The role of dopamine in the dorsomedial striatum in general and outcome-selective Pavlovian-instrumental transfer

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

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

Sex differences in incentive motivation and the relationship to the development and maintenance of alcohol use disorders

Despite considerable evidence of higher rates of alcohol use disorders (AUDs) in men than in women, there is a dearth of research into the underlying causes of this disparity. As the gap in high risk drinking between men and women closes, it is critical to disentangle the biological factors that may place men and women at different risk for the development of AUDs as well as AUD-associated health problems. While sex differences in alcohol drinking have been reported in animal models and in human alcoholics, it increasingly seems that consummatory behavior may be dissociated from propensity toward inflexible and cue-elicited drug seeking and taking that characterize alcohol use disorders. While much of this work was initially performed in males a growing, yet limited, body of literature suggests that there are sex differences in both cue reactivity, and further, the relationship between cue reactivity and the maintenance of addictive behavior, indicating that males may be at greater risk for the development of a subset of addiction-related behaviors independent of alcohol consumption. Here, we will review the current literature on sex effects on the relationship between incentive motivation and addictive behavior and discuss unanswered questions that we expect will inform the development of individualized and sex-specific treatment and prevention strategies for AUDs. We believe that a greater understanding of how sex interacts with in cue reactivity to independently mediate the drug taking and risk for the development of uncontrolled drug or alcohol-seeking and -taking will inform the development of individualized treatment and prevention strategies for addiction.

Modulation of cue-triggered reward seeking by cholinergic signaling in the dorsomedial striatum

The dorsomedial striatum (DMS) has been strongly implicated in flexible, outcome-based decision making, including the outcome-specific Pavlovian-to-instrumental transfer effect (PIT), which measures the tendency for a reward-predictive cue to preferentially motivate actions that have been associated with the predicted reward over actions associated with different rewards. Although the neurochemical underpinnings of this effect are not well understood, there is growing evidence that striatal acetylcholine signaling may play an important role. This study investigated this hypothesis by assessing the effects of intra-DMS infusions of the nicotinic antagonist mecamylamine or the muscarinic antagonist scopolamine on expression of specific PIT in rats. These treatments produced dissociable behavioral effects. Mecamylamine infusions enhanced rats' tendency to use specific cue-elicited outcome expectations to select whichever action was trained with the predicted outcome, relative to their performance when tested after vehicle infusions. In contrast, scopolamine infusions appeared to render instrumental performance insensitive to this motivational influence of reward-paired cues. These drug treatments had no detectable effect on conditioned food cup approach behavior, indicating that they selectively perturbed cue-guided action selection without producing more wide-ranging alterations in behavioral control. Our findings reveal an important role for DMS acetylcholine signaling in modulating the impact of cue-evoked reward expectations on instrumental action selection.

Learning and Motivational Processes Contributing to Pavlovian-Instrumental Transfer and Their Neural Bases: Dopamine and Beyond

Pavlovian stimuli exert a range of effects on behavior from simple conditioned reflexes, such as salivation, to altering the vigor and direction of instrumental actions. It is currently accepted that these distinct behavioral effects stem from two sources (i) the various associative connections between predictive stimuli and the component features of the events that these stimuli predict and (ii) the distinct motivational and cognitive functions served by cues, particularly their arousing and informational effects on the selection and performance of specific actions. Here, we describe studies that have assessed these latter phenomena using a paradigm that has come to be called Pavlovian-instrumental transfer. We focus first on behavioral experiments that have described distinct sources of stimulus control derived from the general affective and outcome-specific predictions of conditioned stimuli, referred to as general transfer and specific transfer, respectively. Subsequently, we describe research efforts attempting to establish the neural bases of these transfer effects, largely in the afferent and efferent connections of the nucleus accumbens (NAc) core and shell. Finally, we examine the role of predictive cues in examples of aberrant stimulus control associated with psychiatric disorders and addiction.

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.

Stimulus-specific and differential distribution of activated extracellular signal-regulated kinase in the nucleus accumbens core and shell during Pavlovian-instrumental transfer

The ability of reward-predictive cues to potentiate reward-seeking behavior--a phenomenon termed Pavlovian--instrumental transfer (PIT)--depends on the activation of extracellular signal-regulated kinase (ERK) in the nucleus accumbens (NAc). Here, we utilized immunohistochemistry to investigate the subregional pattern of ERK activation during PIT, and the contribution of different elements in the PIT condition to the distribution of ERK signaling in the NAc of rats. We found that the occurrence of reward-seeking behavior (lever pressing) did not affect ERK activation in either the core or the shell of the NAc. In contrast, presentation of the reward-predictive cue (auditory conditioned stimulus) caused a significant increase in ERK activation in both subregions of the NAc, with the effect being slightly more robust in the core than the shell. Different from the pattern evoked by the reward-predictive cue, presentation of the reward itself (food pellets) had no effect on ERK activation in the core but caused a pronounced increase in ERK activation in the shell. Taken together, our results demonstrate that ERK signaling in the NAc during PIT involves both the core and the shell and is driven by the conditioned cue irrespective of whether the situation permits engagement in reward-seeking behavior. Furthermore, our results show that the subregional distribution of ERK signaling in the NAc evoked by rewards differs from that evoked by cues that predict them. The stimulus-specific differential pattern of ERK signaling described here may present the molecular complement to stimulus-specific increases in NAc cell firing reported previously.

Dopamine or opioid stimulation of nucleus accumbens similarly amplify cue-triggered 'wanting' for reward: entire core and medial shell mapped as substrates for PIT enhancement

Pavlovian cues [conditioned stimulus (CS+)] often trigger intense motivation to pursue and consume related reward [unconditioned stimulus (UCS)]. But cues do not always trigger the same intensity of motivation. Encountering a reward cue can be more tempting on some occasions than on others. What makes the same cue trigger more intense motivation to pursue reward on a particular encounter? The answer may be the level of incentive salience ('wanting') that is dynamically generated by mesocorticolimbic brain systems, influenced especially by dopamine and opioid neurotransmission in the nucleus accumbens (NAc) at that moment. We tested the ability of dopamine stimulation (by amphetamine microinjection) vs. mu opioid stimulation [by d-Ala, nMe-Phe, Glyol-enkephalin (DAMGO) microinjection] of either the core or shell of the NAc to amplify cue-triggered levels of motivation to pursue sucrose reward, measured with a Pavlovian-Instrumental Transfer (PIT) procedure, a relatively pure assay of incentive salience. Cue-triggered 'wanting' in PIT was enhanced by amphetamine or DAMGO microinjections equally, and also equally at nearly all sites throughout the entire core and medial shell (except for a small far-rostral strip of shell). NAc dopamine/opioid stimulations specifically enhanced CS+ ability to trigger phasic peaks of 'wanting' to obtain UCS, without altering baseline efforts when CS+ was absent. We conclude that dopamine/opioid stimulation throughout nearly the entire NAc can causally amplify the reactivity of mesocorticolimbic circuits, and so magnify incentive salience or phasic UCS 'wanting' peaks triggered by a CS+. Mesolimbic amplification of incentive salience may explain why a particular cue encounter can become irresistibly tempting, even when previous encounters were successfully resisted before.

What and when to "want"? Amygdala-based focusing of incentive salience upon sugar and sex

RATIONALE

Amygdala-related circuitry helps translate learned Pavlovian associations into appetitive and aversive motivation, especially upon subsequent encounters with cues.

OBJECTIVES

We asked whether μ-opioid stimulation via microinjections of the specific agonist D-Ala(2), N-MePhe(4), Gly-ol)-enkephalin (DAMGO) in central nucleus of amygdala (CeA), or the adjacent basolateral amygdala (BLA) would magnify sucrose or sex "wanting", guided by available cues.

MATERIALS AND METHODS

CeA or BLA DAMGO enhancement of cue-triggered "wanting" was assessed using Pavlovian to instrumental transfer (PIT). Unconditioned food "wanting" was measured via intake, and male sexual "wanting" for an estrous female was measured in a sexual approach test. Sucrose hedonic taste "liking" was measured in a taste reactivity test.

RESULTS

CeA (but not BLA) DAMGO increased the intensity of phasic peaks in instrumental sucrose seeking stimulated by Pavlovian cues over precue levels in PIT, while suppressing seeking at other moments. CeA DAMGO also enhanced food intake, as well as sexual approach and investigation of an estrous female by males. DAMGO "wanting" enhancements were localized to CeA, as indicated by "Fos plume"-based anatomical maps for DAMGO causation of behavioral effects. Despite increasing "wanting", CeA DAMGO decreased the hedonic impact or "liking" for sucrose in a taste reactivity paradigm.

CONCLUSIONS

CeA μ-opioid stimulation specifically enhances incentive salience, which is dynamically guided to food or sex by available cues.

Striatal dopamine depletion in rats produces variable effects on contingency detection: task-related influences

Dopamine (DA) depletion of the posterior dorsomedial striatum (pDMS) can impair the capability of rats to detect changes in the causal efficacy of actions. Here we sought to characterize in more detail the effects of pDMS DA depletions on contingency detection as a function of different contingency degradation training protocols. In experiment 1, sham controls and rats with pDMS DA depletions received limited contingency degradation training (4 days) that involved an invariable and high degree of degradation to one of two contingencies controlling instrumental choice behaviour. The results demonstrated that lesioned rats were insensitive to contingency manipulations both during contingency degradation training and in the subsequent extinction test. Experiment 2 further indicated that rats with pDMS DA depletion subjected to extended contingency degradation training (12 days) became sensitive to contingency manipulations during the training phase but not in the subsequent extinction test. In experiment 3, an extended but more complex contingency degradation training protocol (12 days) was used that involved a gradual shift from a low to an intermediate and a high degree of contingency degradation rather than a high and invariable degree of contingency degradation as in experiments 1 and 2. Notably, lesioned rats were sensitive to contingency manipulations both during the contingency degradation training phase and in the subsequent extinction test. Thus, pDMS DA depletions can impair the capability to detect changes in the causal efficacy of actions; however, the occurrence and pattern of impairments depend on the contingency degradation training protocol being used.