Measuring peripheral oxytocin and vasopressin in nonhuman primates

Studying the neural and hormonal changes that modulate behavior is critical to understanding social relationships. Of particular interest is measuring oxytocin (OT) and arginine vasopressin (AVP) peripherally, and preferably, non-invasively, in nonhuman primates. Due to these peptides' neural origin and their stimulation of brain areas that influence social behavior, there has been debate whether peripheral measures in blood, urine, and saliva reflect central levels in the brain. This review elucidates the challenges of OT measurement and the solutions that provide valuable data on OT's role in social behavior. This review discusses the recent studies in rhesus macaques which indicate that exogenous OT delivered by nasal spray results in increased OT in cerebrospinal fluid, and it notes the new methodologies that can measure both endogenous and exogenous OT simultaneously, which thereby determine the source of measured OT in biological fluids. Next, this review highlights the utility of measuring urinary OT by summarizing the results of clearance rate studies in humans and marmosets, which characterize the timing that circulating OT enters urine and illustrate that endogenous releasers of OT also increase urinary OT. With the ability to reliably measure OT and AVP in urine and in blood, we can now study free-ranging captive, and non-captive primates to answer questions about the biology of social bonding that were not possible before. One procedural concern that this review also highlights is whether extraction of the peptides prior to assay is needed, as the values are higher in samples that have not been extracted. Studies indicate that extractions eliminate the interfering compounds that cause higher values. Across studies, to ensure the reliability of measuring OT for nonhuman primates, this review makes suggestions based on empirical evidence for how to correctly preserve samples and emphasizes the need to validate each assay for individual species.