PTH and Headache: In this context, provocation studies assessing pericranial and hind paw hypersensitivity after administration of glyceryl trinitrate (GTN) or bright light stress (BLS) have been used to study susceptibility to headache triggers and therefore to investigate the presence of persistent central sensitization, a hallmark of headache chronification, in both migraine [42] and PTH models [34, 41].