Abstracts

Mounting evidence implicates inflammatory mediators in neuronal injury for a growing number of neurological disorders, including epilepsy, and microglial activation initiates this inflammatory cascade. Microglia constitute the first line of defense against pathological changes within the CNS microenvironment. While their role is primarily supportive, microglial responses may cause excessive or chronic irritation of neurons and become harmful.
In adult rats (P30), KA-induced status epilepticus results in hippocampal neurodegeneration, followed by the emergence of chronic recurrent spontaneous seizures. In contrast, neither cell death nor chronic spontaneous seizures occur if KA is given to immature rats, prior to P21. We hypothesize this is, in part, mediated by distinct transcriptional responses of inflammatory genes. While adaptive and critical for recovery, these processes could lead to injury and death if excessive and chronically present. High-density oligonucleotide arrays (8,799 genes) were utilized to investigate transcriptional regulation at five time points following KA-induced status epilepticus (SE) at P15 and P30: 1, 6, 24, 72, and 240 h. Briefly, total RNA from hippocampi were isolated after KA-induced SE at P15 and P30. Microarray analyses were performed followed by qRT-PCR confirmation. Tissue sections from a cohort collected at 1, 24, and 24 h were processed for immunocytochemistry using antibody against Aif1/Iba1 to investigate the extent of microglial activation. Inflammation-related genes comprised the most abundant functional group significantly regulated ([gt]2 fold, p[lt]0.05) following KA-induced SE for both P15 and P30. Seizures at P15 led to a rapid, but transient, induction of genes coding for Aif1/Iba1, Il-1[beta], C1qb, IL-6, C3, Nf[kappa]B, Litaf, IL-1[alpha], vimentin, TGF-[beta], GFAP, C/ebp, and COX-2. In P15, genes upregulated at 1, 24, and 72 h returned to baseline levels by 240 h. In contrast, seizures at P30 induced inflammatory transcripts to a significantly greater extent and for a longer duration. qRT-PCR of these genes demonstrated similar results, but with magnitude differences. At P30, Aif1/Iba1-labelled activated microglia were increased in both the number and size in hippocampus[mdash]peaked at 24h, but remained elevated at 240 h. In contrast, seizures at P15 showed only a modest microglial response at 24 h, which returned to control levels by 240 h. These data show that inflammation-related genes are significantly increased in both juvenile and adult rat hippocampus after KA-induced seizures, but to a greater extent and duration in older animals. Such age-dependent increases are reflected in tissue microglial activation. Neuroinflammation, thus, represents a potential target for therapeutic intervention and suggests a possible causal role for microglia in seizure susceptibility and neuronal death. (Supported by K088NS02068, Davee Foundation, and CURE.)