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a mechanism underlying substance abuse

Substance Abuse

Major aspects of the literature on substance abuse fit particularly well with the motivational model. Although at one time it was argued that drug abuse could be attributed to physiological dependence per se, there has been a substantial move toward the view that ‘‘abuse of reinforcers’’ conveys the essence of drug abuse, either through the termination of distress or dysphoria (negative reinforcement) or through the induction of pleasure or euphoria (positive reinforcement). The present discussion focuses on positive reinforcement as a mechanism underlying substance abuse. Such a mechanism suggests the involvement of Gray’s BAS. It will be recalled that the BAS responds to cues for rewards, activates behavior, generates a positive emotional state, and is associated with the DA pathways that project from the ventral tegmental area to the limbic system (the mesolimbic DA system).

The contribution of appetitive motivation is particularly clear in the case of stimulant drugs (e.g., cocaine, amphetamines) and opioids (e.g., heroin, morphine). Conditioned incentive stimuli (environmental stimuli previously associated with the consumption of these drugs) generate appetitive motivational states and energize drug-seeking behavior. These conditioned incentive stimuli, it is also believed, contribute to relapse— that is, former addicts often relapse in the presence of cues previously associated with drug consumption. Wise adopts a similar position but emphasizes that positive reinforcers elicit forward locomotion. Additionally, the ability to induce ‘‘psychomotor activation’’ is a common mechanism that underlies almost all drugs of abuse—not only stimulants and opioids, but also alcohol, barbiturates, benzodiazepines, cannabis, and others. This description of the common mechanism in Wise’s theory of drug abuse as response to rewards and activating behavior is quite similar to the behavioral functions of Gray’s BAS (and Depue’s BFS). A consensus has emerged concerning the neurochemical mechanisms that mediate the addicting effects of many drugs of abuse: these mechanisms involve the mesolimbic and mesocortical DA pathways. Leshner asserts that ‘‘Virtually all drugs of abuse have common effects, either directly or indirectly, on … the mesolimbic reward system.… Activation of this system appears to be a common element in what keeps drug users taking drugs … all addictive substances affect this circuit.’’

Thus, the construct of appetitive motivation occupies a central position in the current literature on substance abuse, and the neurochemical substrate for these drug effects is the DA pathways associated with the BAS. It is reasonable to further suggest that drugseeking and drug-consuming behavior reflect the outcome of an approach–avoidance conflict, in which the BAS responds to the immediate positive reinforcing properties of the drugs, whereas the BIS responds to the much delayed negative consequences of drug consumption (e.g., loss of job and friends, legal problems, withdrawal distress, social disapproval, etc.). This conflict model explains why a temperament of impulsivity/disinhibition is a strong risk factor for substance abuse. A dominance of the BAS over the BIS would produce both an impulsive temperament and a bias toward the positively reinforcing effects of drugs of abuse over the delayed negative consequences. Therefore, to the extent that this approach–avoidance conflict model applies, appetitive and aversive motivational constructs are particularly useful to understanding substance abuse.

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