In the last few decades, scientists have come to recognize that the gut microbiome isn’t only important for metabolism, but it also significantly influences neurological health. Early gut-brain axis research focused heavily on neurodegenerative disorders such as Parkinson’s disease and Alzheimer’s disease, and researchers now also investigate the relationship between gut microbiota and neuropsychiatric conditions (1).
Drew Kiraly, a physician-scientist specializing in neuroimmune interactions and gut-brain signaling in neuropsychiatric disease at Wake Forest University, was among the first to venture into the connection between the gut microbiome and drug addiction. He began to explore this relationship as a resident in Eric Nestler’s lab at Mount Sinai School of Medicine. “I brought it up to [Nestler], and he’s like, ‘Well, this seems like a sort of crazy idea, but let’s try it,’” Kiraly said.
In a 2016 study published in Scientific Reports, Kiraly reported that changes in gut microbiota in rodents influenced drug seeking behavior in response to cocaine, providing the first evidence of a link between gut microbiota and drug abuse (2). Now, in a recent study published in Neuropsychopharmacology, Kiraly’s group revealed that bacterially derived metabolites influence drug seeking behavior and transcriptional regulation in an animal model of cocaine use and relapse (3). This finding indicates a unique role for gut bacteria metabolites in regulating drug seeking behavior, highlighting the gut microbiome as a new therapeutic target for cocaine use disorder.
To explore the gut microbiome’s effect on drug seeking behavior, the team treated rats with either water containing nonabsorbable antibiotics to establish microbiome depletion or untreated water. After two weeks, they trained the rats to press a lever in order to self-administer cocaine. Each cocaine delivery was paired with a light cue, conditioning the animals to associate the cue with the drug reward. The researchers then performed a test where the amount of drug dispensed with each lever press decreased over time, requiring the rats to press the lever more frequently to obtain the same dose of drug. The researchers found that the antibiotic-treated rats engaged in more lever pressing activity than the control rats.
“That's the story that seems to be [be]coming more and more clear as we've done all these studies with different drugs of abuse,” Kiraly said. “The lower the dose, or the less available the drug is, the more the microbiome depleted animals seek it out.”
To study the gut microbiome’s role in addiction relapse, the group then evaluated the effect of microbiome depletion on cocaine seeking after a period of abstinence. The researchers allowed antibiotic-treated and untreated rats to self-administer cocaine and then separated them from the cocaine source. After 21 days of abstinence, the team reintroduced the lever and light cue but not the drug, simulating exposure to drug-associated triggers that increase the risk of relapse. Again, the rats treated with antibiotics pressed the lever more than the untreated rats, demonstrating that microbiome depletion produces greater cocaine seeking behavior even after prolonged abstinence. “The biggest issue is the propensity to relapse over time,” Kiraly said. “The fact that we're able to look at how the microbiome affects this kind of relapse type behavior, I think is really cool.”
The researchers next set out to identify metabolites that control the behavioral effects. Intestinal analysis revealed that the antibiotic-treated rats showed significantly lower levels of three of the most abundant short chain fatty acids (SCFA), molecules produced by microbes that play an important role in gut-brain signaling (4).
The team then reintroduced the three SCFA — butyrate, acetate, and propionate — to see if this would remove the behavioral effects of antibiotic treatment. They provided rats with either water, water treated with SCFA, or water treated with SCFA and antibiotics and subjected them to the cocaine self-administration and abstinence regimen. Adding SCFA to the antibiotic treatment decreased lever pressing to values similar to the pure water and SCFA only groups. This result indicates that SFCA treatment reversed cocaine seeking behavior due to microbiome depletion.
The lower the dose, or the less available the drug is, the more the microbiome depleted animals seek it out.
- Drew Kiraly, Wake Forest University
To further probe this phenomenon, the group performed RNA sequencing of the nucleus accumbens (a key reward center in the brain) after the cue-induced drug seeking task. Antibiotic treatment negatively affected gene expression related to synapse function and nervous system development, both of which may help regulate drug seeking behavior. They also found that many of these changes disappeared with SCFA treatment.
“I like this paper a lot because they're actually going in the direction of mechanism. They're starting to tease it, which is what we need to be doing,” said Vanessa Sperandio, a microbiologist and immunologist at the University of Wisconsin, Madison who was not involved in the study.
Questions remain regarding how generalizable the study’s findings are. For example, the researchers obtained their results only when they tested male rodents. To address this, Kiraly is currently conducting further experiments to uncover potential sex differences in the relationship between microbial depletion and drug seeking behavior.
In the future, the team intends to study whether the timing of SCFA repletion matters when influencing drug seeking behavior and transcription. They also plan to administer butyrate, acetate, and propionate separately and in different pairings to determine if the observed effects are due to one individual SCFA or a specific combination. Dietary changes that boost SCFA levels may provide a simple and sustainable way to implement this information into treatment, since these molecules are produced during the fermentation of fiber found in many fruits and vegetables.
Kiraly is also interested in exploring the possibility of using the microbial and genetic changes associated with drug seeking as biomarkers for identifying people who may be more prone to relapse. “One of the most frustrating things for me as a psychiatrist is I have almost no objective data to go on when I’m working with the patient. There's no real lab tests or imaging studies or anything else that we can do that will help really risk-stratify people,” Kiraly said. “This idea for me is almost as exciting as treatments, if you can say this is a marker that we could really use in the clinic.”
References
- Emeran, A., Mayer, K.N., & Chen, S. The gut–brain axis. Annu Rev Med 73, 439-453 (2022).
- Kiraly, D. et al. Alterations of the host microbiome affect behavioral responses to cocaine. Sci Rep 6, 35455 (2016).
- Meckel, K.R. et al. Microbial short-chain fatty acids regulate drug seeking and transcriptional control in a model of cocaine seeking. Neuropsychopharmacol (2023). In press.
- Koh, A., De Vadder, F., Kovatcheva-Datchary, P., & Bäckhed, F. From dietary fiber to host physiology: short-chain fatty acids as key bacterial metabolites. Cell 165, 1332-1345 (2016).
