22.4 : 消化管運動に影響を及ぼす薬剤：ドーパミン受容体拮抗薬

Dopamine, a neurotransmitter abundant in the gastrointestinal or GI tract, is involved in GI motility. It binds to D₂ receptors, lowering intragastric pressures, thereby relaxing the lower esophageal sphincter and inhibiting GI motility.

Dopamine receptor antagonists, such as metoclopramide and domperidone, are useful for treating GI motility disorders.

These agents block D₂ receptors in the GI tract, neutralizing dopamine's inhibitory effects. The resulting increase in the lower esophageal sphincter pressure leads to coordinated GI contractions.

Additionally, they block D₂ receptors in the brain's chemoreceptor trigger zone, mitigating nausea and vomiting.

Metoclopramide, specifically, aids gastric emptying in individuals with gastroparesis and serves as an adjuvant for GI diagnostic procedures like endoscopy. 

Their common adverse effects include elevated prolactin levels resulting in galactorrhea or breastmilk production and gynecomastia or swollen breasts in men.

Furthermore, metoclopramide may induce insomnia, anxiety, tremors, involuntary muscle contractions or dystonia, and uncontrollable body movements or dyskinesia.