**3. Mitochondrial impairment as a possible mechanism of ethanol toxicity in the brain**

Mitochondrial alterations associated with binge alcohol drinking has not being described. For example, binge ethanol exposure in early postnatal stage induced defects in insulin and IGF signaling, resulting in impaired motor abilities [126]. Most important, deficiencies in insulin/IGF-1 signaling are associated to oxidative stress, DNA injury, loss of mitochondrial function, and apoptosis [126]. In other studies, rats submitted to binge alcohol drinking protocol showed an increase in the ventricular volume of the brain; reduced levels of N-acetyl aspartate (NAA) and creatine and increased choline-containing compound in the dorsal hippocampus, effects that were recovered after 7 days of alcohol treatment [127]. These metabolic changes could suggest transient deficiencies of mitochondrial NAA production, resulting finally in an impaired energy production of the brain immediately after binge etha-

Currently, the most complete study associated to mitochondrial alterations corresponds to our findings [128]. In our study, adolescent rats PND25, were exposed to binge-like ethanol consumption using the same protocol described previously [128]. The rats were euthanized at 1, 3, or 7 weeks after treatment. Our results show that binge ethanol pretreatment (BEP) triggers alterations in hippocampal cell structure and function [128]. We found increase in oxidized proteins at 1-week posttreatment, which was subsequently restored at 3 and 7 weeks post BEP [128]. Additionally, proteins participating in the regulation of mitochondrial dynamics were affected. Mitochondrial dynamics involves fission and fusion events [129]. Fusion is mediated by the dynamin-related GTPases mitofusins (Mfn1 and Mfn2) and optic atrophy 1 (OPA1), which induce fusion of mitochondrial membranes [130]. Fission is performed by dynamin-related protein 1 (Drp1), which is recruited by the Fis1 protein [129] to the mitochondrial membrane to constrict mitochondria inducing its division [130]. BEP altered Drp1, Fis1, Mfn1, Mfn2, and Opa1 proteins levels [128], suggesting that binge-like ethanol consumption favors an pro-fission state at 1–3 weeks post BEP, and imbalance between fusion and fission proteins was restored but animals with 7 weeks after ethanol treatment showed reduced both mitochondrial fusion and fission

Interestingly, a decrease in ATP production was also reported in our study at 3 and 7 weeks post BEP [128], indicating a specific bioenergetics mitochondrial failure induced by binge ethanol exposure that persists over time. Finally, we observed a delayed increase in the expression of inflammatory markers such as NF-κB, Iba1, and GFAP, 7 weeks post BEP [128], suggesting a late inflammatory response due to adolescent ethanol consumption. Despite the damage present in the hippocampus of BEP rats, we detected the activation of a mechanism that could possibly protect the brain. These mechanisms include decreased expression of proteins that are key to mPTP formation, such as voltage-dependent anion channel (VDAC) and cyclophilin D (Cyp-D) [128], and the increased levels of nuclear factor erythroid-2 related factor 2 (Nrf2) [128], a transcription factor that is activated under stress conditions regulating the expression of antioxidant, anti-inflammatory, and detoxification enzymes that prevent mitochondrial impairment [131]. The increased Nrf2 levels were mainly observed at 3 and 7 weeks [128] suggesting that this signaling pathway could be involved in restoring

nol consumption [127].

370 Mitochondrial Diseases

redox balance.

processes compared to saline treated animals [128].

In the previous sections, we described the effects of ethanol on neuronal cells, focusing our attention on alterations to the mitochondria. In all patterns of ethanol consumption described above, we reported changes associated with mitochondrial injury [11, 27, 132], therefore strongly suggesting that mitochondria is an important mediator of ethanol neurotoxicity and could be considered as a potential therapeutic approaches for treating ethanol-associated disorders. In this section, we will summarize the effects of ethanol linked to mitochondrial function, and finally, we propose a mechanism in which mitochondrial impairment plays a central role in the neurotoxicity induced by ethanol.

**Figure 1.** Alterations of mitochondrial health during ethanol consumption. A representative scheme is summarizing the mitochondrial alterations described in the different patterns of ethanol consumption. The severity of mitochondrial dysfunction is associated with the amount and time of ethanol exposure, starting with an imbalance in redox response, accompanied by decreased ATP production and the opening of the mPTP (1. Acute ethanol toxicity). Additionally, decreased activity of the electron transporter chain and the loss of mitochondrial membrane potential are observed (2. Hangover); following of high Ca+2 concentrations (3. Chronic ethanol toxicity) that lead finally to prolonged opening of mPTP that eventually triggers neuronal death (4. Withdrawal).

**Figure 1** summarizes the mitochondrial alterations described after the different patterns of ethanol consumption (**Figure 1**). Acute ethanol exposure triggers mitochondrial toxicity, increasing ROS production and decreasing antioxidant defenses [38]. In turn, this leads to reduced ATP production [41] and finally to the opening of mPTP [26]. All these events could eventually lead to apoptotic neuronal death [32, 39] (**Figure 1.1**). After ethanol consumption, a hangover condition can be produced [58]. In this state, the mitochondrial alterations described in acute ethanol conditions are also accompanied by a period of reduced activity of the electron respiratory chain complexes [69] and a loss of mitochondrial membrane potential [69] that suggests a major state of mitochondrial dysfunction (**Figure 1.2**).

aggravated, mainly by the high amount of Ca+2 that enters to the mitochondria [110, 111], which leads to a prolonged opening of mPTP [114, 115] and neuronal death [22] (**Figure 1.4**). In summary, the mitochondrion plays a central role in all these patterns of ethanol consumption, and its alterations gradually increase according to the amount and time of etha-

Ethanol Consumption Affects Neuronal Function: Role of the Mitochondria

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

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Binge drinking ethanol is associated with the adolescent population [116]. Diverse molecular alterations have been reported; however, the cellular mechanism involved in this process are unknown [11]. We recently indicated that adolescent binge ethanol consumption triggers cellular damage by a mechanism that probably involves the mitochondria [128]. Also, the mitochondrial alterations persist over time until adulthood [128]. **Figure 2** summarizes the mitochondrial changes induced by adolescent binge ethanol exposure. One week after BEP, the mitochondria showed increased protein oxidation, reduced expression of Cyp-D, and increased expression of proteins involved in mitochondrial fission, suggesting a mitochondrial pro-fission state [128]. Then, 3 weeks after BEP challenging the mitochondrial pro-fission state is less severe, indicated by a lower reduction in the expression of fission proteins; however, a decrease in VDAC protein, and a significant reduction of ATP, is also observed [128]. Finally, during adulthood, the balance in the fission/fusion state is restored, but the dynamics are decreased compared with the control condition. Also, the ATP deficiency persists and becomes even more drastic [128]. Nevertheless, the reduced expression of VDAC and Cyp-D suggests the activation of a protective mechanism that could prevent the opening of mPTP [128]. Altogether, these alterations indicate that mitochondria have an important role in the binge ethanol toxicity in the hippocampus of

We propose that the mitochondrion is the main mediator of ethanol neurotoxicity where mitochondrial alterations reveal the severity of ethanol toxicity. The initial effect of ethanol exposure implicates an imbalance in the cellular redox state, followed by changes in the respiratory complexes from the electron transport chain that leads to the reduction in ATP production and the opening of mPTP. Persistent ethanol consumption also induced the loss of mitochondrial membrane potential and increased Ca+2 entries into the mitochondria that provoke the prolonged opening of mPTP and finally promote neurodegeneration. Interestingly, these mitochondrial alterations ethanol-associated may could occur mainly in glial cells, inducing inflammation and interfering with the glial-neuronal communication in specific brain areas [119]. The hippotalamus, important to ethanol dependence, and the hippocampus, associated to learning and memory, are particularly vulnerable; possibly due to the downregulation of melanocortin system induced by ethanol [119]. Therefore, the description of all these events highlights the importance of maintaining the function of the mitochondria to prevent the harmful effects of ethanol consumption and propose a new potential treatment for the patho-

nol exposure.

adolescent rats.

**4. Future perspectives**

logical condition related to ethanol use and abuse.

When ethanol consumption implicates the ingestion of a high amount of ethanol on repeated occasions and cannot be controlled, this is considered a pattern of chronic ethanol consumption [81–83]. This condition is pathological, and therefore the mitochondrial alterations are also more complex. In addition to redox imbalance [23, 94], deficiency in ATP generation and a loss of mitochondrial membrane potential can be detected by a reduction in the expression of respiratory complexes and increased levels of mitochondrial calcium [23, 94, 95]. Altogether, these events lead to severe loss of mitochondrial function (**Figure 1.3**). Chronic ethanol consumption implicates the development of addictive behaviors; therefore, in the absence of ethanol, those affected present ethanol withdrawal symptoms [87, 101]. In this state, the mitochondrial effects already described in chronic consumption are

**Figure 2.** Ethanol binge-drinking affects mitochondrial structure and function. Binge-like ethanol consumption during the adolescence induces changes in the structure and function of the mitochondria that persist on time until adulthood. One week after binge ethanol pretreatment (BEP), rat hippocampus has increased expression of fission proteins and protein oxidation, accompanied by reduced expression of Cyp-D. Three weeks after BEP, addition is possibly observed decreased ATP production and reduced expression of VDAC. Finally, at adulthood (7 weeks post BEP), the levels of both fission and fusion proteins suggest decreased mitochondrial dynamics, and the deficiency in ATP production is more severe.

aggravated, mainly by the high amount of Ca+2 that enters to the mitochondria [110, 111], which leads to a prolonged opening of mPTP [114, 115] and neuronal death [22] (**Figure 1.4**). In summary, the mitochondrion plays a central role in all these patterns of ethanol consumption, and its alterations gradually increase according to the amount and time of ethanol exposure.

Binge drinking ethanol is associated with the adolescent population [116]. Diverse molecular alterations have been reported; however, the cellular mechanism involved in this process are unknown [11]. We recently indicated that adolescent binge ethanol consumption triggers cellular damage by a mechanism that probably involves the mitochondria [128]. Also, the mitochondrial alterations persist over time until adulthood [128]. **Figure 2** summarizes the mitochondrial changes induced by adolescent binge ethanol exposure. One week after BEP, the mitochondria showed increased protein oxidation, reduced expression of Cyp-D, and increased expression of proteins involved in mitochondrial fission, suggesting a mitochondrial pro-fission state [128]. Then, 3 weeks after BEP challenging the mitochondrial pro-fission state is less severe, indicated by a lower reduction in the expression of fission proteins; however, a decrease in VDAC protein, and a significant reduction of ATP, is also observed [128]. Finally, during adulthood, the balance in the fission/fusion state is restored, but the dynamics are decreased compared with the control condition. Also, the ATP deficiency persists and becomes even more drastic [128]. Nevertheless, the reduced expression of VDAC and Cyp-D suggests the activation of a protective mechanism that could prevent the opening of mPTP [128]. Altogether, these alterations indicate that mitochondria have an important role in the binge ethanol toxicity in the hippocampus of adolescent rats.
