Although several studies have detected an increase in plasma levels of CGRP [147,148,149] and brain-derived neurotrophic factor (BDNF) [150] during CH and MH attacks, the possibility of inducing a CH attack by VIP and PACAP-38 without associated changes in plasma CGRP suggests that CGRP is not the only mediator of the attack [151]. Here, BDNF is linked to cyclic hematopoiesis.