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The Pursuit of Pleasure: When Does it Become Pathological?




Activation of brain reward systems can be considered a natural component of normal behavior. Indeed, brain reward systems serve to direct the organism's behavior toward goals that are normally beneficial and promote survival of the individual (e.g., food and water intake) or the species (e.g., reproductive behavior) as suggested by Troland's (1928) concept of beneception. The notion that the brain influences behavior is not particularly radical for twentieth century scientists nor is the notion that many rewards activate such mechanisms through various sense modalities such as taste or touch. The direct chemical activation of these reward pathways does not in itself represent any severe departure from the normal control reward systems exert over behavior. Inhalation of a substance (e.g., nicotine) is no less natural than the ingestion of sugar, although the former has no direct survival value to the organism nor to the species. But both involve activation of brain reward mechanisms and both may be subjectively experienced as pleasurable in humans. So what constitutes the pathological control of behavior termed "addiction"? Certainly not the fact that a substance activates a brain reward system nor the fact that this same system may be involved in the potent reward produced by addictive drugs. Simple activation of brain reward systems does not constitute addiction! Rather, the extreme control of behavior?exemplified by a deterioration in the ability of normal rewards to govern behavior (termed motivational toxicity)?is the distinguishing feature of an addiction. Some drugs quickly and uniformly exert extreme control over behavior (e.g., cocaine, heroin), while other substances exert a much less potent influence on behavior (e.g., moderate alcohol consumption, occasional nicotine use). The fact that a chemical (e.g., nicotine) influences behavior does not constitute addiction any more than the chemical reaction that produces a taste (e.g., sugar-associated sweetness) which influences behavior constitutes addiction.

Motivational toxicity is apparent when rewards normally effective in influencing behavior lose their ability to motivate the organism. This is typically seen in human drug addicts that neglect formerly potent rewards (e.g., career, sex) and focus their behavior on the acquisition and ingestion of drug. The neural mechanisms responsible for this disruption of the motivational hierarchy have not been identified; one potential mechanism involves decreased dopaminergic function following chronic drug use (see Bozarth, 1989). In a reward system with decreased dopaminergic function, natural rewards that activate reward processes much less potently than some drug rewards (e.g., cocaine, heroin) may lose their abilities to engage the organism's behavior. In contradistinction, direct pharmacological activation of a reward system dominates the organism's motivational hierarchy at the expense of other rewards that promote survival. The ensuing motivational toxicity distinguishes drug addiction from simple drug activation of reward mechanisms. Motivational toxicity may develop from neuroadaptive responses to chronic intake of some drugs, but it is not a general property of chemical activation of brain reward mechanisms.

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