Single dose of rapamycin rapidly reduces autism-like traits in adult mice

A new study published in the journal Nature Communications suggests that exposing mice to mild inflammation during pregnancy triggers brain and behavioral changes in their offspring that resemble autism spectrum disorder. The research provides evidence that a single dose of the immunosuppressive drug rapamycin can rapidly but temporarily reverse these symptoms in adult mice by altering brain function rather than physical structure.

Autism spectrum disorder is a complex condition associated with diverse changes in brain development, behavior, and sensory processing. One known risk factor is maternal immune activation, which occurs when a pregnant mother experiences an infection or inflammation. This inflammatory response can alter the development of the fetal brain. In both human and animal studies, such early immune events are linked to later neurodevelopmental differences, including increased brain volume in early life, altered social behaviors, and heightened sensitivity to sensory input.

At the cellular level, many of these changes are associated with the overactivation of the mTOR pathway. The mTOR pathway is a biological signaling network that regulates cell growth, division, and survival. When this system is hyperactive, it tends to lead to abnormal synapse formation, an imbalance between excitatory and inhibitory brain signals, and an increased susceptibility to seizures.

“We have a longstanding interest in the mTOR system in brain development and autism spectrum disorder,” said study authors Harley Kornblum, director of the UCLA Intellectual and Developmental Disabilities Research Center and a professor of psychiatry, pediatrics, and pharmacology; Neil Harris, a professor of neurosurgery; and Janel Le Belle, an associate professor of neurosurgery. “Our earlier study also led by Dr. Le Belle showed that mild maternal inflammation in a strain of mouse activated the mTOR system and resulted in many behaviors reminiscent of autism and that are also found in genetic mouse models in which we know that the affected genes cause autism in people.”

Rapamycin is an established drug that inhibits the mTOR pathway. It is commonly used in medicine to prevent organ transplant rejection. In previous animal research, treating young mice with rapamycin over several weeks prevented the physical brain abnormalities associated with certain genetic mutations linked to autism.

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“We were interested in trying treatment with the mTOR inhibitor rapamycin, which is used in children with disorders that activate the mTOR pathway,” the researchers added. However, chronic use of rapamycin can suppress the immune system and inhibit healthy growth. The authors designed this study to observe what happens when adult mice receive a short-acting, single dose of the drug, focusing on immediate functional changes in the brain rather than long-term structural remodeling.

The authors triggered a mild maternal inflammatory response in pregnant mice by injecting them with lipopolysaccharide, a bacterial compound that induces an immune reaction. This low dose was designed to provoke a response without making the pregnant mice noticeably ill. The offspring were then raised to early adulthood or older adulthood, creating experimental groups of male and female mice to compare against a control group that received a harmless saline solution.

The researchers first evaluated the physical and molecular characteristics of the offspring exposed to maternal inflammation. They tracked brain weights from birth to 200 days old for 16 mice per group. The exposed mice experienced mild brain overgrowth early in life compared to the control group, but by day 200, this growth had slowed, resulting in brain weights slightly below the control average.

The exposed mice also maintained chronically elevated levels of immune proteins, known as cytokines, in their blood. Their brain tissue showed persistent overactivation of the mTOR pathway. To see if immune cells were driving the ongoing issues, the researchers depleted microglia, a type of brain immune cell, in some mice. This reduction improved behaviors in young adult mice but failed to help older adult mice, suggesting that other functional mechanisms maintain the behavioral traits later in life.

To test behavior, the scientists observed groups of 26 mice in open field tests. The exposed mice spent roughly twice as much time engaging in repetitive behaviors, such as grooming and circling, compared to the control mice. When the researchers administered a single injection of rapamycin at a dose of 5 milligrams per kilogram, the repetitive behaviors in the exposed mice dropped to match the levels seen in the control mice within two hours.

“We were very surprised by the rapidity of the effects of rapamycin,” the researchers told PsyPost. “We expected that if the mTOR system was still activated in the adult, it would be influencing the structure of how brain cells connect with each other, which would mean that anticipated effects would take longer than just a few hours.”

This behavioral rescue was temporary, as the repetitive behaviors returned to their previous elevated levels 72 hours later. The authors also tested daily injections over five weeks in groups of 10 mice. They found that the mice developed a tolerance to the drug, leading to a gradual loss of its behavioral benefits.

The study also measured sensory over-responsivity, a common trait where individuals are highly sensitive to touch or sound. Using groups of eight mice, the researchers tested tactile avoidance by placing the animals in a box with both smooth and rough floor surfaces. The exposed mice actively avoided the rough-textured floor, spending less time there than the control mice. Following a single dose of rapamycin, the exposed mice increased their time spent on the rough floor, indicating a normalization of their sensory tolerance.

Sensory sensitivities can disrupt daily life and exacerbate other challenges. “Our results point to a significant role of the sensory system in our mouse model and its correction with rapamycin,” the authors noted. “Sensory symptoms are known to be highly disabling in autism and now there is some evidence that abnormalities in sensory responsiveness may contribute to many of the behaviors that we don’t normally view as being mediated by the sensory system.”

Because sensory and behavioral changes often relate to how brain cells fire, the researchers examined the electrical activity of individual brain cells. They analyzed brain slices from 24 mice per group, focusing on pyramidal neurons in the sensory cortex. The neurons from the exposed mice exhibited a higher frequency and amplitude of spontaneous electrical discharges compared to the control neurons, indicating hyper-excitability. Treating the mice or the brain slices with rapamycin quickly reduced this hyperactivity.

To test this hyper-excitability in living animals, the researchers administered a seizure-inducing chemical to groups of eight mice. All eight exposed mice experienced visible seizures at a high dose, compared to only two out of eight control mice. Rapamycin administration lowered the severity of the seizure scores in the exposed group.

To observe brain-wide communication, the authors used functional magnetic resonance imaging to scan 16 mice per group. This technique measures functional connectivity, which tracks how different brain regions synchronize their activity. The exposed mice displayed higher levels of connectivity than the control mice, particularly between sensory processing areas and subcortical regions like the thalamus. Following the two-hour rapamycin treatment, this hyper-connectivity decreased in the sensory cortex and reorganized across the brain, restoring the functional network to a state that closely resembled the control mice.

This neural reorganization provides evidence that specific circuits remain adaptable in adult mice. The average person should take away the idea “[t]hat some behaviors associated with autism can improve, even after the brain has matured,” the authors said. “Furthermore, we have identified the pathways (connections) in the brain that are influenced in this mouse model and that are altered with rapamycin.”

Finally, the researchers analyzed gene expression in the brain cells. They found that the exposed mice had altered activity in genes related to ion channels, which control the flow of electrical charges in and out of cells. After the acute rapamycin treatment, the expression of genes associated with brain cell excitability and autism risk quickly shifted back toward typical levels. This indicates that the drug works by rapidly adjusting the molecular balance of excitation and inhibition.

The findings from this study rely on a specific animal model of maternal inflammation, and physiological responses seen in mice do not directly translate to human neurodevelopment. Because rapamycin has strong immunosuppressive properties, it is not a practical daily treatment for behavioral or sensory symptoms in humans.

“We don’t believe that rapamycin or its close relatives that are currently used clinically will be the ‘cure’ for autism,” the authors cautioned. “First, our effects were temporary and wore off after several treatments. Second, as a class of medications, they can have significant side effects, especially immunosuppression.” They emphasized that they “would not recommend treatment with these medications outside of the conditions for which they are approved.”

Instead of acting as a direct treatment, the drug helps reveal underlying mechanisms for scientists to target. “We look at our findings as a fulcrum to further study autism-associated behaviors and symptoms and how they can be treated,” the authors explained.

Moving forward, the research team plans to explore other interventions. “In one set of studies we are trying to discover the downstream molecular pathways by which rapamycin acts so that we can potentially develop new therapies,” they said. “In the other approach, we are investigating the neural pathways that are misregulated in our model to see if these pathways can be functionally manipulated by therapeutic means, such as transcranial magnetic stimulation.”

The study, “Acute rapamycin treatment reveals distinct mechanisms of dysfunction in a maternal inflammation mouse model,” was authored by JE Le Belle, M. C. Condro, C. Cepeda, KD Oikonomou, K. Tessema, L. Dudley, J. Schoenfield, R. Kawaguchi, D. Geschwind, AJ Silva, Z. Zhang, K. Shokat, NG Harris, and HI Kornblum.