Preventive effects of dexmedetomidine on the development of cognitive dysfunction following systemic inflammation in aged rats.

Purpose: In the present study, we examined whether and by what mechanisms dexmedetomidine (DMED) prevents the development of systemic inflammation (SI)-induced cognitive dysfunction in aged rats.
Methods: Animals received a single intraperitoneal (i.p.) injection of either 5.0 mg/kg lipopolysaccharide (LPS) or vehicle. LPS-treated rats were further divided into three groups: early DMED, late DMED, or midazolam (MDZ) treatment (n = 12 each). Seven days after LPS injection, cognitive function was evaluated using a novel object recognition task, followed by measurement of hippocampal levels of proinflammatory cytokines and Toll-like receptor 4 (TLR-4) expression. For ex vivo experiments, microglia were isolated from the hippocampus for assessment of cytokine response to LPS.
Results: LPS-treated rats showed memory deficits, hippocampal neuroinflammation, and TLR-4 upregulation as compared to saline-treated animals. However, early DMED treatment was able to attenuate these SI-induced neurocognitive changes, whereas no benefits were observed in the MDZ and late DMED treatment groups. In ex vivo experiments, early DMED treatment prevented the development of SI-induced excessive microglial hyperactivation, which was blocked by the nonspecific α ₂ -adrenergic receptor (AR) antagonist atipamezole or the specific α _2A -AR antagonist BRL-44408, but not by the specific α _2B/C -AR antagonist ARC-239. On the other hand, neither DMED nor MDZ had a direct effect on LPS-induced release of pro-inflammatory cytokines from hippocampal microglia at clinically relevant concentrations.
Conclusion: Our findings highlight that treatment with DMED during, but not after, peripheral SI can prevent subsequent hippocampal neuroinflammation, overexpression of TLR-4 in microglia, and cognitive dysfunction, as mediated by the α _2A -AR signaling pathway.