Distinct roles for direct and indirect pathway striatal neurons in reinforcement

Operant Training for Highly Palatable Food Alters Translating Messenger RNA in Nucleus Accumbens D2 Neurons and Reveals a Modulatory Role of Ncdn

BACKGROUND

Highly palatable food triggers behavioral responses including strong motivation. These effects involve the reward system and dopamine neurons, which modulate neurons in the nucleus accumbens (NAc). The molecular mechanisms underlying the long-lasting effects of highly palatable food on feeding behavior are poorly understood.

METHODS

We studied the effects of 2-week operant conditioning of mice with standard or isocaloric highly palatable food. We investigated the behavioral responses and dendritic spine modifications in the NAc. We compared the translating messenger RNA in NAc neurons identified by the type of dopamine receptors they express, depending on the kind of food and training. We tested the consequences of invalidation of an abundant downregulated gene, Ncdn.

RESULTS

Operant conditioning for highly palatable food increased motivation for food even in well-fed mice. In wild-type mice, free choice between regular and highly palatable food increased weight compared with access to regular food only. Highly palatable food increased spine density in the NAc. In animals trained for highly palatable food, translating messenger RNAs were modified in NAc neurons expressing dopamine D2 receptors, mostly corresponding to striatal projection neurons, but not in neurons expressing D1 receptors. Knockout of Ncdn, an abundant downregulated gene, opposed the conditioning-induced changes in satiety-sensitive feeding behavior and apparent motivation for highly palatable food, suggesting that downregulation may be a compensatory mechanism.

CONCLUSIONS

Our results emphasize the importance of messenger RNA alterations in D2 striatal projection neurons in the NAc in the behavioral consequences of highly palatable food conditioning and suggest a modulatory contribution of Ncdn downregulation.

A dopamine mechanism for reward maximization

Individual survival and evolutionary selection require biological organisms to maximize reward. Economic choice theories define the necessary and sufficient conditions, and neuronal signals of decision variables provide mechanistic explanations. Reinforcement learning (RL) formalisms use predictions, actions, and policies to maximize reward. Midbrain dopamine neurons code reward prediction errors (RPE) of subjective reward value suitable for RL. Electrical and optogenetic self-stimulation experiments demonstrate that monkeys and rodents repeat behaviors that result in dopamine excitation. Dopamine excitations reflect positive RPEs that increase reward predictions via RL; against increasing predictions, obtaining similar dopamine RPE signals again requires better rewards than before. The positive RPEs drive predictions higher again and thus advance a recursive reward-RPE-prediction iteration toward better and better rewards. Agents also avoid dopamine inhibitions that lower reward prediction via RL, which allows smaller rewards than before to elicit positive dopamine RPE signals and resume the iteration toward better rewards. In this way, dopamine RPE signals serve a causal mechanism that attracts agents via RL to the best rewards. The mechanism improves daily life and benefits evolutionary selection but may also induce restlessness and greed.

Treadmill exercise training inhibits morphine CPP by reversing morphine effects on GABA neurotransmission in D2-MSNs of the accumbens-pallidal pathway in male mice

Relapse is a major challenge in the treatment of drug addiction, and exercise has been shown to decrease relapse to drug seeking in animal models. However, the neural circuitry mechanisms by which exercise inhibits morphine relapse remain unclear. In this study, we report that 4-week treadmill training prevented morphine conditioned place preference (CPP) expression during abstinence by acting through the nucleus accumbens (NAc)-ventral pallidum (VP) pathway. We found that neuronal excitability was reduced in D2-dopamine receptor-expressing medium spiny neurons (D2-MSNs) following repeated exposure to morphine and forced abstinence. Enhancing the excitability of NAc D2-MSNs via treadmill training decreased the expression of morphine CPP. We also found that the effects of treadmill training were mediated by decreasing enkephalin levels and that restoring opioid modulation of GABA neurotransmission in the VP, which increased neurotransmitter release from NAc D2-MSNs to VP, decreased morphine CPP. Our findings suggest the inhibitory effect of exercise on morphine CPP is mediated by reversing morphine-induced neuroadaptations in the NAc-to-VP pathway.

Fundamental Sex Differences in Cocaine-Induced Plasticity of Dopamine D1 Receptor- and D2 Receptor-Expressing Medium Spiny Neurons in the Mouse Nucleus Accumbens Shell

Background

Cocaine-induced plasticity in the nucleus accumbens shell of males occurs primarily in dopamine D1 receptor-expressing medium spiny neurons (D1R-MSNs), with little if any impact on dopamine D2 receptor-expressing medium spiny neurons (D2R-MSNs). In females, the effect of cocaine on accumbens shell D1R- and D2R-MSN neurophysiology has yet to be reported, nor have estrous cycle effects been accounted for.

Methods

We used a 5-day locomotor sensitization paradigm followed by a 10- to 14-day drug-free abstinence period. We then obtained ex vivo whole-cell recordings from fluorescently labeled D1R-MSNs and D2R-MSNs in the nucleus accumbens shell of male and female mice during estrus and diestrus. We examined accumbens shell neuronal excitability as well as miniature excitatory postsynaptic currents (mEPSCs).

Results

In females, we observed alterations in D1R-MSN excitability across the estrous cycle similar in magnitude to the effects of cocaine in males. Furthermore, cocaine shifted estrous cycle-dependent plasticity from intrinsic excitability changes in D1R-MSNs to D2R-MSNs. In males, cocaine treatment produced the anticipated drop in D1R-MSN excitability with no effect on D2R-MSN excitability. Cocaine increased mEPSC frequencies and amplitudes in D2R-MSNs from females in estrus and mEPSC amplitudes of D2R-MSNs from females in diestrus. In males, cocaine increased both D1R- and D2R-MSN mEPSC amplitudes with no effect on mEPSC frequencies.

Conclusions

Overall, while there are similar cocaine-induced disparities regarding the relative excitability of D1R-MSNs versus D2R-MSNs between the sexes, this is mediated through reduced D1R-MSN excitability in males, whereas it is due to heightened D2R-MSN excitability in females.

N-acetylcysteine attenuates accumbal core neuronal activity in response to morphine in the reinstatement of morphine CPP in morphine extinguished rats

Numerous studies have suggested that N-acetylcysteine (NAC), has the potential to suppress drug craving in people with substance use disorder and reduce drug-seeking behaviors in animals. The nucleus accumbens (NAc) plays a crucial role in the brain's reward system, with the nucleus accumbens core (NAcore) specifically implicated in compulsive drug seeking and relapse. In this study, we aimed to explore the impact of subchronic NAC administration during the extinction period and acute NAC administration on the electrical activity of NAcore neurons in response to a priming dose of morphine in rats subjected to extinction from morphine-induced place preference (CPP).We conducted single-unit recordings in anesthetized rats on the reinstatement day, following the establishment of morphine-induced conditioned place preference (7 mg/kg, s.c., 3 days), and subsequent drug-free extinction. In the subchronically NAC-treated groups, rats received daily injections of either NAC (50 mg/kg; i.p.) or saline during the extinction period. On the reinstatement day, we recorded the spontaneous activity of NAcore neurons for 15 min, administered a priming dose of morphine, and continued recording for an additional 45 min. While morphine excited most recorded neurons in saline-treated rats, it failed to alter firing rates in NAC-treated rats that had received NAC during the extinction period. For acutely NAC-treated animals, we recorded the baseline activity of NAcore neurons for 10 min before administering a single injection of either NAC (50 mg/kg; i.p.) or saline in rats with no treatment during the extinction. Following 30 min of recording and a priming dose of morphine (1 mg/kg, s.c.), the recording continued for an additional 30 min. The firing activity of NAcore neurons did not show significant changes after morphine or NAC injection. In conclusion, our findings emphasize that daily NAC administration during the extinction period significantly attenuates the morphine-induced increase in firing rates of NAcore neurons during the reinstatement of morphine CPP. However, acute NAC injection does not produce the same effect. These results suggest that modulating glutamate transmission through daily NAC during extinction may effectively inhibit the morphine place preference following the excitatory effects of morphine on NAcore neurons.

Drugs of abuse hijack a mesolimbic pathway that processes homeostatic need

Drugs of abuse are thought to promote addiction in part by "hijacking" brain reward systems, but the underlying mechanisms remain undefined. Using whole-brain FOS mapping and in vivo single-neuron calcium imaging, we found that drugs of abuse augment dopaminoceptive ensemble activity in the nucleus accumbens (NAc) and disorganize overlapping ensemble responses to natural rewards in a cell type-specific manner. Combining FOS-Seq, CRISPR-perturbation, and single-nucleus RNA sequencing, we identified Rheb as a molecular substrate that regulates cell type-specific signal transduction in NAc while enabling drugs to suppress natural reward consumption. Mapping NAc-projecting regions activated by drugs of abuse revealed input-specific effects on natural reward consumption. These findings characterize the dynamic, molecular and circuit basis of a common reward pathway, wherein drugs of abuse interfere with the fulfillment of innate needs.

Activation of nucleus accumbens projections to the ventral tegmental area alters molecular signaling and neurotransmission in the reward system

The nucleus accumbens (NAc) and the ventral tegmental area (VTA) are integral brain regions involved in reward processing and motivation, including responses to drugs of abuse. Previously, we have demonstrated that activation of NAc-VTA afferents during the acquisition of cocaine conditioned place preference (CPP) reduces the rewarding properties of cocaine and diminished the activity of VTA dopamine neurons. In the current study, we examined the impact of enhancing these inhibitory inputs on molecular changes and neurotransmission associated with cocaine exposure. Our results unveiled significant reductions in extracellular signal-regulated kinase (ERK) levels in the VTA and medial prefrontal cortex (mPFC) of both cocaine-treated groups compared with the saline control group. Furthermore, optic stimulation of NAc-VTA inputs during cocaine exposure decreased the expression of GluA1 subunit of AMPA receptor in the VTA and mPFC. Notably, in the NAc, cocaine exposure paired with optic stimulation increased ERK levels and reduced GluA1 phosphorylation at Ser845 as compared with all other groups. Additionally, both cocaine-treated groups exhibited decreased levels of GluA1 phosphorylation at Ser831 in the NAc compared with the saline control group. Moreover, cocaine exposure led to reduced ERK, GluA1, and GluA1 phosphorylation at Ser845 and Ser831 in the mPFC. Augmentation of GABAergic tone from the NAc during cocaine conditioning mitigated changes in GluA1 phosphorylation at Ser845 in the mPFC but reduced ERK, GluA1, and GluA1 phosphorylation at Ser831 compared with the saline control group. Interestingly, enhancing GABAergic tone during saline conditioning decreased GluA1 phosphorylation at Ser831 compared with the saline control group in the mPFC. Our findings highlight the influence of modulating inhibitory inputs from the NAc to the VTA on molecular signaling and glutamatergic neurotransmission in cocaine-exposed animals. Activation of these inhibitory inputs during cocaine conditioning induced alterations in key signaling molecules and AMPA receptor, providing valuable insights into the neurobiological mechanisms underlying cocaine reward and cocaine use disorder. Further exploration of these pathways may offer potential therapeutic targets for the treatment of substance use disorder.

D1 and D2 medium spiny neurons in the nucleus accumbens core have distinct and valence-independent roles in learning

At the core of value-based learning is the nucleus accumbens (NAc). D1- and D2-receptor-containing medium spiny neurons (MSNs) in the NAc core are hypothesized to have opposing valence-based roles in behavior. Using optical imaging and manipulation approaches in mice, we show that neither D1 nor D2 MSNs signal valence. D1 MSN responses were evoked by stimuli regardless of valence or contingency. D2 MSNs were evoked by both cues and outcomes, were dynamically changed with learning, and tracked valence-free prediction error at the population and individual neuron level. Finally, D2 MSN responses to cues were necessary for associative learning. Thus, D1 and D2 MSNs work in tandem, rather than in opposition, by signaling specific properties of stimuli to control learning.

HIV-1 Tat and morphine interactions dynamically shift striatal monoamine levels and exploratory behaviors over time

Despite the advent of combination anti-retroviral therapy (cART), nearly half of people infected with HIV treated with cART still exhibit HIV-associated neurocognitive disorders (HAND). HAND can be worsened by co-morbid opioid use disorder. The basal ganglia are particularly vulnerable to HIV-1 and exhibit higher viral loads and more severe pathology, which can be exacerbated by co-exposure to opioids. Evidence suggests that dopaminergic neurotransmission is disrupted by HIV exposure, however, little is known about whether co-exposure to opioids may alter neurotransmitter levels in the striatum and if this in turn influences behavior. Therefore, we assayed motor, anxiety-like, novelty-seeking, exploratory, and social behaviors, and levels of monoamines and their metabolites following 2 weeks and 2 months of Tat and/or morphine exposure in transgenic mice. Morphine decreased dopamine levels, but significantly elevated norepinephrine, the dopamine metabolites dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), and the serotonin metabolite 5-hydroxyindoleacetic acid, which typically correlated with increased locomotor behavior. The combination of Tat and morphine altered dopamine, DOPAC, and HVA concentrations differently depending on the neurotransmitter/metabolite and duration of exposure but did not affect the numbers of tyrosine hydroxylase-positive neurons in the mesencephalon. Tat exposure increased the latency to interact with novel conspecifics, but not other novel objects, suggesting the viral protein inhibits exploratory behavior initiation in a context-dependent manner. By contrast, and consistent with prior findings that opioid misuse can increase novelty-seeking behavior, morphine exposure increased the time spent exploring a novel environment. Finally, Tat and morphine interacted to affect locomotor activity in a time-dependent manner, while grip strength and rotarod performance were unaffected. Together, our results provide novel insight into the unique effects of HIV-1 Tat and morphine on monoamine neurochemistry that may underlie their divergent effects on motor and exploratory behavior.

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.

Sex difference in the effect of environmental enrichment on food restriction-induced persistence of cocaine conditioned place preference and mechanistic underpinnings

Psychosocial and environmental factors, including loss of natural reward, contribute to the risk of drug abuse. Reward loss has been modeled in animals by removal from social or sexual contact, transfer from enriched to impoverished housing, or restriction of food. We previously showed that food restriction increases the unconditioned rewarding effects of abused drugs and the conditioned incentive effects of drug-paired environments. Mechanistic studies provided evidence of decreased basal dopamine (DA) transmission, adaptive upregulation of signaling downstream of D1 DA receptor stimulation, synaptic upscaling and incorporation of calcium-permeable AMPA receptors (CP-AMPARs) in medium spiny neurons (MSNs) of nucleus accumbens (NAc). These findings align with the still evolving 'reward deficiency' hypothesis of drug abuse. The present study tested whether a compound natural reward that is known to increase DA utilization, environmental enrichment, would prevent the persistent expression of cocaine conditioned place preference (CPP) otherwise observed in food restricted rats, along with the mechanistic underpinnings. Because nearly all prior investigations of both food restriction and environmental enrichment effects on cocaine CPP were conducted in male rodents, both sexes were included in the present study. Results indicate that environmental enrichment curtailed the persistence of CPP expression, decreased signaling downstream of the D1R, and decreased the amplitude and frequency of spontaneous excitatory postsynaptic currents (EPSCs) in NAc MSNs of food restricted male, but not female, rats. The failure of environmental enrichment to significantly decrease food restriction-induced synaptic insertion of CP-AMPARs, and how this may accord with previous pharmacological findings that blockade of CP-AMPARs reverses behavioral effects of food restriction is discussed. In addition, it is speculated that estrous cycle-dependent fluctuations in DA release, receptor density and MSN excitability may obscure the effect of increased DA signaling during environmental enrichment, thereby interfering with development of the cellular and behavioral effects that enrichment produced in males.

Role of nucleus accumbens D1-type medium spiny neurons in the expression and extinction of sign-tracking

While sign-tracking, also known as autoshaping, has been studied for many decades, only recently has the tendency to show sign-tracking behavior been linked to the development and persistence of addiction. Sign-tracking is dependent upon dopamine activity in the nucleus accumbens (NAc). The NAc is comprised predominantly of medium spiny projection neurons (MSN) that can be differentiated by their D1-like or D2-like dopamine receptor expression. Here we determined how reducing activity of D1-type MSNs in the NAc affects the expression and extinction of sign-tracking. To address this, we transfected the NAc of transgenic male and female rats that selectively express Cre recombinase in D1-type MSNs with a DIO viral vector expressing hM4Di. Cre- rats were given the same viral infusion but did not express the hM4Di receptor and therefore served as controls. Rats were then conditioned to associate lever presentations with pellet delivery. After sign-tracking was established, all rats were administered clozapine-n-oxide (CNO) prior to three additional conditioning sessions to assess the effects of NAc D1-MSNs inhibition on sign-tracking in the presence of reward. CNO treatment did not alter the expression of sign-tracking in Cre+ or Cre- rats. Next rats underwent extinction training where lever presentations occurred without pellet delivery and all rats received a CNO injection prior to each extinction session. In these extinction conditions, Cre+ rats exhibited robust extinction of sign-tracking across sessions, whereas Cre- rats did not. To determine if D1-MSN inhibition merely produced a temporary cessation of sign-tracking or instead had facilitated a persistent loss of sign-tracking, we evaluated the reemergence of sign-tracking in a test for reconditioning. During testing, reintroduction of the CS-US pairing did not promote the reemergence of sign-tracking in Cre+ rats, but restored sign-tracking in Cre- rats. Thus, chemogenetic inhibition of NAc D1-MSNs promoted extinction of sign-tracking. Collectively, these data suggest that D1-MSNs play an important role in resistance to extinction that typifies sign-tracking behavior.

NMDA- and 6-OHDA-induced Lesions in the Nucleus Accumbens Differently Affect Maternal and Infanticidal Behavior in Pup-naïve Female and Male Mice

Virgin and pups-naïve female and male adult mice display two opposite responses when they are exposed to pups for the first time. While females generally take care of the pups, males attack them. Since the nucleus accumbens (NA), and its dopaminergic modulation, is critical in integrating information and processing reward and aversion, we investigated if NMDA- and 6-OHDA-induced lesions, damaging mostly NA output and dopaminergic inputs respectively, affected female maternal behavior (MB) or male infanticidal behavior (IB) in mice. Our results revealed minor or no effects of both smaller and larger NMDA-induced lesions in MB and IB. On the other hand, while 6-OHDA-induced lesions in females reduced the incidence of full MB (12.5% 6-OHDA vs. 85.7% SHAM) increasing the latency to retrieve the pups, those lesions did not affect IB in males. There were no differences in locomotor and exploratory activity between the lesioned- and SHAM- females. Despite those lesions did not induce any major effect on IB, NMDA-lesioned males spent less time in the central area of an open field, while dopaminergic-lesioned males showed reduced number of rearing and peripheral crosses. The current study shows that an intact NA is not necessary for the expression of MB and IB. However, dopaminergic inputs to NA play different role in MB and IB. While damaging dopaminergic terminals into the NA did not affect IB, it clearly delayed the more flexible and rewarding expression of parental behavior.

Dopaminergic reinforcement in the motor system: Implications for Parkinson's disease and deep brain stimulation

Millions of people suffer from dopamine-related disorders spanning disturbances in movement, cognition and emotion. These changes are often attributed to changes in striatal dopamine function. Thus, understanding how dopamine signalling in the striatum and basal ganglia shapes human behaviour is fundamental to advancing the treatment of affected patients. Dopaminergic neurons innervate large-scale brain networks, and accordingly, many different roles for dopamine signals have been proposed, such as invigoration of movement and tracking of reward contingencies. The canonical circuit architecture of cortico-striatal loops sparks the question, of whether dopamine signals in the basal ganglia serve an overarching computational principle. Such a holistic understanding of dopamine functioning could provide new insights into symptom generation in psychiatry to neurology. Here, we review the perspective that dopamine could bidirectionally control neural population dynamics, increasing or decreasing their strength and likelihood to reoccur in the future, a process previously termed neural reinforcement. We outline how the basal ganglia pathways could drive strengthening and weakening of circuit dynamics and discuss the implication of this hypothesis on the understanding of motor signs of Parkinson's disease (PD), the most frequent dopaminergic disorder. We propose that loss of dopamine in PD may lead to a pathological brain state where repetition of neural activity leads to weakening and instability, possibly explanatory for the fact that movement in PD deteriorates with repetition. Finally, we speculate on how therapeutic interventions such as deep brain stimulation may be able to reinstate reinforcement signals and thereby improve treatment strategies for PD in the future.

Dorsal hippocampus to nucleus accumbens projections drive reinforcement via activation of accumbal dynorphin neurons

The hippocampus is pivotal in integrating emotional processing, learning, memory, and reward-related behaviors. The dorsal hippocampus (dHPC) is particularly crucial for episodic, spatial, and associative memory, and has been shown to be necessary for context- and cue-associated reward behaviors. The nucleus accumbens (NAc), a central structure in the mesolimbic reward pathway, integrates the salience of aversive and rewarding stimuli. Despite extensive research on dHPC→NAc direct projections, their sufficiency in driving reinforcement and reward-related behavior remains to be determined. Our study establishes that activating excitatory neurons in the dHPC is sufficient to induce reinforcing behaviors through its direct projections to the dorso-medial subregion of the NAc shell (dmNAcSh). Notably, dynorphin-containing neurons specifically contribute to dHPC-driven reinforcing behavior, even though both dmNAcSh dynorphin- and enkephalin-containing neurons are activated with dHPC stimulation. Our findings unveil a pathway governing reinforcement, advancing our understanding of the hippocampal circuity's role in reward-seeking behaviors.

An opponent striatal circuit for distributional reinforcement learning

Machine learning research has achieved large performance gains on a wide range of tasks by expanding the learning target from mean rewards to entire probability distributions of rewards - an approach known as distributional reinforcement learning (RL)1. The mesolimbic dopamine system is thought to underlie RL in the mammalian brain by updating a representation of mean value in the striatum2,3, but little is known about whether, where, and how neurons in this circuit encode information about higher-order moments of reward distributions4. To fill this gap, we used high-density probes (Neuropixels) to acutely record striatal activity from well-trained, water-restricted mice performing a classical conditioning task in which reward mean, reward variance, and stimulus identity were independently manipulated. In contrast to traditional RL accounts, we found robust evidence for abstract encoding of variance in the striatum. Remarkably, chronic ablation of dopamine inputs disorganized these distributional representations in the striatum without interfering with mean value coding. Two-photon calcium imaging and optogenetics revealed that the two major classes of striatal medium spiny neurons - D1 and D2 MSNs - contributed to this code by preferentially encoding the right and left tails of the reward distribution, respectively. We synthesize these findings into a new model of the striatum and mesolimbic dopamine that harnesses the opponency between D1 and D2 MSNs5-15 to reap the computational benefits of distributional RL.

Updating the striatal-pallidal wiring diagram

The striatal and pallidal complexes are basal ganglia structures that orchestrate learning and execution of flexible behavior. Models of how the basal ganglia subserve these functions have evolved considerably, and the advent of optogenetic and molecular tools has shed light on the heterogeneity of subcircuits within these pathways. However, a synthesis of how molecularly diverse neurons integrate into existing models of basal ganglia function is lacking. Here, we provide an overview of the neurochemical and molecular diversity of striatal and pallidal neurons and synthesize recent circuit connectivity studies in rodents that takes this diversity into account. We also highlight anatomical organizational principles that distinguish the dorsal and ventral basal ganglia pathways in rodents. Future work integrating the molecular and anatomical properties of striatal and pallidal subpopulations may resolve controversies regarding basal ganglia network function.

Nucleus accumbens local circuit for cue-dependent aversive learning

Response to threatening environmental stimuli requires detection and encoding of important environmental features that dictate threat. Aversive events are highly salient, which promotes associative learning about stimuli that signal this threat. The nucleus accumbens is uniquely positioned to process this salient, aversive information and promote motivated output, through plasticity on the major projection neurons in the brain area. We describe a nucleus accumbens core local circuit whereby excitatory plasticity facilitates learning and recall of discrete aversive cues. We demonstrate that putative nucleus accumbens substance P release and long-term excitatory plasticity on dopamine 2 receptor-expressing projection neurons are required for cue-dependent fear learning. Additionally, we find that fear learning and recall is dependent on distinct projection neuron subtypes. Our work demonstrates a critical role for nucleus accumbens substance P in cue-dependent aversive learning.

Calcitonin receptor signaling in nucleus accumbens D1R- and D2R-expressing medium spiny neurons bidirectionally alters opioid taking in male rats

The high rates of relapse associated with current medications used to treat opioid use disorder (OUD) necessitate research that expands our understanding of the neural mechanisms regulating opioid taking to identify molecular substrates that could be targeted by novel pharmacotherapies to treat OUD. Recent studies show that activation of calcitonin receptors (CTRs) is sufficient to reduce the rewarding effects of addictive drugs in rodents. However, the role of central CTR signaling in opioid-mediated behaviors has not been studied. Here, we used single nuclei RNA sequencing (snRNA-seq), fluorescent in situ hybridization (FISH), and immunohistochemistry (IHC) to characterize cell type-specific patterns of CTR expression in the nucleus accumbens (NAc), a brain region that plays a critical role in voluntary drug taking. Using these approaches, we identified CTRs expressed on D1R- and D2R-expressing medium spiny neurons (MSNs) in the medial shell subregion of the NAc. Interestingly, Calcr transcripts were expressed at higher levels in D2R- versus D1R-expressing MSNs. Cre-dependent viral-mediated miRNA knockdown of CTRs in transgenic male rats was then used to determine the functional significance of endogenous CTR signaling in opioid taking. We discovered that reduced CTR expression specifically in D1R-expressing MSNs potentiated/augmented opioid self-administration. In contrast, reduced CTR expression specifically in D2R-expressing MSNs attenuated opioid self-administration. These findings highlight a novel cell type-specific mechanism by which CTR signaling in the ventral striatum bidirectionally modulates voluntary opioid taking and support future studies aimed at targeting central CTR-expressing circuits to treat OUD.

Tonic dopamine and biases in value learning linked through a biologically inspired reinforcement learning model

A hallmark of various psychiatric disorders is biased future predictions. Here we examined the mechanisms for biased value learning using reinforcement learning models incorporating recent findings on synaptic plasticity and opponent circuit mechanisms in the basal ganglia. We show that variations in tonic dopamine can alter the balance between learning from positive and negative reward prediction errors, leading to biased value predictions. This bias arises from the sigmoidal shapes of the dose-occupancy curves and distinct affinities of D1- and D2-type dopamine receptors: changes in tonic dopamine differentially alters the slope of the dose-occupancy curves of these receptors, thus sensitivities, at baseline dopamine concentrations. We show that this mechanism can explain biased value learning in both mice and humans and may also contribute to symptoms observed in psychiatric disorders. Our model provides a foundation for understanding the basal ganglia circuit and underscores the significance of tonic dopamine in modulating learning processes.

AKAP150 from nucleus accumbens dopamine D1 and D2 receptor-expressing medium spiny neurons regulates morphine withdrawal

Dopamine D1 receptor-expressing medium spiny neurons (D1R-MSNs) and dopamine D2 receptor-expressing MSNs (D2R-MSNs) in nucleus accumbens (NAc) have been demonstrated to show different effects on reward and memory of abstinence. A-kinase anchoring protein 150 (AKAP150) expression in NAc is significantly upregulated and contributes to the morphine withdrawal behavior. However, the underlying mechanism of AKAP150 under opioid withdrawal remains unclear. In this study, AKAP150 expression in NAc is upregulated in naloxone-precipitated morphine withdrawal model, and knockdown of AKAP150 alleviates morphine withdrawal somatic signs and improves the performance of conditioned place aversion (CPA) test. AKAP150 in NAc D1R-MSNs is related to modulation of the performance of morphine withdrawal CPA test, while AKAP150 in NAc D2R-MSNs is relevant to the severity of somatic responses. Our results suggest that AKAP150 from D1R-MSNs or D2R-MSNs in NAc contributes to the developmental process of morphine withdrawal but plays different roles in aspects of behavior or psychology.

Action Sequence Learning Is Impaired in Genetically Modified Mice with the Suppressed GABAergic Transmission from the Thalamic Reticular Nucleus to the Thalamus

The thalamic reticular nucleus (TRN) is a thin sheet of GABAergic neurons surrounding the thalamus, and it regulates the activity of thalamic relay neurons. The TRN has been reported to be involved in sensory gating, attentional regulation, and some other functions. However, little is known about the contribution of the TRN to sequence learning. In the present study, we examined whether the TRN is involved in reward-based learning of action sequence with no eliciting stimuli (operant conditioning), by analyzing the performance of male and female Avp-Vgat-/- mice (Vgatflox/flox mice crossed to an Avp-Cre driver line) on tasks conducted in an operant box having three levers. Our histological and electrophysiological data demonstrated that in adult Avp-Vgat-/- mice, vesicular GABA transporter (VGAT) was absent in most TRN neurons and the GABAergic transmission from the TRN to the thalamus was largely suppressed. The performance on a task in which mice needed to press an active lever for food reward showed that simple operant learning of lever pressing and learning of win-stay and lose-shift strategies are not affected in Avp-Vgat-/- mice. In contrast, the performance on a task in which mice needed to press three levers in a correct order for food reward showed that learning of the order of lever pressing (action sequence learning) was impaired in Avp-Vgat-/- mice. These results suggest that the TRN plays an important role in action sequence learning.

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.

Stable sound decoding despite modulated sound representation in the auditory cortex

The activity of neurons in the auditory cortex is driven by both sounds and non-sensory context. To investigate the neuronal correlates of non-sensory context, we trained head-fixed mice to perform a two-alternative-choice auditory task in which either reward or stimulus expectation (prior) was manipulated in blocks. Using two-photon calcium imaging to record populations of single neurons in the auditory cortex, we found that both stimulus and reward expectation modulated the activity of these neurons. A linear decoder trained on this population activity could decode stimuli as well or better than predicted by the animal's performance. Interestingly, the optimal decoder was stable even in the face of variable sensory representations. Neither the context nor the mouse's choice could be reliably decoded from the recorded neural activity. Our findings suggest that, in spite of modulation of auditory cortical activity by task priors, the auditory cortex does not represent sufficient information about these priors to exploit them optimally. Thus, the combination of rapidly changing sensory information with more slowly varying task information required for decisions in this task might be represented in brain regions other than the auditory cortex.

Computational mechanisms underlying latent value updating of unchosen actions

Current studies suggest that individuals estimate the value of their choices based on observed feedback. Here, we ask whether individuals also update the value of their unchosen actions, even when the associated feedback remains unknown. One hundred seventy-eight individuals completed a multi-armed bandit task, making choices to gain rewards. We found robust evidence suggesting latent value updating of unchosen actions based on the chosen action's outcome. Computational modeling results suggested that this effect is mainly explained by a value updating mechanism whereby individuals integrate the outcome history for choosing an option with that of rejecting the alternative. Properties of the deliberation (i.e., duration/difficulty) did not moderate the latent value updating of unchosen actions, suggesting that memory traces generated during deliberation might take a smaller role in this specific phenomenon than previously thought. We discuss the mechanisms facilitating credit assignment to unchosen actions and their implications for human decision-making.

Competing neural representations of choice shape evidence accumulation in humans

Making adaptive choices in dynamic environments requires flexible decision policies. Previously, we showed how shifts in outcome contingency change the evidence accumulation process that determines decision policies. Using in silico experiments to generate predictions, here we show how the cortico-basal ganglia-thalamic (CBGT) circuits can feasibly implement shifts in decision policies. When action contingencies change, dopaminergic plasticity redirects the balance of power, both within and between action representations, to divert the flow of evidence from one option to another. When competition between action representations is highest, the rate of evidence accumulation is the lowest. This prediction was validated in in vivo experiments on human participants, using fMRI, which showed that (1) evoked hemodynamic responses can reliably predict trial-wise choices and (2) competition between action representations, measured using a classifier model, tracked with changes in the rate of evidence accumulation. These results paint a holistic picture of how CBGT circuits manage and adapt the evidence accumulation process in mammals.

Nucleus Accumbens Local Circuit for Cue-Dependent Aversive Learning

Response to threatening environmental stimuli requires detection and encoding of important environmental features that dictate threat. Aversive events are highly salient which promotes associative learning about stimuli that signal this threat. The nucleus accumbens is uniquely positioned to process this salient, aversive information and promote motivated output, through plasticity on the major projection neurons in the brain area. We uncovered a nucleus accumbens core local circuit whereby excitatory plasticity facilitates learning and recall of discrete aversive cues. We demonstrate that putative nucleus accumbens substance P release and long-term excitatory plasticity on dopamine 2 receptor expressing projection neurons is required for cue-dependent fear learning. Additionally, we found fear learning and recall were dependent on distinct projection-neuron subtypes. Our work demonstrates a critical role for Nucleus Accumbens substance P in cue-dependent aversive learning.

Striatal Dopamine Signals and Reward Learning

We are constantly bombarded by sensory information and constantly making decisions on how to act. In order to optimally adapt behavior, we must judge which sequences of sensory inputs and actions lead to successful outcomes in specific circumstances. Neuronal circuits of the basal ganglia have been strongly implicated in action selection, as well as the learning and execution of goal-directed behaviors, with accumulating evidence supporting the hypothesis that midbrain dopamine neurons might encode a reward signal useful for learning. Here, we review evidence suggesting that midbrain dopaminergic neurons signal reward prediction error, driving synaptic plasticity in the striatum underlying learning. We focus on phasic increases in action potential firing of midbrain dopamine neurons in response to unexpected rewards. These dopamine neurons prominently innervate the dorsal and ventral striatum. In the striatum, the released dopamine binds to dopamine receptors, where it regulates the plasticity of glutamatergic synapses. The increase of striatal dopamine accompanying an unexpected reward activates dopamine type 1 receptors (D1Rs) initiating a signaling cascade that promotes long-term potentiation of recently active glutamatergic input onto striatonigral neurons. Sensorimotor-evoked glutamatergic input, which is active immediately before reward delivery will thus be strengthened onto neurons in the striatum expressing D1Rs. In turn, these neurons cause disinhibition of brainstem motor centers and disinhibition of the motor thalamus, thus promoting motor output to reinforce rewarded stimulus-action outcomes. Although many details of the hypothesis need further investigation, altogether, it seems likely that dopamine signals in the striatum might underlie important aspects of goal-directed reward-based learning.

Asymmetric cortical projections to striatal direct and indirect pathways distinctly control actions

The striatal direct and indirect pathways constitute the core for basal ganglia function in action control. Although both striatal D1- and D2-spiny projection neurons (SPNs) receive excitatory inputs from the cerebral cortex, whether or not they share inputs from the same cortical neurons, and how pathway-specific corticostriatal projections control behavior remain largely unknown. Here using a new G-deleted rabies system in mice, we found that more than two-thirds of excitatory inputs to D2-SPNs also target D1-SPNs, while only one-third do so vice versa. Optogenetic stimulation of striatal D1- vs. D2-SPN-projecting cortical neurons differently regulate locomotion, reinforcement learning and sequence behavior, implying the functional dichotomy of pathway-specific corticostriatal subcircuits. These results reveal the partially segregated yet asymmetrically overlapping cortical projections on striatal D1- vs. D2-SPNs, and that the pathway-specific corticostriatal subcircuits distinctly control behavior. It has important implications in a wide range of neurological and psychiatric diseases affecting cortico-basal ganglia circuitry.

Low Dopamine D2 Receptor Expression Drives Gene Networks Related to GABA, cAMP, Growth and Neuroinflammation in Striatal Indirect Pathway Neurons

Background

A salient effect of addictive drugs is to hijack the dopamine reward system, an evolutionarily conserved driver of goal-directed behavior and learning. Reduced dopamine type 2 receptor availability in the striatum is an important pathophysiological mechanism for addiction that is both consequential and causal for other molecular, cellular, and neuronal network differences etiologic for this disorder. Here, we sought to identify gene expression changes attributable to innate low expression of the Drd2 gene in the striatum and specific to striatal indirect medium spiny neurons (iMSNs).

Methods

Cre-conditional, translating ribosome affinity purification (TRAP) was used to purify and analyze the translatome (ribosome-bound messenger RNA) of iMSNs from mice with low/heterozygous or wild-type Drd2 expression in iMSNs. Complementary electrophysiological recordings and gene expression analysis of postmortem brain tissue from human cocaine users were performed.

Results

Innate low expression of Drd2 in iMSNs led to differential expression of genes involved in GABA (gamma-aminobutyric acid) and cAMP (cyclic adenosine monophosphate) signaling, neural growth, lipid metabolism, neural excitability, and inflammation. Creb1 was identified as a likely upstream regulator, among others. In human brain, expression of FXYD2, a modulatory subunit of the Na/K pump, was negatively correlated with DRD2 messenger RNA expression. In iMSN-TRAP-Drd2HET mice, increased Cartpt and reduced S100a10 (p11) expression recapitulated previous observations in cocaine paradigms. Electrophysiology experiments supported a higher GABA tone in iMSN-Drd2HET mice.

Conclusions

This study provides strong molecular evidence that, in addiction, inhibition by the indirect pathway is constitutively enhanced through neural growth and increased GABA signaling.

Decision-making under ambiguity and risk and executive functions in Parkinson's disease patients: A scoping review of the studies investigating the Iowa Gambling Task and the Game of Dice

Evidence shows that patients affected by Parkinson's disease (PD) display the tendency toward making risky choices. This is due, at least in part, to the pathophysiological characteristics of the disease that affects neural areas underlying decision making (DM), in which a pivotal role is played by nonmotor corticostriatal circuits and dopamine. Executive functions (EFs), which can be impaired by PD as well, may sustain optimal choices in DM processes. However, few studies have investigated whether EFs can support PD patients to make good decisions. Adopting the scoping review approach, the present article is designed to deepen the cognitive mechanisms of DM under conditions of ambiguity and risk (that are conditions common to everyday life decisions) in PD patients without impulse control disorders. We focused our attention on the Iowa Gambling Task and the Game of Dice Task, because they are the most commonly used and reliable tasks to assess DM under ambiguity and under risk, respectively, and analyzed the performances in such tasks and their relationships with EFs tests in PD patients. The analysis supported the relationships between EFs and DM performance, especially when a higher cognitive load is required to make optimal decisions, as it happens under conditions of risk. Possible knowledge gaps and further research directions are suggested to better understand DM mechanisms in PD sustaining patients' cognitive functioning and preventing negative consequences in everyday life derived from suboptimal decisions.

Ketamine metabolite alleviates morphine withdrawal-induced anxiety via modulating nucleus accumbens parvalbumin neurons in male mice

Opioid withdrawal generates extremely unpleasant physical symptoms and negative affective states. A rapid relief of opioid withdrawal-induced anxiety has obvious clinical relevance but has been rarely reported. We have shown that injection of ketamine metabolite (2R,6R)-hydroxynorketamine (HNK) leads to a rapid alleviation of anxiety-like behaviors in male mice undergoing chronic morphine withdrawal. Here we investigated the contribution of nucleus accumbens shell (sNAc) parvalbumin (PV)-neurons to this process. Chronic morphine withdrawal was associated with higher intrinsic excitability of sNAc PV-neurons via reduced voltage-dependent potassium currents. Chemogenetic inhibition of sNAc PV-neurons reversed the enhanced excitability of PV-neurons and anxiety-like behaviors in these morphine withdrawal male mice, while activation of sNAc PV-neurons induced anxiety-like behaviors in naive male mice. (2R,6R)-HNK reversed the altered potassium currents and intrinsic excitability of sNAc PV-neurons. Our findings demonstrate an important contribution of sNAc PV-neurons to modulating morphine withdrawal-induced anxiety-like behaviors and rapid relief of anxiety-like behaviors by (2R,6R)-HNK, this newly identified target may have therapeutic potentials in treating opioid addiction and anxiety disorders.

Linking input- and cell-type-specific synaptic plasticity to the reinforcement of alcohol-seeking behavior

The reinforcement of voluntary alcohol-seeking behavior requires dopamine-dependent long-term synaptic plasticity in the striatum. Specifically, the long-term potentiation (LTP) of direct-pathway medium spiny neurons (dMSNs) in the dorsomedial striatum (DMS) promotes alcohol drinking. However, it remains unclear whether alcohol induces input-specific plasticity onto dMSNs and whether this plasticity directly drives instrumental conditioning. In this study, we found that voluntary alcohol intake selectively strengthened glutamatergic transmission from the medial prefrontal cortex (mPFC) to DMS dMSNs in mice. Importantly, mimicking this alcohol-induced potentiation by optogenetically self-stimulating mPFC→dMSN synapse with an LTP protocol was sufficient to drive the reinforcement of lever pressing in operant chambers. Conversely, induction of a post-pre spike timing-dependent LTD at this synapse time-locked to alcohol delivery during operant conditioning persistently decreased alcohol-seeking behavior. Our results establish a causal relationship between input- and cell-type-specific corticostriatal plasticity and the reinforcement of alcohol-seeking behavior. This provides a potential therapeutic strategy to restore normal cortical control of dysregulated basal ganglia circuitries in alcohol use disorder.

Stable sound decoding despite modulated sound representation in the auditory cortex

The activity of neurons in the auditory cortex is driven by both sounds and non-sensory context. To investigate the neuronal correlates of non-sensory context, we trained head-fixed mice to perform a two-alternative choice auditory task in which either reward or stimulus expectation (prior) was manipulated in blocks. Using two-photon calcium imaging to record populations of single neurons in auditory cortex, we found that both stimulus and reward expectation modulated the activity of these neurons. A linear decoder trained on this population activity could decode stimuli as well or better than predicted by the animal's performance. Interestingly, the optimal decoder was stable even in the face of variable sensory representations. Neither the context nor the mouse's choice could be reliably decoded from the recorded neural activity. Our findings suggest that in spite of modulation of auditory cortical activity by task priors, auditory cortex does not represent sufficient information about these priors to exploit them optimally and that decisions in this task require that rapidly changing sensory information be combined with more slowly varying task information extracted and represented in brain regions other than auditory cortex.

Reward expectation enhances action-related activity of nigral dopaminergic and two striatal output pathways

Neurons comprising nigrostriatal system play important roles in action selection. However, it remains unclear how this system integrates recent outcome information with current action (movement) and outcome (reward or no reward) information to achieve appropriate subsequent action. We examined how neuronal activity of substantia nigra pars compacta (SNc) and dorsal striatum reflects the level of reward expectation from recent outcomes in rats performing a reward-based choice task. Movement-related activity of direct and indirect pathway striatal projection neurons (dSPNs and iSPNs, respectively) were enhanced by reward expectation, similarly to the SNc dopaminergic neurons, in both medial and lateral nigrostriatal projections. Given the classical basal ganglia model wherein dopamine stimulates dSPNs and suppresses iSPNs through distinct dopamine receptors, dopamine might not be the primary driver of iSPN activity increasing following higher reward expectation. In contrast, outcome-related activity was affected by reward expectation in line with the classical model and reinforcement learning theory, suggesting purposive effects of reward expectation.

CBGTPy: An extensible cortico-basal ganglia-thalamic framework for modeling biological decision making

Here we introduce CBGTPy, a virtual environment for designing and testing goal-directed agents with internal dynamics that are modeled off of the cortico-basal-ganglia-thalamic (CBGT) pathways in the mammalian brain. CBGTPy enables researchers to investigate the internal dynamics of the CBGT system during a variety of tasks, allowing for the formation of testable predictions about animal behavior and neural activity. The framework has been designed around the principle of flexibility, such that many experimental parameters in a decision making paradigm can be easily defined and modified. Here we demonstrate the capabilities of CBGTPy across a range of single and multi-choice tasks, highlighting the ease of set up and the biologically realistic behavior that it produces. We show that CBGTPy is extensible enough to apply to a wide range of experimental protocols and to allow for the implementation of model extensions with minimal developmental effort.

Drugs of abuse hijack a mesolimbic pathway that processes homeostatic need

Addiction prioritizes drug use over innate needs by "hijacking" brain circuits that direct motivation, but how this develops remains unclear. Using whole-brain FOS mapping and in vivo single-neuron calcium imaging, we find that drugs of abuse augment ensemble activity in the nucleus accumbens (NAc) and disorganize overlapping ensemble responses to natural rewards in a cell-type-specific manner. Combining "FOS-Seq", CRISPR-perturbations, and snRNA-seq, we identify Rheb as a shared molecular substrate that regulates cell-type-specific signal transductions in NAc while enabling drugs to suppress natural reward responses. Retrograde circuit mapping pinpoints orbitofrontal cortex which, upon activation, mirrors drug effects on innate needs. These findings deconstruct the dynamic, molecular, and circuit basis of a common reward circuit, wherein drug value is scaled to promote drug-seeking over other, normative goals.

Epigenomic profiling of mouse nucleus accumbens at single-cell resolution

The nucleus accumbens (NAc) is a key brain region involved in reward processing and is linked to multiple neuropsychiatric conditions such as substance use disorder, depression, and chronic pain. Recent studies have begun to investigate NAc gene expression at a single-cell resolution, however, our understanding of the cellular heterogeneity of the NAc epigenomic landscape remains limited. In this study, we utilize single-nucleus assay for transposase-accessible chromatin using sequencing (snATAC-seq) to map cell-type-specific differences in chromatin accessibility in the NAc. Our findings not only reveal the transcription factors and putative gene regulatory elements that may contribute to these cell-type-specific epigenomic differences but also provide a valuable resource for future studies investigating epigenomic changes that occur in neuropsychiatric disorders.

Inhibition of the dorsomedial striatal direct pathway is essential for the execution of action sequences

Contrary to the previous notion that the dorsomedial striatum (DMS) is crucial for acquiring new learning, accumulated evidence has suggested that the DMS also plays a role in the execution of already learned action sequences. Here, we examined how the direct and indirect pathways in the DMS regulate action sequences using a task that requires animals to press a lever consecutively. Cell-type-specific bulk Ca2+ recording revealed that the direct pathway was inhibited at the time of sequence execution. The sequence-related response was blunted in trials where the sequential behaviors were disrupted. Optogenetic activation at the sequence start caused distraction of action sequences without affecting motor function or memory of the task structure. By contrast with the direct pathway, the indirect pathway was slightly activated at the start of the sequence, but the optogenetic suppression of such sequence-related signaling did not impact the behaviors. These results suggest that the inhibition of the DMS direct pathway promotes sequence execution potentially by suppressing the formation of a new association.

Rewiring of Prelimbic Inputs to the Nucleus Accumbens Core Underlies Cocaine-Induced Behavioral Sensitization

BACKGROUND

Unbalanced activity of medium spiny neurons (MSNs) of the direct and indirect pathways mediates reward-related behaviors induced by addictive drugs. Prelimbic (PL) input to MSNs in the nucleus accumbens core (NAcC) plays a key role in cocaine-induced early locomotor sensitization (LS). However, the adaptive plastic changes at PL-to-NAcC synapses underlying early LS remain unclear.

METHODS

Using transgenic mice and retrograde tracing, we identified NAcC-projecting pyramidal neurons (PNs) in the PL cortex based on the expression of dopamine receptor types (D1R or D2R). To examine cocaine-induced alterations in PL-to-NAcC synapses, we measured excitatory postsynaptic current amplitudes evoked by optostimulation of PL afferents to MSNs. Riluzole was chosen to test the effects of PL excitability on cocaine-induced changes of PL-to-NAcC synapses.

RESULTS

NAcC-projecting PNs were segregated into D1R- and D2R-expressing PNs (D1- and D2-PNs, respectively), and their excitability was opposingly regulated by respective dopamine agonists. Both D1- and D2-PNs exhibited balanced innervation of direct MSNs and indirect MSNs in naïve animals. Repeated cocaine injections resulted in biased synaptic strength toward direct MSNs through presynaptic mechanisms in both D1- and D2-PNs, although D2R activation reduced the D2-PN excitability. Under group 1 metabotropic glutamate receptors coactivation, however, D2R activation enhanced the D2-PN excitability. The cocaine-induced rewiring accompanied LS, and both rewiring and LS were precluded by PL infusion of riluzole, which reduced the intrinsic excitability of PL neurons.

CONCLUSIONS

These findings indicate that cocaine-induced rewiring of PL-to-NAcC synapses correlates well with early behavioral sensitization and that rewiring and LS can be prevented by riluzole-induced reduction of excitability of PL neurons.

Reliability of gamified reinforcement learning in densely sampled longitudinal assessments

Reinforcement learning is a core facet of motivation and alterations have been associated with various mental disorders. To build better models of individual learning, repeated measurement of value-based decision-making is crucial. However, the focus on lab-based assessment of reward learning has limited the number of measurements and the test-retest reliability of many decision-related parameters is therefore unknown. In this paper, we present an open-source cross-platform application Influenca that provides a novel reward learning task complemented by ecological momentary assessment (EMA) of current mental and physiological states for repeated assessment over weeks. In this task, players have to identify the most effective medication by integrating reward values with changing probabilities to win (according to random Gaussian walks). Participants can complete up to 31 runs with 150 trials each. To encourage replay, in-game screens provide feedback on the progress. Using an initial validation sample of 384 players (9729 runs), we found that reinforcement learning parameters such as the learning rate and reward sensitivity show poor to fair intra-class correlations (ICC: 0.22-0.53), indicating substantial within- and between-subject variance. Notably, items assessing the psychological state showed comparable ICCs as reinforcement learning parameters. To conclude, our innovative and openly customizable app framework provides a gamified task that optimizes repeated assessments of reward learning to better quantify intra- and inter-individual differences in value-based decision-making over time.

Reactivation of Early-Life Stress-Sensitive Neuronal Ensembles Contributes to Lifelong Stress Hypersensitivity

Early-life stress (ELS) is one of the strongest lifetime risk factors for depression, anxiety, suicide, and other psychiatric disorders, particularly after facing additional stressful events later in life. Human and animal studies demonstrate that ELS sensitizes individuals to subsequent stress. However, the neurobiological basis of such stress sensitization remains largely unexplored. We hypothesized that ELS-induced stress sensitization would be detectable at the level of neuronal ensembles, such that cells activated by ELS would be more reactive to adult stress. To test this, we leveraged transgenic mice to genetically tag, track, and manipulate experience-activated neurons. We found that in both male and female mice, ELS-activated neurons within the nucleus accumbens (NAc), and to a lesser extent the medial prefrontal cortex, were preferentially reactivated by adult stress. To test whether reactivation of ELS-activated ensembles in the NAc contributes to stress hypersensitivity, we expressed hM4Dis receptor in control or ELS-activated neurons of pups and chemogenetically inhibited their activity during experience of adult stress. Inhibition of ELS-activated NAc neurons, but not control-tagged neurons, ameliorated social avoidance behavior following chronic social defeat stress in males. These data provide evidence that ELS-induced stress hypersensitivity is encoded at the level of corticolimbic neuronal ensembles.SIGNIFICANCE STATEMENT Early-life stress enhances sensitivity to stress later in life, yet the mechanisms of such stress sensitization are largely unknown. Here, we show that neuronal ensembles in corticolimbic brain regions remain hypersensitive to stress across the life span, and quieting these ensembles during experience of adult stress rescues stress hypersensitivity.

An open-source platform for head-fixed operant and consummatory behavior

Head-fixed behavioral experiments in rodents permit unparalleled experimental control, precise measurement of behavior, and concurrent modulation and measurement of neural activity. Here, we present OHRBETS (Open-Source Head-fixed Rodent Behavioral Experimental Training System; pronounced 'Orbitz'), a low-cost, open-source platform of hardware and software to flexibly pursue the neural basis of a variety of motivated behaviors. Head-fixed mice tested with OHRBETS displayed operant conditioning for caloric reward that replicates core behavioral phenotypes observed during freely moving conditions. OHRBETS also permits optogenetic intracranial self-stimulation under positive or negative operant conditioning procedures and real-time place preference behavior, like that observed in freely moving assays. In a multi-spout brief-access consumption task, mice displayed licking as a function of concentration of sucrose, quinine, and sodium chloride, with licking modulated by homeostatic or circadian influences. Finally, to highlight the functionality of OHRBETS, we measured mesolimbic dopamine signals during the multi-spout brief-access task that display strong correlations with relative solution value and magnitude of consumption. All designs, programs, and instructions are provided freely online. This customizable platform enables replicable operant and consummatory behaviors and can be incorporated with methods to perturb and record neural dynamics in vivo.

Accelerated social representational drift in the nucleus accumbens in a model of autism

Impaired social interaction is one of the core deficits of autism spectrum disorder (ASD) and may result from social interactions being less rewarding. How the nucleus accumbens (NAc), as a key hub of reward circuitry, encodes social interaction and whether these representations are altered in ASD remain poorly understood. We identified NAc ensembles encoding social interactions by calcium imaging using miniaturized microscopy. NAc population activity, specifically D1 receptor-expressing medium spiny neurons (D1-MSNs) activity, predicted social interaction epochs. Despite a high turnover of NAc neurons modulated by social interaction, we found a stable population code for social interaction in NAc which was dramatically degraded in Cntnap2-/- mouse model of ASD. Surprisingly, non-specific optogenetic inhibition of NAc core neurons increased social interaction time and significantly improved sociability in Cntnap2-/- mice. Inhibition of D1- or D2-MSNs showed reciprocal effects, with D1 inhibition decreasing social interaction and D2 inhibition increasing interaction. Therefore, social interactions are preferentially, specifically and dynamically encoded by NAc neurons and social representations are degraded in this autism model.

Role of dopamine D1 receptor in the modulation of memory consolidation by passive and self-administered heroin and associated conditioned stimuli

It has been proposed that opiates modulate memory consolidation, but recent work has indicated that this effect may be mediated by how the drug is experienced (i.e., passive injections vs. self-administration). Because the dopamine (DA) D1 receptor is involved in processing of learning signals and attribution of salience to events experienced by an organism, two studies in male Sprague-Dawley rats tested the effect of blocking this receptor on modulation of memory consolidation by passive and self-administered heroin, in addition to conditioned memory modulation by heroin-paired cues. Using the object location memory task, Study 1 employed SCH23390 (0, 0.05, 0.10 mg/kg, SC) to modulate enhancement of memory consolidation induced by post-training injections of heroin (1 mg/kg, SC) as well as by exposure to the environment paired with heroin injections (6 pairings, 1 h each, 1 mg/kg). Study 2 was conducted in rats that could self-administer heroin (0.05 mg/kg/infusion, IV) and tested whether SCH23390 (0 and 0.1 mg/kg, SC) could prevent memory modulation induced by a change in schedule of self-administration (from fixed to variable ratio). It was found that while repeated passive injections of heroin retained their enhancing effect on memory, when self-administered, heroin enhanced consolidation of object location memory only at the beginning of self-administration and after a change in schedule. Importantly, SCH23390 blocked memory modulation by heroin when passively administered and when the drug was self-administered on a novel schedule. SCH23390 also blocked conditioned memory modulation induced by post-training exposure to heroin-paired cues. Taken together, these results suggest that modulation of memory consolidation by unconditioned and conditioned opiate reinforcers involve a D1-dependent mechanism of salience attribution linked to the anticipation of drug effects.

Exercise therapy for chronic pain: How does exercise change the limbic brain function?

We are exposed to various external and internal threats which might hurt us. The role of taking flexible and appropriate actions against threats is played by "the limbic system" and at the heart of it there is the ventral tegmental area and nucleus accumbens (brain reward system). Pain-related fear causes excessive excitation of amygdala, which in turn causes the suppression of medial prefrontal cortex, leading to chronification of pain. Since the limbic system of chronic pain patients is functionally impaired, they are maladaptive to their situations, unable to take goal-directed behavior and are easily caught by fear-avoidance thinking. We describe the neural mechanisms how exercise activates the brain reward system and enables chronic pain patients to take goal-directed behavior and overcome fear-avoidance thinking. A key to getting out from chronic pain state is to take advantage of the behavioral switching function of the basal nucleus of amygdala. We show that exercise activates positive neurons in this nucleus which project to the nucleus accumbens and promote reward behavior. We also describe fear conditioning and extinction are affected by exercise. In chronic pain patients, the fear response to pain is enhanced and the extinction of fear memories is impaired, so it is difficult to get out of "fear-avoidance thinking". Prolonged avoidance of movement and physical inactivity exacerbate pain and have detrimental effects on the musculoskeletal and cardiovascular systems. Based on the recent findings on multiple bran networks, we propose a well-balanced exercise prescription considering the adherence and pacing of exercise practice. We conclude that therapies targeting the mesocortico-limbic system, such as exercise therapy and cognitive behavioral therapy, may become promising tools in the fight against chronic pain.

Disentangling the role of NAc D1 and D2 cells in hedonic eating

Overeating is driven by both the hedonic component ('liking') of food, and the motivation ('wanting') to eat it. The nucleus accumbens (NAc) is a key brain center implicated in these processes, but how distinct NAc cell populations encode 'liking' and 'wanting' to shape overconsumption remains unclear. Here, we probed the roles of NAc D1 and D2 cells in these processes using cell-specific recording and optogenetic manipulation in diverse behavioral paradigms that disentangle reward traits of 'liking' and 'wanting' related to food choice and overeating in healthy mice. Medial NAc shell D2 cells encoded experience-dependent development of 'liking', while D1 cells encoded innate 'liking' during the first food taste. Optogenetic control confirmed causal links of D1 and D2 cells to these aspects of 'liking'. In relation to 'wanting', D1 and D2 cells encoded and promoted distinct aspects of food approach: D1 cells interpreted food cues while D2 cells also sustained food-visit-length that facilitates consumption. Finally, at the level of food choice, D1, but not D2, cell activity was sufficient to switch food preference, programming subsequent long-lasting overconsumption. By revealing complementary roles of D1 and D2 cells in consumption, these findings assign neural bases to 'liking' and 'wanting' in a unifying framework of D1 and D2 cell activity.

The indirect pathway of the basal ganglia promotes transient punishment but not motor suppression

Optogenetic stimulation of Adora2a receptor-expressing spiny projection neurons (A2A-SPNs) in the striatum drives locomotor suppression and transient punishment, results attributed to activation of the indirect pathway. The sole long-range projection target of A2A-SPNs is the external globus pallidus (GPe). Unexpectedly, we found that inhibition of the GPe drove transient punishment but not suppression of movement. Within the striatum, A2A-SPNs inhibit other SPNs through a short-range inhibitory collateral network, and we found that optogenetic stimuli that drove motor suppression shared a common mechanism of recruiting this inhibitory collateral network. Our results suggest that the indirect pathway plays a more prominent role in transient punishment than in motor control and challenges the assumption that activity of A2A-SPNs is synonymous with indirect pathway activity.

Neuronal glutamate transporters control reciprocal inhibition and gain modulation in D1 medium spiny neurons

Understanding the function of glutamate transporters has broad implications for explaining how neurons integrate information and relay it through complex neuronal circuits. Most of what is currently known about glutamate transporters, specifically their ability to maintain glutamate homeostasis and limit glutamate diffusion away from the synaptic cleft, is based on studies of glial glutamate transporters. By contrast, little is known about the functional implications of neuronal glutamate transporters. The neuronal glutamate transporter EAAC1 is widely expressed throughout the brain, particularly in the striatum, the primary input nucleus of the basal ganglia, a region implicated with movement execution and reward. Here, we show that EAAC1 limits synaptic excitation onto a population of striatal medium spiny neurons identified for their expression of D1 dopamine receptors (D1-MSNs). In these cells, EAAC1 also contributes to strengthen lateral inhibition from other D1-MSNs. Together, these effects contribute to reduce the gain of the input-output relationship and increase the offset at increasing levels of synaptic inhibition in D1-MSNs. By reducing the sensitivity and dynamic range of action potential firing in D1-MSNs, EAAC1 limits the propensity of mice to rapidly switch between behaviors associated with different reward probabilities. Together, these findings shed light on some important molecular and cellular mechanisms implicated with behavior flexibility in mice.

Mitofusin-2 in nucleus accumbens D2-MSNs regulates social dominance and neuronal function

The nucleus accumbens (NAc) is a brain hub regulating motivated behaviors, including social competitiveness. Mitochondrial function in the NAc links anxiety with social competitiveness, and the mitochondrial fusion protein mitofusin 2 (Mfn2) in NAc neurons regulates anxiety-related behaviors. However, it remains unexplored whether accumbal Mfn2 levels also affect social behavior and whether Mfn2 actions in the emotional and social domain are driven by distinct cell types. Here, we found that subordinate-prone highly anxious rats show decreased accumbal Mfn2 levels and that Mfn2 overexpression promotes dominant behavior. In mice, selective Mfn2 downregulation in NAc dopamine D2 receptor-expressing medium spiny neurons (D2-MSNs) induced social subordination, accompanied by decreased accumbal mitochondrial functions and decreased excitability in D2-MSNs. Instead, D1-MSN-targeted Mfn2 downregulation affected competitive ability only transiently and likely because of an increase in anxiety-like behaviors. Our results assign dissociable cell-type specific roles to Mfn2 in the NAc in modulating social dominance and anxiety.