8 Comparison of oxytocin and pro-inflammatory cytokines in saliva and plasma in the post-partum mare as potential biomarkers of well-being

Abstract

Although evaluating cortisol concentrations is a common method to assess stress in many species including horses, interpretation of results in these studies has proven challenging. Therefore, researchers are investigating alternative analytes as biomarkers of well-being. Historically thought to serve primary roles in parturition and lactation, oxytocin (OT) has been shown to act as a neurotransmitter to promote prosocial behavior and buffer stress. Pro-inflammatory cytokines (PICs) have been shown in humans to influence production of neurotransmitters such as serotonin and dopamine, implicating their potential role in psychosocial stress. Analyte levels are often measured in blood, but this sampling procedure can result in acute stress responses that may influence experimental results and conclusions. Evaluating analytes in saliva can serve as a less-invasive method to assess biomarkers of well-being. The purpose of this study was to determine the feasibility of measuring OT and PICs in saliva as an alternative to plasma sampling in horses. As studies have demonstrated that OT and PICs are elevated during parturition and lactation, 4 postpartum Quarter Horse mares (15.0 ± 0.82 yr) were chosen for this study. Plasma (P) and saliva (S) samples were collected 30 min post-foaling (P1, S1) and 30 min post first suckling (P2, S2). Samples were analyzedfor OT, IL1-β, IL-8, IL-6, and TNF-α via electrochemiluminescent (ECL) assay (Salimetrics). Data were analyzed using the PROC GLM and PROC CORR in SAS v 9.4. Mean analyte concentrations, with the exception of IL-8, differed by sample type (S OT = 45.16 ± 36.65 pg/mL, P OT = 96.10 ± 41.0 pg/mL, P < 0.05; S IL1-β = 13.98 ± 2.03 pg/mL, P IL1- β = 1.01 ± 0.18 pg/mL, P < 0.01; S IL-6 = 0.35 ± 0.14 pg/mL, P IL-6 = 1.32 ± 0.46 pg/mL, P < 0.01, S TNF-α = 0.36 ± 0.16 pg/mL, P TNF- α = 3.40 ± 0.86 pg/mL, P < 0.01). There were no differences in analyte concentrations due to time point. Though not significant (P > 0.5), time point comparisons across sample type showed moderate to strong Pearson correlations among OT (S1-P1; r = −0.56; S2-P2; r = −0.47), IL-8 (S1-P1; r = −0.69), IL-6 (S1-P1; r = 0.82), and TNF-α (S1-P1; r = 0.48). The established roles of OT and PICs in prosocial behavior and psychosocial stress suggest that measurement of these analytes may provide a targeted approach to assess biopsychosocial responses. While this assay has not yet been validated for use in horses, this study illustrates that PICs can be quantified in equine saliva which supports this less-invasive approach for studying biomarkers of well-being in horses.