High Frequency Oscillations (HFOs) are categorized as Ripples (80-250Hz), Fast Ripples (250-500 Hz), or Very Fast Ripples (> 500 Hz). Pathologic HFOs (pHFOs) are postulated to be potential biomarkers of network disarray and are implicated within the seizure onset zone in epilepsy.

Previous research demonstrates that the CD40L-CD40 system regulates neuroinflammation and seizure susceptibility. This study aims to contextualize the network level role of CD40L-CD40 during seizures.

16-channel multielectrode silicon probes across the corticohippocampal axis were chronically implanted in adult male CD40 knockout mice (CD40KO) and respective genetic control mice (WT). The pentylenetetrazole (PTZ) model of seizure was used to induce epileptiform network activity. Local field potentials (LFP) at specified depths (channel) and time intervals were recorded in freely moving animals before and after administration of PTZ. Continuous recordings were segmented and processed for offline analysis to capture signal morphology and frequency analysis [δ(0.5-4 Hz), θ(4-8Hz), α(8-12Hz), β(12-30 Hz), γ(30-80Hz), HFOs (80-500Hz)] using signal analysis software. An automatic detection toolbox was used to identify putative HFO events using a Short Time Energy algorithm. Putative HFOs were manually reviewed using the raw signal in parallel to digital filters and spectral analyses. Following euthanasia, hippocampal levels of NR2B were evaluated using ELISA.

Network activation results in a dynamic high-power shift of frequency band distribution in both WT and CD40KO mice. CD40KO mice have considerably smaller θ/δ ratios during normal activity and have limited γ band shift compared to WT, consistent with reduction of hippocampal NR2b (NMDA sub-unit) expression during seizures. Preliminary data demonstrates that CD40 deficiency attenuates HFO rate by over 80% and eliminates the generation of state dependent pHFO.

The CD40-CD40L complex triggers a downstream cascade which alters neuronal network activation and  promotes generation of pathological HFO during ictogenesis though an upregulation of excitatory NMDA receptors.