Animal models of drug craving

Abstinence-dependent transfer of lithium chloride-induced sucrose aversion to a sucrose-paired cue in rats

RATIONALE

Responding for a drug- or sucrose-paired cue increases over forced abstinence (incubation of craving). If the incentive value of a cue depends on the incentive value of the primary reward, devaluing the primary reward should reduce cue reactivity.

OBJECTIVES

We investigated whether conditioned taste aversion (CTA) to sucrose would transfer to a sucrose-paired cue after 1 or 30 days of forced abstinence and whether CTA after 1 day of forced abstinence would affect incubation of craving.

MATERIALS AND METHODS

Rats self-administered 10% sucrose paired with a tone + light cue for 10 days. After 1 (Exp.1) or 30 (Exp.2) days of forced abstinence, rats received two home-cage pairings of sucrose with either LiCl (65 mg/kg, IP) to produce CTA or saline as a control. Two days later, rats responded for the cue alone. The following day, sucrose consumption was assessed in the same operant conditioning chamber. Exp.1 rats were tested again 1 month later to determine if CTA would affect incubation of craving.

RESULTS

Exp.1: CTA after 1 day of forced abstinence did not attenuate cue reactivity when tested immediately after CTA, nor did the treatment affect incubation of craving or incubation of sucrose consumption. Exp.2: CTA after 1 month of forced abstinence resulted in a significant reduction in cue reactivity.

CONCLUSION

The incentive values of sucrose and the conditioned representation of sucrose increase over an extended period of forced abstinence. This incubation appears to facilitate the transfer of an aversion to the primary reward to the conditioned cue.

Dopamine signaling in the nucleus accumbens of animals self-administering drugs of abuse

Abuse of psychoactive substances can lead to drug addiction. In animals, addiction is best modeled by drug self-administration paradigms. It has been proposed that the crucial common denominator for the development of drug addiction is the ability of drugs of abuse to increase extracellular concentrations of dopamine in the nucleus accumbens (NAcc). Studies using in vivo microdialysis and chronoamperometry in the behaving animal have demonstrated that drugs of abuse increase tonic dopamine concentrations in the NAcc. However, it is known that dopamine neurons respond to reward-related stimuli on a subsecond timescale. Thus, it is necessary to collect neurochemical information with this level of temporal resolution, as achieved with in vivo fast-scan cyclic voltammetry (FSCV), to fully understand the role of phasic dopamine release in normal behavior and drug addiction. We review studies that investigated the effects of drugs of abuse on NAcc dopamine levels in freely moving animals using in vivo microdialysis, chronoamperometry, and FSCV. After a brief introduction of dopamine signal transduction and anatomy and a section on current theories on the role of dopamine in natural goal-directed behavior, a discussion of techniques for the in vivo assessment of extracellular dopamine in behaving animals is presented. Then, we review studies using these techniques to investigate changes in phasic and tonic dopamine signaling in the NAcc during (1) response-dependent and -independent administration of abused drugs, (2) the presentation of drug-conditioned stimuli and operant behavior in self-administration paradigms, (3) drug withdrawal, and (4) cue-induced reinstatement of drug seeking. These results are then integrated with current ideas on the role of dopamine in addiction with an emphasis on a model illustrating phasic and tonic NAcc dopamine signaling during different stages of drug addiction. This model predicts that phasic dopamine release in response to drug-related stimuli will be enhanced over stimuli associated with natural reinforcers, which may result in aberrant goal-directed behaviors contributing to drug addiction.

Neural substrates of psychostimulant withdrawal-induced anhedonia

Psychostimulant drugs have powerful reinforcing and hedonic properties and are frequently abused. Cessation of psychostimulant administration results in a withdrawal syndrome characterized by anhedonia (i.e., an inability to experience pleasure). In humans, psychostimulant withdrawal-induced anhedonia can be debilitating and has been hypothesized to play an important role in relapse to drug use. Hence, understanding the neural substrates involved in psychostimulant withdrawal-induced anhedonia is essential. In this review, we first summarize the theoretical perspectives of psychostimulant withdrawal-induced anhedonia. Experimental procedures and measures used to assess anhedonia in experimental animals are also discussed. The review then focuses on neural substrates hypothesized to play an important role in anhedonia experienced after termination of psychostimulant administration, such as with cocaine, amphetamine-like drugs, and nicotine. Both neural substrates that have been extensively investigated and some that need further evaluation with respect to psychostimulant withdrawal-induced anhedonia are reviewed. In the context of reviewing the various neurosubstrates of psychostimulant withdrawal, we also discuss pharmacological medications that have been used to treat psychostimulant withdrawal in humans. This literature review indicates that great progress has been made in understanding the neural substrates of anhedonia associated with psychostimulant withdrawal. These advances in our understanding of the neurobiology of anhedonia may also shed light on the neurobiology of nondrug-induced anhedonia, such as that seen as a core symptom of depression and a negative symptom of schizophrenia.

Reduction by the Positive Allosteric Modulator of the GABA(B) Receptor, GS39783, of Alcohol Self-Administration in Sardinian Alcohol-Preferring Rats Exposed to the "Sipper" Procedure

The present study was designed to evaluate (a) alcohol self-administration behavior of selectively bred, Sardinian alcohol-preferring (sP) rats exposed to the so-called "sipper" procedure (characterized by the temporal separation between alcohol-seeking and -taking phases), and (b) the effect of the positive allosteric modulator of the GABA(B) receptor, GS39783, on alcohol self-administration in sP rats exposed to this procedure. To this end, sP rats were initially trained to lever-respond under a reinforcement requirement (RR) 55 (RR55) for alcohol. Achievement of RR55 resulted in the 20-min presentation of the alcohol (15%, v/v)-containing sipper bottle. Once stable levels of lever-responding and alcohol consumption were reached, rats were treated with 0, 25, 50, and 100 mg/kg GS39783 (i.g.) 60 min before the self-administration session. Rats displayed robust alcohol-seeking (as suggested by relatively short latencies to the first lever-response and high frequencies of lever-responding) and -taking (as suggested by alcohol intakes averaging approximately 1.5 g/kg) behaviors. Pretreatment with GS39783 inhibited both alcohol-seeking (the number of rats achieving RR55 and the mean RR value were virtually halved) and -taking (the amount of self-administered alcohol was reduced by approximately 60%). The results of the present study suggest the power of the "sipper" procedure in triggering high levels of alcohol-seeking and -taking behavior in sP rats. Further, these results extend to this additional procedure of alcohol self-administration the capacity of GS39783 to reduce the motivational properties of alcohol and alcohol consumption in sP rats.

Effect of stress on opioid-seeking behavior: evidence from studies with rats

Studies concerned with the relation between exposure to stress and the behavioral effects of opioid agonists in animal models of drug use are reviewed. These studies, which primarily utilized male rats, indicate that under certain conditions short-term mild stressors increase self-administration of opioid drugs and reinstate herein-seeking behavior following a drug-free period. On the other hand, there is evidence that long-term chronic inescapable stressors and severe acute stressors reduce the reinforcing effects of morphine as measured by a conditioned place preference procedure and decrease the behavioral effects of other positive reinforcers. The results of the studies reviewed suggest that stressors are important modulators of opioid-taking behavior, especially during drug-free periods. The implications of these findings to the understanding of the neurobiology of relapse to opioid-seeking behavior and for strategies for medication development to prevent relapse to heroin are discussed.

How conditioned stimuli acquire the ability to activate VTA dopamine cells: a proposed neurobiological component of reward-related learning

The ability to learn about conditioned stimuli (CS) associated with rewards is a crucial adaptive mechanism. Activity in the mesocorticolimbic dopamine (DA) system, as well as in the ventral tegmental area (VTA), is correlated with responding to and learning about CSs. The mechanism by which VTA neurons become activated by signals associated with conditioned stimuli is not fully understood. Our model suggests that NMDA receptor stimulation in the VTA allows originally weak glutamate signals carrying information about environmental stimuli, coincident with strong excitation correlated with primary rewards, to be strengthened and thereby acquire the ability to activate VTA neurons in themselves, producing approach. Furthermore, once synaptic strengthening occurs, the model suggests that NMDA receptor stimulation in VTA is not necessary for the expression of reward-related learning. In this review we survey evidence that VTA cells respond to cues associated with primary rewards, that this responding is acquired, and that the VTA possesses the attributes to function as a site of integration of signals of primary and conditioned stimuli.

CRF system recruitment mediates dark side of compulsive eating

Dieting to control body weight involves cycles of deprivation from palatable food that can promote compulsive eating. The present study shows that rats withdrawn from intermittent access to palatable food exhibit overeating of palatable food upon renewed access and an affective withdrawal-like state characterized by corticotropin-releasing factor-1 (CRF(1)) receptor antagonist-reversible behaviors, including hypophagia, motivational deficits to obtain less palatable food, and anxiogenic-like behavior. Withdrawal was accompanied by increased CRF expression and CRF(1) electrophysiological responsiveness in the central nucleus of the amygdala. We propose that recruitment of anti-reward extrahypothalamic CRF-CRF(1) systems during withdrawal from palatable food, analogous to abstinence from abused drugs, may promote compulsive selection of palatable food, undereating of healthier alternatives, and a negative emotional state when intake of palatable food is prevented.

c-Fos expression associated with reinstatement of cocaine-seeking behavior by response-contingent conditioned cues

The capability of cocaine cues to generate craving in cocaine-dependent humans, even after extended abstinence, is modeled in rats using cue reinstatement of extinguished cocaine-seeking behavior. We investigated neural activity associated with incentive motivational effects of cocaine cues using c-fos mRNA and Fos protein expression as markers. Unlike preceding studies, we used response-contingent presentation of discrete cues to elicit cocaine seeking. Rats were first trained to press a lever, resulting in cocaine reinforcement and light and tone cues. Rats then underwent extinction training, during which lever presses decreased. On the test day, rats either received response-contingent cocaine cues or received no cues. The cues reinstated extinguished cocaine-seeking behavior on the test day. In general, cue-elicited c-fos mRNA and protein expression were similar and both were enhanced in the prefrontal cortex, ventral tegmental area (VTA), dorsal striatum, and nucleus accumbens. Cues elicited more widespread Fos protein expression relative to our previous research in which cues were presented noncontingently without prior extinction training, including increases in the VTA, substantia nigra, ventral subiculum, and lateral entorhinal cortex. We also observed a correlation between cocaine-seeking behavior and Fos in the agranular insula (AgI) and basolateral amygdala (BLA). The findings suggest that connections between BLA and AgI play a role in cue-elicited incentive motivation for cocaine and that reinstatement of cocaine seeking by response-contingent cues activates a similar corticolimbic circuit as that observed with other modes of cue presentation; however, activation of midbrain and ventral hippocampal regions may be unique to reinstatement by response-contingent cues.

A meta-analytic investigation of the relationship between attentional bias and subjective craving in substance abuse

Theoretical models of addiction suggest that attentional bias for substance-related cues should be associated with self-reported craving. The authors evaluated the strength of the association by performing a meta-analysis on 68 independent data sets from which correlation coefficients between subjective craving and attentional bias indices were derived. Additional stratified analyses were conducted to identify any variables that might moderate the association between craving and attentional bias. The primary meta-analysis indicated a significant, albeit weak (r=.19), association between attentional bias and craving. Stratified analyses revealed that the association was larger for illicit drug and caffeine craving than for alcohol and tobacco craving, larger for direct measures of attention (eye movement measures and event-related potential measures) than for indirect behavioral measures of attentional bias, and larger when craving strength was high than when it was low (all ps<.05). The size of the correlation did not differ among patients in treatment and individuals who were not seeking treatment. These results suggest that attentional bias and craving are related phenomena, although the relationship is generally modest and appears to be moderated by various factors. Theoretical implications are discussed.

Behavioral momentum of cocaine self-administration: effects of frequency of reinforcement on resistance to extinction

Persistent drug seeking is a defining property of substance abuse and is generally thought to depend, in part, on exposure to drug-associated contexts. Behavioral momentum theory provides a set of methods and a theoretical framework for understanding how stimulus contexts contribute to the persistence of operant behavior. Earlier research has extended behavioral momentum theory to alcohol self-administration, but not to intravenous drug self-administration. This experiment extended behavioral momentum theory to cocaine self-administration by examining the effects of frequency of cocaine reinforcement in a context on resistance to extinction. Rats self-administered 0.32 mg/kg infusions of cocaine in a multiple schedule of reinforcement arranging two distinct contexts. Responding in a Rich context was reinforced by cocaine infusions at a higher frequency (i.e. variable interval 120 s) and in a Lean context at a lower frequency (variable interval 360 s). After establishment of responding in the two contexts, resistance to extinction was examined. Preextinction response rates for cocaine were similar in the Rich and Lean contexts. Nonetheless, relative resistance to extinction was greater in the Rich context than in the Lean context. The difference in resistance to extinction in the two contexts was well described by a quantitative model of behavioral momentum. These results suggest that the frequency of drug reinforcement in a context contributes to the persistence of drug seeking in that context, and that behavioral momentum theory might be useful for understanding how drug-associated contexts contribute to the persistence of drug seeking.

Inhibition of fatty acid amide hydrolase reduces reinstatement of nicotine seeking but not break point for nicotine self-administration--comparison with CB(1) receptor blockade

RATIONALE

The endocannabinoid system has been recently identified as having critical involvement in drug taking and relapse phenomenon for various drugs of abuse and notably nicotine. The endocannabinoid system consists of endocannabinoids (such as anandamide), their target receptors (mostly cannabinoid CB(1) receptors), and the enzymes that degrade those endocannabinoids (fatty-acid-amide-hydrolase (FAAH) for anandamide). It has been recently identified that the utility of rimonabant for smoking cessation may be limited by its psychiatric side effects. Therefore, there is a great need to develop alternative ways of modulating the cannabinoid system that will be better tolerated.

OBJECTIVE

The aim of the study was to explore the effect of inhibiting FAAH enzyme by URB597 on nicotine self-administration under a progressive ratio schedule and reinstatement of nicotine seeking, in comparison with the effect of the CB(1) antagonist rimonabant.

RESULTS

Rimonabant, but not URB597, dose-dependently reduced the break point for nicotine self-administration, an effect that was stable over repeated administrations. Rimonabant and URB597 significantly decreased the reinstatement of nicotine seeking induced either by presentation of nicotine-associated stimuli or by nicotine priming.

CONCLUSIONS

These results indicate that the integrity of the CB(1) receptors is necessary for the incentive motivation of the rats for nicotine and that FAAH inhibition may be as effective as CB(1) receptor blockade to prevent reinstatement of nicotine seeking. Since FAAH inhibition present antidepressant and anxiolytic properties in rodents, targeting the FAAH may represent a novel strategy to prevent relapse for tobacco smoking that may be better tolerated than rimonabant.

Correlates of individual differences in compensatory nicotine self-administration in rats following a decrease in nicotine unit dose

RATIONALE

The ability of tobacco harm reduction strategies to produce significant reductions in toxin exposure is limited by compensatory increases in smoking behavior. Characterizing factors contributing to the marked individual variability in compensation may be useful for understanding this phenomenon and assessing the feasibility of harm reduction interventions.

OBJECTIVE

The objective of the study was to use an animal model of human compensatory smoking that involves a decrease in unit dose supporting nicotine self-administration (NSA) to examine potential contributors to individual differences in compensation.

METHODS

Rats were trained for NSA during daily 23-h sessions at a unit dose of 0.06 mg/kg/inf until responding was stable. The unit dose was then reduced to 0.03 mg/kg/inf for at least 10 sessions. Following reacquisition of NSA at the training dose and extinction, single-dose nicotine pharmacokinetic parameters were determined.

RESULTS

Decreases in nicotine intake following dose reduction were proportionally less than the decrease in unit dose, indicating partial compensation. Compensatory increases in infusion rates were observed across the course of the 23-h sessions. The magnitude of compensation differed considerably between rats. Rats exhibiting the highest baseline infusion rates exhibited the lowest levels of compensation. Nicotine pharmacokinetic parameters were not significantly correlated with compensation. Infusion rates immediately returned to pre-reduction levels when baseline conditions were restored.

CONCLUSIONS

These findings provide initial insights into correlates of individual differences in compensation following a reduction in nicotine unit dose. The present assay may be useful for characterizing mechanisms and potential consequences of the marked individual differences in compensatory smoking observed in humans.

Dorsal hippocampal regulation of memory reconsolidation processes that facilitate drug context-induced cocaine-seeking behavior in rats

Exposure to a cocaine-paired context increases the propensity for relapse in cocaine users and prompts cocaine-seeking behavior in rats. According to the reconsolidation hypothesis, upon context re-exposure, established cocaine-related associations are retrieved and can become labile. These associations must undergo reconsolidation into long-term memory to effect enduring stimulus control. The dorsal hippocampus (DH), dorsolateral caudate-putamen and dorsomedial prefrontal cortex are critical for the expression of context-induced cocaine seeking, and these brain regions may also play a role in the reconsolidation of cocaine-related memories that promote this behavior. To test this hypothesis, rats were trained to press a lever for unsignaled cocaine infusions (0.2 mg/infusion, i.v.) in a distinct environmental context (cocaine-paired context), followed by extinction training in a different context (extinction context). Rats were then re-exposed to the cocaine-paired context for 15 min in order to reactivate cocaine-related memories or received comparable exposure to a novel unpaired context. Immediately thereafter, rats received bilateral microinfusions of the protein synthesis inhibitor anisomycin, the sodium channel blocker tetrodotoxin or vehicle into one of the above brain regions. After additional extinction training in the extinction context, reinstatement of cocaine-seeking behavior (i.e., non-reinforced lever presses) was assessed in the cocaine-paired context. Tetrodotoxin, but not anisomycin, administered into the DH inhibited drug context-induced cocaine-seeking behavior in a memory reactivation-dependent manner. Other manipulations failed to alter this behavior. These findings suggest that the DH facilitates the reconsolidation of associative memories that maintain context-induced cocaine-seeking behavior, but it is not the site of anisomycin-sensitive memory restabilization per se.

Anti-relapse medications: preclinical models for drug addiction treatment

Addiction is a chronic relapsing brain disease and treatment of relapse to drug-seeking is considered the most challenging part of treating addictive disorders. Relapse can be modeled in laboratory animals using reinstatement paradigms, whereby behavioral responding for a drug is extinguished and then reinstated by different trigger factors, such as environmental cues or stress. In this review, we first describe currently used animal models of relapse, different relapse triggering factors, and the validity of this model to assess relapse in humans. We further summarize the growing body of pharmacological interventions that have shown some promise in treating relapse to psychostimulant addiction. Moreover, we present an overview on the drugs tested in cocaine or methamphetamine addicts and examine the overlap of existing preclinical and clinical data. Finally, based on recent advances in our understanding of the neurobiology of relapse and published preclinical data, we highlight the most promising areas for future anti-relapse medication development.

Effects of cannabinoid drugs on the reinforcing properties of food in gestationally undernourished rats

Involvement of the endocannabinoids in hyperphagia has been demonstrated, however, behavioral characterization of its role in food reinforcement is limited. The present study investigated whether 2-arachidonoyl glycerol, an endocannabinoid ligand, and rimonabant, a CB1 antagonist, change the reinforcing properties of food in gestationally undernourished rats (a putative model of obesity) vs controls. Albino dams were food deprived by 0 to 45% of their free-feeding weights up to day 18 of their gestational period. Their offspring were allowed to free-feed until postnatal day 75. Then, behavior of the offspring was placed under progressive ratio schedules of sucrose reinforcement. After baseline data were established, intraperitoneal injections of 2-AG (0.03-3.75 mg/kg), and rimonabant (SR141716, 0.3-3.0 mg/kg) were administered and compared across group. Results show gestationally undernourished (GU) rats as adults weighed less than controls at the time of testing and female offspring allowed to free-feed for over 35 weeks exhibited lower body weights than controls. Under baseline, GU rats had lower breakpoints than controls. 2-AG and rimonabant significantly increased and decreased, respectively, breakpoint and responses made per session, suggesting involvement of the cannabinoid system in food reinforcement. When comparing peak doses of 2-AG on breakpoint, gestationally undernourished rats exhibited lower peak doses than controls. These data suggest that under the gestation deprivation method employed, GU rats were thinner and had lower food reinforcer efficacy than controls, and may have heightened sensitivity to 2-AG.

Assessing the validity of current mouse genetic models of obsessive-compulsive disorder

Obsessive-compulsive disorder (OCD) is a disorder characterized by unwanted and intrusive thoughts, images, or impulses and/or repetitive behavior. OCD is a major cause of disability; however, the genetic factors and pathophysiological mechanisms underlying this complex, heterogeneous disorder remain largely unknown. During the past decade, a number of putative mouse genetic models of OCD have been developed for the purpose of studying the neural mechanisms underlying this disorder and developing novel treatments. This review presents and evaluates these experimental preparations to date. Models using knockout or transgenic approaches, as well as those examining variation in genetically diverse populations, are evaluated and discussed.

Unrestricted access to methamphetamine or cocaine in the past is associated with increased current use

Laboratory animals allowed to self-administer stimulants for extended periods of time escalate drug intake compared to animals that self-administer under temporally limited conditions. To our knowledge, this phenomenon has not been systematically investigated in humans. We interviewed 106 (77 male, 29 female) methamphetamine (Meth) and 96 (81 male, 15 female) cocaine (Coc) users to determine if they had experienced discrete period(s) of unrestricted access to unlimited quantities of Meth or Coc in the past. Fifty-eight Meth users and 53 Coc users reported having a discrete period of unrestricted access in the past, but not in the present. Meth-using participants with a prior history of unrestricted access reported significantly more current Meth use, compared to Meth users with no prior history of unrestricted access. Specifically, these participants reported more days used in the past 30 d, more days of use per week, greater use per day and greater total use per week (p<0.05 for each). Coc-using participants with a prior history of unrestricted access also reported significantly more current Coc use, compared to Coc users with no prior history of unrestricted access. This was true across all measures of current use for these participants, including more days used in the past 30 d, more days of use per week, greater use per day, and higher total use per week (p<0.02 for each). Taken together, these results suggest that a history of unrestricted access to stimulants is associated with long-lasting increases in stimulant use.

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.

Dynamics of neuronal circuits in addiction: reward, antireward, and emotional memory

Drug addiction is conceptualized as chronic, relapsing compulsive use of drugs with significant dysregulation of brain hedonic systems. Compulsive drug use is accompanied by decreased function of brain substrates for drug positive reinforcement and recruitment of brain substrates mediating the negative reinforcement of motivational withdrawal. The neural substrates for motivational withdrawal ("dark side" of addiction) involve recruitment of elements of the extended amygdala and the brain stress systems, including corticotropin-releasing factor and norepinephrine. These changes, combined with decreased reward function, are hypothesized to persist in the form of an allostatic state that forms a powerful motivational background for relapse. Relapse also involves a key role for the basolateral amygdala in mediating the motivational effects of stimuli previously paired with drug seeking and drug motivational withdrawal. The basolateral amygdala has a key role in mediating emotional memories in general. The hypothesis argued here is that brain stress systems activated by the motivational consequences of drug withdrawal can not only form the basis for negative reinforcement that drives drug seeking, but also potentiate associative mechanisms that perpetuate the emotional state and help drive the allostatic state of addiction.

Increased operant responding for ethanol in male C57BL/6J mice: specific regulation by the ERK1/2, but not JNK, MAP kinase pathway

RATIONALE

Extracellular signal-regulated protein kinase (ERK(1/2)) is a member of the mitogen-activated protein kinase (MAPK) signaling pathway and a key molecular target for ethanol (EtOH) and other drugs of abuse.

OBJECTIVE

The aim of the study was to assess the role of two MAPK pathways, ERK(1/2) and c-Jun N-terminal kinase (JNK), on the modulation of EtOH and sucrose self-administration.

MATERIALS AND METHODS

C57BL/6J mice were trained to lever press on a fixed-ratio 4 schedule with 9% EtOH/2% sucrose, or 2% sucrose, as the reinforcer. In experiments 1 and 2, mice were injected with the MEK(1/2) inhibitor SL 327 (0-100 mg/kg) and the JNK inhibitor AS 6012452 (0-56 mg/kg) prior to self-administration. In experiment 3, SL 327 (0-100 mg/kg) was administered prior to performance on a progressive ratio (PR) schedule of EtOH reinforcement. In experiment 4, SL 327 and AS 601245 were injected 2 h before a locomotor test.

RESULTS

SL 327 (30 mg/kg) significantly increased EtOH self-administration without affecting locomotion. Higher doses of SL 327 and AS 601245 reduced EtOH-reinforced responding and locomotor activity. Reductions of both ligands on sucrose self-administration were due to decreases in motor activity. SL 327 pretreatment had no effect on PR responding.

CONCLUSIONS

ERK(1/2) activity is more directly involved in modulating the reinforcing properties of EtOH than JNK activity due to its selective potentiation of EtOH-reinforced responding. The specificity of this effect to EtOH self-administration, rather than sucrose self-administration, suggests that the mechanism by which ERK(1/2) increases EtOH-reinforced responding does not generalize to all reinforcing solutions and is not due to increased motivation to consume EtOH.

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.

Motivation for sucrose in sated rats is predicted by low anxiety-like behavior

Anxiety has been implicated in obesity and in the overconsumption of highly palatable foods such as those high in fat, sugar, or both. Also, the novelty-seeking trait has been associated with failure in weight-loss programs. The aim of this study was to investigate the associations of experimental anxiety and the self-administration of sucrose and high fat pellets in non-food deprived rats across different operant schedules. Male Wistar rats were subjected to the elevated plus-maze test (EPM) of anxiety-like behavior. The rats were tested for fixed ratio 5 (FR5) and progressive ratio (PR) operant responding for 50% sucrose, 95% sucrose, and high-fat pellets. PR active lever press response for 95% sucrose, but not the other pellet types, was correlated to % time spent on open arms (P=0.019) in the EPM. On the FR5 schedule, activity (closed arm entries) was correlated to the self-administration of 50% sucrose (P=0.027) and high-fat (P=0.002). This indicates an association of novelty-induced activity and self-administration of palatable food in sated rats, as well as a specific association of PR lever press response for 95% sucrose and low anxiety-like behavior. It has been argued that such active lever press response on PR may be interpreted as craving for the reinforcer; thus, our findings indicate an inverse relationship of experimental anxiety and craving for sucrose. This connection may have implications for human situations, since anxiety and novelty-seeking have been associated with obesity and failure in weight-loss programs.

The phenomenon of drug craving

The phenomenology of drug craving has become the focus of much research within addictive disorders because of the belief that desire plays a role in maintaining the addiction. Many of the studies have focused on the activation of neural pathways, particularly within the dopamine system in response to specific events or stimuli. While many of these studies have focused on a particular drug of choice, little has been done across addictive disorders. This article will present and review phenomena that induce drug craving, as well as delineate precise neural pathways which are activated during craving and specific neurobiological markers which are associated with an increased risk for drug craving and other forms of addictive behavior.

Effects of nicotine in experimental animals and humans: an update on addictive properties

Tobacco use through cigarette smoking is the leading preventable cause of death in the developed world. Nicotine, a psychoactive component of tobacco, appears to play a major role in tobacco dependence, but the reinforcing effects of nicotine have often been difficult to demonstrate directly in controlled studies with laboratory animals or human subjects. Here we update our earlier review published in Psychopharmacology (Berl) in 2006 on findings obtained with various procedures developed to study dependence-related behavioral effects of nicotine in experimental animals and humans. Results obtained with drug self-administration, conditioned place preference, subjective reports of nicotine effects and nicotine discrimination indicate that nicotine can function as an effective reinforcer of drug-seeking and drug-taking behavior both in experimental animals and humans under appropriate conditions. Interruption of chronic nicotine exposure produces ratings of drug withdrawal and withdrawal symptoms that may contribute to relapse. Difficulties encountered in demonstrating reinforcing effects of nicotine under some conditions, relative to other drugs of abuse, may be due to weaker primary reinforcing effects of nicotine, to aversive effects produced by nicotine, or to a more critical contribution of environmental stimuli to the maintenance of drug-seeking and drug-taking behavior with nicotine than with other drugs of abuse. Several recent reports suggest that other chemical substances inhaled along with nicotine in tobacco smoke may play a role in sustaining smoking behavior. However, conflicting results have been obtained with mice and rats and these findings have not yet been validated in nonhuman primates or human subjects. Taken together, these findings suggest that nicotine acts as a typical drug of abuse in experimental animals and humans in appropriate situations.

Medication-related pharmacological manipulations of nicotine self-administration in the rat maintained on fixed- and progressive-ratio schedules of reinforcement

RATIONALE

The use of animal models to study existing medications for smoking cessation can elucidate the mechanism(s) of action of cessation agents and further validate the models for medication development.

OBJECTIVE

The objective of the study was to evaluate the response of nicotine self-administration (NSA) to pharmacological agents related to the smoking cessation medication bupropion and to nicotine dosing mimicking nicotine replacement on fixed-ratio (FR) and progressive-ratio (PR) schedules of reinforcement.

MATERIALS AND METHODS

NSA was maintained at a nicotine dose of 30 microg/kg/infusion i.v. in rats trained on FR5 and PR40% schedules. Pharmacological manipulations related to bupropion were examined by treating animals with a dopamine reuptake inhibitor [GBR 12909 (GBR)], a norepinephrine reuptake inhibitor [nisoxetine (NIS)], and a nicotinic antagonist [dihydro-beta-erythroidine (DHbetaE)]. The effect of nicotine replacement was examined on the PR schedule by chronic dosing with osmotic minipumps.

RESULTS

Significant treatment effects occurred with NIS and combinations of NIS-DHbetaE and with GBR on response rates. Chronic nicotine dosing reduced self-administration. The two schedules yielded different results with some treatments.

CONCLUSIONS

Noradrenergic-nicotinic cholinergic interactions and enhanced responding consequent to dopamine reuptake inhibition may be part of the complex behavioral pharmacology of bupropion-like compounds. Observation of differential results with the two schedules has implication for the use of self-administration techniques to elaborate the mechanisms of dependence as well as drug discovery.

Removing obstacles in neuroscience drug discovery: the future path for animal models

Despite great advances in basic neuroscience knowledge, the improved understanding of brain functioning has not yet led to the introduction of truly novel pharmacological approaches to the treatment of central nervous system (CNS) disorders. This situation has been partly attributed to the difficulty of predicting efficacy in patients based on results from preclinical studies. To address these issues, this review critically discusses the traditional role of animal models in drug discovery, the difficulties encountered, and the reasons why this approach has led to suboptimal utilization of the information animal models provide. The discussion focuses on how animal models can contribute most effectively to translational medicine and drug discovery and the changes needed to increase the probability of achieving clinical benefit. Emphasis is placed on the need to improve the flow of information from the clinical/human domain to the preclinical domain and the benefits of using truly translational measures in both preclinical and clinical testing. Few would dispute the need to move away from the concept of modeling CNS diseases in their entirety using animals. However, the current emphasis on specific dimensions of psychopathology that can be objectively assessed in both clinical populations and animal models has not yet provided concrete examples of successful preclinical-clinical translation in CNS drug discovery. The purpose of this review is to strongly encourage ever more intensive clinical and preclinical interactions to ensure that basic science knowledge gained from improved animal models with good predictive and construct validity readily becomes available to the pharmaceutical industry and clinical researchers to benefit patients as quickly as possible.

Relapse to drug seeking following prolonged abstinence: the role of environmental stimuli

Successful treatment of drug addiction must involve relapse prevention informed by our understanding of the neurobiological bases of drug relapse. In humans, exposure to drug-associated environmental stimuli can elicit drug craving and relapse. Because exposure to drug-paired stimuli similarly induces drug-seeking behavior in laboratory animals, several animal models of drug relapse have been developed. Here, we review animal models of cue-induced drug relapse and critically evaluate their validity and utility in addressing human relapse behaviors.

Roles of hippocampal NMDA receptors and nucleus accumbens D1 receptors in the amphetamine-produced conditioned place preference in rats

There are glutamatergic projections from the hippocampus to the nucleus accumbens (NAc), which regulate DA transmission in this structure. To be precise, the ventral hippocampal (VH) glutamatergic neurons project to the nucleus accumbens shell region (NAcSh), whereas the dorsal hippocampus (DH) sends glutamatergic projections to the nucleus accumbens core region (NAcC). This study investigates the roles of hippocampal N-methyl-D-aspartate (NMDA) glutamate receptors and NAc type 1 dopamine receptor (D1) in amphetamine-produced conditioned place preference (AMPH-CPP) in rats. Our earlier reports showed that AMPH-CPP results in the enhancement of hippocampal CaMKII activity and it can be impaired by NMDA antagonist (AP5). In this study AMPH-CPP did not alter the NAc CaMKII activity, although AMPH-CPP was impaired by a blockade of D1 receptors (SCH23390) during conditioning. Moreover, inactivation of hippocampal area (dorsal hippocampus or ventral hippocampus) impaired AMPH-CPP, but its effect was diminished by the activation of D1 receptors in accumbal region (NAc core or NAc shell). By inactivating both DH and NAc core resulted in the disruption of rat's CPP expression. However, the impaired CPP expression was recovered during the next testing session, suggesting the disruption of CPP expression was a short term effect. Moreover, the disruption of CPP expression was not exhibited if NAc core was not inactivated. Interestingly, the rats that received activation in VH but an inactivation in NAc shell before testing show impaired CPP expression compared to those received inactivation in both VH and NAc shell. DH activation plus an inactivation in NAc core before testing show a significantly higher rate of the weakening of AMPH-CPP expression. Similarly, an activation of VH plus an inactivation of NAc shell before testing also show a statistically significant lower CPP score on tests 3 and 4. These results, taken together, indicate that NMDA receptor activation in DH and VH have different enhancing effects on the AMPH-CPP as their innervations onto the different NAc regions are essential for AMPH-CPP establishment. If the deterioration of AMPH-CPP expression (or extinction process) resembles the formation of new learning, then this active process might have been facilitated by the hippocampal NMDA receptor activations during testing.

Contextual stimuli modulate extinction and reinstatement in rodents self-administering intravenous nicotine

RATIONALE

Discrete cues, such as drug-associated paraphernalia, play an important role in tobacco smoking and relapse, an effect that can be modelled in the nicotine-seeking behaviour of laboratory animals. However, the role of contextual stimuli (i.e. the drug taking environment) within nicotine dependence is less clear. The present study investigated the effects of contextual stimuli on nicotine detoxification and relapse.

MATERIALS AND METHODS

Male hooded Lister rats were trained to self-administer nicotine (0.03 mg/kg/infusion) in one of two distinct environmental contexts: transparent walls and rod floor or checkerboard walls and grid floor. Extinction of drug-seeking behaviour, either in the acquisition context or alternate context, was achieved by removing both nicotine infusions and response-contingent cues. The two contexts were then presented with or without nicotine priming and response-contingent cue presentation.

RESULTS

The initial rate of extinction was quicker in a novel environment compared to in the same context as training, although similar low levels of responding were eventually reached. Nicotine priming and re-presentation of cues resulted in significant reinstatement of nicotine-seeking behaviour, but there was a trend towards a reduction in this effect when conducted in a novel environment. In addition, re-presentation of the acquisition context after extinction in the alternate context produced a significant reinstatement of nicotine-seeking behaviour without the need for nicotine priming and re-presentation of cues.

CONCLUSIONS

Contextual stimuli are capable of modulating the extinction and reinstatement of nicotine-seeking behaviour, and exposure to environments previously associated with smoking may lead to an increased risk of relapse. Context is an additional factor that could be targeted when developing smoking cessation strategies. For example, the long-term success of cue exposure might be improved by conducting treatment in multiple settings.

Sex differences in non-reinforced responding for cocaine

Prior studies report no sex differences in cocaine consumption during maintenance of self-administration. We find female rats show poorer lever discrimination during acquisition of self-administration. Now, we test whether female rats show greater non-reinforced or ineffective responding (presses during infusion and time-out periods as well as inactive lever presses) than male rats during maintenance of cocaine self-administration (.0625-1.0 mg/kg/infusion) in Experiment 1. Persistence of responding during extinction when saline-replaced cocaine was also examined. Whether response differences reflect sex differences in movements under a non-drug condition was tested in Experiment 2. Because cocaine may affect lever press rates differentially between sexes, we examined the effects of cocaine (.3-30 mg/kg; IP) on responding for food in Experiment 3. Cocaine consumption does not differ between female and male rats. However, females respond more during infusion and time-out periods and during extinction than males. There is no sex difference in movements and high cocaine doses decrease responding for food more in female vs. male rats. That females engage in more ineffective responding may represent heightened "craving" and cannot be explained by increased movements or cocaine-stimulated increases in lever pressing. In contrast, responding for cocaine in males appears driven by drug delivery.

Region-specific involvement of AMPA/Kainate receptors in Fos protein expression induced by cocaine-conditioned cues

This study investigated the effects of the AMPA/Kainate receptor antagonist, NBQX, on cue-elicited cocaine-seeking behavior and concomitant changes in Fos protein expression. After cocaine self-administration training, rats underwent 24 days of abstinence during which they were exposed daily either to the self-administration environment with response-contingent cues previously paired with cocaine infusions available (Extinction group) or to an alternate environment (No Extinction group). Subsequently, rats were tested for cocaine-seeking behavior (i.e., operant responses without cocaine reinforcement) elicited by the cocaine-associated cues after pretreatment with either vehicle or NBQX (10 mg/kg, IP). NBQX markedly attenuated cue-elicited cocaine-seeking behavior relative to vehicle pretreatment in the No Extinction group and also decreased cue-elicited Fos protein expression in a region-specific manner in the anterior cingulate and orbitofrontal cortices, basolateral amygdala, nucleus accumbens core, and dorsal caudate-putamen, suggesting involvement of AMPA glutamate systems in specific subregions of the neuronal circuitry activated by cocaine cues.

Intermittent access to preferred food reduces the reinforcing efficacy of chow in rats

Intermittent, extended access to preferred diets increases their intake. However, the effects of such access on the acceptance and reinforcing efficacy of otherwise satisfying alternatives is less known. To investigate the role of nonnutritional contributions to the hypophagia that follows removal of preferred food, male Wistar rats were fed a chow diet (Chow A/I), preferred to their regular chow (Chow), which was equally consumed under 1-choice conditions to an even more preferred chocolate-flavored, sucrose-rich diet (Preferred). Rats then learned to obtain Chow A/I pellets under a progressive ratio schedule of reinforcement and were assigned to two matched groups. Each week, one group (n = 15) was diet-cycled, receiving Chow A/I for 5 days followed by the Preferred diet for 2 days. Controls received Chow A/I daily (n = 14). Progressive ratio sessions were performed daily during the 5 days that all subjects received Chow A/I in the home cage. Across 5 wk, diet-cycled rats progressively ate less of the otherwise palatable Chow A/I diet. Hypophagia was not due to greater prior intake or weight gain, motor impairment, or facilitated satiation and was associated with changes in progressive ratio performance that suggested a reduced reinforcing efficacy of the Chow A/I diet in diet-cycled animals. By week 4, diet-cycled animals began to overeat the preferred diet, especially during the first 6 h of renewed access, resembling a deprivation effect. The results suggest that intermittent access to highly preferred food, as practiced by many restrained eaters, may progressively decrease the acceptability of less palatable foods, and may promote relapse to more rewarding alternatives.

Acupuncture for the treatment of drug addiction

Over the last three decades there has been an increasing interest in acupuncture treatment of substance abuse around the world. Three important steps can be identified in this field. Dr. Wen of Hong Kong was the first (1972) to report that acupuncture at 4 body points and 2 ear points combined with electrical stimulation can relieve opioid withdrawal signs in the addicts. The second major step was made by Dr. M. Smith in New York, the head of the National Acupuncture Detoxification Association (NADA) of the USA, who finalized a protocol (1985), using only ear points without electrical stimulation for the treatment of drug abuse. The recent advance in this field was made by Dr. Han of the Peking University, Beijing, who characterized a protocol (2005), using electrical stimulation of identified frequencies on body points to ameliorate heroin withdrawal signs and prevent relapse of heroin use. In this review, the efficacy of acupuncture and related techniques for the treatment of drug dependence in experimental settings and clinical practice will be reviewed, and the possible mechanisms underlying this effect be discussed.

The effects of chronic versus acute desipramine on nicotine withdrawal and nicotine self-administration in the rat

RATIONALE

Nicotine withdrawal is characterized by depression-like symptomatology that may be mediated by dysregulations in norepinephrine transmission. These aversive aspects of nicotine withdrawal and the rewarding effects of nicotine play major roles in maintaining nicotine dependence.

OBJECTIVES

The aim of this work was to evaluate the effects of desipramine (DMI), a preferential norepinephrine reuptake inhibitor and antidepressant, on preclinical models of nicotine dependence in rats.

MATERIALS AND METHODS

A rate-independent current-intensity discrete-trial threshold intracranial self-stimulation procedure was used to assess brain reward function during nicotine withdrawal induced by cessation of nicotine infusion via subcutaneous osmotic mini pumps (3.16 mg/kg/day, base). Nicotine withdrawal was also measured by somatic signs of withdrawal. DMI was administered acutely (2 or 5 mg/kg, salt) during nicotine/saline withdrawal. In other naïve rats, chronic DMI treatment via mini pump (15 mg/kg/day, salt) began after 7 days of nicotine/saline exposure and continued during administration of nicotine/saline for 14 days and during nicotine/saline withdrawal. Additional rats acquired intravenous nicotine- or food-maintained responding, were prepared with DMI/vehicle-containing mini pumps, and self-administered nicotine or food during 12 days of DMI/vehicle exposure.

RESULTS

Acute DMI administration had no effect on threshold elevations observed in nicotine-withdrawing rats. Chronic DMI administration prevented the reward threshold elevations and the increased somatic signs of nicotine withdrawal. Although chronic DMI significantly decreased nicotine self-administration, it also decreased food-maintained responding.

CONCLUSIONS

The results suggest that norepinephrine reuptake inhibitors may be effective anti-smoking treatments that reduce the anhedonic depression-like and somatic components of nicotine withdrawal and may alter the rewarding effects of nicotine and food.

Addiction and the brain antireward system

A neurobiological model of the brain emotional systems has been proposed to explain the persistent changes in motivation that are associated with vulnerability to relapse in addiction, and this model may generalize to other psychopathology associated with dysregulated motivational systems. In this framework, addiction is conceptualized as a cycle of decreased function of brain reward systems and recruitment of antireward systems that progressively worsen, resulting in the compulsive use of drugs. Counteradaptive processes, such as opponent process, that are part of the normal homeostatic limitation of reward function fail to return within the normal homeostatic range and are hypothesized to repeatedly drive the allostatic state. Excessive drug taking thus results in not only the short-term amelioration of the reward deficit but also suppression of the antireward system. However, in the long term, there is worsening of the underlying neurochemical dysregulations that ultimately form an allostatic state (decreased dopamine and opioid peptide function, increased corticotropin-releasing factor activity). This allostatic state is hypothesized to be reflected in a chronic deviation of reward set point that is fueled not only by dysregulation of reward circuits per se but also by recruitment of brain and hormonal stress responses. Vulnerability to addiction may involve genetic comorbidity and developmental factors at the molecular, cellular, or neurocircuitry levels that sensitize the brain antireward systems.

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.

The neurocircuitry of addiction: an overview

Drug addiction presents as a chronic relapsing disorder characterized by persistent drug-seeking and drug-taking behaviours. Given the significant detrimental effects of this disease both socially and economically, a considerable amount of research has been dedicated to understanding a number of issues in addiction, including behavioural and neuropharmacological factors that contribute to the development, loss of control and persistence of compulsive addictive behaviours. In this review, we will give a broad overview of various theories of addiction, animal models of addiction and relapse, drugs of abuse, and the neurobiology of drug dependence and relapse. Although drugs of abuse possess diverse neuropharmacological profiles, activation of the mesocorticolimbic system, particularly the ventral tegmental area, nucleus accumbens, amygdala and prefrontal cortex via dopaminergic and glutamatergic pathways, constitutes a common pathway by which various drugs of abuse mediate their acute reinforcing effects. However, long-term neuroadaptations in this circuitry likely underlie the transition to drug dependence and cycles of relapse. As further elucidated in more comprehensive reviews of various subtopics on addiction in later sections of this special issue, it is anticipated that continued basic neuroscience research will aid in the development of effective therapeutic interventions for the long-term treatment of drug-dependent individuals.

Neuropharmacology of alcohol addiction

Despite the generally held view that alcohol is an unspecific pharmacological agent, recent molecular pharmacology studies demonstrated that alcohol has only a few known primary targets. These are the NMDA, GABA(A), glycine, 5-hydroxytryptamine 3 (serotonin) and nicotinic ACh receptors as well as L-type Ca(2+) channels and G-protein-activated inwardly rectifying K(+) channels. Following this first hit of alcohol on specific targets in the brain, a second wave of indirect effects on a variety of neurotransmitter/neuropeptide systems is initiated that leads subsequently to the typical acute behavioural effects of alcohol, ranging from disinhibition to sedation and even hypnosis, with increasing concentrations of alcohol. Besides these acute pharmacodynamic aspects of alcohol, we discuss the neurochemical substrates that are involved in the initiation and maintenance phase of an alcohol drinking behaviour. Finally, addictive behaviour towards alcohol as measured by alcohol-seeking and relapse behaviour is reviewed in the context of specific neurotransmitter/neuropeptide systems and their signalling pathways. The activity of the mesolimbic dopaminergic system plays a crucial role during the initiation phase of alcohol consumption. Following long-term, chronic alcohol consumption virtually all brain neurotransmission seems to be affected, making it difficult to define which of the systems contributes the most to the transition from controlled to compulsive alcohol use. However, compulsive alcohol drinking is characterized by a decrease in the function of the reward neurocircuitry and a recruitment of antireward/stress mechanisms comes into place, with a hypertrophic corticotropin-releasing factor system and a hyperfunctional glutamatergic system being the most important ones.

gamma-Hydroxybutyric acid (GHB) suppresses alcohol's motivational properties in alcohol-preferring rats

gamma-Hydroxybutyric acid (GHB) reduces alcohol drinking, promotes abstinence from alcohol, suppresses craving for alcohol, and ameliorates alcohol withdrawal syndrome in alcoholics. At preclinical level, GHB suppresses alcohol withdrawal signs and alcohol intake in rats. The present study was designed to investigate whether GHB administration was capable of affecting alcohol's motivational properties (the possible animal correlate of human craving for alcohol) in selectively bred Sardinian alcohol-preferring rats. To this aim, rats were initially trained to lever press for alcohol (15%, vol/vol) under a procedure of operant, oral alcohol self-administration (fixed ratio 4 in 30-min daily sessions). Once responding for alcohol had stabilized, rats were divided into two groups and allocated to two independent experiments. Experiment 1 assessed the effect of GHB (0, 25, 50, and 100mg/kg, i.p.) on breakpoint for alcohol, defined as the lowest response requirement not achieved by each rat when exposed to a single-session progressive ratio schedule of reinforcement. Experiment 2 assessed the effect of GHB (0, 25, 50, and 100mg/kg, i.p.) on single-session extinction responding for alcohol (alcohol was absent and unreinforced responding was recorded). Breakpoint and extinction responding for alcohol are reliable indexes of alcohol's motivational strength. In Experiment 1, all doses of GHB reduced--by approximately 20% in comparison to saline-treated rats--breakpoint for alcohol. In Experiment 2, administration of 25, 50, and 100mg/kg GHB reduced--by approximately 25%, 40%, and 50%, respectively, in comparison to saline-treated rats--extinction responding for alcohol. Conversely, no dose of GHB altered breakpoint and extinction responding for sucrose (3%, wt/vol) in two independent subsets of Sardinian alcohol-preferring rats. Together, these data suggest that GHB administration specifically suppressed alcohol's motivational properties in Sardinian alcohol-preferring rats. These results are consistent with the anticraving properties of GHB observed in clinical studies.

Goal- and signal-directed incentive: conditioned approach, seeking, and consumption established with unsweetened alcohol in rats

RATIONALE

Many theories of addictive behavior propose that cues signaling drug administration influence the likelihood of drug-taking and drug-seeking behavior.

OBJECTIVES

We investigated the behavioral impact of cues associated with unsweetened ethanol and their interaction with responding maintained by ethanol self-administration. Our goal was to establish the influence of such cues on ethanol seeking.

MATERIALS AND METHODS

The experiment used a matching contingency and saccharin-fading procedure to establish equal levels of responding to two spatially distinct levers using unsweetened 10% ethanol solution. After ethanol self-administration was established, a brief cue light located alternately over each lever location was either paired or unpaired (control) with the opportunity to consume the same ethanol solution. Finally, self-administration was re-established, and the effect of the cue was measured in a transfer design.

RESULTS

The reaction to lights paired with the opportunity to ingest unsweetened ethanol had three main effects: (1) induction of operant behavior reinforced by ethanol, (2) stimulation of ethanol-seeking behavior (drinker entries), and (3) cue-directed approach and contact behavior (i.e. autoshaping or sign-tracking). Cue-directed behavior to the light interacted with choice behavior in a manner predicted by the location of the cue light, enhancing responding only when the approach response did not interfere with the operant response.

CONCLUSIONS

These findings replicate and extend the effects of Pavlovian conditioning on ethanol-seeking and support-conditioned incentive theories of addictive behavior. Signals for ethanol influence spatial choice behavior and may be relevant to attentional bias shown to alcohol-associated stimuli in humans.

Cue-induced reinstatement of nicotine-seeking behavior in rats: effect of bupropion, persistence over repeated tests, and its dependence on training dose

RATIONALE

The motivational effects of nicotine-associated cues have been demonstrated in animal studies. However, it is unknown whether the effectiveness of nicotine cues in reinstating nicotine-seeking varies with the extent of prior nicotine self-administration. In addition, the issue of whether bupropion (an FDA-approved smoking cessation medication) interferes with the conditioned incentive of nicotine cues remains to be addressed.

OBJECTIVE

This study determined the relationship of cue-reinstated nicotine-seeking and the levels of prior self-administration and examined the effect of bupropion on cue-induced reinstatement of nicotine-seeking in comparison with that on self-administration.

MATERIALS AND METHODS

Male Sprague-Dawley rats were trained in daily 1-h sessions to intravenously self-administer nicotine at different doses (0, 0.015, 0.03, 0.06 mg/kg/infusion) and to associate an auditory/visual cue with each nicotine delivery. After extinction, three reinstatement tests at 15 day intervals were conducted with re-presentation of the cue without nicotine delivery. In separate groups of rats trained with 0.03 mg/kg/infusion nicotine, bupropion (0, 10, 20, 40 mg/kg) was intraperitoneally administered to different groups before the reinstatement and in a within-subject design before the self-administration tests.

RESULTS

Cue-induced reinstatement of active lever responses was observed at all nicotine doses in the first reinstatement test, but at only the two highest doses during the second and third tests. The magnitude of reinstatement was positively correlated with level of prior responding for nicotine. Bupropion pretreatment decreased nicotine self-administration but enhanced cue-reinstated nicotine-seeking.

CONCLUSIONS

These results demonstrate the positive correlation of cue-reinstated nicotine-seeking with prior responding for nicotine self-administration and the persistence of the cue effect after taking higher doses of nicotine. The results of pharmacological tests suggest that although it is able to help achieve smoking cessation, bupropion may have little clinical benefit for the prevention of relapse associated with exposure to environmental smoking cues.

Impulsive choice and impulsive action predict vulnerability to distinct stages of nicotine seeking in rats

BACKGROUND

Although heavy smoking has been associated with impulsivity in humans, it is not clear whether poor impulse control represents a risk factor in the etiology of nicotine dependence.

METHODS

To address this issue, rats were selected on the basis of individual differences in impulsivity in the delayed reward task (impulsive choice) and the 5-choice serial reaction time task (impulsive action). Subsequently, rats were subjected to a nicotine self-administration (SA) paradigm tailored to measure the motivational properties of nicotine and nicotine-associated stimuli. In separate groups, differences in electrically evoked dopamine release in slice preparations obtained from several mesolimbic brain regions were determined.

RESULTS

Impulsive action was associated with an enhanced motivation to initiate and maintain nicotine SA. In contrast, impulsive choice predicted a diminished ability to inhibit nicotine seeking during abstinence and an enhanced vulnerability to relapse upon re-exposure to nicotine cues. Impulsive action was associated with reduced dopamine release in the accumbens core and impulsive choice with reduced dopamine release in accumbens core, shell, and medial prefrontal cortex.

CONCLUSIONS

The strong association between sub-dimensions of impulsivity and nicotine SA implies that interventions aimed to improve impulse control might help to reduce susceptibility to nicotine dependence and/or lead to successful smoking cessation.