In a new study published in the journal Cell Reports, scientists have mapped how chronic exposure to ketamine, a drug commonly used for anesthesia and increasingly for treating depression, impacts the dopamine system in the brains of mice.
The findings provide evidence of significant structural plasticity in the brain’s dopaminergic system in response to chronic ketamine exposure. In other words, the study shows that repeated use of ketamine can lead to major changes in the areas of the brain that deal with dopamine, a neurotransmitter that plays a pivotal role in mood, motivation, and reward systems.
But why focus on ketamine?
Ketamine, known for over 50 years as a dissociative anesthetic, has gained attention in recent times for its fast-acting antidepressant properties. However, its antidepressant effect is short-lived, which often necessitates continuous or long-term treatment. This extended use raises concerns about potential side effects. Given ketamine’s broad clinical significance and the risk associated with its long-term use, especially at higher doses, it is crucial to comprehensively understand the drug’s impact on the brain.
“Our primary motivation was to better understand whether and how long-term drug use alters the neural circuits in the brain. In this study, we focused on the impact of ketamine across the entire brain,” said study author Raju Tomer, an associate professor of biological science at Columbia University.
“Ketamine has been used as a dissociative anesthetic for over 50 years and has found applications in various clinical contexts, including as a transformative antidepressant. However, the antidepressant effect of ketamine is transient, lasting only about a week, thus necessitating long-term maintenance treatment. This continuous usage poses significant risks for side effects. Therefore, we aimed to develop whole-brain mapping techniques to comprehensively and objectively assess how varying doses of ketamine affect the brain at a subcellular level and to understand the potential underlying molecular mechanisms.”
The study involved experimentation with male mice, specifically bred for this type of research. The mice were 8-10 weeks old and were subjected to varying doses of ketamine or a saline control for different durations (1, 5, and 10 days). The ketamine doses used were 30 milligrams per kilogram of body weight and 100 milligrams per kilogram, mimicking both therapeutic and high-dose recreational use.
The researchers employed sophisticated brain-clearing and staining techniques, which allowed them to visualize and analyze neurons in the entire brain with high resolution. They used a light-sheet microscopy method, a cutting-edge technology that provides detailed 3D images of brain tissue. The data analysis involved processing large datasets (around 100 Terabytes), which required the development of specialized software for accurate and comprehensive analysis.
By focusing on the entire dopaminergic system in mice and examining the drug’s effects on neuron populations and their projections, the study aimed to provide a comprehensive view of ketamine’s diverse effects on brain function and structure.
After 10 days of exposure, the brains of mice treated with lower doses of ketamine showed an increase in the number of dopamine-related neurons in certain areas of the hypothalamus. This part of the brain plays a key role in regulating many vital processes, including mood, hunger, and sleep. In contrast, higher doses of ketamine led to a decrease in neuron numbers in specific midbrain regions. These areas are associated with controlling behavioral states, indicating that higher ketamine doses could significantly impact these brain functions.
The study also revealed that chronic ketamine exposure altered the patterns of neuronal projections – the paths neurons use to communicate across different brain areas. Notably, there was an increase in the density of these projections in areas related to higher-order cognitive functions, such as the prefrontal cortex, which is involved in decision-making and social behavior.
Conversely, regions involved in processing auditory and spatial information displayed a decrease in neuronal projections. This finding suggests that ketamine could rewire the brain’s communication pathways, potentially leading to changes in how information is processed and integrated.
“The restructuring of the brain’s dopamine system that we see after repeated ketamine use may be linked to cognitive behavioral changes over time,” explained co-author Malika Datta.
Another significant discovery was the involvement of untranslated messenger RNA (mRNA) in the brain’s response to ketamine. In neurons, untranslated mRNA is a form of genetic information not immediately used for protein production. The researchers found that these untranslated mRNAs play a role in the brain’s adaptability to chronic ketamine exposure.
Specifically, they provide a reserve that can be rapidly utilized to modulate the number of dopamine-producing neurons in response to the drug. This mechanism indicates a complex layer of regulation within the brain that allows it to adapt quickly to external stimuli, like drug exposure.
The comprehensive analysis provided a detailed view of how ketamine alters the brain’s structure on a wide scale. The increase in neuron counts in some hypothalamic areas and the decrease in midbrain regions highlight a divergent effect of ketamine, affecting different brain regions in opposite ways. This divergence suggests that ketamine’s impact on the brain is highly complex and region-specific.
“Prolonged use of ketamine can profoundly reshape the brain’s neural circuitry,” Tomer told PsyPost. “This study also shows that drugs can affect similar types of cells in different ways across various brain regions, highlighting the need for targeted delivery methods.”
These findings have significant implications for our understanding of brain function and the treatment of brain disorders. By showing how a psychoactive substance like ketamine can induce widespread changes in the brain, this research contributes to the understanding of the neural underpinnings of mental health conditions like depression and schizophrenia. It also underscores the importance of considering the dosage and long-term use when using drugs like ketamine therapeutically.
“Ketamine rapidly resolves depression in many patients with treatment resistant depression, and it is being investigated for longer term use to prevent the relapse of depression,” said co-author Bradley Miller, a Columbia psychiatrist and neuroscientist. “This study reveals how ketamine rewires the brain with repeated use. This is an essential step for developing targeted treatments that effectively treat depression without some of the unwanted side effects of ketamine.”
While the study shows changes in dopamine-related brain regions, it’s important to remember that these regions also contain neurons involved in producing other neurotransmitters. Therefore, the observed changes might not be exclusively related to dopamine.
“This study specifically investigated the impact of ketamine on the brain’s dopamine systems,” Tomer explained. “Future research is needed to explore its effects on other systems in the brain. Additionally, further detailed investigation into the specific neural and molecular mechanisms that enable structural remodeling and associated behavioral changes is necessary.”
The study, “Whole-brain mapping reveals the divergent impact of ketamine on the dopamine system“, was authored by Malika S. Datta, Yannan Chen, Shradha Chauhan, Jing Zhang, Estanislao Daniel De La Cruz, Cheng Gong, and Raju Tomer.
