6. Addiction Through the Lens of Epigenetics

6. Addiction Through the Lens of Epigenetics


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     Seemingly overnight, Amy Winehouse was catapulted into the limelight with her massively successful single, “Rehab.” Rather than seeing the lyrics of this famed chorus as a cry for help, they were received as an ironic joke by the public, an invitation to ridicule Winehouse for anything related to her drug abuse and addiction. In 2011, the tone changed when Amy Winehouse was found dead on the floor of her apartment from an apparent alcohol overdose. What many people didn’t know until the documentary, Amy, came out in 2015 was that she had been clean for weeks before her overdose, and that this pattern of relapse had been occurring for quite a while. This breeds the question: Why are addicts so prone to relapse, even after long periods of sobriety? Does the saying, “Once an addict, always an addict” have a biological truth?

Two professors at the University of Michigan, Kent C. Berridge and Terry Robinson, have a compelling theory about addiction. You may have heard of dopamine, the neurotransmitter, as the “feel good” molecule.While this is not necessarily inaccurate, Berridge and Robinson’s research shows that it’s reductive to think about dopamine as the “pleasure” molecule, and one of the main areas of the brain that it operates in (the nucleus accumbens) as the “pleasure center”. Grouping it together as a singular pleasure center doesn’t distinguish between the “liking”–or hedonistic pleasure of something–and the “wanting,” or the learned, motivated state of mind towards that special something. In reality, one may be able to exist without the other. Ever felt the excitement of a Friday, itching for a night out on the town? Ever been disappointed with how the night played out? Our anticipation and wanting of these experiences that we expect to be exciting and pleasurable sometimes maybe more powerful than the pleasure derived from the experiences themselves.

In order for researchers to distinguish wanting and liking in the brain, they recorded dopamine levels—the“feel good” molecule of the brain—in the nucleus accumbens (“pleasure center”) of rats as they performed certain behaviors. The rats were given sugar water–a tasty treat for a rat — and they saw what other researchers had always seen: an increase in dopamine at the moment of consumption. Then, they trained the rats to associate the sugary water with a red light, so whenever the red light went on, the rats knew their treat was on its way. What they found next came as a major scientific discovery: When the red light was presented, they saw an increase in dopamine; however, when the rat then consumed the actual reward (the sugar water), there was no increase in dopamine. The supposed “feel good” molecule, dopamine, was only being released in anticipation of the sugary snack, not from the actual experience of it.

Thus, dopamine appears to act as a “neural currency”, or a dollar value in the brain, that assigns value to stimuli that we’ve had past experiences with based on their hedonistic/beneficial properties. When the same stimulus is anticipated—for example, when it’s preceded by some sensory cue such as the red light in the experiment—the brain can interpret that “dollar” value previously assigned to the stimulus, and make a decision about what to do with it. 

Our brain encodes experiences with drugs in the same way, memories of these pleasurable experiences and the environmental cues associated with them are recorded with the neural currency of our brain, dopamine.These memories persist and the motivated behavior, or “wanting,” towards the reward can be easily reinstated by a number of triggers (or cues, such as the red light in the experiment) associated with the reward. This may explain why relapse occurs even after long periods of sobriety: The “wanting” persists without the “liking,” and can be evoked by objects associated with past drug use such as pipes, syringes, other forms of paraphernalia, and even settings or places in which the drug use commonly took place. Think of a bad breakup, either from a partner or friend. It takes a while to get over, let’s say a couple of months. But, even once you think you’ve moved on, you see, hear, or even smell something that reminds you of that person, and it can dredge up the emotions from that relationship. It can make you want that relationship back, even if you didn’t actually like it when it ended. A parallel can be drawn between this sensation and that of an addict seeing the things they associated with their drug use, dredging up the feeling and longing associated with it, and thereby making them more prone to relapse.

This neural currency model, which is analogous to the brain assigning a monetary value to to our experiences, not only shapes the way we experience drugs and whether or not we “want” them, but new research suggests that drugs may be changing the landscape of our brain’s reward circuitry.

While many people have heard about “addiction genes” that are passed from generation to generation,few have heard about the more recently uncovered process of epigenetics. In biology, the central dogma is, “DNA is transcribed into mRNA, and mRNA is translated into proteins that serve some function.” Jargon aside, you can think of this as your DNA being the master set of instructions to building all of the bits and pieces that make your existence possible. mRNA is like a mechanic that uses the instructions in the DNA to build the machines, or proteins, that carry out many of the processes in your body that make you uniquely you.

All of your body’s cells contain this master set of instructions—also known as the genome—but only some of those instructions are used in each cell in your body. For example, your eye cells don’t necessarily use the same set of instructions—that is, genes—as your skin cells because they have to perform different functions. So there has to be a way to essentially ignore the instructions that they don’t need, and pay attention to the ones they do; this is where the field of epigenetics begins.

“Epi,” meaning “above,” is indicative of the fact that the epigenome and the study of it, epigenetics, refers to the structure above the level of the DNA sequence (the unchanging order of the genes, or instructions). You can think of the epigenome as a comprehensive library of the instructions in our DNA, with walls and walls of bound books, each book containing a different set of instructions, or genes, present in the genome.

Just as a library is maintained and regulated by librarians, our genome is regulated by molecular machines, or proteins, that perform various functions. If all the pages in the library were to be unbound, it would be chaos. Papers would be all over the place, and it would be impossible to quickly access the correct set of instructions. Thus, our DNA is bound into these “books” so that it is organized and can be easily labeled with tags that indicate whether or not the book is important.

This is essentially what the epigenome does for each of our cells: It not only compacts the DNA so that it can fit into the cells in the first place— (without the epigenome, our DNA is around 1 meter long!—but it also allows cells to focus on the instructions, or genes, that they need, and ignore the ones they don’t, acting as a filing system for our genome. Using the previous example: The epigenomic library of an eye cell would be organized differently than that of a skin cell, in turn allowing each cell type to access the genes that they need and ignore the ones they don’t. 

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Figure 1: DNA methylation. Orange M’s are like “IGNORE” flags.

One mechanism that is central to the organization of this epigenomic library and the study of epigenetics is DNA methylation, which is when genes are marked, or “flagged”, with a chemical tag called a methyl group. Although DNA methylation may have complex effects, for our purposes, we can think of methylated genes as being silenced or repressed (a set of instructions with a sticky-note that says “IGNORE”), and de-methylated genes as active, or ready to be read.

But epigenetics isn’t only used to help differentiate one cell type from another. Scientists are uncovering the dynamic nature of the epigenome with exciting findings that show how environmental factors, such as maternal care in childhood or drug use, can induce changes in the ability of certain “instructions” (or genes) to be read, which may consequently alter the way we would normally respond to these environmental factors in the future.

Jian Feng and Eric Nestler, two researchers at Mount Sinai Medical Center in New York, provide strong evidence for this argument. Last year, they published a paper in the highly respected scientific journal, Nature,which shows how cocaine use can alter the instructions, or genes, being read in the brain. Specifically, they found that rats repeatedly treated with cocaine showed a decrease in the levels of a “molecular machine,” or a type of protein known as an enzyme, called TET1 in the nucleus accumbens. This is the same area of the brain we discussed earlier that mediates reward and motivated behavior by interpreting levels of the neural currency, dopamine. Essentially, there was something about repeated use of cocaine that caused a reduction in the levels ofthe molecular machine, TET1. 

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Figure 2: Conditioned place preference paradigm.

Previous studies have shown that TET1 removes these IGNORE flags, but Feng and Nestler’s findings suggest that TET1’s function may not be fully understood. They found that after repeated cocaine use and the associated decrease in TET1, there was a decrease in the IGNORE flags. But, if TET1 removes these IGNORE flags as previous studies suggest, then a decrease in TET1 should result in less flags being removed, and therefore an increase in the number of flags overall. Feng and Nestler’s results show that either the function of TET1 is not as clear as we once thought, or that there is something else going on with cocaine use that causes these seemingly contradictory results.

But why does it matter that repeated cocaine use in rats leads to this decrease in TET1? Well, Feng and Nestler’s research also suggests that TET1 is involved in the“rewarding” properties of drugs. They used an experimental set up, one commonly performed in behavioral neuroscience to determine a rat’s preference between two substances,called a “conditioned place preference paradigm”. In this case, they were determining the rats’ preferences for either cocaine or salt water. Basically, the rats are administered cocaine in one box and salt water in another distinct box; they will spend significantly more time in the box that has the substance they prefer, or, will spend roughly equal time in each box if there is no real preference. In order to assess the role of TET1 in reward, they “deleted” the TET1 “book” in the rats, which means that the rats no longer had the set of instructions to make TET1 in their nucleus accumbens. These so-called “knockout” rats showed a robust preference for the box in which they were administered cocaine. However, when TET1was injected back into the nucleus accumbens of the “knock out” rats, essentially restoring TET1’s function, they found that the preference for the box that they received cocaine in significantly decreased. Thus, something about having larger amounts of this enzyme (or molecular machine), TET1, reduced the rewarding properties of cocaine.

So again, why does it matter that repeated cocaine use in rats leads to a decrease in TET1, which is likely involved in DNA methylation? If cocaine use results in a decrease in TET1, and lower levels of TET1 result in a stronger preference for cocaine, then perhaps cocaine itself is able to increase a rat’s preference for cocaine. To draw a parallel, here’s a question: If you like potato chips, how many times have you been able to eat just one?Probably not very often. The first chip reminds you of how much and why you like them, and the rest of the bag naturally follows. In a more extreme version, repeated cocaine use may be increasing how much these rats like or want cocaine in the future; they want to finish the rest of the bag, and then move onto the next.

And this brings us back to Berridge and Robinson’s theory of addiction. Feng and Nestler found that the epigenetic modifications they observed in response to cocaine use remained even a month after cocaine use stopped. Thus, if these epigenetic changes (or changes in the IGNORE flags on sets of instructions)are the result of a down regulation of TET1 after cocaine use, then cocaine itself may have the capacity to change how we interact with cocaine, long after the cessation of its use. And since this was a relatively persistent state of decreased TET1 and decreased IGNORE flags. This may partly account for why the risk of relapse is so enduring: Lasting changes to addicts’ IGNORE flags, or their epigenome, have increased the “neural currency” assigned to cocaine with each use, and thus their “wanting” for cocaine increases as well. This increased “wanting” may endure long through periods of sobriety, and later be triggered by stimuli associated with the earlier cocaine use. When the “wanting”, or desire, for that drug becomes too strong, relapse becomes more likely. 

It may not be terribly surprising that cocaine can induce changes that last for months, but there’s no way a father’s cocaine use can affect his future son’s experience with the drug… right? 

Wrong. In 2013, Robert Pierce and Fair Vassoler at University of Pennsylvania’s Perelman School of Medicine published a study in, once again, Nature, that showed just that.

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Figure 3: Self administration example set up.

They took male rats, and allowed them to “self-administer” cocaine injections whenever they pressed a lever. Basically, they could get high as much as they wanted for 60 days, which is the same amount of time it takes for new sperm to develop. The 60-day time period was critical because they wanted to see what effects cocaine use would have on the sperm of these rats and, in turn, on the offspring of these rats.

After the 60 days, they mated the male rats with females and looked at the rewarding properties of cocaine in their pups. They found that the male offspring self-administered cocaine significantly less than their sisters and control rats. In other words, the father’s cocaine use altered the way his sperm developed such that it induced lasting changes in only his sons that caused their brain to receive little reward from the experience of getting high. It makes you wonder if these rats actually get less high from cocaine, and if these rats are less likely to develop an addiction to cocaine. 

Pierce and Vassoler were curious with how the father rat’s sperm could pass on this aversion to cocaine to its male offspring. BDNF, or brain derived neurotrophic factor, is an extremely important protein: It helps to keep the brain healthy by improving communication between neurons. Neurons communicate through “contact points”called synapses, and with increased BDNF, this communication system can be fine-tuned and optimized. Pierce and Vassoler found that the sons of fathers that used cocaine had an increase in the levels of this essential protein,BDNF. Because BDNF fine-tunes communication between neurons, its possible that the increased levels seen in these male offspring may be the difference between a rat that is able to more successfully control its appetite for cocaine as opposed to one that is more susceptible to addiction.

Still, you may be wondering: How did the father’s sperm pass on a decreased preference for cocaine to his son that the father himself didn’t have? In this case, the apple fell further from the tree. In analyzing the sperm of the cocaine addicted father, they found an increase in activating chemical tags, ones that promote the reading of books of instructions, or genes. Somehow, the use of cocaine affects the rat’s developing sperm such that there is an increase in the expression of proteins that tag genes with this activating chemical flag. In turn, these proteins can flag other genes to be read, increasing the levels of those as well, one of which is the protein involved in healthy communication between neurons, BDNF.  Although the mechanism still remains a mystery, their findings highlight that while drug use obviously affects the individual user, it may also impact their children’s preferences for the drug as well. In this case, cocaine use induced lasting changes that impacted the male offspring to find cocaine about as rewarding as salty water, which is not very rewarding at all.

While it may seem tempting for all of the guys reading this to go out and find all of the cocaine you can—you know, for the sake of your future sons—we suggest you hold off on that. The findings we’ve discussed are undeniably intriguing and exciting, but these are controlled studies in rats looking at a sliver of the impact cocaine use has on our biology. And while rats are great models for understanding these complicated biological systems,they still aren’t humans; we can’t assume that everything directly translates from rat to human. Additionally,epigenetic modifications like chemical tags on DNA are dynamic, so it’s also possible that changes induced by some environmental stimuli may be reversed over time.

We instead present this information to prompt reflection on how we all view both addicts and addiction.As is often the case with addicts we don’t have any personal connection with, we tend to categorize them as harmful, helpless, and in some ways, less human. The public treated Amy Winehouse like an object and used her addiction as a source of entertainment, only sympathizing once she died. How can we change that timeline,instead empathizing at the onset of addiction rather than after it’s too late? 

 The mentality surrounding addicts extends beyond individual cases, like Amy Winehouse, and into systemic issues. Take the justice system: nearly half of the people in prison are there because of drug-related offenses, and the majority of these are not your Pablo Escobars of the world, but rather people caught with personal amounts of addictive substances. The estimated cost of these drug-related incarcerations adds up to $51 billion dollars a year. So why do we throw addicts into prison in lieu of getting the treatment they need? Wouldn’t it be more cost effective and ethical to treat small drug offenders, lessening their likelihood to end up back in the system? 

These findings highlight the importance of recognizing that while addiction can arise from the initial choice to try a drug, its maintenance is likely rooted in molecular changes (such as epigenetic changes) that may endure even after long periods of sobriety, making relapse a persistent challenge. There is clearly a complex relationship between nature and nurture, an ever-evolving exchange where our environment and decisions have the potential to change our own, and even our offspring’s, biological nature. Stigmatizing addicts is to assume we understand this relationship. We don’t, and nobody fully does; therefore, we can’t attribute addicts’ behavior purely to choice.

Although dispelling the stigmas associated with addicts is an uphill battle, we hope that, through this podcast and increasing the general awareness of the neurological nuances of addiction, we can contribute to the larger conversation. Knowledge is the first step to understanding, and understanding is the first step towards greater sympathy at both the interpersonal and policy level, manifesting in more sensitive, open, and effective support systems. And if nothing else, we hope that this podcast inspires an appreciation for not only the complexity that underlies addiction, but the complexity that underlies human nature as a whole.  By pausing to consider these intricacies, we can move away from making impulsive assumptions about others and towards a greater sense of communal empathy. 

If you found our discussion on the epigenetics of addiction interesting, keep the conversation going with the next article and podcast by Mark and Aditya.

References

Day, Jeremy J., Mitchell F. Roitman, R. Mark Wightman, and Regina M. Carelli. “Associative Learning Mediates Dynamic Shifts in Dopamine Signaling in the Nucleus Accumbens.” Nature Neuroscience Nat Neurosci 10.8 (2007): 1020-028. Web.

Federal Bureau of Prisons. (n.d.). Retrieved April, 2016, from https://www.bop.gov/about/statistics/statistics_inmate_offenses.jsp

Feng, Jian, Ningyi Shao, Keith E. Szulwach, Vincent Vialou, Jimmy Huynh, Chun Zhong, Thuc Le, Deveroux Ferguson, Michael E. Cahill, Yujing Li, Ja Wook Koo, Efrain Ribeiro, Benoit Labonte, Benjamin M. Laitman, David Estey, Victoria Stockman, Pamela Kennedy, Thomas Couroussé, Isaac Mensah, Gustavo Turecki, Kym F. Faull, Guo-Li Ming, Hongjun Song, Guoping Fan, Patrizia Casaccia, Li Shen, Peng Jin, and Eric J. Nestler. “Role of Tet1 and 5-hydroxymethylcytosine in Cocaine Action.” Nature Neuroscience Nat Neurosci 18.4 (2015): 536-44. Web.

Robinson, Terry E, and Kent C Berridge. “The Incentive Sensitization Theory of Addiction: Some Current Issues.” Philosophical Transactions of the Royal Society B: Biological Sciences 363.1507 (2008): 3137–3146. PMC. Web. 19 Mar. 2016.

Sledge, M. (n.d.). The Drug War And Mass Incarceration By The Numbers. Retrieved April, 2016, from http://www.huffingtonpost.com/2013/04/08/drug-war-mass-incarceration_n_3034310.html

Vassoler, Fair M., Samantha L. White, Heath D. Schmidt, Ghazaleh Sadri-Vakili, and R. Christopher Pierce. “Epigenetic Inheritance of a Cocaine-resistance Phenotype.” Nature Neuroscience Nat Neurosci 16.1 (2012): 42-47. Web.

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