The conditioned place preference (CPP) is a widely used behavioral model which can evaluate the motivational properties such as the rewarding and aversive effects of drugs and natural substances as well (Suzuki 1999). Furthermore, it has been used to study the neural mechanism underlying the conditioned reward. For long, the rewarding properties of drugs were assessed by the conventional self-administration method. In the late 70’s, CPP paradigm revolutionized the methodological procedure to evaluate the motivational properties of drugs to compensate the methodological and interpretative ambiguities associated with the self-administration model. Since then, CPP has become one of the most frequently used models even surpassing the conventional self-administration method. Most importantly, it has opened avenues for understanding the neural mechanism of rewarding and aversive effects of drug substances, and for screening drugs for abuse liability.
In place conditioning tasks, animals are introduced to an apparatus having two distinct chambers, either through a doorway or smaller connecting chamber. Distinctions are made between these two chambers based on visual and tactile cues, including wall color and floor texture, but in some cases, other elements such as olfactory cues may also be used.
Subsequently, the affective state of the rodents is altered by the administration of a drug, or change in a physiological state. On alternate days, the drug or physiological state is paired with the other environment. After this conditioning trail, the animals are free to explore all the chambers, and they may depict an increase or decrease in the time spent in the chamber that was previously conditioned with the drug substance. A conditioned place preference (CPP) is said to occur if the animals spend considerably more time in the drug-paired compartment than the vehicle-paired compartment. This shift in preference may be attributed to the rewarding properties of the drug substances or the physiological situation that has evoked an approach response (Schechter & Calcagnetti 1993).
On the other hand, if the animals spend considerably longer time in the vehicle-paired compartment rather than the drug-treated compartment, then this is considered a conditioned place aversion (CPA). This reduced time spent in the drug conditioned chamber may be attributed to the aversive properties of the drug substances or the physiological situation that has evoked an avoidance response. Usually, drugs of abuse like ethanol, cocaine produce CPP; while other drugs that elicit aversive effects, such as lithium chloride, produce CPA.
In CPP, the primary stimulus (drug) serves as an unconditioned stimulus (UCS). When it is paired with a secondary stimulus (visual, tactile, or olfactory cue) which acts as a conditioned stimulus (CS), an approach or avoidance behavior for the paired environment is elicited. Drug-induced CPP is based on the principle that when a primary reinforcer is paired with the secondary stimulus, the conditioned stimulus attains secondary reinforcing properties, which are apparently established due to a Pavlovian contingency. Interestingly, the CPP paradigm is not only restricted to drug substances, but the approach or aversive behavior can also be established using food (Bechara et.al 1992; Swerdiow et.al 1983), copulation (Miller et.al 1987), or water (Agmo et.al 1992) as primary reinforcers.