Daily exposure to the popular herbicide glyphosate at levels deemed safe by government regulators increases anxiety and alters the gut microbiome in animal models. Researchers observed that rats that drank the chemical became hypersensitive to harmless objects and sounds. This change in behavior was accompanied by abnormal changes in specific areas of the brain that caused fear. The new study was published in the journal Frontiers in Toxicology.
Glyphosate is the active ingredient in many commercial herbicides used around the world. It destroys weeds by blocking specific chemical pathways found in plants and bacteria. Because human and animal cells lack this unique biological mechanism, regulators previously concluded that the chemical was completely harmless to mammals. The Environmental Protection Agency currently sets a safe daily limit for long-term human exposure at 2.0 milligrams per kilogram of body weight.
In recent years, medical experts have noticed a correlation between heavy use of herbicides and human mental health. Past laboratory experiments have also shown that rodents exposed to chemicals exhibit excessive fear. However, in these previous experiments, the animals received very high daily intakes, up to 500 milligrams per kilogram of body weight. The health effects of long-term exposure to levels far below what the government considers safe remained largely untested.
The team of scientists wanted to see if chronic exposure to this government-approved dose resulted in changes in emotional processing. Researchers Mauricio Caceres-Chacón, Demetrio Sierra-Mercado and colleagues from the University of Puerto Rico School of Medicine led the study. They suspected that low but continuous administration of chemicals could gradually affect the brain networks responsible for fear. They also theorized that the chemical could disrupt the healthy balance of bacteria in the gastrointestinal tract, which has a major impact on the mental health of humans and animals.
To test these ideas, the research team added herbicides to the drinking water of adult male rats. The researchers controlled the concentration so that the animals consistently received a precise threshold dose of 2.0 milligrams per kilogram of body weight each day. Another matched control group of rats received normal filtered water. The total exposure period lasted 16 weeks, during which the scientists performed various psychological tests to measure the animals’ emotional states.
At four weeks, the researchers placed the rats in an open space and measured their general comfort level. At this early stage of the experiment, the scientists saw no clear behavioral differences between the two groups of animals. Both groups explored the center of the test area for approximately the same amount of time. The lack of early effects suggests that the chemical may take a long time to accumulate in the body before causing changes in brain function.
By the 10th week of the experiment, unnatural changes in behavior appeared. The researchers tested rats in a maze that was elevated above the ground. The maze had some paths that were enclosed and protected, and some that were completely open and exposed to the room. Animals that drank the herbicide spent much less time exploring open trails than control animals, a standard indicator of heightened anxiety in rodents.
After four weeks, the researchers looked at how the animals responded to the new experience. They introduced a completely harmless new object into the rodent’s experimental environment. Rats usually investigate new items out of natural curiosity and a desire to forage. Exposed animals spent much less time exploring unfamiliar objects and more time freezing in place or cuddling around the test space.
When the researchers introduced new, unfamiliar rats into the space, the herbicide-exposed rodents interacted with their living mates at completely normal levels. This specific detail showed that this chemical does not completely suppress social behavior. Rather, it appears that the animals incorrectly treated inanimate objects as potential threats. Avoidance of harmless novelty is a behavior frequently associated with trauma and hyperarousal disorders in humans.
During the final week of testing, the scientists played short, neutral audio tones to the animals. When hearing these innocuous sounds, exposed rats froze in place much more often than control rats. But when the researchers combined the sound with a mild shock to the foot, both groups responded with the same fear level. The experiment revealed that the chemical caused the animals to misinterpret neutral stimuli as dangerous, without changing their normal responses to actual physical threats.
To find out how the herbicide caused this fearful state, scientists examined the rats’ brain tissue. They looked at cell activity in several areas known to process fear and emotion, including the amygdala and parts of the prefrontal cortex. Results from these brain regions were not statistically significant. This means that the chemical does not seem to overstimulate or damage their baseline function.
However, the research team found that cell activity was increased in a specific area called the bed nucleus of the terminal line. This deep brain structure is deeply involved in interpreting threats and promoting general anxiety. Hyperactivity in this region was completely consistent with behavioral observations that the rats were acting overly cautious. Physical changes in this area of the brain caused the animals to fundamentally misinterpret a safe environment as dangerous.
In addition to the brain, the scientists also analyzed the animals’ digestive tracts. Using genetic sequencing of fecal samples, they discovered that exposed rats had unnatural changes in their gut microbiota. Specifically, the animals lost most of the group of bacteria known as Lactobacilli. This bacterial family relies heavily on manganese, a nutrient that herbicides tend to bind and remove from circulation.
Loss of lactic acid bacteria provides a strong biological explanation for mood changes in rodents. These specific gut bacteria are essential for processing food proteins into the chemical messenger serotonin. Serotonin regulates mood and, in a healthy brain, functions to calm areas that cause anxiety. Without enough good bacteria, serotonin production decreases, making your brain more susceptible to an irrational fear response.
The researchers found some limitations in their experimental design. Only male rats were observed in this study, leaving open questions about how female physiology responds to exposure to the same chemicals. Male and female rodents often exhibit different behavioral coping strategies when dealing with fear and stress. Additionally, their microbiomes may respond differently to environmental toxins, meaning future studies should include both men and women to build a complete picture of chemical risks.
Furthermore, the scientists only measured the intestinal bacteria in the animals’ feces at the end of the 16-week period. This particular measurement method does not capture the bacterial populations that live deep within the protective mucus layer of the intestine. Checking the composition of the gastrointestinal tract at multiple points during an experiment could help researchers know exactly when bacterial die-off occurs. By tracking the timeline more closely, we can see whether the gut changes occurred before or after the behavioral changes.
Despite these boundaries, this experiment demonstrated a clear biological mechanism by which mental state is altered by routine chemical exposure. Regulatory agencies established initial safety limits based on the assumption that animal biology is unaffected by the herbicide’s mode of action. This chemical indirectly shapes brain health because it targets the bacteria that live in your intestinal tract. The resulting loss of mood-stabilizing brain chemicals suggests that environmental safety thresholds may need to be thoroughly reevaluated.
The study, “Exposure to the herbicide glyphosate triggers inappropriate threat responses and alters gut microbial composition,” was authored by Mauricio Cáceres Chacón, Osmary Martinez Guzman, Héctor A. Haddock Martinez, Alecdiel Figueroa Perez, Sian Rodríguez Rosado, and Jarenis. Suárez-Perez, Raul Y. Ramos-Sanchez, Filipa Godoy-Vitorino, and Demetrio Sierra-Mercado.
