Alcoholism is a chronic disease characterized by trouble controlling the consumption of alcohol, dependence (needing to consume more to achieve the same effects), and withdrawal upon rapid cessation of drinking.Mayo Clinic Staff. (2014, December). Alcoholism. Available at http://www.mayoclinic.org/diseases-conditions/alcoholism/basics/definition/con-20020866 According to ARDI reports approximately 88,000 people had alcohol-related deaths in the United States between the years of 2006 and 2010.Centers for Disease Control and Prevention. (2013). Alcohol and Public Health: Alcohol Related Disease Impact (ARDI). Available athttp://nccd.cdc.gov/DPH_ARDI/Default.aspx. Furthermore, chronic alcohol use is consistently the third leading cause of death in the United States.Centers for Disease Control and Prevention. (2014, November). Alcohol and Public Health: Data, Trends, and Maps. Available at https://www.cdc.gov/alcohol/data-stats.htm In consequence, research has sought to determine the factors responsible for the development and persistence of alcoholism. From this research, several molecular and epigenetic mechanisms have been discovered.

Background Alcoholism is characterized by a wide range of symptoms including compulsive alcohol seeking and consumption, tolerance (resistance to the effects of alcohol after repeated consumption), and withdrawal symptoms such as irritability, profuse sweating, and uncontrollable shaking upon rapid cessation of drinking.Mayo Clinic Staff. (2014, December). Alcoholism: Symptoms. Available at http://www.mayoclinic.org/diseases-conditions/alcoholism/basics/symptoms/con-20020866 There is not a specific test for diagnosing alcoholism; however, patient questionnaires and medical screenings for ailments typically associated with alcoholism, such as cirrhosis, heart problems, and pancreatitis, are often used as diagnostic tools.Mayo Clinic Staff. (2014, December). Alcoholism: Tests and Diagnosis. Available at http://www.mayoclinic.org/diseases-conditions/alcoholism/basics/tests-diagnosis/con-20020866Kahan M (April 1996). "Identifying and managing problem drinkers". Can Fam Physician 42, 661–71 Several factors influence the development of alcoholism including genetic predisposition and environmental stressors such as grief, stress, depression, and anxiety.Gifford, M. (2010). Alcohol Use and Abuse. In Alcoholism (p. 14). Santa Barbara, Calif.: Greenwood Press/ABC-CLIO. In coordination with these factors, molecular and epigenetic mechanisms influence the progression toward alcoholism.

Mechanisms

An increased propensity for alcoholism has been associated with stress-related anxiety and dysphoria, a state of general unease or dissatisfaction. The experience of various types of stress, including severe acute stress and chronic stress, can lead to the onset of dysphoria. Ethanol consumption promotes the release of dopamine into the nucleus accumbens (NAc) which is translated as a “reward".Bolanos, C. A. and Nestler, E. J. (2004). Neurotrophic mechanisms in drug addiction. Neuromolecular Medicine, 5(1), 69-83. Thus, to cope with negative emotions, individuals often turn to alcohol as a form of temporary self-medication.Bolton, J.M., Robinson, J., Sareen, J. (2009). Self-medication of mood disorders with alcohol and drugs in the National Epidemiologic Survey on Alcohol and Related Conditions. Journal of Affective Disorders, 115(3), 367–375. Unfortunately, repeated ethanol use results in diminishing returns which prompts increased intake and dependence. Research has and continues to investigate the molecular and epigenetic mechanisms underlying the downward spiral of alcoholism.

Molecular Mechanisms

Receptors

Several receptors directly interact with ethanol to promote a cascade of signaling. N-methyl-D-aspartate (NMDA) receptors are glutamate receptors particularly important in long-term potentiation in neurons. These receptors have been linked to ethanol use. Acute ethanol exposure, a brief period of ethanol use, inhibits Ca2+ flow through NMDA receptors in the hippocampus, the brain structure particularly important in memory formation.Lovinger, D.M., White, G., Weight, F.F. (1989). Ethanol inhibits NMDA-activated ion current in hippocampal neurons. Science, 243, 1721–1724. A specific subunit of NMDA receptors, NR2B, shows particularly high sensitivity to ethanol as exemplified by increased NR2B expression in response to ethanol.Smothers, C.T., Clayton, R., Blevins, T., Woodward, J.J. (2001). Ethanol sensitivity of recombinant human N-methyl-D-aspartate receptors. Neurochem Int. 38, 333–340.Nagy, J. (2004). The NR2B subtype of NMDA receptor: a potential target for the treatment of alcohol dependence. Curr Drug Targets CNS Neurol Disord, 3, 169–179.Rani, C.S., Qiang, M., Ticku, M.K. (2005). Potential role of cAMP response element-binding protein in ethanol-induced N-methylD-aspartate receptor 2B subunit gene transcription in fetal mouse cortical cells. Mol Pharmacol, 67, 2126–2136. Another family of receptors, metabotropic glutamate receptors (mGluR), may also contribute by activating MAPK pathways and increasing intracellular Ca2+. Antagonism of mGluR5 showed a decrease in ethanol consumption suggesting mGluR5's role in alcoholism.Gass, J.T. and Olive, M.F. (2009) Role of protein kinase C epsilon (PKCe) in the reduction of ethanol reinforcement due to mGluR5 antagonism in the nucleus accumbens shell. Psychopharmacology (Berl) 204:587–597Schroeder, J.P., Overstreet, D.H., Hodge, C.W. (2005) The mGluR5 antagonist MPEP decreases operant ethanol self-administration during maintenance and after repeated alcohol deprivations in alcohol-preferring (P) rats. Psychopharmacology (Berl), 179, 262–270McMillen, B.A., Crawford, M.S., Kulers, C.M., Williams, H.L. (2005) Effects of a metabotropic, mGlu5, glutamate receptor antagonist on ethanol consumption by genetic drinking rats. Alcohol, 40, 494–497.Hodge, C.W., Miles, M.F., Sharko, A.C., Stevenson, R.A., Hillmann, J.R., Lepoutre, V., Besheer, J., Schroeder, J.P. (2006) The mGluR5 antagonist MPEP selectively inhibits the onset and maintenance of ethanol self-administration in C57BL/6J mice. Psychopharmacology (Berl) 183:429–438. Furthermore, voltage-gated calcium channels (VGCCs) were shown to be inhibited by ethanol resulting in reduced influx of Ca2+.Mullikin-Kilpatrick, D., Mehta, N.D., Hildebrandt, J.D., Treistman, S.N. (1995) Gi is involved in ethanol inhibition of L-type calcium channels in undifferentiated but not differentiated PC-12 cells. Mol Pharmacol, 47, 997–1005. Yet, repeated ethanol intake, or chronic ethanol use, increases expression of the slow-inactivating L-type VGCCs known to sustain Ca2+ influx.Katsura, M., Shibasaki, M., Hayashida, S., Torigoe, F., Tsujimura, A., Ohkuma, S. (2006) Increase in expression of a1 and a2/d1 subunits of L-type high voltage-gated calcium channels after sustained ethanol exposure in cerebral cortical neurons. J Pharmacol Sci, 102, 221–230.Walter, H.J., McMahon, T., Dadgar, J., Wang, D., Messing, R.O. (2000) Ethanol regulates calcium channel subunits by protein kinase C d-dependent and -independent mechanisms. J Biol Chem, 275, 25717–25722. When these channels are inhibited with an antagonist, ethanol consumption is reduced.Gardell, L.R., Reid, L.D., Boedeker, K.L., Liakos, T.M., Hubbell, C.L. (1997) Isradipine and naltrexone in combination with isradipine interact with a period of abstinence to reduce rats’ intakes of an alcoholic beverage. Alcohol Clin Exp Res, 21, 1592–1598.Rezvani, A.H. and Janowsky, D.S. (1990) Decreased alcohol consumption by verapamil in alcohol preferring rats. Prog Neuropsychopharmacol Biol Psychiatry, 14, 623–631.De Beun, R., Schneider, R., Klein, A., Lohmann, A., De Vry, J. (1996) Effects of nimodipine and other calcium channel antagonists in alcohol-preferring AA rats. Alcohol, 13, 263–271.

Adenylyl Cyclase

Adenylyl cyclase (AC) plays a role in ethanol induced signaling pathways. Acute ethanol may increase AC activity resulting in increased levels of cAMP and altered activity of cAMP targets.Hatta, S., Saito, T., Ohshika, H. (1994) Effects of ethanol on the function of G proteins in rat cerebral cortex membranes. Alcohol Alcohol Suppl, 29, 45–51. Of the cAMP targets, protein kinase A (PKA) has been associated with ethanol use. While acute ethanol use increases the activity of AC, chronic use tends to desensitize AC such that more simulation, increased ethanol consumption, is required to elicit the same response.Mochly-Rosen, D., Chang, F.H., Cheever, L., Kim, M., Diamond, I., Gordon, A.S. (1988) Chronic ethanol causes heterologous desensitization of receptors by reducing as messenger RNA. Nature, 333, 848–850Tabakoff, B., Whelan, J.P., Ovchinnikova, L., Nhamburo, P., Yoshimura, M., Hoffman, P.L. (1995) Quantitative changes in G proteins do not mediate ethanol-induced downregulation of adenylyl cyclase in mouse cerebral cortex. Alcohol Clin Exp Res, 19, 187–194.

Kinases

Ethanol transduction pathways involve several protein kinases known to phosphorylate substrates linked to alcoholism, namely cAMP response element-binding protein (CREB). CREB plays a central role in ethanol responses making its activation an important step in the pathway. Some of the kinase families currently linked to alcoholism are Ca2+/calmodulin-dependent protein kinases (CaMKs), protein kinase A (PKA), and mitogen-activated protein kinases (MAPKs).Constantinescu, A., Diamond, I., Gordon, A.S. (1999). Ethanolinduced translocation of cAMP-dependent protein kinase to the nucleus. Mechanism and functional consequences. J Biol Chem, 274, 26985–26991.Ron, D., Jurd, R. (2005). The ‘‘ups and downs’’ of signaling cascades in addiction. Sci STKE, 308, re14.Lee, A.M. and Messing, R.O. (2008) Protein kinases and addiction. Ann N Y Acad Sci, 1141, 22–57.

CamK

Rapid changes in Ca2+ concentration, influenced by receptors such as those described above, regulate the activity of CaMKs. Withdrawal following chronic ethanol use as well as voluntary ethanol intake, consuming ethanol when both an ethanol solution and water are offered, in rats showed a decrease in CaMKIV and consequently p-CREB.Pandey, S.C., Roy, A., Zhang, H. (2003). The decreased phosphorylation of cyclic adenosine monophosphate (cAMP) response element binding (CREB) protein in the central amygdala acts as a molecular substrate for anxiety related to ethanol withdrawal in rats. Alcoholism: Clinical and Experimental Research, 27(3), 396–409.Pandey, S.C., Roy, A., Mittal, N. (2001). Effects of chronic ethanol intake and its withdrawal on the expression and phosphorylation of the CREB gene transcription factor in rat cortex. J Pharmacol Exp Ther, 296, 857–868.Misra, K., Roy, A., Pandey, S.C. (2001). Effects of voluntary ethanol intake on the expression of Ca2?/calmodulin-dependent protein kinase IV and on CREB expression and phosphorylation in the rat nucleus accumbens. NeuroReport, 12, 4133–4137.Li, J., Bian, W.L., Xie, G.Q., Cui, S.Z., Wu, M.L., Li, Y.H., Que, L.L., Yuan, X.R. (2008). Chronic ethanol intake-induced changes in open-field behavior and calcium/calmodulin-dependent protein kinase IV expression in nucleus accumbens of rats: naloxone reversal. Acta Pharmacol Sin, 29, 646–652. In contrast, ethanol activates CaMKII resulting in phosphorylation of CaMKII targets such as BK potassium channels.Mahadev, K., Chetty, C.S., Vemuri, M.C. (2001). Effect of prenatal and postnatal ethanol exposure on Ca2+/calmodulin-dependent protein kinase II in rat cerebral cortex. Alcohol, 23, 183–188.Liu, J., Asuncion-Chin, M., Liu, P., Dopico, A.M. (2006). CaM kinase II phosphorylation of slo Thr107 regulates activity and ethanol responses of BK channels. Nat Neurosci, 9, 41–49.

PKA