Effects of different concentration and duration time of isoflurane on acute and long-term neurocognitve function of young adult C57BL/6 mouse

Postoperative cognitive dysfunction (POCD) is a decline in cognitive performance after a surgery with anaesthesia. The exact reasons of surgery and/or anaesthesia resulting in POCD are unclear. The aim of this study is to investigate the effects of different concentration and duration time of isoflurane anaesthesia on cognitive performance and cellular mechanisms involved in learning and memory function. In present work, young adult male C57BL/6 mice (age: 8 weeks) were anaesthetized by different concentration isoflurane in 100% oxygen for different duration time (Mice in group I₁ received 0.7% isoflurane 0.5 h, mice in I₂ received 0.7% isoflurane 2 h, mice in I₃ received 1.4% isoflurane 2 h, and mice in I₄ received 1.4% isoflurane 4 h). Non-anaesthetized mice served as control group (I₀). Spatial learning was assessed at 10 days post-anesthesia in Morris water maze (MWM). Hippocampal protein expressions of activated caspase 3, NMDA receptor subunit NR2B, and extracellular-signal regulated kinase (ERK) 1/2 were evaluated 24 hours and 2 weeks post anesthesia. Protein expression of activated caspase3 was detected acute elevated in I₃ (24 h post-anesthesia) and acute and long-term elevated in I₄ (24 hours and 2 weeks post-anesthesia). There was no significant difference between I₁, I₂ and control group. Protein expressions of NR2B showed an acute and long-term increasement in I₁ and I₂, decreasement in I₄, and an acute decline, then returned to normal in I₃ compared to control group. The ratio of phosopho-ERK1/2 to total-ERK showed an acute increasement in I₁ and I₂, then came to normal 2 weeks post anesthesia compared to control group, meanwhile, we detected an acute and long-term decline in I₃ and I₄. In MWM test, mice in I₁ and I₂ showed cognitive improvement, mice in I₃ showed similar to control group, while mice in I₄ demonstrated cognitive impairment, which were approximately corresponding to the changes of protein expression of NR2B and activation of ERK1/2. The present data suggested the following: (1) Isoflurane may cause neurotoxicity by inducing caspase activation and apoptosis with the anesthetic concentration increased and duration prolonged. (2) Low concentration of isoflurane in 2 hours can induce a hippocampus-specific elevation of NR2B subunit composition and ratio of p-ERK1/2 to total ERK1/2, produce hippocampal-dependent cognitive improvement. While high concentration of isoflurane exceeding 4 hours may induce a decline of NR2B and ratio of pERK1/2 to ERK1/2, then result in cognitive impairment.