Supplementary Material for: Changes in Visual Long-term Potentiation Show Preserved Cyclicity in Human Females Taking Combined Oral Contraceptives

Introduction: The combined oral contraceptive pill (COC) is often employed to address physical and neurological symptoms in menstrual cycle-related disorders by suppressing shifts in endogenous gonadal hormone fluctuations. Symptom persistence, especially in the lead up to the hormone free interval (HFI) suggests an underlying neurobiological mechanism of preserved cycling. Our study utilised a non-invasive method of visually inducing long-term potentiation (LTP) to index changes in neural plasticity in the absence of hormonal fluctuations. Methods: Visually induced LTP was recorded using electroencephalography in 24 healthy female COC users across three sessions; day 3 and 21 during active hormone pills, and day 24 during the HFI. The Daily Record of the Severity of Problems (DRSP) questionnaire tracked premenstrual symptoms. Dynamic causal modelling (DCM) was used to elucidate the neural connectivity and receptor activity changes associated with LTP across different days of COC. Results: Visually induced LTP was greater on day 21 than day 3 (p=0.011) and was localised to the P2 visually evoked potential. There was no effect of the HFI (day 24) on LTP. DCM of differences between day 3 and 21 showed changes to inhibitory interneuronal gating of LTP in cortical layer VI. The DRSP only showed a significant increase in symptoms in the HFI, meaning the LTP result appeared more sensitive to cyclicity. Conclusions: This study provides objective evidence of preserved cyclicity in COC users through enhanced LTP on day 21 compared to day 3 of a 28-day COC regimen, indicating that relatively higher excitation in the brain despite peripheral gonadal suppression may underlie and exacerbate menstrual cycle-related disorders.

引言：复方口服避孕药（combined oral contraceptive pill, COC）常通过抑制内源性性腺激素波动，用于缓解月经周期相关疾病的躯体及神经系统症状。若症状持续存在，尤其是在无激素间期（hormone free interval, HFI）前出现的症状，则提示存在保留周期节律的潜在神经生物学机制。本研究采用非侵入性视觉诱导长时程增强（long-term potentiation, LTP）的方法，在无激素波动的状态下量化神经可塑性的变化。
方法：本研究对24名健康的复方口服避孕药使用者开展3次脑电图记录，分别为活性激素服药期的第3天、第21天，以及无激素间期的第24天。采用《症状严重程度每日记录表》（Daily Record of the Severity of Problems, DRSP）追踪经前期症状。通过动态因果模型（Dynamic causal modelling, DCM）解析不同复方口服避孕药给药日与长时程增强相关的神经连接及受体活性变化。
结果：视觉诱导的长时程增强在第21天显著强于第3天（p=0.011），且定位于P2视觉诱发电位。无激素间期对长时程增强无显著影响。对比第3天与第21天的动态因果模型分析显示，大脑皮层第六层的抑制性中间神经元对长时程增强的门控机制发生改变。症状严重程度每日记录表仅在无激素间期出现症状显著升高，提示长时程增强检测对周期节律的敏感性更高。
结论：本研究通过对比28天复方口服避孕药给药方案中第21天与第3天的长时程增强水平，为复方口服避孕药使用者存在保留的周期节律提供了客观证据，表明尽管外周性腺轴已被抑制，但大脑内相对更高的兴奋水平可能是月经周期相关疾病的潜在诱因并加重相关症状。