Addictive drugs as reinforcers: multiple partial actions on memory systems.
This review applies some new experimental findings and theoretical ideas about how reinforcers act on the neural mechanisms of learning and memory to the problem of how addictive drugs affect behaviour. A basic assumption of this analysis is that all changes in behaviour, including those involved in drug addiction and the initiation of drug self-administration, require the storage of new information in the nervous system. Animal studies suggest that such information is processed in several (this review deals with three) more or less independent learning and memory systems in the mammalian brain. Reinforcers can interact with these systems in three ways: they activate neural substrates of observable approach or escape responses, they produce unobservable internal states that can be perceived as rewarding or aversive, and they modulate or enhance the information stored in each of the memory systems. It is suggested that each addictive drug maintains its own self-administration by mimicking some subset of these actions. Evidence supporting the notion of multiple memory systems and data on the actions of several drugs (amphetamine, cocaine, nicotine, alcohol and morphine) on these systems are briefly reviewed. The utility of the concept of "reward" for understanding the effects of drugs on behaviour is discussed. Evidence demonstrating actions of drugs on multiple neural substrates of reinforcement suggests that no single factor is likely to explain either addictive behaviour in general or self-administration in particular. Some of the findings on the development and maintenance of self-administration by animals of the five exemplar drugs are discussed in the context of these ideas.