Dopaminergic Regulation of Nucleus Accumbens Cholinergic Interneurons Demarcates Susceptibility to Cocaine Addiction

A cocaine-activated ensemble exerts increased control over behavior while decreasing in size

BACKGROUND

Substance use disorder (SUD) is characterized by long-lasting changes in reward-related brain regions, such as the nucleus accumbens (NAc). Previous work has shown that cocaine exposure induces plasticity in broad, genetically-defined cell types in the NAc; however, in response to a stimulus, only a small percent of neurons are transcriptionally active - termed an ensemble. Here, we identify an Arc-expressing neuronal ensemble that has a unique trajectory of recruitment and causally controls drug self-administration after repeated, but not acute, cocaine exposure.

METHOD

Using Arc-CreERT2 transgenic mice, we expressed transgenes in Arc+ ensembles activated by cocaine exposure [either acute (1 x 10mg/kg IP), or repeated (10 x 10mg/kg IP)]. Using genetic, optical, and physiological recording and manipulation strategies, we assessed the contribution of these ensembles to behaviors associated with SUD.

RESULTS

Repeated cocaine exposure reduced the size of the ensemble, while simultaneously increasing its control over behavior. Neurons within the repeated cocaine ensemble were hyperexcitable and their optogenetic excitation was sufficient for reinforcement. Finally, lesioning the repeated cocaine, but not acute cocaine, ensemble blunted cocaine self-administration. Thus, repeated cocaine exposure reduced the size of the ensemble while simultaneously increasing its contributions to drug reinforcement.

CONCLUSIONS

We show that repeated, but not acute, cocaine exposure induces a physiologically distinct ensemble characterized by the expression of the immediate early gene Arc, that is uniquely capable of modulating reinforcement behavior.

Activation of CGRP neurons in the parabrachial nucleus suppresses addictive behavior

Punishment such as electric shock or physical discipline employs a mixture of physical pain and emotional distress to induce behavior modification. However, a neural circuit that produces behavior modification by selectively focusing the emotional component, while bypassing the pain typically induced by peripheral nociceptor activation, is not well studied. Here, we show that genetically silencing the activity of neurons expressing calcitonin gene-related peptide (CGRP) in the parabrachial nucleus blocks the suppression of addictive-like behavior induced by footshock. Furthermore, activating CGRP neurons suppresses not only addictive behavior induced by self-stimulating dopamine neurons but also behavior resulting from self-administering cocaine, without eliciting nocifensive reactions. Moreover, among multiple downstream targets of CGRP neurons, terminal activation of CGRP in the central amygdala is effective, mimicking the results of cell body stimulation. Our results indicate that unlike conventional electric footshock, stimulation of CGRP neurons does not activate peripheral nociceptors but effectively curb addictive behavior.

Cholinergic Interneurons in the Accumbal Shell Region Regulate Binge Alcohol Self-Administration in Mice: An In Vivo Calcium Imaging Study

Recently, we and others have shown that manipulating the activity of cholinergic interneurons (CIN) present in the NAc can modulate binge alcohol consumption. The present study is designed to examine the relationship between binge alcohol consumption and the activity of the CIN in real time by using an in vivo microendoscopic technique. We hypothesized that mice exposed to Drinking in the Dark (DID)-a recognized mouse model for binge drinking-would exhibit increased activity in the accumbal shell region (NAcSh). To test this hypothesis, male mice expressing Cre-recombinase in the cholinergic neurons were exposed to binge alcohol consumption (alcohol group), employing the DID method, and utilized in vivo calcium imaging to observe CIN activity in real time during alcohol consumption. The control (sucrose) group was exposed to 10% (w/v) sucrose. As compared to sucrose, mice in the alcohol group displayed a significant increase in the frequency and amplitude of discharge activity, which was measured using calcium transients in the CIN present in the NAcSh. In summary, our findings suggest that the activity of CIN in the NAcSh plays a crucial role in alcohol self-administration. These results emphasize the potential significance of targeting CIN activity as a therapeutic approach for addressing AUD.

Distinct prefrontal projection activity and transcriptional state conversely orchestrate social competition and hierarchy

Social animals compete for limited resources, resulting in a social hierarchy. Although different neuronal subpopulations in the medial prefrontal cortex (mPFC), which has been mechanistically implicated in social dominance behavior, encode distinct social competition behaviors, their identities and associated molecular underpinnings have not yet been identified. In this study, we found that mPFC neurons projecting to the nucleus accumbens (mPFC-NAc) encode social winning behavior, whereas mPFC neurons projecting to the ventral tegmental area (mPFC-VTA) encode social losing behavior. High-throughput single-cell transcriptomic analysis and projection-specific genetic manipulation revealed that the expression level of POU domain, class 3, transcription factor 1 (Pou3f1) in mPFC-VTA neurons controls social hierarchy. Optogenetic activation of mPFC-VTA neurons increases Pou3f1 expression and lowers social rank. Together, these data demonstrate that discrete activity and gene expression in separate mPFC projections oppositely orchestrate social competition and hierarchy.

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.

The FAAH inhibitor URB597 reduces cocaine intake during conditioned punishment and mitigates cocaine seeking during withdrawal

The endocannabinoid system is prominently implicated in the control of cocaine reinforcement due to its relevant role in synaptic plasticity and neurotransmitter modulation in the mesocorticolimbic system. The inhibition of fatty acid amide hydrolase (FAAH), and the resulting increase in anandamide and other N-acylethanolamines, represents a promising strategy for reducing drug seeking. In the present study, we aimed to assess the effects of the FAAH inhibitor URB597 (1 mg/kg) on crucial features of cocaine addictive-like behaviour in mice. Therefore, we tested the effects of URB597 on acquisition of cocaine (0.6 mg/kg/inf) self-administration, compulsive-like cocaine intake and cue-induced drug-seeking behaviour during withdrawal. URB597 reduced cocaine intake under conditioned punishment while having no impact on acquisition. This result was associated to increased cannabinoid receptor 1 gene expression in the ventral striatum and medium spiny neurons activation in the nucleus accumbens shell. Moreover, URB597 mitigated cue-induced drug-seeking behaviour during prolonged abstinence and prevented the withdrawal-induced increase in FAAH gene expression in the ventral striatum. In this case, URB597 decreased activation of medium spiny neurons in the nucleus accumbens core. Our findings evidence the prominent role of endocannabinoids in the development of cocaine addictive-like behaviours and support the potential of FAAH inhibition as a therapeutical target for the treatment of cocaine addiction.

Critical appraisal and systematic review of genes linked with cocaine addiction, depression and anxiety

Recent lifestyle changes have resulted in tremendous peer pressure and mental stress, and increased the incidences of chronic psychological disorders; like addiction, depression and anxiety (ADA). In this context, the stress-tolerance levels vary amongst individuals and genetic factors play prominent roles. Vulnerable individuals may often be drawn towards drug-addiction to combat stress. This systematic review critically appraises the relationship of various genetic factors linked with the incidences of ADA development. For coherence, we focused solely on cocaine as a substance of abuse in this study. Online scholarly databases were used to screen pertinent literature using apt keywords; and the final retrieval included 42 primary-research articles. The major conclusion drawn from this systematic analysis states that there are 51 genes linked with the development of ADA; and 3 (BDNF, PERIOD2 and SLC6A4) of them are common to all the three aspects of ADA. Further, inter-connectivity analyses of the 51 genes further endorsed the central presence of BDNF and SLC6A4 genes in the development of ADA disorders. The conclusions derived from this systematic study pave the way for future studies for the identification of diagnostic biomarkers and drug targets; and for the development of novel and effective therapeutic regimens against ADA.

Drug reinforcement impairs cognitive flexibility by inhibiting striatal cholinergic neurons

Addictive substance use impairs cognitive flexibility, with unclear underlying mechanisms. The reinforcement of substance use is mediated by the striatal direct-pathway medium spiny neurons (dMSNs) that project to the substantia nigra pars reticulata (SNr). Cognitive flexibility is mediated by striatal cholinergic interneurons (CINs), which receive extensive striatal inhibition. Here, we hypothesized that increased dMSN activity induced by substance use inhibits CINs, reducing cognitive flexibility. We found that cocaine administration in rodents caused long-lasting potentiation of local inhibitory dMSN-to-CIN transmission and decreased CIN firing in the dorsomedial striatum (DMS), a brain region critical for cognitive flexibility. Moreover, chemogenetic and time-locked optogenetic inhibition of DMS CINs suppressed flexibility of goal-directed behavior in instrumental reversal learning tasks. Notably, rabies-mediated tracing and physiological studies showed that SNr-projecting dMSNs, which mediate reinforcement, sent axonal collaterals to inhibit DMS CINs, which mediate flexibility. Our findings demonstrate that the local inhibitory dMSN-to-CIN circuit mediates the reinforcement-induced deficits in cognitive flexibility.

Accumbal Histamine Signaling Engages Discrete Interneuron Microcircuits

BACKGROUND

Central histamine (HA) signaling modulates diverse cortical and subcortical circuits throughout the brain, including the nucleus accumbens (NAc). The NAc, a key striatal subregion directing reward-related behavior, expresses diverse HA receptor subtypes that elicit cellular and synaptic plasticity. However, the neuromodulatory capacity of HA within interneuron microcircuits in the NAc remains unknown.

METHODS

We combined electrophysiology, pharmacology, voltammetry, and optogenetics in male transgenic reporter mice to determine how HA influences microcircuit motifs controlled by parvalbumin-expressing fast-spiking interneurons (PV-INs) and tonically active cholinergic interneurons (CINs) in the NAc shell.

RESULTS

HA enhanced CIN output through an H2 receptor (H2R)-dependent effector pathway requiring Ca2+-activated small-conductance K+ channels, with a small but discernible contribution from H1Rs and synaptic H3Rs. While PV-IN excitability was unaffected by HA, presynaptic H3Rs decreased feedforward drive onto PV-INs via AC-cAMP-PKA (adenylyl cyclase-cyclic adenosine monophosphate-protein kinase A) signaling. H3R-dependent plasticity was differentially expressed at mediodorsal thalamus and prefrontal cortex synapses onto PV-INs, with mediodorsal thalamus synapses undergoing HA-induced long-term depression. These effects triggered downstream shifts in PV-IN- and CIN-controlled microcircuits, including near-complete collapse of mediodorsal thalamus-evoked feedforward inhibition and increased mesoaccumbens dopamine release.

CONCLUSIONS

HA targets H1R, H2R, and H3Rs in the NAc shell to engage synapse- and cell type-specific mechanisms that bidirectionally regulate PV-IN and CIN microcircuit activity. These findings extend the current conceptual framework of HA signaling and offer critical insight into the modulatory potential of HA in the brain.

Transcriptional control of nucleus accumbens neuronal excitability by retinoid X receptor alpha tunes sensitivity to drug rewards

The complex nature of the transcriptional networks underlying addictive behaviors suggests intricate cooperation between diverse gene regulation mechanisms that go beyond canonical activity-dependent pathways. Here, we implicate in this process a nuclear receptor transcription factor, retinoid X receptor alpha (RXRα), which we initially identified bioinformatically as associated with addiction-like behaviors. In the nucleus accumbens (NAc) of male and female mice, we show that although its own expression remains unaltered after cocaine exposure, RXRα controls plasticity- and addiction-relevant transcriptional programs in both dopamine receptor D1- and D2-expressing medium spiny neurons, which in turn modulate intrinsic excitability and synaptic activity of these NAc cell types. Behaviorally, bidirectional viral and pharmacological manipulation of RXRα regulates drug reward sensitivity in both non-operant and operant paradigms. Together, this study demonstrates a key role for NAc RXRα in promoting drug addiction and paves the way for future studies of rexinoid signaling in psychiatric disease states.

CREB Binding at the Zfp189 Promoter Within Medium Spiny Neuron Subtypes Differentially Regulates Behavioral and Physiological Adaptations Over the Course of Cocaine Use

BACKGROUND

Over the course of chronic drug use, brain transcriptional neuroadaptation is thought to contribute to a change in drug use behavior over time. The function of the transcription factor CREB (cAMP response element binding protein) within the nucleus accumbens (NAc) has been well documented in opposing the rewarding properties of many classes of drugs, yet the gene targets through which CREB causally manifests these lasting neuroadaptations remain unknown. Here, we identify zinc finger protein 189 (Zfp189) as a CREB target gene that is transcriptionally responsive to acute and chronic cocaine use within the NAc of mice.

METHODS

To investigate the role of the CREB-Zfp189 interaction in cocaine use, we virally delivered modified clustered regularly interspaced short palindromic repeats (CRISPR)/dCas9 constructs capable of selectively localizing CREB to the Zfp189 gene promoter in the NAc of mice.

RESULTS

We observed that CREB binding to the Zfp189 promoter increased Zfp189 expression and diminished the reinforcing responses to cocaine. Furthermore, we showed that NAc Zfp189 expression increased within D1 medium spiny neurons in response to acute cocaine but increased in both D1- and D2-expressing medium spiny neurons in response to chronic cocaine. CREB-mediated induction of Zfp189 potentiated electrophysiological activity of D1- and D2-expressing medium spiny neurons, recapitulating the known effect of CREB on these neurons. Finally, targeting CREB to the Zfp189 promoter within NAc Drd2-expressing neurons, but not Drd1-expressing neurons, was sufficient to diminish cocaine-conditioned behaviors.

CONCLUSIONS

Together, these findings point to the CREB-Zfp189 interaction within the NAc Drd2+ neurons as a molecular signature of chronic cocaine use that is causal in counteracting the reinforcing effects of cocaine.

Cell-type specific synaptic plasticity in dorsal striatum is associated with punishment-resistance compulsive-like cocaine self-administration in mice

Addiction-related compulsion-like behavior can be modeled in rodents with drug self-administration (SA) despite harmful consequences. Recent studies suggest that the potentiation of glutamatergic transmission at the orbitofrontal cortex (OFC) to dorsal striatum (DS) synapses drives the transition from controlled to compulsion-like SA. However, the timing of the induction of this synaptic plasticity remains elusive. Here, mice were first allowed to intravenously self-administer cocaine. When mice had to endure a risk of electrical foot shock, only a fraction persevered in cocaine SA. In these persevering mice, we recorded high A/N ratios (AMPA-R/NMDA-R: α-amino-3hydroxy-5-methyl-4-isoxazolepropionic acid receptor/N-methyl-D-aspartate receptor) in both types of spiny projection neurons (i.e., D1 and D2 dopamine receptor-expressing SPNs). By contrast, when we prepared slices at the end of the acquisition period, in all mice, the A/N was high in D1R- but not D2R-SPNs. These results indicate that the transition to compulsion-like cocaine SA emerges during the punishment sessions, where synapses onto D2R-SPNs are strengthened. In renouncing individuals, the cocaine-evoked strengthening in D1R-SPNs is lost. Our study thus reveals the cell-type specific sequence of the induction of plasticity that eventually may cause compulsion-like SA.

Dopamine D2 receptors in nucleus accumbens cholinergic interneurons increase impulsive choice

Impulsive choice, often characterized by excessive preference for small, short-term rewards over larger, long-term rewards, is a prominent feature of substance use and other neuropsychiatric disorders. The neural mechanisms underlying impulsive choice are not well understood, but growing evidence implicates nucleus accumbens (NAc) dopamine and its actions on dopamine D2 receptors (D2Rs). Because several NAc cell types and afferents express D2Rs, it has been difficult to determine the specific neural mechanisms linking NAc D2Rs to impulsive choice. Of these cell types, cholinergic interneurons (CINs) of the NAc, which express D2Rs, have emerged as key regulators of striatal output and local dopamine release. Despite these relevant functions, whether D2Rs expressed specifically in these neurons contribute to impulsive choice behavior is unknown. Here, we show that D2R upregulation in CINs of the mouse NAc increases impulsive choice as measured in a delay discounting task without affecting reward magnitude sensitivity or interval timing. Conversely, mice lacking D2Rs in CINs showed decreased delay discounting. Furthermore, CIN D2R manipulations did not affect probabilistic discounting, which measures a different form of impulsive choice. Together, these findings suggest that CIN D2Rs regulate impulsive decision-making involving delay costs, providing new insight into the mechanisms by which NAc dopamine influences impulsive behavior.

Acute and chronic effects by nicotine on striatal neurotransmission and synaptic plasticity in the female rat brain

Introduction

Tobacco use is in part a gendered activity, yet neurobiological studies outlining the effect by nicotine on the female brain are scarce. The aim of this study was to outline acute and sub-chronic effects by nicotine on the female rat brain, with special emphasis on neurotransmission and synaptic plasticity in the dorsolateral striatum (DLS), a key brain region with respect to the formation of habits.

Methods

In vivo microdialysis and ex vivo electrophysiology were performed in nicotine naïve female Wistar rats, and following sub-chronic nicotine exposure (0.36 mg/kg free base, 15 injections). Locomotor behavior was assessed at the first and last drug-exposure.

Results

Acute exposure to nicotine ex vivo depresses excitatory neurotransmission by reducing the probability of transmitter release. Bath applied nicotine furthermore facilitated long-term synaptic depression induced by high frequency stimulation (HFS-LTD). The cannabinoid 1 receptor (CB1R) agonist WIN55,212-2 produced a robust synaptic depression of evoked potentials, and HFS-LTD was blocked by the CB1R antagonist AM251, suggesting that HFS-LTD in the female rat DLS is endocannabinoid mediated. Sub-chronic exposure to nicotine in vivo produced behavioral sensitization and electrophysiological recordings performed after 2-8 days abstinence revealed a sustained depression of evoked population spike amplitudes in the DLS, with no concomitant change in paired pulse ratio. Rats receiving sub-chronic nicotine exposure further demonstrated an increased neurophysiological responsiveness to nicotine with respect to both dopaminergic- and glutamatergic signaling. However, a tolerance towards the plasticity facilitating property of bath applied nicotine was developed during sub-chronic nicotine exposure in vivo. In addition, the dopamine D2 receptor agonist quinpirole selectively facilitate HFS-LTD in slices from nicotine naïve rats, suggesting that the tolerance may be associated with changes in dopaminergic signaling.

Conclusion

Nicotine produces acute and sustained effects on striatal neurotransmission and synaptic plasticity in the female rat brain, which may contribute to the establishment of persistent nicotine taking habits.

Loss of cannabinoid receptor 2 promotes α-Synuclein-induced microglial synaptic pruning in nucleus accumbens by modulating the pCREB-c-Fos signaling pathway and complement system

The disruption of nucleus accumbens (NAc) function impacts mood and learning behavior in α-Synucleinopathy, in which microglial synaptic pruning plays a pivotal role in modulating the neuropathologic progression. Available literature documents that in microglia, the activation of cannabinoid receptor 2 (CB2R) decreases inflammation, but it remains obscured regarding the roles of CB2R in microglia-mediated synaptic pruning in the NAc during the neuropathological progression of α-Synucleinopathy. We adopted the fibrillar α-Synuclein (α-Syn) treatment to characterize the effect of genetic CB2R deletion on microglial function and the signaling pathway. CB2R knockout (CB2^-/-) mice and wild-type (CB2^+/+) mice were divided into the α-Syn or saline treatment groups. Biochemical and microscopy approaches, including immunofluorescence, real-time PCR, and western blotting, were employed to assess the changes in homeostasis of synaptic pruning in NAc under the α-Syn-induced microglia. Moreover, the underlying mechanisms of CB2R on α-Syn induced microglial activity was assessed in vitro. After the injection of α-Syn into the NAc, distinct microglial morphological changes and M1 phenotype transformation were observed between CB2^-/- and CB2^+/+ mice. Meanwhile, after the α-Syn treatment, CB2^-/- mice showed an increased upregulation of CD68 protein and IL-1β mRNA but decreased brain-derived neurotrophic factor (BDNF) and TGF-β mRNA compared with CB2^+/+ mice. Additionally, CB2^-/- microglia after the treatment showed a highly enriched complement 3a receptor (C3aR) producing excessive pruning of cholinergic synapses but less engulfment of dopaminergic synapses. Mechanistically, the loss of CB2R function in the α-Syn stimulation triggered c-Fos activation in microglia, but not in neurons. Further inhibition of microglial CB2R functions under α-Syn stimulation activated the phosphorylated cAMP-response element-binding protein (pCREB)-c-Fos, which was closely related to the C3aR upregulation. Our results reveal a critical and mechanistic role of CB2R in altering the microglial function and its value in the homeostasis of synaptic circuits in the NAc under the α-Syn pathology.

Synaptic ensembles between raphe and D1R-containing accumbens shell neurons underlie postisolation sociability in males

Social animals expend considerable energy to maintain social bonds throughout their life. Male and female mice show sexually dimorphic behaviors, yet the underlying neural mechanisms of sociability and their dysregulation during social disconnection remain unknown. Dopaminergic neurons in dorsal raphe nucleus (DRN^TH) is known to contribute to a loneliness-like state and modulate sociability. We identified that activated subpopulations in DRN^TH and nucleus accumbens shell (NAc^sh) during 24 hours of social isolation underlie the increase in isolation-induced sociability in male but not in female mice. This effect was reversed by chemogenetically and optogenetically inhibiting the DRN^TH-NAc^sh circuit. Moreover, synaptic connectivity among the activated neuronal ensembles in this circuit was increased, primarily in D₁ receptor-expressing neurons in NAc^sh. The increase in synaptic density functionally correlated with elevated dopamine release into NAc^sh. Overall, specific synaptic ensembles in DRN^TH-NAc^sh mediate sex differences in isolation-induced sociability, indicating that sex-dependent circuit dynamics underlie the expression of sexually dimorphic behaviors.

A novel cholinergic projection from the lateral parabrachial nucleus and its role in methamphetamine-primed conditioned place preference

Drug relapse is a big clinical challenge in the treatment of addiction, but its neural circuit mechanism is far from being fully understood. Here, we identified a novel cholinergic pathway from choline acetyltransferase-positive neurons in the external lateral parabrachial nucleus (eLPB^ChAT) to the GABAergic neurons in the central nucleus of the amygdala (CeA^GABA) and explored its role in methamphetamine priming-induced reinstatement of conditioned place preference. The anatomical structure and functional innervation of the eLPB^ChAT–CeA^GABA pathway were investigated by various methods such as fluorescent micro-optical sectioning tomography, virus-based neural tracing, fibre photometry, patch-clamp and designer receptor exclusively activated by a designer drug. The role of the eLPB^ChAT–CeA^GABA pathway in methamphetamine relapse was assessed using methamphetamine priming-induced reinstatement of conditioned place preference behaviours in male mice. We found that the eLPB^ChAT neurons mainly projected to the central nucleus of the amygdala. A chemogenetic activation of the eLPB^ChAT neurons in vitro or in vivo triggered the excitabilities of the CeA^GABA neurons, which is at least in part mediated via the cholinergic receptor system. Most importantly, the chemogenetic activation of either the eLPB^ChAT neurons or the eLPB^ChAT neurons that project onto the central nucleus of the amygdala decreased the methamphetamine priming-induced reinstatement of conditioned place preference in mice. Our findings revealed a previously undiscovered cholinergic pathway of the eLPB^ChAT–CeA^GABA and showed that the activation of this pathway decreased the methamphetamine priming-induced reinstatement of conditioned place preference.

Biomarkers of Relapse in Cocaine Use Disorder: A Narrative Review

Introduction: Cocaine use disorder is a chronic disease with severe consequences and a high relapse rate. There is a critical need to explore the factors influencing relapse in order to achieve more efficient treatment outcomes. Furthermore, there is a great need for easy-to-measure, repeatable, and valid biomarkers that can predict treatment response or relapse. Methods: We reviewed the available literature on the Pubmed database concerning the biomarkers associated with relapse in CUD, including central nervous system-derived, genetic, immune, oxidative stress, and “other” biomarkers. Results: Fifty-one articles were included in our analysis. Twenty-five imaging brain anatomic and function assessment studies, mostly using fMRI, examined the role of several structures such as the striatum activity in abstinence prediction. There were fewer studies assessing the use of neuropsychological factors, neurotrophins, or genetic/genomic factors, immune system, or oxidative stress measures to predict abstinence. Conclusion: Several biomarkers have been shown to have predictive value. Prospective studies using combined multimodal assessments are now warranted.

Cholinergic interneurons mediate cocaine extinction in male mice through plasticity across medium spiny neuron subtypes

Cholinergic interneurons (ChINs) in the nucleus accumbens (NAc) have been implicated in the extinction of drug associations, as well as related plasticity in medium spiny neurons (MSNs). However, since most previous work relied on artificial manipulations, whether endogenous acetylcholine signaling relates to drug associations is unclear. Moreover, despite great interest in the opposing effects of dopamine on MSN subtypes, whether ChIN-mediated effects vary by MSN subtype is also unclear. Here, we find that high endogenous acetylcholine event frequency correlates with greater extinction of cocaine-context associations across male mice. Additionally, extinction is associated with a weakening of glutamatergic synapses across MSN subtypes. Manipulating ChIN activity bidirectionally controls both the rate of extinction and the associated plasticity at MSNs. Our findings indicate that NAc ChINs mediate drug-context extinction by reducing glutamatergic synaptic strength across MSN subtypes, and that natural variation in acetylcholine signaling may contribute to individual differences in extinction.

Fleming et al. show that individual differences in nucleus accumbens (NAc) acetylcholine signaling correlate with extinction of a cocaine-context association. Manipulations of NAc cholinergic interneuron activity support a model where acetylcholine release weakens glutamatergic presynaptic strength at NAc D1R and D2R medium spiny neurons, promoting cocaine-context extinction.

Nicotine and opioid co-dependence: Findings from bench research to clinical trials

Concomitant use of tobacco and opioids represents a growing public health concern. In fact, the mortality rate due to smoking-related illness approaches 50% among SUD patients. Cumulative evidence demonstrates that the vulnerability to drugs of abuse is influenced by behavioral, environmental, and genetic factors. This review explores the contribution of genetics and neural mechanisms influencing nicotine and opioid reward, respiration, and antinociception, emphasizing the interaction of cholinergic and opioid receptor systems. Despite the substantial evidence demonstrating nicotine-opioid interactions within the brain and on behavior, the currently available pharmacotherapies targeting these systems have shown limited efficacy for smoking cessation on opioid-maintained smokers. Thus, further studies designed to identify novel targets modulating both nicotinic and opioid receptor systems may lead to more efficacious approaches for co-morbid nicotine dependence and opioid use disorder.

Resilience to the effects of social stress on vulnerability to developing drug addiction

We review the still scarce but growing literature on resilience to the effects of social stress on the rewarding properties of drugs of abuse. We define the concept of resilience and how it is applied to the field of drug addiction research. We also describe the internal and external protective factors associated with resilience, such as individual behavioral traits and social support. We then explain the physiological response to stress and how it is modulated by resilience factors. In the subsequent section, we describe the animal models commonly used in the study of resilience to social stress, and we focus on the effects of chronic social defeat (SD), a kind of stress induced by repeated experience of defeat in an agonistic encounter, on different animal behaviors (depression- and anxiety-like behavior, cognitive impairment and addiction-like symptoms). We then summarize the current knowledge on the neurobiological substrates of resilience derived from studies of resilience to the effects of chronic SD stress on depression- and anxiety-related behaviors in rodents. Finally, we focus on the limited studies carried out to explore resilience to the effects of SD stress on the rewarding properties of drugs of abuse, describing the current state of knowledge and suggesting future research directions.

Key transcription factors mediating cocaine-induced plasticity in the nucleus accumbens

Repeated cocaine use induces coordinated changes in gene expression that drive plasticity in the nucleus accumbens (NAc), an important component of the brain's reward circuitry, and promote the development of maladaptive, addiction-like behaviors. Studies on the molecular basis of cocaine action identify transcription factors, a class of proteins that bind to specific DNA sequences and regulate transcription, as critical mediators of this cocaine-induced plasticity. Early methods to identify and study transcription factors involved in addiction pathophysiology primarily relied on quantifying the expression of candidate genes in bulk brain tissue after chronic cocaine treatment, as well as conventional overexpression and knockdown techniques. More recently, advances in next generation sequencing, bioinformatics, cell-type-specific targeting, and locus-specific neuroepigenomic editing offer a more powerful, unbiased toolbox to identify the most important transcription factors that drive drug-induced plasticity and to causally define their downstream molecular mechanisms. Here, we synthesize the literature on transcription factors mediating cocaine action in the NAc, discuss the advancements and remaining limitations of current experimental approaches, and emphasize recent work leveraging bioinformatic tools and neuroepigenomic editing to study transcription factors involved in cocaine addiction.

Cocaine shifts dopamine D2 receptor sensitivity to gate conditioned behaviors

Cocaine addiction is a chronic, relapsing disorder characterized by maladaptation in the brain mesolimbic and nigrostriatal dopamine system. Although changes in the properties of D2-receptor-expressing medium spiny neurons (D2-MSNs) and connected striatal circuits following cocaine treatment are known, the contributions of altered D2-receptor (D2R) function in mediating the rewarding properties of cocaine remain unclear. Here, we describe how a 7-day exposure to cocaine alters dopamine signaling by selectively reducing the sensitivity, but not the expression, of nucleus accumbens D2-MSN D2Rs via an alteration in the relative expression and coupling of G protein subunits. This cocaine-induced reduction of D2R sensitivity facilitated the development of the rewarding effects of cocaine as blocking the reduction in G protein expression was sufficient to prevent cocaine-induced behavioral adaptations. These findings identify an initial maladaptive change in sensitivity by which mesolimbic dopamine signals are encoded by D2Rs following cocaine exposure.

Addiction-Associated Genetic Variants Implicate Brain Cell Type- and Region-Specific Cis-Regulatory Elements in Addiction Neurobiology

Recent large genome-wide association studies have identified multiple confident risk loci linked to addiction-associated behavioral traits. Most genetic variants linked to addiction-associated traits lie in noncoding regions of the genome, likely disrupting cis-regulatory element (CRE) function. CREs tend to be highly cell type-specific and may contribute to the functional development of the neural circuits underlying addiction. Yet, a systematic approach for predicting the impact of risk variants on the CREs of specific cell populations is lacking. To dissect the cell types and brain regions underlying addiction-associated traits, we applied stratified linkage disequilibrium score regression to compare genome-wide association studies to genomic regions collected from human and mouse assays for open chromatin, which is associated with CRE activity. We found enrichment of addiction-associated variants in putative CREs marked by open chromatin in neuronal (NeuN+) nuclei collected from multiple prefrontal cortical areas and striatal regions known to play major roles in reward and addiction. To further dissect the cell type-specific basis of addiction-associated traits, we also identified enrichments in human orthologs of open chromatin regions of female and male mouse neuronal subtypes: cortical excitatory, D1, D2, and PV. Last, we developed machine learning models to predict mouse cell type-specific open chromatin, enabling us to further categorize human NeuN+ open chromatin regions into cortical excitatory or striatal D1 and D2 neurons and predict the functional impact of addiction-associated genetic variants. Our results suggest that different neuronal subtypes within the reward system play distinct roles in the variety of traits that contribute to addiction.SIGNIFICANCE STATEMENT We combine statistical genetic and machine learning techniques to find that the predisposition to for nicotine, alcohol, and cannabis use behaviors can be partially explained by genetic variants in conserved regulatory elements within specific brain regions and neuronal subtypes of the reward system. Our computational framework can flexibly integrate open chromatin data across species to screen for putative causal variants in a cell type- and tissue-specific manner for numerous complex traits.

Single Dose of Amphetamine Induces Delayed Subregional Attenuation of Cholinergic Interneuron Activity in the Striatum

Psychostimulants such as amphetamine (AMPH) target dopamine (DA) neuron synapses to engender drug-induced plasticity. While DA neurons modulate the activity of striatal (Str) cholinergic interneurons (ChIs) with regional heterogeneity, how AMPH affects ChI activity has not been elucidated. Here, we applied quantitative fluorescence imaging approaches to map the dose-dependent effects of a single dose of AMPH on ChI activity at 2.5 and 24 h after injection across the mouse Str using the activity-dependent marker phosphorylated ribosomal protein S6 (p-rpS6240/244). AMPH did not affect the distribution or morphology of ChIs in any Str subregion. While AMPH at either dose had no effect on ChI activity after 2.5 h, ChI activity was dose dependently reduced after 24 h specifically in the ventral Str/nucleus accumbens (NAc), a critical site of psychostimulant action. AMPH at either dose did not affect the spontaneous firing of ChIs. Altogether this work demonstrates that a single dose of AMPH has delayed regionally heterogeneous effects on ChI activity, which most likely involves extra-Str synaptic input.

Consolidating the Circuit Model for Addiction

Addiction is a disease characterized by compulsive drug seeking and consumption observed in 20-30% of users. An addicted individual will favor drug reward over natural rewards, despite major negative consequences. Mechanistic research on rodents modeling core components of the disease has identified altered synaptic transmission as the functional substrate of pathological behavior. While the initial version of a circuit model for addiction focused on early drug adaptive behaviors observed in all individuals, it fell short of accounting for the stochastic nature of the transition to compulsion. The model builds on the initial pharmacological effect common to all addictive drugs-an increase in dopamine levels in the mesolimbic system. Here, we consolidate this early model by integrating circuits underlying compulsion and negative reinforcement. We discuss the genetic and epigenetic correlates of individual vulnerability. Many recent data converge on a gain-of-function explanation for circuit remodeling, revealing blueprints for novel addiction therapies.