Abstract
Spatiotemporal Dynamics of Optogenetically-Induced and Spontaneous Seizure Transitions in Primary Generalized Epilepsy.
Wagner FB, Truccolo W, Wang J, Nurmikko AV. J Neurophysiol. 2015;113(7):2321–41.
Transitions into primary generalized epileptic seizures occur abruptly and synchronously across the brain. Their potential triggers remain unknown. We used optogenetics to causally test the hypothesis that rhythmic population bursting of excitatory neurons in a local neocortical region can rapidly trigger absence seizures. Most previous studies have been purely correlational, and it remains unclear whether epileptiform events induced by rhythmic stimulation (e.g., sensory/electrical) mimic actual spontaneous seizures, especially regarding their spatiotemporal dynamics. In this study, we used a novel combination of intracortical optogenetic stimulation and microelectrode array recordings in freely moving WAG/Rij rats, a model of absence epilepsy with a cortical focus in the somatosensory cortex (SI). We report three main findings: 1) Brief rhythmic bursting, evoked by optical stimulation of neocortical excitatory neurons at frequencies around 10 Hz, induced seizures consisting of self-sustained spike-wave discharges (SWDs) for about 10% of stimulation trials. The probability of inducing seizures was frequency-dependent, reaching a maximum at 10 Hz. 2) Local field potential power before stimulation and response amplitudes during stimulation both predicted seizure induction, demonstrating a modulatory effect of brain states and neural excitation levels. 3) Evoked responses during stimulation propagated as cortical waves, likely reaching the cortical focus, which in turn generated self-sustained SWDs after stimulation was terminated. Importantly, SWDs during induced and spontaneous seizures propagated with the same spatiotemporal dynamics. Our findings demonstrate that local rhythmic bursting of excitatory neurons in neocortex at particular frequencies, under susceptible ongoing brain states, is sufficient to trigger primary generalized seizures with stereotypical spatiotemporal dynamics.
Commentary
The mechanisms of ictogenesis and seizure propagation are still largely unexplored. Despite intense interest, these crucial aspects of seizure dynamics have been beyond the capabilities of our technology. Recording such activity requires a combination of high spatial and temporal resolution, and it is only in the past few years that techniques are becoming available to begin approaching these questions. Optical imaging has high spatial resolution but is still not fast enough to follow all seizure dynamics; electrodes are very fast but have limited spatial sampling. However, both are continually improving. Recent developments in optical imaging can now characterize the dynamics of individual spikes.1, 2 Microelectrode arrays (MEA) on rigid probes3, 4 and flexible thin film arrays5 are being used to monitor cortical activity during seizures. As these technologies converge on the combination of high spatial and temporal resolution, we can begin answering some of these longstanding questions about seizure dynamics. The article by Wagner and colleagues is a major step forward in the development of methods to map and compare the initiation and propagation of different seizures.
Wagner et al. combined optical and electrode technologies to investigate how primary generalized seizures begin. Generalized seizures appear to occur suddenly and simultaneously over the majority of the brain and can be triggered by physiological inputs such as photic stimulation or hyperventilation. Yet there is mounting evidence that these “generalized” seizures may actually initiate in focal regions of cortex.6 In the current study, the authors sought to determine whether they could trigger absence seizures in epileptic rats by activating a small, unilateral cortical focus. Researchers have known of this effect for decades with electrical stimulation,7 but in the current work, the authors were able to target neurons selectively with optogenetics, assuring that the trigger was solely affecting excitatory neurons. Using WAG/Rij rats, they injected C1V1 opsins (rAAV5/CaMKIIα) into a few millimeters of layers V/VI in the left S1 region. They then implanted two multichannel electrode arrays and a light source to stimulate the C1V1 channels. To evoke seizures, they presented 10 Hz light pulses onto the cortical surface. They were able to induce spike-wave seizures reliably in response to the light stimuli, showing that focal stimulation of excitatory neurons induced generalized seizures.
But the most significant contribution of this paper was not simply demonstrating this physiological effect. After seeing that their intervention produced “absence seizures,” the authors sought to answer a much more sophisticated question: How do we know if the induced seizures are the same as the spontaneous ones? This question is important at all levels of epilepsy research. In both human and animal epilepsy, great care is taken to categorize different seizure types and determine if there are any events that are not stereotypical. We assume that seizures with a similar semiology and EEG pattern arise from the same process and involve the same tissue. Such conclusions are qualitative and difficult to prove when categorizing spontaneous seizures but much more problematic for induced seizures. An electrical stimulus, or any other experimental intervention, potentially excites neural pathways differently and produces seizures that might be distinct from spontaneous ones. This phenomenon is well known in human cortical mapping, where stimuli can produce seizures or afterdischarges that are not necessarily related to the patient's spontaneous events. This observation has led some experts to conclude that induced and spontaneous seizures are inherently different. In this paper, the authors hypothesized that their induced seizures were “the same” as the spontaneous ones, a statement that required significant effort to prove.
Wagner and colleagues designed their optogenetic stimulus to simulate a natural seizure pathway that putatively initiates absence seizures. While it may seem an oxymoron that a spontaneous, generalized event can be triggered on demand by a focal stimulus, the concept has strong experimental validation. Recent work suggests that any brain is capable of producing seizures, given the appropriate conditions. Jirsa and colleagues recently showed that the transition to focal seizures is governed by the balance between an inherent seizure threshold and the noisy inputs to the system, which can push the system past threshold.8 In that work, many potential stimuli could trigger a focal seizure, which would then have similar dynamics regardless of the original cause. The same theoretical concept applies here to absence seizures—any “natural” stimulus pathway that pushes the system past the seizure threshold should produce the same type of seizure. Using this idea, Wagner and colleagues wanted to prove that they had identified an endogenous seizure pathway and were controlling the onset of naturally occurring seizures.
To compare the different seizures, the authors chose to quantify the seizure dynamics with high spatial and temporal resolution. They used MEA recordings to create a two-dimensional movie of each seizure, showing how electrode voltages changed on the scale of milliseconds and tens of micrometers. This allowed them to map specific locations where the seizures began and determine how the seizure spreads to neighboring electrodes. Just as in previous work with MEA,3–5 they found complex dynamics across the multiple electrodes that would have been unseen on a typical, larger EEG electrode. They then quantified the time-course and direction of this propagation, creating a detailed description of the spatiotemporal dynamics of each seizure discharge, and used rigorous statistics to compare the induced and spontaneous events. They recorded hundreds of events and found very consistent results in each of their animals. Their final results are convincing: Even with an untrained eye, it is easy to see that each animal has a distinct propagation pattern that is essentially identical in that animal's induced and spontaneous seizures.
New technology is now poised to explore the intricate dynamics of seizures. But higher resolution comes with a price—it becomes more and more necessary to develop quantitative methods to map brain activity. In this paper, the authors have developed a new tool to map seizure initiation and propagation that is elegant both statistically and visually. This tool allows the investigators to conclude that they have discovered and modulated an endogenous seizure trigger, which has intriguing implications for future epilepsy research.