic control of impulsivity in gambling disorder

Gambling disorder (GD) is major public health issue. The disorder is often characterized by elevated impulsivity with evidence from analogous substance use disorders underlining prominent roles of brain monoamines in addiction susceptibility and outcome. Critically, GD allows the study of addiction mechanisms without the confounder of the effects of chronic substances. Here, we assessed the roles of striatal dopamine transporter binding and extrastriatal serotonin transporter binding in GD as a function of impulsivity using [¹²³ I]FP-CIT SPECT imaging in 20 older adults with GD (DSM-5 criteria; mean age 64 years) and 40 non-GD age- and sex-matched controls. We focused on GD in older individuals because there are prominent age-related changes in neurotransmitter function and because there are no reported neuroimaging studies of GD in older adults. Volume-of-interest-based and voxelwise analyses were performed. GD patients scored clearly higher on impulsivity and had higher tracer binding in the ventromedial prefrontal cortex than controls (p < 0.001), likely reflecting serotonin transporter activity. The binding in the medial prefrontal cortex positively correlated with impulsivity over the whole sample (r = 0.62, p < 0.001) as well as separately in GD patients (r = 0.46, p = 0.04) and controls (r = 0.52, p < 0.001). Striatal tracer binding, reflecting dopamine transporter activity was also positively correlated with impulsivity but showed no group differences. These findings highlight the role of prefrontal serotonergic function in GD and impulsivity. They identify cerebral coordinates of a potential target for neuromodulation for both GD and high impulsivity, a core phenotypic dimensional cognitive marker in addictions.

Raphe serotonin projections dynamically regulate feeding behavior through targeting inhibitory circuits from rostral zona incerta to paraventricular thalamus

OBJECTIVE

Rostral zona incerta (ZIR) evokes feeding by sending GABA transmission to paraventricular thalamus (PVT). Although central serotonin (5-HT) signaling is known to play critical roles in the regulation of food intake and eating disorders, it remains unknown whether raphe 5-HT neurons functionally innervate ZIR-PVT neural pathway for feeding control. Here, we sought to reveal how raphe 5-HT signaling regulates both ZIR and PVT for feeding control.

METHODS

We used retrograde neural tracers to map 5-HT projections in Sert-Cre mice and slice electrophysiology to examine the mechanism by which 5-HT modulates ZIR GABA neurons. We also used optogenetics to test the effects of raphe-ZIR and raphe-PVT 5-HT projections on feeding motivation and food intake in mice regularly fed, 24 h fasted, and with intermittent high-fat high-sugar (HFHS) diet. In addition, we applied RNAscope in situ hybridization to identify 5-HT receptor subtype mRNA in ZIR.

RESULTS

We show raphe 5-HT neurons sent projections to both ZIR and PVT with partial collateral axons. Photostimulation of 5-HT projections inhibited ZIR but excited PVT neurons to decrease motivated food consumption. However, both acute food deprivation and intermittent HFHS diet downregulated 5-HT inhibition on ZIR GABA neurons, abolishing the inhibitory regulation of raphe-ZIR 5-HT projections on feeding motivation and food intake. Furthermore, we found high-level 5-HT1a and 5-HT2c as well as low-level 5-HT7 mRNA expression in ZIR. Intermittent HFHS diet increased 5-HT7 but not 5-HT1a or 5-HT2c mRNA levels in the ZIR.

CONCLUSIONS

Our results reveal that raphe-ZIR 5-HT projections dynamically regulate ZIR GABA neurons for feeding control, supporting that a dynamic fluctuation of ZIR 5-HT inhibition authorizes daily food intake but a sustained change of ZIR 5-HT signaling leads to overeating induced by HFHS diet.

How do real animals account for the passage of time during associative learning?

Animals routinely learn to associate environmental stimuli and self-generated actions with their outcomes such as rewards. One of the most popular theoretical models of such learning is the reinforcement learning (RL) framework. The simplest form of RL, model-free RL, is widely applied to explain animal behavior in numerous neuroscientific studies. More complex RL versions assume that animals build and store an explicit model of the world in memory. To apply these approaches to explain animal behavior, typical neuroscientific RL models make implicit assumptions about how real animals represent the passage of time. In this perspective, I explicitly list these assumptions and show that they have several problematic implications. I hope that the explicit discussion of these problems encourages the field to seriously examine the assumptions underlying timing and reinforcement learning. (PsycInfo Database Record (c) 2022 APA, all rights reserved).

Importin α3 (KPNA3) Deficiency Augments Effortful Reward-Seeking Behavior in Mice

Importin α3 (Gene: Kpna3, the ortholog of human Importin α4) is a member of the importin α family and participates in nucleocytoplasmic transport by forming trimeric complexes between cargo proteins and importin β1. Evidence from human studies has indicated that single nucleotide polymorphisms (SNP) in the KPNA3 gene are associated with the occurrence of several psychiatric disorders accompanied by abnormal reward-related behavior, including schizophrenia, major depression, and substance addiction. However, the precise roles of importin α3 in controlling reward processing and motivation are still unclear. In this study, we evaluated the behavioral effects of Kpna3 knockout (KO) in mice on performance in touchscreen operant chamber-based tasks evaluating simple (fixed-ratio) and effortful (progressive-ratio) reward-seeking behaviors. While Kpna3 KO mice showed no significant differences in operant reward learning on a fixed-ratio schedule, they demonstrated significantly increased motivation (increased break point) to instrumentally respond for sucrose on a progressive-ratio schedule. We additionally measured the number of c-Fos-positive cells, a marker of neural activity, in 20 regions of the brain and identified a network of brain regions based on their interregional correlation coefficients. Network and graph-theoretic analyses suggested that Kpna3 deficiency enhanced overall interregional functional connectivity. These findings suggest the importance of Kpna3 in motivational control and indicate that Kpna3 KO mice may be an attractive line for modeling motivational abnormalities associated with several psychiatric disorders.

A neurobiological perspective on social influence: Serotonin and social adaptation

Humans are inherently social beings. Being suggestible to each other's expectations enables pro-social skills that are crucial for social learning and adaptation. Despite its high relevance for psychiatry, the neurobiological mechanisms underlying social adaptation are still not well understood. This review therefore provides a conceptual framework covering various distinct mechanisms underlying social adaptation and explores the neuropharmacology - in particular the role of the serotonin (5-HT) system - modulating these mechanisms. This article therefore reviews empirical results on social influence processing and reconciles them with recent findings from psychedelic research on social processing to elucidate neurobiological and neuropharmacological underpinnings of social adaptation. Various computational, neurobiological, and neurochemical processes are involved in distinct mechanisms underlying social adaptation such as the multisensory process of social information integration that is crucial for the forming of self-representation and representations of social norms. This is again associated with self- and other-perception during social interactions as well as value-based decision making that guides our behaviour in daily interactions. We highlight the critical role of 5-HT in these processes and suggest that 5-HT can facilitate social learning and may represent an important target for treating psychiatric disorders characterized by impairments in social functioning. This framework also has important implications for psychedelic-assisted therapy as well as for the development of novel treatment approaches and future research directions.

A Role for Serotonin in Modulating Opposing Drive and Brake Circuits of Impulsivity

Impulsivity generally refers to a deficit in inhibition, with a focus on understanding the neural circuits which constitute the “brake” on actions and gratification. It is likely that increased impulsivity can arise not only from reduced inhibition, but also from a heightened or exaggerated excitatory “drive.” For example, an action which has more vigor, or is fueled by either increased incentive salience or a stronger action-outcome association, may be harder to inhibit. From this perspective, this review focuses on impulse control as a competition over behavioral output between an initially learned response-reward outcome association, and a subsequently acquired opposing inhibitory association. Our goal is to present a synthesis of research from humans and animal models that supports this dual-systems approach to understanding the behavioral and neural substrates that contribute to impulsivity, with a focus on the neuromodulatory role of serotonin. We review evidence for the role of serotonin signaling in mediating the balance of the “drive” and “brake” circuits. Additionally, we consider parallels of these competing instrumental systems in impulsivity within classical conditioning processes (e.g., extinction) in order to point us to potential behavioral and neural mechanisms that may modulate the competing instrumental associations. Finally, we consider how the balance of these competing associations might contribute to, or be extracted from, our experimental assessments of impulsivity. A careful understanding of the underlying behavioral and circuit level contributions to impulsivity is important for understanding the pathogenesis of increased impulsivity present in a number of psychiatric disorders. Pathological levels of impulsivity in such disorders are likely subserved by deficits in the balance of motivational and inhibitory processes.

Altered medial prefrontal cortex and dorsal raphé activity predict genotype and correlate with abnormal learning behavior in a mouse model of autism-associated 2p16.3 deletion

2p16.3 deletion, involving NEUREXIN1 (NRXN1) heterozygous deletion, substantially increases the risk of developing autism and other neurodevelopmental disorders. We have a poor understanding of how NRXN1 heterozygosity impacts on brain function and cognition to increase the risk of developing the disorder. Here we characterize the impact of Nrxn1α heterozygosity on cerebral metabolism, in mice, using ¹⁴C‐2‐deoxyglucose imaging. We also assess performance in an olfactory‐based discrimination and reversal learning (OB‐DaRL) task and locomotor activity. We use decision tree classifiers to test the predictive relationship between cerebral metabolism and Nrxn1α genotype. Our data show that Nrxn1α heterozygosity induces prefrontal cortex (medial prelimbic cortex, mPrL) hypometabolism and a contrasting dorsal raphé nucleus (DRN) hypermetabolism. Metabolism in these regions allows for the predictive classification of Nrxn1α genotype. Consistent with reduced mPrL glucose utilization, prefrontal cortex insulin receptor signaling is decreased in Nrxn1α^+/− mice. Behaviorally, Nrxn1α^+/− mice show enhanced learning of a novel discrimination, impaired reversal learning and an increased latency to make correct choices. In addition, male Nrxn1α^+/− mice show hyperlocomotor activity. Correlative analysis suggests that mPrL hypometabolism contributes to the enhanced novel odor discrimination seen in Nrxn1α^+/− mice, while DRN hypermetabolism contributes to their increased latency in making correct choices. The data show that Nrxn1α heterozygosity impacts on prefrontal cortex and serotonin system function, which contribute to the cognitive alterations seen in these animals. The data suggest that Nrxn1α^+/− mice provide a translational model for the cognitive and behavioral alterations seen in autism and other neurodevelopmental disorders associated with 2p16.3 deletion.

5-HT2C receptor perturbation has bidirectional influence over instrumental vigour and restraint

The serotonin (5-HT) system, particularly the 5-HT2C receptor, has consistently been implicated in behavioural control. However, while some studies have focused on the role 5-HT2C receptors play in regulating motivation to work for reward, others have highlighted its importance in response restraint. To date, it is unclear how 5-HT transmission at this receptor regulates the balance of response invigoration and restraint in anticipation of future reward. In addition, it remains to be established how 5-HT2C receptors gate the influence of internal versus cue-driven processes over reward-guided actions. To elucidate these issues, we investigated the effects of administering the 5-HT2C receptor antagonist SB242084, both systemically and directly into the nucleus accumbens core (NAcC), in rats performing a Go/No-Go task for small or large rewards. The results were compared to the administration of d-amphetamine into the NAcC, which has previously been shown to promote behavioural activation. Systemic perturbation of 5-HT2C receptors-but crucially not intra-NAcC infusions-consistently boosted rats' performance and instrumental vigour on Go trials when they were required to act. Concomitantly, systemic administration also reduced their ability to withhold responding for rewards on No-Go trials, particularly late in the holding period. Notably, these effects were often apparent only when the reward on offer was small. By contrast, inducing a hyperdopaminergic state in the NAcC with d-amphetamine strongly impaired response restraint on No-Go trials both early and late in the holding period, as well as speeding action initiation. Together, these findings suggest that 5-HT2C receptor transmission, outside the NAcC, shapes the vigour of ongoing goal-directed action as well as the likelihood of responding as a function of expected reward.

Local and distributed cortical markers of effort expenditure during sustained goal pursuit

The adaptive adjustment of behavior in pursuit of desired goals is critical for survival. To accomplish this complex feat, individuals must weigh the potential benefits of a given action against time, energy, and resource costs. Here, we examine brain responses associated with willingness to exert physical effort during the sustained pursuit of desired goals. Our analyses reveal a distributed pattern of brain activity in aspects of ventral medial prefrontal cortex that tracks with trial-level variability in effort expenditure. Indicating the brain represents echoes of effort at the point of feedback, whole-brain searchlights identified signals reflecting past effort expenditure in medial and lateral prefrontal cortices, encompassing broad swaths of frontoparietal and dorsal attention networks. These data have important implications for our understanding of how the brain's valuation mechanisms contend with the complexity of real-world dynamic environments with relevance for the study of behavior across health and disease.

Increased serotonin prevents compulsion in addiction

[Figure: see text].

Neurochemical substrates linked to impulsive and compulsive phenotypes in addiction: A preclinical perspective

Drug compulsion manifests in some but not all individuals and implicates multifaceted processes including failures in top-down cognitive control as drivers for the hazardous pursuit of drug use in some individuals. As a closely related construct, impulsivity encompasses rash or risky behaviour without foresight and underlies most forms of drug taking behaviour, including drug use during adverse emotional states (i.e., negative urgency). While impulsive behavioural dimensions emerge from drug-induced brain plasticity, burgeoning evidence suggests that impulsivity also predates the emergence of compulsive drug use. Although the neural substrates underlying the apparently causal relationship between trait impulsivity and drug compulsion are poorly understood, significant advances have come from the interrogation of defined limbic cortico-striatal circuits involved in motivated behaviour and response inhibition, together with chemical neuromodulatory influences from the ascending neurotransmitter systems. We review what is presently known about the neurochemical mediation of impulsivity, in its various forms, and ask whether commonalities exist in the neurochemistry of compulsive drug-motivated behaviours that might explain individual risk for addiction.

Multiple facets of serotonergic modulation

The serotonergic system of the central nervous system (CNS) has been implicated in a broad range of physiological functions and behaviors, such as cognition, mood, social interaction, sexual behavior, feeding behavior, sleep-wake cycle and thermoregulation. Serotonin (5-hydroxytryptamine, 5-HT) establishes a plethora of interactions with neurochemical systems in the CNS via its numerous 5-HT receptors and autoreceptors. The facets of this control are multiple if we consider the molecular actors playing a role in the autoregulation of 5-HT neuron activity including the 5-HT_1A, 5-HT_1B, 5-HT_1D, 5-HT_2B, 5-HT₇ receptors as well as the serotonin transporter. Moreover, extrinsic loops involving other neurotransmitters giving the other 5-HT receptors the possibility to impact 5-HT neuron activity. Grasping the complexity of these interactions is essential for the development of a variety of therapeutic strategies for cognitive defects and mood disorders. Presently we can illustrate the plurality of the mechanisms and only conceive that these 5-HT controls are likely not uniform in terms of regional and neuronal distribution. Our understanding of the specific expression patterns of these receptors on specific circuits and neuronal populations are progressing and will expand our comprehension of the function and interaction of these receptors with other chemical systems. Thus, the development of new approaches profiling the expression of 5-HT receptors and autoreceptors should reveal additional facets of the 5-HT controls of neurochemical systems in the CNS.