They have shown that mice that grow up without microbes (germ - free mice) were more sensitive to visceral pain stimuli. (sciencedaily.com)
These findings are apparently not specific for visceral pain since studies applying somatic pain stimuli show similar results. (content.iospress.com)
Subtitle: Effects of negative emotions and stress on the neural processing of visceral pain stimuli: Current findings from imaging studies (content.iospress.com)