Region-specific Effects of Early-life Status Epilepticus on the Adult Hippocampal CA3 - Medial Entorhinal Cortex Circuitry In vitro: Focus on Interictal Spikes and Concurrent High-frequency Oscillations

Immature Status Epilepticus Alters the Temporal Relationship between Hippocampal Interictal Epileptiform Discharges and High-frequency Oscillations

The aim was to investigate the long-term effects of a single episode of immature Status Epilepticus (SE) on the excitability of the septal and temporal hippocampus in vitro, by studying the relationship between interictal-like epileptiform discharges (IEDs) and high-frequency oscillations (HFOs; Ripples, Rs and Fast Ripples, FRs). A pentylenetetrazol-induced Status Epilepticus-(SE)-like generalized seizure was induced at postnatal day 20 in 22 male and female juvenile rats, sacrificed >40 days later to prepare hippocampal slices. Spontaneous IEDs induced by Mg2+-free ACSF were recorded from the CA3 area of temporal (T) or septal (S) slices. Recordings were band-pass filtered off-line revealing Rs and FRs and a series of measurements were conducted, with mean values compared with those obtained from age-matched controls (CTRs). In CTR S (vs T) slices, we recorded longer R & FR durations, a longer HFO-IED temporal overlap, higher FR peak power and more frequent FR initiation preceding IEDs (% events). Post-SE, in T slices all types of events duration (IED, R, FR) and the time lag between their onsets (R-IED, FR-IED, R-FR) increased, while FR/R peak power decreased; in S slices, the IED 1st population spike and the FR amplitudes, the R and FR peak power and the (percent) events where Rs or FRs preceded IEDs all decreased. The CA3 IED-HFO relationship offers insights to the septal-to-temporal synchronization patterns; its post-juvenile-SE changes indicate permanent modifications in the septotemporal excitability gradient. Moreover, these findings are in line to region-specific regulation of various currents post-SE, as reported in literature.

High-frequency oscillations (250-500 Hz) in animal models of Alzheimer's disease and two animal models of epilepsy

OBJECTIVE

To test the hypothesis that high-frequency oscillations (HFOs) between 250 and 500 Hz occur in mouse models of Alzheimer's disease (AD) and thus are not unique to epilepsy.

METHODS

Experiments were conducted in three mouse models of AD: Tg2576 mice that simulate a form of familial AD, presenilin 2 knock-out (PS2KO) mice, and the Ts65Dn model of Down's syndrome. We recorded HFOs using wideband (0.1-500 Hz, 2 kHz) intra-hippocampal and cortical surface electroencephalography (EEG) at 1 month until 24 months of age during wakefulness, slow wave sleep (SWS), and rapid eye movement (REM) sleep. In addition, interictal spikes (IISs) and seizures were analyzed for the possible presence of HFOs. Comparisons were made to the intra-hippocampal kainic acid and pilocarpine models of epilepsy.

RESULTS

We describe for the first time that hippocampal and cortical HFOs are a new EEG abnormality in AD mouse models. HFOs occurred in all transgenic mice but no controls. They were also detectable as early as 1 month of age and prior to amyloid beta plaque neuropathology. HFOs were most frequent during SWS (vs REM sleep or wakefulness). Notably, HFOs in the AD and epilepsy models were indistinguishable in both spectral frequency and duration. HFOs also occurred during IISs and seizures in the AD models, although with altered spectral properties compared to isolated HFOs.

SIGNIFICANCE

Our data demonstrate that HFOs, an epilepsy biomarker with high translational value, are not unique to epilepsy and thus not disease specific. Our findings also strengthen the idea of hyperexcitability in AD and its significant overlap with epilepsy. HFOs in AD mouse models may serve as an EEG biomarker, which is detectable from the scalp and thus amenable to noninvasive detection in people at risk for AD.

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 Mg²⁺ -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 Mg²⁺ -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 Mg²⁺ -free ACSF vs in 4-AP. R peak frequency and power were higher in Mg²⁺ -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 Mg²⁺ -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.

Inhibition of 2-Oxoglutarate Dehydrogenase as a Chemical Model of Acute Hypobaric Hypoxia

Both hypoxia and inhibition of 2-oxoglutarate dehydrogenase complex (OGDHC) are known to change cellular amino acid pools, but the quantitative comparison of the metabolic and physiological outcomes has not been done. We hypothesize that OGDHC inhibition models metabolic changes caused by hypoxia, as both perturb the respiratory chain function, limiting either the NADH (OGDHC inhibition) or oxygen (hypoxia) supply. In the current study, we quantify the changes in the amino acid metabolism after OGDHC inhibition in the highly sensitive to hypoxia cerebellum and compare them to the earlier characterized changes after acute hypobaric hypoxia. In addition, the associated physiological effects are characterized and compared. A specific OGDHC inhibitor succinyl phosphonate (SP) is shown to act similar to hypoxia, increasing levels of many amino acids in the cerebellum of non-pregnant rats, without affecting those in the pregnant rats. Compared with hypoxia, stronger effects of SP in non-pregnant rats are observed on the levels of cerebellar amino acids, electrocardiography (ECG), and freezing time. In pregnant rats, hypoxia affects ECG and behavior more than SP, although none of the stressors significantly change the levels of cerebellar amino acids. The biochemical differences underlying the different physiological actions of SP and hypoxia are revealed by correlation analysis of the studied parameters. The negative correlations of cerebellar amino acids with OGDHC and/or tryptophan, shown to arise after the action of SP and hypoxia, discriminate the overall metabolic action of the stressors. More negative correlations are induced in the non-pregnant rats by hypoxia, and in the pregnant rats by SP. Thus, our findings indicate that the OGDHC inhibition mimics the action of acute hypobaric hypoxia on the cerebellar amino acid levels, but a better prediction of the physiological outcomes requires assessment of integral network changes, such as increases in the negative correlations among the amino acids, OGDHC, and/or tryptophan.