Operant Conditioning With Drug Reinforcers

A large body of data shows that virtually all drugs of dependence in human beings act as rein-forcers for animals in operant-conditioning situations. Typical studies on the reinforcing properties of drugs involve rats or monkeys fitted with venous catheters, through which a drug can be administered directly. Responses directed toward an object, such as a lever, result in infusions of the drug.

The basic finding of such studies has been that a wide variety of abused drugs—including COCAINE, Morphine, heroin, ¿-Amphetamine, pentobarbi-

tal, and ALCOHOL—all serve to establish and maintain operant behaviors in animals. Other drugs with a lesser abuse potential in humans—such as aspirin, tricyclic antidepressants, hallucinogens, and opioid mixed agonist/antagonists—fail to support responding.

The degree to which a given drug of abuse reinforces behavior appears to depend more on the schedule of reinforcement of the drug than on its intrinsic properties. A schedule of REINFORCEMENT refers to the pattern of access provided to the reinforcing event. For example, ratio schedules require an animal to make some predetermined number of responses before a reinforcer is given. Yet interval schedules are set up so that responses are effective at producing a reinforcer only after a delay following the previous one. Reinforcers in ratio schedules depend solely on the number of responses made; therefore, these schedules typically result in higher response rates than interval schedules in which responses made too early are ineffective. Because reinforcement schedules largely determine the rate of responding in a given situation, the abuse potential of the various drugs cannot be reliably assessed by comparing how quickly animals respond for each substance.

Other techniques for making such comparisons are available, however, and one technique for comparing the reinforcing properties of various substances involves calculating for each a so-called breaking point under a fixed ratio schedule of reinforcement. A fixed ratio schedule requires that an animal make a fixed number of responses (the ratio) for each reinforcement received. For a given drug dose, the breaking point is reached when a ratio too high to support responding is required. The breaking point achieved with the highest tolerable dose of a drug is often taken to be an index of that drug's reinforcing properties. Drugs with the highest breaking point are considered to be the most reinforcing and hence to have the highest abuse potential. Of drugs studied with such a procedure, cocaine appears to have the highest breaking point. For example, in some experiments, animals have been willing to press a lever up to 12,000 times for a single dose of cocaine.

Choice experiments provide a second means for comparing the reinforcing properties of two different drugs. Animals in such designs are typically given a choice between two responses, each of which leads to infusions of a different drug. A

preference for one response is taken to indicate a preference for the drug associated with that response. A finding of particular interest from such studies has been that cocaine appears to be preferred not only to a number of other drugs but also to nondrug rewards such as food and social contact (Johanson, 1984)—but it is important to vary the other reinforcers as well. Animals will choose less cocaine when the amount of food provided is greater or tastier.

Far fewer systematic data on the reinforcing properties of drugs have been collected with human subjects. A number of experiments have shown that human subjects will work for tokens exchangeable for Opioids, alcohol, pentobarbital, DIAZEPAM (Valium), and ¿-amphetamine. In addition, drug-abusing individuals will reliably produce arbitrary responses in a laboratory for immediate access to their drugs of choice. For example, heroin addicts will repeatedly push a button to receive heroin injections and cocaine users will choose to perform responses leading to cocaine injections over responses that yield injections of saline (Henning-field, Lukas, & Bigelow, 1986).

In sum, a body of both animal and human data now exists that documents the way drugs of abuse can act as potent reinforcing events. The pattern of drug use exhibited by an individual user, however, appears to depend as much on the schedule of drug availability as on the particular properties of the chosen drug. Therefore, predicting patterns of drug taking by humans will require a better understanding of the parameters of drug availability that exist in the real world.

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