Peripheral biomarkers and clinical correlates of post-traumatic stress disorder (PTSD) and major depressive episode in patients with bipolar disorder versus healthy controls

Background: Trauma related and post-traumatic stress syndromes have been subject of increasing attention in psychiatric research in most recent decades. Post-Traumatic Stress Disorder (PTSD) is a severe psychiatric disorder typically occurring after exposure to a traumatic event, causing chronic psychological suffering and leading to an often chronic and invalid course. According to the current DSM-5 classification, PTSD is characterized by 4 clusters of psychiatric symptoms, namely intrusion (criterion B), avoidance (criterion C), negative alterations in cognitions and mood (criterion D), alterations in arousal and reactivity (criterion E). Some neurobiological mechanisms might play a significant role on the development of PTSD after traumatic experiences in these subjects. The biological heterogeneity and variable symptoms presentation of PTSD suggest the need for biomarkers that reflect multiple biological measures. The sequential responses to recurrent and chronic stress by the hypothalamic–pituitary–adrenal (HPA) axis and the autonomic nervous system are considered to play a significant role in the onset and progression of PTSD. Decreased activity of the HPA axis and parasympathetic nervous system, along with increased activity of the sympathetic nervous system, have been observed in PTSD, which may lead to increased levels of proinflammatory cytokines. Such heightened activity of the immune system may cause alterations in the structure and function of brain regions through changes in levels of serotonin metabolites, direct neurotoxic effects of cytokines, oxidative stress, and decreasing some specific neurotrophins (as Brain Derived Neurotrophic Factor – BDNF). Furthermore, proinflammatory cytokines induced by stress and traumatic event exposure have been also implicated in the upregulation of the indoleamine 2,3-dioxygenase (IDO), which is a crucial enzyme in the kynurenine (KYN) pathway, a metabolite of tryptophan (TRP) degradation. The activation of IDO can result in the decrease of serotonin (5-HT) concentration and the impairment of TRP metabolic fluxes, including those related to KYN and quinolinic acid (QUIN), that have been involved in the NMDA neurotransmission and possible neurotoxicity. Clinical and epidemiological studies have often remarked the strong association between PTSD and other mental disorders, particularly bipolar disorder (BD), with increased PTSD prevalence rates among these patients with respect to the general population and consequent greater symptoms severity, number of hospitalizations and worsening in quality of life. Patients with BD have been reported to be particularly exposed to lifetime traumatic events as well as being more prone to report post-traumatic stress symptoms across the lifespan even when exposed to minor, even if often multiple, lifetime potentially traumatic events. On the counterparts, the same biochemical parameters mentioned above have also been investigated in patients with mood disorders, particularly major depression, with findings highlighting dysregulation in HPA axis, immune and oxidative stress systems, TRP metabolism and neurotrophins. Scant and conflicting data are currently available in the literature about potential biomarkers of PTSD, and even less on possible comparisons of PTSD and depressive state biomarkers in subjects affected by BD. Aims: The aim of the present investigation was to evaluate potential biochemical markers of PTSD in a group of BD subjects in euthymic phase (PTSD group), compared with a group of subjects with BD with a major depressive episode (DEP group) and a healthy control group (CTL group). In particular, we aimed to analyze, among the different groups of subjects, the circulating concentrations of: 1) 5-HT; 2) TRP, KYN, and QUIN, in order to evaluate the main TRP metabolic pathways; 3) melatonin (particularly its urinary metabolite 6-OH-melatonin-S); 4) plasmatic (PPP) and intraplatelet (PLT) BDNF; 5) IL-6 and IL-1β, as the most investigated and frequently reported altered cytokines in PTSD; 6) superoxide dismutase (SOD), catalase (CAT) and total thiols, as a potential marker of oxidative-stress and impaired antioxidant processes in this population; 7) cortisol, as principal effector of HPA axis that is involved in stress-related reactions. Moreover, we aimed at evaluating the possible correlations between clinical features, as measured by the psychometric scales, and biochemical parameters. Methods: A sample of patients with BD in euthymic phase with PTSD (PTSD group) or with a major depressive episode (DEP group) were recruited among patients followed at the Psychiatric Clinic of the University of Pisa, whereas unrelated controls (CTL group) were recruited on a voluntary basis. All subjects underwent a psychiatric assessment and concomitant biological samplings. The psychometric instruments included: the Structured Clinical Interview for DSM-5 (SCID-5), the Hamilton Depression Rating Scale (HAM-D), the Young Mania Rating Scale (YMRS), the Impact of Event Scale-Revised (IES-R), the Mood Spectrum-Self-Report lifetime version (MOODS-SR), the Trauma and Loss Spectrum-Self Report lifetime version (TALS-SR), and the Work and Social Adjustment Scale (WSAS). A sample of peripheral venous blood, besides a sample of nighttime and first morning urines were withdrawn from all the subjects and then processed for obtaining the different analytical specimen for biochemical assessments: the platelet poor plasma (PPP), platelet pellets, serum and sediment-free urines. All biological parameters under investigation were measured by means of dedicated Enzyme-linked immunosorbent assays (ELISA) or other clinical chemistry procedures. Biochemical evaluations were performed at the Laboratory of Biochemistry of Department of Pharmacy, University of Pisa. Results: Our results showed significantly lower TRP levels in the DEP group than in the CTL one. Both PTSD and DEP groups exhibited lower 5-HT levels when compared with CTL group, and DEP group reported lower 5-HT levels than PTSD ones. Further, DEP group showed significantly higher QUIN concentrations and lower 6-OH-Melatonin-S levels than the CTL group. The intraplatelet (PLT) BDNF levels resulted significantly lower in the DEP group only when compared with the CTL one. Furthermore, both PTSD and DEP groups showed lower SOD and Total Thiols levels than CTL ones, while PTSD patients reported also lower CAT levels with respect the CTL group. IL-6 levels were found significantly higher in the PTSD and DEP groups with respect to the CTL one. Urinary cortisol levels resulted significantly higher in the PTSD group when compared with DEP and CTL ones. Finally, no differences emerged in the IL-1β, KYN and plasmatic (PPP) BDNF levels among groups. Specific patterns of association were found among acute and lifetime post-traumatic stress or mood symptoms and biochemical variables. Finally, discriminant and logistic regression analyses contributed to identify a subset of biochemical variables more associated to both groups of BD patients with respect to the CTL one, whereas specific biological variables were found more closely associated to the PTSD group than the DEP one. Conclusions: To the best of our knowledge, this is the first study investigating and comparing possible peripheral biomarkers of PTSD and major depressive episode in patients with BD versus healthy controls. Our results suggest the key role of a chronic low-grade inflammatory state in BD patients, both with comorbid PTSD and in a major depressive episode, probably related to a dysregulation in HPA axis and cortisol release, with an increase in proinflammatory cytokines including IL-6. This seems to be predominant in patients with BD with comorbid PTSD, with a higher oxidative distress as measured by the decrease in circulating CAT, SOD and total thiols. Conversely, 5-HT and TRP metabolic pathway, including both KYN shunt and melatonin production, seem to be more markedly altered in patients with BD in a major depressive episode. Our findings point out the role of neurobiological substrates in BD patients, with specific state biomarkers of mental disorder, related to the cross-sectional symptomatology (PTSD and major depressive episode), suggesting peculiar neurobiological pathway in PTSD and depression. This is fundamental to allow the development of new specific tailored psychopharmacological treatments for these psychopathological conditions.