Enhanced fear recall and emotional arousal in rats recovering from chronic variable stress

Abstract

Emergence of posttraumatic-like behaviors following chronic trauma is of interest given the rising prevalence of combat-related posttraumatic stress disorder (PTSD). Stress associated with combat usually involves chronic traumatization, composed of multiple, single episode events occurring in an unpredictable fashion. In this study, we investigated whether rats recovering from repeated trauma in the form of chronic variable stress (CVS) express posttraumatic stress-like behaviors and dysregulated neuroendocrine responses. Cohorts of Long-Evans rats underwent a 7 day CVS paradigm followed by behavioral and neuroendocrine testing during early (16 h post CVS) and delayed (7 day) recovery time points. A fear conditioning-extinction-reminder shock paradigm revealed that CVS induces exaggerated fear recall to reminder shock, suggestive of potentiated fear memory. Rats with CVS experience also expressed a delayed expression of fearful arousal under aversive context, however, social anxiety was not affected during post-CVS recovery. Persistent sensitization of the hypothalamic-pituitary-adrenocorticotropic response to a novel acute stressor was observed in CVS exposed rats. Collectively, our data are consistent with the constellation of symptoms associated with posttraumatic stress syndrome, such as re-experiencing, and arousal to fearful contexts. The CVS-recovery paradigm may be useful to simulate trauma outcomes following chronic traumatization that is often associated with repeated combat stress.

Fig. 1

Schematic of experimental timeline. Rats were acclimated to the vivarium for one week after arrival. Rats were weighed and assigned to CVS or control groups. Rats were exposed to a one week CVS paradigm or were gently handled during the period of CVS exposure. Independent cohorts of control and CVS exposed animals were tested for various behavioral tests or neuroendocrine response at either early (16 hr) or delayed (7d) post CVS recovery.

Fig. 2

CVS animals express sensitization of conditioned fear and fear memory recall as well as impaired extinction. (A) Fear conditioning, extinction and reinstatement training and testing schedule. At 7 days post CVS rats were administered three shocks for conditioning followed by measurement of conditioned fear and extinction (Test 1–Test 4). To assess fear memory recall, a single reminder shock was administered 48 hr after the last extinction test. Post shock freezing and contextual freezing in the absence of shock was measured for 5 min. (B) Percent freezing ± SEM per minute in groups of control and CVS rats exposed to context 24 hr after conditioning. (C) Extinction of conditioned fear in groups of control and CVS rats between Test 1 to Test 4. Mean percent freezing ± SEM over five minutes was measured. (D) Mean post shock percent freezing ± SEM following initial conditioning shocks and reminder shock in control and CVS rats. * p<0.05 versus control responses (n= 6–7 per group)

Fig. 3

Elevated Plus Maze testing reveals delayed expression of fear associated arousal in rats exposed to CVS. (A) EPM testing schedule schematic. Testing at early and delayed recovery was conducted under bright light or low light conditions using separate cohorts of CVS and control animals for each condition. (B) Open arm time in CVS and Control groups tested under bright (left) and low (right) light conditions. Data as shown as mean ± SEM represented as percentage of control. (C) Panel shows motor activity, freezing, grooming and rearing outcomes measured for bright light conditions in CVS and control rats at early and delayed recovery. Data are represented as mean ± SEM values * p<0.05 versus control; # trends for significance p=0.08 (open arm time) and p=0.053 (rearing) n = 10–12 animals/group.

Fig. 4

Social interaction is not affected by CVS exposure. (A) Social Interaction testing schedule schematic. (B) Panel shows active interaction, rearing, grooming and duration of fights in CVS and control rats at early and delayed recovery. Data are represented as mean ± SEM values; n = 12 animals/group, * p<0.05 versus control

Fig. 5

Rats exposed to chronic variable stress exhibit sensitized plasma corticosterone response to a novel acute acoustic stressor. (A) Acute stress response testing schedule schematic (B) Plasma corticosterone levels at 30, 60 and 120 min after initiation of stressor at early (left panel) or delayed (right panel) recovery post CVS. Values represent mean ± SEM; n=10–12 animals per group, * p< 0.05 versus control group.