Abstinence from chronic cocaine self-administration alters striatal dopamine systems in rhesus monkeys

Prolonged Exposure to Cocaine Self-Administration Results in a Continued Progression of Alterations in Functional Activity in a Nonhuman Primate Model

Background

Studies of nonhuman primates with exposures of up to 100 days of cocaine self-administration (SA) have provided evidence that the central effects of cocaine progress over time. These durations of cocaine exposure, however, may be insufficient to capture the extent of the neurobiological alterations observed in cocaine users, many of whom use the drug for years. The goal of the present study was to determine whether 1.5 years of cocaine SA would result in further progression of alterations in functional brain activity.

Methods

Adult male rhesus monkeys were exposed to 300 sessions of high-dose cocaine SA over 1.5 years. Following the final session rates of local cerebral glucose utilization (LCGU) were assessed with the 2-[¹⁴C]-deoxyglucose method and compared to rates of LCGU in control monkeys who responded for food reinforcement. In addition, LCGU in these animals was compared to a previously published group of monkeys that had self-administered cocaine or food for 100 sessions over a 4-5 month period.

Results

Compared to 100 days of exposure, 300 days of cocaine SA further reduced LCGU in the post-commissural striatum and produced reductions in areas unaffected by the shorter duration of exposure, such as the hypothalamus, all of the amygdala, and large expanses of cortex.

Conclusions

These findings demonstrate a clear progression of the impact of cocaine on functional activity with increasing durations of drug experience and have important implications for the development of potential strategies for the treatment of cocaine use disorder.

Residual deficits in functional brain activity after chronic cocaine self-administration in rhesus monkeys

Previous studies in humans and in animals have shown dramatic effects of cocaine on measures of brain function that persist into abstinence. The purpose of this study was to examine the neurobiological consequences of abstinence from cocaine, using a model that removes the potential confound of cocaine cues. Adult male rhesus monkeys self-administered cocaine (0.3 mg/kg/injection; N = 8) during daily sessions or served as food-reinforcement controls (N = 4). Two times per week, monkeys were placed in a neutral environment and presented with a cartoon video for ~30 min, sometimes pre- and sometimes post-operant session, but no reinforcement was presented during the video. After ~100 sessions and when the cocaine groups had self-administered 900 mg/kg cocaine, the final experimental condition was a terminal 2-[14C]-deoxyglucose procedure, which occurred in the neutral (cartoon video) environment; for half of the monkeys in each group, this occurred after 1 day of abstinence and for the others after 30 days of abstinence. Rates of local cerebral glucose metabolism were measured in 57 brain regions. Global rates of cerebral metabolism were significantly lower in animals 1 day and 30 days post-cocaine self-administration when compared to those of food-reinforced controls. Effects were larger in 30- vs. 1-day cocaine abstinence, especially in prefrontal, parietal and cingulate cortex, as well as dorsal striatum and thalamus. Because these measures were obtained from monkeys while in a neutral environment, the deficits in glucose utilization can be attributed to the consequences of cocaine exposure and not to effects of conditioned stimuli associated with cocaine.

Cocaine hydrolase blocks cocaine-induced dopamine transporter trafficking to the plasma membrane

Cocaine blocks dopamine uptake via dopamine transporter (DAT) on plasma membrane of neuron cells and, as a result, produces the high and induces DAT trafficking to plasma membrane which contributes to the drug seeking or craving. In this study, we first examined the dose dependence of cocaine-induced DAT trafficking and hyperactivity in rats, demonstrating that cocaine at an intraperitoneal dose of 10 mg/kg or higher led to redistribution of most DAT to the plasma membrane while inducing significant hyperactivity in rats. However, administration of 5-mg/kg cocaine (ip) did not significantly induce DAT trafficking or hyperactivity in rats. So the threshold (intraperitoneal) dose of cocaine that can significantly induce DAT trafficking or hyperactivity should be between 5 and 10 mg/kg. These data suggest that when a cocaine dose is high enough to induce significant hyperactivity, it can also significantly induce DAT trafficking to the plasma membrane. Further, the threshold brain cocaine concentration required to induce significant hyperactivity and DAT trafficking was estimated to be ~2.0 ± 0.8 μg/g. Particularly, for treatment of cocaine abuse, previous studies demonstrated that an exogenous cocaine-metabolizing enzyme, for example, CocH3-Fc(M3), can effectively block cocaine-induced hyperactivity. However, it was unknown whether an enzyme could also effectively block cocaine-induced DAT trafficking to the plasma membrane. This study demonstrates, for the first time, that the enzyme is also capable of effectively blocking cocaine from reaching the brain even with a lethal dose of 60-mg/kg cocaine (ip) and, thus, powerfully preventing cocaine-induced physiological effects such as the hyperactivity and DAT trafficking.

Increased cocaine motivation in tree shrews is modulated by striatal dopamine D1 receptor-mediated upregulation of Cav 1.2

The progressively increased motivation for cocaine during abstinence is closely associated with the dysfunction of dopamine (DA) system. As DA receptors also dynamically regulate L-type calcium channels (LTCCs), in this study we examined how DA receptors (D1R or D2R) and LTCCs (Ca_v 1.2 or Ca_v 1.3) exert their influences on cocaine-seeking in a tree shrew (Tupaia belangeri chinensis) model. First, we demonstrated the 'incubation' effect by showing tree shrews exhibited a significantly higher seeking behaviour on withdrawal day (WD) 45 than on WD1. Then, we confirmed that longer abstinence period induced higher D1R expression in the nucleus accumbens (NAc). Next, we showed that LTCCs in the NAc participated in drug seeking. Moreover, Ca_v 1.2 expression in the NAc was increased on WD45, and disruption of the Ca_v 1.2 inhibited drug seeking. Finally, we found that D1R antagonist blocked the increase of Ca_v 1.2 on drug-seeking test. Collectively, these findings suggest that D1R-mediated upregulation of Ca_v 1.2 is involved in the incubation of cocaine craving.

Orbitofrontal cortex and dorsal striatum functional connectivity predicts incubation of opioid craving after voluntary abstinence

We recently introduced a rat model of incubation of opioid craving after voluntary abstinence induced by negative consequences of drug seeking. Here, we used resting-state functional MRI to determine whether longitudinal functional connectivity changes in orbitofrontal cortex (OFC) circuits predict incubation of opioid craving after voluntary abstinence. We trained rats to self-administer for 14 d either intravenous oxycodone or palatable food. After 3 d, we introduced an electric barrier for 12 d that caused cessation of reward self-administration. We tested the rats for oxycodone or food seeking under extinction conditions immediately after self-administration training (early abstinence) and after electric barrier exposure (late abstinence). We imaged their brains before self-administration and during early and late abstinence. We analyzed changes in OFC functional connectivity induced by reward self-administration and electric barrier-induced abstinence. Oxycodone seeking was greater during late than early abstinence (incubation of oxycodone craving). Oxycodone self-administration experience increased OFC functional connectivity with dorsal striatum and related circuits that was positively correlated with incubated oxycodone seeking. In contrast, electric barrier-induced abstinence decreased OFC functional connectivity with dorsal striatum and related circuits that was negatively correlated with incubated oxycodone seeking. Food seeking was greater during early than late abstinence (abatement of food craving). Food self-administration experience and electric barrier-induced abstinence decreased or maintained functional connectivity in these circuits that were not correlated with abated food seeking. Opposing functional connectivity changes in OFC with dorsal striatum and related circuits induced by opioid self-administration versus voluntary abstinence predicted individual differences in incubation of opioid craving.

Clinical potential of a rationally engineered enzyme for treatment of cocaine dependence: Long-lasting blocking of the psychostimulant, discriminative stimulus, and reinforcing effects of cocaine

It is a grand challenge to develop a truly effective treatment of substance use disorder (SUD), particularly for cocaine and other drugs without an FDA-approved treatment available, because a truly effective therapy must effectively block the drug's physiological and reinforcing effects during the entire period of treatment in order to achieve the long-time abstinence required by the FDA. Whether a biologic, such as monoclonal antibody, vaccine, or therapeutic enzyme, can be truly effective for SUD treatment or not has been the subject of extensive debate. The main debate question is whether a biologic, particularly an exogenous enzyme, can effectively block the drug's reinforcing effect. In this report, we demonstrate that a modest dose of a recently redesigned long-acting cocaine hydrolase, CocH3-Fc(M6), can be used to effectively block the psychostimulant, discriminative stimulus, and reinforcing effects of cocaine for a sufficiently long period of time. For example, a dose of 3 mg/kg CocH3-Fc(M6) completely blocked the discriminative stimulus and reinforcing effects for 24/25 days and continued to significantly attenuate/decrease the cocaine effects for at least 29 days in rats. All the animal data consistently suggest that the long-acting cocaine hydrolase is a truly promising candidate of enzyme therapy for treatment of cocaine use disorder.

The neural interface between negative emotion regulation and motivation for change in cocaine dependent individuals under treatment

BACKGROUND

Emotion regulation is important for cocaine addiction treatment success, particularly during early abstinence. In addition, the neural underpinnings of emotion processing overlap with those of motivation and goal-directed behavior. We examined if the neural underpinnings of emotion maintenance and its regulation correlate with cocaine treatment motivation.

METHODS

Forty-five cocaine dependent individuals (CDIs) starting outpatient treatment in a public specialized addiction treatment clinic in Granada (Spain) underwent fMRI scans while performing a Reappraisal task, and completed the University of Rhode Island Change Assessment Scale (URICA), to measure treatment motivation. We conducted correlation analyses to examine the association between emotion maintenance and regulation related brain activation and URICA's Readiness to Change scores. We also explored links between Emotional reports during the fMRI reappraisal task, duration of abstinence, and anxiety and depression symptoms.

RESULTS

Readiness to Change scores were positively correlated with activations in the right dorsolateral prefrontal and right parietal cortices, the midbrain (p ≤ 0.001, cluster extents ≥109 voxels), and basolateral amygdala (P_FWE-SVC<0.05), while negatively with emotion maintenance related activation in the same cortical areas and activations in the dorsomedial frontal cortex, the nucleus accumbens and the left fusiform gyrus. Emotional reactivity negatively correlated with right dorsolateral prefrontal cortex reappraisal related activation (r= -0.40, p = 0.007), and the Regulate score positively correlated with the left fusiform gyrus emotion maintenance related activation (r = 0.31, p = 0.04).

CONCLUSIONS

Emotional related activation in frontoparietal, accumbens, fusiform, amygdala and midbrain regions engaged during emotion regulation and its maintenance correlate with early treatment motivation in CDIs.

The Neuroscience of Drug Reward and Addiction

Drug consumption is driven by a drug's pharmacological effects, which are experienced as rewarding, and is influenced by genetic, developmental, and psychosocial factors that mediate drug accessibility, norms, and social support systems or lack thereof. The reinforcing effects of drugs mostly depend on dopamine signaling in the nucleus accumbens, and chronic drug exposure triggers glutamatergic-mediated neuroadaptations in dopamine striato-thalamo-cortical (predominantly in prefrontal cortical regions including orbitofrontal cortex and anterior cingulate cortex) and limbic pathways (amygdala and hippocampus) that, in vulnerable individuals, can result in addiction. In parallel, changes in the extended amygdala result in negative emotional states that perpetuate drug taking as an attempt to temporarily alleviate them. Counterintuitively, in the addicted person, the actual drug consumption is associated with an attenuated dopamine increase in brain reward regions, which might contribute to drug-taking behavior to compensate for the difference between the magnitude of the expected reward triggered by the conditioning to drug cues and the actual experience of it. Combined, these effects result in an enhanced motivation to "seek the drug" (energized by dopamine increases triggered by drug cues) and an impaired prefrontal top-down self-regulation that favors compulsive drug-taking against the backdrop of negative emotionality and an enhanced interoceptive awareness of "drug hunger." Treatment interventions intended to reverse these neuroadaptations show promise as therapeutic approaches for addiction.

Regional elevations in microglial activation and cerebral glucose utilization in frontal white matter tracts of rhesus monkeys following prolonged cocaine self-administration

It has been shown that exposure to cocaine can result in neuroinflammatory responses. Microglia, the resident CNS immune cells, undergo a transition to an activated state when challenged. In rodents, and possibly humans, cocaine exposure activates microglia. The goal of this study was to assess the extent and magnitude of microglial activation in rhesus monkeys with an extensive history of cocaine self-administration. Male rhesus monkeys (N = 4/group) were trained to respond on a fixed-interval 3-min schedule of food or 0.3 mg/kg/injection cocaine presentation (30 reinforcers/session) for 300 sessions. At the end of the final session, monkeys were administered 2-[¹⁴C]deoxyglucose intravenously and 45 min later euthanized. Brain sections were used for autoradiographic assessments of glucose utilization and for microglia activation with [³H]PK11195, a marker for the microglial 18-kDa translocator protein. There were no group differences in gray matter [³H]PK11195 binding, while binding was significantly greater in cocaine self-administration animals as compared to food controls in 8 of the 11 white matter tracts measured at the striatal level. Binding did not differ from control at other levels. There were also significant increases in white matter local cerebral glucose utilization at the striatal level, which were positively correlated with [³H]PK11195 binding. The present findings demonstrate an elevation in [³H]PK11195 binding in forebrain white matter tracts of nonhuman primates with a prolonged history of cocaine self-administration. These elevations were also associated with greater cerebral metabolic rates. These data suggest that white matter deficits may contribute to behavioral, motivational, and cognitive impairments observed in cocaine abusers.

Dopamine and addiction: what have we learned from 40 years of research

Among the neurotransmitters involved in addiction, dopamine (DA) is clearly the best known. The critical role of DA in addiction is supported by converging evidence that has been accumulated in the last 40 years. In the present review, first we describe the dopaminergic system in terms of connectivity, functioning and involvement in reward processes. Second, we describe the functional, structural, and molecular changes induced by drugs within the DA system in terms of neuronal activity, synaptic plasticity and transcriptional and molecular adaptations. Third, we describe how genetic mouse models have helped characterizing the role of DA in addiction. Fourth, we describe the involvement of the DA system in the vulnerability to addiction and the interesting case of addiction DA replacement therapy in Parkinson's disease. Finally, we describe how the DA system has been targeted to treat patients suffering from addiction and the result obtained in clinical settings and we discuss how these different lines of evidence have been instrumental in shaping our understanding of the physiopathology of drug addiction.

Model systems for analysis of dopamine transporter function and regulation

The dopamine transporter (DAT) plays a critical role in dopamine (DA) homeostasis by clearing transmitter from the extraneuronal space after vesicular release. DAT serves as a site of action for a variety of addictive and therapeutic reuptake inhibitors, and transport dysfunction is associated with transmitter imbalances in disorders such as schizophrenia, attention deficit hyperactive disorder, bipolar disorder, and Parkinson disease. In this review, we describe some of the model systems that have been used for in vitro analyses of DAT structure, function and regulation, and discuss a potential relationship between transporter kinetic values and membrane cholesterol.

Presumed structural and functional neural recovery after long-term abstinence from cocaine in male military veterans

Cumulative evidence suggests that cocaine use could alter the structure and function of different brain systems. However, the extent to which the altered brain structure and function possibly recover over time after cocaine abstinence remains less clear. The present study examines 39 male military veterans with different stages of cocaine addiction and long-term abstinence (from 1 year up to 30 years) and evaluates plausible changes in brain structure and function of specific brain regions that sub-serve addictions. These include the striatum that is involved in cocaine reward; the lateral prefrontal cortex (especially the dorsolateral PFC) that plays a major role in inhibitory control; the insula, which has been implicated in craving; and the medial orbitofrontal (OFC) and ventromedial prefrontal cortex (VMPFC) shown to play key roles in foresight and decision-making. The results suggest that there are differences in both brain structure (gray matter volume, GMV) and function between cocaine USERS and CONTROLS, with USERS showing plausible relative strengthening in neural systems for processing reward and craving, and relative weakening in neural systems involved in inhibitory control and decision-making. Examination of possible neural changes after abstinence suggests that presumed recovery occurs mostly in neural systems related to reward, craving, and inhibitory control, but to a lesser extent in neural systems related to decision-making. Given the limitations of the data in terms of a small sample size, as well as the lack of certainty about occasional use in the abstinent group, these results may be considered as preliminary. However, they are compelling in that they suggest that male military veterans cocaine USERS are indefinitely at a higher risk compared to CONTROLS for making lapses in judgment and decision-making leading to possible relapse, if reward salience and craving become more intense. Understanding the neurobiology of long-term cocaine abstinence in vulnerable populations and beyond could help devising better therapeutic strategies that prevent relapse.

Cocaine Self-Administration Produces Long-Lasting Alterations in Dopamine Transporter Responses to Cocaine

Cocaine addiction is a debilitating neuropsychiatric disorder characterized by uncontrolled cocaine intake, which is thought to be driven, at least in part, by cocaine-induced deficits in dopamine system function. A decreased ability of cocaine to elevate dopamine levels has been repeatedly observed as a consequence of cocaine use in humans, and preclinical work has highlighted tolerance to cocaine's effects as a primary determinant in the development of aberrant cocaine taking behaviors. Here we determined that cocaine self-administration in rats produced tolerance to the dopamine transporter-inhibiting effects of cocaine in the nucleus accumbens core, which was normalized following a 14 or 60 d abstinence period; however, although these rats appeared to be similar to controls, a single self-administered infusion of cocaine at the end of abstinence, even after 60 d, fully reinstated tolerance to cocaine's effects. A single cocaine infusion in a naive rat had no effect on cocaine potency, demonstrating that cocaine self-administration leaves the dopamine transporter in a "primed" state, which allows for cocaine-induced plasticity to be reinstated by a subthreshold cocaine exposure. Further, reinstatement of cocaine tolerance was accompanied by decreased cocaine-induced locomotion and escalated cocaine intake despite extended abstinence from cocaine. These data demonstrate that cocaine leaves a long-lasting imprint on the dopamine system that is activated by re-exposure to cocaine. Further, these results provide a potential mechanism for severe cocaine binge episodes, which occur even after sustained abstinence from cocaine, and suggest that treatments aimed at transporter sites may be efficacious in promoting binge termination following relapse.

SIGNIFICANCE STATEMENT

Tolerance is a DSM-V criterion for substance abuse disorders. Abusers consistently show reduced subjective effects of cocaine concomitant with reduced effects of cocaine at its main site of action, the dopamine transporter (DAT). Preclinical literature has shown that reduced cocaine potency at the DAT increases cocaine taking, highlighting the key role of tolerance in addiction. Addiction is characterized by cycles of abstinence, often for many months, followed by relapse, making it important to determine possible interactions between abstinence and subsequent drug re-exposure. Using a rodent model of cocaine abuse, we found long-lasting, possibly permanent, cocaine-induced alterations to the DAT, whereby cocaine tolerance is reinstated by minimal drug exposure, even after recovery of DAT function over prolonged abstinence periods.

Chronic oral methylphenidate treatment reversibly increases striatal dopamine transporter and dopamine type 1 receptor binding in rats

Previously, we created an 8-h limited-access dual bottle drinking paradigm to deliver methylphenidate (MP) to rats at two dosages that result in a pharmacokinetic profile similar to patients treated for attention deficit hyperactivity disorder. Chronic treatment resulted in altered behavior, with some effects persisting beyond treatment. In the current study, adolescent male Sprague-Dawley rats were split into three groups at four weeks of age: control (water), low-dose MP (LD), and high-dose MP (HD). Briefly, 4 mg/kg (low dose; LD) or 30 mg/kg (high dose; HD) MP was consumed during the first hour, and 10 mg/kg (LD) or 60 mg/kg (HD) MP during hours two through eight. Following three months of treatment, half of the rats in each group (n = 8-9/group) were euthanized, and remaining rats went through a 1-month abstinence period, then euthanized. In vitro receptor autoradiography was performed to quantify binding levels of dopamine transporter (DAT), dopamine type 1 (D1R)-like receptors, and dopamine type 2 (D2R)-like receptors using [3H] WIN35,428, [3H] SCH23390, and [3H] Spiperone, respectively. Immediately following treatment, HD MP-treated rats had increased DAT and D1R-like binding in several subregions of the basal ganglia, particularly more caudal portions of the caudate putamen, which correlated with some previously reported behavioral changes. There were no differences between treatment groups in any measure following abstinence. These findings suggest that chronic treatment with a clinically relevant high dose of MP results in reversible changes in dopamine neurochemistry, which may underlie some effects on behavior.

Reduced activity in functional networks during reward processing is modulated by abstinence in cocaine addicts

Cocaine addiction is characterized by alterations in motivational and cognitive processes. Recent studies have shown that some alterations present in cocaine users may be related to the activity of large functional networks. The aim of this study was to investigate how these functional networks are modulated by non-drug rewarding stimuli in cocaine-dependent individuals. Twenty abstinent cocaine-dependent and 21 healthy matched male controls viewed erotic and neutral pictures while undergoing a functional magnetic resonance imaging scan. Group independent component analysis was then performed in order to investigate how functional networks were modulated by reward in cocaine addicts. The results showed that cocaine addicts, compared with healthy controls, displayed diminished modulation of the left frontoparietal network in response to erotic pictures, specifically when they were unpredicted. Additionally, a positive correlation between the length of cocaine abstinence and the modulation of the left frontoparietal network by unpredicted erotic images was found. In agreement with current addiction models, our results suggest that cocaine addiction contributes to reduce sensitivity to rewarding stimuli and that abstinence may mitigate this effect.

Neural Correlates of Exposure to Cocaine Cues in Rhesus Monkeys: Modulation by the Dopamine Transporter

BACKGROUND

A major goal of treatments for cocaine addiction is to reduce relapse-associated cravings, which are typically induced by environmental stimuli associated with cocaine use and related to changes in dopamine neurotransmission.

METHODS

The present study used an animal model of cocaine seeking to determine functional consequences of cue exposure using fluorodeoxyglucose positron emission tomography and to relate findings to juvenile levels of dopamine transporter and D₂-like receptor availabilities determined before any drug exposure. Adult male rhesus monkeys (N = 11) self-administered cocaine (0.2 mg/kg per injection) under a second-order schedule of reinforcement, in which responding was maintained by conditioned reinforcers. Positron emission tomography scans assessing glucose utilization, a marker of functional activation, were conducted during cocaine-cue responding and food-reinforced responding in a context where cocaine was never available.

RESULTS

Compared with the noncocaine condition, we found significant functional activation in the medial prefrontal cortex, anterior cingulate, precuneus region of the parietal cortex, and striatum-findings similar to those reported in humans who abuse cocaine. Furthermore, these functional activations in the prefrontal, cingulate, and parietal cortex measured during cocaine-cue responding were significantly correlated with juvenile measures of dopamine transporter availability, whereas no significant relationship with prior D₂-like receptor availability was observed in any brain region.

CONCLUSIONS

The similarity between the present findings and findings in humans who use cocaine supports the use of this model for examination of factors that affect the development and intensity of cue-induced drug seeking and provides evidence for potential biomarkers for the evaluation of potential treatments (behavioral and pharmacologic) for cocaine abuse.

Synaptic mechanisms underlying persistent cocaine craving

Although it is challenging for individuals with cocaine addiction to achieve abstinence, the greatest difficulty is avoiding relapse to drug taking, which is often triggered by cues associated with prior cocaine use. This vulnerability to relapse persists for long periods (months to years) after abstinence is achieved. Here, I discuss rodent studies of cue-induced cocaine craving during abstinence, with a focus on neuronal plasticity in the reward circuitry that maintains high levels of craving. Such work has the potential to identify new therapeutic targets and to further our understanding of experience-dependent plasticity in the adult brain under normal circumstances and in the context of addiction.

Longitudinal changes of dopamine transporters in heroin users during abstinence

RATIONALE

Chronic exposure to heroin results in decreased dopamine transporter levels. Jitai tablets, a traditional Chinese medicine, have been effective at increasing striatal dopamine transporter availability after 6 months of treatment. However, it remains unknown how long the heroin-induced impairment persists and whether dopamine transporter can be normalized following long-term abstinence or treatment.

OBJECTIVES

This study was to evaluate the time course of dopamine transporter changes in heroin users undergoing long-term abstinence and treatment with Jitai tablets for 1 year.

METHODS

Single-photon emission computed tomography using [(99m)Tc]TRODAT-1 was performed on 64 heroin users and 20 healthy subjects to assess striatal dopamine transporter availability at baseline, 3, 6, and 12 months. Heroin users were randomly assigned to treatment with either placebo or Jitai tablets. Depression and anxiety scores were measured before each imaging session.

RESULTS

Compared with healthy controls, significant reduction in dopamine transporter availability was found in heroin users at baseline in both the right (by ∼ 31.6%) and left striatum (by ∼ 33.2%). At 6 months, dopamine transporter availability was significantly higher in Jitai tablet-treated group than placebo group in the bilateral striatum (p < 0.01). At 12 months, dopamine transporter levels in both groups were upregulated substantially from baseline but still not recovered to normal levels in the left striatum (p < 0.05). Depression and anxiety scores significantly decreased at 3, 6, and 12 months (p < 0.05).

CONCLUSIONS

Our results confirmed that heroin abuse induces pronounced, long-term reduction in dopamine transporter. Treatment with Jitai tablets appears to stimulate recovery.

Imaging the D3 dopamine receptor across behavioral and drug addictions: Positron emission tomography studies with [(11)C]-(+)-PHNO

Chronic drug use has been associated with dopaminergic abnormalities, detectable in humans with positron emission tomography (PET). Among these, a hallmark feature is low D2 dopamine receptor availability, which has been linked to clinical outcomes, but has not yet translated into a therapeutic strategy. The D3 dopamine receptor on the other hand has gained increasing attention, as, in contrast to D2, chronic exposure to drugs has been shown to up-regulate this receptor subtype in preclinical models of addiction-a phenomenon linked to dopamine system sensitization and drug-seeking. The present article summarizes the literature to date in humans, suggesting that the D3 receptor may indeed contribute to core features of addiction such as impulsiveness and cognitive impairment. A particularly useful tool in investigating this question is the PET imaging probe [(11)C]-(+)-PHNO, which binds to D2/3 dopamine receptors but has preferential affinity for D3. This technique has been used to demonstrate D3 up-regulation in humans, and can be applied to assess pharmacological interventions for development of D3-targeted strategies in addiction treatment.

Effects of abstinence from chronic cocaine self-administration on nonhuman primate dorsal and ventral noradrenergic bundle terminal field structures

Repeated exposure to cocaine is known to dysregulate the norepinephrine system, and norepinephrine has also been implicated as having a role in abstinence and withdrawal. The goal of this study was to determine the effects of exposure to cocaine self-administration and subsequent abstinence on regulatory elements of the norepinephrine system in the nonhuman primate brain. Rhesus monkeys self-administered cocaine (0.3 mg/kg/injection, 30 reinforcers/session) under a fixed-interval 3-min schedule of reinforcement for 100 sessions. Animals in the abstinence group then underwent a 30-day period during which no operant responding was conducted, followed by a final session of operant responding. Control animals underwent identical schedules of food reinforcement and abstinence. This duration of cocaine self-administration has been shown previously to increase levels of norepinephrine transporters (NET) in the ventral noradrenergic bundle terminal fields. In contrast, in the current study, abstinence from chronic cocaine self-administration resulted in elevated levels of [(3)H]nisoxetine binding to the NET primarily in dorsal noradrenergic bundle terminal field structures. As compared to food reinforcement, chronic cocaine self-administration resulted in decreased binding of [(3)H]RX821002 to α2-adrenoceptors primarily in limbic-related structures innervated by both dorsal and ventral bundles, as well as elevated binding in the striatum. However, following abstinence from responding for cocaine binding to α2-adrenoceptors was not different than in control animals. These data demonstrate the dynamic nature of the regulation of norepinephrine during cocaine use and abstinence, and provide further evidence that the norepinephrine system should not be overlooked in the search for effective pharmacotherapies for cocaine dependence.

Increased Sensitivity to Cocaine Self-Administration in HIV-1 Transgenic Rats is Associated with Changes in Striatal Dopamine Transporter Binding

Cocaine abuse in HIV patients accelerates the progression and severity of neuropathology, motor impairment and cognitive dysfunction compared to non-drug using HIV patients. Cocaine and HIV interact with the dopamine transporter (DAT); however, the effect of their interaction on DAT binding remains understudied. The present study compared the dose-response functions for intravenous self-administration of cocaine and heroin between male HIV-1 transgenic (HIV-1 Tg) and Fischer 344 rats. The cocaine and heroin dose-response functions exhibit an inverted U-shape for both HIV-1 Tg and F344 rats. For cocaine, the number of infusions for each dose on the ascending limb was greater for HIV-1 Tg versus F344 rats. No significant changes in the heroin dose-response function were observed in HIV-1 Tg animals. Following the conclusion of self-administration experiments, DAT binding was assessed in striatal membranes. Saturation binding of the cocaine analog [(125)I] 3β-(4-iodophenyl)tropan-2β-carboxylic acid methyl ester ([(125)I]RTI-55) in rat striatal membranes resulted in binding curves that were best fit to a two-site binding model, allowing for calculation of dissociation constant (Kd) and binding density (Bmax) values that correspond to high- and low-affinity DAT binding sites. Control HIV-1 Tg rats exhibited a significantly greater affinity (i.e., decrease in Kd value) in the low-affinity DAT binding site compared to control F344 rats. Furthermore, cocaine self-administration in HIV-1 Tg rats increased low-affinity Kd (i.e., decreased affinity) compared to levels observed in control F344 rats. Cocaine also increased low-affinity Bmax in HIV-1 Tg rats as compared to controls, indicating an increase in the number of low-affinity DAT binding sites. F344 rats did not exhibit any change in high- or low-affinity Kd or Bmax values following cocaine or heroin self-administration. The increase in DAT affinity in cocaine HIV-1 Tg rats is consistent with the leftward shift of the ascending limb of the cocaine dose-response curve observed in HIV-1 Tg vs. F344 rats, and has major implications for the function of cocaine binding to DAT in HIV patients. The absence of HIV-related changes in heroin intake are likely due to less dopaminergic involvement in the mediation of heroin reward, further emphasizing the preferential influence of HIV on dopamine-related behaviors.

Abstinence duration modulates striatal functioning during monetary reward processing in cocaine patients

Pre-clinical and clinical studies in cocaine addiction highlight alterations in the striatal dopaminergic reward system that subserve maintenance of cocaine use. Using an instrumental conditioning paradigm with monetary reinforcement, we studied striatal functional alterations in long-term abstinent cocaine-dependent patients and striatal functioning as a function of abstinence and treatment duration. Eighteen patients and 20 controls underwent functional magnetic resonance imaging during a Monetary Incentive Delay task. Region of interest analyses based on masks of the dorsal and ventral striatum were conducted to test between-group differences and the functional effects in the cocaine group of time (in months) with no more than two lapses from the first time patients visited the clinical service to seek treatment at the scanning time (duration of treatment), and the functional effects of the number of months with no lapses or relapses at the scanning session time (length of abstinence). We applied a voxel-wise and a cluster-wise FWE-corrected level (pFWE) at a threshold of P < 0.05. The patient group showed lower activation in the right caudate during reward anticipation than the control group. The regression analyses in the patients group revealed a positive correlation between duration of treatment and brain activity in the left caudate during reward anticipation. Likewise, length of abstinence negatively correlated with brain activity in the bilateral nucleus accumbens during monetary outcome processing. In conclusion, caudate and nucleus accumbens show a different brain response pattern to non-drug rewards during cocaine addiction, which can be modulated by treatment success.

Functional consequences of cocaine re-exposure after discontinuation of cocaine availability

Cocaine users exhibit a wide range of behavioral impairments accompanied by brain structural, neurochemical and functional abnormalities. Metabolic mapping studies in cocaine users and animal models have shown extensive functional alterations throughout the striatum, limbic system, and cortex. Few studies, however, have evaluated the persistence of these effects following cessation of cocaine availability. The purpose of this study, therefore, was to assess the functional effects of re-exposure to cocaine in nonhuman primates after the discontinuation of cocaine self-administration for 30 or 90 days, using the quantitative autoradiographic 2-[14C]deoxyglucose (2DG) method. Rhesus monkeys self-administered cocaine (fixed interval 3-min schedule, 30 infusions per session, 0.3 mg/kg/infusion) for 100 sessions followed by 30 (n=4) or 90 days (n=3) during which experimental sessions were not conducted. Food-reinforced control animals (n=5) underwent identical schedules of reinforcement. Animals were then re-exposed to cocaine or food for one final session and the 2DG method applied immediately after session completion. Compared to controls, re-exposure to cocaine after 30 or 90 day drug-free periods resulted in lower rates of glucose utilization in ventral and dorsal striatum, prefrontal and temporal cortex, limbic system, thalamus, and midbrain. These data demonstrate that vulnerability to the effects of cocaine persists for as long as 90 days after cessation of drug use. While there was some evidence for recovery (fewer brain areas were affected by cocaine re-exposure at 90 days as compared to 30 days), this was not uniform across regions, thus suggesting that recovery occurs at different rates in different brain systems.

Chronic alcohol disrupts dopamine receptor activity and the cognitive function of the medial prefrontal cortex

Dopamine (DA) receptors in the medial prefrontal cortex (mPFC) exert powerful effects on cognition by modulating the balance between excitatory and inhibitory neurotransmission. The present study examined the impact of chronic intermittent ethanol (CIE) exposure on cognitive function and DA receptor-mediated neurotransmission in the rat mPFC. Consistent with alterations in executive function in alcoholics, CIE-exposed rats exhibited deficits in behavioral flexibility in an operant set-shifting task. Since alterations in dopaminergic neurotransmission in the mPFC have been implicated in a number of behavioral disorders including addiction, studies were then performed in the adult acute slice preparation to examine changes in DA receptor function in the mPFC following CIE exposure. In slices obtained from control rats, DA receptor stimulation was observed to exert complex actions on neuronal firing and synaptic neurotransmission that were not only dependent upon the particular receptor subtype but also whether it was a pyramidal cell or a fast-spiking interneuron. In contrast to slices from control rats, there was a near complete loss of the modulatory actions of D2/D4 receptors on cell firing and neurotransmission in slices obtained immediately, 1 and 4 weeks after the last day of CIE exposure. This loss did not appear to be associated with changes in receptor expression. In contrast, CIE exposure did not alter D1 receptor function or mGluR1 modulation of firing. These studies are consistent with the suggestion that chronic alcohol exposure disrupts cognitive function at least in part through disruption of D2 and D4 receptor signaling in mPFC.

Regionally-specific alterations in myelin proteins in nonhuman primate white matter following prolonged cocaine self-administration

BACKGROUND

Neuroimaging studies of cocaine users have demonstrated white matter abnormalities associated with behavioral measures of impulsivity and decision-making deficits. The underlying bases for this dysregulation in white matter structure and function have yet to be determined. The aim of the present studies was to investigate the influence of prolonged cocaine self-administration on the levels of myelin-associated proteins and mRNAs in nonhuman primate white matter.

METHODS

Rhesus monkeys (N=4) self-administered cocaine (0.3mg/kg/inj, 30 reinforcers per session) for 300 sessions. Control animals (N=4) responded for food. Following the final session monkeys were euthanized and white matter tissue at three brain levels was processed for immunoblotting analysis of proteolipid protein (PLP) and myelin basic protein (MBP), as well as for in situ hybridization histochemical analysis of PLP and MBP mRNAs.

RESULTS

Both MBP and PLP immunoreactivities in white matter at the level of the precommissural striatum were significantly lower in tissue from monkeys self-administering cocaine as compared to controls. No significant differences were seen for either protein at the levels of the prefrontal cortex or postcommissural striatum. In addition, no differences were observed in expression of mRNA for either protein.

CONCLUSIONS

These preliminary findings, in a nonhuman model of prolonged cocaine self-administration, provide further evidence that compromised myelin may underlie the deficits in white matter integrity described in studies of human cocaine users.

The effects of rearing environment and chronic methylphenidate administration on behavior and dopamine receptors in adolescent rats

Rearing young rodents in socially isolated or environmentally enriched conditions has been shown to affect numerous components of the dopamine system as well as behavior. Methylphenidate (MPH), a commonly used dopaminergic agent, may affect animals differently based on rearing environment. Here we examined the interaction between environment and chronic MPH treatment at clinically relevant doses, administered via osmotic minipump. Young Sprague Dawley rats (PND 21) were assigned to environmentally enriched, pair-housed, or socially isolated rearing conditions, and treated with either 0, 2, 4, or 8 mg/kg/day MPH for 3 weeks. At the end of the treatment period, animals were tested for locomotor activity and anxiety-like behavior. The densities of D1-like and D2-like receptors were measured in the striatum using in vitro receptor autoradiography. Locomotor activity and anxiety-like behavior were increased in isolated animals compared to pair-housed and enriched animals. The density of D1-like receptors was greater in isolated animals, but there were no differences between groups in D2-like receptor density. Finally, there were no effects of MPH administration on any reported measure. This study provides evidence for an effect of early rearing environment on the dopamine system and behavior, and also suggests that MPH administration may not have long-term consequences.

When a good taste turns bad: Neural mechanisms underlying the emergence of negative affect and associated natural reward devaluation by cocaine

An important feature of cocaine addiction in humans is the emergence of negative affect (e.g., dysphoria, irritability, anhedonia), postulated to play a key role in craving and relapse. Indeed, the DSM-IV recognizes that social, occupational and/or recreational activities become reduced as a consequence of repeated drug use where previously rewarding experiences (e.g., food, job, family) become devalued as the addict continues to seek and use drug despite serious negative consequences. Here, research in the Carelli laboratory is reviewed that examined neurobiological mechanisms that may underlie these processes using a novel animal model. Oromotor responses (taste reactivity) were examined as rats learned that intraoral infusion of a sweet (e.g., saccharin) predicts impending but delayed access to cocaine self-administration. We showed that rats exhibit aversive taste reactivity (i.e., gapes/rejection responses) during infusion of the sweet paired with impending cocaine, similar to aversive responses observed during infusion of quinine, a bitter tastant. Critically, the expression of this pronounced aversion to the sweet predicted the subsequent motivation to self-administer cocaine. Electrophysiology studies show that this shift in palatability corresponds to an alteration in nucleus accumbens (NAc) cell firing; neurons that previously responded with inhibition during infusion of the palatable sweet shifted to excitatory activity during infusion of the cocaine-devalued tastant. This excitatory response profile is typically observed during infusion of quinine, indicating that the once palatable sweet becomes aversive following its association with impending but delayed cocaine, and NAc neurons encode this aversive state. We also review electrochemical studies showing a shift (from increase to decrease) in rapid NAc dopamine release during infusion of the cocaine-paired tastant as the aversive state developed, again, resulting in responses similar to quinine infusion. Collectively, our findings suggest that cocaine-conditioned cues elicit a cocaine-need state that is aversive, is encoded by a distinct subset of NAc neurons and rapid dopamine signaling, and promotes cocaine-seeking behavior. Finally, we present data showing that experimentally induced abstinence (30 days) exacerbates this natural reward devaluation by cocaine, and this effect is correlated with a greater motivation to lever press during extinction. Dissecting the neural mechanisms underlying these detrimental consequences of addiction is critical since it may lead to novel treatments that ameliorate negative affective states associated with drug use and decrease the drive (craving) for the drug. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

Recovering from cocaine: insights from clinical and preclinical investigations

Cocaine remains one of the most addictive substances of abuse and one of the most difficult to treat. Although increasingly sophisticated experimental and technologic advancements in the last several decades have yielded a large body of clinical and preclinical knowledge on the direct effects of cocaine on the brain, we still have a relatively incomplete understanding of the neurobiological processes that occur when drug use is discontinued. The goal of this manuscript is to review both clinical and preclinical data related to abstinence from cocaine and discuss the complementary conclusions that emerge from these different levels of inquiry. This commentary will address observed alterations in neural function, neural structure, and neurotransmitter system regulation that are present in both animal models of cocaine abstinence and data from recovering clinical populations. Although these different levels of inquiry are often challenging to integrate, emerging data discussed in this commentary suggest that from a structural and functional perspective, the preservation of cortical function that is perhaps the most important biomarker associated with extended abstinence from cocaine.

Sensitization of hypervigilance effects of cocaine can be induced by NK3 receptor activation in marmoset monkeys

BACKGROUND

Cocaine is a widely abused drug which can result in the establishment of addiction. The neurokinin3-receptor (NK3-R) has been linked to cocaine addiction by genetic, epigenetic, and pharmacological studies suggesting that a cocaine-induced increase in NK3-R signaling may contribute to the establishment of cocaine addiction-related behaviors.

METHODS

Here we measured cocaine-induced sensitization of vigilance- and locomotor behaviors in marmoset monkeys (Callithrix penicillata) in an open field.

RESULTS

We found a sensitization of vigilance-related, but not locomotor behaviors after repeated cocaine (7mg/kg, i.p.) treatment. There was a cross-sensitization for scan frequency, but not of glance frequency, both vigilance-related behaviors, after repeated treatment with the NK3-R agonist senktide (0.2mg/kg, i.p.) given for 7 days, after a cocaine challenge (5mg/kg, i.p.).

CONCLUSIONS

These data suggest that in marmoset monkeys, repeated cocaine treatment leads to a sensitization of vigilance-related behaviors, which have a prominent role in spontaneously expressed activities in this species, but not of locomotor activity. Repeated activation of NK3-Rs can mimic some of the behavioral sensitization effect and may thus contribute to the establishment of cocaine related behaviors.

Enhanced midbrain response at 6-month follow-up in cocaine addiction, association with reduced drug-related choice

Drug addiction is characterized by dysregulated dopamine neurotransmission. Although dopamine functioning appears to partially recover with abstinence, the specific regions that recover and potential impact on drug seeking remain to be determined. Here we used functional magnetic resonance imaging (fMRI) to study an ecologically valid sample of 15 treatment-seeking cocaine addicted individuals at baseline and 6-month follow-up. At both study sessions, we collected fMRI scans during performance of a drug Stroop task, clinical self-report measures of addiction severity and behavioral measures of cocaine seeking (simulated cocaine choice); actual drug use in between the two study sessions was also monitored. At 6-month follow-up (compared with baseline), we predicted functional enhancement of dopaminergically innervated brain regions, relevant to the behavioral responsiveness toward salient stimuli. Consistent with predictions, whole-brain analyses revealed responses in the midbrain (encompassing the ventral tegmental area/substantia nigra complex) and thalamus (encompassing the mediodorsal nucleus) that were higher (and more positively correlated) at follow-up than baseline. Increased midbrain activity from baseline to follow-up correlated with reduced simulated cocaine choice, indicating that heightened midbrain activations in this context may be marking lower approach motivation for cocaine. Normalization of midbrain function at follow-up was also suggested by exploratory comparisons with active cocaine users and healthy controls (who were assessed only at baseline). Enhanced self-control at follow-up was suggested by a trend for the commonly hypoactive dorsal anterior cingulate cortex to increase response during a drug-related context. Together, these results suggest that fMRI could be useful in sensitively tracking follow-up outcomes in drug addiction.

The neurobiology of reward and cognitive control systems and their role in incentivizing health behavior

OBJECTIVE

This article reviews the neurobiology of cognitive control and reward processes and addresses their role in the treatment of addiction. We propose that the neurobiological mechanisms involved in treatment may differ from those involved in the etiology of addiction and consequently are worthy of increased investigation.

METHOD

We review the literature on reward and control processes and evidence of differences in these systems in drug addicted individuals. We also review the relatively small literature on neurobiological predictors of abstinence.

RESULTS

We conclude that prefrontal control systems may be central to a successful recovery from addiction. The frontal lobes have been shown to regulate striatal reward-related processes, to be among the regions that predict treatment outcome, and to show elevated functioning in those who have succeeded in maintaining abstinence.

CONCLUSION

The evidence of the involvement of the frontal lobes in recovery is consistent with the hypothesis that recovery is a distinct process that is more than the undoing of those processes involved in becoming addicted and a return to the pre-addiction state of the individual. The extent to which these frontal systems are engaged by treatment interventions may contribute to their efficacy.

Reward circuitry dysfunction in psychiatric and neurodevelopmental disorders and genetic syndromes: animal models and clinical findings

This review summarizes evidence of dysregulated reward circuitry function in a range of neurodevelopmental and psychiatric disorders and genetic syndromes. First, the contribution of identifying a core mechanistic process across disparate disorders to disease classification is discussed, followed by a review of the neurobiology of reward circuitry. We next consider preclinical animal models and clinical evidence of reward-pathway dysfunction in a range of disorders, including psychiatric disorders (i.e., substance-use disorders, affective disorders, eating disorders, and obsessive compulsive disorders), neurodevelopmental disorders (i.e., schizophrenia, attention-deficit/hyperactivity disorder, autism spectrum disorders, Tourette's syndrome, conduct disorder/oppositional defiant disorder), and genetic syndromes (i.e., Fragile X syndrome, Prader-Willi syndrome, Williams syndrome, Angelman syndrome, and Rett syndrome). We also provide brief overviews of effective psychopharmacologic agents that have an effect on the dopamine system in these disorders. This review concludes with methodological considerations for future research designed to more clearly probe reward-circuitry dysfunction, with the ultimate goal of improved intervention strategies.

Reward-related dorsal striatal activity differences between former and current cocaine dependent individuals during an interactive competitive game

Cocaine addiction is characterized by impulsivity, impaired social relationships, and abnormal mesocorticolimbic reward processing, but their interrelationships relative to stages of cocaine addiction are unclear. We assessed blood-oxygenation-level dependent (BOLD) signal in ventral and dorsal striatum during functional magnetic resonance imaging (fMRI) in current (CCD; n = 30) and former (FCD; n = 28) cocaine dependent subjects as well as healthy control (HC; n = 31) subjects while playing an interactive competitive Domino game involving risk-taking and reward/punishment processing. Out-of-scanner impulsivity-related measures were also collected. Although both FCD and CCD subjects scored significantly higher on impulsivity-related measures than did HC subjects, only FCD subjects had differences in striatal activation, specifically showing hypoactivation during their response to gains versus losses in right dorsal caudate, a brain region linked to habituation, cocaine craving and addiction maintenance. Right caudate activity in FCD subjects also correlated negatively with impulsivity-related measures of self-reported compulsivity and sensitivity to reward. These findings suggest that remitted cocaine dependence is associated with striatal dysfunction during social reward processing in a manner linked to compulsivity and reward sensitivity measures. Future research should investigate the extent to which such differences might reflect underlying vulnerabilities linked to cocaine-using propensities (e.g., relapses).

Neural correlates of stress-induced and cue-induced drug craving: influences of sex and cocaine dependence

OBJECTIVE

Although stress and drug cue exposure each increase drug craving and contribute to relapse in cocaine dependence, no previous research has directly examined the neural correlates of stress-induced and drug cue-induced craving in cocaine-dependent women and men relative to comparison subjects.

METHOD

Functional MRI was used to assess responses to individualized scripts for stress, drug/alcohol cue and neutral-relaxing-imagery conditions in 30 abstinent cocaine-dependent individuals (16 women, 14 men) and 36 healthy recreational-drinking comparison subjects (18 women, 18 men).

RESULTS

Significant three-way interactions between diagnostic group, sex, and script condition were observed in multiple brain regions including the striatum, insula, and anterior and posterior cingulate. Within women, group-by-condition interactions were observed involving these regions and were attributable to relatively increased regional activations in cocaine-dependent women during the stress and, to a lesser extent, neutral-relaxing conditions. Within men, group main effects were observed involving these same regions, with cocaine-dependent men demonstrating relatively increased activation across conditions, with the main contributions from the drug and neutral-relaxing conditions. In men and women, subjective drug-induced craving measures correlated positively with corticostriatal-limbic activations.

CONCLUSIONS

In cocaine dependence, corticostriatal-limbic hyperactivity appears to be linked to stress cues in women, drug cues in men, and neutral-relaxing conditions in both. These findings suggest that sex should be taken into account in the selection of therapies in the treatment of addiction, particularly those targeting stress reduction.

Addiction circuitry in the human brain

A major challenge in understanding substance-use disorders lies in uncovering why some individuals become addicted when exposed to drugs, whereas others do not. Although genetic, developmental, and environmental factors are recognized as major contributors to a person's risk of becoming addicted, the neurobiological processes that underlie this vulnerability are still poorly understood. Imaging studies suggest that individual variations in key dopamine-modulated brain circuits, including circuits involved in reward, memory, executive function, and motivation, contribute to some of the differences in addiction vulnerability. A better understanding of the main circuits affected by chronic drug use and the influence of social stressors, developmental trajectories, and genetic background on these circuits is bound to lead to a better understanding of addiction and to more effective strategies for the prevention and treatment of substance-use disorders.

fMRI of cocaine self-administration in macaques reveals functional inhibition of basal ganglia

Disparities in cocaine-induced neurochemical and metabolic responses between human beings and rodents motivate the use of non-human primates (NHP) to model consequences of repeated cocaine exposure in human subjects. To characterize the functional response to cocaine infusion in NHP brain, we employed contrast-enhanced fMRI during both non-contingent injection of drug and self-administration of cocaine in the magnet. Cocaine robustly decreased cerebral blood volume (CBV) throughout basal ganglia and motor/pre-motor cortex and produced subtle functional inhibition of prefrontal cortex. No brain regions exhibited significant elevation of CBV in response to cocaine challenge. Theses effects in NHP brain are opposite in sign to the cocaine-induced fMRI response in rats, but consistent with previous measurements in NHP based on glucose metabolism. Because the striatal ratio of D2 to D1 receptors is larger in human beings and NHP than rats, we hypothesize that the inhibitory effects of D2 receptor binding dominate the functional response in primates, whereas excitatory D1 receptor stimulation predominates in the rat. If the NHP accurately models the human response to cocaine, downregulation of D2 receptors in human cocaine-abusing populations can be expected to blunt cocaine-induced functional responses, contributing to the weak and variable fMRI responses reported in human basal ganglia following cocaine infusion.

Group II metabotropic glutamate receptors in the striatum of non-human primates: dysregulation following chronic cocaine self-administration

A growing body of evidence has demonstrated a role for group II metabotropic glutamate receptors (mGluRs) in the reinforcing effects of cocaine. These receptors are important given their location in limbic-related areas, and their ability to control the release of glutamate and other neurotransmitters. They are also potential targets for novel pharmacotherapies for cocaine addiction. The present study investigated the impact of chronic cocaine self-administration (9.0mg/kg/session for 100 sessions, 900 mg/kg total intake) on the densities of group II mGluRs, as assessed with in vitro receptor autoradiography, in the striatum of adult male rhesus monkeys. Binding of [(3)H]LY341495 to group II mGluRs in control animals was heterogeneous, with a medial to lateral gradient in binding density. Significant elevations in the density of group II mGluRs following chronic cocaine self-administration were measured in the dorsal, central and ventral portions of the caudate nucleus (P<0.05), compared to controls. No differences in receptor density were observed between the groups in either the putamen or nucleus accumbens. These data demonstrate that group II mGluRs in the dorsal striatum are more sensitive to the effects of chronic cocaine exposure than those in the ventral striatum. Cocaine-induced dysregulation of the glutamate system, and its consequent impact on plasticity and synaptic remodeling, will likely be an important consideration in the development of novel pharmacotherapies for cocaine addiction.

Individual Differences in Nucleus Accumbens Dopamine Receptors Predict Development of Addiction-Like Behavior: A Computational Approach

Clinical and experimental observations show individual differences in the development of addiction. Increasing evidence supports the hypothesis that dopamine receptor availability in the nucleus accumbens (NAc) predisposes drug reinforcement. Here, modeling striatal-midbrain dopaminergic circuit, we propose a reinforcement learning model for addiction based on the actor-critic model of striatum. Modeling dopamine receptors in the NAc as modulators of learning rate for appetitive—but not aversive—stimuli in the critic—but not the actor—we define vulnerability to addiction as a relatively lower learning rate for the appetitive stimuli, compared to aversive stimuli, in the critic. We hypothesize that an imbalance in this learning parameter used by appetitive and aversive learning systems can result in addiction. We elucidate that the interaction between the degree of individual vulnerability and the duration of exposure to drug has two progressive consequences: deterioration of the imbalance and establishment of an abnormal habitual response in the actor. Using computational language, the proposed model describes how development of compulsive behavior can be a function of both degree of drug exposure and individual vulnerability. Moreover, the model describes how involvement of the dorsal striatum in addiction can be augmented progressively. The model also interprets other forms of addiction, such as obesity and pathological gambling, in a common mechanism with drug addiction. Finally, the model provides an answer for the question of why behavioral addictions are triggered in Parkinson's disease patients by D2 dopamine agonist treatments.

Dopamine receptor expression and distribution dynamically change in the rat nucleus accumbens after withdrawal from cocaine self-administration

Dopamine receptors (DARs) in the nucleus accumbens (NAc) are critical for cocaine's actions but the nature of adaptations in DAR function after repeated cocaine exposure remains controversial. This may be due in part to the fact that different methods used in previous studies measured different DAR pools. In the present study, we used a protein crosslinking assay to make the first measurements of DAR surface expression in the NAc of cocaine-experienced rats. Intracellular and total receptor levels were also quantified. Rats self-administered saline or cocaine for 10 days. The entire NAc, or core and shell subregions, were collected one or 45 days later, when rats are known to exhibit low and high levels of cue-induced drug seeking, respectively. We found increased cell surface D1 DARs in the NAc shell on the first day after discontinuing cocaine self-administration (designated withdrawal day 1, or WD1) but this normalized by WD45. Decreased intracellular and surface D2 DAR levels were observed in the cocaine group. In shell, both measures decreased on WD1 and WD45. In core, decreased D2 DAR surface expression was only observed on WD45. Similarly, WD45 but not WD1 was associated with increased D3 DAR surface expression in the core. Taking into account many other studies, we suggest that decreased D2 DAR and increased D3 DAR surface expression on WD45 may contribute to enhanced cocaine-seeking after prolonged withdrawal, although this is likely to be a modulatory effect, in light of the mediating effect previously demonstrated for AMPA-type glutamate receptors.

The endogenous opioid system: a common substrate in drug addiction

Drug addiction is a chronic brain disorder leading to complex adaptive changes within the brain reward circuits that involve several neurotransmitters. One of the neurochemical systems that plays a pivotal role in different aspects of addiction is the endogenous opioid system (EOS). Opioid receptors and endogenous opioid peptides are largely distributed in the mesolimbic system and modulate dopaminergic activity within these reward circuits. Chronic exposure to the different prototypical drugs of abuse, including opioids, alcohol, nicotine, psychostimulants and cannabinoids has been reported to produce significant alterations within the EOS, which seem to play an important role in the development of the addictive process. In this review, we will describe the adaptive changes produced by different drugs of abuse on the EOS, and the current knowledge about the contribution of each component of this neurobiological system to their addictive properties.

Regulation of the dopamine transporter: aspects relevant to psychostimulant drugs of abuse

Dopaminergic signaling in the brain is primarily modulated by dopamine transporters (DATs), which actively translocate extraneuronal dopamine back into dopaminergic neurons. Transporter proteins are highly dynamic, continuously trafficking between plasmalemmal and endosomal membranes. Changes in DAT membrane trafficking kinetics can rapidly regulate dopaminergic tone by altering the number of transporters present at the cell surface. Various psychostimulant DAT ligands-acting either as amphetamine-like substrates or cocaine-like nontranslocated inhibitors-affect transporter trafficking, triggering rapid insertion or removal of plasmalemmal DATs. In this review, we focus on the effects of psychostimulants of addiction (particularly D-methamphetamine and cocaine) on DAT regulation and membrane trafficking, with an emphasis on how these drugs may influence intracellular signaling cascades and transporter-associated scaffolding proteins to affect DAT regulation. In addition, we consider involvement of presynaptic receptors for dopamine and other ligands in DAT regulation. Finally, we discuss possible implications of transporter regulation to the putative toxicity of several substituted amphetamine derivatives commonly used as recreational drugs, as well as to the design of therapeutics for cocaine addiction.

Dopamine D1 receptors in cocaine dependence measured with PET and the choice to self-administer cocaine

The goal of this study was to determine D(1) receptor availability in human cocaine-dependent (CD) subjects and matched healthy controls (HCs). In addition, the CD subjects performed cocaine self-administration sessions in order to explore the association between D(1) receptor availability and cocaine-seeking behavior. Twenty-five CD subjects (40+/-4 years, 19M/6 F) and 23 matched HCs (38+/-4 years, 19M/4F) were scanned with PET and the radiotracer [(11)C]NNC 112. During the cocaine self-administration sessions, CD volunteers were given the choice to self-administer cocaine (0, 6, and 12 mg) or to receive a monetary voucher worth $5. D(1) receptor availability was measured in the limbic, associative, and sensori-motor striatum in addition to cortical brain regions. No difference in D(1) receptor availability was seen between the two groups. A negative association was seen between D(1) receptor BP(ND) in the limbic striatum and the choice for the 6 mg dose of cocaine (r=-0.47, p=0.02, corrected for age). These results do not support the hypothesis that cocaine dependence is associated with a reduction in D(1) receptor availability in the striatum. However, within the CD subjects, low D(1) receptor availability in the ventral striatum was associated with the choice to self-administer cocaine, suggesting that low D(1) receptor availability may be associated with an increased risk of relapse in cocaine dependence.