Abstract
A Novel Focal Seizure Pattern Generated in Superficial Layers of the Olfactory Cortex
Uva L, Saccucci S, Chikhladze M, Tassi L, Gnatkovsky V, Milesi G, Morbin M, de Curtis M. J Neurosci 2017;37:3544–3554.
Seizure patterns identified in focal epilepsies caused by diverse etiologies are likely due to different pathogenic mechanisms. We describe here a novel, region-specific focal seizure pattern that mimics seizure activity observed in a subpopulation of patients submitted to presurgical monitoring with intracerebral electrodes. Distinctive seizure-like events (SLEs) are induced in the olfactory regions by acute treatment of both tangential brain slices and the isolated guinea pig brain with the potassium channel blocker 4-aminopyridine. Analysis of field potentials, intracellular activities, and extracellular potassium changes demonstrates that SLEs in the piriform cortex initiate in the superficial layer 1 lacking principal neurons with an activity-dependent increase of extracellular potassium. SLE progression (but not onset) does not require the participation of synaptic transmission and is mediated by diffusion of potassium to deep cortical layers. The novel seizure pattern here described is not observed in other cortical regions; it is proposed to rely on the peculiar organization of the superficial piriform cortex layers, which are characterized by unmyelinated axons and perisynaptic astroglial envelopes. This study reveals a sequence of ictogenic events in the olfactory cortex that were never described before in other cortical structures and supports the notion that altered potassium homeostasis and unmyelinated fibers may represent a potential vehicle for focal ictogenesis.
Commentary
For 40 years, in vitro brain slices have been used as a tool to help understand ictal brain activity (1). Seizure-like events (SLEs), which approximate various forms of human ictal EEG activity, can be evoked in vitro by blocking K+ channels, removing extracellular Mg2+, and other manipulations (2). Just as human ictal activity can vary with epilepsy syndrome, multiple types of SLEs can be evoked depending on the precipitating manipulation and the in vitro preparation used. The hippocampal and hippocampal/entorhinal cortical slices have been the workhorses of in vitro modeling of ictal-like activity. Using these preparations, ictal EEG patterns seen in human temporal lobe epilepsy, such as hypersynchronous and low-voltage fast activity, can be evoked in vitro (3). By studying in vitro SLEs, scientists have identified important cellular, molecular, and circuit level changes that occur during ictal activity. The role of K+ buffering, intracellular Cl− homeostasis, and purinergic signaling (just to name few) have been elucidated using in vitro models of ictal activity. Excitingly, more complex ex vivo models have been developed, including the thalamocortical slice (4), whole hippocampal preparation (5), and the in toto guinea pig brain (6). These preparations contain more intact neuronal connections, generate unique types of SLEs, and are thought to more closely approximate in vivo brain activity.
Strikingly, Uva and colleagues have been able to generate a novel type of SLE in the piriform cortex (PC) that closely resembles ictal activity seen in frontal lobe in human epilepsy, utilizing one of the most intact in vitro preparations, the in toto guinea pig brain. The in toto preparation itself is difficult to believe: The entire brain is carefully removed and perfused via the vascular system, allowing nearly complete connectivity in both local and long-range neuronal networks. This allows the study of ictal activity under circumstances much closer to the in vivo situation but with complete experimental access to the brain. Brief infusion of the K+ channel antagonist, 4-AP, causes SLEs in the in toto preparation that have been extensively studied in limbic circuits. In this study, the piriform cortex, a brain region associated with olfaction and known to generate ictal activity, was examined. The authors report a novel SLE event, which closely replicates human ictal activity seen commonly in the frontal lobe of patients who underwent intracranial recording for epilepsy resection surgery at the Claudio Munari Epilepsy Surgery Center. In contrast to classic temporal lobe seizures characterized by low-voltage fast activity progressing into irregular spiking and periodic bursting, this SLE has a unique signature: 1) Pre-SLE is fast activity (30 Hz) nested within periodic slow oscillations (PSO) at 0.1–0.5 Hz, 2) Phase 1 shows large amplitude extracellular upward potential, 3) Phase 2 is fast activity of small amplitude (30–60 Hz) superimposed on a plateau potential, and 4) Phase 3 has large-amplitude population spiking ending with a large amplitude negative slow deflection. Like other SLEs, postictal depression follows. This is an exciting advance for the field because no other in vitro model generates this clinically relevant type of SLE. Thus, this model offers an opportunity to understand the ictogenic mechanisms that contribute to this type of human frontal cortical epilepsy and may differ from limbic and neocortical temporal lobe seizures.
In fact, the authors have made significant strides forward in understanding the ictogenic mechanisms at play in this model. They nicely demonstrate that ictal activity arises from unconstrained synaptic activity in layer I (L1), which leads to rapid local accumulation of extracellular K+. This initiates a K+ wave that spreads to deep cortical layers, drives the activity of deep layer neurons, and generates the later phases of the SLE. Interestingly, the initiation, but not propagation, of the SLE is dependent on glutamatergic neurotransmission, underscoring the role of K+ in later stages of the SLE. One of the most interesting findings of this study is that the ictal onset occurs in L1, an area that is largely free of cell soma. In the PC, L1 consists of unmyelinated axons that make synaptic connections onto the dendrites of L2/3 pyramidal neurons and the astrocytic processes that surround them. By blocking K+ channels with 4-AP, action potentials are broadened, neuronal repolarization is inhibited, and synaptic transmission is amplified. This enhancement of neuronal activity begins a positive-feedback loop by driving further K+ efflux from postsynaptic glutamate receptors (7) and the subsequent neuronal activation. This is augmented by the lack of myelination, which normally acts to both physically restrict the spatial spread of extracellular K+ and disperse extracellular K+ through astrocytes (8). This enhanced synaptic and neuronal activity may also reduce glutamate uptake by astrocytes (9), further contributing to a positive-feedback mechanism. The authors pointed out that areas of reduced myelination occur in both TLE (10) and in focal cortical dysplasias (11), and that reduced potassium buffering is thought to be a significant contributor to ictogenesis and epileptogenesis (12).
This study has a number of interesting implications: First, focal K+ accumulation driven by synaptic activity is not a commonly considered mechanism of ictogenesis. Recent advances have emphasized the importance of emergent properties of neuronal circuits, compromised inhibition, and synaptic dysfunction in epilepsy. Perhaps something as simple as elevated extracellular K+ contributes more than we appreciate to generating seizures. Astrocytic, neuronal, and vascular systems all contribute to K+ homeostasis and have been implicated in seizure onset. Perhaps K+ lies at the confluence of these systems and acts as a common seizure-inducing trigger. Second, the findings force those of us accustomed to using the brain slice preparation to revisit the limits of our system. While the use of an in toto approach may not be realistic for all, there is no doubt that any conclusions drawn from slice experiments must be complemented by appropriate larger scale approaches. Multiple technological advances in multisite recording, in vivo imaging, and computer modeling (13) allow us to zoom out from the limitations of in vitro preparations to consider how our findings manifest in larger scale systems. Last, the work presented here challenges the idea that certain types of brain activity seen in humans cannot be replicated in model species. Clearly, there is no guarantee that the mechanisms proposed in this paper to drive this SLE subtype are the same that drive the corresponding ictal activity in people. Nevertheless, the fact that an extremely complex and unique human EEG pattern can be replicated in a model system provides motivation to continue our efforts to understand epilepsy using simpler model systems.