Divergent acute release of CGRP and PACAP from mouse spinal cord tissue: implications for migraine neurobiology

Calcitonin gene-related peptide (CGRP) and pituitary adenylate cyclase-activating polypeptide (PACAP) are both implicated in migraine, but whether they undergo similar acute release from primary sensory terminals remains uncertain. We compared stimulus-evoked CGRP-like immunoreactivity (CGRP-LI) and PACAP-like immunoreactivity (PACAP-LI) in superfused mouse spinal cord tissue. Spinal cords from 14 male C57BL/6 mice were pooled and distributed among six superfusion chambers per stimulus. After two baseline fractions, tissues were exposed to capsaicin (10 μM) or high K+ (40 mM KCl), followed by a post-stimulation fraction. Capsaicin and high K+ produced marked CGRP responses, with mean stimulation-minus-baseline increases of 44.89 and 47.19 pg/mg, respectively, both Holm-adjusted p<0.001. In contrast, PACAP responses were −0.12 and 0.009 pg/mg, with Holm-adjusted p=0.742 and p=0.888. The geometric mean stimulation-to-baseline responses were 23.23-fold and 18.57-fold for CGRP, compared with 0.86-fold and 1.00-fold for PACAP. The corresponding CGRP-to-PACAP ratios of fold responses were 26.99 and 18.51. These findings show a marked dissociation between acute CGRP and PACAP release in this pooled ex vivo spinal cord preparation. They suggest that PACAP should not be assumed to behave as a CGRP-like co-transmitter during acute stimulation of primary sensory neuron (PSN) terminals.