Regulation of astrocyte nitric oxide synthase type II expression by ATP and glutamate involves loss of transcription factor binding to DNA

Is there a role for nitric oxide in methamphetamine-induced dopamine terminal degeneration?

Methamphetamine (METH) abuse results in long-term damage to the dopaminergic system, manifesting as decreases in dopamine (DA) tissue content, DA transporter binding, as well as tyrosine hydroxylase and vesicular monoamine transporter immunostaining. However, the exact cascade of events that ultimately result in this damage has not been clearly elucidated. One factor that has been heavily implicated in METH-induced DA terminal degeneration is the production of nitric oxide (NO). Unfortunately, many of the studies attempting to clarify the role of NO in METH-induced neurotoxicity have been confounded by issues such as the disruption of METH-induced hyperthermia, preventing the formation of strong conclusions. As a result, there is a body of work suggesting that NO is sufficient for METH-induced neurotoxicity, while other studies suggest that NO does not play a role in METH-induced degeneration of DA nerve terminals. This review summarizes the existing studies investigating the role of NO in METH-induced neurotoxicity, and argues that while NO may be necessary for METH-induced neurotoxicity, it is not sufficient. Finally, important areas of future investigation are highlighted and discussed.

Cross-talk between constitutive and inducible NO synthase: an update

Inducible nitric oxide synthase (iNOS) is expressed upon exposure of some cell types to bacterial lipopolysaccharides (LPS) and/or a variety of proinflammatory cytokines. The authors present an overview of some of the recent findings further supporting the notion that this response takes place after an early decline in constitutive nitric oxide (NO) levels (i.e., NO released by constitutive NOS, cNOS). This response is indeed critical for allowing activation of the transcription factor NF-kappaB. Thus, generation of NO by cNOS represents a limiting factor for iNOS expression. Some of the physiological and pathological implications of the cross-talk between these two NOS isoforms are discussed. In addition, the results of recent studies are summarized, suggesting possible mechanisms whereby LPS and/or proinflammatory cytokines may cause inhibition of cNOS.

Inducible nitric oxide synthase subserves cholinergic vasodilation in retina

In this paper, we investigate the role of muscarinic acetylcholine receptor (mAChR) activity in the regulation of inducible (i) nitric oxide synthase (iNOS) expression and activity. The signaling pathway involved is also examined. These experiments also provide a link between mAChR activation and the nitric oxide (NO)-dependent regulation of retinal vascular diameter. The diameter of the retinal vessels at a distance of 1 disc diameter from the center of the optic disc was measured in rats using digital retinal photography, and both iNOS-mRNA gene expression and NOS were specifically measured using RT-PCR and [U-(14)C] citrulline assays, respectively. Stimulation of M(1) and M(3) mAChR with carbachol caused an increase in vessel diameter, in iNOS-mRNA levels and in NOS activity in the retina. Aminoguanidine, an inhibitor of iNOS, attenuated all these effects. Inhibitors of phospholipase C (PLC) and protein kinase C (PKC) but not calcium/calmodulin (CaM) prevented the muscarinic-dependent increase in iNOS-mRNA levels. The results obtained suggest that the activation of mAChR increases retinal vessel diameters by increasing the production of nitric oxide (NO) through iNOS activation and iNOS-mRNA gene expression. The mechanism appears to occur secondarily to stimulation of PLC and PKC enzymatic activity.

Intrinsic regulation of brain inflammatory responses

It is now well accepted that inflammatory responses in brain contribute to the genesis and evolution of damage in neurological diseases, trauma, and infection. Inflammatory mediators including cytokines, cell adhesion molecules, and reactive oxygen species including NO are detected in human brain and its animal models, and interventions that reduce levels or expression of these agents provide therapeutic benefit in many cases. Although in some cases, the causes of central inflammatory responses are clear--for example those due to viral infection in AIDS dementia, or those due to the secretion of proinflammatory substances by activated lymphocytes in multiple sclerosis--in other conditions the factors that allow the initiation of brain inflammation are not well understood; nor is it well known why brain inflammatory activation is not as well restricted as it is in the periphery. The concept is emerging that perturbation of endogenous regulatory mechanisms could be an important factor for initiation, maintenance, and lack of resolution of brain inflammation. Conversely, activation of intrinsic regulatory neuronal pathways could provide protection in neuroinflammatory conditions. This concept is the extension of the principle of "central neurogenic neuroprotection" formulated by Donald Reis and colleagues, which contends the existence of neuronal circuits that protect the brain against the damage initiated by excitotoxic injury. In this paper we will review work initiated in the Reis laboratory establishing that activation of endogenous neural circuits can exert anti-inflammatory actions in brain, present data suggesting that these effects could be mediated by noradrenaline, and summarize recent studies suggesting that loss of noradrenergic locus ceruleus neurons contributes to inflammatory activation in Alzheimer's disease.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Novel insight into the mechanisms involved in the regulation of the m1 muscarinic receptor, iNOS and nNOS mRNA levels

In this paper we have determined the different signaling pathways involved in M(1) muscarinic acetylcholine receptor (mAChR)-dependent stimulation of m1 mAChRs, neural and inducible isoforms of nitric oxide synthase (nNOS and iNOS)-mRNA gene expression of rat frontal cortex. Carbachol-stimulation of M(1) mAChRs exerts an increase in m1 mAChR-mRNA, activation of phosphoinositide (PI) turnover, translocation of protein kinase C (PKC) and stimulation of NOS activity. Inhibitors of phospholipase C (PLC), calcium/calmodulin and NOS, but not guanylate cyclase, prevent the carbachol-dependent increase of m1 mAChR-mRNA levels. These inhibitors also attenuate the muscarinic receptor-dependent increase in nNOS and iNOS mRNA levels. These results suggest that carbachol-activation of M(1) mAChRs increases m1 mAChR, nNOS and iNOS mRNA levels associated with increased production of nitric oxide (NO). The mechanism appears to occur secondarily to stimulation of PI turnover via PLC activation. This in turn, triggers a cascade reaction involving calcium/calmodulin and PKC, leading to activation of NOS. On the basis of our results, the activation of M(1) mAChRs appears to induce nNOS and iNOS expression and, reciprocally, the activator of NOS up-regulates m1 mAChR gene expression. These results may contribute to a better understanding of the effects and side effects of cholinomimetic treatment in patients with neurodegenerative diseases.

Induction of COX-2 and reactive gliosis by P2Y receptors in rat cortical astrocytes is dependent on ERK1/2 but independent of calcium signalling

The present study has been aimed at characterizing the ATP/P2 receptor (and transductional pathways) responsible for the morphological changes induced in vitro by alphabetamethyleneATP on rat astrocytes obtained from cerebral cortex, a brain area highly involved in neurodegenerative diseases. Exposure of cells to this purine analogue resulted in elongation of cellular processes, an event reproducing in vitro a major hallmark of in vivo reactive gliosis. alphabetamethyleneATP-induced gliosis was prevented by the P2X/P2Y blocker pyridoxalphosphate-6-azophenyl-2'-4'-disulfonic acid, but not by the selective P2X antagonist 2',3'-O-(2,4,6-trinitrophenyl)-ATP, ruling out a role for ligand-gated P2X receptors. Conversely, the Gi/Go protein inactivator pertussis toxin completely prevented alphabetamethyleneATP-induced effects. No effects were induced by alphabetamethyleneATP on intracellular calcium concentrations. RT-PCR and western blot analysis showed that alphabetamethyleneATP-induced gliosis involves up-regulation of cyclooxygenase-2 (but not lipooxygenase). Also this effect was fully prevented by pyridoxalphosphate-6-azophenyl-2'-4'-disulfonic acid. Experiments with inhibitors of mitogen-activated protein kinases (MAPK) suggest that extracellular signal regulated protein kinases (ERK)1/2 mediate both cyclooxygenase-2 induction and the associated in vitro gliosis. These findings suggest that purine-induced gliosis involves the activation of a calcium-independent G-protein-coupled P2Y receptor linked to ERK1/2 and cyclooxygenase-2. Based on the involvement of cyclooxygenase-2 and inflammation in neurodegenerative diseases, these findings open up new avenues in the identification of novel biological targets for the pharmacological manipulation of neurodegeneration.

P2Y and P2X purinoceptor mediated Ca2+ signalling in glial cell pathology in the central nervous system

Activation of purinoceptors by extracellular ATP is an important component of the glial response to injury in the central nervous system (CNS). ATP has been shown to evoke raised cytosolic [Ca(2+)] in astrocytes, oligodendrocytes, and microglia, the three major glial cell types in the CNS. Glial cells express a heterogenous collection of metabotropic P2Y and ionotropic P2X purinoceptors, which respectively mobilise Ca(2+) from intracellular stores and trigger Ca(2+) influx across the plasmalemma. It is likely that different receptors have distinct roles in glial cell physiology and pathology. Our studies on optic nerve glia in situ indicate that P2Y(1) and P2Y(2/4) receptors are activated at low ATP concentrations, suggesting they are the predominant purinoceptors mediating physiological Ca(2+) signalling. Glia also express P2X(1) and P2X(3) purinoceptors, which mediate fast, rapidly desensitising current and may also be important in signalling. At high concentrations, such as occur in CNS injury, ATP induces large and prolonged increases in glial [Ca(2+)](i) with a primary role for P2Y purinoceptors and inositol trisphosphate (IP(3))-dependent release of Ca(2+) from intracellular stores. In addition, we found that high concentrations of ATP activated a significant P2X component that did not desensitise or saturate and was dependent on extracellular Ca(2+). These are characteristic properties of the P2X(7) subtype, and we provide in situ evidence that application of the P2X(7) receptor agonist benzoyl-benzoyl ATP (BzATP) evokes raised [Ca(2+)](i) in optic nerve glia, and that the dye YO-PRO-1, which passes through pore-forming P2X(7) receptors, is taken up by astrocytes, oligodendrocytes and microglia. Glia also express P2X(2) and P2X(4) receptors that are also pore-forming in the presence of sustained high ATP concentrations and which may also be important in the glial injury response. There is evidence that activation of P2 purinoceptors is a key step in triggering reactive changes in glial cells, including expression of immediate early genes, induction of extracellular signal regulated kinase and cyclooxygenase-2, synthesis of phospholipase A(2), release of arachidonic acid, production of prostaglandins and release of interleukins. We show that the ATP-mediated increase in glial [Ca(2+)](i) is potentiated by arachidonic acid and reduced by the inhibition of phospholipase A(2) inhibition. Together, the results implicate ATP as a primary signalling molecule in glial cells and indicate specific roles for P2Y and P2X purinoceptors in glial cell pathology.

Role of astrocytes in pathogenesis of ischemic brain injury

Astrocytes play an important role in the homeostasis of the CNS both in normal conditions and after ischemic injury. The swelling of astrocytes is observed during and several seconds after brain ischemia. Then ischemia stimulates sequential morphological and biochemical changes in glia and induces its proliferation. Reactive astrocytes demonstrate stellate morphology, increased glial fibrillary acidic protein (GFAP) immunoreactivity, increased number of mitochondria as well as elevated enzymatic and non-enzymatic antioxidant activities. Astrocytes can re-uptake and metabolize glutamate and in this way they control its extracellular concentration. The ability of astrocytes to protect neurons against the toxic action of free radicals depends on their specific energy metabolism, high glutathione level, increased antioxidant enzyme activity (catalase, superoxide dismutase, glutathione peroxidase) and overexpression of antiapoptotic bcl-2 gene. Astrocytes produce cytokines (TNF-alpha, IL-1, IL-6) involved in the initiation and maintaining of immunological response in the CNS. In astrocytes, like in neurones, ischemia induces the expression of immediate early genes: c-fos, c-jun, fos B, jun B, jun D, Krox-24, NGFI-B and others. The protein products of these genes modulate the expression of different proteins, both destructive ones and those involved in the neuroprotective processes.

The dual personality of NO

In the body, nitric oxide (NO) is an important physiological regulator of functions such as vasodilatation and neurotransmission. Under pathological conditions, high concentrations of NO can be either beneficial(e.g. anti-bacterial, anti-parasitic and anti-viral) or detrimental; NO can therefore be considered a double-edged sword. When manipulating NO levels clinically, attention should be paid to minimize the negative effects and maximize the beneficial effects of NO. This article highlights recent evidence that supports the complexity of the regulatory mechanisms that lead to sophisticated endogenous NO production.

Nitric oxide: an inhibitor of NF-kappaB/Rel system in glial cells

Nitric oxide (NO) has been reported to regulate NF-kappaB, one of the best-characterized transcription factors playing important roles in many cellular responses to a large variety of stimuli. NO has been suggested to induce or inhibit the activation of NF-kappaB, its effect depending, among others, on the cell type considered. In this review, the inhibitory effect of NO on NF-kappaB (and subsequent suppression of NF-kappaB-dependent gene expression) in glial cells is reported. In particular, exogenous and endogenous NO has been observed to keep NF-kappaB suppressed, thus preventing the expression of NF-kappaB-induced genes, such as inducible NO synthase itself or HIV-1 long terminal repeat. Furthermore, the possible molecular mechanisms of NO-mediated NF-kappaB inhibition are discussed. More specifically, NO has been reported to suppress NF-kappaB activation inducing and stabilizing the NF-kappaB inhibitor, IkappaB-alpha. On the other hand, NO may inhibit NF-kappaB DNA binding through S-nitrosylation of cysteine residue (i. e., Cys62) of the p50 subunit. As a whole, a novel concept that the balance of intracellular NO levels may control the induction of NF-kappaB in glial cells has been hypothesized.

Implication of glutamate in the expression of inducible nitric oxide synthase after oxygen and glucose deprivation in rat forebrain slices

Nitric oxide synthesis by inducible nitric oxide synthase (iNOS) has been postulated to contribute to ischemia-reperfusion neurotoxicity. The expression of this enzyme has been demonstrated in cells present in the postischemic brain. The mechanisms of iNOS expression after cerebral ischemia are a subject of current research. We therefore decided to investigate whether glutamate, which is released in ischemia and is implicated in neurotoxicity, might be involved in the mechanisms by which oxygen and glucose deprivation (OGD) leads to the expression of iNOS in rat forebrain slices. In this model, we have shown previously that 20 min of OGD causes the expression of iNOS. We have now found that the NMDA receptor antagonist MK-801 blocks the expression of iNOS, suggesting that the activation of the NMDA subtype of glutamate receptor is implicated in the mechanisms that lead to the expression of this isoform. Moreover, we have found that glutamate alone could trigger the induction process, as shown by the appearance of a Ca(2+)-independent NOS activity and by the detection of iNOS mRNA and protein in slices exposed to glutamate. Glutamate-dependent iNOS expression was concentration-dependent and was blocked by EGTA and by the inhibitors of nuclear factor kappaB (NF-kappaB) activation pyrrolidine dithiocarbamate and MG132. In addition, glutamate induced NF-kappaB translocation to the nucleus, an effect that was inhibited by MG132. Taken together, our data suggest that activation of NMDA receptors by glutamate released in ischemia is involved in the expression of iNOS in rat forebrain slices via a Ca(2+)-dependent activation of the transcription factor NF-kappaB. To our knowledge, this is the first report showing an implication of excitatory amino acids in the expression of iNOS caused by ischemia.

Production of nitric oxide by glial cells: regulation and potential roles in the CNS

Roles proposed for nitric oxide (NO) in CNS pathophysiology are increasingly diverse and range from intercellular signaling, through necrotic killing of cells and invading pathogens, to the involvement of NO in apoptosis and tissue remodeling. In vitro evidence and observations from experimental animal models of a variety of human neuropathologies, including stroke, indicate that glial cells can produce NO. Regulation of at least one of the NO synthase genes (NOS-2) in glia has been well described; however, apart from hints emerging out of co-culture studies and extrapolation based upon the reactivity of NO, we are a long way from identifying functions for glial-derived NO in the CNS. Although the assumption is that NO is very often cytotoxic, it is evident that NO production does not always equate with tissue damage, and that both the cellular source of NO and the timing of NO production are important factors in terms of its effects. With the development of strategies to transfer or manipulate expression of the NOS genes in specific cells in situ, the ability to deliver NO into the CNS via long-lived chemical donors, and the emergence of more selective NOS inhibitors, an appreciation of the significance of glial-derived NO will change.

Changes in GluR4 expression induced by metabotropic receptor activation in radial glia cultures

The expression of neurotransmitter receptors in glial cells has suggested a regulatory role of these cells in synaptic function. In radial glia, glutamate receptors elicit a cascade from the membrane to the nucleus and a consequent change in gene expression. In order to gain insight into this process, we address the question of whether receptor activation leads to changes in the repertoire of AMPA/KA glutamate receptor subunits in Bergmann and Müller glial cells. Of the subunits investigated, only GluR4 was up-regulated in Bergmann glial cells both at mRNA and protein levels. In contrast, in Müller glial cells Glu treatment leads to a reduction in GluR4 mRNA and protein expression. Both effects are receptor-mediated and must probably involve group I of metabotropic glutamate receptors. Accordingly, using Northern blot analysis and RT-PCR we detected the expression of both mGluR1 and mGluR5 transcripts in the cultured cells. Our results confirm that glutamate receptors in Bergmann and Müller cells modulate gene expression and further strengthen a plausible role of glial cells in long-lasting changes in the central nervous system.

Expression of nitric oxide synthase-2 in glia associated with CNS pathology

This chapter discusses the expression of nitric oxide synthase-2 (NOS-2) in glia associated with central nervous system (CNS) pathology. The production of nitric oxide (NO) in the nervous system is catalyzed by three, highly homologous isoforms of NO synthase (NOS). NOS-2, the dimeric, heme-containing, soluble protein whose activity is independent of a rise in intracellular calcium, is variously termed ‘inducible,’ ‘immunologic,’ and ‘macrophage NOS (macNOS).’ Nitric oxide inhibits not only NOS-2 activity but also regulates the level of NOS-2 messenger RNA (mRNA) expression through a mechanism involving NF-^K B. There is specific evidence for the glial expression of NOS-2 associated with neuronal injury and infection of the CNS and in neurodegenerative and demyelinating diseases. Direct injury in the CNS results in a reactive gliosis, characterized by the induction of the glial fibrillary acidic protein gene and changes in astrocyte morphology.