Abstract
The neuropeptide calcitonin gene-related peptide (CGRP) from the trigeminal ganglion has been established as a key player in the pathogenesis of migraine. In this study, we provide evidence that the responsiveness of neuronal CGRP receptors is strongly enhanced in vitro and in vivo by expression of human receptor activity-modifying protein-1 (hRAMP1), an obligatory subunit of the CGRP receptor. We first demonstrated that activation of CGRP receptors on cultured trigeminal ganglion neurons increased endogenous CGRP mRNA levels and promoter activity. The promoter activation was cAMP dependent and blocked by the antagonist BIBN4096BS [1-piperidinecarboxamide, N-[2-[[5-amino-l-[[4-(4-pyridinyl)-l-piperazinyl]carbonyl]pentyl]amino]-1-[(3,5-dibromo-4-hydroxyphenyl)methyl]-2-oxoethyl]-4-(1,4-dihydro-2-oxo-3(2H)-quinazolinyl)], a new antimigraine drug. Gene transfer using an adenoviral hRAMP1 expression vector increased the maximal production of cAMP by 1.8 +/- 0.2-fold and decreased the EC50 to 2.3 +/- 0.8 nM from 9.0 +/- 5.9 nM and 15.6 +/- 5.2 nM in uninfected and control-infected cultures, respectively. To establish whether RAMP1 is limiting in vivo as indicated from the culture studies, a transgenic mouse expressing hRAMP1 in the nervous system was generated. After CGRP injection into the whiskerpad, the hRAMP1 transgenic mice displayed 2.2 +/- 0.2-fold greater plasma extravasation, which is a measure of neurogenic inflammation. These results demonstrate that RAMP1 is functionally rate limiting for CGRP receptor activity in the trigeminal ganglion, which raises the possibility that elevated RAMP1 might sensitize some individuals to CGRP actions in migraine.