**3.1 Epileptogenicity**

Epileptogenicity is the development and extension of tissue capable of generating spontaneous seizures, resulting in the development of an epileptic disorder or progression after the disorder is established [81]. In general, there are three phases to acquire epileptogenicity. First, brain-damaging insult (stroke, traumatic brain injury and central nervous system infections etc.) occurs (acute phase). Second, brain acquires epileptogenicity during a certain period of time (latent period), and third, as a result spontaneous recurrent seizures occur (chronic phase). In order to elucidate the mechanism of acquiring epileptogenicity, it is very important to study what occurs in the brain during latent period.

Risk factors, especially relevant to clinical practice, for acquiring epileptogenicity and subsequent development of PSE are summarized in **Table 3**.

PSE: post-stroke epilepsy, PET: positron emission tomography.

Animal experiment of stroke model is very helpful to elucidate epileptogenicity. Several studies directly evaluate neuronal and glial activities after stroke by using electrophysiological and histological measures. Early seizures evoked by traumatic brain injury and stroke can be suppressed by short-term prophylactic administration of antiepileptic drugs but it doesn't alter the incidence of PSE [83]. Therefore, targeting only neurons may be insufficient to prevent epileptogenicity; the glial involvement in the process of epileptogenesis after experimental stroke should be reviewed. Changes in neuro-glial syncytium during the course of acquiring epileptogenesis are shown in **Figure 1**.

Astrocytes form extensive gap junctions composed of connexin (Cx) 43 with other astrocytes and play a central role of neuron–glia syncytium [84]. Astrocyte regulates neural activities by removing excessive extracellular potassium at synapses and transports them into regions of low potassium concentration via gap junction [85]. Cx 43 also forms hemichannels in the astrocyte. Hemichannels allow the exchange of ions and molecules between the cytoplasm and the


**Table 3.**

*Risk factors for acquiring epileptogenicity after stroke [7, 10, 25, 31, 32, 82].*

*Cerebrovascular Disease; A Leading Cause of Epilepsy DOI: http://dx.doi.org/10.5772/intechopen.95119*

#### **Figure 1.**

*Neuro-glial syncytium in the course of acquiring epileptogenesis. (A) Physiological state. A neuron generates physi0logical action potentials in neuro-glial syncytium. As a result of neural firing, extracellular potassium concentration is elevated. Kir 4.1 channels in the astrocyte mediate spatial potassium burring and regulates neural activities by transporting them into regions of low potassium concentration such as blood vessels. (B) Acute phase. In the acute phase of ischemic stroke, ischemic changes of neuron and glia gradually appear in the ischemic lesion. Neurons and astrocyte/microglia cells in the peri-ischemic lesion are activated. Activated microglia called M1 secretes cytokines (e.g. TNF-*α*, IL-1*β*), which in turn inhibit gap junctional communication and increase hemichannel activity in astrocytes. The release of molecules, such as ATP and glutamate, damages adjacent neurons and glia cells. Inhibition of gap junction leads to dysfunction of spatial potassium buffering, which further provokes neural firing. Electrophysiologically, neuronal activities lead to increase of spatial potassium, and DC shifts appear as a result of exceeding the capacity of buffering. (C) Latent period. Necrotic region in the ischemic lesion is cleaned by microglia and gliosis occurs in per-ischemic lesion. Inflammation, mainly caused by M1 phenotype, continues in this period and further damages neurons and astrocytes. (D) Chronic phase. In the chronic phase of ischemic stroke, inflammation subsides, and epileptogenicity is acquired. Ischemic lesion is replaced by fibrillary astrocyte and peri-ischemic lesion is occupied by gliosis. Dysfunction of neuro-glial syncytium reaches the pathological state which generates spontaneous epileptic activities. Electrophysiologically, DC shifts precede neural firing. N: neuron, A: astrocyte, M: microglia, TNF-*α*: tumor necrosis factor-alpha, IL-1*β*: interleukin 1 beta, ATP: adenosine triphosphate, DC: direct current, Cx43: connexin 43.*

extracellular medium, which physiologically regulate neuronal activity as well as synaptic strength and plasticity [86] and also pathologically be activated by inflammation [87].

#### **3.2 Acute phase**

Epileptiform activities after middle cerebral artery (MCA) occlusion were first recorded using surface conventional EEG in the rat experiment by Hartings et al. [88]. 55–90% of animals had epileptiform activities 33–50 minutes after stroke which exacerbated brain injury [89–91]. High frequency oscillations (HFOs) are involved with epileptogenic region in intractable epileptic patients (e.g. focal

cortical dysplasia) and rat pilocarpine model of temporal lobe epilepsy [92]. In rat stroke model, HFOs were also observed 5–15 seconds before run of theta activities consisted of sharp positive spikes followed by longer negative waves and terminated at the onset of the discharge after ischemia [93]. Direct current (DC) shifts are associated with a steady increase of the extracellular potassium concentration, which matches to the intracellular voltage variations of glial cells [94]. In rat MCA occlusion model, a highly significant linear correlation is reported between the number of depolarization and the infarct size at peri-infarct region, and DC shifts are also recorded [95], which may indicate exceeding of potassium concentration to the buffering capacity of astrocyte.

Histological changes immediately after stroke are reported by Ramírez-Sánchez et al. [96]. When rats were subjected to 90 minutes MCA occlusion followed by 23 hours of reperfusion, neuronal cells in the peri-infart cortex, cornu ammonis (CA) 1, and dentate gyrus (DG) areas were decreased, and widespread reactive astrogliosis in both of the cortex and the hippocampus (CA1, CA3, and DG areas) was observed 24 hours after ischemia.

Therefore, in acute phase of epileptogenesis, stroke immediately damages neurons and glia cells, and provokes neuronal epileptic activity. Furthermore, as a result of destruction of cells and excessive neural firing, the extracellular potassium concentration increases beyond glial potassium buffering capacity, which may lead to a vicious circle of further neural firing.

#### **3.3 Latent period and chronic phase**

Chronic phase of the aged rat post-stroke brain is reported by Titova et al. [97]. They evaluated ischemic lesions at 28 days induced by 5o minutes right MCA occlusion in aged rat (18 month-old). In ischemic lesion, extensive glial scar and apoptotic neurons were found and phagocytic macrophages/microglia cells were seen in the peri-lesional rim. The brain irradiation possibly affects normal post-stroke microglia signaling and prevents following activation of inflammatory cascade mechanisms [98]. When proton irradiation was performed at the heads of aged rat ten days prior to right MCA occlusion, chronic phagocytosis and T-lymphocyte infiltration in the brain were reduced, and formation of glio-vascular complexes, neuronal viability, neovascularization were improved in the peri-lesional zone, and neurological severity scoring were improved [97]. These data clearly demonstrated that, in addition to direct damage to brain by stroke itself, subsequent inflammation also damages neuron, astrocyte, vessel and neural function.

In the central nervous system, microglia is a major player in the brain inflammation. Stroke activates microglia which is called M1 phenotype, which secretes inflammatory cytokine like interleukin (IL) and tumor necrosis factor (TNF)-α [99]. Astrocyte also secretes inflammatory cytokines (e.g. TNF-α, IL-1β) via Cx43-hemichannel in the MCA occlusion model [100], whereas Gap19, a selective Cx43-hemichannel inhibitor, exhibits neuroprotective effects on cerebral ischemia/ reperfusion via suppression of the expression of Cx43 and toll-like receptor 4 pathway-relevant proteins, and prevention of the overexpression of TNF-α and IL-1β in astrocyte. An obvious improvement in neurological scores and infarct volume reduction were observed in Gap19-treated mice after brain ischemia induced by MCA occlusion [100].

Inflammatory cytokines, specifically IL-1β and TNF-α, are involved in inhibition of gap junctional communication and increase of hemichannel activity in astrocytes [87]. Inhibition of gap junction communication impedes potassium buffering which promotes neural firing. Increased hemichannel activity allows the release of molecules such as adenosine triphosphate, glutamate, nicotinamide adenine

*Cerebrovascular Disease; A Leading Cause of Epilepsy DOI: http://dx.doi.org/10.5772/intechopen.95119*

dinucleotide, glutathione, and prostaglangin E2 [101–105]. These molecules are toxic to adjacent cells and finally lead to neuronal and glial death [106, 107]. These insults to the central nervous system tissue trigger a range of molecular, morphological, and functional changes of astrocytes called reactive gliosis [108], which is one of the most important pathogenic steps of spontaneous seizure [92].

As for electrophysiological study of PSE model, few data are available because only 10–20% of post-stroke rats develop spontaneous seizure [109]. Brain damage induced by brain injury, axotomy, or toxic substance in addition to stroke also activates microglia [99], therefore, electrophysiological pathology induced by inflammation of other causes is probably plausible as that is induced by stroke. The mechanism of epileptogenesis after status epilepticus model is well studied. Pilocarpine, a muscarinic acetylcholine agonist, induces status epilepticus, and after a certain latent period surviving rats acquire epileptogenesis [110]. This rat model also shows HFOs and DC shifts during seizure at acute phase [92]. Gliosis occurs in 8–12 weeks after pilocarpine injection [111], and spatial potassium buffering function at hippocampus is impaired [112]. EEG recording during epileptic seizures at chronic phase of this model rat showed DC shifts preceding HFOs and conventional ictal EEG patterns, which may be the result of dysfunction of astrocyte extracellular potassium buffering [92]. Minocycline is a second-generation tetracycline and has potent anti-inflammatory effects independent of its antimicrobial action. Minocycline attenuates spontaneous recurrent seizures following pilocarpine-induced status epilepticus, and inhibits the status epilepticus-induced microglial activation and overproduction of IL-1β and TNF-α in the hippocampal CA1 and the adjacent cortex, without affecting astrocyte activation. In addition, minocycline prevents the status epilepticus-induced neuronal loss [113].
