**2.1. Acute ethanol consumption**

Excessive ethanol use results in brain intoxication, leading to motor and behavior alterations, and eventually to death as a consequence of the depressive effects on the central nervous system (CNS) [6]. These effects result in simultaneous alterations in neuronal circuits including the prefrontal cortex, which controls behavior [7]; the cerebellum, which regulates movement and coordination [8]; the frontal lobe, which controls emotions [9]; the reticular activating system, which determines the sleep-wake cycle [10]; the hippocampus, which mediates learning and memory [8]; and the medulla, which controls vital functions [6]. Ethanol intoxication induces cellular damage and neuronal death [11]. The precise mechanism participating in ethanol toxicity in the brain is unknown. However, at the cellular level, ethanol impairs the neurotransmitters signaling [12]. Also, ethanol promotes reactive oxygen species (ROS) production [13] and activates inflammatory processes [14]. Altogether these events could be

Mitochondria are dynamic organelles, which regulate the production of ATP, redox balance, and calcium homeostasis in the neuron [15]. Interestingly, many effectors described in ethanol toxicity are directly or indirectly related to mitochondria. Mitochondria are the main source of ROS in the brain, and they are mainly affected by the oxidative damage induced by ethanol intoxication [16]. Likewise, dysfunctional mitochondria play a role in inducing proinflammatory events [17]. Finally, during synaptic process, ATP production and calcium buffering capacity produced by mitochondria are critical [18, 19]; therefore, mitochondrial injury may

In fact, evidence suggests that ethanol produces catastrophic changes in the mitochondria of organs such as liver [20] and heart [21], and over the last decade, many studies have reported the toxicity of ethanol to the brain's mitochondria [22, 23]. Briefly, ethanol increases ROS production [23], alters mitochondrial respiration [23, 24], impairs ATP production [22, 25], and eventually induces cell death by opening the mitochondrial permeability transition pore

In this chapter, we will discuss the effects of ethanol toxicity in the brain, focusing on the mitochondria. We will describe the specific ethanol-induced alterations to mitochondrial integrity,

**1.** Acute ethanol toxicity, corresponding to the consumption of high ethanol concentrations

**2.** Hangover, a common term to describe the physical effects following excessive ethanol consumption. Veisalgia is the uncommonly used medical name for this condition [25].

**3.** Chronic ethanol consumption, a condition where ethanol intake lasts 3 months or more. High concentrations of ethanol consumed over time can trigger ethanol use disorder (AUD),

**4.** Ethanol withdrawal, a condition observed in individuals who have consumed a high amount of ethanol for a prolonged period followed by cessation in ethanol intake. Com-

mon symptoms are anxiety and shakiness, seizures, and eventually death [22].

dynamics, and bioenergetics in different scenarios of ethanol exposure including:

for a short period, spanning over hours, and even days [26].

responsible for ethanol-induced damage in the brain.

362 Mitochondrial Diseases

have severe consequences on neuronal communication.

(mPTP) [26], observed both *in vitro* and *in vivo* [27].

commonly called alcoholism [23].

Acute ethanol consumption refers to a high ingestion of ethanol at a rate faster than that at which the body can metabolize it. Acute ethanol intoxication leads to brain injury, resulting in significant alterations of brain structure and function, and induces neuronal apoptosis and neurodegeneration in mouse, rat, and cellular models [11, 29–33].

Although several studies have tried to explain how acute ethanol administration induces brain injury, data on pathophysiology and underlying molecular and cellular mechanisms are still insufficient [34]. One of the possible theories was shown *in vitro*. Acute ethanol exposure induces neuronal apoptosis and changes to the neuronal structure, which could be related to the development of mature synapses and lead to a deficit in brain development [35]. Another alternative is that ethanol triggers inflammatory processes by activating toll-like receptor (TLR4) signaling and down-regulating autophagy pathways that trigger cell death [36]. However, and most importantly, oxidative stress appears to be the main mechanism for explaining brain injury mediated by ethanol. Oxidative stress is significantly increased following ethanol administration in several animal and cellular models [32, 37–39].

Rats exposed to ethanol through gavage administration showed an increase in inflammatory and oxidative stress markers in the brain 1 h post ethanol exposure [38]. Also, pre-treatment for 3 days with 150 mg/kg of antioxidant vitamin E decreased inflammation, an effect that is not observed with other antioxidants such as *N*-acetylcysteine (NAC) or selenium [38]. Similarly, ethanol-related increases in ROS generation are a prime factor in ethanol neurotoxicity. Primary cortical neurons treated with 2.5 mg/mL ethanol for 24 h elicit a rapid onset of oxidative stress, which resulted in mitochondria-dependent apoptotic cell death both in cultured fetal rat cortical neurons and during embryonic development [32, 39]. Also, ethanol downregulates protective cellular antioxidant content in this neuronal model, thus seriously disturbing the cellular redox state [32]. Indeed, pretreatment with NAC increased cellular glutathione and prevented apoptosis, suggesting that oxidative stress precedes a cascade of events mediated by mitochondria. Prevention of apoptosis with NAC antioxidant supports the role of oxidative stress in neuronal death [32].

The mitochondria appear to be a major target of ethanol toxicity in the brain. Some studies suggest that disturbances of the integrity of the mitochondrial membrane are essential for ethanol-induced cell death in mitochondria isolated from ethanol-exposed fetal brains [40]. For example, ethanol treatment for 24 h decreased ATP production and apparently impaired mitochondrial function [41]. Also, ethanol treatment reduced peroxisome proliferatoractivated receptor-gamma coactivator 1alpha (PGC-1α) promoter activity and expression. PCG-1α [41] is a transcriptional coactivator of peroxisome proliferator-activated gamma receptors (PPARγ) that regulates energy metabolism and mitochondrial biogenesis in the brain [42, 43]. These ethanol-mediated changes in PCG-1α could be involved in mitochondrial dysfunction and oxidative damage. In contrast, the overexpression of PCG-1α has shown protective effects against ethanol-induced neuronal death and toxicity [41].

Acetaldehyde, an ethanol metabolite, is the leading cause of hangover [66]. Acetaldehyde causes steatohepatitis, a condition characterized by inflammation of the liver with fat accumulation in this organ, hepatic cirrhosis, and downregulation of aldehyde dehydrogenase 2 (ALD2) expression due to mitochondrial dysfunction [67]. In the brain, an association between

Ethanol Consumption Affects Neuronal Function: Role of the Mitochondria

http://dx.doi.org/10.5772/intechopen.71611

During the hangover state, alterations like the impairment in motor performance have been associated with mitochondrial dysfunction [68]. In mice studies using the ethanol hangover onset protocol, animals showed impaired motor performance that could be the result of disturbed motor control [68]. Because motor performance is associated with the cerebellar function, the effects of the hangover were evaluated in cerebellum. Mitochondria isolated from the cerebellum of AHO mice revealed that at the onset of ethanol hangover, malate-glutamateand succinate-supported oxygen uptake is increased, accompanied by the decreased activity of mitochondrial I–III and IV respiratory complexes and reduced mitochondrial membrane potential [69]. Additionally, the opening of mPTP was reported [69]. Furthermore, the activity of antioxidant enzymes was also differentially affected; superoxide dismutase (SOD) and the monoamine oxidase (MAO) enzyme showed increased activity [69], whereas both catalase (CAT) and glutathione peroxidase (GPx) had decreased activity [69]. In this same context, neuronal nitric oxide synthase (nNOS) expression was reduced [69], indicating a specific

In the brain cortex, isolated mitochondria had increased hydrogen peroxide (H2

in ethanol hangover conditions [68]. Moreover, mitochondrial activity of I–III, II–III, and IV respiratory complexes and the membrane potential are reduced in hangover [68]. Also, in mitochondria from the brain cortex, NO production and NOS expression were decreased [68], and synaptic mitochondrial function was significantly affected [68]. Finally, mitochondria from the brain cortex of hangover mice are more prone to suffer damage, due to the opening

Interestingly, hangover provokes an imbalance in cellular redox homeostasis in isolated mitochondria in AHO mice brain cortex [70]. Decreased activity of both CAT and SOD enzymes accompanied by increased MAO activity was reported in mitochondria from both nonsynaptic extracts and synaptosome, whereas the GSH/GSSG ratio was decreased only in synaptosome mitochondria, with a reduction in both GPx and glutathione reductase (GR)

both mitochondrial pools [70], and treatment with diphenyl (a MAO inhibitor) prevented this increase in AHO in both non synaptic and synaptic mitochondria [70]. Altogether, these results show that ethanol hangover produces an imbalance in mitochondrial redox state, indicated by an overproduction of ROS and a decrease of antioxidant agents [70]. This evidence is consistent with previous studies that described oxidative stress as a key element of the hangover syndrome and suggests that antioxidants could suppress the toxicity caused by ethanol [71]. In fact, the importance of the antioxidant imbalance was confirmed by the administration of natural products to treat hangover mainly in liver and

O2

levels were higher in

O2

) levels

365

hangover and mitochondrial dysfunction has been proposed [68–70].

effect of ethanol against oxidative defenses in hangover.

of mPTP with dramatic consequences to neural cell survival [68].

and an increase in glutathione S-transferase (GST) [70]. Also, H2

brain [72].

Several reports indicate that acute ethanol induces neuronal death [44]. Bax is a proapoptotic protein that when it is activated, it is translocated to the mitochondrial membrane. In primary neuronal cultures, Bax dimerizes with protein inhibitors of apoptosis, such as Bcl-2 and Bcl-xL, in response to ethanol exposure, leading to cell death [44]. Ethanol exposure disrupts the mitochondrial membrane potential, increases ROS production, and finally induced apoptosis in cerebellar granule cells [45–48]. Interestingly, Bax could also interact with the mPTP [49], a high-conductance mitochondrial channel, involved in the mitochondrial permeability to ions and small solutes [50–52]. These result in a reduction in mitochondrial potential [53], dysregulated calcium homeostasis [54], increased ROS formation [55], decreased ATP production [56], and eventually lead to neuronal death [53].

Interestingly, ethanol treatment did not induce the mPTP opening in the isolated mitochondria from mice brain, suggesting that ethanol does not affect mitochondrial health directly in neurons [26]. Further studies showed that ethanol administration transiently decreased the oxygen consumption rate of the mouse brain, an effect that disappeared 24 h after the ethanol treatment ceased [26]. However, this impairment of mitochondrial respiratory function could contribute to spatial learning and memory impairment observed in young mice [57], and to the deficit in the nociceptive response, showed in infant mice [26]. Interestingly, mice treatment with mPTP inhibitors, such as cyclosporine A and nortriptyline, before the first ethanol injection improved the behavior of ethanol-exposed animals [26], highlighting the importance of mPTP opening in acute ethanol intoxication, despite that probably is an indirect effect.

All these evidences indicate that acute ethanol exposure induces an increase in the production of ROS that finally could lead to mitochondrial dysfunction through the opening of mPTP.
