The COX-2 inhibitor parecoxib is neuroprotective but not antiepileptogenic in the pilocarpine model of temporal lobe epilepsy

Abstract

The enzyme cyclooxygenase-2 (COX-2), which catalyzes the production of pro-inflammatory prostaglandins, is induced in the brain after various insults, thus contributing to brain inflammatory processes involved in the long-term consequences of such insults. Mounting evidence supports that inflammation may contribute to epileptogenesis and neuronal injury developing after brain insults. Anti-inflammatory treatments, such as selective COX-2 inhibitors, may thus constitute a novel approach for anti-epileptogenesis or disease-modification after brain injuries such as head trauma, cerebral ischemia or status epilepticus (SE). However, recent rat experiments with prophylactic administration of two different COX-2 inhibitors after SE resulted in conflicting results. In the present study, we evaluated whether treatment with parecoxib, a pro-drug of the highly potent and selective COX-2 inhibitor valdecoxib, alters the long-term consequences of a pilocarpine-induced SE in rats. Parecoxib was administered twice daily at 10 mg/kg for 18 days following SE. Five weeks after termination of treatment, spontaneous recurrent seizures were recorded by continuous video/EEG monitoring. Prophylactic treatment with parecoxib prevented the SE-induced increase in prostaglandin E₂ and reduced neuronal damage in the hippocampus and piriform cortex. However, the incidence, frequency or duration of spontaneous seizures developing after SE or the behavioral and cognitive alterations associated with epilepsy were not affected by parecoxib. Only the severity of spontaneous seizures was reduced, indicating a disease-modifying effect. These results substantiate that COX-2 contributes to neuronal injury developing after SE, but inhibition of COX-2 is no effective means to modify epileptogenesis.

Introduction

There is accumulating experimental and clinical evidence that activation of inflammatory pathways is a common factor contributing to the pathogenesis of seizures in various forms of epilepsy of different etiologies (Vezzani and Granata, 2005, Vezzani and Baram, 2007, Choi and Koh, 2008, Vezzani et al., 2008). Several lines of evidence suggest a key role of inflammation in the development of epilepsy (a process termed epileptogenesis) and the initiation of seizures (Vezzani and Granata, 2005, Takemiya et al., 2007, Vezzani and Baram, 2007, Choi and Koh, 2008, Vezzani et al., 2008, Rijkers et al., 2009): (a) Common risk factors for symptomatic epilepsy (such as CNS tumors, head injury, stroke, complex febrile seizures, status epilepticus [SE]) are accompanied by variable levels of inflammation. (b) Infections (systemic or CNS) are well-recognized etiologic triggers for seizures. (c) Inflammation in epilepsy is not restricted to different types of encephalitis associated with seizures but occurs also in common types of human epilepsy such as temporal lobe epilepsy (TLE) and malformations of cortical development. (d) Both seizures and SE induce pro-inflammatory cytokines and down-stream inflammatory mediators. (e) Systemic or CNS inflammation affect the integrity of the blood–brain barrier (BBB), enhance neuronal excitability, decrease seizure threshold and may exacerbate seizure-induced brain injury. (f) BBB disruption by inflammatory processes may lead to epileptogenesis by disturbance of brain homeostasis. (g) And, finally, several anti-inflammatory drugs have been reported to exert antiepileptic actions. However, opposing results have been published as well, so that more research is needed to fully establish the role of inflammation in seizure generation and epilepsy (Rijkers et al., 2009).

Among the various inflammatory mediators that may be involved in the processes leading to epilepsy, cytokines such as interleukin 1β (IL-1β) and prostaglandins (PGs) such as PGE₂ are thought to play a particular role (Cole-Edwards and Bazan, 2005, Vezzani and Granata, 2005, Takemiya et al., 2007, Vezzani and Baram, 2007, Choi and Koh, 2008, Vezzani et al., 2008). Cyclooxygenase (COX) is the rate-limiting enzyme in PG synthesis and is a major target of nonsteroidal anti-inflammatory drugs (NSAIDs) (Takemiya et al., 2007). Two isoforms of COX enzymes have been identified: the constitutively expressed COX-1 and the inducible, highly regulated COX-2, which is the predominant COX isoform expressed in the brain. Induction of COX-2 in the brain has been shown to facilitate epileptogenesis and contribute to neuronal damage in rat models of TLE (Cole-Edwards and Bazan, 2005, Takemiya et al., 2007, Kulkarni and Dhir, 2009). It is thus tempting to speculate that COX-2 inhibitors exert antiepileptogenic and neuroprotective effects in such models. If so, COX-2 inhibitors may offer a new strategy for preventing neuronal damage and epileptogenesis after brain insults such as traumatic brain injury, focal cerebral ischemia or SE. Such brain insults are typically followed by a latent (or silent) period, during which pharmacological modulation of epileptogenic processes may allow to prevent or modify the development of epilepsy, which is a major clinical goal in people at risk (Kelley et al., 2009).

The first evidence that COX-2 inhibitors may be interesting in this regard was reported by Jung et al. in 2006. They showed that prolonged administration of the COX-2 inhibitor celecoxib after a pilocarpine-induced SE in rats prevents neuronal damage in the hippocampus and, more importantly, reduces the incidence, frequency and duration of spontaneous recurrent seizures, indicating an antiepileptogenic or disease-modifying effect. The study of Jung et al. (2006) prompted us to perform similar experiments with the highly selective COX-2 inhibitor parecoxib because celecoxib has been reported to exert also COX-2 independent actions (Kang et al., 2005, Grosch et al., 2006, Miyamoto et al., 2006), which may have been involved in the antiepileptogenic effects reported by Jung et al. (2006). During the course of our experiments with parecoxib in the pilocarpine model of TLE (first data were presented at a conference in March 2009 and published as an abstract: Polascheck et al., 2009), a second study using a COX-2 inhibitor in a rat TLE model was published (Holtman et al., 2009). In apparent contrast to the study of Jung et al., 2006, Holtman et al., 2009 did not find any evidence of antiepileptogenic, disease-modifying or neuroprotective effects of prolonged treatment with the COX-2 inhibitor SC58236 after an electrically-induced SE. In the present study, we partially confirm the findings of Jung et al. (2006), thus allowing to evaluate which experimental factors may affect the outcome of such studies. Furthermore, in view of the well-known neuroprotective effects of COX-2 inhibitors such as celecoxib and parecoxib (Kunz and Oliw, 2001, Candelario-Jalil et al., 2003, Scali et al., 2003, Kunz et al., 2005, Kunz et al., 2006, Hewett et al., 2006, Jung et al., 2006, Kelsen et al., 2006, Reksidler et al., 2007), we included a behavioral and cognitive test battery in the present study because TLE is known to be associated with psychopathology and cognitive impairment which, at least in part, are thought to be a consequence of neuronal damage in the hippocampal formation and other parts of the limbic system (LaFrance et al., 2008).