Absence of gut microbiota influences lipopolysaccharide-induced behavioral changes in mice

• Lack of gut microbiota alters LPS-induced depressive-like behaviors.
• Germ-free mice present impaired increase of TNF and microglial cells activation within the hippocampus after LPS stimulus.
• Animals lacking gut microbiota display an elevated basal level of dorsal raphe activation.

Changes in the microbiota composition of gastrointestinal tract are emerging as potential players in the physiopathology of neuropsychiatric disorders. In the present work we evaluated the relationship between the absence of gut microbiota and neuroinflammatory mechanisms in a murine model of LPS-induced behavioral alterations. Germ-free (GF) or conventional male mice received a single i.p. injection of lipopolysaccharide (LPS i.p.; 0.83 mg/Kg) or PBS, and after 24 h they were tested for depressive-like behaviors (forced swimming test, tail suspension test – TST, or sucrose preference test – SPT). After behavioral evaluation, animals were analyzed for possible changes in neuroplasticity by means of BDNF, NGF and cytokines levels in prefrontal cortex and hippocampus, and the expression of Iba-1 (microglial activation marker) in the hippocampus, and the cellular activity marker, ΔFosB, in the dorsal raphe nucleus. In conventional mice, LPS induced depressive-like behaviors. LPS-induced changes were followed by up-regulation of the expression of TNF and Iba-1 in the hippocampus. The same effects were not observed in GF mice. Behavioral effects of LPS were not observed in GF mice submitted to TST. GF mice present a lower response to the anhedonia-like effect induced by LPS when compared to conventional animals (SPT). There was up-regulation of ΔFosB in the dorsal raphe nucleus in the absence of gut microbiota, events not influenced by LPS treatment. Our results suggest that gut-microbiota interactions influence depressive-like behaviors, raphe nucleus activation and activation of pro-inflammatory mechanisms within the hippocampus.