Fear conditioning studies in rodents allow us to assess vulnerability factors which might underlie fear-based psychopathology such as post-traumatic stress disorder (PTSD). Despite PTSD being more prevalent in females than males, very few fear conditioning studies in rodents have tested females. Our study assessed fear conditioning and extinction in male and female rats using both fear-potentiated startle and freezing behavior as measures. Rats were trained to fear cues that predicted the occurrence of shock and then subsequently exposed to an extinction training procedure where the cue was presented repeatedly in the absence of shock. Retention of the extinction memory was assessed the next day. Our results showed that females exhibited less retention of fear extinction, but only when measured by fear-potentiated startle. Our results highlight the importance of using multiple indices of fear behavior, particularly when comparing sexes on measures of extinction learning.Not all individuals who experience trauma during their lifetime will develop post-traumatic stress disorder (PTSD). One of the greatest risk factors for developing this disorder is being female, with the lifetime prevalence rate twice as high for women as it is for men (Kessler et al. 1995;Kilpatrick et al. 2013). Fear conditioning studies in rodents allow us to study the behavioral and neural mechanisms that may underlie PTSD (Parsons and Ressler 2013), yet very few fear conditioning studies in rodents have used females as subjects (Lebron-Milad and Milad 2012). Despite the increased prevalence of PTSD and other fear and anxiety-based disorders in females, nonhuman animal studies testing for sex differences in fear conditioning have largely failed to capture this difference.Fear conditioning studies typically use a procedure in which a discrete cue (e.g., auditory stimulus) signals the occurrence of an aversive stimulus, usually a brief shock. Under normal circumstances, animals learn to fear the discrete cue associated with shock and will also learn to fear the context (i.e., place) in which shock occurs. A handful of studies have directly compared animals of both sexes on tests of cued and contextual fear conditioning. Some of these reports have shown that while males and females exhibit equivalent levels of fear conditioning to discrete cues (Maren et al. 1994;Baran et al. 2009;Milad et al. 2009a;Barker and Galea 2010;Gruene et al. 2015a;Fenton et al. 2016), males often show evidence of better contextual conditioning (Maren et al. 1994;Chang et al. 2009;Barker and Galea 2010). Other studies have tested for sex differences in fear extinction learning, a form of learning through which animals learn to inhibit responding to cues that once signaled an aversive event. Over the last several years, the study of fear extinction has received considerable attention (Milad and Quirk 2012) due in part to the fact that humans with PTSD display impaired extinction (Milad et al. 2009b;Norrholm et al. 2011). Recent findings in rodents are mixed with regard t...

Prior studies suggest that levels of ovarian hormones may affect learning and memory in rats, including studies of fear conditioning and extinction. We previously showed that female rats show reduced retention of extinction compared to males when measuring fear-potentiated startle, but not when measuring freezing behavior. One commonly reported observation in studies of freezing behavior is that rats with increased levels of estradiol during extinction learning show better retention of extinction than rats given extinction training when levels of estradiol are low. Here, we tested the hypothesis that fear extinction retention in a fear-potentiated startle paradigm in females is influenced by levels gonadal hormones, which we had not accounted for in our original report. We used the fear-potentiated startle paradigm to test if extinction learning was affected by estrous phase, ovariectomy, or acute systemic injections of estradiol in ovariectomized rats. We report that neither the expression nor extinction of fear-potentiated startle differed in rats given extinction training in proestrus compared to those in metestrus. Removal of the ovaries had no effect on fear acquisition or extinction learning as assessed by fear-potentiated startle. Finally, systemic injections of estradiol given to ovariectomized rats before extinction training had no effect on the expression of fear or the retention of extinction. Our findings suggest that the effect of female gonadal hormones on fear conditioning and extinction may depend on the measure of fear employed or by the parameters used to study fear learning.