Inflammation induced at different developmental stages affects differently the range of microglial reactivity and the course of seizures evoked in the adult rat

Highlights

• LPS-induced inflammation reduces susceptibility to seizures in the rat.

• LPS-induced inflammation decreases reactive microglia transformations.

• Preconditioning as the long-term effect of systemic inflammation

Abstract

Background

In the brain, inflammation occurs following a variety of types of brain damage, including epileptic seizures. Proinflammatory cytokines, like IL-1β or TNFα, can increase neuronal excitability and initiate spontaneous seizures or epileptogenesis. Recent studies indicate that the effects can be attenuated or even abolished in animals subjected to inflammation-inducing treatments at earlier developmental stages, termed “preconditioning”. Immunocompetent microglial cells display particular sensitivity to subtle brain pathologies showing a morphological continuum from resting to reactive forms. Following inflammation, multiple ramified processes of resting microglia become gradually shorter, and the cells transform into macrophages. Parameters of the morphological variations were used here as indicators of the nervous tissue reactivity to seizures in adult rats experiencing inflammation at earlier stages of postnatal development.

Methods

Systemic inflammation was induced with lipopolysaccharide (LPS) in 6-day-old or 30-day-old rats. In two-month-old survivors of the inflammatory status, seizures were evoked with pilocarpine injection. The seizure intensity was scored during a six-hour continuous observation period following the injection. Brain sections were immunostained for Iba1 to visualize microglia. Thereafter, morphology of microglial cells located in the hippocampal formation was analyzed using parameters such as solidity, circularity, ramification index, and area.

Results

In naïve rats, seizure-induced transformations of microglial cells were reflected by strong changes in the parameters of their morphology. However, in the adult rats pretreated with LPS on their 6th or 30th postnatal days, the seizure-induced changes were significantly reduced, and microglial morphology remained significantly closer to normal. Significant amelioration of the acute phase of seizures was observed only when inflammation was induced in 30-day-old, but not in 6-day-old, rats.

Conclusions

The results confirm previous reports that moderate inflammation protects the nervous tissue from subsequent damage by reducing influences of proinflammatory factors on reactive glial cells. The young-age inflammation may have age-dependent effects on susceptibility to seizures induced in adulthood.

This article is part of a Special Issue entitled “Status Epilepticus”.

Introduction

Inflammation in the brain is associated with an increased presence of cytokines and inflammatory mediators, which are normally undetectable or present in very low concentrations [1]. During the inflammatory status, endothelial and blood-derived cells, microglia, and astrocytes are involved in enhanced cytokine production resulting, in turn, in activation of metalloproteinases and catabolism of arachidonic acid [2], [3], [4]. Proinflammatory factors such as IL-1β or TNFα can directly increase neuronal excitability and facilitate spontaneous seizure activity [5]. These findings highlight a possible critical importance of inflammation for the development of epilepsy. Studies in transgenic mice with increased expression of inflammatory cytokines like IL-1β, TNFα, or IL-6 showed increases in seizure intensity and neurodegenerative changes. On the other hand, in patients suffering from seizures and in corresponding animal models, increased levels of proinflammatory factors were also observed [1], [5]. Thus, inflammation may initiate epileptogenesis and epilepsy, which in turn may evoke inflammation [6], [7]. Recent experimental studies point to the fact that, depending on the time period after their administration or the developmental stage, these factors may have beneficial, adverse, or completely neutral effects [8].

One of the best characterized and reliable methods to quickly elicit strong inflammatory response in the CNS is the administration of lipopolysaccharide (LPS), a component of the outer wall of Gram-negative bacteria, which mimics systemic infection [1]. A generalized inflammation induced with LPS prior to experimental cerebral hypoxia significantly reduces its negative effects [8] and is evidence in support of the concept of preconditioning. Its neuroprotective and dose-dependent effect was observed within the period from 24 h to 14 days following LPS administration (therapeutic window) [9], [10]. Depending on the experimental model, the effective dose was from 0.02 to 1.0 mg/kg b.w. [11], [12]. The LPS protective action has a mechanism which is not yet understood and seems to occur via proinflammatory cytokines such as TNFα or IL-1β [8], [13]. It appears reasonable to hypothesize that LPS could also exert a protective influence on hippocampal neurons affected by seizures in animal models of epilepsy. Nevertheless, in rats treated with LPS 72 h before seizure induction, no amelioration in the course of seizures was observed. However, a decrease of neurodegenerative processes occurred in CA1, CA3, and DG regions of the hippocampal formation [14]. Stenzel-Poore et al. [15] reported that LPS administration following a stroke suppressed the microglia and monocyte activation.

The purpose of this study was to verify whether a previously experienced generalized inflammation would affect susceptibility to seizures. We were, however, not focused on an effective LPS dosage or the therapeutic window but on the long-term effects of LPS-induced inflammation. In this case, it could be assumed that inflammation itself could no longer be a factor directly influencing the seizure reactivity because, according to predefined indicators, it disappeared a long time earlier. This directly influencing factor might rather be permanent postinflammatory changes in the nervous tissue. To monitor these changes, we selected the reactive morphological transformation of microglia as particularly sensitive to subtle pathologies of the nervous system [16].

Another aim of this study was to determine whether the generalized inflammation at various developmental stages would affect the susceptibility to seizures induced during adulthood differently and whether the functional change would be accompanied by respective variations of microglial morphology.