VIP and cyclic hematopoiesis: Moreover, significantly increased CGRP and VIP levels were found in the plasma obtained from the ipsilateral external jugular vein during spontaneous attacks of ECH patients [19], and in the extracerebral circulation both during spontaneous and nitroglycerine-induced attack phase of CH [20, 30], whereas lower plasma SP levels were detected during histamine-induced and spontaneously occurring CH attacks when compared with controls [31].