Epigenetics and Drug Abuse: Implications for the Biological Underpinnings of Abuse

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Introduction

Drug addiction is a severe psychological disorder characterized by the inability to control impulsive drug seeking and using, despite health and incarceration risks (McQuown & Wood, 2010). Additionally, this disorder is characterized by the inability to control protruding thoughts related to substance and cravings, or intense urges to use that do not end until the substance is used again (Goldman, Oroszi, & Ducci, 2005). Approximately 23.5 million people in the United States are addicted to alcohol and other substances, such as opiates, stimulants, and hallucinogens (National Institute on Drug Abuse, 2012). Unfortunately, chronic drug use is associated with severe disruptions in the ability to fulfill obligations at school, home, and work (American Psychiatric Association, 2013). Drug addiction is also characterized by severe withdrawal symptoms, such as nausea, physical illness, vomiting, and other symptoms after drug discontinuance. Lastly, roughly 60% of drug addicts relapse after withdrawal and abstinence (Volkow & Li, 2004). This phenomenon can occur months or years after the individual stops using drugs.

Even though classical conditioning is involved in drug addiction, biological factors are also involved because 20-60% of abuse risk are due to biological and genetic factors (Nielson et al., 2013). Specifically, many drugs of abuse, including opiates and stimulants, all act on the brain’s mesolimbic limbic system, including the ventral tegmental area, nucleus accumbens, amygdala, hippocampus, and prefrontal cortex (DeVries & Shippenberg, 2002). All of these drugs act on this system by increasing dopamine concentrations and association between drug use and reward (Kalivas & Volkow, 2005). Resultantly, the individual’s neural system is motivated to administer drugs, consistently remembers memories associated with drug use, and is consistently seeking drugs (Kalivas, 1993).

However, recent research has demonstrated drugs of abuse also act on genes in the mesolimbic system through epigenetics. Epigenetics is the study of how environmental factors alters gene expression without changing genetic structure (Wong, Mill, & Fernandes, 2011). The two main ways this occurs is through gene activation or silencing by DNA methylation or histone modification (Mazzio & Soliman, 2012). DNA methylation is where DNA transcription is inhibited through the addiction of a methyl group whereas histone modification is where DNA transcription is either activated or inhibited through histones opening or closing (Mazzio & Soliman, 2012). Histone methylation is where the histone tightens/closes to prevent DNA transcription while acetylation loosens/opens the histone for DNA activation (Mazzio & Soliman, 2012).

Two of the main genes/transcription factors in drug addiction are CREB and FosB. CREB is a transcription factor involved in gene plasticity, growth, and long-term memory (Chao & Nestler, 2002). This transcription factor is over-expressed in the nucleus accumbens in drug abuse and may lead to tolerance by decreasing the euphoric effects associated with the drug (Chao & Nestler, 2002). It may also be involved in drug withdrawal because CREB decreases after drug discontinuance and causes symptoms of anxiety and panic (McPherson & Lawrence, 2007). Alternatively, FosB is a transcription factor involved in cell proliferation, differentiation, and transformation (McQuown & Wood, 2010). FosB is over-expressed in cocaine/heroin addiction and heavily acetylates dopamine production on D1 receptors in the striatum (Chao & Nestler, 2012). Resultantly, it is involved in compulsive drug seeking and relapse. Fos is especially involved in drug relapse because it is overexpressed in the nucleus accumbens for weeks or months after drug withdrawal (Chao & Nestler, 2012). Ultimately, this paper discusses the characteristics of drug use, neural structures and epigenetic processes involved in addiction, and several biological treatments for addictions.

Characteristics of Drug Addiction

Drug addiction is a chronic psychological and biological disorder characterized by impulsive drug use and seeking, despite adverse and harmful consequences (McQuown & Wood, 2010). Individuals addicted to a substance or drug display an impaired ability to control drug intake and seeking behavior, experience strong cravings to use the drug and intruding thoughts to use that do not disappear until the substance is used again (Goldman, Oroszi, & Ducci, 2005). Lastly, addiction is described as the inability to fulfill obligations at school, work, or home, consistent substance use despite psychological and/or physical complications, disruptions in social or familial relationships, becoming physically dependent on the drug, and experiencing withdrdawal symptoms after disuse (American Psychiatric Association, 2013).

Approximately 23.5 million people in the United States are addicted to alcohol and substances such as opiates, stimulants, and hallucinogens (National Institute on Drug Abuse, 2012). Extensive drug abuse decreases the average life span by eight years, accounts for 590,000 deaths in the US every year, and causes physical/mental injury and illness for 40 million individuals (Goldman, Oroszi, & Ducci, 2005; Volkow & Li, 2004). Psychiatric illnesses are also highly correlated with substance abuse. Roughly 42% of individuals with a substance use disorder have a co-morbid psychiatric disorder, such as schizophrenia, bipolar disorder, anxiety, or depression (SAMHSA, 2017). Finally, roughly 40-60% of substance use disorders are influenced by genetic and familial factors (Li, Mao, & Wei, 2008).

Substance abuse begins with the individual experiencing the euphoric, stimulating, or sedative effects, such as happiness, well-being, and pleasure associated with opiates, hallucinogens, and stimulants (Cami & Farre, 2003). With repeated administrations of a substance, the individual becomes physically dependent and cannot function without it. If an individual stop taking the substance, they will experience withdrawal symptoms, such as headaches and nausea (Goldman, Oroszi, & Ducci, 2005). To prevent experiencing these symptoms, the individual will continue taking the drug.

Besides physical dependence, the individual experiences tolerance to the drug and need higher doses to experience euphoria (Cami & Farre, 2003). This is generally due to the neurotransmitters upregulating and becoming less sensitive to the drugs (Wong, Mill, & Fernandes, 2011). If large doses of the drug are not administered, the individual will also experience withdrawal and evasive symptoms. To eliminate withdrawal symptoms, the individual may actively seek higher doses of the drug.

However, drug addiction is different from tolerance and dependence through the behavior of actively seeking drugs, experiencing intrusive thoughts about administering and using drugs, and the inability to control the behavior and thoughts of using (Kalivas & O’Brien, 2008). The individual also experiences cravings, or strong emotional memories associated with drug use, and cannot stop obsessing over the drug or control the desire to use. To cease these thoughts, the individual spends the majority of time actively seeking their drug(s) of abuse (Kalivas & O’Brien, 2008). This results in the individual forgetting about their responsibilities and aversive side effects of drugs, such as death or illness and incarceration.

Classical conditioning is also involved in drug addiction. The majority of drug addicts abuse in a specific location to administer (Siegel, 1984). With continued administration, the drug (unconditional stimulus) is paired with a needle, bedroom, or another environmental setting or tool (conditioned stimulus) to create the conditioned response of feeling euphoria and relief (Siegel, 1984). These settings and feelings are then stored and remembered in the hippocampus and prefrontal cortex. When exposed to these stimuli or situations, the individual immediately experiences craving, remembers past experiences of abuse, and is motivated to use/administer drugs again. These memories are long-lasting and can induce cravings years after the individual used and became abstinent (Sell et al., 2000). This also influences 40-60% of abusers to relapse after withdrawal (Sell et al., 2000). Ultimately, drug abuse is characterized by the impulsive behavior of seeking/using drugs despite aversive effects and inability to control thoughts and feelings associated with drugs.

Neurological Structures and Pathways Involved in Drug Abuse

Nucleus Accumbens

Drugs of abuse primarily act on the mesolimbic system, a system controlling learning and goal-directed behavior associated with reward in animals and humans. Neural structures involved in this system include the nucleus accumbens, ventral tegmental area, amygdala, and prefrontal cortex (Kalivas, 1993). The nucleus accumbens (NAc) is involved in reinforcement and goal-directed behaviors such as reproduction, exercise, feeding, and other pleasurable activities (DeVries & Shippenberg, 2002). This structure is also involved in the formation of learned associations between an event and environmental stimuli (Kalivas & Volkow, 2005). Drugs of abuse, such as cocaine or heroin act on the NAc by activating dopamine or blocking GABA receptors to increase dopamine levels. After drugs are administered, the NAc is activated and sends dopamine to the amygdala and prefrontal cortex where the event is processed as an emotionally important event (Kalivas & Volkow, 2005). The prefrontal cortex remembers the event of administering drugs and sends glutamate to the NAc. This action tells the NAc the event is remembered and continue the behavior of administering drugs (Salamone, Cousins, & Snyder, 1997). This action also increases the association between certain stimuli, such as needles, and the euphoric effects of drug use.

Ventral Tegmental Area

The ventral tegmental area (VTA) contains one of the highest concentrations of dopaminergic neurotransmitters in the brain and is involved in reward based behavior, cognition, and fear. After drug administration, dopaminergic neurotransmitters are activated and the VTA sends dopamine to the nucleus accumbens, amygdala, and prefrontal cortex where the event is processed (Salamone, Cousins, & Snyder, 1997). After the prefrontal cortex sends glutamate to the NAc, the VTA receives GABA from the NAc (Salamone, Cousins, & Snyder, 1997). When this occurs, the VTA reintegrates the event and outcome as being important and increases behavioral output (Salamone, Cousins, & Snyder, 1997). Resultantly, behavior associated with drug use is increased and strengthened.

Amygdala and Prefrontal Cortex

The amygdala is one of the main structures involved in the expression of emotion and fear (Pinel, 2014). After drug administration, the VTA and NAc sends dopamine to the amygdala where the event is processed as a pleasurable event (Kalivas & Volkow, 2005). Dopamine is then sent to the hippocampus and prefrontal cortex where the event and emotion associated with it is stored in long-term memory.

Lastly, the prefrontal cortex is involved in executive functioning, goal-directed behavior, emotion, problem solving, and encoding the association between an event and environmental stimuli (Kalivas & Volkow, 2005). After drug administration, the ventral tegmental area, amygdala, and nucleus accumbens sends dopamine to the prefrontal cortex (Kalivas & Volkow, 2005). The nucleus accumbens informs the prefrontal cortex drug administration is an important event/behavior and should be remembered (Kalivas & Volkow, 2005). The prefrontal cortex then sends glutamate to the nucleus accumbens telling it remembers the event and continue administering drugs. After several administrations, these behaviors and memories are strengthened and the individual displays increased/compulsive drug seeking behavior.

Epigenetics

The human genome is composed of billions of sequences containing information that controls gene expression (Mazzio & Soliman, 2012). Epigenetics is the study of how biological and environmental factors change gene expression without changing the genetic structure of the gene itself (Holliday, 2006). Unlike gene mutations, epigenetics does not alter an organisms genotype, but can alter their phenotype by turning genes on and off during the life span (Wong, Mill, & Fernandes, 2011). Additionally, epigenetics can reverse regulation of gene expression, is essential for normal cell development and differentiation, and allows for long-term gene regulation without changing the genome (Wong, Mill, & Fernandes, 2011). Epigenetics accomplishes this by acting on the epigenome, the second layer of information on DNA containing a non-static arrangement of histone scaffolding surrounding the DNA that controls the genomic processes of turning a gene on or off (Wong, Mill, & Fernandes, 2011). Nutritional, chemical, environmental, and psychosocial factors can influence the onset of epigenetics and differentiation of gene expression. For example, poor nutrition, inadequate access to healthcare, racial disparities, and low economic standing has been shown to increase the risk of diabetes and obesity due to DNA methylation of the leptin gene (Mazzio & Soliman, 2012). The main factors epigenetics occur include DNA and histone methylation, and chromatin alterations (Wong, Mill, & Fernandes, 2011).

DNA Methylation

DNA methylation is the prominent method of turning a gene off (Phillips, 2008). Methylation usually occurs on the cytosine and guanine bases (also known as the CpG dinucleotide sites) of DNA (Phillips, 2008). Specifically, this occurs by covalently attaching a methyl group to the 5 end of the DNA molecule (Mazzio & Soliman, 2012). Attaching a methyl group to the CpG site is able to inhibit and silence the gene by preventing DNA from binding to promotor sites, terminates transcription initiation, and prevents RNA polymerase from attaching to transcription sites and activating DNA replication (Mazzio & Soliman, 2012). Methylation can also occur by DNA methyltransferases, such as DNMT3A attaching itself to methyl-CpG-binding proteins, such as MeCP2 or MBD2 (Mazzio & Soliman, 2012). When this occurs, these proteins constrict and deactivate histone deacetylases, such as HDAC1, involved in DNA and RNA replication (Mazzio & Soliman, 2012). Lastly, DNMT3A can influence DNA methylation by attaching itself to the replication fork and prevents RNA from replicating (Mazzio & Soliman, 2012).

Once binding to the CpG sites have occurred, methylation begins influencing inheritance patterns by small non-coding RNA (miRNA) silencing and degrading RNA; preventing RNA from maturating promotor areas associated with RNA replication. These non-coding RNA molecules can also induce DNA methylation by inhibiting and constricting histone proteins, such as MeCP2 (Mazzio & Soliman, 2012). Ultimately, DNA methylation is the process of genes becoming inhibited and silenced. The other process influencing DNA methylation is chromatin structure. Chromatin are structures in the cell containing chromosomes, RNA, and DNA (Holliday, 2006). When chromatin is open and/or relaxed, transition factors can attach to it and induce DNA/RNA transcription. However, when chromatin is closed, transcription cannot occur. DNA methyltransferases and proteins, such as DNMT3A and MeCP2 attach themselves to chromatin and close them (Mazzio & Soliman, 2012). By doing this, DNA and RNA cannot be replicated and the gene is inhibited (Mazzio & Soliman, 2012).

Histone Modification

Histones are the proteins DNA wrap around to form nucleotides and chromatin (Wong, Mill, & Fernandes, 2011). All histones are composed of two H2A/H2B dimer cores and H3/H4 tetramers that wrap the DNA base pairs (Mazzio & Soliman, 2012). Additionally, each histone is comprised of N-terminal tails and give them a unique histone marker and code. These markers are then used to determine how strong DNA is wrapped around it and how stable the histone is or chance it will be modified (Mazzio & Soliman, 2012).

The process of histone modification involves genetic expression being activated or de-activated after the histones have been synthesized (Wong, Mill, & Fernandes, 2011). The most common forms of histone modifications include methylation and acetylation (Mazzio & Soliman, 2012). Histone acetylation is the process of the N-terminal tail becoming lose and the histone becoming unstable (Mazzio & Soliman, 2012). When this occurs, the DNA and RNA on the histone are exposed and DNA is transcribed by transcription factors and RNA polymerase factors (Wong, Mill, & Fernandes, 2011).

Alternatively, histone methylation occurs when a methyl group is added to the histone; causing the histone to close (Wong, Mill, & Fernandes, 2011). When this occurs, the DNA tightens around the histone and transcription factors have an extremely difficult time attaching themselves to DNA. Resultantly, gene expression is inhibited (Wong, Mill, & Fernandes, 2011).

The majority of histone modifications are performed or mediated by histone acetyltransferases (HATs) or histone deacetylases (HDACs). HATs are usually involved in histone acetylation by removing the positive charge on the histones N-terminus (Wong, Mill, & Fernandes, 2011). When this happens, the DNA become loose and transcription factors/RNA polymerase are able to attach to it and the gene is activated (Wong, Mill, & Fernandes, 2011). Oppositely, HDACs are involved in histone methylation and deacetylation by tightening DNA on the histone and histone proteins surrounding the DNA (Wong, Mill, & Fernandes, 2011). Transcription, as a result, does not occur.

Epigenetics and Drug Abuse

Epigenetics has also been observed in chronic drug abuse. One of the first studies demonstrating the role of epigenetics in drug use was cocaine inhibiting MeCP2, a major protein activating DNA methylation, in the nucleus accumbens of rats (Nielsen et al., 2013). In one study, the authors found acute administration of psychostimulants, such as cocaine, inhibited the MeCP2 protein on genes responsible for dopamine production through phosphorylation of a Ser421 amino acid on the MeCP2 transcription site (Nielsen et al., 2013). Resultantly, genes expressing dopamine production are overexpressed and an influx of dopamine is released into the nucleus accumbens (Nielsen et al., 2013). Rats were then heavily motivated to seek and use cocaine (Nielsen et al., 2013). However, when MeCP2 was over expressed in the nucleus accumbens, rats were significantly less likely to seek or use heroin (Nielsen et al., 2013). Ultimately, this demonstrated psychostimulants are able to inhibit genes on dopaminergic neurotransmitters, and increase drug seeking behavior in rats.

Common Drugs of Abuse

Two of the most commonly abused drugs are opiates and stimulants. Roughly 7.5 million people in the United States were addicted to cocaine and 700,000 to heroin in 2012 (Foundation for a Drug Free World, 2012). Both drugs act on the dopaminergic system in the mesolimbic system of the brain (Kopnisky & Hyman, 2002). However, they act on these systems differently. Heroin and other stimulants activate dopamine release by binding to u and k receptors (Kopnisky & Hyman, 2002 ). All of these receptors are G-coupled protein receptors and are activated by blocking calcium channels and activating potassium channels (Kopnisky & Hyman, 2002 ). Once activated, dopamine is released into the ventral tegmental area, medulla, and nucleus accumbens. Lastly, heroin and other opiates increase by inhibiting GABAergic receptors (Kopnisky & Hyman, 2002).

Oppositely, cocaine and other stimulants increase dopamine release through blocking GABA reuptake for dopamine, serotonin, and norepinephrine (Kopnisky & Hyman, 2012 ). Resultantly, they increase dopamine in the synaptic cleft.

Drug Induced Epigenetics

Cocaine and heroin impact DNA methylation, and histone acetylation/methylation. For example, cocaine acetylates the H3 and H4 histone complexes in the forebrain, amygdala, prefrontal cortex, and the striatal cortex (Gordino, Jayanthi, & Cadet, 2015). Resultantly, DNA transcription occurs, dopamine levels increase, and the individual displays behavioral sensitization (Gordino, Jayanthi, & Cadet, 2015). This is further supported by rats injected with HDACs, or transcription factors inhibiting DNA transcription, decrease the rats drug intake and drug-seeking behavior. Cocaine-induced acetylation can also increase behavioral activation in rats by decreasing HDAC2 concentrations in the prefrontal cortex and increasing H4 acetylation in the dorsal striatum (Gordino, Jayanthi, & Cadet, 2015). By doing this, rats quickly increase their cocaine intake.

Cocaine also impacts and alters DNA methylation. For example, acute and chronic cocaine administration is able to suppress and inhibit the DNMT and DNMT1 transcription factors in the nucleus accumbens. The DNMT transcription factors are involved in DNA methylation and inhibit the transcription and replication of DNA (Gordino, Jayanthi, & Cadet, 2015). Unfortunately, blocking this gene this enhances drug-seeking behavior in rats. However, DNA methylation is increased after drug discontinuance. This may be a possible explanation for withdrawal symptoms occurring after cocaine use (Gordino, Jayanthi, & Cadet, 2015).

Stimulants also alter DNA methylation by acting on the MeCP2 transcription factor. This is one of the main methyl-CpG binding protein and transcription factors used in DNA methylation and inhibition of DNA transcription (Gordino, Jayanthi, & Cadet, 2015). However, cocaine and other stimulants inhibit MeCP2 in the nucleus accumbens and prefrontal cortex. By doing this, cocaine activates and downregulates the brain derived neurotrophic factor (BDNF) gene, a gene crucial for the formation of long-term memories in the prefrontal cortex and hippocampus (Gordino, Jayanthi, & Cadet, 2015). These genes are crucial in the formation of long-term memories associated with euphoria and drug use in the hippocampus. When these genes are over-activated, memories associated with drug use are quickly formed and maintained. Since BDNF genes continue to be activated after drug use and withdrawal, memories associated with drug use continue to plague the individual’s mind (Gordino, Jayanthi, & Cadet, 2015). Ultimately, cocaine activates genes associated with long-term memories seen in drug use.

Common Genes Stimulants and Opiates Act On

However, two of the most prominent transcription factors in cocaine and heroin use are the CREB and FOS factors in the nucleus accumbens, ventral tegmental area, and hippocampus (Nielson et al., 2013). CREB (cAMP response element binding protein) is a transcription factor related gene that activates DNA by attaching itself on the C-terminal base (Lonze & Ginty, 2002). This transcription factor is involved in neuronal growth, precursors, survival, and gene expression in the peripheral nervous system (Lonze & Ginty, 2002). Additionally, these transcription factors are involved in cell and gene plasticity, and short and long-term memory in the central nervous system, specifically in the hippocampus (Kandel, 2001).

CREB was first identified in cocaine and heroin addiction during the 1990’s in the locus coeruleus, a structure found in the fourth ventricle pons involved in stress and panic. (Koob et al., 1993). When cocaine is chronically administered, CAMP (cyclic adenosine monophosphate), a second messenger derived from ATP, is significantly over produced in the locus coeruleus (Chao & Nestler, 2002). Consequently, CREB is then phosphorylated from cAMP and protein kinase A (PKA) and over-expressed in the coeruleus (Chao & Nestler, 2002). CREB then acts as a histone acetyltransferase (HAT) on histones for dopamine production and release; resulting in an increase of dopamine in the coeruleus (Koob et al., 1993). This then reduces stress and panic in the individual.

However, CREB may be a critical factor involved in opiate and stimulant dependence and withdrawal (Chao & Nestler, 2002). After discontinuing opiates or stimulants, rats and people have been reported to display an increase in stress and display physical withdrawal symptoms (i.e. nausea, vomiting, and craving) (Chao & Nestler, 2002). Additionally, CREB is upregulated and decreases in concentration after drug discontinuance (Lonze & Ginty, 2002). Since CREB significantly increases during drug use and decreases after drug continuation, it is believed CREB is involved in withdrawal symptoms in opiate and stimulant use.

CREB has also been found and studied in the nucleus accumbens (NAc). When cocaine or heroin enters the NAc, the influx of dopamine activates CAMP receptors (McPherson & Lawrence, 2007). This occurs because dopaminergic receptors are G-protein coupled receptors that manage and regulate CAMP messaging (McPherson & Lawrence, 2007). CAMP is then able to phosphorylate CREB on the Ser133 dimer on the histone. Resultantly, CREB acts as a HAT acetylate and activates DNA replication.

Additionally, CREB upregulates CAMP and activates the ATF-1 (Activating Factor-1) transcription factor (McPherson & Lawrence, 2007). This also activates DNA/RNA replication and activation on dopamine genes. Lastly, CREB upregulates the prodynorphin (PDYN) gene, a gene that processes and releases opioid-based peptides (NCBI, 2017). This gene is also involved in pain management. When this gene is upregulated, it decreases opioid peptides (Gieryk et al., 2009). When this occurs, opioid receptors are not activated and the individual does experience the euphoric effects activated opioid receptors bring. Instead, the individual or rat experiences anxiety and panic (Gieryk et al., 2009). This is a possible explanation why CREB is related to drug dependency and withdrawal. With high concentrations and activation of CREB, opiates and stimulants lose their euphoric effects and the individual becomes tolerant to the drug (McPherson & Lawrence, 2007). Resultantly, they need higher concentrations of the drug (American Psychiatric Association, 2013). Additionally, the majority of addicted individuals lose euphoria and pleasure to naturalistic rewards, such as food and sexual intercourse (McPherson & Lawrence, 2007). Part of this observation may be due to increased levels of CREB in the nucleus accumbens inhibiting PDYN activation and expression. Ultimately, CREB may be involved in drug dependency because they reduce pleasure and euphoria after chronic administration.

Several studies have studied the effects CREB has on opiate and cocaine use. In one study, the authors taught rats to self-administer cocaine and either injected them with a CREB knock-out gene or mutant CREB gene (Larsen et al., 2011). They also had the rats go through cocaine withdrawal and reinstated drug use using a conditioned place-preference paradigm. At the end of the study, the authors found rats injected cocaine without the gene significantly less than rats injected with a mutant CREB gene (Larsen et al., 2011). Rats injected with the mutant gene were also significantly more likely to administer higher doses of cocaine than those with the knockout gene. Additionally, rats injected with the mutant CREB gene were significantly more likely to display cocaine reinstatement than those with a knock-out gene (Larsen et al., 2011).

This study demonstrated CREB influences the maintenance and reinstatement of cocaine administration. It is believed mutant CREB increased drug use in rats because it desensitizes them and decreases euphoric effects for these rats (Larsen et al., 2011). Resultantly, they become tolerant and need higher doses to experience the same effects (Larsen et al., 2011). Alternatively, rats with the knock-out gene could administer similar doses and experience similar effects (Larsen et al., 2011). Rats given the mutant gene were also more likely to relapse because CREB decreases after withdrawal and increases panic, anxiety, and the physical symptoms associated with withdrawal. To decrease these experiences, the rats reinstated cocaine use (Larsen et al., 2011). Ultimately, this study suggests CREB may play a factor in drug relapse seen in animals and humans.

Fos, particularly FosB, is the second transcription factor involved in cocaine and heroin drug use. This transcription factor is one of the main genes involved in the activation of the Activator Protein-1 (AP-1), a protein involved in the activation and over expression of genes (McQuown & Wood, 2010). FosB is also involved in cell proliferation, differentiation, and transformation. The FosB is activated by the accumulation of the transcription factor c-fos in the nucleus accumbens and striatal cortex (Chao & Nestler, 2012). Immediately after cocaine or heroin use, c-fos is released into the nucleus accumbens. However, FosB is not released and accumulated until chronic drug use (Nielson et al., 2013). This is because CREB activates FosB phosphorylates and activates FosB on its H4 acetylation site (Nielson et al., 2013). When this occurs, FosB deactivates the c-fos factors by activating the HDAC deacetylation factor (Nielson et al., 2013).

Even though FosB is not activated or transcribed until chronic administration, it has an extremely important role in cocaine and heroin drug abuse. FosB has been demonstrated to significantly increase dopaminergic levels of the dopamine D1 receptors in rat’s nucleus accumbens and striatal cortex (Nielson et al., 2013). Due to the increase of dopamine in the NAc, FosB has been demonstrated to significantly increase heroin and cocaine intake in rats and increase behavior sensitization towards drug use (Chao & Nestler, 2012). Additionally, FosB has been shown to be involved in compulsive drug seeking seen in many drug addicted rats and humans (Chao & Nestler, 2012). FosB has been demonstrated to increase the euphoric effect of drug use as evident by rats injected with a mutant FosB gene significantly administer more cocaine or heroin than rats who do not (Chao & Nestler, 2012). Lastly, FosB have been demonstrated to be involved in compulsive drug-seeking behavior because rats injected with a FosB knock-out gene after withdrawal were significantly less likely to reinstate drug use (McQuown & Wood, 2010).  Since the FosB transcription factor continues to be activated after drug discontinuance and withdrawal, it may have a large role in chronic drug seeking in addicts who have not used in weeks (Chao & Nestler, 2012).

Extensive research has been performed to investigate how FosB impact drug use, withdrawal, and reinstatement in cocaine and heroin use. One study looked at how cocaine use impacted FosB activity in rats. In this study, the authors conditioned rats to self-administer cocaine and measured FosB activity weeks after drug use (Hiroi et al., 1997). Rats then went through withdrawal and half the animals were given a FosB knock-out gene before drug reinstatement (Hiroi et al., 1997). Drug use and drug-seeking behavior was then measured in the rats.

At the end of the study, the authors found high concentrations of FosB weeks after rats started administering cocaine (Hiroi et al., 1997). However, they also found rats given the FosB knockout gene were significantly less likely to use or display compulsive drug-seeking behavior than rats not given the gene (Hiroi et al., 1997). This study demonstrated FosB is a transcription factor heavily involved in compulsive drug seeking and behavior sensitivity. Additionally, FosB can remain in the brain weeks after withdrawal and may be involved in relapse of drug use in animals and humans (Hiroi et al., 1997). However, genes deactivating the FosB gene can decrease compulsive drug seeking after withdrawal.

Treatment Possibilities

There is relatively high evidence opiates and stimulants alter genetic activation and suppression of various genes in the mesolimbic system of the brain. Specifically, epigenetics has been observed to influence drug reward, drug-seeking behavior and sensitization, craving, tolerance, and relapse (Nielson et al., 2013). For example, CREB has been observed to be over-activated and expressed in the nucleus accumbens and locus coeruleus (Chao & Nestler, 2012). However, this dramatic increase in CREB has been observed to decrease the euphoric and positive effects of cocaine and heroin (Gieryk et al., 2009). This is primarily due to CREB upregulating genes such as PDYN that activates opioid receptors  (Gieryk et al., 2009). Due to this, CREB may be involved in drug tolerance because people become conditioned to the drug and need higher doses to experience euphoria. Additionally, when rats are given a mutant CREB gene, they administer significantly more cocaine than rats given a knock-out gene (Larsen et al., 2011).

Rats with mutant CREB genes are also significantly more likely to administer cocaine after drug withdrawal (Larsen et al., 2011). This is generally due to CREB concentration decreases after drug use and causes symptoms of panic and anxiety. However, rats with knock-out CREB genes are significantly less likely to administer high doses of cocaine or reinstate use after withdrawal (Larsen et al., 2011). Therefore, injecting knock-out genes into potential or at-risk drug addicts because it will decrease drug dependence (Larsen et al., 2011). This may also be an effective method for drug addicts because it decreases drug reinstatement after withdrawal.

FosB is the other major transcription factor involved in cocaine and heroin addiction. This factor is especially involved in chronic use and relapse because it does not appear in the nucleus accumbens until weeks after drug use and remains in the brain months after drug abstinence and withdrawal (Nielson et al., 2013). When FosB is expressed, it excites the active transition-1 (AT1) transcription factor involved in histone H3 and H4 acetylation for dopamine synthesis and BDSM gene involved in long-term memory in the hippocampus (McQuown & Wood, 2010). Due to this, FosB is believed to be involved in compulsive drug-seeking behavior/use and relapse (McQuown & Wood, 2010).

Rats injected with a mutant FosB gene were significantly more likely to administer large doses of cocaine or heroin than those injected with a Fos knock-out gene (Hiroi et al., 1997). Additionally, these rats were significantly more likely to display compulsive lever-pressing and reinstate drug use after 30 days of withdrawal (Hiroi et al., 1997). Since FosB knock-out genes have been shown to decrease compulsive drug-seeking behavior and drug reinstatement in rats, injecting FosB knockout genes could be a possible biological/pharmaceutical treatment for addiction. However, more research is needed on how FosB genes work on human models.

Conclusion

Drug addiction is a chronic psychological and biological disorder characterized by the inability to control the urge to crave, seek, or use drugs and intruding thoughts about drug use (American Psychiatric Association, 2013). Genetic factors heavily involved in drug addiction and account for 20-60% of risk (Nielson et al., 2013). Epigenetics is also involved in drug addiction because the CREB and FosB transcription factors are over-expressed and activated with chronic drug use (Larsen et al., 2011). Drugs are able to influence gene activity by disabling DNA methylation or activating histone acetylation (Larsen et al., 2011). CREB is believed to influence drug addiction by decreasing the euphoric effects associated with drug use by upregulating D1 and opioid receptors and creating symptoms of panic and anxiety after drug abstinence (Chao & Nestler, 2012). Alternatively, FosB over-activates D1 dopamine receptors to cause an influx of dopamine in the striatum (Chao & Nestler, 2012). Due to this, FosB is believed to be involved in compulsive drug seeking and use. Fos is also involved in drug relapse because it remains in the brain weeks or months after drug withdrawal and may cause individuals to compulsively use in stressful situations (Chao & Nestler, 2012). Since these two genes are over-activated, two possible biological therapies to decrease drug use is to knock-out and deactivate these two genes. Ultimately, epigenetics is involved in drug abuse and needs to be further understood to decrease drug addiction and relapse in the United States.

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