ADCYAP1 and Headache: It is hypothesized that although vasoactive intestinal peptide (VIP) shares the receptors VPAC1 and VPAC2 with PACAP, it is unable to induce robust degranulation, which might explain why it does not cause headache whereas PACAP activity at VPAC1 and VPAC2 does cause headache (Baun et al., 2012).