Impaired synaptic plasticity and decreased glutamatergic neuron excitability induced by SIRT1/BDNF downregulation in the hippocampal CA1 region are involved in postoperative cognitive dysfunction

Abstract

Background: Postoperative cognitive dysfunction (POCD) is a common complication after anesthesia/surgery, especially among elderly patients, and poses a significant threat to their postoperative quality of life and overall well-being. While it is widely accepted that elderly patients may experience POCD following anesthesia/surgery, the exact mechanism behind this phenomenon remains unclear. Several studies have indicated that the interaction between silent mating type information regulation 2 homologue 1 (SIRT1) and brain-derived neurotrophic factor (BDNF) is crucial in controlling cognitive function and is strongly linked to neurodegenerative disorders. Hence, this research aims to explore how SIRT1/BDNF impacts cognitive decline caused by anesthesia/surgery in aged mice.

Methods: Open field test (OFT) was used to determine whether anesthesia/surgery affected the motor ability of mice, while the postoperative cognitive function of 18 months old mice was evaluated with Novel object recognition test (NORT), Object location test (OLT) and Fear condition test (FC). The expressions of SIRT1 and other molecules were analyzed by western blot and immunofluorescence staining. The hippocampal synaptic plasticity was detected by Golgi staining and Long-term potentiation (LTP). The effects of SIRT1 and BDNF overexpression as well as chemogenetic activation of glutamatergic neurons in hippocampal CA1 region of 18 months old vesicular glutamate transporter 1 (VGLUT1) mice on POCD were further investigated.

Results: The research results revealed that older mice exhibited cognitive impairment following intramedullary fixation of tibial fracture. Additionally, a notable decrease in the expression of SIRT1/BDNF and neuronal excitability in hippocampal CA1 glutamatergic neurons was observed. By increasing levels of SIRT1/BDNF or enhancing glutamatergic neuron excitability in the CA1 region, it was possible to effectively mitigate synaptic plasticity impairment and ameliorate postoperative cognitive dysfunction.

Conclusions: The decline in SIRT1/BDNF levels leading to changes in synaptic plasticity and neuronal excitability in older mice could be a significant factor contributing to cognitive impairment after anesthesia/surgery.

Keywords: General anesthesia; Hippocampus; Postoperative cognitive dysfunction; SIRT1; Synaptic plasticity.

Fig. 1

Anesthesia/surgery induced cognitive impairment in mice. Behavioral tests were performed on the second and third days after tibial fracture surgery to assess the cognitive function of the mice. A Diagram of intramedullary fixation of tibial fracture. B The behavioral timeline. C Experiment diagram of the OFT. D Representative track diagrams in the OFT. E Total moving distance of mice in the OFT (n = 10). F The average speed of mice in the OFT (n = 10). G Experiment diagram of the OLT. H Representative track diagrams in the OLT. I Total object exploration time in the OLT (n = 10). J The percentage of time spent exploring new location objects in the OLT **p < 0.01 (n = 10). K Experiment diagram of the NORT. L Representative track diagrams in the NORT. M Total object exploration time in the NORT (n = 10). N The percentage of time spent exploring new objects in the NORT **p < 0.01 (n = 10). O Experiment diagram of the FC. P Percentage of freezing time of mice during the FC training (n = 10). Q Percentage of freezing time of mice during the FC context test *p < 0.05 (n = 10). R Percentage of freezing time of mice during the FC tone test (n = 10)

Fig. 2

Effects of anesthesia/surgery on SIRT1/BDNF expression in the hippocampal CA1 region of POCD mice. A Representative Western blot protein bands. B The level of SIRT1 in the hippocampal CA1 region of POCD mice was decreased **p < 0.01 (n = 8). C The level of BDNF in the hippocampal CA1 region of POCD mice was decreased **p < 0.01 (n = 8). D Representative images of SIRT1 and CaMKIIα. E Representative images of SIRT1 and GAD67. F Representative images of BDNF and CaMKIIα. G Representative images of BDNF and GAD67. Scale bar, 100 μm. H Quantitation of the co-labeling rate of SIRT1 and CaMKIIα **p < 0.01 (n = 6). I Quantitation of the co-labeling rate of BDNF and CaMKIIα **p < 0.01 (n = 6). J Quantitation of the co-labeling rate of SIRT1 and GAD67 (n = 6). K Quantitation of the co-labeling rate of BDNF and GAD67 (n = 6)

Fig. 3

Impairment of synaptic plasticity in the hippocampal CA1 region of POCD mice. A Golgi staining images of 4 × in the hippocampus under electron microscope. Scale bar, 500 μm. B Profile images of Golgi staining of neurons in the hippocampal CA1 region. 20 × with camera tracings and 60 × for spine counting. Scale bar, 100 μm for 20 ×; 10 μm for 60 ×. C Quantification of dendritic intersections *p < 0.05, **p < 0.01 (n = 18). D Quantitation of the dendritic spine density **p < 0.01 (n = 18). E Quantitation of the total dendritic length **p < 0.01 (n = 18). F The sample image shows stimulation given at the location of the lateral Schaffer branch and recorded in the CA1 region. G LTP recorded in the CA1 region of the hippocampus. The arrow indicates the point in time of the TBS. H Average field excitatory postsynaptic potential (fEPSP) slope during the last 20 min after TBS. **p < 0.01 (n = 5)

Fig. 4

Decreased excitability of glutamatergic neurons in the hippocampal CA1 region of POCD mice. A Fluorescence images show the effective expression of the AAV-CaMKIIα vector in the hippocampal CA1 region and the schematic diagram of glutamatergic neurons in the hippocampal CA1 region recorded under microscope in acute brain sections. B Schematic diagram of action potential induced by depolarization current. C Fring rate of action potentials evoked by depolarizing current pulses of 0–200 pA **p < 0.01 (n = 15). D Threshold current used to excite the first action potential **p < 0.01 (n = 15). E Comparison of resting membrane potential between the two groups of mice (n = 15)

Fig. 5

Effects of SIRT1 overexpression in hippocampal CA1 glutamatergic neurons on SIRT1/BDNF expression, synaptic plasticity and neuronal excitability. A Fluorescence images showed that the AAV-SIRT1 vector was effectively expressed in the CA1 region of the hippocampus. B Representative Western blot protein bands. C Quantitative results show that the level of SIRT1 in the hippocampal CA1 region of SIRT1-overexpression mice was increased **p < 0.01 (n = 8). SIRT1 expression in each sample was normalized to that of GAPDH. The SIRT1 level of control mice was set to 1 for quantification. D Quantitative results show that the level of BDNF in the hippocampal CA1 region of SIRT1-overexpression mice was increased **p < 0.01 (n = 8). BDNF expression in each sample was normalized to that of GAPDH. The BDNF level of control mice was set to 1 for quantification. E Representative images of SIRT1 and CaMKIIα. Scale bar, 100 μm. F Quantitation of the co-labeling rate of SIRT1 and CaMKIIα **p < 0.01 (n = 6). G Representative images of BDNF and CaMKIIα. Scale bar, 100 μm. H Quantitation of the co-labeling rate of BDNF and CaMKIIα *p < 0.05 (n = 6). I Representative images of c-Fos and CaMKIIα. Scale bar, 100 μm. J Quantitation of the co-labeling rate of c-Fos and CaMKIIα **p < 0.01 (n = 6). K Profile images of Golgi staining of neurons in the hippocampal CA1 region. 20 × with camera tracings and 60 × for spine counting. Scale bar, 100 μm for 20 ×; 10 μm for 60 ×. L Quantification of dendritic bifurcations *p < 0.05, **p < 0.01 (n = 18). M Quantitation of the dendritic spine density **p < 0.01 (n = 18). N Quantitation of the total dendritic length *p < 0.05 (n = 18)

Fig. 6

Overexpression of SIRT1 in glutamatergic neurons in the hippocampal CA1 region can improve postoperative cognitive dysfunction. A The behavioral timeline. B Representative track diagrams in the OFT. C Total moving distance of mice in the OFT (n = 10). D The average speed of mice in the OFT (n = 10). E Representative track diagrams in the OLT. F Total object exploration time in the OLT (n = 10). G The percentage of time spent exploring new location objects in the OLT **p < 0.01 (n = 10). H Representative track diagrams in the NORT. I Total object exploration time in the NORT (n = 10). J The percentage of time spent exploring new objects in the NORT **p < 0.01 (n = 10). K Percentage of freezing time of mice during the FC training (n = 10). L Percentage of freezing time of mice during the FC context test *p < 0.05 (n = 10). M Percentage of freezing time of mice during the FC tone test (n = 10)

Fig. 7

Effects of BDNF overexpression in hippocampal CA1 glutamergic neurons on BDNF expression, synaptic plasticity and neuronal excitability. A Fluorescence images showed that the AAV-BDNF vector was effectively expressed in the CA1 region of the hippocampus. B Representative Western blot protein bands. C Quantitative results show that the level of BDNF in hippocampal CA1 region of BDNF-overexpression mice was increased **p < 0.01 (n = 8). BDNF expression in each sample was normalized to that of GAPDH. The BDNF level of control mice was set to 1 for quantification. D Representative images of BDNF and CaMKIIα. Scale bar, 100 μm. E Quantitation of the co-labeling rate of BDNF and CaMKIIα **p < 0.01 (n = 6). F Representative images of c-Fos and CaMKIIα. Scale bar, 100 μm. G Quantitation of the co-labeling rate of c-Fos and CaMKIIα *p < 0.05 (n = 6). H Profile images of Golgi staining of neurons in the hippocampal CA1 region. 20 × with camera tracings and 60 × for spine counting. Scale bar, 100 μm for 20 ×; 10 μm for 60 ×. I Quantification of dendritic bifurcations *p < 0.05, **p < 0.01 (n = 18). J Quantitation of the dendritic spine density **p < 0.01 (n = 18). K Quantitation of the total dendritic length **p < 0.01 (n = 18)

Fig. 8

Overexpression of BDNF in glutamatergic neurons in the hippocampal CA1 region can improve postoperative cognitive dysfunction. A The behavioral timeline. B Representative track diagrams in the OFT. C Total moving distance of mice in the OFT (n = 10). D The average speed of mice in the OFT (n = 10). E Representative track diagrams in the OLT. F Total object exploration time in the OLT (n = 10). G The percentage of time spent exploring new location objects in the OLT **p < 0.01 (n = 10). H Representative track diagrams in the NORT. I Total object exploration time in the NORT (n = 10). J The percentage of time spent exploring new objects in the NORT **p < 0.01 (n = 10). K Percentage of freezing time of mice during the FC training (n = 10). L Percentage of freezing time of mice during the FC context test *p < 0.05 (n = 10). M Percentage of freezing time of mice during the FC tone test (n = 10)

Fig. 9

Effect of specific activation of hippocampal CA1 glutamatergic neurons on synaptic plasticity. A Fluorescence images showed that the chemogenetics vector was effectively expressed in the CA1 region of the hippocampus. B The schematic diagram of glutamatergic neurons in the hippocampal CA1 region recorded under microscope in acute brain sections. C In vitro infusion of CNO (10 mM) increased the firing frequency of glutamatergic neurons in the hippocampal CA1 region of AAV-hM3D(Gq)-injected POCD mice. D Representative colabeling images of c-Fos and CaMKIIα. Scale bar, 100 μm. E Quantitation of the co-labeling rate of c-Fos and CaMKIIα **p < 0.01 (n = 6). F LTP recorded in the CA1 region of the hippocampus. The arrow indicates the point in time of the TBS. G Average fEPSP slope during the last 20 min after TBS *p < 0.05 (n = 5)

Fig. 10

Specific activation of glutamatergic neurons in hippocampal CA1 significantly improved postoperative cognitive dysfunction in mice. A The behavioral timeline. B Representative track diagrams in the OFT. C Total moving distance of mice in the OFT (n = 10). D The average speed of mice in the OFT (n = 10). E Representative track diagrams in the OLT. F Total object exploration time in the OLT (n = 10). G The percentage of time spent exploring new location objects in the OLT **p < 0.01 (n = 10). H Representative track diagrams in the NORT. I Total object exploration time in the NORT (n = 10). J The percentage of time spent exploring new objects in the NORT **p < 0.01 (n = 10). K Percentage of freezing time of mice during the FC training (n = 10). L Percentage of freezing time of mice during the FC context test *p < 0.05 (n = 10). M Percentage of freezing time of mice during the FC tone test (n = 10)