A Novel Individual-based Determination of Postoperative Cognitive Dysfunction in Mice

Postoperative cognitive dysfunction (POCD) is a significant clinical issue. Aging is a risk factor for POCD. It is known that not every patient develops POCD. This situation shall be similar in animals. Determination of POCD is individual-based in humans but group-based in animal studies. This difference prevents effective evaluation of biomarkers and interventions for POCD in preclinical studies. The objective of this study was to determine whether individual animal could be assessed for POCD by a system similar to that for patients. Seven-week old CD1 and 18-month old C57BL/6 male mice were subjected to right carotid arterial exposure under isoflurane anesthesia. Mice were evaluated by Barnes maze and fear conditioning either post-surgery alone or both prior to surgery and post-surgery. Surgery increased the time to identify the target box in Barnes maze when tested one day or 8 days after the training sessions and reduced freezing behavior in fear conditioning test. This phenomenon occurred in 7-week old animals with and without evaluation before the surgery and in 18-month old mice evaluated before and after surgery. Based on the method and criteria used for a human whose cognition was evaluated before and after surgery to assess individual decline of cognition, 7 in 21 mice in the surgical group and 1 in 21 mice in control group of 7-week old mice had cognitive dysfunction. Among 18-month old mice, 13 in 21 mice in the surgical group and 2 in 20 mice in the control group had cognitive dysfunction. The incidence of cognitive dysfunction in mice with surgery was higher than that in control mice no matter whether young adult (P = 0.045) or old mice (P < 0.001) were considered. These results indicate that surgery induces POCD in mice. Individual animal-based assessment can be used to identify animals with POCD.