Published November 27, 2019
A paper published Oct. 9 in Science Advances reveals the genetic basis of relapse, one of the most detrimental — but poorly understood — behaviors related to addiction.
Craig T. Werner, PhD, is first author on the paper, which demonstrates how prolonged abstinence after cocaine use drives gene changes that promote relapse.
“The fact that addiction is a lifelong disease suggests that something in the brain is maintaining that vulnerability to relapse, but until now, we didn’t see how gene expression changed during abstinence to contribute to this behavior,” Werner says.
The study begins to explain why abstinence — both in humans and in preclinical rodent relapse models — intensifies the craving for cocaine over time, facilitated by drug-associated cues. Those cues can be anything that is associated with the rewarding properties of the drug, whether it’s a physical location, fellow addicts or drug paraphernalia.
“The novelty of our current paper is that it begins to reveal what happens on a cellular basis; what genes are turned on and off during a prolonged period of abstinence,” Dietz says.
The new paper reports on a preclinical study with laboratory rodents who learned to self-administer cocaine and were then withdrawn from the drug.
The research focused on the nucleus accumbens, a center of reward and motivation in the brain. The researchers looked specifically at molecules known as chromatin remodelers that can facilitate or prevent access to DNA, ultimately affecting gene expression.
They found that chromatin remodeler INO80 causes changes in gene expression that enhances cocaine-craving behavior. While there was no change in expression of INO80 after one day of abstinence, there was a significant increase after 30 days of abstinence.
The findings also reveal how a class of proteins called E3 ubiquitin ligases facilitates relapse. Currently the subject of well-funded research in the pharmaceutical industry for psychiatric and neurologic diseases, E3 ubiquitin ligase proteins have yet to be well-explored in terms of their relevance to addiction and relapse.
The current study reveals that an E3 ubiquitin ligase called TRIM3 tags INO80 with a ubiquitin chain, indicating to the cell to degrade INO80. Researchers found that a decrease in TRIM3 likely underlies the observed increase in INO80 (less TRIM3 means less INO80 being degraded).
“When there’s less TRIM3 and more INO80, that leads to an increase in drug craving, but when there’s more TRIM3 and less INO80, drug-craving behavior is reduced,” Werner says.
They found this effect whether they manipulated INO80 directly, or through the alterations of TRIM3 levels.
“INO80 affects some combination of target genes,” Werner says. “In the future, we want to find out which of those downstream genes are contributing to drug-craving behaviors. This would be a big first step into this really new territory of how gene expression maintains relapse vulnerability.”
While potential therapeutic targets are, of course, the ultimate goal, the researchers emphasize that their study is allowing critical pieces of the addiction puzzle to fall into place.
“Part of the problem with addiction is that we don’t know exactly what has changed in the brain,” Dietz says. “Yes, we have found a potential therapeutic target, but the target itself is being modified by the addicted state. Until we have a better picture of what has changed with any one aspect of addiction, we will be stuck in terms of developing better treatments. This finding shines a light on the muddy waters of the neurobiology of addiction.”
Other co-authors on the October paper who are or were associated with the Department of Pharmacology and Toxicology and who worked in the Dietz lab at the time of the research are:
Other co-authors are from the University of California-Bakersfield, the Icahn School of Medicine at Mount Sinai and Massachusetts General Hospital.
A paper published in September found that exposure to heroin sharply reduces levels of the protein necessary for developing and maintaining the brain’s synapses. The development of addiction relapse is directly related to the impact that reductions in this protein, called drebrin, have on specific cells involved in the brain’s pleasure-seeking/reward pathways.
The paper, one of the first to trace the pathophysiology of addiction relapse, was published online on Sept. 12 in Nature Communications.
“Very few research studies have examined the molecular mechanisms of heroin relapse and there is almost nothing published about the specific cell types that these changes occur in,” Dietz says. “These findings lead us to a better understanding of the neurobiology of relapse to opiates. In combination with other findings, the research will hopefully provide avenues toward treatments that can prevent relapse behaviors.”
Most currently available treatments are replacement therapies, none of which address the fundamental changes that occur in addiction and lead to relapse, which remains an intractable issue.
“This study is among the first in the field of heroin addiction to thoroughly investigate the role of a protein from the very detailed level of gene regulation, to changes in neuronal structure and function, all the way to behavior,” says Martin, first author on the September paper.
Martin, a postdoctoral associate in the Department of Physiology and Biophysics and research assistant in the Department of Pharmacology and Toxicology, earned her doctorate in pharmacology and toxicology earlier this year and received the prestigious Blueprint Diversity Specialized Predoctoral to Postdoctoral Advancement in Neuroscience Award from the NIH.
Dietz and his colleagues have focused much of their research on relapse after opiate addiction and withdrawal and the structural plasticity in the brain that they cause. He was recently awarded more than $2 million from the National Institutes of Health (NIH) to continue research on drebrin and other potential targets for treating drug addiction.
Drebrin was of interest because loss of the protein has been previously implicated in brain diseases, such as Alzheimer’s disease and Down syndrome.
“Since drebrin is responsible for developing and maintaining synapses, we wondered if it was also involved in addiction to drugs of abuse, ultimately leading to relapse,” Dietz says.
In experiments with rodents, the UB team determined that exposure to heroin and morphine reduced drebrin levels in the nucleus accumbens, a key part of the brain’s reward pathway.
Researchers found that opiate exposure causes synaptic rewiring in this part of the brain, as well as a decrease in drenditic spines, the protrusions on neurons that play key roles in neuronal transmission, learning and memory.
“Opiates fundamentally change how the brain communicates with itself,” Dietz says.
They found that the reduction in drebrin levels is regulated by changes in how an enzyme called HDAC2 facilitates access to the DNA. In addition, the study demonstrates that these changes occur exclusively in a specific type of cell within the nucleus accumbens — known as D1 — which contains medium spiny neurons, the type of cells that make up this part of the reward center.
“Restoring drebrin back to normal levels in these specific brain cells was sufficient to reduce relapse behaviors,” Dietz says.
The research provides a critical and understudied insight into the mechanisms behind addiction and relapse behaviors, which in combination with future studies may lead to a novel and effective treatment to prevent relapse.
“Our lab is focused on improving our understanding of the neurobiology of addiction and relapse so that we can figure out the best way to target these pathways for future therapeutic use,” Dietz says.
Werner, Mitra, Wang, Gobira and Stewart are also co-authors on the September paper.
Other co-authors on the September paper associated with the Jacobs School are:
Other co-authors are from the University of California-Bakersfield, Massachusetts General Hospital and the University of Maryland School of Medicine.
The research detailed in both papers was funded by the National Institute on Drug Abuse of the NIH.