Relaxin-3 is a neuropeptide that is implicated in the regulation of stress responses and memory. The elucidation of its precise physiological role(s) has, however, been hampered by cross-activation of the relaxin-2 receptor, RXFP1, in the brain. The current study undertook to develop analogues of human relaxin-3 (H3 relaxin) that can selectively bind and activate its receptor, RXFP3. We developed a high-affinity selective agonist (analogue 2) by removal of the intra-A chain disulfide bond and deletion of 10 residues from the N terminus of the A chain. Further truncation of this analogue from the C terminus of the B chain to Cys(B22) and addition of an Arg(B23) led to a high-affinity, RXFP3-selective, competitive antagonist (analogue 3). Central administration of analogue 2 in rats increased food intake, which was blocked by prior coadministration of analogue 3. These novel RXFP3-selective peptides represent valuable pharmacological tools to study the physiological roles of H3 relaxin/RXFP3 systems in the brain and important leads for the development of novel compounds for the treatment of affective and cognitive disorders.

Brain regions that regulate fluid satiation are not well characterized, yet are essential for understanding fluid homeostasis. We found that oxytocin-receptor-expressing neurons in the parabrachial nucleus of mice (OxtrPBN neurons) are key regulators of fluid satiation. Chemogenetic activation of OxtrPBN neurons robustly suppressed noncaloric fluid intake, but did not decrease food intake after fasting or salt intake following salt depletion; inactivation increased saline intake after dehydration and hypertonic saline injection. Under physiological conditions, OxtrPBN neurons were activated by fluid satiation and hypertonic saline injection. OxtrPBN neurons were directly innervated by oxytocin neurons in the paraventricular hypothalamus (OxtPVH neurons), which mildly attenuated fluid intake. Activation of neurons in the nucleus of the solitary tract substantially suppressed fluid intake and activated OxtrPBN neurons. Our results suggest that OxtrPBN neurons act as a key node in the fluid satiation neurocircuitry, which acts to decrease water and/or saline intake to prevent or attenuate hypervolemia and hypernatremia.