The overall goal of this procedure is to elicit Pavlovian conditioned approach behavior in rats. This procedure can help answer key questions in the field of behavioral neuroscience, such as why some individuals can resist the temptation to pursue reward cues while others cannot. The main advantage of this procedure is that it allows us to measure the incentive motivational value of cues separately from their predictive value.
Demonstrating this procedure will be Christopher Fitzpatrick, a neuroscience graduate student from my laboratory. This protocol utilizes commercial operant conditioning chambers in sound-attenuating cabinets with ventilation fans to provide ambient noise. Within each chamber, equip a pellet magazine with an infrared sensor for head entries.
Position it in the front wall of each chamber, three centimeters above the floor, and attach a dispenser to deliver 45-milligram banana-flavored food pellets. Next, in a counterbalanced manner, position a retractable lever about 2.5 centimeters to either side of the receptacle and about 3 centimeters above the floor. The lever should have an LED cue light to make it more attractive.
To calibrate the lever, place a 10-gram weight on it and adjust the calibration screw until the weight pushes the lever down. Then, attach a red light to the roof of the chamber above the receptacle. This will increase the conspicuousness of the illuminated lever.
This completes the chamber preparations. Order adult male Sprague Dawley rats that are between 250 and 300 grams, and pair-house them on arrival. Leave the rats undisturbed for at least two days in the housing colony to acclimate.
The colony should be maintained on a 12-hour light-dark cycle, and the rats should have ad libitum access to food and water. After an initial acclimation period, handle each rat for at least one minute per day, for a couple of days. On the two days before pre-training, give each rat 15 banana-flavored pellets daily.
Divide these pellets evenly on separate sides of the home cage. For this protocol, create a pre-training program. The programming is done in MEDState Notation.
The manual is available online. Write a program to deliver 50 food pellets on a variable-time 30-second schedule five minutes after the start of the program. The interval is on average 30 seconds but varies randomly between zero and 60 seconds.
During this time, the red house light should be on, but the retractable lever should remain retracted for the duration of the program. Do the pre-training during the light cycle the day before PCA training. Place each rat into an operant chamber.
Close and latch the chamber door. Close the sound-attenuating cabinet door, and immediately run the program. As soon as the program is completed, remove the rats from the chambers.
Record the number of pellets consumed. Any rats that do not consume all of the pellets should be excluded from further testing. In the software, write a training program to run 25 trials over about 40 minutes, beginning one minute after the start of the program.
During each trial, have the lever illuminate and extend into the chamber for eight seconds. The interior red light is always on. When the lever retracts, have one food pellet dispense independently of the rat's response.
Separate each trial on a variable-time 90-second schedule. The interval is on average 90 seconds but varies randomly between 30 and 150 seconds. Run the PCA program just as the pre-training was run.
After removing the rats, record the number of pellets consumed, and exclude rats that do not consume all of the pellets. Now, using data collected by the program, calculate the PCA index scores. Male rats underwent five PCA training sessions, during which the number, latency, and probability of magazine entries and lever presses were recorded.
For each daily PCA training session, PCA index scores were calculated from these variables. The PCA index scores from the last two training sessions were used to categorize the rats as sign trackers, goal trackers, or as having an intermediate phenotype. Sign trackers primarily approached and directed their behavior toward the reward-related lever cue.
On the PCA index, their scores ranged from 5 to 1. In contrast, goal trackers primarily approached and directed their behavior toward the source of reward delivery, the pellet magazine. On the PCA index, their scores ranged from 5 to 1.
The intermediate phenotype does not bias either behavior. After watching this video, you should have a good understanding of how to elicit Pavlovian conditioned approach in rats.
