Side effects of control treatment can conceal experimental data when studying stress responses to injection and psychological stress in mice.

Routine laboratory procedures, such as handling or transporting animals or carrying out injections on animals, are stressful for animals but are necessary in many pre-clinical studies. Here, the authors show that multiple injections of the non-toxic vehicle cyclodextrin moderately increased plasma corticosterone concentrations in female BALB/c mice. Additionally, male BALB/c mice that had received a single intraperitoneal injection of harmless saline had an increased glucocorticoid response to a second saline injection. The authors found that female mice that had been exposed to an acute psychological stress session had a decreased glucocorticoid response to a second homotypic stressor. In contrast, multiple psychological stress sessions led to increased glucocorticoid release in female mice. Acute injection(s) of saline in male mice and of cyclodextrin in female mice led to transient lymphocytopenia. Further analysis showed that repeated stress-induced lymphocytopenia is glucocorticoid-dependent. The authors conclude that laboratory stress can affect physiological parameters in mice, potentially altering study results.