Neural mechanisms of the nucleus accumbens circuit in reward and aversive learning

Three paradoxes related to the mode of action of pramipexole: The path from D2/D3 dopamine receptor stimulation to modification of dopamine-modulated functions

Pramipexole, a D2/D3 dopamine receptor agonist, is used to treat the motor symptoms of Parkinson's disease, caused by degeneration of the dopaminergic nigrostriatal pathway. There are three paradoxes associated with its mode of action. Firstly, stimulation of D2/D3 receptors leads to neuronal inhibition, although pramipexole does not inhibit but promotes some dopamine-modulated functions, such as locomotion and reinforcement. Secondly, another dopamine-modulated function, arousal, is not promoted but inhibited by pramipexole, leading to sedation. Thirdly, pramipexole-evoked sedation is associated with an increase in pupil diameter, although sedation is expected to cause pupil constriction. To resolve these paradoxes, the path from stimulation of D2/D3 receptors to the modification of dopamine-modulated functions has been tracked. The functions considered are modulated by midbrain dopaminergic nuclei: locomotion - substantia nigra pars compacta (SNc), reinforcement/motivation - ventral tegmental area (VTA), sympathetic activity (as reflected in pupil function) - VTA; arousal - ventral periaqueductal grey (vPAG), with contributions from VTA and SNc. The application of genetics-based molecular techniques (optogenetics and chemogenetics) has enabled tracing the chains of neurones from the dopaminergic nuclei to their final targets executing the functions. The functional neuronal circuits linked to the D2/D3 receptors in the dorsal and ventral striata, stimulated by inputs from SNc and VTA, respectively, may explain how neuronal inhibition induced by pramipexole is translated into the promotion of locomotion, reinforcement/motivation and sympathetic activity. As the vPAG may increase arousal mainly by stimulating cortical D1 dopamine receptors, pramipexole would stimulate only presynaptic D2/D3 receptors on vPAG neurones, curtailing their activity and leading to sedation.

Mu-opioid receptor knockout on Foxp2-expressing neurons reduces aversion-resistant alcohol drinking

Mu-opioid receptors (MORs) in the amygdala and striatum are important in addictive and rewarding behaviors. The transcription factor Foxp2 is a genetic marker of intercalated (ITC) cells in the amygdala and a subset of striatal medium spiny neurons (MSNs), both of which express MORs in wild-type mice and are neuronal subpopulations of potential relevance to alcohol-drinking behaviors. For the current series of studies, we characterized the behavior of mice with genetic deletion of the MOR gene Oprm1 in Foxp2-expressing neurons (Foxp2-Cre/Oprm1fl/fl). Male and female Foxp2-Cre/Oprm1fl/fl mice were generated and heterozygous Cre+ (knockout) and homozygous Cre- (control) animals were tested for aversion-resistant alcohol consumption using an intermittent access (IA) task, operant responding for a sucrose reward, conditioned place aversion (CPA) to morphine withdrawal, and locomotor sensitization to morphine. The results demonstrate that deletion of MOR on Foxp2-expressing neurons renders mice more sensitive to quinine-adulterated ethanol (EtOH). Mice with the deletion (vs. Cre- controls) also consumed less alcohol during the final sessions of the IA task, responded less for sucrose under an FR3 schedule, and were less active at baseline and following morphine injection. Foxp2-MOR deletion did not impair the ability to learn to respond for reward or develop a conditioned aversion to morphine withdrawal. Together, these investigations demonstrate that Foxp2-expressing neurons may be involved in escalation of alcohol consumption and the development of compulsive-like alcohol drinking.

Modulation of Nicotine-Associated Behaviour in Rats By μ-Opioid Signals from the Medial Prefrontal Cortex to the Nucleus Accumbens Shell

Nicotine addiction is a concern worldwide. Most mechanistic investigations are on nicotine substance dependence properties based on its pharmacological effects. However, no effective therapeutic treatment has been established. Nicotine addiction is reinforced by environments or habits. We demonstrate the neurobiological basis of the behavioural aspect of nicotine addiction. We utilized the conditioned place preference to establish nicotine-associated behavioural preferences (NABP) in rats. Brain-wide neuroimaging analysis revealed that the medial prefrontal cortex (mPFC) was activated and contributed to NABP. Chemogenetic manipulation of µ-opioid receptor positive (MOR+) neurons in the mPFC or the excitatory outflow to the nucleus accumbens shell (NAcShell) modulated the NABP. Electrophysiological recording confirmed that the MOR+ neurons directly regulate the mPFC-NAcShell circuit via GABAA receptors. Thus, the MOR+ neurons in the mPFC modulate the formation of behavioural aspects of nicotine addiction via direct excitatory innervation to the NAcShell, which may provide new insight for the development of effective therapeutic strategies.

Dysconnectivity of the Nucleus Accumbens and Amygdala in Youths with Thought Problems: A Dimensional Approach

Background: Youths with thought problems (TP) are at risk to develop psychosis and obsessive-compulsive disorder (OCD). Yet, the pathophysiological mechanisms underpinning TP are still unclear. Functional magnetic resonance imaging (fMRI) studies have shown that striatal and limbic alterations are associated with psychosis-like and obsessive-like symptoms in individuals at clinical risk for psychosis, schizophrenia, and OCD. More specifically, nucleus accumbens (NAcc) and amygdala are mainly involved in these associations. The current study aims to investigate the neural correlates of TP in youth populations using a dimensional approach and explore potential cognitive functions and neurotransmitters associated with it. Methods: Seed-to-voxels functional connectivity analyses using NAcc and amygdala as regions-of-interest were conducted with resting-state fMRI data obtained from 1360 young individuals, and potential confounders related to TP such as anxiety and cognitive functions were included as covariates in multiple regression analyses. Replicability was tested in using an adult cohort. In addition, functional decoding and neurochemical correlation analyses were performed to identify the associated cognitive functions and neurotransmitters. Results: The altered functional connectivities between the right NAcc and posterior parahippocampal gyrus, between the right amygdala and lateral prefrontal cortex, and between the left amygdala and the secondary visual area were the best predictors of TP in multiple regression model. These functional connections are mainly involved in social cognition and reward processing. Conclusions: The results show that alterations in the functional connectivity of the NAcc and the amygdala in neural pathways involved in social cognition and reward processing are associated with severity of TP in youths.

Robust and replicable functional brain signatures of 22q11.2 deletion syndrome and associated psychosis: a deep neural network-based multi-cohort study

A major genetic risk factor for psychosis is 22q11.2 deletion (22q11.2DS). However, robust and replicable functional brain signatures of 22q11.2DS and 22q11.2DS-associated psychosis remain elusive due to small sample sizes and a focus on small single-site cohorts. Here, we identify functional brain signatures of 22q11.2DS and 22q11.2DS-associated psychosis, and their links with idiopathic early psychosis, using one of the largest multi-cohort data to date. We obtained multi-cohort clinical phenotypic and task-free fMRI data from 856 participants (101 22q11.2DS, 120 idiopathic early psychosis, 101 idiopathic autism, 123 idiopathic ADHD, and 411 healthy controls) in a case-control design. A novel spatiotemporal deep neural network (stDNN)-based analysis was applied to the multi-cohort data to identify functional brain signatures of 22q11.2DS and 22q11.2DS-associated psychosis. Next, stDNN was used to test the hypothesis that the functional brain signatures of 22q11.2DS-associated psychosis overlap with idiopathic early psychosis but not with autism and ADHD. stDNN-derived brain signatures distinguished 22q11.2DS from controls, and 22q11.2DS-associated psychosis with very high accuracies (86-94%) in the primary cohort and two fully independent cohorts without additional training. Robust distinguishing features of 22q11.2DS-associated psychosis emerged in the anterior insula node of the salience network and the striatum node of the dopaminergic reward pathway. These features also distinguished individuals with idiopathic early psychosis from controls, but not idiopathic autism or ADHD. Our results reveal that individuals with 22q11.2DS exhibit a highly distinct functional brain organization compared to controls. Additionally, the brain signatures of 22q11.2DS-associated psychosis overlap with those of idiopathic early psychosis in the salience network and dopaminergic reward pathway, providing substantial empirical support for the theoretical aberrant salience-based model of psychosis. Collectively, our findings, replicated across multiple independent cohorts, advance the understanding of 22q11.2DS and associated psychosis, underscoring the value of 22q11.2DS as a genetic model for probing the neurobiological underpinnings of psychosis and its progression.

Nucleus accumbens neurons dynamically respond to appetitive and aversive associative learning

To survive, individuals must learn to associate cues in the environment with emotionally relevant outcomes. This association is partially mediated by the nucleus accumbens (NAc), a key brain region of the reward circuit that is mainly composed by GABAergic medium spiny neurons (MSNs), that express either dopamine receptor D1 or D2. Recent studies showed that both populations can drive reward and aversion, however, the activity of these neurons during appetitive and aversive Pavlovian conditioning remains to be determined. Here, we investigated the relevance of D1- and D2-neurons in associative learning, by measuring calcium transients with fiber photometry during appetitive and aversive Pavlovian tasks in mice. Sucrose was used as a positive valence unconditioned stimulus (US) and foot shock was used as a negative valence US. We show that during appetitive Pavlovian conditioning, D1- and D2-neurons exhibit a general increase in activity in response to the conditioned stimuli (CS). Interestingly, D1- and D2-neurons present distinct changes in activity after sucrose consumption that dynamically evolve throughout learning. During the aversive Pavlovian conditioning, D1- and D2-neurons present an increase in the activity in response to the CS and to the US (shock). Our data support a model in which D1- and D2-neurons are concurrently activated during appetitive and aversive conditioning.

The cortico-striatal circuitry in autism-spectrum disorders: a balancing act

The basal ganglia are major targets of cortical inputs and, in turn, modulate cortical function via their projections to the motor and prefrontal cortices. The role of the basal ganglia in motor control and reward is well documented and there is also extensive evidence that they play a key role in social and repetitive behaviors. The basal ganglia influence the activity of the cerebral cortex via two major projections from the striatum to the output nuclei, the globus pallidus internus and the substantia nigra, pars reticulata. This modulation involves a direct projection known as the direct pathway and an indirect projection via the globus pallidus externus and the subthalamic nucleus, known as the indirect pathway. This review discusses the respective contribution of the direct and indirect pathways to social and repetitive behaviors in neurotypical conditions and in autism spectrum disorders.

The role of orexinergic system in the regulation of cataplexy

Loss of orexin/hypocretin causes serious sleep disorder; narcolepsy. Cataplexy is the most striking symptom of narcolepsy, characterized by abrupt muscle paralysis induced by emotional stimuli, and has been considered pathological activation of REM sleep atonia system. Clinical treatments for cataplexy/narcolepsy and early pharmacological studies in narcoleptic dogs tell us about the involvement of monoaminergic and cholinergic systems in the control of cataplexy/narcolepsy. Muscle atonia may be induced by activation of REM sleep-atonia generating system in the brainstem. Emotional stimuli may be processed in the limbic systems including the amygdala, nucleus accumbens, and medial prefrontal cortex. It is now considered that orexin/hypocretin prevents cataplexy by modulating the activity of different points of cataplexy-inducing circuit, including monoaminergic/cholinergic systems, muscle atonia-generating systems, and emotion-related systems. This review will describe the recent advances in understanding the neural mechanisms controlling cataplexy, with a focus on the involvement of orexin/hypocretin system, and will discuss future experimental strategies that will lead to further understanding and treatment of this disease.

Enhancing reinforcement learning models by including direct and indirect pathways improves performance on striatal dependent tasks

A major advance in understanding learning behavior stems from experiments showing that reward learning requires dopamine inputs to striatal neurons and arises from synaptic plasticity of cortico-striatal synapses. Numerous reinforcement learning models mimic this dopamine-dependent synaptic plasticity by using the reward prediction error, which resembles dopamine neuron firing, to learn the best action in response to a set of cues. Though these models can explain many facets of behavior, reproducing some types of goal-directed behavior, such as renewal and reversal, require additional model components. Here we present a reinforcement learning model, TD2Q, which better corresponds to the basal ganglia with two Q matrices, one representing direct pathway neurons (G) and another representing indirect pathway neurons (N). Unlike previous two-Q architectures, a novel and critical aspect of TD2Q is to update the G and N matrices utilizing the temporal difference reward prediction error. A best action is selected for N and G using a softmax with a reward-dependent adaptive exploration parameter, and then differences are resolved using a second selection step applied to the two action probabilities. The model is tested on a range of multi-step tasks including extinction, renewal, discrimination; switching reward probability learning; and sequence learning. Simulations show that TD2Q produces behaviors similar to rodents in choice and sequence learning tasks, and that use of the temporal difference reward prediction error is required to learn multi-step tasks. Blocking the update rule on the N matrix blocks discrimination learning, as observed experimentally. Performance in the sequence learning task is dramatically improved with two matrices. These results suggest that including additional aspects of basal ganglia physiology can improve the performance of reinforcement learning models, better reproduce animal behaviors, and provide insight as to the role of direct- and indirect-pathway striatal neurons.

α4-Containing GABAA Receptors on DRD2 Neurons of the Nucleus Accumbens Mediate Instrumental Responding for Conditioned Reinforcers and Its Potentiation by Cocaine

Extrasynaptic GABAA receptors (GABAARs) composed of α4, β, and δ subunits mediate GABAergic tonic inhibition and are potential molecular targets in the modulation of behavioral responses to natural and drug rewards. These GABAARs are highly expressed within the nucleus accumbens (NAc), where they influence the excitability of the medium spiny neurons. Here, we explore their role in modulating behavioral responses to food-conditioned cues and the behavior-potentiating effects of cocaine. α4-Subunit constitutive knock-out mice (α4-/-) showed higher rates of instrumental responding for reward-paired stimuli in a test of conditioned reinforcement (CRf). A similar effect was seen following viral knockdown of GABAAR α4 subunits within the NAc. Local infusion of the α4βδ-GABAAR-preferring agonist THIP (4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol; Gaboxadol) into the NAc had no effect on responding when given alone but reduced cocaine potentiation of responding for conditioned reinforcers in wild-type, but not α4-/- mice. Finally, specific deletion of α4-subunits from dopamine D2, but not D1, receptor-expressing neurons (DRD2 and DRD1 neurons), mimicked the phenotype of the constitutive knockout, potentiating CRf responding, and blocking intra-accumbal THIP attenuation of cocaine-potentiated CRf responding. These data demonstrate that α4-GABAAR-mediated inhibition of DRD2 neurons reduces instrumental responding for a conditioned reinforcer and its potentiation by cocaine and emphasize the importance of GABAergic signaling within the NAc in mediating the effects of cocaine.

IPAC integrates rewarding and environmental memory during the acquisition of morphine CPP

The association between rewarding and drug-related memory is a leading factor for the formation of addiction, yet the neural circuits underlying the association remain unclear. Here, we showed that the interstitial nucleus of the posterior limb of the anterior commissure (IPAC) integrated rewarding and environmental memory information by two different receiving projections from ventral tegmental area (VTA) and nucleus accumbens shell region (NAcSh) to mediate the acquisition of morphine conditioned place preference (CPP). A projection from the VTA GABAergic neurons (VTAGABA) to the IPAC lateral region GABAergic neurons (IPACLGABA) mediated the effect of morphine rewarding, whereas the pathway from NAcSh dopamine receptor 1-expressing neurons (NAcShD1) to the IPAC medial region GABAergic neurons (IPACMGABA) was involved in the acquisition of environmental memory. These findings demonstrated that the distinct IPAC circuits VTAGABA→IPACLGABA and NAcShD1R→IPACMGABA were attributable to the rewarding and environmental memory during the acquisition of morphine CPP, respectively, and provided the circuit-based potential targets for preventing and treating opioid addiction.

Nucleus Accumbens Core Dopamine D2 Receptor-Expressing Neurons Control Reversal Learning but Not Set-Shifting in Behavioral Flexibility in Male Mice

The ability to use environmental cues to flexibly guide responses is crucial for adaptive behavior and is thought to be controlled within a series of cortico-basal ganglia-thalamo-cortical loops. Previous evidence has indicated that different prefrontal cortical regions control dissociable aspects of behavioral flexibility, with the medial prefrontal cortex (mPFC) necessary for the ability to shift attention to a novel strategy (set-shifting) and the orbitofrontal cortex (OFC) necessary for shifting attention between learned stimulus-outcome associations (reversal learning). The nucleus accumbens (NAc) is a major downstream target of both the mPFC and the OFC; however, its role in controlling reversal learning and set-shifting abilities is still unclear. Here we investigated the contribution of the two major NAc neuronal populations, medium spiny neurons expressing either dopamine D1 or D2 receptors (D1-/D2-MSNs), in guiding reversal learning and set-shifting in an attentional set-shifting task (ASST). Persistent inhibition of neurotransmitter release from NAc D2-MSNs, but not D1-MSNs, resulted in an impaired ability for reversal learning, but not set-shifting in male mice. These findings suggest that NAc D2-MSNs play a critical role in suppressing responding toward specific learned cues that are now associated with unfavorable outcomes (i.e., in reversal stages), but not in the suppression of more general learned strategies (i.e., in set-shifting). This study provides further evidence for the anatomical separation of reversal learning and set-shifting abilities within cortico-basal ganglia-thalamo-cortical loops.

Psycho-Neuro-Endocrine-Immunological Basis of the Placebo Effect: Potential Applications beyond Pain Therapy

The placebo effect can be defined as the improvement of symptoms in a patient after the administration of an innocuous substance in a context that induces expectations regarding its effects. During recent years, it has been discovered that the placebo response not only has neurobiological functions on analgesia, but that it is also capable of generating effects on the immune and endocrine systems. The possible integration of changes in different systems of the organism could favor the well-being of the individuals and go hand in hand with conventional treatment for multiple diseases. In this sense, classic conditioning and setting expectations stand out as psychological mechanisms implicated in the placebo effect. Recent advances in neuroimaging studies suggest a relationship between the placebo response and the opioid, cannabinoid, and monoaminergic systems. Likewise, a possible immune response conditioned by the placebo effect has been reported. There is evidence of immune suppression conditioned through the insular cortex and the amygdala, with noradrenalin as the responsible neurotransmitter. Finally, a conditioned response in the secretion of different hormones has been determined in different studies; however, the molecular mechanisms involved are not entirely known. Beyond studies about its mechanism of action, the placebo effect has proved to be useful in the clinical setting with promising results in the management of neurological, psychiatric, and immunologic disorders. However, more research is needed to better characterize its potential use. This review integrates current knowledge about the psycho-neuro-endocrine-immune basis of the placebo effect and its possible clinical applications.

Obesity and Cerebral Blood Flow in the Reward Circuitry of Youth With Bipolar Disorder

BACKGROUND

Bipolar disorder (BD) is associated with elevated body mass index (BMI) and increased rates of obesity. Obesity among individuals with BD is associated with more severe course of illness. Motivated by previous research on BD and BMI in youth as well as brain findings in the reward circuit, the current study investigates differences in cerebral blood flow (CBF) in youth BD with and without comorbid overweight/obesity (OW/OB).

METHODS

Participants consisted of youth, ages 13-20 years, including BD with OW/OB (BDOW/OB; n = 25), BD with normal weight (BDNW; n = 55), and normal-weight healthy controls (HC; n = 61). High-resolution T1-weighted and pseudo-continuous arterial spin labeling images were acquired using 3 Tesla magnetic resonance imaging. CBF differences were assessed using both region of interest and whole-brain voxel-wise approaches.

RESULTS

Voxel-wise analysis revealed significantly higher CBF in reward-associated regions in the BDNW group relative to the HC and BDOW/OB groups. CBF did not differ between the HC and BDOW/OB groups. There were no significant region of interest findings.

CONCLUSIONS

The current study identified distinct CBF levels relating to BMI in BD in the reward circuit, which may relate to underlying differences in cerebral metabolism, compensatory effects, and/or BD severity. Future neuroimaging studies are warranted to examine for changes in the CBF-OW/OB link over time and in relation to treatment.

Discovery of a macromolecular complex mediating the hunger suppressive actions of cocaine: Structural and functional properties

Cocaine not only increases brain dopamine levels but also activates the sigma1 receptor (σ1 R) that in turn regulates orexigenic receptor function. Identification of interactions involving dopamine D1 (D1 R), ghrelin (GHS-R1a ), and σ1 receptors have been addressed by biophysical techniques and a complementation approach using interfering peptides. The effect of cocaine on receptor functionality was assayed by measuring second messenger, cAMP and Ca2+ , levels. The effect of acute or chronic cocaine administration on receptor complex expression was assayed by in situ proximity ligation assay. In silico procedures were used for molecular model building. σ1 R KO mice were used for confirming involvement of this receptor. Upon identification of protomer interaction and receptor functionality, a unique structural model for the macromolecular complex formed by σ1 R, D1 R, and GHS-R1a is proposed. The functionality of the complex, able to couple to both Gs and Gq proteins, is affected by cocaine binding to the σ1 R, as confirmed using samples from σ1 R-/- mice. The expression of the macromolecular complex was differentially affected upon acute and chronic cocaine administration to rats. The constructed 3D model is consistent with biochemical, biophysical, and available structural data. The σ1 R, D1 R, and GHS-R1a complex constitutes a functional unit that is altered upon cocaine binding to the σ1 R. Remarkably, the heteromer can simultaneously couple to two G proteins, thus allowing dopamine to signal via Ca2+ and ghrelin via cAMP. The anorexic action of cocaine is mediated by such complex whose expression is higher after acute than after chronic administration regimens.

Modulation of stimulated dopamine release in rat nucleus accumbens shell by GABA in vitro: Effect of sub-chronic phencyclidine pretreatment

Dopamine signaling in nucleus accumbens (NAc) is modulated by γ-aminobutyric acid (GABA), acting through GABA-A and GABA-B receptors: dysregulation of GABAergic control of dopamine function may be important in behavioral deficits in schizophrenia. We investigated the effect of GABA-A (muscimol) and GABA-B (baclofen) receptor agonists on electrically stimulated dopamine release. Furthermore, we explored whether drug-induced changes were disrupted by pretreatment with phencyclidine, which provides a well-validated model of schizophrenia. Using brain slices from female rats, fast-scan cyclic voltammetry was used to measure electrically stimulated dopamine release in NAc shell. Both muscimol and baclofen caused concentration-dependent attenuation of evoked dopamine release: neither effect was changed by dihydro-β-erythroidine, a nicotinic acetylcholine receptor antagonist, or the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), precluding indirect mechanisms using these transmitter systems in the GABAergic actions. In slices taken from rats pretreated with phencyclidine, the attenuation of evoked dopamine release by baclofen was abolished, but the attenuation by muscimol was unaffected. Since phencyclidine pretreatment was followed by drug-free washout period of at least a week, the drug was not present during recording. Therefore, disruption of GABA-B modulation of dopamine is due to long-term functional changes resulting from the treatment, rather than transient changes due to the drug's presence at test. This enduring dysregulation of GABA-B modulation of accumbal dopamine release provides a plausible mechanism through which GABA dysfunction influences accumbal dopamine leading to behavioral changes seen in schizophrenia and may provide a route for novel therapeutic strategies to treat the condition.

Dopaminergic Regulation of Reward System Connectivity Underpins Pain and Emotional Suffering in Migraine

Purpose

It has been suggested that reward system dysfunction may account for emotion and pain suffering in migraine. However, there is a lack of evidence whether the altered reward system connectivity is directly associated with clinical manifestations, including negative affect and ictal pain severity and, at the molecular level, the dopamine (DA) D2/D3 receptors (D2/3Rs) signaling implicated in encoding motivational and emotional cues.

Patients and Methods

We acquired resting-state functional MRI from interictal episodic migraine (EM) patients and age-matched healthy controls, as well as positron emission tomography (PET) with [¹¹C]raclopride, a selective radiotracer for DA D2/3Rs, from a subset of these participants. The nucleus accumbens (NAc) was seeded to measure functional connectivity (FC) and DA D2/3Rs availability based on its essential involvement in pain-related aversive/reward functions. Associations of the brain measures with positive/negative affect and ictal pain severity were also assessed.

Results

Compared with controls, the EM group showed weaker right NAc connectivity with areas implicated in pain and emotional regulation, such as the amygdala, rostral anterior cingulate cortex, hippocampus, and thalamus; but showed stronger left NAc connectivity with the dorsolateral prefrontal cortex and lingual gyrus. Moreover, among the altered NAc connectivities, only right NAc-amygdala connectivity was inversely correlated with DA D2/3Rs availability in migraine patients (diagnostic group-by-D2/3Rs interaction p < 0.007). At a clinical level, such weaker NAc-amygdala connectivity was associated with lower interictal positive affect and greater ictal pain severity over the head and facial extension area (pain area and intensity number summation, PAINS).

Conclusion

Together, our findings suggest that altered reward system connectivity, specifically between the NAc and amygdala, might be affected by endogenous DA D2/3Rs signaling, and such process might be a neural mechanism that underlies emotional and pain suffering in episodic migraineurs.

Motivational competition and the paraventricular thalamus

Although significant progress has been made in understanding the behavioral and brain mechanisms for motivational systems, much less is known about competition between motivational states or motivational conflict (e.g., approach - avoidance conflict). Despite being produced under diverse conditions, behavior during motivational competition has two signatures: bistability and metastability. These signatures reveal the operation of positive feedback mechanisms in behavioral selection. Different neuronal architectures can instantiate this selection to achieve bistability and metastability in behavior, but each relies on circuit-level inhibition to achieve rapid and stable selection between competing tendencies. Paraventricular thalamus (PVT) is identified as critical to this circuit level inhibition, resolving motivational competition via its extensive projections to local inhibitory networks in the ventral striatum and extended amygdala, enabling adaptive responding under motivational conflict.

The contribution of medium spiny neuron subtypes in the nucleus accumbens core to compulsive-like ethanol drinking

Compulsive alcohol use, or drinking that persists despite negative or aversive consequences, is a defining characteristic of alcohol use disorder. Here, chemogenetic technology (i.e. Designer Receptors Exclusively Activated by Designer Drugs; DREADDs) was used to inhibit or excite the NAc core or selectively inhibit D₁-or D₂ receptor-expressing neurons in the NAc core to understand the role of the NAc core and how these subpopulations of neurons may influence compulsive-like ethanol (EtOH) drinking using C57BL/6J, Drd1-cre, and Drd2-cre male and female mice. Compulsive-like EtOH drinking was modeled with a two-bottle choice, drinking in the dark paradigm. The major finding of this study was that mice decreased compulsive-like EtOH intake when the NAc core was inhibited and there was no change of EtOH + quinine intake when the NAc core was excited. Interestingly, inhibition of D₁-or D₂ receptor-expressing neurons did not alter compulsive-like EtOH intake. Control experiments showed that NAc core excitation and selective inhibition of D₁-or D₂-receptor-expressing neurons had no effect on baseline EtOH drinking, intake of water, or intake of quinine-adulterated water. CNO reduced amphetamine-induced locomotion in the D₁-^CRE+ (but not the D₂^CRE+) group in a control experiment. Finally, pharmacological antagonism of D₁ and D₂ receptors together, but not separately, reduced quinine-resistant EtOH drinking. These results suggest that the NAc core is a critical region involved in compulsive-like EtOH consumption, and that both D₁-and D₂ receptor-expressing medium spiny neurons participate in controlling this behavior.

Loss of β-arrestin2 in D2 cells alters neuronal excitability in the nucleus accumbens and behavioral responses to psychostimulants and opioids

Psychostimulants and opioids increase dopamine (DA) neurotransmission, activating D1 and D2 G protein-coupled receptors. β-arrestin2 (βarr2) desensitizes and internalizes these receptors and initiates G protein-independent signaling. Previous work revealed that mice with a global or cell-specific knockout of βarr2 have altered responses to certain drugs; however, the effects of βarr2 on the excitability of medium spiny neurons (MSNs), and its role in mediating the rewarding effects of drugs of abuse are unknown. D1-Cre and D2-Cre transgenic mice were crossed with floxed βarr2 mice to eliminate βarr2 specifically in cells containing either D1 (D1^βarr2-KO ) or D2 (D2^βarr2-KO ) receptors. We used slice electrophysiology to characterize the role of βarr2 in modulating D1 and D2 nucleus accumbens MSN intrinsic excitability in response to DA and tested the locomotor-activating and rewarding effects of cocaine and morphine in these mice. Eliminating βarr2 attenuated the ability of DA to inhibit D2-MSNs and altered the DA-induced maximum firing rate in D1-MSNs. While D1^βarr2-KO mice had mostly normal drug responses, D2^βarr2-KO mice showed dose-dependent reductions in acute locomotor responses to cocaine and morphine, attenuated locomotor sensitization to cocaine, and blunted cocaine reward measured with conditioned place preference. Both D2^βarr2-KO and D1^βarr2-KO mice displayed an enhanced conditioned place preference for the highest dose of morphine. These results indicate that D1- and D2-derived βarr2 functionally contribute to DA-induced changes in MSN intrinsic excitability and behavioral responses to psychostimulants and opioids dose-dependently.

Substance use disorders and chronic itch

Chronic pruritus is one dermatologic manifestation of an underlying substance use disorder. Recent literature has uncovered similarities between the general neurologic mechanisms of addiction and chronic itch, largely involving activation of the dopaminergic reward circuits within the brain and imbalances between mu and kappa opioid receptor activation. It is likely that the use of specific drugs, like central nervous system stimulants and opioids, results in further activation and imbalances within these pathways, perpetuating both addiction and pruritus simultaneously. Opioid users often present to dermatology clinics with a generalized pruritus, whereas individuals using central nervous system stimulants like cocaine and methylenedioxymethamphetamine (MDMA), as well as legally prescribed drugs like treatments for attention deficit hyperactivity disorder, frequently complain of crawling, delusional infestation-like sensations underneath the skin. Because of these overlapping mechanisms and similar clinical presentations to many other chronically itchy conditions, it is necessary for dermatologists to consider and investigate an underlying substance use disorder to effectively treat these patients.

MeCP2 in cholinergic interneurons of nucleus accumbens regulates fear learning

Methyl-CpG-binding protein 2 (MeCP2) encoded by the MECP2 gene is a transcriptional regulator whose mutations cause Rett syndrome (RTT). Mecp2-deficient mice show fear regulation impairment; however, the cellular and molecular mechanisms underlying this abnormal behavior are largely uncharacterized. Here, we showed that Mecp2 gene deficiency in cholinergic interneurons of the nucleus accumbens (NAc) dramatically impaired fear learning. We further found that spontaneous activity of cholinergic interneurons in Mecp2-deficient mice decreased, mediated by enhanced inhibitory transmission via α2-containing GABA_A receptors. With MeCP2 restoration, opto- and chemo-genetic activation, and RNA interference in ChAT-expressing interneurons of the NAc, impaired fear retrieval was rescued. Taken together, these results reveal a previously unknown role of MeCP2 in NAc cholinergic interneurons in fear regulation, suggesting that modulation of neurons in the NAc may ameliorate fear-related disorders.

Neurotransmission through dopamine D1 receptors is required for aversive memory formation and Arc activation in the cerebral cortex

Dopaminergic neurotransmission is considered to play an important role not only in reward-based learning, but also in aversive learning. Here, we investigated the role of dopaminergic neurotransmission via dopamine D1 receptors (D1Rs) in aversive memory formation in a passive avoidance test using D1R knockdown (KD) mice, in which the expression of D1Rs can conditionally and reversibly be controlled by doxycycline (Dox) treatment. We also performed whole-brain imaging after aversive footshock stimulation in activity-regulated cytoskeleton protein (Arc)-dVenus D1RKD mice, which were crossbred from Arc-dVenus transgenic mice and D1RKD mice, to examine the distribution of Arc-controlled dVenus expression in the hippocampus and cerebral cortex during aversive memory formation. Knockdown of D1R expression following Dox treatment resulted in impaired performance in the passive avoidance test and was associated with a decrease in dVenus expression in the cerebral cortex (visual, somatosensory, and motor cortices), but not the hippocampus, compared with control mice without Dox treatment. These findings indicate that D1R-mediated dopaminergic transmission is critical for aversive memory formation, specifically by influencing Arc expression in the cerebral cortex.

The non-receptor tyrosine kinase Pyk2 modulates acute locomotor effects of cocaine in D1 receptor-expressing neurons of the nucleus accumbens

The striatum is critical for cocaine-induced locomotor responses. Although the role of D1 receptor-expressing neurons is established, underlying molecular pathways are not fully understood. We studied the role of Pyk2, a non-receptor, calcium-dependent protein-tyrosine kinase. The locomotor coordination and basal activity of Pyk2 knock-out mice were not altered and major striatal protein markers were normal. Cocaine injection increased Pyk2 tyrosine phosphorylation in mouse striatum. Pyk2-deficient mice displayed decreased locomotor response to acute cocaine injection. In contrast, locomotor sensitization and conditioned place preference were normal. Cocaine-activated ERK phosphorylation, a signaling pathway essential for these late responses, was unaltered. Conditional deletion of Pyk2 in the nucleus accumbens or in D1 neurons reproduced decreased locomotor response to cocaine, whereas deletion of Pyk2 in the dorsal striatum or in A2A receptor-expressing neurons did not. In mice lacking Pyk2 in D1-neurons locomotor response to D1 agonist SKF-81297, but not to an anticholinergic drug, was blunted. Our results identify Pyk2 as a regulator of acute locomotor responses to psychostimulants. They highlight the role of tyrosine phosphorylation pathways in striatal neurons and suggest that changes in Pyk2 expression or activation may alter specific responses to drugs of abuse, or possibly other behavioral responses linked to dopamine action.

Zona incerta projection neurons and GABAergic and GLP-1 mechanisms in the nucleus accumbens are involved in the control of gastric function and food intake

OBJECTIVE

Our aim was to explore the effect of γ-aminobutyric acid (GABA) signaling in the nucleus accumbens (NAc) on promoting gastric function and food intake through glucagon-like peptide 1 (GLP-1)-sensitive gastric distension (GD) neurons under the regulatory control of the zona incerta (ZI).

METHODS

GABA neuronal projections were traced using retrograde tracing following fluorescence immunohistochemistry. An extracellular electrophysiological recording method was used to observe the firing of neurons in the NAc. HPLC was used to quantify the GABA and glutamate levels in the NAc after electrical stimulation of the ZI. Gastric functions including gastric motility and secretion, as well as food intake, were measured after the administration of different concentrations of GABA in the NAc or electrical stimulation of the ZI.

RESULTS

Some of the GABA-positive neurons arising from the ZI projected to the NAc. Some GABA-A receptor (GABA-AR)-immunoreactive neurons in the NAc were also positive for GLP-1 receptor (GLP-1R) immunoreactivity. The firing of most GLP-1-sensitive GD neurons was decreased by GABA infusion in the NAc. Intra-NAc GABA administration also promoted gastric function and food intake. The responses induced by GABA were partially blocked by the GABA-AR antagonist bicuculline (BIC) and weakened by the GLP-1R antagonist exendin 9-39 (Ex9). Electrical stimulation of the ZI changed the firing patterns of most GLP-1-sensitive GD neurons in the NAc and promoted gastric function and food intake. Furthermore, these excitatory effects induced by electrical stimulation of the ZI were weakened by preadministration of BIC in the NAc.

CONCLUSION

Retrograde tracing and immunohistochemical staining showed a GABAergic pathway from the ZI to the NAc. GABAergic and GLP-1 mechanisms in the NAc are involved in the control of gastric function and food intake. In addition, the interaction (direct or indirect) between the ZI and these NAc mechanisms is involved in the control of gastric function and food intake.

microRNA-15b contributes to depression-like behavior in mice by affecting synaptic protein levels and function in the nucleus accumbens

Major depression is a prevalent affective disorder characterized by recurrent low mood. It presumably results from stress-induced deteriorations of molecular networks and synaptic functions in brain reward circuits of genetically-susceptible individuals through epigenetic processes. Epigenetic regulator microRNA-15b inhibits neuronal progenitor proliferation and is up-regulated in the medial prefrontal cortex of mice that demonstrate depression-like behavior, indicating the contribution of microRNA-15 to major depression. Using a mouse model of major depression induced by chronic unpredictable mild stress (CUMS), here we examined the effects of microRNA-15b on synapses and synaptic proteins in the nucleus accumbens of these mice. The application of a microRNA-15b antagomir into the nucleus accumbens significantly reduced the incidence of CUMS-induced depression and reversed the attenuations of excitatory synapse and syntaxin-binding protein 3 (STXBP3A)/vesicle-associated protein 1 (VAMP1) expression. In contrast, the injection of a microRNA-15b analog into the nucleus accumbens induced depression-like behavior as well as attenuated excitatory synapses and STXBP3A/VAMP1 expression similar to the down-regulation of these processes induced by the CUMS. We conclude that microRNA-15b-5p may play a critical role in chronic stress-induced depression by decreasing synaptic proteins, innervations, and activities in the nucleus accumbens. We propose that the treatment of anti-microRNA-15b-5p may convert stress-induced depression into resilience.

Nucleus accumbens pathways control cell-specific gene expression in the medial prefrontal cortex

The medial prefrontal cortex (mPFC) is a critical component of a cortico-basal ganglia-thalamo-cortical loop regulating limbic and cognitive functions. Within this circuit, two distinct nucleus accumbens (NAc) output neuron types, dopamine D1 or D2 receptor-expressing neurons, dynamically control the flow of information through basal ganglia nuclei that eventually project back to the mPFC to complete the loop. Thus, chronic dysfunction of the NAc may result in mPFC transcriptomal changes, which in turn contribute to disease conditions associated with the mPFC and basal ganglia. Here, we used RNA sequencing to analyse differentially expressed genes (DEGs) in the mPFC following a reversible neurotransmission blocking technique in D1 or D2 receptor-expressing NAc neurons, respectively (D1-RNB, or D2-RNB). Gene Set Enrichment Analysis revealed that gene sets of layer 5b and 6 pyramidal neurons were enriched in DEGs of the mPFC downregulated in both NAc D1- and D2-RNB mice. In contrast, gene sets of layer 5a pyramidal neurons were enriched in upregulated DEGs of the mPFC in D1-RNB mice, and downregulated DEGs of the mPFC in D2-RNB mice. These findings reveal for the first time that NAc output pathways play an important role in controlling mPFC gene expression.

Evidence and explanation for the involvement of the nucleus accumbens in pain processing

The nucleus accumbens (NAc) is a subcortical brain structure known primarily for its roles in pleasure, reward, and addiction. Despite less focus on the NAc in pain research, it also plays a large role in the mediation of pain and is effective as a source of analgesia. Evidence for this involvement lies in the NAc’s cortical connections, functions, pharmacology, and therapeutic targeting. The NAc projects to and receives information from notable pain structures, such as the prefrontal cortex, anterior cingulate cortex, periaqueductal gray, habenula, thalamus, etc. Additionally, the NAc and other pain-modulating structures share functions involving opioid regulation and motivational and emotional processing, which each work beyond simply the rewarding experience of pain offset. Pharmacologically speaking, the NAc responds heavily to painful stimuli, due to its high density of μ opioid receptors and the activation of several different neurotransmitter systems in the NAc, such as opioids, dopamine, calcitonin gene-related peptide, γ-aminobutyric acid, glutamate, and substance P, each of which have been shown to elicit analgesic effects. In both preclinical and clinical models, deep brain stimulation of the NAc has elicited successful analgesia. The multi-functional NAc is important in motivational behavior, and the motivation for avoiding pain is just as important to survival as the motivation for seeking pleasure. It is possible, then, that the NAc must be involved in both pleasure and pain in order to help determine the motivational salience of positive and negative events.

Differential rewarding effects of electrical stimulation of the lateral hypothalamus and parabrachial complex: Functional characterization and the relevance of opioid systems and dopamine

BACKGROUND

Since the discovery of rewarding intracranial self-stimulation by Olds and Milner, extensive data have been published on the biological basis of reward. Although participation of the mesolimbic dopaminergic system is well documented, its precise role has not been fully elucidated, and some authors have proposed the involvement of other neural systems in processing specific aspects of reinforced behaviour.

AIMS AND METHODS

We reviewed published data, including our own findings, on the rewarding effects induced by electrical stimulation of the lateral hypothalamus (LH) and of the external lateral parabrachial area (LPBe) - a brainstem region involved in processing the rewarding properties of natural and artificial substances - and compared its functional characteristics as observed in operant and non-operant behavioural procedures.

RESULTS

Brain circuits involved in the induction of preferences for stimuli associated with electrical stimulation of the LBPe appear to functionally and neurochemically differ from those activated by electrical stimulation of the LH.

INTERPRETATION

We discuss the possible involvement of the LPBe in processing emotional-affective aspects of the brain reward system.

Nucleus accumbens connectivity at rest is associated with alcohol consumption in young male adults

Alcohol consumption during adolescence might impede normal brain development, while more excessive drinking during this period poses a risk for developing alcohol use disorder. Here it was tested whether nucleus accumbens (NAcc) resting-state functional connectivity could be associated with lifetime drinking behavior in young adults, and whether it could predict their alcohol consumption during a one-year follow-up period. The current investigation was part of the bicentric Learning and Alcohol Dependence (LeAD) population-based prospective cohort study. One hundred and eighty-four 18-year-old male social drinking volunteers without a lifetime diagnosis of psychotic, bipolar, or alcohol use disorder were recruited from the general population. Seed-based resting-state functional connectivity was calculated for the bilateral NAcc in each participant. Across the group, the association between NAcc functional connectivity and lifetime alcohol consumption was assessed (p < .05, whole-brain FWE-corrected). Individual connectivity values were then extracted from regions that demonstrated a significant association to predict drinking behavior during a one-year follow-up period (n = 143), correcting for lifetime alcohol consumption. Weaker connectivity between the left NAcc and bilateral dorsolateral prefrontal cortex, inferior frontal gyrus, left caudate nucleus, left putamen, and left insula was associated with greater lifetime alcohol consumption, as well as with greater alcohol consumption during the one-year follow-up period. Our findings underscore the relevance of fronto-striatal connectivity to the field of alcohol research. Impaired prefrontal cognitive control might mediate excessive drinking behavior and may prove a promising biomarker for risk of future alcohol (ab)use.

mRNA and miRNA profiles in the nucleus accumbens are related to fear memory and anxiety induced by physical or psychological stress

Anxiety is presumably driven by fear memory. The nucleus accumbens involves emotional regulation. Molecular profiles in the nucleus accumbens related to stress-induced fear memory remain elucidated. Fear memory in mice was induced by a paradigm of social defeat. Physical and psychological stress was delivered to an intruder that was attacked by an aggressive resident. Meanwhile, an observer experienced psychological stress by seeing aggressor attacks. The nucleus accumbens tissues from intruder and observer mice that appear fear memory and anxiety as well as control mice were harvested for analyses of mRNA and miRNA profiles by high throughput sequencing. In the nucleus accumbens of intruders and observers with fear memory and anxiety, genes encoding AdrRα, AChRM2/3, GluRM2/8, HrR1, SSR, BDNF and AC are upregulated, while genes encoding DR3/5, PR2, GPγ8 and P450 are downregulated. Physical and/or psychological stress leads to fear memory and anxiety likely by molecules relevant to certain synapses. Moreover, there are differential expressions in genes that encode GABAR_A, 5-HTR1/5, CREB3, AChRM2, RyR, Wnt and GPγ13 in the nucleus accumbens from intruders versus observers. GABAergic, serotonergic and cholinergic synapses as well as calcium, Wnt and CREB signaling molecules may be involved in fear memory differently induced by psychological stress and physical/psychological stress. The data from analyzing mRNA and miRNA profiles are consistent. Some molecules are validated by qRT-PCR and dual luciferase reporter assay. Fear memory and anxiety induced by the mixture of physical and psychological stress or psychological stress appear influenced by complicated molecular mechanisms in the nucleus accumbens.

Consequences of VGluT3 deficiency on learning and memory in mice

The aim of the present study was to test the hypothesis that vesicular glutamate transporter 3 (VGluT3) deficiency is associated with cognitive impairments. Male VGluT3 knockout (KO) and wild type (WT) mice were exposed to a behavioral test battery covering paradigms based on spontaneous exploratory behavior and reinforcement-based learning tests. Reversal learning was examined to test the cognitive flexibility. The VGluT3 KO mice clearly exhibited the ability to learn. The social recognition memory of KO mice was intact. The y-maze test revealed weaker working memory of VGluT3 KO mice. No significant learning impairments were noticed in operant conditioning or holeboard discrimination paradigm. In avoidance-based learning tests (Morris water maze and active avoidance), KO mice exhibited slightly slower learning process compared to WT mice, but not a complete learning impairment. In tests based on simple associations (operant conditioning, avoidance learning) an attenuation of cognitive flexibility was observed in KO mice. In conclusion, knocking out VGluT3 results in mild disturbances in working memory and learning flexibility. Apparently, this glutamate transporter is not a major player in learning and memory formation in general. Based on previous characteristics of VGluT3 KO mice we would have expected a stronger deficit. The observed hypolocomotion did not contribute to the mild cognitive disturbances herein reported, either.

The Virtual-Environment-Foraging Task enables rapid training and single-trial metrics of rule acquisition and reversal in head-fixed mice

Behavioural flexibility is an essential survival skill, yet our understanding of its neuronal substrates is still limited. While mouse research offers unique tools to dissect the neuronal circuits involved, the measurement of flexible behaviour in mice often suffers from long training times, poor experimental control, and temporally imprecise binary (hit/miss) performance readouts. Here we present a virtual-environment task for mice that tackles these limitations. It offers fast training of vision-based rule reversals (~100 trials per reversal) with full stimulus control and continuous behavioural readouts. By generating multiple non-binary performance metrics per trial, it provides single-trial estimates not only of response accuracy and speed, but also of underlying processes like choice certainty and alertness (discussed in detail in a companion paper). Based on these metrics, we show that mice can predict new task rules long before they are able to execute them, and that this delay varies across animals. We also provide and validate single-trial estimates of whether an error was committed with or without awareness of the task rule. By tracking in unprecedented detail the cognitive dynamics underlying flexible behaviour, this task enables new investigations into the neuronal interactions that shape behavioural flexibility moment by moment.

Preproenkephalin-expressing ventral pallidal neurons control inhibitory avoidance learning

The ventral pallidum (VP) is a critical component of the basal ganglia neurocircuitry regulating learning and decision making; however, its precise role in controlling associative learning of environmental stimuli conditioned to appetitive or aversive outcomes is still unclear. Here, we investigated the expression of preproenkephalin, a polypeptide hormone previously shown to be expressed in nucleus accumbens neurons controlling aversive learning, within GABAergic and glutamatergic VP neurons. Next, we explored the behavioral consequences of chemicogenetic inhibition or excitation of preproenkephalin-expressing VP neurons on associative learning of reward- or aversion-paired stimuli in autoshaping and inhibitory avoidance tasks, respectively. We reveal for the first time that preproenkephalin is expressed predominantly in GABAergic rather than glutamatergic VP neurons, and that excitation of these preproenkephalin-expressing VP neurons was sufficient to impair inhibitory avoidance learning. These findings indicate the necessity for inhibition of preproenkephalin-expressing VP neurons for avoidance learning, and suggest these neurons as a potential therapeutic target for psychiatric disorders associated with maladaptive aversive learning.

Optogenetic investigation of neural mechanisms for alcohol-use disorder

Optogenetic techniques have been widely used in the study of neuropsychiatric diseases such as anxiety, depression, and drug addiction. Cell-type specific targeting of optogenetic tools to neurons has contributed to a tremendous understanding of the function of neural circuits for future treatment of neuropsychiatric disorders. Though optogenetics has been widely used in many research areas, the use of optogenetic tools to uncover and elucidate neural circuit mechanisms of alcohol's actions in the brain are still developing. Here in this review article, we will provide a basic introduction to optogenetics and discuss how these optogenetic experimental approaches can be used in alcohol studies to reveal neural circuit mechanisms of alcohol's actions in regions implicated in the development of alcohol addiction.

[Homeostatic regulation of basal ganglia circuit for flexible behavior]

Behavioral inflexibility has been reported in various psychiatric disorders including drug addiction and schizophrenia. Considerable evidence has demonstrated a critical role for the basal ganglia in the flexibility of behavioral strategies. These processes are guided by the activity of two discrete neuron types, dopamine D1- or D2-receptor expressing medium spiny neurons (D1-/D2-MSNs) in the basal ganglia circuit. We used a reversible neurotransmission blocking technique to examine the role of D1- and D2-MSNs in the acquisition and reversal learning of a place discrimination task in the IntelliCage. We demonstrated that D1- and D2-MSNs do not mediate the acquisition of the task, but that suppression of activity in D2-MSNs impairs reversal learning and increased perseverative errors. Additionally, global knockout of the dopamine D2L receptor isoform produced a similar behavioral phenotype to D2-MSN-blocked mice. We also showed that D2L receptors are necessary for visual discrimination and reversal learning. These results suggest that D2L receptors and D2-MSNs have a critical role in the homeostatic regulation of basal ganglia circuit for flexible behaviors.

Research Domain Criteria: Cutting Edge Neuroscience or Galen's Humors Revisited?

The Research Domain Criteria (RDoC) scheme has guided the research agenda of the National Institute of Mental Health for the past decade. The essence of RDoC is its dimensional conception of mental illness, with the assumption that psychopathology is a manifestation of extremes along axes of neuropsychological variation. Research, it follows, should emphasize normal neuropsychological function and its associated neurocircuitry. We argue that RDoC, dressed in terms of modern neurobiology, is in fact a return to the humoral theory of Galen, a dimensional approach in which physical and mental health requires a balance of the four basic bodily humors (blood, black bile, yellow bile, and phlegm). The RDoC/Galenic approach may be useful in understanding those conditions best understood as extremes along a continuum, such as personality disorders. However, we contend that for the most severe psychiatric disorders - categorically defined diseases such as schizophrenia, bipolar disorder, and autism - RDoC's Galenic dimensionalism is a retreat from the biomedical approach that seeks to find rational therapeutic targets by identifying etiologic factors and pathogenic pathways. Abandoning this medical model now, in the context of remarkable advances in genetics, neuroimaging, and neuroscience, is a major setback for the advancement of scientific psychiatry.

Cell type-specific activation of mitogen-activated protein kinase in D1 receptor-expressing neurons of the nucleus accumbens potentiates stimulus-reward learning in mice

Medium spiny neurons (MSN) in the nucleus accumbens (NAc) are a fundamental component of various aspects of motivated behavior. Although mitogen-activated protein kinase (MAPK) signaling plays a crucial role in several types of learning, the cell type-specific role of MAPK pathway in stimulus-reward learning and motivation remains unclear. We herein investigated the role of MAPK in accumbal MSNs in reward-associated learning and memory. During the acquisition of Pavlovian conditioning, the number of phosphorylated MAPK1/3-positive cells was increased significantly and exclusively in the NAc core by 7-days of extensive training. MAPK signaling in the respective D1R- and D2R-MSNs was manipulated by transfecting an adeno-associated virus (AAV) plasmid into the NAc of Drd1a-Cre and Drd2-Cre transgenic mice. Potentiation of MAPK signaling shifted the learning curve of Pavlovian conditioning to the left only in Drd1a-Cre mice, whereas such manipulation in D2R-MSNs had negligible effects. In contrast, MAPK manipulation in D2R-MSNs of the NAc core significantly increased motivation for food rewards as found in Drd1a-Cre mice. These results suggest that MAPK signaling in the D1R-MSNs of NAc core plays an important role in stimulus-reward learning, while MAPK signaling in both D1R- and D2R-MSNs is involved in motivation for natural rewards.

Deficits in mesolimbic reward pathway underlie social interaction impairments in children with autism

Lack of interest in social interaction is a hallmark of autism spectrum disorder. Animal studies have implicated the mesolimbic reward pathway in driving and reinforcing social behaviour, but little is known about the integrity of this pathway and its behavioural consequences in children with autism spectrum disorder. Here we test the hypothesis that the structural and functional integrity of the mesolimbic reward pathway is aberrant in children with autism spectrum disorder, and these aberrancies contribute to the social interaction impairments. We examine structural and functional connectivity of the mesolimbic reward pathway in two independent cohorts totalling 82 children aged 7-13 years with autism spectrum disorder and age-, gender-, and intelligence quotient-matched typically developing children (primary cohort: children with autism spectrum disorder n = 24, typically developing children n = 24; replication cohort: children with autism spectrum disorder n = 17, typically developing children n = 17), using high angular resolution diffusion-weighted imaging and functional MRI data. We reliably identify white matter tracts linking-the nucleus accumbens and the ventral tegmental area-key subcortical nodes of the mesolimbic reward pathway, and provide reproducible evidence for structural aberrations in these tracts in children with autism spectrum disorder. Further, we show that structural aberrations are accompanied by aberrant functional interactions between nucleus accumbens and ventral tegmental area in response to social stimuli. Crucially, we demonstrate that both structural and functional circuit aberrations in the mesolimbic reward pathway are related to parent-report measures of social interaction impairments in affected children. Our findings, replicated across two independent cohorts, reveal that deficits in the mesolimbic reward pathway contribute to impaired social skills in childhood autism, and provide fundamental insights into neurobiological mechanisms underlying reduced social interest in humans.

A rodent brain-machine interface paradigm to study the impact of paraplegia on BMI performance

BACKGROUND

Most brain machine interfaces (BMI) focus on upper body function in non-injured animals, not addressing the lower limb functional needs of those with paraplegia. A need exists for a novel BMI task that engages the lower body and takes advantage of well-established rodent spinal cord injury (SCI) models to study methods to improve BMI performance.

NEW METHOD

A tilt BMI task was designed that randomly applies different types of tilts to a platform, decodes the tilt type applied and rights the platform if the decoder correctly classifies the tilt type. The task was tested on female rats and is relatively natural such that it does not require the animal to learn a new skill. It is self-rewarding such that there is no need for additional rewards, eliminating food or water restriction, which can be especially hard on spinalized rats. Finally, task difficulty can be adjusted by making the tilt parameters.

RESULTS

This novel BMI task bilaterally engages the cortex without visual feedback regarding limb position in space and animals learn to improve their performance both pre and post-SCI.Comparison with Existing Methods: Most BMI tasks primarily engage one hemisphere, are upper-body, rely heavily on visual feedback, do not perform investigations in animal models of SCI, and require nonnaturalistic extrinsic motivation such as water rewarding for performance improvement. Our task addresses these gaps.

CONCLUSIONS

The BMI paradigm presented here will enable researchers to investigate the interaction of plasticity after SCI and plasticity during BMI training on performance.

Estradiol and the control of feeding behavior

This review lays out the evidence for the role of E2 in homeostatic and hedonic feeding across several species. While significant effort has been expended on homeostatic feeding research, more studies for hedonic feeding need to be conducted (i.e. are there increases in meal size and enhanced motivation to natural food rewards). By identifying the underlying neural circuitry involved, one can better delineate the mechanisms by which E2 influences feeding behavior. By utilizing more selective neural targeting techniques, such as optogenetics, significant progress can be made toward this goal. Together, behavioral and physiological techniques will help us to better understand neural deficits that can increase the risk for obesity in the absence of E2 (menopause) and aid in developing therapeutic strategies.

Nucleus accumbens controls wakefulness by a subpopulation of neurons expressing dopamine D1 receptors

Nucleus accumbens (NAc) is involved in behaviors that depend on heightened wakefulness, but its impact on arousal remains unclear. Here, we demonstrate that NAc dopamine D₁ receptor (D₁R)-expressing neurons are essential for behavioral arousal. Using in vivo fiber photometry in mice, we find arousal-dependent increases in population activity of NAc D₁R neurons. Optogenetic activation of NAc D₁R neurons induces immediate transitions from non-rapid eye movement sleep to wakefulness, and chemogenetic stimulation prolongs arousal, with decreased food intake. Patch-clamp, tracing, immunohistochemistry, and electron microscopy reveal that NAc D₁R neurons project to the midbrain and lateral hypothalamus, and might disinhibit midbrain dopamine neurons and lateral hypothalamus orexin neurons. Photoactivation of terminals in the midbrain and lateral hypothalamus is sufficient to induce wakefulness. Silencing of NAc D₁R neurons suppresses arousal, with increased nest-building behaviors. Collectively, our data indicate that NAc D₁R neuron circuits are essential for the induction and maintenance of wakefulness.

The nucleus accumbens regulates many behaviours that depend on arousal. Here the authors show that dopamine D₁ receptor neurons in the nucleus accumbens can directly regulate wakefulness.

GABAergic neurons in nucleus accumbens are correlated to resilience and vulnerability to chronic stress for major depression

Background

Major depression, persistent low mood, is one of common psychiatric diseases. Chronic stressful life is believed to be a major risk factor that leads to dysfunctions of the limbic system. However, a large number of the individuals with experiencing chronic stress do not suffer from major depression, called as resilience. Endogenous mechanisms underlying neuronal invulnerability to chronic stress versus major depression are largely unknown. As GABAergic neurons are vulnerable to chronic stress and their impairments is associated with major depression, we have examined whether the invulnerability of GABAergic neurons in the limbic system is involved in resilience.

Results

GABAergic neurons in the nucleus accumbens from depression-like mice induced by chronic unpredictable mild stress appear the decreases in their GABA release, spiking capability and excitatory input reception, compared with those in resilience mice. The levels of decarboxylase and vesicular GABA transporters decrease in depression-like mice, but not resilience.

Materials and Methods

Mice were treated by chronic unpredictable mild stress for three weeks. Depression-like behaviors or resilience was confirmed by seeing whether their behaviors change significantly in sucrose preference, Y-maze and forced swimming tests. Mice from controls as well as depression and resilience in response to chronic unpredictable mild stress were studied in terms of GABAergic neuron activity in the nucleus accumbens by cell electrophysiology and protein chemistry.

Conclusions

The impairment of GABAergic neurons in the nucleus accumbens is associated with major depression. The invulnerability of GABAergic neurons to chronic stress may be one of cellular mechanisms for the resilience to chronic stress.

Opioid and nondopamine reward circuitry and state-dependent mechanisms

A common notion is that essentially all addictive drugs, including opioids, activate dopaminergic pathways in the brain reward system, and the inappropriate use of such drugs induces drug dependence. However, an opioid reward response is reportedly still observed in several models of dopamine depletion, including in animals that are treated with dopamine blockers, animals that are subjected to dopaminergic neuron lesions, and dopamine-deficient mice. The intracranial self-stimulation response is enhanced by stimulants but reduced by morphine. These findings suggest that dopaminergic neurotransmission may not always be required for opioid reward responses. Previous findings also indicate the possibility that dopamine-independent opioid reward may be observed in opioid-naive states but not in opioid-dependent states. Therefore, a history of opioid use should be considered when evaluating the dopamine dependency of opioid reward.

Anxiety-like behavior and neuropeptide receptor expression in male and female prairie voles: The effects of stress and social buffering

Strong social support can negate negative health outcomes - an effect defined as 'social buffering'. In the present study, using the socially monogamous prairie vole (Microtus ochrogaster), we examined whether the presence of a bonded partner during a stressful event can reduce stress responses. Adult, pair-bonded female and male voles were assigned into experimental groups that were either handled (Control), experienced a 1-h immobilization (IMO) stress alone (IMO-Alone), or experienced IMO with their partner (IMO-Partner). Thereafter, subjects were tested for anxiety-like behavior, and brain sections were subsequently processed for oxytocin receptor (OTR) and vasopressin V1a-type receptor (V1aR) binding. Our data indicate that while IMO stress significantly decreased the time that subjects spent in the open arms of an elevated plus maze, partner's presence prevented this behavioral change - this social buffering on anxiety-like behavior was the same for both male and female subjects. Further, IMO stress decreased OTR binding in the nucleus accumbens (NAcc), but a partner's presence dampened this effect. No effects were found in V1aR binding. These data suggest that the neuropeptide- and brain region-specific OTR alterations in the NAcc may be involved in both the mediation and social buffering of stress responses. Some sex differences in the OTR and V1aR binding were also found in selected brain regions, offering new insights into the sexually dimorphic roles of the two neuropeptides. Overall, our results suggest a potential preventative approach in which the presence of social interactions during a stressor may buffer typical negative outcomes.

Repetitive behaviors in autism are linked to imbalance of corticostriatal connectivity: a functional connectivity MRI study

The neural underpinnings of repetitive behaviors (RBs) in autism spectrum disorders (ASDs), ranging from cognitive to motor characteristics, remain unknown. We assessed RB symptomatology in 50 ASD and 52 typically developing (TD) children and adolescents (ages 8-17 years), examining intrinsic functional connectivity (iFC) of corticostriatal circuitry, which is important for reward-based learning and integration of emotional, cognitive and motor processing, and considered impaired in ASDs. Connectivity analyses were performed for three functionally distinct striatal seeds (limbic, frontoparietal and motor). Functional connectivity with cortical regions of interest was assessed for corticostriatal circuit connectivity indices and ratios, testing the balance of connectivity between circuits. Results showed corticostriatal overconnectivity of limbic and frontoparietal seeds, but underconnectivity of motor seeds. Correlations with RBs were found for connectivity between the striatal motor seeds and cortical motor clusters from the whole-brain analysis, and for frontoparietal/limbic and motor/limbic connectivity ratios. Division of ASD participants into high (n = 17) and low RB subgroups (n = 19) showed reduced frontoparietal/limbic and motor/limbic circuit ratios for high RB compared to low RB and TD groups in the right hemisphere. Results suggest an association between RBs and an imbalance of corticostriatal iFC in ASD, being increased for limbic, but reduced for frontoparietal and motor circuits.