Social Peptides: Measuring Urinary Oxytocin and Vasopressin in a Home Field Study of Older Adults at Risk for Dehydration

Oxytocin (OT) and vasopressin (AVP) are two closely related peptides with well-known physiological functions, including contractile and antidiuretic properties. They play a key role in regulating blood pressure and in facilitating the allostatic response to external stressors (Gimpl & Fahrenholz, 2001; Szczepanska-Sadowska, 2008). Importantly, for the National Social Life and Aging Project (NSHAP), both of these hormones, but in particular OT, have also been shown to play an important role in mediating and modulating social behaviors such as pair-bonding, attachment, sexuality, and care-taking (Reyes & Mateo, 2008; Wismer Fries, Ziegler, Kurian, Jacoris, & Pollak, 2005). The social role of OT and AVP in humans has been examined primarily in small convenience samples due to the special challenges associated with protein hormone measurement. There is no single ideal experimental protocol for discerning the relationships between these neuropeptides, physiology, and behavior among the everyday lives of people living in their homes. Here, we present a method for gathering data from a large representative population of community-dwelling older adults living in the United States. For reasons detailed below, this is a conservative method, risking failing to detect associations between social peptides, health, and social life. Thus, if analysts do detect such relationships, their work will call for further study in more controlled laboratory settings designed to reveal underlying mechanisms. Studies of central action of these social peptides, including brain binding and production, require highly invasive measurements of cerebrospinal fluid or post-mortem brain analysis. Animal models are optimal. Novel radiolabeled ligands in positron emission tomography could be developed. In these approaches, the neuroendocrinology is sophisticated and focus on where the peptides are having their immediate effects, but the behavior and physiology is far removed from the natural variation of everyday life and the human subjects who participate are not representative of the larger population. Measuring peripheral levels of OT and AVP affords better behavioral measurement, yet does not directly measure at the site of action. OT and AVP are released from the posterior pituitary into the blood stream, from where they diffuse in saliva or pool in urine. There is evidence, albeit currently debated, that peripheral release is coordinated with central release of OT and that peripheral measures correlate to endocrine expression in the brain, although this is not necessarily the case for AVP (Carter et al., 2007; Wotjak et al., 1998). Moreover, peripheral measures pose their own challenges. Neuropeptides are delicate hormones with a short half-life. Blood draws provide both the most acute and most invasive peripheral measurement, and the sample must be immediately treated with an anticoagulant, centrifuged cold, and frozen for storage, impractical in our large survey with field interviewers who are not medical personnel. Saliva is perhaps the least invasive measurement, but quantities of OT are very low, provide the most challenging results, and great care must be taken to immediately freeze the sample to protect the peptides. Although saliva reflects OT released across several minutes, it correlates closely to OT measured in plasma (Hoffman, Brownley, Hamer, & Bulik, 2012). Urine provides a biological sample that integrates neuropeptide concentrations pooled across a longer time-span of at least 1hr, the time frame of our home interview. Urine is less invasive than blood draws thus lowering the barrier to participation and has less challenging handling requirements since the pH environment of the urine helps to stabilize the protein (Anestis, 2010). Urinary OT and AVP have been successfully used to measure effects of social interactions on peptide secretion in laboratory settings (Moses & Steciak, 1986; Polito, Goldstein, Sanchez, Cool, & Morris, 2006; Seltzer & Ziegler, 2007; Seltzer, Ziegler, & Pollak, 2010). Urine specimens are therefore a useful noninvasive method of gathering biomeasures during a field study of psychosocial measures and health, especially large-scale population surveys in the home. However, the short half-life, challenging storage requirements, including immediate and sustained low temperatures pose special challenges for handling in the field. A method to measure OT and AVP in large populations in the field is needed to test and advance the understanding of the social and health properties of these peptides. Levels of any urinary hormone must be standardized based on creatinine levels to control for variation in urine concentration. Creatinine is secreted at a steady rate from creatine degradation in muscle tissue, and its levels directly reflect urine concentration (Barr et al., 2004; Cuthbertson, 1944). In addition to altered kidney function, the thirst response decreases as individuals age, leading to reduced fluid consumption and greater risk of dehydration among older adults. For example, dehydration is a leading cause of hospitalization for individuals older than 65 years (Lavizzo-Mourey, 1987; Sheehy, Perry, & Cromwell, 1999). We describe here a field method for collecting urine pooled during highly standardized home interviews of older adults and measuring urinary OT, AVP, and creatinine. In addition to standardizing urine concentration, we show that creatinine, a marker of mild dehydration in this home dwelling population, can be used to solve issues of assay interference. Finally, we determined the effects of two freeze-thaw cycles on the assayed levels of these peptides.