Date of Award


Document Type


Degree Name

Master of Arts (MA)



First Advisor

Donna Toufexis


Recent research has shown that there is a significant sex difference in the rate of habit acquisition in rats, with intact females acquiring the behavior with fewer action-outcome reinforcer pairings than intact males. This difference is shown to be related to the presence of cyclic estrogen and progesterone during habit behavior acquisition training. Specifically, the presence of progesterone has been shown to be crucial in early habit acquisition. However, the mechanism of action through which progesterone is influencing this behavior is unknown. Additionally, this project explores the impacts of long-term hormonal (levonorgestrel - progestin based) birth control on habit behavior in intact female rats. There is a significant lack of research surrounding the cognitive and behavioral effects of hormonal birth control on the individuals utilizing it. This work begins to fill in this gap and hopes to increase awareness and understanding surrounding the greater impacts of hormonal birth control.

Experiment I of this thesis investigates how progesterone modulates this behavior acquisition through the administration cyclic high levels of estrogen and Medroxy-progesterone during habit behavior acquisition. Medroxy-progesterone is a form of progesterone that is not metabolized into neuroactive metabolites and augments the impacts of progesterone working only through the classic progesterone receptors. Habit behavior was not observed in the rats given this hormone treatment, thus implicating a different mechanism of action responsible for early habit behavior acquisition. It is hypothesized that a neuroactive metabolite, such as allopregnanolone, is imperative for early habit acquisition through its modulatory impact on GABAA receptors.

Experiments II and III examine the effect of levonorgestrel implants on habit behavior in intact female rats. Female rats were implanted with subcutaneous capsules that would slow-release levonorgestrel over the course of the experiment. Experiment II explores the impact of levonorgestrel on the acquisition rate of habit behavior. Female rats were undertrained, and they only received 100 action-outcome reinforcer pairings – the threshold for intact females to exhibit habitual responding is ~120 action-outcome pairings. The rats with levonorgestrel implants did not display habitual responding, suggesting that levonorgestrel does not accelerate the rate of habit acquisition. Experiment III investigates the effect of levonorgestrel on overtrained female rats, and if its presence would delay habit acquisition. The rats were trained to 200 action-outcome reinforcer pairings, where habitual responding has been consistently seen in intact females. However, the rats treated with levonorgestrel did not show habit behavior, thus indicating that the presence of levonorgestrel is delaying habit behavior acquisition.



Number of Pages

53 p.