Calcitonin gene-related peptides: their binding sites and receptor accessory proteins in adult mammalian skeletal muscles

Abstract

This work addresses the presence, pharmacological properties, and anatomical localization of calcitonin gene-related peptide-α (CGRPα) binding sites and the receptor’s accessory proteins in endplate-enriched and non-endplate muscle membrane samples from adult rat gracilis muscles. We examined the binding of 125I-[Tyr 0]-CGRPα, the competitive binding of CGRPα analogs, the immunohistochemical localization of the receptor’s accessory proteins, and Western blots of the receptor component protein. Results show that: (a) 125I-[Tyr 0]-CGRPα binding is saturable, specific, and consistent with the presence of a homogeneous population of binding sites (Hill coefficients=1.0) in endplate and non-endplate samples exhibiting dissociation constants of 0.39 nM and 0.38 nM, respectively; (b) the density of binding sites in the endplate samples (71.0 fmoles/mg protein) is considerably higher than that in their non-endplate counterparts (34.6 fmoles/mg protein); (c) unlabeled CGRPα, hCGRP8-37 and calcitonin compete with the radioligand with the same order of potency in the endplate and non-endplate samples; and (d) the localization of the receptor accessory proteins, including the receptor activity-modifying protein (RAMP1) and the receptor component protein (RCP), for the most part matches that of the motor end-plates. Thus, gracilis muscles express CGRPα-specific binding sites which are predominantly localized in the muscle’s motor endplate regions where RAMP1, RCP, CGRPα, acetylcholine receptors, and acetylcholinesterase are detected in high concentrations. These findings imply that the CGRPα binding sites reflect the presence of physiologically functional receptors with a pharmacological profile consistent with that of the CGRPα receptor type 1 (CGRP1). When considered together with earlier studies on the same neuromuscular preparation, the present work further suggests that the motoneuron-dependent trophic control of acetylcholine receptors and acetylcholinesterase in skeletal muscle endplates is partly mediated by nerve-derived CGRPα activating specific receptors which are highly sensitive to the truncated peptide hCGRP8-37.