Limbic and motor circuitry underlying footshock-induced reinstatement of cocaine-seeking behavior

Executive Control and Striatal Resting-State Network Interact with Risk Factors to Influence Treatment Outcomes in Alcohol-Use Disorder

Alterations within mesocorticolimbic terminal regions commonly occur with alcohol use disorder (AUD). As pathological drug-seeking behavior may arise as a consequence of alcohol-induced neuroadaptations, it is critical to understand how such changes increase the likelihood of relapse. This report examined resting-state functional connectivity (RSFC) using both a seed-based and model-free approach in individuals in treatment for AUD and how dysregulation of network connectivity contributes to treatment outcomes. In order to provide a mechanism by which neural networks promote relapse, interactive effects of mesocorticolimbic connectivity and AUD risk factors in treatment completers and non-completers were examined. AUD group showed stronger RSFC between striatum, insula, and anterior cingulate cortex than controls. Within the AUD group, non-completers compared to completers showed enhanced RSFC between (1) striatum-insula, (2) executive control network (ECN)-amygdala, and (3) basal ganglia/salience network and striatum, precuneus, and insula. Completers showed enhanced RSFC between striatum-right dorsolateral prefrontal cortex. Furthermore, completers and non-completers differed in relationships between RSFC and relapse risk factors, where non-completers exhibited positive associations between craving intensity and RSFC of striatum-insula and ECN-amygdala. These findings provide evidence for interactions between corticolimbic connectivity in AUD and craving and establish an important link between network connectivity and dynamic risk factors that contribute to relapse. Results demonstrate that relapse vulnerability is attributed to craving dysregulation manifested by enhanced connectivity in striato-limbic regions and diminished corticostriatal connectivity.

Ventral Pallidum Output Pathways in Context-Induced Reinstatement of Alcohol Seeking

Ventral pallidum (VP) is a well-established locus for the reinforcing effects of drugs of abuse and reinstatement of drug seeking. However, VP neurons are at the origin of multiple output pathways, with strong projections to ventral tegmental area (VTA), subthalamic nucleus (STN), lateral hypothalamus, among others, and the roles of these VP output pathways in reinstatement of drug seeking remain poorly understood. Here we addressed these issues using a combination of neuroanatomical tracing and chemogenetic approaches. First, using dual-retrograde tracing, we show that VP neurons projecting to either VTA or STN are recruited during context-induced reinstatement of extinguished alcohol seeking in rats. Then, using chemogenetics, we show modulation of context-induced reinstatement and reacquisition of alcohol seeking via designer receptors exclusively activated by designer drugs excitation or inhibition of the VP. To determine the causal roles of VP → VTA and VP → STN pathways in context-induced reinstatement and reacquisition we used a chemogenetic disconnection approach and show that silencing either the VP → VTA or VP → STN pathways is sufficient to reduce both reinstatement and reacquisition of alcohol seeking. Moreover, these disconnections also each reduced responding and motivation during a progressive ratio test but had no effect on locomotor activity. Together, these results show that multiple ventral pallidal output pathways contribute to relapse to alcohol seeking.

SIGNIFICANCE STATEMENT

Ventral pallidum (VP) serves important roles in reward and motivation and is a critical node in the neural circuitry for reinstatement of drug seeking. Despite being a common locus for different forms of reinstatement, fundamental aspects of neural circuitry for these VP contributions to reinstatement of drug seeking remain unknown. Here we used a combination of neuroanatomical tracing and chemogenetic approaches to map the VP output pathways for context-induced reinstatement and reacquisition of alcohol seeking. We show that VP output pathways to the subthalamic nucleus and also to the ventral tegmental area are necessary for these forms of reinstatement.

How contexts promote and prevent relapse to drug seeking

The contexts where drugs are self-administered play an important role in regulating persistent drug taking and in relapse to such taking after periods of abstinence. Here, we review the behavioral and brain mechanisms enabling contexts to promote and prevent relapse to drug seeking. We review the key brain structures, their neuropharmacology and their connectivity. We discuss the similarities and differences between the mechanisms for context-induced reinstatement of drug seeking vs. other forms of relapse to drug seeking in animal models and we highlight the numerous deficits in our understanding. We emphasize that current understanding, although significant, defies explanations in terms of models at the level of brain structures and their connectivity. Rather, we show that there is significant functional compartmentalization and segregation within these structures during reinstatement and extinction of drug seeking that parallels their anatomical segregation into circuits and channels. A key challenge is to recognize this complexity, understand how these circuits and channels are organized, as well as understand how different modes of activity of ensembles of neurons within them promote abstinence or relapse to drug seeking.

Diversity of Dopaminergic Neural Circuits in Response to Drug Exposure

Addictive substances are known to increase dopaminergic signaling in the mesocorticolimbic system. The origin of this dopamine (DA) signaling originates in the ventral tegmental area (VTA), which sends afferents to various targets, including the nucleus accumbens, the medial prefrontal cortex, and the basolateral amygdala. VTA DA neurons mediate stimuli saliency and goal-directed behaviors. These neurons undergo robust drug-induced intrinsic and extrinsic synaptic mechanisms following acute and chronic drug exposure, which are part of brain-wide adaptations that ultimately lead to the transition into a drug-dependent state. Interestingly, recent investigations of the differential subpopulations of VTA DA neurons have revealed projection-specific functional roles in mediating reward, aversion, and stress. It is now critical to view drug-induced neuroadaptations from a circuit-level perspective to gain insight into how differential dopaminergic adaptations and signaling to targets of the mesocorticolimbic system mediates drug reward. This review hopes to describe the projection-specific intrinsic characteristics of these subpopulations, the differential afferent inputs onto these VTA DA neuron subpopulations, and consolidate findings of drug-induced plasticity of VTA DA neurons and highlight the importance of future projection-based studies of this system.

NMDA antagonist MK 801 in nucleus accumbens core but not shell disrupts the restraint stress-induced reinstatement of extinguished cocaine-conditioned place preference in rats

Relapse is a common feature of cocaine addiction. In rodents, it can be elicited by cues, stress or the drug. Restraint stress-induced reinstatement of cocaine-conditioned place preference (CPP) is a useful model to study the mechanisms involved in stress-induced relapse of drug-seeking behavior. There is evidence that the glutamate NMDA receptors are critically involved in drug- and cue-induced reinstatement of seeking behavior and drug-CPP responses. The aim of this study was to investigate the contribution of NMDA receptors within core vs. shell nucleus accumbens (NAc) subregions to restraint stress-induced reinstatement of extinguished cocaine-CPP. After extinction of cocaine-conditioned preference, animals were administered MK 801 systemically or directly into intra-core or intra-shell, and restrained for 30min or left undisturbed in their home-cages. First, we demonstrated that restraint stress-induced reinstatement of extinguished cocaine-CPP depends on the duration of restraint as well as on the context in which it is applied. Second, this effect was blocked by systemic MK 801 administration either before or after restraint. Third, intra-core but not intra-shell administration abrogated the restraint stress-induced reinstatement. These findings show that NMDA receptors within NAc core, but not shell, play a critical role in restraint stress-induced reinstatement of cocaine-CPP.

Medial Prefrontal Cortical Dopamine Responses During Operant Self-Administration of Sweetened Ethanol

BACKGROUND

Medial prefrontal cortex (mPFC) dysfunction is present in heavy alcohol consumers. Dopamine signaling in mPFC is associated with executive functioning and affects drinking behavior; however, direct measurement of extracellular mPFC dopamine during appetitive and consummatory ethanol (EtOH) self-administration behavior has not been reported.

METHODS

We used in vivo microdialysis in freely behaving, adult, male, Long Evans rats to determine extracellular dopamine concentration in the mPFC during operant self-administration of an EtOH-plus-sucrose or sucrose solution. The model separated appetitive/seeking from consummatory phases of the operant session. Dopamine was also monitored in an untrained handling control group, and dialysate EtOH was measured in the EtOH-drinking group.

RESULTS

Home cage baseline dopamine was lower in rats that experienced a week of drinking sweetened EtOH compared with sucrose-drinking and handling controls. Transfer into the operant chamber and the initiation of consumption stimulated a relatively higher change in dopamine over baseline in the sweetened EtOH group compared with sucrose and handling controls. However, all groups show a dopamine response during transfer into the operant chamber, and the sucrose group had a relatively higher change in dopamine over baseline during initiation of consumption compared with handling controls. The time courses of dopamine and EtOH in the mPFC differ in the EtOH-consuming rats.

CONCLUSIONS

Differences in extracellular mPFC dopamine between EtOH drinkers compared with control groups suggest that mPFC dopamine is involved in the mechanism of operant self-administration of sweetened EtOH and sucrose. Furthermore, the increase in dopamine during consumption is consistent with a role of mPFC dopamine in reward prediction.

Signals from the Fourth Dimension Regulate Drug Relapse

Despite the enormous societal burden of alcohol and drug addiction and abundant research describing drug-induced maladaptive synaptic plasticity, there are few effective strategies for treating substance use disorders. Recent awareness that synaptic plasticity involves astroglia and the extracellular matrix is revealing new possibilities for understanding and treating addiction. We first review constitutive corticostriatal adaptations that are elicited by and shared between all abused drugs from the perspective of tetrapartite synapses, and integrate recent discoveries regarding cell type-specificity in striatal neurons. Next, we describe recent discoveries that drug-seeking is associated with transient synaptic plasticity that requires all four synaptic elements and is shared across drug classes. Finally, we prognosticate how considering tetrapartite synapses can provide new treatment strategies for addiction.

mTOR signalling in the nucleus accumbens shell is critical for augmented effect of TFF3 on behavioural response to cocaine

Neuropeptides play important roles in modulating the rewarding value of abused drugs. Trefoil factor 3 (TFF3) was recently reported to modulate withdrawal syndrome of morphine, but the effects of TFF3 on the cocaine-induced behavioral changes are still elusive. In the present study, cocaine-induced hyperlocomotion and conditioned place preference (CPP) rat paradigms were provided to investigate the role of TFF3 in the reward response to cocaine. High-performance liquid chromatography (HPLC) analysis was used to analyse the dopamine concentration. The results showed that systemic TFF3 administration (0.1 mg/kg i.p.) significantly augmented cocaine- induced hyperlocomotion and CPP formation, without any effects on locomotor activity and aversive or rewarding effects per se. TFF3 significantly augmented the increment of the dopamine concentration in the NAc and the activity of the mTOR signalling pathway induced by acute cocaine exposure (10 mg/kg, i.p.) in the NAc shell, but not the core. The Intra-NAc shell infusion of rapamycin blocked TFF3-induced hyperactivity in cocaine-treatment rats. These findings indicated that TFF3 could potentiate behavioural response to cocaine, which may be associated with regulating dopamine concentration. Furthermore, the findings indicated that mTOR signalling pathway in the NAc shell is important for TFF3-induced enhancement on the cocaine-induced behavioral changes.

Effects of prazosin and doxazosin on yohimbine-induced reinstatement of alcohol seeking in rats

RATIONALE AND OBJECTIVES

Alpha-1 adrenoceptor antagonists, such as prazosin, show promise in treating alcoholism. In rats, prazosin reduces alcohol self-administration and relapse induced by footshock stress and the alpha-2 antagonist yohimbine, but the processes involved in these effects of prazosin are not known. Here, we present studies on the central mechanisms underlying the effects of prazosin on yohimbine-induced reinstatement of alcohol seeking.

METHODS

In experiment 1, we trained rats to self-administer alcohol (12 % w/v, 1 h/day), extinguished their responding, and tested the effects of prazosin, administered ICV (2 and 6 nmol) or systemically (1 mg/kg) on yohimbine (1.25 mg/kg)-induced reinstatement. In experiment 2, we determined potential central sites of action by analyzing effects of prazosin (1 mg/kg) on yohimbine (1.25 mg/kg)-induced Fos expression. In experiment 3, we determined the effects of doxazosin (1.25, 2.5, and 5 mg/kg), an alpha-1 antagonist with a longer half-life on yohimbine-induced reinstatement.

RESULTS

Yohimbine-induced reinstatement of alcohol seeking was reduced significantly by ICV and systemic prazosin (50 and 69 % decreases, respectively). Systemic prazosin reduced yohimbine-induced Fos expression in the prefrontal cortex, accumbens shell, ventral bed nucleus of the stria terminalis, and basolateral amygdala (46-67 % decreases). Doxazosin reduced yohimbine-induced reinstatement of alcohol seeking (78 % decrease).

CONCLUSIONS

Prazosin acts centrally to reduce yohimbine-induced alcohol seeking. The Fos mapping study suggests candidate sites where it may act. Doxazosin is also effective in reducing yohimbine-induced reinstatement. These data provide information on the mechanisms of alpha-1 antagonists on yohimbine-induced alcohol seeking and indicate their further investigation for the treatment of alcoholism.

Inhibition of Lactate Transport Erases Drug Memory and Prevents Drug Relapse

BACKGROUND

Drug memories that associate drug-paired stimuli with the effects of abused drugs contribute to relapse. Exposure to drug-associated contexts causes consolidated drug memories to be in a labile state, during which manipulations can be given to impair drug memories. Although substantial evidence demonstrates the crucial role of neuronal signaling in addiction, little is known about the contribution of astrocyte-neuron communication.

METHODS

Rats were trained for cocaine-induced conditioned place preference (CPP) or self-administration and microinjected with the glycogen phosphorylation inhibitor 1,4-dideoxy-1,4-imino-D-arabinitol into the basolateral amygdala (BLA) immediately after retrieval. The concentration of lactate was measured immediately after retrieval via microdialysis, and the CPP score and number of nosepokes were recorded 24 hours later. Furthermore, we used antisense oligodeoxynucleotides to disrupt the expression of astrocytic lactate transporters (monocarboxylate transporters 1 and 2) in the BLA after retrieval, tested the expression of CPP 1 day later, and injected L-lactate into the BLA 15 minutes before retrieval to rescue the effects of the oligodeoxynucleotides.

RESULTS

Injection of 1,4-dideoxy-1,4-imino-D-arabinitol into the BLA immediately after retrieval prevented the subsequent expression of cocaine-induced CPP, decreased the concentration of lactate in the BLA, and reduced the number of nosepokes for cocaine self-administration. Disrupting the expression of monocarboxylate transporters 1 and 2 in the BLA also caused subsequent deficits in the expression of cocaine-induced CPP, which was rescued by pretreatment with L-lactate.

CONCLUSIONS

Our results suggest that astrocyte-neuron lactate transport in the BLA is critical for the reconsolidation of cocaine memory.

Contemporary approaches to neural circuit manipulation and mapping: focus on reward and addiction

Tying complex psychological processes to precisely defined neural circuits is a major goal of systems and behavioural neuroscience. This is critical for understanding adaptive behaviour, and also how neural systems are altered in states of psychopathology, such as addiction. Efforts to relate psychological processes relevant to addiction to activity within defined neural circuits have been complicated by neural heterogeneity. Recent advances in technology allow for manipulation and mapping of genetically and anatomically defined neurons, which when used in concert with sophisticated behavioural models, have the potential to provide great insight into neural circuit bases of behaviour. Here we discuss contemporary approaches for understanding reward and addiction, with a focus on midbrain dopamine and cortico-striato-pallidal circuits.

MicroRNAs Modulate Interactions between Stress and Risk for Cocaine Addiction

Exposure to stress increases vulnerability to drug abuse, as well as relapse liability in addicted individuals. Chronic drug use alters stress response in a manner that increases drug seeking behaviors and relapse. Drug exposure and withdrawal have been shown to alter stress responses, and corticosteroid mediators of stress have been shown to impact addiction-related brain function and drug-seeking behavior. Despite the documented interplay between stress and substance abuse, the mechanisms by which stress exposure and drug seeking interact remain largely unknown. Recent studies indicate that microRNAs (miRNA) play a significant role in stress modulation as well as addiction-related processes including neurogenesis, synapse development, plasticity, drug acquisition, withdrawal and relapse. MiRNAs are short non-coding RNAs that function as bidirectional epigenetic modulators of gene expression through imperfect sequence targeted degradation and/or translational repression of mRNAs. They serve as dynamic regulators of CNS physiology and pathophysiology, and facilitate rapid and long-lasting changes to complex systems and behaviors. MiRNAs function in glucocorticoid signaling and the mesolimbic dopamine reward system, as well as mood disorders related to drug withdrawal. The literature suggests miRNAs play a pivotal role in the interaction between exposures to stress, addiction-related processes, and negative affective states resulting from extended drug withdrawal. This manuscript reviews recent evidence for the role of miRNAs in the modulation of stress and cocaine responses, and discusses potential mediation of the interaction of these systems by miRNAs. Uncovering the mechanism behind the association of stress and drug taking has the potential to impact the treatment of drug abuse and prevention of relapse. Further comprehension of these complex interactions may provide promising new targets for the treatment of drug addiction.

Transitionality in addiction: A "temporal continuum" hypotheses involving the aberrant motivation, the hedonic dysregulation, and the aberrant learning

Addiction is a chronic compulsion and relapsing disorder. It involves several brain areas and circuits, which encode vary functions such as reward, motivation, and memory. Drug addiction is defined as a "pathological pattern of use of a substance", characterized by the loss of control on drug-taking-related behaviors, the pursuance of those behaviors even in the presence of negative consequences, and a strong motivated activity to assume substances. Three different theories guide experimental research on drug addiction. Each of these theories consider singles features, such as an aberrant motivation, a hedonic dysregulation, and an aberrant habit learning as the main actor to explain the entire process of the addictive behaviors. The major goal of this study is to present a new hypotheses of transitionality from a controlled use to abuse of addictive substances trough the overview of the three different theories, considering all the single features of each single theory together on the same "temporal continuum" from use to abuse of addictive substances. Recently, it has been suggested that common neural systems may be activated by natural and pharmacological stimuli, raising the hypotheses that binge-eating disorders could be considered as addictive behaviors. The second goal of this study is to present evidences in order to highlight a possible psycho-bio-physiological superimposition between drug and "food addiction". Finally, interesting questions are brought up starting from last findings about a theoretical/psycho-bio-physiological superimposition between drug and "food addiction" and their possibly same transitionality along the same "temporal continuum" from use to abuse of addictive substances in order to investigate new therapeutic strategies based on new therapeutic strategies based on the individual moments characterizing the transition from the voluntary intake of substances to the maladaptive addictive behavior.

URB597 inhibits oxidative stress induced by alcohol binging in the prefrontal cortex of adolescent rats

Heavy episodic drinking (binging), which is highly prevalent among teenagers, results in oxidative damage. Because the prefrontal cortex (PFC) is not completely mature in adolescents, this brain region may be more vulnerable to the effects of alcohol during adolescence. As endocannabinoids may protect the immature PFC from the harmful effects of high doses of alcohol, this study investigated the effect of the fatty acid amide hydrolase (FAAH) inhibitor URB597 on oxidative stress induced by acute or chronic binge alcohol intake in adolescent rats. At 40min after intraperitoneal pre-treatment with URB597 (0.3mg/kg) or vehicle (Veh), ethanol (EtOH; 3 or 6g/kg, intragastrically) or distilled water (DW) was administered in 3 consecutive sessions (acute binging) or 3 consecutive sessions over 4 weeks (chronic binging). Oxidative stress in PFC slices in situ was measured by dihydroethidium fluorescence staining. At the higher EtOH dose (6g/kg), pre-treatment with URB597 significantly reduced (p<0.01) the production of superoxide anions in the PFC after acute (42.8% decrease) and chronic binge EtOH consumption (44.9% decrease) compared with pre-treatment with Veh. As URB597 decreases anandamide metabolism, this evidence shows an antioxidant effect of endocannabinoids to suppress acute and chronic binge alcohol intake-induced oxidative stress in the PFC of adolescent rats.

Going and stopping: Dichotomies in behavioral control by the prefrontal cortex

The rodent dorsal medial prefrontal cortex (PFC), specifically the prelimbic cortex (PL), regulates the expression of conditioned fear and behaviors interpreted as reward-seeking. Meanwhile, the ventral medial PFC, namely the infralimbic cortex (IL), is essential to extinction conditioning in both appetitive and aversive domains. Here we review evidence that supports, or refutes, this "PL-go/IL-stop" dichotomy. We focus on the extinction of conditioned fear and the extinction and reinstatement of cocaine- or heroin-reinforced responding. We then synthesize evidence that the PL is essential for developing goal-directed response strategies, while the IL supports habit behavior. Finally, we propose that some functions of the orbital PFC parallel those of the medial PFC in the regulation of response selection. Integration of these discoveries may provide points of intervention for inhibiting untethered drug seeking in drug use disorders, failures in extinction in Post-traumatic Stress Disorder, or co-morbidities between the two.

The Human BNST: Functional Role in Anxiety and Addiction

The consequences of chronic stress on brain structure and function are far reaching. Whereas stress can produce short-term adaptive changes in the brain, chronic stress leads to long-term maladaptive changes that increase vulnerability to psychiatric disorders, such as anxiety and addiction. These two disorders are the most prevalent psychiatric disorders in the United States, and are typically chronic, disabling, and highly comorbid. Emerging evidence implicates a tiny brain region-the bed nucleus of the stria terminalis (BNST)-in the body's stress response and in anxiety and addiction. Rodent studies provide compelling evidence that the BNST plays a central role in sustained threat monitoring, a form of adaptive anxiety, and in the withdrawal and relapse stages of addiction; however, little is known about the role of BNST in humans. Here, we review current evidence for BNST function in humans, including evidence for a role in the production of both adaptive and maladaptive anxiety. We also review preliminary evidence of the role of BNST in addiction in humans. Together, these studies provide a foundation of knowledge about the role of BNST in adaptive anxiety and stress-related disorders. Although the field is in its infancy, future investigations of human BNST function have tremendous potential to illuminate mechanisms underlying stress-related disorders and identify novel neural targets for treatment.

Stress-Induced Reinstatement of Drug Seeking: 20 Years of Progress

In human addicts, drug relapse and craving are often provoked by stress. Since 1995, this clinical scenario has been studied using a rat model of stress-induced reinstatement of drug seeking. Here, we first discuss the generality of stress-induced reinstatement to different drugs of abuse, different stressors, and different behavioral procedures. We also discuss neuropharmacological mechanisms, and brain areas and circuits controlling stress-induced reinstatement of drug seeking. We conclude by discussing results from translational human laboratory studies and clinical trials that were inspired by results from rat studies on stress-induced reinstatement. Our main conclusions are (1) The phenomenon of stress-induced reinstatement, first shown with an intermittent footshock stressor in rats trained to self-administer heroin, generalizes to other abused drugs, including cocaine, methamphetamine, nicotine, and alcohol, and is also observed in the conditioned place preference model in rats and mice. This phenomenon, however, is stressor specific and not all stressors induce reinstatement of drug seeking. (2) Neuropharmacological studies indicate the involvement of corticotropin-releasing factor (CRF), noradrenaline, dopamine, glutamate, kappa/dynorphin, and several other peptide and neurotransmitter systems in stress-induced reinstatement. Neuropharmacology and circuitry studies indicate the involvement of CRF and noradrenaline transmission in bed nucleus of stria terminalis and central amygdala, and dopamine, CRF, kappa/dynorphin, and glutamate transmission in other components of the mesocorticolimbic dopamine system (ventral tegmental area, medial prefrontal cortex, orbitofrontal cortex, and nucleus accumbens). (3) Translational human laboratory studies and a recent clinical trial study show the efficacy of alpha-2 adrenoceptor agonists in decreasing stress-induced drug craving and stress-induced initial heroin lapse.

CB1 receptor antagonism blocks stress-potentiated reinstatement of cocaine seeking in rats

RATIONALE

Under some conditions, stress, rather than directly triggering cocaine seeking, potentiates reinstatement to other stimuli, including a subthreshold cocaine dose. The mechanisms responsible for stress-potentiated reinstatement are not well defined. Endocannabinoid signaling is increased by stress and regulates synaptic transmission in brain regions implicated in motivated behavior.

OBJECTIVES

The objective of this study was to test the hypothesis that cannabinoid type 1 receptor (CB1R) signaling is required for stress-potentiated reinstatement of cocaine seeking in rats.

METHODS

Following i.v. cocaine self-administration (2 h access/day) and extinction in male rats, footshock stress alone does not reinstate cocaine seeking but reinstatement is observed when footshock is followed by an injection of an otherwise subthreshold dose of cocaine (2.5 mg/kg, i.p.). CB1R involvement was tested by systemic administration of the CB1R antagonist AM251 (0, 1, or 3 mg/kg, i.p.) prior to testing for stress-potentiated reinstatement.

RESULTS

Stress-potentiated reinstatement was blocked by both 1 and 3 mg/kg AM251. By contrast, AM251 only attenuated food-reinforced lever pressing at the higher dose (i.e., 3 mg/kg) and did not affect locomotor activity at either dose tested. Neither high-dose cocaine-primed reinstatement (10 mg/kg, i.p.) nor footshock stress-triggered reinstatement following long-access cocaine self-administration (6 h access/day) was affected by AM251 pretreatment. Footshock stress increased concentrations of both endocannabinoids, N-arachidonylethanolamine and 2-arachidonoylglycerol, in regions of the prefrontal cortex.

CONCLUSIONS

These findings demonstrate that footshock stress increases prefrontal cortical endocannabinoids and stress-potentiated reinstatement is CB1R-dependent, suggesting that CB1R is a potential therapeutic target for relapse prevention, particularly in individuals whose cocaine use is stress-related.

Ventral Tegmental Area Afferents and Drug-Dependent Behaviors

Drug-related behaviors in both humans and rodents are commonly thought to arise from aberrant learning processes. Preclinical studies demonstrate that the acquisition and expression of many drug-dependent behaviors involves the ventral tegmental area (VTA), a midbrain structure comprised of dopamine, GABA, and glutamate neurons. Drug experience alters the excitatory and inhibitory synaptic input onto VTA dopamine neurons, suggesting a critical role for VTA afferents in mediating the effects of drugs. In this review, we present evidence implicating the VTA in drug-related behaviors, highlight the diversity of neuronal populations in the VTA, and discuss the behavioral effects of selectively manipulating VTA afferents. Future experiments are needed to determine which VTA afferents and what neuronal populations in the VTA mediate specific drug-dependent behaviors. Further studies are also necessary for identifying the afferent-specific synaptic alterations onto dopamine and non-dopamine neurons in the VTA following drug administration. The identification of neural circuits and adaptations involved with drug-dependent behaviors can highlight potential neural targets for pharmacological and deep brain stimulation interventions to treat substance abuse disorders.

Differential roles of medial prefrontal subregions in the regulation of drug seeking

The prefrontal cortex plays an important role in shaping cognition and behavior. Many studies have shown that medial prefrontal cortex (mPFC) plays a key role in seeking, extinction, and reinstatement of cocaine seeking in rodent models of relapse. Subregions of mPFC appear to play distinct roles in these behaviors, such that the prelimbic cortex (PL) is proposed to drive cocaine seeking and the infralimbic cortex (IL) is proposed to suppress cocaine seeking after extinction. This dichotomy of mPFC function may be a general attribute, as similar dorsal-ventral distinctions exist for expression vs. extinction of fear conditioning. However, other results indicate that the role of mPFC neurons in reward processing is more complex than a simple PL-seek vs. IL-extinguish dichotomy. Both PL and IL have been shown to drive and inhibit drug seeking (and other types of behaviors) depending on a range of factors including the behavioral context, the drug-history of the animal, and the type of drug investigated. This heterogeneity of findings may reflect multiple subcircuits within each of these PFC areas supporting unique functions. It may also reflect the fact that the mPFC plays a multifaceted role in shaping cognition and behavior, including those overlapping with cocaine seeking and extinction. Here we discuss research leading to the hypothesis that dorsal and ventral mPFC differentially control drug seeking and extinction. We also present recent results calling the absolute nature of a PL vs. IL dichotomy into question. Finally, we consider alternate functions for mPFC that correspond less to response execution and inhibition and instead incorporate the complex cognitive behavior for which the mPFC is broadly appreciated.

Antagonism of GABA-B but not GABA-A receptors in the VTA prevents stress- and intra-VTA CRF-induced reinstatement of extinguished cocaine seeking in rats

Stress-induced reinstatement of cocaine seeking requires corticotropin releasing factor (CRF) actions in the ventral tegmental area (VTA). However the mechanisms through which CRF regulates VTA function to promote cocaine use are not fully understood. Here we examined the role of GABAergic neurotransmission in the VTA mediated by GABA-A or GABA-B receptors in the reinstatement of extinguished cocaine seeking by a stressor, uncontrollable intermittent footshock, or bilateral intra-VTA administration of CRF. Rats underwent repeated daily cocaine self-administration (1.0 mg/kg/ing; 14 × 6 h/day) and extinction and were tested for reinstatement in response to footshock (0.5 mA, 0.5" duration, average every 40 s; range 10-70 s) or intra-VTA CRF delivery (500 ng/side) following intra-VTA pretreatment with the GABA-A antagonist, bicuculline, the GABA-B antagonist, 2-hydroxysaclofen or vehicle. Intra-VTA bicuculline (1, 10 or 20 ng/side) failed to block footshock- or CRF-induced cocaine seeking at either dose tested. By contrast, 2-hydroxysaclofen (0.2 or 2 μg/side) prevented reinstatement by both footshock and intra-VTA CRF at a concentration that failed to attenuate food-reinforced lever pressing (45 mg sucrose-sweetened pellets; FR4 schedule) in a separate group of rats. These data suggest that GABA-B receptor-dependent CRF actions in the VTA mediate stress-induced cocaine seeking and that GABA-B receptor antagonists may have utility for the management of stress-induced relapse in cocaine addicts.

Animal models of drug relapse and craving: From drug priming-induced reinstatement to incubation of craving after voluntary abstinence

High rates of relapse to drug use during abstinence is a defining feature of drug addiction. In abstinent drug users, drug relapse is often precipitated by acute exposure to the self-administered drug, drug-associated cues, stress, as well as by short-term and protracted withdrawal symptoms. In this review, we discuss different animal models that have been used to study behavioral and neuropharmacological mechanisms of these relapse-related phenomena. In the first part, we discuss relapse models in which abstinence is achieved through extinction training, including the established reinstatement model, as well as the reacquisition and resurgence models. In the second part, we discuss recent animal models in which drug relapse is assessed after either forced abstinence (e.g., the incubation of drug craving model) or voluntary (self-imposed) abstinence achieved either by introducing adverse consequences to ongoing drug self-administration (e.g., punishment) or by an alternative nondrug reward using a discrete choice (drug vs. palatable food) procedure. We conclude by briefly discussing the potential implications of the recent developments of animal models of drug relapse after voluntary abstinence to the development of medications for relapse prevention.

Dopamine D3 receptor binding of (18)F-fallypride: Evaluation using in vitro and in vivo PET imaging studies

Identification of dopamine D3 receptors (D3R) in vivo is important to understand several brain functions related to addiction. The goal of this work was to identify D3R binding of the dopamine D2 receptor (D2R)/D3R imaging agent, (18)F-fallypride. Brain slices from male Sprague-Dawley rats (n = 6) and New Zealand White rabbits (n = 6) were incubated with (18)F-fallypride and D3R selective agonist (R)-7-OH-DPAT (98-fold D3R selective). Rat slices were also treated with BP 897 (68-fold D3R selective partial agonist) and NGB 2904 (56-fold D3R selective antagonist). In vivo rat studies (n = 6) were done on Inveon PET using 18-37 MBq (18)F-fallypride and drug-induced displacement by (R)-7-OH-DPAT, BP 897 and NGB 2904. PET/CT imaging of wild type (WT, n = 2) and D2R knock-out (KO, n = 2) mice were carried out with (18)F-fallypride. (R)-7-OH-DPAT displaced binding of (18)F-fallypride, both in vitro and in vivo. In vitro, at 10 nM (R)-7-OH-DPAT, (18)F-fallypride binding in the rat ventral striatum (VST) and dorsal striatum (DST) and rabbit nucleus accumbens were reduced by ∼10-15%. At 10 μM (R)-7-OH-DPAT all regions in rat and rabbit were reduced by ≥85%. In vivo reductions for DST and VST before and after (R)-7-OH-DPAT were: low-dose (0.015 mg kg(-1)) DST -22%, VST -29%; high-dose (1.88 mg kg(-1)) DST -58%, VST -77%, suggesting D3R/D2R displacement. BP 897 and NGB 2904 competed with (18)F-fallypride in vitro, but unlike BP 897, NGB 2904 did not displace (18)F-fallypride in vivo. The D2R KO mice lacked (18)F-fallypride binding in the DST. In summary, our findings suggest that up to 20% of (18)F-fallypride may be bound to D3R sites in vivo.

Epigenetics and memory: causes, consequences and treatments for post-traumatic stress disorder and addiction

Understanding the interaction between fear and reward at the circuit and molecular levels has implications for basic scientific approaches to memory and for understanding the etiology of psychiatric disorders. Both stress and exposure to drugs of abuse induce epigenetic changes that result in persistent behavioral changes, some of which may contribute to the formation of a drug addiction or a stress-related psychiatric disorder. Converging evidence suggests that similar behavioral, neurobiological and molecular mechanisms control the extinction of learned fear and drug-seeking responses. This may, in part, account for the fact that individuals with post-traumatic stress disorder have a significantly elevated risk of developing a substance use disorder and have high rates of relapse to drugs of abuse, even after long periods of abstinence. At the behavioral level, a major challenge in treatments is that extinguished behavior is often not persistent, returning with changes in context, the passage of time or exposure to mild stressors. A common goal of treatments is therefore to weaken the ability of stressors to induce relapse. With the discovery of epigenetic mechanisms that create persistent molecular signals, recent work on extinction has focused on how modulating these epigenetic targets can create lasting extinction of fear or drug-seeking behavior. Here, we review recent evidence pointing to common behavioral, systems and epigenetic mechanisms in the regulation of fear and drug seeking. We suggest that targeting these mechanisms in combination with behavioral therapy may promote treatment and weaken stress-induced relapse.

Reinforcement principles for addiction medicine; from recreational drug use to psychiatric disorder

The transition from recreational drug use to addiction can be conceptualized as a pathological timeline whereby the psychological mechanisms responsible for disordered drug use evolve from positive reinforcement to favor elements of negative reinforcement. Abused substances (ranging from alcohol to psychostimulants) are initially ingested at regular occasions according to their positive reinforcing properties. Importantly, repeated exposure to rewarding substances sets off a chain of secondary reinforcing events, whereby cues and contexts associated with drug use may themselves become reinforcing and thereby contribute to the continued use and possible abuse of the substance(s) of choice. Indeed, the powerful reinforcing efficacy of certain drugs may eclipse that of competing social rewards (such as career and family) and lead to an aberrant narrowing of behavioral repertoire. In certain vulnerable individuals, escalation of drug use over time is thought to drive specific molecular neuroadaptations that foster the development of addiction. Research has identified neurobiological elements of altered reinforcement following excessive drug use that comprise within-circuit and between-circuit neuroadaptations, both of which contribute to addiction. Central to this process is the eventual potentiation of negative reinforcement mechanisms that may represent the final definitive criterion locking vulnerable individuals into a persistent state of addiction. Targeting the neural substrates of reinforcement likely represents our best chances for therapeutic intervention for this devastating disease.

The bed nucleus of the stria terminalis regulates ethanol-seeking behavior in mice

Drug-associated stimuli are considered important factors in relapse to drug use. In the absence of drug, these cues can trigger drug craving and drive subsequent drug seeking. One structure that has been implicated in this process is the bed nucleus of the stria terminalis (BNST), a chief component of the extended amygdala. Previous studies have established a role for the BNST in cue-induced cocaine seeking. However, it is unclear if the BNST underlies cue-induced seeking of other abused drugs such as ethanol. In the present set of experiments, BNST involvement in ethanol-seeking behavior was assessed in male DBA/2J mice using the conditioned place preference procedure (CPP). The BNST was inhibited during CPP expression using electrolytic lesions (Experiment 1), co-infusion of GABAA and GABAB receptor agonists muscimol and baclofen (M+B; Experiment 2), and activation of inhibitory designer receptors exclusively activated by designer drugs (hM4Di-DREADD) with clozapine-N-oxide (CNO; Experiment 3). The magnitude of ethanol CPP was reduced significantly by each of these techniques. Notably, infusion of M+B (Exp. 2) abolished CPP altogether. Follow-up studies to Exp. 3 showed that ethanol cue-induced c-Fos immunoreactivity in the BNST was reduced by hM4Di activation (Experiment 4) and in the absence of hM4Di, CNO did not affect ethanol CPP (Experiment 5). Combined, these findings demonstrate that the BNST is involved in the modulation of cue-induced ethanol-seeking behavior.

Drug predictive cues activate aversion-sensitive striatal neurons that encode drug seeking

Drug-associated cues have profound effects on an addict's emotional state and drug-seeking behavior. Although this influence must involve the motivational neural system that initiates and encodes the drug-seeking act, surprisingly little is known about the nature of such physiological events and their motivational consequences. Three experiments investigated the effect of a cocaine-predictive stimulus on dopamine signaling, neuronal activity, and reinstatement of cocaine seeking. In all experiments, rats were divided into two groups (paired and unpaired), and trained to self-administer cocaine in the presence of a tone that signaled the immediate availability of the drug. For rats in the paired group, self-administration sessions were preceded by a taste cue that signaled delayed drug availability. Assessments of hedonic responses indicated that this delay cue became aversive during training. Both the self-administration behavior and the immediate cue were subsequently extinguished in the absence of cocaine. After extinction of self-administration behavior, the presentation of the aversive delay cue reinstated drug seeking. In vivo electrophysiology and voltammetry recordings in the nucleus accumbens measured the neural responses to both the delay and immediate drug cues after extinction. Interestingly, the presentation of the delay cue simultaneously decreased dopamine signaling and increased excitatory encoding of the immediate cue. Most importantly, the delay cue selectively enhanced the baseline activity of neurons that would later encode drug seeking. Together these observations reveal how cocaine cues can modulate not only affective state, but also the neurochemical and downstream neurophysiological environment of striatal circuits in a manner that promotes drug seeking.

Cocaine-conditioned odor cues without chronic exposure: Implications for the development of addiction vulnerability

Adolescents are highly vulnerable to addiction and are four times more likely to become addicted at first exposure than at any other age. The dopamine D1 receptor, which is typically overexpressed in the normal adolescent prefrontal cortex, is involved in drug cue responses and is associated with relapse in animal models. In human drug addicts, imaging methods have detected increased activation in response to drug cues in reward- and habit-associated brain regions. These same methods can be applied more quantitatively to rodent models. Here, changes in neuronal activation in response to cocaine-conditioned cues were observed using functional magnetic resonance imaging in juvenile rats that were made to over-express either D1 receptors or green fluorescent protein by viral-mediated transduction. Reduced activation was observed in the amygdala and dopamine cell body regions in the low cue-preferring/control juvenile rats in response to cocaine cues. In contrast, increased activation was observed in the dorsal striatum, nucleus accumbens, prefrontal cortex, and dopamine cell bodies in high cue-preferring/D1 juveniles. The increase in cue salience that is mediated by increased D1 receptor density, rather than excessive cocaine experience, appears to underlie the transition from aversion to reward in cue-induced neural response and may form the basis for habit-forming vulnerability.

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Increased D1 receptors in prefrontal cortex increase BOLD in addiction regions.

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Cocaine-associated cues activated the amygdala when cocaine was preferred.

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Cocaine cues deactivated the amygdala in the absence of cocaine preference.

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Genetic engineering can be used to isolate functional responses in neural circuitry.

The ventral pallidum: Subregion-specific functional anatomy and roles in motivated behaviors

The ventral pallidum (VP) plays a critical role in the processing and execution of motivated behaviors. Yet this brain region is often overlooked in published discussions of the neurobiology of mental health (e.g., addiction, depression). This contributes to a gap in understanding the neurobiological mechanisms of psychiatric disorders. This review is presented to help bridge the gap by providing a resource for current knowledge of VP anatomy, projection patterns and subregional circuits, and how this organization relates to the function of VP neurons and ultimately behavior. For example, ventromedial (VPvm) and dorsolateral (VPdl) VP subregions receive projections from nucleus accumbens shell and core, respectively. Inhibitory GABAergic neurons of the VPvm project to mediodorsal thalamus, lateral hypothalamus, and ventral tegmental area, and this VP subregion helps discriminate the appropriate conditions to acquire natural rewards or drugs of abuse, consume preferred foods, and perform working memory tasks. GABAergic neurons of the VPdl project to subthalamic nucleus and substantia nigra pars reticulata, and this VP subregion is modulated by, and is necessary for, drug-seeking behavior. Additional circuits arise from nonGABAergic neuronal phenotypes that are likely to excite rather than inhibit their targets. These subregional and neuronal phenotypic circuits place the VP in a unique position to process motivationally relevant stimuli and coherent adaptive behaviors.

The ins and outs of the striatum: role in drug addiction

Addiction is a chronic relapsing disorder characterized by the loss of control over drug intake, high motivation to obtain the drug, and a persistent craving for the drug. Accumulating evidence implicates cellular and molecular alterations within cortico-basal ganglia-thalamic circuitry in the development and persistence of this disease. The striatum is a heterogeneous structure that sits at the interface of this circuit, receiving input from a variety of brain regions (e.g., prefrontal cortex, ventral tegmental area) to guide behavioral output, including motor planning, decision-making, motivation and reward. However, the vast interconnectivity of this circuit has made it difficult to isolate how individual projections and cellular subtypes within this circuit modulate each of the facets of addiction. Here, we review the use of new technologies, including optogenetics and DREADDs (Designer Receptors Exclusively Activated by Designer Drugs), in unraveling the role of the striatum in addiction. In particular, we focus on the role of striatal cell populations (i.e., direct and indirect pathway medium spiny neurons) and striatal dopaminergic and glutamatergic afferents in addiction-related plasticity and behaviors.

Neural mechanisms regulating different forms of risk-related decision-making: Insights from animal models

Over the past 20 years there has been a growing interest in the neural underpinnings of cost/benefit decision-making. Recent studies with animal models have made considerable advances in our understanding of how different prefrontal, striatal, limbic and monoaminergic circuits interact to promote efficient risk/reward decision-making, and how dysfunction in these circuits underlies aberrant decision-making observed in numerous psychiatric disorders. This review will highlight recent findings from studies exploring these questions using a variety of behavioral assays, as well as molecular, pharmacological, neurophysiological, and translational approaches. We begin with a discussion of how neural systems related to decision subcomponents may interact to generate more complex decisions involving risk and uncertainty. This is followed by an overview of interactions between prefrontal-amygdala-dopamine and habenular circuits in regulating choice between certain and uncertain rewards and how different modes of dopamine transmission may contribute to these processes. These data will be compared with results from other studies investigating the contribution of some of these systems to guiding decision-making related to rewards vs. punishment. Lastly, we provide a brief summary of impairments in risk-related decision-making associated with psychiatric disorders, highlighting recent translational studies in laboratory animals.

Gray-matter volume, midbrain dopamine D2/D3 receptors and drug craving in methamphetamine users

Dysfunction of the mesocorticolimbic system has a critical role in clinical features of addiction. Despite evidence suggesting that midbrain dopamine receptors influence amphetamine-induced dopamine release and that dopamine is involved in methamphetamine-induced neurotoxicity, associations between dopamine receptors and gray-matter volume have been unexplored in methamphetamine users. Here we used magnetic resonance imaging and [(18)F]fallypride positron emission tomography, respectively, to measure gray-matter volume (in 58 methamphetamine users) and dopamine D2/D3 receptor availability (binding potential relative to nondisplaceable uptake of the radiotracer, BPnd) (in 31 methamphetamine users and 37 control participants). Relationships between these measures and self-reported drug craving were examined. Although no difference in midbrain D2/D3 BPnd was detected between methamphetamine and control groups, midbrain D2/D3 BPnd was positively correlated with gray-matter volume in the striatum, prefrontal cortex, insula, hippocampus and temporal cortex in methamphetamine users, but not in control participants (group-by-midbrain D2/D3 BPnd interaction, P<0.05 corrected for multiple comparisons). Craving for methamphetamine was negatively associated with gray-matter volume in the insula, prefrontal cortex, amygdala, temporal cortex, occipital cortex, cerebellum and thalamus (P<0.05 corrected for multiple comparisons). A relationship between midbrain D2/D3 BPnd and methamphetamine craving was not detected. Lower midbrain D2/D3 BPnd may increase vulnerability to deficits in gray-matter volume in mesocorticolimbic circuitry in methamphetamine users, possibly reflecting greater dopamine-induced toxicity. Identifying factors that influence prefrontal and limbic volume, such as midbrain BPnd, may be important for understanding the basis of drug craving, a key factor in the maintenance of substance-use disorders.

Aversive stimuli drive drug seeking in a state of low dopamine tone

BACKGROUND

Stressors negatively impact emotional state and drive drug seeking, in part, by modulating the activity of the mesolimbic dopamine system. Unfortunately, the rapid regulation of dopamine signaling by the aversive stimuli that cause drug seeking is not well characterized. In a series of experiments, we scrutinized the subsecond regulation of dopamine signaling by the aversive stimulus, quinine, and tested its ability to cause cocaine seeking. Additionally, we examined the midbrain regulation of both dopamine signaling and cocaine seeking by the stress-sensitive peptide, corticotropin releasing factor (CRF).

METHODS

Combining fast-scan cyclic voltammetry with behavioral pharmacology, we examined the effect of intraoral quinine administration on nucleus accumbens dopamine signaling and hedonic expression in 21 male Sprague-Dawley rats. We tested the role of CRF in modulating aversion-induced changes in dopamine concentration and cocaine seeking by bilaterally infusing the CRF antagonist, CP-376395, into the ventral tegmental area (VTA).

RESULTS

We found that quinine rapidly reduced dopamine signaling on two distinct time scales. We determined that CRF acted in the VTA to mediate this reduction on only one of these time scales. Further, we found that the reduction of dopamine tone and quinine-induced cocaine seeking were eliminated by blocking the actions of CRF in the VTA during the experience of the aversive stimulus.

CONCLUSIONS

These data demonstrate that stress-induced drug seeking can occur in a terminal environment of low dopamine tone that is dependent on a CRF-induced decrease in midbrain dopamine activity.

Repeated social defeat stress enhances the anxiogenic effect of bright light on operant reward-seeking behavior in rats

Repeated stress can trigger episodes of depression, along with symptoms of anhedonia and anxiety. Although often modeled separately, anxiogenic factors potently modulate hedonic, or appetitive, behavior. While repeated stress can increase anxiety and decrease appetitive behavior, it is not clear whether repeated stress can influence the impact of anxiogenic factors on appetitive behavior. This study tests whether repeated stress shifts behavior in a task that measures anxiogenic-appetitive balance. To test this, adult male rats were trained to lever press for sucrose pellet reward, and the effect of anxiogenic bright light on this behavior was measured. The impact of the bright light anxiogenic stimulus on lever pressing was compared between groups exposed to either daily repeated social defeat stress or control handling. We found that repeated stress reduced exploration in the open field and decreased social interaction, but had minimal effect on baseline lever pressing for reward. Repeated stress substantially enhanced the effect of anxiogenic bright light on lever pressing. This effect was greater two days after the last stress exposure, and began to diminish within two weeks. These data demonstrate that the anxiogenic and anhedonic features induced by repeated stress can be separately measured, and that the impact of anxiogenic stimuli can be greatly enhanced after repeated stress, even in the face of appetitive drive. The data also demonstrate that some apparent anhedonic-like effects of repeated stress can be due to increased sensitivity to anxiogenic stimuli, and may reflect an imbalance in an appetitive approach-withdrawal continuum.

Anisomycin in the medial prefrontal cortex reduces reconsolidation of cocaine-associated memories in the rat self-administration model

We tested the hypothesis that infusion of anisomycin into the medial prefrontal cortex (mPFC) disrupts the reconsolidation of a cocaine-associated memory in the rat cocaine self-administration model. Male Sprague-Dawley rats were trained to lever press for cocaine self-administration (0.5 mg/kg/infusion) along with a cue light presentation on an FR1 followed by an FR3 schedule of reinforcement for 2 h/day. Rats were then given extinction sessions or an equivalent forced abstinence period followed by a 5 min memory reactivation session during which time they received an ip cocaine injection (10 mg/kg, ip) and were allowed to press for contingent cue light presentation. Immediately after reactivation, they were administered an intra-mPFC infusion of vehicle or anisomycin. Two additional control groups received extinction and either no memory reactivation and intra-mPFC infusions as above or intra-mPFC infusions 6 h after memory reactivation. A fourth group received forced abstinence and intra-mPFC infusions immediately after memory reactivation. Combined cocaine + cue-induced reinstatement was given 2-3 days (early) and 8-12 days (late) later. Rats given anisomycin in the Extinction + Reactivation demonstrated decreased reinstatement, while anisomycin treatment did not alter behavior in any of the other three groups. These results suggest that extinction training may recruit the mPFC such that it renders the memory susceptible to disruption by anisomycin. These findings have implications for using extinction training prior to or in conjunction with other therapies, including reconsolidation disruption, to enhance prefrontal control over drug-seeking behavior.

Medial prefrontal cannabinoid CB1 receptors modulate consolidation and extinction of cocaine-associated memory in mice

RATIONALE

Cannabinoid CB1 receptors are implicated in various forms of learning and memory, including acquisition and reinstatement of cocaine-associated memory. However, roles of CB1 receptors in consolidation and extinction processes of cocaine-associated memory and the brain areas potentially involved remain unknown.

OBJECTIVE

This study examined the effect of rimonabant, a CB1 receptor antagonist, administered systemically or directly into the medial prefrontal cortex (mPFC) on memory consolidation and extinction of cocaine-induced conditioned place preference (CPP).

MATERIALS AND METHODS

Male C57BL/6J mice were trained to acquire cocaine-induced CPP. Rimonabant (0.1-3 mg/kg, i.p. or 1.5 μg bilaterally in the mPFC) or vehicle was administered either immediately after each CPP training (consolidation) or forced extinction (extinction) trial. Cocaine-induced CPP was tested after training, extinction, or cocaine priming.

RESULTS

Systemic or intra-mPFC administration of rimonabant impaired consolidation of CPP induced by a high dose (20 or 40 mg/kg) of cocaine but facilitated that induced by a low dose (2.5, 5, or 10 mg/kg). Moreover, systemic or intra-mPFC administration of rimonabant enhanced extinction of CPP memory induced by a high-dose (20 mg/kg) cocaine.

CONCLUSION

Our results suggest that antagonism of CB1 receptors in the mPFC bidirectionally modulates consolidation but facilitates extinction of cocaine-induced CPP memory. Therefore, CB1 receptor blockade with the concomitant extinction behavioral procedure may hint important therapeutic intervention strategies for the heavy cocaine addicts in a clinical setting.

Frontostriatal systems comprising connections between ventral medial prefrontal cortex and nucleus accumbens subregions differentially regulate motor impulse control in rats

RATIONALE

Deficits in impulse control are prevalent in several neuropsychiatric disorders that are based on impaired frontostriatal communication. The ventral medial prefrontal cortex (vmPFC) and the nucleus accumbens (NAc) are key substrates of impulse control in rats. The NAc core and shell are considered to be differentially involved suggesting a functional distinction between the connections of the vmPFC and particular NAc subregions concerning impulse control.

OBJECTIVES/METHODS

In the present study, simultaneous inactivation of the rats' vmPFC and NAc core or shell by contralateral microinfusion of the GABAA receptor agonist muscimol was used to investigate their relevance for impulse control in the five-choice serial reaction time task (5-CSRTT).

RESULTS

Disconnection of the vmPFC and NAc shell produced specific impairments in inhibitory control, indicated by significantly increased premature responding and an enhanced number of time-out responses, closely resembling the effects of bilateral inactivation of either the vmPFC or NAc shell previously reported using the same task. In contrast, disconnection of the vmPFC and NAc core only slightly increased the rate of omissions and latency of reward collection indicating attentional and motivational deficits.

CONCLUSIONS

Our results extend previous findings indicating the functional specialisation of frontostriatal networks and show a differential contribution of specific vmPFC-NAc connections to behavioural control depending on the NAc subregion. We conclude that the regulation of impulse control in rats requires an intact connection between the vmPFC and the NAc shell, while the vmPFC-NAc core projection seems to be of minor importance.

The rostromedial tegmental nucleus modulates behavioral inhibition following cocaine self-administration in rats

Recent findings suggest that the mesolimbic dopamine neurons, known to promote cocaine-seeking behavior, are strongly inhibited by a newly characterized region of the midbrain known as the rostromedial tegmental nucleus (RMTg). The RMTg appears to be involved in generating reward-prediction error signals and inhibition of motivated behaviors, suggesting its potential involvement in the extinction of cocaine seeking as well. Therefore, to address this question, male Sprague-Dawley rats underwent surgeries for implantation of catheters and cannulas targeted at the RMTg. After cocaine self-administration, rats underwent modified extinction training. Pre- or post-training intra-RMTg microinjections of the allosteric AMPA receptor potentiator PEPA during the first 5 days of extinction training appeared to enhance the retention of the extinction learning. Following the extinction training, rats underwent cue-induced reinstatement or an 'inactivation-alone' extinction tests. RMTg inactivation before a cue-induced reinstatement session or inactivation alone before a standard extinction session increased overall lever pressing. To determine whether these effects generalized to other motivated behaviors, additional experiments examining food-seeking behavior were also conducted. The results from the food-seeking experiments indicate that PEPA microinjections into the RMTg did not influence the extinction of food seeking and that, at least in rats that had not been given PEPA during the extinction learning experiments, RMTg inactivation had no effect on lever pressing during the cue-induced reinstatement or inactivation-alone tests. These findings suggest that the RMTg provides general behavioral inhibition and is potentially involved in learning to extinguish cocaine-seeking behavior in rats.

Identifying candidate drivers of alcohol dependence-induced excessive drinking by assembly and interrogation of brain-specific regulatory networks

BACKGROUND

A systems biology approach based on the assembly and interrogation of gene regulatory networks, or interactomes, was used to study neuroadaptation processes associated with the transition to alcohol dependence at the molecular level.

RESULTS

Using a rat model of dependent and non-dependent alcohol self-administration, we reverse engineered a global transcriptional regulatory network during protracted abstinence, a period when relapse rates are highest. We then interrogated the network to identify master regulator genes that mechanistically regulate brain region-specific signatures associated with dependent and non-dependent alcohol self-administration. Among these, the gene coding for the glucocorticoid receptor was independently identified as a master regulator in multiple brain regions, including the medial prefrontal cortex, nucleus accumbens, central nucleus of the amygdala, and ventral tegmental area, consistent with the view that brain reward and stress systems are dysregulated during protracted abstinence. Administration of the glucocorticoid antagonist mifepristone in either the nucleus accumbens or ventral tegmental area selectively decreased dependent, excessive, alcohol self-administration in rats but had no effect on non-dependent, moderate, alcohol self-administration.

CONCLUSIONS

Our study suggests that assembly and analysis of regulatory networks is an effective strategy for the identification of key regulators of long-term neuroplastic changes within specific brain regions that play a functional role in alcohol dependence. More specifically, our results support a key role for regulatory networks downstream of the glucocorticoid receptor in excessive alcohol drinking during protracted alcohol abstinence.

Orexin/hypocretin role in reward: implications for opioid and other addictions

Addiction is a devastating disorder that affects 15.3 million people worldwide. While prevalent, few effective treatments exist. Orexin receptors have been proposed as a potential target for anti-craving medications. Orexins, also known as hypocretins, are neuropeptides produced in neurons of the lateral and dorsomedial hypothalamus and perifornical area, which project widely throughout the brain. The absence of orexins in rodents and humans leads to narcolepsy. However, orexins also have an established role in reward seeking. This review will discuss some of the original studies describing the roles of the orexins in reward seeking as well as specific works that were presented at the 2013 International Narcotics Research Conference. Orexin signalling can promote drug-induced plasticity of glutamatergic synapses onto dopamine neurons of the ventral tegmental area (VTA), a brain region implicated in motivated behaviour. Additional evidence suggests that orexin signalling can also promote drug seeking by initiating an endocannabinoid-mediated synaptic depression of GABAergic inputs to the VTA, and thereby disinhibiting dopaminergic neurons. Orexin neurons co-express the inhibitory opioid peptide dynorphin. It has been proposed that orexin in the VTA may not mediate reward per se, but rather occludes the 'anti-reward' effects of dynorphin. Finally, orexin signalling in the prefrontal cortex and the central amygdala is implicated in reinstatement of reward seeking. This review will highlight recent work describing the role of orexin signalling in cellular processes underlying addiction-related behaviours and propose novel hypotheses for the mechanisms by which orexin signalling may impart drug seeking.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2.

Stress and VTA synapses: implications for addiction and depression

While stressful experiences are a part of everyone's life, they can also exact a major toll on health. Stressful life experiences are associated with increased substance abuse, and there exists significant co-morbidity between mental illness and substance use disorders [N.D. Volkow & T.K. Li (2004) Nat. Rev. Neurosci., 5, 963-970; G. Koob & M.J. Kreek (2007) Am. J. Psych., 164, 1149-1159; R. Sinha (2008) Annals N.Y. Acad. Sci., 1141, 105-130]. The risk for development of mood or anxiety disorders after stress is positively associated with the risk for substance use disorders [R. Sinha (2008) Annals N.Y. Acad. Sci., 1141, 105-130], suggesting that there are common substrates for vulnerability to addictive and affective disorders. Understanding the molecular and physiological substrates of stress may lead to improved therapeutic interventions for the treatment of substance use disorders and mental illnesses.

Deep brain stimulation of the nucleus accumbens shell attenuates cue-induced reinstatement of both cocaine and sucrose seeking in rats

Stimuli previously associated with drug taking can become triggers that can elicit craving and lead to relapse of drug-seeking behavior. Here, we examined the influence of deep brain stimulation (DBS) in the nucleus accumbens shell on cue-induced reinstatement of cocaine seeking, an animal model of relapse. Rats were allowed to self-administer cocaine (0.254 mg, i.v.) for 2 h daily for 21 days, with each infusion of cocaine being paired with a cue light. After 21 days of self-administration, cocaine-taking behavior was extinguished by replacing cocaine with saline in the absence of the cue light. Next, during the reinstatement phase, DBS was administered bilaterally into the nucleus accumbens shell through bipolar stainless steel electrodes immediately prior to re-exposure to cues previously associated with cocaine reinforcement. DBS continued throughout the 2 h reinstatement session. Parallel studies examined the influence of accumbens shell DBS on reinstatement induced by cues previously associated with sucrose reinforcement. Results indicated that DBS of the nucleus accumbens shell significantly attenuated cue-induced reinstatement of cocaine and sucrose seeking. Together, these results indicate that DBS of the accumbens shell disrupts cue-induced reinstatement associated with both a drug and a natural reinforcer.

Kappa opioid receptor activation potentiates the cocaine-induced increase in evoked dopamine release recorded in vivo in the mouse nucleus accumbens

Behavioral stressors increase addiction risk in humans and increase the rewarding valence of drugs of abuse including cocaine, nicotine and ethanol in animal models. Prior studies have established that this potentiation of drug reward was mediated by stress-induced release of the endogenous dynorphin opioids and subsequent kappa opioid receptor (KOR) activation. In this study, we used in vivo fast scan cyclic voltammetry to test the hypothesis that KOR activation before cocaine administration might potentiate the evoked release of dopamine from ventral tegmental (VTA) synaptic inputs to the nucleus accumbens (NAc) and thereby increase the rewarding valence of cocaine. The KOR agonist U50488 inhibited dopamine release evoked by either medial forebrain bundle (MFB) or pedunculopontine tegmental nucleus (PPTg) activation of VTA inputs to the shell or core of the mouse NAc. Cocaine administration increased the dopamine response recorded in either the shell or core evoked by either MFB or PPTg stimulation. Administration of U50488 15 min before cocaine blocked the conditioned place preference (CPP) to cocaine, but only significantly reduced the effect of cocaine on the dopamine response evoked by PPTg stimulation to NAc core. In contrast, administration of U50488 60 min before cocaine significantly potentiated cocaine CPP and significantly increased the effects of cocaine on the dopamine response evoked by either MFB or PPTg stimulation, recorded in either NAc shell or core. Results of this study support the concept that stress-induced activation of KOR by endogenous dynorphin opioids may enhance the rewarding valence of drugs of abuse by potentiating the evoked dopamine response.

Poststress block of kappa opioid receptors rescues long-term potentiation of inhibitory synapses and prevents reinstatement of cocaine seeking

BACKGROUND

Dopaminergic neurons in the ventral tegmental area of the brain are an important site of convergence of drugs and stress. We previously identified a form of long-term potentiation of gamma-aminobutyric acid (GABA)ergic synapses on these neurons (LTPGABA). Our studies have shown that exposure to acute stress blocks this LTP and that reversal of the block of LTPGABA is correlated with prevention of stress-induced reinstatement of cocaine-seeking behavior.

METHODS

Sprague-Dawley rats were subjected to cold-water swim stress. Midbrain slices were prepared following stress, and whole-cell patch clamp recordings of inhibitory postsynaptic currents were performed from ventral tegmental area dopamine neurons. Antagonists of glucocorticoid receptors and kappa opioid receptors (κORs) were administered at varying time points after stress. Additionally, the ability of a kappa antagonist administered following stress to block forced swim stress-induced reinstatement of cocaine self-administration was tested.

RESULTS

We found that an acute stressor blocks LTPGABA for 5 days after stress through a transient activation of glucocorticoid receptors and more lasting contribution of κORs. Even pharmacological block of κORs beginning 4 days after stress has occurred reversed the block of LTPGABA. Administration of a κORs antagonist following stress prevents reinstatement of cocaine-seeking behavior.

CONCLUSIONS

A brief stressor produces changes in the reward circuitry lasting several days. Our findings reveal roles for glucocorticoid receptors and κORs as mediators of the lasting effects of stress on synaptic plasticity. κORs antagonists reverse the neuroadaptations underlying stress-induced drug-seeking behavior and may be useful in the treatment of cocaine addiction.

Excessive disgust caused by brain lesions or temporary inactivations: mapping hotspots of the nucleus accumbens and ventral pallidum

Disgust is a prototypical type of negative affect. In animal models of excessive disgust, only a few brain sites are known in which localized dysfunction (lesions or neural inactivations) can induce intense 'disgust reactions' (e.g. gapes) to a normally pleasant sensation such as sweetness. Here, we aimed to map forebrain candidates more precisely, to identify where either local neuronal damage (excitotoxin lesions) or local pharmacological inactivation (muscimol/baclofen microinjections) caused rats to show excessive sensory disgust reactions to sucrose. Our study compared subregions of the nucleus accumbens shell, ventral pallidum, lateral hypothalamus, and adjacent extended amygdala. The results indicated that the posterior half of the ventral pallidum was the only forebrain site where intense sensory disgust gapes in response to sucrose were induced by both lesions and temporary inactivations (this site was previously identified as a hedonic hotspot for enhancements of sweetness 'liking'). By comparison, for the nucleus accumbens, temporary GABA inactivations in the caudal half of the medial shell also generated sensory disgust, but lesions never did at any site. Furthermore, even inactivations failed to induce disgust in the rostral half of the accumbens shell (which also contains a hedonic hotspot). In other structures, neither lesions nor inactivations induced disgust as long as the posterior ventral pallidum remained spared. We conclude that the posterior ventral pallidum is an especially crucial hotspot for producing excessive sensory disgust by local pharmacological/lesion dysfunction. By comparison, the nucleus accumbens appears to segregate sites for pharmacological disgust induction and hedonic enhancement into separate posterior and rostral halves of the medial shell.

Effects of the 5-HT2C receptor agonist CP809101 in the amygdala on reinstatement of cocaine-seeking behavior and anxiety-like behavior

Serotonin 2C receptor (5-HT2CR) agonists attenuate reinstatement of cocaine-seeking behavior. These receptors are found throughout the limbic system, including the basolateral amygdala (BlA), which is involved in forming associations between emotional stimuli and environmental cues, and the central amygdala (CeA), which is implicated in the expression of conditioned responding to emotional stimuli. This study investigated whether 5-HT2CRs in the amygdala are involved in cue and cocaine-primed reinstatement of cocaine-seeking behavior. Rats were trained to self-administer cocaine (0.75 mg/kg, i.v.) which that was paired with light and tone cues, and then subsequently they underwent daily extinction training. Rats then received bilateral microinfusions of the 5-HT2CR agonist CP809101 (0.01-1.0 μg/0.2 μl/side) into either the BlA or CeA prior to tests for cue or cocaine-primed (10 mg/kg, i.p.) reinstatement. Rats were also tested for CP809101 effects on anxiety-like behavior on the elevated plus-maze (EPM). Surprisingly, intra-BlA CP809101 had no effect on cue reinstatement, though it did increase anxiety-like behavior on the EPM. Intra-CeA infusions of CP809101 attenuated cocaine-primed reinstatement, an effect that was prevented with concurrent administration of the 5-HT2CR antagonist SB242084 (0.1 μg/0.2 μl/side). CP809101 had no effect on cue reinstatement or anxiety-like behavior on the EPM. These findings suggest that 5-HT2CRs in the BlA modulate anxiety, whereas those in the CeA modulate incentive motivational effects induced by cocaine priming injections.

Prefrontal gamma-aminobutyric acid type A receptor insertion controls cue-induced relapse to nicotine seeking

BACKGROUND

Current smoking cessation therapies offer limited success, as relapse rates remain high. Nicotine, which is the major component of tobacco smoke, is thought to be primarily responsible for the addictive properties of tobacco. However, little is known about the molecular mechanisms underlying nicotine relapse, hampering development of more effective therapies. The objective of this study was to elucidate the role of medial prefrontal cortex (mPFC) glutamatergic and gamma-aminobutyric acid (GABA)ergic receptors in controlling relapse to nicotine seeking.

METHODS

Using an intravenous self-administration model, we studied glutamate and gamma-aminobutyric acid receptor regulation in the synaptic membrane fraction of the rat mPFC following extinction and cue-induced relapse to nicotine seeking. Subsequently, we locally intervened at the level of GABAergic signaling by using a mimetic peptide of the GABA receptor associated protein-interacting domain of GABA type A (GABAA) receptor subunit γ2 (TAT-GABAγ2) and muscimol, a GABAA receptor agonist.

RESULTS

Alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid and N-methyl-D-aspartate receptors were not regulated after the 30-min relapse test. However, GABAA receptor subunits α1 and γ2 were upregulated, and interference with GABAA receptor insertion in the cell membrane using the TAT-GABAγ2 peptide in the dorsal mPFC, but not the ventral mPFC, significantly increased responding during relapse. Increasing GABAA transmission with muscimol in the dorsal and ventral mPFC attenuated relapse.

CONCLUSIONS

These data indicate that cue-induced relapse entails a GABAergic plasticity mechanism that limits nicotine seeking by restoring inhibitory control in the dorsal mPFC. GABAA receptor-mediated neurotransmission in the dorsal mPFC constitutes a possible future therapeutic target for maintaining smoking abstinence.