7. Neuroepigenetics of Drug Addiction: Marijuana

7. Neuroepigenetics of Drug Addiction: Marijuana

Drugs are bad…or so we’ve been told over and over. With the topic of marijuana legalization making its way to the national and global spotlight, hard evidence and straightforward facts cannot come too soon. Colorado, Washington, and Oregon have already legalized the recreational use of marijuana, and it seems inevitable that others will soon follow suit. Pot, dope, ganja, trees, that sticky icky icky—whatever you want to call it—is becoming more accepted in both medical and social settings, but we worry that the research done on its properties and effects is not enough well known. Here, we will share research that demonstrates early exposure to marijuana can make long-lasting changes to the brain, making one more susceptible to later substance abuse.

An estimated 23.5 million Americans, or one in every ten Americans over the age of twelve, are directly impacted by drug addiction. General interest from the scientific community and general public has exploded in anticipation of policy changes concerning marijuana. This is evidenced by a fivefold increase in research publications relating to cannabis in the past twenty years (Figure 1). As we take steps toward complete legalization of marijuana, it is crucial to fully understand the impact of marijuana usage, particularly at younger ages. Thus, proper regulation of marijuana from a legal and health standpoint can be ironed out. For example, what should be the legal age to purchase marijuana? What steps need to be taken prior to purchasing the substance? How much of the substance can be purchased at once? These are all key details of marijuana legalization that need to be thoroughly evaluated prior to nationwide legislation being passed.

It is also important to note that other legal abusive drugs, such as alcohol and tobacco, have caused irreversible health consequences. We must avoid making the same mistakes again when it comes to marijuana legalization.

Think of the body as a complicated machine that has an instruction manual, detailing how we are put together in order to function properly. Each cell in the body has the full instruction manual. However, liver cells only read the portion of the manual with instructions on how the liver needs to be constructed and function. Liver cells will tuck away instructions about the heart and have faith in the heart cells to do their part of the job. The instruction text within the manual (genome) can be thought of as DNA. Imagine DNA as a long string wrapped around small tennis ball-like proteins called histones within each and every cell. DNA needs to be wrapped around these histones so it can be condensed within the nucleus of each cell (it is over 1 meter long!). How tightly packed the DNA is wrapped around the histones makes instructions needed for protein production more or less available.

Epigenetics is the study of how environmental or external inputs (like parental care, drug use, or nutrition) can alter the genome at the molecular level to influence the expression of genes. The genome is essentially a blueprint for life with details on how an organism can function. Without adjusting the sequence of the DNA—or the details of the instruction—in the genome, several modifications can be made to our DNA. These modifications are like tags used to mark which regions of the DNA are “on” or “off”.

Adding tags to the DNA changes whether the blueprint to make a certain molecular machine (the protein) is read or if it is closed off like chapters, the pages of which have been redacted or clipped together. Since proteins are the “do-ers” of our cells, adding or removing tags—opening or closing the chapters of the book of DNA—changes the working of the brain, and, because our brains determine how we behave, they also change our behaviors. Such behaviors may include a less intense stress response or an increased likelihood for drug addiction. Neuroepigenetics is the field that studies how cells in the nervous system, which are called neurons, respond and react to life experiences (childcare, stress) and the environment (pollution, drug exposure) and how this changes our brains and mental health.

Two major tags can be added to DNA that generally have opposing effects on information exposure. Using a stoplight as an analogy for these DNA tags, DNA methylation is like a red light, while histone acetylation is like a green light. Thus, instructions that are “methylated” are not read or carried out by the cell (red light). On the other hand, histone acetylation, which is the addition of an acetyl group to the tennis ball-like histone proteins that DNA is wrapped around inside cells, actually allows the “string” to be more loosely wrapped around the “tennis balls”, making it easier for the instructions to be read and executed by the cell (green light). These antagonist epigenetic tags can be triggered by life experiences, which can result in certain details of the instructions, called genes, being repressed or activated. This can potentially lead to altered organism behavior. See Figure 2 for a visual of these epigenetic modifications. Please reference Sam’s essay for further information.

The brain processes rewards, like ice cream or an A+ grade, primarily through something called the mesolimbic dopamine circuit, which can control motivations to consume drugs. Dopamine is frequently referred to as the “motivation” molecule. It is the primary signal within the brain for motivation, although it has many other functions as well. The details of the mesolimbic dopamine circuit are specified in the Incentive-Sensitization Theory, the chief theory behind the mechanism of drug addiction. This theory hypothesizes that repeated drug consumption can change neuronal connections, which leaves individuals more vulnerable to become addicts. Essentially, taking drugs changes the structure of the cells in specific brain regions that reinforce an organism’s behavior of seeking and consuming drugs. This perpetual cycle leads to drug addiction.

In 2007, researchers found that adolescent exposure to THC—the primary active ingredient in marijuana—has a long-lasting effect on motivation to abuse opiates like heroin in the future. A 2012 study found that a key gene, called Penk, may be responsible for this effect. Researchers saw that higher Penk in rats was associated with increased intake of heroin, and the opposite was true as well—reduced Penk activity resulted in decreased intake. Adolescent exposure to THC increased heroin intake as well (Figure 3).  It does this, at least partly, by decreasing the methylation (turning off the red light) at the Penk gene. As a result, the DNA is freed up, causing more Penk proteins to be made. Penk proteins, which can be thought of as the children of the Penk gene, are key mediators of the mesolimbic dopamine circuitry, meaning motivation for reward or drug may be increased in those who are exposed to THC as adolescents.

Additionally, when adolescent rats exposed to THC did not have a fully functioning Penk gene (releases fewer Penk proteins than normal), they self-administrated significantly less heroin (Figure 4). In other words, once THC was unable to cause Penk to be overexpressed, the effect on heroin intake was mitigated. Thus, increased levels of the Penk gene causes greater intake of heroin, and since THC exposure during adolescence increases Penk levels by blocking its methylation (turning off the red light), it can be inferred that adolescent THC exposure causes increased behavioral vulnerability for heroin and likely other opioids later in life.

Another study, published in 2011, exposed pregnant females to THC and measured the resulting effect on a dopamine receptor (DRD2) in their offspring. THC exposure was linked to reduced mRNA expression of DRD2 in the brain of two-day-old rat pups and, most interestingly, this reduction persisted into adulthood through the epigenetic red light of methylation (Figure 5). Reduced DRD2 protein levels are commonly seen in adult drug abusers, so fetal exposure to cannabis may build the groundwork for vulnerability to addiction and other psychiatric disorders down the road.

In 2015, Watson et al performed a genome-wide methylation study (test looking at the entire instruction manual for markers of red light) seeking to find the extent to which THC during adolescence can affect the next generation. One group’s parents were exposed to THC in adolescence in order to demonstrate group variation in DNA methylation (prevalence of red lights). By comparing the epigenetic differences in the brain of these rats, the authors identified 1027 differentially methylated regions. This means that some genes that would normally be “off” are “on,” while some that normally would be “on” are now “off” after adolescent THC exposure. This study does not itself demonstrate anything overly drastic or specific, but it provides a wealth of data that can be mined for making connections in the future.

There is undeniably a need for more research to be conducted on this topic, though the studies that have been done encourage caution when it comes to marijuana usage, especially during the developmental and adolescent years. These studies suggest that taking marijuana during adolescence subsequently makes heroin more rewarding or addictive through the lasting epigenetic changes it makes on the Penk methylation state and reward pathways as a whole. The notion that marijuana is a gateway drug may in fact have a degree of truth, as the data so far suggests that adolescent and fetal exposure to THC has long-lasting effects on the epigenome, making one more vulnerable to substance abuse. Increased research is necessary to ensure the recent legalization movement does not have unwelcome consequences on individuals’ mental and physical health.



Figure 1: Number of PubMed Articles Involving Cannabis Research
Figure 1: Number of PubMed Articles Involving Cannabis Research (Szutorisz)


Figure 2: Epigenetic Modifications to DNA and Histones
Figure 2: Epigenetic Modifications to DNA and Histones (http://www.epibeat.com/wp-content/uploads/2013/03/3.png)


Figure 3: Penk overexpression and THC exposure positively correlate with heroin self-administration.
Figure 3: Penk overexpression and THC exposure positively correlate with heroin self-administration (Tomasiewicz).


Figure 4: Knockdown of Penk in NAcsh with microRNA reduces heroin self-administration.
Figure 4: Knockdown of Penk in NAcsh with microRNA reduces heroin self-administration (Tomasiewicz).


Figure 5: DRD2 mRNA levels in rats exposed to THC in utero on pre-natal day 2 (A) and pre-natal day 62 (B)
Figure 5: DRD2 mRNA levels in rats exposed to THC in utero on pre-natal day 2 (A) and pre-natal day 62 (B) (DiNieria et.al.)


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