Innate immune reaction in response to seizures: implications for the neuropathology associated with epilepsy

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Abstract

In the present study, the expression of pro-inflammatory transcripts was assessed across the brain of mice having undertaken pilocarpine-induced seizures. Pilocarpine-induced marked neurodegeneration and demyelination in multiple regions of the forebrain. The pattern of genes encoding toll-like receptor type 2 (TLR2) and IκBα (index of NF-κB activation) was associated with the neurodegenerating areas, but this was not the case for the mRNA encoding other inflammatory proteins. Scattered tumor necrosis factor-alpha (TNF-α)-expressing cells were found across brain, whereas the signals for monocyte-chemoattractant protein-1 and microsomal prostaglandin mPGES E synthase were robust in thalamus and cerebral cortex and weak in the hippocampus and amygdala. TLR2 and TNF-α transcripts were expressed mainly in microglia/macrophages. Cyclooxygenase-2 was induced specifically in the hippocampus and piriform cortex. A low increase in interleukin-12 mRNA was detected in the brain, but the signal for interferon gamma (IFN-γ) remained undetectable. Although pro-inflammatory markers were induced in a different manner across the CNS, their patterns were not characteristic of those caused by other inflammatory challenges, such as endotoxin. These data suggest a different mechanism involved in regulating the innate immune reaction in response to seizures and could have direct implications for the neuropathology associated with epilepsy.

Introduction

Seizures and epilepsy are manifested through complex changes in the central nervous system (CNS). The consequences of seizures include excitotoxic cell death (either through necrotic or apoptotic pathways), which is followed by reorganization of different populations of cells in the hippocampus and various thalamic and amygdala nuclei Gwinn et al., 2002, Humphrey et al., 2002, Leite et al., 2002, Scharfman et al., 2002. Together, these events lead to the clinical manifestations associated with the disease. In models of induced seizures, using agents such as kainic acid, pilocarpine, and kindling, these changes can be seen early following the first induced seizure, and are exacerbated in time, leading to recurrent seizures. Furthermore, inflammatory processes, such as the appearance of pro-inflammatory cytokines [e.g., interleukin-1β (IL-1β) and tumor necrosis factor-alpha (TNF-α)], have been observed in the brain following seizure activity Shandra et al., 2002, Vezzani et al., 2002. Inflammatory signals have also been reported in clinical cases of epilepsy Crespel et al., 2002, Lorenz, 2001, and of course, febrile seizures (Virta et al., 2002) and immune molecules have already been associated with predisposition to epilepsy (Kanemoto et al., 2000).

Pro-inflammatory signals, such as cytokines and prostaglandins (PGs), play key roles during many neuropathologies, such as cancers, infections, neurodegenerative diseases, and neuropsychiatric conditions. Following this, it has been theorized that the inflammatory response could be mediating some of the central changes seen in seizure activity (Jankowsky and Patterson, 2001). However, little is known about the role of inflammation during seizures. For example, the nature of cells involved in the production of cytokines and inflammatory molecules is unknown, as is whether these cells are from central (microglia, endothelial cells, and astrocytes) or peripheral (invading monocytes) origin. Furthermore, it has not been established whether this response is purely innate, or if there are some elements of the adaptive immunity. Another interesting debate has arisen to determine if the presence of cytokines in the brain during seizure has neuroprotective Albensi, 2001, Hamano et al., 2002, McLaughlin et al., 2003 or neurodegenerative Eriksson et al., 2000, Troy et al., 2002, Vezzani et al., 1999, Vezzani et al., 2000 consequences.

In the following study, we attempted to elucidate the immune response during acute pilocarpine seizures, which is associated with rapid and localized cell death (Poirier et al., 2000). Through in situ hybridization (ISH), we looked at various markers of innate [cytokines, prostaglandin (PG) pathway enzymes, toll-like receptor type 2 (TLR2)] and adaptive [interferon gamma (IFN-γ)] immunity, and through a combination of ISH and immunocytochemistry (ICC), we determined the nature of the cells releasing some of the pro-inflammatory signals, namely TLR2 and TNF-α. TLR2 is a cell surface receptor that binds components of the Gram-positive bacterial cell walls and has been found to be a reliable marker of microglial activation during inflammation in the CNS Laflamme et al., 2001, Soulet and Rivest, 2003, while TNF-α is a pro-inflammatory cytokine that is quickly released following inflammatory stimuli Nadeau and Rivest, 1999, Nadeau and Rivest, 2002. Furthermore, cell death and morphology, as well as the appearance of invading cells in the brain, were also studied to assess the cellular cast involved in seizure-induced neurological damage. Together, these experiments should provide an accurate temporal and spatial description of the innate immune response that accompanies pilocarpine-induced seizures.

Section snippets

Animals

Adult male CD-1 mice (Charles River Canada, St. Constant, Québec, Canada, approximately 22–25 g b.w.) were housed individually and acclimated to standard laboratory conditions (12-h light/dark cycle; lights on at 0700 and off at 1900) with free access to mouse chow and water. Animal experiments were conducted according to Canadian Council on Animal Care guidelines, as administered by the Laval University Animal Care Committee. A total of 36 mice were assigned to the treatment or vehicle group

Pilocarpine-induced seizures

Of the 30 mice receiving pilocarpine, 80% (n = 24) developed appreciable seizures. Some animals (n = 4) suffered from fatal seizures and were not included in this study. Of those that developed seizures and survived, only mice (n = 13) sustaining stage 3 seizures or higher showed signs of neuronal cell death and generalized inflammation (see below). Seizures were interrupted with the injection of diazepam 3 h after the systemic administration of pilocarpine. Therefore, the data presented in

Discussion

It has long been established that seizures and epilepsy are accompanied by severe neuropathology involving cell death Humphrey et al., 2002, Poirier et al., 2000 and inflammation Crespel et al., 2002, Virta et al., 2002. The use of pilocarpine-induced seizures offers a model of acute (as well as recurrent) seizures associated with similar neuropathology to epilepsy. This study demonstrated that seizures induced by pilocarpine in mice yielded a pattern of neuronal death in the hippocampus,

Acknowledgements

We are grateful to Dr. Y. Imai (National Institute of Neuroscience, Kodaira, Tokyo, Japan) for the gift of iba1 antisera, Dr. A. Israel (Institut Pasteur, Paris, France) for the mouse IκBα cDNA, Dr. D. Radzioch (McGill University, Montréal, Canada) for the plasmid containing the mouse TNF-α cDNA, Dr. I. Campbell (The Scripps Research Institute, La Jolla, CA) for the mouse IFNγ cDNA, Dr. K. Pahan (University of Nebraska, Lincoln, NE, USA) for the mouse IL-12p40 cDNA, Dr. Y. Guan (Vanderbilt

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