Estrogen replacement in ovariectomized rats during acquisition delays the habit formation consistently observed in intact female rats with the same level of instrumental training

Date of Completion


Document Type

Honors College Thesis



Thesis Type

College of Arts and Science Honors, Honors College

First Advisor

Donna Toufexis, Ph.D.

Second Advisor

Allison Anacker, Ph.D.

Third Advisor

Genell Mikkelson, CNM, MSN


estrogen, striatum, dopamine, learning, sex differences


Sex differences in the neurobiological mechanisms of learning have become evident in the literature over the years. Estrogen, a potent sex hormone, has effects on brain structure and neurotransmitter release throughout the brain, including dopamine in the dorsal striatum. Previous findings in our lab have demonstrated a sex difference in habit formation such that female rats require less reinforcer exposures to develop a habitual behavior compared to males. To determine if estrogen plays a critical role in this effect in females, we ovariectomized female rats and replaced naturally cycling estrogen with a silastic tube that constantly released low non-biologically inactive 17beta-estradiol (E2), in order to maintain the presence of the estrogen receptor. During acquisition, one half of the females were given 2 sub-cutaneous injections of E2 at proestrous levels. Training was done to 160 reinforcer exposures, a level at which intact females reliably exhibit habitual responding. After reward devaluation, we found that both ovariectomized (OVX) low E2 and OVX + cycling E2 responded in a goal-directed manner. These results suggest that neither low constant estrogen, nor cycling high estrogen are producing the habitual behavior that is observed at this training level in intact females.

Creative Commons License

Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 License
This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 License.

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