Mucosal Serotonin Signaling Is Altered in Chronic Constipation but Not in Opiate-Induced Constipation

OBJECTIVES: Changes in mucosal serotonin (5-HT) signaling have been detected in a number of functional and inflammatory disorders of the gastrointestinal (GI) tract. This study was undertaken to determine whether chronic constipation (CC) is associated with disordered 5-HT signaling and to evaluate whether constipation caused by opiate use causes such changes. METHODS: Human rectal biopsy samples were obtained from healthy volunteers, individuals with idiopathic CC, and individuals taking opiate medication with or without occurrence of constipation. EC cells were identified by 5-HT immunohistochemistry. 5-HT content and release levels were determined by enzyme immunoassay, and mRNA levels for the synthetic enzyme tryptophan hydroxylase-1 (TpH-1) and serotonin-selective reuptake transporter (SERT) were assessed by quantitative real-time reverse transcription PCR. RESULTS: CC was associated with increases in TpH-1 transcript, 5-HT content, and 5-HT release under basal and stimulated conditions, whereas EC cell numbers and SERT transcript levels were not altered. No changes in these elements of 5-HT signaling were detected in opiate-induced constipation (OIC). CONCLUSIONS: These findings demonstrate that CC is associated with a pattern of altered 5-HT signaling that leads to increased 5-HT availability but does not involve a decrease in SERT expression. It is possible that increased 5-HT availability due to increased synthesis and release contributes to constipation due to receptor desensitization. Furthermore, the finding that elements of 5-HT signaling were not altered in the mucosa of individuals with OIC indicates that constipation as a condition does not lead to compensatory changes in 5-HT synthesis, release, or signal termination.