Preoperative environment enrichment preserved neuroligin 1 expression possibly via epigenetic regulation to reduce postoperative cognitive dysfunction in mice

Abstract

Aims: Postoperative cognitive dysfunction (POCD) is a common and significant syndrome. Our previous studies have shown that surgery reduces dendritic arborization and spine density and that environment enrichment (EE) reduces POCD. Neuroligin 1 is a postsynaptic protein involved in the formation of postsynaptic protein complex. This study was designed to determine the role of neuroligin 1 in the protection of EE against POCD and the mechanisms for EE to affect neuroligin 1 expression.

Methods: Eight-week-old C57BL/6J male mice with or without EE for 3, 7, or 14 days had right carotid artery exposure under isoflurane anesthesia. An anti-neuroligin 1 antibody at 1.5 µg/mouse was injected intracerebroventricularly at one and two weeks before the surgery. Mice were subjected to the Barnes maze and fear conditioning tests from one week after the surgery. Cerebral cortex and hippocampus were harvested after surgery.

Results: Mice with surgery had poorer performance in the Barnes maze and fear conditioning tests than control mice. EE for 2 weeks, but not EE for 3 or 7 days, improved the performance of surgery mice in these tests. Surgery reduced neuroligin 1 in the hippocampus. Preoperative EE for 2 weeks attenuated this reduction. The anti-neuroligin 1 antibody worsened the performance of mice with surgery plus EE in the Barnes maze and fear conditioning tests. Surgery increased histone deacetylase activity and decreased the acetylated histone in the hippocampus. EE attenuated these surgery effects.

Conclusion: Our results suggest that preoperative EE for 2 weeks reduces POCD. This effect may be mediated by preserving neuroligin 1 expression via attenuating surgery-induced epigenetic dysregulation in the brain.

Keywords: environment enrichment; histone deacetylase; neuroligin 1; postoperative cognitive dysfunction.

FIGURE 1

Diagram of time line of experiments. (A) Time line of experiments determining the length of EE needed for improving learning and memory in mice with surgery. (B) Time line of experiments determining the role of neuroligin 1 in the EE effects on mice with surgery. BM: Barnes maze, BMlt: Barnes maze long‐term memory, BMst: Barnes maze short‐term memory, FC: fear conditioning, inacti: heat‐inactivated anti‐neuroligin 1 antibody, anti: anti‐neuroligin 1 antibody, NLGN1: neuroligin 1

FIGURE 2

EE reduced learning and memory impairment in mice with surgery. Mice were subjected to various experimental conditions and tested in the Barnes maze and fear conditioning paradigms starting 7 days after surgery. (A) Performance in the training sessions of Barnes maze test. (B) Performance in the memory phase of Barnes maze test. (C) Performance in fear conditioning paradigm. Results are median ± interquartile range (n = 25) with the presence of individual animal data in the bar graphs. Results were analyzed by two‐way or one‐way repeated measures analysis of variance for panel A and by one‐way analysis of variance on ranks followed by the Tukey test for panels B and C. * indicates p < 0.05 for the comparisons of values of day 1 with those of day 2, values of day 1 with those of day 3 and values of day 1 with those of day 4 for all groups except for the comparisons of values of day 1 with those of day 2 for the EE14 and Surg+EE3 groups. EE3: EE for 3 days, EE7: EE for 7 days, EE14: EE for 14 days, Surg: surgery

FIGURE 3

EE increased neuroligin 1 in the hippocampus. Mice had EE or SE for 14 days. Their hippocampus and cerebral cortex were harvested 24 h after the completion of EE. (A) Representative images of western blotting of hippocampal samples. (B) Representative images of western blotting of cerebral cortical samples. (C) Quantitative data of neuroligin 1. (D) Quantitative data of acetylated histone H3. (E) Quantitative data of acetylated histone H4. (F) HDAC activity. Results are mean ± S.D. (n = 12) with the presence of individual animal data in the bar graphs. Results were analyzed by t‐test. EE14: EE for 14 days

FIGURE 4

EE attenuated the decrease of neuroligin 1 in the hippocampus of mice with surgery. Mice had EE or SE for 14 days before they had surgery. Their hippocampus and cerebral cortex were harvested 24 h after the surgery. (A) Representative images of Western blotting of hippocampal samples. (B) Representative images of western blotting of cerebral cortical samples. (C) Quantitative data of neuroligin 1. (D) Quantitative data of acetylated histone H3. (E) Quantitative data of acetylated histone H4. (F) HDAC activity. Results are mean ± S.D. (n = 12) with the presence of individual animal data in the bar graphs. Results were analyzed by one‐way analysis of variance followed by the Tukey test. EE14: EE for 14 days

FIGURE 5

Neuroligin 1 contributed to the protection of EE against surgery‐induced learning and memory impairment. Mice were subjected to various experimental conditions and tested in the Barnes maze and fear conditioning paradigms starting 7 days after surgery. (A) Performance in the training sessions of Barnes maze test. (B) Performance in the memory phase of Barnes maze test. (C) Performance in fear conditioning paradigm. Results are median ±interquartile range (n = 14) with the presence of individual animal data in the bar graphs. Results were analyzed by two‐way or one‐way repeated measures analysis of variance for panel A and by one‐way analysis of variance on ranks followed by the Tukey test for panels B and C. * indicates p < 0.05 for the comparisons of values of day 1 with those of day 2, values of day 1 with those of day 3 and values of day 1 with those of day 4 for all groups except for the comparisons of values of day 1 with those of day 2 for the control and EE+surg+anti groups. EE14: EE for 14 days, Surg: surgery, Inacti: heat‐inactivated anti‐neuroligin 1 antibody, Anti: anti‐neuroligin 1 antibody