Is it true that opioids in higher doses have antiemetic effects vs lower doses because it depresses the vomiting center?

yes: High doses of some opioids may decrease n/v by interacting with receptors in the vomiting center, rather than the ctz (chemoreceptor trigger zone ).The relationship between opioids use and n/v is complex. The vomiting center in the medulla oblongata, receives input from 4 areas; chemoreceptor trigger zone gi tract vestibular apparatus -temporal lobe cerebral cortex.