Real time mapping of rat midbrain neural circuitry using auditory evoked potentials

Epilepsy as a dynamical system, a most needed paradigm shift in epileptology

The idea of the epileptic brain being highly excitable and facilitated to synchronic activity has guided pharmacological treatment since the early twentieth century. Although tackling epilepsy's seizure-prone feature, by tonically modifying overall circuit excitability and/or connectivity, the last 50 years of drug development has not seen a substantial improvement in seizure suppression of refractory epilepsies. This review presents a new conceptual framework for epilepsy in which the temporal dynamics of the disease plays a more critical role in both its understanding and therapeutic strategies. The repetitive epileptiform pattern (characteristic during ictal activity) and other well-defined electrographic signatures (i.e., present during the interictal period) are discussed in terms of the sequential activation of the circuit motifs. Lessons learned from the physiological activation of neural circuitry are used to further corroborate the argument and explore the transition from proper function to a state of instability. Furthermore, the review explores how interfering in the temporally dependent abnormal connectivity between circuits may work as a therapeutic approach. We also review the use of probing stimulation to access network connectivity and evaluate its power to determine transitional states of the dynamical system as it moves towards regions of instability, especially when conventional electrographic monitoring is proven inefficient. Unorthodox cases, with little or no scalp electrographic correlate, in which ictogenic circuitry and/or seizure spread is temporally restricted to neurovegetative, cognitive, and motivational areas are shown as possible explanations for sudden death in epilepsy (SUDEP) and other psychiatric comorbidities. In short, this review presents a paradigm shift in the way that we address the disease and is aimed to encourage debate rather than narrow the rationale epilepsy is currently engaged in. This article is part of the Special Issue "NEWroscience 2018".

Morphofunctional alterations in the olivocochlear efferent system of the genetic audiogenic seizure-prone hamster GASH:Sal

The genetic audiogenic seizure hamster (GASH:Sal) is a model of a form of reflex epilepsy that is manifested as generalized tonic-clonic seizures induced by external acoustic stimulation. The morphofunctional alterations in the auditory system of the GASH:Sal that may contribute to seizure susceptibility have not been thoroughly determined. In this study, we analyzed the olivocochlear efferent system of the GASH:Sal from the organ of Corti, including outer and inner hair cells, to the olivocochlear neurons, including shell, lateral, and medial olivocochlear (LOC and MOC) neurons that innervate the cochlear receptor. To achieve this, we carried out a multi-technical approach that combined auditory hearing screenings, scanning electron microscopy, morphometric analysis of labeled LOC and MOC neurons after unilateral Fluoro-Gold injections into the cochlea, and 3D reconstruction of the lateral superior olive (LSO). Our results showed that the GASH:Sal exhibited higher auditory brain response (ABR) thresholds than their controls, as well as absence of distortion-product of otoacoustic emissions (DPOAEs) in a wide range of frequencies. The ABR and DPOAE results also showed differences between the left and right ears, indicating asymmetrical hearing alterations in the GASH:Sal. These alterations in the peripheral auditory activity correlated with morphological alterations. At the cochlear level, the scanning electron microscopy analysis showed marked distortions of the stereocilia from basal to apical cochlear turns in the GASH:Sal, which were not observed in the control hamsters. At the brainstem level, MOC, LOC, and shell neurons had reduced soma areas compared with control animals. This LOC neuron shrinkage contributed to reduction in the LSO volume of the GASH:Sal as shown in the 3D reconstruction analysis. Our study demonstrated that the morphofunctional alterations of the olivocochlear efferent system are innate components of the GASH:Sal, which might contribute to their susceptibility to audiogenic seizures. This article is part of a Special Issue entitled "Genetic and Reflex Epilepsies, Audiogenic Seizures and Strains: From Experimental Models to the Clinic".

Avoiding spectral leakage in objective detection of auditory steady-state evoked responses in the inferior colliculus of rat using coherence

Local field potentials (LFP) are bioelectric signals recorded from the brain that reflect neural activity in a high temporal resolution. Separating background activity from that evoked by specific somato-sensory input is a matter of great clinical relevance in neurology. The coherence function is a spectral coefficient that can be used as a detector of periodic responses in noisy environments. Auditory steady-state responses to amplitude-modulated tones generate periodic responses in neural networks that may be accessed by means of coherence between the stimulation signal and the LFP recorded from the auditory pathway. Such signal processing methodology was applied in this work to evaluate in vivo, anaesthetized Wistar rats, activation of neural networks due to single carrier sound stimulation frequencies, as well as to evaluate the effect of different modulating tones in the evoked responses. Our results show that an inappropriate choice of sound stimuli modulating frequencies can compromise coherence analysis, e.g. misleading conclusions due to mathematical artefact of signal processing. Two modulating frequency correction protocols were used: nearest integer and nearest prime number. The nearest prime number correction was successful in avoiding spectral leakage in the coherence analysis of steady-state auditory response, as predicted by Monte Carlo simulations.

An electrographic analysis of the synchronous discharge patterns of GEPR-9s generalized seizures

Previous results from our Laboratory have shown a synchronous discharge pattern (less than 1 ms apart) in monopolar recordings from electrodes placed in the cortex, inferior colliculus, and medulla of seizing GEPR-9s. However, the wave morphology of the ictal EEG is quite different for electrodes placed in different anatomical structures. These results lead us to hypothesize that wave morphology was indicative of neural circuitry involved in the GEPR9 seizure and that volume conduction was accounting for synchronous epileptiform EEG pattern. We decided to approach the problem by using a set of two experiments. Experiment 1: Perform a complete precollicular transection in GEPR-9s before inducing seizure in order to observe changes in EEG morphology after forebrain circuitry removal. Experiment 2: A novel methodological approach using a three-dimensional bipolar array enabled the reconstruction of a vector indicative of to which direction is voltage increasing. Such time-varying vector is indicative of the source direction of the high-amplitude epileptiform EEG signal. By placing such an array of electrodes, used to record the 3 bipolar EEGs, in the forebrain, midbrain, and hindbrain, we were able to use a simple intersection method to infer source localization. Our results suggest that the slow wave component of the GEPR9 epileptiform ictal EEG pattern is associated with a midbrain-forebrain circuit while the spike component is associated with a midbrain-hindbrain substrate. These results are supported by experiment 1 in which only the spike component of EEG remained after the precollicular transection.

Identification of cardiac rhythm features by mathematical analysis of vector fields

Automated techniques for locating cardiac arrhythmia features are limited, and cardiologists generally rely on isochronal maps to infer patterns in the cardiac activation sequence during an ablation procedure. Velocity vector mapping has been proposed as an alternative method to study cardiac activation in both clinical and research environments. In addition to the visual cues that vector maps can provide, vector fields can be analyzed using mathematical operators such as the divergence and curl. In the current study, conduction features were extracted from velocity vector fields computed from cardiac mapping data. The divergence was used to locate ectopic foci and wavefront collisions, and the curl to identify central obstacles in reentrant circuits. Both operators were applied to simulated rhythms created from a two-dimensional cellular automaton model, to measured data from an in situ experimental canine model, and to complex three-dimensional human cardiac mapping data sets. Analysis of simulated vector fields indicated that the divergence is useful in identifying ectopic foci, with a relatively small number of vectors and with errors of up to 30 degrees in the angle measurements. The curl was useful for identifying central obstacles in reentrant circuits, and the number of velocity vectors needed increased as the rhythm became more complex. The divergence was able to accurately identify canine in situ pacing sites, areas of breakthrough activation, and wavefront collisions. In data from human arrhythmias, the divergence reliably estimated origins of electrical activity and wavefront collisions, but the curl was less reliable at locating central obstacles in reentrant circuits, possibly due to the retrospective nature of data collection. The results indicate that the curl and divergence operators applied to velocity vector maps have the potential to add valuable information in cardiac mapping and can be used to supplement human pattern recognition.

A critical review on the participation of inferior colliculus in acoustic-motor and acoustic-limbic networks involved in the expression of acute and kindled audiogenic seizures

The main goal of this article is to review the key role that the inferior colliculus plays in the expression of acoustic-motor and acoustic-limbic integration involved, respectively, in acute and chronic audiogenic seizures. In order to put this in context, we will review the behavioral characterization of acute and chronic audiogenic seizures, neuroanatomical substrates, neurochemistry, neuropharmacology, electrophysiology, as well as the cellular and molecular mechanisms involved in their expression. Secondly, we will also correlate our results, collected from audiogenic seizures susceptible rats, before and after the genetic selection of our own audiogenic susceptible strain, and from those sensitized by lesions or drug microinjections, with those pertinent from the international literature. In brief, genetic or sensitized animals express acute audiogenic seizures as a wild running behavior preceding the onset of tonic-clonic seizures. The latter can have several presentations including opistotonus and fore- and hindlimb tonic hyperextensions, followed by clonic convulsions of fore- and hindlimbs. Chronic (kindled) audiogenic seizures change this behavioral expression, with similar patterns such as those present in temporal lobe epileptic seizures, intermingled with the original audiogenic seizure pattern, which is known to be dependent on brainstem networks.