Brief anesthesia by isoflurane alters plasma corticosterone levels distinctly in male and female rats: Implications for tissue collection methods.

Euthanasia by anesthetic agents is commonly performed prior to tissue collection in order to minimize pain and distress to the animal. However, depending on their mechanism of action as well as administration regimen, different methods of anesthesia may trigger an acute stress response through engaging the hypothalamic-pituitary-adrenal (HPA) axis, which can impact numerous other physiological processes that the researcher may wish to examine as endpoints. We investigated the effects of the commonly used anesthetic agent isoflurane on two different endpoints related to the stress response: plasma corticosterone levels and gene expression of the glucocorticoid receptor (GR) as well as several of its regulators including FK506-binding protein 51 (Fkbp5) in the hippocampus of male and female rats. Our results indicate that brief exposure to anesthesia by isoflurane prior to decapitation can alter plasma corticosterone levels differentially in male and female rats within minutes without impacting gene expression in the hippocampus. We conclude that collection methods can influence stress-related physiological endpoints in female rats and the potential influence of even brief anesthesia as well as sex differences in response to anesthesia should be evaluated during the experimental design process and data interpretation. This finding is particularly important in light of new NIH standards regarding sex and reproducibility, and care should be taken to be certain that sex differences in endpoints of interest are not an artifact of sex differences in response to collection paradigms.