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

Disruption of positive- and negative-feature morphine interoceptive occasion setters by dopamine receptor agonism and antagonism in male and female rats

RATIONALE

Internally perceived stimuli evoked by morphine administration can form Pavlovian associations such that they can function as occasion setters (OSs) for externally perceived reward cues in rats, coming to modulate reward-seeking behaviour. Though much research has investigated mechanisms underlying opioid-related reinforcement and analgesia, neurotransmitter systems involved in the functioning of opioids as Pavlovian interoceptive discriminative stimuli remain to be disentangled despite documented differences in the development of tolerance to analgesic versus discriminative stimulus effects.

OBJECTIVES

Dopamine has been implicated in many opioid-related behaviours, so we aimed to investigate the role of this neurotransmitter in expression of morphine occasion setting.

METHODS

Male and female rats were assigned to positive- (FP) or negative-feature (FN) groups and received an injection of morphine or saline before each training session. A 15-s white noise conditioned stimulus (CS) was presented 8 times during every training session; offset of this stimulus was followed by 4-s access to liquid sucrose on morphine, but not saline, sessions for FP rats. FN rats learned the reverse contingency. Following stable discrimination, rats began generalization testing for expression of morphine-guided sucrose seeking after systemic pretreatment with different doses of the non-selective dopamine receptor antagonist, flupenthixol, and the non-selective dopamine receptor agonist, apomorphine, combined with training doses of morphine or saline in a Latin-square design.

RESULTS

The morphine discrimination was acquired under both FP and FN contingencies by males and females. Neither flupenthixol nor apomorphine at any dose substituted for morphine, but both apomorphine and flupenthixol disrupted expression of the morphine OS. This inhibition was specific to sucrose seeking during CS presentations rather than during the period before CS onset and, in the case of apomorphine more so than flupenthixol, to trials on which access to sucrose was anticipated.

CONCLUSIONS

Our findings lend support to a mechanism of occasion setting involving gating of CS-induced dopamine release rather than by direct dopaminergic modulation by the morphine stimulus.

Generalization of a positive-feature interoceptive morphine occasion setter across the rat estrous cycle

INTRODUCTION

Interoceptive stimuli elicited by drug administration acquire conditioned modulatory properties of the induction of conditioned appetitive behaviours by exteroceptive cues. This effect may be modeled using a drug discrimination task in which the drug stimulus is trained as a positive-feature (FP) occasion setter (OS) that disambiguates the relation between an exteroceptive light conditioned stimulus (CS) and a sucrose unconditioned stimulus (US). We previously reported that females are less sensitive to generalization of a FP morphine OS than males, so we investigated the role of endogenous ovarian hormones in this difference.

METHODS

Male and female rats received intermixed injections of 3.2 mg/kg morphine or saline before each daily training session. Training consisted of 8 presentations of the CS, each followed by access to sucrose on morphine, but not saline sessions. Following acquisiton, rats were tested for generalization of the morphine stimulus to 0, 1.0, 3.2, and 5.4 mg/kg morphine. Female rats were monitored for estrous cyclicity using vaginal cytology throughout the study.

RESULTS

Both sexes acquired stable drug discrimination. A gradient of generalization was measured across morphine doses and this behaviour did not differ by sex, nor did it differ across the estrous cycle in females.

CONCLUSIONS

Morphine generalization is independent of fluctuations in levels of sex and endogenous gonadal hormones in females under these experimental conditions.

Animal Models of the Behavioral Symptoms of Substance Use Disorders

To more effectively manage substance use disorders, it is imperative to understand the neural, genetic, and psychological underpinnings of addictive behavior. To contribute to this understanding, considerable efforts have been made to develop translational animal models that capture key behavioral characteristics of addiction on the basis of DSM5 criteria of substance use disorders. In this review, we summarize empirical evidence for the occurrence of addiction-like behavior in animals. These symptoms include escalation of drug use, neurocognitive deficits, resistance to extinction, exaggerated motivation for drugs, increased reinstatement of drug seeking after extinction, preference for drugs over nondrug rewards, and resistance to punishment. The occurrence of addiction-like behavior in laboratory animals has opened the opportunity to investigate the neural, genetic, and psychological background of key aspects of addiction, which may ultimately contribute to the prevention and treatment of substance use disorders.

Double dissociation between actions of dopamine D1 and D2 receptors of the ventral and dorsolateral striatum to produce reinstatement of cocaine seeking behavior

One of the hallmarks of addiction is the enduring vulnerability to relapse. Following repeated use, cocaine (COC) induces neuroadaptations within the dopamine (DA) system, arguably underlying several aspects of COC-seeking behavior. Peripheral stimulation of D2, but not D1, receptors induces relapse. However, where in the brain these effects occur is still matter of debate. The D1 and D2 receptors (D1R; D2R) are highly expressed in the nucleus accumbens (NAcc) and the dorsolateral striatum (DLS), but their specific involvement in the reinstatement of COC-seeking remains elusive. We assessed the reinstating effects of intracerebral infusions of agonists of D1R (SKF82958) or D2R (quinelorane) within the NAcc or DLS of rats after extinction of COC self-administration (COC SA). To assess whether we could block peripheral D2 agonist (quinelorane) induced reinstatement, we simultaneously infused either a D1R (SCH23390) or a D2R (raclopride) antagonist within the NAcc or DLS. When infused into the NAcc, but not into the DLS, SKF82958 induced reinstatement of COC-seeking; conversely, quinelorane had no effect when injected into the NAcc, but induced reinstatement when infused into the DLS while the D1R agonist has no effect. While administration of raclopride into the NAcc or DLS impedes the reinstating effect of a systemic quinelorane injection, the infusion of SCH23390 into the NAcc or DLS surprisingly, blocks the reinstatement induced by the peripheral D2R stimulation. Our results point to a double dissociation between D1R and D2R of the NAcc and DLS, highlighting their complex interactions within both structures, in the reinstatement of COC-seeking behavior.

Arc reactivity in accumbens nucleus, amygdala and hippocampus differentiates cue over context responses during reactivation of opiate withdrawal memory

Opiate withdrawal induces an early aversive state which can be associated to contexts and/or cues, and re-exposure to either these contexts or cues may participate in craving and relapse. Nucleus accumbens (NAC), hippocampus (HPC) and basolateral amygdala (BLA) are crucial substrates for acute opiate withdrawal, and for withdrawal memory retrieval. Also HPC and BLA interacting with the NAC are suggested to respectively mediate the processing of context and cue representations of drug-related memories. Here we used a paradigm of conditioned suppression of operant food seeking, allowing to differentiate context and cue related responses, to study the influence of withdrawal memories on operant behavior and the underlying neural substrates. catFISH for Arc mRNA expression was used to discriminate cellular responses during context and cue (flashing light) periods in this paradigm. We show that reactivation of the memory of the negative affective state of withdrawal suppresses active lever pressing for food, and this conditioned suppression is generalized to the context. Interestingly the behavioral responses during the context and cue light periods are associated with differential Arc mRNA activations within the NAC, BLA, and HPC. Indeed both periods led to NAC shell activation whereas the NAC core was responsive only following the cue light period. Moreover, BLA and HPC were more responsive during cue-light and context period respectively. These data further support the already reported differential role of these brain structures on cue vs context-induced reinstatement of operant behaviors, and highlight the existence of common mechanisms for the processing of positive and aversive emotional memories.

Behavioral sensitization of the reinforcing value of food: What food and drugs have in common

Sensitization is a basic property of the nervous system whereby repeated exposure to a stimulus results in an increase in responding to that stimulus. This increase in responding contributes to difficulty with treatment of drug abuse, as stimuli associated with substance use become signals or triggers for drug craving and relapse. Our work over the past decade has applied the theoretical framework of incentive sensitization to overeating. We have shown, in several studies, that lean adults do not commonly demonstrate behavioral sensitization after repeated exposure to snack food, but a subset of obese adults reliably does. This review will discuss this change in behavioral response to repeated consumption of snack food in obese individuals and apply the theoretical framework of incentive sensitization to drugs of abuse to high fat/high sugar snack foods. We will also show data that suggest that behavioral sensitization to repeated administration of snack food is predictive of weight gain, which may enhance its utility as a diagnostic tool for identifying at-risk individuals for obesity. Finally, we will discuss the future directions of this line of research, including studying the phenomenon in children and adolescents and determining if similar principles can be used to increase motivation to eat healthier food. A combination of reductions in unhealthy food intake and increases and healthy food intake is necessary to reduce obesity rates and improve health.

Lack of effect of ethanol on cocaine prime-induced reinstatement of extinguished cocaine self-administration in rhesus monkeys

Cocaine and alcohol are commonly co-abused for reasons that are incompletely understood. Laboratory animal studies have suggested that, although the reinforcing effects of low cocaine doses are increased following chronic ethanol (EtOH) consumption, acute EtOH administration does not consistently alter cocaine self-administration. The present study examined whether EtOH influences another abuse-related effect of cocaine: reinstatement of extinguished responding. Rhesus monkeys that had previously consumed EtOH for 8 weeks (2.0 g/kg over 1 h, 5 days/week) self-administered up to 10 injections per day of 0.1 mg/kg cocaine under a fixed-interval 300-s schedule. After responding had been extinguished by substituting saline for cocaine, a pre-session infusion of saline or EtOH (0.5 or 1.0 g/kg, intravenously over 10 min) was followed by a 'priming' injection of saline or cocaine (intravenously). Responding was increased significantly by priming injections of cocaine, but not saline. EtOH infusions neither reinstated behavior when administered before a saline prime nor altered the priming effect of cocaine. The inability of EtOH to alter the response-reinstating ability of cocaine provides further evidence for a lack of acute behavioral interactions between cocaine and EtOH.

Effects of the dopamine/norepinephrine releaser phenmetrazine on cocaine self-administration and cocaine-primed reinstatement in rats

RATIONALE

Like other monoamine releasers such as D-amphetamine, chronic treatment with phenmetrazine can attenuate cocaine self-administration in monkeys.

OBJECTIVES

The present studies extended this finding to rodents and to cocaine-primed reinstatement, a putative laboratory animal model of relapse.

METHODS

In experiment 1, rats self-administered food pellets or injections of 0.19 mg/kg cocaine (i.v.) under a progressive-ratio schedule. When responding was stable, subcutaneous osmotic pumps were implanted containing saline or (+)-phenmetrazine (25 or 50 mg/kg per day). In experiment 2, rats self-administered injections of 0.75 mg/kg cocaine under a fixed-ratio 1 schedule in daily 6-h sessions. When responding was stable, rats were removed from the self-administration environment for 7 days and treated continuously with saline, 5 mg/kg per day D-amphetamine or phenmetrazine (25 or 50 mg/kg per day) via osmotic pumps. Rats were then returned to the self-administration context while treatment continued, and responding was extinguished by removing response-contingent stimulus changes and cocaine injections. Once responding was extinguished, reinstatement tests were conducted using cocaine injections (10 mg/kg i.p.).

RESULTS

Phenmetrazine decreased self-administration of cocaine, but not food pellets, during the 14-day treatment period; effects persisted for several days after treatment was discontinued. Moreover, cocaine-induced increases in responding during the reinstatement test were attenuated by D-amphetamine and both phenmetrazine doses.

CONCLUSIONS

These results extend the study of the effects of phenmetrazine on cocaine self-administration to a rodent model, and provide further support for the use of monoamine releasers as agonist medications for cocaine abuse.

Distinct behavioral phenotypes in ethanol-induced place preference are associated with different extinction and reinstatement but not behavioral sensitization responses

Conditioned place preference (CPP) is a model to study the role of drug conditioning properties. In outbred strains, individual variability may affect some behavioral measures. However, there are few studies focusing on understanding how different phenotypes of ethanol conditioned behavior may influence its extinction, reinstatement, and behavioral adaptation measures. We used male Swiss Webster mice to study different phenotypes related to ethanol conditioning strength, reinstatement and behavioral sensitization. Mice went through a CPP procedure with ethanol (2.2 g/kg, i.p.). After that, one group of mice was submitted to repeated extinction sessions, while another group remained in their home cages without any drug treatment. Mice went through environmental and ethanol priming (1.0 g/kg, i.p.) reinstatement tests. Ethanol priming test reinstated the conditioned behavior only in the animals kept in the home-cage during the abstinence period. Besides, the ethanol conditioned behavior strength was positively correlated with the time required to be extinguished. In the second set of experiments, some mice went through a CPP protocol followed by behavioral sensitization (five i.p. administrations of ethanol 2.2 g/kg or saline per week, for 3 weeks) and another group of mice went through sensitization followed by CPP. No positive correlation was observed between ethanol CPP strength and the intensity of behavioral sensitization. Considering that different phenotypes observed in CPP strength predicted the variability in other CPP measures, we developed a statistics-based method to classify mice according to CPP strength to be used in the evaluation of ethanol conditioning properties.

Habituation of reinforcer effectiveness

In this paper we propose an integrative model of habituation of reinforcer effectiveness (HRE) that links behavioral- and neural-based explanations of reinforcement. We argue that HRE is a fundamental property of reinforcing stimuli. Most reinforcement models implicitly suggest that the effectiveness of a reinforcer is stable across repeated presentations. In contrast, an HRE approach predicts decreased effectiveness due to repeated presentation. We argue that repeated presentation of reinforcing stimuli decreases their effectiveness and that these decreases are described by the behavioral characteristics of habituation (McSweeney and Murphy, 2009; Rankin etal., 2009). We describe a neural model that postulates a positive association between dopamine neurotransmission and HRE. We present evidence that stimulant drugs, which artificially increase dopamine neurotransmission, disrupt (slow) normally occurring HRE and also provide evidence that stimulant drugs have differential effects on operant responding maintained by reinforcers with rapid vs. slow HRE rates. We hypothesize that abnormal HRE due to genetic and/or environmental factors may underlie some behavioral disorders. For example, recent research indicates that slow-HRE is predictive of obesity. In contrast ADHD may reflect “accelerated-HRE.” Consideration of HRE is important for the development of effective reinforcement-based treatments. Finally, we point out that most of the reinforcing stimuli that regulate daily behavior are non-consumable environmental/social reinforcers which have rapid-HRE. The almost exclusive use of consumable reinforcers with slow-HRE in pre-clinical studies with animals may have caused the importance of HRE to be overlooked. Further study of reinforcing stimuli with rapid-HRE is needed in order to understand how habituation and reinforcement interact and regulate behavior.

Extinguished cocaine cues increase drug seeking when presented simultaneously with a non-extinguished cocaine cue

BACKGROUND

Previous research with non-drug reinforcers has shown that simultaneously presenting (compounding) an extinguished cue with another cue formerly associated with the same reinforcer can increase rates of cue-controlled behavior. The present study investigated whether an extinguished cocaine cue would energize cocaine seeking when presented simultaneously with another cocaine cue. This study also investigated whether extinction could be enhanced by subjecting an extinguished cocaine cue to further extinction after administration of reinstating injections of cocaine.

METHODS

Rats were first trained to self-administer cocaine in the presence of three different cues. Then, one of the cues was subjected to the standard extinction treatment. Another cue was subjected to a modified extinction treatment where additional extinction sessions were preceded by non-contingent cocaine injections. The third cue was not extinguished.

RESULTS

The cue subjected to standard extinction ceased to control cocaine seeking when presented alone, but significantly increased cocaine seeking when compounded with the non-extinguished cocaine cue. The cocaine cue subjected to the modified extinction treatment also significantly increased cocaine seeking occasioned by the non-extinguished cocaine cue.

CONCLUSIONS

Extending results of previous studies involving non-drug stimuli, the present study showed that extinguished cocaine cues can enhance cocaine seeking when compounded with other cocaine cues. These results illustrate the persistence of drug cues in controlling behavior despite extinction and highlight the need for developing treatments that eliminate this residual energizing capacity that survives extinction.

Drug wanting: behavioral sensitization and relapse to drug-seeking behavior

Repeated exposure to drugs of abuse enhances the motor-stimulant response to these drugs, a phenomenon termed behavioral sensitization. Animals that are extinguished from self-administration training readily relapse to drug, conditioned cue, or stress priming. The involvement of sensitization in reinstated drug-seeking behavior remains controversial. This review describes sensitization and reinstated drug seeking as behavioral events, and the neural circuitry, neurochemistry, and neuropharmacology underlying both behavioral models will be described, compared, and contrasted. It seems that although sensitization and reinstatement involve overlapping circuitry and neurotransmitter and receptor systems, the role of sensitization in reinstatement remains ill-defined. Nevertheless, it is argued that sensitization remains a useful model for determining the neural basis of addiction, and an example is provided in which data from sensitization studies led to potential pharmacotherapies that have been tested in animal models of relapse and in human addicts.

The neuropharmacology of relapse to food seeking: methodology, main findings, and comparison with relapse to drug seeking

Relapse to old, unhealthy eating habits is a major problem in human dietary treatments. The mechanisms underlying this relapse are unknown. Surprisingly, until recently this clinical problem has not been systematically studied in animal models. Here, we review results from recent studies in which a reinstatement model (commonly used to study relapse to abused drugs) was employed to characterize the effect of pharmacological agents on relapse to food seeking induced by either food priming (non-contingent exposure to small amounts of food), cues previously associated with food, or injections of the pharmacological stressor yohimbine. We also address methodological issues related to the use of the reinstatement model to study relapse to food seeking, similarities and differences in mechanisms underlying reinstatement of food seeking versus drug seeking, and the degree to which the reinstatement procedure provides a suitable model for studying relapse in humans. We conclude by discussing implications for medication development and future research. We offer three tentative conclusions: (1)The neuronal mechanisms of food-priming- and cue-induced reinstatement are likely different from those of reinstatement induced by the pharmacological stressor yohimbine. (2)The neuronal mechanisms of reinstatement of food seeking are possibly different from those of ongoing food-reinforced operant responding. (3)The neuronal mechanisms underlying reinstatement of food seeking overlap to some degree with those of reinstatement of drug seeking.

Conditioned place preference induced by social play behavior: parametrics, extinction, reinstatement and disruption by methylphenidate

In this study, we investigated behavioral factors underlying conditioned place preference (CPP) induced by social interaction in adolescent rats. We found that the magnitude of socially-induced CPP depended on the social motivation of the animals and on the amount of training. After extinction, socially-induced CPP could be reinstated by a single reconditioning session. Treatment with methylphenidate, which disrupts social play behavior in adolescent rats, but not social exploratory behavior, prevented the development of socially-induced CPP. Interestingly, methylphenidate by itself induced CPP. These data demonstrate that: 1. social interaction is rewarding in adolescent rats; 2. appetitive and mnemonic factors influence the development of socially-induced CPP; 3. comparable to drug-induced CPP, socially-induced CPP can be extinguished and reinstated; 4. social play is likely to be the most rewarding aspect of social interaction in adolescent rats; 5. social context influences the subjective effects of methylphenidate.

Effects of extended cocaine conditioning in the reinstatement of place preference

Rats allowed extended access to cocaine self-administration develop a number of symptoms of addiction, such as greater susceptibility to drug-induced relapse. Using the conditioned place preference (CPP), the number of conditioning training sessions was increased in order to augment exposure to contextual cues associated with the effects of a drug. Mice were conditioned with a steady dose of 6 or 25 mg/kg of cocaine for 4, 8, 12, 16, 20 or 40 days. Weekly sessions of extinction followed the establishment of preference, after which a priming dose of cocaine was administered to reinstate the extinguished preference. The magnitude of the place preference effect was equal in all groups, independently of the number of conditioning sessions. The persistence of the place preference was not related with the number of sessions. Higher responsiveness to reinstatement of the extinguished preference occurred only with an intermediate number of conditioning sessions. In this way, the relation between the number of training sessions and vulnerability to relapse appeared to follow an inverted U-shaped function. Our results suggest that increasing the number of conditioning sessions from 12 to up to 16, without increasing the amount of drug administered, can be of great use in the study of vulnerability to relapse.

A reliable method to study cue-, priming-, and stress-induced reinstatement of cocaine self-administration in mice

RATIONALE

Cocaine addiction is a relapsing psychiatric disorder with a high prevalence in developed countries. To date, the reinstatement model has been difficult to implement in mice. The design of an appropriate reinstatement model in mice is required in order to use genetically modified animals with the aim of clarifying the mechanisms involved in cocaine relapse.

OBJECTIVES

Our aim was to develop an appropriate model of reinstatement of cocaine-seeking behavior and to investigate the factors that can trigger this reinstatement by using an operant intravenous self-administration procedure in mice. Discrete cues, priming injection of cocaine, and exposure to stress were the stimuli used to reinstate cocaine-seeking behavior.

MATERIAL AND METHODS

Mice were trained to acquire intravenous self-administration of cocaine (1 mg/kg per infusion) on a fixed ratio 1 (FR1) schedule of reinforcement. After achieving the acquisition criteria, animals were led to extinguish the operant behavior. Subsequently, under extinction conditions, mice were tested after the administration of a cocaine priming injection (10 mg/kg i.p.), the presentation of a light cue associated with cocaine administration, or the exposure to a stressful situation (0.21 mA electric footshock).

RESULTS

Under our experimental conditions the three stimuli successfully reinstated an extinguished cocaine-seeking behavior. Reexposure to cocaine effects by a priming injection was revealed as the strongest stimulus, capable of reinstating cocaine-seeking behavior.

CONCLUSIONS

The effective reinstatement model that we have developed will become a useful tool for future understanding of the neurobiological basis of cocaine addiction and relapse, specifically, with the use of genetically modified mice.