**2.3.5 Immune response**

610 Pharmacology

The developmental generation of astrocytes tends to occur after the initial production of neurons in many CNS regions (Sofroniew & Vinters, 2010). During development of the brain, astrocytes (radial glia) take part in guiding the migration of developing axons and certain neuroblasts (Powel & Geller, 1999). In addition, substantive evidence is accumulating that astrocytes are essential for the formation and function of developing synapses by releasing molecular signals such as thrombospondin (Barres, 2008; Christopherson et al., 2005). Astrocytes appear also to influence developmental synaptic pruning by releasing signals that induce expression of complement C1q in synapses and

Together with brain microvascular endothelial cells astrocytes create the blood-brain barrier that protects the brain from toxic substances in the blood, supplies the brain tissues with nutrients, and filters harmful substances from the brain back to the bloodstream, enabling the proper environment in the CNS. Astrocytes may regulate endothelial cell metabolism, and vasoconstriction and vasodilatation by producing substances with angiogenic properties, such as endothelial growth factor (Proia et al., 2008), ATP (Leybaert et al., 2004), and arachidonic acid, prostagladins and nitric oxide, (Gabryel et al., 2007; Sofroniew & Vinters, 2010), that can increase or decrease CNS blood vessel diameter and blood flow in a coordinated manner. Moreover, astrocytes may be primary mediators of changes in local CNS blood flow in response to changes in neuronal activity (Koehler et al., 2009). Thus, astrocytes play important functions at the level of arterioles where blood flow is controlled, at the level of capillaries where blood-brain barrier is located and at the level of blood

Astrocytes play a number of other functions which are crucial for the maintenance of homeostasis and neuronal function. They provide energy supply to neurons and coordinate metabolic reactions. Astrocytes are the principal storage sites of glycogen granules in CNS. The greatest accumulation of astrocytic glycogen occurs in areas of high synaptic density, and its utilisation can sustain neuronal activity during hypoglicemia and during periods of

Astrocytes regulate the external chemical environment by removing excess ions notably potassium, regulate brain cell volume, and participate in recycling neurotransmitters released during synaptic transmission by expressing high levels of transporters for neurotransmitters such as glutamate, GABA, histamine and glycine, that serve to clear the neurotransmitters from the synaptic space. Astrocytes also represent the major site for the

There is accumulating evidence that astrocytes play direct roles in synaptic transmission through the regulated release of synaptically active molecules including glutamate, purines

**2.3 The role of astroytes in the central nerve system** 

**2.3.1 Astrocytes and development of central nerve system** 

thereby tag them for elimination by microglia (Barres, 2008).

**2.3.2 Blood-brain barrier and regulation of blood flow** 

immune cells (Leybaert et al., 2004).

**2.3.4 Synapse function** 

**2.3.3 Energy, metabolism and homeostasis** 

high neuronal activity (Sofroniew & Vinters, 2010).

detoxification or bioactivation of neurotoxins (Perdan et al., 2009).

Astrocytes importantly contribute to creation of immune response in the brain. They are an important source of several cytokines and neurotrophic factors in the CNS that have a crucial immunoregulatory role and also promote neuronal survival and neurite growth (Lipnik-Štangelj, 2006). Moreover, cytokines have an impact on neurotoxicity, synaptic transmission and synaptic plasticity in the brain (Allan & Rothwell, 2001). Activation of astrocytes leads to up-regulation of pro-inflammatory cytokines like interleukin-1 beta (IL-1beta), tumour necrosis factor alpha (TNF-alpha), interleukin-6 (IL-6), and inducible nitric oxide synthase (iNOS), and cyclooxygenase 2 (COX2) (Gonzalez & Salido, 2009; Sofroniew & Vinters, 2010).

Research into the actions of IL-1beta in the brain initially focused on its role in host defence responses to systemic disease. IL-1beta can also elicit an array of responses which could inhibit, exacerbate or induce neuronal damage and death (Gonzalez & Salido, 2009).

TNF-alpha has an important function in neurotoxicity, synaptic transmission and synaptic plasticity. It influences homeostatic synaptic scaling by inducing the insertion of AMPA receptors at post-synaptic membranes (Stellwagen & Malenka, 2006). In addition, TNF-alpha may have a pivotal role in augmenting intracerebral immune responses and inflammatory demyelination due to its diverse functional effects on glial cells, such as oligodendrocytes and astrocytes themselves (Šarc et al., 2011).

Unlike TNF-alpha, which is a prototypical pro-inflammatory cytokine, IL-6 affects inflammation and neuronal regeneration via a number of mechanisms. In this sense, besides its immunoregulatory role, IL-6 can also promote neuronal survival and neurite growth.IL-6 can be induced by a variety of molecules including IL-1beta, TNF-alpha, transforming growth factor-beta and prostaglandins, and many other mediators such as beta-amyloid, interferon-g and IL-4 can potentiate these primary inducers, highlighting the complex nature of IL-6 modulation (Šarc et al., 2011; Gonzalez & Salido, 2009).

#### **2.4 Reactive gliosis and glial scar formation**

After brain injury, such as a stroke or trauma, astrocytes become reactive, and can undergo to profound proliferation, forming gliosis near or at the site of damage. Astrocyte activity is marked by hypertrophy, resulting in an expression of protein such as glial fibrillary acidic protein (GFAP), adhesion molecules and antigen presenting capabilities, including major histocompatibility antigens. Reactive astrocytes represent an obstacle preventing establishment of normal neural contact and circuitry. On the other hand, reactive astrocytes produce a myriad of neurotoxic substances in various brain pathologies (Mori et al., 2006).

Ethanol Toxicity in the Brain: Alteration of Astroglial Cell Function 613

releases ATP, to intense tissue injury and cell death that release various intracellular molecules

It is becoming clear that different molecular, morphological, and functional changes in reactive astrocytes are specifically controlled by inter- and intra-cellular signalling mechanisms that reflect the specific contexts of the stimuli and produce specific and gradated responses of reactive astrogliosis. For a long, reactive gliosis and scar formation have been recognized as the main impediment to functional recovery after CNS injury or disease. This absolutely negative viewpoint of reactive astrogliosis is no longer tenable and it is now clear from many different lines of experimental evidence that there is a normal process of reactive astrogliosis that exerts essential beneficial functions and does not do harm. As reviewed in detail elsewhere, many studies using transgenic and experimental animal models provide compelling evidence that reactive astrocytes protect CNS cells and tissue by uptake of potentially excitotoxic glutamate, protection from oxidative stress via glutathione production (Dringen et al., 2000), neuroprotection via adenosine release (Lin et al., 2008), protection from NH4 toxicity (Rao et al., 2005), neuroprotection by degradation of amyloid-beta peptides (Koistinaho et al., 2004), facilitating blood brain barrier repair, reducing vasogenic edema after trauma (Bush et al., 1999), stroke or obstructive hydrocephalus, stabilizing extracellular fluid and ion balance and reducing seizure threshold (Zador et al., 2009), and limiting the spread of inflammatory cells or infectious agents from areas of damage or disease into healthy CNS parenchyma (Bush et al., 1999;

The deleterious effects of ethanol in CNS could result either from a direct toxic effect of ethanol or from an indirect effect involving its metabolites and/or ROS generation. Ethanol can induce several cellular reactions which result in a modification of cellular redox status that can severely affect the cell's capacity to be protected against the endogenous production of ROS (Gonthier et al., 2004). The consequences derived from the effects of ethanol on cellular structures would end in a morphological and functional impairment of cellular physiology. Among brain cells, astrocytes seem less vulnerable than neurons, but their impairment can

In the CNS, astrocytes represent the major cellular localisation of ethanol metabolism, and have been postulated to protect neurons from ethanol-induced oxidative stress (Watts et al., 2005). The exact enzymatic mechanism responsible for ethanol oxidation in the brain is not

Ethanol is normally metabolised in the liver to acetaldehyde by the alcohol dehydrogenase reaction, and acetaldehyde can be further metabolised to acetic acid via aldehyde dehydrogenase reaction. The last step in the pathway is the conversion of acetic acid to acetyl-Co-A. Although theoretically the activity of the latter enzyme is high enough to cope with the rate at which ethanol is oxidized by alcohol dehydrogenase, there is a limit to the rate at which the reaction can continue and can therefore lead to accumulation of acetaldehyde, which is toxic for most tissues, including CNS. Thus, there is always a build

dramatically affect neurons because of their protective role towards neurons.

that signal intense tissue damage (Sofroniew, 2009).

Voskuhl et al. 2009).

clear yet.

**3. Ethanol in the central nerve system** 

**3.1 Ethanol metabolism in the brain** 

Although reactive astrogliosis is used widely as a pathological hallmark of diseased CNS tissue, definitions of reactive astrogliosis can vary considerably among authors and there are no widely accepted categories of intensity or severity. Recently proposed definition encompasses four key features: (1) reactive astrogliosis is a spectrum of potential molecular, cellular and functional changes in astrocytes that occur in response to all forms and severities of CNS injury and disease including subtle perturbations, (2) the changes undergone by reactive astrocytes vary with severity of the insult along a gradated continuum of progressive alterations in molecular expression, progressive cellular hypertrophy, and in severe cases, proliferation and scar formation, (3) the changes of reactive astrogliosis are regulated in a context-specific manner by inter- and intracellular signalling molecules, (4) the changes undergone during reactive astrogliosis have the potential to alter astrocyte activities both through gain and loss of function that can impact both beneficially and detrimentally on surrounding neural and non-neural cells. Of particular interest as regards function of reactive astrocytes, is recent evidence that reactive astrogliosis and glial scar formation play essential roles in regulating CNS inflammation (Sofroniew, 2009).

In response to different kind of stimulation, reactive astrocytes can make many different kinds of molecules with either pro- or anti-inflammatory potential (John et al., 2003). There is a normal process of reactive astrogliosis and glial scar formation that exerts various beneficial functions including protecting neural cells and function, restricting the spread of inflammation, and promoting tissue repair.

On the contrary, in a manner analogous to inflammation, reactive astrogliosis also has the potential to exert detrimental effects. For example, reactive astrocytes can be stimulated by specific signalling cascades to gain of detrimental effects such as exacerbating inflammation via cytokine production (Brambilla et al., 2009), producing neurotoxic levels of ROS (Hamby et al., 2006), releasing potentially excitotoxic glutamate (Takano et al., 2005), potential contribution to seizure genesis (Tian et al., 2005), compromising blood brain barrier function due to VEGF-production (Argaw et al., 2009), causing cytotoxic edema during trauma and stroke (Zador et al., 2009), and contributing to chronic pain (Milligan et al., 2009).

#### **2.4.1 Molecular mechanisms of reactive gliosis and scar formation**

Many different types of intercellular signalling molecules are able to trigger reactive astrogliosis or to regulate specific aspects of reactive astrogliosis, including large polypeptide growth factors and cytokines such as IL-1, IL6, IL-10, TNF-alpha, tumour growth factor beta (TGF-beta), mediators of innate immunity such as lipopolysaccharide and other Toll-like receptor ligands, neurotransmitters such as glutamate and noradrenalin, purines such as ATP, ROS including nitric oxide (NO), hypoxia and glucose deprivation, products associated with neurodegeneration such as beta-amyloid, molecules associated with systemic metabolic toxicity such as NH4, and regulators of cell proliferation such endothelin-1, as reviewed in detail elsewhere (Sofroniew, 2009). Such molecular mediators of reactive astrogliosis can be released by all cell types in CNS tissue, including neurons, microglia, oligodendrocyte lineage cells, pericytes, endothelia, and astrocytes, in response to all forms of CNS insults, ranging from subtle cellular perturbations that release some of the specific factors just listed, to cell stretching as might be encountered during acceleration/deceleration CNS injury and which

Although reactive astrogliosis is used widely as a pathological hallmark of diseased CNS tissue, definitions of reactive astrogliosis can vary considerably among authors and there are no widely accepted categories of intensity or severity. Recently proposed definition encompasses four key features: (1) reactive astrogliosis is a spectrum of potential molecular, cellular and functional changes in astrocytes that occur in response to all forms and severities of CNS injury and disease including subtle perturbations, (2) the changes undergone by reactive astrocytes vary with severity of the insult along a gradated continuum of progressive alterations in molecular expression, progressive cellular hypertrophy, and in severe cases, proliferation and scar formation, (3) the changes of reactive astrogliosis are regulated in a context-specific manner by inter- and intracellular signalling molecules, (4) the changes undergone during reactive astrogliosis have the potential to alter astrocyte activities both through gain and loss of function that can impact both beneficially and detrimentally on surrounding neural and non-neural cells. Of particular interest as regards function of reactive astrocytes, is recent evidence that reactive astrogliosis and glial scar formation play essential roles in regulating CNS inflammation

In response to different kind of stimulation, reactive astrocytes can make many different kinds of molecules with either pro- or anti-inflammatory potential (John et al., 2003). There is a normal process of reactive astrogliosis and glial scar formation that exerts various beneficial functions including protecting neural cells and function, restricting the spread of

On the contrary, in a manner analogous to inflammation, reactive astrogliosis also has the potential to exert detrimental effects. For example, reactive astrocytes can be stimulated by specific signalling cascades to gain of detrimental effects such as exacerbating inflammation via cytokine production (Brambilla et al., 2009), producing neurotoxic levels of ROS (Hamby et al., 2006), releasing potentially excitotoxic glutamate (Takano et al., 2005), potential contribution to seizure genesis (Tian et al., 2005), compromising blood brain barrier function due to VEGF-production (Argaw et al., 2009), causing cytotoxic edema during trauma and

Many different types of intercellular signalling molecules are able to trigger reactive astrogliosis or to regulate specific aspects of reactive astrogliosis, including large polypeptide growth factors and cytokines such as IL-1, IL6, IL-10, TNF-alpha, tumour growth factor beta (TGF-beta), mediators of innate immunity such as lipopolysaccharide and other Toll-like receptor ligands, neurotransmitters such as glutamate and noradrenalin, purines such as ATP, ROS including nitric oxide (NO), hypoxia and glucose deprivation, products associated with neurodegeneration such as beta-amyloid, molecules associated with systemic metabolic toxicity such as NH4, and regulators of cell proliferation such endothelin-1, as reviewed in detail elsewhere (Sofroniew, 2009). Such molecular mediators of reactive astrogliosis can be released by all cell types in CNS tissue, including neurons, microglia, oligodendrocyte lineage cells, pericytes, endothelia, and astrocytes, in response to all forms of CNS insults, ranging from subtle cellular perturbations that release some of the specific factors just listed, to cell stretching as might be encountered during acceleration/deceleration CNS injury and which

stroke (Zador et al., 2009), and contributing to chronic pain (Milligan et al., 2009).

**2.4.1 Molecular mechanisms of reactive gliosis and scar formation** 

(Sofroniew, 2009).

inflammation, and promoting tissue repair.

releases ATP, to intense tissue injury and cell death that release various intracellular molecules that signal intense tissue damage (Sofroniew, 2009).

It is becoming clear that different molecular, morphological, and functional changes in reactive astrocytes are specifically controlled by inter- and intra-cellular signalling mechanisms that reflect the specific contexts of the stimuli and produce specific and gradated responses of reactive astrogliosis. For a long, reactive gliosis and scar formation have been recognized as the main impediment to functional recovery after CNS injury or disease. This absolutely negative viewpoint of reactive astrogliosis is no longer tenable and it is now clear from many different lines of experimental evidence that there is a normal process of reactive astrogliosis that exerts essential beneficial functions and does not do harm. As reviewed in detail elsewhere, many studies using transgenic and experimental animal models provide compelling evidence that reactive astrocytes protect CNS cells and tissue by uptake of potentially excitotoxic glutamate, protection from oxidative stress via glutathione production (Dringen et al., 2000), neuroprotection via adenosine release (Lin et al., 2008), protection from NH4 toxicity (Rao et al., 2005), neuroprotection by degradation of amyloid-beta peptides (Koistinaho et al., 2004), facilitating blood brain barrier repair, reducing vasogenic edema after trauma (Bush et al., 1999), stroke or obstructive hydrocephalus, stabilizing extracellular fluid and ion balance and reducing seizure threshold (Zador et al., 2009), and limiting the spread of inflammatory cells or infectious agents from areas of damage or disease into healthy CNS parenchyma (Bush et al., 1999; Voskuhl et al. 2009).
