Astrocytes rather than neurones mediate interleukin-1beta dependent nitric oxide and superoxide radical release in primary hypothalamic rat cell cultures

Chronic estrogen affects TIDA neurons through IL-1β and NO: effects of aging

Women are chronically exposed to estrogens through oral contraceptives, hormone replacement therapy or environmental estrogens. We hypothesized that chronic exposure to low levels of estradiol-17β (E2) can induce inflammatory and degenerative changes in the tuberoinfundibular dopaminergic (TIDA) system leading to reduced dopamine synthesis and hyperprolactinemia. Young (Y; 3–4 months) and middle-aged (MA; 10–12 months) Sprague-Dawley rats that were intact or ovariectomized (OVX) were either sham-implanted or implanted with a slow-release E2 pellet (20 ng E2/day for 90 days). To get mechanistic insight, adult 3- to 4-month-old WT, inducible nitric oxide synthase (iNOS) and IL-1 receptor (IL-1R) knockout (KO) mice were subjected to a similar treatment. Hypothalamic areas corresponding to the TIDA system were analyzed. E2 treatment increased IL-1β protein and nitrate levels in the arcuate nucleus of intact animals (Y and MA). Nitration of tyrosine hydroxylase in the median eminence increased with E2 treatment in both intact and OVX animals. There was no additional effect of age. This was accompanied by a reduction in dopamine levels and an increase in prolactin in intact animals. E2 treatment increased nitrate and reduced dopamine levels in the hypothalamus and increased serum prolactin in WT mice. In contrast, the effect of E2 on nitrate levels was blocked in IL-1R KO mice and the effect on dopamine and prolactin were blocked in iNOS KO animals. Taken together, these results show that chronic exposure to low levels of E2 decreases TIDA activity through a cytokine-nitric oxide-mediated pathway leading to hyperprolactinemia and that aging could promote these degenerative changes.

Glutamate transporter EAAT2 expression is up-regulated in reactive astrocytes in human periventricular leukomalacia

The major neuropathological correlate of cerebral palsy in premature infants is periventricular leukomalacia (PVL), a disorder of the immature cerebral white matter. Cerebral ischemia leading to excitotoxicity is thought to be important in the pathogenesis of this disorder, implying a critical role for glutamate transporters, the major determinants of extracellular glutamate concentration. Previously, we found that EAAT2 expression is limited primarily to premyelinating oligodendrocytes early in development and is rarely observed in astrocytes until >40 weeks. In this study, we analyzed the expression of EAAT2 in cerebral white matter from PVL and control cases. Western blot analysis suggested an up-regulation of EAAT2 in PVL compared with control cases. Single- and double-label immunocytochemistry showed a significantly higher percentage of EAAT2-immunopositive astrocytes in PVL (51.8% +/- 5.6%) compared with control white matter (21.4% +/- 5.6%; P = 0.004). Macrophages in the necrotic foci in PVL also expressed EAAT2. Premyelinating oligodendrocytes in both PVL and control cases expressed EAAT2, without qualitative difference in expression. The previously unrecognized up-regulation of EAAT2 in reactive astrocytes and its presence in macrophages in PVL reported here may reflect a response to either hypoxic-ischemic injury or inflammation.

Inhibition of small-conductance Cl- channels by the interleukin-1beta-stimulated production of superoxide in rabbit gastric parietal cells

We have shown previously that the G protein-coupled production of superoxide anion (O2-) leads to closure of small-conductance Cl- channels (0.3-0.4 pS) in the basolateral membrane of rabbit parietal cells. In the present study, effects of interleukin-1beta (IL-1beta) on the Cl- channel were investigated. In the whole-cell patch-clamp recording, IL-1beta (0.3-10 ng ml-1) inhibited the whole-cell Cl- current recorded from a parietal cell within isolated rabbit gastric glands. Variance noise analysis of the whole-cell Cl- current showed that the single channel conductance of the Cl- channel that is sensitive to IL-1beta is 0.37 pS. The IL-1beta (1 ng ml-1)-induced decrease of the Cl- current was abolished by anti-IL-1beta antibody (2 microg ml-1), recombinant IL-1 receptor antagonist (500 ng ml-1), GDPbetaS (500 microM) and superoxide dismutase (100 units ml-1), a scavenger of O2-. Northern blot analysis showed that the mRNA of the IL-1 receptor was selectively expressed in rabbit gastric parietal cells. In the dihydrofluorescein diacetate-loaded single parietal cells in gastric glands, IL-1beta (0.3-10 ng ml-1) stimulated the production of oxygen radicals. Y-27632 (1-10 microM), a specific Rho-kinase inhibitor, and fluvastatin (10 microM), an indirect inhibitor for Rho proteins, significantly inhibited the IL-1beta-induced effects on the channel activity and production of oxygen radicals. IL-1beta (0.3-10 ng ml-1) activated Rho in the parietal cells. These results indicate that IL-1beta binds to the IL-1 receptor of gastric parietal cells and inhibits the small-conductance Cl- channel via the G protein-mediated Rho/Rho-kinase-dependent production of O2-.

Arginine metabolism and the synthesis of nitric oxide in the nervous system

The biochemistry and physiology of L-arginine have to be reconsidered in the light of the recent discovery that the amino acid is the only substrate of all isoforms of nitric oxide synthase (NOS). Generation of nitric oxide, NO, a versatile molecule in signaling processes and unspecific immune defense, is intertwined with synthesis, catabolism and transport of arginine which thus ultimately participates in the regulation of a fine-tuned balance between normal and pathophysiological consequences of NO production. The complex composition of the brain at the cellular level is reflected in a complex differential distribution of the enzymes of arginine metabolism. Argininosuccinate synthetase (ASS) and argininosuccinate lyase which together can recycle the NOS coproduct L-citrulline to L-arginine are expressed constitutively in neurons, but hardly colocalize with each other or with NOS in the same neuron. Therefore, trafficking of citrulline and arginine between neurons necessitates transport capacities in these cells which are fulfilled by well-described carriers for cationic and neutral amino acids. The mechanism of intercellular exchange of argininosuccinate, a prerequisite also for its proposed function as a neuromodulator, remains to be elucidated. In cultured astrocytes transcription and protein expression of arginine transport system y(+) and of ASS are upregulated concomittantly with immunostimulant-mediated induction of NOS-2. In vivo ASS-immunoreactivity was found in microglial cells in a rat model of brain inflammation and in neurons and glial cells in the brains of Alzheimer patients. Any attempt to estimate the contributions of arginine transport and synthesis to substrate supply for NOS has to consider competition for arginine between NOS and arginase, the latter enzyme being expressed as mitochondrial isoform II in nervous tissue. Generation of NOS inhibitors agmatine and methylarginines is documented for the nervous system. Suboptimal supply of NOS with arginine leads to production of detrimental peroxynitrite which may result in neuronal cell death. Data have been gathered recently which point to a particular role of astrocytes in neural arginine metabolism. Arginine appears to be accumulated in astroglial cells and can be released after stimulation with a variety of signals. It is proposed that an intercellular citrulline-NO cycle is operating in brain with astrocytes storing arginine for the benefit of neighbouring cells in need of the amino acid for a proper synthesis of NO.

Cytokines and oxidative signalling in skeletal muscle

A growing body of literature indicates that cytokines regulate skeletal muscle function, including gene expression and adaptive responses. Tumour necrosis factor-alpha (TNF-alpha) is the cytokine most prominently linked to muscle pathophysiology and, therefore, has been studied most extensively in muscle-based systems. TNF-alpha is associated with muscle catabolism and loss of muscle function in human diseases that range from cancer to heart failure, from arthritis to AIDS. Recent advances have established that TNF-alpha causes muscle weakness via at least two mechanisms, accelerated protein loss and contractile dysfunction. Protein loss is a chronic response that occurs over days to weeks. Changes in gene expression required for TNF-alpha induced catabolism are regulated by the transcription factor nuclear factor-kappaB which is essential for the net loss of muscle protein caused by chronic TNF-alpha exposure. Contractile dysfunction is an acute response to TNF-alpha stimulation, developing over hours and resulting in decreased force production. Both actions of TNF-alpha involve a rapid rise in endogenous oxidants as an essential step in post-receptor signal transduction. These oxidants appear to include reactive oxygen species derived from mitochondrial electron transport. Such information provides insight into the cellular and molecular mechanisms of TNF-alpha action in skeletal muscle and establishes a scientific basis for continued research into cytokine signalling.