Colocalization of CGRP with 5-HT_1B/1D receptors and substance P in trigeminal ganglion neurons in rats

Vasodilatation in the dura mater has been implicated in migraine pathogenesis. Anti-migraine triptan drugs block vasodilatation by binding to 5-HT_1B/1D receptors localized on the peripheral sensory terminals and dural blood vessel smooth muscles. Previous studies suggest that calcitonin gene-related peptide (CGRP) released from Aδ-fibres plays a more important role than substance P (SP) released from C-fibres in inducing dural vasodilatation and that one of the antimigraine mechanisms of triptan drugs is inhibiting 'CGRP release. In the present study, the relationship between CGRP and 5-HT_1B/1D receptors, and between CGRP and SP in the trigeminal ganglion neurons in rats was examined by double immunohistochemical staining. CGRP, 5-HT_1B, 5-HT_1D and SP-positive trigeminal ganglion neurons were all predominantly small and medium-sized. In the trigeminal ganglia, ≅ 50% of CGRP-positive neurons were 5-HT_1B positive. Similarly, ≅ 55% of CGRP-positive neurons were 5-HT_1D immunoreactive. Approximately 50% of CGRP-positive neurons were SP-positive, while 93% of SP-positive neurons were CGRP-positive, suggesting that nearly all SP-positive neurons also contain CGRP. The fibre types of the 5-HT_1B- and 5-HT_1D-positive neurons were further investigated with an antibody against the A-fibre marker 200-kDa neurofilaments (NF200). Approximately 46% of the 5-HT_1B-positive and 43% of the 5-HT_1D-positive trigeminal ganglion neurons were also NF200 positive, indicating that many A-fibre trigeminal neurons express 5-HT_1B or 5-HT_1D receptors. These results support the hypothesis that one important action of antimigraine drugs is the inhibition of CGRP release and that Aδ-fibres may play an important role in migraine pathogenesis.