Cytokines in the nervous system with emphasis on interleukin-1 receptor-mediated activity

Abstract

Interleukin-1 (IL-1) is one of the most important pro-inflammatory cytokines and plays a pivotal role in the neuroinflammation associated with certain forms of neurodegeneration, such as epilepsy, cerebral ischemia, trauma, and Alzheimer's disease (AD). Also, it was suggested that IL-1 is involved in physiological processes, namely modulation of sleep, glucose and lipid metabolism, and neuroendocrine regulation, as well as modulation of learning and memory.

The objectives of the present work were to study the role of the IL- 1 system in the pathogenesis of neurodegenerative disorders such as epilepsy and cerebral ischemia, as well as the influence of central IL-1 signalling on brain morphology, expression of AD-related proteins, and inflammatory markers, and on different aspects of behaviour, activation of hypothalamo-pituitary-adrenal (HPA)-axis, and brain monoamines. It is well known that hypothermia protects against neurodegeneration induced by excitotoxic brain damage, while hyperthermia has an aggravating effect.

In Paper I and II, experimental excitotoxicity was induced by peripheral administration of the glutamate analogue kainic acid (KA) in rats, a well known model for human temporal lobe epilepsy, which leads to epileptic seizures, temperature changes, and specific brain damage characterized by neuronal cell death and glial activation, as well as induction of inflammatory cytokines. The aims were to study the mechanisms underlying KA-induced core temperature changes, i.e. initial hypothermia followed by a longer hyperthermic effect, with regard to the subtypes of glutamate receptors and the involvement of IL-1, as well as interleukin-6 (IL6) and IL-1 receptor antagonist (IL-1ra).

We found that KA-induced changes in core temperature are not dependent upon NMDA-receptors activation and are unlikely to be a result of increased motor activity since dizocilpine, a non-competitive antagonist of NMDA-receptors, blocked KA-induced behavioural, but not temperature changes (Paper I). The levels of IL-1beta, IL-6 and IL-1ra in the hypothalamus did not seem to correlate with KA-induced changes in core temperature, except for the increase in IL-1beta in the hypothalamus, at 2 h after KA-injection. The ratios between IL-1beta and IL-1ra or IL-6 were increased at the time points when the core temperatures were modified, suggesting that imbalances between cytokine levels may contribute to KA-induced core temperature changes (Paper II).

The role of IL- 1 receptor-mediated activity in the brain, upon focal cerebral ischemia and under normal conditions, was investigated in a previously developed transgenic mouse strain with central blockade of IL-1 signalling, due to the brain-directed overexpression of human soluble IL-1ra (Tg hsIL-1ra) (Paper III-V). A model of permanent focal cerebral ischemia was established in the Tg hsIL-1ra mice and analysis of neurological scores, lesion size, and inflammatory markers showed similar results in Tg hsIL-1ra (heterozygotic) and wild type mice, indicating that the brain-directed blockade of IL-1 signalling was not neuroprotective in this model of permanent focal cerebral ischemia in mice (Paper III).

Analysis of the phenotype of the Tg hsIL-ra mice indicated that there were no significant compensatory changes of IL-1beta, IL-6, or tumour necrosis factor-alpha (TNF-alpha) in the brain. However, the marked reduction in area density and brain volume in the Tg hsIL-1ra (homozygotic) mice indicates that IL-1 signalling in the brain affects CNS development. Furthermore, the expression of amyloid precursor protein (APP) was lower in heterozygotic, but not homozygotic male Tg hsIL-1ra mice (Paper IV). In addition, we observed modulation of different aspects of behaviour, and of the expression of dopamine (DA), serotonin (5-HT) and their metabolites in the brain of Tg hsIL-1ra mice, but no effect on the activation of the HPA-axis, reflected in the serum levels of corticosterone (Paper V).

Studies on the behavioural phenotype revealed higher locomotor activity, decreased habituation for locomotion, and lower anxiety levels in the Tg hsIL-1ra mice. The levels of DA, 5-HT, and their metabolites were decreased in certain brain regions of the Tg hsIL-1ra mice. IL-1 signalling in the brain seems to have important roles, both in the basic functionality of the CNS, but also in the pathogenesis of excitototoxic neurodegeneration, suggesting that the manipulation of IL-1 system can have promising therapeutic implications.