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.