Olfactory Three-Needle Electroacupuncture Improved Synaptic Plasticity and Gut Microbiota of SAMP8 Mice by Stimulating Olfactory Nerve

Objective To investigate the effects and mechanisms of olfactory three-needle(OTN)electroacupuncture(EA)stimulation of the olfactory system on cognitive dysfunction,synaptic plasticity,and the gut microbiota in senescence-accelerated mouse prone 8(SAMP8)mice.Methods Thirty-six SAMP8 mice were randomly divided into the SAMP8(P8),SAMP8+OTN(P8-OT),and SAMP8+nerve transection+OTN(P8-N-OT)groups according to a random number table(n=12 per group),and 12 accelerated senescence-resistant(SAMR1)mice were used as the control(R1)group.EA was performed at the Yintang(GV 29)and bilateral Yingxiang(LI 20)acupoints of SAMP8 mice for 4 weeks.The Morris water maze test,transmission electron microscopy,terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling(TUNEL)staining,Nissl staining,Golgi staining,Western blot,and 16S rRNA sequencing were performed,respectively.Results Compared with the P8 group,OTN improved the cognitive behavior of SAMP8 mice,inhibited neuronal apoptosis,increased neuronal activity,and attenuated hippocampal synaptic dysfunction(P<0.05 or P<0.01).Moreover,the expression levels of synaptic plasticity-related proteins N-methyl-D-aspartate receptor 1(NMDAR1),NMDAR2B,synaptophysin(SYN),and postsynaptic density protein-95(PSD95)in hippocampus were increased by OTN treatment(P<0.05 or P<0.01).Furthermore,OTN greatly enhanced the brain-derived neurotrophic factor(BDNF)/cAMP-response element binding(CREB)signaling and phosphatidylinositol 3-kinase/protein kinase B/mammalian target of rapamycin(PI3K/AKT/mTOR)signaling compared with the P8 group(P<0.05 or P<0.01).However,the neuroprotective effect of OTN was attenuated by olfactory nerve truncation.Compared with the P8 group,OTN had a very limited effect on the fecal microbial structure and composition of SAMP8 mice,while specifically increased the genera Oscillospira and Sutterella(P<0.05).Interestingly,the P8-N-OT group showed an abnormal fecal microbiota with higher microbialα-diversity,Firmicutes/Bacteroidetes ratio and pathogenic bacteria(P<0.05 or P<0.01).Conclusions OTN improved cognitive deficits and hippocampal synaptic plasticity by stimulating the olfactory nerve and activating the BDNF/CREB and PI3K/AKT/mTOR signaling pathways.Although the gut microbiota was not the main therapeutic target of OTN for Alzheimer’s disease,the olfactory nerve was essential to maintain the homeostasis of gut microbiota.