Marijuana use may change brain structure in adolescents

In an effort to understand the implications of marijuana use on adolescent brain development, researchers at Johns Hopkins Medicine have discovered alarming alterations in brain immune cells. 

Their findings suggest that THC, the primary psychoactive compound in marijuana, may have detrimental effects on these cells – which are crucial for teenage brain growth.

Microglial cells 

The team, led by Dr. Atsushi Kamiya, focused on the effects of THC on microglial cells. Microglia, a specialized subset of immune cells located in the central nervous system, play pivotal roles in neuron-to-neuron communication, immune response, and overall healthy brain development.

During the adolescent years, microglial cells are particularly significant. They assist in brain maturation, impacting social and cognitive functions through synapse pruning and the release of chemical transmitters. 

Structural changes 

The researchers theorized that any structural changes to these cells could disrupt the brain’s intricate wiring and messaging system, especially in teenagers.

To investigate, the team used genetically engineered mice that mirrored human genetic risks for psychiatric disorders. Alongside a control group of normal mice, both sets were either administered daily injections of THC or a benign saline solution. 

Following 30 days of injections and a three-week rest period, behavioral tests were conducted to evaluate the mice’s psychosocial development.

Programmed cell death 

The study revealed that mice exposed to THC underwent a spike in microglial apoptosis, a form of programmed cell death. More notably, genetically modified mice treated with THC showed a 33% higher reduction in microglial cells compared to the normal mice injected with THC. 

The brain’s prefrontal cortex, which is responsible for memory, social behavior, and decision-making, was the most affected region.

The team concluded that a decrease in healthy microglia might elevate abnormal cell signaling and communication. Supporting this theory, genetically altered mice exposed to THC scored 40% lower in social memory tests than their saline-injected counterparts.

Study implications 

The implications of the study are vast, especially considering the surge in both recreational and medical marijuana usage in recent years. 

“This kind of study is critical right now because marijuana is becoming more mainstream, and we are just beginning to understand how it affects the brain immune cells,” said study lead author Dr. Yuto Hasegawa.

Dr. Kamiya said that while the study results from genetically engineered mice cannot be applied directly to what happens in a human brain, studies in animals suggest there may be long lasting and negative effects of marijuana use during adolescence.

“More research is needed, but we strongly advise caution in marijuana use by teenagers,” said Dr. Kamiya.

Future research 

According to the researchers, the next step for these studies is to pinpoint exactly how microglial abnormality affects neuron function at the molecular level. From a clinical perspective, they hope to use these findings to explore how marijuana exposure contributes to schizophrenia and other psychiatric disorders.

This study was funded by the National Institutes of Health, the Mitsui Sumitomo Insurance Welfare Foundation, the Brain & Behavior Research Foundation, and the Korea Brain Research Institute.

The study is published in the journal Nature Communications.

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