<prompt> Activation of the following type of receptors present on myenteric neurons by metoclopramide is primarily responsible for the enhanced acetylcholine release and improving gastric motility:Muscarine M1Serotonergic 5-HT3Serotonergic 5-HT4Dopaminergic D2  <response>: Ref: KDT 6e p646, 647 Gastric motility is increased by ACh. Release of this neurotransmitter is enhanced by 5HT4 receptor stimulation and 5HT3 and D2 receptor antagonism. 