VIP and Cluster headache: Similarly, pituitary adenylate cyclase activating polypeptide (PACAP), another neuropeptide in the vasoactive intestinal peptide (VIP) family, demonstrated to have a role in migraine and cluster headache possibly via the parasympathetic cranial outflow pathway in the sphenopalatine ganglion (SPG) and superior salivatory nucleus (SSN) [27,28,29,30], has also been shown to provoke migraine-like attacks when infused into patients with migraine [31].