DIFFERENTIAL CALCITONIN-GENE-RELATED PEPTIDE (CGRP) AND AMYLIN BINDING-SITES IN NUCLEUS-ACCUMBENS AND LUNG - POTENTIAL MODELS FOR STUDYING CGRP AMYLIN RECEPTOR SUBTYPES/

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 s hown to display similar effects on in vitro and in vivo carbohydrate m etabolism. The present study was initiated to identify and characteriz e the binding sites for these peptides in lung and nucleus accumbens m embranes prepared from pig and guinea pig. Both tissues in either spec ies displayed high-affinity (2-[I-125]iodohistidyl(10)) humanCGRP alph a ([I-125]hCGRP alpha) binding (IC50 = 0.4-7.7 nM), which was displace d by hCGRP(8-37)alpha with equally high affinity (IC50 = 0.4-7.3 nM). High-affinity binding for [I-125]Bolton-Hunter human amylin ([I-125]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 calcitoni n competed for amylin binding sites with high affinity (IC50 = 0.1 nM) but was poor in competing for amylin binding in lung membranes, Rat a mylin(8-37) competed for [I-125]hCGRP alpha binding with higher affini ty (IC50 = 5.4 nM) compared with [I-125]BH-h-amylin binding (IC50 = 20 0 nM) in porcine nucleus accumbens, whereas in guinea pig nucleus accu mbens, the IC50 values for rat amylin(8-37) were 117 and 12 nM against [I-125]hcGRP alpha and [I-125]BH-h-amylin, respectively. Also, functi onal studies evaluating the activation of adenylate cyclase and genera tion of cyclic AMP in response to these agonists indicated that hCGRP alpha (EC(50) = 0.3 nM), h-amylin (EC(50) = 150 nM), and salmon calcit onin (EC(50) = 1,000 nM) activated adenylate cyclase, resulting in inc reased cyclic AMP production in porcine lung membranes that was antago nized by hCGRP(8-37)alpha. The affinity of hCGRP(8-37)alpha was simila r for all three peptides. The cyclic AMP responses to amylin and salmo n calcitonin were significantly (p < 0.05) lower than that of hCGRP al pha and not additive, suggesting that they are acting as partial agoni sts 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 hCGRP alpha in activatin g lung adenylate cyclase. None of these peptides activated adenylate c yclase in membranes prepared from the nucleus accumbens of both specie s. The data from these studies demonstrate both species and tissue dif ferences in the existence of distinct CGRP and amylin binding sites an d present a potential opportunity to study further CGRP and amylin rec eptor subtypes.