The effects of lipopolysaccharide exposure on social interaction, cytokine expression, and alcohol consumption in male and female mice

Abstract

Much recent research has demonstrated a role of inflammatory pathways in depressive-like behavior and excess alcohol consumption. Lipopolysaccharide (LPS) is a cell wall component of gram-negative bacteria that can be used to trigger a strong inflammatory response in rodents in a preclinical research setting to study the mechanisms behind this relationship. In our study, we exposed male and female mice to LPS and assessed depressive-like behavior using the social interaction (SI) test, alcohol consumption in the two-bottle choice procedure, and expression of inflammatory mediators using quantitative PCR. We found that LPS administration decreased SI in female mice but had no significant impact on male mice when assessed 24 h after injection. LPS resulted in increased proinflammatory cytokine expression in both male and female mice; however, some aspects of the cytokine upregulation observed was greater in female mice as compared to males. A separate cohort of male and female mice underwent drinking for 12 days before receiving a saline or LPS injection, which we found to increase alcohol intake in both males and females. We have previously observed a role of the neurokinin-1 receptor (NK1R) in escalated alcohol intake, and in the inflammatory and behavioral response to LPS. The NK1R is the endogenous target of the neuropeptide SP, and this system has wide ranging roles in depression, anxiety, drug/alcohol seeking, pain, and inflammation. Thus, we administered a NK1R antagonist prior to alcohol access. This treatment reduced escalated alcohol consumption in female mice treated with LPS but did not affect drinking in males. Taken together, these results indicate that females are more sensitive to some physiological and behavioral effects of LPS administration, but that LPS escalates alcohol consumption in both sexes. Furthermore, NK1R antagonism can reduce alcohol consumption that is escalated by LPS treatment, in line with our previous findings.

Keywords: Alcohol; Cytokines; Inflammation; Neurokinin-1 receptor; Sex differences.

Figure 1.

Timeline of experiments.

Figure 2.. LPS reduces social interaction in female mice.

Shown is the time (in seconds) that male and female mice spent in the interaction zone during the SI test. Main effects of treatment (p=0.002), and interaction effect (p=0.04) were observed. **p<0.01 compared to saline treated females.

Figure 3.. TNFα and IL6 expression following LPS injection.

Expression of IL-6 and TNFα proinflammatory cytokines in the hippocampus (HPC), prefrontal cortex (PFC), and ventral striatum (VS), a-c Expression of IL-6 in the HPC, PFC, and VS. d-f Expression of TNF in the HPC, PFC, and VS. Data expressed as fold change in expression compared to saline treated mice of same sex. *p<0.05, **p<0.01, ***p<0.001 compared to saline treatment. + p<0.05 compared to male mice treated with LPS.

Figure 4.. Body Weights after LPS treatment.

Body weights were tracked for 7 days following injection of LPS. **p<0.01, ***p<0.001 male saline versus male LPS; ###p<0.001 female saline versus female LPS.

Figure 5.. Alcohol consumption following LPS injection.

Ethanol consumption, averaged over the three-day periods immediately before and after LPS injection, were compared in male and female mice with a treatment of saline or LPS (main effects of treatment p<0.0001 and sex: p<0.0001). *p<0.05, **p<0.01.

Figure 6.. Effects of NK1R antagonist on alcohol consumption.

Alcohol consumption is reported in g/kg consumed over the 24 hours following treatment. a LPS treated mice were given pretreatment injections with vehicle or NK1R antagonist (main effects of antagonist treatment: p=0.03, sex: p=0.003, interaction: p=0.04). b Saline treated mice were given pretreatment injections with vehicle or NK1R antagonist (main effects of antagonist treatment p=0.0005, sex: p=0.005). *p<0.05, **p<0.01, ***p<0.001.