Neural and psychological mechanisms underlying appetitive learning: links to drug addiction

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Abstract

The complexity of drug addiction mirrors the complexity of the psychological processes that motivate animals to work for any reinforcer, be it a natural reward or a drug. Here, we review the role of the nucleus accumbens, together with its dopaminergic and cortical innervation, in responding to reinforcement. One important contribution made by the nucleus accumbens is to the process through which neutral stimuli, once paired with a reinforcer such as a drug, have the capacity to motivate behaviour. This process may be one of several contributing to addiction, and it may be amenable to pharmacological intervention.

Section snippets

Introduction: the psychology of action

The study of motivated action is the study of instrumental conditioning — the process by which animals alter their behaviour when there is a contingency between their behaviour and a reinforcing outcome [1]. Instrumental conditioning is neither psychologically nor neurally simple. Rats and humans exhibit goal-directed action, which is based upon knowledge of the contingency between one’s actions and their outcomes, and knowledge of the value of those outcomes; these two psychological processes

Neural underpinnings of instrumental conditioning

Several limbic cortical and subcortical structures play a role in assessing the value of reinforcers and of stimuli that predict them, and in actions directed at obtaining those reinforcers or stimuli [7]. The contribution of key elements of this circuitry, including the amygdala and orbitofrontal cortex, are discussed by Holland and Gallagher and Matsumoto and Tanaka (this volume), and in this review we will therefore restrict out discussion primarily to the role of the nucleus accumbens

Differences between drugs of abuse and natural reinforcement

Clearly, ‘natural’ reinforcers, such as food and sex, have their effects in very similar ways to ‘artificial’ reinforcers, such as drugs of abuse; there may be no sharp dividing line between the two. A behavioural economic view would liken addiction to that situation in which demand for a particular reinforcer (drug) has become relatively ‘inelastic’ — that is, it remains high in the face of increased price or other costs associated with drug-taking 57., 58., 59., 60.. Yet this could represent

Conclusions

Key questions remain as to what extent food and drug reinforcement are qualitatively or quantitatively different. Is sensitisation of a motivational system a specific property of drugs of abuse that gives them value above that of alternative reinforcers and renders the user hypersensitive to ‘wanting’ induced by environmental cues signalling the availability of those drugs? Does compulsive (or habitual) responding develop as a consequence of sensitisation, can it develop in the absence of

Update

Two recent studies have shed further light on the way in which habits are acquired and/or expressed. Rats with lesions of the dorsolateral striatum do not acquire habits normally [86], while the infralimbic cortex may suppress goal-directed actions once behaviour has become habitual [87].

References and recommended reading

Papers of particular interest, published within the annual period of review, have been highlighted as:

  • of special interest

  • ••

    of outstanding interest

Acknowledgements

The work was supported by a UK Medical Research Council Programme Grant and the Medical Research Council Cambridge Centre for Behavioural and Clinical Neuroscience.

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