Sepsis-induced brain dysfunction receives increasing attention since it directly causes brain damage [
9,
10] and correlates well with morbidity and mortality rates of systemic septicemia [
2]. Despite this fact, the established treatment protocols for patients suffering from sepsis or septic shock lack a specific neuroprotective approach, and the therapeutic strategy mainly focuses on antimicrobial drugs and stabilization of cardiovascular parameters. A prerequisite for the development of improved treatment approaches is the availability of appropriate animal models that use tools and techniques relevant to the clinical setting. In this study we aimed to investigate the mutual relation of CBF changes, EEG and brain metabolism in experimentally induced sepsis in rats. We correlated our findings to inflammatory gene transcription and histological analysis of neuronal loss as well as micro- and astroglial activation.
Induction of experimental sepsis by LPS did not significantly alter systolic blood pressure, heart rate or the calculated shock index at 24 h. Thus, LPS application did not cause a profound septic shock syndrome but rather resulted in slight changes reminiscent of a hyperdynamic circulatory status which can be observed in early stages of sepsis, where a marked peripheral vasodilatation is offset by a substantial increase in cardiac output resulting in little or no change of mean arterial blood pressure [
11]. Of note, a mean arterial blood pressure within normal limits may resemble more closely the situation of patients who are treated with vasopressants if septic shock is present. Since the lower limit of cerebral autoregulation in rats is around 50 mmHg [
12,
13], it seems unlikely that the CBF reduction found in the present study is caused by systemic cardiovascular changes, but rather results from impaired cerebral microcirculation as recently reported in a similar model [
14].
During human sepsis, EEG changes are common and the degree of EEG abnormalities is associated with the clinical severity and the prognosis of SE [
5]. Although EEG changes found in septic rats were not as pronounced as those described in septic patients [
5], our study yielded comparable results, revealing a generalized slowing of overall EEG activity and a significant decrease of alpha activity. These findings confirm previous observations that LPS-induced sepsis in rats reduced EEG activity in frequency bands ≥ 8 Hz [
15] up to 12 h after LPS administration. In the latter study, a significant increase for frequencies between 2 and 4 Hz was only observed within the first 6 h of the experiment, as in our study where no significant changes of low frequency bands were detected at 24 h. Interestingly, both serotype and LPS concentration of the latter study were different to our experimental protocol, suggesting that the observed effects on EEG activity are independent from these factors. Paralleling the LPS-induced EEG changes, microPET
in vivo analysis of cerebral glucose uptake, a metabolic process that has been linked to neuronal activity in rodents [
16], was significantly reduced in all cortical areas examined. Explorative data analysis showed a significant correlation of slowing of the EEG activity and the decrease of regional CBF found in septic animals. Reduction of regional CBF might therefore, at least in part, be causative for the observed EEG-changes considered to reflect SE. Alternatively, the induction of sepsis may have caused substantial brain dysfunction through both, systemic release [
17] as well as local generation of inflammatory molecules by peripheral immune cells or locally activated microglia. Such an activation of microglial cells and astrocytes, which both can serve as major source of inflammatory molecules has been well documented in rodent models of SE [
18] and brains of septic patients [
9]. In this study, we found that microglial activation in the cortex was associated with a significant increase of inflammatory gene transcription of Il-1β, TNFα, TGF-β, iNOS and MCP-1. In addition, we observed a reduction of total and neuronal cell number in the cortex and hippocampus, even so we can not exclude that this may be caused by swelling of the brain. Of note, both cytokines, TNF-α and Il-1β, have been reported to affect neuronal function [
4,
19,
20] or survival [
21,
22]. Similarly, iNOS-derived NO can down regulate neuronal activity [
23,
24], and it has been shown that neuronal viability is remarkably sensitive to sustained iNOS dependent NO generation [
25‐
27]. Thus, inflammatory molecules in concert with inflammation-triggered NO generation may directly impair neuronal function and cause neurodegeneration during SE. Neuronal cell death and a reduction of neuronal activity – as evidenced by reduced cerebral glucose utilization and EEG slowing, in turn, may negatively regulate the local CBF since the latter is directly coupled to the activity of the neighboring neurons [
28,
29]. It is therefore likely that during SE, cerebral microcirculatory failure, systemic and local inflammation and neuronal activity mutually influence and promote each other.