Influence of cue-conditioning on acquisition, maintenance and relapse of cocaine intravenous self-administration

Individual variations in motives for nicotine self-administration in male rats: evidence in support for a precision psychopharmacology

The significant heterogeneity in smoking behavior among smokers, coupled with the inconsistent efficacy of approved smoking cessation therapies, supports the presence of individual variations in the mechanisms underlying smoking. This emphasizes the need to shift from standardized to personalized smoking cessation therapies. However, informed precision medicine demands precision fundamental research. Tobacco smoking is influenced and sustained by diverse psychopharmacological interactions between nicotine and environmental stimuli. In the classical experimental rodent model for studying tobacco dependence, namely intravenous self-administration of nicotine, seeking behavior is reinforced by the combined delivery of nicotine and a discrete cue (nicotine+cue). Whether self-administration behavior is driven by the same psychopharmacological mechanisms across individual rats remains unknown and unexplored. To address this, we employed behavioral pharmacology and unbiased cluster analysis to investigate individual differences in the mechanisms supporting classical intravenous nicotine self-administration (0.04 mg/kg/infusion) in male outbred Sprague-Dawley rats. Our analysis identified two clusters: one subset of rats sought nicotine primarily for its reinforcing effects, while the second subset sought nicotine to enhance the reinforcing effects of the discrete cue. Varenicline (1 mg/kg i.p.) reduced seeking behavior in the former group, whereas it tended to increase in the latter group. Crucially, despite this fundamental qualitative difference revealed by behavioral manipulation, the two clusters exhibited quantitatively identical nicotine+cue self-administration behavior. The traditional application of rodent models to study the reinforcing and addictive effects of nicotine may mask individual variability in the underlying motivational mechanisms. Accounting for this variability could significantly enhance the predictive validity of translational research.

Modulation of cue value and the augmentation of heroin seeking in chronically food-restricted male rats under withdrawal

Food restriction augments drug seeking in abstinent rats. The underlying motivational mechanisms, however, remain unclear. We hypothesized that caloric restriction enhances the incentive value attributed to drug-associated cues and, in turn, augments drug seeking. Male rats were trained to lever-press for heroin, and then moved to the animal colony for a forced-abstinence period. Rats were maintained on free access to food (Sated) or subjected to 14 days of food restriction (FDR). In a series of experiments, we assessed the effect of food-restriction on the incentive value of heroin-associated cues. Tests included performance under a progressive ratio (PR) schedule of reinforcement maintained by heroin-associated cues, acquisition of a novel operant response reinforced by drug-associated cues, effect of food-restriction on operant response reinforced by neutral cues, acquisition of a novel operant response reinforced by drug-associated or neutral cues, and the effect of food-restriction on operant response reinforced by drug-associated or neutral cues, under a discrete choice procedure. Food-restriction did not change breakpoints in PR maintained by heroin-associated cues. FDR rats acquired the novel response at a greater level compared to the Sated group. Food-restriction-induced increase in novel-response rate was observed for both heroin-paired and the neutral cue. Responding for a heroin-associated cue was greater than for the neutral cue in both Sated and FDR groups. Response rate for the neutral cue, however, was greater in the FDR versus Sated group. Our findings suggest that food restriction increases the conditioned motivational properties of environmental stimuli, including, but not exclusive to, heroin-paired cues.

The ventral tegmental area dopamine to lateral amygdala projection supports cocaine cue associative learning

Learning and memory mechanisms are critically involved in drug craving and relapse. Environmental cues paired with repeated drug use acquire incentive value such that exposure to the cues alone can trigger craving and relapse. The amygdala, particularly the lateral amygdala (LA), underlies cue-related learning processes that assign valence to environmental stimuli including drug-paired cues. Evidence suggests that the ventral tegmental area (VTA) dopamine (DA) projection to the LA participates in encoding reinforcing effects that act as a US in conditioned cue reward-seeking as DA released in the amygdala is important for emotional and behavioral functions. Here we used chemogenetics to manipulate these VTA DA inputs to the LA to determine the role of this projection for acquisition of drug-cue associations and reinstatement of drug-seeking. We found inhibiting DA input to the LA during cocaine self-administration slowed acquisition and weakened the ability of the previously cocaine-paired cue to elicit cocaine-seeking. Conversely, exciting the projection during self-administration boosted the salience of the cocaine-paired cue as indicated by enhanced responding during cue-induced reinstatement. Importantly, interfering with DA input to the LA had no impact on the ability of cocaine to elicit a place preference or induce reinstatement in response to a priming cocaine injection. Overall, we show that manipulation of projections underlying DA signaling in the LA may be useful for developing therapeutic interventions for substance use disorders.

Prelimbic cortical projections to rostromedial tegmental nucleus play a suppressive role in cue-induced reinstatement of cocaine seeking

The prelimbic (PL) region of prefrontal cortex has been implicated in both driving and suppressing cocaine seeking in animal models of addiction. We hypothesized that these opposing roles for PL may be supported by distinct efferent projections. While PL projections to nucleus accumbens core have been shown to be involved in driving reinstatement of cocaine seeking, PL projections to the rostromedial tegmental nucleus (RMTg) may instead suppress reinstatement of cocaine seeking, due to the role of RMTg in behavioral inhibition. Here, we used a functional disconnection approach to temporarily disrupt the PL-RMTg pathway during cue- or cocaine-induced reinstatement. Male Sprague Dawley rats self-administered cocaine during daily 2-h sessions for ≥10 days and then underwent extinction training. Reinstatement of extinguished cocaine seeking was elicited by cocaine-associated cues or cocaine prime. Prior to reinstatement, rats received microinjections of the GABA agonists baclofen/muscimol (1/0.1 mM) into unilateral PL and the AMPA receptor antagonist NBQX (1 mM) into contralateral or ipsilateral RMTg. Functional disconnection of PL-RMTg via contralateral inactivation markedly increased cue-induced reinstatement, but did not increase cocaine-induced reinstatement or drive reinstatement of extinguished cocaine seeking in the absence of cues or cocaine. Enhanced cue-induced reinstatement was also observed with ipsilateral inactivation of PL and RMTg, but not with unilateral inactivation of PL or RMTg alone, indicating that both ipsilateral and contralateral projections from PL to RMTg have an inhibitory influence on behavior. These data further support a suppressive role for PL in cocaine seeking by implicating PL efferent projections to RMTg in inhibiting cue-induced reinstatement.

Associative processes in addiction relapse models: A review of their Pavlovian and instrumental mechanisms, history, and terminology

Animal models of relapse to drug-seeking have borrowed heavily from associative learning approaches. In studies of relapse-like behaviour, animals learn to self-administer drugs then receive a period of extinction during which they learn to inhibit the operant response. Several triggers can produce a recovery of responding which form the basis of a variety of models. These include the passage of time (spontaneous recovery), drug availability (rapid reacquisition), extinction of an alternative response (resurgence), context change (renewal), drug priming, stress, and cues (reinstatement). In most cases, the behavioural processes driving extinction and recovery in operant drug self-administration studies are similar to those in the Pavlovian and behavioural literature, such as context effects. However, reinstatement in addiction studies have several differences with Pavlovian reinstatement, which have emerged over several decades, in experimental procedures, associative mechanisms, and terminology. Interestingly, in cue-induced reinstatement, drug-paired cues that are present during acquisition are omitted during lever extinction. The unextinguished drug-paired cue may limit the model’s translational relevance to cue exposure therapy and renders its underlying associative mechanisms ambiguous. We review major behavioural theories that explain recovery phenomena, with a particular focus on cue-induced reinstatement because it is a widely used model in addiction. We argue that cue-induced reinstatement may be explained by a combination of behavioural processes, including reacquisition of conditioned reinforcement and Pavlovian to Instrumental Transfer. While there are important differences between addiction studies and the behavioural literature in terminology and procedures, it is clear that understanding associative learning processes is essential for studying relapse.

Amphetamine maintenance therapy during intermittent cocaine self-administration in rats attenuates psychomotor and dopamine sensitization and reduces addiction-like behavior

D-amphetamine maintenance therapy shows promise as a treatment for people with cocaine addiction. Preclinical studies using Long Access (LgA) cocaine self-administration procedures suggest D-amphetamine may act by preventing tolerance to cocaine's effects at the dopamine transporter (DAT). However, Intermittent Access (IntA) cocaine self-administration better reflects human patterns of use, is especially effective in promoting addiction-relevant behaviors, and instead of tolerance, produces psychomotor, incentive, and neural sensitization. We asked, therefore, how D-amphetamine maintenance during IntA influences cocaine use and cocaine's potency at the DAT. Male rats self-administered cocaine intermittently (5 min ON, 25 min OFF x10; 5-h/session) for 14 sessions, with or without concomitant D-amphetamine maintenance therapy during these 14 sessions (5 mg/kg/day via s.c. osmotic minipump). We then assessed responding for cocaine under a progressive ratio schedule, responding under extinction and cocaine-primed reinstatement of drug seeking. We also assessed the ability of cocaine to inhibit dopamine uptake in the nucleus accumbens core using fast scan cyclic voltammetry ex vivo. IntA cocaine self-administration produced psychomotor (locomotor) sensitization, strong motivation to take and seek cocaine, and it increased cocaine's potency at the DAT. D-amphetamine co-administration suppressed the psychomotor sensitization produced by IntA cocaine experience. After cessation of D-amphetamine treatment, the motivation to take and seek cocaine was also reduced, and sensitization of cocaine's actions at the DAT was reversed. Thus, treatment with D-amphetamine might reduce cocaine use by preventing sensitization-related changes in cocaine potency at the DAT, consistent with an incentive-sensitization view of addiction.

Nicotine reduction does not alter essential value of nicotine or reduce cue-induced reinstatement of nicotine seeking

Reduction of nicotine content in tobacco products is a regulatory control strategy intended to decrease smoking dependence, and is hypothesized to produce gradual reductions of nicotine intake. Rats were initially trained to self-administer 0.06 mg/kg/infusion nicotine (Phase 1), which was followed by a threshold procedure to determine nicotine demand via a behavioral economics (BE) paradigm (Phase 2). Rats then either self-administered the training dose (high dose group), or were switched to a low dose of nicotine (0.001 mg/kg/infusion; low dose group) in Phase 3. Both groups then underwent a second threshold procedure and demand curves were re-determined (Phase 4). In Phase 5, responding for nicotine was extinguished over the course of 21 sessions. Cue-induced reinstatement was then evaluated (Phase 6). Rats in the low dose group maintained a steady amount of infusions, and thus, did not compensate for nicotine reduction. Rats in the low dose group also showed similar demand elasticity and nicotine seeking (Phase 6) compared to the high dose group, indicating that nicotine reduction did not decrease nicotine demand or seeking. Further, both groups displayed resistance to extinction, indicating that nicotine reduction did not facilitate extinction learning. These results suggest that although compensation of intake does not occur, decreasing the dose of nicotine does not alter nicotine reinforcement value or relapse vulnerability. Further, these results indicate persistence of nicotine-motivated behavior after self-administration of a low nicotine dose. Translationally, these results suggest that alternative strategies may be needed to achieve positive smoking cessation outcomes.

Varenicline Targets the Reinforcing-Enhancing Effect of Nicotine on Its Associated Salient Cue During Nicotine Self-administration in the Rat

Nicotine is acknowledged as the key addictive compound of tobacco. Varenicline (Champix^® or Chantix^®), mainly acting as a partial agonist at the α4β2 nicotinic receptor, is an approved smoking cessation pharmacotherapy, although with efficacy limited to a portion of smokers. Smokers differ in the motives that drive their drug seeking and Varenicline might be more efficient in some groups more than others. Studies in rodents revealed that nicotine-seeking is strongly supported by complex interactions between nicotine and environmental cues, and notably the ability of nicotine to enhance the reinforcing properties of salient environmental stimuli. It is not yet understood whether the decrease of nicotine-seeking by acute Varenicline in rats results from antagonism of the primary reinforcing effects of nicotine, of the reinforcement-enhancing effect of nicotine on cues, or of a combination of both. Thanks to a protocol that allows assessment of the reinforcement-enhancing effect of nicotine on cues during self-administration in rats, we showed that Varenicline targets both nicotine reinforcing effects and reinforcement-enhancing effect of nicotine on cues. Importantly, individual variations in the latter determined the amplitude of acute Varenicline-induced decrease in seeking. These results suggest that Varenicline might be more beneficial in smokers who are more sensitive to nicotine effects on surrounding stimuli.

An extended history of drug self-administration results in multiple sources of control over drug seeking behavior

It is widely recognized that across the development of drug addiction, cues associated with drug use come to exert increasing control over drug seeking and taking behaviors. However, there remain gaps in our knowledge regarding how the different types of drug related cues affect drug seeking and taking behaviors, and how the emergence of cue control over these behaviors relates to the onset of drug seeking compulsions. This paper reviews the literature on drug self-administration in animals to address these gaps. It first identifies the different types of cues that acquire control over reward seeking behavior generally, and examines whether the same types of cues acquire control over drug seeking behavior specifically. It then examines how the role of drug related cues in motivating and reinforcing drug seeking behavior changes across an extended drug-taking history, with a particular focus on the case of nicotine. The evidence reviewed shows that, after an extended history of drug taking, drug seeking behaviors are controlled by contextual cues associated with the development of drug seeking habits, response contingent cues that accompany delivery of the drug, as well as internal states that correlate with levels of drug intake. These multiple sources of control over drug seeking are discussed in relation to the generation of an addicted phenotype in animal models and the hypothesized progression from internal control over drug use to compulsive drug seeking.

A neuronal activation correlate in striatum and prefrontal cortex of prolonged cocaine intake

Maladaptive changes in the involvement of striatal and frontal cortical regions in drug use are thought to underlie the progression to habitual drug use and loss of cognitive control over drug intake that occur with accumulating drug experience. The present experiments focus on changes in neuronal activity in these regions associated with short-term (10 days) and long-term (60 days) self-administration of cocaine. Quantitative in situ hybridization for the immediate early gene Mkp1 was combined with statistical parametric mapping to assess the distribution of neuronal activity. We hypothesized that neuronal activity in striatum would increase in its dorsal part and that activity in frontal cortex would decrease with prolonged cocaine self-administration experience. Expression of Mkp1 was profoundly increased after cocaine self-administration, and the magnitude of this effect was greater after short-term compared to long-term self-administration. Increased neuronal activity was seen in both dorsal and ventral sectors of the striatum after 10 days exposure to cocaine. However, enhanced activity was restricted to dorsomedial and dorsocentral striatum after 60 days cocaine self-administration. In virtually all medial prefrontal and most orbitofrontal areas, increased expression of Mkp1 was observed after 10 days of cocaine taking, whereas after 60 days, enhanced expression was restricted to caudal parts of medial prefrontal and caudomedial parts of orbitofrontal cortex. Our data reveal functional changes in cellular activity in striatum and frontal cortex with increasing cocaine self-administration experience. These changes might reflect the neural processes that underlie the descent from recreational drug taking to compulsive cocaine use.

Repeated MDMA administration increases MDMA-produced locomotor activity and facilitates the acquisition of MDMA self-administration: role of dopamine D2 receptor mechanisms

RATIONALE

Repeated exposure to ±3, 4-methylenedioxymethamphetamine (MDMA) produces sensitization to MDMA-produced hyperactivity, but the mechanisms underlying the development of this sensitized response or the relationship to the reinforcing effects of MDMA is unknown.

OBJECTIVES

This study determined the effect of a sensitizing regimen of MDMA exposure on the acquisition of MDMA self-administration and investigated the role of dopamine D₂ receptor mechanisms.

METHODS

Rats received the selective D₂ antagonist, eticlopride (0.0 or 0.3 mg/kg, i.p.) and MDMA (0.0 or 10.0 mg/kg, i.p.) during a five-day pretreatment regimen. Two days following the final session, the locomotor activating effects of MDMA (5 mg/kg, i.p.) and the latency to acquisition of MDMA self-administration were determined.

RESULTS

Pretreatment with MDMA enhanced the locomotor activating effects of MDMA and facilitated the acquisition of MDMA self-administration. Administration of eticlopride during MDMA pretreatment completely blocked the development of sensitization to MDMA-produced hyperactivity but failed to significantly alter the facilitated acquisition of MDMA self-administration. Pretreatment with eticlopride alone facilitated the acquisition of self-administration.

CONCLUSIONS

These data suggest that repeated MDMA exposure sensitized both the locomotor activating and reinforcing effects of MDMA. Activation of D₂ receptors during MDMA pretreatment appears critical for the development of sensitization to MDMA-produced hyperactivity. The role of D₂ receptor mechanisms in the development of sensitization to the reinforcing effects of MDMA is equivocal.

Extended nicotine self-administration increases sensitivity to nicotine, motivation to seek nicotine and the reinforcing properties of nicotine-paired cues

An array of pharmacological and environmental factors influence the development and maintenance of tobacco addiction. The nature of these influences likely changes across the course of an extended smoking history, during which time drug seeking can become involuntary and uncontrolled. The present study used an animal model to examine the factors that drive nicotine-seeking behavior after either brief (10 days) or extended (40 days) self-administration training. In Experiment 1, extended training increased rats' sensitivity to nicotine, indicated by a leftward shift in the dose-response curve, and their motivation to work for nicotine, indicated by an increase in the break point achieved under a progressive ratio schedule. In Experiment 2, extended training imbued the nicotine-paired cue with the capacity to maintain responding to the same high level as nicotine itself. However, Experiment 3 showed that the mechanisms involved in responding for nicotine or a nicotine-paired cue are dissociable, as treatment with the partial nicotine receptor agonist, varenicline, suppressed responding for nicotine but potentiated responding for the nicotine-paired cue. Hence, across extended nicotine self-administration, pharmacological and environmental influences over nicotine seeking increase such that nicotine seeking is controlled by multiple sources, and therefore highly resistant to change.

Design and development of a modified runway model of mouse drug self-administration

The present study established a novel mouse model of a runway drug self-administration in our laboratory. The operant runway apparatus consisted of three long runways arranged in a zig-zag manner. The methodology consisted of six distinct phases: habituation, preconditioning, conditioning, post-conditioning, extinction and reinstatement. The effects of saline were compared with escalating doses of either ethanol (0.5–4.0 g/kg, i.p), heroin (5–40 mg/kg, i.p), or nicotine (0.1–0.5mg/kg, i.p) administered in the goal box during the conditioning phase (day 1 to day 5). A significant decrease in the time of trained (conditioned) mice to reach the goal box confirmed the subjects’ motivation to seek those drugs on day 6 (expression). The mice were then subjected to non-rewarded extinction trials for 5 days over which run times were significantly increased. After 5 days of abstinence, a priming dose of ethanol or heroin (1/5th of maximum dose used in conditioning) significantly reinstated the drug-seeking behavior. These results suggest that the modified runway model can serve as a powerful behavioral tool for the study of the behavioral and neurobiological bases of drug self-administration and, as such, is appropriate simple but powerful tool for investigating the drug-seeking behavior of laboratory mice.

Contrasting Roles of Dopamine and Noradrenaline in the Motivational Properties of Social Play Behavior in Rats

Social play behavior, abundant in the young of most mammalian species, is thought to be important for social and cognitive development. Social play is highly rewarding, and as such, the expression of social play depends on its pleasurable and motivational properties. Since the motivational properties of social play have only sporadically been investigated, we developed a setup in which rats responded for social play under a progressive ratio schedule of reinforcement. Dopaminergic neurotransmission plays a key role in incentive motivational processes, and both dopamine and noradrenaline have been implicated in the modulation of social play behavior. Therefore, we investigated the role of dopamine and noradrenaline in the motivation for social play. Treatment with the psychostimulant drugs methylphenidate and cocaine increased responding for social play, but suppressed its expression during reinforced play periods. The dopamine reuptake inhibitor GBR-12909 increased responding for social play, but did not affect its expression, whereas the noradrenaline reuptake inhibitor atomoxetine decreased responding for social play as well as its expression. The effects of methylphenidate and cocaine on responding for social play, but not their play-suppressant effects, were blocked by pretreatment with the dopamine receptor antagonist α-flupenthixol. In contrast, pretreatment with the α2-adrenoceptor antagonist RX821002 prevented the play-suppressant effect of methylphenidate, but left its effect on responding for social play unaltered. In sum, the present study introduces a novel method to study the incentive motivational properties of social play behavior in rats. Using this paradigm, we demonstrate dissociable roles for dopamine and noradrenaline in social play behavior: dopamine stimulates the motivation for social play, whereas noradrenaline negatively modulates the motivation for social play behavior and its expression.

Individual variation in the motivational properties of a nicotine cue: sign-trackers vs. goal-trackers

RATIONALE

Individuals vary in the extent to which they attribute incentive salience to reward cues. Discrete food and drug (cocaine and opioid) cues become more attractive, eliciting approach toward them, and more "wanted," in that they serve as more effective conditioned reinforcers, in some rats (sign-trackers, STs), than in others (goal-trackers, GTs).

OBJECTIVES

We asked whether there is similar variation in the extent to which a cue associated with a drug from another class, nicotine, acquires incentive motivational properties.

METHODS

First, a Pavlovian conditioned approach procedure was used to identify rats that attribute incentive salience to a food cue (i.e., STs and GTs). We then measured the extent to which a cue (a light) paired with intravenous nicotine injections acquired two properties of an incentive stimulus: (1) the ability to elicit approach toward it, and (2) the ability to act as a conditioned reinforcer.

RESULTS

In contrast to previous findings with food, cocaine, and opioid cues, we found that the nicotine cue was equally attractive in STs and GTs, eliciting dose-dependent approach behavior in both. However, the nicotine cue was a more effective conditioned reinforcer in STs than in GTs.

CONCLUSIONS

We suggest the dissociation between these two measures of incentive salience attribution may be related to the fact that when present (as in the test of Pavlovian approach), nicotine can act as a potent "incentive amplifier," and by this action, nicotine may render cues especially salient for all animals.

Effect of yohimbine on reinstatement of operant responding in rats is dependent on cue contingency but not food reward history

Yohimbine is an alpha-2 adrenoceptor antagonist that has been used in numerous studies as a pharmacological stressor in rodents, monkeys and humans. Recently, yohimbine has become the most common stress manipulation in studies on reinstatement of drug and food seeking. However, the wide range of conditions under which yohimbine promotes reward seeking is significantly greater than that of stressors like intermittent footshock. Here, we addressed two fundamental questions regarding yohimbine's effect on reinstatement of reward seeking: (1) whether the drug's effect on operant responding is dependent on previous reward history or cue contingency, and (2) whether yohimbine is aversive or rewarding under conditions typically used in reinstatement studies. We also used in vivo microdialysis to determine yohimbine's effect on dopamine levels in nucleus accumbens (NAc) and medial prefrontal cortex (mPFC). We found that the magnitude of yohimbine-induced (0.5, 1.0, 2.0 mg/kg) operant responding during the reinstatement tests was critically dependent on the contingency between lever pressing and discrete tone-light cue delivery but not the previous history with food reward during training. We also found that yohimbine (2 mg/kg) did not cause conditioned place aversion. Finally, we found that yohimbine modestly increased dopamine levels in mPFC but not NAc. Results suggest that yohimbine's effects on operant responding in reinstatement studies are likely independent of the history of contingent self-administration of food or drug rewards and may not be related to the commonly assumed stress-like effects of yohimbine.

Loss of feedback inhibition via D2 autoreceptors enhances acquisition of cocaine taking and reactivity to drug-paired cues

A prominent aspect of drug addiction is the ability of drug-associated cues to elicit craving and facilitate relapse. Understanding the factors that regulate cue reactivity will be vital for improving treatment of addictive disorders. Low availability of dopamine (DA) D2 receptors (D2Rs) in the striatum is associated with high cocaine intake and compulsive use. However, the role of D2Rs of nonstriatal origin in cocaine seeking and taking behavior and cue reactivity is less understood and possibly underestimated. D2Rs expressed by midbrain DA neurons function as autoreceptors, exerting inhibitory feedback on DA synthesis and release. Here, we show that selective loss of D2 autoreceptors impairs the feedback inhibition of DA release and amplifies the effect of cocaine on DA transmission in the nucleus accumbens (NAc) in vitro. Mice lacking D2 autoreceptors acquire a cued-operant self-administration task for cocaine faster than littermate control mice but acquire similarly for a natural reward. Furthermore, although mice lacking D2 autoreceptors were able to extinguish self-administration behavior in the absence of cocaine and paired cues, they exhibited perseverative responding when cocaine-paired cues were present. This enhanced cue reactivity was selective for cocaine and was not seen during extinction of sucrose self-administration. We conclude that low levels of D2 autoreceptors enhance the salience of cocaine-paired cues and can contribute to the vulnerability for cocaine use and relapse.

Orexin-1 receptor signaling increases motivation for cocaine-associated cues

The orexin/hypocretin system is involved in multiple cocaine addiction processes that involve drug-associated environmental cues, including cue-induced reinstatement of extinguished cocaine seeking and expression of conditioned place preference. However, the orexin system does not play a role in several behaviors that are less cue-dependent, such as cocaine-primed reinstatement of extinguished cocaine seeking and low-effort cocaine self-administration. We hypothesized that cocaine-associated cues, but not cocaine alone, engage signaling at orexin-1 receptors (OX1Rs), and this cue-engaged OX1R signaling increases motivation for cocaine. Motivation for cocaine was measured in Sprague-Dawley rats with behavioral-economic demand curve analysis after pretreatment with the OX1R antagonist SB-334867 (SB) or vehicle with and without light + tone cues. Demand for cocaine was higher when cocaine-associated cues were present, and SB only reduced cocaine demand in the presence of these cues. We then investigated whether cocaine demand was linked to the cued reinstatement of cocaine seeking, as both procedures are partially driven by cocaine-associated cues in an orexin-dependent manner. SB blocked cue-induced reinstatement behavior, and baseline demand predicted SB efficacy with the largest effect in high-demand animals, i.e. animals with the greatest cue-dependent behavior. We conclude that OX1R signaling increases the reinforcing efficacy of cocaine-associated cues but not that of cocaine alone. This supports our view that orexin plays a prominent role in the ability of conditioned cues to activate motivational responses.

Loss of dopaminergic nigrostriatal neurons accounts for the motivational and affective deficits in Parkinson's disease

Parkinson's disease (PD) involves the degeneration of dopaminergic (DA) neurons in the substantia nigra pars compacta (SNc) that is thought to cause the classical motor symptoms of this disease. However, motivational and affective impairments are also often observed in PD patients. These are usually attributed to a psychological reaction to the general motor impairment and to a loss of some of the neurons within the ventral tegmental area (VTA). We induced selective lesions of the VTA and SNc DA neurons that did not provoke motor deficits, and showed that bilateral dopamine loss within the SNc, but not within the VTA, induces motivational deficits and affective impairments that mimicked the symptoms of PD patients. Thus, motivational and affective deficits are a core impairment of PD, as they stem from the loss of the major group of neurons that degenerates in this disease (DA SNc neurons) and are independent of motor deficits.

Individual variation in resisting temptation: implications for addiction

When exposed to the sights, sounds, smells and/or places that have been associated with rewards, such as food or drugs, some individuals have difficulty resisting the temptation to seek out and consume them. Others have less difficulty restraining themselves. Thus, Pavlovian reward cues may motivate maladaptive patterns of behavior to a greater extent in some individuals than in others. We are just beginning to understand the factors underlying individual differences in the extent to which reward cues acquire powerful motivational properties, and therefore, the ability to act as incentive stimuli. Here we review converging evidence from studies in both human and non-human animals suggesting that a subset of individuals are more "cue reactive", in that certain reward cues are more likely to attract these individuals to them and motivate actions to get them. We suggest that those individuals for whom Pavlovian reward cues become especially powerful incentives may be more vulnerable to impulse control disorders, such as binge eating and addiction.

Cue-evoked cocaine "craving": role of dopamine in the accumbens core

Drug-associated cues can acquire powerful motivational control over the behavior of addicts, and can contribute to relapse via multiple, dissociable mechanisms. Most preclinical models of relapse focus on only one of these mechanisms: the ability of drug cues to reinforce drug-seeking actions following a period of extinction training. However, in addicts, drug cues typically do not follow seeking actions; they precede them. They often produce relapse by evoking a conditioned motivational state ("wanting" or "craving") that instigates and/or invigorates drug-seeking behavior. Here we used a conflict-based relapse model to ask whether individual variation in the propensity to attribute incentive salience to reward cues predicts variation in the ability of a cocaine cue to produce conditioned motivation (craving) for cocaine. Following self-administration training, responding was curtailed by requiring rats to cross an electrified floor to take cocaine. The subsequent response-independent presentation of a cocaine-associated cue was sufficient to reinstate drug-seeking behavior, despite the continued presence of the adverse consequence. Importantly, there were large individual differences in the motivational properties of the cocaine cue, which were predicted by variation in the propensity to attribute incentive salience to a food cue. Finally, a dopamine antagonist injected into the nucleus accumbens core attenuated, and amphetamine facilitated, cue-evoked cocaine seeking, implicating dopamine signaling in cocaine cue-evoked craving. These data provide a promising preclinical approach for studying sources of individual variation in susceptibility to relapse due to conditioned craving and implicate mesolimbic dopamine in this process.

Sensory reinforcement as a predictor of cocaine and water self-administration in rats

RATIONALE

The ability of locomotor activity in a novel environment (Loco) and visual stimulus reinforcement (VSR) to predict acquisition of responding for cocaine and water reinforcers in the absence of explicit audiovisual signals was evaluated.

METHODS

In Experiment 1 (Exp 1), rats (n = 60) were tested for VSR, followed by Loco, and finally acquisition of responding for cocaine (0.3 mg/kg/inf). In Experiment 2 (Exp 2), rats (n = 32) were tested for VSR, followed by Loco, and finally acquisition of responding for water (0.01 mL/reinforcer).

RESULTS

There were three main findings. First, Loco and VSR were significantly associated (Exp 1: r = 0.49, p < 0.00; Exp 2: r = 0.35, p < 0.05). Second, neither Loco (r = .00, p = 0.998) nor VSR (r = -0.12, p = 0.352) predicted acquisition of cocaine SA. Third, in the subgroup of animals that acquired cocaine SA, VSR (r = 0.41, p < 0.01) but not Loco (r = 0.28, p = 0.10) was positively associated with operant responding for cocaine. Both Loco and VSR (Loco: r = 0.37, p < 0.04; VSR: r = 0.51, p < 0.00) were positively associated with operant responding for water reinforcers.

CONCLUSIONS

The results indicate that VSR is at least as good a predictor of cocaine reinforced responding as Loco. VSR was predictive of operant responding for both drug and water reinforcers, while Loco was found to be predictive of responding only for water reinforcers. In studies that present visual stimuli in association with drug delivery, Loco may be predicting acquisition of responding for VSR rather than drug.

Neural processing of a cocaine-associated odor cue revealed by functional MRI in awake rats

Using an olfactory conditioning procedure, brain stimulation reward threshold measurements, and functional magnetic resonance imaging (fMRI), we investigated brain stimulation reward threshold change and fMRI neural activation in response to a cocaine-associated odor cue. In the first brain stimulation experiment, over 10 days of rate-frequency curve-shift testing, rats were administered intravenous cocaine (1.0mg/kg) paired with a contextual cue of peppermint odor previously placed in the operant chamber or they were given vehicle treatment (no cocaine) in the presence of no olfactory cue. Following a 14-day drug-free rest period, rats were again given the rate-frequency curve-shift threshold test with or without the odor cue. In a second experiment, rats were similarly conditioned with a peppermint odor but with intraperitoneally delivered cocaine (10mg/kg). After a 14 day rest period, rats were imaged on a 7-T MRI for their blood oxygen level dependent (BOLD) in response to the cocaine-paired peppermint odor versus an unpaired neutral lemon odor. In the brain stimulation experiment, expected significant reward threshold shifts were produced by cocaine and, importantly, about half that level of shift was produced by the paired contextual olfactory cue. In the fMRI experiment, the insular cortex showed a significantly greater BOLD activation in cocaine-treated versus saline-treated animals to the olfactory cue, but not with the unpaired lemon scent. These data are in agreement with previous studies suggesting a role of the insular cortex in attributing reward value (positive or negative) to conditioned odor stimuli.

Association between locomotor response to novelty and light reinforcement: sensory reinforcement as a rodent model of sensation seeking

BACKGROUND

The human personality trait of sensation seeking (SS) indicates an attraction to novel sensations and experiences, and is associated with greater likelihood of drug abuse. In rodents, locomotor activity in a novel environment (Loco) has been found to predict drug self-administration (SA), and has been hypothesized to be a translational model of human SS. Previously, we reported (Gancarz et al., 2011) that high responder (HR) animals responded more than low responder (LR) animals to produce a response contingent light onset. The primary goal of this paper was a detailed analysis of the association between Loco and light contingent responding in a large sample of rats (n = 93).

METHODS

Male rats were pre-exposed to dark operant test chambers for ten 30 min sessions and baseline levels of responding (snout poking) were determined. The pre-exposure phase was followed by 6 sessions during which active responding produced a visual sensory reinforcer (VSR; 5 s light onset) according to a variable interval 1 min schedule of reinforcement. After completion of the VSR phase, Loco was tested.

RESULTS

The activating effects (total responding) of light were associated with Loco, but the response guiding effects (proportion of active responding) of the light were not. In addition, HR rats habituated more slowly in both the VSR and Loco tests than LR rats.

CONCLUSIONS

These data indicate that VSR measures aspects of the rodent’s response to novel sensations and experiences that are not detected by Loco. These data provide some evidence for the use of light reinforcement as an animal model of SS.

Novel cues reinstate cocaine-seeking behavior and induce Fos protein expression as effectively as conditioned cues

Cue reinstatement of extinguished cocaine-seeking behavior is a widely used model of cue-elicited craving in abstinent human addicts. This study examined Fos protein expression in response to cocaine cues or to novel cues as a control for activation produced by test novelty. Rats were trained to self-administer cocaine paired with either a light or a tone cue, or received yoked saline and cue presentations, and then underwent daily extinction training. They were then tested for reinstatement of extinguished cocaine-seeking behavior elicited by response-contingent presentations of either the cocaine-paired cue or a novel cue (that is, tone for those trained with a light or vice versa). Surprisingly, conditioned and novel cues both reinstated responding and increased Fos similarly in most brain regions. Exceptions included the anterior cingulate, which was sensitive to test cue modality in saline controls and the dorsomedial caudate-putamen, where Fos was correlated with responding in the novel, but not conditioned, cue groups. In subsequent experiments, we observed a similar pattern of reinstatement in rats trained and tested for sucrose-seeking behavior, whereas rats trained and tested with the cues only reinstated to a novel, and not a familiar, light or tone. The results suggest that novel cues reinstate responding to a similar extent as conditioned cues regardless of whether animals have a reinforcement history with cocaine or sucrose, and that both types of cues activate similar brain circuits. Several explanations as to why converging processes may drive drug and novel cue reinforcement and seeking behavior are discussed.

A new criterion for acquisition of nicotine self-administration in rats

BACKGROUND

Acquisition of nicotine self-administration in rodents is relatively difficult to establish and measures of acquisition rate are sometimes confounded by manipulations used to facilitate the process. This study examined acquisition of nicotine self-administration without such manipulations and used mathematical modeling to define the criterion for acquisition.

METHODS

Rats were given 20 daily 2-h sessions occurring 6 days/week in chambers equipped with active and inactive levers. Each active lever press resulted in nicotine reinforcement (0-0.06 mg/kg, IV) and retraction of both levers for a 20-s time out, whereas inactive lever presses had no consequences. Acquisition was defined for individual rats by the higher likelihood of reinforcers obtained across sessions fitting a logistic over a constant function according to the corrected Akaike Information Criterion (AICc).

RESULTS

For rats that acquired self-administration, an AICc-based multi-model comparison demonstrated that the asymptote (highest number of reinforcers/session) and mid-point of the acquisition curve (h; the number of sessions necessary to reach half the asymptote) varied by nicotine dose, with both exhibiting a negative relationship (the higher the dose, the lower number of reinforcers and the lower h).

CONCLUSIONS

The modeling approach used in this study provides a way of defining acquisition of nicotine self-administration that takes advantage of all data from individual subjects and the procedure used is sensitive to dose differences in the absence of manipulations that influence acquisition (e.g., food restriction, prior food reinforcement, conditioned reinforcers).

Cue effects on methylphenidate self-administration in rats

Associations between drugs and the stimuli paired with drugs have been proposed as primary factors in drug addiction and relapse. Previous research has found cues paired with drug infusions are important for many classes of drugs. The purpose of the present experiment was to determine if a cue light was necessary to engender reliable self-administration of methylphenidate (MPH), which is a widely prescribed drug for attention deficit hyperactivity disorder. Rats were given access to MPH (0.3 mg/kg/infusion) or saline for self-administration. Half of the rats in each group had infusions paired with a cue light, whereas the other half did not. Two additional groups of rats received MPH infusions noncontingently; one group's lever pressing turned on the cue light, and the other group's lever pressing had no consequence. Both MPH and the cue functioned as weak reinforcers on their own. The group that lever pressed for MPH paired with a cue light pressed significantly more for MPH than any other group, indicating that the cue and MPH had a synergistic effect on self-administration when combined. Taken together, these results indicate that MPH has reinforcing properties on its own, but that environmental cues also play an important role in enhancing MPH self-administration.

Rapid delivery of cocaine facilitates acquisition of self-administration in rats: an effect masked by paired stimuli

In general, faster infusions of cocaine are more likely to support behavior related to abuse than are slower infusions. However, some studies of cocaine self-administration in rats have failed to support this finding, possibly because the effect was masked by other factors. One such factor may be the pairing of a stimulus with the infusion, a procedure that is known to facilitate acquisition of drug self-administration. We compared fast and slow infusions by allowing groups of rats to acquire cocaine self-administration at a dose of 1mg/kg/infusion, delivered over different durations (1.8 or 100 s). Two groups were trained with either short or long infusions paired with a visual stimulus change (lights off), and two other groups were trained with short or long durations but with no stimulus change. Both groups trained with a paired stimulus acquired cocaine self-administration. With no stimulus change, the rats trained with the 1.8-s infusion acquired cocaine self-administration at a rate comparable to the two groups that were trained with a paired stimulus. However, most rats in the group trained with the 100-s infusion that was not accompanied by a stimulus change failed to acquire cocaine self-administration. The stimulus itself did not support responding. These results indicate that infusing a given dose of cocaine over a longer duration reduces its ability to support self-administration, but drug-paired stimuli can partially mask this effect by enhancing the effectiveness of slow infusions.

Involvement of the dorsal subiculum and rostral basolateral amygdala in cocaine cue extinction learning in rats

Memory system circuitry may regulate how cues associated with cocaine are extinguished, and understanding neurosubstrates of extinction may lead to the development of improved treatment strategies for cocaine addiction. Sites within the hippocampus and amygdala were investigated for their role in regulating cocaine cue extinction learning. Initially, rats were trained to self-administer cocaine under a second-order reinforcement schedule (cocaine and cocaine cues present) followed by a 2-week abstinence period. Using lidocaine, rats next underwent bilateral inactivation of the dorsal subiculum (dSUB) or rostral basolateral amygdala (rBLA), asymmetric inactivation of the dSUB and rBLA, unilateral inactivation of the dSUB or rBLA, or ipsilateral inactivation of the dSUB and rBLA prior to cocaine cue extinction training sessions (only cocaine cues present) on two consecutive days. Relative to vehicle, bilateral and asymmetric lidocaine treatments in the dSUB and rBLA slowed cocaine cue extinction learning. Specifically, vehicle-treated rats exhibited a significantly larger difference in responding from Day 1 to Day 2 of extinction training than lidocaine-treated rats. In comparison, unilateral or ipsilateral lidocaine treatments in the dSUB and rBLA did not slow cocaine cue extinction learning. Rats treated with lidocaine and vehicle exhibited a similar difference in responding from Day 1 to Day 2 of extinction training. These results indicate that sites within the hippocampus and amygdala need to be functionally active simultaneously in at least one brain hemisphere for acquisition of cocaine cue extinction learning. These results further suggest that a serial circuit within each hemisphere mediates acquisition of cocaine cue extinction learning.

Locomotor activity in a novel environment predicts both responding for a visual stimulus and self-administration of a low dose of methamphetamine in rats

There is evidence that visual stimuli used to signal drug delivery in self-administration procedures have primary reinforcing properties, and that drugs of abuse enhance the reinforcing properties of such stimuli. Here, we explored the relationships between locomotor activity, responding for a visual stimulus, and self-administration of methamphetamine (METH). Rats were classified as high or low responders based on activity levels in a novel locomotor chamber and were subsequently tested for responding to produce a visual stimulus followed by self-administration of a low dose of METH (0.025 mg/kg/infusion) paired with the visual stimulus. High responder rats responded more for the visual stimulus than low responder rats indicating that the visual stimulus was reinforcing and that operant responding for a visual stimulus has commonalities with locomotor activity in a novel environment. Similarly, high responder rats responded more for METH paired with a visual stimulus than low responder rats. Because of the reinforcing properties of the visual stimulus, it was not possible to determine if the rats were responding to produce the visual stimulus, METH or the combination. We speculate that responding to produce sensory reinforcers may be a measure of sensation seeking. These results indicate that visual stimuli have unconditioned reinforcing effects which may have a significant role in acquisition of drug self-administration, a role that is not yet well understood.

Reward sensitivity: issues of measurement, and achieving consilience between human and animal phenotypes

Reward is a concept fundamental to discussions of drug abuse and addiction. The idea that altered sensitivity to either drug-reward, or to rewards in general, contributes to, or results from, drug-taking is a common theme in several theories of addiction. However, the concept of reward is problematic in that it is used to refer to apparently different behavioural phenomena, and even to diverse neurobiological processes (reward pathways). Whether these different phenomena are different behavioural expressions of a common underlying process is not established, and much research suggests that there may be only loose relationships among different aspects of reward. Measures of rewarding effects of drugs in humans often depend upon subjective reports. In animal studies, such insights are not available, and behavioural measures must be relied upon to infer rewarding effects of drugs or other events. In such animal studies, but also in many human methods established to objectify measures of reward, many other factors contribute to the behaviour being studied. For that reason, studying the biological (including genetic) bases of performance of tasks that ostensibly measure reward cannot provide unequivocal answers. The current overview outlines the strengths and weaknesses of current approaches that hinder the conciliation of cross-species studies of the genetics of reward sensitivity and the dysregulation of reward processes by drugs of abuse. Some suggestions are made as to how human and animal studies may be made to address more closely homologous behaviours, even if those processes are only partly able to isolate 'reward' from other factors contributing to behavioural output.

Reinstated ethanol-seeking in rats is modulated by environmental context and requires the nucleus accumbens core

The reinstatement of ethanol (EtOH)-seeking induced by an EtOH-predictive light-tone stimulus is enhanced in an environment associated with prior EtOH self-administration (SA) compared with a context associated with EtOH unavailability (Tsiang & Janak, 2006). Here we hypothesized that EtOH-seeking would be elicited by the conditioned sensory stimulus properties of EtOH and that this reinstatement would be similarly modulated by context. We also determined whether pharmacologically inactivating the nucleus accumbens (NAc), a key structure in relapse circuitry, would attenuate reinstated EtOH-seeking. Rats lever-pressed for oral EtOH (10% v/v) in operant conditioning chambers distinguished by specific visual, olfactory and tactile stimuli. Responding was then extinguished by withholding EtOH in a different context. EtOH-seeking, expressed as elevated responding without EtOH delivery, was subsequently tested by presenting an oral EtOH prime (two aliquots of 0.1 mL EtOH) in either the extinction or the prior EtOH-SA context. Rats received a microinfusion (0.3 microL/hemisphere) of saline or GABA agonists (muscimol/baclofen) into the NAc core or shell immediately before the reinstatement test. Robust EtOH-seeking was observed in the prior EtOH-SA but not the extinction context in saline-pretreated rats. This effect was significantly attenuated by inactivating the NAc core but not shell. Conversely, NAc shell inactivation significantly elevated lever-pressing in the extinction context. These data suggest that the sensory stimulus properties of oral EtOH can reinstate EtOH-seeking when experienced in the appropriate context and that functional activity in the NAc core is required for this effect. In contrast, the shell may normally inhibit incorrect behavioral responses.

Distinct contributions of dopamine receptors in the nucleus accumbens core or shell to established cocaine reinforcement under a second-order schedule

Nucleus accumbens dopamine is implicated in the primary and conditioned reinforcing properties of abused drugs. In the present study, specific impairments in responding for intravenous cocaine (0.3 mg/inf/0.1 ml/5 s) under a fixed-ratio 1 (FR-1) or second-order schedule (FI 15 min (FR10:S)) were investigated following infusion of the dopamine antagonist, alpha-flupenthixol, into either the nucleus accumbens core or shell. Infusion of alpha-flupenthixol into the core decreased cocaine intake under the FR-1 and second-order schedules. By comparison, blockade of nucleus accumbens shell dopamine receptors increased cocaine intake under the FR-1 schedule. Under the second-order schedule, cocaine intake and the number of responses was decreased. Effects on responding were more apparent after self-administered cocaine, when impairments in the latency to receive cocaine infusions were no longer evident. These results are discussed with reference to a role for nucleus accumbens shell dopamine in instrumental responding, and a role of nucleus accumbens core dopamine in incentive motivation, perhaps under the control of contextual stimuli.

Effect of drug-paired exteroceptive stimulus presentations on methamphetamine reinstatement in rats

The purpose of the present study was to examine the impact of drug-paired cues on methamphetamine reinstatement. Three groups of rats were trained to self-administer 0.1 mg/kg/infusion methamphetamine. Each methamphetamine infusion was accompanied by a 6 s flashing light+tone stimulus (cues). After training, the groups were then given 12, daily extinction sessions either with or without response-contingent drug-paired cues and then tested for 1 mg/kg i.p. methamphetamine priming-induced reinstatement either with or without cues. Methamphetamine priming significantly reinstated drug-appropriate responding regardless of whether response-contingent cues were omitted during both extinction and testing, presented during both extinction and testing, or omitted during extinction but presented during reinstatement testing. The group in which cues were omitted during extinction and presented during reinstatement exhibited significantly greater reinstatement than did the other two groups. A separate group of rats was also tested demonstrating that response-contingent presentation of previously methamphetamine-paired cues alone, without methamphetamine priming, significantly reinstated drug-appropriate responding. These data show that methamphetamine priming produces a robust reinstatement effect which can be influenced by drug-paired cues.

A conflict rat model of cue-induced relapse to cocaine seeking

RATIONALE AND OBJECTIVE

Relapse to drug use in humans can be induced by exposure to drug-associated cues. The ability of drug cues to provoke 'relapse' has been studied in laboratory animals using a reinstatement model in which resumption of drug seeking is assessed after extinction of drug-reinforced responding. In this model, there are no adverse consequences of drug-seeking behavior. However, in humans, abstinence is often self-imposed, and relapse episodes likely involve making a choice between the desire for the drug and the negative consequences of pursuing it (a conflict situation). In this paper, we describe a conflict model of cue-induced relapse in rats that approximate the human condition.

MATERIALS AND METHODS

Rats were trained to lever press for cocaine; infusions were paired with a discrete light cue. An 'electric barrier' was then introduced by electrifying the floor area near the levers. Responding decreased over days with increasing shock intensities, until the rats did not approach the levers for 3 days. Subsequently, the effect of intermittent noncontingent light-cue presentations on resumption of lever responding (relapse) was assessed in extinction tests, with the electric barrier remaining activated; during testing, lever presses led to contingent light-cue presentations.

RESULTS

Noncontingent cue exposure led to resumption of lever presses during the relapse tests in 14 of the 24 rats. Surprisingly, 24 h later, 11 of the 24 rats resumed lever responding in a subsequent post-noncontingent cue test under similar extinction conditions. Large individual differences in responding were observed during both tests.

CONCLUSIONS

At its current stage of development, the conflict relapse model appears particularly suitable for studying individual differences in cue-induced relapse to cocaine seeking or factors that promote this relapse.

AMPA-receptor GluR1 subunits are involved in the control over behavior by cocaine-paired cues

The learning processes underlying the formation of drug-cue associations involve changes in synaptic transmission mediated by AMPA receptors. Here, we examine the consequences of targeted deletion of the gene encoding GluR1 subunits of AMPA receptors (gria1 knockouts (KO)) on cocaine self-administration and on the ability of cocaine-paired cues to affect cocaine-seeking in mice. Cocaine self-administration was unaffected by gria1 deletion, as was the ability of a cocaine-paired cue to reinstate responding following extinction, following either a 3 or a 66 day delay. However, gria1 KOs over-responded during extinction. The KOs were unable initially to learn a new response to access a cue previously conditioned to cocaine (conditioned reinforcement (CR)), although a second test 2 months later revealed that this was a transient deficit. These studies indicate that GluR1-containing AMPA-receptors are not involved in cocaine self-administration, cue-induced reinstatement of cocaine seeking, or incubation of the cocaine seeking response. In order to understand the specificity of the deficits in CR responding, a parallel study was performed with food reward. As with cocaine, there were no effects of gria1 deletion on food self-administration or cue-induced reinstatement, and KOs over-responded during extinction. However, even immediately after instrumental training for food, KO mice demonstrated responding for CR, suggesting that the CR deficit observed with a cocaine cue is specific to drug reward. These data indicate that GluR1-containing AMPA receptors are important in stimulus reward learning, though the method of cue-reward association formation, the reward class, and the behavioral end point are critical variables in determining their involvement.

Motivational control of heroin seeking by conditioned stimuli associated with withdrawal and heroin taking by rats

The authors investigated the impact of conditioned withdrawal on drug seeking by training rats to work for a heroin infusion on a seeking-taking schedule, which required responding on a seeking lever in order to gain the opportunity to self-administer the drug by a single response on a taking lever. Following the establishment of opiate dependence, a conditioned stimulus (CS) that had been previously paired with naloxone-precipitated withdrawal suppressed heroin seeking in extinction. However, when the rats had prior experience of heroin taking in the presence of the withdrawal CS, drug seeking was elevated in the presence of this stimulus. The authors conclude that the conditioned motivation of drug seeking in withdrawal depends on previous association of the CS with drug taking.

Effect of methamphetamine self-administration on tyrosine hydroxylase and dopamine transporter levels in mesolimbic and nigrostriatal dopamine pathways of the rat

RATIONALE AND OBJECTIVES

Many studies have examined the effect of experimenter-delivered methamphetamine on the mesolimbic and nigrostriatal dopamine pathways. In contrast, little is known about the effect of methamphetamine self-administration on these neuronal pathways. We studied the effect of methamphetamine self-administration on two key regulators of dopamine transmission, tyrosine hydroxylase (TH), and dopamine transporter (DAT), in components of the mesolimbic and nigrostriatal dopamine pathways.

METHODS

Rats self-administered methamphetamine (0.1 mg/kg per infusion, fixed-ratio-1 reinforcement schedule) or saline (control condition) for 9 h/day over 10 days. The brains of these rats were collected after 1 or 30 days of forced abstinence and the expression levels of TH and DAT were assayed by in situ, hybridization and western blot.

RESULTS

TH mRNA and protein levels were increased in the ventral tegmental area (VTA, the cell body region of the mesolimbic dopamine system) and the substantia nigra pars compacta (SNC, the cell body region of the nigrostriatal dopamine system) after 1 day, but not 30 days, of forced abstinence from methamphetamine. In contrast, methamphetamine self-administration had no effect on TH protein levels in dopaminergic terminals located in the nucleus accumbens and caudate-putamen. In addition, methamphetamine self-administration had no effect on DAT mRNA levels in the VTA.

CONCLUSIONS

Results suggest that extended daily access to self-administered methamphetamine results in a transient, short-lasting effect on mesolimbic and nigrostriatal dopamine neurons of the rat brain.

Increased vulnerability to nicotine self-administration and relapse in alcohol-naive offspring of rats selectively bred for high alcohol intake

The prevalence of smoking in human alcoholics is substantially higher than in the general population, and results from twin studies suggest that a shared genetic vulnerability underlies alcohol and nicotine addiction. Here, we directly tested this hypothesis by examining nicotine-taking behavior in alcohol-naive offspring of alcohol-preferring (P) rats and alcohol-nonpreferring (NP) rats that had been selectively bred for high and low alcohol intake. The self-administration of intravenous nicotine (0.015-0.060 mg/kg per infusion) in P rats was more than twice than that of NP rats. Nicotine seeking induced by reexposure to nicotine cues in extinction tests was also substantially greater in P rats than in NP rats. In a subsequent relapse test, priming nicotine injections reinstated drug seeking in P rats but not NP rats. P rats also self-administered higher amounts of oral sucrose (1-20%) than NP rats, a finding consistent with previous reports. In contrast, self-administration of intravenous cocaine (0.1875-1.125 mg/kg per infusion) was remarkably similar in the P and NP rats; however, P-NP differences in cocaine seeking emerged in subsequent extinction and cocaine priming-induced reinstatement tests. In both cases, lever responding was higher in P rats than in NP rats. Thus, alcohol-naive offspring of rats genetically selected for high alcohol intake are highly susceptible to nicotine self-administration and relapse, and this susceptibility is not likely caused by general reward deficits in NP rats. The present findings provide experimental evidence for the hypothesis that a shared genetic determinant accounts for the co-abuse of nicotine and alcohol.

Neuropsychopharmacology of drug seeking: Insights from studies with second-order schedules of drug reinforcement

Second-order schedules of reinforcement model complex chains of responding for rewards such as food or drugs. Derived from studies of conditioned reinforcement, an important feature of these schedules is that responding is maintained by the response-dependent presentation of conditioned stimuli. They are thus well-suited to the study of the influences over drug seeking exerted by drug-associated stimuli. In the present review, we summarise studies investigating the neurobiology and neuropsychopharmacology of responding for cocaine under a second-order schedule of reinforcement. We conclude that limbic-striatal circuitries underlie drug seeking measured in this way. Emphasis is placed on potential interactions between structures within these subsystems by reviewing neuropsychopharmacological studies in which antagonists selective for either glutamate or dopamine receptors have been infused directly into limbic, cortical and striatal areas.

The dissociation of heroin-seeking patterns induced by contextual, discriminative, or discrete conditioned cues in a model of relapse to heroin in rats

RATIONALE

The role of heroin-related stimuli in motivating the resumption of heroin use is not fully understood.

OBJECTIVES

The objective was to characterize the relative importance of drug-related contextual stimuli, discriminative stimuli (DS), or discrete conditioned stimuli (CSs) on drug seeking when rats were reintroduced into the operant context after withdrawal.

METHODS

Nose-poke responding by male rats was reinforced with intravenous heroin (0.05 mg/kg per infusion, 4-h session daily) under a progressive ratio schedule of reinforcement for 14 days. Each session began with the illumination of a green light in the active hole that served as DS. Each earned heroin injection was paired with a 5-s compound cue light and the sound of the infusion pump that served as the discrete CSs.

RESULTS

Response rates of heroin seeking induced by the contextual stimuli were comparable to the average rates of responding during self-administration training, but rates induced by either DS or CSs were greater than those induced by the contextual stimuli alone (P<0.05). The responding induced by contingent presentations of CSs was higher than that of DS after extinction of instrumental behavior. The drug seeking induced by CSs can be maintained after 3 days extinction with DS in the original context, although the responding elicited by DS cannot be recovered after 3 days of extinction with CSs.

CONCLUSIONS

The relapse to drug seeking can be elicited separately by environmental cues, heroin-predictive DS, or discrete CSs in the same rat after withdrawal.

Nucleus accumbens dopamine release is necessary and sufficient to promote the behavioral response to reward-predictive cues

The nucleus accumbens is part of the neural circuit that controls reward-seeking in response to reward-predictive cues. Dopamine release in the accumbens is essential for the normal functioning of this circuit. Previous studies have shown that injection of dopamine receptor antagonists into the accumbens severely impairs an animal's ability to perform operant behaviors specified by predictive cues. Furthermore, excitations and inhibitions of accumbens neurons evoked by such cues are abolished by inactivation of the ventral tegmental area, the major dopaminergic input to the accumbens. These results indicate that dopamine is necessary to elicit neural activity in the accumbens that drives the behavioral response to cues. Here we show that accumbens dopamine release is causal to the rats' reward-seeking behavioral response by demonstrating that dopamine in this structure is both necessary and sufficient to promote the appropriate behavioral response to reward-predictive cues.

Differential effects of dopaminergic agents on locomotor sensitisation and on the reinstatement of cocaine-seeking and food-seeking behaviour

RATIONALE

The dopaminergic pathways are involved in natural and drug reward related processes.

OBJECTIVES

To compare the respective involvement of the dopaminergic receptors D1, D2 and D3 in natural-seeking versus drug-seeking behaviour and evaluate any concomitant expression of locomotor sensitisation.

METHODS

In separate experiments, male Wistar rats were trained to self-administer cocaine (0.25 mg/infusion) or to press a lever to obtain food pellets. Following a prolonged period of extinction, reinstatement of lever responding was measured following non-contingent food delivery, cocaine (15 mg/kg), D1-like (SKF 82958, 0.25 mg/kg), D2-like (quinelorane, 0.25 mg/kg) and D3-like (7-OHDPAT, 0.25 mg/kg) agonists. To demonstrate parallel expression of behavioural sensitisation, locomotor activity was recorded during the reinstatement sessions.

RESULTS

Cocaine and quinelorane administrations reinstated cocaine-seeking behaviour and induced the expression of locomotor sensitisation, whereas SKF 82958 and 7-OHDPAT had either no effect or non-specific effects. In the food-seeking experiment, we found that quinelorane and 7-OHDPAT did not reinstate lever pressing. Cocaine increased responding on both active and inactive levers, whereas SKF 82958 had a more specific effect with higher responding on the previously food-associated lever.

CONCLUSIONS

Our results indicate that expression of locomotor sensitisation and reinstatement of cocaine-seeking but not food-seeking behaviours are in part supported by common dopaminergic substrates, among which the D2 receptors play a crucial role.

Contrasting effects of dopamine and glutamate receptor antagonist injection in the nucleus accumbens suggest a neural mechanism underlying cue-evoked goal-directed behavior

Discriminative stimuli (DSs) inform animals that reward can be obtained contingent on the performance of a specific behavior. Such stimuli reinstate drug-seeking behavior, evoke dopamine release in the nucleus accumbens (NAc) and excite and inhibit specific subpopulations of NAc neurons. Here we show in rats that DSs can reinstate food-seeking behavior. In addition, we compare the effects of injecting dopamine receptor antagonists into the NAc with those of general NAc inactivation on the performance of a DS task. Selective antagonism of D1 receptors reduced responding to the DS and increased the latency to respond, whereas general inactivation of NAc neuronal activity increased the latency to respond to the DS and increased behaviors extraneous to the task, such as responding in the absence of cues and responding on the inactive lever. Based on these results and our previous findings that NAc neuronal responses to DSs are dependent on the ventral tegmental area, we propose a model for the functional role of NAc neurons in controlling behavioral responses to reward-predictive stimuli.

Cocaine self-administration selectively decreases noradrenergic regulation of metabotropic glutamate receptor-mediated inhibition in dopamine neurons

Stimulant drugs of abuse have several effects on neural activity, including altering the excitability of dopamine neurons via the noradrenergic and glutamatergic systems. Thus, an interaction between noradrenergic and glutamatergic systems may play a role in drug-seeking behavior. Although many of the direct pharmacological effects of psychostimulants on dopamine neuron physiology are well established, the neurophysiological bases of drug-seeking behavior have yet to be fully elucidated. The present study measured short-term (3 d) and long-term (14 d) access to cocaine, by self-administration or passive exposure, and the regulation of metabotropic glutamate receptor (mGluR)-mediated inhibition of dopamine cells in rat midbrain slices. The results indicated that alpha-adrenoreceptor modulation of the mGluR-mediated inhibition is selectively reduced in animals that self-administered cocaine for 3 d. This effect was not observed in slices from either yoked cocaine animals, which were given cocaine in an amount and pattern equal to that used for the self-administering animals, or saline control animals. However, after 14 d of cocaine, alpha-adrenoreceptor regulation of the mGluR-mediated inhibition was equally reduced in both self-administering and yoked cocaine animals relative to saline controls. The results suggest that alpha-adrenoreceptor regulation of the mGluR-mediated inhibition is an adaptive cellular mechanism involved in early cocaine self-administration that is distinct from a direct pharmacological effect of cocaine on dopamine neurons. The noradrenergic system could therefore serve to alter the reward value of stimuli that have significant effects on dopamine neuron firing pattern through mGluRs.

Repeated nicotine exposure enhances responding with conditioned reinforcement

RATIONALE

Stimuli associated with a reinforcer (e.g., an addictive drug) can acquire conditioned reinforcing effects. Clinical observations indicate that smoking depends strongly upon conditioned reinforcement (i.e., cues support smoking behavior); however, little is known about the effects of repeated nicotine exposure on these processes.

OBJECTIVE

This study investigated the consequences of prior repeated nicotine exposure on responding with conditioned reinforcement and on the potentiation of conditioned reinforcement by intra-NAc amphetamine infusion.

METHODS

Rats received repeated saline or nicotine injections (0.35 mg/kg; 15 days) and were, following 3 days of withdrawal, trained to associate a tone + light stimulus with water reinforcement for 10 days. Animals were subsequently tested on acquisition of a new instrumental response with conditioned reinforcement (i.e., 14 days after the final nicotine injection). In additional experiments, animals received an infusion of amphetamine (10 microg per side) prior to the conditioned reinforcement test.

RESULTS

Prior repeated nicotine exposure produced a behaviorally specific enhancement of responding with conditioned reinforcement. Furthermore, repeated nicotine pretreatment also augmented the potentiation of conditioned reinforcement by intra-NAc amphetamine.

CONCLUSIONS

These findings demonstrate that prior repeated nicotine exposure augments the control over behavior by a conditioned reinforcer. Such long-lasting alterations in incentive motivational processes produced by repeated nicotine exposure may depend on drug-induced neuroadaptations in dopamine-regulated signaling within limbic-striatal brain regions that could underly persistent and compulsive aspects of addiction.

Conditioned reinforcing properties of stimuli paired with self-administered cocaine, heroin or sucrose: implications for the persistence of addictive behaviour

Conditioned environmental stimuli are known to be important determinants of drug seeking. Traditional models of drug seeking under the control of conditioned stimuli have focused on the ability of conditioned reinforcers either to reinstate extinguished responding or to maintain prolonged chains of drug seeking under second-order schedules. These models have consistently suggested that it is the conditioned reinforcing, rather than other, effects of Pavlovian drug stimuli that most profoundly influence drug seeking. However, the impact of drug-associated conditioned reinforcers has not been studied directly and in isolation, not least because the instrumental seeking response is invariably the same as that which was previously reinforced with the drug itself. The purpose of the present study was, therefore, to investigate the conditioned reinforcing properties of drug-paired CSs using an acquisition of a new response procedure in which an animal learns to make a new instrumental response reinforced solely by the CS. It was found that CSs paired with either cocaine, heroin or sucrose supported the rapid acquisition of lever pressing for the CS that persisted over months of repeated, intermittent testing. Furthermore, rats did not acquire the lever press response when the CS was not paired with drug, suggesting that for this stimulus to acquire conditioned reinforcing properties, it must be predictively associated with the drug's effect. Moreover, lever pressing for the CS could not be explained as coincidental to an over-riding Pavlovian approach response to the location of the lever, since animals also acquired discriminated lever pressing when the CS was above the opposite, inactive lever. Extinction decreased responding with conditioned reinforcement, but only when the CS-US association was devalued prior to, and not after, acquisition of the lever press response, providing evidence for the establishment of habitual CS-maintained responding that may explain the persistence of drug-seeking responses in animal models of addiction and relapse.

Selective induction of c-Fos immunoreactivity in the prelimbic cortex during reinstatement of heroin seeking induced by acute food deprivation in rats

We previously reported that acute 1-day food deprivation reinstates heroin seeking in rats. The goal of the present study was to begin identifying brain sites potentially involved in this effect. For this purpose, we measured, by immunohistochemistry, the expression of c-Fos following a test for food deprivation-induced reinstatement. Groups of rats (n=9-10 per group) were trained to lever-press for heroin (0.05-0.1mg/kg/infusion) or saline for 10 days (9 h/day); each infusion was paired with a cue light. Rats were then given 10 days of extinction during which the heroin and saline syringes were removed. Next, a test for reinstatement was conducted after exposure to 0 (baseline) or 1-day food deprivation. During training, lever pressing for heroin increased over days, while responding for saline infusions paired with the cue light decreased over time. During extinction, responding on the heroin-paired lever decreased over time, while responding on the saline-paired lever remained low. In heroin-trained rats, food deprivation induced a large increase in responding on the lever associated with drug infusions. Surprisingly, food deprivation also modestly increased responding in the saline-trained rats. Food deprivation selectively increased c-Fos immunoreactivity (IR) in the prelimbic cortex of heroin-trained, but not saline-trained, rats (n=4 per condition). Food deprivation also increased c-Fos IR in both heroin- and saline-trained rats in the basolateral amygdala and the ventrolateral bed nucleus of stria terminalis (BNST), but had no effect on c-Fos expression in the dorsolateral BNST, cingulate cortex, nucleus accumbens, and central amygdala. These results raise the possibility that the prelimbic cortex is involved in food deprivation-induced reinstatement of heroin seeking.

Comparison of reinstatement of ethanol- and sucrose-seeking by conditioned stimuli and priming injections of allopregnanolone after extinction in rats

RATIONALE AND OBJECTIVES

Understanding the mechanism of relapse provoked by conditioned and unconditioned stimuli is critical to improving treatments for alcoholism. This study compared the reinstatement of alcohol- or sucrose-seeking by conditioned stimuli and priming injections of the neuroactive steroid, allopregnanolone (ALLO).

METHODS

Rats were trained to lever-press for 0.1 ml of 10% ethanol or 5% sucrose solutions. Responding was then extinguished, and subjects were tested for reinstatement of lever-press responding. The effects of priming injections of 0, 1.0, 3.0 and 7.5 mg/kg ALLO were determined in subjects trained to self-administer ethanol, and the response-reinstating effects of priming injections of 3.0 mg/kg ALLO were compared with those of conditioned cue presentation in subjects trained to self-administer either ethanol or sucrose.

RESULTS

Priming injections of ALLO dose-dependently reinstated previously extinguished responding for ethanol, as shown by increased responding on the active (ethanol) lever. Contingent presentation of cues previously associated with the reinforcer increased the number of active lever-presses for both ethanol- and sucrose- trained subjects. In contrast, pretreatment with 3.0 mg/kg ALLO increased the number of active lever-presses for subjects that were trained to self-administer ethanol, but not sucrose.

CONCLUSIONS

ALLO promotes responding for ethanol, but not sucrose, following a period of abstinence, suggesting that GABA(A) receptor modulation may contribute to processes involved in reinstatement of ethanol-seeking behavior. In contrast, conditioned stimuli reinstate previously extinguished ethanol- and sucrose-seeking behavior, indicating that the mechanisms that subserve cue-induced reinstatement do not depend upon the nature of the positive reinforcer.