The chemoreceptor trigger zone contains dopamine D2 receptors, serotonin 5-HT3 receptors, opioid receptors, acetylcholine receptors, and receptors for substance P. Stimulation of different receptors are involved in different pathways leading to emesis. In the final common pathway, substance P, which activates the neurokinin-1 receptor, appears to be involved.[3] Additionally, the vagal and enteric nervous system inputs transmit information regarding the state of the gastrointestinal system. Irritation of the GI mucosa by chemotherapy, radiation, distention, or acute infectious gastroenteritis activates the 5-HT3 receptors of these inputs.[4] It is now widely known that cytotoxic chemotherapeutic agents cause a detectable increase in blood levels of serotonin (5-HT) and its major metabolite, 5-Hydroxyindoleacetic acid (5-HIAA).[5] The presence of these chemicals in the blood activate 5-HT3 receptors in the chemoreceptor trigger zone, in turn releasing substance P, which activates NK1 receptors to cause an emetic response (vomiting).