Behavioral and cognitive impact of early life stress: Insights from an animal model

Abstract

Background: Children subjected to traumatic events during childhood are reported to exhibit behavioral and cognitive deficits later in life, often leading to post-traumatic stress disorder (PTSD) and major depression. Interestingly, some children continue to remain normal despite being exposed to the same risk factors. These trauma-related behavioral and cognitive profiles across different stages of life are not well understood. Animal studies can offer useful insights.

Objective: The goal of this study was to determine the impact of early life exposure to traumatic events on behavioral and cognitive profile in rats by tracking the behavior of each rat at different ages.

Methods: We utilized the single prolonged stress (SPS), a rodent model of PTSD, to study the effects of early life stress. Male Sprague-Dawley rats were exposed to SPS on post-natal day (PND) 25. Tests to assess anxiety- and depression-like behavior, as well as learning and memory function were performed at PND32, 60 and 90.

Results: Rats exposed to SPS exhibited both anxiety- and depression-like behavior at PND32. And, short-term (STM) but not long-term memory (LTM) was impaired. Rats exposed to SPS at PND60 exhibited anxiety- but not depression-like behavior. STM but not LTM was impaired. Rats exposed to SPS at PND90 exhibited fearful (as indicated by elevated plus maze test) but not an overall anxiety-like behavior (in light and dark test). These rats also displayed significant depression-like behavior with no changes in STM or LTM. Interestingly, when data was further analyzed, two subsets of PND90 rats exposed to SPS were identified, "susceptible": with depression-like behavior and "resilient": without depression-like behavior. Importantly, while resilient group expressed early signs of anxiety- (at PND32 and PND60) and depression-like behavior (at PND32), these behavioral deficits were absent at PND90. On the other hand, susceptible PND90 rats exposed to SPS expressed later onset of anxiety-like behavior (at PND60), while depression-like phenotype was evident only later on at PND90.

Conclusions: Our findings suggest that early life stress caused co-occurrence of anxiety and depression-like behavior at PND32 (mimics human early-adolescent period). This co-occurrence was lost at PND60 with demonstration of anxiety- but not depression-like behavior. Later, depression but not anxiety-like behavior was observed at PND90. It seems that behavioral adaptations occur at the critical PND60 stage (mimics human late-adolescent period), where behavioral and cognitive switching occurs, thereby, expressing susceptible and resilient phenotypes.

Keywords: Early life stress; PTSD; Psychological stress; Resilience; Trauma.

Figure 1. Schematic representation of the experimental regimen

Male Sprague-Dawley consolidated litters were acclimatized for one week and separated from mother at PND21. The rats were then subjected to single prolonged stress (SPS: 2 h restraint stress, 20 min forced swim stress and 2–3 min anesthesia) at PND25 as previously published (Patki, Li et al. 2014) with some modifications. Behavior tests including light and dark (LD), elevated plus maze (EPM) and forced swim test (FST) were conducted using our published protocols (Patki, Salvi et al. 2015). Short-term (STM) and long-term memory (LTM) was examined using radial arm water maze (RAWM) test according to our previously published protocols (Patki, Salvi et al. 2015). The same set of behavior tests were carried out at PND32 (LD at PND32, EPM at PND33, FST at PND34, STM at PND35, and LTM at PND36), PND60 (LD at PND60, EPM at PND61, FST at PND62, STM at PND63, and LTM at PND64), and PND90 (LD at PND90, EPM at PND91, FST at PND92, STM at PND93, and LTM at PND94). Rats were sacrificed after the conclusion of all behavior tests at PND90 and at PND95.

Figure 2. Examination of anxiety- and depression-like behavior and short and long term memory function tests conducted at PND32

Male Sprague-Dawley rats were subjected to control or SPS exposures at PND25. One week later, behavior and cognitive tests were conducted at PND32. Total time spent in the light compartment in light-dark (LD) test (A) and in the open arms in the elevated plus maze (EPM) test (B) measured anxiety-like behavior respectively. Total immobility time in the Forced swim test (FST) was used to examine depression-like behavior (C). Number of errors made in the short-term memory (STM) test (D) and long-term memory (LTM) test (E) examined learning and memory function in radial arm water maze apparatus (RAWM) comprising of six swim paths. Group designations: Control exposures (CON: open bars, n = 25 rats); early life single prolonged stress (SPS: black bars, n = 55 rats). (*) indicates significantly different from control at p < 0.05. Bars represent means ±SEM.

Figure 3. Examination of anxiety- and depression-like behavior and short and long term memory function tests conducted at PND60

Male Sprague-Dawley rats were subjected to control or SPS exposures at PND25. Five weeks later, behavior and cognitive tests were conducted at PND60. Total time spent in the light compartment in light-dark (LD) test (A) and in the open arms in the elevated plus maze (EPM) test (B) measured anxiety-like behavior. Total immobility time in the Forced swim test (FST) was used to examine depression-like behavior (C). Number of errors made in the short-term memory (STM) test (D) and long-term memory (LTM) test (E) examined learning and memory function in the radial arm water maze apparatus (RAWM) comprising of six swim paths. Group designations: Control exposures (CON: open bars, n = 25 rats); early life single prolonged stress (SPS: black bars, n = 55 rats). (*) indicates significantly different from control at p < 0.05. Bars represent means ±SEM.

Figure 4. Examination of anxiety- and depression-like behavior and short and long term memory function tests conducted at PND90

Male Sprague-Dawley rats were subjected to control or SPS exposures at PND25, and behavioral tests at PND32 and 60. Nine weeks after SPS exposure, behavior and cognitive tests were conducted at PND90. Total time spent in the light compartment in light-dark (LD) test (A) and in the open arms in the elevated plus maze (EPM) test (B) measured anxiety-like behavior. Total immobility time in the Forced swim test (FST) was used to examine depression-like behavior (C). Number of errors made in the short-term memory (STM) test (D) and long-term memory (LTM) test (E) examined learning and memory function in radial arm water maze apparatus (RAWM) comprising of six swim paths. Group designations: Control exposures (CON: open bars, n = 25 rats); early life single prolonged stress (SPS: black bars, n = 55 rats). (*) indicates significantly different from control at p < 0.05. Bars represent means ±SEM.

Figure 5. Cluster analysis of depression-like behavior at PND90

PND90 FST data was examined using K-mean cluster analysis with IBM SPSS (IBM, Armonk, NY) (panel I). Two clusters were identified in cluster test at PND90 as a function of immobility time in FST (panel II). Control group n = 25; Resilient group n = 28; Susceptible group n = 27. (*) indicates significantly different from control as well as resilient groups at p < 0.05.

Figure 6. Retrospective data analysis of susceptible and resilient rats

Performance of susceptible and resilient PND90 rats was traced back in FST (panel I) and LD (panel II) tests at PND32 (A and D), 60 (B and E) and 90 (C and F). A diagrammatical representation of occurrence of either anxiety- and depression-like behavior or their cooccurrence is provided in different colors (panel IIIG). Presence of anxiety-like behavior is indicated in green and its absence in black. Presence of depression-like behavior is indicated in maroon and its absence in black, while cooccurrence of the two behaviors is indicated in yellow. A black colored dotted box in the center indicates a potential behavior switch period (panel IIIG). Group designations: Control exposures (CON: open bars, n = 25 rats); Resilient (Grey bars, n = 28 rats); Susceptible (Black bars, n = 27 rats). (*) Significantly different at p < 0.05. Bars represent means ±SEM.