Extinction of drug- and withdrawal-paired cues in animal models: relevance to the treatment of addiction

Transcriptional characterization of cocaine withdrawal versus extinction within nucleus accumbens

Substance use disorder is characterized by a maladaptive imbalance wherein drug seeking persists despite negative consequences or drug unavailability. This imbalance correlates with neurobiological alterations some of which are amplified during forced abstinence, thereby compromising the capacity of extinction-based approaches to prevent relapse. Cocaine use disorder (CUD) exemplifies this phenomenon in which neurobiological modifications hijack brain reward regions such as the nucleus accumbens (NAc) to manifest craving and withdrawal-like symptoms. While increasing evidence links transcriptional changes in the NAc to specific phases of addiction, genome-wide changes in gene expression during withdrawal vs. extinction (WD/Ext) have not been examined in a context- and NAc-subregion-specific manner. Here, we used cocaine self-administration (SA) in rats combined with RNA-sequencing (RNA-seq) of NAc subregions (core and shell) to transcriptionally profile the impact of experiencing withdrawal in the home cage or in the previous drug context or experiencing extinction training. As expected, home-cage withdrawal maintained drug seeking in the previous drug context, whereas extinction training reduced it. By contrast, withdrawal involving repetitive exposure to the previous drug context increased drug-seeking behavior. Bioinformatic analyses of RNA-seq data revealed gene expression patterns, networks, motifs, and biological functions specific to these behavioral conditions and NAc subregions. Comparing transcriptomic analysis of the NAc of patients with CUD highlighted conserved gene signatures, especially with rats that were repetitively exposed to the previous drug context. Collectively, these behavioral and transcriptional correlates of several withdrawal-extinction settings reveal fundamental and translational information about potential molecular mechanisms to attenuate drug-associated memories.

Restoring GABAB receptor expression in the ventral tegmental area of methamphetamine addicted mice inhibits locomotor sensitization and drug seeking behavior

Repeated exposure to psychostimulants such as methamphetamine (METH) induces neuronal adaptations in the mesocorticolimbic dopamine system, including the ventral tegmental area (VTA). These changes lead to persistently enhanced neuronal activity causing increased dopamine release and addictive phenotypes. A factor contributing to increased dopaminergic activity in this system appears to be reduced GABAB receptor-mediated neuronal inhibition in the VTA. Dephosphorylation of serine 783 (Ser783) of the GABAB2 subunit by protein phosphatase 2A (PP2A) appears to trigger the downregulation GABAB receptors in psychostimulant-addicted rodents. Therefore, preventing the interaction of GABAB receptors with PP2A using an interfering peptide is a promising strategy to restore GABAB receptor-mediated neuronal inhibition. We have previously developed an interfering peptide (PP2A-Pep) that inhibits the GABAB receptors/PP2A interaction and thereby restores receptor expression under pathological conditions. Here, we tested the hypothesis that restoration of GABAB receptor expression in the VTA of METH addicted mice reduce addictive phenotypes. We found that the expression of GABAB receptors was significantly reduced in the VTA and nucleus accumbens but not in the hippocampus and somatosensory cortex of METH-addicted mice. Infusion of PP2A-Pep into the VTA of METH-addicted mice restored GABAB receptor expression in the VTA and inhibited METH-induced locomotor sensitization as assessed in the open field test. Moreover, administration of PP2A-Pep into the VTA also reduced drug seeking behavior in the conditioned place preference test. These observations underscore the importance of VTA GABAB receptors in controlling addictive phenotypes. Furthermore, this study illustrates the value of interfering peptides targeting diseases-related protein-protein interactions as an alternative approach for a potential development of selective therapeutic interventions.

Effectiveness of Reducing Craving in Alcohol Use Disorder Using a Serious Game (SALIENCE): Randomized Controlled Trial

BACKGROUND

Alcohol use disorder (AUD) has become a major global health problem. Therapy for this condition is still a great challenge. Recently, it has become increasingly evident that computer-based training is a valuable addition to the treatment of addictive disorders.

OBJECTIVE

This study aims to evaluate the web-based serious game SALIENCE (Stop Alcohol in Everyday Life-New Choices and Evaluations) as an add-on therapy for AUD. It combines the cue-exposure therapy approach with elements of decision-making training, enhanced by interactive panoramic images. The effects of SALIENCE training on levels of craving, attention, and cognitive bias are investigated.

METHODS

In a randomized controlled trial, 62 participants with AUD undergoing 3 weeks of an extended alcohol detoxification program were randomly allocated to an intervention and a control group. A total of 49 individuals (mean age 44.04 y; 17/49, 35% female) completed all sessions and were included in the analysis. Only pretreatment data were available from the other 13 patients. Participants answered questionnaires related to alcohol consumption and craving and completed neuropsychological tasks at the beginning of the study and 2 weeks later to evaluate levels of attention and cognitive biases. During the 2-week period, 27 of the participants additionally performed the SALIENCE training for 30 minutes 3 times a week, for a total of 6 sessions.

RESULTS

We observed a significant decrease in craving in both groups: the control group (mean 15.59, SD 8.02 on the first examination day vs mean 13.18, SD 8.38 on the second examination day) and the intervention group (mean 15.19, SD 6.71 on the first examination day vs mean 13.30, SD 8.47 on the second examination day; F1,47=4.31; P=.04), whereas the interaction effect was not statistically significant (F1,47=0.06; P=.80). Results of the multiple linear regression controlling for individual differences between participants indicated a significantly greater decrease in craving (β=4.12; t36=2.34; P=.03) with the SALIENCE intervention. Participants with lower drinking in negative situations reduced their craving (β=.38; t36=3.01; P=.005) more than people with higher drinking in negative situations.

CONCLUSIONS

The general effectiveness of SALIENCE training as an add-on therapy in reducing alcohol craving was not confirmed. Nevertheless, taking into account individual differences (gender, duration of dependence, stress, anxiety, and drinking behavior in different situations), it was shown that SALIENCE training resulted in a larger reduction in craving than without. Notably, individuals who rarely consume alcohol due to negative affect profited the most from SALIENCE training. In addition to the beneficial effect of SALIENCE training, these findings highlight the relevance of individualized therapy for AUD, adapted to personal circumstances such as drinking motivation.

TRIAL REGISTRATION

ClinicalTrials.gov NCT03765476; https://clinicaltrials.gov/show/NCT03765476.

Morphine-withdrawal aversive memories and their extinction modulate H4K5 acetylation and Brd4 activation in the rat hippocampus and basolateral amygdala

Chromatin modification is a crucial mechanism in several important phenomena in the brain, including drug addiction. Persistence of drug craving and risk of relapse could be attributed to drug-induced epigenetic mechanisms that seem to be candidates explaining long-lasting drug-induced behaviour and molecular alterations. Histone acetylation has been proposed to regulate drug-seeking behaviours and the extinction of rewarding memory of drug taking. In this work, we studied the epigenetic regulation during conditioned place aversion and after extinction of aversive memory of opiate withdrawal. Through immunofluorescence assays, we assessed some epigenetic marks (H4K5ac and p-Brd4) in crucial areas related to memory retrieval -basolateral amygdala (BLA) and hippocampus-. Additionally, to test the degree of transcriptional activation, we evaluated the immediate early genes (IEGs) response (Arc, Bdnf, Creb, Egr-1, Fos and Nfkb) and Smarcc1 (chromatin remodeler) through RT-qPCR in these nuclei. Our results showed increased p-Brd4 and H4K5ac levels during aversive memory retrieval, suggesting a more open chromatin state. However, transcriptional activation of these IEGs was not found, therefore suggesting that other secondary response may already be happening. Additionally, Smarcc1 levels were reduced due to morphine chronic administration in BLA and dentate gyrus. The activation markers returned to control levels after the retrieval of aversive memories, revealing a more repressed chromatin state. Taken together, our results show a major role of the tandem H4K5ac/p-Brd4 during the retrieval of aversive memories. These results might be useful to elucidate new molecular targets to improve and develop pharmacological treatments to address addiction and to avoid drug relapse.

[Conditioning of the immune system-Already clinically usable?]

The brain and the immune system permanently exchange information via various neuronal and humoral signaling pathways. This communication network forms the basis for controlling peripheral immune functions via associative learning or conditioning processes. Establishing a learned immune reaction, an immunomodulatory drug that represents the unconditioned stimulus (US) is paired with a new odor or taste stimulus. Re-presentating this previously neutral odor or taste stimulus, its now functions as a conditioned stimulus (CS) and triggers reactions in the immune system similar to those formerly induced by the drug used as US. Using different learning protocols, it was possible to condition immunopharmacological effects in animal disease models, such as lupus erythematosus, contact allergy or rheumatoid arthritis, thereby reducing disease symptoms. Preliminary experimental studies in healthy volunteers and patients confirmed a possible clinical use of learned immune responses with the aim of using associative learning protocols as complementary measures to pharmacological interventions in clinical practice in order to reduce drug doses and thus undesirable drug side effects while maintaining therapeutic efficacy. However, there is still a great need for further research to understand the mechanisms of learned immune responses in preclinical studies and to optimize the associative learning processes for using them in the clinical routine in studies with healthy volunteers and patients.

Male and female rats show opiate withdrawal-induced place aversion and extinction in a Y-maze paradigm

Gender differences have been observed in the vulnerability to drug abuse and in the different stages of the addictive process. In opiate dependence, differences between sexes have been shown in humans and laboratory animals in various phases of opiate addiction, especially in withdrawal-associated negative affective states. Using a Y-maze conditioned place aversion paradigm, we investigated potential sex differences in the expression and extinction of the aversive memory of precipitated opiate withdrawal state in morphine-dependent rats. No significant difference between sexes was observed in the occurrence of withdrawal signs following naloxone injection during conditioning. Moreover, opiate withdrawal memory expression and extinction following repeated testing was demonstrated in both male and female rats, with no significant differences between sexes. Finally, we report spontaneous recovery following extinction of opiate withdrawal memory. Altogether these data provide further evidence that persistent withdrawal-related memories may be strong drivers of opiate dependence, and demonstrate that both males and females can be used in experimental rodent cohorts to better understand opiate-related effects, reward, aversive state of withdrawal, abstinence and relapse.

Hippocampal and amygdalar increased BDNF expression in the extinction of opioid-induced place preference

The opioid crisis was exacerbated during the COVID-19 pandemic in the United States with alarming statistics about overdose-related deaths. Current treatment options, such as medication assisted treatments, have been unable to prevent relapse in many patients, whereas cue-based exposure therapy have had mixed results in human trials. To improve patient outcomes, it is imperative to develop animal models of addiction to understand molecular mechanisms and identify potential therapeutic targets. We previously found increased brain derived neurotrophic factor (bdnf) transcript in the ventral striatum/nucleus accumbens (VS/NAc) of rats that extinguished morphine-induced place preference. Here, we expand our study to determine whether BDNF protein expression was modulated in mesolimbic brain regions of the reward system in animals exposed to extinction training. Drug conditioning and extinction sessions were followed by Western blots for BDNF in the hippocampus (HPC), amygdala (AMY) and VS/NAc. Rears, as a measure of withdrawal-induced anxiety were also measured to determine their impact on extinction. Results showed that animals who received extinction training and successfully extinguished morphine CPP significantly increased BDNF in the HPC when compared to animals deprived of extinction training (sham-extinction). This increase was not significant in animals who failed to extinguish (extinction-resistant). In AMY, all extinction-trained animals showed increased BDNF, regardless of behavior phenotype. No BDNF modulation was observed in the VS/NAc. Finally, extinction-trained animals showed no difference in rears regardless of extinction outcome, suggesting that anxiety elicited by drug withdrawal did not significantly impact extinction of morphine CPP. Our results suggest that BDNF expression in brain regions of the mesolimbic reward system could play a key role in extinction of opioid-induced maladaptive behaviors and represents a potential therapeutic target for future combined pharmacological and extinction-based therapies.

Rapastinel accelerates loss of withdrawal signs after repeated morphine and blunts relapse to conditioned place preference

The purpose of the present study was to evaluate the efficacy of rapastinel, an allosteric modulator of NMDA receptor function, to accelerate the loss of opioid withdrawal symptoms and blunt or prevent relapse to morphine conditioned place preference (CPP) in rats. Two studies were conducted. In study 1, adult and adolescent male and female rats were treated with increasing doses of morphine (5 mg/kg, bid to 25 mg/kg bid) for 5 days. On day 6 animals were treated with naloxone (1 mg/kg) and withdrawal was assessed. They were then treated with saline or rapastinel (5 mg/kg) on days 6 and 8, and withdrawal was assessed on day 9. Rapastinel treated animals exhibited significantly lower levels of withdrawal signs on day 9. No sex or age differences were observed. In Study 2, CPP for morphine was established in adult rats (males and females) by 4 daily pairings with saline and morphine (am/pm alternation). They were tested for CPP on day 5, and then treated with rapastinel (5 mg/kg) or saline daily on days 6-10 of extinction. On day 11 they received a final dose of rapastinel or saline followed by extinction trial. On day 12, animals received 1 mg/kg of morphine and were tested for relapse. Rapastinel did not affect extinction of CPP, but rapastinel-treated animals spent significantly less time in the previously morphine-paired side than saline-treated animals during the relapse trial. These findings of accelerated loss of withdrawal signs and blunted relapse to CPP suggest that rapastinel could provide an adjunctive therapy for opioid dependence during initiation of pharmacotherapy for opioid dependence.

Reward Responsiveness, Learning, and Valuation Implicated in Problematic Pornography Use — a Research Domain Criteria Perspective

Purpose of Review

Problematic pornography use (PPU) describes a pattern of behavior characterized by excessive time spent using or thinking about pornography and continued use despite negative consequences. To help advance the understanding of transdiagnostic underlying psychological and neurobiological mechanisms in PPU, we aim to review existing evidence on these mechanisms focusing on positive valence systems within the transdiagnostic Research Domain Criteria (RDoC) framework.

Recent Findings

Reward anticipation processes seem to be increased in individuals with PPU symptoms when they anticipate sexual stimuli compared with other rewards. Studies further suggest that the initial neural and attentional responses to sexual rewards compared with different control stimuli are also increased in individuals with PPU symptoms, as are conditioned responses in sexual reward learning paradigms. Sexual reward valuation studies point towards an increased neural value differentiation with increasing PPU symptoms.

Summary

The current state of evidence indicates that positive valence systems are altered in persons with PPU. This framework of organizing evidence may aid in elucidating PPU development and maintenance as well as planning future studies.

Deep brain stimulation for opioid use disorder: A systematic review of preclinical and clinical evidence

Opioid use disorder (OUD) is a chronic and complex disease characterized by repeated relapses and remissions. Deep brain stimulation (DBS) has been discussed again and again as a potentially helpful neuromodulatory procedure in this context. In this review, for the first time, we intended to systematically identify the positive and negative effects of DBS in human and animal models of opioid dependence to assess the viability of DBS as a treatment of OUD. Eligible studies were incorporated by a comprehensive literature search and evaluated through proper methodological quality assessment tools. Findings showed that the nucleus accumbens was the most stimulated brain target in human and animal studies, and DBS was applied chiefly in the form of high-frequency stimulation (HFS). DBS administration effectively reduced opioid craving and consumption in human and animal subjects dependent on opioids. DBS represents a valuable alternative strategy for treating intractable opioid addiction. Based on our systematic literature analysis, research efforts in this field should be continued.

A spatiotemporal increase of neuronal activity accompanies the motivational effect of wheel running in mice

Spatiotemporal patterns of neuronal activity underlying the motivational effect of rotating running wheels (RWs) in rodents remain largely undetermined. Here, we investigated changes of neuronal activity among brain regions associated with motivation across different intensities of motivation for RWs in mice. Daily exposure to RWs gradually increased rotation number, then became stable after approximately 3 weeks. Immunohistochemical analyses revealed that the number of c-Fos (a neuronal activity marker)-positive cells increased in the medial prefrontal cortex (mPFC), core and shell of the nucleus accumbens (NAc), dorsal striatum (Str), and lateral septum (LS) at day 1, day 9, and days 20-24, in a time-dependent manner. Additionally, despite exposure to locked RWs for over 7 days after establishing stable rotation with 3-week RW access, increased c-Fos expression was still observed in most of these brain areas. Furthermore, daily overnight RW access developed stable rotation by day 6, with high and low rotation numbers at the start and end of the overnight session, respectively. The number of c-Fos-positive cells at the start of RW rotation was significantly higher than at the end of RW rotation in most brain regions. Furthermore, after establishing stable rotation, the number of c-Fos-positive cells increased in the mPFC and shell of the NAc of mice that only observed RWs. These findings suggest that the subareas of the mPFC and NAc may be critically involved in the motivational effects of RW rotations.

Portable Neuroimaging-Guided Noninvasive Brain Stimulation of the Cortico-Cerebello-Thalamo-Cortical Loop—Hypothesis and Theory in Cannabis Use Disorder

Background: Maladaptive neuroplasticity-related learned response in substance use disorder (SUD) can be ameliorated using noninvasive brain stimulation (NIBS); however, inter-individual variability needs to be addressed for clinical translation. Objective: Our first objective was to develop a hypothesis for NIBS for learned response in SUD based on a competing neurobehavioral decision systems model. The next objective was to develop the theory by conducting a computational simulation of NIBS of the cortico-cerebello-thalamo-cortical (CCTC) loop in cannabis use disorder (CUD)-related dysfunctional “cue-reactivity”—a construct closely related to “craving”—that is a core symptom. Our third objective was to test the feasibility of a neuroimaging-guided rational NIBS approach in healthy humans. Methods: “Cue-reactivity” can be measured using behavioral paradigms and portable neuroimaging, including functional near-infrared spectroscopy (fNIRS) and electroencephalogram (EEG) metrics of sensorimotor gating. Therefore, we conducted a computational simulation of NIBS, including transcranial direct current stimulation (tDCS) and transcranial alternating current stimulation (tACS) of the cerebellar cortex and deep cerebellar nuclei (DCN) of the CCTC loop for its postulated effects on fNIRS and EEG metrics. We also developed a rational neuroimaging-guided NIBS approach for the cerebellar lobule (VII) and prefrontal cortex based on a healthy human study. Results: Simulation of cerebellar tDCS induced gamma oscillations in the cerebral cortex, while transcranial temporal interference stimulation induced a gamma-to-beta frequency shift. A preliminary healthy human study (N = 10) found that 2 mA cerebellar tDCS evoked similar oxyhemoglobin (HbO) response in the range of 5 × 10⁻⁶ M across the cerebellum and PFC brain regions (α = 0.01); however, infra-slow (0.01–0.10 Hz) prefrontal cortex HbO-driven phase–amplitude-coupled (PAC; 4 Hz, ±2 mA (max)) cerebellar tACS evoked HbO levels in the range of 10⁻⁷ M that were statistically different (α = 0.01) across these brain regions. Conclusion: Our healthy human study showed the feasibility of fNIRS of cerebellum and PFC and closed-loop fNIRS-driven ctACS at 4 Hz, which may facilitate cerebellar cognitive function via the frontoparietal network. Future work needs to combine fNIRS with EEG for multi-modal imaging for closed-loop NIBS during operant conditioning.

Effects of retrieval-extinction training on internet gaming disorder

BACKGROUND AND AIMS

Internet gaming disorder (IGD) leads to serious impairments in cognitive functions, and lacks of effective treatments. Cue-induced craving is a hallmark feature of this disease and is associated with addictive memory elements. Memory retrieval-extinction manipulations could interfere with addictive memories and attenuate addictive syndromes, which might be a promising intervention for IGD. The aims of this study were to explore the effect of a memory retrieval-extinction manipulation on gaming cue-induced craving and reward processing in individuals with IGD.

METHODS

A total of 49 individuals (mean age: 20.52 ± 1.58) with IGD underwent a memory retrieval-extinction training (RET) with a 10-min interval (R-10min-E, n = 24) or a RET with a 6-h interval (R-6h-E, n = 25) for two consecutive days. We assessed cue-induced craving pre- and post-RET, and at the 1- and 3-month follow-ups. The neural activities during reward processing were also assessed pre- and post-RET.

RESULTS

Compared with the R-6h-E group, gaming cravings in individuals with IGD were significantly reduced after R-10min-E training at the 3-month follow-up (P < 0.05). Moreover, neural activities in the individuals with IGD were also altered after R-10min-E training, which was corroborated by enhanced reward processing, such as faster responses (P < 0.05) and stronger frontoparietal functional connectivity to monetary reward cues, while the R-6h-E training had no effects.

DISCUSSION AND CONCLUSIONS

The two-day R-10min-E training reduced addicts' craving for Internet games, restored monetary reward processing in IGD individuals, and maintained long-term efficacy.

Extended amygdala, conditioned withdrawal and memory consolidation

Opioid withdrawal can be associated to environmental cues through classical conditioning. Exposure to these cues can precipitate a state of conditioned withdrawal in abstinent subjects, and there are suggestions that conditioned withdrawal can perpetuate the addiction cycle in part by promoting the storage of memories. This review discusses evidence supporting the hypothesis that conditioned withdrawal facilitates memory consolidation by activating a neurocircuitry that involves the extended amygdala. Specifically, the central amygdala, the bed nucleus of the stria terminalis, and the nucleus accumbens shell interact functionally during withdrawal, mediate expression of conditioned responses, and are implicated in memory consolidation. From this perspective, the extended amygdala could be a neural pathway by which drug-seeking behaviour performed during a state of conditioned withdrawal is more likely to become habitual and persistent.

Instrumental and Pavlovian Mechanisms in Alcohol Use Disorder

Purpose of Review

Current theories of alcohol use disorders (AUD) highlight the importance of Pavlovian and instrumental learning processes mainly based on preclinical animal studies. Here, we summarize available evidence for alterations of those processes in human participants with AUD with a focus on habitual versus goal-directed instrumental learning, Pavlovian conditioning, and Pavlovian-to-instrumental transfer (PIT) paradigms.

Recent Findings

The balance between habitual and goal-directed control in AUD participants has been studied using outcome devaluation or sequential decision-making procedures, which have found some evidence of reduced goal-directed/model-based control, but little evidence for stronger habitual responding. The employed Pavlovian learning and PIT paradigms have shown considerable differences regarding experimental procedures, e.g., alcohol-related or conventional reinforcers or stimuli.

Summary

While studies of basic learning processes in human participants with AUD support a role of Pavlovian and instrumental learning mechanisms in the development and maintenance of drug addiction, current studies are characterized by large variability regarding methodology, sample characteristics, and results, and translation from animal paradigms to human research remains challenging. Longitudinal approaches with reliable and ecologically valid paradigms of Pavlovian and instrumental processes, including alcohol-related cues and outcomes, are warranted and should be combined with state-of-the-art imaging techniques, computational approaches, and ecological momentary assessment methods.

Learned Immunosuppressive Placebo Response Attenuates Disease Progression in a Rodent Model of Rheumatoid Arthritis

OBJECTIVE

Patients with chronic inflammatory autoimmune diseases benefit from a broad spectrum of immunosuppressive and antiproliferative medication available today. However, nearly all of these therapeutic compounds have unwanted toxic side effects. Recent knowledge about the neurobiology of placebo responses indicates that associative learning procedures can be utilized for dose reduction in immunopharmacotherapy while simultaneously maintaining treatment efficacy. This study was undertaken to examine whether and to what extent a 75% reduction of pharmacologic medication in combination with learned immunosuppression affects the clinical outcome in a rodent model of type II collagen-induced arthritis.

METHODS

An established protocol of taste-immune conditioning was applied in a disease model of chronic inflammatory autoimmune disease (type II collagen-induced arthritis) in rats, where a novel taste (saccharin; conditioned stimulus [CS]) was paired with an injection of the immunosuppressive drug cyclosporin A (CSA) (unconditioned stimulus [US]). Following conditioning with 3 CS/US pairings (acquisition), the animals were immunized with type II collagen and Freund's incomplete adjuvant. Fourteen days later, at the first occurrence of clinical symptoms, retrieval was started by presenting the CS together with low-dose CSA as reminder cues to prevent the conditioned response from being extinguished.

RESULTS

This "memory-updating" procedure stabilized the learned immune response and significantly suppressed disease progression in immunized rats. Clinical arthritis score and histologic inflammatory symptoms (both P < 0.05) were significantly diminished by learned immunosuppression in combination with low-dose CSA (25% of the full therapeutic dose) via β-adrenoceptor-dependent mechanisms, to the same extent as with full-dose (100%) pharmacologic treatment.

CONCLUSION

These results indicate that learned immunosuppression appears to be mediated via β-adrenoceptors and might be beneficial as a supportive regimen in the treatment of chronic inflammatory autoimmune diseases by diminishing disease exacerbation.

Pavlovian Conditioning of Immunological and Neuroendocrine Functions

The phenomenon of behaviorally conditioned immunological and neuroendocrine functions has been investigated for the past 100 yr. The observation that associative learning processes can modify peripheral immune functions was first reported and investigated by Ivan Petrovic Pavlov and his co-workers. Their work later fell into oblivion, also because so little was known about the immune system’s function and even less about the underlying mechanisms of how learning, a central nervous system activity, could affect peripheral immune responses. With the employment of a taste-avoidance paradigm in rats, this phenomenon was rediscovered 45 yr ago as one of the most fascinating examples of the reciprocal functional interaction between behavior, the brain, and peripheral immune functions, and it established psychoneuroimmunology as a new research field. Relying on growing knowledge about efferent and afferent communication pathways between the brain, neuroendocrine system, primary and secondary immune organs, and immunocompetent cells, experimental animal studies demonstrate that cellular and humoral immune and neuroendocrine functions can be modulated via associative learning protocols. These (from the classical perspective) learned immune responses are clinically relevant, since they affect the development and progression of immune-related diseases and, more importantly, are also inducible in humans. The increased knowledge about the neuropsychological machinery steering learning and memory processes together with recent insight into the mechanisms mediating placebo responses provide fascinating perspectives to exploit these learned immune and neuroendocrine responses as supportive therapies, the aim being to reduce the amount of medication required, diminishing unwanted drug side effects while maximizing the therapeutic effect for the patient’s benefit.

The Opioid-Addicted Tetrapartite Synapse

Opioid administration in preclinical models induces long-lasting adaptations in reward and habit circuitry. The latest research demonstrates that in the nucleus accumbens, opioid-induced excitatory synaptic plasticity involves presynaptic and postsynaptic elements as well as adjacent astroglial processes and the perisynaptic extracellular matrix. We outline opioid-induced modifications within each component of the tetrapartite synapse and provide a neurobiological perspective on how these adaptations converge to produce addiction-related behaviors in rodent models. By incorporating changes observed at each of the excitatory synaptic compartments into a unified framework of opioid-induced glutamate dysregulation, we highlight new avenues for restoring synaptic homeostasis that might limit opioid craving and relapse vulnerability.

α-Conotoxin TxIB: A Uniquely Selective Ligand for α6/α3β2β3 Nicotinic Acetylcholine Receptor Attenuates Nicotine-Induced Conditioned Place Preference in Mice

α-Conotoxin TxIB is a specific antagonist of α6/α3β2β3(α6β2*) nicotinic acetylcholine receptor (nAChR) with an IC₅₀ of 28 nM. Previous studies have shown that α6β2* nAChRs are abundantly expressed in midbrain dopaminergic neurons and play an important role in mediating the mechanism of nicotine and other drugs reward effect. It provided important targets for the development of anti-addiction drugs. The present study evaluated the pharmacological activity of TxIB in vivo with conditioned place preference (CPP) model, which were induced by subcutaneous injection (s.c.) of nicotine (NIC, 0.5 mg/kg). α-Conotoxin TxIB inhibited the expression and reinstatement of CPP in mice dose-dependently, but had no significant effect on locomotor activity. The concentrations of dopamine (DA), γ-aminobutyric acid (GABA) and noradrenaline (NE) in different brain regions were measured by enzyme-linked immunosorbent assay (ELISA). We found that TxIB could inhibit the concentrations of DA, GABA and NE in different brain regions (such as nucleus accumbens (NAc), hippocampus (HIP) and prefrontal cortex (PFC)) in NIC-induced mice. The concentrations of DA and NE were decreased in ventral tegmental area (VTA), while GABA had little change. The current work described the inhibition activity of TxIB in NIC-induced CPP, suggesting that α6β2* nAChR-targeted compound may be a promising drug for nicotine addiction treatment.

Neuroplasticity transcript profile of the ventral striatum in the extinction of opioid-induced conditioned place preference

Persistent drug-seeking behavior has been associated with deficits in neural circuits that regulate the extinction of addictive behaviors. Although there is extensive data that associates addiction phases with neuroplasticity changes in the reward circuit, little is known about the underlying mechanisms of extinction learning of opioid associated cues. Here, we combined morphine-conditioned place preference (CPP) with real-time polymerase chain reaction (RT-PCR) to identify the effects of extinction training on the expression of genes (mRNAs) associated with synaptic plasticity and opioid receptors in the ventral striatum/nucleus accumbens (VS/NAc). Following morphine extinction training, we identified two animal subgroups showing either extinction (low CPP) or extinction-resistance (high CPP). A third group were conditioned to morphine but did not receive extinction training (sham-extinction; high CPP). RT-PCR results showed that brain derived neurotrophic factor (Bdnf) was upregulated in rats showing successful extinction. Conversely, the lack of extinction training (sham-extinction) upregulated genes associated with kinases (Camk2g), neurotrophins (Ngfr), synaptic connectivity factors (Ephb2), glutamate neurotransmission (Grm8) and opioid receptors (μ1, Δ1). To further identify genes modulated by morphine itself, comparisons with their saline-counterparts were performed. Results revealed that Bdnf was consistently upregulated in the extinction group. Alternatively, widespread gene modulation was observed in the group with lack of extinction training (i.e. Drd2, Cnr1, Creb, μ1, Δ1) and the group showing extinction resistance (i.e. Crem, Rheb, Tnfa). Together, our study builds on the identification of putative genetic markers for the extinction learning of drug-associated cues.

Arc reactivity in accumbens nucleus, amygdala and hippocampus differentiates cue over context responses during reactivation of opiate withdrawal memory

Opiate withdrawal induces an early aversive state which can be associated to contexts and/or cues, and re-exposure to either these contexts or cues may participate in craving and relapse. Nucleus accumbens (NAC), hippocampus (HPC) and basolateral amygdala (BLA) are crucial substrates for acute opiate withdrawal, and for withdrawal memory retrieval. Also HPC and BLA interacting with the NAC are suggested to respectively mediate the processing of context and cue representations of drug-related memories. Here we used a paradigm of conditioned suppression of operant food seeking, allowing to differentiate context and cue related responses, to study the influence of withdrawal memories on operant behavior and the underlying neural substrates. catFISH for Arc mRNA expression was used to discriminate cellular responses during context and cue (flashing light) periods in this paradigm. We show that reactivation of the memory of the negative affective state of withdrawal suppresses active lever pressing for food, and this conditioned suppression is generalized to the context. Interestingly the behavioral responses during the context and cue light periods are associated with differential Arc mRNA activations within the NAC, BLA, and HPC. Indeed both periods led to NAC shell activation whereas the NAC core was responsive only following the cue light period. Moreover, BLA and HPC were more responsive during cue-light and context period respectively. These data further support the already reported differential role of these brain structures on cue vs context-induced reinstatement of operant behaviors, and highlight the existence of common mechanisms for the processing of positive and aversive emotional memories.

Cue Exposure Therapy for Alcohol Use Disorders: What Can Be Learned from Exposure Therapy for Anxiety Disorders?

Background: Cue Exposure Therapy (CET) has shown efficacy for treating alcohol use disorders (AUDs). Exposure therapy is highly effective for treating anxiety. Both techniques involve repeated, controlled exposures to alcohol or fear-related stimuli. Objectives: We considered the mechanisms of CET for AUDs by comparing it to exposure therapy for anxiety. Method: We conducted a narrative review contrasting theoretical literature examining the mechanisms of CET versus exposure therapy for anxiety. We reviewed RCTs and acute laboratory paradigms examining CET for AUDs. We considered common areas of emerging research, including the use of d-Cycloserine (DCS) and virtual reality (VR). Results: We found evidence that exposure therapy and CET at least partially achieve their effects through extinction learning. We found evidence that CET for AUDs is effective, with comparable benefits to other effective psychosocial treatments. DCS and VR have shown some limited success for augmenting CET for AUDs, so further research is needed to determine their value. Conclusions: There are theoretical and practical similarities between exposure to fear cues and cues of addiction, especially regarding extinction learning. However, these processes are also unique, particularly regarding the differing motivational properties of fear versus reward-related stimuli. We propose that unlike exposure for anxiety, CET takes effect by increasing self-control with each unreinforced exposure. We consider reasons for CET's limited use for AUDs, including its lower acceptability to clients and clinicians. We also note the limited evidence for CET for other substance use disorders, highlighting the need for continued investigation into its mechanisms and efficacy.

Brain and Cognition for Addiction Medicine: From Prevention to Recovery Neural Substrates for Treatment of Psychostimulant-Induced Cognitive Deficits

Addiction to psychostimulants like cocaine, methamphetamine, and nicotine poses a continuing medical and social challenge both in the United States and all over the world. Despite a desire to quit drug use, return to drug use after a period of abstinence is a common problem among individuals dependent on psychostimulants. Recovery for psychostimulant drug-dependent individuals is particularly challenging because psychostimulant drugs induce significant changes in brain regions associated with cognitive functions leading to cognitive deficits. These cognitive deficits include impairments in learning/memory, poor decision making, and impaired control of behavioral output. Importantly, these drug-induced cognitive deficits often impact adherence to addiction treatment programs and predispose abstinent addicts to drug use relapse. Additionally, these cognitive deficits impact effective social and professional rehabilitation of abstinent addicts. The goal of this paper is to review neural substrates based on animal studies that could be pharmacologically targeted to reverse psychostimulant-induced cognitive deficits such as impulsivity and impairment in learning and memory. Further, the review will discuss neural substrates that could be used to facilitate extinction learning and thus reduce emotional and behavioral responses to drug-associated cues. Moreover, the review will discuss some non-pharmacological approaches that could be used either alone or in combination with pharmacological compounds to treat the above-mentioned cognitive deficits. Psychostimulant addiction treatment, which includes treatment for cognitive deficits, will help promote abstinence and allow for better rehabilitation and integration of abstinent individuals into society.

Modulating reconsolidation and extinction to regulate drug reward memory

Drug addiction is an aberrant memory that shares the same memory processes as other memories. Brief exposure to drug-associated cues could result in reconsolidation, a hypothetical process during which original memory could be updated. In contrast, longer exposure times to drug-associated cues could trigger extinction, a process that decreases the conditioned responding. In this review, we discuss the pharmacological and non-pharmacological manipulations on the reconsolidation and extinction that could be used to interfere with drug reward memories. Pharmacological agents such as β-adrenergic receptor antagonist propranolol can interfere with reconsolidation to disrupt drug reward memory. Pharmacological agents such as the NMDA receptor glycine site agonists d-cycloserine and d-serine can facilitate extinction and then attenuate the expression of drug reward memory. Besides pharmacological interventions, drug-free behavioral approaches by utilizing the reconsolidation and extinction, such as 'post-retrieval extinction' and 'UCS-retrieval extinction', are also effective to erase or inhibit the recall of drug reward memory. Taken together, pharmacological modulation and non-pharmacological modulation of reconsolidation and extinction are promising approaches to regulate drug reward memory and prevent relapse.

Divergent prefrontal dopaminergic mechanisms mediate drug- and fear-associated cue extinction during adolescence versus adulthood

Cue-associated learning is vital to guiding behaviour for survival. Adolescence represents a key developmental stage for perturbations in cue-related learning, including a characteristic deficit in cue extinction learning. The present review summarizes evidence from animal and human literature that cue extinction is critically mediated by prefrontal dopamine, a system that undergoes dramatic reorganization during adolescence. We propose that extinction learning and memory is governed by a developmentally dynamic balance of dopamine receptors in the prefrontal cortex, which changes across adolescence into adulthood. This is contrary to the previous idea that extinction deficits during adolescence reflect inefficiency in the same neural circuitry as adults. This leads to proposal of the novel theory that cue extinction involves divergent prefrontal dopaminergic mechanisms depending on the age of extinction.

Memory retrieval in addiction: a role for miR-105-mediated regulation of D1 receptors in mPFC neurons projecting to the basolateral amygdala

Background

Drug addiction is a chronic brain disorder characterized by the compulsive use of drugs. The study of chronic morphine-induced adaptation in the brain and its functional significance is of importance to understand the mechanism of morphine addiction. Previous studies have found a number of chronic morphine-induced adaptive changes at molecular levels in the brain. A study from our lab showed that chronic morphine-induced increases in the expression of D1 receptors at presynaptic terminals coming from other structures to the basolateral amygdala (BLA) played an important role in environmental cue-induced retrieval of morphine withdrawal memory. However, the neurocircuitry where the increased D1 receptors are located and how chronic morphine increases D1 receptor expression in specific neurocircuits remain to be elucidated.

Results

Our results show that chronic morphine induces a persistent increase in D1 receptor expression in glutamatergic terminals of projection neurons from the medial prefrontal cortex (mPFC) to the BLA, but has no influence on D1 receptor expression in projection neurons from the hippocampus or the thalamus to the BLA. This adaptation to chronic morphine is mediated by reduced expression of miR-105 in the mPFC, which results in enhanced D1 receptor expression in glutamatergic terminals of projection neurons from the mPFC to the BLA. Ex vivo optogenetic experiments show that a chronic morphine-induced increase in D1 receptor expression in glutamatergic terminals of projection neurons from the mPFC to the BLA results in sensitization of the effect of D1 receptor agonist on presynaptic glutamate release. mPFC to BLA projection neurons are activated by withdrawal-associated environmental cues in morphine-withdrawal rats, and overexpression of miR-105 in the mPFC leads to reduced D1 receptor induction in response to chronic morphine in glutamatergic terminals of the projection neurons from the mPFC to the BLA, and a reduction in place aversion conditioned by morphine withdrawal.

Conclusions

These results suggest that chronic morphine use induces a persistent increase in D1 receptors in glutamatergic terminals of projection neurons from the mPFC to the BLA via downregulation of miR-105 in the mPFC, and that these adaptive changes contribute to environmental cue-induced retrieval of morphine withdrawal memory.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-017-0467-2) contains supplementary material, which is available to authorized users.

The Role of Serine Racemase in the Pathophysiology of Brain Disorders

The N-methyl-d-aspartate receptor (NMDAR) is unique in requiring two agonists to bind simultaneously to open its cation channel: the neurotransmitter, glutamate, and the coagonists, glycine, or d-serine. The Snyder laboratory was the first to clone serine racemase (SR), the enzyme that synthesizes d-serine, and to localize it immunocytochemically. Our laboratory has focused on the role of d-serine in brain disorders. Silencing the expression of SR, a risk gene for schizophrenia (SCZ), in mice (SR-/-), results in a phenotype that closely resembles SCZ including: cortical atrophy, reduced dendritic spine density and complexity, downregulation of parvalbumin-positive cortical GABAergic neurons, and cognitive impairments. This pathology can be reversed by treatment of SR-/- mice with d-serine in adulthood. SR-/- mice also exhibit abnormal response toward abusable substances, such as stimulants. They show reduced behavioral sensitization to d-amphetamine, but fail to extinguish it. Place preference to cocaine is altered, and the hedonic response to it is profoundly impaired as assessed by intracranial self-stimulation. d-cycloserine, a partial agonist at the NMDAR glycine modulatory site, shows therapeutic benefit for treating pathologic anxiety in combination with behavioral therapies. Studies in vitro with cortical culture and in vivo with middle cerebral artery occlusion show that silencing SR provides substantial protection against ischemic neuronal death. Finally, the switch of SR expression from neurons to reactive astrocytes after closed head trauma accounts for the reduced in vivo neuroplasticity, electroencephalogram abnormalities, and cognitive impairments.

Prefrontal Glx and GABA concentrations and impulsivity in cigarette smokers and smoking polysubstance users

Glutamate and GABA play an important role in substance dependence. However, it remains unclear whether this holds true for different substance use disorders and how this is related to risk-related traits such as impulsivity. We, therefore, compared Glx (as a proxy measure for glutamate) and GABA concentrations in the dorsal anterior cingulate cortex (dACC) of 48 male cigarette smokers, 61 male smoking polysubstance users, and 90 male healthy controls, and investigated the relationship with self-reported impulsivity and substance use. Glx and GABA concentrations were measured using proton Magnetic Resonance Spectroscopy. Impulsivity, smoking, alcohol and cocaine use severity and cannabis use were measured using self-report instruments. Results indicate a trend towards group differences in Glx. Post-hoc analyses showed a difference between smokers and healthy controls (p=0.04) and a trend towards higher concentrations in smoking polysubstance users and healthy controls (p=0.09), but no differences between smokers and smoking polysubstance users. dACC GABA concentrations were not significantly different between groups. Smoking polysubstance users were more impulsive than smokers, and both groups were more impulsive than controls. No significant associations were observed between dACC neurotransmitter concentrations and impulsivity and level and severity of smoking, alcohol or cocaine use or the presence of cannabis use. The results indicate that differences in dACC Glx are unrelated to type and level of substance use. No final conclusion can be drawn on the lack of GABA differences due to assessment difficulties. The relationship between dACC neurotransmitter concentrations and cognitive impairments other than self-reported impulsivity should be further investigated.

Naloxone effects on extinction of ethanol- and cocaine-induced conditioned place preference in mice

RATIONALE

Previous studies found that naloxone (NLX) facilitated choice extinction of ethanol conditioned place preference (CPP) using long (60 min) test sessions, but there is little information on the variables determining this effect.

OBJECTIVES

These studies examined repeated exposure to NLX during extinction of ethanol- or cocaine-induced CPP using both short and long tests.

METHODS

DBA/2J mice were injected with NLX (0 or 10 mg/kg) before three 10- or 60-min choice extinction tests (experiment 1). All mice received a final 60-min test without NLX. Post-test NLX was given in experiment 2. Experiment 3 tested whether NLX would affect a forced extinction procedure. Experiment 4 tested its effect on extinction of cocaine-induced CPP.

RESULTS

Pre-test (but not post-test) injections of NLX-facilitated choice extinction of ethanol CPP at both test durations. Pre-test NLX also facilitated forced extinction. However, pre-test NLX had no effect on choice extinction of cocaine CPP.

CONCLUSIONS

Extinction test duration is not critical for engaging the opioid system during ethanol CPP extinction (experiment 1). Moreover, NLX's effect does not depend on CPP expression during extinction, just exposure to previously conditioned cues (experiment 3). The null effect of post-test NLX eliminates a memory consolidation interpretation (experiment 2) and the failure to alter cocaine CPP extinction argues against alteration of general learning or memory processes (experiment 4). Overall, these data suggest that the endogenous opioid system mediates a conditioned motivational effect that normally maintains alcohol-induced seeking behavior, which may underlie the efficacy of opiate antagonists in the treatment of alcoholism.

AMPA receptors control fear extinction through an Arc-dependent mechanism

Activity-regulated cytoskeleton-associated protein (Arc) supports fear memory through synaptic plasticity events requiring actin cytoskeleton rearrangements. We have previously shown that reducing hippocampal Arc levels through antisense knockdown leads to the premature extinction of contextual fear. Here we show that the AMPA receptor antagonist CNQX elevates hippocampal Arc levels during extinction and blocks extinction that can be rescued by reducing Arc. Increasing Arc levels with CNQX also overcomes the actin-destabilizing properties of cytochalasin D and promotes extinction. Therefore, extinction is dependent on AMPA-mediated reductions of Arc via a mechanism consistent with a role for Arc in stabilizing the actin cytoskeleton to constrain extinction.

Applications and limitations of behaviorally conditioned immunopharmacological responses

The importance of placebo responses for the treatment of various medical conditions has increasingly been recognized, whereas knowledge and systematic application in clinical settings are still sparse. One possible application for placebo responses in pharmacotherapy is given by learning paradigms, such as behaviorally conditioned immunosuppression, aiming at drug dose reduction while maintaining therapeutic efficacy of drug treatment. In an established learning paradigm of conditioned taste aversion/avoidance (CTA) in both, rats and humans, respectively, a novel-tasting drinking solution (conditioned stimulus, CS) is paired with an injection of the immunosuppressive drug cyclosporine A (CsA) as unconditioned stimulus (US). The conditioned response, evoked by re-presenting the CS alone at a later time, is reflected by avoidance behavior of consuming the solution (conditioned taste aversion; CTA) and a diminished interleukin (IL)-2 and interferon (IFN)-γ cytokine production as well as mRNA expression of rat splenic T cells or human peripheral T lymphocytes, closely mimicking the immunosuppressive effects of CsA. However, due to unreinforced CS-re-exposure conditioned responses progressively decreases over time (extinction), reflecting a considerable challenge for potential clinical applications of this learned immunosuppression. The present article discusses and critically reviews actual approaches, applications but also limitations of learning paradigms in immune pharmacotherapy.

Recent developments in the behavioural and pharmacological enhancement of extinction of drug seeking

One of the principal barriers to overcoming addiction is the propensity to relapse, even after months or years of abstinence. Relapse can be precipitated by cues and contexts associated with drug use; thus, decreasing the conditioned properties of these cues and contexts may assist in preventing relapse. The predictive power of drug cues and contexts can be reduced by repeatedly presenting them in the absence of the drug reinforcer, a process known as extinction. The potential of extinction to limit relapse has generated considerable interest and research over the past few decades. While pre-clinical animal models suggest extinction learning assists relapse prevention, treatment efficacy is often lacking when extinction learning principles are translated into clinical trials. Conklin and Tiffany (Addiction, 2002) suggest the lack of efficacy in clinical practice may be due to limited translation of procedures demonstrated through animal research and propose several methodological improvements to enhance extinction learning for drug addiction. This review will examine recent advances in the behavioural and pharmacological manipulation of extinction learning, based on research from pre-clinical models. In addition, the translation of pre-clinical findings-both those suggested by Conklin and Tiffany () and novel demonstrations from the past 13 years-into clinical trials and the efficacy of these methods in reducing craving and relapse, where available, will be discussed. Finally, we highlight areas where promising pre-clinical models have not yet been integrated into current clinical practice but, if applied, could improve upon existing behavioural and pharmacological methods.

Effects of sex and remifentanil dose on rats' acquisition of responding for a remifentanil-conditioned reinforcer

Opioid-conditioned reinforcement is thought to exacerbate opioid abuse and dependence. Sex/gender can influence opioid abuse behaviors, but the effects of sex/gender on opioid-conditioned reinforcement, specifically, are unclear. In this study, we compared new-response acquisition with opioid-conditioned reinforcement in male and female rats. First, separate groups received response-independent remifentanil injections (0.0-32.0 μg/kg, intravenous) and presentations of a light-noise stimulus. In the experimental groups, injections and stimulus presentations always co-occurred [paired Pavlovian conditioning (PAV)]; in the control groups, the two occurred independently of each other (random PAV). Next, in the instrumental acquisition (ACQ) sessions, two novel nose-poke manipulanda were introduced. All animals (regardless of sex, dose, and PAV type) could respond in the active nose-poke, which produced the stimulus alone, or in the inactive nose-poke. Both males and females dose-dependently acquired nose-poke responding (active>inactive) after paired PAV, but not after random PAV. Therefore, the stimulus was a conditioned reinforcer. We identified three sex differences. First, only females acquired responding after paired PAV with 32.0 μg/kg remifentanil. Second, using a progressive ratio schedule for ACQ, both sexes acquired responding, but females made significantly more active responses. Third, when a single session of PAV was conducted, only males acquired responding. Thus, rats' sex interacts with pharmacological and environmental factors to determine opioid-conditioned reinforcement.

Open-field exposure facilitates consummatory extinction

During extinction, the organism learns that a conditioned stimulus or a conditioned response is no longer associated with an unconditioned stimulus, and as a consequence, a decrement in the response is presented. The exposure to novel situations (e.g. exploration of a novel open field) has been used widely to modulate (i.e. either enhance or deteriorate) learning and memory. The aim of the present study was to test whether open-field exposure could modulate consummatory extinction. The results indicated that open-field exposure accelerated the extinction response (i.e. experimental animals provided novelty exposure had lower consummatory behavior than control animals) when applied before - but not after - the first extinction trial, or when applied before the second extinction trial. The results suggest that environmental treatments such as novelty exposure provide a valuable, nonpharmacological alternative to potentially modulate extinction processes.

Bidirectional Modulation of Extinction of Drug Seeking by Deep Brain Stimulation of the Ventral Striatum

BACKGROUND

Recent research in humans and rodents has explored the use of deep brain stimulation (DBS) of the ventral capsule/ventral striatum (VS) as a possible treatment for drug addiction. However, the optimum electrode placement and optimum DBS parameters have not been thoroughly studied. Here we varied stimulation sites and frequencies to determine whether DBS of the VS could facilitate the extinction of morphine-induced conditioned place preference in rats.

METHODS

Rats were implanted with DBS electrodes in the dorsal or ventral subregions of the VS and trained to the morphine conditioned place preference. Subsequently, rats received extinction sessions over 9 days, combined with 60 min of either high- (130 Hz) or low- (20 Hz) frequency DBS. To study circuit-wide activations after DBS of the VS, c-fos immunohistochemistry was performed in regions involved in the extinction of drug-seeking behaviors.

RESULTS

High-frequency DBS of the dorsal-VS impaired both extinction training and extinction memory, whereas high-frequency DBS of the ventral-VS had no effect. In contrast, low-frequency DBS of the dorsal-VS strengthened extinction memory when tested 2 or 9 days after the cessation of stimulation. Both DBS frequencies increased c-fos expression in the infralimbic prefrontal cortex, but only low-frequency DBS increased c-fos expression in the basal amygdala and the medial portion of the central amygdala.

CONCLUSIONS

Our results suggest that low-frequency (rather than high-frequency) DBS of the dorsal-VS strengthens extinction memory and may be a potential adjunct for extinction-based therapies for treatment-refractory opioid addiction.

Aversive Counterconditioning Attenuates Reward Signaling in the Ventral Striatum

Appetitive conditioning refers to the process of learning cue-reward associations and is mediated by the mesocorticolimbic system. Appetitive conditioned responses are difficult to extinguish, especially for highly salient reward such as food and drugs. We investigate whether aversive counterconditioning can alter reward reinstatement in the ventral striatum in healthy volunteers using functional magnetic resonance imaging (fMRI). In the initial conditioning phase, two different stimuli were reinforced with a monetary reward. In the subsequent counterconditioning phase, one of these stimuli was paired with an aversive shock to the wrist. In the following extinction phase, none of the stimuli were reinforced. In the final reinstatement phase, reward was reinstated by informing the participants that the monetary gain could be doubled. Our fMRI data revealed that reward signaling in the ventral striatum and ventral tegmental area following reinstatement was smaller for the stimulus that was counterconditioned with an electrical shock, compared to the non-counterconditioned stimulus. A functional connectivity analysis showed that aversive counterconditioning strengthened striatal connectivity with the hippocampus and insula. These results suggest that reward signaling in the ventral striatum can be attenuated through aversive counterconditioning, possibly by concurrent retrieval of the aversive association through enhanced connectivity with hippocampus and insula.

Exaggerated acquisition and resistance to extinction of avoidance behavior in treated heroin-dependent men

OBJECTIVE

Addiction is often conceptualized as a behavioral strategy for avoiding negative experiences. In rodents, opioid intake has been associated with abnormal acquisition and extinction of avoidance behavior. Here, we tested the hypothesis that these findings would generalize to human opioid-dependent subjects.

METHOD

Adults meeting DSM-IV criteria for heroin dependence and treated with opioid medication (n = 27) and healthy controls (n = 26) were recruited between March 2013 and October 2013 and given a computer-based task to assess avoidance behavior. For this task, subjects controlled a spaceship and could either gain points by shooting an enemy spaceship or hide in safe areas to avoid on-screen aversive events. Hiding duration during different periods of the task was used to measure avoidance behavior.

RESULTS

While groups did not differ on escape responding (hiding) during the aversive event, heroin-dependent men (but not women) made more avoidance responses during a warning signal that predicted the aversive event (analysis of variance, sex × group interaction, P = .007). Heroin-dependent men were also slower to extinguish the avoidance response when the aversive event no longer followed the warning signal (P = .011). This behavioral pattern resulted in reduced opportunity to obtain reward without reducing risk of punishment. Results suggest that, in male patients, differences in avoidance behavior cannot be easily explained by impaired task performance or by exaggerated motor activity.

CONCLUSIONS

This study provides evidence for abnormal acquisition and extinction of avoidance behavior in opioid-dependent patients. Interestingly, data suggest that abnormal avoidance is demonstrated only by male patients. Findings shed light on cognitive and behavioral manifestations of opioid addiction and may facilitate development of therapeutic approaches to help affected individuals.

Manipulating a "cocaine engram" in mice

Experience with drugs of abuse (such as cocaine) produces powerful, long-lasting memories that may be important in the development and persistence of drug addiction. The neural mechanisms that mediate how and where these cocaine memories are encoded, consolidated and stored are unknown. Here we used conditioned place preference in mice to examine the precise neural circuits that support the memory of a cocaine-cue association (the "cocaine memory trace" or "cocaine engram"). We found that a small population of neurons (∼10%) in the lateral nucleus of amygdala (LA) were recruited at the time of cocaine-conditioning to become part of this cocaine engram. Neurons with increased levels of the transcription factor CREB were preferentially recruited or allocated to the cocaine engram. Ablating or silencing neurons overexpressing CREB (but not a similar number of random LA neurons) before testing disrupted the expression of a previously acquired cocaine memory, suggesting that neurons overexpressing CREB become a critical hub in what is likely a larger cocaine memory engram. Consistent with theories that coordinated postencoding reactivation of neurons within an engram or cell assembly is crucial for memory consolidation (Marr, 1971; Buzsáki, 1989; Wilson and McNaughton, 1994; McClelland et al., 1995; Girardeau et al., 2009; Dupret et al., 2010; Carr et al., 2011), we also found that post-training suppression, or nondiscriminate activation, of CREB overexpressing neurons impaired consolidation of the cocaine memory. These findings reveal mechanisms underlying how and where drug memories are encoded and stored in the brain and may also inform the development of treatments for drug addiction.

Effects of pramipexole on the acquisition of responding with opioid-conditioned reinforcement in the rat

RATIONALE

Dopamine D3 receptor-preferring ligands may be able to modify the conditioned reinforcing effects of drug-associated stimuli. In evaluating the effects of these compounds, it is important to clarify the extent to which responding depends on (1) conditioned reinforcement vs. other behavioral mechanisms and (2) dopamine D3 vs. D2 receptor activity.

OBJECTIVES

To use behaviorally stringent new-response acquisition procedures to characterize the effects of the D3-preferring agonist, pramipexole, on the conditioned reinforcing effects of a stimulus paired with the opioid agonist, remifentanil.

METHODS

First, in Pavlovian conditioning (PAV) sessions, rats received response-independent IV injections of remifentanil and presentations of a light-noise stimulus. In separate groups, injections and stimuli either always co-occurred ("paired PAV") or occurred with no consistent relationship ("random PAV" control). Next, in instrumental acquisition (ACQ) sessions, all animals could respond in two nose-poke manipulanda: an active nose-poke, which produced the stimulus alone, or an inactive nose-poke. Pramipexole was injected SC prior to ACQ sessions with or without pretreatments of the D3-preferring antagonist, SB-277011A, or the D2-preferring antagonist, L-741,626.

RESULTS

After paired PAV, but not random PAV, rats acquired nose-poke responding during ACQ (i.e., active > inactive). Pramipexole dose-dependently increased active responding without changing inactive responding. Pramipexole-induced increases in responding were blocked by pretreatment with L-741,626, but not SB-277011A.

CONCLUSIONS

Pramipexole specifically enhanced remifentanil-conditioned reinforcement: active responding was selectively increased only after the stimulus was paired with remifentanil. Although pramipexole is D3-preferring, the antagonist effects obtained presently suggest an important role for the D2 receptor in opioid-conditioned reinforcement.

Persistence of a hyperthermic sign-reversal during nitrous oxide inhalation despite cue-exposure treatment with and without a drug-onset cue

We asked whether chronic tolerance and the hyperthermic sign-reversal induced by repeated 60% N2O exposures could be extinguished using a cue-exposure paradigm. Rats received 18 N2O administrations in a total calorimetry system that simultaneously measures core temperature (Tc), metabolic heat production (HP), and body heat loss (HL). Each exposure entailed a 2-h baseline period followed by a 1.5-h N2O exposure. The 18 drug exposures induced a robust intra-administration hyperthermia in which the initial hypothermic effect of N2O inverted to a significant hyperthermic sign-reversal during N2O inhalation due primarily to an acquired robust increase in HP. The rats were then randomized to one of three extinction procedures (n=8/procedure) over a 20-d interval: 1) a N2O-abstinent home-cage group (HC) that received only the usual animal care; 2) a cue-exposure group (CEXP) in which the animals were placed in the calorimeter 8 times but received no N2O; and 3) a drug-onset-cue group (DOC) in which animals received a brief N2O exposure in the calorimeter that mimicked the first 3 min of an actual 60% N2O trial. Following the extinction sessions, all rats received a 60% N2O test trial and Tc, HP and HL were assessed. The hyperthermic sign-reversal remained fully intact during the test trial, with no significant differences observed among groups in any post-baseline change in any thermal outcome. These data suggest that cue exposure may not be an efficacious strategy to reduce sign-reversals that develop with chronic drug use.

Role of cues and contexts on drug-seeking behaviour

Environmental stimuli are powerful mediators of craving and relapse in substance-abuse disorders. This review examined how animal models have been used to investigate the cognitive mechanisms through which cues are able to affect drug-seeking behaviour. We address how animal models can describe the way drug-associated cues come to facilitate the development and persistence of drug taking, as well as how these cues are critical to the tendency to relapse that characterizes substance-abuse disorders. Drug-associated cues acquire properties of conditioned reinforcement, incentive motivation and discriminative control, which allow them to influence drug-seeking behaviour. Using these models, researchers have been able to investigate the pharmacology subserving the behavioural impact of environmental stimuli, some of which we highlight. Subsequently, we examine whether the impact of drug-associated stimuli can be attenuated via a process of extinction, and how this question is addressed in the laboratory. We discuss how preclinical research has been translated into behavioural therapies targeting substance abuse, as well as highlight potential developments to therapies that might produce more enduring changes in behaviour.