LIPOPOLYSACCHARIDE-INDUCED INFLAMMATORY RESPONSES INCREASED DYADIC SOCIAL INTERACTION IN MALE BUT NOT FEMALE MICE
Authors: Yamamoto *Mizuki , Hayashi Kazuko , Kanayama Masashi , Tagashira Kyoko , Matsushima Shuntaro , Kasahara Haruki , Inoue Hiroto , Toda Koji 
Published: February 2025

Abstract

Background
Depression is a major mental disorder. Accumulating evidence suggests that inflammatory processes are closely correlated to the pathophysiology of depression. Inflammatory responses are indexed by elevated systemic cytokines, such as IL-1β, IL-6, and TNF-α. Proinflammatory cytokines are produced predominantly by activated macrophages and are involved in the up-regulation of inflammatory responses. Increased levels of cytokines were associated with the degree of depression. Intraperitoneal injection of lipopolysaccharide (LPS) is known to cause inflammatory responses and is widely used in animal models of inflammatory depression. LPS is present in the cell membranes of gram-negative bacteria. When the bacteria die in the animal's body, LPS is released from the bacteria and acts on toll- like receptors. As a result, it activates immune cells and triggers an inflammatory response by inducing cytokines. In LPS-treated animals, increased levels of cytokines were observed in the blood and the brain. In addition, decreased body weight, decreased spontaneous locomotor activity, increased immobility time in forced-swim and tail-suspension tests, and decreased sucrose preference were also observed after the LPS injection. These existing findings support the validity of LPS-treated animals as an animal model of depression.

Aims & Objectives
Despite the close relationship between social behavior and depression, how LPS-induced inflammatory responses affect social behavior remains unclear. Here, we examined the effects of inflammatory responses on social and non-social behavior in C57BL/6J mice after the intraperitoneal injection of LPS.

Method
We validated that the intraperitoneal injection of LPS (1.0 mg/kg) caused inflammatory responses by (1) quantifying cytokines with ELISA and (2) counting immune cells with flow cytometry 1.5 and 3 hours after the LPS injection. We examined the effects of inflammatory responses on dyadic interaction 24 hours after the LPS injection. In addition, we validated existing findings of LPS-induced changes of body weight and non-social behavior in open-field and sucrose preference tests.

Results
By the immunological examinations with ELISA and flow cytometry, we found prominent increases of IL- 1β, IL-6, and TNF-α and decreases of T cells, B cells, neutrophils, and monocytes. There was a sex difference in the baseline number of B cells, neutrophils, and monocytes. Through the behavioral assessments, we found that LPS-induced inflammatory responses increased contact time and decreased dyadic distance in familiar and unfamiliar males. Meanwhile, these differences were not found in female dyads. LPS injection caused decreased body weight, locomotor activities, and fecal outputs in an open- field test and decreased not only sucrose preference but also the overall number of licking in a sucrose preference task in both male and female mice.

Discussion & Conclusion
Our results suggest that social behavior is modulated by LPS-induced inflammatory responses in a sex- dependent manner. The increased contact time of dyadic interaction in males could not be explained by weight loss or decreased locomotion, orofacial movements, and reward sensitivities because these results were also observed in females. Our findings pave the way to understanding the relationship between immune responses and social behavior.