Reduction of morphine abstinence in mice with a mutation in the gene encoding CREB

Neural Network Connectivity Following Opioid Dependence is Altered by a Common Genetic Variant in the µ-Opioid Receptor, OPRM1 A118G

Opioid use disorder is a chronic, relapsing disease associated with persistent changes in brain plasticity. A common single nucleotide polymorphism (SNP) in the µ-opioid receptor gene, OPRM1 A118G, is associated with altered vulnerability to opioid addiction. Reconfiguration of neuronal connectivity may explain dependence risk in individuals with this SNP. Mice with the equivalent Oprm1 variant, A112G, demonstrate sex-specific alterations in the rewarding properties of morphine and heroin. To determine whether this SNP influences network-level changes in neuronal activity, we compared FOS expression in male and female mice that were opioid-naive or opioid-dependent. Network analyses identified significant differences between the AA and GG Oprm1 genotypes. Based on several graph theory metrics, including small-world analysis and degree centrality, we show that GG females in the opioid-dependent state exhibit distinct patterns of connectivity compared to other groups of the same genotype. Using a network control theory approach, we identified key cortical brain regions that drive the transition between opioid-naive and opioid-dependent brain states; however, these regions are less influential in GG females leading to sixfold higher average minimum energy needed to transition from the acute to the dependent state. In addition, we found that the opioid-dependent brain state is significantly less stable in GG females compared to other groups. Collectively, our findings demonstrate sex- and genotype-specific modifications in local, mesoscale, and global properties of functional brain networks following opioid exposure and provide a framework for identifying genotype differences in specific brain regions that play a role in opioid dependence.

Ameliorating effects of Acanthopanax koreanum extract and components on nicotine dependence and withdrawal symptoms

Tobacco smoking is a serious health problem in society. While smoking rates are declining, smoking remains a serious risk to national health. Currently, there are several medications available to aid in smoking cessation. However, these medications have the disadvantages of low success rates in smoking cessation and various side effects. Therefore, natural-based smoking cessation aids are being suggested as a good alternative due to their accessibility and minimal side effects. The roots and stems of Acanthopanax koreanum (AK) Nakai, a plant that is native to Jeju Island, South Korea, have traditionally been used as tonic and sedatives. Moreover, eleutheroside B and chlorogenic acid are the main components of AK stem extract. In the present study, we investigated the effect of 70% ethanol AK extract and its components on ameliorating nicotine dependence and withdrawal symptoms by using behavioural tests in mice. In addition, alterations in the dopaminergic and DRD1-EPAC-ERK-CREB pathways were observed using dopamine ELISA and western blotting using mouse brains. Our findings demonstrate that the AK extract and its components effectively mitigated the effects of nicotine treatment in behavioural tests. Furthermore, it normalized the dopamine concentration and the expression level of nicotine acetylcholine receptor α7. Additionally, it was observed that AK extract and its components led to the normalization of DRD1, ERK and CREB expression levels. These results indicate that AK extract exhibits effects in ameliorating nicotine dependence behaviour and alleviating withdrawal symptoms. Moreover, EB and CGA are considered potential marker components of AK extract.

AKAP150 from nucleus accumbens dopamine D1 and D2 receptor-expressing medium spiny neurons regulates morphine withdrawal

Dopamine D1 receptor-expressing medium spiny neurons (D1R-MSNs) and dopamine D2 receptor-expressing MSNs (D2R-MSNs) in nucleus accumbens (NAc) have been demonstrated to show different effects on reward and memory of abstinence. A-kinase anchoring protein 150 (AKAP150) expression in NAc is significantly upregulated and contributes to the morphine withdrawal behavior. However, the underlying mechanism of AKAP150 under opioid withdrawal remains unclear. In this study, AKAP150 expression in NAc is upregulated in naloxone-precipitated morphine withdrawal model, and knockdown of AKAP150 alleviates morphine withdrawal somatic signs and improves the performance of conditioned place aversion (CPA) test. AKAP150 in NAc D1R-MSNs is related to modulation of the performance of morphine withdrawal CPA test, while AKAP150 in NAc D2R-MSNs is relevant to the severity of somatic responses. Our results suggest that AKAP150 from D1R-MSNs or D2R-MSNs in NAc contributes to the developmental process of morphine withdrawal but plays different roles in aspects of behavior or psychology.

Adinazolam, a Benzodiazepine-Type New Psychoactive Substance, Has Abuse Potential and Induces Withdrawal Symptoms in Rodents

Adinazolam (ADZ) is a benzodiazepine-type new psychoactive substance (NPS) with anxiolytic, anticonvulsant, and antidepressant effects. High ADZ doses have been reported to impair psychomotor performance and memory; however, the abuse potential and drug dependence of ADZ have not yet been fully investigated. In this study, we evaluated whether ADZ has abuse potential and leads to drug dependence and withdrawal symptoms. The intravenous self-administration (IVSA) test revealed that ADZ (0.01, 0.03, and 0.1 mg/kg/infusion) was self-administered significantly above vehicle levels, suggesting the reinforcing effect of ADZ. Furthermore, we revealed that treatment discontinuation following chronic ADZ administration (3 and 6 mg/kg) caused several somatic withdrawal symptoms in mice, including body tremor. Moreover, it induced motivational withdrawal signs, such as anxiety-related behavior in the elevated plus maze (EPM) test and memory deficits in the Y-maze test. After the IVSA test, an enzyme-linked immunosorbent assay (ELISA) showed that ADZ administration significantly increased the dopamine contents in the thalamus, nucleus accumbens (NAc), and ventral tegmental area (VTA). This finding was also supported by the results of the Western blot. Taken together, our results suggest that ADZ has abuse potential and can lead to drug dependence and withdrawal syndrome.

All Hands on Deck: We Need Multiple Approaches To Uncover the Neuroscience behind the Opioid Overdose Crisis

Opioid use disorder (OUD) affects millions of people throughout the United States, yet there are only three Food and Drug Administration-approved pharmacological treatments. Though these treatments have been shown to be effective, the number of overdose deaths continues to rise. The increase of fentanyl, fentanyl analogs, and adulterants in the illicit drug supply has further complicated treatment strategies. Preclinical researchers strive to model OUD to better understand this complicated disorder, and this research is a critical enabler for the development of novel treatments. As a result, there are many different preclinical models of OUD. Often, researchers form strong opinions on what they believe to be the "best" model to mimic the human condition. Here, we argue that researchers should be supportive of multiple models to promote new perspectives and discoveries and always consider the trends in human opioid use when designing preclinical studies. We describe the benefits of contingent and noncontingent models as well as models of opioid withdrawal and how each of these can help illuminate different components of OUD.

Hypocretin/Orexin Interactions with Norepinephrine Contribute to the Opiate Withdrawal Syndrome

We previously found that human heroin addicts and mice chronically exposed to morphine exhibit a significant increase in the number of detected hypocretin/orexin (Hcrt)-producing neurons. However, it remains unknown how this increase affects target areas of the hypocretin system involved in opioid withdrawal, including norepinephrine containing structures locus coeruleus (LC) and A1/A2 medullary regions. Using a combination of immunohistochemical, biochemical, imaging, and behavioral techniques, we now show that the increase in detected hypocretin cell number translates into a significant increase in hypocretin innervation and tyrosine hydroxylase (TH) levels in the LC without affecting norepinephrine-containing neuronal cell number. We show that the increase in TH is completely dependent on Hcrt innervation. The A1/A2 regions were unaffected by morphine treatment. Manipulation of the Hcrt system may affect opioid addiction and withdrawal.SIGNIFICANCE STATEMENT Previously, we have shown that the hypothalamic hypocretin system undergoes profound anatomic changes in human heroin addicts and in mice exposed to morphine, suggesting a role of this system in the development of addictive behaviors. The locus coeruleus plays a key role in opioid addiction. Here we report that the hypothalamic hypocretin innervation of the locus coeruleus increases dramatically with morphine administration to mice. This increase is correlated with a massive increase in tyrosine hydroxylase expression in locus coeruleus. Elimination of hypocretin neurons prevents the tyrosine hydroxylase increase in locus coeruleus and dampens the somatic and affective components of opioid withdrawal.

Mu opioid receptors on vGluT2-expressing glutamatergic neurons modulate opioid reward

The role of Mu opioid receptor (MOR)‐mediated regulation of GABA transmission in opioid reward is well established. Much less is known about MOR‐mediated regulation of glutamate transmission in the brain and how this relates to drug reward. We previously found that MORs inhibit glutamate transmission at synapses that express the Type 2 vesicular glutamate transporter (vGluT2). We created a transgenic mouse that lacks MORs in vGluT2‐expressing neurons (MORflox‐vGluT2cre) to demonstrate that MORs on the vGluT2 neurons themselves mediate this synaptic inhibition. We then explored the role of MORs in vGluT2‐expressing neurons in opioid‐related behaviors. In tests of conditioned place preference, MORflox‐vGluT2cre mice did not acquire place preference for a low dose of the opioid, oxycodone, but displayed conditioned place aversion at a higher dose, whereas control mice displayed preference for both doses. In an oral consumption assessment, these mice consumed less oxycodone and had reduced preference for oxycodone compared with controls. MORflox‐vGluT2cre mice also failed to show oxycodone‐induced locomotor stimulation. These mice displayed baseline withdrawal‐like responses following the development of oxycodone dependence that were not seen in littermate controls. In addition, withdrawal‐like responses in these mice did not increase following treatment with the opioid antagonist, naloxone. However, other MOR‐mediated behaviors were unaffected, including oxycodone‐induced analgesia. These data reveal that MOR‐mediated regulation of glutamate transmission is a critical component of opioid reward.

Interruption of continuous opioid exposure exacerbates drug-evoked adaptations in the mesolimbic dopamine system

Drug-evoked adaptations in the mesolimbic dopamine system are postulated to drive opioid abuse and addiction. These adaptations vary in magnitude and direction following different patterns of opioid exposure, but few studies have systematically manipulated the pattern of opioid administration while measuring neurobiological and behavioral impact. We exposed male and female mice to morphine for one week, with administration patterns that were either intermittent (daily injections) or continuous (osmotic minipump infusion). We then interrupted continuous morphine exposure with either naloxone-precipitated or spontaneous withdrawal. Continuous morphine exposure caused tolerance to the psychomotor-activating effects of morphine, whereas both intermittent and interrupted morphine exposure caused long-lasting psychomotor sensitization. Given links between locomotor sensitization and mesolimbic dopamine signaling, we used fiber photometry and a genetically encoded dopamine sensor to conduct longitudinal measurements of dopamine dynamics in the nucleus accumbens. Locomotor sensitization caused by interrupted morphine exposure was accompanied by enhanced dopamine signaling in the nucleus accumbens. To further assess downstream consequences on striatal gene expression, we used next-generation RNA sequencing to perform genome-wide transcriptional profiling in the nucleus accumbens and dorsal striatum. The interruption of continuous morphine exposure exacerbated drug-evoked transcriptional changes in both nucleus accumbens and dorsal striatum, dramatically increasing differential gene expression and engaging unique signaling pathways. Our study indicates that opioid-evoked adaptations in brain function and behavior are critically dependent on the pattern of drug administration, and exacerbated by interruption of continuous exposure. Maintaining continuity of chronic opioid administration may, therefore, represent a strategy to minimize iatrogenic effects on brain reward circuits.

Allostatic Changes in the cAMP System Drive Opioid-Induced Adaptation in Striatal Dopamine Signaling

Opioids are powerful addictive agents that alter dopaminergic influence on reward signaling in medium spiny neurons (MSNs) of the nucleus accumbens. Repeated opioid exposure triggers adaptive changes, shifting reward valuation to the allostatic state underlying tolerance. However, the cellular substrates and molecular logic underlying such allostatic changes are not well understood. Here, we report that the plasticity of dopamine-induced cyclic AMP (cAMP) signaling in MSNs serves as a cellular substrate for drug-induced allostatic adjustments. By recording cAMP responses to optically evoked dopamine in brain slices from mice subjected to various opioid exposure paradigms, we define profound neuronal-type-specific adaptations. We find that opioid exposure pivots the initial hyper-responsiveness of D1-MSNs toward D2-MSN dominance as dependence escalates. Presynaptic dopamine transporters and postsynaptic phosphodiesterases critically enable cell-specific adjustments of cAMP that control the balance between opponent D1-MSN and D2-MSN channels. We propose a quantitative model of opioid-induced allostatic adjustments in cAMP signal strength that balances circuit activity.

Muntean et al. examine how opioid exposure influences cyclic AMP (cAMP) responses to dopamine in striatal medium spiny neurons (MSNs). They describe allostatic adaptations in the processing of dopaminergic signals by D1-MSN and D2-MSN populations as opioid administration progresses from acute exposure to chronic use, and they define molecular elements contributing to the process.

The circadian gene, Per2, influences methamphetamine sensitization and reward through the dopaminergic system in the striatum of mice

Drug addiction is a chronic and relapsing brain disorder, influenced by complex interactions between endogenous and exogenous factors. Per2, a circadian gene, plays a role in drug addiction. Previous studies using Per2-knockout mice have shown a role for Per2 in cocaine, morphine and alcohol addiction. In the present study, we investigated the role of Per2 in methamphetamine (METH) addiction using Per2-overexpression and knockout mice. We observed locomotor sensitization responses to METH administration, and rewarding effects using a conditioned place preference test. In addition, we measured expression levels of dopamine and dopamine-related genes (monoamine oxidase A, DA receptor 1, DA receptor 2, DA active transporter, tyrosine hydroxylase and cAMP response element-binding protein 1) in the striatum of the mice after repeated METH treatments, using qRT-PCR. Per2-overexpressed mice showed decreased locomotor sensitization and rewarding effects of METH compared to the wildtype mice, whereas the opposite was observed in Per2 knockout mice. Both types of transgenic mice showed altered expression levels of dopamine-related genes after repeated METH administration. Specifically, we observed lower dopamine levels in Per2-overexpressed mice and higher levels in Per2-knockout mice. Taken together, Per2 expression levels may influence the addictive effects of METH through the dopaminergic system in the striatum of mice.

Opioid Neurobiology, Neurogenetics and Neuropharmacology in Zebrafish

Despite the high prevalence of medicinal use and abuse of opioids, their neurobiology and mechanisms of action are not fully understood. Experimental (animal) models are critical for improving our understanding of opioid effects in vivo. As zebrafish (Danio rerio) are increasingly utilized as a powerful model organism in neuroscience research, mounting evidence suggests these fish as a useful tool to study opioid neurobiology. Here, we discuss the zebrafish opioid system with specific focus on opioid gene expression, existing genetic models, as well as its pharmacological and developmental regulation. As many human brain diseases involve pain and aberrant reward, we also summarize zebrafish models relevant to opioid regulation of pain and addiction, including evidence of functional interplay between the opioid system and central dopaminergic and other neurotransmitter mechanisms. Additionally, we critically evaluate the limitations of zebrafish models for translational opioid research and emphasize their developing utility for improving our understanding of evolutionarily conserved mechanisms of pain-related, addictive, affective and other behaviors, as well as for fostering opioid-related drug discovery.

Distinct regulation pattern of Egr-1, BDNF and Arc during morphine-withdrawal conditioned place aversion paradigm: Role of glucocorticoids

Negative affective aspects of opiate abstinence contribute to the persistence of substance abuse. Importantly, interconnected brain areas involved in aversive motivational processes, such as the ventral tegmental area (VTA) and medial prefrontal cortex (mPFC), become activated when animals are confined to withdrawal-paired environments. In the present study, place aversion was elicited in sham and adrenalectomized (ADX) animals by conditioned naloxone-precipitated drug withdrawal following exposure to chronic morphine. qPCR was employed to detect the expression of brain derived neurotrophic factor (Bdnf) and the immediate early genes (IEG) early growth response 1 (Egr-1) and activity-regulated cytoskeletal-associated protein (Arc) mRNAs in the VTA and mPFC at different time points of the conditioned place aversion (CPA) paradigm: after the conditioning phase and after the test phase. Sham + morphine rats exhibited robust CPA, which was impaired in ADX + morphine animals. Egr-1 and Arc were induced in the VTA and mPFC after morphine-withdrawal conditioning phase. Furthermore, Bdnf expression was enhanced in the VTA during the test phase. Bdnf induction seemed to be glucocorticoid-dependent, given that was correlated with HPA axis function and was not observed in morphine-dependent ADX animals. In addition, BDNF regulation and function was opposite in the VTA and mPFC during aversive-withdrawal memory retrieval. Our results suggest that IEGs and BDNF in these brain regions may play key roles in mediating the negative motivational component of opiate withdrawal.

Targeting Phosphodiesterases in Pharmacotherapy for Substance Dependence

Substance dependence is a chronic relapsing brain disorder associated with adaptational changes in synaptic plasticity and neuronal functions. The high levels of substance consumption and relapse rate suggest more reliable medications are in need to better address the underlying causes of this disease. It has been well established that the intracellular second messengers cyclic AMP (cAMP) and cyclic GMP (cGMP) and their signaling systems play an important role in the molecular mechanisms of substance taking behaviors. On this basis, the phosphodiesterase (PDE) superfamily, which crucially controls cyclic nucleotide levels by catalyzing their hydrolysis, has been proposed as a novel class of therapeutic targets for substance use disorders. This chapter reviews the expression patterns of PDEs in the brain with regard to neural structures underlying the dependent process and highlights available evidence for a modulatory role of PDEs in substance dependence.

STEP signaling pathway mediates psychomotor stimulation and morphine withdrawal symptoms, but not for reward, analgesia and tolerance

Striatal-enriched protein tyrosine phosphatase (STEP) is abundantly expressed in the striatum, which strongly expresses dopamine and opioid receptors and mediates the effects of many drugs of abuse. However, little is known about the role of STEP in opioid receptor function. In the present study, we generated STEP-targeted mice carrying a nonsense mutation (C230X) in the kinase interaction domain of STEP by screening the N-ethyl-N-nitrosourea (ENU)-driven mutant mouse genomic DNA library and subsequent in vitro fertilization. It was confirmed that the C230X nonsense mutation completely abolished functional STEP protein expression in the brain. STEP(C230X-/-) mice showed attenuated acute morphine-induced psychomotor activity and withdrawal symptoms, whereas morphine-induced analgesia, tolerance and reward behaviors were unaffected. STEP(C230X-/-) mice displayed reduced hyperlocomotion in response to intrastriatal injection of the μ-opioid receptor agonist DAMGO, but the behavioral responses to δ- and κ-opioid receptor agonists remained intact. These results suggest that STEP has a key role in the regulation of psychomotor action and physical dependency to morphine. These data suggest that STEP inhibition may be a critical target for the treatment of withdrawal symptoms associated with morphine.

Emerging targets for addiction neuropharmacology: From mechanisms to therapeutics

Drug abuse represents a considerable burden of disease and has enormous economic impacts on societies. Over the years, few medications have been developed for clinical use. Their utilization is endowed with several limitations, including partial efficacy or significant side effects. On the other hand, the successful advancement of these compounds provides an important proof of concept for the feasibility of drug development programs in addiction. In recent years, a wealth of information has been generated on the psychological mechanisms, genetic or epigenetic predisposing factors, and neurobiological adaptations induced by drug consumption that interact with each other to contribute to disease progression. It is now clear that addiction develops through phases, from initial recreational use to excessive consumption and compulsive drug seeking, with a shift from positive to negative reinforcement driving motivated behaviors. A greater understanding of these mechanisms has opened new vistas in drug development programs. Researchers' attention has been shifted from investigation of classical targets associated with reward to biological substrates responsible for negative reinforcement, impulse loss of control, and maladaptive mechanisms resulting from protracted drug use. From this research, several new biological targets for the development of innovative therapies have started to emerge. This chapter offers an overview of targets currently under scrutiny for the development of new medications for addiction. This work is not exhaustive but rather it provides a few examples of how this research has advanced in recent years by virtue of studies carried out in our laboratory.

Molecular Changes in Opioid Addiction: The Role of Adenylyl Cyclase and cAMP/PKA System

For centuries, opiate analgesics have had a considerable presence in the treatment of moderate to severe pain. While effective in providing analgesia, opiates are notorious in exerting many undesirable adverse reactions. The receptor targets and the intracellular effectors of opioids have largely been identified. Furthermore, much of the mechanisms underlying the development of tolerance, dependence, and withdrawal have been delineated. Thus, there is a focus on developing novel compounds or strategies in mitigating or avoiding the development of tolerance, dependence, and withdrawal. This review focuses on the adenylyl cyclase and cyclic adenosine 3,5-monophosphate (cAMP)/protein kinase A (AC/cAMP/PKA) system as the central player in mediating the acute and chronic effects of opioids. This chapter also reviews the neuronal adaptive changes in the locus coeruleus, amygdala, periaqueductal gray, and ventral tegmental area induced by acute and chronic actions of opioid because these neuronal adaptive changes in these regions may underlie the behavioral changes observed in opiate users and abusers.

CP-154,526 Modifies CREB Phosphorylation and Thioredoxin-1 Expression in the Dentate Gyrus following Morphine-Induced Conditioned Place Preference

Corticotropin-releasing factor (CRF) acts as neuro-regulator of the behavioral and emotional integration of environmental and endogenous stimuli associated with drug dependence. Thioredoxin-1 (Trx-1) is a functional protein controlling the redox status of several proteins, which is involved in addictive processes. In the present study, we have evaluated the role of CRF1 receptor (CRF1R) in the rewarding properties of morphine by using the conditioned place preference (CPP) paradigm. We also investigate the effects of the CRF1R antagonist, CP-154,526, on the morphine CPP-induced activation of CRF neurons, CREB phosphorylation and Trx expression in paraventricular nucleus (PVN) and dentate gyrus (DG) of the mice brain. CP-154,526 abolished the acquisition of morphine CPP and the increase of CRF/pCREB positive neurons in PVN. Moreover, this CRF1R antagonist prevented morphine-induced CRF-immunoreactive fibers in DG, as well as the increase in pCREB expression in both the PVN and DG. In addition, morphine exposure induced an increase in Trx-1 expression in DG without any alterations in PVN. We also observed that the majority of pCREB positive neurons in DG co-expressed Trx-1, suggesting that Trx-1 could activate CREB in the DG, a brain region involved in memory consolidation. Altogether, these results support the idea that CRF1R antagonist blocked Trx-1 expression and pCREB/Trx-1 co-localization, indicating a critical role of CRF, through CRF1R, in molecular changes involved in morphine associated behaviors.

Drug withdrawal conceptualized as a stressor

Drug withdrawal is often conceptualized as an aversive state that motivates drug-seeking and drug-taking behaviors in humans. Stress is more difficult to define, but is also frequently associated with aversive states. Here we describe evidence for the simple theory that drug withdrawal is a stress-like state, on the basis of common effects on behavioral, neurochemical, and molecular endpoints. We also describe data suggesting a more complex relationship between drug withdrawal and stress. As one example, we will highlight evidence that, depending on drug class, components of withdrawal can produce effects that have characteristics consistent with mood elevation. In addition, some stressors can act as positive reinforcers, defined as having the ability to increase the probability of a behavior that produces it. As such, accumulating evidence supports the general principles of opponent process theory, whereby processes that have an affective valence are followed in time by an opponent process that has the opposite valence. Throughout, we identify gaps in knowledge and propose future directions for research. A better understanding of the similarities, differences, and overlaps between drug withdrawal and stress will lead to the development of improved treatments for addiction, as well as for a vast array of neuropsychiatric conditions that are triggered or exacerbated by stress.

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.

Frustrated expected reward induces differential transcriptional changes in the mouse brain

Frustration represents a particular aspect of the addictive process that is related to loss of control when the expected reward is not obtained. We aim to study the consequences of frustrated expected reward on gene expression in the mouse brain. For this purpose, we used an operant model of frustration using palatable food as reward combined with microarrays. Transcriptomic profiles of frontal cortex, ventral striatum and hippocampus were analysed in five groups of mice: (1) positive control receiving palatable food and the cue light as conditioned stimulus; (2) frustrated group only receiving the cue light; (3) extinction learning group that did not receive palatable food nor the light; (4) negative control that never received the reinforcer nor the light during the whole experiment; and (5) yoked that received palatable food passively. Gene expression changes produced by frustration were revealed in the frontal cortex and ventral striatum, but not in the hippocampus. Most of the changes, such as the modification of the dopamine-DARPP-32 signalling pathway, were common in both areas and estimated to have neuronal origin. Extinction learning induced transcriptional changes only in the ventral striatum, with most genes showing down-regulation and without alteration in the dopamine-DARPP-32 signalling pathway. Active palatable food-seeking behaviour induced changes in gene expression in ventral striatum mainly affecting cell communication. In conclusion, frustration behaviour-induced changes in frontal cortex and ventral striatum mainly related to dopamine-DARPP-32 signalling that could play an important role in the loss of behavioural control during the addictive processes.

Adverse health consequences of performance-enhancing drugs: an Endocrine Society scientific statement

Despite the high prevalence of performance-enhancing drug (PED) use, media attention has focused almost entirely on PED use by elite athletes to illicitly gain a competitive advantage in sports, and not on the health risks of PEDs. There is a widespread misperception that PED use is safe or that adverse effects are manageable. In reality, the vast majority of PED users are not athletes but rather nonathlete weightlifters, and the adverse health effects of PED use are greatly underappreciated. This scientific statement synthesizes available information on the medical consequences of PED use, identifies gaps in knowledge, and aims to focus the attention of the medical community and policymakers on PED use as an important public health problem. PED users frequently consume highly supraphysiologic doses of PEDs, combine them with other PEDs and/or other classical drugs of abuse, and display additional associated risk factors. PED use has been linked to an increased risk of death and a wide variety of cardiovascular, psychiatric, metabolic, endocrine, neurologic, infectious, hepatic, renal, and musculoskeletal disorders. Because randomized trials cannot ethically duplicate the large doses of PEDs and the many factors associated with PED use, we need observational studies to collect valid outcome data on the health risks associated with PEDs. In addition, we need studies regarding the prevalence of PED use, the mechanisms by which PEDs exert their adverse health effects, and the interactive effects of PEDs with sports injuries and other high-risk behaviors. We also need randomized trials to assess therapeutic interventions for treating the adverse effects of PEDs, such as the anabolic-androgen steroid withdrawal syndrome. Finally, we need to raise public awareness of the serious health consequences of PEDs.

Association of CREB1 gene polymorphism with drug seeking behaviour in eastern Indian addicts

cAMP response element binding protein (CREB) is a major transcription factor which plays an important role in a wide array of cellular functions. CREB also has a significant function in developing substance abuse. A study was undertaken to identify the single nucleotide polymorphisms (SNP) at selective areas of CREB1 gene in heroin as well as in alcohol addicts in comparison with control population. One hundred and forty control subjects, 112 heroin and 102 alcoholics, all male and residing in Kolkata, a city in eastern India participated in the study. SNPs from several exonic regions of CREB1 gene were assessed to investigate possible associations with addiction. One SNP in exon 3, rs35349697, demonstrated a significant correlation with opioid addiction as well as with alcohol addiction. A novel SNP, also located in exon 3, was identified which showed epistatic interaction with rs35349697 to decrease susceptibility to narcotic addiction in the population. The study is the first report on the identification of a role of CREB1 gene polymorphism with addiction.

Genome-wide association study identifies a potent locus associated with human opioid sensitivity

Opioids, such as morphine and fentanyl, are widely used as effective analgesics for the treatment of acute and chronic pain. In addition, the opioid system has a key role in the rewarding effects of morphine, ethanol, cocaine and various other drugs. Although opioid sensitivity is well known to vary widely among individual subjects, several candidate genetic polymorphisms reported so far are not sufficient for fully understanding the wide range of interindividual differences in human opioid sensitivity. By conducting a multistage genome-wide association study (GWAS) in healthy subjects, we found that genetic polymorphisms within a linkage disequilibrium block that spans 2q33.3-2q34 were strongly associated with the requirements for postoperative opioid analgesics after painful cosmetic surgery. The C allele of the best candidate single-nucleotide polymorphism (SNP), rs2952768, was associated with more analgesic requirements, and consistent results were obtained in patients who underwent abdominal surgery. In addition, carriers of the C allele in this SNP exhibited less vulnerability to severe drug dependence in patients with methamphetamine dependence, alcohol dependence, and eating disorders and a lower 'Reward Dependence' score on a personality questionnaire in healthy subjects. Furthermore, the C/C genotype of this SNP was significantly associated with the elevated expression of a neighboring gene, CREB1. These results show that SNPs in this locus are the most potent genetic factors associated with human opioid sensitivity known to date, affecting both the efficacy of opioid analgesics and liability to severe substance dependence. Our findings provide valuable information for the personalized treatment of pain and drug dependence.

15 years of genetic approaches in vivo for addiction research: Opioid receptor and peptide gene knockout in mouse models of drug abuse

The endogenous opioid system is expressed throughout the brain reinforcement circuitry, and plays a major role in reward processing, mood control and the development of addiction. This neuromodulator system is composed of three receptors, mu, delta and kappa, interacting with a family of opioid peptides derived from POMC (β-endorphin), preproenkephalin (pEnk) and preprodynorphin (pDyn) precursors. Knockout mice targeting each gene of the opioid system have been created almost two decades ago. Extending classical pharmacology, these mutant mice represent unique tools to tease apart the specific role of each opioid receptor and peptide in vivo, and a powerful approach to understand how the opioid system modulates behavioral effects of drugs of abuse. The present review summarizes these studies, with a focus on major drugs of abuse including morphine/heroin, cannabinoids, psychostimulants, nicotine or alcohol. Genetic data, altogether, set the mu receptor as the primary target for morphine and heroin. In addition, this receptor is essential to mediate rewarding properties of non-opioid drugs of abuse, with a demonstrated implication of β-endorphin for cocaine and nicotine. Delta receptor activity reduces levels of anxiety and depressive-like behaviors, and facilitates morphine-context association. pEnk is involved in these processes and delta/pEnk signaling likely regulates alcohol intake. The kappa receptor mainly interacts with pDyn peptides to limit drug reward, and mediate dysphoric effects of cannabinoids and nicotine. Kappa/dynorphin activity also increases sensitivity to cocaine reward under stressful conditions. The opioid system remains a prime candidate to develop successful therapies in addicted individuals, and understanding opioid-mediated processes at systems level, through emerging genetic and imaging technologies, represents the next challenging goal and a promising avenue in addiction research. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

Human Mas-related G protein-coupled receptors-X1 induce chemokine receptor 2 expression in rat dorsal root ganglia neurons and release of chemokine ligand 2 from the human LAD-2 mast cell line

Primate-specific Mas-related G protein-coupled receptors-X1 (MRGPR-X1) are highly enriched in dorsal root ganglia (DRG) neurons and induce acute pain. Herein, we analyzed effects of MRGPR-X1 on serum response factors (SRF) or nuclear factors of activated T cells (NFAT), which control expression of various markers of chronic pain. Using HEK293, DRG neuron-derived F11 cells and cultured rat DRG neurons recombinantly expressing human MRGPR-X1, we found activation of a SRF reporter gene construct and induction of the early growth response protein-1 via extracellular signal-regulated kinases-1/2 known to play a significant role in the development of inflammatory pain. Furthermore, we observed MRGPR-X1-induced up-regulation of the chemokine receptor 2 (CCR2) via NFAT, which is considered as a key event in the onset of neuropathic pain and, so far, has not yet been described for any endogenous neuropeptide. Up-regulation of CCR2 is often associated with increased release of its endogenous agonist chemokine ligand 2 (CCL2). We also found MRGPR-X1-promoted release of CCL2 in a human connective tissue mast cell line endogenously expressing MRGPR-X1. Thus, we provide first evidence to suggest that MRGPR-X1 induce expression of chronic pain markers in DRG neurons and propose a so far unidentified signaling circuit that enhances chemokine signaling by acting on two distinct yet functionally co-operating cell types. Given the important role of chemokine signaling in pain chronification, we propose that interruption of this signaling circuit might be a promising new strategy to alleviate chemokine-promoted pain.

Thioredoxin-1 was required for CREB activity by methamphetamine in rat pheochromocytoma cells

Methamphetamine (METH) is one of the most commonly abused agents by illicit-drug users. Thioredoxin-1 (Trx-1) plays important biological roles both in intra- and extracellular compartments, including in regulation of various intracellular molecules via thiol redox control. In this study, we found that Trx-1 was induced by METH in rat pheochromocytoma PC12 cells. Furthermore, PI3K/Akt pathway was involved in METH-induced increase of Trx-1 expression. An increase in phosphorylated cAMP response element-binding protein (CREB) was also observed after exposure of PC12 cells to METH, which was inhibited by a PI3K inhibitor, LY294002. In addition, the siRNA targeted toTrx-1 reduced the level of phosphorylated CREB by METH, suggesting Trx-1 is necessary for increased activity of CREB by METH. The results obtained in this study showed that Trx-1 might play a role in the actions of METH.

Glucocorticoid receptors participate in the opiate withdrawal-induced stimulation of rats NTS noradrenergic activity and in the somatic signs of morphine withdrawal

BACKGROUND AND PURPOSE

Recent evidence suggests that glucocorticoid receptor (GR) is a major molecular substrate of addictive properties of drugs of abuse. Hence, we performed a series of experiments to further characterize the role of GR signalling in opiate withdrawal-induced physical signs of dependence, enhanced noradrenaline (NA) turnover in the hypothalamic paraventricular nucleus (PVN) and tyrosine hydroxylase (TH) phosphorylation (activation) as well as GR expression in the nucleus of the solitary tract noradrenergic cell group (NTS-A₂).

EXPERIMENTAL APPROACH

The role of GR signalling was assessed by i.p. pretreatment of the selective GR antagonist, mifepristone. Rats were implanted with two morphine (or placebo) pellets. Six days later, rats were pretreated with mifepristone or vehicle 30 min before naloxone and physical signs of abstinence, NA turnover, TH activation, GR expression and the hypothalamus-pituitary-adrenocortical axis activity were measured using HPLC, immunoblotting and RIA.

KEY RESULTS

Mifepristone alleviated the somatic signs of naloxone-induced opiate withdrawal. Mifepristone attenuated the increase in the NA metabolite, 3-methoxy-4-hydroxyphenylethylen glycol (MHPG), in the PVN, and the enhanced NA turnover observed in morphine-withdrawn rats. Mifepristone antagonized the TH phosphorylation at Ser³¹ and the expression of c-Fos expression induced by morphine withdrawal. Finally, naloxone-precipitated morphine withdrawal induced up-regulation of GR in the NTS.

CONCLUSIONS AND IMPLICATIONS

These results suggest that the physical signs of opiate withdrawal, TH activation and stimulation of noradrenergic pathways innervating the PVN are modulated by GR signalling. Overall, the present data suggest that drugs targeting the GR may ameliorate stress and aversive effects associated with opiate withdrawal.

An emerging new paradigm in opioid withdrawal: a critical role for glia-neuron signaling in the periaqueductal gray

The chronic use of opiates (i.e., narcotics such as the natural derivatives of opium including morphine or codeine) or opioids (i.e., semisynthetic derivatives of opium and other molecules that activate opioid receptors) induces dependence, which is associated with various specific behavioral and somatic signs after their withdrawal or after the administration of an opioid antagonist. Among the brain regions implicated in opiate dependence and withdrawal, the periaqueductal gray area (PAG) appears to be critical in regulating the complex signs and symptoms of opioid withdrawal. Numerous neurochemical mechanisms in the PAG have been identified that may contribute to the opioid withdrawal syndrome. Accumulating evidence suggests that glial activation leading to the release of proinflammatory molecules acting on neurons is important in the complex syndrome of opioid dependence and withdrawal. This paper focuses on the recent advances in our understanding of the vital role that glia-neuron interactions play in opioid dependence and withdrawal within the PAG. We summarize those neurochemical mechanisms associated with opioid withdrawal including the recently defined importance of TNFα release from activated glial cells that communicate with TNF receptors on PAG neurons.

Interactions between the cannabinoid and dopaminergic systems: evidence from animal studies

There is a prominent role of the cannabinoid system to control basal ganglia function, in respect to reward, psychomotor function and motor control. Cannabinoid dysregulations might have a pathogenetic role in dopamine- and basal ganglia related neuropsychiatric disorders, such as drug addiction, psychosis, Parkinson's disease and Huntington's disease. This review highlights interactions between cannabinoids, and dopamine, to modulate neurotransmitter release and synaptic plasticity in the context of drug addiction, psychosis and cognition. Modulating endocannabinoid function, as a plasticity based therapeutic strategy, in the above pathologies with particular focus on cannabinoid receptor type 1 (CB1 receptor) antagonists/inverse agonists, is discussed. On the basis of the existing literature and of new experimental evidence presented here, CB1 receptor antagonists might be beneficial in disease states associated with hedonic dysregulation, and with cognitive dysfunction in particular in the context of psychosis. It is suggested that this effects might be mediated via a hyperglutamatergic state through metabotropic glutamate activation. Indications for endocannabinoid catabolism inhibitors in psychiatric disorders, that might be CB1 receptor independent and might involve TRPV1 receptors, are also discussed.

Hypothalamic orexin--a neurons are involved in the response of the brain stress system to morphine withdrawal

Both the hypothalamus-pituitary-adrenal (HPA) axis and the extrahypothalamic brain stress system are key elements of the neural circuitry that regulates the negative states during abstinence from chronic drug exposure. Orexins have recently been hypothesized to modulate the extended amygdala and to contribute to the negative emotional state associated with dependence. This study examined the impact of chronic morphine and withdrawal on the lateral hypothalamic (LH) orexin A (OXA) gene expression and activity as well as OXA involvement in the brain stress response to morphine abstinence. Male Wistar rats received chronic morphine followed by naloxone to precipitate withdrawal. The selective OX1R antagonist SB334867 was used to examine whether orexins' activity is related to somatic symptoms of opiate withdrawal and alterations in HPA axis and extended amygdala in rats dependent on morphine. OXA mRNA was induced in the hypothalamus during morphine withdrawal, which was accompanied by activation of OXA neurons in the LH. Importantly, SB334867 attenuated the somatic symptoms of withdrawal, and reduced morphine withdrawal-induced c-Fos expression in the nucleus accumbens (NAc) shell, bed nucleus of stria terminalis, central amygdala and hypothalamic paraventricular nucleus, but did not modify the HPA axis activity. These results highlight a critical role of OXA signalling, via OX1R, in activation of brain stress system to morphine withdrawal and suggest that all orexinergic subpopulations in the lateral hypothalamic area contribute in this response.

Molecular determinants of the spacing effect

Long-term memory formation is sensitive to the pattern of training sessions. Training distributed over time (spaced training) is superior at generating long-term memories than training presented with little or no rest interval (massed training). This spacing effect was observed in a range of organisms from invertebrates to humans. In the present paper, we discuss the evidence supporting cyclic-AMP response element-binding protein 2 (CREB), a transcription factor, as being an important molecule mediating long-term memory formation after spaced training. We also review the main upstream proteins that regulate CREB in different model organisms. Those include the eukaryotic translation initiation factor (eIF2α), protein phosphatase I (PP1), mitogen-activated protein kinase (MAPK), and the protein tyrosine phosphatase corkscrew. Finally, we discuss PKC activation and protein synthesis and degradation as mechanisms by which neurons decode the spacing intervals.

Protein kinase C phosphorylates the cAMP response element binding protein in the hypothalamic paraventricular nucleus during morphine withdrawal

BACKGROUND AND PURPOSE

Exposure to drugs of abuse or stress results in adaptation in the brain involving changes in gene expression and transcription factors. Morphine withdrawal modulates gene expression through various second-messenger signal transduction systems. Here, we investigated changes in activation of the transcription factor, cAMP-response element binding protein (CREB), in the hypothalamic paraventricular nucleus (PVN) and the kinases that may mediate the morphine withdrawal-triggered activation of CREB and the response of the hypothalamic-pituitary-adrenocortical (HPA) axis after naloxone-induced morphine withdrawal.

EXPERIMENTAL APPROACH

The effects of morphine dependence and withdrawal, phosphorylated CREB (pCREB), corticotrophin-releasing factor (CRF) expression in the PVN and HPA axis activity were measured using immunoblotting, immunohistochemistry and radioimmunoassay in controls and in morphine-dependent rats, withdrawn with naloxone and pretreated with vehicle, calphostin C, chelerythrine (inhibitors of protein kinase C (PKC) or SL-327 [inhibitor of extracellular signal regulated kinase (ERK) kinase]. In addition, changes in PKCα and PKCγ immunoreactivity were measured after 60 min of withdrawal.

KEY RESULTS

In morphine-withdrawn rats, pCREB immunoreactivity was increased within CRF immunoreactive neurons in the PVN and plasma corticosterone levels were raised. SL-327, at doses that reduced the augmented pERK levels in the PVN, did not attenuate the rise in pCREB immunoreactivity or plasma corticosterone secretion. In contrast, PKC inhibition reduced the withdrawal-triggered rise in pCREB, pERK1/2 and corticosterone secretion.

CONCLUSIONS AND IMPLICATIONS

PKC mediated, in part, both CREB activation and the HPA response to morphine withdrawal. The ERK kinase/ERK pathway might not be necessary for either activation of CREB or HPA axis hyperactivity.

Genetic approaches to investigate the role of CREB in neuronal plasticity and memory

In neurons, the convergence of multiple intracellular signaling cascades leading to cAMP-responsive element-binding protein (CREB) activation suggests that this transcription factor plays a critical role in integrating different inputs and mediating appropriate neuronal responses. The nature of this transcriptional response depends on both the type and strength of the stimulus and the cellular context. CREB-dependent gene expression has been involved in many different aspects of nervous system function, from embryonic development to neuronal survival, and synaptic, structural, and intrinsic plasticity. Here, we first review the different methodological approaches used to genetically manipulate CREB activity and levels in neurons in vivo in the adult brain, including recombinant viral vectors, mouse transgenesis, and gene-targeting techniques. We then discuss the impact of these approaches on our understanding of CREB's roles in neuronal plasticity and memory in rodents. Studies combining these genetic approaches with electrophysiology and behavior provide strong evidence that CREB is critically involved in the regulation of synaptic plasticity, intrinsic excitability, and long-term memory formation. These findings pave the way for the development of novel therapeutic strategies to treat memory disorders.

Inhibitory role of inducible cAMP early repressor (ICER) in methamphetamine-induced locomotor sensitization

Background

The inducible cyclic adenosine monophosphate (cAMP) early repressor (ICER) is highly expressed in the central nervous system and functions as a repressor of cAMP response element-binding protein (CREB) transcription. The present study sought to clarify the role of ICER in the effects of methamphetamine (METH).

Methods and Findings

We tested METH-induced locomotor sensitization in wildtype mice, ICER knockout mice, and ICER I-overexpressing mice. Both ICER wildtype mice and knockout mice displayed increased locomotor activity after continuous injections of METH. However, ICER knockout mice displayed a tendency toward higher locomotor activity compared with wildtype mice, although no significant difference was observed between the two genotypes. Moreover, compared with wildtype mice, ICER I-overexpressing mice displayed a significant decrease in METH-induced locomotor sensitization. Furthermore, Western blot analysis and quantitative real-time reverse transcription polymerase chain reaction demonstrated that ICER overexpression abolished the METH-induced increase in CREB expression and repressed cocaine- and amphetamine-regulated transcript (CART) and prodynorphin (Pdyn) expression in mice. The decreased CART and Pdyn mRNA expression levels in vivo may underlie the inhibitory role of ICER in METH-induced locomotor sensitization.

Conclusions

Our data suggest that ICER plays an inhibitory role in METH-induced locomotor sensitization.

A polymorphism of the CREB binding protein (CREBBP) gene is a risk factor for addiction

Unequivocal evidences have implicated c-AMP response element binding protein (CREB) in drug addiction. Recent reports indicate that the CREB binding protein (CREBBP), a transcription co-activator, may also be involved in the sensitivity to drugs of abuse. We undertook studies on the single nucleotide polymorphisms (SNP) at selective areas of CREBBP gene in heroin as well as in alcohol addicts and compared them with that in normal population. One hundred fifty healthy controls, one hundred thirty heroin addict and one hundred ten alcohol addicts, all males, Bengali-Hindu, and residing in Kolkata, a city in eastern India, participated in the study. DNA prepared from blood drawn from the subjects was PCR amplified for the regions corresponding to exon 3 and 22 of CREBBP gene followed by sequencing. Three SNPs identified in the population were analyzed to find out the association of these SNPs with addiction. One SNP, rs3025684 in intron 21 having the contig position of 3795363, showed association with addiction. The genotype frequencies of the SNP were significantly different between opioid dependent cases and controls (χ(2)=20.28, p<0.0001) as well as between alcoholics and controls (χ(2)=13.60, p=0.0011). Our studies suggest that rs3025684 polymorphism may be a possible risk factor for developing addiction.

CRF₂ mediates the increased noradrenergic activity in the hypothalamic paraventricular nucleus and the negative state of morphine withdrawal in rats

BACKGROUND AND PURPOSE

Recent evidence suggests that corticotropin-releasing factor (CRF) receptor signalling is involved in modulating the negative symptoms of opiate withdrawal. In this study, a series of experiments were performed to further characterize the role of CRF-type 2 receptor (CRF₂) signalling in opiate withdrawal-induced physical signs of dependence, hypothalamus-pituitary-adrenal (HPA) axis activation, enhanced noradrenaline (NA) turnover in the hypothalamic paraventricular nucleus (PVN) and tyrosine hydroxylase (TH) phosphorylation (activation), as well as CRF₂ expression in the nucleus of the solitary tract-A₂ noradrenergic cell group (NTS-A₂).

EXPERIMENTAL APPROACH

The contribution of CRF₂ signalling in opiate withdrawal was assessed by i.c.v. infusion of the selective CRF₂ antagonist, antisauvagine-30 (AS-30). Rats were implanted with two morphine (or placebo) pellets. Six days later, rats were pretreated with AS-30 or saline 10 min before naloxone and the physical signs of abstinence, the HPA axis activity, NA turnover, TH activation and CRF₂ expression were measured using immunoblotting, RIA, HPLC and immunohistochemistry.

KEY RESULTS

Rats pretreated with AS-30 showed decreased levels of somatic signs of naloxone-induced opiate withdrawal, but the corticosterone response was not modified. AS-30 attenuated the increased production of the NA metabolite, 3-methoxy-4-hydroxyphenylglycol, as well as the enhanced NA turnover observed in morphine-withdrawn rats. Finally, AS-30 antagonized the TH phosphorylation at Serine40 induced by morphine withdrawal.

CONCLUSIONS AND IMPLICATIONS

These results suggest that physical signs of opiate withdrawal, TH activation and stimulation of noradrenergic pathways innervating the PVN are modulated by CRF₂ signalling. Furthermore, they indicate a marginal role for the HPA axis in CRF₂-mediation of opiate withdrawal.

The role of TNFα in the periaqueductal gray during naloxone-precipitated morphine withdrawal in rats

Tolerance and dependence are common complications of long-term treatment of pain with opioids, which substantially limit the long-term use of these drugs. The mechanisms underlying these phenomena are poorly understood. Studies have implicated the midbrain periaqueductal gray (PAG) in the pathogenesis of morphine withdrawal, and recent evidence suggests that proinflammatory cytokines in the PAG may play an important role in morphine withdrawal. Here we report that chronic morphine withdrawal-induced upregulation of glial fibrillary acidic protein (GFAP), tumor necrosis factor alpha (TNFα) and phosphorylation of ERK1/2 (pERK1/2) in the caudal ventrolateral PAG (vlPAG). Microinjection of recombinant TNFα into the vlPAG followed by intraperitoneal naloxone resulted in morphine withdrawal-like behavioral signs, and upregulation of pERK1/2, expression of Fos, and phosphorylation of cAMP response element binding (pCREB) protein. We used a herpes simplex virus (HSV)-based vector expressing p55 soluble TNF receptor (sTNFR) microinjected into the PAG to examine the role of the proinflammatory cytokine TNFα in the PAG in the naloxone-precipitated withdrawal response. Microinjection of HSV vector expressing sTNFR into the PAG before the start of morphine treatment significantly reduced the naloxone-precipitated withdrawal behavioral response and downregulated the expression of GFAP and TNFα in astrocytes of the PAG. TNFR type I colocalized with neuronal pERK1/2. Microinjection of HSV vector expressing sTNFR into the PAG also significantly reduced the phosphorylation of both ERK1/2 and CREB, and reduced Fos immunoreactivity in neurons of the PAG following naloxone-precipitated withdrawal. These results support the concept that proinflammatory cytokines expressed in astrocytes in the PAG may play an important role in the pathogenesis of morphine withdrawal response.

Changes in ensemble activity of hippocampus CA1 neurons induced by chronic morphine administration in freely behaving mice

The hippocampus plays an important role in the formation of new memories and spatial navigation. Recently, growing evidence supports the view that it is also involved in addiction to opiates and other drugs. Theoretical and experimental studies suggest that hippocampal neural-network oscillations at specific frequencies and unit firing patterns reflect information of learning and memory encoding. Here, using multichannel recordings from the hippocampal CA1 area in behaving mice, we investigated the phase correlations between the theta (4-10 Hz) and gamma (40-100 Hz) oscillations, and the timing of spikes modulated by these oscillations. Local field potentials and single unit recordings in the CA1 area of mice receiving chronic morphine treatment revealed that the power of the theta rhythm was strongly increased; at the same time, the theta frequency during different behavioral states shifted markedly, and the characteristic coupling of theta and gamma oscillations was altered. Surprisingly, though the gamma oscillation frequency changed, the power of gamma lacking theta did not. Moreover, the timing of pyramidal cell spikes relative to the theta rhythm and the timing of interneuron spikes relative to the gamma rhythm changed during chronic morphine administration. Furthermore, these responses were impaired by a selective D1/D5 receptor antagonist intra-hippocampus injection. These results indicate that chronic morphine administration induced the changes of ensemble activity in the CA1 area, and these changes were dependent on local dopamine receptor activation.

Effects of the cell type-specific ablation of the cAMP-responsive transcription factor in noradrenergic neurons on locus coeruleus firing and withdrawal behavior after chronic exposure to morphine

Repeated exposure to opiates leads to cellular and molecular changes and behavioral alterations reflecting a state of dependence. In noradrenergic neurons, cyclic AMP (cAMP)-dependent pathways are activated during opiate withdrawal, but their contribution to the activity of locus coeruleus noradrenergic neurons and behavioral manifestations remains controversial. Here, we test whether the cAMP-dependent transcription factors cAMP responsive element binding protein (CREB) and cAMP-responsive element modulator (CREM) in noradrenergic neurons control the cellular markers and the physical signs of morphine withdrawal in mice. Using the Cre/loxP system we ablated the Creb1 gene in noradrenergic neurons. To avoid adaptive effects because of compensatory up-regulation of CREM, we crossed the conditional Creb1 mutant mice with a Crem-/- line. We found that the enhanced expression of tyrosine hydroxylase normally observed during withdrawal was attenuated in CREB/CREM mutants. Moreover, the withdrawal-associated cellular hyperactivity and c-fos expression was blunted. In contrast, naloxone-precipitated withdrawal signs, such as jumping, paw tremor, tremor and mastication were preserved. We conclude by a specific genetic approach that the withdrawal-associated hyperexcitability of noradrenergic neurons depends on CREB/CREM activity in these neurons, but does not mediate several behavioral signs of morphine withdrawal.

The nuclear transcription factor CREB: involvement in addiction, deletion models and looking forward

Addiction involves complex physiological processes, and is characterised not only by broad phenotypic and behavioural traits, but also by ongoing molecular and cellular adaptations. In recent years, increasingly effective and novel techniques have been developed to unravel the molecular implications of addiction. Increasing evidence has supported a contribution of the nuclear transcription factor CREB in the development of addiction, both in contribution to phenotype and expression in brain regions critical to various aspects of drug-seeking behaviour and drug reward. Abstracting from this, models have exploited these data by removing the CREB gene from the developing or developed mouse, to crucially determine its impact upon addiction-related processes. More recent models, however, hold greater promise in unveiling the contribution of CREB to disorders such as addiction.

Tolerance and withdrawal from prolonged opioid use in critically ill children

OBJECTIVE

After prolonged opioid exposure, children develop opioid-induced hyperalgesia, tolerance, and withdrawal. Strategies for prevention and management should be based on the mechanisms of opioid tolerance and withdrawal.

PATIENTS AND METHODS

Relevant manuscripts published in the English language were searched in Medline by using search terms "opioid," "opiate," "sedation," "analgesia," "child," "infant-newborn," "tolerance," "dependency," "withdrawal," "analgesic," "receptor," and "individual opioid drugs." Clinical and preclinical studies were reviewed for data synthesis.

RESULTS

Mechanisms of opioid-induced hyperalgesia and tolerance suggest important drug- and patient-related risk factors that lead to tolerance and withdrawal. Opioid tolerance occurs earlier in the younger age groups, develops commonly during critical illness, and results more frequently from prolonged intravenous infusions of short-acting opioids. Treatment options include slowly tapering opioid doses, switching to longer-acting opioids, or specifically treating the symptoms of opioid withdrawal. Novel therapies may also include blocking the mechanisms of opioid tolerance, which would enhance the safety and effectiveness of opioid analgesia.

CONCLUSIONS

Opioid tolerance and withdrawal occur frequently in critically ill children. Novel insights into opioid receptor physiology and cellular biochemical changes will inform scientific approaches for the use of opioid analgesia and the prevention of opioid tolerance and withdrawal.

Chronic morphine treatment impaired hippocampal long-term potentiation and spatial memory via accumulation of extracellular adenosine acting on adenosine A1 receptors

Chronic exposure to opiates impairs hippocampal long-term potentiation (LTP) and spatial memory, but the underlying mechanisms remain to be elucidated. Given the well known effects of adenosine, an important neuromodulator, on hippocampal neuronal excitability and synaptic plasticity, we investigated the potential effect of changes in adenosine concentrations on chronic morphine treatment-induced impairment of hippocampal CA1 LTP and spatial memory. We found that chronic treatment in mice with either increasing doses (20-100 mg/kg) of morphine for 7 d or equal daily dose (20 mg/kg) of morphine for 12 d led to a significant increase of hippocampal extracellular adenosine concentrations. Importantly, we found that accumulated adenosine contributed to the inhibition of the hippocampal CA1 LTP and impairment of spatial memory retrieval measured in the Morris water maze. Adenosine A(1) receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine significantly reversed chronic morphine-induced impairment of hippocampal CA1 LTP and spatial memory. Likewise, adenosine deaminase, which converts adenosine into the inactive metabolite inosine, restored impaired hippocampal CA1 LTP. We further found that adenosine accumulation was attributable to the alteration of adenosine uptake but not adenosine metabolisms. Bidirectional nucleoside transporters (ENT2) appeared to play a key role in the reduction of adenosine uptake. Changes in PKC-alpha/beta activity were correlated with the attenuation of the ENT2 function in the short-term (2 h) but not in the long-term (7 d) period after the termination of morphine treatment. This study reveals a potential mechanism by which chronic exposure to morphine leads to impairment of both hippocampal LTP and spatial memory.