Abstract
Non-invasive measurements of glucocorticoid (GC) concentrations, including cortisol and corticosterone, serve as reliable indicators of adrenocortical activities and physiological stress loads in a variety of species. As an alternative to invasive analyses based on plasma, GC concentrations in saliva still represent single-point-of-time measurements, suitable for studying short-term or acute stress responses, whereas fecal GC metabolites (FGMs) reflect overall stress loads and stress responses after a species-specific time frame in the long-term. In our study species, the domestic guinea pig, GC measurements are commonly used to indicate stress responses to different environmental conditions, but the biological relevance of non-invasive measurements is widely unknown. We therefore established an experimental protocol based on the animals’ natural stress responses to different environmental conditions and compared GC levels in plasma, saliva, and fecal samples during non-stressful social isolations and stressful two-hour social confrontations with unfamiliar individuals. Plasma and saliva cortisol concentrations were significantly increased directly after the social confrontations, and plasma and saliva cortisol levels were strongly correlated. This demonstrates a high biological relevance of GC measurements in saliva. FGM levels measured 20 h afterwards, representing the reported mean gut passage time based on physiological validations, revealed that the overall stress load was not affected by the confrontations, but also no relations to plasma cortisol levels were detected. We therefore measured FGMs in two-hour intervals for 24 h after another social confrontation and detected significantly increased levels after four to twelve hours, reaching peak concentrations already after six hours. Our findings confirm that non-invasive GC measurements in guinea pigs are highly biologically relevant in indicating physiological stress responses compared to circulating levels in plasma in the short- and long-term. Our approach also underlines the importance of detailed investigations on how to use and interpret non-invasive measurements, including the determination of appropriate time points for sample collections.
Highlights
Non-invasive measurements of steroid hormones reliably reflect endocrine states and have become common tools in biological and biomedical researchHow to cite this article Nemeth et al (2016), Non-invasive cortisol measurements as indicators of physiological stress responses in guinea pigs
Concentrations of fecal GC metabolites (FGMs) measured in the +20 h samples showed significant differences between and within the experimental conditions (Fig. 1C) and were generally lower during the three days of social confrontations compared to the social isolation period
Saliva cortisol and FGM levels measured in samples adjusted to the appropriate gut passage time were both significantly increased in response to the social confrontations and were highly correlated to the actual circulating cortisol levels in plasma
Summary
Non-invasive measurements of steroid hormones reliably reflect endocrine states and have become common tools in biological and biomedical researchHow to cite this article Nemeth et al (2016), Non-invasive cortisol measurements as indicators of physiological stress responses in guinea pigs. Glucocorticoids (GCs), including cortisol and corticosterone, measured non-invasively in saliva or fecal samples can reliably indicate an individual’s hypothalamic-pituitary-adrenal (HPA) axis activity and physiological stress responses (Sheriff et al, 2011; Cook, 2012). Beyond the importance of careful physiological and biological validations of non-invasive GC measurement techniques (Goymann, 2012; Touma & Palme, 2005; Cook, 2012), relations to the biologically active circulating concentrations in plasma should be tested to determine their biological relevance. If non-invasively measured GC levels predict and resemble those in plasma, they can be used to reliably indicate the strength of physiological stress responses and related, well-described physiological actions of GCs (Munck, Guyre & Holbrook, 1984; Blanchard, McKittrick & Blanchard, 2001; McEwen, 2008)
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