Neuropsychopharmacology of drug seeking: Insights from studies with second-order schedules of drug reinforcement

Understanding the shift to compulsion in addiction: insights from personality traits, social factors, and neurobiology

Compulsion stands as a central symptom of drug addiction; however, only a small fraction of individuals who use drugs exhibit compulsive characteristics. Differences observed in Sign-trackers (ST) and Goal-trackers (GT) during Pavlovian conditioning may shed light on individual variances in drug addiction. Here, we focus on the behavioral attributes, formation processes, and neural mechanisms underlying ST and how they drive addiction toward compulsivity in humans. We will explore addiction from three interconnected levels: individual personality traits, social factors, and neurobiology. Furthermore, we distinguish between the processes of sensitization and habituation within ST. These nuanced distinctions across various aspects of addiction will contribute to our understanding of the addiction development process and the formulation of targeted preventive strategies.

Pan-striatal reduction in the expression of the astrocytic dopamine transporter precedes the development of dorsolateral striatum dopamine-dependent incentive heroin seeking habits

The emergence of compulsive drug-seeking habits, a hallmark feature of substance use disorder, has been shown to be predicated on the engagement of dorsolateral striatal control over behaviour. This process involves the dopamine-dependent functional coupling of the anterior dorsolateral striatum (aDLS) with the nucleus accumbens core, but the mechanisms by which this coupling occurs have not been fully elucidated. The striatum is tiled by a syncytium of astrocytes that express the dopamine transporter (DAT), the level of which is altered in individuals with heroin use disorder. Astrocytes are therefore uniquely placed functionally to bridge dopamine-dependent mechanisms across the striatum. Here we tested the hypothesis that exposure to heroin influences the expression of DAT in striatal astrocytes across the striatum before the development of DLS-dependent incentive heroin seeking habits. Using Western-blot, qPCR, and RNAscope™, we measured DAT protein and mRNA levels in whole tissue, culture and in situ astrocytes from striatal territories of rats with a well-established cue-controlled heroin seeking habit and rats trained to respond for heroin or food under continuous reinforcement. Incentive heroin seeking habits were associated with a reduction in DAT protein levels in the anterior aDLS that was preceded by a heroin-induced reduction in DAT mRNA and protein in astrocytes across the striatum. Striatal astrocytes were also shown to be susceptible to direct dopamine- and opioid-induced downregulation of DAT expression. These results suggest that astrocytes may critically regulate the striatal dopaminergic adaptations that lead to the development of incentive heroin seeking habits.

Functional neuroanatomy of monoaminergic systems in the basolateral nuclear complex of the amygdala: Neuronal targets, receptors, and circuits

This review discusses neuroanatomical aspects of the three main monoaminergic systems innervating the basolateral nuclear complex (BNC) of the amygdala (serotonergic, noradrenergic, and dopaminergic systems). It mainly focuses on immunohistochemical (IHC) and in situ hybridization (ISH) studies that have analyzed the relationship of specific monoaminergic inputs and their receptors to specific neuronal subtypes in the BNC in order to better understand the anatomical substrates of the monoaminergic modulation of BNC circuitry. First, light and electron microscopic IHC investigations identifying the main BNC neuronal subpopulations and characterizing their local circuitry, including connections with discrete PN compartments and other INs, are reviewed. Then, the relationships of each of the three monoaminergic systems to distinct PN and IN cell types, are examined in detail. For each system, the neuronal targets and their receptor expression are discussed. In addition, pertinent electrophysiological investigations are discussed. The last section of the review compares and contrasts various aspects of each of the three monoaminergic systems. It is concluded that the large number of different receptors, each with a distinct mode of action, expressed by distinct cell types with different connections and functions, should offer innumerable ways to subtlety regulate the activity of the BNC by therapeutic drugs in psychiatric diseases in which there are alterations of BNC monoaminergic modulatory systems, such as in anxiety disorders, depression, and drug addiction. It is suggested that an important area for future studies is to investigate how the three systems interact in concert at the neuronal and neuronal network levels.

Effects of sex and estrous cycle on intravenous oxycodone self-administration and the reinstatement of oxycodone-seeking behavior in rats

Introduction

The increasing misuse of both prescription and illicit opioids has culminated in a national healthcare crisis in the United States. Oxycodone is among the most widely prescribed and misused opioid pain relievers and has been associated with a high risk for transition to compulsive opioid use. Here, we sought to examine potential sex differences and estrous cycle-dependent effects on the reinforcing efficacy of oxycodone, as well as on stress-induced or cue-induced oxycodone-seeking behavior, using intravenous (IV) oxycodone self-administration and reinstatement procedures.

Methods

In experiment 1, adult male and female Long-Evans rats were trained to self-administer 0.03 mg/kg/inf oxycodone according to a fixed-ratio 1 schedule of reinforcement in daily 2-h sessions, and a dose-response function was subsequently determined (0.003-0.03 mg/kg/inf). In experiment 2, a separate group of adult male and female Long-Evans rats were trained to self-administer 0.03 mg/kg/inf oxycodone for 8 sessions, followed by 0.01 mg/kg/inf oxycodone for 10 sessions. Responding was then extinguished, followed by sequential footshock-induced and cue-induced reinstatement tests.

Results

In the dose-response experiment, oxycodone produced a typical inverted U-shape function with 0.01 mg/kg/inf representing the maximally effective dose in both sexes. No sex differences were detected in the reinforcing efficacy of oxycodone. In the second experiment, the reinforcing effects of 0.01-0.03 mg//kg/inf oxycodone were significantly attenuated in females during proestrus/estrus as compared to metestrus/diestrus phases of the estrous cycle. Neither males nor females displayed significant footshock-induced reinstatement of oxycodone seeking, but both sexes exhibited significant cue-induced reinstatement of oxycodone seeking at magnitudes that did not differ either by sex or by estrous cycle phase.

Discussion

These results confirm and extend previous work suggesting that sex does not robustly influence the primary reinforcing effects of oxycodone nor the reinstatement of oxycodone-seeking behavior. However, our findings reveal for the first time that the reinforcing efficacy of IV oxycodone varies across the estrous cycle in female rats.

Effects of sex and estrous cycle on intravenous oxycodone self-administration and the reinstatement of oxycodone-seeking behavior in rats

The increasing misuse of both prescription and illicit opioids has culminated in a national healthcare crisis in the United States. Oxycodone is among the most widely prescribed and misused opioid pain relievers and has been associated with a high risk for transition to compulsive opioid use. Here, we sought to examine potential sex differences and estrous cycle-dependent effects on the reinforcing efficacy of oxycodone, as well as on stress-induced or cue-induced oxycodone-seeking behavior, using intravenous (IV) oxycodone self-administration and reinstatement procedures. In experiment 1, adult male and female Long-Evans rats were trained to self-administer 0.03 mg/kg/inf oxycodone according to a fixed-ratio 1 schedule of reinforcement in daily 2-hr sessions, and a dose-response function was subsequently determined (0.003-0.03 mg/kg/inf). In experiment 2, a separate group of adult male and female Long-Evans rats were trained to self-administer 0.03 mg/kg/inf oxycodone for 8 sessions, followed by 0.01 mg/kg/inf oxycodone for 10 sessions. Responding was then extinguished, followed by sequential footshock-induced and cue-induced reinstatement tests. In the dose-response experiment, oxycodone produced a typical inverted U-shape function with 0.01 mg/kg/inf representing the maximally effective dose in both sexes. No sex differences were detected in the reinforcing efficacy of oxycodone. In the second experiment, the reinforcing effects of 0.01-0.03 mg//kg/inf oxycodone were significantly attenuated in females during proestrus/estrus as compared to metestrus/diestrus phases of the estrous cycle. Neither males nor females displayed significant footshock-induced reinstatement of oxycodone seeking, but both sexes exhibited significant cue-induced reinstatement of oxycodone seeking at magnitudes that did not differ either by sex or by estrous cycle phase. These results confirm and extend previous work suggesting that sex does not robustly influence the primary reinforcing effects of oxycodone nor the reinstatement of oxycodone-seeking behavior. However, our findings reveal for the first time that the reinforcing efficacy of IV oxycodone varies across the estrous cycle in female rats.

Enduring disruption of reward and stress circuit activities by early-life adversity in male rats

In humans, early-life adversity (ELA) such as trauma, poverty, and chaotic environment is linked to increased risk of later-life emotional disorders including depression and substance abuse. These disorders involve underlying disruption of reward circuits and likely vary by sex. Accordingly, we previously found that ELA leads to anhedonia for natural rewards and cocaine in male rodents, whereas in females ELA instead increases vulnerability to addiction-like use of opioid drugs and palatable food. While these findings suggest that ELA-induced disruption of reward circuitry may differ between the sexes, the specific circuit nodes that are influenced by ELA in either sex remain poorly understood. Here, in adult male Sprague-Dawley rats, we ask how ELA impacts opioid addiction-relevant behaviors that we previously tested after ELA in females. We probe potential circuit mechanisms in males by assessing opioid-associated neuronal activation in stress and reward circuit nodes including nucleus accumbens (NAc), amygdala, medial prefrontal cortex (mPFC), and paraventricular thalamus. We find that ELA diminishes opioid-seeking behaviors in males, and alters heroin-induced activation of NAc, PFC, and amygdala, suggesting a potential circuit-based mechanism. These studies demonstrate that ELA leads to behavioral and neurobiological disruptions consistent with anhedonia in male rodents, unlike the increased opioid seeking we previously saw in females. Our findings, taken together with our prior work, suggest that men and women could face qualitatively different mental health consequences of ELA, which may be essential for individually tailoring future intervention strategies.

Neurodevelopmental origins of substance use disorders: Evidence from animal models of early-life adversity and addiction

Addiction is a chronic relapsing disorder with devastating personal, societal, and economic consequences. In humans, early-life adversity (ELA) such as trauma, neglect, and resource scarcity are linked with increased risk of later-life addiction, but the brain mechanisms underlying this link are still poorly understood. Here, we focus on data from rodent models of ELA and addiction, in which causal effects of ELA on later-life responses to drugs and the neurodevelopmental mechanisms by which ELA increases vulnerability to addiction can be determined. We first summarize evidence for a link between ELA and addiction in humans, then describe how ELA is commonly modeled in rodents. Since addiction is a heterogeneous disease with many individually varying behavioral aspects that may be impacted by ELA, we next discuss common rodent assays of addiction-like behaviors. We then summarize the specific addiction-relevant behavioral phenotypes caused by ELA in male and female rodents and discuss some of the underlying changes in brain reward and stress circuits that are likely responsible. By better understanding the behavioral and neural mechanisms by which ELA promotes addiction vulnerability, we hope to facilitate development of new approaches for preventing or treating addiction in those with a history of ELA.

The influence of conditioned stimuli on [11C]-(+)-PHNO PET binding in tobacco smokers after a one week abstinence

Stimuli previously paired with drugs of dependence can produce cravings that are associated with increased dopamine (DA) levels in limbic and striatal brain areas. Positron Emission Tomography (PET) imaging with [¹¹C]-(+)-PHNO allows for a sensitive measurement of changes in DA levels. The purpose of the present study was to investigate changes in DA levels, measured with PET imaging with [¹¹C]-(+)-PHNO, in regions of interest in smokers who had maintained abstinence for 7–10 days. Participants (N = 10) underwent two PET scans on separate days, during which they viewed either smoking-related or neutral images, in counterbalanced order. Craving was measured with the 12-item Tobacco Craving Questionnaire (TCQ) and the Questionnaire on Smoking Urges-Brief (QSU-B). Compared to neutral cues, smoking cues did not increase craving. There were no changes in [¹¹C]-(+)-PHNO binding in the cue condition compared to the neutral condition for most regions of interest (ventral pallidum, globus pallidus, limbic striatum, associative striatum, sensorimotor striatum). However, binding potential in the substantia nigra was greater in the smoking-cue condition, indicating decreased synaptic dopamine. There is a potential change of DA level occurring in midbrain following the presentation of smoking-related cues. However, this preliminary finding would need to be validated with a larger sample.

Extra-striatal D2/3 receptor availability in youth at risk for addiction

The neurobiological traits that confer risk for addictions remain poorly understood. However, dopaminergic function throughout the prefrontal cortex, limbic system, and upper brainstem has been implicated in behavioral features that influence addiction vulnerability, including poor impulse control, and altered sensitivity to rewards and punishments (i.e., externalizing features). To test these associations in humans, we measured type-2/3 dopamine receptor (DA_2/3R) availability in youth at high vs. low risk for substance use disorders (SUDs). In this study, N = 58 youth (18.5 ± 0.6 years) were recruited from cohorts that have been followed since birth. Participants with either high (high EXT; N = 27; 16 F/11 M) or low pre-existing externalizing traits (low EXT; N = 31; 20 F/11 M) underwent a 90-min positron emission tomography [¹⁸F]fallypride scan, and completed the Barratt Impulsiveness Scale (BIS-11), Substance Use Risk Profile scale (SURPS), and Sensitivity to Punishment (SP) and Sensitivity to Reward (SR) questionnaire. We found that high vs. low EXT trait participants reported elevated substance use, BIS-11, SR, and SURPS impulsivity scores, had a greater prevalence of psychiatric disorders, and exhibited higher [¹⁸F]fallypride binding potential (BP_ND) values in prefrontal, limbic and paralimbic regions, even when controlling for substance use. Group differences were not evident in midbrain dopamine cell body regions, but, across all participants, low midbrain BP_ND values were associated with low SP scores. Together, the results suggest that altered DA_2/3R availability in terminal extra-striatal and dopamine cell body regions might constitute biological vulnerability traits, generating an EXT trajectory for addictions with and without co-occurring alterations in punishment sensitivity (i.e., an internalizing feature).

Role of glucocorticoids on noradrenergic and dopaminergic neurotransmission within the basolateral amygdala and dentate gyrus during morphine withdrawal place aversion

Aversive memories related to drug withdrawal can generate a motivational state leading to compulsive drug taking. However, the mechanisms underlying the generation of these withdrawal memories remain unclear. Limbic structures, such as the basolateral amygdala (BLA) and the dentate gyrus (DG) of the hippocampus, play a crucial role in the negative affective component of morphine withdrawal. Given the prominent role of glucocorticoids (GCs), noradrenaline (NA), and dopamine (DA) in memory-related processes, in the present study, we employed the conditioned place aversion (CPA) paradigm to uncover the role of GCs on NA and DA neurotransmission within the BLA and NA neurotransmission within the DG during opiate-withdrawal conditioning (memory formation consolidation), and after reexposure to the conditioned environment (memory retrieval). We observed that adrenalectomy impaired naloxone-induced CPA. Memory retrieval was associated with an increase in dihydroxyphenylacetic acid (DOPAC) levels in the BLA in morphine-addicted animals in a GC-independent manner. Importantly, NA turnover was related with the expression of withdrawal physical signs during the conditioning phase and with locomotor activity during the test phase. On the other hand, reduced DA concentration in the BLA was correlated with the CPA score. Our results indicate that while noradrenergic system is more associated with the somatic consequences of withdrawal, dopaminergic neurotransmission modulates the affective state. Nevertheless, it seems necessary that both systems work together with GCs to enable aversive-memory formation and recall.

CREST in the Nucleus Accumbens Core Regulates Cocaine Conditioned Place Preference, Cocaine-Seeking Behavior, and Synaptic Plasticity

Epigenetic mechanisms result in persistent changes at the cellular level that can lead to long-lasting behavioral adaptations. Nucleosome remodeling is a major epigenetic mechanism that has not been well explored with regards to drug-seeking behaviors. Nucleosome remodeling is performed by multi-subunit complexes that interact with DNA or chromatin structure and possess an ATP-dependent enzyme to disrupt nucleosome-DNA contacts and ultimately regulate gene expression. Calcium responsive transactivator (CREST) is a transcriptional activator that interacts with enzymes involved in both histone acetylation and nucleosome remodeling. Here, we examined the effects of knocking down CREST in the nucleus accumbens (NAc) core on drug-seeking behavior and synaptic plasticity in male mice as well as drug-seeking in male rats. Knocking down CREST in the NAc core results in impaired cocaine-induced conditioned place preference (CPP) as well as theta-induced long-term potentiation in the NAc core. Further, similar to the CPP findings, using a self-administration procedure, we found that CREST knockdown in the NAc core of male rats had no effect on instrumental responding for cocaine itself on a first-order schedule, but did significantly attenuate responding on a second-order chain schedule, in which responding has a weaker association with cocaine. Together, these results suggest that CREST in the NAc core is required for cocaine-induced CPP, synaptic plasticity, as well as cocaine-seeking behavior.SIGNIFICANCE STATEMENT This study demonstrates a key role for the role of Calcium responsive transactivator (CREST), a transcriptional activator, in the nucleus accumbens (NAc) core with regard to cocaine-induced conditioned place preference (CPP), self-administration (SA), and synaptic plasticity. CREST is a unique transcriptional regulator that can recruit enzymes from two different major epigenetic mechanisms: histone acetylation and nucleosome remodeling. In this study we also found that the level of potentiation in the NAc core correlated with whether or not animals formed a CPP. Together the results indicate that CREST is a key downstream regulator of cocaine action in the NAc.

Adenosine A2A receptor and ERK-driven impulsivity potentiates hippocampal neuroblast proliferation

Dampened adenosine A_2A receptor (A_2AR) function has been implicated in addiction through enhancement of goal-directed behaviors. However, the contribution of the A_2AR to the control of impulsive reward seeking remains unknown. Using mice that were exposed to differential reward of low rate (DRL) schedules during Pavlovian-conditioning, second-order schedule discrimination, and the 5-choice serial reaction time task (5-CSRTT), we demonstrate that deficits of A_2AR function promote impulsive responses. Antagonism of the A_2AR lowered ERK1 and ERK2 phosphorylation in the dorsal hippocampus (dHip) and potentiated impulsivity during Pavlovian-conditioning and the 5-CSRTT. Remarkably, inhibition of ERK1 and ERK2 phosphorylation by U0126 in the dHip prior to Pavlovian-conditioning exacerbated impulsive reward seeking. Moreover, we found decreased A_2AR expression, and reduced ERK1 and ERK2 phosphorylation in the dHip of equilibrative nucleoside transporter type 1 (ENT1^-/-) null mice, which displayed exacerbated impulsivity. To determine whether impulsive response behavior is associated with hippocampal neuroblast development, we investigated expression of BrdU⁺ and doublecortin (DCX⁺) following 5-CSRTT testing. These studies revealed that impulsive behavior driven by inhibition of the A_2AR is accompanied by increased neuroblast proliferation in the hippocampus.

Label-Free Neuroproteomics of the Hippocampal-Accumbal Circuit Reveals Deficits in Neurotransmitter and Neuropeptide Signaling in Mice Lacking Ethanol-Sensitive Adenosine Transporter

The neural circuit of the dorsal hippocampus (dHip) and nucleus accumbens (NAc) contributes to cue-induced learning and addictive behaviors, as demonstrated by the escalation of ethanol-seeking behaviors observed following deletion of the adenosine equilibrative nucleoside transporter 1 (ENT1^-/-) in mice. Here we perform quantitative LC-MS/MS neuroproteomics in the dHip and NAc of ENT1^-/- mice. Using Ingenuity Pathway Analysis, we identified proteins associated with increased long-term potentiation, ARP2/3-mediated actin cytoskeleton signaling and protein expression patterns suggesting deficits in glutamate degradation, GABAergic signaling, as well as significant changes in bioenergetics and energy homeostasis (oxidative phosphorylation, TCA cycle, and glycolysis). These pathways are consistent with previously reported behavioral and biochemical phenotypes that typify mice lacking ENT1. Moreover, we validated decreased expression of the SNARE complex protein VAMP1 (synaptobrevin-1) in the dHip as well as decreased expression of pro-dynorphin (PDYN), neuroendocrine convertase (PCSK1), and Leu-Enkephalin (dynorphin-A) in the NAc. Taken together, our proteomic approach provides novel pathways indicating that ENT1-regulated signaling is essential for neurotransmitter release and neuropeptide processing, both of which underlie learning and reward-seeking behaviors.

A Decade of Orexin/Hypocretin and Addiction: Where Are We Now?

One decade ago, our laboratory provided the first direct evidence linking orexin/hypocretin signaling with drug seeking by showing that activation of these neurons promotes conditioned morphine-seeking behavior. In the years since, contributions from many investigators have revealed roles for orexins in addiction for all drugs of abuse tested, but only under select circumstances. We recently proposed that orexins play a fundamentally unified role in coordinating "motivational activation" under numerous behavioral conditions, and here we unpack this hypothesis as it applies to drug addiction. We describe evidence collected over the past 10 years that elaborates the role of orexin in drug seeking under circumstances where high levels of effort are required to obtain the drug, or when motivation for drug reward is augmented by the presence of external stimuli like drug-associated cues/contexts or stressors. Evidence from studies using traditional self-administration and reinstatement models, as well as behavioral economic analyses of drug demand elasticity, clearly delineates a role for orexin in modulating motivational, rather than the primary reinforcing aspects of drug reward. We also discuss the anatomical interconnectedness of the orexin system with wider motivation and reward circuits, with a particular focus on how orexin modulates prefrontal and other glutamatergic inputs onto ventral tegmental area dopamine neurons. Last, we look ahead to the next decade of the research in this area, highlighting the recent FDA approval of the dual orexin receptor antagonist suvorexant (Belsomra®) for the treatment of insomnia as a promising sign of the potential clinical utility of orexin-based therapies for the treatment of addiction.

Evaluation of the "Pipeline" for Development of Medications for Cocaine Use Disorder: A Review of Translational Preclinical, Human Laboratory, and Clinical Trial Research

Cocaine use disorder is a persistent public health problem for which no widely effective medications exist. Self-administration procedures, which have shown good predictive validity in estimating the abuse potential of drugs, have been used in rodent, nonhuman primate, and human laboratory studies to screen putative medications. This review assessed the effectiveness of the medications development process regarding pharmacotherapies for cocaine use disorder. The primary objective was to determine whether data from animal and human laboratory self-administration studies predicted the results of clinical trials. In addition, the concordance between laboratory studies in animals and humans was assessed. More than 100 blinded, randomized, fully placebo-controlled studies of putative medications for cocaine use disorder were identified. Of the 64 drugs tested in these trials, only 10 had been examined in both human and well-controlled animal laboratory studies. Within all three stages, few studies had been conducted for each drug and when multiple studies had been conducted conclusions were sometimes contradictory. Overall, however, there was good concordance between animal and human laboratory results when the former assessed chronic drug treatment. Although only seven of the ten reviewed drugs showed fully concordant results across all three types of studies reviewed, the analysis revealed several subject-related, procedural, and environmental factors that differ between the laboratory and clinical trial settings that help explain the disagreement for other drugs. The review closes with several recommendations to enhance translation and communication across stages of the medications development process that will ultimately speed the progress toward effective pharmacotherapeutic strategies for cocaine use disorder.

A Selective Role for Lmo4 in Cue-Reward Learning

The ability to use environmental cues to predict rewarding events is essential to survival. The basolateral amygdala (BLA) plays a central role in such forms of associative learning. Aberrant cue-reward learning is thought to underlie many psychopathologies, including addiction, so understanding the underlying molecular mechanisms can inform strategies for intervention. The transcriptional regulator LIM-only 4 (LMO4) is highly expressed in pyramidal neurons of the BLA, where it plays an important role in fear learning. Because the BLA also contributes to cue-reward learning, we investigated the role of BLA LMO4 in this process using Lmo4-deficient mice and RNA interference. Lmo4-deficient mice showed a selective deficit in conditioned reinforcement. Knockdown of LMO4 in the BLA, but not in the nucleus accumbens, recapitulated this deficit in wild-type mice. Molecular and electrophysiological studies identified a deficit in dopamine D2 receptor signaling in the BLA of Lmo4-deficient mice. These results reveal a novel, LMO4-dependent transcriptional program within the BLA that is essential to cue-reward learning.

Sigma-1 receptor mediates acquisition of alcohol drinking and seeking behavior in alcohol-preferring rats

Sigma-1 receptor (Sig-1R) has been proposed as a novel therapeutic target for drug and alcohol addiction. We have shown previously that Sig-1R agonists facilitate the reinforcing effects of ethanol and induce binge-like drinking, while Sig-1R antagonists on the other hand block excessive drinking in genetic and environmental models of alcoholism, without affecting intake in outbred non-dependent rats. Even though significant progress has been made in understanding the function of Sig-1R in alcohol reinforcement, its role in the early and late stage of alcohol addiction remains unclear. Administration of the selective Sig-1R antagonist BD-1063 dramatically reduced the acquisition of alcohol drinking behavior as well as the preference for alcohol in genetically selected TSRI Sardinian alcohol preferring (Scr:sP) rats; the treatment had instead no effect on total fluid intake, food intake or body weight gain, proving selectivity of action. Furthermore, BD-1063 dose-dependently decreased alcohol-seeking behavior in rats trained under a second-order schedule of reinforcement, in which responding is maintained by contingent presentation of a conditioned reinforcer. Finally, an innate elevation in Sig-1R protein levels was found in the nucleus accumbens of alcohol-preferring Scr:sP rats, compared to outbred Wistar rats, alteration which was normalized by chronic, voluntary alcohol drinking. Taken together these findings demonstrate that Sig-1R blockade reduces the propensity to both acquire alcohol drinking and to seek alcohol, and point to the nucleus accumbens as a potential key region for the effects observed. Our data suggest that Sig-1R antagonists may have therapeutic potential in multiple stages of alcohol addiction.

Skewed by Cues? The Motivational Role of Audiovisual Stimuli in Modelling Substance Use and Gambling Disorders

The similarity between gambling disorder (GD) and drug addiction has recently been recognized at the diagnostic level. Understanding the core cognitive processes involved in these addiction disorders, and in turn their neurobiological mechanisms, remains a research priority due to the enormous benefits such knowledge would have in enabling effective treatment design. Animal models can be highly informative in this regard. Although numerous rodent behavioural paradigms that capture different facets of gambling-like behaviour have recently been developed, the motivational power of cues in biasing individuals towards risky choice has so far received little attention despite the central role played by drug-paired cues in successful laboratory models of chemical dependency. Here, we review some of the comparatively simple paradigms in which reward-paired cues are known to modulate behaviour in rodents, such as sign-tracking, Pavlovian-to-instrumental transfer and conditioned reinforcement. Such processes are thought to play an important role in mediating responding for drug reward, and the need for future studies to address whether similar processes contribute to cue-driven risky choice is highlighted.

Optogenetic excitation of central amygdala amplifies and narrows incentive motivation to pursue one reward above another

Choosing one reward above another is important for achieving adaptive life goals. Yet hijacked into excessive intensity in disorders such as addiction, single-minded pursuit becomes maladaptive. Here, we report that optogenetic channelrhodopsin stimulation of neurons in central nucleus of amygdala (CeA), paired with earning a particular sucrose reward in rats, amplified and narrowed incentive motivation to that single reward target. Therefore, CeA rats chose and intensely pursued only the laser-paired sucrose reward while ignoring an equally good sucrose alternative. In contrast, reward-paired stimulation of basolateral amygdala did not hijack choice. In a separate measure of incentive motivation, CeA stimulation also increased the progressive ratio breakpoint or level of effort exerted to obtain sucrose reward. However, CeA stimulation by itself failed to support behavioral self-stimulation in the absence of any paired external food reward, suggesting that CeA photo-excitation specifically transformed the value of its external reward (rather than adding an internal reinforcement state). Nor did CeA stimulation by itself induce any aversive state that motivated escape. Finally, CeA stimulation also failed to enhance 'liking' reactions elicited by sucrose taste and did not simply increase the general motivation to eat. This pattern suggests that CeA photo-excitation specifically enhances and narrows incentive motivation to pursue an associated external reward at the expense of another comparable reward.

Dopamine D3 receptors in the basolateral amygdala and the lateral habenula modulate cue-induced reinstatement of nicotine seeking

Dopamine D3 receptors are implicated in cue-induced relapse to drug seeking. We have previously shown that systemic administration of a selective D3 antagonist reduces cue-induced reinstatement of nicotine seeking in rats. The current study sought to investigate potential neural substrates mediating this effect. The D3 antagonist SB-277011-A (0.01-1 μg/0.5 μl/side) infused into the basolateral amygdala or the lateral habenula, but not the nucleus accumbens, significantly attenuated cue-induced reinstatement of nicotine seeking. Moreover, infusion of SB-277011-A (1 μg/0.5 μl/side) into the basolateral amygdala or lateral habenula had no effect on food self-administration. Together with the finding that systemic SB-277011-A had no effect on extinction responding, this suggests that the effects observed here were on reinstatement and cue seeking, and not due to nonspecific motor activation or contextual-modified residual responding. The further finding of binding of [(125)I]7-OH-PIPAT to D3 receptors in the lateral habenula and in the basolateral amygdala is consistent with an important role of D3 receptors in these areas in nicotine seeking. It was also found that systemic administration of the selective D2 antagonist L741626 decreased cue-induced reinstatement, consistent with a role of D2 and D3 receptors in modulating this behavior. The current study supports an important role for D3 receptors in the basolateral amygdala and lateral habenula in cue-induced reinstatement.

Peroxisome proliferator-activated receptor (PPAR) agonists as promising new medications for drug addiction: preclinical evidence

This review examines the growing literature on the role of peroxisome proliferator-activated receptors (PPARs) in addiction. There are two subtypes of PPAR receptors that have been studied in addiction: PPAR-α and PPAR-γ. The role of each PPAR subtype in common models of addictive behavior, mainly pre-clinical models, is summarized. In particular, studies are reviewed that investigated the effects of PPAR-α agonists on relapse, sensitization, conditioned place preference, withdrawal and drug intake, and effects of PPAR-γ agonists on relapse, withdrawal and drug intake. Finally, studies that investigated the effects of PPAR agonists on neural pathways of addiction are reviewed. Taken together these preclinical data indicate that PPAR agonists are promising new medications for drug addiction treatment.

Individual variation in resisting temptation: implications for addiction

When exposed to the sights, sounds, smells and/or places that have been associated with rewards, such as food or drugs, some individuals have difficulty resisting the temptation to seek out and consume them. Others have less difficulty restraining themselves. Thus, Pavlovian reward cues may motivate maladaptive patterns of behavior to a greater extent in some individuals than in others. We are just beginning to understand the factors underlying individual differences in the extent to which reward cues acquire powerful motivational properties, and therefore, the ability to act as incentive stimuli. Here we review converging evidence from studies in both human and non-human animals suggesting that a subset of individuals are more "cue reactive", in that certain reward cues are more likely to attract these individuals to them and motivate actions to get them. We suggest that those individuals for whom Pavlovian reward cues become especially powerful incentives may be more vulnerable to impulse control disorders, such as binge eating and addiction.

Cocaine-induced reinstatement of a conditioned place preference in developing rats: involvement of the d2 receptor

Reinstatement of conditioned place preferences have been used to investigate physiological mechanisms mediating drug-seeking behavior in adolescent and adult rodents; however, it is still unclear how psychostimulant exposure during adolescence affects neuron communication and whether these changes would elicit enhanced drug-seeking behavior later in adulthood. The present study determined whether the effects of intra-ventral tegmental area (VTA) or intra-nucleus accumbens septi (NAcc) dopamine (DA) D2 receptor antagonist infusions would block (or potentiate) cocaine-induced reinstatement of conditioned place preferences. Adolescent rats (postnatal day (PND 28–39)) were trained to express a cocaine place preference. The involvement of D2 receptors on cocaine-induced reinstatement was determined by intra-VTA or intra-NAcc infusion of the DA D2 receptor antagonist sulpiride (100 μM) during a cocaine-primed reinstatement test (10 mg/kg cocaine, i.p.). Infusion of sulpiride into the VTA but not the NAcc blocked reinstatement of conditioned place preference. These data suggest intrinsic compensatory mechanisms in the mesolimbic DA pathway mediate responsivity to cocaine-induced reinstatement of a conditioned place preference during development.

What and when to "want"? Amygdala-based focusing of incentive salience upon sugar and sex

RATIONALE

Amygdala-related circuitry helps translate learned Pavlovian associations into appetitive and aversive motivation, especially upon subsequent encounters with cues.

OBJECTIVES

We asked whether μ-opioid stimulation via microinjections of the specific agonist D-Ala(2), N-MePhe(4), Gly-ol)-enkephalin (DAMGO) in central nucleus of amygdala (CeA), or the adjacent basolateral amygdala (BLA) would magnify sucrose or sex "wanting", guided by available cues.

MATERIALS AND METHODS

CeA or BLA DAMGO enhancement of cue-triggered "wanting" was assessed using Pavlovian to instrumental transfer (PIT). Unconditioned food "wanting" was measured via intake, and male sexual "wanting" for an estrous female was measured in a sexual approach test. Sucrose hedonic taste "liking" was measured in a taste reactivity test.

RESULTS

CeA (but not BLA) DAMGO increased the intensity of phasic peaks in instrumental sucrose seeking stimulated by Pavlovian cues over precue levels in PIT, while suppressing seeking at other moments. CeA DAMGO also enhanced food intake, as well as sexual approach and investigation of an estrous female by males. DAMGO "wanting" enhancements were localized to CeA, as indicated by "Fos plume"-based anatomical maps for DAMGO causation of behavioral effects. Despite increasing "wanting", CeA DAMGO decreased the hedonic impact or "liking" for sucrose in a taste reactivity paradigm.

CONCLUSIONS

CeA μ-opioid stimulation specifically enhances incentive salience, which is dynamically guided to food or sex by available cues.

Chronic morphine treatment switches the effect of dopamine on excitatory synaptic transmission from inhibition to excitation in pyramidal cells of the basolateral amygdala

Dopaminergic signaling in the basolateral amygdala (BLA) is important for drug-stimulus learning that triggers relapse to drug-seeking behavior. However, little is known about adaptive changes in this signaling pathway upon chronic morphine treatment. In this paper, we observed the influence of chronic morphine treatment on the effect of dopamine (DA) on the excitatory transmission in the pyramidal cells of BLA in slices with the whole-cell patch-clamp method. We also studied its mechanism and significance with pharmacological approaches combined with biochemical and behavioral techniques. The results showed that chronic morphine exposure switched the effect of DA on the excitatory synaptic transmission from inhibition to excitation; the chronic morphine-induced switching action on the effect of DA was due to its influence on D1 receptors; the site of the effect of chronic morphine treatment on D1 receptors was at presynaptic locus; chronic morphine treatment induced a significant increase in the amount of D1 receptor expression in the synaptosomes and synaptosomal membrane fraction from BLA; the enhancement of presynaptic glutamate release by D1 receptor agonist upon chronic morphine treatment was dependent on the activation of cAMP-dependent protein kinase; and the intra-BLA injection of D1 receptor antagonist canceled the conditioned place aversion (CPA) in morphine-dependent rats. In conclusion, chronic morphine treatment switches the effect of DA on the excitatory synaptic transmission from inhibition to excitation by the presynaptic D1 receptor amount increase-mediated glutamate release in the pyramidal cells of BLA and the blockade of D1 receptors in BLA cancels CPA in morphine-dependent rats.

Multiple roles for orexin/hypocretin in addiction

Orexins/hypocretins are hypothalamic peptides involved in arousal and wakefulness, but also play a critical role in drug addiction and reward-related behaviors. Here, we review the roles played by orexins in a variety of animal models of drug addiction, emphasizing both commonalities and differences for orexin's involvement in seeking of the major classes of abused drugs, as well as food. One common theme that emerges is an involvement of orexins in drug seeking triggered by external stimuli (e.g., cues, contexts or stressors). We also discuss the functional neuronal circuits in which orexins are embedded, and how these circuits mediate addiction-related behaviors, with particular focus on the role of orexin and glutamate interactions within the ventral tegmental area. Finally, we attempt to contextualize the role of orexins in reward by discussing ways in which these peptides, expressed in only a few thousand neurons in the brain, can have such wide-ranging effects on behavior.

Alcohol-related effects on automaticity due to experimentally manipulated conditioning

BACKGROUND

The use of alcohol is associated with various forms of automatic processing, such as approach tendencies and attentional biases, which may play a role in addictive behavior. The development of such automaticity has generally occurred well before subjects perform tasks designed to detect them. Although it seems plausible that this development involves some form of alcohol-related conditioning, this process is not usually included in the experimental procedure.

METHODS

The development of automaticity involving alcoholic or nonalcoholic stimuli was experimentally manipulated via a conditioning task. Subjects were presented with pairs of stimuli from a set of 4 stimuli: 2 pictures of alcoholic beverages, and 2 pictures of nonalcoholic beverages. One of the alcoholic and 1 of the nonalcoholic beverages was associated with reward, the other stimuli with punishment. Subjects had to learn to select the rewarded stimuli from pairs of 1 rewarded and 1 punished stimulus. The task, thus experimentally established reward versus punishment stimulus-response-outcome associations, for alcoholic and for nonalcoholic stimuli. Subsequently, a cued reversal task was used to test automaticity involving alcoholic versus nonalcoholic, and rewarded versus punished stimuli.

RESULTS

An association was found between heavier drinking and an alcohol-related conditioning bias: heavier drinkers had more difficulty overcoming a conditioned response when it involved selecting a previously punished nonalcoholic stimulus over a previously rewarded alcoholic stimulus.

CONCLUSIONS

The study provided novel information on secondary reinforcement involving alcoholic stimuli: heavier drinkers may more easily develop automaticity related to alcohol-reward contingencies. This may have implications for interventions and the interpretation of findings concerning alcohol-related automatic processing.

Neural Changes Developed during the Extinction of Cocaine Self-Administration Behavior

The high rate of recidivism in cocaine addiction after prolonged periods of abstinence poses a significant problem for the effective treatment of this condition. Moreover, the neurobiological basis of this relapse phenomenon remains poorly understood. In this review, we will discuss the evidence currently available regarding the neurobiological changes during the extinction of cocaine self-administration. Specifically, we will focus on alterations in the dopaminergic, opioidergic, glutamatergic, cholinergic, serotoninergic and CRF systems described in self-administration experiments and extinction studies after chronic cocaine administration. We will also discuss the differences related to contingent versus non-contingent cocaine administration, which highlights the importance of environmental cues on drug effects and extinction. The findings discussed in this review may aid the development of more effective therapeutic approaches to treat cocaine relapse.

D-cycloserine facilitates extinction of cocaine self-administration in C57 mice

INTRODUCTION

Cocaine is a highly addictive drug of abuse for which there are currently no medications. In rats and mice d-cycloserine (DCS), a partial NMDA agonist, accelerates extinction of cocaine seeking behavior. Since cues delay extinction here, we evaluated the effects d-cycloserine in extinction with and without the presence of cues.

METHODS

Two doses of DCS (15 and 30 mg/kg) were studied in C57 mice. Mice self-administered cocaine (1 mg/kg) for 2 weeks and then underwent a 20-day extinction period where DCS was administered i.p. immediately following each daily session. Extinction was conducted in some mice with the presence of cocaine-paired cues; while others were in the absence of these cues.

RESULTS

DCS treated mice (either dose) showed significantly reduced lever pressing during extinction with cue exposures when compared with vehicle treated mice. Without cues, animals showed much lower levels of lever pressing but the differences between vehicle and DCS were not significant.

CONCLUSION

DCS accelerated extinction with the presence of cues, but there were no differences on extinction without cues as compared with vehicle. These findings are consistent with DCS disrupting the memory process associated with the cues. Since drug cues are significantly involved in relapse, these findings support research to assess the therapeutic potential of DCS in cocaine addiction.

Acute effects of nicotine amplify accumbal neural responses during nicotine-taking behavior and nicotine-paired environmental cues

Nicotine self-administration (SA) is maintained by several variables, including the reinforcing properties of nicotine-paired cues and the nicotine-induced amplification of those cue properties. The nucleus accumbens (NAc) is implicated in mediating the influence of these variables, though the underlying neurophysiological mechanisms are not yet understood. In the present study, Long-Evans rats were trained to self-administer nicotine. During SA sessions each press of a lever was followed by an intravenous infusion of nicotine (30 µg/kg) paired with a combined light-tone cue. Extracellular recordings of single-neuron activity showed that 20% of neurons exhibited a phasic change in firing during the nicotine-directed operant, the light-tone cue, or both. The phasic change in firing for 98% of neurons was an increase. Sixty-two percent of NAc neurons additionally or alternatively showed a sustained decrease in average firing during the SA session relative to a presession baseline period. These session decreases in firing were significantly less prevalent in a group of neurons that were activated during either the operant or the cue than in a group of neurons that were nonresponsive during those events (referred to as task-activated and task-nonactivated neurons, respectively). Moreover, the session decrease in firing was dose-dependent for only the task-nonactivated neurons. The data of the present investigation provide supportive correlational evidence for two hypotheses: (1) excitatory neurophysiological mechanisms mediate the NAc role in cue-maintenance of nicotine SA, and (2) a differential nicotine-induced inhibition of task-activated and task-nonactivated neurons mediates the NAc role in nicotine-induced amplification of cue effects on nicotine SA.

Dopamine D1 receptors within the basolateral amygdala mediate heroin-induced conditioned immunomodulation

This study investigates the role of basolateral amygdala (BLA) dopamine in heroin-induced conditioned immunomodulation. Animals underwent conditioning in which heroin administration was repeatedly paired with placement into a conditioning chamber. Six days after the final conditioning session animals were returned to the chamber and received intra-BLA microinfusions of dopamine, D(1) or D(2), antagonist. Antagonism of D(1), but not D(2), receptors within the BLA blocked the suppressive effect of heroin-associated environmental stimuli on iNOS, TNF-α and IL-1β. This study is the first to demonstrate that the expression of heroin's conditioned effects on proinflammatory mediators require dopamine D(1) receptors within the BLA.

A molecular dissociation between cued and contextual appetitive learning

In appetitive Pavlovian learning, animals learn to associate discrete cues or environmental contexts with rewarding outcomes, and these cues and/or contexts can potentiate an ongoing instrumental response for reward. Although anatomical substrates underlying cued and contextual learning have been proposed, it remains unknown whether specific molecular signaling pathways within the striatum underlie one form of learning or the other. Here, we show that while the striatum-enriched isoform of adenylyl cyclase (AC5) is required for cued appetitive Pavlovian learning, it is not required for contextual appetitive learning. Mice lacking AC5 (AC5KO) could not learn an appetitive Pavlovian learning task in which a discrete signal light predicted reward delivery, yet they could form associations between context and either natural or drug reward, which could in turn elicit Pavlovian approach behavior. However, unlike wild-type (WT) mice, AC5KO mice could not use these Pavlovian conditioned stimuli to potentiate ongoing instrumental behavior in a Pavlovian-to-instrumental transfer paradigm. These data suggest that AC5 is specifically required for learning associations between discrete cues and outcomes in which the temporal relationship between conditioned stimulus (CS) and unconditioned stimulus (US) is essential, while alternative signaling mechanisms may underlie the formation of associations between context and reward. In addition, loss of AC5 compromises the ability of both contextual and discrete cues to modulate instrumental behavior.

Cocaine effects on mouse incentive-learning and human addiction are linked to alpha2 subunit-containing GABAA receptors

Because GABA(A) receptors containing alpha2 subunits are highly represented in areas of the brain, such as nucleus accumbens (NAcc), frontal cortex, and amygdala, regions intimately involved in signaling motivation and reward, we hypothesized that manipulations of this receptor subtype would influence processing of rewards. Voltage-clamp recordings from NAcc medium spiny neurons of mice with alpha2 gene deletion showed reduced synaptic GABA(A) receptor-mediated responses. Behaviorally, the deletion abolished cocaine's ability to potentiate behaviors conditioned to rewards (conditioned reinforcement), and to support behavioral sensitization. In mice with a point mutation in the benzodiazepine binding pocket of alpha2-GABA(A) receptors (alpha2H101R), GABAergic neurotransmission in medium spiny neurons was identical to that of WT (i.e., the mutation was silent), but importantly, receptor function was now facilitated by the atypical benzodiazepine Ro 15-4513 (ethyl 8-amido-5,6-dihydro-5-methyl-6-oxo-4H-imidazo [1,5-a] [1,4] benzodiazepine-3-carboxylate). In alpha2H101R, but not WT mice, Ro 15-4513 administered directly into the NAcc-stimulated locomotor activity, and when given systemically and repeatedly, induced behavioral sensitization. These data indicate that activation of alpha2-GABA(A) receptors (most likely in NAcc) is both necessary and sufficient for behavioral sensitization. Consistent with a role of these receptors in addiction, we found specific markers and haplotypes of the GABRA2 gene to be associated with human cocaine addiction.

Role of the orbitofrontal cortex and dorsal striatum in regulating the dose-related effects of self-administered cocaine

Little is known regarding which neural systems regulate dose-related changes in responses maintained by self-administered cocaine. This empirical question is important because elucidating neural systems engaged in this process could provide clues for effectively treating cocaine addiction. It has been suggested that different cocaine doses represent reinforcers of differing magnitudes, implicating the dorsal striatum or orbitofrontal cortex as important. Rats were trained to self-administer 1.0 mg/kg cocaine under a fixed-interval based second-order schedule. Next, cocaine unit doses (0.1-3.0 mg/kg) were each non-systematically available for a 5-day block of sessions. Tests (1h) were conducted on day 3 (vehicle) and day 5 (100 microg lidocaine) of each block. Lidocaine inactivation of the lateral dorsal striatum had no effect on dose-related responding or cocaine intake. In contrast, when doses along the ascending limb were available for self-administration, lidocaine inactivation of the lateral orbitofrontal cortex caused reductions in responding and cocaine intake, resulting in overall flattening of dose-response curves. This included reductions during the entire 1-h test sessions and during the interval immediately following the first cocaine infusion of test sessions. Lidocaine inactivation of the lateral orbitofrontal cortex did not alter responding during the first cocaine-free interval of test sessions, but increased the latency to the first infusion. Collectively, the findings suggest that when the amount of experience with different cocaine unit doses is limited to a few sessions, the lateral orbitofrontal cortex regulates the dose-related effects of self-administered cocaine, likely by processing information pertaining to the reinforcing value of each unit dose.

Which cue to "want?" Central amygdala opioid activation enhances and focuses incentive salience on a prepotent reward cue

The central nucleus of the amygdala (CeA) helps translate learning into motivation, and here, we show that opioid stimulation of CeA magnifies and focuses learned incentive salience onto a specific reward cue (pavlovian conditioned stimulus, or CS). This motivation enhancement makes that cue more attractive, noticeable, and liable to elicit appetitive and consummatory behaviors. To reveal the focusing of incentive salience, we exploited individual differences in an autoshaping paradigm in which a rat prefers to approach, nibble, and sniff one of two reward-associated stimuli (its prepotent stimulus). The individually prepotent cue is either a predictive CS+ that signals reward (8 s metal lever insertion) or instead the metal cup that delivers sucrose pellets (the reward source). Results indicated that CeA opioid activation by microinjection of the mu agonist DAMGO (0.1 microg) selectively and reversibly enhanced the attractiveness of whichever reward CS was that rat's prepotent cue. CeA DAMGO microinjections made rats more vigorously approach their particular prepotent CS and to energetically sniff and nibble it in a nearly frenzied consummatory manner. Only the prepotent cue was enhanced as an incentive target, and alternative cues were not enhanced. Conversely, inactivation of CeA by muscimol microinjection (0.25 microg) suppressed approach, nibbles, and sniffs of the prepotent CS. Confirming modulation of incentive salience, unconditioned food intake was similarly increased by DAMGO microinjection and decreased by muscimol in CeA. We conclude that opioid neurotransmission in CeA helps determine which environmental stimuli become most "wanted," and how "wanted" they become. This may powerfully guide reward-seeking behavior.

Parallel and interactive learning processes within the basal ganglia: relevance for the understanding of addiction

In this review we discuss the evidence that drug addiction, defined as a maladaptive compulsive habit, results from the progressive subversion by addictive drugs of striatum-dependent operant and Pavlovian learning mechanisms that are usually involved in the control over behaviour by stimuli associated with natural reinforcement. Although mainly organized through segregated parallel cortico-striato-pallido-thalamo-cortical loops involved in motor or emotional functions, the basal ganglia, and especially the striatum, are key mediators of the modulation of behavioural responses, under the control of both action-outcome and stimulus-response mechanisms, by incentive motivational processes and Pavlovian associations. Here we suggest that protracted exposure to addictive drugs recruits serial and dopamine-dependent, striato-nigro-striatal ascending spirals from the nucleus accumbens to more dorsal regions of the striatum that underlie a shift from action-outcome to stimulus-response mechanisms in the control over drug seeking. When this progressive ventral to dorsal striatum shift is combined with drug-associated Pavlovian influences from limbic structures such as the amygdala and the orbitofrontal cortex, drug seeking behaviour becomes established as an incentive habit. This instantiation of implicit sub-cortical processing of drug-associated stimuli and instrumental responding might be a key mechanism underlying the development of compulsive drug seeking and the high vulnerability to relapse which are hallmarks of drug addiction.

Relapse to drug seeking following prolonged abstinence: the role of environmental stimuli

Successful treatment of drug addiction must involve relapse prevention informed by our understanding of the neurobiological bases of drug relapse. In humans, exposure to drug-associated environmental stimuli can elicit drug craving and relapse. Because exposure to drug-paired stimuli similarly induces drug-seeking behavior in laboratory animals, several animal models of drug relapse have been developed. Here, we review animal models of cue-induced drug relapse and critically evaluate their validity and utility in addressing human relapse behaviors.

Targeting endogenous mu- and delta-opioid receptor systems for the treatment of drug addiction

Drug addiction is a chronic, relapsing disorder that is characterized by a compulsion to take drug regardless of the adverse consequences that may ensue. Although the involvement of mesoaccumbal dopamine neurons in the initiation of drug abuse is well-established, neuroadaptations within the limbic cortical- striatopallidal circuit that occur as a consequence of repeated drug use are thought to lead to the behavioral dysregulation that characterizes addiction. Opioid receptors and their endogenous ligands are enriched in brain regions comprising this system and are, thus, strategically located to modulate neurotransmission therein. This article will review data suggesting an important role of mu-opioid receptor (MOPr) and delta opioid receptor (DOPr) systems in mediating the rewarding effects of several classes of abused drugs and that aberrant activity of these opioid systems may not only contribute to the behavioral dysregulation that characterizes addiction but to individual differences in addiction vulnerability.

Dopaminergic innervation of pyramidal cells in the rat basolateral amygdala

Dopaminergic (DA) inputs to the basolateral nuclear complex of the amygdala (BLC) are critical for several important functions, including reward-related learning, drug-stimulus learning, and fear conditioning. Despite the importance of the DA projection to the BLC, very little is known about which neuronal subpopulations are innervated. The present study utilized dual-labeling immunohistochemistry at the electron microscopic level to examine DA inputs to pyramidal cells in the anterior basolateral amygdalar nucleus (BLa) in the rat. DA axon terminals and BLa pyramidal cells were labeled using antibodies to tyrosine hydroxylase (TH) and calcium/calmodulin-dependent protein kinase II (CaMK), respectively. Serial section reconstructions of TH-positive (TH+) terminals were performed to determine the extent to which these axon terminals formed synapses versus non-synaptic appositions in the BLa. Our results demonstrate that at least 77% of TH+ terminals form synapses in the BLa, and that 90% of these synapses are with pyramidal cells. The distal dendritic compartment received the great majority of these synaptic contacts, with CaMK+ distal dendrites and spines receiving one-third and one-half, respectively, of all synaptic inputs to pyramidal cells. Many spines receiving innervation from TH+ terminals also received asymmetrical synaptic inputs from putative excitatory terminals. In addition, TH+ terminals often formed non-synaptic appositions with axon terminals, most of which were putatively excitatory in that they were CaMK+ and/or made asymmetrical synapses. Thus, using CaMK as a marker, the present study demonstrates that pyramidal cells, especially their distal dendritic compartments, are the primary targets of dopaminergic inputs to the basolateral amygdala.

Roles of the nucleus accumbens and amygdala in the acquisition and expression of ethanol-conditioned behavior in mice

Although progress has been made identifying the neural areas underlying the primary reinforcing effects of ethanol, few studies have examined the neural areas mediating ethanol-induced conditioned effects. Previous work using the conditioned place preference (CPP) procedure implicates the ventral tegmental area (VTA) (Bechtholt and Cunningham, 2005), but the downstream neural areas modulating the conditioned rewarding effects of ethanol have not been identified. Although the nucleus accumbens (Acb) and the amygdala (Amy), which both receive dopamine innervation from the VTA, have been implicated in the primary reinforcing effects of ethanol, the roles these areas play in ethanol-conditioned behaviors are unknown. In the present set of experiments, we use the CPP procedure along with selective electrolytic lesions to examine the neural areas underlying the acquisition and expression of ethanol conditioned behavior. In the acquisition experiment, male DBA/2J mice received bilateral lesions of the Acb or Amy before CPP training. In the expression experiments, mice received bilateral lesions of the Acb, Acb shell, Acb core, and Amy, or unilateral lesions of the Amy after training but before testing. Lesions of the Acb and Amy before training disrupted acquisition and expression of ethanol CPP. However, when given after training, only lesions of the Amy disrupted expression, whereas lesions of the Acb core facilitated loss of responding, of ethanol CPP. For the first time, these results demonstrate the role of the Acb and Amy in the acquisition and expression of ethanol-induced conditioned reward.

Dynorphin and the pathophysiology of drug addiction

Drug addiction is a chronic relapsing disease in which drug administration becomes the primary stimulus that drives behavior regardless of the adverse consequence that may ensue. As drug use becomes more compulsive, motivation for natural rewards that normally drive behavior decreases. The discontinuation of drug use is associated with somatic signs of withdrawal, dysphoria, anxiety, and anhedonia. These consequences of drug use are thought to contribute to the maintenance of drug use and to the reinstatement of compulsive drug use that occurs during the early phase of abstinence. Even, however, after prolonged periods of abstinence, 80-90% of human addicts relapse to addiction, suggesting that repeated drug use produces enduring changes in brain circuits that subserve incentive motivation and stimulus-response (habit) learning. A major goal of addiction research is the identification of the neural mechanisms by which drugs of abuse produce these effects. This article will review data showing that the dynorphin/kappa-opioid receptor (KOPr) system serves an essential function in opposing alterations in behavior and brain neurochemistry that occur as a consequence of repeated drug use and that aberrant activity of this system may not only contribute to the dysregulation of behavior that characterizes addiction but to individual differences in vulnerability to the pharmacological actions of cocaine and alcohol. We will provide evidence that the repeated administration of cocaine and alcohol up-regulates the dynorphin/KOPr system and that pharmacological treatments that target this system may prove effective in the treatment of drug addiction.

Role of the prelimbic cortex in the acquisition, re-acquisition or persistence of responding for a drug-paired conditioned reinforcer

The medial prefrontal cortex has been suggested to play a role in drug addiction due to its involvement in the reinstatement of drug seeking. In the present study, the role of the prelimbic cortex in persistent responding maintained by the earned presentations of a drug-paired conditioned reinforcer was studied. Temporary inactivation of the prelimbic, prefrontal cortex of rats had no effect on this persistent response, but did impair its initial acquisition, maintained by the drug-paired conditioned reinforcer. The lesion also impaired re-acquisition of this response after extinction by omission of the contingent conditioned reinforcer. These results suggest that the prelimbic cortex has a selective role in the acquisition, or re-acquisition, of instrumental responses for drug-paired conditioned reinforcers, that may be important in relapse to drug seeking. Anatomical controls with placements in the infralimbic cortex showed longer-lasting impairments in the acquisition of this response, consistent with the suggestion that the prelimbic and infralimbic cortices mediate different aspects of behavior, with the infralimbic being more specialized for habits. The implications of the present findings toward the understanding of drug seeking and relapse behaviors and the separate brain systems that may underlie them are discussed.

Black agouti (ACI) rats show greater drug- and cue-induced reinstatement of methamphetamine-seeking behavior than Fischer 344 and Lewis rats

Fischer 344 (F344) and Lewis (LEW) rats differ in methamphetamine self-administration (SA) and methamphetamine-induced reinstatement of previously extinguished behavior. We sought to determine whether genetic background also influences methamphetamine reinforcement efficacy, conditioned reinstatement, and methamphetamine-primed reinstatement of responding in F344, LEW, and Black Agouti (ACI) rats. We implanted rats with jugular catheters and trained them to self-administer methamphetamine (0.06 mg/kg/infusion) under a progressive ratio (PR) schedule of reinforcement during daily 2-h SA sessions. A compound stimulus (light+tone; LT) was paired with each infusion. Dose-dependent intake was determined for each rat. Rats then entered the extinction phase of the experiment where responding resulted in no programmed consequences. Following extinction sessions, rats underwent conditioned reinstatement testing. For conditioned reinstatement, rats received response-contingent presentations of the LT and no methamphetamine. Last, methamphetamine-primed reinstatement test sessions where conducted where subjects received experimenter delivered infusions of methamphetamine (0.06, 0.12, or 0.24 mg/kg). The strains did not differ in PR responding across the doses tested. The ACI rats demonstrated the highest behavioral output during extinction training, conditioned- and methamphetamine-primed reinstatement of previously extinguished behavior compared to the other strains. These data suggest that genetic background differentially influences extinction, conditioned reinstatement and methamphetamine-primed reinstatement in rats.

Ethanol-associated cues produce general pavlovian-instrumental transfer

BACKGROUND

Conditioned stimuli are thought to play an important role in maintaining ethanol use and inducing relapse. Therefore, understanding the mechanisms through which such stimuli trigger ethanol seeking is of interest in the study and treatment of alcoholism.

METHODS

This series of experiments examined the impact of ethanol-associated cues on ethanol-seeking behavior using a Pavlovian-instrumental transfer design. Rats received Pavlovian training in which an auditory stimulus predicted ethanol (10%) delivery. In a separate instrumental training phase, animals were trained to press a lever for ethanol. In the test phase, the impact of the stimulus on instrumental performance was assessed in extinction by presenting the stimulus while animals were free to perform the lever-press response. Experiment 2 assessed the selectivity of the transfer effect; rats received training with 2 auditory stimuli which predicted either ethanol or sucrose (2%) delivery and were trained to perform 2 instrumental responses, one earning ethanol and the other earning sucrose. Finally, Experiment 3 examined the selectivity of PIT using 2 natural rewards (sucrose and polycose).

RESULTS

The results from Experiment 1 show that ethanol supports excitatory conditioning and that ethanol-associated cues facilitate instrumental performance for ethanol. When the selectivity of the transfer effect was examined in Experiment 2, the ethanol-paired stimulus was found to have a general excitatory effect on reward-seeking behavior, affecting both ethanol-directed and sucrose-directed responding equally. In contrast, the sucrose-paired stimulus had a selective effect, elevating sucrose-directed responding only. Experiment 3 confirms that selective transfer is observed when 2 natural rewards are used to reinforce responding.

CONCLUSIONS

These data provide further evidence that ethanol-associated cues can drive ethanol-seeking behaviors. Because ethanol-associated cues also enhanced seeking behavior for a nonalcohol reward, these results additionally suggest that the modulation of reward-directed behaviors by cues associated with ethanol versus natural rewards may rely on different behavioral and neural mechanisms.

Does response-contingent access to cocaine reinstate previously extinguished cocaine-seeking behavior in C57BL/6J mice?

Inbred strains of mice are valuable tools for determining the impact of genes and the environment on behavior. However, use of mice in intravenous (iv) cocaine self-administration (SA) extinction/reinstatement paradigms has yielded mixed results. Mice do demonstrate significant conditioned reinstatement but do not significantly reinstate previously extinguished cocaine-seeking behavior when passively primed with cocaine. We tested the hypothesis that C57BL/6J (B6) mice would reinstate previously extinguished cocaine-seeking behavior when provided with response-contingent access to conditioned cues and fixed doses of cocaine. Male B6 mice were implanted with jugular catheters and trained to lever press for cocaine infusions. Each infusion was paired with a compound stimulus (light and tone; LT). Following 14 days of SA, subjects underwent extinction training--responding resulted in no programmed consequences. After at least 5 extinction sessions, cue-primed reinstatement was tested (LT-test). For the LT-test, mice received response-contingent presentations of the LT. After the LT-test, subjects returned to extinction training. Once responding decreased to extinction criteria, cocaine priming began. During cocaine priming, mice had response-contingent access to saline, 1.05, 3.5, or 17.5 mg/kg cocaine. Response-contingent presentations of the LT significantly reinstated cocaine-seeking behavior in the mice. Response-contingent access to cocaine dose-dependently reinstated responding. Our results suggest that response-contingent access to cocaine is a robust method for modeling cocaine craving and relapse in mice.

Activation of group II metabotropic glutamate receptors in the nucleus accumbens shell attenuates context-induced relapse to heroin seeking

Using a rat relapse model, we previously reported that re-exposing rats to a drug-associated context, following extinction of operant responding in a different context, reinstates heroin seeking. In an initial pharmacological characterization, we found that the mGluR2/3 agonist LY379268, which acts centrally to reduce evoked glutamate release, attenuates context-induced reinstatement of heroin seeking when injected systemically or into the ventral tegmental area, the cell body region of the mesolimbic dopamine system. Here, we tested whether injections of LY379268 into the nucleus accumbens (NAc), a terminal region of the mesolimbic dopamine system, would also attenuate context-induced reinstatement of heroin seeking. Rats were trained to self-administer heroin; drug infusions were paired with a discrete tone-light cue. Subsequently, lever pressing was extinguished in the presence of the discrete cue in a context that differed from the drug self-administration context in terms of visual, auditory, tactile, and circadian cues. After extinction of responding, LY379268 was injected to different groups of rats into the NAc core or shell or into the caudate-putamen, a terminal region of the nigrastriatal dopamine system. Injections of LY379268 into the NAc shell (0.3 or 1.0 microg) dose-dependently attenuated context-induced reinstatement of heroin seeking. Injections of 1.0 microg of LY379268 into the NAc core had no effect, while a higher dose (3.0 microg) decreased this reinstatement. Injections of LY379268 (3.0 microg) 1.5 mm dorsal from the NAc core into the caudate-putamen were ineffective. Results suggest an important role of glutamate transmission in the NAc shell in context-induced reinstatement of heroin seeking.

Behavioral and neural mechanisms of compulsive drug seeking

Not the mere procurement and use of drugs, but the fact that patterns of seeking and taking become compulsive after prolonged drug use is a defining characteristic of drug addiction. Development of a therapy that targets the compulsive aspects of drug use and thus addresses addiction at its core would therefore be very desirable. In the present review, we will discuss animal studies that attempt to model loss of control over drug use. Furthermore, we will try to put these studies in a theoretical perspective, and discuss the hypothesized underlying neural and behavioral mechanisms.