Canine and Feline Epilepsy. Luisa De Risio
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 ruled out in these cases.
Does inflammation cause cell loss?
Available studies suggest that seizure-related or injury-related inflammation might contribute to cell loss and synaptic reorganization, which are important mediators of the development of hyperexcitable circuits that lead to epilepsy after insults such as status epilepticus or TBI in the adult rodent brain (Bartfai and Schultzberg, 1993; Buckmaster and Dudek, 1997; Pitkanen and Sutula, 2002). Inflammation is induced rapidly following such insults, preceding neurodegeneration in lesional models of seizures (Rizzi et al., 2003; Ravizza and Vezzani, 2006). This finding is consistent with the idea that inflammation augments cell death, which is further supported by data from studies involving injection of inflammatory mediators together with excitotoxic stimuli (Allan et al., 2005). Activation of microglia and astrocytes and production of cytokines and PGE2 can occur in seizure models where cell loss is not detected in immature or adult rodents (Vezzani et al., 1999, 2000a; Rizzi et al., 2003; Kovacs et al., 2006; Dube et al., 2010). Such observations suggest that rather than being a consequence of cell loss, seizure-induced brain inflammation can contribute to cell death (Vezzani and Baram, 2007). Additional interactions between inflammation and cell death in the context of epilepsy have been observed. Brain injury, such as TBI, causes tissue inflammation that seems to contribute to both cell death and long-term hyperexcitability (Clausen et al., 2009; Longhi et al., 2009). In the context of CNS injury (for example, in chronic neurodegenerative diseases or acute stroke), inflammation can have a neuroprotective role (Liesz et al., 2009; Schwartz and Shechter, 2010). Indeed, whether micro-glia, macrophages and/or T cells are destructive or neuroprotective seems to depend on their activation status, which is orchestrated by the specific inflammatory environment (Rothwell, 1989; Schwartz and Shechter, 2010). This balance, together with the specific brain regions in which inflammation develops, might account for the relatively low incidence of seizures in other neurological disorders associated with brain inflammation (Vezzani et al., 2013).
Mechanistic insights
Several established and novel mechanisms could mediate the effects of inflammatory mediators on neuronal excitability and epilepsy. Some of these mechanisms could be involved in the precipitation and recurrence of seizures, while others are implicated in the development of epileptogenesis (Vezzani and Baram, 2007). These mechanisms constitute potential molecular targets for drug design, and are briefly summarized here. As discussed above, IL-1β and HMGB1 activate convergent signalling cascade through binding to IL-1R1 and TLR4, respectively (Akira et al., 2001; Perkins, 2007; Hoebe and Beutler, 2008). The downstream pathways activated by these ligands converge with the TNF pathways at the transcription factor NFκB, which regulates the synthesis of chemokines, cytokines, enzymes (for example, COX-2) and receptors (for example, TLRs, IL-1R1, and TNF p55 and p75 receptors) (Gilmore, 2006). This transcriptional pathway modulates the expression of genes involved in neurogenesis, cell death and survival, and in synaptic molecular reorganization and plasticity (processes that occur concomitantly with epileptogenesis in experimental models) (Buckmaster and Dudek, 1997; Pitkanen and Lukasiuk, 2009).
Immune and anti-inflammatory therapies
If immune mechanisms and inflammation do indeed have a role in the generation of seizures, immune-modulating and anti-inflammatory therapies