The main result of the present study is that, despite very high cytokine concentrations during experimental endotoxemia, no indications were found that acute systemic inflammation results in increased levels of BSPs and deterioration of CFs in humans in vivo
. In addition, a group level quantitative EEG analysis showed a higher state of alertness that correlated with cortisol concentrations. Nevertheless, the concomitant improvement in CFTs turned out to represent a practice effect as a similar improvement was observed in subjects who did not receive LPS. Although the increased alpha activity in the central region of the brain correlated with the improvement of working memory in the LPS group, it appears conceivable that this correlation may also be present in the control group during the repeated CFTs, but this finding needs to be confirmed. Interestingly, the one subject with a transient mild encephalopathic episode on EEG, that is category 2 following the score used by Young and colleagues [33
], showed that this was not associated with objective cognitive dysfunction. In addition, this subject had one of the lowest LPS-induced proinflammatory cytokine responses of the whole group, arguing against a cytokine-mediated effect.
Although experimental endotoxemia in young humans without any co-morbidity mimics the pathophysiological changes in septic patients in many ways, important differences also exist. For example, TNF-α concentrations found during experimental endotoxemia are much higher than in septic patients, whereas other cytokines are released to a lesser extent and some inflammatory mediators found in septic patients are not induced during experimental endotoxemia [39
]. It appears likely that the relatively mild insult and short duration of elevated cytokine levels during experimental endotoxemia does account for the increase in cortisol concentration and observed stimulating effects on the brain, but may not reflect the neurotoxic effects of inflammatory mediators present in septic patients. In addition, age and the pre-existing neurological situation is likely to be important. Healthy elderly people show a more pronounced inflammatory response during experimental endotoxemia [40
] and pre-existing micro-glial inflammation primes the brain for development of cognitive impairment in non-infectious and infectious central nervous system dysfunction [41
]. Therefore, although our study shows that a short duration of very high cytokine levels is not associated with brain dysfunction it does not exclude the possible effects of cytokines on neurons in older ICU patients with co-morbidities.
Cortisol secretion is related to electroencephalographic alertness [13
]. We showed a significant correlation between the elevated levels of cortisol and the change in occipital peak frequency. It is likely that this higher state of alertness was due to the LPS-induced inflammation with feelings of sickness resulting in a stress hormone-driven 'flight-fight' response [42
], which is also associated with increased cortisol. This appears to be a short-lived effect, because chronically elevated levels of glucocorticoids result in a deterioration of CF [43
]. As a result of this, it is possible that in the septic patient the stimulating effect of stress hormones on the brain is overshadowed by the neurotoxic effect of persistently elevated level of cytokines and other mediators. In septic patients, levels of some proinflammatory cytokines are not as high as in the LPS model, but the duration of the elevated cytokine level is much longer [44
]. If these cytokines play a role in the sepsis-associated encephalopathy, it is apparently not the absolute peak concentration of the proinflammatory cytokine that is of importance. Presumably, sustained elevated levels of cytokines are more important in the development of organ failure and brain dysfunction in sepsis. In accordance, chronic small increases in proinflammatory cytokine levels due to polymorphisms were found to be associated with decreased brain function [10
]. Naturally, other not yet identified mediators of inflammation that may be increased in septic patients but not during experimental endotoxemia may also account for brain dysfunction observed in septic patients.
In previous studies with much lower doses of LPS (0.2 to 0.8 ng/kg), with little systemic inflammatory response, conflicting effects on CFs were reported [22
]. Compared with experiments with 0.2 ng/kg, improvement of working memory was shown in a study with 10 healthy volunteers with a dose of 0.8 ng/kg LPS [22
]. In these studies, cortisol level and cytokines increased slightly, compared with our results [22
], which is associated with dysfunction of other organs [24
]. Furthermore, a potential problem in the studies with low doses of LPS was that no correction for practice effect was performed while practice effects during CFT are common, especially in situations with short test-retest intervals. Our study demonstrates that the observed improvement in CFs after LPS infusion in all domains was due to a practice effect. Without the use of a control group and the measurement of practice effect results are bound to be misinterpreted. Our results suggest that a short-term inflammation does not influence practice effect or lead to a significant deterioration or improvement of CFs.
The observed relations between EEG changes and inflammatory markers indicate a higher state of inflammation-induced alertness. Higher dosages of LPS result in higher levels of cytokines [23
] and more elevated levels of cortisol result in a higher state of alertness [13
]. The higher state of alertness during endotoxemia is possibly a so-called fight and flight response, rather than being due to the increased cytokine concentrations.
Although it is tempting to speculate, due to the observational nature of the present study we cannot conclude whether or not the anti-inflammatory innate immune response, measured by IL-10, exerts a protective effect on the brain, and this correlation needs further study. In addition, the pathophysiological mechanism by which systemic inflammation results in the observed decrease of NSE is not clear. Increased levels of NSE are associated with deterioration of CF after cardiac surgery [46
]. Also, increased NSE levels are associated with brain injury in septic patients, but an association between NSE and CFs in septic patients has not been examined.