Serotonin and stress: protective or malevolent actions in the biobehavioral response to repeated trauma?

Maternal oxytocin administration mitigates nociceptive, social, and epigenetic impairments in adolescent offspring exposed to perinatal trauma

Adverse childhood experiences (ACEs) alter brain development, leading to vulnerability for chronic pain, mental health disorders, and suicidality. These effects often emerge during adolescence. Importantly, ACEs can occur prenatally, including when exposed to in utero intimate partner violence (IPV) or postnatally as maternal neglect. Maternal social support has demonstrated promise in the mitigation of ACE-related deficits. Oxytocin, which has a role in social-bonding and stress regulation, serves as a suitable surrogate for social support in preclinical studies. Therefore, we aimed to explore the effects of oxytocin on alleviating social deficits, nociception, and epigenetic changes resulting from models that aimed to mimic the stress normally induced following exposure to two ACEs: IPV in utero and maternal neglect. During pregnancy, dams were randomly assigned to experience the model of IPV or a sham insult. Following birth, offspring from the IPV group underwent 10 days of maternal separation. Dams received three days of oxytocin therapy while nursing. In adolescence, half of the offspring underwent a plantar surgery to induce pain. Overall, in adolescence, rats exposed to the ACEs exhibited increased nociceptive sensitivity and aberrant social interactions, particularly among males, further suggesting that ACEs can increase an individual's risk for chronic pain. The ACEs changed gene expression related to social behaviour and neuroplasticity. Maternal oxytocin normalized pain, social, and gene changes, while oxytocin levels in offspring correlated with nociceptive sensitivity. Although ACEs have enduring consequences, the outcomes are modifiable, and oxytocin may be a robust and implementable therapeutic capable of attenuating early adversity.

Increased stress vulnerability in the offspring of socially isolated rats: Behavioural, neurochemical and redox dysfunctions

Stressful events during pregnancy impact on the progeny neurodevelopment. However, little is known about preconceptional stress effects. The rat social isolation represents an animal model of chronic stress inducing a variety of dysfunctions. Moreover, social deprivation during adolescence interferes with key neurodevelopmental processes. Here, we investigated the development of behavioural, neurochemical and redox alterations in the male offspring of socially isolated female rats before pregnancy, reared in group (GRP) or in social isolation (ISO) from weaning until young-adulthood. To this aim, females were reared in GRP or in ISO conditions, from PND21 to PND70, when they were mated. Their male offspring was housed in GRP or ISO conditions through adolescence and until PND70, when passive avoidance-PA, novel object recognition-NOR and open field-OF tests were performed. Levels of noradrenaline (NA), serotonin (5-HT), 5-hydroxyindoleacetic acid (5-HIAA), glutamate (GLU) and GABA were assessed in the prefrontal cortex (PFC). Moreover, cortical ROS levels were quantified, as well as NF-kB and the NADPH oxidase NOX2 expression, redox status (expressed as GSH:GSSG ratio) and SOD1 amount. A significant decrease of the latency time in the PA was observed in the offspring of ISO females. In the NOR test, while a significant increase in the exploratory activity towards the novel object was observed in the offspring of GRP females, no significant differences were found in the offspring of ISO females. No significant differences were found in the OF test among experimental groups. Theoffspring of ISO females showed increased NA and 5-HIAA levels, whereas in the offspring persistently housed in isolation condition from weaninguntil adulthood, we detected reduced 5-HT levels and ehnanced 5-HIAA amount. No significant changes in GLU concentrations were detected, while decreased GABA content was observed in the offspring of ISO females exposed to social isolation. Increased ROS levels as well as reduced NF-κB, NOX2 expression were detected in the offspring of ISO females. This was accompanied by reduced redox status and enhanced SOD1 levels. In conclusion, our results suggest that female exposure to chronic social stress before pregnancy might have a profound influence on the offspring neurodevelopment in terms of cognitive, neurochemical and redox-related alterations, identifying this specific time window for possible preventive and therapeutic strategies.

Lateralization of the 5-HT1A receptors in the basolateral amygdala in metabolic and anxiety responses to chronic restraint stress

Behavioral and functional studies describe hemispheric asymmetry in anxiety and metabolic behaviors in responses to stress. However, no study has reported serotonergic receptor (the 5-HT1A receptor) lateralization in the basolateral amygdala (BLA) in vivo on anxiety and metabolic behaviors under stress. In the present study, the effect of unilateral and bilateral suppression of the 5-HT1A receptor in the BLA on anxiety, and metabolic responses to chronic restraint stress was assessed. Male Wistar rats 7 days after cannulation into the BLA received chronic restraint stress for 14 consecutive days. 20 minutes before induction of stress, WAY-100-635 (selective 5-HT1A antagonist) or sterile saline (vehicle) was administered either uni- or bi-laterally into the BLA. Behavioral (elevated plus maze; EPM, and open field test), and metabolic parameter studies were performed. Results showed that stress causes a significant increase in weight gain compared to control. In the non-stress condition, the left and bilaterally, and in the stress condition the right, left, and both sides, inhibition of 5-HT1A in the BLA reduced weight gain. In the restraint stress condition, only inhibition of the 5-HT1A receptor in the left BLA led to decreased food intake compared to the control group. In stress conditions, inhibition of the 5-HT1A receptor on the right, left, and bilateral BLA increased water intake compared to the stress group. Inhibition of the 5-HT1A receptor on the left side of the BLA by WAY-100-635 induced anxiety-like behaviors in stressed rats. Similarly, WAY-100-635 on the left BLA effectively caused anxiety-like behaviors in both EPM and open field tests in the control animals. In conclusion, it seems that 5-HT1A receptors in the left BLA are more responsible for anxiety-like behaviors and metabolic changes in responses to stress.

Vicarious Social Defeat Increases Conditioned Rewarding Effects of Cocaine and Ethanol Intake in Female Mice

Stress is a critical factor in the development of mood and drug use disorders. The social defeat model is not appropriate for female rodents due to their low level of aggression. Therefore, a robust female model of social stress needs to be developed and validated. The aim of the present study was to unravel the long-lasting effects of vicarious social defeat (VSD) on the conditioned rewarding effects of cocaine and ethanol intake in female mice. Although VSD seems to be a good model for inducing behavioral and physiologic endophenotypes induced by stress, there are no studies to date that characterize the effect of VSD on cocaine or alcohol use. The results confirm that VSD females showed an increase in corticosterone levels after a vicarious experience while also displaying an increase in anxiety- and anhedonic-like behaviors. Three weeks after the last VSD, vicariously defeated female mice showed an increased developed preference for a non-effective dose of cocaine in the conditioned place preference (CPP) paradigm and showed an increase in ethanol intake. Our results suggest that female mice vicariously experience a state of distress through the social observation of others suffering from adverse events, confirming the use of VSD as a valid model to study the response to social stress in females. The fact that VSD in females induced a comparable behavioral phenotype to that observed in physically defeated males could indicate a relationship with the higher rate of psychopathologies observed in women. Notwithstanding, more studies are needed to dissect the neurobiological and behavioral peculiarities of the female response to social stress.

A tale of two transmitters: serotonin and histamine as in vivo biomarkers of chronic stress in mice

Background

Stress-induced mental illnesses (mediated by neuroinflammation) pose one of the world’s most urgent public health challenges. A reliable in vivo chemical biomarker of stress would significantly improve the clinical communities’ diagnostic and therapeutic approaches to illnesses, such as depression.

Methods

Male and female C57BL/6J mice underwent a chronic stress paradigm. We paired innovative in vivo serotonin and histamine voltammetric measurement technologies, behavioral testing, and cutting-edge mathematical methods to correlate chemistry to stress and behavior.

Results

Inflammation-induced increases in hypothalamic histamine were co-measured with decreased in vivo extracellular hippocampal serotonin in mice that underwent a chronic stress paradigm, regardless of behavioral phenotype. In animals with depression phenotypes, correlations were found between serotonin and the extent of behavioral indices of depression. We created a high accuracy algorithm that could predict whether animals had been exposed to stress or not based solely on the serotonin measurement. We next developed a model of serotonin and histamine modulation, which predicted that stress-induced neuroinflammation increases histaminergic activity, serving to inhibit serotonin. Finally, we created a mathematical index of stress, S_i and predicted that during chronic stress, where S_i is high, simultaneously increasing serotonin and decreasing histamine is the most effective chemical strategy to restoring serotonin to pre-stress levels. When we pursued this idea pharmacologically, our experiments were nearly identical to the model’s predictions.

Conclusions

This work shines the light on two biomarkers of chronic stress, histamine and serotonin, and implies that both may be important in our future investigations of the pathology and treatment of inflammation-induced depression.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-022-02508-9.

Pharmacological Implications of Adjusting Abnormal Fear Memory: Towards the Treatment of Post-Traumatic Stress Disorder

Post-traumatic stress disorder (PTSD) is a unique clinical mental abnormality presenting a cluster of symptoms in which patients primarily experience flashbacks, nightmares and uncontrollable thoughts about the event that triggered their PTSD. Patients with PTSD may also have comorbid depression and anxiety in an intractable and long-term course, which makes establishing a comprehensive treatment plan difficult and complicated. The present article reviews current pharmacological manipulations for adjusting abnormal fear memory. The roles of the central monoaminergic systems (including serotonin, norepinephrine and dopamine) within the fear circuit areas and the involvement of the hypothalamic-pituitary-adrenal (HPA) axis and glucocorticoid receptor (GR) are explored based on attempts to integrate current clinical and preclinical basic studies. In this review, we explain how these therapeutic paradigms function based on their connections to stages of the abnormal fear memory process from condition to extinction. This may provide useful translational interpretations for clinicians to manage PTSD.

Decreased kynurenine pathway potentiate resilience to social defeat effect on cocaine reward

The kynurenine (KYN) pathway of tryptophan (TRP) degradation is activated by stress and inflammatory factors. It is now well established that social stress induces the activation of the immune system, with central inflammation and KYN metabolism being two of the main factors linking stress with depression. The aim of the present study was to evaluate the long-lasting changes in the KYN pathway induced by social defeat (SD) associated with the resilience or susceptibility to an increase in the conditioned rewarding effects of cocaine. Mice were exposed to repeated SD and 3 weeks later, a conditioned place preference (CPP) induced by a subthreshold dose of cocaine (1.5 mg/kg) was developed. KYN levels in plasma, cerebellum, hippocampus, striatum and limbic forebrain were studied at the end of the CPP procedure. Changes in the KYN pathway after exposure to pharmacological (oxytocin and indomethacin) and environmental interventions (environmental enrichment) were also evaluated. Our results showed that defeated susceptible (SD-S) mice had higher conditioning scores than resilient mice (SD-R). In addition, although KYN concentration was elevated in all defeated mice, SD-R mice showed smaller increases in KYN concentration in the cerebellum than SD-S mice. Oxytocin or Indomethacin treatment before SD normalized cocaine-induced CPP, although the increase in the KYN pathway was maintained. However, environmental enrichment before SD normalized cocaine-induced CPP and prevented the increase in the KYN pathway. The present study highlights the role of the KYN pathway and anti-inflammatory drugs acting on TRP metabolism as pharmacological targets to potentiate resilience to social stress effects.

Post-weaning Social Isolated Flinders Sensitive Line Rats Display Bio-Behavioural Manifestations Resistant to Fluoxetine: A Model of Treatment-Resistant Depression

Treatment-resistant depression (TRD) complicates the management of major depression (MD). The underlying biology of TRD involves interplay between genetic propensity and chronic and/or early life adversity. By combining a genetic animal model of MD and post-weaning social isolation rearing (SIR), we sought to produce an animal that displays more severe depressive- and social anxiety-like manifestations resistant to standard antidepressant treatment. Flinders Sensitive Line (FSL) pups were social or isolation reared from weaning [postnatal day (PND) 21], receiving fluoxetine (FLX) from PND 63 (10 mg/kg × 14 days), and compared to Sprague Dawley (SD) controls. Depressive-, anxiety-like, and social behaviour were assessed from PND 72 in the forced swim test (FST) and social interaction test (SIT). Post-mortem cortico-hippocampal norepinephrine (NE), serotonin (5-HT), and dopamine (DA), as well as plasma interleukin 6 (IL-6), tumour necrosis factor alpha (TNF-α), corticosterone (CORT), and dopamine-beta-hydroxylase (DBH) levels were assayed. FSL rats displayed significant cortico-hippocampal monoamine disturbances, and depressive- and social anxiety-like behaviour, the latter two reversed by FLX. SIR-exposed FSL rats exhibited significant immobility in the FST and social impairment which were, respectively, worsened by or resistant to FLX. In SIR-exposed FSL rats, FLX significantly raised depleted NE and 5-HT, significantly decreased DBH and caused a large effect size increase in DA and decrease in CORT and TNF-α. Concluding, SIR-exposed FSL rats display depressive- and social anxiety-like symptoms that are resistant to, or worsened by, FLX, with reduced plasma DBH and suppressed cortico-hippocampal 5-HT, NE and DA, all variably altered by FLX. Exposure of a genetic animal model of MD to post-weaning SIR results in a more intractable depressive-like phenotype as well as changes in TRD-related biomarkers, that are resistant to traditional antidepressant treatment. Given the relative absence of validated animal models of TRD, these findings are especially promising and warrant study, especially further predictive validation.

Adapting to Stress: Understanding the Neurobiology of Resilience

There is significant variation in the way individuals react and respond to extreme stress and adversity. While some individuals develop psychiatric conditions such as posttraumatic stress disorder or major depressive disorder, others recover from stressful experiences without displaying significant symptoms of psychological ill-health, demonstrating stress-resilience. To understand why some individuals exhibit characteristics of a resilient profile, the interplay between neurochemical, genetic, and epigenetic processes over time needs to be explained. In this review, we examine the hormones, neuropeptides, neurotransmitters, and neural circuits associated with resilience and vulnerability to stress-related disorders. We debate how this increasing body of knowledge could also be useful in the creation of a stress-resilient profile. Additionally, identification of the underlying neurobiological components related to resilience may offer a contribution to improved approaches toward the prevention and treatment of stress-related disorders.

Exploring a post-traumatic stress disorder paradigm in Flinders sensitive line rats to model treatment-resistant depression II: response to antidepressant augmentation strategies

OBJECTIVE

Post-traumatic stress disorder (PTSD) displays high co-morbidity with major depression and treatment-resistant depression (TRD). Earlier work demonstrated exaggerated depressive-like symptoms in a gene×environment model of TRD and an abrogated response to imipramine. We extended the investigation by studying the behavioural and monoaminergic response to multiple antidepressants, viz. venlafaxine and ketamine with/without imipramine.

METHODS

Male Flinders sensitive line (FSL) rats, a genetic model of depression, were exposed to a time-dependent sensitisation (TDS) model of PTSD and compared with stress naive controls. 7 days after the TDS procedures, immobility and coping (swimming and climbing), behaviours in the forced swim test (FST) as well as hippocampal and cortical 5-hydroxyindoleacetic acid (5HIAA) and noradrenaline (NA) levels were analysed. Response to imipramine, venlafaxine and ketamine treatment (all 10 mg/kg×7 days) alone and in combination were subsequently studied.

RESULTS

TDS exacerbated depressive-like behaviour of FSL rats in the FST. Imipramine, venlafaxine and ketamine were ineffective as monotherapy in TDS-exposed FSL rats. However, combining imipramine with either venlafaxine or ketamine resulted in significant anti-immobility effects and enhanced coping behaviours. Only ketamine+imipramine (frontal-cortical 5HIAA and NA), ketamine alone (frontal-cortical and hippocampal NA) and venlafaxine+imipramine (frontal-cortical NA) altered monoamine responses versus untreated TDS-exposed FSL rats.

CONCLUSION

Exposure of FSL rats to TDS inhibits antidepressant response at behavioural and neurochemical levels. Congruent with TRD, imipramine plus venlafaxine or ketamine overcame treatment resistance in these animals. These data further support the hypothesis that exposure of FSL rats to a PTSD-like paradigm produces a valid animal model of TRD and warrants further investigation.

Exploring a post-traumatic stress disorder paradigm in Flinders sensitive line rats to model treatment-resistant depression I: bio-behavioural validation and response to imipramine

OBJECTIVE

Co-morbid depression with post-traumatic stress disorder (PTSD) is often treatment resistant. In developing a preclinical model of treatment-resistant depression (TRD), we combined animal models of depression and PTSD to produce an animal with more severe as well as treatment-resistant depressive-like behaviours.

METHODS

Male Flinders sensitive line (FSL) rats, a genetic animal model of depression, were exposed to a stress re-stress model of PTSD [time-dependent sensitisation (TDS)] and compared with stress-naive controls. Seven days after TDS stress, depressive-like and coping behaviours as well as hippocampal and cortical noradrenaline (NA) and 5-hydroxyindoleacetic acid (5HIAA) levels were analysed. Response to sub-chronic imipramine treatment (IMI; 10 mg/kg s.c.×7 days) was subsequently studied.

RESULTS

FSL rats demonstrated bio-behavioural characteristics of depression. Exposure to TDS stress in FSL rats correlated negatively with weight gain, while demonstrating reduced swimming behaviour and increased immobility versus unstressed FSL rats. IMI significantly reversed depressive-like (immobility) behaviour and enhanced active coping behaviour (swimming and climbing) in FSL rats. The latter was significantly attenuated in FSL rats exposed to TDS versus unstressed FSL rats. IMI reversed reduced 5HIAA levels in unstressed FSL rats, whereas exposure to TDS negated this effect. Lowered NA levels in FSL rats were sustained after TDS with IMI significantly reversing this in the hippocampus.

CONCLUSION

Combining a gene-X-environment model of depression with a PTSD paradigm produces exaggerated depressive-like symptoms that display an attenuated response to antidepressant treatment. This work confirms combining FSL rats with TDS exposure as a putative animal model of TRD.

Intensity-Dependent Auditory Evoked Potential Defines Subgroups of Patients with PTSD: A Multimodality Imaging Study

Post Traumatic Stress Disorder (PTSD) is a complex and heterogeneous psychiatric disorder. Patients with chronic PTSD present with significant psychiatric co-morbidities. Current treatment guidelines recommend the use of selective serotonin reuptake inhibitors in PTSD patients, but only 50–60% of these patients respond favorably. To improve the response rate, understanding the underlying pathophysiology is necessary. Serotonergic transmission has been implicated in PTSD and the intensity-dependent auditory evoked potential (IDAEP) is presumed to reflect central serotonergic activity, thereby providing an opportunity to more precisely characterize PTSD patient populations and possibly predict response to treatment. We studied nineteen patients diagnosed with PTSD, matched for age and medications. Based on the stimulus intensity relationship, the patients were divided into two groups: augmenters and reducers. These groups were further evaluated using resting state quantitative EEG and HMPAO-based SPECT brain perfusion acquired simultaneously. The imaging data were correlated with behavioral measures characterizing symptoms of PTSD and measurements of mood state. The augmenters differed from the reducers in the resting state quantitative EEG and SPECT perfusion measures. The differences were primarily localized to the right hemisphere. There were significant differences in measures of PTSD symptoms, but not in overall measures of depression. Item analysis of depression measures showed a significant difference between the augmenters and the reducers in reported sleep difficulties, which correlated with the reported anxiety measures. Objective separation of patients into subgroups based on the IDAEP contributes to the understanding of PTSD biological substrate and can potentially lead to more effective treatment strategies.

Behavioral Activity and Some Markers of Posttraumatic Stress Disorder among Serotoninergic System Indicators and Glucocorticoid Metabolizing Enzymes in Rats with Different Duration of Hexenal Sleep

Post-traumatic stress disorder was imitated in rats with long and short hexenal sleep by exposure to cat odor. Rats with long hexenal sleep demonstrated the highest sensitivity to posttraumatic stress disorders and developed anxiety and depressive disorders. The duration of hexenal sleep correlated with changes in markers of post-traumatic stress disorder, e.g. activity of 11β-hydroxysteroid dehydrogenase-2 in the liver of non-stressed animals and serotonin and monoamine oxidase A activity in the brain of stressed animals.

Escitalopram reversed the traumatic stress-induced depressed and anxiety-like symptoms but not the deficits of fear memory

RATIONALE

Posttraumatic stress disorder (PTSD) is a trauma-induced mental disorder characterised by fear extinction dysfunction in which fear circuit monoamines are possibly associated. PTSD often coexists with depressive/anxiety symptoms, and selective serotonin reuptake inhibitors (SSRIs) are recommended to treat PTSD. However, therapeutic mechanisms of SSRIs underlying the PTSD fear symptoms remain unclear.

OBJECTIVES

Using a rodent PTSD model, we examined the effects of early SSRI intervention in mood and fear dysfunctions with associated changes of monoamines within the fear circuit areas.

METHODS

A 14-day escitalopram (ESC) regimen (5 mg/kg/day) was undertaken in two separate experiments in rats which previously received a protocol of single prolonged stress (SPS). In experiment 1, sucrose preference and elevated T-maze were used to index anhedonia depression and avoidance/escape anxiety profiles. In experiment 2, the percentage of freezing time was measured in a 3-day fear conditioning paradigm. At the end of our study, tissue levels of serotonin (5-HT) in the medial prefrontal cortex, amygdala, hippocampus, and striatum were measured in experiment 1, and the efflux levels of infralimbic (IL) monoamines were measured in experiment 2.

RESULTS

In experiment 1, ESC corrected both behavioural (depression/anxiety) and neurochemical (reduced 5-HT tissue levels in amygdala/hippocampus) abnormalities. In experiment 2, ESC was unable to correct the SPS-impaired retrieval of fear extinction. In IL, ESC increased the efflux level of 5-HT but failed to reverse SPS-reduced dopamine (DA) and noradrenaline (NA).

CONCLUSIONS

PTSD-induced mood dysfunction is psychopathologically different from PTSD-induced fear disruption in terms of disequilibrium of monoamines within the fear circuit areas.

Mechanisms Underlying Footshock and Psychological Stress-Induced Abrupt Awakening From Posttraumatic "Nightmares"

BACKGROUND

Posttraumatic nightmares are a highly prevalent and distressing symptom of posttraumatic stress disorder (PTSD), but have been the subject of limited phenomenological investigations.

METHODS

We utilized a communication box to establish PTSD symptoms in rats through exposure to footshock stress (FS) and psychological stress (PS). The immunohistochemical test and high-performance liquid chromatography with electrochemical detection were used to detect the activity and monoamine levels in the rats' arousal systems.

RESULTS

Twenty-one days after traumatic stress, 14.17% of FS and 12.5% of PS rats exhibited startled awakening, and the same rats showed hyperfunction of the locus coeruleus/noradrenergic system and hypofunction of the perifornical nucleus/orexinergic system. Changes in serotonin levels in the dorsal raphe nucleus showed opposite trends in the FS and PS rats that were startled awake. No differences were found in other sleep/arousal systems.

CONCLUSION

These results suggest that different clinically therapeutic strategies should be considered to treat different trauma-induced posttraumatic nightmares.

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.

A role for 5-HT1A receptors in the basolateral amygdala in the development of conditioned defeat in Syrian hamsters

The basolateral nucleus of the amygdala (BLA) is a key brain region regulating behavioral changes following stressful events, including social defeat. Previous research has shown that activation of serotonin (5-HT) 1A receptors in the BLA reduces conditioned fear and anxiety-like behavior. The objective of this study was to test whether 5-HT1A receptors in the BLA contribute to conditioned defeat in male Syrian hamsters (Mesocricetus auratus). We tested whether injection of the selective 5-HT1A receptor agonist flesinoxan (400 ng, 800 ng, or 1200 ng in 200 nl saline) into the BLA prior to social defeat would reduce the acquisition of conditioned defeat, and whether a similar injection prior to testing would reduce the expression of conditioned defeat. We also tested whether injection of the selective 5-HT1A receptor antagonist WAY-100635 (400 ng or 1600 ng in 200 nl saline) into the BLA prior to social defeat would enhance the acquisition of conditioned defeat, and whether a similar injection prior to testing would enhance the expression of conditioned defeat. We found that injection of flesinoxan into the BLA decreased both the acquisition and expression of conditioned defeat. However, injection of WAY-100635 into the BLA did not alter the acquisition or expression of conditioned defeat. These data indicate that pharmacological activation of 5-HT1A receptors in the BLA is sufficient to impair the acquisition and expression of conditioned defeat. Our results suggest that pharmacological treatments that activate 5-HT1A receptors in the BLA are capable of reducing the development of stress-induced changes in behavior.

Efficacy of chronic antidepressant treatments in a new model of extreme anxiety in rats

Animal models of anxious disorders found in humans, such as panic disorder and posttraumatic stress disorder, usually include spontaneous and conditioned fear that triggers escape and avoidance behaviors. The development of a panic disorder model with a learned component should increase knowledge of mechanisms involved in anxiety disorders. In our ethological model of extreme anxiety in the rat, forced apnea was combined with cold water vaporization in an inescapable situation. Based on the reactions of vehicle controls, behaviors involved in paroxysmic fear were passive (freezing) and active (jumping) reactions. Our results show that subchronic fluoxetine (5 mg/kg, IP, 21 days) and imipramine (10 mg/kg, IP, 14 days) administration alleviated freezing and jumping behaviors, whereas acute fluoxetine (1 mg/kg, IP) provoked opposite effects. Acute low dose of diazepam (1 mg/kg, IP) was not effective, whereas the higher dose of 3 mg/kg, IP, and clonazepam (1 mg/kg, IP) only had an effect on jumping. Paroxysmic fear generated in this experimental condition may therefore mimic the symptomatology observed in patients with anxiety disorders.

Single prolonged stress: toward an animal model of posttraumatic stress disorder

Although selective serotonin reuptake inhibitors (SSRIs) are reported to be effective in decreasing posttraumatic stress disorder (PTSD) symptoms, a subgroup of PTSD patients remain chronically symptomatic and maintain conditioned fear responses to traumatic stimuli. In this context, the establishment of an appropriate animal model of PTSD is necessary to promote better understanding of the mechanisms of the disorder and to facilitate the development of more effective therapeutic alternatives to SSRIs. Although no single widely accepted animal model of PTSD has been established to date, the single prolonged stress (SPS) animal model has been partially validated as a model for PTSD. SPS rats mimic the pathophysiological abnormalities and behavioral characteristics of PTSD, such as enhanced anxiety-like behavior and glucocorticoid negative feedback, and they exhibit the expected therapeutic response to paroxetine on enhanced fear memory. In addition, SPS rats exhibit enhanced freezing in response to contextual fear conditioning, and impaired extinction of fear memory, which is alleviated by D-cycloserine. The enhanced consolidation and impaired extinction of fear memory found in SPS rats suggests that this model has additional value because recent studies of PTSD indicate that memory abnormalities are a central feature. In this study, we summarize the behavioral and pathophysiological PTSD-like symptoms in SPS, focusing on memory abnormalities, and evaluate the validity of SPS as an animal model of PTSD.

Noradrenergic and serotonergic mechanisms in the neurobiology of posttraumatic stress disorder and resilience

Posttraumatic stress disorder (PTSD) is characterized mainly by symptoms of re-experiencing, avoidance and hyperarousal as a consequence of catastrophic and traumatic events that are distinguished from ordinary stressful life events. Although extensive research has already been done, the etiology of PTSD remains unclear. Research on the impact of trauma on neurobiological systems can be expected to inform the development of treatments that are directed specifically to symptoms of PTSD. During the past 25 years there has been a dramatic increase in the knowledge about noradrenergic and serotonergic mechanisms in stress response, PTSD and more recently in resilience and this knowledge has justified the use of antidepressants with monoaminergic mechanisms of action for patients with PTSD. Nevertheless, available treatments of PTSD are only to some extent effective and enhanced understanding of the neurobiology of PTSD may lead to the development of improved treatments for these patients. In the present review, we aim to close existing gaps between basic research in psychopathology, neurobiology and treatment development with the ultimate goal to translate basic research into clinically relevant findings which may directly benefit patients with PTSD.

Stress and re-stress increases conditioned taste aversion learning in rats: possible frontal cortical and hippocampal muscarinic receptor involvement

Symptoms of posttraumatic stress disorder are often precipitated by sensory cues in the form of visual, auditory, olfactory and gustatory "flashbacks" resulting in enhanced fear-memory consolidation and the characteristic symptoms of re-experiencing, avoidance and hyper-arousal. Single prolonged stress with and without re-stress have been used to explore the neurobiology of this disorder, particularly with respect to contextual conditioning and spatial memory impairment. However, less work has been done regarding associative sensory-related memories linked to aversive events. Although growing evidence supports a role for cholinergic pathways in stress, this has not been studied in the above animal models. We studied the effects of single prolonged stress with and without re-stress on conditioned taste aversion learning in rats, together with differential analysis of frontal cortical and hippocampal [3H]-quinuclidinyl benzylate ([3H]-QNB) muscarinic receptor binding. Single prolonged stress with and without re-stress both enhanced associative sensory aversion learning 7 days after stressor-taste pairing, although re-stress did not strengthen this response. Increased cortical and hippocampal muscarinic receptor density (Bmax) was found 7 days after single prolonged stress with re-stress, although receptor affinity remained unaltered. Frontal cortical and hippocampal muscarinic receptor changes may thus underlie conditioned taste aversion learning in rats exposed to stress and re-stress. These data suggest that it may be useful to study the role of cholinergic pathways in mediating associative memory in psychiatric disorders such as posttraumatic stress disorder.

Rats exposed to traumatic stress bury unfamiliar objects--a novel measure of hyper-vigilance in PTSD models?

Electric shocks lead to lasting behavioral deficits in rodents, and as such are often used to model post-traumatic stress disorder (PTSD) in the laboratory. Here we show that a single exposure of rats to 3 mA-strong shocks results in a marked social avoidance that lasts at least 28 days; moreover, the response intensifies over time. In an attempt to study the impact of cue reminders on the behavior of shocked rats, we administered shocks in the presence of a highly conspicuous, 10 cm-large object. This object was introduced into the home cage of rats 28 days after shock exposure. Shocked rats manipulated the object considerably less than controls. More importantly, however, the object was buried by shocked rats. This behavior was virtually absent in controls. The response strongly depended on the intensity of shocks, and was robust. Rats shocked with 3 mA currents spent 40% of time burying the object, which was often hardly visible at the end of the 5 min test. Subsequent experiments demonstrated that the response was not cue-specific as unfamiliar objects were also buried. Rats are well known to bury dangerous objects; the shock-prod burying test of anxiety is based on this response. Behavioral similarities with this test and the differences from the marble-burying behavior of mice suggest that traumatized rats bury unfamiliar objects in defense, and the response can be interpreted as a sign of hyper-vigilance. We further suggest that object burying can be used as a sign of hyper-vigilance in models of PTSD.

Modulation of stress consequences by hippocampal monoaminergic, glutamatergic and nitrergic neurotransmitter systems

Several findings relate the hippocampal formation to the behavioural consequences of stress. It contains a high concentration of corticoid receptors and undergoes plastic modifications, including decreased neurogenesis and cellular remodelling, following stress exposure. Various major neurotransmitter systems in the hippocampus are involved in these effects. Serotonin (5-HT) seems to exert a protective role in the hippocampus and attenuates the behavioural consequences of stress by activating 5-HT1A receptors in this structure. These effects may mediate the therapeutic actions of several antidepressants. The role of noradrenaline is less clear and possibly depends on the specific hippocampal region (dorsal vs. ventral). The deleterious modifications induced in the hippocampus by stress might involve a decrease in neurotrophic factors such as brain derived neurotrophic factor (BDNF) following glutamate N-methyl-D-aspartate (NMDA) receptor activation. In addition to glutamate, nitric oxide (NO) could also be related to these effects. Systemic and intra-hippocampal administration of nitric oxide synthase (NOS) inhibitors attenuates stress-induced behavioural consequences. The challenge for the future will be to integrate results related to these different neurotransmitter systems in a unifying theory about the role of the hippocampus in mood regulation, depressive disorder and antidepressant effects.

Increased hippocampal nitric oxide synthase activity and stress responsiveness after imipramine discontinuation: role of 5HT 2A/C-receptors

Chronic depressive illness may cause shrinkage of the hippocampus with stress-induced release of glutamate and nitric oxide possibly causally linked to this pathology. Poor antidepressant compliance may contribute to this pathology as well as to long term morbidity. However, antidepressant withdrawal-associated symptoms in depressed patients often reflect hyperserotonergia. The effect of chronic imipramine (IMI; 15 mg/kg/d ip x 3wks) treatment and withdrawal on swim stress responsiveness was studied in Sprague-Dawley rats together with assay of hippocampal NO synthase (NOS) activity. The dependence of any biobehavioral changes following IMI withdrawal on 5HT(2A/C) receptor-mediated events was studied using the 5HT(2A/C) receptor antagonist, ritanserin (RIT; 4 mg/kg/day ip x 7 days), administered alone or during IMI withdrawal. IMI significantly inhibited the situational stress response to forced swimming while also significantly decreasing NOS activity. IMI withdrawal was associated with a significant increase in swim immobility together with a significant increase in NOS activity compared to both control and IMI-treated groups. RIT re-established the anti-immobility effects and reversed NOS hyper-function during IMI withdrawal, although alone it increased NOS activity. Antidepressant discontinuation therefore increases stress responsiveness together with disinhibition of hippocampal NOS through a mechanism involving 5HT(2A/C) receptor activation. The resulting increased nitrergic activity may have significant implications for depressive illness and its treatment.

Cortical/hippocampal monoamines, HPA-axis changes and aversive behavior following stress and restress in an animal model of post-traumatic stress disorder

Post-traumatic stress disorder (PTSD) is characterized by monoaminergic and hypothalamic-pituitary-adrenal (HPA)-axis abnormalities. Understanding monoamine-HPA-axis responses following stress and restress may provide a greater understanding of the neurobiology of PTSD and of its treatment. Hippocampal and frontal cortex serotonin, noradrenaline and dopamine, plasma corticosterone and aversive behavior were studied in rats on day 1 and day 7 post acute stress (AS = sequential restraint stress, swim stress and halothane exposure), and on day 1 and day 7 post restress (RS = swim stress). After AS, there was an early increase in both avoidant behavior and corticosterone (1 h after stress), with subsequent normalisation (day 7), suggesting an adequate adaptive response to the stressor. However, restress (RS) evoked a significant early HPA-axis hyporesponsiveness (1 h after RS) and a later significant increase in avoidant behavior on day 7 post RS. Hippocampal serotonin, noradrenaline and dopamine concentrations were unchanged 1 h post AS, but were significantly raised on day 7 post AS. Restress, however, reduced serotonin and noradrenaline levels 1 h after and on day 7 post RS, respectively, while dopamine was unchanged. In the frontal cortex only dopamine levels were altered, being significantly elevated 1 h after AS, and reduced on day 7 post RS. AS and RS thus differently effect the HPA-axis, evoking regional-specific brain monoamine changes that underlie maladaptive behavior and other post stress-related sequelae.

Early life trauma decreases glucocorticoid receptors in rat dentate gyrus upon adult re-stress: reversal by escitalopram

Early exposure to adverse experiences may lead to specific changes in hippocampal glucocorticoid function resulting in abnormalities within the hypothalamic-adrenal axis. Given interactions between the neuroendocrine and central serotonergic systems, we hypothesized that exposure to early trauma would lead to abnormal hypothalamic-adrenal axis activity that would be normalized by pretreatment with a selective serotonin re-uptake inhibitor. Hypothalamic-adrenal axis function was assessed by determining basal corticosterone levels and hippocampal glucocorticoid receptor immunoreactivity. Rats were subjected to a triple stressor on postnatal day 28, and again to a single swim re-stress session on postnatal day 35 and postnatal day 60. On postnatal day 61 i.e. 24 h after the last re-stress, trunk blood was collected for serum corticosterone determinations and hippocampal tissue was collected for immunohistochemistry of glucocorticoid receptors. Escitalopram (5mg/kg) or saline vehicle was administered from postnatal day 47-postnatal day 60 via osmotic mini-pumps. Animals exposed to early life trauma showed an increase in basal corticosterone levels, and a significant decrease in the ratio of glucocorticoid receptor positive cells to total cells in the hilus, granule cell layer and the dentate gyrus. Both the increase in basal corticosterone and decrease in glucocorticoid receptor immunoreactivity were reversed by escitalopram pretreatment. These data confirm alterations in hypothalamic-adrenalaxis function that may stem from decreases in glucocorticoid receptor levels, in response to early adverse experiences, and demonstrate that these alterations are reversed by serotonin re-uptake inhibitor pretreatment.

Involvement of the NMDA receptor, NO-cyclic GMP and nuclear factor K-beta in an animal model of repeated trauma

Post-traumatic stress disorder (PTSD) may be associated with shrinkage of the hippocampus, with glutamate release causally related to these events. Recent animal studies strongly implicate activation of the nitric oxide (NO)-cascade in anxiety and stress. Using an animal model of repeated trauma, the effect of stress was investigated on the hippocampal NO-cGMP signalling pathway, specifically the release of nitrogen oxides (NOx) and its modulation by NMDA receptor-, NO-, cGMP- and nuclear factor K-beta (NFK-beta)-selective drugs. Immediately after stress, rats received the glutamate NMDA receptor antagonist, memantine (MEM; 5 mg/kg i.p./d), the NO synthase inhibitor, 7-nitroindazole sodium salt (7-NINA; 20 mg/kg i.p./d), the cGMP-specific PDE inhibitor, sildenafil (SIL; 10 mg/kg i.p./d) or the NFkappa-beta antagonist, pyrollidine dithiocarbamate (PDTC; 70 mg/kg i.p./d), for 7 days. Stress significantly increased hippocampal NOx on day 7 post-stress, which was blocked by either 7-NINA or PDTC, while MEM was without effect. SIL, however, significantly augmented stress-induced NOx accumulation. Increased cGMP therefore acts as a protagonist in driving stress-related events, while both nNOS (neuronal NOS) and iNOS (inducible/immunological NOS) may represent a therapeutic target in preventing the effects of severe stress. The value of NMDA receptor antagonism, however, appears limited in this model.

Nitric oxide as inflammatory mediator in post-traumatic stress disorder (PTSD): evidence from an animal model

Post-traumatic stress disorder (PTSD) is a severe anxiety disorder that may develop after experiencing or witnessing a traumatic event. Recent clinical evidence has suggested the involvement of neurodegenerative pathology in the illness, particularly with brain imaging studies revealing a marked reduction in hippocampal volume. Of greater significance is that these anatomical changes appear to be positively correlated with the degree of cognitive deficit noted in these patients. Stress-induced increases in plasma cortisol have been implicated in this apparent atrophy. Although not definitive, clinical studies have observed a marked suppression of plasma cortisol in PTSD. The basis for hippocampal neurodegeneration and cognitive decline therefore remains unclear. Stress and glucocorticoids increase glutamate release, which is recognized as an important mediator of glucocorticoid-induced neurotoxicity. Recent preclinical studies have also noted that glutamate and nitric oxide (NO) play a causal role in anxiety-related behaviors. Because of the prominent role of NO in neuronal toxicity, cellular memory processes, and as a neuromodulator, nitrergic pathways may have an important role in stress-related hippocampal degenerative pathology and cognitive deficits seen in patients with PTSD. This paper reviews the preclinical evidence for involvement of the NO-pathway in PTSD, and emphasizes studies that have addressed these issues using time-dependent sensitization - a putative animal model of PTSD.