Multiple affinity and guanine nucleotide sensitive forms of the calcitonin gene related peptide (CGRP) receptor

Calcitonin gene related peptide (CGRP) is a novel neuropeptide with an impressive array of biological actions consistent with its diverse tissue distribution and suggested role as neurotransmitter or neuromodulator. Binding sites for CGRP with properties consistent with those of receptors are present in both central and peripheral tissues. Radioligand binding studies were performed to investigate the fundamental processes underlying CGRP receptor activation and signaling following agonist occupancy of the receptor. These studies documented the existence of a selective, high affinity, and homogeneous population of binding sites for CGRP in membranes prepared from central and various peripheral tissues. The affinity of [¹²⁵I]CGRP for these sites was regulated by GTP or its stable analog GTPγS, indicating coupling of CGRP receptors to G-protein(s). Kinetic studies documented the existence of the CGRP receptor in multiple affinity states when both coupled to and uncoupled from G-proteins(s). These findings suggest that CGRP occupancy of its receptor induces conformational changes in the receptor that may be involved in its coupling to G-proteins and that the resulting ligand-receptor-G-protein ternary complex exists in multiple affinity conformational states. It seems likely that the multiple affinity states of the CGRP receptor ternary complex are involved differentially in signaling by and desensitization of the receptor. This evidence for agonist-induced conformational changes in a G-protein-coupled receptor prior to its coupling with G-protein(s) and for the existence of the ligand-receptor-G-protein ternary complex in multiple affinity conformational states is novel and extends our current understanding of the nature of the processes involved in agonist-dependent activation of G-protein-coupled receptors.