that compete with cytokines to bind their receptors, such as IL-1 receptor antagonist protein (IL-1ra), and decoy receptors that bind cytokines and chemokines but are incapable of signalling, thereby acting as molecular traps to prevent such ligands from interacting with biologically active receptors (Mantovani et al., 2001; Dinarello, 2009). Proteins that inhibit cytokine-induced signal transduction (for example, suppressor of cytokine signalling proteins) or transcription (for example, Nurr1-CoREST or activity transcription factor 3), as well as an array of soluble mediators with anti-inflammatory activities (such as IL-10 and TGF-β), are produced concomitantly with proinflammatory molecules to resolve inflammation (Blobe et al., 2000; Khuu et al., 2007; Baker et al., 2009). For example, glucocorticoids, via activation of glucocorticoid receptors and, consequently, down-regulation of nuclear factor-κB (NFκB) and activator protein 1 activity, inhibit innate immune responses and, hence, act as an endogenous anti-inflammatory feedback system. Proinflammatory cytokines are powerful enhancers of glucocorticoid levels in adrenal glands via corticotropin-releasing hormone and adrenocorticotropic hormone (ACTH). Glucocorticoids also elicit immunosuppressive effects through inhibition of leukocyte extravasation from the vasculature, and through regulation of T helper cell differentiation (Sapolsky et al., 1987; Elenkov et al., 1999). The CNS can also negatively regulate the inflammatory response in a reflexive manner, using the efferent activity of the vagus nerve to inhibit release of proinflammatory molecules from tissue macrophages (Vezzani et al., 2000a, 2011; Tracey, 2002).
Do seizures cause inflammation?
In adult rats and mice, induction of recurrent short seizures or single prolonged seizures (status epilepticus; defined as a seizure lasting >30 min) by chemoconvulsants or electrical stimulation triggers rapid induction of inflammatory mediators in brain regions of seizure activity onset and propagation (Vezzani et al., 2000a, 2011; Crespel et al., 2002). Immunohistochemical studies on rodent brains after induction of status epilepticus demonstrated subsequent waves of inflammation during the epileptogenic process (that is, the process underlying the onset and chronic recurrence of spontaneous seizures after an initial precipitating event), involving various cell populations. Findings from these and other studies show that proinflammatory cytokines (IL-1β, TNF and IL-6) are first expressed in activated microglia and astrocytes, and cytokine receptor expression is up-regulated in microglia, astrocytes and neurons (Vezzani and Granata, 2005). These initial events are followed by the induction of cyclooxygenase-2 (COX-2) and, hence, prostaglandins, and up-regulation of components of the complement system in microglia, astrocytes and neurons (Yoshikawa et al., 2006; Aronica et al., 2007; Kulkarni and Dhir, 2009; Xu et al., 2009).
In addition to the molecules mentioned above, chemokines and their receptors are produced – predominantly in neurons and in activated astrocytes – days to weeks after status epilepticus (Wu et al., 2008; Xu et al., 2009; Fabene et al., 2010). An ensuing wave of inflammation is induced in brain endothelial cells by seizures, and includes up-regulation of IL-1β and its receptor IL-1R1, the complement system, and adhesion molecules (P-selectin, E-selectin, intercellular adhesion molecule 1 (ICAM) and vascular cell adhesion molecule 1) (Vezzani and Granata, 2005; Aronica et al., 2007; Fabene et al., 2008; Vezzani et al., 2011). The presumed cascade of events leading to this vascular inflammation involves seizure-induced activation of perivascular glia, which produce and release cytokines and prostaglandins. Importantly, no peripheral immune cells or blood-derived inflammatory molecules are required for vascular inflammation, as such events have been replicated in vitro in isolated guinea pig brain undergoing seizure activity (Vezzani and Granata, 2005; Vezzani et al., 2011).
The presence of inflammation originating from the brain might promote the recruitment of peripheral inflammatory cells. Indeed, chemokines expressed by neurons and glia and in the cerebrovasculature following seizures might direct blood leukocytes into the brain, which would be consistent with the reported emergence of granulocytes during epileptogenesis, and sparse T lymphocytes in chronic epileptic tissue from TLE models and humans (Ravizza et al., 2008). As in human epileptic brain specimens, brain tissue from rodents with experimental chronic TLE contains both activated astrocytes and microglia expressing inflammatory mediators (Crespel et al., 2002; Dube et al., 2007; Ravizza et al., 2008). Evidence for brain vessel inflammation associated with BBB breakdown is also prevalent (Fabene et al., 2008). A recent veterinary study evaluated the relationship of microglial activation to seizure-induced neuronal death in the cerebral cortex of Shetland sheepdogs with familial epilepsy (Sakurai et al., 2013). Cadavers of ten Shetland sheepdogs from the same family (six dogs with seizures and four dogs without seizures) and four age-matched unrelated Shetland sheepdogs were evaluated. Samples of brain tissues were collected after euthanasia and sectioned for H&E staining and immunohistochemical analysis. Evidence of seizure-induced neuronal death was detected exclusively in samples of cerebral cortical tissue from the dogs with familial epilepsy in which seizures had been observed. The seizure-induced neuronal death was restricted to tissues from the cingulate cortex and sulci surrounding the cerebral cortex. In almost the same locations as where seizure-induced neuronal death was identified, microvessels appeared longer and more tortuous and the number of microvessels was greater than in the dogs without seizures and control dogs. Immunohistochemical results for neurons and glial cells (astrocytes and microglia) were positive for vascular endothelial growth factor, and microglia positive for ionized calcium-binding adapter molecule 1 were activated (i.e. had swollen cell bodies and long processes) in almost all the same locations as where seizure-induced neuronal death was detected. Double-label immunofluorescence techniques revealed that the activated microglia had positive results for TNF-α, IL-6 and vascular endothelial growth factor receptor 1. These findings were not observed in the cerebrum of dogs without seizures, whether the dogs were from the same family as those with epilepsy or were unrelated to them. The suggested conclusion of this study was that microglial activation induced by vascular endothelial growth factor and associated pro-inflammatory cytokine production may accelerate seizure-induced neuronal death in dogs with epilepsy (Sakurai et al., 2013).
The findings discussed above show that brain inflammation induced by status epilepticus develops further during epileptogenesis and demonstrate that this phenomenon persists in chronic epileptic tissue, thereby supporting the idea that inflammation might be intrinsic to, and perhaps a biomarker of, the epileptogenic process (Dube et al., 2007).
Does inflammation cause seizures?
Although the functions of many inflammatory mediators remain unresolved, clear evidence exists for an active role for IL-1β, TNF, IL-6, prostaglandin E2 (PGE2) and the complement cascade in seizure generation and exacerbation (Xiong et al., 2003). Seizure activity leads to the production of inflammatory molecules that, in turn, affect seizure severity and recurrence, and this action takes place through mechanisms distinct from the transcriptional events traditionally activated during systemic inflammation. Cerebrospinal fluid studies in children and animal models have implicated the release of endogenous cytokines, especially IL-1β, in the generation of febrile seizures and, possibly, in the development of epilepsy after febrile seizures (Haspolat et al., 2002; Virta et al., 2002; Dube et al., 2005; Heida and Pittman, 2005; Vezzani et al., 2013).
A positive feedback pathway has been identified in rat models between seizure activity and the presence of inflammation (Vezzani et al., 2011). However, the role of inflammation in epilepsy in veterinary medicine has really only been described clinically in cats with hippocampal necrosis (Fatzer et al., 2000). Hippocampal lesions of 38 cats with seizures have been described and seemed to reflect different stages of disease consisting of acute neuronal degeneration to complete malacia, affecting mainly the layer of the large pyramidal cells but sometimes also the neurons of the dentate gyrus and the piriform lobe. The clinical, neuropathologic and epidemiologic findings suggest that the seizures in these cats were triggered by primary structural brain damage, perhaps resulting from excitotoxicity, but secondary inflammation cannot be
