Abstract Body: Background: A damaged stroke area can affect both local and connected brain regions, leading to network-wide disruptions in brain functions. Increasing evidence shows that estrogen, particularly 17β-estradiol, plays a protective role after ischemic stroke. Understanding how hormonal levels impact brain stimulation may shed light on varying responses to neuromodulation therapies across estrous cycle phases. Previously, we demonstrated that optogenetic stimulations in the ipsilesional primary motor cortex (iM1) enhance functional recovery in male mice. In this study, we aim to expand upon these findings by exploring functional recovery and post-stroke neuroplasticity changes in in female mice.

Methods: Female C57BL/6 mice (7-9 weeks old) underwent stereotaxic surgery to express channelrhodopsins-2 and implant an optical fiber in iM1. Four weeks later, transient middle cerebral artery occlusion (MCAO) was induced. Optogenetic stimulation was initiated on day 5 post-stroke and continued for 10 days. Estrous cycles were monitored via vaginal swabbing and estradiol ELISA measurements. Rotating beam tests were performed at pre-stroke, 4, 7, 10, and 14 days post-stroke (PD4, PD7, PD10, and PD14). Infarct size, astrocytic activation and microglia/macrophage activation were assessed.

Results: Rotating beam tests showed that 71.4% of the stimulated mice significantly recovered by PD10 (p<0.01). We observed irregular hormonal cycles post-stroke, with the diestrus phase noticeably longer in majority of mice. Stimulated mice exhibit a trend toward better recovery when in the diestrus/metestrus phase at the start of stimulation on PD5. Optogenetic stimulation appeared to reset the hormonal cycles, though some irregularities remained during stimulation period. Additionally, recovery varied during estrous cycle stages, with optogenetic stimulation enhancing motor function during specific phases. At PD10, improved recovery was significantly correlated with low estradiol levels (R² = 0.61, p<0.05), and most recovered mice were in the estrus phase by PD14.

Conclusions: These results highlight the importance of considering hormonal cycles in post-stroke brain stimulations. Ongoing studies are investigating the effects of stimulation on hormonal cycles in relation to post-stroke plasticity. Our data suggest that tailoring brain stimulations to specific estrous hormonal phases may optimize recovery outcomes.