The neuropeptide hormone oxytocin (OT) is released peripherally and centrally and has been implicated in both physiology and behavior, especially sociosexual behaviors. Knowledge of OT levels in blood or other sources would be useful but these are rarely reported. Radioimmunoassay following extraction is the most commonly used method for measuring OT but is not ideal for use in small mammals in which blood volumes and concentrations of OT are low. Here we report a chemical and biological validation for a commercially available enzyme immunoassay for OT in unextracted plasma. In addition, comparisons of OT were made across species to allow comparison of the monogamous prairie vole (Microtus ochrogaster (Wagner, 1842)) to the polygynous Sprague Dawley rat. These species were chosen because OT plays a role in the formation of social bonds and we predicted that the highly social prairie vole would have higher plasma OT than the less social rat. Results of this comparison confirmed our hypothesis. Further, OT was significantly higher in females than in males in both species. Our results indicate that this enzyme immunoassay can be used to assay plasma OT in rodents and that the predicted correlations exist between plasma OT and gender as well as species-typical social behavior.

Aging of the reproductive system has been studied in numerous vertebrate species. Although there are wide variations in reproductive strategies and hormone cycle components, many of the fundamental changes that occur during aging are similar. Evolutionary hypotheses attempt to explain why menopause occurs, whereas cellular hypotheses attempt to explain how it occurs. It is commonly believed that a disruption in the hypothalamic-pituitary-gonadal axis is responsible for the onset of menopause. Data exist to demonstrate that the first signs of menopause occur at the level of the brain or the ovary. Thus, finding an appropriate and representative animal model is especially important for the advancement of menopause research. In primates, there is a gradual decline in the function of the hypothalamic-pituitary-gonadal (HPG) axis ultimately resulting in irregularities in menstrual cycles and increasingly sporadic incidence of ovulation. Rodents also exhibit a progressive deterioration in HPG axis function; however, they also experience a period of constant estrus accompanied by intermittent ovulations, reduced progesterone levels, and elevated circulating estradiol levels. It is remarkable to observe that females of other classes also demonstrate deterioration in HPG axis function and ovarian failure. Comparisons of aging in various taxa provide insight into fundamental biological mechanisms of aging that could underlie reproductive decline.

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