Involvement of central relaxin-3 signalling in sodium (salt) appetite

What is the central question of this study? Sodium appetite is controlled by conserved neuronal transmitter-receptor systems. Here, we tested the contribution made by relaxin family peptide 3 receptor (RXFP3), the cognate G-protein-coupled receptor for the neuropeptide relaxin-3. What is the main finding and its importance? Intracerebroventricular infusion of an RXFP3 antagonist reduced in a dose-dependent manner the volume of 0.3 m NaCl consumed by sodium-depleted C57Bl/6J (wild-type) mice. This effect was absent in sodium-depleted Rxfp3 knockout mice, and RXFP3 antagonist infusion did not alter water consumption in wild-type mice subjected to multiple thirst tests, indicating both the pharmacological and the physiological specificity of observed effects. Our findings identify endogenous relaxin-3-RXFP3 signalling as a modulator of sodium appetite. Overconsumption of highly salted foods is common in Western diets and contributes significantly to metabolic disorders such as hypertension, renal dysfunction and diabetes. Sodium appetite, or the desire of terrestrial animals to seek and consume sodium-containing salts, is a behaviour mediated by a set of evolutionarily conserved neuronal systems. In these studies, we tested whether this instinctive behavioural drive is influenced by the G-protein-coupled relaxin family peptide 3 receptor (RXFP3), the cognate receptor for the neuropeptide relaxin-3, because relaxin-3-RXFP3 signalling can modulate arousal, motivation and ingestive behaviours. Intracerebroventricular (i.c.v.) infusion of the selective RXFP3 antagonist, R3(B1-22)R, reduced in a dose-dependent manner the volume of 0.3 m NaCl solution consumed when offered to sodium-depleted C57Bl/6J wild-type mice, relative to vehicle-treated control animals. Notably, i.c.v. R3(B1-22)R infusion did not alter 0.3 m NaCl consumption relative to vehicle in sodium-depleted Rxfp3 knockout mice, confirming the pharmacological specificity of this effect. Furthermore, i.c.v. R3(B1-22)R did not alter the volume of water consumed by wild-type mice in three tests where water drinking was the normal physiological response, suggesting that the ability of R3(B1-22)R to reduce activated salt appetite is specific and not due to a generalized reduction in drinking behaviour. These findings identify, for the first time, that endogenous relaxin-3-RXFP3 signalling is a powerful mediator of salt appetite in mice and further elucidate the functional role of the relaxin-3-RXFP3 system in the integrative control of motivated behaviours.