Behavioral and neural mechanisms of compulsive drug seeking

An animal model of compulsive food-taking behaviour

The increase in the incidence of obesity and eating disorders has promoted research aimed at understanding the aetiology of abnormal eating behaviours. Apart from metabolic factors, obesity is caused by overeating. Clinical reports have led to the suggestion that some individuals may develop addictive-like behaviours when consuming palatable foods, and compulsive eating plays a similar dominant role in obesity as compulsive drug taking does in drug addiction. The progress made in the development of treatment strategies for obesity is limited, in part, because the physiological and neurological causes and consequences of compulsive eating behaviour are not clearly understood and cannot readily be studied in human subjects. We have developed experimental approaches that reflect the functioning of the components of eating control, including compulsive food taking in rats. Rats that are given free choice between standard chow and a palatable, chocolate-containing 'Cafeteria Diet' (CD) develop distinct signs of compulsive food taking that appear at an early stage. These include the inability to adapt intake behaviour in periods of limited or bitter-tasting CD access, continued food intake during resting phases and changes in fine structure of feeding (duration, distribution and recurrence of feeding bouts). The model will help examine the neurobiological underpinnings of compulsive food seeking and food taking and provides a possibility to study the effects of novel anti-obesity compounds on compulsive eating and other components of food-taking behaviour in detail. For future use of genetic models, the possibility of a transfer to a mouse was discussed.

Neurobiology of apathy in Alzheimer's disease

Apathy is considered the most frequent neuropsychiatric disturbance in dementia and its outcome is generally deleterious. Apathy can be related to a dysfunction of the anatomical-system that supports the generation of voluntary actions, namely the prefrontal cortex and/or the prefrontal-subcortical circuits. In Alzheimer's disease, pathological and neuroimaging data indicate that apathy is likely due to a dysfunction of the medial prefrontal cortex. Accordingly, in this review article, we propose a pathophysiological model to explain apathetic behavior in Alzheimer's disease, combining data from neuroimaging, neuropathology and experimental research on the role of orbito-frontal cortex, anterior cingulate cortex, basal ganglia and dopamine in decision-making neurobiology.

Long-lasting sensitization of reward-directed behavior by amphetamine

Exposure to psychostimulant drugs of abuse such as amphetamine can result in long-lasting "sensitization" of reward-directed behavior, such that subjects display enhancements in behavior directed by and toward rewards and reward-predictive cues (i.e. "incentive sensitization"). The purpose of these experiments was to determine the degree to which such sensitization resulting from chronic amphetamine exposure influences both appetitive and consummatory food-motivated behavior. Adult male Long-Evans rats received daily i.p. injections of D-amphetamine (2.0 mg/kg) or saline vehicle for five consecutive days. This amphetamine exposure regimen produced lasting sensitization to the acute locomotor stimulant effect of the drug. One month after drug exposure rats were tested for instrumental responding (lever pressing) for food reward under various response schedules. Two months after drug exposure, rats were tested for food consumption in a discriminative Pavlovian context-potentiated eating task, involving pairings of one context with food and another context with no food. Amphetamine exposed rats showed significantly greater instrumental responding for food reward than saline controls, particularly under conditions of high response ratios. In the potentiated eating task, testing under conditions of food satiation revealed that amphetamine exposed rats ate significantly more than saline controls in the food-paired context. These experiments demonstrate that amphetamine exposure can cause enduring increases in both appetitive and consummatory aspects of natural reward-directed behavior. Such long-lasting incentive sensitization could account in part for the propensity for relapse in drug addiction, as well as for reported enhancements in non-drug reward-related behavior.

Long-term haloperidol treatment (but not risperidone) enhances addiction-related behaviors in mice: role of dopamine D2 receptors

The high prevalence of psychostimulant abuse observed in schizophrenic patients may be related to the development of mesolimbic dopaminergic supersensitivity (MDS) or nigrostriatal dopaminergic supersensitivity (NDS) in response to the chronic blockade of dopamine receptors produced by typical neuroleptic treatment. We compared the effects of withdrawal from long-term administration of the typical neuroleptic haloperidol (Hal) and/or the atypical agent risperidone (Ris) on MDS and NDS, behaviorally evaluated by amphetamine-induced locomotor stimulation (AILS) and apomorphine-induced stereotypy (AIS) in mice, respectively. We further evaluated the duration of MDS and investigated the specific role of dopamine D2 receptors in this phenomenon by administering the D2 agonist quinpirole (Quin) to mice withdrawn from long-term treatment with these neuroleptics. Withdrawal (48 hours) from long-term (20 days) Hal (0.5 mg/kg i.p.) (but not 0.5 mg/kg Ris i.p.) treatment potentiated both AILS and AIS. Ris co-administration abolished the potentiation of AILS and AIS observed in Hal-withdrawn mice. Ten days after withdrawal from long-term treatment with Hal (but not with Ris or Ris + Hal), a potentiation in AILS was still observed. Only Hal-withdrawn mice presented an attenuation of locomotor inhibition produced by Quin. Our data suggest that the atypical neuroleptic Ris has a pharmacological property that counteracts the compensatory MDS and NDS developed in response to the chronic blockade of dopamine receptors imposed by Ris itself or by typical neuroleptics such as Hal. They also indicate that MDS may be long lasting and suggest that an upregulation of dopamine D2 receptors in response to long-term treatment with the typical neuroleptic is involved in this phenomenon.

Development of pharmacotherapies for drug addiction: a Rosetta stone approach

Current pharmacotherapies for addiction represent opportunities for facilitating treatment and are forming a foundation for evaluating new medications. Furthermore, validated animal models of addiction and a surge in understanding of neurocircuitry and neuropharmacological mechanisms involved in the development and maintenance of addiction - such as the neuroadaptive changes that account for the transition to dependence and the vulnerability to relapse - have provided numerous potential therapeutic targets. Here, we emphasize a 'Rosetta Stone approach', whereby existing pharmacotherapies for addiction are used to validate and improve animal and human laboratory models to identify viable new treatment candidates. This approach will promote translational research and provide a heuristic framework for developing efficient and effective pharmacotherapies for addiction.

Withdrawal from free-choice high-fat high-sugar diet induces craving only in obesity-prone animals

INTRODUCTION

Vulnerability for weight gain is an individual trait. Obese people undertake dieting, but permanent weight loss is difficult to attain due to repeated phases of relapse to excess consumption.

MATERIALS AND METHODS

In this study, male Wistar rats were trained to operantly self-administer pellets followed by free-choice access in the homecage to high-fat high-sugar (HFHS) diet consisting of 30% sucrose, lard, standard rodent chow and water. Animals were divided into obesity-prone (OP) and obesity-resistant (OR) groups based on relative weight gain compared to normally fed controls despite equal consumption of HFHS.

RESULTS AND DISCUSSION

After 4 weeks of HFHS access, OP and OR animals did not differ in motivation for food pellets in terms of progressive ratio break point, lever pressing or response rate. However, upon discontinuation of the HFHS diet, differences between the OP and OR groups were noted. OP animals increased their motivation (i.e. craving) during the second withdrawal week and reduced time spent in the centre of an open field (increased anxiety) compared to the OR animals. Both OP and OR animals consumed less of the standard rodent chow during the first week of withdrawal when compared to normally fed controls. But, while the OR animals quickly returned to control levels of food consumption, OP animals continued to consume less standard rodent chow.

CONCLUSION

The results show for the first time that withdrawal from free-choice HFHS induces craving that is specific to the OP animals and suggests that OP individuals may have withdrawal symptoms that are similar to those induced by addictive drugs.

Which cue to "want?" Central amygdala opioid activation enhances and focuses incentive salience on a prepotent reward cue

The central nucleus of the amygdala (CeA) helps translate learning into motivation, and here, we show that opioid stimulation of CeA magnifies and focuses learned incentive salience onto a specific reward cue (pavlovian conditioned stimulus, or CS). This motivation enhancement makes that cue more attractive, noticeable, and liable to elicit appetitive and consummatory behaviors. To reveal the focusing of incentive salience, we exploited individual differences in an autoshaping paradigm in which a rat prefers to approach, nibble, and sniff one of two reward-associated stimuli (its prepotent stimulus). The individually prepotent cue is either a predictive CS+ that signals reward (8 s metal lever insertion) or instead the metal cup that delivers sucrose pellets (the reward source). Results indicated that CeA opioid activation by microinjection of the mu agonist DAMGO (0.1 microg) selectively and reversibly enhanced the attractiveness of whichever reward CS was that rat's prepotent cue. CeA DAMGO microinjections made rats more vigorously approach their particular prepotent CS and to energetically sniff and nibble it in a nearly frenzied consummatory manner. Only the prepotent cue was enhanced as an incentive target, and alternative cues were not enhanced. Conversely, inactivation of CeA by muscimol microinjection (0.25 microg) suppressed approach, nibbles, and sniffs of the prepotent CS. Confirming modulation of incentive salience, unconditioned food intake was similarly increased by DAMGO microinjection and decreased by muscimol in CeA. We conclude that opioid neurotransmission in CeA helps determine which environmental stimuli become most "wanted," and how "wanted" they become. This may powerfully guide reward-seeking behavior.

Brain activities associated with gaming urge of online gaming addiction

The aim of this study was to identify the neural substrates of online gaming addiction through evaluation of the brain areas associated with the cue-induced gaming urge. Ten participants with online gaming addiction and 10 control subjects without online gaming addiction were tested. They were presented with gaming pictures and the paired mosaic pictures while undergoing functional magnetic resonance imaging (fMRI) scanning. The contrast in blood-oxygen-level dependent (BOLD) signals when viewing gaming pictures and when viewing mosaic pictures was calculated with the SPM2 software to evaluate the brain activations. Right orbitofrontal cortex, right nucleus accumbens, bilateral anterior cingulate and medial frontal cortex, right dorsolateral prefrontal cortex, and right caudate nucleus were activated in the addicted group in contrast to the control group. The activation of the region-of-interest (ROI) defined by the above brain areas was positively correlated with self-reported gaming urge and recalling of gaming experience provoked by the WOW pictures. The results demonstrate that the neural substrate of cue-induced gaming urge/craving in online gaming addiction is similar to that of the cue-induced craving in substance dependence. The above-mentioned brain regions have been reported to contribute to the craving in substance dependence, and here we show that the same areas were involved in online gaming urge/craving. Thus, the results suggest that the gaming urge/craving in online gaming addiction and craving in substance dependence might share the same neurobiological mechanism.

The Roman high- and low-avoidance rat lines differ in the acquisition, maintenance, extinction, and reinstatement of intravenous cocaine self-administration

The selective breeding of Roman high- (RHA) and low-avoidance (RLA) rats for, respectively, rapid vs extremely poor acquisition of avoidant behavior in a shuttlebox has produced two phenotypes that differ in temperament traits, in mesocortical/mesolimbic dopamine system function, and in the behavioral and neurochemical responses to the acute and repeated administration of psychostimulants and opiates. The phenotypic traits of the RHA line predict higher susceptibility, compared with RLA rats, to the reinforcing properties of addictive substances like cocaine. The present study was designed to compare the acquisition, maintenance, reinstatement of drug-seeking after long-term extinction, and reacquisition of intravenous cocaine self-administration (SA) behavior in the Roman lines. Compared with RLA rats, the rates of responding during cocaine SA acquisition were higher, extinction from cocaine SA was prolonged, and drug-induced reinstatement of cocaine-seeking behavior was more robust in RHA rats. Moreover, only RHA rats reacquired extinguished lever-pressing activity when a low reinforcing dose of cocaine was available. These findings are consistent with the view that subjects with genetically determined high responsiveness to the acute and chronic (ie, sensitizing) effects of psychostimulants, such as RHA rats, also display a higher propensity to self -administer cocaine. Further comparative studies in the Roman lines, using SA paradigms that distinguish mere drug-taking from the compulsive and uncontrolled drug use that characterizes addiction in humans, may eventually help to characterize the relationships among genotype, temperament traits, and neurobiological mechanisms involved in the individual vulnerability to cocaine addiction.

Augmentation of morphine-induced sensitization but reduction in morphine tolerance and reward in delta-opioid receptor knockout mice

Studies in experimental animals have shown that individuals exhibiting enhanced sensitivity to the locomotor-activating and rewarding properties of drugs of abuse are at increased risk for the development of compulsive drug-seeking behavior. The purpose of the present study was to assess the effect of constitutive deletion of delta-opioid receptors (DOPr) on the rewarding properties of morphine as well as on the development of sensitization and tolerance to the locomotor-activating effects of morphine. Locomotor activity testing revealed that mice lacking DOPr exhibit an augmentation of context-dependent sensitization following repeated, alternate injections of morphine (20 mg/kg; s.c.; 5 days). In contrast, the development of tolerance to the locomotor-activating effects of morphine following chronic morphine administration (morphine pellet: 25 mg: 3 days) is reduced relative to WT mice. The conditioned rewarding effects of morphine were reduced significantly in DOPrKO mice as compared to WT controls. Similar findings were obtained in response to pharmacological inactivation of DOPr in WT mice, indicating that observed effects are not due to developmental adaptations that occur as a consequence of constitutive deletion of DOPr. Together, these findings indicate that the endogenous DOPr system is recruited in response to both repeated and chronic morphine administration and that this recruitment serves an essential function in the development of tolerance, behavioral sensitization, and the conditioning of opiate reward. Importantly, they demonstrate that DOPr has a distinct role in the development of each of these drug-induced adaptations. The anti-rewarding and tolerance-reducing properties of DOPr antagonists may offer new opportunities for the treatment and prevention of opioid dependence as well as for the development of effective analgesics with reduced abuse liability.

Cocaine activates Homer1 immediate early gene transcription in the mesocorticolimbic circuit: differential regulation by dopamine and glutamate signaling

Homer proteins are intracellular scaffolding proteins that, among glutamate receptors, selectively bind to group1 metabotropic glutamate receptors and regulate their trafficking and intracellular signaling. Homer proteins have been implicated in synaptic and behavioral plasticity, including drug-seeking behavior after cocaine treatment. Homer1 gene activation leads to transcription of a variant mRNA (Homer1a), which functions as an immediate early gene. Homer1a competes with the constitutive Homer proteins (Homer1b/c/d, Homer2a/b, Homer3) for binding to group1 metabotropic glutamate and IP3 receptors. Binding of Homer1a to these proteins disrupts their association with the intracellular signaling scaffold and modulates receptor function. In this study, using RT-PCR, activation of Homer1a mRNA transcription in response to acute and repeated administration of cocaine was characterized in prefrontal cortex, nucleus accumbens, and ventral tegmental area, three mesocorticolimbic nuclei of the rat brain. Moreover, the dopaminergic and glutamatergic regulation of Homer1 gene activation by cocaine was investigated. Acute cocaine rapidly and transiently activated transcription of Homer1a mRNA in all three nuclei. However, repeated administration of cocaine was not effective in inducing the Homer1a mRNA transcription after various withdrawal times ranging from 2 h to 3 weeks. The acute cocaine-mediated activation of Homer1 gene was regulated by D1 but not D2 dopamine receptors. The blockade of AMPA or NMDA glutamate receptors did not prevent cocaine-mediated activation of Homer1 gene in the three mesocorticolimbic nuclei. These data indicate that acute administration of cocaine transiently activates Homer1 gene producing the immediate early gene Homer1a mRNA in the three mesocorticolimbic nuclei of the rat brain. Activation of Homer1 gene may contribute to the cocaine-mediated synaptic and behavioral plasticity.

Stress, alcohol and drug interaction: an update of human research

A challenging question that continues unanswered in the field of addiction is why some individuals are more vulnerable to substance use disorders than others. Numerous risk factors for alcohol and other drugs of abuse, including exposure to various forms of stress, have been identified in clinical studies. However, the neurobiological mechanisms that underlie this relationship remain unclear. Critical neurotransmitters, hormones and neurobiological sites have been recognized, which may provide the substrates that convey individual differences in vulnerability to addiction. With the advent of more sophisticated measures of brain function in humans, such as functional imaging technology, the mechanisms and neural pathways involved in the interactions between drugs of abuse, the mesocorticolimbic dopamine system and stress systems are beginning to be characterized. This review provides a neuroadaptive perspective regarding the role of the hormonal and brain stress systems in drug addiction with a focus on the changes that occur during the transition from occasional drug use to drug dependence. We also review factors that contribute to different levels of hormonal/brain stress activation, which has implications for understanding individual vulnerability to drug dependence. Ultimately, these efforts may improve our chances of designing treatment strategies that target addiction at the core of the disorder.

Locomotor sensitization to cocaine is associated with distinct pattern of glutamate receptor trafficking to the postsynaptic density in prefrontal cortex: early versus late withdrawal effects

Glutamatergic neurotransmission plays an important role in the behavioral and molecular plasticity observed in cocaine mediated locomotor sensitization. Recent studies show that glutamatergic signaling is regulated by receptor trafficking, synaptic localization, and association with scaffolding proteins. The trafficking of the glutamate receptors was investigated in the dorsal and ventral prefrontal cortex at 1 and 21 days after repeated cocaine administration which produced robust locomotor sensitization. A subcellular fractionation technique was used to isolate the cellular synaptosomal fraction containing the postsynaptic density. At early withdrawal, the prefrontal cortex displayed a reduction in the synaptosomal content of the AMPA and NMDA receptor subunits. In contrast, after extended withdrawal, there was a significant increase in the trafficking of the receptors into the synaptosomal compartment. These changes were accompanied by corresponding trafficking of the postsynaptic glutamatergic scaffolding proteins. Thus, enhanced trafficking of glutamate receptors from cytosolic to synaptosomal compartment is associated with prolonged withdrawal from repeated exposure to cocaine and may have functional consequences for the synaptic and behavioral plasticity.

Overlapping neuronal circuits in addiction and obesity: evidence of systems pathology

Drugs and food exert their reinforcing effects in part by increasing dopamine (DA) in limbic regions, which has generated interest in understanding how drug abuse/addiction relates to obesity. Here, we integrate findings from positron emission tomography imaging studies on DA's role in drug abuse/addiction and in obesity and propose a common model for these two conditions. Both in abuse/addiction and in obesity, there is an enhanced value of one type of reinforcer (drugs and food, respectively) at the expense of other reinforcers, which is a consequence of conditioned learning and resetting of reward thresholds secondary to repeated stimulation by drugs (abuse/addiction) and by large quantities of palatable food (obesity) in vulnerable individuals (i.e. genetic factors). In this model, during exposure to the reinforcer or to conditioned cues, the expected reward (processed by memory circuits) overactivates the reward and motivation circuits while inhibiting the cognitive control circuit, resulting in an inability to inhibit the drive to consume the drug or food despite attempts to do so. These neuronal circuits, which are modulated by DA, interact with one another so that disruption in one circuit can be buffered by another, which highlights the need of multiprong approaches in the treatment of addiction and obesity.

DECISION UTILITY, THE BRAIN, AND PURSUIT OF HEDONIC GOALS

How do brain representations of the utility of a hedonic goal guide decisions about whether to pursue it? Our focus here will be on brain mechanisms of reward utility operating at particular decision moments in life. Moments such as when you encounter an image, sound, scent or other cue associated in your past with a particular reward; or perhaps just vividly imagine that cue. Such a cue can often trigger a sudden motivational urge to pursue that goal, and sometimes a decision to do so. In drug addicts trying to quit, a cue for the addicted drug might trigger urges that rise to compulsive levels of intensity, despite prior commitments to abstain, leading to the decision to relapse into taking the drug again. Normal or addicted, the urge and decision may well have been lacking immediately before the cue was encountered. The decision to pursue the cued reward might never have happened if the cue had not been encountered. Why can such cues momentarily dominate decision making? The answer involves brain mesolimbic dopamine mechanisms that amplify the incentive salience of reward cues, selectively elevating decision utility to trigger "wanting" for the goal. We describe affective neuroscience studies of brain limbic generators of "wanting" that shed light on how cues trigger pursuit of their goals, both normally and even under intense conditions of irrational goal pursuit.

Context-specific reversal of cocaine sensitization by the CB1 cannabinoid receptor antagonist rimonabant

The CB(1) cannabinoid receptor is implicated in the rewarding properties of many drugs of abuse, including cocaine. While CB(1) receptor involvement in the acute rewarding properties of cocaine is controversial, CB(1) antagonists such as SR141716 (rimonabant) have clearly been found to prevent cue- and cocaine-elicited reinstatement of cocaine self-administration in rodents. Here we demonstrate the novel involvement of CB(1) receptors in the maintenance of behavioral sensitization to cocaine in C57BL/6 mice. Consistent with previous reports, the induction of locomotor sensitization following repeated daily cocaine was not prevented by systemic pretreatment of either rimonabant, Delta(9)-tetrahydrocannabinol (THC), or a 1:1 mixture of THC and cannabidiol (CBD). In contrast, established cocaine sensitization was markedly disrupted following subchronic treatment with rimonabant alone. This effect was notably context-dependent, in that rimonabant did not diminish established cocaine sensitization if delivered in the home cage, but only if the rimonabant-injected mice were exposed to activity chambers previously paired with cocaine. These findings are consistent with CB(1) receptor involvement in conditioned cocaine-seeking behaviors, and further suggest that endocannabinoid (eCB)-mediated synaptic plasticity may act specifically within drug-paired environments to maintain cocaine-directed behavioral responses.

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.

Neural substrates of cognitive inflexibility after chronic cocaine exposure

Cognitive changes in addicts and animals exposed to addictive drugs have been extensively investigated over the past decades. One advantage of studying addiction using cognitive paradigms is that neural processing in addicts or drug-exposed animals can be compared to that in normal subjects. Tests of cognitive flexibility that measure the ability to change responding to a previously rewarded or punished stimulus are of potential interest in the study of addiction, because addiction can itself be viewed as an inability to change responding to stimuli previously associated with drug reward. One such test is reversal learning, which is impaired in cocaine addicts and animals that have chronically self-administered or been exposed to cocaine. A circuit including orbitofrontal cortex, basolateral amygdala and striatum subserves reversal learning. In rats that have been previously exposed to cocaine, neurons in these regions show selective and distinct changes in how they encode information during reversal learning. These changes suggest that in these rats, orbitofrontal cortex loses the ability to signal expected outcomes, and basolateral amygdala becomes inflexible in its encoding of cue significance. These changes could explain cocaine-induced impairments to cognitive flexibility and may have theoretical importance in addiction.

Beta-endorphin and drug-induced reward and reinforcement

Although drugs of abuse have different acute mechanisms of action, their brain pathways of reward exhibit common functional effects upon both acute and chronic administration. Long known for its analgesic effect, the opioid beta-endorphin is now shown to induce euphoria, and to have rewarding and reinforcing properties. In this review, we will summarize the present neurobiological and behavioral evidences that support involvement of beta-endorphin in drug-induced reward and reinforcement. Currently, evidence supports a prominent role for beta-endorphin in the reward pathways of cocaine and alcohol. The existing information indicating the importance of beta-endorphin neurotransmission in mediating the reward pathways of nicotine and THC, is thus far circumstantial. The studies described herein employed diverse techniques, such as biochemical measurements of beta-endorphin in various brain sites and plasma, and behavioral measurements, conducted following elimination (via administration of anti-beta-endorphin antibodies or using mutant mice) or augmentation (by intracerebral administration) of beta-endorphin. We suggest that the reward pathways for different addictive drugs converge to a common pathway in which beta-endorphin is a modulating element. Beta-endorphin is involved also with distress. However, reviewing the data collected so far implies a discrete role, beyond that of a stress response, for beta-endorphin in mediating the substance of abuse reward pathway. This may occur via interacting with the mesolimbic dopaminergic system and also by its interesting effects on learning and memory. The functional meaning of beta-endorphin in the process of drug-seeking behavior is discussed.

Imaging dopamine's role in drug abuse and addiction

Dopamine is involved in drug reinforcement but its role in addiction is less clear. Here we describe PET imaging studies that investigate dopamine's involvement in drug abuse in the human brain. In humans the reinforcing effects of drugs are associated with large and fast increases in extracellular dopamine, which mimic those induced by physiological dopamine cell firing but are more intense and protracted. Since dopamine cells fire in response to salient stimuli, supraphysiological activation by drugs is experienced as highly salient (driving attention, arousal, conditioned learning and motivation) and with repeated drug use may raise the thresholds required for dopamine cell activation and signaling. Indeed, imaging studies show that drug abusers have marked decreases in dopamine D2 receptors and in dopamine release. This decrease in dopamine function is associated with reduced regional activity in orbitofrontal cortex (involved in salience attribution; its disruption results in compulsive behaviors), cingulate gyrus (involved in inhibitory control; its disruption results in impulsivity) and dorsolateral prefrontal cortex (involved in executive function; its disruption results in impaired regulation of intentional actions). In parallel, conditioning triggered by drugs leads to enhanced dopamine signaling when exposed to conditioned cues, which then drives the motivation to procure the drug in part by activation of prefrontal and striatal regions. These findings implicate deficits in dopamine activity-inked with prefrontal and striatal deregulation-in the loss of control and compulsive drug intake that results when the addicted person takes the drugs or is exposed to conditioned cues. The decreased dopamine function in addicted individuals also reduces their sensitivity to natural reinforcers. Therapeutic interventions aimed at restoring brain dopaminergic tone and activity of cortical projection regions could improve prefrontal function, enhance inhibitory control and interfere with impulsivity and compulsive drug administration while helping to motivate the addicted person to engage in non-drug related behaviors.

Dissociation of alcohol-seeking and consumption under a chained schedule of oral alcohol reinforcement in baboons

BACKGROUND

Initiation and maintenance of compulsive alcohol drinking involves a sequence of behaviors which occur in the presence of environmental cues. Animal models using chained schedules of alcohol reinforcement may be useful for examining the complex interactions between cues and alcohol-seeking and -consumption.

METHODS

Four baboons self-administered alcohol under a 3-component chained schedule of reinforcement; distinct cues were presented in the context of different behavioral contingencies associated with gaining access to 4% w/v alcohol (alcohol-seeking) and concluding with alcohol self-administration. First, the response strength of alcohol-related seeking responses was evaluated using a between-sessions progressive ratio (PR) procedure in which the response requirement to initiate the final contingency and gain access to the daily supply of alcohol was increased each session. The highest response requirement completed that resulted in alcohol access was defined as the breaking point (BP). Second, water was substituted for alcohol and PR procedures were repeated. The effects of increasing the "seeking" response requirement on subsequent alcohol or water consumption were also determined.

RESULTS

When alcohol was available, operant responses to gain access to and self-administer alcohol were maintained. When water was substituted for alcohol, alcohol-related cues continued to maintain alcohol-seeking responses. However, higher BPs, higher rates of self-administration and higher volumes of intake occurred when alcohol was available compared with water. Increasing the response requirement to gain access to alcohol did not reduce alcohol consumption (total alcohol intake).

CONCLUSIONS

These results show that alcohol-related cues maintained alcohol-seeking even after a prolonged period of only water availability. Cue-maintained alcohol-seeking behavior can be dissociated from subsequent alcohol consumption.

Cocaine-induced reinstatement in rats: evidence for a critical role of cocaine stimulus properties

RATIONALE

The exact behavioral nature of drug-induced reinstatement of drug seeking is still debated. As an incentive, the drug can have general facilitatory influences on appetitive behaviors. As an interoceptive stimulus, the drug can acquire discriminative properties and control behavior.

OBJECTIVE

This study assessed the relative contribution of the incentive versus discriminative properties of cocaine in food-seeking reinstatement.

METHODS

In Experiment 1, eight groups of rats were trained to press a lever for food pellets and experienced cocaine (0, 5, 10, or 15 mg/kg; i.p.), either during the operant conditioning sessions or 4 h after, in another environment without food access. In Experiment 2, to dissociate the role of the operant response per se from the consummatory response, two groups of rats experienced food consumption under cocaine (10 mg/kg; i.p.) either during operant conditioning sessions or during alternate sessions of free access to the food. Then, for both experiments, food pellets were withheld and cocaine injections ceased (extinction). The reinstating effects of noncontingent cocaine (10 mg/kg; i.p.) and food pellet delivery were assessed. Locomotor activity was recorded to probe expression of behavioral sensitization.

RESULTS

Cocaine reinstated lever pressing only in rats having previously performed the operant responses under cocaine. In contrast, food pellet delivery reinstated lever pressing independently of rats' history with cocaine. Locomotor sensitization was evidenced for all cocaine-pre-exposed rats, dissociating sensitization from reinstatement.

CONCLUSIONS

When present during operant conditioning, the stimulus "cocaine" acquires conditioned properties which can then promote reinstatement of the extinguished behavior.

Level of operant training rather than cocaine intake predicts level of reinstatement

RATIONALE

Extended cocaine self-administration has been shown to potentiate reinstatement. This increased vulnerability to relapse could be attributed not only to extended cocaine exposure but also to extended operant training.

OBJECTIVE

This study was aimed at determining the influence of different operant training histories on cocaine-induced reinstatement when cocaine intake is kept constant.

MATERIALS AND METHODS

Cocaine intake and operant training were dissociated by using experimental procedures generating different histories of operant training but almost identical histories of cocaine intake. Rats were first trained to self-administer cocaine at a classical unit dose (250 microg/inf, FR1), then in independent groups, the level of operant response was changed for the next 20 sessions by changing either the unit dose available (83, 250, or 750 microg/inf, Experiment 1) or the fixed ratio required (FR-1, FR-3, or FR-10, Experiment 2). Then, all rats were tested for reinstatement with different priming doses of cocaine (0, 5, 10, and 15 mg/kg; i.p.) at an early and late stage of an extinction period.

RESULTS

Level of responding during training predicts the level of reinstatement later on, independently of the amount of cocaine consumed. High FR requirement and low unit dose access led to higher level of reinstatement at early and late stage of the extinction period, respectively.

CONCLUSIONS

This study shows that the level of operant responding required to maintain optimal cocaine intake directly influences later levels of reinstatement. This finding suggests that environmental constrains that make drug-taking demanding and effortful may increase the vulnerability to relapse.

Cocaine seeking habits depend upon dopamine-dependent serial connectivity linking the ventral with the dorsal striatum

A neuroanatomical principle of striatal organization has been established through which ventral domains, including the nucleus accumbens, exert control over dorsal striatal processes mediated by so-called "spiraling," striato-nigro-striatal, circuitry. We have investigated the functional significance of this circuitry in the control over a cocaine-seeking habit by using an intrastriatal disconnection procedure that combined a selective, unilateral lesion of the nucleus accumbens core and infusion of a dopamine receptor antagonist into the contralateral dorsolateral striatum, thereby disrupting striato-midbrain-striatal serial connectivity bilaterally. We show that this disconnection selectively decreased drug-seeking behavior in rats extensively trained under a second-order schedule of cocaine reinforcement. These data thereby define the importance of interactions between ventral and dorsal domains of the striatum, mediated by dopaminergic transmission, in the neural mechanisms underlying the development and performance of cocaine-seeking habits that are a key characteristic of drug addiction.

Hedonic Homeostatic Dysregulation as a Driver of Drug-Seeking Behavior

Drug addiction can be defined by a compulsion to seek and take drug and loss of control in limiting intake, and the excessive drug taking derives from multiple motivational mechanisms. One such mechanism is the emergence of a negative emotional state when access to the drug is prevented, reflecting hedonic homeostatic dysregulation. Excessive drug taking then results in part via the construct of negative reinforcement. The negative emotional state that drives such negative reinforcement is hypothesized to derive from dysregulation of key neurochemical elements involved in reward and stress within basal forebrain structures, including the ventral striatum and extended amygdala. Specific neurochemical elements in these structures include not only decreases in reward neurotransmission, such as decreases in dopamine and opioid peptide function in the ventral striatum, but also recruitment of brain stress systems, such as corticotropin-releasing factor (CRF), in the extended amygdala. Chronic exposure or extended access to self-administration of all major drugs of abuse produces during abstinence increases in reward thresholds, increases in aversive anxiety-like responses, increases in extracellular levels of CRF in the central nucleus of the amygdala, and increases in drug self-administration. CRF receptor antagonists block excessive drug intake produced by dependence. A combination of decreased reward system function and increased brain stress response system function is hypothesized to be responsible for hedonic homeostatic dysregulation that drives drug seeking behavior in dependence. Such hedonic dysregulation is hypothesized to extend into protracted abstinence to provide a residual negative emotional state that enhances the salience of cues eliciting drug seeking and relapse.

Neural correlates of inflexible behavior in the orbitofrontal-amygdalar circuit after cocaine exposure

Addiction is characterized by compulsive or inflexible behavior, observed both in the context of drug-seeking and in contexts unrelated to drugs. One possible contributor to these inflexible behaviors may be drug-induced dysfunction within circuits that support behavioral flexibility, including the basolateral amygdala (ABL) and the orbitofrontal cortex (OFC). Here we describe data demonstrating that chronic cocaine exposure causes long-lasting changes in encoding properties in the ABL and the OFC during learning and reversal in an odor-guided task. In particular, these data suggest that inflexible encoding in ABL neurons may be the proximal cause of cocaine-induced behavioral inflexibility, and that a loss of outcome-expectant encoding in OFC neurons could be a more distal contributor to this impairment. A similar mechanism of drug-induced orbitofrontal-amygdalar dysfunction may cause inflexible behavior when animals and addicts are exposed to drug-associated cues and contexts.

Dynorphin and the pathophysiology of drug addiction

Drug addiction is a chronic relapsing disease in which drug administration becomes the primary stimulus that drives behavior regardless of the adverse consequence that may ensue. As drug use becomes more compulsive, motivation for natural rewards that normally drive behavior decreases. The discontinuation of drug use is associated with somatic signs of withdrawal, dysphoria, anxiety, and anhedonia. These consequences of drug use are thought to contribute to the maintenance of drug use and to the reinstatement of compulsive drug use that occurs during the early phase of abstinence. Even, however, after prolonged periods of abstinence, 80-90% of human addicts relapse to addiction, suggesting that repeated drug use produces enduring changes in brain circuits that subserve incentive motivation and stimulus-response (habit) learning. A major goal of addiction research is the identification of the neural mechanisms by which drugs of abuse produce these effects. This article will review data showing that the dynorphin/kappa-opioid receptor (KOPr) system serves an essential function in opposing alterations in behavior and brain neurochemistry that occur as a consequence of repeated drug use and that aberrant activity of this system may not only contribute to the dysregulation of behavior that characterizes addiction but to individual differences in vulnerability to the pharmacological actions of cocaine and alcohol. We will provide evidence that the repeated administration of cocaine and alcohol up-regulates the dynorphin/KOPr system and that pharmacological treatments that target this system may prove effective in the treatment of drug addiction.

Some conceptual problems with the classical theory of behaviour

Behaviour is usually assumed to depend on the reach of a critical intensity--termed reactivity threshold--by its motivation. This view represents a simple, predictive theoretical framework in ethology and animal psychology. However, it is here argued that only the influence of an isolated motivation on behaviour can be explained that way; that such a view fails to account for behaviour when several motivations are jointly activated. Upon analysis, the classical theory of behaviour (CTB) proves to be under-specified and thus leads to three conceptual problems that make it logically inconsistent for the study of multiple motivations. A revision of the CTB, called anticipatory dynamics model (ADM), is then developed in order to bring a theoretical solution to these conceptual problems. The ADM hypothesizes that an organism's motivational interactions are due to the limitation of the organism's attentional resources.

Evidence for sugar addiction: behavioral and neurochemical effects of intermittent, excessive sugar intake

[Avena, N.M., Rada, P., Hoebel B.G., 2007. Evidence for sugar addiction: Behavioral and neurochemical effects of intermittent, excessive sugar intake. Neuroscience and Biobehavioral Reviews XX(X), XXX-XXX]. The experimental question is whether or not sugar can be a substance of abuse and lead to a natural form of addiction. "Food addiction" seems plausible because brain pathways that evolved to respond to natural rewards are also activated by addictive drugs. Sugar is noteworthy as a substance that releases opioids and dopamine and thus might be expected to have addictive potential. This review summarizes evidence of sugar dependence in an animal model. Four components of addiction are analyzed. "Bingeing," "withdrawal," "craving" and "cross-sensitization" are each given operational definitions and demonstrated behaviorally with sugar bingeing as the reinforcer. These behaviors are then related to neurochemical changes in the brain that also occur with addictive drugs. Neural adaptations include changes in dopamine and opioid receptor binding, enkephalin mRNA expression and dopamine and acetylcholine release in the nucleus accumbens. The evidence supports the hypothesis that under certain circumstances rats can become sugar dependent. This may translate to some human conditions as suggested by the literature on eating disorders and obesity.

Relations among functional systems in behavior analysis

This paper proposes that an organism's integrated repertoire of operant behavior has the status of a biological system, similar to other biological systems, like the nervous, cardiovascular, or immune systems. Evidence from a number of sources indicates that the distinctions between biological and behavioral events is often misleading, engendering counterproductive explanatory controversy. A good deal of what is viewed as biological (often thought to be inaccessible or hypothetical) can become publicly measurable variables using currently available and developing technologies. Moreover, such endogenous variables can serve as establishing operations, discriminative stimuli, conjoint mediating events, and maintaining consequences within a functional analysis of behavior and need not lead to reductionistic explanation. I suggest that explanatory misunderstandings often arise from conflating different levels of analysis and that behavior analysis can extend its reach by identifying variables operating within a functional analysis that also serve functions in other biological systems.

The debate over dopamine's role in reward: the case for incentive salience

INTRODUCTION

Debate continues over the precise causal contribution made by mesolimbic dopamine systems to reward. There are three competing explanatory categories: 'liking', learning, and 'wanting'. Does dopamine mostly mediate the hedonic impact of reward ('liking')? Does it instead mediate learned predictions of future reward, prediction error teaching signals and stamp in associative links (learning)? Or does dopamine motivate the pursuit of rewards by attributing incentive salience to reward-related stimuli ('wanting')? Each hypothesis is evaluated here, and it is suggested that the incentive salience or 'wanting' hypothesis of dopamine function may be consistent with more evidence than either learning or 'liking'. In brief, recent evidence indicates that dopamine is neither necessary nor sufficient to mediate changes in hedonic 'liking' for sensory pleasures. Other recent evidence indicates that dopamine is not needed for new learning, and not sufficient to directly mediate learning by causing teaching or prediction signals. By contrast, growing evidence indicates that dopamine does contribute causally to incentive salience. Dopamine appears necessary for normal 'wanting', and dopamine activation can be sufficient to enhance cue-triggered incentive salience. Drugs of abuse that promote dopamine signals short circuit and sensitize dynamic mesolimbic mechanisms that evolved to attribute incentive salience to rewards. Such drugs interact with incentive salience integrations of Pavlovian associative information with physiological state signals. That interaction sets the stage to cause compulsive 'wanting' in addiction, but also provides opportunities for experiments to disentangle 'wanting', 'liking', and learning hypotheses. Results from studies that exploited those opportunities are described here.

CONCLUSION

In short, dopamine's contribution appears to be chiefly to cause 'wanting' for hedonic rewards, more than 'liking' or learning for those rewards.

Behavioural assessment of drug reinforcement and addictive features in rodents: an overview

Some psychoactive drugs are abused because of their ability to act as reinforcers. As a consequence behavioural patterns (such as drug-seeking/drug-taking behaviours) are promoted that ensure further drug consumption. After prolonged drug self-administration, some individuals lose control over their behaviour so that these drug-seeking/taking behaviours become compulsive, pervading almost all life activities and precipitating the loss of social compatibility. Thus, the syndrome of addictive behaviour is qualitatively different from controlled drug consumption. Drug-induced reinforcement can be assessed directly in laboratory animals by either operant or non-operant self-administration methods, by classical conditioning-based paradigms such as conditioned place preference or sign tracking, by facilitation of intracranial electric self-stimulation, or, alternatively by drug-induced memory enhancement. In contrast, addiction cannot be modelled in animals, at least as a whole, within the constraints of the laboratory. However, various procedures have been proposed as possible rodent analogues of addiction's major elements including compulsive drug seeking, relapse, loss of control/impulsivity, and continued drug consumption despite negative consequences. This review provides an extensive overview and a critical evaluation of the methods currently used for studying drug-induced reinforcement as well as specific features of addictive behaviour. In addition, comic strips that illustrate behavioural methods used in the drug abuse field are provided given for free download under http://www.zi-mannheim/psychopharmacology.de.