Individual Differences in Conditioned Fear and Extinction in Female Rats

Abstract

The inability to extinguish a traumatic memory is a key aspect of post-traumatic stress disorder (PTSD). While PTSD affects 10–20% of individuals who experience a trauma, women are particularly susceptible to developing the disorder. Despite this notable female vulnerability, few studies have investigated this particular resistance to fear extinction observed in females. Similar to humans, rodent models of Pavlovian fear learning and extinction show a wide range of individual differences in fear learning and extinction, although female rodents are considerably understudied. Therefore, the present study examined individual differences in fear responses, including freezing behavior and ultrasonic vocalizations (USVs), of female Long–Evans rats during acquisition of fear conditioning and cued fear extinction. Similar to prior studies in males, female rats displayed individual variation in freezing during cued fear extinction and were divided into extinction competent (EC) and extinction resistant (ER) phenotypes. Differences in freezing between ER and EC females were accompanied by shifts in rearing during extinction, but no darting was seen in any trial. Freezing behavior during fear learning did not differ between the EC and ER females. Vocalizations emitted in the 22 and 50 kHz ranges during fear learning and extinction were also examined. Unlike vocalizations seen in previous studies in males, very few 22 kHz distress vocalizations were emitted by female rats during fear acquisition and extinction, with no difference between ER and EC groups. Interestingly, all female rats produced significant levels of 50 kHz USVs, and EC females emitted significantly more 50 kHz USVs than ER rats. This difference in 50 kHz USVs was most apparent during initial exposure to the testing environment. These results suggest that like males, female rodents show individual differences in both freezing and USVs during fear extinction, although females appear to vocalize more in the 50 kHz range, especially during initial periods of exposure to the testing environment, and emit very few of the 22 kHz distress calls that are typically observed in males during fear learning or extinction paradigms. Overall, these findings show that female rodents display fear behavior repertoires divergent from males.

ChAT::Cre transgenic rats show sex‐dependent altered fear behaviors, ultrasonic vocalizations and cholinergic marker expression

“…The current findings also show diverging USV and freezing behaviors in male and female rats. Similar to our prior results, 43 females showed more 50 kHz USVs during the exposure to the testing context prior to presentation of the tones (or shock) than males, but males showed more 22 kHz distress USVs during the toneshock pairings or conditioned tone presentations than females.…”

“…3.1 | Differences in conditioned freezing in male and female transgenic ChAT::Cre rats Breeding of the transgenic ChAT::Cre+ males with wildtype females yielded 41 offspring that were tested in a cue-conditioned fear and extinction protocol (see Figure 1; as described in 17,42,43 ). The groups Not all rats emitted 22 kHz USVs, even during fear learning.…”

“…Note that due to an issue with the software, only 19 tones were used in extinction learning and recall trials, rather than the 20 tones used in previous reports. 17,42,43 Freezing behavior during each trial was measured using Freezescan software (CleverSys, Inc, Reston, VA) using settings to detect freezing as the absence of movement except for respiration. Freezing was evaluated over time using the percent of time spent freezing in each one-minute period that included the tone and the subsequent inter-tone interval.…”

“…Though contextual fear learning was not significantly altered in transgenic ChAT::Cre male rats in a previous study, 30 in the present study we examined both contextual and cue‐conditioned fear recall, cued fear extinction, and generalization to a novel tone in both male and female transgenic ChAT::Cre rats. Since the transgenic ChAT::Cre line was developed using Long Evans outbred rats, 26 we used a Pavlovian fear conditioning and extinction paradigm we have previously validated in both male and female Long Evans rats 17,42,43 . The current study expands upon prior analysis of transgenic ChAT::Cre+ rats to compare both freezing behavior and ultrasonic vocalizations (USVs; 22 kHz and 50 kHz) in transgenic (ChAT::Cre+) male and female rats to their wildtype (Cre‐) littermates.…”

Darting across space and time: Parametric modulators of sex-biased conditioned fear responses

“…Furthermore, Long Evans rats (Totty et al 2021), and both FVBB6 F1 hybrid and C57BL/6J mice (Borkar et al 2020; Fadok et al 2017; Hersman et al 2020) display darting-like active flight behaviors in response to conditioning with a serial compound stimulus, a behavior that is amenable to extinction training. Reports in which darting is not observed (Tryon et al 2021) may be explained by brevity of conditioning (3 CS-US pairings), and the intensity of the US (1 mA). As we report here, darting typically emerges on or after the 5th tone presentation and occurs less frequently in animals receiving 1 mA shocks compared to milder ones.…”