Single-Prolonged Stress: A Review of Two Decades of Progress in a Rodent Model of Post-traumatic Stress Disorder

Brain-wide mapping of c-fos expression in the single prolonged stress model and the effects of pretreatment with ACH-000029 or prazosin

Post-traumatic stress disorder (PTSD) is a mental health condition that is triggered by a stressful event, with symptoms including exaggerated startle response, intrusive traumatic memories and nightmares. The single prolonged stress (SPS) is a multimodal stress protocol that comprises a sequential exposure to physical restraint, forced swimming, predator scent and ether anesthesia. This procedure generates behavioral and neurobiological alterations that resemble clinical findings of PTSD, and thus it is commonly used to model the disease in rodents. Here, we applied c-fos mapping to produce a comprehensive view of stress-activated brain regions in mice exposed to SPS alone or to SPS after oral pretreatment with the serotonin-noradrenaline receptor dual modulator ACH-000029 or the α1-adrenergic blocker prazosin. The SPS protocol evoked c-fos expression in several brain regions that control the stress-anxiety response, including the central and medial amygdala, the bed nucleus of the stria terminalis, the pallidum, the paraventricular hypothalamus, the intermediodorsal, paraventricular and central medial thalamic nuclei, the periaqueductal gray, the lateral habenula and the cuneiform nucleus. These effects were partially blocked by pretreatment with prazosin but completely prevented by ACH-000029. Collectively, these findings contribute to the brain-wide characterization of neural circuits involved in PTSD-related stress responses. Furthermore, the identification of brain areas regulated by ACH-000029 and prazosin revealed regions in which SPS-induced activation may depend on the combined or isolated action of the noradrenergic and serotonergic systems. Finally, the dual regulation of serotonin and α1 receptors by ACH-000029 might represent a potential pharmacotherapy that can be applied in the peri-trauma or early post-trauma period to mitigate the development of symptoms in PTSD patients.

Rodent models of post-traumatic stress disorder: behavioral assessment

Although the etiology and expression of psychiatric disorders are complex, mammals show biologically preserved behavioral and neurobiological responses to valent stimuli which underlie the use of rodent models of post-traumatic stress disorder (PTSD). PTSD is a complex phenotype that is difficult to model in rodents because it is diagnosed by patient interview and influenced by both environmental and genetic factors. However, given that PTSD results from traumatic experiences, rodent models can simulate stress induction and disorder development. By manipulating stress type, intensity, duration, and frequency, preclinical models reflect core PTSD phenotypes, measured through various behavioral assays. Paradigms precipitate the disorder by applying physical, social, and psychological stressors individually or in combination. This review discusses the methods used to trigger and evaluate PTSD-like phenotypes. It highlights studies employing each stress model and evaluates their translational efficacies against DSM-5, validity criteria, and criteria proposed by Yehuda and Antelman's commentary in 1993. This is intended to aid in paradigm selection by informing readers about rodent models, their benefits to the clinical community, challenges associated with the translational models, and opportunities for future work. To inform PTSD model validity and relevance to human psychopathology, we propose that models incorporate behavioral test batteries, individual differences, sex differences, strain and stock differences, early life stress effects, biomarkers, stringent success criteria for drug development, Research Domain Criteria, technological advances, and cross-species comparisons. We conclude that, despite the challenges, animal studies will be pivotal to advances in understanding PTSD and the neurobiology of stress.

Forget the stress: retrograde amnesia for the stress-induced impairment of extinction retrieval

We investigated whether cycloheximide (CHX) would induce amnesia for the stress-induced impairment of extinction retrieval. First, a single restraint stress session was demonstrated to impair extinction retrieval, but not fear conditioning. A second experiment showed that when CHX was administered immediately after restraint, rats exhibited significant extinction retrieval at test (i.e., retrograde amnesia for the stress). In a third experiment, the stress session impaired various amounts of extinction durations, suggesting that the stress inhibited extinction retrieval rather than enhancing the original fear learning. These results suggest memories for acute stress are susceptible to disruption, which could have clinical implications.

Oxytocin receptor antagonist reverses the blunting effect of pair bonding on fear learning in monogamous prairie voles

Social relationships among spouses, family members, and friends are known to affect physical and mental health. In particular, long-lasting bonds between socio-sexual partners have profound effects on cognitive, social, emotional, and physical well-being. We have previously reported that pair bonding in monogamous prairie voles (Microtus ochrogaster) is prevented by a single prolonged stress (SPS) paradigm, which causes behavioral and endocrine symptoms resembling post-traumatic stress disorder (PTSD) patients in rats (Arai et al., 2016). Since fear memory function is crucial for anxiety-related disorders such as PTSD, we investigated the effects of pair bonding on fear learning in prairie voles. We applied an SPS paradigm to male prairie voles after the cohabitation with a male (cage-mate group) or female (pair-bonded group). The cage-mate group, but not the pair-bonded group, showed enhanced fear response in a contextual fear conditioning test following the SPS treatment. Immunohistochemical analyses revealed that cFos-positive cells in the central amygdala were increased in the pair-bonded group after the contextual fear conditioning test and that oxytocin immunoreactivity in the paraventricular nucleus of the hypothalamus was significantly higher in the pair-bonded group than the cage-mate group. This pair-bonding dependent blunting of fear memory response was confirmed by a passive avoidance test, another fear-based learning test. Interestingly, intracerebroventricular injection of an oxytocin receptor antagonist 30 min before the passive avoidance test blocked the blunting effect of pair bonding on fear learning. Thus, pair bonding between socio-sexual partners results in social buffering in the absence of the partner, blunting fear learning, which may be mediated by oxytocin signaling.

High Behavioral Sensitivity to Carbon Dioxide Associates with Enhanced Fear Memory and Altered Forebrain Neuronal Activation

There is considerable interest in pre-trauma individual differences that may contribute to increased risk for developing post-traumatic stress disorder (PTSD). Identification of underlying vulnerability factors that predict differential responses to traumatic experiences is important. Recently, the relevance of homeostatic perturbations in shaping long-term behavior has been recognized. Sensitivity to CO₂ inhalation, a homeostatic threat to survival, was shown to associate with the later development of PTSD symptoms in veterans. Here, we investigated whether behavioral sensitivity to CO₂ associates with PTSD-relevant behaviors and alters forebrain fear circuitry in mice. Mice were exposed to 5% CO₂ or air inhalation and tested one week later on acoustic startle and footshock contextual fear conditioning, extinction and reinstatement. CO₂ inhalation evoked heterogenous freezing behaviors (high freezing CO₂-H and low freezing CO₂-L) that significantly associated with fear conditioning and extinction behaviors. CO₂-H mice elicited potentiated conditioned fear and delayed extinction while behavioral responses in CO₂-L mice were similar to the air group. Persistent neuronal activation marker ΔFosB immunostaining revealed altered regional neuronal activation within the hippocampus, amygdala and medial pre-frontal cortex that correlated with conditioned fear and extinction. Inter-regional co-activation mapping revealed disruptions in the coordinated activity of hippocampal dentate-amygdala-infralimbic regions and infralimbic-prelimbic associations in CO₂-H mice that may explain their enhanced fear phenotype. In conclusion, our data support an association of behavioral sensitivity to interoceptive threats such as CO₂ with altered fear responding to exteroceptive threats and suggest that "CO₂-sensitive" individuals may be susceptible to developing PTSD.

Stathmin Regulates Spatiotemporal Variation in the Memory Loop in Single-Prolonged Stress Rats

Posttraumatic stress disorder (PTSD) is closely related to brain structures of the memory loop such as the hippocampus, amygdala, and medial prefrontal cortex (mPFC). The fear gene stathmin plays an important role in regulating fear memory. However, whether the fear gene stathmin is related to fear memory loop anomalies caused by PTSD is unclear. A single-prolonged stress (SPS) rat model of PTSD was constructed. Wistar rats were randomly divided into 5 groups: the control group, SPS 1-day group, SPS 4-day group, SPS 7-day group, and SPS 14-day group. Then, we measured the protein and mRNA expression of stathmin, p-stathmin (Ser16, Ser25, Ser38, and Ser63), β-tubulin, and MAP-1B in the hippocampus, amygdala, and mPFC in the 5 groups by immunohistochemistry, Western blotting, and qRT-PCR. The expression of the stathmin protein in the hippocampus, mPFC, and amygdala of the rat memory loop decreased gradually in the SPS 1-day group, the SPS 4-day group, and the SPS 7-day group, in which it was the lowest, and then increased. The trend of the expression of stathmin mRNA in the three areas of the memory loop was consistent with the trend of the expression of the stathmin protein. The trend of the protein expression of p-stathmin (Ser25 and Ser38) was opposite of that of stathmin; it reached a peak on the 7th day, and then decreased in the hippocampus. The protein expression of p-stathmin (Ser63) showed the same trend in the mPFC. The protein and mRNA expression of β-tubulin and MAP-1B was consistent with that of p-stathmin; it reached a peak on the 7th day, and then decreased in the rat hippocampus, mPFC, and amygdala. Stathmin in the memory loop, especially in the hippocampus, regulates microtubule structure through its phosphorylation at Ser25 and Ser38 and thereby participates in the mediation of fear memory abnormalities in PTSD.

The serotonergic and alpha-1 adrenergic receptor modulator ACH-000029 ameliorates anxiety-like behavior in a post-traumatic stress disorder model

Post-traumatic stress disorder (PTSD) is a severe chronic mental illness that develops in individuals exposed to life-threatening trauma and is characterized by hyperarousal, flashbacks and nightmares. The serotonergic (5-HT) and noradrenergic (NE) systems are deeply involved in the pathogenesis of PTSD. We have previously reported a novel anxiolytic compound, ACH-000029, that modulates 5-HT and α1-adrenergic receptors and induces acute anxiolytic-like effects in rodents. Here, we investigated the potential of ACH-000029 to prevent anxiety-like behavior in the single prolonged stress (SPS) PTSD model. Mice were subjected to the SPS procedure, followed by a 7-day treatment with ACH-000029 and, for comparison, with the α1-adrenergic antagonist prazosin. Animals were behaviorally assessed using social interaction, elevated plus maze and open field tests. Interestingly, treatment with ACH-000029 but not with prazosin ameliorated the SPS-induced sociability impairment and anxiety-like behavior. The brain-wide c-fos mapping, used as a surrogate for brain activity, indicated the brain structures that were altered by SPS and putatively involved in the anxiolytic-like effect of ACH-000029. The SPS protocol produced long-lasting impairment of regions involved in stress-anxiety response, such as the amygdala, prefrontal cortex, globus pallidus and superior colliculus. ACH-000029 treatment reversed the SPS-induced c-fos changes in the globus pallidus, lateral septum and entorhinal cortex and exclusively modulated c-fos levels in subregions from the retrosplenial cortex, cerebellum, superior colliculus and ventromedial hypothalamus. These results support the hypothesis that the dual regulation of 5-HT and α1-adrenergic receptors is required to alleviate PTSD symptoms and suggest a possible role of ACH-000029 as a PTSD treatment.

Role of the dorsal periaqueductal gray in posttraumatic stress disorder: mediation by dopamine and neurokinin

In susceptible individuals, exposure to intensely traumatic life events can lead to the development of posttraumatic stress disorder (PTSD), including long-term dysregulation of the contextual processing of aversive stimuli, the overgeneralization of learned fear, and impairments in the ability to learn or respond to safety signals. The neuropathophysiological changes that underlie PTSD remain incompletely understood. Attention has focused on forebrain structures associated with fear processing. Here we consider evidence from human and animal studies that long-lasting changes in functional connectivity between the midbrain periaqueductal gray (dPAG) and amygdala may be one of the precipitating events that contribute to PTSD. Long-lasting neuroplastic changes in the dPAG can persist after a single aversive stimulation and are pharmacologically labile. The early stage (at least up to 24 h post-stimulation) involves neurokinin-1 receptor-mediated events in the PAG and amygdala and is also regulated by dopamine, both of which are mainly involved in transferring ascending aversive information from the dPAG to higher brain structures, mainly the amygdala. Changes in the functional connectivity within the dPAG-amygdala circuit have been reported in PTSD patients. We suggest that further investigations of plasticity and pharmacology of the PAG-amygdala network provide a promising target for understanding pathophysiological circuitry that underlies PTSD in humans and that dopaminergic and neurokininergic drugs may have a potential for the treatment of psychiatric disorders that are associated with a dysfunctional dPAG.

Late glucocorticoid receptor antagonism changes the outcome of adult life stress

BACKGROUND

Stressors activate a wide spectrum of interacting hormonal and neuronal systems resulting in behavioral and physiological responses, with consequences for the development of psychopathology. Several recent studies demonstrated that treatment with the glucocorticoid receptor (GR) antagonist RU486 during adulthood normalized effects of early life stress. We aimed to evaluate the potential of RU486 to reverse stress-induced changes in an animal model of adult stress.

METHOD

We employed the single-prolonged stress (SPS) model as a multimodal stress exposure protocol in male rats. SPS rats and unstressed controls were treated with RU486 on days 8, 9, 10 after stress exposure and the effects of treatment were evaluated after another 4 days. We determined body weight gain, corticosterone levels, behavioral reactivity in anxiety tests, and brain gene expression of c-fos, corticosteroid receptors, drivers of the stress response and genes (epi-)genitally linked to PTSD.

RESULTS

RU486 affected body weight gain, corticosterone levels and open field behavior only in SPS rats. RU486 had history-independent effects in reducing fear in the elevated plus maze and fear conditioning behavior. Gene expression analysis showed a diversity of in- and interdependent effects of stress and RU486.

CONCLUSION

The effects of RU486 applied 1 week after stress and measured 4 days after treatment demonstrate that in the state of post-SPS the GR-dependence of homeostatic processes has changed. This suggests that GR-mediated processes are part of allostatic regulation after adult stress. The normalization of a number of SPS-effects after RU486 treatment reinforces the potential of targeting GR for treatment of stress-related psychopathologies.

Preclinical neuroimaging of gene-environment interactions in psychiatric disease

Psychiatric disease is one of the leading causes of disability worldwide. Despite the global burden and need for accurate diagnosis and treatment of mental illness, psychiatric diagnosis remains largely based on patient-reported symptoms, allowing for immense symptomatic heterogeneity within a single disease. In renewed efforts towards improved diagnostic specificity and subsequent evaluation of treatment response, a greater understanding of the underlying of the neuropathology and neurobiology of neuropsychiatric disease is needed. However, dissecting these mechanisms of neuropsychiatric illness in clinical populations are problematic with numerous experimental hurdles limiting hypothesis-driven studies including genetic confounds, variable life experiences, different environmental exposures, therapeutic histories, as well as the inability to investigate deeper molecular changes in vivo . Preclinical models, where many of these confounding factors can be controlled, can serve as a crucial experimental bridge for studying the neurobiological origins of mental illness. Furthermore, although behavioral studies and molecular studies are relatively common in these model systems, focused neuroimaging studies are very rare and represent an opportunity to link the molecular changes in psychiatric illness with advanced quantitative neuroimaging studies. In this review, we present an overview of well-validated genetic and environmental models of psychiatric illness, discuss gene-environment interactions, and examine the potential role of neuroimaging towards understanding genetic, environmental, and gene-environmental contributions to psychiatric illness.

Common pathways and communication between the brain and heart: connecting post-traumatic stress disorder and heart failure

Psychiatric illnesses and cardiovascular disease (CVD) contribute to significant overall morbidity, mortality, and health care costs, and are predicted to reach epidemic proportions with the aging population. Within the Veterans Administration (VA) health care system, psychiatric illnesses such as post-traumatic stress disorder (PTSD) and CVD such as heart failure (HF), are leading causes of hospital admissions, prolonged hospital stays, and resource utilization. Numerous studies have demonstrated associations between PTSD symptoms and CVD endpoints, particularly in the Veteran population. Not only does PTSD increase the risk of HF, but this relationship is bi-directional. Accordingly, a VA-sponsored conference entitled "Cardiovascular Comorbidities in PTSD: The Brain-Heart Consortium" was convened to explore potential relationships and common biological pathways between PTSD and HF. The conference was framed around the hypothesis that specific common systems are dysregulated in both PTSD and HF, resulting in a synergistic acceleration and amplification of both disease processes. The conference was not intended to identify all independent pathways that give rise to PTSD and HF, but rather identify shared systems, pathways, and biological mediators that would be modifiable in both disease processes. The results from this conference identified specific endocrine, autonomic, immune, structural, genetic, and physiological changes that may contribute to shared PTSD-CVD pathophysiology and could represent unique opportunities to develop therapies for both PTSD and HF. Some recommendations from the group for future research opportunities are provided.

Investigating the reciprocal relationships between locomotor sensitization to ethanol and PTSD-like clusters in DBA/2J mice

BACKGROUND

Post-traumatic stress disorder (PTSD) and alcohol use disorder (AUD) are two conditions that co-occur frequently. The mechanistic explanations of this co-morbidity are still unclear. The goal of this study was twofold. First to investigate whether PTSD reduces the threshold for the acquisition of ethanol sensitization in an animal model of PTSD. Then to investigate whether ethanol sensitization modulates the expression of PTSD.

METHODS

152 female inbred DBA/2 J mice were submitted to an inescapable footshock paradigm to induce a PTSD-like condition (PTSDLC) and to a paradigm of locomotor sensitization to ethanol. In a first experiment, mice were submitted to the PTSDLC and then repeatedly injected with either saline, 1 g/kg ethanol or 2 g/kg ethanol. Their sensitization to the locomotor stimulant effects of ethanol was then tested in an open field. In a second experiment, mice were first sensitized to the locomotor stimulant effects of ethanol and then tested for their behavioral response to PTSDLC.

RESULTS

In the first experiment, PTSDLC failed to induce a significant locomotor sensitization at the subthreshold dose of 1 g/kg ethanol. However, with 2 g/kg ethanol, a stronger ethanol sensitization was observed in mice submitted to the footshock relative to the control group. In the second experiment, ethanol sensitization increased only some of the behavioral clusters of PTSDLC, namely the fear generalization in a new context.

CONCLUSION

PTSDLC did not reduce the dose threshold for the acquisition of ethanol sensitization but strengthened the development of ethanol sensitization with effective doses. This suggests that PTSD might interact with one of the mechanisms underlying the development of alcohol sensitization. When the relationship between ethanol sensitization and PTSDLC is tested in the reverse direction, the present study only shows a significant effect of ethanol administration on the "sensitized fear" PTSD cluster.

Elevated dopamine signaling from ventral tegmental area to prefrontal cortical parvalbumin neurons drives conditioned inhibition

Conditioned inhibition is an important process to suppress learned responses for optimal adaptation, but its underlying biological mechanism is poorly understood. Here we used safety learning (SL)/fear discrimination after fear conditioning as a conditioned inhibition model because it demonstrates the essential properties of summation and retardation. Activity of the dorsomedial prefrontal cortex (dmPFC) parvalbumin (PV) neurons bidirectionally regulates spiking levels of dmPFC excitatory neurons and fear states. Responses to safety cues are increased in dopaminergic (DA) neurons in the ventral tegmental area (VTA) and in PV neurons in dmPFC after SL. Plasticity in the VTA is implicated, since SL requires activation of N-methyl-d-aspartate receptors. Furthermore, in a posttraumatic stress disorder model, impaired SL is associated with impaired potentiation of VTA DA neuron activity. Our results demonstrate a DA-dependent learning process that targets prefrontal inhibitory neurons for suppression of learned responses, and have implications for the pathogenesis and treatment of various psychiatric diseases.

The anxiolytic-like effects of puerarin on an animal model of PTSD

Post traumatic stress disorder (PTSD) is a mental illness that affected numerous people. The anti-PTSD-like effects of puerarin is unknown, although the antidepressant- and anxiolytic- like effects of puerarin have been reported. The PTSD behavioral deficits in rats were induced by single prolonged stress (SPS), mainly including the reduced time/entries in the open arms and the elevated time/entries in the closed arms in elevated plus maze test, increased freezing duration in contextual fear paradigm and lowered time/entries in the central zone in open field test. However, the behavioral deficits were attenuated by puerarin (50 and 100 mg/kg) without affecting the locomotor activity. For the evaluation of mechanism, the decreased levels of progesterone, allopregnanolone, and the increased levels of corticosterone, corticotropin releasing hormone, and adrenocorticotropic hormone in the brain or serum were induced by SPS, which is blocked by puerarin. In summary, the anti-PTSD-like effects of puerarin were associated with biosynthesis of neurosteroids and normalized levels of stress hormones in HPA axis.

Dopamine D2/D3 receptor agonists attenuate PTSD-like symptoms in mice exposed to single prolonged stress

Medications that enhance dopaminergic neurotransmission can be useful in the pharmacotherapy of posttraumatic stress disorder (PTSD), which manifests as fearful memory retrieval, anxiety and depression. We examined the effects of subchronic (15 days) treatment with select dopaminergic medications, including bromocriptine, modafinil, dihydrexidine, rotigotine and pramipexole, in a mouse model of PTSD induced by single prolonged stress (mSPS). The potential antidepressant-like and anxiolytic effects of the medications were measured by the forced swim test (FST) and the elevated plus maze (EPM) test, respectively. In addition, we studied the effects of these medications on memory retrieval in an auditory fear conditioning (FC) test, on ultrasonic vocalizations (USVs) induced by restraint stress, and on spontaneous locomotor activity (SLA). We report that a single exposure to a severe and complex set of stressors several days before testing increased immobility time in the FST and freezing in the FC paradigm and reduced the time spent in the open arms of the EPM. The stressed mice also displayed increased USVs, especially the short type. While none of the tested dopamine-mimetics exhibited anxiolytic-like effects, rotigotine produced antidepressant-like activity specifically in the mSPS-exposed animals. Moreover, both rotigotine and pramipexole shortened the duration of freezing in the fear conditioning test, but only in the mSPS-exposed mice. This study supports the hypothesis that the activation of dopaminergic D2/D3 receptors may be a promising pharmacotherapy for PTSD.

Sex differences in the regulation of brain IL-1β in response to chronic stress

Elevations in brain interleukin-1 beta (IL-1β) during chronic stress exposure have been implicated in behavioral and cognitive impairments associated with depression and anxiety. Two critical regulators of brain IL-1β production during times of stress are glucocorticoids and catecholamines. These hormones work in opposition to one another to inhibit (via glucocorticoid receptors) or stimulate (via beta-adrenergic receptors: β-AR) IL-1β production. While chronic stress often heightens both corticosterone and catecholamine levels, it remains unknown as to how chronic stress may affect the "yin-yang" balance between adrenergic stimulation and glucocorticoid suppression of brain IL-1β. To investigate this further, male and female rats underwent 4 days of stress exposure or served as non-stressed controls. On day 5, animals were administered propranolol (β-AR antagonist), metyrapone (a glucocorticoid synthesis inhibitor), vehicle, or both drugs and brain IL-1β mRNA was measured by rtPCR in limbic brain areas. In males, administration of propranolol had no effect on IL-1β expression in non-stressed controls but significantly reduced IL-1β in the hippocampus and amygdala of chronically stressed animals. In females, propranolol significantly reduced IL-1β in the amygdala and hypothalamus of both control and stressed rats. In male rats, metyrapone treatment significantly increased IL-1β mRNA regardless of stress treatment in all brain areas, while in female rats metyrapone only increased IL-1β in the hypothalamus. Interestingly, propranolol treatment blocked the metyrapone-induced increase in brain IL-1β indicating the increase in brain IL-1β following metyrapone treatment was due to increase β-AR activation. Additional studies revealed that metyrapone significantly increases norepinephrine turnover in the hypothalamus and medial prefrontal cortex in male rats and that microglia appear to be the cell type contributing to the production of IL-1β. Overall, data reveal that stress exposure in male rats affects the regulation of brain IL-1β by the norepinephrine-β-AR pathway, while stress had no effect in the regulation of brain IL-1β in female rats.

Social isolation rearing-induced anxiety and response to agomelatine in male and female rats: Role of corticosterone, oxytocin, and vasopressin

BACKGROUND

The chronobiotic antidepressant, agomelatine, acts via re-entrainment of circadian rhythms. Earlier work has demonstrated late-life anxiety and reduced corticosterone in post-weaning social isolation reared (SIR) rats. Agomelatine was anxiolytic in this model but did not reverse hypocortisolemia. Reduced corticosterone or cortisol (in humans) is well-described in anxiety states, although the anxiolytic-like actions of agomelatine may involve targeting another mechanism. Central oxytocin and vasopressin exert anxiolytic and anxiogenic effects, respectively, and are subject to circadian fluctuation, while also showing sex-dependent differences in response to various challenges.

AIMS AND METHODS

If corticosterone is less involved in the anxiolytic-like actions of agomelatine in SIR rats, we wondered whether effects on vasopressin and oxytocin may mediate these actions, and whether sex-dependent effects are evident. Anxiety as assessed in the elevated plus maze, as well as plasma vasopressin, oxytocin, and corticosterone were analyzed in social vs SIR animals receiving sub-chronic treatment with vehicle or agomelatine (40 mg/kg/day intraperitoneally at 16:00) for 16 days.

RESULTS

Social isolation rearing induced significant anxiety together with increased plasma vasopressin levels, but decreased corticosterone and oxytocin. While corticosterone displayed sex-dependent changes, vasopressin, and oxytocin changes were independent of sex. Agomelatine suppressed anxiety as well as reversed elevated vasopressin in both male and female rats and partially reversed reduced oxytocin in female but not male rats.

CONCLUSION

SIR-associated anxiety later in life involves reduced corticosterone and oxytocin, and elevated vasopressin. The anxiolytic-like effects of agomelatine in SIR rats predominantly involve targeting of elevated vasopressin.

Single-Prolonged Stress Impairs Prefrontal Cortex Control of Amygdala and Striatum in Rats

Medial prefrontal cortex (mPFC), amygdala, and striatum neurocircuitry has been shown to play an important role in post-traumatic stress disorder (PTSD) pathology in humans. Clinical studies show hypoactivity in the mPFC and hyperactivity in the amygdala and striatum of PTSD patients, which has been associated with decreased mPFC glutamate levels. The ability to refine neurobiological characteristics of PTSD in an animal model is critical in furthering our mechanistic understanding of the disease. To this end, we exposed male rats to single-prolonged stress (SPS), a validated model of PTSD, and hypothesized that traumatic stress would differentially activate mPFC subregions [prelimbic (PL) and infralimbic (IL) cortices] and increase striatal and amygdalar activity, which would be associated with decreased mPFC glutamate levels. in vivo, neural activity in the subregions of the mPFC, amygdala, and striatum was measured using manganese-enhanced magnetic resonance imaging (MEMRI), and glutamate and N-acetylaspartate (NAA) levels in the mPFC and the dorsal striatum (dSTR) were measured using proton magnetic resonance spectroscopy (1H-MRS) longitudinally, in rats exposed to SPS or control conditions. As hypothesized, SPS decreased MEMRI-based neural activity in the IL, but not PL, cortex concomitantly increasing activity within the basolateral amygdala (BLA) and dorsomedial striatum (dmSTR). 1H-MRS studies in a separate cohort revealed SPS decreased glutamate levels in the mPFC and increased NAA levels in the dSTR. These results confirm previous findings that suggest SPS causes mPFC hypoactivation as well as identifies concurrent hyperactivation in dmSTR and BLA, effects which parallel the clinical neuropathology of PTSD.

New neurons restore structural and behavioral abnormalities in a rat model of PTSD

Post-traumatic stress disorder (PTSD) has been associated with anxiety, memory impairments, enhanced fear, and hippocampal volume loss, although the relationship between these changes remain unknown. Single-prolonged stress (SPS) is a model for PTSD combining three forms of stress (restraint, swim, and anesthesia) in a single session that results in prolonged behavioral effects. Using pharmacogenetic ablation of adult neurogenesis in rats, we investigated the role of new neurons in the hippocampus in the long-lasting structural and behavioral effects of SPS. Two weeks after SPS, stressed rats displayed increased anxiety-like behavior and decreased preference for objects in novel locations regardless of the presence or absence of new neurons. Chronic stress produced by daily restraint for 2 or 6 hr produced similar behavioral effects that were also independent of ongoing neurogenesis. At a longer recovery time point, 1 month after SPS, rats with intact neurogenesis had normalized, showing control levels of anxiety-like behavior. However, GFAP-TK rats, which lacked new neurons, continued to show elevated anxiety-like behavior and enhanced serum corticosterone response to anxiogenic experience. Volume loss in ventral CA1 region of the hippocampus paralleled increases in anxiety-like behavior, occurring in all rats exposed to SPS at the early time point and only rats lacking adult neurogenesis at the later time point. In chronic stress experiments, volume loss occurred broadly throughout the dentate gyrus and CA1 after 6-hr daily stress but was not apparent in any hippocampal subregion after 2-hr daily stress. No effect of SPS was seen on cell proliferation in the dentate gyrus, but the survival of young neurons born a week after stress was decreased. Together, these data suggest that new neurons are important for recovery of normal behavior and hippocampal structure following a strong acute stress and point to the ventral CA1 region as a potential key mediator of stress-induced anxiety-like behavior.

A Rodent Model of Exposure Therapy: The Use of Fear Extinction as a Therapeutic Intervention for PTSD

The symptoms of post-traumatic stress disorder (PTSD) include cognitive impairment related to medial prefrontal cortical dysfunction. Indeed, a deficit of cognitive flexibility, i.e., an inability to modify previously learned thoughts and behaviors based on changes in the environment, may underlie many of the other symptoms of PTSD, such as changes in mood, hyper-arousal, intrusive thoughts, exaggerated and over-generalized fear, and avoidance behavior. Cognitive-behavioral therapies target the cognitive dysfunction observed in PTSD patients, training them to recalibrate stress-related perceptions, interpretations and responses. Preclinically, the extinction of conditioned fear bears resemblance to one form of cognitive therapy, exposure therapy, whereby an individual learns, through repeated exposure to a fear-provoking stimulus in a safe environment, that the stimulus no longer signals imminent threat, and their fear response is suppressed. In this review article, we highlight recent findings from our lab using fear extinction as a preclinical model of exposure therapy in rodents exposed to chronic unpredictable stress (CUS). We specifically focus on the therapeutic effects of extinction on stress-compromised set-shifting as a measure of cognitive flexibility, and active vs. passive coping behavior as a measure of avoidance. Finally, we discuss mechanisms involving activity and plasticity in the medial prefrontal cortex (mPFC) necessary for the therapeutic effects of extinction on cognitive flexibility and active coping.

Modelling posttraumatic stress disorders in animals

Animal models of posttraumatic stress disorder are useful tools to reveal the neurobiological basis of the vulnerability to traumatic events, and to develop new treatment strategies, as well as predicting treatment response contributing to personalized medicine approach. Different models have different construct, face and predictive validity and they model different symptoms of the disease. The most prevalent models are the single prolonged stress, electric foot-shock and predator odor. Freezing as 're-experiencing' in cluster B and startle as 'arousal' in cluster E according to DSM-5 are the most frequently studied parameters; however, several other symptoms related to mood, cognitive and social skills are part of the examinations. Beside behavioral characteristics, symptoms of exaggerated sympathetic activity and hypothalamic-pituitary-adrenocortical axis as well as signs of sleep disturbances are also warranted. Test battery rather than a single test is required to describe a model properly and the results should be interpreted in a comprehensive way, e.g. creating a z-score. Research is shifting to study larger populations and identifying the features of the resilient and vulnerable individuals, which cannot be easily done in humans. Incorporation of the "three hit theory" in animal models may lead to a better animal model of vulnerability and resilience. As women are twice as vulnerable as men, more emphasize should be taken to include female animals. Moreover, hypothesis free testing and big data analysis may help to identify an array of biomarkers instead of a single variable for identification of vulnerability and for the purpose of personalized medicine.

Cognitive Flexibility Training Improves Extinction Retention Memory and Enhances Cortical Dopamine With and Without Traumatic Stress Exposure

Stress exposure can cause lasting changes in cognition, but certain individual traits, such as cognitive flexibility, have been shown to reduce the degree, duration, or severity of cognitive changes following stress. Both stress and cognitive flexibility training affect decision making by modulating monoamine signaling. Here, we test the role cognitive flexibility training, and high vs. low cognitive flexibility at the individual level, in attenuating stress-induced changes in memory and monoamine levels using the single prolonged stress (SPS) rodent model of traumatic stress in male Sprague-Dawley rats. Exposure to SPS can heighten fear responses to conditioned cues (i.e., freezing) after a fear association has been extinguished, referred to as a deficit in extinction retention. This deficit is thought to reflect an impairment in context processing that is characteristic of posttraumatic stress disorder (PTSD). During a cognitive flexibility training we assessed individual variability in cognitive skills and conditioned rats to discriminately use cues in their environment. We found that cognitive flexibility training, alone or followed by SPS exposure, accelerated extinction learning and decreased fear responses over time during extinction retention testing, compared with rats not given cognitive flexibility training. These findings suggest that cognitive flexibility training may improve context processing in individuals with and without traumatic stress exposure. Individual performance during the reversal phase of the cognitive flexibility training predicted subsequent context processing; individuals with high reversal performance exhibited a faster decrease in freezing responses during extinction retention testing. Thus, high reversal performance predicted enhanced retention of extinction learning over time and suggests that cognitive flexibility training may be a strategy to promote context processing. In a brain region vital for maintaining cognitive flexibility and fear suppression, the prelimbic cortex (PLC), cognitive flexibility training also lastingly enhanced dopamine (DA) and norepinephrine (NE) levels, in animals with and without traumatic stress exposure. In contrast, cognitive flexibility training prior to traumatic stress exposure decreased levels of DA and its metabolites in the striatum, a region mediating reflexive decision making. Overall, our results suggest that cognitive flexibility training can provide lasting benefits by enhancing extinction retention, a hallmark cognitive effect of trauma, and prelimbic DA, which can maintain flexibility across changing contexts.

Post Traumatic Stress Disorder and Substance Use Disorder as Two Pathologies Affecting Memory Reactivation: Implications for New Therapeutic Approaches

In the present review, we provide evidence indicating that although post traumatic stress disorder (PTSD) and substance use disorder (SUD) are two distinct pathologies with very different impacts on people affected by these chronic illnesses, they share numerous common characteristics, present high rates of co-morbidity, and may result from common physiological dysfunctions. We propose that these pathologies result from hyper reactivity to reminders, and thus should be considered as two disorders of memory, treated as such. We review the different possibilities to intervene on pathological memories such as extinction therapy and reconsolidation blockade. We also introduce new therapeutic avenues directly indicate by our recent proposal to replace the consolidation/reconsolidation hypothesis by the integration concept. State dependency and emotional remodeling are two innovative treatments that have already provided encouraging results. In summary, this review shows that the discovery of reactivation-dependent memory malleability has open new therapeutic avenues based on the reprocessing of pathological memories, which constitute promising approaches to treat PTSD and SUD.

Single Prolonged Stress as a Prospective Model for Posttraumatic Stress Disorder in Females

Sex plays an important role in susceptibility to stress triggered disorders. Posttraumatic Stress disorder (PTSD), a debilitating psychiatric disorder developed after exposure to a traumatic event, is two times more prevalent in women than men. However, the vast majority of animal models of PTSD, including single prolonged stress (SPS), were performed mostly with males. Here, we evaluated SPS as an appropriate PTSD model for females in terms of anxiety, depressive symptoms and changes in gene expression in the noradrenergic system in the brain. In addition, we examined intranasal neuropeptide Y (NPY) as a possible treatment in females. Female rats were subjected to SPS and given either intranasal NPY or vehicle in two separate experiments. In the first experiment, stressed females were compared to unstressed controls on forced swim test (FST) and for levels of expression of several genes in the locus coeruleus (LC) 12 days after SPS exposure. Using a separate cohort of animals, experiment two examined stressed females and unstressed controls on the elevated plus maze (EPM) and LC gene expression 7 days after SPS stressors. SPS led to increased anxiety-like behavior on EPM and depressive-like behavior on FST. Following FST, the rats displayed elevated tyrosine hydroxylase (TH), CRHR1 and Y1R mRNA levels in the LC, consistent with increased activation of the noradrenergic system. The expression level of these mRNAs was unchanged following EPM, except Y1R. Intranasal NPY at the doses shown to be effective in males, did not prevent development of depressive or anxiety-like behavior or molecular changes in the LC. The results indicate that while SPS could be an appropriate PTSD model for females, sex differences, such as response to NPY, are important to consider.

Untangling PTSD and TBI: Challenges and Strategies in Clinical Care and Research

PURPOSE OF REVIEW

Traumatic brain injury (TBI) and post-traumatic stress disorder (PTSD) can result from similar injuries and can result in similar symptoms, such as problems with sleep, concentration, memory, and mood. Although PTSD and persistent sequelae due to a TBI (PST) have generally been viewed as pragmatically confounded but conceptually separable entities, we examine emerging evidence emphasizing the breadth of overlap in both clinical presentation and underlying pathophysiology between PST and PTSD.

RECENT FINDINGS

New evidence underscores the poor specificity of symptoms to etiology and emphasizes the potential, after both physical brain injury and traumatic stress, for changes in each of the three interacting systems that coordinate the body's response to the experience or expectation of major injury-the immune, endocrine, and neuromodulatory neurotransmitter systems. A view of PTSD and PST sharing common pathophysiologic elements related to the CNS response to acute injury or threat carries important implications for research and clinical care.

Rosmarinic acid reverses the deleterious effects of repetitive stress and tat protein

Human immunodeficiency virus type 1 (HIV) has infected more than 40 million people worldwide and is associated with central nervous system (CNS) disruption in at least 30% of these persons. The use of highly active antiretroviral therapy (HAART) has significantly reduced the systemic immunopathology associated with HIV, but the occurrence of neurological disorders continues to be reported in notable numbers. The present study evaluated the potential of rosmarinic acid to reverse the detrimental effects of an intracerebral injection of the viral protein tat. Control and tat-injected rats were also subjected to repetitive restrain stress (RRS) for 28 days, 6 h per day, to investigate whether subsequent stress exposure would worsen the effects of tat. 14 days after the initiation of RRS, animals were treated with rosmarinic acid (10 mg/kg given intraperitoneally) daily until the end of the stress exposure period. We assessed locomotor activity and anxiety-like behavioral changes. We also measured plasma corticosterone levels and quantified the expression of mineralocorticoid receptors (MR), glucocorticoid receptors (GR) and brain-derived neurotrophic factor (BDNF) in the hippocampus. Rosmarinic acid attenuated anxiety-like behavior induced by tat and stress, reduced plasma corticosterone levels and increased the expression of hippocampal GR, MR and BDNF when compared to controls. These results suggest that rosmarinic acid may reverse the anxiogenic effect of HIV-1 viral protein tat and related stress through modulation of the hypothalamic-pituitary-adrenal axis and hippocampal neurotrophic factor levels.

Voluntary exercise or systemic propranolol ameliorates stress-related maladaptive behaviors in female rats

Many people will experience at least one traumatic event in their lifetime, with up to 20% developing Post-Traumatic Stress Disorder (PTSD) or PTSD-like symptoms. In addition, the likelihood that females will develop PTSD after trauma is more than twice that of males. Despite its prevalence, current treatment strategies for trauma victims are limited and substantial portions of affected individuals remain resistant to treatment, suggesting that additional interventions are necessary. Using an animal model of traumatic stress, the present studies tested the hypothesis that either voluntary exercise and/or administration of the adrenergic beta-receptor antagonist propranolol, would ameliorate stress-related maladaptive behaviors. In Study 1 four groups of female rats were exposed to a sequence of stressors that included anesthesia, restraint, forced swim, exposure to predator scent and fear conditioning. Rats then underwent re-exposure sessions in which stress-related conditioned stimuli were presented. In addition to re-exposure, stressed rats were treated with propranolol (10 mg/kg) and/or given the opportunity to engage in voluntary wheel running intermittently for 4 weeks. Stress-associated maladaptive behavior was assessed using the elevated plus and open field mazes and fear memory tests. Cognitive ability was assessed using a novel odor recognition task. A main effect of exercise on behaviors related to anxiety and resilience was observed, but neither a main effect of propranolol nor a synergistic effect of propranolol and exercise were observed. Neither stress induction nor treatment influenced recognition memory. In contrast, in Study 2 in which the timing and dosage of propranolol (0.25-2.0 mg/kg), and the number and timing of re-exposure sessions were adjusted, propranolol produced both a reduction in anxiety-like behaviors as well as resilience to a subsequent stressor. These results are consistent with the notion that combining re-exposure therapy with additional interventions is beneficial for female trauma victims. Furthermore, the findings support the view that in pre-clinical models, voluntary exercise, which bolsters hippocampal function and propranolol, which affects amygdala-dependent memory reconsolidation and peripheral noradrenergic signaling, can ameliorate stress-related symptoms.

Gait and Conditioned Fear Impairments in a Mouse Model of Comorbid TBI and PTSD

STUDY OBJECTIVES

Traumatic brain injury (TBI) and posttraumatic stress disorder (PTSD) commonly cooccur. Approaches to research and treatment of these disorders have been segregated, despite overlapping symptomology. We and others have hypothesized that comorbid TBI + PTSD generates worse symptoms than either condition alone. We present a mouse model of comorbid TBI + PTSD to further explore this condition.

METHODS

A mouse model of TBI + PTSD was generated using the single prolonged stress (SPS) protocol in combination with the controlled cortical impact (CCI) protocol. This resulted in four experimental groups: control, TBI, PTSD, and TBI + PTSD. Behavioral phenotyping included gait analysis, contextual fear conditioning, acoustic startle response, and prepulse inhibition.

RESULTS

Mice in the TBI + PTSD group showed a significantly impaired gait compared to their counterparts with TBI alone as well as control mice. Mice in the TBI + PTSD group showed significantly impaired contextual fear recall compared to controls. Prepulse inhibition testing revealed intact acoustic startle and auditory sensory gating.

CONCLUSIONS

These results indicate that SPS paired with CCI in mice produces unique behavioral impairments in gait and fear recall that are not present in either condition alone. Further studies are underway to examine additional behavioral, physiological, and pathological phenotypes in this combined model of TBI + PTSD.

Sex Differences in Nociceptin/Orphanin FQ Peptide Receptor-Mediated Pain and Anxiety Symptoms in a Preclinical Model of Post-traumatic Stress Disorder

Nociceptin/Orphanin FQ (N/OFQ) is a neuropeptide that modulates pain transmission, learning/memory, stress, anxiety, and fear responses via activation of the N/OFQ peptide (NOP or ORL1) receptor. Post-traumatic stress disorder (PTSD) is an anxiety disorder that may arise after exposure to a traumatic or fearful event, and often is co-morbid with chronic pain. Using an established animal model of PTSD, single-prolonged stress (SPS), we were the first to report that NOP receptor antagonist treatment reversed traumatic stress-induced allodynia, thermal hyperalgesia, and anxiety-like behaviors in male Sprague-Dawley rats. NOP antagonist treatment also reversed SPS-induced serum and CSF N/OFQ increase and circulating corticosterone decrease. The objective of this study was to examine the role of the NOP receptor in male and female rats subjected to traumatic stress using Wistar wild type (WT) and NOP receptor knockout (KO) rats. The severity of co-morbid allodynia was assessed as change in paw withdrawal threshold (PWT) to von Frey and paw withdrawal latency (PWL) to radiant heat stimuli, respectively. PWT and PWL decreased in male and female WT rats within 7 days after SPS, and remained decreased through day 28. Baseline sensitivity did not differ between genotypes. However, while male NOP receptor KO rats were protected from SPS-induced allodynia and thermal hypersensitivity, female NOP receptor KO rats exhibited tactile allodynia and thermal hypersensitivity to the same extent as WT rats. Male NOP receptor KO rats had a lower anxiety index (AI) than WT, but SPS did not increase AI in WT males. In contrast, SPS significantly increased AI in WT and NOP receptor KO female rats. SPS increased circulating N/OFQ levels in male WT, but not in male NOP receptor KO, or WT or KO female rats. These results indicate that the absence of the NOP receptor protects males from traumatic-stress-induced allodynia and hyperalgesia, consistent with our previous findings utilizing a NOP receptor antagonist. However, female NOP receptor KO rats experience allodynia, hyperalgesia and anxiety-like symptoms to the same extent as WT females following SPS. This suggests that endogenous N/OFQ-NOP receptor signaling plays an important, but distinct, role in males and females following exposure to traumatic stress.