Timing differences between HFOs and interictal epileptiform discharges generated in vitro by different mechanisms in rat hippocampal slices: A novel approach

OBJECTIVE: To investigate the effect of generating mechanism on the relationship between interictal‐like epileptiform discharges (IEDs) and the underlying High Frequency Oscillations (HFOs; Ripples, R, and Fast Ripples, FR). METHODS: Synchronous spontaneous IEDs were recorded from the CA1 area of hippocampal slices from adult rats, perfused by Mg(2+)‐free ACSF (n = 41slices/14 animals) or 4‐aminopyridine (50 μM, n = 37slices/16 animals); IED filtering revealed Rs and FRs and several metrics were calculated and compared (amplitude, duration, relative onset, time lag, % overlap, peak frequency, peak power, FR/R). RESULTS: Longer IEDs and higher 1st Population Spike (PS) amplitude in Mg(2+)‐free ACSF (vs 4‐AP; P < .001, P < .001) correlated with longer duration and higher amplitude Rs (P < .0001, P = .001) and longer duration FRs (P < .001). In both media, Rs and FRs appeared before IED onset with Rs preceding FRs; R‐ and FR‐IED lag (P = .008, P = .01) as well as R‐FR lag (P = .04) were significantly longer in Mg(2+)‐free ACSF vs in 4‐AP. R peak frequency and power were higher in Mg(2+)‐free ACSF, while no such differences were observed in FRs. Inter‐model differences were mostly reflected in Rs, not FRs, suggesting that mechanisms unique to R generation are more active in Mg(2+)‐free ACSF vs in 4‐AP. FRs appeared to contribute equally to IEDs irrespective of generating mechanism. SIGNIFICANCE: Several of the metrics used, particularly those regarding the timing between HFOs and IEDs, appear to correlate with the synchronizing mechanism and we propose that they may be useful when investigating antiepileptic substance effects on neuronal network activity.