Behavioral Diversity Across Classic Rodent Models Is Sex-Dependent

Abstract

Symptoms of trauma and stressor related disorders such as post-traumatic stress disorder (PTSD) often develop well after the traumatic experience has occurred, and so identifying early predictors of risk or resilience is important for the implementation of interventional therapies. For example, passive coping strategies such as tonic immobility and peritraumatic dissociation during the trauma itself are risk factors for the developments of PTSD, especially in women. However, discrete, sex-specific coping responses that predict later outcomes in animal models have not been rigorously defined. Recently, we identified an active, escape-like response exhibited primarily by a subset of female rats in a classic auditory fear conditioning task ("darting"). Here, we asked whether darting during conditioning predicted active responding in a single forced swim (SFS) session to study the potential for darting to reflect a trait-like behavioral strategy that translated across stress models. Male and female Sprague-Dawley (SD) rats were tested in auditory fear conditioning acquisition and memory tests to identify Darters, and then a 15-min SFS 2 weeks later. We observed a significant effect of sex in conditioned freezing behavior, with males exhibiting greater freezing than females across conditioning and testing trials in comparison to females. However, females demonstrated higher velocities in response to shock presentations, and were more likely to exhibit darting behavior in response to the conditioned stimulus (CS). In SFS measures, females engaged in active behaviors such as climbing, head shaking, and diving in greater proportions than males, while males spent more time immobile throughout testing. Despite females exhibiting a more diverse behavioral repertoire in both tests, Darters did not differ from Non-darters in any SFS measure. These results suggest that the propensity to dart does not reflect a simple hyperactivity, and that despite conceptual overlap across the two tests (inescapable stress exposure and the ability to measure active vs. passive coping), the behavioral strategies engaged by an individual animal in each are likely driven by discrete mechanisms. We discuss potential challenges in interpretation of standard behavioral outcomes in classic models across the sexes, and consider the potential need for novel models that better tap into motivational states in females.

Keywords: coping; darting; fear conditioning; forced swim; sex differences.

Figure 1

Sex differences in active and passive behavior during fear conditioning. (A) Females froze less than males during fear conditioning tone presentations. (B) Females froze less than males during a tone test in a new context 24 h after fear conditioning. (C) Females exhibited greater shock responses than males during fear conditioning, as measured by the maximum velocity reached. (D) Proportions of male and female cohorts that exhibited darting behavior. (E) Maximum tone velocity differed between female Darters and both males and female Non-Darters. *p < 0.05 main effect of sex; **p < 0.01 main effect of sex; ***p < 0.001 main effect of sex; ****p < 0.0001 adjusted post hoc, Darters vs. both males and female Non-darters.

Figure 2

Sex differences in active and passive behavior during the forced swim test (FST). (A) Females exhibited less immobility than males in the last two single forced swim (SFS) time bins. (B) Females spent more time climbing than males during the first 5 min of the SFS. (C) Females exhibited more head shaking than males during early SFS time points. (D) Proportions of male and female cohorts that exhibited diving behavior. (E) Among Divers, males and females did not differ in time spent diving. *p < 0.05 adjusted post hoc males vs. females; **p < 0.01 adjusted post hoc males vs. females.

Figure 3

Darter and Non-darter females did not differ in the SFS. No statistically significant differences were observed between Darters and Non-darters in measures of immobility (A), climbing (B), headshakes (C), or time spent diving (D).