Decreased excitatory and increased inhibitory transmission in the hippocampal CA1 drive neuroinflammation-induced cognitive impairments in mice

Neuroinflammation is one of crucial pathogenic mechanisms underlying Alzheimer’s disease, sepsis-associated encephalopathy, and postoperative cognitive dysfunction. These diseases or conditions are often accompanied by typical clinical manifestations of cognitive impairments, including impaired learning and memory but underlying mechanisms are unknown. Hence, effective treatments are not available. In the current study, mice received intraperitoneal administrations of LPS (0.5 mg/kg, daily, Escherichia coli O55:B5) for seven consecutive days and after which, different cohorts were used for behavioral assessments with open field, Y maze, and novel object recognition test or for electrophysiology recordings of mEPSC, mIPSC or LTP in ex vivo preparations. Their hippocampi were harvested for immunostaining or Western blotting of PSD95, vGLUT1, vGAT, gephyrin, PV, and SST. In vivo optical fiber calcium recording was used to evaluate the neuronal excitability. During the early stage of neuroinflammation induced by LPS, there was a decrease of excitatory afferent synapses and transmission in the CA1. During the later stage of neuroinflammation, there was an increase of inhibitory afferent synapses and transmission in the CA1, resulting in excessive inhibition on excitatory neurons. Both of them contributed to the decreased hippocampal neuronal excitability and impaired LTP, ultimately leading to cognitive impairments. Overexpression of CREB in the early stage or inactivation of PV-positive interneurons in the later stage in the CA1 both improved cognitive impairments. Our work suggests that negating decreased excitatory and increased inhibitory afferent in the hippocampus may improve cognitive impairments relate to neuroinflammation associated with neurological diseases.