The reductions of MMA blood flow, TRPV1 and CGRP immunoreactivity, and TRPV1 and TRPA1 mRNA expression, are all consistent with the hypothesis that TRPA1 receptors, co-expressed with TRPV1, play a critical role in environmental irritant-induced headache [8], [18], [55]. Here, TRPA1 is linked to Headache.