Slow-wave-modulated high-frequency oscillations reveal early network abnormalities in pre-pathological 5XFAD mice

Alzheimer's disease (AD) is characterized by progressive disruption of large-scale neural networks, leading to abnormal brain oscillatory activity, yet sensitive electrophysiological markers of the earliest disease stages remain limited. Here, we investigated intracranial EEG (iEEG) biomarkers of early network dysfunction in the 5XFAD mouse model of AD, with a particular focus on the pre-plaque stage. iEEG recordings were obtained from 5XFAD and wild-type mice at 1 month (young), 3-4 months (adult), and 10-12 months (old). Spectral power distribution, epileptiform activity, and phase-amplitude coupling (PAC) were analyzed. Young 5XFAD mice exhibited detectable amyloid-β species in cortical and hippocampal regions in the absence of amyloid plaques, neuroinflammation, or neuronal loss, indicating an early pathological stage. Despite the lack of overt histopathology, young 5XFAD mice already showed reduced low-frequency power and increased high-frequency oscillation (HFO) power. Similar spectral alterations were observed in adult animals, together with prominent epileptiform activity and seizures. Notably, we demonstrate for the first time that pathological HFO activity in 5XFAD mice is associated with altered phase-amplitude coupling (PAC) with slow oscillations, characterized by enhanced coupling between HFO amplitude and delta phase during interictal periods. In young mice, increased coupling between fast-ripple activity and theta phase was also detected, indicating that cross-frequency network dysfunction emerges before plaque deposition. In contrast, old 5XFAD mice showed no consistent differences in iEEG spectral features relative to age-matched controls. These results show that increased HFO activity and its dysregulated coupling to low-frequency rhythms represent early and sensitive electrophysiological signatures of cortical hyperexcitability and network disruption in AD, preceding overt amyloid plaque formation and neurodegeneration, and highlight the translational potential of combined HFO power and PAC metrics as biomarkers for early-stage AD.