Figure 4. A, Immunohistochemistry of colonic 5-HT₄ receptor expression. 5-HT₄ receptor staining is pronounced mainly in epithelial cells (arrows). B, The gray value of 5-HT₄ receptor immunoreactivity in different groups. 5-HT₄ receptor expression was significantly decreased in the control group compared to the hypersensitivity group (^*P < 0.05, ANOVA). 5-HT₄ receptor expression was significantly increased in the fluoxetine group compared to the hypersensitivity group (^#P < 0.01, ANOVA).