In the present study, we limited confounders such as the ages, underlying disease, the type of surgery and comorbidities known to affect the prevalence of PND, and enrolled healthy adult subjects undergoing anesthesia and surgery. The present results demonstrated that plasma NFL was significantly increased, but Aβ42 and Aβ40 were decreased after anesthesia and surgery. The changes of biomarkers of neuronal injury might be attributed to the changes of multiple inflammatory mediators after surgery.
NFL is the major constituents of the neuronal cytoskeleton , which is detected in the cerebrospinal fluid (CSF) and that increased CSF NFL levels are associated with neuronal injury in some neurodegenerative disorders, such as multiple sclerosis, HIV infection, and Alzheimer disease . Plasma NFL levels are associated with the severity of injured and/or degenerating neurons and correlated highly with CSF levels [18, 19]. In the present study, we found the NFL levels in plasma increased rapidly in response to anesthesia and surgery. In general, our results of NFL were in agreement with those reported by Evered et al , who measured in elderly surgical patients. In addition, our results confirmed a strong positive correlation between NFL levels and duration of anesthesia/surgery. Since the measurement of serum NFL may be useful to assess the severity of neuronal injury following traumatic brain injury  and ischemic stroke , PND is believed to be multifactorial and involves age and healthy state, and NFL is sensitive for neuro injury, our results from healthy adult subjects suggest that anesthesia and surgery are responsible for the neurotoxicity, although DiMeglio’s study showed that serum NFL concentration did not change significantly after cardiac surgery .
Neuritic plaques containing Aβ and neurofibrillary tangles consisting of tau protein are the primary neuropathologic criteria for AD diagnosis . Both Aβ42 and Aβ40 isoforms in the CSF and/or blood have been used as biomarkers for the identification of the earliest stages of the certain forms of AD [9, 25-27]. Evered’s study showed that plasma levels of Aβ42 and Aβ40 were significantly lower in patients with POCD at 3 months than those without POCD after cardiac surgery . The lower plasma levels of Aβ may indicate premorbid neurodegenerative disease as Aβ deposited selectively in the brain . In the present study, the levels of Aβ42 and Aβ40 decreased over time after surgery, which remain lower levels at 48 hours postoperatively. There are studies showing that the level of plasma Aβ is associated with age [15, 30]. Moreover, the first signs of AD pathology and cognitive decline may occur from around 50-60 years of age [31, 32]. The decreased blood Aβ of Evered’s results may be involved in age, since the mean age of the patients enrolled was 68.0 years . Our findings showed lower level of plasma Aβ obtained from young healthy adults, which are unlikely attributed to Aβ accumulation. A recent study showed that exposure to surgery with general anesthesia during adult life did not induce increased Aβ deposition in brain . In addition, Pikwer’s study showed that there were no significant effects on Aβ in CSF after surgery and anesthesia (5 hours after induction) . These data suggest that surgery and anesthesia may be involved in the complex mechanisms of Aβ metabolism, which remain to be explored.
Tau protein is primarily localized in CNS neurons and contributes to axonal integrity, which is considered as an important biomarker for neurodegenerative disease and brain injury . Preclinical and clinical studies suggest that anesthesia and/or surgery have effect on this biomarker of AD, the changes of which might be associated with PND . Previous studies have showed that blood tau increased rapidly from baseline postoperatively , and even remained elevated at 7 days and three months  in the elderly. In the present study, we did not detect a difference in levels of tau over time. A similar finding has been reported by Pikwer et al. . They demonstrated that anesthesia and surgery have no effects on tau in adults. These results suggest that tau metabolism may be related to age in that aging brains are vulnerable to anesthesia and surgery.
Surgery and anesthesia can trigger a systemic inflammatory response which is coordinated by the immune system and mediated by endogenous mediators such as inflammatory cytokines . For example, Hirsch’s study showed that statistically significant changes compared to baseline were present in IL-5, IL-6, IL-8, IL-10, monocyte chemotactic protein (MCP)-1, MIP-1α in plasma of patients undergoing major knee surgery who received spinal anesthesia with intravenous sedation (propofol) . There are other studies showing that the serum levels of cytokines, such as IL-1β, IL-2, IL-6 and IL-8, increased after anesthesia and surgery [39-42]. In the present study, we observed dynamic changes over time after anesthesia and surgery in specific plasma inflammatory biomarkers, including IL-6, IL-7, IL-8, IL-10, TNF-α, I-TAC (CXCL11) and MIP-1β. The inflammatory mediators were increased and peaked at 3 hours, and many of the cytokines were not restored baseline at 48 hours postoperatively. IL-10 trajectory matched other proinflammatory biomarkers in plasma, which is an anti-inflammatory cytokine that maintains the balance of the immune response . The data suggests a substantial activation of key pathways of the immune response following surgery and anesthesia. Moreover, we further observed that there were correlations between plasma inflammatory mediators (IL-6, IL-7, IL-8, IL-10) and duration of surgery and anesthesia. These results suggest that plasma levels of IL-6, IL-7, IL-8, IL-10 may be useful markers of the magnitude of surgical stress response to trauma and injury. In general, our results were consistent with previous studies, although there were contradictory results obtained in the pattern and extent of inflammatory response , which may be due to different types of surgery, underlying diseases, research methods and age of subjects.
Neuroinflammation has become a key hallmark of neurological complications including PND . Surgery is known to induce a systemic inflammatory response causing blood-brain barrier breakdown and then triggering neuroinflammation [38, 45, 46]. In the present study, we demonstrated that there was a positive correlation between plasma concentrations of NFL and inflammatory cytokines (IL-6 and IL-8). The changes of biomarkers of neuronal injury might be attributed to increase of inflammatory cytokines IL-6 and IL-8. These findings lead to possibility that systemic inflammation might have profound impact on the brain after anesthesia and surgery. Experimental and clinical studies have suggested that different anesthetics may modulate immune signaling pathways, which can directly cause immune suppression by influencing the functions of immunocompetent cells and inflammatory mediator gene expression and secretion [47, 48]. Volatile anesthetics have been thought to have anti-inflammatory properties [49, 50]. In addition, a latest research showed that general anesthesia (sevoflurane) without surgery in healthy volunteers did not provoke an inflammatory state or neuronal injury in the early hours after exposure . Another study in healthy subjects without any surgical intervention or other nociceptive stimuli demonstrated that propofol exerted a partly pro-inflammatory but also slightly anti-inflammatory effect on the immune system . Since anesthetics may have anti-inflammatory effects on the immune system, our results obtained in healthy subjects suggest that systemic inflammation induced by operative trauma might be main culprit causing neuronal injury.
Although anesthesia and surgery have been proposed to increase the incidence of PND [7, 52], we did not observe a corresponding behavioral phenotype to the changes we measured in a short period. Whether the changes have a prolonged effect on brain and cause cognitive dysfunction remained to be evaluated. Because of anesthesia and the accompanying surgical interventions, we cannot differentiate any adverse effects of the surgical trauma from those specifically associated with the use of anesthetic agents. Moreover, due to invasiveness and low availability of the testing CSF biomarkers, the biomarkers of systemic inflammation might not be able to evaluate neuroinflammation, although systemic inflammatory processes have been linked to brain homeostasis and brain injury . The causal relationship between the changes of plasma cytokines and neuronal damage will be required to determine. Clinical studies designed properly could elucidate the effects of inflammation on the pathogenesis of PND. The use of animals in research is also essential to further elucidate the underlying cellular mechanisms.