Acute episodes of predator exposure in conjunction with chronic social instability as an animal model of post-traumatic stress disorder

Stress controversies: post-traumatic stress disorder, hippocampal volume, gastroduodenal ulceration*

Stress in mammals triggers a neuroendocrine response mediated by the hypothalamic-pituitary-adrenal axis and the autonomic nervous system. Increased activity of these two systems induces behavioural, cardiovascular, endocrine and metabolic cascades that enable the individual to fight or flee and cope with the stress. Our understanding of stress and stress-response mechanisms is generally robust. Here, however, we review three themes that remain controversial and perhaps deserve further scrutiny and investigation before they achieve canonical status. The themes are, first, hypocortisolaemia in post-traumatic stress disorder (PTSD). A reduction rather than a stress-induced increase in adrenal glucocorticoid levels, as seen in major depressive disorder (MDD), is puzzling and furthermore is not a consistent feature of PTSD. Overall, studies on PTSD show that glucocorticoid levels may be normal or higher or lower than normal. The second theme concerns the reduction in volume of the hippocampus in MDD attributed to the neurotoxicity of hypercortisolaemia. Again, as for hypocortisolaemia in PTSD, reduced hippocampal volume in MDD has been found in some but not all studies. Third, the discovery of a causal association between Helicobacter pylori and peptic ulcers apparently brought to an end the long-held view that peptic ulceration was caused predominantly by stress. However, recent studies suggest that stress can cause peptic ulceration in the absence of H. pylori. Predictably, the aetiological pendulum of gastric and duodenal ulceration has swung from 'all stress' to 'all bacteria' followed by a sober realisation that both factors may play a role. This raises the question as to whether stress and H. pylori interact, and if so how? All three controversies are of clinical significance, pose fundamental questions about stress mechanisms and offer important areas for future research.

Beneficial Effects of Tianeptine on Hippocampus-Dependent Long-Term Memory and Stress-Induced Alterations of Brain Structure and Function

Tianeptine is a well-described antidepressant which has been shown to prevent stress from producing deleterious effects on brain structure and function. Preclinical studies have shown that tianeptine blocks stress-induced alterations of neuronal morphology and synaptic plasticity. Moreover, tianeptine prevents stress from impairing learning and memory, and, importantly, demonstrates memory-enhancing properties in the absence of stress. Recent research has indicated that tianeptine works by normalizing glutamatergic neurotransmission, a mechanism of action that may underlie its effectiveness as an antidepressant. These findings emphasize the value in focusing on the mechanisms of action of tianeptine, and specifically, the glutamatergic system, in the development of novel pharmacotherapeutic strategies in the treatment of depression.

Enhanced fear recall and emotional arousal in rats recovering from chronic variable stress

Emergence of posttraumatic-like behaviors following chronic trauma is of interest given the rising prevalence of combat-related posttraumatic stress disorder (PTSD). Stress associated with combat usually involves chronic traumatization, composed of multiple, single episode events occurring in an unpredictable fashion. In this study, we investigated whether rats recovering from repeated trauma in the form of chronic variable stress (CVS) express posttraumatic stress-like behaviors and dysregulated neuroendocrine responses. Cohorts of Long-Evans rats underwent a 7 day CVS paradigm followed by behavioral and neuroendocrine testing during early (16 h post CVS) and delayed (7 day) recovery time points. A fear conditioning-extinction-reminder shock paradigm revealed that CVS induces exaggerated fear recall to reminder shock, suggestive of potentiated fear memory. Rats with CVS experience also expressed a delayed expression of fearful arousal under aversive context, however, social anxiety was not affected during post-CVS recovery. Persistent sensitization of the hypothalamic-pituitary-adrenocorticotropic response to a novel acute stressor was observed in CVS exposed rats. Collectively, our data are consistent with the constellation of symptoms associated with posttraumatic stress syndrome, such as re-experiencing, and arousal to fearful contexts. The CVS-recovery paradigm may be useful to simulate trauma outcomes following chronic traumatization that is often associated with repeated combat stress.

CRF receptor blockade prevents initiation and consolidation of stress effects on affect in the predator stress model of PTSD

Post traumatic stress disorder (PTSD) is a chronic anxiety disorder initiated by an intensely threatening, traumatic event. There is a great need for more efficacious pharmacotherapy and preventive treatments for PTSD. In animals, corticotropin-releasing factor (CRF) and the CRF1 receptor play a critical role in behavioural and neuroendocrine responses to stress. We tested the hypothesis that CRF1 activation is required for initiation and consolidation of long-term effects of trauma on anxiety-like behaviour in the predator exposure (predator stress) model of PTSD. Male C57BL6 mice were treated with the selective CRF1 antagonist CRA0450 (2, 20 mg/kg) 30 min before or just after predator stress. Long-term effects of stress on rodent anxiety were measured 7 d later using acoustic startle, elevated plus maze (EPM), light/dark box, and hole-board tests. Predator stress increased startle amplitude and delayed startle habituation, increased time in and decreased exits from the dark chamber in the light/dark box test, and decreased risk assessment in the EPM. CRF1 antagonism had limited effects on these behaviours in non-stressed controls, with the high dose decreasing risk assessment in the EPM. However, in stressed animals CRF1 antagonism blocked initiation and consolidation of stressor effects on startle, and returned risk assessment to baseline levels in predator-stressed mice. These findings implicate CRF1 activation in initiation and post-trauma consolidation of predator stress effects on anxiety-like behaviour, specifically on increased arousal as measured by exaggerated startle behaviours. These data support further research of CRF1 antagonists as potential prophylactic treatments for PTSD.

A critical review of chronic stress effects on spatial learning and memory

The purpose of this review is to evaluate the effects of chronic stress on hippocampal-dependent function, based primarily upon studies using young, adult male rodents and spatial navigation tasks. Despite this restriction, variability amongst the findings was evident and how or even whether chronic stress influenced spatial ability depended upon the type of task, the dependent variable measured and how the task was implemented, the type and duration of the stressors, housing conditions of the animals that include accessibility to food and cage mates, and duration from the end of the stress to the start of behavioral assessment. Nonetheless, patterns emerged as follows: For spatial memory, chronic stress impairs spatial reference memory and has transient effects on spatial working memory. For spatial learning, however, chronic stress effects appear to be task-specific: chronic stress impairs spatial learning on appetitively motivated tasks, such as the radial arm maze or holeboard, tasks that evoke relatively mild to low arousal components from fear. But under testing conditions that evoke moderate to strong arousal components from fear, such as during radial arm water maze testing, chronic stress appears to have minimal impairing effects or may even facilitate spatial learning. Chronic stress clearly impacts nearly every brain region and thus, how chronic stress alters hippocampal spatial ability likely depends upon the engagement of other brain structures during behavioral training and testing.

Tianeptine: an antidepressant with memory-protective properties

The development of effective pharmacotherapy for major depression is important because it is such a widespread and debilitating mental disorder. Here, we have reviewed preclinical and clinical studies on tianeptine, an atypical antidepressant which ameliorates the adverse effects of stress on brain and memory. In animal studies, tianeptine has been shown to prevent stress-induced morphological sequelae in the hippocampus and amygdala, as well as to prevent stress from impairing synaptic plasticity in the prefrontal cortex and hippocampus. Tianeptine also has memory-protective characteristics, as it blocks the adverse effects of stress on hippocampus-dependent learning and memory. We have further extended the findings on stress, memory and tianeptine here with two novel observations: 1) stress impairs spatial memory in adrenalectomized (ADX), thereby corticosterone-depleted, rats; and 2) the stress-induced impairment of memory in ADX rats is blocked by tianeptine. These findings are consistent with previous research which indicates that tianeptine produces anti-stress and memory-protective properties without altering the response of the hypothalamic-pituitary-adrenal axis to stress. We conclude with a discussion of findings which indicate that tianeptine accomplishes its anti-stress effects by normalizing stress-induced increases in glutamate in the hippocampus and amygdala. This finding is potentially relevant to recent research which indicates that abnormalities in glutamatergic neurotransmission are involved in the pathogenesis of depression. Ultimately, tianeptine’s prevention of depression-induced sequelae in the brain is likely to be a primary factor in its effectiveness as a pharmacological treatment for depression.

Salivary glands as the source of plasma brain-derived neurotrophic factor in stressed rats engaged in biting behavior

Brain-derived neurotrophic factor (BDNF) is crucial for the survival and differentiation of the central and peripheral nervous systems. Recently, BDNF has been reported to exert broader biological activity on non-neural cells. A previous study examined the effect of immobilization stress on BDNF and its receptor tyrosine receptor kinase B in male rat submandibular glands. In the present study, we found that the rat submandibular gland is the major source of plasma BDNF during acute immobilization stress. Biting modulates the mRNA and protein levels of BDNF in the rat hippocampus, so we also investigated whether the plasma BDNF concentration is influenced by biting. Two hours of acute immobilization stress significantly increased the amount of BDNF mRNA within the rat submandibular glands. Moreover, allowing biting behavior for the second half of the 2-h stress exposure significantly increased the amount of salivary gland BDNF mRNA relative to stress alone. Similar results were found with plasma BDNF concentrations under the same conditions. We confirmed that biting during stress attenuates the increases in plasma adrenocorticotropic hormone and corticosterone concentrations, but this was not dependent on the submandibular glands. Increased BDNF, mRNA and protein expressions were observed in salivary duct cells as a result of immobilization stress and biting behavior, as demonstrated by real-time polymerase chain reaction, immunohistochemistry, western blotting, and enzyme-linked immunosorbent assay. Taken together, the findings indicate that the submandibular glands evidently contribute to the increase in plasma BDNF upon biting.

The relevance of epigenetics to PTSD: implications for the DSM-V

Epigenetic modifications, such as DNA methylation, can occur in response to environmental influences to alter the functional expression of genes in an enduring and potentially, intergenerationally transmissible manner. As such, they may explain interindividual variation, as well as the long-lasting effects of trauma exposure. Although there are currently no findings that suggest epigenetic modifications that are specific to posttraumatic stress disorder (PTSD) or PTSD risk, many recent observations are compatible with epigenetic explanations. These include recent findings of stress-related gene expression, in utero contributions to infant biology, the association of PTSD risk with maternal PTSD, and the relevance of childhood adversity to the development of PTSD. The relevance of epigenetic mechanisms to formulations of PTSD for the fifth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-V) is described.

Chronic stress induces dendritic atrophy in the rat medial geniculate nucleus: effects on auditory conditioning

Chronic stress induces dendritic atrophy in the inferior colliculus (IC, auditory mesencephalon) and impairs auditory avoidance conditioning. The aim of this study was to determine in Golgi preparations and in cued fear conditioning whether stress affects other auditory components, like the thalamic medial geniculate nucleus (MG) or the posterior thalamic nucleus (PO), in Sprague-Dawley rats. Chronic restraint stress produced a significant dendritic atrophy in the MG (stress: 407+/-55 microm; control: 808+/-120 microm; p<0.01) but did not affect auditory fear conditioning. The last result was in apparent contrast with the fact that stress impairs both the acquisition of auditory avoidance conditioned responses and the dendritic structure in two major nuclei of the auditory system. In order to analyze this disagreement, we investigated whether the stress-related freezing to tone occurring in the fear conditioning protocol corresponded to a conditioned or an unconditioned fear response, using changes in tone instead of light throughout conditioning trials. Chronic stress significantly enhanced visual fear conditioning in stressed animals compared to controls (stress: 58.9+/-8.42%, control: 23.31+/-8.01%; p<0.05), but this fear enhancement was related to unconditioned fear. Conversely, chronic stress did not affect the morphology of the PO (subserving both auditory and somatosensory information) or the corresponding auditory and somatosensory unconditioned responses (acoustic startle response and escape behavior). Our results suggest that the auditory conditioned stimulus can be processed in part independently of the IC and MG in the stressed animals, and sent to the amygdala via the PO inducing unconditioned fear. Comparable alterations could be produced in major depression.