Glyphosate increases anxiety‐like behavior and threat response to novel neutral stimuli

Abstract

Glyphosate is the active ingredient in the most used herbicides worldwide. Glyphosate was initially considered safe for mammals because it acts by inhibiting a metabolic route that is not present in mammals. Recent studies have correlated an increase in the diagnosis of anxiety with the increased use of glyphosate. Studies in rodents have shown that high doses of glyphosate increase anxiety‐like behaviors. Glyphosate exposure is regulated by the Environmental Protection Agency, which has established a chronic reference dose for glyphosate of 2.0mg/kg daily. The effect of this dose on anxiety has not been explored. Therefore, we aimed to assess the effect of prolonged oral consumption of glyphosate‐contaminated drinking water on anxiety‐like behaviors. Furthermore, we also aim to assess the effect of glyphosate on threat behaviors in response to novel, neutral stimuli. To achieve this, rats were given access to glyphosate‐contaminated drinking water ad libitum. Water was prepared for a target dose of 2.0mg/kg daily. Control rats received filtered drinking water. No differences were observed in the volume of water consumed. After 4 weeks of exposure, we assessed for anxiety‐like behavior using the open field test. No difference was observed in the amount of time spent in the center (glyphosate: 40.42.68, control: 49.01; p = 0.2601). After an additional 10 weeks of exposure, anxiety‐like behavior was reassessed in the elevated plus maze. Here, glyphosate decreased the time spent in the open arms (glyphosate: 48.95, control: 103.7; p=0.0104). Next, after 4 more weeks of exposure, animals were assessed for threat response in an open field containing a novel object in the center. Glyphosate decreased time spent exploring the novel object (glyphosate: 44.35, control: 89.69; p=0.0365). Lastly, after a total of 16 weeks of exposure, animals were exposed to 5 repetitions of a novel auditory tone (30s, 4kHz, 77dB; 3 min intertrial interval) within a familiar context to further explorer threat response. Threat response was measured as time spent freezing during the tone. We observed that glyphosate increased overall freezing to a novel tone (glyphosate: 14.05, control: 4.949; p=0.0201). For these reasons, we conclude that glyphosate exposure increases anxiety‐like behaviors as well as negative valence interpretation of different types of neutral, novel stimuli. Future directions include performing immunohistochemistry on brain regions involved in anxiety‐like behaviors such as the basolateral amygdala and bed nucleus of the stria terminalis.