Plasma oxytocin measured by LC-MS/MS varies with life stage, sex, and obesity in mice

ObjectiveOxytocin (Oxt) assessment in plasma is challenging, and available data are contradictory. We aimed to assess circulating Oxt in mice by a validated nano-liquid chromatography/mass-spectrometry (nLC-MS/MS) protocol, combined with Oxt hypothalamic expression in different sex, life stages, and in diet-induced obesity. MethodsWe assessed plasma Oxt by nLC-MS/MS, Oxt hypothalamic expression by qPCR, and Oxt-immunoreactive neuron and fiber densities by immunohistochemistry and morphometric analyses in C57BL/6 mice at 21 and 60 days of life (p21 and p60, respectively). Mice in normo-fed condition and following 12 weeks of high-fat diet (HFD) were studied alongside food intake and hypothalamic expression of its regulators. ResultsCirculating Oxt does not vary based on sex at p21 and p60 but increases with aging. While hypothalamic Oxt mRNA expression followed the same trend across both sexes, Oxt neuron and fiber densities exhibited a similar trend only in females. Plasma vasopressin (Avp) followed Oxt trend in females but was opposite in males and was not mirrored by Avp mRNA hypothalamic expression. HFD-fed females were more resistant to weight gain compared to males and displayed higher Oxt plasma levels and hypothalamic expression. Sex dimorphism in food intake and hypothalamic expression of Avp and of key anorexigenic and orexigenic neuropeptides was detected. ConclusionsOxt plasma levels are higher in adulthood compared to weaning in mice of both sexes who displayed similar concentrations. Oxt plasma levels are mirrored by Oxt hypothalamic expression. In obesity, females display a lower increase in body weight but higher Oxt plasma levels than males. SignificanceThis study delineates the pattern of Oxt plasma levels across various age, sex and in obesity in mice. We overcome some limitations of previous research, which were often confounded by numerous factors, by reliably measuring circulating Oxt and integrating this with parallel analyses of Oxt gene expression, and Oxt neuron and immunoreactive fiber densities. Critically, this is among the few studies to employ LC-MS/MS specifically in mice. This novel, integrated perspective is relevant given the compelling evidence for Oxts role as an anti-obesity agent and its potential as a therapeutic target. A precise understanding of Oxt regulation in different conditions, backed by reliable methodologies, is indispensable for the rational development of effective treatments for obesity and related endocrine diseases.