All procedures in this study were approved by the Institutional Animal Care and Use Committee (IACUC) at Bryn Mawr College. Obtain IACUC or otherwise applicable regulatory approval before ordering laboratory animals and beginning experimentation.
1. Animal preparation
2. Vaginal lavage
NOTE: Gonadal hormones (i.e., estrogen and progesterone) are known to affect the stress response and cognition28,29,30. These hormones fluctuate over the estrous cycle of female rats31. If interested in tracking the estrous cycle of freely cycling female rodents to correlate with stress or cognitive flexibility data, collect vaginal lavage as described below. Representative data considering estrous cycle stage are not provided.
3. Equipment and software
4. Stress procedures
5. Training
NOTE: This paradigm is modified from the operant set-shifting procedure developed by Floresco et al. such that it can be completed in 3 days19. Training procedures for rats require 3 days (1 day to learn each task as described below). It is rare that a rat does not learn these tasks. If a rat fails to learn each task, it should be excluded from the final study. See Figure 1A for a visual depiction of the training paradigm described below.
6. Testing
NOTE: See Figure 1B for a visual depiction of the testing paradigm described below.
7. Behavioral analysis
NOTE: The data acquired for each animal on the test day are automatically recorded and saved by the computer, as long as a MED-PC script for each task been initiated and allowed to complete (see supplementary materials for MED-PC scripts).
8. Brain substrates
The adapted automated operant strategy shifting paradigm outlined above was used to determine if repeated restraint stress affects cognition in male and female Sprague Dawley rats. Representative behavioral data are described in Figure 2 below. In short, control and repeatedly restrained rats performed this operant strategy shifting test, which consisted of a series of tasks: side discrimination, side reversal, and light discrimination.
Trials to criterion for each task are depicted in Figure 2A. Typically, better performance on each task was represented by a reduced number of trials to criterion. These data indicate that, following acute restraint, males completed the side reversal task in significantly fewer trials than unstressed, control males. Conversely, stressed females required a significantly greater number of trials to complete the side reversal task. These results suggest that males exhibited improved performance following stress, whereas females exhibited impaired performance. In the light discrimination task, stress increased the number of trials to criterion compared to control females, thereby impairing performance in females but not males in this task.
The total number of errors made for each attention task is depicted in Figure 2B. Consistent with the number of trials to criterion, stressed males made significantly fewer errors than control males, whereas stressed females made more errors in the side reversal task. Furthermore, in the light discrimination task, females also made significantly more errors. In sum, these data suggest that repeated stress improves cognitive performance in males but impairs cognitive performance in females.
Total errors were further categorized into perseverative or regressive errors in Figure 2C (for a distinction between these two types of errors, refer to section 7 of the protocol). Interestingly, stressed males made fewer perseverative errors in the side reversal task than control males. On the other hand, in both the side reversal and light discrimination tasks, stressed females made a greater number of perseverative errors than control females. There were no differences between the treatment groups in the number of regressive errors made during either task.
Omissions in each trial and time to reach criterion are shown in Figure 2D (for more information on how these were calculated, refer to section 7 of the protocol). These measures were evaluated in the side reversal task only, as this task exhibited the largest sex differences. Stressed females made a higher percentage of omissions compared to all other treatment groups. In addition, while stress appeared to decrease the time to complete the side reversal task in males, stress prolonged completion of the task in females. In sum, repeated stress impaired cognitive flexibility in females but not males.
Brain substrates underlying cognitive flexibility are depicted in Figure 3. As stark sex differences were observed in the side reversal task, the brain areas underlying this task were examined to determine whether they displayed similar sex differences in neural activity. As previously discussed, lesion studies have indicated that the orbitofrontal cortex (OFC) mediates the side reversal task34. Thus, c-fos, a measure of neural activation37, was labeled in the OFC at 30 min after the completion of strategy shifting, which should have reflected performance in the side reversal task38. However, it is possible that OFC may also play a role in the extradimensional strategy shifting component of this task39. Thus, it is important to perform the sacrifice at the appropriate time to reflect brain activity during a particular task within the operant strategy shifting paradigm. Here, stress induced a significant increase in neuronal activation in the OFC of males compared to controls. However, stress induced a significant decrease in neuronal activation in the OFC of females compared to controls. Furthermore, in males, OFC activation and trials to criterion were negatively correlated; specifically, higher OFC activation was associated with fewer trials to criterion. In contrast, there was no correlation between OFC activation and performance in females, suggesting that the OFC was disengaged during these performances.
Figure 1: Schematic of the operant strategy shifting paradigm during training and test days. Please click here to view a larger version of this figure.
Figure 2: Representative behavioral data from operant strategy shifting paradigm. (A) Trials to criterion for each task on test day. In the side reversal task, stress improved performance in males but impaired performance in females. In the light discrimination task, stress weakened performance in females, while it did not affect males. (B) Number of errors for each task on test day. Stress reduced the number of errors made in males but increased errors in females in both side reversal and light discrimination tasks. (C) Perseverative and regressive error categorization. Stress decreased perseverative errors made in males but increased perseverative errors made in females in both side reversal and light discrimination tasks. (D) Percent trials omitted and time to criterion in the side reversal task. Stress increased the percent omissions in female rats. Stress decreased the time required by males but increased the time required by females to complete the task. Statistics were calculated using two-way ANOVA followed by Tukey’s t-test (n = 12 rats per group; error bars represent SEM; #p ≤ 0.10, *p < 0.05). This figure has been modified from a previous publication17. Please click here to view a larger version of this figure.
Figure 3: Representative neural activation after operant strategy shifting paradigm. (A) OFC activation after strategy shifting task. Representative images of immunohistochemical 3,3’-diaminobenzidine (DAB) staining using an antibody against c-fos in the OFC visualized using brightfield microscopy, then quantified. Stress significantly increased activation (demonstrated by the number of c-fos-expressing cells) in the OFC of males, while it decreased activation in females. Scale bar in bottom-right image panel represents 200 µm. Statistics were calculated using two-way ANOVA followed by Tukey’s t-test (n = 12 rats per group, 6–8 sections of OFC analyzed per rat; error bars represent SEM; *p < 0.05). (B) Trials to criterion in the side reversal task correlated with OFC activation. Males displayed a significant negative correlation, whereas females did not. Please click here to view a larger version of this figure.
3 inch glass pipette eye droppers | Amazon | 4306-30-012LC | For vaginal lavage |
Alcohol Wipes | VWR | 15648-990 | To clean trays in set shifting boxes between rats |
Biotin-SP-conjugated AffiniPure Donkey Anti-Mouse lgG (H+L), minimal cross reaction to bovine, chicken, goat, guinea pig, hamster, horse, human, rabbit, sheep serum proteins | Jackson ImmunoResearch | 715-065-150 | All other DAB protocol staining materials are standard buffers/DAB and are not specified here, as this is not the main focus of the methods paper |
C-fos mouse monoclonal primary antibody | AbCam | ab208942 | To stain neural activation in brain areas after set shifting |
Dustless Food Pellets | Bio Serv | F0021 | For set shifting boxes (dispenser for reward) |
GraphPad Prism | Used for data analysis | ||
Leica DM4 B Microscope and associated imaging software | Leica | Lots of different parts for the microscope and work station, for imaging lavage and/or cfos | |
MatLab | Software; code to help analyze set shifting data, available upon request. | ||
Med-PC Software Suite | Med Associates | SOF-736 | Software; uses codes to operate operant chambers |
Operant Chambers | Med PC | MED-008-B2 | Many different parts for the chamber set up and software to work with it; we also wrote a separate code for set shifting, available upon request. |
Rat Bedding | Envigo | T.7097 | |
Rat Chow | Envigo | T.2014.15 | |
Restraint Devices | Bryn Mawr College | Made by our shop | For stress exposure; specifications available upon request. |
Scribbles 3d fabric paint | Amazon | 54139 | For vaginal lavage |
Sprague Dawley Rats | Envigo | At least D65 Males and Females | |
VWR Superfrost Plus Micro Slide | VWR | 48311-703 | For vaginal lavage and/or brain slices/staining for c-fos |
Stress affects cognitive function. Whether stress enhances or impairs cognitive function depends on several factors, including the 1) type, intensity, and duration of the stressor; 2) type of cognitive function under study; and 3) timing of the stressor in relation to learning or executing the cognitive task. Furthermore, sex differences among the effects of stress on cognitive function have been widely documented. Described here is an adaptation of an automated operant strategy shifting paradigm to assess how variations in stress affect cognitive flexibility in male and female Sprague Dawley rats. Specifically, restraint stress is used before or after training in this operant-based task to examine how stress affects cognitive performance in both sexes. Particular brain areas associated with each task in this automated paradigm have been well-established (i.e., the medial prefrontal cortex and orbitofrontal cortex). This allows for targeted manipulations during the experiment or the assessment of particular genes and proteins in these regions upon completion of the paradigm. This paradigm also allows for the detection of different types of performance errors that occur after stress, each of which has defined neural substrates. Also identified are distinct sex differences in perseverative errors after a repeated restraint stress paradigm. The use of these techniques in a preclinical model may reveal how stress affects the brain and impairs cognition in psychiatric disorders, such as post-traumatic stress disorder (PTSD) and major depressive disorder (MDD), which display marked sex differences in prevalence.
Stress affects cognitive function. Whether stress enhances or impairs cognitive function depends on several factors, including the 1) type, intensity, and duration of the stressor; 2) type of cognitive function under study; and 3) timing of the stressor in relation to learning or executing the cognitive task. Furthermore, sex differences among the effects of stress on cognitive function have been widely documented. Described here is an adaptation of an automated operant strategy shifting paradigm to assess how variations in stress affect cognitive flexibility in male and female Sprague Dawley rats. Specifically, restraint stress is used before or after training in this operant-based task to examine how stress affects cognitive performance in both sexes. Particular brain areas associated with each task in this automated paradigm have been well-established (i.e., the medial prefrontal cortex and orbitofrontal cortex). This allows for targeted manipulations during the experiment or the assessment of particular genes and proteins in these regions upon completion of the paradigm. This paradigm also allows for the detection of different types of performance errors that occur after stress, each of which has defined neural substrates. Also identified are distinct sex differences in perseverative errors after a repeated restraint stress paradigm. The use of these techniques in a preclinical model may reveal how stress affects the brain and impairs cognition in psychiatric disorders, such as post-traumatic stress disorder (PTSD) and major depressive disorder (MDD), which display marked sex differences in prevalence.
Stress affects cognitive function. Whether stress enhances or impairs cognitive function depends on several factors, including the 1) type, intensity, and duration of the stressor; 2) type of cognitive function under study; and 3) timing of the stressor in relation to learning or executing the cognitive task. Furthermore, sex differences among the effects of stress on cognitive function have been widely documented. Described here is an adaptation of an automated operant strategy shifting paradigm to assess how variations in stress affect cognitive flexibility in male and female Sprague Dawley rats. Specifically, restraint stress is used before or after training in this operant-based task to examine how stress affects cognitive performance in both sexes. Particular brain areas associated with each task in this automated paradigm have been well-established (i.e., the medial prefrontal cortex and orbitofrontal cortex). This allows for targeted manipulations during the experiment or the assessment of particular genes and proteins in these regions upon completion of the paradigm. This paradigm also allows for the detection of different types of performance errors that occur after stress, each of which has defined neural substrates. Also identified are distinct sex differences in perseverative errors after a repeated restraint stress paradigm. The use of these techniques in a preclinical model may reveal how stress affects the brain and impairs cognition in psychiatric disorders, such as post-traumatic stress disorder (PTSD) and major depressive disorder (MDD), which display marked sex differences in prevalence.