Infant mice fed soy-based formulas exhibit alterations in anxiety-like behaviours and the 5-HT system

Genistein (GEN) is a phytoestrogen with oestrogen-like activity found in many plants. Classified as an endocrine disruptor, GEN is potentially hazardous, particularly during developmental stages. It induces alterations in anxious behaviour, fertility, and energy metabolism, alongside modifications in specific brain circuits. As the serotonin (5-HT) system is critically involved in many of these behaviours, we hypothesised that some of GEN's behavioural effects might results from disruptions in the development of the 5-HT system. To test this, we examined the impact of early postnatal exposure to GEN at a dose of 50 mg/kg body weight, mimicking the exposure level of infants consuming soy-based formulas, on anxiety-related behaviours and 5-HT neuronal populations in the raphe nucleus. Male and female CD1 mice were treated orally with GEN or a vehicle during the first 8 days of life. On postnatal day 60, one cohort underwent anxiety behaviour testing, while another was euthanised for immunohistochemical analysis. Behavioural testing revealed that male control mice exhibited higher anxiety levels than females, whereas GEN exposure produced sex-specific effects: anxiolytic in males and anxiogenic in females. Immunohistochemical analysis of the raphe nuclei demonstrated significant alterations in 5-HT neuronal numbers in GEN-treated animals. Specifically, GEN exposure affected dorsal and median raphe 5-HT neuronal populations in a sexually dimorphic manner, with females showing a reduction and males an increase in 5-HT neurones compared to controls. These findings indicate that the regulation of anxiety-related behaviours and the 5-HT system are key targets of early phytoestrogen exposure at levels comparable to those in soy-based infant formulas.