Date of Award

Spring 2020

Degree Type



Pharmacology, Physiology and Neuroscience

Director of Thesis

Marlene A. Wilson, PhD

Second Reader

James R. Fadel, PhD


Although many people experience traumatic events, only 10%-20% go on to develop post- traumatic stress disorder (PTSD). Women are particularly susceptible, with a prevalence rate double that of men, even when controlling for type of trauma exposure. This disparity suggests the existence of distinct neurobiological processes, particularly related to sex, that predispose some individuals to be more resistant to extinguishing learned fear. Similar differences exist in rodent fear conditioning and extinction, though female rodents are considerably understudied. We hypothesized that female rodents would exhibit individual differences in fear extinction similar to those that we have observed previously in males. The present study examined freezing behavior, plus both 22 kHz and 50 kHz ultrasonic vocalizations (USVs) of female Long Evans rats (N=14) during acquisition of fear conditioning and cued fear extinction. Similar to prior studies in males, rats were divided into extinction competent (EC) and extinction resistant (ER) phenotypes based on a median split of freezing behavior during the last ten tone presentations of the extinction trial. Similar to males, in females freezing behavior during fear learning did not differ between EC and ER groups, but there was a difference during cued fear extinction. During fear acquisition trials, all female rats produced 50 kHz USVs, which were emitted mostly during the unconditioned period prior to the tone-shock pairings. During fear acquisition, EC rats also emitted significantly more 50 kHz USVs than ER rats during the unconditioned freezing period and in total throughout the trial. Both number and duration of 50 kHz calls differed between EC and ER groups over time. Only about half the female rats exhibited “distress” USVs in the 22 kHz range, and only during tone-shock pairings during fear acquisition and tone presentations during extinction learning. There was no significant difference in number of 22 kHz USVs between ER and EC groups. These results suggest that like males, female rodents show individual differences in both freezing and vocalizations during fear learning and extinction, although males appear to vocalize more in the 22 kHz range while females show a distinct pattern and higher number of 50 kHz USVs.

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