HTR4 and diabetes mellitus: To investigate whether 5-HT4R-mediated protection from diabetes-driven TJ barrier disruption is involved in 5-HT4R-promoted expression of TJ proteins, we examined the effects of a 5-HT4R agonist on occludin, claudin-1, and ZO-1 expressions in the diabetic colon by IF staining and western blot analysis.