Binding profile of a selective calcitonin gene-related peptide (CGRP) receptor antagonist ligand, [125I-Tyr]hCGRP8-37, in rat brain and peripheral tissues

Abstract

The calcitonin gene-related peptide (CGRP) C-terminal fragment human CGRP8-37 acts as a potent antagonist of various in vitro and in vivo effects of CGRP. Its iodinated counterpart, [125I-Tyr] hCGRP8-37, binds with high affinity (KD values between 7.5 x 10(-11)-2.1 x 10(-10) M) to what is apparently a single class of CGRP receptors in brain, atrium and vas deferens membrane preparations. The relative potencies of various CGRP-related fragments and analogs in competing for [125I-Tyr]hCGRP8-37 binding sites were similar in these three preparations, with hCGRP alpha being the most potent competitor, followed by unlabeled hCGRP8-37, the linear agonist [acetamidomethyl-cysteine2,7] hCGRP alpha, and finally by rat amylin-amide and salmon calcitonin. Competition profiles suggested the existence of a single affinity site (except in the case of hCGRP alpha, for which competition binding data were best fitted to two apparent affinity constants in all three tissues), whereas rat amylin-amide revealed two affinity constants in the rat brain. Guanylylimidodiphosphate (100 microM) failed to alter specific [125I-Tyr]hCGRP8-37 binding in the various tissues studied here. Quantitative receptor autoradiography in the rat brain revealed [125I-Tyr]hCGRP8-37 binding sites mostly concentrated in the nucleus accumbens (shell), caudate putamen (tail), amygdaloid body, pontine nuclei, cerebellum and inferior olive, whereas lower quantities of sites were present in the olfactory tubercle, nucleus accumbens (core), medial geniculate, superior colliculus, temporal cortex, inferior colliculus, lateral lemniscus, vestibular nuclei and principal sensory trigeminal nucleus.(ABSTRACT TRUNCATED AT 250 WORDS)