Sex differences in mouse models of fear inhibition: Fear extinction, safety learning, and fear-safety discrimination

Abstract

Background and purpose: Women are overrepresented in post-traumatic stress disorder (PTSD), a mental disorder characterised by ineffective inhibition of fear. The use of male animals dominates preclinical studies, which may contribute to a lack of understanding as to why this disparity exists. Thus, the current study explores sex differences in three mouse models of fear inhibition.

Experimental approach: All experiments tested male and female C57Bl/6J mice. Experiment 1 employed two fear conditioning protocols, in which tones were paired with footshocks of differing intensity (moderate or intense). Fear recall and extinction were tested subsequently. In Experiment 2, safety learning was investigated. Tones were explicitly unpaired with footshocks during safety conditioning. Recall of safety learning was tested 24 hr later. Experiment 3 assessed a model of fear-safety discrimination. Cued stimuli were paired or never paired with footshocks during fear and safety conditioning, respectively. Discrimination between stimuli was assessed 24 hr later.

Key results: In fear extinction, males, compared to females, responded with greater fear in sessions most proximal to conditioning but subsequently showed a more rapid fear extinction over time. Sex differences were not observed during safety learning. During fear-safety discrimination, both males and females discriminated between stimuli; however, males revealed a greater level of freezing to stimuli.

Conclusion and implications: The current study provides evidence that sex differences influence fear but not safety-based behaviour in C57Bl/6J mice. These findings indicate that processing of fear, but not safety, may play a greater role in sex differences observed for PTSD.

Linked articles: This article is part of a themed section on The Importance of Sex Differences in Pharmacology Research. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.21/issuetoc.

Figure 1

Schematic representations of the fear inhibition paradigms used in the current study

Figure 2

Sex differences in a fear extinction model (mean ± SEM per protocol, males, n = 10; females, n = 10): (a) Jumping behaviour during fear conditioning. Jumping increased significantly among male and female mice allocated to the intense protocol only. (b) Freezing across three epochs of contextual fear recall. For the intense protocol only, males, compared with females, presented with significantly greater freezing. (c) Freezing during the initial fear extinction session. For the intense protocol only, freezing was significantly different between males and females. (d) Freezing across fear extinction sessions tested over time (freezing to the CS+ averaged within session). For the intense protocol only, males, when compared with females, demonstrated a much steeper fear extinction curve. Freezing in the presence of the CS+ during the initial fear extinction session were averaged and included as Session 1 in fear extinction over time data. *P<0.05, significant main effects; ^# P<0.05, significant differences between sexes within session

Figure 3

Sex differences in safety learning recall (mean ± SEM males, n = 14; females, n = 14): (a) pattern of discrimination between the CS− and post CS− illustrated in the safety recall session. Freezing response to stimuli are plotted in the order presented during testing. (b) Freezing response to the CS− and Post CS− period averaged within session. Mice (males and females) demonstrated significant differences in freezing response between stimuli. (c) Males and females did not differ in general levels of freezing %. *P<0.05, significant main effect. ns: non‐significant

Figure 4

Sex differences in fear–safety discrimination (mean ± SEM males, n = 10; females, n = 10): (a) the pattern of discrimination between the CS+ and CS− demonstrated in the fear and safety recall session. Freezing responses to stimuli are plotted in the order they were presented during testing. (b) Freezing, averaged within session, was greater for the CS+ than the CS− and were higher in (c) males than in females. *P<0.05, significant main effects