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A Mouse Model of Stress-Enhanced Fear Learning Demonstrates Extinction-Sensitive and Extinction-Resistant Effects of Footshock Stress
Abstract
Stressful experiences ca cause long-lasting sensitization of fear and anxiety that extends beyond the circumstances of the initial trauma. The neural mechanisms of these stress effects have been studied extensively in rats using the stress-enhanced fear learning (SEFL) paradigm, in which exposure to footshock stress potentiates subsequent fear conditioning. Here we establish a mouse version of the SEFL. Male and female 129s6 mice received four 1-mA foot shocks or equivalent context exposure without shock. Shock exposure induced Pavlovian fear conditioning to the shock context and produced three more general effects: (1) suppression of open field exploration, (2) potentiated unconditioned fear of a novel tone stimulus, and (3) enhanced fear conditioning in a novel context. To determine whether these effects of footshock stress reflect generalized Pavlovian fear conditioning versus nonassociative fear sensitization, some mice received extinction training in the footshock stress context, which reduced contextual fear to the levels of unstressed control mice. Extinction restored normal open field exploration, suggesting that this effect of stress reflects generalized Pavlovian fear. In contrast, extinction failed to attenuate stress-enhanced fear, indicating that stress-enhanced fear is nonassociative and mechanistically distinct from Pavlovian fear conditioning. The effects of footshock stress were similar in male and female mice, although female mice displayed larger acute responses to fear-inducing stimuli than did males. The results demonstrate that footshock stress influences emotional behavior through distinct associative and nonassociative mechanisms, which likely involve unique neural underpinnings.
... While multiple-shock fear conditioning like that used in SEFL produces significant generalized fear (Poulos et al., 2016a), there are several reasons to discount the generalization account. First, the extinction of context fear to the stress context does not mitigate the sensitization of fear to a novel context (Hassien et al., 2020;Long & Fanselow, 2012;Rau et al., 2005). Moreover, the resulting associative memory of stress induction is required for neither the induction nor maintenance of SEFL. ...
... Pre-exposure to repeated footshock sensitizes fear responding to future footshock and potentiates responding to other strong stimuli. For example, footshock stress also potentiates freezing to high decibel tone and white noise stimuli (Hassien et al., 2020;Perusini et al., 2016) and enhances reactivity to white noise in the stress context (Hoffman et al., 2022). ...
... Perusini and colleagues (2016) demonstrated that SEFL induction in rats could produce an anxiety-like change in exploratory behaviors in a modified open field (Perusini et al., 2016). Interestingly, Hassien et al (2020) showed in mice that SEFL induction decreased locomotion in the open field, an effect that was abolished by the extinction of conditioned fear to the stress induction context and suggested that this stress-enhanced anxiety-like behavior is mediated by associative memory related to the stress context. However, while the above results were obtained in adult stressed animals, infant stressed rats exhibit long-lasting increases in the elevated plus maze as adults even in the absence of the stress induction memory, suggesting nonassociative contributions (Poulos et al., 2014). ...
Extreme stress can cause long-lasting changes in affective behavior manifesting in conditions such as post-traumatic stress disorder (PTSD). Understanding the biological mechanisms that govern trauma-induced behavioral dysregulation requires reliable and rigorous pre-clinical models that recapitulate multiple facets of this complex disease. For decades, Pavlovian fear conditioning has been a dominant paradigm for studying the effects of trauma through an associative learning framework. However, severe stress also causes long-lasting nonassociative fear sensitization, which is often overlooked in Pavlovian fear conditioning studies. This paper synthesizes recent research on the stress-enhanced fear learning (SEFL) paradigm, a valuable rodent model that can dissociate associative and nonassociative effects of stress. We discuss evidence that the SEFL paradigm produces nonassociative fear sensitization that is distinguishable from Pavlovian fear conditioning. We also discuss key biological variables, such as age and sex, neural circuit mechanisms, and crucial gaps in knowledge. We argue that nonassociative fear sensitization deserves more attention within current PTSD models and that SEFL provides a valuable complement to Pavlovian conditioning research on trauma-related pathology.
... Prior shock exposure can sensitize the animal to subsequently respond more fearfully to otherwise neutral stimuli. While nonassociative controls were once commonplace (Rescorla 1967;Mackintosh 1974), they are very infrequently run, and nonassociative contribution to freezing is often not considered (although see Siegmund and Wotjak 2007a,b;Hassien et al. 2020). ...
... Prior studies have reported sex differences in tone freezing; however, the extent to which these differences are driven by associative versus nonassociative factors is unknown (Moore et al. 2010;Ter Horst et al. 2012;Day et al. 2020;Hassien et al. 2020). To characterize sex differences in associative versus nonassociative fear learning and memory, we used five different fear conditioning protocols: two associative protocols (Trace and Delay) and three nonassociative protocols (Sensitization, Explicitly Unpaired, and No Shock). ...
... The extent to which a stimulus produces a fear response in females more so than in males may impair performance, either by directly competing with proper object investigation or by stress-induced impairments in cognition and/or synaptic plasticity Wood et al. 2001;Kajantie and Phillips 2006;Zitman and Richter-Levin 2013). Two previous studies have reported evidence consistent with an increase in nonassociative fear in female rodents, suggesting this is a robust effect worthy of more direct investigation (Ter Horst et al. 2012;Hassien et al. 2020). Hippocampal-specific deletion of the mineralocorticoid receptor produced deficits in fear extinction in female but not male mice, suggesting an important role for the stress response in mediating sex differences in fear expression (Ter Horst et al. 2012). ...
Inclusion of male and female subjects in behavioral neuroscience research requires a concerted effort to characterize sex differences in standardized behavioral assays. Sex differences in hippocampus-dependent assays have been widely reported but are still poorly characterized. In the present study, we conducted a parametric analysis of spontaneous alternation, object recognition, and fear conditioning in a commonly used control strain, C57BL/6NTac. Our findings show largely similar performance between males and females across the majority of behavioral end points. However, we identified an important difference in nonassociative fear sensitization, whereby females showed an enhanced fear response to the 75-dB tone that is used as the conditional stimulus. In addition, we observed an impairment in object location performance in females that was ameliorated by more extensive habituation to handling. Together, these findings argue that sex differences in nonassociative fear responses to both novel auditory cues and novel objects need to be considered when designing and interpreting cognitive assays in C57BL/6 mice. Furthermore, this elevated fear sensitization could serve as a novel approach to model the increased incidence of anxiety disorders in women.
... Prior shock exposure can sensitize the animal to subsequently respond more fearfully to otherwise neutral stimuli. While nonassociative controls were once commonplace (Rescorla 1967;Mackintosh 1974), they are very infrequently run, and nonassociative contribution to freezing is often not considered (although see Siegmund and Wotjak 2007a,b;Hassien et al. 2020). ...
... Prior studies have reported sex differences in tone freezing; however, the extent to which these differences are driven by associative versus nonassociative factors is unknown (Moore et al. 2010;Ter Horst et al. 2012;Day et al. 2020;Hassien et al. 2020). To characterize sex differences in associative versus nonassociative fear learning and memory, we used five different fear conditioning protocols: two associative protocols (Trace and Delay) and three nonassociative protocols (Sensitization, Explicitly Unpaired, and No Shock). ...
... The extent to which a stimulus produces a fear response in females more so than in males may impair performance, either by directly competing with proper object investigation or by stress-induced impairments in cognition and/or synaptic plasticity Wood et al. 2001;Kajantie and Phillips 2006;Zitman and Richter-Levin 2013). Two previous studies have reported evidence consistent with an increase in nonassociative fear in female rodents, suggesting this is a robust effect worthy of more direct investigation (Ter Horst et al. 2012;Hassien et al. 2020). Hippocampal-specific deletion of the mineralocorticoid receptor produced deficits in fear extinction in female but not male mice, suggesting an important role for the stress response in mediating sex differences in fear expression (Ter Horst et al. 2012). ...
Noradrenergic neurons of the locus coeruleus (LC-NE) are known to play an important role in arousal, emotion, memory and cognition. In the present study, we use fiber photometry combined with chemogenetic and optogenetic approaches to demonstrate a previously unrecognized role for LC-NE neurons in the modulation of feeding. We show that endogenous activity of LC-NE neurons is enhanced while approaching food and suppressed during feeding. These changes in LC activity during feeding behavior are attenuated as mice approach satiety, demonstrating that nutritional state modulates LC responses to food. Direct activation of LC-NE neurons results in the suppression of feeding. Further, activation of an LC projection to the lateral hypothalamus also suppresses feeding. Together, these findings demonstrate a direct causal role for LC-NE activity in the modulation of feeding.
... This heightened response is resistant to massed extinction occurring 10 min or 72 h following initial learning (Long and Fanselow, 2012). Further, enhanced fear conditioning to the novel context persisted even when responding to the stress context was reduced through extinction (i.e., exposure to the chamber without presentation of the shock; see also Hassien et al., 2020). Interestingly, certain effects of stress-enhanced fear learning (i.e., increased later fear conditioning and unconditional fear to a loud noise stimulus) seem dependent on non-associative mechanisms, whereas others (i.e., reduced open field exploration) seem dependent on associative mechanisms (Hassien et al., 2020). ...
... Further, enhanced fear conditioning to the novel context persisted even when responding to the stress context was reduced through extinction (i.e., exposure to the chamber without presentation of the shock; see also Hassien et al., 2020). Interestingly, certain effects of stress-enhanced fear learning (i.e., increased later fear conditioning and unconditional fear to a loud noise stimulus) seem dependent on non-associative mechanisms, whereas others (i.e., reduced open field exploration) seem dependent on associative mechanisms (Hassien et al., 2020). This suggests that some behavioral effects of stress-enhanced fear learning are reflective of generalized fear responding rather than changes caused by stress or sensitization per se. ...
Stress can negatively impact brain function and behaviors across the lifespan. However, stressors during adolescence have particularly harmful effects on brain maturation, and on fear and social behaviors that extend beyond adolescence. Throughout development, social behaviors are refined and the ability to suppress fear increases, both of which are dependent on amygdala activity. We review rodent literature focusing on developmental changes in social and fear behaviors, cortico-amygdala circuits underlying these changes, and how this circuitry is altered by stress. We first describe changes in fear and social behaviors from adolescence to adulthood and parallel developmental changes in cortico-amygdala circuitry. We propose a framework in which maturation of cortical inputs to the amygdala promote changes in social drive and fear regulation, and the particularly damaging effects of stress during adolescence may occur through lasting changes in this circuit. This framework may explain why anxiety and social pathologies commonly co-occur, adolescents are especially vulnerable to stressors impacting social and fear behaviors, and predisposed towards psychiatric disorders related to abnormal cortico-amygdala circuits.
... This general finding suggests that the effects of stress during adolescence may be more readily reversible by extinction. More work is clearly needed on developmental effects of SEFL and on the conditions in which extinction weakens SEFL (Hassien et al. 2020;Rau et al. 2005;Williams et al. 2019). Additionally, we were underpowered to determine if extinction had sexspecific effects on SEFL and thus further work is needed to address this possibility. ...
Rationale
Intensely stressful experiences can lead to long-lasting changes in appetitive and aversive behaviors. In humans, post-traumatic stress disorder increases the risk of comorbid appetitive disorders including addiction and obesity. We have previously shown that an acute stressful experience in adult male rats suppresses motivation for natural reward.
Objectives
We examine the impact of sex and age on the effects of intense stress on action-based (instrumental) and stimulus-based (Pavlovian) motivation for natural reward (food).
Methods
Rats received 15 unsignaled footshocks (stress) in a single session followed by appetitive training and testing in a distinct context. In Experiment 1, stress occurred in either adolescence (PN28) or adulthood (PN70) with appetitive training and testing beginning on PN71 for all rats. In Experiment 2, stress and appetitive training/testing occurred in adolescence.
Results
Acute stress in adolescent females suppressed instrumental motivation assessed with progressive ratio testing when testing occurred in late adolescence or in adulthood, whereas in males stress in adolescence did not suppress instrumental motivation. Acute stress in adulthood did not alter instrumental motivation. In contrast, Pavlovian motivation assessed with single-outcome Pavlovian-to-instrumental transfer (SO-PIT) was consistently enhanced in females following adolescent or adult stress. In males, however, stress in adolescence had no effect, whereas stress in adulthood attenuated SO-PIT.
Conclusions
Acute stress in adolescence or adulthood altered instrumental motivation and stimulus-triggered Pavlovian motivation in a sex and developmentally specific manner. These findings suggest that the persistent effects of acute stress on Pavlovian and instrumental motivational processes differ in females and males, and that males may be less vulnerable to the deleterious effects of intense stress during adolescence on appetitive motivation.
... Conversely, males submitted to the same acute stressor acquired SAA like control animals but demonstrated increased ARs and ITRs during extinction. Finally, consistent with previous reports, we did not observe a sex difference in the effects of SEFL on contextual freezing (Poulos et al., 2015, Hassien et al., 2020Gonzalez et al., 2021). Thus, we can conclude that acute stress facilitated SAA in a sex-dependent manner that is specific to avoidance and is not related to changes in defensive freezing or Pavlovian fear conditioning. ...
Post-traumatic stress disorder (PTSD) is a debilitating disorder characterized by excessive fear, hypervigilance, and avoidance of thoughts, situations or reminders of the trauma. Among these symptoms, relatively little is known about the etiology of pathological avoidance. Here we sought to determine whether acute stress influences avoidant behavior in adult male and female rats. We used a stress procedure (unsignaled footshock) that is known to induce long-term sensitization of fear and potentiate aversive learning. Rats were submitted to the stress procedure and, one week later, underwent two-way signaled active avoidance conditioning (SAA). In this task, rats learn to prevent an aversive outcome (shock) by performing a shuttling response when exposed to a warning signal (tone). We found that acute stress significantly enhanced SAA acquisition rate in females, but not males. Female rats exhibited significantly greater avoidance responding on the first day of training relative to controls, reaching similar levels of performance by the second day. Males that underwent the stress procedure showed similar rates of acquisition to controls but exhibited resistance to extinction. This was manifest as both elevated avoidance and intertrial responding across extinction days relative to non-stressed controls, an effect that was not observed in females. In a second experiment, acute stress sensitized footshock unconditioned responses in males, not females. However, males and females exhibited similar levels of stress-enhanced fear learning (SEFL), which was expressed as sensitized freezing to a shock-paired context. Together, these results reveal that acute stress facilitates SAA performance in both male and female rats, though the nature of this effect is different in the two sexes. We did not observe sex differences in SEFL, suggesting that the stress-induced sex difference in performance was selective for instrumental avoidance. Future work will elucidate the neurobiological mechanisms underlying the differential effect of stress on instrumental avoidance in male and female rats.
... Here, we found an overall decrease in exploration in unknown contexts (i.e., habituation phase of the NOR and avoidance test) in both fear conditioned groups without a notable impact of increased number of footshocks. A lessened exploration in an open field after CFC (or CFC-based protocols) has previously been interpreted as increased fear generalization [40][41][42]. Thus, we interpreted reduced exploration as indicative of fear generalization, especially since in the PTSD group that received the highest number of shocks, the mice that traveled the least in the OF, were also the ones that exhibited the highest freezing response during the first re-exposure to the conditioned context. ...
Posttraumatic stress disorder (PTSD) is a widespread fear-related psychiatric affection associated with fear extinction impairments and important avoidance behaviors. Trauma-related exposure therapy is the current first-hand treatment for PTSD, yet it needs to be improved to shorten the time necessary to reach remission and increase responsiveness. Additional studies to decipher the neurobiological bases of extinction and effects on PTSD-like symptoms could therefore be of use. However, a PTSD-like animal model exhibiting pronounced PTSD-related phenotypes even after an extinction training directly linked to the fearful event is necessary. Thus, using a contextual fear conditioning model of PTSD, we increased the severity of stress during conditioning to search for effects on extinction acquisition and on pre- and post-extinction behaviors. During conditioning, mice received either two or four electrical shocks while a control group was constituted of mice only exposed to the context. Stressed mice exhibited important fear generalization, high fear reaction to the context and selective avoidance of a contextual reminder even after the extinction protocol. Increasing the number of footshocks did not induce major changes on these behaviors.
... Additionally, the present results share some similarities with stress-enhanced fear learning (Poulos et al., 2015;Perusini et al., 2016;Hassien et al., 2020), where prior experience with many inescapable footshocks exacerbates subsequent fear learning. This outcome is likely not the case here as, again, presenting several weak shocks actually reduces conditional freezing. ...
Fear memory formation and retention rely on the activation of distributed neural circuits. The basolateral amygdala (BLA) and ventral hippocampus (VH) in particular are two regions that support contextual fear memory processes and share reciprocal connections. The VH → BLA pathway is critical for increases in fear after initial learning, in both fear renewal following extinction learning and during fear generalization. This raises the possibility that functional changes in VH projections to the BLA support increases in learned fear. In line with this, fear can also be increased with alterations to the original content of the memory via reconsolidation, as in fear elevation procedures. However, very little is known about the functional changes in the VH → BLA pathway supporting reconsolidation-related increases in fear. In this study, we used in vivo extracellular electrophysiology to examine the functional neuronal changes within the BLA and in the VH → BLA pathway as a result of fear elevation and standard fear retrieval procedures. Elevated fear expression was accompanied by higher BLA spontaneous firing compared to a standard fear retrieval condition. Across a range of stimulation frequencies, we also found that VH stimulation evoked higher BLA firing following fear elevation compared to standard retrieval. These results suggest that fear elevation is associated with an increased capacity of the VH to drive neuronal activity in the BLA, highlighting a potential circuit involved in strengthening existing fear memories.
... What is clear is that, comparable baseline levels of generalized fear prior to the SEFL single shock exposure between all groups in the extinction experiment (Fig. 3F) demonstrates that augmented SEFL following chronic stress is not simply the result of new fear compounding with preexisting augmented levels of unextinguished fear. Our findings mirror those of others describing an associative, extinction-sensitive component of SEFL acting on generalized stressor-context fear versus a non-associative, extinction-resistant component (Hassien et al., 2020). It is also noteworthy that complete extinction of stressor context fear had no effect on SEFL within the acute-day-1 stress group (data not shown), which, unlike the acute-day-15 stress group, displayed comparable levels of (panel D, days 16-21), and context test (panel E, day 23). ...
Firm conclusions regarding the differential effects of the maladaptive consequences of acute versus chronic stress on the etiology and symptomatology of stress disorders await a model that isolates chronicity as a variable for studying the differential effects of acute versus chronic stress. This is because most previous studies have confounded chronicity with the total amount of stress. Here, we have modified the stress-enhanced fear learning (SEFL) protocol, which models some aspects of posttraumatic stress disorder (PTSD) following an acute stressor, to create a chronic variant that does not have this confound. Comparing results from this new protocol to the acute protocol, we found that chronic stress further potentiates enhanced fear-learning beyond the nonassociative enhancement induced by acute stress. This additional component is not observed when the unconditional stimulus (US) used during subsequent fear learning is distinct from the US used as the stressor, and is enhanced when glucose is administered following stressor exposure, suggesting that it is associative in nature. Furthermore, extinction of stressor-context fear blocks this additional associative component of SEFL as well as reinstatement of generalized fear, suggesting reinstatement of generalized fear may underlie this additional SEFL component.
... Group differences in fear extinction have been studied before, particularly using models of stress-enhanced fear learning [51][52][53]. In these models, a prior 'traumatic' event results in the enhancement of fear learning and deficits in fear mitigation upon extinction [54]. ...
Fear-related pathologies are among the most prevalent psychiatric conditions, having inappropriate learned fear and resistance to extinction as cardinal features. Exposure therapy represents a promising therapeutic approach, the efficiency of which depends on inter-individual variation in fear extinction learning, which neurobiological basis is unknown. We characterized a model of extinction learning, whereby fear-conditioned mice were categorized as extinction (EXT)-success or EXT-failure, according to their inherent ability to extinguish fear. In the lateral amygdala, GluN2A-containing NMDAR are required for LTP and stabilization of fear memories, while GluN2B-containing NMDAR are required for LTD and fear extinction. EXT-success mice showed attenuated LTP, strong LTD and higher levels of synaptic GluN2B, while EXT-failure mice showed strong LTP, no LTD and higher levels of synaptic GluN2A. Neurotrophin 3 (NT3) infusion in the lateral amygdala was sufficient to rescue extinction deficits in EXT-failure mice. Mechanistically, activation of tropomyosin receptor kinase C (TrkC) with NT3 in EXT-failure slices attenuated lateral amygdala LTP, in a GluN2B-dependent manner. Conversely, blocking endogenous NT3-TrkC signaling with TrkC-Fc chimera in EXT-success slices strengthened lateral amygdala LTP. Our data support a key role for the NT3-TrkC system in inter-individual differences in fear extinction in rodents, through modulation of amygdalar NMDAR composition and synaptic plasticity.
... Legend: ♂ = male; ♀ = female; ↑ = increase; ↓ = reduction, impairment; AA = active avoidance; AFC = auditory fear conditioning; CFC = contextual fear conditioning; DC = discriminative conditioning with reward and auditory fear stimuli; FS = inescapable or unpredictable footshock stress; LFC = fear conditioning using light as conditioned stimulus; ND = not described; RS = restraint stress (immobilization); SPS = multimodal single prolonged stress; STIA = step-through inhibitory avoidance; TS = tail shock stress; US = unconditioned stimulus (shock). Hoffman et al., 2010;Poulos et al., 2015;Sillivan et al., 2017;Lim et al., 2018;Conoscenti and Fanselow, 2019;Briggs and McMullen, 2020;Hassien et al., 2020;Kulp et al., 2020;Gonzalez et al., 2021;Novaes et al., 2021;Przybyl et al., 2021;Torres-Rodriguez et al., 2023) are relatively less frequent but appear consistent. The potential influence of the estrous cycle in stress-enhanced fear learning protocols is still unknown. ...
Fear conditioning and avoidance tasks usually elicit adaptive aversive memories. Traumatic memories are more intense, generalized, inflexible, and resistant to attenuation via extinction- and reconsolidation-based strategies. Inducing and assessing these dysfunctional, maladaptive features in the laboratory are crucial to interrogating posttraumatic stress disorder's neurobiology and exploring innovative treatments. Here we analyze over 350 studies addressing this question in adult rats and mice. There is a growing interest in modeling several qualitative and quantitative memory changes by exposing already stressed animals to freezing- and avoidance-related tests or using a relatively high aversive training magnitude. Other options combine aversive/fearful tasks with post-acquisition or post-retrieval administration of one or more drugs provoking neurochemical or epigenetic alterations reported in the trauma aftermath. It is potentially instructive to integrate these procedures and incorporate the measurement of autonomic and endocrine parameters. Factors to consider when defining the organismic and procedural variables, partially neglected aspects (sex-dependent differences and recent vs. remote data comparison) and suggestions for future research (identifying reliable individual risk and treatment-response predictors) are discussed.
... In the case of post-encounter defense, the context where the single shock occurred reflects the cue that signals an aversive encounter. This finding is consistent with a recent paper by Hassien et al. demonstrating that a prior footshock stressor supports both associative and non-associative fear (Hassien et al., 2020) as well as our SEFL model of fear sensitization (Perusini et al., 2016;Poulos et al., 2015;Rajbhandari et al., 2018). ...
In order to effectively thwart predation, antipredator defensive behaviors must be matched to the current spatio-temporal relationship to the predator. We have proposed a model where different defensive responses are organized along a predatory imminence continuum (PIC). The PIC is a behavior system organized as a sequence of innately programmed behavioral modes, each representing a different interaction with the predator or threat. Ranging from low threat to predator contact, the PIC categorizes defense modes as pre-encounter, post-encounter, and circa-strike, corresponding to states of anxiety, fear, and panic, respectively. This experiment examined if the same significant stressor caused overexpression of all defensive responses along the PIC, including anxiety-like behavior, freezing, and panic-like responses. Female and male mice were exposed to acute stress that consisted of a series of ten pseudorandomly presented unsignaled footshocks (or no shocks). Mice were subsequently tested on a battery of tasks to assess stress effects on pre-encounter (anxiety-like), post-encounter (fear), and circa-strike (panic-like) behaviors. Results revealed that following stress, mice exhibited increased anxiety-like behavior shown through reduced average velocity within a modified open field. Furthermore, stressed mice showed increased fear following a single footshock in a new context as well as an increase in reactivity to white noise in the original stress context, with stressed mice exhibiting a more robust circa-strike-like response than controls. Therefore, significant stress exposure influenced the defensive states of anxiety, fear, and panic across the predatory imminence continuum. This research could therefore reveal how such responses become maladaptive following traumatic stress in humans.
... Exposure therapy involves modifying one's cognitive appraisal of fear by utilizing fear extinction (FE) learning, a form of safety learning that consists of forming a new association between a conditioned stimulus and conditioned response [12,13]. FE in rodents mimics some of the beneficial effects of exposure therapy [14,15]. During FE, rodents learn that a previously conditioned stimulus (e.g., a tone) no longer signals an aversive stimulus (e.g., footshock). ...
Current pharmacotherapies for posttraumatic stress disorder (PTSD) and major depressive disorder (MDD) are ineffective for many patients, and often do not restore cognitive dysfunction associated with these disorders. Behavioral therapies, such as exposure therapy, can be effective for treatment-resistant patients. The mechanisms underlying exposure therapy are not well-understood. Fear extinction as an intervention after chronic stress can model the beneficial effects of exposure therapy in rats. Extinction requires neuronal activity and protein synthesis in the infralimbic (IL) cortex for its beneficial effects. We hypothesized that extinction requires Brain-Derived Neurotrophic Factor (BDNF) activity in the IL cortex to reverse stress-induced cognitive flexibility impairments. Extinction learning reversed set-shifting deficits induced by Chronic Unpredictable Stress (CUS), tested 24 h after extinction. Blocking BDNF signaling in the IL cortex during extinction by local administration of a neutralizing antibody prevented the beneficial effects of extinction on set shifting after stress. Extinction induced activation of the BDNF TrkB receptor, and signaling pathways associated with BDNF (Akt and Erk). Administration of exogenous BDNF into IL cortex in the absence of extinction was sufficient to reverse the effects of stress on set shifting. The effects of extinction were prevented by blocking either Erk or Akt signaling in the IL cortex, whereas the effects of exogenous BDNF were dependent on Erk, but not Akt, signaling. Our observations suggest that BDNF-Erk signaling induced by extinction underlies plastic changes that can reverse or counteract the effects of chronic stress in the IL cortex.
... For infant SEFL, enhanced contextual fear conditioning occurs months after the initial stressful experience and in the absence of memory for the context in which ELS was experienced (Poulos et al. 2014;Quinn et al. 2014). SEFL protocols have been used to model adult or infant trauma in rats (e.g., Rau et al. 2009;Poulos et al. 2014;Quinn et al. 2014) and have been extended to adult mice (Sillivan et al. 2017;Hassien et al. 2020;Pennington et al. 2020). However, to our knowledge, no studies have established the use of acute, infant footshock as a model of ELS in mice.We sought to characterize the effects of acute, infant trauma exposure across several types of learning in adult mice. ...
Early life stress (ELS) experiences can cause changes in cognitive and affective functioning. This study examined the persistent effects of a single traumatic event in infancy on several adult behavioral outcomes in male and female C57BL/6J mice. Mice received 15 footshocks in infancy and were tested for stress-enhanced fear learning, extinction learning, discrimination and reversal learning, and novel object recognition. Infant trauma potentiated fear learning in adulthood and produced resistance to extinction but did not influence other behaviors, suggesting restricted effects of infant trauma on behaviors reliant on cortico-amygdala circuitry.
Patients diagnosed with posttraumatic stress disorder (PTSD) present with a spectrum of debilitating anxiety symptoms resulting from exposure to trauma. Women are twice as likely to be diagnosed with anxiety and PTSD compared to men; however, the reason for this vulnerability remains unknown. We conducted four experiments where we first demonstrated a female vulnerability to stress‐enhanced fear learning (SEFL) with a moderate, acute early life stress (aELS) exposure (4 footshocks in a single session), compared to a more intense aELS exposure (15 footshocks in a single session) where males and females demonstrated comparable SEFL. Next, we demonstrated that this female vulnerability does not result from differences in footshock reactivity or contextual fear conditioning during the aELS exposure. Finally, using gonadectomy or sham surgeries in adult male and female rats, we showed that circulating levels of gonadal steroid hormones at the time of adult fear conditioning do not explain the female vulnerability to SEFL. Additional research is needed to determine whether this vulnerability can be explained by organizational effects of gonadal steroid hormones or differences in sex chromosome gene expression. Doing so is critical for a better understanding of increased female vulnerability to certain psychiatric diseases.
Exposure to acute and chronic stress has significant effects on the basic mechanisms of associative learning and memory. Stress can both impair and enhance associative learning depending on type, intensity, and persistence of the stressor, the subject's sex, the context that the stress and behavior is experienced in, and the type of associative learning taking place. In some cases, stress can cause or exacerbate the maladaptive behavior that underlies numerous psychiatric conditions including anxiety disorders, obsessive-compulsive disorder, post-traumatic stress disorder, substance use disorder, and others. Therefore, it is critical to understand how the varied effects of stress, which may normally facilitate adaptive behavior, can also become maladaptive and even harmful. In this review, we highlight several findings of associative learning and decision-making processes that are affected by stress in both human and non-human subjects and how they are related to one another. An emerging theme from this work is that stress biases behavior towards less flexible strategies that may reflect a cautious insensitivity to changing contingencies. We consider how this inflexibility has been observed in different associative learning procedures and suggest that a goal for the field should be to clarify how factors such as sex and previous experience influence this inflexibility.
In two experiments, we adapted the stress-enhanced fear learning approach to evaluate the persistent effects of acute stress on appetitive learning and motivation in adult male Long Evans rats. In Experiment 1, we found that exposure to a battery of footshocks in one context had no effect on the acquisition, extinction, or contextual renewal of an appetitive Pavlovian discrimination in different contexts. However, when rats were subsequently trained to respond on a progressive ratio instrumental schedule, rats with a history of shock showed lower response rates and progressive ratio break points. Extinction of the shock-associated context had little effect on progressive ratio responding. In Experiment 2, we replicated this instrumental responding deficit with a continuous reinforcement schedule when the Pavlovian phases did not intervene in the time between shock and instrumental testing. Our findings here demonstrate that highly stressful acute experiences produce long-lasting deficits in instrumental motivation for food in male rats.
Flexible calibration of threat responding in accordance with the environment is an adaptive process that allows an animal to avoid harm while also maintaining engagement of other goal-directed actions. This calibration process, referred to as threat response regulation, requires an animal to calculate the probability that a given encounter will result in a threat so they can respond accordingly. Here we review the neural correlates of two highly studied forms of threat response suppression: extinction and safety conditioning. We focus on how relative levels of certainty or uncertainty in the surrounding environment alter the acquisition and application of these processes. We also discuss evidence indicating altered threat response regulation following stress exposure, including enhanced fear conditioning, and disrupted extinction and safety conditioning. To conclude, we discuss research using an animal model of coping that examines the impact of stressor controllability on threat responding, highlighting the potential for previous experiences with control, or other forms of coping, to protect against the effects of future adversity.
Disruption of persistent, stress-associated memories is relevant for treating posttraumatic stress disorder (PTSD) and related syndromes, which develop in a subset of individuals following a traumatic event. We previously developed a stress-enhanced fear learning (SEFL) paradigm in inbred mice that produces PTSD-like characteristics in a subset of mice, including persistently enhanced memory and heightened cFos in the basolateral amygdala complex (BLC) with retrieval of the remote (30-day-old) stress memory. Here, the contribution of BLC microRNAs (miRNAs) to stress-enhanced memory was investigated because of the molecular complexity they achieve through their ability to regulate multiple targets simultaneously. We performed small-RNA sequencing (smRNA-Seq) and quantitative proteomics on BLC tissue collected from mice 1 month after SEFL and identified persistently changed microRNAs, including mir-135b-5p, and proteins associated with PTSD-like heightened fear expression. Viral-mediated overexpression of mir-135b-5p in the BLC of stress-resilient animals enhanced remote fear memory expression and promoted spontaneous renewal 14 days after extinction. Conversely, inhibition of BLC mir-135b-5p in stress-susceptible animals had the opposite effect, promoting a resilient-like phenotype. mir-135b-5p is highly conserved across mammals and was detected in post mortem human amygdala, as well as human serum samples. The mir-135b passenger strand, mir-135b-3p, was significantly elevated in serum from PTSD military veterans, relative to combat-exposed control subjects. Thus, miR-135b-5p may be an important therapeutic target for dampening persistent, stress-enhanced memory and its passenger strand a potential biomarker for responsivity to a mir-135-based therapeutic.
Learned fear often relapses after extinction, suggesting that extinction training generates a new memory that coexists with the original fear memory; however, the mechanisms governing the expression of competing fear and extinction memories remain unclear. We used activity-dependent neural tagging to investigate representations of fear and extinction memories in the dentate gyrus. We demonstrate that extinction training suppresses reactivation of contextual fear engram cells while activating a second ensemble, a putative extinction engram. Optogenetic inhibition of neurons that were active during extinction training increased fear after extinction training, whereas silencing neurons that were active during fear training reduced spontaneous recovery of fear. Optogenetic stimulation of fear acquisition neurons increased fear, while stimulation of extinction neurons suppressed fear and prevented spontaneous recovery. Our results indicate that the hippocampus generates a fear extinction representation and that interactions between hippocampal fear and extinction representations govern the suppression and relapse of fear after extinction.
Even following long periods of abstinence, individuals with anxiety disorders have high rates of relapse to drugs of abuse. Although many current models of relapse demonstrate effects of acute stress on drug-seeking, most of these studies examine stressful experiences that occur in close temporal and physical proximity to the reinstatement test. Here, we assess the effects of a stressful experience in one context on fear and drug-seeking in a different context. We adapt the stress-enhanced fear learning procedure to examine impacts on drug-seeking long after the stressful experience occurred. We find massive footshock in a distinct environment produced an acute increase in corticosterone, long-term hyper-responsivity to a single shock in different contexts with extensive histories of drug-seeking behaviors, enhancements in cocaineinduced conditioned place preference in mice, and persistent enhancements in cue-induced reinstatement of methamphetamine-seeking behavior in rats. Together, these experiments demonstrate that an acute trauma causes persistent changes in responsivity to mild stressors and drug-seeking behavior in other contexts, which mirrors aspects of the comorbidity between post-traumatic stress disorder and substance use disorders. These behavioral approaches provide novel procedures for investigating basic mechanisms underlying this comorbidity and they provide powerful tools for testing preclinical pharmacological and behavioral interventions.
There is growing interest in generalization of learned contextual fear, driven in part by the hypothesis that mood and anxiety disorders stem from impaired hippocampal mechanisms of fear generalization and discrimination. However, there has been relatively little investigation of the behavioral and procedural mechanisms that might control generalization of contextual fear. We assessed the relative contribution of different contextual features to context fear generalization and characterized how two common conditioning protocols-foreground (uncued) and background (cued) contextual fear conditioning-affected context fear generalization. In one experiment, mice were fear conditioned in context A, and then tested for contextual fear both in A and in an alternate context created by changing a subset of A's elements. The results suggest that floor configuration and odor are more salient features than chamber shape. A second experiment compared context fear generalization in background and foreground context conditioning. Although foreground conditioning produced more context fear than background conditioning, the two procedures produced equal amounts of generalized fear. Finally, results indicated that the order of context tests (original first versus alternate first) significantly modulates context fear generalization, perhaps because the original and alternate contexts are differentially sensitive to extinction. Overall, results demonstrate that context fear generalization is sensitive to procedural variations and likely reflects the operation of multiple interacting psychological and neural mechanisms.
Stress-enhanced fear learning (SEFL) refers to the long-lasting nonassociative sensitization produced by intense stress (e.g., repeated and unpredictable footshock) that results in increased fear learning to a mild conditioning regimen (e.g., one shock). SEFL experiments suggest that one component of posttraumatic behavior is inappropriately strong fear conditioning occurring to relatively mild stressors. Past reports of SEFL have used the same intensity (1 mA) of footshock to cause both the sensitization and conditioning of new fear. SEFL would be a particularly problematic component of posttrauma behavior if intense stress results in substantial fear conditioning under conditions that would not normally support conditioning. Therefore, we determined if SEFL occurred when the conditioning shock was substantially milder than the SEFL-inducing shock. The results indicate that exposure to a sensitizing regimen of shock can convert a mild footshock that normally does not support measurable levels of fear conditioning into one that causes substantial learned fear. Moreover, as the intensity of single footshock increases, so does the capacity of the prior stressor to contribute to the sensitization of fear responses. Consistent with prior studies, males acquired and retained a greater level of fear conditioning than female rats, however the level of sensitization did not differ between sexes. (PsycINFO Database Record (c) 2015 APA, all rights reserved).
Women have a greater risk of developing posttraumatic stress disorder (PTSD) after exposure to trauma. Although sleep abnormalities have been implicated in the development of PTSD, gender differences in sleep soon after a traumatic event have not been investigated. This secondary analysis examined sleep characteristics using polysomnography in 13 female and 22 male trauma patients within a month of their traumatic injuries and assessed PTSD symptoms at 2-months post-injury. Results revealed more wake after sleep onset in women who developed PTSD compared to men who developed PTSD. Women with subsequent PTSD also had less total sleep time than women without subsequent PTSD. Findings suggest possible contributions of impaired sleep maintenance to the development of PTSD in women.
Traumatic fear memories can be inhibited by behavioral therapy for humans, or by extinction training in rodent models, but
are prone to recur. Under some conditions, however, these treatments generate a permanent effect on behavior, which suggests
that emotional memory erasure has occurred. The neural basis for such disparate outcomes is unknown. We found that a central
component of extinction-induced erasure is the synaptic removal of calcium-permeable α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate
receptors (AMPARs) in the lateral amygdala. A transient up-regulation of this form of plasticity, which involves phosphorylation
of the glutamate receptor 1 subunit of the AMPA receptor, defines a temporal window in which fear memory can be degraded by
behavioral experience. These results reveal a molecular mechanism for fear erasure and the relative instability of recent
memory.
Meta-analyses of studies yielding sex-specific risk of potentially traumatic events (PTEs) and posttraumatic stress disorder (PTSD) indicated that female participants were more likely than male participants to meet criteria for PTSD, although they were less likely to experience PTEs. Female participants were more likely than male participants to experience sexual assault and child sexual abuse, but less likely to experience accidents, nonsexual assaults, witnessing death or injury, disaster or fire, and combat or war. Among victims of specific PTEs (excluding sexual assault or abuse), female participants exhibited greater PTSD. Thus, sex differences in risk of exposure to particular types of PTE can only partially account for the differential PTSD risk in male and female participants.
The AMPA receptor subunit glutamate receptor 1 (GluR1 or GluR-A) contributes to amygdala-dependent emotional learning. It remains unclear, however, to what extent different amygdala pathways depend on GluR1, or other AMPA receptor subunits, for proper synaptic transmission and plasticity, and whether GluR1-dependent long-term potentiation (LTP) is necessary for auditory and contextual fear conditioning. Here, we dissected the role of GluR1 and GluR3 (GluR-C) subunits in AMPA receptor-dependent amygdala LTP and fear conditioning using knock-out mice (GluR1-/- and GluR3-/-). We found that, whereas LTP at thalamic inputs to lateral amygdala (LA) projection neurons and at glutamatergic synapses in the basal amygdala was completely absent in GluR1-/- mice, both GluR1 and GluR3 contributed to LTP in the cortico-LA pathway. Because both auditory and contextual fear conditioning were selectively impaired in GluR1-/- but not GluR3-/- mice, we conclude that GluR1-dependent synaptic plasticity is the dominant form of LTP underlying the acquisition of auditory and contextual fear conditioning, and that plasticity in distinct amygdala pathways differentially contributes to aversive conditioning.
While stressful life events are an important cause of psychopathology, most individuals exposed to adversity maintain normal psychological functioning. The molecular mechanisms underlying such resilience are poorly understood. Here, we demonstrate that an inbred population of mice subjected to social defeat can be separated into susceptible and unsusceptible subpopulations that differ along several behavioral and physiological domains. By a combination of molecular and electrophysiological techniques, we identify signature adaptations within the mesolimbic dopamine circuit that are uniquely associated with vulnerability or insusceptibility. We show that molecular recapitulations of three prototypical adaptations associated with the unsusceptible phenotype are each sufficient to promote resistant behavior. Our results validate a multidisciplinary approach to examine the neurobiological mechanisms of variations in stress resistance, and illustrate the importance of plasticity within the brain's reward circuits in actively maintaining an emotional homeostasis.
In contextual fear conditioning, footshock is given in a context, and re-exposure to this context elicits the conditional defensive response of freezing, a reliable behavioral index of conditional fear. Normally, the amount of contextual freezing is directly proportional to the number of shocks an animal receives in the context. However, pre-exposure to a stressor can produce an enhancement in conditional freezing. Pre-exposure to repeated footshock in one context produces an enhancement of conditional freezing to cues associated with a single shock in a second distinct context. This model of stress-enhanced fear learning (SEFL) can be utilized to study how stress affects learning of future aversive events. The experiments in this paper characterize the magnitude and longevity of SEFL. In the first experiment, the number of footshocks given during the pre-exposure session was varied and conditional fear to the single shock was assessed. Pre-exposure to 1 shock did not produce an enhancement in fear learning in the second context, but pre-exposure to 4 or 15 shocks did. The time-course of the enhancement was examined in the next two experiments. These experiments show that SEFL persists for at least 3 months.
There are sex differences in the prevalence and presentation of many psychiatric disorders. For example, posttraumatic stress disorder (PTSD) and major depression are more common in women than men, and women with these disorders present with more hyperarousal symptoms than men. In contrast, attention deficit hyperactivity (ADHD) and schizophrenia are more common in men than women, and men with these disorders have increased cognitive deficits compared to women. A shared feature of the aforementioned psychiatric disorders is the contribution of stressful events to their onset and/or severity. Here we propose that sex differences in stress responses bias females towards hyperarousal and males towards cognitive deficits. Evidence from clinical and preclinical studies is detailed. We also describe underlying neurobiological mechanisms. For example, sex differences in stress receptor signaling and trafficking in the locus coeruleus-arousal center are detailed. In learning circuits, evidence for sex differences in dendritic morphology is provided. Finally, we describe how evaluating sex-specific mechanisms for responding to stress in female and male rodents can lead to better treatments for stress-related psychiatric disorders.
Background
The limited neurobiological understanding of PTSD has been partially attributed to the need for improved animal models. Stress-enhanced fear learning (SEFL) in rodents recapitulates many PTSD-associated behaviors, including stress-susceptible (SS) and –resilient (SR) subgroups in outbred rats. Identification of subgroups requires additional behavioral phenotyping, a confound to mechanistic studies.
Methods
We employed a SEFL paradigm in inbred male and female C57BL/6 that combines acute stress with fear conditioning to precipitate “traumatic” memories. Extinction and long-term retention of extinction were examined after SEFL. Further characterization of SEFL effects on male mice was performed with additional behavioral tests, determination of regional activation by Fos immunofluorescence and RNA-sequencing of the basolateral amygdala (BLA).
Results
Stressed animals displayed persistently elevated freezing during extinction. While more uniform in females, SEFL produced male subgroups with differential susceptibility that were identified without post-training phenotyping. Additional phenotyping of males revealed PTSD-associated behaviors, including extinction-resistant fear memory, hyperarousal, generalization and dysregulated corticosterone in SS males. Altered Fos activation was also seen in the infralimbic cortex and BLA of SS males after remote memory retrieval. Key behavioral outcomes, including susceptibility, were replicated by two independent laboratories. RNA-sequencing of the BLA revealed transcriptional divergence between the male subgroups, including genes with reported polymorphic association to PTSD patients.
Conclusions
This SEFL model provides a tool for development of PTSD therapeutics that is compatible with the growing number of mouse-specific resources. Furthermore, use of an inbred strain allows for investigation into epigenetic mechanisms that are expected to critically regulate susceptibility and resilience.
Fear promotes adaptive responses to threats. However, when the level of fear is not proportional to the level of threat, maladaptive fear-related behaviors characteristic of anxiety disorders result. Post-traumatic stress disorder develops in response to a traumatic event, and patients often show sensitized reactions to mild stressors associated with the trauma. Stress-enhanced fear learning (SEFL) is a rodent model of this sensitized responding, in which exposure to a 15-shock stressor nonassociatively enhances subsequent fear conditioning training with only a single trial. We examined the role of corticosterone (CORT) in SEFL. Administration of the CORT synthesis blocker metyrapone prior to the stressor, but not at time points after, attenuated SEFL. Moreover, CORT co-administered with metyrapone rescued SEFL. However, CORT alone without the stressor was not sufficient to produce SEFL. In these same animals, we then looked for correlates of SEFL in terms of changes in excitatory receptor expression. Western blot analysis of the basolateral amygdala (BLA) revealed an increase in the GluA1 AMPA receptor subunit that correlated with SEFL. Thus, CORT is permissive to trauma-induced changes in BLA function.Neuropsychopharmacology advance online publication, 2 September 2015; doi:10.1038/npp.2015.224.
Unlabelled:
Adult hippocampal neurogenesis is believed to support hippocampus-dependent learning and emotional regulation. These putative functions of adult neurogenesis have typically been studied in isolation, and little is known about how they interact to produce adaptive behavior. We used trace fear conditioning as a model system to elucidate mechanisms through which adult hippocampal neurogenesis modulates processing of aversive experience. To achieve a specific ablation of neurogenesis, we generated transgenic mice that express herpes simplex virus thymidine kinase specifically in neural progenitors and immature neurons. Intracerebroventricular injection of the prodrug ganciclovir caused a robust suppression of neurogenesis without suppressing gliogenesis. Neurogenesis ablation via this method or targeted x-irradiation caused an increase in context conditioning in trace but not delay fear conditioning. Data suggest that this phenotype represents opposing effects of neurogenesis ablation on associative and nonassociative components of fear learning. Arrest of neurogenesis sensitizes mice to nonassociative effects of fear conditioning, as evidenced by increased anxiety-like behavior in the open field after (but not in the absence of) fear conditioning. In addition, arrest of neurogenesis impairs associative trace conditioning, but this impairment can be masked by nonassociative fear. The results suggest that adult neurogenesis modulates emotional learning via two distinct but opposing mechanisms: it supports associative trace conditioning while also buffering against the generalized fear and anxiety caused by fear conditioning.
Significance statement:
The role of adult hippocampal neurogenesis in fear learning is controversial, with some studies suggesting neurogenesis is needed for aspects of fear learning and others suggesting it is dispensable. We generated transgenic mice in which neural progenitors can be selectively and inducibly ablated. Our data suggest that adult neurogenesis supports fear learning through two distinct mechanisms: it supports the ability to learn associations between traumatic events (unconditioned stimuli) and predictors (conditioned stimuli) while also buffering against nonassociative, anxiogenic effects of a traumatic experience. As a result, arrest of neurogenesis can enhance or impair learned fear depending on intensity of the traumatic experience and the extent to which it recruits associative versus nonassociative learning.
Traumatic experience can result in life-long changes in the ability to cope with future stressors and emotionally salient events. These experiences, particularly during early development, are a significant risk factor for later life anxiety disorders such as posttraumatic stress disorder (PTSD). However, because traumatic experience typically results in strong episodic memories, it is not known whether such long-term memories are necessary for particular features of PTSD, such as enhanced fear and anxiety. Here, we used a fear conditioning procedure in juvenile rats before maturation of the neural systems supporting declarative memory to assess the necessity of early memory to the later life development of PTSD-related symptoms.
Nineteen-day old rats were exposed to unpredictable and inescapable footshocks, and fear memory for the shock context was assessed during adulthood. Thereafter, adult animals were either exposed to single-trial fear conditioning or elevated plus maze or sacrificed for basal diurnal corticosterone and quantification of neuronal glucocorticoid and neuropeptide Y receptors.
Early trauma exposed rats displayed stereotypic footshock reactivity, yet by adulthood, hippocampus-dependent contextual fear-related memory was absent. However, adult rats showed sensitized fear learning, aberrant basal circadian fluctuations of corticosterone, increased amygdalar glucocorticoid receptors, decreased time spent in the open arm of an elevated plus maze, and an odor aversion associated with early-life footshocks.
These results suggest that traumatic experience during developmental periods of hippocampal immaturity can promote lifelong changes in symptoms and neuropathology associated with human PTSD, even if there is no explicit memory of the early trauma.
Major depression is a complex psychiatric disorder characterized by affective, cognitive, and physiological impairments that lead to maladaptive behavior. The high lifetime prevalence of this disabling condition, coupled with limitations in existing medications, make necessary the development of improved therapeutics. This requires animal models that allow investigation of key biological correlates of the disorder. Described in this unit is the unpredictable chronic mild stress mouse model that is used to screen for antidepressant drug candidates. Originally designed for rats, this model has been adapted for mice to capitalize on the advantages of this species as an experimental model, including inter-strain variability, which permits an exploration of the contribution of genetic background, the ability to create transgenic animals, and lower cost. Thus, by combining genetic features and socio-environmental chronic stressful events, the unpredictable, chronic mild stress model in mice can be used to study the etiological and developmental components of major depression, and to identify novel treatments for this condition. Curr. Protoc. Pharmacol. 61:5.65.1-5.65.17 © 2013 by John Wiley & Sons, Inc.
Previous studies showed that the mineralocorticoid receptor (MR) is needed for behavioral flexibility in a fear conditioning paradigm. Female mice with forebrain-specific deletion of the MR gene (MR(CaMKCre) ) were unable to show extinction of contextual fear, and could not discriminate between cue and context fear unlike control mice. In the present study, male and female (MR(CaMKCre) ) mice and control littermates were used to study sex-specific fear conditioning, memory performance and extinction. The fear conditioning paradigm assessed both context- and cue-related fear within one experimental procedure. We observed that at the end of the conditioning all mice acquired the fear-motivated response. During the first minutes of the memory test, both male and female MR(CaMKCre) mice remembered and feared the context more than the control mice. Furthermore, female MR(CaMKCre) mice were not able to extinguish this memory even on the second day of memory testing. The female mutants also could not discriminate between cue (more freezing) and context periods (less freezing). In contrast, male MR(CaMKCre) mice and the controls showed extinction and were capable to discriminate, although the MR(CaMKCre) mice needed more time before they started extinction. These findings further support the relevance of MR for behavioral flexibility and extinction of fear-motivated behavior. In conclusion, the loss of MR in the forebrain results in large differences in emotional and cognitive behaviors between female and male mice, which suggests a role of this receptor in the female prevalence of stress- and anxiety-regulated disorders.
A common approach to the clinical treatment of Posttraumatic Stress Disorder (PTSD) has focused on the facilitation of fear extinction through cognitive behavioural therapy that involves both safe exposure to the trauma-related cues and subsequent changes in conditioned stimulus–unconditioned stimulus (CS–US) contingency expectations. PTSD symptoms can be tracked back to pathologically modified associative fear, hyperarousal and a time-dependent fear generalization. We have used a mouse model of PTSD that is based on a brief exposure to an inescapable foot shock in order to investigate the influence of early (starting 1 day after the shock) and late (starting 1 month after the shock) extinction training. Both early and late extinction training led to a long-lasting reduction of contextual and generalized fear, but only early extinction caused an amelioration of hyperarousal. Consequently, our results suggest early post-shock intervention as a successful strategy for reducing hyperarousal in the aftermath of a trauma.
This is a review of the relevant empirical literature concerning the DSM-IV-TR diagnostic criteria for PTSD. Most of this work has focused on Criteria A1 and A2, the two components of the A (Stressor) Criterion. With regard to A1, the review considers: (a) whether A1 is etiologically or temporally related to the PTSD symptoms; (b) whether it is possible to distinguish "traumatic" from "non-traumatic" stressors; and (c) whether A1 should be eliminated from DSM-5. Empirical literature regarding the utility of the A2 criterion indicates that there is little support for keeping the A2 criterion in DSM-5. The B (reexperiencing), C (avoidance/numbing) and D (hyperarousal) criteria are also reviewed. Confirmatory factor analyses suggest that the latent structure of PTSD appears to consist of four distinct symptom clusters rather than the three-cluster structure found in DSM-IV. It has also been shown that in addition to the fear-based symptoms emphasized in DSM-IV, traumatic exposure is also followed by dysphoric, anhedonic symptoms, aggressive/externalizing symptoms, guilt/shame symptoms, dissociative symptoms, and negative appraisals about oneself and the world. A new set of diagnostic criteria is proposed for DSM-5 that: (a) attempts to sharpen the A1 criterion; (b) eliminates the A2 criterion; (c) proposes four rather than three symptom clusters; and (d) expands the scope of the B-E criteria beyond a fear-based context. The final sections of this review consider: (a) partial/subsyndromal PTSD; (b) disorders of extreme stress not otherwise specified (DESNOS)/complex PTSD; (c) cross- cultural factors; (d) developmental factors; and (e) subtypes of PTSD.
Freezing to a tone following auditory fear conditioning is commonly considered as a measure of the strength of the tone-shock association. The decrease in freezing on repeated nonreinforced tone presentation following conditioning, in turn, is attributed to the formation of an inhibitory association between tone and shock that leads to a suppression of the expression of fear. This study challenges these concepts for auditory fear conditioning in mice. We show that acquisition of conditioned fear by a few tone-shock pairings is accompanied by a nonassociative sensitization process. As a consequence, the freezing response of conditioned mice seems to be determined by both associative and nonassociative memory components. Our data suggest that the intensity of freezing as a function of footshock intensity is primarily determined by the nonassociative component, whereas the associative component is more or less categorical. We next demonstrate that the decrease in freezing on repeated nonreinforced tone presentation following conditioning shows fundamental properties of habituation. Thus, it might be regarded as a habituation-like process, which abolishes the influence of sensitization on the freezing response to the tone without affecting the expression of the associative memory component. Taken together, this study merges the dual-process theory of habituation with the concept of classical fear conditioning and demonstrates that sensitization and habituation as two nonassociative learning processes may critically determine the expression of conditioned fear in mice.
The ability to change behavior likely depends on the selective strengthening and weakening of brain synapses. The cellular models of synaptic plasticity, long-term potentiation (LTP) and depression (LTD) of synaptic strength, can be expressed by the synaptic insertion or removal of AMPA receptors (AMPARs), respectively. We here present an overview of studies that have used animal models to show that such AMPAR trafficking underlies several experience-driven phenomena-from neuronal circuit formation to the modification of behavior. We argue that monitoring and manipulating synaptic AMPAR trafficking represents an attractive means to study cognitive function and dysfunction in animal models.
Spontaneous recovery from extinction is one of the most basic phenomena of Pavlovian conditioning. Although it can be studied by using a variety of designs, some procedures are better than others for identifying the involvement of underlying learning processes. A wide range of different learning mechanisms has been suggested as being engaged by extinction, most of which have implications for the nature of spontaneous recovery. However, despite the centrality of the notion of spontaneous recovery to the understanding of extinction, the empirical literature on its determinants is relatively sparse and quite mixed. Its very ubiquity suggests that spontaneous recovery has multiple sources.
The authors present a multidimensional meta-analysis of studies published between 1980 and 2003 on psychotherapy for PTSD.
Data on variables not previously meta-analyzed such as inclusion and exclusion criteria and rates, recovery and improvement rates, and follow-up data were examined.
Results suggest that psychotherapy for PTSD leads to a large initial improvement from baseline. More than half of patients who complete treatment with various forms of cognitive behavior therapy or eye movement desensitization and reprocessing improve. Reporting of metrics other than effect size provides a somewhat more nuanced account of outcome and generalizability.
The majority of patients treated with psychotherapy for PTSD in randomized trials recover or improve, rendering these approaches some of the most effective psychosocial treatments devised to date. Several caveats, however, are important in applying these findings to patients treated in the community. Exclusion criteria and failure to address polysymptomatic presentations render generalizability to the population of PTSD patients indeterminate. The majority of patients posttreatment continue to have substantial residual symptoms, and follow-up data beyond very brief intervals have been largely absent. Future research intended to generalize to patients in practice should avoid exclusion criteria other than those a sensible clinician would impose in practice (e.g., schizophrenia), should avoid wait-list and other relatively inert control conditions, and should follow patients through at least 2 years.
Fear is an adaptive response that initiates defensive behavior to protect animals and humans from danger. However, anxiety disorders, such as Posttraumatic Stress Disorder (PTSD), can occur when fear is inappropriately regulated. Fear conditioning can be used to study aspects of PTSD, and we have developed a model in which pre-exposure to a stressor of repeated footshock enhances conditional fear responding to a single context-shock pairing. The experiments in this chapter address interpretations of this effect including generalization and summation or fear, inflation, and altered pain sensitivity. The results of these experiments lead to the conclusion that pre-exposure to shock sensitizes conditional fear responding to similar less intense stressors. This sensitization effect resists exposure therapy (extinction) and amnestic (NMDA antagonist) treatment. The pattern predicts why in PTSD patients, mild stressors cause reactions more appropriate for the original traumatic stressor and why new fears are so readily formed in these patients. This model can facilitate the study of neurobiological mechanisms underlying sensitization of responses observed in PTSD.
We describe five cases of traumatic asphyxiation injury, each meeting diagnostic criteria for posttraumatic stress disorder (PTSD) and characterized by a range of postinjury cognitive impairment. Four patients exhibited dense retrograde amnesia, including absence of conscious memory for the traumatic event. Appreciation of these asphyxiation cases, which involve temporally extended trauma exposure, may help resolve arguments regarding the possibility of co-occurrence of PTSD and neurological amnesia based exclusively on observations of much briefer duration events (specifically, motor vehicle crashes). These five cases also provide evidence that cognitive symptoms of PTSD can develop in the absence of conscious memory for the event.
The widely accepted stress-diathesis hypothesis of depression postulates that genetic factors contribute to biological vulnerability. Based on this concept, the unpredictable chronic mild stress (UCMS) animal model was developed. Most effects of UCMS can be reversed by antidepressant agents, illustrating a strong predictive validity. In rodents, UCMS also has good face validity as it can elicit depression-like symptoms. While abundant for rats, the UCMS literature on mice is relatively limited. Reports sometimes are contradictory, making it difficult to establish a clear profile of stress-induced depression-like behaviors in mice. As different groups often use different strains for their experiments, differential strain susceptibility to UCMS may provide at least a partial explanation of these discrepancies. Moreover, differences in testing methodology add another level of complexity. Very little is known about the role of genetic factors and their interactions with the environment in the development of stress-induced behavioral changes relevant to depression, though recent studies unequivocally demonstrated the effects of specific gene polymorphisms on stress-induced depressive symptoms, as well as the effects of stress on gene expression. In the present study, we investigated the effects of UCMS on a battery of different tests measuring anxiety and depression-like behaviors in three behaviorally and genetically distinct inbred strains. The goals of these experiments are to obtain a clearer behavioral profile of genetically/phenotypically distant mouse strains after UCMS treatment and to evaluate the limitations and strengths of the UCMS model in mice.
The pathomechanisms of posttraumatic stress disorder (PTSD) are still unknown, but both fear conditioning and stress sensitisation are supposed to play a crucial role. Hence, valid animal models that model both associative and non-associative components of fear will facilitate elucidation of the biological substrates of the illness, and to develop novel and specific approaches for its prevention and therapy. Here we applied a single electric footshock to C57BL/6N (B6N) and C57BL/6JOla (B6JOla) mice and recorded the conditioned response to contextual trauma reminders (associative fear), the sensitised reaction to a neutral tone in a novel environment (non-associative fear, hyperarousal), social interaction and various emotional behaviours using Modified Holeboard, Test for Novelty-Induced Suppression of Feeding and Forced Swimming Test, after different incubation times (1, 14, 28 days). Freezing generally increased as a function of shock intensity. In B6N mice, sensitised fear was maximal 28 days after trauma and was accompanied by signs of emotional blunting and social withdrawal. B6JOla mice, in contrast, were less susceptible to develop PTSD-like symptoms. The phenotype of B6N exhibited high behavioural variance, allowing distinction between vulnerable and resilient individuals. Only in vulnerable B6N mice, chronic fluoxetine treatment - initiated after an incubation period of 28 days - ameliorated sensitised fear. This new mouse model fulfils common criteria for face and predictive validity and can be used to investigate the biological correlates of individual fear susceptibility, as well as the impact and interrelationship of associative and non-associative fear components in the development and maintenance of PTSD.
Both classical fear conditioning and fear sensitization have been implicated in the development of Posttraumatic Stress Disorder (PTSD), but little is known about their interaction or interdependency. Therefore, we administered the NMDA receptor antagonist AP5 or vehicle bilaterally into the dorsal hippocampus of C57BL/6N mice 15 min before administration of an electric footshock. One month later mice were tested for their conditioned fear reaction to the shock context (associative fear memory) and for their sensitized fear response to a neutral tone in a new context (hyperarousal). AP5-treated animals exhibited only half as much conditioned, but equal amounts of sensitized fear, compared to vehicle-treated mice, demonstrating that hyperarousal does not depend on associative fear memory about the aversive encounter, but solely on sensitization induced by the inescapable footshock. This result points to the need for a 'desensitization' approach in PTSD therapy, as sensitization and classical conditioning seem to be independent processes in this psychiatric disorder.
- M J Friedman
- P A Resick
- R A Bryant
- C R Brewin
M.J. Friedman, P.A. Resick, R.A. Bryant, C.R. Brewin, Considering PTSD for DSM-5,
Depress. Anxiety 28 (9) (2010) 750-769.