Background: Brain inflammation has been implicated as a critical mechanism responsible for the progression of neurodegeneration and characterized by glial cell activation accompanied by production of inflammation-related cytokines and chemokines. Growing evidence also suggests that metabolites derived from docosahexaenoic acid (DHA) have anti-inflammatory and pro-resolving effects; however, the possible role of N-docosahexaenoylethanolamine (synaptamide), an endogenous neurogenic and synaptogenic metabolite of DHA, in inflammation, is largely unknown. (The term "synaptamide" instead of "DHEA" was used for N-docosahexaenoylethanolamine since DHEA is a widely used and accepted term for the steroid, dehydroepiandrosterone.) In the present study, we tested this possibility using a lipopolysaccharide (LPS)-induced neuroinflammation model both in vitro and in vivo.
Methods: For in vitro studies, we used P3 primary rat microglia and immortalized murine microglia cells (BV2) to assess synaptamide effects on LPS-induced cytokine/chemokine/iNOS (inducible nitric oxide synthase) expression by quantitative PCR (qPCR) and enzyme-linked immunosorbent assay (ELISA). To evaluate in vivo effects, mice were intraperitoneally (i.p.) injected with LPS followed by synaptamide, and expression of proinflammatory mediators was measured by qPCR and western blot analysis. Activation of microglia and astrocyte in the brain was examined by Iba-1 and GFAP immunostaining.
Results: Synaptamide significantly reduced LPS-induced production of TNF-α and NO in cultured microglia cells. Synaptamide increased intracellular cAMP levels, phosphorylation of PKA, and phosphorylation of CREB but suppressed LPS-induced nuclear translocation of NF-κB p65. Conversely, adenylyl cyclase or PKA inhibitors abolished the synaptamide effect on p65 translocation as well as TNF-α and iNOS expression. Administration of synaptamide following LPS injection (i.p.) significantly reduced neuroinflammatory responses, such as microglia activation and mRNA expression of inflammatory cytokines, chemokine, and iNOS in the brain.
Conclusions: DHA-derived synaptamide is a potent suppressor of neuroinflammation in an LPS-induced model, by enhancing cAMP/PKA signaling and inhibiting NF-κB activation. The anti-inflammatory capability of synaptamide may provide a new therapeutic avenue to ameliorate the inflammation-associated neurodegenerative conditions.
Keywords: CCL2; Cytokine; Docosahexaenoic acid; FAAH; GFAP; Iba-1; Microglia; NF-κB; Synaptamide; iNOS.