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It is becoming increasingly recognized that post-traumatic stress disorder (PTSD) can be acquired vicariously from witnessing traumatic events. Recently, we published an animal model called the “Trauma witness model” (TWM) which mimics PTSD-like symptoms in rats from witnessing daily traumatic events (social defeat of cage mate) . Our TWM does not result in any physical injury. This is a major procedural advantage over the typical intruder paradigm in which it is difficult to delineate the inflammatory response of tissue injury and the response elicited from emotional distress. Using TWM paradigm, we examined behavioral and cognitive effects in rats  however, the long-term persistence of PTSD-like symptoms or a time-course of these events (anxiety and depression-like behaviors and cognitive deficits) and the contribution of olfactory and auditory stress vs visual reinforcement were not examined. This study demonstrates that some of the features of PTSD-like symptoms in rats are reversible after a significant time lapse of the witnessing of traumatic events. We also have established that witnessing is critical to the PTSD-like phenotype and cannot be acquired solely due to auditory or olfactory stresses.
Approximately 40% of the North American population is exposed to at least one traumatic event during their lifetime . Exposure to traumatic events may be short-lived (e.g., a car accident) or sustained and repeated (e.g., sexual abuse). Several studies have reported strong association between experiencing of traumatic events and developing post-traumatic disorder (PTSD) [15, 21]. While the link between experiencing traumatic events and PTSD seems well established, the impact of witnessing traumatic events and acquiring PTSD is not clear. This is alarming as approximately 25% to 30% of individuals who witness a traumatic event may also develop PTSD and other forms of mental disorders including depression . Approximately half of the individuals who develop PTSD continue to suffer from its effects several years later . Recently, we published an animal model called the “Trauma witness model” (TWM) which mimics majority of PTSD-like symptoms in rats from witnessing daily traumatic events (social defeat of cage mate) . Using TWM paradigm, we examined behavioral and cognitive effects in rats  however, the long-term persistence of PTSD-like symptoms (anxiety and depression-like behaviors and cognitive deficits) and the contribution of olfactory and auditory stress vs visual reinforcement were not examined.
Several studies have reported that PTSD-like behaviors last at least 3–4 weeks after experiencing traumatic events [1, 23] and that olfactory or auditory stress alone cause cognitive and behavioral impairments in animals [18, 19]. Limited information is however available regarding the witnessing aspects of trauma and persistence of PTSD-like behaviors . Recently, Warren et al. had reported that deficits in social avoidance and sucrose preference are salient 1 month after emotional stress of witnessing traumatic events, perhaps indicating that neuronal adaptations underlying these behavioral effects build over time, in the absence of continued stress [8, 10, 21]. In the present study, we have addressed the question of whether the behavioral and cognitive deficits seen in our TWM model are long-lasting or a result of acute stress reaction due to witnessing traumatic events.
Male Sprague Dawley (SD) rats (225–250 g) were used as controls or intruders, and male Long-Evans (LE) retired breeders (400–500 g) served as residents (Charles River, Wilmington, MA). Rats were singly housed with a 12-h light, 12-h dark cycle (lights on at 0700 h) in a climate-controlled room with ad libitum food and water. All experiments were conducted in accordance with the NIH guidelines using approved protocols from the University of Houston Animal Care Committee.
Successful application of chronic social defeat stress to SD rats was dependent on appropriate selection of LE rats with consistent levels of aggressive behaviors, as determined from the 3-d screening process as published by us [13, 14].
The social defeat model used in the present study was modified from the resident-intruder model originally developed by Miczek . As we previously published , this paradigm consisted of 7 encounters, carried out for 7 consecutive days, with an aggressive male Long Evans (LE) rat. Each intruder (Sprague Dawley) was defeated by 7 different resident LE rats [5, 7]. After defeat, a plexiglass partition with holes was placed in the cage to avoid direct physical contact between the LE and intruder. This partition allowed visual, auditory, and olfactory interactions for the remainder of the 30-min session (Fig. 1A). Social defeat was observed by the SD rat present outside the cage, initiating a freezing response. Two more bouts of social defeat were performed with 5-min separation, in order to reinforce the visual stress in the TW rat. Controls were placed behind a wire partition in a novel cage and another SD rat present outside the cage for 30 min daily. TW and SD rats were housed in separate cages after each social defeat exposure. Behavioral assessments were performed at 1, 2, 6 and 8 weeks post trauma witness. The experiments were conducted three times (experiment 1: n=4 rats/group, experiment 2: n=8 rats/group, experiment 3: n=10 rats/group).
A Sprague-Dawley male rat (intruder rat) was put into the home cage of a large male LE rat (resident rat) resulting in social defeat of the intruder. During this process occasional vocalization sounds were made by the intruder. Brief sounds were also made when the intruder hit the walls of the central enclosure in an effort to escape attacks and avoid defeat. These sounds were audible to the cage mate (another small Sprague-Dawley male rat) of the intruder rat. The cage mate could hear the noise but not see the defeat process as the central enclosure was covered with opaque black paper (Fig. 5).
Small Sprague-Dawley rat was allowed to smell but not see the LE rat. This was done either by placing the LE rats in a blacked-out central enclosure, or by putting LE fur and urine inside the enclosure. No social defeat was done (Fig. 5).
Rats were placed in the center of the OF (60×40 cm) and left free to explore the arena for 15 min and movement quantified using Opto-Varimax Micro Activity Meter v2.00 system (Optomax, Columbus Instruments; OH) [16, 20]. Total activity, ambulatory activity and distance covered were examined.
The light-dark box consisted of a light and a dark compartment separated with a single opening for passage from one compartment to the other and total time spent in the lit area was recorded [16, 20].
A standard rat elevated plus-maze was obtained from Med Associates Inc., (St. Albans, VT). The rat’s movements were tracked manually. The observer was blinded to the group classification to avoid bias. Each session was started by placing the rat in the central area facing the open arms of the maze and lasted 5 min. The amount of time the rat spent in the open arms was noted .
Rats were individually introduced into a 25°C water tank for 5 min. During this time the rat assumes an immobile posture, marked by motionless floating and the cessation of struggling. The total time spent immobile was recorded [3, 6]. FST was performed 72 h after 1, 2, 6 and 8 week TW/AS/OS protocols.
A black circular water filled pool with six swim paths were used . First set of six learning trials (trials # 1–6) were followed by a five min rest and another set of six learning trials (trials # 7–12). Rats were tested for short-term memory 30 min after the end of 12th trial and returned to their home cages and 24h later subjected to long-term memory test.
Data are expressed as mean ± SEM. Significance was determined by unpaired student t-test (GraphPad Software, Inc. San Diego, CA). A value of p<0.05 was considered significant.
Control rats spent more time in the light compartment, when compared to TW rats 1 week (CON: 93 ± 8, TW: 43 ± 6, t=4.6, p=0.03), 2 weeks (CON: 105 ± 9, TW: 51 ± 14, t=3.1, p=0.001) and 6 weeks (CON: 61 ± 4, TW: 19 ± 8, t=3.1, p=0.04) after witnessing social defeat of their cage-mate, but after 8 weeks (CON: 29 ± 18, TW: 21 ± 5, t=0.4, p=0.69), anxiety-like behavior was not different from the controls (Fig. 1B, ,2A,2A, ,3A,3A, ,4A).4A). The amount of time control rats spent in the open arms of EPM test was significantly higher when compared to TW rats 1 week (CON: 72 ± 2, TW: 29 ± 3, t=7.3, p=0.04), 2 weeks (CON: 116 ± 14, TW: 76 ± 4, t=2.9, p=0.009) and 6 weeks (CON: 49 ± 9, TW: 18 ± 7, t=3.1, p=0.04) after witnessing social defeat of their cage-mate, but after 8 weeks (CON: 71 ± 8, TW: 63 ± 7, t=0.8, p=0.45), anxiety-like behavior was not different from the control group (Fig. 1C, ,2B,2B, ,3B,3B, ,4B).4B). In the open-field test, TW rats had lower total activity than their matched controls following 1 week (CON: 4785±344, TW: 3905±196, P=0.04, t=2.21, df=18), 2 weeks (CON: 3952±198, TW: 2414±212, P=0.002, t=4.31, df=18), and 6 weeks (CON: 4441±132, TW: 3708±173, P=0.04, t=2.41, df=18) of trauma witness protocol (Fig. 1D, ,2C,2C, ,3C,3C, ,4C).4C). TW rats also demonstrated lower ambulatory activity than their matched controls following 1 week (CON: 4008±318, TW: 3215±183, P=0.04, t=2.15, df=18), 2 weeks (CON: 2932±216, TW: 2308±100, P=0.017, t=2.73, df=18), and 6 weeks (CON: 3764±150, TW: 3174±122, P=0.03, t=2.19, df=18) of trauma witness (Fig. 1E, ,2D,2D, ,3D,3D, ,4D).4D). Less distance was travelled by TW rats in the open-field when compared to their matched controls following 1 week (CON: 3876±107, TW: 3100±116, P=0.04, t=2.19, df=15), 2 weeks (CON: 3629±209, TW: 2243±178, P=0.002, t=3.94, df=15), and 6 weeks (CON: 4295±49, TW: 3437±112, P=0.01, t=3.92, df=15) of trauma witness protocol (Fig. 1F, ,2E,2E, ,3E,3E, ,4E).4E). No difference was observed between TW rats and their matched controls in any of the parameters of the OFT after 8 week of trauma witness protocol. In the FST, TW rats spent more time being immobile when compared to their matched controls after 1 week (CON: 61 ± 15, TW: 129 ± 6, t=4.2, p=0.001), 2 weeks (CON: 11 ± 4, TW: 61 ± 4, t=8.0, p=0.005) and 6 weeks (CON: 32 ± 12, TW: 63 ± 22, t=3.1, p=0.04) of trauma witness protocol, but even after 8 weeks (CON: 33 ± 11, TW: 68 ± 12, t=3.1, p=0.05) the depression-like behavior existed in TW rats (Fig. 1G, ,2F,2F, ,3F,3F, ,4F4F).
TW rats made comparable errors when compared to their matched controls in the STM test at 1, 2, 6 and 8 weeks after TWM (Fig. 1H, ,2G,2G, ,3G,3G, ,4G).4G). On the other hand, in the LTM, TW rats committed significantly higher number of errors when compared to their matched controls at 1 week (CON: 0.7 ± 0.7, TW: 3.8 ± 0.9, t=2.9, p=0.04), 2 weeks (CON: 1 ± 0.5, TW: 4.7 ± 1.2, t=2.9, p=0.04) and 6 weeks (CON: 0.3 ± 0.3, TW: 3 ± 1.1, t=3.2, p=0.04) after witnessing social defeat of their cage-mate, but after 8 weeks (CON: 2.3 ± 0.8, TW: 2.3 ± 1.2, t=0, p=0.96), the LTM of TW rats was not different from their matched controls (Fig. 1I, ,2H,2H, ,3H,3H, ,4H).4H). Thus, witnessing social defeat did not significantly affect STM but the LTM consolidation was affected in these rats until 6 weeks after witnessing social defeat.
In separate experiments, we addressed the issue of the visual experience as a key component in induction of majority of the PTSD-like symptoms. Our results suggest that auditory stress (Fig. 5(i)A–I) arising from hearing sounds (ultrasonic vocalizations) of defeat process or olfactory stress (Fig. 5(ii)A–I) from smelling resident odor, fur and urine do not lead to key PTSD-like behaviors, including anxiety and depression-like behaviors or STM or LTM in rats, suggesting that visually witnessing traumatic events is essential for development of PTSD-like behaviors in rats.
We have previously shown that rats witnessing social defeat of cage-mates for 7 consecutive days developed majority of the PTSD-like behavioral (anxiety and depression-like) and cognitive deficits . In this study, we have validated the robustness of this animal model and have established that some of the features of PTSD-like symptoms in rats are reversible after a significant time lapse of the witnessing of traumatic events. We also have established that witnessing is critical to the PTSD-like phenotype and cannot be acquired solely via application of auditory or olfactory stresses.
Our results suggest that anxiety-like behaviors persisted until after 6 weeks of termination of TWM but seemed to be reversible beyond this time period when examined after 8 weeks of TWM. Similarly, the LTM impairments lasted until 6 weeks post TWM but also were reversed after 8 weeks of TWM. These results seem to follow the “Dual–Branch Hypothesis of PTSD”, previously put forth by Kamprath et al who had suggested aversive encounters (physical or witness) lead to simultaneous formation of associative and non-associative memories [9,], and both memory components must be inhibited for maintenance of PTSD-like symptoms. Gradual desensitization and habituation processes are thought to slowly decrease the response to an aversive stimulus. Our 8 week TWM data clearly suggests an involvement of habituation and/or desensitization processes. Interestingly, depression-like behavior persisted 8 weeks after the trauma witness protocol. This is not surprising considering that depression affects several neural circuits and mechanisms as compared to anxiety or memory function, and hence could take more time to normalize. Perhaps, 8 week time lapse of TWM is not enough to cause depressive recovery.
Another attractive feature of this study is the that we have tested the importance of visually witnessing the traumatic effects rather than only hearing the social defeat (ultrasonic vocalizations, auditory stress) or smelling the predator odor and urine (olfactory stress). Both auditory as well as olfactory stress in itself had no effect on anxiety and depression-like behaviors or memory when tested 2 weeks after subjecting them to auditory or olfactory stress. This supports the fact that visually witnessing the traumatic event is an important aversive stimulus for the development of PTSD-like symptoms in rats.
In summary, witnessing traumatic events but not directly experiencing those lead to behavioral and cognitive deficits, and some of the deficits including anxiety and long-term memory impairment last at least up to 6 weeks after conclusion of TWM. Witnessing is an integral part of the TWM as auditory and olfactory stresses are not enough to cause anxiety and depression-like behaviors which are the key components of PTSD-like phenotype. Our results underline the importance of stress sensitization and associative learning as explained by the dual-branch hypothesis. Finally, we suggest that TWM is an excellent model for studying vicarious trauma.
Funding was provided by 1R15MH093918-01A1 grant awarded to S.S. G.P and S.S conceived and designed the experiments and wrote the manuscript. G.P, A.S and H.L conducted research and analyzed the data.
All authors read and approved the final manuscript and declare no conflict of interest.
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