Central Neuropeptide Receptors Involved in Water Balance: Application to Apelin

Because G-protein-coupled receptors (GPCRs) constitute excellent putative therapeutic targets, functional characterization of orphan GPCRs through identification of their endogenous ligands has great potential for drug discovery. In an attempt to identify a receptor specific for angiotensin III, we have cloned, by homology from a rat brain cDNA library, a GPCR that shares 90% amino acid sequence identity with the human orphan APJ (putative receptor protein related to the angiotensin receptor AT1) receptor and 31% with the rat AT1A angiotensin receptor. In 1998, the endogenous ligand for the human orphan APJ receptor, i.e., apelin, was isolated from bovine stomach extracts. Apelin, a bioactive peptide, naturally occurs in the brain and plasma as 13 (pE13F) and 17 amino acid (K17F) fragments of a 77 amino acid precursor. The APJ receptor binds with high affinity K17F and pE13F but not the shorter N-terminal-deleted apelin fragments. This receptor is negatively coupled to adenylate cyclase and internalizes following stimulation with K17F and pE13F. Apelin and its receptor are both widely distributed in the brain and are highly expressed in the supraoptic and paraventricular hypothalamic nuclei. Dual labeling studies demonstrate that, within these two types of nuclei, apelin and its receptor co-localize with vasopressin (AVP) in magnocellular neurons. In lactating rodents, characterized by increases in synthesis and release of AVP, central injection of apelin inhibits the phasic electrical activity of AVP neurons and reduces the secretion of AVP in the bloodstream, resulting in aqueous diuresis. Apelin may thus be considered as a natural inhibitor of the anti-diuretic effect of AVP. Moreover, water deprivation, which increases systemic AVP release, decreases plasma apelin concentrations and induces apelin storage inside magnocellular neurons, thereby avoiding the inhibitory action of apelin on AVP release. Thus apelin and AVP are conversely regulated to optimize systemic AVP release and prevent additional water loss at the kidney level.