TNF and metabolic disease: We hypothesized that this could be the case based on previous findings that high-fat diet/metabolic disease are associated with increased inflammation, including microglia activity in the hippocampus (Kang et al., 2016; Ledreux et al., 2016; Yu et al., 2019), and that microglial-associated inflammation (i.e., via release of inflammatory cytokines such as IL-1β, TNF-α, and IL-6) impairs AHN in rodents (Ekdahl et al., 2003; Monje et al., 2003).