Region-specific expression of the interleukin-1 system in rat brain following endotoxin challenge and excitotoxic neurodegeneration

Abstract

Traditionally, up-regulation and secretion of cytokines are associated with peripheral immune and inflammatory responses. There is now evidence that cytokines also exert various actions in the central nervous system (CNS). Cytokines, such as interleukin-1 (IL-1), are implicated in several human neurological disorders, for example bacterial meningitis, stroke, epilepsy and Alzheimer's disease.

One cytokine family is the IL-1 system consisting of the two agonists IL-1[alpha] and IL-1ß, the endogenous IL-1 receptor antagonist (IL-1ra), and the enzyme caspase-1 (interleukin-1ß converting enzyme), responsible for generating mature and biologically active interleukin-1ß. Lately, the key questions have concerned which cell types are capable of producing these cytokines within the CNS, and what is the functional role of the components of the IL-1 system in the CNS. Our studies were therefore focussed on analyzing the expression pattern and site of synthesis of the components of the IL-1 system in the rat CNS, and to investigate how their expression patterns are interrelated, following endotoxin challenge and following excitotoxic brain damage induced by the glutamate analogue kainic acid.

Following peripheral endotoxin injection, cytokine production occurs mainly in peripherally derived monocytes in the circumventricular organs (regions lacking the blood-brain-barrier) and pituitary gland. IL-1ß is the predominant form expressed in these regions, whereas the expression of IL-1[alpha] and IL-1ra mRNA is lower. These findings support the view that peripherally derived IL-1ß may play a role in the induction of centrally mediated responses, such as sickness symptoms, fever and activation of the hypothalamus-pituitary-adrenal axis.

Brain damage following systemic kainic acid injection was characterized by the occurrence of degenerating neurons at 5 h post-injection, as observed by cresyl violet staining or in situ end labelling (ISEL). A more severe stage of neurodegeneration was seen at 24 h, particularly in the hippocampus, thalamus, amygdala, and in the piriform, perirhinal and entorhinal cortex.

The expression of IL-1ß and IL-1ra mRNA, as detected by in situ hybridization histochemistry (ISHH), occurred in brain regions showing neurodegenerative changes. The IL-10 mRNA signal was detected at 2 h and the highest expression was seen at 12 h, while the IL-1ra mRNA signal was first observed at 5 h and highest at 24 h. The cytokines were mainly observed in microglial cells. RT-PCR analysis indicated that caspase-1 mRNA was up-regulated at 12 h in the hippocampus and amygdala, and that the predominant form of IL-1ra mRNA in the brain is the secreted isoform of IL1ra.

Kainic acid induced protein expression of IL-1ß, IL-1ra and caspase-1, as detected by immunohistochemistry, was compared with the distribution of ISEL cells. The cells immunoreactive to IL-1ß and IL-1ra proteins and the ISEL-cells were localized in the same regions, while the induced expression of caspase-1 had a more limited distribution and was only found in some of these regions. The degree of IL-1ß expression and appearance of ISEL-cells followed the same temporal profile and regional distribution. The expression profile of caspase-1 was delayed compared to that of IL-1ß and ISEL-cells. The IL-1 system proteins were also mainly observed in microglial cells.

Taken together these results show that in response to systemic kainic acid administration, induction of IL-1ß and IL-1ra, and to certain extent caspase-1, was found in the regions showing neuronal degeneration. This suggests that the IL-1 system may play an important role in the neuropathology during excitotoxic conditions. The fact that microglia are the main site of synthesis indicate that the IL-1 system is part of the inflammatory response following excitotoxic brain damage. Induction of IL-1 ß was accompanied by induction of IL-1ra and caspase-1, which both affect the activity of the IL-1 system, suggesting that they may represent targets for therapeutic interventions to reduce brain inflammatory responses.

The findings that NMDA-receptor antagonists inhibit IL-10 and IL-lra mRNA synthesis induced by kainic acid suggests that NMDA-receptor activation may be involved in triggering cytokine synthesis following excitotoxic brain damage.