Endogenous kappa-opioid mediation of stress-induced potentiation of ethanol-conditioned place preference and self-administration

A review of kappa opioid receptor antagonists and their clinical trial landscape

Myriad signaling pathways are implicated in neuropsychiatric disorders, yet many mechanisms are unknown and current treatment options are limited. The intriguing dynorphin/kappa opioid receptor (KOR) system that is widely distributed throughout the brain appears to be essential in regulating many physiological and pathophysiological processes. This review explores up to date advances on the relationship between the dynorphin/KOR system with a particular focus on the KOR antagonist compounds tested as clinical candidates that could offer potential treatment options for CNS disorders.

Emerging medications and pharmacological treatment approaches for substance use disorders

Medications to treat substance use disorders (SUDs) remain suboptimal or, in the case of stimulants and cannabis, non-existent. Many factors have contributed to this paucity, including the biological complexity of addiction, regulatory challenges, and a historical lack of enthusiasm among pharmaceutical companies to commit resources to this disease space. Despite these headwinds, the recent opioid crisis has highlighted the devastating consequences of SUDs for both individuals and society, stimulating urgent efforts to identify novel treatment approaches. In addition, several neurobiological systems have been recently implicated in unique aspects of drug reward, opening the door to candidate medications with novel mechanisms of action. Here, we provide an overview of efforts to target several of these new systems, with a focus on those that are the subject of ongoing clinical trials as well as being areas of interest among the authors' research groups (MHJ, MTB, NAE). Specifically, we discuss new classes of medications targeting the serotonin 2A receptor (i.e., psychedelics), glucagon-like peptide 1 receptor, cannabidiol, dynorphin/kappa opioid receptor, orexin/hypocretin, and oxytocin receptor systems, as well as emergent approaches for modulating the more canonical dopaminergic system via agonist therapies for stimulant use disorders. Collectively, innovations in this space give reason for optimism for an improved therapeutic landscape for substance use disorders in the near future.

Differential c-Fos Response in Neurocircuits Activated by Repeated Predator Stress in Male and Female C57BL/6J Mice with Stress Sensitive or Resilient Alcohol Intake Phenotypes

Comorbidity of post-traumatic stress disorder (PTSD) and alcohol use disorder (AUD) worsens the prognosis for each of these individual disorders. The current study aimed to identify neurocircuits potentially involved in regulation of PTSD-AUD comorbidity by mapping expression of c-Fos in male and female C57BL/6J mice following repeated predator stress (PS), modeled by exposure to dirty rat bedding. In experiment 1, the levels of c-Fos in the paraventricular nucleus of the hypothalamus (PVH) and the nucleus accumbens shell were higher after the second PS vs the first PS, indicating a sensitized response to this stressor. Additional brain regions showed varied sex-dependent and independent regulation by the two consecutive PS exposures. In experiment 2, mice that increased voluntary alcohol consumption following four exposures to PS (Sensitive subgroup) showed higher c-Fos induction in the PVH, piriform cortex and ventromedial hypothalamus than mice that decreased consumption following these exposures (Resilient subgroup). In contrast to these brain regions, c-Fos was higher in the anterior olfactory nucleus of Resilient vs Sensitive mice. Taken together, these data demonstrate that repeated PS exposure and voluntary alcohol consumption increase neuronal activity across neurocircuits in which specific components depend on the vulnerability of individual mice to these stressors. Increased PVH activity observed across both experiments suggests this brain area as a potential mediator of PS-induced increases in alcohol consumption. Future investigations of specific neuronal populations within the PVH activated by PS, and manipulation of these specific neuronal populations, could improve our understanding of the mechanisms leading to PTSD-AUD comorbidity.

A critical review of effects on ethanol intake of the dynorphin/kappa opioid receptor system in the extended amygdala: From inhibition to stimulation

The dynorphin (DYN)/kappa opioid receptor (KOR) system has increasingly been investigated as a possible pharmacotherapeutic target for alcohol use disorder, but findings on the direction of its effects have been mixed. Activation of KORs by DYN has been shown to elicit dysphoric effects, and the DYN/KOR system has canonically been considered particularly important in driving alcohol intake through negative reinforcement in dependent states. However, this review also highlights its activity in opposing the positive reinforcement that drives alcohol intake at earlier stages. Both DYN and KORs are concentrated in the extended amygdala, a set of interconnected regions that includes the bed nucleus of the stria terminalis, central nucleus of the amygdala, and nucleus accumbens shell. This review focuses on the role of the DYN/KOR system in the extended amygdala in ethanol use. It begins by examining the effects of ethanol on the expression of DYN/KOR in the extended amygdala, expression of DYN/KOR in alcohol-preferring and alcohol-avoiding animals, and the effects of knocking out DYN/KOR genes on ethanol intake. Then, it examines the effects on ethanol use in both dependent and nondependent states from systemic pharmacological manipulations of DYN/KOR and from specific manipulation of this system in regions of the extended amygdala. We propose that greater expression and binding of DYN/KOR, by reducing the positive reinforcement that drives early stages of intake, initially acts to prevent the escalation of ethanol drinking. However, prolonged, binge-like, or intermittent ethanol intake enhances levels of DYN/KOR in the extended amygdala such that the system ultimately facilitates the negative reinforcement that drives later stages of ethanol drinking. This review highlights the potential of the DYN/KOR system as a target that can affect different outcomes across different stages of ethanol drinking and the development of alcohol use disorder.

Contribution of the opioid system to depression and to the therapeutic effects of classical antidepressants and ketamine

Major depressive disorder (MDD) afflicts approximately 5 % of the world population, and about 30-50 % of patients who receive classical antidepressant medications do not achieve complete remission (treatment resistant depressive patients). Emerging evidence suggests that targeting opioid receptors mu (MOP), kappa (KOP), delta (DOP), and the nociceptin/orphanin FQ receptor (NOP) may yield effective therapeutics for stress-related psychiatric disorders. As depression and pain exhibit significant overlap in their clinical manifestations and molecular mechanisms involved, it is not a surprise that opioids, historically used to alleviate pain, emerged as promising and effective therapeutic options in the treatment of depression. The opioid signaling is dysregulated in depression and numerous preclinical studies and clinical trials strongly suggest that opioid modulation can serve as either an adjuvant or even an alternative to classical monoaminergic antidepressants. Importantly, some classical antidepressants require the opioid receptor modulation to exert their antidepressant effects. Finally, ketamine, a well-known anesthetic whose extremely efficient antidepressant effects were recently discovered, was shown to mediate its antidepressant effects via the endogenous opioid system. Thus, although opioid system modulation is a promising therapeutical venue in the treatment of depression further research is warranted to fully understand the benefits and weaknesses of such approach.

The life and times of endogenous opioid peptides: Updated understanding of synthesis, spatiotemporal dynamics, and the clinical impact in alcohol use disorder

The opioid G-protein coupled receptors (GPCRs) strongly modulate many of the central nervous system structures that contribute to neurological and psychiatric disorders including pain, major depressive disorder, and substance use disorders. To better treat these and related diseases, it is essential to understand the signaling of their endogenous ligands. In this review, we focus on what is known and unknown about the regulation of the over two dozen endogenous peptides with high affinity for one or more of the opioid receptors. We briefly describe which peptides are produced, with a particular focus on the recently proposed possible synthesis pathways for the endomorphins. Next, we describe examples of endogenous opioid peptide expression organization in several neural circuits and how they appear to be released from specific neural compartments that vary across brain regions. We discuss current knowledge regarding the strength of neural activity required to drive endogenous opioid peptide release, clues about how far peptides diffuse from release sites, and their extracellular lifetime after release. Finally, as a translational example, we discuss the mechanisms of action of naltrexone (NTX), which is used clinically to treat alcohol use disorder. NTX is a synthetic morphine analog that non-specifically antagonizes the action of most endogenous opioid peptides developed in the 1960s and FDA approved in the 1980s. We review recent studies clarifying the precise endogenous activity that NTX prevents. Together, the works described here highlight the challenges and opportunities the complex opioid system presents as a therapeutic target.

Cocaine self-administration augments kappa opioid receptor system-mediated inhibition of dopamine activity in the mesolimbic dopamine system

Prior studies examining the effects of cocaine on the dynorphin/kappa opioid receptor (Dyn/KOR) system primarily focus on non-contingent cocaine exposure, but the effects of self-administration, which more closely reflects human drug-taking behaviors, are not well studied. In this study we characterized the effects of escalated intravenous cocaine self-administration on the functional state of the Dyn/KOR system and its interaction with mesolimbic dopamine signaling. Rats self-administered cocaine in an extended access, limited intake cocaine procedure, in which animals obtained 40 infusions per day (1.5 mg/kg/inf) for 5 consecutive days to ensure comparable consumption levels. Following single day tests of cue reactivity and progressive ratio responding, quantitative real-time polymerase chain reaction was used to measure levels of Oprk and Pdyn transcripts in the ventral tegmental area and nucleus accumbens. Additionally, after self-administration, ex vivo fast-scan cyclic voltammetry in the NAc was used to examine the ability of the KOR agonist U50,488 to inhibit dopamine release. We found that KOR-induced inhibition of dopamine release was enhanced in animals that self-administered cocaine compared to controls, suggesting upregulated Dyn/KOR activity after cocaine self-administration. Furthermore, expression levels of Pdyn in the nucleus accumbens and ventral tegmental area, and Oprk in the nucleus accumbens, were elevated in cocaine animals compared to controls. Additionally, Pdyn expression in the nucleus accumbens was negatively correlated with progressive ratio breakpoints, a measure of motivation to self-administer cocaine. Overall, these data suggest that cocaine self-administration elevates KOR/Dyn system activity in the mesolimbic dopamine pathway.

Combined Chronic Oral Methylphenidate and Fluoxetine Treatment During Adolescence: Effects on Behavior

BACKGROUND

Attention Deficit Hyperactivity Disorder (ADHD) can be comorbid with depression, often leading to the prescription of both methylphenidate (MP) and selective serotonin reuptake inhibitor (SSRI) antidepressants, such as fluoxetine (FLX). Moreover, these drugs are often misused as cognitive enhancers. This study examined the effects of chronic oral co-administration of MP and FLX on depressive- and anxiety-like behaviors.

METHODS

Adolescent rats received daily either water (control), MP, FLX, or the combination of MP plus FLX in their drinking water over the course of 4 weeks.

RESULTS

Data analysis shows a decrease in food consumption and body weight for rats exposed to FLX or the combination of MP and FLX. Sucrose consumption was significantly greater in FLX or MP+FLX groups compared to controls. FLX-treated rats showed no effect in the elevated plus maze (EPM; open arm time) and forced swim test (FST; latency to immobility). However, rats treated with the combination (MP+FLX) showed significant anxiolytic-like and anti-depressive-like behaviors (as measured by EPM and FST), as well as significant increases in overall activity (distance traveled in open field test). Finally, the combined MP+FLX treatment induced a decrease in anxiety and depressive- like behaviors significantly greater than the response from either of these drugs alone.

CONCLUSION

These behavioral results characterize the long-term effects of these drugs (orally administered) that are widely co-administered and co-misused and provide important insight into the potential neurobiological and neurochemical effects. Future research will determine the potential risks of the long-term use of MP and FLX together.

Adverse Effects of Repeated, Intravenous Morphine on Recovery after Spinal Cord Injury in Young, Male Rats Are Blocked by a Kappa Opioid Receptor Antagonist

Immediately following spinal cord injury (SCI) patients experience pain associated with injury to the spinal cord and nerves as well as with accompanying peripheral injuries. This pain is usually treated with opioids, and most commonly with morphine. However, in a rodent model we have shown that, irrespective of the route of administration, morphine administered in the acute phase of SCI undermines long-term locomotor recovery. Our previous data suggest that activation of kappa opioid receptors (KORs) mediates these negative effects. Blocking KORs with norbinaltorphimine (norBNI), prior to a single dose of epidural morphine, prevented the morphine-induced attenuation of locomotor recovery. Because numerous cellular changes occur with chronic opioid administration compared with a single dose, the current study tested whether norBNI was also effective in a more clinically relevant paradigm of repeated, intravenous morphine administration after SCI. We hypothesized that blocking KOR activation during repeated, intravenous morphine administration would also protect recovery. Supporting this hypothesis, we found that blocking KOR activation in young, male rats prevented the negative effects of morphine on locomotor recovery, although neither norBNI nor morphine had an effect on long-term pain at the doses used. We also found that norBNI treatment blocked the adverse effects of morphine on lesion size. These data suggest that a KOR antagonist given in conjunction with morphine may provide a clinical strategy for effective analgesia without compromising locomotor recovery after SCI.

An analog of [d-Trp]CJ-15,208 exhibits kappa opioid receptor antagonism following oral administration and prevents stress-induced reinstatement of extinguished morphine conditioned place preference

Opioid use disorder (OUD) relapse rates are discouragingly high, underscoring the need for new treatment options. The macrocyclic tetrapeptide natural product CJ-15,208 and its stereoisomer [d-Trp]CJ-15,208 demonstrate kappa opioid receptor (KOR) antagonist activity upon oral administration which prevents stress-induced reinstatement of cocaine-seeking behavior. In order to further explore the structure-activity relationships and expand the potential therapeutic applications of KOR antagonism for the treatment of OUD, we screened a series of 24 analogs of [d-Trp]CJ-15,208 with the goal of enhancing KOR antagonist activity. From this screening, analog 22 arose as a compound of interest, demonstrating dose-dependent KOR antagonism after central and oral administration lasting at least 2.5 h. In further oral testing, analog 22 lacked respiratory, locomotor, or reinforcing effects, consistent with the absence of opioid agonism. Pretreatment with analog 22 (30 mg/kg, p.o.) prevented stress-induced reinstatement of extinguished morphine conditioned place preference and reduced some signs of naloxone-precipitated withdrawal in mice physically dependent on morphine. Collectively, these data support the therapeutic potential of KOR antagonists to support abstinence in OUD and ameliorate opioid withdrawal.

Traumatic Stress-Induced Vulnerability to Addiction: Critical Role of the Dynorphin/Kappa Opioid Receptor System

Substance use disorders (SUD) may emerge from an individual’s attempt to limit negative affective states and symptoms linked to stress. Indeed, SUD is highly comorbid with chronic stress, traumatic stress, or post-traumatic stress disorder (PTSD), and treatments approved for each pathology individually often failed to have a therapeutic efficiency in such comorbid patients. The kappa-opioid receptor (KOR) and its endogenous ligand dynorphin (DYN), seem to play a key role in the occurrence of this comorbidity. The DYN/KOR function is increased either in traumatic stress or during drug use, dependence acquisition and DYN is released during stress. The behavioural effects of stress related to the DYN/KOR system include anxiety, dissociative and depressive symptoms, as well as increased conditioned fear response. Furthermore, the DYN/KOR system is implicated in negative reinforcement after the euphoric effects of a drug of abuse ends. During chronic drug consumption DYN/KOR functions increase and facilitate tolerance and dependence. The drug-seeking behaviour induced by KOR activation can be retrieved either during the development of an addictive behaviour, or during relapse after withdrawal. DYN is known to be one of the most powerful negative modulators of dopamine signalling, notably in brain structures implicated in both reward and fear circuitries. KOR are also acting as inhibitory heteroreceptors on serotonin neurons. Moreover, the DYN/KOR system cross-regulate with corticotropin-releasing factor in the brain. The sexual dimorphism of the DYN/KOR system could be the cause of the gender differences observed in patients with SUD or/and traumatic stress-related pathologies. This review underlies experimental and clinical results emphasizing the DYN/KOR system as common mechanisms shared by SUD or/and traumatic stress-related pathologies, and suggests KOR antagonist as a new pharmacological strategy to treat this comorbidity.

An updated assessment of the translational promise of G-protein-biased kappa opioid receptor agonists to treat pain and other indications without debilitating adverse effects

Kappa opioid receptor (κOR) agonists lack the abuse liability and respiratory depression effects of clinically used mu opioid receptor (μOR) analgesics and are hypothesized to be safer alternatives. However, κOR agonists have limiting adverse effects of their own, including aversion, sedation, and mood effects, that have hampered their clinical translation. Studies performed over the last 15 years have suggested that these adverse effects could result from activation of distinct intracellular signaling pathways that are dependent on β-arrestin, whereas signaling downstream of G protein activation produces antinociception. This led to the hypothesis that agonists biased away from β-arrestin signaling would have improved therapeutic windows over traditional unbiased agonists and allow for clinical development of analgesic G-protein-biased κOR agonists. Given a recent controversy regarding the benefits of G-protein-biased μOR agonists, it is timely to reassess the therapeutic promise of G-protein-biased κOR agonists. Here we review recent discoveries from preclinical κOR studies and critically evaluate the therapeutic windows of G-protein-biased κOR agonists in each of the adverse effects above. Overall, we find that G-protein-biased κOR agonists generally have improved therapeutic window relative to unbiased agonists, although frequently study design limits strong conclusions in this regard. However, a steady flow of newly developed biased κOR agonists paired with recently engineered behavioral and molecular tools puts the κOR field in a prime position to make major advances in our understanding of κOR function and fulfill the promise of translating a new generation of biased κOR agonists to the clinic.

KOR Control over Addiction Processing: An Exploration of the Mesolimbic Dopamine Pathway

Drug addiction is a complex, persistent, and chronically relapsing neurological disorder exacerbated by acute and chronic stress. It is well known that the dynorphin/kappa opioid receptor (KOR) system regulates stress perception and responsivity, while the mesolimbic dopamine system plays a role in reward and reinforcement associated with alcohol and substance use disorders. Interestingly, the dopamine and dynorphin/KOR systems are highly integrated in mesolimbic areas, with KOR activation leading to inhibition of dopamine release, further altering the perception of reinforcing and aversive stimuli. Chronic or repeated exposure to stress or drugs potentiates KOR function ultimately contributing to a hypodopaminergic state. This hypodopaminergic state is one of the hallmarks of hyperkatifeia, defined as the hypersensitivity to emotional distress that is exacerbated during drug withdrawal and abstinence. The relationship between stress and drug addiction is bidirectional; repeated/chronic stress promotes pro-addictive behaviors, and repeated cycles of drug exposure and withdrawal, across various drug classes, produces stress. Neuroadaptations driven by this bidirectional relationship ultimately influence the perception of the reinforcing value of rewarding stimuli. In this chapter, we address the involvement of the dopamine and dynorphin/KOR systems and their interactions in shaping reinforcement value processing after drug and stress exposure, as well as a combinatorial impact of both drugs and stress.

The Buprenorphine Analogue BU10119 Attenuates Drug-Primed and Stress-Induced Cocaine Reinstatement in Mice

There are no Food and Drug Administration-approved medications for cocaine use disorder, including relapse. The μ-opioid receptor (MOPr) partial agonist buprenorphine alone or in combination with naltrexone has been shown to reduce cocaine-positive urine tests and cocaine seeking in rodents. However, there are concerns over the abuse liability of buprenorphine. Buprenorphine's partial agonist and antagonist activity at the nociception receptor (NOPr) and κ-opioid receptor (KOPr), respectively, may contribute to its ability to inhibit cocaine seeking. Thus, we hypothesized that a buprenorphine derivative that exhibits antagonist activity at MOPr and KOPr with enhanced agonist activity at the NOPr could provide a more effective treatment. Here we compare the pharmacology of buprenorphine and two analogs, BU10119 and BU12004, in assays for antinociception and for cocaine- and stress-primed reinstatement in the conditioned place preference paradigm. In vitro and in vivo assays showed that BU10119 acts as an antagonist at MOPr, KOPr, and δ-opioid receptor (DOPr) and a partial agonist at NOPr, whereas BU12004 showed MOPr partial agonist activity and DOPr, KOPr, and NOPr antagonism. BU10119 and buprenorphine but not BU12004 lessened cocaine-primed reinstatement. In contrast, BU10119, BU12004, and buprenorphine blocked stress-primed reinstatement. The selective NOPr agonist SCH221510 but not naloxone decreased cocaine-primed reinstatement. Together, these findings are consistent with the concept that NOPr agonism contributes to the ability of BU10119 and buprenorphine to attenuate reinstatement of cocaine-conditioned place preference in mice. The findings support the development of buprenorphine analogs lacking MOPr agonism with increased NOPr agonism for relapse prevention to cocaine addiction. SIGNIFICANCE STATEMENT: There are no Food and Drug Administration-approved medications for cocaine use disorder. Buprenorphine has shown promise as a treatment for cocaine relapse prevention; however, there are concerns over the abuse liability of buprenorphine. Here we show a buprenorphine analogue, BU10119, which lacks μ-opioid receptor agonism and inhibits cocaine-primed and stress-primed reinstatement in a conditioned place-preference paradigm. The results suggest the development of BU10119 for the management of relapse to cocaine seeking.

Vulnerability factors for mephedrone-induced conditioned place preference in rats-the impact of sex differences, social-conditioning and stress

RATIONALE

Mephedrone is a frequently overused drug of abuse that belongs to the group of novel psychoactive substances. Although its mechanism of action, as well as toxic and psychoactive effects, has been widely studied, the role of different factors that could contribute to the increased vulnerability to mephedrone abuse is still poorly understood.

OBJECTIVES

The aim of the presented study was to assess the impact of several factors (sex differences, social-conditioning, and chronic mild unpredictable stress - CMUS) on the liability to mephedrone-induced reward in Wistar rats.

METHODS

The rewarding effects of mephedrone in male and female rats were assessed using the conditioned place preference (CPP) procedure. Furthermore, the impact of social factor and stress was evaluated in male rats using social-CPP and CMUS-dependent CPP, respectively.

RESULTS

Mephedrone induced classic-CPP in female (10 mg/kg), as well as in male (10 and 20 mg/kg) rats. However, the impact of mephedrone treatment during social-CPP was highly dose-dependent as the rewarding effects of low dose of mephedrone (5 mg/kg; non-active in classic-CPP) were potentiated when administered during social-conditioning. Interestingly, social-conditioning with a higher dose of 20 mg/kg (that induced classic-CPP) was able to reverse these effects. Finally, CMUS potentiated rewarding effects of a low dose of mephedrone (5 mg/kg) and increased the level of corticosterone in rats' prefrontal cortex and hippocampus.

CONCLUSIONS

Altogether, the presented results give new insight into possible factors underlying the vulnerability to mephedrone abuse and can serve as a basis for further studies assessing mechanisms underlying observed effects.

The pharmacological stressor yohimbine, but not U50,488, increases responding for conditioned reinforcers paired with ethanol or sucrose

RATIONALE

Environmental stimuli paired with alcohol can function as conditioned reinforcers (CRfs) and trigger relapse to alcohol-seeking. In animal models, pharmacological stressors can enhance alcohol consumption and reinstate alcohol-seeking, but it is unknown whether stress can potentiate the conditioned reinforcing properties of alcohol-paired stimuli.

OBJECTIVES

We examined whether the pharmacological stressors, the α-2 adrenoreceptor antagonist yohimbine (vehicle, 1.25, 2.5 mg/kg; IP) and the K-opioid receptor agonist U50,488 (vehicle, 1.25, 2.5 mg/kg; SC), increase responding for conditioned reinforcement, and if their effects interact with the nature of the reward (alcohol vs. sucrose). We subsequently examined the effects of yohimbine (vehicle, 1.25, 2.5 mg/kg; IP) on responding for sensory reinforcement.

METHODS

Male Long-Evans underwent Pavlovian conditioning, wherein a tone-light conditioned stimulus (CS) was repeatedly paired with 12% EtOH or 21.7% sucrose. Next, tests of responding for a CRf were conducted. Responding on the CRf lever delivered the CS, whereas responding on the other lever had no consequences. In a separate cohort of rats, the effects of yohimbine on responding just for the sensory reinforcer were examined.

RESULTS

Both doses of yohimbine, but not U50,488, increased responding for conditioned reinforcement. This enhancement occurred independently of the nature of the reward used during Pavlovian conditioning. Yohimbine-enhanced responding for a CRf was reproducible and stable over five tests; it even persisted when the CS was omitted. Both doses of yohimbine also increased responding for sensory reinforcement.

CONCLUSIONS

Yohimbine increases operant responding for a variety of sensory and conditioned reinforcers. This enhancement may be independent of its stress-like effects.

Pain-induced alterations in the dynorphinergic system within the mesocorticolimbic pathway: Implication for alcohol addiction

Latest studies have revealed that pain negatively impacts on reward processing and motivation leading to negative affective states and stress. These states not only reduce quality of life of patients by increasing the appearance of psychiatric comorbidities, but also have an important impact on vulnerability to drug abuse, including alcohol. In fact, clinical, epidemiological but also preclinical studies have revealed that the presence of pain is closely related to alcohol use disorders (AUDs). All this evidence suggests that pain is a factor that increases the risk of suffering AUD, predicting heavy drinking behavior and relapse drinking in those patients with a previous history of AUD. The negative consequences of chronic pain and its impact on stress and AUD are likely mediated by alterations in the central nervous system, especially in the stress and reward systems. Therefore, pain and stress impact on dopaminergic mesolimbic pathway can lead to an increase in drug abuse liability. In this mini review we analyze the interaction between pain, stress, and alcohol addiction, and how dynamic changes in the kappa opioid system might play a crucial role in the development of compulsive alcohol drinking in chronic pain patients.

Behavioral, neurobiological, and neurochemical mechanisms of ethanol self-administration: A translational review

Alcohol use disorder has multiple characteristics including excessive ethanol consumption, impaired control over drinking behaviors, craving and withdrawal symptoms, compulsive seeking behaviors, and is considered a chronic condition. Relapse is common. Determining the neurobiological targets of ethanol and the adaptations induced by chronic ethanol exposure is critical to understanding the clinical manifestation of alcohol use disorders, the mechanisms underlying the various features of the disorder, and for informing medication development. In the present review, we discuss ethanol's interactions with a variety of neurotransmitter systems, summarizing findings from preclinical and translational studies to highlight recent progress in the field. We then describe animal models of ethanol self-administration, emphasizing the value, limitations, and validity of commonly used models. Lastly, we summarize the behavioral changes induced by chronic ethanol self-administration, with an emphasis on cue-elicited behavior, the role of ethanol-related memories, and the emergence of habitual ethanol seeking behavior.

mTORC1 pathway is involved in the kappa opioid receptor activation-induced increase in excessive alcohol drinking in mice

KOP-r agonist U50,488H produces strong aversion and anxiety/depression-like behaviors that enhance alcohol intake and promote alcohol seeking and relapse-like drinking in rodents. Mammalian target of rapamycin complex 1 (mTORC1) pathway in mouse striatum is highly involved in excessive alcohol intake and seeking, and in the U50,488H-induced conditioned place aversion. Therefore, we hypothesized that KOP-r activation increases alcohol consumption through the mTORC1 activation. This study focuses on: (1) how chronic excessive alcohol drinking (4-day drinking-in-the-dark paradigm followed by 3-week chronic intermittent access drinking paradigm [two-bottle choice, 24-h access every other day]) affected nuclear transcript levels of the mTORC1 pathway genes in mouse nucleus accumbens shell (NAcs), using transcriptome-wide RNA sequencing analysis; and (2) whether selective mTORC1 inhibitor rapamycin could alter excessive alcohol drinking and prevent U50,488H-promoted alcohol intake. Thirteen nuclear transcripts of mTORC1 pathway genes showed significant up-regulation in the NAcs, with two genes down-regulated, after excessive alcohol drinking, suggesting the mTORC1 pathway was profoundly disrupted. Single administration of rapamycin decreased alcohol drinking in a dose-dependent manner. U50,488H increased alcohol drinking, and pretreatment with rapamycin, at a dose lower than effective doses, blocked the U50,488H-promoted alcohol intake in a dose-dependent manner, indicating a mTORC1-mediated mechanism. Our results provide supportive and direct evidence relevant to the transcriptional profiling of the critical mTORC1 genes in mouse NAc shell: with functional and pharmacological effects of rapamycin, altered nuclear transcripts in the mTORC1 signaling pathway after excessive alcohol drinking may contribute to increased alcohol intake triggered by KOP-r activation.

Crosstalk Between Kappa Opioid and Dopamine Systems in Compulsive Behaviors

The strength of goal-oriented behaviors is regulated by midbrain dopamine neurons. Dysfunctions of dopaminergic circuits are observed in drug addiction and obsessive-compulsive disorder. Compulsive behavior is a feature that both disorders share, which is associated to a heightened dopamine neurotransmission. The activity of midbrain dopamine neurons is principally regulated by the homeostatic action of dopamine through D2 receptors (D2R) that decrease the firing of neurons as well as dopamine synthesis and release. Dopamine transmission is also regulated by heterologous neurotransmitter systems such as the kappa opioid system, among others. Much of our current knowledge of the kappa opioid system and its influence on dopamine transmission comes from preclinical animal models of brain diseases. In 1988, using cerebral microdialysis, it was shown that the acute activation of the Kappa Opioid Receptors (KOR) decreases synaptic levels of dopamine in the striatum. This inhibitory effect of KOR opposes to the facilitating influence of drugs of abuse on dopamine release, leading to the proposition of the use of KOR agonists as pharmacological therapy for compulsive drug intake. Surprisingly, 30 years later, KOR antagonists are instead proposed to treat drug addiction. What may have happened during these years that generated this drastic change of paradigm? The collected evidence suggested that the effect of KOR on synaptic dopamine levels is complex, depending on the frequency of KOR activation and timing with other incoming stimuli to dopamine neurons, as well as sex and species differences. Conversely to its acute effect, chronic KOR activation seems to facilitate dopamine neurotransmission and dopamine-mediated behaviors. The opposing actions exerted by acute versus chronic KOR activation have been associated with an initial aversive and a delayed rewarding effect, during the exposure to drugs of abuse. Compulsive behaviors induced by repeated activation of D2R are also potentiated by the sustained co-activation of KOR, which correlates with decreased synaptic levels of dopamine and sensitized D2R. Thus, the time-dependent activation of KOR impacts directly on dopamine levels affecting the tuning of motivated behaviors. This review analyzes the contribution of the kappa opioid system to the dopaminergic correlates of compulsive behaviors.

GPCR and Alcohol-Related Behaviors in Genetically Modified Mice

G protein-coupled receptors (GPCRs) constitute the largest class of cell surface signaling receptors and regulate major neurobiological processes. Accordingly, GPCRs represent primary targets for the treatment of brain disorders. Several human genetic polymorphisms affecting GPCRs have been associated to different components of alcohol use disorder (AUD). Moreover, GPCRs have been reported to contribute to several features of alcohol-related behaviors in animal models. Besides traditional pharmacological tools, genetic-based approaches mostly aimed at deleting GPCR genes provided substantial information on how key GPCRs drive alcohol-related behaviors. In this review, we summarize the alcohol phenotypes that ensue from genetic manipulation, in particular gene deletion, of key GPCRs in rodents. We focused on GPCRs that belong to fundamental neuronal systems that have been shown as potential targets for the development of AUD treatment. Data are reviewed with particular emphasis on alcohol reward, seeking, and consumption which are behaviors that capture essential aspects of AUD. Literature survey indicates that in most cases, there is still a gap in defining the intracellular transducers and the functional crosstalk of GPCRs as well as the neuronal populations in which their signaling regulates alcohol actions. Further, the implication of only a few orphan GPCRs has been so far investigated in animal models. Combining advanced pharmacological technologies with more specific genetically modified animals and behavioral preclinical models is likely necessary to deepen our understanding in how GPCR signaling contributes to AUD and for drug discovery.

A potential role for microglia in stress- and drug-induced plasticity in the nucleus accumbens: A mechanism for stress-induced vulnerability to substance use disorder

Stress is an important risk factor for the development of substance use disorder (SUD). Exposure to both stress and drugs abuse lead to changes in synaptic plasticity and stress-induced alterations in synaptic plasticity may contribute to later vulnerability to SUD. Recent developmental neuroscience studies have identified microglia as regulators of synaptic plasticity. As both stress and drugs of abuse lead to microglial activation, we propose this as a potential mechanism underlying their ability to change synaptic plasticity. This review focuses on three components of synaptic plasticity: spine density, brain-derived neurotrophic factor (BDNF) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor expression. Their roles in addiction, stress, and development will be reviewed, as well as possible mechanisms by which microglia could regulate their function. Potential links between stress, vulnerability to addiction, and microglial activity will be explored.

Combination of Clinically Utilized Kappa-Opioid Receptor Agonist Nalfurafine With Low-Dose Naltrexone Reduces Excessive Alcohol Drinking in Male and Female Mice

BACKGROUND

Nalfurafine is the first clinically approved kappa-opioid receptor (KOP-r) agonist as an antipruritus drug with few side effects in humans (e.g., sedation, depression, and dysphoria). No study, however, has been done using nalfurafine on alcohol drinking in rodents or humans.

METHODS

We investigated whether nalfurafine alone or in combination with mu-opioid receptor (MOP-r) antagonist naltrexone changed excessive alcohol drinking in male and female C57BL/6J (B6) mice subjected to a chronic intermittent-access drinking paradigm (2-bottle choice, 24-hour access every other day) for 3 weeks. Neuronal proopiomelanocortin enhancer (nPE) knockout mice with brain-specific deficiency of beta-endorphin (endogenous ligand of MOP-r) were used as a genetic control for the naltrexone effects.

RESULTS

Single administration of nalfurafine decreased alcohol intake and preference in both male and female B6 mice in a dose-dependent manner. Pretreatment with nor-BNI (a selective KOP-r antagonist) blocked the nalfurafine effect on alcohol drinking, indicating a KOP-r-mediated mechanism. Pharmacological effects of a 5-dosing nalfurafine regimen were further evaluated: The repeated nalfurafine administrations decreased alcohol consumption without showing any blunted effects, suggesting nalfurafine did not develop a tolerance after the multidosing regimen tested. Nalfurafine did not produce any sedation (spontaneous locomotor activity), anhedonia-like (sucrose preference test), anxiety-like (elevated plus maze test), or dysphoria-like (conditioned place aversion test) behaviors, suggesting that nalfurafine had few side effects. Investigating synergistic effects between low-dose naltrexone and nalfurafine, we found that single combinations of nalfurafine and naltrexone, at doses lower than individual effective dose, profoundly decreased excessive alcohol intake in both sexes. The effect of nalfurafine on decreasing alcohol consumption was confirmed in nPE-/- mice, suggesting independent mechanisms by which nalfurafine and naltrexone reduced alcohol drinking.

CONCLUSION

The clinically utilized KOP-r agonist nalfurafine in combination with low-dose naltrexone has potential in alcoholism treatment.

Dynorphin-kappa opioid receptor activity in the central amygdala modulates binge-like alcohol drinking in mice

Although previous research has demonstrated a role for kappa opioid receptor-mediated signaling in escalated alcohol consumption associated with dependence and stress exposure, involvement of the dynorphin/kappa opioid receptor (DYN/KOR) system in binge-like drinking has not been fully explored. Here we used pharmacological and chemogenetic approaches to examine the influence of DYN/KOR signaling on alcohol consumption in the drinking-in-the-dark (DID) model of binge-like drinking. Systemic administration of the KOR agonist U50,488 increased binge-like drinking (Experiment 1) while, conversely, systemic administration of the KOR antagonist nor-BNI reduced drinking in the DID model (Experiment 2). These effects of systemic KOR manipulation were selective for alcohol as neither drug influenced consumption of sucrose in the DID paradigm (Experiment 3). In Experiment 4, administration of the long-acting KOR antagonist nor-BNI into the central nucleus of the amygdala (CeA) decreased alcohol intake. Next, targeted "silencing" of DYN+ neurons in the CeA was accomplished using a chemogenetic strategy. Cre-dependent viral expression in DYN+ neurons was confirmed in CeA of Pdyn-IRES-Cre mice and functionality of an inhibitory (hM4Di) DREADD was validated (Experiment 5). Activating the inhibitory DREADD by CNO injection reduced binge-like alcohol drinking, but CNO injection did not alter alcohol intake in mice that were treated with control virus (Experiment 6). Collectively, these results demonstrate that DYN/KOR signaling in the CeA contributes to excessive alcohol consumption in a binge-drinking model.

Biobehavioral Interactions Between Stress and Alcohol

In this review, the effects of stress on alcohol drinking are discussed. The interactions between biological stress systems and alcohol drinking are examined, with a focus on the hypothalamic pituitary adrenal axis, corticotropin releasing factor, dynorphin, neuropeptide Y, and norepinephrine systems. Findings from animal models suggest that these biological stress systems may be useful targets for medications development for alcohol use disorder and co-occurring stress-related disorders in humans.

Predator odor stress blunts alcohol conditioned aversion

Alcohol use disorder is highly co-morbid with traumatic stress disorders in humans, and dually diagnosed individuals cite negative affective symptoms as a primary reason for drinking alcohol. Therefore, it is reasonable to hypothesize that traumatic stress history increases the rewarding properties and/or blunts the aversive properties of alcohol. We used a place conditioning procedure to test the rewarding/aversive properties of alcohol in adult male Wistar rats with or without a traumatic stress (i.e., predator odor exposure) history, and with or without an alcohol drinking history. Because extended amygdala regions have documented roles in stress, reward, and stress-induced changes in reward, we also tested the effect of acute alcohol on CREB phosphorylation (pCREB) and striatal-enriched protein tyrosine phosphatase (STEP) expression in central amygdala (CeA) and bed nucleus of stria terminalis (BNST). Our results show that a moderate alcohol dose (1.0 g/kg) produces conditioned place aversion (CPA) that is blunted by stress history but is not affected by alcohol drinking history, and this effect differed in pair-housed versus single-housed rats. Stress history reduced pCREB expression in BNST of rats with and without an alcohol drinking history. Finally, acute alcohol effects on pCREB and STEP expression in CeA were positively associated with preference for the alcohol-paired chamber. These data suggest that stress history reduces the aversive properties of moderate alcohol doses, and that alcohol aversion is associated with acute alcohol effects on pCREB and STEP expression in the extended amygdala.

Kappa opioid receptors mediate yohimbine-induced increases in impulsivity in the 5-choice serial reaction time task

Dynorphin (DYN), and its receptor, the kappa opioid receptor (KOR) are involved in drug seeking and relapse but the mechanisms are poorly understood. One hypothesis is that DYN/KOR activation promotes drug seeking through increased impulsivity, because many stimuli that induce DYN release increase impulsivity. Here, we systematically compare the effects of drugs that activate DYN/KOR on performance on the 5-choice serial reaction time task (5-CSRTT), a test of sustained attention and impulsivity. In Experiment 1, we determined the effects of U50,488 (0, 2.5, 5 mg/kg), yohimbine (0, 1.25, 2.5 mg/kg), and nicotine (0, 0.15, 0.3 mg/kg) on 5-CSRTT performance. In Experiment 2, we determined the effects of alcohol (0, 0.5, 1.0, 1.5 g/kg) on 5-CSRTT performance before and after voluntary, intermittent alcohol exposure. In Experiment 3, we determined the potential role of KOR in the pro-impulsive effects of yohimbine (1.25 mg/kg) and nicotine (0.3 mg/kg) by the prior administration of the KOR antagonist nor-BNI (10 mg/kg). Premature responding, the primary measure of impulsivity, was reduced by U50,488 and alcohol, but these drugs had a general suppressive effect. Yohimbine and nicotine increased premature responding. Yohimbine-, but not nicotine-induced increases in premature responding were blocked by nor-BNI, suggesting that impulsivity induced by yohimbine is KOR dependent. This may suggests a potential role for KOR-mediated increases in impulsivity in yohimbine-induced reinstatement.

Anti-stress neuropharmacological mechanisms and targets for addiction treatment: A translational framework

Stress-related substance use is a major challenge for treating substance use disorders. This selective review focuses on emerging pharmacotherapies with potential for reducing stress-potentiated seeking and consumption of nicotine, alcohol, marijuana, cocaine, and opioids (i.e., key phenotypes for the most commonly abused substances). I evaluate neuropharmacological mechanisms in experimental models of drug-maintenance and relapse, which translate more readily to individuals presenting for treatment (who have initiated and progressed). An affective/motivational systems model (three dimensions: valence, arousal, control) is mapped onto a systems biology of addiction approach for addressing this problem. Based on quality of evidence to date, promising first-tier neurochemical receptor targets include: noradrenergic (α1 and β antagonist, α2 agonist), kappa-opioid antagonist, nociceptin antagonist, orexin-1 antagonist, and endocannabinoid modulation (e.g., cannabidiol, FAAH inhibition); second-tier candidates may include corticotropin releasing factor-1 antagonists, serotonergic agents (e.g., 5-HT reuptake inhibitors, 5-HT3 antagonists), glutamatergic agents (e.g., mGluR2/3 agonist/positive allosteric modulator, mGluR5 antagonist/negative allosteric modulator), GABA-promoters (e.g., pregabalin, tiagabine), vasopressin 1b antagonist, NK-1 antagonist, and PPAR-γ agonist (e.g., pioglitazone). To address affective/motivational mechanisms of stress-related substance use, it may be advisable to combine agents with actions at complementary targets for greater efficacy but systematic studies are lacking except for interactions with the noradrenergic system. I note clinically-relevant factors that could mediate/moderate the efficacy of anti-stress therapeutics and identify research gaps that should be pursued. Finally, progress in developing anti-stress medications will depend on use of reliable CNS biomarkers to validate exposure-response relationships.

Age as a factor in stress and alcohol interactions: A critical role for the kappa opioid system

The endogenous kappa opioid system has primarily been shown to be involved with a state of dysphoria and aversion. Stress and exposure to drugs of abuse, particularly alcohol, can produce similar states of unease and anxiety, implicating the kappa opioid system as a target of stress and alcohol. Numerous behavioral studies have demonstrated reduced sensitivity to manipulations of the kappa opioid system in early life relative to adulthood, and recent reports have shown that the kappa opioid system is functionally different across ontogeny. Given the global rise in early-life stress and alcohol consumption, understanding how the kappa opioid system responds and adapts to stress and/or alcohol exposure differently in early life and adulthood is imperative. Therefore, the objective of this review is to highlight and discuss studies examining the impact of early-life stress and/or alcohol on the kappa opioid system, with focus on the documented neuroadaptations that may contribute to future vulnerability to stress and/or increase the risk of relapse. We first provide a brief summary of the importance of studying the effects of stress and alcohol during early life (prenatal, neonatal/juvenile, and adolescence). We then discuss the literature on the effects of stress or alcohol during early life and adulthood on the kappa opioid system. Finally, we discuss the few studies that have shown interactions between stress and alcohol on the kappa opioid system and provide some discussion about the need for studies investigating the development of the kappa opioid system.

Alcohol-induced conditioned place preference negatively correlates with anxiety-like behavior in adolescent mice: inhibition by a neurokinin-1 receptor antagonist

RATIONALE

Although alcohol use disorder and anxiety disorders are highly comorbid in humans, controversy remains regarding whether anxiety predisposes individuals to alcohol reward, and the relationship with neurokinin-1 receptor (NK1R) is unclear.

OBJECTIVES

The objectives of the study are to investigate the association between anxiety-like behavior and alcohol-induced conditioned place preference (CPP) and to examine the effect of NK1R antagonist L-703,606 on this preference and levels of NK1R protein in different brain regions in adolescent mice.

METHODS

The anxiety-like behavior of adolescent male C57BL/6 mice was assessed using the elevated plus maze (EPM) test, and the animals were then allocated into high-anxiety mouse (HAM) and low-anxiety mouse (LAM) groups based on the percent of open arm time (OT%). After the reinforcement of ethanol was established by alcohol-induced CPP (2 g/kg), NK1R expression was quantified in the hippocampus, prefrontal cortex, and amygdala. Finally, the effect of L-703,606 (10 mg/kg) on the alcohol-induced CPP was examined.

RESULTS

LAM showed a greater ethanol preference (P = 0.004) and a higher level of NK1R protein in the hippocampus (P = 0.026) than HAM group. Interestingly, the CPP score positively correlated with OT% (r = 0.520, P = 0.016) and the level of NK1R protein (r = 0.476, P = 0.029) in the hippocampus. Moreover, L-703,606 attenuated alcohol-induced CPP (P < 0.001) in both groups.

CONCLUSIONS

The present results highlight the negative correlation between anxiety-like behavior and the propensity for alcohol and the critical role for NK1R in alcohol reward in adolescent mice. Importantly, the NK1R antagonist L-703,606 might be a promising therapeutic target for alcohol use disorder.

Predator-based psychosocial stress model of PTSD differentially influences voluntary ethanol consumption depending on methodology

Post-traumatic stress disorder (PTSD) is a debilitating psychological disorder typified by diagnostic symptom clusters including hyperarousal, avoidance, negative cognitions and mood, and intrusive re-experiencing of the traumatic event. Patients with PTSD have been reported to self-medicate with alcohol to ameliorate hyperarousal symptoms associated with the disorder. Research utilizing rodent models of PTSD to emulate this behavioral phenomenon has thus far yielded inconsistent results. In the present study, we examined the effects of a predator-based psychosocial stress model of PTSD on voluntary ethanol consumption. In the first of two experiments, following exposure to a 31-day stress or control paradigm, rats were singly housed during the dark cycle with free access to 1% sucrose solution or 10% ethanol, which was also sweetened with 1% sucrose. Over the course of a 20-day period of ethanol access, stressed rats consumed significantly less ethanol than non-stressed rats. These counterintuitive results prompted the completion of a second experiment which was identical to the first, except rats were also exposed to the two-bottle paradigm for 20 days before the stress or control paradigm. In the second experiment, after the stress manipulation, stressed rats exhibited significantly greater ethanol preference than non-stressed rats. These findings suggest that prior exposure to ethanol influences the subsequent effect of stress on ethanol intake. They also validate the use of the present model of PTSD to examine potential mechanisms underlying stress-related changes in ethanol-seeking behavior.

Involvement of Activated Brain Stress Responsive Systems in Excessive and "Relapse" Alcohol Drinking in Rodent Models: Implications for Therapeutics

Addictive diseases, including addiction to alcohol, pose massive public health costs. Addiction is a chronic relapsing disease caused by both the direct effects induced by drugs and persistent neuroadaptations at the molecular, cellular, and behavioral levels. These drug-type specific neuroadaptations are brought on largely by the reinforcing effects of drugs on the central nervous system and environmental stressors. Results from animal experiments have demonstrated important interactions between alcohol and stress-responsive systems. Addiction to specific drugs such as alcohol, psychostimulants, and opioids shares some common direct or downstream effects on the brain's stress-responsive systems, including arginine vasopressin and its V1b receptors, dynorphin and the κ-opioid receptors, pro-opiomelanocortin/β-endorphin and the μ-opioid receptors, and the endocannabinoids. Further study of these systems through laboratory-based and translational research could lead to the discovery of novel treatment targets and the early optimization of interventions (for example, combination) for the pharmacologic therapy of alcoholism.

Adolescent rats fail to demonstrate a LiCl-induced pre-exposure effect: Implications for the balance of drug reward and aversion in adolescence

Adolescents display weaker taste avoidance induced by both abused and non-abused drugs than adults. Drug history attenuates avoidance learning in adults (the drug pre-exposure effect), but little is known about this phenomenon in adolescents. Given that the weaker taste avoidance in adolescence is thought to be a function of their relative insensitivity to the drug's aversive effects, it might be expected that the drug pre-exposure effect would be weaker in adolescents given that for some drugs this effect is mediated by associative blocking that depends on the association of environmental cues with the drug's aversive effects. To address this, in the present studies male adolescent (Experiment 1) and adult (Experiment 2) rats were given five spaced injections of LiCl prior to subsequent taste avoidance conditioning with LiCl. Consistent with past reports, adolescents displayed weaker taste avoidance than adults. While adults displayed attenuated LiCl-induced taste avoidance following LiCl pre-exposure, adolescents showed no evidence of this pre-exposure. This work is consistent with the view that adolescents are relatively insensitive to the aversive effects of drugs, an insensitivity potentially important in subsequent intake of drugs of abuse given that such intake is a function of the balance of their rewarding and aversive effects.

Role of κ-Opioid Receptors in the Bed Nucleus of Stria Terminalis in Reinstatement of Alcohol Seeking

κ-Opioid receptors (KORs) and their endogenous ligand dynorphin are involved in stress-induced alcohol seeking but the mechanisms involved are largely unknown. We previously showed that systemic injections of the KOR agonist U50,488, which induce stress-like aversive states, reinstate alcohol seeking after extinction of the alcohol-reinforced responding. Here, we used the neuronal activity marker Fos and site-specific injections of the KOR antagonist nor-BNI and U50,488 to study brain mechanisms of U50,488-induced reinstatement of alcohol seeking. We trained male Long-Evans rats to self-administer alcohol (12% w/v) for 23-30 days. After extinction of the alcohol-reinforced responding, we tested the effect of U50,488 (0, 1.25, 2.5, and 5 mg/kg) on reinstatement of alcohol seeking. Next, we correlated regional Fos expression with reinstatement induced by the most effective U50,488 dose (5 mg/kg). Based on the correlational Fos results, we determined the effect of bed nucleus of the stria terminalis (BNST) injections of nor-BNI (4 μg/side) on U50,488-induced reinstatement of alcohol seeking, and reinstatement induced by injections of U50,488 (0, 0.3, 1, and 3 μg/side) into the BNST. U50,488-induced reinstatement of alcohol seeking was associated with increased Fos expression in multiple brain areas, including the BNST, where it was significantly correlated with lever pressing. U50,488-induced reinstatement was blocked by BNST nor-BNI injections, and BNST U50,488 injections partially mimicked the drug's systemic effect on reinstatement. Our data indicate that the BNST is a critical site for U50,488-induced reinstatement of alcohol seeking and suggest that KOR/dynorphin mechanisms in this brain area play a key role in stress-induced alcohol seeking.

Contribution of Dynorphin and Orexin Neuropeptide Systems to the Motivational Effects of Alcohol

Understanding the neural systems that drive alcohol motivation and are disrupted in alcohol use disorders is of critical importance in developing novel treatments. The dynorphin and orexin/hypocretin neuropeptide systems are particularly relevant with respect to alcohol use and misuse. Both systems are strongly associated with alcohol-seeking behaviors, particularly in cases of high levels of alcohol use as seen in dependence. Furthermore, both systems also play a role in stress and anxiety, indicating that disruption of these systems may underlie long-term homeostatic dysregulation seen in alcohol use disorders. These systems are also closely interrelated with one another - dynorphin/kappa opioid receptors and orexin/hypocretin receptors are found in similar regions and hypocretin/orexin neurons also express dynorphin - suggesting that these two systems may work together in the regulation of alcohol seeking and may be mutually disrupted in alcohol use disorders. This chapter reviews studies demonstrating a role for each of these systems in motivated behavior, with a focus on their roles in regulating alcohol-seeking and self-administration behaviors. Consideration is also given to evidence indicating that these neuropeptide systems may be viable targets for the development of potential treatments for alcohol use disorders.

Traumatic stress affects alcohol‑drinking behavior through cocaine‑ and amphetamine‑regulated transcript 55‑102 in the paraventricular nucleus in rats

Cumulative evidence has suggested an association between stress and alcohol self‑administration; however, less is known about the role of traumatic stress in alcohol drinking behavior. It has been reported that cocaine‑ and amphetamine‑regulated transcript (CART) 55‑102 may be involved in mediating stress responses and regulating reward and reinforcement. The aim of the present study was to evaluate the role of CART 55‑102 in alcohol drinking behavior of rats in the presence or absence of traumatic stress. Alcohol drinking behavior was examined using the two‑bottle choice drinking paradigm (one bottle contained 10% alcohol and the other contained filtered water), which was initiated 1, 3 and 7 days post‑trauma (T1, T3 and T7), for 14 days in rats; the control group was initiated from T0. The results indicated that exposure to trauma significantly increased alcohol consumption and preference, particularly drinking from T3. Immunohistochemistry revealed that the lowest level of CART 55‑102 immunoreactivity within the paraventricular nucleus (PVN) was exhibited in the T3 group. Additionally, an intra‑PVN injection of CART 55‑102 attenuated alcohol‑drinking behavior in a dose‑dependent manner, in the T3 group. Furthermore, the significant increase in circulating adrenocorticotrophic hormone (ACTH) and corticosterone (CORT) concentrations in the T3 group were inhibited by CART 55‑102 administration to the PVN, in particular CORT levels were significantly decreased. Positive correlations between alcohol preference and ACTH and CORT levels were also observed. These results indicated that CART 55‑102 in the PVN serves an inhibitory role in traumatic stress‑induced alcohol drinking behavior, possibly through disturbing hypothalamus‑pituitary‑adrenal axis hyperactivity.

Antagonism of the kappa opioid receptor attenuates THC-induced place aversions in adult male Sprague-Dawley rats

RATIONALE

Prior research with transgenic mice in which the kappa opioid receptor (KOR) has been suppressed or activated suggests that the aversive effects of THC are mediated by activity of this receptor subtype. If the activity of the KOR system is responsible for mediating the THC's aversive effects, then selective antagonism of the KOR by norBNI should block such aversive effects. To test this hypothesis, rats were pretreated with norBNI 24h prior to place conditioning with THC to assess its effect on the acquisition of THC-induced place aversions.

METHODS

In Experiment 1, rats pretreated with norBNI (0 or 15mg/kg) were exposed 24h later to one side of a place conditioning chamber and injected with THC (0, 0.56, 1 and 3.2mg/kg). On the next day, they were injected with vehicle and placed on the opposite side of the chamber. This was repeated for a total of five cycles followed by a test of the animal's aversion to the THC-paired side. In Experiment 2, rats were pretreated with norBNI (0 or 30mg/kg) prior to place conditioning 24h later with THC (0 or 3.2mg/kg).

RESULTS

In Experiment 1, THC produced dose-dependent place aversions that were unaffected by norBNI (15mg/kg). In Experiment 2, THC induced significant place aversions that were fully attenuated by norBNI (30mg/kg).

CONCLUSIONS

Although 15mg/kg norBNI was ineffective in antagonizing the aversive effects of THC, 30mg/kg norBNI blocked the ability of THC to induce a place aversion. The results of the latter assessment are consistent with prior research with transgenic manipulations of the KOR and provide further evidence for the role of the KOR system in the aversive properties of THC.

Dynorphin/Kappa Opioid Receptor Signaling in Preclinical Models of Alcohol, Drug, and Food Addiction

The dynorphin/kappa opioid receptor (KOR) system is implicated in the "dark side" of addiction, in which stress exacerbates maladaptive responses to drug and alcohol exposure. For example, acute stress and acute ethanol exposure result in an elevation in dynorphin, the KOR endogenous ligand. Activation of KORs results in modulation of several neurotransmitters; however, this chapter will focus on its regulatory effects on dopamine in mesolimbic areas. Specifically, KOR activation has an inhibitory effect on dopamine release, thereby influencing reward processing. Repeated stimulation of KORs, for example, via chronic drug and/or stress exposure, results in increased function of the dynorphin/KOR system. This augmentation in KOR function shifts the homeostatic balance in favor of an overall reduction in dopamine signaling via either by reducing dopamine release or by increasing dopamine transporter function. This chapter examines the effects of chronic ethanol exposure on KOR function and the downstream effects on dopamine transmission. Additionally, the impact of chronic cocaine exposure and its effects on KOR function will be explored. Further, KORs may also be involved in driving excessive consumption of food, contributing to the risk of developing obesity. While some studies have shown that KOR agonists reduce drug intake, other studies have shown that antagonists reduce addiction-like behaviors, demonstrating therapeutic potential. For example, KOR inhibition reduces ethanol intake in dependent animals, motivation to self-administer cocaine in chronic stress-exposed animals, and food consumption in obese animals. This chapter will delve into the mechanisms by which modulation of the dynorphin/KOR system may be therapeutic.

The Role of NFkB in Drug Addiction: Beyond Inflammation

Aims

Nuclear factor kappa light chain enhancer of activated B cells (NFkB) is a ubiquitous transcription factor well known for its role in the innate immune response. As such, NFkB is a transcriptional activator of inflammatory mediators such as cytokines. It has recently been demonstrated that alcohol and other drugs of abuse can induce NFkB activity and cytokine expression in the brain. A number of reviews have been published highlighting this effect of alcohol, and have linked increased NFkB function to neuroimmune-stimulated toxicity. However, in this review we focus on the potentially non-immune functions of NFkB as possible links between NFkB and addiction.

Methods

An extensive review of the literature via Pubmed searches was used to assess the current state of the field.

Results

NFkB can induce the expression of a diverse set of gene targets besides inflammatory mediators, some of which are involved in addictive processes, such as opioid receptors and neuropeptides. NFkB mediates complex behaviors including learning and memory, stress responses, anhedonia and drug reward, processes that may lie outside the role of NFkB in the classic neuroimmune response.

Conclusions

Future studies should focus on these non-immune functions of NFkB signaling and their association with addiction-related processes.

Influence of stress associated with chronic alcohol exposure on drinking

Stress is commonly regarded as an important trigger for relapse and a significant factor that promotes increased motivation to drink in some individuals. However, the relationship between stress and alcohol is complex, likely changing in form during the transition from early moderated alcohol use to more heavy uncontrolled alcohol intake. A growing body of evidence indicates that prolonged excessive alcohol consumption serves as a potent stressor, producing persistent dysregulation of brain reward and stress systems beyond normal homeostatic limits. This progressive dysfunctional (allostatic) state is characterized by changes in neuroendocrine and brain stress pathways that underlie expression of withdrawal symptoms that reflect a negative affective state (dysphoria, anxiety), as well as increased motivation to self-administer alcohol. This review highlights literature supportive of this theoretical framework for alcohol addiction. In particular, evidence for stress-related neural, physiological, and behavioral changes associated with chronic alcohol exposure and withdrawal experience is presented. Additionally, this review focuses on the effects of chronic alcohol-induced changes in several pro-stress neuropeptides (corticotropin-releasing factor, dynorphin) and anti-stress neuropeptide systems (nocicepton, neuropeptide Y, oxytocin) in contributing to the stress, negative emotional, and motivational consequences of chronic alcohol exposure. Studies involving use of animal models have significantly increased our understanding of the dynamic stress-related physiological mechanisms and psychological underpinnings of alcohol addiction. This, in turn, is crucial for developing new and more effective therapeutics for treating excessive, harmful drinking, particularly stress-enhanced alcohol consumption. This article is part of the Special Issue entitled "Alcoholism".

The long-term effects of stress and kappa opioid receptor activation on conditioned place aversion in male and female California mice

Psychosocial stress leads to the activation of kappa opioid receptors (KORs), which induce dysphoria and facilitate depression-like behaviors. However, less is known about the long-term effects of stress and KORs in females. We examined the long-term effects of social defeat stress on the aversive properties of KOR activation in male and female California mice (Peromyscus californicus) using a conditioned place aversion paradigm. Female California mice naïve to social defeat, formed a place aversion following treatment with 2.5mg/kg of the KOR agonist U50,488, but females exposed to defeat did not form a place aversion to this dose. This supports the finding by others that social defeat weakens the aversive properties of KOR agonists. In contrast, both control and stressed males formed an aversion to 10mg/kg of U50,488. We also examined EGR1 immunoreactivity, an indirect marker of neuronal activity, in the nucleus accumbens (NAc) and found that stress and treatment with 10mg/kg of U50,488 increased EGR1 immunoreactivity in the NAc core in females but reduced activation in males. The effects of stress and U50,488 on EGR1 were specific to the NAc, as we found no differences in the bed nucleus of the stria terminalis. In summary, our data indicate important sex differences in the long-term effects of stress and indicate the need for further study of the molecular mechanisms mediating the behavioral effects of KOR in both males and females.

Role of the Dynorphin/Kappa Opioid Receptor System in the Motivational Effects of Ethanol

Evidence has demonstrated that dynorphin (DYN) and the kappa opioid receptor (KOR) system contribute to various psychiatric disorders, including anxiety, depression, and addiction. More recently, this endogenous opioid system has received increased attention as a potential therapeutic target for treating alcohol use disorders. In this review, we provide an overview and synthesis of preclinical studies examining the influence of alcohol (ethanol [EtOH]) exposure on DYN/KOR expression and function, as well as studies examining the effects of DYN/KOR manipulation on EtOH's rewarding and aversive properties. We then describe work that has characterized effects of KOR activation and blockade on EtOH self-administration and EtOH dependence/withdrawal-related behaviors. Finally, we address how the DYN/KOR system may contribute to stress-EtOH interactions. Despite an apparent role for the DYN/KOR system in motivational effects of EtOH, support comes from relatively few studies. Nevertheless, review of this literature reveals several common themes: (i) rodent strains genetically predisposed to consume more EtOH generally appear to have reduced DYN/KOR tone in brain reward circuitry; (ii) acute and chronic EtOH exposure typically up-regulate the DYN/KOR system; (iii) KOR antagonists reduce behavioral indices of negative affect associated with stress and chronic EtOH exposure/withdrawal; and (iv) KOR antagonists are effective in reducing EtOH consumption, but are often more efficacious under conditions that engender high levels of consumption, such as dependence or stress exposure. These results support the contention that the DYN/KOR system plays a significant role in contributing to dependence- and stress-induced elevation in EtOH consumption. Overall, more comprehensive analyses (on both behavioral and mechanistic levels) are needed to provide additional insight into how the DYN/KOR system is engaged and adapts to influence the motivation effects of EtOH. This information will be critical for the development of new pharmacological agents targeting KORs as promising novel therapeutics for alcohol use disorders and comorbid affective disorders.

Behavioral and Biochemical Impact of Chronic Unpredictable Mild Stress on the Acquisition of Nicotine Conditioned Place Preference in Rats

Addiction is a chronic psychiatric disease which represents a global problem, and stress can increase drug addiction and relapse. Taking into account frequent concomitance of nicotine dependence and stress, the purpose of the present study was to assess behavioral and biochemical effects of chronic unpredictable mild stress (CUMS) exposure on nicotine reward in rats measured in the conditioned place preference (CPP) paradigm. Rats were submitted to the CUMS for 3 weeks and conditioned with nicotine (0.175 mg/kg) for 2 or 3 days. Our results revealed that only CUMS-exposed animals exhibited the CPP after 2 days of conditioning indicating that stressed rats were more sensitive to the rewarding properties of nicotine and that chronic stress exacerbates nicotine preference. Administration of metyrapone (50 mg/kg), a glucocorticosteroid antagonist, and imipramine (15 mg/kg), an antidepressant, abolished nicotine CPP in stressed rats after 2 days of conditioning. The biochemical experiments showed increased markers of oxidative stress after nicotine conditioning for 2 and 3 days, while the CUMS further potentiated pro-oxidative effects of nicotine. Moreover, metyrapone reversed oxidative changes caused by stress and nicotine, while imipramine was not able to overwhelm nicotine- and stress-induced oxidative damages; however, it could exert antioxidant effect if administered repeatedly. The results suggest that recent exposure to a stressor may augment the rewarding effects of nicotine through anhedonia- and stress-related mechanisms. Our study contributes to the understanding of behavioral and biochemical stress-induced modification of the rewarding effects of nicotine on the basis of the development of nicotine dependence.

Synergistic blockade of alcohol escalation drinking in mice by a combination of novel kappa opioid receptor agonist Mesyl Salvinorin B and naltrexone

Mesyl Salvinorin B (MSB) is a potent selective kappa opioid receptor (KOP-r) agonist that has potential for development as an anti-psychostimulant agent with fewer side-effects (e.g., sedation, depression and dysphoria) than classic KOP-r agonists. However, no such study has been done on alcohol. We investigated whether MSB alone or in combination with naltrexone (mu-opioid receptor antagonist) altered voluntary alcohol drinking in both male and female mice. Mice, subjected to 3weeks of chronic escalation drinking (CED) in a two-bottle choice paradigm with 24-h access every other day, developed rapid escalation of alcohol intake and high preference. We found that single, acute administration of MSB dose-dependently reduced alcohol intake and preference in mice after 3-week CED. The effect was specific to alcohol, as shown by the lack of any effect of MSB on sucrose or saccharin intake. We also used the drinking-in-the-dark (DID) model with limited access (4h/day) to evaluate the pharmacological effect of MSB after 3weeks of DID. However, MSB had no effect on alcohol drinking after 3-week DID. Upon investigation of potential synergistic effects between naltrexone and MSB, we found that acute administration of a combination of MSB and naltrexone reduced alcohol intake profoundly after 3-week CED at doses lower than those individual effective doses. Repeated administrations of this combination showed less tolerance development than repeated MSB alone. Our study suggests that the novel KOP-r agonist MSB both alone and in combination with naltrexone shows potential in alcoholism treatment models.

A Trigger for Opioid Misuse: Chronic Pain and Stress Dysregulate the Mesolimbic Pathway and Kappa Opioid System

Pain and stress are protective mechanisms essential in avoiding harmful or threatening stimuli and ensuring survival. Despite these beneficial roles, chronic exposure to either pain or stress can lead to maladaptive hormonal and neuronal modulations that can result in chronic pain and a wide spectrum of stress-related disorders including anxiety and depression. By inducing allostatic changes in the mesolimbic dopaminergic pathway, both chronic pain and stress disorders affect the rewarding values of both natural reinforcers, such as food or social interaction, and drugs of abuse. Despite opioids representing the best therapeutic strategy in pain conditions, they are often misused as a result of these allostatic changes induced by chronic pain and stress. The kappa opioid receptor (KOR) system is critically involved in these neuronal adaptations in part through its control of dopamine release in the nucleus accumbens. Therefore, it is likely that changes in the kappa opioid system following chronic exposure to pain and stress play a key role in increasing the misuse liability observed in pain patients treated with opioids. In this review, we will discuss how chronic pain and stress-induced pathologies can affect mesolimbic dopaminergic transmission, leading to increased abuse liability. We will also assess how the kappa opioid system may underlie these pathological changes.

Accumbal μ-Opioid Receptors Modulate Ethanol Intake in Alcohol-Preferring Alko Alcohol Rats

BACKGROUND

The nucleus accumbens shell is a key brain area mediating the reinforcing effects of ethanol (EtOH). Previously, it has been shown that the density of μ-opioid receptors in the nucleus accumbens shell is higher in alcohol-preferring Alko Alcohol (AA) rats than in alcohol-avoiding Alko Non-Alcohol rats. In addition, EtOH releases opioid peptides in the nucleus accumbens and opioid receptor antagonists are able to modify EtOH intake, all suggesting an opioidergic mechanism in the control of EtOH consumption. As the exact mechanisms of opioidergic involvement remains to be elucidated, the aim of this study was to clarify the role of accumbal μ- and κ-opioid receptors in controlling EtOH intake in alcohol-preferring AA rats.

METHODS

Microinfusions of the μ-opioid receptor antagonist CTOP (0.3 and 1 μg/site), μ-opioid receptor agonist DAMGO (0.03 and 0.1 μg/site), nonselective opioid receptor agonist morphine (30 μg/site), and κ-opioid receptor agonist U50488H (0.3 and 1 μg/site) were administered via bilateral guide cannulas into the nucleus accumbens shell of AA rats that voluntarily consumed 10% EtOH solution in an intermittent, time-restricted (90-minute) 2-bottle choice access paradigm.

RESULTS

CTOP (1 μg/site) significantly increased EtOH intake. Conversely, DAMGO resulted in a decreasing trend in EtOH intake. Neither morphine nor U50488H had any effect on EtOH intake in the used paradigm.

CONCLUSIONS

The results provide further evidence for the role of accumbens shell μ-opioid receptors but not κ-opioid receptors in mediating reinforcing effects of EtOH and in regulating EtOH consumption. The results also provide support for views suggesting that the nucleus accumbens shell has a major role in mediating EtOH reward.