Like humans, not all rats develop the same level of addiction. The researchers studied genetic and molecular differences that may underlie these differences.
Millions of people in the United States suffer from drug addictions.1 “There’s a lot of stigma related to addiction,” said Francesca Telese, a neuroscientist at the University of California, San Diego. “Not everyone realizes that … it’s a disease like many other complex diseases.”
To understand addiction, researchers typically look at neurons firing or molecular signatures across the brain, but these tools cannot nail down specific addiction-related molecules in different cell types. To get a more detailed understanding of addiction’s molecular underpinnings, in a study recently published in Nature Genetics Telese and her collaborators used single cell resolution technologies to better understand cocaine addiction in a diverse pool of rats.2 Focusing on the amygdala, the part of the brain involved in emotions and memories, they found that many molecular markers of addiction relate to how cells generate and use energy, which may point to a treatment for addiction-related behaviors.
Researchers typically use genetically identical rats in their studies, which do not reflect how addiction behaviors vary between individuals. The new study, co-led by Telese and Graham McVicker, a geneticist at the Salk Institute, instead used genetically diverse rats. “That’s a challenge,” said Elizabeth Heller, a neuroscientist at the University of Pennsylvania who was not involved in the study. “But the authors overcame it brilliantly and made this enormous contribution to our understanding of these strains’ genetics and gene expression profiles.”
The researchers first gave the rats cocaine, and the animals learned how to self-administer the drug. Eventually, they developed either high or low levels of addiction. To study how addiction in the absence of drugs affected the brain, the researchers next withheld the drug to make the rats go through withdrawal. By using single-nuclear RNA-seq and ATAC-seq to measure gene expression in the rats’ amygdalas, Telese and McVicker’s team determined how accessible different parts of the DNA were in each individual cell. This allowed them to survey which genes were the most active in different cell types and pair this information with measurements of the rats’ drug-taking behaviors and neuronal signaling.
The researchers found that rats with stronger addiction had differences in genes related to energy production and usage. “The way cells use energy is very important for addiction related behaviors,” said Telese. One way that energy could affect brain cells is by changing GABA neurotransmitter signaling—the main way that neurons inhibit signal transmission—which had been implicated in addiction by previous studies.3 When the researchers measured GABA signaling, it was elevated in the addicted rats’ inhibitory neurons.
The scientist thought that changes in energy usage could also affect how cellular machinery works. For example, some proteins called pioneer transcription factors can bind to DNA and change its conformation to affect gene expression, which requires energy. Looking at the ATAC-seq data, the researchers observed that the DNA regions that were easier to access were places where pioneer transcription factors bind. The team hypothesized that this could be another way that energy is involved in addiction, and genetic differences between individuals could change the transcription factors’ activity and the strength of the resulting addiction.
While this study focuses on cocaine addiction, Telese was encouraged to see a recent pre-print that found similar changes in metabolism and energy in monkey and human models of opioid addiction.4 “Not all drugs are the same, but the hallmarks of addiction are the same,” Telese said. “It’s possible that this is a common mechanism in different addictions, but of course we need to test it.”
McVicker hopes to expand the study to more rats to capture wider diversity in genetics and addiction behaviors, which could help the researchers find genetic markers that predict whether an individual will become addicted. “We absolutely have to identify [genetic markers] in order to help figure out who is most susceptible in the population and maybe be able to provide prophylactic support,” Heller said.
The researchers are seeking a potential drug that can target these addiction-related pathways in humans. In this study, the team tested a molecule that blocks the glyoxalase 1 enzyme, which is involved in GABAergic signaling, and found that it reduced the rats’ tendency to seek out and use drugs after a drug-free period. To translate to humans, Heller thought that the therapy may need to be more targeted to avoid affecting other behaviors controlled by this type of signaling, but she emphasized the importance of pursuing drugs to treat addictions. “We have no therapeutic options,” Heller said. “Moving into this space is extremely important for the patient population.”
McVicker agreed, adding that genetic studies may also make a dent in the existing stigmas surrounding addiction. “Our study [shows] that there are molecular underpinnings that relate to your genetics,” said McVicker. “When we think about that epidemic of drug addiction, we should be thinking of treating it like it’s any other disease.”
References:
- Substance Abuse and Mental Health Services Administration. Key Substance Use and Mental Health Indicators in the United States: Results from the 2021 National Survey on drug use and health. U.S. Department of Health and Human Services. https://www.samhsa.gov/data/sites/default/files/reports/rpt39443/2021NSDUHFFRRev010323.pdf
- Zhou JL, et al. Single-nucleus genomics in outbred rats with divergent cocaine addiction-like behaviors reveals changes in amygdala GABAergic inhibition. Nature Genetics. 2023;26(11):1868-1879.
- Nutt DJ & Nestor LJ. The GABA system and addiction. In: Addiction (2nd ed.) Oxford, United Kingdom: Oxford University Press; 2018. p. 55-68.
- Phan BN, et al. Single nuclei transcriptomics in human and non-human primate striatum implicates neuronal DNA damage and proinflammatory signaling in opioid use disorder. bioRxiv. 2023.
