Characterization of calcitonin gene-related peptide (CGRP) receptors and their receptor-activity-modifying proteins (RAMPs) in human brain microvascular and astroglial cells in culture

1. In the present study, we examined the expression of the CGRP receptor-activity-modifying proteins (RAMP1, RAMP2 and RAMP3) and receptor component protein (RCP) in human brain astrocytes (AST), cerebromicrovascular endothelial (EC) and smooth muscle (SMC) cells in culture. Further, we pharmacologically characterized CGRP receptors in these cells by assessing the potency of the CGRP receptor antagonists h-αCGRP 8-37 and the new non-peptide compound BIBN4096BS to block the production of cAMP elicited by CGRP 1 and CGRP 2 receptor agonists. 2. AST, EC and SMC all expressed mRNAs for RAMP1, RAMP2 and RCP. In contrast, message for RAMP3 was detected in AST, but not in SMC and in only one out of four preparations of EC. 3. h-αCGRP, h-βCGRP and [Cys (Et) 2,7]-h-αCGRP exerted concentration-dependent production of cAMP in all cultures, with a maximal effect at 25–50 nM (20–60-fold increase from basal levels). In contrast, 50 nM [Cys (Acm) 2,7]-h-αCGRP only induced a weak stimulatory effect on cAMP formation, especially in SMC and AST (1.5- and 5-fold increase above baseline, respectively). 4. h-αCGRP 8-37 and BIBN4096BS concentration-dependently inhibited cAMP formation evoked by CGRP receptor agonists. Depending on the agonists used, h-αCGRP 8-37 distinguished two different CGRP receptors for which it exhibited low (pIC 50≤6.4) and high (pIC 50∼7.3) affinity, respectively. BIBN4096BS was much more potent (>2.5 orders of magnitude) than h-αCGRP 8-37. Further, BIBN4096BS was able to discriminate three different CGRP receptor sites for which it exhibited low (pIC 50∼9.3–9.9), intermediate (pIC 50∼10.9), and a very high (pIC 50∼13.7) affinity, respectively. Together, these results suggest the presence of CGRP 1 and/or CGRP 2 receptors in human brain AST, EC and SMC, and of an additional population of CGRP receptors in AST, possibly associated to the combined expression of RAMP3 and RCP in these cells, for which BIBN4096BS exhibits an exquisitely high affinity.