High frequency head impact exposure changes hippocampal sharp-wave ripple architecture.

Repeated head impact in sports leads to chronic cognitive and neurobehavior deficits even in the absence of brain pathology. High-frequency head impact (HFHI) in mice causes a chronic change to the synaptic properties in core hippocampal circuits, and causes no cell death, no axonal damage, no tau or amyloid accumulation, and no inflammation, yet results in impaired cognitive function. It is unknown how HFHI affects intrinsic plasticity events and if neural biomarkers of HFHI can be detected. Sharp-wave ripples (SWR) are hippocampal population events consisting of a sharp wave (1-30 Hz) and associated ripple oscillation (120-220 Hz). SWR are strongly associated with memory and are an established biomarker for cognition and memory. To characterize the effect of HFHI on SWR, we prepared acute slices 24 hours after HFHI and used field recordings to characterize hippocampal SWR. Physiological SWR were present in both sham and HFHI mice, and their architecture was grossly intact. We did not detect pathological ripples. Quantification of SWR features showed a decreased amplitude and power of SWR in HFHI brains compared to sham. Further analysis of the ripple oscillations found decreased number of ripple cycles within each event, and reduced ripple power in HFHI brains. These data show that HFHI alters hippocampal SWR architecture, reducing metrics including SWR amplitude and power, which may play a role in impaired cognition in this mouse model.