Cortical spreading depression (CSD) induction and propagation in the rat cortex. (A) CSD was induced with topical application of 1 μl 3 M KCl (or artificial cerebrospinal fluid for sham) onto the dura via a posterior burr hole. An anterior hole was used for CSD recording. A total of 36 rats were used. Of these, 26 rats were used for multiple CSD experiments including 14 for CSD induction and 12 for sham. To minimize the animal use, three of 14 rats in the CSD group and three of 12 in the sham group were also used for measuring calcitonin gene-related peptide (CGRP) levels in addition to CGRP mRNA. In the single CSD group, 10 rats were used with five for CSD and sham, respectively. (B) A representative trace showing CSD propagation and magnitude (indicated as area under the curve, AUC, grey lines).