Ripples e fast ripples na epileptogênese: caracterização das oscilações de alta frequência após o estado epiléptico

Abstract

High-frequency oscillations (HFOs) are spontaneous (80-500 Hz), transient and fast (20-100 ms) oscillations observed in cortical structures in mammals. Recorded mainly during slow-wave sleep (SWS) and quiet waking, physiological HFOs (ripples, between 80 and 200 Hz) participate in sensorial perception and memory consolidation. In parallel, pathological HFOs (above 200 Hz) occur in brain regions involved in the seizure initiation in individuals with epilepsy. In this work, we used electrophysiological recordings and an animal model of status epilepticus (SE) to study the expression of HFOs associated with epileptogenesis in mice. Animals were implanted with deep electrodes, bilaterally, in the hippocampus and retrosplenial cortex. We also implanted a guide-cannula in the right dorsal hippocampus for local administration of pilocarpine. After the recording of the sleep-wake cycle (baseline; minimum of 2 days, 5 h/day), animals received a single intrahippocampal dose of pilocarpine (560 µg/site in 800 nL) for the induction of status epilepticus (SE) during video-electrophysiology recordings. After SE, animals were recorded in four moments in time: one day after SE (SE+1), two days after SE (SE+2), between 7-14 days after SE (SE+7) and between 15 and 30 days (SE+30). The recordings were processed to identify ripples and pathological HFOs in windows of slow wave sleep and during the two initial hours of the SE, respectively. We identified 1,689 ripples in 6 animals. We observed that ripple rate of occurrence decreased after SE (F[4,20]=4.34, p=0.01; ANOVA), as well as oscillation frequency of ripples (F[4,20]=5.39, p=0.003; ANOVA). We did not identify statistical differences regarding power and duration of ripples after the SE. Interestingly, we observed a significant correlation between the reduction in frequency of ripples and the severity of the SE (SE+2, R= -0.82, p= 0.05; Spearman), as well as between the rate of ripples and the severity of the SE (SE+2, R= -0.94, p= 0.01; Spearman). As to the pathological HFOs, we did not identify a single oscillation above 200 Hz in the baseline period. Curiously, pathological HFOs were detected in the first seconds of the first seizure of the SE in all recorded animals (N=6). Similar events were detected on day SE+1 in two animals. Most of the pathological HFOs occurred coupled with high-amplitude ictal spikes. The pathological HFOs showed higher oscillation frequency (t(38)=-8.8, p< 0.001), power (t(38)=-15.5, p< 0.001) and duration (t(38)=-4.6, p< 0.001, testes t de Welch), when compared to ripples. As far as we know, this work is the first study to extensively describe the evolution of the electrophysiological profile of ripples during epileptogenesis. Our results show that ripple alterations are partially explained by SE severity. We also show that pathological HFOs occur in the first seconds after the beginning of SE, suggesting that these oscillations do not need structural reorganization for its expression. Our hypothesis is that pathological HFOs result from a sustained depolarization of a neuronal population which recurrent (feedback) inhibition is functional, contributing with synchronized firing of action potentials in a neuronal population. Taken together, we observed that ripples and pathological HFOs display distinct dynamics, during and after the SE, that can help to comprehend the temporal evolution of epilepsy.