Sepsis and its Effects on Behavior in Mouse Models

Sepsis, also known colloquially as septicemia or blood poisoning, is a serious condition in which the bloodstream becomes infected with such a high concentration of pathogens (most common gram-negative bacteria) that the body’s immune response to the infection becomes severe enough to cause tissue damage and eventually death to the sufferer. In response to infection, body temperature rises causing fever, blood pressure drops, and breathing rate increases. Lack of oxygen supply leads to widespread organ damage and eventually failure.

Sepsis is typically treated by administering antibiotics to remove the pathogens while infusing intravenous fluids to raise blood pressure. Untreated or unresponsive sepsis progresses to septic shock, where blood pressure remains dangerously low despite any efforts to raise it with intravenous fluids. Antibiotics become useless at this stage, as they simply increase the circulation of bacterial antigens in the bloodstream. The patient often then dies, most frequently from heart failure or respiratory failure.

Sepsis and septic shock, the latter especially, have a poor prognosis: sepsis results in death about 25% of the time, septic shock about 50% of the time. Mortality is higher in the very young, very old, and those with weak immune systems. The number of cases of sepsis around the globe each year is not known, but is estimated to be in the tens of millions; over a million US citizens suffer sepsis each year, and it is the most expensive medical condition treated in the US, with an annual cost of over $20 billion^[1].

Given the high prevalence and gravity of the condition, it is vital to learn as much about sepsis as possible, in order to devise more accurate diagnoses and more effective treatments. Through the effects of the immune system on brain function, sepsis can have a number of marked behavioral consequences, and mouse models have proven useful in the elucidation of these consequences. In this article, we will discuss the behavioral effects of sepsis in mouse models.

Sepsis and Anxiety

One major phenomenon that sepsis has been associated with in mouse models is an increase in behaviors indicating anxiety. A study published in 2012^[2] reports elevated “anxiety-like” behavior in mice with polymicrobial sepsis (i.e. caused by multiple pathogens)  induced through surgical puncturing of the small intestine. Increased anxiety-like behavior compared to control mice was observed in both the elevated plus maze test and contextual fear conditioning test. The results from the elevated plus maze were corroborated by a very similar Turkish study in 2015^[3].

In the elevated plus maze, mice are placed into a plus-shaped apparatus raised above the ground with two open arms exposed to the air, and two closed arms. Mice that spend less time on the open arms compared to controls, as the septic mice did in this study, are deemed to have higher levels of anxiety. In the contextual fear conditioning test, mice are taught to associate an electric shock with a glass enclosure; when mice are later returned to the enclosure in the absence of shocks, they will exhibit periodic “freezing”, which will be recorded as more frequent in mice with increased anxiety.

Bacterial lipopolysaccharide (LPS) is a large sugar found on the outside of bacteria and is the most potent mediator for sepsis. A 2015 study from Ireland^[4] examined the behavior of mice with sepsis induced by injection of LPS, and corroborated the 2012 study’s observation of increased anxiety. In a usual paradigm, the septic mice buried more novel objects (marbles) than controls, which was interpreted as increased anxiety towards these novel objects. Elevated anxiety was also seen in the elevated plus maze, and the open field test, where time spent in the inner as opposed to the outer portion of an enclosure is seen as indicative of anxiety.

Sepsis and Depression

Sepsis in mouse models has also been associated with depressive-like behaviors, as seen in the 2015 study mentioned above^[4]. Mice with LPS induced sepsis displayed a lower preference for glucose than controls, a sign of anhedonia (a decreased capacity to experience positive effect).

The septic mice also remained immobile more frequently during the tail suspension test, a test where mice are hung upside down from their tail; this is a distressing situation that compels them to try and wriggle free, and remaining immobile here is seen as a sign of decreased motivation. However, no significant difference in immobility was seen in the forced swimming test, where mice are placed into a large tub of water (mice hate water, and will swim frantically to try and escape it). The results with regard to depression are thus not entirely clear-cut.

Although, a Chinese study conducted in 2016 also observed depressive symptoms in mice with sepsis induced by LPS^[5]. The mice in this study showed the same anhedonic behavior in the sucrose preference test. The Chinese scientists hypothesized that the depression had resulted from a decrease in serotonin levels in the inflamed mouse brain, which was in turn caused by natural killer (NK) cells (a kind of immune cell). In support of this hypothesis, they demonstrated that depleting the NK cells mitigated the mice’s depressive-like behavior.

It has been shown that challenging the mouse immune system with LPS, even if the concentration is not high enough to trigger sepsis, can still engender depressive-like symptoms. A 2011 study from a group of Austrian researchers^[6] showed that mice injected with LPS exhibited signs of depression in both the forced swimming test and sucrose preference test. Interestingly, results varied significantly between strains of mice and were also profoundly affected by whether the mice were housed in groups or alone, indicating the complex nature of their immune response.

Sepsis and Memory

In addition to its consequences for mood, sepsis in mice also appears to have marked consequences for cognitive function and behaviors such as learning and memory. A 2015 study^[7] looking at mice with sepsis induced through surgical ligation of the small intestine found deficits with these mice compared to controls in the novel object recognition test. Mice usually spend less time investigating an object they have seen before than a newly introduced one, and so abnormalities in this time difference are interpreted as a sign of memory deficit.

The same apparent failure to recognize a previously unseen object was observed in a Brazilian study published in 2007^[8], with sepsis here also induced by small intestinal ligation. In this study, rats were used instead of mice, although one would expect the results to be comparable given the use of the same experimental paradigms, and the proximity in evolutionary lineage between those two genera.

Another study from 2007^[9] observed septic mice (with sepsis induced by LPS injection) exhibiting memory deficits in the radial maze test. In this paradigm, a mouse is placed in the middle of a maze with a number of arms, and has to learn the location of a piece of food in one of the arms; failure to more quickly retrieve food when the mouse is placed back into the maze in future trials is seen as a failure of learning and memory.

A 2008 review paper^[10] urges caution on interpreting the cognitive effects of sepsis. The authors note that prior studies have found that mice with LPS induced sepsis take longer than controls in the Morris water maze, a test of spatial memory in which mice must learn the location of a raised platform in order to escape a pool of water. The latency could be interpreted as a deficit in spatial memory, but the review authors suggest that it is more likely to be linked with the emotional and motor consequences of the sepsis.

Sepsis and Social Behavior

The consequences of sepsis in mouse models also extends to social behavior. A 2012 study^[11] examined social exploration in mice suffering from sepsis induced by ligation of the small intestine. Mice have a tendency to sniff, make contact with and fight with other mice they have not encountered before. The scientists here observed that sniffing and physical contact were both significantly reduced in septic mice, and fighting behavior was completely absent.

A Chinese study published in 2017^[12] also reports reduced capacity for social interaction in septic mice, this time with mice whose sepsis was induced by LPS injection. Since the conclusions of this study on social interaction were inferred in the context of a novel object recognition test, they are perhaps a little less robust than those of the 2012 study.

Sepsis and Sleep

One study conducted in 2012^[13] noted how immune dysfunction can be linked with the circadian rhythm (natural sleep-wake cycle) in mice, and so investigated the effects of LPS injection induced sepsis on sleep in mouse models. The researchers found that just a single treatment of LPS had noticeable long-term effects on the mouse circadian rhythm, including disruption of the “clock” proteins that regulate tiredness via a part of the brain called the suprachiasmatic nucleus.

Sepsis and Locomotion

The motor effects of a disorder can often be difficult to tease apart from its effects on mood and emotion. As mentioned above, mice with sepsis were seen to exhibit greater immobility in the tail suspension test, a result which could be interpreted as a sign of motor deficits, greater depression, or both. One test which is more specific to motor deficits is the rotarod test, in which mice must move across a rotating rod; mice which fall from the rod more quickly or at lower speeds of rotation are considered to have motor deficits.

A 2012 study^[14] used the rotarod test with mice whose sepsis had been induced through small intestinal ligation and found no significant difference between their performance and that of controls. This suggests the sepsis did not cause the mice to develop noticeable motor deficits and impugns the 2008 review’s interpretation of the Morris water maze data.

However, these conclusions conflict with data obtained from an Irish study published in 2009, with experiments performed by the authors of the 2008 review^[15]. Here, mice with sepsis induced by LPS injection into the perineum performed worse than controls on the horizontal bar and inverted screen tests. The horizontal bar assesses each mouse’s ability to balance while walking on a narrow platform, and the inverted screen assays muscular strength by looking at how long each mouse can cling to an upside-down wooden frame.

In addition, the Turkish study published in 2015 observed decreased locomotion by septic mice in the open field test. In this test, each mouse is placed separately within a box surrounded by high walls, with the floor divided into a number of squares. Movement is measured between squares, as well as the time spent in each square and each area of the enclosure.

The effects of sepsis on locomotor behavior and capabilities in mice seem inconclusive. Different studies have employed different experimental paradigms, making it harder to compare their results. It may be that the apparently contradictory data show that sepsis only affects certain forms of locomotor capability (such as balance) but not others (such as endurance). Attempts at exact replication of prior experiments would be crucial to figuring out what is causing the apparent discrepancy. It would be surprising if sepsis did not cause motor deficits in mice, given that reduced oxygen perfusion ought to have a marked effect on muscular and nervous function.

Conclusion

Studies using a broad range of experimental techniques have made it clear that sepsis can have marked behavioral effects in mouse models. These effects can include increased anxiety and depression, as well as deficits in learning and memory, locomotion and social behavior. The experimental data helps us to better understand the pathology and consequences of sepsis in humans, to allow for the development of superior diagnoses and treatments.

References

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