Serotonin increases gut motility. There are pro-motility drugs that stimulate 5HT4 receptors, such as prucalopride.
Dopamine decreases gut motility, since dopamine antagonizes acetylcholine, which is the main neurotransmitter in the digestive system. There are pro-motility drugs that block dopamine, which increases acetylcholine concentrations, such as domperidone.