Date of Award

2015

Document Type

Thesis

Degree Name

Master of Science

College

College of Arts and Sciences

Program

Biology, MS

First Advisor

Pamela S. Coburn-Litvak

Second Advisor

David Mbungu

Third Advisor

Brenda Anderson

Abstract

Problem

Stress influences an organism’s physiological systems via an inverted u-shaped curve: An optimum amount of stress will optimize body functions, but too little stress or too much stress for long periods of time can impair body functions. Researchers have been very interested in exploring the mechanisms that may “delay the tipping point” between the positive and negative effects of stress. A rightward shift in the stress curve would allow one to maintain optimal performance even at higher or more prolonged stress levels. The molecular and cellular mechanisms that underlie this rightward shift could result in resilience, clinically defined as the ability to endure stress without sustaining damage, or even to benefit from experiencing stress. The experiments described in this thesis investigate two potential mechanisms of resilience. The first mechanism is “stress inoculation,” in which previous exposure to a stressor “inoculates” an organism to respond more effectively to subsequent stressors. Recent studies suggest that controllable stress, even if the organism cannot predict when the stress will occur (thus called “unpredictable/controllable stress” or UST), may cause a rightward shift in the stress curve. The second mechanism is physical fitness that may improve the ability to cope with stress through molecular and cellular changes in the body.

Method

Experiment #1 (Stress inoculation): Thirty-three male Sprague-Dawley rats were in housing platforms for 21 days; 15 rats were exposed to unpredictable/controllable stress (UST), and 18 rats were not exposed to stressful stimuli as a control group. After 21 days, spatial memory and strategies were assessed on the Barnes maze under high stress conditions.

Experiment #2 (Physical Fitness): A pilot study was conducted on 22 subjects (12 females, 10 males). Thirty human subjects (15 females, 15 males) were recruited among the freshmen taking HLED 120, Fit for Life, at Andrews University. Physical fitness was assessed with the MicroFit® FAS-2 system, a FDA-registered medical device. The students’ chronic stress levels were assessed with ICSRLE (Inventory of College Student Recent Life Experiences), and their depression and anxiety levels with DASS 21 (Depression Anxiety Stress Scale). Cognitive performance was assessed with two memory tasks: an object location task thought to be dependent on the hippocampus, and an n-back test thought to be dependent on the prefrontal cortex. Finally, the physiological stress response to the acute, cognitive stressors (performing the n-back test) was assessed by changes in salivary cortisol, heart rate, and systolic/diastolic blood pressure.

Results

Experiment #1: UST rats took less time to find the goal box on the Barnes maze (p

Experiment #2 (Effects of physical fitness, stress, and depression and anxiety on memory): While higher fitness levels tended to be associated with better hippocampal memory scores (p=0.15, d=0.7), it did not affect prefrontal cortex-dependent memory (“n-back different”: p=0.286; “n-back same”: p=0.411. A significant, positive correlation was seen between ICSRLE and DASS 21 (p

Conclusion

Experiment #1 (Stress inoculation as an active resilience mechanism): The results of this study are consistent with the “stress inoculation hypothesis.” Exposure to unpredictable / controllable stress for 21 days causes “stress inoculation,” causing neural and behavioral adaptations that may represent a rightward shift of the stress curve. This would explain the optimal performance of UST with new environmental stressors on the Barnes maze.

Experiment #2 (Physical activity as an active resilience mechanism): The results of this pilot study partially support the original hypotheses, but they indicate directions for future studies. First, more subjects should be added (at least 54) to validate the current statistical results. Second, more rigorous spatial memory tasks may be needed in order to avoid “ceiling effects.”

Subject Area

Stress (Physiology), Stress (Psychology), Rats--Effect of stress on, Rats--Exercise

Creative Commons License

Creative Commons Attribution-No Derivative Works 4.0 International License
This work is licensed under a Creative Commons Attribution-No Derivative Works 4.0 International License.

DOI

https://dx.doi.org/10.32597/theses/71/

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