Animal models of post-traumatic stress disorder: face validity

MicroRNAs as potential biomarkers for diagnosis of post-traumatic stress disorder

Post-traumatic stress disorder is a mental disorder caused by exposure to severe traumatic life events. Currently, there are no validated biomarkers or laboratory tests that can distinguish between trauma survivors with and without post-traumatic stress disorder. In addition, the heterogeneity of clinical presentations of post-traumatic stress disorder and the overlap of symptoms with other conditions can lead to misdiagnosis and inappropriate treatment. Evidence suggests that this condition is a multisystem disorder that affects many biological systems, raising the possibility that peripheral markers of disease may be used to diagnose post-traumatic stress disorder. We performed a PubMed search for microRNAs (miRNAs) in post-traumatic stress disorder (PTSD) that could serve as diagnostic biomarkers and found 18 original research articles on studies performed with human patients and published January 2012 to December 2023. These included four studies with whole blood, seven with peripheral blood mononuclear cells, four with plasma extracellular vesicles/exosomes, and one with serum exosomes. One of these studies had also used whole plasma. Two studies were excluded as they did not involve microRNA biomarkers. Most of the studies had collected samples from adult male Veterans who had returned from deployment and been exposed to combat, and only two were from recently traumatized adult subjects. In measuring miRNA expression levels, many of the studies had used microarray miRNA analysis, miRNA Seq analysis, or NanoString panels. Only six studies had used real time polymerase chain reaction assay to determine/validate miRNA expression in PTSD subjects compared to controls. The miRNAs that were found/validated in these studies may be considered as potential candidate biomarkers for PTSD and include miR-3130-5p in whole blood; miR-193a-5p, -7113-5p, -125a, -181c, and -671-5p in peripheral blood mononuclear cells; miR-10b-5p, -203a-3p, -4488, -502-3p, -874-3p, -5100, and -7641 in plasma extracellular vesicles/exosomes; and miR-18a-3p and -7-1-5p in blood plasma. Several important limitations identified in the studies need to be taken into account in future studies. Further studies are warranted with war veterans and recently traumatized children, adolescents, and adults having PTSD and use of animal models subjected to various stressors and the effects of suppressing or overexpressing specific microRNAs.

Agmatine diminishes behavioral and endocrine alterations in a rat model of post-traumatic stress disorder

Post-traumatic stress disorder (PTSD), is a severe anxiety disorder characterized by associative fear conditioning. Single prolonged stress (SPS) is a widely accepted reliable animal model to stimulate PTSD. Agmatine is an endogenous neuromodulator of stress; however, its effect on PTSD remains to be investigated. This study explored the role of agmatine in conditioned fear response (CFR) in PTSD and highlighted the role of imidazoline receptors in the effect of agmatine. Intra-cerebroventricular (icv) surgery was done in order to facilitate drug administration. Animals were subjected to SPS. Agmatine and the involvement of imidazoline receptors (I1 and I2) were assessed for their effect in fear conditioning apparatus. During weeks 1, 2, and 3, in CFR, agmatine (40 µg/rat, icv) showed significantly decreased freezing time whereas other doses of agmatine (10 and 20 µg/rat, icv). Imidazoline (I1 and I2) receptor agonists Moxonidine (25 µg/rat, icv) and 2-BFI, (10 µg/rat, icv) respectively, at their sub-effective doses, with a submaximal dose of agmatine (20 µg/rat, icv) significantly decreased the altered freezing time during weeks 1, 2 and 3 compared to SPS animals. Moreover, the effective dose of agmatine (40 µg/rat, icv) with imidazoline (I1 and I2) receptor antagonists Efaroxan (10 µg/rat, icv) and Idazoxan (4 µg/rat, icv) respectively does not reversed the effect of agmatine on freezing. Agmatine and its combination with I1 and I2 agonists, normalized the altered freezing behavior, corticosterone level, organ coefficient of adrenal gland, neuroinflammatory and neurotrophic factor due to SPS during CFR projecting its strong therapeutic effect in SPS induced PTSD.

ErbB4 precludes the occurrence of PTSD-like fear responses by supporting the bimodal activity of the central amygdala

Post-traumatic stress disorder (PTSD) often arises after exposure to traumatic events and is characterized by dysregulated fear responses. Although the associations of erb-b2 receptor tyrosine kinase 4 (ErbB4) with various neuropsychiatric diseases, including schizophrenia and bipolar disorder, have been widely examined, the physiological roles of ErbB4 in PTSD and fear responses remain unclear. Using Cre-dependent ErbB4 knockout (KO) mice, we observed that PTSD-like fear behaviors emerged in ErbB4-deficient mice, particularly in inhibitory neurons. Specifically, the loss of ErbB4 in somatostatin-expressing (SST+) neurons was sufficient to induce PTSD-like fear responses. We also adopted the CRISPR/Cas9 system for region-specific KO of ErbB4, which revealed that ErbB4 deletion in SST+ neurons of the lateral division of the amygdala (CeL) caused elevated anxiety and PTSD-like fear generalization. Consistent with its physiological role, ErbB4 expression was diminished in CeLSST neurons from mice that exhibited PTSD-like phenotypes. While fear On and Off cells identified in the CeL displayed distinct responses to conditioned and novel cues, as previously shown, the selectivity of those On and Off cells was compromised in SSTErbB4-/- and stressed mice, which displayed strong fear generalization. Therefore, the bimodal activity that CeL On/Off cells display is likely required for proper discrimination of fearful stimuli from ambient stimuli, which should be sustained by the presence of ErbB4. Taken together, our data substantiate the correlation between PTSD-like fear responses and ErbB4 expression in CeLSST neurons and further underscore the functional effects of ErbB4 in CeLSST neurons, supporting the bimodal responses of CeL neurons.

Insomnia-related rodent models in drug discovery

Despite the widespread prevalence and important medical impact of insomnia, effective agents with few side effects are lacking in clinics. This is most likely due to relatively poor understanding of the etiology and pathophysiology of insomnia, and the lack of appropriate animal models for screening new compounds. As the main homeostatic, circadian, and neurochemical modulations of sleep remain essentially similar between humans and rodents, rodent models are often used to elucidate the mechanisms of insomnia and to develop novel therapeutic targets. In this article, we focus on several rodent models of insomnia induced by stress, diseases, drugs, disruption of the circadian clock, and other means such as genetic manipulation of specific neuronal activity, respectively, which could be used to screen for novel hypnotics. Moreover, important advantages and constraints of some animal models are discussed. Finally, this review highlights that the rodent models of insomnia may play a crucial role in novel drug development to optimize the management of insomnia.

Social-Single Prolonged Stress affects contextual fear conditioning in male and female Wistar rats: Molecular insights in the amygdala

Stress exposure can lead to post-traumatic stress disorder (PTSD) in male and female rats. Social-Single Prolonged Stress (SPS) protocol has been considered a potential PTSD model. This study aimed to pharmacologically validate the Social-SPS as a PTSD model in male and female rats. Male and female Wistar rats (60-day-old) were exposed to Social-SPS protocol and treated with fluoxetine (10 mg/Kg) or saline solution intraperitoneally 24 h before euthanasia. Two cohorts of animals were used; for cohort 1, male and female rats were still undisturbed until day 7 post-Social-SPS exposure, underwent locomotor and conditioned fear behaviors, and were euthanized on day 9. Animals of cohort 2 were subjected to the same protocol but were re-exposed to contextual fear behavior on day 14. Results showed that fluoxetine-treated rats gained less body weight than control and Social-SPS in both sexes. Social-SPS effectively increased the freezing time in male and female rats on day eight but not on day fourteen. Fluoxetine blocked the increase of freezing in male and female rats on day 8. Different mechanisms for fear behavior were observed in males, such as Social-SPS increased levels of glucocorticoid receptors and Beclin-1 in the amygdala. Social-SPS was shown to increase the levels of NMDA2A, GluR-1, PSD-95, and CAMKII in the amygdala of female rats. No alterations were observed in the amygdala of rats on day fourteen. The study revealed that Social-SPS is a potential PTSD protocol applicable to both male and female rats.

Effects of Repetitive Mild Traumatic Brain Injury on Corticotropin-Releasing Factor Modulation of Lateral Habenula Excitability and Motivated Behavior

Mild traumatic brain injury (mTBI) is a significant health burden due to mTBI-related chronic debilitating cognitive and psychiatric morbidities. Recent evidence from our laboratory suggests a possible dysregulation within reward/motivational circuit function at the level of a subcortical structure, the lateral habenula (LHb), where we demonstrated a causal role for hyperactive LHb in mTBI-induced motivational deficits in self-care grooming behavior in young adult male mice when exposed to mTBI injury during late adolescence (at ~8 weeks old). Here we extended this observation by further characterizing neurobehavioral effects of this repetitive closed head injury model of mTBI in both young adult male and female mice on LHb excitability, corticotropin releasing factor (CRF) modulation of LHb activity, and behavioral responses of motivation to self-care behavior, and approach versus avoidance behavior in the presence of a social- or threat-related stimulus. We show that mTBI increases LHb spontaneous tonic activity in female mice similar to what we previously observed in male mice as well as promoting LHb neuronal hyperexcitability and hyperpolarization-induced LHb bursting in both male and female mice. Interestingly, mTBI only increases LHb intrinsic excitability in male mice coincident with higher levels of the hyperpolarization-activated cation currents (HCN/Ih) and reduces levels of the M-type potassium currents while potentiating M-currents without altering intrinsic excitability in LHb neurons of female mice. Since persistent dysregulation of brain CRF systems is suggested to contribute to chronic psychiatric morbidities and that LHb neurons are highly responsive to CRF, we then tested whether LHb CRF subsystem becomes engaged following mTBI. We found that in vitro inhibition of CRF receptor type 1 (CRFR1) within the LHb normalizes mTBI-induced enhancement of LHb tonic activity and hyperexcitability in both sexes, suggesting that an augmented intra-LHb CRF-CRFR1-mediated signaling contributes to the overall LHb hyperactivity following mTBI. Behaviorally, mTBI diminishes motivation for self-care grooming in female mice as in male mice. mTBI also alters defensive behaviors in the looming shadow task by shifting the innate defensive behaviors towards more passive action-locking rather than escape behaviors in response to an aerial threat in both male and female mice as well as prolonging the latency to escape responses in female mice. While, this model of mTBI reduces social preference in male mice, it induces higher social novelty seeking during the novel social encounters in both male and female mice. Overall, our study provides further translational validity for the use of this preclinical model of mTBI for investigation of mTBI-related reward circuit dysfunction and mood/motivation-related behavioral deficits in both sexes while uncovering a few sexually dimorphic neurobehavioral effects of this model that may differentially affect young males and females when exposed to this type of mTBI injury during late adolescence.

Rodent tests of depression and anxiety: Construct validity and translational relevance

Behavioral testing constitutes the primary method to measure the emotional states of nonhuman animals in preclinical research. Emerging as the characteristic tool of the behaviorist school of psychology, behavioral testing of animals, particularly rodents, is employed to understand the complex cognitive and affective symptoms of neuropsychiatric disorders. Following the symptom-based diagnosis model of the DSM, rodent models and tests of depression and anxiety focus on behavioral patterns that resemble the superficial symptoms of these disorders. While these practices provided researchers with a platform to screen novel antidepressant and anxiolytic drug candidates, their construct validity-involving relevant underlying mechanisms-has been questioned. In this review, we present the laboratory procedures used to assess depressive- and anxiety-like behaviors in rats and mice. These include constructs that rely on stress-triggered responses, such as behavioral despair, and those that emerge with nonaversive training, such as cognitive bias. We describe the specific behavioral tests that are used to assess these constructs and discuss the criticisms on their theoretical background. We review specific concerns about the construct validity and translational relevance of individual behavioral tests, outline the limitations of the traditional, symptom-based interpretation, and introduce novel, ethologically relevant frameworks that emphasize simple behavioral patterns. Finally, we explore behavioral monitoring and morphological analysis methods that can be integrated into behavioral testing and discuss how they can enhance the construct validity of these tests.

Unveiling Adolescent Suicidality: Holistic Analysis of Protective and Risk Factors Using Multiple Machine Learning Algorithms

Adolescent suicide attempts are on the rise, presenting a significant public health concern. Recent research aimed at improving risk assessment for adolescent suicide attempts has turned to machine learning. But no studies to date have examined the performance of stacked ensemble algorithms, which are more suitable for low-prevalence conditions. The existing machine learning-based research also lacks population-representative samples, overlooks protective factors and their interplay with risk factors, and neglects established theories on suicidal behavior in favor of purely algorithmic risk estimation. The present study overcomes these shortcomings by comparing the performance of a stacked ensemble algorithm with a diverse set of algorithms, performing a holistic item analysis to identify both risk and protective factors on a comprehensive data, and addressing the compatibility of these factors with two competing theories of suicide, namely, The Interpersonal Theory of Suicide and The Strain Theory of Suicide. A population-representative dataset of 173,664 Norwegian adolescents aged 13 to 18 years (mean = 15.14, SD = 1.58, 50.5% female) with a 4.65% rate of reported suicide attempt during the past 12 months was analyzed. Five machine learning algorithms were trained for suicide attempt risk assessment. The stacked ensemble model significantly outperformed other algorithms, achieving equal sensitivity and a specificity of 90.1%, AUC of 96.4%, and AUCPR of 67.5%. All algorithms found recent self-harm to be the most important indicator of adolescent suicide attempt. Exploratory factor analysis suggested five additional risk domains, which we labeled internalizing problems, sleep disturbance, disordered eating, lack of optimism regarding future education and career, and victimization. The identified factors provided stronger support for The Interpersonal Theory of Suicide than for The Strain Theory of Suicide. An enhancement to The Interpersonal Theory based on the risk and protective factors identified by holistic item analysis is presented.

MDMA-assisted psychotherapy for PTSD: Growing evidence for memory effects mediating treatment efficacy

The application of MDMA in conjunction with psychotherapy has in recent years seen a resurgence of clinical, scientific, and public interest in the treatment of posttraumatic stress disorder (PTSD). Clinical trials have shown promising safety and efficacy, but the mechanisms underlying this treatment form remain largely unestablished. This article explores recent preclinical and clinical evidence suggesting that the treatment's efficacy may be influenced by the mnemonic effects of MDMA. We review data on the effects of MDMA on fear extinction and fear reconsolidation and the utility of these processes for PTSD treatment. We corroborate our findings by incorporating research from cognitive psychology and psychopharmacology and offer recommendations for future research.

Reconfiguration of brain-wide neural activity after early life adversity

Early life adversity (ELA) predisposes individuals to both physical and mental disorders lifelong. How ELA affects brain function leading to this vulnerability is under intense investigation. Research has begun to shed light on ELA effects on localized brain regions within defined circuits. However, investigations into brain-wide neural activity that includes multiple localized regions, determines relationships of activity between regions and identifies shifts of activity in response to experiential conditions is necessary. Here, we performed longitudinal manganese-enhanced magnetic resonance imaging (MEMRI) to image the brain in normally reared or ELA-exposed adults. Images were captured in the freely moving home cage condition, and short- and long-term after naturalistic threat. Images were analyzed with new computational methods, including automated segmentation and fractional activation or difference volumes. We found that neural activity was increased after ELA compared to normal rearing in multiple brain regions, some of which are involved in defensive and/or reward circuitry. Widely distributed patterns of neural activity, "brain states", and their dynamics after threat were altered with ELA. Upon acute threat, ELA-mice retained heightened neural activity within many of these regions, and new hyperactive responses emerged in monoaminergic centers of the mid- and hindbrain. Nine days after acute threat, heightened neural activity remained within locus coeruleus and increased within posterior amygdala, ventral hippocampus, and dorso- and ventromedial hypothalamus, while reduced activity emerged within medial prefrontal cortical regions (prelimbic, infralimbic, anterior cingulate). These results reveal that functional imbalances arise between multiple brain-systems which are dependent upon context and cumulative experiences after ELA.

The Evolving Role of Animal Models in the Discovery and Development of Novel Treatments for Psychiatric Disorders

Historically, animal models have been routinely used in the characterization of novel chemical entities (NCEs) for various psychiatric disorders. Animal models have been essential in the in vivo validation of novel drug targets, establishment of lead compound pharmacokinetic to pharmacodynamic relationships, optimization of lead compounds through preclinical candidate selection, and development of translational measures of target occupancy and functional target engagement. Yet, with decades of multiple NCE failures in Phase II and III efficacy trials for different psychiatric disorders, the utility and value of animal models in the drug discovery process have come under intense scrutiny along with the widespread withdrawal of the pharmaceutical industry from psychiatric drug discovery. More recently, the development and utilization of animal models for the discovery of psychiatric NCEs has undergone a dynamic evolution with the application of the Research Domain Criteria (RDoC) framework for better design of preclinical to clinical translational studies combined with innovative genetic, neural circuitry-based, and automated testing technologies. In this chapter, the authors will discuss this evolving role of animal models for improving the different stages of the discovery and development in the identification of next generation treatments for psychiatric disorders.

The role of hippocampal CaMKII in resilience to trauma-related psychopathology

Traumatic stress exposure can form persistent trauma-related memories. However, only a minority of individuals develop post-traumatic stress disorder (PTSD) symptoms upon exposure. We employed a rat model of PTSD, which enables differentiating between exposed-affected and exposed-unaffected individuals. Two weeks after the end of exposure, male rats were tested behaviorally, following an exposure to a trauma reminder, identifying them as trauma 'affected' or 'unaffected.' In light of the established role of hippocampal synaptic plasticity in stress and the essential role of Ca2+/calmodulin-dependent protein kinase II (CaMKII) in hippocampal based synaptic plasticity, we pharmacologically inhibited CaMKII or knocked-down (kd) αCaMKII (in two separate experiments) in the dorsal dentate gyrus of the hippocampus (dDG) following exposure to the same trauma paradigm. Both manipulations brought down the prevalence of 'affected' individuals in the trauma-exposed population. A day after the last behavioral test, long-term potentiation (LTP) was examined in the dDG as a measure of synaptic plasticity. Trauma exposure reduced the ability to induce LTP, whereas, contrary to expectation, αCaMKII-kd reversed this effect. Further examination revealed that reducing αCaMKII expression enables the formation of αCaMKII-independent LTP, which may enable increased resilience in the face of a traumatic experience. The current findings further emphasize the pivotal role dDG has in stress resilience.

Anxiety, fear, panic: An approach to assessing the defensive behavior system across the predatory imminence continuum

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.

Pharmacological strategies for post-traumatic stress disorder (PTSD): From animal to clinical studies

Post-traumatic stress disorder (PTSD) is a disabling psychiatric condition with a critical familiar, personal, and social impact. Patients diagnosed with PTSD show various symptoms, including anxiety, depression, psychotic episodes, and sleep disturbances, complicating their therapeutic management. Only sertraline and paroxetine, two selective serotonin reuptake inhibitors, are approved by different international agencies to treat PTSD. In addition, these drugs are generally combined with psychotherapy to achieve positive results. However, these pharmacological strategies present limited efficacy. Nearly half of the PTSD patients do not experience remission of symptoms, possibly due to the high prevalence of psychiatric comorbidities. Therefore, in clinical practice, other off-label medications are common, even though the effectiveness of these drugs needs to be further investigated. In this line, antipsychotics, antiepileptics, adrenergic blockers, benzodiazepines, and other emerging pharmacological agents have aroused interest as potential therapeutic tools to improve some specific symptoms of PTSD. Thus, this review is focused on the most widely used drugs for the pharmacological treatment of PTSD with a translational approach, including clinical and preclinical studies, to emphasize the need to develop safer and more effective medications.

TrkA-cholinergic signaling modulates fear encoding and extinction learning in PTSD-like behavior

Recent studies have suggested that the use of cognitive enhancers as adjuncts to exposure-based therapy in individuals suffering from post-traumatic stress disorder (PTSD) may be beneficial. Brain cholinergic signaling through basal forebrain projections to the hippocampus is an established pathway mediating fear response and cognitive flexibility. Here we employed a genetic strategy to enhance cholinergic activity through increased signaling of the NGF receptor TrkA. This strategy leads to increased levels of the marker of cholinergic activation, acetylcholine synthesizing enzyme choline acetyltransferase, in forebrain cholinergic regions and their projection areas such as the hippocampus. Mice with increased cholinergic activity do not display any neurobehavioral abnormalities except a selective attenuation of fear response and lower fear expression in extinction trials. Reduction in fear response is rescued by the GABA antagonist picrotoxin in mutant mice, and, in wild-type mice, is mimicked by the GABA agonist midazolam suggesting that GABA can modulate cholinergic functions on fear circuitries. Importantly, mutant mice also show a reduction in fear processing under stress conditions in a single prolonged stress (SPS) model of PTSD-like behavior, and augmentation of cholinergic signaling by the drug donepezil in wild-type mice promotes extinction learning in a similar SPS model of PTSD-like behavior. Donepezil is already in clinical use for the treatment of dementia suggesting a new translational application of this drug for improving exposure-based psychotherapy in PTSD patients.

Sotalol Treatment may Interfere With Retrieval, Expression, and/or Reconsolidation Processes Thus Disrupting Traumatic Memories in a Post-Traumatic Stress Disorder Mice Model

The processes by which fear memory is encoded, consolidated, and re-consolidated are extremely complex and appear to require the release of stress hormones, especially adrenaline (AD). AD improves contextual fear memory, acting specifically on peripheral β2-adrenoceptors. Propranolol (peripheral and central β-adrenoceptor antagonist) treatment was shown to prevent post-traumatic stress disorder (PTSD) development and reduce its symptoms. However, propranolol has several side effects. Thus, we aimed to evaluate if sotalol (a peripheral β-adrenoceptor antagonist) treatment interferes with retrieval, expression, and/or reconsolidation of traumatic memories in a validated mice model that mimics the signs/symptoms of PTSD, thus intending to decrease them. Female mice were induced with PTSD following an established protocol. Sotalol (2.0 mg/kg) or vehicle were administered on days 2, 7, and 14. The percentage of freezing was calculated, and behavioral tests were carried out. Catecholamines in plasma were quantified by HPLC with electrochemical detection. Quantitative real-time polymerase chain reaction (qPCR) was used to evaluate mRNA expression of NR4A family genes in hippocampus. Following the submission of the animals to the same aversive context on days 2, 7, and 14, sotalol-treated mice exhibited significant less freezing behavior. In the elevated plus-maze test, the time spent and number of entries in the open arms, and total arm entries were increased in sotalol-treated mice. Also, the light-dark transition test revealed higher time spent, number of transitions to the light, and total number of transitions in sotalol-treated mice. Moreover, plasma AD was significantly decreased in sotalol-treated mice. On day 14, sotalol-treated mice exhibited a decrease in mRNA expression of Nr4a1 in the hippocampus. In conclusion, in PTSD mice model, sotalol appears to decrease traumatic memories and anxiety-like behavior, probably due to a decrease in peripheral adrenergic activity, which influences traumatic memories. The effects of sotalol upon re-exposure to the traumatic context may be consistent with interference in the retrieval, expression, and/or reconsolidation processes of contextual traumatic memory, resulting in a long-term reduction of PTSD symptoms and signs. The decreased Nr4a1 mRNA expression in the hippocampal formation may be crucial for these mice to develop diminished traumatic contextual memories after sotalol therapy in PTSD.

Interleukine-17 Modulates Neurogenesis and Behavior Following Exposure to Trauma in Mice

Post-traumatic stress disorder (PTSD) is a psychiatric disorder accompanied by deficits in cognitive and social skills. Adult hippocampal neurogenesis is a lifelong phenomenon, with new neurons being formed in the granular cell layer of the dentate gyrus. Impaired neurogenesis is associated with multiple behavioral disorders including Alzheimer's disease and schizophrenia. PTSD patients often present hippocampal atrophy and animal models clearly present impaired neurogenesis. Previous studies on PTSD patients demonstrated elevated levels of Th17 cells and plasma levels of the pro-inflammatory cytokine interleukin-17A (IL-17A). Since IL-17A can impair neurogenesis in mice, we thus hypothesized that decreasing the serum levels of IL-17A will increase hippocampal neurogenesis and alleviate symptoms in a murine model of PTSD. Surprisingly, our results showed that attempting to neutralize IL-17A with an antibody resulted in increased serum levels of IL-17A, while targeting IL-23, the upstream regulator of IL-17, did lower the levels of IL-17A in trauma-exposed mice. As expected, increased levels of serum IL-17A (in anti-IL-17A treated mice) resulted in impaired neurogenesis, reflected by reduced number of proliferating Ki67⁺ neural progenitors and newly formed DCX⁺ neurons, which was correlated with increased expression of Hes1. Nevertheless, increased maturation was noted by the expression of Slit2 and Ache. In contrast, treatment with anti-IL-23 indeed resulted in increased neurogenesis. Behaviorally, both treatments did not affect trauma-related freezing behavior but did affect trauma-related social deficits. Unexpectedly, increased levels of serum IL-17A (in anti-IL-17A treated mice) prevented social deficits in trauma-exposed mice while anti-IL-23 exacerbated these deficits. We thus conclude that IL-17 is involved in regulating neurogenesis following exposure to stress but may be important in maintaining social behavior.

Exposure to footshock stress downregulates antioxidant genes and increases neuronal apoptosis in an Aβ(1-42) rat model of Alzheimer's disease

Post-traumatic stress disorder (PTSD) is a neuropsychiatric disorder that develops from exposure to trauma, mostly when normal psychological mechanisms fail. Studies have shown that people who have PTSD are susceptible to developing dementia, mostly Alzheimer's disease (AD), suggesting common underlying risk factors in the comorbidity. However, data elucidating links between these conditions is scarce. Here we show that footshock stress exacerbates AD-like pathology. To induce a trauma-like condition, the rats were exposed to multiple intense footshocks followed by a single reminder. This was followed by bilateral intrahippocampal lesions with amyloid-beta (Aβ) (1-42), to model AD-like pathology. We found that footshocks increased anxiety behavior and impaired fear memory extinction in Aβ(1-42) lesioned rats. We also found a reduced expression of nuclear factor erythroid 2-related factor 2 (Nrf2), NAD (P) H: quinone oxidoreductase 1 (NQO1), heme oxygenase-1 (HO-1), and an increased expression of Kelch-like ECH-associated protein 1 (Keap1) in the amygdala and hippocampus. Furthermore, oxidative stress level was sustained, which was associated with increased apoptosis in the amygdala and hippocampus. Our finding suggests that AD-like pathology can induce oxidative changes in the amygdala and hippocampus, which can be exaggerated by footshock stress.

Repeated cocaine exposure prior to fear conditioning induces persistency of PTSD-like symptoms and enhancement of hippocampal and amygdala cell density in male rats

Pre- and post-trauma drug use can interfere with recovery from post-traumatic stress disorder (PTSD). However, the biological underpinnings of this interference are poorly understood. Here we examined the effect of pre-fear conditioning cocaine self-administration on PTSD-like symptoms in male rats, and defined impairment of fear extinction as difficulty to recover from PTSD. We also examined cell density changes in brain regions suspected of being involved in resistance to PTSD recovery. Before footshock stress testing, rats were trained to self-administer cocaine during 20 consecutive days, after which they were exposed to footshocks, while other rats continued to self-administer cocaine until the end of the experiment. Upon assessment of three PTSD-like symptoms (fear during situational reminders, anxiety-like behavior, and impairment of recognition memory) and fear extinction learning and memory, changes in cell density were measured in the medial prefrontal cortex, hippocampus, and amygdala. Results show that pre-footshock cocaine exposure did not affect fear during situational reminders. Fear conditioning did not lead to an increase in cocaine consumption. However, in footshock stressed rats, cocaine induced a reduction of anxiety-like behavior, an aggravation of recognition memory decline, and an impairment of extinction memory. These behavioral alterations were associated with increased cell density in the hippocampal CA1, CA2, and CA3 regions and basolateral amygdala, but not in the medial prefrontal cortex. Our findings suggest that enhancement of cell density in the hippocampus and amygdala may be changes associated with drug use, interfering with PTSD recovery.

The advent of fear conditioning as an animal model of post-traumatic stress disorder: Learning from the past to shape the future of PTSD research

Translational research on post-traumatic stress disorder (PTSD) has produced limited improvements in clinical practice. Fear conditioning (FC) is one of the dominant animal models of PTSD. In fact, FC is used in many different ways to model PTSD. The variety of FC-based models is ill defined, creating confusion and conceptual vagueness, which in turn impedes translation into the clinic. This article takes a historical and conceptual approach to provide a comprehensive picture of current research and help reorient the research focus. This work historically reviews the variety of models that have emerged from the initial association of PTSD with FC, highlighting conceptual pitfalls that have limited the translation of animal research into clinical advances. We then provide some guidance on how future translational research could benefit from conceptual and technological improvements to translate basic findings in patients. This objective will require transdisciplinary approaches and should involve physicians, engineers, philosophers, and neuroscientists.

Behavioral profiling reveals an enhancement of dentate gyrus paired pulse inhibition in a rat model of PTSD

We recently introduced behavioral profiling as a translational approach to increase the validity of animal models of posttraumatic stress disorder (PTSD). Behavioral profiling utilizes the response of a 'normal population' of control animals and compares the performance of animals with a history of traumatic stress in different behavioral tests that can capture PTSD-like symptoms. Thus, affected, PTSD-like individuals can be subdivided from resilient trauma-exposed animals. While in our recent study we focused mainly on tests for activity and anxiety, we now expand the behavioral tests battery and include also fear memory and extinction tasks as well as a spatial object recognition test in our behavioral profiling approach. Utilizing underwater trauma as the traumatic event, we found that only a small subset of animals exposed to underwater trauma showed lasting increases in anxiety-like behavior and heightened emotional memory formation. Adding juvenile stress as a model for childhood adversity increased the prevalence of such affected animals and furthermore and induced additional cognitive deficits in a subgroup of such emotionally affected individuals. In addition, multiple affected individual rats displayed increased local circuit activity in the dorsal dentate gyrus, as measured in vivo with paired pulse protocols in anesthetized animals. Together, our findings highlight behavioral profiling, refined by including multiple behavioral tests, as a valid tool to identify PTSD-like vs. resilient individual animals and further suggest that enhanced local inhibition in specific circuits of the dorsal dentate gyrus may be associated with the observed symptoms.

A CRHR1 antagonist prevents synaptic loss and memory deficits in a trauma-induced delirium-like syndrome

Older patients with severe physical trauma are at high risk of developing neuropsychiatric syndromes with global impairment of cognition, attention, and consciousness. We employed a thoracic trauma (TxT) mouse model and thoroughly analyzed age-dependent spatial and temporal posttraumatic alterations in the central nervous system. Up to 5 days after trauma, we observed a transient 50% decrease in the number of excitatory synapses specifically in hippocampal pyramidal neurons accompanied by alterations in attention and motor activity and disruption of contextual memory consolidation. In parallel, hippocampal corticotropin-releasing hormone (CRH) expression was highly upregulated, and brain-derived neurotrophic factor (BDNF) levels were significantly reduced. In vitro experiments revealed that CRH application induced neuronal autophagy with rapid lysosomal degradation of BDNF via the NF-κB pathway. The subsequent synaptic loss was rescued by BDNF as well as by specific NF-κB and CRH receptor 1 (CRHR1) antagonists. In vivo, the chronic application of a CRHR1 antagonist after TxT resulted in reversal of the observed histological, molecular, and behavioral alterations. The data suggest that neuropsychiatric syndromes (i.e., delirium) after peripheral trauma might be at least in part due to the activation of the hippocampal CRH/NF-κB/BDNF pathway, which results in a dramatic loss of synaptic contacts. The successful rescue by stress hormone receptor antagonists should encourage clinical trials focusing on trauma-induced delirium and/or other posttraumatic syndromes.

A novel arousal-based individual screening reveals susceptibility and resilience to PTSD-like phenotypes in mice

Translational animal models for studying post-traumatic stress disorder (PTSD) are valuable for elucidating the poorly understood neurobiology of this neuropsychiatric disorder. These models should encompass crucial features, including persistence of PTSD-like phenotypes triggered after exposure to a single traumatic event, trauma susceptibility/resilience and predictive validity. Here we propose a novel arousal-based individual screening (AIS) model that recapitulates all these features. The AIS model was designed by coupling the traumatization (24 h restraint) of C57BL/6 J mice with a novel individual screening. This screening consists of z-normalization of post-trauma changes in startle reactivity, which is a measure of arousal depending on neural circuits conserved across mammals. Through the AIS model, we identified susceptible mice showing long-lasting hyperarousal (up to 56 days post-trauma), and resilient mice showing normal arousal. Susceptible mice further showed persistent PTSD-like phenotypes including exaggerated fear reactivity and avoidance of trauma-related cue (up to 75 days post-trauma), increased avoidance-like behavior and social/cognitive impairment. Conversely, resilient mice adopted active coping strategies, behaving like control mice. We further uncovered novel transcriptional signatures driven by PTSD-related genes as well as dysfunction of hypothalamic–pituitary–adrenal axis, which corroborated the segregation in susceptible/resilient subpopulations obtained through the AIS model and correlated with trauma susceptibility/resilience. Impaired hippocampal synaptic plasticity was also observed in susceptible mice. Finally, chronic treatment with paroxetine ameliorated the PTSD-like phenotypes of susceptible mice. These findings indicate that the AIS model might be a new translational animal model for the study of crucial features of PTSD. It might shed light on the unclear PTSD neurobiology and identify new pharmacological targets for this difficult-to-treat disorder.

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The AIS model includes highly requested features necessary to shape a translational PTSD animal model.

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Susceptible mice identified through the AIS model exhibited persistent PTSD-like phenotypes.

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Resilient mice identified through the AIS model adopted active coping strategies.

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The AIS model revealed molecular adaptations underlying trauma susceptibility/resilience.

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The AIS model meets the criterion of predictive validity by exclusively using susceptible mice.

Hippocampal GABAergic interneurons and their co-localized neuropeptides in stress vulnerability and resilience

Studies in humans and rodents suggest a critical role for the hippocampal formation in cognition and emotion, but also in the adaptation to stressful events. Successful stress adaptation promotes resilience, while its failure may lead to stress-induced psychopathologies such as depression and anxiety disorders. Hippocampal architecture and physiology is shaped by its strong control of activity via diverse classes of inhibitory interneurons that express typical calcium binding proteins and neuropeptides. Celltype-specific opto- and chemogenetic intervention strategies that take advantage of these biochemical markers have bolstered our understanding of the distinct role of different interneurons in anxiety, fear and stress adaptation. Moreover, some of the signature proteins of GABAergic interneurons have a potent impact on emotion and cognition on their own, making them attractive targets for interventions. In particular, neuropeptide Y is a promising endogenous agent for mediating resilience against severe stress. In this review, we evaluate the role of the major types of interneurons across hippocampal subregions in the adaptation to chronic and acute stress and to emotional memory formation.

Short-term sleep deprivation immediately after contextual conditioning inhibits BDNF signaling and disrupts memory consolidation in predator odor trauma mice model of PTSD

Post-traumatic stress disorder (PTSD) is a debilitating neuropsychiatric illness affecting > 7 million people every year in the US. Recently, we have shown that the mouse model of predator odor trauma (POT) displayed contextual conditioning and core features of PTSD including sleep disturbances (hyperarousal) and retrieval of traumatic memories following exposure to objective reminders (re-experiencing). PTSD is a disorder of memory function. Since memory consolidation requires the expression of BDNF along with an activation of MAPK/pERK signaling pathway in limbic brain structures (hippocampus and amygdala) and sleep favors memory consolidation, we hypothesized that short-term sleep deprivation (SD, 3 h), immediately after contextual conditioning will attenuate molecular correlates of memory consolidation, sleep disturbances, and memory consolidation. We performed two experiments in adult male C57BL/6J mice to test our hypothesis. Experiment 1 determined the effects of SD on contextual conditioning and changes in sleep wakefulness. Experiment 2 determined the effects of SD on contextual conditioning-induced changes in the expression of BDNF and pERK in hippocampus and amygdala. SD immediately after contextual conditioning (POT + SD group) significantly attenuated sleep disturbances, memory retrieval, and expression of pERK and BDNF in the hippocampus and amygdala as compared to POT-SD group (no SD after contextual conditioning). No significant differences were observed between POT + SD, NOC-SD (no contextual conditioning + no SD), and NOC + SD (no contextual conditioning + SD) groups. Memory consolidation requires sleep and the expression of pERK and BDNF in hippocampus and amygdala immediately after contextual conditioning in POT model of PTSD in mice.

Fear response-based prediction for stress susceptibility to PTSD-like phenotypes

Most individuals undergo traumatic stresses at some points in their life, but only a small proportion develop stress-related disorders such as anxiety diseases and posttraumatic stress disorder (PTSD). Although stress susceptibility is one determinant of mental disorders, the underlying mechanisms and functional implication remain unclear yet. We found that an increased amount of freezing that animals exhibited in the intertrial interval (ITI) of a stress-enhanced fear learning paradigm, predicts ensuing PTSD-like symptoms whereas resilient mice show ITI freezing comparable to that of unstressed mice. To examine the behavioral features, we developed a systematic analytical approach for ITI freezing and stress susceptibility. Thus, we provide a behavioral parameter for prognosis to stress susceptibility of individuals in the development of PTSD-like symptoms as well as a new mathematical means to scrutinize freezing behavior.

Animal models for the discovery of novel drugs for post-traumatic stress disorder

INTRODUCTION

Existing pharmacological treatments for PTSD are limited and have been used primarily because of their effectiveness in other psychiatric conditions. To generate novel, PTSD specific pharmacotherapy, researchers must utilize animal models to assess the efficacy of experimental drugs.

AREAS COVERED

This review includes a discussion of factors that should be considered when developing an animal model of PTSD, as well as descriptions of the most commonly used models. Researchers have utilized physical stressors, psychological stressors, or a combination of the two to induce PTSD-like physiological and behavioral sequelae in animals. Such models have provided researchers with a valuable tool to examine the neurobiological mechanisms underlying the condition.

EXPERT OPINION

PTSD is a heterogeneous disorder that manifests as different symptom clusters in different individuals. Thus, there cannot be a one-size-fits-all approach to modeling the disorder in animals. Preclinical investigators must adopt a concentrated effort aimed at modeling specific PTSD subtypes and the distinct symptom profiles that result from specific types of human trauma. Moreover, researchers have focused so much on modeling a single PTSD syndrome in animals that studies examining only specific facets of the disorder are largely ignored. Future research employing animal models of PTSD requires greater focus on the nuances of PTSD.

Acquisition of remifentanil self-administration: Enhanced in female rats but no effect of adolescent stress exposure

Animal models of acquisition have been vital in shaping our understanding of vulnerability factors that influence susceptibility to drugs of abuse. Decades of research substantiates a number of biological, environmental, and behavioral factors that predict vulnerability - many of which have been important in the development of early intervention efforts in humans. The goal of the present study was to examine the acquisition of a synthetic opioid derivative in 66 adult male and female Long-Evans rats following histories of stress exposure during adolescence. Stress-exposed rats were subjected to a mild stress paradigm, which included alternating exposure to synthetic fox feces and physical restraint for eight days. Following stress induction and assessment, all rats were implanted with intravenous catheters in order to self-administer remifentanil (1 μm/kg/infusion) with no prior operant training. Acquisition of remifentanil self-administration was measured over 15 days. Findings indicate that regardless of stress condition, female rats acquired remifentanil self-administration sooner and emitted more active lever presses than males. Stress exposed animals exhibited increased anxiety-like response compared to the control group following exposure to stress, operationalized as decreased exploratory behavior on an Elevated Plus Maze. However, these effects were not expressed as significant differences in self-administration by stress. Together, these findings indicate that sex differences are evident in remifentanil self-administration.

Acute and Post-Traumatic Stress Disorders: A biased nervous system

Acute stress disorder and post-traumatic stress disorder are generally triggered by an exceptionally intense threat. The consequences of this traumatogenic situation are explored here in chronological order, from exposure to the threat to development of symptoms. Such a situation may disrupt the equilibrium between two fundamental brain circuits, referred to as the "defensive" and "cognitive". The defensive circuit triggers the stress response as well as the formation of implicit memory. The cognitive circuit triggers the voluntary response and the formation of explicit autobiographical memory. During a traumatogenic situation, the defensive circuit could be over-activated while cognitive circuit is under-activated. In the most severe cases, overactivation of the defensive circuit may cause its brutal deactivation, resulting in dissociation. Here, we address the underlying neurobiological mechanisms at every scale: from neurons to behaviors, providing a detailed explanatory model of trauma.

Computational causal discovery for post-traumatic stress in police officers

This article reports on a study aimed to elucidate the complex etiology of post-traumatic stress (PTS) in a longitudinal cohort of police officers, by applying rigorous computational causal discovery (CCD) methods with observational data. An existing observational data set was used, which comprised a sample of 207 police officers who were recruited upon entry to police academy training. Participants were evaluated on a comprehensive set of clinical, self-report, genetic, neuroendocrine and physiological measures at baseline during academy training and then were re-evaluated at 12 months after training was completed. A data-processing pipeline-the Protocol for Computational Causal Discovery in Psychiatry (PCCDP)-was applied to this data set to determine a causal model for PTS severity. A causal model of 146 variables and 345 bivariate relations was discovered. This model revealed 5 direct causes and 83 causal pathways (of four steps or less) to PTS at 12 months of police service. Direct causes included single-nucleotide polymorphisms (SNPs) for the Histidine Decarboxylase (HDC) and Mineralocorticoid Receptor (MR) genes, acoustic startle in the context of low perceived threat during training, peritraumatic distress to incident exposure during first year of service, and general symptom severity during training at 1 year of service. The application of CCD methods can determine variables and pathways related to the complex etiology of PTS in a cohort of police officers. This knowledge may inform new approaches to treatment and prevention of critical incident related PTS.

The effect of traumatic-like stress exposure on alterations in the temporal social behavior of a rodent population

Though the relationship between traumatic stress and social behavior, which has been explored for years, is dynamic and largely estimated between dyads, little is known about the causal effects of traumatic stress exposure on the time-dependent dynamic alterations in the social behaviors on a large-group level. We thus investigated the effect of a single prolonged stress (SPS) exposure, a classical animal model that recapitulates posttraumatic stress disorder (PTSD)-like symptoms in rodents, on the spatiotemporal, social behavior changes within a large group of cohabiting rats. One-half of thirty-two Sprague-Dawley rats were assigned to the experimental group and subjected to SPS treatment administered two weeks after baseline social behavior recording; the other half served as the controls. Each group of rats (n = 16) was housed in one of two large custom-made cylinders. We used an automatic tracking system to record the behavioral indices of social behavior of the rats before SPS exposure, on the SPS exposure day, during a 7-day-long quiescent period after SPS treatment, as well as during subsequent behavioral test days. In addition to SPS-induced PTSD-like behaviors, SPS induced a time-dependent, oscillating change in active/passive social behaviors that lasted for 3 weeks. SPS treatment decreased active social behaviors (especially affiliative behaviors) but increased passive social behaviors (e.g. huddling) immediately following stress exposure. Increased active social interactions were observed during the early phase after SPS treatment; while increased passive social behaviors were observed during the late phase after SPS treatment. These dynamic changes were repeatedly observed when the rats underwent subsequent stressful behavioral tests and challenges. SPS induced a long-term, time-dependent oscillating change in indices of the social behavior. These changes may serve as an adaptive mechanism, and their manifestations critically depended on the time course following the traumatic stress exposure.

Region-specific involvement of interneuron subpopulations in trauma-related pathology and resilience

Only a minority of trauma-exposed individuals develops Posttraumatic stress disorder (PTSD) and active processes may support trauma resilience. Individual behavioral profiling allows investigating neurobiological alterations related to resilience or pathology in animal models of PTSD and is utilized here to examine the activation of different interneuron subpopulations of the dentate gyrus-amygdala system associated with trauma resilience or pathology. To model PTSD, rats were exposed to juvenile stress combined with underwater trauma (UWT) in adulthood. Four weeks later, individual anxiety levels were assessed in the elevated plus maze test for classifying rats as highly anxious 'affected' vs. 'non-affected', i.e. behaving as control animals. Analyzing the activation of specific interneuron subpopulations in the dorsal and ventral dentate gyrus (DG), the basolateral (BLA) and central amygdala by immunohistochemical double-labeling for cFos and different interneuron markers, revealed an increased activation of cholecystokinin (CCK)-positive interneurons in the ventral DG, together with increased activation of parvalbumin- and CCK-positive interneurons in the BLA of affected trauma-exposed rats. By contrast, increased activation of neuropeptide Y (NPY)-positive interneurons was observed in the dorsal DG of trauma-exposed, but non-affected rats. To test for a direct contribution of NPY in the dorsal DG to trauma resilience, a local shRNA-mediated knock down was performed after UWT. Such a treatment significantly reduced the prevalence of resilient animals. Our results suggest that distinct interneuron populations are associated with resilience or pathology in PTSD with high regional specificity. NPY within the dorsal DG was found to significantly contribute to trauma resilience.

The propensity for re-triggered predation fear in a prey fish

Variation in predation risk can drive variation in fear intensity, the length of fear retention, and whether fear returns after waning. Using Trinidadian guppies, we assessed whether a low-level predation threat could easily re-trigger fear after waning. First, we show that background risk induced neophobia after either multiple exposures to a low-level threat or a single exposure to a high-level threat. However, a single exposure to the low-level threat had no such effect. The individuals that received multiple background exposures to the low-level threat retained their neophobic phenotype over an 8-day post-risk period, and this response was intensified by a single re-exposure to the low-level threat on day 7. In contrast, the neophobia following the single high-level threat waned over the 8-day period, but the single re-exposure to the low-level threat on day 7 re-triggered the neophobic phenotype. Thus, despite the single low-level exposure being insufficient to induce neophobia, it significantly elevated existing fear and re-triggered fear that had waned. We highlight how such patterns of fear acquisition, retention, and rapid re-triggering play an important role in animal ecology and evolution and outline parallels between the neophobic phenotype in fishes and dimensions of post-traumatic stress in humans.

Repeated exposure of naïve and peripheral nerve-injured mice to a snake as an experimental model of post-traumatic stress disorder and its co-morbidity with neuropathic pain

Confrontation of rodents by natural predators provides a number of advantages as a model for traumatic or stressful experience. Using this approach, one of the aims of this study was to investigate a model for the study of post-traumatic stress disorder (PTSD)-related behaviour in mice. Moreover, because PTSD can facilitate the establishment of chronic pain (CP), and in the same way, patients with CP have an increased tendency to develop PTSD when exposed to a traumatic event, our second aim was to analyse whether this comorbidity can be verified in the new paradigm. C57BL/6 male mice underwent chronic constriction injury of the sciatic nerve (CCI), a model of neuropathic CP, or not (sham groups) and were submitted to different threatening situations. Threatened mice exhibited enhanced defensive behaviours, as well as significantly enhanced risk assessment and escape behaviours during context reexposure. Previous snake exposure reduced open-arm time in the elevated plus-maze test, suggesting an increase in anxiety levels. Sham mice showed fear-induced antinociception immediately after a second exposure to the snake, but 1 week later, they exhibited allodynia, suggesting that multiple exposures to the snake led to increased nociceptive responses. Moreover, after reexposure to the aversive environment, allodynia was maintained. CCI alone produced intense allodynia, which was unaltered by exposure to either the snake stimuli or reexposure to the experimental context. Together, these results specifically parallel the behavioural symptoms of PTSD, suggesting that the snake/exuvia/reexposure procedure may constitute a useful animal model to study PTSD.

Temporally variable predation risk and fear retention in Trinidadian guppies

Predation fear is a unifying theme across vertebrate taxa. Here, we explored how the frequency and duration of predation risk affects postrisk fear behavior in Trinidadian guppies. We first exposed individuals to visual cues of potential predators for 3 days, either frequently (6×/day) or infrequently (1×/day). Each exposure lasted for either a relatively brief (5 min) or long (30 min) duration, whereas a control group consisted of no risk exposures. One day later, we quantified guppy behavior. All background risk treatments induced a fear response toward a novel odor (i.e., neophobia), and individuals previously exposed to frequent bouts of brief risk showed elevated baseline fear. Although neophobic responses were initially similar across risk treatments (1 day later), retention of this response differed. After 8 days, only individuals previously exposed to brief bouts of risk (both frequent and infrequent) maintained neophobic responses, whereas their initially higher level of baseline fear remained elevated but was no longer significantly different from the control. These results increase our understanding of temporal factors that affect the intensity and retention of fear that persists after risk exposure, which may have applications across vertebrates in relation to problems with fearful phenotypes.

Cannabinoids as therapeutics for PTSD

Post-traumatic stress disorder (PTSD) is a complex disorder that involves dysregulation of multiple neurobiological systems. The traumatic stressor plays a causal role in producing psychological dysfunction and the pattern of findings suggests that the hypothalamic-pituitary-adrenal (HPA) axis, which is instrumental for stress adaptation, is critically dysfunctional in PTSD. Given the lack of understanding of the basic mechanisms and underlying pathways that cause the disorder and its heterogeneity, PTSD poses challenges for treatment. Targeting the endocannabinoid (ECB) system to treat mental disorders, and PTSD in particular, has been the focus of research and interest in recent years. The ECB system modulates multiple functions, and drugs enhancing ECB signaling have shown promise as potential therapeutic agents in stress effects and other psychiatric and medical conditions. In this review, we focus on the interaction between the ECB-HPA systems in animal models for PTSD and in patients with PTSD. We summarize evidence supporting the use of cannabinoids in preventing and treating PTSD in preclinical and clinical studies. As the HPA system plays a key role in the mediation of the stress response and the pathophysiology of PTSD, we describe preclinical studies suggesting that enhancing ECB signaling is consistent with decreasing PTSD symptoms and dysfunction of the HPA axis. Overall, we suggest that a pharmacological treatment targeted at one system (e.g., HPA) may not be very effective because of the heterogeneity of the disorder. There are abnormalities across different neurotransmitter systems in the pathophysiology of PTSD and none of these systems function uniformly among all patients with PTSD. Hence, conceptually, enhancing ECB signaling may be a more effective avenue for pharmacological treatment.

Chronic repeated predatory stress induces resistance to quinine adulteration of ethanol in male mice

BACKGROUND

Trauma related psychiatric disorders, such as posttraumatic stress disorder (PTSD), and alcohol use disorder (AUD) are highly comorbid illnesses that separately present an opposing, sex-specific pattern, with increased prevalence of PTSD in females and increased prevalence of AUD diagnoses in males. Likewise, PTSD is a risk factor in the development of AUD, with conflicting data on the impact of sex in the comorbid development of both disorders. Because the likelihood of experiencing more than one traumatic event is high, we aim to utilize chronic repeated predatory stress (CRPS) to query the extent to which sex interacts with CRPS to influence alcohol consumption, or cessation of consumption.

METHODS

Male (n = 16) and female (n = 15) C57BL/6 J mice underwent CRPS or daily handling for two weeks during adolescence (P35-P49) and two weeks during adulthood (P65-P79). Following the conclusion of two rounds of repeated stress, behavior was assessed in the open field. Mice subsequently underwent a two-bottle choice intermittent ethanol access (IEA) assessment (P90-131) with the options of 20 % ethanol or water. After establishing drinking behavior, increasing concentrations of quinine were added to the ethanol to assess the drinking response to adulteration of the alcohol.

RESULTS

CRPS increased fecal corticosterone concentrations and anxiety-like behaviors in the open field in both male and female mice as compared to control mice that had not been exposed to CRPS. Consistent with previous reports, we observed a sex difference in alcohol consumption such that females consumed more ethanol per gram of body mass than males. In addition, CRPS reduced alcohol aversion in male mice such that higher concentrations of quinine were necessary to reduce alcohol intake as compared to control mice. CRPS did not alter alcohol-related behaviors in female mice.

CONCLUSION

Collectively, we demonstrate that repeated CRPS can induce anxiety-like behavior in both sexes but selectively influences the response to ethanol adulteration in males.

An ecological framework of neophobia: from cells to organisms to populations

Neophobia is the fear of novel stimuli or situations. This phenotype has recently received much ecological attention, primarily in the context of decision making. Here, we explore neophobia across biological levels of organisation, first describing types of neophobia among animals and the underlying causes of neophobia, highlighting high levels of risk and uncertainty as key drivers. We place neophobia in the framework of Error Management Theory and Signal Detection Theory, showing how increases in overall risk and uncertainty can lead to costly non-responses towards novel threats unless individuals lower their response threshold and become neophobic. We then discuss how neophobic behaviour translates into population and evolutionary consequences before introducing neophobia-like processes at the cellular level, where some phenomena such as allergy and autoimmunity can parallel neophobic behaviour. Finally, we discuss neophobia attenuation, considering how a sudden change in the environment from dangerous to safe can lead to problematic over-responses (i.e. the 'maladaptive defensive carry-over' hypothesis), and discuss treatment methods for such over-responses. We anticipate that bridging the concept of neophobia with a process-centered perspective can facilitate a transfer of insight across organisational levels.

The predator odor avoidance model of post-traumatic stress disorder in rats

Individuals with post-traumatic stress disorder avoid trauma-related stimuli and exhibit blunted hypothalamic-pituitary-adrenal axis response at the time of trauma. Our laboratory uses predator odor (i.e. bobcat urine) stress to divide adult Wistar rats into groups that exhibit high (avoiders) or low (nonavoiders) avoidance of a predator odor-paired context, modeling the fact that not all humans exposed to traumatic events develop psychiatric conditions. Male avoiders exhibit lower body weight gain after stress, as well as extinction-resistant avoidance that persists after a second stress exposure. These animals also show attenuated hypothalamic-pituitary-adrenal axis response to predator odor that predicts subsequent avoidance of the odor-paired context. Avoiders exhibit unique brain activation profiles relative to nonavoiders and controls (as measured by Fos immunoreactivity), and higher corticotropin-releasing factor levels in multiple brain regions. Furthermore, avoider rats exhibit escalated and compulsive-like alcohol self-administration after traumatic stress. Here, we review the predator odor avoidance model of post-traumatic stress disorder and its utility for tracking behavior and measuring biological outcomes predicted by avoidance. The major strengths of this model are (i) etiological validity with exposure to a single intense stressor, (ii) established approach distinguishing individual differences in stress reactivity, and (iii) robust behavioral and biological phenotypes during and after trauma.

Animal models of post-traumatic stress disorder and novel treatment targets

Understanding the neurobiological basis of post-traumatic stress disorder (PTSD) is fundamental to accurately diagnose this neuropathology and offer appropriate treatment options to patients. The lack of pharmacological effects, too often observed with the most currently used drugs, the selective serotonin reuptake inhibitors (SSRIs), makes even more urgent the discovery of new pharmacological approaches. Reliable animal models of PTSD are difficult to establish because of the present limited understanding of the PTSD heterogeneity and of the influence of various environmental factors that trigger the disorder in humans. We summarize knowledge on the most frequently investigated animal models of PTSD, focusing on both their behavioral and neurobiological features. Most of them can reproduce not only behavioral endophenotypes, including anxiety-like behaviors or fear-related avoidance, but also neurobiological alterations, such as glucocorticoid receptor hypersensitivity or amygdala hyperactivity. Among the various models analyzed, we focus on the social isolation mouse model, which reproduces some deficits observed in humans with PTSD, such as abnormal neurosteroid biosynthesis, changes in GABAA receptor subunit expression and lack of pharmacological response to benzodiazepines. Neurosteroid biosynthesis and its interaction with the endocannabinoid system are altered in PTSD and are promising neuronal targets to discover novel PTSD agents. In this regard, we discuss pharmacological interventions and we highlight exciting new developments in the fields of research for novel reliable PTSD biomarkers that may enable precise diagnosis of the disorder and more successful pharmacological treatments for PTSD patients.

Laurent-Arriot K, Tsirka SE, Bergold PJ. Increased Behavioral Deficits and Inflammation in a Mouse Model of Co-Morbid Traumatic Brain Injury and Post-Traumatic Stress Disorder

Comorbid post-traumatic stress disorder with traumatic brain injury (TBI) produce more severe affective and cognitive deficits than PTSD or TBI alone. Both PTSD and TBI produce long-lasting neuroinflammation, which may be a key underlying mechanism of the deficits observed in co-morbid TBI/PTSD. We developed a model of co-morbid TBI/PTSD by combining the closed head (CHI) model of TBI with the chronic variable stress (CVS) model of PTSD and examined multiple behavioral and neuroinflammatory outcomes. Male C57/Bl6 mice received sham treatment, CHI, CVS, CHI then CVS (CHI → CVS) or CVS then CHI (CVS → CHI). The CVS → CHI group had deficits in Barnes maze or active place avoidance not seen in the other groups. The CVS → CHI, CVS and CHI → CVS groups displayed increased basal anxiety level, based on performance on elevated plus maze. The CVS → CHI had impaired performance on Barnes Maze, and Active Place Avoidance. These performance deficits were strongly correlated with increased hippocampal Iba-1 level an indication of activated MP/MG. These data suggest that greater cognitive deficits in the CVS → CHI group were due to increased inflammation. The increased deficits and neuroinflammation in the CVS → CHI group suggest that the order by which a subject experiences TBI and PTSD is a major determinant of the outcome of brain injury in co-morbid TBI/PTSD.

Pavlovian Olfactory Fear Conditioning: Its Neural Circuity and Importance for Understanding Clinical Fear-Based Disorders

Odors have proven to be the most resilient trigger for memories of high emotional saliency. Fear associated olfactory memories pose a detrimental threat of potentially transforming into severe mental illness such as fear and anxiety-related disorders. Many studies have deliberated on auditory, visual and general contextual fear memory (CFC) processes; however, fewer studies have investigated mechanisms of olfactory fear memory. Evidence strongly suggests that the neuroanatomical representation of olfactory fear memory differs from that of auditory and visual fear memory. The aim of this review article is to revisit the literature regarding the understanding of the neurobiological process of fear conditioning and to illustrate the circuitry of olfactory fear memory.

A Systematic Review of Interleukin-1β in Post-Traumatic Stress Disorder: Evidence from Human and Animal Studies

Pro-inflammatory cytokines, such as interleukin (IL)-1β, have been implicated as underlying pathophysiological mechanisms and potential biomarkers of post-traumatic stress disorder (PTSD). This systematic review examines data regarding IL-1β production/concentration in human and animal studies of PTSD. In accordance with PRISMA guidelines, relevant articles from PubMed were reviewed from inception until July 10, 2017. Nineteen studies were eligible for inclusion. Animal studies demonstrated increased hippocampal IL-1β in rodent models of PTSD. Several immunomodulatory drugs were shown to reduce elevated IL-1β levels and anxiety-like behaviors in animals. Human cross-sectional studies showed contradictory results; serum and plasma IL-1β concentrations in PTSD patients were either elevated or did not differ from control groups. In vitro IL-1β production by stimulated cells demonstrated no difference between PTSD and control participants, although spontaneous in vitro production of IL-1β was increased in the PTSD group. The findings from 2 longitudinal studies were inconsistent. Given the conflicting findings, it is premature to consider IL-1β as a biomarker of PTSD. Anti-inflammatory agents may reduce IL-1β, and be a potential basis for future therapeutic agents in PTSD treatment. More longitudinal research is needed to better understand the role of IL-1β in the development and/or maintenance of PTSD.

Animal Models of PTSD: The Socially Isolated Mouse and the Biomarker Role of Allopregnanolone

Post-traumatic stress disorder (PTSD) is a debilitating undertreated condition that affects 8%-13% of the general population and 20%-30% of military personnel. Currently, there are no specific medications that reduce PTSD symptoms or biomarkers that facilitate diagnosis, inform treatment selection or allow monitoring drug efficacy. PTSD animal models rely on stress-induced behavioral deficits that only partially reproduce PTSD neurobiology. PTSD heterogeneity, including comorbidity and symptoms overlap with other mental disorders, makes this attempt even more complicated. Allopregnanolone, a neurosteroid that positively, potently and allosterically modulates GABAA receptors and, by this mechanism, regulates emotional behaviors, is mainly synthesized in brain corticolimbic glutamatergic neurons. In PTSD patients, allopregnanolone down-regulation correlates with increased PTSD re-experiencing and comorbid depressive symptoms, CAPS-IV scores and Simms dysphoria cluster scores. In PTSD rodent models, including the socially isolated mouse, decrease in corticolimbic allopregnanolone biosynthesis is associated with enhanced contextual fear memory and impaired fear extinction. Allopregnanolone, its analogs or agents that stimulate its synthesis offer treatment approaches for facilitating fear extinction and, in general, for neuropsychopathologies characterized by a neurosteroid biosynthesis downregulation. The socially isolated mouse model reproduces several other deficits previously observed in PTSD patients, including altered GABAA receptor subunit subtypes and lack of benzodiazepines pharmacological efficacy. Transdiagnostic behavioral features, including expression of anxiety-like behavior, increased aggression, a behavioral component to reproduce behavioral traits of suicidal behavior in humans, as well as alcohol consumption are heightened in socially isolated rodents. Potentials for assessing novel biomarkers to predict, diagnose, and treat PTSD more efficiently are discussed in view of developing a precision medicine for improved PTSD pharmacological treatments.

Individual variability in behavior and functional networks predicts vulnerability using an animal model of PTSD

Only a minority of individuals experiencing trauma subsequently develop post-traumatic stress disorder (PTSD). However, whether differences in vulnerability to PTSD result from a predisposition or trauma exposure remains unclear. A major challenge in differentiating these possibilities is that clinical studies focus on individuals already exposed to trauma without pre-trauma conditions. Here, using the predator scent model of PTSD in rats and a longitudinal design, we measure pre-trauma brain-wide neural circuit functional connectivity, behavioral and corticosterone responses to trauma exposure, and post-trauma anxiety. Freezing during predator scent exposure correlates with functional connectivity in a set of neural circuits, indicating pre-existing circuit function can predispose animals to differential fearful responses to threats. Counterintuitively, rats with lower freezing show more avoidance of the predator scent, a prolonged corticosterone response, and higher anxiety long after exposure. This study provides a framework of pre-existing circuit function that determines threat responses, which might directly relate to PTSD-like behaviors.

Stress Across Generations: DNA Methylation as a Potential Mechanism Underlying Intergenerational Effects of Stress in Both Post-traumatic Stress Disorder and Pre-clinical Predator Stress Rodent Models

Although most humans will experience some type of traumatic event in their lifetime only a small set of individuals will go on to develop post-traumatic stress disorder (PTSD). Differences in sex, age, trauma type, and comorbidity, along with many other elements, contribute to the heterogenous manifestation of this disorder. Nonetheless, aberrant hypothalamus-pituitary-adrenal (HPA) axis activity, especially in terms of cortisol and glucocorticoid receptor (GR) alterations, has been postulated as a tenable factor in the etiology and pathophysiology of PTSD. Moreover, emerging data suggests that the harmful effects of traumatic stress to the HPA axis in PTSD can also propagate into future generations, making offspring more prone to psychopathologies. Predator stress models provide an ethical and ethologically relevant way to investigate tentative mechanisms that are thought to underlie this phenomenon. In this review article, we discuss findings from human and laboratory predator stress studies that suggest changes to DNA methylation germane to GRs may underlie the generational effects of trauma transmission. Understanding mechanisms that promote stress-induced psychopathology will represent a major advance in the field and may lead to novel treatments for such devastating, and often treatment-resistant trauma and stress-disorders.

Region- and time-dependent gene regulation in the amygdala and anterior cingulate cortex of a PTSD-like mouse model

Posttraumatic stress disorder is developed by exposure to a threatening and/or a horrifying event and characterized by the presence of anxiety, hyperarousal, avoidance, and sleep abnormality for a prolonged period of time. To elucidate the potential molecular mechanisms, we constructed a mouse model by electric foot shock followed by situational reminders and performed transcriptome analysis in brain tissues. The stressed mice acquired anxiety-like behavior after 2 weeks and exaggerated startle response after 4 weeks. Avoidance latency and freezing behavior were sustained up to 5 weeks post stress and abnormal static behavior was observed during the sleep period. RNA sequencing was performed in two of the emotional regulatory regions, anterior cingulate cortex and amygdala, at 2 and 5 weeks post stress. More than 1000 differentially expressed genes were identified at 2 weeks in both regions. The number of the regulated genes remained constant in amygdala at 5 weeks post stress, whereas those in anterior cingulate cortex were plummeted. Although synaptic remodeling and endocrine system were the most enriched signaling pathways in both anterior cingulate cortex and amygdala, the individual gene expression profile was regulated in a region- and time-dependent manner. In addition, several genes associated with PTSD involved in Hypothalamic-Pituitary-Adrenal axis were differentially regulated. These findings suggested that global gene expression profile was dynamically regulated in accordance with the disease development stage, and therefore targeting the distinct signaling molecules in different region and development stage might be critical for effective treatment to PTSD.

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.