Review. Neurobiological mechanisms for opponent motivational processes in addiction

The conceptualization of drug addiction as a compulsive disorder with excessive drug intake and loss of control over intake requires motivational mechanisms. Opponent process as a motivational theory for the negative reinforcement of drug dependence has long required a neurobiological explanation. Key neurochemical elements involved in reward and stress within basal forebrain structures involving the ventral striatum and extended amygdala are hypothesized to be dysregulated in addiction to convey the opponent motivational processes that drive dependence. Specific neurochemical elements in these structures include not only decreases in reward neurotransmission such as dopamine and opioid peptides in the ventral striatum, but also recruitment of brain stress systems such as corticotropin-releasing factor (CRF), noradrenaline and dynorphin in the extended amygdala. Acute withdrawal from all major drugs of abuse produces increases in reward thresholds, anxiety-like responses and extracellular levels of CRF in the central nucleus of the amygdala. CRF receptor antagonists block excessive drug intake produced by dependence. A brain stress response system is hypothesized to be activated by acute excessive drug intake, to be sensitized during repeated withdrawal, to persist into protracted abstinence and to contribute to stress-induced relapse. The combination of loss of reward function and recruitment of brain stress systems provides a powerful neurochemical basis for the long hypothesized opponent motivational processes responsible for the negative reinforcement driving addiction.     

A cannabinoid mechanism in relapse to cocaine seeking

Treatment of cocaine addiction is hampered by high rates of relapse even after prolonged drug abstinence. This relapse to compulsive cocaine use can be triggered by re-exposure to cocaine, by re-exposure to stimuli previously associated with cocaine or by exposure to stress. In laboratory rats, similar events reinstate cocaine seeking after prolonged withdrawal periods, thus providing a model to study neuronal mechanisms underlying the relapse to cocaine. The endocannabinoid system has been implicated in a number of neuropsychiatric conditions, including drug addiction. The active ingredient of marijuana, Delta9-tetrahydrocannabinol, activates the mesolimbic dopamine (DA) reward system and has rewarding effects in preclinical models of drug abuse. We report here that the synthetic cannabinoid agonist, HU210 (ref. 13), provokes relapse to cocaine seeking after prolonged withdrawal periods. Furthermore, the selective CB1 receptor antagonist, SR141716A (ref. 14), attenuates relapse induced by re-exposure to cocaine-associated cues or cocaine itself, but not relapse induced by exposure to stress. These data reveal an important role of the cannabinoid system in the neuronal processes underlying relapse to cocaine seeking, and provide a rationale for the use of cannabinoid receptor antagonists for the prevention of relapse to cocaine use.     

Cocaine withdrawal causes delayed dysregulation of stress genes in the hippocampus

Relapse, even following an extended period of withdrawal, is a major challenge in substance abuse management. Delayed neurobiological effects of the drug during prolonged withdrawal likely contribute to sustained vulnerability to relapse. Stress is a major trigger of relapse, and the hippocampus regulates the magnitude and duration of stress responses. Recent work has implicated hippocampal plasticity in various aspects of substance abuse. We asked whether changes in stress regulatory mechanisms in the hippocampus may participate in the neuroadaptations that occur during prolonged withdrawal. We therefore examined changes in the rat stress system during the course of withdrawal from extended daily access (5-hours) of cocaine self-administration, an animal model of addiction. Tissue was collected at 1, 14 and 28 days of withdrawal. Plasma corticosterone levels were determined and corticosteroid receptors (GR, MR, MR/GR mRNA ratios) and expression of other stress-related molecules (HSP90AA1 and HSP90AB1 mRNA) were measured in hippocampal subfields using in situ hybridization. Results showed a delayed emergence of dysregulation of stress genes in the posterior hippocampus following 28 days of cocaine withdrawal. This included increased GR mRNA in DG and CA3, increased MR and HSP90AA1 mRNA in DG, and decreased MR/GR mRNA ratio in DG and CA1. Corticosterone levels progressively decreased during the course of withdrawal, were normalized following 28 days of withdrawal, and were correlated negatively with GR and positively with MR/GR mRNA ratio in DG. These results suggest a role for the posterior hippocampus in the neuroadaptations that occur during prolonged withdrawal, and point to a signaling partner of GR, HSP90AA1, as a novel dysregulated target during cocaine withdrawal. These delayed neurobiological effects of extended cocaine exposure likely contribute to sustained vulnerability to relapse.     

Disruption of the CRF/CRF1 receptor stress system exacerbates the somatic signs of opiate withdrawal

Escape from the extremely stressful opiate withdrawal syndrome may motivate opiate seeking and taking. The corticotropin-releasing factor receptor-1 (CRF1) pathway mediates behavioral and endocrine responses to stress. Here, we report that genetic inactivation (CRF1-/-) as well as pharmacological antagonism of the CRF/CRF1 receptor pathway increased and prolonged the somatic expression of opiate withdrawal. Opiate-withdrawn CRF1-/- mice also showed aberrant CRF and dynorphin expression in the paraventricular nucleus of the hypothalamus (PVN) and the striatum, indicating profound impairments in stress-responsive brain circuitry. Intake of nonstressful amounts of corticosterone effectively reduced the exaggerated somatic reactions of CRF1-/- mice to opiate withdrawal. Exogenous corticosterone also restored "wild-type-like" patterns of CRF and dynorphin gene expression in the PVN and the striatum of opiate-withdrawn CRF1-/- mice, respectively. The present findings unravel a key role for the hypothalamus-pituitary-adrenal (HPA) system and brain extra-hypothalamic CRF/CRF1 receptor circuitry in somatic, molecular, and endocrine alterations induced by opiate withdrawal.     

Drug-induced Alterations in the Extracellular Signal-regulated Kinase (ERK) Signalling Pathway: Implications for Reinforcement and Reinstatement

Drug addiction, characterized by high rates of relapse, is recognized as a kind of neuroadaptive disorder. Since the extracellular signal-regulated kinase (ERK) pathway is critical to neuroplasticity in the adult brain, understanding the role this pathway plays is important for understanding the molecular mechanism underlying drug addiction and relapse. Here, we review previous literatures that focus on the effects of exposure to cocaine, amphetamine, Δ⁹-tetrahydrocannabinol (THC), nicotine, morphine, and alcohol on ERK signaling in the mesocorticolimbic dopamine system; these alterations of ERK signaling have been thought to contribute to the drug’s rewarding effects and to the long-term maladaptation induced by drug abuse. We then discuss the possible upstreams of the ERK signaling pathway activated by exposure of drugs of abuse and the environmental cues previously paired with drugs. Finally, we argue that since ERK activation is a key molecular process in reinstatement of conditioned place preference and drug self-administration, the pharmacological manipulation of the ERK pathway is a potential treatment strategy for drug addiction. Haifeng Zhai and Yanqin Li contributed equally to this paper.     

Kinase interest you in treating incubated cocaine‐craving? A hypothetical model for treatment intervention during protracted withdrawal from cocaine

A diagnostic criterion for drug addiction, persistent drug‐craving continues to be the most treatment‐resistant aspect of addiction that maintains the chronic, relapsing, nature of this disease. Despite the high prevalence of psychomotor stimulant addiction, there currently exists no FDA‐approved medication for craving reduction. In good part, this reflects our lack of understanding of the neurobiological underpinnings of drug‐craving. In humans, cue‐elicited drug‐craving is associated with the hyperexcitability of prefrontal cortical regions. Rodent models of cocaine addiction indicate that a history of excessive cocaine‐taking impacts excitatory glutamate signaling within the prefrontal cortex to drive drug‐seeking behavior during protracted withdrawal. This review summarizes evidence that the capacity of cocaine‐associated cues to augment craving in highly drug‐experienced rats relates to a withdrawal‐dependent incubation of glutamate release within prelimbic cortex. We discuss how stimulation of mGlu1/5 receptors increases the activational state of both canonical and noncanonical intracellular signaling pathways and present a theoretical molecular model in which the activation of several kinase effectors, including protein kinase C, extracellular signal‐regulated kinase and phosphoinositide 3‐kinase (PI3K) might lead to receptor desensitization to account for persistent cocaine‐craving during protracted withdrawal. Finally, this review discusses the potential for existing, FDA‐approved, pharmacotherapeutic agents that target kinase function as a novel approach to craving intervention in cocaine addiction.     

Role of Corticotropin-Releasing Factor (CRF) Receptor-1 on the Catecholaminergic Response to Morphine Withdrawal in the Nucleus Accumbens (NAc)

Stress induces the release of the peptide corticotropin-releasing factor (CRF) into the ventral tegmental area (VTA), and also increases dopamine (DA) levels in brain regions receiving dense VTA input. Since the role of stress in drug addiction is well established, the present study examined the possible involvement of CRF1 receptor in the interaction between morphine withdrawal and catecholaminergic pathways in the reward system. The effects of naloxone-precipitated morphine withdrawal on signs of withdrawal, hypothalamo-pituitary-adrenocortical (HPA) axis activity, dopamine (DA) and noradrenaline (NA) turnover in the nucleus accumbens (NAc) and activation of VTA dopaminergic neurons, were investigated in rats pretreated with vehicle or CP-154,526 (selective CRF1R antagonist). CP-154,526 attenuated the increases in body weight loss and suppressed some of withdrawal signs. Pretreatment with CRF1 receptor antagonist resulted in no significant modification of the increased NA turnover at NAc or plasma corticosterone levels that were seen during morphine withdrawal. However, blockade of CRF1 receptor significantly reduced morphine withdrawal-induced increases in plasma adrenocorticotropin (ACTH) levels, DA turnover and TH phosphorylation at Ser40 in the NAc. In addition, CP-154,526 reduced the number of TH containing neurons expressing c-Fos in the VTA after naloxone-precipitated morphine withdrawal. Altogether, these results support the idea that VTA dopaminergic neurons are activated in response to naloxone-precipitated morphine withdrawal and suggest that CRF1 receptors are involved in the activation of dopaminergic pathways which project to NAc.     

A role for brain stress systems in addiction

Drug addiction is a chronically relapsing disorder characterized by compulsion to seek and take drugs and has been linked to dysregulation of brain regions that mediate reward and stress. Activation of brain stress systems is hypothesized to be key to the negative emotional state produced by dependence that drives drug seeking through negative reinforcement mechanisms. This review explores the role of brain stress systems (corticotropin-releasing factor, norepinephrine, orexin [hypocretin], vasopressin, dynorphin) and brain antistress systems (neuropeptide Y, nociceptin [orphanin FQ]) in drug dependence, with emphasis on the neuropharmacological function of extrahypothalamic systems in the extended amygdala. The brain stress and antistress systems may play a key role in the transition to and maintenance of drug dependence once initiated. Understanding the role of brain stress and antistress systems in addiction provides novel targets for treatment and prevention of addiction and insights into the organization and function of basic brain emotional circuitry.     

Synaptic plasticity in drug reward circuitry

Drug addiction is a major public health issue worldwide. The persistence of drug craving coupled with the known recruitment of learning and memory centers in the brain has led investigators to hypothesize that the alterations in glutamatergic synaptic efficacy brought on by synaptic plasticity may play key roles in the addiction process. Here we review the present literature, examining the properties of synaptic plasticity within drug reward circuitry, and the effects that drugs of abuse have on these forms of plasticity. Interestingly, multiple forms of synaptic plasticity can be induced at glutamatergic synapses within the dorsal striatum, its ventral extension the nucleus accumbens, and the ventral tegmental area, and at least some of these forms of plasticity are regulated by behaviorally meaningful administration of cocaine and/or amphetamine. Thus, the present data suggest that regulation of synaptic plasticity in reward circuits is a tractable candidate mechanism underlying aspects of addiction.     

Cdk5在可卡因诱导的药物成瘾中的作用

细胞周期素依赖性蛋白激酶5(cyclin dependent kinase-5,Cdk5)是细胞周期素蛋白激酶之一,具有很多磷酸化底物,其激动剂p35和p39特异存在于神经系统(CNS)。因此,Cdk5在神经系统中的功能尤为突出,成为神经科学研究热点。目前研究较多的是Cdk5在可卡因诱导的药物成瘾中的作用。在可卡因所致药物成瘾过程中,多巴胺系统,ΔFosB,神经元突触可塑性等发挥重要作用。Cdk5与这些分子相互作用,所以,Cdk5与可卡因诱导所致药物成瘾密切相关。阐明其与药物成瘾的联系,探索新的以Cdk5为靶向的药物,将可能成为成瘾治疗的有效手段。综述了在可卡因诱导的药物成瘾中Cdk5作用,以及Cdk5与相关的信号转导分子之间的相互调节。     

Psychostimulant-Induced Endoplasmic Reticulum Stress and Neurodegeneration

The endoplasmic reticulum (ER) is a subcellular organelle that ensures proper protein folding process. The ER stress is defined as cellular conditions that disturb the ER homeostasis, resulting in accumulation of unfolded and/or misfolded proteins in the lumen of the ER. The presence of these proteins within the ER activates the ER stress response, known as unfolded protein response (UPR), to restore normal functions of the ER. However, under the severe and/or prolonged ER stress, UPR initiates apoptotic cell death. Psychostimulants such as cocaine, amphetamine, and methamphetamine cause the ER stress and/or apoptotic cell death in regions of the brain related to drug addiction. Recent studies have shown that the ER stress in response to psychostimulants is linked to behavioral sensitization and that the psychostimulant-induced ER stress signaling cascades are closely associated with the pathogenesis of the neurodegenerative diseases. Therefore, this review was conducted to improve understanding of the functional role of the ER stress in the addiction as well as neurodegenerative diseases. This would be helpful to facilitate development of new therapeutic strategies for the drug addiction and/or neurodegenerative diseases caused or exacerbated by exposure to psychostimulants.     

Review. The incentive sensitization theory of addiction: some current issues

We present a brief overview of the incentive sensitization theory of addiction. This posits that addiction is caused primarily by drug-induced sensitization in the brain mesocorticolimbic systems that attribute incentive salience to reward-associated stimuli. If rendered hypersensitive, these systems cause pathological incentive motivation (‘wanting’) for drugs. We address some current questions including: what is the role of learning in incentive sensitization and addiction? Does incentive sensitization occur in human addicts? Is the development of addiction-like behaviour in animals associated with sensitization? What is the best way to model addiction symptoms using animal models? And, finally, what are the roles of affective pleasure or withdrawal in addiction?     

Dopamine-dependent responses to cocaine depend on corticotropin-releasing factor receptor subtypes

The effects on locomotor response to cocaine challenge, acquisition of cocaine conditioned place preference and cocaine-induced dopamine (DA) release in nucleus accumbens and ventral tegmental area by the non-specific corticotropin-releasing factor (CRF) receptors antagonist alpha-helical CRF, the selective CRF receptor subtype 1 antagonist CP-154,526 and the selective CRF receptor subtype 2 antagonist anti-sauvagine-30 (AS-30) were investigated in rats. Both alpha-helical CRF (10 microg, i.c.v.) and CP-154,526 (3 microg, i.c.v.) decreased the cocaine-induced distance travelled, whereas AS-30 (3 microg, i.c.v.) did not show such an effect. The CRF receptor antagonists also have significant effects on stereotype counts induced by cocaine injection, in which the alpha-helical CRF or CP-154,526 but not AS-30 did significantly reduce the stereotype counts. alpha-Helical CRF (10 microg) prior to each injection of cocaine blocked cocaine conditioned place preference with no significant difference observed in the time spent in the drug-paired side between post- and pre-training and both 1 and 3 microg CP-154,526 also had significant inhibitory effects on cocaine-induced place preference. However, pre-treatment with an i.c.v. infusion of AS-30 (1 or 3 microg) prior to each injection of cocaine did not affect the acquisition of conditioned place preference. The alpha-helical CRF and CP-154,526 reduced extracellular DA levels of nucleus accumbens and ventral tegmental area in response to the injection of cocaine. However, both alpha-helical CRF and CP-154,526 did not modify extracellular DA levels under basal conditions. In contrast, the i.c.v. infusion of AS-30 had no effects on either the basal DA or the cocaine-induced increase in DA release in nucleus accumbens and ventral tegmental area. These findings demonstrate that activation of the CRF receptor is involved in behavioral and neurochemical effects of cocaine challenge and cocaine reward and that the role of CRF receptor subtypes 1 and 2 in cocaine-induced locomotion, reward and DA release is not identical. The CRF receptor subtype 1 is largely responsible for the action of the CRF system on cocaine locomotion and reward. These results suggest that the CRF receptor antagonist, particularly the CRF receptor subtype 1 antagonist, might be of some value in the treatment of cocaine addiction and cocaine-related behavioral disorders.     

Corticotropin-releasing factor requires CRF binding protein to potentiate NMDA receptors via CRF receptor 2 in dopamine neurons

Stress increases addictive behaviors and is a common cause of relapse. Corticotropin-releasing factor (CRF) plays a key role in the modulation of drug taking by stress. However, the mechanism by which CRF modulates neuronal activity in circuits involved in drug addiction is poorly understood. Here we show that CRF induces a potentiation of NMDAR (N-methyl-D-aspartate receptor)-mediated synaptic transmission in dopamine neurons of the ventral tegmental area (VTA). This effect involves CRF receptor 2 (CRF-R2) and activation of the phospholipase C (PLC)-protein kinase C (PKC) pathway. We also find that this potentiation requires CRF binding protein (CRF-BP). Accordingly, CRF-like peptides, which do not bind the CRF-BP with high affinity, do not potentiate NMDARs. These results provide evidence of the first specific roles for CRF-R2 and CRF-BP in the modulation of neuronal activity and suggest that NMDARs in the VTA may be a target for both drugs of abuse and stress.     

Transforming Growth Factor Beta Receptor 1 Is Increased following Abstinence from Cocaine Self-Administration,but Not Cocaine Sensitization

The addicted phenotype is characterized as a long-lasting, chronically relapsing disorder that persists following long periods of abstinence, suggesting that the underlying molecular changes are stable and endure for long periods even in the absence of drug. Here, we investigated Transforming Growth Factor-Beta Type I receptor (TGF-β R1) expression in the nucleus accumbens (NAc) following periods of withdrawal from cocaine self-administration (SA) and a sensitizing regimen of non-contingent cocaine. Rats were exposed to either (i) repeated systemic injections (cocaine or saline), or (ii) self-administration (cocaine or saline) and underwent a period of forced abstinence (either 1 or 7 days of drug cessation). Withdrawal from cocaine self-administration resulted in an increase in TGF-β R1 protein expression in the NAc compared to saline controls. This increase was specific for volitional cocaine intake as no change in expression was observed following a sensitizing regimen of experimenter-administered cocaine. These findings implicate TGF-β signaling as a novel potential therapeutic target for treating drug addiction.     

Role of NMDA receptors in dopamine neurons for plasticity and addictive behaviors

A single exposure to drugs of abuse produces an NMDA receptor (NMDAR)-dependent long-term potentiation (LTP) of AMPA receptor (AMPAR) currents in DA neurons; however, the importance of LTP for various aspects of drug addiction is unclear. To test the role of NMDAR-dependent plasticity in addictive behavior, we genetically inactivated functional NMDAR signaling exclusively in DA neurons (KO mice). Inactivation of NMDARs results in increased AMPAR-mediated transmission that is indistinguishable from the increases associated with a single cocaine exposure, yet locomotor responses to multiple drugs of abuse were unaltered in the KO mice. The initial phase of locomotor sensitization to cocaine is intact; however, the delayed sensitization that occurs with prolonged cocaine withdrawal did not occur. Conditioned behavioral responses for cocaine-testing environment were also absent in the KO mice. These findings provide evidence for a role of NMDAR signaling in DA neurons for specific behavioral modifications associated with drug seeking behaviors.     

Sex Differences between CRF1 Receptor Deficient Mice following Naloxone-Precipitated Morphine Withdrawal in a Conditioned Place Aversion Paradigm: Implication of HPA Axis

Background

Extinction period of positive affective memory of drug taking and negative affective memory of drug withdrawal, as well as the different response of men and women might be important for the clinical treatment of drug addiction. We investigate the role of corticotropin releasing factor receptor type one (CRF1R) and the different response of male and female mice in the expression and extinction of the aversive memory.

Methodology/Principal Finding

We used genetically engineered male and female mice lacking functional CRF1R. The animals were rendered dependent on morphine by intraperitoneally injection of increasing doses of morphine (10–60 mg/kg). Negative state associated with naloxone (1 mg/kg s.c.)-precipitated morphine withdrawal was examined by using conditioned place aversion (CPA) paradigm. No sex differences for CPA expression were found in wild-type (n = 29) or CRF1R knockout (KO) mice (n = 29). However, CRF1R KO mice presented less aversion score than wild-type mice, suggesting that CRF1R KO mice were less responsive than wild-type to continuous associations between drug administration and environmental stimuli. In addition, CPA extinction was delayed in wild-type and CRF1R KO male mice compared with females of both genotypes. The genetic disruption of the CRF1R pathway decreased the period of extinction in males and females suggesting that CRF/CRF1R is implicated in the duration of aversive memory. Our results also showed that the increase in adrenocorticotropic hormone (ACTH) levels observed in wild-type (n = 11) mice after CPA expression, were attenuated in CRF1R KO mice (n = 10). In addition, ACTH returned to the baseline levels in males and females once CPA extinction was finished.

Conclusion/Significance

These results suggest that, at least, CPA expression is partially due to an increase in plasma ACTH levels, through activation of CRF1R, which can return when CPA extinction is finished.     

Cues paired with either rapid or slower self-administered cocaine injections acquire similar conditioned rewarding properties

The faster drugs of abuse reach the brain, the more addictive they can be. It is not known why this is. Environmental stimuli associated with drugs can promote the development and persistence of addiction by invigorating and precipitating drug-seeking behaviour. We determined, therefore, whether cues associated with the self-administration of rapidly delivered cocaine (injected intravenously over 5 versus 90 seconds) would acquire greater conditioned rewarding properties, as assessed by the performance of an operant response reinforced solely by the cues. Rats nose-poked for intravenous cocaine infusions delivered either over 5 or 90 seconds. Discrete visual cues accompanied each infusion. The rats could then press a lever to obtain the cues--now a conditioned reward--or an inactive lever. Rats in both the 5- and 90-second groups pressed more on the active versus inactive lever following extensive (24 sessions) but not following limited (3 sessions) self-administration training. There were no group differences in this behaviour. Following withdrawal from cocaine self-administration, lever discrimination progressively abated in both groups and was lost by withdrawal day 30. However, the rewarding properties of the cues were not "forgotten" because on withdrawal days 32-33, amphetamine selectively enhanced active-lever pressing, and did so to a similar extent in both groups. Thus, cues paired with rapid or slower cocaine delivery acquire similar conditioned rewarding properties. We conclude, therefore, that the rapid delivery of cocaine to the brain promotes addiction by mechanisms that might not involve a greater ability of drug cues to control behaviour.     

Estradiol: A key biological substrate mediating the response to cocaine in female rats

A consistent finding in drug abuse research is that males and females show differences in their response to drugs of abuse. In women, increased plasma estradiol is associated with increased vulnerability to the psychostimulant and reinforcing effects of drugs of abuse. Our laboratory has focused on the role of estradiol in modulating the response to cocaine. We have seen that ovariectomy increases the locomotor response to a single cocaine injection, whereas estradiol exacerbates the locomotor response to repeated cocaine administration. Cocaine-induced sensitization of brain activity, as measured by fMRI, is also dependent on plasma estradiol. Moreover, we observed that although all ovariectomized rats show conditioned place preference to cocaine, it is more robust in ovariectomized rats with estradiol.Opioid receptors are enriched in brain regions associated with pleasure and reward. We find that in females, the effectiveness of kappa opioid agonists in decreasing the locomotor response to repeated cocaine varies with plasma estradiol. We also find that estradiol regulates the density of mu opioid receptors in brains areas associated with reward. These data hint that in females, estradiol modulates the behavioral effects of cocaine by regulating mu and kappa opioid signaling in mesocorticolimbic brain structures. Identifying the mechanisms that mediate differences in vulnerability to drugs of abuse may lead to effective therapeutic strategies for the treatment and prevention of addiction and relapse. We encourage health practitioners treating persons addicted to drugs to consider gender differences in response to particular pharmacotherapies, as well the sex steroid milieu of the patient.