Social dominance in monkeys: dopamine D2 receptors and cocaine self-administration

Apathy predicts hedonic but not craving response to cocaine

Cocaine-induced craving has been implicated in the maintenance of ongoing cocaine use and is presumed to be mediated by enhanced synaptic availability of monoamines, including dopamine. Apathy is a neuropsychiatric syndrome that is associated with hypodopaminergic functioning and is neurobiologically distinct from depression. Apathy has been observed to be prevalent during the initial phases of abstinence in cocaine-dependent individuals. In the current report, we sought to investigate the relationship between apathy, depression, and craving in response to an acute intravenous administration of cocaine. To this end, sixteen non-treatment seeking volunteers were evaluated. Following acute administration of cocaine (40 mg, IV), patients with low apathy scores exhibited increased craving, whereas patients with high apathy scores exhibited decreased craving. In addition, patients with high apathy scores exhibited increased ratings of the subjective measure of "High", suggesting that high apathy predicts a greater hedonic response in dependence. Self-reported ratings of depression did not account for the observed differences. The data reveal that cocaine-induced craving is not ubiquitous, and may not play a critical role in the maintenance of cocaine dependence. Overall, the findings suggest that apathy predicts hedonic but not craving response to cocaine.

Mapping dopamine D2/D3 receptor function using pharmacological magnetic resonance imaging

RATIONALE

Regulation of dopamine release and synthesis occurs via pre-synaptic dopamine (DA) D2/D3 autoreceptors (DARs). Mapping of DAR function in vivo is difficult and is usually best assessed using invasive measures of DA release, such as microdialysis at discrete sites. We wished to show that pharmacological magnetic resonance imaging (phMRI) may prove useful for this purpose.

OBJECTIVE

To demonstrate that the relative cerebral blood volume (rCBV) changes induced by amphetamine can be modulated by DA D2 receptor antagonists and agonists in a manner consistent with modulation of DAR function and to compare these effects with microdialysis.

METHODS

We used phMRI with iron oxide contrast agents to map changes in rCBV in response to an amphetamine challenge, pre-treatment and post-treatment with varying doses of the D2 antagonist eticlopride and the D2 agonist quinpirole. We also compared the effects of D2 antagonism using microdialysis measurements of DA release.

RESULTS

Antagonism of D2 receptors with eticlopride potentiated rCBV changes induced by amphetamine in the nucleus accumbens and caudate putamen in a dose-dependent manner. The amphetamine-induced increase in rCBV in the accumbens in animals pre-treated with eticlopride was paralleled by a similar percentage increase in DA release measured by means of microdialysis. Conversely, agonism of D2 receptors using quinpirole reduced amphetamine-induced rCBV changes in the caudate putamen and nucleus accumbens. The effects of both quinpirole and eticlopride on amphetamine-induced rCBV changes were largest in the nucleus accumbens.

CONCLUSIONS

These results suggest that phMRI may potentially prove useful to map DAR function non-invasively in multiple brain regions simultaneously.

Structural violence and schizophrenia

Despite clear evidence of a substantial biological basis to schizophrenia, there is also evidence that social, economic and political factors have considerable relevance to the clinical features, treatment and outcome of the illness. Individuals from lower socio-economic groups have an earlier age at first presentation and longer durations of untreated illness, both of which are associated with poor outcome. Individuals with schizophrenia are over-represented in the homeless population. Migration is associated with increased rates of mental illness, including schizophrenia, and this relationship appears to be mediated by psycho-social factors, including difficulties establishing social capital in smaller migrant groups. Individuals with schizophrenia are substantially over-represented amongst prison populations, and imprisonment increases the disability and stigma associated with mental illness, and impedes long-term recovery. The adverse effects of these social, economic and societal factors, along with the social stigma of mental illness, constitute a form of 'structural violence' which impairs access to psychiatric and social services and amplifies the effects of schizophrenia in the lives of sufferers. As a result of these over-arching social and economic factors, many individuals with schizophrenia are systematically excluded from full participation in civic and social life, and are constrained to live lives that are shaped by stigma, isolation, homelessness and denial of rights. There are urgent needs for (1) the development of enhanced aetiological models of schizophrenia, which elucidate the interactions between genetic risk and social environment, and can better inform bio-psycho-social approaches to treatment; (2) a renewal of emphasis on the United Nations' "Principles for the Protection of Persons with Mental Illness" and related legislative measures in individual countries; and (3) continued study and examination of the impact of social, economic and political structures on the clinical features and outcome of mental illnesses.

Conditioned place preference associates with the mRNA expression of diazepam binding inhibitor in brain regions of the addicted rat during withdrawal

The diazepam binding inhibitor (DBI) modulating the functions of the GABAA receptors is involved in the maladaptation of neural system during using opiate, but its role in opiate dependence is not fully understood. Using conditioned place preference (CPP) rat model and in situ hybridization technique, we examined the correlation between opiate dependence and the mRNA expression of DBI. We found that chronic morphine treatment enabled CPP and increased the DBI mRNA expression in crucial brain regions of addiction. Withdrawal for 3 days caused significant physical signs and further increased the DBI mRNA expression. Both the DBI mRNA and CPP expression remained significantly high but physical signs were at control level in the animals withdrawal for 6 days. Remarkably, the DBI mRNA expressions in the CA1 region of the hippocampus (CA1), ventral tagmental area (VTA), nucleus accumbens (NAc) and amygdala (AMG) were positively correlated to CPP during the periods from withdrawal for 3 days to withdrawal for 6 days. These findings suggest that DBI may play a role in both physical and psychological dependence of opiates.

The epidemiology, pathophysiology, and management of psychosocial risk factors in cardiac practice: the emerging field of behavioral cardiology

Observational studies indicate that psychologic factors strongly influence the course of coronary artery disease (CAD). In this review, we examine new epidemiologic evidence for the association between psychosocial risk factors and CAD, identify pathologic mechanisms that may be responsible for this association, and describe a paradigm for studying positive psychologic factors that may act as a buffer. Because psychosocial risk factors are highly prevalent and are associated with unhealthy lifestyles, we describe the potential role of cardiologists in managing such factors. Management approaches include routinely screening for psychosocial risk factors, referring patients with severe psychologic distress to behavioral specialists, and directly treating patients with milder forms of psychologic distress with brief targeted interventions. A number of behavioral interventions have been evaluated for their ability to reduce adverse cardiac events among patients presenting with psychosocial risk factors. Although the efficacy of stand-alone psychosocial interventions remains unclear, both exercise and multifactorial cardiac rehabilitation with psychosocial interventions have demonstrated a reduction in cardiac events. Furthermore, recent data suggest that psychopharmacologic interventions may also be effective. Despite these promising findings, clinical practice guidelines for managing psychosocial risk factors in cardiac practice are lacking. Thus, we review new approaches to improve the delivery of behavioral services and patient adherence to behavioral recommendations. These efforts are part of an emerging field of behavioral cardiology, which is based on the understanding that psychosocial and behavioral risk factors for CAD are not only highly interrelated, but also require a sophisticated health care delivery system to optimize their effectiveness.

Drug addiction: the neurobiology of behaviour gone awry

Drug addiction manifests as a compulsive drive to take a drug despite serious adverse consequences. This aberrant behaviour has traditionally been viewed as bad "choices" that are made voluntarily by the addict. However, recent studies have shown that repeated drug use leads to long-lasting changes in the brain that undermine voluntary control. This, combined with new knowledge of how environmental, genetic and developmental factors contribute to addiction, should bring about changes in our approach to the prevention and treatment of addiction.

PET imaging of striatal dopamine D2 receptors in nonhuman primates: Increases in availability produced by chronic raclopride treatment

Previous research using positron emission tomography (PET) in monkeys has shown that attaining social dominance can result in increased DA D2 receptor availability and attenuated sensitivity to the reinforcing effects of cocaine. The present study utilized a within-subjects design to determine whether chronic treatment with the D2 receptor antagonist raclopride could similarly increase D2 receptor availability. Using the D2-selective radioligand [(18)F]fluoroclebopride (FCP), three adult male cynomolgus monkeys were scanned before and after chronic treatment with raclopride (0.01 mg/kg per h for 30 +/- 1 day) administered by a subcutaneous osmotic pump. Food-reinforced operant behavior was assessed during treatment. A transitory decrease in responding was observed during the initial eight days of raclopride treatment. Tolerance developed by the tenth session, and responding remained at baseline levels for the duration of treatment and after treatment was discontinued. Averaged across monkeys, chronic raclopride administration increased FCP distribution volume ratios (DVRs) between 12 and 20% in the caudate nucleus, putamen, and anterior cingulate cortex. When monkeys were re-scanned 9-12 months after termination of raclopride treatment, FCP DVRs remained elevated in two subjects, and decreased below baseline levels in the third monkey. Considering the reported 2% test/retest variability for FCP, these findings indicate that chronic treatment with a D2 receptor antagonist can produce large increases in D2 receptor availability as measured with PET. Individual differences in rates of recovery were observed, such that the increases in DVR persisted in two of three subjects.

Potential for Unintended Consequences of Environmental Enrihment for Laboratory Animals and Research Results

Many aspects of the research animal's housing environment are controlled for quality and/or standardization. Of recent interest is the potential for environmental enrichment to have unexpected consequences such as unintended harm to the animal, or the introduction of variability into a study that may confound the experimental data. The effects of enrichment provided to nonhuman primates, rodents, and rabbits are described to illustrate that the effects can be numerous and may vary by strain and/or species. Examples of parameters measured where no change is detected are also included because this information provides an important counterpoint to studies that demonstrate an effect. In addition, this review of effects and noneffects serves as a reminder that the provision of enrichment should be evaluated in the context of the health of the animal and research goals on a case-by-case basis. It should also be kept in mind that the effects produced by enrichment are similar to those of other components of the animal's environment. Although it is unlikely that every possible environmental variable can be controlled both within and among research institutions, more detailed disclosure of the living environment of the subject animals in publications will allow for a better comparison of the findings and contribute to the broader knowledge base of the effects of enrichment.

Effects of the kappa opioid agonist U50,488 and the kappa opioid antagonist nor-binaltorphimine on choice between cocaine and food in rhesus monkeys

RATIONALE

Selective kappa opioid receptor agonists usually decrease cocaine self-administration in procedures that use rate-based measures of reinforcement; however, the rate-altering effects of kappa agonists complicate interpretation of these findings.

OBJECTIVES

To evaluate the effects of the selective kappa agonist U50,488 and the selective kappa antagonist nor-binaltorphimine (nor-BNI) on concurrent choice between cocaine and food in rhesus monkeys. The concurrent-choice procedure provides a rate-independent measure of the relative reinforcing effects of cocaine in comparison with food.

METHODS

Four rhesus monkeys were trained to respond under a concurrent-choice schedule for food (1-g pellets) or cocaine (0-0.1 mg/kg per injection). Saline and increasing doses of U50,488 (0.0032-0.1 mg/kg per h) were administered by pseudo-continuous i.v. infusion (one infusion every 20 min) during sequential 3-day blocks. In a separate experiment, monkeys were treated with nor-BNI (3.2 mg/kg, i.v.), and cocaine choice was re-determined during pseudo-continuous infusion with saline or U50,488 (0.1 mg/kg per h).

RESULTS

During saline treatment, cocaine maintained a dose-dependent and monotonic increase in cocaine choice. Monkeys responded primarily for food when low cocaine doses were available (0-0.01 mg/kg per injection) and primarily for cocaine when higher cocaine doses were available (0.032-0.1 mg/kg per injection). U50,488 produced a dose-dependent increase in cocaine choice, manifested as leftward shifts in the cocaine-choice, dose-effect curve. U50,488 also dose-dependently decreased overall response rates. Nor-BNI did not alter cocaine choice, but it attenuated the effects of U50,488.

CONCLUSIONS

These results suggest that continuous treatment with U50,488 produces a kappa receptor-mediated increase in the relative reinforcing effects of cocaine in comparison with food.

Ethological analyses of crayfish behavior: a new invertebrate system for measuring the rewarding properties of psychostimulants

Recent investigations in invertebrate neurobiology have opened up a new line of research into the basic behavioral, neurochemical and genomic alterations that accompany psychostimulant drug exposure. However, the extent to which such findings relate to changes in motivational and learning processes, such as those that typify drug addictions, remains unclear. The present study addressed this issue in the crayfish, Orconectes rusticus. The first set of experiments demonstrated that intramuscular injections of cocaine and amphetamine have robust and distinguishable effects on crayfish behavior. In the second part of the study, the reinforcing properties of psychostimulants were tested in a series of conditioned place preference experiments. Amphetamine and, to a lesser extent, cocaine were both found to serve as rewards when their intra-circulatory infusion was coupled to a distinct visual environment. The monoaminergic regulation of behavior has been extensively studied in decapod crustaceans and the present experiments demonstrated that (mammalian) drugs of abuse, capable of interfering with monoamine chemistry, are similarly rewarding to crayfish. Behavioral studies in crayfish can provide a complementary approach to using other invertebrate species in addiction research.

Assessment of the relative reinforcing strength of cocaine in socially housed monkeys using a choice procedure

Position in the social hierarchy can influence brain dopamine function and cocaine reinforcement in nonhuman primates during early cocaine exposure. With prolonged exposure, however, initial differences in rates of cocaine self-administration between dominant and subordinate monkeys dissipate. The present studies used a choice procedure to assess the relative reinforcing strength of cocaine in group-housed male cynomolgus monkeys with extensive cocaine self-administration histories. Responding was maintained under a concurrent fixed-ratio 50 schedule of food and cocaine (0.003-0.1 mg/kg per injection) presentation. Responding on the cocaine-associated lever increased as a function of cocaine dose in all monkeys. Although response distribution was similar across social rank when saline or relatively low or high cocaine doses were the alternative to food, planned t tests indicated that cocaine choice was significantly greater in subordinate monkeys when choice was between an intermediate dose (0.01 mg/kg) and food. When a between-session progressive-ratio procedure was used to increase response requirements for the preferred reinforcer (either cocaine or food), choice of that reinforcer decreased in all monkeys. The average response requirement that produced a shift in response allocation from the cocaine-associated lever to the food-associated lever was higher in subordinates across cocaine doses, an effect that trended toward significance (p = 0.053). These data indicate that despite an extensive history of cocaine self-administration, most subordinate monkeys were more sensitive to the relative reinforcing strength of cocaine than dominant monkeys.

Characterization of dopamine D1 and D2 receptor function in socially housed cynomolgus monkeys self-administering cocaine

RATIONALE

Social rank has been shown to influence dopamine (DA) D(2) receptor function and vulnerability to cocaine self-administration in cynomolgus monkeys. The present studies were designed to extend these findings to maintenance of cocaine reinforcement and to DA D(1) receptors.

OBJECTIVE

Examine the effects of a high-efficacy D(1) agonist on an unconditioned behavior (eyeblinking) and a low-efficacy D(1) agonist on cocaine self-administration, as well as the effects of cocaine exposure on D(2) receptor function across social ranks, as determined by positron emission tomography (PET).

METHODS

Effects of the high-efficacy D(1) agonist SKF 81297 and cocaine (0.3-3.0 mg/kg) on spontaneous blinking were characterized in eight monkeys during 15-min observation periods. Next, the ability of the low-efficacy D(1) agonist SKF 38393 (0.1-17 mg/kg) to decrease cocaine self-administration (0.003-0.1 mg/kg per injection, IV) was assessed in 11 monkeys responding under a fixed-ratio 50 schedule. Finally, D(2) receptor levels in the caudate and putamen were assessed in nineteen monkeys using PET.

RESULTS

SKF 81297, but not cocaine, significantly increased blinking in all monkeys, with slightly greater potency in dominant monkeys. SKF 38393 dose-dependently decreased cocaine-maintained response rates with similar behavioral potency and efficacy across social rank. After an extensive cocaine self-administration history, D(2) receptor levels did not differ across social ranks.

CONCLUSIONS

These results suggest that D(1) receptor function is not substantially influenced by social rank in monkeys from well-established social groups. While an earlier study showed that dominant monkeys had higher D(2) receptor levels and were less sensitive to the reinforcing effects of cocaine during initial exposure, the present findings indicate that long-term cocaine use changed D(2) receptor levels such that D(2) receptor function and cocaine reinforcement were not different between social ranks. These findings suggest that cocaine exposure attenuated the impact of social housing on DA receptor function.

Activator of G protein signaling 3: a gatekeeper of cocaine sensitization and drug seeking

Chronic cocaine administration reduces G protein signaling efficacy. Here, we report that the expression of AGS3, which binds to GialphaGDP and inhibits GDP dissociation, was upregulated in the prefrontal cortex (PFC) during late withdrawal from repeated cocaine administration. Increased AGS3 was mimicked in the PFC of drug-naive rats by microinjecting a peptide containing the Gialpha binding domain (GPR) of AGS3 fused to the cell permeability domain of HIV-Tat. Infusion of Tat-GPR mimicked the phenotype of chronic cocaine-treated rats by manifesting sensitized locomotor behavior and drug seeking and by increasing glutamate transmission in nucleus accumbens. By preventing cocaine withdrawal-induced AGS3 expression with antisense oligonucleotides, signaling through Gialpha was normalized, and both cocaine-induced relapse to drug seeking and locomotor sensitization were prevented. When antisense oligonucleotide infusion was discontinued, drug seeking and sensitization were restored. It is proposed that AGS3 gates the expression of cocaine-induced plasticity by regulating G protein signaling in the PFC.

Cocaine dependence and d2 receptor availability in the functional subdivisions of the striatum: relationship with cocaine-seeking behavior

Striatal dopamine D2 receptors have been implicated in the neurobiology of cocaine addiction. Previous imaging studies showed reduced striatal D2 receptor availability in chronic cocaine abusers, and animal studies suggested that low D2 receptor availability promotes cocaine self-administration. Here, D2 receptor availability was assessed with positron emission tomography (PET) and [11C]raclopride in the limbic, associative, and sensori-motor subdivisions of the striatum in 17 recently detoxified chronic cocaine-dependent (CCD) subjects and 17 matched healthy control (HC) subjects. In addition, the relationship between regional D2 receptor availability and behavioral measures obtained in cocaine self-administration sessions was investigated in CCD subjects. [11C]Raclopride binding potential was significantly reduced by 15.2% in the limbic striatum, 15.0% in the associative striatum, and 17.1% in the sensori-motor striatum in CCD subjects compared to HC subjects. In CCD subjects, no relationship was detected between D2 availability in striatal regions and either the positive effects of smoked cocaine or the choice of cocaine over an alternative reinforcer (money) following a priming dose of cocaine (a laboratory model of relapse). Thus, this study confirms previous reports of a modest decrease in D2 receptor availability in CCD subjects, and establishes that this decrease is generalized throughout the striatum. However, this study failed to demonstrate a relationship between D2 receptor availability and cocaine-induced cocaine-taking behavior. Additional research is warranted to unravel potential neurobiological traits that might confer vulnerability to relapse in detoxified CCD subjects.

Dopamine in drug abuse and addiction: results from imaging studies and treatment implications

The involvement of dopamine in drug reinforcement is well recognized but its role in drug addiction is much less clear. Imaging studies have shown that the reinforcing effects of drugs of abuse in humans are contingent upon large and fast increases in dopamine that mimic but exceed in the intensity and duration those induced by dopamine cell firing to environmental events. In addition, imaging studies have also documented a role of dopamine in motivation, which appears to be encoded both by fast as well as smooth DA increases. Since dopamine cells fire in response to salient stimuli, the supraphysiological activation by drugs is likely to be experienced as highly salient (driving attention, arousal conditioned learning and motivation) and may also reset the thresholds required for environmental events to activate dopamine cells. Indeed, imaging studies have shown that in drug-addicted subjects, dopamine function is markedly disrupted (decreases in dopamine release and in dopamine D2 receptors in striatum) and this is associated with reduced activity of the orbitofrontal cortex (neuroanatomical region involved with salience attribution and motivation and implicated in compulsive behaviors) and the cingulate gyrus (neuroanatomical region involved with inhibitory control and attention and implicated in impulsivity). However, when addicted subjects are exposed to drug-related stimuli, these hypoactive regions become hyperactive in proportion to the expressed desire for the drug. We postulate that decreased dopamine function in addicted subjects results in decreased sensitivity to nondrug-related stimuli (including natural reinforcers) and disrupts frontal inhibition, both of which contribute to compulsive drug intake and impaired inhibitory control. These findings suggest new strategies for pharmacological and behavioral treatments, which focus on enhancing DA function and restoring brain circuits disrupted by chronic drug use to help motivate the addicted subject in activities that provide alternative sources of reinforcement, counteract conditioned responses, enhance their ability to control their drive to take drugs and interfere with their compulsive administration.

The expanding effects of cocaine: studies in a nonhuman primate model of cocaine self-administration

Although neuroimaging investigations in human cocaine abusers have provided important insights into the brain changes that accompany drug use, the interpretation of reports in human abusers can be very difficult. Studies in nonhuman primates provide a way to systematically evaluate the structural and functional adaptations engendered by cocaine self-administration without the confounds of human research. Functional activity, measured with metabolic mapping methods, and markers of the dopamine system, assessed autoradiographically, were evaluated over the course of chronic cocaine self-administration (5 days, 3.3 months, and 15-22 months). Within the striatum the topography of these responses shifts dramatically over time. Changes in functional activity and alterations in the dopamine system occupy larger and larger portions of dorsal and ventral striatum with increasing durations of cocaine exposure. The growing impact of cocaine suggests that the elements of the behavioral repertoire outside of the influence of cocaine become smaller and smaller with increasing durations of exposure to drug use resulting in cocaine's dominance over all aspects of the addict's life.

Regulation of neuromodulator receptor efficacy—implications for whole-neuron and synaptic plasticity

Membrane receptors for neuromodulators (NM) are highly regulated in their distribution and efficacy-a phenomenon which influences the individual cell's response to central signals of NM release. Even though NM receptor regulation is implicated in the pharmacological action of many drugs, and is also known to be influenced by various environmental factors, its functional consequences and modes of action are not well understood. In this paper we summarize relevant experimental evidence on NM receptor regulation (specifically dopamine D1 and D2 receptors) in order to explore its significance for neural and synaptic plasticity. We identify the relevant components of NM receptor regulation (receptor phosphorylation, receptor trafficking and sensitization of second-messenger pathways) gained from studies on cultured cells. Key principles in the regulation and control of short-term plasticity (sensitization) are identified, and a model is presented which employs direct and indirect feedback regulation of receptor efficacy. We also discuss long-term plasticity which involves shifts in receptor sensitivity and loss of responsivity to NM signals. Finally, we discuss the implications of NM receptor regulation for models of brain plasticity and memorization. We emphasize that a realistic model of brain plasticity will have to go beyond Hebbian models of long-term potentiation and depression. Plasticity in the distribution and efficacy of NM receptors may provide another important source of functional plasticity with implications for learning and memory.

Neurobiologic processes in drug reward and addiction

Neurophysiologic processes underlie the uncontrolled, compulsive behaviors defining the addicted state. These"hard-wired"changes in the brain are considered critical for the transition from casual to addictive drug use. This review of preclinical and clinical (primarily neuroimaging) studies will describe how the delineation between pleasure, reward, and addiction has evolved as our understanding of the biologic mechanisms underlying these processes has progressed. Although the mesolimbic dopaminergic efflux associated with drug reward was previously considered the biologic equivalent of pleasure, dopaminergic activation occurs in the presence of unexpected and novel stimuli (either pleasurable or aversive) and appears to determine the motivational state of wanting or expectation. The persistent release of dopamine during chronic drug use progressively recruits limbic brain regions and the prefrontal cortex, embedding drug cues into the amygdala (through glutaminergic mechanisms) and involving the amygdala, anterior cingulate, orbitofrontal cortex, and dorsolateral prefrontal cortex in the obsessive craving for drugs. The abstinent, addicted brain is subsequently primed to return to drug use when triggered by a single use of drug, contextual drug cues, craving, or stress, with each process defined by a relatively distinct brain region or neural pathway. The compulsive drive toward drug use is complemented by deficits in impulse control and decision making, which are also mediated by the orbitofrontal cortex and anterior cingulate. Within this framework, future targets for pharmacologic treatment are suggested.

The addicted human brain viewed in the light of imaging studies: brain circuits and treatment strategies

Imaging studies have provided evidence of how the human brain changes as an individual becomes addicted. Here, we integrate the findings from imaging studies to propose a model of drug addiction. The process of addiction is initiated in part by the fast and high increases in DA induced by drugs of abuse. We hypothesize that this supraphysiological effect of drugs trigger a series of adaptations in neuronal circuits involved in saliency/reward, motivation/drive, memory/conditioning, and control/disinhibition, resulting in an enhanced (and long lasting) saliency value for the drug and its associated cues at the expense of decreased sensitivity for salient events of everyday life (including natural reinforcers). Although acute drug intake increases DA neurotransmission, chronic drug consumption results in a marked decrease in DA activity, associated with, among others, dysregulation of the orbitofrontal cortex (region involved with salience attribution) and cingulate gyrus (region involved with inhibitory control). The ensuing increase in motivational drive for the drug, strengthened by conditioned responses and the decrease in inhibitory control favors emergence of compulsive drug taking. This view of how drugs of abuse affect the brain suggests strategies for intervention, which might include: (a) those that will decrease the reward value of the drug of choice; (b) interventions to increase the saliency value of non-drug reinforcers; (c) approaches to weaken conditioned drug behaviors; and (d) methods to strengthen frontal inhibitory and executive control. Though this model focuses mostly on findings from PET studies of the brain DA system it is evident that other neurotransmitters are involved and that a better understanding of their roles in addiction would expand the options for therapeutic targets.

Advances in nonhuman primate alcohol abuse and alcoholism research

Advances in our understanding of the biological basis of alcohol abuse and alcoholism and the development of prevention and therapeutic intervention require appropriate animal models. Nonhuman primates are important to the study of complex biomedical disease processes. Genetic, anatomical, physiological, and behavioral similarities to humans offer unique opportunities for translational research along with the advantage of a degree of experimental control that is not possible in human studies. The purpose of this review is to outline the approaches taken with nonhuman primates as subjects in alcohol research and to highlight our current understanding of data on organismal variables that can be uniquely studied in these complex organisms. We review literature on alcohol self-administration to provide an integrative framework for discussion of progress in 2 important areas of research. Designs that incorporate self-administration provide a context for studying excessive alcohol consumption, including the organismal and environmental factors that influence risk for heavy drinking. We then review the use of monkeys to identify aspects of adverse biomedical consequences that follow excessive alcohol consumption. One of the primary conclusions to be drawn from this review is that nonhuman primates are a central part of the translational bridge in alcohol research, providing powerful and unique opportunities for experimental work that can address the biomedical complexities of alcohol abuse and alcoholism.

Effect of environmental stressors on opiate and psychostimulant reinforcement, reinstatement and discrimination in rats: a review

Studies in humans suggest that exposure to life stressors is correlated with compulsive drug abuse and relapse to drugs during periods of abstinence. The behavioral and neurobiological mechanisms involved in the effect of stress on drug abuse, however, are not known. Here, we review data from studies using preclinical models in rats on the effect of environmental stressors on opiate and psychostimulant reinforcement, as measured by the intravenous drug self-administration and conditioned place preference procedures, on relapse to these drugs, as measured by the reinstatement procedure, and on the subjective effects of these drugs, as measured by the drug discrimination procedure. The results of the studies reviewed here suggest that while stressors are important modulators of the behavioral effects of opiate and psychostimulant drugs, the effect of stress on behavior in these animal models is stressor-specific, and to some degree, procedure- and drug-class-specific. The review of studies on the neurobiological mechanisms underlying stress-drug interactions in these animal models indicate that central noradrenaline and extrahypothalamic corticotropin-releasing factor mediate the effect of one form of stress (intermittent footshock) on reinstatement of opiate and psychostimulant seeking after prolonged drug-free periods. At present, however, little is known about the neuronal events that mediate the effect of environmental stressors on opiate and psychostimulant reinforcement or discrimination. The broader implications of the data reviewed here for future research and for the treatment of opiate and psychostimulant addiction are briefly discussed.

Confinement but not microgravity alters NMDA NR1 receptor expression in rat inner ear ganglia

Space flight produces changes in neuronal activity in the vestibular system. We studied the protein expression of the NMDA receptor subunit NR1 in the vestibular ganglia of rats exposed to microgravity for 17 days, beginning on postnatal day 8, as part of the NASA Neurolab mission. As a control, we studied the cochlear ganglia in the same way. NR1 expression in rats that had experienced microgravity (flight-FLT rats) was compared with that in two types of ground control. One control consisted of rats housed in regular cage conditions (VIV, vivarium); the other, asynchronous ground control (AGC), consisted of rats kept in cages similar to those used in flight (animal enclosure module, AEM), requiring no human care. After 8 days of flight, NR1 levels in the vestibular and cochlear neurons were similar in FLT, VIV and AGC rats. In contrast, 8 h after landing, the FLT and VIV animals showed similar, normal levels of NR1 staining, whereas the ganglia of the AGC animals displayed only very faint staining. Thus, microgravity did not modify NR1 expression in vestibular neurons. The lower levels of NR1 expression in the vestibular and cochlear neurons of AGC rats suggest an effect of confinement for 17 days in AEMs on the ground.

Cultural objects modulate reward circuitry

Using event-related fMRI we investigated the rewarding properties of cultural objects (cars) signaling wealth and social dominance. It has been shown recently that reward mechanisms are involved in the regulation of social relations like dominance and social rank. Based on evolutionary considerations we hypothesized that sports cars in contrast to other categories of cars, e.g. limousines and small cars, are strong social reinforcers and would modulate the dopaminergic reward circuitry. Twelve healthy male subjects were studied with fMRI while viewing photographs of different car classes followed by an attractivity rating. Behaviorally sports cars were rated significantly more attractive than limousines and small cars. Our fMRI results revealed significantly more activation in ventral striatum, orbitofrontal cortex, anterior cingulate and occipital regions for sports cars in contrast to other categories of cars. We could thus demonstrate that artificial cultural objects associated with wealth and social dominance elicit activation in reward-related brain areas.

Brain DA D2 receptors predict reinforcing effects of stimulants in humans: replication study

We had shown that striatal DA D2 receptors levels predicted the reinforcing responses to the psychostimulant drug methylphenidate in nondrug-abusing subjects. Here, we assessed the replicability of this finding. We measured D2 receptors with PET and [(11)C]raclopride (twice to determine stability) in seven nondrug-abusing subjects to assess if they predicted the self-reports of "drug-liking" to intravenous methylphenidate (0.5 mg/kg). DA D2 measures were significantly correlated with "drug-liking" in both evaluations (r = 0.82 and r = 0.78); subjects with the lowest levels reported the higher ratings of "drug-liking" and vice versa. These results replicate our previous findings and provide further evidence that striatal DA D2 receptors modulate reinforcing responses to stimulants in humans and may underlie predisposition for drug self-administration.

Modeling addiction: trusted experimental approaches are tried in new applications

The EuroConference/Neurochemistry Winter Conference on Modeling Addiction, held on 6-11 April 2002 in Soelden, Austria(1) explored the question of how various experimental approaches, ranging from those at the level of molecular biology to human behavior, reflect human patterns of drug abuse and dependence and whether these various approaches constitute suitable predictive or homologous models to test novel pharmacotherapies.

Role of dopamine in drug reinforcement and addiction in humans: results from imaging studies

The involvement of dopamine (DA) in drug reinforcement is well established, but much less in known about its contribution to addiction. We have used positron emission tomography to investigate in humans the role of DA in drug reinforcement, addiction and drug vulnerability. We have shown that during drug intoxication increases in striatal DA are associated with the drug's reinforcing effects only if the DA changes occur rapidly. These results corroborate the relevance of drug-induced DA increases and of pharmacokinetics in the rewarding effects of drugs in humans. During withdrawal, we have shown significant reductions in DA D(2) receptors and in DA release in drug abusers, which is likely to result in decreased sensitivity to non-drug-related reinforcing stimuli. The DA D(2) reductions were associated with decreased activity in the orbitofrontal cortex, which we postulate is one of the mechanisms underlying compulsive drug administration in the addict. In fact, during craving the orbitofrontal cortex becomes hyperactive in proportion to the desire for the drug. In non-drug-abusing subjects striatal DA D(2) receptors levels predicted the reinforcing responses to stimulant drugs, providing evidence that striatal DA D(2) receptors modulate reinforcing responses to stimulants in humans and may contribute to the predisposition for drug self-administration.

Repeated sucrose access influences dopamine D2 receptor density in the striatum

A decrease in D2 dopamine receptor subtype (D2R) binding in the striatum has been reported in obese individuals and drug addicts. We examined D2R density in the striatum of food-restricted rats that had contingent access to food with different incentive values. Results showed that animals receiving limited access to 0.3 M sucrose paired 2 h with a chow meal for 7 days had a significantly lower D2R binding in nucleus accumbens shell and dorsolateral striatum compared with animals that had limited access to chow. There was no differential binding, however, in the accumbens core in any of the groups. These findings indicate that feeding conditions and sucrose intake influence D2R density specifically in subregions of the striatum.

Role of dopamine D2-like receptors in cocaine self-administration: studies with D2 receptor mutant mice and novel D2 receptor antagonists

Dopamine receptor subtypes have been classified generally as D1-like (e.g., D1, D5) or D2-like (D2, D3, D4), and converging evidence suggests that D2-like receptors may be especially important in mediating the abuse-related effects of cocaine. However, it has been difficult to differentiate the roles of the D2-like receptor subtypes in the behavioral effects of cocaine because of the relatively low selectivity of drugs for D2, D3, and D4 receptors in vivo. The goal of the present series of studies was to investigate the contributions of D2-like receptor subtypes in the reinforcing effects of cocaine using new genetic and pharmacological tools. First, we evaluated cocaine self-administration behavior, and related effects of nonselective D2-like drugs, in mutant mice that lack the D2 receptor but express D3 and D4 receptors. When high doses of cocaine on the descending limb of the cocaine dose-effect function were available, D2 mutant mice self-administered at higher rates than their heterozygous or wild-type littermates, but the ascending limb of the cocaine dose-effect function did not differ between genotypes. Elevated rates of drug intake were not attributable to nonspecific increases in response rate, because response rates maintained by presentation of a range of food concentrations were significantly lower in D2 mutant mice than in wild-type mice. In wild-type mice, pretreatment with the D2-like antagonist eticlopride increased rates of self-administration of high doses of cocaine, and the D2-like agonist quinelorane served as a positive reinforcer when substituted for cocaine. However, these effects of eticlopride and quinelorane were not observed in mice that lacked the D2 receptor. Next, we compared the effects of novel antagonists selective for different D2 receptor subtypes on cocaine self-administration behavior in outbred rats. In rats, a D2 selective antagonist increased rates of self-administration of high doses of cocaine and also combinations of cocaine and the D2-like agonist quinelorane, whereas D3/D4 antagonists were ineffective. Collectively, these findings suggest that the D2 receptor is not necessary for cocaine self-administration, but this receptor subtype is involved in mechanisms that limit rates of high-dose cocaine self-administration. Our results also suggest that D3 and D4 receptors do not play major roles in the modulation of cocaine self-administration by D2-like drugs.