Prior infection exacerbates postoperative cognitive dysfunction in aged rats

Older patients may experience persisting postoperative cognitive dysfunction (POCD), which is considered to largely depend on surgery-induced (neuro)inflammation. We hypothesize that inflammatory events before surgery could predispose patients to POCD. When part of our aged rats developed Mycoplasma pulmonis, this presented the unique opportunity to investigate whether a pulmonary infection before surgery influences surgery-induced neuroinflammation and POCD. Male 18-mo-old Wistar rats that had recovered from an active mycoplasma infection (infection) and control rats (healthy) were subjected to abdominal surgery and jugular vein catheterization under general anesthesia (surgery) or remained naïve (control). In postoperative week 2, behavioral tests were performed to assess cognitive performance and exploratory behavior. The acute systemic inflammatory response was investigated by measuring plasma IL-6 and IL-12. In the hippocampus, prefrontal cortex and striatum, microglial activity, neurogenesis, and concentrations of IL-6, IL-12, IL1B, and brain-derived neurotropic factor on postoperative day 14 were determined. Rats still showed signs of increased neuroinflammatory activity, as well as cognitive and behavioral changes, 3 wk after the symptoms of infection had subsided. Rats that had experienced infection before surgery exhibited a more generalized and exacerbated postoperative cognitive impairment compared with healthy surgery rats, as well as a prolonged increase in systemic cytokine levels and increased microglial activation in the hippocampus and prefrontal cortex. These findings support the hypothesis that an infection before surgery under general anesthesia exacerbates POCD. Future studies are necessary to determine whether the found effects are aging specific and to investigate the magnitude and time course of this effect in a controlled manner.