Drugs Affecting GI Tract Motility: Serotonin Receptor Agonists

Serotonin, also called 5-hydroxytryptamine (5-HT), is a neurotransmitter released by enterochromaffin cells in response to chemical or mechanical stimuli. It stimulates sensory neurons in the myenteric plexus and vagal pathways, triggering peristalsis—coordinated muscle contractions that move food through the GI tract. Serotonin also activates 5-HT4 receptors on excitatory motor neurons, increasing acetylcholine release and enhancing motility throughout the stomach, small intestine, and colon.

5-HT4 agonists like prucalopride and cisapride bind to 5-HT4 receptors on excitatory motor neurons, enhancing acetylcholine release at the neuromuscular junction. This increases the strength and frequency of colonic contractions, particularly high-amplitude propagating contractions in the proximal colon. These enhanced contractions propel fecal contents more effectively, reducing transit time and relieving chronic constipation.

Cisapride is a rapidly acting 5-HT4 agonist but carries serious risk of fatal cardiac arrhythmias, limiting its availability in the United States to restricted protocols. Prucalopride is a more selective 5-HT4 agonist with higher affinity, approved for chronic constipation treatment in women, primarily in Europe and Canada. While prucalopride causes fewer cardiac complications, it may produce side effects including nausea, abdominal pain, diarrhea, and headaches.

The peristaltic reflex involves multiple serotonin receptors within the enteric nervous system. 5-HT1p and 5-HT4 receptors on intrinsic sensory neurons, along with 5-HT3 receptors on extrinsic neurons, coordinate the wave-like muscle contractions. Additionally, 5-HT1 receptors at the gastric fundus generate nitric oxide, causing smooth muscle relaxation that facilitates food movement through the digestive tract.

High-amplitude propagating contractions (HAPCs) are powerful, coordinated muscle movements in the proximal colon characterized by contractions in longitudinal muscles paired with relaxation of circular muscles. These contractions are stimulated when 5-HT4 receptors on excitatory motor neurons are activated, increasing acetylcholine release. HAPCs effectively propel fecal contents through the colon, making them essential for normal bowel function and constipation relief.

Tegaserod is a serotonin receptor agonist developed to treat GI dysmotility, but its availability is severely restricted. In the United States, tegaserod is available only as an emergency investigational new drug, meaning it cannot be prescribed for routine clinical use. This limitation reflects regulatory concerns about safety or efficacy compared to other serotonin agonists like prucalopride.

Over 90 percent of the body's total serotonin is synthesized and stored in the gastrointestinal tract, primarily by enterochromaffin cells. This high concentration underscores serotonin's profound influence on digestive processes and GI motility. The abundance of serotonin in the GI tract makes it an ideal target for therapeutic interventions addressing motility disorders and constipation.