Development of PTSD-like behavior in adult mice after observing an acute traumatic event

SummaryIn human post-traumatic stress disorder (PTSD), a major psychiatry challenge is how diverse stress reactions emerge after a protracted symptom-free period. Here, we study the behavioral development in mice isolated after observing an aggressive encounter inflicted upon their pair-housed partners and compared the results with those in multiple control paradigms. Compared with mice plainly isolated, mice isolated following the acute witnessing social stress gradually developed a wide range of long-term differences of their physiological conditions, spontaneous behaviors, and social interactions, including paradoxical results if interpreted in traditional ways. To address this developmental diversity, we applied fine-scale behavioral analysis to standard behavioral tests and showed that the seemingly sudden emergent behavioral differences developed gradually. Mice showed different developmental patterns in different zones of a behavior testing apparatus. However, the results of the fine-scale analysis together with state-space behavioral characterization allow a consistent interpretation of the seemingly conflicting observations among multiple tests. Interestingly, these behavioral differences were not observed if the aggressive encounter happened to a stranger mouse. Additionally, traumatized mice showed rebound responses to their partners after the long separation. In contrast, mice pair-housed with their attacked partners after the aggressive encounters still showed a difference in social interactions, while a difference in spontaneous behaviors did not occur. Accordingly, we propose that social relationship is the single common factor underlying the otherwise independent development of behavioral differences in this mouse paradigm and that the gained insights could have parallels in human PTSD development.

Learning your own strength: winner and loser effects should change with age and experience

Winner and loser effects, in which the outcome of an aggressive encounter influences the tendency to escalate future conflicts, have been documented in many taxa, but we have limited understanding of why they have evolved. One possibility is that individuals use previous victories and defeats to assess their fighting ability relative to others. We explored this idea by modelling a population of strong and weak individuals that do not know their own strength, but keep track of how many fights they have won. Under these conditions, adaptive behaviour generates clear winner and loser effects: individuals who win fights should escalate subsequent conflicts, whereas those who lose should retreat from aggressive opponents. But these effects depend strongly on age and experience. Young, naive individuals should show highly aggressive behaviour and pronounced loser effects. For these inexperienced individuals, fighting is especially profitable because it yields valuable information about their strength. Aggression should then decline as an individual ages and gains experience, with those who lose fights becoming more submissive. Older individuals, who have a better idea of their own strength, should be more strongly influenced by victories than losses. In conclusion, we predict that both aggressiveness and the relative magnitude of winner and loser effects should change with age, owing to changes in how individuals perceive their own strength.

Liver injury after an aggressive encounter in male mice

Acute and intense psychological stressors induce cell damage in several organs, including the heart and the liver. Much less is known about social stress. In male mice, aggressive behavior is the most common social stressor. It is remarkable that upon fighting, submandibular salivary glands release a number of peptides into the bloodstream including epidermal growth factor (EGF). We showed previously that released EGF protects the heart from cell damage in this particular stressful situation. Here, we studied the effect of an aggressive encounter on the liver and whether EGF has a similar effect on this organ. An aggressive encounter in male mice caused inflammatory response and a transient increase in plasma alanine and aspartate transaminase activities. At 3 h, focal infiltration of neutrophils was observed in liver parenchyma. These cells accumulate on eosinophilic hepatocytes, which may correspond to dying cells. A few hours later, evidence of necrotic lesion was observed. Surgical excision of submandibular glands, sialoadenectomy, did not prevent the rise in plasma EGF concentration and did not affect the increase in plasma transaminase activities. Neither did the administration of tyrphostin AG-1478 (inhibitor of EGF receptor kinase) alter the increase in plasma alanine transaminase activity. However, it did enhance the rise in both aspartate transaminase and creatine kinase activity, suggesting heart damage. We conclude that an aggressive encounter causes mild liver damage and that released EGF does not protect this organ, in contrast to its effect on the heart.