Susceptibility and Resilience to Posttraumatic Stress Disorder–like Behaviors in Inbred Mice

Abstract

Background The limited neurobiological understanding of PTSD has been partially attributed to the need for improved animal models. Stress-enhanced fear learning (SEFL) in rodents recapitulates many PTSD-associated behaviors, including stress-susceptible (SS) and –resilient (SR) subgroups in outbred rats. Identification of subgroups requires additional behavioral phenotyping, a confound to mechanistic studies. Methods We employed a SEFL paradigm in inbred male and female C57BL/6 that combines acute stress with fear conditioning to precipitate “traumatic” memories. Extinction and long-term retention of extinction were examined after SEFL. Further characterization of SEFL effects on male mice was performed with additional behavioral tests, determination of regional activation by Fos immunofluorescence and RNA-sequencing of the basolateral amygdala (BLA). Results Stressed animals displayed persistently elevated freezing during extinction. While more uniform in females, SEFL produced male subgroups with differential susceptibility that were identified without post-training phenotyping. Additional phenotyping of males revealed PTSD-associated behaviors, including extinction-resistant fear memory, hyperarousal, generalization and dysregulated corticosterone in SS males. Altered Fos activation was also seen in the infralimbic cortex and BLA of SS males after remote memory retrieval. Key behavioral outcomes, including susceptibility, were replicated by two independent laboratories. RNA-sequencing of the BLA revealed transcriptional divergence between the male subgroups, including genes with reported polymorphic association to PTSD patients. Conclusions This SEFL model provides a tool for development of PTSD therapeutics that is compatible with the growing number of mouse-specific resources. Furthermore, use of an inbred strain allows for investigation into epigenetic mechanisms that are expected to critically regulate susceptibility and resilience.

To Bend and Not Break: The Neurobiology of Stress, Resilience, and Recovery

“…The two groups show differences not only in the primary behavioral test, but also in other PTSD-related behaviors (e.g., hyperarousal), stress hormone levels, markers of synaptic activation, and RNA expression in brain regions associated with learning and memory. Promisingly, differences in RNA expression between the two groups largely corresponded to genes previously described in human genetic studies of PTSD (9). Fascinatingly, female mice do not segregate into “stress-resistant” and “stress-sensitive” groups and instead respond homogenously as stress-sensitive—thereby suggesting that sex may be another complicating factor.…”

“…In this issue of Biological Psychiatry , Sillivan et al . (9) describe a mouse model that they have developed to isolate the biologic determinants of resilience. Doing so required overcoming two principal shortcomings of previous animal models.…”

“…Sillivan et al .’s (9) animal model allows us to dissect a topic like resilience more deeply—and the more we do so, the more nuanced our understanding becomes. We now see that even when genetic and environmental factors are tightly controlled, subtle biological differences may still predispose individuals to having different responses to stress.…”

Brain circuit dysfunction in post-traumatic stress disorder: from mouse to man

“…Commonly used behavioural paradigms that have been used to assess PTsD-like symptoms in animal models include chronic social defeat stress, inescapable foot shock, early-life stress and stress-enhanced fear learning 221,222 . These tests have different advantages and disadvantages, which have recently been reviewed 221 ; however, they all suffer from a lack of robustness and from low construct validity.…”

Assaying Fear Memory Discrimination and Generalization: Methods and Concepts

“…Approximately 30 days between training and testing has been consistently shown to increase both cued and contextual fear memory generalization, although shorter retention intervals have been reported (Lynch et al, 2013). There is also now a growing body of evidence to suggest that both chronic alcohol exposure and stress exposure promote generalized fear responses in rats and mice (Bender, Otamendi, Calfa, & Molina, 2018;Elliott & Richardson, 2019;Scarlata et al, 2019;Sillivan et al, 2017;Stephens et al, 2005). Finally, context and cued exposure before and after fear learning is another extrinsic factor that can determine the degree of generalization and discrimination.…”