Involvement of acute neuroinflammation in postoperative delirium-like cognitive deficits in rats.

Purpose: The purpose of this study was to investigate the age-, time-, and brain region-dependent postoperative neuroinflammatory trajectory, and its association with neurocognitive outcomes in rats.

Methods: Adult and aged rats were randomly assigned to one of three groups: control, isoflurane anesthesia alone, and isoflurane anesthesia with abdominal surgery. On either postoperative day 2 (early phase) or 7 (late phase), all rats were tested for trace and context fear memory retention after acquisition of trace fear conditioning. Freezing behavior was used as an index of fear memory. Following the cognitive testing, the levels of pro-inflammatory cytokines in several brain regions were measured using enzyme-linked immunosorbent assay (n = 8 in each group).

Results: In the early postoperative period, surgery under isoflurane anesthesia induced acute neuroinflammation along with related trace and context memory dysfunction. Such acute neuroinflammatory responses were comparably observed in both adult and aged animals, whereas the aged rats were more likely to exhibit behavioral changes. On the other hand, in the late postoperative period, neither neuroinflammation in all tested brain regions nor concomitant memory decline were found in adult animals. Significant neuroinflammation was detected only in the hippocampus of aged rats, which was associated with context, but not trace memory dysfunction.

Conclusion: Our findings indicate that surgery-induced acute, transient, brain-wide neuroinflammation may be involved in the pathogenesis of the postoperative delirium-like cognitive deficits in rats. Furthermore, neuroinflammation may convert from acute to chronic in an age- and hippocampal-specific manner, likely resulting in the development of sustained cognitive dysfunction.