Primary trigeminal afferents are the main source for stimulus-induced CGRP release into jugular vein blood and CSF

Background: Administration of inflammatory soup (IS) leads to a significant release of calcitonin gene-related peptide (CGRP). Whether IS-induced CGRP release originates in primary or secondary neurons of the trigeminovascular system has not been clarified. Methods: We determined CGRP release into the external jugular vein and in cerebrospinal fluid (CSF) following intracisternal IS administration using an in vivo rat model. We further performed polymerase chain reaction (PCR) and immunohistochemistry of the trigeminal ganglion and brainstem (trigeminal nucleus caudalis). To further elucidate a primary vs. secondary origin, experiments were repeated after neonatal capsaicin treatment (NCT) as this treatment destroys primary trigeminal afferents. Results: IS-induced CGRP release into the external jugular vein and CSF were significantly reduced after NCT in both compartments but inhibition was more pronounced in jugular vein blood than in CSF. Baseline CGRP levels were not affected by NCT. PCR results show that following NCT, CGRP mRNA was significantly reduced in the trigeminal ganglion but not in the brainstem. Immunohistochemistry of the TG and brainstem support these results. Conclusions: We conclude that resting state CGRP levels can be maintained after trigeminal denervation of the meninges. However, for functional purposes primary trigeminal afferents are mandatory as they are the major source for stimulus-induced CGRP release.