Which neurotransmitter is most closely associated with vomiting?

A.  Noradrenaline
B.  Calcitonin gene-related peptide (CGRP)
C.  Acetylcholine (Ach)
D.  Substance P
E.  Vasoactive intestinal peptide

The answer is D.

Substance P (SP)-neurokinin-1 (NK1) receptor pathways are involved in the pathophysiology of emesis and depression. It is a potent vaso-dilator mediated by endothelial release of NO. Substance P is involved in central and peripheral pain transmission; however, although SP antagonists have shown beneficial results in treatment of depression and nausea, their antinociceptive properties have been disappointing. Aprepitant is a SP antagonist used for che-motherapy-induced nausea and vomiting.

Source: Hargreaves, J Clin Psychiatry 2002