Conducting behavioral procedures to assess impulsivity in rats provides insight into the environmental, neurophysiological, and neurochemical basis of impulsivity, that is, about the factors that trigger and underlie those behaviors. This protocol allow the study of impulsivity with a good balance of efficiency and accessibility, and instruction of data suitable for quantitative analysis. The performance in these protocols may be considered as an analog of impulsiveness in humans.
Hence, human versions of them serve for the diagnosis and treatment of related psychiatric disorders such as ADHD or pathological gambling. Demonstrating the procedure with Patsy Moreno will be Gustavo Lago, a technician, and Pablo Saavedra, an undergraduate student from our laboratory. After beginning the food restriction regime, add 60 food pellets to the food receptacle to habituate food neophobia before introducing the rats to the conditioning chambers without the initiation of any protocol for 30 minutes to habituate the exploring responses.
After the habituation stage, introduce the rats into the conditioning chambers for two additional daily 30-minute sessions of delivering a food pellet every 45 seconds to help the rats identify the source of the pellets. For intertemporal choice and differential reinforcement of low rates of responding, or DRL protocols, place one or two levers into the chamber and deliver food for every level press in conjunction with a free food pellet delivery every 45 seconds. For an intertemporal choice protocol, select the values for the delay and the magnitude of the reward.
For example, choices for the larger-later, or LL alternative, deliver five food pellets after a 20-second fixed delay and choices for the small-sooner, or SS alternative, deliver one food pellet immediately. Select a finishing criterion, and end the sessions automatically after the completion of the specified criterion, for example, after 40 choice trials or after 50 minutes. Combine each alternative with a lever within the conditioning chamber, counterbalancing the laterality of the alternatives among the subjects, and make SS and LL alternatives available upon the accomplishment of a variable interval schedule.
In free choice trials, when one of the levers is pressed after a certain interval has elapsed, the associated alternative will be activated. Both levers will be retracted, and the consequence associated with the SS or LL alternatives after the accomplishment of a variable interval schedule of reinforcement will be activated. After reward delivery, perform a time-out condition, adjusting the duration of this condition to equate the average duration of the intertrial intervals for both alternatives.
The next choice trial begins after the completion of the time-out. If a subject selects one alternative for two consecutive trials, the program will determine that the next trial will be a forced trial of the remaining alternative, that is, in the next trial, both levers will be available, but only one will operate to ensure that the subject experiences the outcomes associated with both alternatives. For DRL programming, select the value of the minimum time after which responding will produce a reward.
After the beginning of a session or after any lever-press response, start a countdown timer from the selected time value to zero. If the subject displays a response before the timer reaches the value of zero, the timer will reset so that the subject must wait for a new opportunity to obtain a reward. If subject emit a response after the timer reaches the value of zero, deliver a food pellet.
The timer will be reset after two seconds to allow the animal to consume the pellet. For feature-negative discrimination, first select stimuli durations, intertrial interval durations, and finishing criteria for the sessions. After presenting a stimulus A in A-plus trials, deliver a food pellet.
After presenting the same stimulus A accompanied by an additional stimulus X in AX-minus trials, do not deliver any food. After setting up the protocols in the computer software, clean the inner walls, ceiling, and grill floor of the operant chambers with an appropriate disinfectant solution to remove odors from the previous sessions or previous studies. Use the computer to manually activate and monitor the crucial inputs and outputs to make sure they work and check that the food dispenser holds enough food to be delivered during the session.
Move the housing cages with the rats inside close to the conditioning chambers, and open the housing cage. Gently place eat rat in its corresponding conditioning chamber before closing the conditioning chambers in the isolating shells. Initiate the program and wait until the program is finished.
If the data is not saved automatically, save the output files of the session in the computer drive or elsewhere. When the program is finished, gently return the rats to their corresponding housing cages and give complementary food to the animals according to the selected food restriction regime. In this intertemporal choice procedure, the log-ratio of the lever response rate associated with the SS alternative was higher in spontaneously hypertensive rats compared to wild-type rats, suggesting the ADHD model rats prefer an immediate reward at the expense of a richer but delayed alternative.
In this representative DRL protocol experiment, the longitudinal data of a single rat with a 10-second temporal restraint on responding demonstrates that over time, the animal acquired experience in the task, eventually learning to respond around 10 seconds. This pharmacological experiment tested whether impulsive performance in a DRL procedure with a temporal restriction of 10 seconds was decreased with a treatment consisting of low levels of an antipsychotic drug. Blue density plots represent the distribution of responses when saline is administered, and salmon plots represent the distribution of such responses when haloperidol was administered.
The upper embedded plots show response rates, and lower ones show reward rates for saline and haloperidol with the same color code as the former plots. It can be noted that the response rate was reduced for three to five subjects, specifically for those subjects with a high proportion of burst responses in the saline condition. Typically, in the feature-negative discrimination protocol, responding during the single stimulus plus food trials and during the compound stimulus without food trials do not differ substantially in early sessions.
However, after a few sessions, the rats respond differentially in both types of trials, revealing that the addition of a second stimulus counteracts the response tendency controlled by the first. Importantly, the subjects exhibit quite robust individual differences in both types of trials. Take care that the appropriate protocols are loaded in the corresponding conditioning chambers, that the rats are correctly assigned to each chamber, and that all of the chambers are closed.
After a baseline procedure has been completed, behavioral, pharmacological, or surgical treatments could be administered in order to test their effect on performance. The results are not always conclusive, so researchers might want to consider whether the observed data are dependent upon the selected parameters or are in fact induced by confounding factors. Be sure to handle the rats appropriately to avoid harming the rats and to avoid the rats harming you.
