The attribution of incentive salience to a stimulus that signals an intravenous injection of cocaine

The Continuing Challenges of Studying Parallel Behaviours in Humans and Animal Models

The use of animal models continues to be essential for carrying out research into clinical phenomena, including addiction. However, the complexity of the clinical condition inevitably means that even the best animal models are inadequate, and this may go some way to account for the apparent failures of discoveries from animal models, including the identification of potential novel therapies, to translate to the clinic. We argue here that it is overambitious and misguided in the first place to attempt to model complex, multifacetted human disorders such as addiction in animals, and especially in rodents, and that all too frequently "validity" of such models is limited to superficial similarities, referred to as "face validity", that reflect quite different underlying phenomena and biological processes from the clinical situation. Instead, a more profitable approach is to identify (a) well-defined intermediate human behavioural phenotypes that reflect defined, limited aspects of, or contributors to, the human clinical disorder, and (b) to develop animal models that are homologous with those discrete human behavioural phenotypes in terms of psychological processes, and underlying neurobiological mechanisms. Examples of past and continuing weaknesses and suggestions for more limited approaches that may allow better homology between the test animal and human condition are made.

Effects of an ethanol-paired conditioned stimulus on responding for ethanol suppressed by a conditioned-taste-aversion

Ethanol-Paired Conditioned Stimuli (CS) can increase ethanol-responding either in extinction or occurring at low rates late in a session. To examine the generality of CS-induced increases in ethanol-responding, we examined whether a CS could increase responding suppressed by Conditioned-Taste-Aversion (CTA), which presumably suppresses responding by changing ethanol's valence from positive to negative. Rats were trained to respond for ethanol under a Random Interval (RI) schedule. We then removed the lever and paired Random-Time ethanol deliveries with illumination of a stimulus light (i.e., CS) for 10 sessions. Results were compared with a Truly Random Control group, in which the light and ethanol deliveries occurred independently. In a subsequent experiment, rats were treated similarly, except the light served as a discriminative stimulus, as the lever was extended and ethanol deliveries were available under a RI during light presentations. After this training, the lever was returned and rats again responded for ethanol. Subsequently, sessions were followed by LiCl administration. When responding reached low levels, LiCl administration stopped and the light was occasionally illuminated during the session. Responding during the light presentation was compared to responding during the period preceding light presentation. Responding partially recovered across 10 sessions and was greater during light presentations than in the period before it in all three groups. Increases were not reliably different between the groups, indicating that explanations for these increases such as CS-induced increases in motivation or approach toward the light are unlikely to be correct. The most likely explanation for these light-induced increases is that during sessions in which the light had been presented previously, LiCl had never been presented and thus, the light had come to signal that ethanol was safe to drink.

Reward prediction-errors weighted by cue salience produces addictive behaviours in simulations, with asymmetrical learning and steeper delay discounting

Dysfunction in learning and motivational systems are thought to contribute to addictive behaviours. Previous models have suggested that dopaminergic roles in learning and motivation could produce addictive behaviours through pharmacological manipulations that provide excess dopaminergic signalling towards these learning and motivational systems. Redish (2004) suggested a role based on dopaminergic signals of value prediction error, while (Zhang et al., 2009) suggested a role based on dopaminergic signals of motivation. However, both models present significant limitations. They do not explain the reduced sensitivity to drug-related costs/negative consequences, the increased impulsivity generally found in people with a substance use disorder, craving behaviours, and non-pharmacological dependence, all of which are key hallmarks of addictive behaviours. Here, we propose a novel mathematical definition of salience, that combines aspects of dopamine's role in both learning and motivation within the reinforcement learning framework. Using a single parameter regime, we simulated addictive behaviours that the (Zhang et al., 2009; Redish, 2004) models also produce but we went further in simulating the downweighting of drug-related negative prediction-errors, steeper delay discounting of drug rewards, craving behaviours and aspects of behavioural/non-pharmacological addictions. The current salience model builds on our recently proposed conceptual theory that salience modulates internal representation updating and may contribute to addictive behaviours by producing misaligned internal representations (Kalhan et al., 2021). Critically, our current mathematical model of salience argues that the seemingly disparate learning and motivational aspects of dopaminergic functioning may interact through a salience mechanism that modulates internal representation updating.

Separating desire from prediction of outcome value

Individuals typically want what they expect to like, often based on memories of previous positive experiences. However, in some situations desire can decouple completely from memories and from learned predictions of outcome value. The potential for desire to separate from prediction arises from independent operating rules that control motivational incentive salience. Incentive salience, or 'wanting', is a type of mesolimbic desire that evolved for adaptive goals, but can also generate maladaptive addictions. Two proof-of-principle examples are presented here to show how motivational 'wanting' can soar above memory-based predictions of outcome value: (i) 'wanting what is remembered to be disgusting', and (ii) 'wanting what is predicted to hurt'. Consequently, even outcomes remembered and predicted to be negatively aversive can become positively 'wanted'. Similarly, in human addictions, people may experience powerful cue-triggered cravings for outcomes that are not predicted to be enjoyable.

Hierarchical cue control of cocaine seeking in the face of cost

RATIONALE

Addiction is characterized by intermittent drug seeking despite rising costs. This behavior is heavily influenced by environmental stimuli that signal drug availability and reinforce drug seeking.

OBJECTIVE

To establish the relationship between three key aspects of human drug use in rats: the intermittent, binge nature of drug intake, the motivational conflict of drug seeking in the face of escalating negative costs, and the ability of different drug cues to interact to modulate relapse.

METHODS

Male and female rats were trained to self-administer cocaine on an intermittent access schedule, where brief drug-availability states were signaled by a shift in the ambient lighting of the environment, and cocaine infusions were signaled by a separate proximal discrete cue. Rats then went through a conflict procedure, where foot shock intensity associated with cocaine seeking was escalated until intake was suppressed. We then completed relapse tests where the drug-delivery cue was noncontingently presented alone, or in the context of dynamic drug-availability state transitions.

RESULTS

Intermittent access spurred psychomotor sensitization and binge-like cocaine intake. The intensity of binge-like drug taking during training was predictive of later drug seeking despite escalating costs during conflict. In relapse tests, the ability of a proximal discrete drug cue to trigger relapse was gated by the presence of a global cue signaling drug-availability state transitions.

CONCLUSIONS

Our results suggest that the pattern of drug intake plays a role in many features of addiction, including modifying an individual's willingness to endure high costs associated with drug seeking. Furthermore, our studies indicate that drug-related sensory information can be hierarchically organized to exert a dynamic modulating influence on drug-seeking motivation.

Investigating discriminative stimulus modulation of opioid seeking after conflict-induced abstinence in sign- and goal-tracking rats

RATIONALE

Discriminative stimuli (DS) are cues that predict reward availability. DS are resistant to extinction and motivate drug seeking even after long periods of abstinence. Previous studies have demonstrated that sign-tracking (ST) and goal-tracking (GT) differences in Pavlovian approach predict distinct cue-modulated vulnerabilities to cocaine reinstatement. GT rats show heightened reinstatement to contextual and DS, while ST rats show heightened reinstatement to discrete stimuli. Here we examine whether DS modulate reinstatement after electric barrier-induced abstinence and whether tracking-related relapse vulnerabilities generalize to opioid relapse.

OBJECTIVES

We examine whether DS-modulated reinstatement to fentanyl seeking persists in the presence of reduced adverse consequences after electric barrier-induced abstinence. We also examine whether tracking differences predict the magnitude of DS-modulated reinstatement of fentanyl seeking after electric barrier-induced abstinence.

METHODS

We used Pavlovian lever autoshaping (PLA) training to determine sign-, goal-, and intermediate tracking groups in male and female Sprague Dawley rats. We then trained rats in a DS model of intermittent fentanyl self-administration, and extinguished drug seeking by imposing an electric barrier of increasing intensity. We then measured the level of DS-modulated reinstatement in the presence of a reduced electric barrier intensity.

RESULTS

We report that DS strongly modulate fentanyl seeking after electric barrier-induced abstinence. DS-modulation of fentanyl acquisition, electric barrier-induced abstinence, and reinstatement was similar for sign- and goal-tracking groups.

CONCLUSIONS

Discriminative stimuli powerfully motivate opioid seeking, despite continued aversive consequences. Pavlovian approach differences do not predict the level of DS-modulated reinstatement to fentanyl seeking after conflict-induced abstinence.

The Role of Context Conditioning in the Reinstatement of Responding to an Alcohol-Predictive Conditioned Stimulus

Re-exposure to an unconditioned stimulus (US) can reinstate extinguished conditioned responding elicited by a conditioned stimulus (CS). We tested the hypothesis that the reinstatement of responding to an appetitive CS is driven by an excitatory association formed between the US and the context that the US was ingested in during US re-exposure. Male, Long-Evans rats were acclimated to drinking alcohol (15%, v/v) in the home-cage, then trained to associate an auditory CS with an alcohol-US that was delivered into a fluid port for oral intake. During subsequent extinction sessions, the CS was presented as before, but without alcohol. After extinction, rats were re-exposed to alcohol as in training, but without the CS (alcohol re-exposure). 24h later at test, the CS was presented as in training, but without alcohol. First, we tested the effect of extinguishing the context-alcohol association, formed during alcohol re-exposure, on reinstatement. Conducting four context extinction sessions across four days (spaced extinction) after the alcohol re-exposure session did not impact reinstatement. However, four context extinction sessions conducted across two days (massed extinction) prevented reinstatement. Next, we conducted alcohol re-exposure in a context that either differed from, or was the same as, the test context. One alcohol re-exposure session in a different context did not affect reinstatement, however, three alcohol re-exposure sessions in a different context significantly reduced reinstatement during the first CS trial. These results partially support the view that a context-US association formed during US re-exposure drives the reinstatement of responding to an appetitive, alcohol-predictive CS.

Nicotine Enhances Goal-Tracking in Ethanol and Food Pavlovian Conditioned Approach Paradigms

Rationale

Nicotine promotes alcohol intake through pharmacological and behavioral interactions. As an example of the latter, nicotine can facilitate approach toward food- and alcohol-associated stimuli ("sign-tracking") in lever-Pavlovian conditioned approach (PavCA) paradigms. However, we recently reported that nicotine can also enhance approach toward locations of reward delivery ("goal-tracking") triggered by ethanol-predictive stimuli when the location of ethanol delivery is non-static (i.e., a retractable sipper bottle).

Objective

To determine whether the non-static nature of the reward location could have biased the development of goal-tracking in our previous study (Loney et al., 2019); we assessed the effect of nicotine in a lever-PavCA paradigm wherein the location of ethanol delivery was static (i.e., a stationary liquid receptacle). Then, to determine whether nicotine's enhancement of goal-tracking is unique to ethanol-predictive stimuli, we assessed the effect of systemic nicotine on approach triggered by food-predictive stimuli in a lever-PavCA paradigm.

Methods

Long-Evans rats were used in two PavCA experiments wherein a lever predicted the receipt of ethanol (15% vol/vol; experiment 1) or food (experiment 2) into a stationary receptacle. Prior to testing, rats were administered nicotine (0.4 mg/kg subcutaneously) or saline systemically.

Results

In both experiments, nicotine increased measures of goal-tracking, but not sign-tracking.

Conclusion

Nicotine can facilitate approach to reward locations without facilitating approach to reward-predictive stimuli. As such, conceptualization of the mechanisms by which nicotine affects behavior must be expanded to explain an enhancement of goal-tracking by nicotine.

Incentive motivation: 'wanting' roles of central amygdala circuitry

The central nucleus of amygdala (CeA) mediates positively-valenced reward motivation as well as negatively-valenced fear. Optogenetic or neurochemical stimulation of CeA circuitry can generate intense incentive motivation to pursue and consume a paired natural food, sex, or addictive drug reward, and even create maladaptive 'wanting what hurts' such as attraction to a shock rod. Evidence indicates CeA stimulations selectively amplify incentive motivation ('wanting') but not hedonic impact ('liking') of the same reward. Further, valence flips can occur for CeA contributions to motivational salience. That is, CeA stimulation can promote either incentive motivation or fearful motivation, even in the same individual, depending on situation. These findings may carry implications for understanding CeA roles in neuropsychiatric disorders involving aberrant motivational salience, ranging from addiction to paranoia and anxiety disorders.

Ethanol-paired stimuli can increase reinforced ethanol responding

While ethanol-paired stimuli are frequently postulated to increase drinking motivation and thus increase ethanol responding and precipitate relapse, no study has demonstrated increases in ethanol-reinforced responding following presentation of an ethanol-paired stimulus that had not previously been part of a contingent relationship. Previous studies have shown that food-paired stimuli can increase food responding that is at low rates and increase food consumption in food-sated rats. In Experiment 1, we show that an ethanol-paired stimulus can increase ethanol responding that is at low levels late in the experimental session, presumably due to satiation. However, these increases may have resulted from either associative or non-associative mechanisms. In Experiment 2, we compared the effects of an ethanol-paired stimulus to those of the same stimulus in a Truly-Random-Control group. In a Truly-Random-Control, the stimulus and ethanol each are presented on independent random schedules, and thus any differences between the effects of the stimulus in the experimental and control groups is likely attributable to the association between the stimulus and ethanol. The stimulus increased ethanol-reinforced responding in both the experimental and control groups, but these increases were greater in the experimental than the control group. Thus, both stimulus-change and the pairing of the stimulus with ethanol may result in increases in ethanol-reinforced responding.

From sign-tracking to attentional bias: Implications for gambling and substance use disorders

Sign-tracking behavior in Pavlovian autoshaping is known to be a relevant index of the incentive salience attributed to reward-related cues. Evidence has accumulated to suggest that animals that exhibit a sign-tracker phenotype are especially vulnerable to addiction and relapse due to their proneness to attribute incentive salience to drug cues, and their relatively weak cognitive and attentional control over their behavior. Interestingly, sign-tracking is also influenced by reward uncertainty in a way that may promote gambling disorder. Research indicates that reward uncertainty sensitizes sign-tracking responses and favors the development of a sign-tracker phenotype, compatible with the conditioned attractiveness of lights and sounds in casinos for problem gamblers. The study of attentional biases in humans (an effect akin to sign-tracking in animals) leads to similar observations, notably that the propensity to develop attraction for conditioned stimuli (CSs) is predictive of addictive behavior. Here we review the literature on drug addiction and gambling disorder, highlighting the similarities between studies of sign-tracking and attentional biases.

Understanding Addiction Using Animal Models

Drug addiction is a neuropsychiatric disorder with grave personal consequences that has an extraordinary global economic impact. Despite decades of research, the options available to treat addiction are often ineffective because our rudimentary understanding of drug-induced pathology in brain circuits and synaptic physiology inhibits the rational design of successful therapies. This understanding will arise first from animal models of addiction where experimentation at the level of circuits and molecular biology is possible. We will review the most common preclinical models of addictive behavior and discuss the advantages and disadvantages of each. This includes non-contingent models in which animals are passively exposed to rewarding substances, as well as widely used contingent models such as drug self-administration and relapse. For the latter, we elaborate on the different ways of mimicking craving and relapse, which include using acute stress, drug administration or exposure to cues and contexts previously paired with drug self-administration. We further describe paradigms where drug-taking is challenged by alternative rewards, such as appetitive foods or social interaction. In an attempt to better model the individual vulnerability to drug abuse that characterizes human addiction, the field has also established preclinical paradigms in which drug-induced behaviors are ranked by various criteria of drug use in the presence of negative consequences. Separation of more vulnerable animals according to these criteria, along with other innate predispositions including goal- or sign-tracking, sensation-seeking behavior or impulsivity, has established individual genetic susceptibilities to developing drug addiction and relapse vulnerability. We further examine current models of behavioral addictions such as gambling, a disorder included in the DSM-5, and exercise, mentioned in the DSM-5 but not included yet due to insufficient peer-reviewed evidence. Finally, after reviewing the face validity of the aforementioned models, we consider the most common standardized tests used by pharmaceutical companies to assess the addictive potential of a drug during clinical trials.

Effects of rat strain and method of inducing ethanol drinking on Pavlovian-Instrumental-Transfer with ethanol-paired conditioned stimuli

Ethanol-paired conditioned stimuli (CSs) are widely thought to invigorate ethanol responding, and thus, precipitate relapse to drinking. However, preclinical studies investigating this issue using Pavlovian-Instrumental-Transfer (PIT) procedures have had mixed results, with some studies finding PIT while others did not. The studies failing to show PIT used Lewis rats and induced ethanol drinking using a post-prandial drinking procedure. The present experiments examined whether either of these two variables influenced the magnitude of PIT observed. In the first experiment, ethanol drinking in Lewis rats was induced using either sucrose fading or post-prandial drinking. In the second experiment, ethanol drinking was induced using post-prandial drinking in either Long-Evans Hooded or Lewis rats. In both experiments, rats were trained to respond for ethanol under a random interval schedule. Subsequently with the lever removed, 2-min light presentations were paired with ethanol deliveries. Finally, with the lever returned, the effect of light presentations on responding was tested while responding was in extinction. Light presentations similarly affected responding in Lewis rats regardless of the method of drinking induction. Likewise, light presentations similarly affected responding in both Lewis and Long-Evans Hooded rats. Neither ethanol induction method nor rat strain affected the magnitude of PIT observed, and thus, neither likely explains previous failures to observe PIT with ethanol-maintained behavior.

Sex differences in nicotine-enhanced Pavlovian conditioned approach in rats

Background

Nicotine exposure enhances Pavlovian conditioned approach (PCA), or the learned approach to reward-predictive cues. While females show elevated approach to conditioned stimuli compared to males, potentially indicating heightened addiction vulnerability, it is unknown how sex may interact with nicotine to influence approach behavior. Additionally, brain-derived neurotrophic factor (BDNF) levels can be altered significantly after repeated nicotine exposure, suggesting a potential mechanism contributing to nicotine-induced behavioral phenotypes. The present study investigated the role of sex on nicotine-induced changes to stimulus-response behavior and associated BDNF protein levels.

Methods

Male and female rats were exposed to nicotine (0.4 mg/kg, subcutaneously) or saline 15 min prior to each PCA session. PCA training consisted of 29 sessions of 15 trials, in which a 30-s cue presentation ended concurrently with a sucrose reward (20% w/v in water, 100 μL), and a 120-s variable intertrial interval occurred between trials. Approach behavior to the cue and reward receptacle was recorded. Preference toward the reward receptacle indicated a goal-tracking phenotype, and preference toward the cue indicated a sign-tracking phenotype, demonstrating that the cue had gained incentive salience. Twenty-four hours after the last PCA session, brain tissue was collected and BDNF levels were measured in the basolateral amygdala, orbitofrontal cortex, and nucleus accumbens using Western blot analysis.

Results

Nicotine exposure enhanced both sign- and goal-tracking conditioned approach, and females showed elevated sign-tracking compared to males. There were no sex-by-drug interactions on conditioned approach. Day-to-day variability in conditioned approach was similar between sexes. In contrast to prior studies, neither repeated exposure to nicotine nor sex significantly affected BDNF expression.

Conclusions

Drug-naïve females exhibited heightened sign-tracking compared to males, and nicotine enhanced conditioned approach similarly in males and females. Further, non-significant changes to BDNF expression in brain regions highly associated with PCA indicate that BDNF is unlikely to drive nicotine-enhanced conditioned behavior.

Nicotine pre-treatment reduces sensitivity to the interoceptive stimulus effects of commonly abused drugs as assessed with taste conditioning paradigms

BACKGROUND

Drug pre-exposure attenuates sensitivity to the interoceptive stimulus properties of additional subsequently administered drugs in drug-induced conditioned taste avoidance (CTA) and conditioned place preference (CPP) paradigms. Specifically, nicotine, commonly used in conjunction with other addictive substances, attenuates acquisition of ethanol and caffeine CTAs and morphine-induced CPP.

METHODS

Because nicotine use is comorbid with a number of substance use disorders, we systematically examined the effects of nicotine pre-exposure on two different conditioning paradigms involving integration of the interoceptive stimulus properties of multiple commonly abused drugs, in male and female rats, designed to examine both the aversive and reinforcing properties of these drugs.

RESULTS

Nicotine dose-dependently interfered with acquisition of CTA to passively administered morphine, ethanol, and cocaine, but not lithium chloride, demonstrating that the effects of nicotine are not simply a matter of reduced orosensory processing or an inability to learn such associations. Moreover, nicotine-treated rats required higher doses of drug in order to develop CTA and did not show increased acceptance of the taste of self-administered ethanol compared with saline-treated rats.

CONCLUSIONS

These data demonstrate that nicotine pre-exposure attenuates sensitivity to the stimulus effects of multiple drugs in two conditioning paradigms, in a manner which is consistent with a reduced ability to integrate the interoceptive properties of abused drugs. Through reducing these stimulus properties of drugs of abuse, concomitant nicotine use may result in a need to increase either the frequency or strength of doses during drug-taking, thus likely contributing to enhanced addiction liability in smokers.

Cocaine Directly Impairs Memory Extinction and Alters Brain DNA Methylation Dynamics in Honey Bees

Drug addiction is a chronic relapsing behavioral disorder. The high relapse rate has often been attributed to the perseverance of drug-associated memories due to high incentive salience of stimuli learnt under the influence of drugs. Drug addiction has also been interpreted as a memory disorder since drug associated memories are unusually enduring and some drugs, such as cocaine, interfere with neuroepigenetic machinery known to be involved in memory processing. Here we used the honey bee (an established invertebrate model for epigenomics and behavioral studies) to examine whether or not cocaine affects memory processing independently of its effect on incentive salience. Using the proboscis extension reflex training paradigm we found that cocaine strongly impairs consolidation of extinction memory. Based on correlation between the observed effect of cocaine on learning and expression of epigenetic processes, we propose that cocaine interferes with memory processing independently of incentive salience by directly altering DNA methylation dynamics. Our findings emphasize the impact of cocaine on memory systems, with relevance for understanding how cocaine can have such an enduring impact on behavior.

Nicotine-enhanced Pavlovian conditioned approach is resistant to omission of expected outcome

Conditioned stimuli contribute to the resilience of nicotine addiction in that nicotine-associated cues can influence smokers and promote relapse. These stimuli are thought to acquire incentive motivational properties through a Pavlovian mechanism, and this phenomenon can be measured in animals by observing conditioned approach to the conditioned stimulus (sign-tracking) or to the location of unconditioned stimulus delivery (goal-tracking). Goal-tracking is thought to be more flexible than sign-tracking in response to changes in expected outcome. Nicotine exposure can increase the expression of conditioned responses, and we hypothesized that animals exposed to nicotine would also exhibit less flexible conditioned responses after a change in the expected unconditioned stimulus. Adult male rats were exposed to nicotine (0.4mg/kg, s.c.) or saline before Pavlovian conditioned approach training sessions. After training, animals underwent test sessions that reduced (water substitution) or withheld (omission) the unconditioned stimulus (US, 20% sucrose). As expected, nicotine enhanced sign- and goal-tracking. Water substitution moderately and nonspecifically reduced both sign- and goal-tracking in all rats. In contrast, US omission only reduced goal-tracking, with robust effects in saline-exposed rats and smaller effects in nicotine-exposed rats. These data support the hypothesis that both sign-tracking and nicotine exposure confer behavioral inflexibility under US omission.

Optogenetic Central Amygdala Stimulation Intensifies and Narrows Motivation for Cocaine

Addiction is often characterized by intense motivation for a drug, which may be narrowly focused at the expense of other rewards. Here, we examined the role of amygdala-related circuitry in the amplification and narrowing of motivation focus for intravenous cocaine. We paired optogenetic channelrhodopsin (ChR2) stimulation in either central nucleus of amygdala (CeA) or basolateral amygdala (BLA) of female rats with one particular nose-poke porthole option for earning cocaine infusions (0.3 mg/kg, i.v.). A second alternative porthole earned identical cocaine but without ChR2 stimulation. Consequently, CeA rats quickly came to pursue their CeA ChR2-paired cocaine option intensely and exclusively, elevating cocaine intake while ignoring their alternative cocaine alone option. By comparison, BLA ChR2 pairing failed to enhance cocaine motivation. CeA rats also emitted consummatory bites toward their laser-paired porthole, suggesting that higher incentive salience made that cue more attractive. A separate progressive ratio test of incentive motivation confirmed that CeA ChR2 amplified rats' motivation, raising their breakpoint effort price for cocaine by 10-fold. However, CeA ChR2 laser on its own lacked any reinforcement value: laser by itself was never self-stimulated, not even by the same rats in which it amplified motivation for cocaine. Conversely, CeA inhibition by muscimol/baclofen microinjections prevented acquisition of cocaine self-administration and laser preference, whereas CeA inhibition by optogenetic halorhodopsin suppressed cocaine intake, indicating that CeA circuitry is needed for ordinary cocaine motivation. We conclude that CeA ChR2 excitation paired with a cocaine option specifically focuses and amplifies motivation to produce intense pursuit and consumption focused on that single target.SIGNIFICANCE STATEMENT In addiction, intense incentive motivation often becomes narrowly focused on a particular drug of abuse. Here we show that pairing central nucleus of amygdala (CeA) optogenetic stimulation with one option for earning intravenous cocaine makes that option almost the exclusive focus of intense pursuit and consumption. CeA stimulation also elevated the effort cost rats were willing to pay for cocaine and made associated cues become intensely attractive. However, we also show that CeA laser had no reinforcing properties at all when given alone for the same rats. Therefore, CeA laser pairing makes its associated cocaine option and cues become powerfully attractive in a nearly addictive fashion.

Central GLP-1 receptor activation modulates cocaine-evoked phasic dopamine signaling in the nucleus accumbens core

Drugs of abuse increase the frequency and magnitude of brief (1-3s), high concentration (phasic) dopamine release events in terminal regions. These are thought to be a critical part of drug reinforcement and ultimately the development of addiction. Recently, metabolic regulatory peptides, including the satiety signal glucagon-like peptide-1 (GLP-1), have been shown to modulate cocaine reward-driven behavior and sustained dopamine levels after cocaine administration. Here, we use fast-scan cyclic voltammetry (FSCV) to explore GLP-1 receptor (GLP-1R) modulation of dynamic dopamine release in the nucleus accumbens (NAc) during cocaine administration. We analyzed dopamine release events in both the NAc shell and core, as these two subregions are differentially affected by cocaine and uniquely contribute to motivated behavior. We found that central delivery of the GLP-1R agonist Exendin-4 suppressed the induction of phasic dopamine release events by intravenous cocaine. This effect was selective for dopamine signaling in the NAc core. Suppression of phasic signaling in the core by Exendin-4 could not be attributed to interference with cocaine binding to one of its major substrates, the dopamine transporter, as cocaine-induced increases in reuptake were unaffected. The results suggest that GLP-1R activation, instead, exerts its suppressive effects by altering dopamine release - possibly by suppressing the excitability of dopamine neurons. Given the role of NAc core dopamine in the generation of conditioned responses based on associative learning, suppression of cocaine-induced dopamine signaling in this subregion by GLP-1R agonism may decrease the reinforcing properties of cocaine. Thus, GLP-1Rs remain viable targets for the treatment and prevention of cocaine seeking, taking and relapse.

Orbitofrontal participation in sign- and goal-tracking conditioned responses: Effects of nicotine

Pavlovian conditioned stimuli can acquire incentive motivational properties, and this phenomenon can be measured in animals using Pavlovian conditioned approach behavior. Drugs of abuse can influence the expression of this behavior, and nicotine in particular exhibits incentive amplifying effects. Both conditioned approach behavior and drug abuse rely on overlapping corticolimbic circuitry. We hypothesize that the orbitofrontal cortex (OFC) regulates conditioned approach, and that one site of nicotine action is in the OFC where it reduces cortical output. To test this, we repeatedly exposed rats to 0.4 mg/kg nicotine (s.c.) during training and then pharmacologically inactivated the lateral OFC or performed in vivo electrophysiological recordings of lateral OFC neurons in the presence or absence of nicotine. In Experiment 1, animals were trained in a Pavlovian conditioning paradigm and behavior was evaluated after inactivation of the OFC by microinfusion of the GABA agonists baclofen and muscimol. In Experiment 2, we monitored phasic firing of OFC neurons during Pavlovian conditioning sessions. Nicotine reliably enhanced conditioned responding to the conditioned cue, and inactivation of the OFC reduced conditioned responding, especially the sign-tracking response. OFC neurons exhibited phasic excitations to cue presentation and during goal tracking, and nicotine acutely blunted this phasic neuronal firing. When nicotine was withheld, both conditioned responding and phasic firing in the OFC returned to the level of controls. These results suggest that the OFC is recruited for the expression of conditioned responses, and that nicotine acutely influences this behavior by reducing phasic firing in the OFC.

Sex and Adolescent Ethanol Exposure Influence Pavlovian Conditioned Approach

BACKGROUND

Alcohol use among adolescents is widespread and a growing concern due to long-term behavioral deficits, including altered Pavlovian behavior, that potentially contribute to addiction vulnerability. We tested the hypothesis that adolescent intermittent ethanol (AIE) exposure alters Pavlovian behavior in males and females as measured by a shift from goal-tracking to sign-tracking. Additionally, we investigated GLT-1, an astrocytic glutamate transporter, as a potential contributor to a sign-tracking phenotype.

METHODS

Male and female Sprague-Dawley rats were exposed to AIE (5 g/kg, intragastric) or water intermittently 2 days on and 2 days off from postnatal day (P) 25 to 54. Around P70, animals began 20 daily sessions of Pavlovian conditioned approach (PCA), where they learned that a cue predicted noncontingent reward delivery. Lever pressing indicated interaction with the cue, or sign-tracking, and receptacle entries indicated approach to the reward delivery location, or goal-tracking. To test for effects of AIE on nucleus accumbens (NAcc) excitatory signaling, we isolated membrane subfractions and measured protein levels of the glutamate transporter GLT-1 after animals completed behavior as a measure of glutamate homeostasis.

RESULTS

Females exhibited elevated sign-tracking compared to males with significantly more lever presses, faster latency to first lever press, and greater probability to lever press in a trial. AIE significantly increased lever pressing while blunting goal-tracking, as indicated by fewer cue-evoked receptacle entries, slower latency to receptacle entry, and lower probability to enter the receptacle in a trial. No significant sex-by-exposure interactions were observed in sign- or goal-tracking metrics. Moreover, we found no significant effects of sex or exposure on membrane GLT-1 expression in the NAcc.

CONCLUSIONS

Females exhibited enhanced sign-tracking compared to males, while AIE decreased goal-tracking compared to control exposure. Our findings support the hypothesis that adolescent binge ethanol can shift conditioned behavior from goal- to cue-directed in PCA, especially in females.

Individual Differences in the Attribution of Incentive Salience to a Pavlovian Alcohol Cue

Individual differences exist in the attribution of incentive salience to conditioned stimuli associated with food. Here, we investigated whether individual differences also manifested with a Pavlovian alcohol conditioned stimulus (CS). We compiled data from five experiments that used a Pavlovian autoshaping paradigm and tests of conditioned reinforcement. In all experiments, male, Long-Evans rats with unrestricted access to food and water were acclimated to 15% ethanol. Next, rats received Pavlovian autoshaping training, in which a 10 s presentation of a retractable lever served as the CS and 0.2 mL of 15% ethanol served as the unconditioned stimulus (US). Finally, rats underwent conditioned reinforcement tests in which nose-pokes to an active aperture led to brief presentations of the lever-CS, but nose-pokes to an inactive aperture had no consequence. Rats were categorized as sign-trackers, goal-trackers and intermediates based on a response bias score that reflected their tendencies to sign-track or goal-track at different times during training. We found that distinct groups of rats either consistently interacted with the lever-CS (“sign-trackers”) or routinely approached the port during the lever-CS (“goal-trackers”) across a majority of the training sessions. However, some individuals (“shifted sign-trackers”) with an early tendency to goal-track later shifted to comparable asymptotic levels of sign-tracking as the group identified as sign-trackers. The lever-CS functioned as a conditioned reinforcer for sign-trackers and shifted sign-trackers, but not for goal-trackers. These results provide evidence of robust individual differences in the extent to which a Pavlovian alcohol cue gains incentive salience and functions as a conditioned reinforcer.

Effects of an ethanol-paired CS on responding for ethanol and food: Comparisons with a stimulus in a Truly-Random-Control group and to a food-paired CS on responding for food

Motivational increases due to exposure to alcohol-paired Conditioned Stimuli (CS) are central to some accounts of alcoholism. However, few studies isolate a stimulus's function as a CS from its other potential functions. Pavlovian-Instrumental-Transfer (PIT) procedures isolate a stimulus's function as a CS from its other functions. Though there are several relevant studies using PIT, knowledge gaps exist. Particularly, it is not clear that an alcohol-paired CS will increase alcohol seeking compared to the same stimulus in a Truly-Random-Control group, nor whether such increases are specific to alcohol seeking. To address these knowledge gaps in Experiment 1, rats responded for ethanol (0.1 ml 8% w/v) under an RI 30-sec schedule, then the lever was removed and half the rats had ethanol delivered during occasional 120-sec light presentations, while the remainder had ethanol and the light presented under independent RT schedules. Later the lever was returned and the light was presented during responding in extinction (PIT test). Following this test, levers were again removed and the light was presented without ethanol (light extinction), following again by a PIT test. Responding in the two groups during light presentations did not differ in either PIT test. Experiment 2 repeated Experiment 1 using food instead of ethanol. In Experiment 2, responding during light presentations increased in the paired group. In Experiment 3, rats were trained on a concurrent FR schedule of food and ethanol delivery. Ethanol was delivered following 5 responses and the response requirement for food adjusted so that similar numbers of food and ethanol deliveries were obtained. Subsequently, rats underwent conditioning, control and testing procedures identical to those in Experiment 1. In Experiment 3, the ethanol-paired CS increased ethanol-responding, but not food-responding. These results are most easily interpreted as changes in responding resulting from CS-elicited behavior rather than motivational changes. This interpretation is more compatible with some descriptions of the role of an alcohol-paired CS in alcoholism than others.

Less is more: prolonged intermittent access cocaine self-administration produces incentive-sensitization and addiction-like behavior

RATIONALE

Contemporary animal models of cocaine addiction focus on increasing the amount of drug consumption to produce addiction-like behavior. However, another critical factor is the temporal pattern of consumption, which in humans is characterized by intermittency, both within and between bouts of use.

OBJECTIVE

To model this, we combined prolonged access to cocaine (∼70 days in total) with an intermittent access (IntA) self-administration procedure and used behavioral economic indicators to quantify changes in motivation for cocaine.

RESULTS

IntA produced escalation of intake, a progressive increase in cocaine demand (incentive-sensitization), and robust drug- and cue-induced reinstatement of drug-seeking behavior. We also asked whether rats that vary in their propensity to attribute incentive salience to reward cues (sign-trackers [STs] vs. goal-trackers [GTs]) vary in the development of addiction-like behavior. Although STs were more motivated to take cocaine after limited drug experience, after IntA, STs and GTs no longer differed on any measure of addiction-like behavior.

CONCLUSIONS

Exposure to large quantities of cocaine is not necessary for escalation of intake, incentive-sensitization, or other addiction-like behaviors (IntA results in far less total cocaine consumption than 'long access' procedures). Also, the ST phenotype may increase susceptibility to addiction, not because STs are inherently susceptible to incentive-sensitization (perhaps all individuals are at risk), but because this phenotype promotes continued drug use, subjecting them to incentive-sensitization. Thus, the pharmacokinetics associated with the IntA procedure are especially effective in producing a number of addiction-like behaviors and may be valuable for studying associated neuroadaptations and for assessing individual variation in vulnerability.

Conditioned stimuli's role in relapse: preclinical research on Pavlovian-Instrumental-Transfer

RATIONALE AND OBJECTIVE

Pavlovian learning is central to many theories of addiction. In these theories, stimuli paired with drug ingestion become conditioned stimuli (CS) and subsequently elicit drug-seeking and drug-taking. However, in most relevant studies, Pavlovian and instrumental learning are confounded. This confound may be avoided in Pavlovian-Instrumental-Transfer (PIT) procedures. In PIT, Pavlovian and instrumental learning are established separately and then combined. In order to better understand the role of CSs in addiction, we review the relevant studies using PIT.

FINDINGS

We identified seven articles examining PIT effects of ethanol- or cocaine-paired CSs. Under at least one condition, six of these articles reported CS-elicited increases in responding previously maintained by drug. However, the only study using the optimal control condition failed to find a CS-elicited increase. Two studies examining CS specificity found the CS also increased responding maintained by a different reinforcer. Two studies examined if CSs elicit increases in actual drug-taking. Both failed to find CS-elicited increases, i.e., no study shows CS-elicited increases in actual drug-taking. Further, CS-elicited increases in extinguished responding are short-lived.

CONCLUSIONS

These findings are not entirely consistent with Pavlovian learning playing a central role in addiction. However, design issues can explain most of these inconsistencies. Studies without these design issues are needed. Additionally, existing theories hypothesize drug-paired CSs increase drug-taking by increasing motivation, by eliciting conditioned responses that make drug-seeking more probable, or by a combination of these. Work distinguishing between these mechanisms would also be useful.

Conditioned cortical reactivity to cues predicting cigarette-related or pleasant images

Through Pavlovian conditioning, reward-associated neutral stimuli can acquire incentive salience and motivate complex behaviors. In smokers, cigarette-associated cues may induce cravings and trigger smoking. Understanding the brain mechanisms underlying conditioned responses to cigarette-associated relative to other inherently pleasant stimuli might contribute to the development of more effective smoking cessation treatments that emphasize the rehabilitation of reward circuitry. Here we measured brain responses to geometric patterns (the conditioned stimuli, CSs) predicting cigarette-related, intrinsically pleasant and neutral images (the unconditioned stimuli, USs) using event-related potentials (ERPs) in 29 never-smokers, 20 nicotine-deprived smokers, and 19 non-deprived smokers. Results showed that during US presentation, cigarette-related and pleasant images prompted higher cortical positivity than neutral images over centro-parietal sensors between 400 and 800ms post-US onset (late positive potential, LPP). The LPP evoked by pleasant images was significantly larger than the LPP evoked by cigarette images. During CS presentation, ERPs evoked by geometric patterns predicting pleasant and cigarette-related images had significantly larger amplitude than ERPs evoked by CSs predicting neutral images. These effects were maximal over right parietal sites between 220 and 240ms post-CS onset and over occipital and frontal sites between 308 and 344ms post-CS onset. Smoking status did not modulate these effects. Our results show that stimuli with no intrinsic reward value (e.g., geometric patterns) may acquire rewarding properties through repeated pairings with established reward cues (i.e., cigarette-related, intrinsically pleasant).

Implicit measures of "wanting" and "liking" in humans

Incentive Sensitization Theory (IST; e.g., Robinson and Berridge, 1993. Brain Res. Rev., 18, 291; Robinson and Berridge, 2003 Trends Neurosci., 26, 507) suggests that a common dopamine system that deals with incentive salience attribution is affected by different types of drugs. Repeated drug use will sensitize this neural system, which means that drugs increasingly trigger the experience of incentive salience or "wanting". Importantly, Robinson and Berridge stress that there is a dissociation between drug "wanting" (the unconscious attribution of incentive salience) and drug "liking" (the unconscious hedonic experience when one consumes drugs). Whereas the former plays an essential role in the development and maintenance of drug addiction, the latter does not. Although this model was based mainly on research with non-human animals, more recently the dissociation between "wanting" and "liking" has been examined in humans as well. A widely used and promising means of studying these processes are behavioral implicit measures such as the Implicit Association Test (IAT), the Approach-Avoidance Task (AAT), different types of Stimulus-Response Compatibility (SRC) tasks, and Affective Simon Tasks (AST). IST makes the clear prediction that (1) there should be a positive correlation between indices of "wanting" (e.g., drug consumption) and implicit "wanting" scores. Similarly, there should be a positive correlation between indices of "liking" (e.g., various expressions of subjective pleasure) and implicit "liking" scores; (2) there should be higher "wanting" scores in substance abusers or frequent substance users compared to non-users or infrequent users, and there should be no differences in "liking" between these groups (or even less "liking" in frequent substance users); (3) manipulations of "wanting" should affect implicit "wanting" scores whereas manipulations of "liking" should affect implicit "liking" scores. However, studies that tested these hypotheses did not produce equivocal results. To shed light on these discrepancies, we first discuss the different definitions of "wanting" and "liking" and the different tests that have been used to assess these processes. Then, we discuss whether it is reasonable to assume that these tests are valid measures of "wanting" and "liking" and we review correlational, quasi-experimental, and experimental studies that inform us about this issue. Finally, we discuss the future potential of implicit measures in research on IST and make several recommendations to improve both theory and methodology.

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.

Individual differences in the influence of task-irrelevant Pavlovian cues on human behavior

Pavlovian-to-instrumental transfer (PIT) refers to the process of a Pavlovian reward-paired cue acquiring incentive motivational proprieties that drive choices. It represents a crucial phenomenon for understanding cue-controlled behavior, and it has both adaptive and maladaptive implications (i.e., drug-taking). In animals, individual differences in the degree to which such cues bias performance have been identified in two types of individuals that exhibit distinct Conditioned Responses (CR) during Pavlovian conditioning: Sign-Trackers (ST) and Goal-Trackers (GT). Using an appetitive PIT procedure with a monetary reward, the present study investigated, for the first time, the extent to which such individual differences might affect the influence of reward-paired cues in humans. In a first task, participants learned an instrumental response leading to reward; then, in a second task, a visual Pavlovian cue was associated with the same reward; finally, in a third task, PIT was tested by measuring the preference for the reward-paired instrumental response when the task-irrelevant reward-paired cue was presented, in the absence of the reward itself. In ST individuals, but not in GT individuals, reward-related cues biased behavior, resulting in an increased likelihood to perform the instrumental response independently paired with the same reward when presented with the task-irrelevant reward-paired cue, even if the reward itself was no longer available (i.e., stronger PIT effect). This finding has important implications for developing individualized treatment for maladaptive behaviors, such as addiction.

Drug versus sweet reward: greater attraction to and preference for sweet versus drug cues

Despite the unique ability of addictive drugs to directly activate brain reward circuits, recent evidence suggests that drugs induce reinforcing and incentive effects that are comparable to, or even lower than some nondrug rewards. In particular, when rats have a choice between pressing a lever associated with intravenous cocaine or heroin delivery and another lever associated with sweet water delivery, most respond on the latter. This outcome suggests that sweet water is more reinforcing and attractive than either drug. However, this outcome may also be due to the differential ability of sweet versus drug levers to elicit Pavlovian feeding-like conditioned responses that can cause involuntary lever pressing, such as pawing and biting the lever. To test this hypothesis, rats first underwent Pavlovian conditioning to associate one lever with sweet water (0.2% saccharin) and a different lever with intravenous cocaine (0.25 mg) or heroin (0.01 mg). Choice between these two levers was then assessed under two operant choice procedures: one that permitted the expression of Pavlovian-conditioned lever press responses during choice, the other not. During conditioning, Pavlovian-conditioned lever press responses were considerably higher on the sweet lever than on either drug lever, and slightly greater on the heroin lever than on the cocaine lever. Importantly, though these differences in Pavlovian-conditioned behavior predicted subsequent preference for sweet water during choice, they were not required for its expression. Overall, this study confirms that rats prefer the sweet lever because sweet water is more reinforcing and attractive than cocaine or heroin.

The attribution of incentive salience to Pavlovian alcohol cues: a shift from goal-tracking to sign-tracking

Environmental stimuli that are reliably paired with alcohol may acquire incentive salience, a property that can operate in the use and abuse of alcohol. Here we investigated the incentive salience of Pavlovian alcohol cues using a preclinical animal model. Male, Long-Evans rats (Harlan) with unrestricted access to food and water were acclimated to drinking 15% ethanol (v/v) in their home-cages. Rats then received Pavlovian autoshaping training in which the 10 s presentation of a retractable lever served as the conditioned stimulus (CS) and 15% ethanol served as the unconditioned stimulus (US) (0.2 ml/CS; 12 CS presentations/session; 27 sessions). Next, in an operant test of conditioned reinforcement, nose pokes into an active aperture delivered presentations of the lever-CS, whereas nose pokes into an inactive aperture had no consequences. Across initial autoshaping sessions, goal-tracking behavior, as measured by entries into the fluid port where ethanol was delivered, developed rapidly. However, with extended training goal-tracking diminished, and sign-tracking responses, as measured by lever-CS activations, emerged. Control rats that received explicitly unpaired CS and US presentations did not show goal-tracking or sign-tracking responses. In the test for conditioned reinforcement, rats with CS-US pairings during autoshaping training made more active relative to inactive nose pokes, whereas rats in the unpaired control group did not. Moreover, active nose pokes were positively correlated with sign-tracking behavior during autoshaping. Extended training may produce a shift in the learned properties of Pavlovian alcohol cues, such that after initially predicting alcohol availability they acquire robust incentive salience.

Motivational Processes Underlying Substance Abuse Disorder

Drug addiction is a syndrome of dysregulated motivation, evidenced by intense drug craving and compulsive drug-seeking behavior. In the search for 'common neurobiological substrates of addiction to different classes of drugs, behavioral neuroscientists have attempted to determine the neural basis for a number of motivational concepts and describe how they are changed by repeated drug use. Here, we describe these concepts and summarize previous work describing three major neural systems that play distinct roles in different conceptual aspects of motivation: (1) a nigrostriatal system that is involved in two forms of instrumental learning, (2) a ventral striatal system that is involved in Pavlovian incentive motivation and negative reinforcement, and (3) frontal cortical areas that regulate decision making and motivational processes. Within striatal systems, drug addiction can involve a transition from goal-oriented, incentive processes to automatic, habit-based responding. In the cortex, weak inhibitory control is a predisposing factor to, as well as a consequence of, repeated drug intake. However, these transitions are not absolute, and addiction can occur without a transition to habit-based responding, occurring as a result of the overvaluation of drug outcomes and hypersensitivity to incentive properties of drug-associated cues. Finally, we point out that addiction is not monolithic and can depend not only on individual differences between addicts, but also on the neurochernical action of specific drug classes.

Sign-tracking predicts increased choice of cocaine over food in rats

The purpose of this study was to determine whether the tendency to sign-track to a food cue was predictive of rats' choice of cocaine over food. First, rats were trained on a procedure where insertion of a retractable lever was paired with food. A sub-group of rats - sign-trackers - primarily approached and contacted the lever, while another sub-group - goal-trackers - approached the site of food delivery. Rats were then trained on a choice task where they could choose between an infusion of cocaine (1.0 mg/kg) and a food pellet (45 mg). Sign-trackers chose cocaine over food significantly more often than did goal-trackers. These results support the incentive-salience theory of addiction and add to a growing number of studies which suggest that sign-trackers may model an addiction-prone phenotype.

Effects of ethanol exposure during adolescence or in adulthood on Pavlovian conditioned approach in Sprague-Dawley rats

Human studies have shown that adolescents who repeatedly use alcohol are more likely to be dependent on alcohol and are more likely to suffer from psychological problems later in life. There has been limited research examining how ethanol exposure in adolescence might contribute to later abuse or addiction in adulthood. The present experiment examined effects of intermittent ethanol exposure during adolescence on sign-tracking behavior in adulthood, indexed by a Pavlovian conditioned approach (PCA) task wherein an 8s lever presentation served as a cue predicting subsequent delivery of a flavored food pellet. Although no response was required for food delivery, after multiple pairings, 1 of 2 different responses often emerged during the lever presentation: goal tracking (head entries into the food trough) or sign tracking (engagement with the lever when presented). Sign tracking is thought to reflect the attribution of incentive salience to reward-paired cues and has been previously correlated with addiction-like behaviors. Following the last PCA session, blood samples were collected for analysis of post-session corticosterone levels. Sixty-two rats (n = 10-12/group) were pseudo-randomly assigned to 1 of 2 intragastric (i.g.) exposure groups (water or 4 g/kg ethanol) or a non-manipulated (NM) control group. Animals were intubated with ethanol or water every other session from postnatal session (PND) 28-48 or PND 70-90. Rats were then tested in adulthood (PND 71-79 or PND 113-122) on the PCA task. Animals exposed chronically to ethanol during adolescence exhibited significantly higher levels of sign-tracking behavior in adulthood than NM and water-treated animals, and showed higher corticosterone than NM control animals. These effects were not seen after comparable ethanol exposure in adulthood. These results suggest that adolescent alcohol exposure has long-term consequences on the expression of potential addiction-relevant behaviors in adulthood.

Applying incentive sensitization models to behavioral addiction

The incentive sensitization theory is a promising model for understanding the mechanisms underlying drug addiction, and has received support in animal and human studies. So far the theory has not been applied to the case of behavioral addictions like Gambling Disorder, despite sharing clinical symptoms and underlying neurobiology. We examine the relevance of this theory for Gambling Disorder and point to predictions for future studies. The theory promises a significant contribution to the understanding of behavioral addiction and opens new avenues for treatment.

Can antipsychotic treatment contribute to drug addiction in schizophrenia?

Individuals with schizophrenia are at very high risk for drug abuse and addiction. Patients with a coexisting drug problem fare worse than patients who do not use drugs, and are also more difficult to treat. Current hypotheses cannot adequately account for why patients with schizophrenia so often have a co-morbid drug problem. I present here a complementary hypothesis based on evidence showing that chronic exposure to antipsychotic medications can induce supersensitivity within the brain's dopamine systems, and that this in turn can enhance the rewarding and incentive motivational effects of drugs and reward cues. At the neurobiological level, these effects of antipsychotics are potentially linked to antipsychotic-induced increases in the striatal levels of dopamine D2 receptors and D2 receptors in a high-affinity state for dopamine, particularly at postsynaptic sites. Antipsychotic-induced dopamine supersensitivity and enhanced reward function are not inevitable consequences of prolonged antipsychotic treatment. At least two parameters appear to promote these effects; the use of antipsychotics of the typical class, and continuous rather than intermittent antipsychotic exposure, such that silencing of dopaminergic neurotransmission via D2/3 receptors is unremitting. Thus, by inducing forms of neural plasticity that facilitate the ability of drugs and reward cues to gain control over behaviour, some currently used treatment strategies with typical antipsychotics might contribute to compulsive drug seeking and drug taking behaviours in vulnerable schizophrenia patients.

The form of a conditioned stimulus can influence the degree to which it acquires incentive motivational properties

There is considerable individual variation in the extent to which food- and drug-associated cues (conditioned stimuli, CSs) acquire incentive salience, as indicated by whether they elicit approach towards them, and/or act as conditioned reinforcers. Here we asked whether this variation is influenced by properties of the CS itself. In rats, we assessed both the attractiveness and conditioned reinforcing properties of two CSs: a manipulable lever CS versus an auditory (tone) CS. There was considerable individual variation in the extent to which a lever CS acquired incentive motivational properties, as indicated by whether it became attractive (evoked a sign-tracking or goal-tracking conditioned response) or acted as a conditioned reinforcer. However, with a tone CS all rats learned a goal-tracking response, and the tone CS was an equally effective conditioned reinforcer in sign-trackers and goal-trackers. Even when presented in compound (a lever-tone CS), the two elements of the compound differentially acquired motivational properties. In contrast, amphetamine and stress potentiated the conditioned reinforcing properties of both visual and auditory CSs similarly in rats that primarily sign-tracked or goal-tracked. We conclude that variation in the to the ability of CSs to acquire incentive salience, and thus their ability to act as incentive stimuli capable of motivating behavior, is determined in part by properties of the CS itself.

Pavlovian-conditioned alcohol-seeking behavior in rats is invigorated by the interaction between discrete and contextual alcohol cues: implications for relapse

INTRODUCTION

Drug craving can be independently stimulated by cues that are directly associated with drug intake (discrete drug cues), as well as by environmental contexts in which drug use occurs (contextual drug cues). We tested the hypothesis that the context in which a discrete alcohol-predictive cue is experienced can influence how robustly that cue stimulates alcohol-seeking behavior.

METHODS

Male, Long-Evans rats received Pavlovian discrimination training (PDT) sessions in which one conditioned stimulus (CS+; 16 trials/session) was paired with ethanol (0.2 mL/CS+) and a second stimulus (CS-; 16 trials/session) was not. PDT occurred in a specific context, and entries into a fluid port where ethanol was delivered were measured during each CS. Next, rats were acclimated to an alternate (nonalcohol) context where cues and ethanol were withheld. Responses to the nonextinguished CS+ and CS- were then tested without ethanol in the alcohol-associated PDT context, the nonalcohol context or a third, novel context.

RESULTS

Across PDT the CS+ elicited more port entries than the CS-, indicative of Pavlovian discrimination learning. At test, the CS+ elicited more port entries than the CS- in all three contexts: however, alcohol seeking driven by the CS+ was more robust in the alcohol-associated context. In a separate experiment, extinguishing the context-alcohol association did not influence subsequent CS+ responding but reduced alcohol seeking during non-CS+ intervals during a spontaneous recovery test.

CONCLUSION

These results indicate that alcohol-seeking behavior driven by a discrete Pavlovian alcohol cue is strongly invigorated by an alcohol context, and suggest that contexts may function as excitatory Pavlovian conditioned stimuli that directly trigger alcohol-seeking behavior.

Pavlovian conditioned approach, extinction, and spontaneous recovery to an audiovisual cue paired with an intravenous heroin infusion

RATIONALE

Novel stimuli paired with exposure to addictive drugs can elicit approach through Pavlovian learning. While such approach behavior, or sign tracking, has been documented for cocaine and alcohol, it has not been shown to occur with opiate drugs like heroin. Most Pavlovian conditioned approach paradigms use an operandum as the sign, so that sign tracking can be easily automated.

OBJECTIVES

We were interested in assessing whether approach behavior occurs to an audiovisual cue paired with an intravenous heroin infusion. If so, would this behavior exhibit characteristics of other Pavlovian conditioned behaviors, such as extinction and spontaneous recovery?

METHODS

Rats were repeatedly exposed to an audiovisual cue, similar to that used in standard self-administration models, along with an intravenous heroin infusion. Sign tracking was measured in an automated fashion by analyzing motion pixels within the cue zone during each cue presentation.

RESULTS

We were able to observe significant sign tracking after only five pairings of the conditioned stimulus (CS) with the unconditioned stimulus (US). This behavior rapidly extinguished over 2 days, but exhibited pronounced spontaneous recovery 3 weeks later.

CONCLUSIONS

We conclude that sign tracking measured by these methods exhibits all the characteristics of a classically conditioned behavior. This model can be used to examine the Pavlovian component of drug memories, alone, or in combination with self-administration methods.

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.

On the motivational properties of reward cues: Individual differences

Cues associated with rewards, such as food or drugs of abuse, can themselves acquire motivational properties. Acting as incentive stimuli, such cues can exert powerful control over motivated behavior, and in the case of cues associated with drugs, they can goad continued drug-seeking behavior and relapse. However, recent studies reviewed here suggest that there are large individual differences in the extent to which food and drug cues are attributed with incentive salience. Rats prone to approach reward cues (sign-trackers) attribute greater motivational value to discrete localizable cues and interoceptive cues than do rats less prone to approach reward cues (goal-trackers). In contrast, contextual cues appear to exert greater control over motivated behavior in goal-trackers than sign-trackers. It is possible to predict, therefore, before any experience with drugs, in which animals specific classes of drug cues will most likely reinstate drug-seeking behavior. The finding that different individuals may be sensitive to different triggers capable of motivating behavior and producing relapse suggests there may be different pathways to addiction, and has implications for thinking about individualized treatment. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

Prior haloperidol, but not olanzapine, exposure augments the pursuit of reward cues: implications for substance abuse in schizophrenia

Drug abuse and addiction are excessively common in schizophrenia. Chronic antipsychotic treatment might contribute to this comorbidity by inducing supersensitivity within the brain's dopamine system. Dopamine supersensitivity can enhance the incentive motivational properties of reward cues, and reward cues contribute to the maintenance and severity of drug addiction. We have shown previously that rats withdrawn from continuous haloperidol (HAL) treatment (via subcutaneous minipump) develop dopamine supersensitivity and pursue reward cues more vigorously than HAL-naive rats following an amphetamine (AMPH) challenge. Atypical antipsychotic drugs are thought to be less likely than typicals to produce dopamine supersensitivity. Thus, we compared the effects of HAL and the atypical antipsychotic olanzapine (OLZ) on the pursuit of reward cues. Rats were trained to associate a light-tone cue with water then treated with HAL or OLZ. Following antipsychotic withdrawal, we assessed AMPH-induced enhancement of lever pressing for the cue. Withdrawal from HAL, but not from OLZ, enhanced this effect. HAL, but not OLZ, also enhanced AMPH-induced psychomotor activation and c-fos mRNA expression in the caudate-putamen. Thus, prior HAL, but not OLZ, enhanced conditioned reward following an AMPH challenge, and this was potentially linked to enhanced behavioral sensitivity to AMPH and AMPH-induced engagement of the caudate-putamen. These findings suggest that HAL, but not an atypical like OLZ, modifies reward circuitry in ways that increase responsiveness to reward cues. Because enhanced responsiveness to reward cues can promote drug-seeking behavior, it should be investigated whether atypical antipsychotics might be a preferential option in schizophrenic patients at risk for drug abuse or addiction.

A classically conditioned cocaine cue acquires greater control over motivated behavior in rats prone to attribute incentive salience to a food cue

RATIONALE

Cues associated with rewards bias attention towards them and can motivate drug-seeking and drug-taking behavior. There is, however, considerable individual variation in the extent to which cues associated with rewards acquire motivational properties. For example, only in some rats does a localizable food cue become attractive, eliciting approach towards it, and "wanted", in the sense that it serves as an effective conditioned reinforcer.

OBJECTIVES

We asked whether the propensity of animals to attribute incentive salience to a food cue predicts the extent to which a classically conditioned cocaine cue acquires incentive motivational properties.

METHODS

First, a Pavlovian conditioned approach procedure was used to identify rats prone to attribute incentive salience to a food cue. We then measured the extent to which a classically conditioned cocaine cue acquired two properties of an incentive stimulus: (1) the ability to elicit approach towards it, and (2) the ability to reinstate drug-seeking behavior, using an extinction-reinstatement procedure (i.e., to act as a conditioned reinforcer).

RESULTS

We found that a classically conditioned cocaine cue became more attractive, in that it elicited greater approach toward it, and more desired, in that it supported more drug-seeking behavior under extinction conditions, in individuals prone to attribute incentive salience to a food cue.

CONCLUSIONS

We conclude that rats vary in their propensity to attribute incentive salience to both food and cocaine cues, and it is possible to predict, prior to any drug experience, in which rats a cocaine cue will acquire the strongest motivational control over behavior.

The effect of nicotine on sign-tracking and goal-tracking in a Pavlovian conditioned approach paradigm in rats

RATIONALE

Nicotine (NIC) potently increases operant responding for non-NIC reinforcers, and this effect may depend on drug-mediated increases in incentive motivation. According to this hypothesis, NIC should also potently increase approach to Pavlovian-conditioned stimuli associated with rewards.

OBJECTIVE

The present studies explored the effects of NIC on Pavlovian-conditioned approach responses.

METHOD

To do so, liquid dippers were used to deliver an unconditioned stimulus (US; 0.1 ml sucrose) after presentation of a conditioned stimulus (CS; 30 s illumination of a stimulus light)-both the CS and US were presented in receptacles equipped to monitor head entries.

RESULTS

In experiment 1, the CS and US were presented in the same receptacle, but NIC pretreatment (0.4 mg/kg base) did not increase conditioned approach responses. Delivery of the sucrose US was then shifted to receptacle in a different location. All rats learned to approach the new US location (goal-tracking) at similar rates. Approach to the CS receptacle (sign-tracking) declined for saline-pretreated rats, but NIC pretreatment increased sign-tracking. In experiment 2, NIC pretreatment increased sign-tracking when the CS and US were spatially separated during acquisition. In experiment 3, NIC pretreatments were replaced with saline, but the effect of NIC persisted for an additional 24 test sessions.

CONCLUSION

The findings suggest that NIC increases incentive motivation and that this effect is long-lasting, persisting beyond the pharmacological effects of NIC.

From prediction error to incentive salience: mesolimbic computation of reward motivation

Reward contains separable psychological components of learning, incentive motivation and pleasure. Most computational models have focused only on the learning component of reward, but the motivational component is equally important in reward circuitry, and even more directly controls behavior. Modeling the motivational component requires recognition of additional control factors besides learning. Here I discuss how mesocorticolimbic mechanisms generate the motivation component of incentive salience. Incentive salience takes Pavlovian learning and memory as one input and as an equally important input takes neurobiological state factors (e.g. drug states, appetite states, satiety states) that can vary independently of learning. Neurobiological state changes can produce unlearned fluctuations or even reversals in the ability of a previously learned reward cue to trigger motivation. Such fluctuations in cue-triggered motivation can dramatically depart from all previously learned values about the associated reward outcome. Thus, one consequence of the difference between incentive salience and learning can be to decouple cue-triggered motivation of the moment from previously learned values of how good the associated reward has been in the past. Another consequence can be to produce irrationally strong motivation urges that are not justified by any memories of previous reward values (and without distorting associative predictions of future reward value). Such irrationally strong motivation may be especially problematic in addiction. To understand these phenomena, future models of mesocorticolimbic reward function should address the neurobiological state factors that participate to control generation of incentive salience.

Neural correlates of the formation and retention of cocaine-induced stimulus-reward associations

BACKGROUND

Cocaine can elicit drug-seeking behavior for drug-predicting stimuli, even after a single stimulus-cocaine pairing. Although orbitofrontal cortex is thought to be important during encoding and maintenance of stimulus-reward value, we still lack a comprehensive model of the neural circuitry underlying this cognitive process.

METHODS

We studied the conditioned effects of cocaine with monkey functional magnetic resonance imaging and classical conditioning by pairing a visual shape (conditioning stimulus [CS+]) with a noncontingent cocaine infusion; a control stimulus was never paired. We correlated the behavioral preference of the monkey for the CS+, as measured offline, with the activity induced by the CS+ relative to the control stimulus as function of time.

RESULTS

We observed that during formation of stimulus-cocaine associations strong CS+-induced functional magnetic resonance imaging activations emerged in frontal cortex that correlated significantly with behavioral CS+ preference. Afterward, CS+ preference correlated only with activity in early visual cortex. Control experiments suggest that these findings cannot be explained by increased familiarity for the CS+.

CONCLUSIONS

Our findings suggest a complex interaction between frontal and occipital cortex during cocaine conditioning. Frontal cortex is important for establishing novel representations of stimulus valence when cocaine is used as reinforcer, whereas early visual cortex is involved in retaining these cocaine-stimulus associations.

Quantifying individual variation in the propensity to attribute incentive salience to reward cues

If reward-associated cues acquire the properties of incentive stimuli they can come to powerfully control behavior, and potentially promote maladaptive behavior. Pavlovian incentive stimuli are defined as stimuli that have three fundamental properties: they are attractive, they are themselves desired, and they can spur instrumental actions. We have found, however, that there is considerable individual variation in the extent to which animals attribute Pavlovian incentive motivational properties ("incentive salience") to reward cues. The purpose of this paper was to develop criteria for identifying and classifying individuals based on their propensity to attribute incentive salience to reward cues. To do this, we conducted a meta-analysis of a large sample of rats (N = 1,878) subjected to a classic Pavlovian conditioning procedure. We then used the propensity of animals to approach a cue predictive of reward (one index of the extent to which the cue was attributed with incentive salience), to characterize two behavioral phenotypes in this population: animals that approached the cue ("sign-trackers") vs. others that approached the location of reward delivery ("goal-trackers"). This variation in Pavlovian approach behavior predicted other behavioral indices of the propensity to attribute incentive salience to reward cues. Thus, the procedures reported here should be useful for making comparisons across studies and for assessing individual variation in incentive salience attribution in small samples of the population, or even for classifying single animals.