Cocaine-Induced Synaptic Alterations in Thalamus to Nucleus Accumbens Projection

Pivotal role of orexin signaling in the posterior paraventricular nucleus of the thalamus during the stress-induced reinstatement of oxycodone-seeking behavior

BACKGROUND

The orexin (OX) system has received increasing interest as a potential target for treating substance use disorder. OX transmission in the posterior paraventricular nucleus of the thalamus (pPVT), an area activated by highly salient stimuli that are both reinforcing and aversive, mediates cue- and stress-induced reinstatement of reward-seeking behavior. Oral administration of suvorexant (SUV), a dual OX receptor (OXR) antagonist (DORA), selectively reduced conditioned reinstatement of oxycodone-seeking behavior and stress-induced reinstatement of alcohol-seeking behavior in dependent rats.

AIMS

This study tested whether OXR blockade in the pPVT with SUV reduces oxycodone or sweetened condensed milk (SCM) seeking elicited by conditioned cues or stress.

METHODS

Male Wistar rats were trained to self-administer oxycodone (0.15 mg/kg, i.v., 8 h/day) or SCM (0.1 ml, 2:1 dilution [v/v], 30 min/day). After extinction, we tested the ability of intra-pPVT SUV (15 µg/0.5 µl) to prevent reinstatement of oxycodone or SCM seeking elicited by conditioned cues or footshock stress.

RESULTS

The rats acquired oxycodone and SCM self-administration, and oxycodone intake correlated with signs of physical opioid withdrawal, confirming dependence. Following extinction, the presentation of conditioned cues or footshock elicited reinstatement of oxycodone- and SCM-seeking behavior. Intra-pPVT SUV blocked stress-induced reinstatement of oxycodone seeking but not conditioned reinstatement of oxycodone or SCM seeking or stress-induced reinstatement of SCM seeking.

CONCLUSIONS

The results indicate that OXR signaling in the pPVT is critical for stress-induced reinstatement of oxycodone seeking, further corroborating OXRs as treatment targets for opioid use disorder.

Neural responses to gaming content on social media in young adults

Excessive gaming can impair both mental and physical health, drawing widespread public and clinical attention, especially among young generations. People are now more exposed to gaming-related content on social media than before, and this exposure may have a significant impact on their behavior. However, the neural mechanisms underlying this effect remain unexplored. Using functional magnetic resonance imaging (fMRI), this study aimed to investigate the neural activity induced by gaming-related content on social media among young adults casually playing online games. While being assessed by fMRI, the participants watched gaming-related videos and neutral (nongaming) videos on social media. The gaming-related cues significantly activated several brain areas, including the medial prefrontal cortex, posterior cingulate cortex, hippocampus, thalamus, superior/middle temporal gyrus, precuneus and occipital regions, compared with the neutral cues. Additionally, the participants' gaming desire levels positively correlated with a gaming-related cue-induced activation in the left orbitofrontal cortex and the right superior temporal gyrus. These findings extend previous studies on gaming cues and provide useful information to elucidate the effects of gaming-related content on social media in young adults. Continued research using real-world gaming cues may help improve our understanding of promoting gaming habits and provide support to individuals vulnerable to gaming addiction.

Paraventricular thalamus to nucleus accumbens circuit activation decreases long-term relapse of alcohol-seeking behaviour in male mice

BACKGROUND

Some studies have highlighted the crucial role of aversion in addiction treatment. The pathway from the anterior paraventricular thalamus (PVT) to the shell of the nucleus accumbens (NAc) has been reported as an essential regulatory pathway for processing aversion and is also closely associated with substance addiction. However, its impact on alcohol addiction has been relatively underexplored. Therefore, this study focused on the role of the PVT-NAc pathway in the formation and relapse of alcohol addiction-like behaviour, offering a new perspective on the mechanisms of alcohol addiction.

RESULTS

The chemogenetic inhibition of the PVT-NAc pathway in male mice resulted in a notable decrease in the establishment of ethanol-induced conditioned place aversion (CPA), and NAc-projecting PVT neurons were recruited due to aversive effects. Conversely, activation of the PVT-NAc pathway considerably impeded the formation of ethanol-induced conditioned place preference (CPP). Furthermore, during the memory reconsolidation phase, activation of this pathway effectively disrupted the animals' preference for alcohol-associated contexts. Whether it was administered urgently 24 h later or after a long-term withdrawal of 10 days, a low dose of alcohol could still not induce the reinstatement of ethanol-induced CPP.

CONCLUSIONS

Our results demonstrated PVT-NAc circuit processing aversion, which may be one of the neurobiological mechanisms underlying aversive counterconditioning, and highlighted potential targets for inhibiting the development of alcohol addiction-like behaviour and relapse after long-term withdrawal.

Inhibition of the glutamatergic PVT-NAc projections attenuates local anesthetic-induced neurotoxic behaviors

INTRODUCTION

Local anesthetic-induced neurotoxicity contributes to perioperative nerve damage; however, the underlying mechanisms remain unclear. Here, we investigated the role of the paraventricular thalamus (PVT)-nucleus accumbens (NAc) projections in neurotoxicity induced by ropivacaine, a local anesthetic agent.

METHODS

Ropivacaine (58 mg/kg, intraperitoneal administration) was used to construct the local anesthetic systemic toxicity (LAST) mice model. We first identified neural projections from the PVT to the NAc through the expression of a retrograde tracer and virus. The inhibitory viruses (rAAV-EF1α-DIO-hm4D(Gi)-mCherry-WPREs: AAV2/retro and rAAV-CaMKII-CRE-WPRE-hGh: AAV2/9) were injected into the mice model to assess the effects of the specific inhibition of the PVT-NAc pathway on neurological behaviors in the presence of clozapine-N-oxide. The inhibition of the PVT-NAc pathway was evaluated by immunofluorescence staining of c-Fos-positive neurons and Ca2+ signals in CaMKIIa neurons.

RESULTS

We successfully identified a circuit connecting the PVT and NAc in C57BL/6 mice. Ropivacaine administration induced the activation of the PVT-NAc pathway and seizures. Specific inhibition of NAc-projecting CaMKII neurons in the PVT was sufficient to inhibit the neuronal activity in the NAc, which subsequently decreased ropivacaine-induced neurotoxicity.

CONCLUSION

These results reveal the presence of a dedicated PVT-NAc circuit that regulates local anesthetic-induced neurotoxicity and provide a potential mechanistic explanation for the treatment and prevention of LAST.

Accumbal-thalamic connectivity and associated glutamate alterations in human cocaine craving: A state-dependent rs-fMRI and 1H-MRS study

Craving is a core symptom of cocaine use disorder and a major factor for relapse risk. To date, there is no pharmacological therapy to treat this disease or at least to alleviate cocaine craving as a core symptom. In animal models, impaired prefrontal-striatal signalling leading to altered glutamate release in the nucleus accumbens appear to be the prerequisite for cocaine-seeking. Thus, those network and metabolic changes may constitute the underlying mechanisms for cocaine craving and provide a potential treatment target. In humans, there is recent evidence for corresponding glutamatergic alterations in the nucleus accumbens, however, the underlying network disturbances that lead to this glutamate imbalance remain unknown. In this state-dependent randomized, placebo-controlled, double-blinded, cross-over multimodal study, resting state functional magnetic resonance imaging in combination with small-voxel proton magnetic resonance spectroscopy (voxel size: 9.4 × 18.8 × 8.4 mm3) was applied to assess network-level and associated neurometabolic changes during a non-craving and a craving state, induced by a custom-made cocaine-cue film, in 18 individuals with cocaine use disorder and 23 healthy individuals. Additionally, we assessed the potential impact of a short-term challenge of N-acetylcysteine, known to normalize disturbed glutamate homeostasis and to thereby reduce cocaine-seeking in animal models of addiction, compared to a placebo. We found increased functional connectivity between the nucleus accumbens and the dorsolateral prefrontal cortex during the cue-induced craving state. However, those changes were not linked to alterations in accumbal glutamate levels. Whereas we additionally found increased functional connectivity between the nucleus accumbens and a midline part of the thalamus during the cue-induced craving state. Furthermore, obsessive thinking about cocaine and the actual intensity of cocaine use were predictive of cue-induced functional connectivity changes between the nucleus accumbens and the thalamus. Finally, the increase in accumbal-thalamic connectivity was also coupled with craving-related glutamate rise in the nucleus accumbens. Yet, N-acetylcysteine had no impact on craving-related changes in functional connectivity. Together, these results suggest that connectivity changes within the fronto-accumbal-thalamic loop, in conjunction with impaired glutamatergic transmission, underlie cocaine craving and related clinical symptoms, pinpointing the thalamus as a crucial hub for cocaine craving in humans.

Valence and salience encoding by parallel circuits from the paraventricular thalamus to the nucleus accumbens

The anterior and posterior subregions of the paraventricular thalamus (aPVT and pPVT, respectively) play unique roles in learned behaviors, from fear conditioning to alcohol/drug intake, potentially through differentially organized projections to limbic brain regions including the nucleus accumbens medial shell (mNAcSh). Here, we found that the aPVT projects broadly to the mNAcSh and that the aPVT-mNAcSh circuit encodes positive valence, such that in vivo manipulations of the circuit modulated both innately programmed and learned behavioral responses to positively and negatively valenced stimuli, particularly in females. Further, the endogenous activity of aPVT presynaptic terminals in the mNAcSh was greater in response to positively than negatively valenced stimuli, and the probability of synaptic glutamate release from aPVT neurons in the mNAcSh was higher in females than males. In contrast, we found that the pPVT-mNAcSh circuit encodes stimulus salience regardless of valence. While pPVT-mNAcSh circuit inhibition suppressed behavioral responses in both sexes, circuit activation increased behavioral responses to stimuli only in males. Our results point to circuit-specific stimulus feature encoding by parallel PVT-mNAcSh circuits that have sex-dependent biases in organization and function.

Neural and Molecular Investigation into the Paraventricular Thalamic-Nucleus Accumbens Circuit for Pain Sensation and Non-opioid Analgesia

The paucity of medications with novel mechanisms for pain treatment combined with the severe adverse effects of opioid analgesics has led to an imperative pursuit of non-opioid analgesia and a better understanding of pain mechanisms. Here, we identify the putative glutamatergic inputs from the paraventricular thalamic nucleus to the nucleus accumbens (PVT^Glut→NAc) as a novel neural circuit for pain sensation and non-opioid analgesia. Our in vivo fiber photometry and in vitro electrophysiology experiments found that PVT^Glut→NAc neuronal activity increased in response to acute thermal/mechanical stimuli and persistent inflammatory pain. Direct optogenetic activation of these neurons in the PVT or their terminals in the NAc induced pain-like behaviors. Conversely, inhibition of PVT^Glut→NAc neurons or their NAc terminals exhibited a potent analgesic effect in both naïve and pathological pain mice, which could not be prevented by pretreatment of naloxone, an opioid receptor antagonist. Anterograde trans-synaptic optogenetic experiments consistently demonstrated that the PVT^Glut→NAc circuit bi-directionally modulates pain behaviors. Furthermore, circuit-specific molecular profiling and pharmacological studies revealed dopamine receptor 3 as a candidate target for pain modulation and non-opioid analgesic development. Taken together, these findings provide a previously unknown neural circuit for pain sensation and non-opioid analgesia and a valuable molecular target for developing future safer medication.

Membrane excitability of nucleus accumbens neurons gates the incubation of cocaine craving

After drug withdrawal, a key factor triggering relapse is progressively intensified cue-associated drug craving, termed incubation of drug craving. After withdrawal from cocaine self-administration, incubation of cocaine craving develops more reliably in rats compared to mice. This species difference provides an opportunity to determine rat-specific cellular adaptations, which may constitute the critical mechanisms that contribute to incubated cocaine craving in humans. Expression of incubated cocaine seeking is mediated, in part, by cocaine-induced cellular adaptations in medium spiny neurons (MSNs) within the nucleus accumbens (NAc). In rats, decreased membrane excitability in NAc MSNs is a prominent cellular adaptation, which is induced after cocaine self-administration and lasts throughout prolonged drug withdrawal. Here, we show that, similar to rats, mice exhibit decreased membrane excitability of dopamine D₁ receptor (D1)-, but not D₂ (D2)-, expressing MSNs within the NAc shell (NAcSh) after 1 d withdrawal from cocaine self-administration. However, in contrast to rats, this membrane adaptation does not persist in mice, diminishing after 45-d withdrawal. We also find that restoring the membrane excitability of NAcSh MSNs after cocaine withdrawal decreases cocaine seeking in rats. This suggests that drug-induced membrane adaptations are essential for behavioral expression of incubated cocaine craving. In mice, however, experimentally inducing hypoactivity of D1 NAcSh MSNs after cocaine withdrawal does not alter cocaine seeking, suggesting that MSN hypo-excitability alone is insufficient to increase cocaine seeking. Together, our results demonstrate an overall permissive role of cocaine-induced hypoactivity of NAcSh MSNs in gating increased cocaine seeking after prolonged cocaine withdrawal.

Perioperative sleep deprivation activates the paraventricular thalamic nucleus resulting in persistent postoperative incisional pain in mice

Background

The duration of postsurgical pain is closely correlated with perioperative stress. Most patients suffer short-term sleep disorder/deprivation before and/or after surgery, which leads to extended postsurgical pain by an undetermined mechanism. The paraventricular thalamus (PVT) is a critical area that contributes to the regulation of feeding, awakening, and emotional states. However, whether the middle PVT is involved in postoperative pain or the extension of postoperative pain caused by perioperative sleep deprivation has not yet been investigated.

Methods

We established a model of postoperative pain by plantar incision with perioperative rapid eye movement sleep deprivation (REMSD) 6 h/day for 3 consecutive days in mice. The excitability of the CaMKIIα⁺ neurons in the middle PVT (mPVT^CaMKIIα) was detected by immunofluorescence and fiber photometry. The activation/inhibition of mPVT^CaMKIIα neurons was conducted by chemogenetics.

Results

REMSD prolonged the duration of postsurgical pain and increased the excitability of mPVT^CaMKIIα neurons. In addition, mPVT^CaMKIIα neurons showed increased excitability in response to nociceptive stimuli or painful conditions. However, REMSD did not delay postsurgical pain recovery following the ablation of CaMKIIα neurons in the mPVT. The activation of mPVT^CaMKIIα neurons prolonged the duration of postsurgical pain and elicited anxiety-like behaviors. In contrast, inhibition of mPVT^CaMKIIα neurons reduced the postsurgical pain after REMSD.

Conclusion

Our data revealed that the CaMKIIα neurons in the mPVT are involved in the extension of the postsurgical pain duration induced by REMSD, and represented a novel potential target to treat postoperative pain induced by REMSD.

Gut colonization by Proteobacteria alters host metabolism and modulates cocaine neurobehavioral responses

Gut-microbiota membership is associated with diverse neuropsychological outcomes, including substance use disorders (SUDs). Here, we use mice colonized with Citrobacter rodentium or the human γ-Proteobacteria commensal Escherichia coli HS as a model to examine the mechanistic interactions between gut microbes and host responses to cocaine. We find that cocaine exposure increases intestinal norepinephrine levels that are sensed through the bacterial adrenergic receptor QseC to promote intestinal colonization of γ-Proteobacteria. Colonized mice show enhanced host cocaine-induced behaviors. The neuroactive metabolite glycine, a bacterial nitrogen source, is depleted in the gut and cerebrospinal fluid of colonized mice. Systemic glycine repletion reversed, and γ-Proteobacteria mutated for glycine uptake did not alter the host response to cocaine. γ-Proteobacteria modulated glycine levels are linked to cocaine-induced transcriptional plasticity in the nucleus accumbens through glutamatergic transmission. The mechanism outline here could potentially be exploited to modulate reward-related brain circuits that contribute to SUDs.

Structural and functional organization of the midline and intralaminar nuclei of the thalamus

The midline and intralaminar nuclei of the thalamus form a major part of the "limbic thalamus;" that is, thalamic structures anatomically and functionally linked with the limbic forebrain. The midline nuclei consist of the paraventricular (PV) and paratenial nuclei, dorsally and the rhomboid and nucleus reuniens (RE), ventrally. The rostral intralaminar nuclei (ILt) consist of the central medial (CM), paracentral (PC) and central lateral (CL) nuclei. We presently concentrate on RE, PV, CM and CL nuclei of the thalamus. The nucleus reuniens receives a diverse array of input from limbic-related sites, and predominantly projects to the hippocampus and to "limbic" cortices. The RE participates in various cognitive functions including spatial working memory, executive functions (attention, behavioral flexibility) and affect/fear behavior. The PV receives significant limbic-related afferents, particularly the hypothalamus, and mainly distributes to "affective" structures of the forebrain including the bed nucleus of stria terminalis, nucleus accumbens and the amygdala. Accordingly, PV serves a critical role in "motivated behaviors" such as arousal, feeding/consummatory behavior and drug addiction. The rostral ILt receives both limbic and sensorimotor-related input and distributes widely over limbic and motor regions of the frontal cortex-and throughout the dorsal striatum. The intralaminar thalamus is critical for maintaining consciousness and directly participates in various sensorimotor functions (visuospatial or reaction time tasks) and cognitive tasks involving striatal-cortical interactions. As discussed herein, while each of the midline and intralaminar nuclei are anatomically and functionally distinct, they collectively serve a vital role in several affective, cognitive and executive behaviors - as major components of a brainstem-diencephalic-thalamocortical circuitry.

Dual medial prefrontal cortex and hippocampus projecting neurons in the paraventricular nucleus of the thalamus

The paraventricular nucleus (PVT) of the midline thalamus is a critical higher-order cortico-thalamo-cortical integration site that plays a critical role in various behaviors including reward seeking, cue saliency, and emotional memory. Anatomical studies have shown that PVT projects to both medial prefrontal cortex (mPFC) and hippocampus (HC). However, dual mPFC-HC projecting neurons which could serve a role in synchronizing mPFC and HC activity during PVT-dependent behaviors, have not been explored. Here we used a dual retrograde adenoassociated virus (AAV) tracing approach to characterize the location and proportion of different projection populations that send collaterals to mPFC and/or ventral hippocampus (vHC) in rats. Additionally, we examined the distribution of calcium binding proteins calretinin (CR) and calbindin (CB) with respect to these projection populations in PVT. We found that PVT contains separate populations of cells that project to mPFC, vHC, and those that innervate both regions. Interestingly, dual mPFC-HC projecting cells expressed neither CR nor CB. Topographically, CB+ and CR+ containing cells clustered around dual projecting neurons in PVT. These results are consistent with the features of dual mPFC-vHC projecting cells in the nucleus reuniens (RE) and suggestive of a functional mPFC-PVT-vHC system that may support mPFC-vHC interactions in PVT-dependent behaviors.

Inhibition of a cortico-thalamic circuit attenuates cue-induced reinstatement of drug-seeking behavior in "relapse prone" male rats

RATIONALE

Relapse often occurs when individuals are exposed to stimuli or cues previously associated with the drug-taking experience. The ability of drug cues to trigger relapse is believed to be a consequence of incentive salience attribution, a process by which the incentive value of reward is transferred to the reward-paired cue. Sign-tracker (ST) rats that attribute enhanced incentive value to reward cues are more prone to relapse compared to goal-tracker (GT) rats that primarily attribute predictive value to such cues.

OBJECTIVES

The neurobiological mechanisms underlying this individual variation in relapse propensity remains largely unexplored. The paraventricular nucleus of the thalamus (PVT) has been identified as a critical node in the regulation of cue-elicited behaviors in STs and GTs, including cue-induced reinstatement of drug-seeking behavior. Here we used a chemogenetic approach to assess whether "top-down" cortical input from the prelimbic cortex (PrL) to the PVT plays a role in mediating individual differences in relapse propensity.

RESULTS

Chemogenetic inhibition of the PrL-PVT pathway selectively decreased cue-induced reinstatement of drug-seeking behavior in STs, without affecting behavior in GTs. In contrast, cocaine-primed drug-seeking behavior was not affected in either phenotype. Furthermore, when rats were characterized based on a different behavioral phenotype-locomotor response to novelty-inhibition of the PrL-PVT pathway had no effect on either cue- or drug-induced reinstatement.

CONCLUSIONS

These results highlight an important role for the PrL-PVT pathway in vulnerability to relapse that is consequent to individual differences in the propensity to attribute incentive salience to discrete reward cues.

Substance abuse and neurotransmission

The number of people who suffer from a substance abuse disorder has continued to rise over the last decade; particularly, the number of drug-related overdose deaths has sharply increased during the COVID-19 pandemic. Converging lines of clinical observations, supported by imaging and neuropsychological performance testing, have demonstrated that substance abuse-induced dysregulation of neurotransmissions in the brain is critical for development and expression of the addictive properties of abused substances. Recent scientific advances have allowed for better understanding of the neurobiological processes that mediates drugs of abuse and addiction. This chapter presents the past classic concepts and the recent advances in our knowledge about how cocaine, amphetamines, opioids, alcohol, and nicotine alter multiple neurotransmitter systems, which contribute to the behaviors associated with each drug. Additionally, we discuss the interactive effects of HIV-1 or COVID-19 and substance abuse on neurotransmission and neurobiological pathways. Finally, we introduce therapeutic strategies for development of pharmacotherapies for substance abuse disorders.

Cocaine-induced projection-specific and cell type-specific adaptations in the nucleus accumbens

Cocaine craving, seeking, and relapse are mediated, in part, by cocaine-induced adaptive changes in the brain reward circuits. The nucleus accumbens (NAc) integrates and prioritizes different emotional and motivational inputs to the reward system by processing convergent glutamatergic projections from the medial prefrontal cortex, basolateral amygdala, ventral hippocampus, and other limbic and paralimbic brain regions. Medium spiny neurons (MSNs) are the principal projection neurons in the NAc, which can be divided into two major subpopulations, namely dopamine receptor D1- versus D2-expressing MSNs, with complementing roles in reward-associated behaviors. After cocaine experience, NAc MSNs exhibit complex and differential adaptations dependent on cocaine regimen, withdrawal time, cell type, location (NAc core versus shell), and related input and output projections, or any combination of these factors. Detailed characterization of these cellular adaptations has been greatly facilitated by the recent development of optogenetic/chemogenetic techniques combined with transgenic tools. In this review, we discuss such cell type- and projection-specific adaptations induced by cocaine experience. Specifically, (1) D1 and D2 NAc MSNs frequently exhibit differential adaptations in spinogenesis, glutamatergic receptor trafficking, and intrinsic membrane excitability, (2) cocaine experience differentially changes the synaptic transmission at different afferent projections onto NAc MSNs, (3) cocaine-induced NAc adaptations exhibit output specificity, e.g., being different at NAc-ventral pallidum versus NAc-ventral tegmental area synapses, and (4) the input, output, subregion, and D1/D2 cell type may together determine cocaine-induced circuit plasticity in the NAc. In light of the projection- and cell-type specificity, we also briefly discuss ensemble and circuit mechanisms contributing to cocaine craving and relapse after drug withdrawal.

Astrocytes in cocaine addiction and beyond

Drug addiction remains a key biomedical challenge facing current neuroscience research. In addition to neural mechanisms, the focus of the vast majority of studies to date, astrocytes have been increasingly recognized as an "accomplice." According to the tripartite synapse model, astrocytes critically regulate nearby pre- and postsynaptic neuronal substrates to craft experience-dependent synaptic plasticity, including synapse formation and elimination. Astrocytes within brain regions that are implicated in drug addiction exhibit dynamic changes in activity upon exposure to cocaine and subsequently undergo adaptive changes themselves during chronic drug exposure. Recent results have identified several key astrocytic signaling pathways that are involved in cocaine-induced synaptic and circuit adaptations. In this review, we provide a brief overview of the role of astrocytes in regulating synaptic transmission and neuronal function, and discuss how cocaine influences these astrocyte-mediated mechanisms to induce persistent synaptic and circuit alterations that promote cocaine seeking and relapse. We also consider the therapeutic potential of targeting astrocytic substrates to ameliorate drug-induced neuroplasticity for behavioral benefits. While primarily focusing on cocaine-induced astrocytic responses, we also include brief discussion of other drugs of abuse where data are available.

Kappa opioid receptor modulation of excitatory drive onto nucleus accumbens fast-spiking interneurons

The dynorphin/kappa opioid receptor (KOR) system within the nucleus accumbens (NAc) contributes to affective states. Parvalbumin fast-spiking interneurons (PV-FSIs), a key component of feedforward inhibition, participate in integration of excitatory inputs to the NAc by robustly inhibiting select populations of medium spiny output neurons, therefore greatly influencing NAc dependent behavior. How the dynorphin/KOR system regulates feedforward inhibition in the NAc remains unknown. Here, we elucidate the molecular mechanisms of KOR inhibition of excitatory transmission onto NAc PV-FSIs using a combination of whole-cell patch-clamp electrophysiology, optogenetics, pharmacology, and a parvalbumin reporter mouse. We find that postsynaptic KOR stimulation induces long-term depression (LTD) of excitatory synapses onto PV-FSI by stimulating the endocytosis of AMPARs via a PKA and calcineurin-dependent mechanism. Furthermore, KOR regulation of PV-FSI synapses are input specific, inhibiting thalamic but not cortical inputs. Finally, following acute stress, a protocol known to elevate dynorphin/KOR signaling in the NAc, KOR agonists no longer inhibit excitatory transmission onto PV-FSI. In conclusion, we delineate pathway-specific mechanisms mediating KOR control of feedforward inhibitory circuits in the NAc and provide evidence for the recruitment of this system in response to stress.

Silent Synapses in Cocaine-Associated Memory and Beyond

Glutamatergic synapses are key cellular sites where cocaine experience creates memory traces that subsequently promote cocaine craving and seeking. In addition to making across-the-board synaptic adaptations, cocaine experience also generates a discrete population of new synapses that selectively encode cocaine memories. These new synapses are glutamatergic synapses that lack functionally stable AMPARs, often referred to as AMPAR-silent synapses or, simply, silent synapses. They are generated de novo in the NAc by cocaine experience. After drug withdrawal, some of these synapses mature by recruiting AMPARs, contributing to the consolidation of cocaine-associated memory. After cue-induced retrieval of cocaine memories, matured silent synapses alternate between two dynamic states (AMPAR-absent vs AMPAR-containing) that correspond with the behavioral manifestations of destabilization and reconsolidation of these memories. Here, we review the molecular mechanisms underlying silent synapse dynamics during behavior, discuss their contributions to circuit remodeling, and analyze their role in cocaine-memory-driven behaviors. We also propose several mechanisms through which silent synapses can form neuronal ensembles as well as cross-region circuit engrams for cocaine-specific behaviors. These perspectives lead to our hypothesis that cocaine-generated silent synapses stand as a distinct set of synaptic substrates encoding key aspects of cocaine memory that drive cocaine relapse.

Calcium-permeable AMPA receptor activity and GluA1 trafficking in the basolateral amygdala regulate operant alcohol self-administration

Addiction is viewed as maladaptive glutamate-mediated neuroplasticity that is regulated, in part, by calcium-permeable AMPA receptor (CP-AMPAR) activity. However, the contribution of CP-AMPARs to alcohol-seeking behavior remains to be elucidated. We evaluated CP-AMPAR activity in the basolateral amygdala (BLA) as a potential target of alcohol that also regulates alcohol self-administration in C57BL/6J mice. Operant self-administration of sweetened alcohol increased spontaneous EPSC frequency in BLA neurons that project to the nucleus accumbens as compared with behavior-matched sucrose controls indicating an alcohol-specific upregulation of synaptic activity. Bath application of the CP-AMPAR antagonist NASPM decreased evoked EPSC amplitude only in alcohol self-administering mice indicating alcohol-induced synaptic insertion of CP-AMPARs in BLA projection neurons. Moreover, NASPM infusion in the BLA dose-dependently decreased the rate of operant alcohol self-administration providing direct evidence for CP-AMPAR regulation of alcohol reinforcement. As most CP-AMPARs are GluA1-containing, we asked if alcohol alters the activation state of GluA1-containing AMPARs. Immunocytochemistry results showed elevated GluA1-S831 phosphorylation in the BLA of alcohol as compared with sucrose mice. To investigate mechanistic regulation of alcohol self-administration by GluA1-containing AMPARs, we evaluated the necessity of GluA1 trafficking using a TET-ON AAV encoding a dominant-negative GluA1 c-terminus (GluA1ct) that blocks activity-dependent synaptic delivery of native GluA1-containing AMPARs. GluA1ct expression in the BLA reduced alcohol self-administration with no effect on sucrose controls. These results show that CP-AMPAR activity and GluA1 trafficking in the BLA mechanistically regulate the reinforcing effects of sweetened alcohol. Pharmacotherapeutic targeting these mechanisms of maladaptive neuroplasticity may aid medical management of alcohol use disorder.

The paraventricular thalamus provides a polysynaptic brake on limbic CRF neurons to sex-dependently blunt binge alcohol drinking and avoidance behavior in mice

Bed nucleus of the stria terminalis (BNST) neurons that synthesize corticotropin-releasing factor (CRF) drive binge alcohol drinking and anxiety. Here, we found that female C57BL/6J mice binge drink more than males and have greater basal BNSTCRF neuron excitability and synaptic excitation. We identified a dense VGLUT2 + synaptic input from the paraventricular thalamus (PVT) that releases glutamate directly onto BNSTCRF neurons but also engages a large BNST interneuron population to ultimately inhibit BNSTCRF neurons, and this polysynaptic PVTVGLUT2-BNSTCRF circuit is more robust in females than males. Chemogenetic inhibition of the PVTBNST projection promoted binge alcohol drinking only in female mice, while activation reduced avoidance behavior in both sexes. Lastly, repeated binge drinking produced a female-like phenotype in the male PVT-BNSTCRF excitatory synapse without altering the function of PVTBNST neurons per se. Our data describe a complex, feedforward inhibitory PVTVGLUT2-BNSTCRF circuit that is sex-dependent in its function, behavioral roles, and alcohol-induced plasticity.

Circuit and neuropeptide mechanisms of the paraventricular thalamus across stages of alcohol and drug use

The paraventricular nucleus of the thalamus (PVT) is a midline thalamic brain region that has emerged as a critical circuit node in the regulation of behaviors across domains of affect and motivation, stress responses, and alcohol- and drug-related behaviors. The influence of the PVT in this diverse array of behaviors is a function of its ability to integrate and convey information about salience and valence through its connections with cortical, hypothalamic, hindbrain, and limbic brain regions. While understudied to date, recent studies suggest that several PVT efferents play critical and complex roles in drug and alcohol-related phenotypes. The PVT is also the site of signaling for many neuropeptides released from the synaptic terminals of distal inputs and local neuropeptidergic neurons within. While there is some evidence that neuropeptides including orexin, neurotensin, substance P, and cocaine and amphetamine-related transcript (CART) signal in the PVT to regulate alcohol/drug intake and reinstatement, there remains an overall lack of understanding of the roles of neuropeptides in the PVT in addiction-related behaviors, especially in a circuit-specific context. In this review, we present the current status of preclinical research regarding PVT circuits and neuropeptide modulation of the PVT in three aspects of the addiction cycle: reward/acquisition, withdrawal, and relapse, with a focus on alcohol, opioids (particularly morphine), and psychostimulants (particularly cocaine). Given the PVT's unique position within the broader neural landscape, we further discuss the potential ways in which neuropeptides may regulate these behaviors through their actions upon PVT circuits.

All-optical approaches to studying psychiatric disease

Improvements in all-optical means of monitoring and manipulating neural activity have generated new ways of studying psychiatric disease. The combination of calcium imaging techniques with optogenetics to concurrently record and manipulate neural activity has been used to create new disease models that link distinct circuit abnormalities to specific disease dimensions. These approaches represent a new path towards the development of more effective treatments, as they allow researchers to identify circuit manipulations that normalize pathological network activity. In this review we highlight the utility of all-optical approaches to generate new psychiatric disease models where the specific circuit abnormalities associated with disease symptomology can be assessed in vivo and in response to manipulations designed to normalize disease states. We then outline the principles underlying all-optical interrogations of neural circuits and discuss practical considerations for experimental design.

Thalamo-Nucleus Accumbens Projections in Motivated Behaviors and Addiction

The ventral striatum, also called nucleus accumbens (NAc), has long been known to integrate information from cortical, thalamic, midbrain and limbic nuclei to mediate goal-directed behaviors. Until recently thalamic afferents have been overlooked when studying the functions and connectivity of the NAc. However, findings from recent studies have shed light on the importance and roles of precise Thalamus to NAc connections in motivated behaviors and in addiction. In this review, we summarize studies using techniques such as chemo- and optogenetics, electrophysiology and in vivo calcium imaging to elucidate the complex functioning of the thalamo-NAc afferents, with a particular highlight on the projections from the Paraventricular Thalamus (PVT) to the NAc. We will focus on the recent advances in the understanding of the roles of these neuronal connections in motivated behaviors, with a special emphasis on their implications in addiction, from cue-reward association to the mechanisms driving relapse.

Neuropathic pain generates silent synapses in thalamic projection to anterior cingulate cortex

ABSTRACT

Pain experience can change the central processing of nociceptive inputs, resulting in persistent allodynia and hyperalgesia. However, the underlying circuit mechanisms remain underexplored. Here, we focus on pain-induced remodeling of the projection from the mediodorsal thalamus (MD) to the anterior cingulate cortex (ACC), a projection that relays spinal nociceptive input for central processing. Using optogenetics combined with slice electrophysiology, we detected in male mice that 7 days of chronic constriction injury (CCI; achieved by loose ligation of the sciatic nerve) generated AMPA receptor (AMPAR)-silent glutamatergic synapses within the contralateral MD-to-ACC projection. AMPAR-silent synapses are typically GluN2B-enriched nascent glutamatergic synapses that mediate the initial formation of neural circuits during early development. During development, some silent synapses mature and become "unsilenced" by recruiting and stabilizing AMPARs, consolidating and strengthening the newly formed circuits. Consistent with these synaptogenic features, pain-induced generation of silent synapses was accompanied by increased densities of immature dendritic spines in ACC neurons and increased synaptic weight of GluN2B-containing NMDA receptors (NMDARs) in the MD-to-ACC projection. After prolonged (∼30 days) CCI, injury-generated silent synapses declined to low levels, which likely resulted from a synaptic maturation process that strengthens AMPAR-mediated MD-to-ACC transmission. Consistent with this hypothesis, viral-mediated knockdown of GluN2B in ACC neurons, which prevented pain-induced generation of silent synapses and silent synapse-mediated strengthening of MD-to-ACC projection after prolonged CCI, prevented the development of allodynia. Taken together, our results depict a silent synapse-mediated mechanism through which key supraspinal neural circuits that regulate pain sensitivity are remodeled to induce allodynia and hyperalgesia.

The Paraventricular Thalamus: A Potential Sensor and Integrator of Emotionally Salient Early-Life Experiences

Early-life experiences influence a broad spectrum of behaviors throughout the lifespan that contribute to resilience or vulnerability to mental health disorders. Yet, how emotionally salient experiences early in life are encoded, stored, and processed and the mechanisms by which they influence future behaviors remain poorly understood. The paraventricular nucleus of the thalamus (PVT) is a key structure in modulating positive and negative experiences and behaviors in adults. However, little is known of the PVT's role in encoding and integrating emotionally salient experiences that occur during neonatal, infancy, and childhood periods. In this review, we (1) describe the functions and connections of the PVT and its regulation of behavior, (2) introduce novel technical approaches to elucidating the role of the PVT in mediating enduring changes in adult behaviors resulting from early-life experiences, and (3) conclude that PVT neurons of neonatal rodents are engaged by both positive and negative emotionally salient experiences, and their activation may enduringly govern future behavior-modulating PVT activity during emotionally salient contexts.

Input-specific modulation of murine nucleus accumbens differentially regulates hedonic feeding

Hedonic feeding is driven by the "pleasure" derived from consuming palatable food and occurs in the absence of metabolic need. It plays a critical role in the excessive feeding that underlies obesity. Compared to other pathological motivated behaviors, little is known about the neural circuit mechanisms mediating excessive hedonic feeding. Here, we show that modulation of prefrontal cortex (PFC) and anterior paraventricular thalamus (aPVT) excitatory inputs to the nucleus accumbens (NAc), a key node of reward circuitry, has opposing effects on high fat intake in mice. Prolonged high fat intake leads to input- and cell type-specific changes in synaptic strength. Modifying synaptic strength via plasticity protocols, either in an input-specific optogenetic or non-specific electrical manner, causes sustained changes in high fat intake. These results demonstrate that input-specific NAc circuit adaptations occur with repeated exposure to a potent natural reward and suggest that neuromodulatory interventions may be therapeutically useful for individuals with pathologic hedonic feeding.

The paraventricular nucleus of the thalamus: an integrative node underlying homeostatic behavior

Early anatomical evidence suggested that the paraventricular nucleus of the thalamus (PVT) regulates arousal, as well as emotional and motivated behaviors. We discuss recent studies using modern techniques which now confirm and expand the involvement of the rodent PVT in these functions. Despite the emerging notion that the PVT is implicated in various behavioral processes, a recurrent theme is that activity in this brain region depends on internal state information arriving from the hypothalamus and brainstem, and is influenced by prior experience. We propose that the primary function of the PVT is to detect homeostatic challenges by integrating information about prior experiences, competing needs, and internal state to guide adaptive behavioral responses aimed at restoring homeostasis.

Influences of Stress and Sex on the Paraventricular Thalamus: Implications for Motivated Behavior

The paraventricular nucleus of the thalamus (PVT) is a critical neural hub for the regulation of a variety of motivated behaviors, integrating stress and reward information from environmental stimuli to guide discrete behaviors via several limbic projections. Neurons in the PVT are activated by acute and chronic stressors, however several roles of the PVT in behavior modulation emerge only following repeated stress exposure, pointing to a role for hypothalamic pituitary adrenal (HPA) axis modulation of PVT function. Further, there may be a reciprocal relationship between the PVT and HPA axis in which chronic stress-induced recruitment of the PVT elicits an additional role for the PVT to regulate motivated behavior by modulating HPA physiology and thus the neuroendocrine response to stress itself. This complex interaction may make the PVT and its role in influencing motivated behavior particularly susceptible to chronic stress-induced plasticity in the PVT, especially in females who display increased susceptibility to stress-induced maladaptive behaviors associated with neuropsychiatric diseases. Though literature is describing the sex-specific effects of acute and chronic stress exposure on HPA axis activation and motivated behaviors, the impact of sex on the role of the PVT in modulating the behavioral and neuroendocrine response to stress is less well established. Here, we review what is currently known regarding the acute and chronic stress-induced activation and behavioral role of the PVT in male and female rodents. We further explore stress hormone and neuropeptide signaling mechanisms by which the HPA axis and PVT interact and discuss the implications for sex-dependent effects of chronic stress on the PVT's role in motivated behaviors.

The Contribution of Thalamic Nuclei in Salience Processing

The brain continuously receives diverse information about the external environment and changes in the homeostatic state. The attribution of salience determines which stimuli capture attention and, therefore, plays an essential role in regulating emotions and guiding behaviors. Although the thalamus is included in the salience network, the neural mechanism of how the thalamus contributes to salience processing remains elusive. In this mini-review, we will focus on recent advances in understanding the specific roles of distinct thalamic nuclei in salience processing. We will summarize the functional connections between thalamus nuclei and other key nodes in the salience network. We will highlight the convergence of neural circuits involved in reward and pain processing, arousal, and attention control in thalamic structures. We will discuss how thalamic activities represent salience information in associative learning and how thalamic neurons modulate adaptive behaviors. Lastly, we will review recent studies which investigate the contribution of thalamic dysfunction to aberrant salience processing in neuropsychiatric disorders, such as drug addiction, posttraumatic stress disorder (PTSD), and schizophrenia. Based on emerging evidence from both human and rodent research, we propose that the thalamus, different from previous studies that as an information relay, has a broader role in coordinating the cognitive process and regulating emotions.

Cocaine Triggers Astrocyte-Mediated Synaptogenesis

BACKGROUND

Synaptogenesis is essential in forming new neurocircuits during development, and this is mediated in part by astrocyte-released thrombospondins (TSPs) and activation of their neuronal receptor, α2δ-1. Here, we show that this developmental synaptogenic mechanism is utilized during cocaine experience to induce spinogenesis and the generation of AMPA receptor-silent glutamatergic synapses in the adult nucleus accumbens shell (NAcSh).

METHODS

Using multidisciplinary approaches including astrocyte Ca²⁺ imaging, genetic mouse lines, viral-mediated gene transfer, and operant behavioral procedures, we monitor the response of NAcSh astrocytes to cocaine administration and examine the role of astrocytic TSP-α2δ-1 signaling in cocaine-induced silent synapse generation as well as the behavioral impact of astrocyte-mediated synaptogenesis and silent synapse generation.

RESULTS

Cocaine administration acutely increases Ca²⁺ events in NAcSh astrocytes, while decreasing astrocytic Ca²⁺ blocks cocaine-induced generation of silent synapses. Furthermore, knockout of TSP2, or pharmacological inhibition or viral-mediated knockdown of α2δ-1, prevents cocaine-induced generation of silent synapses. Moreover, disrupting TSP2-α2δ-1-mediated spinogenesis and synapse generation in NAcSh decreases cue-induced cocaine seeking after withdrawal from cocaine self-administration and cue-induced reinstatement of cocaine seeking after drug extinction.

CONCLUSIONS

These results establish that silent synapses are generated by an astrocyte-mediated synaptogenic mechanism in response to cocaine experience and embed critical cue-associated memory traces that promote cocaine relapse.

AMPA and NMDA Receptor Trafficking at Cocaine-Generated Synapses

Cocaine experience generates AMPA receptor (AMPAR)-silent synapses in the nucleus accumbens (NAc), which are thought to be new synaptic contacts enriched in GluN2B-containing NMDA receptors (NMDARs). After drug withdrawal, some of these synapses mature by recruiting AMPARs, strengthening the newly established synaptic transmission. Silent synapse generation and maturation are two consecutive cellular steps through which NAc circuits are profoundly remodeled to promote cue-induced cocaine seeking after drug withdrawal. However, the basic cellular processes that mediate these two critical steps remains underexplored. Using a combination of electrophysiology, viral-mediated gene transfer, and confocal imaging in male rats as well as knock-in (KI) mice of both sexes, our current study characterized the dynamic roles played by AMPARs and NMDARs in generation and maturation of silent synapses on NAc medium spiny neurons after cocaine self-administration and withdrawal. We report that cocaine-induced generation of silent synapses not only required synaptic insertion of GluN2B-containing NMDARs, but also, counterintuitively, involved insertion of AMPARs, which subsequently internalized, resulting in the AMPAR-silent state on withdrawal day 1. Furthermore, GluN2B NMDARs functioned to maintain these cocaine-generated synapses in the AMPAR-silent state during drug withdrawal, until they were replaced by nonGluN2B NMDARs, a switch that allowed AMPAR recruitment and maturation of silent synapses. These results reveal dynamic interactions between AMPARs and NMDARs during the generation and maturation of silent synapses after cocaine experience and provide a mechanistic basis through which new synaptic contacts and possibly new neural network patterns created by these synapses can be manipulated for therapeutic benefit.SIGNIFICANCE STATEMENT Studies over the past decade reveal a critical role of AMPA receptor-silent, NMDA receptor-containing synapses in forming cocaine-related memories that drive cocaine relapse. However, it remains incompletely understood how AMPA and NMDA receptors traffic at these synapses during their generation and maturation. The current study characterizes a two-step AMPA receptor trafficking cascade that contributes to the generation of silent synapses in response to cocaine experience, and a two-step NMDA receptor trafficking cascade that contributes to the maturation of these synapses after cocaine withdrawal. These results depict a highly regulated cellular procedure through which nascent glutamatergic synapses are generated in the adult brain after drug experience and provide significant insight into the roles of glutamate receptors in synapse formation and maturation.

Nucleus accumbens fast-spiking interneurons in motivational and addictive behaviors

The development of drug addiction is associated with functional adaptations within the reward circuitry, within which the nucleus accumbens (NAc) is anatomically positioned as an interface between motivational salience and behavioral output. The functional output of NAc is profoundly altered after exposure to drugs of abuse, and some of the functional changes continue to evolve during drug abstinence, contributing to numerous emotional and motivational alterations related drug taking, seeking, and relapse. As in most brain regions, the functional output of NAc is critically dependent on the dynamic interaction between excitation and inhibition. One of the most prominent sources of inhibition within the NAc arises from fast-spiking interneurons (FSIs). Each NAc FSI innervates hundreds of principal neurons, and orchestrates population activity through its powerful and sustained feedforward inhibition. While the role of NAc FSIs in the context of drug addiction remains poorly understood, emerging evidence suggests that FSIs and FSI-mediated local circuits are key targets for drugs of abuse to tilt the functional output of NAc toward a motivational state favoring drug seeking and relapse. In this review, we discuss recent findings and our conceptualization about NAc FSI-mediated regulation of motivated and cocaine-induced behaviors. We hope that the conceptual framework proposed in this review may provide a useful guidance for ongoing and future studies to determine how FSIs influence the function of NAc and related reward circuits, ultimately leading to addictive behaviors.

Psychostimulant-Induced Adaptations in Nucleus Accumbens Glutamatergic Transmission

Carrying different aspects of emotional and motivational signals, glutamatergic synaptic projections from multiple limbic and paralimbic brain regions converge to the nucleus accumbens (NAc), in which these arousing signals are processed and prioritized for behavioral output. In animal models of drug addiction, some key drug-induced alterations at NAc glutamatergic synapses underlie important cellular and circuit mechanisms that promote subsequent drug taking, seeking, and relapse. With the focus of cocaine, we review changes at NAc glutamatergic synapses that occur after different drug procedures and abstinence durations, and the behavioral impact of these changes.

Cortical and Thalamic Interaction with Amygdala-to-Accumbens Synapses

The nucleus accumbens shell (NAcSh) regulates emotional and motivational responses, a function mediated, in part, by integrating and prioritizing extensive glutamatergic projections from limbic and paralimbic brain regions. Each of these inputs is thought to encode unique aspects of emotional and motivational arousal. The projections do not operate alone, but rather are often activated simultaneously during motivated behaviors, during which they can interact and coordinate in shaping behavioral output. To understand the anatomic and physiological bases underlying these interprojection interactions, the current study in mice of both sexes focused on how the basolateral amygdala projection (BLAp) to the NAcSh regulates, and is regulated by, projections from the medial prefrontal cortex (mPFCp) and paraventricular nucleus of the thalamus (PVTp). Using a dual-color SynaptoTag technique combined with a backfilling spine imaging strategy, we found that all three afferent projections primarily targeted the secondary dendrites of NAcSh medium spiny neurons, forming putative synapses. We detected a low percentage of BLAp contacts closely adjacent to mPFCp or PVTp presumed synapses, and, on some rare occasions, the BLAp formed heterosynaptic interactions with mPFCp or PVTp profiles or appeared to contact the same spines. Using dual-rhodopsin optogenetics, we detected signs of dendritic summation of BLAp with PVTp and mPFCp inputs. Furthermore, high-frequency activation of BLAp synchronous with the PVTp or mPFCp resulted in a transient enhancement of the PVTp, but not mPFCp, transmission. These results provide anatomic and functional indices that the BLAp interacts with the mPFCp and PVTp for informational processing within the NAcSh.SIGNIFICANCE STATEMENT The nucleus accumbens regulates emotional and motivational responses by integrating extensive glutamatergic projections, but the anatomic and physiological bases on which these projections integrate and interact remain underexplored. Here, we used dual-color synaptic markers combined with backfilling of nucleus accumbens medium spiny neurons to reveal some unique anatomic alignments of presumed synapses from the basolateral amygdala, medial prefrontal cortex, and paraventricular nucleus of thalamus. We also used dual-rhodopsin optogenetics in brain slices, which reveal a nonlinear interaction between some, but not all, projections. These results provide compelling anatomic and physiological mechanisms through which different glutamatergic projections to the nucleus accumbens, and possibly different aspects of emotional and motivational arousal, interact with each other for final behavioral output.

A Synthetic Likelihood Solution to the Silent Synapse Estimation Problem

Functional features of synaptic populations are typically inferred from random electrophysiological sampling of small subsets of synapses. Are these samples unbiased? Here, we develop a biophysically constrained statistical framework to address this question and apply it to assess the performance of a widely used method based on a failure-rate analysis to quantify the occurrence of silent (AMPAR-lacking) synapses. We simulate this method in silico and find that it is characterized by strong and systematic biases, poor reliability, and weak statistical power. Key conclusions are validated by whole-cell recordings from hippocampal neurons. To address these shortcomings, we develop a simulator of the experimental protocol and use it to compute a synthetic likelihood. By maximizing the likelihood, we infer silent synapse fraction with no bias, low variance, and superior statistical power over alternatives. Together, this generalizable approach highlights how a simulator of experimental methodologies can substantially improve the estimation of physiological properties.

Homeostatic regulation of reward via synaptic insertion of calcium-permeable AMPA receptors in nucleus accumbens

The incentive effects of food and related cues are determined by stimulus properties and the internal state of the organism. Enhanced hedonic reactivity and incentive motivation in energy deficient subjects have been demonstrated in animal models and humans. Defining the neurobiological underpinnings of these state-based modulatory effects could illuminate fundamental mechanisms of adaptive behavior, as well as provide insight into maladaptive consequences of weight loss dieting and the relationship between disturbed eating behavior and substance abuse. This article summarizes research of our laboratory aimed at identifying neuroadaptations induced by chronic food restriction (FR) that increase the reward magnitude of drugs and associated cues. The main findings are that FR decreases basal dopamine (DA) transmission, upregulates signaling downstream of the D1 DA receptor (D1R), and triggers synaptic incorporation of calcium-permeable AMPA receptors (CP-AMPARs) in the nucleus accumbens (NAc). Selective antagonism of CP-AMPARs decreases excitatory postsynaptic currents in NAc medium spiny neurons of FR rats and blocks the enhanced rewarding effects of d-amphetamine and a D1R, but not a D2R, agonist. These results suggest that FR drives CP-AMPARs into the synaptic membrane of D1R-expressing MSNs, possibly as a homeostatic response to reward loss. FR subjects also display diminished aversion for contexts associated with LiCl treatment and centrally infused cocaine. An encompassing, though speculative, hypothesis is that NAc synaptic incorporation of CP-AMPARs in response to food scarcity and other forms of sustained reward loss adaptively increases incentive effects of reward stimuli and, at the same time, diminishes responsiveness to aversive stimuli that have potential to interfere with goal pursuit.

Off-Target Influences of Arch-Mediated Axon Terminal Inhibition on Network Activity and Behavior

Archaerhodopsin (ArchT)-mediated photoinhibition of axon terminals is commonly used to test the involvement of specific long-range neural projections in behavior. Although sustained activation of this opsin in axon terminals has the unintended consequence of enhancing spontaneous vesicle release, it is unclear whether this desynchronized signaling is consequential for ArchT’s behavioral effects. Here, we compare axon terminal and cell body photoinhibition of nucleus accumbens (NAc) afferents to test the utility of these approaches for uncovering pathway-specific contributions of neural circuits to behavior. First, in brain slice recordings we confirmed that ArchT photoinhibition of glutamatergic axons reduces evoked synaptic currents and increases spontaneous transmitter release. A further consequence was increased interneuron activity, which served to broadly suppress glutamate input via presynaptic GABA_B receptors. In vivo, axon terminal photoinhibition increased feeding and reward-seeking behavior irrespective of the afferent pathway targeted. These behavioral effects are comparable to those obtained with broad inhibition of NAc neurons. In contrast, cell body inhibition of excitatory NAc afferents revealed a pathway-specific contribution of thalamic input to feeding behavior and amygdala input to reward-seeking under extinction conditions. These findings underscore the off-target behavioral consequences of ArchT-mediated axon terminal inhibition while highlighting cell body inhibition as a valuable alternative for pathway-specific optogenetic silencing.

The hypocretin (orexin) system: from a neural circuitry perspective

Hypocretin/orexin neurons are distributed restrictively in the hypothalamus, a brain region known to orchestrate diverse functions including sleep, reward processing, food intake, thermogenesis, and mood. Since the hypocretins/orexins were discovered more than two decades ago, extensive studies have accumulated concrete evidence showing the pivotal role of hypocretin/orexin in diverse neural modulation. New method of viral-mediated tracing system offers the possibility to map the monosynaptic inputs and detailed anatomical connectivity of Hcrt neurons. With the development of powerful research techniques including optogenetics, fiber-photometry, cell-type/pathway specific manipulation and neuronal activity monitoring, as well as single-cell RNA sequencing, the details of how hypocretinergic system execute functional modulation of various behaviors are coming to light. In this review, we focus on the function of neural pathways from hypocretin neurons to target brain regions. Anatomical and functional inputs to hypocretin neurons are also discussed. We further briefly summarize the development of pharmaceutical compounds targeting hypocretin signaling. This article is part of the special issue on Neuropeptides.

Timing of Morphine Administration Differentially Alters Paraventricular Thalamic Neuron Activity

The paraventricular thalamic nucleus (PVT) is a brain region involved in regulating arousal, goal-oriented behaviors, and drug seeking, all key factors playing a role in substance use disorder. Given this, we investigated the temporal effects of administering morphine, an opioid with strongly addictive properties, on PVT neuronal function in mice using acute brain slices. Here, we show that morphine administration and electrophysiological recordings that occur during periods of animal inactivity (light cycle) elicit increases in PVT neuronal function during a 24-h abstinence time point. Furthermore, we show that morphine-induced increases in PVT neuronal activity at 24-h abstinence are occluded when morphine administration and recordings are performed during an animals' active state (dark cycle). Based on our electrophysiological results combined with previous findings demonstrating that PVT neuronal activity regulates drug-seeking behaviors, we investigated whether timing morphine administration with periods of vigilance (dark cycle) would decrease drug-seeking behaviors in an animal model of substance use disorder. We found that context-induced morphine-seeking behaviors were intact regardless of the time morphine was administered (e.g., light cycle or dark cycle). Our electrophysiological results suggest that timing morphine with various states of arousal may impact the firing of PVT neurons during abstinence. Although, this may not impact context-induced drug-seeking behaviors.

Projection-Specific Potentiation of Ventral Pallidal Glutamatergic Outputs after Abstinence from Cocaine

The ventral pallidum (VP) is a central node in the reward system that is strongly implicated in reward and addiction. Although the majority of VP neurons are GABAergic and encode reward, recent studies revealed a novel glutamatergic neuronal population in the VP [VP neurons expressing the vesicular glutamate transporter 2 (VP_VGluT2)], whose activation generates aversion. Withdrawal from drugs has been shown to induce drastic synaptic changes in neuronal populations associated with reward, such as the ventral tegmental area (VTA) or nucleus accumbens neurons, but less is known about cocaine-induced synaptic changes in neurons classically linked with aversion. Here, we demonstrate that VP_VGluT2 neurons contact different targets with different intensities, and that cocaine conditioned place preference (CPP) training followed by abstinence selectively potentiates their synapses on targets that encode aversion. Using whole-cell patch-clamp recordings combined with optogenetics in male and female transgenic mice, we show that VP_VGluT2 neurons preferentially contact aversion-related neurons, such as lateral habenula neurons and VTA GABAergic neurons, with minor input to reward-related neurons, such as VTA dopamine and VP GABA neurons. Moreover, after cocaine CPP and abstinence, the VP_VGluT2 input to the aversion-related structures is potentiated, whereas the input to the reward-related structures is depressed. Thus, cocaine CPP followed by abstinence may allow VP_VGluT2 neurons to recruit aversion-related targets more readily and therefore be part of the mechanism underlying the aversive symptoms seen after withdrawal.SIGNIFICANCE STATEMENT The biggest problem in drug addiction is the high propensity to relapse. One central driver for relapse events is the negative aversive symptoms experienced by addicts during withdrawal. In this work, we propose a possible mechanism for the intensification of aversive feelings after withdrawal that involves the glutamatergic neurons of the ventral pallidum. We show not only that these neurons are most strongly connected to aversive targets, such as the lateral habenula, but also that, after abstinence, their synapses on aversive targets are strengthened, whereas the synapses on other rewarding targets are weakened. These data illustrate how after abstinence from cocaine, aversive pathways change in a manner that may contribute to relapse.

Silent synapses dictate cocaine memory destabilization and reconsolidation

Cocaine-associated memories are persistent, but, upon retrieval, become temporarily destabilized and vulnerable to disruptions, followed by reconsolidation. To explore the synaptic underpinnings for these memory dynamics, we studied AMPA receptor (AMPAR)-silent excitatory synapses, which are generated in the nucleus accumbens by cocaine self-administration, and subsequently mature after prolonged withdrawal by recruiting AMPARs, echoing acquisition and consolidation of cocaine memories. We show that, upon memory retrieval after prolonged withdrawal, the matured silent synapses become AMPAR-silent again, followed by re-maturation ~6 hr later, defining the onset and termination of a destabilization window of cocaine memories. These synaptic dynamics are controlled by Rac1, with decreased and increased Rac1 activities opening and closing, respectively, the silent synapse-mediated destabilization window. Preventing silent synapse re-maturation within the destabilization window decreases cue-induced cocaine seeking. Thus, cocaine-generated silent synapses constitute a discrete synaptic ensemble dictating the dynamics of cocaine-associated memories and can be targeted for memory disruption.

Gender-related differences in neural responses to gaming cues before and after gaming: implications for gender-specific vulnerabilities to Internet gaming disorder

Backgrounds

More males than females play video games and develop problems with gaming. However, little is known regarding how males and females who game on the Internet may differ with respect to neural responses to gaming cues.

Methods

Behavioral and functional magnetic resonance imaging (fMRI) data were recorded from 40 female and 68 male Internet gamers. This study included three components including participation in a pre-gaming cue-craving task, 30 min of online gaming and a post-gaming cue-elicited-craving task. Group differences were examined at pre-gaming, post-gaming and post- vs pre-gaming times. Correlations between brain responses and behavioral performance were calculated.

Results

Gaming-related cues elicited higher cravings in male vs female subjects. Prior to gaming, males demonstrated greater activations in the striatum, orbitofrontal cortex (OFC), inferior frontal cortex and bilateral declive. Following gaming, male subjects demonstrated greater activations in the medial frontal gyrus and bilateral middle temporal gyri. In a post–pre comparison, male subjects demonstrated greater thalamic activation than did female subjects.

Conclusions

Short-term gaming elicited in males vs females more craving-related activations to gaming cues. These results suggest neural mechanisms for why males may be more vulnerable than females in developing Internet gaming disorder.

The paraventricular thalamic nucleus: A key hub of neural circuits underlying drug addiction

Drug addiction is a chronic relapsing brain disease characterized by compulsive, out-of-control drug use and the appearance of negative somatic and emotional consequences when drug access is prevented. The limited efficacy of treatment urges researchers toward a deeper understanding of the neural mechanism of drug addiction. Brain circuits that regulate reward and motivation are considered to be the neural substrate of drug addiction. An increasing body of literature indicates that the paraventricular thalamic nucleus (PVT) could serve as a key node in the neurocircuits that control goal-directed behaviors. In this review, we summarize the anatomical and functional evidence that the PVT regulates drug-related behaviors. The PVT receives extensive inputs from the brainstem and hypothalamus, and is reciprocally connected with the limbic system. Neurons in the PVT are recruited by drug exposure as well as cues and context associated with drug taking. Pathway-specific perturbation studies have begun to decipher the precise role of PVT circuits in drug-related behaviors. We also highlight recent findings about the involvement of neural plasticity of the PVT pathways in drug addiction and provide perspectives on future studies.

The thalamus in drug addiction: from rodents to humans

Impairments in response inhibition and salience attribution (iRISA) have been proposed to underlie the clinical symptoms of drug addiction as mediated by cortico-striatal-thalamo-cortical networks. The bulk of evidence supporting the iRISA model comes from neuroimaging research that has focused on cortical and striatal influences with less emphasis on the role of the thalamus. Here, we highlight the importance of the thalamus in drug addiction, focusing on animal literature findings on thalamic nuclei in the context of drug-seeking, structural and functional changes of the thalamus as measured by imaging studies in human drug addiction, particularly during drug cue and non-drug reward processing, and response inhibition tasks. Findings from the animal literature suggest that the paraventricular nucleus of the thalamus, the lateral habenula and the mediodorsal nucleus may be involved in the reinstatement, extinction and expression of drug-seeking behaviours. In support of the iRISA model, the human addiction imaging literature demonstrates enhanced thalamus activation when reacting to drug cues and reduced thalamus activation during response inhibition. This pattern of response was further associated with the severity of, and relapse in, drug addiction. Future animal studies could widen their field of focus by investigating the specific role(s) of different thalamic nuclei in different phases of the addiction cycle. Similarly, future human imaging studies should aim to specifically delineate the structure and function of different thalamic nuclei, for example, through the application of advanced imaging protocols at higher magnetic fields (7 Tesla).This article is part of a discussion meeting issue 'Of mice and mental health: facilitating dialogue between basic and clinical neuroscientists'.

Cocaine-Induced Structural Plasticity in Input Regions to Distinct Cell Types in Nucleus Accumbens

BACKGROUND

The nucleus accumbens (NAc) is a brain region implicated in pathological motivated behaviors such as drug addiction and is composed predominantly of two discrete populations of neurons, dopamine receptor-1- and dopamine receptor-2-expressing medium spiny neurons (D1-MSNs and D2-MSNs, respectively). It is unclear whether these populations receive inputs from different brain areas and whether input regions to these cell types undergo distinct structural adaptations in response to the administration of addictive drugs such as cocaine.

METHODS

Using a modified rabies virus-mediated tracing method, we created a comprehensive brain-wide monosynaptic input map to NAc D1- and D2-MSNs. Next, we analyzed nearly 2000 dendrites and 125,000 spines of neurons across four input regions (the prelimbic cortex, medial orbitofrontal cortex, basolateral amygdala, and ventral hippocampus) at four separate time points during cocaine administration and withdrawal to examine changes in spine density in response to repeated intraperitoneal cocaine injection in mice.

RESULTS

D1- and D2-MSNs display overall similar input profiles, with the exception that D1-MSNs receive significantly more input from the medial orbitofrontal cortex. We found that neurons in distinct brain areas projecting to D1- and D2-MSNs display different adaptations in dendritic spine density at different stages of cocaine administration and withdrawal.

CONCLUSIONS

While NAc D1- and D2-MSNs receive input from similar brain structures, cocaine-induced spine density changes in input regions are quite distinct and dynamic. While previous studies have focused on input-specific postsynaptic changes within NAc MSNs in response to cocaine, these findings emphasize the dramatic changes that occur in the afferent input regions as well.

Genetic loss of GluN2B in D1-expressing cell types enhances long-term cocaine reward and potentiation of thalamo-accumbens synapses

Transient upregulation of GluN2B-containing NMDA receptors (R) in the nucleus accumbens (NAc) is proposed as an intermediate to long-term AMPAR plasticity associated with persistent cocaine-related behaviors. However, cell type- and input-specific contributions of GluN2B underlying lasting actions of cocaine remain to be elucidated. We utilized GluN2B cell type-specific knockouts and optogenetics to deconstruct the role of GluN2B in cocaine-induced NAc synaptic and behavioral plasticity. While reward learning was unaffected, loss of GluN2B in D1 dopamine receptor-expressing cells (D1) led to prolonged retention of reward memory. In control mice, prefrontal cortex (PFC)-D1(+) NAc AMPAR function was unaffected by cocaine exposure, while midline thalamus (mThal)-D1(+) NAc AMPAR function was potentiated but diminished after withdrawal. In D1-GluN2B^-/- mice, the potentiation of mThal-D1(+) NAc AMPAR function persisted following withdrawal, corresponding with continued expression of cocaine reward behavior. These data suggest NAc GluN2B-containing NMDARs serve a feedback role and may weaken reward-related memories.

Metaplasticity at the addicted tetrapartite synapse: A common denominator of drug induced adaptations and potential treatment target for addiction

In light of the current worldwide addiction epidemic, the need for successful therapies is more urgent than ever. Although we made substantial progress in our basic understanding of addiction, reliable therapies are lacking. Since 40-60% of patients treated for substance use disorder return to active substance use within a year following treatment discharge, alleviating the vulnerability to relapse is regarded as the most promising avenue for addiction therapy. Preclinical addiction research often focuses on maladaptive synaptic plasticity within the reward pathway. However, drug induced neuroadaptations do not only lead to a strengthening of distinct drug associated cues and drug conditioned behaviors, but also seem to increase plasticity thresholds for environmental stimuli that are not associated with the drug. This form of higher order plasticity, or synaptic metaplasticity, is not expressed as a change in the efficacy of synaptic transmission but as a change in the direction or degree of plasticity induced by a distinct stimulation pattern. Experimental addiction research has demonstrated metaplasticity after exposure to multiple classes of addictive drugs. In this review we will focus on the concept of synaptic metaplasticity in the context of preclinical addiction research. We will take a closer look at the tetrapartite glutamatergic synapse and outline forms of metaplasticity that have been described at the addicted synapse. Finally we will discuss the different potential avenues for pharmacotherapies that target glutamatergic synaptic plasticity and metaplasticity. Here we will argue that aberrant metaplasticity renders the reward seeking circuitry more rigid and hence less able to adapt to changing environmental contingencies. An understanding of the molecular mechanisms that underlie this metaplasticity is crucial for the development of new strategies for addiction therapy. The correction of drug-induced metaplasticity could be used to support behavioral and pharmacotherapies for the treatment of addiction.

Synaptic Plasticity in the Nucleus Accumbens: Lessons Learned from Experience

Synaptic plasticity contributes to behavioral adaptations. As a key node in the reward pathway, the nucleus accumbens (NAc) is important for determining motivation-to-action outcomes. Across animal models of motivation including addiction, depression, anxiety, and hedonic feeding, selective recruitment of neuromodulatory signals and plasticity mechanisms have been a focus of physiologists and behaviorists alike. Experience-dependent plasticity mechanisms within the NAc vary depending on the distinct afferents and cell-types over time. A greater understanding of molecular mechanisms determining how these changes in synaptic strength track with behavioral adaptations will provide insight into the process of learning and memory along with identifying maladaptations underlying pathological behavior. Here, we summarize recent findings detailing how changes in NAc synaptic strength and mechanisms of plasticity manifest in various models of motivational disorders.

Synaptic Plasticity in the Bed Nucleus of the Stria Terminalis: Underlying Mechanisms and Potential Ramifications for Reinstatement of Drug- and Alcohol-Seeking Behaviors

The bed nucleus of the stria terminalis (BNST) is a component of the extended amygdala that shows significant changes in activity and plasticity through chronic exposure to drugs and stress. The region is critical for stress- and cue-induced reinstatement of drug-seeking behaviors and is thus a candidate region for the plastic changes that occur in abstinence that prime addicted patients for reinstatement behaviors. Here, we discuss the various forms of long-term potentiation (LTP) and long-term depression (LTD) in the rodent BNST and highlight the way that these changes in excitatory transmission interact with exposure to alcohol and other drugs of abuse, as well as other stressors. In addition, we highlight potential areas for future research in this area, including investigating input- and cell-specific bidirectional changes in activity. As we continue to accrue foundational knowledge in the mechanisms and effects of plasticity in the BNST, molecular targets and treatment strategies that are relevant to reinstatement behaviors will also begin to emerge. Here, we briefly discuss the effects of catecholamine receptor modulators on synaptic plasticity in the BNST due to the role of norepinephrine in LTD and dopamine on the short-term component of LTP as well as the role that signaling at these receptors plays in reinstatement of drug- and alcohol-seeking behaviors. We hope that insights gained on the specific changes in plasticity that occur within the BNST during abstinence from alcohol and other drugs of abuse will provide insight into the biological underpinnings of relapse behavior in human addicts and inform future treatment modalities for addiction that tackle this complex biological problem.