On the other hand, while the existence of neurotransmitters and neuropeptides related to postganglionic parasympathetic fibers (CGRP [60], nitric oxide synthase, VIP, and PACAP-38 [12]) is known, the results of two studies that attempted to investigate the exact mechanism of the trigemino-autonomic reflex suggest that peripheral activation, both afferent and efferent, is not sufficient to produce a CH attack [61,62]. This evidence concerns the gene VIP and cyclic hematopoiesis.