Posttraumatic stress disorder (PTSD) is a condition caused by severe stress and subsequent chronic distress, which is generally associated with a life-threatening experience, such as a natural disaster, military combat, traffic accidents, incurable disease or a personal tragedy. PTSD may result in neuropsychological [1
], cardiovascular [3
], gastrointestinal, metabolic, endocrine, and even oncological diseases [4
]. Current pharmacological treatment for PTSD is not adequately effective and may cause serious side effects.
Although incidence of PTSD is increasing, approximately 60–80% of humans and animals exposed to severe stress do not develop PTSD [5
]. This individual resistance to PTSD is apparently based on genetically determined potency of endogenous defense systems also called stress-limiting systems, which include heat shock proteins (HSPs), antioxidants, nitric oxide, prostaglandins, and serotonergic, GABAergic and other systems [8
Endogenous defense systems can be enhanced by intermittent hypoxia conditioning (IHC), which has been shown to be efficient, safe, and free of side effects [8
]. In particular, IHC is highly cardio-, vaso-, and neuroprotective [11
]. In clinical and experimental studies, IHC prevented behavioral disorders and cerebral neuronal damage in epilepsy [13
], Parkinson disease [14
], and Alzheimer’s disease [12
]. IHC preconditioning [16
] and postconditioning [17
] were shown to abolish development of stress-induced anxiety in the rat “stress-restress” model of PTSD. Recently we found that rats treated with IHC prior to predator stress exhibited significantly less anxiety-like behavior, a lesser drop of plasma corticosterone, and reduced structural signs of adrenal gland dystrophy, all signs characteristic of PTSD [18
]. In fact, IHC exerted a robust anti-stress effect in naïve rats, as evidenced from less anxiety during an elevated X-maze test [18
PTSD is known to be associated with both mental disorders and considerable physical comorbidity, including cardiovascular, gastrointestinal, respiratory, musculoskeletal, renal, and autoimmune diseases [20
]. New PTSD comorbidities have been discovered frequently, and each of these comorbidities requires a specific treatment. At the present time, possibilities for preventing PTSD-associated comorbidities are very limited. The major objectives of this study were to identify specific PTSD-induced damages to the rat heart, liver, and brain and then to evaluate the possibility of protecting these organs with IHC, a non-pharmacological approach. Results showed that IHC prevented PTSD-associated morphological injuries in the heart and liver. IHC also alleviated depletion of hepatic glycogen, increases in activities of the injury markers, AST and ALT, in blood and liver, and also reduced disorders of norepinephrine metabolism in the cerebral cortex.
Exposure of rats to predator stress is a model of human PTSD [25
]. As expected, these PTSD rats had increased anxiety-like behavior in the elevated X-maze test that resulted in a computed AI significantly higher than of control rats. In stressed PTSD+IHC rats, AI was decreased compared to that of PTSD rats, i.e., IHC produced significant protection from PTSD. This result confirms our earlier study [18
]. In that earlier study, we performed the X-maze test to demonstrate that PTSD did not develop in the IHC rats exposed to predator stress in contrast to non-IHC rats exposed to the same stress.
PTSD has been shown to positively correlate with a number of cardiovascular diseases, including coronary heart disease, myocardial ischemia, and infarction [24
], hypertension [29
], tachycardia [29
], and stroke [30
]. PTSD is an independent risk factor for CVDs [27
] and increases the risk of sudden cardiac death [32
Our histological findings agree with Reznik’s prior description of PTSD-mediated ischemic injury of the myocardium of PTSD rats [34
]. Loss of myocardial cross-striation was primarily due to focal I-disk destruction, as has been observed in early human myocardial infarction [19
]. In Figure 3
, ischemic myocardial damage was visible on polarized photographs as A disks merging into a solid, white-lightened conglomerates, specifically indicating focal myofibrillar disaggregation and lysis. In an earlier study, using light microscopy and hematoxylin-eosin staining or staining for glycogen, we observed chromatin homogenization in cardiomyocyte nuclei, interstitial edema, and decreased glycogen content in hearts of PTSD rats [35
]. IHC in absence of stress produced no signs of myocardial injury, and it completely prevented the PTSD-associated morphological alterations. IHC in the absence of stress produced no signs of myocardial injury, and it completely prevented the PTSD-associated morphological alterations.
Previously, Liu et al. [36
] found morphological changes characteristic of apoptosis in the myocardium of PTSD rats. These changes included chromatin pyknosis, with chromatin clotted around the nuclear membrane or dispersed in the nucleoplasm. Myocardial fibers were also ruptured, disorganized, and infiltrated by inflammatory cells. The authors suggested that the mechanism of PTSD-induced myocardial damage involved endoplasmic reticulum stress. Yuan et al. [37
] reported that IHC prevented cardiac dysfunction and apoptosis by suppressing endoplasmic reticulum stress in ischemic myocardium. Data obtained by Wang and Si [38
] showed that IHC protected myocardium from reperfusion injury by inhibiting myocardial apoptosis.
Histological signs of heart injury seen in PTSD rats were not observed in PTSD+IHC. While IHC cardioprotection likely involves multiple mechanisms, a role of IHC preventing myocardial oxidative stress has been well documented [11
]. In the present study, we showed that PTSD potentiated oxidative stress in the myocardium, which was evident from increased concentrations of LP products and carbonylated proteins. IHC reduced these markers of oxidative stress.
An additional, although indirect marker for ischemic damage to the heart, was the elevated activities of hepatic aminotransferases, AST and ALT, in the blood, since, previously, a clinical study [39
] reported that elevated AST and ALT were inversely associated with subclinical myocardial damage and impaired function. This was indicated by elevated highly sensitive cardiac troponin T and n-terminal pro-brain natriuretic peptide. Also, elevated AST and ALT are considered unfavorable prognostic factors for patients with myocardial infarction as they reflect more severe myocardial damage and dysfunction [40
PTSD-induced increased transaminases in liver tissue generally indicate a liver injury, even in the absence of symptoms [43
]. Severe acute or chronic psychological stress [44
] has been shown to be associated with increased ALT and AST activities and hepatic inflammation [47
]. Patients who developed PTSD following acute myocardial infarction had significantly higher plasma levels of ALT, AST, and alkaline phosphatase, and PTSD severity was a strong predictor for transaminase level [44
]. Moreover, severe stress is associated with development of non-alcoholic fatty liver disease and ballooning degeneration of hepatocytes [48
Glucose is stored as glycogen in the cytoplasm of liver and serves as the main depot source that maintains blood glucose homeostasis. During stress-induced increases in energy demand, glucagon triggers glycogen release from the liver stores and its transformation into glucose [50
]. Depletion of hepatic glycogen occurs in chronic stress, including PTSD [49
]. In humans with PTSD, the glycogen depletion may contribute to development of metabolic syndrome and its associated cardiovascular risk factors, including obesity, type 2 diabetes, dyslipidemia, and hypertension [51
]. Survivors of the earthquake and tsunami following accident at the Fukushima Daiichi Nuclear Power Plant who developed PTSD after evacuation [53
] had a significantly higher prevalence of hepatobiliary enzyme abnormality (HEA), which was defined as high ALT, AST, or GTP [53
]. Pharmacological treatment of PTSD is likely to further damage the liver [53
We observed that IHC significantly alleviated the stress-induced increases in AST and ALT activities and hepatic glycogen depletion. These results are consistent with data from studies showing pronounced hepatoprotective effect of direct and remote ischemic preconditioning [57
], as evident from more normal activities of hepatic enzymes and reduced apoptosis of hepatocytes [58
]. IHC, employed as in the current study, has been shown to have a hepatoprotective effect in rats exposed to high doses of alcohol [8
]. The most likely mechanism of the hepatoprotection produced by hypoxic or ischemic conditioning is alleviation of oxidative stress induced by the detrimental factor due to increased activity of antioxidant enzymes [8
]. The present study supports this view, since we observed reduced oxidative stress markers in PTSD+IHD rats. Another possible mechanism for the IHC-induced facilitation of glycogen accumulation in hepatocytes was proposed by Lebkova et al. [63
] and Hzhehots’kyĭ et al. [62
]. They suggested that during hypoxic periods, glycogen synthesis switches to using fatty acids as the primary substrate, which maintains the energy homeostasis in hepatocytes.
Stress-induced catecholamine release is detrimental to the cerebral cortex, especially prefrontal cortex (PFC) and hippocampus [64
], since high concentrations of norepinephrine (NE) impair the cortical function via α1-adrenoceptors. Thus, α1-antagonists can be protective in PTSD [65
]. NE-induced damage is associated with impaired cortex cognitive function, which correlates with morphological changes, such as dendritic spine loss, dendritic atrophy, and grey matter loss in medial PFC [66
]. Stress induced by traumatic events and resultant PTSD are recognized as a risk factor for Alzheimer’s disease [68
]. The PTSD-induced NE accumulation results from suppressed activity and expression of monoaminoxidase A (MAO-A), a key mediator of biogenic amine metabolism [70
]. Our finding of increased NE and reduced MAO-A activity in the cortex of rats with experimental PTSD are consistent with these data. A cause for suppression of MAO-A activity in PTSD is decreased levels of glucocorticoids, which are normally responsible for activation of MAO-A expression [71
]. The decrease in glucocorticoids, specifically corticosterone, is, at least partially, due to dystrophy of adrenal glands as previously observed in experimental PTSD [18
]. Exposure of rats to IHC significantly decreased the NE accumulation and increased the MAO-A activity in the brain cortex. These protective effects could have contributed to the lessening of anxiety behavior of PTSD+IHC rats in the X-maze. A possible mechanism for IHC prevention of the PTSD-related brain damage is alleviation of adrenal gland dystrophy and dysfunction [18
], which leads to improvement of corticosterone production [18
] and, thereby, improvement of MAO-A activity and restriction of NE accumulation in the cortex.
IHC has been previously shown to exert anti-stress [8
], antidepressant [72
], and neuroprotective effects [15
]. For example, IHC prevented stress-induced stomach ulcers [8
]; the impairment of memory and brain neurodegeneration in experimental Alzheimer’s disease [12
]; depressive anxiety-like behavior and apoptosis of brain hippocampal neurons [74
] in conditions associated with cerebral damage, such as experimental Alzheimer’s disease [12
] and chronic alcohol consumption [75
]. Also, pre- and postconditioning with moderate hypobaric hypoxia prevented development of experimental PTSD in a stress-restress PTSD model [16
]. All these detrimental conditions, including severe stress and PTSD [76
], depression [77
], Alzheimer’s disease [78
], and chronic alcohol consumption [75
], are associated with increased oxidative stress, and the authors of the studies that have demonstrated a beneficial effect of IHC suggested restriction of oxidative stress in the brain to be a major factor of IHC protection. Consistently, in the present study, experimental PTSD resulted in increased content of LP products and carbonylated proteins in the brain cortex, whereas IHC significantly decreased these oxidative stress markers, which indicated alleviation of oxidative stress as a beneficial effect of hypoxic conditioning.
In conclusion, this study has demonstrated that experimental PTSD is associated with multiorgan pathology. Results showed that the heart, liver, and brain are target organs for PTSD. IHC is a promising means for prevention of PTSD-induced damages of these target organs. The beneficial effect of IHC is at least partially due to restriction of generalized, non-specific increase in oxidative stress induced by PTSD in blood and tissues. Future clinical studies should consider IHC as a means to lessen or prevent PTSD-induced comorbidities.