What is the primary action of Ondansetron?

Ondansetron is primarily used as an antiemetic medication, particularly effective in preventing nausea and vomiting caused by chemotherapy, radiation therapy, or postoperative conditions. The key mechanism through which it exerts its effects is by blocking serotonin receptors, specifically the 5-HT3 receptors in the central nervous system and gastrointestinal tract.

By inhibiting these receptors, Ondansetron prevents the action of serotonin, a neurotransmitter that can trigger signals for nausea and vomiting. This action is crucial for patients undergoing treatments that typically induce nausea and helps in managing their symptoms effectively.

The other options describe mechanisms or effects that are not relevant to Ondansetron's primary function. For instance, it does not operate on dopamine receptors, nor does it specifically stimulate appetite or increase gastric motility. These actions are associated with different drug classes or treatments altogether.