The Detrimental Effects of Lipopolysaccharide-induced Neuroinflammation on Adult Hippocampal Neurogenesis Depend on the Duration of the Pro-inflammatory Response

Adult hippocampal neurogenesis is a finely tuned process regulated by extrinsic factors. Neuroinflammation is a hallmark of several pathological conditions underlying dysregulation of neurogenesis. In animal models, lipopolysaccharide (LPS)-induced neuroinflammation leads to a neurogenic decrease mainly associated to the early inflammatory response. However, it is not well understood how the neuroinflammatory response progresses over time and if neurogenesis continues to be diminished during the late neuroinflammatory response. Moreover, it is unknown if repeated intermittent administration of LPS along time induces a greater reduction in neurogenesis. We administered one single intraperitoneal injection of LPS or saline or four repeated injections (one per week) of LPS or saline to young-adult mice. A cohort of new cells was labeled with three 5-bromo-2-deoxyuridine injections (one per day) 4 days after the last LPS injection. We evaluated systemic and neuroinflammation-associated parameters and compared the effects of the late neuroinflammatory response on neurogenesis induced by each protocol. Our results show that 1) a single LPS injection leads to a late pro-inflammatory response characterized by microglial activation, moderate astrocytic reaction and increased interleukin-6 levels. This response correlates in time with decreased neurogenesis and 2) a repeated intermittent injection of LPS does not elicit a late pro-inflammatory response although activated microglia persists. The latter profile is not accompanied by a continued long-term hippocampal neurogenic decrease. Hereby, we provide evidence that the neuroinflammatory response is a dynamic process that progresses in a milieu-dependent manner and does not necessarily lead to a neurogenic decrease, highlighting the complex interaction between the immune system and neurogenesis.