Conditioned Place Preference A

procedure called conditioned place preference has been used to study the ''rewarding'' effects of drugs. The procedure is designed to ask the question ''When given a choice, will an animal prefer an environment in which it has experienced a drug to one in which it has not?'' To answer this question, an animal is placed in an experimental chamber that is divided into two compartments that are different in some way. For example, they may have different floors and/or distinctive odors. Initially, the animal is placed in the chamber for several preconditioning trials and the time spent in each compartment is measured. Usually, a rat exhibits some preference for one or the other side in these trials. At this point, the experimenter can do one of two things—(1) modify the compartments in some way, perhaps by changing the lighting, so that equal time is spent in the two chambers before proceeding (balanced procedure), or (2) go ahead with the experiment with unequal preferences (unbalanced procedure). With either procedure, conditioning trials are conducted next.

To run conditioning trials, a barrier is placed in the middle of the chamber that does not allow the animal to switch sides. The drug of interest is then administered to the animal and it is confined to one compartment for usually fifteen to thirty minutes. If the unbalanced procedure is used, the animal is usually placed in the compartment that was initially avoided. A second group may be given a placebo (a substance that has no effect) under these same conditions or a placebo may be given to these same animals before placing them in the second compartment in alternating sessions. In this way, the effect of the drug is associated with a particular environment. After several—three to ten— conditioning sessions, the animal is placed in the chamber without being given the drug, and the door is removed so that the animal can spend time in either compartment. The length of time spent in each chamber is recorded and used as a measure of preference for that chamber.

The hypothesis underlying this sort of experiment is that the length of time spent in an environment should increase if that environment is associated with the effects of a drug of abuse. In fact, many studies have shown that this does happen with drugs such as HEROIN, COCAINE, and AMPHETAMINES. In the balanced procedure, animals spend more time in the drug-associated side than in the other side. In the unbalanced procedure, the animals spend more time in the drug-associated side than they did previously, but only rarely demonstrate an actual preference for it. As would be expected, preference is greater with higher doses of the drug and does not occur with placebo injections. In addition, it does not occur with drugs that are not typically abused, such as antipsychotic drugs, antidepressant drugs, and opioid antagonists. Thus, it seems likely that the technique measures a drug effect that is related to drug abuse.

Like other models for studying drug abuse, conditioned place preference has strengths and weak nesses. Among its strengths is that animals are tested in a drug-free state. Therefore, the measure of preference is not influenced by the direct effects of drugs. The procedure can be done with drug injections given by routes other than intravenous, therefore surgical preparation is not involved. Moreover, the procedure is rapid, with maximum effect usually evident within three conditioning sessions.

The major weakness relates to the drug effects that it is measuring. Since drug administration is not due to the behavior of the animal (i.e., self-administration), it is by definition not a reinforcing effect. Although many of the same drugs that are self-administered induce place preferences, it is not clear whether the drug effect studied in conditioned place preference is the same as that studied in procedures that directly measure reinforcing effects. Another weakness is that is it not known whether it is meaningful to compare drugs in terms of their ability to engender place preferences. That is, if drug X induces a greater place preference than drug Y, does it have more abuse potential? Finally, because the procedure involves the simple behavioral response of moving from one chamber to another, it is not known whether it can be used to study some of the complex behavioral variables that are known to be determinants of drug self-administration. Despite these ambiguities, however, the simplicity of the procedure makes it likely that it will continue to be useful for studying drug abuse.

(SEE ALSO: Abuse Liability of Drugs; Reinforcement)

BIBLIOGRAPHY

Bozarth, M. A. (1987). Conditioned place preference: A parametric analysis using systemic heroin injections. In Methods of assessing the reinforcing properties of abused drugs, pp. 241-273. New York. SpringerVerlag.

Hoffman, D. C. (1989). The use of place conditioning in studying the neuropharmacology of drug reinforcement. Brain Research Bulletin, 23, 373-387.

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