There are examples of physiological conditions under which thirst is inappropriately exaggerated, and the mechanisms for these paradoxical ingestive behaviors remain unknown. We are interested in thirst mechanisms across the female life cycle and have identified a novel mechanism through which ingestive behavior may be activated. We discovered a previously unrecognized endogenous hypothalamic peptide, phoenixin (PNX), identified physiologically relevant actions of the peptide in brain and pituitary gland to control reproductive hormone secretion in female rodents, and in the process identified the previously orphaned G protein-coupled receptor Gpr173 to be a potential receptor for the peptide. Labeled PNX binding distribution in brain parallels areas known to be important in ingestive behaviors as well in areas where gonadal steroids feedback to control estrous cyclicity (Stein LM, Tullock CW, Mathews SK, Garcia-Galiano D, Elias CF, Samson WK, Yosten GLC, Am J Physiol Regul Integr Comp Physiol 311: R489–R496, 2016). We have demonstrated upregulation of Gpr173 during puberty, fluctuations across the estrous cycle, and, importantly, upregulation during the last third of gestation. It is during this hypervolemic, hyponatremic state that both vasopressin secretion and thirst are inappropriately elevated in humans. Here, we show that central administration of PNX stimulated water drinking in both males and females under ad libitum conditions, increased water drinking after overnight fluid deprivation, and increased both water and 1.5% NaCl ingestion under fed and hydrated conditions. Importantly, losartan pretreatment blocked the effect of PNX on water drinking, and knockdown of Gpr173 by use of short interfering RNA constructs significantly attenuated water drinking in response to overnight fluid deprivation. These actions, together with the stimulatory action of PNX on vasopressin secretion, suggest that this recently discovered neuropeptide may impact the recruitment of critically important neural circuits through which ingestive behaviors and endocrine mechanisms that maintain fluid and electrolyte homeostasis are regulated.

Recent work identified Gpr160 as a candidate receptor for cocaine- and amphetamine-regulated transcript peptide (CARTp) and described its role in pain modulation. The aims of the present study were to determine if Gpr160 is required for the CARTp’s ability to reduce food intake and water intake and to initially identify the distribution of Gpr160-like immunoreactivity (Gpr160ir) in the rat brain. A passive immunoneutralization approach targeting Gpr160 was used to block the behavioral effects of a pharmacological dose of CARTp in the fourth cerebroventricle (4V) of rats and to determine the importance of endogenously produced CARTp in the control of ingestive behaviors. Passive immunoneutralization of Gpr160 in the 4V blocked the actions of CARTp to inhibit food intake and water intake. Blockade of Gpr160 in the 4V, independent of pharmacological CART treatment, caused an increase in both overnight food intake and water intake. The decrease in food intake, but not water intake, caused by central injection of CARTp was demonstrated to be interrupted by prior administration of a glucagon-like peptide 1 (GLP-1) receptor antagonist. Gpr160ir was observed in several, distinct sites throughout the rat brain, where CARTp staining has been described. Importantly, Gpr160ir was observed to be present in both neuronal and nonneuronal cell types. These data support the hypothesis that Gpr160 is required for the anorexigenic actions of central CARTp injection and extend these findings to water drinking. Gpr160ir was observed in both neuronal and nonneuronal cell types in regions known to be important in the multiple pharmacological effects of CARTp, identifying those areas as targets for future compromise of function studies.