Cocaine craving, euphoria, and self-administration: a preliminary study of the effect of catecholamine precursor depletion

Simultaneous cocaine and ethanol self-administration promotes a sex-dimorphic pattern in drug-seeking behaviour and alters plasma amino acid profile related to glutamate homeostasis in young adult rats

The concurrent use of cocaine and alcohol is highly prevalent in Western countries and carries a substantial risk of relapse in recovering process. Craving, characterized as a strong desire for consuming drugs, is a core feature of cocaine and ethanol use disorders and presents a significant challenge to maintaining abstinence and preventing relapse. The primary objective of this study was to explore the behavioural patterns associated with the incubation of drug-seeking for a cocaine-ethanol combination and, secondarily, to examine the plasma amino acid profile that might be associated with this combination. To achieve this, we employed an extended-access intravenous self-administration model over 10 sessions with both substances, followed by withdrawal periods of 2 and 30 days in young adult rats in both sexes. After the subsequent drug-seeking test, changes in plasma amino acid concentrations were examined. Cocaine and ethanol co-administration resulted in a lower consumption rate among rats compared to those consuming cocaine alone. However, both groups exhibited incubation of drug-seeking behaviour. Sex differences were observed in self-administration patterns and in the incubation of drug-seeking behaviour. Notably, after 30 days of withdrawal, alterations were detected in plasma levels of several amino acids, including glutamine, glycine, serine, threonine, and glutamate, which are associated with glutamate homeostasis. This study aims to contribute to our understanding of potential changes in the plasma amino acid profile in cocaine users who also consume ethanol, particularly during abstinence and craving incubation.

The Incentive-Sensitization Theory of Addiction 30 Years On

The incentive-sensitization theory (IST) of addiction was first published in 1993, proposing that (a) brain mesolimbic dopamine systems mediate incentive motivation ("wanting") for addictive drugs and other rewards, but not their hedonic impact (liking) when consumed; and (b) some individuals are vulnerable to drug-induced long-lasting sensitization of mesolimbic systems, which selectively amplifies their "wanting" for drugs without increasing their liking of the same drugs. Here we describe the origins of IST and evaluate its status 30 years on. We compare IST to other theories of addiction, including opponent-process theories, habit theories of addiction, and prefrontal cortical dysfunction theories of impaired impulse control. We also address critiques of IST that have been raised over the years, such as whether craving is important in addiction and whether addiction can ever be characterized as compulsive. Finally, we discuss several contemporary phenomena, including the potential role of incentive sensitization in behavioral addictions, the emergence of addiction-like dopamine dysregulation syndrome in medicated Parkinson's patients, the role of attentional capture and approach tendencies, and the role of uncertainty in incentive motivation.

Pharmacological Treatments for Methamphetamine Use Disorder: Current Status and Future Targets

The illicit use of the psychostimulant methamphetamine (METH) is a major concern, with overdose deaths increasing substantially since the mid-2010s. One challenge to treating METH use disorder (MUD), as with other psychostimulant use disorders, is that there are no available pharmacotherapies that can reduce cravings and help individuals achieve abstinence. The purpose of the current review is to discuss the molecular targets that have been tested in assays measuring the physiological, the cognitive, and the reinforcing effects of METH in both animals and humans. Several drugs show promise as potential pharmacotherapies for MUD when tested in animals, but fail to produce long-term changes in METH use in dependent individuals (eg, modafinil, antipsychotic medications, baclofen). However, these drugs, plus medications like atomoxetine and varenicline, may be better served as treatments to ameliorate the psychotomimetic effects of METH or to reverse METH-induced cognitive deficits. Preclinical studies show that vesicular monoamine transporter 2 inhibitors, metabotropic glutamate receptor ligands, and trace amine-associated receptor agonists are efficacious in attenuating the reinforcing effects of METH; however, clinical studies are needed to determine if these drugs effectively treat MUD. In addition to screening these compounds in individuals with MUD, potential future directions include increased emphasis on sex differences in preclinical studies and utilization of pharmacogenetic approaches to determine if genetic variances are predictive of treatment outcomes. These future directions can help lead to better interventions for treating MUD.

Optogenetic hedonic hotspots in orbitofrontal cortex and insula: causing enhancement of sweetness 'liking'

Hedonic hotspots are brain subregions that causally amplify the hedonic impact of palatable tastes, measured as increases in affective orofacial 'liking' reactions to sweetness. Previously, two cortical hedonic hotspots in orbitofrontal cortex and insula were identified by neurochemical stimulation using opioid or orexin microinjections. Here we used optogenetic stimulation in rats as an independent neurobiological technique for activating cortical hedonic hotspots to identify hedonic functions and map boundaries. We report that channelrhodopsin stimulations within rostral orbitofrontal and caudal insula hotspots doubled the number of hedonic 'liking' reactions elicited by sucrose taste. This independently confirms their robust functional identity as causal amplifiers of hedonic 'liking' and confirms their anatomical boundaries. Additionally, we confirmed an intervening suppressive hedonic coldstrip, to stretching from caudal orbitofrontal cortex to rostral insula. By contrast to localized hedonic hotspots for 'liking' enhancement, motivational 'wanting' for reward, measured as laser self-stimulation, was mediated by more widely distributed anatomical sites.

The influence of drug class on reward in substance use disorders

In the United States, the societal costs associated with drug use surpass $500 billion annually. The rewarding and reinforcing properties that drive the use of these addictive substances are typically examined concerning the neurobiological effects responsible for their abuse potential. In this review, terms such as "abuse potential," "drug," and "addictive properties" are used due to their relevance to the methodological, theoretical, and conceptual framework for understanding the phenomenon of drug-taking behavior and the associated body of preclinical and clinical literature. The use of these terms is not intended to cast aspersions on individuals with substance use disorders (SUD). Understanding what motivates substance use has been a focus of SUD research for decades. Much of this corpus of work has focused on the shared effects of each drug class to increase dopaminergic transmission within the central reward pathways of the brain, or the "reward center." However, the precise influence of each drug class on dopamine signaling, and the extent thereof, differs considerably. Furthermore, the aforementioned substances have effects on several neurobiological targets that mediate and modulate their addictive properties. The current manuscript sought to review the influence of drug class on the rewarding effects of each of the major pharmacological classes of addictive drugs (i.e., psychostimulants, opioids, nicotine, alcohol, and cannabinoids). Our review suggests that even subtle differences in drug effects can result in significant variability in the subjective experience of the drug, altering rewarding and other reinforcing effects. Additionally, this review will argue that reward (i.e., the attractive and motivational property of a stimulus) alone is not sufficient to explain the abuse liability of these substances. Instead, abuse potential is best examined as a function of both positive and negative reinforcing drug effects (i.e., stimuli that the subject will work to attain and stimuli that the subject will work to end or avoid, respectively). Though reward is central to drug use, the factors that motivate and maintain drug taking are varied and complex, with much to be elucidated.

Pharmacological Treatments for Cocaine Craving: What Is the Way Forward? A Systematic Review

BACKGROUND

cocaine craving is a core feature of cocaine use disorder and remains a critical challenge for abstinence and relapse prevention. This review summarizes the anti-craving efficacy of pharmacotherapies tested for cocaine use disorder, in the context of randomized-controlled clinical trials.

OBJECTIVES

we assessed the databases of the U.S. National Library of Medicine, Google Scholar, and PsycINFO, without date restrictions up to August 2022, to identify relevant studies.

STUDY ELIGIBILITY CRITERIA, PARTICIPANTS, AND INTERVENTIONS

we included double-blinded randomized-controlled trials investigating pharmacotherapies for cocaine craving and/or cocaine use disorder whose outcomes included cocaine craving.

STUDY APPRAISAL AND SYNTHESIS METHODS

Two authors screened studies' titles and abstracts for inclusion, and both read all the included studies. We systematically gathered information on the following aspects of each study: title; author(s); year of publication; sample size; mean age; sample characteristics; study set-ting; whether participants were treatment-seeking; study design; craving measures; study interventions; drop-out rates; and other relevant outcomes.

RESULTS

Overall, we appraised 130 clinical trials, including 8137 participants. We further considered the drugs from the studies that scored equal to or greater than six points in the quality assessment. There was a correlation between craving and cocaine use outcomes (self-reports, timeline follow-back or urinary benzoylecgonine) in the vast majority of studies. In the short-term treatment, acute phenylalanine-tyrosine depletion, clonidine, fenfluramine, meta-chlorophenylpiperazine (m-CPP) and mecamylamine presented promising effects. In the long term, amphetamine, biperiden, carbamazepine, lisdexamfetamine, lorcaserin, methamphetamine, mirtazapine, pioglitazone, progesterone, guanfacine, levodopa, nefazodone presented promising anti-craving effects. Unfortunately, the highly tested medications were not successful in most of the trials, as follows: propranolol in the short term; amantadine, aripiprazole, bromocriptine, citicoline, ketamine, modafinil, olanzapine, topiramate in the long term. The remaining 52 medications had no positive anti-craving outcomes.

LIMITATIONS

Our review was limited by high heterogeneity of craving assessments across the studies and by a great range of pharmacotherapies. Further, the majority of the studies considered abstinence and retention in treatment as the main outcomes, whereas craving was a secondary outcome and some of the studies evaluated patients with cocaine use disorder with comorbidities such as opioid or alcohol use disorder, schizophrenia, bipolar disorder or attention deficit hyperactivity. Lastly, most of the studies also included non-pharmacological treatments, such as counseling or psychotherapy.

CONCLUSIONS

There is a direct association between craving and cocaine use, underscoring craving as an important treatment target for promoting abstinence among persons with cocaine use disorder. Clonidine, fenfluramine and m-CPP showed to be promising medications for cocaine craving in the short-term treatment, and amphetamine, biperiden, carbamazepine, lisdexamfetamine, lorcaserin, methamphetamine, mirtazapine, pioglitazone, progesterone, guanfacine, levodopa, nefazodone in the long-term treatment.

Suboxone: History, controversy, and open questions

There are more than 200 opioid overdose deaths each day in the US. In combating this epidemic we look to available treatment tools. Here, we find only three medications approved by the Food and Drug Administration (FDA) for the treatment of opioid use disorder. Of the three, buprenorphine is of particular importance due to its reduced overdose potential as a partial opioid agonist. Evidence supports its clinical equivalence to its full agonist cousin methadone, and suggests that it is better slated for long-term treatment of opioid use disorder compared to the non-selective opioid antagonist naltrexone. Buprenorphine is most popularized within Suboxone, a medication which also contains the non-selective opioid antagonist naloxone. The naloxone has no additional effect when the drug is taken as instructed, as it is intended to prevent diversion in those that would attempt to inject the medication. While Suboxone is regarded by some as the future of medical treatment, others have expressed concerns. This review aims to explore the history, controversy, and open questions that surround buprenorphine and its most prescribed variation, Suboxone. These include its pharmacological, legislative, and social history, alternative indications, efficacy as a treatment of opioid use disorder, and more. Armed with this information, the reader will have a more in-depth and holistic understanding of the medication's place in their community.

Computational approaches to predicting treatment response to obesity using neuroimaging

Obesity is a worldwide disease associated with multiple severe adverse consequences and comorbid conditions. While an increased body weight is the defining feature in obesity, etiologies, clinical phenotypes and treatment responses vary between patients. These variations can be observed within individual treatment options which comprise lifestyle interventions, pharmacological treatment, and bariatric surgery. Bariatric surgery can be regarded as the most effective treatment method. However, long-term weight regain is comparably frequent even for this treatment and its application is not without risk. A prognostic tool that would help predict the effectivity of the individual treatment methods in the long term would be essential in a personalized medicine approach. In line with this objective, an increasing number of studies have combined neuroimaging and computational modeling to predict treatment outcome in obesity. In our review, we begin by outlining the central nervous mechanisms measured with neuroimaging in these studies. The mechanisms are primarily related to reward-processing and include "incentive salience" and psychobehavioral control. We then present the diverse neuroimaging methods and computational prediction techniques applied. The studies included in this review provide consistent support for the importance of incentive salience and psychobehavioral control for treatment outcome in obesity. Nevertheless, further studies comprising larger sample sizes and rigorous validation processes are necessary to answer the question of whether or not the approach is sufficiently accurate for clinical real-world application.

Unraveling the Temporal Dynamics of Reward Signals in Music-Induced Pleasure with TMS

Music's ability to induce feelings of pleasure has been the subject of intense neuroscientific research lately. Prior neuroimaging studies have shown that music-induced pleasure engages cortico-striatal circuits related to the anticipation and receipt of biologically relevant rewards/incentives, but these reports are necessarily correlational. Here, we studied both the causal role of this circuitry and its temporal dynamics by applying transcranial magnetic stimulation (TMS) over the left dorsolateral PFC combined with fMRI in 17 male and female participants. Behaviorally, we found that, in accord with previous findings, excitation of fronto-striatal pathways enhanced subjective reports of music-induced pleasure and motivation, whereas inhibition of the same circuitry led to the reduction of both. fMRI activity patterns indicated that these behavioral changes were driven by bidirectional TMS-induced alteration of fronto-striatal function. Specifically, changes in activity in the NAcc predicted modulation of both hedonic and motivational responses, with a dissociation between pre-experiential versus experiential components of musical reward. In addition, TMS-induced changes in the fMRI functional connectivity between the NAcc and frontal and auditory cortices predicted the degree of modulation of hedonic responses. These results indicate that the engagement of cortico-striatal pathways and the NAcc, in particular, is indispensable to experience rewarding feelings from music.SIGNIFICANCE STATEMENT Neuroimaging studies have shown that music-induced pleasure engages cortico-striatal circuits involved in the processing of biologically relevant rewards. Yet, these reports are necessarily correlational. Here, we studied both the causal role of this circuitry and its temporal dynamics by combining brain stimulation over the frontal cortex with functional imaging. Behaviorally, we found that excitation and inhibition of fronto-striatal pathways enhanced and disrupted, respectively, subjective reports of music-induced pleasure and motivation. These changes were associated with changes in NAcc activity and NAcc coupling with frontal and auditory cortices, dissociating between pre-experimental versus experiential components of musical reward. These results indicate that the engagement of cortico-striatal pathways, and the NAcc in particular, is indispensable to experience rewarding feeling from music.

Striatal activation to monetary reward is associated with alcohol reward sensitivity

One well-known phenotypic risk factor for the development of alcohol use disorder is sensitivity to the rewarding effects of alcohol. In the present study, we examined whether individuals who are sensitive to alcohol reward are also sensitive to nondrug rewards, thereby reflecting a broader individual difference risk factor. Specifically, we tested the hypothesis that subjective response to acute rewarding effects of alcohol would be related to neural activation during monetary reward receipt relative to loss (in the absence of alcohol). Community-recruited healthy young social drinkers (N = 58) completed four laboratory sessions in which they received alcohol (0.8 g/kg) and placebo in alternating order under double-blind conditions, providing self-report measures of subjective response to alcohol at regular intervals. At a separate visit 1-3 weeks later, they completed a reward-guessing game, the 'Doors' task, during fMRI in a drug-free state. Participants who reported greater motivation (i.e., wanting) to consume more alcohol after a single moderate dose of alcohol also exhibited greater neural activation in the bilateral ventral caudate and the nucleus accumbens during reward receipt relative to loss. Striatal activation was not related to other subjective ratings including alcohol-induced sedation, stimulation, or pleasure (i.e., feeling, liking). Our study is the first to show that measures of alcohol reward are related to neural indices of monetary reward in humans. These results support growing evidence that individual differences in responses to drug and nondrug reward are linked and together form a risk profile for drug use or abuse, particularly in young adults.

Using pharmacological manipulations to study the role of dopamine in human reward functioning: A review of studies in healthy adults

Dopamine (DA) plays a key role in reward processing and is implicated in psychological disorders such as depression, substance use, and schizophrenia. The role of DA in reward processing is an area of highly active research. One approach to this question is drug challenge studies with drugs known to alter DA function. These studies provide good experimental control and can be performed in parallel in laboratory animals and humans. This review aimed to summarize results of studies using pharmacological manipulations of DA in healthy adults. 'Reward' is a complex process, so we separated 'phases' of reward, including anticipation, evaluation of cost and benefits of upcoming reward, execution of actions to obtain reward, pleasure in response to receiving a reward, and reward learning. Results indicated that i) DAergic drugs have different effects on different phases of reward; ii) the relationship between DA and reward functioning appears unlikely to be linear; iii) our ability to detect the effects of DAergic drugs varies depending on whether subjective, behavioral, imaging measures are used.

The pathophysiological mechanisms of motivational deficits in Parkinson's disease

Parkinson's disease (PD) is a progressive degenerative disorder that leads to disabling motor symptoms and a wide variety of neuropsychiatric symptoms. Apathy is the most common psychiatric disorder in the early stages of untreated PD and can be defined as a hypodopaminergic syndrome, which also includes anxiety and depression. Apathy is also considered the core feature of the parkinsonian triad (apathy, anxiety and depression) of behavioural non-motor signs, including a motivational deficit. Moreover, apathy is recognised as a distinct chronic neuropsychiatric behavioural disorder based on specific diagnostic criteria. Given the prevalence of apathy in approximately 40% of the general Parkinson's disease population, this appears to be a contributing factor to dementia in PD; also, apathy symptoms are factors that potentially contribute to morbidity, leading to a major impairment of health-related quality of life, thus stressing the importance of understanding the pathophysiology of this disease. Several studies have clearly established a prominent role for DA-mediated signals in PD apathy. However, synergistic interaction between dopaminergic impairment resulting from the neurodegenerative process and deep brain stimulation of the subthalamic nucleus may cause or exacerbate apathy. Furthermore, serotoninergic mechanism signalling is also likely to be of importance in this pathophysiology.

Liking, wanting, and the incentive-sensitization theory of addiction

Rewards are both "liked" and "wanted," and those 2 words seem almost interchangeable. However, the brain circuitry that mediates the psychological process of "wanting" a particular reward is dissociable from circuitry that mediates the degree to which it is "liked." Incentive salience or "wanting," a form of motivation, is generated by large and robust neural systems that include mesolimbic dopamine. By comparison, "liking," or the actual pleasurable impact of reward consumption, is mediated by smaller and fragile neural systems, and is not dependent on dopamine. The incentive-sensitization theory posits the essence of drug addiction to be excessive amplification specifically of psychological "wanting," especially triggered by cues, without necessarily an amplification of "liking." This is because of long-lasting changes in dopamine-related motivation systems of susceptible individuals, called "neural sensitization." A quarter-century after its proposal, evidence has continued to grow in support the incentive-sensitization theory. Further, its scope is now expanding to include diverse behavioral addictions and other psychopathologies. (PsycINFO Database Record

Dopamine Depletion Reduces Food-Related Reward Activity Independent of BMI

Reward sensitivity and possible alterations in the dopaminergic-reward system are associated with obesity. We therefore aimed to investigate the influence of dopamine depletion on food-reward processing. We investigated 34 female subjects in a randomized placebo-controlled, within-subject design (body mass index (BMI)=27.0 kg/m(2) ±4.79 SD; age=28 years ±4.97 SD) using an acute phenylalanine/tyrosine depletion drink representing dopamine depletion and a balanced amino acid drink as the control condition. Brain activity was measured with functional magnetic resonance imaging during a 'wanting' and 'liking' rating of food items. Eating behavior-related traits and states were assessed on the basis of questionnaires. Dopamine depletion resulted in reduced activation in the striatum and higher activation in the superior frontal gyrus independent of BMI. Brain activity during the wanting task activated a more distributed network than during the liking task. This network included gustatory, memory, visual, reward, and frontal regions. An interaction effect of dopamine depletion and the wanting/liking task was observed in the hippocampus. The interaction with the covariate BMI was significant in motor and control regions but not in the striatum. Our results support the notion of altered brain activity in the reward and prefrontal network with blunted dopaminergic action during food-reward processing. This effect is, however, independent of BMI, which contradicts the reward-deficiency hypothesis. This hints to the hypothesis suggesting a different or more complex mechanism underlying the dopaminergic reward function in obesity.

Roles of "Wanting" and "Liking" in Motivating Behavior: Gambling, Food, and Drug Addictions

The motivation to seek out and consume rewards has evolutionarily been driven by the urge to fulfill physiological needs. However in a modern society dominated more by plenty than scarcity, we tend to think of motivation as fueled by the search for pleasure. Here, we argue that two separate but interconnected subcortical and unconscious processes direct motivation: "wanting" and "liking." These two psychological and neuronal processes and their related brain structures typically work together, but can become dissociated, particularly in cases of addiction. In drug addiction, for example, repeated consumption of addictive drugs sensitizes the mesolimbic dopamine system, the primary component of the "wanting" system, resulting in excessive "wanting" for drugs and their cues. This sensitizing process is long-lasting and occurs independently of the "liking" system, which typically remains unchanged or may develop a blunted pleasure response to the drug. The result is excessive drug-taking despite minimal pleasure and intense cue-triggered craving that may promote relapse long after detoxification. Here, we describe the roles of "liking" and "wanting" in general motivation and review recent evidence for a dissociation of "liking" and "wanting" in drug addiction, known as the incentive sensitization theory (Robinson and Berridge 1993). We also make the case that sensitization of the "wanting" system and the resulting dissociation of "liking" and "wanting" occurs in both gambling disorder and food addiction.

Deep Brain Stimulation for Obesity: From a Theoretical Framework to Practical Application

Obesity remains a pervasive global health problem. While there are a number of nonsurgical and surgical options for treatment, the incidence of obesity continues to increase at an alarming rate. The inability to curtail the growing rise of the obesity epidemic may be related to a combination of increased food availability and palatability. Research into feeding behavior has yielded a number of insights into the homeostatic and reward mechanisms that govern feeding. However, there remains a gap between laboratory investigations of feeding physiology in animals and translation into meaningful treatment options for humans. In addition, laboratory investigation may not be able to recapitulate all aspects of human food consumption. In a landmark pilot study of deep brain stimulation (DBS) of the lateral hypothalamic area for obesity, we found that there was an increase in resting metabolic rate as well as a decreased urge to eat. In this review, the authors will review some of the work relating to feeding physiology and research surrounding two nodes involved in feeding homeostasis, nucleus accumbens (NAc) and hypothalamus, and use this to provide a framework for future investigations of DBS as a viable therapeutic modality for obesity.

Effects of context-drug learning on synaptic connectivity in the basolateral nucleus of the amygdala in rats

Context-drug learning produces structural and functional synaptic changes in the circuitry of the basolateral nucleus of the amygdala (BLA). However, how the synaptic changes translated to the neuronal targets was not established. Thus, in the present study, immunohistochemistry with a cell-specific marker and the stereological quantification of synapses was used to determine if context-drug learning increases the number of excitatory and inhibitory/modulatory synapses contacting the gamma-aminobutyric acid (GABA) interneurons and/or the pyramidal neurons in the BLA circuitry. Amphetamine-conditioned place preference increased the number of asymmetric (excitatory) synapses contacting the spines and dendrites of pyramidal neurons and the number of multisynaptic boutons contacting pyramidal neurons and GABA interneurons. Context-drug learning increased asymmetric (excitatory) synapses onto dendrites of GABA interneurons and increased symmetric (inhibitory or modulatory) synapses onto dendrites but not perikarya of these same interneurons. The formation of context-drug associations alters the synaptic connectivity in the BLA circuitry, findings that have important implications for drug-seeking behavior.

Dopamine and glutamate interaction mediates reinstatement of drug-seeking behavior by stimulation of the ventral subiculum

BACKGROUND

Drug addiction is a chronic brain disease characterized by recurrent episodes of relapse to drug-seeking/-taking behaviors. The ventral subiculum, the primary output of the hippocampus, plays a critical role in mediating drug-seeking behavior.

METHODS

A d-amphetamine intravenous self-administration rat model was employed along with focal electrical stimulation of the ventral subiculum (20 Hz/200 pulses) to examine its role in reinstatement of drug-seeking behavior. Dopamine efflux in the nucleus accumbens was measured by in vivo microdialysis and subsequent HPLC-ED analyses. Pharmacological antagonism of dopamine and ionotropic glutamate receptors locally within the nucleus accumbens was employed to assess the role of glutamate and dopamine in reinstatement of drug-seeking behavior induced by stimulation of the ventral subiculum.

RESULTS

Here, we demonstrate that reinstatement of drug-seeking behavior following extinction of d-amphetamine self-administration by rats was induced by electrical stimulation in the ventral subiculum but not the cortex. This reinstatement was accompanied by a significant increase in dopamine efflux in the nucleus accumbens and was disrupted by microinfusion of a dopamine D1 or D2 antagonist into the nucleus accumbens. Inhibition of N-methyl-D-aspartate or non- N-methyl-D-aspartate receptors had no effect on the reinstatement induced by ventral subiculum stimulation, whereas co-infusion of D1 and N-methyl-D-aspartate antagonists at formerly ineffective doses prevented drug-seeking behavior.

CONCLUSIONS

These data support the hypothesis that dopamine/glutamate interactions within the ventral striatum related to memory processes are involved in relapse to addictive behavior.

Applying incentive sensitization models to behavioral addiction

The incentive sensitization theory is a promising model for understanding the mechanisms underlying drug addiction, and has received support in animal and human studies. So far the theory has not been applied to the case of behavioral addictions like Gambling Disorder, despite sharing clinical symptoms and underlying neurobiology. We examine the relevance of this theory for Gambling Disorder and point to predictions for future studies. The theory promises a significant contribution to the understanding of behavioral addiction and opens new avenues for treatment.

Amphetamine self-administration attenuates dopamine D2 autoreceptor function

Dopamine D2 autoreceptors located on the midbrain dopaminergic neurons modulate dopamine (DA) neuron firing, DA release, and DA synthesis through a negative-feedback mechanism. Dysfunctional D2 autoreceptors following repeated drug exposure could lead to aberrant DA activity in the ventral tegmental area (VTA) and projection areas such as nucleus accumbens (NAcc), promoting drug-seeking and -taking behavior. Therefore, it is important to understand molecular mechanisms underlying drug-induced changes in D2 autoreceptors. Here, we reported that 5 days of amphetamine (AMPH) self-administration reduced the ability of D2 autoreceptors to inhibit DA release in the NAcc as determined by voltammetry. Using the antibody-capture [(35)S]GTPγS scintillation proximity assay, we demonstrated for the first time that midbrain D2/D3 receptors were preferentially coupled to Gαi2, whereas striatal D2/D3 receptors were coupled equally to Gαi2 and Gαo for signaling. Importantly, AMPH abolished the interaction between Gαi2 and D2/D3 receptors in the midbrain while leaving striatal D2/D3 receptors unchanged. The disruption of the coupling between D2/D3 receptors and Gαi2 by AMPH is at least partially explained by the enhanced RGS2 (regulator of G-protein signaling 2) activity resulting from an increased RGS2 trafficking to the membrane. AMPH had no effects on the midbrain expression and trafficking of other RGS proteins such as RGS4 and RGS8. Our data suggest that midbrain D2/D3 receptors are more susceptible to AMPH-induced alterations. Reduced D2 autoreceptor function could lead to enhanced DA signaling and ultimately addiction-related behavior. RGS2 may be a potential non-dopaminergic target for pharmacological intervention of dysfunctional DA transmission and drug addiction.

Dopamine ups and downs in vulnerability to addictions: a neurodevelopmental model

Addictions are commonly presaged by problems in childhood and adolescence. For many individuals this starts with the early expression of impulsive risk-taking, social gregariousness, and oppositional behaviors. Here we propose that these early diverse manifestations reflect a heightened ability of emotionally salient stimuli to activate dopamine pathways that foster behavioral approach. If substance use is initiated, these at-risk youth can also develop heightened responses to drug-paired cues. Through conditioning and drug-induced sensitization, these effects strengthen and accumulate, leading to responses that exceed those elicited by other rewards. At the same time, cues not paired with drug become associated with comparatively lower dopamine release, accentuating further the difference between drug and non-drug rewards. Together, these enhancing and inhibiting processes steer a pre-existing vulnerability toward a disproportionate concern for drugs and drug-related stimuli. Implications for prevention and treatment are discussed.

Tyrosine-free amino acid mixtures reduce physiologically-evoked release of dopamine in a selective and activity-dependent manner

Depletion of the catecholamine precursor tyrosine using tyrosine-free amino acid mixtures is an important tool in neuropsychological studies, and often considered dopamine selective on the basis of neuropharmacological studies. However, little is known of the effects of tyrosine depletion when catecholamine neurons are activated physiologically. Here we investigated the effect of tyrosine-free amino acid mixtures on catecholamine release evoked in vivo using a stimulation paradigm aimed to approximate the phasic firing pattern of these neurons that accompanies cognitive and behavioural change. Dopamine and noradrenaline release was monitored by microdialysis in rat medial prefrontal cortex (mPFC) and striatum (chloral hydrate anaesthesia, perfusion medium containing 1 µM cocaine). Electrical stimulation of the medial forebrain bundle (MFB) caused a short-lasting, frequency-dependent increase in dopamine and noradrenaline. A full tyrosine-free amino acid mixture reduced the release of dopamine in mPFC and striatum, across a range of stimulation frequencies, and the effect was greater as stimulation frequency increased. Similar results were obtained using a smaller tyrosine-free amino acid mixture. In the same experiments showing decreased dopamine, neither tyrosine-free mixture of amino acids significantly altered stimulation-evoked release of noradrenaline. These results show that tyrosine depletion using tyrosine-free amino acid mixtures causes a selective, activity-dependent decrease in dopamine release when dopamine neurons are driven physiologically.

Dopamine and light: dissecting effects on mood and motivational states in women with subsyndromal seasonal affective disorder

BACKGROUND

Despite evidence that bright light can improve mood, the neurobiology remains poorly understood. Some evidence implicates the catecholamines. In the present study, we measured the effects of transiently decreasing dopamine (DA) synthesis on mood and motivational states in healthy women with mild seasonal mood changes who were tested in either bright or dim light.

METHODS

On 2 test days, participants slept overnight in a light-controlled room. On the morning of each session, half of the participants awoke to gradual increases of bright light, up to 3000 lux, and half to dim light (10 lux). For all participants, DA was reduced on 1 of the test days using the acute phenylalanine/tyrosine depletion (APTD) method; on the other day, they ingested a nutritionally balanced control mixture (BAL). Beginning 4 hours postingestion, participants completed subjective mood questionnaires, psychological tests and a progressive ratio breakpoint task during which they worked for successive units of $5.

RESULTS

Thirty-two women participated in our study. The APTD lowered mood, agreeableness, energy and the willingness to work for monetary reward. The effects on energy and motivation were independent of light, while the effects on mood and agreeableness were seen in the dim condition only, being prevented by bright light.

LIMITATIONS

Acute phenylalanine/tyrosine depletion might affect systems other than DA. The sample size was small.

CONCLUSION

These results suggest that increased DA function may be responsible for some of the beneficial effects of light, while adding to the evidence that the neurobiology of mood and motivational states can be dissociated.

Striatal ups and downs: their roles in vulnerability to addictions in humans

Susceptibility to addictive behaviors has been related to both increases and decreases in striatal function. Both profiles have been reported in humans as well as in animal models. Yet, the mechanisms underlying these opposing effects and the manner in which they relate to the behavioral development and expression of addiction remain unclear. In the present review of human studies, we describe a number of factors that could influence whether striatal hyper- or hypo-function is observed and propose a model that integrates the influence of these opposite responses on the expression of addiction related behaviors. Central to this model is the role played by the presence versus absence of addiction related cues and their ability to regulate responding to abused drugs and other rewards. Striatal function and incentive motivational states are increased in the presence of these cues and decreased in their absence. Alternations between these states might account for the progressive narrowing of interests as addictions develop and point to relevant processes to target in treatment.

Cocaine cue-induced dopamine release in amygdala and hippocampus: a high-resolution PET [¹⁸F]fallypride study in cocaine dependent participants

Drug-related cues are potent triggers for relapse in people with cocaine dependence. Dopamine (DA) release within a limbic network of striatum, amygdala and hippocampus has been implicated in animal studies, but in humans it has only been possible to measure effects in the striatum. The objective here was to measure drug cue-induced DA release in the amygdala and hippocampus using high-resolution PET with [(18)F]fallypride. Twelve cocaine-dependent volunteers (mean age: 39.6 ± 8.0 years; years of cocaine use: 15.9 ± 7.4) underwent two [(18)F]fallypride high-resolution research tomography-PET scans, one with exposure to neutral cues and one with cocaine cues. [(18)F]Fallypride non-displaceable-binding potential (BPND) values were derived for five regions of interest (ROI; amygdala, hippocampus, ventral limbic striatum, associative striatum, and sensorimotor striatum). Subjective responses to the cues were measured with visual analog scales and grouped using principal component analysis. Drug cue exposure significantly decreased BPND values in all five ROI in subjects who had a high-, but not low-, craving response (limbic striatum: p=0.019, associative striatum: p=0.008, sensorimotor striatum: p=0.004, amygdala: p=0.040, and right hippocampus: p=0.025). Individual differences in the cue-induced craving response predicted the magnitude of [(18)F]fallypride responses within the striatum (ventral limbic: r=0.581, p=0.048; associative: r=0.589, p=0.044; sensorimotor: r=0.675, p=0.016). To our knowledge this study provides the first evidence of drug cue-induced DA release in the amygdala and hippocampus in humans. The preferential induction of DA release among high-craving responders suggests that these aspects of the limbic reward network might contribute to drug-seeking behavior.

Dopamine and food addiction: lexicon badly needed

Over the last few years, the concept of food addiction has become a common feature in the scientific literature, as well as the popular press. Nevertheless, the use of the term addiction to describe pathological aspects of food intake in humans remains controversial, and even among those who affirm the validity of the concept, there is considerable disagreement about its utility for explaining the increasing prevalence of obesity throughout much of the world. An examination of the literature on food addiction indicates that mesolimbic and nigrostriatal dopamine systems often are cited as mechanisms that contribute to the establishment of food addiction. However, in reviewing this literature, it is important to have a detailed consideration of the complex nature of dopaminergic involvement in motivational processes. For example, although it is often stated that mesolimbic dopamine mediates reward, there is no standard or consistent technical meaning of this term. Moreover, there is a persistent tendency to link dopamine transmission with pleasure or hedonia, as opposed to other aspects of motivation or learning. The present article provides a critical discussion of some aspects of the food addiction literature, viewed through the lens of recent findings and current theoretical views of dopaminergic involvement in food motivation. Furthermore, compulsive food intake and binge eating will be considered from an evolutionary perspective, in terms of the motivational subsystems that are involved in adaptive patterns of food consumption and seeking behaviors and a consideration of how these could be altered in pathological conditions.

From prediction error to incentive salience: mesolimbic computation of reward motivation

Reward contains separable psychological components of learning, incentive motivation and pleasure. Most computational models have focused only on the learning component of reward, but the motivational component is equally important in reward circuitry, and even more directly controls behavior. Modeling the motivational component requires recognition of additional control factors besides learning. Here I discuss how mesocorticolimbic mechanisms generate the motivation component of incentive salience. Incentive salience takes Pavlovian learning and memory as one input and as an equally important input takes neurobiological state factors (e.g. drug states, appetite states, satiety states) that can vary independently of learning. Neurobiological state changes can produce unlearned fluctuations or even reversals in the ability of a previously learned reward cue to trigger motivation. Such fluctuations in cue-triggered motivation can dramatically depart from all previously learned values about the associated reward outcome. Thus, one consequence of the difference between incentive salience and learning can be to decouple cue-triggered motivation of the moment from previously learned values of how good the associated reward has been in the past. Another consequence can be to produce irrationally strong motivation urges that are not justified by any memories of previous reward values (and without distorting associative predictions of future reward value). Such irrationally strong motivation may be especially problematic in addiction. To understand these phenomena, future models of mesocorticolimbic reward function should address the neurobiological state factors that participate to control generation of incentive salience.

The role of dopaminergic transmission through D1-like and D2-like receptors in amphetamine-induced rat ultrasonic vocalizations

RATIONALE

Systemic amphetamine (AMPH) administration increases the rate of 50-kHz ultrasonic vocalizations (USVs) in adult rats and preferentially enhances the 'trill' subtype; these effects of AMPH critically depend on noradrenergic transmission, but the possible contributions of dopamine are unclear.

OBJECTIVE

To assess the role of dopamine in 50-kHz USVs emitted drug-free and following systemic AMPH administration.

METHODS

Adult male Long-Evans rats pre-selected for high AMPH-induced calling rates were tested with AMPH (1 mg/kg, intraperitoneal (IP)) and saline following pretreatment with the following dopamine receptor antagonists: SCH 23390 (0.005-0.02 mg/kg, subcutaneous (SC)), SCH 39166 (0.03-0.3 mg/kg, SC), haloperidol (0.1, 0.2 mg/kg, IP), sulpiride (20-80 mg/kg, SC), raclopride (0.1-0.5 mg/kg, SC), clozapine (4 mg/kg, SC), risperidone (0.5 mg/kg, SC), and pimozide (1 mg/kg, IP). The dopamine and noradrenaline reuptake inhibitors (GBR 12909 and nisoxetine, respectively) were also tested, alone and in combination.

RESULTS

SCH 23390, SCH 39166, haloperidol, and raclopride dose-dependently inhibited vocalizations under AMPH and suppressed the proportion of trill calls. Sulpiride, however, had no discernable effect on call rate or profile, even at a high dose that reduced locomotor activity. Single doses of clozapine, risperidone, and pimozide all markedly decreased calling under saline and AMPH. Finally, GBR 12909 and nisoxetine failed to promote 50-kHz USVs detectably or alter the subtype profile, when tested alone or in combination.

CONCLUSIONS

The rate of 50-kHz USVs and the call subtype profile following systemic AMPH administration depends on dopaminergic neurotransmission through D1-like and D2-like receptors. However, inhibiting dopamine and/or noradrenaline reuptake appears insufficient to induce calling.

The effect of acute tyrosine phenylalanine depletion on emotion-based decision-making in healthy adults

Despite interest in dopamine's role in emotion-based decision-making, few reports of the effects of dopamine manipulations are available in this area in humans. This study investigates dopamine's role in emotion-based decision-making through a common measure of this construct, the Iowa Gambling Task (IGT), using Acute Tyrosine Phenylalanine Depletion (ATPD). In a between-subjects design, 40 healthy adults were randomized to receive either an ATPD beverage or a balanced amino acid beverage (a control) prior to completing the IGT, as well as pre- and post-manipulation blood draws for the neurohormone prolactin. Together with conventional IGT performance metrics, choice selections and response latencies were examined separately for good and bad choices before and after several key punishment events. Changes in response latencies were also used to predict total task performance. Prolactin levels increased significantly in the ATPD group but not in the control group. However, no significant group differences in performance metrics were detected, nor were there sex differences in outcome measures. However, the balanced group's bad deck latencies speeded up across the task, while the ATPD group's latencies remained adaptively hesitant. Additionally, modulation of latencies to the bad decks predicted total score for the ATPD group only. One interpretation is that ATPD subtly attenuated reward salience and altered the approach by which individuals achieved successful performance, without resulting in frank group differences in task performance.

The mysterious motivational functions of mesolimbic dopamine

Nucleus accumbens dopamine is known to play a role in motivational processes, and dysfunctions of mesolimbic dopamine may contribute to motivational symptoms of depression and other disorders, as well as features of substance abuse. Although it has become traditional to label dopamine neurons as "reward" neurons, this is an overgeneralization, and it is important to distinguish between aspects of motivation that are differentially affected by dopaminergic manipulations. For example, accumbens dopamine does not mediate primary food motivation or appetite, but is involved in appetitive and aversive motivational processes including behavioral activation, exertion of effort, approach behavior, sustained task engagement, Pavlovian processes, and instrumental learning. In this review, we discuss the complex roles of dopamine in behavioral functions related to motivation.

Effects of amphetamine on reactivity to emotional stimuli

RATIONALE

Most studies of the reinforcing effects of stimulants have focused on the drugs' capacity to induce positive mood (i.e., euphoria). However, recent findings suggest drugs may also alter emotional reactivity to external stimuli, and that this may occur independently of direct effects on mood.

OBJECTIVES

We aimed to examine effects of D: -amphetamine, a prototypic stimulant, on self-reported and psychophysiological reactivity to emotional stimuli as well as overall subjective mood. We predicted that amphetamine would enhance reactivity to pleasant stimuli, particularly, stimuli with social content and that these effects would be independent of the drug's direct effects on mood.

METHODS

Over three sessions, 36 healthy normal adults received placebo, D: -amphetamine 10 and 20 mg under counterbalanced double-blind conditions. At each session, emotional reactivity to standardized positive, neutral, and negative pictures with and without social content was measured in self-reports and facial muscles sensitive to emotional state. Drug effects on cardiovascular variables and subjective mood were also measured.

RESULTS

Amphetamine produced euphoria, feelings of drug effect, and increased blood pressure. Most notably, amphetamine enhanced self-reported positive reactions to all pictures and psychophysiological reactions to positive pictures. These effects were not significantly related to drug-induced mood changes. Contrary to our hypothesis, effects of amphetamine on emotional reactivity were not moderated by social content.

CONCLUSIONS

This study demonstrates a previously unexamined and potentially reinforcing effect of stimulant drugs in humans, distinct from more typically measured euphorigenic effects, and suggests new areas of research in stimulant abuse risk and adaptations occurring during drug dependence.

α- and β-Adrenergic receptors differentially modulate the emission of spontaneous and amphetamine-induced 50-kHz ultrasonic vocalizations in adult rats

Amphetamine (AMPH) increases adult rat 50-kHz ultrasonic vocalizations, preferentially promoting frequency-modulated (FM) calls that have been proposed to reflect positive affect. The main objective of this study was to investigate a possible noradrenergic contribution to AMPH-induced calling. Adult male Long-Evans rats were tested with AMPH (1 mg/kg intraperitoneal) or saline combined with various systemic pretreatments: clonidine (α2 adrenergic agonist), prazosin (α1 antagonist), atipamezole (α2 antagonist), propranolol, betaxolol, and/or ICI 118,551 (β1/β2, β1, and β2 antagonists, respectively), nadolol (β1/β2 antagonist, peripheral only), or NAD-299 (5HT(1A) antagonist). In addition, effects of cirazoline (α1 adrenergic agonist) and cocaine (0.25-1.5 mg/kg intravenous) were studied alone. AMPH-induced calling was suppressed by low-dose clonidine and prazosin. Cirazoline and atipamezole did not significantly affect calling rate. Propranolol, without affecting the call rate, dose dependently promoted 'flat' calls under AMPH while suppressing 'trills,' thus reversing the effects of AMPH on the 'call subtype profile.' This effect of propranolol seemed to be mediated by simultaneous inhibition of CNS β1 and β2 rather than by 5HT(1A) receptors. Finally, cocaine elicited fewer calls than did AMPH, but produced the same shift in the call subtype profile. Taken together, these results reveal differential drug effects on flat vs trill vs other FM 50-kHz calls. These findings highlight the value of detailed call subtype analyses, and show that 50-kHz calls are associated with adrenergic α1- and β-receptor mechanisms. These preclinical findings suggest that noradrenergic contributions to psychostimulant subjective effects may warrant further investigation.

The dopamine augmenter L-DOPA does not affect positive mood in healthy human volunteers

Dopamine neurotransmission influences approach toward rewards and reward-related cues. The best cited interpretation of this effect proposes that dopamine mediates the pleasure that commonly accompanies reward. This hypothesis has received support in some animal models and a few studies in humans. However, direct assessments of the effect of transiently increasing dopamine neurotransmission have been largely limited to the use of psychostimulant drugs, which elevate brain levels of multiple neurotransmitters in addition to dopamine. In the present study we tested the effect of more selectively elevating dopamine neurotransmission, as produced by administration of the immediate dopamine precursor, L-DOPA (0, 100/25, 200/50 mg, Sinemet), in healthy human volunteers. Neither dose altered positive mood. The results suggest that dopamine neurotransmission does not directly influence positive mood in humans.

Ethanol drinking-in-the-dark facilitates behavioral sensitization to ethanol in C57BL/6J, BALB/cByJ, but not in mu-opioid receptor deficient CXBK mice

BACKGROUND

Neuroplasticity associated with drug-induced behavioral sensitization has been associated with excessive drug pursuit and consumption characteristic of addiction. Repeated intraperitoneal (ip) injections of ethanol (EtOH) can induce psychomotor sensitization in mice. In terms of its clinical relevance, however, it is important to determine whether this phenomenon can also be produced by voluntary EtOH consumption.

METHODS

The present investigation used a drinking-in-the-dark (DID) methodology to induce high levels of EtOH drinking in mice; EtOH replaces water for 2 or 4h, starting 3h after the beginning of the dark cycle. Animals followed a 3-week DID protocol prior to an evaluation of EtOH-induced locomotor activity (acute and repeated EtOH). For the first week, animals had access to 20% EtOH. On weeks 2 and 3, different concentrations of EtOH (10, 20 or 30%) were used. Three different inbred strains of mice were used: C57BL/6J (B6), BALB/cByJ (BALB), and CXBK. The CXBK mouse line was used because of its reduced expression and functioning of brain mu-opioid receptors, which have been suggested to participate in the development of EtOH-induced sensitization. B6 and BALB mice were used as controls.

RESULTS

B6 and CXBK mice presented comparable levels of EtOH drinking (approx. 3g/kg in 2h), that were higher than those showed by BALB. All animals, regardless of genotype, adjusted volume of EtOH intake to obtain stable g/kg of EtOH across concentrations. Previous EtOH DID produced (B6) or potentiated (BALB) sensitization to EtOH; this effect was not seen in CXBK. Western blot analysis showed a reduced number of mu-opioid receptors in several brain regions of CXBK as compared to that of B6 and BALB mice.

CONCLUSIONS

In summary, here we show that the DID methodology can be used to trigger EtOH-induced neuroplasticity supporting psychomotor sensitization, a process that might require participation of mu-opioid receptors.

Acute phenylalanine/tyrosine depletion reduces motivation to smoke cigarettes across stages of addiction

The neurobiology of tobacco use is poorly understood, possibly in part because the relevant mechanisms might differ depending on past nicotine exposure and degree of addiction. In the present study we investigated whether these factors might affect the role of dopamine (DA). Using the acute phenylalanine/tyrosine depletion method (APTD), DA synthesis was transiently decreased in three groups of abstinent smokers (n=47): (1) early low-frequency smokers, who had smoked a maximum of five cigarettes per day for less than one year, (2) stable low-frequency smokers smoking at the same level as early low-frequency smokers for at least 3 years, and (3) stable high-frequency smokers, who smoked a minimum of 10 or more cigarettes per day for at least 5 years. Motivation to obtain tobacco was measured using a progressive ratio breakpoint schedule for nicotine-containing and de-nicotinized cigarettes. Compared with a nutritionally balanced control mixture, APTD decreased the self-administration of nicotine-containing cigarettes, and this occurred in all three groups of smokers. The results suggest that DA influenced the willingness to sustain effort for nicotine reward, and this was seen in participants at all three levels of cigarette addiction. In the transition from sporadic to addicted use, the role of DA in the motivation to seek drug may change less than previously proposed.

Neuroscience of behavioral and pharmacological treatments for addictions

Although substantial advances have been made in behavioral and pharmacological treatments for addictions, moving treatment development to the next stage may require novel ways of approaching addictions, particularly ways based on new findings regarding the neurobiological underpinnings of addictions that also assimilate and incorporate relevant information from earlier approaches. In this review, we first briefly review theoretical and biological models of addiction and then describe existing behavioral and pharmacologic therapies for the addictions within this framework. We then propose new directions for treatment development and targets that are informed by recent evidence regarding the heterogeneity of addictions and the neurobiological contributions to these disorders.

Role of dopamine–adenosine interactions in the brain circuitry regulating effort-related decision making: insights into pathological aspects of motivation

Brain dopamine, particularly in the nucleus accumbens, has been implicated in activational aspects of motivation and effort-related processes. Accumbens dopamine depletions reduce the tendency of rats to work for food, and alter effort-related decision making, but leave aspects of food motivation such as appetite intact. Recent evidence indicates that the purine neuromodulator adenosine, largely through actions on adenosine A2A receptors, also participates in regulating effort-related processes. Adenosine A2A antagonists can reverse the effects of dopamine D2 antagonists on effort-related choice, and intra-accumbens injections of adenosine A2A agonists produce effects that are similar to those induced by accumbens dopamine depletion or antagonism. These studies have implications for the understanding and treatment of energy-related disorders such as anergia and fatigue in psychiatry and neurology.

Liking and wanting of drug and non-drug rewards in active cocaine users: the STRAP-R questionnaire

Few studies have examined the subjective value attributed to drug rewards specifically as it compares with the value attributed to primary non-drug rewards in addicted individuals. The objective of this study is to assess 'liking' and 'wanting' of expected 'drug' rewards as compared to 'food' and 'sex' while respondents report about three different situations ('current', and hypothetical 'in general', and 'under drug influence'). In all, 20 cocaine-addicted individuals (mean abstinence = 2 days) and 20 healthy control subjects were administered the STRAP-R (Sensitivity To Reinforcement of Addictive and other Primary Rewards) questionnaire after receiving an oral dose of the dopamine agonist methylphenidate (20 mg) or placebo. The reinforcers' relative value changed within the addicted sample when reporting about the 'under drug influence' situation (drug > food; otherwise, drug < food). This change was highest in the addicted individuals with the youngest age of cocaine use onset. Moreover, 'drug' 'wanting' exceeded 'drug' 'liking' in the addicted subjects when reporting about this situation during methylphenidate. Thus, cocaine-addicted individuals assign the highest subjective valence to 'drug' rewards but only when recalling cue-related situations. When recalling this situation, they also report higher 'drug' 'wanting' than hedonic 'liking', a motivational shift that was only significant during methylphenidate. Together, these valence shifts may underlie compulsive stimulant abuse upon pharmacological or behavioural cue exposure in addicted individuals. Additional studies are required to assess the reliability of the STRAP-R in larger samples and to examine its validity in measuring the subjective value attributed to experienced reinforcers or in predicting behaviour.

Dopamine precursor depletion improves punishment prediction during reversal learning in healthy females but not males

INTRODUCTION

The neurotransmitter dopamine has frequently been implicated in reward processing but is also, increasingly, implicated in punishment processing. We have previously shown that both patients with Parkinson's disease and healthy individuals with low dopamine (DA) synthesis are better at reversal learning based on punishment than reward. Here, we extend these prior findings by examining the effects of artificially reducing DA synthesis in healthy individuals performing this previously employed task.

METHODS

In a double-blind, placebo-controlled crossover design, we applied the acute tyrosine and phenylalanine depletion (ATPD) procedure to reduce global DA synthesis in 15 female and 14 male subjects. Each subject performed the reward- and punishment-based reversal-learning paradigm.

RESULTS

There was a significant three-way interaction between ATPD, the valence of the outcome signalling reversal and the gender of the participants. Examination of punishment and reward-based reversals separately revealed that this was driven by a significant improvement in punishment processing in female but not male subjects following DA depletion.

CONCLUSIONS

Reducing DA synthesis in healthy individuals shifted sensitivity of performance from reward to punishment processing. Gender differences in DA synthesis might underlie the selectivity of this effect to female subjects. Such gender biases may go some way towards explaining the gender biases in certain psychiatric disorders such as depression and Parkinson's disease.

Cocaine craving and use during daily life

RATIONALE

Craving is often assumed to cause ongoing drug use and relapse and is a major focus of addiction research. However, its relationship to drug use has not been adequately documented.

OBJECTIVES

The aim of this study was to investigate the relationship between craving and drug use in real time and in the daily living environments of drug users.

METHODS

In a prospective, longitudinal, cohort design (ecological momentary assessment), 112 cocaine-abusing individuals in methadone maintenance treatment rated their craving and mood at random times (two to five times daily, prompted by electronic diaries) as they went about their everyday activities. They also initiated an electronic diary entry each time they used cocaine. Drug use was monitored by thrice-weekly urine testing.

RESULTS

During periods of urine-verified cocaine use, ratings of cocaine craving increased across the day and were higher than during periods of urine-verified abstinence. During the 5 h prior to cocaine use, ratings of craving significantly increased. These patterns were not seen in ratings of heroin craving or mood (e.g., feeling happy or bored).

CONCLUSIONS

Cocaine craving is tightly coupled to cocaine use in users' normal environments. Our findings provide previously unavailable support for a relationship that has been seriously questioned in some theoretical accounts. We discuss what steps will be needed to determine whether craving causes use.

Review. Psychological and neural mechanisms of relapse

Relapse, the resumption of drug taking after periods of abstinence, remains the major problem for the treatment of addiction. Even when drugs are unavailable for long periods or when users are successful in curbing their drug use for extended periods, individuals remain vulnerable to events that precipitate relapse. Behavioural studies in humans and laboratory animals show that drug-related stimuli, drugs themselves and stressors are powerful events for the precipitation of relapse. Molecular, neurochemical and anatomical studies have identified lasting neural changes that arise from mere exposure to drugs and other enduring changes that arise from learning about the relationship between drug-related stimuli and drug effects. Chronic drug exposure increases sensitivity of some systems of the brain to the effects of drugs and stressful events. These changes, combined with those underlying conditioning and learning, perpetuate vulnerability to drug-related stimuli. Circuits of the brain involved are those of the mesocorticolimbic dopaminergic system and its glutamatergic connections, and the corticotropin-releasing factor and noradrenergic systems of the limbic brain. This paper reviews advances in our understanding of how these systems mediate the effects of events that precipitate relapse and of how lasting changes in these systems can perpetuate vulnerability to relapse.

Affective neuroscience of pleasure: reward in humans and animals

INTRODUCTION

Pleasure and reward are generated by brain circuits that are largely shared between humans and other animals.

DISCUSSION

Here, we survey some fundamental topics regarding pleasure mechanisms and explicitly compare humans and animals.

CONCLUSION

Topics surveyed include liking, wanting, and learning components of reward; brain coding versus brain causing of reward; subjective pleasure versus objective hedonic reactions; roles of orbitofrontal cortex and related cortex regions; subcortical hedonic hotspots for pleasure generation; reappraisals of dopamine and pleasure-electrode controversies; and the relation of pleasure to happiness.

Conditioned cues and the expression of stimulant sensitization in animals and humans

Repeated intermittent exposure to psychostimulants can lead to long-lasting sensitization of the drugs' behavioral and biochemical effects. Such findings have figured importantly in recent theories of drug addiction proposing that sensitized nucleus accumbens (NAcc) dopamine (DA) overflow in particular acts in concert with other alterations in the neurochemistry of this nucleus to promote drug seeking and self-administration. Yet, experiments in rodents, non-human primates and humans have not always detected behavioral or biochemical sensitization following drug exposure, bringing into doubt the utility of this model. In an effort to reconcile apparent discrepancies in the literature, this review assesses conditions that might affect the expression of sensitization during testing. Specifically, the role played by conditioned cues is reviewed. A number of reports strongly support a potent and critical role for conditioned stimuli in the expression of sensitization. Findings suggest that stimuli associated either with the presence or absence of drug can respectively facilitate or inhibit sensitized responding. It is concluded that the presence or absence of such stimuli during testing for sensitization in animal and human studies could significantly affect the results obtained. It is necessary to consider this possibility especially when interpreting the results of studies that fail to observe sensitized responding.

The role of dopamine in alcohol self-administration in humans: individual differences

OBJECTIVE

To clarify dopamine's role in alcohol self-administration in a heterogeneous sample of drinkers using acute phenylalanine/tyrosine depletion (APTD).

METHODS

Sixteen men with variable drinking histories were characterized on their ethanol-induced cardiac response, a marker previously proposed to index dopamine system reactivity and vulnerability to alcohol abuse. During separate sessions participants were administered (i) a nutritionally balanced (BAL) amino acid (AA) mixture, (ii) a mixture lacking the dopamine precursors, phenylalanine and tyrosine, and (iii) APTD followed by the dopamine precursor, L-DOPA. Five hours after AA administration, participants could earn units of alcohol using a progressive ratio breakpoint task.

RESULTS

Alcohol self-administration was reduced in the APTD and APTD+L-DOPA conditions relative to the BAL condition. In both cases the changes were predicted by ethanol-induced cardiac change.

CONCLUSIONS

The motivation to drink is likely regulated by more than one neurobiological mechanism. Individual differences in cardiac responsivity to ethanol might provide a peripheral marker of responsiveness to pharmacological manipulations of dopamine.

The acetylcholinesterase inhibitor rivastigmine does not alter total choices for methamphetamine, but may reduce positive subjective effects, in a laboratory model of intravenous self-administration in human volunteers

A human laboratory model of intravenous methamphetamine self-administration may facilitate study of putative treatments for methamphetamine addiction. We conducted a double-blind, placebo-controlled, between groups investigation of the acetylcholinesterase (AChE) inhibitor rivastigmine in non-treatment-seeking volunteers who met criteria for methamphetamine abuse or dependence. Safety and subjective effects data derived from days 1-10 of this protocol are described in a separate publication. In this report, we describe self-administration outcomes in participants randomized to treatment with rivastigmine (0 mg, N=7; 1.5 mg, N=6; 3 mg, N=9); data that were collected on days 11-15 of the inpatient protocol. On day 11, participants sampled two infusions of methamphetamine (0 and 30 mg, i.v.). On days 12-15, participants made ten choices each day to receive an infusion of either methamphetamine (3 mg, IV) or saline or a monetary alternative ($0.05-$16). The study design allowed for evaluation of differences in behavior on days in which infusions were performed by the physician (experimenter-administered) versus by the participant using a PCA pump (self-administered), and when monetary alternatives were presented in either ascending or descending sequence. The data show that rivastigmine (1.5 and 3 mg), as compared to placebo, did not significantly alter total choices for methamphetamine (p=0.150). Importantly, the number of infusion choices was greater when methamphetamine was available then when saline was available (p<0.0001), and the number of money choices was greater when saline was available then when methamphetamine was available (p<0.0001). The total number of choices for methamphetamine was not altered as a function of a participant's preferred route of methamphetamine use (p=0.57), and did not differ significantly whether they were experimenter-administered or self-administered (p=0.30). In addition, total choices for methamphetamine were similar made when money was available in an ascending versus descending sequence (p=0.49). The participants' years of methamphetamine use, recent use of methamphetamine (in the past 30 days), or baseline craving (indexed here as "Desire") on the day of the self-administration task were not predictive of number of choices for methamphetamine. In a subset of participants (N=8) for which data was available, individual dose of methamphetamine (3 x 3 mg, i.v.) produced significant increases in positive subjective effects, and a preliminary analysis revealed that 3 mg rivastigmine was associated with reductions in these responses, as compared to placebo. In summary, the current report indicates that there were no effects of rivastigmine on total choices for methamphetamine, that there were low levels of methamphetamine self-administration but these were 8 times greater than saline, and that choice behavior was insensitive to alternative reinforcers. In addition, we showed that rivastigmine may reduce the positive subjective effects produced by methamphetamine during self-administration.