Antagonism of peripheral inflammation reduces the severity of status epilepticus

Abstract

Status epilepticus (SE) is one of the most serious manifestations of epilepsy. Systemic inflammation and damage of blood-brain barrier (BBB) are etiologic cofactors in the pathogenesis of pilocarpine SE while acute osmotic disruption of the BBB is sufficient to elicit seizures. Whether an inflammatory-vascular-BBB mechanism could apply to the lithium–pilocarpine model is unknown. LiCl facilitated seizures induced by low-dose pilocarpine by activation of circulating T-lymphocytes and mononuclear cells. Serum IL-1β levels increased and BBB damage occurred concurrently to increased theta EEG activity. These events occurred prior to SE induced by cholinergic exposure. SE was elicited by lithium and pilocarpine irrespective of their sequence of administration supporting a common pathogenetic mechanism. Since IL-1β is an etiologic trigger for BBB breakdown and its serum elevation occurs before onset of SE early after LiCl and pilocarpine injections, we tested the hypothesis that intravenous administration of IL-1 receptor antagonists (IL-1ra) may prevent pilocarpine-induced seizures. Animals pre-treated with IL-1ra exhibited significant reduction of SE onset and of BBB damage. Our data support the concept of targeting systemic inflammation and BBB for the prevention of status epilepticus.

Introduction

In spite of the advancements in therapeutic interventions for seizure disorders, 102,000 to 152,000 individuals are affected each year in the United States (with 126,000 to 195,000 status epilepticus episodes), and over 22,000 to 42,000 deaths per year occur in patients with status epilepticus (SE) (Shorvon et al., 2008). There is increasing evidence that systemic inflammation may have significant effects of normal brain function and may in particular lead to epileptic seizures and SE (Vezzani and Granata, 2005, Marchi et al., 2007b, Marchi et al., 2007a, Uva et al., 2008, Galic et al., 2008). For example, fever is the most common correlate of immune activation against pathogens, and hyperthermia has been for a long time recognized as a powerful trigger for seizures. More recently, clinical (Elkassabany et al., 2008, Marchi et al., 2007a, Korn et al., 2005) and translational (Korn et al., 2005, Tomkins et al., 2007, Ivens et al., 2007) studies have shown that the gatekeeper at the brain-systemic circulation interface, the blood-brain barrier, may be a crucial etiological player in epilepsy (Oby and Janigro, 2006). These concepts have been expanded to a popular model of limbic epilepsy, namely the pilocarpine model (Marchi et al., 2007b, Uva et al., 2008). Pilocarpine caused acute peripheral pro-inflammatory changes leading to blood-brain barrier (BBB) leakage prior to SE (Marchi et al., 2007b). These studies also demonstrated that it is unlikely that cholinergic drugs such as pilocarpine may act directly on neurons since pilocarpine was relatively impermeant across the BBB. Consistent with this hypothesis is the fact than when directly applied to the brain, pilocarpine failed to produce electrographic seizures (Marchi et al., 2007b, Uva et al., 2008). While converging evidence points to the BBB failure as a trigger for limbic seizures and cholinergic SE, it is not clear whether these transient episodes of BBB dysfunction are truly necessary for seizure development. This is particularly intriguing in the muscarinic models of SE where pilocarpine exerts two independent effects, one aimed at BBB integrity (Marchi et al., 2007b) and one promoting gamma oscillations in the hippocampus and related structures (Uva et al., 2008). Furthermore, it is not currently known if induction of BBB leakage by non-cholinergic means prior to pilocarpine exposure is equally effective in facilitating the latter epileptogenic actions of the drug.

To test the effect of a pro-inflammatory stimulus prior to administration of seizure promoting agents we used the lithium–pilocarpine model, where pretreatment with the psychoactive ion allows to reduce the minimal epileptogenic dosage of pilocarpine to 1/10th of the dosage required in absence of pretreatment (Cavalheiro et al., 2006). Lithium salt compounds are widely employed in the treatment of manic-depressive psychosis. Patients receiving lithium therapy often demonstrate an unexplained increase in white blood cell counts, with granulocytosis being the most common finding (Carmen et al., 1993). Pronounced EEG changes were also reported (Helmchen and Kanowski, 1971).

In the presented study we tested the hypothesis that increased peripheral immunological mediators and blood-brain barrier disruption are mechanism of lithium's pro-epileptogenic effects. We also tested the efficacy of anti-inflammatory molecules in reducing the onset SE induced by cholinergic activation.