can also be inhibited post-transcriptionally by mirna, double-stranded rna, typically formed from hairpin structures, that is used to inhibit translation of proteins. after processing by the rna interference (rnai) molecules drosha and dicer, a single, guide-strand is loaded into the rna induced silencing complex (risc) which is used to bind mrna. this binding suppresses protein synthesis and sometimes initiates mrna degradation. the epigenetic link to several ethanol related molecules has been established. as discussed before, acute ethanol exposure tends to increase creb and p-creb levels while withdrawal after chronic ethanol use is associated with decreased creb and p-creb. also, creb recruits the cbp, a hat. increased creb and cbp activity at the bdnf promoter have been associated with decreased h3 methylation and increased h3 acetylation at lysine 9. in concordance, histone acetylation, particularly at the bdnf promoter ii, increases bdnf expression. similarly, bdnf exon iv expression following depolarization is increased and is associated with increased histone acetylation, reduced dna methylation and reduced mecp2 binding at the bdnf promoter. these changes would tend to increase bdnf expression during acute ethanol exposure. conversely, since creb levels and subsequent cbp recruitment fall during withdrawal, these types of epigenetic changes would likely reverse upon withdrawal after chronic ethanol use. in particular, lack of cbp likely results in decreased acetylation of the bdnf promoter. another layer of regulation modulates the activity of mecp2 via the protein rack1. rack1 at h3 and h4 inhibits mecp2 binding and promotes histone acetylation; thus, resulting in increased bdnf expression. chronic stress, often linked with a propensity to alcoholism, increases h3 methylation near bdnf promoters which inhibits transcription. to oppose this process, antidepressants have been shown to reduce histone methylation, increase h3 acetylation at the bdnf promoter, and reduce levels of hdac5. recall that hdacs remove acetyl groups and are associated with heterochromatin formation. nr2b is also influenced by epigenetic mechanisms. recall that this ndma receptor subunit shows increased expression after ethanol exposure. decreased cpg methylation of the nr2b gene is associated with chronic, but not acute, ethanol exposure. thus, the increase in nr2b expression in chronic ethanol exposed rats may be mediated by a more open chromatin structure. bk potassium channels are another target: mirna-9 has been shown to target bk channel transcripts and may influence ethanol tolerance. this will be discussed in more detail in the tolerance section. finally, a link has been found between ethanol use and histone acetylation during development. after ethanol exposure, adolescent rats showed increased h3 and h4 acetylation in reward centers of the brain, such as the frontal cortex and nucleus accumbens. this effect was not seen in adult rats. thus, brain chromatin remodeling that increases gene expression in reward centers of developing brains may contribute to an increased propensity toward alcoholism upon and after ethanol exposure. tolerance tolerance is a lessened response to ethanol after repeated or prolonged ethanol exposure or consumption. in mammals, tolerance can form within minutes or over longer periods of time. ghezzi et al. (2014) speculated that tolerance occurs due to a homeostatic mechanism that resists environmental changes. however, homeostatis does not explain how tolerance influences alcohol addiction in many cases. epigenetic alterations, including phosphorylation, methylation, acetylation, mirna, and chromatin remodeling, may help explain the cases not explained by homeostatic mechanisms. these epigenetic mechanisms have been studied in rodents. acute tolerance was shown to be controlled by changes in the bk channel's phosphorylation state. acute tolerance is defined as ethanol tolerance that appears during an ethanol experience. phosphorylation of bk channels by pka is needed for ethanol potentiation of the channel. alcohol can change the phosphorylation patterns to characterize alcohol-tolerant bk channels. in addition, in rat magnocellular neurons it was shown that mirna contributes to rapid and chronic ethanol tolerance by altering the expression of many proteins. rapid tolerance is defined as tolerance produced following a single ethanol exposure. chronic tolerance is tolerance resulting from repeated exposure. ethanol exposure upregulates mir-9, a mirna that binds to some bk channel mrna transcripts in their 3'-utr. the binding of mir-9 causes the degradation of the mrna. the bk channel mrnas targeted by mir-9 are those that contain an alternatively-spliced exon that has been called alcorex. mrnas that contain this exon produce bk channels that respond strongly to ethanol (high potentiation channels). on the other hand, bk channel mrna that contains the alternative exon that has been named strex is used to produce channels that are relatively ethanol insensitive (low potentiation channels). mir-9 specifically degrades the transcripts encoding high potentiationchannels, leaving behind mostly alcohol-resistant channels. the reason this mechanism is not used for acute tolerance is because tolerance depends on the modified protein synthesis, which takes time. the effect histone acetylation on bk channel expression and alcohol tolerance has been studied by ghezzi et al. (2014) using the drosophila. the drosophila gene that encodes bk channels is called slowpoke (slo). after ethanol sedation of the flies occurs, acetyl groups are added to histones within the slo promoter region. acetylation exposes the slo promoter to creb, which enhances slo protein expression. when the flies are exposed to alcohol again, it takes a shorter period of time to recover from sedation and net neural excitability is enhanced. this shows that a tolerance has been built due to increased slo product. this can also be accomplished by histone deacetylase inhibition that also causes increased slo gene expression. if the ethanol does not sedate the flies, or slo expression is not induced, tolerance does not occur. the effect the slo gene has on tolerance appears to differ between species. for example, when slo expression increases in c. elegans, the worms become more sensitive to ethanol as opposed to in drosophilia where a tolerance is built. while the response to increased slo expression differs, the slo gene is involved with tolerance in every species. whether or not tolerance and alcoholism are related is still under debate. further work must be done in order to find a connection. the closest link between the two is that mirna is able to regulate expression of multiple genes, and alcoholism is influenced by multiple genes. if a relationship is found, studying predisposition to alcoholism will become a possibility, which could lead to therapeutic targets for alcoholism. treatment current treatments treatments for alcoholism aim to end ethanol consumption and provide social support to prevent relapse. in some cases, sedating medications (benzodiazepines) may be necessary to prevent and/or reduce withdrawal symptoms. these benzodiazepines are only prescribed for a short period time to aid with withdrawal symptoms, for they too can become addictive. some other commonly used drugs on the market include: disulfiram disulfiram (trade name antabuse) creates a sudden acute response to ethanol consumption by inhibiting an enzyme that metabolizes ethanol. in the liver, ethanol is metabolized by alcohol dehydrogenease to produce acetaladehye which is then metabolized by acetaldehyde dehydrogenease to produce acetic acid. disulfiram interferes with the acetaldehyde dehydrogenease preventing acetaldehyde metabolism. this buildup of acetaldehyde is responsible for immediate “hangover” effects such as vomiting, nausea, and headaches. naltrexone naltrexone (trade name revia) is an opioid antagonist that is thought to work by interfering with the dopaminergic mesolimbic (or reward) pathway associated with risk-reward evaluation in the brain. this pathway starts at the ventral tegmental area and makes its way to the nucleus accumbens. addictions of many drugs, including ethanol is commonly linked to this pathway. naltrexone is thought to block or attenuate opioid stimulation. acamprosate acamprosate (trade name campral) is believed to stabilize chemical balances in the brain that ethanol withdrawal would normally induce. in chronic ethanol users, ethanol binds to gaba receptors causing the down-regulation of gaba receptors; which are now less sensitive to the inhibitory gaba neurotransmitter. acamprosate is thought to act as a gaba agonist that relieves anxiety associated with ethanol withdrawal, thereby promoting the discontinue use of ethanol. these drug treatments are often coupled with social support through counseling, rehabilitation centers, and support groups. these social systems aid in dealing with the underlying social and psychological issues related to ethanol addiction. hdac inhibitors chronic alcohol use up-regulates hdacs through oxidative stress in the brain; this leads to decreased histone acetylation and decreased npy expression, especially in the amygdala. the lower levels of npy are associated with increased ethanol consumption and increased anxiety in periods of ethanol withdrawal. trichostatin a (tsa) is an hdac inhibitor which has been shown to reverse these histone acetylation and npy deficits by preventing and reversing hdac up-regulation. tsa acted as an anxiolytic as it was able to reduce anxiety associated with ethanol withdrawal. suberanilohydroxamic acid or saha is an hdac inhibitor that reduces the motivation of rats to consume and/or seek ethanol, saha reduced binge drinking in rats by ending ethanol consumption periods sooner than in non-saha rats. saha was selective for reducing ethanol seeking but not sucrose seeking. dnmt inhibitors in alcoholics, certain regions of the amygdala are associated with higher levels of dna methyltransferases. 5-azacitidine (5-azac) in mice reduced excessive ethanol consumption. 5-azac decreases dna methylation by inhibiting the activity of dna methyltransferases. these results suggest ethanol consumption increases dnmt activity and that this histone modification can be reversed by dnmt inhibitors.