ADCYAP1 and migraine disorder: Later human studies by Tuka et al. provided the first evidence that PACAP-38 concentration in the plasma of migraineurs is significantly lower in the interictal period as compared with that of healthy volunteers, but increases during migraine attacks relative to the attack-free period; the difference in the plasma levels of PACAP-38 in migraineurs in the two periods indicate that this peptide is involved in the development of the attacks [25].