Activation of mu-opioid receptors in the gut influences anxiety related behaviour in female rats

Background: The enteric nervous system (ENS) is an important pathway through which the gut communicates with the brain. However, there is little information on how modulating ENS activity influences higher cognitive function. The objective of this experiment was to investigate if modulating the ENS affects anxiety related behaviour in rats. Loperamide, a μ-opioid receptor agonist that does not cross the blood brain barrier, was used to manipulate ENS function in a rat model. 

Methods: Male and female Sprague-Dawley rats were treated with an acute dose of loperamide, or control solution and anxiety related behaviour assessed using the Open Field and Elevated-Plus maze tests. Gene expression in the caecum, prefrontal cortex, hippocampus, and amygdala was assessed by RNAseq, and caecal microbiota composition determined by shotgun metagenome sequencing.

Results: In female rats, loperamide treatment significantly decreased total distance moved (P=0.04) and frequency of supported rearing (P=0.03) compared to controls, which indicate a decrease in exploratory behaviour and increased anxiety. However, loperamide did not significantly alter behaviour in male rats. Similarly, loperamide did not alter gene expression in any brain region in male rats. However, in female rats, loperamide significantly altered expression of 7 genes in the hippocampus (FDR<0.05). In contrast, loperamide altered caecum tissue gene expression and microbiome composition in both male and female rats (FDR<0.05).

Conclusion: Modulation of the ENS can alter behaviour in a rodent model, with differential effects between male and female rats in the brain, and changes in caecal tissue gene expression and microbiome composition for both sexes.