Abstract
Exposure to milk caseins beyond the normal weaning age in rats (21 days) was shown to impact the development of the opioid system, a key player in emotional regulation. Further, milk alters the gut microbiome and evidence suggests the gut microbiota can modify brain function and behaviour. Given such evidence, it is hypothesised exposure to milk casein and its bioactive breakdown product, beta casomorphin-7 (BCM-7) beyond weaning age in rats results in disruptions to brain neurochemistry and behaviour via a microbiome-gut-brain axis mechanism. This work aimed to investigate this hypothesis using in vivo behavioural, neurochemical, gut microbial and metabonomic studies. The behavioural results showed that exposure to casein from postnatal day 21-26 resulted in an increase in ‘depressive-like’ behaviour as measured by the forced swim test (FST) and that BCM-7 is not the only casein derived product driving such behavioural impairments. Quantitative autoradiography revealed the casein induced depressive phenotype was concomitant with changes to oxytocin and opioid receptors in regions of the brain associated with mood. Fluorescence in situ hybridization showed an increase in Clostridium Histolyticum in the gut suggesting a microbiome-gut-brain axis role in the casein induced changes reported. In line with this, antibiotic knock-down of gut microbial activity not only prevented the development of the casein induced depressive phenotype, but also caused a casein independent antidepressive-like effect as assessed by the FST. Lastly, metabonomic analysis revealed an increase in gut microbial metabolites and disruptions to choline and energy metabolism in response to prolonged casein exposure, which have been implicated in mood disorders. Collectively, these findings suggest casein exposure beyond weaning age results in neurochemical and emotional impairments via a potential microbiome-gut-brain axis mechanism. This indicates that prolonged breastfeeding periods may cause mood disorders in humans and highlight the need for more comprehensive guidelines regarding an appropriate weaning age.