$$\rightleftharpoonup{xx}$$
$$\longleftharp{xx}$$,
$$\longrightharp{xx}$$,
The Conditioned Place Preference (CPP) paradigm offers a simple way of assessing the conditioned reward induced by diverse stimuli1,2, and has been used broadly to study the conditioned rewarding effects of addictive drugs3. It is based on Pavlovian conditioning, evaluating the motivational value of drug-associated environmental cues for maintaining addictive behavior4. In this model, environmental cues acquire secondary appetitive properties (conditioned rewarding effects) when paired with a primary reinforcer3. For example, an initially neutral place (such as the color of one compartment in the CPP cage) is paired with the specific effects of a drug of abuse during some conditioning sessions5, while another compartment is associated with the injection of a vehicle. Following conditioning, if the animal spends more time in the compartment previously associated with the drug, it is assumed that CPP has developed3. The establishment of the preference is achieved when the animal gives a positive value to the environmental cues linked to the drug, which is the primary reinforcer. Consequently, animals will perform behavioral drug-seeking responses in response to those contextual cues6. The CPP model permits the evaluation of the rewarding properties of subthreshold doses of the drug, showing whether animals in a specific condition (e.g., having suffered from social defeat previously) are more vulnerable and sensitive to doses that are not effective in naive animals7.
The CPP model has also been used to evaluate extinction/reinstatement as an animal model to study relapse3, which is the aim of the present protocol. There are three different phases: acquisition, extinction, and reinstatement (Figure 1). In the CPP reinstatement model, animals first acquire the CPP for a drug-paired compartment, and then they perform several extinction sessions. We define extinction as the moment in which the animal reduces its behavioral responses of approximation to a conditioned rewarding stimulus that has been removed (e.g., the drug)8. During the extinction sessions, animals explore the compartments in the absence of the drug, so that the acquired preference is gradually attenuated9. An important issue to consider is that the behavioral change that the animal exhibits during extinction (the progressive decrease in the time spent in the drug-paired compartment) can be due to new learning processes that compete with the previous learned response, or due to a decrease in the internal motivational state of the subject3. Finally, the reestablishment of the place preference through the context or drug cues would be our model of reinstatement1.
Administering a priming injection of the associated drug can reinstate the preference, which is considered a reestablishment of the approximation to the contextual cues. Drug priming reinstatement occurs due to the persistent memory of the pleasurable effects of the drug, which induced craving and motivates animals to seek the environmental cues related to reward.
Some advantages of the CPP reinstatement model are that the procedure is non-invasive (in contrast with self-administration, which requires surgery), inexpensive, and simple. In addition, this model has a good criterion validity, as it mimics well what occurs in humans10,11, inducing reinstatement with stimuli that induce relapse, such as re-exposure to the drug12,13 or stress14.
There are other techniques such as the extinction – reinstatement model of intravenous self-administration. Here, animals press a lever to self-administer the drug, which permits the evaluation of the operant response of the animal, compulsivity, and motivation14,15,16. The main advantage of the CPP over self-administration procedures is that CPP reinstatement is considered to reflect the reactivation of the incentive-motivational value of the context stimuli paired with the drug, consisting of the reappearance of the approach behavior to the context17. Moreover, non-drug stimuli, such as stress, can also induce reinstatement18,19. For example, one self-administration study described no effects on reinstatement of heroin intake in rats after a foot shock or restraint stress20. Authors discussed that it was unsuccessful because those stressors were tested outside the self-administration chamber in a different context. In contrast, when using the CPP model of reinstatement, there was a clear re-establishment of morphine-induced CPP after using the same stressors, and applied in a different context to that of the CPP and at different times (0 and 15 min after stress exposure)18.
Several studies in the literature have shown different ways of drug and stress-induced reinstatement. On the one hand, drug-induced reinstatement has been reported in rats and mice using morphine5,21,22,23, cocaine24,25, amphetamine26,27, ethanol28,29, and 3,4-Methylenedioxymethamphetamine (MDMA)30. On the other hand, exposure to stress may be a determining factor in vulnerability to drug abuse. Stress is known to increase the rewarding effects of drugs7,31,32 and their role in relapse is well established33,34. For example, defeat in social interactions with a conspecific reinstates morphine and cocaine CPP18,19. In addition, animals exposed to repeated social defeat are more vulnerable to the conditioned rewarding effects of subthreshold doses of cocaine, and reinstate the preference with very low doses of cocaine7.
Application of the CPP reinstatement model is a useful and sensitive way to evaluate vulnerability to relapse in animals, and permits the assessment of different subtle environmental manipulations, which are the main triggers that threaten human relapse, such as drug- or stress-induced reinstatement.