Effects of genetic deletion of endogenous opioid system components on the reinstatement of cocaine-seeking behavior in mice

Endogenous opioid system modulates conditioned cocaine reward in a sex-dependent manner

Cocaine predictive cues and contexts exert powerful control over behaviour and can incite cocaine seeking and taking. This type of conditioned behaviour is encoded within striatal circuits, and these circuits and behaviours are, in part, regulated by opioid peptides and receptors expressed in striatal medium spiny neurons. We previously showed that augmenting levels of the opioid peptide enkephalin in the striatum facilitates acquisition of cocaine conditioned place preference (CPP), while opioid receptor antagonists attenuate expression of cocaine CPP. However, whether striatal enkephalin is necessary for acquisition of cocaine CPP and maintenance during extinction remains unknown. To address this, we generated mice with a targeted deletion of enkephalin from dopamine D2-receptor expressing medium spiny neurons and tested them in a cocaine CPP paradigm. Low striatal enkephalin levels did not attenuate acquisition of CPP. However, expression of preference, assessed after acute administration of the opioid receptor antagonist naloxone, was blocked in females, regardless of genotype. When saline was paired with the cocaine context during extinction sessions, females, regardless of genotype, extinguished preference faster than males, and this was prevented by naloxone when paired with the cocaine context. We conclude that while striatal enkephalin is not necessary for acquisition, expression, or extinction of cocaine CPP, expression and extinction of cocaine preference in females is mediated by an opioid peptide other than striatal enkephalin. The unique sensitivity of females to opioid antagonists suggests sex should be a consideration when using these compounds in the treatment of cocaine use disorder.

Histone H3 dopaminylation in nucleus accumbens, but not medial prefrontal cortex, contributes to cocaine-seeking following prolonged abstinence

Enduring patterns of epigenomic and transcriptional plasticity within the mesolimbic dopamine system contribute importantly to persistent behavioral adaptations that characterize substance use disorders (SUD). While drug addiction has long been thought of as a disorder of dopamine (DA) neurotransmission, therapeutic interventions targeting receptor mediated DA-signaling have not yet resulted in efficacious treatments. Our laboratory recently identified a non-canonical, neurotransmission-independent signaling moiety for DA in brain, termed dopaminylation, whereby DA itself acts as a donor source for the establishment of post-translational modifications (PTM) on substrate proteins (e.g., histone H3 at glutamine 5; H3Q5dop). In our previous studies, we demonstrated that H3Q5dop plays a critical role in the regulation of neuronal transcription and, when perturbed within monoaminergic neurons of the ventral tegmental area (VTA), critically contributes to pathological states, including relapse vulnerability to both psychostimulants (e.g., cocaine) and opiates (e.g., heroin). Importantly, H3Q5dop is also observed throughout the mesolimbic DA reward pathway (e.g., in nucleus accumbens/NAc and medial prefrontal cortex/mPFC, which receive DA input from VTA). As such, we investigated whether H3Q5dop may similarly be altered in its expression in response to drugs of abuse in these non-dopamine-producing regions. In rats undergoing extended abstinence from cocaine self-administration (SA), we observed both acute and prolonged accumulation of H3Q5dop in NAc, but not mPFC. Attenuation of H3Q5dop in NAc during drug abstinence reduced cocaine-seeking and affected cocaine-induced gene expression programs associated with altered dopamine signaling and neuronal function. These findings thus establish H3Q5dop in NAc, but not mPFC, as an important mediator of cocaine-induced behavioral and transcriptional plasticity during extended cocaine abstinence.

Striatal enkephalin supports maintenance of conditioned cocaine reward during extinction

Drug predictive cues and contexts exert powerful control over behavior and can incite drug seeking and taking. This association and the behavioral output are encoded within striatal circuits, and regulation of these circuits by G-protein coupled receptors affects cocaine-related behaviors. Here, we investigated how opioid peptides and G-protein coupled opioid receptors expressed in striatal medium spiny neurons (MSNs) regulate conditioned cocaine seeking. Augmenting levels of the opioid peptide enkephalin in the striatum facilitates acquisition of cocaine conditioned place preference (CPP). In contrast, opioid receptor antagonists attenuate cocaine CPP and facilitate extinction of alcohol CPP. However, whether striatal enkephalin is necessary for acquisition of cocaine CPP and maintenance during extinction remains unknown. We generated mice with a targeted deletion of enkephalin from dopamine D2-receptor expressing MSNs (D2-PenkKO) and tested them for cocaine CPP. Low striatal enkephalin levels did not attenuate acquisition or expression of CPP; however, D2-PenkKOs showed faster extinction of cocaine CPP. Single administration of the non-selective opioid receptor antagonist naloxone prior to preference testing blocked expression of CPP selectively in females, but equally between genotypes. Repeated administration of naloxone during extinction did not facilitate extinction of cocaine CPP for either genotype, but rather prevented extinction in D2-PenkKO mice. We conclude that while striatal enkephalin is not necessary for acquisition of cocaine reward, it maintains the learned association between cocaine and its predictive cues during extinction learning. Further, sex and pre-existing low striatal enkephalin levels may be important considerations for use of naloxone in treating cocaine use disorder.

Dopamine D2 receptors bidirectionally regulate striatal enkephalin expression: Implications for cocaine reward

Low dopamine D2 receptor (D2R) availability in the striatum can predispose for cocaine abuse; though how low striatal D2Rs facilitate cocaine reward is unclear. Overexpression of D2Rs in striatal neurons or activation of D2Rs by acute cocaine suppresses striatal Penk mRNA. Conversely, low D2Rs in D2-striatal neurons increases striatal Penk mRNA and enkephalin peptide tone, an endogenous mu-opioid agonist. In brain slices, met-enkephalin and inhibition of enkephalin catabolism suppresses intra-striatal GABA transmission. Pairing cocaine with intra-accumbens met-enkephalin during place conditioning facilitates acquisition of preference, while mu-opioid receptor antagonist blocks preference in wild-type mice. We propose that heightened striatal enkephalin potentiates cocaine reward by suppressing intra-striatal GABA to enhance striatal output. Surprisingly, a mu-opioid receptor antagonist does not block cocaine preference in mice with low striatal D2Rs, implicating other opioid receptors. The bidirectional regulation of enkephalin by D2R activity and cocaine offers insights into mechanisms underlying the vulnerability for cocaine abuse.

Low striatal D2 receptor levels are associated with cocaine abuse. Dai et al. bidirectionally alter striatal D2 receptor levels to probe the downstream mechanisms underlying this abuse liability. They provide evidence that enhanced enkephalin tone resulting from low D2 receptors is associated with suppressed intra-striatal GABA and potentiated cocaine reward.

The role of enkephalinergic systems in substance use disorders

Enkephalin, an endogenous opioid peptide, is highly expressed in the reward pathway and may modulate neurotransmission to regulate reward-related behaviors, such as drug-taking and drug-seeking behaviors. Drugs of abuse also directly increase enkephalin in this pathway, yet it is unknown whether or not changes in the enkephalinergic system after drug administration mediate any specific behaviors. The use of animal models of substance use disorders (SUDs) concurrently with pharmacological, genetic, and molecular tools has allowed researchers to directly investigate the role of enkephalin in promoting these behaviors. In this review, we explore neurochemical mechanisms by which enkephalin levels and enkephalin-mediated signaling are altered by drug administration and interrogate the contribution of enkephalin systems to SUDs. Studies manipulating the receptors that enkephalin targets (e.g., mu and delta opioid receptors mainly) implicate the endogenous opioid peptide in drug-induced neuroadaptations and reward-related behaviors; however, further studies will need to confirm the role of enkephalin directly. Overall, these findings suggest that the enkephalinergic system is involved in multiple aspects of SUDs, such as the primary reinforcing properties of drugs, conditioned reinforcing effects, and sensitization. The idea of dopaminergic-opioidergic interactions in these behaviors remains relatively novel and warrants further research. Continuing work to elucidate the role of enkephalin in mediating neurotransmission in reward circuitry driving behaviors related to SUDs remains crucial.

Analyses of polymorphisms of intron 2 of OPRK1 (kappa-opioid receptor gene) in association with opioid and cocaine dependence diagnoses in an African-American population

RATIONALE

The dynorphin/kappa-opioid receptor (KOR) system (encoded by PDYN and OPRK1 genes respectively) is highly regulated by repeated exposure to drugs of abuse, including mu-opioid agonists and cocaine. These changes in the dynorphin/KOR system can then influence the rewarding effects of these drugs of abuse. Activation of the dynorphin/KOR system is also thought to have a role in the pro-addictive effects of stress. Recent in vitro assays showed that the OPRK1 intron 2 may function as a genomic enhancer in the regulation KOR expression, and contains a glucocorticiod-responsive sequence site. We hypothesize that SNPs in intron 2 of OPRK1 are associated with categorical opioid or cocaine dependence diagnoses, as well as with dimensional aspects of drug use (i.e., magnitude of drug exposure).

METHODS

This study includes 577 subjects ≥ 18 years old, with African ancestry (AA) from the USA. They were divided into three groups: 152 control subjects, 142 persons with lifetime opioid dependence diagnosis (OD), and 283 subjects with lifetime cocaine dependence diagnosis (CD). Five SNPs (rs16918909, rs7016778, rs997917, rs6473797, rs10111937) that span 10 Kb nucleotides in intron 2 of OPRK1 were used for the association analyses. Genotyping was performed with the Smokescreen® array or sequencing of PCR-amplified DNA fragments. Association analyses for OD and CD diagnoses and the OPRK1 intron 2 alleles were carried out with Fisher's exact test. The Kreek-McHugh-Schluger-Kellogg (KMSK) scales were used for dimensional measure of maximum exposure to specific drugs, using Mann-Whitney tests.

RESULTS

Two SNPs, rs997917 and rs10111937 showed point-wise significant allelic association (p < 0.05) with CD diagnosis, and rs10111937 showed a point-wise significance in association with OD. None of these single SNP associations with categorical diagnoses were significant after correction for multiple testing (p_corr > 0.05). However, significant associations of several genotype patterns (diplotypes) were found with cocaine dependence, but none for opioid dependence. The most significant genotype pattern with cocaine dependence diagnosis occurred for rs6473797 and rs10111937 (p_corr = 0.036, odds ratio = 1.92, FDR < 0.05), and survived correction for multiple testing. Dimensional analyses with KMSK scores show that persons with either rs997917 or rs10111937 variants had greater exposure to cocaine, compared to those with prototype allele (Mann-Whitney tests, point-wise).

CONCLUSIONS

This study provides additional support of potential importance of regulatory regions of intron 2 of the OPRK1 gene in development of cocaine and opioid dependence diagnoses, in a population with African-American ancestry.

Cell-type- and region-specific modulation of cocaine seeking by micro-RNA-1 in striatal projection neurons

The persistent and experience-dependent nature of drug addiction may result in part from epigenetic alterations, including non-coding micro-RNAs (miRNAs), which are both critical for neuronal function and modulated by cocaine in the striatum. Two major striatal cell populations, the striato-nigral and striato-pallidal projection neurons, express, respectively, the D1 (D1-SPNs) and D2 (D2-SPNs) dopamine receptor, and display distinct but complementary functions in drug-evoked responses. However, a cell-type-specific role for miRNAs action has yet to be clarified. Here, we evaluated the expression of a subset of miRNAs proposed to modulate cocaine effects in the nucleus accumbens (NAc) and dorsal striatum (DS) upon sustained cocaine exposure in mice and showed that these selected miRNAs were preferentially upregulated in the NAc. We focused on miR-1 considering the important role of some of its predicted mRNA targets, Fosb and Npas4, in the effects of cocaine. We validated these targets in vitro and in vivo. We explored the potential of miR-1 to regulate cocaine-induced behavior by overexpressing it in specific striatal cell populations. In DS D1-SPNs miR-1 overexpression downregulated Fosb and Npas4 and reduced cocaine-induced CPP reinstatement, but increased cue-induced cocaine seeking. In DS D2-SPNs miR-1 overexpression reduced the motivation to self-administer cocaine. Our results indicate a role of miR1 and its target genes, Fosb and Npas4, in these behaviors and highlight a precise cell-type- and region-specific modulatory role of miR-1, illustrating the importance of cell-specific investigations.

Reduced cue-induced reinstatement of cocaine-seeking behavior in Plcb1 +/- mice

Cocaine addiction causes serious health problems, and no effective treatment is available yet. We previously identified a genetic risk variant for cocaine addiction in the PLCB1 gene and found this gene upregulated in postmortem brains of cocaine abusers and in human dopaminergic neuron-like cells after an acute cocaine exposure. Here, we functionally tested the contribution of the PLCB1 gene to cocaine addictive properties using Plcb1+/- mice. First, we performed a general phenotypic characterization and found that Plcb1+/- mice showed normal behavior, although they had increased anxiety and impaired short-term memory. Subsequently, mice were trained for operant conditioning, self-administered cocaine for 10 days, and were tested for cocaine motivation. After extinction, we found a reduction in the cue-induced reinstatement of cocaine-seeking behavior in Plcb1+/- mice. After reinstatement, we identified transcriptomic alterations in the medial prefrontal cortex of Plcb1+/- mice, mostly related to pathways relevant to addiction like the dopaminergic synapse and long-term potentiation. To conclude, we found that heterozygous deletion of the Plcb1 gene decreases cue-induced reinstatement of cocaine-seeking, pointing at PLCB1 as a possible therapeutic target for preventing relapse and treating cocaine addiction.

Epigenetic and Transcriptional Control of the Opioid Prodynorphine Gene: In-Depth Analysis in the Human Brain

Neuropeptides serve as neurohormones and local paracrine regulators that control neural networks regulating behavior, endocrine system and sensorimotor functions. Their expression is characterized by exceptionally restricted profiles. Circuit-specific and adaptive expression of neuropeptide genes may be defined by transcriptional and epigenetic mechanisms controlled by cell type and subtype sequence-specific transcription factors, insulators and silencers. The opioid peptide dynorphins play a critical role in neurological and psychiatric disorders, pain processing and stress, while their mutations cause profound neurodegeneration in the human brain. In this review, we focus on the prodynorphin gene as a model for the in-depth epigenetic and transcriptional analysis of expression of the neuropeptide genes. Prodynorphin studies may provide a framework for analysis of mechanisms relevant for regulation of neuropeptide genes in normal and pathological human brain.

Hippocampal mu opioid receptors are modulated following cocaine self-administration in rat

Cocaine addiction is a complex pathology induced by long-term brain changes. Understanding the neurochemical changes underlying the reinforcing effects of this drug of abuse is critical for reducing the societal burden of drug addiction. The mu opioid receptor plays a major role in drug reward. This receptor is modulated by chronic cocaine treatment in specific brain structures, but few studies investigated neurochemical adaptations induced by voluntary cocaine intake. In this study, we investigated whether intravenous cocaine-self administration (0.33 mg/kg/injection, fixed-ratio 1 [FR1], 10 days) in rats induces transcriptional and functional changes of the mu opioid receptor in reward-related brain regions. Epigenetic processes with histone modifications were examined for two activating marks, H3K4Me3, and H3K27Ac. We found an increase of mu opioid receptor gene expression along with a potentiation of its functionality in hippocampus of cocaine self-administering animals compared to saline controls. Chromatin immunoprecipitation followed by qPCR revealed no modifications of the histone mark H3K4Me3 and H3K27Ac levels at mu opioid receptor promoter. Our study highlights the hippocampus as an important target to further investigate neuroadaptive processes leading to cocaine addiction.

Cocaine Self-administration Regulates Transcription of Opioid Peptide Precursors and Opioid Receptors in Rat Caudate Putamen and Prefrontal Cortex

The brain opioid system plays an important role in cocaine reward. Altered signaling in the opioid system by chronic cocaine exposure contributes to cocaine-seeking and taking behavior. The current study investigated concurrent changes in the gene expression of multiple components in rat brain opioid system following cocaine self-administration. Animals were limited to 40 infusions (1.5 mg/kg/infusion) within 6 h per day for five consecutive days. We then examined the mRNA levels of opioid receptors including mu (Oprm), delta (Oprd), and kappa (Oprk), and their endogenous opioid peptide precursors including proopiomelanocortin (Pomc), proenkephalin (Penk), prodynorphin (Pdyn) in the dorsal striatum (CPu) and the prefrontal cortex (PFC) 18 h after the last cocaine infusion. We found that cocaine self-administration significantly increased the mRNA levels of Oprm and Oprd in both the CPu and PFC, but had no effect on Oprk mRNA levels in either brain region. Moreover, cocaine had a greater influence on the mRNA levels of opioid peptide precursors in rat CPu than in the PFC. In the CPu, cocaine self-administration significantly increased the mRNA levels of Penk and Pdyn and abolished the mRNA levels of Pomc. In the PFC, cocaine self-administration only increased Pdyn mRNA levels without changing the mRNA levels of Pomc and Penk. These data suggest that cocaine self-administration influences the expression of multiple genes in the brain opioid system, and the concurrent changes in these targets may underlie cocaine-induced reward and habitual drug-seeking behavior.

A dopamine-induced gene expression signature regulates neuronal function and cocaine response

Drugs of abuse elevate dopamine levels in the nucleus accumbens (NAc) and alter transcriptional programs believed to promote long-lasting synaptic and behavioral adaptations. Here, we leveraged single-nucleus RNA-sequencing to generate a comprehensive molecular atlas of cell subtypes in the NAc, defining both sex-specific and cell type-specific responses to acute cocaine experience in a rat model system. Using this transcriptional map, we identified an immediate early gene expression program that is up-regulated following cocaine experience in vivo and dopamine receptor activation in vitro. Multiplexed induction of this gene program with a large-scale CRISPR-dCas9 activation strategy initiated a secondary synapse-centric transcriptional profile, altered striatal physiology in vitro, and enhanced cocaine sensitization in vivo. Together, these results define the transcriptional response to cocaine with cellular precision and demonstrate that drug-responsive gene programs can potentiate both physiological and behavioral adaptations to drugs of abuse.

Analysis of Evidence for the Combination of Pro-dopamine Regulator (KB220PAM) and Naltrexone to Prevent Opioid Use Disorder Relapse

Blum's laboratory first showed the benefits of naloxone or narcotic antagonists in the treatment of alcohol dependence. This seminal work published in Nature in the early 70's, in conjunction with many other studies, later served as the basis for the development of the narcotic antagonist (NTX) now used to treat both alcohol and opioid dependence. In 2006 an extended-release injectable of Naltrexone (XR-NTX) was approved by the FDA. Naltrexone is a relatively weak antagonist of κ- and δ-receptors and is also a potent μ-receptor antagonist. Dosages of naltrexone that effectively reduce opioid and alcohol consumption also actively block μ-receptors, but chronically down-regulate mesolimbic dopamine release. While studies show benefit especially in the short term, there is ongoing evidence that the retention and compliance with NTX are not sufficient to characterize adherence as high. However, extended-release NTX opioid treatment is associated with superior outcomes including less likely relapse (defined as daily use), and much longer time to relapse despite higher rates of concurrent non-opioid substance use like cocaine. Regarding long-term extended-release injectable (XR-NTX) for opioid dependence; there was higher compliance with Opioid Use Disorder (OUD) than for Alcohol Use Disorder (AUD.). Consideration of modalities in combination with XR-NTX is imperative. Research by Blum., et al. showed that a combination of Naltrexone and a pro-dopamine regulator neuro-nutrient (KB220) significantly prevented opioid relapse. Thus, early identification of addiction vulnerability with the Genetic Addiction Risk Score (GARS™) a panel of polymorphic risk alleles from ten reward circuitry genes will provide valuable information especially as it relates to genetically guided therapy with the KB220 neuro nutrient termed 'Precision Addiction Management".

Intravenous cocaine self-administration in a panel of inbred mouse strains differing in acute locomotor sensitivity to cocaine

RATIONALE

Initial sensitivity to drugs of abuse often predicts subsequent use and abuse, but this relationship is not always observed in human studies. Moreover, studies examining the relationship between initial locomotor sensitivity and the rewarding and reinforcing effects of drugs in animal models have also been equivocal. Understanding the relationship between initial drug effects and propensity to continue use, potentially resulting in the development of a substance use disorder, may help to identify key targets for prevention and treatment.

OBJECTIVES

We examined intravenous cocaine self-administration in a set of mouse strains that were previously identified to be at the phenotypic extremes for cocaine-induced locomotor activation to determine if initial locomotor sensitivity predicted acquisition, extinction, dose response, or progressive ratio (PR) breakpoint.

METHODS

We selected eight inbred mouse strains based on locomotor sensitivity to 20 mg/kg cocaine. These strains, designated as low and high responders, were tested in an intravenous self-administration paradigm that included acquisition of 0.5 mg/(kg*inf) under a FR1 schedule, extinction, re-acquisition, dose response to 0.125, 0.25, 0.5, 1, and 2 mg/(kg*inf), and progressive ratio.

RESULTS

We observed overall differences in self-administration behavior between high and low responders. Low responders self-administered less cocaine and had lower breakpoints under the PR schedule. However, we also observed strain differences within each group. Self-administration in the low responder, LG/J, more closely resembled the behavior of the high-responding group, and the high responder, P/J, had self-administration behavior that more closely resembled the low-responding group.

CONCLUSIONS

We conclude that acute cocaine-induced locomotor activation does predict self-administration behavior, but in a strain-specific manner. These data support the idea that genetic background influences the relationship among addiction-related behaviors.

Extinction and reinstatement of an operant responding maintained by food in different models of obesity

A major problem in treating obesity is the high rate of relapse to abnormal food-taking habits after maintaining an energy balanced diet. Alterations of eating behavior such as compulsive-like behavior and lack of self-control over food intake play a critical role in relapse. In this study, we used an operant paradigm of food-seeking behavior on two different diet-induced obesity models, a free-choice chocolate-mixture diet and a high-fat diet with face validity for a rapid development of obesity or for unhealthy food regularly consumed in our societies. A reduced operant performance and motivation for the hedonic value of palatable chocolate pellets was revealed in both obesity mouse models. However, only mice exposed to high-fat diet showed an increased compulsive-like behavior in the absence of the reinforcer further characterized by impaired operant learning, enhanced impulsivity and intensified inflexibility. We used principal component analysis to globally identify the specific behaviors responsible for the differences among diet groups. Learning impairment and inflexible behaviors contributed to a first principal component, explaining the largest proportion of the variance in the high-fat diet mice phenotype. Reinforcement, impulsion and compulsion were the main contributors to the second principal component explaining the differences in the chocolate-mixture mice behavioral phenotype. These behaviors were not exclusive of chocolate group because some high-fat individuals showed similar values on this component. These data indicate that extended access to hypercaloric diets differentially modifies operant behavior learning, behavioral flexibility, impulsive-like and compulsive-like behavior, and these effects were dependent on the exposure to each specific diet.

Enkephalin as a Pivotal Player in Neuroadaptations Related to Psychostimulant Addiction

Enkephalin expression is high in mesocorticolimbic areas associated with psychostimulant-induced behavioral and neurobiological effects, and may also modulate local neurotransmission in this circuit network. Psychostimulant drugs, like amphetamine and cocaine, significantly increase the content of enkephalin in these brain structures, but we do not yet understand the specific significance of this drug-induced adaptation. In this review, we summarize the neurochemical and molecular mechanism of psychostimulant-induced enkephalin activation in mesocorticolimbic brain areas, and the contribution of this opioid peptide in the pivotal neuroadaptations and long-term behavioral changes underlying psychostimulant addiction. There is evidence suggesting that adaptive changes in enkephalin content in the mesocorticolimbic circuit, induced by acute and chronic psychostimulant administration, may represent a key initial step in the long-term behavioral and neuronal plasticity induced by these drugs.

Role of DOR in neuronal plasticity changes promoted by food-seeking behaviour

Several lines of evidence support that food overconsumption may be related to the role of the endogenous opioid system in the control of food palatability. The opioid system, and particularly the delta opioid receptor (DOR), plays a crucial role in the regulation of food rewarding properties. In our study, we used operant conditioning maintained by chocolate-flavoured pellets to investigate the role of DOR in the motivation for palatable food and the structural plasticity changes promoted by this behaviour. For this purpose, we evaluated the specific role of this receptor in the behavioural and neuroplastic changes induced by palatable food in the prefrontal cortex (PFC), hippocampus (HCP) and nucleus accumbens (NAc) in constitutive knockout (KO) mice deficient in DOR. Mutant mice and their wild-type littermates were trained to obtain chocolate-flavoured pellets on fixed ratio 1 (FR1), FR5 and progressive ratio (PR) schedule of reinforcement. No significant differences between genotypes were revealed on operant behaviour acquisition in FR1. DOR knockout mice displayed lower number of active lever-presses than wild-type mice on FR5, and a similar decrease was revealed in DOR KO mice in the breaking point during the PR. This operant training to obtain palatable food increased dendritic spine density in the PFC, HCP and NAc shell of wild-type, but these plasticity changes were abolished in DOR KO mice. Our results support the hypothesis that DOR regulates the reinforcing effects and motivation for palatable food through neuroplastic changes in specific brain reward areas.

Endogenous opioid system: a promising target for future smoking cessation medications

BACKGROUND

Nicotine addiction continues to be a health challenge across the world. Despite several approved medications, smokers continue to relapse. Several human and animal studies have evaluated the role of the endogenous opioid system as a potential target for smoking cessation medications.

METHODS

In this review, studies that have elucidated the role of the mu (MORs), delta (DORs), and kappa (KORs) opioid receptors in nicotine reward, nicotine withdrawal, and reinstatement of nicotine seeking will be discussed. Additionally, the review will discuss discrepancies in the literature and therapeutic potential of the endogenous opioid system, and suggest studies to address gaps in knowledge with respect to the role of the opioid receptors in nicotine dependence.

RESULTS

Data available till date suggest that blockade of the MORs and DORs decreased the rewarding effects of nicotine, while activation of the MORs and DORs decreased nicotine withdrawal-induced aversive effects. In contrast, activation of the KORs decreased the rewarding effects of nicotine, while blockade of the KORs decreased nicotine withdrawal-induced aversive effects. Interestingly, blockade of the MORs and KORs attenuated reinstatement of nicotine seeking. In humans, MOR antagonists have shown benefits in select subpopulations of smokers and further investigation is required to realize their full therapeutic potential.

CONCLUSION

Future work must assess the influence of polymorphisms in opioid receptor-linked genes in nicotine dependence, which will help in both identifying individuals vulnerable to nicotine addiction and the development of opioid-based smoking cessation medications. Overall, the endogenous opioid system continues to be a promising target for future smoking cessation medications.

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.

Responses to drugs of abuse and non-drug rewards in leptin deficient ob/ob mice

RATIONALE

Although leptin receptors are found in hypothalamic nuclei classically associated with homeostatic feeding mechanisms, they are also present in brain regions known to regulate hedonic-based feeding, natural reward processing, and responses to drugs of abuse. The ob/ob mouse is deficient in leptin signaling, and previous work has found altered mesolimbic dopamine signaling and sensitivity to the locomotor activating effects of amphetamine in these mice.

OBJECTIVES

We directly assessed responses to three drugs of abuse and non-drug rewards in the leptin-deficient ob/ob mouse.

METHODS

Ob/ob mice were tested in assays of sweet preference, novelty seeking, and drug reward/reinforcement.

RESULTS

In assays of novelty seeking, novel open field activity and operant sensation seeking were reduced in ob/ob mice, although novel object interaction and novel environment preference were comparable to wild types. We also found that ob/ob mice had specific phenotypes in regard to cocaine: conditioned place preference for 2.5 mg/kg was increased, while the locomotor response to 10 mg/kg was reduced, and cocaine self-administration was the same as wild types. Ob/ob mice also acquired self-administration of the potent opioid remifentanil, but breakpoints for the drug were significantly reduced. Finally, we found significant differences in ethanol drinking in ob/ob mice that correlated negatively with body weight and positively with operant sensation seeking.

CONCLUSIONS

In conclusion, ob/ob mice displayed task-specific deficits in novelty seeking and dissociable differences in reward/reinforcement associated with cocaine, remifentanil, and ethanol.

Strain and cocaine-induced differential opioid gene expression may predispose Lewis but not Fischer rats to escalate cocaine self-administration

The aim of the present study was to investigate alterations in gene expression of opioid system components induced by extended access (18 h) cocaine self-administration and to determine the impact of genetic background in the vulnerability to escalate cocaine intake. Comparing two inbred rat strains, we previously reported that Lewis rats progressively escalated cocaine consumption compared to Fischer rats, in a new translational model of intravenous cocaine self-administration, which included 14 sessions of 18-h operant sessions in which rats were allowed to select the cocaine unit dose to self-administer. We compare here Fischer and Lewis rats in the gene expression of endogenous opioid peptides (Pomc, Penk, Pdyn) and cognate receptors (Oprm, Oprk and Oprd) in reward-related brain regions, after exposure to either cocaine self-administration or yoked-saline, in the aforementioned translational paradigm. We performed a correlation analysis between the mRNA level, found in the Dorsal Striatum (DS), Nucleus accumbens (NAcc) shell and core respectively, and individual cocaine intake. Our findings show that the gene expression of all the aforementioned opioid genes exhibit strain-dependent differences in the DS, in absence of cocaine exposure. Also, different strain-specific cocaine-induced mRNA expression of Oprm and Oprk was found in DS. Only few differences were found in the ventral parts of the striatum. Moreover, gene expression level of Pdyn, Penk, Oprk, and Oprm in the DS was significantly correlated with cocaine intake only in Fischer rats. Overall, these data shed light on potential genetic differences which may predispose of subjects to initiate and escalate cocaine consumption.

Endogenous opiates and behavior: 2014

This paper is the thirty-seventh consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2014 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (endogenous opioids and receptors), and the roles of these opioid peptides and receptors in pain and analgesia (pain and analgesia); stress and social status (human studies); tolerance and dependence (opioid mediation of other analgesic responses); learning and memory (stress and social status); eating and drinking (stress-induced analgesia); alcohol and drugs of abuse (emotional responses in opioid-mediated behaviors); sexual activity and hormones, pregnancy, development and endocrinology (opioid involvement in stress response regulation); mental illness and mood (tolerance and dependence); seizures and neurologic disorders (learning and memory); electrical-related activity and neurophysiology (opiates and conditioned place preferences (CPP)); general activity and locomotion (eating and drinking); gastrointestinal, renal and hepatic functions (alcohol and drugs of abuse); cardiovascular responses (opiates and ethanol); respiration and thermoregulation (opiates and THC); and immunological responses (opiates and stimulants). This paper is the thirty-seventh consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2014 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (endogenous opioids and receptors), and the roles of these opioid peptides and receptors in pain and analgesia (pain and analgesia); stress and social status (human studies); tolerance and dependence (opioid mediation of other analgesic responses); learning and memory (stress and social status); eating and drinking (stress-induced analgesia); alcohol and drugs of abuse (emotional responses in opioid-mediated behaviors); sexual activity and hormones, pregnancy, development and endocrinology (opioid involvement in stress response regulation); mental illness and mood (tolerance and dependence); seizures and neurologic disorders (learning and memory); electrical-related activity and neurophysiology (opiates and conditioned place preferences (CPP)); general activity and locomotion (eating and drinking); gastrointestinal, renal and hepatic functions (alcohol and drugs of abuse); cardiovascular responses (opiates and ethanol); respiration and thermoregulation (opiates and THC); and immunological responses (opiates and stimulants).

Delta Opioid Receptors: Learning and Motivation

Delta opioid receptor (DOR) displays a unique, highly conserved, structure and an original pattern of distribution in the central nervous system, pointing to a distinct and specific functional role among opioid peptide receptors. Over the last 15 years, in vivo pharmacology and genetic models have allowed significant advances in the understanding of this role. In this review, we will focus on the involvement of DOR in modulating different types of hippocampal- and striatal-dependent learning processes as well as motor function, motivation, and reward. Remarkably, DOR seems to play a key role in balancing hippocampal and striatal functions, with major implications for the control of cognitive performance and motor function under healthy and pathological conditions.

Effects of an opioid (proenkephalin) polymorphism on neural response to errors in health and cocaine use disorder

Chronic exposure to drugs of abuse perturbs the endogenous opioid system, which plays a critical role in the development and maintenance of addictive disorders. Opioid genetics may therefore play an important modulatory role in the expression of substance use disorders, but these genes have not been extensively characterized, especially in humans. In the current imaging genetics study, we investigated a single nucleotide polymorphism (SNP) of the protein-coding proenkephalin gene (PENK: rs2609997, recently shown to be associated with cannabis dependence) in 55 individuals with cocaine use disorder and 37 healthy controls. Analyses tested for PENK associations with fMRI response to error (during a classical color-word Stroop task) and gray matter volume (voxel-based morphometry) as a function of Diagnosis (cocaine, control). Results revealed whole-brain Diagnosis×PENK interactions on the neural response to errors (fMRI error>correct contrast) in the right putamen, left rostral anterior cingulate cortex/medial orbitofrontal cortex, and right inferior frontal gyrus; there was also a significant Diagnosis×PENK interaction on right inferior frontal gyrus gray matter volume. These interactions were driven by differences between individuals with cocaine use disorders and controls that were accentuated in individuals carrying the higher-risk PENK C-allele. Taken together, the PENK polymorphism-and potentially opioid neurotransmission more generally-modulates functioning and structural integrity of brain regions previously implicated in error-related processing. PENK could potentially render a subgroup of individuals with cocaine use disorder (i.e., C-allele carriers) more sensitive to mistakes or other related challenges; in future studies, these results could contribute to the development of individualized genetics-informed treatments.

Interactions of the opioid and cannabinoid systems in reward: Insights from knockout studies

The opioid system consists of three receptors, mu, delta, and kappa, which are activated by endogenous opioid peptides (enkephalins, endorphins, and dynorphins). The endogenous cannabinoid system comprises lipid neuromodulators (endocannabinoids), enzymes for their synthesis and their degradation and two well-characterized receptors, cannabinoid receptors CB1 and CB2. These systems play a major role in the control of pain as well as in mood regulation, reward processing and the development of addiction. Both opioid and cannabinoid receptors are coupled to G proteins and are expressed throughout the brain reinforcement circuitry. Extending classical pharmacology, research using genetically modified mice has provided important progress in the identification of the specific contribution of each component of these endogenous systems in vivo on reward process. This review will summarize available genetic tools and our present knowledge on the consequences of gene knockout on reinforced behaviors in both systems, with a focus on their potential interactions. A better understanding of opioid-cannabinoid interactions may provide novel strategies for therapies in addicted individuals.

Novel approaches for the treatment of psychostimulant and opioid abuse - focus on opioid receptor-based therapies

INTRODUCTION

Psychostimulant and opioid addiction are poorly treated. The majority of abstinent users relapse back to drug-taking within a year of abstinence, making 'anti-relapse' therapies the focus of much current research. There are two fundamental challenges to developing novel treatments for drug addiction. First, there are three key stimuli that precipitate relapse back to drug-taking: stress, presentation of drug-conditioned cue, taking a small dose of drug. The most successful novel treatment would be effective against all three stimuli. Second, a large number of drug users are poly-drug users: taking more than one drug of abuse at a time. The ideal anti-addiction treatment would, therefore, be effective against all classes of drugs of abuse.

AREAS COVERED

In this review, the authors discuss the clinical need and animal models used to uncover potential novel treatments. There is a very broad range of potential treatment approaches and targets currently being examined as potential anti-relapse therapies. These broadly fit into two categories: 'memory-based' and 'receptor-based' and the authors discuss the key targets here within.

EXPERT OPINION

Opioid receptors and ligands have been widely studied, and research into how different opioid subtypes affect behaviours related to addiction (reward, dysphoria, motivation) suggests that they are tractable targets as anti-relapse treatments. Regarding opioid ligands as novel 'anti-relapse' medication targets, research suggests that a 'non-selective' approach to targeting opioid receptors will be the most effective.