Mild environmental stress inoculation promotes resilience to anxiety in female rats.

Stress inoculation refers to exposure to mild stressors that promote resilience to future stress. However, the optimal intensity and nature of prior stress required for this effect remain unclear. In this study, we developed a novel stress paradigm, environmental chronic mild stress (ECMS), by modifying the chronic mild stress model and evaluated its stress inoculation effects on behavioural and physiological stress responses in female rats. Under ECMS, rats were exposed to multiple environments that reduced the comfort of their living conditions without inducing hunger, thirst, or direct physical distress. We also examined the interaction between stress inoculation and intake of an antidepressive bacterium, Lactococcus cremoris H61. Both 8 weeks of ECMS exposure and H61 supplementation increased time spent in the center area of the open field. Furthermore, ECMS increased total distance travelled and decreased self-grooming frequency under anxiety-inducing conditions. Although ECMS elevated serum corticosterone levels, this effect was mitigated by long-term treatment with strain H61. These findings suggest that ECMS exposure in female rats is an effective stress inoculation model for anxiety. Although ECMS overactivated the hypothalamic-pituitary-adrenal axis, its combination with strain H61 produced a complementary effect, enabling stress resilience without added physiological burden.