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Method Article
Here, we present a protocol to elicit Pavlovian conditioned approach behavior in rats. This procedure can be used to measure individual differences in the tendency to approach and attribute incentive salience to reward-related cues and investigate addiction vulnerability.
Cues that are contingently paired with unconditioned, rewarding stimuli can acquire rewarding properties themselves through a process known as the attribution of incentive salience, or the transformation of neutral stimuli into attractive, "wanted' stimuli capable of motivating behavior. Pavlovian conditioned approach (PCA) develops after the response-independent presentation of a conditioned stimulus (CS; e.g., a lever) that predicts the delivery of an unconditioned stimulus (US; e.g., a food pellet) and can be used to measure incentive salience. During training, three patterns of conditioned responses (CRs) can develop: sign-tracking behavior (CS-directed CR), goal-tracking behavior (US-directed CR), and an intermediate response (both CRs). Sign-trackers attribute incentive salience to reward-related cues and are more vulnerable to cue-induced reinstatement of drug-seeking as well as other addiction-related behaviors, making PCA a potentially valuable procedure for studying addiction vulnerability. Here, we describe materials and methods used to elicit PCA behavior from rats as well as analyze and interpret PCA behavior in individual experiments.
The transition of drug use to addiction involves complex interactions between Pavlovian and instrumental learning1,2. During drug-taking and drug-seeking behaviors, actions and outcomes are learned through instrumental processes; however, relationships between stimuli (e.g., drug-related cues) and rewards are also learned through Pavlovian processes. Pavlovian cues acquire predictive value, but they can also acquire incentive motivational value3, whereby they become attractive, desirable, and capable of promoting and maintaining reward-seeking behavior4.
Many procedures investigating reward learning in animals, however, do not permit the disentanglement of predictive versus incentive motivational learning. In drug self-administration procedures, for example, both instrumental and Pavlovian contingencies are typically employed, such that rats learn to perform an action (e.g., nose-poking, lever-pressing, etc.) in order to receive an outcome (i.e., intravenous drug infusion). The rewarding outcome is also paired with Pavlovian stimuli (e.g., illumination of a nose-poke port, cue light presentation, sound of the infusion pump, interoceptive feeling of fluid delivery into the bloodstream, etc.). It is often unclear in these procedures whether cues are supporting goal-directed actions simply because of their predictive relationship with the reward, or whether cues have acquired incentive motivational properties of their own.
In order to experimentally isolate the incentive motivational value from the predictive value of Pavlovian stimuli, our laboratory uses a Pavlovian conditioned approach (PCA) procedure in order to identify rats that attribute incentive salience to reward-related cues. During a training session, a conditioned stimulus (CS; e.g., a lever) response-independently predicts the delivery of an unconditioned stimulus (US; e.g., a food pellet). Over the course of multiple training sessions, three phenotypes develop: sign-tracking (CS-directed conditioned response [CR]), goal-tracking (US-directed CR), and an intermediate response (both CRs). For goal-trackers (GTs), the CS is utilized as a predictor of reward delivery; however, for sign-trackers (STs), the CS is attributed with incentive salience, becoming attractive and desirable. In this review, we outline the equipment, setup, and data processing necessary to perform a PCA procedure. In addition, we provide representative results of PCA training, outline important experimental considerations, and discuss the putative utility of PCA procedures in investigating addiction and other neuropsychiatric disorders.
All procedures have been approved by the University Committee on the Use and Care of Animals (UCUCA; University of Michigan, Ann Arbor, MI).
1. Equipment and Software
2. PCA Training
3. Data Processing
We have found that 5-7 daily sessions of PCA training is sufficient to phenotype rats as STs, GTs, and intermediate responders (IRs), although rats can be trained further based on the needs of a particular laboratory or experiment. Phenotyping is based on PCA index scores, which are calculated by averaging the response bias, latency score, and probability difference of individual sessions as previously described in the protocol. Phenotypes are determined by averaging the PCA index scores ...
PCA training can be used to determine individual variation in the tendency to attribute incentive salience to conditioned cues, which has been argued to be an important component of addiction vulnerability. For example, it has previously been demonstrated that STs attribute incentive salience to both food and drug cues6, and both food- and drug-related cues activate similar brain regions within an amygdalo-striatal-thalamic circuit in STs7. Moreover, STs are more impulsive8, vulnerable to...
The authors have nothing to disclose.
This work was funded by the University of Michigan Department Of Psychiatry (U032826; JDM), the National Institute on Drug Abuse (NIDA; K08 DA037912-01; JDM) and the Department of Defense (DoD) National Defense Science and Engineering Graduate (NDSEG) Fellowship (CJF).
Name | Company | Catalog Number | Comments |
Standard Modular Operant Test Chamber with Modified Top for Rat | Med Associates, Inc. | ENV-008CT | Top has access slot for drug delivery leashes or other harness systems; comes with ENV-008CTC (Cover Insert) for access slot |
Expanded PVC Sound Attenuating Cubicle | Med Associates, Inc. | ENV-022V | |
PCI Operating Package for up to Sixteen Chambers | Med Associates, Inc. | MED-SYST-16 | Based on the number of operant chambers, MED-SYST-8 (PCI Operating Package for up to Eight Chambers) is also available |
SmartCtrl | Med Associates, Inc. | DIG-716P2 | |
Universal Cable, 25′ (7.6 m) | Med Associates, Inc. | SG-210CB | Based on individual needs, SG-210CB-50 (Universal Cable, 50′ [15.2 m]) is also available |
18″ (45.7 cm) 3-Pin Mini-Molex | Med Associates, Inc. | SG-216A | Based on individual needs, SG-216A-2 (2′ [61.0 cm] 3-Pin Mini-Molex) is also available |
Power Cable, 25′ (7.6 m) | Med Associates, Inc. | SG-210CP-25 | |
Stainless Steel Grid Floor for Rat | Med Associates, Inc. | ENV-005 | Based on preference, ENV-005A (Stainless Steel Grid Floor for Mouse) is also available |
Reusable Waste Pan | Med Associates, Inc. | ENV-007-P3 | |
Filler Panel Package for Standard Modular Test Chamber | Med Associates, Inc. | ENV-008-FP | |
Modular Pellet Dispenser for Rat, 45 mg | Med Associates, Inc. | ENV-203M-45 | |
Pellet Receptacle, Trough Type | Med Associates, Inc. | ENV-200R2M | Based on preference, ENV-200R1M (Pellet Receptable, Cup Type) is also available |
Retractable Lever | Med Associates, Inc. | ENV-112CM | Calibrated to 25 g; needs to be adjusted to 10 g before Pavlovian conditioned approach training |
Retractable Lever Cue Light | Med Associates, Inc. | ENV-112CML | |
House Light for Rat | Med Associates, Inc. | ENV-215M | |
100 mA Replacement Bulbs, Pack of 10 | Med Associates, Inc. | SG-800R | Red-colored; for use with ENV-215M (House Light for Rat) |
Pellets, 45 mg, Primate Purified Diet, Banana Flavor | Bio-Serv | F0059 | 50,000/box |
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