CTZ

 Chemoreceptor trigger zone [CTz] are the area present in the brain which also consist of Vomit activation center, when they are activated and produces a vomit sensation.

There are some barrier present in brain i.e. Blood Brain Barrier which restrict the drug from the systemic circulation towards brain and where as these barrier are absent in area of Chemoreceptor Trigger Zone CTz, some of chemical molecule like (adrenaline,opoids,dopamine) which are restricted towards brain may come in contact of CTz and produce nausea and vomit sensation. Some of toxic molecule may interact to brain through CTz as there is no barrier present.

Function of CTz 

CTz relay information about amount of Emetic agent (which cause vomit) in the blood and trigger receptor of vomit center to genrate a response of vomit.

The main neurotransmitter that aids in this process are Serotonin,Endorphin,Histamine etc.

How CTz Communicate ?

CTz contacts with VOMIT CENTRE by  mainly 2 types of receptors MU , DELTA receptors which comes in contact with drugs mainly OPOIDS when there level exceeds in the body the receptor got the signal and cause sensation of vomiting.

Source :

Essential of Medical Pharmacology by KD Tripathi

https://en.wikipedia.org/wiki/Chemoreceptor_trigger_zone