Differential Calcitonin Gene-Related Peptide (CGRP) and Amylin Binding Sites in Nucleus Accumbens and Lung: Potential Models for Studying CGRP/Amylin Receptor Subtypes

Abstract: Calcitonin gene-related peptide (CGRP), a 37-amino-acid peptide, is a member of a small family of peptides including amylin or islet amyloid polypeptide and salmon calcitonin. These related peptides have been shown to display similar effects on in vitro and in vivo carbohydrate metabolism. The present study was initiated to identify and characterize the binding sites for these peptides in lung and nucleus accumbens membranes prepared from pig and guinea pig. Both tissues in either species displayed high-affinity (2-[¹²⁵I]iodohistidyl¹⁰)humanCGRPα ([¹²⁵I]hCGRPα) binding (IC₅₀ = 0.4–7.7 nM), which was displaced by hCGRP_8–37α with equally high affinity (IC₅₀ = 0.4–7.3 nM). High-affinity binding for [¹²⁵I]Bolton-Hunter human amylin ([¹²⁵I]BH-h-amylin) was also observed in these tissues (IC₅₀ = 0.2–6.0 nM). In membranes from the nucleus accumbens of both species, salmon calcitonin competed for amylin binding sites with high affinity (IC₅₀ = 0.1 nM) but was poor in competing for amylin binding in lung membranes. Rat amylin_8–37 competed for [¹²⁵I]hCGRPα binding with higher affinity (IC₅₀ = 5.4 nM) compared with [¹²⁵I]BH-h-amylin binding (IC₅₀ = 200 nM) in porcine nucleus accumbens, whereas in guinea pig nucleus accumbens, the IC₅₀ values for rat amylin_8–37 were 117 and 12 nM against [¹²⁵I]hCGRPα and [¹²⁵I]BH-h-amylin, respectively. Also, functional studies evaluating the activation of adenylate cyclase and generation of cyclic AMP in response to these agonists indicated that hCGRPα (EC₅₀ = 0.3 nM), h-amylin (EC₅₀ = 150 nM), and salmon calcitonin (EC₅₀ = 1,000 nM) activated adenylate cyclase, resulting in increased cyclic AMP production in porcine lung membranes that was antagonized by hCGRP_8–37α. The affinity of hCGRP_8–37α was similar for all three peptides. The cyclic AMP responses to amylin and salmon calcitonin were significantly (p < 0.05) lower than that of hCGRPα and not additive, suggesting that they are acting as partial agonists at the same CGRP1-type receptor in porcine lung membranes. Similar observations were made for guinea pig lung membranes. However, human amylin and salmon calcitonin were weaker than hCGRPα in activating lung adenylate cyclase. None of these peptides activated adenylate cyclase in membranes prepared from the nucleus accumbens of both species. The data from these studies demonstrate both species and tissue differences in the existence of distinct CGRP and amylin binding sites and present a potential opportunity to study further CGRP and amylin receptor subtypes.