Attenuation of intestinal peristalsis with age is attributed to decreased sensitivity of receptors in the enteric nervous system.

Constipation and other digestive disorders are common in older adults. The autonomic nervous system plays a critical role in regulating digestive motility in the intestinal tract. However, studies on age-related changes in autonomic function and receptor expression in the intestinal tract are limited. In this study, we examined the expression of neurotransmitter receptors in the autonomic nervous system and the effects of acetylcholine and β-agonists on intestinal contraction and relaxation in the jejunum of aged rats. Jejunal sections collected from male and female Wistar/ST rats aged 4, 11, and 18 months were analyzed. Immunohistochemical staining and enzyme-linked immunosorbent assay were used to measure the expression of muscarinic acetylcholine receptors (CHRM2 and CHRM3) and β-adrenergic receptors (β2-ADR and β3-ADR). The effects of acetylcholine, isoproterenol, and mirabegron were assessed in the isolated jejunum for each age group. There was no significant difference in CHRM2 receptor expression among the age groups; however, CHRM3 receptor expression decreased with age. Additionally, the sensitivity to acetylcholine-induced contractile responses decreased with age. Although β2-ADR receptor expression did not differ among the age groups, β3-ADR receptor expression increased with age. Despite this, the relaxation response to isoproterenol and mirabegron decreased with age. Our study revealed an age-related decrease in CHRM3 expression and the contractile response to acetylcholine in the small intestine of rats. Although β-ADR expression, particularly β3-ADR, increased with age, the relaxation response to β-adrenergic agonists gradually decreased.