A double-hit model of stress dysregulation in rats: implications for limbic corticosteroid receptors and anxious behavior under amitriptyline treatment

Early Life Stress Negatively Impacts Spatial Learning Acquisition and Increases Hippocampal CA1 Microglial Activation After a Mild Traumatic Brain Injury in Adult Male Rats

Early life stress (ELS) affects neurogenesis and spatial learning, and increases neuroinflammation after a pediatric mild traumatic brain injury (mTBI). Previous studies have shown that ELS has minimal effects in juveniles but shows age-dependent effects in adults. Hence, we aimed to evaluate the effects of ELS in adult male rats after an mTBI. Maternal separation for 180 min per day (MS180) during the first 21 post-natal (P) days was used as the ELS model. At P110, the rats were subjected to a mild controlled cortical impact injury (2.6 mm) or sham surgery. Spatial learning was evaluated in the Morris water maze (MWM) 14 days after surgery and both microglial activation and neurogenesis were quantified. The results indicate that MS180 + mTBI, but not control (CONT) + mTBI, rats show deficiencies in the acquisition of spatial learning. mTBI led to comparable increases in microglial activation in both the hilus and cortical regions for both groups. However, MS180 + mTBI rats exhibited a greater increase in microglial activation in the ipsilateral CA1 hippocampus subfield compared with CONT + mTBI. Interestingly, for the contralateral CA1 region, this effect was observed exclusively in MS180 + mTBI. ELS and mTBI independently caused a decrease in hippocampal neurogenesis and this effect was not increased further in MS180 + mTBI rats. The findings demonstrate that ELS and mTBI synergistically affect cognitive performance and neuroinflammation, thus supporting the hypothesis that increased inflammation resulting from the combination of ELS and mTBI could underlie the observed effects on learning.

Early life stress aggravates disease pathogenesis in mice with experimental autoimmune encephalomyelitis: Support for a two-hit hypothesis of multiple sclerosis etiology

Vision problems are one of the earliest diagnosed symptoms of multiple sclerosis (MS). The onset and progression of vision loss and the underlying pathogenesis in MS may be influenced by cumulative psychophysiological stress. Here, we used a two-hit model of stress in female mice to determine if early life stress (ELS, the first hit) influences the response to an immunization that induces experimental autoimmune encephalomyelitis (EAE, the second hit) later in life. We hypothesized that ELS caused by animal transportation from a vendor during early postnatal development represents a co-factor which can exacerbate the clinical severity of EAE. Indeed, adult EAE mice with a history of ELS displayed more severe clinical signs and delayed recovery compared to non-stressed EAE mice. ELS also diminished visual acuity measured by optokinetic responses, as well as locomotion and exploratory behaviours in EAE mice. Notably, ELS accelerated vision loss and caused earlier onset of visual impairments in EAE. Exacerbated functional impairments in stressed EAE mice were highly correlated with circulating corticosterone levels. The findings show that the progression of induced EAE in adulthood can be significantly impacted by adverse early life experiences. These observations emphasize the importance of comprehensive behavioural testing, including non-motor functions, to enhance the translational value of preclinical animal models of MS. Moreover, shipment stress of laboratory animals should be considered a necessary variable in preclinical MS research. The consideration of cumulative lifetime stresses provides a new perspective of MS pathogenesis within a personalized medicine framework.

Amitriptyline Improves Cognitive and Neuronal Function in a Rat Model that Mimics Dementia with Lewy Bodies

Dementia with Lewy bodies (DLB), a highly prevalent neurodegenerative disorder, causes motor and cognitive deficits. The main pathophysiologies of DLB are glutamate excitotoxicity and accumulation of Lewy bodies comprising α-synuclein (α-syn) and β-amyloid (Aβ). Amitriptyline (AMI) promotes expression of glutamate transporter-1 and glutamate reuptake. In this study, we measured the effects of AMI on behavioral and neuronal function in a DLB rat model. We used rivastigmine (RIVA) as a positive control. To establish the DLB rat model, male Wistar rats were stereotaxically injected with recombinant adenoassociated viral vector with the SNCA gene (10μg/10μL) and Aβ (5μg/2.5μL) into the left ventricle and prefrontal cortex, respectively. AMI (10mg/kg/day, i.p.), RIVA (2mg/kg/day, i.p.), or saline was injected intraperitoneally after surgery. From the 29^th day, behavioral tests were performed to evaluate the motor and cognitive functions of the rats. Immunohistochemical staining was used to assess neuronal changes. We measured the α-syn level, number of newborn cells, and neuronal density in the hippocampus and in the nigrostriatal dopaminergic system. The DLB group exhibited deficit in object recognition. Both the AMI and RIVA treatments reversed these deficits. Histologically, the DLB rats exhibited cell loss in the substantia nigra pars compacta and in the hippocampal CA1 area. AMI reduced this cell loss, but RIVA did not. In addition, the DLB rats exhibited a lower number of newborn cells and higher α-syn levels in the dentate gyrus (DG). AMI did not affect α-syn accumulation but recovered neurogenesis in the DG of the rats, whereas RIVA reversed the α-syn accumulation but did not affect neurogenesis in the rats. We suggest that AMI may have potential for use in the treatment of DLB.

The Effects of Furosemide on Behavioral and Hormonal Parameters in Male and Female Mice Subjected to Immobilization and Cold-Water Stress

Introduction

The diuretic agent furosemide (FUR, 25 and 50 mg/kg) has been shown in a single report to act as an anti-stressor agent in two models of acute stress in mice, viz. electric foot-shock stress and immobilization (IMS). The present work aimed to investigate the possible anti-stressor action of FUR on two models of acute stress in mice, cold-water stress (CWS) and IMS, and tried to determine whether gender has any impact on the effect of FUR.

Methods

FUR (40 mg/kg) was injected intraperitoneally, and after 30 minutes, mice were subjected to CWS (4°C for three minutes) or IMS (fixing movement for two and a half hrs using adhesive tape). Motor and exploratory activities, neuromuscular coordination, and thermal nociception were then tested. Blood was collected from the mice and used to measure the concentrations of three stress hormones (corticosterone, epinephrine and prolactin).

Results

Mice subjected to CWS and IMS had significantly reduced motor and exploratory activities, neuromuscular coordination, and increased nociception. CWS and IMS also significantly increased the plasma concentrations of the three hormones. FUR pretreatment significantly mitigated these stress-induced hormonal changes. There was no significant sex difference when CWS or IMS was applied.

Discussion

IMS and CWS stimuli in male and female mice caused significant elevations in the plasma concentrations of corticosterone, epinephrine, and prolactin, accompanied by a significant reduction of motor and exploratory activities, neuromuscular coordination, and thermal nociception. There were no sex differences when IMS was applied. In stressed mice, prior administration of FUR (40 mg/kg) significantly decreased the concentrations of stress hormones, and this effect significantly mitigated the stress-induced behavioural and motor changes.

Early Life Stress Preceding Mild Pediatric Traumatic Brain Injury Increases Neuroinflammation but Does Not Exacerbate Impairment of Cognitive Flexibility during Adolescence

Early life stress (ELS) followed by pediatric mild traumatic brain injury (mTBI) negatively impacts spatial learning and memory and increases microglial activation in adolescent rats, but whether the same paradigm negatively affects higher order executive function is not known. Hence, we utilized the attentional set-shifting test (AST) to evaluate executive function (cognitive flexibility) and to determine its relationship with neuroinflammation and hypothalamic-pituitary-adrenal (HPA) axis activity after pediatric mTBI in male rats. ELS was induced via maternal separation for 180 min per day (MS180) during the first 21 post-natal (P) days, while controls (CONT) were undisturbed. At P21, fully anesthetized rats received a mild controlled cortical impact (2.2 mm tissue deformation at 4 m/sec) or sham injury. AST was evaluated during adolescence on P35-P40 and cytokine expression and HPA activity were analyzed on P42. The data indicate that pediatric mTBI produced a significant reversal learning deficit on the AST versus sham (p < 0.05), but that the impairment was not exacerbated further by MS180. Additionally, ELS produced an overall elevation in set-loss errors on the AST, and increased hippocampal interleukin (IL)-1β expression after TBI. A significant correlation was observed in executive dysfunction and IL-1β expression in the ipsilateral pre-frontal cortex and hippocampus. Although the combination of ELS and pediatric mTBI did not worsen executive function beyond that of mTBI alone (p > 0.05), it did result in increased hippocampal neuroinflammation relative to mTBI (p < 0.05). These findings provide important insight into the susceptibility to incur alterations in cognitive and neuroimmune functioning after stress exposure and TBI during early life.

Sex-Dependent Modulation of Acute Stress Reactivity After Early Life Stress in Mice: Relevance of Mineralocorticoid Receptor Expression

Early life stress (ELS) is considered a major risk factor for developing psychopathology. Increasing evidence points towards sex-dependent dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis as a contributing mechanism. Additionally, clinical studies suggest that the mineralocorticoid receptor (MR) may further confer genetic vulnerability/resilience on a background of ELS. The link between ELS, sex and the HPA axis and how this interacts with MR genotype is understudied, yet important to understand vulnerability/resilience to stress. We used the early life-limited nesting and bedding model to test the effect of ELS on HPA properties in adult female and male mice carrying a forebrain-specific heterozygous knockout for MR. Basal HPA axis activity was measured by circadian peak and nadir corticosterone levels, in addition to body weight and weight of stress-sensitive tissues. HPA axis reactivity was assessed by mapping corticosterone levels after 10 min immobilization. Additionally, we measured the effects of ELS on steroid receptor [MR and glucocorticoid receptor (GR)] levels in the dorsal hippocampus and medial prefrontal cortex (mPFC) with western blot. Finally, behavioral reactivity towards a novel environment was measured as a proxy for anxiety-like behavior. Results show that HPA axis activity under rest conditions was not affected by ELS. HPA axis reactivity after immobilization was decreased by ELS in females and increased, at trend-level in males. This effect in females was further exacerbated by low expression of the MR. We also observed a sex*ELS interaction regarding MR and GR expression in the dorsal hippocampus, with a significant upregulation of MR in males only. The sex-dependent interaction with ELS was not reflected in the behavioral response to novel environment and time spent in a sheltered compartment. We did find increased locomotor activity in all groups after a history of ELS, which attenuated after 4 h in males but not females regardless of condition. Our findings support that ELS alters HPA axis functioning sex-dependently. Genetic predisposition to low MR function may render females more susceptible to the harmful effect of ELS whereas in males low MR function promotes resilience. We propose that this model may be a useful tool to investigate the underlying mechanisms of sex-dependent and genetic vulnerability/resilience to stress-related psychopathology.

Maternal separation induces long-term changes in mineralocorticoid receptor in rats subjected to chronic stress and treated with tianeptine

PURPOSE

The aim of this study was to analyze whether early maternal separation would result in long-term, persistent alterations in stress response in adulthood, altering mineralocorticoid receptor immunoreactivity (MR-ir) in the dorsal hippocampal areas [CA1, CA2, CA3 and dentate gyrus (DG)], paraventricular nucleus of the hypothalamus and medial and central nucleus of the amygdala, key structures involved in stress response regulation. We also analyzed whether chronic treatment with the antidepressant tianeptine reverses these possible changes.

MATERIAL AND METHODS

Male Wistar rats were subjected to daily maternal separation for 4.5 h during 3 weeks or left undisturbed. As adults, they were exposed to chronic stress during 24 days or left undisturbed, and they were also daily treated with tianeptine (10 mg/kg i.p.) or isotonic solution.

RESULTS

In the CA2 and DG areas of the dorsal hippocampus, there was an increase in MR-ir in non-maternally separated and chronic stressed groups. Tianeptine raised MR-ir in the CA3. In the DG, control and maternally separated + chronic stress groups treated with tianeptine showed more MR-ir than their respective vehicle groups. In the paraventricular nucleus, tianeptine decreased MR-ir in non-separated groups, but not in maternally separated rats.

CONCLUSIONS

Our results support findings that early-life events induce long-term changes in stress response regulation, persistent into adulthood, which are manifested during challenges in later life, and that treatment with tianeptine, which tends to attenuate the hypothalamus-pituitary-adrenal axis dysregulation, depends on the individual experience of each rat.

Current Status of Animal Models of Posttraumatic Stress Disorder: Behavioral and Biological Phenotypes, and Future Challenges in Improving Translation

Increasing predictability of animal models of posttraumatic stress disorder (PTSD) has required active collaboration between clinical and preclinical scientists. Modeling PTSD is challenging, as it is a heterogeneous disorder with ≥20 symptoms. Clinical research increasingly utilizes objective biological measures (e.g., imaging, peripheral biomarkers) or nonverbal behaviors and/or physiological responses to complement verbally reported symptoms. This shift toward more-objectively measurable phenotypes enables refinement of current animal models of PTSD, and it supports the incorporation of homologous measures across species. We reviewed >600 articles to examine the ability of current rodent models to probe biological phenotypes of PTSD (e.g., sleep disturbances, hippocampal and fear-circuit dysfunction, inflammation, glucocorticoid receptor hypersensitivity) in addition to behavioral phenotypes. Most models reliably produced enduring generalized anxiety-like or depression-like behaviors, as well as hyperactive fear circuits, glucocorticoid receptor hypersensitivity, and response to long-term selective serotonin reuptake inhibitors. Although a few paradigms probed fear conditioning/extinction or utilized peripheral immune, sleep, and noninvasive imaging measures, we argue that these should be incorporated more to enhance translation. Data on female subjects, on subjects at different ages across the life span, or on temporal trajectories of phenotypes after stress that can inform model validity and treatment study design are needed. Overall, preclinical (and clinical) PTSD researchers are increasingly incorporating homologous biological measures to assess markers of risk, response, and treatment outcome. This shift is exciting, as we and many others hope it not only will support translation of drug efficacy from animal models to clinical trials but also will potentially improve predictability of stage II for stage III clinical trials.