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

Amelioration by Withania somnifera of neurobehavioural and immunological markers in time dependent sensitization induced post traumatic stress disorder in rats

AIMS AND OBJECTIVES

Posttraumatic stress disorder (PTSD) is a complex neuropsychiatric pathophysiology with an unmet need for safe, effective, and sustainable therapeutic modalities. Thus, the present study evaluated the effects of Withaniasomnifera (WS, Ashwagandha) on an experimental model of PTSD in rats.

MATERIALS AND METHODS

Wistar rats (200-250 g) were used and time-dependent sensitization (TDS) was used as the experimental model of PTSD. Standardized WS root extract (100 and 300 mg/kg, p.o. for 15 days) was administered with TDS and their effects were observed on neurobehavioral (anxiety) and brain cytokines, corticosterone, and oxidative stress markers.

RESULTS

Exposure to TDS resulted in anxiogenic behavior in the elevated plus maze (EPM) test, i.e., reductions in open arm entries and open arm time, as compared to the control group. Pretreatment with WS extract (100 and 300 mg/kg × 14 days) attenuated the TDS-induced anxiogenic activity in a dose-related manner, and these WS effects were comparable to those seen after the comparator drug fluoxetine (10 mg/kg). Assay of brain homogenates showed that TDS also resulted in elevations in brain interleukin-6 and reduction in corticosterone levels in both the hippocampus and prefrontal cortex (PFC), which were reversed after WS pretreatments. Further, WS pretreatment also reversed the TDS-induced changes in brain oxidative stress markers, namely elevated malondialdehyde and reduced glutathione levels in both the hippocampus and PFC.

CONCLUSION

These results suggest that WS could have potential as a therapeutic agent for treating PTSD by attenuating anxiogenesis, neuroimmune axis activation, and oxidative stress.

Rodent models of post-traumatic stress disorder: behavioral assessment

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

CB1 cannabinoid receptor-mediated plasticity of GABAergic synapses in the mouse insular cortex

The insular cortex plays pivotal roles in taste learning. As cellular mechanisms of taste learning, long-term potentiation (LTP) at glutamatergic synapses is well studied. However, little is known about long-term changes of synaptic efficacy at GABAergic synapses in the insular cortex. Here, we examined the synaptic mechanisms of long-term plasticity at GABAergic synapses in layer V pyramidal neurons of the mouse insular cortex. In response to a prolonged high-frequency stimulation (HFS), GABAergic synapses displayed endocannabinod (eCB)-mediated long-term depression (LTD_GABA). When cannabinoid 1 receptors (CB1Rs) were blocked by a CB1R antagonist, the same stimuli caused LTP at GABAergic synapses (LTP_GABA) which was mediated by production of nitric oxide (NO) via activation of NMDA receptors. Intriguingly, NO signaling was necessary for the induction of LTD_GABA. In the presence of leptin which blocks CB1 signaling, the prolonged HFS caused LTP_GABA which was mediated by NO signaling. These results indicate that long-term plasticity at GABAergic synapses in the insular cortex can be modulated by combined effects of eCB and NO signaling. These forms of GABAergic synaptic plasticity in the insular cortex may be crucial synaptic mechanisms in taste learning.

Enhanced Contextual Fear Memory and Elevated Astroglial Glutamate Synthase Activity in Hippocampal CA1 BChE shRNA Knockdown Mice

Butyrylcholinesterase (BChE) efficiently hydrolyzes acetylcholine (ACh) at high concentrations when acetylcholinesterase (AChE) is substrate-inhibited. Recent studies have shown that BChE also has a function that is independent of ACh, but it has not been fully explored. Low BChE expression is accompanied with higher stress-induced aggression and ghrelin levels in stress models, and BChE knockout mice exhibit cognitive and memory impairments. However, the role of BChE in posttraumatic stress disorder (PTSD) remains unclear. In the present study, we investigated the role of BChE in contextual fear memory and its regulatory effect on the expression of factors related to the glutamate (Glu)-glutamine (Gln) cycle via knockdown studies. We used AAVs and lentiviruses to knockdown BChE expression in the mouse hippocampal CA1 region and C8D1A astrocytes. Our behavioral data from those mice injected with AAV-shBChE in the hippocampal CA1 region showed strengthened fear memory and increased dendritic spine density. Elevated Glu levels and glutamine synthetase (GS) enzyme activity were detected in contextual fear conditioned-BChE knockdown animals and astrocytes. We observed that an AAV-shBChE induced lowering of BChE expression in the hippocampus CA1 region enhanced contextual fear memory expression and promoted the astrocytic Glu-Gln cycle but did not elevate ACh-hydrolyzing activity. This study provides new insight into the regulatory role of BChE in cognition and suggests potential target for stress-related psychiatric disorder such as PTSD where patients experience fear after exposure to severe life-threatening traumatic events.

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

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

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

Post-traumatic stress disorder (PTSD) is a common, costly, and often debilitating psychiatric condition. However, the biological mechanisms underlying this disease are still largely unknown or poorly understood. Considerable evidence indicates that PTSD results from dysfunction in highly-conserved brain systems involved in stress, anxiety, fear, and reward. Pre-clinical models of traumatic stress exposure are critical in defining the neurobiological mechanisms of PTSD, which will ultimately aid in the development of new treatments for PTSD. Single prolonged stress (SPS) is a pre-clinical model that displays behavioral, molecular, and physiological alterations that recapitulate many of the same alterations observed in PTSD, illustrating its validity and giving it utility as a model for investigating post-traumatic adaptations and pre-trauma risk and protective factors. In this manuscript, we review the present state of research using the SPS model, with the goals of (1) describing the utility of the SPS model as a tool for investigating post-trauma adaptations, (2) relating findings using the SPS model to findings in patients with PTSD, and (3) indicating research gaps and strategies to address them in order to improve our understanding of the pathophysiology of PTSD.

Using the Single Prolonged Stress Model to Examine the Pathophysiology of PTSD

The endurance of memories of emotionally arousing events serves the adaptive role of minimizing future exposure to danger and reinforcing rewarding behaviors. However, following a traumatic event, a subset of individuals suffers from persistent pathological symptoms such as those seen in posttraumatic stress disorder (PTSD). Despite the availability of pharmacological treatments and evidence-based cognitive behavioral therapy, a considerable number of PTSD patients do not respond to the treatment, or show partial remission and relapse of the symptoms. In controlled laboratory studies, PTSD patients show deficient ability to extinguish conditioned fear. Failure to extinguish learned fear could be responsible for the persistence of PTSD symptoms such as elevated anxiety, arousal, and avoidance. It may also explain the high non-response and dropout rates seen during treatment. Animal models are useful for understanding the pathophysiology of the disorder and the development of new treatments. This review examines studies in a rodent model of PTSD with the goal of identifying behavioral and physiological factors that predispose individuals to PTSD symptoms. Single prolonged stress (SPS) is a frequently used rat model of PTSD that involves exposure to several successive stressors. SPS rats show PTSD-like symptoms, including impaired extinction of conditioned fear. Since its development by the Liberzon lab in 1997, the SPS model has been referred to by more than 200 published papers. Here we consider the findings of these studies and unresolved questions that may be investigated using the model.

Lithium-induced malaise does not interfere with adaptation of the hypothalamic-pituitary-adrenal axis to stress

We have recently demonstrated that adaptation of the hypothalamic-pituitary-adrenal (HPA) axis to repeated exposure to a stressor does not follow the rules of habituation and can be fully expressed after a single experience with severe stressors. In the present work we tested the hypothesis that adaptation could be impaired if animals experience malaise during initial exposure to the stressor. To this end, animals were allowed to drink saccharin for 30min before being exposed for 3h to immobilization on boards (IMO), a severe stressor; then they were given either saline or lithium ip after the first hour of IMO. Stress-naïve rats followed exactly the same procedure except IMO. Exposure to IMO caused a strong activation of the HPA axis whereas the effect of lithium was modest. Both IMO and lithium administration resulted in conditioned taste aversion to saccharin when evaluated 4days later. When all animals were exposed to IMO 6days later, reduced HPA response and less impact on body weight was observed in the two groups previously exposed to IMO as compared with stress-naïve rats. Therefore, lithium administration during the first IMO exposure did not affect adaptation of the HPA axis and weight gain. These results indicate that malaise per se only weakly activated the HPA axis and argue against the hypothesis that signs of physical malaise during exposure to the stressor could impair HPA adaptation.

Severe, multimodal stress exposure induces PTSD-like characteristics in a mouse model of single prolonged stress

Appropriate animal models of posttraumatic stress disorder (PTSD) are needed because human studies remain limited in their ability to probe the underlying neurobiology of PTSD. Although the single prolonged stress (SPS) model is an established rat model of PTSD, the development of a similarly-validated mouse model emphasizes the benefits and cross-species utility of rodent PTSD models and offers unique methodological advantages to that of the rat. Therefore, the aims of this study were to develop and describe a SPS model for mice and to provide data that support current mechanisms relevant to PTSD. The mouse single prolonged stress (mSPS) paradigm, involves exposing C57Bl/6 mice to a series of severe, multimodal stressors, including 2h restraint, 10 min group forced swim, exposure to soiled rat bedding scent, and exposure to ether until unconsciousness. Following a 7-day undisturbed period, mice were tested for cue-induced fear behavior, effects of paroxetine on cue-induced fear behavior, extinction retention of a previously extinguished fear memory, dexamethasone suppression of corticosterone (CORT) response, dorsal hippocampal glucocorticoid receptor protein and mRNA expression, and prefrontal cortex glutamate levels. Exposure to mSPS enhanced cue-induced fear, which was attenuated by oral paroxetine treatment. mSPS also disrupted extinction retention, enhanced suppression of stress-induced CORT response, increased mRNA expression of dorsal hippocampal glucocorticoid receptors and decreased prefrontal cortex glutamate levels. These data suggest that the mSPS model is a translationally-relevant model for future PTSD research with strong face, construct, and predictive validity. In summary, mSPS models characteristics relevant to PTSD and this severe, multimodal stress modifies fear learning in mice that coincides with changes in the hypothalamo-pituitary-adrenal (HPA) axis, brain glucocorticoid systems, and glutamatergic signaling in the prefrontal cortex.

Stress, trauma and PTSD: translational insights into the core synaptic circuitry and its modulation

Evidence is considered as to whether behavioral criteria for diagnosis of post-traumatic stress disorder (PTSD) are applicable to that of traumatized animals and whether the phenomena of acquisition, extinction and reactivation of fear behavior in animals are also successfully applicable to humans. This evidence suggests an affirmative answer in both cases. Furthermore, the deficits in gray matter found in PTSD, determined with magnetic resonance imaging, are also observed in traumatized animals, lending neuropsychological support to the use of animals to probe what has gone awry in PTSD. Such animal experiments indicate that the core synaptic circuitry mediating behavior following trauma consists of the amygdala, ventral-medial prefrontal cortex and hippocampus, all of which are modulated by the basal ganglia. It is not clear if this is the case in PTSD as the observations using fMRI are equivocal and open to technical objections. Nevertheless, the effects of the basal ganglia in controlling glutamatergic synaptic transmission through dopaminergic and serotonergic synaptic mechanisms in the core synaptic circuitry provides a ready explanation for why modifying these mechanisms delays extinction in animal models and predisposes towards PTSD. In addition, changes of brain-derived neurotrophic factor (BDNF) in the core synaptic circuitry have significant effects on acquisition and extinction in animal experiments with single nucleotide polymorphisms in the BDNF gene predisposing to PTSD.

Fear memory formation can affect a different memory: fear conditioning affects the extinction, but not retrieval, of conditioned taste aversion (CTA) memory

The formation of fear memory to a specific stimulus leads to subsequent fearful response to that stimulus. However, it is not apparent whether the formation of fear memory can affect other memories. We study whether specific fearful experience leading to fear memory affects different memories formation and extinction. We revealed that cued fear conditioning, but not unpaired or naïve training, inhibited the extinction of conditioned taste aversion (CTA) memory that was formed after fear conditioning training in rats. Fear conditioning had no effect on retrieval of CTA memory but specifically impaired its extinction. Extinguished fear memory, after fear extinction training, had no effect on future CTA memory extinction. Fear conditioning had no effect on CTA memory extinction if CTA memory was formed before fear conditioning. Conditioned taste aversion had no effect on fear conditioning memory extinction. We conclude that active cued fear conditioning memory can affect specifically the extinction, but not the formation, of future different memory.

Single prolonged stress enhances hippocampal glucocorticoid receptor and phosphorylated protein kinase B levels

Animal models of posttraumatic stress disorder (PTSD) can explore neurobiological mechanisms by which trauma enhances fear and anxiety reactivity. Single prolonged stress (SPS) shows good validity in producing PTSD-like behavior. While SPS-induced behaviors have been linked to enhanced glucocorticoid receptor (GR) expression, the molecular ramifications of enhanced GR expression have yet to be identified. Phosphorylated protein kinase B (pAkt) is critical for stress-mediated enhancement in general anxiety and memory, and may be regulated by GRs. However, it is currently unknown if pAkt levels are modulated by SPS, as well as if the specificity of GR and pAkt related changes contribute to anxiety-like behavior after SPS. The current study set out to examine the effects of SPS on GR and pAkt protein levels in the amygdala and hippocampus and to examine the specificity of these changes to unconditioned anxiety-like behavior. Levels of GR and pAkt were increased in the hippocampus, but not amygdala. Furthermore, SPS had no effect on unconditioned anxiety-like behavior suggesting that generalized anxiety is not consistently observed following SPS. The results suggest that SPS-enhanced GR expression is associated with phosphorylation of Akt, and also suggest that these changes are not related to an anxiogenic phenotype.

Glucocorticoid-cholinergic interactions in the dorsal striatum in memory consolidation of inhibitory avoidance training

Extensive evidence indicates that glucocorticoid hormones act in a variety of brain regions to enhance the consolidation of memory of emotionally motivated training experiences. We previously reported that corticosterone, the major glucocorticoid in the rat, administered into the dorsal striatum immediately after inhibitory avoidance training dose-dependently enhances memory consolidation of this training. There is also abundant evidence that the intrinsic cholinergic system of the dorsal striatum is importantly involved in memory consolidation of inhibitory avoidance training. However, it is presently unknown whether these two neuromodulatory systems interact within the dorsal striatum in the formation of long-term memory. To address this issue, we first investigated in male Wistar rats whether the muscarinic receptor agonist oxotremorine administered into the dorsal striatum immediately after inhibitory avoidance training enhances 48 h retention of the training. Subsequently, we examined whether an attenuation of glucocorticoid signaling by either a systemic administration of the corticosterone-synthesis inhibitor metyrapone or an intra-striatal infusion of the glucocorticoid receptor (GR) antagonist RU 38486 would block the memory enhancement induced by oxotremorine. Our findings indicate that oxotremorine dose-dependently enhanced 48 h retention latencies, but that the administration of either metyrapone or RU 38486 prevented the memory-enhancing effect of oxotremorine. In the last experiment, corticosterone was infused into the dorsal striatum together with the muscarinic receptor antagonist scopolamine immediately after inhibitory avoidance training. Scopolamine blocked the enhancing effect of corticosterone on 48 h retention performance. These findings indicate that there are mutual interactions between glucocorticoids and the striatal cholinergic system in enhancing the consolidation of memory of inhibitory avoidance training.

The change in muscarinic receptor subtypes in different brain regions of rats treated with fluoxetine or propranolol in a model of post-traumatic stress disorder

This study shows the possible contribution of muscarinic receptors in the pathophysiology of post-traumatic stress disorder. Sprague-Dawley rats of both sexes were exposed to dirty cat litter (trauma) for 10 min and the protocol was repeated 1 week later with a trauma reminder (clean litter). The rats also received intraperitoneal fluoxetine (2.5, 5 or 10 mg/kg/day), propranolol (10 mg/kg/day) or saline for 7 days between two exposure sessions. Functional behavioral experiments were performed using elevated plus maze, following exposure to trauma reminder. Western blot analyses for M(1), M(2), M(3), M(4) and M(5) receptor proteins were employed in the homogenates of the hippocampus, the frontal cortex and the amygdaloid complex. The anxiety indices increased from 0.63±0.02 to 0.89±0.04 in rats exposed to the trauma reminder. The freezing times were also recorded as 47±6 and 133±12 s, in control and test animals respectively. Fluoxetine or propranolol treatments restored the increases in the anxiety indices and the freezing times. Female rats had higher anxiety indices compared to males. Western blot data showed increases in M(2) and M(5) expression in the frontal cortex. Expression of M(1) receptors increased and M(4) subtype decreased in the hippocampus. In the amygdaloid complex of rats, we also detected a down-regulation of M(4) receptors. Fluoxetine and propranolol only corrected the changes occurred in the frontal cortex. These results may imply that muscarinic receptors are involved in this experimental model of post-traumatic stress disorder.

The NMDA receptor/nitric oxide pathway: a target for the therapeutic and toxic effects of lithium

Although lithium has largely met its initial promise as the first drug discovered in the modern era of psychopharmacology, to date no definitive mechanism for its effects has been established. It has been proposed that lithium exerts its therapeutic effects by interfering with signal transduction through G-protein-coupled receptor (GPCR) pathways or direct inhibition of specific targets in signaling systems, including inositol monophosphatase and glycogen synthase kinase-3 (GSK-3). Recently, increasing evidence has suggested that N-methyl-D-aspartate receptor (NMDAR)/nitric oxide (NO) signaling could mediate some lithium-induced responses in the brain and peripheral tissues. However, the probable role of the NMDAR/NO system in the action of lithium has not been fully elucidated. In this review, we discuss biochemical, preclinical/behavioral and physiological evidence that implicates NMDAR/NO signaling in the therapeutic effect of lithium. NMDAR/NO signaling could also explain some of side effects of lithium.

Increased stress-evoked nitric oxide signalling in the Flinders sensitive line (FSL) rat: a genetic animal model of depression

Stress engenders the precipitation and progression of affective disorders, while stress-related release of excitatory mediators is implicated in the degenerative pathology observed especially in the hippocampus of patients with severe depression. Nitric oxide (NO) release following stress-evoked N-methyl-d-aspartate (NMDA) receptor activation modulates neurotransmission, cellular memory and neuronal toxicity. We have investigated the Flinders rat (FSL/FRL), a genetic animal model of depression, regarding the response of the hippocampal nitrergic system following exposure to an escapable stress/inescapable stress (ES-IS) paradigm. Hippocampal tissue from naive FSL/FRL rats and those exposed to ES-IS were studied with respect to constitutive nitric oxide synthase (cNOS) activity and neuronal nitric oxide synthase (nNOS) protein levels, as well as transcript expression of upstream regulatory proteins in the NMDA-NO signalling pathway, including NMDAR1, nNOS, CAPON, PIN and PSD95. Within stress-naive animals, no differences in hippocampal cNOS activity and nNOS expression or PIN were evident in FSL and FRL rats, although transcripts for NMDAR1 and CAPON were increased in FSL rats. Within the group of ES-IS animals, we found an increase in total hippocampal cNOS activity, nNOS protein levels and mRNA expression in FSL vs. FRL rats, together with an increase in PSD95 transcripts, and a reduction in PIN. In conclusion, ES-IS enhanced hippocampal cNOS activity in FSL rats, but not FRL rats, confirming the NMDA-NO cascade as an important vulnerability factor in the depressive phenotype of the FSL rat.

The interaction between acute oligomer Abeta(1-40) and stress severely impaired spatial learning and memory

In this study, we investigated whether stress can enhance the toxicity of oligomer Abeta(1-40) in the mouse brain. Stress was applied to the animals, consisting of a 2-day inescapable foot shock followed by 3-weekly situation reminders (SRs). We found that stress significantly affected not only the amygdala-dependent (anxiety) but also the hippocampal-dependent (spatial learning and memory) behaviors through the oxidative damage caused in these two regions. However, oligomer Abeta(1-40) treatment alone did not induce behavioral impairment. In addition, combined oligomer Abeta(1-40) and stress treatment increased the glucocorticoid receptor (GR)/mineralocorticoid receptor (MR) ratio and the expression of corticotrophin releasing factor 1 (CRF-1) receptor in the hippocampus. Changes in the components of the hypothalamic-pituitary-adrenal (HPA) axis, such as the GR/MR ratio and CRF-1 level, were observed, accompanied by increasing Abeta accumulation, oxidative stress, nuclear transcription factor (NF-kappaB) hypoactivity, and apoptotic signaling in the hippocampus, and decreasing calbindin D28K and NMDA receptor 2A/2B (NR2A/2B) in the hippocampus, along with alteration of the cholinergic neurons (ChAT) in the medium septum/diagnoid band (MS/DB), noradrenergic neurons (TH) in the locus coeruleus (LC), and serotonergic neurons (5-HT) in the Raphe nucleus. Therefore, apoptosis and synaptic dysfunction in the hippocampus severely induced the impairment of spatial learning and memory. These results suggest that stress may play an important role in the early stages of Alzheimer's disease (AD), and an antioxidant strategy might be a potential therapeutic approach for stress-mediated disorders.