Role of Ventral Subiculum in Context-Induced Relapse to Alcohol Seeking after Punishment-Imposed Abstinence

Involvement of Pre-limbic Cortex-Nucleus accumbens projections in Context-Induced alcohol seeking

Alcohol use disorder (AUD) remains a critical public health issue worldwide, characterized by high relapse rates often triggered by contextual cues. This research investigates the neural mechanisms behind context-induced reinstatement of alcohol-seeking behavior, focusing on the nucleus accumbens and its interactions with the prelimbic cortex, employing Male Long-Evans rats in an ABA renewal model. In our experimental setup, rats were trained to self-administer 10 % ethanol in Context A, followed by extinction of lever pressing in the presence of discrete cues in Context B. The context-induced reinstatement of ethanol-seeking was then assessed by re-exposing rats to Context A or B under extinction conditions, aiming to simulate the environmental cues' influence on relapse behaviors. Three experiments were conducted: Experiment 1 utilized Fos-immunohistochemistry to examine neuronal activation in the nucleus accumbens; Experiment 2 applied the baclofen + muscimol inactivation technique to probe the functional importance of the nucleus accumbens core; Experiment 3 used Fos-immunofluorescence along with Retrobeads injection to investigate activation of neurons projecting from the prelimbic cortex to the nucleus accumbens core. Our findings revealed significant increases in Fos-immunoreactive nuclei within the nucleus accumbens core and shell during the reinstatement phase in Context A, underscoring the environment's potent effect on ethanol-seeking behavior. Additionally, inactivation of the nucleus accumbens core markedly reduced reinstatement, and there was a notable activation of neurons from the prelimbic cortex to the nucleus accumbens core in the ethanol-associated context. These results highlight the critical role of the nucleus accumbens core and its corticostriatal projections in the neural circuitry underlying context-driven ethanol seeking.

Examining a punishment-related brain circuit with miniature fluorescence microscopes and deep learning

In humans experiencing substance use disorder (SUD), abstinence from drug use is often motivated by a desire to avoid some undesirable consequence of further use: health effects, legal ramifications, etc. This process can be experimentally modeled in rodents by training and subsequently punishing an operant response in a context-induced reinstatement procedure. Understanding the biobehavioral mechanisms underlying punishment learning is critical to understanding both abstinence and relapse in individuals with SUD. To date, most investigations into the neural mechanisms of context-induced reinstatement following punishment have utilized discrete loss-of-function manipulations that do not capture ongoing changes in neural circuitry related to punishment-induced behavior change. Here, we describe a two-pronged approach to analyzing the biobehavioral mechanisms of punishment learning using miniature fluorescence microscopes and deep learning algorithms. We review recent advancements in both techniques and consider a target neural circuit.

Understanding sex differences and the translational value of models of persistent substance use despite negative consequences

Persistent substance use despite negative consequences is a key facet of substance use disorder. The last decade has seen the preclinical field adopt the use of punishment to model adverse consequences associated with substance use. This has largely involved the pairing of drug use with either electric foot shock or quinine, a bitter tastant. Whilst at face value, these punishers may model aspects of the physical and psychological consequences of substance use, such models are yet to assist the development of approved medications for treatment. This review discusses progress made with animal models of punishment to understand the behavioral consequences of persistent substance use despite negative consequences. We highlight the importance of examining sex differences, especially when the behavioral response to punishment changes following drug exposure. Finally, we critique the translational value these models provide for the substance use disorder field.

Augmenting glutamatergic, but not dopaminergic, activity in the nucleus accumbens shell disrupts responding to a discrete alcohol cue in an alcohol context

Discrete alcohol cues and contexts are relapse triggers for people with alcohol use disorder exerting particularly powerful control over behaviour when they co-occur. Here, we investigated the neural substrates subserving the capacity for alcohol-associated contexts to elevate responding to an alcohol-predictive conditioned stimulus (CS). Specifically, rats were trained in a distinct 'alcohol context' to respond by entering a fluid port during a discrete auditory CS that predicted the delivery of alcohol and were familiarized with a 'neutral context' wherein alcohol was never available. When conditioned CS responding was tested by presenting the CS without alcohol, we found that augmenting glutamatergic activity in the nucleus accumbens (NAc) shell by microinfusing α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) reduced responding to an alcohol CS in an alcohol, but not neutral, context. Further, AMPA microinfusion robustly affected behaviour, attenuating the number, duration and latency of CS responses selectively in the alcohol context. Although dopaminergic inputs to the NAc shell were previously shown to be necessary for CS responding in an alcohol context, here, chemogenetic excitation of ventral tegmental area (VTA) dopamine neurons and their inputs to the NAc shell did not affect CS responding. Critically, chemogenetic excitation of VTA dopamine neurons affected feeding behaviour and elevated c-fos immunoreactivity in the VTA and NAc shell, validating the chemogenetic approach. These findings enrich our understanding of the substrates underlying Pavlovian responding for alcohol and reveal that the capacity for contexts to modulate responding to discrete alcohol cues is delicately underpinned by the NAc shell.

Chemogenetic inhibition of the ventral hippocampus but not its direct projection to the prelimbic cortex attenuates context-specific operant responding

Previous work has demonstrated the importance of the prelimbic cortex (PL) in contextual control of operant behavior. However, the associated neural circuitry responsible for providing contextual information to the PL is not well understood. In Pavlovian fear conditioning the ventral hippocampus (vH) and its projection to the PL have been shown to be important in supporting the effects of context on learning. The present experiments used chemogenetic inhibition of the direct vH-PL projection or the vH to determine involvement in expression of context-specific operant behavior. Rats were injected with an inhibitory DREADD (hM4Di) or mCherry-only into the vH, and subsequently trained to perform a lever press response for a food pellet in a distinct context. The DREADD ligand clozapine-n-oxide (CNO) was then delivered directly into the PL (experiment 1) and then systemically (experiment 2) prior to tests of the response in the training context as well as an equally familiar but untrained context. vH (systemic CNO) but not vH-PL (intra-PL CNO) inhibition was found to attenuate operant responding in its acquisition context. A third experiment, using the same rats, showed that chemogenetic inhibition of vH also reduced Pavlovian contextual fear. The present results suggest that multisynapatic connections between the vH and PL may be responsible for integration of contextual information with operant behavior.

Antagonists of the stress and opioid systems restore the functional connectivity of the prefrontal cortex during alcohol withdrawal through divergent mechanisms

Chronic alcohol consumption leads to dependence and withdrawal symptoms upon cessation, contributing to persistent use. However, the brain network mechanisms by which the brain orchestrates alcohol withdrawal and how these networks are affected by pharmacological treatments remain elusive. Recent work revealed that alcohol withdrawal produces a widespread increase in coordinated brain activity and a decrease in modularity of the whole-brain functional network using single-cell whole-brain imaging of immediate early genes. This decreased modularity and functional hyperconnectivity are hypothesized to be novel biomarkers of alcohol withdrawal in alcohol dependence, which could potentially be used to evaluate the efficacy of new medications for alcohol use disorder. However, there is no evidence that current FDA-approved medications or experimental treatments known to reduce alcohol drinking in animal models can normalize the changes in whole-brain functional connectivity. In this report, we tested the effect of R121919, a CRF1 antagonist, and naltrexone, an FDA-approved treatment for alcohol use disorder, on whole-brain functional connectivity using the cellular marker FOS combined with graph theory and advanced network analyses. Results show that both R121919 and naltrexone restored the functional connectivity of the prefrontal cortex during alcohol withdrawal, but through divergent mechanisms. Specifically, R121919 increased FOS activation in the prefrontal cortex, partially restored modularity, and normalized connectivity, particularly in CRF1-rich regions, including the prefrontal, pallidum, and extended amygdala circuits. On the other hand, naltrexone decreased FOS activation throughout the brain, decreased modularity, and increased connectivity overall except for the Mu opioid receptor-rich regions, including the thalamus. These results identify the brain networks underlying the pharmacological effects of R121919 and naltrexone and demonstrate that these drugs restored different aspects of functional connectivity of the prefrontal cortex, pallidum, amygdala, and thalamus during alcohol withdrawal. Notably, these effects were particularly prominent in CRF1- and Mu opioid receptors-rich regions highlighting the potential of whole-brain functional connectivity using FOS as a tool for identifying neuronal network mechanisms underlying the pharmacological effects of existing and new medications for alcohol use disorder.

Modeling methamphetamine use disorder and relapse in animals: short- and long-term epigenetic, transcriptional., and biochemical consequences in the rat brain

Methamphetamine use disorder (MUD) is a neuropsychiatric disorder characterized by binge drug taking episodes, intervals of abstinence, and relapses to drug use even during treatment. MUD has been modeled in rodents and investigators are attempting to identify its molecular bases. Preclinical experiments have shown that different schedules of methamphetamine self-administration can cause diverse transcriptional changes in the dorsal striatum of Sprague-Dawley rats. In the present review, we present data on differentially expressed genes (DEGs) identified in the rat striatum following methamphetamine intake. These include genes involved in transcription regulation, potassium channel function, and neuroinflammation. We then use the striatal data to discuss the potential significance of the molecular changes induced by methamphetamine by reviewing concordant or discordant data from the literature. This review identified potential molecular targets for pharmacological interventions. Nevertheless, there is a need for more research on methamphetamine-induced transcriptional consequences in various brain regions. These data should provide a more detailed neuroanatomical map of methamphetamine-induced changes and should better inform therapeutic interventions against MUD.

Molecular fingerprints in the hippocampus of alcohol seeking during withdrawal

Alcohol use disorder (AUD) is characterized by pathological motivation to consume alcohol and cognitive inflexibility, leading to excessive alcohol seeking and use. Due to limited understanding of the molecular basis of the disease, there are few pharmacological interventions available to combat AUD. In this study, we aimed to investigate the molecular correlates of impaired extinction of alcohol seeking during alcohol withdrawal using a mouse model of AUD implemented in the automated IntelliCage social system. This model enabled us to distinguish between animals exhibiting AUD-prone and AUD-resistant phenotypes, based on the presence of ≥ 2 or < 2 criteria of AUD, respectively. We utilized new generation RNA sequencing to identify genes that were differentially expressed in the hippocampus and amygdala of mice meeting ≥ 2 or < 2 criteria, as these brain regions are implicated in alcohol motivation, seeking, consumption and the cognitive inflexibility characteristic of AUD. To complement the sequencing studies, we conducted ex vivo electrophysiology experiments. Our findings revealed significant dysregulation of the hippocampal genes associated with the actin cytoskeleton and synaptic function, including actin binding molecule cofilin, during alcohol withdrawal in mice meeting ≥ 2 criteria compared to those meeting < 2 criteria. Moreover, this dysregulation was accompanied by impaired synaptic transmission in the molecular layer of the hippocampal dentate gyrus (ML-DG). Additionally, we demonstrated that overexpression of cofilin in the polymorphic layer of the hippocampal dentate gyrus (PoDG) inhibited ML-DG synapses, increased motivation to seek alcohol, impaired extinction of alcohol seeking and increased correlation between AUD behaviors, resembling the phenotype observed in mice meeting ≥ 2 criteria. Overall, our study uncovers a novel mechanism linking increased hippocampal cofilin expression with the AUD phenotype.

Blocking μ-opioid receptors attenuates reinstatement of responding to an alcohol-predictive conditioned stimulus through actions in the ventral hippocampus

The µ-opioid system is involved in the reinstatement of responding that is immediately evoked by alcohol-predictive cues. The extent of its involvement in reinstatement observed in a new model that evaluates the delayed effects of re-exposure to alcohol, however, is unclear. The current study investigated the role of µ-opioid receptors (MORs) in the delayed reinstatement of an extinguished, Pavlovian conditioned response that was evoked 24 h after alcohol re-exposure. Female and male Long-Evans rats received Pavlovian conditioning in which a conditioned stimulus (CS) was paired with the delivery of an appetitive unconditioned stimulus (US; Experiments 1, 2, 4: 15% v/v alcohol; Experiment 3: 10% w/v sucrose) that was delivered into a fluid port for oral intake. During subsequent extinction sessions, the CS was presented as before but without the US. Next, the US was delivered but without the CS. A reinstatement test was conducted 24 h later, during which the CS was presented in the absence of the US. Silencing MORs via systemic naltrexone (0.3 or 1.0 mg/kg) attenuated reinstatement of port entries elicited by an alcohol-CS, but not those elicited by a sucrose-CS. Finally, blocking MORs in the ventral hippocampus via bilateral microinfusion of D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP; 2.5 or 5.0 µg/hemisphere) prevented reinstatement of port alcohol-CS port entries. These data show that MORs are involved in the delayed reinstatement of a Pavlovian conditioned response in an alcohol-specific manner. Importantly, these data illustrate, for the first time, that MORs in the ventral hippocampus are necessary for responding to an alcohol-predictive cue.

Molecular fingerprints in the hippocampus of alcohol seeking during withdrawal

Alcohol use disorder (AUD) is characterized by excessive alcohol seeking and use. Here, we investigated the molecular correlates of impaired extinction of alcohol seeking using a multidimentional mouse model of AUD. We distinguished AUD-prone and AUD-resistant mice, based on the presence of ≥ 2 or < 2 criteria of AUD and utilized RNA sequencing to identify genes that were differentially expressed in the hippocampus and amygdala of mice meeting ≥ 2 or < 2 criteria, as these brain regions are implicated in alcohol motivation, seeking, consumption and the cognitive inflexibility characteristic of AUD. Our findings revealed dysregulation of the genes associated with the actin cytoskeleton, including actin binding molecule cofilin, and impaired synaptic transmission in the hippocampi of mice meeting ≥ 2 criteria. Overexpression of cofilin in the polymorphic layer of the dentate gyrus (PoDG) inhibited ML-DG synapses, increased motivation to seek alcohol and impaired extinction of alcohol seeking, resembling the phenotype observed in mice meeting ≥ 2 criteria. Overall, our study uncovers a novel mechanism linking increased hippocampal cofilin expression with the AUD phenotype.

Role of Piriform Cortex and Its Afferent Projections in Relapse to Fentanyl Seeking after Food Choice-Induced Voluntary Abstinence

We previously demonstrated a role of piriform cortex (Pir) in relapse to fentanyl seeking after food choice-induced voluntary abstinence. Here, we used this model to further study the role of Pir and its afferent projections in fentanyl relapse. We trained male and female rats to self-administer palatable food pellets for 6 d (6 h/day) and fentanyl (2.5 µg/kg/infusion, i.v.) for 12 d (6 h/day). We assessed relapse to fentanyl seeking after 12 voluntary abstinence sessions, achieved through a discrete choice procedure between fentanyl and palatable food (20 trials/session). We determined projection-specific activation of Pir afferents during fentanyl relapse with Fos plus the retrograde tracer cholera toxin B (injected into Pir). Fentanyl relapse was associated with increased Fos expression in anterior insular cortex (AI) and prelimbic cortex (PL) neurons projecting to Pir. We next used an anatomical disconnection procedure to determine the causal role of these two projections (AI→Pir and PL→Pir) in fentanyl relapse. Contralateral but not ipsilateral disconnection of AI→Pir projections decreased fentanyl relapse but not reacquisition of fentanyl self-administration. In contrast, contralateral but not ipsilateral disconnection of PL→Pir projections modestly decreased reacquisition but not relapse. Fluorescence-activated cell sorting and quantitative PCR data showed molecular changes within Pir Fos-expressing neurons associated with fentanyl relapse. Finally, we found minimal or no sex differences in fentanyl self-administration, fentanyl versus food choice, and fentanyl relapse. Our results indicate that AI→Pir and PL→Pir projections play dissociable roles in nonreinforced relapse to fentanyl seeking versus reacquisition of fentanyl self-administration after food choice-induced voluntary abstinence.SIGNIFICANCE STATEMENT We previously showed a role of Pir in fentanyl relapse after food choice-induced voluntary abstinence in rats, a procedure mimicking human abstinence or a significant reduction in drug self-administration because of the availability of alternative nondrug rewards. Here, we aimed to further characterize the role of Pir in fentanyl relapse by investigating the role of Pir afferent projections and analyzing molecular changes in relapse-activated Pir neurons. We identified dissociable roles of two Pir afferent projections (AI→Pir and PL→Pir) in relapse to fentanyl seeking versus reacquisition of fentanyl self-administration after voluntary abstinence. We also characterized molecular changes within Pir Fos-expressing neurons associated with fentanyl relapse.

Molecular Profiling of VGluT1 AND VGluT2 Ventral Subiculum to Nucleus Accumbens Shell Projections

The nucleus accumbens shell is a critical node in reward circuitry, encoding environments associated with reward. Long-range inputs from the ventral hippocampus (ventral subiculum) to the nucleus accumbens shell have been identified, yet their precise molecular phenotype remains to be determined. Here we used retrograde tracing to identify the ventral subiculum as the brain region with the densest glutamatergic (VGluT1-Slc17a7) input to the shell. We then used circuit-directed translating ribosome affinity purification to examine the molecular characteristics of distinct glutamatergic (VGluT1, VGluT2-Slc17a6) ventral subiculum to nucleus accumbens shell projections. We immunoprecipitated translating ribosomes from this population of projection neurons and analysed molecular connectomic information using RNA sequencing. We found differential gene enrichment across both glutamatergic projection neuron subtypes. In VGluT1 projections, we found enrichment of Pfkl, a gene involved in glucose metabolism. In VGluT2 projections, we found a depletion of Sparcl1 and Dlg1, genes known to play a role in depression- and addiction-related behaviours. These findings highlight potential glutamatergic neuronal-projection-specific differences in ventral subiculum to nucleus accumbens shell projections. Together these data advance our understanding of the phenotype of a defined brain circuit.

Effect of Selective Lesions of Nucleus Accumbens µ-Opioid Receptor-Expressing Cells on Heroin Self-Administration in Male and Female Rats: A Study with Novel Oprm1-Cre Knock-in Rats

The brain µ-opioid receptor (MOR) is critical for the analgesic, rewarding, and addictive effects of opioid drugs. However, in rat models of opioid-related behaviors, the circuit mechanisms of MOR-expressing cells are less known because of a lack of genetic tools to selectively manipulate them. We introduce a CRISPR-based Oprm1-Cre knock-in transgenic rat that provides cell type-specific genetic access to MOR-expressing cells. After performing anatomic and behavioral validation experiments, we used the Oprm1-Cre knock-in rats to study the involvement of NAc MOR-expressing cells in heroin self-administration in male and female rats. Using RNAscope, autoradiography, and FISH chain reaction (HCR-FISH), we found no differences in Oprm1 expression in NAc, dorsal striatum, and dorsal hippocampus, or MOR receptor density (except dorsal striatum) or function between Oprm1-Cre knock-in rats and wildtype littermates. HCR-FISH assay showed that iCre is highly coexpressed with Oprm1 (95%-98%). There were no genotype differences in pain responses, morphine analgesia and tolerance, heroin self-administration, and relapse-related behaviors. We used the Cre-dependent vector AAV1-EF1a-Flex-taCasp3-TEVP to lesion NAc MOR-expressing cells. We found that the lesions decreased acquisition of heroin self-administration in male Oprm1-Cre rats and had a stronger inhibitory effect on the effort to self-administer heroin in female Oprm1-Cre rats. The validation of an Oprm1-Cre knock-in rat enables new strategies for understanding the role of MOR-expressing cells in rat models of opioid addiction, pain-related behaviors, and other opioid-mediated functions. Our initial mechanistic study indicates that lesioning NAc MOR-expressing cells had different effects on heroin self-administration in male and female rats.SIGNIFICANCE STATEMENT The brain µ-opioid receptor (MOR) is critical for the analgesic, rewarding, and addictive effects of opioid drugs. However, in rat models of opioid-related behaviors, the circuit mechanisms of MOR-expressing cells are less known because of a lack of genetic tools to selectively manipulate them. We introduce a CRISPR-based Oprm1-Cre knock-in transgenic rat that provides cell type-specific genetic access to brain MOR-expressing cells. After performing anatomical and behavioral validation experiments, we used the Oprm1-Cre knock-in rats to show that lesioning NAc MOR-expressing cells had different effects on heroin self-administration in males and females. The new Oprm1-Cre rats can be used to study the role of brain MOR-expressing cells in animal models of opioid addiction, pain-related behaviors, and other opioid-mediated functions.

Acute food deprivation-induced relapse to heroin seeking after short and long punishment-imposed abstinence in male rats

RATIONAL

Stress is a major trigger for drug relapse in humans and animal models, even after prolonged abstinence. However, animal models for stress-induced relapse were criticized for the lack of predictive and face validity.

OBJECTIVES

Here we investigated the effect of acute food deprivation stress in a novel stress-induced relapse model using voluntary, punishment-imposed abstinence from heroin. We also performed a detailed characterization of the development of punishment-imposed abstinence.

METHODS

Male rats were trained to self-administered heroin (0.1 mg/kg/infusion) for 2 weeks, using the seeking-taking chained schedule. Pressing the 'seeking' lever led to the insertion of the 'taking' lever and pressing the take lever resulted in heroin infusion. Following self-administration training, rats were exposed to 8 or 21 days of heroin-seeking punishment. During punishment, 30% of the completed seek links resulted in a mild escalating footshock instead of take lever presentation. Next, rats were tested for heroin seeking under extinction conditions after 24 h of food deprivation and sated conditions.

RESULTS

Probabilistic punishment of seeking lever responses resulted in gradual suppression of heroin seeking and taking. Exposure to food-deprivation stress induced a robust relapse to heroin seeking after short and long punishment-imposed abstinence periods, without significant effects of time, i.e., no incubation of heroin seeking. Individual differences were observed in the development of punishment-induced abstinence and stress-induced relapse.

CONCLUSIONS

These results suggest that stress is a reliable trigger to relapse even after a prolonged period of punishment-induced, voluntary abstinence.

Learning processes in relapse to alcohol use: lessons from animal models

RATIONALE

Alcohol use is reliably preceded by discrete and contextual stimuli which, through diverse learning processes, acquire the capacity to promote alcohol use and relapse to alcohol use.

OBJECTIVE

We review contemporary extinction, renewal, reinstatement, occasion setting, and sex differences research within a conditioning framework of relapse to alcohol use to inform the development of behavioural and pharmacological therapies.

KEY FINDINGS

Diverse learning processes and corresponding neurobiological substrates contribute to relapse to alcohol use. Results from animal models indicate that cortical, thalamic, accumbal, hypothalamic, mesolimbic, glutamatergic, opioidergic, and dopaminergic circuitries contribute to alcohol relapse through separable learning processes. Behavioural therapies could be improved by increasing the endurance and generalizability of extinction learning and should incorporate whether discrete cues and contexts influence behaviour through direct excitatory conditioning or occasion setting mechanisms. The types of learning processes that most effectively influence responding for alcohol differ in female and male rats.

CONCLUSION

Sophisticated conditioning experiments suggest that diverse learning processes are mediated by distinct neural circuits and contribute to relapse to alcohol use. These experiments also suggest that gender-specific behavioural and pharmacological interventions are a way towards efficacious therapies to prevent relapse to alcohol use.

Ventral Subiculum Inputs to Nucleus Accumbens Medial Shell Preferentially Innervate D2R Medium Spiny Neurons and Contain Calcium Permeable AMPARs

Ventral subiculum (vSUB) is the major output region of ventral hippocampus (vHIPP) and sends major projections to nucleus accumbens medial shell (NAcMS). Hyperactivity of the vSUB-NAcMS circuit is associated with substance use disorders and the modulation of vSUB activity alters drug seeking and drug reinstatement behavior in rodents. However, to the best of our knowledge, the cell type-specific connectivity and synaptic transmission properties of the vSUB-NAcMS circuit have never been directly examined. Instead, previous functional studies have focused on total ventral hippocampal (vHIPP) output to NAcMS without distinguishing vSUB from other subregions of vHIPP, including ventral CA1 (vCA1). Using ex vivo electrophysiology, we systematically characterized the vSUB-NAcMS circuit with cell type- and synapse-specific resolution in male and female mice and found that vSUB output to dopamine receptor type-1 (D1R) and type-2 (D2R) expressing medium spiny neurons (MSNs) displays a functional connectivity bias for D2R MSNs. Furthermore, we found that vSUB-D1R and vSUB-D2R MSN synapses contain calcium-permeable AMPA receptors in drug-naive mice. Finally, we find that, distinct from other glutamatergic inputs, cocaine exposure selectively induces plasticity at vSUB-D2R synapses. Importantly, we directly compared vSUB and vCA1 output to NAcMS and found that vSUB synapses are functionally distinct and that vCA1 output recapitulated the synaptic properties previously ascribed to vHIPP. Our work highlights the need to consider the contributions of individual subregions of vHIPP to substance use disorders and represents an important first step toward understanding how the vSUB-NAcMS circuit contributes to the etiologies that underlie substance use disorders.SIGNIFICANCE STATEMENT Inputs to nucleus accumbens (NAc) dopamine receptor type 1 (D1R) and D2R medium spiny neurons (MSNs) are critically involved in reward seeking behavior. Ventral subiculum (vSUB) provides robust synaptic input to nucleus accumbens medial shell (NAcMS) and activity of this circuit is linked to substance use disorders. Despite the importance of the vSUB to nucleus accumbens circuit, the functional connectivity and synaptic transmission properties have not been tested. Here, we systematically interrogated these properties and found that basal connectivity and drug-induced plasticity are biased for D2R medium spiny neurons. Overall, we demonstrate that this circuit is distinct from synaptic inputs from other brain regions, which helps to explain how vSUB dysfunction contributes to the etiologies that underlie substance use disorders.

The BDNF Val68Met polymorphism causes a sex specific alcohol preference over social interaction and also acute tolerance to the anxiolytic effects of alcohol, a phenotype driven by malfunction of BDNF in the ventral hippocampus of male mice

BACKGROUND

The brain-derived neurotrophic factor (BDNF) Valine 66 to Methionine human polymorphism results in impaired activity-dependent BDNF release and has been linked to psychiatric disorders including depression and anxiety. We previously showed that male knock-in mice carrying the mouse Methionine homolog (Met68BDNF) exhibit excessive and compulsive alcohol drinking behaviors as compared to the wild-type Val68BDNF mice.

OBJECTIVE

Here, we set out to determine the potential mechanism for the heightened and compulsive alcohol drinking phenotypes detected in Met68BDNF mice.

RESULTS

We found that male, but not female Met68BDNF mice exhibit social anxiety-like behaviors. We further show that male Met68BDNF mice exhibit a preference for alcohol over social interaction. In contrast, alcohol place preference without an alternative social reward, is similar in male Met68BDNF and Val68BDNF mice. Since the Met68BDNF mice show social anxiety phenotypes, we tested whether alcohol reliefs anxiety similarly in Met68BDNF and Val68BDNF mice and found that male, but not female Met68BDNF mice are insensitive to the acute anxiolytic action of alcohol. Finally, we show that this acute tolerance to alcohol-dependent anxiolysis can be restored by overexpressing wild-type Val68BDNF in the ventral hippocampus (vHC) of Met68BDNF mice.

CONCLUSIONS

Together, our results suggest that excessive alcohol drinking in the Met68BDNF may be attributed, in part, to heighted social anxiety and a lack of alcohol-dependent anxiolysis, a phenotype that is associated with malfunction of BDNF signaling in the vHC of male Met68BDNF mice.

Alcohol dependence and the ventral hippocampal influence on alcohol drinking in male mice

Examining neural circuits underlying persistent, heavy drinking provides insight into the neurobiological mechanisms driving alcohol use disorder. Facilitated by its connectivity with other parts of the brain such as the nucleus accumbens (NAc), the ventral hippocampus (vHC) supports many behaviors, including those related to reward seeking and addiction. These studies used a well-established mouse model of alcohol (ethanol) dependence. After surgery to infuse DREADD-expressing viruses (hM4Di, hM3Dq, or mCherry-only) into the vHC and position guide cannula above the NAc, male C57BL/6J mice were treated in the CIE drinking model that involved repeated cycles of chronic intermittent alcohol (CIE) vapor or air (CTL) exposure alternating with weekly test drinking cycles in which mice were offered alcohol (15% v/v) 2 h/day. Additionally, smaller groups of mice were evaluated for either cFos expression or glutamate release using microdialysis procedures. In CIE mice expressing inhibitory (hM4Di) DREADDs in the vHC, drinking increased as expected, but CNO (3 mg/kg intraperitoneally [i.p.]) given 30 min before testing did not alter alcohol intake. However, in CTL mice expressing hM4Di, CNO significantly increased alcohol drinking (∼30%; p < 0.05) to levels similar to the CIE mice. The vHC-NAc pathway was targeted by infusing CNO into the NAc (3 or 10 μM/side) 30 min before testing. CNO activation of the pathway in mice expressing excitatory (hM3Dq) DREADDs selectively reduced consumption in CIE mice back to CTL levels (∼35-45%; p < 0.05) without affecting CTL alcohol intake. Lastly, activating the vHC-NAc pathway increased cFos expression and evoked significant glutamate release from the vHC terminals in the NAc. These data indicate that reduced activity of the vHC increases alcohol consumption and that targeted, increased activity of the vHC-NAc pathway attenuates excessive drinking associated with alcohol dependence. Thus, these findings indicate that the vHC and its glutamatergic projections to the NAc are involved in excessive alcohol drinking.

Role of ventral subiculum neuronal ensembles in incubation of oxycodone craving after electric barrier-induced voluntary abstinence

High relapse rate is a key feature of opioid addiction. In humans, abstinence is often voluntary due to negative consequences of opioid seeking. To mimic this human condition, we recently introduced a rat model of incubation of oxycodone craving after electric barrier-induced voluntary abstinence. Incubation of drug craving refers to time-dependent increases in drug seeking after cessation of drug self-administration. Here, we used the activity marker Fos, muscimol-baclofen (GABAa + GABAb receptor agonists) global inactivation, Daun02-selective inactivation of putative relapse-associated neuronal ensembles, and fluorescence-activated cell sorting of Fos-positive cells and quantitative polymerase chain reaction to demonstrate a key role of vSub neuronal ensembles in incubation of oxycodone craving after voluntary abstinence, but not homecage forced abstinence. We also used a longitudinal functional magnetic resonance imaging method and showed that functional connectivity changes in vSub-related circuits predict opioid relapse after abstinence induced by adverse consequences of opioid seeking.

Activation of the dorsal septum increases alcohol consumption in male C57BL/6J mice

Binge drinking is a common pattern of excessive alcohol consumption associated with Alcohol Use Disorder (AUD) and unraveling the neurocircuitry that promotes this type of drinking is critical to the development of novel therapeutic interventions. The septal region was once a focal point of alcohol research yet has seen limited study over the last decade in relation to binge drinking. Numerous studies point to involvement of the dorsal septum (dSep) in excessive drinking and withdrawal, but few studies have manipulated this region in the context of binge drinking behavior. The present experiments were primarily designed to determine the effect of chemogenetic manipulation of the dSep on binge-like alcohol drinking in male and female C57BL/6J mice. Mice received bilateral infusion of AAVs harboring hM4Di, hM3Dq, or mCherry into the dSep and subjects were challenged with systemic administration of clozapine-N-oxide (CNO) and vehicle in the context of binge-like alcohol consumption, locomotor activity, and sucrose drinking. CNO-mediated activation (hM3Dq) of the dSep resulted in increased binge-like alcohol consumption, locomotor activity, and sucrose intake in males. DSep activation promoted sucrose drinking in female mice, but alcohol intake and locomotor activity were unaffected. Conversely, silencing (hM4Di) of the dSep modestly decreased locomotor activity in males and did not influence alcohol or sucrose intake in either sex. These data support a role for the dSep in promoting binge-like drinking behavior in a sex-dependent fashion and suggests a broad role for the region in the modulation of general appetitive behaviors and locomotor activity.

Examining ventral subiculum and basolateral amygdala projections to the nucleus accumbens shell: Differential expression of VGLuT1, VGLuT2 and VGaT in the rat

Projections to the striatum are well-identified. For example, in the ventral striatum, two major inputs to the medial nucleus accumbens shell include the ventral subiculum and basolateral amygdala. However, the chemical phenotype(s) of these projection neurons remain unclear. In this study, we examined amygdalostriatal and corticostriatal connectivity in rats using injections of the retrograde tracer cholera toxin b into the nucleus accumbens shell. To determine the neurotransmitter identity of projection neurons, we combined retrograde tracing with RNAscope in-situ hybridization, using mRNA probes against vesicular transporters associated with glutamatergic (VGluT1 - Slc17a7, VGluT2 - Slc17a6) or GABAergic (VGaT - Slc32a1) neurotransmission. Confocal imaging was used to examine vesicular transporter mRNA expression in the ventral subiculum and basolateral amygdala inputs to the nucleus accumbens shell. Both projections contained mostly VGluT1-expressing neurons. Interestingly, almost a quarter of ventral subiculum to nucleus accumbens shell projections co-expressed VGluT1 and VGluT2 compared to a relatively small number (∼3%) that were co-expressed in basolateral amygdala to nucleus accumbens shell afferents. However, almost a quarter of basolateral amygdala to nucleus accumbens shell projections were VGaT-positive. These findings highlight the diverse proportions of glutamatergic and GABAergic afferents in two major projections to the nucleus accumbens shell and raise important questions for functional studies.

Role of anterior insula cortex in context-induced relapse of nicotine-seeking

Tobacco use is the leading cause of preventable death worldwide, and relapse during abstinence remains the critical barrier to successful treatment of tobacco addiction. During abstinence, environmental contexts associated with nicotine use can induce craving and contribute to relapse. The insular cortex (IC) is thought to be a critical substrate of nicotine addiction and relapse. However, its specific role in context-induced relapse of nicotine-seeking is not fully known. In this study, we report a novel rodent model of context-induced relapse to nicotine-seeking after punishment-imposed abstinence, which models self-imposed abstinence through increasing negative consequences of excessive drug use. Using the neuronal activity marker Fos we find that the anterior (aIC), but not the middle or posterior IC, shows increased activity during context-induced relapse. Combining Fos with retrograde labeling of aIC inputs, we show projections to aIC from contralateral aIC and basolateral amygdala exhibit increased activity during context-induced relapse. Next, we used fiber photometry in aIC and observed phasic increases in aIC activity around nicotine-seeking responses during self-administration, punishment, and the context-induced relapse tests. Next, we used chemogenetic inhibition in both male and female rats to determine whether activity in aIC is necessary for context-induced relapse. We found that chemogenetic inhibition of aIC decreased context-induced nicotine-seeking after either punishment- or extinction-imposed abstinence. These findings highlight the critical role nicotine-associated contexts play in promoting relapse, and they show that aIC activity is critical for this context-induced relapse following both punishment and extinction-imposed abstinence.

Chemogenetics as a neuromodulatory approach to treating neuropsychiatric diseases and disorders

Chemogenetics enables precise, non-invasive, and reversible modulation of neural activity via the activation of engineered receptors that are pharmacologically selective to endogenous or exogenous ligands. With recent advances in therapeutic gene delivery, chemogenetics is poised to support novel interventions against neuropsychiatric diseases and disorders. To evaluate its translational potential, we performed a scoping review of applications of chemogenetics that led to the reversal of molecular and behavioral deficits in studies relevant to neuropsychiatric diseases and disorders. In this review, we present these findings and discuss the potential and challenges for using chemogenetics as a precision medicine-based neuromodulation strategy.

Increased responsiveness to punishment of cocaine self-administration after experience with high punishment

One behavioral feature of drug addiction is continued drug use despite awareness that this causes negative consequences. Attempts to model this feature in animals typically involve punishing drug self-administration with electrical footshock to identify individuals whose drug use is differently suppressed by punishment. Here we sought to further study individual responsiveness of drug use to punishment in rats self-administering intravenous cocaine. Rats were first trained during several weeks to self-administer cocaine under a fixed-ratio 3 schedule of reinforcement. Then, their self-administration behavior was punished with increasing intensity of footshock (i.e., from 0.1 mA to 0.9 mA, every 30 min). With increasing intensity of punishment, rats first continued to self-administer cocaine before eventually stopping near completely. When retested, however, drug use became more responsive to punishment and was suppressed by a low and initially ineffective footshock intensity (i.e., 0.1 mA). This increase in responsiveness to punishment was seen in all individuals tested, albeit with varying degrees, and was acquired after one single experience with an intensity of punishment that near completely suppressed drug self-administration. Mere passive, non-contingent exposure to the same intensity, however, had no such effect. Once acquired, increased responsiveness to punishment persisted during at least one month when rats were tested every week, but not every day. Finally, increased responsiveness to punishment was not observed after exposure to a non-painful form of punishment (i.e., histamine). Overall, this study reveals that initial responsiveness of drug use to punishment can change rapidly and persistently with experience. We discuss several possible mechanisms that may account for this change in punishment responsiveness and also draw some of the implications and future perspectives for research on animal models of compulsion-like behavior.

Drinking and smoking polygenic risk is associated with childhood and early-adulthood psychiatric and behavioral traits independently of substance use and psychiatric genetic risk

Alcohol drinking and tobacco smoking are hazardous behaviors associated with a wide range of adverse health outcomes. In this study, we explored the association of polygenic risk scores (PRS) related to drinks per week, age of smoking initiation, smoking initiation, cigarettes per day, and smoking cessation with 433 psychiatric and behavioral traits in 4498 children and young adults (aged 8-21) of European ancestry from the Philadelphia neurodevelopmental cohort. After applying a false discovery rate multiple testing correction accounting for the number of PRS and traits tested, we identified 36 associations related to psychotic symptoms, emotion and age recognition social competencies, verbal reasoning, anxiety-related traits, parents' education, and substance use. These associations were independent of the genetic correlations among the alcohol-drinking and tobacco-smoking traits and those with cognitive performance, educational attainment, risk-taking behaviors, and psychopathology. The removal of participants endorsing substance use did not affect the associations of each PRS with psychiatric and behavioral traits identified as significant in the discovery analyses. Gene-ontology enrichment analyses identified several neurobiological processes underlying mechanisms of the PRS associations we report. In conclusion, we provide novel insights into the genetic overlap of smoking and drinking behaviors in children and young adults, highlighting their independence from psychopathology and substance use.

Alcohol Seeking Under Risk of Punishment Is Associated With Activation of Cortical and Subcortical Brain Regions

In humans, stimuli associated with alcohol availability can provoke relapse during abstinence. In this study, we investigated the role of discriminative stimuli (DS) in the control of alcohol seeking in two types of behavioral tests. The first test examined the ability of an alcohol-associated DS to promote alcohol seeking (relapse) after punishment-imposed abstinence in the presence of a different DS. Following this, we tested whether the differentially associated DS can promote and suppress alcohol self-administration in a within-session discrimination task. During the within-session discrimination task, we also tested the rate of alcohol self-administration when two DS are presented in a compound. We first trained Long-Evans male rats (n = 24) to self-administer alcohol in the presence of one DS (reward-associated discriminative stimulus, rewDS) and then punished that behavior in the presence of a different DS (punishment-associated discriminative stimulus, punDS). On the test, we found that rats tested with the rewDS showed higher alcohol seeking than rats tested with the punDS. This result shows that a single Cue DS can promote alcohol seeking in a manner comparable to contexts. Subsequently, we trained 16 of these rats in a within-session trial-based discrimination task, comprised of intervening 2-min trials of rewDS, punDS, or conflict with rewDS and punDS in compound and a reduced probability of punishment. We found that alcohol self-administration is bi-directionally regulated by the rewDS and punDS. In conflict trials, alcohol self-administration was at a rate that was intermediate between the rewDS and punDS trials. In a final test, rats were presented with one of the three trial conditions and perfused for Fos immunohistochemistry. We found Fos expression was higher in the rats tested in the conflict condition in three interconnected sub-cortical brain regions. This study demonstrated the important role that alcohol-associated DS plays an important role in promoting relapse to alcohol seeking after punishment-imposed abstinence. We also implemented a within-session discrimination task that allows for the study of alcohol seeking under motivational conflict, which may be relevant for alcohol use despite negative consequences. The results from the Fos data suggest that higher alcohol seeking in approach-avoidance motivational conflict is associated with activation of sub-cortical regions but not cortical regions.

Individual differences in the engagement of habitual control over alcohol seeking predict the development of compulsive alcohol seeking and drinking

Excessive drinking is an important behavioural characteristic of alcohol addiction, but not the only one. Individuals addicted to alcohol crave alcoholic beverages, spend time seeking alcohol despite negative consequences and eventually drink to intoxication. With prolonged use, control over alcohol seeking devolves to anterior dorsolateral striatum, dopamine-dependent mechanisms implicated in habit learning and individuals in whom alcohol seeking relies more on these mechanisms are more likely to persist in seeking alcohol despite the risk of punishment. Here, we tested the hypothesis that the development of habitual alcohol seeking predicts the development of compulsive seeking and that, once developed, it is associated with compulsive alcohol drinking. Male alcohol-preferring rats were pre-exposed intermittently to a two-bottle choice procedure and trained on a seeking-taking chained schedule of alcohol reinforcement until some individuals developed punishment-resistant seeking behaviour. The associative basis of their seeking responses was probed with an outcome-devaluation procedure, early or late in training. After seeking behaviour was well established, subjects that had developed greater resistance to outcome devaluation (were more habitual) were more likely to show punishment-resistant (compulsive) alcohol seeking. These individuals also drank more alcohol, despite quinine adulteration, even though having similar alcohol preference and intake before and during instrumental training. They were also less sensitive to changes in the contingency between seeking responses and alcohol outcome, providing further evidence of recruitment of the habit system. We therefore provide direct behavioural evidence that compulsive alcohol seeking emerges alongside compulsive drinking in individuals who have preferentially engaged the habit system.

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.

Animal Models of Drug Relapse and Craving after Voluntary Abstinence: A Review

Relapse to drug use during abstinence is a defining feature of addiction. During the last several decades, this clinical scenario has been studied at the preclinical level using classic relapse/reinstatement models in which drug seeking is assessed after experimenter-imposed home-cage forced abstinence or extinction of the drug-reinforced responding in the self-administration chambers. To date, however, results from studies using rat relapse/reinstatement models have yet to result in Food and Drug Administration-approved medications for relapse prevention. The reasons for this state of affairs are complex and multifaceted, but one potential reason is that, in humans, abstinence is often self-imposed or voluntary and occurs either because the negative consequences of drug use outweigh the drug's rewarding effects or because of the availability of nondrug alternative rewards that are chosen over the drug. Based on these considerations, we and others have recently developed rat models of relapse after voluntary abstinence, achieved either by introducing adverse consequences to drug taking (punishment) or seeking (electric barrier) or by providing mutually exclusive choices between the self-administered drug and nondrug rewards (palatable food or social interaction). In this review, we provide an overview of these translationally relevant relapse models and discuss recent neuropharmacological findings from studies using these models. We also discuss sex as a biological variable, future directions, and clinical implications of results from relapse studies using voluntary abstinence models. Our main conclusion is that the neuropharmacological mechanisms controlling relapse to drug seeking after voluntary abstinence are often different from the mechanisms controlling relapse after home-cage forced abstinence or reinstatement after extinction. SIGNIFICANCE STATEMENT: This review describes recently developed rat models of relapse after voluntary abstinence, achieved either by introducing adverse consequences to drug taking or seeking or by providing mutually exclusive choices between the self-administered drug and nondrug rewards. This review discusses recent neuropharmacological findings from studies using these models and discusses future directions and clinical implications.

Muscarinic M4 and M5 receptors in the ventral subiculum differentially modulate alcohol seeking versus consumption in male alcohol-preferring rats

BACKGROUND AND PURPOSE

Muscarinic acetylcholine receptors mediate alcohol consumption and seeking in rats. While M₄ and M₅ receptors have recently been implicated to mediate these behaviours in the striatum, their role in other brain regions remain unknown. The ventral tegmental area (VTA) and ventral subiculum (vSub) both densely express M₄ and M₅ receptors and modulate alcohol-seeking, via their projections to the nucleus accumbens shell (AcbSh).

EXPERIMENTAL APPROACH

In Indiana alcohol-preferring (iP) male rats, we examined Chrm4 (M₄ ) and Chrm5 (M₅ ) expression in the VTA and vSub following long-term alcohol consumption and abstinence using RT-qPCR. Using a combination of retrograde tracing and RNAscope, we examined the localisation of Chrm4 and Chrm5 on vSub cells that project to the AcbSh. Using selective allosteric modulators, we examined the functional role of M₄ and M₅ receptors within the vSub in alcohol consumption, context-induced alcohol-seeking, locomotor activity, and food/water consumption.

KEY RESULTS

Long-term alcohol and abstinence dysregulated the expression of genes for muscarinic receptors in the vSub, not in the VTA. Chrm4 was down-regulated following long-term alcohol and abstinence, while Chrm5 was up-regulated following long-term alcohol consumption. Consistent with these data, a positive allosteric modulator (VU0467154) of intra-vSub M₄ receptors reduced context-induced alcohol-seeking, but not motivation for alcohol self-administration, while M₅ receptor negative allosteric modulator (ML375) reduced initial motivation for alcohol self-administration, but not context-induced alcohol-seeking.

CONCLUSION AND IMPLICATIONS

Collectively, our data highlight alcohol-induced cholinergic dysregulation in the vSub and distinct roles for M₄ and M₅ receptor allosteric modulators to reduce alcohol consumption or seeking.

Chronic intermittent ethanol promotes ventral subiculum hyperexcitability via increases in extrinsic basolateral amygdala input and local network activity

The hippocampus, particularly its ventral domain, can promote negative affective states (i.e. stress and anxiety) that play an integral role in the development and persistence of alcohol use disorder (AUD). The ventral hippocampus (vHC) receives strong excitatory input from the basolateral amygdala (BLA) and the BLA-vHC projection bidirectionally modulates anxiety-like behaviors. However, no studies have examined the effects of chronic alcohol on the BLA-vHC circuit. In the present study, we used ex vivo electrophysiology in conjunction with optogenetic approaches to examine the effects of chronic intermittent ethanol exposure (CIE), a well-established rodent model of AUD, on the BLA-vHC projection and putative intrinsic vHC synaptic plasticity. We discovered prominent BLA innervation in the subicular region of the vHC (vSub). CIE led to an overall increase in the excitatory/inhibitory balance, an increase in AMPA/NMDA ratios but no change in paired-pulse ratios, consistent with a postsynaptic increase in excitability in the BLA-vSub circuit. CIE treatment also led to an increase in intrinsic network excitability in the vSub. Overall, our findings suggest a hyperexcitable state in BLA-vSub specific inputs as well as intrinsic inputs to vSub pyramidal neurons which may contribute to the negative affective behaviors associated with CIE.

Recent Perspectives on Sex Differences in Compulsion-Like and Binge Alcohol Drinking

Alcohol use disorder remains a substantial social, health, and economic problem and problem drinking levels in women have been increasing in recent years. Understanding whether and how the underlying mechanisms that drive drinking vary by sex is critical and could provide novel, more targeted therapeutic treatments. Here, we examine recent results from our laboratories and others which we believe provide useful insights into similarities and differences in alcohol drinking patterns across the sexes. Findings for binge intake and aversion-resistant, compulsion-like alcohol drinking are considered, since both are likely significant contributors to alcohol problems in humans. We also describe studies regarding mechanisms that may underlie sex differences in maladaptive alcohol drinking, with some focus on the importance of nucleus accumbens (NAcb) core and shell regions, several receptor types (dopamine, orexin, AMPA-type glutamate), and possible contributions of sex hormones. Finally, we discuss how stressors such as early life stress and anxiety-like states may interact with sex differences to contribute to alcohol drinking. Together, these findings underscore the importance and critical relevance of studying female and male mechanisms for alcohol and co-morbid conditions to gain a true and clinically useful understanding of addiction and neuropsychiatric mechanisms and treatment.

BEHAVIORAL AND NEUROBIOLOGICAL MECHANISMS OF PAVLOVIAN AND INSTRUMENTAL EXTINCTION LEARNING

This article reviews the behavioral neuroscience of extinction, the phenomenon in which a behavior that has been acquired through Pavlovian or instrumental (operant) learning decreases in strength when the outcome that reinforced it is removed. Behavioral research indicates that neither Pavlovian nor operant extinction depends substantially on erasure of the original learning but instead depends on new inhibitory learning that is primarily expressed in the context in which it is learned, as exemplified by the renewal effect. Although the nature of the inhibition may differ in Pavlovian and operant extinction, in either case the decline in responding may depend on both generalization decrement and the correction of prediction error. At the neural level, Pavlovian extinction requires a tripartite neural circuit involving the amygdala, prefrontal cortex, and hippocampus. Synaptic plasticity in the amygdala is essential for extinction learning, and prefrontal cortical inhibition of amygdala neurons encoding fear memories is involved in extinction retrieval. Hippocampal-prefrontal circuits mediate fear relapse phenomena, including renewal. Instrumental extinction involves distinct ensembles in corticostriatal, striatopallidal, and striatohypothalamic circuits as well as their thalamic returns for inhibitory (extinction) and excitatory (renewal and other relapse phenomena) control over operant responding. The field has made significant progress in recent decades, although a fully integrated biobehavioral understanding still awaits.

Neural Substrates and Circuits of Drug Addiction

Drug addiction is a chronic relapsing disorder, and a significant amount of research has been devoted to understand the factors that contribute to the development, loss of control, and persistence of compulsive addictive behaviors. In this review, we provide an overview of various theories of addiction to drugs of abuse and the neurobiology involved in elements of the addiction cycle. Specific focus is devoted to the role of the mesolimbic pathway in acute drug reinforcement and occasional drug use, the role of the mesocortical pathway and associated areas (e.g., the dorsal striatum) in escalation/dependence, and the contribution of these pathways and associated circuits to conditioned responses, drug craving, and loss of behavioral control that may underlie drug relapse. By enhancing the understanding of the neurobiological factors that mediate drug addiction, continued preclinical and clinical research will aid in the development of novel therapeutic interventions that can serve as effective long-term treatment strategies for drug-dependent individuals.

Motivational competition and the paraventricular thalamus

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

Differential importance of nucleus accumbens Ox1Rs and AMPARs for female and male mouse binge alcohol drinking

Alcohol use disorder exhausts substantial social and economic costs, with recent dramatic increases in female problem drinking. Thus, it is critically important to understand signaling differences underlying alcohol consumption across the sexes. Orexin-1 receptors (Ox1Rs) can strongly promote motivated behavior, and we previously identified Ox1Rs within nucleus accumbens shell (shell) as crucial for driving binge intake in higher-drinking male mice. Here, shell Ox1R inhibition did not alter female mouse alcohol drinking, unlike in males. Also, lower dose systemic Ox1R inhibition reduced compulsion-like alcohol intake in both sexes, indicating that female Ox1Rs can drive some aspects of pathological consumption, and higher doses of systemic Ox1R inhibition (which might have more off-target effects) reduced binge drinking in both sexes. In contrast to shell Ox1Rs, inhibiting shell calcium-permeable AMPA receptors (CP-AMPARs) strongly reduced alcohol drinking in both sexes, which was specific to alcohol since this did not reduce saccharin intake in either sex. Our results together suggest that the shell critically regulates binge drinking in both sexes, with shell CP-AMPARs supporting intake in both sexes, while shell Ox1Rs drove drinking only in males. Our findings provide important new information about sex-specific and -general mechanisms that promote binge alcohol intake and possible targeted therapeutic interventions.

Cooperative synaptic and intrinsic plasticity in a disynaptic limbic circuit drive stress-induced anhedonia and passive coping in mice

Stress promotes negative affective states, which include anhedonia and passive coping. While these features are in part mediated by neuroadaptations in brain reward circuitry, a comprehensive framework of how stress-induced negative affect may be encoded within key nodes of this circuit is lacking. Here, we show in a mouse model for stress-induced anhedonia and passive coping that these phenomena are associated with increased synaptic strength of ventral hippocampus (VH) excitatory synapses onto D1 medium spiny neurons (D1-MSNs) in the nucleus accumbens medial shell (NAcmSh), and with lateral hypothalamus (LH)-projecting D1-MSN hyperexcitability mediated by decreased inwardly rectifying potassium channel (IRK) function. Stress-induced negative affective states are prevented by depotentiation of VH to NAcmSh synapses, restoring Kir2.1 function in D1R-MSNs, or disrupting co-participation of these synaptic and intrinsic adaptations in D1-MSNs. In conclusion, our data provide strong evidence for a disynaptic pathway controlling maladaptive emotional behavior.

Tracing goes viral: Viruses that introduce expression of fluorescent proteins in chemically-specific neurons

Over the last century, there has been great progress in understanding how the brain works. In particular, the last two decades have been crucial in gaining more awareness over the complex functioning of neurotransmitter systems. The use of viral vectors in neuroscience has been pivotal for such development. Exploiting the properties of viral particles, modifying them according to the research needs, and making them target chemically-specific neurons, techniques such as optogenetics and chemogenetics have been developed, which could lead to a giant step toward gene therapy for brain disorders. In this review, we aim to provide an overview of some of the most widely used viral techniques in neuroscience. We will discuss advantages and disadvantages of these methods. In particular, attention is dedicated to the pivotal role played by the introduction of adeno-associated virus and the retrograde tracer canine-associated-2 Cre virus in order to achieve optimal visualization, and interrogation, of chemically-specific neuronal populations and their projections.

Orexin-1 receptor signaling within the lateral hypothalamus, but not bed nucleus of the stria terminalis, mediates context-induced relapse to alcohol seeking

BACKGROUND

The lateral hypothalamic orexin (hypocretin) system has a well-established role in the motivation for reward. This has particular relevance to substance use disorders since orexin-1 receptors play a critical role in alcohol-seeking behavior, acting at multiple nodes in relapse-associated networks.

AIMS

This study aimed to further our understanding of the role of orexin-1 receptor signaling within the lateral hypothalamus and bed nucleus of the stria terminalis, specifically in context-induced relapse to alcohol-seeking following punishment-imposed abstinence.

METHODS

We trained inbred male alcohol-preferring rats to self-administer alcohol in one environment or context (Context A) and subsequently punished their alcohol-reinforced lever presses in a different environment (Context B) using contingent foot shock punishment. Finally, we tested rats for relapse-like behavior in either context following systemic, intra-lateral hypothalamus or intra-bed nucleus of the stria terminalis orexin-1 receptor antagonism with SB-334867.

RESULTS/OUTCOMES

We found that systemic orexin-1 receptor antagonism significantly reduced alcohol-seeking in both contexts. Intra-lateral hypothalamus orexin-1 receptor antagonism significantly reduced alcohol-seeking in Context A whereas intra-bed nucleus of the stria terminalis orexin-1 receptor antagonism had no effect on alcohol-seeking behavior.

CONCLUSIONS/INTERPRETATION

Our results suggest a role for the orexin-1 receptor system in context-induced relapse to alcohol-seeking. Specifically, intra-lateral hypothalamus orexin microcircuits contribute to alcohol-seeking.

Dynamic c-Fos changes in mouse brain during acute and protracted withdrawal from chronic intermittent ethanol exposure and relapse drinking

Alcohol dependence promotes neuroadaptations in numerous brain areas, leading to escalated drinking and enhanced relapse vulnerability. We previously developed a mouse model of ethanol dependence and relapse drinking in which repeated cycles of chronic intermittent ethanol (CIE) vapor exposure drive a significant escalation of voluntary ethanol drinking. In the current study, we used this model to evaluate changes in neuronal activity (as indexed by c‐Fos expression) throughout acute and protracted withdrawal from CIE (combined with or without a history of ethanol drinking). We analyzed c‐Fos protein expression in 29 brain regions in mice sacrificed 2, 10, 26, and 74 hours or 7 days after withdrawal from 5 cycles of CIE. Results revealed dynamic time‐ and brain region‐dependent changes in c‐Fos activity over the time course of withdrawal from CIE exposure, as compared with nondependent air‐exposed control mice, beginning with markedly low expression levels upon removal from the ethanol vapor chambers (2 hours), reflecting intoxication. c‐Fos expression was enhanced during acute CIE withdrawal (10 and 26 hours), followed by widespread reductions at the beginning of protracted withdrawal (74 hours) in several brain areas. Persistent reductions in c‐Fos expression were observed during prolonged withdrawal (7 days) in prelimbic cortex, nucleus accumbens shell, dorsomedial striatum, paraventricular nucleus of thalamus, and ventral subiculum. A history of ethanol drinking altered acute CIE withdrawal effects and caused widespread reductions in c‐Fos that persisted during extended abstinence even without CIE exposure. These data indicate that ethanol dependence and relapse drinking drive long‐lasting neuroadaptations in several brain regions.

Improving translation of animal models of addiction and relapse by reverse translation

Critical features of human addiction are increasingly being incorporated into complementary animal models, including escalation of drug intake, punished drug seeking and taking, intermittent drug access, choice between drug and non-drug rewards, and assessment of individual differences based on criteria in the fourth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV). Combined with new technologies, these models advanced our understanding of brain mechanisms of drug self-administration and relapse, but these mechanistic gains have not led to improvements in addiction treatment. This problem is not unique to addiction neuroscience, but it is an increasing source of disappointment and calls to regroup. Here we first summarize behavioural and neurobiological results from the animal models mentioned above. We then propose a reverse translational approach, whose goal is to develop models that mimic successful treatments: opioid agonist maintenance, contingency management and the community-reinforcement approach. These reverse-translated 'treatments' may provide an ecologically relevant platform from which to discover new circuits, test new medications and improve translation.

Expression of a heroin contextually conditioned immune effect in male rats requires CaMKIIα-expressing neurons in dorsal, but not ventral, subiculum and hippocampal CA1

The physiological and motivational effects of heroin and other abused drugs become associated with environmental (contextual) stimuli during repeated drug use. As a result, these contextual stimuli gain the ability to elicit drug-like conditioned effects. For example, after context-heroin pairings, exposure to the heroin-paired context alone produces similar effects on peripheral immune function as heroin itself. Conditioned immune effects can significantly exacerbate the adverse health consequences of heroin use. Our laboratory has shown that exposure to a heroin-paired context suppresses lipopolysaccharide (LPS)-induced splenic nitric oxide (NO) production in male rats, and this effect is mediated in part by the dorsal hippocampus (dHpc). However, specific dHpc output regions, whose efferents might mediate conditioned immune effects, have not been identified, nor has the contribution of ventral hippocampus (vHpc) been investigated. Here, we evaluated the role of CaMKIIα-expressing neurons in the dHpc and vHpc main output regions by expressing G_i-coupled designer receptors exclusively activated by designer drugs (DREADDs) under a CaMKIIα promoter in the dorsal subiculum and CA1 (dSub, dCA1) or ventral subiculum and CA1 (vSub, vCA1). After context-heroin conditioning, clozapine-N-oxide (CNO, DREADD agonist) or vehicle was administered systemically prior to heroin-paired context (or home-cage control) exposure and LPS immune challenge. Chemogenetic inhibition of CaMKIIα-expressing neurons in dHpc, but not vHpc, output regions attenuated the expression of conditioned splenic NO suppression. These results establish that the main dHpc output regions, the dSub and dCA1, are critical for this context-heroin conditioned immune effect.

Dissociable mesolimbic dopamine circuits control responding triggered by alcohol-predictive discrete cues and contexts

Context can influence reactions to environmental cues and this elemental process has implications for substance use disorder. Using an animal model, we show that an alcohol-associated context elevates entry into a fluid port triggered by a conditioned stimulus (CS) that predicted alcohol (CS-triggered alcohol-seeking). This effect persists across multiple sessions and, after it diminishes in extinction, the alcohol context retains the capacity to augment reinstatement. Systemically administered eticlopride and chemogenetic inhibition of ventral tegmental area (VTA) dopamine neurons reduce CS-triggered alcohol-seeking. Chemogenetically silencing VTA dopamine terminals in the nucleus accumbens (NAc) core reduces CS-triggered alcohol-seeking, irrespective of context, whereas silencing VTA dopamine terminals in the NAc shell selectively reduces the elevation of CS-triggered alcohol-seeking in an alcohol context. This dissociation reveals new roles for divergent mesolimbic dopamine circuits in the control of responding to a discrete cue for alcohol and in the amplification of this behaviour in an alcohol context.

Resurgence of punishment-suppressed cocaine seeking in rats

Alternative reinforcement-based treatments are among the most effective for reducing substance abuse. However, relapse often occurs when alternative reinforcement ends. Relapse following the loss of alternative reinforcement is called resurgence. An animal model has been used to study basic factors that may ultimately reduce resurgence but uses drug unavailability (i.e., extinction) to reduce drug seeking. In humans, drug abstinence is thought to be a product of aversive consequences associated with drug use rather than extinction. This discrepancy is important because the environmental and neurobiological factors involved in relapse may differ between punished and extinguished behavior. Experiment 1 evaluated resurgence of previously punished cocaine seeking. In Phase 1, rats earned cocaine for pressing levers. In Phase 2, cocaine remained available, but lever pressing also produced mild foot shocks while an alternative response produced food pellets for 1 group but not for another group. In Phase 3, alternative reinforcement and punishment were removed and resurgence of cocaine seeking occurred only in rats previously exposed to alternative reinforcement. In Experiment 2, resurgence was evaluated similarly, except that consequences of cocaine seeking (i.e., punishment and cocaine) remained available during Phase 3. Resurgence did not occur in either group during Experiment 2. The animal models of resurgence developed herein could increase translational utility and improve examination of the environmental and neurobiological factors underlying resurgence of drug seeking. (PsycInfo Database Record (c) 2020 APA, all rights reserved).

Combining designer receptors exclusively activated by designer drugs and neuroimaging in experimental models: A powerful approach towards neurotheranostic applications

The combination of chemogenetics targeting specific brain cell populations with in vivo imaging techniques provides scientists with a powerful new tool to study functional neural networks at the whole-brain scale. A number of recent studies indicate the potential of this approach to increase our understanding of brain function in health and disease. In this review, we discuss the employment of a specific chemogenetic tool, designer receptors exclusively activated by designer drugs, in conjunction with non-invasive neuroimaging techniques such as PET and MRI. We highlight the utility of using this multiscale approach in longitudinal studies and its ability to identify novel brain circuits relevant to behaviour that can be monitored in parallel. In addition, some identified shortcomings in this technique and more recent efforts to overcome them are also presented. Finally, we discuss the translational potential of designer receptors exclusively activated by designer drugs in neuroimaging and the promise it holds for future neurotheranostic applications.

Role of Projections between Piriform Cortex and Orbitofrontal Cortex in Relapse to Fentanyl Seeking after Palatable Food Choice-Induced Voluntary Abstinence

We recently developed a rat model of relapse to drug seeking after food choice-induced voluntary abstinence. Here, we used this model to study the role of the orbitofrontal cortex (OFC) and its afferent projections in relapse to fentanyl seeking. We trained male and female rats to self-administer palatable food pellets for 6 d (6 h/d) and intravenous fentanyl (2.5 μg/kg/infusion) for 12 d (6 h/d). We assessed relapse to fentanyl seeking after 13-14 voluntary abstinence days, achieved through a discrete choice procedure between fentanyl infusions and palatable food (20 trials/d). In both sexes, relapse after food choice-induced abstinence was associated with increased expression of the activity marker Fos in the OFC. Pharmacological inactivation of the OFC with muscimol plus baclofen (50 + 50 ng/side) decreased relapse to fentanyl seeking. We then determined projection-specific activation of OFC afferents during the relapse test by using Fos plus the retrograde tracer cholera toxin B (injected into the OFC). Relapse to fentanyl seeking was associated with increased Fos expression in the piriform cortex (Pir) neurons projecting to the OFC, but not in projections from the basolateral amygdala and thalamus. Pharmacological inactivation of the Pir with muscimol plus baclofen decreased relapse to fentanyl seeking after voluntary abstinence. Next, we used an anatomical disconnection procedure to determine whether projections between the Pir and OFC are critical for relapse to fentanyl seeking. Unilateral muscimol plus baclofen injections into the Pir in one hemisphere plus unilateral muscimol plus baclofen injections into the OFC in the contralateral, but not ipsilateral, hemisphere decreased relapse. Our results identify Pir-OFC projections as a new motivation-related pathway critical to relapse to opioid seeking after voluntary abstinence.SIGNIFICANCE STATEMENT There are few preclinical studies of fentanyl relapse, and these studies have used experimenter-imposed extinction or forced abstinence procedures. In humans, however, abstinence is often voluntary, with drug available in the drug environment but forgone in favor of nondrug alternative reinforcers. We recently developed a rat model of drug relapse after palatable food choice-induced voluntary abstinence. Here, we used classical pharmacology, immunohistochemistry, and retrograde tracing to demonstrate a critical role of the piriform and orbitofrontal cortices in relapse to opioid seeking after voluntary abstinence.