**4. Conclusions**

Active avoidance requires attentiveness, memory, and, arguably, anxiety (Mowrer, 1940; Miller, 1948; Solomon & Wynne, 1954; Hoffman & Fleshler, 1962; Dinsmoor, 1977). Others have utilized active avoidance procedures as a tool to study chronic stress effects on physiology (Forsyth, 1968; Forsyth, 1969; Natelson et al., 1976; Natelson et al., 1977), but an explicit connection between the behavioral symptoms of PTSD needed to be tested. In both experiments, an initial increase in startle sensitivity occurred in response to the initial experiences with shock. This supports our contention that an acute increase in startle sensitivity occurs as a function of shock exposure – not startle responsivity. In other words, the level of vigilance is changed for a period of time following shock exposures. Conversely, startle responsivity, general arousal, increases as the avoidance response is refined and can even persist into extinction sessions. This supports our and others' past work that found inescapable shock only increases startle reactivity after some period of time has passed since the acute exposure to the shock (in the range of days)(Manion et al., 2007; Servatius et al., 1995; Beck et al., 2002), unless the subject under study is female (Beck & Servatius, 2005; Beck et al., 2008). Thus, we also replicated that sex difference by failing to find any increases in startle responsivity in female rats following shock exposure (Beck & Servatius, 2005; Beck et al., 2008).

One could surmise that the strain and sex differences in startle sensitivity and responsivity during later acquisition and through extinction occur as a function of differences in learning controllability over their general environment. Controllability over stressful situations cause different long-term effects, but these effects are commonly thought to be regarded as a means to buffer the deleterious effects of stress (Baratta et al., 2007). This work questions that general assumption, and, in fact, suggests for some individuals diagnosed with PTSD, hyperarousal could be the result of being overly controlling and not "testing the waters" to see if perceived warning signals still predict aversive events. This appears to be the male pattern, which may vary in expression across strain, but is most consistent in the WKY rats. For the females, some have proposed emotional numbing in PTSD patients is actually a result of hyperarousal to negative valenced stimuli being contrasted to a lack of arousal to positive stimuli (Litz et al., 1997; Litz & Gray, 2002). Our data do not support that relationship, but the reduction in startle sensitivity in female WKY rats appears to parallel documented lower startle reactions in female PTSD patients with comorbid depressive symptoms (Medina et al., 2001). WKY rats have been touted as a model for depression and maybe this sex-specific response reflects some aspect of those characteristics (Pare & Redei, 1993). Overall, these divergent changes in arousal and vigilance are likely a bi-product of

in acquisition in both avoidance and yoked rats. These differences persisted into extinction and following the termination of training. Because each yoked rat experienced the same stimuli as an avoidance-trained rat, they should have learned the predictability of the stimuli. However, what is not learned is that they have any controllability. The poorer learning by the avoidance-trained SD rats in the yoked-avoidance experiment would conform to this idea. Because they did not acquire the response to the same level as those in the earlier experiment, this may have led them to also fail to show any difference in startle responsivity from their yoked controls as training progressed. Observed differences in avoidance-trained WKY rats suggest they too may have similar increased arousal with the

Active avoidance requires attentiveness, memory, and, arguably, anxiety (Mowrer, 1940; Miller, 1948; Solomon & Wynne, 1954; Hoffman & Fleshler, 1962; Dinsmoor, 1977). Others have utilized active avoidance procedures as a tool to study chronic stress effects on physiology (Forsyth, 1968; Forsyth, 1969; Natelson et al., 1976; Natelson et al., 1977), but an explicit connection between the behavioral symptoms of PTSD needed to be tested. In both experiments, an initial increase in startle sensitivity occurred in response to the initial experiences with shock. This supports our contention that an acute increase in startle sensitivity occurs as a function of shock exposure – not startle responsivity. In other words, the level of vigilance is changed for a period of time following shock exposures. Conversely, startle responsivity, general arousal, increases as the avoidance response is refined and can even persist into extinction sessions. This supports our and others' past work that found inescapable shock only increases startle reactivity after some period of time has passed since the acute exposure to the shock (in the range of days)(Manion et al., 2007; Servatius et al., 1995; Beck et al., 2002), unless the subject under study is female (Beck & Servatius, 2005; Beck et al., 2008). Thus, we also replicated that sex difference by failing to find any increases in startle responsivity in female rats following shock exposure (Beck & Servatius, 2005; Beck

One could surmise that the strain and sex differences in startle sensitivity and responsivity during later acquisition and through extinction occur as a function of differences in learning controllability over their general environment. Controllability over stressful situations cause different long-term effects, but these effects are commonly thought to be regarded as a means to buffer the deleterious effects of stress (Baratta et al., 2007). This work questions that general assumption, and, in fact, suggests for some individuals diagnosed with PTSD, hyperarousal could be the result of being overly controlling and not "testing the waters" to see if perceived warning signals still predict aversive events. This appears to be the male pattern, which may vary in expression across strain, but is most consistent in the WKY rats. For the females, some have proposed emotional numbing in PTSD patients is actually a result of hyperarousal to negative valenced stimuli being contrasted to a lack of arousal to positive stimuli (Litz et al., 1997; Litz & Gray, 2002). Our data do not support that relationship, but the reduction in startle sensitivity in female WKY rats appears to parallel documented lower startle reactions in female PTSD patients with comorbid depressive symptoms (Medina et al., 2001). WKY rats have been touted as a model for depression and maybe this sex-specific response reflects some aspect of those characteristics (Pare & Redei, 1993). Overall, these divergent changes in arousal and vigilance are likely a bi-product of

adoption and use of active avoidance behavior.

**4. Conclusions** 

et al., 2008).

strain and sex differences in learning processes that are involved in forming predictive associations under conditions where some level of stress is involved (Wood & Shors, 1998; Ricart et al., 2011a; Ricart et al., 2011b; Beck et al., 2011). When the requirement to cope is brought to the fore, these inherent differences in learning processes can be seen both in their rate to acquire an avoidance coping response (i.e. gaining control) and any subsequent resistance to cease that response (maintaining control).

The problems associated with anxiety disorders, such as PTSD, are multifaceted and variable. In part, this is because different individuals perceive and cope with stressors differently. Active-avoidance behavior, using a lever-press, is rarely uniform within a group of animals, and, as evidenced from the data from our lab; the resulting effects on startle responses can be variable in when they emerge over time. Yet, the variability caused by having a subject-controlled manipulation of stressor exposure is important for understanding the disorder. Controllability may selectively influence certain individuals in a manner that causes increases in general arousal whereas others are not so affected. Moreover, when we consider vulnerability factors, such as demonstrated by the WKY rats (behavioral inhibition and higher baseline startle responses), individual differences in coping with stressor and different rates of acquisition of avoidant strategies should occur – as in the human condition. Gaining an understanding of the relationship between different symptoms, as demonstrated here between avoidance and arousal, will provide us with the knowledge to broaden our expectations for how different populations may develop the symptoms of PTSD. As shown here, our data suggest that the acute experience of pain is not sufficient to immediately increase startle responsivity, and it may not be a good marker for tracking the development of PTSD. Our data suggests that the avoidance process is already well-acquired when this other symptom becomes evident. As has been suggested from the clinical literature, increased expression of avoidance may be a very good marker for tracking the development of the disorder (Karamustafalioglu et al., 2006; O'Donnell et al., 2007). Further, breaking the adoption and utilization of avoidance strategies may lead to a reduction in general arousal (at least in males); therefore, some non-pharmacological therapeutic approaches (e.g. cognitive-behavioral therapy) may have beneficial effects on these two core features of PTSD. Additional research is required to better understand and track the developmental course of symptom expression in different subpopulations (e.g. women) such that our animal model systems can be better tailored to reflect the cascade of changes occurring in those people, especially for those at risk for developing the symptoms of PTSD.
