5-HT2A receptor antagonism by MDL 11,939 during inescapable stress prevents subsequent exaggeration of acoustic startle response and reduced body weight in rats

Integrated proteomics and connectivity map-based analysis reveal compounds with a potential antiviral effect against Japanese encephalitis virus infection in a mouse model

Japanese encephalitis virus (JEV) is the leading causative agent of viral encephalitis in India and contributes to a significant disease burden in South Asian countries. However, no antiviral treatment is available against JEV-induced encephalitis, highlighting the urgent need for novel therapeutic approaches. Repurposing or repositioning drugs was found to be more economical and practical in the current drug development scenario. The present study aimed to develop a host-directed strategy through a computational drug repurposing approach. As part of the strategy, we first generated a dynamic signature of differentially expressed JEV infection-associated proteins in mice brains through a semiquantitative proteomics approach. With the help of the Connectivity Map (CMap) analysis, we narrowed down the lists of drugs with a high negative CMap score (-70 or lower). Based on the CMap score, we chose the top three compounds (Tipifarnib, Ly303511 and MDL11939) with CMap scores of -91.83, -88.18 and -91.15, respectively. The antiviral potential of these three compounds was further compared in both JEV-infected mouse neuroblastoma cells and C57BL/6 mice. Oral administration of Ly303511 and MDL11939, alone or in combination, showed improved outcomes (e.g. delayed death, increased survival, and less viral load than Tipifarnib alone or combined). The JEV-infected mice survived upon drug treatment, effectively reducing viral load and reversing the antiviral signature. Our results highlight Ly303511 and MDL11939 as promising host-targeted inhibitors of JEV infection and pathogenesis. Moreover, our results favor the combination of Ly303511 and MDL11939 therapy to improve clinical symptoms and reduce JEV-induced damage, thus warranting inclusion in clinical studies.

Pimavanserin reverses multiple measures of anxiety in a rodent model of post-traumatic stress disorder

Pimavanserin is a highly selective 5-HT_2A inverse agonist in current medical use. Prior studies suggest that 5-HT_2A serotonin receptors may play a role in anxiety and emotional memory. Therefore, pimavanserin was tested in a rat model of PTSD to determine whether it might ameliorate PTSD-like symptoms. The lifetime prevalence of PTSD is estimated to be 125% higher in women than men. Consequently, in an effort to create a robust model of PTSD that was more representative of human PTSD prevalence, 20-week old female rats of the emotionally hyperreactive Lewis strain were used for these studies. The rats were single-housed and exposed twice to restraint stress coupled with predator odor or to a sham-stressed condition. Twenty days after the second stress or sham-stress exposure, rats were injected with saline alone or with 0.3 or 1.0 mg/kg pimavanserin, doses that were confirmed to substantially block 5-HT_2A receptor activity in this study without causing any non-specific behavioral or adverse effects. One hour later, rats were tested for anxiety through acoustic startle response, the elevated plus-maze and three parameters of open field behavior. Five days later, blood was sampled for plasma corticosterone. The stressed/saline-injected rats had higher anxiety scores and corticosterone levels than sham-stressed/saline-injected rats. Pimavanserin significantly and generally dose-dependently reversed these persistent stress effects, but had no significant effect on the behavioral measures in normal, non-stressed rats. These results, consistent with a role for the 5-HT_2A receptor, suggest that pimavanserin might have potential to reduce some consequences of traumatic stress.

Post-traumatic stress disorder: a psychiatric disorder requiring urgent attention

Post-traumatic stress disorder (PTSD) is a severe and heterogenous psychiatric disorder that was first defined as a mental disorder in 1980. Currently, the Diagnostic and Statistical Manual of Mental Disorders Fifth Edition (DSM-5) and the International Classification of Diseases 11th Edition (ICD-11) offer the most widely accepted diagnostic guidelines for PTSD. In both diagnostic categories, experiencing a traumatic event (TE) is the necessary criterion for diagnosing PTSD. The TEs described in the DSM-5 include actual or threatened death, serious injury, sexual violence, and other extreme stressors, either directly or indirectly. More than 70% of adults worldwide are exposed to a TE at least once in their lifetime, and approximately 10% of individuals develop PTSD after experiencing a TE. The important features of PTSD are intrusion or re-experiencing fear memories, pervasive sense of threat, active avoidance, hyperarousal symptoms, and negative alterations of cognition and mood. Individuals with PTSD have high comorbidities with other psychiatric diseases, including major depressive disorder, generalized anxiety disorder, and substance use disorder. Multiple lines of evidence suggest that the pathophysiology of PTSD is complex, involving abnormal neural circuits, molecular mechanisms, and genetic mechanisms. A combination of both psychotherapy and pharmacotherapy is used to treat PTSD, but has limited efficacy in patients with refractory PTSD. Because of the high prevalence, heavy burden, and limited treatments, PTSD is a psychiatric disorder that requires urgent attention. In this review, we summarize and discuss the diagnosis, prevalence, TEs, pathophysiology, and treatments of PTSD and draw attention to its prevention.

Decreased BDNF in female but not male rats after exposure to stress: a sex-sensitive rat model of stress?

The literature has consistently emphasized a relationship between chronic stress and depression as well as the involvement of brain-derived neurotrophic factor (BDNF). It is also well known that there are gender disparities with regard to depression. However, there has been a lack of biobehavioral experimental investigations of these relationships with regard to the role of BDNF in sex differences in response to stress. It was hypothesized that stress (chronic unpredictable mild stress [CUMS], shock stress [SS]) would result in greater deleterious alterations in behavior (open field activity [OFA]) and biological (serum BDNF, body weight [BW]) indices of depression for female rats as compared to male rats. Subjects consisted of 79 Sprague-Dawley rats with 11-16 rats per each condition. CUMS consisted of 14 d of stress whereby on each stress day, rats were exposed to 20-min periods of predator stress and unpredictable environmental stress. SS consisted of a 2-h per day session of immobilization and tail-shocks repeated for three consecutive days. Serum BDNF was collected via trunk blood and quantitated using commercial enzyme-linked immunosorbent assay (ELISA). There were pronounced sex differences with regard to stress-induced behavioral and biological alterations. Both stressors decreased vertical activity (VA) (i.e. increased depressive-related behavior) and SS decreased serum BDNF in female rats, but not in male rats. Findings indicate a potential relationship between depressive-like behaviors and BDNF after exposure to stress. The clear sex differences in stress responding emphasize the need for more stress research that involves male and female subjects. Lay summary Stress decreased vertical activity (VA) in female but not male rats while shock stress (SS) decreased serum BDNF in female but not male rats. VA was positively correlated with serum BDNF for female rats. These findings suggest sex differences in response to stress.

Increases in dendritic spine density in BLA without metabolic changes in a rodent model of PTSD

Imaging studies have shown abnormal amygdala function in patients with posttraumatic stress disorder (PTSD). In addition, alterations in synaptic plasticity have been associated with psychiatric disorders and previous reports have indicated alterations in the amygdala morphology, especially in basolateral (BLA) neurons, are associated with stress-related disorders. Since, some individuals exposed to a traumatic event develop PTSD, the goals of this study were to evaluate the early effects of PTSD on amygdala glucose metabolism and analyze the possible BLA dendritic spine plasticity in animals with different levels of behavioral response. We employed the inescapable footshock protocol as an experimental model of PTSD and the animals were classified according to the duration of their freezing behavior into distinct groups: "extreme behavioral response" (EBR) and "minimal behavioral response". We evaluated the amygdala glucose metabolism at baseline (before the stress protocol) and immediately after the situational reminder using the microPET and the radiopharmaceutical ¹⁸F-FDG. The BLA dendritic spines were analyzed according to their number, density, shape and morphometric parameters. Our results show the EBR animals exhibited longer freezing behavior and increased proximal dendritic spines density in the BLA neurons. Neither the amygdaloid glucose metabolism, the types of dendritic spines nor their morphometric parameters showed statistically significant differences. The extreme behavior response induced by this PTSD protocol produces an early increase in BLA spine density, which is unassociated with either additional changes in the shape of spines or metabolic changes in the whole amygdala of Wistar rats.

REBUS and the Anarchic Brain: Toward a Unified Model of the Brain Action of Psychedelics

This paper formulates the action of psychedelics by integrating the free-energy principle and entropic brain hypothesis. We call this formulation relaxed beliefs under psychedelics (REBUS) and the anarchic brain, founded on the principle that-via their entropic effect on spontaneous cortical activity-psychedelics work to relax the precision of high-level priors or beliefs, thereby liberating bottom-up information flow, particularly via intrinsic sources such as the limbic system. We assemble evidence for this model and show how it can explain a broad range of phenomena associated with the psychedelic experience. With regard to their potential therapeutic use, we propose that psychedelics work to relax the precision weighting of pathologically overweighted priors underpinning various expressions of mental illness. We propose that this process entails an increased sensitization of high-level priors to bottom-up signaling (stemming from intrinsic sources), and that this heightened sensitivity enables the potential revision and deweighting of overweighted priors. We end by discussing further implications of the model, such as that psychedelics can bring about the revision of other heavily weighted high-level priors, not directly related to mental health, such as those underlying partisan and/or overly-confident political, religious, and/or philosophical perspectives. SIGNIFICANCE STATEMENT: Psychedelics are capturing interest, with efforts underway to bring psilocybin therapy to marketing authorisation and legal access within a decade, spearheaded by the findings of a series of phase 2 trials. In this climate, a compelling unified model of how psychedelics alter brain function to alter consciousness would have appeal. Towards this end, we have sought to integrate a leading model of global brain function, hierarchical predictive coding, with an often-cited model of the acute action of psychedelics, the entropic brain hypothesis. The resulting synthesis states that psychedelics work to relax high-level priors, sensitising them to liberated bottom-up information flow, which, with the right intention, care provision and context, can help guide and cultivate the revision of entrenched pathological priors.

Updates in PTSD Animal Models Characterization

Post-traumatic stress disorder (PTSD) is a chronic, debilitating mental disorder afflicting more than 7% of the US population and 12% of military service members. Since the Afghanistan and Iraq wars, thousands of US service members have returned home with PTSD. Despite recent progress, the molecular mechanisms underlying the pathology of PTSD are poorly understood. To promote research on PTSD (especially its molecular mechanisms) and to set a molecular basis for discovering novel medications for this disorder, well-validated animal models are needed. However, to develop PTSD animal models is a challenging process, due to predisposing factors such as physiological, behavioral, emotional, and cognitive changes that emerge after trauma. Currently, there is no well-validated animal model of PTSD, although several stress paradigms mimic the behavioral symptoms and neurological alterations seen in PTSD. In this chapter, we will provide an overview of animal models of PTSD including learned helplessness, footshock, restraint stress, inescapable tail shock, single-prolonged stress, underwater trauma, social isolation, social defeat, early-life stress, and predator-based stress. We emphasize rodent models because they reproduce some of the behavioral and biotical phenotypes seen in PTSD. We will also present data showing that homologous biological measures are increasingly incorporated in studies to assess markers of risk and therapeutic response in these models. Therefore, PTSD animal models may be refined in hopes of capitalizing on the understanding of the molecular mechanisms and delivering tools in order to develop new and more efficacious treatments for PTSD.

Corticosterone mitigates the stress response in an animal model of PTSD

Activation of glucocorticoid receptor signaling in the stress response to traumatic events has been implicated in the pathogenesis of stress-associated psychiatric disorders such as post-traumatic stress disorder (PTSD). Elevated startle response and hyperarousal are hallmarks of PTSD, and are generally considered to evince fear (DSM V). To further examine the efficacy of corticosterone in treating hyperarousal and elevated fear, the present study utilized a learned helplessness stress model in which rats are restrained and subjected to tail shock for three days. These stressed rats develop a delayed long-lasting exaggeration of the acoustic startle response (ASR) and retarded body weight growth, similar to symptoms of PTSD patients (Myers et al., 2005; Speed et al., 1989). We demonstrate that both pre-stress and post-stress administration of corticosterone (3 mg/kg/day) mitigates a subsequent exaggeration of the ASR measured 14 days after cessation of the stress protocol. Furthermore, the mitigating efficacy of pre-stress administration of corticosterone (3 mg/kg/day for three days) appeared to last significantly longer, up to 21 days after the cessation of the stress protocol, in comparison to that of post-stress administration of corticosterone. However, pre-stress administration of corticosterone at 0.3 mg/kg/day for three days did not mitigate stress-induced exaggeration of the ASR measured at both 14 and 21 days after the cessation of the stress protocol. In addition, pre-stress administration of corticosterone (3 mg/kg/day for three days) mitigates the retardation of body weight growth otherwise resulting from the stress protocol. Congruently, co-administration of the corticosterone antagonist RU486 (40 mg/kg/day for three days) with corticosterone (3 mg/kg/day) prior to stress diminished the mitigating efficacy of the exogenous corticosterone on exaggerated ASR and stress-retarded body weight. The relative efficacy of pre versus post administration of corticosterone and high versus low dose of corticosterone on stress-induced exaggeration of innate fear response and stress-retarded body weight growth indicate that exogenous corticosterone administration within an appropriate time window and dosage are efficacious in diminishing traumatic stress induced pathophysiological processes. Clinical implications associated with the efficacy of prophylactic and therapeutic corticosterone therapy for mitigating symptoms of PTSD are discussed, particularly in relation to diminishing hyperarousal and exaggerated innate fear response.

Association between initial morphine intake and body weight change, acoustic startle reflex and drug seeking in rats

RATIONALE

Although chronic use of opiates can induce physical dependence and addiction, individual differences contributing to these symptoms are largely unknown.

OBJECTIVES

Using intravenous morphine self-administration (MSA), we investigated whether individual differences in drug intake are associated with weight change, acoustic startle reflex (ASR), pre-pulse inhibition (PPI), and drug seeking during spontaneous withdrawal.

METHODS

Male Sprague-Dawley rats self-administered morphine (0.5 mg/kg/infusion) or saline for 3 weeks (4-6 h/day, 5 days/week) and drug intake and body weight were monitored daily. The ASR and the PPI (baseline, 1 day and 1 week) and drug seeking (1 week) were measured during spontaneous withdrawal.

RESULTS

Morphine animals did not gain weight (101 % ± 0.69), while the control animals did (115 %  ± 1.06) after 3 weeks of self-administration. The ASR and the PPI were not significantly different between morphine and saline animals in 1-day or 1-week withdrawal. However, individual differences in initial (first 10 min), but not total (4-6 h), morphine intake of the daily sessions were positively correlated with weight change (r = 0.437, p = 0.037) and drug seeking (r = 0.424, p = 0.035) while inversely correlated with the ASR (r = -0.544, p = 0.005) in 1-week withdrawal from chronic morphine.

CONCLUSIONS

A subgroup of animals that self-administered a larger amount of morphine at the beginning of the daily sessions exhibited subsequent weight gain, reduced ASR, and enhanced drug seeking in morphine withdrawal. Thus, individual differences in initial morphine intake may reveal a novel behavioral phenotype in opioid addiction.

Activation of 5-HT2a receptors in the basolateral amygdala promotes defeat-induced anxiety and the acquisition of conditioned defeat in Syrian hamsters

Conditioned defeat is a model in Syrian hamsters (Mesocricetus auratus) in which normal territorial aggression is replaced by increased submissive and defensive behavior following acute social defeat. The conditioned defeat response involves both a fear-related memory for a specific opponent as well as anxiety-like behavior indicated by avoidance of novel conspecifics. We have previously shown that systemic injection of a 5-HT2a receptor antagonist reduces the acquisition of conditioned defeat. Because neural activity in the basolateral amygdala (BLA) is critical for the acquisition of conditioned defeat and BLA 5-HT2a receptors can modulate anxiety but have a limited effect on emotional memories, we investigated whether 5-HT2a receptor modulation alters defeat-induced anxiety but not defeat-related memories. We injected the 5-HT2a receptor antagonist MDL 11,939 (0 mM, 1.7 mM or 17 mM) or the 5-HT2a receptor agonist TCB-2 (0 mM, 8 mM or 80 mM) into the BLA prior to social defeat. We found that injection of MDL 11,939 into the BLA impaired acquisition of the conditioned defeat response and blocked defeat-induced anxiety in the open field, but did not significantly impair avoidance of former opponents in the Y-maze. Furthermore, we found that injection of TCB-2 into the BLA increased the acquisition of conditioned defeat and increased anxiety-like behavior in the open field, but did not alter avoidance of former opponents. Our data suggest that 5-HT2a receptor signaling in the BLA is both necessary and sufficient for the development of conditioned defeat, likely via modulation of defeat-induced anxiety.

Serum and amygdala microRNA signatures of posttraumatic stress: fear correlation and biomarker potential

Exposure to acute traumatic stress can cause permanent changes in neurological circuitry and may lead to the development of an anxiety disorder known as posttraumatic stress disorder (PTSD). Current diagnosis of PTSD is based on clinical or behavioral symptom assessment, however, these are not definitive due to overlapping symptoms with other psychiatric disorders or mild traumatic brain injury (mTBI). No FDA approved diagnostic tests or biomarkers are currently available for diagnosis of PTSD. Recently, circulating miRNAs have emerged as novel biomarkers of many diseases. In this study, we have examined the altered expression of serum and amygdala miRNAs in an animal model of PTSD. Differentially expressed and statistically significant miRNAs in serum were validated for their presence in amygdala of corresponding animals. A panel of nine stress-responsive miRNAs viz., miR-142-5p, miR-19b, miR-1928, miR-223-3p, miR-322∗, miR-324, miR-421-3p and miR-463∗ and miR-674∗ were identified, and may have potential as biomarker(s) for PTSD. Further validations by bioinformatics and system biology approaches indicate that five miRNAs such as miR-142-5p, miR-19b, miR-1928, miR-223 and miR-421-3p may play a potential role in the regulation of genes associated with delayed and exaggerated fear. To the best of our knowledge, this is the first report demonstrating the plausibility of using circulating miRNAs as biomarkers of PTSD.

Acute social defeat does not alter cerebral 5-HT2A receptor binding in male Wistar rats

It has been hypothesized that effects of uncontrollable stress on serotonin receptor expression contribute to the etiology of stress-related disorders like depression. While the serotonin-2A receptors (5-HT2A R) are thought to be important in this context, only few studies examined effects of stress on this receptor subtype. In this study, we therefore assessed acute and long-term changes in 5HT2A R binding after social defeat stress in rats. Male Wistar rats were subjected to social defeat by placing them in the home cage of an aggressive, dominant Long Evans rat. Acute social defeat suppressed growth, but did not affect anxiety-like behavior in an open field test. A positron emission tomography scan with the 5-HT2A R tracer [11C]MDL 100907 1 day and 3 weeks after defeat did not show significant changes in receptor binding. To verify these results, [3H]MDL 100907 binding assays were performed in homogenates of prefrontal cortex and hippocampus, which also did not indicate any changes in Bmax or Kd . These findings do not support the hypothesis that changes in 5-HT2A R function are a vital mechanism through which uncontrollable stress contributes to stress-related pathologies such as depression. It remains to be determined whether effects of stress on 5HT2A R binding depend on the nature of the stressor or on the characteristics of the rat strain.

Impact of repeated stress on traumatic brain injury-induced mitochondrial electron transport chain expression and behavioral responses in rats

A significant proportion of the military personnel returning from Iraq and Afghanistan conflicts have suffered from both mild traumatic brain injury (mTBI) and post-traumatic stress disorder. The mechanisms are unknown. We used a rat model of repeated stress and mTBI to examine brain activity and behavioral function. Adult male Sprague-Dawley rats were divided into four groups: Naïve; 3 days repeated tail-shock stress; lateral fluid percussion mTBI; and repeated stress followed by mTBI (S-mTBI). Open field activity, sensorimotor responses, and acoustic startle responses (ASRs) were measured at various time points after mTBI. The protein expression of mitochondrial electron transport chain (ETC) complex subunits (CI-V) and pyruvate dehydrogenase (PDHE1α1) were determined in four brain regions at day 7-post mTBI. Compared to Naïves, repeated stress decreased horizontal activity; repeated stress and mTBI both decreased vertical activity; and the mTBI and S-mTBI groups were impaired in sensorimotor and ASRs. Repeated stress significantly increased CI, CII, and CIII protein levels in the prefrontal cortex (PFC), but decreased PDHE1α1 protein in the PFC and cerebellum, and decreased CIV protein in the hippocampus. The mTBI treatment decreased CV protein levels in the ipsilateral hippocampus. The S-mTBI treatment resulted in increased CII, CIII, CIV, and CV protein levels in the PFC, increased CI level in the cerebellum, and increased CIII and CV levels in the cerebral cortex, but decreased CI, CII, CIV, and PDHE1α1 protein levels in the hippocampus. Thus, repeated stress or mTBI alone differentially altered ETC expression in heterogeneous brain regions. Repeated stress followed by mTBI had synergistic effects on brain ETC expression, and resulted in more severe behavioral deficits. These results suggest that repeated stress could have contributed to the high incidence of long-term neurologic and neuropsychiatric morbidity in military personnel with or without mTBI.

Predator odor stress alters corticotropin-releasing factor-1 receptor (CRF1R)-dependent behaviors in rats

Humans with stress-related anxiety disorders exhibit increases in arousal and alcohol drinking, as well as altered pain processing. Our lab has developed a predator odor stress model that produces reliable and lasting increases in alcohol drinking. Here, we utilize this predator odor stress model to examine stress-induced increases in arousal, nociceptive processing, and alcohol self-administration by rats, and also to determine the effects of corticotropin-releasing factor-1 receptors (CRF1Rs) in mediating these behavioral changes. In a series of separate experiments, rats were exposed to predator odor stress, then tested over subsequent days for thermal nociception in the Hargreaves test, acoustic startle reactivity, or operant alcohol self-administration. In each experiment, rats were systemically injected with R121919, a CRF1R antagonist, and/or vehicle. Predator odor stress increased thermal nociception (i.e., hyperalgesia) and acoustic startle reactivity. Systemic administration of R121919 reduced thermal nociception and hyperarousal in stressed rats but not unstressed controls, and reduced operant alcohol responding over days. Stressed rats exhibited increased sensitivity to the behavioral effects of R121919 in all three tests, suggesting up-regulation of brain CRF1Rs number and/or function in stressed rats. These results suggest that post-stress alcohol drinking may be driven by a high-nociception high-arousal state, and that brain CRF1R signaling mediates these stress effects.

Biomarkers in an animal model for revealing neural, hematologic, and behavioral correlates of PTSD

Identification of biomarkers representing the evolution of the pathophysiology of Post Traumatic Stress Disorder (PTSD) is vitally important, not only for objective diagnosis but also for the evaluation of therapeutic efficacy and resilience to trauma. Ongoing research is directed at identifying molecular biomarkers for PTSD, including traumatic stress induced proteins, transcriptomes, genomic variances and genetic modulators, using biologic samples from subjects' blood, saliva, urine, and postmortem brain tissues. However, the correlation of these biomarker molecules in peripheral or postmortem samples to altered brain functions associated with psychiatric symptoms in PTSD remains unresolved. Here, we present an animal model of PTSD in which both peripheral blood and central brain biomarkers, as well as behavioral phenotype, can be collected and measured, thus providing the needed correlation of the central biomarkers of PTSD, which are mechanistic and pathognomonic but cannot be collected from people, with the peripheral biomarkers and behavioral phenotypes, which can.

Our animal model of PTSD employs restraint and tail shocks repeated for three continuous days - the inescapable tail-shock model (ITS) in rats. This ITS model mimics the pathophysiology of PTSD ^{17, 7, 4, 10}. We and others have verified that the ITS model induces behavioral and neurobiological alterations similar to those found in PTSD subjects ^{17, 7, 10, 9}. Specifically, these stressed rats exhibit (1) a delayed and exaggerated startle response appearing several days after stressor cessation, which given the compressed time scale of the rat's life compared to a humans, corresponds to the one to three months delay of symptoms in PTSD patients (DSM-IV-TR PTSD Criterian D/E ¹³), (2) enhanced plasma corticosterone (CORT) for several days, indicating compromise of the hypothalamopituitary axis (HPA), and (3) retarded body weight gain after stressor cessation, indicating dysfunction of metabolic regulation.

The experimental paradigms employed for this model are: (1) a learned helplessness paradigm in the rat assayed by measurement of acoustic startle response (ASR) and a charting of body mass; (2) microdissection of the rat brain into regions and nuclei; (3) enzyme-linked immunosorbent assay (ELISA) for blood levels of CORT; (4) a gene expression microarray plus related bioinformatics tools ¹⁸. This microarray, dubbed rMNChip, focuses on mitochondrial and mitochondria-related nuclear genes in the rat so as to specifically address the neuronal bioenergetics hypothesized to be involved in PTSD.

Targeting memory processes with drugs to prevent or cure PTSD

INTRODUCTION

Post-traumatic stress disorder (PTSD) is a chronic debilitating psychiatric disorder resulting from exposure to a severe traumatic stressor and an area of great unmet medical need. Advances in pharmacological treatments beyond the currently approved SSRIs are needed.

AREAS COVERED

Background on PTSD, as well as the neurobiology of stress responding and fear conditioning, is provided. Clinical and preclinical data for investigational agents with diverse pharmacological mechanisms are summarized.

EXPERT OPINION

Advances in the understanding of stress biology and mechanisms of fear conditioning plasticity provide a rationale for treatment approaches that may reduce hyperarousal and dysfunctional aversive memories in PTSD. One challenge is to determine if these components are independent or reflect a common underlying neurobiological alteration. Numerous agents reviewed have potential for reducing PTSD core symptoms or targeted symptoms in chronic PTSD. Promising early data support drug approaches that seek to disrupt dysfunctional aversive memories by interfering with consolidation soon after trauma exposure, or in chronic PTSD, by blocking reconsolidation and/or enhancing extinction. Challenges remain for achieving selectivity when attempting to alter aversive memories. Targeting the underlying traumatic memory with a combination of pharmacological therapies applied with appropriate chronicity, and in combination with psychotherapy, is expected to substantially improve PTSD treatment.

A double dissociation in the effects of 5-HT2A and 5-HT2C receptors on the acquisition and expression of conditioned defeat in Syrian hamsters

Previous research indicates that serotonin enhances the development of stress-induced changes in behavior, although it is unclear which serotonin receptors mediate this effect. 5-HT2 receptors are potential candidates because activation at these receptors is associated with increased fear and anxiety. In this study, we investigated whether pharmacological treatments targeting 5-HT2 receptors would alter the acquisition and expression of conditioned defeat. Conditioned defeat is a social defeat model in Syrian hamsters in which individuals display increased submissive and defensive behavior and a loss of territorial aggression when tested with a novel intruder 24 hours after an acute social defeat. The nonselective 5-HT2 receptor agonist mCPP (0.0, 0.3, 1.0, or 3.0 mg/kg) was injected either prior to social defeat training or prior to conditioned defeat testing. Also, the 5-HT2A receptor antagonist MDL 11,939 (0.0, 0.5, or 2.0 mg/kg) was injected either prior to social defeat training or prior to conditioned defeat testing. Injection of mCPP prior to testing increased the expression of conditioned defeat, but injection of mCPP prior to training did not alter the acquisition of conditioned defeat. Conversely, injection of MDL 11,939 prior to training reduced the acquisition of conditioned defeat, but injection of MDL 11,939 prior to testing did not alter the expression of conditioned defeat. Our data suggest that mCPP activates 5-HT2C receptors during testing to enhance the display of submissive and defensive behavior, whereas MDL 11,939 blocks 5-HT2A receptors during social defeat to disrupt the development of the conditioned defeat response. In sum, these results suggest that serotonin acts at separate 5-HT2 receptors to facilitate the acquisition and expression of defeat-induced changes in social behavior.

5-hydroxytryptamine 2C receptors in the dorsal striatum mediate stress-induced interference with negatively reinforced instrumental escape behavior

Uncontrollable stress can interfere with instrumental learning and induce anxiety in humans and rodents. While evidence supports a role for serotonin (5-HT) and serotonin 2C receptors (5-HT(2C)R) in the behavioral consequences of uncontrollable stress, the specific sites of action are unknown. These experiments sought to delineate the role of 5-HT and 5-HT(2C)R in the dorsal striatum (DS) and the lateral/basolateral amygdala (BLA) in the expression of stress-induced instrumental escape deficits and exaggerated fear, as these structures are critical to instrumental learning and fear behaviors. Using in vivo microdialysis, we first demonstrated that prior uncontrollable, but not controllable, stress sensitizes extracellular 5-HT in the dorsal striatum, a result that parallels prior work in the BLA. Additionally, rats were implanted with bi-lateral cannula in either the DS or the BLA and exposed to uncontrollable tail shock stress. One day later, rats were injected with 5-HT(2C)R antagonist (SB242084) and fear and instrumental learning behaviors were assessed in a shuttle box. Separately, groups of non-stressed rats received an intra-DS or an intra-BLA injection of the 5-HT(2C)R agonist (CP809101) and behavior was observed. Intra-DS injections of the 5-HT(2C)R antagonist prior to fear/escape tests completely blocked the stress-induced interference with instrumental escape learning; a partial block was observed when injections were in the BLA. Antagonist administration in either region did not influence stress-induced fear behavior. In the absence of prior stress, intra-DS administration of the 5-HT(2C)R agonist was sufficient to interfere with escape behavior without enhancing fear, while intra-BLA administration of the 5-HT(2C)R agonist increased fear behavior but had no effect on escape learning. Results reveal a novel role of the 5-HT(2C)R in the DS in the expression of instrumental escape deficits produced by uncontrollable stress and demonstrate that the involvement of 5-HT(2C)R activation in stress-induced behaviors is regionally specific.