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

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 dramatically affect neurons because of their protective role towards neurons.

#### **3.1 Ethanol metabolism in the brain**

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 clear yet.

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

Ethanol Toxicity in the Brain: Alteration of Astroglial Cell Function 615

primary cultures of rat astrocytes. Ethanol reduced endogenous levels of acive RhoA due to an increase in the activity of small Rho GTP-ases, reduced phosphoinositides levels and

Ethanol presents as well morphological effects on the developing adolescent brain. There were clear effects immediately and long after drinking cessation of a chronic ethanol administration on two neurotransmitter systems (the serotoninergic and nitrergic), which decreased, and the astrocytic cytoskeleton and neuron, which increased and decreased, respectively (Evrard et al., 2006). The authors concluded that drinking cessation can partially ameliorate the ethanol-induced morphological changes on neurons and astrocytes

Cholesterol is an essential component of cell membranes and plays an important rule in signal transduction. There are evidences that cholesterol homeostasis may be affected by ethanol, and this may be involved in neurotoxicity (Guizzetti & Costa, 2007). Indeed, the pathogenesis of Alzheimer's disease has been linked to altered cholesterol homeostasis in the brain. Several functions are carried out by cholesterol and are important for brain development, such as glial cell proliferation, synaptogenesis, neuronal survival and neurite outgrowth. In addition, the brain contains high level of cholesterol, mostly synthesized in situ. Furthermore, astrocytes produce large amounts of cholesterol that can be released by

It has been shown that chronic ethanol consumption affects the synaptic structure. The density of dendritic spines was found lower in the nucleus accumbens, and depicted an upregulation of a subunit of the NMDA receptor. The up-regulated NMDA receptor subunit is a splice variant isoform which is required for membrane-bound trafficking or anchoring into a spine synaptic site. These changes, evoked by ethanol, demonstrated an alteration of micro

Adermark and Loviger (2006) showed that ethanol inhibits a Ca2+-insensitive K+ channel activity, and affects gap junction coupling, demonstrating that astrocytes play a critical role in brain K+ homeostasis, and that ethanol effects on astrocytic function could influence

Finally, despite most of the investigations on the effects of ethanol have been performed following its addition to tissue or cell cultures, an interesting study has shown excessive activation of glutamatergic neurotransmission in the cerebral cortex following ethanol withdrawal and its contribution to significant behavioural disturbances and to alcohol craving. These effects were related to the activity of the enzyme glutamine synthetase,

Brain tissue is particularly vulnerable to oxidative damage, possibly due to its high consumption of oxygen and the consequent generation of high quantities of ROS during

which converts released glutamate to glutamine (Miguel-Hidalgo, 2006).

these cells and utilized by neurons to form synapses (Gonzalez & Salido, 2009).

induced changes in the dynamics and organization of the actin cytoskeleton.

but cannot fully return it to the basal state.

**3.2.2 The effects of ethanol on cholesterol homeostasis** 

**3.2.3 The effects of ethanol on synaptic structure** 

circuitry for glutamate reception (Zhou et al., 2007).

**3.2.4 Ethanol and glial oxidative stress** 

neuronal activity.

up of acetaldehyde which passes out from the liver into the blood, and this acetaldehyde is responsible or some of the unpleasant symptoms of alcohol excess. Once in the bloodstream, the acetaldehyde can also cross the blood-brain barrier and attack the CNS.

In the brain, ethanol can be metabolized by catalase, cytochrome P450 2E1, and alcoholdehydrogenase, with catalase, playing a pivotal role among the others (Gonzalez et al., 2007).

On the other hand, ethanol also induces up-regulation of antioxidant defences by increasing the enzymatic activities of superoxide-dismutase, catalase, and glutathione-peroxidase (Eysseric et al., 2000; Rathinam et al., 2006). The expression of heat shock proteins like HSP70 (Russo et al., 2001), which have a protective and stabilizing effect on stress-induced injury, is also induced by ethanol. Altogether, this would confer to astrocytes a survival advantage preventing oxidative damage.

#### **3.2 Ethanol influence on astrocyte function**

Ethanol has several targets in astrocytes and other cell types, impairing cellular redox status, cell growth and differentiation, interfering with the stimulatory effect of trophic factors or altering the expression of cytoskeletal proteins. In addition, ethanol induces astroglial activation, associated with up-regulation of several pro-inflammatory cytokines, that contribute to neuroinflammation, neurodegeneration and cell apoptosis (Alfonso-Loeches et al., 2010; Šarc & Lipnik-Štangelj, 2009).

#### **3.2.1 The effects of ethanol on developing central nerve system**

Ethanol is a known teratogen and has been implicated in the etiology of human fetal alcohol syndrome, which is characterized by distinct craniofacial abnormalities such as microcephaly, agnathia, and ocular aberrations. Prenatal ethanol exposure induces functional abnormalities during brain development affecting neurogenesis and gliogenesis. Thus, ethanol cases a number of changes in several neurochemical systems. Astrocytes are predominant source of postnatal retinoic acid synthesis in the cerebellum, and this acid shows teratogenic effects responsible for the fetal alcohol syndrome.

McCaffery et al. (2004) showed that ethanol could stimulate retinoic acid synthesis leading to abnormal embryonic concentrations of this morphogen and, thus, ethanol could represent a major cause of fetal alcohol syndrome. Additionally, increased sensitivity of glutamate receptors and enhanced trans-membrane transport of glutamate has been observed in the presence of ethanol. This was in relationship to the increase in the expression of the excitatory amino acid transporters EAAT1 and EAAT2. Thus, glutamatergic system is affected by ethanol, which can be viewed as a maladaptive process that disposes the developing brain to fetal alcohol syndrome (Zink et al. 2004).

Furthermore, ethanol affects the synthesis, intracellular transport, distribution, and secretin of N-glicoproteins in different cell types, including astrocytes and neurons (Braza-Boils t al., 2006). Glicoproteins, such as adhesion molecules and growth factors, participate in the regulation of nervous system development. Thus, the alteration in the glycosylation process induced by ethanol could be a key mechanism involved in the teratogenic effects of ethanol exposure on brain development. Further studies by Martinez et al., (2007) showed that longterm ethanol treatment substantially impairs glycosylation and membrane trafficking in

up of acetaldehyde which passes out from the liver into the blood, and this acetaldehyde is responsible or some of the unpleasant symptoms of alcohol excess. Once in the bloodstream,

In the brain, ethanol can be metabolized by catalase, cytochrome P450 2E1, and alcoholdehydrogenase, with catalase, playing a pivotal role among the others (Gonzalez et al., 2007). On the other hand, ethanol also induces up-regulation of antioxidant defences by increasing the enzymatic activities of superoxide-dismutase, catalase, and glutathione-peroxidase (Eysseric et al., 2000; Rathinam et al., 2006). The expression of heat shock proteins like HSP70 (Russo et al., 2001), which have a protective and stabilizing effect on stress-induced injury, is also induced by ethanol. Altogether, this would confer to astrocytes a survival

Ethanol has several targets in astrocytes and other cell types, impairing cellular redox status, cell growth and differentiation, interfering with the stimulatory effect of trophic factors or altering the expression of cytoskeletal proteins. In addition, ethanol induces astroglial activation, associated with up-regulation of several pro-inflammatory cytokines, that contribute to neuroinflammation, neurodegeneration and cell apoptosis (Alfonso-Loeches et

Ethanol is a known teratogen and has been implicated in the etiology of human fetal alcohol syndrome, which is characterized by distinct craniofacial abnormalities such as microcephaly, agnathia, and ocular aberrations. Prenatal ethanol exposure induces functional abnormalities during brain development affecting neurogenesis and gliogenesis. Thus, ethanol cases a number of changes in several neurochemical systems. Astrocytes are predominant source of postnatal retinoic acid synthesis in the cerebellum, and this acid

McCaffery et al. (2004) showed that ethanol could stimulate retinoic acid synthesis leading to abnormal embryonic concentrations of this morphogen and, thus, ethanol could represent a major cause of fetal alcohol syndrome. Additionally, increased sensitivity of glutamate receptors and enhanced trans-membrane transport of glutamate has been observed in the presence of ethanol. This was in relationship to the increase in the expression of the excitatory amino acid transporters EAAT1 and EAAT2. Thus, glutamatergic system is affected by ethanol, which can be viewed as a maladaptive process that disposes the

Furthermore, ethanol affects the synthesis, intracellular transport, distribution, and secretin of N-glicoproteins in different cell types, including astrocytes and neurons (Braza-Boils t al., 2006). Glicoproteins, such as adhesion molecules and growth factors, participate in the regulation of nervous system development. Thus, the alteration in the glycosylation process induced by ethanol could be a key mechanism involved in the teratogenic effects of ethanol exposure on brain development. Further studies by Martinez et al., (2007) showed that longterm ethanol treatment substantially impairs glycosylation and membrane trafficking in

the acetaldehyde can also cross the blood-brain barrier and attack the CNS.

**3.2.1 The effects of ethanol on developing central nerve system** 

shows teratogenic effects responsible for the fetal alcohol syndrome.

developing brain to fetal alcohol syndrome (Zink et al. 2004).

advantage preventing oxidative damage.

al., 2010; Šarc & Lipnik-Štangelj, 2009).

**3.2 Ethanol influence on astrocyte function** 

primary cultures of rat astrocytes. Ethanol reduced endogenous levels of acive RhoA due to an increase in the activity of small Rho GTP-ases, reduced phosphoinositides levels and induced changes in the dynamics and organization of the actin cytoskeleton.

Ethanol presents as well morphological effects on the developing adolescent brain. There were clear effects immediately and long after drinking cessation of a chronic ethanol administration on two neurotransmitter systems (the serotoninergic and nitrergic), which decreased, and the astrocytic cytoskeleton and neuron, which increased and decreased, respectively (Evrard et al., 2006). The authors concluded that drinking cessation can partially ameliorate the ethanol-induced morphological changes on neurons and astrocytes but cannot fully return it to the basal state.

#### **3.2.2 The effects of ethanol on cholesterol homeostasis**

Cholesterol is an essential component of cell membranes and plays an important rule in signal transduction. There are evidences that cholesterol homeostasis may be affected by ethanol, and this may be involved in neurotoxicity (Guizzetti & Costa, 2007). Indeed, the pathogenesis of Alzheimer's disease has been linked to altered cholesterol homeostasis in the brain. Several functions are carried out by cholesterol and are important for brain development, such as glial cell proliferation, synaptogenesis, neuronal survival and neurite outgrowth. In addition, the brain contains high level of cholesterol, mostly synthesized in situ. Furthermore, astrocytes produce large amounts of cholesterol that can be released by these cells and utilized by neurons to form synapses (Gonzalez & Salido, 2009).
